One-dimensional titanium dioxide nanomaterials: nanotubes. - PDF Download Free (2024)

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One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes Kiyoung Lee,† Anca Mazare,† and Patrik Schmuki* Department of Materials Science WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany 5. Modification of TiO2 Nanotubes 5.1. Doping of Anodic TiO2 Nanotubes 5.2. Doping of Hydrothermal Tubes 5.3. Self-Doping/Magneli Phases/Black Titania 5.3.1. Self-Doping 5.3.2. Magneli Phases 5.3.3. Black Titania 5.4. Conversion of Tubes 5.5. Particle Decoration, Heterojunctions, Charge Transfer Catalysis 5.6. Self-Assembled Monolayers (SAMs) 5.6.1. Wettability 6. Applications of TiO2 Nanotubes 6.1. Dye-Sensitized Solar Cells 6.1.1. Dye-Sensitized Solar Cells with 1-D Nanostructures 6.1.2. Dye-Sensitized Solar Cells with SelfOrganized TiO2 Nanotubes 6.1.3. Annealing Effects 6.1.4. Geometry Effects (Tube Length, Diameter, Wall Thickness, and Corrugation) 6.1.5. Front-Side DSSCs 6.1.6. Doped TiO2 Nanotubes for DSSCs 6.2. Photocatalysis 6.2.1. Some Key Factors for the Photocatalytic Efficiency of TiO2 Nanotubes 6.2.2. Annealing 6.2.3. TiO2 Tube Length, Diameter, and Type 6.2.4. Applied Voltage 6.2.5. Doping 6.2.6. Degradation of Pollutants and Undesired Biological Entities 6.2.7. Water Splitting 6.2.8. CO2 Reduction 6.2.9. Membranes 6.3. Ion-Intercalation (Insertion) Devices 6.3.1. Electrochromic Devices 6.3.2. Li-Ion Batteries 6.4. Sensors 6.4.1. Response and Sensitivity 6.4.2. Preparation of Sensing Devices from TiO2 Nanotube Arrays 6.4.3. Temperature Effects 6.4.4. Improved Sensing Performance 6.4.5. Free-Standing TiO2 Nanotube Arrays 6.4.6. Single TiO 2 Nanotubes as Sensing Devices

CONTENTS 1. Introduction 2. Growth Techniques for TiO2 Nanotubes 2.1. Overview 2.2. Deposition into/onto Templates 2.2.1. Anodic Aluminum Oxide (AAO) 2.2.2. Molecular or Molecular Assembly Templating 2.3. Titanium Oxide Nanotubes by Hydrothermal Reaction 2.4. Self-Organizing Anodic TiO2 Nanotube Arrays 2.5. Electrospinning 3. Ordered TiO2 Nanotube Arrays 3.1. Electrochemical Aspects of Anodic Growth of Nanotube Layers 3.2. Why Is a Tubular Shape Formed? 3.3. Factors Influencing the Morphology of the Anodic Film 3.4. Advanced Morphologies of Anodic Nanotubes 3.4.1. Bamboo, Branched Stacks, Multilayers 3.4.2. Free-Standing Membranes 3.5. Maximizing Ordering 3.6. Theoretical Considerations to Self-Organizing Anodization 4. Properties of TiO2 Nanotubes 4.1. Crystal Structure and Compositional Aspects 4.1.1. Hydrothermal Tubes 4.1.2. Anodic Tubes 4.1.3. Low Temperature Annealing 4.1.4. TEM Artifacts 4.2. Electronic and Optical Properties 4.2.1. Size Confinement 4.2.2. Conductivity 4.2.3. Defects, Ti3+, and Oxygen Vacancies 4.2.4. Electrochemistry, Photoelectroctrochemistry © 2014 American Chemical Society

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Special Issue: 2014 Titanium Dioxide Nanomaterials

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Received: February 10, 2014 Published: August 14, 2014 9385

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Chemical Reviews 6.5. Memristive Behavior 6.6. Supercapacitors 6.7. Biomedical Applications 6.7.1. TiO2 Nanotube Interactions with Cells 6.7.2. TiO2 Nanotubes for Improved Cell Interactions 6.7.3. Antibacterial Behavior 6.7.4. Drug Delivery and Release of Other Payloads 6.7.5. Hydroxyapatite Formation 7. Summary Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments References

Review

tremendous interest due to other specific advantages or features, including geometric factors such as surface area, size exclusion effects, defined diffusion behavior, biological interactions, or directional charge and ion transport properties. In this context, a most relevant geometric quality is provided by nanotubes that are produced as arrays, that is, in an aligned form perpendicular to the substrate. In this case, the directionality of the ensemble provides inherent advantages, for example, as large-scale oriented electrode in photoelectrochemistry (solar cells, photocatalysts) or as highly sizedefined bioactive coating. These aligned TiO2 nanotube arrays that can be grown by self-organizing anodization (or template filling) have created enormous interest as reflected in a vigorous publication output over the past few years. Figure 1 gives a

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1. INTRODUCTION In 1991, Iijima reported on the formation of carbon nanotubes1 using a simple arc discharge reaction that led to an arrangement of the material in some micrometer long tubes with a diameter of only a few nanometers; this turned out to be a milestone in materials science and technology.2−7 Only a few years later, a range of transition metal oxides8−17 were reported to form nanotubular arrangements when oxide powder was heated in hot alkaline solutions, either by simple refluxing or under mild pressure in an autoclave.8−17 Over the past decades, nanotubular geometries stimulated immerse research activity, which is evident from the over 100 000 papers published on “nanotubes” up to 2013. The main reason for this interest is, except for scientific curiosity, the anticipated economic impact in the form of applications that are based on specific physical and chemical features of these 1D (or extremely high aspect ratio) structures. Among the transition metal oxides, particularly the ability to produce the classic wide band gap semiconducting TiO2 in form of nanotubes found immediate interest, mainly due to the perspective of using the structures to enhance the properties in Grätzel-type solar cells18 and photocatalytic materials.19 These two photoelectrochemical applications made TiO2 the most studied functional oxide over the past 30 years. Nevertheless, other features of TiO2 (besides its classic use as a pigment), such as its excellent biocompatibility as well as ion intercalation properties, contribute largely to the high interest in this material. The first hydrothermal TiO2 nanotubes and carbon nanotubes have in common that they essentially consist of a rolled atomic or molecular plane with (in the ideal case) monomolecular layer thickness.20 These nanotube geometries may be considered (in a physical sense) as “true” nanotubes, as quantum size and dimensionality may fully be effective on physical and chemical properties, such as electron mobility, optical band gap, and surface reactivity.21−36 In contrast to hydrothermal (titanate) tubes that consist only of one or a few atom layer thick wall, a considerable range of synthesis techniques leads to high aspect ratio TiO2 tubular shapes that may be hundreds of micrometers long and some 10−1000 nm in diameter, but with a wall thickness that is typically in a range of ∼10−100 nm. Although in this case no considerable quantum size effects occur, these nanotubes attract

Figure 1. Research trend: Number of research article publications in the field of TiO2 nanotubes separated by different fabrication approaches (self-organizing anodization, hydrothermal synthesis, and other approaches) in the period 2000−2012 according to “Scopus (Elsevier)”.

comparison of the publication activities broken down according to tube type over the last 20 years. From this compilation, not only it is apparent that overall an almost exponential increase in work in this field can be observed, but also the fact that currently a vast majority of work deals with self-organized anodic TiO2 nanotube arrays. Except for work toward improved synthesis conditions to tailor geometry, structure, organization, or modification (doping, band gap engineering, decoration), virtually every application of TiO2 that has been based on nanoparticulate forms of titania is being examined using nanotubular geometries. Scientifically even more exciting are, of course, new aspects that arise from the specific geometry and its fine-tuning. Figure 2 gives schematically an overview over the most important (realized and anticipated) beneficial features of using TiO2 nanotubes or nanotube arrays. Classic 1D quantum size effects on electronic properties may lead to reduced electron scattering (or in an extreme case to ballistic transport). Also, extreme surface curvature may result in modification of chemical and physical properties. These effects may be exploited in virtually all electrical or photoelectrochemical arrangements (sensors, solar cells, photoreactors). The fact that diffusion length for minority carriers (holes) lies within the range of the tube wall thickness, and the comparably long electron lifetime in TiO2, allow orthogonal carrier separation (hole to the wall, electron to the back contact). Tube arrays enable core−shell structures (carrier separation) or interdigi9386

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Figure 2. Beneficial features of nanotubes: (a) size confinement (directional or ballistic charge transport), (b) atomically curved surface (modified chemical and physical properties), (c) electron−hole separation (higher efficiency of charge separation devices), (d) ion intercalation (low diffusion length in batteries, electrochromic devices), (e) p−n junction and core−shell structures (efficient charge separation), (f) harvesting functionality (for light absorption of DSSCs, chemical sensors), (g) interdigitated electrode structure (e.g., for memristive devices), and (h) small confined volumes with high observation length (e.g., high-sensitivity/low analyte volume sensing). Reproduced with permission from ref 50. Copyright 2010 WileyVCH Verlag GmbH & Co.

overview of the most explored and prospective applications of nanotubular TiO2.

tated electrode assemblies, as well as decoration of the walls with, for example, light harvesting or sensing elements, while keeping well-defined diffusion pathways for charge carriers or ion intercalation (ion insertion batteries). The definition of tight compartments, “nano-test-tubes”, combined with a high observation length provides platforms for low volume/high sensitivity sensing (e.g., high optical contrast in light absorption or fluorescence emission). Because of its high biocompatibility, TiO2 nanotubular structures are explored in various biomedical applications, such as nanosize defined biocompatible coatings (TiO2 is the prime coating material for biomedical implants) or drugdelivery devices. A number of excellent reviews have been written mainly on specific aspects of TiO2 nanotubes, such as dealing with hydrothermal tubes,16,17,37 anodic tube synthesis, applications and self-organization,38−44 or specific applications such as in solar cells,45,46 sensing,47 photocatalysis,48,49 or biomedical use.50,51 In this Review, we try to give a comprehensive and most up to date view of the field, with an emphasis on the currently most investigated anodic TiO2 nanotube arrays. We will first give an overview of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, and then deal with physical and chemical properties of TiO2 nanotubes and techniques to modify them. Finally, we will provide an

2. GROWTH TECHNIQUES FOR TiO2 NANOTUBES 2.1. Overview

Over the past 20 years, a considerable number of different strategies to synthesize TiO2 nanotubes or TiO2 nanotube arrays have been elaborated (Figure 3 provides an overview of typical morphologies and characteristic features of tubes synthesized by different approaches). Roughly one may divide the main routes into templating, hydrothermal, and anodic selforganization approaches. Templates to form tubular structures may be single high aspect-ratio molecules (such as cellulose), molecular rod-like assemblies (e.g., micelles), or defined organized nanostructures (such as ordered porous alumina or track-etch membranes).20,52−62 These templates then are coated or decorated with various deposition approaches (such as sol−gel or atomic layer deposition (ALD)63−65) to form TiO2 in a nanotubular form. Such composite structures may be used while the TiO2 is in/on the template, but most frequently the template is removed (selectively dissolved, evaporated, decomposed) to form “free” nanotubes, nanotube assemblies, or tube-powder. Template-free approaches are based mainly on either hydro/solvothermal methods (where typically titanium oxide particles are autoclaved in NaOH to delaminate to titanate units and finally reassemble in the shape of 9387

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Figure 3. Overview of differently synthesized TiO2 nanotubes: (a) schematic synthesis path, (b) typical morphology in TEM or SEM images, and (c) characteristic features of TiO2 nanotubes formed by different synthetic approaches. TiO2 nanotubes are fabricated by (3.1) hydrothermal method (Reproduced with permission from ref 105. Copyright 2004 The Royal Society of Chemistry), (3.2) template assisted formation (Reproduced with permission from ref 71. Copyright 2002 American Chemical Society. Reproduced with permission from ref 89. Copyright 2013 IOP Publishing Ltd.), (3.3) anodic self-organization (Reproduced with permission from ref 40. Copyright 2011 Wiley-VCH Verlag GmbH & Co.), and (3.4) electrospinning (Reproduced with permission from ref 163. Copyright 2010 Koji Nakane and Nobuo Ogata. Originally published in ref 163 under CC BY-NC-SA 3.0 license. Available online at: http://dx.doi.org/10.5772/8155).

tubes),16,17,20,53 or nanotube arrays that form by self-organized electrochemical anodization of metallic titanium, typically in dilute fluoride-electrolytes.40,41,66−69 One should note that Ti precursor/molecular template-based processes and hydrothermal approaches result in single tubes or loose agglomerates of tubes or bundles that are dispersed in a solution where often a wide distribution of tube lengths is obtained. To make use of the structures in electrically contacted devices, the tubes are usually compacted to layers (similar to powders) on an electrode surface. However, this leads to an arbitrary orientation of the nanotubes on the electrode, and this, in turn, eliminates many advantages of the onedimensional nature of the structure (e.g., providing a 1D direct electron path to the electrode). Using aligned templates or selforganizing electrochemical anodization leads to an array of oxide nanotubes oriented perpendicular to the substrate surface (such as in Figure 3, 2.b or 3.b). The tubes in the template can relatively easily be contacted by metal deposition. In the case of anodic tubes, the tube layers are directly attached to the metal

surface and thus are already electrically connected. Additionally, electrochemical anodization allows one to coat virtually any shape of Ti (and other metal) surfaces with a dense and defined nanotube layer. In the case of templates, the form of the template (molecule or aligned structure) determines to a large extent if electrodes with a back-contact perpendicular to the tubes can be obtained. We will discuss the main techniques to obtain the main types of TiO2 nanotubes in more detail below. 2.2. Deposition into/onto Templates

2.2.1. Anodic Aluminum Oxide (AAO). Historically, the first effort to produce titania nanotubes was probably the work by Hoyer et al.,70 who used an electrochemical deposition method in an ordered alumina template. His electrodeposition approach was based on a TiCl3 solution that was hydrolyzed and electrodeposited as a polymerized oxyhydroxide. nTiOH2 + + mH 2O → [Ti IVOx (OH)4 − 2x ]n + 3nH+ + ne− 9388

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titanate (PZT, PbZr0.52Ti0.48O3) and barium titanate (BaTiO3) have been produced in this manner.91 2.2.2. Molecular or Molecular Assembly Templating. TiO2 nanoparticles or nanorods are also prepared by using micelle-templates of appropriate surfactants above their critical micelle concentration (the surfactant molecules aggregate and disperse in liquid, to so-called spherical or rod-like micelles, which are used as template for TiO2 preparation). In this approach, nanotube formation is mostly carried out using water containing reverse micelles with a cylindrical shape. The Ti precursor can then react at the micelle surface, and after removal of the surfactant (burn off), a nanotube structure is obtained.92,93 Usually TiCl4 or Ti-alkoxide solutions are employed as the Ti-precursor. Using certain H2O:micelle ratios allows one to vary the dimensions of the nanotubes.92,93 Nevertheless, only low aspect ratio tubes can generally be obtained by this technique. A range of other methods involve the use or synthesis of other fibrous or rod-like templates. For example, Kobayashi et al. reported the preparation of TiO2 nanotubes using gelation of an organogelator to a template that is coated using titanium alkoxides and alcohols.94,95 The organogelator used was a cyclohexane derivative that was specially synthesized for this purpose. The outer and inner diameters of the TiO2 nanotubes obtained were 150−600 and 50−300 nm, respectively. Other examples are TiO2 hollow nanostructures that are formed using cotton fiber as a template.96 Here, chemical deposition of a TiO2-precursor onto the cellulose template is used, and the cotton thereafter can easily be burnt-off, forming pure hollow TiO2 nanostructures.96

Nowadays, to create aligned nanostructures (such as nanowires, nanotubes) by templating, more frequently than electrodeposition, other filling approaches are employed such as sol−gel techniques,20,52−62,71 or more recently atomic layer deposition (ALD).63−65 The former approach is a common path of TiO2 synthesis, which is based on the hydrolysis reactions of Ti-alkoxide, TiCl4, TiF4 precursors followed by condensation reactions (i.e., a gel-type polymeric Ti−O−Ti chain is developed, which further hydrolyzes and thus results in TiO2 precipitates). For example, TiO2 sol can be sucked into the pores of an alumina template, and after an appropriate heat treatment the alumina template can selectively be dissolved.58 Various modifications of this process have been reported (for example, see refs 52,71−73). In ALD, surfaces of templates (such as porous alumina) can be coated conformably with one atomic layer after the other by using alternating cycles of exposure to a titania precursor (such as TiCl4, Ti(OiPr)4), followed by purging and hydrolysis.63−65 Most typical templates are porous alumina, ion track-etch channels, or occasionally ordered nanowires or rods, such as ZnO nanowires.70,74−78 The classic template for the synthesis of a variety of aligned nanomaterials is porous anodic aluminum oxide (AAO), which can be produced with a hexagonal pattern of nanopores in a long-range, virtually perfect order. Experimental details of the fabrication of highly ordered alumina pore arrays can be found, for example, in refs 79−81. To date, it is possible to fabricate well-defined selfordered porous alumina with interpore distances between 10 and 500 nm,79,82,83 with aspect ratios >1000. If electrodeposition is used for filling, one needs to consider that alumina is an insulating material, and thus the thin barrier oxide (the pore bottom) has a high resistance. Therefore, prior to electrodeposition, the pore bottom is usually thinned or removed. For this, wet chemical etching of the anodic alumina film using a diluted phosphoric acid solution (also used as a pore widening treatment), or in situ thinning of the pore bottom using stepwise lowering of the anodic voltage at the end of anodization is typically employed.84,85 For electrodeposition, usually pulsed current is used to overcome the remaining resistance of the barrier layer at the pore bottom, and to take into account diffusion processes during deposition. Here, in pulse breaks, the cation concentration gradient established during a deposition pulse is balanced by diffusion from the electrolyte to the pore bottom.86 Alternatively, the barrier layer can be entirely removed by chemical means. The resulting through-hole porous layer (membrane) may then be PVD coated with a metal, such as Pt, Au, Ag, to establish a contact for conventional electrodeposition approaches.87 That is, DC electrodeposition can then be used to fill the porous channels starting from the bottom. For TiO2 electrodeposition to form tubes, for example, a TiF4 precursor deposited into a AAO template can be used,76,77,88 as shown in Figure 3, 2.a. Another approach of synthesizing NTs in AAO templates is based on polymer wetting.90 It is based on the observation that a low surface energy polymer preferentially wets the walls of pores of a material that has a high surface energy, such as Al2O3. To form oxide nanotubes in an alumina template, suitable oxide precursor compounds are mixed with polymers, and then this mixture is used in the wetting process, and after template removal (and polymer dissolution or burning off), the ceramic structure remains in a tubular form. For instance, ferroelectric and piezoelectric oxide nanotubes such as lead zirconate

2.3. Titanium Oxide Nanotubes by Hydrothermal Reaction

In 1998, Kasuga et al.20 reported for the first time on the hydrothermal synthesis of TiO2 nanotubes. In general, the approach is based on alkaline treatment of a titanium oxide precursor, which may be rutile,53,97 anatase, commercial P25,97−99 or amorphous TiO2.100 The powders or crystallites are typically heated in a NaOH solution with a concentration between 4 and 20 mol/L in an autoclave at temperatures between 100 and 180 °C for several (1−2) days.97,101 The formation of nanotubes is facilitated with an increase in NaOH concentration102 and temperature.97,101 At higher temperatures, nano fibers and ribbons can be formed.103 NaOH can be replaced by KOH, which allows one to increase the temperature to 200 °C.99 LiOH, however, forms more stable LiTiO2 compounds rather than oxide sheets or tubes.104 After the alkaline treatment, usually the resulting powders are washed with water and 0.1 mol/L HCl aqueous solution until the pH value of the washing solution is lower than 7, and subsequently powders are filtered and dried at various temperatures.53,97−101 Figure 3, 1.b shows an HRTEM image of TiO2 nanotubes, produced by Bavykin et al.105 The inner diameter of hydrothermal nanotubes usually ranges from 2 to 20 nm. The tubes generally have a multiwalled morphology. The distance between the wall-layers is approximately 0.72 nm in the protonated form. They are generally open-ended and have a constant diameter along their lengths.37 Sizes and shapes depend on the synthetic conditions and on the size and structure of the used titanium and titanium oxide raw materials.37,106 Generally, for higher hydrothermal temperatures and larger substrate precursors, longer tubes of up to several micrometers can be obtained.107 Size and shape can be also influenced using other experimental conditions. Final tubes 9389

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tend to agglomerate, but can be dispersed into aqueous colloidal solutions. Mechanistic details of hydrothermal titanium oxide nanotube formation are discussed somewhat controversially in the literature. Kasuga et al.20,53 believed that the nanotubes were formed in the washing step containing the hydrochloric acid. In 1999 they proposed the following formation mechanism: By treating the raw material with aqueous NaOH solution, some Ti−O−Ti bonds are broken, and Ti−O−Na and Ti−OH bonds are formed. In the washing step, the Na+ ions in the Ti− O−Na bonds, present in the alkali-treated specimen, are exchanged by H+, exfoliating the material to a sheet-like structure. By treating the material with HCl solution, Ti−OH bonds react with the acid and water to form new Ti−O−Ti bonds and anatase is formed. In this step, the titanium oxide sheets convert to anatase nanotubes by folding.53 Other authors later supported this conclusion.11 On the other hand, the formation of nanotubes was found by Du et al.54 (2001) to occur even without washing the as-formed product with hydrochloric acid. The authors concluded that the nanotubes were formed in the hydrothermal treatment step. Since then, most work reported that the nanotube shape is created during the hydrothermal alkali reaction.15,107−111 This is also supported by recent publications.99,103 Wang et al.112 concluded that the as-synthesized nanotubes were anatase rather than titanate. They proposed a formation mechanism as shown in Figure 4a−e.112 In this approach, NaOH initially disturbs the crystalline structure (Figure 4a) of raw anatase TiO2 crystals (Figure 4b). The free octahedra reassemble to link together by sharing edges, with the formation of hydroxyl bridges between the Ti ions resulting in a zigzag structure (Figure 4c), leading to growth along the [100] direction of anatase. Lateral growth occurs in the [001] direction and results in the formation of two-dimensional crystalline sheets (Figure 4d). To saturate dangling bonds and reduce the surface to volume ratio, the crystalline sheets roll-up, lowering the total energy; the result, seen in Figure 4e, is anatase TiO2 nanotubes.112 Other mechanisms involving scrolling singlelayer nanosheets13,113 or curving of conjoined nanosheets (Figure 4f−h)105 were also proposed in the literature. It may well be that different experimental conditions are responsible for different findings. A detailed analysis by Sekino et al.114 for an alkali treatment using refluxing at 110 °C leads to findings as illustrated in Figure 5a and b. The figures show a sequence of typical XRD spectra and TEM images for samples taken during chemical processing. After the alkaline treatment, the product mainly consists of amorphous and crystalline phase corresponding to sodium titanate (Na2TiO3, Figure 5a2), but no clear morphological features can be observed (Figure 5b1). After water and HCl treatment (Figure 5a3 and b2), sodium titanate disappears completely and a low crystallinity phase is observed. In this step, a nanometer-sized TiO2 nanosheet-like morphology is obtained. Subsequent water washing leads to a clear nanotube morphology (with an open-end structure and with individual tubes, Figure 5b3 and b4). The outer and inner diameters of the tubes are around 8−10 and 5−7 nm, respectively, and the length is of several hundred to several micrometers. In this process, the size of the obtained nanotubes does not depend on the starting materials, or whether KOH is used as a reaction solution rather than NaOH. If the hydrothermal synthesis is carried out in an autoclave with a higher pressure during the process,97 not only TiO2 but also Ti metal can be used as the source material for oxide

Figure 4. Schematic illustration of hydrothermal TiO2 nanotube formation: (a)−(e) mechanism of hydrothermal formation of TiO2 nanotubes involving delamination of starting crystal into sheets (a/d) and TiO62− units (a/b), reassembly (c/d), and scrolling (e). Reproduced with permission from ref 112. Copyright 2004 Copyright 2004 Materials Research Society. (f)−(h) Three different types of loop closing: (f) snail, (g) onion, and (h) concentric. Reproduced with permission from ref 105. Copyright 2004 The Royal Society of Chemistry.

nanotubes.110 That is, titanium is chemically oxidized in the alkaline solution prior to tube formation. Mostly a higher degree of size control, especially thick nanotubes, can be synthesized at higher temperatures. In addition, natural mineral sources can also be used for nanotube synthesis.115 Overall, while some dispute exists about mechanistic details and the composition of titanate tubes (see also section on structure and properties), the hydrothermal method is a versatile approach scalable to large synthesis batches, and it is the only approach based on forming TiO2 nanotubes with a wall thickness in the range of atomic sheets. 2.4. Self-Organizing Anodic TiO2 Nanotube Arrays

Another most simple, low-cost, and straightforward approach to fabricate titania nanotubes is self-organizing anodization of Timetal substrates under specific electrochemical conditions.40 Typical examples of such type of tubes are shown in Figure 3, 3.b and Figure 6d. Mostly such tubes are formed in dilute 9390

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direct doping the tube oxide by using alloys, with defined amounts of a secondary desired doping-metal, for anodization. In the context of anodic treatments, it should be mentioned that except for self-organizing tube-formation in fluorideelectrolytes, also photoelectrochemical etch-channels,147,148 or self-organized channel structures (obtained in hot glycerol electrolyte)149−153 have been reported. Furthermore, also some chloride or perchlorate containing electrolytes134,154,155 can be used to grow anodic TiO2 nanotubes; however, this type of tubes typically grows in the form of bundles on the Ti surface (see Figure 6g).156,157 These tubes usually can reach few tens to few hundreds micrometer length within tens of seconds. In this case, the anodization voltage must be sufficiently high to create a local breakdown of the oxide film that then represents the nucleus for the tubebundle growth. This so-called “rapid breakdown anodization” can also be extended to other materials such as W and even to alloys such as Ti−Nb, Ti−Zr, and Ti−Ta.158 Although the process is very fast and thus is useful for producing large amounts of nanotubes in a short time, due to a lack of geometry control (uniform length, diameter control), these NTs received far less attention in comparison with self-organized TiO2 nanotube arrays. The latter, in comparison, show a very high adjustability of geometry, and, as a result, these self-organizing tube layers have been over the last years the most widely investigated TiO2 nanotube morphology. Because of this, we will emphasize these self-organized tube arrays and provide more details on growth mechanisms and critical growth factors in section 3. 2.5. Electrospinning

A range of other approaches to form TiO2 nanotubes have been reported, and in particular electrospinning is certainly worth mentioning. In this process, a strong electric field is used to pull a thin jet out of a drop of polymer solution or melt through a suitable nozzle. The jet then is deposited in form of a nanofiber.159 TiO2 nanotubes are obtained, for example, by using titanate precursors to coat the fiber, and after polymerizing the precursor to TiO2, applying a thermal treatment to remove (decompose) the organic fiber.160−162 An example of such nanotubes is shown in Figure 3, 4.b.163 Most elegant is a simultaneous coating of the fiber while spinning. Li and Xia164 reported the formation of TiO2 hollow-nanofibers (nanotubes) by electrospinning of an ethanol solution (containing Ti tetraisopropoxide/polyvinylpyrrolidone) and of a heavy mineral oil or polymer through a coaxial, two-capillary spinneret, followed by selective removal of the cores and calcination in air. Under optimum conditions, TiO2 nanotubes are formed with continuous and uniform structures (several centimeters range and well-separated single tubes). Another example is the work of Nakane et al.165 in 2007 that used electrospinning to form precursor nanofibers of poly(vinyl alcohol) (PVA)−titanium compound hybrids, which then were calcinated to TiO2 nanotubes. In the report, they examined the crystal structure and morphological change with different heat treatment processes. By heat treatment from 400 to 600 °C, anatase phase was obtained, and above 600 °C anatase/rutile mixed phases were formed. The specific surface areas evaluated by BET decreased with increasing heat treatment temperature due to sintering of the tube structures. In general, electrospinning allows producing nanotubes with extremely large aspect ratios. The nanofibers typically have a diameter range from a few ten nanometers to a few

Figure 5. Crystal structure (a) and morphology evolution (b) during hydrothermal TiO2 nanotube formation: (a) XRD spectra and (b) TEM images. (a1) Anatase-type TiO2 raw material, (a2 and b1) after alkaline reflux (10 M NaOH, 110 °C, 24 h), (a3 and b2) after 0.1 M HCl treatment and washing, and (a4, b3, and b4) final product. Reproduced with permission from ref 114. Copyright 2010 SpringerVerlag.

fluoride electrolytes under several 10 V of anodization potential. In their most elaborated way, they form highly selforganized hexagonal arrangements as in Figure 6d. Interestingly, first reports by Assefpour-Dezfuly et al.66 in 1984 and later by Zwilling et al.67 in 1999 on the formation of self-organized porous/tubular TiO2 structures using anodization of Ti and some alloys in fluoride-based electrolytes were widely overlooked, and the finding was mainly ascribed to Grimes68 in 2001. However, all of these TiO2 layers, including early follow-up work,69 were far from perfect; that is, they showed a considerable degree of inhom*ogeneity and were limited to tubes with length of about 500 nm. Later work showed significantly improved control over length, diameter, ordering, and composition by the use of pH mediation,116 and particularly by the introduction of nonaqueous electrolytes.117−119 It is noteworthy that fluoride-based electrolytes were then also found to be an extremely versatile tool to grow ordered anodic oxide nanostructures on other metals, such as Hf,120,121 Zr,122−127 Fe,128−130 Nb,131,132 V,133 W,134−137 Ta,138−142 Co,143 and even Si.144−146 Not only pure Ti, but a full range of alloys can also be used to form nanotubes; this turned out to become a unique way of 9391

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Figure 6. Anodic self-organized TiO2 nanotube formation: (a) Schematic of growth sequence of TiO2 nanotube growth. (b,c) Oxide growth by field aided transport either to compact oxide or tubular structures in the presence of fluorides (rapid fluoride migration leads to accumulation at the metal−oxide interface). (d) SEM images of typical TiO2 nanotubes taken at the top, from the fractures in the middle, and close to the bottom of a tube layer, illustrating the gradient in the tube-wall thickness, the inner opening diameter increase, and the increasing intertube spacing from bottom to top. Reproduced with permission from ref 205. Copyright 2007 Wiley-VCH Verlag GmbH & Co. (e) Typical current−time (j−t) characteristics after a voltage step in the absence and presence of fluoride ions in the electrolyte. Either compact oxide, CO (fluoride free), or porous/tubular metal oxide, PO formation (fluoride containing), with different morphological stages (I−III). The inset shows typical linear sweep voltammograms (j−U curves) for different fluoride concentrations resulting in either electropolished metal (EP, high fluoride concentration), compact oxide (CO, very low fluoride concentration), or tube formation (PO, intermediate fluoride concentration). (f) SEM image of the fluoride-rich layer at the bottom of nanotubes. Reproduced with permission from ref 40. Copyright 2011 Wiley-VCH Verlag GmbH & Co. (g) TiO2 nanotubes prepared by rapid breakdown anodization in chloride containing electrolyte. Reproduced with permission from refs 156 and 157. Copyright 2007 Wiley-VCH Verlag GmbH & Co. and Copyright 2011 Elsevier B.V.

3.1. Electrochemical Aspects of Anodic Growth of Nanotube Layers

micrometers. The ability to obtain such high aspect ratios rests on the fact that electrospinning is, like an extrusion processes, a continuous process. Moreover, by extension of the coaxial spinneret system multishell nanostructures can be formed. In addition, it is possible to use this method to obtain nanofibers/ nanotubes, with specific surface topologies.166−168

Electrochemical anodization as such is a century old process mostly used in industry, to create “thick” compact or porous oxide layers on the surface of a metal substrate. It is carried out typically in an electrochemical cell, as illustrated in Figure 3, 3.a, containing a suitable electrolyte, with the metal of interest as a working electrode (anode), and an inert counter electrode (usually platinum or carbon). Upon applying a sufficiently high anodic voltage to the metal of interest M, it is oxidized to Mz+ (eq 1) that either forms a metal oxide, MOz/2 (eq 2a), or is solvatized and then dissolved in the electrolyte (eq 2b). As a counter reaction, protons are reduced to produce hydrogen gas at the cathode (eq 3).

3. ORDERED TiO2 NANOTUBE ARRAYS Electrochemical formation of ordered TiO2 nanotube arrays such as that shown in Figure 3, 3 and Figure 6 is based on anodization of a metal in an electrolyte under conditions, where self-organization is established. The key to the “right” electrochemical conditions is an optimized steady-state situation of electrochemical oxide formation and chemical oxide dissolution. These conditions and key factors affecting them will be discussed in the following.

M → Mz + + z e− 9392

(1)

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z H 2O → MOz /2 + z H+ + z e− 2

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z+ Mz + + solv → M solv

(2b)

z H2 2

(3)

z H + + z e− →

and eq 4. Experimentally one observes that this layer is then partially penetrated by nanoscopic etch channels and a porous initiation layer is formed.40 Porosification increases the surface area of the electrode, and consequently in region-II a rise in current occurs. Underneath this initial layer, stable pore growth is initiated in region-III, and the current then reaches a virtually constant value reflecting the establishment of a steady-state situation between dissolution and formation of oxide. In a typical self-ordering tube formation sequence, first the chemical dissolution is nonuniform (region-II) and the initiated pores grow progressively in a tree-like fashion. As a result, the individual pores start interfering with each other, and will be competing for the available current. This leads, under optimized conditions, to a situation where the pores equally share the available current, and self-ordering under steady-state conditions is established (region III). Here, oxide is continuously formed at the bottom under high field conditions, but part of the bottom layer is permanently dissolved, leading to a thinner oxide at the tube bottom than the corresponding final high field value. The steady-state current density is typically in the order of some mA/cm2. In addition to affecting the TiO2 dissolution according to eq 5, fluoride ions, due to their small ion size, also migrate under the constant applied field through the growing porous oxide layer toward the metal, and form a fluoride-rich layer at the metal/oxide interface, as illustrated in Figure 6f. The formation of this fluoride-rich layer is the most likely origin of the formation of a nanotube, rather than a nanopore morphology, as discussed in the next section, but it is also the cause for a reduced adherence of oxide nanotube layers on the metal substrate.173−176 A large number of fluoride-based electrolyte compositions have, over the past 10 years, been explored for tube formation such as mixtures of organic solvents (mainly EG, glycerol, DMSO) with H2O and fluoride sources such as HF, NH4F, BF4−. Even ionic liquids containing BF4−177−180 were reported to successfully form tubes. It is noteworthy that the formation of reaction products also increases the conductivity of the electrolyte and thereby increases the growth rate of the nanotubes (namely in the case of low conducting organic electrolytes). Anodic growth of nanotubes takes place mainly by the competition of reaction 2a, reactions 5 and 6. The purely chemical dissolution (eq 5) leads to permanent thinning of the tube walls, which is the strongest for the longest time exposed tube tops (Figure 6a). After extended times, thinning to zero wall thickness prevents further overall growth of the nanotubes.40,116,181

(2a)

Regarding the question if an oxide layer is formed (eq 2a) or dissolution (eq 2b) dominates, thermodynamic aspects such as solubility products and oxide stability (that can be taken, for example, from Pourbaix-diagrams)169 together with respective reaction rates need to be considered. In an electrolyte where the oxide is insoluble and no other side reactions occur, mainly reaction 2a dominates; that is, a high oxide formation efficiency is obtained. Anodic oxidation processes (thickening of the oxide) typically follow a high field law of the form: I = A exp(BE) = A exp(BΔU /d)

(4)

where I is the current, ΔU is the voltage across the oxide, d is the layer thickness, and E is the electric field.170,171 A and B are experimental constants. The key process is based on the field effect on ions migrating through the oxide layer as illustrated in Figure 6b. As the transport of M+ ions outward and of O2− ions inward is controlled by the applied field, with E = ΔU/d, with an increasing film thickness the field (and thus the current, if a constant voltage is applied) drops exponentially. Finally, the field is lowered to an extent that it is not able to significantly promote ion transport any longer and the film reaches a final thickness. If, however, a certain degree of solubility of the oxide is provided and an equilibrium of film formation and dissolution can be established, a considerable ion and electron flux is maintained in a steady-state situation. For example, solvatization of Ti4+ can be realized by the formation of fluorocomplexes such as in eqs 5 and 6. One may consider pure chemical dissolution of the oxide (eq 5) or direct complexation of high-field transported cations at the oxide electrolyte interface (eq 6); often this process is called ejection of transported cations to the electrolyte: H+

MO2 + 6F− ⎯→ ⎯ [MF6]2 − + 2H 2O

(5)

M 4 + + 6F− → [MF6]2 −

(6)

Figure 6e schematically illustrates the observed electrochemistry represented in i/E and i/t curves for three cases of oxide “solubilities”. First, if metal oxidation forms ions that are immediately and completely solvatized (eq 2b), no oxide film is observed; this case is commonly described as active corrosion or electropolishing (EP). Second, one may obtain the formation of a stable (insoluble) compact metal oxide according to eq 2a, and in accordance with the high field law (eq 4). Third, if there is a competition between solvatization and oxide formation, where the former reaction can be promoted by the addition of a suitable agent (such as F− in the TiO2 case, Figure 6b and c), the established steady-state situation often leads to the formation of a porous oxide. If formation and dissolution are in an optimum range, highly selforganized oxide pore arrangement or nanotube formation is possible.44,172 In this case, a typical current versus time curve as shown in Figure 6e for porous oxide (PO) is obtained, and it can be divided into three different regions.38,40 In region-I, the current is decreasing exponentially as a result of coverage of the anodized surface with an oxide film according to reaction 2a

3.2. Why Is a Tubular Shape Formed?

In general, self-organizing anodization leads to hexagonal nanoporous cells (a honeycomb structure) due to the competitive space-filling nature of pore growth. The role model is self-organized porous alumina formed in acidic solution182−197 and in neutral fluoride containing solutions.198 In contrast, for TiO2, virtually under all self-organizing conditions a tubular shape is formed (rather than a porous morphology). This can be attributed mainly to the composition of cell boundaries; that is, the main difference of porous alumina and self-ordered TiO2 nanotube layers is that in the case of TiO2 nanotubes, the cell boundaries can be etched (dissolved) under the applied electrochemical conditions. As mentioned above, a specific feature of fluoride electrolytes is 9393

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Figure 7. Critical parameters for anodic TiO2 nanotube formation: (a,b) Diagrams of different morphologies formed during anodization in fluoride containing electrolyte depending on (a) applied voltage and water concentration in electrolyte (Reproduced with permission from ref 204. Copyright 2010 Wiley-VCH Verlag GmbH & Co.) and (b) applied voltage, hydrodynamic parameters, and F− concentration in electrolyte (Reproduced with permission from ref 207. Copyright 2012 Elsevier B.V.). (c−e) Influence of applied voltage on (c,d) nanotube diameter and (e) growth rate shown for classic anodization in a fluoride containing ethylene glycol electrolyte and the effect of complexing additives EDTA and lactic acid. Data taken from various sources.

that under high field oxide growth conditions, the small F− ions may migrate several times faster through an oxide layer than O2−, and thus a fluoride-rich layer is formed at the metal/oxide interface (schematically shown in Figure 6c). In addition to classical analytical techniques,173,174 this fluoride-rich layer can be observed as a haze at the bottom of the tubes such as shown in Figure 6f. H2O easily dissolves the fluoride-rich layers between TiO2 “cells”, and, as a consequence, a tubular shape is formed. This occurs under a wide range of anodization conditions (Figure 7a).40,174 In case of porous alumina, even if formed in fluoride electrolytes, a porous morphology is obtained, as Al-fluorides are not easily soluble in H2O.198 Additionally, one may consider differences in the specific volume, structure, and surface termination that contribute to separation into tubes. For alumina, it is, however, noteworthy that Chu et al.199 reported cell boundary etching of porous alumina to form a tubular layer. Moreover, there are a few examples that produced alumina nanotubes by direct anodization.200−202 An example is the work of Lee et al.200,201 that used pulsed anodization process. In contrast, self-organizing anodization of titanium leads only under very specific conditions to true porous layers, for example, if the water content in the electrolyte is very low.203 As illustrated in Figure 6a and d, in many cases a transition from a porous to a tubular morphology is apparent along the self-organized oxide layer. That is, the bottom of the tubes has an ordered hexagonal nanoporous appearance, while toward the top, clearly a separated tube structure can be seen (due to different exposure times of top and bottom to the electrolyte). Not only the exact composition of the electrolyte, but also the

anodization voltage affects the pore to tube transition;204 this can be ascribed to a field effect on the fluoride ion mobility (faster or slower accumulation) and stress effects (electrostriction),204 which affect the degree of fluoride accumulation at the metal/oxide interface. 3.3. Factors Influencing the Morphology of the Anodic Film

In practice, anodization of Ti in fluoride electrolytes can result in various morphologies. Formation of well-defined nanotubular structures depends on a number of factors such as the applied potential, the temperature of the electrolyte, the concentration of fluorides, etc.38,40,44 Figure 7a and b shows the regions of existence for nanotubes versus other morphologies such as nanopores, nanosponge, and compact oxide layers for a range of anodization parameters. Typically, high water contents204 and high hydrodynamic flow in the electrolyte181,206,207 favor a transition to a sponge structure rather than a nanotube structure. Under otherwise optimized tube formation conditions, the tube diameter can be controlled with the applied voltage.38,40,181 The tube length can, over a considerable range, be adjusted by the anodization time until a steady-state between the growth of nanotubes at the bottom and the chemical etching at the nanotube top is established.38,116,181 The control of tube length and diameter depends strongly on the electrolyte (see Figure 7c−e). In aqueous electrolytes, typically tubes from ∼10 nm diameter to ∼100 nm diameter can be formed by applying voltages between 1 and 25 V in electrolytes with 0.1−0.5 wt % F−,208,209 as shown in Figure 7c. In organic electrolytes, such as the most typical ethylene glycol 9394

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with 0−13 wt % of H2O and 400 nm), while no H2 evolution by commercial anatase TiO2 could be observed under visible light.473,479 Theoretical simulations support that the width of the band gap is related to the concentration of Ti3+ or OV. It was further suggested that the high concentration of OV could break the selection rules for indirect transitions, resulting in an enhanced absorption for photon energy below the band gap.478 Electrochemical reduction has also been used to reduce TiO2 and fabricate self-doped TiO2. Several studies investigated the effect of hydrogen loading by cathodic electrochemical treatment of various TiO2 forms, such as single crystals,468 sputtered layers,480 thermal oxides,481 and anodic nanotubes.482−484 For single crystal TiO2, hydrogen can be incorporated into the rutile lattice electrochemically. Depth profiling electron-stimulated desorption (ESD) shows a high density of hydrogen in a shallow surface layer.481 With strong cathodic reduction,481 hydrogen penetrates deeper into the TiO2 electrode, and an increased amount of hydroxy and/or oxyhydroxy groups was found by XPS. Moreover, hierarchical TiO2 nanotubes were reduced electrochemically and were reported to show a remarkably improved and stable water 9412

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creating a high density of oxygen vacancies that serve as electron donors. In contrast, only a mild enhancement on PEC water splitting was found for hydrogen treated TiO2 anatase nanotubes. On the other hand, it was also found that hydrogenated TiO2 nanotubes show considerably enhanced capacitive properties for supercapacitors, which are attributed to the higher carrier density and an increased density of hydroxyl groups.498 Most recent work by Liu et al.473 shows, however, a remarkable activation of TiO2 nanotubes for noble metal-free photocatalytic H2 generation under open circuit conditions. Hoang et al.499 reported on a synergistic effect using a hydrogenation and nitration cotreatment of TiO2 nanowire (NW) array that improves the water photooxidation performance. The two-step hydrogenation and nitration cotreated rutile TiO2 wires show visible light (>420 nm) photocurrent that accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. From EPR spectroscopy, the concentration of Ti3+ species is significantly higher than for samples treated solely with ammonia. It is believed that Ti3+ enrichment by annealing in H2 atmosphere also is the origin of higher N doping level observed for these tubes after traditional nitration.499 At current, the treatment from Chen and Mao16,479,495 and derivatives of it are widely explored for TiO2 nanotubes and similar structures.

H2 yield remained unchanged without catalyst regeneration, indicating an excellent stability for the black TiO2. Under the same experimental conditions, no H2 was produced from the unmodified white TiO2 nanocrystals loaded with Pt. Using only visible light illumination, the rate of photocatalytic H 2 production is however considerably lower. From TEM images, the formation of a core−shell structure was observed where a highly disordered surface layer (approximately 1 nm thick) with hydrogen dopants surrounded a crystalline (anatase) core.479 XRD diffraction peaks indicated that the black TiO2 was highly crystallized anatase. Raman spectroscopy used to examine structural changes in the TiO2 nanocrystals showed that new bands emerged for the black TiO2, in addition to the broadening of the anatase Raman peaks. From XPS results there was no detectable Ti3+ found, and a broader peak of O 1s at 530.9 eV (for the H2-treated samples) was attributed to Ti−OH species. The onset of optical absorption of the black hydrogenated TiO2 nanocrystals was found at about 1.0 eV (approximately 1200 nm), together with an abrupt change in both the reflectance and the absorbance spectra at approximately 1.54 eV (806.8 nm). By valence band XPS, the density of states (DOS) of the valence band of TiO2 nanocrystals was evaluated. For the black TiO2 nanocrystals, the valence band maximum energy blue-shifts toward the vacuum level by approximately −0.92 eV. From FTIR reflectance spectra, the strength of the terminal O−H mode is reduced after hydrogenation of TiO2.495 By 1H NMR measurements, small and sharp resonances were observed for the black TiO2, suggesting that the hydrogen concentration is low and there are dynamical exchanges between hydrogen in the different environments.495 Unlike in the case of traditionally doped TiO2, Chen et al. considered not Ti3+/OV defects to be responsible for the longwavelength absorption of black TiO2, but assigned this effect to the formation of the disordered phase around the crystalline anatase nanoparticle core.16,479,495 The dramatic color change was ascribed to the optical gap of the black TiO2 nanocrystals that was substantially narrowed by intraband transitions. Additionally, the engineered disordered phase is perceived to provide trapping sites for photogenerated carriers and prevent them from rapid recombination, thus promoting electron transfer and photocatalytic reactions. The authors compared DFT, without disorder, where defects yielded a gap state in TiO2 nanocrystals, about 0.5 eV below the conduction band minimum. With DFT that considers lattice disorder, the presence of midgap electronic states leads to a band gap of ∼1.8 eV. Follow-up work used various reduction treatments, mainly without pressure, to achieve a visible response. For example, black TiO2 nanoparticles obtained through a one-step reduction/crystallization process also exhibit a crystalline core/disordered shell morphology.496 With valence band XPS, these TiO2 nanoparticles exhibit a band gap of 1.85 eV, which well matches with visible light absorption. However, in this case, the presence of Ti3+ was confirmed by EPR;496 that is, visible light absorption may be attributed to the classic Ti3+ formation. A similar simple annealing treatment (without pressure)497 in hydrogen was used to reduce TiO2 nanowires and nanotubes. The results showed a fundamental improvement for photoelectrochemical (PEC) water splitting of rutile TiO2 nanowires.497 The hydrogen treatment was found to increase the donor density in TiO2 nanowires by 3 orders of magnitude, via

5.4. Conversion of Tubes

TiO2 nanotubes can comparably easily be converted to a perovskite oxide by hydrothermal treatments.500 Perovskite materials, such as lead titanate (PbTiO3), barium titanate (BaTiO3), strontium titanate (SrTiO3), and lead−zirconium titanate (PbZrTiO3), show a variety of interesting piezoelectric or ferroelectric properties.501−507 Particularly conversion to other photocatalytically active (semiconductive) materials such as SrTiO3500,504−507 or bismuth titanate508,509 can extend the range of potential applications considerably, such as toward capacitors, actuators, electrochromics, gas-sensors, photocatalysts, biotemplates, and various electronic applications.501−509 Furthermore, hydrothermal treatments can also be used to convert the ordered nanotubular layers into other geometries.510 5.5. Particle Decoration, Heterojunctions, Charge Transfer Catalysis

Decoration of TiO2 nanotubes with nanoparticles (metals, semiconductors, polymers) is frequently used to achieve property improvements. Main effects include: (i) heterojunction formation that changes the surface band bending (metal clusters or other semiconductors), (ii) suitable surface states are created for enhanced charge transfer with surroundings, (iii) catalytic effects for chemical reactions (e.g., Pt for H2 evolution, RuO2, IrO2 for O2 evolution), and (iv) surface plasmon effects that lead to field enhancement in the vicinity of metal particles and thus allow, for example, for a more efficient charge harvesting. If particle decoration is used to introduce locally on the TiO2 surface variations in the band bending, a similar effect as by applying an external potential can be reached but under “opencircuit conditions’’ (for example, metal particles can pin the Fermi level locally corresponding to their work function; see Figures 28 and 29f). The geometric range of the effect depends mainly on the nature of the particle (i.e., its work function) and the doping concentration of the TiO2. 9413

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Figure 23. Self-assembled monolayer (SAM) modification of TiO2 surfaces: (a) Attachment of SAMs to −OH terminated surfaces.581 (b) Immobilization of proteins (R) via linker SAMs (protein not to scale).581 (c) SEM image of microscopic wetting of TiO2 nanotube surfaces, showing that wetting takes place preferentially between tubes. Reprinted with permission from ref 590. Copyright 2010 The Electrochemistry Society.

techniques.515,516,522 Ag/TiO2 or Au/TiO2 nanotubes show a significantly higher photocatalytic activity as compared to plain nanotubes.516 Ag decorated tubes were also found to enhance the performance of DSSCs significantly.523 Oxide nanoparticle decoration of TiO2 nanotubes by, for example, WO3,28 or tungstates,524 Cu2O,525−527 Fe2O3,528 CuInS2,529 ZnO,530−532 Bi2O3,533 ZnTe,534 or TiO2529,535 has been obtained by slow hydrolysis of precursors electrochemically, or by CVD, PVD deposition. One of the most followed up schemes to establishing useful p−n heterojunctions (Cu2O− TiO2) for solid-state solar energy devices is, however, the electrochemical deposition of Cu2O.536 Nevertheless, it should be noted that for many applied compounds, for II−VI type of materials or Cu2O, the long-time stability in photoelectrochemical applications must be questioned, not only due to corrosion or photocorrosion, but also due to the instability of some of the cocatalysts under applied voltage. An elegant decoration approach for anodic nanotubes with noble metal particles is using low concentration Ti−X (X = Au,Pt) alloys27,537 that can provide very uniform particle densities and defined particle diameters. To increase the surface area in form of hierarchical structures, mainly hydrolysis of TiCl4 is used that leads to layers of TiO2 nanoparticles with 2−3 nm diameter that decorate the inside and outside of the tube walls.535 In this case, the beneficial effect is a surface area increase; if a similar treatment is used to deposit WO3 nanoparticles, additionally junction formation between TiO2 and the misaligned bands of WO3 can be beneficially exploited.28 More recent work deals with tube decoration using C60,538 graphene,539 Ag/AgCl, or AgBr540,541 to enhance mainly their photocatalytic activity. Decoration with nickel oxide nanoparticles has recently been shown to exhibit significant photoelectrochemical activity under visible light (possibly by charge injection from NiO states to the conduction band of TiO2).542 A most simple but very successful approach for

For TiO2 nanotubes, a range of approaches for decoration with foreign materials (metals or metal oxides) have been reported. Electrodeposition reactions into TiO2 nanotubes essentially provide a very versatile tool to fill or decorate oxide nanotubes.25 Complete filling of the empty tube space on the substrate is, however, not as straightforward as, for example, in the case of alumina,86 because of the semiconductive nature of TiO2,25 that for crystalline tubes under cathodic bias a forward biased Schottky function is established (i.e., almost metallic conductivity is established). Nevertheless, several filling-by-electrodeposition approaches have been reported. After a first approach of Cu electrodeposition in amorphous tubes25 to establish a p−n junction, further attempts involved tube layers that were lifted off from the metal substrate, opened at the bottom, and the oxide tubes were filled from an evaporatednoble-metal contact by electrodeposition297 (in analogy to a treatment used for porous alumina).287 Of course, this treatment does not lead to an interdigitated structure. Cathodic metal deposition into intact crystalline tube layers in their most functional anatase or rutile form was only reported recently using more elaborate deposition techniques.26 Complete filling of tubes with polymers is easier, as the deposition usually occurs under anodic conditions. In this case, a reverse biased junction (TiO2/electrolyte) is providing the insulating properties needed for easy bottom-to-top deposition. For example, electrodeposition of conductive organic polymers (polypyrrole, polyaniline, PEDOT, etc.) can even be tuned to selectively fill the intertube space or additionally the inner tube cavity dependent on the applied conditions.511−514 Only partial decoration of TiO2 nanotubes by noble metal nanoparticles (such as Au, Ag, Pt, Pd, AuPd) is very frequently carried out to achieve cocatalyst effects.515−520 Ag or Pt nanoparticles can be deposited on the tube wall by photocatalytically reducing Ag or Pt compounds on a TiO2 surface by UV illumination.515,521 Other metal nanoparticles are preferably deposited by UHV evaporation or chemical reduction 9414

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particle decoration is filling the TiO2 nanotubes with a suspension of magnetic (Fe3O4) nanoparticles.543 TiO2 nanotubes can also be decorated by narrow band gap semiconductors, such as CdS, CdSe, PbS quantum dots.544−548 These quantum dots can be deposited on the nanotube wall electrochemically, by sequential chemical bath deposition methods, or by chemical treatment in the presence of Cdprecursors. Such CdS/CdSe quantum dots have band gap values of 2−2.4 eV (i.e., they absorb visible light) and can inject the excited electron into the TiO2 conduction band, that is, essentially act as sensitizers. There are also reports on sensitizing TiO2 nanotubes with conducting (semiconducting) polymers549 for solar energy conversion; however, it must be expected that such structures fail fairly quickly due to the photocatalytic degradation of the polymer.

monolayer without any interaction, and the self-assembly follows Langmuir adsorption kinetics.560 Nevertheless, also multilayer adsorption can be observed; in this case, the coverage first approaches a constant value (monolayer coverage) and subsequently increases again. Multilayer adsorption is best described by the BET model.561 For TiO2, most typical is the use of n-octadecylphosphonic acid under surface water split-off. Ethoxy- and methoxysilanes release ethanol or methanol upon condensation to TiO2,562 shifting the equilibrium to the covalently bound state at elevated reaction temperatures.563 Carboxylic acids also react with surface −OH and typically represent the anchoring groups for the organic dyes in dye-sensitized TiO2 solar cells.18,564−566 For the latter applications, it should be noted that even though the quality of the monolayer (packing density, attachment strength) is in the order phosphonate > silane > carboxylate, also the charge transfer reactions across the attached functional group are important. For example, in DSSCs, charge transfer from a dye molecule to the TiO2 conduction band is significantly faster for COO− groups than for silanes.40 To build up a several stage functionalized surface, linker molecules that carry two terminal functional groups are commonly used. Prominent examples are amine terminated silanes. These linker SAMs find extensive application in a wide variety of both industrial and research-oriented applications, ranging from adhesion promotion of polymer films on glass,567,568 fiberglass-epoxy composites,569,570 and attachment of (noble) metal nanoparticles to silica substrates571 to biomedical applications. For the latter, specifically 3-aminopropyltriethoxysilane (APTES) is used in lab-on-a-chip applications,572,573 or as bioactive linker to promote protein adhesion to oxide surfaces relevant in implant technology.574 For example, the enzyme horseradish peroxidase (HRP) was coupled to TiO2 nanotubes via APTES and used for model drug release applications245 as well as for the determination of the protein activity by ToF-SIMS.575 Figure 23b shows the attachment of proteins or other biomolecules to various typical linker SAMs: pure APTES (A) can bind the protein via free carboxylic acid groups (amino acids Asp, Glu); in combination with glutaraldehyde (AG)562 or ascorbic acid (vitamin C, AV),245,575 a free amino group of the protein (amino acids Arg, Asn, Gln, Lys) coordinates to the linker; carbonyldiimidazole (CDI, C)576 also couples via free amine groups and is nearly completely replaced by the protein; that is, the protein is adsorbed in close distance to the surface; HUPA, 11hydroxyundecylphosphonic acid (H)577 is a long chain linker molecule that provides a certain degree of steric freedom to the protein. The latter is adsorbed via free carboxylic acid groups. Proteins can also bind to the pristine oxide,578,579 as the interaction with the surface is strong enough to withstand surfactant rinsing;580 that is, the protein may even form a covalent bond with the oxide. It could be shown that the efficiency of the protein coatings immobilized on TiO2 is strongly dependent on the choice of bioactive linker SAM, with HUPA and CDI producing the most active protein coatings.581 Gao et al.582 modified TiO2 nanotube arrays with APTES for the immobilization of an antibody to develop an ultrasensitive immunosensor system. Carboxyalkylphosphonic acid SAM modified TiO2 nanotube surfaces constituted a highly sensitive fluorescence immunoassay for the detection of human cardiac troponin I as low as 0.1 pg mL−1 without the use of enzymatic amplification.583

5.6. Self-Assembled Monolayers (SAMs)

The properties of TiO2 nanotubes can further be modified by decoration via defined monolayer coatings (SAMs), to tailor various properties of the surface, such as the wettability,550−552 change the charge transfer properties, biological interactions,553,554 to tailor morphology (e.g., when obtaining TiO2 nanotubes by ALD),555 or to trigger reactions (such as payload release). 245,543,556 Attaching organic molecules is most straightforward by self-assembly of molecules from the gaseous or liquid phase. Typically attached molecules have a polar functional group and an organic tail. The attachment to the substrate can be based on covalent or noncovalent bonding. TiO2 as many other metal oxide surfaces are in ambient conditions at least partially terminated with hydroxyl groups.557 This can be exploited to anchor monolayers by condensation reactions of a functional group. Various reactive groups can strongly interact with −OH terminated surfaces: carboxylic acids, esters, siloxanes, and phosphonic acids can attach to the surface via condensation, chlorosilanes (and potentially also acyl chlorides) via elimination of HCl.558 Amines can adsorb to a metal oxide surface via either formation of peptide-like bonds with the metal oxide or by interaction of the positively charged NH3+-group with the underlying substrate. Examples of the SAM adsorption process (reaction) are displayed in Figure 23a. The initial adsorption of molecules occurs randomly with no systematic orientation of the organic chains. At low concentrations, submonolayers with a high degree of disorder and defect density are produced. At higher concentrations, a denser coverage with increased order of adsorbates and erected organic tails (e.g., hydrocarbon chains) will be obtained. According to Helmy et al., phosphonic acids and silanes (chlorosilanes, siloxanes) are especially suited to modify TiO2 surfaces, with phosphonic acids adsorbing faster and forming more stable SAMs than silanes, even though a comparable final coverage is reached.558 Silanes with reactive groups, that is, chloro-, methoxy-, and ethoxysilanes, are converted to hydroxysilanes in contact with water. While silanes show an insular growth pattern with cross-linking of neighboring molecules, phosphonic acids initially adsorb randomly, forming ordered monolayers with a higher surface concentration.558 In this case, typically the maximum coverage is limited by the amount of available adsorption sites; that is, the density of −OH groups on the surface determines the adsorbate density.559 The mechanism of SAM formation is dependent on the interaction of the adsorbates with each other: in most cases adsorption data show that the molecules adsorb as 9415

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1991 Grätzel and O’Regan reported probably the most significant achievement that is a first fully functional solar cell device that they called dye-sensitized solar cell (DSSC) and operated at 11% of solar light conversion efficiency.18,602,603 The photoelectrode is based on a 5−15 μm thick layer of compacted TiO2 nanoparticles coated on a conductive glass electrode. The nanoparticles are modified with a monolayer of attached Ru−bipyridyl molecules that act as visible light absorber that inject light excited electrons from the dye’s LUMO into the conduction band of the TiO2 as illustrated in Figure 24. To refill the electrons of the dye, an iodine redox

Hydrothermal TiO2 nanotubes have been used as adsorbent for organic dyes and organic vapors.584 Modified with amines, they were found to be attractive adsorbents for CO2 fixation that can be regenerated readily and energy-efficiently by temperature-programmed desorption.585 It is noteworthy that bifunctional molecules, such as APTES with a silane and an opposing amino group, show different affinities of either functional group to amorphous, anatase, and rutile polycrystalline surfaces.556 Dependent on the strength of the interaction with the substrate, various drug and other payload release processes can be achieved, for example, voltage induced,400 by simple immersion in a solvent,586,587 and by irradiation with UV light245 and X-rays.588 5.6.1. Wettability. Organic modification of nanotubes combined with their photocatalytic properties was further used to tune the wettability properties of TiO2 nanotube surfaces.550,551 Pristine nanotube layers (amorphous or crystalline) are superhydrophilic; only when treated with a suitable monolayer do they become superhydrophobic, with the maximum achievable contact angle depending on the tube diameter.550,551 The overall wettability behavior is in accord with the Cassie−Baxter model.589 In typical photocatalytic reactions of monolayers on TiO2 or TiO2 nanotubes with UV light, chain scission occurs, which makes the surface increasingly hydrophilic with the duration of illumination. Chain scission was observed to occur between the functional group of the SAM and the substrate for irradiation of siloxane or phosphonic acid SAMs on TiO2, indicating strong, covalent bonding with the substrate.581,588 Superhydrophobic tubes are the basic material for the fabrication of amphiphilic nanotubes;245 organic solvents are needed to fill them with a liquid, for example, an electrolyte. Of interest in this context is, however, the observation that on the microscopic level, all TiO2 nanotube layers (nonmodified and modified) show preferential wetting on the outer wall (the intertubular space) rather than on the inside (see Figure 23c).590 This observation is in line with those for dry anatase tubes: the inside of the tubes is not easily filled by aqueous electrolytes.591 Another elegant way to adjust the wettability of nanotube layers is by applying mixed monolayers with a different degree of polarity or even actively switchable polarity. Such mixed monolayers of N-(3triethoxysilyl)propylferrocenecarboxamide and perfluorotriethoxysilane were used to demonstrate electrical redox switching of attached ferrocene molecules and thus to induce alterations of the wettability on TiO2 nanotube layers accordingly.592

Figure 24. Schematic diagram of DSSCs: (a) principle of a dyesensitized solar cell; (b) different configuration using nanoparticle and nanotube layers, front-side illuminated (left) and back-side illuminated (right) (1, platinized FTO; 2, iodine electrolyte; 3, TiO2 layer; 4, FTO substrate; and 5, Ti metal).

6. APPLICATIONS OF TiO2 NANOTUBES electrolyte is used that itself then is rereduced at a platinized counter electrode. In these solar cells, the TiO2 particle network plays only the role of an electron transport medium to the back contact.595−599,604 Over the years, most of the efforts for enhancing conversion efficiency have targeted the optimization of suitable dyes605−607 and optimizing metal oxide materials and structures. Key to a high efficiency are the time scales of the individual processes, as illustrated in Figure 24. After light induces an electron excitation in the dye from the highest occupied molecular orbital (hom*o) to the lowest unoccupied molecular orbital (LUMO), the excited electrons are injected from the dye’s LUMO to the conduction band of TiO2 in a femto- to picosecond time scale. The oxidized dye molecules are reduced by the electrolyte redox reaction within nano-

6.1. Dye-Sensitized Solar Cells

One of the most investigated applications of TiO2 nanotubes is in Grätzel-type dye-sensitized solar cells (DSSCs). This type of solar cell has a considerable history involving the observation of photoelectric effect on sensitized silver halide in the 1870s.593,594 In the 1960s, the work of Gerischer and Tributsch on organic dye-sensitized semiconductive metal oxides showed a visible range photoresponse,595−597 and the work of Spitler and Calvin reported that excited electrons from rose Bengal dye can be injected into the conduction band of ZnO (although only of a quantum efficiency of 4 × 10−3).598,599 In 1985, Grätzel et al. reported on an efficient photovoltaic system using TiO2 nanoparticles and Ru(bpy)32+ complex600 that showed 80% quantum efficiency under visible light irradiation,601 and in 9416

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Figure 25. Key factors affecting efficiency of TiO2 nanotube-based DSSCs: Comparison of solar cell efficiencies for (a) different annealing temperatures, (b,c) different tube lengths for (b) back side illuminated and (c) front side illuminated cell configuration, and (d) different tube diameter at same length for back-side illuminated cell configuration. Data are taken from various sources.45,76,96,217,262,265,345,364,615,627,631,633,644,655,657,659,663−669

nanotube structures led to a higher short circuit current density than commercial Degussa P-25, not only due to higher dye loading but also due to a significant enhancement of the electron transport kinetics.57 As a result, a solar cell efficiency of 4.88% with a 4 μm thick nanotube-powder film layer was obtained. Such early results considerably stimulated further investigations of one-dimensional nanostructures in DSSCs. More recent examples of using templating are TiO2 hollow nanostructures that are formed on a cotton template. After the template was burned off, the one-dimensional open morphology and high porosity provide a relatively high specific surface area (BET = 52 m2/g) for dye-loading and good diffusional access of the electrolyte, resulting in a conversion efficiency as high as 7.15%.96 Another typical approach to fabricate advanced DSSCs is based on the use of hydrothermal TiO2 nanostructures.20,53 Hydrothermally formed TiO2 nanostructures generally have high specific surface area (with a BET over 100 m2/g) that allows a high dye loading that finally leads to conversion efficiencies for DSSCs that range from 6.7−8.9%.618−623 However, the specific surface area of hydrothermally formed titanate nanostructures is drastically decreased by the required heat treatments.623 Furthermore, the formed TiO2 nanostructures are obtained as a powder, slurry, or paste, and typically need to be deposited on a conductive glass substrate by doctor blading, screen-printing, or electrophoretic deposition.619 When using such deposition techniques, the one-dimensional nanostructure layers are oriented randomly, and due to this irregular arrangement the merit of one-dimensionality is to a large extent lost. As outlined before, a most directional charge transport is expected in an aligned arrangement of nanotubes perpendicular

seconds. However, electron transport rates through the TiO2 and the diffusion rates within the electrolyte are comparably slow (micro- to milliseconds). This is the reason the overall cell efficiency is to a large extent determined by the electron transport rate.608 Electron transport competes with the recombination within the TiO2, and with the dye and the electrolyte. Often this is characterized by the definition of a charge collection efficiency (ηcc), which can be estimated from the electron transport constant (τc) and recombination rate (τr) constant according to ηcc = 1 −

τc τr

(9)

6.1.1. Dye-Sensitized Solar Cells with 1-D Nanostructures. Generally, the electron transport rate in TiO2 nanoparticles is considered to be comparably slow due to surface states, intrinsic TiO2 defects, and grain boundaries, which play a role as electron trapping and recombination sites.609−613 To overcome the drawback of TiO2 nanoparticles (mainly to provide direct and less defective electron pathways to the back contact), one-dimensional TiO2 nanostructures such as nanorods, nanowires, and nanotubes have been considered as substitutes in photoanodes in dye-sensitized solar cells.609−615 Approaches involve the use of various nanotube geometries mainly produced by anodic self-organization and template assisted methods.57,70,74−78,96,616,617 One of the earliest attempts was the use of nanotube powders that consisted of nanotubes formed by a surfactant template assisted technique.57 The individual single-crystalline TiO2 nanotube structures had a pore diameter of 5−10 nm and a length of approximately 30−500 nm. The use of such 9417

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solar cell characteristics, mainly by an improved short circuit current and a higher open circuit potential, which result in a higher overall conversion efficiency.345,663 Figure 25a shows a compilation of various literature data of cell efficiency, for solar cells fabricated with different TiO2 nanotubes annealed at different temperatures. The increase in conversion efficiency with higher annealing temperatures is generally explained by the formation of anatase with an increasing crystallinity that finally leads to improved electron diffusion coefficients and lifetimes.628,629 On the other hand, as mentioned earlier, when TiO2 nanotube layers are annealed on their metallic substrate due to direct thermal oxidation of the underlying metal, thermal rutile layers are formed at the metal/tube interface.372 Generally, the higher is the annealing temperature, the thicker are these thermal rutile layers.362,372 As these rutile layers have lower electron mobility than anatase layers, the presence of thick layers considerably decreases the solar cell efficiency. Finally, at even higher temperatures (over 600 °C), the tube structures sinter and collapse. An optimal annealing temperature range of anodic TiO2 nanotubes on Ti metal substrates for DSSCs is generally found at 400−550 °C.173,345 However, Huang et al.361 reported that TiO2 nanotube membranes produced as described in section 3.4 (and thus Ti metal substrate free) could be annealed at temperatures higher than 700 °C without rutile phase formation (Figure 25a). Such TiO2 anatase nanotube layers were reported to show a 4 times faster electron transport than nanotubes annealed at 400 °C. The fast electron transport for high temperature annealed tubes mainly contributes to an enhanced cell efficiency (50% higher efficiency), even though the amount of absorbed dye was found to be 30% lower than that for tubes annealed at 400 °C. 6.1.4. Geometry Effects (Tube Length, Diameter, Wall Thickness, and Corrugation). As expected, the geometry of TiO2 nanotube layers considerably affects the resulting solar cell efficiency. Diameter and length of tube layers influence the surface area and thus the specific dye loading but also influence light reflection, internal light management, and electrolyte diffusion effects; this makes a direct prediction of the resulting efficiency not always straightforward. In the following, we discuss most influential geometry factors using wherever possible data where only one parameter at the time was investigated. Regarding tube length, in principle, by increasing thickness of anodic TiO2 nanotube layers the specific surface area increases and the conversion efficiency should accordingly be improved until electron diffusion limits are evaluated. Nevertheless, thick oxide layers, >20 μm grown in the common EG electrolyte, often show only a weak adherence to Ti metal substrate, and thus frequently a drop in efficiency is reported in the literature as shown in Figure 25b and c. From these data, an optimum length of TiO2 nanotubes for DSSCs has, in early works, been considered as 15−20 μm.45,233,280,345,630 However, recently So et al.233 reported ultrafast anodic growth of TiO2 nanotubes in a lactic acid additive containing electrolyte that can be grown to lengths >100 μm. These nanotubes show a considerably higher mechanical stability even for thick layers.233,631 For these tubes, the optimal nanotube layer thickness for a maximum solar cell efficiency is ∼40 μm. The solar cell efficiency is 20% higher than for 15 μm thick nanotubes due to 2.6 times higher dye loading. From Figure 25d, it is clear that also the diameter of TiO2 nanotubes is an important factor influencing the final solar cell

to the surface, that is, to the back contact. Therefore, many aligned templates have been used for the fabrication of TiO2 nanorods or tubes and used for DSSCs. Porous alumina membranes70,74−78,616 and ZnO nanorods/wires617 have very frequently served for this purpose. Overall, DSSCs produced using such TiO2 nanotubular structures typically show a 3−5% conversion efficiency.74−78,616,617 However, the fabrication process of such a template assisted TiO2 nanostructure is relatively complicated, and to reach the step to use the tubes in functional DSSCs takes a comparably long time. 6.1.2. Dye-Sensitized Solar Cells with Self-Organized TiO2 Nanotubes. Because of the simple synthesis, anodically formed self-organized TiO2 nanotube structures have been considered one of the most promising approaches to achieve vertically oriented fast electron pathways.614,615,624,625 The first attempt of using anodic TiO2 nanotubes in dye-sensitized solar cells was reported in 2005 by Macak et al.564 However, the aspect ratio and type of nanotubes in that report were not suitable for use in DSSCs (100 nm of diameter and 500 nm of tube length). It showed only 3.3% of incident photon to energy conversion efficiency (IPCE) in visible range and 0.036% of conversion efficiency in a fully fabricated DSSC.45 Over time, anodic TiO2 nanostructures have been improved; in particular, a smooth tube wall and high aspect ratio116,117,211,626 led to a drastically enhanced solar cell performance.45,217,345,627 Empirically, the conversion efficiencies of self-organized TiO2 nanotubes are highly related to geometry, crystal structures, cell fabrication, etc., as shown in Figure 25. However, there are some general important findings in TiO2 nanoparticle layers and nanotubes: (i) Zhu et al.615 investigated the electron mobility in DSSCs by measuring electron transport times and recombination rates. These authors found that the electron transport times in TiO2 nanoparticle-based and nanotube-based DSSCs are similar, due to a similar average crystal size being present in tube walls as in nanoparticles.615 Nevertheless, the recombination times in TiO2 nanotubes were found to be 10 times slower than for TiO2 nanoparticle layers; this results in a 25% higher charge collection efficiency for TiO2 nanotube layers as compared to TiO2 nanoparticle layers. (ii) Jennings al.624 reported the estimated electron diffusion length of TiO2 nanotubes in DSSCs to be in the order of 100 μm, based on measurements of electron diffusion coefficients and lifetimes. These measurements were carried out on a 20 μm thick TiO2 nanotube layer where a charge collection efficiency of close to 100% was obtained. The results were extrapolated to longer tubes using experimental data and numerical evaluation of electron transport and trapping properties in TiO2 nanotube-based DSSCs.624 These findings indicated that nanotube layers considerably thicker than 20 μm could be used for optimized nanotube-based solar cells. The authors, however, observed that for higher layer thicknesses delamination of nanotube layers from the substrate occurred. It was only much more recent work that established anodization procedures to obtain considerably more robust (better adherent) nanotube layers.233 In the following, we will discuss some key factors that strongly affect nanotube-based solar cells. 6.1.3. Annealing Effects. As-formed anodic TiO2 nanotubes are amorphous and need to be annealed (preferably to anatase) to show a sufficient electron conductivity for use in DSSCs. In general, by increasing annealing temperature the crystallinity of TiO2 nanotubes is increased; this affects the final 9418

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efficiency.345,632−635 In direct comparison of different diameter nanotubes in DSSCs applications, small diameter nanotubes show generally a higher cell efficiency due to a higher specific surface area and accordingly a higher dye loading.345,632 However, although small diameter aligned nanotube structures show clearly beneficial effects in DSSC electrodes, the growth of very small diameter nanotubes (such as 15 nm) with length >2 μm is difficult.632 It is, however, interesting to note that anodically grown one-dimensional TiO2 nanoporous structures show much lower dye loading (62%) and cell efficiency (70%) than nanotube structures, even if the diameter and length of tube/pore layers are similar.632 Recognizing the importance of surface area led to various tube geometry modifications. Additional gain has been reported for bamboo type nanotubes (as shown in Figure 10a and b).227,228 Such modulated TiO2 bamboo nanotube structures can show higher cell efficiency due to a higher surface area and resulting higher dye loading (50%).228 Additionally, Yip et al. revealed that such bamboo type TiO2 nanotubes on conventional nanotube layers can be used as photonic crystal layers.636 Such optical properties can also be used to enhance the overall solar cell efficiency of nanotube-based DSSC.636 The overall conversion efficiency of TiO2 nanotube-based solar cells is typically not fully matching the performance of classical nanoparticle based cells. A main reason is that TiO2 nanotubes on Ti metal substrate show a considerably lower specific surface area than nanoparticles (BETNT = 20−30 m2/g, BETNP = 50−150 m2/g). The most straightforward approach to improve the specific surface area is surface modification with small TiO2 nanoparticle layers.535,637−642 Typically, a significant enhancement of surface area of the nanotubes can be achieved by nanotube wall decoration with a so-called TiCl4 hydrolysis treatment.280 By this treatment, nanotube structures can be uniformly coated with 20−30 nm of TiO2 nanoparticles with ∼3 nm individual particle size (Figure 26). In comparison with bare TiO2 nanotube electrodes, the overall efficiencies are enhanced by 20−30%.280,643,644 This reflects that the specific surface area is indeed the most important parameter for cell efficiency enhancement. So far the highest reported solar cell efficiency using classic anodic tubes under back-side illumination approaches with TiCl4 treated nanotube structures is 7.6%.643 Another approach to enhance the efficiency of tubes is the use of single wall, instead of double wall, tube morphologies. As mentioned earlier (Figure 14c−e), annealing of single walled tubes leads to comparably larger crystallite size with considerably higher electrical conductivity as compared to conventional tubes.259 If such tubes are used together with an appropriate TiCl4 treatment, efficiencies up to 8.14% can be achieved.217 Another simple approach to improve geometry factors is growing nanotubes on already structured metal substrates such as Ti metal wire, mesh, and bifacial (TiO2 nanotubes grown on both metal surfaces) substrate.645−652 Such structures may provide higher specific surface area than TiO2 nanotubes on flat metal substrate. However, cell fabrication process is complicated for these 3D structures, and reliable cell fabrication is difficult. Frequently, it is also found that the solar cell performance is strongly affected by the morphology of the tube tops; an open tube top seems to be of a significant advantage.222,614,643,653 6.1.5. Front-Side DSSCs. Anodic TiO2 layers that are formed on a metal substrate have the drawback for optimized

Figure 26. Effect of TiCl4 treatment (TiO2 nanoparticle decoration) on TiO2 nanotubes: Top and cross-sectional SEM images of (a,c) before and (b,d) after TiCl4 treatment of TiO2 nanotubes. Reproduced with permission from ref 535. Copyright 2009 Elsevier B.V. (e) TEM image of TiCl4 treated TiO2 nanotubes. Reproduced with permission from ref 45. Copyright 2010 The Royal Society of Chemistry. (f) Comparison of solar cell efficiency before and after TiCl4 treatment for back-side illuminated cell configuration.

DSSCs fabrication that they can only be used directly (on the metal) in a back-side illumination configuration (Figure 24b). This back-side illuminated cell configuration leads to loss of photons by light absorption in the electrolyte and by reflection at the Pt coated counter electrode.654,655 The cell efficiency difference between front- and back-side illumination configurations with TiO2 nanotube layers is estimated at 20− 50%.656,657 To fabricate front-side illuminated DSSCs with TiO2 nanotubes, the most straightforward way is the growth of TiO2 nanotubes on a transparent conductive oxide (TCO) glass such as FTO or ITO.658 For this, a thin Ti metal layer needs to be deposited first by sputtering or evaporation on the TCO.338,656,658,659 The key parameter to achieve suitable TiO2 nanotube layers on such substrates is the adherence between Ti metal and the TCO. An optimum thickness of TiO2 nanotubes in front-side illuminated DSSCs is considered to be 15−20 μm. In other words, to grow optimized TiO2 nanotube layers on TCO glass, Ti metal layers need to be deposited to a thickness of approximately 5−8 μm (due to the volume expansion of metal to oxide of a factor of 2−3 during anodization).338 Using such a layer, the highest reported cell efficiency in a front-side illuminated configuration is 6.9% (including a TiCl4 treatment).659 Nevertheless, this efficiency is still considerably far from conventional nanoparticle-based DSSCs, and lower than the reported values for back-side illuminated TiO2 nanotubebased DSSCs. This may indicate that the quality of the deposited Ti metal on TCO glass substrate governs the critical 9419

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In a general scheme of a photocatalytic reaction (as shown in Figure 27), TiO2 absorbs light of a wavelength > Eg, and

properties of TiO2 nanotube layers in view of solar cell efficiency. Because of these difficulties, indirect approaches have been considered to build reliable front-side configuration DSSCs. Most investigated are approaches that detach the TiO2 nanotube layer from the Ti metal substrate and transfer the free-standing nanotube layers on a TCO glass substrate. Means to detach TiO2 nanotubes from Ti metal have been described in section 3.4. To fabricate DSSCs with such free-standing TiO2 nanotube membranes, the membrane structures need to be attached on the TCO substrate. For gluing the nanotube layer on the TCO substrate, TiO2 nanoparticle paste or Ti alkoxide are usually used. A general finding from these investigations38,262,660,666 is that a strong binding to the nanoparticle glue is needed, light reflection at the nanotube/nanoparticle interface needs to be minimized, and that a maximized redox electrolyte diffusion into nanotubes/nanoparticle structure should be achieved. To optionally satisfy these conditions, the bottom of the TiO2 nanotubes should be opened.660 Under such optimum conditions, the best conversion efficiency of front-side illuminated TiO2 nanotube-based DSSCs is 8.0% without additional particle decoration,361 9.1% after a TiCl4 treatment,660 and 9.8% after using bottom opened membranes.627 Nevertheless, it should be mentioned that TiO2 nanoparticle layer under nanotubes in NT/NP/FTO solar cell configuration also highly contributes to the overall conversion efficiency, and only few studies clearly separate the effects of tube layers from nanoparticle layers.266 6.1.6. Doped TiO2 Nanotubes for DSSCs. Another direction to improve conversion efficiency is to increase the electric conductivity of TiO2 nanotubes by introducing low concentration (less than 1 at. %) of doping elements such as niobium,661 tantalum,630 and ruthenium.662 By simple anodization of Ti alloys (Ti−Nb, Ti−Ta, and Ti−Ru), metal-doped TiO2 nanotube structures can be obtained. Such low concentration of metal dopant in TiO2 nanotubes mainly helps to reduce the recombination rate by a faster electron transport.661 If the concentration of the dopant in the structures is too high, usually the beneficial effects are lost. Under optimum doping condition, the cell efficiencies were reported to be enhanced by 15−35% as compared to nondoped nanotubes.630,661,662

Figure 27. Scheme of photoinduced processes at a TiO2 semiconductor/electrolyte interface: Light (hν) excites valence band electron to conduction band. Electron and hole react with environment acceptor (A) and/or donor (D). Acceptor and donor species are reduced and oxidized (=photocatalytic reactions). Competing reactions are recombination and trapping of electrons and holes (=reducing photocatalytic efficiency). Gray boxes give typical reactants and reaction products in photocatalytic reactions on TiO2. Reprinted with permission from ref 48. Copyright 2012 Wiley-VCH Verlag GmbH & Co.

electron/hole pairs are generated in the conduction and valence band, respectively. These excited charge carriers can then have different fates: (i) They can separate, travel on their respective bands, and finally transfer to the surrounding and react with the red−ox species. This would be the desired photocatalytic pathway. (ii) They may recombine by a direct band-to-band transition or via trap (localized) states in the gap, either in the bulk or at the surface. (iii) If the holes reach the surface they may form an oxidized state of the semiconductor, which can be detrimental (for many semiconductors such as CdS, Si, etc., hole accumulation can lead to full oxidation, e.g., Si0 → Si4+, and this may finally lead to semiconductor dissolution [photocorrosion]). This problem hardly occurs for TiO2 due to a favorable electronic structure. The thermodynamic feasibility of a photocatalytic reaction is given by the positions of the valence and conduction bands relative to the red−ox levels in the environment (as illustrated in Figure 27). From an application viewpoint, the most important reactions are the transfer of valence band electrons to H2O, H+, or O2 and the transfer of holes to H2O, OH−, or organic species. If we consider Figure 27 and an aqueous environment, then the transfer of conduction band electrons may lead to the production of H2. However, if O2 is present in the electrolyte, the conduction band electrons may “prefer” to react with O2 (compare red−ox potentials in Figure 28 to form superoxide, hydrogen peroxide, or water). For the valence band holes, except for a reaction with OH− or H2O to form O2, also OH· formation may occur and is often the desired reaction, for pollution degradation. In this case, formed OH· radicals are able to virtually decompose all organics to CO2 + H2O. Nevertheless, if the H2O concentration is comparably small, valence band holes may also be transferred directly to the organics and lead to their decomposition. A maximum efficiency for the photocatalytic reaction (looking at it from the semiconductor side) is when all charge carriers react with the species from the surroundings rather than recombine.

6.2. Photocatalysis

Ever since the ground-breaking work of Fujishima and Honda in 1972,19 TiO2 is regarded as the key photocatalytic material. Here, the semiconductive nature of TiO2 is used to absorb UV light and thus create charge carriers (electrons and holes) that then individually react with their environment. Most photocatalytic investigations focus on: (i) the conversion of sunlight directly into an energy carrier (H2), (ii) the degradation or conversion of unwanted environmental pollutants, and (iii) to some extent, photocatalytic organic synthesis reactions. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use.417 This is because TiO2 combines a very high stability against photocorrosion with comparably low cost. Not only the electronic properties of a material, but also its structure and morphology can have a considerable influence on its photocatalytic performance. Therefore, in recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention, for use as a photoelectrode. 9420

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Geometry for a defined chemical or electronic gradient or junction fabrication is provided. 6.2.1. Some Key Factors for the Photocatalytic Efficiency of TiO2 Nanotubes. The most important factors that influence the photocatalysis of TiO2 nanotubes are the crystallinity, length, diameter of the tubes, together with compositional effects. In early reports, less defined tubes were used just to show photocatalytic activity, but it could nevertheless be demonstrated that the nanotube layers can have a higher efficiency than comparable compacted nanoparticle layers.671 In general, it is found that also for a photocatalytic use of nanotube layers, for low reactant concentrations a Langmuir−Hinshelwood kinetics holds,48 and that sufficient solution agitation (in most cases) prevents that reactant diffusion effects (onto the tube layers) play a significant role. As for particles, and as expected from a point of zero charge of TiO2 of approximately 6−7, for acidic pH typically a better adsorption of, for example, COO−-containing molecules (for example dyes) is observed, and typically at least slightly increased photocatalytic kinetics is observed.672 In the following, some comparably well-studied parameters are discussed for TiO2 nanotubes. 6.2.2. Annealing. As-formed (amorphous) TiO2 nanotubes show a significantly lower photocatalytic activity than tubes annealed to anatase or rutile.29,358,359,626,673 Figure 29a shows the comparison of the photocatalytic activity of TiO2 nanotubes annealed at different temperatures and environments.674 The photocatalytic activity increases with increasing temperature (above 300 °C), first due to anatase formation at 300 °C and secondary due to crystallinity.345,358 Above ∼500 °C, rutile phase starts forming with the highest photocatalytic activity for tubes annealed at ∼650 °C (Figure 29a), that is, when a mixed anatase/rutile structure is present. These results are, in this general form, manifold confirmed,671,675−677 but it should be considered what light source is used for excitation, for example, a broad spectral UV/vis lamp (such as a solar simulator) or pure UV (e.g., a laser). This is of special importance because a solar simulator spectrum possesses a strong intensity in the range of 3.0−3.2 eV. In other words, the small difference in band gap between rutile and anatase considerably influences the results; this is not the case if a deep UV source is used. In this case, explanations in terms of an anatase/rutile junction due to band offsets are more plausible than simple light absorption arguments.48,678 As mentioned before, if the annealing temperature is higher than 650 °C, the tubes start to collapse, and the lower photocatalytic activity is rather due to the destruction of the tubes than to a high rutile content.173 If the annealing process takes place under slightly reducing atmospheres (Ar), a somewhat increased activity can be observed as a result of Ti3+ formation. The effect of Ti3+ formation has been attributed to a higher conductivity (better charge separation) or the formation of surface states that facilitate charge transfer.674 Recently, so-called “water annealing” was reported to convert amorphous TiO2 nanotubes to crystalline material,366 and it is similar to some other low temperature approaches.674 In these approaches, conversion to anatase is only partial as shown in Figure 14g, and the efficiency in photocatalytic or solar cell applications remains far below thermal annealing.674 6.2.3. TiO2 Tube Length, Diameter, and Type. The photocatalytic activity of TiO2 nanotubes, as of other TiO2 morphologies, is commonly investigated by dye decomposition measurements, using dyes such as methylene blue or acid

Figure 28. Redox potential and work function: Relative positions of various red−ox couples and work functions of various metals relative to the band-edges of TiO2. Reprinted with permission from ref 48. Copyright 2012 Wiley-VCH Verlag GmbH & Co.

However, hole and electron transfer may thermodynamically be possible but in many cases are sluggish and thus a slow photocatalytic kinetics is obtained. Therefore, frequently cocatalysts such as Pt, Pd for electron transfer or RuO2, IrO2 for hole transfer are used. The thermodynamic feasibility of reactions is slightly different for anatase and rutile TiO2. In the classic potential− pH diagram composed by Fujishima et al.559 (Figure 31a), the conduction band of anatase lies at more negative redox potentials than for rutile, and the valence band edges for both phases are at similar energies (redox potentials). However, a recent re-evaluation of band alignment shows an inverse conduction band offset (Figure 31b) and an according shift in the valence band positions.670 Here, both data are presented as still some ambiguity about exact band edge positions exists. These clearly need to be resolved to obtain a consistent picture on the thermodynamics of photocatalytic reactions on anatase and rutile TiO2. Most photocatalytic applications are carried out either with TiO2 nanoparticle suspensions, that is under open circuit conditions (electron and hole transfer occur from the same particle), or in photoelectrochemical two electrode configuration where TiO2 is generally used as a photoanode together with an inert or catalytic cathode such as Pt, C, etc. In this latter case, classically compacted nanoparticle electrodes have been used. However, over the past years, nanotube geometries and particularly anodic TiO2 nanotube layers gained a lot of interest due to various potential advantages: For anodic self-organized tubes, a key advantage is the fact that they are fabricated from the metal; that is, no immobilization process is needed, and the tubes are directly used as back contacted photoelectrodes. Directionality for charge separation, that is, as described in Figure 2, orthogonal separation of charge transport, can be exploited. Easy control of the photocatalytic size (diameter, length) is provided. Controlled doping via substrate can be achieved. 9421

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Figure 29. Key factors affecting the activity of anodic TiO2 nanotube-based photocatalysts: Photocatalytic degradation of AO7 for different (a) annealing conditions (temperature and atmosphere) using TiO2 nanotubes of thickness ∼1.5 μm. Reproduced with permission from ref 674. Copyright 2012 Springer-Verlag. (b,c) Tube formation and nanotube thickness, TiO2 nanotubes grown in (b) glycerol and (c) ethylene glycerolbased electrolyte. Reproduced with permission from ref 452. Copyright 2010 Wiley-VCH Verlag GmbH & Co. (d) Photocatalytic degradation of AO7 with doped nanotubes (mixed oxides) and particle decoration on TiO2 nanotubes. Reproduced with permission from ref 40. Copyright 2011 Wiley-VCH Verlag GmbH & Co. (e) Energy diagram of n-type TiO2 semiconductor for the case Ef,sc > Ef,redox and applying an anodic bias (+ΔU) that leads to an increase in band bending and an increase in the space charge layer width (W). Reproduced with permission from ref 48. Copyright 2012 Wiley-VCH Verlag GmbH & Co. (f) Schematic of effect of noble metal particle on a n-type semiconductor surface (TiO2) in a band diagram. Reproduced with permission from ref 48. Copyright 2012 Wiley-VCH Verlag GmbH & Co.

circuit decomposition to a large extent depends on the amount of absorbed light). In the case of ethylene glycol-based tubes, higher degradation kinetics of AO7 is observed for higher length of nanotubes until ∼16 μm (as shown in Figure 29c). This is in line with several other investigations by various researchers.505,681−684 However, there are a number of investigations that report either a maximum in the photocatalytic activity for tube layer thicknesses around 3−7 μm,151,681,684 or the absence of an influence of the tube length.685 Some discrepancies exist also for the influence of tube diameter. Several reports find no significant influence,48,683 but other works report a maximum

orange 7 (AO7). First experiments were carried out for tubes grown in aqueous electrolytes,671,679−681 and it was observed that they may be more efficient than comparable Degussa P25 layers.671 An overview of more recent investigations on the photocatalytic activity for different lengths and types of TiO2 nanotubes has recently been published.48 Some typical results for two types of nanotubes (water-based rough tubes and ethylene glycol-based smooth tubes) are shown in Figure 29b and c. In both cases, a strong increase in the degradation kinetics of AO7 can be observed with increasing tube length48,671 up to a certain limit (this is expected as the open 9422

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Figure 29b and c, it can be seen that tubes grown in aqueous electrolytes are more active than tubes grown in ethylene glycol electrolytes; this has been ascribed to ripple formation on the tube walls for water grown tubes, which may affect charge carrier trapping.35 Another factor that is crucial is the topgeometry of the tubes,279,293,687,688 not only because it can affect the electronic properties of the tubes but can also strongly influence the reflectivity of a nanotube layer.48 Besides self-organized nanotubes, there are also reports about other forms of self-organized structures such as self-organized mesoporous TiO2.150 These structures were termed “titania mesosponge” (TMS) or “nanochannelar” structures. These TMS layers, when formed, can contain significant crystallinity (anatase and anatase/rutile) and when annealed can show enhanced photocatalytic activity as compared to P25 layers or TiNT layers, depending on layer thickness and annealing conditions.149 6.2.4. Applied Voltage. Using an applied voltage to carry out photocatalytic reactions on a TiO2-based photoelectrode dates back to the early 1990s when Kamat et al.689,690 reported electrochemically assisted photocatalytic degradation of organic pollutant. By applying anodic potentials to the TiO2 electrode, charge separation in the increased field of the Schottky barrier is accelerated, and holes are driven more efficiently to the surface, enhancing the photocatalytic reactivity (Figure 29e).305,400,691 A similar behavior was also shown for nanotube electrodes400,692 and has been confirmed several times.672,674,693 Additionally, at higher anodic voltages, Schottky barrier breakdown can occur, and that leads to valence band ionization694 and hole generation even in the absence of light. Such a “dark photocatalysis” approach may be particularly useful in environments where the use of UV light is hampered, for example, in MEMS devices or lab on a chip that require a “photocatalytic” reaction or a self-cleaning step in the dark.400

Figure 30. Photocatalytic activity of hydrothermal TiO2 nanotubes and comparison with P25 nanoparticles. Effect of several modifications of TiO2 on photocatalytic H2 production in glycerol/water mixture solution under UV/vis (17% of UV and 83% of visible) light illumination (Vsolution = 400 mL, glycerol concentration = 10 vol %, catalyst amount = 0.4 g). Reproduced with permission from ref 409. Copyright 2012 John Wiley & Son, Ltd.

efficiency at around ∼100 nm,682,684 or other trends.686 These discrepancies can be attributed to the fact that it is very difficult to vary tube length independently from tube diameter (e.g., compare refs 48,681). Other very relevant morphological features of tubes seem to be their side wall morphology35 or tube top features.279,293,687,688 For example, when comparing

Figure 31. Band positions of TiO2 anatase and rutile: (a) Energy bands of TiO2 to relative redox potentials of water as a function of pH according to Fujishima et al.559 Reproduced with permission from ref 559. Copyright 2008 Elsevier B.V. (b) Alternative model for relative positions of valence and conduction band for the anatase and rutile interface according to David et al.670 Reproduced with permission from ref 670. Copyright 2013 Macmillan Publishers Ltd. 9423

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some extent.726−729 TiO2 photocatalysis is considered to be effective in sterilization effects using bacteria such as Escherichia coli (E. coli), Methicillin-resistant Staphylococcus aureus (MRSA 11D 1677), and Pseudomonas aaruguuinosa (IFO 13736). For these bacteria, TiO2 substrates were reported to have a strong antibacterial effect,730,731 even under very weak UV light.729 This type of photocatalytic effect was also investigated in view of cancer cells.726,732 6.2.7. Water Splitting. The use of TiO2 photoelectrodes (or TiO2 suspensions) to produce hydrogen from water has been highly investigated over the past decades, as, in principle, using TiO2 the photogenerated e− and h+ can react with H2O to form H2 and O2; that is, direct splitting of water can basically be achieved. As shown in Figure 31, for TiO2, at the conduction band the red−ox potentials for O2 → O2− and H + → 1/2H2 are very close, meaning that H2 generation and O2− formation are typically competing. At the valence band, O2 can be formed from water via various pathways including radicals that can react to O2. The reaction rates of the photocatalytic processes on pure TiO2 in water are typically limited by the kinetics of the charge transfer process to a suitable red−ox species. Therefore, at the conduction band, often catalysts such as Pt are used to promote H2 evolution, and at the valence band O2 evolution catalysts such as IrO2 or RuO2733−735 and/or hole capture agents such as CH3OH are used to promote the overall reaction rate. For water splitting, TiO2 is most efficient as a photoanode in a photoelectrochemical arrangement,559 that is, using TiO2 as a photoanode, under an applied voltage (coupled with a suitable cathode). This is because the slow cathodic H2 evolution reaction can be performed on a separate ideal electrode (such as Pt) and the reaction can be “aided” by an applied voltage (inducing band bending and thus efficient carrier separation). Photoanodes based on TiO2 nanotube layers have been reported to be more promising than nanoparticulate layers due to their well-defined geometry,31,151,516,671,672,695,736−741 and the feasibility to easily incorporate cocatalysts and dopants.31,377,434,435,695,742 Particularly promising results regarding the alterations of TiO2 nanotubes have been reported regarding RuO2 by in situ doping (i.e., growth from Ru-containing alloys). Already very low concentrations can cause a significant increase in the photoelectrochemical water splitting efficiency. In this context, it should be mentioned that an often neglected key point in photoelectrochemical arrangements is that the majority of carriers (electrons) have to travel through the TiO2 layer to the back contact of the photoanode, that is, electron lifetime, and, in particular, conductivity within the TiO2 structure becomes a very important factor for the overall efficiency. Therefore, doping nanotubular layers with appropriate elements such as Nb (in low concentrations) was also found very efficient to increase the water splitting efficiency in photoelectrochemical arrangements31,434,435 (see Figure 32). 6.2.8. CO2 Reduction. Photocatalytic reactions on TiO2 have also been examined for the reduction of undesired highly stable molecules such as carbon dioxide (CO2). Photocatalytic reduction of CO2 in the presence of H2O on suitable semiconductors can lead to the formation of desired products such as CH4, CH3OH, HCHO, and other higher carbon chain molecules.743,744 Therefore, photocatalytic conversion of CO2 is considered in view of climate remediation by reducing the greenhouse gas stresses while producing useful chemicals as a product. Anpo et al.745−747 studied the anchored CO2 and H2O

6.2.5. Doping. Several types of doped TiO2 nanotubes have been explored for photocatalytic reactions.48,695 A compilation of the photocatalytic activity in view of organic degradation for various mixed oxide tube layers is shown in Figure 29d. In contrast to Al doping (one of the most efficient additives inducing carrier recombination),452,695 WO3 and MoO3 mixed oxide tubes show a strongly enhanced photocatalytic activity as compared to nondoped tubes for the degradation of AO7 dye. The highly beneficial effect for W and Mo cannot be explained by a better charge transport in the tubes but must be ascribed to modification of the band or surface state distribution of the doped nanotubes.696−698 Graphene−TiO2 nanotubes539 showed higher photocatalytical activity than normal unmodified TiO2 nanotubes. Hydrothermal nanotubes can be easily doped (see section 5.2) during hydrothermal treatment in view of enhancing photocatalytic activity. Fe doped TiO2 nanotubes showed increased activity on the photodegradation of methyl orange.458 Pt and N doped nanotubes409 showed a higher activity than nanoparticles, as indicated in Figure 30. Gadolinium and nitrogen codoped TiO2 nanotubes463 have been shown to possess higher catalytic activity in the Rhodamine B degradation reaction (the presence of Gd3+ leads to higher cystallinity,699 can sensitize the surface of the nanotube,463,700 and enhances the photocatalytic activity of TiO2 in the visible light region). An increase in the photocatalytic activity was also observed for C,N,S-tridoped TiO2 nanotubes.467 Silica-coated nanotubes407 annealed at 650 °C showed higher photoactivity than nanoparticles. Titania nanotubes modified with 4 wt % WO328,408 and annealed at 380 °C also enhanced photocatalytic activity, as compared to nondoped materials. 6.2.6. Degradation of Pollutants and Undesired Biological Entities. Several toxic organic compounds, such as organochlorine compounds,697,701−704 aromatic pesticides,705 PCB, dioxins,706,707 DDT,697 azo dyes,528,708−717 and others,718 can be degraded relatively fast by photocatalysis, leaving small traces of intermediates. Several of these processes were also explored using TiO2 nanotubes.693,704,719,720 Similarly, TiO2 nanotube layers were used for the destruction of gaseous pollutants as irritants,684,709 or the photoreduction of Cr6+.721 A considerable advantage of tubes over powders is their easy applicability in static flow-through reactors, possibly including the application of an aiding voltage. Nevertheless, this approach has hardly been explored up to now. Early investigations on hydrothermal nanotubes by Liu et al.722 showed that this type of TiO2 nanotubes has a better photocatalytic activity than nanoparticles for the degradation of methylcyclohexane. On the other hand, Thennarasu et al.723 showed that nanostructures obtained by hydrothermal treatment (titanate nanotubes and nanoribbons) have no significant photocatalytic efficiency for rhodamine B degradation prior to calcination. Nevertheless, already calcination at 150 °C (24 or 72 h) showed promising results. Hydrothermal TiO2 nanotubes annealed at 400 °C724 were also used for degradation of brilliant red X-3B from aqueous solution. The influence of sodium on the photocatalytic properties has also been analyzed.725 For example, at low annealing temperatures (75%). That is, such ordered structures reach close to an ideal Li2Ti2O4 stoichiometry that

Figure 37. Anodic TiO2 nanotubes for Li-ion batteries: (a) Areal capacity and (b) normalized areal capacity versus the current density for different thicknesses of anodic self-organized TiO2 nanotubes.

considered that the detailed anodization conditions for these data are different). Nevertheless, if the data are given as normalized areal capacities against tube length (areal capacity/ tube length), similar values are obtained (see Figure 37b). In addition to tube geometry, top morphology and highly aligned TiO2 nanotubes have also been considered for enhancing the capacity in Li-ion batteries. The main aim is to reach a higher diffusion rate of electrolyte. Recently, Wei et 9428

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Yu et al.815 reported 10 μm thick TiO2 tubes that were modified with Fe2O3 particles by dip-coating using a FeCl3 solution, thereby increasing the nominal capacitance from 300 to 600 μA h cm−2. For 1 μm long TiO2 nanotubes with electrodeposited Co3O4 and NiO submicrometer particles, Keyeremateng et al.816 reported an increase in the capacitance from 22 to 100 μA h cm−2 for Co3O4, and from 22 to 90 μA h cm−2 for NiO, respectively. Another approach by Fan et al.793 used a 5 μm thick TiO2 nanotube array that was covered by Co3O4 via photo deposition from a K2Co(CO3)2 solution; an improvement from 120 to 400 μA h cm−2 was obtained. Ortiz et al.817 used a 2 μm thick nanotube array to grow several micrometers big Sn-oxide crystals on the top of the tube surface, which led to a capacitance increase from 95 to 140 μA h cm−2. Alternatively, ALD coating of nonconductive amorphous TiO2 nanotubes with ZnO led to a capacity enhancement from 74 to 170 μA h cm−2.800

al.788 investigated highly aligned anodic TiO2 nanotube structures formed by two-step anodization, followed by a wall-thinning process using chemical dissolution. After 100 charge and discharge cycles, the modified anatase TiO2 nanotubes show an areal capacity of 460 μA h cm−2 at 0.05 mA cm−2 that is more than twice higher than for conventional single step anodized nanotubes (200 μA h cm−2 at 0.05 mA cm−2).296,788 Another important approach to achieve better battery performance is to modify TiO2 nanotubes with other active materials. Most frequently explored is decoration with noble metals, Ag,782,794,795 or secondary transition metal oxides, Snoxide, 796−798 ZnO, 799,800 Co 3 O 4 , 801−803 NiO, 804−806 Fe2O3,807−809 MoO3,813 or nonmetallic doping materials, C,455 N,420 S.810 Such modifications increase Li-intercalation properties, increase the conductivity overall or locally,778,811,812 or increase pseudo capacitive contributions.782,794,813,814 From the compilation of the areal capacity for various modified anodic TiO2 nanotubes (Figure 38), it is evident that different modification techniques lead to much higher areal capacity than bare TiO2 anodic nanotubes.

6.4. Sensors

TiO2 nanotubes have also been explored to a considerable extent for sensing applications, and in particular toward gas sensing. Many approaches target the use of TiO2 in quantitative or qualitative analysis, with a maximum sensitivity, toward one specified type of gas.818 The unique properties of titanium dioxide such as a high chemical stability, high temperature resistance, combined with its semiconducting behavior make this material a very promising candidate for sensing devices. One of the first TiO2 sensing layers was reported in 1983 by Logothetis and Kaiser819 for high-temperature oxygen sensing to monitor and control the combustion process of the air−fuel mixture of internal combustion engines. In 1989, the first report on hydrogen gas sensing showed compact crystalline TiO2 to have a good sensitivity combined with a relatively fast response and recovery behavior.820,821 In 1999, Dutta et al.822 used TiO2 nanoparticles instead of SnO2, which was at that time the most extensively investigated semiconducting metal oxide for CO gas sensing,373 the main advantage of TiO2 being its stability in exhaust pipe environments. Meanwhile, another important advantage of TiO2 over SnO2 is that defined nanostructures, such as nanotubes, nanoparticles, nanowires, or other nanoporous assemblies, can easily be obtained by a variety of fabrication methods.823 From density functional theory (DFT) calculations, the defect structure of the TiO2 surface is found to be a crucial factor to sense, for example, SO2 and CO2 gas molecules.824,825 According to theory, it is possible to estimate the number of oxygen vacancies by measuring the change of resistivity when the sample is brought into CO containing environment.825 The high surface to volume ratio of nanomaterials results in a sensing response that is enhanced by several orders of magnitude as compared to flat compact surfaces.826 In addition, TiO2 possesses suitable band edge positions to detect a number of gases. The reaction with a red−ox active gas species injects or consumes electrons from the TiO2. A variety of sensing strategies can be used that are based on field-effect transistors, electrochemical phenomena, fluorescence, acoustic wave speed, and, most characteristic for metal oxide semiconductors, simply the electrical resistance. The response r of a sensor device can be defined as the relative change of the resistivity:827

Figure 38. TiO2 nanotube modifications for Li-ion batteries: Normalized areal capacity versus the current density for different modifications of anodic TiO2 nanotube electrodes.

The earliest attempts to enhance the nanotube performance with a noble metal coating were reported by Fang et al.782 in 2008. Anodic TiO2 nanotubes can be decorated with silver nanoparticles via simple dip-coating deposition in an AgNO3 solution. This additional treatment increases the capacitance of the tubes from 90 to 110 mAh g−1.782 For example, Guan et al.795 have attempted electrodeposition of Ag nanoparticles on bamboo type nanotubes. For an optimized electrodeposition, an impressive increase of the capacitance values of more than a factor of 2 (from 55 to 131 μA h cm−2) could be observed.795 To deposit secondary metal oxides on TiO2 nanotubes, several techniques are used such as dip-coating, sputtering, electrodeposition, photodeposition, or atomic layer deposition.

r= 9429

(R − R 0) R0

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Figure 39. Sensors: Schematic representation of sensing mechanisms for ceramic-based sensor devices. (a) Change in the Schottky barrier between semiconductor and metal contact when active gas arrives in that area. (b) Gas triggers change in the conductivity of the grain surfaces. (c) Gas triggers change in contact resistivity between two touching grains.

where R represents the resistance depending on the concentration of the tested gas and R0 is the reference resistance, meaning the resistance for normalized conditions (e.g., in an atmosphere of N2, dry air, or under ambient conditions). The sensitivity S of the sensor device is the dependence of the response to a change in the concentration M: S=

1 ∂R ∂r ∂ (R − R 0) ∂R = = ∝ R0 R 0 ∂M ∂M ∂M ∂M

All reactions mentioned above take place at the surface of the semiconductor. As nanostructured sensors exhibit a high surface area, accordingly they have a high fraction of electron-depleted zones. The relative change of the resistivity is therefore much higher when atoms are adsorbed to the surface, than in macroscopic sensors where the electrondepleted surface layer is only a small fraction of the material. A maximum of sensitivity can be achieved when the pattern size of the nanostructure (e.g., thickness of the tube wall) is in the range of the Debye length of the material. Another benefit of nanostructures is the possibility to minimize the size of sensor devices by orders of magnitude, which is desirable for very large-scale integration and the reduction of power consumption in view of heating. 6.4.1. Response and Sensitivity. The main focus of most of the recent studies is to further decrease the lower limit of detection (LOD) of sensing devices. However, one of the fundamental challenges of sensors is to combine high sensitivity with extensive dynamic range. This is also the case when designing sensors from TiO2 nanotubes as illustrated in Figure 40. It shows the relative response and the sensitivity of two different titanium oxide nanotube array-based gas sensors; the data are taken from work of Li et al.47 and Lee et al.835 In general, the relative change of resistivity can be calculated according to eq 12, and the sensitivity is estimated by its slope. While the array in Figure 40a is grown by anodic oxidation, the TiO2 array in Figure 40b is fabricated by atomic layer deposition (ALD) into a template of porous anodic aluminum oxide. The anodic nanotubes have an outer diameter of 70 nm and a length of 5 μm and are examined at 215 °C for H2 sensing, while the ALD deposited nanopillars have a length of 750 nm and a diameter of 250 nm and are examined at 100 °C. When these sensors are compared, a considerable difference can be observed. While the sensing device in Figure 40a exhibits a relatively linear sensing behavior, allowing for a quantitative sensing of H2, the device in Figure 40b shows a high sensitivity; in this case, minimal concentrations of H2 can be detected but only over a small concentration range and only with a semiquantitative accuracy. This illustrates that TiO2 structure tuning in the nanometer range and sensor operation

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In general, the entire resistance change of the metal/metal oxide sensing devices can be attributed to changes of the Schottky barrier at the interface to the deposited metal contact due to changes in surface doping (or surface states) or due to changes in the neck-conductivity at a multigrain structure.828 These effects are induced when atoms and molecules are physisorbed or chemisorbed to the surface of the sensitive material or are absorbed to the metal.826 The degree of physisorption and chemisorption can be controlled by heating the sensor element. At temperatures higher than 100 °C, typically the influence of physisorption can be neglected. Because most ceramic-based sensors are n-type semiconductors, their conductivity is increased if they are exposed to acidic gases (which inject electrons into the semiconductor surface) and decreased when exposed to basic gases. These changes of the electrical transport mechanism hold for single crystal materials. Anodic nanotube walls do not consist of a single crystal, but are built up from crystallites. In general, a change of the electron-depleted zones influences the conductivity at grain boundaries. A schematic picture of these three effects is shown in Figure 39. The chemical reactions that are leading to the loss and the injection of electrons can be described by the following chemical equations: reduction: R + MO−O− → MO + RO + e−

(14)

which causes the injection of an electron, while the oxidation: MO−O− + O + e− → MO−O−2

leads to consumption of an electron.

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826

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measurements essentially deliver a “resistivity” along the direction of the tubes. To determine the resistivity perpendicular to the growth direction of the tubes, the TiO2 nanotube array has to be grown or transferred on an insulating substrate. Various geometries of anodic TiO2 nanotube layers were examined by Perillo and Rodriguez843 in 2012. They used a glycerol NH4F electrolyte and a 2 h 550 °C heat treatment to produce tubes of 50, 90, 110, 150, 200, to 240 nm diameters. While the diameter is significantly increased, the tube wall thickness increased only slightly from 20 to 30 nm. The gas sensing behavior was tested with 400 ppm ethanol and 150 ppm ammonia gas at different relative humidities of 40% and 90%, at room temperature. The authors found that the tube size does not have a significant impact on the sensing behavior. However, a low humidity leads for both gases to a drastically improved response. While in relative wet air a relative response for ammonia of 0.06 can be detected, in drier air a relative response of 0.4 is observed. The use of amorphous TiO2 nanotubes, grown in a (NH4)2SO4 water-based electrolyte with 0.5 wt % NH4F for the use of O2 sensing, was reported by Lu et al.832 in 2008. The 2.3 μm long amorphous tubes with a 40 nm wall thickness showed a relatively good linear O2 sensing behavior between 200 ppm and 20% at 100 °C. This corresponds to a maximal sensitivity of 170. In contrast, crystalline anatase TiO2 nanotubes with comparable morphology showed only a maximal sensitivity of 20. The mechanism behind this phenomenon is the change of the charge carrier concentration on the nanotube surfaces. Amorphous nanotubes are more disordered and contain more sensing-active defects such as oxygen vacancies373 but often show a decreased long-term stability and reversibility. A very interesting sensing material is TiO2−B.858 An important inherent advantage of this material is the crystal structure, which exhibits intrinsically abundant surface states and oxygen vacancies. If the material is used to synthesize nanowires (with a diameter of 20−50 nm), humidity in the range of 5−95% relative humidity could successfully be detected.858 6.4.3. Temperature Effects. In most sensing studies, temperatures around 200−500 °C are used. This is due to the fact that at temperatures close to room temperature, metal oxide sensors will show cross-sensitivity for humidity along with the sensing gas. Therefore, the very first step toward selective sensitivity is heating. While water from the atmosphere is physisorbed, oxygen is chemisorbed, that is, is stronger adsorbed. When the sensor is heated to 100−200 °C, the water (including surface adsorbed layers) is released so that the influence of humidity can be neglected. Nevertheless, there are strong efforts to make reliable sensor devices to operate at room temperature.859 For example, Palacios-Padrós et al.840 managed to detect H2 with a concentration of 9 ppm at a temperature of only 80 °C using a SnO2-nanotube array as a sensing device. Chen et al.834 built a room-temperature hydrogen sensor based on highly ordered TiO2 nanotube arrays using a crystallized mixed anatase and rutile phase. For hydrogen sensing tests, the arrays were contacted with Pt electrodes, and H2 concentrations in dry ambient air between 10 and 3000 ppm were used. Even at the lowest concentration, the conductance shows a well-defined peak with a response time of 53 s. Up to a concentration of 1500 ppm, the change in resistivity is reversible; however, at higher values a different behavior is observed. Not only is the resistivity not recovering

Figure 40. Comparison of response and sensitivity in H2 sensing using different TiO2 nanotube arrays: (a) anodic TiO2 nanotubes with a length of 5 μm at a working temperature of 215 °C; (b) TiO2 nanotube produced by atomic layer deposition into anodic aluminum oxide nanopores, at sensing temperature of 100 °C. Data taken from refs 47 and 835.

conditions can lead to drastic changes in LOD and the dynamic response that can be achieved. 6.4.2. Preparation of Sensing Devices from TiO2 Nanotube Arrays. The first anodic TiO2 nanotube sensor for sensing H2 was reported by Varghese et al.829−831 in 2003. The nanotube layers were grown on a titanium foil in a waterbased electrolyte. The lowest detection limit for H2 was 100 ppm. Up to now, studies report on the use of TiO2 nanotubesbased sensors in the detection of different gases, for example, oxygen (200 ppm of O2 at 100 °C, 10 ppm of O2 in CO2 at 600 °C),832,833 hydrogen (10 ppm of H2 at 25 °C in dry air),47,834−841 carbon monoxide (100 ppm of CO at 200 °C),822,835,842 ammonia (150 ppm of NH3 at room temperature),843,844 ethanol (400 ppm of CH3CH2OH at room temperature)842,845−847 and formaldehyde (10 ppm of HCHO at room temperature),847 nitric oxide (0.97−97 ppm of NO at room temperature),848 nitrogen dioxide (0.97−97 ppm of NO2 at room temperature),848 sulfur dioxide (50 ppm of SO2 at 200 °C),824,849 thionyl fluoride (50 ppm of SOF2 at 200 °C), sulfuryl fluoride (50 ppm of SO2F2 at 200 °C), hydrogen peroxide (3 × 10−5 mol L−1 H2O2 in solution),850 sulfur hexafluoride (99.999% SF6 at 200 °C),849 and humidity (11− 95%).851 Different techniques can be used to obtain TiO2 nanotube electrodes for sensing. While tubes prepared by electrochemical anodization are already backcontacted by the Ti-substrate, top contacts are generally metals such as Pt,851,852 Au,853,854 Ag,849,853,854 Al,855 stainless steel,843 or Cu856 that can be sputtered or evaporated in different configurations on the array of TiO2 nanotubes.857 In two terminal measurements, an overlap of dc-resistance and Schottky diode behavior will be seen (if no ohmic contact is used). Pure dc-conductivity requires four terminal measurements. Four- and two-point 9431

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example, hydrogen gas split-up into hydrogen atoms can be facilitated. This leads to an increased interaction of adsorbed dissociated hydrogen with TiO2, and results therefore in an enhanced hydrogen sensing performance.864 Jiang et al.865 showed in 2013 the decoration of TiO2 nanotubes with Ag for H2O2 detection. Fifteen nanometer Ag particles were deposited on the walls of the nanotubes by electrophoretic deposition. As compared to plain TiO2, the decorated nanotubes showed a 3 times enhanced sensitivity (up to 184.24 mA/M·cm2) and a low detection limit (85.6 nM).865 Modified hydrothermal titania nanotubes with Pt−Ir by MOCVD were reported by Colindres et al.863 The nanotubes with a length of several hundred micrometers and a diameter of 7−10 nm were decorated with Pt and Ir particles. While Ir or Pt forms single particles during the deposition, a bimetallic particle was grown when both precursors were used. The combination of TiO2 nanotubes with the Pt−Ir nanoparticle mixture showed the highest activity, when tested for the detection of cyclohexene. Pd and Pt nanoparticle decoration for H2 sensing was also used by Han et al.866 in 2007. Mixing the precursors in the right concentrations led to the hydrothermal synthesis of Pd and Pt decorated titanate nanotubes with a diameter of 100 nm and with small, dispersed noble metal particles. The best sensing was obtained by a mixture of Pd and Pt on titanate nanotubes. For all temperatures, the mixed nanoparticles showed an enhanced performance as compared to single metal decoration; the best sensing performance was achieved at 250 °C. The rate of oxidation reactions on Pd/Pt decorated nanotube surfaces was found to be almost twice as high as compared to other catalysts.866 In the context of nonenzymatic glucose sensing, Gao et al.861 reported a significant improvement of the anodic current density for glucose oxidation when carbonized TiO2 nanotube arrays (TiOxCy NTs) are decorated with Ni(OH)2 nanoparticles without the aid of a polymer binder.861 Improved properties such as a wider linear range, a low detection limit, fast response, and long-term stability were observed. Another efficient way to modify the physical and chemical properties of TiO2 is to dope or alloy the semiconductor system. The sensing sensitivity or the sensing stability can be improved by adding different dopants, for example, Ni,855 Rh,820 Nb,867 WO3,868 ZnO,869 Ga2O3,870 Al2O3, and V2O5.871 While in most cases thin films in form of nanoparticles are used as sensing layer, only few reports with doped or alloyed TiO2 nanotube arrays are published. Li et al.872 used a NiTi alloy (50.8 at. % Ni) and further annealing treatment (425 °C for 1 h) after forming the oxide nanotubes in a nonaqueous ethylene glycol/glycerol electrolyte. The grown nanotubes (diameter of 65 nm, length of 500 nm) were used to fabricate a sensor device that could detect 1000 ppm hydrogen at room and elevated temperatures. Alloying of the substrate led to TiO2 mixed oxides and was perceived to result in a change of the semiconducting behavior; an amount of approximately 7 at. % of Ni was reported to show a p-type behavior, which resulted in a resistance increase during H2 sensing.872 The same group showed the doping of TiO2 nanotubes by anodizing a TiAl6 V4 alloy.871 Al and V doping were reported to lead to a reduction of the band gap, with a p-type sensing behavior for hydrogen as compared to nondoped TiO2 nanotube films.871 The authors found the annealing temperature to have a significant influence on the sensing behavior of

to its original value, but also the induced change in resistivity for higher concentrations gets smaller. It is suggested that this behavior is related to the saturation of the nanotube surface with hydrogen. However, with this device it seems possible to analyze hydrogen gas in dry air up to concentrations of 1500 ppm without heating. Lee et al.835 conducted a study of the sensing behavior at different temperatures of a device made from ALD-deposited TiO2 nanotubes. The relative resistivity and response time at different temperatures are shown in Figure 41a and b. The relative response shows a maximum at 100 °C, while the response time decreases with increasing temperature.

Figure 41. Temperature effect for H2 sensing: (a) Response at various hydrogen concentrations and (b) response times of anodic TiO2 nanotube sensors to 1000 ppm hydrogen as a function of sensing temperature. Reproduced with permission from ref 835. Copyright 2011 Elsevier B.V.

6.4.4. Improved Sensing Performance. To enhance the sensing performance, in particular the sensitivity and response of TiO2 nanotube arrays, considerable efforts have been made over the last two decades. Preferential doping, decoration of nanoparticles, and optimizing sensing parameters were explored for different gases.373,818,823,826,860 The sensing properties of TiO2 nanotubes can be drastically enhanced when the arrays are decorated with secondary materials such as nanoparticles of Ni(OH)2 (glucose),861 Au (H2O2),850 Ag (O2),862 Pt (benzene, H2),838,863 Ir (benzene),863 PtIr (benzene),863 or Pd (H2).848 The most important effects of such a decoration are a change in the Schottky barrier due to the different workfunctions of the used material as well as chemical catalysis effects. In the presence of Pd or Pt nanoclusters on the surface of TiO2, for 9432

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the anodic nanostructures. The best performance was observed for samples annealed at 450 °C, where the saturation response of the films was more than twice the value of the 550 °C annealed sample (sensing conditions: 200 °C, 1000 ppm) and a response at room temperature was obtained. In 2010, Moon et al.873 showed the beneficial effect of Pd decoration on TiO2 nanofibers produced by electro spinning. The relative response of pure TiO2 at 200 °C for 2.8 ppm of NO2 is 30%, while the Pd doped fibers have a response of 55%. Even more impressive is the sensing ability for very low concentrations of 0.16 ppm of NO2 at 180 °C for the Pd doped sensor.873 6.4.5. Free-Standing TiO2 Nanotube Arrays. Another approach to enhance the intrinsic sensing properties of TiO2 nanotube arrays is to remove them from their substrates as described in section 3.4 and open the bottom to build a flowthrough sensor. Chen et al.259 reported large-scale free-standing nanotube arrays of 7−50 μm thickness by ultrasonic splitting of as-anodized TiO2 nanotube films. For such a nanotube array of a thickness of 25 μm, the relative resistivity was found to double when the film was free-standing.259 This significant increase in the relative resistivity may justify the comparably large effort of producing free-standing membranes. 6.4.6. Single TiO2 Nanotubes as Sensing Devices. The ultimate miniaturization of a sensing device is achieved when only a single nanotube (or a few nanotubes) is used as a sensing device. Such a single TiNT sensing device can be expected to have a very short response time, because there is no need for the gas to diffuse into a complex structure. Nevertheless, only very few reports on sensing with single TiO2 nanotubes can be found. Techniques as electron beam-induced deposition and electron beam lithography have to be used to fabricate electrical contacts to a nanotube of only a few micrometers length.32 The high resistivity observed for a single TiO2 nanotube in these reports (in contrast to section 4.2) requires an advanced system for electrical measurements. Lee et al.874 report the fabrication of a single TiO2 nanotube device produced by ALD into a AAO template for biochemical sensing, where contacts were fabricated with electron beam-induced deposition and electron beam lithography as mentioned above. The resistance of the TiO2 nanotube is obtained in a two terminal measurement before and after the tube surface was functionalized with glycine (C2H5NO2), lysine (C6H14N2O2), or γ-aminobutyric acid (GABA, C4H9NO2). In all three cases, the resistance reaches a final value after 12 h of amino acid treatment, and a relative change of resistivity of 2.88 (lysine), 2.19 (GABA), and 0.065 (glycine) is found.

Figure 42. Memristive effect with TiO2 layer: (a) I−V curve of a memristive TiO2 nanotube/Pt layer as shown in the cross-sectional SEM image of (b) formed at the bottom of a TiO2 nanotube stump. Resistive switching in the I−V curves occurs at approximately ±1 V. The inset shows I−V curves for conductive and resistive state (obtained after voltage pulses ±1.5 V). Reproduced with permission from ref 237. Copyright 2013 Elsevier B.V.

can cycle the voltage and notices either a very shallow or steep IV curve (inset in Figure 42a). In other words, applying a sufficiently high voltage step (above threshold potential), depending on the sign of the voltage, a higher or lower resistivity state is established in the oxide. Works by Yang et al.878 in 2008 and follow-up work879−882 on thin TiO2 layers demonstrate convincingly that the effect is associated with the presence of TiOx (suboxides) in the oxide layer. However, it is generally accepted that the memristive effect is based on mobility of oxygen vacancies or Ti3+, that is, that some degree of defectiveness of the film is required883 to cause such switching effects. The concept is that vacancies, originally present in a layer of TiO2, can be moved across the oxide using a sufficiently high applied voltage (field). A negative electrode attracts vacancies; a positive electrode rejects vacancies. A sufficiently connected vacancy path (percolating) throughout the oxide can lead to a conductive path in the oxide (often described as a conductive filament); as a result, the oxide as a whole shows high electron conductivity. By a sufficiently high reverse pulse, vacancies are repelled from the positive electrode, and the oxide as a whole shows a high resistivity. In other words, by sufficiently high voltage pulses that open or cut conductive filaments, one can switch the resistivity state forth and back reversibly, and hence this effect can be used as a data

6.5. Memristive Behavior

In 2008, Strukov et al.875 (revisiting some earlier electrochemical876 and theoretical877 work) demonstrated that a thin TiO2 (TiOx) film sandwiched between two platinum contacts shows a voltage-dependent on/off resistive switching. In such a memristive device, the conductivity of the TiO2 layer depends strongly on the history of previous voltages applied. The effect is illustrated in the I−V curve in Figure 42a. If voltage is applied to a Pt/TiO2/Pt sandwich, such as across a TiO2 nanotube bottom (shown in Figure 42b), in the positive direction, the current increases quite steeply; however, once a threshold voltage is passed, the current drops almost instantly and remains low in the reverse cycle until a lower threshold voltage is reached where the current suddenly increases again. If the voltages are kept below the upper and lower thresholds, one 9433

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Figure 43. Size-effect of TiO2 nanotubes diameter on cell interactions: (a,b) Fluorescence images of adherent rat mesenchymal stem cells (GFPlabeled) on TiO2 nanotubes of 15 and 100 nm diameters. Three days after plating an identical density of 5000 cells/cm−2 on the two surfaces, a strongly different cell density and cell spreading is obtained for 15 nm (a) and 100 nm (b). Reproduced with permission from ref 948. Copyright 2011 The Royal Society of Chemistry. (c) Comparison of cell activity for mesenchymal stem cells, primary human osteoblasts, osteoclasts, and endothelial cells for different TiO2 nanotube diameter (3 days after seeding). (d,e) Model illustrating how nanotubes of different diameter may affect formation of focal contacts. Reproduced with permission from ref 208. Copyright 2007 American Chemical Society.

energy density than conventional capacitors and a higher power density than batteries. On the basis of charge-storage mechanism, two types of supercapacitors can be distinguished. The first one is called electrochemical double layer capacitor (EDLC) that is based on the charge separation at electrode/ electrolyte interface without a Faradaic reaction. In this case, electrodes are mostly made of high surface-area carbon materials (e.g., CNT, active carbon, carbon aerogel).890−892 The second type of electrodes involves redox reaction, metal oxides such as MnO2, RuO2, SnO2, NiO, etc., or redox charge within conductive polymers. In this case, Faradaic processes, that is, redox-state switching in the oxide, occur.893−897 TiO2 nanotube structures have been considered as promising materials for the second type of supercapacitors due to their combination of a high specific surface area and a defined ion and charge transport directionality, as well as for their semiconductive properties and chemical stability. However, the reported capacitance of TiO2 nanotubes is not as high as for other active materials such as RuO2, NiO, or conductive polymer nanostructures. Nevertheless, some reports on the capacitance of anodic TiO2 nanotubes discuss considerably high values (911 μF cm−1 at 1 mV s−1), which are significantly higher than reported for nanopowder-based electrodes (181 μF cm−1 at 1 mV s−1).898,899 To enhance the capacitance, hydrogenated,900 nitrided TiO2 nanotubes901,902 or annealed TiO 2 in Ar atmosphere903 were used; such modifications of TiO2 nanotubes lead mainly to an increase of the electric conductivity.900−904 Nevertheless, the absolute capacitance value of TiO2 nanotubes is still not comparable with conventional materials due to the relatively low electric conductivity of TiO2. Another approach to use self-organized TiO2 nanotubes for supercapacitors is to use the nanotubes as template of active material. In other words, most active materials such as RuO2 or

storage element. Such memristor effects have attracted tremendous attention for use in nonvolatile memories. For a rapid and high magnitude memristive switching with a fast fieldaided transport, thin oxide layers (some nm-thickness) are preferred, as they create sufficiently high fields already at low voltages. In 2011, Szot et al.884 suggested an alternative to vacancy mobility, that is, that memristive switching might take place in amorphous TiO2 by formation of magneli-phases. On the basis of their XRD results, it was observed that under applied voltage, crystalline filaments of Ti5O9 or other magneli-phases are formed. Also, using this explanation it seems clear that the formation of conducting channels is localized and not uniform along the whole metal/TiO2 interface. Frequently used methods to fabricate thin film memristors involve atomic layer deposition or sputtering of TiO2 (or derivatives of it), followed by an adequate heat treatment (to adjust the vacancy concentration), and finally establishing a top contact (mostly Pt).878,885−889 For TiO2 nanotube layers, reports exist on the observation of memristic effects using entire layers of amorphous nanotubes889 or highly defined nanotubes with conformally metal filled bottoms,236,237 as shown in Figure 42b, where only the ∼30 nm thick tube bottom is used to create a memristive response. More recently, Liu et al.26 showed that reliable switching can also be achieved using crystalline (anatase) TiO2 nanotubes that are exposed to a reductive treatment in Ar/H2 atmosphere. These findings are in line with explanations given by Yang,878 that in any case suboxide or vacancies in the film must be present to achieve a memristive response. 6.6. Supercapacitors

Nanostructures, such as nanotubes, are also of high interest in electrochemical capacitors, supercapacitors or ultracapacitors. These devices are a focus of large interest due to their higher 9434

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Figure 44. (a−c) Effect of neighboring 15 nm/100 nm diameter tubes on cell activity shown on fluorescence micrographs of GFP labeled MSC on concise 15 nm/100 nm patterns: (a) 500 μm wide stripe pattern after 1 day in culture, (b) after 3 days in culture, and (c) cell numbers on isolated tube surfaces (15 nm, 100 nm, respectively) and on 15 nm/100 nm stripe pattern. Reproduced with permission from ref 934. Copyright 2012 Elsevier Ltd. (d) Effect of immobilized growth factors with schematic illustration of BMP-2 immobilization by covalent reaction of an amino group of the protein with grafted CDI. Fluorescence micrographs show that differentiation of MSCs to osteoblasts is strongly supported on 15 nm BMP-2coated nanotubes but much less on uncoated nanotubes as indicated by osteocalcin staining. No osteogenic differentiation occurred on 100 nm nanotubes. Reproduced with permission from ref 950. Copyright 2012 Wiley-VCH Verlag GmbH & Co.

hydrothermal nanotubes have been molded to pellets or tested in composites for in vitro or in vivo experiments.98 6.7.1. TiO2 Nanotube Interactions with Cells. The influence of TiO2 nanotubes on living cells (their growth, proliferation, and differentiation) has been intensely investigated in ongoing nanotube research.50,208,554,923−927 A key finding is that for cells on anodic TiO2 layers, the nanotube diameter significantly affects virtually any aspect in cell viability (Figure 43). In vitro/in vivo biocompatibility of 1-D titania nanostructures has been tested with mesenchymal stem cells, hematopoietic stem cells, endothelial cells, osteoblasts, and osteoclasts (eukaryotic cells), with tubes varying from 10−150 nm in diameter.209,923 The effect of different nanotube diameter on cell adhesion was first reported in 2007,208 showing that mesenchymal stem cells react in a very pronounced way to the nanotube diameter. Fifteen nanometer diameter nanotubes were shown to strongly promote cell adhesion, proliferation, and differentiation, whereas 100 nm diameter nanotubes were found to be detrimental, inducing programmed cell death (apoptosis). Furthermore, osteogenic differentiation of mesenchymal stem cells was stimulated on 15 nm but impaired on 100 nm diameter nanotubes. Nevertheless, it is worth mentioning that other studies yielded conflicting results (see, e.g., Oh et al.923): (i) small diameter nanotubes (30 nm) promoted adhesion without noticeable differentiation, and (ii) larger diameter nanotubes (70−100 nm) induced osteogenic differentiation. In the course of reasoning such different findings, a number of critical factors were screened such as the role of TiO2 crystallinity, residual fluorides remaining in the nanotubes after formation, surface pretreatment, or the cell type used.554,924−927 However, investigations of these factors further supported the beneficial effect of 15 nm diameter nanotubular layers and the universal nature of the cell stimulating influence (see Figure 43a and b). Similar size-dependent responses (Figure 43c) were present not

NiO and conductive polymers cannot be directly grown in selforganized nanotube structures. By coating or filling the active materials on/into TiO2 nanotubes, several groups target the fabrication of electrodes with an enhanced specific capacitance.905−911 6.7. Biomedical Applications

The use of TiO2 nanotubes toward biomedical applications is still in a very early stage, but the inherent biotolerance of TiO2,912,913 combined with a nanotubular geometry, bear considerable potential.50,208,914 Numerous possibilities are explored, toward advanced tissue engineering, novel drug delivery systems, coatings that are antibacterial, or enhanced osseointegration of a biomedical implant.50,51,226,915−918 Titanium and titanium alloys are one of the most important biomaterials, due to the high biocompatibility and corrosion resistance of TiO2 layers in biological environments.912 Implant surface chemistry and morphology on the micro- and nanoscale were widely found to affect biointeractions.919 In the past decade, interest in terms of length scales shifted from micrometer to nanometer surface topographies.919−921 Of all surface nanopatterning techniques, self-organized anodic oxidation is one of the most convenient methods to induce controllable topography and chemistry directly on biomaterials surfaces; that is, coating of an implant with an ordered nanotube layer presents a viable option of achieving a welldefined and controllable nanotopography. In this context, anodic 1-D nanotubular structures have been explored in the view of cell interactions, hydroxyapatite formation, and even in some in vivo tests. Anodizing allows control over the dimension of the nanotubes and can be used to easily coat complex shapes;922 thus it can be directly applied to coat an implant material. Titania nanotubes obtained by the hydrothermal method have the disadvantage of being obtained finally in powder form17 and thus cannot be directly obtained aligned perpendicular to the implant surface. Nevertheless, 9435

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A different approach using TiO2 nanotube layers and cells interactions concerns macrophage cells in the expectation of controlling and optimizing the inflammatory response to a Ti implant surface.945−947 A nanotube size dependence of macrophage adhesion and proliferation was suggested,947 but possible optimization of the inflammatory response is hard to conclude without further in vivo tests. 6.7.2. TiO2 Nanotubes for Improved Cell Interactions. Efforts were also undertaken to combine the nanotopography provided by TiO2 nanotube surfaces with surface functionalization, by the immobilization or attachment of bioactive molecules (proteins, peptides, enzymes).948−951 Of potential interest is the functionalization of surfaces with biomolecules, which have been shown to be involved in bone development and regeneration during fracture healing, such as growth factors. Some examples of significant growth factors that were successfully used for implant surfaces and have also been used for TiO2 nanotube modification948−950 include: EGF that influences the regulation of cellular proliferation or survival of osteoblasts, and members of the TGFβ and BMP family that induce osteoinduction and increase the activity of bone cells including collagen synthesis.952−955 TiO2 nanotubes were decorated with growth factors using epidermal growth factors (EGF),948 vascular endothelial growth factors (VEGF), 951 or bone morphogenetic protein 2 (BMP2).948−950 Immobilization of EGF948 was found to affect the response on 100 nm nanotubes, specifically enabling the cells seeded on the 100 nm nanotubes to attach and proliferate. Immobilization of VEGF951 proved more beneficial when performed via heparin−VEGF interaction, presenting higher bioactivity and also inhibiting bacterial adhesion (S. aureus). Immobilization of BMP-2 can be performed by polydopamine,949 by an amino-functional organizilane (APTES),956 or by carbonyldiimidazol.948,950 BMP-2 functionalized nanotubes via polydopamine proved beneficial for cell proliferation and differentiation (higher osteocalcin and osteopontin levels),949 while the uncoated nanotubes showed the clear size effects of small diameter nanotubes. BMP-2 functionalized nanotubes via covalent immobilization with carbonyldiimidazol (CDI)948 (schematic is shown in Figure 44d) did not induce beneficial conditions on cell adhesion and proliferation, but led to enhanced osteogenic differentiation on 15 nm nanotubes and to chondrogenic differentiation on 100 nm nanotubes (while rescuing MSCs from apoptosis generally occurring on uncoated 100 nm nanotubes);950 see Figure 44d. Functionalization with CDI948,950 maintained the surface structure of the nanotubes without evidence of blocking the nanotubes, whereas some authors956 reported functionalization via APTES to alter the surface structure, a blocking of the tube tops. Further approaches consist of loading different biocompatible materials inside the nanotubular structure to increase its bioactivity and osseointegration, for example, gelatin-coated gold nanoparticles.957 Recent work98 using nanotubes obtained via sol−gel method and hydrothermally modified to calcium doped titanate nanotubes showed their beneficial use for bone regeneration; in vivo tests in a rat’s femur put into evidence a good bioactivity in terms of bone regeneration speed. After the development of the antiosteoporosis drug strontium ranelate, strontium has received significant clinical interest, and, in this respect, Zhao et al.958 have used a Sr

only for mesenchymal stem cells, but also for hematopoietic stem cells, endothelial cells, osteoblasts, and osteoclasts. It was shown that the size effect clearly dominates over nanotube crystal structure (amorphous, anatase, or rutile), fluoride content, as well as to some extent over the wetting properties.554,924,925 Even by changing the substrate material to other valve metals (e.g., ZrO2), size effects similar to those for TiO2 nanotubes were observed.924 Generally, models to explain these findings are based on integrin interactions with the nanotopography or the nature of the adsorbed proteins.928−931 The main hypothesis for the clear size effect was related to integrin clustering in the cell membrane leading to a focal adhesion complex with a size of about 10 nm in diameter, thus being a perfect fit to the tube openings of about 15 nm, as depicted in Figure 43d and e, whereas nanotubes larger than 70 nm in diameter do not support focal contact formation and thus trigger apoptosis. An alternative approach to account for the adhesion of cells to nanotubular structures is based on modeling of the charge distribution at the nanotube tops.932,933 The attraction between a negatively charged nanotubular surface and a negatively charged osteoblast is assumed to be mediated by charged proteins (proteins with a quadrupolar internal charge distribution such as fibronectin, vitronectin).932 Some authors concluded that smaller diameter nanotubes have on average more sharp convex edges per unit area than larger diameter tubes; therefore, a stronger binding affinity is present on smaller diameter nanotubular surfaces.933 An interesting experiment934 using micropatterns with defined areas of stimulating 15 nm diameter tubes within apoptotic 100 nm tube environments showed that the micropatterned mixed 15 and 100 nm nanotube surfaces responded initially in line with tests on monodiameter surfaces;208 however, the extracellular matrix (ECM) produced by “active” cells on regions of small tube diameters led to a spreading of cells to neighboring “unfavorable” larger nanotube regions, enabling after some time settling of vital cells on the 100 nm nanotube patterns (Figure 44a−c). Similar effects934 were observed for nanotubular layers with less well-defined long-range order (as for nanotubes obtained in water-HF electrolytes). Because of the ECM spreading effect, the defects present in nanotube layers act strongly as point of attachment and activation of cells; thus such defects are a crucial experimental factor. In vivo experiments with anodic nanotubular surfaces in adult domestic pigs demonstrated that nanotubular surfaces can enhance collagen type I and BMP-2 expression922 and that a higher bone contact can be established if implants are coated with nanotubes. Furthermore, recent studies935 pointed out that nanotube diameter can be designed to support cellular functions of osteoblasts and osteoclasts in vivo, including differentiation and protein expression, and therefore offer a powerful tool for the controlled formation of peri-implant bone around medical implant devices. TiO2 nanotubes showed potential applications as bloodcontacting implant materials, presenting a good hemocompatibility.936−938 As it is possible to obtain nanotubular structures also on titanium alloys,40,939 it is a straightforward method of additionally improving the in vitro adhesion, proliferation, or osseointegration of nanostructured surfaces. Such alloys include Ti-6Al-4 V,257 Ti-6Al-7Nb,940,941 Ti26Nb13Ta4.6Zr,250,940 or binary alloys such as TiZr,942,943 TiTa,944 etc. 9436

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cytotoxicity. By further controlling the Ag release rate, it may be possible to achieve an optimum effect, that is, to accomplish both long-term antibacterial ability and biointegration (but no cytotoxicity). Other less commonly investigated inorganic antimicrobial agents that may be suitable for active coatings on titanium implants are copper, fluorine, calcium, zinc, and nitrogen. For example, incorporation of low doses of Zn by hydrothermal treatment in anodized nanotubular layers led to good antibacterial effects and enhanced osseointegration.972,973 As mentioned before, immobilization of VEGF inhibited S. aureus bacteria adhesion (due to a highly hydrophilic and negatively charged surface).951 A preferred method for decreasing bacteria adhesion could be loading the TiO2 nanotubes with antibiotics (such as gentamicin,916 vancomycin, cephalotin, etc.); however, it is difficult to ensure a constant release rate, as will be discussed in the following section. 6.7.4. Drug Delivery and Release of Other Payloads. The geometry of TiO2 nanotubes suggests that their surface may be used as a drug-delivery capsule by separating and stabilizing the nanotubular layers or as a drug-eluting coating on biomedical devices. Nanoscale encapsulated ferromagnetic structures can be transported (or held) at pretargeted locations within the human body using an external magnetic field. The goal is to perform a specific bioactive function with good precision regarding time and place. Shrestha et al.543 showed that TiO2 nanotubes can be completely filled with magnetic Fe3O4 particles and thus became magnetically guidable. Using suitable linker molecules, the nanotubes can be coated with drugs; a schematic of the release mechanism is shown in Figure 45a. Apart from UV reactions, drug release can also be triggered electrically (voltage

loaded nanotubular structure produced by hydrothermally transforming the titania nanotubes into SrTiO3, to demonstrate no cytotoxicity and good osteogenic activity. Investigation into surface chemistry modifications in terms of surface wettability alterations555 indicated no significant influence on the size-dependent cell behavior over large observation times. Nevertheless, drastic changes in the wetting behavior of TiO2 nanotubes from superhydrophilic to superhydrophobic (i.e., by means of octadecylphosphonic acid, a selfassembled monolayer)959 induced modifications in the adsorption of characteristic ECM proteins and improved the attachment of mesenchymal stem cells. Comparing superhydrophilic conditions with superhydrophobic conditions, in the latter case, adhesion became independent of the nanotube diameter; however, this effect was only of a temporary nature (3 days).959 Superhydrophobic nanotubular surfaces obtained with SAMs showed an improved blood compatibility.960 Overall, in all cases studied and with all possible modifications of the nanotubular layers (of surface chemistry, substrate material, and wettability), it seems evident that the observed size effects dominate over surface chemistry of the nanotubes as long as the material is not actively cytotoxic (e.g., Ag). 6.7.3. Antibacterial Behavior. The size effects of TiO2 nanotubes on interactions with cells also pose the question of using nanotopography to create bacteria-repellant surfaces. However, until now only few studies have addressed the influence of nanostructured titanium surfaces on bacterial interactions. One key aspect related to implant materials consists of the fact that cells have to compete with bacteria for the surface of the implant (also known as a “race for the surface”).961 Once bacteria attach to an implant surface, a biofilm will form with time,962 and bacteria present in the biofilm cannot be replaced by cells and are difficult to eradicate.963 As compared to interactions between cells and implant surfaces, interactions involving bacteria are not as well understood.931,964 Existing research indicates a similar trend for bacteria, as for cells, that larger diameter nanotubes (60 or 80 nm) decrease the number of live bacteria (S. aureus and S. epidermidis) as compared to lower diameter (20 nm) nanotubes.965,966 However, adhesion of bacteria (S. aureus, S. epidermidis, and P. aeruginosa) on nanotubes (60−70 nm) suggested that the increase in bacterial attachement (as compared to conventional or nanorough titanium) could be explained by a decreased number of living bacteria and a large number of adherent dead bacteria, the latter helping in the adhesion of subsequent live bacteria.967 Other approaches for decreasing bacterial interactions are based on using titanium alloys containing elements that could inhibit bacteria (e.g., zirconium) or using the nanotubes for active coatings (to release preincorporated bactericidal agents as silver ions, growth factors/chemokines/peptides). Grigorescu et al.968 suggest that small diameter nanotubes on Ti50Zr alloy have higher antibacterial effect against E. coli as compared to larger diameter. The active coating method has been extensively studied for TiO2 nanotubes. An often used bactericidal agent is Ag, which can be easily incorporated in nanotubes.969−971 Incorporation of silver in anodic nanotubes969,970 and hydrothermally obtained hydrogen titanate nanotubes971 showed promising results in view of constructing bacteriostatic materials with long-term silver ion release capability, however showing some

Figure 45. Drug delivery principles using TiO2 nanotubes: (a) Diagram showing the release principle of active molecules (model drug) from the functionalized magnetic TiO2 nanotubes upon irradiation with UV light. A fluorescent dye (active molecule) was attached to the TiO2 nanotubes with a siloxane linker. Reproduced with permission from ref 543. Copyright 2009 Wiley-VCH Verlag GmbH & Co. (b) Schematic of the amphiphilic TiO2 nanotube arrays fabricated by a two-step anodization procedure combined with hydrophobic monolayer modification after the first step. The outer hydrophobic barrier provides an efficient cap to the drug inside the nanotube, providing also a controlled drug release after photocatalytic cap removal. Reproduced with permission from ref 245. Copyright 2009 American Chemistry Society. 9437

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induce catalysis)400 or by X-rays543 allowing in vivo treatments through living tissue. TiO2 nanotubes filled with magnetic particles can be used directly for photocatalytic reactions with cells or tissue, such as site-selective killing of cancer cells.543 Kalbacova et al.732 showed the possible use of nanotubes as photocatalyst for killing of cancer cells, the only drawback being that there is a need for direct access of UV light to the TiO2 nanotubes. With regard to drug eluting coatings based on titania nanotubes, reported data are conflicting. Some reports show that for loading with pacl*taxel, sirolimus, and BSA, there are elusion time constants of minutes,974,975 while for similar morphologies and drugs, the elusion time is of days or weeks.976 Despite promising possibilities for local drug release on titanium-based implant materials, the release kinetics from these drug loaded TiO2 nanotubes are not highly controllable. It seems that a facile approach for ensuring the use of TiO2 for drug delivery is by simple physical adsorption or by deposition of such drugs from simulated body fluid (SBF), for example, penicillin/streptomycin, dexamethasone, etc.975 An increased drug elution was observed in the case of drug deposition from SBF;975 however, in this case, the nanotubular structure was not retained, and it is not clear if the noticed effect was due to the drug or to a possible calcium phosphate coating. The most difficult aspect is to be able to create a drug delivery system, which can allow a controlled delivery (release kinetics). The key point is the membrane (interface), which allows the drug elution. A more complex system using an amphiphilic TiO2 nanotubular structure consisting of nanotubes that provide a hydrophobic cap (monolayer) that does not allow water (body fluid) to enter into the nanotubes unless the cap is opened by a photocatalytic interaction was developed;226,245 the concept is schematically represented in Figure 45b. By this approach, the hydrophobic layer avoids leaching of the hydrophilic drug, and the opening of the hydrophobic layer (achieved by UV induced chain scission of attached organic monolayers) would lead to washing out by the body fluids of the hydrophilic drugs loaded into the nanotubes. Recently, with the same goal of controlled delivery, the drugloaded nanotubular layers were capped with biopolymer. For example, coating the structure with chitosan or other polymers, and based on the thickness, properties, and degradability, a controllable and sustained drug release could be achieved.977 Nanocarriers used for designing of nanoparticle drug delivery systems could also be integrated into the nanotubular layers.977 In vivo tests of a drug delivery system consisting of N-acetyl cysteine (NAC)-loaded TiO2 nanotubes used as dental implants in rats978 indicated that NAC delivery from the nanotubular titania implant led to a higher degree of osseointegration. 6.7.5. Hydroxyapatite Formation. A key factor for a successful osseointegration of biomedical implants (e.g., dental screws, hip-replacements) is hydroxyapatite formation leading to a bone-binding ability of biomaterials (hydroxyapatite exhibits bioactive behavior and integrates into living tissue resulting in a physicochemical bond between implant and bone). For Ti-based biomaterials, the high bone-binding ability has been assigned to a spontaneous modification of the passive Ti surface by calcium and phosphate ions during exposure to a biological environment.979 The mechanism of hydroxyapatite (HAp) formation is sequential: first, Ca2+ is adsorbed around a surface OH group

(or oxide ion), and then the HPO42− group is adsorbed to form the apatite layer.980 In the literature, it has been established that surface hydroxyl groups such as Ti−OH are efficient inducers for apatite formation.981,982 Consequently, as the as-grown amorphous nanotube layers contain a high amount of Ti−OH groups on their surface,983 it would be expected that they are most efficient for apatite formation (particularly more efficient than the layers obtained by annealing at high temperatures with a drastically lower amount of surface hydroxide). However, it was shown that it takes more time to initiate apatite formation on an amorphous surface than on anatase or a mixture of anatase and rutile.984 In other words, other more effective factors (e.g., crystal structure or surface morphology effects) must override the hydroxide effect. Regarding TiO2 nanotubular structures, three important aspects have to be mentioned: (i) the nanotubular structure clearly enhances the formation of apatite compared with a flat surface; (ii) transforming the tubes from amorphous to crystalline structure facilitates the formation of apatite; and (iii) a mixed anatase/rutile nanotube structure is even more efficient than a plain anatase structure.984−988 The formation and growth of apatite precipitates on nanotubular surfaces might be more hom*ogeneous and faster due to a higher number of nuclei formed in the initiation stage, whereas the apatite growth on flat TiO2 proceeds in a more heterogeneous, mushroom-like manner.986 Further examinations reported that the key factors for hydroxyapatite are crystallographic structure, geometry, porosity, or the presence of foreign elements in titania. Different geometry titania nanotube surfaces have also been investigated with respect to the growth kinetics of hydroxyapatite, reporting an acceleration of the hydroxyapatite growth.984−988 In all studied cases, when crystalline forms of titania are used instead of amorphous titania, an improvement of hydroxyapatite growth was observed.984,986,989 Furthermore, a nanotube size influence on hydroxyapatite growth was also observed (indicating 100 nm diameter nanotubes are best for HAp formation).986 Using nanolithographic approaches, it was possible to construct well-defined microstructures,988 where the micropatterned titania nanotubes are surrounded with compact oxide; based on the different hydroxyapatite formation rates, by immersion in simulated body fluid one can preferentially grow hydroxyapatite only on the nanotubular layers, thus being able to deposit hydroxyapatite in designed locations on biomedical devices.990,991 Calcium phosphate coatings (leading to hydroxyapatite) can also be obtained by alternating immersion methods (AIM) (e.g., cycles of alternating immersions in Ca(OH)2 and (NH4)2HPO4).915,987,992 It was shown that TiO2 nanotube layers are a highly suitable host for such synthetic hydroxyapatite coatings formed by AIM, leading to a uniform deposition of HAp and even loading the primer HAp inside the nanotubes.987 Obviously, AIM enhances apatite formation in SBF environments for TiO2 nanotubes. However, most striking are the acceleration effects on amorphous nanotubes when under the same SBF exposure conditions and in the absence of AIM primer no apatite forms, whereas in the presence of AIM primer, several micrometer thick hydroxyapatite layers can be obtained.987 The apatite growth rate for amorphous AIMtreated nanotubes was higher than on AIM-treated or AIM-free anatase/rutile nanotube layers.987,992 9438

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Still a large potential for property improvement exists, for example, by reducing defects in anodic tubes, formation of singe crystalline tube walls, shrinkage of tube walls to achieve electronic size effects, or optimized doping and modification approaches. Further improvement of ordering may allow the use of TiO2 nanotube arrays, and thus the high refractive index of TiO2, in even more applications, such as in photonic guiding structures. In addition to reviewing current knowledge and perspectives to these exciting nanostructures, we hope to have reflected to the readers some of the fascination involved in the research in this field.

Compact and hom*ogeneous HAp coatings on the TNTs substrate can be easily prepared by electrochemical deposition.993−995 When electrodeposition occurs in the presence of a magnetic field, the crystal orientation, shape, and size of HA particles were influenced by the intensities and directions of applied magnetic field that can accelerate the nucleation rate of HA crystals.995 To achieve more uniform HA coating with higher bond strength, alkali treatments in NaOH before deposition of HA can be used.994 Following the alkali treatments, titanates are formed on the top of titania nanotubes, enhancing the formation of HA during the electrodeposition process,994 or during immersion in simulated body fluids.996 HAp crystals can be synthesized under hydrothermal conditions, for example, using urea as precipitation agent to control the HAp growth and to mediate its morphology.997 Under these conditions,997 nanotubular surfaces favor prismlike hexagonal HAp crystallization. TiO2 nanotubes obtained by anodization can be useful also as template for deposition of mixed coating based on HAp and other materials, for example, carbon nanotubes,998 improving the bonding strength of the material. Furthermore, TiO2 nanotubes obtained by hydrothermal methods could be used for obtaining composite coatings with bioactive materials. An example of the potential of hydrothermally obtained nanotubes is by Ca-nanotubes, which are molded to a shape and immersed for only 1 day in simulated body fluid, inducing deposition of apatite crystals.98 Indifferent of the method used to obtain nanotubes on the implant surface, it was shown that the nanotubes contribute to increasing surface area necessary for the coating deposition, acting as anchor and enhancing hydroxyapatite nucleation and growth. Because of these aspects, TiO2 nanotubes present an interesting approach to increase osseointegration, enhance bond strength, and reduce interfacial failure of implants.

AUTHOR INFORMATION Corresponding Author

*E-mail: [emailprotected]. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest. Biographies

7. SUMMARY In this Review, we have tried to give an overview on the state of the art of research on TiO2 nanotubes, their formation, properties, and applications. Currently, this is a rapidly expanding research area, where methods for the fabrication of increasingly defined TiO2 nanotubes and aligned arrays produced by anodic and ordered template techniques make fast progress. In this Review, we deal with the most common attempts to produce nanotubular TiO2, that is, hydrothermal tubes as well as templated structures and, with a certain emphasis, anodic nanotube arrays. For anodic tubes, the understanding of the interactions of anodization parameters, self-ordering, tube morphology, composition, and structure evolves daily and leads to ever refined morphologies and to a largely improved control over the growth of such structures. In parallel, theory and modeling of the anodic self-organizing process enable not only further understanding but also entirely new perspectives for property modification (e.g., models describing plastic flow of the oxide during formation). In approaches based on templating, certainly the current wide use of ALD techniques provides not only a new platform to increasingly defined structures but also may be one key to further improving tube decoration (cladding) to form core− shell structures, with unprecedented definition. Many fields of classic TiO2 applications have been explored using tubular structures, such as their use in dye-sensitized solar cells, photocatalysis, or as biomedical coatings, and many attempts have already led to very promising features and findings.

Kiyoung Lee obtained his M.Sc. degree in Chemical Engineering from Inha University, Korea, in 2009 and Ph.D. degree in Materials Science and Engineering from University of Erlangen-Nuremberg, Germany, in 2013. He is currently carrying out postdoctoral fellowship in the group of Prof. Schmuki at the University of Erlangen-Nuremberg in Germany. His research interests include functional nanomaterial synthesis by electrochemical processes with the direction of application in energy conversion and storage devices.

Anca Mazare studied Chemical Engineering at Politechnica University of Bucharest, Romania, and obtained her Ph.D. in Chemistry from 9439

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Politechnica University of Bucharest, Romania, in 2012. She joined the group of Prof. Schmuki at the University of Erlangen-Nuremberg, Germany, in 2012 as a postdoctoral fellow. Her research interests include the synthesis of semiconductor nanomaterials and their application in various energy-related and biomedical fields.

Patrik Schmuki studied physical chemistry at the University of Basel and obtained his Ph.D. from the ETH Zurich in 1992. From 1994 to 1997 he worked at Brookhaven National Laboratory, U.S., and the Institute for Microstructural Sciences of the National Research Council of Canada. From 1997 until 2000 he was an associate professor for microstructuring materials at EPFL, Switzerland, and since 2000 he has been full professor and head of the Institute for Surface Science at the Materials Science Department of the University of Erlangen-Nuremberg. His current research interests cover electrochemistry and materials science at the nanoscale, with a particular focus on functional materials and the control of self-assembly processes.

ACKNOWLEDGMENTS We would like to acknowledge DFG and the Erlangen DFG cluster of excellence (EAM), and the European Research Council (ERC) for financial support, and Manuela Killian, Robin Kirchgeorg, Christopher Schneider, Ning Liu, Sabina Grigorescu, Alexei Tighineanu, Ole Pfoch, Nabeen Shrestha, Sergiu Albu, and Chong Yong Lee for their valuable contributions. REFERENCES (1) Iijima, S. Nature 1991, 354, 56. (2) Baughman, R. H.; Zakhidov, A. A.; De Heer, W. A. Science 2002, 297, 787. (3) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (4) Rao, C. N. R.; Nath, M. Dalton Trans. 2003, 1. (5) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. (6) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (7) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (8) Spahr, M. E.; Stoschitzki-Bitterli, P.; Nesper, R.; Haas, O.; Novák, P. J. Electrochem. Soc. 1999, 146, 2780. (9) Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985. (10) Krumeich, F.; Muhr, H.-J.; Niederberger, M.; Bieri, F.; Schnyder, B.; Nesper, R. J. Am. Chem. Soc. 1999, 121, 8324. (11) Sun, X.; Li, Y. Chem.Eur. J. 2003, 9, 2229. (12) Bavykin, D. V.; Friedrich, J. M.; Walsh, F.C. Adv. Mater. 2006, 18, 2807. (13) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. 9440

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