DOI: 10.1002/cphc.201000276

Doped TiO2 and TiO2 Nanotubes: Synthesis andApplicationsYoon-Chae Nah, Indhumati Paramasivam, and Patrik Schmuki*[a]

2698 � 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 2698 – 2713

1. Introduction

TiO2 is one of the most studied semiconductor materials, as isevident from more than 50 000 papers published over the past40 years on this material (with a still strongly increasing publi-cation rate over the past decade). In contrast to classic semi-conductors such as Si, efforts are not primarily driven by themicroelectronic device industry, but by the materials sciencecommunity. In the center of interest is a set of virtually uniquefunctional properties (that make the material stand out fromother transition-metal oxides and classical group IV, III–V, or II–VI semiconductors). TiO2-based materials have a superior posi-tion as biomaterials, in photocatalytic applications, and in dye-sensitized solar cells (DSSCs).

The key to the latter two applications lies in the electronicstructure of the material with a large band gap of �3 eV, suita-ble band-edge positions for many redox reactions, a compara-bly high lifetime of excited electrons, and an exceptional pho-tocorrosion resistance. While the large band gap is crucial formany applications, it also limits the use for solar-irradiation-driven applications (only 7 % of solar energy is provided in thespectral range <400 nm, that is, under super-band-gap energyconditions of TiO2). Therefore, intense efforts focus on modify-ing the electronic properties of TiO2 by band-gap engineering(narrowing the optical band gap), doping the bulk, or by suita-ble modification of the TiO2 surface (sensitization or junctionformation) to create a visible-light response.

In this Review, we will mainly discuss these efforts in termsof photocatalysis and progress towards a higher efficiency. Theconcept of photocatalytic reactions is illustrated in Figure 1 a.Upon absorbing a photon with energy larger than the bandgap of TiO2, electrons are excited from the valence band to theconduction band, which creates electron–hole pairs. Thesecharge carriers migrate to the TiO2 surface and react with theenvironment—that is, molecules (in solution or the gas phase)that are adsorbed on the surface. The photocatalytic activity ofa semiconductor is limited by i) the light absorption properties(absorption coefficient and spectral range), ii) the electron–holerecombination rate, and iii) the reduction and oxidation reac-tion rates on the surface by ejected electrons and holes. Awide range of possibilities for photocatalytic reactions havebeen explored.[1–3] Most spectacular is direct water splitting,that is, Red = OH� , Ox = O2�, Red’= H2, Ox’= H+ , which is thephotoconversion of H2O to H2 and O2. In self-cleaning or envi-ronmental degradation applications, Red and Ox may be pollu-tants in the environment (mostly hydrocarbons that are de-

composed to CO2 and H2O). In aqueous or wet environmentsthis process usually involves one or more radicals or intermedi-ate species such as OHC, O2

� , H2O2, or O2.This principle could be obtained from a number of classical

semiconductors or metal oxides (Figure 1 b). However, somespecific features such as the absolute and relative band posi-tions—discussed later in this Review—make TiO2 the primematerial of choice. Another point that is crucial for photocata-lysts is a large surface area that leads to faster surface photoca-talytic reaction rates. For many applications, nanoscale TiO2 isused either as isolated nanoparticles (e.g. , suspensions) or ascompacted or sintered photoanodes of nanoparticles (Fig-ure 2 a).[4] Most recently, a novel 1D nanotube structure, a TiO2

nanotube array, has been shown to be highly competitive andin many cases favorable to achieve enhanced photocatalyticperformance (Figure 2 b).[4–9] In such photoanodes chargetransfer through the assembled nanostructure is crucial, that is,the situation significantly differs from that of isolated particlesin solution.

TiO2 nanotubes with different sizes and geometrical shapescould be prepared using various physical and chemical synthe-sis routes.[10–12] Among them, the cheapest and most straight-forward approaches that lead to ordered nanostructures (tubesand sponges) are anodization techniques.[5–8, 13] During theanodization process, for tube growth the electrochemical con-ditions such as the applied potential, anodization time, and pHof electrolyte can be used to control the resulting tube diame-ter, tube length, and overall morphology.[5, 6, 9] As the synthesisroute for these self-organized nanotubular arrays is the electro-chemical anodization (oxidation) of a metal foil, some entirelynew doping principles become feasible (e.g. , using an alloy ofTiX for anodization, with X being the doping species). TheReview will give an overview of the state of current activitiesand some underlying principles of classical doping approaches,but will also discuss these issues in the light of recent advan-ces in the formation of TiO2 nanotubes—which not only allowthe use of TiO2 in novel and highly defined geometries butalso allow specific doping effects.

[a] Y.-C. Nah, I. Paramasivam, P. SchmukiDepartment of Materials Science, WW4-LKOUniversity of Erlangen-NurembergMartensstr. 7, 91058 Erlangen (Germany)Fax: (+ 49) 9131-852-7582E-mail : schmuki@ww.uni-erlangen.de

TiO2 is one of the most investigated compounds in contempo-rary materials science. Due to a set of virtually unique electron-ic properties, it finds intense use in photoelectrochemical ap-plications such as photocatalysis or solar cells. The main draw-back in view of direct exploitation of solar-light-based effectsis its large band gap of >3 eV. Visible-light-activated TiO2 canbe prepared by doping (band-gap engineering) through incor-poration or decoration with other metal ions, nonmetal ions,and semiconductors. Most recently, efforts in TiO2 research

have been even more intensified by the finding of self-organ-ized nanotubular oxide architectures that can be prepared bya simple but optimized anodization of Ti metal surfaces. Thesenanotubular geometries provide large potential for enhancedand novel functional features. This Review examines dopedTiO2 and in particular TiO2 nanotubes. Various types of dopants,doping methods, and applications of modified TiO2 nanotubesare discussed.

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Doped TiO2 and TiO2 Nanotubes

Applications of TiO2 nanotubes in DSSCs and electrochromicdevices will only be briefly discussed as some recent reviewsare available.[6, 7, 9]

2. Properties of TiO2

2.1. Structural Properties

Essentially TiO2 is known in three main crystalline forms: rutile,anatase, and brookite. Figure 3 shows the unit cell structures

of the practically most important crystal structures: rutile andanatase TiO2. These two structures can be described in termsof chains of TiO6 octahedra, in which each Ti4 + ion is surround-ed by six O2� ions. The two crystal structures can be dividedby the distortion of each octahedron and by the assembly pat-tern of the octahedra chains. In rutile, the octahedron shows aslight orthorhombic distortion, whereas in anatase the octahe-dron is significantly distorted so that its symmetry is lowerthan orthorhombic. In the rutile structure, each octahedron isin contact with ten neighboring octahedrons (two sharingedge oxygen pairs and eight sharing corner oxygen atoms),while, in the anatase structure, each octahedron is in contactwith eight neighbors (four sharing an edge and four sharing acorner). These differences in lattice structures cause differentelectronic properties between the two forms of TiO2.

Rutile has minimum free energy compared to the anatasephase, hence given the necessary activation energy it willtransform into the rutile phase under most conditions.[14–16]

Temperatures at which the anatase-to-rutile transformationtakes place are in the range of 300–500 8C, as they dependupon several factors, including impurities present in the ana-tase, primary particle size, texture, and strain in the struc-ture.[15, 17, 18]

A new set of crystal structures of TiO2 has been developedby several synthetic methods. Magneli phases are a substoi-chiometric composition of TiO2, which follow the formulaTinO2n�1 (4<n<10). It is known that several forms from thisseries show high electrical conductivity at room temperatureand high corrosion resistance in aqueous electrolyte.[19–22] Thecrystal structure of the Magneli phases can be regarded asrutile-type slabs of infinite extension and different thickness,which are separated by shear planes with a corundum-likeatomic arrangement.[23] When changing from TiO2 to Ti2O3, thed-band occupation across the series increases and the elec-tronic structure changes, which is dependent on tempera-ture.[24] Since the high-pressure phase of TiO2 was first pre-pared by Dachille and Roy, many researchers have focused oninvestigating phase relations in the TiO2 system.[25] McQueenet al. reported that TiO2 underwent a phase transition begin-ning at 33 GPa and finishing at 100 GPa, with a reduction of20 vol %.[26] They found that after compression to 75 GPa, thesample formed a structure with a mixture of rutile and a-PbO2

type.[27] Preparation of a metastable TiO2 (B) structure from thelayered alkali-metal titanate K2Ti4O9 was first reported byMarchand et al.[28] Further research by other groups has shownthat TiO2 (B) can be prepared from several layered titanate pre-cursors.[29] The structure of TiO2 (B), like that of its layered start-ing material, is composed of corrugated sheets of edge- andcorner-sharing TiO6 octahedra, but the sheets are joined to-gether to form a 3D framework.[30] Compared to the stableforms of TiO2, TiO2 (B) shows enhanced electrochemical andcatalytic properties.[31–33] Additionally, it should be noted thatin many sol–gel or electrochemical formation processes, TiO2 ismainly formed as an amorphous material.[6, 9, 16]

Yoon-Chae Nah obtained a B.S. in Ma-

terials Science and Engineering at

Korea University in 2000 and a Ph.D. at

Gwangju Institute of Science and Tech-

nology (GIST) in 2007. From 2007 to

2009 he worked at the University of Er-

langen-Nuremberg with Prof. Dr. Patrik

Schmuki and he is currently working

on Li-ion batteries at LG Chem Re-

search Park. His research interests in-

clude (photo-)electrochemical devices

for energy conversion and storage.

Indhumati Paramasivam received her

B.Tech. degree in Chemical and Elec-

trochemical Engineering from the Cen-

tral Electro Chemical Research Institute

(CECRI, a CSIR Institute), Karaikudi,

India, in 2000. She then graduated

with an M.Sc. in Chemical Engineering

from the University of Erlangen-Nur-

emberg, Germany, in 2004. Since 2006

she has been pursuing her Ph.D.

degree at Prof. Patrik Schmuki’s labora-

tory, Department of Materials Science,

University of Erlangen. Her research interests are photocatalysis

using nanostructured materials, their band-gap engineering, and

environmental pollutant degradation.

Patrik Schmuki studied Physical

Chemistry at the University of Basel,

and obtained his Ph.D. from ETH-

Zurich in 1992. From 1994 to 1997 he

worked at Brookhaven National Labo-

ratory, USA, and the Institute for Micro-

structural Sciences of the NRC, Canada.

From 1997 to 2000 he was an Associ-

ate Professor for Microstructuring Ma-

terials at EPFL, Switzerland, and since

2000 he has been Full Professor and

head of the Institute for Surface Sci-

ence at the Materials Science Department of the University of Er-

langen-Nuremberg. His current research interests cover electro-

chemistry and materials at the nanoscale—with particular focus on

functional materials and the control of self-assembly.

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P. Schmuki et al.

2.2. Electronic Properties

The electronic structure of TiO2 has been explored by using avariety of theoretical approaches and experimental tech-niques.[34–40] TiO2 is an n-type semiconductor that has a con-duction band with an edge of low energy formed by thevacant Ti4+ d bands, and a valence band where the upperedge is formed of the O2�-filled pp bands. The band-gapenergy of indirect electron transition is 3.0 and 3.2 eV for rutileand anatase, respectively.[18, 41–43] Apart from these definite

values of the band gap, the relative position compared toredox potentials of some species in media are of great impor-tance for numerous functional applications of TiO2 (Figure 1 b).For example, to split water by illumination with light, the posi-tion of the conduction edge of the material should lie higherthan that of the species to be reduced (H+/H2) and the va-lence-band edge should be located lower that of than the spe-cies to be oxidized (H2O/O2). TiO2 fulfills these basic require-ments relatively well compared to other metal oxides.

A crucial point is also the formation of Ti3+ species. Heattreatment of TiO2 in the absence of oxidizing species or invacuum or inert gas environments usually leads to release ofO2 from the material and formation of Ti3 + (at least in the sur-face near region). The formed reduced material shows visible-light absorption and enhanced conductivity. This effect can beregarded as a doping band (or high density of localized donorstates) that is introduced to TiO due to the formation of Ti3 +

(Ti3+ being the electron-donor species located close to theconduction band of TiO2). It extends about 0.5 eV deep underthe original TiO2 conduction band. By such thermal treatmentsand under typical conditions, approximately 1 % of the lattice

Figure 1. a) Schematic representation of the photocatalysis mechanism on TiO2. b) Electronic band structure of different metal oxides and relative band-edgeposition to electrochemical scale.

Figure 2. Scanning electron microscopy (SEM) images of photoanodes con-sisting of a) compacted TiO2 nanoparticles and b) self-organized TiO2 nano-tubes grown on a Ti foil by optimized anodization.[5, 6, 9]

Figure 3. Schematic of crystal structure of rutile (left) and anatase TiO2

(right).

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Doped TiO2 and TiO2 Nanotubes

atoms may be reduced to Ti3+—this process is reversible uponexposure to air or O2. The oxygen vacancies formed by reduc-tion of TiO2 act as color centers that induce an enhanced visi-ble-light absorption.

In nanotubular geometries, the electronic properties of TiO2

are even more important, because they determine how effi-ciently electrons can be transferred along the 1D path. This isillustrated in Figure 4, which shows the conductivity of a nano-

tubular layer under different annealing conditions. At low tem-peratures the resistivity of the tubes increases due to dehydra-tion, then decreases due to crystallization. Up to 450 8C ana-tase is formed; at higher temperatures the conversion to themore resistive rutile phase decreases the overall resistivityagain.[9, 44] Several other methods are used to characterize theelectronic properties of TiO2 nanotubes. For photoanodes, theband-gap energy can be obtained by measuring the photocur-rent, and capacitance measurements can be used to estimatethe doping concentration and the flat-band potential.[16, 45–48]

For TiO2 nanotubes, as-anodized and heat-treated sampleshave a similar band gap of 3.2 eV.[16, 45] However, a different ki-netic behavior is shown when photoelectrons are generatedby external irradiation, which can be monitored by photocur-rent transients.[47, 49] For the anatase TiO2 nanotube layer, an ini-tial fast increase of the current is recorded, followed by a grad-ual decrease of the photocurrent with time by a recombinationprocess, while for the amorphous nanotubes the photocurrentcontinuously increases with time (seconds), which can be at-tributed to a slow trap-filling process. Moreover, it should beconsidered that TiO2 with different particle sizes, together withsurface area, may have a band gap comparable with that ofthe bulk TiO2 catalyst due to size effects.[15, 50–53]

2.3. Electrochemical Properties

TiO2 behaves in electrochemical I–V curves mostly as a typicaln-type semiconductor with a current-blocking characteristic in

the anodic direction and a current-passing behavior in thecathodic direction.[1] In the context of doping and applications,a particularly important feature is the fact that upon sufficientcathodic polarization, Ti3+ in the TiO2 lattice can be producedelectrochemically by reduction of TiO2 according to Equa-tion (1):

TiO2 þ e� Yþ�! TiIIIYO2 ð1Þ

In this case, reduction is accompanied by insertion (intercala-tion) of small cations (H+ , Li+ , etc.) into the lattice and a visiblecolor change. This color change (usually dark blue to brown) isassociated with the formation of a highly doped material byformation of Ti3 + species in the lattice. The latter effect can beexploited for switchable electrochromic devices as the electro-chemical reduction and oxidation is reversible (the switchingtime for nanoscale dimensions is fractions of seconds). This re-duction-induced self-doping of TiO2 can, however, also be usedto significantly increase the conductivity in TiO2, for examplefor electrodeposition reactions.[54]

The semiconductive nature of course provides the basis forredox reactions on TiO2.[55] For TiO2, photoelectrochemicalredox reactions are much more important than dark redox re-actions. For example, let us consider the photocatalytic split-ting of water into H2 and O2 in view of Figure 1. When TiO2 ab-sorbs light with energy larger than the band gap, electronsand holes are generated in the conduction and valence bands,respectively. The electrons and holes are used for redox reac-tions in which water molecules are reduced by the electronsto form H2 and oxidized by the holes to form O2. In order toachieve the reaction, the relative energetic positions of theconduction and valence bands are important. The conductionband has to be more negative than the reduction potential ofH+/H2 (0 V vs normal hydrogen electrode, NHE), whereas thevalence band has to be more positive than the oxidation po-tential of O2/H2O (1.23 V vs NHE). This, however, only indicatesthat in principle this reaction should be thermodynamicallypossible on the substrate. For the kinetics (hydrogen produc-tion rate), many other factors such as charge separation, mobi-lity, and lifetime of photogenerated electrons and holes arevery important.[56] These factors are strongly affected by thebulk properties of the material such as crystallinity. Surfaceproperties, such as surface states, surface chemical groups, andsurface area, are also important.

Nevertheless, in terms of the relative band-gap energy posi-tions one may deduce that if the TiO2 band gap could be engi-neered to be smaller (to achieve a higher visible response [insolar applications]), one also needs to take care that desiredelectron-exchange reactions with the environment are still fea-sible. That is, if the conduction band is lowered too much,transfer to H+ may not be possible any longer. In analogy, ifthe valence band is raised too much, the production of O2 (orin pollutant degradation reactions the formation of OHC radi-cals via valence-band holes, which requires around + 2 V vsNHE) may become impossible.

Figure 4. Two-point conductivity measurements of TiO2 nanotube layers an-nealed at different temperatures in air[44] (measured through the layer by de-positing an Al dot on the surface).

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3. Approaches To Dope TiO2 and TiO2Nanotubes

3.1. Bulk and Surface Doping

To modify the electronic and optical properties of TiO2 by in-corporating impurities, doping or band-gap engineering princi-ples are employed as shown in Figure 5. As mentioned, the

band gap of TiO2 is formed by Ti 3d states and O 2p levels,with the lower edge of the conduction band being formed byTi dxy states and the upper edge of the conduction band beingformed by O 2p states (for both anatase and rutile). Modifica-tion of the band gap can involve strategies such as mixingconduction-band states with lower levels (for example by Ti3 +

formation, WO3 alloying, H+ uptake; Figure 3 a), thus lifting thevalence band (for example N incorporation and possibly C dop-ing). Modification of the optical and electrical properties canalso involve introducing new energy bands (Figure 5 d) or lo-calized states (Figure 4 e) in the band gap of TiO2. Several ap-proaches for TiO2 modification by doping have been proposedin the literature, which cover a wide range of elements, suchas metal-ion implantation including transition-metal ions intoTiO2 (Cu, Co, Ni, V, Cr, Mn, Fe, Mo, Nb, Ru, Au, Ag, Pt),[57–60] non-metals (N, S, C, B, P, I, F),[61–66] and composites of TiO2 with sem-iconductors having lower band-gap energy. Various methodshave been introduced to prepare doped TiO2 by liquid- or gas-phase chemical methods[67–70] or physical methods.[59, 71, 72]

The electronic structures, that is, the densities of state (DOS,examples are shown in Figure 6), of doped TiO2 have been an-alyzed by theoretical calculations.[51, 72–76] At the beginning oftheoretical analysis, a cluster approach by molecular orbital cal-culations was extensively employed.[68, 77] These cluster calcula-tions, however, did not lead to unambiguous conclusions re-garding the effects of doping on the electronic structure.[73]

Most reliable results were obtained using a super-cell approachfor band calculations that can deal with defective crystal sys-tems such as that induced by dopant incorporation.[78–80]

Recent theoretical studies have shown that the modificationof the band structure of TiO2 can also be achieved by usingnonmetal (anionic) dopants.[51, 72, 74–76] Asahi et al. calculated theelectronic band structures of anatase TiO2 with different dop-ants such as C, N, F, P, or S.[51] In this study, the substitutionaldoping of N was the most effective in the band-gap narrowingbecause its p states mixed with O 2p states. Although thequestion of whether nonmetal doping achieves band-gap en-gineering and the extent of narrowing is in much debate, alarge number of the reports agree that nonmetal doping indu-ces an enhancement in the visible-light photoactivity ofTiO2.[51, 66, 81–89]

Considerable efforts have also been made to investigate theestablishment of true doping effects. For metal (cationic) dop-ants such as V, Cr, Mn, Fe, or Co, it was found that an electron-occupied level formed and the electrons were localized aroundthe dopant.[73] As the atomic number of the dopant increased,the localized level shifted to lower energy and the electrondensities around the dopant were large in the valence bandand small in the conduction band compared to the case ofpure TiO2. While several authors have reported that metal-iondoping enhances the photoactivity of TiO2, there are also con-siderable drawbacks such as thermal instability and shorterelectron–hole lifetime.[90]

3.2. Synthesis of Doped TiO2

Doped TiO2 nanostructures can typically be prepared by:

1) treating final or growing TiO2 nanomaterials in a solution ormelt of the doping species

2) thermal treatments in gas atmospheres of the doping spe-cies

3) co-sputtering, or sputtering in an atmosphere of thedoping species

4) high-energy ion implantation.

For anodic TiO2 nanotubes (that grow from the metal byelectrochemical oxidation) additionally there is the possibilityto:

5) use substrates of a suitable alloy6) try to incorporate active electrolytic species, which is essen-

tially similar to (1) but electrochemical aid may be used.

While approach (1) is often successfully used in precipitation,hydrothermal, or sol–gel processes while growing crystallites,post-processing seems to be successful only in some specificareas—particularly when the TiO2 crystallites are in the fewnanometers range.[81, 91, 92] Sol–gel processes[93–95] are one of themost versatile methods to prepare nanosized materials. Typicalpreparation procedures by the sol–gel method involve a titani-um precursor, such as titanium isopropoxide (TIP), tetrabutylorthotitanate (TBOT), or titanium tetrachloride, which is mixedwith the dissolved dopant precursor, followed by hydrolysisperformed at room or elevated temperature. The precipitate isdried, usually at a temperature in the range from 80 to 110 8C,

Figure 5. Schematic illustration of the possibilities to alter the TiO2 bandstructure: a) pristine TiO2, b) conduction band (CB) lowering by state mixing,c) valence band (VB) increase by state mixing, d) creation of discrete band inthe band gap, e) discrete doping level, and f) surface doping (sensitization).Eg = band-gap energy.

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Doped TiO2 and TiO2 Nanotubes

pulverized to obtain a xerogel, and calcined in air at tempera-tures from 200 to 600 8C.

The sol–gel method is a very successful technique to pre-pare N-doped TiO2 nanoparticles, because it affords simplicityin controlling the nitrogen doping level and particle size bysimple variations in the experimental conditions, such as hy-drolysis rate, solution pH, and solvent systems. Various hydroly-sis approaches have been developed[91, 92, 96–98] for the directbinding of N to the central Ti ion. Appropriate sintering re-moves most of the organic residues on the surface and retainssufficiently high N-doping levels in the resulting nanoparticles.

Approach (2) is frequently used for nitrogen or carbondoping and employs mainly treatments in NH3, urea, CO, oracetylene.[85, 86, 99] N-doped TiO2 was, for example, obtained by

heating TiO2 under an NH3 stream at high temperature.[85, 100–102]

A high-temperature NH3 treatment was also successful for TiO2

nanotubes.[85] In salt melts TiO2 nanotubes can be reduced tometallic nanomaterial.[103]

Approach (3) is the classic approach used by Asahi when re-porting for the first time about successful N doping.[51] Thistype of N-doped TiO2 shows a strong improvement over pureTiO2 in view of visible-light and optical absorption.

Approach (4) is indeed a most reliable way to truly incorpo-rate any species into the TiO2 lattice at low to medium dopinglevels. The drawbacks are that maximum (reasonable) fluencesof high-energy accelerators (operating at 50–1000 keV) aresuited to 1017–1018 ions cm�2 and the implantation depth (intothe substrate) is limited to some micrometers with typically a

Figure 6. Examples of TiO2 doping with different elements based on theory. a) Total DOS of doped TiO2 and b) the projected DOS into the doped anion sites,for the dopants F, N, C, S, and P located at a substitutional site for an O atom in the anatase TiO2 crystal (eight TiO2 units per cell). Ni-doped: N doping at aninterstitial site ; Ni + s-doped: doping at both substitutional and interstitial sites. c,d) Total DOS of S- and F-doped TiO2. Panel (a) reprinted with permission fromthe American Association for the Advancement of Science and panels (b)–(d) reprinted with permission from Elsevier.

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somewhat inhomogeneous dopant distribution in the sub-strate depth. Nevertheless, this approach has been reported tolead to most effective doping of TiO2 nanotubes.[83] Defectsproduced by ion bombardment, which lead to a decrease inthe photoconversion efficiency, can be annealed out after heattreatment. This results in an N-doped crystalline anatase nano-tube structure with strongly enhanced photoresponse in boththe UV and visible range.

The latter is typical for high-energy ion implantation. Usuallyit leads to implant species in interstitial positions and requirespost-implantation thermal treatment to diminish (anneal out)structural damage (amorphization) that is caused by the im-plantation process. It should be noted that in the case of nano-tubular geometries, high-dose ion implantation can lead to dis-integration of the tubular structure.[84]

Approaches (5) and (6) that are specific to anodic oxidationwill be discussed in more detail in the section on TiO2 nano-tube growth.

The most successful approach to influence the optical prop-erties of TiO2 in a desired way currently seems to be N doping,followed by the more disputed case of C doping.[66] For N-doped TiO2, it was firstly described that the spectral sensitiza-tion of TiO2 in the visible range was due to NOx impurities.[104]

Later, Asashi et al. reported the band-gap narrowing of titani-um dioxide by nitrogen doping.[51] These highly active N-doped TiO2 films were obtained by sputtering in a N2/Ar ambi-ent atmosphere. The most efficient N doping of TiO2 nanotubelayers was achieved as mentioned above by using ion implan-tation.[83]

A specific challenge is that the various approaches to nitro-gen doping lead to different states of nitrogen in the TiO2 bulkor on the TiO2 surface. Moreover, several of the different stateslead to a visible photoresponse. This may be best illustratedby looking at the X-ray photoelectron spectroscopy (XPS) N 1speak after doping treatments. Proper ion implantation of N+

and annealing lead to a peak at 396 eV[83] that is in line withresults from sputtering TiO2 in N environments,[51] or obtainedfor titanium nitrides.[105]

Wet treatment in, for example, amine-based solutions typi-cally lead to peaks at around 400 or even 402 eV, and thesewere also found to be active under visible-light illumina-tion.[16, 81, 98] These peak positions in many cases can be inter-preted as surface doping (sensitization) with, for example, anN–C compound.[16]

Most ambiguous are N-doping reactions that result in anXPS N 1s peak at �400 eV. This peak position is also found forN2 adsorbed on TiO2. Although several groups reported suc-cessful N doping with this peak position, most investigationsdo not show visible photocurrent but the results are based onlight absorption measurements. Such results are, however, noproof of successful doping (involving electronic coupling) butonly show that a species is present that absorbs photons. Itshould also be mentioned that testing the success of N dopingby visible photocatalytic activity is somewhat questionable ifthe photocatalytic reaction is indeed valence-band dominated(h+ transfer to the electrolyte). As N doping raises the valence-band edge, the h+ leave the semiconductor with a lower

energy (less anodic redox potential) that is possibly not suffi-cient to oxidize H2O to OHC or to achieve direct decompositionof the organic material. The fact that in some cases a visiblephotocatalytic effect (mostly very mild) was found for N-dopedmaterial could then be attributed to another reaction path. Inparticular, in these cases O2

� generation via the conductionband may be the remaining rate-controlling path.

Another critical point is that typically N doping leads tosome <2 % of N in the structure. Although this corresponds toa comparably very high doping concentration for a classicalsemiconductor, it is problematic to assume that this concentra-tion is sufficient to raise the bulk valence-band level by>0.5 eV (in traditional band-gap-engineered materials, e.g. ,GaAlAs and InAlP, substantially higher concentrations of thethird element are required). In other words, a picture corre-sponding to Figure 5 e (high density of localized states) seemsmore appropriate to describe the situation of N-doped materi-al. This may be illustrated by the following somewhat contro-versial findings. In general, there are three different opinionsregarding the band structure modification by doping. Asahiand co-workers reported that the substitutional doping of Nwas the most effective in the band-gap narrowing becauseN 2p states mix with O 2p states, due to the fact that their en-ergies are very close.[51] By the decrease in the band gap, theonset of the absorption spectra red-shifted to longer wave-lengths. On the contrary, Irie et al.[106] state that in TiO2, oxygensites substituted by nitrogen form an isolated impurity levelabove the valence band rather than narrowing the band gap.They found that electrons both in the valence band and im-purity levels are excited when illuminated with UV light,whereas visible light excites only electrons in the impuritylevels. Finally, oxygen vacancies are also reported to be respon-sible for the band modification. Ihara et al.[107] concluded thatoxygen-deficient sites are formed in the grain boundary andplay an important role in showing a visible-light response.Other groups[108] have also produced visible-light-active TiO2

catalysts by using a variety of procedures; they concluded thatthe visible-light photoresponse was due to oxygen vacancieswhich give rise to a donor level located below the conductionband.

The second most used approach is C doping. C-doped TiO2

has been obtained by heating titanium carbide[87–89] or by an-nealing TiO2 under CO gas flow or acetylene at high tempera-tures.[86, 109] Most spectacular are recent results on C doping ofTiO2 nanotubes using acetylene, where it was shown that notonly successful doping can be achieved[86] but also that TiO2

nanotubes at a sufficiently high temperature can be convertedto a TiOC material that contains Magnelli-type oxides, and thusa semimetallic conductivity at about the level of graphite canbe induced. Such substrates seem to be extremely promisingin view of electrochemical redox reactions.

In spite of a lot of preparation methods, it remains difficultto unify the conclusions on doping effects. For example, it wasshown that ion implantation of V, Cr, Ni, and Mn metals in-duced the shift of the absorption edge towards longer wave-lengths, which increased the visible-light photoresponse ofTiO2,[110] while it was also reported that Cr doping by a chemi-

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cal method decreases the photoreactivity under both visibleand UV illumination.[77] Different doping methods can inducedifferent valence states of the dopants. For example, the incor-porated S from thiourea was reported to lead to S4 + or S6 +

states,[62] while direct heating of TiS2 or sputtering led to theS2� anion.[111] Although a considerable number of other dop-ants have been reported in the literature, such as phospho-rus,[95, 112] iodine,[113] fluorine,[114] manganese, cerium, seleni-um,[115] tin, zinc,[116] etc. , in many cases clear evidence for sub-stantial contributions on an optical or electronic level are notunambiguously provided.

3.3. Particle Decoration or Surface Activation

An approach related to doping is the decoration of TiO2 surfa-ces with particles that lead to enhanced surface catalytic activi-ty by: i) junction formation that provides either a change inband bending or provides suitable energy levels for charge in-jection (e.g. , in DSSCs) ;[7, 117, 118] ii) catalytic effects that facilitatecharge-transfer reactions such as the formation of O2

� speciesand OHC radicals to decompose organics;[119, 120] and iii) surfaceplasmon effects, for example leading to field enhancement inthe vicinity of metal particles and thus allowing more efficientcharge transfer.[121]

In the case of anodic TiO2 nanotubes, activation towards im-proved photocatalytic activity was reported by Ag, Au, and Ptnanoparticle decoration,[119, 121–123] and filling with secondaryscavenging materials such as zeolites.[123] Moreover, visible-light activation towards photocatalytic and photoelectrochemi-cal activity was observed by decorating with Ni oxide nanopar-ticles by simple chemical-bath precipitation.[124] Also, Ag-deco-rated tubes significantly enhanced the DSSC performance.[117]

Improved surface reactivity by WO3 particles for electrochromicdevices was achieved due to junction formation between TiO2

and the misaligned WO3 bands.[125] TiO2 nanotubes decoratedwith narrow-band-gap semiconductors, such as CdS, CdSe, andPbS quantum dots, are useful in visible-light absorption andlead to a higher solar-cell efficiency of up to �4 %.[126]

The most popular surface activation scheme for TiO2 is dyesensitization, which is mostly used for solar cells[127]—for TiO2

nanotubes this was also successfully demonstrated.[128, 129]

4. Specific Aspects of Anodic TiO2 Nanotubes

Anodization of Ti to form oxide layers has been investigatedfor many decades. Under almost all electrolyte conditions(most acids, salt solutions, organic solvents), a compact oxidelayer is obtained (Figure 7 a) with a linearly increasing thicknesswith applied voltage. However, in 1999, Zwilling et al. reportedthe formation of self-organized TiO2 nanotubes which were ob-tained by anodization of Ti in a fluoride-containing electro-lyte.[130] These authors called the structure porous (in analogyto ordered porous Al), but in the light of today’s findings thisis only semantics. Their SEM images clearly show a tubularstructure, and, moreover, a significant amount of recent workshows that transition from pores (connected oxide body

around a pore) to tubes (separated oxide body around a pore)is of a gradual nature.[131–136]

However, this first generation of nanotube layers, also con-firmed by subsequent work,[16, 137] grown in acidic aqueouselectrolytes was limited in thickness to �500 nm and showedconsiderable disorder. Significant achievements were made byMacak et al. who introduced the second and third generationof nanotubes by optimizing the pH of the electrolyte[138, 139]

and by introducing nonaqueous electrolytes.[140, 141] Today,almost ideally hexagonally ordered[142] TiO2 nanotube arraysseveral 100 mm in length can be grown with tube diametersranging from 10 to >200 nm.[143–145]

Various modifications of the electrochemical parameters(pulsing, stepping) allow for novel morphologies such as nano-bamboo, nanolace,[146, 147] stacked layers,[103, 148, 149] branchedtubes,[147, 150] and multiwalled tubes,[151] as well as the formationof amphiphilic TiO2 nanotube stacks.[152] Under specific condi-tions, self-ordering at two length scales can be obtained.[153–156]

The formation of these self-ordered structures in fluoride-con-taining electrolytes is not limited to Ti, but was also successful-ly demonstrated for other transition metals, such as Zr,[157–160]

Hf,[161, 162] W,[163, 164] Nb,[165, 166] and Ta,[167, 168] and a full range ofalloys, such as TiNb,[169] TiTa,[168, 170] TiZr,[103, 171] TiW,[172] TiMo,[173]

and TiAl.[174] Recent reviews on the growth of these differentsystems are available.[5, 6, 9]

TiO2 nanotube layers were successfully doped by classicaldoping approaches with N,[83–85] C,[86] and Cr[49] by using ion im-plantation or thermal treatments. More recently, nanoporousWO3 layers were doped with N by using NH3 treatment.[175]

Examples of successful N and C doping of TiO2 nanotubesare presented in Figure 8. Figure 8 a shows photocurrent spec-tra of nitrogen-doped TiO2 nanotube layers obtained by ionimplantation. The inset shows a spectrum to evaluate theband-gap energy. Compared to undoped TiO2 (band gap at3.15 eV), the effect of nitrogen implantation in the TiO2 nano-

Figure 7. a) Schematic representation of the anodization process of Ti metalforming either a compact TiO2 layer or (under specific optimized conditions)a self-organized TiO2 nanotube layer[5, 6, 9] and b) several methods for dopingTiO2 layers.

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tube layer is twofold. First, a significant drop in the magnitudeof the photocurrent is observed, and second a substantial sub-band-gap photoresponse is measured down to approximately2.2 eV. Most important is, however, that thermal annealing ofthe sample after implantation leads to an enhancement of thephotocurrent in the visible and UV range by a factor of 8. Alsovery remarkable is that this reannealing step increases the UVresponse to values even larger than those for the undopedsample.[83]

Figure 8 b shows the visible-light response of carbon-dopedTiO2 nanotube layers.[86] The inset shows a band gap of thecarbon-doped TiO2 nanotubular array from the (jphhn)1/2 versusenergy plot. Clearly, the behavior exhibits two linear parts. Onerefers to the band-gap onset energy of anatase TiO2 at 3.16 eV.The other shows the deep photocurrent response down to1.5 eV due to the carbon doping.

The fact that the tubes grow by oxidation of a substrateallows the use of alloyed metal substrates to achieve mixed-oxide layers. Dopant materials such as N[105] or oxides ofW,[125, 172] Mo,[173] Al,[176, 177] Ta,[167] Zr,[103, 178] Nb,[179] etc. were incor-porated into the TiO2 structure. Interesting features of nano-tube morphologies can be obtained, such as two size scaleself-organization.[153–156] Furthermore, the ionic or electronic

properties in TiO2 can be modified by alloying. To achieve suc-cessful formation of a highly ordered nanotube layer under op-timized anodizing conditions, sometimes the anodization con-ditions need to be adjusted.

Some examples of alloy doping are shown in Figure 9. Com-posite TiO2–WO3/MoO3/Nb2O5 has been successfully fabricatedusing alloy anodization. In all cases the nanotubes show a

straight wall morphology and are vertically well aligned on thesubstrate (Figure 9 a). In all three cases, highly improved ion in-sertion and electrochromic properties are shown even whenonly small amounts, such as 0.2 at % WO3, are present.[9] In par-ticular, these composite oxides have much better photocatalyt-ic properties (Figure 9 b). Compared with pure TiO2 nanotubes(TiNT), W-, Nb-, or Mo-doped layers (Ti0.2W, Ti9W, Ti7Mo) showfaster catalytic kinetics, while Al-doped TiO2 (TiAl) hardly pres-ents catalytic effects. Doping with W or Mo can enhance thephotocatalytic effects due to favorable electron transfer fromTiO2, which results in a decrease of recombination of electron–hole pairs. Apart from doping with certain species, porous WO3

itself exhibited enhanced photocurrent conversion.[180]

Also spectacular are the results from oxide nanotubesgrown on TiNb[169, 179] alloys (Figure 10). These TiO2–Nb2O5

mixed-oxide tubes showed high intercalation activity and sta-bility due to the fact that the Nb ions widen the host TiO2 lat-tice. This makes the process not only faster, but also allows

Figure 8. Photocurrent spectra of the anatase TiO2 nanotubular layer aftera) N doping[86] and b) C doping.[19, 86] The insets show a band-gap determina-tion of the doped TiO2 nanotubular array from a (jphhg)1/2 versus energy plot.

Figure 9. a) SEM images of TiO2–WO3, TiO2–Nb2O5, and TiO2–MoO3 nanotubelayers grown by alloy anodization. b) Photocatalytic properties of dopedTiO2 nanotubes compared with pure TiO2 nanotube layers.

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both H+ and Li+ ions and also much larger Na+ ions to be in-tercalated into the lattice.[179]

5. Applications of Modified TiO2 Nanotubes

5.1. Photocatalysts

TiO2 nanotubes can have a higher photocatalytic reactivitythan a comparable nanoparticulate layer.[181] Various factorsmay be responsible for this effect (optimized reaction geome-try for charge transfer, UV absorption characteristics over thetube, solution diffusion effects). Later work showed that parti-cle decoration with Ag or Au, or applying an external anodicvoltage, can be used to even further increase the photocatalyt-ic activity.[121, 182, 183]

As mentioned, various mixed-oxide tube layers, such asTiNb, TiMo, and TiW,[120, 179, 184] were also explored and a compi-

lation of some photocatalytic re-sults are shown in Figure 9. Visi-ble photocatalysis was shownfor TiW oxide tubes[120] and tosome degree for water split-ting[109, 185–188] using carbon-doped tubular material (al-though there is considerable dis-pute on its effective-ness[66, 185–188]).

A most spectacular possibilityto use TiO2 nanotubes in photo-catalytic applications is to pro-duce freestanding flow-throughmembranes,[145, 189] as they allowfor extremely defined photocata-lytic interactions (highly definedinteraction times) combinedwith a filtration ability.

TiO2 photocatalysis can alsobe used to induce chain scissionin attached organic monolay-ers.[148, 190, 191] This can be used tocreate extremely well definedwettability on surfaces,[190–192] orto liberate terminal payload mol-ecules from the surface upon UVillumination.[148, 182, 193] Thus, forexample, defined drug-releasesystems can be construct-ed.[148, 182, 193]

Common to the approach isthat photocatalytic chain scissionoccurs after the anchoring group(mostly silane or phosphonate).A novel concept in the field ofphotocatalysis is the combina-tion of the TiO2 nanotube geom-etry with molecular selectivebinding units, for example zeo-

lites.[123] For biomedical purposes, X-ray-induced photocatalysiswas used to liberate drugs from the surfaces of TiO2 nano-tubes,[192] or to directly kill cancer cells.[194] Additionally, it wasrecently also demonstrated that electron-beam-induced “pho-tocatalytic” reactions can occur in the vacuum of a SEM instru-ment using ionic liquids as a (nonvolatile) solvent.[195, 196]

Shrestha et al.[193] showed that TiO2 nanotubes can be filledwith magnetic Fe3O4 and thus be magnetically guided to de-sired locations. Such tubes can then be easily coated withdrugs which can be released optically.[182] Similarly, Song et al.showed a filling and release scheme based on amphiphilictube layers.[148, 197]

In the context of photocatalytic reactions it should also bementioned that a similar reaction scheme can be triggered inthe absence of light on anatase TiO2 and TiO2 nanotube surfa-ces (if sufficiently doped).[182] If a voltage is applied to the ma-terial it can cause anodic Schottky barrier breakdown,[198, 199]

Figure 10. a) XRD patterns of TiO2 and TiO2/Nb nanotubes after annealing at 450 8C for 1 h in air showing a shiftof the anatase peak due to lattice expansion.[169] b) Density functional theory (DFT) calculations for TiO2 and theTiO2/Nb unit cell indicating lattice expansion. c) Electrochromic contrast measurements for Nb-doped and pristineTiO2 nanotubes showing enhanced properties for H+ and Li+ lattice insertion (d) for the Nb-doped tubes.[169]

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that is, valence-band holes are created that react with the envi-ronment in a similar manner to photogenerated holes (OHC

radical formation, destruction of organics and organic mono-layers).

5.2. Dye-Sensitized Solar Cells

Dye sensitization of TiO2 was extensively investigated around1970. After the work of Gerischer and Tributsch in 1968,[200] thefirst report on Ru-bipyridyl sensitization on TiO2 appeared in1980.[201] In 1991, Gr�tzel and O’Regan[202] used this principle tofabricate a full DSSC, in which the photon-absorber layer wasmade of TiO2 nanoparticulates that were dye-coated.

A potential source for losses in nanoparticulate DSSCs is car-rier recombination at grain boundaries (due to the presence oftrapping states) and long carrier diffusion path (random walk)effects through the TiO2 network. This could be counteractedby replacing the TiO2 nanoparticulate photoanode with a TiO2

nanotubular layer. Since the first effort to dye-sensitize TiO2

nanotube arrays,[203] where efficiencies of only 0.036 % werereached, approximately 5 % solar-cell conversion efficiency hasbeen reached for tubular-based systems. The literature, howev-er, is sometimes misleading, as in some work pure TiO2 nano-tubes are used[17, 146, 204, 205] while other work uses nanotubesthat additionally were treated with TiCl4 (leading to decorationwith TiO2 nanoparticles) or mixed with TiO2 nanoparticles, forexample.[129, 206, 207] Of course only in investigations where plainTiO2 nanotubes were used can the observed effects (efficiency,dye loading, transport times, reaction kinetics) be unambigu-ously ascribed to the nanotubes; in mixed cases the effectsmay even be dominated by the TiO2 nanoparticles. For pureTiO2 nanotube layers the record reported efficiency currentlystands at 5.2 %[208]—for mixtures with nanoparticles at 7 %.[206]

The most important issues and factors for using TiO2 tubes inDSSCs were recently discussed and reviewed.[7, 17] As discussedin several works,[17, 129, 146, 205] the overall efficiency-limiting factorin TiO2 nanotube-based solar cells is specific dye loading, thatis, the specific surface area of the tubes (BET �30 m2g�1) isconsiderably smaller than that of comparable nanoparticulatelayers (BET �80–100 m2g�1). Several strategies to increase thespecific surface area in tube systems have been explored, suchas the above-mentioned TiCl4 treatment[129, 206, 207, 209] or increas-ing the surface area of the tube walls by creating bamboo-type structures or double-walled nanotubes.[17, 146] Very criticalis also the geometry of the tube tops.[204, 208, 210] Recently, Tsu-chiya et al.[163] introduced a new concept by using silver deco-ration of TiO2 nanotubes which led to an enhancement of theconversion efficiency in DSSCs. Another type of disorganizedbut rapidly growing TiO2 nanotube, produced by so-calledrapid-breakdown anodization (RBA), also shows promising im-provement on the photoconversion efficiency.[211, 212] In thiscontext, however, it should also be noted that very recently, ananodic self-organized TiO2 mesosponge/nanochannel layer hasbeen reported that has a significantly higher specific surfacearea than tubular layers, which seems to be capable of outper-forming nanotube layers in the field of TiO2-based solar cellsand other applications.[8, 13, 118]

5.3. Electrochromic Devices

TiO2 has been widely explored as an active electrode of elec-trochromic devices. Electrochromism can be defined as theability of a material to undergo color change upon electro-chemical oxidation or reduction. Electrochromic devices canvary optical absorption using a low voltage. A small voltageapplied to the windows will cause them to become colored;reversing the voltage causes them to be bleached.[213] Sometransition-metal oxides, such as MnO2, WO3, Nb2O5, MoO3, andTiO2, provide an excellent host lattice for ion intercalation devi-ces, for example, Li-ion batteries or electrochromic devi-ces.[214–217] These devices rely on a reversible uptake (upon ap-plying or releasing an electric field) of small ions such as H+

and Li+ into interstitial positions of the metal oxide.After the first report on electrochromic switching using TiO2

nanotubes,[213] considerable follow-up work improved the con-trast, switching time, threshold voltage, and cyclability byusing doped (mixed oxide) TiO2 nanotubes.[125, 164, 169, 172, 173, 218]

Other work showed that TiO2 nanotube layers can be lifted offfrom a metallic Ti substrate and transferred onto conductingglass,[219] or complete anodization of thin Ti layers on conduct-ing glass can be prepared[220] to construct effective transparentelectrochromic devices.

5.4. Other Applications and Aspects

A very important application of Ti surface modification is inbiomedical materials. About 40 % of today’s biomedical im-plant materials are based on Ti or Ti alloys.[221] TiO2 nanotubularsurfaces are ideal for studying and applying size effects withliving matter or biorelevant species.[222–231] These studies indi-cated a universal beneficial effect of 15-nm-diameter nanotubelayers on bioactivity.

The 3D structure is also optimal for embedding precursorsto hydroxyapatite (HAp) formation[228, 229] that additionally pro-mote specific HAp nucleation. In vivo experiments with adultdomestic pigs showed that nanotube-structured implant surfa-ces can indeed influence bone formation and development byenhancing osteoblast function, and that higher implant bonecontacts can be established if the implants are coated withnanotube layers.[232]

In terms of using TiO2 or TiO2 nanotubes doped with C ascatalysts for visible-light water-splitting reactions,[233, 234] the re-sults are highly ambiguous, as well discussed by Murphy.[66] Interms of gas sensing, TiO2 layers have been shown to have ahigh sensitivity to CO, H2, and NOx gases particularly as nano-particulate films.[235–237] This has also been demonstrated forTiO2 nanotube layers.[57, 238] In terms of in-solution oxygen sens-ing for Au nanoparticles supported on TiO2 nanotube layers, astrongly enhanced reaction rate with O2 in aqueous solutionwas found.[239] Considerable support effects were also observedfor methanol oxidation reactions, not only for supporting Pt/Pd on TiO2 nanotubes[122] but also for “semimetallic” tubes thatshowed an efficiency enhancement of 700 %[19]—these findingsare very promising towards applications in methanol fuel cells(i.e. , as a substitute for carbon-based supports). An approach

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that ideally exploits the high aspect ratio of the nanotubulargeometry is the use as so-called self-cleaning nano test-tubes[240] for fluorescence sensing. One may also use interfer-ence fringes occurring in reflection spectra, by tailoring theside wall rugosity of the tubes[197] as a gas- or liquid-basedsensor.

A point that clearly should be addressed but has not exten-sively been studied yet is the mechanical stability of the tubelayers.[241] While general adhesion is reported to be good, thepresence of a fluoride-rich layer at the oxide/metal interface[9]

affects the adhesion of nanotube layers, for example, whenbending the substrate.[8, 13] This can be improved by carboniza-tion of the tubes,[241] but even better adhesion for anodiclayers can be obtained by using titania mesosponge[8,13] ornanochannel layers.[242]

6. Summary and Outlook

We have reviewed the fabrication, properties, and selected ap-plications of doped TiO2 materials—with an emphasis on self-organized nanotube layers formed by anodization. The tremen-dous effort regarding TiO2 materials has resulted in a lot ofdata in terms of synthesis, properties, modifications, and appli-cations. However, the main challenge with doped TiO2 photo-catalysts remains that the photocatalytic activity under visiblelight is lower than that under ultraviolet light. Therefore, fur-ther development of these photocatalysts is needed. A majorarea of future research is the development of optimized dop-ants and doping strategies. A particularly promising route isprovided by self-ordered TiO2-based nanotubes, as the growthfrom an alloy and the anodization process allow for new,hardly explored doping techniques combined with an ex-tremely high degree of geometry control in reactive TiO2 sys-tems.

Keywords: doping · nanotubes · photocatalysis ·photochemistry · titanium dioxide

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Received: April 6, 2010Published online on July 20, 2010

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