Indhumati Paramasivam

A Review of Photocatalysis using Self‐organized TiO2 Nanotubes and Other Ordered Oxide Nanostructures

Photocatalytic approaches, that is the reaction of light-produced charge carriers at a semiconductor surface with their environment, currently attract an extremely wide scientific interest. This is to a large extent due to the high expectations: i) to convert sunlight directly into an energy carrier (H(2)), ii) to stimulate chemical synthetic reactions, or iii) to degrade unwanted environmental pollutants. Since the early reports in 1972, TiO(2) has been the most investigated photocatalytic material by far; this originates from its outstanding electronic properties that allow for a wide range of applications. Not only the material, but also its structure and morphology, can have a considerable influence on the photocatalytic performance of TiO(2). In recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention. The present Review focuses on TiO(2) nanotube arrays (and similar structures) that grow by self-organizing electrochemistry (highly aligned) from a Ti metal substrate. Herein, the growth, properties, and applications of these tubes are discussed, as well as ways and means to modify critical tube properties. Common strategies are addressed to improve the performance of photocatalysts such as doping or band-gap engineering, co-catalyst decoration, junction formation, or applying external bias. Finally, some unique applications of the ordered tube structures in various photocatalytic approaches are outlined.

Doped TiO2 and TiO2 nanotubes: synthesis and applications.

TiO(2) is one of the most investigated compounds in contemporary materials science. Due to a set of virtually unique electronic properties, it finds intense use in photoelectrochemical applications such as photocatalysis or solar cells. The main drawback in view of direct exploitation of solar-light-based effects is its large band gap of >3 eV. Visible-light-activated TiO(2) can be prepared by doping (band-gap engineering) through incorporation or decoration with other metal ions, nonmetal ions, and semiconductors. Most recently, efforts in TiO(2) research have been even more intensified by the finding of self-organized nanotubular oxide architectures that can be prepared by a simple but optimized anodization of Ti metal surfaces. These nanotubular geometries provide large potential for enhanced and novel functional features. This Review examines doped TiO(2) and in particular TiO(2) nanotubes. Various types of dopants, doping methods, and applications of modified TiO(2) nanotubes are discussed.

Morphological instability leading to formation of porous anodic oxide films

Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.

Anodic Formation of Thick Anatase TiO2 Mesosponge Layers for High-Efficiency Photocatalysis

We report a process for the fabrication of an anatase TiO(2) mesosponge (TMS) layer by an optimized Ti anodization process in a hot glycerol electrolyte followed by a suitable etching process. Such layers can easily be grown to >10 microm thickness and have regular channels and structural features in the 5-20 nm range. The layers show high photocatalytic activity and are mechanically very robust. The layers therefore open new pathways to the wide field of TiO(2)(anatase) applications.

WO3/TiO2 Nanotubes with Strongly Enhanced Photocatalytic Activity

Initiated by the work of Fujishima and Honda in 1972, over the past decades, the use of particulate or colloidal semiconductors in solutions has extensively been studied for the photocatalytic oxidation of organic waste and pollutants in water. By far the most studied material is TiO2, as it is considered to represent the most suitable photocatalyst, in view of effectiveness and stability against photodecomposition. Typically these photocatalytic systems are used in form of nanoparticles, either freely suspended in solution or compacted to a robust photoelectrode. Earlier studies mainly focused on geometric parameters of the particles that influence the photocatalytic activity, such as surface area, size distribution in solution, as well as the TiO2 crystal structure, which was found to play a crucial role. Later, various approaches were reported to enhance the photocatalytic activity of TiO2, for example, by decorating the surface of TiO2 nanoparticles with noble metals such as Pt, Pd, Ag, Au, and so forth. More recently, improved photocatalytic activity was reached by modifying TiO2 particles with other oxides (such as CrxOy, FexOy, VxOy, MoOx, WOx, etc.). The effect of noble particle decoration was mainly interpreted in terms of a facilitated contribution of the photoexcited electrons in the photocatalytic reaction producing, for example, superoxide from O2 dissolved in aqueous electrolytes, while decoration with other oxides particles may influence the rate of charge transfer to the environment via surface states or junction formation. In general, two main reactions are considered to be relevant for the photocatalytic activity of TiO2: 1) the generation of valence-band holes that upon ejection to the environment (electrolyte) have an oxidative power sufficient to oxidize almost any organic material and 2) conduction-band electrons ejected to the electrolyte that may form reactive superoxides. Most recently, advanced geometries of TiO2 have been increasingly explored, in particular self-ordered TiO2 nanotubes (TiNT) have attracted wide attention due to the high level of geometrical definition combined with a high surface area (for an overview see references [23–26]). Such self-ordered TiO2 nanotubular layers can easily be grown on Ti metal sheets by a simple but optimized electrochemical anodization in F containing electrolytes. 28] Investigations of their photocatalytic properties have shown that these tubular layers can be more efficient than classical nanoparticulate layers of a comparable thickness. Self-ordered oxide nanotubes cannot only be grown on pure Ti, but also on other transition metals such as Mo, W, Ta, Nb, and so forth, and a full range of Ti alloys including TiW, TiNb, TiAl, TiMo, TiTa. In the present work, we demonstrate a very strong effect of tungsten addition to the TiO2 nanotubes in terms of their photocatalytic activity. For this, different TiW alloys (Ti0.2at%W (Ti0.2W) and Ti9at%W (Ti9W)) as well as pure Ti were anodized to form 2 mm-long self-organized tube layers as shown in Figure 1. To achieve these self-organized layers different anodization conditions had to be applied as outlined in the Supporting Information. For all cases, comparable dimensions of nanotubular layers with a tube length between 2.2 mm to 2.6 mm and a diameter (obtained from SEM cross sections) between 85 nm to 100 nm were used. For the TiW alloys (Figure 1a–d), a thin porous initiation layer is present on the top of the highly ordered nanotubes, as visible in the cross-sectional images of Figure 1b and d. Figure 1e and f show for comparison, the top view and cross section of pure TiO2 nanotube layers. The top layer can be removed, but this was found to not affect the results strongly. [a] I. Paramasivam, Y.-C. Nah, C. Das, N. K. Shrestha, Prof. Dr. P. Schmuki Department of Materials Science WW-4 Institute of Corrosion and Surface Science (LKO) University of Erlangen—N rnberg Martensstr.7, 91058 Erlangen (Germany) Fax: (+49)9131-852-7575 E-mail : schmuki@ww.uni-erlangen.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000397. It contains experimental descriptions, XRD patterns of TiO2-WO3 nanotubes at 350 8C and 550 8C, and EDX of elemental compositions for Ti9W and Ti0.2W.

Nitrogen doping of nanoporous WO3 layers by NH3 treatment for increased visible light photoresponse

Nanoporous WO(3) layers were grown by electrochemical anodization of W in a fluoride containing electrolyte. These layers were exposed to a thermal treatment in NH(3) to achieve nitrogen doping of the material. The morphology, crystal structure, composition and photoresponse of pure and nitrogen doped WO(3) were compared using scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and photoelectrochemical measurements. The results clearly show that successful nitrogen doping into WO(3) layers can be achieved by controlling the temperature and time during the NH(3) treatment. Most importantly, it is demonstrated that for the nitrogen doped WO(3) layers the photocurrent is significantly enhanced in the visible light region.

Multilayer TiO2–Nanotube Formation by Two-Step Anodization

In this work we report on the growth of a closely stacked double layer of a self-organized TiO 2 nanotubes. For that we first anodize Ti in acidic electrolyte containing hydrofluoric acid to form thin nanotube layers. Afterwards we start a second anodization in a different electrolyte, a glycerol/NH 4 F mixture. This procedure allows us to grow the second layer directly underneath the first one. From scanning electron microscopy and transmission electron microscopy investigations we revealed that the second growth occurs via the tube bottoms of the first layer. These stacked multilayers generate new possibilities to vertically tailor the properties of the self-organized TiO 2 nanotube layers.

MoO3 in self-organized TiO2 nanotubes for enhanced photocatalytic activity.

Self-organized nanostructures (nanotubes and nanopores) consisting of TiO2-MoO3 were grown on Ti-Mo substrates with different Mo contents (7 wt% Mo, 20 wt% Mo, and 35 wt% Mo) by electrochemical anodization. The photocatalytic activity of these oxide layers was compared with TiO2 nanotubes. The results show clearly that MoO3 doping of approx. 7 wt% strongly enhances the achievable photodegradation ability of titania nanotubes. The findings bear considerable significance for pollutant degradation and other photocatalytic applications of TiO2. TiO2 is one of the most important photocatalysts owing to its suitable band gap, nontoxicity, excellent chemical and photo corrosion resistance, and low price. Semiconductorbased photocatalysis has received over the past decades considerable attention for its potential applications in renewable energy-conversion devices, degradation of organics in environmental pollutants, as well as in biomedicine. For many practical photocatalytic applications, high surface area electrodes are desired. This is established typically by using nanostructures of TiO2, traditionally fabricated by compacted or sintered nanoparticles or most recently by using layers of self-organized TiO2 nanotubes. [3,4,6,7] Such self-organized TiO2 nanotubes (TiNT) can be grown by anodization of metallic Ti, directly on the substrate. This is particularly interesting as certain geometries of TiO2 nanotube layers have been reported to show a higher photocatalytic activity than comparable nanoparticulate systems. Even a further enhancement of the photocatalytic activity of self-organized TiO2 nanotube layers can be achieved by filling the tubes with secondary scavenger materials or decorating the nanotubes with metal or oxide nanoparticles. These attempts mostly target a retardation of the recombination of photogenerated charge by junction formation, surface passivation, or enhancing the electron transfer rate to the electrolyte. Recently, various transition-metal oxides have been used in combination with TiO2. [12,13] In these composite systems, owing to the different band gap, junction formation can provide efficient separation of electron-hole pairs and thus the composite oxide material may promote the photocatalytic reactions. As nanotubes of TiO2 composites with various metal oxides can be grown by using alloyed Ti substrates , we investigated in this work the feasibility of using MoO3 as cocatalyst in anodic TiO2 layers in order to enhance the photocatalytic activity. To achieve this, self-organized composite nanostructures (nanotubular and nanoporous morphologies) of TiO2-MoO3 were grown on the Ti-Mo substrates with different Mo content by electrochemical anodization. Their photocatalytic activity was then compared with neat TiO2 nanotube layers. Figure 1 shows scanning electron microscope (SEM) images of the top and cross section of the oxide layers formed on the different Ti-Mo alloys after anodization in ethylene glycol containing 0.05m HF at 50 V for 35 min, 2 h, and 3 h for Ti7Mo (7 wt% Mo), Ti20Mo (20 wt% Mo), and Ti35Mo (35 wt% Mo), respectively. It is clear that there is a homogeneous growth of nanotubes on the Ti7Mo sample whereas porous oxide layers are formed on the two alloys with a Mo content higher than 7 wt%. The different anodization times were selected to form a constant thickness of the oxide layer in each case resulting in about 0.8 mm. The reason for the longer anodization time that is necessary to grow the same thickness of the oxide layer in the case of samples containing higher Mo content (i.e. Ti20Mo and Ti35Mo) is the slower growth rate of nanostructures with increasing Mo content. The chemical composition of the oxide layers was investigated using EDX analysis. Figure 2 shows EDX spectra of a) Ti7Mo, b) Ti20Mo, and c) Ti35Mo sam[a] P. Agarwal, I. Paramasivam, Dr. N. K. Shrestha, Prof. Dr. P. Schmuki Institute for Surface Science and Corrosion (LKO) Department of Materials Science (WW4) Freiderich-Alexander University Martensstrasse 7 D-91508, Erlangen (Germany) Fax: (+49)9131-852-7582 E-mail : schmuki@ww.uni-erlangen.de [] On leave from: Indian Institute of Technology Roorkee (IITR) Roorkee 247667, Uttranchal (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.200900369.

Optimizing TiO2 Nanotube Top Geometry for Use in Dye-Sensitized Solar Cells

Recombination dynamics: For TiO(2) nanotube-based dye-sensitized solar cells, the efficiency can be drastically enhanced by a synergetic effect that occurs when using nanowire-ended nanotubes in combination with an adequate nanoparticle decoration (see figure).

Some critical factors for photocatalysis on self-organized TiO2 nanotubes

In the present work, different intrinsic and extrinsic parameters are investigated that affect the photocatalytic activity of self-organized TiO2 nanotube layers. Particularly, the influence of annealing temperature and annealing atmosphere, the influence of different gas purging in the electrolyte, and the effect of applied voltage on the photocatalytic degradation rates of acid orange (AO7) are discussed. We find that the effect of the reducing gas atmosphere dominates over the anatase/rutile ratio in activating the nanotube layers. Moreover, we show that the effect of different gas purging (Ar and O2) of the electrolyte affects the reaction rate twofold: (1) by providing electron acceptor states and also by (2) a different change in the red–ox potential, i.e., the band bending in TiO2. By an external anodic voltage, the reaction rates can be increased drastically due to increased band bending. Nevertheless, the magnitude of the effect is also affected by the presence or absence of O2 in the electrolyte.