Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

TiO 2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO 2 photocatalysis. In this review, recent developments in the area of TiO 2 photocatalysis research, in terms of new materials from a structural design perspective, have been summarized. The dimensionality associated with the structure of a TiO 2 material can affect its properties and functions, including its photocatalytic performance, and also more specifically its surface area, adsorption, reflectance, adhesion, and carrier transportation properties. We provide a brief introduction to the current situation in TiO 2 photocatalysis, and describe structurally controlled TiO 2 photocatalysts which can be classified into zero-, one-, two-, and three-dimensional structures. Furthermore, novel applications of TiO 2 surfaces for the fabrication of wettability patterns and for printing are discussed.

TiO2 photocatalysis: Design and applications

A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications

The present paper gives an overview and review on self-organized TiO2 nanotube layers and other transition metal oxide tubular structures grown by controlled anodic oxidation of a metal substrate We describe mechanistic aspects of the tube growth and discuss the electrochemical conditions that need to be fulfilled in order to synthesize these layers Key properties of these highly ordered, high aspect ratio tubular layers are discussed In the past few years, a wide range of functional applications of the layers have been explored ranging from photocatalysis, solar energy conversion, electrochromic effects over using the material as a template or catalyst support to applications in the biomedical field A comprehensive view on state of the art is provided

TiO2 nanotubes: Self-organized electrochemical formation, properties and applications

Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum and silicon have attracted significant interest in recent years. While scientific thrust is often directed towards the elucidation of the principles of the self-organization phenomena, technological efforts target applications based on the direct use of the high surface area, for example, for sensing 6] or controlled catalysis, exploit the optical properties in photonic crystals, waveguides, or in 3D arranged Bragg-stack type of reflectors. The highly organized structures may be used indirectly as templates for the deposition of other materials such as metals, semiconductors, or polymers. Over the past few years, nanoporous TiO2 structures have also been formed by electrochemical anodization of titanium. Although several applications have been proposed, a wider impact of these structures has been hampered by the fact that the layers could only been grown to a limiting thickness of a few hundreds of nanometers. Herein we demonstrate for the first time how high-aspectratio, self-organized, TiO2 films can be grown by tailoring the electrochemical conditions during titanium anodization. Figure 1 shows scanning electron microscope (SEM) images of self-organized porous titanium oxide formed to a thickness of approximately 2.5 mm in 1m (NH4)2SO4 electrolyte containing 0.5 wt.% NH4F. From the SEM images it is evident that the self-organized regular porous structure consists of pore arrays with a uniform pore diameter of approximately 100 nm and an average spacing of 150 nm. It is also clear that pore mouths are open on the top of the layer while on the bottom of the structure the tubes are closed by presence of an about 50-nm thick barrier layer of TiO2. The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO2 by localized acidification at the pore bottom while a protective environment is maintained along the pore walls and at the pore mouth. In our previous work in HF and NaF solutions it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO2 at the pore bottom and the chemical dissolution of this TiO2 in an F ion containing solution (Figure 2). The solubility of TiO2 in HF, forming [TiF6] 2 , is essential for pore formation, however, it is also the reason that previous attempts to form porous layers in HF electrolytes always resulted in layer thicknesses in the range of some 100 nm. We tackled the problem by controlling the self-induced acidification of the pore bottom that is caused by the electrochemical dissolution of the metal (Figure 2 a–c). Main reason for the localized acidification is the oxidation and hydrolysis of elemental titanium [Eq. (1), in Figure 2]. The chemical dissolution rate of TiO2 is highly dependent on the pH value (see Figure 2 d). Using a numerical simulation of the relevant ion fluxes we can construct the pH profile in the pore (such as in Figure 2b), in other words, the ideal ion flux for the desired pH profile can then be determined. Furthermore we can tune the dissolution rate by the dissolution current. In other words, using a buffered neutral solution as electrolyte and adjusting the anodic current flow to an ideal value, acid can be created where it is needed, that is, at the pore bottom, while higher pH values are established at the pore mouth as a result of migration and diffusion effects of the pH buffer species (NH4F, (NH4)2SO4). Assuming equilibrium, the flux of dissolving species (leading to acidification at the pore bottom) and the flux of buffering species are equal. The calculations show that for the experimental conditions given in Figure 1 the pH value at the pore bottom is around 2 and increase to about 5 at the pore mouth, this corresponds to a drop in the local chemical etch rate of about 20 times. We used a voltage-sweep technique to achieve a steadystate current and to establish the desired pH profile. The reason to use a voltage-sweep technique rather than a Figure 1. SEM images of porous titanium oxide nanotubes. The crosssectional (a), top (b), and bottom (c) views of a 2.5-mm thick selforganized porous layer. The titanium sample was anodized up to 20 V in 1m (NH4)2SO4 + 0.5 wt. % NH4F using a potential sweep from open-circuit potential to 20 V with sweep rate 0.1 Vs . The average pore diameter is approximately 100 nm and the average pore spacing is approximately 150 nm.

High‐Aspect‐Ratio TiO2 Nanotubes by Anodization of Titanium

Self‐Organized TiO2 Nanotube Layers as Highly Efficient Photocatalysts

The formation of self-organized porous titania nanotubes is achieved by electrochemical anodization under specific experimental conditions. In present work, the formation of porous titania nanotubes on titanium substrates is investigated in several SO42−/F− based electrolytes. The presence of some non-porous layers covering the porous layers and accompanying the pore growth is observed. We discuss in details the influence of different electrolyte composition on the structure of self-organized porous layers, investigate the conditions for ideal pore growth. SEM investigations and XRD, AES and EDX surface analyses are carried out to characterize the self-organized porous layers. The results show that using SO42−/F− electrolytes with different cations can drastically influence the final morphology of the self-organized porous nanotubes. We furthermore show that the nanotubes consist of TiO2 and that they remain unchanged when annealed.

Jan M. Macak

Papers

TiO2 nanotubes: Self-organized electrochemical formation, properties and applications

High‐Aspect‐Ratio TiO2 Nanotubes by Anodization of Titanium

Self‐Organized TiO2 Nanotube Layers as Highly Efficient Photocatalysts

Influence of different fluoride containing electrolytes on the formation of self-organized titania nanotubes by Ti anodization