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

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

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

We report the first demonstration of hydrogen treatment as a simple and effective strategy to fundamentally improve the performance of TiO2 nanowires for photoelectrochemical (PEC) water splitting. Hydrogen-treated rutile TiO2 (H:TiO2) nanowires were prepared by annealing the pristine TiO2 nanowires in hydrogen atmosphere at various temperatures in a range of 200–550 °C. In comparison to pristine TiO2 nanowires, H:TiO2 samples show substantially enhanced photocurrent in the entire potential window. More importantly, H:TiO2 samples have exceptionally low photocurrent saturation potentials of −0.6 V vs Ag/AgCl (0.4 V vs RHE), indicating very efficient charge separation and transportation. The optimized H:TiO2 nanowire sample yields a photocurrent density of ∼1.97 mA/cm2 at −0.6 V vs Ag/AgCl, in 1 M NaOH solution under the illumination of simulated solar light (100 mW/cm2 from 150 W xenon lamp coupled with an AM 1.5G filter). This photocurrent density corresponds to a solar-to-hydrogen (STH) efficiency of ∼1...

Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

Recent achievements in the construction of surfaces with special wettabilities, such as superhydrophobicity, superhydrophilicity, superoleophobicity, superoleophilicity, superamphiphilicity, superamphiphobicity, superhydrophobicity/superoleophilicity, and reversible switching between superhydrophobicity and superhydrophilicity, are presented. Particular attention is paid to superhydrophobic surfaces created via various methods and surfaces with reversible superhydrophobicity and superhydrophilicity that are driven by various kinds of external stimuli. The control of the surface micro-/nanostructure and the chemical composition is critical for these special properties. These surfaces with controllable wettability are of great importance for both fundamental research and practical applications.

Design and Creation of Superwetting/Antiwetting Surfaces

Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 microm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2-3 microm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.

Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications

Remarkable surface wettability transition occurs with an inducement of ultraviolet (UV) for aligned ZnO nanorod films. The inorganic oxide films, which show super-hydrophobicity (left), become super-hydrophilic (right) when exposed to UV illumination. After the films are placed in the dark, the wettability evolves back to super-hydrophobicity. This reversible effect is ascribed to the cooperation of the surface photosensitivity and the aligned nanostructure. Such special property will greatly extend the applications of ZnO films.

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Reversible super-hydrophobicity to super-hydrophilicity transition of aligned ZnO nanorod films

In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO2−polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results rel...

Xinjian Feng

Papers

Design and Creation of Superwetting/Antiwetting Surfaces

Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications

Reversible super-hydrophobicity to super-hydrophilicity transition of aligned ZnO nanorod films

Recent Advances in the Use of TiO2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry

Superhydrophobic Aligned Polystyrene Nanotube Films with High Adhesive Force