In the past few decades, there has been a wide research interest in titanium dioxide (TiO2) nanomaterials due to their applications in photocatalytic hydrogen generation and environmental pollution removal. Improving the optical absorption properties of TiO2 nanomaterials has been successfully demonstrated to enhance their photocatalytic activities, especially in the report of black TiO2 nanoparticles. The recent progress in the investigation of black TiO2 nanomaterials has been reviewed here, and special emphasis has been given on their fabrication methods along with their various chemical/physical properties and applications.

Black titanium dioxide (TiO2) nanomaterials

Radiation damage in nanostructured materials

Photocatalytic hydrogen production and pollutant degradation provided both great opportunities and challenges in the field of sustainable energy and environmental science. Over the past few decades, we have witnessed fast growing interest and efforts in developing new photocatalysts, improving catalytic efficiency and exploring the reaction mechanism at the atomic and molecular levels. Owing to its relatively high efficiency, nontoxicity, low cost and high stability, TiO2 becomes one of the most extensively investigated metal oxides in semiconductor photocatalysis. Fundamental studies on well characterized single crystals using ultrahigh vacuum based surface science techniques could provide key microscopic insight into the underlying mechanism of photocatalysis. In this review, we have summarized recent progress in the photocatalytic chemistry of hydrogen, water, oxygen, carbon monoxide, alcohols, aldehydes, ketones and carboxylic acids on TiO2 surfaces. We focused this review mainly on the rutile TiO2(110) surface, but some results on the rutile TiO2(011), anatase TiO2(101) and (001) surfaces are also discussed. These studies provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting field. At the end of this review, we have discussed how these studies can help us to develop new photocatalysis models.

Elementary photocatalytic chemistry on TiO2 surfaces

Oxygen vacancy plays an important role in many fields, such as photocatalysis, energy storages, electro-catalysis, which has been widely investigated both by experiment and theoretical calculations. Herein, this review presents the new type and advanced methods to detect the existence of oxygen vacancies in nanomaterials, such as, electron paramagnetic resonance, synchrotron radiation-based X-ray absorption fine structure, X-ray photoelectron spectroscopy and positron annihilation spectroscopy measurements. In addition, we provide some perspectives on the challenge and new direction for future research in oxygen vacancy. We hope that this minireview can offer some useful contributions to the future development of defective nanomaterials for diverse application.

An overview of advanced methods for the characterization of oxygen vacancies in materials

The photocatalytic activity of TiO2 has aroused a broad range of research effort since 1972. Although TiO2 has a very high efficiency in utilizing ultraviolet light, its overall solar activity is very limited due to its wide bandgap (≈3.0−3.2 eV). This is a bottleneck for TiO2 to be applied in the areas ranging from visible-light photocatalysis and photovoltaics to photo-electrochemistry and sensors. Recently, the emergence of black TiO2 nanomaterial has triggered world-wide research interest, because of its substantially enhanced solar absorption and the improved photocatalytic activities. Here, a variety of synthetic strategies of black TiO2 are outlined, and the structural and chemical features, band structures and electronic properties of the black TiO2 nanomaterials are described in details, along with their photocatalytic performances as well as some other new applications.

Progress in Black Titania: A New Material for Advanced Photocatalysis

O2 adsorption and dissociation on a hydrogenated anatase (101) surface are studied with first-principle calculations coupled with the nudged elastic band (NEB) method. H atoms on the anatase (101) surface or at subsurface sites can increase the absolute values of the O2 adsorption energy. The O2 dissociation barriers on an anatase surface with two H atoms at the subsurface sites or with a H surface adatom and a subsurface atom are much lower than that of the dissociation on a surface with H adatoms on the (101) surface. After the dissociation, OH, H2O, and O adatoms may form on the surface. Because it is not difficult for H adatoms on the surface to diffuse to the subsurface sites, surface H doping atoms are very useful to reduce the O2 dissociation barrier. The anatase particles with hydrogenated (101) surface are efficient catalysts to oxidize the adsorbed toxic gas molecule.

O2 Adsorption and Dissociation on A Hydrogenated Anatase (101) Surface

Gallium antisite defect and residual acceptors in undoped GaSb

Self-healing mechanism of irradiation defects near Σ = 11(113) grain boundary in copper

O2 molecule adsorption and dissociation on an anatase (101) surface with a subsurface Ti interstitial atom are studied using density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method. The subsurface Ti interstitial atom can facilitate O2 adsorption and dissociation. When a single O2 molecule is adsorbed onto the surface, after dissociation, the two O adatoms are strongly bonded to the surface and no longer active on the surface. Different from the case of a single O2 molecule dissociation, two active dissociated oxygen atoms are generated when two O2 molecules are adsorbed onto the surface near a subsurface Ti interstitial atom. These two active oxygen atoms can oxidize the toxic gases adsorbed onto the surface. For this situation, the subsurface Ti interstitial atom may be a key factor to improve the catalytic performance of TiO2.

O2 adsorption and dissociation on an anatase (101) surface with a subsurface Ti interstitial

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO2 anatase (001) surface is studied by first-principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O2 molecule is high enough to break the O=O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O=O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.

Zhu Wang

Papers

O2 Adsorption and Dissociation on A Hydrogenated Anatase (101) Surface

Gallium antisite defect and residual acceptors in undoped GaSb

Self-healing mechanism of irradiation defects near Σ = 11(113) grain boundary in copper

O2 adsorption and dissociation on an anatase (101) surface with a subsurface Ti interstitial

Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface