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

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

Anatase hierarchical TiO2 with innovative designs (hollow microspheres with exposed high-energy {001} crystal facets, hollow microspheres without {001} crystal facets, and solid microspheres without {001} crystal facets) were synthesized via a one-pot hydrothermal method and characterized. Based on these materials, gas sensors were fabricated and used for gas-sensing tests. It was found that the sensor based on hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets exhibited enhanced acetone sensing properties compared to the sensors based on the other two materials due to the exposing of high-energy {001} crystal facets and special hierarchical hollow structure. First-principle calculations were performed to illustrate the sensing mechanism, which suggested that the adsorption process of acetone molecule on TiO2 surface was spontaneous, and the adsorption on high-energy {001} crystal facets would be more stable than that on the normally exposed {101} crystal facets. Further c...

Enhanced Gas-Sensing Properties of the Hierarchical TiO2 Hollow Microspheres with Exposed High-Energy {001} Crystal Facets

First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles

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

Adsorption and diffusion studies of an O adatom on TiO2 anatase surfaces with first principles calculations

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.

Qin Liu

Papers

First principles study of the adsorption of a NO molecule on N-doped anatase nanoparticles

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

Adsorption and diffusion studies of an O adatom on TiO2 anatase surfaces with first principles calculations

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

Interstitial clusters on Σ = 11(113) grain boundary in copper: Geometric structure, stability, and ability to annihilate vacancies