Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental 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

Graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be produced on a large scale at low cost. Thus, it has been regarded as an important component for making various functional composite materials. Especially, graphene-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. This critical review summarizes the recent progress in the design and fabrication of graphene-based semiconductor photocatalysts via various strategies including in situ growth, solution mixing, hydrothermal and/or solvothermal methods. Furthermore, the photocatalytic properties of the resulting graphene-based composite systems are also discussed in relation to the environmental and energy applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation and photocatalytic disinfection. This critical review ends with a summary and some perspectives on the challenges and new directions in this emerging area of research (158 references).

Graphene-based semiconductor photocatalysts

A review on g-C3N4-based photocatalysts

Photocatalytic water splitting represents a promising strategy for clean, low-cost, and environmental-friendly production of H2 by utilizing solar energy. There are three crucial steps for the photocatalytic water splitting reaction: solar light harvesting, charge separation and transportation, and the catalytic H2 and O2 evolution reactions. While significant achievement has been made in optimizing the first two steps in the photocatalytic process, much less efforts have been put into improving the efficiency of the third step, which demands the utilization of cocatalysts. To date, cocatalysts based on rare and expensive noble metals are still required for achieving reasonable activity in most semiconductor-based photocatalytic systems, which seriously restricts their large-scale application. Therefore, seeking cheap, earth-abundant and high-performance cocatalysts is indispensable to achieve cost-effective and highly efficient photocatalytic water splitting. This review for the first time summarizes all the developed earth-abundant cocatalysts for photocatalytic H2- and O2-production half reactions as well as overall water splitting. The roles and functional mechanism of the cocatalysts are discussed in detail. Finally, this review is concluded with a summary, and remarks on some challenges and perspectives in this emerging area of research.

Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting

Mesporous surface-fluorinated TiO2 (F−TiO2) powders of anatase phase with high photocatalytic activity are prepared by a one-step hydrothermal strategy in a NH4HF2−H2O−C2H5OH mixed solution with tetrabutylorthotitanate (Ti(OC4H9)4, TBOT) as precursor. The prepared samples are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption−desorption isotherms, UV−vis absorption spectroscopy, and transmission electron microscopy. The production of hydroxyl radicals (•OH) on the surface of UV-illuminated TiO2 is detected by a photoluminescence (PL) technique with use of terephthalic acid as a probe molecule. The photocatalytic activity is evaluated by photocatalytic oxidation decomposition of acetone in air under UV light illumination. The results show that the photocatalytic activity of F−TiO2 powders is obviously higher than that of pure TiO2 and commercial Degussa P25 (P25) powders due to the fact that the strong electron-withdrawing ability of the surface ≡Ti−F groups reduces the rec...

Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment

Graphene, a single layer of graphite, possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area, and can be obtained on a large scale at low cost. Thus, it has been regarded as an excellent catalyst support. Recently, graphene-based semiconductor photocatalysts have attracted more attention due to their enhanced photocatalytic activity. In this work, hierarchical macro/mesoporpous TiO2–graphene composites with low loadings (0–0.2 wt.%) of graphene were first produced by a simple one-step hydrothermal method using tetrabutyl titanate as the titanium precursor. The prepared composite samples presented enhanced photocatalytic activity in photodegradation of acetone in air. Graphene content exhibited an obvious influence on photocatalytic activity and the optimal graphene addition content was determined. At the optimal graphene concentration (0.05 wt.%), the prepared composites showed the highest photocatalytic activity, exceeding that of pure TiO2 and Degussa P-25 by a factor of 1.7 and 1.6, respectively. The enhanced photocatalytic activity is due to graphene as an excellent electron acceptor and transporter, thus reducing the recombination of charge carriers and enhancing the photocatalytic activity. The transient photocurrent response experiment further confirmed the transfer of photogenerated electrons from TiO2 to graphene and the suggested mechanism.

Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air

Nitrogen self-doped nanosized TiO2 sheets with exposed {001} facets for enhanced visible-light photocatalytic activity

A highly efficient photocatalytic system for hydrogen production by a robust hydrogenase mimic in an aqueous solution.

We present a blueprint for aromatic C–H functionalization via a combination of photocatalysis and cobalt catalysis and describe the utility of this strategy for benzene amination and hydroxylation. Without any sacrificial oxidant, we could use the dual catalyst system to produce aniline directly from benzene and ammonia, and phenol from benzene and water, both with evolution of hydrogen gas under unusually mild conditions in excellent yields and selectivities.

Wenguang Wang

Papers

Enhancement of Photocatalytic Activity of Mesporous TiO2 Powders by Hydrothermal Surface Fluorination Treatment

Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2–graphene composites for photodegradation of acetone in air

Nitrogen self-doped nanosized TiO2 sheets with exposed {001} facets for enhanced visible-light photocatalytic activity

A highly efficient photocatalytic system for hydrogen production by a robust hydrogenase mimic in an aqueous solution.

Photocatalytic Hydrogen-Evolution Cross-Couplings: Benzene C-H Amination and Hydroxylation.