This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor–semiconductor heterojunction, the semiconductor–metal heterojunction, the semiconductor–carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems are also discussed in relation to the environmental and energy applications, such as degradation of pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research.

Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2–), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption–desorption equilibrium at the TiO2–solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity...

Generation and Detection of Reactive Oxygen Species in Photocatalysis

Core/shell structured C3N4/BiPO4 photocatalyst is fabricated via a facile ultrasonic dispersion method. The thickness of the shell may be controlled by tuning the amount of C3N4 in the dispersion, which determines the enhanced level of photocatalytic activity. The optimum photocatalytic activity of C3N4/BiPO4 at a weight ratio of 4% (C3N4/BiPO4) under UV irradiation is almost 4.5 times as high as that of reference P25 (TiO2) and 2.5 times of BiPO4. More attractively, the dramatic visible light photocatalytic activity is generated due to the C3N4 loaded. The enhancement in performance is demonstrated to be the match of lattice and energy level between the C3N4 and BiPO4. This match facilitates the separation and transfer of photogenerated electron–hole pairs at the heterojunction interfaces and may be important for other core/shell structured materials. In addition, this method is expected to be extended for other C3N4 loaded materials.

Dramatic Activity of C3N4/BiPO4 Photocatalyst with Core/Shell Structure Formed by Self-Assembly

A high photocatalytic BiPO4 with a novel nonmetal oxy acid structure is synthesized by a hydrothermal method. BiPO4 photocatalyst has an optical indirect band gap of 3.85 eV. In a comparison of BiPO4 with that of TiO2 (P25, Degussa), it is found that the photocatalytic activity of BiPO4 is twice that of TiO2 (P25, Degussa) for the degradation of methylene blue (MB) dye, while the BET surface of BiPO4 is just one tenth of that of P25. Both the high position of the valence band and the high separation efficiency of electron−hole pairs result in the high photocatalytic activity. The inductive effect of PO43− helps the e−/h+ separation, which plays an important role in its excellent photocatalytic activity. It may extend to the synthesis of other inorganic nonmetal salts of oxy photocatalysts with suitable band gap and high activity for the environmental purification of organic pollutants in aqueous solution.

New Type of BiPO4 Oxy-Acid Salt Photocatalyst with High Photocatalytic Activity on Degradation of Dye

One of the simplest methods for splitting water into H2 and O2 with solar energy entails the use of a particulate-type semiconductor photocatalyst. To harness solar energy efficiently, a new water-splitting photocatalyst that is active over a wider range of the visible spectrum has been developed. In particular, a complex perovskite-type oxynitride, LaMgxTa1−xO1+3xN2−3x (x≥1/3), can be employed for overall water splitting at wavelengths of up to 600 nm. Two effective strategies for overall water splitting were developed. The first entails the compositional fine-tuning of a photocatalyst to adjust the bandgap energy and position by forming a series of LaMgxTa1−xO1+3xN2−3x solid solutions. The second method is based on the surface coating of the photocatalyst with a layer of amorphous oxyhydroxide to control the surface redox reactions. By combining these two strategies, the degradation of the photocatalyst and the reverse reaction could be prevented, resulting in successful overall water splitting.

A Complex Perovskite-Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm

Bi2WO6 photocatalyst was hybridized by graphite-like C3N4via facile chemisorption. After hybridization with C3N4, the photocatalytic activity of Bi2WO6 was obviously enhanced. A C3N4/Bi2WO6 photocatalyst hybridized with monolayer C3N4 exhibited the highest photocatalytic activity which was 68.9% higher than that of pure Bi2WO6. The enhanced photocatalytic activity of the C3N4/Bi2WO6 photocatalysts could be attributed to the synergetic effect between C3N4 and Bi2WO6. The photogenerated holes on the valence band of Bi2WO6 could transfer to the highest occupied molecular orbital of C3N4via the well developed interface, causing rapid photoinduced charge separation and enhancing the photocatalytic activity.

Enhancement of photocatalytic activity of Bi2WO6 hybridized with graphite-like C3N4

A simple method allows the preparation of core/shell photocatalysts with spatially separated co-catalysts for efficient water splitting. The high activity was attributed to the core/shell structure and separated co-catalysts that assisted separation and collection of the electrons and holes at the respective co-catalysts, owing to active rectification of electron and hole transport (see picture; Eg =2.1 eV).

Chengsi Pan

Papers

Dramatic Activity of C3N4/BiPO4 Photocatalyst with Core/Shell Structure Formed by Self-Assembly

New Type of BiPO4 Oxy-Acid Salt Photocatalyst with High Photocatalytic Activity on Degradation of Dye

A Complex Perovskite-Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm

Enhancement of photocatalytic activity of Bi2WO6 hybridized with graphite-like C3N4

Core/Shell Photocatalyst with Spatially Separated Co-Catalysts for Efficient Reduction and Oxidation of Water†