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

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

Solar energy is an alternative, sustainable energy source for mankind. Finding a convenient way to convert sunlight energy into chemical energy is a key step towards realizing large-scale solar energy utilization. Owing to their structural regularity and synthetic tunability, metal–organic frameworks (MOFs) provide an interesting platform to hierarchically organize light-harvesting antennae and catalytic centers to achieve solar energy conversion. Such photo-driven catalytic processes not only play a critical role in the solar to chemical energy conversion scheme, but also provide a novel methodology for the synthesis of fine chemicals. In this review, we summarize the fundamental principles of energy transfer and photocatalysis and provide an overview of the latest progress in energy transfer, light-harvesting, photocatalytic proton and CO2 reduction, and water oxidation using MOFs. The applications of MOFs in organic photocatalysis and degradation of model organic pollutants are also discussed.

Metal–organic frameworks for artificial photosynthesis and photocatalysis

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

Gold nanorods and their plasmonic properties

When Alpha met Beta: a tuneable α-β surface phase junction on Ga(2)O(3) can significantly improve photocatalytic overall water splitting into H(2) and O(2) over individual α-Ga(2)O(3) or β-Ga(2)O(3) surface phases. This enhanced photocatalytic performance is mainly attributed to the efficient charge separation and transfer across the α-β phase junction.

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3†

The hydrogenation of CO2 was investigated over a wide range of reaction conditions, using two Pd/γ-Al2O3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼100%, respectively). Turnover rates for CO and CH4 formation were both higher over 5% Pd/Al2O3 with a larger average Pd particle size than those over 0.5% Pd/Al2O3 with a smaller average particle size. The selectivity to methane (22–40%) on 5% Pd/Al2O3 was higher by a factor of 2–3 than that on 0.5% Pd/Al2O3. The drastically different rate expressions and apparent energies of activation for CO and CH4 formation led us to conclude that reverse water gas shift and CO2 methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO2 and H2 pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the pr...

Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies

Kinetic modeling and transient DRIFTS–MS studies of CO2 methanation over Ru/Al2O3 catalysts

A composite Sr2TiO4/SrTiO3(La,Cr) heterojunction photocatalyst has been prepared by a simple in situ polymerized complex method. Upon Pt cocatalyst loading, this catalyst shows higher photocatalytic activity towards hydrogen production than individual SrTiO3(La,Cr) and Sr2TiO4(La,Cr) in the presence of methanol sacrificial reagent. Microscopic morphology studies show that well defined heterojunctions are formed by matching the lattice fringes of SrTiO3(La,Cr) and Sr2TiO4(La,Cr), and Pt was preferentially loaded on the surface of the Sr2TiO4(La,Cr) component in the composite Sr2TiO4/SrTiO3(La,Cr) photocatalyst. XPS and EPR analyses show that the composite photocatalyst also has the lowest amount of Cr6+ electron trapping sites. Band structure analysis by combining absorption spectroscopy and Mott–Schottky plots shows that, in the composite photocatalyst, the photogenerated electrons and holes tend to migrate from SrTiO3(La,Cr) to Sr2TiO4(La,Cr) and from Sr2TiO4(La,Cr) to SrTiO3(La,Cr), respectively. This kind of band structure can facilitate charge transfer and separation driven by the minor potential difference between the two components, which is further confirmed by the observation of long lived electrons in the time resolved FT-IR spectroscopic study. It is concluded that the superior photocatalytic activity of the composite heterojunction photocatalyst is due to efficient charge transfer and separation by well defined heterojunctions formed between SrTiO3(La,Cr) and Sr2TiO4(La,Cr), preferential loading of Pt nanoparticles on the Sr2TiO4(La,Cr) component, and the lowest amount of Cr6+ in the composite photocatalyst. The tailored design and synthesis of the composite heterojunction structure is a promising approach for the improvement of the photocatalytic activity of a photocatalyst.

Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation

The fabrication of surface phase junctions has proven to be an efficient strategy for the enhancement of photocatalytic activity; however, some questions about these systems are not yet well understood. In this study, the photocatalytic overall water splitting reaction was achieved on photocatalysts with pure and mixed phase compositions of cubic γ-Ga2O3 and monoclinic β-Ga2O3. All the Ga2O3 photocatalysts can split water stoichiometrically into H2 and O2; however, the phase-mixed γ/β-Ga2O3 photocatalyst with a small amount of β phase shows the lowest activity. This is opposite from that of the reported α/β-Ga2O3 system in which the phase-mixed α/β-Ga2O3 photocatalyst with a small amount of β phase shows much higher photocatalytic activity than the individual phases. Much more disordered structure is found between the γ and β phases in the γ/β-Ga2O3 photocatalyst with low content of β phase due to the defective spinel structure of γ phase. Spectroscopic characterization and theoretical calculations indica...

Xiang Wang

Papers

Photocatalytic Overall Water Splitting Promoted by an α–β phase Junction on Ga2O3†

Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies

Kinetic modeling and transient DRIFTS–MS studies of CO2 methanation over Ru/Al2O3 catalysts

Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation

Effect of Phase Junction Structure on the Photocatalytic Performance in Overall Water Splitting: Ga2O3 Photocatalyst as an Example