Trends in Chemistry
ReviewEmerging Photocatalysts for Hydrogen Evolution
Section snippets
Hydrogen Evolution via PWS
Concerns regarding the depletion of fossil fuels and the increase of environmental pollution have aroused a sense of urgency in developing renewable and clean energy technologies. Hydrogen gas (H2) has been regarded as one of the cleanest energy carriers for such purposes, since when used in a fuel cell, for example, it produces only water, electricity, and heat. However, H2 is primarily created by energy-consuming and environmentally unsustainable hydrocracking of fossil fuels through refinery
Fundamentals of H2 Evolution from PWS
Ideally, H2 evolution from PWS is based on the fact that water is split directly into H2 and O2 on the surface of a photocatalyst (e.g., a semiconductor) 5, 6. For reference, Box 1 summarizes the three major steps underlying PWS. Thermodynamically, water splitting is a four-electron process, requiring a Gibbs free energy of 237 kJ mol-1 7, 8, 9, 10, 11 that corresponds to 1.23 eV and ∼1000 nm in wavelength for light activation. Therefore, >50% of energy is available from natural sunlight for
Semiconductors
Semiconductors are the most widely investigated photocatalysts [51]. The chemical composition, electronic band structure, and morphology significantly affect the resulting catalyst performance. In the various kinds of metal-based semiconductor photocatalysts, metal cations with d0 or d10 electronic configurations are usually introduced to construct CBs, whereas the VBs usually comprise nonmetal elements (e.g., N, O, S, Se).
Cocatalysts for Photocatalytic H2 Evolution
Reduction cocatalysts are commonly used for photocatalytic H2 evolution to provide redox reaction sites and lower activation energies. Noble metals (e.g., Pt, Rh, Pd) are widely used [19]; however, their scarcity largely limits their promise towards large-scale, practical applications. Accordingly, various non-noble metal cocatalysts have been explored as alternatives. Metal chalcogenides (e.g., MoS2, NiS, CuS) have emerged as promising cocatalysts [55]. Excellent H2-evolution kinetics are
Improving Key Contributors to Photocatalytic H2 Evolution
Photocatalytic H2 evolution depends strongly on the photocatalyst light-absorption, charge-transfer, and surface-reaction properties. In this section, we briefly describe recent progress on improving these key properties.
Photocatalytic H2-evolution activity is strongly related to catalyst light absorption, which dictates the number of excited charges created for all of the photocatalytic reactions. Dopants, dye sensitizers, and defect-related color centers are widely used to enhance light
Concluding Remarks
In recent years, considerable progress has been made in photocatalytic H2 evolution. However, several critical scientific challenges remain in the areas of efficiency improvement, mechanistic understanding, and practicality for both fundamental research and large-scale photocatalytic H2 evolution (see Outstanding Questions). Of course, a major goal is it to achieve continuous, high-efficiency, and sustainable photocatalytic H2 evolution from water without the assistance of electron donors and
Acknowledgments
This work was supported by the Natural Science Foundation of China (No 21703046) and the Ministry of Science and Technology of China (No 2016YFF0203803 and 2016YFA0200902). X.C. appreciates support from the US National Science Foundation (DMR-1609061) and the College of Arts and Sciences, University of Missouri-Kansas City.
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