Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Two-dimensional (2D) heterostructures with the Z-scheme can significantly improve the recombination of the photogenerated charge pairs and increase the overpotential compared with the corresponding monolayers. Based on first-principles calculations, we demonstrate the photocatalytic performance with the Z-scheme of three 2D AuSe/SnS heterostructures identified from nine stacking types of AuSe and SnS monolayers using their formation energies. The thermal stabilities of the chosen heterostructures are assured by molecular dynamics simulations. The band alignment and built-in electric field support the heterostructures to drive the photocatalytic hydrogen and oxidation evolution reactions with the direct Z-scheme. Moreover, the optical absorption of both AuSe and SnS monolayers is obviously observed in the visible light and UV ranges, indicating their excellent response to solar light. The highest solar-to-hydrogen energy conversion efficiency of the heterostructures can reach 23.96%. The change in Gibbs free energy in the hydrogen evolution reaction with the heterostructures is in the range of 0.63–0.97 eV, indicating that these reactions are not difficult to carry out. Therefore, the present AuSe/SnS heterostructures could be promising candidates for producing hydrogen from photocatalytic water splitting with the Z-scheme driven by solar light.

Two-dimensional heterostructures of AuSe/SnS for the photocatalytic hydrogen evolution reaction with a Z-scheme

Plasmonic metal@semiconductor core–shell nanoparticles (CSNPs) are considered as promising candidates for artificial photosynthesis. Herein, Au@CeO2 CSNPs are hydrothermally fabricated for photocatalytic hydrogen evolution reaction (HER). CSNPs deliver superior HER performance compared to free CeO2. In particular, Au@CeO2-18 model (shell thickness of 18 nm) produces an HER rate of 4.05 μmol mg–1 h–1, which is ∼10 times higher than that of pure CeO2 (0.40 μmol mg–1 h–1) under visible-light. Additionally, Au@CeO2-18 photocatalyst demonstrates long-term stability after five repetitive runs, at which point it only loses approximately 5% of the activity, while core-free CeO2 decreases by 37.5 %. Such improvements are attributed to the electronic interactions between Au and CeO2, which not only enriches Ce3+ active sites to narrow bandgap of ceria toward visible, but also increases the affinity for hydrogen ions on the CSNPs surface. Moreover, localized surface plasmon resonance is light-excited and decays to efficiently produce hot-carrier to drive catalytic reactions.

Insightful understanding of hot-carrier generation and transfer in plasmonic Au@CeO2 core–shell photocatalysts for light-driven hydrogen evolution improvement

The design and application of semiconductor-based photocatalytic technology has a significant impact on addressing the world’s energy crisis and environmental degradation by transforming solar energy into chemicals and fuels. Semiconductor-based photocatalyst, ceria (CeO2), which possesses unique 4f electrons, is superior to other metal oxide semiconductors owing to its defect-rich structure, multivalence state, surface oxygen vacancy, photostability, and eco-friendly properties. This critical review is taken up to report the basic mechanistic details of CeO2-based photocatalysis as well as several methods for boosting the activity and stability of this group of photocatalysts. Additionally, numerous photocatalytic applications of CeO2, such as H2 generation by water splitting and CO2 reduction to alcohols and hydrocarbon fuels, are introduced and summarized. In the end, a concise summary of the challenges and potential areas for future research in the synthesis of CeO2-based photocatalysts are also provided. This review is anticipated to provide directions for developing innovative CeO2-based photocatalysts with low-cost and high efficiency, which have the promising application possibilities to fulfill the pressing needs of alternative fuels. 

Ceria-based photocatalysts in water-splitting for hydrogen production and carbon dioxide reduction

The interaction between CeO2-GO or CeO2-rGO and gold as co-catalysts were here investigated for solar H2 production by photoreforming of glycerol. The materials were prepared by a solar photoreduction/deposition method, where in addition to the activation of CeO2 the excited electrons were able to reduce the gold precursor to metallic gold and the GO into rGO. The presence of gold was fundamental to boost the H2 production, whereas the GO or the rGO extended the visible-light activity of cerium oxide (as confirmed by UV-DRS). Furthermore, the strong interaction between CeO2 and Au (verified by XPS and TEM) led to good stability of the CeO2-rGO-Au sample with the evolved H2 that increased during five consecutive runs of glycerol photoreforming. This catalytic behaviour was ascribed to the progressive reduction of GO into rGO, as shown by Raman measurements of the photocatalytic runs. The good charge carrier separation obtained with the CeO2-rGO-Au system allowed the simultaneous production of H2 and reduction of GO in the course of the photoreforming reaction. These peculiar features exhibited by these unconventional photocatalysts are promising to propose new solar-light-driven photocatalysts for green hydrogen production.

/pdf/ceo2-rgo-composites-for-photocatalytic-h2-evolution-by-3rchusot.pdf

CeO2-rGO Composites for Photocatalytic H2 Evolution by Glycerol Photoreforming

Photocatalytic overall water splitting into hydrogen and oxygen on RuO2-loaded Sm3+-doped CeO2 was examined. Pure CeO2 showed a negligible small activity, however Sm3+ doped CeO2 with heterogeneous...

Photocatalytic Overall Water Splitting on RuO2-loaded Sm3+-doped CeO2 with Heterogenous Doping Structure

Enhancement of photoexcited charge separation in semiconductor photocatalysts is one of the important subjects to improve the efficiency of energy conversion for photocatalytic overall water splitting into H2 and O2. In this study, we report an efficient separation of photoexcited charge at the interface between non-doped pure CeO2 and Y3+-doped CeO2 phases on particle surfaces with heterogeneous doping structure. Neither non-doped pure CeO2 and homogeneously Y3+-doped CeO2 gave activities for photocatalytic H2 and O2 production under ultraviolet light irradiation, meaning that both single phases showed little activity. On the other hand, Y3+-heterogeneously doped CeO2 of which the surface was composed of non-doped pure CeO2, and Y3+-doped CeO2 phases exhibited remarkable photocatalytic activities, indicating that the interfacial heterostructure between non-doped pure CeO2 and Y3+-doped CeO2 phases plays an important role for the activation process. The role of the interface between two different phases for activated expression was investigated by selective photo-reduction and oxidation deposition techniques of metal ion, resulting that the interface between two phases become an efficient separation site of photoexcited charge. Electronic band structures of both phases were investigated by the spectroscopic method, and then a mechanism of charge separation is discussed.

https://www.mdpi.com/1996-1944/14/2/350/pdf

Efficient Separation of Photoexcited Charge at Interface between Pure CeO2 and Y3+-Doped CeO2 with Heterogonous Doping Structure for Photocatalytic Overall Water Splitting.

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Photocatalytic Overall Water Splitting on RuO2-loaded Sm3+-doped CeO2 with Heterogenous Doping Structure

Efficient Separation of Photoexcited Charge at Interface between Pure CeO2 and Y3+-Doped CeO2 with Heterogonous Doping Structure for Photocatalytic Overall Water Splitting.