PdPt/SrTiO3:Al-catalysed redox-selective photoreduction of unsaturated carboxylic acids using minimal electron-donor and water.

We developed a semiconductor photocatalyst, Pd-Pt alloy nanoparticle-loaded, Al-doped SrTiO3 (PdPt/STO:Al), for photoreduction of unsaturated carboxylic acids. Due to the cooperative STO:Al surface and Pd-Pt alloy nanoparticles, the catalyst dispersed in water provided highly redox-selective photoreduction against oxidative degradation of starting materials/products and against reductive evolution of H2, where minimal glycolic acid worked as an efficient electron-donating fuel.

Aliovalent metal cation doping of La5Ti2AgO7S5 particles for improved photocatalytic and photoelectrochemical water splitting.

The doping of photocatalytic La5Ti2AgO7S5 particles with aliovalent metal cations was investigated. The incorporation of lower-valence Al3+ cations at Ti4+ sites improved the photocatalytic hydrogen evolution activity. In addition, the anodic photocurrent during oxygen evolution was increased upon adding higher-valence Ta5+ ions. The effect of doping on the carrier density in the photocatalytic particles was also examined in this work.

An Artificial Leaf with Patterned Photocatalysts for Sunlight-Driven Water Splitting.

Plant leaves can turn entirely absorbed light into chemical energy due to their spatially separated photosystems I and II in the thylakoid membrane that enables unidirectional Z-scheme type charge transfer between them. In artificial systems that mimic leaves, a lack of spatial and interfacial control of active units (i.e., hydrogen evolution photocatalyst/HEP and oxygen evolution photocatalyst/OEP) introduces competitive charge transfer channels between them, resulting in deficient Z-scheme type charge transfer. Herein, we demonstrate that a patterned photocatalyst sheet, namely, an artificial leaf, comprising an ordered and separated distribution of the OEP and HEP strips on a conductive substrate, achieves unidirectional Z-scheme type charge transfer as the leaves do. It represents a next-generation photocatalytic system that mimics the leaves to bring breakthrough in photocatalytic over water splitting performance with the combination of highly active HEP and OEP photocatalysts, opening up a promising avenue toward solar energy conversion by artificial photosynthesis.

Best Practices for Assessing Performance of Photocatalytic Water Splitting Systems.

Photocatalytic water splitting has become a very popular research subject in recent years. Consequently, it is important to report appropriately standardized experimental data, so that each researcher can properly understand the results generated by others. However, experimental methods and measures of photocatalytic performance are not yet sufficiently systematic. In the present manuscript, experimental procedures and standardization of photocatalytic performance are described in relation to the basic theory of photocatalytic water splitting.

Impact of polymorphism vs. shape of titania nanocrystals on the hydrogen evolution reaction.

Herein, we investigated the impact of polymorphism vs. dimension control of titania nanocrystals towards hydrogen generation. Two different forms of titania nanoparticles have been synthesized following the solvothermal method, leading to the formation of two distinct physicochemical features. Detailed structural, morphological, and optical studies revealed that the formation of titania nanorods correspond to rutile while granular particles correspond to the anatase phase. Among various titania polymorphs, anatase is well known for its superior photocatalytic activity; however, to our surprise, the as-synthesized rutile nanorods exhibited higher catalytic activity in comparison to anatase spheres, and hydrogen evolution was considerably enhanced after the addition of a minute amount of Pt as the co-catalyst. Thus, despite the higher catalytic activity of anatase, the enhanced hydrogen evolution of rutile nanorods may be related to the creation of a 1D structure. Our study highlights the importance of considering not only TiO2 polymorphism but also shape and dimension in optimizing photocatalytic H2 production.

Simultaneous Structural and Electronic Engineering on Bi- and Rh-co-doped SrTiO3 for Promoting Photocatalytic Water Splitting.

Activating metal ion-doped oxides as visible-light-responsive photocatalysts requires intricate structural and electronic engineering, a task with inherent challenges. In this study, we employed a solid (template)-molten (dopants) reaction to synthesize Bi- and Rh-codoped SrTiO3 (SrTiO3 : Bi,Rh) particles. Our investigation reveals that SrTiO3 : Bi,Rh manifests as single-crystalline particles in a core (undoped)/shell (doped) structure. Furthermore, it exhibits a well-stabilized Rh3+ energy state for visible-light response without introducing undesirable trapping states. This precisely engineered structure and electronic configuration promoted the generation of high-concentration and long-lived free electrons, as well as facilitated their transfer to cocatalysts for H2 evolution. Impressively, SrTiO3 : Bi,Rh achieved an exceptional apparent quantum yield (AQY) of 18.9 % at 420 nm, setting a new benchmark among Rh-doped-based SrTiO3 materials. Furthermore, when integrated into an all-solid-state Z-Scheme system with Mo-doped BiVO4 and reduced graphene oxide, SrTiO3 : Bi,Rh enabled water splitting with an AQY of 7.1 % at 420 nm. This work underscores the significance of simultaneous structural and electronic engineering and introduces the solid-molten reaction as a viable approach for this purpose.

Carbon Nanotubes as a Solid-State Electron Mediator for Visible-Light-Driven Z-Scheme Overall Water Splitting.

So-called Z-scheme systems, which typically comprise an H2 evolution photocatalyst (HEP), an O2 evolution photocatalyst (OEP), and an electron mediator, represent a promising approach to solar hydrogen production via photocatalytic overall water splitting (OWS). The electron mediator transferring photogenerated charges between the HEP and OEP governs the performance of such systems. However, existing electron mediators suffer from low stability, corrosiveness to the photocatalysts, and parasitic light absorption. In the present work, carbon nanotubes (CNTs) were shown to function as an effective solid-state electron mediator in a Z-scheme OWS system. Based on the high stability and good charge transfer characteristics of CNTs, this system exhibited superior OWS performance compared with other systems using more common electron mediators. The as-constructed system evolved stoichiometric amounts of H2 and O2 at near-ambient pressure with a solar-to-hydrogen energy conversion efficiency of 0.15%. The OWS reaction was also promoted in the case that this CNT-based Z-scheme system was immobilized on a substrate. Hence, CNTs are a viable electron mediator material for large-scale Z-scheme OWS systems.

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

Interfacial Design of Particulate Photocatalyst Materials for Green Hydrogen Production.

Green hydrogen production using particulate photocatalyst materials has attracted much attention in recent years because this process could potentially lead to inexpensive and scalable solar-to-chemical energy conversion systems. Although the development of efficient particulate photocatalysts enabling one-step overall water splitting (OWS) with solar-to-hydrogen efficiencies in excess of 10 % remains challenging, promising photocatalyst candidates exhibiting OWS activity have been demonstrated. This review provides a comprehensive introduction to the solar-to-hydrogen energy conversion process of semiconductor photocatalyst materials and highlights recent advances in photocatalytic OWS via both one-step and two-step photoexcitation processes. The review also covers recent developments in the photocatalytic OWS of SrTiO3, including the establishment of large-scale photocatalytic systems, interfacial design using cocatalysts to enhance water splitting activity, and its photoelectrochemical (PEC) properties at the electrified solid/liquid interface. In addition, there is a special focus on visible-light-absorbing oxynitride and oxysulfide particulate photocatalysts with absorption edges near 600 nm. Methods for photocatalyst preparation and surface modification, as well as PEC properties, are also discussed. The semiconductor properties of particulate photocatalysts obtained from photoelectroanalytical evaluations using particulate photoelectrodes are evaluated. This review is intended to provide guidelines for the future development of particulate photocatalysts capable of efficient and stable OWS.

Precise analyses of photoelectrochemical reactions on particulate Zn0.25Cd0.75Se photoanodes in nonaqueous electrolytes using Ru bipyridyl complexes as a probe.

Recombination of photoexcited carriers at interface states is generally believed to strongly govern the photoelectrochemical (PEC) performance of semiconductors in electrolytes. Sacrificial reagents (e.g., methanol or Na2SO3) are often used to assess the ideal PEC performance of photoanodes in cases of minimised interfacial recombination kinetics as well as accelerated surface reaction kinetics. However, varying the sacrificial reagents in the electrolyte means simultaneously changing the equilibrium potential and the number of electrons required to perform the sacrificial reaction, and thus the thermodynamic and kinetic aspects of the PEC reactions cannot be distinguished. In the present study, we propose an alternative methodology to experimentally evaluate the energy levels of interfacial recombination centres that can reduce PEC performance. We prepare nonaqueous electrolytes containing three different Ru complexes with different bipyridyl ligands; redox reactions of Ru complexes represent one-electron processes with similar charge transfer rates and diffusion coefficients. Therefore, the Ru complexes can serve as a probe to isolate and evaluate only the thermodynamic aspects of PEC reactions. Recombination centres at the interface between a nonaqueous electrolyte and a Zn0.25Cd0.75Se particulate photoanode are elucidated using this method as a model case. The energy level at which photocorrosion proceeds is also determined.

Nanoparticulate TiN Loading to Promote Z-Scheme Water Splitting Using a Narrow-Bandgap Nonoxide-Based Photocatalyst Sheet.

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO2N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5Ti2Cu0.9Ag0.1O7S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2N with CoOx provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420 nm. A photocatalyst sheet incorporating CoOx-loaded SrTaO2N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx-loaded SrTaO2N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications.

Liquid metal-embraced photoactive films for artificial photosynthesis.

The practical applications of solar-driven water splitting pivot on significant advances that enable scalable production of robust photoactive films. Here, we propose a proof-of-concept for fabricating robust photoactive films by a particle-implanting technique (PiP) which embeds semiconductor photoabsorbers in the liquid metal. The strong semiconductor/metal interaction enables resulting films efficient collection of photogenerated charges and superior photoactivity. A photoanode of liquid-metal embraced BiVO4 can stably operate over 120 h and retain ~ 70% of activity when scaled from 1 to 64 cm2. Furthermore, a Z-scheme photocatalyst film of liquid-metal embraced BiVO4 and Rh-doped SrTiO3 particles can drive overall water splitting under visible light, delivering an activity 2.9 times higher than that of the control film with gold support and a 110 h stability. These results demonstrate the advantages of the PiP technique in constructing robust and efficient photoactive films for artificial photosynthesis.

Efficient and stable visible-light-driven Z-scheme overall water splitting using an oxysulfide H2 evolution photocatalyst.

So-called Z-scheme systems permit overall water splitting using narrow-bandgap photocatalysts. To boost the performance of such systems, it is necessary to enhance the intrinsic activities of the hydrogen evolution photocatalyst and oxygen evolution photocatalyst, promote electron transfer from the oxygen evolution photocatalyst to the hydrogen evolution photocatalyst, and suppress back reactions. The present work develop a high-performance oxysulfide photocatalyst, Sm2Ti2O5S2, as an hydrogen evolution photocatalyst for use in a Z-scheme overall water splitting system in combination with BiVO4 as the oxygen evolution photocatalyst and reduced graphene oxide as the solid-state electron mediator. After surface modifications of the photocatalysts to promote charge separation and redox reactions, this system is able to split water into hydrogen and oxygen for more than 100 hours with a solar-to-hydrogen energy conversion efficiency of 0.22%. In contrast to many existing photocatalytic systems, the water splitting activity of the present system is only minimally reduced by increasing the background pressure to 90 kPa. These results suggest characteristics suitable for applications under practical operating conditions.

Sub-50 nm perovskite-type tantalum-based oxynitride single crystals with enhanced photoactivity for water splitting.

A long-standing trade-off exists between improving crystallinity and minimizing particle size in the synthesis of perovskite-type transition-metal oxynitride photocatalysts via the thermal nitridation of commonly used metal oxide and carbonate precursors. Here, we overcome this limitation to fabricate ATaO2N (A = Sr, Ca, Ba) single nanocrystals with particle sizes of several tens of nanometers, excellent crystallinity and tunable long-wavelength response via thermal nitridation of mixtures of tantalum disulfide, metal hydroxides (A(OH)2), and molten-salt fluxes (e.g., SrCl2) as precursors. The SrTaO2N nanocrystals modified with a tailored Ir-Pt alloy@Cr2O3 cocatalyst evolved H2 around two orders of magnitude more efficiently than the previously reported SrTaO2N photocatalysts, with a record solar-to-hydrogen energy conversion efficiency of 0.15% for SrTaO2N in Z-scheme water splitting. Our findings enable the synthesis of perovskite-type transition-metal oxynitride nanocrystals by thermal nitridation and pave the way for manufacturing advanced long-wavelength-responsive particulate photocatalysts for efficient solar energy conversion.

Criteria for Efficient Photocatalytic Water Splitting Revealed by Studying Carrier Dynamics in a Model Al-doped SrTiO3 Photocatalyst.

Overall water splitting (OWS) using semiconductor photocatalysts is a promising method for solar fuel production. Achieving a high quantum efficiency is one of the most important prerequisites for photocatalysts to realize high solar-to-fuel efficiency. In a recent study (Nature 2020, 58, 411-414), a quantum efficiency of almost 100 % has been achieved in an aluminum-doped strontium titanate (SrTiO3 : Al) photocatalyst. Herein, using the SrTiO3 : Al as a model photocatalyst, we reveal the criteria for efficient photocatalytic water splitting by investigating the carrier dynamics through a comprehensive photoluminescence study. It is found that the Al doping suppresses the generation of Ti3+ recombination centers in SrTiO3 , the surface band bending facilitates charge separation, and the in situ photo-deposited Rh/Cr2 O3 and CoOOH co-catalysts render efficient charge extraction. By suppressing photocarrier recombination and establishing a facile charge separation and extraction mechanism, high quantum efficiency can be achieved even on photocatalysts with a very short (sub-ns) intrinsic photocarrier lifetime, challenging the belief that a long carrier lifetime is a fundamental requirement. Our findings could provide guidance on the design of OWS photocatalysts toward more efficient solar-to-fuel conversion.

An Oxysulfide Photocatalyst Evolving Hydrogen with an Apparent Quantum Efficiency of 30 % under Visible Light.

Photocatalytic water splitting is a simple means of converting solar energy into storable hydrogen energy. Narrow-band gap oxysulfide photocatalysts have attracted much attention in this regard owing to the significant visible-light absorption and relatively high stability of these compounds. However, existing materials suffer from low efficiencies due to difficulties in synthesizing these oxysulfides with suitable degrees of crystallinity and particle sizes, and in constructing effective reaction sites. The present work demonstrates the production of a Gd2 Ti2 O5 S2 (λ<650 nm) photocatalyst capable of efficiently driving photocatalytic reactions. Single-crystalline, plate-like Gd2 Ti2 O5 S2 particles with atomically ordered surfaces were synthesized by flux and chemical etching methods. Ultrafine Pt-IrO2 cocatalyst particles that promoted hydrogen (H2 ) and oxygen (O2 ) evolution reactions were subsequently loaded on the Gd2 Ti2 O5 S2 while ensuring an intimate contact by employing a microwave-heating technique. The optimized Gd2 Ti2 O5 S2 was found to evolve H2 from an aqueous methanol solution with a remarkable apparent quantum efficiency of 30 % at 420 nm. This material was also stable during O2 evolution in the presence of a sacrificial reagent. The results presented herein demonstrates a highly efficient narrow-band gap oxysulfide photocatalyst with potential applications in practical solar hydrogen production.

Transition-metal (oxy)nitride photocatalysts for water splitting.

Solar-driven water splitting based on particulate semiconductor materials is studied as a technology for green hydrogen production. Transition-metal (oxy)nitride photocatalysts are promising materials for overall water splitting (OWS) via a one- or two-step excitation process because their band structure is suitable for water splitting under visible light. Yet, these materials suffer from low solar-to-hydrogen energy conversion efficiency (STH), mainly because of their high defect density, low charge separation and migration efficiency, sluggish surface redox reactions, and/or side reactions. Their poor thermal stability in air and under the harsh nitridation conditions required to synthesize these materials makes further material improvements difficult. Here, we review key challenges in the two different OWS systems and highlight some strategies recently identified as promising for improving photocatalytic activity. Finally, we discuss opportunities and challenges facing the future development of transition-metal (oxy)nitride-based OWS systems.

Flux-Assisted Synthesis of Y2 Ti2 O5 S2 for Photocatalytic Hydrogen and Oxygen Evolution Reactions.

Photocatalytic water splitting is an ideal means of producing hydrogen in a sustainable manner, and developing highly efficient photocatalysts is a vital aspect of realizing this process. The photocatalyst Y2 Ti2 O5 S2 (YTOS) is capable of absorbing at wavelengths up to 650 nm and exhibits outstanding thermal and chemical durability compared with other oxysulfides. However, the photocatalytic performance of YTOS synthesized using the conventional solid-state reaction (SSR) process is limited owing to the large particle sizes and structural defects associated with this synthetic method. Herein, we report the synthesis of YTOS particles by a flux-assisted technique. The enhanced mass transfer efficiency in the flux significantly reduced the preparation time compared with the SSR method. In addition, the resulting YTOS showed improved photocatalytic H2 and O2 evolution activity when loaded with Rh and Co3 O4 co-catalysts, respectively. These improvements are attributed to the reduced particle size and enhanced crystallinity of the material as well as the slower decay of photogenerated carriers on a nanosecond to sub-microsecond time range. Further optimization of this flux-assisted method together with suitable surface modification is expected to produce high-quality YTOS crystals with superior photocatalytic activity.