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.

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 SrTaO2 N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5 Ti2 Cu0.9 Ag0.1 O7 S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2 N 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 SrTaO2 N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx -loaded SrTaO2 N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2 N 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.

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.

Physicochemical insights into semiconductor properties of a semitransparent tantalum nitride photoanode for solar water splitting.

The self-conductivity of tantalum nitride (Ta3N5) thin film-based semitransparent photoanodes was found to promote the current originating from the photoelectrochemical oxygen evolution reaction (PEC OER) without a conducting substrate. With surface modification by the NiFeOx-electrocatalyst, an optimized Ta3N5 thin film fabricated directly on a transparent insulating quartz substrate generated a photocurrent density of ∼5.9 ± 0.1 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode under simulated AM 1.5G solar illumination. The correlation between the PEC OER performance of NiFeOx-modified Ta3N5 photoanodes and the electrical properties of Ta3N5 thin films was investigated based on the Hall effect measurements. By changing the nitridation conditions, these properties can be tuned so that the higher the Hall mobility (0.2 to 1.7 cm2 V-1 s-1) and the lower the carrier concentration (1020 to 1019 cm-3). The surface chemical states of Ta3N5 thin films were investigated using X-ray photoelectron spectroscopy as a means of evaluating surface oxygen impurities and nitrogen vacancies, which may correlate with the PEC OER performance and the electrical properties of the material.

Strategies for Improving the Photocatalytic Hydrogen Evolution Reaction of Carbon Nitride-Based Catalysts.

Due to the depletion of fossil fuels and their-related environmental issues, sustainable, clean, and renewable energy is urgently needed to replace fossil fuel as the primary energy resource. Hydrogen is considered as one of the cleanest energies. Among the approaches to hydrogen production, photocatalysis is the most sustainable and renewable solar energy technique. Considering the low cost of fabrication, earth abundance, appropriate bandgap, and high performance, carbon nitride has attracted extensive attention as the catalyst for photocatalytic hydrogen production in the last two decades. In this review, the carbon nitride-based photocatalytic hydrogen production system, including the catalytic mechanism and the strategies for improving the photocatalytic performance is discussed. According to the photocatalytic processes, the strengthened mechanism of carbon nitride-based catalysts is particularly described in terms of boosting the excitation of electrons and holes, suppressing carriers recombination, and enhancing the utilization efficiency of photon-excited electron-hole. Finally, the current trends related to the screening design of superior photocatalytic hydrogen production systems are outlined, and the development direction of carbon nitride for hydrogen production is clarified.

Co-doping of a La5Ti2Cu0.9Ag0.1O7S5 photocatalyst (λ < 700 nm) with Ga and Al to enhance photocatalytic H2 evolution.

La5Ti2Cu0.9Ag0.1O7S5 (LTCA) (λ < 700 nm) can function as a photocatalyst for H2 evolution. Co-doping LTCA with Ga3+ and Al3+ at Ti4+ sites effectively enhanced the H2 evolution activity of LTCA, yielding an apparent quantum efficiency of 18% at 420 nm. The activity of this material was greater than that previously reported for Ga-doped LTCA by a factor of 1.6. Such activity enhancement is attributed to increasing the population of long-lived photogenerated electrons and facilitating the electron transfer to the cocatalyst. This work significantly improved the LTCA-based photocatalyst for H2 evolution, making it a promising material for future application in non-sacrificial Z-scheme water splitting.

Narrow-Band-Gap Particulate Photocatalysts for One-Step-Excitation Overall Water Splitting.

ConspectusSunlight-driven one-step-excitation overall water splitting (OWS) using a single particulate photocatalyst is a simple and cost-effective approach to producing sustainable hydrogen on a large scale, providing an important impetus to achieving a carbon-neutral society. Technoeconomic studies have determined that a minimum solar-to-hydrogen (STH) energy conversion efficiency of 5% must be achieved to allow this process to be economically competitive. Meeting this goal will require the fabrication of particulate photocatalysts comprising composites of semiconductors and cocatalysts that are sufficiently active under sunlight. A one-step-excitation OWS system based on a metal oxide semiconductor having a wide bandgap was first reported in 1980, and the performance of such systems has been improved significantly over the past decade. In particular, work by the authors' group increased the apparent quantum yield (AQY) obtainable for ultraviolet (UV)-active SrTiO3 to more than 90% in 2020. However, the STH conversion efficiency of a photocatalyst that absorbs only UV light (that is, λ < 400 nm) is limited to 1.7% even at an AQY of unity. It is therefore highly important to develop one-step-excitation OWS processes utilizing narrow bandgap photocatalysts having absorption edge wavelengths equal to or longer than 500 nm. Such systems would be expected to meet the desired 5% STH energy conversion efficiency once a constant AQY of approximately 63% is obtained.This Account summarizes the development and application of narrow-band-gap (oxy)nitride and oxysulfide photocatalysts in the authors' laboratory that are able to split water in response to wavelengths as high as 500 to 650 nm via single-step photoexcitation. At first, the authors briefly recount the key steps required to progress from the initial utilization of a UV-active SrTiO3 photocatalyst as an OWS-active material to the realization of an AQY of almost unity. Multiple design and refinement strategies applied to both the semiconductor and cocatalysts associated with this benchmark photocatalyst are summarized, providing insights into the rational design of narrow-band-gap OWS-active photocatalysts. Furthermore, the necessity, target, and current status of developing narrow-band-gap OWS-active photocatalysts are discussed, followed by a comprehensive discussion of progress in the fabrication of OWS-active (oxy)nitride and oxysulfide photocatalysts with absorption edge wavelengths at up to the range of 500-650 nm in our laboratory. Specific examples are used to show the importance of several factors. First, adjusting the properties of the semiconducting material based on designing appropriate precursors, optimizing the synthetic conditions and aliovalent doping is described. Second, loading of efficient dual cocatalysts is examined. Lastly, the effectiveness of coating the particulate photocatalysts with surface nanolayers is addressed. Deficits related to the performance of present-day photocatalysts are also evaluated. Expectations with regard to future improvements of (oxy)nitride- and oxysulfide-based photocatalysts as a means of increasing the AQY are considered. The strategies summarized in this Account are expected to promote the development of nonsacrificial long-wavelength-responsive photosynthesis systems using water as a hydrogen/oxygen source.

Reduction and Diffusion of Cr-Oxide Layers into P25, BaLa4Ti4O15, and Al:SrTiO3 Particles upon High-Temperature Annealing.

Chromium oxide (Cr2O3) is a beneficial metal oxide used to prevent the backward reaction in photocatalytic water splitting. The present work investigates the stability, oxidation state, and the bulk and surface electronic structure of Cr-oxide photodeposited onto P25, BaLa4Ti4O15, and Al:SrTiO3 particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr2O3 on the surface of P25 and Al:SrTiO3 particles and Cr(OH)3 on BaLa4Ti4O15. After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO2), the Cr2O3 layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa4Ti4O15, Cr(OH)3 converts to Cr2O3 upon annealing and diffuses slightly into the particles. However, for Al:SrTiO3, the Cr2O3 remains stable at the surface of the particles. The diffusion here is due to the strong metal-support interaction effect. In addition, some of the Cr2O3 on the P25, BaLa4Ti4O15, and Al:SrTiO3 particles is reduced to metallic Cr after annealing. The effect of Cr2O3 formation and diffusion into the bulk on the surface and bulk band gaps is investigated with electronic spectroscopy, electron diffraction, DRS, and high-resolution imaging. The implications of the stability and diffusion of Cr2O3 for photocatalytic water splitting are discussed.

A perspective on two pathways of photocatalytic water splitting and their practical application systems.

Photocatalytic water splitting has been widely studied as a means of converting solar energy into hydrogen as an ideal energy carrier in the future. Systems for photocatalytic water splitting can be divided into one-step excitation and two-step excitation processes. The former uses a single photocatalyst while the latter uses a pair of photocatalysts to separately generate hydrogen and oxygen. Significant progress has been made in each type of photocatalytic water splitting system in recent years, although improving the solar-to-hydrogen energy conversion efficiency and constructing practical technologies remain important tasks. This perspective summarizes recent advances in the field of photocatalytic overall water splitting, with a focus on the design of photocatalysts, co-catalysts and reaction systems. The associated challenges and potential approaches to practical solar hydrogen production via photocatalytic water splitting are also presented.

Mechanistic Insights into Enhanced Hydrogen Evolution of CrOx /Rh Nanoparticles for Photocatalytic Water Splitting.

The hydrogen evolution reaction (HER) of Rh nanoparticles (RhNP) coated with an ultrathin layer of Cr-oxides (CrOx ) was investigated as a model electrode for the Cr2 O3 /Rh-metal core-shell-type cocatalyst system for photocatalytic water splitting. The CrOx layer was electrodeposited over RhNP on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The CrOx layer on RhNP facilitates the electron transfer process at the CrOx /RhNP interface, leading to the increased current density for the HER. Impedance spectroscopic analysis revealed that the CrOx layer transferred protons via the hopping mechanism to the RhNP surface for HER. In addition, CrOx restricted electron transfer from the FTO to the electrolyte and/or RhNP and suppressed the backward reaction by limiting oxygen migration. This study clarifies the crucial role of the ultrathin CrOx layer on nanoparticulate cocatalysts and provides a cocatalyst design strategy for realizing efficient photocatalytic water splitting.

Impact of Ball Milling on the Hydrogen Evolution Performance of Cu2Sn0.38Ge0.62S3 Photocatalytic Particles Synthesized via a Flux Method.

Ball milling has been shown empirically to produce fine photocatalytic particles from large bulky particles but to drastically reduce the photocatalytic activity of such material during water splitting due to mechanical damage to the photocatalyst surfaces. If the damaged photocatalyst surfaces could be removed or reconstructed, the reduced particle sizes resulting from milling would be expected to provide enhanced photocatalytic activity. In the present study, fine particles of crystalline Cu2Sn0.38Ge0.62S3 (CTGS), which is responsive to long wavelength light up to the near-infrared region, were synthesized by a flux method and subsequent ball milling. A photocathode made of such particles showed significantly enhanced photoelectrochemical (PEC) performance under simulated sunlight while the photocatalytic hydrogen evolution activity of a powder suspension system made from the same material exhibited a typical decrease. The CTGS crystalline particles synthesized using the flux method were found to be highly crystalline but to have relatively large micrometer-scale sizes. Ball milling reduced the particle size but produced an amorphous coating of oxidized species that lowered the photocatalytic activity of the powder suspension system. Typical surface modifications of a photocathode made from this material, consisting of wet chemical processes, also served as an etching treatment to successfully remove the minimally crystalline surface layer and provide greater PEC activity. These data suggest the benefits of combining flux crystal growth with ball milling and the appropriate chemical etching process to obtain high-crystallinity fine photocatalytic particles responsive to long wavelength light with improved PEC hydrogen evolution activity.

Overall Water Splitting by a SrTaO2N-Based Photocatalyst Decorated with an Ir-Promoted Ru-Based Cocatalyst.

The development of narrow-bandgap photocatalysts for one-step-excitation overall water splitting (OWS) remains a critical challenge in the field of solar hydrogen production. SrTaO2N is a photocatalytic material having a band structure suitable for OWS under visible light (λ ≤ 600 nm). However, the presence of defects in the oxynitride and the lack of cocatalysts to promote simultaneous hydrogen and oxygen evolution make it challenging to realize OWS using this material. The present work demonstrates a SrTaO2N-based particulate photocatalyst for OWS. This photocatalyst, which was composed of single crystals, was obtained by nitriding SrCl2 and Ta2O5 together with NaOH, with the latter added to control the formation of defects. The subsequent loading of bimetallic RuIrOx nanoparticles accelerated charge separation and allowed the SrTaO2N photocatalyst to exhibit superior OWS activity. This research presenting the strategies of controlling the oxygen sources and promoting the cocatalyst function is expected to expand the range of potential OWS-active oxynitride photocatalysts and permit the design of efficient cocatalysts for photocatalytic OWS.

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode.

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta3N5 thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta3N5 as a leading performer among visible-light-responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Exploring the Photocorrosion Mechanism of a Photocatalyst.

Photoelectrochemical (PEC) water splitting using Ta3N5 anodes shows a high solar-to-hydrogen (STH) efficiency approaching 10%. However, the long-term stability of gas evolution should be improved for the commercial utilization of PEC water-splitting technology. Herein, we examined the photocurrent degradation of photoanodes prepared by uniformly loading a NiFeOx cocatalyst onto a Ta3N5 semiconductor. Although spectroscopic analysis showed that the degradation was attributable to the formation of an oxide layer, several oxide growth kinetic laws and mechanisms are known. We theoretically derived the photocurrent kinetic laws instead of the oxide growth kinetic laws by generalizing the Cabrera-Mott oxidation theory of metal oxidation in air to apply it to photocorrosion. The measured photocurrent kinetics are fully consistent with the theoretical kinetic laws. We show that ion drift due to charging of the oxide layer limits oxide growth even though uniform cocatalyst loading is designed to prevent self-oxidation of Ta3N5.