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The use of density functional theory (DFT) to calculate the optical properties of materials frequently leads to a predicted energy bandgap that is narrower than that experimentally determined. When the energy bandgap is incorrectly evaluated by DFT, the resulting absorption coefficient must be adjusted to give the correct value, in particular in the vicinity of the energy bandgap. Recently, a method has been developed whereby the dielectric coefficient for a material calculated using DFT is blueshifted and its amplitude is scaled such that the scaled function satisfies the same moment sum rule as the unshifted dielectric coefficient. However, while the moment sum rule is a necessary condition for correctly predicting the optical properties, it is not a sufficient condition. In the present work, it is shown that this method of scaling the imaginary part of the dielectric coefficient is based on the fact that the optical conductivity obtained using the fluctuation–dissipation (FD) theorem can be shifted to adjust the energy bandgap. This assumes that the energy dispersion is accurately calculated using DFT, although the energy bandgap is shifted. This shift is taken into account simply by modifying the energy associated with inter-band transitions in an expression for the electron conductivity obtained using the FD theorem within an independent particle approximation. The effectiveness of this method is illustrated by predicting the optical properties of particulate oxysulfide photocatalysts recently shown to promote visible-light-driven overall water splitting. [ABSTRACT FROM AUTHOR]

Overall water splitting, evolving hydrogen and oxygen in a 2:1 stoichiometric ratio, using particulate photocatalysts is a potential means of achieving scalable and economically viable solar hydrogen production. To obtain high solar energy conversion efficiency, the quantum efficiency of the photocatalytic reaction must be increased over a wide range of wavelengths and semiconductors with narrow bandgaps need to be designed. However, the quantum efficiency associated with overall water splitting using existing photocatalysts is typically lower than ten per cent.sup.1,2. Thus, whether a particulate photocatalyst can enable a quantum efficiency of 100 per cent for the greatly endergonic water-splitting reaction remains an open question. Here we demonstrate overall water splitting at an external quantum efficiency of up to 96 per cent at wavelengths between 350 and 360 nanometres, which is equivalent to an internal quantum efficiency of almost unity, using a modified aluminium-doped strontium titanate (SrTiO.sub.3:Al) photocatalyst.sup.3,4. By selectively photodepositing the cocatalysts Rh/Cr.sub.2O.sub.3 (ref. .sup.5) and CoOOH (refs. .sup.3,6) for the hydrogen and oxygen evolution reactions, respectively, on different crystal facets of the semiconductor particles using anisotropic charge transport, the hydrogen and oxygen evolution reactions could be promoted separately. This enabled multiple consecutive forward charge transfers without backward charge transfer, reaching the upper limit of quantum efficiency for overall water splitting. Our work demonstrates the feasibility of overall water splitting free from charge recombination losses and introduces an ideal cocatalyst/photocatalyst structure for efficient water splitting. Water splitting with an internal quantum efficiency of almost unity is achieved using a modified semiconductor photocatalyst that selectively promotes the hydrogen and oxygen evolution reactions on separate crystal facets., Author(s): Tsuyoshi Takata [sup.1] , Junzhe Jiang [sup.2] , Yoshihisa Sakata [sup.2] , Mamiko Nakabayashi [sup.3] , Naoya Shibata [sup.3] , Vikas Nandal [sup.4] , Kazuhiko Seki [sup.4] , Takashi [...]

The creation of effective and clean energy storage technologies has advanced dramatically due to growing worldwide worries over the depletion of fossil fuels and environmental issues. Energy storage and conversion systems have emerged as a crucial component with the development of numerous electronic devices, enabling the devices to function for extended periods of time. Due to their enormous surface area, strong electrical conductivity, and ion transport, 3D self‐supported nanoarchitectures associated with electrochemical energy storage (EES) devices can offer alternatives for enhancing the performance and advancement of existing EES systems. Here, we present the results of our findings regarding the design, production, and use of self‐supported 3D nanostructures in energy storage and conversion systems such as supercapacitors, batteries, solar cells, and fuel cells. A variety of advanced 3D nanomaterials may be readily created on an immense scale using various synthetic techniques, and they have a great deal of potential for use as electrodes in energy storage and conversion devices with much better performance. These fabrication techniques include electrochemical deposition, electrospinning, chemical precipitation, spray pyrolysis, sol‐gel method, hydrothermal method, chemical vapor deposition, and so forth. [ABSTRACT FROM AUTHOR]

With near unity quantum efficiency and operational stability surpassing 250 days in outdoor conditions, aluminum‐doped SrTiO3 (Al:SrTiO3) with tailored cocatalysts is one of the promising photocatalysts for scalable solar H2 production. Nevertheless, mechanistic insights behind Al‐doping and Rh cocatalyst‐induced enhanced overall water splitting (OWS) efficiency are not well elucidated. Herein, detailed charge carrier dynamics from sub‐picosecond to milliseconds are unveiled for Al:SrTiO3 by transient (optical and microwave probe) spectroscopy measurements. The obtained transients are rationalized using a theoretical model considering bimolecular recombination, trapping and detrapping processes. Due to a decrease in an n‐type doping density, Al doping of SrTiO3 significantly prolongs bulk carrier lifetime from 50 ns to 12.5 µs (consistent with the previous report). The crucial electron extraction process by the Rh cocatalyst located on the surface from Al:SrTiO3 occurs well before the decay of charge carriers. In contrast, µs‐long electron extraction time observed in SrTiO3 is significantly slower than tens of ns bulk carrier lifetime, thus reducing the photocatalytic OWS reaction. Complementary analysis in conjunction with in situ charge carrier dynamics in water interface addresses the mechanistic insight into Al‐doping‐induced enhancement of OWS activity. Correlating material properties, carrier dynamics and photocatalytic activity is expected to help design next‐generation photocatalysts via dopant and/or defects engineering for efficient solar‐fuel production. [ABSTRACT FROM AUTHOR]

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 Y2Ti2O5S2 (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 Co3O4 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. [ABSTRACT FROM AUTHOR]

The use of stibnite (Sb2S3) as a light-harvesting material in thin film solar cells has received considerable research interest during the transition of the millennium. However, the use of perovskite diminished the research in the field, and the potential of antimony Chalcogenides [Sb2(S,Se)3] was not explored thoroughly. Although these materials also provide bandgap tuning like perovskite, by varying the composition of S and Se, it is not as popular as perovskite for the fabrication of solar cells mainly because of the low efficiency of the solar cells based on it. In this paper, we present a landscape of the functional role of various device parameters on the performance of Sb2(S,Se)3-based solar cells. For this purpose, we first calibrate the optoelectronic model used for simulation with the experimental results from the literature. The model is then subjected to parametric variations to explore the performance metrics for this class of solar cells. Our results show that despite the belief that the open circuit voltage is independent of contact layers' doping in proper band-aligned carrier selective thin film solar cells, here we observe otherwise and the open circuit voltage is indeed dependent on the doping density of the contact layers. Using the detailed numerical simulation and analytical model, we further identify the performance optimization route for Sb2(S,Se)3-based thin film solar cells. [ABSTRACT FROM AUTHOR]

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 Cr2O3/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. [ABSTRACT FROM AUTHOR]

Demonstration of high efficiency large area cells with excellent stability is an important requirement towards the commercialization of perovskite solar cells (PSCs). With reports of high quality perovskite grains, it is evident that the performance of such large area cells will be strongly influenced by phenomena like carrier recombination and ion migration at grain boundaries (GBs). Here, we develop a modeling framework to address performance limitations due to GBs in large area PSCs. Through detailed numerical simulations, we show that photo-carrier recombination has a nontrivial dependence on the orientation of GBs. We find that ions at GBs lead to significant performance improvement through field effect passivation, which is influenced by critical parameters like density and polarity of ions, and the location of GBs. These results could have interesting implications toward long term stability and hence are relevant for the performance optimization of large area polycrystalline based thin film solar cells. [ABSTRACT FROM AUTHOR]

Y2Ti2O5S2, a stable oxysulfide photocatalyst with a bandgap energy of 1.9 eV, has been studied for the purpose of H2 production via sunlight-driven water splitting. Although this material absorbs a wide range of wavelengths, the light-driven hydrogen evolution activity of Y2Ti2O5S2 remains low due to ineffective charge carrier separation. The present study improved the charge separation in Y2Ti2O5S2via heterogeneous doping with lower valency Sc3+ at Ti4+ sites. This doping combined with flux- and etching-treatments incorporated Sc3+ ions into the Y2Ti2O5S2 surface without degrading the bulk crystallinity of the material or generating by-products. The optimized Sc-doped Y2Ti2O5S2 was 3.4 times more active than pristine Y2Ti2O5S2 during photocatalytic H2 evolution. Transient diffuse-reflectance spectroscopy and theoretical modelling established that Sc doping provided a shallower trap state distribution above the valence band of Y2Ti2O5S2 and facilitated hole detrapping. This effect promoted surface oxidation reactions and thereby enhanced the photocatalytic properties of the oxysulfide. [ABSTRACT FROM AUTHOR]

Visible-light responsive photocatalytic materials are expected to be deployed for practical use in photocatalytic water splitting. One of the promising materials as a p-type semiconductor, oxysulfides, was investigated in terms of the local charge carrier behavior for each particle by using a home-built time-resolved microscopic technique in combination with clustering analysis. We could differentiate electron and hole trapping to the surface states and the following recombination on a micron-scale from the nanosecond to microsecond order. The map of the charge carrier type revealed that charge trapping sites for electrons and holes were spatially separated within each particle/aggregate. Furthermore, the effect of the rhodium cocatalyst was recognized as a new electron pathway, trapping to the rhodium site and the following recombination, which was delayed compared with the original electron recombination process. The Rh effect was discussed based on the phenomenological simulation, revealing a possible reason for the decay was due to the anisotropic diffusion of charge carriers in oxysulfides or the interfacial energy barrier at the interface. [ABSTRACT FROM AUTHOR]

There are multiple requirements for maximizing the efficiency of photoelectrochemical overall water splitting using a single n‐type semiconducting photoanode. Many visible‐light‐responsive photoanode materials have been developed but overall water splitting by such materials without external bias has hardly been demonstrated. Herein, the aim is to scale the impact of performance improvement by an n–n heterojunction photoanode comprising an n‐type thin modification surface layer over an n‐type photon absorber, using a finite element method. For the n‐type photon absorber, the semiconductor properties are initially set to those of a well‐studied visible–light‐absorbing Ta3N5, and key properties are varied. It is demonstrated that an appropriate donor density and thickness of the thin surface layer induce adequate energy band bending in the photon absorber, resulting in a dramatic reduction of the onset potential of the photoanode. Thus, even though the quality of the photoanode is poor (e.g., the donor density is high (≈1 × 1019 cm−3)), the optimized surface layer enables the n–n heterojunction photoanode to facilitate bias‐free solar water splitting. These exhaustive investigations could provide design guidelines for obtaining effective photoanodes using an n‐type surface modification layer, such as cocatalysts and hole collection layers. [ABSTRACT FROM AUTHOR]