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The intrinsic carrier dynamics of cuprous oxide Cu2O are known to have a crucial influence on photocatalytic performances. The photoactivity of rhombic dodecahedral Cu2O with dominant 110 facets RD Cu2O is demonstrated to surpass that of cubic Cu2O with 100 surfaces CB Cu2O . Time resolved microwave conductivity TRMC measurements reveal the higher carrier mobility of RD Cu2O when compared to CB Cu2O. Additionally, modulated surface photovoltage SPV measurements further supported the better charge separation efficiency of RD Cu2O. Although CB Cu2O exhibited more pronounce SPV signals, the homogeneous distribution of electrical fields drives the majority charge inward and led to detrimental charge recombination. In contrast, the weak SPV signals for RD Cu2O were attributed to a modulated distribution of charges towards the facets and facet boundaries, demonstrating a better charge separation. This study shows that carrier dynamics and defect density should also be regarded as facet dependent properties that can have deciding influence on the photocatalytic activity

Angewandte Chemie / International edition e202218850 (2023). doi:10.1002/anie.202218850 special issue: "Hot Topic: Water Splitting", Published by Wiley-VCH, Weinheim

Covalent organic frameworks (COFs) have emerged as an important class of organic semiconductors and photocatalysts for the hydrogen evolution reaction (HER)from water. To optimize their photocatalytic activity, typically the organic moieties constituting the frameworks are considered and the most suitable combinations of them are searched for. However, the effect of the covalent linkage between these moieties on the photocatalytic performance has rarely been studied. Herein, we demonstrate that donor‐acceptor (D‐A) type imine‐linked COFs can produce hydrogen with a rate as high as 20.7 mmol g−1 h−1 under visible light irradiation, upon protonation of their imine linkages. A significant red‐shift in light absorbance, largely improved charge separation efficiency, and an increase in hydrophilicity triggered by protonation of the Schiff‐base moieties in the imine‐linked COFs, are responsible for the improved photocatalytic performance., Protonation of imine‐linked COFs yields significant variations of their (opto)electronic properties and results in a largely enhanced performance in photocatalytic hydrogen evolution from water. This is attributed to an enhanced light absorption ability, charge separation efficiency, and hydrophilicity of imine‐linked COFs upon protonation.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO4(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO4(010) surface stabilizes the formation of a small electron polaron on the VO4 tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V4+ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO

Direct photoelectrochemical (PEC) solar water splitting has the potential to be a key element in a sustainable energy supply chain. However, integrated PEC systems based on metal oxides still lack the high efficiencies required for large-scale, economically feasible applications. A main obstacle for the realization of higher solar-to-hydrogen efficiencies is the appropriate design of the semiconductor–catalyst and semiconductor–electrolyte interfaces. Thus, a more accurate understanding of the energy loss mechanisms and the driving forces that determine the charge separation, transport and recombination of electrons and holes in a PEC device would be instrumental for the selection of the most appropriate design routes. In this context we highlight a common misconception within the PEC research community, which is to consider the built-in electrical field at the solid/liquid interface as essential for charge separation. We subsequently emphasize the established viewpoint within the photovoltaic research community that the gradient of the electrochemical potential is the principle driving force for charge separation and efficient solar energy conversion. Based on this realization, we argue that improved contact design in PEC devices should be one of the main research directions in the design of PEC devices. To address this challenge, we take a closer look at how optimized contacts have been constructed so far and present potential design approaches which can be used to further improve the performance of PEC devices., publishedVersion

The spatial distribution and concentration of lanthanide activator and sensitizer dopant ions are of key importance for the luminescence color and efficiency of upconverting nanoparticles (UCNPs). Quantifying dopant ion distributions and intermixing, and correlating them with synthesis methods require suitable analytical techniques. Here, X-ray photoelectron spectroscopy depth-profiling with tender X-rays (2000-6000 eV), providing probe depths ideally matched to UCNP sizes, is used to measure the depth-dependent concentration ratios of Er

Water splitting in photoelectrochemical cells is a promising technology to produce solar hydrogen. Fe2TiO5 pseudobrookite with a bandgap of around 2 eV absorbs the predominant visible range of the ...

Recently, α-SnWO4 attracted attention as a material to be used as a top absorber in a tandem device for photoelectrochemical water splitting due to its nearly optimum band gap of ∼1.9 eV and an ear...

Monoclinic copper vanadate (n-type Cu2V2O7) thin film photoanodes were prepared for the first time by spray pyrolysis and evaluated for photoelectrochemical (PEC) water oxidation. The spray pyrolys...

We assess a tandem photoelectrochemical cell consisting of a W:BiVO4 photoanode top absorber and a CuBi2O4 photocathode bottom absorber for overall solar water splitting. We show that the W:BiVO4 photoanode oxidizes water and produces oxygen at potentials ≥0.7 V vs RHE when CoPi is added as a cocatalyst. However, the CuBi2O4 photocathode does not produce a detectable amount of hydrogen from water reduction even when Pt or RuOx is added as a cocatalyst because the photocurrent primarily goes toward photocorrosion of CuBi2O4 rather than proton reduction. Protecting the CuBi2O4 photocathode with a CdS/TiO2 heterojunction and adding RuOx as a cocatalyst prevents photocorrosion and allows for photoelectrochemical production of hydrogen at potentials ≤0.3 V vs RHE. A tandem photoelectrochemical cell composed of a W:BiVO4/CoPi photoanode and a CuBi2O4/CdS/TiO2/RuOx photocathode produces hydrogen which can be detected under illumination at an applied bias of ≥0.4 V. Since the valence band of BiVO4 and conduction band of CuBi2O4 are adequately positioned to oxidize water and reduce protons, we hypothesize that the applied bias is required to overcome the relatively low photovoltages of the photoelectrodes, that is, the relatively low quasi-Fermi level splitting within BiVO4 and CuBi2O4. This work is the first experimental demonstration of hydrogen production from a BiVO4-CuBi2O4-based tandem cell and it provides important insights into the significance of photovoltage in tandem devices for overall water splitting, especially for cells containing CuBi2O4 photocathodes.

BiVO4 thin films for use as photoelectrodes for solar water splitting are prepared by pulsed laser deposition (PLD), a powerful technique to synthesize compact multinary metal oxide films with high...

Photoelectrochemical water splitting in near-neutral pH conditions offers a safe and sustainable way to produce solar fuels, but operation at near-neutral pH is challenging because of the added concentration overpotentials due to mass-transport limitations of protons and hydroxide ions. Understanding the extent of this limitation is essential in designing a highly efficient solar fuel conversion device. In the present study, the local pH between the anode and cathode in a water splitting cell is monitored in situ using fluorescence pH sensor foils. By this direct visualization, we confirm that supporting buffer ions effectively suppress local pH changes, and we show that electrochemical reactions induce natural electrolyte convection in a non-stirred cell. The observed electrolyte convection at low current densities (

P-type copper vanadate (Cu5V2O10) photoelectrodes made by spray pyrolysis were developed and evaluated as a potential photocathode material for photoelectrochemical (PEC) water splitting. Using fluorine-doped tin oxide as a substrate, highly phase-pure p-Cu5V2O10 thin films were obtained after annealing at 550 °C for 4 hours in air. Cu5V2O10 has a small bandgap energy in the range of 1.8–2.0 eV, allowing it to absorb visible light and making it potentially interesting for solar water splitting applications. The p-Cu5V2O10 films were characterized by photoelectrochemical techniques in order to provide insight into the critical PEC properties such as the flat-band potential, chemical stability, and incident photon-to-current efficiency (IPCE). The best-performing films showed a photocurrent density of up to 0.5 mA cm−2 under AM1.5 simulated sunlight, and an IPCE value up to 14% for 450 nm light at 0.8 VRHE with H2O2 as an electron scavenger. Despite the narrow band gap and suitable conduction band edge position for PEC H2 production, these p-type films were unstable under constant illumination in aqueous electrolyte (pH 6.8) due to the reduction and dissolution of Cu. Based on our findings, the suitability of Cu5V2O10 as a photocathode material for photoelectrochemical water splitting is critically discussed.

Metal oxide semiconductors are promising as photoanodes for solar water splitting, but they typically suffer from poor charge transfer properties due to the slow surface reaction kinetics for oxygen evolution. To overcome this, their surfaces are usually modified by depositing earth-abundant, efficient, and inexpensive water oxidation co-catalysts. While this effort has been successful in enhancing the photoelectrochemical performance, a true understanding of the nature of the improvement is still under discussion. This is due to the fact that the co-catalyst can have multiple functionalities, e.g., accelerating charge transfer, passivating surface states, or modifying band bending. Disentangling these factors is challenging, but necessary to obtain a full understanding of the enhancement mechanism and better design the semiconductor/co-catalyst interface. In this study, we investigate the role of atomic layer deposited (ALD) MnOx co-catalysts and their thickness in the photoelectrochemical performance of BiVO4 photoanodes. Modified MnOx/BiVO4 samples with an optimum thickness of ∼4 nm show higher photocurrent (a factor of >3) as well as lower onset potential (by ∼100 mV) compared to the bare BiVO4. We combine spectroscopic and photoelectrochemical measurements to unravel the different roles of MnOx and explain the photocurrent trend as a function of the thickness of MnOx. X-ray photoelectron spectroscopy (XPS) studies reveal that the surface band bending of BiVO4 is modified after the addition of MnOx, therefore reducing surface recombination. At the same time, increasing the thickness of MnOx beyond the optimal 4 nm provides shunting pathways, as shown by energy dispersive X-ray scanning transmission electron microscopy (EDX-STEM) and redox electrochemistry. This cancels out the band bending effect, which explains the observed photocurrent trend. Therewith, this study provides additional insights into the understanding of the charge transfer processes occurring at the semiconductor–catalyst interface.

The unique possibilities of rapid thermal processing RTP for overcoming two significant challenges in the development of oxide thin film photoelectrodes are demonstrated. The first is the need to exceed normal temperature limits for glass based F SnO2 substrates FTO, amp; 8764;550 C to achieve the desired density, crystallinity, and low defect concentrations in metal oxides. Flash heating of Ta2O5, TiO2, and WO3 photoelectrodes to 850 C is possible without damaging the FTO. RTP heating rate dependencies suggest that the emission spectrum of the RTP lamp, which blue shifts with increasing heating power, can significantly influence the crystallization behavior of wide bandgap photoelectrodes amp; 8805;1.8 eV . The second challenge is avoiding the formation of structural defects, trap states, grain boundaries, and phase impurities, which can be particularly difficult in multinary metal oxides. RTP treatment of amp; 945; SnWO4, a promising photoanode material, resulted in an increase in grain size and favorable crystallographic reorientation, culminating in a new performance record

A previously developed sustainable immobilization concept for photocatalysts based on cellulose as a renewable support material was applied for the photocatalytic hydrogenation of acetophenone (ACP) to 1-phenyl ethanol (PE). Four different TiO

A surface treatment process, named ReTreat, is presented, and is shown to enhance the adhesion of electroplated metals on Sn containing transparent conducting oxides TCOs . The ReTreat process uses Zn powders, FeSO4 and glycine buffered aqueous solutions pH 3 to 5 in order to regulate a controlled and uniform conversion of SnO2 surfaces to SnO, Sn metal, and FexSny alloys. These surface metallic and intermetallic layers selectively enrich the electroplating of metallic films including Ni, Au, and Ag . Subsequently, the process has been used to fabricate thermally stable metal films on rigid FTO coated glass and flexible ITO coated PET substrates. Standardized testing confirms that the metallic coatings exhibit sufficient adhesion to the underlying TCO with high thermal stability and tolerance to flexural strain. A reaction mechanism for the heterogenous surface treatment is deduced from X ray diffraction, X ray photoelectron spectroscopy, and in situ transmittance measurements. These investigations show how the process parameters e.g., Zn powders, FeSO4 concentration, pH, and TCO type impact the reaction rate, morphology, and composition of the treated TCO surface. This report provides detailed insights necessary for the future implementation of this innovative surface treatment, which has the prospect to be a customary process for electroplating onto Sn containing TCOs

Ultrathin Bi2O3 films were grown by thermal- and plasma-enhanced ALD by using [Bi(tmhd)3] as bismuth precursor. The use of an O2 plasma instead of H2O as oxidant increases the growth per cycle (GPC...

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO4 photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm−2 at 1.23 VRHE (from original 4.01 mA cm−2) for a single photoanode configuration, and 6.22 mA cm−2 at 1.23 VRHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance. While photoelectrochemical water splitting offers a low-cost, integrated means to generate fuel from light, poor charge carrier transport limits performances. Here, authors embed laser-synthesized colloids in bismuth vanadate photoanodes to boost charge carrier mobilities and enhance photocurrents.

Molybdenum sulfide, MoSx, is considered as attractive hydrogen evolution catalyst since it is free of noble metals and shows a low overpotential. Especially, amorphous molybdenum sulfide has attracted attention because of its high catalytic activity. However, the catalytic mechanism of the hydrogen evolution reaction is not yet fully understood. Therefore, in our study, layers of MoSx were deposited by reactive magnetron sputtering, varying the substrate temperature in the range from room temperature (RT) to 500 °C. The morphology and structure of the films change significantly as a function of temperature, from an amorphous to a highly textured 2H-MoS2 phase. The highest catalytic activity was found for amorphous layers deposited at RT, showing an overvoltage of 180 mV at a current density of −10 mA cm–2 in a 0.5 M sulfuric acid electrolyte (pH 0.3) after electrochemical activation. As detected by Raman spectroscopy, the RT deposited catalyst consists of [Mo3S13]2– and [Mo3S12]2– entities which are inter...