Influence of SnWO4, SnW3O9, and WO3 Phases in Tin Tungstate Films on Photoelectrochemical Water Oxidation.

An essential step toward enabling the production of renewable and cost-efficient fuels is an improved understanding of the performance of energy conversion materials. In recent years, there has been growing interest in ternary metal oxides. Particularly, α-SnWO4 exhibited promising properties for application to photoelectrochemical (PEC) water splitting. However, the number of corresponding studies remains limited, and a deeper understanding of the physical and chemical processes in α-SnWO4 is necessary. To date, charge-carrier generation, separation, and transfer have not been exhaustively studied for SnWO4-based photoelectrodes. All of these processes depend on the phase composition, not only α-SnWO4 but also on the related phases SnW3O9 and WO3, as well as on their spatial distributions resulting from the coating synthesis. In the present work, these processes in different phases of tin tungstate films were investigated by transient surface photovoltage (TSPV) spectroscopy to complement the analysis of the applicability of α-SnWO4 thin films for practical PEC oxygen evolution. Pure α-SnWO4 films exhibit higher photoactivities than those of films containing secondary SnW3O9 and WO3 phases due to the higher recombination of charge carriers when these phases are present.

Photoelectrochemical Water Oxidation over Novel Semiconducting Zinc-Based Metal-Thiolate Framework.

Designing an efficient catalyst for a sustainable photoelectrochemical water oxidation reaction is very challenging in the context of renewable energy research. Here, we have introduced a new semiconducting porous zinc-thiolate framework via successful stitching of an "N" donor linker with a triazine-based tristhiolate secondary building unit in the overall architecture. The introduction of both linker and tristhiolate ligand synergistically modifies the architecture by making it a rigid, crystalline, three-dimensional, thermally stable, and porous framework. Our novel zinc-thiolate framework is used as an n-type semiconductor as revealed from the solid-state UV-vis DRS spectroscopic analysis, ac and dc conductivity analysis, and Mott-Schottky plot. This n-type semiconductor-based zinc-thiolate framework is utilized in the photoelectrochemical water oxidation reaction. It displayed a very high efficiency for a visible-light-driven oxygen evolution reaction (OER) in a KOH medium using standard Ag/AgCl as the reference electrode. The superiority of this material was further revealed from the low onset potential (0.822 mV vs RHE), high photocurrent density (0.204 mA cm-2), good stability, and high O2 evolution rate (77 µmol g-1 of oxygen evolution within 2 h), and a good efficiency (ABPE 0.42%, IPCE 29.6% and APCE 34.5%). Furthermore, the porosity in the overall framework seems to be a blessing to the photoelectrochemical performance due to better mass diffusion of the electrolyte. A detailed mechanism for the OER reaction was analyzed through density functional theory analysis suggesting the potential future of this Zn-thiolate framework for achieving a high efficiency in the sustainable water oxidation reaction.

Transient Surface Photovoltage Spectroscopy of (NH4)2Mo3S13/WSe2 Thin-Film Photocathodes for Photoelectrochemical Hydrogen Evolution.

Hydrogen produced from solar energy has the potential to replace petroleum in the future. To this respect, there is a need in the abandoned and efficient materials that can continuously split water molecules using solar energy. In this report, an ammonium thiomolybdate (ATM: (NH4)2Mo3S13) is evaluated as a p-type semiconductor film photocathode for hydrogen evolution reaction. The ATM thin films are prepared by spin-coating on fluorine-doped tin oxide substrates, and their structural, morphological, optical, photoelectrical, and photoelectrochemical (PEC) properties are studied. Transient surface photovoltage (TSPV) spectroscopy and spectroscopic ellipsometry indicate the band gap Eg = 1.9 eV for the ATM thin films. Furthermore, the photovoltage of the ATM thin films measured by TSPV is correlated to the photocurrents measured by the PEC characterization that can be used to evaluate the material potential for hydrogen generation. The films exhibit a low photocurrent density of 46 µA cm-2 at 0 VRHE. However, its combination with WSe2 thin-film photocathodes results in a significant increase in photocurrent density up to 4.6 mA cm-2 at 0 VRHE (100 times). The reason for such a strong charge carrier transfer effect for ATM/WSe2 heterojunction photocathodes is studied by TSPV spectroscopy that allows a comprehensive evaluation of potential photovoltaic materials toward PEC hydrogen production. Furthermore, the photovoltage generated by a WSe2 thin film is 30 times lower than that of its single crystal, which indicates that the quality of WSe2 thin films should be improved for faster PEC hydrogen evolution.

Photoelectrochemical Water-Splitting Using CuO-Based Electrodes for Hydrogen Production: A Review.

The cost-effective, robust, and efficient electrocatalysts for photoelectrochemical (PEC) water-splitting has been extensively studied over the past decade to address a solution for the energy crisis. The interesting physicochemical properties of CuO have introduced this promising photocathodic material among the few photocatalysts with a narrow bandgap. This photocatalyst has a high activity for the PEC hydrogen evolution reaction (HER) under simulated sunlight irradiation. Here, the recent advancements of CuO-based photoelectrodes, including undoped CuO, doped CuO, and CuO composites, in the PEC water-splitting field, are comprehensively studied. Moreover, the synthesis methods, characterization, and fundamental factors of each classification are discussed in detail. Apart from the exclusive characteristics of CuO-based photoelectrodes, the PEC properties of CuO/2D materials, as groups of the growing nanocomposites in photocurrent-generating devices, are discussed in separate sections. Regarding the particular attention paid to the CuO heterostructure photocathodes, the PEC water splitting application is reviewed and the properties of each group such as electronic structures, defects, bandgap, and hierarchical structures are critically assessed.

Facile synthesis of sphere-like structured ZnIn2S4-rGO-CuInS2 ternary heterojunction catalyst for efficient visible-active photocatalytic hydrogen evolution.

Photocatalysis is a promising approach for generating hydrogen, an eco-friendly and cost-effective fuel. It is hypothesized that the ternary catalyst ZnIn2S4-rGO-CuInS2, prepared by ultrasonication method, should be effective for optimized photocatalytic hydrogen generation in a Na2S/Na2SO3-water mixture. The as-synthesized catalyst was characterized using various surface analytical and optical techniques. Field-emission scanning electron microscopy and high-resolution transmission electron microscopy analyses revealed that marigold-like structured ZnIn2S4 and layer-structured CuInS2 were dispersed on the reduced graphene oxide sheets. The ternary ZnIn2S4-rGO-CuInS2 system showed enhanced photocatalytic H2 production compared to pure ZnIn2S4, CuInS2, ZnIn2S4-rGO, CuInS2-rGO, and ZnIn2S4-CuInS2 catalysts under visible light illumination. The fabricated ZnIn2S4-rGO-CuInS2 catalyst afforded hydrogen generation of 2531 µmol/g after 5 h. The enhanced performance of the ZnIn2S4-rGO-CuInS2 catalyst originates from the synergetic effect with rGO as the electron transfer medium, and is confirmed by photocurrent density and photoluminescence measurements that indicate reduced recombination between the excited electron and hole pairs, and fast electron transfer in the ternary composite. The excellent performance of the ZnIn2S4-rGO-CuInS2 catalyst for up to three consecutive cycles was demonstrated in cyclic stability tests under visible-light illumination.

Construction of visible-light driven Bi2MoO6-rGO-TiO2 photocatalyst for effective ofloxacin degradation.

Photocatalytic removal is more appropriate for the destruction of organic contaminants. The ternary Bi2MoO6-reduced graphene oxide (rGO)-TiO2 catalyst was synthesized using a simple hydrothermal method, and various surface analytical optical techniques were analyzed. The photocatalytic decomposition efficiency of the Bi2MoO6-rGO-TiO2 composite was 92.3% higher than those of pure and binary photocatalysts. The effects of operational parameters, such as catalyst ratio, catalyst variation, rGO ratio variation, and pH value variation were also analyzed. The as-prepared ternary photocatalyst exhibited low photoluminescence and high photocurrent density, which suppressed photon-induced electron and hole (h+) recombination and effective charge separation. The study demonstrated that rGO has excellent electron transfer performance and enhanced photocatalytic reaction stability. The perfect cycling stability of Bi2MoO6-rGO-TiO2 was retained even after five consecutive cycles on the photocatalytic degradation reaction performance. In this study, we propose a decomposition performance mechanism for ofloxacin degradation that underwent visible-light irradiation.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation.

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Photoelectrochemical Strategy for Discrimination of Microbial Pathogens Using Conjugated Polymers.

A photoelectrochemical (PEC) biosensor for facile and sensitive identification of pathogenic microorganisms was developed. Cationic poly(phenylene vinylene) derivative (PPV) as photoelectrochemical active species was modified on the electrode. Under light irradiation, PPV could be excited and generate efficient photocurrent. PPV also had the ability to bind with negatively charged membrane of pathogenic microorganisms, which hindered the electron transfer between electrode and electrolyte. As a result, the photocurrent would decrease obviously. For E. coli, B. subtilis and C. albicans, the photocurrent density was reduced by 18, 33 and 59 %, respectively. Based on the reduction degree of the photocurrent after capturing different types of species of pathogenic microorganisms, a PEC sensor for discrimination of pathogenic microorganisms was realized.

Preparation of nanoporous BiVO4/TiO2/Ti film through electrodeposition for photoelectrochemical water splitting.

A nanoporous BiVO4/TiO2/Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C for 2 h. The electrodeposition of BiOI film was carried out at different times (10, 30, 100, 500 and 1000 s) in Bi(NO3)3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO4/TiO2/Ti heterojunction film were examined by using different characterization techniques. UV-vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO4/TiO2/Ti film is lower than that of TiO2/Ti. The photocatalytic efficiency of BiVO4/TiO2/Ti heterojunction film was higher compared to that of the TiO2/Ti film owing to the longer transient decay time for BiVO4/TiO2/Ti film (3.2 s) than that of TiO2/Ti film (0.95 s) in our experiment. The BiVO4/TiO2/Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm-2 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti film (1000 s) under irradiation.

Simple and facile ultrasound-assisted fabrication of Bi(2)O2CO(3)/g-C(3)N(4) composites with excellent photoactivity.

Bi2O2CO3/g-C3N4 (BOC/CN) composites photocatalyst was fabricated via a facile ultrasonic-assisted method. The crystal structure, morphology, optical and photocatalytic properties of the as-prepared samples were characterized by various analytical techniques. The results indicated that the Bi2O2CO3 nanoflakes grew on the surface of the g-C3N4 nanosheets, forming closely contacted interfaces between the Bi2O2CO3 and the g-C3N4 component. BOC/CN composites with 50wt% of g-C3N4 showed the optimal photoactivity for the degradation of RhB under visible light, which was approximately 2.2 times higher than that of pure g-C3N4 and 7 times of pure Bi2O2CO3, respectively. The enhanced performance of the BOC/CN composites was mainly attributed to a synergistic effect including the accelerated separation and migration of photogenerated charge carriers, demonstrated by Photoluminescence (PL), electrochemical impedance spectra (EIS) and photocurrent density. Finally, a possible photocatalytic mechanism was proposed based on the experimental results. It is expected that such a facile route method could provide new insights into fabricating other g-C3N4-based composite photocatalysts for environmental remediation.

Synthesis of novel AuPd nanoparticles decorated one-dimensional ZnO nanorod arrays with enhanced photoelectrochemical water splitting activity.

The vertically aligned one-dimensional (1D) ZnO nanorod arrays decorated with AuPd alloy nanoparticles have been synthesized with ZnO nanorod arrays as template via a mild hydrothermal method. In this work, the as-prepared AuPd/ZnO nanorod arrays demonstrated high light-harvesting efficiency. The microstructures, morphologies and chemical properties of the obtained AuPd/ZnO composite photocatalyst were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS). The photoelectrochemical (PEC) performances of as-obtained AuPd/ZnO nanorod arrays were examined, and the photocurrent density was up to 0.98mAcm(-2) at 0.787V versus Ag/AgCl, which was about 2.4 times higher than the pure ZnO sample. A possible photocatalytic mechanism of the AuPd/ZnO hybrid nanostructures under the simulated sunlight irradiation was proposed to guide further improvement of other desirable materials. According to the above experiment results, it can be clearly found that AuPd/ZnO composite nanorod arrays showed excellent PEC performance and had promising applications in the utilization of solar energy.

Impact of Capacitive Effect and Ion Migration on the Hysteretic Behavior of Perovskite Solar Cells.

In the past five years, perovskite solar cells (PSCs) based on organometal halide perovskite have exhibited extraordinary photovoltaic (PV) performance. However, the PV measurements of PSCs have been widely recognized to depend on voltage scanning condition (hysteretic current density-voltage [J-V] behavior), as well as on voltage treatment history. In this study, we find that varied PSC responses are attributable to two causes. First, capacitive effect associated with electrode polarization provides a slow transient non-steady-state photocurrent that modifies the J-V response. Second, modification of interfacial barriers induced by ion migration can modulate charge-collection efficiency so that it causes a pseudo-steady-state photocurrent, which changes according to previous voltage conditioning. Both phenomena are strongly influenced by ions accumulating at outer interfaces, but their electrical and PV effects are different. The time scale for decay of capacitive current is on the order of seconds, whereas the slow redistribution of mobile ions requires several minutes.

Ag2S/CdS/TiO2 Nanotube Array Films with High Photocurrent Density by Spotting Sample Method.

Ag2S/CdS/TiO2 hybrid nanotube array films (Ag2S/CdS/TNTs) were prepared by selectively depositing a narrow-gap semiconductor-Ag2S (0.9 eV) quantum dots (QDs)-in the local domain of the CdS/TiO2 nanotube array films by spotting sample method (SSM). The improvement of sunlight absorption ability and photocurrent density of titanium dioxide (TiO2) nanotube array films (TNTs) which were obtained by anodic oxidation method was realized because of modifying semiconductor QDs. The CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs fabricated by uniformly depositing the QDs into the TNTs via the successive ionic layer adsorption and reaction (SILAR) method were synthesized, respectively. The X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectrum (XPS) results demonstrated that the Ag2S/CdS/TNTs prepared by SSM and other films were successfully prepared. In comparison with the four films of TNTs, CdS/TNTs, Ag2S/TNTs, and Ag2S/CdS/TNTs by SILAR, the Ag2S/CdS/TNTs prepared by SSM showed much better absorption capability and the highest photocurrent density in UV-vis range (320~800 nm). The cycles of local deposition have great influence on their photoelectric properties. The photocurrent density of Ag2S/CdS/TNTs by SSM with optimum deposition cycles of 6 was about 37 times that of TNTs without modification, demonstrating their great prospective applications in solar energy utilization fields.

Resonant multiple light scattering for enhanced photon harvesting in dye-sensitized solar cells.

A new benchmark for DSSC performances is set using a novel dye and fabricating a very efficient resonant light-scattering device with a high photocurrent and good stability.