Fast Charge Carrier Research Articles

Pd2+-coordinated polybenzimidazole membranes with fast and selective ion transport for alkaline aqueous organic redox flow battery

The advancement of aqueous organic redox-flow batteries (AORFBs) is impeded by the lack of efficient, low-resistance, and highly selective ion-conducting membranes (ICMs). Although polybenzimidazole (PBI) is one of the most promising low-cost non-fluorinated ion-conducting membranes, it remains challenging to design stable PBI membranes with fast and selective ion transport channels. Here, we engineer the chemical structure of PBI and regulate the ion transport channels to prepare highly conductive and selective Pd2+-coordinated membranes with grafting and crosslinking structures. In this design, the positively charged quaternary ammonium groups on the side chain improve the conductivity of OH−, while the continuous cross-linked network formed by ionic bonds between quaternary ammonium groups and deprotonated imidazole groups significantly enhances the membrane's mechanical properties. Furthermore, the coordination of Pd2+ with PBI and plasticization of PBI chain by trifluoroacetate anions expand the molecular free volume while inducing local contraction. Consequently, the QPBI-xPBI-Pd membrane enables fast charge carrier transport and suppresses the diffusion of active substances. The AORFBs assembled with the QPBI-10PBI-Pd membranes exhibit high coulombic efficiency (99.6 %) and energy efficiency (77.67 %) at 100 mA cm−2, maintaining excellent stability for 500 cycles. This study provides an innovative strategy to design high-performance PBI membranes for RFBs, thereby advancing the viability of RFBs in the realm of large-scale energy storage technologies.

Synthesis and characterization of Zn-In-S composites for photocatalytic benzyl alcohol oxidation and nitrobenzene reduction

Photocatalytic transformation of organics is a promising alternative to conventional synthetic methodologies. ZnIn2S4 is active in photocatalytic redox reactions, but its performance is hindered by fast charge carrier recombination, and optimizing its properties through synthesis or modification is complicated. Herein, Zn-In-S based composites were prepared via a facile solvothermal method by varying the ratio of sulfide precursor (TAA) with a fixed Zn/ In molar ratio of 1: 2. The phase composition of the obtained materials depends on the amount of TAA. At lower ratios, composites consisting of crystalline In(OH)3 and unknown ZnxInySz phase were formed. Pure-phase ZnIn2S4 was obtained when the feed molar ratio of zinc and TAA was 1: 6 or higher. The photocatalytic oxidation of benzyl alcohol (BA) and reduction of nitrobenzene (NB) in a coupled system were applied to evaluate the activity of Zn-In-S-based composites. Under visible light irradiation, the ZnxInySz/ In(OH)3 composite (ZIS14) synthesized with a molar ratio of Zn to TAA of 1: 4 exhibited boosted and optimal performance compared to the bare ZnIn2S4 and other samples. After 5 hours of irradiation, the BA conversion and benzaldehyde (BAD) yield were 57 % and 55 %, respectively, and the NB conversion and aniline (AN) yield were 70 % and 35 %, individually, outperforming previous studies under similar experimental conditions. This work not only elucidates the photoredox process of BA and NB in one system, but also has significant implications for understanding the complex hydro/solvothermal processes involved in the fabrication of multicomponent sulfides.

Enabling extreme low-temperature proton pseudocapacitor with tailored pseudocapacitive electrodes and antifreezing electrolytes engineering

A critical current challenge in the development of proton pseudocapacitors is developing antifreezing electrolytes and high-performance proton storage materials in extreme environments. Here, we design an antifreezing proton pseudocapacitor using a high-concentration phosphoric acid electrolyte and pseudocapacitive electrode materials, which delivers an outstanding rate capacity and cycle life below − 40 °C. Comprehensive physical characterization techniques and theoretical simulations demonstrate that the solvation structure of the electrolyte defines the freezing point and electrochemical stability window, which is crucial for achieving fast charge carrier mobility and avoiding side effects. The pseudocapacitive hydrated tungsten oxide anodes and Prussian blue analogue cathodes with their open crystal structures achieve fast proton transport and storage, resulting in excellent electrochemical performance from − 60 to 25 °C (1000 cycles with no capacity fading at − 40 °C). Benefiting from the synergistic effect between the electrolyte and electrode materials, the fabricated proton pseudocapacitors exhibit remarkable low-temperature electrochemical performance. It achieves a maximum energy density of 39 Wh kg−1 and a broadened electrochemical window from 1.7 V at 25 °C to 2.3 V at − 60 °C. This work provides a comprehensive strategy to develop high-performance proton energy storage devices under ultralow-temperature conditions.

Design of CdS/Covalent Organic Framework as an efficient heterojunction photocatalyst for enhanced photocatalytic H2 evolution

In this study, we design a CdS/Covalent Organic Framework (COF) heterojunction to enhance the photocatalytic H2 evolution. The results demonstrate that the 30% CdS/TTCOF heterojunction exhibits the highest H2 production activity of 21.8 mmol g−1 under visible light, surpassing that of pure CdS and TTCOF by 8.1 and 18.2 times, respectively. Additionally, time-resolved fluorescence decay and steady-state photoluminescence spectra indicate that the enhanced photocatalytic performance of the CdS/TTCOF heterojunction can be ascribed to fast charge carrier transfer between CdS and TTCOF.

Effective redox reaction in a three-body smart photocatalyst through multi-interface modulation of organic semiconductor junctioned with metal and inorganic semiconductor

Forming metal/semiconductor heterojunctions is a well-known strategy to enhance photo-reaction through interface band bending. However, challenges remain in simultaneously minimizing fast charge carrier recombination and avoiding potential photo-corrosion in photocatalysts for high efficiency and long lifetime application in energy, environment, and green chemistry. We here design and propose a new strategy by which multi-interface band structure modulation occurs in a three-body photocatalyst, exemplified by Ag/PDA@Ag2S (PDA: polydopamine). Experimental analyses including in-situ x-ray photoelectron spectroscopy (XPS) and time-resolved photoluminescence (TRPL) spectroscopy together with projected density of states studies suggest an S-scheme heterojunction formation in the PDA@Ag2S, which greatly enhances the separation of photogenerated carriers. Consequently, we demonstrate that this multibody photocatalyst exhibits superior stability and selective oxidation of a series of ionic dyes under solar-simulated irradiation. The hydrogen generation efficiency (reduction) is also remarkably enhanced compared to virgin Ag/Ag2S photocatalysts. We believe this work brings a new insight into catalyst design.

Constructing Heptazine-COF@TiO2 Heterojunction Photocatalysts for Efficient Photodegradation of Acetaminophen under Visible Light.

Constructing heterojunction photocatalysts are widely applied to boost the photocatalytic activity of materials. Here, a novel covalent organic framework (COF) material with heptazine units was developed and hybridized with TiO2 nano particles (NPs) to fabricate the Heptazine-COF@TiO2 photocatalysts for acetaminophen (AAP) photodegradation. The successfully assembled heptazine unit endows the Heptazine-COF with outstanding semiconductor property (optical bandgap is 2.53 eV). The synthesized Heptazine-COF@TiO2 hybrids is proved to have the heterojunction structure with high visible light activity and fast charge-carrier mobility, and exhibits better performance in photodegradation of AAP under visible light. The excellent photodegradation efficiency (rate constant: 0.758 min-1) and high reusability (rate constant: 0.452 min-1 in the 6th cycles) of the optimized sample outperform the traditional inorganic photocatalysts and other heterojunction photocatalysts. In addition, these photocatalysts present universal degradation activity for other dyes and antibiotics.

Selective vanadium etching and in-situ formation of δ-Bi2O3 on m-BiVO4 with g-C3N4 nanosheets for photocatalytic degradation of antibiotic tetracycline

Antibiotics play a vital role in treating microbial illnesses, but their presence in water bodies is a global concern. Despite the potential of polymeric graphitic carbon nitrate (g-C3N4) as a photocatalyst for wastewater treatment, its effectiveness is hindered by a fast charge carrier recombination rate. To overcome this challenge, we have developed a novel approach involving the surface etching-oxidation technique to introduce δ-Bi2O3 onto m-BiVO4. Subsequently, the modified δ-Bi2O3/m-BiVO4 (BBV) was synergistically coupled with g-C3N4 (CN) to create a hybrid photocatalyst (BBVCN). The photocatalytic activity of BBVCN was evaluated using tetracycline hydrochloride (TC) as a model wastewater antibiotic pollutant, demonstrating a 92 % degradation of TC within a 1 h reaction time. Total organic carbon analysis demonstrated a TC removal efficiency of 76 %, indicating significant mineralization by BBVCN.

MBene with Redox-Active Terminal Groups for an Energy-Dense Cascade Aqueous Battery.

Two-dimensional (2D) transition metal borides (MBenes), new members of the 2D materials family, hold great promise for use in the electrocatalytic and energy storage fields because of their high specific area, high chemical activity, and fast charge carrier mobility. Although various types of MBenes are reported, layered MBenes featuring redox-active terminal groups for high energy output are not yet produced. A facile and energy-efficient method for synthesizing MBenes equipped with redox-active terminal groups for cascade Zn||I2 batteries is presented. Layered MBenes have ordered metal vacancies and ─Br terminal groups, enabling the sequential reactions of I-/I0 and Br-/Br0. The I2-hosting MBene-Br cathode results in a specific energy as high as 485.8Whkg-1 at 899.7Wkg-1 and a specific power as high as 6007.7Wkg-1 at 180.2Whkg-1, far exceeding the best records for Zn||I2 batteries. The results of this study demonstrate that the challenges of MBene synthesis can be overcome and reveal an efficient path for producing high-performance redox-active electrode materials for energy-dense cascade aqueous batteries.

Light-Induced Ammonia Generation over Defective Carbon Nitride Modified with Pyrite

The challenge to mitigate climate change and to limit global warming to well below 2 °C according to the Paris Agreement, requires a reduction of greenhouse gas emissions not only in transportation, but also in industry.[1] The Haber-Bosch process for the production of ammonia, NH3, is one of the most important industrial processes, but significantly contributes to the world’s annual CO2 emissions.[2] Photocatalytic reduction of dinitrogen (NRR) offers a sustainable alternative for the synthesis of ammonia. Some of the highest activities so far were achieved with carbon nitride (C3N4) photocatalysts, especially after the introduction of nitrogen vacancies.[3] Carbon nitrides are earth abundant, non-toxic, low-cost polymeric materials, with a suitable band gap for visible light absorption. To overcome the inherently fast charge carrier recombination that diminishes the efficiency, heterojunctions with a second photocatalyst are widely investigated.[4] Another common approach in the design of photocatalyst for the NRR is nature-inspired. Herein, mainly sulphur, iron and molybdenum containing materials are investigated, inspired by the composition of the active centre in nitrogenase enzymes.[5]In this contribution we present composite materials of FeS2 and vacancy-rich VN-C3N4, thereby combining both strategies of heterojunction formation and bio-inspiration, as well as that of defect engineering in C3N4, to increase the number of active sites.[6] The composites yield significantly higher amounts of ammonia after light irradiation for 7 h, compared to both FeS2 and VN-C3N4 alone. Hydrogen is produced as the only by-product. A combination of control experiments and material characterisation before and after light irradiation revealed that iron in FeS2 coordinates to VN-C3N4 at the defect site. Cyano-groups at these sites are activated via π-back-donation and subsequently converted into ammonia. Thus we could prove that – at least initially – ammonia is generated not from the N2 gas, but from the C3N4-framework itself.

Improving the photocatalytic activity of graphitic carbon nitride through ionic liquid-mediated thermal annealing

Graphitic carbon nitride (g-C3N4) has shown a great promise in photocatalytic hydrogen evolution. However, the catalytic activity of g-C3N4 derived from nitrogen-rich compounds is still limited by fast charge carrier recombination and slow surface reaction dynamics. Herein, a series of primary g-C3N4 materials were prepared by thermal polymerization of melamine and/or urea, and then thermally treated with the involvement of different ionic liquids under altering heat conditions. Imidazole-containing ionic liquids, such as 1-propyl-3-methylimidazolium chloride ([PMIm]Cl), acted as an effective additive to mediate the thermal annealing behavior of primary g-C3N4, leading to the dissociation of aggregates into porous nanosheets with varying element compositions and with increased surface area. Regardless of little change in optical absorption and electronic structure, the resulting porous g-C3N4 nanosheets promoted the separation and transfer of photogenerated charges and modified surface hydrogen desorption. Hydrogen evolution tests confirmed that the catalytic activity of g-C3N4 from the co-polymerization of melamine and urea was enhanced by ∼3.4 and ∼7.4 times upon [PMIm]Cl-free and -mediated thermal annealing, respectively. This work suggests the potential effect of the additive involvement in the thermal annealing on the photocatalytic activity of g-C3N4.

Plasmonic Au nanoparticles sensitized ZnO/CuO heterostructure for efficient photoelectrochemical water splitting

Hydrogen production via photoelectrochemical water splitting is a promising route to convert solar energy into chemical fuel, reducing energy crises and boosting environmental health. Here, we report the fabrication of ternary ZnO/CuO/Au heterostructures by electrodeposition and chemical bath deposition methods. The heterojunction formed can enhance the optical absorption, and photogenerated electrons and holes can efficiently separate due to the built-in electric field, which reduces recombination losses. Au plasmons incorporated in ZnO/CuO heterojunction enhance the optical absorption and promote fast free charge carrier transport at the interface through the surface plasmon resonance (SPR) effect. The ZnO/CuO/Au photoanode exhibits a photocurrent density of 1.08 mA/cm2 at 1.6 V Vs RHE, two times higher than pristine ZnO. The ZnO/CuO/Au photoanode exhibits the lowest charge transfer resistance of 600 Ω from electrochemical impedance spectra(EIS). The Bode plot revealed that the most extended lifetime of 8.69 μs is observed for ZnO/CuO/Au photoanode, which is larger than pristine ZnO. The synthesized thin films showed good stability and reusability for the photooxidation of water.

Synergistic silver doping and N vacancy promoting photocatalytic performances of carbon nitride for pollutant oxidation and hydrogen production

Carbon nitride (C3N4) has attracted immense interest as a low-cost, non-toxic and naturally abundant raw material. However, graphite-like phase carbon nitride (g-C3N4) still suffers from poor visible light absorption, fast charge carrier recombination, slow electron mobility and relatively fewer surface-active sites. In this work, we synthesized silver-doped N vacancy-rich carbon nitride (AgCN) with convoluted ultra-thin lamellar layers from Ag precursors and melamine-cyanuric acid monomers using a self-assembly supramolecular strategy. AgCN exhibited excellent photocatalytic performance and stability. The introduction of N vacancies disrupted the off-domain π-bonds and weakened the conjugation effect of the triazine ring elements. The large specific surface area of the convoluted ultra-thin lamellar structure helps suppress the aggregation of active silver centers, and the Ag-N2C2 bond acts as a bridge for photoexcited charge transfer to promote the separation and transfer of photogenerated electron/hole pairs for surface redox reactions. As a result, AgCN exhibited excellent photocatalytic performance for photodegradation of rhodamine B (RhB) and hydrogen production (1.69 mmol g-1h−1), well outperforming the pristine CN. Density flooding theory (DFT) calculations revealed the improved conductivity and efficient separation of electron-hole pairs in AgCN at the excited state, generating superoxide radicals, singlet oxygen and holes.

A Practical Approach Toward Highly Reproducible and High-Quality Perovskite Films Based on an Aging Treatment.

Solution processing of hybrid perovskite semiconductors is a highly promising approach for the fabrication of cost-effective electronic and optoelectronic devices. However, challenges with this approach lie in overcoming the controllability of the perovskite film morphology and the reproducibility of device efficiencies. Here, a facile and practical aging treatment (AT) strategy is reported to modulate the perovskite crystal growth to produce sufficiently high-quality perovskite thin films with improved homogeneity and full-coverage morphology. The resulting AT-films exhibit fewer defects, faster charge carrier transfer/extraction, and suppressed non-radiative recombination compared with reference. The AT-devices achieve a noticeable improvement in the reproducibility, operational stability, and photovoltaic performance of devices, with the average efficiency increased by 16%. It also demonstrates the feasibility and scalability of AT strategy in optimizing the film morphology and device performance for other perovskite components including MAPbI3 , (MAPbBr3 )15 (FAPbI3 )85 , and Cs0.05 (MAPbBr3 )0.17 (FAPbI3 )0.83 . This method opens an effective avenue to improve the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Clay-based 1D-2D halloysite&g-C3N4 nanostructured meat floss for photocatalytic hydrogen evolution

Graphitic carbon nitride (g-C3N4) has drawn extensive attention with some features including visible-light response as non-metallic semiconductor, low cost in raw material and green pollution-free for environment, but suffers from some issues such as fast charge carriers’ recombination, easy aggregation, etc. In this work, the 1D-2D HNTs&g–C3N4–X binary materials similar to meat floss pattern in a series of halloysite loading amounts are designed via a facile electrostatic self-assembly strategy with debris g-C3N4 after cell pulverizing treatment and HNTs that outwardly modified by cetyltrimethylammonium bromide (CTAB) as the building blocks. The halloysite-mediated satellite-core material displays a photocatalytic of H2 evolution performance with the highest evolution rate of 137.0 μmol g−1 h−1 in visible light condition with no co-catalysts, and is ∼3.4 times that of bulk g-C3N4, mainly benefiting from the reduced nanometer size of debris g-C3N4 and enhanced interface dispersion ability by HNTs, resulting in ameliorative separation efficiency of photogenerated charge carriers. This research conclusively provides the new perspective towards the performance enhancement of water splitting of g-C3N4 in raw clay mineral modification mode and broadens the applications of mineral-based composite in the renewable energy utilization field.

Metal/bacteria cellulose nanofiber bilayer membranes for high-performance hydrovoltaic electric power generation

Hydrovoltaic devices that produce electricity from water represent a promising solution for green energy harvesting. Hydrovoltaic power generators based on various emerging nanostructured materials have shown great potential in water-enabled electricity generation. However, the development of high-performance and practical hydrovoltaic devices remains limited because of low electric power generation, high cost of precursor materials, and complicated fabrication processes. In this study, we developed a novel metal-coated bacteria cellulose nanofiber bilayer membrane (MBCBM) for high-performance hydrovoltaic power-generation devices. The top side of the MBCBM has metal-bacteria cellulose (BC) nanofibers that serve as a conducting electrode for fast charge carrier collection, whereas the bottom side has BC nanofibers that serve as hydrovoltaic materials for high efficient energy generation. A Schottky barrier was incorporated into the hydrovoltaic device, which enhanced the electric power output. Experiments revealed that the optimized single-MBCBM based hydrovoltaic device generated a maximum voltage of 0.935 V, current of 7.51 mA, and power output of 6.07 mW with a 50 μl electrolyte solution. The hybrid membrane and device design concept is expected to effectively utilize practical sustainable and clean energy sources for Internet of Things (IoT) devices and self-powered wearable devices in next-generation electronics.

Electrospinning Photocatalysis Meet In Situ Irradiated XPS: Recent Mechanisms Advances and Challenges.

Producing solar fuels over photocatalysts under light irradiation is a considerable way to alleviate energy crises and environmental pollution. To develop the yields of solar fuels, photocatalysts with broad light absorption, fast charge carrier migration, and abundant reaction sites need to be designed. Electrospun 1D nanofibers with large specific areas and high porosity have been widely used in the efficient production of solar fuels. Nevertheless, it is challenging to do in-depth mechanism research on electrospun nanofiber-based photocatalysts since there are multiple charge transfer routes and various reaction sites in these systems. Here, the basic principles of electrospinning and photocatalysis are systemically discussed. Then, the different roles of electrospun nanofibers played in recent research to boost photocatalytic efficiency are highlighted. It is noteworthy that the working principles and main advantages of in situ irradiated photoelectron spectroscopy (ISI-XPS), a new technique to investigate migration routes of charge carriers and identify active sites in electrospun nanofibers based photocatalysts, are summarized for the first time. At last, a brief summary on the future orientation of photocatalysts based on electrospun nanofibers as well as the perspectives on the development of the ISI-XPS technique are also provided.

Heterojunction architecture of Ce2S3 nanocubes with ZnxCd1-xS photocatalyst enables efficient hydrogen evolution

Heterojunction coupling with narrow bandgap semiconductor can effectively promote light absorption, inhibits photoinduced carrier recombination and enhances redox performance. Herein, a novel Ce2S3 nanocube was synthesized via a high-temperature sulfating method, then ZnxCd1-xS (ZCS) was grown on Ce2S3 cube to form a tight junction for the photocatalytic hydrogen evolution (PHE) activity. The experimental results confirm that this novel composite vastly boosts the redox performance due to superior visible light absorbance, narrow bandgap and fast charge carrier separation via a Z-scheme mechanism. Consequently, the optimized Ce2S3/ZCS composite presents the best PHE rate of 35.35 mmol h−1 g−1 without an obvious reduction in efficiency for cycling experiments, which has a 4.8-fold increase over Pt/ZCS. This strategy proposes a novel method for optimizing ZCS-based photocatalysis by introducing Ce2S3 component to improve the property of PHE.

Recent Progress in Conjugated Polymers-Based Donor–Acceptor Semiconductor Materials for Photocatalytic Hydrogen Evolution from Water Splitting

Exploration of high-efficiency stabilization and abundant source-conjugated polymers semiconductor materials with suitable molecular orbital energy levels has always been a hot topic in the field of photocatalytic hydrogen evolution (PHE) from water splitting. In the recent years, constructing the intramolecular donor–acceptor (D–A)-conjugated architecture copolymers has been proved as one of the most excellent photocatalyst modification tactics for optimizing the PHE properties because of unique advantages, including easy regulate band-gap position, fast transfer charge carrier in the intramolecular architecture, superior sunlight absorption capacity and range, large interfacial areas, and so forth. Therefore, in this minireview, we summarize the latest research progress of D–A architecture semiconductor materials for PHE from water splitting. First, we briefly overview the fundamental description and principles for the construction D–A heterostructures in the photocatalytic system. After that, the application of D–A architecture photocatalyst for PHE reaction over different classes of organic semiconductors have been discussed in detail. At last, the present development prospects and future potential challenges of D–A architecture materials are proposed. We hope this minireview has some parameter values for the further developments of intermolecular special structured organic semiconductor material in the future PHE research.

An overview on ZnO-based sonophotocatalytic mitigation of aqueous phase pollutants

Over the past several decades, the increase in industrialization provoked the discharge of harmful pollutants into the environment, affecting human beings and ecosystems. ZnO-based photocatalysts seem to be the most promising photocatalysts for treating harmful pollutants. However, fast charge carrier recombination, photo corrosion, and long reaction time are the significant factors that reduce the photoactivity of ZnO-based photocatalysts. In order to enhance the photoactivity of such photocatalysts, a combined process i.e., sonocatalysis + photocatalysis = sonophotocatalysis was used. Sonophotocatalysis is one of several different AOP methods that have recently drawn considerable interest, as it produces high reactive oxygen species (ROS) which helps in the oxidation of pollutants by acoustic cavitation. This combined technique enhanced the overall efficiency of the individual method by overcoming its limiting factors. The current review aims to present the theoretical and fundamental aspects of sonocatalysis and photocatalysis along with a detailed discussion on the benefits that can be obtained by the combined process i.e., US + UV (sonophotocatalysis). Also, we have provided a comparison of the excellent performance of ZnO to that of the other metal oxides. The purpose of this study is to discuss the literature concerning the potential applications of ZnO-based sonophotocatalysts for the degradation of pollutants i.e., dyes, antibiotics, pesticides, phenols, etc. That are carried out for future developments. The role of the produced ROS under light and ultrasound stimulation and the degradation mechanisms that are based on published literature are also discussed. In the end, future perspectives are suggested, that are helpful in the development of the sonophotocatalysis process for the remediation of wastewater containing various pollutants.

Enhanced hydrophilicity and promoted charge transfer in covalent triazine frameworks/sepiolite complexed via hydrogen bonding for visible-light driven degradation of antibiotics

Featuring strong visible-light absorption, tunable band gaps, structural diversity, and fully conjugated nitrogen-rich structures, covalent triazine frameworks (CTFs) are emerging as a class of promising semiconductor photocatalysts. However, limited by their poor hydrophilicity and fast charge carrier recombination, their photocatalytic efficiency in water is far lower than expected. Herein, a hybrid composite of sepiolite (SEP) and CTF with a tunable density was synthesized via an in situ CF3SO3H-promoted strategy. The resultant SEP@Th-CTF 2:1 composite exhibited excellent photocatalytic efficiency in the degradation of antibiotics under visible light, which can be attributed to its improved charge transfer and enhanced hydrophilicity. The interaction between sepiolite and Th-CTF via hydrogen bonding results in the transformation of photogenerated electron from Th-CTF to antibiotics. The combination of sepiolite with CTFs can improve photocatalytic performance, which is instructive to broaden the application of CTFs as semiconductors for efficient solar energy conversion in a water medium.