Application In Photoelectrochemical Water Splitting Research Articles

Polymerization Improvement of Graphitic Carbon Nitride Films Derived from Melamine and Thiourea.

Deposition of low-cost, efficient, and environmentally friendly graphitic carbon nitride (g-CN) films as photoanodes is a crucial step for constructing photoelectrochemical (PEC) cells and exploring their PEC performance. Currently, the improvement of the photocurrent density of g-CN films is badly needed for their practical applications in PEC water splitting. Enhancing the g-CN crystallinity by optimizing their synthesis conditions only through screening appropriate reactant precursors is insufficient for this purpose. Herein, using melamine and thiourea precursors with mass ratio 5:1, the degree of polymerization of g-CN thin films is successfully improved by a thermal vapor condensation method. The obtained pure g-CN exhibits a remarkably enhanced photocurrent density of 404.4µA cm-2 at 1.23V versus reversible hydrogen electrodes. Theoretical calculations reveal that the continuous attachment of small carbodiimide (HN═C═NH) mainly generated by thiourea to the melamine matrix facilitates the formation of large-area conjugated structure, which fundamentally determines better charge carrier separation and transfer thereby enhancing the PEC performance. This work realizes the synthesis of well-polymerized g-CN films with improved PEC activity and offers a computational understanding for the nucleation and growth mechanism of the polycrystalline g-CN.

NiO@GaN nanorods-based core-shell heterostructure for enhanced photoelectrochemical water splitting via efficient charge separation

Remarkable properties of III-V semiconductors, particularly GaN nanostructured based photoelectrodes offers a potential attention in the field of photoelectrochemical water splitting (PEC-WS) for clean and sustainable hydrogen production, due to its wide bandgap, magnificent optoelectrical properties. However, the presence of inevitable surface states in GaN nanorods (NRs) leads to low solar-to-hydrogen (STH) conversion efficiency with poor stability, thereby severely limiting their practical application in PEC-WS, which can be effectively alleviated by constructing a hybrid heterostructures. Herein, we present the development of interfacial engineering of a type-II core-shell heterostructure based on p-NiO nanoparticles (NPs) loaded on n-GaN NRs photoelectrodes for PEC-WS. We assessed the impact of the NiO NPs shell density on core GaN NRs, finding that the optimized NiO@GaN NRs photoelectrode achieved a photocurrent density (Jph) of 1.38 mA/cm² at 1.23 V versus RHE and an excellent applied bias photo-to-current conversion efficiency (ABPE) of ∼0.39 %, which was 3.8 (Jph) and 4.8 (ABPE)-fold times higher than the pristine GaN NRs photoanode under 1-Sun illumination. The type-II p-n heterojunction band alignment between core-shell NiO@GaN NRs photoelectrode effectively reduced the photogenerated carrier recombination rate through surface states passivation and boost the light absorption and harvesting capacity. This facilitates a significant charge separation and transfer at the photoanode/electrolyte interface leading to enhanced redox reactions, resulting in improved PEC-WS and STH performances. These findings offer a promising strategy to design and fabricate highly efficient III-V hybrid heterostructure photoelectrodes for futuristic PEC-WS-based green energy applications.

Copper-Surface-Mediated Synthesis of sp2 Carbon-Conjugated Covalent Organic Framework Photocathodes for Photoelectrochemical Hydrogen Evolution.

Sp2-carbon (sp2-c) covalent organic frameworks (COFs), featuring distinctive π-conjugated network structures, facilitate the migration of photo-generated carriers, rendering them exceptionally appealing for applications in photoelectrochemical water splitting. However, owing to the powdery nature of COFs, leaving anchor the sp2-c COFs powder tightly onto a conductive substrate challenging. Here, we propose a method for preparing photoactive substance-conductive substrate integrated photocathodes through copper surface-mediated knoevenagel polycondensation (Cu-SMKP), this approach results in a uniform and stable sp2-c COF film, directly grown on commercial copper foam (COFTh-Cu). The COFTh-Cu demonstrates a high H2-evolution photocurrent density of 56 μA cm-2 at 0.3 V vs. RHE, sustaining stability for 12 h. The as-prepared COFTh-Cu represents a 4.5-fold increase in current density compared to traditional spin-coating methods and outperforms most COF photocathodes without cocatalysts. This innovative copper surface-mediated approach for preparing photocathodes opens up a crucial pathway towards the realization of highly active COF photocathodes.

Enhancing PEC hydrogen generation efficiency: Robust ZnO-GaN hierarchical nanowires loaded with NiS for superior charge separation and transportation

We present a method for the sequential fabrication of a three-dimensional hierarchical structure, comprising ZnO nanowires (NWs) grown atop Vapor-Liquid-Solid (VLS) GaN NWs using Metal Organic Chemical Vapor Deposition (MOCVD) for applications in photoelectrochemical water splitting (PEC-WS). To enhance the PEC performance and photostability of the system, we performed NiS anchoring. The optimized 60 NiS/ZnO-GaN hierarchical nanowires (HNWs) exhibited a remarkable 23-fold enhancement (1.62 %) in photoconversion efficiency at 0.75 V vs reversible hydrogen electrode (RHE) compared to GaN NWs passivated with ZnO deposited by atomic layer deposition (ALD) (0.07 %), alongside an 18.9-fold increase in photocurrent density (7.6 mAcm−2 @ 1.23 V vs RHE). Additionally, these hierarchical structures demonstrated exceptional photostability over a period of 10 hours (36000 s) at 0.8 V vs RHE. The hierarchical assembly is crucial in achieving high photoconversion efficiency and photostability. The rational design of hierarchical NW photoanodes synergistically enhances both photocurrent and photostability, paving the way for facile integration of II-VI with III-V materials and potentially unlocking advancements in next-generation energy harvesting and conversion devices.

Immobilization of Prussian Blue Nanoparticles Onto Au‐Modified ZnIn2S4 Photoanode for Efficient Photoelectrochemical Water Splitting

AbstractZnIn2S4 has been regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting. However, its severe charge recombination and low carrier transport efficiency limit practical applications in PEC water splitting. A novel ZnIn2S4/Au/PB photoanode was prepared by using the in‐situ photodeposition of Au nanoparticles and electrodeposition of Prussian blue (PB) nanoparticles onto ZnIn2S4 nanosheets. The photocurrent density of the prepared ternary photoanode reached 0.42 mA/cm2 at 1.23 VRHE, which is 6 times higher than pure ZnIn2S4. The introduction of Au nanoparticles remarkably improved the efficiency of carrier transport and separation. Additionally, the application of PB onto ZnIn2S4 resulted in an extended spectrum of light absorption. The incident photon‐to‐current efficiency (IPCE) value recorded was 24.6 % at 350 nm. This improvement can be attributed to the formation of a Z‐type heterojunction.

Advancements in cadmium-based photoanodes for photoelectrochemical water splitting: A short review

Photoelectrochemical (PEC) water splitting, which directly converts sunlight into storable hydrogen fuel, has emerged as a promising technology in pursuing renewable energy. This process relies on semiconductor materials functioning as photoelectrodes to harness solar energy and drive water electrolysis into hydrogen and oxygen. However, PEC systems’ efficiency and cost-effectiveness face formidable challenges, primarily centred around developing robust and efficient photoanodes. Cadmium-based catalysts, belonging to the II-VI n-type semiconductor family, exhibit exceptional properties conducive to PEC applications, such as tunable band gaps and superior light absorption capabilities. Yet, their practical utility has been hindered by issues related to photocorrosion and inadequate charge carrier separation. Recent breakthroughs in cadmium-based photoanodes have addressed these limitations through innovative strategies. In regards to this matter, this review provides a comprehensive overview of recent advancements in cadmium-based photoanodes, shedding light on their innovative applications in PEC water splitting.

Perovskite BaSnO3 nanoparticles for solar-driven bi-functional photocatalytic activity: PEC water splitting and Wastewater treatment

The highly crystalline n-type BaSnO3 nanoparticles with size ∼30 nm, a larger surface area of ∼15.11 m2·g−1, and a wide optical band gap of 3.24 eV have been synthesized using a co-precipitation route for applications in photoelectrochemical water splitting and organic dye degradation. The spin-coated films of BSO nanoparticles photoanodes exhibited a photocurrent density of 20 μA·cm−2 at a forward bias potential of 0.6 V, a donor carrier density of 2.56 × 1018 cm−3 with a flat band potential of −0.82 V, and four-fold increment in the photocurrent density at a small bias voltage of 0.3 V, under AM 1.5 illumination (100 mW·cm−2). Pseudo-first-order kinetic study showed 96.49 and 95.38 % degradation performance of the Methylene Blue and Crystal Violet dyes at rate constant of 0.0128 and 0.0127 min−1, respectively, under constant irradiation for 240 min in the presence of BSO NPs, which is the highest degradation efficiency reported so far for BSO photocatalyst. The scavenger study revealed the significant role of h+ and ·OH− radicals in the degradation process. Liquid chromatography-mass spectroscopy provides intermediates obtained during the photocatalytic degradation of dyes. The present study focuses on the bifunctional photocatalytic performance of the BSO NPs as a potential candidate for PEC water splitting and wastewater treatment.

Chemical synthesis of Bi[formula omitted]Sb[formula omitted]S3 (x = 0 to 2) thin films for photoelectrochemical water splitting

We report the synthesis of BixSb2−xS3 (x = 0 - 2) thin films on ITO-coated glass substrates via Bi-doping of Sb2S3 using chemical bath deposition (CBD). The structural, microstructural, elemental, optical, and photoelectrochemical (PEC) properties of the synthesized films were investigated. Elemental analysis confirmed the inclusion of Bi and a reduction in the Sb content in the BixSb2−xS3 films with an increase in the Bi-dopant concentration. X-ray diffraction analysis revealed the transformation of pristine Sb2S3 into Bi0.76Sb1.24S3 and Bi2S3 when the Bi-doping concentration in the films was increased from x = 0 to 2. The films crystallized in an orthorhombic structure. The lattice parameters varied, and the crystallite size decreased from 42.6 to 19.6 nm as x increased from 0 to 2 for the BixSb2−xS3 films. Optical absorption studies revealed a reduction in the bandgap of BixSb2−xS3 from 1.68 to 1.55 eV with an increase in x from 0 to 2. PEC measurements revealed significant photoresponses of the BixSb2−xS3 photoanodes synthesized with x = 0, 1.0, and 2.0. The Bi0.76Sb1.24S3 photoanode synthesized with x = 1.0 exhibited a stable photocurrent of 1 mA/cm2 over a period of 630 s under light illumination owing to its low charge-transfer resistance and good charge-carrier separation. Thus, we demonstrated the synthesis of BixSb2−xS3 photoanodes using a simple and inexpensive CBD method for PEC water-splitting applications.

Construction of type-II SnO2/InGaN nanorods heterostructure toward high photoelectrochemical performance

Exploring highly efficient and stable photoelectrode material is essential for high-performance photoelectrochemical (PEC) water-splitting applications. III-nitride semiconductors, particularly InGaN, have been considered as prospective materials for PEC hydrogen evolution. However, their surface states and other recombination centers, which enhance the charge recombination kinetics, are bottlenecks for the high PEC performance. In this work, we report the construction of type-II heterojunction by sputter depositing SnO2 on InGaN nanorods (NRs) to promote interfacial carrier transport and thereby enhance PEC performance. The energy band offsets at the SnO2/InGaN NRs interface were analyzed by x-ray photoelectron spectroscopy. Type-II heterojunction was defined at the SnO2/InGaN NRs interface with a valence band offset of 0.77 eV and conduction band offset of 0.25 eV. The photocurrent density of the SnO2/InGaN NRs photoanode is 7.09 mA/cm2 at 0.77 V vs Ag/AgCl electrode with 80 nm SnO2 thickness, which is ∼14-fold higher than that of the pristine InGaN NRs photoanode. Furthermore, the applied bias photo-to-current efficiency of SnO2/InGaN NRs photoanode records 3.36% at 0.77 V vs Ag/AgCl electrode. The enhanced PEC performance is mainly ascribed to the formation of high-quality SnO2/InGaN NRs heterojunction that enforces the directional charge transfer and substantially boosts the separation of photogenerated electron–hole pairs at the interface of InGaN NRs and SnO2. Overall, this work sheds light on the promising strategy to design and fabricate III-nitride nanostructures-based photoelectrodes for feasible PEC water-splitting applications.

Integrating Bi2MoO6 nanoflakes into TiO2 photoanode for efficient charge separation in photoelectrochemical water splitting

Achieving highly effective water splitting based on conventional TiO2 photoanode is challenging due to its poor visible light response and low electrical conductivity. Herein, a Bi2MoO6/TiO2 heterojunction with a suitable band structure is constructed to broaden light absorption and accelerate oxidation kinetics. In particular, the highest photocurrent density (0.35 mA cm−2 at 1.23 V vs. RHE) of Bi2MoO6/TiO2 is 2.1 times higher than that of pristine TiO2. The integrated Bi2MoO6 nanoflakes enrich more reactive sites and charge transfer tunnels. The surface hole injection efficiency and electron lifetime of Bi2MoO6/TiO2 are 1.4 and 5.5 times larger than those of bare TiO2, respectively. Additionally, the activation energy and charge transfer resistance of Bi2MoO6/TiO2 electrodes are decreased remarkably, dramatically promoting the separation of electron-hole pairs. This work offers a novel strategy for TiO2-based heterojunction with great potential for application in photoelectrochemical water splitting.

In-situ construction of 2D/1D Bi2O2S/Bi2S3 heterojunction from the topological transformation of Bi2O2S with significantly enhanced photoelectrochemical performance

Bi2O2S has gradually attracted the interest of researchers due to its excellent carrier mobility and light absorption property. However, its application in photoelectrochemical water splitting is very limited. In this work, hydrothermal treatment is performed on the obtained Bi2O2S film. Bi2S3 nanowires are in-situ evolved by the topological transformation of Bi2O2S, and heterojunction with atomic-level contact interface is constructed. The photocurrent density of Bi2O2S/Bi2S3 photoelectrode at 1.23 V vs. RHE is 3.82 mA/cm2, which is 14 times that of monomer Bi2O2S. The Bi2O2S/Bi2S3 heterojunction co-shared by Bi atoms has a close contact coupling interface, which stimulates stronger interfacial charge transfer. In addition, the 2D/1D morphology of the synthesized Bi2O2S nanosheets combined with Bi2S3 nanowires by fine tuning builds a channel for fast carrier transport. This study provides reference and inspiration for the preparation of highly active heterostructures, and expands new paths for the design and application of Bi2O2S based photoelectrodes.

Transferable Highly (001) Oriented Sb2Se3 Nanorod Films on Flexible Substrates for Innovative Optoelectronic Devices

AbstractThin films of antimony chalcogenides, Sb2X3 (X = S, Se, or SxSe1−x), comprised of (001) oriented nanorods are highly desirable for optoelectronic applications owing to their light trapping capacity and highly efficient charge transport along the optimally aligned quasi‐1D (Sb4X6)n ribbons. Here, highly (001) oriented Sb2Se3 nanorod (Sb2Se3NR) layers are obtained via a novel template growth method, and transfer these layers to functional substrates via a polymer‐assisted technique. Transferable layers overcome the current challenges associated with achieving oriented growth directly on the desired substrate. Using a combination of a low‐temperature processable SnO2 nanoparticle transport layer and encapsulation/etching techniques photodetector devices are fabricated with excellent electrical contact. The optimized transferred devices show broad range photoresponse with signal‐to‐noise ratios up to 105 and responsivities reaching 0.96 mA W−1. Importantly, access to low‐temperature processing enables the fabrication of flexible devices. Based on this platform, a proof‐of‐concept self‐powered flexible heart rate monitor is fabricated. The achievement of transferable Sb2Se3NR layers introduced in this work is expected to be readily applicable to generate new (001) Sb2Se3 device architectures that may otherwise not be achievable via direct deposition, with application in photoelectrochemical water splitting, battery electrodes, and solar cells.

Advances in MXene-based photoanodes for water-splitting

MXene, composed of two-dimensional transition metal carbides/nitrides, has excellent potential as photoanode material for photoelectrochemical water-splitting applications. Its exceptional physical and chemical properties, including high electrical conductivity, wide bandgap, and high surface area, make it a promising material for photoelectrochemical applications. In this regard, a comprehensive review of recent studies on fabrication strategies for MXene-based photoanodes and their performance in photoelectrochemical water-splitting is necessary. Thus, this review paper provides a comprehensive overview of MXene-based photoanodes for photoelectrochemical water-splitting, focusing on the fundamental properties of MXenes, including their structure, physical and chemical features, as well as synthesis strategies. The review also discusses recent progress in MXene-based photoanodes in photoelectrochemical water-splitting, highlighting their superior performance. Remarkably, coupling MXenes with other semiconductors such as titanium dioxide, bismuth vanadate, zinc oxide, ferric oxide, cadmium sulfide, and InGaN nanorods led to enhancement in light absorption efficiency, photocurrent density up to 7.27 x10-3 A cm−2, charge transfer and separation efficiency, and stability. Finally, the paper discusses future perspectives and potential applications of MXene-based photoanodes in photoelectrochemical water-splitting. This review provides valuable insights into the current state of knowledge and offers an outline for future research in this area.

Integration of Ag Plasmonic Metal and WO3/InGaN Heterostructure for Photoelectrochemical Water Splitting.

In this study, a Ag/WO3/InGaN hybrid heterostructure was successfully developed by sputtering and molecular beam epitaxy techniques, to obtain unique Ag nanospheres adorned with cauliflower-like WO3 nanostructure over the InGaN nanorods (NRs). Exploiting the localized surface plasmon resonance of Ag, the Ag/WO3/InGaN heterostructure exhibited superior photoabsorption ability in the visible region (400-700 nm) of the solar spectrum, with a surface plasmon resonance band centered around 440 nm. Comprehensive analysis through photoluminescence spectroscopy, photocurrent measurements, and electrochemical impedance spectroscopy revealed that the Ag/WO3/InGaN hybrid heterostructure significantly enhances the charge carrier separation and transfer kinetics leading to improved overall photoelectrochemical (PEC) performance. The photocurrent density of the Ag/WO3/InGaN photoanode is 1.17 mA/cm2, which is about 2.72 times higher than that of pure InGaN NRs under visible light irradiation. The photoanode exhibited excellent stability for about 12 h. From the study, it has been found that the maximum applied bias photon-to-current efficiency (ABPE) is ∼1.67% at the applied bias of 0.6 V. The improved PEC water splitting efficiency of the Ag/WO3/InGaN photoanode is attributed to the synergistic effects of localized surface plasmon resonance (LSPR), efficient charge carrier separation and transport, and the presence of a Schottky junction. Consequently, the plasmonic metal-assisted heterojunction-based semiconductor Ag/WO3/InGaN demonstrates immense potential for practical applications in photoelectrochemical water splitting.

Fabrication of Homogeneous Nanoporous Structure on 4H-/6H-SiC Wafer Surface via Efficient and Eco-Friendly Electrolytic Plasma-Assisted Chemical Etching.

Nanoporous single-crystal silicon carbide (SiC) is widely used in various applications such as protein dialysis, as a catalyst support, and in photoanodes for photoelectrochemical water splitting. However, the fabrication of nano-structured SiC is challenging owing to its extreme chemical and mechanical stability. This study demonstrates a highly-efficient, open-circuit electrolytic plasma-assisted chemical etching (EPACE) method without aggressive fluorine-containing reactants. The EPACE method enables the nano-structuring of SiC via a plasma-enveloped microtool traversing over the target material in an electrolyte bath. Through process design, EPACE readily produces a uniform nanoporous layer on a 4H-SiC wafer in KOH aqueous solution, with adjustable pore diameters in the range 40-130nm. Plasma diagnosis by optical emission spectrometry (OES) and surface microanalysis reveal that EPACE realizes a nanoporous structure by electrolytic plasma-assisted oxidation and subsequent thermochemical reduction of an oxide. An increase in voltage or a decrease in etch gap intensifies the plasma and improves the etching efficiency. The maximum etch rate and depth reach 540nm min-1 and 10µm, respectively, demonstrating the significant potential of the approach as a time-saving and sustainable nanofabrication method for industrial applications. Further, the effectiveness of the fabricated SiC nanoporous structure for application in photoelectrochemical water splitting is demonstrated.

Recent progress of graphitic carbon nitride films and their application in photoelectrochemical water splitting

This review summarizes the progress in the synthesis of CN electrodes spanning from top-down and bottom-up categories, carrier dynamics modulation, and surface water oxidation reaction kinetics.

Enhancing the activity and stability of Cu2O nanorods via coupling with a NaNbO3/SnS2 heterostructure for photoelectrochemical water-splitting

We synthesized a stable copper-based heterostructure catalyst, NaNbO3/SnS2/Cu2O for photoelectrochemical water-splitting applications with improved activity, stability, and inhibited photocorrosion in Cu2O.

Chemically grown Bi2S3 nanorod films for hydrogen evolution

Bi2S3 nanorod films were grown on ITO-coated glass substrates through chemical bath deposition (CBD) and annealing in a sulfur atmosphere. The as-deposited films were amorphous/nanocrystalline, with a particle size of 20 nm and a direct optical band gap of 1.87 eV. Upon annealing at 350 °C, the films exhibited a nanorod morphology with a length of 300 nm. Further increasing the temperature from 400 to 450 °C resulted in an increased diameter of nanorods. The direct optical band gap decreased from 1.68 to 1.47 eV upon increasing the annealing temperature from 350 to 400 °C. Photoelectrochemical (PEC) measurements showed that the nanorod films grown on ITO-coated glass substrates exhibited significantly increased PEC activity owing to their nanorod structures. The Bi2S3 nanorod films formed at 400 °C exhibited a maximum photocurrent density of 6.1 mA/cm2 at 1 V, which was 2.5 times higher than that of the as-deposited films. The enhancement in the photocurrent density could be due to the effective visible-light absorption of Bi2S3 nanorods as a result of the increased crystallinity and decreased band gap. This study demonstrates the synthesis route involving a simple and inexpensive CBD method of Bi2S3 nanorod films for the optimized PEC water-splitting applications.

Recent Progress on Semiconductor Heterojunction‐Based Photoanodes for Photoelectrochemical Water Splitting

For photoelectrochemical (PEC) water splitting, the utilization of semiconductor heterojunctions as building blocks for photoanodes allows for high light absorption, effective charge separation, and superior redox capability, greatly boosting the solar energy conversion efficiency. This review mainly focuses on the construction of heterojunction photoanodes, improvement strategies of carrier transmission, and their application in PEC water splitting. First, a series of carrier dynamics characterization methods are introduced to reveal the principle and significance of promoting carrier transport in heterojunctions. Then, from the perspective of the mechanism of promoting the separation and transport of charge carriers, several strategies are summarized and analyzed, including the micro/nanostructure, energy band structure, photothermal effect, piezoelectric effect, pyroelectric effect, ferroelectric effect, and intermediate layer. Finally, the challenges and opportunities for heterojunction photoanodes in PEC reactions are explained clearly, which points the way forward for the development of heterojunctions.

Architecture lattice‐matched cauliflower‐like CuO/ZnO p–n heterojunction toward efficient water splitting

AbstractBACKGROUNDIn photoelectrochemical (PEC) water splitting, hydrogen and oxygen can be dissociated from water molecules on semiconductors by directly utilizing clean renewable solar energy. The key bottleneck of PEC water splitting is to design new types of photoelectrodes with long‐term stability and high PEC performance.RESULTSHere, cauliflower‐shaped p–n CuO/ZnO heterojunction photocathodes with lattice matching were synthesized using one‐step electrodeposition and heat treatment. The results showed that the deposition of CuO and ZnO took place simultaneously. The proportion of CuO and ZnO varied with the electrodeposition time and calcination temperature and duration. The parallel lattice‐matched CuO/ZnO heterojunction enhanced the separation and migration of photogenerated charge carriers, which accomplished a photocurrent density value of approximately −1.8 mA cm−2 at a bias of 0 VRHE in 0.5 mol L−1 Na2SO4 solution.CONCLUSIONSThis work could provide a simplified strategy for the synthesis of p–n copper‐based heterogeneous photocathodes for application in PEC water splitting. © 2021 Society of Chemical Industry (SCI).