A Plasma‐Triggered O−S Bond and P−N Junction Near the Surface of a SnS 2 Nanosheet Array to Enable Efficient Solar Water Oxidation

Rapid Surface Reconstruction of In2S3 Photoanode via Flame Treatment for Enhanced Photoelectrochemical Performance

Surface reconstruction, reorganizing the surface atoms or structure, is a promising strategy to manipulate materials' electrical, electrochemical, and surface catalytic properties. Herein, a rapid surface reconstruction of indium sulfide (In2S3) is demonstrated via a high-temperature flame treatment to improve its charge collection properties. The flame process selectively transforms the In2S3 surface into a diffusionless In2O3 layer with high crystallinity. Additionally, it controllably generates bulk sulfur vacancies within a few seconds, leading to surface-reconstructed In2S3 (sr-In2S3). When using those sr-In2S3 as photoanode for photoelectrochemical water splitting devices, these dual functions of surface In2O3/bulk In2S3 reduce the charge recombination in the surface and bulk region, thus improving photocurrent density and stability. With optimized surface reconstruction, the sr-In2S3 photoanode demonstrates a significant photocurrent density of 8.5 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (RHE), marking a 2.5-fold increase compared to pristine In2S3 (3.5 mA cm-2). More importantly, the sr-In2S3 photoanode exhibits an impressive photocurrent density of 7.3 mA cm-2 at 0.6 V versus RHE for iodide oxidation reaction. A practical and scalable surface reconstruction is also showcased via flame treatment. This work provides new insights for surface reconstruction engineering in sulfide-based semiconductors, making a breakthrough in developing efficient solar-fuel energy devices.

Substitutional Cd Dopant as Photohole Transfer Mediator Boosting Photoelectrochemical Solar Energy Conversion of 2D Cd-ZnIn2 S4 Photoanode

Fast recombination dynamics of photocarriers competing with sluggish surface photohole oxidation kinetics severely restricts the photoelectrochemical (PEC) conversion efficiency of photoanode. Here, a defect engineering strategy is developed to regulate photohole transfer and interfacial injection dynamics of 2D ZnIn2 S4 (ZIS). Via selectively introducing substitutional Cd dopant at Zn sites of the ZIS basal plane, energy band structure and surface electrochemical activity are successfully modulated in the Cd-doped ZIS (Cd-ZIS) nanosheet array photoanode. Comprehensive characterizations manifest that a shallow acceptor level induced by Cd doping and superior electrochemical activity make surface Cd dopants simultaneously act as capture centers and active sites to mediate photohole dynamics at the reaction interface. In depth photocarrier dynamics analysis demonstrates that highly efficient photohole capture of Cd dopants brings about effective space separation of photocarriers and acceleration of surface reaction kinetics. Therefore, the optimum 2D Cd-ZIS achieves excellent PEC solar energy conversion efficiency with a photocurrent density of 5.1 mA cm-2 at 1.23 VRHE and a record of applied bias photon-to-current efficiency (ABPE) of 3.0%. This work sheds light on a microstructure design strategy to effectively regulate photohole dynamics for the next-generation semiconducting PEC photoanodes.

Dual Heterojunctions and Nanobowl Morphology Engineered BiVO4 Photoanodes for Enhanced Solar Water Splitting

Photoelectrochemical (PEC) water splitting represents an attractive strategy to realize the conversion from solar energy to hydrogen energy, but severe charge recombination in photoanodes significantly limits the conversion efficiency. Herein, a unique BiVO4 (BVO) nanobowl (NB) heterojunction photoanode, which consists of [001]-oriented BiOCl underlayer and BVO nanobowls containing embedded BiOCl nanocrystals, is fabricated by nanosphere lithography followed by in situ transformation. Experimental characterizations and theoretical simulation prove that nanobowl morphology can effectively enhance light absorption while reducing carrier diffusion path. Density functional theory (DFT) calculations show the tendency of electron transfer from BVO to BiOCl. The [001]-oriented BiOCl underlayer forms a compact type II heterojunction with the BVO, favoring electron transfer from BVO through BiOCl to the substrate. Furthermore, the embedded BiOCl nanoparticles form a bulk heterojunction to facilitate bulk electron transfer. Consequently, the dual heterojunctions engineered BVO/BiOCl NB photoanode exhibits attractive PEC performance toward water oxidation with an excellent bulk charge separation efficiency of 95.5%, and a remarkable photocurrent density of 3.38 mA cm-2 at 1.23 V versus reversible hydrogen electrode, a fourfold enhancement compared to the flat BVO counterpart. This work highlights the great potential of integrating dual heterojunctions engineering and morphology engineering in fabricating high-performance photoelectrodes toward efficient solar conversion.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

Extending conducting channels in Fe-N-C by interfacial growth of CNTs with minimal metal loss for efficient ORR electrocatalysis

Achieving a high electrocatalytic performance using a completely metal-free electrocatalyst, preferably based on only carbonaceous materials, remains a challenge. Alternatively, an efficient composite of a carbon nanostructure and a non-noble metal with minimum dependence on a metal holds immense potential. Although single-atom catalysis brings superior performance, its complex synthetic strategy limits its large-scale implementation. Previous investigation has shown that atomic dispersion (Fe-Nx-C) is accompanied by higher metal-loss compared to nanoparticle formation (Fe-NPs-N-C). Therefore, to achieve minimum metal loss, we first incorporated iron nanoparticles (Fe NPs) to N-doped carbon (N-C) and then exposed them to a cheap carbon source, melamine at high temperature, resulting in the growth of carbon nanotubes (CNTs) catalysed by those Fe NPs loaded on N-C (Fe-NPs-N-C). Thermogravimetric analysis showed that the metal-retention in the composite is higher than that in the bare carbon nanotube and even the atomically dispersed Fe-active sites on N-C. The composite material (Fe-NPs-N-C/CNT) shows a high half-wave potential (0.89 V vs. RHE) which is superior to that of commercial Pt/C towards the oxygen reduction reaction (ORR). The enhanced activity is attributed to the synergistic effect of high conductivity of CNTs and active Fe-sites as the composite exceeds the individual electrocatalytic performance shown by Fe-CNTs & Fe-NPs-N-C, and even that of atomically dispersed Fe-active sites on N-C.

Porphyrins-Assisted Cocatalyst Engineering with CoOV Bond in BiVO4 Photoanode for Efficient Oxygen Evolution Reaction

The application of photoelectrochemical (PEC) water splitting is limited by the sluggish surface oxygen evolution reaction (OER) kinetics. OER kinetics can be effectively improved through cocatalyst engineering. However, the tardy transfer process and serious recombination of carriers are the key factors restricting the cocatalyst development. Taking BiVO4 as an example, a Co-modified heme film rich in large conjugated ring structures is introduced onto the photoanode surface using a solvothermal method. This film functions as an efficient cocatalyst. It considerably reduces the surface overpotential, promotes the transfer of photogenerated holes, and boosts the kinetics of OER by specifically affecting the formation of OOH*. Simultaneously, the formed CoOV bonds induce strong interaction at the photoanode/cocatalyst interfaces, reducing the recombination of photogenerated carriers. Consequently, the onset potential of the optimized photoanode decreases from 0.45 to 0.07 V and the photocurrent density at 1.23 V versus reversible hydrogen electrode boosts to 5.3 mA cm-2 . This work demonstrates a facile strategy for designing cocatalysts to obtain rapid hole transfer capability and reduced carrier recombination for improved PEC performance.

Constructing NCuS Interface Chemical Bonds over SnS2 for Efficient Solar-Driven Photoelectrochemical Water Splitting

The restricted charge transfer and slow oxygen evolution reaction (OER) dynamics tremendously hamper the realistic implementation of SnS₂ photoanodes for photoelectrochemical (PEC) water splitting. Here, a novel strategy is developed to construct interfacial NCuS bonds between NC skeletons and SnS₂ (CuNC@SnS₂ ) for efficient PEC water splitting. Compared with SnS₂ , the PEC activity of CuNC@SnS₂ photoelectrode is tremendously heightened, obtaining a current density of 3.40 mA cm² at 1.23 V_RHE with a negatively shifted onset potential of 0.04 V_RHE , which is 6.54 times higher than that of SnS₂ . The detailed experimental characterizations and theoretical calculation demonstrate that the interfacial NCuS bonds enhance the OER kinetic, reduce the surface overpotential, facilitate the separation of photon-generated carriers, and provide a fast transmission channel for electrons. This work presents a new approach for modulating charge transfer by interfacial bond design in heterojunction photoelectrodes toward promoting PEC performance and solar energy application.

Interfacial Bi-S bonds modulate band alignment for efficient solar water oxidation

Introducing suitable interfacial chemical bonds into heterojunctions can increase the charge carrier density, propel the charge separation, and facilitate interfacial charge extraction in photoanodes for photoelectrochemical (PEC) water oxidation. However, tuning chemical bonds at heterojunction interfaces and elucidating their influences on band alignment and the associated evolution of PEC performance remain elusive. Herein, Bi-S bonds were introduced into the interface of a CdIn₂S₄ (CIS)/Bi₂WO₆ (BWO) heterojunction. In situ irradiated X-ray photoelectron spectroscopy and electron spin resonance signals confirm that the Bi-S bond transforms the band alignment from type II to the direct Z-scheme, significantly enhancing the carrier separation efficiency. Theoretical calculations show that the Bi-S bond not only acts as an atomic-level charge transfer channel, but also changes the migration pathway and distance within the heterojunction. As a result, the optimized CIS/BWO photoanode exhibits a relatively high PEC performance of 4.25 mA cm^-2 at 1.23 V vs. RHE (V_RHE) and a low onset potential of 0.30 V_RHE. This work presents a new avenue to construct comprehensively improved photoanodes by tuning the interfacial structures at the atomic level.

Porous silver microrods by plasma vulcanization activation for enhanced electrocatalytic carbon dioxide reduction

Metal electrode is considered as an ideal candidate for electrocatalytic carbon dioxide (CO₂) reduction considering its excellent chemical stability, application potential and eco-friendly properties. Optimization process such as morphological control, non-metallic doping, alloying is widely studied to improve the efficiency of metal electrodes. In this work, we successfully improved the CO₂ reduction performance of silver using a facile plasma vulcanization treatment. The obtained sulfide derived silver (Ag) porous microrods (SD-AgPMRs) are optimized from both morphology and composition aspects, and demonstrates high Faradaic efficiency and partial current density for carbon monoxide (CO) production at low potentials. The larger specific surface area of porous microrod structure and the improved adsorption energy of important intermediates in comparison with Ag foil are realized by introduction of sulfur (S) atoms after plasma vulcanization activation, as suggested by density functional theory (DFT) calculations. This work presents a novel strategy to optimize metal electrocatalysts for CO₂ reduction as well as to improve catalysis in other fields.

Hydrophilic polypyrrole and g-C3N4 co-decorated ZnO nanorod arrays for stable and efficient photoelectrochemical water splitting

Hydrophilic polypyrrole and g-C3N4 co-decorated ZnO nanorod arrays were synthesized for stable and efficient photoelectrochemical water splitting.

Plasma-Induced Defect Engineering and Cation Refilling of NiMoO4 Parallel Arrays for Overall Water Splitting

Developing highly active water splitting electrocatalysts with ordered micro/nanostructures and uniformly distributed active sites can meet the increasing requirement for sustainable energy storage/utilization technologies. However, the stability of complicated structures and active sites during a long-term process is also a challenge. Herein, we fabricate a novel approach to create sufficient atomic defects via N₂ plasma treatment onto parallel aligned NiMoO₄ nanosheets, followed by refilling of these defects via heterocation dopants and stabilizing them by annealing. The parallel aligned nanosheet arrays with an open structure and quasi-two-dimensional long-range diffusion channels can accelerate the mass transfer at the electrolyte/gas interface, while the incorporation of Fe/Pt atoms into defect sites can modulate the local electronic environment and facilitate the adsorption/reaction kinetics. The optimized Pt-NP-NMC/CC-5 and Fe-NP-NMC/CC-10 electrodes exhibit low overpotentials of 71 and 241 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, and the assembled device reveals a low voltage of 1.55 V for overall water splitting. This plasma-induced high-efficiency defect engineering and coupled active site stabilization strategy can be extended to large-scale fabrication of high-end electrocatalysts.

Heterostructures between a tin-based intermetallic compound and a layered semiconductor for gas sensing

SnS2 nanoplates are used as sacrificial templates to facilitate the in situ growth of intermetallic compound Pt3Sn nanoparticles. The Pt3Sn/SnS2 heterostructures show promise for selective NO2 sensing due to the favored gas adsorption and gas-solid charge transfer on Pt3Sn, combined with the optimized film conductance and formation of ohmic-type Pt3Sn/SnS2 heterointerfaces.

Atomic Sandwiched p-n Homojunctions

Semiconductor p-n junctions have been explored and applied in photoelectrochemical (PEC) water splitting, but serious carrier recombination and sluggish oxygen evolution reaction (OER) dynamics have demanded further progress in p-n junction photoelectrode design. Here, via a controllable NH₃ treatment, we construct sandwiched p-n homojunctions in three-unit-cells n-type SnS₂ (n-SnS₂ ) nanosheet arrays using nitrogen (N) as acceptor dopants. The optimal N-doped n-SnS₂ (pnp-SnS₂ ) with such unique structure achieves a record photocurrent density of 3.28 mA cm^-2 , which is 21 times as high as that of n-SnS₂ and the highest value among all the SnS₂ photoanodes reported so far. Moreover, the stoichiometric O₂ and H₂ evolution from water was achieved with Faradaic efficiencies close to 100 %. The superior performance could be attributed to the facilitated electron-hole separation/transfer, accelerated surface OER kinetics, prolonged carrier lifetime, and improved structural stability.

Atomically thin photoanode of InSe/graphene heterostructure

Achieving high-efficiency photoelectrochemical water splitting requires a better understanding of ion kinetics, e.g., diffusion, adsorption and reactions, near the photoelectrode's surface. However, with macroscopic three-dimensional electrodes, it is often difficult to disentangle the contributions of surface effects to the total photocurrent from that of various factors in the bulk. Here, we report a photoanode made from a InSe crystal monolayer that is encapsulated with monolayer graphene to ensure high stability. We choose InSe among other photoresponsive two-dimensional (2D) materials because of its unique properties of high mobility and strongly suppressing electron-hole pair recombination. Using the atomically thin electrodes, we obtained a photocurrent with a density >10 mA cm^-2 at 1.23 V versus reversible hydrogen electrode, which is several orders of magnitude greater than other 2D photoelectrodes. In addition to the outstanding characteristics of InSe, we attribute the enhanced photocurrent to the strong coupling between the hydroxide ions and photo-generated holes near the anode surface. As a result, a persistent current even after illumination ceased was also observed due to the presence of ions trapped holes with suppressed electron-hole recombination. Our results provide atomically thin materials as a platform for investigating ion kinetics at the electrode surface and shed light on developing next-generation photoelectrodes with high efficiency.

Toward Understanding the Enhanced Pseudocapacitive Storage in 3D SnS/MXene Architectures Enabled by Engineered Surface Reactions

The optimization of three-dimensional (3D) MXene-based electrodes with desired electrochemical performances is highly demanded. Here, a precursor-guided strategy is reported for fabricating the 3D SnS/MXene architecture with tiny SnS nanocrystals (≈5 nm in size) covalently decorated on the wrinkled Ti₃ C₂ T_x nanosheets through Ti-S bonds (denoted as SnS/Ti₃ C₂ T_x -O). The formation of Ti-S bonds between SnS and Ti₃ C₂ T_x was confirmed by extended X-ray absorption fine structure (EXAFS). Rather than bulky SnS plates decorated on Ti₃ C₂ T_x (SnS/Ti₃ C₂ T_x -H) by one-step hydrothermal sulfidation followed by post annealing, this SnS/Ti₃ C₂ T_x -O presents size-dependent structural and dynamic properties. The as-formed 3D hierarchical structure can provide short ion-diffusion pathways and electron transport distances because of the more accessible surface sites. In addition, benefiting from the tiny SnS nanocrystals that can effectively improve Na⁺ diffusion and suppress structural variation upon charge/discharge processes, the as-obtained SnS/Ti₃ C₂ T_x -O can generate pseudocapacitance-dominated storage behavior enabled by engineered surface reactions. As predicted, this electrode exhibits an enhanced Na storage capacity of 565 mAh g^-1 at 0.1 A g^-1 after 75 cycles, outperforming SnS/Ti₃ C₂ T_x -H (336 mAh g^-1 ), SnS (212 mAh g^-1 ), and Ti₃ C₂ T_x (104 mAh g^-1 ) electrodes.