Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode

Engineering the microenvironment of electron transport layers with nickle single-atom sites for boosting photoelectrochemical performance

Advances in the rational design of semiconductor-electrocatalyst photoelectrodes provide robust driving forces for improving energy conversion and quantitative analysis, while a deep understanding of elementary processes remains underwhelming due to the multistage interfaces involved in semiconductor/electrocatalyst/electrolyte. To address this bottleneck, we have constructed carbon-supported nickel single atoms (Ni SA@C) as an original electron transport layer with catalytic sites of Ni-N4 and Ni-N2O2. This approach illustrates the combined effect of photogenerated electron extraction and the surface electron escape ability of the electrocatalyst layer in the photocathode system. Theoretical and experimental studies reveal that Ni-N4@C, with excellent oxygen reduction reaction catalytic activity, is more beneficial for alleviating surface charge accumulation and facilitating electrode-electrolyte interfacial electron-injection efficiency under a similar built-in electric field. This instructive method enables us to engineer the microenvironment of the charge transport layer for steering the interfacial charge extract and reaction kinetics, providing a great prospect for atomic scale materials to enhance photoelectrochemical performance.

Manipulating the surface states of BiVO4 through electrochemical reduction for enhanced PEC water oxidation

Bismuth vanadate (BiVO₄) is a prospective candidate for photoelectrochemical (PEC) water oxidation, but its commercial application is limited due to the serious surface charge recombination. In this work, we propose a novel and effective electrochemical reduction strategy combined with co-catalyst modification to manipulate the surface states of the BiVO₄ photoanode. Specifically, an ultrathin amorphous structure is formed on the surface of BiVO₄ after electrochemical reduction ascribed to the breaking of the surface metal-O bonds. Photoelectrochemical measurements and first-principles calculation show that the electrochemical reduction treatment can effectively reduce the surface energy, thereby passivating the recombined surface states (r-ss) and increasing the mobility of photogenerated holes. In addition, the FeOOH co-catalyst layer further increases the intermediate surface states (i-ss) of BiVO₄, stabilizes the surface structure and enhances its PEC performance. Benefiting from the superior charge transfer efficiency and the excellent water oxidation kinetics, the -0.8/BVO/Fe photoanode achieves 2.02 mA cm^-2 photocurrent at 1.23 V_RHE (2.4 times that of the original BiVO₄); meanwhile, the onset potential shifts 90 mV to the cathode. These results provide a new surface engineering tactic to modify the surface states of semiconductor photoanodes for high-efficiency PEC water oxidation.

Hydrothermal Synthesized CoS2 as Efficient Co-Catalyst to Improve the Interfacial Charge Transfer Efficiency in BiVO4

The bare surface of BiVO4 photoanode usually suffers from extremely low interfacial charge transfer efficiency which leads to a significantly suppressed photoelectrochemical water splitting performance. Various strategies, including surface modification and the loading of co-catalysts, facilitate the interface charge transfer process in BiVO4. In this study, we demonstrate that CoS2 synthesized from the hydrothermal method can be used as a high-efficient co-catalyst to sufficiently improve the interface charge transfer efficiency in BiVO4. The photoelectrochemical water splitting performance of BiVO4 was significantly improved after CoS2 surface modification. The BiVO4/CoS2 photoanode achieved an excellent photocurrent density of 5.2 mA/cm2 at 1.23 V versus RHE under AM 1.5 G illumination, corresponding to a 3.7 times enhancement in photocurrent compared with bare BiVO4. The onset potential of the BiVO4/CoS2 photoanode was also negatively shifted by 210 mV. The followed systematic combined optical and electrochemical characterization results reveal that the interfacial charge transfer efficiency of BiVO4 was largely improved from less than 20% to more than 70% due tor CoS2 surface modification. The further surface carrier dynamics study performed using an intensity modulated photocurrent spectroscopy displayed a 6–10 times suppression in surface recombination rate constants for CoS2 modified BiVO4, which suggests that the key reason for the improved interfacial charge transfer efficiency possibly originates from the passivated surface states due to the coating of CoS2.

Observation of 4th-order water oxidation kinetics by time-resolved photovoltage spectroscopy

Artificial photo-driven water oxidation has been proposed over half a century through a four-charge involved multiple-step oxygen evolution process. However, the knowledge of the intrinsic activity, such as the rate-law of the water oxidation reactions, has been inadequately studied. Up to date, the highest order reported is the third one under photoelectrochemical condition. In this work, we identified the fourth-order charge decay reactions on hematite by using a time-resolved surface photovoltage probe technique. A theoretical turnover frequency (TOF) > 100 nm^-2·s^-1 can be expected for O₂ molecules when the hole density >0.1 nm^-2. This work demonstrates a facile and robust method to investigate the high-order reaction kinetics. More excitingly, this research built the bridge between the rate-law, rate-determining step, and energy barrier of intermediates.

Highly efficient hamburger-like nanostructure of a triadic Ag/Co3O4/BiVO4 photoanode for enhanced photoelectrochemical water oxidation

The combination of a semiconductor heterojunction and oxygen evolution cocatalyst (OEC) is an important strategy to improve photoelectrochemical (PEC) water oxidation. Herein, a novel hamburger-like nanostructure of a triadic photoanode composed of BiVO4 nanobulks, Co3O4 nanosheets and Ag nanoparticles (NPs), that is, Ag/Co3O4/BiVO4, was designed. In our study, an interlaced 2D ultrathin p-type Co3O4 OEC layer was introduced onto n-type BiVO4 to form a p-n Co3O4/BiVO4 heterojunction with an internal electric field (IEF) in order to facilitate charge transport. Then the modification with Ag NPs can significantly facilitate the separation and transport of photogenerated carriers through the surface plasma resonance (SPR) effect, inhibiting the electron-hole recombination. The resulting Ag/Co3O4/BiVO4 photoanodes exhibit largely enhanced PEC water oxidation performance: the photocurrent density of the ternary photoanode reaches up to 1.84 mA cm-2 at 1.23 V vs. RHE, which is 4.60 times higher than that of the pristine BiVO4 photoanode. The IPCE value is 2.83 times higher than that of the pristine BiVO4 at 400 nm and the onset potential has a significant cathodic shift of 550 mV for the ternary well-constructed photoanode.

Role of Alkali Metal in BiVO4 Crystal Structure for Enhancing Charge Separation and Diffusion Length for Photoelectrochemical Water Splitting

Alkali metal (Na or K) doping in BiVO₄ was examined systematically for enhancing bulk charge separation and transport in addition to improving charge transfer from the surface. The alkali metal-doped BiVO₄ thin film photoanodes having nanostructured porous grain surface morphology exhibited better photocurrent density than pristine BiVO₄. In particular, Na:BiVO₄/Fe:Ni/Co-Pi photoanode showed a significantly improved photocurrent of 3.2 ± 0.15 mA·cm^-2 in 0.1 M K₂HPO₄ electrolyte at 1.23 V_RHE under 1 sun illumination. The depth-dependent Doppler broadening spectroscopy measurements confirmed the significant reduction in Bi- and V-based defect density with Na metal doping, and this led to a higher bulk diffusion length of charge pairs (four times that of the pristine one). Na doping led to reduced surface defects resulting in improved surface charge transfer based on cyclic voltammetry experiments. The density functional theory calculations confirmed the improved performance in Na-doped BiVO₄ photoanodes achieved through interband formation and reduction in the band gap.

Multilevel Bipolar Electroforming-Free Resistive Switching Memory Based on Silicon Oxynitride

Resistive random-access memory (RRAM) devices are fabricated by utilizing silicon oxynitride (SiOxNy) thin film as a resistive switching layer. A SiOxNy layer is deposited on a p+-Si substrate and capped with a top electrode consisting of Au/Ni. The SiOxNy-based memory device demonstrates bipolar multilevel operation. It can switch interchangeably between all resistance states, including direct SET switching from a high-resistance state (HRS) to an intermediate-resistance state (IRS) or low-resistance state (LRS), direct RESET switching process from LRS to IRS or HRS, and SET/RESET switching from IRS to LRS or HRS by controlling the magnitude of the applied write voltage signal. The device also shows electroforming-free ternary nonvolatile resistive switching characteristics having RHRS/RIRS > 10, RIRS/RLRS > 5, RHRS/RLRS > 103, and retention over 1.8 × 104 s. The resistive switching mechanism in the devices is found to be combinatory processes of hopping conduction by charge trapping/detrapping in the bulk SiOxNy layer and filamentary switching mode at the interface between the SiOxNy and Ni layers.

Electrodeposited Co-Substituted LaFeO3 for Enhancing the Photoelectrochemical Activity of BiVO4

Co-substituted LaFeO₃ was electrodeposited on the surface of BiVO₄ as a co-catalyst to enhance the water splitting performance. Compared to bare BiVO₄, the BiVO₄/Co-LaFeO₃ composite photoanode shows a water oxidation photocurrent of 3.4 mA/cm² at 1.23 V versus reverse hydrogen electrode, accompanied by a notable cathodic shift in the onset potential for 300 mV. Combined optical and electrochemical characterizations show that the solid/electrolyte charge transfer efficiency of BiVO₄ are dramatically improved by the incorporation of Co-substituted LaFeO₃. From the surface kinetic study of charge carriers by intensity-modulated photocurrent spectroscopy, a suppressed surface recombination rate constant is observed and the enhanced photoelectrochemical water splitting performance observed in the BiVO₄/Co-LaFeO₃ photoanode is attributed to the surface passivation effect of Co-substituted LaFeO₃.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Hydrogen (H2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

Reversible Charge Transfer and Adjustable Potential Window in Semiconductor/Faradaic Layer/Liquid Junctions

Semiconductor/Faradaic layer/liquid junctions have been widely used in solar energy conversion and storage devices. However, the charge transfer mechanism of these junctions is still unclear, which leads to inconsistent results and low performance of these devices in previous studies. Herein, by using Fe₂O₃ and Ni(OH)₂ as models, we precisely control the interface structure between the semiconductor and the Faradaic layer and investigate the charge transfer mechanism in the semiconductor/Faradaic layer/liquid junction. The results suggest that the short circuit severely restricts the performance of the junction for both solar water splitting cells and solar charging supercapacitors. More importantly, we also find that the charge-discharge potential window of a Faradaic material sensitively depends on the energy band positions of a semiconductor, which provides a new way to adjust the potential window of a Faradaic material. These new insights offer guidance to design high-performance devices for solar energy conversion and storage.