Unveiling the Influence of Sulfur Doping on Photoelectrochemical Performance in BiVO4/FeOOH Heterostructures.

BiVO4 is a promising photoanode for photoelectrochemical (PEC) water splitting but suffers from high charge carrier recombination and sluggish surface water oxidation kinetics that limit its efficiency. In this work, a model of sulfur-incorporated FeOOH cocatalyst-loaded BiVO4 was constructed. The composite photoanode (BiVO4/S-FeOOH) demonstrates an enhanced photocurrent density of 3.58 mA cm-2, which is 3.7 times higher than that of the pristine BiVO4 photoanode. However, the current explanations for the generation of enhanced photocurrent signals through the incorporation of elements and cocatalyst loading remain unclear and require further in-depth research. In this work, the hole transfer kinetics were investigated by using a scanning photoelectrochemical microscope (SPECM). The results suggest that the incorporation of sulfur can effectively improve the charge transfer capacity of FeOOH. Moreover, the oxygen evolution reaction model provides evidence that S-doping can induce a "fast" surface catalytic reaction at the cocatalyst/solution interface. The work not only presents a promising approach for designing a highly efficient photoanode but also offers valuable insights into the role of element doping in the PEC water-splitting system.

Insights into the Enhanced Photoelectrochemical Performance through Construction of the Z-Scheme and Type II Heterojunctions.

Photoelectrochemical (PEC) water splitting technology is a promising strategy toward producing sustainable hydrogen fuel. However, it is an essential bottleneck to reduce severe charge recombination for the improvement of PEC performance. Construction of heterojunction systems, such as Z-scheme and type II heterojunctions, could efficiently boost charge separation, whereas the mechanism of charge separation is still ambiguous. We describe herein a charge transfer system designed with Bi2WO6/Bi2S3 (BWO/BS) as a prototype. In this system, Au nanoparticles act as charge relays to engineer a charge transfer pathway, and the obtained BWO/Au/BS photoanode achieves a remarkable photocurrent density of 0.094 mA cm-2 at 1.23 V versus reversible hydrogen electrode (vs RHE), over approximately 1.2 and 2.3 times larger than those of BWO/BS/Au and BWO, exhibiting long-term photostability. More importantly, scanning photoelectrochemical microscopy (SPECM) and intensity-modulated photocurrent spectroscopy (IMPS) studies are performed to in situ-capture the photogenerated hole during the PEC process. Operando analysis reveals that the Z-scheme BWO/Au/BS system (1.33 × 10-2 cm s-1) exhibits higher charge transfer kinetics compared to the type II BWO/BS/Au heterostructure (0.85 × 10-2 cm s-1) while efficiently suppressing charge recombination for optimized PEC activity. Note that this smart strategy can also be extended to other semiconductor-based photoanodes such as BiVO4. Our study offers an effective pathway for the rational design of highly efficient charge separation for solar conversion based on water splitting.

Significantly Promoting the Photogenerated Charge Separation by Introducing an Oxygen Vacancy Regulation Strategy on the FeNiOOH Co-Catalyst.

Semiconductor/co-catalyst coupling is considered as a promising strategy to enhance the photoelectrochemical (PEC) conversion efficiency. Unfortunately, this model system is faced with a serious interface recombination problem, which limits the further improvement of PEC performances. Here, a FeNiOOH co-catalyst with abundant oxygen vacancies on BiVO4 is fabricated through simple and economical NaBH4 reduction to accelerate hole transfer and achieve efficient electron-hole pair separation. The photocurrent of the BV (BiVO4 )/Vo-FeNiOOH system is more than four times that of pure BV. Importantly, the charge transfer kinetics and charge carrier recombination process are studied by scanning photoelectrochemical microscopy and intensity modulated photocurrent spectroscopy in detail. In addition, the oxygen vacancy regulation proposed is also applied successfully to other semiconductors (Fe2 O3 ), demonstrating the applicability of this strategy.

Interfacial repairing of semiconductor-electrocatalyst interfaces for efficient photoelectrochemical water oxidation.

Photoelectrochemical (PEC) water splitting is an attractive strategy to convert and store of intermittent solar power into fuel energy. However, the detrimental charge recombination of photogenerated electrons and holes severely limits its efficiency. Despite electrocatalyst loading can obviously improve the PEC conversion efficiency, current systems still suffer from high recombination owing to the surface states. Herein, an interface "repairing" strategy is proposed to suppress the recombination at the semiconductor/electrocatalyst interface. NiOx layer acts as an interfacial repairing layer to efficiently extract photogenerated charge carriers and eliminate the surface states via high hole-transfer kinetics rather than as a traditional electrocatalyst. As expected, the resulting repaired system yields an impressive photocurrent density of 4.58 mA cm-2 at 1.23 V (vs. RHE), corresponding to a more than three-fold increase compared to BiVO4 (1.40 mA cm-2). Our work offers an appealing maneuver to improve the water oxidation performance for the semiconductor/electrocatalyst coupling system.

Enhanced Photoelectrochemical Water Splitting on Nickel-Doped Cobalt Phosphate by Modulating both Charge Transfer and Oxygen Evolution Efficiencies.

Detrimental charge recombination at photoanode/electrolyte junctions severely impedes photoelectrochemical (PEC) performance. The deposition of cobalt phosphate (CoPi) onto photoanodes is an efficient approach to achieve high PEC efficiency. However, achieving performances at the required remains a huge challenge, owing to the passivation effect of CoPi. In this study, function-tunable strategy, whereby the passivation role is switched with the activation role, is exploited to modulate PEC performance through simultaneous activation of interface charge transfer and surface catalysis. By depositing nickel-doped CoPi onto a BiVO4 (BV) substrate, the integrated system (BV/Ni1 Co7 Pi) exhibits a remarkable photocurrent density (4.15 mA cm-2 ), which is a 4.6-fold increase relative to BV (0.90 mA cm-2 ). Moreover, the satisfactory performance can be also achieved on α-Fe2 O3 photoanode. These findings provide guidance for improving the efficiency of CoPi on photoanodes for PEC water oxidation.

Insight into interface charge regulation through the change of the electrolyte temperature toward enhancing photoelectrochemical water oxidation.

The desired photoelectrochemical performance can be achieved by temperature regulation, but the nature for this improvement remains a controversial topic. Herein, we employed BiVO4/CoOx as a typical model system, and explored the fate of photogenerated holes at the different interfaces among BiVO4/CoOx/electrolyte by means of intensity modulated photocurrent spectroscopy (IMPS), scanning photoelectrochemical microscopy (SPECM) and traditional electrocatalysis characterization methods. Systematic quantitative analysis of the kinetics of photogenerated holes transfer at the BiVO4/CoOx interface under illumination and surface water oxidation at the CoOx/electrolyte interface in the dark indicates that increasing temperature could not only enhance the surface catalytic reaction kinetics but also facilitate the interfacial charge transfer. As expected, the integrated system exhibited a remarkable photocurrent density of 3.6 mA cm-2 (1.23 VRHE, AM 1.5G, 45 °C), which is approximately 2.1 times higher than that of BiVO4/CoOx (15 °C). This work provides a promising strategy for achieving efficient photoelectrochemical water splitting.