Suppressing photoinduced charge recombination at the BiVO4||NiOOH junction by sandwiching an oxygen vacancy layer for efficient photoelectrochemical water oxidation

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Abstract

Nickel oxyhydroxide (NiOOH) is regarded as one of the promising cocatalysts to enhance the catalytic performance of photoanodes but suffers from serious interfacial charge-carrier recombination at the photoanode||NiOOH interface. In this work, surface-engineered BiVO4 photoanodes are fabricated by sandwiching an oxygen vacancy (Ovac) interlayer between BiVO4 and NiOOH. The surface Ovac interlayer is introduced on BiVO4 by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced Ovac can alleviate the interfacial charge-carrier recombination at the BiVO4||NiOOH junction, resulting in efficient charge separation and transfer efficiencies, while an outer NiOOH layer is coated to prevent the Ovac layer from degradation. As a result, the as-prepared NiOOH-P-BiVO4 photoanode exhibits a high photocurrent density of 3.2 mA cm−2 at 1.23 V vs. RHE under the irradiation of 100 mW/cm2 AM 1.5G simulated sunlight, in comparison to those of bare BiVO4, P-BiVO4, and NiOOH-BiVO4 photoanodes (1.1, 2.1 and 2.3 mA cm−2, respectively). In addition to the superior photoactivity, the 5-h amperometric measurements illustrate improved stability of the surface-engineered NiOOH-P-BiVO4 photoanode. Our work showcases the feasibility of combining cocatalysts with Ovac, for improved photoactivity and stability of photoelectrodes.

Introduction

Bismuth vanadate (BiVO4) is one of the potential candidates for photoelectrochemical (PEC) water splitting to produce hydrogen fuel in the future. The photoanode is endowed with a suitable bandgap structure (the bandgap of 2.4 eV and conduction band edge of 0.02 V vs. RHE for hydrogen production) and moderate charge carrier lifetime [1]. Despite this, unacceptable charge carrier recombination and poor water oxidation kinetics have significantly impaired the PEC performance of this photoanode [2]. To tackle the issue, versatile strategies including doping engineering [3], morphology engineering [4], heterostructure engineering [5], and surface modification of oxygen evolution catalysts (OECs) [6], have been employed to promote the PEC performance of BiVO4 photoanodes.

In general, the overlayer of OECs has been recognized as a useful strategy to accelerate water oxidation reaction and alleviate bulk electron-hole pair recombination. Numerous OECs such as metal phosphates (e.g., CoPi) [7], metal phosphides (CoP) [8], metal oxides (IrO2 and CoOx) [9], and metal (oxy)hydroxides (Ni(OH)2 and NiOOH) [9], [10], have been designed for improving the photoactivity of BiVO4 photoelectrodes. Among these catalysts, metal oxyhydroxides such as iron oxyhydroxide (FeOOH) and nickel oxyhydroxide (NiOOH) have gained much attention because of their prominent catalytic activity, earth abundance, and environmental friendliness [11]. Notably, the modification of NiOOH as an OEC on BiVO4 photoanodes enables a lower onset potential in comparison to FeOOH, which indicates an exceedingly active water oxidation process [6a]. Despite these benefits, considerable interfacial recombination of electron-hole pairs at the BiVO4||NiOOH interface has significantly jeopardized its photoactivity [6a], which raises concerns about applying NiOOH for practical uses.

To eliminate interfacial charge carrier recombination, introducing an oxygen vacancies (Ovac) layer between the BiVO4 photoanode and the NiOOH overlayer may be one of the effective strategies [12]. This is because Ovac has the opportunity to modify the electronic structure and create inter-band states in the forbidden band of BiVO4, resulting in both enhanced conductivity and promoted charge separation [12], [13]. In the meantime, the overlaid cocatalysts may also serve as a protective layer, preventing the degradation of the Ovac layer which is not stable in a highly oxidizing environment [12]. In view of these aspects, we propose that interfacial charge recombination may be suppressed with a rational sandwiched Ovac layer between BiVO4 photoanodes and NiOOH cocatalysts.

Herein, an interfacial Ovac layer has been introduced to BiVO4 photoanodes by a chemical reduction treatment using a mild reducing agent, sodium hypophosphite. The induced Ovac interlayer can decrease interfacial charge carrier recombination, as indicated by the charge separation and transfer efficiencies, while the outer NiOOH layer as active sites for the oxidation of water is loaded to simultaneously prevent the Ovac interlayer from degradation. As a result of the alleviation of the interfacial carrier recombination and the acceleration of water oxidation reaction, the PEC performance of BiVO4 photoanodes can be considerably enhanced. In this work, we provide an effective and facile strategy to coordinate OECs and Ovac, leading to an improved photoactivity of metal oxide semiconductors.

Section snippets

Fabrication of BiVO4, P-BiVO4, NiOOH-BiVO4 and NiOOH-P-BiVO4 films

Bare BiVO4 films on FTO glass (6–9 Ω/sq) were synthesized using a reported electrodeposition method followed by the calcination [6], [14]. The pH of 50 mL DI water was adjusted to 1.7 using 65% HNO3. Next, 3.32 g of KI and 0.97 g of Bi(NO3)3·5H2O were subsequently added and dissolved in the aqueous solution. The as-prepared solution was mixed with 20 mL of 0.23 M p-benzoquinone dissolved in ethanol, which was then used as an electrolyte. FTO substrates (1.5 cm × 3.0 cm) were cleaned by

Results and discussion

As depicted in Scheme 1, a controllable chemical treatment using sodium hypophosphite followed by coating NiOOH as cocatalysts has been employed to modify BiVO4 films. The pristine BiVO4 films were first treated with 0.5 M hypophosphite (NaH2PO2) aqueous solution to prepare hypophosphite-treated BiVO4 (labeled as P-BiVO4). Then, both BiVO4 and P-BiVO4 films were deposited with NiOOH cocatalysts using a pH-adjusting technique to get NiOOH--coated BiVO4 (NiOOH-BiVO4) and NiOOH-coated P-BiVO4

Conclusion

In summary, a chemical treatment method has been utilized to create an Ovac interlayer at the surface of BiVO4 photoanode. The generated Ovac interlayer, sandwiched between the bulk BiVO4 and an OECNiOOH overlayer, can function to reduce interfacial charge recombination. Thereby, the rational construction of the BiVO4/Ovac/NiOOH structure improves charge separation and charge transport efficiencies, giving rise to the enhanced performance of PEC water oxidation. In addition, the chemical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge financial support from the Research Grants Council of Hong Kong (grant no. 21203518, F-CityU106/18 and 9048121), City University of Hong Kong (grant no. 9667229, 7005289, 7005580, 7005720, 9680208, 9667213 and 9052029), Shenzhen Science Technology and Innovation Commission (grant no. R-IND12302) as well as National Natural Science Foundation of China (grant no. 22071070, 21974131 51701159, 52022054, and 51974181).

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