Interface modulation of BiVO4 based photoanode with Bi(III)Bi(V)O4 for enhanced solar water splitting
Graphical abstract
Bi2O4(4 0 0)/BiVO4(0 4 0) heterojunction has been designed through a novel oxidation conversion progress with facilitated interfacial charge transfer. The optimized Bi2O4/Mo-BiVO4/CQDs/Ni-FeOOH photoanode reached a remarkable photocurrent of 6.7 mA/cm2 at 1.23 V vs. RHE under AM 1.5G without using any hole scavenger.
Introduction
For almost five decades, numerous efforts have been devoted to the developing of solar water splitting machineries to produce hydrogen as an ideal energy carrier [1], [2]. However, the overall solar water splitting efficiency is often limited by the oxidation process, which is largely dependent on the physiochemical properties of photoanode materials [3]. Bismuth vanadate (BiVO4) with monoclinic scheelite structure is one of the most studied sustainable photoanode materials due to its relatively small bandgap (around 2.4 eV) and suitable valence band edge position [4], but its PEC performance is still greatly hindered by the severe charge recombination and slow surface catalytic kinetics [5], [6], [7]. In this regard, finding a viable strategy to facilitate charge transport, especially at the interfaces, is of critical importance.
Chemical modification has been exploited to solve the abovementioned problems of BiVO4. For example, a black BiVO4@amorphous TiO2-x photoanode, fabricated through atomic layer deposition and subsequent hydrogen plasma treatment, exhibited enhanced surface reaction kinetics and promoted photostability due to the introduction of the TiO2-x outer layer [8]. Meng et al. soaked a BiVO4 photoanode in borate buffer solution to modify the local catalytic environment with borate layer as both passivator and co-catalyst, which achieved a photocurrent density of 3.5 mA cm−2 at 1.23 V and a cathodically shifted onset potential of 250 mV [9]. Although those surface modification trials minimized the interfacial charge transfer barrier, the approach was accompanied by the surface screening with foreign materials, which restricted the full potential of the BiVO4 photoanode materials and also brought in additional interfacial barrier to some extent.
Recently, a bismuth-based metal oxide, Bi2O4, with mixed valence states (Bi3+ and Bi5+), has been utilized for photocatalytic reactions, such as organic pollutants degradation, sterilization and carbon dioxide reduction [10], [11], [12], [13]. Having a compatible crystal structure with BiVO4 [14], [15], Bi2O4 is potential material to form a coherent heterojunction with BiVO4. In this work, we venture to explore an oxidation conversion method to top Bi2O4 nanoparticles closely on the surface of BiVO4, as shown in Fig. 1a, with the aim to facilitate interfacial charge transport through the well-aligned lattice docking junction. The BiVO4 photoanode is further optimized via Mo doping and co-loading with CQDs as light absorber and Ni-FeOOH as co-catalyst. While the pristine BiVO4 photoanode only produces a photocurrent of 1.3 mA/cm2 at 1.23 V vs. RHE under AM 1.5G irradiation, the optimized Bi2O4/Mo-BiVO4 junction photoanode can achieve 2.7 mA/cm2 under the same conditions, and with further modification, the obtained Bi2O4/Mo-BiVO4/CQDs/Ni-FeOOH raises the value to, astonishingly, 6.7 mA/cm2.
Section snippets
Results and discussions
The pristine BiVO4 photoelectrode was obtained via conversion from a BiOI film prepared via electro-deposition [16], with morphologies shown in Fig. S1. The porous BiVO4 structure consisted of small particles with uniform sizes could be observed from the SEM images. To deposit Bi2O4 on the surface of BiVO4, BiOCl nanosheets were first grown on BiVO4 by a chemical batch deposition method. The details could be found in the experimental section. The amount of BiOCl could be controlled by adjusting
Conclusion
In this work, we have developed a series of strategies to improve the activities of BiVO4 photoanode for solar water oxidation, e.g., by promoting interfacial charge diffusion, surface charge transfer and light absorption range. Most notably, the incorporation of Bi2O4 nanoparticles to construct a smooth lattice-docking heterojunction through the oxidation conversion method has been demonstrated to be crucial for improving the PEC performance of BiVO4. With the improved charge separation
Experimental section
Electro-deposition of BiOI films: The Bi(NO3)3 solution was prepared by dissolving 0.97 g Bi(NO3)3·5H2O (Sigma-Aldrich, 98%) and 3.32 g KI (Sigma-Aldrich, ≥99.0%) in 50 mL deionized (DI) water with vigorous stirring. Then, the solution pH was adjusted to 1.7 with adding 2.0 M HNO3 solution. Afterwards, 20 mL ethanol solution with 0.4972 g dissolved p-benzoquinone (Sigma-Aldrich, ≥98%) was mixed with the previous Bi(NO3)3 solution with stirring for 30 min. Electro-deposition of BiOI films was
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
M. Ma and Z. Xing contributed equally to this work. The authors acknowledge the financial support from the RGC of Hong Kong (GRF Nos. 16312216), Shenzhen Peacock Plan (KQTD2016053015544057) and Nanshan Pilot Plan (LHTD20170001), National Natural Science Foundation of China (21905298, 21972006), Shenzhen Science and Technology Innovation Committee (JCYJ20190807164205542), and Guangdong Basic and Applied Basic Research Foundation (2020A1515010342).
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These authors contributed equally to this work.