Operando time-resolved diffuse reflection spectroscopy: The origins of photocatalytic water-oxidation activity of bismuth vanadate

https://doi.org/10.1016/j.jphotochem.2020.112493Get rights and content

Highlights

  • Physical properties of two types of BiVO4 crystals were investigated.

  • Sub-ns transient diffuse reflection spectra (TDRS) of each BiVO4 powder was recorded.

  • Operando analysis of picosecond TDRS was applied to each BiVO4 aqueous suspension.

  • TDRS signals of monoclinic scheelite crystal are affected by an e/ h+ scavenger.

Abstract

Photocatalytic water splitting has been believed to be a key technology for the construction of a sustainable society based on a renewable energy source. Understanding the exact mechanisms of oxygen evolution by water-splitting reaction, which has been regarded as a bottleneck to realize the water splitting in practical basis, should help us to enhance solar-to-hydrogen energy conversion efficiency. Here, we present a detailed investigation based on two types of bismuth vanadate (BiVO4) crystals. One is a monoclinic scheelite crystal that forms a truncated tetragonal bipyramidal shape (MS), and the other is a tetragonal zircon crystal that forms an octahedral shape (TZ). MS, which has relatively higher photocatalytic activity of water oxidation, rarely adsorbed reduced mediator (Fe2+). Operando time-resolved diffuse reflection spectral measurement demonstrated, for the first time, that the surface holes of MS crystals show extremely high reactivity, while those of TZ crystals show poor reactivity. Our investigation would promote an essential understanding of the photocatalytic reaction mechanism of water oxidation.

Introduction

H2 evolution from water as a raw material is a crucial technique to produce sustainable chemicals [1]. There are two possible methods to generate H2 under CO2-exhaust free condition. One is an electrolysis of water using an electromotive force driven by a solar cell, and the other is a photocatalytic water splitting. The latter is an economically attractive way to produce H2 because of the simplicity of a constructed system [2]. During photocatalytic water-splitting reaction, both O2 evolution by water oxidation and H2 evolution by proton (H+) reduction should occur simultaneously. Here, water oxidation is recognized as a bottleneck reaction because it requires more carriers (4 electrons) than proton reduction (2 electrons) [3,4]. In water-oxidation reaction using semiconductor photocatalysts, unlike a general catalytic reaction, large specific surface area does not always enhance the photocatalytic activity. For instance, photocatalytic O2 evolution from water with rutile-type TiO2 powder crystals has an intimate relation to their crystal sizes or surface defect densities, rather than their specific surface area [5,6]. It is of great importance to identify the factors that determine the rate of water oxidation reaction.

Many researchers have already been trying to scrutinize a number of photocatalysts that can oxidize water with high quantum yield [[7], [8], [9], [10], [11], [12]]. Bismuth vanadate (BiVO4) is one of the most remarkable candidates for water oxidation utilizing visible light [13,14]. This is because its band structure is suitable for water oxidation (valence band maximum lies under H2O/O2 redox potential), while it is unsuitable for hydrogen evolution (conduction band minimum lies beneath H+/H2 redox potential) [15]. Kudo et al. reported that a plate-like BiVO4 with monoclinic scheelite (MS) structure can effectively oxidize water under visible light irradiation [14,16]. Its unique morphology enables the carriers to react effectively with adsorbed molecular species [16]. Some reports claim, using experimental and computational approaches, that these molecular species can be reduced at only {010} surfaces, while they are oxidized at both {110} and {011} surfaces [[16], [17], [18]]. Photoexcited carriers of BiVO4 with MS structure are reported to survive during several seconds, the fact which is claimed to play a crucial role for high activity of water-oxidation [19]. On the other hand, BiVO4 with the other crystal systems (e.g. tetragonal zircon (TZ) structure) hardly show water-oxidation activities, even though their band structures have enough potential for water oxidation [10,11]. There are several reports to investigate the effect of crystal phase of BiVO4 on their photocatalytic activities [14,[20], [21], [22], [23]]. One of the possible reasons to explain the different water oxidation activities between MS and TZ structures is claimed to be due to the different bandgap energies, which indeed should affect the water oxidation activity when the crystals are irradiated with visible light [14]. However, MS structure has much higher photocatalytic activity than TZ structure even though the crystals are irradiated with UV light. Therefore, different band-gap energies cannot be well accounted for to explain this observation. BiVO4 microparticles with tetragonal scheelite (TS) structure were also compared with MS microparticles, both of which show similar band-gap energies [[20], [21], [22]]. The results of hybrid density-functional theory (DFT) calculations predicted that, in TS structure excess holes tend to localize around BiO8 polyhedrons with strong lattice distortion, whereas in MS structure those holes spread over many lattice sites [20]. Therefore, this striking difference should carefully be inspected based on the experimental viewpoint of both physical properties and carrier dynamics.

In this study, we investigate the photoexcited carriers of BiVO4 photocatalyst with MS or TZ structure and compare their behaviors. Our investigation will contribute to an essential understanding of the optical properties of BiVO4 that could be brought out its potential for the application of water oxidation.

Section snippets

Preparation of BiVO4 microparticles

All chemical reagents were purchased from Fuji Film Wako Pure Chemicals and used without further purification. BiVO4 microparticles were synthesized following methods described in a previous report [24]. Briefly, Bi2O3 (50 mmol) and V2O5 (50 mmol) were added to 0.5 or 1.0 M HNO3 aqueous solution. The suspended solution was vigorously stirred at 80 °C for 4 h. The resulting yellow or orange precipitate was filtered, washed with de-ionized water, and dried overnight under vacuum at room

Results and discussion

Fig. 1 shows XRD patterns of the two samples that were prepared by different concentration of aqueous nitric acid solutions. Two types of crystal structures were successfully controlled, which are identified as monoclinic scheelite (MS) and tetragonal zircon (TZ) type structures [27,28]. Fig. 2 shows FE-SEM image of the obtained BiVO4 microparticles. MS forms a truncated tetragonal bipyramidal shape, while TZ forms an octahedral shape. Diffuse reflectance spectra showed that TZ has the

Conclusion

We prepared BiVO4 crystals with monoclinic scheelite (MS) structure that forms a truncated tetragonal bipyramidal shape, which have much higher photocatalytic activity of water oxidation than BiVO4 crystals with tetragonal zircon (TZ) structure that forms an octahedral shape. While both of them have almost the same specific surface area and crystal size, MS has lower amount of adsorbed Fe2+ than that of TZ. This suggests that after the reduction of Fe3+ by electron scavengers, Fe2+ is likely to

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 would like to thank Prof. Ryu Abe at Kyoto University for kind instruction and guidance to perform the experiments of water oxidation. HH thanks JSPS KAKENHI, Grant-in-Aids for Basic Research (B) (No. 16H04181) and Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4LEC)” (No.17H06433 & No. 17H0637) for financial support.

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