Facile one-pot microwave-assisted synthesis of tungsten-doped BiVO4/WO3 heterojunctions with enhanced photocatalytic activity

doi:10.1016/j.materresbull.2020.110783

Materials Research Bulletin

Volume 125, May 2020, 110783

https://doi.org/10.1016/j.materresbull.2020.110783 Get rights and content

Highlights

•
One-pot microwave-assisted synthesis of tungsten-doped bismuth vanadate/tungsten oxide heterojunctions.
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Photocatalytic activity of the heterojunctions was evaluated through hydroxyl radical quantification and photoelectrochemical experiments.
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Correlation between photocatalytic activity and the band-edges alignment of the semiconductor that makes up the heterojunction.

Abstract

Bismuth vanadate (BiVO₄) is considered one of the most successful candidates to be used as photoanode for solar-to-hydrogen conversion. However, its relatively high recombination rates and considered low long-term stability limits its applicability. Doping BiVO₄ with tungsten and developing heterojunctions with inorganic perovskites has emerged as strategies to overcome these issues. However, the development of these materials requires plenty of time and energy. Here, we reported the development of a new synthetic method to prepare heterojunctions of tungsten-doped bismuth vanadate (using 1, 3 and 5 % in mass of W) and tungsten oxide. The heterojunctions were prepared in a one-pot microwave-assisted method that takes only 24 min. Photocatalytic efficiency was determined through quantification of the amount of hydroxyl radical, generated by the catalysts when they are irradiated with simulated sunlight, and through photoelectrochemical experiments. The heterojunctions showed enhanced photocatalytic activity, which was ascribed to the adequate alignment of the band edges potentials of the semiconductors. Also, the tungsten doping decreases the recombination rates. Both effects increase the photogenerated charges lifetime.

Graphical abstract

Introduction

Bismuth vanadate (BiVO₄) is considered one of the most successful candidates to be used as photoanode for solar-to-hydrogen conversion, [1] due to its visible-light sensibility, low band-gap energy (∼2.4 eV), low toxicity and high chemical and physical stability. [[2], [3], [4], [5]] However, it intrinsically suffers from poor charge transport and poor activity for the oxygen evolution reaction, relatively high recombination rates and considered low long-term stability [1,6]. These disadvantages have been overcame through several strategies, among which the tungsten doping is highlighted [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. W-doping causes a modification of either the bulk and surface properties of the BiVO₄ suppressing the electron trapping process, reducing the recombination rates. [13] Another important strategy consists in the creation of heterojunctions between BiVO₄ and tungsten oxide (WO₃). [[21], [22], [23], [24], [25], [26], [27], [28], [29]] WO₃ is very promising due to its visible-light sensibility, low band-gap energy (2.7–3.0 eV), excellent charge transfer properties and stability. [[30], [31], [32], [33]] Indeed, the heterojunction between WO₃ and BiVO₄ is one the best pairs, since the band alignment of them is very favorable; however, as stated before, the limitations of bare BiVO₄ reduces the applicability of this junction. In this context, the heterojunction formed by tungsten-doped bismuth vanadate (W-BiVO₄) and WO₃ is extremely attractive. [14]

Morphology also plays a crucial role in the photocatalytic and photoelectrocatalytic activity of BiVO₄ [[34], [35], [36]], which exhibits rich polymorphism [4,[34], [35], [36], [37], [38]]. Thus, the precise control of morphology and crystalline structure is essential for further development of this kind of catalyst. However, the established protocols of synthesis usually take hours and are very energy consuming, mainly due to the use of conventional heating sources such as oil bath, hot plate and laboratory oven. These sources are slow and inefficient due to the dependence on convection currents or thermic conductivity of several materials, and substances throughout the heat must flows throughout. In this context, the use of microwave irradiation is very attractive since it interacts directly with the solvent molecules producing internal and homogeneous heating. This strategy allows reaching higher heating rates, increasing the reaction velocity, improving the product yields, selectivity and reproducibility [[39], [40], [41], [42], [43]]. The use of this heating strategy for BiVO₄ [[44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]] and WO₃ [[58], [59], [60], [61], [62], [63]] synthesis has been increasing recently. However, it is not of our awareness the use of this strategy to produce the heterojunction of W-BiVO₄/WO₃.

Here we report, for the first time the use of a microwave-assisted method to produce W-BiVO₄/WO₃ heterojunctions, using a one-pot approach that takes only 24 min, in which W-doping was added at 1, 3 and 5% in mass. The photocatalytic effect was investigated through quantification of the amount of hydroxyl radical, generated by the catalysts when they are irradiated with simulated sunlight, and through photoelectrochemical experiments.

Section snippets

Chemicals

All chemicals were of analytical grade and used as received. Ammonium metatungstate hydrate ((NH₄)₆H₂W₁₂O₄₀ · xH₂O), ethylene glycol, bismuth(III) nitrate pentahydrate (Bi(NO₃)₃), ammonium metavanadate (NH₄VO₃) were received from Sigma-Aldrich. Oxalic acid (C₂H₂O₄·2H₂O) was received from Synth. Coumarin (COU) was received from Synth, and 7-hydroxycoumarin (OH−COU) was received from Fluka.

Synthesis of tungsten oxide (WO₃)

The precursor solution was prepared by dissolving 0.142 g of (NH₄)₆H₂W₁₂O₄₀·xH₂O and 3.24 g of oxalic acid

Results and discussion

The crystalline structures of the WO₃, BiVO₄, W(X)-BiVO_4, and the heterojunctions were first evaluated using XRD-diffractometry (Fig. 1). The XRD diffractogram of pure WO₃ (Fig. 1a) shows the peaks ascribed to the orthorhombic perovskite-like structure (ICSD-836); [65] and the pattern observed for pure BiVO₄ (Fig. 1a) is ascribed to monoclinic phase (ICSD-62706). [66] The tungsten-doped materials show the same pattern as pure BiVO₄ with only an additional peak at approximately 28° that can be

Conclusions

Here we described the preparation of heterojunctions composed by bismuth vanadate (and tungsten-doped bismuth vanadate) and tungsten oxide. These materials were synthesized by a one-pot microwave-assisted methodology, which was developed by our group, where the WO₃ nanoparticles were first prepared followed by the deposition of BiVO₄ and W(X)-BiVO₄ (where X = 1, 3 and 5 % in mass of tungsten).

XRD and XPS analysis revealed that the heterojunctions are composed of a mixture of bismuth vanadate,

Author statement

J. S. S. designed and supervised the research. C. H. C. performed the synthesis, the phototocatalytic and photoelectrocatalytic experiments of WO₃, W(X)-BiVO₄ and W(X)-BiVO₄/WO₃ and analyzed the data. B. S. R. and M. K. performed the characterization of the catalysts and analyzed the data. C. C., Z. W. and M. S. performed the transmission electron microscopies studies of the heterojunctions. JSS written the manuscript based on inputs from all authors.

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.

Acknowledgment

This work was supported by FAPESP (grants 2017/11395-7 and 2017/26633-0) and by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Codes 001 and CAPES-Print 88,881.310334/2018-01. The authors also acknowledge Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). We are thankful to LNNano-CNPEM for the use of SEM facilities, and to the Multi users platform (CEM) at UFABC for instrumental facilities.

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Facile one-pot microwave-assisted synthesis of tungsten-doped BiVO4/WO3 heterojunctions with enhanced photocatalytic activity

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals

Synthesis of tungsten oxide (WO3)

Results and discussion

Conclusions

Author statement

Declaration of Competing Interest

Acknowledgment

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Appl. Catal. B

Visible-Light-Driven Photocatalytic Inactivation of E. coli K-12 by Bismuth Vanadate Nanotubes: Bactericidal Performance and Mechanism

Environ. Sci. Technol.

Effects of pH on the hierarchical structures and photocatalytic performance of BiVO4 powders prepared via the microwave hydrothermal method

ACS Appl. Mater. Interfaces

Template-free synthesis of BiVO4 nanostructures: I. Nanotubes with hexagonal cross sections by oriented attachment and their photocatalytic property for water splitting under visible light

Nanotechnology

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ACS Energy Lett.

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J. Chem. Soc. Faraday Trans.

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Facile one-pot microwave-assisted synthesis of tungsten-doped BiVO₄/WO₃ heterojunctions with enhanced photocatalytic activity

Synthesis of tungsten oxide (WO₃)