Elsevier

Ultrasonics Sonochemistry

Volume 68, November 2020, 105233
Ultrasonics Sonochemistry

Ultrasonically controlled growth of monodispersed octahedral BiVO4 microcrystals for improved photoelectrochemical water oxidation

https://doi.org/10.1016/j.ultsonch.2020.105233Get rights and content

Highlights

  • Considerable effect of ultrasonic power on seed growth and morphology of BiVO4.

  • Ultrasonically controlled growth of monodispersed octahedral BiVO4 in short time.

  • Monodisperse BiVO4 microcrystals have single phase with exposed reactive surfaces.

  • Optimized BiVO4 (BV20) has excellent photo efficiency and photostability.

Abstract

The synthesis of facet-controlled structures with precise morphology and exposed reactive surface is one of the key research challenges. We effectively endeavoured to obtain the monodisperse octahedral bismuth vanadate microcrystals with exposed {1 0 1},{2 0 0},{3 1 2} and {0 2 1} dominant facets through an optimized sonochemical assisted hydrothermal process. A pulse sonication (5-s ON and 2-s OFF cycle, 21 W ultrasonic power and 20 kHz ultrasonic frequency) for 30 mins followed by 1 h hydrothermal treatment was found to yield the preferred octahedral morphology. The microscopic and X-ray analysis suggested a potent role of ultrasonic waves for the initial seed growth and its evolution into a well-defined monodisperse microcrystals. The density functional theory (DFT) calculations revealed strongly localized bandgap states with a bandgap of ~2.47 eV. The PEC measurements for water oxidation demonstrated the efficacy of these microcrystals as photoanode. Notably, the optimized octahedral BiVO4 microstructure exhibited a superior performance as evident from photocurrent density ~0.9 mAcm−2 at 1.23 V vs. RHE and %IPCE value of ~22% compared to analogous photoanodes under visible light irradiation.

Introduction

The design of shape-controlled photoactive materials has attracted a considerable amount of research attention since the morphology and crystallinity have potential impacts on the catalytic behaviour [1], [2]. Designing specific morphology by tuning the surface atomic structures can substantially enhance the photochemical processes such as light absorption and charge separations [3], [4], [5]. For example, the morphological and facet tuning of anatase TiO2 grown single-crystal generates more photoresponse due to the large surface area with exposed {0 0 1} facets [2], [6]. The same principle can be extended to related photoactive materials such as metal vanadates [1]. However, controlling the morphology and crystallinity with the enhanced yield in shorter reaction time is a major research challenge particularly for the Platonic shape (tetrahedra, hexahedra, octahedra, etc.) metal oxides, since few articles described their growth compared to the conventional morphologies under specific reaction conditions [7], [8].

Metal vanadates are an emerging class of promising materials for photoassisted catalytic applications. Among various metal vanadates, BiVO4 (BV) is frequently explored due to intriguing optoelectrical characteristics with a bandgap of ~2.40 eV, ionic conductivity, ferroelasticity, acoustooptical and photochromic effects. It is widely accepted that BV is an excellent photoanode in photoelectrochemical (PEC) applications under visible light irradiation. Additionally, attributes such as chemical stability, suitable optical band positions, visible photoresponse, and relatively lower cost make BV a suitable alternative to TiO2 for solar energy conversion and environmental sustainability [2], [9], [10], [11], [12].

Morphologically, BV exists in four polymorphs: pucherite, clinobisvanite, dreyerite, and scheelite-type tetragonal phase [13], [14]. Among these, the scheelite-type tetragonal phase is highly photoactive and explored widely as a photocatalyst [9], [15], [16], [17]. Hence, more efforts must be devoted to growing controlled morphologies with faceted crystallinities for improved PEC performance. Various synthetic techniques have been reported to develop different morphologies of BV such as nanowires [18], [19], [20], nanocones [21], spherical [22] and flower-shaped [23], [24], [25]. However, the platonic morphology for BV is rarely reported, so a synthetic strategy with specific reaction parameters should be adopted to acquire the platonic morphology and crystallinity. Additionally, the first principle density functional theory (DFT) calculations can be useful to determine the characteristics of platonic structures [26], [27], [28], [29], mechanisms, and optical characteristics of BV [30], [31], [32].

In recent years, our research group has developed various bismuth vanadate morphologies and deliberated the effect of various reaction parameters on photoelectrochemical (PEC) water splitting [17], [20], [33], [34], [35]. We synthesized pseudo flower BV microparticles and demonstrated that the calcination temperature played an important role in controlling the morphology and overall PEC performance as evident from the current density (0.7 mAcm−2) [17]. However, we could not attain the particle separation and monodispersion due to the bulk mixing of the precursors. Here, we report the synthesis of highly controlled and photoactive octahedral shaped BV photocatalysts, synthesized via the highly standardized sonochemical assisted hydrothermal approach. Excellent morphology was acquired in a short time by tuning the amplitude of the ultrasonic waves and keeping other parameters constant. In this work, we found pre-sonication and slow addition of reactants to be highly efficient in controlling the morphology of BV with exceptional yield. Additionally, the synthesized microstructures showed enhanced PEC water oxidation compared to our previous studies. The controlled morphology, monodispersity and short reaction time are the notable outcomes of this work with exceptional photostability and photocurrent density.

Section snippets

Sonochemical assisted synthesis of BiVO4

The sonochemical assisted hydrothermal synthesis of BiVO4 microstructures is performed using Branson 450 Digital Sonifier operating at 20 kHz, equipped with Branson 101–147-041 Solid Exponential Horn for Sonifier Cell Disruptor, 1/2″ Tip Diameter, 10–65 Amplitude Range, 10–250 mL volume with varying % amplitude followed by hydrothermal treatment (Fig. S1 and Fig. 1). Initially, Solution A was prepared by mixing 0.1 M Bi(NO3)3·5H2O (Sigma Aldrich) and 0.075 M of Sodium dodecylbenzene sulfonate

Role of ultrasonication

The sonochemical assisted synthesis and dispersion of nanoparticles is a well-established technique [4], [17], [45], [46], [47]. The high-energy ultrasonic waves transferred through the reaction media cause cavitation. Cavitation is the most crucial step during ultrasonication, since the formation, growth, and implosive collapse of microbubbles/cavities in the medium occur in this step. High energy released from collapsing bubbles/cavities generates a localized hotspot region. This region can

Conclusions

We successfully synthesized single-phase monodispersed and highly crystalline octahedral BiVO4 microcrystals via the controlled ultrasonic-assisted hydrothermal method. The pre- and post-ultrasonication along with varying % amplitude, revealed a significant effect on the platonic morphology and crystal size of BV microcrystals. The PEC performance of BV photoanodes is exclusively influenced by the morphology, dispersity, and surface smoothness. We acquired substantial photostability (~1h) and

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

We acknowledge the support of Center of Research Excellence in Nanotechnology (CENT), and Center of Center of Integrative Petroleum Research (CIPR), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, 31261, Saudi Arabia.

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