Ultrathin BiVO₄ nanobelts: controllable synthesis and improved photocatalytic oxidation of water

Abstract

Ultrathin BiVO₄ nanobelts with about 3 nm thickness have been firstly fabricated by a hydrothermal method. It has been discovered that these special nanostructures exhibit much higher photoelectric conversion efficiency as well as water splitting ability than commercial BiVO₄ nanoparticles. It is believed that the one-dimensional ultrathin structure of BiVO₄ nanobelts is of benefit for the separation and transmission of photoexcited electrons due to its short charge transfer pathway and favourable crystalline structure.

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Nanostructure semiconductors have attracted considerable attention under the background of the energy crisis and global climate change due to their promising applications in solar-driven water splitting, carbon dioxide reduction and degradation of pollutants.^1–5 Since the initial demonstration of water splitting using a TiO₂ photoelectrode under ultraviolet light, great effort has been paid to seek for visible-light-responsive photocatalysts for the efficient utilization of solar energy.^6–10 In recent years, ultrathin nanostructures, such as ultrathin PtNi alloy shells,¹¹ ultrathin WS₂ nanoflakes,¹² ultrathin BiOCl nanoplates,¹³ and ultrathin C₃N₄ nanoplates^14, have attracted intense attention due to their unique physical and chemical properties. Besides the wide responsive range of the solar spectrum, there are still many other requirements for an ideal photocatalyst, such as high energy conversion efficiency, high stability, non-toxicity and economic feasibility. In view of these points, monoclinic bismuth vanadate (m-BiVO₄) has been considered as one of the most appealing photocatalysts owing to its fascinating crystal structure and unique properties.^15–18 However, one-dimensional (1D) ultrathin BiVO₄ nanobelts with highly efficient photocatalytic and photoelectrochemical performances have rarely been reported until now.

BiVO₄, an n-type semiconductor with a layered structure and a direct band gap of 2.4 eV, has been extensively investigated over the past several decades because of its applications in pigments, ferroelasticity, ionic conductivity, and so on.^19–22 Recently, BiVO₄ has been proved to be an excellent photocatalyst for photocatalytic evolution of oxygen and degradation of organic pollutants under visible light illumination.^23–25 It is well known that the photocatalytic performances of semiconductors are greatly dependent on their morphologies and structures. However, the m-BiVO₄ crystals fabricated by traditional solid state and solution phase reactions are mainly large size and irregular particles. More recently, various methods, especially wet chemical routes, have been attempted to synthesize BiVO₄ nanoparticles with different morphologies, such as nano-sheets,²⁶ nanotubes,²⁷ nanoplates,²⁸ nanofibers,²⁹ mesoporous,³⁰ nanoellipsoids³¹ and hyperbranched structures.³² Unfortunately, most of the BiVO₄ nanostructures are synthesized in the presence of organic additives or surfactants, which is difficult to remove and thus increase the complexity of scale up production. In addition, ultrathin one-dimensional BiVO₄ with single-crystalline structures have not yet been reported until now, which is especially favourable for the charge separation and transmission due to their ultrathin architecture. Therefore, the surfactant-free production of one-dimensional BiVO₄ ultrathin nanocrystals remains a great challenge.

Herein, we demonstrate for the first time the fabrication of ultrathin one-dimensional BiVO₄ nanobelts in the absence of surfactants and organic additives. The thickness of a BiVO₄ nanobelt is about 2.8 nm and each nanobelt is composed of multiple layers owing to the intrinsic layered structure of BiVO₄. Furthermore, the photocatalytic water splitting properties of BiVO₄ nanobelts are much higher than that of commercial BiVO₄ powders. Meanwhile, the photoelectrochemical experiments demonstrated that the photoconversion performances of BiVO₄ nanobelts are also better than commercial BiVO₄ nanoparticles, which may be ascribed to the shorter charge transfer distance and the better crystalline structure.

Scheme 1 illustrates the fabrication approach of ultrathin BiVO₄ nanobelts. Firstly, NH₄VO₃ was dissolved in ammonia solution (solution A) because it is slight soluble in pure water and BiNO₃ should be dissolved into dilute nitric acid (solution B) since it could hydrolyze to BiONO₃ precipitates in aqueous solution. The major reason why the acid and the alkaline solution were chosen is that they can react slowly and thus release the reactants when solution A was mixed with solution B. Secondly, solution A was added into solution B drop by drop. Along with the consumption of ammonia and nitric acid, NH₄VO₃ and BiNO₃ would react and produce tetragonal BiVO₄ nanoparticles. Then the solution was transferred to a Teflon-lined autoclave and heated at 180 °C for one day. The tetragonal nanoparticles could be transformed to ultrathin monoclinic BiVO₄ nanobelts after the hydrothermal treatment.

Scheme 1 Schematic illustration of the fabrication process of monoclinic BiVO₄ nanobelts.

Fig. 1A and B shows the typical scanning electron microscopy (SEM) images of ultrathin BiVO₄ nanobelts fabricated by hydrothermal method. It can be clearly seen that the morphology and the dimension of BiVO₄ nanobelts are regular and uniform, respectively. The length of the nanobelts can reach to tens of microns and the average width is about 50 nm. In addition, the surfaces of these nanobelts are very smooth and no other secondary nanostructures are observed. Therefore, it can be rationally deduced that this novel method may achieve the mass production of BiVO₄ nanobelts. The UV-vis absorption spectra of ultrathin BiVO₄ nanobelts and nanoparticles have been shown in Fig. 1C, and the inset exhibits the relationship of (αhν)² vs. hν based on the direct transition. It can be observed that the band gap energy of BiVO₄ nanobelts is estimated to be 2.46 eV, which is smaller than that of nanoparticles (2.65 eV). The composition and crystalline structure of BiVO₄ nanocrystals before and after hydrothermal treatment have been examined by X-ray diffraction (XRD) and shown in Fig. 1D. It can be seen that before hydrothermal reaction, the XRD patterns of BiVO₄ is identical to that of pure tetragonal BiVO₄ (JCPDS 14-0133).³³ Additionally, the morphologies of tetragonal BiVO₄ are irregular nanospheres with an average diameter of about 20 nm (Fig. S1†). And after hydrothermal treatment, the tetragonal BiVO₄ would be transformed to monoclinic BiVO₄ phase (JCPDS 75-1866), accompanying the morphology transformation from nanospheres to nanobelts. Thereby, it can be deduced that there must be a first dissolution and then assembling process during the hydrothermal reaction, since the thickness of nanobelts is much smaller than the diameter of nanoparticles.

Fig. 1 (A and B) SEM images of BiVO₄ nanobelts with different magnifications. (C) UV-vis diffuse reflectance spectra and the inset shows photographs and plots of (αhν)² vs. hν. (D) XRD patterns of BiVO₄ nanocrystals before and after hydrothermal treatment.

Transmission electron microscopy (TEM) analysis (Fig. 2A) further demonstrates that the as-prepared products are composed of well-defined ultrathin BiVO₄ nanobelts. The thickness values of BiVO₄ nanobelts are quite small and it is even difficult to distinguish them from the substrate under high magnification (Fig. 2B). The fringe spacing of 1.95 Å in Fig. 2B, which agrees well with the (006) lattice plane, indicates the growth direction of [001] zone axis. Fig. 2C exhibits the HRTEM image of ultrathin BiVO₄ nanobelts, and the fringe spacing of 2.10 Å and 2.11 Å correspond to the (015) and (105) crystal planes of monoclinic BiVO₄, respectively. Therefore, it can be known that high indexed facets are exposed on ultrathin BiVO₄ structures, which may be favorable for their photocatalytic activities. In addition, the atomic force microscopic (AFM) examination has also been performed, which is exhibited in Fig. 2D. It can be easily measured that the thickness of the BiVO₄ nanobelts is no more than 3 nm, which confirms the ultrathin structure of BiVO₄ nanocrystals. Furthermore, the sideview TEM image (Fig. S2†) of BiVO₄ nanobelts indicates that they are multilayer structures, and the average thickness of monolayer is about 9.06 Å, which is much larger than all the fringe spacing of monoclinic BiVO₄. Therefore, it can be deduced that a single ultrathin BiVO₄ nanobelt consists of several monolayers, and the thickness of each BiVO₄ nanobelt depends on the number of the monolayers.

Fig. 2 (A and B) TEM images, (C) HRTEM image and (D) AFM image of ultrathin BiVO₄ nanobelts.

The photocatalytic ability of BiVO₄ nanobelts has been evaluated by oxygen evolution from water splitting under visible light illumination, as illustrated in Fig. 3A. Under visible light illumination, the electron–hole pairs would separate. The Ag⁺ would react with photogenerated electrons and thus reduce the recombination of electron–hole pairs, while the holes would oxide water to oxygen. It can be clearly seen that ultrathin BiVO₄ nanobelts exhibits much higher water oxidation performance than BiVO₄ nanoparticles (Fig. S3†), and the O₂ evolution rate of nanobelts is about 2.5 times higher than that of nanoparticles. Possessing ultrathin structures, the photoexcited electron–hole pairs of BiVO₄ nanobelts can be easily separated and exhausted due to the shorter charge transfer distance, and thus exhibit higher quantum efficiency than BiVO₄ nanoparticles. In addition, the photoelectrochemical properties of BiVO₄ nanobelts and nanoparticles have also been examined (Fig. 3B). The photocurrent density of ultrathin BiVO₄ nanobelts is about 60 μA cm⁻², which is about 4 times larger than that of BiVO₄ nanoparticles. In order to explore the origin of the enhanced PEC performances and discover the interfacial charge transfer properties, electrochemical impedance spectroscopy (EIS) are measured for BiVO₄ nanoparticles and nanobelts. The EIS measurements, which cover the frequency range of 10⁵ to 0.1 Hz using an amplitude of 5 mV at the open circuit potential of the system, are presented as Nyquist plots in Fig. 4. The arch in the Nyquist plot represents the charge transfer kinetics on the working electrode as the diameter of the semicircle reflects the charge transfer resistance. As depicted in Fig. 4, BiVO₄ nanobelts exhibits smaller impedance arc radius than nanoparticles. Thereby, it can be concluded that ultrathin BiVO₄ nanobelts demonstrate better photocatalytic and photoelectrochemical activities than BiVO₄ nanoparticles, which may be ascribed to the improved charge separation efficiency owing to the unique ultrathin architectures.

Fig. 3 (A) Photocatalytic O₂ evolution from an aqueous AgNO₃ solution under visible light illumination and (B) transient photocurrent densities measured with chopped light over ultrathin BiVO₄ nanobelts and nanoparticles.

Fig. 4 Nyquist plots of the electrochemical impedance spectra of BiVO₄ nanoparticles and nanobelts.

Conclusions

In summary, we have demonstrated for the first time the fabrication of ultrathin one-dimensional BiVO₄ nanobelts in the absence of surfactants and organic additives. It has been discovered that these special nanostructures exhibit much higher photoelectric conversion efficiency as well as water splitting ability than commercial BiVO₄ nanoparticles. Thereby, it is believed that the ultrathin one-dimensional structures of BiVO₄ nanobelts are beneficial for the separation and transmission of photoexcited electrons due to the shorter charge transfer distance and the better crystalline degree.

Acknowledgements

This work is financially supported by the “Hundred Talents Program” of the Chinese Academy of Science and National Natural Science Foundation of China (21273255, 21303232) and Youth Innovation Promotion Association (2014380), CAS.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15566a

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