Hydrothermal synthesis of BiVO4: Structural and morphological influence on the photocatalytic activity
Graphical abstract
Highlights
► By a simple hydrothermal method we achieve the BiVO4 photoactive monoclinic phase. ► Best performances were not associated to the surface area values. ► Photoactivity is strongly affected by the crystallite size and morphology. ► Better photocatalytic performances have been obtained for needle-like m-BiVO4. ► Irradiation with low power sunlike lamp leads to notably photodegradation of MB.
Introduction
Nowadays, the environmental problems demand increasingly severe regulations that open up opportunities for novel green photocatalytic routes leading to the alternative materials to traditional TiO2, in order to allow the use of sunlight as the energy source for pollutant abatement [1]. For this scope the use of visible light photons constitutes the key point for a good photocatalyst performance under sunlight conditions. Among the family of new photocatalysts recently proposed, BiVO4 (Eg = 2.40 eV) has been widely reported exhibiting good photocatalytic properties [2]. Bismuth vanadate (BiVO4) has three main crystalline structures: zircon-tetragonal, scheelite-tetragonal and scheelite-monoclinic. BiVO4 can be synthesized by different preparation routes which allow obtaining selectively one of the mentioned structures with different morphologies depending on the preparation method [3], [4], [5], [6], [7]. Regarding to scheelite-like compound, the monoclinic structure is usually obtained by means of high temperature methods. Meanwhile, the tetragonal form is normally achieved by aqueous media methods at low temperature process. However, from the literature data, it arises that the most photoactive phase is the monoclinic one. So, the attempts of achieving this crystalline phase are of great importance. The use of hydrothermal treatments for the preparation of BiVO4 leads to the appearance of the monoclinic phase at mild temperatures with lower crystallite sizes with respect to those obtained from solid state reaction [8], [9]. Thus, Zhang et al. proposed the synthesis of monoclinic BiVO4 nanosheets by means of hydrothermal method assisted by a morphology-directing agent [5]. These nanosheet-shape material exhibits much higher photocatalytic activity for solar photodegradation of rhodamine B than the bulk material. In this sense, m-BiVO4 structure with high surface area was also obtained by means of K2SO4 as inorganic shape-controlling additive. Recently, Xi and Ye reported a novel hydrothermal synthetic procedure to obtain m-BiVO4 nanoplates showing preferential exposition of [0 0 1] facets [10]. Similarly to Bi2MoO6 systems, this preferentially exposed facet seems to produce an enhanced photoactivity for the degradation of organic contaminants as well as for the photocatalytic oxidation of water to O2. By comparison the m-BiVO4 nanorods with [1 0 0] growth direction, the nanoplates showed remarkably higher photoactivity in spite of its lower surface area with respect to the nanorods [11], [12]. This clearly points out the fact that in this material the photoactivity is more related to the surface (or shape) structure than in the surface area. Moreover, Zheng et al. proposed a similar procedure using a Gemini surfactant and producing 3D hierarchical structure [13]. The influence of the particular morphology has been also pointed out by Zhou et al. [14]. They reported the preparation of monoclinic BiVO4 microtubes particles forming flowerlike structures and exhibiting a prominent improvement in the photocatalytic activity. Such notably visible photoactivity seems to be attributed to the distinctive morphology. In spite of the strong influence of morphological aspects, high surface area system is still needed in order to achieve a high photoactive material [15].
In the present paper, we report a simple surfactant free hydrothermal method which allows obtaining BiVO4 monoclinic systems with controlled morphology. The relationship between surface area, structure and morphology with the further photocatalytic activity for methylene blue degradation has been also performed.
Section snippets
Synthesis of photocatalysts
The preparation of BiVO4 was carried out by mixing the corresponding amounts of Bi(NO3)3·5H2O and NH4VO3. Thus, 5 mmol of the Bi precursor was dissolved in 10 ml of glacial acetic acid while the stoichiometric amount of NH4VO3 (5 mmol) was dissolved in 60 ml of bidistilled water. These two solutions were finally mixed to form a yellowish suspension (pH 1 approx.) which is kept under stirring for 1 h. Similar series were prepared but in this case after the precipitation the pH was settled at 5 and 9
Hydrothermal synthesis of BiVO4 with ammonia
The preparation at low pH value leads to well crystallized scheelite BiVO4 structure in the monoclinic phase (JCPDS 14-0688, corresponding to the I2/a space group). However, at the initial state of the hydrothermal treatment at 100 °C a mixture of tetragonal and monoclinic phase can be observed (Fig. 1). In the same direction, samples prepared by hydrothermal method using NH4OH (at pH 5 and pH 9) presented only the monoclinic phase. It is worthy to note that the preparation with NH4OH at pH 9
Conclusions
We have obtained well crystallized m-BiVO4 by a simple surfactant free hydrothermal method. By changing the pH, directing agent and hydrothermal parameters such as temperature and time, different morphologies have been obtained.
The best performance was clearly not associated to the surface area values, and is strongly affected by the crystallite size and morphology. For the different series obtained, better photocatalytic performances have been achieved for m-BiVO4 with needle-like morphology.
Acknowledgments
The financial support by the project P09-FQM-4570 is fully acknowledged. S. Obregón Alfaro thanks CSIC for the concession of a JAE-Pre grant.
References (19)
- et al.
Mater. Chem. Phys.
(2007) - et al.
J. Mol. Catal. A: Chem.
(2006) - et al.
Appl. Surf. Sci.
(2010) - et al.
Appl. Catal. B: Environ.
(2011) - et al.
Nanostructured oxides in photocatalysis
- et al.
Chem. Rev.
(2011) - et al.
Chem. Mater.
(2001) - et al.
J. Phys. Chem. B
(2006) - et al.
Chem. Eur. J.
(2008)