Proceedings Article | 23 May 2018
KEYWORDS: Nanoparticles, Luminescence, Bismuth, Solar cells, Quantum efficiency, Solar radiation, Visible radiation, Crystals, Annealing, Electrons
In recent years, BiVO4 based-materials have attracted much attention because of their fascinating multifunctional physicochemical properties, such as pigmentation, ferroelelasticity, semiconductivity, optical, luminescent and (photo)catalytic properties. Their potential at various applications is exceptionally broad. For example, BiVO4 can be useful in improving photovoltaic cell efficiency through solar spectral conversion by shifting short-wavelength sunlight (ultraviolet and blue) to longer wavelengths (downshifting), or by shifting long-wavelength near-infrared (NIR) radiation to visible light (up-conversion), which provides more radiation in the spectral region wherein the solar cell shows the largest quantum efficiency. Also, BiVO4 has proved to be an excellent material for use (under visible-light illumination) in photocatalytic water splitting and photocatalytic degradation of organic compounds (air/water pollutants). Brilliant yellow color of non-toxic ms-BiVO4 makes it a good commercially available substitute for toxic cadmium- and lead-based yellow pigments.
Herein, colloidal synthesis of BiVO4 nanoparticles and their multifunctional physicochemical properties, such as photoluminescence (down-conversion and up-conversion) emission properties, pigmentation, (photo)catalytic and adsorptive features are presented.
Prepared colloid solutions have been characterized by UV-VIS spectroscopy. Measured absorption spectra, for lower concentrations of the precursors, showed blue shift and calculated band gap of the colloid particles ranged from 3.07 to 3.12 eV. Obtained colloid solutions have been mixed with water, centrifuged, and the residue washed with methanol. X-ray diffraction (XRD) patterns show that the BiVO4 nanoparticles crystallize in pure tetragonal phase. In order to improve their crystallinity and to investigate in more detail the structural and photocatalytic properties, the obtained BiVO4 nanoparticles have been annealed at 450°C for 3 hours. Thermogravimetric analysis has been utilized to obtain the optimal annealing temperature. In annealed samples, tetragonal phase completely transits to monoclinic structure for which it has been reported in the literature to be photocatalytically more active. BET surface analysis showed greater specific surface of the samples obtained from the less concentrated colloids. This difference is even more noticeable after the annealing of the samples. In order to investigate photocatalytic activity of the samples, their ability to degrade organic dyes has been tested.
Photoluminescence spectra of BiVO4 show two broad emission bands in the range of 450–800 nm: greenish-yellow emission band centered at 530 nm and red emission centered at 678 nm under excitation at 425 nm. This luminescence emission is considered to originate from the radiative recombination of photo-generated electrons and holes. Namely, under the UV/Vis irradiation the electron–hole pairs are generated, holes are formed in the Bi (bismuth) 6s orbitals, or hybrid orbitals of Bi 6s and O (oxygen) 2p, while the electrons are elevated to the V (vanadium) 3d orbitals. Photoluminescence spectra of co-doped samples of BiVO4: Ho/Yb show up-conversion properties and characteristic Ho3+ emission bands centered at 542 nm 659 nm and 755 nm under excitation at 980 nm.
BiVO4 nanoparticles as mesoporous material prepared by this technique of synthesis show great photocatalytic activity and further improvements as well as some other applications are to be expected in the future.
