Electrical, optical and visible light-photocatalytic properties of monoclinic BiVO4 nanoparticles synthesized hydrothermally at different pH

Monoclinic BiVO4 nanoparticles were synthesized hydrothermally at pH 0.5, 2.0, 5.0 and 7.0. They were characterized by high resolution scanning electron microscopy or field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction, powder X-ray diffraction, Raman spectroscopy, solid state impedance spectroscopy, UV–visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. While BiVO4 nanoparticles synthesized at pH 0.5 and 2.0 provide perfect semicircular Nyquist plots the nanocrystals prepared at pH 5 show a semicircular arc. The nanocrystals obtained at pH 7 exhibit a quasi-linear dependence of ZIm on ZRe. The absorption edge of BiVO4 nanoparticles is red-shifted on decrease of the pH of synthesis; BiVO4 synthesized at pH 0.5 is an exception. The synthesized nanoparticles show band gap emission at 483 nm and defect emissions at 534 and 578 nm. The band gap emission of BiVO4 nanocrystals synthesized at pH 0.5 is much less than those of others. The photocatalytic activity of BiVO4 nanoparticles decreases with increase of the pH of synthesis and nanocrystalline BiVO4 synthesized at pH 0.5 is an exception. The photocatalytic activities of BiVO4 nanoparticles synthesized at different pH are explained in terms of the charge transfer resistance, band gap energy, photoluminescence due to charge carrier recombination and preferential orientation of 040-plane in BiVO4 nanocrystals synthesized at pH 2.