Facile and ligand-free synthesis, phase transformation, structures and phase-dependent optical properties of BiVO4 nanocrystals

Semiconductor nanocrystals have recently attracted increasing interest because of their fascinating physical and chemical properties. Obtaining nanocrystals with desired structures and properties using a greener and facile protocol is highly desirable but remains a great challenge. In this work, we demonstrate a facile ligand-free strategy for the fabrication of highly photoluminescent BiVO4 nanocrystals under mild conditions. The crystalline structures, local structures, morphological features, chemical compositions, surface chemistry and optical properties of the resulting BiVO4 nanocrystals were explored in detail using several techniques, e.g., X-ray diffraction, Raman spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, elemental distribution technique, Fourier transform infrared spectroscopy, UV-visible absorption spectroscopy and photoluminescence spectroscopy. Furthermore, the underlying mechanism of phase transformation of BiVO4 nanocrystals was elucidated. We show that BiVO4 nanocrystals with monoclinic or tetragonal crystalline phases can be rationally synthesized by simply controlling the acid-base properties of the precursors. The strongest diffraction peaks of BiVO4 nanocrystals with tetragonal and monoclinic phases arise at 2θ = 24.3° and 28.8°, corresponding to (200) and (112) crystal planes, respectively. The strong Raman peaks at 821 and 855 cm−1 are attributed to the VO symmetric stretching mode of monoclinic BiVO4 with Ag symmetry, and the VO symmetric stretching mode of tetragonal BiVO4, respectively. The absorption shoulders of BiVO4 nanocrystals display the blue shift in absorption peaks with respect to bulk BiVO4, indicating distinct quantum confinement effect. More interestingly, the overall emission intensity of monoclinic BiVO4 nanocrystals is lower than tetragonal BiVO4 nanocrystals, manifesting the very low recombination rate of electron-hole pairs in this material. This work provides a more promising strategy with superiorities of low cost in synthesis, environment friendly and adaptive for large scale fabrication with controllable particles size, morphology and stability. The present study would serve as alternative scheme in the fabrication of nanostructures for the advancement of photoelectric technology and its other correlative applications in energy and environment.