Crystallinity and photocatalytic properties of BiVO4/halloysite nanotubes hybrid catalysts for sunlight-driven decomposition of dyes from aqueous solution

The BiVO4/HNTs hybrid photocatalysts were synthesized by liquid phase precipitation using natural halloysite nanotubes (HNTs) as supporter and Bi(NO3)·5H2O as resource of Bi. XRD, scanning electron microscopy (SEM), transmission electron microscopy, HRTEM, x-ray photo electron spectroscopy and UV-Vis DRS were used to characterize the samples prepared at different calcination temperatures, and the effects of crystallization, Brunauer-Emmett-Teller specific surface area and morphological structure on the photoactivity were investigated. Results reveal that increasing calcination temperature can accelerate the transition of BiVO4 from tetragonal to monoclinic and also decrease the surface area of BiVO4/HNTs. The SEM results showed that BiVO4 was successfully coated on HNTs surface with ellipsoid or near rod like morphology, and the obtained BiVO4/HNTs had regular nanotube morphology. HRTEM results showed that, the regular fringe spacing of the lattice planes are about 0.474 and 0.364 nm, which is consistent with the (110) and (200) plane of the monoclinic and tetragonal BiVO4, confirming the exiting of mixed crystal structure in BiVO4/HNTs. BiVO4 with tetragonal phase (80.02%) and monoclinic phase (19.98%) mixed crystal is loaded on the surface of HNTs with calcinations at 400 °C for 2 h. The structure and Si (Al)-O bond of HNTs can be obviously changed over the calcination temperature of 400 °C. The effect of calcination temperature on photocatalytic reactivity of samples was investigated by degradation of dyes (MB, MO and RhB) under simulated solar light. And the sample calcined at 400 °C with the better mixed crystalline structure and larger specific surface area exhibits significant activity with the removal rate of MB and RhB up to 100% within 4 h. The degradation of MB follows the first order kinetic model. BiVO4/HNTs photocatalysts with the band gap of 2.34 eV has higher photocatalytic reaction rate and better sedimentation performance than Degussa P25. The photocatalytic degradation efficiency of BiVO4/HNTs for MB was no significant reduction after four times recycles.