Effect of pH on photocatalytic and photoelectrochemical (PEC) properties of monoclinic bismuth vanadate.

Monoclinic bismuth vanadate (c-BVO) was prepared via simple calcination of solvothermally processed tetrahedral BiVO4. The physicochemical and morphological properties of c-BVO demonstrated the successful synthesis of the photoactive monoclinic phase from the tetrahedral phase, which has low photoactivity properties. The photoactivities of c-BVO were investigated using the photodegradation of methylene blue (MB) and photoelectrochemical (PEC) measurements in acidic (pH = 2.5), neutral (pH = 6.5) and basic (pH = 9.5) media. The photocatalytic activity of c-BVO was increased with increasing pH, achieved 99% MB degradation in the basic condition, compared with 70 and 45% in the neutral and acidic media, respectively. Although the tetrahedral BiVO4 showed mainly adsorption with negligible photodegradation, c-BVO demonstrated both good adsorption and photodegradation activities. The PEC results indicated that the photocurrent density was affected by both pH and applied voltage. Impedance measurements showed faster charge transfer in the neutral condition than in the acidic and basic electrolytes. The incident photon conversion efficiency (IPCE) showed very low activity for tetrahedral BiVO4, but in comparison it was enhanced by 20- and 10-fold for c-BVO in the visible and simulated solar light, respectively.