the overall effect of OVs formation on PEC performance is positive. Moreover, an enhancement of crystallinity in sample BOC-O-300 is not beneficial to the photoelectrochemical properties. A high crystallinity may accelerate the separation and migration of charge carriers. However, this effect seems not to outweigh the depletion of O vacancies defects in these samples, suggesting that carrier mobility in BiOCl is not the rate-limiting step for the photoelectrochemical performance.