The role of surface states during photocurrent switching: Intensity modulated photocurrent spectroscopy analysis of BiVO4 photoelectrodes

Intriguing photo-electrochemical characteristics of BiVO4 photoelectrodes studied in pH-neutral aqueous solutions are reported herein. Indeed, we have observed photocurrent polarity switching, as put in evidence by cyclic voltammetry under chopped illumination conditions. Such unusual behavior was analyzed in detail using Intensity Modulated Photocurrent Spectroscopy (IMPS). At potentials where positive photocurrent was observed, the expected shape of IMPS was recorded, starting in quadrant (IV) at high-frequency (HF) and reaching quadrant (I) at low-frequency (LF) with two well defined semicircles. Surprisingly, in the negative photocurrent region, IMPS started in quadrant (II) at HF and ended in quadrant (III) at LF. Such highly infrequent features were interpreted here as the rotation of the IMPS spectra around the origin of the plot, due to the sign switch of the photocurrent. A model that takes into account the existence of in-band energy states at the surface of BiVO4 has been used in order to account for the experimental results. It was found that (i) the surface state capacitance; (ii) the relaxation time constant associated to surface states; and (iii) the density of in-gap surface states, were all showing a well-marked maximum in the nearby value of the switch potential. This suggests that surface states are more influent in the nearby where photocurrent switch occurs.