In addition, such weak interaction between interfacial water and BiVO 4 (010) surface groups giving the chance to the compensated counterions in electrolyte to have a direct and strong interaction with surface sites. Therefore, the inner sphere coordination formed at BiVO 4 (010)/NaCl surface (see Fig. and). The width of the Helmholtz layer d EDL is about 2.4 Å at low pH and 2.7 Å at high pH, respectively (Fig. ). The values of d EDL were estimated from the surface normal projected distance between the counterions and the nuclei of O br sites. It is found that the adsorbed of Cl -on Bi 7f site at positively charged surface (Fig. ) makes the width of d EDL smaller than that of negatively charged surface. In the reported synchronous illumination XPS (SI-XPS) study, the Cl -concentrates and bonds to Bi on the sur- face can facilitate H 2 O molecule activation, as well as promoting carrier charge separation and accelerating water oxidation reaction . This may be explained by the adsorption of Cl -which shorten the width of d EDL and have an effect on Helmholtz capacitance as seen here.