However, with a potential change from 0 V to +0.9 V the visible absorbance of the NiO film increases substantially (Figure ), initiating at ~0.3 V and becoming even more pronounced from potentials around 0.7 V. It is widely accepted that NiO may show a gray or black color due to a large amount of defects (Ni 3+ ). However, the oxidation potential of Ni 2+ to Ni 3+ is around 1.4 V vs. RHE (reversible hydrogen electrode, i.e. ~0.7 V vs. Ag/AgCl, see Figure ). Therefore, the increase in visible light absorption <0.7 V in Figure is not due to electrochemical oxidation of bulk NiO, which is further confirmed by the XPS results in Figure . Therefore, we believe that surface related hydroxylation phenomena forming different phases of Ni-OH in the IHP are responsible for the observed changes in UV-Vis spectra. The quantity of OH -ions in the IHP, adsorbed on the NiO surface, will increase with a more positive potential, while a negative potential will promote H + adsorption. Furthermore, the extraction/intercalation of H + and OH -ions from/into the NiO films can lead to electrochromic phenomena. Therefore, we assign the changes in the UV-Vis spectra with applied potential to compositional surface modifications associated with the IHP. following excitation at 532 nm as function of the applied external potential.