The Nonlinear Optical Response of Pt(111) Electrodes in Perchloric Acid Solution: Implications for the Potential of Zero Charge

Abstract

Abstract Second harmomic and sum frequency generation (SHG, SFG) are used to investigate the second-order non-linear optical response of a Pt(111) single-crystal electrodes in perchloric acid solution. In the potential window between hydrogen evolution and surface oxidation the SHG signal shows a pronounced minimum at 760 mVRHE and rises linearly with decreasing electrode potential. The potential of the minimum as well as the magnitude of the signal in the potential range of hydrogen adsorption depend on the pH of the electrolyte. The dependence of the SHG signal on excitation frequency in the range of 9000–1200 0cm−1 shows a continuous SHG signal increase to higher frequencies without characteristic surface resonances. The SHG signal is assigned to the excitation of a continuum of electronic levels. The maximum of the signal intensity is observed at potentials close to 0 VRHE, where hydrogen evolution takes place and the surface has a maximum of negative charge. Sum-frequency spectra of CO adsorbed on Pt(111) exhibit the known vibrational signature of terminal and bridge-like coordination and an additional resonant signal. On the same surface, the SHG signal is characterized by a high signal intensity which remains constant up to the CO oxidation potential. The potential dependence of the nonlinear response of the Pt(111)/CO surface as well as of the neat surface in perchloric acid indicates a high sensitivity to the surface charge. As a consequence, a negatively charged surface up to a potential of 600 mVRHE is deduced. Our results are at variance with a value for the potential of zero charge of 0.34 VRHE which was derived from the CO charge displacement method, but in agreement with the value based on the immersion technique (U. W. Hamm et al. , J. Electroanal. Chem. 414 (1996) 85).