Electrochemistry of the Semiconductor-Electrolyte Electrode. I. The Electrical Double Layer

The thermodynamics of the semiconductor-electrolyte electrode (S/C—E electrode) are examined. The following is concluded about the distribution of the p.d. (potential difference) due to excess charge V in the interphase (assuming the dipole distribution to be invariant with semiconductor electron concentration): (i) For a semiconductor, with a nondegenerate free carrier distribution throughout and no surface states, in contact with a concentrated electrolyte solution (such that the Gouy layer p.d.→0), most of V occurs within the semiconductor phase and the p.d. across the compact Helmholtz layer is small (millivolts). Changes in p.d. across the S/C—E electrode caused by an externally applied p.d. will occur mainly in the S/C phase (if the electrolyte is concentrated); (ii) For a S/C—E electrode with a high concentration of ionized surface states of one kind, p.d. caused by excess charge in the Helmholtz layer, H will be large (∼1 volt) and the electrode behaves like a metal electrode. The Helmholtz layer p.d., H, is found to be essentially independent of bulk S/C electron concentration. An applied p.d. is mainly manifest across the Helmholtz layer (when the electrolyte solution is concentrated).