The electrochemistry of the tin oxide semi- conductor-II. a study of the charge-transfer phenomena in the tin oxide semiconductor in solutions of tetra-alkyl ammonium salts

Abstract

The charging processes on n-type tin oxide in aqueous solutions of tetra-alkyl ammonium salts were observed by chronopotentiometry. The tin oxide was coated on Pyrex glass, in thickness of 0·5 − 1 × 103 Å, as determined by the Tolansky method of double reflexion.After the establishment of a steady potential, the tin oxide was charged cathodically and anodically, respectively. The influence of the tetra-alkyl ammonium ions and Cl−, Br− and I− on the I/Ec curves were determined with and without prepolarization from 0–2 V.The I/E curves in the presence of tetra-alkyl ammonium salts at a concentration of 10−3−1 M showed a regular shift of the steady-state potential and the potential of charge-transfer reaction on the semiconductor with the size of the adsorbed ions. There is a linear dependence of potential on the proton affinity of anions in tetramethyl ammonium salts of Cl−, Br−, I−. The same relation with a smaller slope was observed in tetra-ethyl ammonium salts. The potential shift is increasingly in a negative direction with decrease in size of alkyl and tetra-alkyl ammonium ions in the order methyl- ethyl-butyl-methylphenyl, with the same halide. In the case of tetra-alkyl halides, a similar shift of electrode potential was observed, and may be shown in the order of Cl−−Br−−I−. This relationship is interpreted by proton charge-transfer processes on Sn-O-H and Sn-O-alkyl, as well as adsorption of halides.This relationship agrees with a calculation of the rate of charge-transfer processes on the semi-conductor surface and H-bonding of the cassiterite structure and the reaction SnOx + N (Alkyl) ⇆ SnOx N(Alkyl)4+ + e + H+ ⇆ SnOx, − H + N(Alkyl)4+ ⇆ SnOx−y + y(H20).The shift of the reaction to the left or right depends upon the charging rate and the covering of semiconductor surface with halide and alkyl ions respectively.