SCANNING ELECTROCHEMICAL MICROSCOPY .25. APPLICATION TO INVESTIGATIONOF THE KINETICS OF HETEROGENEOUS ELECTRON-TRANSFER AT SEMICONDUCTOR (WSE2 AND SI) ELECTRODES

The measurement of the kinetics of electron transfer in the dark to va rious outer-sphere redox couples at both n- and p-WSe2 electrodes in a queous electrolytes and at n-Si in acetonitrile and methanol by scanni ng electrochemical microscopy (SECM) is reported. The absence of error s due to ohmic resistance, charging currents, and relative insensitivi ty of the method to parallel processes, such as corrosion, is shown to give the SECM approach advantages over more traditional electrochemic al methods of measurement. Only one case, oxidation of RU(NH3)(6)(2+) on p-WSe2, exhibited an apparent transfer coefficient of 1, in agreeme nt with theories that assume an ideal semiconductor/solution interface . The standard heterogeneous rate constant in 0.5 M Na2SO4 was (1.7 +/ - 0.6) X 10(-16) cm s(-1), independent of concentration in the range 1 -5 mM, in rough agreement with theoretical estimates: In general, othe r redox couples showed low values (0.1-0.4) of the transfer coefficien t, including the reduction of Ru(NH3)(6)(3+) on n-WSe2, contrary to th eoretical expectations of 1 for a semiconductor in depletion or 0.5 fo r a degenerate semiconductor. The SECM was also used to observe the di fferent kinetics of Ru(NH3)(6)(2+) oxidation at step edges compared to the van der Waals Surface, At steps, significant oxidation currents w ere observed even with the p-WSe2 biased 0.4 V negative of the flatban d potential. The low apparent transfer coefficients commonly found for dark processes at semiconductors are explained as the result of avera ging over sites of different reactivity (steps,cracks, pits, and the v an der Waals surface). The reduction of ferrocenium and N,N,N',N'-tetr amethyl-1,4-phenylene diamine cation radical (TMPD(.+)) on n-Si in ace tonitrile also exhibited transfer coefficients of about 0.3, whereas t he transfer coefficient for the reduction of TMPD(.+) in methanol was 0.6, in methanol,ferrocenium reacted with Si and so was reduced by a p urely chemical mechanism, with a heterogeneous rate constant greater t han 0.37 cm s(-1) and concurrent etching of the semiconductor.