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

The measurement of the kinetics of electron transfer in the dark to various outer-sphere redox couples at both n- and p-WSe2 electrodes in aqueous electrolytes and at n-Si in acetonitrile and methanol by scanning electrochemical microscopy (SECM) is reported. The absence of errors due to ohmic resistance, charging currents, and relative insensitivity of the method to parallel processes, such as corrosion, is shown to give the SECM approach advantages over more traditional electrochemical methods of measurement. Only one case, oxidation of RU(NH3)(6)(2+) on p-WSe2, exhibited an apparent transfer coefficient of 1, in agreement 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, other redox couples showed low values (0.1-0.4) of the transfer coefficient, including the reduction of Ru(NH3)(6)(3+) on n-WSe2, contrary to theoretical expectations of 1 for a semiconductor in depletion or 0.5 for a degenerate semiconductor. The SECM was also used to observe the different kinetics of Ru(NH3)(6)(2+) oxidation at step edges compared to the van der Waals Surface, At steps, significant oxidation currents were observed even with the p-WSe2 biased 0.4 V negative of the flatband potential. The low apparent transfer coefficients commonly found for dark processes at semiconductors are explained as the result of averaging over sites of different reactivity (steps,cracks, pits, and the van der Waals surface). The reduction of ferrocenium and N,N,N',N'-tetramethyl-1,4-phenylene diamine cation radical (TMPD(.+)) on n-Si in acetonitrile also exhibited transfer coefficients of about 0.3, whereas the transfer coefficient for the reduction of TMPD(.+) in methanol was 0.6, in methanol,ferrocenium reacted with Si and so was reduced by a purely chemical mechanism, with a heterogeneous rate constant greater than 0.37 cm s(-1) and concurrent etching of the semiconductor.