Reaction paths in concurrence: The electrochemical hydrogen reaction on GaAs(111)A- and GaAs(110)-surfaces A quantumchemical approach

We have performed quantumchemical investigations towards a further explanat ion of the reaction mechanism of the hydrogen evolution reaction on semicon ductor electrodes (continuation of Z. Phys. Chem. 210 (1999) 95). Details o f the two-step-mechanism via H-ad-intermediates on both GaAs(111)A- and GaA s(110)-surfaces were studied on two different theoretical foundations. On t he one hand we have made molecular cluster calculations on nb initio Hartre e-Fock-, MP2- or DFT(B3LYP)-level. On the other hand we have performed calc ulation on periodically repeated supercells in the density functional forma lism at special topics of our systems. Effects of solvation and of an appli ed electrode potential on the energy potential profiles along restricted re action paths were investigated. On the (110)-surface a reaction sequence vi a H-ad-intermediates both on the gallium and on the arsenic sites should be possible. For the Volmer step we find a return of the preferred adsorption position from the arsenic site to the gallium site if the applied electrod e potential runs from zero to negative values. Against it, the Heyrovsky st ep is always slightly preferred on the arsenic site, independent on the app lied electrode potential. The hydrogen evolution on local structure motifs with 3-fold-coordinated, sp(3)-hybridised Ga-surface atoms and no concurren ce of As-surface atoms, like on an ideal GaAs(111)A surface structure, is s lightly preferred in comparison to the (110)-surface and a shift of the ons et potential towards a less negative value is expected. For all the treated reaction sequences we have calculated charge injection coefficients, which agree very well with the data from experiments and from macroscopic charge transfer dynamics simulations.