Ke. Pomykal et al., STABILITY OF N-SI CH3OH CONTACTS AS A FUNCTION OF THE REORGANIZATION ENERGY OF THE ELECTRON-DONOR/, Journal of physical chemistry, 99(20), 1995, pp. 8302-8310
Predictions of the Marcus/Gerischer theory for photoelectrode stabilit
y have been investigated experimentally for n-Si/CH3OH photoelectroche
mical cells. Specifically, a semiconductor electrode is predicted to b
e more stable if the reorganization energy of the stabilizing agent is
decreased (in the normal region of the Marcus behavior), thereby incr
easing the rate of minority carrier capture by the stabilizer. This pr
ediction was quantified experimentally by monitoring the branching rat
io between two competing reactions at a semiconductor/liquid interface
: hole transfer from a Si photoanode to the electron donor in solution
vs passivation of the Si photoanode through hole transfer to water. D
eliberate addition of water to n-Si/CH3OH contacts provided a constant
, known passivation pathway that competed with charge transfer to the
stabilizing agent. Dimethyl ferrocene (Me(2)Fc), ruthenium(II) pentaam
mine 4,4'-bipyridine (Ru(NH3)(5)(4,4'-bpy)(2+)), and cobalt(II) tris-(
2,2'-bipyridine) (Co(2,2'-bpy)(3)(2+)) provided three outer sphere ele
ctron donors with very similar standard electrochemical potentials but
varying solvent reorganization energies. At constant electron donor c
oncentration, constant driving force for reaction, constant photocurre
nt density, and constant water concentration in CH3OH, the stability o
f n-Si photoelectrodes decreased in the order Me(2)Fc(+/0) > Ru(NH3)(5
)(4,4'-bpy)(3+/2+) > Co(2,2'-bpy)(3)(3+/2+). This observation can be c
onsistently explained through the theoretically predicted influence of
the minority carrier acceptor reorganization energy on the interfacia
l charge transfer rare constant.