CH3OH and CH3OD/TiO2(110):

 

Interfacial Proton Coupled Electron Transfer

From our theoretical studies about the wet electron in the H2O/ TiO2 system, we found that the excited state wet electron molecular structure is not optimized at the ground state equilibrium geometry. For instance, rotating the bridging –OH from the bend to the surface normal configuration stabilizes the wet electron state by ~0.5eV. Such structure change in the excited stat will not only stabilize wet electron state, but can also modulate the rate of population decay of the wet electron through RCT. So, For long-lived wet electron states we expect to find evidence for such inertial dynamics from deuterium isotope effects on the excited state dynamics.

Such a substantial deuterium isotope effect for the wet electron population decay has been found in the CH3OH/TiO2 system. By using femtosecond laser excitation, we transferred electrons from a rutile titanium dioxide (110) surface into a CH3OH overlayer state that is 2.3 ± 0.2 eV above the Fermi level. The redistributed charge was stabilized within 30 femtoseconds by the inertial motion of substrate ions (polaron formation) and, more slowly, by adsorbate molecules (solvation to wet electron). A pronounced deuterium isotope effect (Fig.1) was found for the decay of the excited wet electron in the CH3OH / TiO2 and CH3OD/ TiO2 system. The RCT rate of electrons excited into a CH3OD monolayer is about 2.2 times slower than for CH3OH under similar conditions. Our calculation found that such a substantial isotopic effect comes from the Proton-Coupled electron transfer at the CH3OH/TiO2 interface. During the solvation process of the excited electrons, substantial changes involving both the rotation of the bridging –OH (green in Fig.2) and the dissociation of CH3OH into methoxy and bridging –OH species (yellow in Fig.2) happen in the molecules. This Proton-Coupled electron transfer make the decay of the excited electrons from a purely electronic process (nonadiabatic) to a correlated response of electrons and protons . The coupling of electron and nuclear motions in ultrafast charge transfer at molecule/semiconductor interface is central to many phenomena, including catalysis, photocatalysis, and molecular electronics.