Invited Paper SS+NS-TuA1
2012 AVS Gaede-Langmuir Award Lecture: Surface Photochemistry on Compact Crystals and on Metal Nanoparticles

Tuesday, October 30, 2012, 2:00 pm, Room 21

Adsorbing a molecule on a substrate changes its photochemistry. I shall briefly review characteristics of surface photochemistry, established mechanisms, and effects such as whether the substrate acts mainly as source or sink of electronic excitations of adsorbates, how long the latter survive, and how effects which influence their localization and delocalization influence the success rate of excitations. For laser excitation, linear and nonlinear response to excitations can occur.

Use of nanoparticles (MNPs) instead of bulk metals further changes surface photochemistry, mainly by changing the substrate optical excitations (e.g. the Mie plasmon of MNPs), and excitation lifetimes and efficiency (by confinement). This will be illustrated by data obtained in the past years in Berlin on NO dimers adsorbed on Ag NPs with varied size (2 to 10 nm) supported on thin alumina films on NiAl single crystals, laser-excited with 2 to 5 eV, with in situ comparison with Ag(111). The main channel is photodesorption of NO; conversion to N₂O + O, and to NO(ad) stabilized by O also occur. Adsorption energies were characterized by TPD, cross sections (PCS) by photo-depletion, and desorbate energy distributions (translation, rotation, vibration) by TOF and REMPI analysis. Linear and nonlinear fluence dependencies of desorption signals have been found with ns and fs laser pulses, respectively. The main changes in NO photodesorption are found in the PCS which are strongly enhanced by plasmon excitation and more weakly by excitation confinement, and show clear size dependences interpreted by counteracting influences. The branching into the minor photoreaction channels is also changed at Ag NPs compared to Ag(111) which is due to varying PCS enhancement factors. The photochemical mechanism, however, as evidenced by state-resolved analysis of the desorbing NO molecules, remains the same – formation of transient negative ions by hot electrons in the substrate - for most of the investigated range (with an exception for high energy and small particles). With fs laser pulses further drastic PCS increases are found even at low fluences at the NPs but not at Ag(111). This nonlinear effect is explained by re-excitation of hot electrons confined in the NPs within a single laser pulse. But even here the individual dynamics stay the same. This action of NPs on the success probability of excitations with essentially unchanged dynamics appears to be the typical behavior for photochemistry on MNPs. Only in an unusual case (Xe/Ag NPs) we have seen a direct influence of plasmon excitation on desorption.

These findings may help in the understanding of photocatalysis on MNPs.