Case studies in surface photochemistry on metal nanoparticles

The authors give a survey of their work on photochemical processes at silver nanoparticles carried out in Berlin in the past decade. Using well established procedures for the preparation of silver nanoparticles (Ag NPs) supported on ultrathin alumina layers on NiAl single crystals, they have investigated the photoreactions of adsorbed (NO)₂ and of Xe induced by laser pulses. The authors examined the influences of photon energy (2.3, 3.5, and 4.7 eV) and polarization, mean particle size (2-10 nm), and pulse length (5 ns and 100 fs) on yields and cross sections, and on photoreaction mechanisms. Comparison with Ag(111) was made throughout. For the NO dimer layer, the authors find general agreement with known results on bulk Ag(111) in terms of possible reactions (NO desorption and NO monomer formation as well as conversion into adsorbed N₂O and O) and predominant mechanism (via transient negative ion formation, TNI); NO desorption is the strongest channel. However, on the NPs, the cross sections show selective enhancement in particular under conditions of excitation of the Mie plasmon due to the field enhancement caused by it, but - more weakly - also under off-resonant conditions which the authors interpret by excitation confinement in the NPs. For ns laser pulses, the desorption yield responds linearly to photon flux so that the cross sections are independent of laser fluence. Using fs laser pulses, nonlinear yield response is found under plasmon excitation which is interpreted as due to re-excitation of hot electrons in the NPs during a single laser pulse. The dynamics of the individual process, however, stay the same under almost all conditions, as indicated by constant energy distributions over translational, rotational, and vibrational energies of the desorbing NO molecules, even in the nonlinear range. Only for the highest photon energy (i.e., off-resonance) and the smallest particles, a new channel is observed with higher translational energy which is believed to proceed via transient positive ions. The branching into the various reaction channels is found to be different on Ag NPs from that on Ag(111) which is ascribed to differing enhancements for the various channels. While these results show that for a typical molecular reaction only the yields are modified on NPs, very different behavior is observed for desorption of adsorbed Xe. Here, low intensity excitation of the Mie plasmon leads to chaotic response which must be due to hot spot formation. As in this case no simple desorption mechanism (via transient negative or positive ions, or direct HOMO-LUMO excitation of the adsorbate) is expected, a direct action of the plasmon excitation is postulated. Some general conclusions are drawn from these case studies.