Photochemistry on metal nanoparticles

There is no question that the size and morphology of MNPs strongly influence the photochemistry on their surfaces. MNPs may offer new adsorption sites and new electronic states, which are absent on single-crystal surfaces. The special photochemistry occurring on MNPs is related to these changes of electronic structure and geometrical parameters (which are, of course, correlated). Different ground and excited electronic states and their lifetimes then result in different dynamics and kinetics on nanostructured surfaces. This survey has shown that the particular photophysical properties of MNPs are diverse and complicated, but as they have been investigated for a considerable time, the accumulated knowledge and understanding is appreciable. The confinement of excitations due to the small size of the MNPs is likely to increase the efficiency of photochemistry. The overwhelming importance of the plasmon excitation, when present, and of the concomitant field enhancement is very obvious; the time evolution following the excitation is understood in principle. The relevant parameters are generally clear, even though their magnitude, relative importance, and interplay are not understood in detail. What is still missing here is to perform all experiments on well-defined samples where size and morphology of the individual particles as well as their arrangement on the support are well determined. While, in principle, a bottom-up approach (build up by soft landing of mass-selected particles with subsequent in situ characterization) might appear more appealing, in practice the (now well-developed) procedures for preparing defined samples by vapor deposition seem to be more promising. Also, use of alloy NPs such as Au-Ag may allow one to tailor plasmon enhancement with specific chemical selectivity for surface photoreactions of interest. In comparison to the progress made in photo physics, work on the photochemistry on well-defined samples is very much in its beginning stage with the number of publications being quite small. Again, it will be of utmost importance to work with well-defined samples. Application of the highly developed laser techniques with their continuing extension to shorter pulse times will certainly be very fruitful. The most promis ing approach will be combined photophysical and photochemical investigations using the same sample to exclude any influence of sample preparation on the transferability of results. Unanswered questions abound. The relative contributions of the various sketched (and maybe of new) mechanisms have to be sorted out and explained and examined for low and high excitation densities. The contributions of plasmon enhancement and detailed influence of hot electron dynamics on the photochemical processes have to be examined for molecules in contact with the MNPs. A mere influence of the field enhancement for molecules not in contact with the MNPs may be important to induce reactions on the oxide. Such an antenna effect of MNPs may be usable to induce spill-over effects of hot electrons, where local injection of e-h pairs into a suitable oxide triggers photochemical reactivity on such oxides, possibly even allowing one to probe the length scale of such transfer processes. In all cases, the plasmonic coupling in an array of MNPs and the connected hotspot formation may lead to peculiar reactive situations. There is no question that this is an exciting research field and that interesting results can be expected soon. © 2006 American Chemical Society.