The Norrish type I photocleavage is an excellent source of strongly reducing free radicals that can be used to convert soluble metal ions into their atomic state that proceed to form nanoparticles. Proton coupled electron transfer (PCeT) is a useful tool to interpret the mechanism for metal ion reduction, a process that in these systems involves multisite PCeT, with proton and electron having separate receiving substrates.

Ablated, "pseudo-naked" gold nanoparticles (AuNPs) catalyze the cis-trans isomerization of substituted azobenzenes. para-Substitution was found to affect the rate of isomerization, suggesting the participation of AuNP-mediated electron transfer in the isomerization mechanism.

Surface plasmon excitation of aqueous colloidal gold nanoparticles with visible light in the presence of H2O2 led to rapid and selective oxidation of sec-phenethyl and benzyl alcohols to acetophenone and benzaldehyde, respectively. Laser drop, light emitting diode, and microwave irradiation have been used as energy sources. Interestingly, sec-phenethyl alcohol conversion was calculated to be 95% in 20 min when monochromatic 530 nm LEDs were used, being as good or better yield than the corresponding laser and microwave techniques. These results demonstrate the versatility of this inexpensive arrangement. Further attention was placed on the possible mechanism for Au nanoparticle plasmon-mediated alcohol oxidations in the presence of H2O2. We propose electron transfer with the nanoparticle surface, as well as the participation of peroxyl and ketyl free radicals as fundamental steps in the reaction pathway.

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

Surface plasmon excitation of gold nanoparticles on ZnO in the presence of an aldehyde, an amine and phenylacetylene led to rapid and selective formation of propargylamines with good yields (50-95%) at room temperature. Plasmon mediated catalysis is the best available route for this ternary coupling.

Surface plasmon excitation of supported gold nanoparticles in the presence of H2O2 leads to selective oxidation of sec-phenethyl and benzyl alcohols to the carbonyl products acetophenone and benzaldehyde, respectively, in the absence of additional solvents. Light-emitting diodes are compared with microwave irradiation as excitation sources. Hydrotalcite, ZnO, and Al2O3 have been chosen as the solid supports. The overall efficiency of the alcohol oxidation was found to be largely dependent on the nature of the support, with hydrotalcite-derived nanocomposites giving the highest conversions to product, yielding 90% acetophenone after 40 min of LED irradiation. The mechanism for plasmon-mediated alcohol oxidation is believed to involve a significant contribution from the support itself, with adsorption of the alcohol substrate and progression of the oxidation reaction being largely facilitated by the basicity of the support used.

Lamellar, or layered, potassium niobium oxide perovskites are a class of underdeveloped semiconductors in organic photocatalysis that offer the inherent advantages of larger particle size and ease of recoverability as compared to traditional semiconductor materials.

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