Modulation of the electron transfer processes in Au-ZnO nanostructures.

Plasmonic nanostructures comprising Au and ZnO nanoparticles synthesized by the spontaneous reduction of HAuCl4 in ethylene glycol were used to assess the possibility of modulating the direction of the electron transfer processes at the interface. One electron UV reduction and visible oxidation of the reversible couple TEMPOL/TEMPOL-H was confirmed by EPR spectroscopy. The apparent quantum yield for TEMPOL-H conversion under continuous wave visible excitation depends on the irradiation wavelength, being 0.57% and 0.27% at 450 ± 12 and 530 ± 12 nm, respectively. These results indicate that both the surface plasmon resonance and the interband transition from the 5d to the 6s level of Au nanoparticles contribute to the visible activity of the nanostructure. In addition, by detecting free electron conduction band electrons in ZnO, after the visible excitation of Au/ZnO nanostructures, we provide direct evidence of the photoexcited electron transfer from gold nanoparticles to ZnO.

Electron transfer from photoexcited TiO2 to chelating alizarin molecules: reversible photochromic effect in alizarin@TiO2 under UV irradiation.

Reduction of alizarin molecules coupled to TiO(2) nanoparticles (A@TiO(2)) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO(2) nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO(2) coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO(2) solar cells is briefly discussed.