Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO(2) nanoparticles under steady-state and laser pulse excitation. Equilibration with the C(60)/C(60)(-) redox couple provides a means to determine the apparent Fermi level of the TiO(2)-Au composite system. The size-dependent shift in the apparent Fermi level of the TiO(2)-Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO(2) --> Au --> C(60) has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and size-dependent catalytic activity of metal-semiconductor nanocomposites.

Noble metal particles of Au, Pt, and Ir were deposited on nanostructured TiO2 film using an electrophoretic approach. The nanocomposite films were characterized by UV-absorption and atomic force microscopy (AFM). The deposition of tetraoctylammonium bromide (TOAB)-capped metal nanoparticles on TiO2 films improved the photocurrent generation and induced a shift in the apparent flat band potential. The TiO2 films modified with TOAB-capped metal nanoparticles were less prone to the electron scavenging by the oxygen in solution. Improved photoelectrochemical performance of semiconductor−metal composite film is attributed to the shift in quasi-Fermi level of the composite to more negative potentials. Continuous irradiation of the composite films over a long period causes photocurrent to decrease as the semiconductor−metal interface undergoes chemical changes. The role of semiconductor−metal composite films in improving the rate of photocatalytic degradation of an azo dye is also discussed.

Photoinduced electron accumulation in ZnO nanoparticles results in the bleaching of the exciton band as well as quenching of green emission. In the absence of an electron scavenger, photogenerated electrons are stored near the conduction band edge and promote charge recombination via a nonradiative process. By exposing the UV-irradiated ZnO suspension to an electron acceptor (O 2 or thionine dye) the stored electrons are discharged and the original excitonic band and the visible emission are restored. Titration of electrons stored in ZnO nanoparticles with an electron acceptor, thionine dye, shows a linear relationship between stored electrons and the emission quenching. When gold nanoparticles are added to pre-UV-irradiated ZnO colloids, only partial recovery of the emission is seen. Pt nanoparticles on the other hand caused almost complete recovery of the quenched emission as the electrons are discharged into the solution. The charge distribution between UV-irradiated ZnO and gold nanoparticles results in equilibration of the Fermi level. Furthermore, the transfer of electrons to the metal nanocore followed by equilibration continues until the Fermi level reaches close to the conduction band edge of ZnO. Basic understanding of the interaction between the semiconductor and metal layers leading to Fermi-level equilibration is important for evaluating the role of noble metals in photocatalytic reactions.

The mechanism of structural transformation during combustion of nickel nitrate (oxidizer)−glycine (fuel) system is investigated by using different in situ techniques, including time-resolved X-ray diffraction (TRXRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) with dynamic mass spectrometry (MS), and high-speed infrared thermal imaging. It is shown that for initial compositions with a relatively large fuel-to-oxidizer ratio (φ), pure Ni phase forms directly in the combustion front. For fuel-lean conditions, only NiO phase can be detected. Analysis of the obtained data, including transmission and scanning electron microscopy (TEM−SEM) studies of the quenched reaction fronts, allows us to suggest the intrinsic mechanism of pure metal formation in the investigated system. It is shown that the combustion front propagates because of the reaction between N 2 O and NH 3 , which are the products of decomposition of the oxidizer and fuel. The excess of NH 3 gas produced in fuel-rich conditions rapidly (<0.2 s) reduces nickel oxide to pure metal in the reaction front.

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