Effect of Gold Loading on Time-Resolved ps Photoluminescence of ZnO
Ahmed Ziani, S. Al-Taweel, M. A. Nadeem, Hicham Idriss
Abstract
Au/ZnO with Au at.% between 0.1 and 3.0, were prepared, analyzed by UV-vis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and other techniques. The bandgap energy of all materials was 3.3 eV, all showed a pronounced plasmon resonance at about 2.5 eV when Au was present. Au deposited on ZnO surfaces had a mean particle size between 2.7 and 4.4 nm (depending on coverage), while ZnO particles had a mean size of 105 nm. XPS Au 4f7/2 indicated that Au is in its metallic state (binding energy at 83.8 ± 0.1 eV) in the investigated series. Time-resolved photoluminescence (TPRL) was conducted to monitor the band to band (e-h) recombination decay signal on ZnO and Au/ZnO. The decay was best fitted with a biexponential function with time constants t1 and t2 ranging from 20 to 25 ps and from 57 to 85 ps, respectively, for the whole series with Au/ZnO showing faster decay rate than ZnO. Fitting the decay function followed by its integration over the investigated time domain then normalizing it with gold atoms provided a useful method to quantify the effect of Au particles on radiative recombination rates of ZnO. The resulting function showed a single exponential decay with respect to Au atoms on the surface of ZnO; y(x) = A exp(−ax), where x is the atomic % of Au on ZnO and a = 2.4. This indicated that charge trapping, per Au atom, is most efficient at very low coverage. Possible reasons for this and links to observed similar results in photocatalytic reactions on noble metal/semiconductors materials are discussed.