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 t
1 and t
2 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.