Correlation between Electronic Structures and Photocatalytic Activities of Nanocrystalline-(Au, Ag, and Pt) Particles on the Surface of ZnO Nanorods

O K-, Au, Ag and Pt L-3-edge X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy, and scanning photoelectron microscopy (SPEM) measurements have been performed to study the correlation between the electronic structures and photocatalytic activities of nanocrystalline (nc)-(Au, Ag, and Pt) particles on the surface of ZnO nanorods (ZnO-NRs). The O K-edge XANES spectra reveal greater occupation of the O 2p orbitals, i.e., a greater negative effective charge of the O ions, in nc-Pt/ZnO-NRs than of nc-(Au, Ag)/ZnO-NRs. This result suggests that nc-Pt particles have weaker photocatalytic activities than those of nc-(Au, Ag) particles on the surface of ZnO-NRs. Well-defined bandgaps of nanoparticle-coated ZnO-NRs increase in the order Au (3.3 eV) -> Pt (3.5 eV) -> Ag (3.6 eV), which can be correlated with an decreasing Pauling's electronegativity and a reduction of the screening effect. The valence-band SPEM measurement of nc-(Au, Ag, and Pt)/ZnO-NRs does not support the general argument that the Fermi levels of the (Au, Ag)/semiconductor composites are shifted toward the conduction-band edge relative to that of the Pt/semiconductor composite.