CO and H2 Activation over g-ZnO Layers and w-ZnO(0001)

Graphene-like ZnO (g-ZnO) nanostructures (NSs) and thin films were prepared on Au(111), and their reactivities toward CO and H-2 were compared with that of wurtzite ZnO (w-ZnO) (0001) single crystals. The interaction and reaction between CO/H-2 and the different types of ZnO surfaces were studied using near-ambient-pressure scanning tunneling microscopy (NAP-STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The reactivity of the w-ZnO(0001) surface toward CO and H-2 was found to be more prominent than those on the surfaces of g-ZnO/Au(111). CO oxidation took place primarily at the edge sites of w-ZnO(0001) and the interface between g-ZnO NSs and Au(111), while g-ZnO thin films on Au(111) appeared to be inert below 600 K. Similarly, the w-ZnO(0001) surface could dissociate H-2 at 300 K, accompanied by a substantial surface reconstruction, while g-ZnO on Au(111) appeared inert for H-2 activation at 300 K. DFT calculations showed that the reactivities of ZnO surfaces toward CO could be related to the formation energy of oxygen vacancy (E-Ovf), which could be related to the charge transfer to lattice oxygen atoms or surface polarity.