A VIBRATIONAL INVESTIGATION OF THE STABILITY, MORPHOLOGY AND SURFACE REACTIVITY OF NIO ON NI(100)

The formation and thermal stability of NiO on Ni(100) have been investigated using high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). Our results indicate that the saturated NiO/Ni(100) layer prepared at 300 K is rather poorly ordered and is thermally unstable at higher temperatures. Heating this NiO/Ni(100) layer to 800 K produces a surface with mixtures of crystalline NiO(100) clusters and c(2 x 2)-O chemisorbed local structures. The long range order of the NiO(100) clusters could be improved by repeated cycles of oxygen adsorption at 300 K followed by heating to 800 K. The NiO(100) clusters obtained after 9 cycles of such dosing-annealing exhibit bulk-like properties, as suggested both by the off-specular EELS measurements and by the experimental observation that the intensities of the multiple loss features follow the expected Poisson distribution. The Ni-O bond strength of the NiO(100) clusters, estimated from the overtone spectra, is approximately 3.6 eV. In addition, the reduction of NiO(100) clusters by H-2 at 800 K has also been investigated. The NiO(100) clusters are reduced preferentially with respect to the c(2 x 2)-O overlayer, resulting in a reduction sequence of NiO(100) --> c(2 x 2)-O --> p(2 x 2)-O --> Ni(100).