Surface EXAFS studies of chromium and titanium upon #alpha#-quartz (0001) surfaces.
In this thesis two studies of reactive metal adsorption upon a low index single
crystal silicon dioxide surface are presented in addition to a study of sulphur
adsorption upon a low index single crystal nickel surface.
Chromium growth upon the a-quartz Si02(0001) (J84xJ84) Rll 0 surface is studied
at three coverages, 0.25±O.08 ML, 0.5±O.16 ML and 1.0±0.33 ML, using surface
extended x-ray absorption fine structure (SEXAFS). SEXAFS measurements, from
the chromium K-edge, recorded at both grazing and normal incidence show that
chromium growth proceeds via the formation of mesoscopic particles with a body
centred cubic (b.c.c.) like structure having an average nearest neighbour Cr-Cr
distance of 2.36±O.03 A. This represents a contraction of 5.6 % from the bulk b.c.c.
lattice spacing of 2.49 A. There is no evidence of a surface reaction between chromium
and the surface oxygen.
SEXAFS was used to study titanium reactional growth on a-quartz (0001)
(J84xJ84) Rll 0 and (lx1). Three nominal coverages were studied, 0.25±O.08 ML,
0.5±O.16 ML and 1.0±O.33 ML. Both normal and grazing incidence SEXAFS data
were recorded and show the formation of a spatially extensive region in which an
interfacial reaction has occurred between surface oxygen and adsorbate titanium
atoms. Coupled with this is the formation of subnanometre titanium clusters. The
metal oxide has nearest neighbour Ti-O distances close to those of both the anatase
and rutile forms of titania with the metallic titanium clusters having a Ti-Ti distance
within experimental error that of bulk hexagonal close packed (h.c.p.) titanium,
A re-examination of the surface geometry of Ni(1l0)c(2x2)S using SEXAFS has
been performed. Data out to an electron wavevector of 9 A-I are analysed with
a new code to assess the influence of multiple scattering. The first shell S-Ni
distance is determined to be 2.20±O.02 A with the next nearest neighbour distance
being 2.29±O.02 A, giving a top-layer Ni expansion of 14±3% relative to
the bulk. The influence of multiple scattering does not significantly alter these
values from earlier studies.