Data courtesy
of:
Dr. David Sauer
INTEL, MS/AL3-66,
5200 N.E. Elam Young Parkway
Hillsborough, OR 97124
Prof. Eric M. Stuve
University of Washington
Department of Chemical
Engineering, BF-10
Seattle, WA 98195
November 1994
Carbon on
Platinum (111)
Sulphur on
Platinum (111)
Carbon Monoxide
on Platinum (111)
Change in the work
function of Pt(111)
as a function of carbon
coverage.
Graphitic carbon adlayers were deposited on Pt(111) by thermal decomposition
of ethylene and the work function of these adlayers was measured with the
KP5000 at room temperature as a function of carbon coverage. The work function
of this system decreases linearly with carbon coverage and reaches a saturated
value around one monolayer of carbon. The linear decrease in the work function
allows for a quick determination of carbon coverage. The decrease in the
work function indicates an electron transfer from the carbon to the platinum
substrate. This may seem surprising since carbon is more electronegative
than platinum and therefore a work function increase might be expected.
If the work function of graphite surface is taken into account, 4.5 eV,
which is much less than that of a clean Pt(111) surface (5.9 eV) the result
no longer seems surprising.
Change in the work
function of Pt(111)
as a function of sulfur
coverage.
The KP 5000 was also used to monitor the work function change of Pt(111)
with sulfur adlayers. The sulfur adlayers were formed by thermal decomposition
of hydrogen sulfide. The work function of the sulfur covered Pt(111) changes
rapidly at small coverages of sulfur and appears to reach a steady value
at the saturation coverage of sulfur (0.33 monolayers). The nonlinear decrease
in the work function indicates a changing dipole moment with increasing
sulfur coverage. As the coverage of the sulfur increases the repulsive
interactions of the sulfur become important thus changing the surface dipole.
The repulsive interactions may be influencing the amount of charge transferred
to the Pt(111) surface.
Changes in the work
function with CO exposure
(P=5 x 10-8 torr)
for increasing sulfur coverages, 
S, on Pt(111).
The work function with CO exposure was monitored in-situ for Pt(111) precovered
with sulfur. On a clean surface the work function of Pt(111) initially
decreases with exposure to CO and then increases. The initial decrease
in the work function has been associated with on-top CO absorption and
the subsequent increase is attributed to the population of bridge sites
[1,2]. As seen in the figure the initial decrease in the work function
with CO exposure is less pronounced as the sulfur pre-coverage increases.
The change in the initial work function changes can be accounted for by
either a decrease in the 5
donation of the CO to platinum or an increase in the 2
back donation of the platinum. An increase in 2
back donation would result in a strengthening of the CO platinum bond.
This is not seen with thermal desorption spectroscopy in which the peak
desorption temperature of CO decreases with sulfur coverage. Therefore,
a decrease in the 5
donation to the platinum seems to be the most likely explanation for the
work function changes due to addition of sulfur.