Ethynylbenzene Monolayers on Gold
A R T I C L E S
2
84.7, 285.3, 286.9, and 289.2 eV. Previous XPS experiments19
The C 1s spectrum (see Supporting Information) is significantly
different to that of the sample prepared using ethynylbenzene.
Peaks for carbon-containing compounds are seen at 289.1,
287.1, and 285.2 eV, which is similar to the sample prepared
using ethynylbenzene; however the observed atom ratios of
1:1.4:18.1, respectively, are markedly different. Furthermore,
significantly more carbon was deposited compared to the sample
prepared using ethynylbenzene. Using the reduction in intensity
of the Au 4f signal to gauge the relative layer thicknesses, a
total carbon to gold atom ratio of 2.4:1 was recorded for this
sample compared to 0.78:1 for the ethynylbenzene-containing
specimen. This behavior has been documented previously where
XPS indicated that carbon- and oxygen-containing contaminants
formed thicker layers than SAMs of long-chain alkanethiol
investigating ethynylbenzene bound to Cu(100) showed C 1s
signals at 283.2 eV, assigned to the two acetylenic carbon atoms
bound to the metal, together with signals at 284.2, 284.5, and
285.2 eV assigned to the carbon atoms of the aromatic ring. In
the current work, the signals at 285.3 eV (C) and 284.7 eV (D)
may be assigned to the carbon atoms of the aromatic ring by
comparison. However, the signal at 289.2 eV (A) is character-
istic of a carbon atom in a carboxylic acid group.20 Assignment
of the peak at 286.9 eV (B) is more speculative although it is
similar in binding energy to the value of 286.6 eV reported for
epoxide carbon atoms in phenyloxirane (styrene oxide) bound
2
1
to Ag(111).
The acetylenic carbons of ethynylbenzene shift to lower
19
24
binding energies of ∼283.2 eV when attached to Cu(100) and
molecules on gold surfaces. It was argued that the formation
2
2
Rh(100) surfaces. A di-σ bond between the alkynyl carbon
atoms and the metal surface is proposed as the likely bonding
mode. In the current work, this mode of bonding is not indicated
due to the absence of peaks below 284 eV. Inspection of the
of thiolate SAMs lowers the free surface energy. Thus, less
contaminant overlayers are physisorbed on the modified gold
surface compared to an unmodified surface.
The O 1s region of the spectrum is also different to that of
the ethynylbenzene sample. Signals at 533.5 and 532.3 eV are
observed while no signal at 531.2 eV is apparent. The carbon
to oxygen ratio is also significantly higher in these specimens.
Another significant difference is a peak in the N 1s region of
the spectrum. These results indicate that the XPS spectra
obtained with ethynylbenzene present are not simply the result
of the ethanol/ammonium hydroxide solution.
2
3
reported XPS spectra for styrene films on silver surfaces
reveals no peaks below 284 eV. DFT calculations indicated
2
1
that the signal assigned to the metal-bound carbon atom is
located at a binding energy that overlaps with those of most of
the aromatic carbon atoms. The current data are consistent with
a similar binding mode. Although we are unable to unambigu-
ously discern the identity of the molecular species, the data are
indicative of a mixture of oxidized aromatic hydrocarbon
species, such as phenylacetic acid and phenyloxirane. However,
we cannot rule the possibility of a surface-bound oxametallo-
cycle, which has been suggested as an intermediate structure
in the oxidation of styrene on Au(111).11
The XPS spectrum also contains signals in the O 1s region.
Deconvolution of the experimental data (see Supporting Infor-
mation) yields three peaks with binding energies of 533.6, 532.6,
and 531.2 eV. These data are consistent with the above
conclusions and suggest that there is more than one oxidized
species on the surface (for example, phenylacetic acid alone
should give only two O 1s signals and phenyloxirane should
give only one).
No signals due to any nitrogen species, which may arise from
the ammonium hydroxide, were observed suggesting that it does
not bind to the gold surface, nor react to form nitrogen-
containing organic compounds that bind to the gold.
An XPS spectrum of the same sample was recorded after
one week of exposure to an ambient atmosphere (away from
light). The spectrum is virtually identical in all respects
indicating that the monolayer is stable over this period of time.
This result also suggests that oxidation of ethynylbenzene occurs
during the deposition phase.
We conclude from the XPS data that the monolayer formed
from ethynylbenzene contains carbon and oxygen only, and that
the bound organic molecules are probably oxidized derivatives
of ethynylbenzene. Furthermore, no monolayer degradation
occurs after exposure to air for one week and the layer is
significantly different to that deposited on gold from the solvent/
base mixture.
Infrared Spectroscopy. Polarization modulation infrared
reflection absorption spectroscopy (PM-IRRAS) was used to
further characterize the films on gold surfaces. Data in the
-1
fingerprint region, 1200-1700 cm was obtained (see Sup-
porting Information); however, outside this region the signal-
to-noise ratio was such that no distinct bands could be reliably
identified. Bands consistent with carboxylate groups are ob-
-
1
served at 1397 and 1607 cm while bands consistent with
25
aromatic ring stretching modes are evident at 1512, 1607, and
-
1
1652 cm . We are more cautious with assignment of the band
-1
at 1250 cm . However a band at similar wavenumber has been
2
6
reported and assigned to ether groups. Although the PM-
IRRAS data does not enable unambiguous identification, the
data are consistent with oxygen-containing organic species
bound to the gold surface.
1H NMR Experiments. As mentioned previously, the oxida-
XPS experiments were performed using gold substrates
immersed in solution at 60 °C for 24 h without ethynylbenzene
to ascertain if the spectra discussed above result from exposure
of the gold surface to the ethanol/ammonium hydroxide solution.
tion of ethynylbenzene by O2 has been reported as well as the
oxidation of styrene catalyzed by planar gold surfaces. H NMR
experiments were performed to ascertain if the oxidation
products detected by XPS result simply from the preparation
conditions. A 1 mM solution of ethynylbenzene in deuterated
ethanol (ethanol-d6) was prepared and subjected to the same
1
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19) Iucci, G.; Carravetta, V.; Altamura, P.; Russo, M. V.; Paolucci, G.; Goldoni,
A.; Polzonetti, G. Chem. Phys. 2004, 302, 43.
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4
64, 388.
(24) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.;
Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
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2
004, 571, 139.
(25) Stoycheva, S.; Himmelhaus, M.; Fick, J.; Kornviakov, A.; Grunze, M.;
Ulman, A. Langmuir 2006, 22, 4170.
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J. AM. CHEM. SOC.
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