Langmuir 1990, 6, 105-113
Acknowledgment. This work is supported by the Air Force Office of Scientific Research, the U S . Department of Energy, the Edison Program of Ohio, the Gas
105
Research Institute, Lehr Precision Inc., the National Institutes of Health, the National Science Foundation, and Pratt-Whitney, Inc.
Articles XPS Studies of Gold Films Prepared from Nonaqueous Gold Colloids Beng Jit Tan, Peter M. A. Sherwood,* and Kenneth J. Klabunde* Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506 Received January 3,1989. I n Final Form: June 23, 1989 XPS studies on gold and gold-palladium films prepared from nonaqueous colloidal solution show evidence of an unusual surface species. This "Au-carbon" species exhibits a positive shift in the core level binding energy. Applying a biasing potential to the sample enabled the separation of the Au 4f peaks of the surface species from those of the bulklike metal particles. The results of the biasing experiment are consistent with the model of the surface metal cluster being negatively charged and coated with a carbonaceous film of solvent fragments. The "Au-carbon" species are not stable to prolonged exposure to ambient conditions, agglomerating to form larger bulklike metal particles. The nature of the substrate on which the film is grown plays an important role in determining the formation of the surface species.
Introduction In recent years, we have been interested in clusters/ particles formed from metal atom accretion in low-temperature Unusual shapes and high reactivities for such particles have been realized, and this is probably due to the formation of surface structural features not usually encountered, since under these conditions particle growth will be kinetically controlled. These studies have led to a new preparative procedure for zero-valent metal heterogeneous catalysts, which we have termed solvated metal atom dispersed (SMAD).'v5 Both monometallic and bimetallic SMAD catalysts have exhibited unusual proper tie^.^" Their behavior can be explained by structural considerations mentioned above (defect sites, etc.) and by the fact that some carbonaceous fragments from the solvent end up incorporated in the particles, which can affect magnetics and electricalg properties as well as catalytic metal-support interactions, and the particle size of the metal c l u ~ t e r s . ~ J ~ (1)Klabunde, K. J.; Efner, H. F.; Murdock, T. D.; Ropple, R. J. Am. Chem. SOC.1976.98. 1021. (2) Klabunde, K: J. In Chemistry of Free Atoms and Particles; Academic: New York, 1980.
(3) Davis, S. C.; Klabunde, K. J. Chem. Reu. 1982,82,153. (4) Tan, B. J.; Klabunde, K. J. 'Solvated Metal Atom Dispersed Catalysts-A Comprehensive Review", to be published. (5) Imizu, Y.; Klabunde, K. J. In Catalysis of Organic Reactions; Augustine, R. L., Ed.; Marcel Dekker: New York, 1985. (6) (a) Klabunde, K. J.; Davis, C.; Hattori, H.; Tanaka, Y. J . Catal. 1978,54,254. (b) Klabunde, K. J.; Tanaka, Y. J . Mol. Catal. 1983,21, 57. (7) (a) Kanai, H.; Tan, B. J.; Klabunde, K. J. Langmuir 1986,2, 760. (b) Li, X.Y.; Klabunde, K. J. Langmuir 1987,3,558.
0743-7463/90/2406-0105$02.50/0
More recently, we have reported on a modification of the SMAD procedure which has allowed the synthesis of stable, nonaqueous metal colloids. In particular, noble metal atoms solvated in polar organics can lead to stable colloidal dispersions of indefinite stability.13-ls The method seems to be wide in scope and allows entry into a series of hitherto unavailable nonaqueous colloidal systems. An additional interesting feature is the ability of some of these dispersions to form metallic films simply by solvent removal. In order to understand these materials better, such as particle formation and film growth, we have chosen a colloidal suspension of gold in acetone as a model. Gold is very suitable for XPS studies and one that we have studied as part of a program to investigate electrode surface oxidation.16 By studying films grown from goldacetone solutions, we hope to learn something about par(8) Davis, s. C.; Severson, s.;Klabunde, K. J. J. Am. Chem. SOC. 1981,103,3024. (9) Davis, S. C.; Klabunde, K. J. J. Am. Chem. SOC.1978,ZOO,5973. (10) Klabunde, K. J.; Ralston, D.; Zoellner, R.; Hattori, H.; Tanaka, Y.J. Catal. 1978,55,213. (11) Matsuo, K.; Klabunde, K. J. J. Org. Chem. 1982,47,843. (12) Ralston, D. H.;Klabunde, K. J. Appl. Catal. 1982,47,843. (13) Lin, S.-T.;Franklin, M. T.; Klabunde, K. J. Langmuir 1986,2, 259. (14) Franklin, M. T.; Klabunde, K. J. In "Living Colloidal Metal Particles From Solvated Metal Atoms: Clustering of Metal Atoms in Organic Media"; High Energy Processes in Organometallic Chemistry; ACS Symposium Series 33, American Chemical Society: Washington, D.C., 1987;pp 246-259. (15) Cardenas-Trivino, G.;Klabunde, K. J.; Dale, E. B. Langmuir 1987,3,986. (16) Sherwood, P.M. A. Chem. SOC.Reu. 1985,14,1.
0 1990 American Chemical Society
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ticle structure a n d the particle to film conversion. We have extended these studies to gold-palladium bimetallic particles as well.
Experimental Section Preparation of Gold Films. Au colloids and Au-Pd colloids were prepared by the metal vapor SMAD method, which has been described Acetone was used as the solvating medium; 0.2-0.3 g (1.0-1.5 mmol) of gold was vaporized over a period of 90 min by using 100 mL (1.35 mol) of acetone. The final product was a purple gold or gold-palladium colloidal s o h tion. The acetone was then removed in vacuo, leaving a film of gold or gold-palladium on the glass walls of the Schlenk ware. SMAD Au films were also prepared on silver foil by the spot dry method. About 2 mL of the colloidal solution was placed on the silver foil and dried by passing a stream of nitrogen over the foil. This was repeated several times until the the entire silver foil was covered by a film of gold or gold-palladium. X P S E x p e r i m e n t s . X-ray photoelectron spectra were obtained by using an AEI ES2OOB spectrometer with Mg Xradiation (hv = 1253.6 eV). The power was 240 W (12 kV and 20 mA), and the vacuum of the sample chamber was in the range 10-8-10-9 Torr. The spectrometer was operated in the FRR mode. The pressure and residual gases in the sample chamber were monitored by a VSW Vacuum Analyst mass spectrometer with a 10% valley resolution (Le., two equal-intensity peaks separated by 1 amu have a valley of 10%) and a mass range of 1-100 amu. The gold film was scraped from the walls of the Schlenk ware and mounted onto the sample probe with the aid of double-sided sticky tape. The film came off the reactor in the form of "flakes". These flakes were then pressed onto the tape so that the sample was more like that of a thin metal plate pressed t o stick t o the tape than of a powder pressed to the tape. This sample arrangement allowed us to perform angleresolved core XPS studies in order to identify any surface species present by examining spectra a t a small (10") takeoff angle. The sample was prepared so that the "flakes" had good electrical contact with the sample block, despite being mounted on tape (caused by overlap of tape-mounted flakes and the copper of the underlying block). Biasing experiments were also carried out by either applying a positive or negative potential to the sample. Aging experiments were carried out starting with a fresh sample. Spectra of the fresh sample were recorded. The sample was then exposed to ambient conditions for 2 days and then a further period of 12 days (14 days total). The experiments reported here were nearly all repeated several times to ensure that artifacts due to poor sample preparation (which might lead to mechanical macroscopic features, e.g., changes related to sample packing and poor electrical contact with the block) could be eliminated. We also recorded the spectra of a gold palladium alloy film prepared by vaporizing a length of commercially prepared 40% Au-60% Pd (by weight) alloy onto double-sided sticky tape stuck to the sample block of the spectrometer probe. The thickness of the film deposited onto the tape was about 100 A. The spectra of gold powder.on a piece of gold foil were also recorded. Spectra were also recorded for a freshly prepared sample examined on a liquid nitrogen cooled cold block. The temperature of the block was kept at 77 K, though the sample surface temperature will be higher due to radiative heating from the X-ray gun (from other work in our laboratory we know that the temperature of the sample surface is a t least as low as 200 K under these conditions). We monitored the gas phase using the quadrupole mass spectrometer attached to the sample chamber. We found the spectrometer vacuum near the sample to contain no acetone vapor or hydrocarbons after pumpdown, and the vacuum appeared very similar to the normal background instrument vacuum (traces of nitrogen and oxygen were presentpresumably due to a tiny leak through the Viton "0"rings of the insertion lock seals. The shaft of the probe acted as a cryopump during this experiment, removing many potential contaminants from the sample surface. Data were collected by using an Apple I1 microcomputer and transferred to an IBM PC/AT for data analysis. The curve fitting was carried out with a nonlinear least-squares curve fit-
Binding Energy ( e V )
Figure 1. Au 4f spectrum of reference bulk gold samples and SMAD Au-acetone film: (a) gold foil; (b) gold powder; (c) gold powder on gold foil; (d) gold SMAD at normal angle; (e) gold SMAD a t surface-sensitive angle. All spectra were taken a t normal angles except for Figure le, in which case the spectrum was taken at surface-sensitive angles. However, the spectra in Figure la-c were unaltered when examined at surface-sensitive angles. ting program with a Gaussian/Lorentzian f u n ~ t i o n . ' ~The binding energies were referenced to the C 1s binding energy of residual hydrocarbon, taken as 284.6 eV.
Results Core Region. 1. Chemically Shifted Species and Surface Charging. In t h i s paper, we present informat i o n for at least two types of species on the SMAD sample surface. One species has a binding energy characteristic of bulk metal; the other species (or species) has a binding energy greater than that of the bulk metal. We report sample biasing experiments which show that this high binding energy species behaves differently from that of the bulk metal on sample biasing. We first wished to establish whether this higher binding energy species was peculiar to the SMAD catalyst or an artifact of certain gold samples. Figure 1 shows the spectra of various gold samples recorded at normal and surface-sensitive takeoff angles (known f r o m previous experiments on our spectrometer to give a considerable magnification of the intensity of a n y surface species). Gold foil (Figure l a ) shows one set of spin-orbit split doublets i n the A u 4f core region. Identical spectra are found for gold powder (which will have gold particles with a surface hydrocarbon film) on tape and gold powder on gold. Gold SMAD at normal angles appears as normal gold but at low takeoff angle shows the shoulder at higher binding energy. The higher binding energy species is thus peculiar to the gold S M A D sample. Figure 2 and Table I show how the spectra can be well f i t t e d to three sets of doublets, t w o of which correspond to the high binding species. Gold oxide is not normally seen on gold sur(17) Sherwood, P. M. A. In "Data Analysis in X-Ray Photoelectron Spectroscopy"; Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1983; pp 445-476.
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Langmuir, Vol. 6, NO.1, 1990 107
Films
A
Au4f
~a
I
b
b
A 92
90
88
86
84
82
80
Binding Energy (eV) Figure 2. Au 4f spectrum of the SMAD Au-acetone film taken at surface-sensitive angles. The spectrum is fitted (solid lines) to the experimental data (*) with three sets of spin-orbit spin doublets (area ratio 3:4) by using a Gaussian-Lorentzian product function. A nonlinear background was taken off from the spectrum before curve fitting the data. Table I. Curve-Fitting Results for Au SMAD Film binding energy, eV" area ratio normal surface-sensitive normal surface-sensitive level angle angle angle angle 4f7,, 83.9 83.8 1.000.05:0.05 1.00.17:0.35
4f,,,
(1.0) 85.8 (1.2) 86.3 (1.2) 87.5 (1.0) 89.5 (1.2) 90.1 (1.2)
(0.9) 85.5 (1.3) 86.3 (1.3) 87.4 (0.9) 89.3 (1.3) 90.0 (1.3)
fwhm value (eV) is in parentheses below the binding energy valve to which it corresponds.
faces but can be formed electrochemically.2s The gold oxide shift is in the right range (1.9 eV) for the observed features; however, our previous work on gold oxideB shows that it is rapidly decomposed in the X-ray beam. No such rapid loss of the high binding energy species was observed in our work. Experiments where the SMAD sample is exposed to an external positive and negative charge are discussed below. A t this point, the reader may wonder whether the high binding energy species is some charging artifact
"7
(18) Shevchik, N.J. J. Phys. F 1975,5,1860. (19) Baird, P. J.; Wagner, L. F.; Fadley, C. S. Phys. Reu. Lett. 1976, ,.I.
J I , 111.
(20) McFeely, F. R.; Stohr, J.; Apai, G.; Wehner, P. S.; Shirley, D. A. Phys. Reu. B 1976,14,3273. (21) (a) Schofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,8,129.(b) Band, I. M.; Kharitonov, Y. I.; Trzhaskovskaya, M. B. A t . Data Nucl. Data Tables 1979,23,443. (22) Castelliani, N. J.; Leroy, D. B.; Lambrecht, W. Chem. Phys. 1985,95,459. (23) Cheung, T. P. P. Surf. Sci. 1984,140,151. (24) Roulet, H.; Mariot, J.-M.; Dufour, G.; Hague, C. F. J. Phrs. F 1980,IO, 1025. (25) De Crescenzi, M. Surf. Sci. 1985,162,838. (26) Wertheim, G. K.: Di Cenzo, S. B.; Buchanan. D. N. E.: Bennette, P. A. Solid State Commun. 1985,53,377. (27) Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom. 1973,2,441. (28) Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1975,71, 298.
90
88
88
84
88
300
285
280
285
280
Binding Energy (e\')
Figure 3. Au 4f and C 1s spectra of the SMAD Au-acetone film freshly prepared and then studied on a probe a t normal angles a t liquid nitrogen temperature (a). Spectra b show the spectra from the film after it has been allowed to warm up to ambient temperature.
associated with small SMAD particles surrounded by a hydrocarbon shell giving rise to different sample charging than the bulk gold. Certainly no such effect is seen for gold powder in our instrument. The high binding energy SMAD peaks do shift considerably with positive bias, suggesting that they are in reasonably good electrical contact with the sample holder. We estimate the surface charge on the SMAD sample of Figure 1 to be less than 1 eV, and thus charging effects would seem not to be able to explain the high binding energy species shift of 1.7 and 2.5 eV from the bulk gold. Differential charging effects are often complex, so there is some uncertainty as to their possible contribution to the shifts of the high binding energy species. Further insight on these high binding energy species is provided by the spectra of a freshly prepared sample mounted on a sample block cooled to liquid nitrogen temperature. Figure 3 (recorded at normal takeoff angle) illustrates that at these low temperatures the Au 4f region shows the high binding energy species forms the most intense feature in the spectrum and that this is accompanied by a feature in the C 1s spectrum that is shifted to higher binding energy. Both these higher binding energy features are lost when the sample is warmed up to room temperature. The higher binding energy species in the Au 4f and C 1s spectra have different intensities. The C Is spectra contains a strong surface hydrocarbon feature which dominates the spectrum. If the high binding energy Au 4f species arises solely from differential charging effects, then the shift in the Au 4f region (about 1.5 eV) would be expected to be the same as that in the C 1s region. There is no species shifted by 1.5 eV in the C 1s region, the main high binding energy species being shifted by about 6 eV from the main C 1s peak. This observation, combined with our observation that the surface charging shift is less than 1 eV, leads us to consider it reasonable to suggest that the high binding feature in the Au 4f region is shifted, in the absence of an applied bias (where the larger shifts appear to be caused by a charging effect), by a difference in the electronic environment of the gold
108 Langmuir, Vol. 6, No. 1, 1990
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C
92
90
00
86
84
82
80
116
108
iaa
92
84
B i n d i n g E n e r g y (eY) Figure 4. Au 4f spectra of a SMAD Au-acetone film undergoing aging. The film was aged in a desiccator. Spectra were collected with the fresh film (a), after 2 days (b), and after 14 days (c).
B i n d i n g E n e r g y (eb') Figure 5. Au 4f spectra of a SMAD Au-acetone film under the influence of an applied positive bias potential. The bias potential applied is indicated on the right-hand side of the diagram.
SMAD particles. For reasons discussed below, we will refer to such higher binding energy gold SMAD species as "Au-carbon" species. Room temperature low takeoff angle experiments (Figure le) show this species to be a surface species. The greater intensity of the Au-carbon species at liquid nitrogen temperatures and the observed diminishing of the intensity of this species over time for room temperature studies suggest that the Au-carbon species converts to bulk goldlike species more rapidly a t the higher temperatures. We will suggest below that the carbon species on the outside of the Au-carbon species will involve mostly hydrocarbon carbon, together with some oxygenated carbon species. Some of the oxygenated carbon species may be associated with the high binding energy tail on the C 1s region, though the most pronounced feature of this region at 6-eV shift represents a large shift for oxygenated carbon. Some of the 6-eVshifted feature may be associated with adsorbed carbon species from the interior of the SMAD a t liquid nitrogen temperatures. There may be some contribution from differential charging to the size of the 6-eV shift. 2. Aging Experiments. Upon aging of the SMAD gold film under ambient conditions, the Au 4f peak intensities of the surface-sensitive species (referred to as Aucarbon species) decreases with time relative to the bulk species. This can be seen in Figure 4 as a decrease in peak heights of the higher binding energy peaks relative to those of the bulk gold corresponding to an area ratio of the Au-carbon species of 0.67, 0.52, and 0.20 after 0, 2, and 14 days, respectively. 3. Biasing Experiments. We have used sample biasing as an additional probe of Au-carbon species for many years. We findz7that nonconducting species can be shifted considerably on biasing, especially when large bias potentials are used. We always find that the behavior on positive biasing and negative biasing is of the same magnitude, with the shift reversed on a change of bias p ~ l a r i t y . ' ~For example, a single-crystal sample of silicon with an air-formed oxide layer gives an XPS spectrum showing both oxide and underlying elemental silicon. Both peaks shift 20 eV (h0.1 eV) on applying a 20-
V bias, the shift being to higher binding energies when a positive bias is used and to lower binding energies when a negative bias is used. Similar results are obtained for a gold powder sample subjected to a 10-V bias. Lateral heterogeneity, which may be present to an extent in the SMAD films, could complicate the behavior of the films to biasing. When the sample was biased with a positive potential, the Au-carbon species did not shift by as much as that of the bulk species; that is, the Au-carbon species lagged behind the bulk species. Figure 5 illustrates how the spectra change with some of the bias voltages used, and Table I1 lists all the results including the results of fitting the spectra. The results shows a linear shift with potential, typical of a situation where one species is more insulating than the other and thus cannot follow the applied bias. The actual shift depends upon the ability of the sample to respond to applied potential changes and the effect that these potential changes have on the secondary electron bombardment within the chamber. This result thus suggests that the Au-carbon species is not as good a conductor as the bulk gold and so lags behind the bulk species, especially at large applied potentials. The biasing effect is reversible; that is, when the biasing potential is switched off, the original spectrum is obtained. The results obtained on biasing the sample with a negative potential were quite different from the results expected for a normal XPS sample. To our knowledge, this is the first time that negative biasing has behaved differently from positive biasing. As expected, the bulk gold peaks shifted with the applied negative potential, but the Au-carbon species showed no shift (other than a small shift in the opposite direction to that expected) with applied potential. Figure 6 and Table I11 show the results. 4. SMAD Gold-Palladium Film. SMAD Au-Pd films were prepared on both on glass and on a silver foil. The sample prepared on glass showed a similar behavior to that seen with the SMAD Au films. The sample prepared on silver showed no effect, illustrating the impor-
XPS Studies
Langmuir, Vol. 6, No. 1, 1990 109
of Gold Films
Table 11. Results of the Application of a Positive Bias Potential to Au SMAD Film at a Surface-Sensitive Angle
level Au 4f7,2
displacement from no applied bias potential, eV
kinetic energy, eV" 1165.6 (1.0) 1162.5 (1.2) 1163.0 (1.2) 1151.9 (1.0) 1153.4 (1.3) 1155.5 (1.3) 1145.6 (0.9) 1147.6 (1.5) 1148.3 (1.5) 1134.3 (1.2) 1137.7 (1.3) 1139.4
bias potential, V 0
+15
+20
+32
~
Table 111. Results of the Application of a Negative Bias Potential to the Au SMAD Film at a Surface-Sensitive Anale displacement kinetic from no bias energy, applied bias level potential, V eV" potential, eV Au 4f7/2 0.0 1166.3 (1.0)
13.7 9.2 -2.0 7.5 20.0 15.2 14.3 -5.0 31.3
23.6
Ob
-10.0
-15.0
I
1I
\
1.3 1.1
-0.3 -4.9
1161.5 (1.3) 1162.5 (1.3) 1163.4 (1.3) 1176.3 (1.1) 1161.2 (1.0) 1162.7 (1.4) 1164.0 (1.5) 1180.9 (0.9) 1160.3
1.6 1.5 1.2 -10.0 1.9 1.3 0.6 -14.6 2.8
(1.2)
1
d
-1.4
(0.9)
24.8
"fwhm value (eV) is in parentheses below the kinetic energy value to which it corresponds. *Bias potential removed at the end of the experiment.
1163.1 (1.2) 1164.6 (1.1) 1164.6 (1.2) 1167.7 (1.0) 1161.8 (1.2) 1162.9 (1.2) 1164.9 (1.2) 1171.2
li
-lov -20.0
1161.9 (1.5) 1163.1 (1.5) 1185.8 (1.0) 1159.6 (1.1) 1160.9 (1.5) 1163.0
2.1
1.5 -19.5 3.5 3.1 1.6
(1.5)
100
92
84
76
68
Binding Energy ( e V )
Figure 6. Au 4f spectra of a SMAD Au-acetone film under the influence of an applied negative bias potential. The bias potential applied is indicated on the right-hand side of the diagram.
tance of the substrate used in forming the SMAD films. The Au 4f spectrum at low takeoff angle showed high binding energy features as before (Figure 7 ) . Positive biasing (Table IV) gave similar results to those of the Au SMAD films. The Pd 3d (Figure 8 and Table V) region
" fwhm value (eV) is in parentheses below the kinetic energy value to which it corresponds. showed high binding energy features, but in this case some PdO contribution was present. No high binding energy features were found in the Au 4f or Pd 3d region of a conventional (non SMAD) 40 Au:60 Pd commercial alloy, implying that alloy effects are not responsible for the observed features. Valence Band Region. Valuable information can be obtained from the valance band spectra. The narrowing of the metal d band as well as the 5d splitting provides information on the bonding in alloys18 and the density of state^.^^'^^ Figure 9 summarizes the valence bands of the SMAD Au film, SMAD Au-Pd film, commercial bulk metals and the 40 Au:60 P d alloy film (non SMAD), while Table VI lists their binding energies and spin-orbit separation of the Au 5d band. The Au 5d bands of the SMAD Au film and SMAD Au-Pd film resemble those of bulk Au metal.
110 Langmuir, Vol. 6, No. 1, 1990
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Table IV. Results of Applying a Positive Potential to Au-PD SMAD Film at a Surface-Sensitive Angle shift from no bias kinetic applied bias level potential, V energy, eV" potential, eV 0
+3.0
+10.0
+15.0
+20.0
1165.6 (1.0) 1164.0 (1.5) 1163.1 (1.5) 1163.1 (1.0) 1162.1 (1.3) 1160.9 (1.3) 1158.2 (1.0) 1159.8 (1.0) 1158.7 (1.0) 1151.6 (1.0) 1153.2 (1.3) 1154.2 (1.3) 1146.7 (1.0) 1149.3 (1.5) 1147.8 (1.5)
C
m
B i n d i n g E n e r g y (eV)
F i g u r e 7. Au 4f XPS spectra of SMAD Au-Pd films (a-c) and bulk Au-Pd alloy film (d): (a) SMAD Au-Pd film taken a t normal angle; (b) SMAD Au-Pd film taken at surface-sensitive angle; (c) SMAD Au-Pd film grown on silver foil; (d) bulk Au-Pd alloy film.
4f7p
335.2
M u E ! 4 3 ? d ! ! d l u o I J p
80
B i n d i n g Energy (eV)
Au
a
F i g u r e 8. P d 3d XPS spectrum of SMAD film and bulk AuP d alloy film. All spectra were taken a t surface-sensitive angles: (a) SMAD P d film; (b) SMAD Au-Pd film; (c) SMAD Au-Pd film grown on Ag foil; (d) bulk Au-Pd alloy film. Table V. Core-Level Binding Energies* (eV) for Various Gold Samples sample Au 4f7/, Au 4fSl2 Pd 3d,,, Pd 3d,/, 0 1s gold foil 83.8 87.5 Au SMAD film Au-Pd SMAD film
2.5 1.9 2.2 7.4
Au-Pd SMAD film on Ag foil 40 Au:60 Pd alloy film Pd film
(1.0) 84.0 (1.0) 83.8 (1.0) 83.9 (1.0) 83.8 (1.1)
(1.0) 87.7 (1.0) 87.5 (1.0) 87.5 (1.0) 87.5 (1.1)
335.3 (1.3) 335.2 (1.4) 335.2 (1.3) 335.4 (1.3)
340.6 (1.3) 340.5 (1.4) 340.4 (1.3) 340.7 (1.3)
532.5 (1.9) 532.5 (2.4) 532.3 (2.4) 532.5 (2.6) 532.5 (2.4)
a Binding energy with respect to C 1s (284.6 eV) spectra taken at normal angle to sample surface. The fwhm value (eV) is in parentheses below the binding energy value to which it corresponds.
4.2 4.4 14.0 10.8 8.9 18.9 14.7 16.1
a fwhm value (eV) is in parentheses below the kinetic energy value to which it corresponds.
No narrowing of the d band or decrease in the 5d band splitting is seen. There is also no appearance of a carbon band. This is due not to the fact that there is no carbon present but rather that the photoelectron cross sections of gold and palladium are so much greater than carbon that the carbon valence band is hidden in the Au and P d bands.21
We have carried out spectral subtraction of the gold valence band from both the SMAD Au-Pd and the bulk Au-Pd spectra. On subtracting the valence band spectrum of bulk Au from the SMAD Au-Pd valence band spectrum, we obtain a difference spectrum that resembles the valence band spectrum of PdO (see Figure 1Oc). Subtracting a SMAD Au valence band spectrum from that of the SMAD Au film gives a similar result. A similar spectral subtraction was performed on the bulk AuPd alloy, resulting in the difference spectrum shown in Figure I l d . Further subtraction of the P d valence band from this difference spectrum results in yet another difference spectrum (Figure l l e ) that resembles the PdO valence band spectrum. Thus spectral subtraction procedures suggest that the SMAD Au-Pd film is composed of Au and PdO, but the bulk alloy is made up of Au, Pd, and PdO. An important difference between the difference spectra obtained above (Figures 1Oc and l l e ) and that of PdO is that the former have a narrower line width than that of the PdO valence band spectrum. This may be due to the effects of bonding in the alloy and the SMAD Au-
X P S Studies of Gold Films
Langmuir, Vol. 6, No. 1, 1990 111
I
J
\
n -
Id
i? 1
0
8
4
0
Binding Energy ( e V )
16 12 8
4
E,-4
Binding Energy (eV)
Figure 9. XPS valence band spectrum: (a) bulk Pd foil; (b) SMAD Au-Pd film grown on Ag foil; (c) SMAD Au-Pd film; (d) SMAD Au film; (e) bulk Au foil.
Figure 10. Spectral subtraction of the valence band of bulk Au (b) from the valence band of SMAD Au-Pd (a). All spectra have a width of 18.3 eV. The difference spectrum, c, resembles the valence band of PdO but has a narrower line width (see discussion in text).
Table VI. Valence Band Parameters
Au foil Au SMAD film Au-Pd SMAD film Au-Pd SMAD film on Ag foil
5.7 5.1 5.4 5.4
3.1 3.0 3.0 3.0
2.6 2.6 2.4 2.4
5.3 5.5 6.9
6.8
Pd film, the PdO being immersed in a matrix of metal rather than in bulk PdO.
Discussion We need to establish the correct cause of the shifts that we have observed in this work. In the absence of an applied external bias, we have found shifts that we have associated with the different electronic environments of the Au-carbon species on the SMAD films. Such shifts will be considered below in terms of the shifts reported for small atom clusters reported by other workers.22-26 When any XPS core level shift is recorded, it is important to be sure that the shift is caused by a real change in chemical environment rather than other factors such as differential surface ~harging,~’ surface oxidation, etc. For reasons given above, we consider a significant part of the shift of the Au-carbon species with no bias is due to differences in electronic environment. On biasing, especially to large potentials, we do see shifts, but we consider that these shifts are caused by electrostatic effects associated with the solvent shell coating of the cluster rather than a normal differential charging effect. In particular, the unusual behavior on negative biasing establishes that our observed shifts are not caused by normal biasing behavior. Further, gold powder in our instrument shows no differential charging on the application of a similar bias. Gold oxide (or hydroxide or oxyhydroxide) can be eliminated for the reasons given above.
I f I
n ,
/
5
8
8
,
I
,
4
,
,
8
,
0
Binding Energy (eV)
Figure 11. Spectral subtraction of the valence band of bulk Au-Pd alloy film: (a) valence band of bulk Au-Pd alloy film; (b) valence band of bulk Au foil; (c) valence band of bulk Pd foil; (d) result of subtracting b from a; (e) result of subtracting c from d; (f) valence band of PdO. 1. Shifts Due t o Electronic Cluster Environment. Our shifts on unbiased samples could arise from the shift associated with small metal particles and/or the shift associated with Au-carbon species. A number of papers have appeared in which shifts are observed as a function of coverage (and hence of cluster size) when a metal is deposited on a relatively inert The interpretations of positive binding shifts of core levels of small metal clusters are rather controversial. The origins of such shifts have been discussed in detail by Wat-
112 Langmuir, Vol. 6, No. I , 1990
Tan et al.
son and c o - w ~ r k e r s .Of ~ ~the many factors discussed, two a system similar but not identical with metal atoms/ have received the most attention, namely, initial-state clusters supported on amorphous carbon. e f f e ~ t s ~ ' *and ~ l final-state effects.z6 Studies of clusters Transmission electron micrographs of our gold colon ~ a r b o n , ~ 'a- l~~~m i n a , ~ ~and - ~ 's i l i ~ a ~have ~ , ~ been ' loids show an average particle diameter of 20 A,14 though reported. Related experiments have involved the implanthere are some smaller particles present. The films formed tation of atoms into matrices such as silica43 and appear optically smooth to the eye. However, SEM shows alumina.44 The observed shifts generally occur toward a bulk film with a surface of some roughness (hills and higher binding energy when the clusters are supported valleys corresponding to 20 A or less building blocks just on relatively inert substrates and toward lower binding before fusing (amalgamating) together). Lee and coenergy when reactive surfaces such as single-crystal metw o r k e r ~have ~ ~ reported that gold clusters with an averals are used as supports. Core surface chemical shifts age diameter of about 19 A will begin to show bulklike have been observed for metals in a high state of purity properties. We believe that the SMAD particles that we under full UHV conditions at surface-sensitive angles. have described as Au-carbon species are smaller than 19 Such shifts are negative shifts and have been reported and thus might be expected to show the binding energy by Citrin and Wertheim,45 Wertheim and B ~ c h a n a n , ~ ~ shifts associated with metal clusters. Our SEM obserand Salmeron and c o - ~ o r k e r s .We ~ ~do not see such shifts vations support the suggestion that these smaller clusin non SMAD samples run under comparable conditions ters are a t the SMAD film surface. Through time, these to our SMAD films (Figure 1). This is not surprising smaller clusters will agglomerate to form larger clusters since such surface shifts are smaller than the shifts seen and become bulklike. Hence, when these films are aged, with the SMAD films and generally require monochrothe ratio of the amount of the Au-carbon species to that matized XPS radiation to be clearly observed. of bulklike gold will decrease. This is reflected in a We believe that our shifts on unbiased samples are due decrease in intensity of the higher binding energy peaks to the shifts associated with small metal particles. Thus, to that of bulk gold peaks in our aging experiment. the core level shifts seen in our SMAD Au and SMAD Though the covaporization and cocondensation of Pd Au-Pd films are very similar to positive core level bindand Au do not seem to cause any spectral changes due ing energy shifts for metals deposited on relatively inert to alloy formation in the Au 4f region, it is difficult for substrates such as amorphous carbon, alumina, and silus to conclude, a t present, from the Au 4f spectra whether ica. We conclude, therefore, that our surface gold spealloy formation did take place or not. This is because cies behave as gold clusters supported on amorphous carAu 4f core level shifts in gold alloys are also toward higher bon. The origin of the amorphous carbon is from the binding en erg^.^'^.^' In fact, Mason30a reported that the way these films are prepared with the SMAD technique. XPS spectra of a Au-Cd alloy and Au supported on amorDuring the SMAD reaction, metal atoms cocondense phous carbon are very similar and that the only signifiwith acetone molecules, forming a frozen matrix of metal cant difference between the alloy and cluster spectra is solvate a t liquid nitrogen temperature. On slow warmin the line width broadening observed in the clusters at ing to room temperature, the cluster formation process low coverages. We do not observe a broader line width begins. In addition to cluster formation, there exist comin the case of the higher binding energy peaks. petitive pathways through which the metal atoms react Alloy formation is usually accompanied by a positive with the solvent molecules, which leads to the formation shift of the Au 5d bands and a decrease in the spin-orof metal organic fragments. Hence, in the final product, bit splitting of the 5d band.30a,49 The Au 5d bands of there exist clusters of gold and/or palladium atoms surthe Au-Pd films do not show such features, and hence rounded by carbonaceous fragments. In essence, we have either Au-Pd alloy formation in our SMAD system did not take place or so little alloy is formed that its presence does not alter the bulk spectral features of the Au (29) Watson, R. E.; Perlman, M. L. Struct. Bonding (Berlin) 1975, 24, 83. 5d band. It is important to note when comparing the (30) (a) Mason, M. G. Phys. Reu. B 1983, 27, 748. (b) Mason, M. spectra of the SMAD Au-Pd films with those of the comG.; Gerenser, L. J.; Lee, S. T. Phys. Reu. Lett. 1977,39, 288. mercial Au-Pd alloy film that the latter do not exhibit (31) (a) Egelhoff, W. F.; Tibbettes, G. G. Phys. Reu. B 1979, 19, 5028. (b) Egelhoff, W. F.; Tibhettes, G. G. Solid State Commun. 1979, the core level shifted peaks seen in the SMAD films. -29. _ ,52 As Figure 8 shows, the Au-carbon species is also present (32) Lee, S. T.; Apai, G.; Mason, M. G.; Benbow, R.; Hurych, Z. in both the SMAD Pd and the SMAD Au-Pd films. Again, Phys. Reu. B 1981,23,505. (33) Hamilton, J. F.; Preuss, D. R.; Apai, G. R. Surf. Sci. 1981, 106, the surface species is not present when the SMAD film 146. is grown on Ag foil just as in the case with the SMAD (34) Oberli, L.; Monot, R.; Marthieu, H. J.; Landolt, D.; Buttet, J. gold films. Also, the Pd 3d spectrum of the commercial Surf. Sci. 1981, 106, 301. Au-Pd alloy film does not show any structure resem(35) Cheung, T. P. P. Surf. Sci. 1983, 127, L129. (36) Wertheim, G. K.; DiCenzo, S. B.; Youngquist, S. E. Phys. Reu. -. bling the positively core level shifted peaks. We will report Lett. 1983,51, 2310. further on the Pd films later when more work has been (37) Liang, K. S.; Salaneck, W. R.; Aksay, I. A. Solid State Commun. 1976, 1 6 329. done. For now, it will suffice to know that surface clus(38) Kohiki, S. Appl. Surf. Sci. 1986, 34, 3786. ters are also present in the SMAD Pd films prepared by (39) Kohiki. S.: Ikeda. S. Phvs. Reu. B 1986, 34. 3786. our technique. (40) Fritsch, A,; Legare, P. Surf. Sci. 1987, 184, L355. 2. Shifts Caused by Sample Biasing Experi(41) Takasu, Y.; Unwin, R.; Tesche, B.; Bradshaw, A. M.; Grunze, M. Surf. Sci. 1978, 77, 219. ments. The most interesting characteristics of our SMAD (42) Murgai, V.; Raaen, S.; Strongin, M.; Garrett, R. F. Phys. Reu. gold films were seen in the biasing experiments. Earlier E 1986,33, 4345. work on SMAD gold colloid showed that the colloid pos(43) Young, V. Y . ;Gibbs, R. A,; Winograd, N. J . Chem. Phys. 1979, 79, 5714. sesses a negative charge.l4,l5 The biasing experiments (44) Gibbs, R. A.; Winograd, N.; Young, V. Y. J . Chem. Phys. 1980, carried out in our present work are consistent with the 72. _. 4799. .. suggestion that the Au colloidal particles are covered by (45) (a) Citrin, P. H.; Wertheim, G. K. Phys. Reu. B 1983,27, 3160. ~
(b) Citrin, P. H.; Wertheim, G. K. Phys. Reu. E 1983,27, 3176. (46) Wertheim, G. K.; Buchanan, D. N. E. Phys. Reo. B 1986, 33,
914. .- ..
(47) Salmeron, M.; Ferrer, S.; Jazzar, M.; Somorjai, G. A. Phys. Reu. B 1983,28, 1158.
(48) Shevchik, N. J. J . Phys. F 1975,5, 1860. (49) Chye, P. W.; Lindau, J.; Pianetta, P.; Garner, C. M.; Spicer, W. E. Phys. Lett. 1977,63A, 387.
Langmuir, Vol. 6, No. 1, 1990 113
X P S Studies of Gold Films
a ) Positive
Bias
"lKd Electrons
b)
Negative Bias
BULK
1
GOLD
Electrons
F i g u r e 12. Model to explain the phenomena observed when a biasing potential is applied to the SMAD Au film. Applying a positive bias is equivalent to drawing electrons away from the surface clusters, while applying a negative bias is equivalent to pumping electrons into the surface clusters. The carbonaceous layer around the surface gold clusters acts as a selective barrier to the direction of electron flow, providing a greater resistance to electron flow from bulk gold to the surface gold clusters as compared to the flow of electrons in the opposite direction.
a negatively charged carbonaceous film surrounding the surface gold clusters. Since the bulk gold is in direct electrical contact with the sample probe, the Au 4f peak of the bulk gold will shift in the same direction and magnitude as the bias potential. This is not so with the surface gold clusters which are surrounded by the negatively charged carbonaceous layer composed of acetone fragments (probably mainly of a hydrocarbon nature, though with some oxygenated species). The negative charge may have been acquired by a scavenging of electrons from the reaction walls, electrodes, and solvent medium.15 The clusters are partially insulated from the biasing potential by the carbonaceous layer. When no bias potential is applied, the insulation provided by this layer is too small to give any noticeable peak shift resulting from surface charging differences since the total surface charge is very small. When the sample is subjected to an applied potential, and the potential is large enough, differences in peak shifts will occur. When a positive potential is applied, the Au-carbon species will have a shift that lags that of the bulk gold. When a negative potential is applied, the Au-carbon species show no shift. This surprising result might be explained by the model in Figure 12. A positive bias will cause a draining of electrons from the sample, while a negative bias will cause electrons to move into the sample. We suggest that if, as we propose, the Au-carbon species carry a negative charge, then this charge would resist the taking up of electrons by the Au-carbon species, explaining the lack of a shift on negative biasing.
It is important to point out that the possible charge on the colloid cannot be determined by the biasing experiments themselves. Biasing effects arise from charges that often arise from the biasing experiment themselves. Our suggestion of a negative outer layer on the cluster is based upon the evidence obtained in our earlier work on SMAD catalysts. We thus find that, on applying a biasing potential, the binding energy difference between the smaller surface metal clusters which are covered by a negatively charged layer of carbonaceous fragments of acetone and the larger bulklike metal particles is greatly enhanced. Lastly, we note that no surface metal clusters are present when the metal films are grown on silver foil. In this case, we need to consider the interaction between the cluster and the substrate as well as the influence of the substrate of the films' growth morphology. Since the negative charge of the layer of carbonaceous fragments is due to ionic charges, we do not think that the differences observed between SMAD films grown on glass and those grown on silver foil are due to charge neutralization. The absence of the the Au-carbon species is probably due to the fact that the silver foil favors the growth of large bulklike metal particles.
Conclusions Metal films prepared from nonaqueous metal colloids by using the SMAD technique result in surface metal clusters. These clusters are negatively charged and protected by carbonaceous fragments which are specifically associated with the SMAD solvent. The increase in Au 4f binding energy for unbiased samples is due to the electronic effects associated with clusters. The shifts in the Au 4f peaks when a biasing potential is applied are due to an electrostatic effect on the photoelectron leaving the sample. The carbonaceous film surrounding the surface metal cluster allows the flow of electrons out of the cluster but does not favor electron flow into these clusters. The growth of these films on silver foil does not result in the formation of surface metal clusters. The Au-carbon species formed during the SMAD synthesis of the Au colloids for the SMAD Au film formation are very unstable toward high vacuum and heat. The SMAD Au-Pd films are made up of Au and PdO while those formed from sputtering techniques using bulk alloy are made of Au, Pd, and PdO. Acknowledgment. Support for this work was provided by the National Science Foundation and 3M, and they are gratefully acknowledged. We thank Ellis Zuckerman for helpful discussions and Susan Antrim for technical assistance. Registry No. Au, 1440-51-5; Au, Pd, 11106-95-9.
