Langmuir 1993, 9, 733-739
733
Infrared and Photoelectron Spectroscopic Studies of Ethyl and Octyl Xanthate Ions Adsorbed on Metallic and Sulfidized Gold Surfaces A. Ihs; K. Uvdal, and B. Liedberg Laboratory of Applied Physics, Department of Physics and Measurement Technology, University of Linkoping, S-58183 Linktiping, Sweden Received September 1,1992. I n Final Form: December 14,1992 Infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) have been used to study the adsorption of ethyl and octyl xanthate ions on metallic gold surfaces and gold surfaces sulfidized by exposure to HzS. The experimental reflection-absorption (R-A) spectra of ethyl xanthate ions adsorbed on gold are compared with the calculated R-A spectrum, which is based on the optical constanta n(v) and k(v) derived from the KBr transmission spectrum of gold(1) ethyl xanthate. A considerable difference in relative intensities between the experimental and calculated R-A spectra is observed. At full monolayer coverage highly ordered structures are formed where the alkyl xanthate ions are coordinated to the surface through both sulfur atoms. The frequencies of the methylene stretching vibrations near 3000 cm-l for octyl xanthate furthermore suggest that the alkyl chain has a fully extended, all trans, zigzag conformation. At submonolayer coverage the orientation and/or conformation of the xanthate ions appears to be changed and the alkyl chains of the octyl xanthate ions are also conformationally disordered. On sulfidized gold surfaces only coverages less than a monolayer could be obtained. This is probably due to blocking of the adsorption sites by adsorbed HzS. The coordination of the xanthate ions to the surface is different than for submonolayer coverages on pure gold, and the alkyl chains are conformationally disordered.
Introduction Alkyl xanthates are commonly used in the flotation of sulfide minerals. The basic action of the alkyl xanthate ions is to adsorb strongly on the mineral surface and change ita wettability properties. Detailed investigations about the surface chemical bonding and alkyl chain conformation (orientation) appear therefore to be well founded and relevant to the development of the flotation process. Accordingly,several studies have been performed on pure metals and sulfide mineral surfaces by, e.g., electrochemical m e t h ~ d s l -and ~ infraredP7 and X-ray photoelectrons14 spectroscopy. The role of oxygen has also been investiRecent infrared investigations on sulfide minerals indicate formation of metal alkyl xanthates on the mineral surface and that the presence of soluble oxidation producta of the mineral is important for the hydrophobization proce~s.~ In this work we have used infrared reflection-absorption spectroscopy (IRAS)l5J6 and X-ray photoelectron spec(1) Woods, R. J. Phys. Chem. 1971, 75 (3), 354. (2) Lezna, R. 0.; de Tacconi, N. R.; Arvia, A. J. J. Electroanal. Chem. 1988,255, 251. (3) Leppinen, J. O.;Yoon,R.-H.;Mielczarski,J. A. Colloids Surf. 1991, 61, 189. (4) Leja, J.; Little, L. H.; Poling, G. W. Trans.-Inst. Min. Metall. 1963, 72, 407. (5) Mielczarski, J.; Leppinen, J. Surf. Sci. 1987, 187,526. (6) Mielczarski, J. A.;Yoon, R. H. J. Phys. Chem. 1989,93, 2034. (7) Persson, P. Thesis, Uppsala 1990. (8) Ranta, L.;Minni,E.;Suoninen, E.; Heimala, S.;Hintikka, V.; Saari, M.; R a s e , J. Appl. Surf. Sci. 1981, 7, 393. (9) Mielczarski,J.; Werfel, F.; Suoninen, E. Appl. Surf. Sci. 1983,17, 160. (10) Mielczarski,J.; Suoninen, E. Surf. Interface Anal. 1984,6 (I), 34. (11) Mielczarski, J. J. Colloid Interface Sci. 1987, 120 (I), 201. (12) Mielczarski, J.; Suoninen, E.; Johansson,L.-S.; Laajalehto,K. Int. J. Miner. Process. 1989,26, 181. (13) Johansson, L.-S.; Juhanoa, J.; Laajalehto, K.; Suoninen, E.; Mielczarski, J. Surf. Interface Anal. 1986, 9, 501. (14) SzBpv61gyi, J., Tildes, A. and Bertbti, I. J. Electron Spectrosc. Relat. Phenom. 1990,50, 239. (15) Francis, S. A.; Ellison, A. H. J. Opt. SOC.Am. 1959, 49, 131. (16) Greenler, R. G.J . Chem. Phys. 1969,50,1963.
troscopy (XPS)17 to study the adsorption of ethyl and octyl xanthate ions on gold. The adsorption of ethyl xanthate ions on metallic copper, sulfidized copper, and cuprous sulfide has been thoroughly investigated by others using infrared spectroscopy and XPS.4*5+11J3 It was observed that multilayer structures of cuprous alkyl xanthate were formed on the copper surface even a t fairly low exposures. Gold is used as the substrate material in this study in order to avoid this type of multilayer formation, because the dissolution/precipitationmechanism observed on copper is not expected to occur on gold. We have also studied the adsorption of ethyl and octyl xanthate ions on gold surfaces that had been sulfidized by exposure to 50 ppm HzS in argon for 1h. '
Experimental Section Gold films, approximately 2000 A thick, were made on silicon (100)slides by electron beam evaporation at a base pressure of Torr and with an evaporation rate of 5 A/s. The slides had the dimensions 40 X 20 X 0.5 mm. The silicon slides were precoated witha 10A thick chromium layer before the evaporation of the gold films in order to improve the adhesion between gold and silicon. The substrates were neither intentionally heated nor cooled during the entire evaporation process. The gold surfaces were sulfidized by placing them in a glass vessel filled with 50 ppm HzS in Nz or Ar for about 1h. A depth profile of a sulfidizedgold surfacewas obtainedby Auger electron spectroscopy (AES)at a background pressure of about 10-9 Torr and with a constant ion beam current. Potassium ethyl and octyl xanthates were synthesized according to the standard procedure from carbon disulfide, potassium hydroxide, and ethanol or octanol, respectively.'*An elemental analysis was done on fresh recrystallized potassium ethyl xanthate in order to check the purity of the material. The result obtained showed that the material contained 22.6 wt % ~~~
(17) Siegbahn, K.;Nordling, C.;Fahlman, A.; Nordenberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergman, T.; Karlsson, %E.; Lindgren, I.; Lindberg, B. In ESCA -Atomic Molecular and Solid State Structure Studied by Means of Electron Spectroscopy;Nova Acta Regiae SOC.Sci. Ups., Ser. IV 1967,20. (18) Du Rietz, C. Sven. Kem. Tidskr. 1957,69,310.
0743-7463/93/2409-0733$04.00/00 1993 American Chemical Society
Ihs et al.
734 Langmuir,Vol. 9,No.3, 1993 (weight percent) C, 9.7 wt % 0,39.9 wt % S, and 22.0 wt % K. The calculated values for a pure material are 22.5 w t % C, 10.0 wt % 0,40.0 wt 7% S, and 24.4 wt % K; i.e., our material contains very little contamination. The purity of the potassium alkyl xanthate salts was always checked before use by recording their IR spectra in KBr and comparing them with the spectrum of the newly synthesized salt. If needed, the salt was recrystallized from acetone, in order to remove inorganic impurities, and precipitated with diethyl ether. Gold(1) ethyl xanthate was precipitated from an excess of potassium ethyl xanthate in aqueous solution with HAuC14accordingto the method of Denko and Anders~on.’~The HAuC14 was purchased from J. M. Speciality products, England (assay 48.5% Au), and the water used was taken from a MilliQ unit. The yellow solid was separated by centrifugation, washed several times with analytical grade diethylether, and dried in vacuo for several days before a spectrum in KBr was recorded. The solutions were prepared in MilliQ water, through which nitrogen had been bubbled in order to remove oxygen. The concentration used in most of the experiments was 10rM. Lower and higher concentrations were also used in some cases. The gold substrates were immediately immersed in the solution after they had been taken out of the evaporation chamber in order to avoid organic contamination from the laboratory atmosphere as much as possible. The substrates were left in the solution for a period of 1-180 min. They were then rinsed with water, blown dry with nitrogen, and inserted as quickly as possible in the spectrometer. The FT-IRunit used was a Bruker IFS 113vFourier transform spectrometer equipped with a Bruker GIR (grazing angle incidence reflection) accessoryaligned at 83O. All the R-A spectra were recorded with the infrared radiation polarized parallel to the plane of incidence. The KBr and R-A spectra were obtained by averaging 200 and 500 interferograms, respectively, at 4 cm-I resolution with a deuterated triglycine sulfate (DTGS) detector. The XPS measurements were carried out with a commercial VG spectrometer using unmonochromatized Mg (Ka) photons (1253.6 eV). The resolution was determined from the full width at half maximum (fwhm) of the Au 4f(7/2) line to be 1.5 eV. In the surface-sensitivemode electrons with a take-off angle of 80’ with respect to the surface normal are collected. The signal from the outermost surface is then enhanced. This enhancement is correlated to the limited escape depth of the photoelectrons.20 In the bulk-sensitive mode photoelectrons with a take-off angle of 30° are collected.
Rssults and Discussion I. Characterization of Sulfidized Gold Surfaces. The gold surfaces were sulfidized according to the method described in the Experimental Section above. A depth profile of sulfidized gold was obtained with Auger electron spectroscopy (AES). The results are only qualitative, and no attempt has been made to determine the true surface composition. Only sulfur and gold were observed on the sulfidized gold surface. No detectable amounts of carbon or oxygen were found, even though the surface had been exposed to air before insertion into the spectrometer. The relative peak-to-peak heights (ptph) of the 150 eV (S) and the 239 eV (Au) peaks are shown in Figure 1. However, the S peak at 150 eV was overlapped by one of the Au peaks. In order to determine the contribution of Au to the 150-eV peak, the relative peak-to-peak height of the Au 150-eV peak and the Au 239-eV peak was obtained from the Auger electron spectrum of a pure Au surface. The ratio (r) obtained was then used to subtract the contribution of Au from the 150-eV peak in the following (19) Denko, C. W.; Andersson, A. K. J . Am. Chem. SOC.1945,67,2241.
(20) Fadley, C. S.;Baird, R. J.; Siekhaus, W.; Novakov, T.;BiegstrBm, S. A. A. J. Electron Spectrosc. 1974, 4, 93.
+Au239
-
+S150
-
way s(15O),tph = (150)ptph- r*Au(239),tph (1) 11. Infrared Spectra. Calculation of R-A Spectra. The experimental R-Aspectra of ethyl xanthate ions (EX) adsorbedon metallic gold are compared with the simulated R-A spectrum that was calculated from the transmissionabsorption spectrum of the gold(1) ethyl xanthate (Au(1)EX)complex (KBr pellet). The reason for calculating a simulated R-A spectrumis to account for peak distortions in the measured R-A spectra that are due to an optical effect.21 Effectively, this optical effect gives rise to an intensity enhancement on the high frequency side of an absorption band and thereby to a peak shift to higher frequencies. The effect is especially pronounced for strong and broad absorptions, whereas weak and sharp bands normally remain unaffected. The R-A spectrum is calculated from the bulk optical constants n(v)and k(v) of the Au(1)EX complex according to a procedure described below and in Figure 2. First, the k ( v ) spectrum is generated from the transmission-absorption spectrum of the complex through the BeerLambert law. The corresponding n(v) spectrum is then calculated using the Kramers-Kronig transformation.22 Through an iterative process, the n(v)spectrum is refined by adjusting the initialk(v) spectrumaccordingtoamethod described by Bertie et aL23 until a good fit is obtained between the calculated and the experimentaltransmissionabsorption spectra. Finally, the obtained n(v) and k(v) spectra are used to simulate the R-A spectrum. The simulation is based on a three-layer model where the adsorbed layer is assumed to be isotropic (randomly oriented molecules) with a thickness of 10 A. Figure 3 shows the experimental Au(1)EXtransmission-absorption spectrum, the fiially obtained n(v)and k ( v ) spectra, and the simulated R-A spectrum. Note that the strongest bands have shifted upward in frequency and also that the relative intensities have changed in the simulated R-A spectrum as compared to the original transmissionabsorption (bulk) spectrum A(v). Ethyl and Octyl Xanthate on Gold. In Figure 4 the simulated R-A spectrum (d) of a 10 A thick layer of Au(1)EX on gold is shown together with the R-A spectra of EX adsorbed on gold after immersion for 10 min in a 1mM solution (a) and for 10 (b) and 1(c) min in a 10 pM solution. The gold surfaces were completely hydrophobic, except after the shortest adsorption time in the 10 pM (21) Allara, D. L.;B a a , A.; Pryde, C. A. Macromolecules 1978, 1 1 , 1215. (22) Allara, D.L.;Nuzzo, R. G. Langmuir 1985, 1, 52. (23) Bertie, J. E.;Eysel, H. H. Appl. Spectrosc. 1985, 39,392.
Xanthate Ions on Gold Surfaces
Langmuir, Vol. 9, No. 3, 1993 735
Record the "bulk" (KBr)
spect" of the model compound A(v)
Effective thickneie
des
-
"
I
5.10-3
t Extract k(v) from A W by using the Beer-Lambert law A(v) = 4x k(v) v 4 s
Use the Kramers-Kronig transformation of k(v) to get n (v)
I1
Calculate the isotropic "bulk" spectrum frop n(v) and k(v) fTT;(v)
A ;
15.10-3
I
Acceptablematch
I\rb A s I
spectrum from n(v) and k(v), and compare it with the experimental R-A spectrum
Figure 2. Block diagram illustrating the iterative KramersKronig procedure used for extraction of the optical constants n(v) and k(v) from a 'bulk" infrared spectrum.
A
1600
1400 1200 1000 WAVENUMBER cm-1
800
Figure 3. (a) Experimental Au(1)EX transmission-absorption
spectrum, (b) k(v) and (c) n(v)spectrum, and (d) simulated R-A spectrum of a 10 A thick layer of Au(1)EX on gold.
solution. In the simulated R-A spectrum there are three strong absorption bands at 1204,1124,and 1056 cm-l. All metal alkyl xanthates have characteristic bands in the region 900-1300 cm-l. However, there has been some disagreementregarding the assignmentsof these bands to
1600
1400 1200 1000 WAVENUMBER cm-l
800
Figure 4. Experimental R-A spectra of EX adsorbed on gold. The concentrations and adsorption times are (a) 1 m M and 10 min, (b) 10 pM and 10 min, and (c) 10 pM and 1 min. The simulated R-A spectrum is shown in part d. certain fundamental vibrations of the alkyl xanthate ion. The problem with making the assignments is that the vibrationsto a large extent are coupled.sn Nevertheless, some bands, although not pure, have a large contribution from a certain normal mode of vibration. An IR spectrum of Au(I)EX, very similar to that obtained by us,has been presented by Leppinen et al.? but no assignments were given. The positions of the absorption bands are, however, close to those observed for copper(1) alkyl xanthates. According to the assignments made by Mielczarski et al.5 the band at 1204 cm-l should have a large contribution from the v, C - 0 4 stretching vibration. This is the only strong band observed in the R-A spectra of EX adsorbed on gold from the 1mM solution (Figure 4a) and from the 10 pM solution when the immersion time was 10 min (Figure4b). The assignmentsof the two absorption bands at 1124 and 1056 cm-l in the simulated R-A spectrum are more uncertain. Comparison with the results obtained for Cu(I)EX, however, indicates that at least the latter absorption band should have a large contribution from the va8 S-C-S vibrational mode. Theoretical normal coordinate calculations made by Colthup et al.25for alkyl xanthates also show that absorption bands in this region have a large contribution from the v, S-C-S vibrational mode. Some contribution from this vibrational mode to the absorption band at 1124 cm-l is also suggested by Mielczarski et al.5 The electrochemical behavior of gold, silver, and their alloys of different compositionsin ethyl xanthate solutions (24) Pilipenko, A. T.;Mel'nikova, N. V. R w s . J. Znorg. Chem 1970,15 (5), 608. (25) Colthup,N. B.;Porter Powel, L. Spectrochim. Acta 1987,43A (3), 317. (26) Ray, A.; Sathyanarayana,D. N.;Prasad Spectrochim. Acta 1978, 29, 1579. (27) Agarwala, U.; Lakehmi; Rao, P. B. Znorg. Chim Acta 1968,2,337.
Zhs et al.
736 Langmuir, Vol. 9,No. 3, 1993 has recently been studied by Leppinen et aL3 Cyclic voltammetry was used for the electrochemical studies, whereas IRAS and XPS were used for structural characterization of the adsorbed species. On gold two species were observed. One was diethyl dixanthogen, which was evidenced by ita characteristic bands at 1270,1248,1112, 1044, and 1030 cm-l in the R-A spectrum. The other species gave rise to a R-A spectrum almost identical to ours. Only one strong band, near 1200 cm-l, appeared in the low frequency region of the spectrum. The disappearance of the bands at 1120 and 1050 cm-l in their R-A spectrum, as compared to an attenuated total reflection spectrum of the Au(1)EX complex, was taken as evidence for the formation of non-xanthate species, possibly decomposed ethyl xanthate ions, on the surface. However, the disappearance of the two bands may not necessarily be due to a decomposition reaction of the ethyl xanthate ions on the surface. A coordination where both sulfur atoms are involved in the binding to the gold surface, as proposed by Lema et aL2in an electrochemical study of the adsorption of ethyl xanthate on gold, will give a transition dipole moment of the ,Y (S-C-S) vibration parallel to the gold surface. According to the so-called surface selection rule the corresponding bands will then disappear in the R-A spectrum. Persson et al.28observed the same intensity behavior for potassiump-methylbenzyl xanthate and potassium p-(trifluoromethy1)benzyl xanthate on gold as we do for ethyl xanthate. Through comparison with structurally well characterized model compounds, such as gold(1) dithioa~etates~~ and gold(1) dithiocarbamate^,^^*^^ it was concluded that a bridging coordination was the most probable. They observed also a sharp transition from a flat to an almost vertical orientation of the molecular skeleton with increasing exposure. Thus, we cannot exclude the possibility that the same type of transition occurs for EX and that the dramatic change in intensity in the 1000-1300 cm-l region should be ascribed to an orientation transition rather than to a decomposition reaction. At the lowest concentration of the solution and the shortest immersion time (Figure 4c) a broad band appears at 1140 cm-l. Probably the coverage is now less than a monolayer. The presence of the 1140-cm-' band could indicate that the ions are differently oriented or have a different conformation (coordination), but we have not been able to make a conclusive assignment of this band. The R-A spectra of octyl xanthate (OX) ions adsorbed on gold are shown in Figure 5. The OX ions form a monolayer on gold much more rapidly than the EX ions. In order to obtain a coverage less than a monolayer, the concentration of the KOX solution had to be decreased to 1 pM (Figure 5c). At the higher concentrations the R-A spectra of OX on gold are almost identical to the R-A spectra of EX on gold. Only one strong band, at 1205 cm-', can be observed in the low frequency region of the spectrum and the coordination and orientation of the head group (-OCSz-) is therefore assumed to be the same as for EX. However, since OX has a longer carbon chain than EX, the high frequency bands just below 3000 cm-l, which correspond to CH2 and CH3 stretching modes, are stronger in the R-A spectra of OX on gold. The frequencies and intensities of these bands can be used to investigate the orientation and conformation of the alkyl chain on the (28) Persson, N.-0.;Uvdal, K.; Liedberg, B.;Hellsten, M.Prog. Colloid Polym. Sci. 1992, 88, 100. (29) Chiari,B.; Piovesana, 0.; Tarantelli, T.;Zanazzi,P. F.Znorg. Chem. 1985, 24, 366. (30) Akerstrom, S. Ark. Kemi 1969, 14, 387. (31) Hesse, R.;Jennische, P. Acta Chen. Scand. 1972, 26, 3855.
AR
I
Ro
i-10-3
A
0
2
I
I
3000
2800
I
I
1400 t2OO WAVENUMBER cm-'
x 0.2
I
1000
Figure 5. Experimental R-A spectra of OX adsorbed on gold. The concentrations and adsorption times are (a) 1mM and 20 min, (b) 10 pM and 10 min, and (c) 1 pM and 10 min. surface. The method used to determine the orientation of the alkyl chains is based on the knowledge of the direction of the transition dipole moment (Mi) of the corresponding vibrational mode (qi) and that the E-field (E)a t the surface is perpendicular to the metal surface. The intensity of an arbitrary absorption band can generally be written
Ii = C I M i *E l2 (2) where C is a constant containing parameters that are specific for the actual experimental setup. To determine the orientation, the following ratio Aij is formed between intensities of corresponding bands in the experimental R-A spectrum and the transmission-absorption spectrum (3) where and I;" (r = reflection, t = transmission) are the intensities for vibration qi and qj, respectively. The functions Mi,z and Mjj are the projections of Mi and Mj, respectively, on the z axis (parallel with the surface normal). We used the u, (CH2) vibrational mode at 2920 (2916) cm-l, the us (CH3) vibrational mode at 2880 (2871)cm-l, and the us (CH2) vibrational mode a t 2851 (2853)cm-l in the R-A (transmission-absorption) spectrum to calculate the orientation of the alkyl chain for a typical monolayer of OX, Figure 5a. The alkyl chains are in this calculation assumed to have a fully extended all trans (zigzag) conformation. TheMi,zvalues for the above CH2 and CH3 modes are deduced through simple geometrical considerations, Figure 6,and are given in Table I. We obtain the angles 0 = 40" and (o = 29O for the full monolayer in Figure 5a, where B and (o are the angles defined in Figure 6.We &*It
Xanthate Ions on Gold Surfaces
Langmuir, Vol. 9,No. 3, 1993 737
I
1,10-3
N
1600 L
I
1200
1000 I
1400
1200
Id00
/ //'
I/
/ / / / / / / / / / / / /'y Figure 6. Definition of the angles 0 and (a used to determine the
orientation of the alkyl chain of OX adsorbed on gold. The directions of the transition dipole moments used for the calculations are also shown in the drawing. Table I. Projection of Mi on the Surface Normal (zAxis) vibration u , b (em-') Mi, vas (CHd 2920 sin 0 sin 4 cos a COB cp + sin a cos e sin (pb VB (CHd 2880 (CHd 2851 COB e sin cp 0 The angles 0 and cpare definedin Figure 6. b The angle a is defined in Figure 6 and is assumed to be 35.5O.
also estimated the value of the angle cp to be expected for an OX molecule coordinated through both alfur atoms so that the S2C-0 axis is perpendicular to the surface. The C-0-C angle used (116') was given by Pilipenko et for the ethyl xanthate ion and the angles around the CH:, carbon atoms were assumed to be tetrahedral. This gave an angle cp = 2 8 . 5 O which is in excellent agreement with the experimentally obtained cp angle. Even though it is an estimate baaed on EX rather than OX, it strongly supports our model where both sulfur atoms are involved in a bridge-like bonding to the gold surface. At the lowest concentrationof the KOX solution (Figure 5c) a band at 1123 cm-l appears in the R-A spectrum. Also, the absorption bands corresponding to the v, (CH2) and vs (CH:,) vibrational modes have shifted to 2928 and 2856 cm-l, respectively, and the relative intensities of all the bands just below 3000 cm-l have changed. The calculated values of both 8 and cp are now about 50°, which implies that the molecules are more tilted. Another, and perhaps more probable, interpretation of the changes in the R-A spectrum (Figure 5c) implied by the rather large upward shift in frequency of the v, (CH3 and vS (CH3 vibrational modes, is a conformational disordering of the alkyl chains. The correlationbetween the frequencyshifts of the C-H stretching modes and the structure of n-alkyl chains has been studied by Snyder et al.32 For totally conformationallydisordered polymethylene(liquidphase) the frequencies were observed to be 6-8 cm-l higher than for the all-trans chains (crystalline phase). Frequency (32) Snyder,R.G.;Strauss, H.L.;Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
1400
3000
2800
L
1600
4
WAVENUMBER cm-1
Figure 7. Experimental R-A spectra of (a) EX and (b) OX adsorbed on gold exposed to 50 ppm HzS in Ar for 1 h. shifts to higher values have also been observed by Allara et al.22in a study of l-alcanoic acids adsorbed on oxidized aluminum surfaces. The possibility that these shifts were caused by conformational disorder was discussed. Thus, since the frequency shifta of the Y, (CH3 and ve (CH2) stretching modes are within the range given above, we believe that the most plausible interpretation of the high frequency region of the R-A spectrum (Figure 5c) is that the alkyl chains are conformationally disordered. Furthermore, the expected value of both 8 and Q for totally disordered alkyl chains should be 4 5 O , i.e., a value close to that obtained above. Ethyl and Octyl Xanthate on Sulfidized Gold. The R-A spectra of EX and OX adsorbed on sulfidized gold are shown in parta a and b of Figure 7,respectively. Fewer xanthate ions are adsorbed on the sulfidized gold surfaces aa compared to the amount adsorbed on the pure gold surfaces,a phenomenon which we believe is due to blocking of the adsorption sites by adsorbed H2S. Even though the coverage of alkyl xanthate ions is less than a monolayer, the R-A spectra differ from the R d spectrum obtained for submonolayer coverage on pure gold (Figure 4c). Only one band at about 1190 cm-l is observed, indicating a different coordination of the xanthate ions on sulfidized gold. In the high frequency region, just below 3000 cm-l, of the R-A spectrum of OX adsorbed on sulfidized gold, the frequencies and relative intensities are very close to those obtained for submonolayer coverage on pure gold. This implies that the alkyl chains are conformationally disordered. 111. X-rayPhotoelectronSpectroscopy. Potassium Ethyl Xanthate (Solid). XPS studies of solid potassium ethyl xanthate have been preeentad earlier by Sz6pviilgyi et aL14 We have repeated the measurements on solid KEX, and the results are used as a reference for the measurementa on EX adsorbed on metallic and sulfidized gold surfaces. A charging effect, which is correlated to the thickness of the sample,was observed for solid KEX during the XPS measurement. All binding energies in the spectra of the solid sample have therefore been lined up with the strongest C(1s) peak in the monolayer spectra. Figure 8 shows the core level XPSspectra of (a) solid KEX (pressed
738 Langmuir, Vol. 9, No. 3,1993
Ihs et al. Table 11. Core Level Binding Energies in eV
WP) 2PlI2 295.6
sample
Dellet idsorbate/Au bulk surface adsorbatels-Au bulk surface
540
530
300
2P3/2 292.8
290 280 BINDING ENERGY (eV)
S(2P) C(lS)
om)
285.0
286.9 286.7
285.4 285.1
163.3 162.9
162.2 161.9
533.7 533.7
84.0 84.0
286.7 286.8
284.7 284.9
162.9 162.9
161.8 161.8
533.4 533.5
83.1 83.8
170
160
Figure 8. XPS core level spectra of (a) solid KEX (pressed pellet) and (b) EX adsorbed on gold. The concentration of the solution was 10 pM and the adsorption time was 20 min. pellet) and (b) EX adsorbed on gold, and the binding energies are summarized in Table 11. Only one state of sulfur is observed in the solid material, i.e. the two sulfur atoms of the alkyl xanthate ion experience the same chemical environment. The sulfur peak consists of a spin split doublet, with the S(2p3/2)and S(2p1/2)binding energies measured to be 162.0 and 163.1 eV, respectively. The S(2p312)sulfur binding energy, 162.0 eV, is identical to what SzBpvBlgyi et a l . I 4 have reported for solid KEX. However, they also observed an undesired sulfate component at 168 eV which is not present in our sulfur spectrum. The C(1s) binding energy peak is asymmetric, with a broadening on the high energy side, and the line shape is very similar to that observed in the previous study of solid KEX.14 There has been a discussionabout how to interpret the C(1s)spectrum and, furthermore, the relative intensity ratio between carbon and sulfur (C(ls)/S(2p)) is unexpectedly high.14 We obtain a total carbon to sulfur ratio C(ls)/S(2p) 2.8, with the cross sections taken into account, which is substancially larger than the expected stoichiometricvalue 1.5. SzBpvBlgyi et al.14 assigned this verylargeC(ls)/S(2p) ratiotoalargeexcessofhydrmbon contamination. Our FTIR studies of solid KEX (KBr pellets), however, give no indication of a large amount of contamination. The FTIR measurementswere performed both before and after the XPS measurements. In the latter case the pellet used for the XPS measurement was crushed, mixed with KBr, ground, and pressed into a new pellet. The IR transmission spectrum obtained for this pellet was identical to the spectrum obtained for fresh recrystallizedKEX. We have also had an independent elemental analysis done on our material (see the Experimental Section), which showed that it contains very little contamination. These results indicate that the large relative intensity ratio between carbon and sulfur (C(ls)/S(2p)) may have another explanation than a large excess of hydrocarbon contamination. We have ongoing detailed studies trying to determine the correct way to deconvolute the C(ls) spectrum and thereby determinethe contribution of hydrocarbon contamination to the C(1s) spectrum. The oxygen binding energy in solid KEX is 533.3 eV
2P312 162.0
533.3
M 4 D 4fip
286.9
2Pip 163.1
530.9
(Figure 8). The peak has also a weak shoulder on the low binding energy side at 530.9 eV which we have not been able to assign. The binding energy peaks at 295.6 and 292.8 eV in Figure 8 correspond to the K(2p1/2) and K(2p3p),respectively. The total amount of carbon relative to potassium (C(ls)/K(Bp)),with the cross sections taken into account, is 3. This is the value to be expected if there is one K+ ion per EX- ion and it supports our previous proposal that the large relative intensity ratio between carbon and sulfur (C(ls)/S(Bp))should not necessarily be ascribed to a large excess of hydrocarbon. One possible explanation for the divergence from the expected stoichiometric value 1.5 of the C(ls)/S(2p) intensity ratio may be a long tail shake up satellite for sulfur not visible above the backgroundmZ0 Ethyl Xanthate Adsorbate. As expected, the signal from K(2p) is gone when KEX is adsorbed on gold. There is no significant change in the peak position of S(2p) and still there is just one type of sulfur. The presence of only one type of sulfur is expected for a coordination to the surface through both sulfur atoms of the EX ion since they will then have the same chemical environment. The relative intensity ratio between carbon and sulfur is even higher for the monolayer on gold as compared to the solid sample. This behavior (increase) is, in fact, not unexpected for the sample preparation technique employed here. The newly evaporated gold surface are inevitably exposed to air for a few minutes before they are immersed in the solution, and that time is enough for hydrocarbons in air to contaminate the surface. However, high values of the relative intensity ratios between carbon and sulfur as well as between fluorine and sulfur have been observed by Persson et al.28in a study of potassium p-methylbenzyl xanthate and potassium p-(trifluoromethy1)benzyl xanthate on gold. They gave several alternative explanations to the very high intensity ratios, especially the F(ls)/S(Bp), and one of them involved a change in cross section of the S(2p) orbital caused by interaction with the substrate. The symmetry properties of the electronic orbitals may also influence the cross section,20and thereby the relative intensity ratios. In case of EX adsorbed on sulfidized gold the relative intensity ratio between carbon and sulfur (C(ls)/S(2p)) has decreased compared to the ratio obtained for EX adsorbed on pure gold. The results obtained from the infrared measurements of EX adsorbed on sulfidized gold indicated that a large number of adsorption sites are blocked by adsorbed HzS, and therefore much less EX is adsorbed than on a pure gold surface. The decrease of the relative intensity ratio between carbon and sulfur for EX adsorbed on sulfidized gold is thus expected. When going from the bulk to the surfacesensitive mode the relative intensity ratio between carbon and sulfur (C(ls)/S(2p))increases from 5.3 to 8.5 in the case of EX on gold and from 2.6 to 8.4 in the case of EX on sulfidized gold. These angle-dependent XPS results, together with the observations that there are no K+ions on the surface and that there is only one type of sulfur, indicate that the
Xanthate Ions on Gold Surfaces
Langmuir, Vol. 9, No. 3, 1993 739
EX ions are preferentially oriented with the sulfur atoms close to the surface and the carbon atoms away from the surface. Conclusions We have investigated the adsorption of ethyl and octyl xanthate ions on gold with Fourier transform infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy (only ethyl xanthate). The experimental R-A spectra of ethyl xanthate ions adsorbed on gold were compared with the simulated R-A spectrum of gold(1) ethyl xanthate, whereby significant differences in intensities of the adsorption bands were observed. In a recent study by Leppinen et aL3the dramatic change in intensity in the 1000-1300 cm-l region was taken as evidence for the formation of a non-xanthatespecies on the gold surface. Another possible interpretation of the experimental R-A spectra is that the alkyl xanthate ions are highly oriented at full monolayer coverage and coordinated to the gold surface through both sulfur atoms. The latter interpretation is supported by calculations of the orientation of the alkyl chain of the octyl xanthate ion adsorbed on a metallic gold surface. The experimentally obtained angle of the alkyl chain against the surface normal (29') is approximately the angle expected for an alkyl xanthate ion coordinated to the surface through both sulfur atoms. Also, only one type of sulfur is observed in the XPS measurements; i.e., both sulfur atoms have the same chemical environment as expected for a coordination to the surface through both sulfur atoms. Furthermore, a bridge-like coordination of the chemisorbed alkyl xanthate ions through the sulfur atoms was proposed in a recent study of the adsorption of p-methylbenzyl and p-(trifluoromethy1)benzyl xanthate ions on metallic gold.28 Thus, we believe that a plausible interpretation of our results is that the alkyl xanthate ions are highly oriented at full monolayer coverage and that they are coordinated to the gold surface through both sulfur atoms (Figure 9). At lower coverages the alkyl xanthate ions appear to have a different orientation and conformation. The conformation of the alkyl chains of adsorbedoctyl xanthate ions changes from fully extended all trans (zigzag) at full monolayer coverage to disordered at lower coverages. This change is evidenced by the upward shift in frequency of the v, (CH3 and v, (CH2) stretching modes. When the gold surface is sulfidized, the amount of adsorbed molecules is reduced, a phenomenon which we believe is due to blocking of adsorption sites by adsorbed H2S. The XPS results confirm this observation, since the
",k yi" 0% Figure 9. Suggested coordination and orientation of OX on gold at full monolayer coverage. The alkyl chain is tilted 29' away from the surface normal and rotated 40' out of the plane of the paper around the axis of the chain.
relative intensity ratio between carbon and sulfur (C(ls)/ S(2p)) had decreased significantly for EX adsorbed on a sulfidized gold surface as compared to EX adsorbed on a metallic gold surface. Similar studies of EX and OX adsorbed on metallic and sulfidized copper, silver and nickel surfaces are in progress and will be published separately. Acknowledgment. This work was financially supported by the Swedish National Board for Industrial and Technical Development (NUTEK). We also wish to thank Mr. A. Robertsson for helping us with the Auger measurements, Dr. N.-0. Persson for the preparation of Au(I)EX, Mr. A. N. Parikh for verifying our calculations of the R-A spectra, and Professor I. Lundstrom for critical reading of the manuscript.
