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Adsorption of Potassium O,O′-Di(para-fluorophenyl) Dithiophosphate on Gold, Silver, and Copper Nils-Ola Persson,† Kajsa Uvdal,‡ Ola Almquist,† Isak Engquist,‡ Hans Kariis,‡ and Bo Liedberg*,‡ Department of Chemistry and Laboratory of Applied Physics, Institute of Physics and Measurement Technology, Linko¨ ping University, S-581 83 Linko¨ ping, Sweden Received March 5, 1999. In Final Form: June 21, 1999 Gold, silver, and copper substrates were immersed in aqueous solutions of a simulant mineral flotation agent, potassium O,O′-di(para-fluorophenyl) dithiophosphate. The adsorbed molecules on gold were studied in detail with infrared reflection-absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and ellipsometry. The most significant peaks in the IRAS spectra were assigned to the appropriate molecular vibrations and their relative intensities were compared with those found in simulated spectra derived from the isotropic optical constants of corresponding metal salts to deduce the binding and orientation. Moreover, intensity ratios of XPS signals were compared at different takeoff angles to reveal the depth distribution of atoms in the dithiophosphate layers. The following modes of adsorption were deduced: The adsorption on gold takes place by the formation of bonds involving the two sulfur atoms of the flotation agent (bridging coordination), regardless of immersion time and solution concentration. A thin and less organized layer is formed at low exposures. Longer adsorption times with more concentrated solutions give a more dense molecular packing and vertical orientation of the molecules on the surface. Adsorption on silver and copper was studied by IRAS. The adsorption proceeded via a dissolution-precipitation mechanism that manifests itself by less pronounced orientation effects. The intensities of the silver and copper IRAS spectra after long immersion times in concentrated solutions also show the formation of multilayers with some persisting long-range molecular ordering.
Introduction Organic xanthates and dithiophosphates are used as collectors in mineral flotation,1 thus their adsorption behavior on metal or metal sulfide surfaces merits an investigation. Studies of organic xanthates (i.e., dithiocarbonates) have been performed earlier by infrared (IR) spectroscopy techniques at our laboratory2-4 and by other groups.5-10 Other techniques used are electrochemistry10-14 and X-ray photoelectron spectroscopy (XPS).2,4,10,15 Although thoroughly characterized, there is still a debate in the literature9 about the binding of xanthates to mineral and metal surfaces. Leppinen et al.10 concluded from IR analyses that adsorption of ethyl xanthate on * Corresponding author. Fax: +46-13 288969. Telephone: +4613 281877. E-mail:
[email protected]. † Department of Chemistry. ‡ Laboratory of Applied Physics. (1) Flotation in Encyclopaedia of Chemical Technology, Vol. 10; KirkOttmer, Eds.; John Wiley & Sons: New York, 1980; p 523. (2) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733. (3) Ihs, A.; Liedberg, B. Langmuir 1994, 10, 734. (4) Persson, N.-O.; Uvdal, K.; Liedberg, B.; Hellsten, M. Prog. Colloid Polym. Sci. 1992, 88, 100. (5) Persson, P. Thesis, Uppsala, Sweden, 1990. (6) Mielczarski, J. A.; Yoon, R. H. J. Phys Chem. 1989, 93, 2034. (7) Mielczarski, J. A.; Leppinen, J. Surface Sci. 1987, 187, 526. (8) Cases, J. M.; De Donato, P.; Kongolo, M.; Michot, L. Colloids Surf. 1989, 36, 323. (9) (a) Mielczarski, J. A.; Mielczarski, E.; Zachwieja, J.; Cases, J. M. Langmuir 1995, 11, 2787; (b) Woods, R.; Yoon, R.-H. Langmuir 1997, 13, 876; (c) Mielczarski, J. A. Langmuir 1997, 13, 878. (10) Leppinen, J. O.; Yoon, R. H.; Mielczarski, J. A. Colloids Surf. 1991, 61, 189. (11) Hyun Thi, O.; Lamache, M.; Bauer, D. Electrochim. Acta 1981, 26, 33. (12) Lezna, R. O.; De Tacconi, N. R.; Arvia, A. J. J. Electroanal. Chem. 1988, 255, 251. (13) Woods, R. J. Phys. Chem. 1971, 75, 354. (14) Groot, D. R. S. Afr. J. Chem. 1984, 37, 103. (15) Mielczarski, J. A.; Suoninen, E.; Johansson, L.-S.; Laajalehto, K. Int. J. Mineral Proc. 1989, 26, 181.
gold at low coverage involved a decomposition reaction of the xanthate molecules, and at higher coverage dixanthogen was the main component formed. We recently offered a different interpretation of the IR spectra of ethyl xanthate on gold at low coverage.2 Application of the surface selection rule16,17 revealed that the disappearance of stretch vibrations of the S-C-S anchoring group in the IR spectrum was caused by an orientation effect (bridging coordination)2,4 and not by a decomposition reaction. The adsorption of xanthates on silver and copper surfaces, on the other hand, has been found to occur via a dissolution-precipitation mechanism that leads to less organized mono- and multilayers.3,7,15 The organic dithiophosphates have not been investigated to the same extent, probably because they are not as frequently used as the xanthates in mineral flotation. Reviews of the dithiophosphate-metal complexes, however, have been given by Wasson et al.18 and Haiduc,19 and a more specialized review on XPS, dealing with metal complexes, has been reported by Hill et al.20 In a review on the adsorption of organic collectors on sulfide minerals, Persson21 reported the crystalline structure of metal salts of organic dithiophosphates. Moreover, Groot14 found from an investigation of adsorption of dithiophosphates on metal surfaces with electrochemical methods that at low potentials relative to the reduction potential, the dithiophosphate anions were adsorbed reversibly, and that at higher potentials the formation of bis(O,O′-diethyl dithiophosphate) [(RO)2(PdS)S-S(PdS)(RO)2] occurred. (16) Francis, S. A.; Ellison, A. H. J. Opt. Soc. Am. 1959, 49, 131. (17) Greenler, R. G. J. Chem. Phys. 1969, 50, 1963. (18) Wasson, J. R.; Woltermann, G. M.; Stocklosa, H. J. Fortschr. Chem. Forsch. 1973, 35, 65. (19) Haiduc, I.; Sowerby, D. B.; Lu, Shao-Fang Polyhedron 1995, 14, 3389. (20) Hill, J. O.; Magee, R. J.; Liesgang, J. Comments Inorg. Chem. 1985, 5, 1. (21) Persson, I. J. Coord. Chem. 1994, 32, 261.
10.1021/la990266n CCC: $18.00 © 1999 American Chemical Society Published on Web 10/12/1999
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Figure 1. Structure of the dithiophosphate potassium O,O′di(para-fluorophenyl) dithophosphate (KFTP).
We present here an infrared reflectionsabsorption spectroscopy (IRAS) and XPS study on the adsorption of the aromatic dithiophosphate potassium O,O′-di(parafluorophenyl) dithiophosphate on the coinage metals gold, silver, and copper. To our knowledge no IRAS investigation of organic dithiophosphate adsorption on metals has been reported before. The molecular structure of potassium O,O′-di(para-fluorophenyl) dithiophosphate is shown in Figure 1. The intensities of several vibrations of the aromatic skeleton can be used to probe variations in molecular orientation and packing with increasing exposure (concentration and immersion time).16,17 This information is crucial for obtaining a deeper understanding of the collector action at surfaces. The XPS method22-24 was used to obtain additional information about the layers. The relative intensity ratios of the elements are calculated and compared with the stoichiometric values. Furthermore, the molecular orientation of well-organized monolayers adsorbed on flat surfaces can be estimated by angledependent XPS [XPS(Θ)].25 The molecular orientation can be further elucidated by studying chemical shifts due to molecular surface interactions, as discussed in earlier works2,4,10,15 and in following sections. Experimental Section Chemicals. Potassium O,O′-di(para-fluorophenyl) dithiophosphate (KFTP) was kindly provided by prof. M. Hellsten at Azko Nobel AB, Stenungsund, Sweden. It was synthesized at Azko Nobel according to the following procedure: para-Fluorophenol and phosphorus pentasulfide in a molar ratio of 4:1 were weighted into a flange flask equipped with a mechanical stirrer, an inlet for nitrogen, and tubing connected to a gas washing bottle that was half-filled with water. The mixture was vigorously stirred and heated to 142 °C. After 2 h at that temperature, the product was withdrawn and analyzed by titration (yield: 85%). After recrystallization in hexane, no impurities could be detected by nuclear magnetic resonance (NMR) spectroscopy (peaks at δ ) 7.3 and 7.1 from Ar-H and 3.5 from -S-H, intensity estimations uncertain because of broadening of the latter peak). To convert the product (O,O′di(para-fluorophenyl) dithiophosphoric acid, HFTP) to KFTP, the HFTP was dissolved in toluene and potassium carbonate was added. The mixture was stirred at room temperature for 1 day. Then, the salt was collected by filtration and dried at 40-50 °C, and then acetone was added. The unreacted potassium carbonate was removed by filtration, and the solution was evaporated to dryness. The purity, estimated by titration, was 81%. The KFTP was further purified at our laboratory by dissolution in hot acetone, filtration, evaporation of most of the acetone, and precipitation by addition of diethyl ether, which gave a white solid. The IR (KBr) spectrum of the solid thus obtained was quite similar to that of the KFTP as received and (22) Fadley, C. S.; Baca, A.; Pryde, C. A. J. Electron Spec. 1974, 4, 93. (23) Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergman, T.; Karlsson, S.-E.; Lindgren, I.; Lindberg, B. Esca-Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, in Nova Acta R. Sci. Ups., Ser. IV, Vol. 20; Almqvist & Wiksells: Uppsala, Sweden, 1967. (24) Fadley, C. S. Basic Concepts of X-ray Photoelectron Spectroscopy; University Hawaii, HI 96822, U. S. A, BK#07464, 1978, 2, 1. (25) Fadley, C. S. Prog. Solid State Chem. 1976, 11, 265.
Persson et al. the compound was then considered as pure, possibly with some inorganic impurities (cf. XPS results). All water used in the adsorption experiments and most syntheses of metal dithiophosphates was taken from a MilliQ unit. Some of the salt preparations were made in distilled water (conductivity, 2-7 µS‚cm-1). The acid gold(III) chloride (J. M. Speciality Products, England), silver nitrate, and copper(II) chloride (Merck, Darmstadt, Germany) used in the preparation of the corresponding metal salts were analytical grade and were used without further purification. Preparation of Metal Salts. The metal salts were prepared, using a standard procedure, by adding water solutions of hydrogen tetrachloroaurate(III), silver nitrate, or copper(II) chloride to water solutions of the organic potassium dithiophosphate.18,26 The resulting solids were then separated by filtration, centrifugation, or extraction with organic solvent. The solid salts were worked up by recrystallization or washing in organic solvent. Details for the individual salts are given later. When dithiophosphate ions (FTP-) reacts with Au(III) or Cu(II) ions, the following redox reactions take place:18,26
Au3+ + 3 FTP- f AuFTP + FTP2 2 Cu2+ + 4 FTP- f 2 CuFTP + FTP2 The resulting dithiophosphogen [) FTP2 ) bis(O,O′-di(parafluorophenyl) dithiophosphate] then must be removed by washing with some nonpolar solvent. In the case of copper dithiophosphate, the redox reaction is slow but it can be followed by observing that the Cu(I) compounds have a brighter color than the corresponding Cu(II) compounds.26 AuFTP. First, 0.03490 g of hydrogen tetrachloroaurate(III) trihydrate (48.54% Au) dissolved in 2 mL of water was added in a dropwise manner into a solution of 0.1057 g of KFTP in 20 mL of water. After allowing the solution to stand for 24 h, the resulting light brown precipitate was filtered off and then washed in 30 mL of water followed by 2 × 15 mL of diisopropyl ether. Then, after drying in a vacuum, 0.0227 g of product was obtained. After further purification by recrystallization in n-pentane, this sample was used for spectroscopy. AgFTP. First, 0.0253 g of AgNO3 dissolved in 3 mL of water was added in a dropwise manner to 0.1048 g of KFTP in 20 mL of water. A yellowish white precipitation was filtered off and washed with 20 mL of diisopropyl ether. Drying under reduced pressure gave 0.0545 g of product, which was stored in the dark. CuFTP. First, 0.0245 g of CuCl2‚2H2O dissolved in 3 mL of water was added in a dropwise manner to 0.099 g of KFTP in 29 mL of water. Because filtration was not possible, the mixture was extracted with 3 × 10 mL of diisopropyl ether. In the ether phase, a white gelatinous precipitate was formed, and more was obtained after cooling. The solid was filtered off, washed with diisopropyl ether, and dried under reduced pressure to yield 0.016 g of product. Preparation of Metal Surfaces. The silicon wafers (100) were obtained from OKMETIC, Espoo, Finland. Silver (99.95%) and gold (99.95%) for the evaporation were obtained from ANA A ¨ delmetall, Helsingborg, Sweden, and copper (>99%) was from Balzers. The wafers were cut into slides; 20 × 40 mm2 for IRAS and ellipsometry, and 10 × 20 mm2 for XPS. The metal films were prepared by electron beam evaporation onto clean silicon single-crystal (100) slides. The slides were cleaned in a 5:1:1 mixture of MilliQ water, ammonia (25% aqueous solution), and hydrogen peroxide (30% aqueous solution) called TL1, heated to 80 °C for 5 min, rinsed in MilliQ water, and blown dry in nitrogen. The slides were primed with an adhesion layer of a 25 Å titanium prior to the deposition of the metals. The metals were evaporated at a rate of 5 Å/s to a thickness of ∼2000 Å. The base pressure in the chamber was 2 × 10-9 mbar, and the pressure during evaporation was held 100 Å. XPS of FTP Films on Gold. The carbon binding energy spectrum of the adsorbate from solution still shows a main sharp, symmetric peak at 284.5 eV and a shoulder at 286.7 eV (Figure 4b). The signal from K(2p) is gone when FTP is adsorbed on gold showing that the salt form no longer exist. The shake-up peak related to the aromatic ring structure is now seen as a weak, broad feature just above the background at ∼292 eV. The fluorine binding energy peak found at 686.7 eV is still sharp and symmetric (Figure 5b). Only one state of sulfur is observed even for FTP adsorbate (Figure 6b). The two sulfur atoms in FTP have the same chemical environment, and because the potassium salt form is gone, this result supports a coordination of FTP to the surface through both sulfur atoms (bridging) as was suggested from the IRAS measurements. This coordination is further supported by the angle-dependent XPS results given later. For the adsorbate, the relative intensity ratio between the C(1s) and S(2p) peaks, with cross section taken into account, is 8.5 (Table 4), which is higher than the stoichiometric value of 6 and more than the 5.6 for the multilayer. The freshly evaporated gold surfaces were shortly exposed to air before they were immersed into solution. Some hydrocarbon contamination for monolayers prepared in this way will always be present because gold has a high affinity for hydrocarbons. Cross-sectional changes of S(2p) caused by interaction with the surface may also occur. The symmetry properties of the electronic orbitals may also influence the cross section.22 The most important reason, however, is molecular orientation. When this relatively large molecule is adsorbed with the sulfur atoms close to the gold surface, the signal from the sulfur atoms will be attenuated compared with those from the aromatic rings because of the limited escape depth of the photoelectrons.26 One state of phosphorus is observed for the FTP adsorbate, as expected (Figure 7b). The relative intensity ratio between the C(1s) and P(2p) peaks, with cross section taken into account, is 11.5 (Table 4), which is close to stoichiometric value but still more than that observed for a multilayer. Angle-dependent measurements were made using photoelectron takeoff angles of 30°, 70°, and 80° with respect to the surface normal of the sample. This method is used to extract information about the orientation of molecules at the surface. High takeoff angles (surface sensitive mode) result in high surface sensitivity, whereas lower takeoff angles, e.g. 30°, provide information from the whole adsorbate (bulk sensitive mode). Angle dependent XPS measurements show a drastic increase of C/P ratio with increasing takeoff angle (Table 4). Even more pronounced is the increase of C/S ratio with increasing takeoff angle. The reverse is true for the C/F ratio, the C/F decreases with increasing takeoff angle. This is in good agreement with molecules oriented with both sulfur atoms close to the gold surface and with the fluorine on the other side of the ring pointing away from the surface. As expected the gold signal drastically decreases with higher takeoff angles. The thickness and orientation patterns of the monomolecular layers just given should be compared with data from X-ray studies performed on potassium O,O′-dibenzyl dithiophosphate39 and gold(I) O,O′-diisopropyl dithiophosphate.36 In the former case, the side of the unit cell, 11-12 Å, should be compared with the thickness of a monomolecular layer at maximum packing density, which
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in our case is 11.5 Å. The molecular arrangement given by those authors leave the benzene rings oriented in a synclinal and antiperiplanar way. In our case, however, the strong bonds between the sulfur and the gold atoms crowd the molecules together in a more erected way, making the monomolecular layers thicker than the half of a double layer in the model of Hazel et al.39 A more recent ab initio calculation on zinc O,O′-diphenyl dithiophosphate on R-Fe2O340 results in a molecular conformation similar to the one observed here. Also, space-filling molecular models can be arranged to support our data. IRAS of FTP on Silver and Copper. In Figure 9 are shown the spectra from FTP adsorbed on silver with the simulated spectra of an isotropic layer of the silver salt, and in Figure 10 are the corresponding data are given for FTP on copper. The input film thicknesses in the simulations were chosen to give IRAS intensities comparable to those of the experimental spectra of the thinnest films. Comparing the spectra with those from molecules adsorbed on gold in Figures 2 and 3 shows that the orientation effects are less significant going from gold to silver and copper. In particular, the peaks at 700-600 cm-1 appear with appreciable intensities in the two latter cases. Also, the intensities in the spectra of the adsorbed molecules relative to the simulated spectra are higher than those on gold (cf. Figure 3 with Figures 9 and 10), and no saturation value of the peak intensities can be observed. A comparison between the simulated RA spectra of the metal complexes and the RA spectra in Figures 9 and 10, reveals that the individual peaks lie at the same wavenumbers within the used resolution (4 cm-1) as was the case with gold (vide supra and ref 4). Furthermore, the spectra of adsorbed layers of dithiophosphates on silver and copper show only minor variations in relative peak intensities compared with the simulated spectra of the corresponding isotropic layers. The adsorption of xanthates on silver and copper is suggested to start with the formation of a monolayer of xanthates with their sulfur atoms bound to the metal surface, followed by the association of multilayers in which the molecules are randomly oriented.3,7,9,15,41 Such a stepwise adsorption would manifest itself as marked orientation effects after short immersion times, especially in solutions of low concentrations. The changes should be most pronounced in the S-P-S vibrations at 750-650 cm-1 region for the dithiophosphates. Upon increasing immersion time and solution concentration, the spectra should approach a more random orientation appearance. We find it difficult to observe spectra on silver with marked orientation effects at lower degrees of adsorption; for example, the PS2 bands in the region 700-600 cm-1 do not disappear as on gold. In the case of copper, these effects are not easily studied because of the presence of a Cu2O band at ∼650 cm-1. The weakening of the peak at 1175 cm-1 of the spectrum taken after 4 min in 1.0 µM solution indicates perhaps that the first layer is preferentially oriented. In our measurements on gold, however, RA spectra of comparable intensity showed stronger orientation effects. We then conclude that highly organized and densely packed monolayers do not form on silver and copper before the formation of multilayers starts. Instead, the continued adsorption takes place via a dissolutionprecipitation process that erodes the metal surface and then prevents the molecules from forming an organized (40) Jiang, S.; Frazier, R.; Yamagushi, E.-S.; Blanco, M.; Dasgupta, S.; Zhou, Y.; Cagin T.; Tang, Y.; Goddard, W. A., III J. Phys Chem B 1997, 101, 7702. (41) Talonen, P.; Rastas, J.; Leppinen, J. Surf. Interface Anal. 1991, 17, 669.
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Figure 9. Simulated RA spectrum of silver O,O′-di(parafluorophenyl) dithiophosphate on silver with layer thickness of 5 Å. The refractive index, n∞, was 1.52 and the salt density was 1.70 g/cm3. Shown in the lower traces are spectra of potassium O,O′-di(para-fluorophenyl) dithiophosphate adsorbed on silver. The concentrations of the immersion solutions and the immersion times are given in the figure. Note the change of intensity scale between the second and third spectra, showing the occurrence of multilayers.
Figure 10. Simulated RA spectrum of copper O,O′-di(parafluorophenyl) dithiophosphate on copper with layer thickness of 5 Å. The refractive index, n∞, was 1.54, and the salt density was 1.55 g/cm3. Shown in the lower traces are spectra of potassium O,O′-di(para-fluorophenyl) dithiophosphate adsorbed on copper. The concentrations of the immersion solutions and the immersion times are given in the figure. Note the change of intensity scale between the second and third spectra, showing the occurrence of multilayers.
structure even at low degrees of adsorption. The exact mechanism of the process just described is not fully understood. However, two alternative models have been discussed:15,41 (1) dissolution of metal ions into the solution followed by complex formation and redeposition on the metal surface, and (2) dissolution of metal ions followed by instantaneous complex formation on the surface. In the second model, the sulfur-containing ion is expected to take an active part in the dissolution process. We believe that the second model is the dominating one, at least for high concentrations. The more intense spectra on silver and copper (cf. Figures 9 and 10, lower parts) show minor orientation pattern. For example, the bands at 1175 and 830 cm-1 are somewhat weakened. This result may be due to the formation of partly ordered multilayers after longer
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adsorption times. Such overlayers have been observed before by Ihs et al.3 for the adsorption of ethyl and octyl xanthates on copper and silver surfaces. In case of copper, a layer of cuprous oxide is inevitably formed on the metal surfaces before they are immersed in the xanthate solution. This formation is supported by XPS measurements,15 Auger spectroscopy measurements,3 and, in the present experiments, by the successive disappearance of a broad band from Cu2O at ∼650 cm-1 at longer immersion times and higher concentration of KFTP (Figure 10). When a metal surface with an oxide layer is immersed in the dithiophosphate solution, the oxide layer dissolves rapidly and a precipitate of CuFTP is formed on the surface. Thus, the oxide may act as a source of Cu(I) ions during the early events of the overall process. Silver surfaces are less prone to air oxidation,3 which may influence the kinetics of the precipitation reaction. Slower kinetics may also affect the structural properties of the adsorbed layer. None of these metal surfaces, however, seems to induce the same degree of molecular orientation of the adsorbed dithiophosphates as the gold surface, which is supposed to remain intact during the adsorption process.2,4 The structure of multilayers on silver and copper may be compared with results from structural analysis of analogous complexes. Copper(I) O,O′-diisopropyl dithiophosphate has been examined using X-ray methods by Lawton et al.42 This compound forms a tetramer, Cu4[(iC3H7O)2PS2]4, which is built up of a central tetrahedron of four copper atoms. Four dithiophosphate ligands are linked to this tetrahedron, which is so arranged as to give the tetramer a symmetry approaching S4. One sulfur atom in each ligand bridges two copper atoms almost symmetrically, and the other forms a bond with a third copper atom. Each phosphorus atom is thus centered more or less above a face of the copper tetrahedron. The two sulfur atoms in one ligand play different roles in the structure and have different P-S bond lengths, the bridging one 2.036 Å and the other 1.972 Å. The overall structure of the tetramer centered around the four Cu atoms is difficult to combine with orientation on the metal surface in contrast to the linear dimer suggested for gold(I) diisopropyldithiophosphate.36 However, the X-ray studies can provide important information regarding the coordination, which in turn may assist in the assignment of the IR spectra. For example, comparing the simulated RA spectrum of AuFTP with that of CuFTP (Table 2) shows that there is one band at 656 cm-1 in the former case, but
two at 649 and 636 cm-1 in the latter case. The doublet may very well reflect the existence of two different P-S bonds in CuFTP. The doublet could also be attributed to the occurrence of both Cu(I) and Cu(II) salts. However, this latter explanation is ruled out by the presence of a similar doublet in the corresponding Ag(I) salt (cf. Figures 9 and 10), where no ambiguity concerning the metal valence state can be maintained. To our knowledge no structure elucidation of any simple organic silver dithiophosphate has been published. However, Drew et al.43 have studied the anion [Ag{S2P(OC2H5)2}2]22- and concluded the existence of both bridging and chelating sulfur atoms. Our spectrum of isotropic AgFTP (Figure 9) indicates better similarity to that of CuFTP (Figure 10) than to that of AuFTP (Figure 2). Furthermore, spectra of FTP adsorbed on silver are more alike those obtained on copper than on gold. We then conclude that the interaction of dithiophosphates with silver surfaces is similar to that with copper surfaces. This conclusion is also in line with earlier results from this laboratory3 and from other groups10,41 concerning the adsorption of various organic xanthates on metal surfaces.
(42) Lawton, S. L.; Rohrbaugh, W. L. Kokotailo, G. T. Inorg. Chem. 1972, 11, 612.
(43) Drew, M. G. B.; Hobson, R. J.; Mumba, P. P. E. M. Rice, D. A. Inorg. Chim. Acta 1988, 142, 301.
Conclusions Adsorption of potassium O,O′-di(para-fluorophenyl) dithiphophosphate on gold, silver, and copper was investigated by IRAS, ellipsometry, and XPS methods. Orientation patterns derived from IRAS spectra and ellipsometric measurements reveal that the adsorption on gold takes place with bonding of both sulfur atoms to gold atoms. The organic part of the molecules changes orientation from a flat to a fully erected conformation with increasing immersion times and solution concentrations. This picture is confirmed by the angle-dependent XPS. The adsorption on silver and copper proceeds via a dissolution-precipitation mechanism, which is probably initiated by the presence of an oxide on the metal surface. The spectral intensities increase with longer immersion times and more concentrated solutions, and no limiting thickness of the layer is achieved. Acknowledgment. We are grateful to staff members at the Laboratory of Applied Physics for preparation some of the metal surfaces and to Prof. M. Hellsten for the synthesis of the dithiophosphate. This work was supported by a grant from the Swedish Research Council for Engineering Sciences. LA990266N
