Langmuir 1996, 12, 5709-5721
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Oxidation of Galena in Acetate Buffer Investigated by Atomic Force Microscopy and Photoelectron Spectroscopy Gunther Wittstock,*,† Ilkka Kartio,‡ Dietmar Hirsch,§ Stefan Kunze,† and Ru¨diger Szargan† University of Leipzig, Institute of Physical and Theoretical Chemistry, Linne´ strasse 2, D-04103 Leipzig, Germany, Laboratory of Materials Science, Department of Applied Physics, University of Turku, Vesilinnantie 5, FIN-20014 Turku, Finland, and Institute of Surface Modification, Permoser Strasse 15, D-04318 Leipzig, Germany Received April 22, 1996X Galena oxidation was investigated by AFM in acetate buffer under potentiostatic control and by photoelectron spectroscopy on potentiostatically pretreated specimens. At +236 mV (SHE) formation of sulfur protrusions could be observed with AFM. XPS showed the formation of elemental sulfur to start at potentials more anodic than +161 mV (SHE). Elemental sulfur could only be retained on the galena surface if sample cooling was started before the beginning of the evacuation in the spectrometer entry chamber. Sulfur-oxygen species could not be detected on galena samples oxidized in acetate buffer even when investigated with synchrotron-excited X-ray photoelectron spectroscopy. AFM images showed two important features: Oxidation starts with a roughening of the sample surface. At slightly more anodic potentials oxidation products are present on the samples as protrusions of 10-200 nm in height and with mutual distances of several hundred nanometers. Two types of sulfur deposits are formed differing in the emergence potential, size, and mutual distance. The formation of such protrusions can only be understood if the reactants for the depositions reach the growing protrusion by diffusion in the liquid phase. Therefore, it is proposed that the process causing the surface roughening is a dissolution of PbS to lead(II) ions and hydrosulfide ions while the deposition reaction is the electrochemical oxidation of hydrosulfide ions to elemental sulfur. By removal of the hydrosulfide ion from the aqueous solution, further dissolution becomes possible at other sample regions. The sulfur formation occurs at distinct points which are not preferentially located at steps. It is likely that the sulfur formation starts at impurity locations. Different impurities may be responsible for different rates of deposit formation, leading to protrusions of different size which however cannot be distinguished by XPS.
Introduction Galena is a natural lead sulfide mineral and constitutes one of the most important sources of lead. It is separated from other metal sulfides and gangue material by selective flotation, which can be achieved by controlling the potential and the pH of the flotation pulp as well as by the choice of a collector which renders the mineral surface hydrophobic, which is a prerequisite for the mineral particles to adhere to the purge gas bubbles.1 Natural galena is a semiconductor with a band gap of about 0.4 eV,2 which results in nearly metallic conductivity at room temperature. Most samples of natural origin show n-type behavior.3 However, as pointed out by a recent study, pieces from one cluster of crystals may behave as highly n-type (probably degenerate, majority), n-type (not degenerate), and slightly p-type.4 Galena crystals can easily be cleaved to expose the (100) face of a cubic facecentered crystal. Because of this property in conjunction with its good conductivity, galena lends itself to electrochemical, spectroscopic, and scanning probe microscopic investigation. * To whom correspondence should be addressed. † University of Leipzig. Phone: (+49-341) 9736460. Fax (+49341) 9736399. E-mail:
[email protected]. ‡ University of Turku. Phone: (+358-2) 3335967. Fax: (+358-2) 333 5070. E-mail:
[email protected]. § Institute of Surface Modification. Phone: (+49-341) 2352727. Fax: (+49-341) 235 2595. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Gaudin, A. M. Flotation, 2nd ed.; McGraw-Hill: New York, 1957. (2) Scanlon, W. W. Solid State Physics; Academic Press: New York, 1959; p 83. (3) Richardson, P. E.; O’Dell, C. S. J. Electrochem. Soc. 1985, 132, 1350. (4) Richardson, P. E.; Yoon, R.-H.; Woods, R.; Buckley, A. N. Int. J. Miner. Process. 1994, 41, 77.
S0743-7463(96)00385-X CCC: $12.00
The importance of surface oxidation of sulfide minerals has been recognized for a long time.5-7 The anodic processes are important for the understanding (and optimization) of the flotation behavior of metal sulfides for several reasons:8 (i) sulfide mineral surfaces become oxidized when they are exposed to oxygen and water during mining and mechanical processing. The surface composition of the mineral has a great influence on the subsequent separation by flotation. (ii) Surface oxidation is considered as the reason for the so-called self-induced flotation reported for galena,9 i.e. a flotation without the addition of a collector under oxidizing conditions. The hydrophobization of the mineral is ascribed to the formation of sulfur-rich overlayers on the galena particles. (iii) Reaction products containing the cation and an oxygencontaining anion (PbO, Pb(OH)2, PbCO3, PbSO4, etc.) are normally hydrophilic. On the other hand, collectors can replace oxygen-containing anions to form metal complexes. In this case, the final hydrophobicity depends on the balance between hydophilic metal-oxygen compounds and hydrophobic metal-collector complexes at the surface. (iv) The mechanism of xanthate chemisorption has been proved to be electrochemical in nature.10 In some cases further oxidation of thiol-type collectors is assumed to (5) (a) Plaksin, I. N.; Shafeev, R. Sh. Dokl. Phys. Chem. (Transl. of Dokl. Akad. Nauk) 1960, 132, 421. (b) Plaksin, I. N.; Shafeev, R. Sh. Trans.sInst. Min. Metall. 1963, 72, 715. (6) Tolun, R.; Kitchener, J. A. Trans.sInst. Min. Metall. 1964, 73, 313. (7) Woods, R. J. Phys. Chem. 1971, 75, 354. (8) Richardson, P. E. In Mineral Surfaces; Vaughan, D. J., Pattrick, R. A. D., Eds.; Chapman & Hall: London, 1995; Vol. 5; pp 261-272. (9) (a) Guy, P. J.; Trahar, W. J. Int. J. Miner. Process. 1984, 12, 15. (b) Guy, P. J.; Trahar, W. J. In Developments in Mineral Processing 6, Flotation of Sulfide Minerals; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; pp 91-109. (10) de Donato, P.; Cases, J. M.; Kongolo, M.; Michot, L.; Burneau, A. Colloids Surf. 1990, 44, 207.
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achieve the hydrophobicity required for flotation.11 (v) If cations are solvated, they can interact with other mineral surfaces. These processes may enable the flotation of an otherwise not floatable mineral12 or corrupt the selectivity for a specific mineral intended by the use of particular flotation conditions.13 Because of these reasons, galena oxidation has been the subject of numerous studies. However, no final agreement could be reached about the mechanisms of oxidation under various conditions and the nature of the reaction products (See ref 8 for a recent review). The simplest experimental situation is found in acidic electrolytes, which do not contain anions that form salts of low solubility with lead ions. In such a situation, lead(II) ions are solvated after oxidation and leave the mineral surface for the bulk phase of the electrolyte, as has been proved by cyclic voltammetry with rotating ring-disk electrodes.4,14 It remains a task to clarify the chemical nature and structure of the sulfur-containing oxidation products formed when the composition is changed beyond the stability range of bulk PbS ((0.1 atom %).15 Partially conflicting experimental evidence has been presented for the formation of elemental sulfur,14,16,17 metal polysulfide,18-20 metal-deficient lead sulfide,21,22 and sulfate.23 Early voltammetric studies assumed that elemental sulfur is formed in acidic solutions if the oxidation exceeds the stability range.14,16,17 Buckley and Woods18 investigated cleaved galena surfaces with X-ray photoelectron spectroscopy (XPS) at 150 K after oxidation. Elemental sulfur could only be identified if samples were treated in solutions of hydrogen peroxide. For air oxidation, Pb(OH)2 was identified as the initial oxidation product from the Pb 4f and O 1s signals. No change of the S 2p signal could be detected. For reasons of stoichiometry, it was concluded that the Pb(OH)2 formation has to be associated with a layer of metal-deficient lead sulfide,18a which might be metastable or stabilized by oxygen outside the stoichiometric range of bulk PbS. However, no deviation from the 1:1 stoichiometry could be detected within the accuracy of the method. This was seen as consistent with the lack of a S 2p binding energy (BE) shift of the metaldeficient surface layer. If the lead-containing oxidation products were dissolved in diluted acetic acid, a considerable enrichment of minor elements (Cu, Sb) was observed (11) Salamy, S. G.; Nixon, J. C. Recent Developments in Mineral Dressing; Institute of Mining and Metallurgy: London, 1953; pp 503516. (12) Finkelstein, N. E.; Allison, S. A. In Flotation A. M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical and Petroleum Engineers: New York, 1976; Vol. 1, Chapter 14. (13) Guy, P. J.; Trahar, W. J. In Developments in Mineral Processing 6, Flotation of Sulfide Minerals; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; pp 61-79. (14) Paul, R. L.; Nicol, M. J.; Diggle, J. W.; Saunders, A. P. Electrochim. Acta 1978, 23, 625. (15) Bloem, J.; Kroger, F. A. Z. Phys. Chem. (NF) 1956, 7, 1. (16) Richardson, P. E.; Maust, E. E. In Flotation A. M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical and Petroleum Engenieers: New York, 1976; Vol. 1, Chapter 12. (17) Gardner, J. R.; Woods, R. J. Electroanal. Chem. 1979, 100, 447. (18) (a) Buckley, A. N.; Woods, R. Appl. Surf. Sci. 1984, 17, 401. (b) Buckley, A. N.; Woods, R. In Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing; Richardson, P. E., Srinivasan, S., Woods, R. Eds.; Electrochemical Society: Pennington, NJ, 1985; pp 286-302. (19) Luttrell, G. H.; Yoon, R.-H. Colloids Surf. 1984, 12, 239. (20) Kartio, I.; Laajalehto, K.; Kaurila, T.; Suoninen, E. Appl. Surf. Sci. 1996, 93, 167. (21) (a) Buckley, A. N.; Woods, R. Colloids Surf. 1991, 59, 307. (b) Buckley, A. N.; Riley, K. W. Surf. Interface Anal. 1991, 17, 655. (22) Buckley, A. N.; Hamilton, I. C.; Woods, R. In Developments in Mineral Processing 6, Flotation of Sulfide Minerals; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; pp 41-60. (23) Fornasiero, D.; Li, F.; Ralston, J.; Smart, R. St. C. J. Colloid Interface Sci. 1994, 164, 333.
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together with a component in the S 2p spectrum which had a BE higher than that of PbS but lower than that of elemental sulfur. From the relative intensities it was concluded that copper sulfides and metal polysulfide cause the additional spectral component. The enrichment of Cu and Sb was presumed to result from mobile minor elements filling vacancies in the metal-deficient layer. These findings did not remain undisputed,8,24 and reinvestigations were published by Buckley and Woods and other groups together with attemps to extend the concept of metal-deficient PbS to the oxidation in solution.22 Besides elemental sulfur and metal-deficient lead sulfide, metal polysulfide was reported to be the oxidation product of galena in acidic solutions.18-20 Polysulfides Sn2- are well-known unbranched sulfur chains which can be considered as the anions of the weak two-basic acid H-S-Sn-2-S-H. These species should account for at least two sulfur signals in photoelectron spectra with higher BE than that of the bulk lead sulfide component. Such spectra have been recorded and assigned for other compound semiconductors.25,26 The terminal sulfur atoms have a BE slightly above that of the bulk lead sulfide component. For all other sulfur atoms of the chain only one signal could be resolved with monochromatized Al KR27 or synchrotron radiation (SR)20 excitation with a slightly lower BE than that of elemental sulfur and a BE clearly above that of the bulk sulfide component as well as that of the terminal atom of the polysulfide chain. Buckley et al.27 also considered the possibility that the terminal sulfur atom may not be distinguishable from the bulk sulfide component, the second sulfur atom may have a slightly higher BE than both bulk sulfide and the terminal atom of polysulfide, and all other sulfur atoms cannot be resolved. In light of the results for polysulfides on other semiconductors, where an accurate assignment is possible, such a designation seems less probable.25,26 While elemental sulfur adheres to the mineral at atmospheric pressure by nonchemical interaction, polysulfides are linked to the metal sulfide lattice by ionic or covalent bonds and are therefore not volatile in UHV at room temperature.18 Fornasiero et al.23 have proposed a detailed mechanism for processes occurring at the galena surface in contact with electrolyte solutions of pH 5. An oxidative dissolution with formation of sulfate is assumed on the basis of the assignment of a shoulder at the high BE side of the Pb 4f peak. However, the corresponding S 2p or S 2s signals could not be measured. Very recently the oxidation of galena in air and in solution has regained a lot of attention because scanning probe microscopic techniques allow the direct monitoring of the changes at the galena surface during oxidation with very high resolution in real space and real time. Atomic resolution both in the lateral (x, y) and vertical (z) dimensions at galena has been demonstrated using scanning tunneling microscopy (STM).28-31 Some of these (24) Walker, G. W.; Richardson, P. E.; Buckley, A. N. Int. J. Miner. Process. 1989, 25, 153. (25) Fukuda, Y.; Suzuki, Y.; Sanada, N.; Sasaki, S.; Ohsawa, T. J. Appl. Phys. 1994, 75, 3059. (26) Chasse´, T.; Peisert, H.; Streubel, P.; Szargan, R. Surf. Sci. 1995, 331-333, 434. (27) Buckley, A. N.; Kravets, I. M.; Shukarev, A. V.; Woods, R. J. Appl. Electrochem. 1994, 24, 513. (28) (a) Eggleston, C. M.; Hochella, M. F. Science 1991, 254, 983. (b) Eggleston, C. M.; Hochella, M. F. Geochim. Cosmochim. Acta 1990, 54, 1511. (c) Hochella, M. F.; Eggleston, C. M.; Ellings, V. B.; Parks, G. A.; Brown, G. E.; Wu, M. C.; Kjoller, K. Am. Mineral. 1989, 74, 1233. (29) (a) Zheng, N. J.; Wilson, I. H.; Knipping, U.; Burt, D. M.; Krinsley, D. H.; Tsong, I. S. T. Phys. Rev. B 1988, 38, 12780. (b) Sharp, T. G.; Zheng, N. J.; Tsong, I. S. T.; Buseck, P. R. Am. Mineral. 1990, 75, 1438. (30) Liao, L.-B.; Shi, N.-C.; Ma, Z.-S.; Bai, C.-L. Chin. Sci. Bull. 1991, 36, 1989. (31) Higgins, S. R.; Hamers, R. J. Surf. Sci. 1995, 324, 263.
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images were recorded under a non-polar organic liquid28 or in vacuum.29 Other groups investigating galena in air or in aqueous solutions report atomic resolution for the z dimension and a considerably lower resolution for the x, y dimensions.32-35 During air oxidation the reaction products have been evident from protrusions on the mineral surface, which grow with time.33 These protrusions seem to be randomly scattered on natural galena, while they tend to be aligned at terrace steps on synthetic galena.34 These findings support earlier assumptions that the oxidation starts at impurity locations or crystal defects rather than on step edges. After the oxidation at an impurity site, neighboring sulfide ions are believed to be activated (so-called neighboring atom effect).32 If impurities or defects are not present (as in synthetic samples), the oxidation proceeds more slowly and step edges become the preferred starting point for that process.34a The in situ STM study of Higgins and Hamers31 with galena potentiostated to the open circuit potential (OCP) in deareated NaClO4, pH 1.7, revealed a continuous dissolution with removal of material from step edges. A series of images under increasingly anodic potentials showed the enhanced removal of material from step edges and the nucleation of pits, preferentially at locations where small deposits (likely impurities) had been detected at the OCP. No indication of depositions was found. It remained open whether they were not formed or formed but not imaged. Kim et al.35 monitored the formation of pits in gas-saturated (O2 and N2) water, pH 3, which they interpreted as congruent oxidative dissolution (formation of Pb2+ and SO42-). No evidence was found for the formation of a sulfur-rich overlayer. In this study the oxidation was carried out in acetate buffer, pH 4.9, under potentiostatic control. In situ atomic force microscopy (AFM) was chosen to monitor changes in surface morphology, because AFM can be used for conducting, semiconducting, and insulating materials. Therefore, AFM is suitable not only to image galena but also to display nonconducting oxidation products like elemental sulfur. The technique developed here will be applicable to sulfide minerals which are nonconducting, too. Since AFM uses an optical detection system, it was hoped to circumvent problems associated with tip-sample interaction which frequently pose a problem in STM investigations. Tip-sample interactions have early been reported for STM imaging of galena in air.28 While such interactions were not observed during STM imaging under aqueous solutions,31 other studies did not verify experimentally the absence of such artifacts. Instead, it was tried to minimize their extend by removing the STM tip from the sample between capturing successive images over time.33-35 This in return creates the problem of relocating the spot originally investigated. Our own results indicate rather severe tip-sample interaction during STM imaging of galena in oxidizing gas atmospheres (air, I2).36 In this study AFM was used in contact mode with low forces applied to continuously monitor the oxidation process in acetate buffer. However, probe-sample interactions are to be reported for AFM of galena in acidic solutions, too. XPS was used to determine the chemical nature of the reaction products as well as to clarify the processes induced (32) Cotterill, G. F.; Bartlett, R.; Hughes, A. E.; Sexton, B. A. Surf. Sci. Lett. 1990, 232, L211. (33) Laajalehto, K.; Smart, R. St. C.; Ralston, J.; Suoninen, E. Appl. Surf. Sci. 1993, 64, 29. (34) (a) Kim, B. S.; Hayes, R. A.; Prestidge, C. A.; Ralston, J.; Smart, R. St. C. Appl. Surf. Sci. 1994, 78, 385. (b) Buckley, A. N.; Woods, R. Appl. Surf. Sci. 1995, 84, 223. (c) Ralston, J. Appl. Surf. Sci 1995, 84, 225. (35) Kim, B. S.; Hayes, R. A.; Prestidge, C. A.; Ralston, J.; Smart, R. St. C. Langmuir 1995, 11, 2554. (36) Wittstock, G.; Hirsch, D.; Uhlig, I. Unpublished results, 1995.
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by the laser light of AFM. An experimental difficulty should be mentioned at this point. Since elemental sulfur is volatile under ultrahigh vacuum (UHV) conditions, samples have to be cooled in order to retain elemental sulfur on the surface. Experimental Section Materials and Reagents. Galena samples were purchased from Wards Natural Science Establishment (New York, NY) and were certified to originate from Brushy Creek, MO. Elemental analysis (X-ray fluorescence analysis)37 showed an elemental composition (by mass) of 85.56% Pb, 13.34% S, 0.5% Si, 0.3% Ga, 0.08% Fe. XPS analysis carried out in the authors’ laboratories also revealed occasionally the presence of Ca and Cl (probably CaCl2 inclusions), C (also on samples cleaved in ultrahigh vacuum and sputtered extensively), and Cu (only after exposure to aqueous solutions, as has been described by Buckley and Woods18) on samples from the same lump as used in this work. Deionized water (Elga, Bucks, U.K., or Christ, Stuttgart, Germany, 18 MΩ cm) was used for the preparation of buffer solutions and for rinsing. Analytical grade acetic acid (Merck, Darmstadt, Germany) and sodium acetate (Fluka, Buchs, Switzerland) were utilized to prepare the buffer of the specified pH. High-purity nitrogen (99.999%) was used to deaerate the electrolyte solutions for electrochemical measurements and sample preparation. Cyclic Voltammetry. For cyclic voltammetric studies a galena crystal was cleaved in air and immediately immersed in acetate buffer in such a way that only the freshly cleaved surface touched the surface of the liquid. The sample was connected as working electrode to an IMd 5 potentiostatic system (Zahner Elektrik GmbH, Kronach, Germany). The electrochemical threeelectrode cell was completed by a saturated calomel electrode (SCE, Sensortechnik Meinsberg, Meinsberg, Germany) as reference electrode and a platinum electrode as counter electrode. All potentials are quoted relative to the standard hydrogen electrode (SHE) in this paper. The SCE has a potential of +241 mV versus the SHE at 298 K. AFM Experiments. A NanoScope III instrument (registered trademark, Digital Instruments Inc. (DI), Santa Barbara, CA) was used to acquire the AFM images. The instrument is supplemented with electrochemical equipment (ECAFM, DI) and an optical microscope (Nikon) which was used during the mounting of samples and the rough positioning of the cantilever. Silicon nitride cantilevers (NanoProbes , DI) with a curvature of about 40 nm and a nominal spring constant of 0.58 or 0.12 N m-1 were used with an applied force between 20 and 60 nN. Galena samples were cleaved in air by a new scalpel blade to obtain flat specimens of about 4 mm × 2 mm × 0.5 mm. They were investigated using the ECAFM setup (DI). The attachment to that unit was accomplished with the help of a locally designed sample stage of magnetic steel (to attach the sample mechanically and electrically to the J scanner) and of a poly(tetrafluoroethylene) (PTFE) body to form the liquid reservoir (Figure 1). The galena crystal was attached to the steel stub with silver paint (Sigma, Deisenhofen, Germany). The polymer body was subsequently screwed on the steel stub in such a way that the surface of the mineral crystal was level with the upper surface of the polymer body. Mechanical strength was added to the silver paint junction by filling the bottom of the sample stage with rubber cement (Marabuwerke GmbH, Tamm, Germany). This also prevents a contact between the electrolyte solution and the steel parts of the cell construction. The cement dried in about 5 min. After adhesion of the cement, final drying and hardening was accelerated by intensive flushing with nitrogen for 3 min. The assembly was then mounted on the AFM scanner unit, and the commercially available part of the electrochemical cell was added as in the standard operational procedures described by the manufacturer. The electrochemical cell was used with a platinum wire as counter electrode and a chlorinated silver wire as reference electrode. This reference electrode has a potential of +421 mV against the SHE in the acetate buffer used. The potential values quoted in this paper were transformed to the SHE reference. (37) The elemental analysis of the galena sample was carried out by the Mining Academy Ostrava, Czech Republic. (38) Schro¨ter, T.; Eckardt, I. PHELPSsprogram for analysis of photoelectron spectra; University of Halle: Halle, Germany, 1994.
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Figure 1. In-house constructed sample stage to fit to the NanoScope III fluid cell. The screw made of steel and the PTFE part fit water-tight. The steel part is magnetically attached to the NanoScope III scanner unit (type A or J). The electrochemical cell was operated by the potentiostat of the electrochemical equipment of the AFM instrument (DI). For work under liquids, acetate buffer deaerated by bubbling nitrogen was introduced into the cell. When galena should be investigated under potentiostatic conditions, it was crucial to connect the potentiostat to the electrodes and to apply a potential where no reactions are anticipated for the galena electrode (-179 mV (SHE) in this work) prior to addition of the electrolyte. Failing to do so resulted in rapid changes of the mineral surface immediately following the contact with the aqueous solution. Imaging was initiated on an area which could be relocated easily by the shape of the steps caused by the cleavage procedure. In preliminary experiments the potential range was bracketed in which changes of the surface morphology can be observed. On a new sample, this range was approached by potential steps of 200 mV. After a decrease of the initial current following the potential step (about 20 s), the applied force was corrected to avoid high forces which could eventually result in the abrasion of formed oxidation products from the surface. Thermal drifts were corrected using the larger steps of the cleavage plane for orientation. Subsequently, one image was recorded usually at each potential with a scan frequency of 2.76 Hz (about 90 s for one image frame with 256 lines). Due to these operations each potential was applied for about 2-3 min. At some points the potential was held intentionally longer to see if observed changes continue with time or if the reaction had already ceased in the moment of the first image recording (e.g. 1-2 min after the potential step). Once the interesting potential range was reached, more anodic potentials were applied in 15 mV increments. This procedure was accompanied by a short visual inspection of the galena sample by the optical microscope to check for probesample interaction due to the AFM laser beam. Such interaction is evident from a decrease of the reflectivity of the galena surface in the vicinity of the cantilever. This phenomenon will be discussed further in the Results and Discussion section of this paper. The light of the optical microscope was switched off before image recording to prevent any additional photochemical effects during imaging. The potential step direction was reversed when the first sign of local sample darkening became apparent. To investigate whether the observed changes of the galena surface could be reversed, selected cathodic potentials were applied subsequently. AFM images were treated by a plane-fit procedure after the experiment. This feature of the NanoScope software removes sample tilts from images. No smoothing or other image enhancement techniques have been applied. A video camera system BL600 (Panasonic, Osaka, Japan) attached to the optical microscope of the AFM setup was used to record optical microphotographs of galena samples treated by the laser beam. XPS Experiments. A Perkin-Elmer PHI 5400 XPS spectrometer was used to record the spectra of potentiostatically pretreated samples. It uses monochromatized Al KR excitation (1486.6 eV) and a hemispherical analyzer with a constant pass energy of 89.45 eV for wide scans (BE range 0-1100 eV) and of 35.75 eV for narrow scans. The photoelectrons were collected at a 45° takeoff angle. The base pressure in the analyzing chamber
Wittstock et al. was in the 10-7 Pa range. The BE scale of the spectrometer was calibrated by using Au 4f7/2 (BE ) 84.0 eV) and Cu 2p3/2 (BE ) 932.7 eV) lines as a reference. The full width at half maximum (fwhm) of the Au 4f7/2 line was 0.9 eV when measured with narrow scan settings. Atomic concentration ratios used in this work were obtained by dividing the intensity of each spectrum by its experimental sensitivity factor provided by the manufacturer of the instrument. The samples for XPS measurements were compact galena pieces of approximately 10 mm × 10 mm × 0.5 mm size fractured from the same lump as the samples used in AFM experiments. A new sample with a fresh surface was provided prior to each experiment by cleaving a cubic piece of galena with a sharp razor blade. A three-electrode cell with a Ag/AgCl/KCl reference electrode, a platinum counter electrode, and the fresh galena surface as working electrode was connected to a AFRDE5 potentiostat (Pine Instruments Inc., Grove City, PA) for the potentiostatic sample preparations. All potential values are transformed to the SHE reference using the experimentally determined potential difference between the Ag/AgCl/KCl electrode and the SCE of -20 mV (+221 mV versus SHE). Acetate buffer, pH 4.9, was used as an electrolyte in all experiments. Prior to each sample preparation, fresh electrolyte solution was purged with nitrogen for at least 20 min to remove most of the dissolved oxygen. The nitrogen outlet was raised above the solution surface just before the sample immersion, maintaining an overpressure to prevent back-diffusion of oxygen. A potential of -89 mV (SHE) was applied to each sample for 5 min before sweeping the potential at a rate of 2 mV s-1 to the desired value. The sample was kept at the treatment potential for 15 min and then removed from the cell, rinsed gently with water, and inserted into the precooling and introduction chamber of the spectrometer. Cooling the sample down to liquid nitrogen temperature before the start of evacuation was considered essential, since elemental sulfur, a possible oxidation product, has been observed to desorb rapidly from a sulfide surface in UHV at ambient temperature.18 Technical details and test results of the precooling system utilized here are presented elsewhere.39 After cooling and evacuation, the sample was transferred into the analysis chamber, maintaining the sample temperature during the transfer. At the measuring stage, the sample was kept at about 130 K throughout the measurement. When considered useful, the sample was remeasured at ambient temperature after letting it warm up overnight in the analysis chamber. Curve fitting of the XPS spectra was performed with the Peak Fitting Module of the software package Origin (version 3.78; Microcal Software, Inc., Northampton, MA). All lines were fitted with a symmetrical pseudo-Voigt function (linear combination of Gaussian and Lorentzian functions), where the Gaussian and Lorentzian components have the same width. When fitting the S 2p spectra, a 1:2 peak area ratio was maintained between the S 2p1/2 and S 2p3/2 components of each doublet. In addition, the iteration limits of the BE difference between the doublet components was set to 1.17-1.20 eV and the fwhm’s of the components were kept equal within a (0.1 eV margin. To investigate the local changes introduced by the laser light of the AFM, a sample area of about 1.5 mm × 1 mm on a cleavage plane of 2 mm × 4 mm was modified by creating a closely packed array of dark spots by directly irradiating many points of that sample area in acetate buffer, pH 4.9, at the OCP. The modified surface was investigated with a small spot XPS setup 3 h after its removal from the AFM cell. An ESCALAB 220iXL spectrometer (VG, East Grinstead, U.K.) was used with a hemispherical analyzer and a pass energy of 50 or 10 eV for survey scans and for individual lines, respectively. The investigated spot area (150 µm × 800 µm) was defined by the incident monochromatized Al KR irradiation (1486.6 eV). The binding energy scale of this spectrometer had been calibrated as indicated above. The fwhm of the Au 4f7/2 line was 0.5 eV at a 10 eV pass (39) Kartio, I.; Laajalehto, K.; Suoninen, E.; Karthe, S.; Szargan, R. Surf. Interface Anal. 1992, 18, 807. (40) Szargan, R.; Karthe, S.; Suoninen, E. Appl. Surf. Sci. 1992, 55, 227. (41) Karthe, S.; Szargan, R.; Suoninen, E. Appl. Surf. Sci. 1993, 72, 157. (42) Mycroft, J. R.; Bancroft, G. M.; McIntyre, N. S.; Lorimer, J. W.; Hill, I. R. J. Electroanal. Chem. 1990, 292, 139. (43) Hyland, M. M.; Bancroft, G. M. Geochim. Cosmochim. Acta 1989, 53, 367.
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b Figure 3. Cyclic voltammogram of a galena electrode, cleaved in air and measured in acetate buffer, pH 4.9, scan rate 20 mV s-1, start of the potential scan at -159 mV toward the anodic direction. The markers indicate the potentials at which the AFM images of Figure 4 were recorded.
Figure 2. (a) Cleaved galena surface measured in air with the height scale 3.5 nm; (b) cleaved galena surface measured in acetate buffer at the open circuit potential with the height scale 84 nm. energy. The pressure in the analysis chamber was E/mV > -200). Since a part of the Pb2+ ions released during the anodic scan are dispersed into the bulk phase of the solution (either by diffusion or by enforced convection for example in rotating-ring disk experiments), a second peak at more cathodic potential (E < -400 mV) is often observed which has been ascribed to the removal of excess sulfur by
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k
n
Figure 4. AFM images of galena cleaved in air and measured in acetate buffer, pH 4.9, under a controlled potential: scan frequency, 2.76 Hz; height scale, 450 nm for parts a and c-n; image b is the same as image a scaled to 11.4 nm; potentials in mV (SHE) (a and b) +21, (c) +221, (d) +236, (e and f) +251, (g) +266, (h) +281, (i) +296, (j) +311, (k) +221, (l) -179, (m and n) -579; part f was recorded 4 min after part e, and part n was recorded 11 min after part m; relocation of specific features within the series can be done relative to the large steps for parts a-l. For parts l-n three prominent protrusions were labeled A, B, and C at the x and y axes for convenient relocation.
reduction to hydrogen sulfide:16,17,21a +2ne-, +2nH+
Sn(0) 98 nH2S
(2)
However, there is evidence that lead ions can be removed from the crystal lattice to form a metal-deficient lead sulfide at potentials around + 200 mV4,22 or in aerated solutions at OCP:18,21
8 Pb1-xS + Pb2+ PbS 9 -
(3)
-2xe
A distinction between the formation of elemental sulfur
and metal-deficient PbS cannot be made on the basis of voltammetric experiments alone. Techniques are required which provide information about the electronic structure of the surface and the surface morphology. The limit of the anodic potential excursion in Figure 3 is the same as that used during the AFM imaging under potentiostatic control. The markers indicate the potentials at which AFM images were recorded. These images are shown in Figure 4. To ease comparison, the images in Figure 4a and c-n have been scaled to the maximum scale within the series. Details of the initial surface are shown in Figure 4b with a smaller height scale. The initial cleavage plane at a potential of +21 mV has height
5716 Langmuir, Vol. 12, No. 23, 1996
variations of 11.4 nm (Figure 4b). The larger steps extending approximately diagonally across the investigated spot originate from the cleavage procedure. The appearance of the cleavage plane is not changed after a potential step to +221 mV (Figure 4c). The first changes in surface morphology are observable at a potential of +236 mV (Figure 4d). Two protrusions with an apparent height of 17 nm are displayed. As the potential is further increased, the protrusions continue to grow (Figure 4eg). Parts e and f of Figure 4 were both recorded at the potential of +251 mV with a 4 min constant polarization between both image recordings. The growth of the protrusions continues at a constant potential with time (height variations: 117 nm and 150 nm in Figure 4e and f, respectively). Therefore, Parts d-j represent snapshots from the changing surface morphology rather than established equilibrium situations. The structures formed at potentials below +266 mV are fairly large and have distances of several hundred nanometers between each other. They are referred to as type I protrusions in this communication. In the potential range (+266)-(+281) mV a new type of protrusion is formed, which is called a type II protrusion here. Their mutual distances are much smaller. While it seems that the growth of type I protrusions has ceased or is at least considerably slowed down (Figure 4g-i), type II protrusions rapidly grow. As evident from Figure 4i, there are areas of the sample which do not have structures of type I or II (i.e. 200 < x/nm < 1200; 1500 < y/nm < 1800). The generally flat morphology of the sample is destroyed at the potential of +311 mV (Figure 4j). The sample area which has already been identified as free from both type I and type II structures also appears significantly smoother than all other sample regions even at this potential. At +311 mV, a darkening of the area, which had been illuminated by laser light scattered at the cantilever, was observed by optical microscopy. The potential was set back to +211 mV, a potential at which no changes of the surface morphology had been observed during the anodic potential excursion (Figure 4k). There are no significant changes compared to Figure 4j. Some protrusions in the foreground have grown even further during the recording of Figure 4j. However, no reversal reaction to the anodic processes monitored in Figure 4d-j is observed which eventually would lead to a restoration of a smooth cleavage plane. Changes in morphology during cathodic treatment are found at -179 mV (Figure 4l). They correspond to the cathodic peak at -139 mV in Figure 3. The protrusions formed in anodic scans are flattened. At even more cathodic potential (-579 mV) the image of the surface changes again (Figure 4m and n). On the basis of previous XPS studies these new flat structures can be assigned to metallic lead.4,20 The lead formation continues with time, as revealed by a comparison of Figure 4n, which was recorded 11 min after Figure 4m at the same potential. Parts d-j of Figure 4 clearly demonstrate the localized deposition of material on the surface of galena during anodic treatment in acetate buffer. Areas between the protrusions become considerable rougher during that oxidative treatment. This second important observation is barely discernible from the reproductions in Figure 4dj, but it can be demonstrated by adjusting the height-togray translation to highlight features of the terraces rather than the large protrusions (Figure 5a-d). Appropriate line scans across the images result in profiles that display this observation in detail (Figure 5e). The growth of protrusions at some points is accompanied by the dissolution of PbS at other parts. The roughening of galena surfaces under oxidative conditions has already been observed with STM.31,35 Higgins and Hamers31 concluded
Wittstock et al.
that this process is not an electrochemical process but a dissolution of PbS (according to eqs 4-6) because the removal of material occurs even 250 mV cathodic of the OCP. Kim et al.35 proposed a mechanism similar to that introduced by Fornasiero et al.23 in which proton adsorption on the galena surface is followed by the adsorption of dissolved oxygen and formation of dissolved lead(II) ions and sulfate. The y extension of the protrusions (perpendicular to the scan direction) seems to be larger than the x extension. If the scan direction is changed by 90°, the protrusions change their shape and have a larger extension in the x direction (which is now perpendicular to the scan direction). This effect is most likely due to a suboptimal calibration of the piezo scanner and a result of the convolution between the tip and object shapes in AFM images. XPS Identification of the Chemical Nature of Oxidation Products. In order to clarify the chemical nature of the product formed during anodic treatment, galena specimens were subjected to a potentiostatic pretreatment at various potentials and subsequently investigated with XPS. S 2p spectra are shown in Figure 6. Four signals can be seen in all spectra pretreated at E > +161 mV. They are grouped in two doublets consisting of partially resolved S 2p3/2 and S 2p1/2 components (intensity ratio 2:1, BE difference 1.15-1.20 eV). For further discussion in this article, photoelectron signal positions are always quoted as the binding energy of the S 2p3/2 peak. The component at 160.7 eV corresponds to sulfide ions in the PbS bulk. Broad features at the low and high BE sides of that peak for the sample pretreated at -150 mV are caused by interference with the Pb 4f energy-loss structures and can be observed on fresh cleavage planes, too (Figure 7 and ref 18). The signal at 163.9 eV originates from the oxidation product. It grows relative to the bulk component at 160.7 eV with increasing anodic potentials. At the sample treated at +381 mV the signal from the bulk sulfide is only measurable as a very weak feature. The BE of the oxidation product corresponds to the BE of elemental sulfur (Table 1). This assignment is further supported by the observation that the oxidation product (Figure 7, middle) is lost in the UHV if a cooled sample warms up to room temperature (Figure 7, top), leaving only the bulk PbS sulfide component. From our experiences, sample cooling has to be started before pumping-down the sample entry system. Cooling the sample in the UHV part and the measurement position alone turned out to be not sufficient to retain lower amounts of elemental sulfur on the samples. The Pb 4f signal is suppressed in the same manner as the S 2p signal of bulk sulfide (Figure 8) if the sample is measured at 130 K. Rewarming the samples to room temperature restores both the bulk sulfide as well as the lead signal (Figure 8) with its energy-loss structure (Figure 7, top). The atomic ratios beween lead and sulfide sulfur are close to unity under all conditions. That proves that the overlayer formed by the oxidation product does not contain lead species. Rewarmed samples do not show any other sulfur-containing oxidation products; in particular there is no evidence for the presence of sulfate or thiosulfate. If lead sulfate or lead thiosulfate were formed, they would dissolve into the electrolyte (pKL(PbSO4) ) 7.80, KL is the dissolution equilibrium constant at 298 K). Fornasiero et al.23 proposed the presence of PbSO4 on a galena slurry after oxidation in a N2-purged solution of pH 5. This was concluded from intensity ratios of different nonresolved Pb 4f, C 1s, and O 1s components, although direct observation of sulfuroxygen species in S 2p and S 2s spectra was not reported.
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Langmuir, Vol. 12, No. 23, 1996 5717
e
Figure 5. Selected profiles of AFM images of Figure 4 during anodic potential change. (e) Profiles along cuts (a) +21 mV for Figure 4a; (b) +221 mV for Figure 4c; (c) +251 mV for Figure 4e; and (d) +281 mV for Figure 4h. The gray scale range corresponds to the apparent height in nanometers above the lowest point of the images: (a) 0-11.4; (b) 0-11.5; (c) 0-16.4; (d) 4-37. Other conditions are the same as in Figure 4.
Their absence was explained with the comparatively low sensitivity of the sulfur lines. In order to rule out that possibility, galena samples which were potentiostatically pretreated in a manner similar to that for the standard XPS experiments were investigated using SXPS. These spectra could not be recorded with sample cooling. Therefore, elemental sulfur is lost to the UHV. S 2p spectra of galena specimens pretreated at different potentials are shown in Figure 9 for an excitation energy of 240 eV. At an excitation energy close to this value, it has been estimated that approximately half of the S 2p signal intensity originates from the very first atomic layer at a pure PbS substrate.20 An additional advantage is the increased ratio of effective cross sections for the S 2p and Pb 4f photoelectron emissions (2.2:1.4) compared to Al KR excitation (0.022: 0.333).44 Minute amounts of sulfate or other sulfur oxygen (44) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1. (45) Kartio, I.; Laajalehto, K.; Suoninen, E. Submitted to Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing/1996; Electrochemical Society: Pennington, NJ.
species, which remain undetected with conventional XPS equipment, are easily detected with SR excitation (ref 20 and Figure 9b). Besides the well resolved doublet for the bulk sulfide, there are no other oxidation products detectable for pretreatment potentials of +141 and +191 mV. Only at potentials of +251 and +311 mV a component is recorded at the high-energy side of the sulfide peak with a contribution of less than 7% to the total S 2p intensity. The line position matches that ascribed to metal polysulfides on galena.20 However, as revealed by the spectra of properly cooled samples, this oxidation product can only be regarded as a side product formed in low amounts compared to elemental sulfur. Since the positions of the additional S 2p contribution on the different specimens are significantly different, it seems more likely that these high-energy components are due to sulfur species associated with impurity cations which may have a slightly different enrichment and concentration on each specimen, even when taken from the same lump. Because there are no components in the BE range 165170 eV, the presence of sulfate or thiosulfate on galena
5718 Langmuir, Vol. 12, No. 23, 1996
Wittstock et al. Table 1. Comparison of Binding Energies of Sulfides and Elemental Sulfur on Different Samples S 2p3/2 BE (eV) sample
ref
Aa
Bb
reference BE (eV)
Selemental on PbS Selemental on PbS Selemental on FeS2 Selemental on FeS2 Selemental bulk Selemental bulk
c 18 40 41 42 43
160.7 160.4 162.6 162.6
163.9 163.5 164.1 164.2 164.0 163.7
Au 4f7/2 ) 84.0 Au 4f7/2 ) 83.8 Au 4f7/2 ) 83.9 Au 4f7/2 ) 84.0 Au 4f7/2 ) 83.9 Au 4f7/2 ) 83.9
a Component from sulfide sulfur. b Component from elemental sulfur. c This work.
Figure 6. XPS measurement of the S 2p region of galena samples potentiostatically treated at various potentials. Final pretreatment potentials (SHE) from bottom to top: +91, +161, +211, +261, +306, and +381 mV (see Experimental Section for details of the potential program). Samples were cooled down to 130 K before the beginning of the evacuation in the sample entry system and kept at this temperature during the measurement. The intensities of the S 2p region were normalized to the largest peak. Figure 8. Atomic concentration ratios between sulfur and lead calculated from the XPS spectra measured at 130 K (b) and at 300 K (4). The atomic ratio between sulfur of bulk PbS and lead is given for comparison (O).
Figure 7. S 2p XPS signals of a freshly cleaved galena sample (bottom), a galena sample potentiostatically pretreated at +306 mV, cooled down to 130 K before the beginning of the pumping (middle), and the same sample after warming up overnight to room temperature (top).
samples potentiostatically oxidized at potentials below +70 mV can be ruled out. If very low amounts of lead sulfate were formed, they would probably have been dissolved in the electrolyte or during rinsing prior to the SXPS measurement. Survey spectra of all samples in Figure 6 were inspected for trace elements. Only in the case of the sample for Figure 6d were lower amounts of copper detected with a
BE of 932.3 eV (Cu 2p3/2), which together with the absence of a satellite corresponds to Cu(I) ions. The formation of Cu(I) polysulfides or metal-deficient copper-lead sulfide may be the reason for the slightly shifted energy position and some signal broadening of the high binding energy component in Figure 6d compared to Figure 6c,e, and f. The AFM images showed the formation of large protrusions on the galena surface. Since the surface is finally covered by an overlayer which does not contain sulfuroxygen species or lead species other than PbaSb, these protrusion can only be elemental sulfur Sn0, metal polysulfide PbSn, or lead-deficient lead sulfide Pb1-xS. The large protrusions are incompatible with a notion of metaldeficient lead sulfide, where the sulfur atoms still occupy their original lattice position. XPS spectra with cooled samples clearly prove that elemental sulfur is the dominant oxidation product. After bringing the samples back to room temperature, the original signal of the cleavage plane is restored because oxidation products are lost to the UHV. Polysulfide would still be present on the sample. Therefore, it is clear that the protrusions seen in the AFM images consist of elemental sulfur. This result is contrary to the earlier study from Buckley and Woods18 in which metal polysulfide or severely metaldeficient lead sulfide was found as the dominant oxidation product when galena was exposed to aerated diluted acetic acid for 35 days. However, during this time a considerable Cu enrichment of the surface was observed. As seen in Figure 6d of this paper, the presence of Cu at the sample surface leads to a sulfur component with a BE close to that expected for metal polysulfide. This fact together with the high amount of copper observed during the long treatment in acetic acid makes the assignment made in ref 18 somehow difficult. The potential of the galena electrode in the aerated acetic acid was later reported to be +230 mV (SHE) in that investigation.18b The same
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Langmuir, Vol. 12, No. 23, 1996 5719
likely that the reaction sequence is started by PbS dissolution, as proposed by Fornasiero et al.:23
PbS h Pb2+ + S2-, pKL ) 26.60
(4)
The sulfide ions will be protonated in acetate buffer, pH 4.9 (KS ) acidity constant at 298 K)
HS- h S2- + H+, pKS ) 12.60
(5)
H2S h HS- + H+, pKS ) 6.99
(6)
This limited dissolution will always occur in slightly acidic solutions. However, at anodic potentials the oxidation of hydrogen sulfide to elemental sulfur
nH2S h Sn(0) + 2nH+ + 2ne-
Figure 9. Synchrotron radiation excited photoelectron spectra (hν ) 240 eV) of the S 2p region of a cleaved galena sample: (a) immediately transferred to the spectrometer; (b) cleaved and stored in air for 18 min; (c-f) potentiostatically oxidized in acetate buffer for 30 min at potentials (SHE) of (c) +141, (d) +191, (e) +251, (e) and (f) +311 mV.
authors found a difference between potentiostatic oxidation and oxidation at OCP and suggested the kinetics of the oxidation process as the reason for it.21 At a potential equal to the OCP quoted in ref 18, considerable amounts of elemental sulfur were found in this study (Figure 6c and d). Unfortunately, ref 21 does not contain any spectra of potentiostatically oxidized samples, which would allow us to resolve this discrepancy further. It was suggested that in the case of oxidation at OCP, Pb ions or impurity ions in the bulk of the sample would have enough time to migrate to the surface and compensate for the lead vacancies, which would promote the formation of a metal-deficient lead sulfide layer. It is, however, not conclusive why an OCP would cause a slower oxidation rate than the very same potential imposed potentiostatically. In another study Buckley et al.27 observed the formation of lead polysulfide, when a galena sample was potentiostated to +200 mV (SHE) in a 2 mM hydrosulfide solution, pH 9.2, for 20 min. Different reaction products can be expected compared to this study because of the high hydrosulfide content in the solution and because the lead-containing oxidation products do not completely dissolve at pH 9.2. The highly localized formation of sulfur on the galena surface has to be associated with a mass transport of reactants to the protrusions. Anion diffusion in ionic crystals cannot provide enough material to form such large protrusions within the time scale of the experiment. Before the onset of the protrusion formation, a roughening of the surface is observed (Figure 5). These processes are likely identical to those seen in STM investigations of galena oxidation.31,35 The dissolution products diffuse and form the protrusions of elemental sulfur. Elemental sulfur would not be imaged by STM because it is insulating in such thick layers. Because of these observations, it seems
(7)
removes one component from the coupled equilibra of eqs 4-6, which will force an increased PbS dissolution to maintain the equilibrium. This reaction sequence is in accordance with the experimental observations: roughening of the galena surface followed by localized deposition of oxidation products identified as elemental sulfur. Different types of elemental sulfur deposits cannot be identified from the XPS spectra that would provide an explanation for the observation of type I and type II sulfur deposits in the AFM images. Because sulfur formation is scattered across the cleavage plane at slightly oxidizing conditions with no correlation to step edges it is likely that it starts at impurity locations. Different types of impurities may enhance the oxidation rate of hydrogen sulfide on the galena surface to a different extent, which could explain the existence of sulfur deposits with differing emergence potentials, mutual distances, and growth rates (type I and II protrusions). It remains the task for further work to quantify the influence of different impurity ions on the oxidation rate of galena and dissolved hydrosulfide ions, respectively. Influence of the AFM Laser Light on Galena Samples. The AFM uses a laser beam (wavelength ) 670 nm corresponding to 1.85 eV, total power ) 5 mW) for the detection of the cantilever deflection. As has been mentioned above a decrease of the reflectivity of the sample around the cantilever was normally observed, when the investigations were carried out in buffer solution at the OCP or at anodic potential. The effect caused by the laser light is shown in Figure 10. The areas illuminated by the scattered laser light lost their reflectivity, while the triangle which was in the shadow of the cantilever retained the high reflection factor. This process was not observed if the sample was potentiostated at cathodic potentials or if imaging was performed in air. As demonstrated by the optical microphotographs in Figure 11, the exact potential at which the effect occurs varies significantly between samples and can be quite different even at various spots on the same cleavage plane. The sample shown in Figure 11 was irradiated without a cantilever. The cantilever reflects most of the intensity during AFM imaging. Therefore, the darkening processes are greatly enhanced when the sample is irradiated directly compared to normal AFM operation. For this cleavage plane -140 mV seemed to be the lowest potential at which an influence of the light could be detected. The local differences in the properties of the galena specimen are well illustrated by the sequence of spots 7-10 in Figure 11. Different photoelectrochemical behavior of galena cleavage planes from one lump was reported by Richardson et al.4 for macroscopic galena samples. The laser beam
5720 Langmuir, Vol. 12, No. 23, 1996
Wittstock et al.
Figure 10. Optical microphotograph of a galena specimen after investigation by AFM at the OCP. The bright triangle is the shadow of the cantilever. Areas darkened were reached by the light scattered at the edges of the cantilever.
allows us to observe the different photochemical behavior of galena cleavage planes on an even smaller scale. For the investigation here, it was crucial to determine whether the processes induced or stimulated by the laser light are different from those caused by an externally imposed potential alone. On one sample, a closely spaced array of laser-treated spots was generated at the OCP. The region modified was about 1.5 mm × 1 mm on a cleavage plane of 2 mm × 4 mm. The area not irradiated was exposed to the buffer solution in the AFM cell but not to the laser light. The modified area was large enough to see the darkening with the naked eye. Comparative XPS spectra are shown in Figure 12 of the light-treated and nontreated sample area. There are no distinct differences between both sample parts. The unmodified area contains some silicon, probably as SiO2 inclusion. The S 2p and Pb 4f signals are nearly identical. Some copper enrichment on the surface is detected (BE(Cu2p3/2) ) 932 eV). Carbonate and considerable amounts of hydrocarbon contamination were also found on that sample. Carbonate formation occurred likely during the sample transfer between AFM and XPS, e.g. after emersion from the acetate buffer. After removal of the sample from the UHV all parts of the sample had regained their shiny appearance. Since sample cooling is not yet available at that instrument, elemental sulfur could not be retained. Since other oxidation products could not be observed in the S 2p spectra, it is assumed that the laser light accelerates the oxidation processes but does not change the reaction pathway. This conclusion is in accordance with observations on other n-type semiconductors. As pointed out by Gerischer46 in a recent review, n-type semiconductors need a mechanism of hole generating for fast oxidation. Light absorption is one mechanism which can enhance the hole formation rate on n-type semiconductors in contact with electrolytes (see Figure 19 of ref 46 for an instructive sketch of the mechanism). Whether the potentials at which the oxidation processes start are shifted to more cathodic values cannot be (46) Gerischer, H. Electrochim. Acta 1990, 35, 1677.
Figure 11. Optical microphotographs of a galena specimen irradiated with the laser of the AFM instrument (without a cantilever) while the sample was held at different potentials. Spots are numbered in the sequence of irradiation. Potentials (SHE) and irradiation time: (a) (1) +21 mV, 150 s (no spot was obtained); (2) +121 mV, 120 s; (3) +141 mV, 120 s; (4) +161 mV, 120 s; (5) +101 mV, 120 s (only very weak spot); (6) +121 mV, 120 s; (b) (1-4) same spots as described for part a; (7) +101 mV, 120 s (only very weak spot); (8) +121 mV, 120 s; (9) +141 mV, 120 s; (10) +161 mV, 120 s (no spot); (11) +161 mV, 120 s; (12) +181 mV , 120 s (no spot); (13) +181 mV , 120 s (no spot).
concluded yet. Since even the regions on the same cleavage plane behave quite differently toward the interaction with light and the effect of light is much smaller when the AFM is operated with the cantilever in the beam path than in the experiments of Figures 11 and 12, it seems difficult to find a general answer to this problem. For these reasons, samples were observed by AFM and between image recording by optical microscopy to ensure that no light-induced darkening has occurred. If the beginning of such a darkening was detected, the anodic potential excursion was immediately terminated. Therefore, and because fairly large amounts of the suspected reaction product are also found in XPS measurements after potentiostatic sample treatment without illumination, we are convinced that AFM images recorded such as those in Figure 4a-h represent the oxidation reaction under potentiostatic control and are not obscured by interfering light-induced reactions. Conclusions Oxidation of galena in acetate buffer was investigated by AFM and photoelectron spectroscopy. The dominant oxidation product found was elemental sulfur. The first
Oxidation of Galena in Acetate Buffer
Langmuir, Vol. 12, No. 23, 1996 5721
samples down to liquid nitrogen temperature before starting the evacuation in the sample entry chamber to retain the elemental sulfur. No sulfur-oxygen species were present at potentiostatically oxidized galena samples even when investigated with SXPS. AFM images showed that oxidation starts with a roughening of the surface. This process is complemented (and probably accelerated) by highly localized sulfur deposition in protrusions which are 10-200 nm high and have mutual distances of several hundred nanometers. Such localized deposition can only be understood if the reactants are transported via diffusion in the liquid phase from the dissolution pits to the growing deposits. The formation of the protrusions is likely to start at impurity locations. In situ AFM was shown to be a valuable technique for flotation-related research on metal sulfides primarily because nonconducting minerals or overlayers can be investigated. This expands the possibilities of the STM technique. The laser light used in the optical detection system of the AFM was found to accelerate the oxidation processes but not to change the reaction pathway.
Figure 12. S 2p region of a galena sample from which one part was modified with laser light (a) while the other part was just in contact with the electrolyte in the AFM cell at the OCP (b): treatment time in solution, 100 min; sample transfer via air (3 h). The sample could be measured at ambient temperature only.
spectroscopic proof was found for the formation of elemental sulfur during potentiostatic oxidation at slightly anodic potentials. It turned out to be crucial to cool the
Acknowledgment. G.W. thanks K.-H. Hallmeier (University of Leipzig, Germany) and C. Hellwig (BESSY GmbH, Berlin, Germany) for technical support during the SXPS measurements as well as T. Scho¨ter and I. Eckardt (University of Halle, Germany) for the program PHELPS. The work was supported by the European Community under contract No. BRE2-CT94-0606. I.K. received a travel grant of the Academy of Finland. G.W., D.H., and S.K. acknowledge a travel grant of the German Academic Exchange Office. S.K. thanks Deutsche Forschungsgemeinschaft for a grant within the graduate course Physical Chemistry of Surfaces and Interfaces. LA960385S
