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Interaction of vapours of mercury with PbS(0 0 1): a study by X-ray photoelectron spectroscopy, RHEED and X-ray absorption spectroscopy
Applied Surface Science 173 (2001) 44±53
Interaction of vapours of mercury with PbS(0 0 1): a study by X-ray photoelectron spectroscopy, RHEED and X-ray absorption spectroscopy Francine Genina, Marc Alnotb, Jean Jacques Ehrhardta,* a
Laboratoire de Chimie-Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS-UHP Nancy 1, 405, rue de Vandoeuvre, F-54600 Villers leÁs Nancy, France b Laboratoire de Physique des MateÂriaux, UMR 7556, CNRS-UHP Nancy 1, Campus Victor Grignard, F-54506 Vandoeuvre leÁs Nancy, France Received 11 September 2000; accepted 28 October 2000
Abstract The interactions of mercury on cleaved and polished galena PbS(0 0 1) were studied at room temperature. The surface of the samples was characterised by X-ray photoelectron spectroscopy (XPS), RHEED and X-ray absorption spectroscopy (EXAFS) before and after interaction with a pressure of 2:5 10ÿ3 mbar of mercury vapours. Sorbed mercury was oxidised to Hg(II), as revealed by the chemical shift observed on the Hg 4f7/2 levels. Since the adsorption of mercury is completely inhibited when the oxidation products of galena cover the surface, the formation of Hg±S bonding is most likely to occur. The amount of adsorbed mercury at saturation was estimated to be about one monolayer, i.e. one atom of mercury per surface atom of sulphur. A 1 1 structure was observed by RHEED for the longest exposures. The analysis of the EXAFS oscillations at Ê and a co-ordination number of 3. A model is the LIII edge of mercury allowed an estimate of the Hg±S bond length of 2.62 A proposed for the structure of the chemisorbed layer of mercury. # 2001 Elsevier Science B.V. All rights reserved. PACS: 82.65 i; 89:60 x Keywords: Adsorption of mercury on galena; Oxidation of galena; XPS; RHEED; EXAFS
1. Introduction Industrial development, including coal ®red power plants and municipal waste incinerators, can release various mercury species into the atmosphere. Although the background mercury in the atmosphere is only about 3±5 ng/m3, the lifetime of mercury in the air can be as long as 1±2 years [1]. Elemental mercury *
Corresponding author. Tel.: 33-83-91-63-00; fax: 33-83-27-54-44. E-mail address: [email protected] (J.J. Ehrhardt).
gas, however, is the dominant form in the plume of power plants, ranging from 92 to 99% of the total mercury concentration in the air. Another example concerns a relatively high concentration of elemental mercury (up to 15 ng/m3) detected in the atmosphere of a tropical Amazonian forest associated with gold mining activity [2]. Despite these very important sources of pollution, few studies have so far addressed the mechanism of the interaction of mercury vapours with surfaces at atomic scale. The adsorption of mercury has been investigated in detail on Ni(1 1 1) in the temperature range 323±403 K and also at 222 K
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 7 4 - 6
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
[3,4]. Surface structures have been suggested for interpreting the various LEED patterns observed as a function of coverage and temperature [3]. Calculations for free and adsorbed mercury layer demonstrated that phase transitions are dominated by Hg±Hg interactions and only slightly affected by the metallic substrate [4]. The overlap of the empty p-orbitals of mercury favours the square ad-layers despite the poorer packing of the square structure compared with the hexagonal structure [4]. The high af®nity of mercury for sulphur has been recognised earlier and can be understood through the Lewis acid±base theory, since Hg(II) is a ``soft'' Lewis acid and should strongly bind to thiol functional groups which are ``soft'' Lewis bases. As a consequence, the sorption of mercury onto sulphides in aqueous phase has been the subject of a large number of reports ([5], and references cited therein) and has sometimes been proposed as a possible method for removing this highly toxic element from aqueous solutions [6±9]. On galena PbS, the studies of Bancroft's group [10±12], on the adsorption of Hg(II) at various pH and ionic strengths, deserve to be mentioned. Adsorption is strongly pH dependent and it was postulated that the hydrolysed Hg(II) species are adsorbed directly on the sulphide groups, probably as a monolayer [10]. Desorption experiments and X-ray photoelectron spectroscopy (XPS) investigations show that many species can be adsorbed on the surface, including HgS [12]. The question arises whether sulphide minerals play a role in the limitation of gaseous mercury in the atmosphere. No straightforward answer can be suggested, since the surface of these minerals may be partly covered by many oxidation products, each of them exhibiting speci®c activities towards mercury adsorption. The sulphide used in the present study, PbS, is one of the sulphide minerals less reactive to air oxidation at room temperature [13]. Mainly lead carbonates, oxides and hydroxides were found in experiments where XPS was used to monitor the surface oxidation products of this mineral. Sulphoxy species were also identi®ed in the earliest stages of galena oxidation in air ([14], and references cited therein). Whatever the actual nature of the oxidation products, it appears that surface sulphatation occurs after very long periods of exposure. For example, the thickness of the sulphate overlayer is still much
45
thinner than the escape depth of electrons with kinetic energy close to 1000 eV after 220 days [13]. Moreover, it is generally accepted that oxidation is a very complex process in which structural defects could play a major role. For example, a recent study by XPS and scanning tunnelling microscopy (STM) of the kinetics of the oxidation of pyrite FeS2(0 0 1) in air shows that 2 days after cleavage in the air, the most reactive sites (steps and sites close to steps) were obviously oxidised, while fairly large areas on the terraces were free of oxidation products [15]. It is therefore believed that the PbS surface exposed to air could also either display unoxidised areas or areas partly covered with oligosulphide species, elemental sulphur or sulphoxy species, at least for periods of several days, thereby offering many possibilities for interaction with mercury atoms. This paper presents a preliminary study of the interaction of mercury vapours with a simple sulphide PbS which will be presented. Mercury uptake and its surface speciation are investigated by XPS. The structure of the full monolayer and the local environment of the adsorbed mercury are analysed using RHEED and EXAFS techniques, respectively. 2. Materials and methods Plates of galena were prepared by the fracture in air of a single crystal (purity higher than 99.9%) obtained from mines in Kansas (USA). After cleavage, the samples were either directly transferred under vacuum for structural analysis by RHEED, or further polished with SiC paper (P400) before mercury exposure and surface analysis. Mercury adsorption experiments were performed at room temperature in a stainless steel reactor with two glass cold ®ngers (Fig. 1). A droplet of high purity mercury (provided by Rhone Alpes Mercure) was introduced in one of the cold ®ngers and the system was pumped down with a rough pump until the vapour pressure of mercury at room temperature 2:5 10ÿ3 mbar was reached. The tap was then closed and the sample introduced in the second glass cold ®nger, the reactor was pumped down with the rough pump protected by a liquid nitrogen cold trap. The base pressure in the reactor was estimated at below 5 10ÿ4 mbar. Galena was exposed to mercury at
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F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 1. Experimental set-up for mercury adsorption.
room temperature and constant pressure by closing the valve connecting the vacuum system and opening the tap of the mercury reservoir. The temperature of the mercury reservoir was kept constant at 228C to control the vapour pressure of mercury during adsorption. After adsorption the samples were rapidly transferred under nitrogen either to the XPS machine for surface analysis or to the RHEED for structural analysis. Analyses were performed at room temperature, which meant that the loosely bound species such as physisorbed mercury or volatile sulphur compounds could not be detected. Some samples were stored in vacuum for few days before analysis by EXAFS. Apart from few preliminary experiments aimed to the study the adsorption kinetics, the PbS surface was cleaned by polishing after each exposure to mercury. The X-ray photoelectron spectrometer was extensively described in a previous paper [16]. Brie¯y, the XPS spectra were obtained using an unmonochromatised Mg Ka (1253.6 eV) photon source, the pressure in the analytical chamber being in the low 10ÿ9 mbar range. The spectrometer work function was adjusted to give a value of 84:0 0:05 eV for the Au 4f7/2 level of metallic gold [17]. Survey scans were recorded using a ®xed pass energy of 90 eV, while narrow scan spectra of the Hg 4f, S 2s, Pb 4f and O 1s levels were recorded in ®xed analyser transmission mode (FAT mode) using a ®xed pass energy of 22 eV. As the S 2p peaks lie on the high binding energy tail of the very
intense Pb 4f peaks, the S 2s signal was preferred for the study of this sulphide. The binding energy of the C 1s level from contamination at 284:6 0:1 eV was used as an internal reference for calibrating every spectrum when charging effects were observed. Calculated cross-sections [18] and estimated inelastic mean free path [19] were used for quanti®cation. The recorded lines were ®tted using a curve-®tting program with Gaussian±Lorentzian shape for Hg 4f and Pb 4f. RHEED patterns were obtained with a 30 keV electron gun (vacuum generator) running at 27 keV (30 mA emission current) at grazing incidence and for two different azimuths. Diffraction diagrams observed on the screen were digitised using a CCD camera and line intensities were analysed with the Video Studio 2.0 software. EXAFS data were acquired at LURE (Laboratoire pour l'Utilisation du Rayonnement ElectromagneÂtique, Orsay, France) on the D44 experimental hutch at the DCI high energy ring (1.8 GeV, lc 3:5 keV) with Si(1 1 1) double crystal monochromator (instrumental bandwidths were calculated at 8.3 eV for LIII Hg edge). Mercury LIII edge data were collected in ¯uorescence mode using a Si/Li detector. Argon-®lled ionisation chambers were used for signal detection in transmission mode for reference compounds HgO and cinnabar HgS (Aldrich, space groups, respectively, Pmnb and P3121). Energy calibration was performed by assigning the top of the white line in absorption edge of a Pb foil to 13 078 eV.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
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EXAFS data analysis was carried out with standard methods using the set of programs WINXAS [20]. After removal of background absorption using a straight line, Hg atomic absorption was modelled by a ®fth degree polynomial and subtracted from the experimental spectra. EXAFS oscillations were obtained using the Heitler±Eisenberger normalisation. The E0 value was chosen as the ®rst in¯ection point in the Hg absorption edge. k2 weighted Fourier Transform (FT) of these oscillations was achieved between Ê ÿ1 using a Kaiser window with t 2:5. 1.4 and 3.0 A Ab initio phase and amplitude function were calculated using the FeFF7.02 code [21]. The re®ned parameters for reference compounds were the Debye Waller factor (s2) and the energy shift (E0). These parameters were subsequently ®xed for analysis of the spectra of PbS with adsorbed mercury, and the coordination number (CN) and interatomic distances were determined. 3. Results and discussion The Hg 4f, Pb 4f, S 2s and O 1s XPS (Mg Ka) spectra are shown Figs. 2±5 for selected experimental conditions for interactions of mercury vapours of with PbS(0 0 1), respectively. Short (b) and longer (c and d) exposures to mercury with the clean surface were investigated as well as the interaction of mercury with PbS oxidised in air at room temperature for 2 h (e). Some structural investigations by RHEED and EXAFS of the adsorbed layer were also performed to obtain a deeper insight into the actual mechanism of the interaction of mercury with PbS(0 0 1). 3.1. Surface characterisation of clean and oxidised PbS(0 0 1) Whatever the experimental protocol for the surface handling, weak O 1s (see, e.g. Fig. 5a) and C 1s signals were always detected, indicating some oxidation and/ or contamination. The best results were obtained after cleavage and polishing. Argon bombardment and surface scraping were attempted to clean the PbS surface in the spectrometer further. These treatments greatly improved the cleanliness of the surface, thereby indicating that contamination is essentially located at the surface. However, the Pb 4f and S 2s levels are generally enlarged, suggesting a distribution of
Fig. 2. Hg 4f XPS (Mg Ka) spectra for mercury adsorption on PbS(0 0 1): (a) before exposure to mercury; after (b) 15 min, (c) 60 min and (d) 115 h exposure to mercury at a pressure of about 2:5 10ÿ3 mbar; (e) the crystal has been ®rst oxidised in the air for 2 h at room temperature then exposed to mercury (2:5 10ÿ3 mbar, 2 h, room temperature).
chemical environments due to the disorder induced by ion sputtering. On this sulphide, it is not possible to apply annealing treatments to improve the surface structure, as usually performed on metals. Scof®eld [18] calculated photoemission cross-sections which were used to determine the surface composition of the galena crystals used in this study prior to mercury adsorption. A sulphur surface de®ciency was always observed, leading to the general formula PbS0:800:05 . A possible explanation for this composition could be the sublimation of volatile compounds such as the initial oxidation products (oligosulphide species, elemental sulphur or sulphoxy species) when the samples are introduced into the spectrometer [14,22]. The Pb 4f7/2 and the S 2s binding energies were identi®ed at 137.4 and 224.8 eV, respectively, on the
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F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 3. Pb 4f XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
clean surface, in agreement with the values previously reported in the literature [23]. After exposure to air at atmospheric pressure and room temperature for 2 h, the PbS surface is obviously oxidised as revealed by a noticeable shoulder at 138.8 eV in the Pb 4f (Fig. 3e) spectra and the intense signal of the O 1s peak centred at 531 eV, assigned to OHÿ groups (Fig. 5e). Moreover, the presence of a sulphate component in the corresponding S 2s spectrum (Fig. 4e) and of an extra peak at 289.0 eV in the C 1s spectrum (not shown) tend to favour a mixture of Pb(OH)2, PbSO4 and PbCO3. These products are frequently cited in previous studies of galena oxidation [14]. 3.2. Adsorption experiments A preliminary experiment to study the kinetics of mercury adsorption on PbS(0 0 1) (room temperature,
Fig. 4. S 2s XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
Hg pressure: 2:5 10ÿ3 mbar) was performed by monitoring the Hg 4f/Pb 4f ratio as a function of exposure time. In the ®rst minutes of exposure, the mercury signal increased very quickly but the Hg 4f/ Pb 4f ratio levelled-off for exposures only longer than a few score hours. After such long exposure, the surface is unaffected by oxidation and can be preserved for long periods of time without signi®cant change. Based on the analysis of the intensities of Hg and Pb signals, in a very crude model of a layer by layer growth without any mercury diffusion in the subsurface, the maximum amount of adsorbed mercury would correspond roughly to the formation of one monolayer. Here the monolayer was de®ned as the quantity equivalent to one mercury atom per surface sulphur atom. The accuracy of this type of calculation is rather low and the amount of mercury adsorbed on the surface must be considered with caution.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
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Table 1 Some selected binding energies of Hg 4f7/2 in various compounds
Fig. 5. O 1s XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
The binding energy of the Hg 4f7/2 level is identi®ed at 100:25 0:2 eV (Fig. 2b±d) whatever the coverage. This ®gure is signi®cantly higher than the 99.2 eV reported for metallic mercury on the same spectrometer. According to the values of the binding energies reported in Table 1 for Hg 4f7/2 on selected reference samples, it is clear that Hg was oxidised, going from Hg(0) in the gas phase to Hg(II) in the adsorbed phase. Note also that mercury adsorption was completely inhibited on an oxidised PbS(0 0 1) surface (Fig. 2e). This is a strong indication of the different surface reactivities of ``clean PbS'' and ``oxidised PbS''. As a consequence, the adsorption sites for mercury are more likely related to the surface sulphur atoms than to oxygen atoms, and the formation of Hg±S bonding can reasonably be assumed. Note that no signi®cant change was observed either in the binding energy or in the shape of the Pb 4f and S
Compound
Hg 4f7/2 binding energy (eV)
References
Hg(0) Hg(0) Hg(0) at 100 K HgO HgO HgS HgS HgS HgSO4
99.85 99.2 99.2 101.0 100.6 101.0 100.9 100.0 100.5
[17] [12] [5] [29] [5] [29] [12] [5] [5]
Hg sorbed on PbS(0 0 1) Hg(II) sorbed on PbS
100.250.2 100.9
This work [12]
2s levels after mercury adsorption on PbS. In fact, the reduction of mercury would imply an electron withdrawal from sulphur atoms that should induce an extra component at the high binding energy side of the S 2s peak, which was not observed. This could be due to the width of the S 2s peaks, that would make it dif®cult to observe of a shift of about 1.7 eV, as theoretically estimated for gold adsorption on galena [24]. 3.3. Structural analysis of the mercury adsorbed layer PbS crystallises with the NaCl structure in which each Pb atom is surrounded by six S atoms. In such a structure, the easy cleavage plane is (0 0 1). In the absence of surface relaxation, one would
Fig. 6. RHEED patterns of (a) freshly cleaved PbS(0 0 1) surface and (b) PbS(0 0 1) covered by about one monolayer of mercury.
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F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 7. EXAFS analysis of LIII edge of mercury adsorption on PbS(0 0 1): (a) FT of the EXAFS function pondered by k2 and limited to the ®rst shell of co-ordination; (b) associated pseudo-function of radial distribution.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
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Fig. 8. Possible arrangement for mercury adsorbed on PbS(0 0 1).
expect a simple square unit cell described by the twodimensional group p4g [25]. The unit cell parameter is Ê and the dense rows of atoms are along direc5.92 A tions {1 0} and {1 1} situated at 458 from each other. The occupied states at the top of the valence band of PbS have S 3p character, these orbitals extending vertically from the surface [26]. The surface structure was con®rmed with the STM image, showing both the S and Pb atoms in the expected positions [27]. The experimental protocol was modi®ed for application to the RHEED experiment due to the dif®cul-
ties of obtaining a clean and well-ordered surface of PbS. A crystal of galena was cleaved into two pieces in order to obtain two (0 0 1) planes. The ®rst was immediately introduced into the RHEED machine and structurally characterised. It is referred to as the clean surface. The second was exposed to mercury 2:5 10ÿ3 mbar for 48 h, which is known to produce a very stable surface covered with about one monolayer. The sample was then transferred and its surface structure characterised. The two main azimuths {1 0} and {1 1} were investigated and the
52
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
diffraction patterns are shown in Fig. 6 before and after interaction with mercury vapours. RHEED patterns obtained on the ``clean'' surface reveal rather well-de®ned diffraction lines, with some modulation probably associated with the roughness of the cleaved surface. The separation between the lines is in agreement with the structure of the (0 0 1) planes, thus excluding a possible surface reconstruction, in agreement with previous observations by LEED [28]. After exposure to mercury the line intensities are much weaker. Despite the poor quality of the pattern, the separation between the lines remains unchanged, and even after careful examination no extra line was detected. This demonstrates the formation of a 1 1 superstructure, i.e. a square structure for the overlayer of mercury with the same lattice parameter as the substrate. As mentioned previously by Singh et al. [4], on metal surfaces, the square structure of adsorbed mercury should be more stable due to the maximum porbital overlap. By contrast, on semi-conductor surfaces, the highly directional bonding may lead to signi®cant overlap of the mercury d-orbitals (lateral interactions) as well as s±p hybrid orbitals, leading to more complex situations. On PbS(0 0 1), however, the S 3p orbitals point vertically from the surface [26] and could strongly favour the square structure for the mercury adsorbed layer. The EXAFS analysis of the LIII edge of mercury (12 292 eV) is shown in Fig. 7. Since the amount of mercury is low, and considering the nearness of the LIII edge of Pb (13 078 eV), this analysis was limited to the ®rst co-ordination shell. The best ®t was obtained for the mercury atoms surrounded by three Ê. sulphur atoms CN 3 at a distance of 2.62 A Attempts to model the EXAFS spectra with Hg±Pb or Hg±O bonding failed, supporting the assumption of the formation of Hg±S bonding. Despite the high uncertainty on the CN, adsorption on the top of the sulphur atoms CN 1 can be de®nitively ruled out. An adsorption of mercury atoms in twofold co-ordinated bonding sites, in between two sulphur atoms CN 2, most likely occurs. However, it must be emphasised that the measured Hg±S bond length is signi®cantly lower than the sum of the atomic radii of Ê ) and S2ÿ (1.84 A Ê ). This array is scheHg2 (1.10 A matically presented in Fig. 8. It would also be consistent with the monolayer coverage de®ned in the previous section.
4. Conclusions This preliminary study of the interaction of mercury with PbS(0 0 1) reveals the possibility of formation of a monolayer of mercury when the surface is almost free of oxidation products such as Pb(OH)2, PbSO4 and PbCO3, which are known to appear belatedly in the oxidation process with air. Adsorption is assumed to proceed through the formation of Hg±S bonding, Ê and the cothe bond length being equal to 2.62 A ordination number estimated to be 3. A structural model with mercury atoms adsorbed in twofold coordination bonding sites is proposed. This is consistent with the 1 1 structure and also with the amount of mercury adsorbed at saturation. Further detailed studies are needed to establish the actual mechanism of the interaction of Hg atoms with PbS. However, it has been clearly demonstrated that the presence of oxidation products on the surface of PbS(0 0 1) completely inhibits the adsorption of elemental mercury, ruling out the possibility that galena reduces the concentration of mercury in the atmosphere by irreversible interaction. Acknowledgements The authors thank J. Lambert (LCPE, Nancy), R. Revel (LURE, Orsay) and C. Ruby (LCPE, Nancy) for their assistance for acquiring XPS and EXAFS spectra and RHEED patterns, respectively.
References [1] R.P. Mason, W.F. Fitzgerald, F.M.M. Morel, Geochem. Cosmochim. Acta 58 (1994) 3191±3198. [2] D. Amouroux, J.C. Wasserman, E. Tessier, O.F.X. Donard, Environ. Sci. Technol. 33 (1999) 3044±3048. [3] N.K. Singh, R.G. Jones, Surf. Sci. 232 (1990) 229±242 and 243±258. [4] N.K. Singh, P.A.D.M.A. Dale, D. Bullet, R.G. Jones, Surf. Sci. 294 (1993) 333±348. [5] P. Bonnissel-Gissinger, Ph.D. Thesis, University Henri Poincare Nancy 1, France, 1997. [6] H.O. Philpps, K.A. Kraus, J. Chromatogr. 17 (1965) 549. [7] J.R. Brown, G.M. Bancroft, R.E.N. McLean, Environ. Sci. Technol. 13 (1979) 1142. [8] K.S.E. Forssberg, H. JoÈnsson, Scand. J. Metall. 10 (1981) 225.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53 [9] G.E. Kislinskaya, I.A. Sheka, N.N. Kosachek, G.M. Krasnova, N.I. Schenk, Sov. Prog. Chem. 47 (1981) 92. [10] G.E. Jean, G.M. Bancroft, Geochem. Comoschim. Acta 50 (1986) 1455±1463. [11] M.M. Hyland, G.M. Bancroft, Geochem. Cosmochim. Acta 53 (1989) 367±372. [12] M.M. Hyland, G.E. Jean, G.M. Bancroft, Geochem. Cosmochim. Acta 54 (1990) 1957±1967. [13] D. Brion, Appl. Surf. Sci. 5 (1980) 133±152. [14] P. Nowak, K. Laajalehto, Appl. Surf. Sci. 157 (2000) 101±111. [15] C.M. Eggleston, J.J. Ehrhardt, W. Stumm, Am. Mineral. 81 (1996) 1036±1056. [16] J.A. Mielczarki, J.M. Cases, M. Alnot, J.J. Ehrhardt, Langmuir 12 (1996) 2519±2530. [17] C.J. Powell, Appl. Surf. Sci. 89 (1995) 141±149. [18] J.H. Scof®eld, J. Electron. Spectrosc. 8 (1976) 129. [19] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2±11. [20] T. Ressler, J. Phys. IV 7 (C2) (1997) 269. [21] A. Ankudinov, Relativistic spin dependent X-ray absorption theory, Ph.D. Thesis, University of Washington, Washington, USA, 1996.
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[22] K. Laajalehto, I. Kartio, M. Heinonen, T. Laiho, Jpn. J. Appl. Phys. 38 (1999) 265±268. [23] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST X-ray Photoelectron Database, Ver. 3.0 (Web version), 2000. [24] U. Becker, M.F. Hochella, D.J. Vaughan, Geochem. Cosmochem. Acta 61 (1997) 3565±3585. [25] M.F. Hochella Jr., Review in Mineralogy, Vol. 23, Mineral± Water Interface Geochemistry, The Mineralogical Society of America, 1990, pp. 87±132. [26] J.A. Tossel, D.J. Vaughan, Theoretical Geochemistry, Applications of Quantum Mechanism in Earth and Mineral Sciences, Oxford University Press, New York, 1992. [27] C.M. Eggleston, M.F. Hochella Jr., Geochem. Cosmochim. Acta 54 (1990) 1511±1517. [28] M.F. Hochella Jr., C.M. Eggleston, V.B. Elings, G.A. Park, G.E. Brown, C.M. Wu, K. Kjoller, Am. Mineral. 74 (1989) 1235±1248. [29] V.I. Nefedov, Y.V. Salyn, P.M. Solozhenkin, G.Y. Pultatov, Surf. Interf. Anal. 2 (1980) 170±172.
Interaction of vapours of mercury with PbS(0 0 1): a study by X-ray photoelectron spectroscopy, RHEED and X-ray absorption spectroscopy Francine Genina, Marc Alnotb, Jean Jacques Ehrhardta,* a
Laboratoire de Chimie-Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS-UHP Nancy 1, 405, rue de Vandoeuvre, F-54600 Villers leÁs Nancy, France b Laboratoire de Physique des MateÂriaux, UMR 7556, CNRS-UHP Nancy 1, Campus Victor Grignard, F-54506 Vandoeuvre leÁs Nancy, France Received 11 September 2000; accepted 28 October 2000
Abstract The interactions of mercury on cleaved and polished galena PbS(0 0 1) were studied at room temperature. The surface of the samples was characterised by X-ray photoelectron spectroscopy (XPS), RHEED and X-ray absorption spectroscopy (EXAFS) before and after interaction with a pressure of 2:5 10ÿ3 mbar of mercury vapours. Sorbed mercury was oxidised to Hg(II), as revealed by the chemical shift observed on the Hg 4f7/2 levels. Since the adsorption of mercury is completely inhibited when the oxidation products of galena cover the surface, the formation of Hg±S bonding is most likely to occur. The amount of adsorbed mercury at saturation was estimated to be about one monolayer, i.e. one atom of mercury per surface atom of sulphur. A 1 1 structure was observed by RHEED for the longest exposures. The analysis of the EXAFS oscillations at Ê and a co-ordination number of 3. A model is the LIII edge of mercury allowed an estimate of the Hg±S bond length of 2.62 A proposed for the structure of the chemisorbed layer of mercury. # 2001 Elsevier Science B.V. All rights reserved. PACS: 82.65 i; 89:60 x Keywords: Adsorption of mercury on galena; Oxidation of galena; XPS; RHEED; EXAFS
1. Introduction Industrial development, including coal ®red power plants and municipal waste incinerators, can release various mercury species into the atmosphere. Although the background mercury in the atmosphere is only about 3±5 ng/m3, the lifetime of mercury in the air can be as long as 1±2 years [1]. Elemental mercury *
Corresponding author. Tel.: 33-83-91-63-00; fax: 33-83-27-54-44. E-mail address: [email protected] (J.J. Ehrhardt).
gas, however, is the dominant form in the plume of power plants, ranging from 92 to 99% of the total mercury concentration in the air. Another example concerns a relatively high concentration of elemental mercury (up to 15 ng/m3) detected in the atmosphere of a tropical Amazonian forest associated with gold mining activity [2]. Despite these very important sources of pollution, few studies have so far addressed the mechanism of the interaction of mercury vapours with surfaces at atomic scale. The adsorption of mercury has been investigated in detail on Ni(1 1 1) in the temperature range 323±403 K and also at 222 K
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 7 4 - 6
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
[3,4]. Surface structures have been suggested for interpreting the various LEED patterns observed as a function of coverage and temperature [3]. Calculations for free and adsorbed mercury layer demonstrated that phase transitions are dominated by Hg±Hg interactions and only slightly affected by the metallic substrate [4]. The overlap of the empty p-orbitals of mercury favours the square ad-layers despite the poorer packing of the square structure compared with the hexagonal structure [4]. The high af®nity of mercury for sulphur has been recognised earlier and can be understood through the Lewis acid±base theory, since Hg(II) is a ``soft'' Lewis acid and should strongly bind to thiol functional groups which are ``soft'' Lewis bases. As a consequence, the sorption of mercury onto sulphides in aqueous phase has been the subject of a large number of reports ([5], and references cited therein) and has sometimes been proposed as a possible method for removing this highly toxic element from aqueous solutions [6±9]. On galena PbS, the studies of Bancroft's group [10±12], on the adsorption of Hg(II) at various pH and ionic strengths, deserve to be mentioned. Adsorption is strongly pH dependent and it was postulated that the hydrolysed Hg(II) species are adsorbed directly on the sulphide groups, probably as a monolayer [10]. Desorption experiments and X-ray photoelectron spectroscopy (XPS) investigations show that many species can be adsorbed on the surface, including HgS [12]. The question arises whether sulphide minerals play a role in the limitation of gaseous mercury in the atmosphere. No straightforward answer can be suggested, since the surface of these minerals may be partly covered by many oxidation products, each of them exhibiting speci®c activities towards mercury adsorption. The sulphide used in the present study, PbS, is one of the sulphide minerals less reactive to air oxidation at room temperature [13]. Mainly lead carbonates, oxides and hydroxides were found in experiments where XPS was used to monitor the surface oxidation products of this mineral. Sulphoxy species were also identi®ed in the earliest stages of galena oxidation in air ([14], and references cited therein). Whatever the actual nature of the oxidation products, it appears that surface sulphatation occurs after very long periods of exposure. For example, the thickness of the sulphate overlayer is still much
45
thinner than the escape depth of electrons with kinetic energy close to 1000 eV after 220 days [13]. Moreover, it is generally accepted that oxidation is a very complex process in which structural defects could play a major role. For example, a recent study by XPS and scanning tunnelling microscopy (STM) of the kinetics of the oxidation of pyrite FeS2(0 0 1) in air shows that 2 days after cleavage in the air, the most reactive sites (steps and sites close to steps) were obviously oxidised, while fairly large areas on the terraces were free of oxidation products [15]. It is therefore believed that the PbS surface exposed to air could also either display unoxidised areas or areas partly covered with oligosulphide species, elemental sulphur or sulphoxy species, at least for periods of several days, thereby offering many possibilities for interaction with mercury atoms. This paper presents a preliminary study of the interaction of mercury vapours with a simple sulphide PbS which will be presented. Mercury uptake and its surface speciation are investigated by XPS. The structure of the full monolayer and the local environment of the adsorbed mercury are analysed using RHEED and EXAFS techniques, respectively. 2. Materials and methods Plates of galena were prepared by the fracture in air of a single crystal (purity higher than 99.9%) obtained from mines in Kansas (USA). After cleavage, the samples were either directly transferred under vacuum for structural analysis by RHEED, or further polished with SiC paper (P400) before mercury exposure and surface analysis. Mercury adsorption experiments were performed at room temperature in a stainless steel reactor with two glass cold ®ngers (Fig. 1). A droplet of high purity mercury (provided by Rhone Alpes Mercure) was introduced in one of the cold ®ngers and the system was pumped down with a rough pump until the vapour pressure of mercury at room temperature 2:5 10ÿ3 mbar was reached. The tap was then closed and the sample introduced in the second glass cold ®nger, the reactor was pumped down with the rough pump protected by a liquid nitrogen cold trap. The base pressure in the reactor was estimated at below 5 10ÿ4 mbar. Galena was exposed to mercury at
46
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 1. Experimental set-up for mercury adsorption.
room temperature and constant pressure by closing the valve connecting the vacuum system and opening the tap of the mercury reservoir. The temperature of the mercury reservoir was kept constant at 228C to control the vapour pressure of mercury during adsorption. After adsorption the samples were rapidly transferred under nitrogen either to the XPS machine for surface analysis or to the RHEED for structural analysis. Analyses were performed at room temperature, which meant that the loosely bound species such as physisorbed mercury or volatile sulphur compounds could not be detected. Some samples were stored in vacuum for few days before analysis by EXAFS. Apart from few preliminary experiments aimed to the study the adsorption kinetics, the PbS surface was cleaned by polishing after each exposure to mercury. The X-ray photoelectron spectrometer was extensively described in a previous paper [16]. Brie¯y, the XPS spectra were obtained using an unmonochromatised Mg Ka (1253.6 eV) photon source, the pressure in the analytical chamber being in the low 10ÿ9 mbar range. The spectrometer work function was adjusted to give a value of 84:0 0:05 eV for the Au 4f7/2 level of metallic gold [17]. Survey scans were recorded using a ®xed pass energy of 90 eV, while narrow scan spectra of the Hg 4f, S 2s, Pb 4f and O 1s levels were recorded in ®xed analyser transmission mode (FAT mode) using a ®xed pass energy of 22 eV. As the S 2p peaks lie on the high binding energy tail of the very
intense Pb 4f peaks, the S 2s signal was preferred for the study of this sulphide. The binding energy of the C 1s level from contamination at 284:6 0:1 eV was used as an internal reference for calibrating every spectrum when charging effects were observed. Calculated cross-sections [18] and estimated inelastic mean free path [19] were used for quanti®cation. The recorded lines were ®tted using a curve-®tting program with Gaussian±Lorentzian shape for Hg 4f and Pb 4f. RHEED patterns were obtained with a 30 keV electron gun (vacuum generator) running at 27 keV (30 mA emission current) at grazing incidence and for two different azimuths. Diffraction diagrams observed on the screen were digitised using a CCD camera and line intensities were analysed with the Video Studio 2.0 software. EXAFS data were acquired at LURE (Laboratoire pour l'Utilisation du Rayonnement ElectromagneÂtique, Orsay, France) on the D44 experimental hutch at the DCI high energy ring (1.8 GeV, lc 3:5 keV) with Si(1 1 1) double crystal monochromator (instrumental bandwidths were calculated at 8.3 eV for LIII Hg edge). Mercury LIII edge data were collected in ¯uorescence mode using a Si/Li detector. Argon-®lled ionisation chambers were used for signal detection in transmission mode for reference compounds HgO and cinnabar HgS (Aldrich, space groups, respectively, Pmnb and P3121). Energy calibration was performed by assigning the top of the white line in absorption edge of a Pb foil to 13 078 eV.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
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EXAFS data analysis was carried out with standard methods using the set of programs WINXAS [20]. After removal of background absorption using a straight line, Hg atomic absorption was modelled by a ®fth degree polynomial and subtracted from the experimental spectra. EXAFS oscillations were obtained using the Heitler±Eisenberger normalisation. The E0 value was chosen as the ®rst in¯ection point in the Hg absorption edge. k2 weighted Fourier Transform (FT) of these oscillations was achieved between Ê ÿ1 using a Kaiser window with t 2:5. 1.4 and 3.0 A Ab initio phase and amplitude function were calculated using the FeFF7.02 code [21]. The re®ned parameters for reference compounds were the Debye Waller factor (s2) and the energy shift (E0). These parameters were subsequently ®xed for analysis of the spectra of PbS with adsorbed mercury, and the coordination number (CN) and interatomic distances were determined. 3. Results and discussion The Hg 4f, Pb 4f, S 2s and O 1s XPS (Mg Ka) spectra are shown Figs. 2±5 for selected experimental conditions for interactions of mercury vapours of with PbS(0 0 1), respectively. Short (b) and longer (c and d) exposures to mercury with the clean surface were investigated as well as the interaction of mercury with PbS oxidised in air at room temperature for 2 h (e). Some structural investigations by RHEED and EXAFS of the adsorbed layer were also performed to obtain a deeper insight into the actual mechanism of the interaction of mercury with PbS(0 0 1). 3.1. Surface characterisation of clean and oxidised PbS(0 0 1) Whatever the experimental protocol for the surface handling, weak O 1s (see, e.g. Fig. 5a) and C 1s signals were always detected, indicating some oxidation and/ or contamination. The best results were obtained after cleavage and polishing. Argon bombardment and surface scraping were attempted to clean the PbS surface in the spectrometer further. These treatments greatly improved the cleanliness of the surface, thereby indicating that contamination is essentially located at the surface. However, the Pb 4f and S 2s levels are generally enlarged, suggesting a distribution of
Fig. 2. Hg 4f XPS (Mg Ka) spectra for mercury adsorption on PbS(0 0 1): (a) before exposure to mercury; after (b) 15 min, (c) 60 min and (d) 115 h exposure to mercury at a pressure of about 2:5 10ÿ3 mbar; (e) the crystal has been ®rst oxidised in the air for 2 h at room temperature then exposed to mercury (2:5 10ÿ3 mbar, 2 h, room temperature).
chemical environments due to the disorder induced by ion sputtering. On this sulphide, it is not possible to apply annealing treatments to improve the surface structure, as usually performed on metals. Scof®eld [18] calculated photoemission cross-sections which were used to determine the surface composition of the galena crystals used in this study prior to mercury adsorption. A sulphur surface de®ciency was always observed, leading to the general formula PbS0:800:05 . A possible explanation for this composition could be the sublimation of volatile compounds such as the initial oxidation products (oligosulphide species, elemental sulphur or sulphoxy species) when the samples are introduced into the spectrometer [14,22]. The Pb 4f7/2 and the S 2s binding energies were identi®ed at 137.4 and 224.8 eV, respectively, on the
48
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 3. Pb 4f XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
clean surface, in agreement with the values previously reported in the literature [23]. After exposure to air at atmospheric pressure and room temperature for 2 h, the PbS surface is obviously oxidised as revealed by a noticeable shoulder at 138.8 eV in the Pb 4f (Fig. 3e) spectra and the intense signal of the O 1s peak centred at 531 eV, assigned to OHÿ groups (Fig. 5e). Moreover, the presence of a sulphate component in the corresponding S 2s spectrum (Fig. 4e) and of an extra peak at 289.0 eV in the C 1s spectrum (not shown) tend to favour a mixture of Pb(OH)2, PbSO4 and PbCO3. These products are frequently cited in previous studies of galena oxidation [14]. 3.2. Adsorption experiments A preliminary experiment to study the kinetics of mercury adsorption on PbS(0 0 1) (room temperature,
Fig. 4. S 2s XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
Hg pressure: 2:5 10ÿ3 mbar) was performed by monitoring the Hg 4f/Pb 4f ratio as a function of exposure time. In the ®rst minutes of exposure, the mercury signal increased very quickly but the Hg 4f/ Pb 4f ratio levelled-off for exposures only longer than a few score hours. After such long exposure, the surface is unaffected by oxidation and can be preserved for long periods of time without signi®cant change. Based on the analysis of the intensities of Hg and Pb signals, in a very crude model of a layer by layer growth without any mercury diffusion in the subsurface, the maximum amount of adsorbed mercury would correspond roughly to the formation of one monolayer. Here the monolayer was de®ned as the quantity equivalent to one mercury atom per surface sulphur atom. The accuracy of this type of calculation is rather low and the amount of mercury adsorbed on the surface must be considered with caution.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
49
Table 1 Some selected binding energies of Hg 4f7/2 in various compounds
Fig. 5. O 1s XPS (Mg Ka) spectra: (a) after cleavage and polishing of the PbS(0 0 1) crystal; (b)±(e) cf. caption of Fig. 2 for the experimental conditions for mercury adsorption.
The binding energy of the Hg 4f7/2 level is identi®ed at 100:25 0:2 eV (Fig. 2b±d) whatever the coverage. This ®gure is signi®cantly higher than the 99.2 eV reported for metallic mercury on the same spectrometer. According to the values of the binding energies reported in Table 1 for Hg 4f7/2 on selected reference samples, it is clear that Hg was oxidised, going from Hg(0) in the gas phase to Hg(II) in the adsorbed phase. Note also that mercury adsorption was completely inhibited on an oxidised PbS(0 0 1) surface (Fig. 2e). This is a strong indication of the different surface reactivities of ``clean PbS'' and ``oxidised PbS''. As a consequence, the adsorption sites for mercury are more likely related to the surface sulphur atoms than to oxygen atoms, and the formation of Hg±S bonding can reasonably be assumed. Note that no signi®cant change was observed either in the binding energy or in the shape of the Pb 4f and S
Compound
Hg 4f7/2 binding energy (eV)
References
Hg(0) Hg(0) Hg(0) at 100 K HgO HgO HgS HgS HgS HgSO4
99.85 99.2 99.2 101.0 100.6 101.0 100.9 100.0 100.5
[17] [12] [5] [29] [5] [29] [12] [5] [5]
Hg sorbed on PbS(0 0 1) Hg(II) sorbed on PbS
100.250.2 100.9
This work [12]
2s levels after mercury adsorption on PbS. In fact, the reduction of mercury would imply an electron withdrawal from sulphur atoms that should induce an extra component at the high binding energy side of the S 2s peak, which was not observed. This could be due to the width of the S 2s peaks, that would make it dif®cult to observe of a shift of about 1.7 eV, as theoretically estimated for gold adsorption on galena [24]. 3.3. Structural analysis of the mercury adsorbed layer PbS crystallises with the NaCl structure in which each Pb atom is surrounded by six S atoms. In such a structure, the easy cleavage plane is (0 0 1). In the absence of surface relaxation, one would
Fig. 6. RHEED patterns of (a) freshly cleaved PbS(0 0 1) surface and (b) PbS(0 0 1) covered by about one monolayer of mercury.
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F. Genin et al. / Applied Surface Science 173 (2001) 44±53
Fig. 7. EXAFS analysis of LIII edge of mercury adsorption on PbS(0 0 1): (a) FT of the EXAFS function pondered by k2 and limited to the ®rst shell of co-ordination; (b) associated pseudo-function of radial distribution.
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
51
Fig. 8. Possible arrangement for mercury adsorbed on PbS(0 0 1).
expect a simple square unit cell described by the twodimensional group p4g [25]. The unit cell parameter is Ê and the dense rows of atoms are along direc5.92 A tions {1 0} and {1 1} situated at 458 from each other. The occupied states at the top of the valence band of PbS have S 3p character, these orbitals extending vertically from the surface [26]. The surface structure was con®rmed with the STM image, showing both the S and Pb atoms in the expected positions [27]. The experimental protocol was modi®ed for application to the RHEED experiment due to the dif®cul-
ties of obtaining a clean and well-ordered surface of PbS. A crystal of galena was cleaved into two pieces in order to obtain two (0 0 1) planes. The ®rst was immediately introduced into the RHEED machine and structurally characterised. It is referred to as the clean surface. The second was exposed to mercury 2:5 10ÿ3 mbar for 48 h, which is known to produce a very stable surface covered with about one monolayer. The sample was then transferred and its surface structure characterised. The two main azimuths {1 0} and {1 1} were investigated and the
52
F. Genin et al. / Applied Surface Science 173 (2001) 44±53
diffraction patterns are shown in Fig. 6 before and after interaction with mercury vapours. RHEED patterns obtained on the ``clean'' surface reveal rather well-de®ned diffraction lines, with some modulation probably associated with the roughness of the cleaved surface. The separation between the lines is in agreement with the structure of the (0 0 1) planes, thus excluding a possible surface reconstruction, in agreement with previous observations by LEED [28]. After exposure to mercury the line intensities are much weaker. Despite the poor quality of the pattern, the separation between the lines remains unchanged, and even after careful examination no extra line was detected. This demonstrates the formation of a 1 1 superstructure, i.e. a square structure for the overlayer of mercury with the same lattice parameter as the substrate. As mentioned previously by Singh et al. [4], on metal surfaces, the square structure of adsorbed mercury should be more stable due to the maximum porbital overlap. By contrast, on semi-conductor surfaces, the highly directional bonding may lead to signi®cant overlap of the mercury d-orbitals (lateral interactions) as well as s±p hybrid orbitals, leading to more complex situations. On PbS(0 0 1), however, the S 3p orbitals point vertically from the surface [26] and could strongly favour the square structure for the mercury adsorbed layer. The EXAFS analysis of the LIII edge of mercury (12 292 eV) is shown in Fig. 7. Since the amount of mercury is low, and considering the nearness of the LIII edge of Pb (13 078 eV), this analysis was limited to the ®rst co-ordination shell. The best ®t was obtained for the mercury atoms surrounded by three Ê. sulphur atoms CN 3 at a distance of 2.62 A Attempts to model the EXAFS spectra with Hg±Pb or Hg±O bonding failed, supporting the assumption of the formation of Hg±S bonding. Despite the high uncertainty on the CN, adsorption on the top of the sulphur atoms CN 1 can be de®nitively ruled out. An adsorption of mercury atoms in twofold co-ordinated bonding sites, in between two sulphur atoms CN 2, most likely occurs. However, it must be emphasised that the measured Hg±S bond length is signi®cantly lower than the sum of the atomic radii of Ê ) and S2ÿ (1.84 A Ê ). This array is scheHg2 (1.10 A matically presented in Fig. 8. It would also be consistent with the monolayer coverage de®ned in the previous section.
4. Conclusions This preliminary study of the interaction of mercury with PbS(0 0 1) reveals the possibility of formation of a monolayer of mercury when the surface is almost free of oxidation products such as Pb(OH)2, PbSO4 and PbCO3, which are known to appear belatedly in the oxidation process with air. Adsorption is assumed to proceed through the formation of Hg±S bonding, Ê and the cothe bond length being equal to 2.62 A ordination number estimated to be 3. A structural model with mercury atoms adsorbed in twofold coordination bonding sites is proposed. This is consistent with the 1 1 structure and also with the amount of mercury adsorbed at saturation. Further detailed studies are needed to establish the actual mechanism of the interaction of Hg atoms with PbS. However, it has been clearly demonstrated that the presence of oxidation products on the surface of PbS(0 0 1) completely inhibits the adsorption of elemental mercury, ruling out the possibility that galena reduces the concentration of mercury in the atmosphere by irreversible interaction. Acknowledgements The authors thank J. Lambert (LCPE, Nancy), R. Revel (LURE, Orsay) and C. Ruby (LCPE, Nancy) for their assistance for acquiring XPS and EXAFS spectra and RHEED patterns, respectively.
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