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STM and XPS investigation of reaction of galena in air
Applied Surface Science 64 (1993) 29-39 North-Holland
aol: o
surface science
STM and XPS investigation of reaction of galena in air Kari L a a j a l e h t o a, R o g e r St.C. S m a r t b, J o h n R a l s t o n b,. a n d E e r o S u o n i n e n
a
a Laboratory of Materials Science, Department of Apphed Physics, Unwerslty of Turku, Itamen, Pttkakatu 1, SF-20520 Turku, Finland b School of Chemical Technology, Unwerstty of South Austraha, The Levels, Adelaide, SA 5095, Austraha
Received 10 June 1992; accepted for publication 7 July 1992
Scanning tunneling microscopy(STM) was used to study the oxidation of stepped (100) PbS surfaces in air over periods up to 270 min. The systematic growth of reaction products as surface features with lateral dimensions < 0.6 nm initially to overlapped regions (> 9 nm diameter) was observed. No clear preference for growth at step edges was found but defect sites in the (100) surface appear to provide initiation points for reaction. X-ray photoelectron spectra (XPS) from PbS after successive reaction times suggest that limited adsorption of peroxide (O-) and hydroxide (OH-) species occurs initially but that the predominant products after 60 rain are carbonate species from CO2 reaction with O- or OH- sites. After 120 rain, hydroxycarbonatespecies are formed but the S 2p spectra show only sulfide at all times, i.e. no sulfur-oxygen species are formed. The combined results provide both structural and compositional evidence for the mechanism of reaction of galena surfaces to PbCO3 "xPb(OH)2 surface products in air
1. Introduction Galena is one of the most common sulfide minerals concentrated by the flotation method. To understand the flotation behaviour, it is essential to know the surface composition of the mineral before the addition of surface-active reagents to the flotation pulp. Previous studies of the oxidation of galena under different conditions using X-ray photoelectron spectroscopy (XPS) have been reported [1,2]. Although the XPS method is very suitable for quantitative surface analysis, its geometrical resolution is restricted. Usually the measured composition is an average from a surface area larger than 0.02 mm 2. On the other hand, scanning tunnelling microscopy (STM) is a surface imaging method capable of atomic resolution in both vertical and horizontal directions. The main disadvantage of STM is that it provides no chemical information. STM images of heavily oxidized (more than one day in air) galena samples have been published
* Author to whom correspondence should be addressed.
recently [3]. Keeping in mind the mineral flotation correlation, it was considered especially interesting to study the very early stages of reaction of galena in air by combining STM and XPS methods. In this way, the composition, topography and distribution of reaction products on the surface were characterised. The galena mineral is a very suitable material for STM studies, because its electrical conductivity is sufficiently high and it is relatively easy to find large single crystals that cleave easily along the (100) crystal plane. Recent work has also shown STM images of (100) galena surfaces close to atomic resolution with information on defects and impurity sites [4,5].
2. Experimental Natural galena crystals, supplied by Wards Natural Science Establishment (New York), originating from Brushy Creek, Missouri, were used in all measurements. According to electron probe microanalysis the mineral contained on average 720 ppm Fe, 110 ppm Zn and 25 ppm Cu as minor impurities.
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
30
K. Laajalehto et al. / S T M and X P S mvesttgatton of reactton o f galena m atr
The STM instrument was an inverted scanner type designed and constructed by Sexton and Cotterill in CSIRO Division of Materials Science and Technology, Melbourne, Australia [6]. The STM tip was cut from P t - R h alloy without electrochemical etching. The PbS sample was attached to the AI holder with conductive silver paint and allowed to dry before cleavage. Cleavage was carried out in air and the sample was immediately transferred to the nitrogen or argon atmosphere in the STM investigation chamber or to the XPS introduction chamber. In both cases the contact time with air was < 1 rain, and usually just a few seconds, before immersion in the inert atmosphere. Thermal equilibration in the chamber, i.e. continuing sample motion, occupied ~ 10 min before images could be recorded. All images were recorded in constant-current mode with a tip bias of + 0.35 V (usually - 0 . 3 5 V) and tunneling current 0.20-0.25 nA. Recording time for an image was 5-10 min. Images are presented after background subtraction but without any smoothing procedures. The location and relocation of suitable areas for analysis presented considerable difficulties. Several cleavages and trials on many areas were often necessary to obtain reliable, reproducible images of areas without large vertical surface features. Drift in images between successive reaction times was also experienced requiring several similar series of experiments for validation. After the first STM image was taken, using the nitrogen atmosphere to maintain the fresh surface, ambient air at ~ 20°C and ~ 60% humidity, was introduced to the chamber and a new image was taken after every 30 min for periods up to 270 min. Samples for XPS experiments were prepared by identical procedures and measured after 0, 60, 120 and 240 min reaction in air. Spectra were recorded using a Perkin-Elmer PHI 5400 electron spectrometer equipped with a monochromatized AI K a X-ray source. A sample region of diameter 0.15 mm from the cleaved galena surface was analysed. The energy scale was calibrated with the AU4fT/2 binding energy (BE) 84.0 eV and Cu2p3/2 BE 932.6 eV lines. The measured line width of the Au 4f7/2 was 0.85 eV for an analyser
pass energy of 35.75 eV. Elemental atomic concentrations were calculated using curve-fitted peak areas and standard sensitivity factors for Pb (4f7/9, 8.329, 4f, 6.968), C ls (0.296), O ls (0.711) and S 2p (0.54).
3. Results and discussion 3.1. STM images Grey scale and 3D plots of freshly cleaved and subsequently air-reacted galena sample are presented in fig. 1. Realignment of the STM tip on the same area presented some difficulties due to very small drift in either the crystal position or tunnelling contact point. Nevertheless, pictures are approximately from the same area. The images show a 2 unit cell step (i.e. ~ 1.2 nm height) traversing the 70 × 70 nm area with relatively fiat ledges above and below this step. The drift can be identified with the help of two vertical defects in the step seen especially well in the grey scale plots in fig. 1. There are a few atomic scale (i.e. < 0.3 nm) defects or reaction sites imaged as vertical protrusions in the first image after cleavage. At higher magnification, they appear as sharp, local changes in the tip distance d (with constant current) on particular sites within a unit cell parameter. They may arise from impurity atoms or sites of initial, rapid oxidation, e.g. vacancies, displaced atoms, etc. Remarkable changes can be seen in the images as a function of time. Systematic growth with time of oxidation products as roughly conical structures are seen on the galena surface. Realignment of successive images, using the step defects shows that each oxidation region grows vertically and laterally with time until, after 170 rain, some overlap of adjacent regions begins to occur. It is also noted in the early images (i.e. 30, 60 min) that, whilst oxidation products do grow on the positions of the initial defects in the first image, they also grow at other apparently undistinguished sites on the ledges. Of particular importance is the observation that they do not grow at the edges of the steps despite the lower coordination of atoms in these sites. After 270 min, the dimensions of the
I(. Laajalehto et al. / STM and XPS mvesttgatton of reaction of galena m atr
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largest reaction structures were ~ 9 nm (diameter) and ~ 4 nm (height), which correspond to about 16 and 7 P b - P b distances, respectively, in the original PbS lattice. The observed patterns, however are likely to be a superposition of topographical changes with changes in tunneling current induced by the lower conductivity of the oxidation products. The approximate relationship between tunnelling current density ( j ) and tunnel distance (d) e2 k 0 J = h 47r2d V e x p ( - 2 k 0 d ), where V = bias (V) and k0(nm) = 0.1025vr~, shows that a change in the work function • of the surface in the region of the oxidation products will induce significant change in d at constant current j. Hence, it is possible that the images overestimate the vertical dimensions of the reacted regions. The horizontal dimensions, ultimately showing overlap of the reacted regions are likely to be more closely correlated to the actual size of the oxidation products. The final image in fig. 1 can be readily correlated with that shown by Cotterill et al. [3] from a galena surface oxidised for 3 days. The continued growth of the products in our images would dearly result in images closely similar to their results. Another series of experiments is shown in fig. 2. A similar systematic growth of oxidation products with time is also seen in this sequence. The most interesting difference, compared to fig. 1, is the shape of the growing patterns of oxidation products on the surface. Samples in both figures are from the same PbS source prepared in the same way but the resulting surface structure indicates chain-like patterns in fig. 2. The step edges are usually considered to be the most energetic sites on the surface and therefore it is assumed that those sites would be more reactive in adsorption and oxidation. On the basis of figs. 1 and 2 this seems not to be the case. A possible explanation is that the oxidation is controlled by the microstructure of the samples, i.e. the amount and distribution of lattice defects, e.g. emerging screw dislocations, missing atoms,
Table 1 XPS analysis of galena samples oxidized in air: atomic concentrations (in %) of the elements after different oxidation times Element Pb S O C
Oxidationtime (min) 0 60 38.8 32.7 35.4 28.6 4.0 8.1 21.7 30.6
120 34.0 24.5 14.5 27.0
240 31.7 22.2 25.4 20 7
impurity atoms (e.g. refs. [4,5]). Variation in this microstructure may then explain the differences in STM images between these and other sets of oxidation patterns. However, more recently, examples of growth patterns have been seen in STM images from other surfaces, e.g. F e / C u (111) deposition [7], leading to separated islands and organised chain structures, in which the predominant influence appears to be repulsion between the growing clusters. This is another possible factor determining the final structures. 3.2. X P S analysis o f galena samples oxidized in air
In order to determine the chemical composition of oxidation products observed in STM images, XPS measurements were carried out on surfaces prepared under closely similar conditions. Table 1 shows the atomic concentrations of the elements after different oxidation times. In general, as expected, the signal from the sulfide substrate decreases and the concentration of oxygen increases as a function of time. The Pb signal varies less systematically since it is incorporated in both substrate and products but it shows an overall decrease with time. Moreover, the ratio P b : S is very far from the 1:1 stoichiometric ratio after long oxidation time. These factors indicate changes in the atom distributions in the surface region. Because the kinetic energies of Pb 4f7/2 and S 2p electrons are almost the same, this effect cannot be due to differences (and errors) in inelastic mean free paths and transmission functions of these electrons or to attenuation in gradually growing overlayers. On the other hand, as discussed in some other publi-
34
K. Laajalehto et al. / STM and XPS investtgatton of reactton of galena m air i
cations [8], it can be difficult to determine the peak area of the S 2p signal correctly, because of the vicinity of the strong Pb4f7/2 line and its associated energy loss structure. This may explain the small deviation from the stoichiometric ratio in the case of the fresh sample, but it is unlikely to explain the clear change in ratio as a function of time. Enrichment of lead relative to sulfur during air-oxidation was also observed by Buckley and Woods [1]. 3.2.1. Pb 4f 7/2 and S 2p spectra The S 2p spectra did not show any other chemical states except sulfide (i.e. $2p3/2 component at 160.7 eV) at any time in the oxidation sequence. This result accords with that of Buckley and Woods [1]. In contrast, in each successive Pb 4f7/2 spectrum, a second component was observed increasing in intensity at a BE near 138.4 eV, i.e. approximately 1.0 eV higher than the original PbS signal (fig. 3, table 2). A second component was also detected in the fresh sample at BE 138.4 eV, where its intensity was only about 2% of the main Pb4fT/2 component. Experiments with galena surfaces scraped in UHV, where a similar, very weak, high BE component can be fitted, suggest that this initial peak on the fresh surface is not completely due to oxidation products but may be partly attributed to multiplet splitting or difficulties in background subtraction. These results show that there are no sulfuroxygen species (e.g. sulfite, sulfate) on the surface, but there is some lead-containing oxidation species already present after < 1 min contact with air which grows with time. The intensity of the Pb 4f7/2 emission at 138.4 eV reached a value of 18% of the sulfide component after 240 min oxidation time. From the Pb 4f emission alone, it is difficult to decide the nature of this additional lead species because most of the oxygen-containing lead compounds have a component in the observed BE region. 3.2.2. O i s and C ls spectra Figs. 4 and 5 and table 2 show the result of curve-fitting procedures applied to the O ls and C ls emissions respectively from fresh and airoxidised galena samples. In the freshly cleaved
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Fig.3. XPSPb4fspectrafromthe galenasurfaceafter:(a) freshcleavage;(b)60mmmair;(c) 120rainin air;(d)240 min in air.Notethe successivegrowthof a highbinding energy(BE)componentnear138.4eV. sample, the amount of oxygen was near the limit of detection which meant that it was not reasonable to try to divide the weak signal into more than one component. This broad emission centered near 532.5 eV can be interpreted as a mixture of adsorbed water, peroxide (O-) and hydroxide-type oxygen [9,10] apparently formed on the surface during the short period (< 1 min) of contact with air between cleavage and insertion into the spectrometer. At this stage, the C ls spectrum (fig. 5) shows only contributions due to normal ubiquitous hydrocarbon contamination at
K. Laajalehto et al. / STM and XPS investigation of reaction of galena in air
284.8 eV (e.g. saturated - C H 2- species) and 286.6 eV (e.g. oxidised - C O - species). After the sample was oxidised for 60 min, the still relatively weak O ls spectrum (fig. 4) could be divided into two components at 531.4 and 533.4 eV. The latter binding energy is usually associated with water molecules either adsorbed or in hydrated structures [9]. The component at 531.4 eV, however, grows in intensity in this and subsequent oxidized samples. It appears to be associated, at least in part, with lead carbonate formation. This conclusion is supported by the •
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parallel growth of a component in the C ls spectra in the BE region 288-289 eV (fig. 5) which is typical for carbonate bonding. The BE of this component is shifted systematically from 288.9 eV (after 60 min) to 288.1 eV (after 240 rain). This behaviour is explained by assuming the formation of oxycarbonate and hydroxycarbonate species on the surface. After 120 min of oxidation we observed an oxide (O 2-) type oxygen component in O ls spectra near 529.4 eV (fig. 4, table 2) and its intensity then increased as a function of time. The O ls BE of 531.5 eV can be attributed
36
K. Laajalehto et al. / STM and XPS mvesugatton of reactton of galena m mr
to peroxide or hydroxide species [10] but the position of the BE of O ls in PbCO 3 is not well documented.
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3.3. XPS spectra o f PbCO 3
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Lead(II) carbonate powder (A.R. grade) on a gold substrate was measured using unmonochromatised M g K a excitation and an analyser pass energy 35.75 eV (fig. 6). A shift of 3.8 eV due to sample charging was observed in all XPS lines. The value of the shift was determined by fixing the C ls BE of hydrocarbon contamination to the value 284.8 eV. In the following discussion we refer to these corrected BE values. The binding energies of all observed components in Pb4f7/2, C l s and O ls spectra and their relative intensities are summarised in table 3. The interpretation of the results is obvious. In the C ls spectrum the components at 284.8 and 287.5 (weak) eV are due to hydrocarbon and oxidised hydrocarbon contamination respectively, whilst the component at 289.0 eV arises from the PbCO 3. Comparing the intensities of the components interpreted as originating only from PbCO 3, we obtained a measured stoichiometry of PbC0 96Oz ss, which is in good agreement with the formal composition. Hence, the O ls BE for PbCO 3 is apparently centred at 531.3 eV, i.e. very close to the BE
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Table 2 XPS line
Time of oxldatton (min) 0 BE (eV)
60 RI (%)
BE (eV)
120 RI (%)
BE (eV)
240 RI (%)
BE (eV)
RI (%)
Pb 4f7/2
137.5 138.4
98 2
137.5 138,4
94 6
137.5 138.4
86 14
137.4 138.4
85 15
S 2p3/2
160.7
100
160.8
100
160.7
100
160.7
100
O Is
532.5
100
531.4 533.4
73 27
529 4 531.3 533.4
6 85 9
529.3 531.2 533.5
9 87 4
C Is
284.8 286.6
90 10
284.7 286.3 288 9
78 13 9
284.7 286.0 288.3
76 12 12
284.8 286.0 288.1
76 6 17
RI = relative intensity
K. Laajalehto et al. / STM and XPS mvesttgation of reaction o f galena m atr Table 3 XPS emission
BE (eV)
RI (%)
FWHM
Pb4f7/2 C ls
138.4 284.8 287.5 289.0 531.3
100 45 6 49 100
1.5 1.6 2.2 1.4 1.6
O ls
37
products, ratioed to the oxidised Pb4f (i.e. BE 138.4 eV) component signal as unity in each case, gives the figures in table 4. Totalled stoichiometries after 60, 120 and 240 min correspond to: PBC145045, PBC07028, and PbCo.aO3.9, respectively. Given the uncertainties in these estimates, the agreement with an approximate stoichiometry of PbCO 3 is not unreasonable. It is also interesting that the earlier oxidation (i.e. 60 min) tends to show a higher proportion of oxygen and carbon relative to lead suggesting attenuation of the Pb signal due to an overlayer of carbonate products as in the mechanism discussed below. Later oxidation (i.e. 120 min) suggests that lead diffuses into the reaction regions as is necessary for the continued growth observed in STM images. The final stages apparently overestimate oxygen compared with PbCO 3. This may indicate that there are some oxide and hydroxide species adsorbed separately on all sample surfaces and that their contribution may superimpose onto the main carbonate emission. Alternatively oxide and hydroxide species may form part of the oxidation products, e.g. PbCO 3 .xPb(OH) 2 or PbCO 3 •yPbO. Initial oxidation of PbS has been previously studied with XPS by Buckley and Woods [1]. They concluded that, during air-oxidation, the surfaces became enriched with lead due to formation of lead hydroxide, oxide and carbonate. As noted above, to identify these different species from the O ls spectra alone is a difficult task. In
observed for the growing oxidation products described above. For the cerrusite mineral (i.e. 0.98PBCO3.0.02PbS), Carbussi and Marabini [11] have also reported a value of 531.4 eV for O ls but they give BE values of 139.2 eV for Pb 4f7/2 and 290.0 eV for C ls which are not consistent with our spectra. However, the large FWHM values of their spectra (i.e. 2.4-2.9 eV) may indicate inhomogeneous charging or defective surfaces despite their use of an electron flood gun to compensate for surface charging.
3.4. Chemicalforms of oxidation products Table 4 summarises the BE and area intensities of the components of the Pb 4f, O Is and C ls curve-fitted XPS spectra from the air-oxidised sequences in figs. 3, 4 and 5, i.e. the galena surface after 60, 120 and 240 min oxidation. The assignment of each component to sulfide, oxide (O2-), peroxide (O-), hydroxide (OH-), carbonate (CO 2-) and molecular water is also indicated. Calculating the stoichiometries of the Pb4f, C ls and O ls components attributed to the oxidised
Table 4 XPS Line
Pb 4f O ls
C ls
Species
Time of oxidation (mm) 60 BE
I
(eV)
(c eV s- i)
S 2ox
137.5 138.4
6363 390
0 2CO3z - , O H - , O H20
531.4 533 4
131 48
-CH2 CO CO3z -
284.7 286.3 288.9
206 25 24
120 Ratio
240
BE
I
Ratio
(eV)
(c eV s-l)
1.0
137.5 138 4
5563 1124
3.3 1.2
529.4 531.3 533.4
18 267 36
1.45
284.7 286.0 288.3
192 25 33
BE
I
(eV)
(c eV s-l)
Ratio
1.0
137.4 138.4
6127 1418
1.0
0.16 2.3 0.30
529.3 531.2 533.5
53 499 18
0.36 3.4 0.12
0.70
284.8 286.0 288.1
200 27 49
0.80
IK Laajalehto et al / STM and XPS mvesttgatton of reactton of galena matr
38
the initial stages of oxidation, Buckley and Woods interpreted the strongest O ls component which, after 15 min in air in their work, was at 530.8 eV as lead hydroxide and chemisorbed water, because there was no evidence of carbonate in the C ls spectrum at that stage. They report no measurements between 15 min and one day of oxidation. After exposure to air for one day, they observed carbonate in the C Is spectrum, but they gave no information about the corresponding oxygen line. According to the results of Buckley and Woods and our results presented here, it seems likely that the O ls binding energies in lead carbonate and lead hydroxide are so near to each other that the differentiation of these compounds can be made only with the help of the C ls spectrum. Unfortunately, very small amounts of carbonate are difficult to observe because of the overlap with carbon contamination and the low sensitivity factor for C ls emission. The predominant product in the oxidation of galena, is however, lead carbonate. This clearly distinguishes this sulfate mineral from the ironcontaining sulfide minerals, i.e. pyrite [12], pyrrhotite [13], chalcopyrite [14], pentlandite [15] and violarite [15] where the predominant oxidation product formed on the surface is ferric hydroxide. The implications of this difference for flotation separation of these sulfide minerals has been discussed elsewhere [16], particularly in relation to the action of cyanide in facilitating selective galena flotation [17] by selective removal of oxidised products from sulfide mineral surfaces.
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3.5. Mechamsm of carbonate formation The mechanism of formation of carbonate and hydroxycarbonate species from C O 2 adsorption on oxide surfaces is well documented (e.g. ref. [18]). For instance, on NiO [19] surfaces, adsorption of O 2 takes places initially, predominantly at low-coordination defect sites, giving O - ions at these sites. Subsequent adsorption of CO 2 produces carboxylate structures in monodentate, bidentate and binuclear modes [18,20] depending on surface preparation and reaction conditions. In this case, where prior oxidation of the sulfide surface is required before CO 2 adsorption, the likely mechanism involves initial O - (or O H - ) adsorption at defect Pb sites (since no oxy-sulfur species are found) on the (100) PbS facets. Apparent activation of adjacent Pb sites for further O - adsorption is accompanied by CO 2 adsorption to form carboxylate groups in separate clusters or islands on the surface. A schematic illustration of a possible sequence is shown in fig. 7.
4. Conclusions STM images of galena (100) crystal planes oxidised in air (0-270 min), show the systematic growth of oxidation products from regions < 0.6 to > 9 nm diameter. Overlap of oxidation regions begins to occur after ~ 180 min. The step edges between the crystal planes apparently show no preference for the beginning of oxidation. Instead, the formation of isolated islands dis-
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7 Schematic representation of: (a) point defect (S 2 - vacancy)site m a PbS surface, (b) adsorption of O- (or OH-) ions on adjacent (actwated) Pb2+ sites; (c) adsorption of CO2 molecules on O- ions to form adsorbed CO2- species. Hydroxylspecies may also be present as adsorbed HCO~- or as separate OH- entities on adjacent sites
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K Laajalehto et al / STM and XPS mvesugatton ofreacuon o f galena m air
tributed randomly over the (100) surface is found. This behaviour may be explained by assuming that different lattice point defects (emerging screw dislocation, missing atoms, impurities, etc.) are the initiation points for oxidation, and there is some evidence for such point defects in initial images. This could also explain the observed differences in the structure of oxidation patterns in different samples. The chemical nature of surface oxidation species was determined with XPS. Soon after cleavage, limited adsorption of peroxide and probably hydroxide species occurs. When oxidation has proceeded for more than 60 min, the formation of lead carbonate was observed and after 120 rain the formation of hydroxycarbonate type species on the surface is consistent with the spectra. No sulfur-oxygen compounds were detected during the time period studied. The surface outside the oxidation regions may have some adsorbed oxygen, water and hydroxide, which was not seen in STM images, but this must constitute much less than a monolayer.
Acknowledgements The main part of this work was done during a research visit by Mr. K. Laajalehto to the University of South Australia. The visit was financially supported by the Academy of Finland and the University of South Australia. Support from the Australian Research Council is also gratefully acknowledged.
References [1] A.N. Buckley and R. Woods, Appl. Surf. Sci. 17 (1984) 401. [2] S Evans and E. Raftery, J. Chem. Soc. Faraday Trans. I, 78 (1982) 3545. [3] G.F. CottenU, R. Bartlett, A.E. Hughes and B.A. Sexton, Surf. Scl. Lett. 232 (1990) L211.
39
[4] M.F Hochella, Jr., C.M. Eggleston, V.B. Elings, G.A. Parks, G.E. Brown, Jr., C.M. Wu and K. Kjoller, Am. Mineral. 74 (1989) 1233. [5] C.M Eggleston and M.F. Hochella, Jr., Geochim. Cosmochim Acta 54 (1990) 1511. [6] B.A. Sexton and G.F. CotteriU, J. Vac. SCL Technol A 7 (1989) 2734. [7] H. Neddermeyer, Scanning tunnehng microscopy on clean and adsorbate covered semiconductor and metal surfaces, Autrahan-German Surface Science Workshop, Springer Progress in Physics Series, Eds. R Lamb, K. Wandelt and R.F. Howe (Springer, Berlin, November 1992, in press). [8] K Laajalehto, P Nowak, A. Pomlanowskl and E. Suomnen, Colloids Surf. 57 (1991) 319. [9] C.D. Wagner, D.A. Zatko and R H. Raymond, Anal. Chem. 52 (1980) 1445, A.F. Carley, S. Rassias and M.W Roberts, Surf. Sci. 135 (1983) 35. [10] M.W. Roberts and R.St C. Smart, J. Chem. Soc. Faraday I, 80 (1984) 2957 [11] F. Carbussi and A.M. Marabinl, J. Chem. Soc Faraday Trans I, 82 (1986) 2043. [12] A N. Buckley and R. Woods, Appl. Surf. Sci. 27 (1987) 437 [13] A.N Buckley and R. Woods, Appl. Surf. Sci. 22/23 (1985) 280; 20 (1985) 472 [14] A N. Buckley and R. Woods, Aust. J. Chem. 37 (1984) 2403. [15] S. Richardson and D.J. Vaughan, Mineral Mag. 53 (1989) 213. [16] C. Prestidge, J. Ralston and R.St C. Smart, The competitive adsorption of cyanide and ethyl xanthate on pyrite and pyrrhoute surfaces, Int. J. Miner. Process., submitted. [17] M.W. Baker, M.C. Pietrobon, J. Ralston and R.St.C. Smart, The enhancement of galena flotation using cyanide, in. Proc. Extract. Met. Conf., Perth, Western Australia, October 1991 (Aus. Inst. Min. Met., Austraha) pp 192-202. [18] L.H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1966); M.L Hair, Infrared Spectroscopy in Surface Chemistry (Arnold, London, 1967). [19] M W. Roberts and R.St C. Smart, Surf. Sci. 103 (1981) 271. [20] E Guglielminotti, L. Cerrutl and E. Borello, Gazz. Chim. Ital. 107 (1977) 447, A. Ueno, J. Hochmuth and C.O. Bennett, J. Catal. 49 (1977) 225; P.C. Gravelle and S.J. Teichner, Adv. Catal. 20 (1969) 167.
aol: o
surface science
STM and XPS investigation of reaction of galena in air Kari L a a j a l e h t o a, R o g e r St.C. S m a r t b, J o h n R a l s t o n b,. a n d E e r o S u o n i n e n
a
a Laboratory of Materials Science, Department of Apphed Physics, Unwerslty of Turku, Itamen, Pttkakatu 1, SF-20520 Turku, Finland b School of Chemical Technology, Unwerstty of South Austraha, The Levels, Adelaide, SA 5095, Austraha
Received 10 June 1992; accepted for publication 7 July 1992
Scanning tunneling microscopy(STM) was used to study the oxidation of stepped (100) PbS surfaces in air over periods up to 270 min. The systematic growth of reaction products as surface features with lateral dimensions < 0.6 nm initially to overlapped regions (> 9 nm diameter) was observed. No clear preference for growth at step edges was found but defect sites in the (100) surface appear to provide initiation points for reaction. X-ray photoelectron spectra (XPS) from PbS after successive reaction times suggest that limited adsorption of peroxide (O-) and hydroxide (OH-) species occurs initially but that the predominant products after 60 rain are carbonate species from CO2 reaction with O- or OH- sites. After 120 rain, hydroxycarbonatespecies are formed but the S 2p spectra show only sulfide at all times, i.e. no sulfur-oxygen species are formed. The combined results provide both structural and compositional evidence for the mechanism of reaction of galena surfaces to PbCO3 "xPb(OH)2 surface products in air
1. Introduction Galena is one of the most common sulfide minerals concentrated by the flotation method. To understand the flotation behaviour, it is essential to know the surface composition of the mineral before the addition of surface-active reagents to the flotation pulp. Previous studies of the oxidation of galena under different conditions using X-ray photoelectron spectroscopy (XPS) have been reported [1,2]. Although the XPS method is very suitable for quantitative surface analysis, its geometrical resolution is restricted. Usually the measured composition is an average from a surface area larger than 0.02 mm 2. On the other hand, scanning tunnelling microscopy (STM) is a surface imaging method capable of atomic resolution in both vertical and horizontal directions. The main disadvantage of STM is that it provides no chemical information. STM images of heavily oxidized (more than one day in air) galena samples have been published
* Author to whom correspondence should be addressed.
recently [3]. Keeping in mind the mineral flotation correlation, it was considered especially interesting to study the very early stages of reaction of galena in air by combining STM and XPS methods. In this way, the composition, topography and distribution of reaction products on the surface were characterised. The galena mineral is a very suitable material for STM studies, because its electrical conductivity is sufficiently high and it is relatively easy to find large single crystals that cleave easily along the (100) crystal plane. Recent work has also shown STM images of (100) galena surfaces close to atomic resolution with information on defects and impurity sites [4,5].
2. Experimental Natural galena crystals, supplied by Wards Natural Science Establishment (New York), originating from Brushy Creek, Missouri, were used in all measurements. According to electron probe microanalysis the mineral contained on average 720 ppm Fe, 110 ppm Zn and 25 ppm Cu as minor impurities.
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
30
K. Laajalehto et al. / S T M and X P S mvesttgatton of reactton o f galena m atr
The STM instrument was an inverted scanner type designed and constructed by Sexton and Cotterill in CSIRO Division of Materials Science and Technology, Melbourne, Australia [6]. The STM tip was cut from P t - R h alloy without electrochemical etching. The PbS sample was attached to the AI holder with conductive silver paint and allowed to dry before cleavage. Cleavage was carried out in air and the sample was immediately transferred to the nitrogen or argon atmosphere in the STM investigation chamber or to the XPS introduction chamber. In both cases the contact time with air was < 1 rain, and usually just a few seconds, before immersion in the inert atmosphere. Thermal equilibration in the chamber, i.e. continuing sample motion, occupied ~ 10 min before images could be recorded. All images were recorded in constant-current mode with a tip bias of + 0.35 V (usually - 0 . 3 5 V) and tunneling current 0.20-0.25 nA. Recording time for an image was 5-10 min. Images are presented after background subtraction but without any smoothing procedures. The location and relocation of suitable areas for analysis presented considerable difficulties. Several cleavages and trials on many areas were often necessary to obtain reliable, reproducible images of areas without large vertical surface features. Drift in images between successive reaction times was also experienced requiring several similar series of experiments for validation. After the first STM image was taken, using the nitrogen atmosphere to maintain the fresh surface, ambient air at ~ 20°C and ~ 60% humidity, was introduced to the chamber and a new image was taken after every 30 min for periods up to 270 min. Samples for XPS experiments were prepared by identical procedures and measured after 0, 60, 120 and 240 min reaction in air. Spectra were recorded using a Perkin-Elmer PHI 5400 electron spectrometer equipped with a monochromatized AI K a X-ray source. A sample region of diameter 0.15 mm from the cleaved galena surface was analysed. The energy scale was calibrated with the AU4fT/2 binding energy (BE) 84.0 eV and Cu2p3/2 BE 932.6 eV lines. The measured line width of the Au 4f7/2 was 0.85 eV for an analyser
pass energy of 35.75 eV. Elemental atomic concentrations were calculated using curve-fitted peak areas and standard sensitivity factors for Pb (4f7/9, 8.329, 4f, 6.968), C ls (0.296), O ls (0.711) and S 2p (0.54).
3. Results and discussion 3.1. STM images Grey scale and 3D plots of freshly cleaved and subsequently air-reacted galena sample are presented in fig. 1. Realignment of the STM tip on the same area presented some difficulties due to very small drift in either the crystal position or tunnelling contact point. Nevertheless, pictures are approximately from the same area. The images show a 2 unit cell step (i.e. ~ 1.2 nm height) traversing the 70 × 70 nm area with relatively fiat ledges above and below this step. The drift can be identified with the help of two vertical defects in the step seen especially well in the grey scale plots in fig. 1. There are a few atomic scale (i.e. < 0.3 nm) defects or reaction sites imaged as vertical protrusions in the first image after cleavage. At higher magnification, they appear as sharp, local changes in the tip distance d (with constant current) on particular sites within a unit cell parameter. They may arise from impurity atoms or sites of initial, rapid oxidation, e.g. vacancies, displaced atoms, etc. Remarkable changes can be seen in the images as a function of time. Systematic growth with time of oxidation products as roughly conical structures are seen on the galena surface. Realignment of successive images, using the step defects shows that each oxidation region grows vertically and laterally with time until, after 170 rain, some overlap of adjacent regions begins to occur. It is also noted in the early images (i.e. 30, 60 min) that, whilst oxidation products do grow on the positions of the initial defects in the first image, they also grow at other apparently undistinguished sites on the ledges. Of particular importance is the observation that they do not grow at the edges of the steps despite the lower coordination of atoms in these sites. After 270 min, the dimensions of the
I(. Laajalehto et al. / STM and XPS mvesttgatton of reaction of galena m atr
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largest reaction structures were ~ 9 nm (diameter) and ~ 4 nm (height), which correspond to about 16 and 7 P b - P b distances, respectively, in the original PbS lattice. The observed patterns, however are likely to be a superposition of topographical changes with changes in tunneling current induced by the lower conductivity of the oxidation products. The approximate relationship between tunnelling current density ( j ) and tunnel distance (d) e2 k 0 J = h 47r2d V e x p ( - 2 k 0 d ), where V = bias (V) and k0(nm) = 0.1025vr~, shows that a change in the work function • of the surface in the region of the oxidation products will induce significant change in d at constant current j. Hence, it is possible that the images overestimate the vertical dimensions of the reacted regions. The horizontal dimensions, ultimately showing overlap of the reacted regions are likely to be more closely correlated to the actual size of the oxidation products. The final image in fig. 1 can be readily correlated with that shown by Cotterill et al. [3] from a galena surface oxidised for 3 days. The continued growth of the products in our images would dearly result in images closely similar to their results. Another series of experiments is shown in fig. 2. A similar systematic growth of oxidation products with time is also seen in this sequence. The most interesting difference, compared to fig. 1, is the shape of the growing patterns of oxidation products on the surface. Samples in both figures are from the same PbS source prepared in the same way but the resulting surface structure indicates chain-like patterns in fig. 2. The step edges are usually considered to be the most energetic sites on the surface and therefore it is assumed that those sites would be more reactive in adsorption and oxidation. On the basis of figs. 1 and 2 this seems not to be the case. A possible explanation is that the oxidation is controlled by the microstructure of the samples, i.e. the amount and distribution of lattice defects, e.g. emerging screw dislocations, missing atoms,
Table 1 XPS analysis of galena samples oxidized in air: atomic concentrations (in %) of the elements after different oxidation times Element Pb S O C
Oxidationtime (min) 0 60 38.8 32.7 35.4 28.6 4.0 8.1 21.7 30.6
120 34.0 24.5 14.5 27.0
240 31.7 22.2 25.4 20 7
impurity atoms (e.g. refs. [4,5]). Variation in this microstructure may then explain the differences in STM images between these and other sets of oxidation patterns. However, more recently, examples of growth patterns have been seen in STM images from other surfaces, e.g. F e / C u (111) deposition [7], leading to separated islands and organised chain structures, in which the predominant influence appears to be repulsion between the growing clusters. This is another possible factor determining the final structures. 3.2. X P S analysis o f galena samples oxidized in air
In order to determine the chemical composition of oxidation products observed in STM images, XPS measurements were carried out on surfaces prepared under closely similar conditions. Table 1 shows the atomic concentrations of the elements after different oxidation times. In general, as expected, the signal from the sulfide substrate decreases and the concentration of oxygen increases as a function of time. The Pb signal varies less systematically since it is incorporated in both substrate and products but it shows an overall decrease with time. Moreover, the ratio P b : S is very far from the 1:1 stoichiometric ratio after long oxidation time. These factors indicate changes in the atom distributions in the surface region. Because the kinetic energies of Pb 4f7/2 and S 2p electrons are almost the same, this effect cannot be due to differences (and errors) in inelastic mean free paths and transmission functions of these electrons or to attenuation in gradually growing overlayers. On the other hand, as discussed in some other publi-
34
K. Laajalehto et al. / STM and XPS investtgatton of reactton of galena m air i
cations [8], it can be difficult to determine the peak area of the S 2p signal correctly, because of the vicinity of the strong Pb4f7/2 line and its associated energy loss structure. This may explain the small deviation from the stoichiometric ratio in the case of the fresh sample, but it is unlikely to explain the clear change in ratio as a function of time. Enrichment of lead relative to sulfur during air-oxidation was also observed by Buckley and Woods [1]. 3.2.1. Pb 4f 7/2 and S 2p spectra The S 2p spectra did not show any other chemical states except sulfide (i.e. $2p3/2 component at 160.7 eV) at any time in the oxidation sequence. This result accords with that of Buckley and Woods [1]. In contrast, in each successive Pb 4f7/2 spectrum, a second component was observed increasing in intensity at a BE near 138.4 eV, i.e. approximately 1.0 eV higher than the original PbS signal (fig. 3, table 2). A second component was also detected in the fresh sample at BE 138.4 eV, where its intensity was only about 2% of the main Pb4fT/2 component. Experiments with galena surfaces scraped in UHV, where a similar, very weak, high BE component can be fitted, suggest that this initial peak on the fresh surface is not completely due to oxidation products but may be partly attributed to multiplet splitting or difficulties in background subtraction. These results show that there are no sulfuroxygen species (e.g. sulfite, sulfate) on the surface, but there is some lead-containing oxidation species already present after < 1 min contact with air which grows with time. The intensity of the Pb 4f7/2 emission at 138.4 eV reached a value of 18% of the sulfide component after 240 min oxidation time. From the Pb 4f emission alone, it is difficult to decide the nature of this additional lead species because most of the oxygen-containing lead compounds have a component in the observed BE region. 3.2.2. O i s and C ls spectra Figs. 4 and 5 and table 2 show the result of curve-fitting procedures applied to the O ls and C ls emissions respectively from fresh and airoxidised galena samples. In the freshly cleaved
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Fig.3. XPSPb4fspectrafromthe galenasurfaceafter:(a) freshcleavage;(b)60mmmair;(c) 120rainin air;(d)240 min in air.Notethe successivegrowthof a highbinding energy(BE)componentnear138.4eV. sample, the amount of oxygen was near the limit of detection which meant that it was not reasonable to try to divide the weak signal into more than one component. This broad emission centered near 532.5 eV can be interpreted as a mixture of adsorbed water, peroxide (O-) and hydroxide-type oxygen [9,10] apparently formed on the surface during the short period (< 1 min) of contact with air between cleavage and insertion into the spectrometer. At this stage, the C ls spectrum (fig. 5) shows only contributions due to normal ubiquitous hydrocarbon contamination at
K. Laajalehto et al. / STM and XPS investigation of reaction of galena in air
284.8 eV (e.g. saturated - C H 2- species) and 286.6 eV (e.g. oxidised - C O - species). After the sample was oxidised for 60 min, the still relatively weak O ls spectrum (fig. 4) could be divided into two components at 531.4 and 533.4 eV. The latter binding energy is usually associated with water molecules either adsorbed or in hydrated structures [9]. The component at 531.4 eV, however, grows in intensity in this and subsequent oxidized samples. It appears to be associated, at least in part, with lead carbonate formation. This conclusion is supported by the •
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B.E. (eV) Fig. 4 XPS O ls spectra from the galena surface as for fig. 3. Note the increasing oxygen percentage of the surface (tables 1 and 2) and the growth of a predominant emission near 531.4 eV.
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parallel growth of a component in the C ls spectra in the BE region 288-289 eV (fig. 5) which is typical for carbonate bonding. The BE of this component is shifted systematically from 288.9 eV (after 60 min) to 288.1 eV (after 240 rain). This behaviour is explained by assuming the formation of oxycarbonate and hydroxycarbonate species on the surface. After 120 min of oxidation we observed an oxide (O 2-) type oxygen component in O ls spectra near 529.4 eV (fig. 4, table 2) and its intensity then increased as a function of time. The O ls BE of 531.5 eV can be attributed
36
K. Laajalehto et al. / STM and XPS mvesugatton of reactton of galena m mr
to peroxide or hydroxide species [10] but the position of the BE of O ls in PbCO 3 is not well documented.
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3.3. XPS spectra o f PbCO 3
54-
Lead(II) carbonate powder (A.R. grade) on a gold substrate was measured using unmonochromatised M g K a excitation and an analyser pass energy 35.75 eV (fig. 6). A shift of 3.8 eV due to sample charging was observed in all XPS lines. The value of the shift was determined by fixing the C ls BE of hydrocarbon contamination to the value 284.8 eV. In the following discussion we refer to these corrected BE values. The binding energies of all observed components in Pb4f7/2, C l s and O ls spectra and their relative intensities are summarised in table 3. The interpretation of the results is obvious. In the C ls spectrum the components at 284.8 and 287.5 (weak) eV are due to hydrocarbon and oxidised hydrocarbon contamination respectively, whilst the component at 289.0 eV arises from the PbCO 3. Comparing the intensities of the components interpreted as originating only from PbCO 3, we obtained a measured stoichiometry of PbC0 96Oz ss, which is in good agreement with the formal composition. Hence, the O ls BE for PbCO 3 is apparently centred at 531.3 eV, i.e. very close to the BE
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Table 2 XPS line
Time of oxldatton (min) 0 BE (eV)
60 RI (%)
BE (eV)
120 RI (%)
BE (eV)
240 RI (%)
BE (eV)
RI (%)
Pb 4f7/2
137.5 138.4
98 2
137.5 138,4
94 6
137.5 138.4
86 14
137.4 138.4
85 15
S 2p3/2
160.7
100
160.8
100
160.7
100
160.7
100
O Is
532.5
100
531.4 533.4
73 27
529 4 531.3 533.4
6 85 9
529.3 531.2 533.5
9 87 4
C Is
284.8 286.6
90 10
284.7 286.3 288 9
78 13 9
284.7 286.0 288.3
76 12 12
284.8 286.0 288.1
76 6 17
RI = relative intensity
K. Laajalehto et al. / STM and XPS mvesttgation of reaction o f galena m atr Table 3 XPS emission
BE (eV)
RI (%)
FWHM
Pb4f7/2 C ls
138.4 284.8 287.5 289.0 531.3
100 45 6 49 100
1.5 1.6 2.2 1.4 1.6
O ls
37
products, ratioed to the oxidised Pb4f (i.e. BE 138.4 eV) component signal as unity in each case, gives the figures in table 4. Totalled stoichiometries after 60, 120 and 240 min correspond to: PBC145045, PBC07028, and PbCo.aO3.9, respectively. Given the uncertainties in these estimates, the agreement with an approximate stoichiometry of PbCO 3 is not unreasonable. It is also interesting that the earlier oxidation (i.e. 60 min) tends to show a higher proportion of oxygen and carbon relative to lead suggesting attenuation of the Pb signal due to an overlayer of carbonate products as in the mechanism discussed below. Later oxidation (i.e. 120 min) suggests that lead diffuses into the reaction regions as is necessary for the continued growth observed in STM images. The final stages apparently overestimate oxygen compared with PbCO 3. This may indicate that there are some oxide and hydroxide species adsorbed separately on all sample surfaces and that their contribution may superimpose onto the main carbonate emission. Alternatively oxide and hydroxide species may form part of the oxidation products, e.g. PbCO 3 .xPb(OH) 2 or PbCO 3 •yPbO. Initial oxidation of PbS has been previously studied with XPS by Buckley and Woods [1]. They concluded that, during air-oxidation, the surfaces became enriched with lead due to formation of lead hydroxide, oxide and carbonate. As noted above, to identify these different species from the O ls spectra alone is a difficult task. In
observed for the growing oxidation products described above. For the cerrusite mineral (i.e. 0.98PBCO3.0.02PbS), Carbussi and Marabini [11] have also reported a value of 531.4 eV for O ls but they give BE values of 139.2 eV for Pb 4f7/2 and 290.0 eV for C ls which are not consistent with our spectra. However, the large FWHM values of their spectra (i.e. 2.4-2.9 eV) may indicate inhomogeneous charging or defective surfaces despite their use of an electron flood gun to compensate for surface charging.
3.4. Chemicalforms of oxidation products Table 4 summarises the BE and area intensities of the components of the Pb 4f, O Is and C ls curve-fitted XPS spectra from the air-oxidised sequences in figs. 3, 4 and 5, i.e. the galena surface after 60, 120 and 240 min oxidation. The assignment of each component to sulfide, oxide (O2-), peroxide (O-), hydroxide (OH-), carbonate (CO 2-) and molecular water is also indicated. Calculating the stoichiometries of the Pb4f, C ls and O ls components attributed to the oxidised
Table 4 XPS Line
Pb 4f O ls
C ls
Species
Time of oxidation (mm) 60 BE
I
(eV)
(c eV s- i)
S 2ox
137.5 138.4
6363 390
0 2CO3z - , O H - , O H20
531.4 533 4
131 48
-CH2 CO CO3z -
284.7 286.3 288.9
206 25 24
120 Ratio
240
BE
I
Ratio
(eV)
(c eV s-l)
1.0
137.5 138 4
5563 1124
3.3 1.2
529.4 531.3 533.4
18 267 36
1.45
284.7 286.0 288.3
192 25 33
BE
I
(eV)
(c eV s-l)
Ratio
1.0
137.4 138.4
6127 1418
1.0
0.16 2.3 0.30
529.3 531.2 533.5
53 499 18
0.36 3.4 0.12
0.70
284.8 286.0 288.1
200 27 49
0.80
IK Laajalehto et al / STM and XPS mvesttgatton of reactton of galena matr
38
the initial stages of oxidation, Buckley and Woods interpreted the strongest O ls component which, after 15 min in air in their work, was at 530.8 eV as lead hydroxide and chemisorbed water, because there was no evidence of carbonate in the C ls spectrum at that stage. They report no measurements between 15 min and one day of oxidation. After exposure to air for one day, they observed carbonate in the C Is spectrum, but they gave no information about the corresponding oxygen line. According to the results of Buckley and Woods and our results presented here, it seems likely that the O ls binding energies in lead carbonate and lead hydroxide are so near to each other that the differentiation of these compounds can be made only with the help of the C ls spectrum. Unfortunately, very small amounts of carbonate are difficult to observe because of the overlap with carbon contamination and the low sensitivity factor for C ls emission. The predominant product in the oxidation of galena, is however, lead carbonate. This clearly distinguishes this sulfate mineral from the ironcontaining sulfide minerals, i.e. pyrite [12], pyrrhotite [13], chalcopyrite [14], pentlandite [15] and violarite [15] where the predominant oxidation product formed on the surface is ferric hydroxide. The implications of this difference for flotation separation of these sulfide minerals has been discussed elsewhere [16], particularly in relation to the action of cyanide in facilitating selective galena flotation [17] by selective removal of oxidised products from sulfide mineral surfaces.
o0o0o0o0
3.5. Mechamsm of carbonate formation The mechanism of formation of carbonate and hydroxycarbonate species from C O 2 adsorption on oxide surfaces is well documented (e.g. ref. [18]). For instance, on NiO [19] surfaces, adsorption of O 2 takes places initially, predominantly at low-coordination defect sites, giving O - ions at these sites. Subsequent adsorption of CO 2 produces carboxylate structures in monodentate, bidentate and binuclear modes [18,20] depending on surface preparation and reaction conditions. In this case, where prior oxidation of the sulfide surface is required before CO 2 adsorption, the likely mechanism involves initial O - (or O H - ) adsorption at defect Pb sites (since no oxy-sulfur species are found) on the (100) PbS facets. Apparent activation of adjacent Pb sites for further O - adsorption is accompanied by CO 2 adsorption to form carboxylate groups in separate clusters or islands on the surface. A schematic illustration of a possible sequence is shown in fig. 7.
4. Conclusions STM images of galena (100) crystal planes oxidised in air (0-270 min), show the systematic growth of oxidation products from regions < 0.6 to > 9 nm diameter. Overlap of oxidation regions begins to occur after ~ 180 min. The step edges between the crystal planes apparently show no preference for the beginning of oxidation. Instead, the formation of isolated islands dis-
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7 Schematic representation of: (a) point defect (S 2 - vacancy)site m a PbS surface, (b) adsorption of O- (or OH-) ions on adjacent (actwated) Pb2+ sites; (c) adsorption of CO2 molecules on O- ions to form adsorbed CO2- species. Hydroxylspecies may also be present as adsorbed HCO~- or as separate OH- entities on adjacent sites
Fig.
K Laajalehto et al / STM and XPS mvesugatton ofreacuon o f galena m air
tributed randomly over the (100) surface is found. This behaviour may be explained by assuming that different lattice point defects (emerging screw dislocation, missing atoms, impurities, etc.) are the initiation points for oxidation, and there is some evidence for such point defects in initial images. This could also explain the observed differences in the structure of oxidation patterns in different samples. The chemical nature of surface oxidation species was determined with XPS. Soon after cleavage, limited adsorption of peroxide and probably hydroxide species occurs. When oxidation has proceeded for more than 60 min, the formation of lead carbonate was observed and after 120 rain the formation of hydroxycarbonate type species on the surface is consistent with the spectra. No sulfur-oxygen compounds were detected during the time period studied. The surface outside the oxidation regions may have some adsorbed oxygen, water and hydroxide, which was not seen in STM images, but this must constitute much less than a monolayer.
Acknowledgements The main part of this work was done during a research visit by Mr. K. Laajalehto to the University of South Australia. The visit was financially supported by the Academy of Finland and the University of South Australia. Support from the Australian Research Council is also gratefully acknowledged.
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