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Identifying chemisorption in the interaction of thiol collectors with sulfide minerals by XPS: adsorption of xanthate on silver and silver sulfide
COLLOIDS
AND E L S E V INN
Colloids and Surfaces A: Physicochemical and Engineering Aspects 104 { 19951295 305
A
SURFACES
Identifying chemisorption in the interaction of thiol collectors with sulfide minerals by XPS: adsorption of xanthate on silver and silver sulfide Alan N. Buckley a,* Ronald Woods
b
CSIRO Di~,ision of Coal and Energy Technolog)', PO Box 136. North R)'de, N.S.W. 2113. Australia b CSIRO Dil~ision of Minerals. PO Bo.~c 124, Port M~,lhourlle. flit. 3207, Australia Received 17 March 1995; accepted 15
Miiy1995
Abstract The adsorption of ethyl xanthate on silver and sulfidised silver surfaces at controlled potentials has been investigated by X-ray photoelectron spectroscopy (XPS). Silver/xanthate and silver sulfide/xanthate were chosen as model systems for investigating the identification of chemisorption of thiol collectors on sulfide minerals by means of core electron energy shifts. There was no discernible effect from submonolayer adsorption on either substrate on the corresponding A g ( M N N t spectrum, even when the presence of adsorbed xanthate was clearly evident from the S 2p spectrum. It was confirmed that the Ag(MNN) electron energies for multilayer silver xanthate were significantly different from those for silver and silver sulfide. The basis for the general proposition, that chemisorption of thiol collectors oll sulfide mineral surfaces might not necessarily give rise to discernible substrate core electron energy shifts, was examined in the context of previously reported investigations of chemisorption of adsorbates from the vapour phase. It was concluded from this appraisal, and from the experimental results for the interaction of xanthate with silver and silver sulfide, that substrate core electron energy shifts on chemisorption are no! always large enough to be detectable by conventional XPS. By contrast, core electron energy shifts for multilayer metal-collector compound formation are clearly discernible. Kerwords: Adsorption: Chemisorption: Sulfide minerals: Thiol: Xanthate: X-ray photoelectron spectroscopy: Collectors
1. Introduction The beneficiation of sulfide ores by flotation is usually effected by i m p a r t i n g h y d r o p h o b i c i t y to the m i n e r a l c o n s t i t u e n t to be c o n c e n t r a t e d by m e a n s of a thiol collector. The a m o u n t s of collector required for this p u r p o s e are usually equivalent to less than m o n o l a y e r coverage of the surface of the particles [ 1 ] . Electrochemical evidence indicates [ 2 ] that, u n d e r typical flotation conditions, thiol * Corresponding author. 0927-7757/95,,'$09.50 ,c9 1995 Elsevier Science B.V. All rights reserved SSDI (/927-7757(95)03279-7
collectors are c h e m i s o r b e d to the metal a t o m s in the surface layer of a sulfide mineral by an a n o d i c reaction: X
--~Xad s -~- e
(I}
with a c o m p l e m e n t a r y c a t h o d i c process: 1/202 + H 2 0 + 2 e
~2OH
{2)
occurring at n e a r b y sites on the surface. S p e c t r o s c o p i c techniques have been a p p l i e d by a n u m b e r of w o r k e r s to characterise the nature of the a d s o r b e d layer, but there has not been general
296
A.N. Buckle),, R. Woods~Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 295 305
agreement as to the interpretation of spectra in relation to chemisorption. Although numerous infrared spectroscopic studies have been carried out on xanthate/sulfide systems, the assumptions made in interpreting the spectra have been questioned recently [-3-5]. X-ray photoelectron spectroscopy (XPS) has also been used to investigate several sulfide mineral/thiol collector systems under conditions which should have limited adsorption to submonolayer coverage [6-9]. In most cases the electron spectra have revealed imperceptible substrate core level shifts resulting from such adsorption, despite clear evidence for a new sulfur species at the surface, but this observation has been explained in different ways. Mielczarski et al. [-6] concluded that the absence of any significant change in the Cu(LMM) Auger spectrum for chalcocite before and after interaction with ethyl xanthate within the potential range where voltammetry indicates chemisorption to have taken place, was evidence for adsorption of xanthate n o t having occurred. Such an argument is based on the assumption that since copper xanthate in molecular, multilayer, or bulk form exhibits Auger peaks at energies clearly different from those of the sulfide substrate (approximately 1 eV), then any adsorption must result in a discernible shift in the Auger electron energies. A new component in the S 2p spectrum from chalcocite in this potential range was attributed to an impurity or to a xanthate decomposition product [6]. By contrast, Buckley and Woods [7] interpreted the negligible difference in the Cu(LMM) Auger spectrum for chalcocite before and after treatment with diethyl dithiophosphate under chemisorpfion conditions as being consistent with submonolayer adsorption. They also considered this finding as confirmation that deposition of molecular or multilayer copper dithiophosphate had not occurred. Chemisorption of the collector was revealed not only by a shifted S 2p component but also by the presence of a P 2p peak. More recently, Contini et al. [-8] observed no change in the Cu(LMM) spectrum from chalcocite before and after immersion for 15 min in 10 -5 mol dm -3 5-methyl-2-mercaptobenzazole, whereas the S 2p and N ls spectra provided a clear indication of adsorption. Contini et al. accepted that these
observations could be explained in terms of submonolayer chemisorption, but would not exclude the possibility that a change in the Cu(LMM) spectrum was not evident because of sensitivity problems they considered could arise from the number of copper atoms linked to the collector being small compared with the number in the mineral surface. If it is accepted that thiol collectors chemisorb to the metal atoms in sulfide surfaces, then in the case of a collector such as 5-methyl-2mercaptobenzazole chemisorbed on chalcocite, the number of copper atoms involved would not be less than the number of sulfur atoms in the collector. Thus, the ratio of copper atoms interacting with the collector to copper atoms interacting only with neighbouring lattice atoms would be equal to or greater than the ratio of collector sulfur atoms to lattice sulfur atoms. Consequently, while any new Cu(LMM) component that might arise as a result of the chemisorption could be slightly less evident than the S 2p component from the adsorbed collector because of masking considerations, it should not be sufficiently less evident to make the situation unclear. The component from the collector in the S 2p spectrum reported by Contini et al. [-8] for treatment in 10 -5 mol dm -3 solution accounted for 30% of the intensity, so that any new component in the Cu(LMM) spectrum arising from the adsorption should have been quite obvious. With respect to the question of masking being able to impair the sensitivity of conventional (K~ X-ray excited) substrate core electron spectra to reveal the effects of adsorbate interaction, there are now numerous reports of substrate core electron spectra being evident after coverage by various organo-sulfur compound monolayers having thickness significantly greater than that corresponding to xanthate flotation collectors. In one recent study, it was found that gold peaks remained detectable from 2 nm thick, well-packed monolayers of derivatised resorcin(4)arenes selfassembled on gold [ 10]. Imperceptible substrate core level binding energy shifts on collector chemisorption have also been observed with the galena/xanthate system. Buckley and Woods [-9,11] found that the Pb 4f spectra for unoxidised galena fracture surfaces, before and after submonolayer adsorption of ethyl or amyl
A.N. Buckler, R. Woods~Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 295 305
xanthate, were indistinguishable, despite clear evidence in the S 2p spectrum for chemisorbed xanthate. The Pb 4f binding energies for molecular (multilayer or bulk} lead xanthate are about 1 eV greater than those for galena [4,5,9,11], so that the presence of this species at the surface of galena would be discernible from the Pb 4f spectrum. It was argued [4,5] however, that because of the relatively low coverage, there would have been insufficient sensitivity to establish unequivocally that no shifted Pb 4f component resulted from xanthate adsorption. In order to counter this contention, a subsequent XPS investigation of the submonolayer chemisorption of n-butyl xanthate on galena fracture surfaces was carried out with monochromatised X-rays and an electron take-off angle of 15:' [ 12]. The use of a low electron takeoff angle increased the surface sensitivity of the measurements, and the use of the longer chain xanthate reduced the differential effect of masking on the S 2p and Pb 4f components arising from the adsorbed collector. Once again, the Pb 4f spectrum for the mineral treated with collector was found to be indistinguishable from that for a fresh fracture surface, whereas the S 2p component arising from the adsorbed xanthate accounted for 35% of the total S 2p intensity [12]. Taking into account the very high signal-to-noise ratio for the Pb 4f spectrum, it was asserted that this observation confirmed a negligible substrate core level binding energy shift on chemisorption. Nevertheless, there has been reluctance for the general proposition that chemisorption could lead to a negligible shift in substrate core electron energies to be accepted. For this reason, the general basis for the hypothesis that chemisorption might not necessarily have a detectable impact on substrate core electron energies will be examined here by consideration of previous studies of chemisorption from the vapour phase.
2. Substrate core electron binding energy shifts upon chemisorption Until 1975, it was generally believed that all substrate core level binding energy shifts resulting from chemisorption were less than 0.3 eV [13 15].
297
Metal surface core level shifts were thought to be very small even for the adsorption of oxygen. One system forming the basis of this conclusion was oxygen adsorption on tungsten, notwithstanding the observed W 4f shift of over 4 eV for bulk WO,~ [15]. Such an assessment probably resulted from a failure to appreciate the relatively low surface sensitivity of conventional XPS for low binding energy electrons. Barrie and Bradshaw [ 16], however, used monochromatised radiation and electron take-off angles as low as 7.5 to show that the W 4f shift for the outermost substrate layer on chemisorption of a monolayer of oxygen was in fact 0.9 eV. Thus, although it had been established that metallic substrate core level shifts were not always small, it was found subsequently, from spectra obtained with synchrotron radiation, that such shifts are indeed very small for some systems. Even highly electropositive or electronegative adsorbates, such as potassium or fluorine, on metallic substrates can give rise to substrate core electron binding energy shifts of less than 0.2 eV [17,18]. When considering substrate shifts, any surface core level shifts in the absence of chemisorption must be taken into account. For example, a clean Ta(110) surface gives rise to a Ta 4f component at approximately +0.35 eV relative to the bulk Ta 4f value [18]. On adsorption of up to a monolayer of an alkali metal, the surface core level shift is increased by less than 0.05 eV [ 18]. In the case of tungsten, the clean W ( l l 0 ) surface W 4f shift (approximately - 0 . 3 5 eV)is decreased by less than 0.05 eV on adsorption of up to a monolayer of alkali metal [18]. Giirer et al. [19] found that the clean Pd(100) surface Pd 3d component is shifted +0.4 eV relative to the bulk level, whereas the surface shift with an oxygen overlayer is increased slightly to +0.55 eV. By contrast, in an investigation of CO adsorption on a palladium {100) surface, Andersen et al. [201 observed the clean surface Pd 3d shift to be --0.43 eV while the shift for Pd atoms bonded to a single CO atom was +0.5 eV (i.e. nearly compensating for the surface shift) relative to the bulk on submonolayer adsorption. The shift for Pd atoms bonded to two CO molecules was about 1 eV [20]. In the case of semiconductors, clean surface core level shifts of the order of 0.3 eV are typical. An
298
A.N. Buckley, R. Woods~Colloids Surjklces A." Physicochem. Eng. Aspects 104 (1995) 295 305
In 4d shift of +0.27eV and an As 3d shift of - 0 . 3 0 e V have been observed for an undoped n-type InAs (110) surface prepared by cleavage in situ, which are values similar to those observed for other II1-V semiconductor (110) surfaces [21]. It is also common for surface reconstruction to take place, even before adsorption, so that there may be more than one surface state having an energy different from that of the bulk [22-25]. For example, two surface Si 2p levels at - 0 . 2 e V and + 0.34 eV are observed at the reconstructed, clean S i ( l l l ) surface [23]. After adsorption of a monolayer of Na, the Si 2p component near +0.32 eV is the only surface state remaining. Thus adsorption of an alkali metal results in only a very small change in the Si 2p spectrum. Similarly small changes in the Si 2p spectrum on adsorption of a submonolayer of A1 have been observed [25]. The situation is further complicated by the fact that not all materials exhibit core level shifts for the clean surface. Lead sulfide, for example, exhibits no measurable surface shifts [26 28], behaviour that Trafas et al. [28] maintain is consistent with the minimal relaxation and charge redistribution expected for the surface of a material with rocksalt structure. Evaporation of Cr, Co, Pd, Au and In onto cleaved (100) PbS surfaces was found to induce both Pb 5d and S 2p binding energy shifts, even for submonolayer coverage, but these shifts were interpreted as being associated with disruption of the substrate surface [28]. For some materials, surface shifts may be present for the non-metal core levels but absent for the metal core levels. With TiN and TiC, for example, surface shifts of 0.57eV for N ls and 0.26eV for C ls were observed, but no surface-shifted Ti 2p level could be detected in either case [29]. Thus it can be concluded that not all materials exhibit metal atom core electron binding energy shifts for clean surfaces, or for chemisorbed systems, of a magnitude sufficient to be detected in conventional XPS, i.e. of at least 0.2eV. Accordingly, when the core electron spectra for a potential adsorbate are consistent with adsorption, the absence of a detectable substrate core electron energy shift does not necessarily indicate that submonolayer chemisorption has not occurred. In principle, there is no reason to expect that this
situation for solid/vapour adsorbate systems should be any different from that for adsorption at the solid/solution interface, although adsorption of a particular species from solution will not necessarily lead to the same adsorbate structure as adsorption from the gas phase. To further explore aqueous adsorption systems, an investigation of the interaction of ethyl xanthate with silver metal and silver sulfide at pH 9 has been undertaken. Silver/xanthate and silver sulfide/xanthate are appropriate model systems for an XPS study of collector chemisorption. Electrochemical studies [30] supported by infrared [ 31,32] investigations of the silver/xanthate system in alkaline and unbuffered solutions have shown that chemisorption precedes silver xanthate formation and have clearly defined the potential regions for monolayer and multilayer adsorption. Furthermore, pure silver metal is essentially nonreactive towards molecular oxygen (apart from adsorption) under ambient conditions [33,34], and, therefore, it is feasible to compare the chemisorption of collector on the unoxidised metal with that on the unoxidised sulfide. Moreover, the kinetic energy of Ag(MNN) Auger electrons is approximately 350 eV, and hence the information obtained through these electrons relates to a surface layer thinner than that derived from Pb 4f or C u ( L M M ) electrons (which have kinetic energies greater than 900 eV). This additional surface sensitivity should offset the masking effect of molecules adsorbed on the metal atoms, relative to the sensitivity associated with S 2p electrons which have a kinetic energy greater than 1 keV when ejected by Mg K s X-rays. It should be noted that for silver compounds, information is usually obtained from the Ag(MNN) electrons rather than Ag 3d electrons because of the greater sensitivity of the former to changes in chemical environment [35]. Infrared studies have shown ethyl and octyl xanthate adsorb on sulfidised silver surfaces to produce adsorbates similar to those on silver metal [31 ]. This suggests that the electrochemical characteristics of the interaction of xanthate with silver sulfide are analogous to those for the corresponding reaction with silver metal. Experiments have been carried out in the present work to confirm this presumption, and to delineate the relevant
A.N. Buckley, R. Woods~Colloids Surjaces A: Physicochem. En~. Aspects 104 (1995) 295 305
potential regions for the silver sulfide/xanthate system. The interaction of thiol collectors with silver sulfide is also of some practical interest, as complex sulfide ores often contain low concentrations of silver sulfide which usually report to the lead flotation concentrate.
3. Experimental details
3.1. Silver sulfide preparation Silver sulfide surfaces were prepared by anodic polarization of silver electrodes at - 0 . 2 5 V for 15 h in 0.01 mol dm 3 sodium sulfide in 0.05 mol dm 3 sodium tetraborate solution. This resulted in a thick sulfide coating having an even black appearance. Silver sulfide produced in this way has a high electronic to ionic conductivity ratio [36] and is free from the minor elements typical of natural mineral sulfides.
3.2. Electrochemical measurements Voltammetric investigations were carried out at 2 1 C in 0.05 mol dm -3 sodium tetraborate solution containing 10 3 mol dm -3 potassium ethyl xanthate that had been de-oxygenated with a stream of pure nitrogen. A PAR 173 potentiostat programmed with a Utah model 0151A sweep generator was used to control the potential, and currents were recorded on a Yew type 3086 X-Y recorder. Potentials were measured against a saturated calomel electrode (SCE) and converted to the standard hydrogen electrode (SHE) scale assuming the SCE has a potential of 0.245 V with respect to the SHE. Fresh surfaces were generated on silver electrodes by abrasion on grade P 1000 silicon carbide paper and were thoroughly rinsed before each experiment. Silver sulfide surfaces were removed from the cell used for their preparation, rinsed thoroughly and immediately transferred to a cell containing de-oxygenated xanthate solution.
299
3.3. X-ray photoelectron spectroscopy X-ray photoelectron and X-ray excited Auger electron spectra of fresh surfaces of pure silver and silver sulfide, before and after adsorption of ethyl xanthate at pH 9, were obtained with a Vacuum Generators ESCA 3 spectrometer at an analyser pass energy of 20eV and a slit width of 2 ram. Under these conditions, the 3d52 photoelectron peak for metallic silver had a full width at half maximum height of 0.9 eV. Nonmonochromatised X-rays from a Mg source operated at 10 kV and 10 mA were used. Spectra were determined with the specimen in an analyser vacuum of 10 ~ Pa, and are shown normalised by the difference between the maximum and minimum number of counts. Binding energies are given relative to a value of 84.0 eV for the Au 4f72 peak from metallic gold. Specimens treated in xanthate solutions were subsequently washed with distilled water and dried in a nitrogen-flushed glove-box attached to the introduction chamber of the spectrometer. These specimens, of area approximately 0.5 cm 2, were cooled to 150 K before the pressure was reduced below 10- 3 Pa, and maintained at this temperature while the spectra were being recorded, in order to avoid any volatilization or decomposition of the adsorbed xanthate. Silver surfaces were prepared by abrasion of specimens on silicon carbide paper and pure silica, before being washed with water and dried under nitrogen. Electrochemically prepared silver sulfide surfaces were washed and dried in a similar way.
4. Results and discussion
4.1. Interaction of xanthate with sih~er and sih:er su!lide electrode surfaces Ethyl xanthate chemisorbs on silver electrodes to form a monolayer as shown in Eq. (1) before silver xanthate develops [30,32] through: Ag+EX
--*AgEX+e
(3)
where EX is C 2 H s O C S 2. This is illustrated by the voltammograms in the upper part of Fig. 1: these voltammograms were recorded for scans taken
A.N. Buckley, R. Woods~ColloidsSurfaces A: Physicochem. Eng. Aspects 104 (1995) 295-305
300
I
I
potential limit. These features can be assigned to chemisorption and bulk silver xanthate formation, respectively, as for silver metal. Thus, as with silver, it is possible to prepare silver sulfide surfaces on which chemisorbed xanthate is present alone and on which silver xanthate is co-deposited. It can be seen from Fig. 1 that the difference in the potential at which silver xanthate forms on the metal and sulfide surface is approximately 0.04 V. A shift to higher potentials of 0.207 V would be expected if the process giving rise to silver xanthate on silver sulfide were:
I
Ag
I 0
Ag2S+2EX
I
-0.6
-0.4
I
I
-0.2 0 Potential / V vs SHE
0.2
Fig. 1. Cyclic voltammograms determined at 20 mV s 1 for silver (Ag) and sulfidised silver (Ag2S) electrodes in pH 9.2 solution containing 10 .3 tool dm -3 ethyl xanthate taken to different upper potential limits. Bars correspond to 10 gA cm 2.
from - 0 . 4 6 V to different upper potential limits. The initial anodic peak at approximately - 0 . 2 V corresponds to chemisorption, see Eq. (1), while the current above approximately - 0 . 1 V arises from bulk compound formation on the chemisorbed monolayer. These processes are reversed on the return scan to yield corresponding cathodic peaks. It is apparent from these voltammograms that it is possible to prepare surfaces electrochemically on which only chemisorbed xanthate is present and on which both the monolayer and bulk silver xanthate are deposited. Cyclic voltammograms for the silver sulfide electrode in the presence of xanthate are also shown in Fig. 1. It can be seen that interaction of xanthate with this electrode gives rise to voltammograms of a similar form to those for silver metal, being characterised by an anodic prewave and a subsequent increase in current close to the upper
--*2AgEX + S°+2e -
(4)
The method of formation of silver sulfide used in the present work, that of electrochemical sulfidisation of a silver substrate, is expected to result in the fl-form of silver sulfide containing excess silver [37]. The silver activity in such silver-rich material is greater than that in stoichiometric silver sulfide. The activity is close to that of the metal when the silver/sulfur ratio reaches 2.002:1, the upper limit of the homogeneity range of fl-Ag2+~S [38]. Thus, a much smaller potential shift than that predicted by Eq. (4) is to be expected. The reaction forming silver xanthate is then more appropriately represented as: Ag2+~S + x E X - --~Ag2+~_~S + xAgEX + xe
(5) The electron spectra for the deposited silver sulfide layer were not discernibly different from those for bulk silver sulfide prepared by precipitation under nitrogen and examined under the same spectrometer conditions [39]. At none of the silver surfaces sulfidised in the sodium sulfide solution was a N a ( K L L ) X-ray excited Auger electron peak observed, confirming the absence of occluded sulfide or polysulfide. The measured values for the Ag 3d5/2, S 2p3/2 and Ag(M4N4.sN4,s) binding and kinetic energies are given in Table 1. The observed Ag 3d5/2 binding energy and Ag(M4N4,sN4,5) kinetic energy for silver are in good agreement with published data [35]. Previously reported values for AgzS are listed in Table 2 together with those for Ag/EX. The energies obtained in the
A.N. Buckley, R. Woods~ColloidsSurflwes A." Physicochem. Eng. Aspects 104 (1995) 295 305
301
Table I Ag 3d and S 2p binding energies, relative to Au 4f7/2-84.0 eV, and Ag{MNN) kinetic energies for silver and silver sulfide substrates before and after treatment with ethyl xanthate at pH 9 System
Photoelectron or Auger electron level
Binding or kinetic energy for clean surface (eV)
Binding or kinetic energy for monolayer leVI
Binding or kinetic energy ['or multilayer leVI
Ag /EX
Ag 3d5,e Ag(M4Na.sN4,51 S 2p3,z Ag 3d5,2 Ag( MaNa,sN
368.2 357.9
368.2 357.9 162.2 368.0 357.0 162.3
368. I 355.7 162.3 36~. I 355.9 162.4
Ag,S/EX
368.0 357.0 161.3
Table 2 Previously published values for Ag 3d and S 2p binding energies and Agl MNNJ kinetic energies, relative to An 4t- := 84.0 cV, for silver sulfide and the Ag/EX system Surface
Ag 3d5/2 (eV)
Ag( M4N4,sNa.s} leV)
S 2p3,z (eVI
Reference
AgeS : AgeS Ages AgeS Ag2S AgEX Ag'EX (multilayer) Ag~EX (monolayerl
368.0 368.2 367.8 368.8 368.2 368.0 368.0 368.2
357.0 356.8 356.5
161.3
[ 39 ] [40] [41 ] [421 [43] [44]
356.2 356.2 358.0
present w o r k are in r e a s o n a b l e a g r e e m e n t with the t a b u l a t e d data.
4.2. X-ray photoelectron spectra of xanthate treated silver smJ'aces The A g ( M N N ) spectra from an u n t r e a t e d silver electrode, an electrode held for 5 min at - 0 . 0 5 V in 10 4 mol d m 3 x a n t h a t e solution, a n d an electrode held for 5 min at + 0 . 0 5 V in 10 4 mol d m 3 x a n t h a t e solution are shown in Fig. 2 spectra (al-{c), respectively. T r e a t m e n t of silver at - 0 . 0 5 V in such a x a n t h a t e solution results in close to m o n o l a y e r c o v e r a g e of x a n t h a t e c h e m i s o r b e d to the surface, whereas t r e a t m e n t at + 0 . 0 5 V p r o duces bulk silver x a n t h a t e in a d d i t i o n to the chemisorbed m o n o l a y e r [ 3 0 ] . It can be seen from Fig. 2 spectra (a) a n d (b) that, a p a r t from a slightly higher b a c k g r o u n d level on the low kinetic energy side of the s p e c t r u m for the x a n t h a t e - t r e a t e d surface,
161.2 161.9 161.4 162.0 162.0 161.5
[44] [44]
which is consistent with the presence of an overlayer, the two spectra are the same within experimental uncertainty. There is no evidence for m u l t i l a y e r silver xanthate, which w o u l d be most clearly revealed in the trough between the MsN4,sN4.s and M4N4.sN4.s peaks in the 356 eV kinetic energy region. By contrast, peaks from both the s u b s t r a t e a n d bulk silver x a n t h a t e are a p p a r e n t in the A g ( M N N ) s p e c t r u m from the surface treated at + 0 . 0 5 V. The Ag 3d s p e c t r u m for the surface b e a r i n g the c h e m i s o r b e d m o n o l a y e r was the same as that for the clean metal surface a p a r t from a slightly higher b a c k g r o u n d on the high b i n d i n g energy side of the former spectrum. The linewidth of the Ag 3d peaks from the surface treated at + 0 . 0 5 V was a b o u t 0.15 eV greater than that for the surface treated at - 0 . 0 5 V, but the s p e c t r u m was shifted by at most 0.1 eV to lower b i n d i n g energy. These o b s e r v a t i o n s are consistent with a m u l t i l a y e r coverage of silver
A.N. Buckley, R. Woods/Colloids Surfaces A: Physic•chem. Eng. Aspects 104 (1995) 295-305
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in Section 2, the absence of a discernible shift is consistent with monolayer chemisorption.
4.3. X-ray photoelectron spectra of xanthate treated silver sulfide surfaces The A g ( M N N ) spectra from a clean sulfidised silver electrode (i.e. silver sulfide surface), a similar electrode maintained for 2 min at - 0 . 0 5 V in 10 4 mol dm -3 xanthate solution, and one held for 5 rain at +0.15 V in 10 4 mol dm -3 xanthate solution are shown in Fig. 4 spectra (a)-(c), respectively. It was established in the voltammetric experiments described in Section 4.1 that, as in the case of silver metal, treatment at - 0 . 0 5 V should have resulted in close to monolayer coverage of
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I
•
eo
•
364
o
oo
(eV)
Fig. 4. Ag(MNN) spectra from a sulfidised silver electrode (a) untreated, (b) treated at -0.05 V for 2 min, and {c) treated at +0.15 V for 5 min, in 10 4 mol dm 3 xanthate solution at pH 9. (a}
c h e m i s o r b e d xanthate, a n d t r e a t m e n t at + 0 . 1 5 V should have p r o d u c e d bulk silver xanthate. The S 2p s p e c t r u m from the surface treated at - 0 . 0 5 V (Fig. 5 s p e c t r u m (b)) confirms the presence of xanthate. The C ls a n d O ls spectra were also consistent with x a n t h a t e a d s o r b e d at the treated surface. The S 2p s p e c t r u m can be fitted with a c o m p o n e n t shifted 1.0 eV to higher b i n d i n g energy from the silver sulfide position, a n d a c c o u n t i n g for a b o u t 30% of the total S 2p intensity. A s s u m i n g that b o t h sulfur a t o m s in each a d s o r b e d x a n t h a t e radical would interact with one silver a t o m of each Ag2S molecule in the silver sulfide surface, a xanthate c o m p o n e n t in the S 2p s p e c t r u m of the intensity o b s e r v e d s h o u l d c o r r e s p o n d to a c o m p o nent in the s u b s t r a t e A g ( M N N ) s p e c t r u m a c c o u n t ing for at least 20% of its intensity if, as expected,
__ [ 170
• e
_]-168
I
h
l
]
i
166
164
162
l~0
158
BINDING
ENERGY
leV~
Fig. 5. S 2p spectra from a sulfidised sih'er electrode (at untreated, (b) treated at -0.05V for 2 rain, and (c) treated at +(I.15 V for 5 min. in 10 4 tool dm 3 xanthate solution at pH 9. the a d d i t i o n a l surface sensitivity of the A g l M N N ) electrons c o m p a r e d with the S 2p electrons c o m p e n s a t e s for any m a s k i n g by x a n t h a t e of the silver a t o m s to which the x a n t h a t e is attached. The c o r r e s p o n d i n g A g ( M N N ) s p e c t r u m (Fig. 4 s p e c t r u m (b}/, however, is the same within experim e n t a l u n c e r t a i n t y as the s p e c t r u m from silver sulfide (Fig. 4 s p e c t r u m ( a l l This o b s e r v a t i o n con-
304
A.N. Buckley, R. Woods/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 295 305
firms the absence of multilayer silver xanthate, since the shift that would be associated with the presence of multilayer silver xanthate is 1.1 eV (Table 1), and a component shifted by this value would be clearly visible. In addition, this observation establishes that any core electron shift on chemisorption must be small. An Ag(MNN) component of the magnitude expected from the S 2p spectrum should be discernible if it were shifted by more than 0.2 eV. The Ag 3d spectrum from the silver sulfide surface bearing chemisorbed xanthate was indistinguishable from that for the clean sulfidised silver surface, whereas the linewidth of the Ag 3d peaks from the sulfide surface covered with bulk silver xanthate was 0.2 eV greater than for the clean sulfidised silver surface (and approximately 0.4 eV greater than that for silver metal). These observations are also consistent with the presence of chemisorbed xanthate at the surface of the sulfidised silver treated at - 0 . 0 5 V. The Ag(MNN), S 2p, C ls and O ls spectra for the sulfidised silver surface treated at +0.15 V all indicate that a relatively thick overlayer of silver xanthate had been formed. In particular, only a very small shoulder from the underlying silver sulfide remained near 161 eV on the low binding energy side of the S 2p spectrum (Fig. 5 spectrum (c)) and no substrate component was evident in the Ag(MNN) spectrum (Fig. 4 spectrum (c)).
5. Concluding remarks There is a significant body of evidence establishing that core level energy shifts upon chemisorption from the vapour phase can be too small to be resolved by conventional XPS. Thus, the previously reported absence of a shift for chemisorption of thiol collectors from the aqueous phase for sulfide minerals is quite reasonable. The model systems silver/ethyl xanthate and silver sulfide/ethyl xanthate have been shown here to behave similarly. In particular, no shift in the Ag(MNN) spectrum for either system was detected when xanthate was chemisorbed on a surface. This contrasts with shifts of greater than 1 eV for silver xanthate on these two substrates. Such a difference
in spectral behaviour provides a means of distinguishing chemisorption from multilayer adsorption of thiol collectors on sulfde mineral surfaces.
Acknowledgement The authors are grateful to Dr Giorgio Contini, Consiglio Nazionale delle Ricerche Istituto per il Trattamento dei Minerali, Rome, for a preprint of Ref. [8].
References [1] A.M. Gaudin, Flotation, McGraw-Hill, New York, 1957. [2] R. Woods and P.E. Richardson, in P. Somasundaran (Ed.), Advances in Mineral Processing: A Half-Century of Progress in Applications of Theory to Practice, Society of Mining Engineers, Littleton, CO, 1986, pp. 154-170. [3] P. Nowak, Colloids Surfaces A: Physicochem. Eng. Aspects, 76 (1993) 65 72. I-4] K. Laajalehto, P. Nowak, A. Pomianowski and E. Suoninen, Colloids Surfaces, 57 (1991) 319 333. 1-5] K. Laajalehto, P. Nowak and E. Suoninen, Int. J. Miner. Process., 37 (1993) 123 147. 1-6] J.A. Mielczarski, J.B. Zachwieja and R.-H. Yoon, SME Ann. Meeting, Salt Lake City, Utah, February 1990, Preprint No. 90-174. [7] A.N. Buckley and R. Woods, J. Electroanal. Chem., 357 (1993) 387-405. [8] G. Contini, K. Laajalehto, E. Suoninen and A.M. Marabini, J. Colloid Interface Sci., 171 (1995) 234 239. 1-9] A.N. Buckley and R. Woods, Int. J. Miner. Process., 28 (1990) 301-311. [10] E.U.T. van Velzen, J.F.J. Engbersen and D.N. Reinhoudt, J. Am. Chem. Soc., 116 (1994) 3597 3598. [11] A.N. Buckley and R. Woods, Colloids Surfaces, 53 (1991) 33-45. [12] A.V. Shchukarev, I.M. Kravets, A.N. Buckley and R. Woods, Int. J. Miner. Process., 41 (1994) 99-114. [13] J.T. Yates, Jr. and N.E. Erickson, Surf. Sci., 44 (1974) 489-514. [14] J.C. Fuggle and D. Menzel, Chem. Phys. Lett., 33 (1975) 37 40. [15] C.R. Brundle, J. Electron Spectrosc., 5 (1974) 291 319. [16] A. Barrie and A.M. Bradshaw, Phys. Lett., 55A (1975) 306 308. [17] P.S. Bagus and G. Pacchioni, Phys. Rev. B, 48 (1993) 15 262 15 273. [18] A.B. Andrews, D.M. Rifle and G.K. Wertheim, Phys. Rev. B, 49 (1994) 8396 8401. [19] E. Gt~rer, K. Klier and G.W. Simmons, Phys. Rev. B, 49 (1994) 14 657-14 665.
A.N. BuckhT, R. Woods~Colloids Surjaces A." Physicochem. Eng. A.~pects 104 (1995) 2(.U 305 [20] J.N. Andersen, M. Qvarford, R. Nyholm, S.L. Sorensen and C. Wigren, Phys. Rev. Lett., 67 (1991) 2822-2825. [21] J.N. Andersen and U.O. Karlsson, Phys. Rev. B, 41 ( 19901 3844 3846. [22] E. Landemark. C.J. Karlsson, L.S.O. Johansson and R.I.G. Uhrberg, Phys. Rev. B, 49 (1994) 16 523 16 533. [23] P.S. Mangat, P. Soukiassian, Y. Huttel and Z. Hurych, J. Vac. Sci. Technol. B, 12 (1994) 2694 2698. [24] M.L. Shek, P.M. Stefan, C. Binns, I. Lindau and W.E. Spicer, Surf. Sci., 115 11982) L81-L85. [25] J.N. Andersen, C. Wigren and U.O. Karlsson, J. Vac. Sci. Technol. B, 9[199112384 2387. [26] G. Paolucci and K.C. Prince. Phys. Rev. B, 41 (19901 385l 3853. [27] h Kartio, K. Laajalehto and E. Suoninen, Colloids Surfaces A: Physicochem. Eng. Aspects, 93 (1994) 149 158. [28] B.M. Trafas, I.M. Vitomirov, C.M. Aldao, Y. Gao, F. Xu, .hH. Weaver and D.L. Partin, Phys. Rev. B, 39 {1989) 3265 3273. [29] El. Johansson, H,I.P. Johansson and K.L. H&kansson, Phys. Rev. B, 48 (1993) 14 520 14 523. [30] R. Woods, C.I. Basilio, D.S. Kim and R.-H. Yoon, ,h Flectroanal. Chem., 328 (1992) 179. [31] A. lhs and B. Liedberg, Langmuir, 10 (1994) 734 740. [32] P. Talonen, J. Rastas and J. Leppinen, Surf. Interface Anal., 17 ( 1991 } 669 674.
31)5
[33] A.W. Czanderna, J. Phys. Chem., 68 11964) 2765 2772. [34] R.B, Grant and R.M. Lambert, Surf. Sci_ 146 119841 256 268. [35] D. Briggs and M.P. Seah (Eds.), Practical Surthce Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester (UKI, 1983, p. 50L [36] M. Koebel. N. Ibl and A.M. Frei, Electrochim. Acla, 19 (1974) 287 295. [37] G.P. Power, I.M. Ritchie and M.T. Wylie, Electrochim. Acta, 26 (19811 1633-1638. [38] H. Schmalzried, Prog. Solid State Chem., 13 11980) 119 157. [39] A.N. Buckley, H.J. Wouterlood and R, Woods, ,1. Appl. Electrochcm., 19 (1989) 744 751. [40] M. Romand, M. Roubin and J.P. l)eloume, J. Electron Spectrosc., 13 (19781 229 242. [41] N.H. Turner, J.S. Murday and D.E. Ramaker, Anal. Chem., 52 (19801 84 92. [42] L.J. Gerenser, K.E. Goppert-Berarducci, R.C Baetzold and J.M. Pochan, J. Chem. Phys., 95 {1991 ) 4641 4649. [43] T.M.H. Saber and A.A. E1 Warraky, Br. Corros. ,I., 23 (1988) 131 133. [44] J.O. Leppinen, R-H. Yoon and J.A. Mielczarski, Colloids Surfaces, ~1 {1991} 189 203.
AND E L S E V INN
Colloids and Surfaces A: Physicochemical and Engineering Aspects 104 { 19951295 305
A
SURFACES
Identifying chemisorption in the interaction of thiol collectors with sulfide minerals by XPS: adsorption of xanthate on silver and silver sulfide Alan N. Buckley a,* Ronald Woods
b
CSIRO Di~,ision of Coal and Energy Technolog)', PO Box 136. North R)'de, N.S.W. 2113. Australia b CSIRO Dil~ision of Minerals. PO Bo.~c 124, Port M~,lhourlle. flit. 3207, Australia Received 17 March 1995; accepted 15
Miiy1995
Abstract The adsorption of ethyl xanthate on silver and sulfidised silver surfaces at controlled potentials has been investigated by X-ray photoelectron spectroscopy (XPS). Silver/xanthate and silver sulfide/xanthate were chosen as model systems for investigating the identification of chemisorption of thiol collectors on sulfide minerals by means of core electron energy shifts. There was no discernible effect from submonolayer adsorption on either substrate on the corresponding A g ( M N N t spectrum, even when the presence of adsorbed xanthate was clearly evident from the S 2p spectrum. It was confirmed that the Ag(MNN) electron energies for multilayer silver xanthate were significantly different from those for silver and silver sulfide. The basis for the general proposition, that chemisorption of thiol collectors oll sulfide mineral surfaces might not necessarily give rise to discernible substrate core electron energy shifts, was examined in the context of previously reported investigations of chemisorption of adsorbates from the vapour phase. It was concluded from this appraisal, and from the experimental results for the interaction of xanthate with silver and silver sulfide, that substrate core electron energy shifts on chemisorption are no! always large enough to be detectable by conventional XPS. By contrast, core electron energy shifts for multilayer metal-collector compound formation are clearly discernible. Kerwords: Adsorption: Chemisorption: Sulfide minerals: Thiol: Xanthate: X-ray photoelectron spectroscopy: Collectors
1. Introduction The beneficiation of sulfide ores by flotation is usually effected by i m p a r t i n g h y d r o p h o b i c i t y to the m i n e r a l c o n s t i t u e n t to be c o n c e n t r a t e d by m e a n s of a thiol collector. The a m o u n t s of collector required for this p u r p o s e are usually equivalent to less than m o n o l a y e r coverage of the surface of the particles [ 1 ] . Electrochemical evidence indicates [ 2 ] that, u n d e r typical flotation conditions, thiol * Corresponding author. 0927-7757/95,,'$09.50 ,c9 1995 Elsevier Science B.V. All rights reserved SSDI (/927-7757(95)03279-7
collectors are c h e m i s o r b e d to the metal a t o m s in the surface layer of a sulfide mineral by an a n o d i c reaction: X
--~Xad s -~- e
(I}
with a c o m p l e m e n t a r y c a t h o d i c process: 1/202 + H 2 0 + 2 e
~2OH
{2)
occurring at n e a r b y sites on the surface. S p e c t r o s c o p i c techniques have been a p p l i e d by a n u m b e r of w o r k e r s to characterise the nature of the a d s o r b e d layer, but there has not been general
296
A.N. Buckle),, R. Woods~Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 295 305
agreement as to the interpretation of spectra in relation to chemisorption. Although numerous infrared spectroscopic studies have been carried out on xanthate/sulfide systems, the assumptions made in interpreting the spectra have been questioned recently [-3-5]. X-ray photoelectron spectroscopy (XPS) has also been used to investigate several sulfide mineral/thiol collector systems under conditions which should have limited adsorption to submonolayer coverage [6-9]. In most cases the electron spectra have revealed imperceptible substrate core level shifts resulting from such adsorption, despite clear evidence for a new sulfur species at the surface, but this observation has been explained in different ways. Mielczarski et al. [-6] concluded that the absence of any significant change in the Cu(LMM) Auger spectrum for chalcocite before and after interaction with ethyl xanthate within the potential range where voltammetry indicates chemisorption to have taken place, was evidence for adsorption of xanthate n o t having occurred. Such an argument is based on the assumption that since copper xanthate in molecular, multilayer, or bulk form exhibits Auger peaks at energies clearly different from those of the sulfide substrate (approximately 1 eV), then any adsorption must result in a discernible shift in the Auger electron energies. A new component in the S 2p spectrum from chalcocite in this potential range was attributed to an impurity or to a xanthate decomposition product [6]. By contrast, Buckley and Woods [7] interpreted the negligible difference in the Cu(LMM) Auger spectrum for chalcocite before and after treatment with diethyl dithiophosphate under chemisorpfion conditions as being consistent with submonolayer adsorption. They also considered this finding as confirmation that deposition of molecular or multilayer copper dithiophosphate had not occurred. Chemisorption of the collector was revealed not only by a shifted S 2p component but also by the presence of a P 2p peak. More recently, Contini et al. [-8] observed no change in the Cu(LMM) spectrum from chalcocite before and after immersion for 15 min in 10 -5 mol dm -3 5-methyl-2-mercaptobenzazole, whereas the S 2p and N ls spectra provided a clear indication of adsorption. Contini et al. accepted that these
observations could be explained in terms of submonolayer chemisorption, but would not exclude the possibility that a change in the Cu(LMM) spectrum was not evident because of sensitivity problems they considered could arise from the number of copper atoms linked to the collector being small compared with the number in the mineral surface. If it is accepted that thiol collectors chemisorb to the metal atoms in sulfide surfaces, then in the case of a collector such as 5-methyl-2mercaptobenzazole chemisorbed on chalcocite, the number of copper atoms involved would not be less than the number of sulfur atoms in the collector. Thus, the ratio of copper atoms interacting with the collector to copper atoms interacting only with neighbouring lattice atoms would be equal to or greater than the ratio of collector sulfur atoms to lattice sulfur atoms. Consequently, while any new Cu(LMM) component that might arise as a result of the chemisorption could be slightly less evident than the S 2p component from the adsorbed collector because of masking considerations, it should not be sufficiently less evident to make the situation unclear. The component from the collector in the S 2p spectrum reported by Contini et al. [-8] for treatment in 10 -5 mol dm -3 solution accounted for 30% of the intensity, so that any new component in the Cu(LMM) spectrum arising from the adsorption should have been quite obvious. With respect to the question of masking being able to impair the sensitivity of conventional (K~ X-ray excited) substrate core electron spectra to reveal the effects of adsorbate interaction, there are now numerous reports of substrate core electron spectra being evident after coverage by various organo-sulfur compound monolayers having thickness significantly greater than that corresponding to xanthate flotation collectors. In one recent study, it was found that gold peaks remained detectable from 2 nm thick, well-packed monolayers of derivatised resorcin(4)arenes selfassembled on gold [ 10]. Imperceptible substrate core level binding energy shifts on collector chemisorption have also been observed with the galena/xanthate system. Buckley and Woods [-9,11] found that the Pb 4f spectra for unoxidised galena fracture surfaces, before and after submonolayer adsorption of ethyl or amyl
A.N. Buckler, R. Woods~Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 295 305
xanthate, were indistinguishable, despite clear evidence in the S 2p spectrum for chemisorbed xanthate. The Pb 4f binding energies for molecular (multilayer or bulk} lead xanthate are about 1 eV greater than those for galena [4,5,9,11], so that the presence of this species at the surface of galena would be discernible from the Pb 4f spectrum. It was argued [4,5] however, that because of the relatively low coverage, there would have been insufficient sensitivity to establish unequivocally that no shifted Pb 4f component resulted from xanthate adsorption. In order to counter this contention, a subsequent XPS investigation of the submonolayer chemisorption of n-butyl xanthate on galena fracture surfaces was carried out with monochromatised X-rays and an electron take-off angle of 15:' [ 12]. The use of a low electron takeoff angle increased the surface sensitivity of the measurements, and the use of the longer chain xanthate reduced the differential effect of masking on the S 2p and Pb 4f components arising from the adsorbed collector. Once again, the Pb 4f spectrum for the mineral treated with collector was found to be indistinguishable from that for a fresh fracture surface, whereas the S 2p component arising from the adsorbed xanthate accounted for 35% of the total S 2p intensity [12]. Taking into account the very high signal-to-noise ratio for the Pb 4f spectrum, it was asserted that this observation confirmed a negligible substrate core level binding energy shift on chemisorption. Nevertheless, there has been reluctance for the general proposition that chemisorption could lead to a negligible shift in substrate core electron energies to be accepted. For this reason, the general basis for the hypothesis that chemisorption might not necessarily have a detectable impact on substrate core electron energies will be examined here by consideration of previous studies of chemisorption from the vapour phase.
2. Substrate core electron binding energy shifts upon chemisorption Until 1975, it was generally believed that all substrate core level binding energy shifts resulting from chemisorption were less than 0.3 eV [13 15].
297
Metal surface core level shifts were thought to be very small even for the adsorption of oxygen. One system forming the basis of this conclusion was oxygen adsorption on tungsten, notwithstanding the observed W 4f shift of over 4 eV for bulk WO,~ [15]. Such an assessment probably resulted from a failure to appreciate the relatively low surface sensitivity of conventional XPS for low binding energy electrons. Barrie and Bradshaw [ 16], however, used monochromatised radiation and electron take-off angles as low as 7.5 to show that the W 4f shift for the outermost substrate layer on chemisorption of a monolayer of oxygen was in fact 0.9 eV. Thus, although it had been established that metallic substrate core level shifts were not always small, it was found subsequently, from spectra obtained with synchrotron radiation, that such shifts are indeed very small for some systems. Even highly electropositive or electronegative adsorbates, such as potassium or fluorine, on metallic substrates can give rise to substrate core electron binding energy shifts of less than 0.2 eV [17,18]. When considering substrate shifts, any surface core level shifts in the absence of chemisorption must be taken into account. For example, a clean Ta(110) surface gives rise to a Ta 4f component at approximately +0.35 eV relative to the bulk Ta 4f value [18]. On adsorption of up to a monolayer of an alkali metal, the surface core level shift is increased by less than 0.05 eV [ 18]. In the case of tungsten, the clean W ( l l 0 ) surface W 4f shift (approximately - 0 . 3 5 eV)is decreased by less than 0.05 eV on adsorption of up to a monolayer of alkali metal [18]. Giirer et al. [19] found that the clean Pd(100) surface Pd 3d component is shifted +0.4 eV relative to the bulk level, whereas the surface shift with an oxygen overlayer is increased slightly to +0.55 eV. By contrast, in an investigation of CO adsorption on a palladium {100) surface, Andersen et al. [201 observed the clean surface Pd 3d shift to be --0.43 eV while the shift for Pd atoms bonded to a single CO atom was +0.5 eV (i.e. nearly compensating for the surface shift) relative to the bulk on submonolayer adsorption. The shift for Pd atoms bonded to two CO molecules was about 1 eV [20]. In the case of semiconductors, clean surface core level shifts of the order of 0.3 eV are typical. An
298
A.N. Buckley, R. Woods~Colloids Surjklces A." Physicochem. Eng. Aspects 104 (1995) 295 305
In 4d shift of +0.27eV and an As 3d shift of - 0 . 3 0 e V have been observed for an undoped n-type InAs (110) surface prepared by cleavage in situ, which are values similar to those observed for other II1-V semiconductor (110) surfaces [21]. It is also common for surface reconstruction to take place, even before adsorption, so that there may be more than one surface state having an energy different from that of the bulk [22-25]. For example, two surface Si 2p levels at - 0 . 2 e V and + 0.34 eV are observed at the reconstructed, clean S i ( l l l ) surface [23]. After adsorption of a monolayer of Na, the Si 2p component near +0.32 eV is the only surface state remaining. Thus adsorption of an alkali metal results in only a very small change in the Si 2p spectrum. Similarly small changes in the Si 2p spectrum on adsorption of a submonolayer of A1 have been observed [25]. The situation is further complicated by the fact that not all materials exhibit core level shifts for the clean surface. Lead sulfide, for example, exhibits no measurable surface shifts [26 28], behaviour that Trafas et al. [28] maintain is consistent with the minimal relaxation and charge redistribution expected for the surface of a material with rocksalt structure. Evaporation of Cr, Co, Pd, Au and In onto cleaved (100) PbS surfaces was found to induce both Pb 5d and S 2p binding energy shifts, even for submonolayer coverage, but these shifts were interpreted as being associated with disruption of the substrate surface [28]. For some materials, surface shifts may be present for the non-metal core levels but absent for the metal core levels. With TiN and TiC, for example, surface shifts of 0.57eV for N ls and 0.26eV for C ls were observed, but no surface-shifted Ti 2p level could be detected in either case [29]. Thus it can be concluded that not all materials exhibit metal atom core electron binding energy shifts for clean surfaces, or for chemisorbed systems, of a magnitude sufficient to be detected in conventional XPS, i.e. of at least 0.2eV. Accordingly, when the core electron spectra for a potential adsorbate are consistent with adsorption, the absence of a detectable substrate core electron energy shift does not necessarily indicate that submonolayer chemisorption has not occurred. In principle, there is no reason to expect that this
situation for solid/vapour adsorbate systems should be any different from that for adsorption at the solid/solution interface, although adsorption of a particular species from solution will not necessarily lead to the same adsorbate structure as adsorption from the gas phase. To further explore aqueous adsorption systems, an investigation of the interaction of ethyl xanthate with silver metal and silver sulfide at pH 9 has been undertaken. Silver/xanthate and silver sulfide/xanthate are appropriate model systems for an XPS study of collector chemisorption. Electrochemical studies [30] supported by infrared [ 31,32] investigations of the silver/xanthate system in alkaline and unbuffered solutions have shown that chemisorption precedes silver xanthate formation and have clearly defined the potential regions for monolayer and multilayer adsorption. Furthermore, pure silver metal is essentially nonreactive towards molecular oxygen (apart from adsorption) under ambient conditions [33,34], and, therefore, it is feasible to compare the chemisorption of collector on the unoxidised metal with that on the unoxidised sulfide. Moreover, the kinetic energy of Ag(MNN) Auger electrons is approximately 350 eV, and hence the information obtained through these electrons relates to a surface layer thinner than that derived from Pb 4f or C u ( L M M ) electrons (which have kinetic energies greater than 900 eV). This additional surface sensitivity should offset the masking effect of molecules adsorbed on the metal atoms, relative to the sensitivity associated with S 2p electrons which have a kinetic energy greater than 1 keV when ejected by Mg K s X-rays. It should be noted that for silver compounds, information is usually obtained from the Ag(MNN) electrons rather than Ag 3d electrons because of the greater sensitivity of the former to changes in chemical environment [35]. Infrared studies have shown ethyl and octyl xanthate adsorb on sulfidised silver surfaces to produce adsorbates similar to those on silver metal [31 ]. This suggests that the electrochemical characteristics of the interaction of xanthate with silver sulfide are analogous to those for the corresponding reaction with silver metal. Experiments have been carried out in the present work to confirm this presumption, and to delineate the relevant
A.N. Buckley, R. Woods~Colloids Surjaces A: Physicochem. En~. Aspects 104 (1995) 295 305
potential regions for the silver sulfide/xanthate system. The interaction of thiol collectors with silver sulfide is also of some practical interest, as complex sulfide ores often contain low concentrations of silver sulfide which usually report to the lead flotation concentrate.
3. Experimental details
3.1. Silver sulfide preparation Silver sulfide surfaces were prepared by anodic polarization of silver electrodes at - 0 . 2 5 V for 15 h in 0.01 mol dm 3 sodium sulfide in 0.05 mol dm 3 sodium tetraborate solution. This resulted in a thick sulfide coating having an even black appearance. Silver sulfide produced in this way has a high electronic to ionic conductivity ratio [36] and is free from the minor elements typical of natural mineral sulfides.
3.2. Electrochemical measurements Voltammetric investigations were carried out at 2 1 C in 0.05 mol dm -3 sodium tetraborate solution containing 10 3 mol dm -3 potassium ethyl xanthate that had been de-oxygenated with a stream of pure nitrogen. A PAR 173 potentiostat programmed with a Utah model 0151A sweep generator was used to control the potential, and currents were recorded on a Yew type 3086 X-Y recorder. Potentials were measured against a saturated calomel electrode (SCE) and converted to the standard hydrogen electrode (SHE) scale assuming the SCE has a potential of 0.245 V with respect to the SHE. Fresh surfaces were generated on silver electrodes by abrasion on grade P 1000 silicon carbide paper and were thoroughly rinsed before each experiment. Silver sulfide surfaces were removed from the cell used for their preparation, rinsed thoroughly and immediately transferred to a cell containing de-oxygenated xanthate solution.
299
3.3. X-ray photoelectron spectroscopy X-ray photoelectron and X-ray excited Auger electron spectra of fresh surfaces of pure silver and silver sulfide, before and after adsorption of ethyl xanthate at pH 9, were obtained with a Vacuum Generators ESCA 3 spectrometer at an analyser pass energy of 20eV and a slit width of 2 ram. Under these conditions, the 3d52 photoelectron peak for metallic silver had a full width at half maximum height of 0.9 eV. Nonmonochromatised X-rays from a Mg source operated at 10 kV and 10 mA were used. Spectra were determined with the specimen in an analyser vacuum of 10 ~ Pa, and are shown normalised by the difference between the maximum and minimum number of counts. Binding energies are given relative to a value of 84.0 eV for the Au 4f72 peak from metallic gold. Specimens treated in xanthate solutions were subsequently washed with distilled water and dried in a nitrogen-flushed glove-box attached to the introduction chamber of the spectrometer. These specimens, of area approximately 0.5 cm 2, were cooled to 150 K before the pressure was reduced below 10- 3 Pa, and maintained at this temperature while the spectra were being recorded, in order to avoid any volatilization or decomposition of the adsorbed xanthate. Silver surfaces were prepared by abrasion of specimens on silicon carbide paper and pure silica, before being washed with water and dried under nitrogen. Electrochemically prepared silver sulfide surfaces were washed and dried in a similar way.
4. Results and discussion
4.1. Interaction of xanthate with sih~er and sih:er su!lide electrode surfaces Ethyl xanthate chemisorbs on silver electrodes to form a monolayer as shown in Eq. (1) before silver xanthate develops [30,32] through: Ag+EX
--*AgEX+e
(3)
where EX is C 2 H s O C S 2. This is illustrated by the voltammograms in the upper part of Fig. 1: these voltammograms were recorded for scans taken
A.N. Buckley, R. Woods~ColloidsSurfaces A: Physicochem. Eng. Aspects 104 (1995) 295-305
300
I
I
potential limit. These features can be assigned to chemisorption and bulk silver xanthate formation, respectively, as for silver metal. Thus, as with silver, it is possible to prepare silver sulfide surfaces on which chemisorbed xanthate is present alone and on which silver xanthate is co-deposited. It can be seen from Fig. 1 that the difference in the potential at which silver xanthate forms on the metal and sulfide surface is approximately 0.04 V. A shift to higher potentials of 0.207 V would be expected if the process giving rise to silver xanthate on silver sulfide were:
I
Ag
I 0
Ag2S+2EX
I
-0.6
-0.4
I
I
-0.2 0 Potential / V vs SHE
0.2
Fig. 1. Cyclic voltammograms determined at 20 mV s 1 for silver (Ag) and sulfidised silver (Ag2S) electrodes in pH 9.2 solution containing 10 .3 tool dm -3 ethyl xanthate taken to different upper potential limits. Bars correspond to 10 gA cm 2.
from - 0 . 4 6 V to different upper potential limits. The initial anodic peak at approximately - 0 . 2 V corresponds to chemisorption, see Eq. (1), while the current above approximately - 0 . 1 V arises from bulk compound formation on the chemisorbed monolayer. These processes are reversed on the return scan to yield corresponding cathodic peaks. It is apparent from these voltammograms that it is possible to prepare surfaces electrochemically on which only chemisorbed xanthate is present and on which both the monolayer and bulk silver xanthate are deposited. Cyclic voltammograms for the silver sulfide electrode in the presence of xanthate are also shown in Fig. 1. It can be seen that interaction of xanthate with this electrode gives rise to voltammograms of a similar form to those for silver metal, being characterised by an anodic prewave and a subsequent increase in current close to the upper
--*2AgEX + S°+2e -
(4)
The method of formation of silver sulfide used in the present work, that of electrochemical sulfidisation of a silver substrate, is expected to result in the fl-form of silver sulfide containing excess silver [37]. The silver activity in such silver-rich material is greater than that in stoichiometric silver sulfide. The activity is close to that of the metal when the silver/sulfur ratio reaches 2.002:1, the upper limit of the homogeneity range of fl-Ag2+~S [38]. Thus, a much smaller potential shift than that predicted by Eq. (4) is to be expected. The reaction forming silver xanthate is then more appropriately represented as: Ag2+~S + x E X - --~Ag2+~_~S + xAgEX + xe
(5) The electron spectra for the deposited silver sulfide layer were not discernibly different from those for bulk silver sulfide prepared by precipitation under nitrogen and examined under the same spectrometer conditions [39]. At none of the silver surfaces sulfidised in the sodium sulfide solution was a N a ( K L L ) X-ray excited Auger electron peak observed, confirming the absence of occluded sulfide or polysulfide. The measured values for the Ag 3d5/2, S 2p3/2 and Ag(M4N4.sN4,s) binding and kinetic energies are given in Table 1. The observed Ag 3d5/2 binding energy and Ag(M4N4,sN4,5) kinetic energy for silver are in good agreement with published data [35]. Previously reported values for AgzS are listed in Table 2 together with those for Ag/EX. The energies obtained in the
A.N. Buckley, R. Woods~ColloidsSurflwes A." Physicochem. Eng. Aspects 104 (1995) 295 305
301
Table I Ag 3d and S 2p binding energies, relative to Au 4f7/2-84.0 eV, and Ag{MNN) kinetic energies for silver and silver sulfide substrates before and after treatment with ethyl xanthate at pH 9 System
Photoelectron or Auger electron level
Binding or kinetic energy for clean surface (eV)
Binding or kinetic energy for monolayer leVI
Binding or kinetic energy ['or multilayer leVI
Ag /EX
Ag 3d5,e Ag(M4Na.sN4,51 S 2p3,z Ag 3d5,2 Ag( MaNa,sN
368.2 357.9
368.2 357.9 162.2 368.0 357.0 162.3
368. I 355.7 162.3 36~. I 355.9 162.4
Ag,S/EX
368.0 357.0 161.3
Table 2 Previously published values for Ag 3d and S 2p binding energies and Agl MNNJ kinetic energies, relative to An 4t- := 84.0 cV, for silver sulfide and the Ag/EX system Surface
Ag 3d5/2 (eV)
Ag( M4N4,sNa.s} leV)
S 2p3,z (eVI
Reference
AgeS : AgeS Ages AgeS Ag2S AgEX Ag'EX (multilayer) Ag~EX (monolayerl
368.0 368.2 367.8 368.8 368.2 368.0 368.0 368.2
357.0 356.8 356.5
161.3
[ 39 ] [40] [41 ] [421 [43] [44]
356.2 356.2 358.0
present w o r k are in r e a s o n a b l e a g r e e m e n t with the t a b u l a t e d data.
4.2. X-ray photoelectron spectra of xanthate treated silver smJ'aces The A g ( M N N ) spectra from an u n t r e a t e d silver electrode, an electrode held for 5 min at - 0 . 0 5 V in 10 4 mol d m 3 x a n t h a t e solution, a n d an electrode held for 5 min at + 0 . 0 5 V in 10 4 mol d m 3 x a n t h a t e solution are shown in Fig. 2 spectra (al-{c), respectively. T r e a t m e n t of silver at - 0 . 0 5 V in such a x a n t h a t e solution results in close to m o n o l a y e r c o v e r a g e of x a n t h a t e c h e m i s o r b e d to the surface, whereas t r e a t m e n t at + 0 . 0 5 V p r o duces bulk silver x a n t h a t e in a d d i t i o n to the chemisorbed m o n o l a y e r [ 3 0 ] . It can be seen from Fig. 2 spectra (a) a n d (b) that, a p a r t from a slightly higher b a c k g r o u n d level on the low kinetic energy side of the s p e c t r u m for the x a n t h a t e - t r e a t e d surface,
161.2 161.9 161.4 162.0 162.0 161.5
[44] [44]
which is consistent with the presence of an overlayer, the two spectra are the same within experimental uncertainty. There is no evidence for m u l t i l a y e r silver xanthate, which w o u l d be most clearly revealed in the trough between the MsN4,sN4.s and M4N4.sN4.s peaks in the 356 eV kinetic energy region. By contrast, peaks from both the s u b s t r a t e a n d bulk silver x a n t h a t e are a p p a r e n t in the A g ( M N N ) s p e c t r u m from the surface treated at + 0 . 0 5 V. The Ag 3d s p e c t r u m for the surface b e a r i n g the c h e m i s o r b e d m o n o l a y e r was the same as that for the clean metal surface a p a r t from a slightly higher b a c k g r o u n d on the high b i n d i n g energy side of the former spectrum. The linewidth of the Ag 3d peaks from the surface treated at + 0 . 0 5 V was a b o u t 0.15 eV greater than that for the surface treated at - 0 . 0 5 V, but the s p e c t r u m was shifted by at most 0.1 eV to lower b i n d i n g energy. These o b s e r v a t i o n s are consistent with a m u l t i l a y e r coverage of silver
A.N. Buckley, R. Woods/Colloids Surfaces A: Physic•chem. Eng. Aspects 104 (1995) 295-305
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xanthate on the silver, and with a Ag 3d5/2 binding energy for silver xanthate no more than 0.1 eV lower than that for silver metal. The intensities of the corresponding S 2p spectra from the treated surfaces (Fig. 3), as well as the form of the C ls and O ls spectra, were also consistent with monolayer chemisorption of xanthate for treatment at - 0 . 0 5 V and multilayer adsorption of silver xanthate for treatment at +0.05 V. Unlike the situation in which xanthate is adsorbed at the surface of galena I-9, 11], however, there was only a very small difference in the S 2p3/2 binding energy for the monolayer and for the multilayer (Table 1). Thus it would appear that any A g ( M N N ) energy shift on xanthate chemisorption is not discernible, and therefore must be smaller than 0.2 eV. Nevertheless, as established
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in Section 2, the absence of a discernible shift is consistent with monolayer chemisorption.
4.3. X-ray photoelectron spectra of xanthate treated silver sulfide surfaces The A g ( M N N ) spectra from a clean sulfidised silver electrode (i.e. silver sulfide surface), a similar electrode maintained for 2 min at - 0 . 0 5 V in 10 4 mol dm -3 xanthate solution, and one held for 5 rain at +0.15 V in 10 4 mol dm -3 xanthate solution are shown in Fig. 4 spectra (a)-(c), respectively. It was established in the voltammetric experiments described in Section 4.1 that, as in the case of silver metal, treatment at - 0 . 0 5 V should have resulted in close to monolayer coverage of
303
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c h e m i s o r b e d xanthate, a n d t r e a t m e n t at + 0 . 1 5 V should have p r o d u c e d bulk silver xanthate. The S 2p s p e c t r u m from the surface treated at - 0 . 0 5 V (Fig. 5 s p e c t r u m (b)) confirms the presence of xanthate. The C ls a n d O ls spectra were also consistent with x a n t h a t e a d s o r b e d at the treated surface. The S 2p s p e c t r u m can be fitted with a c o m p o n e n t shifted 1.0 eV to higher b i n d i n g energy from the silver sulfide position, a n d a c c o u n t i n g for a b o u t 30% of the total S 2p intensity. A s s u m i n g that b o t h sulfur a t o m s in each a d s o r b e d x a n t h a t e radical would interact with one silver a t o m of each Ag2S molecule in the silver sulfide surface, a xanthate c o m p o n e n t in the S 2p s p e c t r u m of the intensity o b s e r v e d s h o u l d c o r r e s p o n d to a c o m p o nent in the s u b s t r a t e A g ( M N N ) s p e c t r u m a c c o u n t ing for at least 20% of its intensity if, as expected,
__ [ 170
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304
A.N. Buckley, R. Woods/Colloids Surfaces A." Physicochem. Eng. Aspects 104 (1995) 295 305
firms the absence of multilayer silver xanthate, since the shift that would be associated with the presence of multilayer silver xanthate is 1.1 eV (Table 1), and a component shifted by this value would be clearly visible. In addition, this observation establishes that any core electron shift on chemisorption must be small. An Ag(MNN) component of the magnitude expected from the S 2p spectrum should be discernible if it were shifted by more than 0.2 eV. The Ag 3d spectrum from the silver sulfide surface bearing chemisorbed xanthate was indistinguishable from that for the clean sulfidised silver surface, whereas the linewidth of the Ag 3d peaks from the sulfide surface covered with bulk silver xanthate was 0.2 eV greater than for the clean sulfidised silver surface (and approximately 0.4 eV greater than that for silver metal). These observations are also consistent with the presence of chemisorbed xanthate at the surface of the sulfidised silver treated at - 0 . 0 5 V. The Ag(MNN), S 2p, C ls and O ls spectra for the sulfidised silver surface treated at +0.15 V all indicate that a relatively thick overlayer of silver xanthate had been formed. In particular, only a very small shoulder from the underlying silver sulfide remained near 161 eV on the low binding energy side of the S 2p spectrum (Fig. 5 spectrum (c)) and no substrate component was evident in the Ag(MNN) spectrum (Fig. 4 spectrum (c)).
5. Concluding remarks There is a significant body of evidence establishing that core level energy shifts upon chemisorption from the vapour phase can be too small to be resolved by conventional XPS. Thus, the previously reported absence of a shift for chemisorption of thiol collectors from the aqueous phase for sulfide minerals is quite reasonable. The model systems silver/ethyl xanthate and silver sulfide/ethyl xanthate have been shown here to behave similarly. In particular, no shift in the Ag(MNN) spectrum for either system was detected when xanthate was chemisorbed on a surface. This contrasts with shifts of greater than 1 eV for silver xanthate on these two substrates. Such a difference
in spectral behaviour provides a means of distinguishing chemisorption from multilayer adsorption of thiol collectors on sulfde mineral surfaces.
Acknowledgement The authors are grateful to Dr Giorgio Contini, Consiglio Nazionale delle Ricerche Istituto per il Trattamento dei Minerali, Rome, for a preprint of Ref. [8].
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A.N. BuckhT, R. Woods~Colloids Surjaces A." Physicochem. Eng. A.~pects 104 (1995) 2(.U 305 [20] J.N. Andersen, M. Qvarford, R. Nyholm, S.L. Sorensen and C. Wigren, Phys. Rev. Lett., 67 (1991) 2822-2825. [21] J.N. Andersen and U.O. Karlsson, Phys. Rev. B, 41 ( 19901 3844 3846. [22] E. Landemark. C.J. Karlsson, L.S.O. Johansson and R.I.G. Uhrberg, Phys. Rev. B, 49 (1994) 16 523 16 533. [23] P.S. Mangat, P. Soukiassian, Y. Huttel and Z. Hurych, J. Vac. Sci. Technol. B, 12 (1994) 2694 2698. [24] M.L. Shek, P.M. Stefan, C. Binns, I. Lindau and W.E. Spicer, Surf. Sci., 115 11982) L81-L85. [25] J.N. Andersen, C. Wigren and U.O. Karlsson, J. Vac. Sci. Technol. B, 9[199112384 2387. [26] G. Paolucci and K.C. Prince. Phys. Rev. B, 41 (19901 385l 3853. [27] h Kartio, K. Laajalehto and E. Suoninen, Colloids Surfaces A: Physicochem. Eng. Aspects, 93 (1994) 149 158. [28] B.M. Trafas, I.M. Vitomirov, C.M. Aldao, Y. Gao, F. Xu, .hH. Weaver and D.L. Partin, Phys. Rev. B, 39 {1989) 3265 3273. [29] El. Johansson, H,I.P. Johansson and K.L. H&kansson, Phys. Rev. B, 48 (1993) 14 520 14 523. [30] R. Woods, C.I. Basilio, D.S. Kim and R.-H. Yoon, ,h Flectroanal. Chem., 328 (1992) 179. [31] A. lhs and B. Liedberg, Langmuir, 10 (1994) 734 740. [32] P. Talonen, J. Rastas and J. Leppinen, Surf. Interface Anal., 17 ( 1991 } 669 674.
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[33] A.W. Czanderna, J. Phys. Chem., 68 11964) 2765 2772. [34] R.B, Grant and R.M. Lambert, Surf. Sci_ 146 119841 256 268. [35] D. Briggs and M.P. Seah (Eds.), Practical Surthce Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester (UKI, 1983, p. 50L [36] M. Koebel. N. Ibl and A.M. Frei, Electrochim. Acla, 19 (1974) 287 295. [37] G.P. Power, I.M. Ritchie and M.T. Wylie, Electrochim. Acta, 26 (19811 1633-1638. [38] H. Schmalzried, Prog. Solid State Chem., 13 11980) 119 157. [39] A.N. Buckley, H.J. Wouterlood and R, Woods, ,1. Appl. Electrochcm., 19 (1989) 744 751. [40] M. Romand, M. Roubin and J.P. l)eloume, J. Electron Spectrosc., 13 (19781 229 242. [41] N.H. Turner, J.S. Murday and D.E. Ramaker, Anal. Chem., 52 (19801 84 92. [42] L.J. Gerenser, K.E. Goppert-Berarducci, R.C Baetzold and J.M. Pochan, J. Chem. Phys., 95 {1991 ) 4641 4649. [43] T.M.H. Saber and A.A. E1 Warraky, Br. Corros. ,I., 23 (1988) 131 133. [44] J.O. Leppinen, R-H. Yoon and J.A. Mielczarski, Colloids Surfaces, ~1 {1991} 189 203.