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An aes study of a copper-iron sulphide mineral
Surface Science 0 North-Holland
60 (1976) 196-210 Publishing Company
ANAESSTUDYOFACOPPER-IRONSULPHIDEMINERAL W. LOSCH and A.J. MONHEMIUS * Prograha de Engenharia Metalurgica e de Materiais, COPPE, Universidade Federal do Rio de Janeiro, Caixa Postal I I91 - ZC-00, 20.000, Rio de Janeiro, RJ, Brazil
Received
17 February
1976; manuscript
received
in final form 25 May 1976
Synthetic bornite, CusFcS4, has been studied by Auger electron spectroscopy. Sputtercleaned bornite shows a sulphur spectrum with three peaks at 138, 147 and 149 eV. These Auger transitions are different from those observed when sulphur is adsorbed on metal surfaces, where the peaks are at 139, 149 and 154 eV. The adsorption of oxygen on the surface of bornite at room temperature results in the formation of a layer of iron oxide and, in addition, the sulphur spectrum loses its fine structure and shows only a single peak at 148 eV. Under the influence of both the ion sputter beam and the electron beam, the surface composition of bornite shows large and rapid changes which are due mainly to movement of mobile Cu+ ions through the lattice, this movement being caused by surface charging effects.
1. Introduction The technique of Auger electron spectroscopy (AES) has been widely applied to the study of the surfaces of metals and alloys, and such studies have given valuable information to metallurgists, particularly concerning oxidation behaviour and variations in surface composition due to diffusion. The surface properties of classical semiconductors have also been well studied. Less attention has been paid however to the surface properties of the semiconducting transition metal sulphides. The behaviour of these materials is of considerable~interest in fields as diverse as extraction metallurgy, solid-state electrochemistry and geochemistry. The ternary Cu-Fe-S system embraces a large number of sulphide compounds of varying complexity, many of which are found as naturally occurring minerals. The mineral bornite, Cu,FeS,, was chosen from this system for study by AES, since it was known that it could be synthesised successfully in a pure and stoichiometric form [l] . Furthermore, bomite is a widely occurring natural mineral and an important raw material for the production of copper metal. Recent leaching studies have shown that large changes in the solid-state composition of this mineral * Present address: Department SW7 2BP, England.
of Metallurgy
and Materials 196
Science,
Imperial
College,
London
W. Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
191
can occur in oxidizing aqueous environments due to rapid removal of part of the copper from the lattice [ 11. The structure of bornite is of the ZnS sphalerite type with layers of interstitial Cuf ions in half of the unoccupied tetrahedral lattice positions [2]. The composition can be represented by: (Cu,FeS,)*-
.2Cu+ .
It is believed that diffusion of the interstitial copper ions leads to the observed changes in the solid-state composition during leaching [ 11.
2. Experimental The experiments were carried out in a commerical Varian AES system, equipped with a cylindrical mirror analyser (CMA) and a coaxial electron gun. A vacuum of approximately 1O-7 Pa (10e9 Torr) was maintained by the use of vacsorb, iongetter, and titanium sublimation pumps so that the system was maintained as free as possible of traces of hydrocarbons. Bakeout was not used in order to avoid possible changes in specimen composition, and all experiments were carried-out at room temperature (20-22°C). Ion bombardment was carried out at a pressure of 8 X lop3 Pa using pre-purified argon (99.998% Ar). Specimens were mounted so that the normal to the surface under examination made an angle of approximately 30” with the incident electron beam and 40” with the ion beam. The diameters of the electron and ion beams at the target were approximately 100 pm and 2 mm, respectively. Auger spectra were recorded using a primary electron beam energy of 2 keV, with a modulation voltage of 5 V (p - p) and, unless otherwise stated, an electron beam current of 10 PA. Sputtering was carried out with an ion beam energy of 1.5 keV and an ion current density of 4 X 10e4 A/cm2. The bornite used was a synthetic material, which had been made by reacting stoichiometric weights of ultra-pure Cu, Fe and S under controlled conditions in a sealed and evacuated silica tube. The bonite thus produced was a solid rod of coarsely-crystalline, brittle material with grain sizes of the order of 3-4 mm. Specimens for Auger examination were obtained simply by cleaving pieces from this rod, in air or under nitrogen. No further surface preparation was carried out. Specimen sizes were of the order of 7-10 mm in diameter.
3. Results 3. I. Adsorption studies The surface of freshly broken bornite is brown in colour. After exposure to air over a period of hours, the colour changes to a characteristic iridescent blue. Initial
198
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
experiments were carried out to determine whether there was a difference in the surface composition of these two types of bornite. Before cleaning by sputtering, both types of surfaces showed large adsorption of carbon and oxygen only. Fig. la shows a typical Auger spectrum of a bornite surface before sputtering. Large peaks of carbon and oxygen are seen, and the peaks of Cu, Fe and S are much diminished as compared with the sputter-cleaned surface, shown in fig. lb. The changes occurring in the surface of blue bornite during sputtering are shown in fig. 2, in which the peak-to-peak intensities of the various elements are plotted as a function of depth, which is expressed as sputtering time, since the sputtering rate of this material is unknown. It may be seen that the concentrations of carbon and oxygen decrease with sputtering time, whereas those of the three constituent elements of bornite increase. Nevertheless, even after one hour of sputtering, both carbon and oxygen were still present. However, in the case of brown bornite, it was found that the peaks of carbon and oxygen disappeared rapidly after a few
dN
dE
-I
(a)
--
Cu
Fe
(b)
Fe c s h 0 Fig. 1. Auger spectra
:
: 200
of bornite
:
: 400
surfaces:
:
: 600
:
: : I 800 Dal
(a) before
sputtering
Energy (eV) (b) after sputtering.
W.Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
199
Fig. 2. Variation of surface composition of blue bornite as a function of time of sputtering.
minutes of sputtering. Thus the only difference observable by AES in the surface compositions of brown and blue bornite is the greater depth of penetration of carbon and oxygen into the surface of the latter. All subsequent experiments were carried out using freshly-broken bornite surfaces. The extremely symmetrical reciprocal behaviour of carbon and sulphur, evident in fig. 2, should be noted. This symmetry was found to be typical whenever carbon adsorption was observed (e.g. see also fig. 8). The observed electron energies of the peaks of Cu, Fe and S in sputter-cleaned bornite, with energies greater than 100 eV, were in good agreement with normally accepted values. The agreement with accepted values was less good for the low energy peaks in the 50 eV region, although the energies of these peaks observed in bornite were the same as those observed when Cu and Fe metal targets were used.
200
W. Losch, A.J. Monhemius
/ AES study
of copper-iron
sulphide mineral
As correct calibration procedures were always carried out, these discrepancies were attributed to the characteristics of the CMA, since the energy calibration of this type of analyser is less reliable in the low energy range [3]. The influence of oxygen adsorption on the structure of the Auger peaks of the
dN
dE
I (a)
5v PP
,
I(--;i Sens. = 0.2
-
Sens =0.5 (b)
Fe I47
I
400
1
803
800
149 I : : I 110 I?0150 I70
I lum
. ENEFSY f eV )
Fig. 3. The influence of oxygen adsorption on the Auger spectrum of bornite: (a) with adsorbed oxygen; (b) after sputtering for 30 see; (c) after sputtering for 5 min.
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
201
constituent elements of bornite was studied in some detail. The low energy peaks were chosen for study, since it is known that the escape depths of Auger electrons in metals depend on their energies [4], with electrons in the 50 eV originating from the first few atom layers, and it was assumed that the same would be true for bornite. In addition, the low energy peaks are valence transitons and thus it was ex-
4
(a)
2
VPP
I
5
VPP
4;. 44
(b)
Fig. 4. Structure of spectra in the low energy region: (a) sputter-cleaned with adsorbed oxygen; (c) iron metal with adsorbed oxygen.
bornite; (b) bornite
202
W. Losch, A.J. Monhemius
/ AES study
of copper--iron
sulphide
mineral
petted that these would be the most likely to show changes due to adsorption. Figs. 3c and 4a show the structure of the peaks of S and of Cu and Fe, respectively, in sputter-cleaned bornite. The copper and iron peaks are similar in shape and peak energies to those of the pure metals. The sulphur peak shows a fine structure consisting of three peaks at 138,147 and 149 eV. Exposure of sputter-cleaned bornite to oxygen at 13 Pa for 15 min at room temperature produced changes in the spectrum. The low energy peak of copper at 57 eV disappeared, and the resulting spectrum, shown in fig. 4b, is almost identical to that of iron metal exposed to oxygen at 10e3 Pa for 20 min at room temperature, shown in fig. 4c. In the case of sulphur, the fine structure disappeared, resulting in a single peak at 148 eV, as shown in fig. 3a. Partial removal of the adsorbed oxygen by sputtering of the oxidised bornite surface for 30 set was sufficient to restore the fine structure of the sulphur spectrum (see fig. 3b), although it was not fully developed until all the oxygen had been removed. The oxidised bornite surface exhibited charging effects. However it was possible to obtain the Auger spectra by increasing the electron beam current. In addition to the structural changes in the low energy region of the spectrum brought about by oxygen adsorption, it was noted that the intensities of the high energy iron peaks were increased, and those of the high energy copper peaks were decreased, as illustrated in figs. 3 and 4. 3.2. Composition studies In general, after sputtering had been applied to the surface of bornite, large and rapid changes occurred in the surface composition when the iron beam was switched off. During this “relaxation”, occurring under electron bombardment, the intensities of the copper peaks increased until saturation values were achieved, whereas those of iron decreased, approximately symmetrically with copper, also reaching constant values. The intensities of the sulphur peaks remained more or less unchanged. These effects are demonstrated in fig. 5, which shows the changes in the intensities of the various peaks as a function of time after cessation of sputtering. The curves are asymptotic in shape, with the largest changes occurring within the first few minutes. The percentage changes in intensities during this relaxation generally lay within the range of 25-50s for both copper and iron. Since there is evidence that the interstitial Cu+ ions in the bornite lattice show a high mobility [ 11, it was assumed that diffusion of copper ions was responsible for these effects. Diffusion of these ions could be affected both by the electron beam and the ion beam and thus the effects of these beams on the relaxation phenomenon were investigated. The peak-to-peak Auger intensity, Ii, of transition, i, is given by [3] : ri = Ai, @i(E) $/i(E) TRpX, D(E) B(E) . The only factor which was changed in the experiment
to be described was the in-
203
W. Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
E,=2keVV,I
=
lopA
SW
,Sens.
2.0
*/is=-_I
”
”
IO
20
50
40
”
45
.
t tmlnl Fig. 5. Changes in surface composition after cessation of sputtering (ion beam 1.5 keV, 4 X lo4 A/cm2, electron beam 2 keV, 10 PA).
tensity of the primary electron beam, i,. The electron energy, E, was kept constant and, provided the same transitions, i, are compared, then the ionization and transition probabilities, pi and tii(E), the escape depth, D(E), the backscattering factor, B(E), the roughness factor, R, the transmission factor, T, and the area A, may all be considered constant. Under these conditions, the expression for Ii reduced to: Ii = C.pXaip , where p is the total density of the material and X, is the atomic fraction of element or. If movement of ions in the lattice occurs, then the density, p, will change. However this variation is the same for all transitions, i, and for all elements, cr, since p is the total density. Thus the expression for Ii may be written as:
ri = Ccolip ,, where c, is the concentration of element (Y. This proportionality was tested by measuring the peak-to-peak intensities on a previously sputtered, fully relaxed, bornite surface, at various primary electron
W. Losch, A.J. Monhemius / AES study of copper-iron
204
sulphide mineral
I10
I IO<
lOO(
;; :
Ep’2keV,TC=0.1
,V,,_$S 100
0
CUJ7
0
cug20
Sons. =
I .o
0
s ,so
smr
’
2.0
V
Fe,,4
Smr
.
0.5
a
Few
Sons
-
I .O
Senr=
0.5
9oc
0
G &
a;; sot
90
so
?OC
70
60C
60
5oc
50
4oc
40
3oc
30
2oc
20
IO
Ioc
5
10
15
20
25
30
35
40
45
50
55
60
Fig. 6. The influence of primary electron beam current ip on peak-to-peak intensities Zi on a relaxed surface.
beam currents. Variations in c,, resulting from changes in the atomic fractions, X,, should give rise to deviations from the linear proportionality Ii =f(i,). In fig. 6, the peak-to-peak intensity values of the various transitions, i, are plotted as a function of primary electron beam current, i,. The linearity of the plots up to approximately 40 PA indicates that, in this region, the composition of the relaxed bornite surface was not influenced by the electron beam. However, above 40 DA, the decrease in the slopes of the iron and sulphur peaks and the increase in the slope of the low energy copper peak indicates that a surface enrichment of copper took place under the influence of the higher electron beam currents. When the ion sputter beam was applied to the relaxed bornite surface, the effects shown in fig. 5 were reversed, i.e. the copper peaks decreased, while the iron peaks increased. Hence the application of the ion beam led to a depletion of copper in the surface. It is likely that the sputtering action of the ion beam will influence the sur-
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
205
face composition due to differential sputtering or “knock-in” effects. However, in the case of bornite, due to the presence of the mobile interstitial copper ions, there will also be an effect due to the repulsion of these ions by the Ar+ ions. Fig. 6 demonstrates that, at low currents (<40 /LA), the electron beam does not alter the composition of the relaxed bornite surface. However, in the unrelaxed state, when the surface is depleted in copper due to the effects of the ion beam, the electron beam then has an influence on the composition. This is illustrated in fig. 7. At time zero in this figure, the ion beam was applied to a fully relaxed bornite surface, simultaneously with the electron beam. As already mentioned, a decrease in the intensity of the copper peak and a corresponding increase in the iron peak is observed. After 14 min, the electron beam was switched off and sputtering continued for 2 min. The electron beam was then switched back on and a spectrum recorded immediately. It is evident that the concentration of copper dropped sharply in the absence of the electron beam, indicating that this beam influenced the surface composition, either by attraction of positive copper ions, or by neutralisation of any excess positive charge at the surface which might have accumulated as a result of ion bombardment. Another experiment further demonstrated the influence of the electron beam. The diameter of the ion beam was approximately twenty times that of the electron beam, so, in this experiment, the relaxation effect after cessation of sputtering
Fig. 7. The influence of the primary electron beam (ip = 10 @A) on peak-to-peak intensities Ii on an unrelaxed surface.
W. Losch, A.J. Monhemius / AES study of copper-iron
206
sulphide mineral
was
observed until the saturation surface composition was achieved. The target was then moved so that the electron beam fell on an area still within the copper depleted surface caused by sputtering. Here the initial surface composition was the same as the initial composition of the area studied first. In other words, in the absence of the electron beam, there had been no rapid change in the surface composition. In order to determine the Auger intensities of a bornite surface undisturbed by sputtering, a specimen was cleaved in ultra-high-vacuum. To do this, the specimen was glued, using leak-sealer, between two small metal tubes. At the instant of cleavEp=2
Ii
BCUST
keV;ip
=IOuA;Vp_p=5
senr. ’
TC=O,l
0.5
oCuS20Swl’.~0.2 200
‘.
OS ,SO
Senr.=l.O
D Fe 44
Senr. * 0.5
0 Fe6wSm?s.
‘0.2
.I50
100
X
Fig. 8. Peak-to-peak ip = 10 MA).
intensities
Ii as a function
of time
t after cleavage in UHV (Ep = 2 keV,
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
207
ing, the pressure rose briefly from 10v7 to 10e5 Pa, perhaps due to a small amount of air trapped behind the glue. This led to rapid carbon adsorption on the freshly cleaved surface and, in order to obtain initial peak-to-peak intensities, an extrapolation to zero time was carried out, as shown in fig. 8. In a repeat of this experiment, the problem of carbon contamination was largely avoided by careful application of the glue, but problems were encountered with charging of the specimen surface, which was probably due to bad electrical contact. This led to unreliable values of peak-to-peak intensities in the low energy range, and so only the Feb5u and Cu920 p eak values were used. These are also plotted in fig. 8 and show reasonable agreement with the results of the first cleavage experiment. Although all of the phenomena reported above were repeatable, the reproducibility of the absolute values of the peak intensities was, in general, rather poor. Differences in intensity values of up to T30% occurred when values from different specimens were compared. However if an experiment was repeated using the same target area, the reproducibility was much better, of the order of +.5%. This was thought to be related to the large grain size of the synthetic bornite, because the diameter of the electron beam was much smaller than the average grain size and the orientation of the grains could have influenced the results.
4. Discussion The influence of adsorption of oxygen on the surface of sputter-cleaned bornite is evidently most marked in the case of iron. A comparison of figs. 4b and 4c shows that, in the low energy range, the spectrum of oxidized bornite is almost identical to that of oxidized iron metal, with both spectra showing two peaks at 39 and 47 eV and 39 and 48 eV, respectively. These spectra closely resemble, in shape and peak energies, that given by Ertl and Wandelt [S] for Fe,O,. Seo and co-workers [6] studied the Auger spectra of a series of iron oxides and, although the spectrum given by them for Fe,04 is similar in shape to that shown in fig. 4c, their peak energies are more closely spaced at 46.5 and 51 eV. Consideration of the energy difference between the peaks, as well as the shape, would indicate the presence of y-FeOOH on the surface of bomite, according to the results of Seo. The composition of the oxide layer on bornite thus remains in doubt, but there is no doubt that it is an iron oxide. This conclusion is reinforced by the changes which occur in the spectrum of oxidized bornite at higher energies. As shown in figs. 3 and 4, the high energy iron peaks increase in intensity, whereas those of copper and sulphur decrease. The formation of this iron oxide layer probably accounts for the charging effects, which, as noted above, were encountered when oxygen was adsorbed on bornite. The spectrum of sulphur in sputter-cleaned bornite, shown in figs. 3. and 9b, which has a fine structure with three peaks at 138, 147 and 149 eV, is different from previously reported sulphur spectra.
208
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
(b)
1
140
I50
I
,
140
150 qGy
Fig. 9. Fine structure of sulphur spectrum: (a) adsorbed on iron with peaks at 139, 149 and 154 eV (b) in bornite with peaks at 138, 147 and 149 eV (Ep = 2 kV, i,, = 15 IA, Vpp = 2 V, s = 0.2).
Sickafus and Steinrisser [7] reported a tine structure for sulphur adsorbed on a nickel metal substrate, but in this case, the peak energies were 139, 149 and 154 eV. The spectrum of sulphur adsorbed on an iron metal substrate, determined in the course of the present work and shown in fig. 9a, is almost identical to the spectrum published by Sickafus for sulphur on nickel. Comparison of the spectrum of sulphur adsorbed on iron (fig. 9a) with that of sulphur in bomite (fig. 9b) shows distinct differences both in peak energies and line shape. Adsorption of oxygen on the surface of bornite causes this fine structure to disappear and results in a single sulphur peak at 148 eV, as shown in fig. 3a. Diffusion of mobile Cu+ ions from the surface under the influence of excess positive charge which could accumulate at the surface during ion bombardment, together with the possibility of differential sputtering rates, can account for the depletion of copper in the surface of bomite observed during sputtering. However the so-called relaxation effect, where copper rapidly diffused back to the surface on cessation of sputtering, requires comment since it is known that ion bombard-
W. Losch, A.J. Morthemius / AES study of copper-iron
w&hide mineral
209
a Pig. 10. Scanning electron bornite.
micrographs
(X $000): (a) freshly cleaved bornite; (b) sputtered
ment of non-metallic materials can cause considerable damage to the crystalline structure and often results in an amorphous surface [S]. In the present work, the structure of bomite after sputtering could not be determined as LEED equipment was not available. The scanning electron micrographs of a freshly cleaved bomite surface and ofasputteredsurface, shown in figs. 10 a and lob, respectively, indicate that considerable surface damage had occurred. It thus seems unlikely that the sputtered surface retained a crystalline structure. Nevertheless, the rapid diffusion of copper during relaxation suggests that either the surface retained a certain crystalline structure within the volume detectable by Auger electrons, or that the copper ions remained highly mobile in the amorphous surface. These two possibilities remain unresolved. The inverse behaviour of the intensities of the ion peaks during relaxation, which decreases as those of copper increase, is attributable mainly to the change in atomic fractions of the elements in the surface caused by the diffusion of copper.
5. Conclusions (1) Sulphur in bomite shows an Auger spectrum with a iine structure comprising three peaks at 138, 147 and 149 eV. In the presence of oxygen adsorption, this fine structure disappears and is replaced by a single peak at 148 eV. (2) The adsorption of oxygen at room temperature on bornite results in the formation of a surface layer of iron oxide. (3) It is shown that both the primary electron beam and the ion sputter beam influence the surface composition of bomite due to the presence of highly mobile Cu+ ions in the crystal lattice.
210
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
Acknowledgements The authors are grateful to Dr. A.R. Burkin of Imperial College, London, gift of the sample of synthetic bomite used in this study.
for the
References [l] G. IJgarte and A.R. Burkin, in: Leaching and Reduction in Hydrometallurgy (IMM, London, 1975). [2] PG. Manning, Can. Mineral. 9 (1967) 85. [3] C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and G.B. Lacrabee (Plenum, New York, 1974). [4] C.R. Brundle, J. Vacuum Sci. Technol. 11 (1974) 212. [5] G. Ertl and K. Wandelt, Surface Sci. 50 (1975) 479. [6] M. Seo, J.B. Lumsden and R.W. Staehle, Surface Sci. 50 (1975) 541. [7 ] E.N. Sickafus and 1;. Steinrisser, J. Vacuum Sci. Technol. 10 (1973) 43. [8] H.M. Naguib and R. Kelly, Radiation Effects 25 (1975) 1.
60 (1976) 196-210 Publishing Company
ANAESSTUDYOFACOPPER-IRONSULPHIDEMINERAL W. LOSCH and A.J. MONHEMIUS * Prograha de Engenharia Metalurgica e de Materiais, COPPE, Universidade Federal do Rio de Janeiro, Caixa Postal I I91 - ZC-00, 20.000, Rio de Janeiro, RJ, Brazil
Received
17 February
1976; manuscript
received
in final form 25 May 1976
Synthetic bornite, CusFcS4, has been studied by Auger electron spectroscopy. Sputtercleaned bornite shows a sulphur spectrum with three peaks at 138, 147 and 149 eV. These Auger transitions are different from those observed when sulphur is adsorbed on metal surfaces, where the peaks are at 139, 149 and 154 eV. The adsorption of oxygen on the surface of bornite at room temperature results in the formation of a layer of iron oxide and, in addition, the sulphur spectrum loses its fine structure and shows only a single peak at 148 eV. Under the influence of both the ion sputter beam and the electron beam, the surface composition of bornite shows large and rapid changes which are due mainly to movement of mobile Cu+ ions through the lattice, this movement being caused by surface charging effects.
1. Introduction The technique of Auger electron spectroscopy (AES) has been widely applied to the study of the surfaces of metals and alloys, and such studies have given valuable information to metallurgists, particularly concerning oxidation behaviour and variations in surface composition due to diffusion. The surface properties of classical semiconductors have also been well studied. Less attention has been paid however to the surface properties of the semiconducting transition metal sulphides. The behaviour of these materials is of considerable~interest in fields as diverse as extraction metallurgy, solid-state electrochemistry and geochemistry. The ternary Cu-Fe-S system embraces a large number of sulphide compounds of varying complexity, many of which are found as naturally occurring minerals. The mineral bornite, Cu,FeS,, was chosen from this system for study by AES, since it was known that it could be synthesised successfully in a pure and stoichiometric form [l] . Furthermore, bomite is a widely occurring natural mineral and an important raw material for the production of copper metal. Recent leaching studies have shown that large changes in the solid-state composition of this mineral * Present address: Department SW7 2BP, England.
of Metallurgy
and Materials 196
Science,
Imperial
College,
London
W. Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
191
can occur in oxidizing aqueous environments due to rapid removal of part of the copper from the lattice [ 11. The structure of bornite is of the ZnS sphalerite type with layers of interstitial Cuf ions in half of the unoccupied tetrahedral lattice positions [2]. The composition can be represented by: (Cu,FeS,)*-
.2Cu+ .
It is believed that diffusion of the interstitial copper ions leads to the observed changes in the solid-state composition during leaching [ 11.
2. Experimental The experiments were carried out in a commerical Varian AES system, equipped with a cylindrical mirror analyser (CMA) and a coaxial electron gun. A vacuum of approximately 1O-7 Pa (10e9 Torr) was maintained by the use of vacsorb, iongetter, and titanium sublimation pumps so that the system was maintained as free as possible of traces of hydrocarbons. Bakeout was not used in order to avoid possible changes in specimen composition, and all experiments were carried-out at room temperature (20-22°C). Ion bombardment was carried out at a pressure of 8 X lop3 Pa using pre-purified argon (99.998% Ar). Specimens were mounted so that the normal to the surface under examination made an angle of approximately 30” with the incident electron beam and 40” with the ion beam. The diameters of the electron and ion beams at the target were approximately 100 pm and 2 mm, respectively. Auger spectra were recorded using a primary electron beam energy of 2 keV, with a modulation voltage of 5 V (p - p) and, unless otherwise stated, an electron beam current of 10 PA. Sputtering was carried out with an ion beam energy of 1.5 keV and an ion current density of 4 X 10e4 A/cm2. The bornite used was a synthetic material, which had been made by reacting stoichiometric weights of ultra-pure Cu, Fe and S under controlled conditions in a sealed and evacuated silica tube. The bonite thus produced was a solid rod of coarsely-crystalline, brittle material with grain sizes of the order of 3-4 mm. Specimens for Auger examination were obtained simply by cleaving pieces from this rod, in air or under nitrogen. No further surface preparation was carried out. Specimen sizes were of the order of 7-10 mm in diameter.
3. Results 3. I. Adsorption studies The surface of freshly broken bornite is brown in colour. After exposure to air over a period of hours, the colour changes to a characteristic iridescent blue. Initial
198
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
experiments were carried out to determine whether there was a difference in the surface composition of these two types of bornite. Before cleaning by sputtering, both types of surfaces showed large adsorption of carbon and oxygen only. Fig. la shows a typical Auger spectrum of a bornite surface before sputtering. Large peaks of carbon and oxygen are seen, and the peaks of Cu, Fe and S are much diminished as compared with the sputter-cleaned surface, shown in fig. lb. The changes occurring in the surface of blue bornite during sputtering are shown in fig. 2, in which the peak-to-peak intensities of the various elements are plotted as a function of depth, which is expressed as sputtering time, since the sputtering rate of this material is unknown. It may be seen that the concentrations of carbon and oxygen decrease with sputtering time, whereas those of the three constituent elements of bornite increase. Nevertheless, even after one hour of sputtering, both carbon and oxygen were still present. However, in the case of brown bornite, it was found that the peaks of carbon and oxygen disappeared rapidly after a few
dN
dE
-I
(a)
--
Cu
Fe
(b)
Fe c s h 0 Fig. 1. Auger spectra
:
: 200
of bornite
:
: 400
surfaces:
:
: 600
:
: : I 800 Dal
(a) before
sputtering
Energy (eV) (b) after sputtering.
W.Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
199
Fig. 2. Variation of surface composition of blue bornite as a function of time of sputtering.
minutes of sputtering. Thus the only difference observable by AES in the surface compositions of brown and blue bornite is the greater depth of penetration of carbon and oxygen into the surface of the latter. All subsequent experiments were carried out using freshly-broken bornite surfaces. The extremely symmetrical reciprocal behaviour of carbon and sulphur, evident in fig. 2, should be noted. This symmetry was found to be typical whenever carbon adsorption was observed (e.g. see also fig. 8). The observed electron energies of the peaks of Cu, Fe and S in sputter-cleaned bornite, with energies greater than 100 eV, were in good agreement with normally accepted values. The agreement with accepted values was less good for the low energy peaks in the 50 eV region, although the energies of these peaks observed in bornite were the same as those observed when Cu and Fe metal targets were used.
200
W. Losch, A.J. Monhemius
/ AES study
of copper-iron
sulphide mineral
As correct calibration procedures were always carried out, these discrepancies were attributed to the characteristics of the CMA, since the energy calibration of this type of analyser is less reliable in the low energy range [3]. The influence of oxygen adsorption on the structure of the Auger peaks of the
dN
dE
I (a)
5v PP
,
I(--;i Sens. = 0.2
-
Sens =0.5 (b)
Fe I47
I
400
1
803
800
149 I : : I 110 I?0150 I70
I lum
. ENEFSY f eV )
Fig. 3. The influence of oxygen adsorption on the Auger spectrum of bornite: (a) with adsorbed oxygen; (b) after sputtering for 30 see; (c) after sputtering for 5 min.
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
201
constituent elements of bornite was studied in some detail. The low energy peaks were chosen for study, since it is known that the escape depths of Auger electrons in metals depend on their energies [4], with electrons in the 50 eV originating from the first few atom layers, and it was assumed that the same would be true for bornite. In addition, the low energy peaks are valence transitons and thus it was ex-
4
(a)
2
VPP
I
5
VPP
4;. 44
(b)
Fig. 4. Structure of spectra in the low energy region: (a) sputter-cleaned with adsorbed oxygen; (c) iron metal with adsorbed oxygen.
bornite; (b) bornite
202
W. Losch, A.J. Monhemius
/ AES study
of copper--iron
sulphide
mineral
petted that these would be the most likely to show changes due to adsorption. Figs. 3c and 4a show the structure of the peaks of S and of Cu and Fe, respectively, in sputter-cleaned bornite. The copper and iron peaks are similar in shape and peak energies to those of the pure metals. The sulphur peak shows a fine structure consisting of three peaks at 138,147 and 149 eV. Exposure of sputter-cleaned bornite to oxygen at 13 Pa for 15 min at room temperature produced changes in the spectrum. The low energy peak of copper at 57 eV disappeared, and the resulting spectrum, shown in fig. 4b, is almost identical to that of iron metal exposed to oxygen at 10e3 Pa for 20 min at room temperature, shown in fig. 4c. In the case of sulphur, the fine structure disappeared, resulting in a single peak at 148 eV, as shown in fig. 3a. Partial removal of the adsorbed oxygen by sputtering of the oxidised bornite surface for 30 set was sufficient to restore the fine structure of the sulphur spectrum (see fig. 3b), although it was not fully developed until all the oxygen had been removed. The oxidised bornite surface exhibited charging effects. However it was possible to obtain the Auger spectra by increasing the electron beam current. In addition to the structural changes in the low energy region of the spectrum brought about by oxygen adsorption, it was noted that the intensities of the high energy iron peaks were increased, and those of the high energy copper peaks were decreased, as illustrated in figs. 3 and 4. 3.2. Composition studies In general, after sputtering had been applied to the surface of bornite, large and rapid changes occurred in the surface composition when the iron beam was switched off. During this “relaxation”, occurring under electron bombardment, the intensities of the copper peaks increased until saturation values were achieved, whereas those of iron decreased, approximately symmetrically with copper, also reaching constant values. The intensities of the sulphur peaks remained more or less unchanged. These effects are demonstrated in fig. 5, which shows the changes in the intensities of the various peaks as a function of time after cessation of sputtering. The curves are asymptotic in shape, with the largest changes occurring within the first few minutes. The percentage changes in intensities during this relaxation generally lay within the range of 25-50s for both copper and iron. Since there is evidence that the interstitial Cu+ ions in the bornite lattice show a high mobility [ 11, it was assumed that diffusion of copper ions was responsible for these effects. Diffusion of these ions could be affected both by the electron beam and the ion beam and thus the effects of these beams on the relaxation phenomenon were investigated. The peak-to-peak Auger intensity, Ii, of transition, i, is given by [3] : ri = Ai, @i(E) $/i(E) TRpX, D(E) B(E) . The only factor which was changed in the experiment
to be described was the in-
203
W. Losch, A.J. Monhemius / AES study of copper-iron sulphide mineral
E,=2keVV,I
=
lopA
SW
,Sens.
2.0
*/is=-_I
”
”
IO
20
50
40
”
45
.
t tmlnl Fig. 5. Changes in surface composition after cessation of sputtering (ion beam 1.5 keV, 4 X lo4 A/cm2, electron beam 2 keV, 10 PA).
tensity of the primary electron beam, i,. The electron energy, E, was kept constant and, provided the same transitions, i, are compared, then the ionization and transition probabilities, pi and tii(E), the escape depth, D(E), the backscattering factor, B(E), the roughness factor, R, the transmission factor, T, and the area A, may all be considered constant. Under these conditions, the expression for Ii reduced to: Ii = C.pXaip , where p is the total density of the material and X, is the atomic fraction of element or. If movement of ions in the lattice occurs, then the density, p, will change. However this variation is the same for all transitions, i, and for all elements, cr, since p is the total density. Thus the expression for Ii may be written as:
ri = Ccolip ,, where c, is the concentration of element (Y. This proportionality was tested by measuring the peak-to-peak intensities on a previously sputtered, fully relaxed, bornite surface, at various primary electron
W. Losch, A.J. Monhemius / AES study of copper-iron
204
sulphide mineral
I10
I IO<
lOO(
;; :
Ep’2keV,TC=0.1
,V,,_$S 100
0
CUJ7
0
cug20
Sons. =
I .o
0
s ,so
smr
’
2.0
V
Fe,,4
Smr
.
0.5
a
Few
Sons
-
I .O
Senr=
0.5
9oc
0
G &
a;; sot
90
so
?OC
70
60C
60
5oc
50
4oc
40
3oc
30
2oc
20
IO
Ioc
5
10
15
20
25
30
35
40
45
50
55
60
Fig. 6. The influence of primary electron beam current ip on peak-to-peak intensities Zi on a relaxed surface.
beam currents. Variations in c,, resulting from changes in the atomic fractions, X,, should give rise to deviations from the linear proportionality Ii =f(i,). In fig. 6, the peak-to-peak intensity values of the various transitions, i, are plotted as a function of primary electron beam current, i,. The linearity of the plots up to approximately 40 PA indicates that, in this region, the composition of the relaxed bornite surface was not influenced by the electron beam. However, above 40 DA, the decrease in the slopes of the iron and sulphur peaks and the increase in the slope of the low energy copper peak indicates that a surface enrichment of copper took place under the influence of the higher electron beam currents. When the ion sputter beam was applied to the relaxed bornite surface, the effects shown in fig. 5 were reversed, i.e. the copper peaks decreased, while the iron peaks increased. Hence the application of the ion beam led to a depletion of copper in the surface. It is likely that the sputtering action of the ion beam will influence the sur-
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
205
face composition due to differential sputtering or “knock-in” effects. However, in the case of bornite, due to the presence of the mobile interstitial copper ions, there will also be an effect due to the repulsion of these ions by the Ar+ ions. Fig. 6 demonstrates that, at low currents (<40 /LA), the electron beam does not alter the composition of the relaxed bornite surface. However, in the unrelaxed state, when the surface is depleted in copper due to the effects of the ion beam, the electron beam then has an influence on the composition. This is illustrated in fig. 7. At time zero in this figure, the ion beam was applied to a fully relaxed bornite surface, simultaneously with the electron beam. As already mentioned, a decrease in the intensity of the copper peak and a corresponding increase in the iron peak is observed. After 14 min, the electron beam was switched off and sputtering continued for 2 min. The electron beam was then switched back on and a spectrum recorded immediately. It is evident that the concentration of copper dropped sharply in the absence of the electron beam, indicating that this beam influenced the surface composition, either by attraction of positive copper ions, or by neutralisation of any excess positive charge at the surface which might have accumulated as a result of ion bombardment. Another experiment further demonstrated the influence of the electron beam. The diameter of the ion beam was approximately twenty times that of the electron beam, so, in this experiment, the relaxation effect after cessation of sputtering
Fig. 7. The influence of the primary electron beam (ip = 10 @A) on peak-to-peak intensities Ii on an unrelaxed surface.
W. Losch, A.J. Monhemius / AES study of copper-iron
206
sulphide mineral
was
observed until the saturation surface composition was achieved. The target was then moved so that the electron beam fell on an area still within the copper depleted surface caused by sputtering. Here the initial surface composition was the same as the initial composition of the area studied first. In other words, in the absence of the electron beam, there had been no rapid change in the surface composition. In order to determine the Auger intensities of a bornite surface undisturbed by sputtering, a specimen was cleaved in ultra-high-vacuum. To do this, the specimen was glued, using leak-sealer, between two small metal tubes. At the instant of cleavEp=2
Ii
BCUST
keV;ip
=IOuA;Vp_p=5
senr. ’
TC=O,l
0.5
oCuS20Swl’.~0.2 200
‘.
OS ,SO
Senr.=l.O
D Fe 44
Senr. * 0.5
0 Fe6wSm?s.
‘0.2
.I50
100
X
Fig. 8. Peak-to-peak ip = 10 MA).
intensities
Ii as a function
of time
t after cleavage in UHV (Ep = 2 keV,
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
207
ing, the pressure rose briefly from 10v7 to 10e5 Pa, perhaps due to a small amount of air trapped behind the glue. This led to rapid carbon adsorption on the freshly cleaved surface and, in order to obtain initial peak-to-peak intensities, an extrapolation to zero time was carried out, as shown in fig. 8. In a repeat of this experiment, the problem of carbon contamination was largely avoided by careful application of the glue, but problems were encountered with charging of the specimen surface, which was probably due to bad electrical contact. This led to unreliable values of peak-to-peak intensities in the low energy range, and so only the Feb5u and Cu920 p eak values were used. These are also plotted in fig. 8 and show reasonable agreement with the results of the first cleavage experiment. Although all of the phenomena reported above were repeatable, the reproducibility of the absolute values of the peak intensities was, in general, rather poor. Differences in intensity values of up to T30% occurred when values from different specimens were compared. However if an experiment was repeated using the same target area, the reproducibility was much better, of the order of +.5%. This was thought to be related to the large grain size of the synthetic bornite, because the diameter of the electron beam was much smaller than the average grain size and the orientation of the grains could have influenced the results.
4. Discussion The influence of adsorption of oxygen on the surface of sputter-cleaned bornite is evidently most marked in the case of iron. A comparison of figs. 4b and 4c shows that, in the low energy range, the spectrum of oxidized bornite is almost identical to that of oxidized iron metal, with both spectra showing two peaks at 39 and 47 eV and 39 and 48 eV, respectively. These spectra closely resemble, in shape and peak energies, that given by Ertl and Wandelt [S] for Fe,O,. Seo and co-workers [6] studied the Auger spectra of a series of iron oxides and, although the spectrum given by them for Fe,04 is similar in shape to that shown in fig. 4c, their peak energies are more closely spaced at 46.5 and 51 eV. Consideration of the energy difference between the peaks, as well as the shape, would indicate the presence of y-FeOOH on the surface of bomite, according to the results of Seo. The composition of the oxide layer on bornite thus remains in doubt, but there is no doubt that it is an iron oxide. This conclusion is reinforced by the changes which occur in the spectrum of oxidized bornite at higher energies. As shown in figs. 3 and 4, the high energy iron peaks increase in intensity, whereas those of copper and sulphur decrease. The formation of this iron oxide layer probably accounts for the charging effects, which, as noted above, were encountered when oxygen was adsorbed on bornite. The spectrum of sulphur in sputter-cleaned bornite, shown in figs. 3. and 9b, which has a fine structure with three peaks at 138, 147 and 149 eV, is different from previously reported sulphur spectra.
208
W. Losch, A.J. Monhemius
/ AES study of copper-iron
sulphide mineral
(b)
1
140
I50
I
,
140
150 qGy
Fig. 9. Fine structure of sulphur spectrum: (a) adsorbed on iron with peaks at 139, 149 and 154 eV (b) in bornite with peaks at 138, 147 and 149 eV (Ep = 2 kV, i,, = 15 IA, Vpp = 2 V, s = 0.2).
Sickafus and Steinrisser [7] reported a tine structure for sulphur adsorbed on a nickel metal substrate, but in this case, the peak energies were 139, 149 and 154 eV. The spectrum of sulphur adsorbed on an iron metal substrate, determined in the course of the present work and shown in fig. 9a, is almost identical to the spectrum published by Sickafus for sulphur on nickel. Comparison of the spectrum of sulphur adsorbed on iron (fig. 9a) with that of sulphur in bomite (fig. 9b) shows distinct differences both in peak energies and line shape. Adsorption of oxygen on the surface of bornite causes this fine structure to disappear and results in a single sulphur peak at 148 eV, as shown in fig. 3a. Diffusion of mobile Cu+ ions from the surface under the influence of excess positive charge which could accumulate at the surface during ion bombardment, together with the possibility of differential sputtering rates, can account for the depletion of copper in the surface of bomite observed during sputtering. However the so-called relaxation effect, where copper rapidly diffused back to the surface on cessation of sputtering, requires comment since it is known that ion bombard-
W. Losch, A.J. Morthemius / AES study of copper-iron
w&hide mineral
209
a Pig. 10. Scanning electron bornite.
micrographs
(X $000): (a) freshly cleaved bornite; (b) sputtered
ment of non-metallic materials can cause considerable damage to the crystalline structure and often results in an amorphous surface [S]. In the present work, the structure of bomite after sputtering could not be determined as LEED equipment was not available. The scanning electron micrographs of a freshly cleaved bomite surface and ofasputteredsurface, shown in figs. 10 a and lob, respectively, indicate that considerable surface damage had occurred. It thus seems unlikely that the sputtered surface retained a crystalline structure. Nevertheless, the rapid diffusion of copper during relaxation suggests that either the surface retained a certain crystalline structure within the volume detectable by Auger electrons, or that the copper ions remained highly mobile in the amorphous surface. These two possibilities remain unresolved. The inverse behaviour of the intensities of the ion peaks during relaxation, which decreases as those of copper increase, is attributable mainly to the change in atomic fractions of the elements in the surface caused by the diffusion of copper.
5. Conclusions (1) Sulphur in bomite shows an Auger spectrum with a iine structure comprising three peaks at 138, 147 and 149 eV. In the presence of oxygen adsorption, this fine structure disappears and is replaced by a single peak at 148 eV. (2) The adsorption of oxygen at room temperature on bornite results in the formation of a surface layer of iron oxide. (3) It is shown that both the primary electron beam and the ion sputter beam influence the surface composition of bomite due to the presence of highly mobile Cu+ ions in the crystal lattice.
210
W. Losch, A.J. Monhemius / AES study of copper-iron
sulphide mineral
Acknowledgements The authors are grateful to Dr. A.R. Burkin of Imperial College, London, gift of the sample of synthetic bomite used in this study.
for the
References [l] G. IJgarte and A.R. Burkin, in: Leaching and Reduction in Hydrometallurgy (IMM, London, 1975). [2] PG. Manning, Can. Mineral. 9 (1967) 85. [3] C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and G.B. Lacrabee (Plenum, New York, 1974). [4] C.R. Brundle, J. Vacuum Sci. Technol. 11 (1974) 212. [5] G. Ertl and K. Wandelt, Surface Sci. 50 (1975) 479. [6] M. Seo, J.B. Lumsden and R.W. Staehle, Surface Sci. 50 (1975) 541. [7 ] E.N. Sickafus and 1;. Steinrisser, J. Vacuum Sci. Technol. 10 (1973) 43. [8] H.M. Naguib and R. Kelly, Radiation Effects 25 (1975) 1.