SURFACE AND INTERFACE ANALYSIS
Surf. Interface Anal. 28, 101–105 (1999)
XPS of Sulphide Mineral Surfaces:
Metal-deficient, Polysulphides, Defects
and Elemental Sulphur
Roger St. C. Smart,* William M. Skinner and Andrea R. Gerson
Ian Wark Research Institute, The Levels Campus, University of South Australia, Mawson Lakes, South Australia 5095
This paper reviews evidence for the assignments of components of the S 2p XPS spectra from sulphide
mineral surfaces under different conditions of preparation, oxidation and reaction. Evidence from other
techniques confirming assignment of high-binding-energy S 2p components to metal-deficient sulphide
surfaces, polysulphides, elemental sulphur and electronic defect structures is considered for specific cases.
Reliable assignment of S 2p3=2 components at 163.6–164.0 eV to elemental sulphur Sn 0 can be confirmed
by evaporative loss at 295 K and/or observation of S–S bonding by x-ray absorption fine structure
(XAFS), x-ray diffraction or vibrational spectroscopy. Assignment to polysulphides Sn 2− at 162.0–163.6 eV
requires confirmation of S–S bonding by XAFS or vibrational spectroscopy. Metal-deficient lattices can be
represented as electronic defects (e.g. vacancies) or restructured surface phases confirmed by diffraction
or XAFS evidence. High-binding-energy S 2p3=2 components can also result from Cu(I) substitution into
ZnS with associated oxidation of sulphur as electronic defect sites without S–S bonding, metal deficiency or
restructuring. This assignment is confirmed by XAFS evidence. Copyright  1999 John Wiley & Sons, Ltd.
KEYWORDS: sulphides; minerals; surfaces; XPS or ESCA
INTRODUCTION
Knowledge of the structure, bonding and electronic states
of sulphur in the surface of metal sulphides is of critical
importance in the control of mineral surface reactivity,
particularly oxidation and adsorption, for mineral separation by flotation and in sulphide mineral leaching for
metal recovery or control of environmental pollution.1 – 3
Reactions at metal sulphide surfaces are equally crucial to
understanding the actions of these materials in catalysis,
solar energy conversion and geochemistry.4,5 In this paper,
we will focus on the evidence for assignment of components of the S 2p XPS spectra from sulphide mineral
surfaces under different conditions of preparation, oxidation and reaction. This consideration of the evidence
for these assignments, however, applies equally to XPS
information from sulphide surfaces in the other areas of
application or study.
The S 2p spectrum of a sulphide S2 ion is known to
contain the S 2p3/2 and the S 2p1/2 spin-orbit doublet separated by 1.2 eV with intensity ratio 2 : 1. The S 2p3/2 component is normally found in the binding energy (BE) range
160.0–161.2 eV, depending on the metal (M D Pb, Zn,
Fe) in the sulphide.6 – 10 In disulphide S2 2 minerals (e.g.
pyrite FeS2 )7,10,11 and on sulphide surfaces after oxidation
reaction, S 2p doublets at higher BEs have been identified
reliably and routinely by many researchers under a variety
of reaction conditions. These high-BE components have
been assigned variously to the four different models summarized in Table 1. In these models it is important to note
* Correspondence to: R. St. C. Smart, Ian Wark Research Institute,
The Levels Campus, University of South Australia, Mawson Lakes,
South Australia 5095.
CCC 0142–2421/99/130101–05 $17.50
Copyright  1999 John Wiley & Sons, Ltd.
that an assignment to a polysulphide Sn 2 species requires
direct evidence of S–S bonding,11,12 that a decrease in M/S
stoichiometric ratio may imply metal deficiency either
as vacancies (defects) in the original host lattice13 or in
restructuring to an altered lattice structure in the surface
layers,14 and that bulk elemental sulphur is expected to
evaporate in ultrahigh vacuum unless the surface is maintained at <200 K.15 In the last case, XPS spectra recorded
at room temperature would not be expected to show S 2p
components attributable to S8 0 species. There is a possibility that Sn 0 species, including S8 0 , may be protected
from evaporation in a vacuum at 298 K by an overlayer
of oxide/hydroxide species, but this proposition has not
been tested.
An assignment of the high-BE components to altered
electronic states can clearly cover any of the previous
three models but, as we will demonstrate in this paper,
there are examples of non-integer oxidation states of
sulphur with high-BE components that cannot be assigned
to polysulphides, metal-deficient sulphides or elemental
sulphur.16 The assignment of S 2p3/2 BEs to any of these
models can be compromised by overlap of the BE range
in which they have been found.
Hence, it is objective of this paper to consider the
reliability of these assignments on the basis of welldocumented literature assignment supported by other techniques or experimental procedures.
DISCUSSION
In the following discussion, particular examples of sulphur
species, mineral surfaces and their reactions have been
selected from the literature, including our own results,
Received 30 November 1998
Accepted 18 December 1998
102
R. St. C. SMART ET AL.
Table 1. Structural models and confirmatory evidence for assignment to XPS high-BE S 2p3=2
components from sulphide mineral surfaces
Confirmationa
Model
1. Polysulphides Sn
2
S S bonding; Bond distances
(XAFS); Vibrations (IR, Raman)
2. Metal-deficient sulphides
(a) Host lattice: Vacancies
Defects
Substitution
(b) New lattice: Restructuring
Surface layers
M S, S S bond distances
M S coordination (XAFS)
Diffraction (XRD, ED)
M/S stoichiometry (XPS, Auger)
Diffusion
3. Elemental sulphur Sn 0
Evaporation at 295 K; S S
distances (XAFS); Vibration (IR,
Raman); S8 0 diffraction (XRD)
4. Electronic states (defects)
Altered electron distributions
(VBXPS); Non-integer M, S
oxidation states (XAFS)
a
XAFS, x-ray absorption fine structure; XRD, x-ray diffraction; ED, electron diffraction; VBXPS, valenceband XPS.
Table 2. Assignment of S 2p3=2 : sulphides (S2− , S2 2− ); polysulphides (Sn 2− ); sulphur (Sn 0 )
Speciesa
Conditions
2
S
S2
S2
Galena PbS, vac. fracture
Sphalerite ZnS, wet polished
Pyrrhotite Fe7 S8 , vac. fracture
S2 2
S4 2
S5 2
Sn 0
Pyrite FeS2 , vacuum fracture
[CuI .S4 /3 ]3
[PtIV .S5 /3 ]2
Electrochemical deposition on
Au, Pt
Sulphur (bulk)
S8 0
XPS S 2p3/2 components (eV)b
160.1 160.7
161.2
161.2(1), 162.2, S2 2 (0.14);
163.0, Sn 2 (0.17)
162.1 162.6
162.0(1); 163.0(1)
161.9(2); 162.8(2); 163.2(1)
161.4 161.8 (interface, S2 );
163.0 163.2 (surface, Sn 2 )
163.6 164.2
Refs
Other techniques
c
Refs
6, 7, 8
17
10
SRXPS
SRXPSc
6, 7
17
7, 10, 11
12
12
19
Raman 379 cm 1
IR, Raman 440 550 cm 1
IR, Raman 440 550 cm 1
No evap. at 295 Kd ;
electrochemical charge transfer
IR, Raman 469 cm 1 ; evap. at
295 Kd
11
12, 17, 18
12, 17, 18
12, 19
11, 12
11
a
Assigned by authors.
Relative to Au 4f7/2 D 83.9 eV; numbers in parentheses are relative intensities.
c
SRXPS, synchrotron radiation XPS.
d
Loss of Sn 0 by evaporation at room temperature in a vacuum.
b
over nearly two decades. The results from the selected
papers have been assessed and validated reliably in many
supporting papers that are too numerous for inclusion.
We believe that these results are widely accepted by
researchers currently working on sulphide mineral surface
reactions using surface analytical techniques.
Table 2 summarizes the S 2p3/2 BE components attributed to sulphide S2 , disulphide S2 2 , polysulfide Sn 2
and sulfur Sn 0 species. There is a progression of increasing
BE through these increasingly oxidized sulphur species
from S2 (160.1–161.2 eV), S2 2 (162.1–162.6 eV), Sn 2
(161.9–163.2 eV) to Sn 0 (163.0–164.2 eV). The polysulphide species (Table 1) have more negative S -like
end sulphur atoms with some residual negative charge
Surf. Interface Anal. 28, 101–105 (1999)
on the next nearest sulphur atoms and chain atoms closer
to neutral S0 (i.e. 163.2–163.6 eV).12 Thin layers of sulphur electrochemically deposited by anodic oxidation of
sulphide species in solution onto gold and platinum surfaces exhibited BEs corresponding to sulphide species
close to the metal interface (i.e. 161.4–161.8 eV). Multilayer deposits gave surfaces with much higher BE (i.e.
163.0–163.2 eV) corresponding to polysulphide (but not
elemental sulphur) species.19 The multilayer-deposited
sulphur films did not exhibit the volatility of bulk elemental sulphur at 298 K, indicating interaction with the underlying metal or metal sulphide. The formation of polysulphide species in solution was inferred from corresponding
electrochemical conditions for analysis of polysulphide
Copyright  1999 John Wiley & Sons, Ltd.
XPS OF SULPHIDE MINERAL SURFACES
species at a rotating disc electrode, i.e. charge transfer. A
Raman spectrum was not recorded in these experiments.
Table 3 now turns to the assignments of high-BE components on mineral surfaces as proposed by a wide selection of authors. The model of oxidation and reaction of
iron-containing sulphides is now well established.1,10,11,14,21
The development of Fe(II)–O species and, on oxidation, an Fe(III)–O overlayer predominantly as hydroxide species proceeds, leaving the underlying surface of
the original mineral increasingly deficient in iron. The
stoichiometry of this underlayer (measured by XPS or
Auger spectrometry) on pyrrhotite can be observed to
change from Fe7 S8 through Fe2 S3 to FeS2 with extended
reaction.10 This, however, does not directly imply that the
lattice structure of the surface layers has altered from that
of the original host lattice or that S–S bonding has been
formed, producing disulphide or polysulphide ions. The
effect of iron (as Fe(II) or Fe(III)) vacancies on the electronic states of neighbouring sulphur atoms has not yet
been substantiated theoretically or experimentally. There
is a contention that the almost 30% of Fe(III) in pristine
pyrrhotite surfaces, demonstrated by Pratt et al.,10 is associated with the formation of S2 2 ions but their presence
has not been confirmed directly by lattice or molecular
structural evidence. X-ray diffraction evidence for surface restructuring of pyrrhotite after acid reaction has
been obtained in one case14 where the defective tetragonal
Fe2 S3 phase was found in the surface layers. In this phase,
103
the [001] zone exhibits linear chains of Sn atoms with an
S–S distance similar to elemental sulphur S8 0 , although
the BE is still 0.2–0.6 eV less than that expected for
bulk elemental sulphur. Disulphide structures, as found in
pyrite, are not formed in this tetragonal phase nor can the
BEs be interpreted as representing polysulphide Sn 2 ions.
Raman spectroscopic evidence for polysulphide Sn 2
molecular structure has been obtained directly from oxidation of pyrite by Mycroft et al.11 However, this observation
could be made only when the anodic potential exceeded
600 mV SCE, whereas the high-BE components of the
S 2p spectra were also observed at potentials well below
this value. Continued oxidation produced elemental sulfur,
confirmed by the Raman peak at 469 cm 1 and evaporative loss at room temperature in a vacuum. The absence
of vibrational evidence for polysulphide and sulphur at
lower anodic potentials may be due to limitations on the
sensitivity of the Raman observations but this also remains
unsubstantiated at this time.
There are two other interesting cases in which substantially increased hydrophicity and hence flotation recovery
without reagents has been demonstrated. The preparation of galena surfaces following oxidation and surface
cleaning by hydrocyclone (cyclosizer) shearing action produced new, intense S 2p3/2 components in the range
162.0–162.8 eV not attributable to elemental sulphur
because the XPS measurements were carried out at room
temperature.13 Similarly, oxidation of chalcopyrite at high
Table 3. Assignment of S 2p3=2 components from sulphide mineral surfaces to polysulphides, metal deficiency and sulphur
XPS S 2p3/2 (eV)b
Speciesa
Fe7 S8 .Po/
Fe7 S8 .Po/
Fe7 S8 .Po/
S2
Conditions
Sn 2
Sn 0
Ref.
164.3(0.05)
10
10
14
Auger Fe2 S3 /Fe(III) O
XRD Fe7 S8 /Fe2 S3
10
14
20
SRXPSc
20
162.5(0.3)
20
SRXPSc
20
163.1(2)
11
Raman 440 550 cm 1 ,
Sn 2
Evap. 295 K,e Sn 0
11
S2 2
FeS2 .Py/
Vac. fracture
Air ox., 6.5 h
5 ð 10 2 M HClO4 ,
20 min, 25 ° C
Vac. fracture
FeS2 .Py/
Air ox., 10 min
FeS2 .Py/
0.6 V SCE,d
30 min, pH 5
3 ð 10 3 M Na2 S,
pH 9.2, 180 K
pH 11, air/O2 , 2 h
161.4(0.2)
161.1(1)
162.1(?)
163.0(?)
3 ð 10 4 M Na2 S,
pH 9.2, 180 K
Ground (ox.), pH 7,
cyclosizer (shear)
161.1(1)
162.1(0.2)
162.8(0.1)
pH 4.9, >0.23 V
SHE
2 ð 10 3 M HS , 0.2 V
SHE, 20 min
>50 mV SHE,
pH 4.6, 5 min
160.7(1)
FeS2 .Py/
CuFeS2
(Chpy)f
CuFeS2
(Chpy)
PbS(Ga)
PbS(Ga)
PbS(Ga)
PbS(Ga)
161.2(1)
161.2(1)
161.1(1)
162.2(0.14)
162.2(0.3)
161.3(0.3)
162.0 (surface)
(0.75)
162.7 (bulk)(1)
162.0(s) (0.5)
162.7(b) (1)
162.5(1)
163.7(0.17)
163.2(0.1)
162.9(2)
162.2(1)
163.6(1)
22
163.6(1)
162.0
162.8(1.5)
160.7(1)
21
21
13
163.9(2)
7
160.4(1)
161.7(0.15)
163.0(0.3)
8
160.7(1)
161.9(0.06)
163.1(0.09)
23
Other techniques
No Sn 0 (evap. 295 K)
Flot. recovery 100%
Evap. 295 K,e Sn 0
Ref.
21
22
21
Contact angle
50° ! 75°
Flot. recovery
18 ! 40%
AFM; evap. 295 K,e
Sn 0
No Sn 0 (evap. 295 K)
13
SRXPS
No Sn 0 (evap. 295 K)
23
7
8
a
Assigned by authors.
Values in parentheses are relative intensities.
c
SRXPS, synchrotron radiation XPS.
d
Raman not observed <0.6 V SCE.
e
Loss of Sn 0 observed by evaporation at room temperature in a vacuum.
f
Chpy.
b
Copyright  1999 John Wiley & Sons, Ltd.
Surf. Interface Anal. 28, 101–105 (1999)
104
R. St. C. SMART ET AL.
pH has long been known to produce very high flotation
recovery and the appearance of new S 2p3/2 components at
162.1 and 163.0 eV not attributable to elemental sulphur.22
In both cases, there is strong evidence that either metal
ion vacancies (i.e. Pb2C or Fe3C , respectively) or polysulphide Sn 2 species are responsible for the increased
hydrophobicity, but no direct evidence is available yet to
discriminate between these two models.
The formation of elemental sulfur Sn 0 , confirmed by
evaporative loss at room temperature, has been demonstrated in reactions under oxidative acid conditions7 and
in reactions with HS and S2 species from solution at
high pH.21 Dramatic evidence of the formation of islands
of elemental sulphur has been provided by AFM imaging of galena oxidising in acid solution. The sulphur
islands are directly correlated with an S 2p3/2 component
at 163.9 eV lost on evaporation at room temperature in a
vacuum.7 These islands only appear at anodic potentials
of >230 mV SHE, a result that is still consistent with
vacancy or polysulphide formation at potentials below this
value.13
In Table 4, the model of altered electronic states of sulphur arising from defects and substitution/reaction (activation) is examined. The reaction of Cu(II) from solution
with sphalerite surfaces has now been shown reliably9,16
to proceed by a 1 : 1 ion exchange of Cu(II) for Zn(II),
followed by reduction to Cu(I) and corresponding oxidation of adjacent sulphur ions. This mechanism is followed at both low (e.g. 5.5) and high (e.g. 8.5–9.2) pH
for solution concentrations up to at least five monolayerequivalents. At solution concentrations in excess of 10
monolayers and pH >8, precipitation of some colloidal
Cu(II) hydroxide can be observed in SIMS imaging and
in x-ray absorption fine structure (XAFS) spectra. This
mechanism at the lower concentrations has been confirmed both in nitrogen- and air-purged solutions.9,16 Using
tuned synchrotron radiation (SRXPS) at monolayer sensitivity, it has been demonstrated that new high-BE components of S 2p3/2 spectra appear first at 162.1 eV and
subsequently at 163.1 eV. Warming the sample from 180
to 295 K does not affect these components and they have
been attributed to polysulphide Sn 2 formation.9
The most recent evidence from XAFS has not found
S–S bonding to be formed in this substitution/reaction
mechanism.16 Instead, the incorporation of Cu(I) into surface Zn sites alters the Cu–S3 local structure to a distorted trigonal planar configuration with oxidation states
(determined from the copper edge in XAFS and structural modelling) corresponding to Cu(0.9) and S( 1.63).16
This assignment is consistent with the XPS evidence of
the 162.1 eV component appearing first as a surface state
in the first monolayer. Secondary ion mass spectroscopy
depth profiles from flat ZnS substrates have demonstrated
that this substitution/reaction proceeds into the bulk of
the lattice structure up to 45 monolayers deep after 18 h
of conditioning. X-ray adsorption fine structure Cu 1s
edge determination and structural modelling of the bulk
CuS4 local structure shows that one of the Cu–S bonds is
effectively broken, reverting to a distorted trigonal planar
structure for the remaining CuS3 bonding with oxidation
states corresponding to Cu(0.8) and S. 1.63/ for the CuS3
structure and S. 0.8/ for the broken Cu–S bond. The last
electronic configuration corresponds to the appearance in
XPS spectra of the 163.1 eV component with bulk substitution/reaction.
Hence, in this model, the high-BE components do not
correspond to metal deficiency, polysulphide or elemental
sulphur. The original host lattice remains intact but is
substituted, creating electronic defects at the sulphur sites.
The presence of the oxidized sulphur sites is sufficient to
impart increased hydrophobicity and flotation recovery24
without the addition of other flotation reagents.
CONCLUSIONS
It is apparent that several different assignments or model
structures can be attributed to the high-BE components
of S 2p XPS spectra of sulphide mineral surfaces after
fracture, polishing, oxidation or reaction. The BE ranges
of S 2p3/2 components of polysulphides, metal-deficient
sulphides, elemental sulphur and defect electronic states
overlap so that assignment cannot be based on this criterion alone. In addition, the electronegativity/electron
affinity and coordination number of neighbouring metal
atoms can have a substantial effect on the sulphur oxidation states and their observed BEs The central issues in
the assignment relate to:
Table 4. Assignment of S 2p3=2 components from sulphide mineral surfaces to activation, defects and substitution
Species
ZnS
Cu/ZnS
Cu/ZnS
Cu/ZnS
Cu/ZnS
Cu/ZnS
Cu(I) defect
Conditions
Wet polished
10 4 M CuSO4 , pH 9.2,
1 h, N2 -purged
10 4 M CuSO4 , pH 9.2,
1 h, air-purged
10 4 M CuSO4 , pH 9.2,
1 h, N2 -purged
10 4 M CuSO4 , pH 9.2,
1 h, air-purged
10 4 M Cu.NO3 /2
pH 5.5, 15 min,
N2 (3.7 ML)
XPS
S2
Ref.
Other techniques
162.4(0.2)
9
9
SRXPSb
Cu(I) XPS;c no Sn 0 (evap. 295 K)
9
9
161.3(1)
162.6(0.3)
9
Cu(I) XPS;c no Sn 0 (evap. 295 K)
9
161.2(1)
162.1(0.6)
163.1(0.6)
9
SRXPS; no Sn 0 (evap. 295 K)
9
161.2(1)
162.1(0.6)
163.1(1.2)
9
SRXPS; no Sn 0 ; S6 2 assignedc
9
162.1
163.1
9
XAFS:d Cu0.9 ; S 1.63 (surface)c
Cu0.8 ; S 1.63 ; S 0.8 (bulk)c
161.2
161.2(1)a
S 2p3/2
BE (eV)
Ref.
16
16
a
Values in parentheses are relative intensities.
SRXPS, synchrotron radiation XPS.
c
Assigned by authors.
d
XAFS, x-ray adsorption fine structure.
b
Surf. Interface Anal. 28, 101–105 (1999)
Copyright  1999 John Wiley & Sons, Ltd.
XPS OF SULPHIDE MINERAL SURFACES
(1) whether S–S bonding exists (as in disulphide ions,
polysulphide ions and elemental sulphur);
(2) whether the lattice structure of the original mineral surface is retained (with vacancies, coordination
defects or substitution) or is restructured to a new surface mineral phase (confirmed by diffraction or XAFS
coordination evidence);
(3) whether elemental sulphur species are present (lost
to vacuum at room temperature), requiring sample
cooling to <200 K for retention;
(4) whether altered electronic states associated with substituted atoms and defect (coordination) sites can be
confirmed (by XAFS or valence band SRXPS) without invoking S–S bonding or lattice restructuring.
On the basis of the present evidence, the following
conclusions concerning the reliability of assignment of
the high-BE S 2p components can be drawn.
(1) Assignment of high-BE components of S 2p3/2 spectra at 163.6–164.0 eV to elemental sulphur requires
105
confirmation by evaporative loss at 295 K and/or S–S
distances (XAFS, x-ray diffraction) or vibrations (IR,
Raman).
(2) Assignment of high-BE components at 162.0–163.6 eV
to polysulphides Sn 2 requires confirmation of S–S
bonding by IR, Raman or XAFS.
(3) Assignment of restructuring in surface layers requires
confirmation by diffraction (x-ray diffraction, electron
diffraction) or XAFS for altered lattice structure, M–S
and/or S–S bond coordination and distances.
(4) Assignment of electronic defect sites as vacancies,
lower coordination sites or substituted sites (activation, impurities) requires confirmation by XAFS
and/or valence-band XPS (SRXPS)
The combination of evidence from XAFS, diffraction, vibrational spectroscopy and possibly valence-band
SRXPS with XPS evidence is likely to become of increasing importance in further studies of the reactions of metal
sulphide surfaces.
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Surf. Interface Anal. 28, 101–105 (1999)