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Photoeffects at the polycrystalline pyrrhotite-electrolyte interface
Solar Energy Materials 20 (1990) 323-340 North-Holland
323
PHOTOEFFECTS AT THE POLYCRYSTALLINE PYRRHOTITEELECTROLYTE INTERFACE A.S. A R I C O a, V. A N T O N U C C I U. R U S S O c a n d N . G I O R D A N O
a,
P.L. A N T O N U C C I
b, D.L. C O C K E
a.,
a
o CNR Institute for Transformation and Storage of Energy, Salita S. Lucia sopra Contesse 39, 1-98126 Messina, Italy b Faculty of Engineerin~ Unioersity of Reggio Calabria, Via Cuzzocrea 48, 1-89100 Reggio Calabria, Italy c Department of Chemistry, Unioersity of Padooa, Via Loredan 4, 1-35131 Padova, Italy
Received 25 August 1989 Polycrystalline layers of pyrrhotite (F%.9S) mixed with PTFE and triton have been produced through a screen-printing method, followed by pressure and thermal (N2) treatments. The effect of the heat treatment as well as of the organic binders on the solid state and surface characteristics of the pyrrhotite powders and electrodes, has been analysed by XRD, XPS and M6ssbauer spectroscopy. The pyrrhotite electrodes showed behavior as photoactive n-type semiconductors giving rise to a photopotential of about 200 mV in I - and Fe 2+ containing solutions. Although the electrochemical corrosion was greatly inhibited by PTFE, the presence of a photocorrosion process was monitored at the illuminated electrode in presence of the redox electrolytes. A discussion is given as to explain the electrode behavior. Hypotheses related with its structure property are made. 1. I n t r o d u c t i o n
T h e p h o t o e l e c t r o c h e m i s t r y of s e m i c o n d u c t o r s is a n i m p o r t a n t a n d g r o w i n g a r e a o f t e c h n o l o g y [1-3]. T h e r e are two m a i n p r o b l e m s b e i n g a d d r e s s e d in this area: the m a n i p u l a t i o n of the b a n d gap a n d the s t a b i l i z a t i o n of t h e e l e c t r o d e a g a i n s t c o r r o sion. I n this c o n t e x t n e w m a t e r i a l s a r e b e i n g i n v e s t i g a t e d a n d studies o n p h o t o e l e c t r o c h e m i c a l b e h a v i o r of l o w - b a n d - g a p s e m i c o n d u c t o r s w i t h f e r r o m a g n e t i c p r o p e r ties have b e e n r e c e n t l y i n i t i a t e d [4,5]. O n e o f these m a t e r i a l s is i r o n m o n o s u l p h i d e which s h o u l d b e e x p l o r e d for its a v a i l a b i l i t y a n d low cost. I r o n m o n o s u l p h i d e exists in n a t u r e m a i n l y as n o n s t o i c h i o m e t r i c F e l _ x S ( p y r r h o tite) with h e x a g o n a l crystal structure (similar to t h a t o f N i A s ) , w h e r e o c t a h e d r a l c o o r d i n a t i o n of i r o n b y s u l p h u r a t o m s occurs [6]. T h e e x t e n t o f x in the p y r r h o t i t e f o r m u l a ranges t y p i c a l l y b e t w e e n 0 a n d 0.2: w h e n x is greater t h a n 0.07, p y r r h o t i t e shows f e r r o m a g n e t i c b e h a v i o r [7]. A l t h o u g h p y r i t e ( F e S 2) has b e e n extensively s t u d i e d in p h o t o e l e c t r o c h e m i c a l cell ( P E C ) a p p l i c a t i o n s [8-10], little w o r k has b e e n m a d e in this field o n F e l _ x S . * Visiting Professor from Department of Chemistry, Texas A&M University, College Station, TX 77843,
USA. 0165-1633/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
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A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
Pyrrhotite is of interest because of its low band gap (0.2 eV) and nonstoichiometry characteristics which need further study in photoelectrochemical investigations. This will supply information about the semiconducting properties of this material, provided that the main drawback limiting its photoelectrochemical behavior (i.e. the corrosion) is suitably controlled. In this context, recent studies of our group [11] dealing with the use of polytetrafluoroethylene (PTFE) as a binder agent for the pyrite powder have given evidence of a good rectifying behavior toward semiconductor dissolution. We present here a photoelectrochemical study of pyrrhotite screen-printed electrodes in aqueous solution containing different redox couples. The photoelectrochemical results supported by solid state characterization (XRD, XPS, MSssbauer spectroscopy), are interpreted in terms of the surface properties and electronic structure. Pyrrhotite electronic structure. In the energy band model of pyrrhotite the S 3p valence band is separated by 0.8 eV from the conduction band [12]. However, the highest states occupied by electrons in the Fe l_x S are the t2g and eg* levels derived from the Fe3d bands. Mt~ssbauer and magnetic susceptibility investigations [13] have shown that the electronic configuration of the crystal field d orbitals in the Fe 2÷ sites is a high spin quintet state i.e. (2t2gT) 3 (3eg*~)2 (2t2g~). Such a configuration results from a large spin-pairing energy P exceeding the energy separation between the two groups of the majority spin (t2ga and eg*a) electron states. The minority spin electrons in the t2gfl levels occupy states near the top of the valence band bridging most of the 0.8 eV gap and so leave only a strip of 0.2 eV at the bottom of the conduction band formed by the lowest empty eg*fl levels [12]. The t2g band formation occurs because, similarly to NiAs, the F e - F e separation along the c axis of the exagonal structure of pyrrhotite is short enough (about 2.84 [14]) to allow a direct overlapping among the t2g orbitals, with a corresponding broadening of the t2g band. The transition temperature from semiconducting to metallic behavior for Fe7S 8 is 558 K and is associated with the fl transformation, i.e. the transition from a magnetically ordered to a paramagnetic phase [15]. Such a temperature level has not been reached in the present work and only the aspects related with the semiconducting properties of the materials are hence outlined.
2. Experimental 2.1. Materials
Pyrrhotite powder was obtained from Cerac with 3N purity. The composition according to the supplier is Fe0:9S. Our XRD analysis confirmed it as hexagonal pyrrhotite. The alumina substrate was from G.E. (96% A1SiMay No. 860). Pd-Ag ink was from Ferro Corp. (No. 3432). The PTFE was Dupont 30N. The Triton X-114 was from BHD Chemicals.
A. S. A ricb et al. / P olycrystalline pyrrhotite - electrolyte interface
325
2.2. Electrode preparation
The pyrrhotite electrodes were prepared as follows: a layer of conductive P d - A g thick film was screen-printed (325 mesh) on an alumina substrate and dried at 1 5 0 ° C for 15 rain followed by sintering at 8 5 0 ° C for 15 rain. Then a paste consisting of a mixture of polycrystalline Fe0.9S powder and P T F E (Teflon 61% w / w ) as a binder, dispersed in a liquid (5% Triton X-114 in H20), was screen-printed through a 400 mesh screen to partially overlap the conductive substrate. The resulting pyrrhotite films were cold pressed at about 9400 k g / c m 2 for a few minutes, to a final thickness of about 100 btm. Afterwards, the electrodes were thermally activated at 200 ° C for three hours in an oxygen-free nitrogen flux (300 cc/rnin) and then cooled down slowly. The electrodes were finished by soldering a Cu wire to an extremity of the conductor layer and covering it with an insulating silicone rubber ( R O T H 5990 Silicon-Kautschuk) to expose only a 0.5 cm 2 of the geometrical Fel_xS surface having a typical sheet resistance of 100 f~/D. 2.3. Electrochemical characterization
Photoelectrochemical measurements were performed using a three-electrode conventional cell connected with an electrochemical set-up made of an A M E L Model 551 potentiostat, an A M E L Model 631 electrometer, an A M E L Model 567 function generator, an A M E L Model 560 interface and a Keithley Model 197 digital multimeter. A saturated calomel electrode (SCE) and a large area Pt disc were used as reference and counter electrodes, respectively. A 300 W Osram light source (310-1000 nm) placed at a distance of 16 cm from the electrode provided 100 m W / c m 2 of irradiation. All the chemicals used were of analytical grade. 2.4. Solid state analysis
Characterization of the pyrrhotite powders was performed before and after the thermal treatment (200 ° C in N 2 for three hours) using a powder diffractometer (Philips) with C u K a radiation (1.5406 A), controlled by a M 24 Olivetti PC and equipped with an automatic peak search program. The diffraction peaks were assigned, according to the ASTM cards, the Fe0.9s (25-410), FeS (11-151), Fe0.914S (25-411) and Fe0.88S (17-200). M~Sssbauer spectra were obtained at room temperature on a conventional constant acceleration spectrometer which utilized a room temperature rhodium matrix cobalt-57 source and was calibrated at room temperature with natural a-iron foil. The materials were finely ground, suspended in vaseline and wrapped in a thin aluminium foil under a rigorously controlled nitrogen atmosphere. The spectra were fit to Lorentzian line shapes by using least-squares computer minimization techniques; the quadrupole doublets were fit as two lines with equal areas and widths, while the lines of the sextets were constrained to have relative intensities 3 : 2 : 1 : 1 : 2 : 3 and widths equal in pairs. o
326
A.S. Aricb et al. / Polycrystalline pyrrhotite electrolyte interface -
Surface analysis was done with a Kratos ES 300 XPS spectrometer using A1 K s radiation at 225 W. The analyzer was run at a constant retarding ratio. The samples were pressed powders for the pyrrhotite or completed electrodes according to the above preparation procedure. Carbon from ambient contamination was used as reference at 285 eV binding energy. Ag3ds/2 at 368 eV was used as a secondary reference.
3. Results The results are presented in three aspects. The materials have been characterized with respect to their bulk, electronic and surface properties before and after activation treatments. The corrosion of the pyrrhotite was moderated by PTFE. The photoelectrochemical behavior has been studied in various electrolytes. These results provided information on the photovoltaic effect and the band level characteristics. 3.1. Characterization
The results of the bulk characterization before and after activation are shown by the XRD spectra of figs. 1 and 2. The diffraction pattern of the Fe0.9S powder showed the characteristic peaks of this hexagonal pyrrhotite (fig. 1). In addition, a lower intensity diffraction peak characteristic of the hexagonal troilite (FeS) reveals the presence of a small amount of this compound. The thermal treatment of the powders, carried out under the same conditions as for the treated electrodes, did not
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A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
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cause significant variations in the powder composition. The diffraction pattern of fig. 2 shows the Feo.9S pyrrhotite to remain the largest compound while the decrease of the peak for Troilite suggests its disappearance. At the same time the appearance of peaks associated with Feo.914S hexagonal pyrrhotite and Feo.ssS monoclinic pyrrhotite disclose that a substantial rearrangement of the iron sites in the Fel_xS superlattice is induced by the thermal treatment in an inert atmosphere. Nevertheless, this should not influence to any great extent the photoelectrochemical properties, as the electronic structure of these compounds is essentially the same. M~ssbauer spectroscopy, here employed as a typical transmission technique, gives information related with the bulk properties and composition of the material. It is necessary to stress that some differences may be found when these results are compared with those obtained by X-ray diffraction analysis. The M~Sssbauer effect spectrum of the "as-received" pyrrhotite (fig. 3a) can be reasonably fit with one quadrupole doublet and four sextets. The parameters of the first of these sextets (see table 1) are typical of a-iron and this component is responsible for about 7% of the total area. The other three sextets are due to either unequivalent iron sites present in the natural pyrrhotite, or to different phases found in this sample. For these sextets, the isomer shifts are all around 0.70 mm s -1, a value which is common to the spectra of natural pyrrhotites [13]: but on the basis of the internal fields, the sextet A of table 1 has been attributed to troilite [16]. The other two sextets may be ascribed either to mixtures of several definite structures or better to undefined situations with randomly distributed iron vacancies. The quadrupole doublet has the same hyperfine parameters as FeS2, but on the basis of the X-ray diffraction results, we consider that these species may be better attributed
A.S. Aric3 et al. / Polycrystalline pyrrhotite-electrolyte inter](tce
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to very small superparamagnetic Fel_xS particles [16,17]. The composition of the "as-received" monosulphide, as determined by the relative areas under each absorption pattern, does not appreciably change upon heating the sample under N 2 (provided that oxygen and moisture are rigorously excluded) (fig. 3b). Small
Table 1 M~Sssbauer parameters for the "as-received" pyrrhotite AEQ (mm/s)
F (mm/s)
Hin t (kOe)
A (7o)
Assignment
(mm/s) 0.31 0.01 0.74 0.76 0.58
0.61 - 0.03 -0.17 --0.11 0.17
0.33 0.20 0.21 0.22 0.25
328 311 298 259
10 7 16 25 42
Fel _xS a) Fe A FeS b) B FeS c) C FeS ¢)
a) Fine particles,
b) Troihte.
c) Pyrrhotite.
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
329
Table 2 M~Sssbauer p a r a m e t e r s for the t h e r m a l l y treated (N2, 200 o C) p y r r h o t i t e
AEQ (mm/s)
/~ (mm/s)
Hint (kOe)
A (%)
Assignment
(mm/s) 0.31 0.02 0.71 0.74 0.66
0.61 - 0.06 - 0.03 0.03 0.10
0.34 0.23 0.20 0.24 0.29
328 309 292 254
10 6 15 26 43
F e 1 - x S a) Fe A FeS b) B FeS c) C FeS O
a) F i n e particles,
b) Troilite.
¢) Pyrrhotite.
variations of the hyperfine parameters (table 2) may be attributed to little rearrangements of the crystal packing.
3.2. Surface analysis (XPS) To obtain a perspective on the nature of the surface of the pyrrhotite electrodes, two samples were examined (by XPS). The first sample, the pyrrhotite material, was pressed flat and examined without further treatment. The second sample was a completed electrode without any electrochemical treatment. The XPS results of the as-received pyrrhotite showed the surface to be partially oxidized to iron sulphates, possibly FeSO 4 and Fe2(SO4) 3. The surface was also possibly partially contaminated with O H - . However, since both the sulfate oxygen and the oxygen from O H - signals appear at approximately the same binding energy, 532 eV, the differentiation of these is not possible. The results from the completed electrode show, however, that the same signal was present and, due to the 200 ° C thermal treatment, O H - can be ruled out. The spectral results for the electrode are shown in figs. 4, 5a-5c and 5e. Fig. 4 is a general 1250 eV scan of the electrode. As can be seen, the surface is comprised only of the components from the electrode preparative process with the exception of oxygen. A small Ag 3d signal appearing at 368 eV results from conductive paste and was used as a calibration aid. The surface of this electrode is composed of Fe, S, F, C and O. The large oxygen signal results from the surface oxidation of the sulphide to sulphate. This can be seen in figs. 5a and 5b. The sight asymmetry in fig. 5b is due to charging. The F ls signal is extensively affected by charging and is broadened and shifted by 2.5 eV (fig. 5c). The high binding energy carbon is at 294 eV (fig. 5d) is also due to the fluorinated hydrocarbon and is shifted as well by 2.5 eV. This charging shift can be used to monitor the electrical connection with the other electrode components, as recently done for other electrodes in our lab [18]. F r o m this spectrum (fig. 5d), it can be stated that the fluorinated hydrocarbon compound is uniformly distributed as far as its interaction with the other conductive electrode components is concerned. The carbon ls spectrum in fig. 5d shows, in addition to the C - F peak, a high binding energy shoulder at about 287 eV which is due to the two types of oxygen from the triton solvent and some carbon contaminant on the
A.S. A rich et aL / Polycrystalline pyrrhotite electrolyte interlace
330
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BINDING ENERGY (~V) Fig. 4. AIKa (1486.6 eV) survey spectrum of the completed pyrrhotite electrode surface before the electrochemical test.
P T F E compound that is charge shifted. A main peak at 285 eV is due to surface carbonaceous carbon on the sulphide component. The oxygen spectrum in fig. 5a shows mainly oxygen associated with sulfate. A little low binding energy shoulder at 531 eV is attributed to some iron oxide. F r o m this spectrum it appears that the main surface oxidation product is sulfate and not iron oxides. The Fe 2p spectrum shown in fig. 5e shows that both Fe 2+ and Fe 3÷ oxidation states are present. The XPS of these electrodes are quite complicated and a thorough surface analysis with all controls of exposure are the subject of a future work.
3.3. Electrochemistry An electrochemical test was first carried out with a mixture of P T F E and triton (activated as in the Fel_xS electrodes) to discern the contribution of said compounds. The polarization curve in 7M K I solution showed no detectable currents. As the P T F E has been found to play an important role in the rectifying behavior of
A.S. Aric6 et al. / Polycrystalline pyrrhotite- electrolyte interface
331
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Fig. 5. XPS spectra of the completed pyrrhotite electrode before the electroehemicN test: (a) 0 ls, (b) S 2p, (c) F ls, (d) C l s and (e) Fe 2p.
A.S. Aric6 et al. / Polycrystalline pyrrhotite- electrolyte interface
332
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the SC towards corrosion (see below), the corrosion behavior of monosulphide electrodes was investigated at the extremes of the pH range (1-13). A significant difference between acidic and alkaline conditions is shown in the cyclic voltammograms (CV) in fig. 6. In alkaline solutions, the onset of the corrosion potential falls at -0.35 V (SCE) and it is followed in the anodic direction by a sharp increase in current with a wide corrosion peak at about -0.15 V (SCE). Following this, the current begins to increase with a much steeper slope when the oxygen evolution potential is reached. This behavior is very close to that found for the pyrrhotite monocrystal by Hamilton et al. [19], at the same pH value and the corrosion was attributed to the prevailing formation of sulphur. The oxidation of dissolved sulphide species and formation of Fe(OH)3 also occurs. The CV profile in an acidic environment (pH = 1 for HaSt4) appeared more flat and showed no peaks even after several cycles. The absence of any remarkable corrosion phenomenon at this pH value was quite surprising and in disagreement with the previous literature. In fact the thermodynamic plot of E h versus pH for the F e - S - H 2 0 system [19], shows that no stability region exists at pH < 3 for pyrrhotite. Such an astounding observation can be attributed in our opinion to the presence of the PTFE chains surrounding the Fe l_xS grains. The polymer chains axe present and available considering that the electrode activation temperature (i.e. 200 ° C) is not sufficient to degrade the polymer chains [20]. Moreover, PTFE can also act as an inhibiting agent against corrosion by imparting hydrophobic properties to the electrode surface. This prevents the attack of the aqueous electrolyte at the edges
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
333
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Fig. 7. Polarization curves of the completed pyrrhotite electrode in 1M (lower curves) and 7M (higher curves) iodide containing solutions (pH = 1). In the dark (dashed curve) and under 100 mW/cm2 of illumination (full curve). and defects (the most susceptible sites toward corrosion) of the sulphide surface. For this reason, an electrolyte of pH = 1 was selected in the present study to avoid corrosion phenomena. In fig. 7 the polarization curves are reported, starting from the rest potential of pyrrhotite in iodide solution, to avoid positive ion injection into the semiconductor surface under cathodic bias. The measurements of the curves were carried out either in the light or in the dark, at two different electrolyte concentrations (i.e. 1M and 7M, the p H was adjusted using HI) with a potential scan rate of 1 m V / s . A significant cathodic shift of the anodic current onset in the light with respect to the dark was revealed as well as observed by changing, the concentration. For this latter parameter a non-Nerstian dependency was found. In 7M iodide the curves showed three distinct peaks, which cathodically shifted in the light to - 0 . 4 7 5 V (peak A), - 0 . 2 5 V (peak B), 0.1 V (peak C), against the SCE. This behavior indeed reflects the presence of a junction at the electrode-electrolyte interface and a distinctive n-type behavior for the Fe 1_x S semiconductor. To identify the peak associated with the photogenerated charge carrier transfer to t h e redox electrolyte we have carried out CV experiments of the Fe l_xS electrodes at higher scan rate (10 m V / s ) in 7M KI and in 0.1M FeC12 solutions. In 7M KI the three anodic peaks were again monitored (fig. 8) but the shape of the voltammogram and the relative peak current values different slightly from those in fig. 7. Peak A appeared in the dark and in the light but it was not followed by a relation counter peak in the cathodic scan. Peak B appeared only in the light and differed from the 1 m V / s voltammogram and was followed by a counter peak in the cathodic scan with a corresponding peak separation not exceeding 0.059 V. The cathodic shift of the formal potential from V~ed was about 0.2 V. Peak C was present in both the light and the dark, with the related counter peaks having a separation of 80 and 100 mV, respectively.
A.S. A rich et al. / Polycrystalline pyrrhotite- electrolyte interface
334
C
B
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P o t e n t i a l / V vs, S C E Fig. 8. Cyclic voltammetry of the completed pyrrhotite electrode in 7M iodide solutions (pH = l). In the dark (dashed curve) and under 100 m W / c m 2 of illumination (full curve).
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A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
335
~9
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Potential/V vs. SCE Fig. 10. Cyclic voltammetryof the pyrrhotite completed electrode in 1M bromide solution (pH = 1), In the dark (dashed curve) and under 100 mW/cm2 (full curve). The CV profile in a solution containing Fe2÷/HC1 solution shows (fig. 9) peak A (both present in the light and in the dark) at the same potential value of the previous voltammogram (fig. 8). Also, in this case, no cathodic peak was recorded. Peak B appeared in the light and in the dark at a more negative value with respect to the redox potential in solution (about 0.2 V). The last sharp increase in current is either due to the electrochemical oxidation of Fe 2÷ to Fe 3+ or the oxidation of the semiconductor which occurs in the same potential range. This obscures the observability of peak C. The Fel_xS semiconductor was also tested in HBr (1M) and the results are shown in fig. 10. The measured redox potential of the electrolyte was 0.8 V (SCE). Significant anodic currents at potential values lower than the redox potential, were detected with no difference in the dark and in the light. N o photocurrent was detected also using higher electrolyte concentrations with addition of KBr.
4. Discussion
The photoelectrochemical results are interesting because it appears that an n-type behavior occurs on our pyrrhotite electrodes and a 0.2 V photovoltaic effect never found in previous solid state work on this material has been discovered. By a comparison of the first three voltammograms (figs. 7-9), it appears that peak A is associated with an irreversible photocorrosion process being present at the same potential value in the different electrolyte environments. Peak B, characterized by a good reversibility in the light, appears to be related to a transfer of the
A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
336
OFF
ON
OFF
ON
ON 50 S ,_--..--
Time Fig. 11. Anodic current for the Fe 1_x S semiconductor in 7M iodide solution (pH = 1), under square wave illumination with an applied electrode potential of 0 V (SCE). The electrode surface is 0.5 cm 2.
photogenerated minority carriers to the redox states in the electrolyte. By applying the potential value of peak B in iodide solution and alternatively illuminating the electrode surface (fig. 11), one observes a sluggish increase in current. This is consistent with the poor quality of the semiconductor and with the preferential absorption of IR radiation of the polychromatic light. The behavior appears sufficiently reversible confirming the occurrence of a photoelectrochemical transfer process. The reversible peak C which appears at V > ~ o x and the electrochemical reaction occurring at higher potential values are related with the electrochemical oxidation of the redox species and with the corrosion of the semiconductor, as outlined above. In order to point out the influence of the additives (PTFE and triton) on the electrode behavior, we gleaned from the surface analytical results some important aspects which require a specific discussion. The starting material (Fe0.9S) shows the presence of surface sulfate formation when exposed to the air environment due to its air sensitive character. There is little indication that iron oxide is present in the spectra of the "as-received" pyrrhotite and completed electrodes. Now it is important to realize that the sulfate on the surface will dissolve into the solution prior to the electrochemical measurements, owing to its high solubility (296 g/g FeSO4 at 25°C) [21]. On the other hand, iron oxides are much less soluble and could contribute to the electrodics during these experiments. The fact that we have found little surface iron oxides assures that the latter is not the case. The charging of the P T F E component is informative in that the broad peak related to carbon-bonded fluorine is attributed to a charging interaction of a uniformly distributed P T F E on the surface. This is confirmed by the absence of differential charging, as it has been seen previously with the P T F E binder [18], where a non-uniform interaction with the conductive components has been observed. Furthermore the absence of O H - may be due to either the heat treatment or to
A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
337
v(scE) --0`5
EFB CORROSION LEVEL
--0.4 --0.3 --0,2 --0.1
C.B.
0 +0.1
EF'SC"
....
_tF,t_t . . . .
+O2 +O.3 V.IL
Fe~.x$ SEMICONDUCTOR ELECTROLYTE
Fig. 12. Schematic diagram of the energetic levels at the Fel_xS electrode-electrolyte interface under operating conditions under illumination. Charge transfer processes occurring in the dark (i.e. tunneling by surface states) and under illumination are indicated.
the PTFE induced hydrophobic surface or both and accounts well for the corrosion considerations discussed above. The behavior of the Fel_xS semiconductor is hence interpreted in terms of the band-like model of the semiconductor-electrolyte junction. Assuming that the flat band potential in KI solution is that of the anodic photocurrent onset and considering an energy gap of 0.2 eV, we have outlined the thermodynamic model as shown in fig. 12. It is known that the flat band potential determination by the onset of photocurrent results is only approximate because it disregards the semiconductor recombination phenomena [22]. In this case however, other available methods such as capacitance measurements of the semiconductor-electrolyte interface have not given reproducible results. On the basis of our results, we have interpreted the charge transfer process at the interface mainly considering mechanisms involving electronic energy levels localized at the surface, within the SC band :gap (surface states) and originated by strong adsorption of electrolyte species on the electrodes. The electrochemical potential for the adsorbed ions is generally quite different from that of the same species in the electrolyte [23], depending on the activity difference in the two phases and the chemisorption energy. We suggest two mechanisms occurring simultaneously during the anodic sweeps: inelastic charge transfer in the electrolyte of the photogenerated valence band holes mediated by trapping in surface states and injection assisted by tunneling of electrons in the conduction band. Some evidence supporting the surface states electrochemical pathway lies on the
338
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
occurrence of the photocurrent onset almost at the same potential range for the KI and FeC12 and the absence of any photocurrent in the HBr redox electrolyte with its potential quite positive with respect to the band-gap level position. Hence, in the latter case, the band-edge unpinning [24] (frequently occurring on small-band-gap semiconductors) is not verified while in I - and Fe 2+ electrolytes photocurrent occurs and depends strongly upon the concentration. Our opinion is that in the presence of appropriate concentrations of these electrolytes, the energy band level position remains pinned by the electronic levels of the surface states. Hence, for the supra-band-edge reactions, the transfer of thermalized photogenerated holes is achieved through their trapping in the surface states lying above the redox level in solution and the successive transfer in the electrolyte. The oxidation in the dark through the surface states occurs when the electrode potential (the Fermi level in the semiconductor) goes down to the corresponding electronic energy levels on the surface. This allows the transfer of electrons in the semiconductor conduction band through a direct mechanism or through deeplying-states-assisted tunneling. As it is known, the iso-energetic transfer by tunneling [25] requires a short width for the space charge region, so it is favoured by a high doping level and a large band bending. For these reasons, the electron tunneling mechanism is the most obviously responsible of the dark currents at V < Vr~dox. To explain the effect of surface states at the F e S 2 - I - electrolyte interface, it has been suggested [9] that the I - hole acceptors are involved in the coordination of unsaturated Fe 2÷ and possibly Fe 3+ surface sites,thus developing a fast charge transfer through an inner coordination sphere mechanism. In our view-point this situation may also occur, even accentuated, for polycrystalline non-stoichiometric iron sulphides such as pyrrhotite. This is possibly also responsible for the lack of rectifying behavior in dark. Furthermore, our results show, especially in the iodide case, that a fast chemical adsorption occurs when the iron sulphide is first immersed in solution (decreased of the electrode potential at open circuit). The observation, upon anodic polarizations in the dark, of oxidation peaks whose potentials remain almost constant with sweeps, whereas the current of these peaks decreases in a way that is inversely proportional to the time of storage at open circuit potential, suggests that the I - adsorbed species are fully oxidized to 13 at V < Vredo xThe formation of iron iodide (FeI2) at the solid-liquid interface is improbable. The thermodynamic stability of this compound (Gf (Fela) = -132.23 K J / m o l [26]) is lower than that of pyrrhotite (Gf(FeTSs) = -748.52 K J / m o l [27]). A similar adsorption process occurs also in the presence of FeCt 2 as redox electrolyte and this suggests that this mechanism is truly related with the surface coordination of local unsaturated Fe 1_xS sites by the ions in the solution, as pointed out above. As for the photocorrosion process, the transfer of photogenerated holes from the SC VB to the redox corrosion level localized in the band gap is described by the following equation: Fe I _x S + 2 h + ~ (1 - x ) Fe2++ S. The redox level of the above equation lies above the Fermi levels of the redox zouples here investigated. In all the cases, it produces the cathodic shift of the
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
339
anodic current onset either in the dark or under illumination. Otherwise, it is difficult to explain this result on the basis of a photovoltaic effect. The absence of the photoeorrosion peak in Br- seems to be related with the fact that, as has been observed in many semiconductors [28], the oxidized corrosion sites may act as recombination sites. This controls the carrier transfer at low band bending. On the other hand, I--induced surface states have been recognized to passivate the surface recombination centers [29] allowing current onset at lower voltages. This is confirmed in fig. 7 by the different photoeurrent onset with different concentrations. The observed shift of peak B in the light of the n-type pyrrhotite is associated with a photovoltaic effect as stated above. Yet, this does not agree with the solid state characteristics of monocrystalline pyrrhotite, according to which the semiconductor should have p-type conduction and an energy gap not exceeding 0.2 eV [12]. To explain such a discrepancy the observed results could be interpreted asserting that pressure and heat treatments in the course of the preparation procedure may have caused significant modifications in the actual crystal lattice, in such a way to increase the crystal field by sulphur atoms a n d / o r spin pairing of the d electrons. As the semiconducting properties of pyrhotite have been interpreted through an antiferromagnetic arrangement of the Fe atomic spins [15] the antiferromagnetic order splits the impurity band levels, that occur in pyrrhotite, due to the iron vacancies and makes the material semiconductive. On the other hand, calculations made by Tossell [30], based on a self-consistent field X-ray scattered wave method have demonstrated that, due to its high compressibility, a spin pairing in the Fe 3d electrons of the iron monosulphide structure under pressure may occur. This phenomenon has been observed by M~ssbauer spectroscopy in magnetic iron sulphides subjected to pressure treatments. The spin-pairing effect is lost when the pressure is released with a more or less rapid kinetics depending upon the pressure value and the morphological characteristics of the material [31]. This phenomenon may take place on our pyrrhotite electrodes during the pressure treatment. The spin-paired arrangement may be stabilized by the successive thermal activation. The pyrrhotite electronic structure FeSI+ x in this case can be considered, as a limiting case, similar to that of a largely substoichiometric pyrite, whose predominant n-type behavior is derived from the S vacancies, i.e. FeS2_ x and photovoltaic effect in I-/I~- have been well established [10,11]. The only explanation which we can give at this time for the maintenance of the spin pairing after the pressure release lies on the presence of PTFE chains surrounding the pyrrhotite grains. Such a hypothesis is speculative, and is based on the evidence that neither corrosion resistance nor photoeffects are recorded in the absence of PTFE a n d / o r pressure treatments. Yet, the MiSssbauer hyperfine parameters of the powders without PTFE and pressure treatment are typical of the natural pyrrhotites and do not show any significant difference that can be attributed to substantial variations of the electronic structure. However, this might not be the case for the treated electrodes. A confirmation of the hypotheses here made could be investigated by M/Sssbauer spectroscopy in a reflectance configuration on PTFE-bonded pyrrhotite electrodes.
340
A.S. Aricb et al. / Polycrystalline pyrrhotite - electro(vte interface
S u c h an a n a l y s i s likely will s u p p l y useful i n f o r m a t i o n on the e l e c t r o n i c s t r u c t u r e of the surface, a n d t h u s o n the o b s e r v e d p h o t o e l e c t r o c h e m i c a l b e h a v i o r . W e are currently planning such experiments.
Acknowledgments T h e a u t h o r s a r e g r a t e f u l to M i s s E. M o d i c a a n d to M r . G. M o n f o r t e for t h e i r cooperation.
References [1] S.U.M. Khan and J.O'M. Bockris, in: Modern Aspects of Electrochemistry, Eds. J.O'M. Bockris, B.E. Conway and R.E. White (Plenum, New York, 1982). [2] H. Gerischer, J. Electroanal. Chem. 150 (1983) 553. [3] A.J. Bard, J. Electroanal. Chem. 168 (1984) 5. [4] M.S. Antonius, M. Etman, M. Guyot and T. Merceron, Mater. Res. Bull. 21 (1986) 1515. [5] M. Etman, T. Merceron and M. Nenmann-SpaUart, J. Electroanal. Chem. 260 (1989) 227. [6] R.A. Robie, P.M. Bethke and K.M. Beardsley, US Geol. Surv. Bull. (t967) No. 1248. [7] G. Gottardi, in: I Minerali, Ed. Boringhieri (Torino, 1984). [8] A. Ennaui, S. Fiechter, W. Jaegermann and H. Tributsch, J. Electrochem. Soc. 132 (1985) 1579. [9] X.P. Li, N. Alonso-Vante and H. Tributsch, J. Electroanal. Chem. 242 (1988) 255. [10] N. Alonso-Vante, G. Chatzitheodorou, S. Fiecbter, N. Mgoduka Poulios and H. Tributsch, Sol. Energy Mater. 18 (1988) 9. [11] V. Antonucci, A.S. Ailco, M. Minutoli and N. Giordano, Extended Abstract No. 359, Electrochemical Society Meeting, Los Angeles, CA, May 1989. [12] S. Sakkopoulos, E. Vitoratos and T. Argyreas, J. Phys. Chem. Solids 45 (1984) 923. [13] J.A. Morice, L.V.C. Rees and D.T. Rickard, J. Inorg. Nucl. Chem. 31 (1969) 3797. [14] J.B. Goodenough, J. Appl. Phys. Suppl. 33 (1962) 1197. [15] S. Sakkopoulos, J. Appl. Phys. 59 (1986) 3540. [16] P. Busiere, A. Newman and B.K. Sharma, Hyerfine Interact. 43 (1988) 1211. [17] P.P. Vaishnava, P.A. Montanu, R.E. Tischer and S.S. Pollack, J. Catal. 78 (1982) 1211. [18] V. Alderncci, V. Recupero, L. Pino, F. Parmigiani, D.L. Cocke and N. Giordano, J. Appl. Electrochem., submitted. [19] I.C. Hamilton and R. Woods, J. Electroanal. Chem. 118 (1981) 327. [20] T. Moil, J. Imahashi, T. Kamo, K. Tamura and Y. Hishinuma, J. Electrochem. So=. 133 (1986) 896. [21] D. Nicliolls, in: Comprehensive inorganic Chemistry, Eds. J.C. Bailar, H.J. Emelens, R. Nyholm and A.F. Trotman-Dickenson (Pergamon, Oxford, 1975). [22] M.A. Buttler, J. Appl. Phys. 68 (1977) 1914. [23] D.S. Ginley and M.A. Butler, in: Photoeffects at Semiconductor-Electrolyte Interfaces, Ed. A.J. Nozik, ACS Symposium Series (American Chemical Society, Washington, 1981) p. 79. [24] J.A. Turner, J. Manassen and A.J. Nozik, in: Photoeffects at Semiconductor-Electrolyte Interfaces, Ed. A.J. Nozik, ACS Symposium Series (American Chemical Society, Washington, 1981) p. 253. [25] J.P. Szabo and M. Cocivera, J. Electroanal. Chem. 239 (1988) 307. [26] A. Khandhar, V.B. Tare and J.B. Wagner, J. Electrochem. Soc. 130 (1983) 880. [27] R.T. Lowson, Chem. Rev. 82 (1982) 461. [28] R.H. Wilson, in: Photoeffects at Semiconductor-Electrolyte Interfaces, Ed. A.J. Nozik, ACS Symposium Series (American Chemical Society, Washington, 1981) p. 103. [29] W.K. Hofmann, H.J. Lewerenz and C. Pettenkofer, Sol. Energy Mater. 17 (1988) 165. [30] J.A. Tossel, J. Chem. Phys. 66 (1977) 5712. [31] C.B. Bargeron, M. Avinor and H.G. Drickamer, Inorg. Chem. 10 (1971) 1338.
323
PHOTOEFFECTS AT THE POLYCRYSTALLINE PYRRHOTITEELECTROLYTE INTERFACE A.S. A R I C O a, V. A N T O N U C C I U. R U S S O c a n d N . G I O R D A N O
a,
P.L. A N T O N U C C I
b, D.L. C O C K E
a.,
a
o CNR Institute for Transformation and Storage of Energy, Salita S. Lucia sopra Contesse 39, 1-98126 Messina, Italy b Faculty of Engineerin~ Unioersity of Reggio Calabria, Via Cuzzocrea 48, 1-89100 Reggio Calabria, Italy c Department of Chemistry, Unioersity of Padooa, Via Loredan 4, 1-35131 Padova, Italy
Received 25 August 1989 Polycrystalline layers of pyrrhotite (F%.9S) mixed with PTFE and triton have been produced through a screen-printing method, followed by pressure and thermal (N2) treatments. The effect of the heat treatment as well as of the organic binders on the solid state and surface characteristics of the pyrrhotite powders and electrodes, has been analysed by XRD, XPS and M6ssbauer spectroscopy. The pyrrhotite electrodes showed behavior as photoactive n-type semiconductors giving rise to a photopotential of about 200 mV in I - and Fe 2+ containing solutions. Although the electrochemical corrosion was greatly inhibited by PTFE, the presence of a photocorrosion process was monitored at the illuminated electrode in presence of the redox electrolytes. A discussion is given as to explain the electrode behavior. Hypotheses related with its structure property are made. 1. I n t r o d u c t i o n
T h e p h o t o e l e c t r o c h e m i s t r y of s e m i c o n d u c t o r s is a n i m p o r t a n t a n d g r o w i n g a r e a o f t e c h n o l o g y [1-3]. T h e r e are two m a i n p r o b l e m s b e i n g a d d r e s s e d in this area: the m a n i p u l a t i o n of the b a n d gap a n d the s t a b i l i z a t i o n of t h e e l e c t r o d e a g a i n s t c o r r o sion. I n this c o n t e x t n e w m a t e r i a l s a r e b e i n g i n v e s t i g a t e d a n d studies o n p h o t o e l e c t r o c h e m i c a l b e h a v i o r of l o w - b a n d - g a p s e m i c o n d u c t o r s w i t h f e r r o m a g n e t i c p r o p e r ties have b e e n r e c e n t l y i n i t i a t e d [4,5]. O n e o f these m a t e r i a l s is i r o n m o n o s u l p h i d e which s h o u l d b e e x p l o r e d for its a v a i l a b i l i t y a n d low cost. I r o n m o n o s u l p h i d e exists in n a t u r e m a i n l y as n o n s t o i c h i o m e t r i c F e l _ x S ( p y r r h o tite) with h e x a g o n a l crystal structure (similar to t h a t o f N i A s ) , w h e r e o c t a h e d r a l c o o r d i n a t i o n of i r o n b y s u l p h u r a t o m s occurs [6]. T h e e x t e n t o f x in the p y r r h o t i t e f o r m u l a ranges t y p i c a l l y b e t w e e n 0 a n d 0.2: w h e n x is greater t h a n 0.07, p y r r h o t i t e shows f e r r o m a g n e t i c b e h a v i o r [7]. A l t h o u g h p y r i t e ( F e S 2) has b e e n extensively s t u d i e d in p h o t o e l e c t r o c h e m i c a l cell ( P E C ) a p p l i c a t i o n s [8-10], little w o r k has b e e n m a d e in this field o n F e l _ x S . * Visiting Professor from Department of Chemistry, Texas A&M University, College Station, TX 77843,
USA. 0165-1633/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
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A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
Pyrrhotite is of interest because of its low band gap (0.2 eV) and nonstoichiometry characteristics which need further study in photoelectrochemical investigations. This will supply information about the semiconducting properties of this material, provided that the main drawback limiting its photoelectrochemical behavior (i.e. the corrosion) is suitably controlled. In this context, recent studies of our group [11] dealing with the use of polytetrafluoroethylene (PTFE) as a binder agent for the pyrite powder have given evidence of a good rectifying behavior toward semiconductor dissolution. We present here a photoelectrochemical study of pyrrhotite screen-printed electrodes in aqueous solution containing different redox couples. The photoelectrochemical results supported by solid state characterization (XRD, XPS, MSssbauer spectroscopy), are interpreted in terms of the surface properties and electronic structure. Pyrrhotite electronic structure. In the energy band model of pyrrhotite the S 3p valence band is separated by 0.8 eV from the conduction band [12]. However, the highest states occupied by electrons in the Fe l_x S are the t2g and eg* levels derived from the Fe3d bands. Mt~ssbauer and magnetic susceptibility investigations [13] have shown that the electronic configuration of the crystal field d orbitals in the Fe 2÷ sites is a high spin quintet state i.e. (2t2gT) 3 (3eg*~)2 (2t2g~). Such a configuration results from a large spin-pairing energy P exceeding the energy separation between the two groups of the majority spin (t2ga and eg*a) electron states. The minority spin electrons in the t2gfl levels occupy states near the top of the valence band bridging most of the 0.8 eV gap and so leave only a strip of 0.2 eV at the bottom of the conduction band formed by the lowest empty eg*fl levels [12]. The t2g band formation occurs because, similarly to NiAs, the F e - F e separation along the c axis of the exagonal structure of pyrrhotite is short enough (about 2.84 [14]) to allow a direct overlapping among the t2g orbitals, with a corresponding broadening of the t2g band. The transition temperature from semiconducting to metallic behavior for Fe7S 8 is 558 K and is associated with the fl transformation, i.e. the transition from a magnetically ordered to a paramagnetic phase [15]. Such a temperature level has not been reached in the present work and only the aspects related with the semiconducting properties of the materials are hence outlined.
2. Experimental 2.1. Materials
Pyrrhotite powder was obtained from Cerac with 3N purity. The composition according to the supplier is Fe0:9S. Our XRD analysis confirmed it as hexagonal pyrrhotite. The alumina substrate was from G.E. (96% A1SiMay No. 860). Pd-Ag ink was from Ferro Corp. (No. 3432). The PTFE was Dupont 30N. The Triton X-114 was from BHD Chemicals.
A. S. A ricb et al. / P olycrystalline pyrrhotite - electrolyte interface
325
2.2. Electrode preparation
The pyrrhotite electrodes were prepared as follows: a layer of conductive P d - A g thick film was screen-printed (325 mesh) on an alumina substrate and dried at 1 5 0 ° C for 15 rain followed by sintering at 8 5 0 ° C for 15 rain. Then a paste consisting of a mixture of polycrystalline Fe0.9S powder and P T F E (Teflon 61% w / w ) as a binder, dispersed in a liquid (5% Triton X-114 in H20), was screen-printed through a 400 mesh screen to partially overlap the conductive substrate. The resulting pyrrhotite films were cold pressed at about 9400 k g / c m 2 for a few minutes, to a final thickness of about 100 btm. Afterwards, the electrodes were thermally activated at 200 ° C for three hours in an oxygen-free nitrogen flux (300 cc/rnin) and then cooled down slowly. The electrodes were finished by soldering a Cu wire to an extremity of the conductor layer and covering it with an insulating silicone rubber ( R O T H 5990 Silicon-Kautschuk) to expose only a 0.5 cm 2 of the geometrical Fel_xS surface having a typical sheet resistance of 100 f~/D. 2.3. Electrochemical characterization
Photoelectrochemical measurements were performed using a three-electrode conventional cell connected with an electrochemical set-up made of an A M E L Model 551 potentiostat, an A M E L Model 631 electrometer, an A M E L Model 567 function generator, an A M E L Model 560 interface and a Keithley Model 197 digital multimeter. A saturated calomel electrode (SCE) and a large area Pt disc were used as reference and counter electrodes, respectively. A 300 W Osram light source (310-1000 nm) placed at a distance of 16 cm from the electrode provided 100 m W / c m 2 of irradiation. All the chemicals used were of analytical grade. 2.4. Solid state analysis
Characterization of the pyrrhotite powders was performed before and after the thermal treatment (200 ° C in N 2 for three hours) using a powder diffractometer (Philips) with C u K a radiation (1.5406 A), controlled by a M 24 Olivetti PC and equipped with an automatic peak search program. The diffraction peaks were assigned, according to the ASTM cards, the Fe0.9s (25-410), FeS (11-151), Fe0.914S (25-411) and Fe0.88S (17-200). M~Sssbauer spectra were obtained at room temperature on a conventional constant acceleration spectrometer which utilized a room temperature rhodium matrix cobalt-57 source and was calibrated at room temperature with natural a-iron foil. The materials were finely ground, suspended in vaseline and wrapped in a thin aluminium foil under a rigorously controlled nitrogen atmosphere. The spectra were fit to Lorentzian line shapes by using least-squares computer minimization techniques; the quadrupole doublets were fit as two lines with equal areas and widths, while the lines of the sextets were constrained to have relative intensities 3 : 2 : 1 : 1 : 2 : 3 and widths equal in pairs. o
326
A.S. Aricb et al. / Polycrystalline pyrrhotite electrolyte interface -
Surface analysis was done with a Kratos ES 300 XPS spectrometer using A1 K s radiation at 225 W. The analyzer was run at a constant retarding ratio. The samples were pressed powders for the pyrrhotite or completed electrodes according to the above preparation procedure. Carbon from ambient contamination was used as reference at 285 eV binding energy. Ag3ds/2 at 368 eV was used as a secondary reference.
3. Results The results are presented in three aspects. The materials have been characterized with respect to their bulk, electronic and surface properties before and after activation treatments. The corrosion of the pyrrhotite was moderated by PTFE. The photoelectrochemical behavior has been studied in various electrolytes. These results provided information on the photovoltaic effect and the band level characteristics. 3.1. Characterization
The results of the bulk characterization before and after activation are shown by the XRD spectra of figs. 1 and 2. The diffraction pattern of the Fe0.9S powder showed the characteristic peaks of this hexagonal pyrrhotite (fig. 1). In addition, a lower intensity diffraction peak characteristic of the hexagonal troilite (FeS) reveals the presence of a small amount of this compound. The thermal treatment of the powders, carried out under the same conditions as for the treated electrodes, did not
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A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
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cause significant variations in the powder composition. The diffraction pattern of fig. 2 shows the Feo.9S pyrrhotite to remain the largest compound while the decrease of the peak for Troilite suggests its disappearance. At the same time the appearance of peaks associated with Feo.914S hexagonal pyrrhotite and Feo.ssS monoclinic pyrrhotite disclose that a substantial rearrangement of the iron sites in the Fel_xS superlattice is induced by the thermal treatment in an inert atmosphere. Nevertheless, this should not influence to any great extent the photoelectrochemical properties, as the electronic structure of these compounds is essentially the same. M~ssbauer spectroscopy, here employed as a typical transmission technique, gives information related with the bulk properties and composition of the material. It is necessary to stress that some differences may be found when these results are compared with those obtained by X-ray diffraction analysis. The M~Sssbauer effect spectrum of the "as-received" pyrrhotite (fig. 3a) can be reasonably fit with one quadrupole doublet and four sextets. The parameters of the first of these sextets (see table 1) are typical of a-iron and this component is responsible for about 7% of the total area. The other three sextets are due to either unequivalent iron sites present in the natural pyrrhotite, or to different phases found in this sample. For these sextets, the isomer shifts are all around 0.70 mm s -1, a value which is common to the spectra of natural pyrrhotites [13]: but on the basis of the internal fields, the sextet A of table 1 has been attributed to troilite [16]. The other two sextets may be ascribed either to mixtures of several definite structures or better to undefined situations with randomly distributed iron vacancies. The quadrupole doublet has the same hyperfine parameters as FeS2, but on the basis of the X-ray diffraction results, we consider that these species may be better attributed
A.S. Aric3 et al. / Polycrystalline pyrrhotite-electrolyte inter](tce
328
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to very small superparamagnetic Fel_xS particles [16,17]. The composition of the "as-received" monosulphide, as determined by the relative areas under each absorption pattern, does not appreciably change upon heating the sample under N 2 (provided that oxygen and moisture are rigorously excluded) (fig. 3b). Small
Table 1 M~Sssbauer parameters for the "as-received" pyrrhotite AEQ (mm/s)
F (mm/s)
Hin t (kOe)
A (7o)
Assignment
(mm/s) 0.31 0.01 0.74 0.76 0.58
0.61 - 0.03 -0.17 --0.11 0.17
0.33 0.20 0.21 0.22 0.25
328 311 298 259
10 7 16 25 42
Fel _xS a) Fe A FeS b) B FeS c) C FeS ¢)
a) Fine particles,
b) Troihte.
c) Pyrrhotite.
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
329
Table 2 M~Sssbauer p a r a m e t e r s for the t h e r m a l l y treated (N2, 200 o C) p y r r h o t i t e
AEQ (mm/s)
/~ (mm/s)
Hint (kOe)
A (%)
Assignment
(mm/s) 0.31 0.02 0.71 0.74 0.66
0.61 - 0.06 - 0.03 0.03 0.10
0.34 0.23 0.20 0.24 0.29
328 309 292 254
10 6 15 26 43
F e 1 - x S a) Fe A FeS b) B FeS c) C FeS O
a) F i n e particles,
b) Troilite.
¢) Pyrrhotite.
variations of the hyperfine parameters (table 2) may be attributed to little rearrangements of the crystal packing.
3.2. Surface analysis (XPS) To obtain a perspective on the nature of the surface of the pyrrhotite electrodes, two samples were examined (by XPS). The first sample, the pyrrhotite material, was pressed flat and examined without further treatment. The second sample was a completed electrode without any electrochemical treatment. The XPS results of the as-received pyrrhotite showed the surface to be partially oxidized to iron sulphates, possibly FeSO 4 and Fe2(SO4) 3. The surface was also possibly partially contaminated with O H - . However, since both the sulfate oxygen and the oxygen from O H - signals appear at approximately the same binding energy, 532 eV, the differentiation of these is not possible. The results from the completed electrode show, however, that the same signal was present and, due to the 200 ° C thermal treatment, O H - can be ruled out. The spectral results for the electrode are shown in figs. 4, 5a-5c and 5e. Fig. 4 is a general 1250 eV scan of the electrode. As can be seen, the surface is comprised only of the components from the electrode preparative process with the exception of oxygen. A small Ag 3d signal appearing at 368 eV results from conductive paste and was used as a calibration aid. The surface of this electrode is composed of Fe, S, F, C and O. The large oxygen signal results from the surface oxidation of the sulphide to sulphate. This can be seen in figs. 5a and 5b. The sight asymmetry in fig. 5b is due to charging. The F ls signal is extensively affected by charging and is broadened and shifted by 2.5 eV (fig. 5c). The high binding energy carbon is at 294 eV (fig. 5d) is also due to the fluorinated hydrocarbon and is shifted as well by 2.5 eV. This charging shift can be used to monitor the electrical connection with the other electrode components, as recently done for other electrodes in our lab [18]. F r o m this spectrum (fig. 5d), it can be stated that the fluorinated hydrocarbon compound is uniformly distributed as far as its interaction with the other conductive electrode components is concerned. The carbon ls spectrum in fig. 5d shows, in addition to the C - F peak, a high binding energy shoulder at about 287 eV which is due to the two types of oxygen from the triton solvent and some carbon contaminant on the
A.S. A rich et aL / Polycrystalline pyrrhotite electrolyte interlace
330
-
O(A) KVV
FI(A) LMM LMV LVV
~2p
,¢
!
v
t
C1S
Will
1200
1000
800
600
400
200
0
BINDING ENERGY (~V) Fig. 4. AIKa (1486.6 eV) survey spectrum of the completed pyrrhotite electrode surface before the electrochemical test.
P T F E compound that is charge shifted. A main peak at 285 eV is due to surface carbonaceous carbon on the sulphide component. The oxygen spectrum in fig. 5a shows mainly oxygen associated with sulfate. A little low binding energy shoulder at 531 eV is attributed to some iron oxide. F r o m this spectrum it appears that the main surface oxidation product is sulfate and not iron oxides. The Fe 2p spectrum shown in fig. 5e shows that both Fe 2+ and Fe 3÷ oxidation states are present. The XPS of these electrodes are quite complicated and a thorough surface analysis with all controls of exposure are the subject of a future work.
3.3. Electrochemistry An electrochemical test was first carried out with a mixture of P T F E and triton (activated as in the Fel_xS electrodes) to discern the contribution of said compounds. The polarization curve in 7M K I solution showed no detectable currents. As the P T F E has been found to play an important role in the rectifying behavior of
A.S. Aric6 et al. / Polycrystalline pyrrhotite- electrolyte interface
331
01s a S2p
)Z
_z
177
173
169
165
161
157
BINDING ENERGY (eV) 538
$42
534
530
526
BINDING ENERGY (eV)
Fls
Cls
d
Z
Z
i
b~
,
692
J
J
688
i
,
684
,
J
680
i
i
i
676
302
i 298
i
BINDING ENERGY (eV)
i 294
T
I 290
I
J 286
v
I 282
•
BINDING ENERGY (*V)
e
Fe 2 p 3
F. 2pj.
746
736
726
1~ ~
7;6
BINDING ENERGY
706
696
(eV)
Fig. 5. XPS spectra of the completed pyrrhotite electrode before the electroehemicN test: (a) 0 ls, (b) S 2p, (c) F ls, (d) C l s and (e) Fe 2p.
A.S. Aric6 et al. / Polycrystalline pyrrhotite- electrolyte interface
332
II
// II
1.2i
,/ I
'7 0.0
/
<
/
jf~
8
/ !
J
! I
//
0.4
/ ///
~J
/ /
T.
/
¢" /
"~
/i
//
f-
.....
/
.-?'~ f"
i
i/
~.
J
i
i ~ /
i .I
I"
0-
>i /.~5."
-0.4-0.6
-o14
'
Poter, t i a l / V
6 vs.
024 F~CE
Fig. 6. Cyclic voltammctry of the completed pyrrhotite electrode in alkaline, pH ~ 13 for KOH addition (dashed curve) and acidic, pH = 1 for H2SO4 addition (point-broken curve) solutions.
the SC towards corrosion (see below), the corrosion behavior of monosulphide electrodes was investigated at the extremes of the pH range (1-13). A significant difference between acidic and alkaline conditions is shown in the cyclic voltammograms (CV) in fig. 6. In alkaline solutions, the onset of the corrosion potential falls at -0.35 V (SCE) and it is followed in the anodic direction by a sharp increase in current with a wide corrosion peak at about -0.15 V (SCE). Following this, the current begins to increase with a much steeper slope when the oxygen evolution potential is reached. This behavior is very close to that found for the pyrrhotite monocrystal by Hamilton et al. [19], at the same pH value and the corrosion was attributed to the prevailing formation of sulphur. The oxidation of dissolved sulphide species and formation of Fe(OH)3 also occurs. The CV profile in an acidic environment (pH = 1 for HaSt4) appeared more flat and showed no peaks even after several cycles. The absence of any remarkable corrosion phenomenon at this pH value was quite surprising and in disagreement with the previous literature. In fact the thermodynamic plot of E h versus pH for the F e - S - H 2 0 system [19], shows that no stability region exists at pH < 3 for pyrrhotite. Such an astounding observation can be attributed in our opinion to the presence of the PTFE chains surrounding the Fe l_xS grains. The polymer chains axe present and available considering that the electrode activation temperature (i.e. 200 ° C) is not sufficient to degrade the polymer chains [20]. Moreover, PTFE can also act as an inhibiting agent against corrosion by imparting hydrophobic properties to the electrode surface. This prevents the attack of the aqueous electrolyte at the edges
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
333
i,6< t~
1.2-
B
,,4.1 N ¢1
Vr~dox O,8-
u
0,4-
0" -0.6
-o),
ti
0.4
Potential/V v~SCE
Fig. 7. Polarization curves of the completed pyrrhotite electrode in 1M (lower curves) and 7M (higher curves) iodide containing solutions (pH = 1). In the dark (dashed curve) and under 100 mW/cm2 of illumination (full curve). and defects (the most susceptible sites toward corrosion) of the sulphide surface. For this reason, an electrolyte of pH = 1 was selected in the present study to avoid corrosion phenomena. In fig. 7 the polarization curves are reported, starting from the rest potential of pyrrhotite in iodide solution, to avoid positive ion injection into the semiconductor surface under cathodic bias. The measurements of the curves were carried out either in the light or in the dark, at two different electrolyte concentrations (i.e. 1M and 7M, the p H was adjusted using HI) with a potential scan rate of 1 m V / s . A significant cathodic shift of the anodic current onset in the light with respect to the dark was revealed as well as observed by changing, the concentration. For this latter parameter a non-Nerstian dependency was found. In 7M iodide the curves showed three distinct peaks, which cathodically shifted in the light to - 0 . 4 7 5 V (peak A), - 0 . 2 5 V (peak B), 0.1 V (peak C), against the SCE. This behavior indeed reflects the presence of a junction at the electrode-electrolyte interface and a distinctive n-type behavior for the Fe 1_x S semiconductor. To identify the peak associated with the photogenerated charge carrier transfer to t h e redox electrolyte we have carried out CV experiments of the Fe l_xS electrodes at higher scan rate (10 m V / s ) in 7M KI and in 0.1M FeC12 solutions. In 7M KI the three anodic peaks were again monitored (fig. 8) but the shape of the voltammogram and the relative peak current values different slightly from those in fig. 7. Peak A appeared in the dark and in the light but it was not followed by a relation counter peak in the cathodic scan. Peak B appeared only in the light and differed from the 1 m V / s voltammogram and was followed by a counter peak in the cathodic scan with a corresponding peak separation not exceeding 0.059 V. The cathodic shift of the formal potential from V~ed was about 0.2 V. Peak C was present in both the light and the dark, with the related counter peaks having a separation of 80 and 100 mV, respectively.
A.S. A rich et al. / Polycrystalline pyrrhotite- electrolyte interface
334
C
B
'rr"
-i-
P o t e n t i a l / V vs, S C E Fig. 8. Cyclic voltammetry of the completed pyrrhotite electrode in 7M iodide solutions (pH = l). In the dark (dashed curve) and under 100 m W / c m 2 of illumination (full curve).
B
1.6-
/
1.2-
A
z:/
~ - 0.6-
,~ o.4-
!o-0.4-
/; o:8
-o~
oi2
o
i2
o~
o6
Potential/V vs,~E Fig. 9. Cyclic voltammetry of the completed pyrrhotite electrode in 0.4M Fe 2+ solution (pH = 1). In the dark (dashed curve) and under 100 m W / c m 2 of illumination (full curve).
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
335
~9
¢0 "0
-i
4.,,
4.2
o
o',,
Potential/V vs. SCE Fig. 10. Cyclic voltammetryof the pyrrhotite completed electrode in 1M bromide solution (pH = 1), In the dark (dashed curve) and under 100 mW/cm2 (full curve). The CV profile in a solution containing Fe2÷/HC1 solution shows (fig. 9) peak A (both present in the light and in the dark) at the same potential value of the previous voltammogram (fig. 8). Also, in this case, no cathodic peak was recorded. Peak B appeared in the light and in the dark at a more negative value with respect to the redox potential in solution (about 0.2 V). The last sharp increase in current is either due to the electrochemical oxidation of Fe 2÷ to Fe 3+ or the oxidation of the semiconductor which occurs in the same potential range. This obscures the observability of peak C. The Fel_xS semiconductor was also tested in HBr (1M) and the results are shown in fig. 10. The measured redox potential of the electrolyte was 0.8 V (SCE). Significant anodic currents at potential values lower than the redox potential, were detected with no difference in the dark and in the light. N o photocurrent was detected also using higher electrolyte concentrations with addition of KBr.
4. Discussion
The photoelectrochemical results are interesting because it appears that an n-type behavior occurs on our pyrrhotite electrodes and a 0.2 V photovoltaic effect never found in previous solid state work on this material has been discovered. By a comparison of the first three voltammograms (figs. 7-9), it appears that peak A is associated with an irreversible photocorrosion process being present at the same potential value in the different electrolyte environments. Peak B, characterized by a good reversibility in the light, appears to be related to a transfer of the
A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
336
OFF
ON
OFF
ON
ON 50 S ,_--..--
Time Fig. 11. Anodic current for the Fe 1_x S semiconductor in 7M iodide solution (pH = 1), under square wave illumination with an applied electrode potential of 0 V (SCE). The electrode surface is 0.5 cm 2.
photogenerated minority carriers to the redox states in the electrolyte. By applying the potential value of peak B in iodide solution and alternatively illuminating the electrode surface (fig. 11), one observes a sluggish increase in current. This is consistent with the poor quality of the semiconductor and with the preferential absorption of IR radiation of the polychromatic light. The behavior appears sufficiently reversible confirming the occurrence of a photoelectrochemical transfer process. The reversible peak C which appears at V > ~ o x and the electrochemical reaction occurring at higher potential values are related with the electrochemical oxidation of the redox species and with the corrosion of the semiconductor, as outlined above. In order to point out the influence of the additives (PTFE and triton) on the electrode behavior, we gleaned from the surface analytical results some important aspects which require a specific discussion. The starting material (Fe0.9S) shows the presence of surface sulfate formation when exposed to the air environment due to its air sensitive character. There is little indication that iron oxide is present in the spectra of the "as-received" pyrrhotite and completed electrodes. Now it is important to realize that the sulfate on the surface will dissolve into the solution prior to the electrochemical measurements, owing to its high solubility (296 g/g FeSO4 at 25°C) [21]. On the other hand, iron oxides are much less soluble and could contribute to the electrodics during these experiments. The fact that we have found little surface iron oxides assures that the latter is not the case. The charging of the P T F E component is informative in that the broad peak related to carbon-bonded fluorine is attributed to a charging interaction of a uniformly distributed P T F E on the surface. This is confirmed by the absence of differential charging, as it has been seen previously with the P T F E binder [18], where a non-uniform interaction with the conductive components has been observed. Furthermore the absence of O H - may be due to either the heat treatment or to
A.S. Aricb et al. / Polycrystalline pyrrhotite- electrolyte interface
337
v(scE) --0`5
EFB CORROSION LEVEL
--0.4 --0.3 --0,2 --0.1
C.B.
0 +0.1
EF'SC"
....
_tF,t_t . . . .
+O2 +O.3 V.IL
Fe~.x$ SEMICONDUCTOR ELECTROLYTE
Fig. 12. Schematic diagram of the energetic levels at the Fel_xS electrode-electrolyte interface under operating conditions under illumination. Charge transfer processes occurring in the dark (i.e. tunneling by surface states) and under illumination are indicated.
the PTFE induced hydrophobic surface or both and accounts well for the corrosion considerations discussed above. The behavior of the Fel_xS semiconductor is hence interpreted in terms of the band-like model of the semiconductor-electrolyte junction. Assuming that the flat band potential in KI solution is that of the anodic photocurrent onset and considering an energy gap of 0.2 eV, we have outlined the thermodynamic model as shown in fig. 12. It is known that the flat band potential determination by the onset of photocurrent results is only approximate because it disregards the semiconductor recombination phenomena [22]. In this case however, other available methods such as capacitance measurements of the semiconductor-electrolyte interface have not given reproducible results. On the basis of our results, we have interpreted the charge transfer process at the interface mainly considering mechanisms involving electronic energy levels localized at the surface, within the SC band :gap (surface states) and originated by strong adsorption of electrolyte species on the electrodes. The electrochemical potential for the adsorbed ions is generally quite different from that of the same species in the electrolyte [23], depending on the activity difference in the two phases and the chemisorption energy. We suggest two mechanisms occurring simultaneously during the anodic sweeps: inelastic charge transfer in the electrolyte of the photogenerated valence band holes mediated by trapping in surface states and injection assisted by tunneling of electrons in the conduction band. Some evidence supporting the surface states electrochemical pathway lies on the
338
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
occurrence of the photocurrent onset almost at the same potential range for the KI and FeC12 and the absence of any photocurrent in the HBr redox electrolyte with its potential quite positive with respect to the band-gap level position. Hence, in the latter case, the band-edge unpinning [24] (frequently occurring on small-band-gap semiconductors) is not verified while in I - and Fe 2+ electrolytes photocurrent occurs and depends strongly upon the concentration. Our opinion is that in the presence of appropriate concentrations of these electrolytes, the energy band level position remains pinned by the electronic levels of the surface states. Hence, for the supra-band-edge reactions, the transfer of thermalized photogenerated holes is achieved through their trapping in the surface states lying above the redox level in solution and the successive transfer in the electrolyte. The oxidation in the dark through the surface states occurs when the electrode potential (the Fermi level in the semiconductor) goes down to the corresponding electronic energy levels on the surface. This allows the transfer of electrons in the semiconductor conduction band through a direct mechanism or through deeplying-states-assisted tunneling. As it is known, the iso-energetic transfer by tunneling [25] requires a short width for the space charge region, so it is favoured by a high doping level and a large band bending. For these reasons, the electron tunneling mechanism is the most obviously responsible of the dark currents at V < Vr~dox. To explain the effect of surface states at the F e S 2 - I - electrolyte interface, it has been suggested [9] that the I - hole acceptors are involved in the coordination of unsaturated Fe 2÷ and possibly Fe 3+ surface sites,thus developing a fast charge transfer through an inner coordination sphere mechanism. In our view-point this situation may also occur, even accentuated, for polycrystalline non-stoichiometric iron sulphides such as pyrrhotite. This is possibly also responsible for the lack of rectifying behavior in dark. Furthermore, our results show, especially in the iodide case, that a fast chemical adsorption occurs when the iron sulphide is first immersed in solution (decreased of the electrode potential at open circuit). The observation, upon anodic polarizations in the dark, of oxidation peaks whose potentials remain almost constant with sweeps, whereas the current of these peaks decreases in a way that is inversely proportional to the time of storage at open circuit potential, suggests that the I - adsorbed species are fully oxidized to 13 at V < Vredo xThe formation of iron iodide (FeI2) at the solid-liquid interface is improbable. The thermodynamic stability of this compound (Gf (Fela) = -132.23 K J / m o l [26]) is lower than that of pyrrhotite (Gf(FeTSs) = -748.52 K J / m o l [27]). A similar adsorption process occurs also in the presence of FeCt 2 as redox electrolyte and this suggests that this mechanism is truly related with the surface coordination of local unsaturated Fe 1_xS sites by the ions in the solution, as pointed out above. As for the photocorrosion process, the transfer of photogenerated holes from the SC VB to the redox corrosion level localized in the band gap is described by the following equation: Fe I _x S + 2 h + ~ (1 - x ) Fe2++ S. The redox level of the above equation lies above the Fermi levels of the redox zouples here investigated. In all the cases, it produces the cathodic shift of the
A.S. Aricb et al. / Polycrystalline pyrrhotite-electrolyte interface
339
anodic current onset either in the dark or under illumination. Otherwise, it is difficult to explain this result on the basis of a photovoltaic effect. The absence of the photoeorrosion peak in Br- seems to be related with the fact that, as has been observed in many semiconductors [28], the oxidized corrosion sites may act as recombination sites. This controls the carrier transfer at low band bending. On the other hand, I--induced surface states have been recognized to passivate the surface recombination centers [29] allowing current onset at lower voltages. This is confirmed in fig. 7 by the different photoeurrent onset with different concentrations. The observed shift of peak B in the light of the n-type pyrrhotite is associated with a photovoltaic effect as stated above. Yet, this does not agree with the solid state characteristics of monocrystalline pyrrhotite, according to which the semiconductor should have p-type conduction and an energy gap not exceeding 0.2 eV [12]. To explain such a discrepancy the observed results could be interpreted asserting that pressure and heat treatments in the course of the preparation procedure may have caused significant modifications in the actual crystal lattice, in such a way to increase the crystal field by sulphur atoms a n d / o r spin pairing of the d electrons. As the semiconducting properties of pyrhotite have been interpreted through an antiferromagnetic arrangement of the Fe atomic spins [15] the antiferromagnetic order splits the impurity band levels, that occur in pyrrhotite, due to the iron vacancies and makes the material semiconductive. On the other hand, calculations made by Tossell [30], based on a self-consistent field X-ray scattered wave method have demonstrated that, due to its high compressibility, a spin pairing in the Fe 3d electrons of the iron monosulphide structure under pressure may occur. This phenomenon has been observed by M~ssbauer spectroscopy in magnetic iron sulphides subjected to pressure treatments. The spin-pairing effect is lost when the pressure is released with a more or less rapid kinetics depending upon the pressure value and the morphological characteristics of the material [31]. This phenomenon may take place on our pyrrhotite electrodes during the pressure treatment. The spin-paired arrangement may be stabilized by the successive thermal activation. The pyrrhotite electronic structure FeSI+ x in this case can be considered, as a limiting case, similar to that of a largely substoichiometric pyrite, whose predominant n-type behavior is derived from the S vacancies, i.e. FeS2_ x and photovoltaic effect in I-/I~- have been well established [10,11]. The only explanation which we can give at this time for the maintenance of the spin pairing after the pressure release lies on the presence of PTFE chains surrounding the pyrrhotite grains. Such a hypothesis is speculative, and is based on the evidence that neither corrosion resistance nor photoeffects are recorded in the absence of PTFE a n d / o r pressure treatments. Yet, the MiSssbauer hyperfine parameters of the powders without PTFE and pressure treatment are typical of the natural pyrrhotites and do not show any significant difference that can be attributed to substantial variations of the electronic structure. However, this might not be the case for the treated electrodes. A confirmation of the hypotheses here made could be investigated by M/Sssbauer spectroscopy in a reflectance configuration on PTFE-bonded pyrrhotite electrodes.
340
A.S. Aricb et al. / Polycrystalline pyrrhotite - electro(vte interface
S u c h an a n a l y s i s likely will s u p p l y useful i n f o r m a t i o n on the e l e c t r o n i c s t r u c t u r e of the surface, a n d t h u s o n the o b s e r v e d p h o t o e l e c t r o c h e m i c a l b e h a v i o r . W e are currently planning such experiments.
Acknowledgments T h e a u t h o r s a r e g r a t e f u l to M i s s E. M o d i c a a n d to M r . G. M o n f o r t e for t h e i r cooperation.
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