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Transformation of amorphous iron oxide films pre-deposited by spray pyrolysis into FeS2-pyrite films
Materials Letters 59 (2005) 734 – 739 www.elsevier.com/locate/matlet
Transformation of amorphous iron oxide films pre-deposited by spray pyrolysis into FeS2-pyrite films B. Ouertania, J. Ouerfellib,1, M. Saadouna, B. BessaRsa,*, M. Hajjia, M. Kanzaric,1, H. Ezzaouiaa, N. Hamdadoud, J.C. Berne`dee a
Institut National de Recherche Scientifique et Technique, Laboratoire de Photovoltaı¨que et des Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisie b Faculte´ des Sciences de Tunis, Laboratoire de Physique de la Matie`re Condense´e, El Manar II-Tunis, Tunisie c Ecole Nationale des Inge´nieurs deTunis, Laboratoire des cellules Photovoltaı¨ques et des Mate´riaux Semiconducteurs, El Manar II–Tunis, Tunisie d Laboratoire de Physique des Mate´riaux et Composants pour lTElectronique, Universite´ dTOran, BP 92208 El Mnaouer Oran, Algeria e Equipe de Physique des Solides pour lTElectronique, Groupe Couches Minces et Mate´riaux Nouveaux, Universite´ de Nantes, FSTN, 2 rue de la Houssinie`re, BP 9209 44322 Nantes Cedex 3, France Received 17 February 2004; accepted 29 June 2004 Available online 16 December 2004
Abstract Amorphous iron oxide films were prepared by spray pyrolysis of FeCl3d 6H2O (0.03 M)-based aqueous solution onto glass substrates heated at 350 8C. Rust red iron oxide films were obtained. Crystallized iron oxide films were achieved after heat treatments in vacuum. Xray diffraction (XRD) patterns show two polycrystalline iron oxide phases. The dominant phase corresponds to Fe2O3 and the minor one corresponds to traces of FeOOH, which transforms immediately to Fe2O3 after further heat treatment. Scanning electron microscopy (SEM) microanalyses show that stoichiometric Fe2O3 films were obtained. Sulphuration at 4508C for 6 h under vacuum (104 Torr) transforms the amorphous iron oxide films into FeS2-pyrite films having good crystallinity. SEM images show that the FeS2 films are granular. The homogeneity and both dimension and distribution of the FeS2 grain sizes were found to depend on the sulphuration temperature of the predeposited amorphous iron oxide films. First optical analyses enabled us to deduce a direct energy band gap of about 0.72 eV. All the prepared FeS2 films show p-type conduction. D 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.50.Nw; 61.66.Fn; 68.37.Hk; 78.40.Fy Keywords: Thin films; Iron oxide; Spray pyrolysis; Pyrite; FeS2; Sulphuration
1. Introduction FeS2-pyrite is an interesting material for solar energy conversion devices such as battery [1–5] and photoelectrochemical solar cells [6] due to its very high optical absorption coefficient (a~105 cm1 for kV700 nm) and its suitable energy band gap [7,8]. Considerable progress has been made since Wfhler [9] first prepared artificial pyrite by reaction of Fe2O3 with liquid sulphur and ammonium chloride in an open system at the 19th century, and succeeded in obtaining small * Corresponding author. Tel.: +216 71 430 160; fax: +216 1 430 934. E-mail address: [email protected] (B. BessaRs). 1 Tel.: +216 71 872 600. 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.06.074
brass-yellow octahedron. The high absorption coefficient, which is two orders of magnitude higher than that of crystalline silicon, would make it possible to allow the use of very thin absorption layers. Due to the various difficulties found in growing large synthetic pieces, strong efforts are nowadays devoted to obtain FeS2 thin films suitable for their application in solar energy conversion in addition to their use as positive electrodes in Al/FeS2 and Li/FeS2 batteries [10,11]. Several methods have been attempted to prepare pyrite thin films: MOCVD [12], spray pyrolysis in a controlled atmosphere [13], reaction of Fe2O3 or Fe3O4 with elemental sulphur [14], reactive ion beam sputtering [15], electrodeposition [16,17] and sulphurization of iron thin films [6]. In this paper, we describe an inexpensive, non toxic
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735
Fig. 1. (a) XRD patterns of the iron oxide films annealed under a vacuum of 104 Pa at different temperatures for 5 h; (b) plotted ASTM JCPDS data 33-663 for Fe2O3.
and easy to manipulate method to prepare pyrite thin films: sulphurization of iron oxide thin films pre-deposited on glass substrates by spray pyrolysis under very simple conditions. A single-phase pyrite film on a glass substrate with very high optical absorption coefficient (~105 cm1) was obtained. The different phases were determined using X-ray diffraction (XRD) with a Siemens D500 powder diffractometer (CuKa radiation) in the usual h/2h geometry. The microstructure of the iron oxide and pyrite films were observed by scanning electron microscopy (SEM).
at 350 8C and the presence of two iron oxide phases: the dominant one corresponds to Fe2O3 and the traces correspond to FeOOH, which were transformed after further thermal treatments to Fe2O3 [18]. Indeed, Diamandescu et al. [18] showed that, for temperatures higher than 350 8C, the microporosity of the iron oxide particles increased; this was attributed to the water loss during the crystallisation. The corresponding dehydration reaction may be written as: 2FeOOHYFe2 O3 þ H2 O: In this work, the traces of FeOOH were ignored. The Fe2O3 peaks intensities correspond to those in JCPDS data (33-663). The SEM microanalyses results showed in Table 1 confirm the stoechiometry of the Fe2O3 phase detected by the XRD technique (Fig. 1).
2. Growth of the iron oxide films 2.1. Experimental procedure An aqueous solution of FeCl3d 6H2O (0.03 M) was prepared and sprayed with a gas vector (N2) on glass substrates, which were placed on a hotplate heated at 350 8C. The as-prepared solution was sprayed on the heated glass substrates during 30 min, with a speed rate of 5 ml/min. 2.2. XRD patterns After spray pyrolysis, rust red amorphous films were obtained. To crystallise the obtained phase, the as-prepared films were annealed in a vacuum (~104 Pa) sealed tube at different temperatures (250 8C, 300 8C, 350 8C and 400 8C) for 5 h. The XRD patterns (Fig. 1) show a good crystallinity
3. Transformation of the amorphous iron oxide to FeS2-pyrite films The following reaction corresponds to the transformation of Fe2O3 iron oxide, into FeS2:
. The negative value of the free enthalpy, corresponding to the reaction, shows that the latter can be made sponta-
Table 1 Microprobe SEM analyses taken in three different zones of a Fe2O3 film thermally treated at 350 8C for 5 h in a vacuum (~104 Pa) sealed tube Zone
Elements
El (wt.%)
Norm (wt.%)
Prec.
Atomic %
Line
O/Fe
1
Fe O Fe O Fe O
65.52 28.50 65.20 27.68 65.03 28.88
65.91 28.67 68.46 29.07 64.69 28.73
2.50 0.55 2.600 0.56 2.62 0.55
37.85 57.48 39.76 58.92 36.81 57.06
K K K K K K
1.519
2 3
line line line line line line
1.482 1.550
736
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
Fig. 2. XRD patterns of FeS2-pyrite thin films prepared under a vacuum of 104 Pa: (a) for 6 h at different sulphuration temperatures, (b) at 450 8C for different sulphuration durations, (c) plotted ASTM: JCPDF data 42-1340 for FeS2.
grain size of the sulphurated films were determined by SEM.
neously in the direct direction (1) in the case of the presence of all the reagents. This reaction leads to the substitution of oxygen by sulphur. Thus, we had the idea to apply this reaction to amorphous iron oxide films that can easily crystallise into FeS2 films. In fact, the substitution mechanism is much more difficult for crystallized materials than for amorphous ones.
3.2. Results and discussion 3.2.1. XRD patterns The optimisation of the pyrite growth parameters was pointed out using XRD patterns. The latter show a singlephase pyrite films (Fig. 2). Fig. 2(a) shows that for duration of 6 h the crystallinity increased with sulphuration temperature in a temperature range of 300 8C–450 8C. A good crystallinity was obtained at 450 8C. At 500 8C, the crystallinity decreases. The peak corresponding to the (200) line is the highest in the tested films then line corresponding the (311) direction. The bar lines in the plot indicate XRD patterns of pyrite powder with the positions and the relative intensities of the diffraction peaks (JCPDS data 42-1340). The XRD patterns show that the as-prepared films crystallised in the expected cubic structure. Significant decrease of the line intensities occurred by increasing the sulphuration temperature up to 450 8C. D. Wan et al. [19] showed the same dependence: great decrease of the intensities of the two highest peaks (200) and (311) by increasing the sulphuration temperature up to 500 8C. Once the sulphuration temperature
3.1. Experimental procedures The prepared amorphous iron oxide layers were sulphured for various temperatures and durations in vacuum (~104 Pa) sealed tubes. The sulphuration parameters such as the temperature and the duration were optimized using XRD. The sulphuration pressure was fixed during all the experience at about 104 Pa. The other sulphuration parameters such as the temperature and the duration were optimized in two steps. In the first step, the duration was fixed arbitrarily to 6 h and the temperature was varied. In the second step, the sulphuration temperature was fixed at 450 8C, at which a good crystallinity was obtained, and the duration was varied. Optical absorption spectra of pyrite thin films grown at the optimum conditions were recorded using a spectrophotometer. The surface morphology and the
Table 2 Microprobe SEM analyses of the FeS2-pyrite thin films synthesised at 450 8C and 500 8C during 6 h under a vacuum of about 104 Pa FeS2 (450 8C)
FeS2 (500 8C)
Zone 1
Zone 2
Zone 1
Zone 2
Elements
Fe
S
Fe
S
Fe
S
Fe
S
El (wt.%) Norm (wt.%) Prec. Atomic% Line
45.18 45.31 2.17 31.84 K line 1.93
49.86 50.15 0.64 61.20 K line
45.25 45.70 2.17 32.63 K line 1.95
50.69 51.19 0.65 63.68 K line
44.42 45.65 2.17 31.83 K line 2.0063
51.15 52.57 0.66 63.86 K line
44.55 46.08 2.68 32.55 K line 2.0003
51.15 52.90 0.67 65.11 K line
S Fe
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
737
optimised and fixed at 450 8C, the duration was varied (2 h, 4 h, 6 h and 8 h). A good crystallinity was obtained for a duration of 6 h (Fig. 2(b)). The pyrite films grown at 450 8C for 6 h seem to be practically stoichiometric. Microprobe SEM analysis (Table 2) shows a slight deficiency of sulphur for films prepared at 450 8C compared to the stoichiometric FeS2. This deficiency can be corrected by a slight rise in the sulphuration temperature (Table 2). 3.2.2. Microstructure morphology of the pyrite films Fig. 3 shows SEM micrographs surface views at different magnifications of pyrite FeS2 films annealed at 450 8C (a and b) and at 500 8C (c and d) under a vacuum (~104 Pa) for 6 h. The surface of the films prepared at 450 8C (Fig. 4(a)) appears granular and homogeneous. At higher magnification (Fig. 4(b)), the surface of these layers appears granular but rather homogeneous with grain size dimensions around 250 nm. For the films prepared at 500 8C (Fig. 4(c)), the surface appears granular but inhomogeneous. Indeed, at higher magnification (Fig. 4(d)), the surface of these layers appears more granular, apparently with two grain size distributions. The largest grains have a size dimension of about 370 nm and the smallest ones of about 70 nm. The microstructure morphology of FeS2 films synthesised at 500 8C (Fig. 4(d)) is similar to that obtained by Hamdadou et al. [6], who used pre-evaporated iron films. The SEM crosssection (Fig. 4) shows that the as-prepared pyrite films have a good adherence to the substrate. An average value
Fig. 4. SEM cross section of view of a FeS2-pyrite film synthesised at 450 8C for 6 h under a vacuum of about 104 Pa.
of the thickness of the films synthesised at 450 8C for 6 h was estimated at about 800 nm. 3.2.3. Optical properties Fig. 5(a) depicts the optical absorption spectra versus photon energy (ht) for FeS2 films sulphurated at 450 8C for 6 h under a vacuum of about 104 Pa. A high absorption coefficient (a~5d 104 cm1) was observed for wavelengths lower than 1000 nm. Plots of (aht)n/2 versus the photon energy ht, with n=1 and n=4 are given in Fig. 5(b,c). The variation of (aht)1/2 vs. ht gives a straight line, suggesting that the FeS2 films have an indirect band gap energy of 0.72 eV. Fig. 5(c) shows that the first direct transition of the FeS2-pyrite thin films, prepared in this work, occurs at about
Fig. 3. Scanning electron microscopy views of FeS2 films sulphurated from sprayed pre-deposited iron oxide layers at 450 8C (a and b) and at 500 8C (c and d) for 6 h under a vacuum of about 104 Pa.
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Fig. 5. (a) Optical absorption coefficient of the FeS2-pyrite thin films synthesised at 450 8C for 6 h in a vacuum of 104 Pa; (b) (ahm)1/2 vs. hm and (c) (ahm)2 vs. hm.
0.87 eV. The values of the optical absorption obtained in this work (Fig. 5(a)) are similar to those obtained by other workers [19].
This work was supported by the Secre´tariat d’Etat a` la Recherche scientifique et a` la technologie. References
4. Conclusion During this work, homogeneous FeS2-pyrite thin films having good crystallinity were synthesized by simple sulphuration of amorphous iron oxide films pre-deposited by spray pyrolysis. Optimum sulphuration temperature and duration were found at 450 8C and 6 h, respectively. The high absorption coefficient (a~5d 104 cm1) and the relatively large grain size (~250 nm) observed for these FeS2-pyrite films may be promising to be tested in photovoltaic and photoelectrochemical applications.
Acknowledgements The authors would like to thank Dr. Ferid M. for its support in XRD patterns.
[1] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonso-Vante, K. Bqker, M. Bronold, C. Hfpfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (1993) 289. [2] M. Bronold, S. Fiechter, C. Pettenkofer, W. Jaegermann, Thin Solid Films 304 (1997) 178 – 182. [3] C. de las Heras, G. Lifante, J. Appl. Phys. 82 (10) (1997) 5132 – 5137. [4] E. Peled, D. Golodnitsky, E. Strauss, J. Lang, Y. Lavi, Electrochim. Acta 43 (10–11) (1998) 1593 – 1599. [5] V. Euert, K.-H. Hock, S. Fiechter, H. Tributsch, Phys. Rev., B 57 (11) (1998) 101. [6] N. Hamdadou, A. Khelil, J.C. Berne`de, Mater. Chem. Phys. 78 (2003) 591 – 601. [7] W. Jaegermann, H. Tributsch, J. Appl. Electrochem. 13 (1983) 743. [8] A. Ennaoui, H. Tributsch, Sol. Cells 13 (1948) 197. [9] W. Wfhler, Liebigs Ann. 17 (1836) 260. [10] T.A. Aselage, E.E. Hellstrom, J. Electrochem. Soc. 134 (1987) 1932. [11] M.B. Clark, in: J.-B. Gabano (Ed.), Lithium Batteries, Academic Press, New York, 1983, p. 115.
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[16] A.S. Arico, V. Antonucci, P.L. Anntonucci, D.L. Cocke, N. Giordano, Electrochim. Acta 36 (1991) 581. [17] S. Nakamura, A. Yamamoto, Sol. Energy Mater. Sol. Cells 65 (2001) 79. [18] L. Diamandescu, et al., Solid State Ionics 101–103 (1997) 591 – 596. [19] D. Wan, Y. Wang, B. Wang, C. Ma, H. Sun, L. Wei, J. Cryst. Growth 253 (2003) 230 – 238.
Transformation of amorphous iron oxide films pre-deposited by spray pyrolysis into FeS2-pyrite films B. Ouertania, J. Ouerfellib,1, M. Saadouna, B. BessaRsa,*, M. Hajjia, M. Kanzaric,1, H. Ezzaouiaa, N. Hamdadoud, J.C. Berne`dee a
Institut National de Recherche Scientifique et Technique, Laboratoire de Photovoltaı¨que et des Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisie b Faculte´ des Sciences de Tunis, Laboratoire de Physique de la Matie`re Condense´e, El Manar II-Tunis, Tunisie c Ecole Nationale des Inge´nieurs deTunis, Laboratoire des cellules Photovoltaı¨ques et des Mate´riaux Semiconducteurs, El Manar II–Tunis, Tunisie d Laboratoire de Physique des Mate´riaux et Composants pour lTElectronique, Universite´ dTOran, BP 92208 El Mnaouer Oran, Algeria e Equipe de Physique des Solides pour lTElectronique, Groupe Couches Minces et Mate´riaux Nouveaux, Universite´ de Nantes, FSTN, 2 rue de la Houssinie`re, BP 9209 44322 Nantes Cedex 3, France Received 17 February 2004; accepted 29 June 2004 Available online 16 December 2004
Abstract Amorphous iron oxide films were prepared by spray pyrolysis of FeCl3d 6H2O (0.03 M)-based aqueous solution onto glass substrates heated at 350 8C. Rust red iron oxide films were obtained. Crystallized iron oxide films were achieved after heat treatments in vacuum. Xray diffraction (XRD) patterns show two polycrystalline iron oxide phases. The dominant phase corresponds to Fe2O3 and the minor one corresponds to traces of FeOOH, which transforms immediately to Fe2O3 after further heat treatment. Scanning electron microscopy (SEM) microanalyses show that stoichiometric Fe2O3 films were obtained. Sulphuration at 4508C for 6 h under vacuum (104 Torr) transforms the amorphous iron oxide films into FeS2-pyrite films having good crystallinity. SEM images show that the FeS2 films are granular. The homogeneity and both dimension and distribution of the FeS2 grain sizes were found to depend on the sulphuration temperature of the predeposited amorphous iron oxide films. First optical analyses enabled us to deduce a direct energy band gap of about 0.72 eV. All the prepared FeS2 films show p-type conduction. D 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.50.Nw; 61.66.Fn; 68.37.Hk; 78.40.Fy Keywords: Thin films; Iron oxide; Spray pyrolysis; Pyrite; FeS2; Sulphuration
1. Introduction FeS2-pyrite is an interesting material for solar energy conversion devices such as battery [1–5] and photoelectrochemical solar cells [6] due to its very high optical absorption coefficient (a~105 cm1 for kV700 nm) and its suitable energy band gap [7,8]. Considerable progress has been made since Wfhler [9] first prepared artificial pyrite by reaction of Fe2O3 with liquid sulphur and ammonium chloride in an open system at the 19th century, and succeeded in obtaining small * Corresponding author. Tel.: +216 71 430 160; fax: +216 1 430 934. E-mail address: [email protected] (B. BessaRs). 1 Tel.: +216 71 872 600. 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.06.074
brass-yellow octahedron. The high absorption coefficient, which is two orders of magnitude higher than that of crystalline silicon, would make it possible to allow the use of very thin absorption layers. Due to the various difficulties found in growing large synthetic pieces, strong efforts are nowadays devoted to obtain FeS2 thin films suitable for their application in solar energy conversion in addition to their use as positive electrodes in Al/FeS2 and Li/FeS2 batteries [10,11]. Several methods have been attempted to prepare pyrite thin films: MOCVD [12], spray pyrolysis in a controlled atmosphere [13], reaction of Fe2O3 or Fe3O4 with elemental sulphur [14], reactive ion beam sputtering [15], electrodeposition [16,17] and sulphurization of iron thin films [6]. In this paper, we describe an inexpensive, non toxic
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
735
Fig. 1. (a) XRD patterns of the iron oxide films annealed under a vacuum of 104 Pa at different temperatures for 5 h; (b) plotted ASTM JCPDS data 33-663 for Fe2O3.
and easy to manipulate method to prepare pyrite thin films: sulphurization of iron oxide thin films pre-deposited on glass substrates by spray pyrolysis under very simple conditions. A single-phase pyrite film on a glass substrate with very high optical absorption coefficient (~105 cm1) was obtained. The different phases were determined using X-ray diffraction (XRD) with a Siemens D500 powder diffractometer (CuKa radiation) in the usual h/2h geometry. The microstructure of the iron oxide and pyrite films were observed by scanning electron microscopy (SEM).
at 350 8C and the presence of two iron oxide phases: the dominant one corresponds to Fe2O3 and the traces correspond to FeOOH, which were transformed after further thermal treatments to Fe2O3 [18]. Indeed, Diamandescu et al. [18] showed that, for temperatures higher than 350 8C, the microporosity of the iron oxide particles increased; this was attributed to the water loss during the crystallisation. The corresponding dehydration reaction may be written as: 2FeOOHYFe2 O3 þ H2 O: In this work, the traces of FeOOH were ignored. The Fe2O3 peaks intensities correspond to those in JCPDS data (33-663). The SEM microanalyses results showed in Table 1 confirm the stoechiometry of the Fe2O3 phase detected by the XRD technique (Fig. 1).
2. Growth of the iron oxide films 2.1. Experimental procedure An aqueous solution of FeCl3d 6H2O (0.03 M) was prepared and sprayed with a gas vector (N2) on glass substrates, which were placed on a hotplate heated at 350 8C. The as-prepared solution was sprayed on the heated glass substrates during 30 min, with a speed rate of 5 ml/min. 2.2. XRD patterns After spray pyrolysis, rust red amorphous films were obtained. To crystallise the obtained phase, the as-prepared films were annealed in a vacuum (~104 Pa) sealed tube at different temperatures (250 8C, 300 8C, 350 8C and 400 8C) for 5 h. The XRD patterns (Fig. 1) show a good crystallinity
3. Transformation of the amorphous iron oxide to FeS2-pyrite films The following reaction corresponds to the transformation of Fe2O3 iron oxide, into FeS2:
. The negative value of the free enthalpy, corresponding to the reaction, shows that the latter can be made sponta-
Table 1 Microprobe SEM analyses taken in three different zones of a Fe2O3 film thermally treated at 350 8C for 5 h in a vacuum (~104 Pa) sealed tube Zone
Elements
El (wt.%)
Norm (wt.%)
Prec.
Atomic %
Line
O/Fe
1
Fe O Fe O Fe O
65.52 28.50 65.20 27.68 65.03 28.88
65.91 28.67 68.46 29.07 64.69 28.73
2.50 0.55 2.600 0.56 2.62 0.55
37.85 57.48 39.76 58.92 36.81 57.06
K K K K K K
1.519
2 3
line line line line line line
1.482 1.550
736
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
Fig. 2. XRD patterns of FeS2-pyrite thin films prepared under a vacuum of 104 Pa: (a) for 6 h at different sulphuration temperatures, (b) at 450 8C for different sulphuration durations, (c) plotted ASTM: JCPDF data 42-1340 for FeS2.
grain size of the sulphurated films were determined by SEM.
neously in the direct direction (1) in the case of the presence of all the reagents. This reaction leads to the substitution of oxygen by sulphur. Thus, we had the idea to apply this reaction to amorphous iron oxide films that can easily crystallise into FeS2 films. In fact, the substitution mechanism is much more difficult for crystallized materials than for amorphous ones.
3.2. Results and discussion 3.2.1. XRD patterns The optimisation of the pyrite growth parameters was pointed out using XRD patterns. The latter show a singlephase pyrite films (Fig. 2). Fig. 2(a) shows that for duration of 6 h the crystallinity increased with sulphuration temperature in a temperature range of 300 8C–450 8C. A good crystallinity was obtained at 450 8C. At 500 8C, the crystallinity decreases. The peak corresponding to the (200) line is the highest in the tested films then line corresponding the (311) direction. The bar lines in the plot indicate XRD patterns of pyrite powder with the positions and the relative intensities of the diffraction peaks (JCPDS data 42-1340). The XRD patterns show that the as-prepared films crystallised in the expected cubic structure. Significant decrease of the line intensities occurred by increasing the sulphuration temperature up to 450 8C. D. Wan et al. [19] showed the same dependence: great decrease of the intensities of the two highest peaks (200) and (311) by increasing the sulphuration temperature up to 500 8C. Once the sulphuration temperature
3.1. Experimental procedures The prepared amorphous iron oxide layers were sulphured for various temperatures and durations in vacuum (~104 Pa) sealed tubes. The sulphuration parameters such as the temperature and the duration were optimized using XRD. The sulphuration pressure was fixed during all the experience at about 104 Pa. The other sulphuration parameters such as the temperature and the duration were optimized in two steps. In the first step, the duration was fixed arbitrarily to 6 h and the temperature was varied. In the second step, the sulphuration temperature was fixed at 450 8C, at which a good crystallinity was obtained, and the duration was varied. Optical absorption spectra of pyrite thin films grown at the optimum conditions were recorded using a spectrophotometer. The surface morphology and the
Table 2 Microprobe SEM analyses of the FeS2-pyrite thin films synthesised at 450 8C and 500 8C during 6 h under a vacuum of about 104 Pa FeS2 (450 8C)
FeS2 (500 8C)
Zone 1
Zone 2
Zone 1
Zone 2
Elements
Fe
S
Fe
S
Fe
S
Fe
S
El (wt.%) Norm (wt.%) Prec. Atomic% Line
45.18 45.31 2.17 31.84 K line 1.93
49.86 50.15 0.64 61.20 K line
45.25 45.70 2.17 32.63 K line 1.95
50.69 51.19 0.65 63.68 K line
44.42 45.65 2.17 31.83 K line 2.0063
51.15 52.57 0.66 63.86 K line
44.55 46.08 2.68 32.55 K line 2.0003
51.15 52.90 0.67 65.11 K line
S Fe
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
737
optimised and fixed at 450 8C, the duration was varied (2 h, 4 h, 6 h and 8 h). A good crystallinity was obtained for a duration of 6 h (Fig. 2(b)). The pyrite films grown at 450 8C for 6 h seem to be practically stoichiometric. Microprobe SEM analysis (Table 2) shows a slight deficiency of sulphur for films prepared at 450 8C compared to the stoichiometric FeS2. This deficiency can be corrected by a slight rise in the sulphuration temperature (Table 2). 3.2.2. Microstructure morphology of the pyrite films Fig. 3 shows SEM micrographs surface views at different magnifications of pyrite FeS2 films annealed at 450 8C (a and b) and at 500 8C (c and d) under a vacuum (~104 Pa) for 6 h. The surface of the films prepared at 450 8C (Fig. 4(a)) appears granular and homogeneous. At higher magnification (Fig. 4(b)), the surface of these layers appears granular but rather homogeneous with grain size dimensions around 250 nm. For the films prepared at 500 8C (Fig. 4(c)), the surface appears granular but inhomogeneous. Indeed, at higher magnification (Fig. 4(d)), the surface of these layers appears more granular, apparently with two grain size distributions. The largest grains have a size dimension of about 370 nm and the smallest ones of about 70 nm. The microstructure morphology of FeS2 films synthesised at 500 8C (Fig. 4(d)) is similar to that obtained by Hamdadou et al. [6], who used pre-evaporated iron films. The SEM crosssection (Fig. 4) shows that the as-prepared pyrite films have a good adherence to the substrate. An average value
Fig. 4. SEM cross section of view of a FeS2-pyrite film synthesised at 450 8C for 6 h under a vacuum of about 104 Pa.
of the thickness of the films synthesised at 450 8C for 6 h was estimated at about 800 nm. 3.2.3. Optical properties Fig. 5(a) depicts the optical absorption spectra versus photon energy (ht) for FeS2 films sulphurated at 450 8C for 6 h under a vacuum of about 104 Pa. A high absorption coefficient (a~5d 104 cm1) was observed for wavelengths lower than 1000 nm. Plots of (aht)n/2 versus the photon energy ht, with n=1 and n=4 are given in Fig. 5(b,c). The variation of (aht)1/2 vs. ht gives a straight line, suggesting that the FeS2 films have an indirect band gap energy of 0.72 eV. Fig. 5(c) shows that the first direct transition of the FeS2-pyrite thin films, prepared in this work, occurs at about
Fig. 3. Scanning electron microscopy views of FeS2 films sulphurated from sprayed pre-deposited iron oxide layers at 450 8C (a and b) and at 500 8C (c and d) for 6 h under a vacuum of about 104 Pa.
738
B. Ouertani et al. / Materials Letters 59 (2005) 734–739
Fig. 5. (a) Optical absorption coefficient of the FeS2-pyrite thin films synthesised at 450 8C for 6 h in a vacuum of 104 Pa; (b) (ahm)1/2 vs. hm and (c) (ahm)2 vs. hm.
0.87 eV. The values of the optical absorption obtained in this work (Fig. 5(a)) are similar to those obtained by other workers [19].
This work was supported by the Secre´tariat d’Etat a` la Recherche scientifique et a` la technologie. References
4. Conclusion During this work, homogeneous FeS2-pyrite thin films having good crystallinity were synthesized by simple sulphuration of amorphous iron oxide films pre-deposited by spray pyrolysis. Optimum sulphuration temperature and duration were found at 450 8C and 6 h, respectively. The high absorption coefficient (a~5d 104 cm1) and the relatively large grain size (~250 nm) observed for these FeS2-pyrite films may be promising to be tested in photovoltaic and photoelectrochemical applications.
Acknowledgements The authors would like to thank Dr. Ferid M. for its support in XRD patterns.
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