Iron Pyrite from Iron Thin Films: Identification of Intermediate Phases

Article pubs.acs.org/JPCC

Iron Pyrite from Iron Thin Films: Identification of Intermediate Phases and Associated Conductivity-type Transitions Antonio Pascual, Satoko Yoda, Mariam Barawi, José M. Clamagirand, José R. Ares, Isabel J. Ferrer,* and Carlos Sánchez MIRE Group, Materials Physics Departent, Autonomous University of Madrid, Madrid 28049, Spain S Supporting Information *

ABSTRACT: In this work, the growth of FeS2 by direct sulfuration of Fe thin films is examined with the purpose of elucidating the nature of the Fe to FeS2 transformation and to state some of its characteristics. To this aim, the film Seebeck coefficient (S) was measured during the whole sulfuration treatment. The S value changes from positive (ca. +8 μV K−1) to negative values (ca. −15 μV K−1) and, then, to very positive (ca. +100 μV K−1) at the end of the sulfuration process. To understand the film transformations and the resulting S evolution (mainly, the cause of its negative values) some Fe films were sulfurated during some selected times and, then, quickly cooled (to prevent changes in the chemical composition of the films) from the corresponding sulfuration temperature to room temperature (RT). Chemical composition and structural analyses of the quenched samples were accomplished at RT. Using the obtained data, it was concluded that the sulfuration transforms the original Fe film into hexagonal pyrrhotite (Fe1−xSH), which partially converts to its orthorhombic (Fe1−xSO) phase through a Néel transformation. Then, both the orthorhombic and hexagonal pyrrhotites react with sulfur to form pyrite (FeS2). These chemical transformations are accompanied by changes of the film conductivity type: from p-type (Fe) to n-type (pyrrhotites) and finally to p-type (FeS2). The intermediate pyrrhotite phases appear to be precursors of the final pyrite phase. Results are discussed in the light of former data on sulfurated thin films and Fe bulk samples.

1. INTRODUCTION Iron pyrite (FeS2) thin films have been extensively prepared to be used as a base material in photovoltaic, photoelectrochemical, and thermoelectric applications.1−15 With an energy band gap of ∼0.95 eV2,16−23 and a high absorption coefficient (α ≥ 5 × 105 cm−1 for λ ≤ 700 nm), pyrite appears to be a suitable material to produce thin solar cells. Values of the thermoelectric figure of merit at different temperatures have been recently published.24 Furthermore, pyrite is a gentle semiconductor from the environmental point of view, and its components are highly abundant in nature.25 However, the extensive use of pyrite in the mentioned applications is currently prevented by several drawbacks of basic and/or experimental nature. In relation to the pyrite growth/production processes, three matters are of particular relevance: type and concentration of created intrinsic point defects, unwanted doping by uncontrolled impurities, and the formation of non-pyrite sulfide phases. In particular, low sulfur-containing sulfides frequently appear when trying to get pyrite.4,26−30 In principle, these phases (some of them with near-metallic behavior) would contribute to worsen the pyrite potential properties. Some investigations, mainly accomplished in sulfurated Fe powder and bulk polycrystalline samples, clearly suggest that some Fe sulfides with low sulfur concentration play a crucial role in the pyrite formation process. Sweeney and Kaplan,31 trying to reproduce some natural processes, found that dissolved Fe and © XXXX American Chemical Society

hydrogen sulfide lead to mackinawite and hexagonal pyrrhotite. These compounds, in the presence of elemental sulfur, produced pyrite. Furthermore, Fe monosulfide (FexS) was found as precursor of FeS2 by Pimenta and Kautek32 on sulfurating Fe bulk polycrystalline plates. The sulfurated sample was formed by a layer of pyrite, followed by another one of monosulfide and finally iron. The Fe monosulfide growth obeys a faster kinetics than the pyrite formation.32 However, on trying with sulfurated Fe thin films the role played by the low sulfur concentration sulfides is not clearly established. The research done by Pimenta and Kautek33,34 on electrodeposited Fe thin films pointed out that iron monosulfide (FexS) plays on these samples the same role as that in sulfurated bulk Fe. However, the experimental evidence they reported in thin films was quite weak. The existence of pyrite precursor phases (and their identification) in the Fe thin film direct sulfuration process has not been deeply investigated up to now, in spite of the wide literature dealing with pyrite thin films. In particular, it must be well-defined which one(s), if any, of the several low sulfur sulfides is responsible for the Fe to FeS2 transition. In this context, it is relevant to quote a recent publication35 dealing with a conductivity-type transition (from Received: May 29, 2014 Revised: September 18, 2014

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dx.doi.org/10.1021/jp505303d | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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profilometer (accuracy ±10 nm). Film structure was identified by glazing angle XRD diffractograms obtained with a Siemens D5000 Automated X-ray diffractometer (Cu Kα radiation, λ = 0.154 18 nm, and a fixed incidence angle of 1.7°). Sulfur-to-iron atomic ratios were determined by EDX analyses with an Oxford software associated with a scanning electron microscope (SEM, Hitachi S-3000 N). All the EDX spectra were obtained with an electron beam energy of 15 keV under normal incidence to the sample surface.

p- to n-type), which takes place on crossing a sulfuration temperature of ∼350 °C. The authors discuss in detail the corresponding electrical resistivity (ρ(t)) responses of the films and conclude that from their experimental results it was not possible to identify the phases (Fe or some low sulfur sulfide) responsible for that behavior. Therefore, a more detailed knowledge of the pyrite thin-film formation process and the compounds therein involved is needed to improve our understanding of its transitory (stoichiometric ratios, lattice parameter, crystalline size, conductivity type, etc.) and permanent characteristics and to reach any one of the wanted applications. “In situ” measurements (i.e., during the sulfuration of the Fe films) are a useful tool to investigate the transformations involved in the iron-topyrite conversion. Changes of several physical/chemical characteristics of the films can be well-investigated.36−39 We now present results obtained by in situ Seebeck coefficient measurements and “ex-situ” compositional (energy-dispersive X-ray spectrometry (EDX)) and structural analyses (X-ray diffraction (XRD)) of the corresponding samples. In situ thermoelectric measurements provide unambiguous information about the conductivity type of the thin film during its sulfuration and may yield some light on the phase transformations that are taking place. These experiments allowed us to conclude that Fe-to-FeS2 thin-film conversion is a nonelemental transformation but that some intermediate phases (pyrrhotite, both hexagonal and orthorhombic) are involved as precursors of pyrite. Furthermore, due to the high temperature n-type (negative Seebeck coefficients) conductivity of the formed pyrrhotites,40 transitions of the conductivity type from p (original Fe film) to n (formed pyrrhotites) and to p (formed pyrite) were clearly recorded during the sulfuration.

3. RESULTS AND DISCUSSION In Figure 1a the temperature profiles of both the sample (TSa) and the sulfur powder (TSu) are presented as a function of the

2. EXPERIMENTAL SECTION Iron thin films with thicknesses of dFe ≅ 80 ± 2 nm were prepared by thermal evaporation of iron powder (99.9%, Goodfellow) on previously outgassed soda lima glasses (23 mm × 9 mm × 1 mm, Corning 7059) at room temperature (RT). Sulfuration of the iron layers has been carried out in a vacuum sealed glass ampule, with sulfur powder (99.99% Merck) inside, placed in a system formed by two different cylindrical furnaces in order to independently regulate the sample and the sulfur source temperatures. Under this configuration, it is possible to control the partial pressure of the S2 specie near the Fe layer, which seems to be the molecule responsible for the iron sulfuration.32,41,42 The sample is placed in a ceramic holder which is located near the heating resistance of the furnace. This geometric configuration creates a thermal gradient (ΔT/Δx ∼1.5−2.0 K cm−1) along the sample what allows to measure S during the sulfuration process. Two thermocouples (K-type) are used to know the temperature difference (ΔT ≈ 3−4K) between the ends of the sample. Two contacts of steel AISI 446 placed near the thermocouples are used to measure the thermoelectric voltage (ΔV). The Seebeck coefficient (S) is obtained as the quotient of the thermoelectric voltage and the temperature difference (S ≈ ΔV/ΔT). We have accepted the Telkes criterion43 about the sign of S. More details of the experimental system are given in refs.36,37,44,45 All the rest of our measurements were accomplished “ex situ” at RT, after cooling the sulfurated (partially or totally) films. In particular, the thicknesses of both the initial iron and the sulfurated films were measured with a Sloan Dektak IIA

Figure 1. (a) Temperature profiles of the sample (TSa) and sulfur source (TSu) during the Fe thin-film sulfuration. (b) Evolution of the S2 partial pressure around the sample. (c) Seebeck coefficient (S) as a function of the sulfuration time; the dotted line, a guide for the eyes, represents the possible evolution of S to its room-temperature value; hollow circles indicate the instants when samples were quenched (sample Nos. 1 to 7, see text). (insets) The initial part of the graph is enlarged. B



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process time (ts) during all the Fe-to-FeS2 transformation. After several hours of the vacuum system operation (∼10−4 Pa) the sample temperature TSa is increased (at ts ≈ 0.35 h) to ∼450 K (the sulfur source remaining at RT) at a heating rate of ∼75 K h−1 during ∼2 h. At this temperature the heating is interrupted during ∼10 min to vacuum seal the glass ampule. Once the ampule is sealed, the sample heating is continued to TSa ≈ 600 K, and the sulfur powder temperature starts (at ts ≈ 1.8 h) to increase from RT to ∼390 K at a heating rate of ∼45 K h−1. The sample reaches its final temperature at ts ≈ 3.5 h, and the sulfur powder reaches its final temperature at ts ≈ 4.5 h. For ts ≥ 4.5 h both temperatures remain constant until ts = 32 h, when the system is cooled down. In Figure 1b the evolution of the S2 partial pressure near the sample is represented as a function of the sulfuration time. It can be seen that the S2 pressure increases from ∼10−6 Pa to ∼1 Pa during ∼4 h. The total sulfur pressure is ∼2.5 Pa when the sample and sulfur temperatures are constant. Under these experimental conditions, the film Seebeck coefficient evolution during its complete transformation (Fe to FeS2) is shown in Figure 1c. As can be seen, initial S is positive and then decreases to negative values, while TSa and TSu (and PS2) are growing, and then it becomes positive until it reaches a typical FeS2 final positive value. The inset of Figure 1c shows details of the S behavior for ts ≤ 5 h. The S change initially observed (ts ≤ 2.8 h), from S ≈ 8 μV K−1 to ∼0 μV K−1, has been identified as due to the heating of the starting Fe film,44,46−48 and it shows the “Fe thin-film annealing.” Once the sulfur powder temperature is increased (and so the S2 partial pressure), S decreases and reaches a value of ca. −12 μV K−1 at ts ≈ 3.1 h. After this stage, S rises to ca. −1 μV K−1 at ts ≈ 4 h, and then it continues to grow but more slowly. For longer sulfuration times, S becomes ∼100 μV K−1 at ts ≈ 20 h, and it shows no change during the rest of the experiment. At ts ≈ 32 h the complete transformation of the Fe film seems to be finished, and the sample is cooled. The S evolution during the cooling time is then due to the fully sulfurated film thermoelectric behavior with temperature. As described in the Experimental Section, the thermal gradient along the sample is created by the heating resistance of the sample furnace, and, therefore, it vanishes a short time after that resistance is switched off, preventing the Seebeck coefficient measurement. For this reason, we obtained the S coefficient of the fully sulfurated film at RT (marked in Figure 1c) and extrapolated the S evolution until that value (dotted line in Figure 1c). It is clear from Figure 1c that the film behaves as an n-type conductor (S < 0) during part of the sulfuration process. This behavior cannot, in principle, be attributed to transformation of Fe into pyrite because, as has been repeatedly published, pyrite behaves as a p-type conductor at room and higher temperatures. Some other compounds must cause the found n-type Seebeck coefficient. To clarify this hypothesis we proceeded in the following way: some films were quenched from the corresponding sulfuration temperature to RT after being partially sulfurated during times corresponding to selected points of the S evolution curve (sample Nos. 1 to 7 marked with hollow circles in Figure 1c). The composition (EDX) and structure (XRD) of these films were investigated at RT. The XRD patterns of those films are shown in Figure 2 with indication of the sample number. Clearly, sample 1 (ts ≈ 2.7 h) is formed exclusively by Fe (JCPDS 06−0696) (Al peak is due to the sample holder), and it proves that the change of S during

Figure 2. Glazing angle XRD spectra of samples 1 to 7. Fe: iron (JCPDS 06−0696); H: hexagonal pyrrhotite;49 O: orthorhombic pyrrhotite;50 P: pyrite (JCPDS 42−1340); M: marcasite (JCPDS 02− 0908); Al: aluminum from the sample holder of the XRD diffractometer.

0 h < ts 1 kPa. All these considerations are strongly supported by several energetic and thermodynamic calculations that show that the formation heat of the low sulfur sulfides is less negative than the pyrite formation heat.32,55,56 In addition to the conclusions related to the chemical composition of the films during their sulfuration, we must emphasize the conductivity-type transition (n- to p-type) observed in our experiments (Figure 1c) for ts > 3 h, which clearly suggests a change of the chemical composition of the film (any possible doping is excluded). This behavior is easily understood by considering that pyrrhotite phases present negative S values40 at the temperatures involved in this work during the sulfuration. The conversion of pyrrhotites to pyrite induces the positive S sign corresponding to FeS2. However, this chemical transformation of the film is not so clearly suggested by S measurements accomplished at RT, which do not show any conductivity-type transition. We measured the RT Seebeck coefficient of the quenched films (samples 1 to 7), and the obtained results are displayed in Figure 6. All the films

Figure 7. Evolution of Seebeck coefficient of pyrrhotite phases with temperature. Circles corresponds to films partially sulfurated (ts ≈ 3.0−3.5 h) and then quickly cooled to RT. Full squares are the S values of a pyrrhotite film during its heating from RT to ∼600 K.

The results we are reporting now may contribute to a better understanding of the formation mechanism of pyrite by Fe (thin films and bulk) sulfuration. This understanding may help to overcome some of the drawbacks pyrite still presents as a material for photovoltaic and thermoelectric applications. On the other hand, and from a more technological point of view, we emphasize the fact that in some sulfuration conditions the transformation of pyrrhotite phases into pyrite may be not fully accomplished. Some traces (hardly detected by conventional analytical techniques mainly if pyrrhotite phases are poorly crystallized) of pyrrhotites may remain in the sample, and pyrrhotite properties (transport, S/Fe ratio, etc.) might considerably differ from those of pure pyrite. In fact, comparison of Figures 1c, 3, and 4 indicates that S is more influenced by the S/Fe ratio than by the film grain structure.

4. CONCLUSIONS We have shown that the transformation of Fe into FeS2 by direct sulfuration of Fe thin films does not take place via an elemental iron−sulfur reaction. The original Fe thin film reacts with sulfur, leading to hexagonal pyrrhotite. When the temperature is high enough, this Fe1−xSH partially transforms into its orthorhombic phase (Néel transformation). Then both orthorhombic and hexagonal pyrrhotites react with sulfur to form pyrite. In parallel, we have shown that the pyrrhotite-topyrite conversion is accompanied by a change of the conductivity type from n-type to p-type (according to Seebeck coefficient measurements accomplished during the Fe sulfuration). At RT all the films appear to be p-type conductors, and the Seebeck coefficients of those formed by pyrrhotites show very low positive values. We have discussed the reasons why the presence of pyrrhotite phases during the Fe thin film sulfuration has been hard to detect. Additionally, the results reported in this work may help to better understand previously published experimental results and to establish relationships between them.

Figure 6. Seebeck coefficient of the sulfurated (quenched) films (1 to 7) measured at RT as a function of the corresponding sulfuration time. Error bars are of the size of the points.

appear to be p-type at RT although the S values change with the sulfuration time. Because of the nearly metallic nature of the Fe pyrrhotites, the Seebeck coefficient becomes more positive on lowering the temperature of the pyrrhotites. This behavior is shown in Figure 7 where the S evolution during cooling of the films partially sulfurated (ts ≈ 3.0−3.5 h) and composed mainly by pyrrhotites is plotted against temperature. The third curve (full squares) shows data of a pyrrhotite film during its heating (obtained in a different experimental system). At RT all these films have a very low positive S value. In the same way and due to the semiconductor character of FeS2, its S coefficient turns to be less positive.24 The room temperature measurements hardly suggest the formation of intermediate pyrrhotite phases.

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ASSOCIATED CONTENT

S Supporting Information *

Structural data of different binary sulfide phases and experimental XRD patterns from different pyrrhotite samples are reported. Furthermore, a brief discussion about the influence of the thickness on the Seebeck coefficient and the E



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stability of monosulfide phases is included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 00 34 497 5027/8579. E-mail: isabel.j.ferrer@ uam.es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by MINECO under Contract No. MAT 2011-22780. Technical assistance from Mr. F. Moreno is gratefully recognized.

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Iron Pyrite from Iron Thin Films: Identification of Intermediate Phases

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