Polycrystalline and single phase FeS₂ films grown by chemical bath deposition

1 Introduction

Iron disulphide (FeS₂) has considerable attraction as a potential candidate for photoelectrochemical and photovoltaic (PV) applications [1, 2] consisting of abundant, cheap and nontoxic elements. Pyrite films exhibit complex behaviour by variation in structural properties. Pyrite and marcasite are two phases of FeS₂. Marcasite has an orthorhombic structure with a band gap of 0.34 eV indicating its unsuitability for PV applications. These days, cubic pyrite is receiving more attention because of its important applications as PV, photoelectrochemical, thermoelectric and cathode materials, though it is susceptible to oxidative degradation [3, 4]. It is a promising PV material as it has a suitable band gap with high optical absorption coefficient (10⁵ cm^-1), with an indirect band gap at 0.95 eV [5, 6] and a direct band gap at 1.38 eV. Moreover, the elements involved in this material are earth abundant and non-toxic. FeS₂ thin films have been prepared by a variety of methods such as metal organic chemical vapour deposition [7], sol-gel method [8], solvothermal process [9], chemical vapour deposition [10], electrodeposition [2], molecular beam deposition [11], spray pyrolysis [1] and chemical bath deposition (CBD) [12, 13]. Of all the methods, CBD is inexpensive, highly feasible and reproducible for large area deposition with less material consumption. So far, many semiconductor materials have been deposited by CBD with its limited reports on the preparation of FeS₂. This method is based on the reaction between dissolved cations and anions in acidic water solution. Triethanolamine, ammonia (NH₃) and ethylene diamine tetraacetic acid (EDTA) are generally used as complexing agents, which eliminate spontaneous precipitation by slowing down the release of metallic ions on dissociation and thereby forming a solid film on the substrate. In this work, we report the effect of bath temperature (T_b) on the properties of chemically deposited FeS₂ films.

2 Materials and methods

Iron sulphate (FeSO₄·9H₂O; Sisco Research Laboratories Pvt. Ltd., Mumbai, Maharashtra, India) and thiourea (CS (NH₂)₂) (Sigma Aldrich, Kushaiguda, Hyderabad, India) were used as precursors with NH₃ (Sigma Aldrich, Kushaiguda, Hyderabad, India) and EDTA (Sigma Aldrich, Kushaiguda, Hyderabad, India) as complexing agents. Three sets of films were prepared by varying the bath temperature (T_b) from 50°C to 70°C. Sodalime glass plates were used as substrates. Non-uniformity and poor adhesion of the films were the common notified problems in CBD. In order to obtain good quality films, the substrates were cleaned in the following procedure: The substrates were thoroughly cleaned with ordinary water, soap solution and distilled water sequentially. The cleaned substrates were then dipped in potassium dichromate solution for 5–6 h. Finally, the substrates were rinsed with distilled water and dried in a hot air oven before placing in a beaker. Chemical bath consisted of 10 ml of FeSO₄·9H₂O and CS(NH₂)₂ and 3 ml and 5 ml of NH₃ and EDTA, respectively. Twenty millilitres of distilled water was added to the bath and stirred well. A deposition time (t_d) of 60 min was maintained for all the depositions. The as-deposited films were removed from the beaker, cleaned with distilled water and dried. Structural analyses of the films were carried out by using a Siefert X-ray diffractometer (model: 3003 TT) using Cu-Kα as a radiation source (wavelength, λ=1.542 A°) and a Raman spectrometer. Surface morphology and chemical composition of the films were examined using a Carl Zeiss scanning electron microscope (SEM) (EVO MA 15) attached to an Oxford instruments (Inca Penta FET x3) X-ray energy analyser. Optical absorption studies were carried out using a Perkin-Elmer Lambda 950 UV-Vis-NIR spectrophotometer. Fourier transform infra red (FTIR) measurements were carried out with a Nicolet Model 400D Spectrometer operating in the wavenumber range 4000–1500 cm^-1.

3 Results and discussion

Iron pyrite (FeS₂) films deposited by CBD method at different bath temperatures are reddish brown in colour. The X-ray diffraction (XRD) patterns of the pyrite films deposited at different temperatures are shown in Figure 1. From the XRD patterns, it was observed that at lower temperature, there were four characteristic peaks of 110 and 220 corresponding to marcasite and 200 and 023 corresponding to pyrite phases of iron sulphide. At 60°C, the 110 peak that correspond to marcasite phase was converted to pyrite phase. At a temperature of 70°C, all the other phases disappeared except pyrite phase. Four peaks corresponding to 111, 200, 311 and 023 [14] planes were observed at this temperature, which indicates that the purity of the films increases with the rise of bath temperature to 70°C. The identified planes are in agreement with the reported Joint Committee on Powder Diffraction Standards (JCPDS)-42-1340 data. The XRD observations were also confirmed by the Raman analysis. The Raman spectra of the deposited films at different bath temperatures are shown in Figure 2.

Figure 1:

XRD patterns of the deposited films at different bath temperatures (M=marcasite, P=pyrite).

Figure 2:

Raman spectra of the deposited films.

The Raman spectra showed a peak present at 217 cm^-1 in the film formed at 50°C, which corresponds to the marcasite phase that was also detected by XRD studies. But at T_b=70°C, this peak disappeared, and the peaks related to pyrite phase alone were present. Figure 3 shows the SEM pictures of FeS₂ films formed at different bath temperatures. The morphological studies showed that films deposited at 50°C had an inhomogeneous distribution of grains on the substrate with small grain size. However, at 70°C, a regular distribution of large grains over the substrate surface was observed. The energy-dispersive X-ray spectroscopy (EDAX) spectra of the films deposited at different bath temperatures are shown in Figure 4. The S/Fe ratio in the films formed at 70°C was found to be 1.95, and this was close to the stoichiometric ratio of FeS₂ (1.97) observed by other workers. Figure 5 represents the FTIR data of FeS₂ films grown in this work. The spectra exhibited a small peak around 3738 cm^-1 due to -OH stretching mode. Another band at 2925 cm^-1 was also observed, which can be assigned to the C-H stretching modes. In addition, the band that appeared at 1774 cm^-1 represents the asymmetric S-O mode corresponding to the sulphate ion.

Figure 3:

SEM photographs at different bath temperatures.

Figure 4:

EDAX spectrum of the film deposited at T_b=70°C.

Figure 5:

FTIR spectra of the films at different bath temperatures.

The optical properties of the films were studied using UV-Vis-NIR spectrophotometer. Figure 6 shows the absorbance versus wavelength spectra recorded in the wavelength range of 400–800 nm. The evaluated absorption coefficient was >10⁵ cm^-1. The films exhibited an indirect as well as a direct band gap. The direct and indirect band gaps of the material were calculated by plotting the graphs of (αhν)^1/2 versus hν and (αhν)² versus hν, respectively. The evaluated direct band gap of the material decreases from 1.47 eV to 1.26 eV with increase of the bath temperature from 50°C to 70°C, which was well matched with the literature values [15].

Figure 6:

Plot of wavelength versus absorbance of the films at different T_b.

4 Conclusion

The iron pyrite thin films were chemically deposited by using aqueous solutions of iron sulphate and thiourea. The layers were formed at various bath temperatures that varied in the range 50°C–70°C. The film deposited at T_b=50°C showed pyrite as well as marcasite phases, while it showed only pyrite phase with cubic structure at 70°C. The later films showed uniform and better surface morphology compared to the films formed at temperatures lower than 70°C. The EDAX analysis revealed the presence of Fe and S with some impurity elements. The FTIR spectrum exhibited peaks related to -OH, C-H and S-O stretching modes. The optical studies indicated a decrease of direct band gap from 1.47 eV to 1.26 eV as T_b increased from 50°C to 70°C.