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Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment
Materials Chemistry and Physics xxx (2014) 1e4
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment Hong Tak Kim, Thao Phuong Ngoc Nguyen, Chang-duk Kim, Chinho Park* School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
h i g h l i g h t s Pure FeS2 NCs were synthesized in sulfur abundant environment using colloidal method. The phase of iron sulfide NCs was transformed from FeS via Fe3S4 to FeS2 with growth time. Fe3S4 acted as the intermediate precursors for the formation of FeS2 NCs. Pure FeS2 NCs showed the band gap of 0.9 eV, indicating the semiconducting nature of FeS2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2014 Received in revised form 25 August 2014 Accepted 22 September 2014 Available online xxx
Pyrite (FeS2) nano-crystals (NCs) were synthesized in an excess sulfur environment using a colloidal hot-injection method, and the phase change behavior of the iron sulfide compounds was investigated. As the growth time increased, the phase of the iron sulfide NCs transformed from mackinawite (FeS) via greigite (Fe3S4) to pyrite (FeS2). Thus, Fe3S4 phases was considered as intermediate precursors on the pathway of FeS2 phases in the reaction between FeS phases and excess sulfur. The elemental ratio of [S/Fe] increased from 1.1 to 2.1 during the phase change, and the shape of the NCs changed from a hexagonal nano-sheet (Fe3S4), via cubic (FeS2), to a cubic-hedral structure (FeS2). Strong absorption peaks in the UVeVis spectra were observed in the FeS2 phase, and its optical band gap was estimated to be ~0.9 eV, indicating the semiconducting nature of pyrite. Consequently, the synthesis of FeS2 in sulfur abundant environment was suitable method to acquire a pure semiconducting FeS2 phases. The reason was thought that the depletion of Fe-element after the formation of FeS2 phases led to the decrease of intermediate phases and the gradual changes from intermediate phases to FeS2 resulted in pure phases. © 2014 Published by Elsevier B.V.
Keywords: Chemical synthesis Crystal structure Electron microscopy Nanostructures Phase transitions
1. Introduction Pyrite (FeS2) is an interesting material with a high absorption coefficient (~105 cm1), low energy band-gap (~0.9 eV) with a direct transition, and p-type semi-conductivity [1e4]. These properties make FeS2 suitable as the absorption layer in photovoltaic applications. Furthermore, FeS2 is one of the most earthabundant materials, and is very cheap. The cost per area is very important in photovoltaic applications because most solar cells require large area to obtain desired energy from the sun-light [5,6]. To date, many researchers have evaluated a range of materials, such as CuInGaSe2 (CIGS), CdTe and Si, for solar cell applications [7,8]. On the other hand, In, Ga, Se, and Te are not only rare on earth but are
* Corresponding author. E-mail address: [email protected] (C. Park).
also expensive. Although Si is applied most-widely to produce electric energy, the cost of Si should be reduced further for the widespread deployment of photovoltaic modules. Therefore, the next-generation materials for photovoltaic applications should be earth-abundant, cheap, and have high performance properties. In this sense, FeS2 is a very attractive material for next-generation solar cells [9,10]. On the other hand, usual as-formed FeS2 materials as a light absorption semiconductor, have serious problems, such as the presence of highly conducting impurity phases including mackinawite (FeS) and greigite (Fe3S4), and poor surface stability due to the decomposition of FeS2 [1,11e13]. These might be a major reason for a high dark current and low open circuit voltage, which decreases the efficiency of FeS2 devices. Therefore, understanding the formation mechanism of FeS2 materials can be helpful in the context of controlling the impurity phases and improving the surface stability.
http://dx.doi.org/10.1016/j.matchemphys.2014.09.024 0254-0584/© 2014 Published by Elsevier B.V.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
2
H.T. Kim et al. / Materials Chemistry and Physics xxx (2014) 1e4
In this study, FeS2 nano-crystals (NCs) were synthesized via a colloidal route in an excess sulfur environment, which enhances the sulfurization process of iron elements. Chemical reactions between iron and sulfur were controlled using a buffer solution of paraffin oil. The phase change of the iron sulfide NCs during synthesis was examined as a function of the growth time. The structural and optical properties of iron sulfide NCs were also studied using standard measurement methods.
2. Materials and methods The iron sulfide NCs were synthesized using a colloidal hotinjection method. Iron (II) chloride tetrahydrate (FeCl2$4H2O, Aldrich 99%) and sulfur (S, Aldrich 98%) were used as the precursor materials, and the mole ratio of Fe:S was kept to 1:6. Octadecylamine (CH3(CH2)16CH2NH2, Aldrich 97%) and diphenyl ether ((C6H5)2O, Aldrich 99%) were used as the capping materials and solvent, respectively. Paraffin liquid (OCI Company Ltd. ultrapure) was used as a buffer solution for the Fe-solution to reduce the chemical reactions. FeCl2$4H2O (100 mg) and octadecylamine (10 g) were dissolved in 40 mL of paraffin liquid at 220 C for 1 h, and S (96 mg) was dissolved in 5 mL of dipenyl ether (5 ml) at 150 C for 1 h, respectively. The prepared solutions were then combined to form the FeS2 NCs in vessel with three-bottle necks. The growth temperature was kept at 205 C, and the solution temperature was measured directly using a K-type thermocouple. The vessel was purged with nitrogen gas before the synthesis process, and then closed to block oxygen during NCs synthesis. After finishing the growth process, the solution was cooled rapidly to room temperature to prevent the excessive chemical reactions. The as-synthesized solutions were centrifuged and washed with methanol and acetone to remove the impurities. The purified NCs were stored in 1-propanol (CH3CH2CH2OH, Aldrich 98%) to prevent oxidation and aggregation between the NCs. The structural properties of iron sulfide NCs were examined by X-ray diffraction (XRD, PANalytical PRO). The shape, selected area electron diffraction (SAED) patterns, and elemental mapping of the NCs were measured by transmission electron microscopy (TEM, Hitachi H-7600). The optical properties of the iron sulfide NCs were measured using a UVeViseNIR spectrometer (Cary 5G). The elemental ratio of the NCs was analyzed by X-ray photoelectron spectroscopy (XPS, VG Microtech MT 500/1).
Fig. 1. X-ray diffraction (XRD) patterns of the iron sulfide NCs at different growth times.
, PFi ¼ Ii
3 X
Ij
ði ¼ 1; 2; 3Þ
(1)
j¼1
where I1, I2, and I3 are the intensity for FeS (101), Fe3S4 (113), and FeS2 (200) peak, respectively. Fig. 2 shows the change of phase fraction for iron sulfide materials as a function of the growth time. The magnitudes of phase fractions of the Fe3S4 and FeS2 phases showed an inverse relationship according to the growth time, indicating the formation of FeS2 from the transformation of Fe3S4. In addition, small amount of FeS phases were decreased gradually to a negligible level with the increase of growth time. This shows that Fe3S4 phases act as intermediate precursors on the pathway to FeS2 phases in the chemical reaction between FeS and excess sulfur. Fig. 3 shows the elemental ratio of S and Fe as a function of the growth time. The amount of sulfur in the iron sulfide materials increased with increasing growth time, in which the sulfur to iron ratio changed from 1.1 to 2.1. The change in the elemental ratio of [S/Fe] suggests that the phase of iron sulfide transformed gradually from FeS to Fe3S4 and then to FeS2. This is consistent with the XRD data, as shown in Figs. 1 and 2. The proposed chemical reactions for the formation of single-phase FeS2 NCs are as follows:
3. Results and discussion Fig. 1 shows XRD patterns of iron sulfide NCs according to the growth time. The phase of the iron sulfide materials changes from the mixed phase of FeS and Fe3S4 to a pure FeS2 phase, when the growth time was increased from 1 h to 5 h. FeS, Fe3S4, and FeS2 had a tetragonal structure (space group: P4-nmm), a cubic structure (space group: Fd-3 m) and a cubic structure (space group: Pa-3), respectively [JCPDS 42-1340, JCPDS 89-1998]. In this case, both FeS and Fe3S4 phases could be observed by XRD up to a growth time of 4 h, and the phase of FeS disappeared when the phase of iron sulfide changes to a pure FeS2 phase at 5 h. This suggests that FeS acts as a precursor material to form Fe3S4 until the end of FeS2 formation. The phase change of the iron sulfides can be described in terms of the phase fraction (PF) for different crystal structures, which defined the ratio of mostly preferred orientation peak for each crystal structure. The phase fraction for each crystal structure is given by:
Fig. 2. The variation of phase fraction (PF) for iron sulfide NCs as a function of the growth time.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
H.T. Kim et al. / Materials Chemistry and Physics xxx (2014) 1e4
3
Fig. 5. (a) UVeViseNIR transmittance spectra at different growth times and (b) optical band-gap at a growth time of 5 h.
Fig. 3. Elemental ratio of Fe, S, and [S/Fe] as a function of the growth time.
Fig. 4. TEM analysis of iron sulfide NCs at different growth times: (a) TEM images of iron sulfide NCs and (b) elemental maps of FeeS components and selected area electron diffraction (SAED) patterns of iron sulfide NCs at the growth time of 1 h, 3 h, and 5 h.
Fe þ S/FeS Saddition
3FeS þ S ! Fe3 S4 Feloss
4FeS ! Fe3 S4 þ Fe Saddition
Fe3 S4 þ 2S ! 3FeS2 Feloss
Fe3 S4 ! 2FeS2 þ Fe
(2) (3) (4) (5) (6)
As shown in Eq. (2), the first reaction between iron and sulfur is the formation of FeS. The as-formed FeS phase then transformed to a Fe3S4 phase by a reaction between FeS phase and sulfur. In addition, Fe loss in the FeS phase is also responsible for the formation of the Fe3S4 phase. The Fe released can produce FeS again because the sulfur addition and Fe loss reactions in the FeS phase are chemically equivalent processes. The formation of Fe3S4 from
reactions (3) and (4) was continued until all Fe-sites were exhausted. The Fe3S4 phase combined gradually with the excess Ssites, which led to a phase transition from Fe3S4 to FeS2. The FeS2 NCs can be described as the form of Fe2þS2 2 , which means that the S-atoms in FeS2 NCs exist in pairs with SeS bonding. Therefore, S atoms combine with Fe3S4 structures in the form of Fe2þFe3þ 2 S4, and the FeS2 NCs due to SeS bonding increases gradually. In this process, the formation of FeS2 due to Fe-loss from Fe3S4 NCs is thermodynamically equivalent to the formation of a FeS2 phase due to S-addition [14,15]. This process makes it possible to establish the FeS2 phase, and Fe in this process acts as another source of FeS phase [15]. As shown in Fig. 1, the FeS phase was observed weakly in all XRD patterns except after 5 h growth, suggesting Fe-loss as the main reason. In addition, conducting impurity phases including FeS and Fe3S4 disappeared at a growth time of 5 h, which means that the FeS2 phase did not decompose to other iron sulfide phases after the formation of FeS2. Therefore, stable FeS2 NCs can be acquired through a suitable chemical reaction path and understanding of these reactions will be needed to remove the conducting impurity phases including FeS and Fe3S4 phases.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
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Fig. 4(a) shows TEM images of the iron sulfide NCs at different growth times. First, the Fe3S4 NCs formed in the shape of hexagonal nano-sheets at a synthesis time of 1 h, and the size of the sheets increased with increasing growth time. The shape of the Fe3S4 NCs changed dramatically to FeS2 NCs with a cubic shape after a synthesis time of 3 h, and the shape of the NCs then changed to a cubichedral structure with the continued growth of the NCs. On the other hand, it is difficult to find the FeS phase in the TEM image. The FeS phase is converted directly to a Fe3S4 phase before sufficient growth of the FeS phase, which is evidenced by the weak intensity of the FeS XRD peak. Fig. 4(b) shows an elemental map of Fe and S in iron sulfide NCs, and the SAED patterns of the NCs at different growth times. The elemental ratio of Fe3S4 and FeS2 was stoichiometric in each NC, which corresponded to the XPS results in Fig. 3. The electron diffraction patterns of Fe3S4 and FeS2 NCs clearly revealed the phase transition of iron sulfide NCs from Fe3S4 to FeS2 NCs. Fig. 5(a) shows the transmittance spectra of iron sulfide NCs at a growth time of 1 h and 5 h. As mentioned above, iron sulfide NCs at 1 h and 5 h had a dominant Fe3S4 and FeS2 phase, respectively. The Fe3S4 NCs at 1 h showed a low transmittance value through the measured wavelength region, and the weak absorption peak around 1200 nm was attributed to the FeS2 phase. This spectrum of the Fe3S4 NCs was similar to metal materials, which means the Fe3S4 NCs has metallic properties. On the other hand, the FeS2 NCs at 5 h showed a strong absorption peak at approximately 1200 nm, which is strongly related to the optical energy band-gap. Fig. 5(b) shows the optical band-gap of FeS2 NCs. The estimated optical band-gap, which was evaluated using Tauc's method [16,17], was ~0.9 eV. This shows that Fe3S4 has metallic behavior, whereas the FeS2 NCs exhibits semiconductor behavior. An understanding of the growth reactions during FeS2 synthesis can be applied to remove the metallic impurity phases from the semiconducting FeS2 phases, which can be used as the absorber layer material in the nextgeneration solar cells. 4. Conclusions In this study, iron sulfide NCs were synthesized using a simple colloidal hot-injection method in an excess sulfur environment, and the formation mechanisms of FeS2 NCs were examined. From chemical reactions between Fe and S, the FeS phase formed first, which transformed to the Fe3S4 phase via S-addition and Fe-loss
processes of FeS phases. The as-formed Fe3S4 phase was transformed to the FeS2 phase through S-addition and Fe-loss process of the Fe3S4 phase and the FeS2 phase finally remains as the chemical reactions are completed. The elemental ratio of [S/Fe] was increased from 1.1 to 2.1, when the growth time increased, and the estimated optical band-gap of the as-synthesized FeS2 NCs was ~0.9 eV. These results show that the metallic FeS and Fe3S4 phase act as intermediate phases to form a semiconducting FeS2 phase. Furthermore, highly conductive impurity phases, such as FeS and Fe3S4, can be removed efficiently by controlling the growth time and adding excess sulfur. Acknowledgments This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20123010010160) funded by the Korea government Ministry of Trade, Industry and Energy. References [1] Y. Bi, Y. Yuan, C.L. Exstrom, S.A. Darveau, I. Huang, Nano Lett. 11 (2012) 4953e4957. €nenkamp, H.J. Wetzel, Mater. Res. 5 [2] R. Schieck, A. Hartmann, S. Fiechter, R. Ke (7) (1990) 1567e1572. [3] H.A. Macpherson, C.R. Stoldt, ACS Nano 6 (10) (2012) 8940e8949. n-Acevedo, M.S. Faber, Y. Tan, R.J. Hamers, S. Jin, Nano Lett. 12 (2012) [4] M. Caba 1977e1982. [5] V. Fthenakis, Renew. Sustain. Energy Rev. 13 (2009) 2746e2750. [6] M. Bazilian, I. Onyeji, M. Liebreich, I. MacGill, J. Chase, J. Shah, D. Gielen, D. Arent, D. Landfear, S. Zhengrong, Renew. Energy 53 (2013) 329e338. [7] A. Luque, S. Hegedus, Handbook of Photovoltaic Science and Engineering, Wiley, Chichester, 2002. [8] D.H. Kim, H.T. Kim, C. Park, Mol. Cryst. Liq. Cryst. 565 (2012) 32e36. [9] P.P. Altermatt, T. Kiesewetter, K. Ellmer, H. Tributsch, Sol. Energy Mater. Sol. Cells 71 (2) (2002) 181e195. [10] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonsovante, K. Buker, M. Bronold, C. Hopfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (4) (1993) 289e370. [11] K. Büker, N. Alonso-Vnate, H. Tributsch, J. Appl. Phys. 72 (12) (1992) 5721e5728. [12] R. Murphy, D.R. Strongin, Sur. Sci. Rep. 64 (2009) 1e45. [13] T.P. Dhakal, L.K. Ganta, D. Vanhart, C.R. Westgate, PVSC 38th IEEE (2012) 170e173. [14] S. Hunger, L.G. Benning, Geochem. Trans. (2007), http://dx.doi.org/10.1186/ 1467-4866-8-1. [15] R.T. Wilkin, H.L. Barnes, Geochimica Cosmochimica Acta 60 (1996) 4167e4179. [16] S. Ilican, M. Caglar, Y. Caglar, Mater. Pol. 25 (3) (2007) 709e718. [17] J. Robertson, Mater. Sci. Eng. R. 37 (2002) 129e281.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment Hong Tak Kim, Thao Phuong Ngoc Nguyen, Chang-duk Kim, Chinho Park* School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
h i g h l i g h t s Pure FeS2 NCs were synthesized in sulfur abundant environment using colloidal method. The phase of iron sulfide NCs was transformed from FeS via Fe3S4 to FeS2 with growth time. Fe3S4 acted as the intermediate precursors for the formation of FeS2 NCs. Pure FeS2 NCs showed the band gap of 0.9 eV, indicating the semiconducting nature of FeS2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 April 2014 Received in revised form 25 August 2014 Accepted 22 September 2014 Available online xxx
Pyrite (FeS2) nano-crystals (NCs) were synthesized in an excess sulfur environment using a colloidal hot-injection method, and the phase change behavior of the iron sulfide compounds was investigated. As the growth time increased, the phase of the iron sulfide NCs transformed from mackinawite (FeS) via greigite (Fe3S4) to pyrite (FeS2). Thus, Fe3S4 phases was considered as intermediate precursors on the pathway of FeS2 phases in the reaction between FeS phases and excess sulfur. The elemental ratio of [S/Fe] increased from 1.1 to 2.1 during the phase change, and the shape of the NCs changed from a hexagonal nano-sheet (Fe3S4), via cubic (FeS2), to a cubic-hedral structure (FeS2). Strong absorption peaks in the UVeVis spectra were observed in the FeS2 phase, and its optical band gap was estimated to be ~0.9 eV, indicating the semiconducting nature of pyrite. Consequently, the synthesis of FeS2 in sulfur abundant environment was suitable method to acquire a pure semiconducting FeS2 phases. The reason was thought that the depletion of Fe-element after the formation of FeS2 phases led to the decrease of intermediate phases and the gradual changes from intermediate phases to FeS2 resulted in pure phases. © 2014 Published by Elsevier B.V.
Keywords: Chemical synthesis Crystal structure Electron microscopy Nanostructures Phase transitions
1. Introduction Pyrite (FeS2) is an interesting material with a high absorption coefficient (~105 cm1), low energy band-gap (~0.9 eV) with a direct transition, and p-type semi-conductivity [1e4]. These properties make FeS2 suitable as the absorption layer in photovoltaic applications. Furthermore, FeS2 is one of the most earthabundant materials, and is very cheap. The cost per area is very important in photovoltaic applications because most solar cells require large area to obtain desired energy from the sun-light [5,6]. To date, many researchers have evaluated a range of materials, such as CuInGaSe2 (CIGS), CdTe and Si, for solar cell applications [7,8]. On the other hand, In, Ga, Se, and Te are not only rare on earth but are
* Corresponding author. E-mail address: [email protected] (C. Park).
also expensive. Although Si is applied most-widely to produce electric energy, the cost of Si should be reduced further for the widespread deployment of photovoltaic modules. Therefore, the next-generation materials for photovoltaic applications should be earth-abundant, cheap, and have high performance properties. In this sense, FeS2 is a very attractive material for next-generation solar cells [9,10]. On the other hand, usual as-formed FeS2 materials as a light absorption semiconductor, have serious problems, such as the presence of highly conducting impurity phases including mackinawite (FeS) and greigite (Fe3S4), and poor surface stability due to the decomposition of FeS2 [1,11e13]. These might be a major reason for a high dark current and low open circuit voltage, which decreases the efficiency of FeS2 devices. Therefore, understanding the formation mechanism of FeS2 materials can be helpful in the context of controlling the impurity phases and improving the surface stability.
http://dx.doi.org/10.1016/j.matchemphys.2014.09.024 0254-0584/© 2014 Published by Elsevier B.V.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
2
H.T. Kim et al. / Materials Chemistry and Physics xxx (2014) 1e4
In this study, FeS2 nano-crystals (NCs) were synthesized via a colloidal route in an excess sulfur environment, which enhances the sulfurization process of iron elements. Chemical reactions between iron and sulfur were controlled using a buffer solution of paraffin oil. The phase change of the iron sulfide NCs during synthesis was examined as a function of the growth time. The structural and optical properties of iron sulfide NCs were also studied using standard measurement methods.
2. Materials and methods The iron sulfide NCs were synthesized using a colloidal hotinjection method. Iron (II) chloride tetrahydrate (FeCl2$4H2O, Aldrich 99%) and sulfur (S, Aldrich 98%) were used as the precursor materials, and the mole ratio of Fe:S was kept to 1:6. Octadecylamine (CH3(CH2)16CH2NH2, Aldrich 97%) and diphenyl ether ((C6H5)2O, Aldrich 99%) were used as the capping materials and solvent, respectively. Paraffin liquid (OCI Company Ltd. ultrapure) was used as a buffer solution for the Fe-solution to reduce the chemical reactions. FeCl2$4H2O (100 mg) and octadecylamine (10 g) were dissolved in 40 mL of paraffin liquid at 220 C for 1 h, and S (96 mg) was dissolved in 5 mL of dipenyl ether (5 ml) at 150 C for 1 h, respectively. The prepared solutions were then combined to form the FeS2 NCs in vessel with three-bottle necks. The growth temperature was kept at 205 C, and the solution temperature was measured directly using a K-type thermocouple. The vessel was purged with nitrogen gas before the synthesis process, and then closed to block oxygen during NCs synthesis. After finishing the growth process, the solution was cooled rapidly to room temperature to prevent the excessive chemical reactions. The as-synthesized solutions were centrifuged and washed with methanol and acetone to remove the impurities. The purified NCs were stored in 1-propanol (CH3CH2CH2OH, Aldrich 98%) to prevent oxidation and aggregation between the NCs. The structural properties of iron sulfide NCs were examined by X-ray diffraction (XRD, PANalytical PRO). The shape, selected area electron diffraction (SAED) patterns, and elemental mapping of the NCs were measured by transmission electron microscopy (TEM, Hitachi H-7600). The optical properties of the iron sulfide NCs were measured using a UVeViseNIR spectrometer (Cary 5G). The elemental ratio of the NCs was analyzed by X-ray photoelectron spectroscopy (XPS, VG Microtech MT 500/1).
Fig. 1. X-ray diffraction (XRD) patterns of the iron sulfide NCs at different growth times.
, PFi ¼ Ii
3 X
Ij
ði ¼ 1; 2; 3Þ
(1)
j¼1
where I1, I2, and I3 are the intensity for FeS (101), Fe3S4 (113), and FeS2 (200) peak, respectively. Fig. 2 shows the change of phase fraction for iron sulfide materials as a function of the growth time. The magnitudes of phase fractions of the Fe3S4 and FeS2 phases showed an inverse relationship according to the growth time, indicating the formation of FeS2 from the transformation of Fe3S4. In addition, small amount of FeS phases were decreased gradually to a negligible level with the increase of growth time. This shows that Fe3S4 phases act as intermediate precursors on the pathway to FeS2 phases in the chemical reaction between FeS and excess sulfur. Fig. 3 shows the elemental ratio of S and Fe as a function of the growth time. The amount of sulfur in the iron sulfide materials increased with increasing growth time, in which the sulfur to iron ratio changed from 1.1 to 2.1. The change in the elemental ratio of [S/Fe] suggests that the phase of iron sulfide transformed gradually from FeS to Fe3S4 and then to FeS2. This is consistent with the XRD data, as shown in Figs. 1 and 2. The proposed chemical reactions for the formation of single-phase FeS2 NCs are as follows:
3. Results and discussion Fig. 1 shows XRD patterns of iron sulfide NCs according to the growth time. The phase of the iron sulfide materials changes from the mixed phase of FeS and Fe3S4 to a pure FeS2 phase, when the growth time was increased from 1 h to 5 h. FeS, Fe3S4, and FeS2 had a tetragonal structure (space group: P4-nmm), a cubic structure (space group: Fd-3 m) and a cubic structure (space group: Pa-3), respectively [JCPDS 42-1340, JCPDS 89-1998]. In this case, both FeS and Fe3S4 phases could be observed by XRD up to a growth time of 4 h, and the phase of FeS disappeared when the phase of iron sulfide changes to a pure FeS2 phase at 5 h. This suggests that FeS acts as a precursor material to form Fe3S4 until the end of FeS2 formation. The phase change of the iron sulfides can be described in terms of the phase fraction (PF) for different crystal structures, which defined the ratio of mostly preferred orientation peak for each crystal structure. The phase fraction for each crystal structure is given by:
Fig. 2. The variation of phase fraction (PF) for iron sulfide NCs as a function of the growth time.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
H.T. Kim et al. / Materials Chemistry and Physics xxx (2014) 1e4
3
Fig. 5. (a) UVeViseNIR transmittance spectra at different growth times and (b) optical band-gap at a growth time of 5 h.
Fig. 3. Elemental ratio of Fe, S, and [S/Fe] as a function of the growth time.
Fig. 4. TEM analysis of iron sulfide NCs at different growth times: (a) TEM images of iron sulfide NCs and (b) elemental maps of FeeS components and selected area electron diffraction (SAED) patterns of iron sulfide NCs at the growth time of 1 h, 3 h, and 5 h.
Fe þ S/FeS Saddition
3FeS þ S ! Fe3 S4 Feloss
4FeS ! Fe3 S4 þ Fe Saddition
Fe3 S4 þ 2S ! 3FeS2 Feloss
Fe3 S4 ! 2FeS2 þ Fe
(2) (3) (4) (5) (6)
As shown in Eq. (2), the first reaction between iron and sulfur is the formation of FeS. The as-formed FeS phase then transformed to a Fe3S4 phase by a reaction between FeS phase and sulfur. In addition, Fe loss in the FeS phase is also responsible for the formation of the Fe3S4 phase. The Fe released can produce FeS again because the sulfur addition and Fe loss reactions in the FeS phase are chemically equivalent processes. The formation of Fe3S4 from
reactions (3) and (4) was continued until all Fe-sites were exhausted. The Fe3S4 phase combined gradually with the excess Ssites, which led to a phase transition from Fe3S4 to FeS2. The FeS2 NCs can be described as the form of Fe2þS2 2 , which means that the S-atoms in FeS2 NCs exist in pairs with SeS bonding. Therefore, S atoms combine with Fe3S4 structures in the form of Fe2þFe3þ 2 S4, and the FeS2 NCs due to SeS bonding increases gradually. In this process, the formation of FeS2 due to Fe-loss from Fe3S4 NCs is thermodynamically equivalent to the formation of a FeS2 phase due to S-addition [14,15]. This process makes it possible to establish the FeS2 phase, and Fe in this process acts as another source of FeS phase [15]. As shown in Fig. 1, the FeS phase was observed weakly in all XRD patterns except after 5 h growth, suggesting Fe-loss as the main reason. In addition, conducting impurity phases including FeS and Fe3S4 disappeared at a growth time of 5 h, which means that the FeS2 phase did not decompose to other iron sulfide phases after the formation of FeS2. Therefore, stable FeS2 NCs can be acquired through a suitable chemical reaction path and understanding of these reactions will be needed to remove the conducting impurity phases including FeS and Fe3S4 phases.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024
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Fig. 4(a) shows TEM images of the iron sulfide NCs at different growth times. First, the Fe3S4 NCs formed in the shape of hexagonal nano-sheets at a synthesis time of 1 h, and the size of the sheets increased with increasing growth time. The shape of the Fe3S4 NCs changed dramatically to FeS2 NCs with a cubic shape after a synthesis time of 3 h, and the shape of the NCs then changed to a cubichedral structure with the continued growth of the NCs. On the other hand, it is difficult to find the FeS phase in the TEM image. The FeS phase is converted directly to a Fe3S4 phase before sufficient growth of the FeS phase, which is evidenced by the weak intensity of the FeS XRD peak. Fig. 4(b) shows an elemental map of Fe and S in iron sulfide NCs, and the SAED patterns of the NCs at different growth times. The elemental ratio of Fe3S4 and FeS2 was stoichiometric in each NC, which corresponded to the XPS results in Fig. 3. The electron diffraction patterns of Fe3S4 and FeS2 NCs clearly revealed the phase transition of iron sulfide NCs from Fe3S4 to FeS2 NCs. Fig. 5(a) shows the transmittance spectra of iron sulfide NCs at a growth time of 1 h and 5 h. As mentioned above, iron sulfide NCs at 1 h and 5 h had a dominant Fe3S4 and FeS2 phase, respectively. The Fe3S4 NCs at 1 h showed a low transmittance value through the measured wavelength region, and the weak absorption peak around 1200 nm was attributed to the FeS2 phase. This spectrum of the Fe3S4 NCs was similar to metal materials, which means the Fe3S4 NCs has metallic properties. On the other hand, the FeS2 NCs at 5 h showed a strong absorption peak at approximately 1200 nm, which is strongly related to the optical energy band-gap. Fig. 5(b) shows the optical band-gap of FeS2 NCs. The estimated optical band-gap, which was evaluated using Tauc's method [16,17], was ~0.9 eV. This shows that Fe3S4 has metallic behavior, whereas the FeS2 NCs exhibits semiconductor behavior. An understanding of the growth reactions during FeS2 synthesis can be applied to remove the metallic impurity phases from the semiconducting FeS2 phases, which can be used as the absorber layer material in the nextgeneration solar cells. 4. Conclusions In this study, iron sulfide NCs were synthesized using a simple colloidal hot-injection method in an excess sulfur environment, and the formation mechanisms of FeS2 NCs were examined. From chemical reactions between Fe and S, the FeS phase formed first, which transformed to the Fe3S4 phase via S-addition and Fe-loss
processes of FeS phases. The as-formed Fe3S4 phase was transformed to the FeS2 phase through S-addition and Fe-loss process of the Fe3S4 phase and the FeS2 phase finally remains as the chemical reactions are completed. The elemental ratio of [S/Fe] was increased from 1.1 to 2.1, when the growth time increased, and the estimated optical band-gap of the as-synthesized FeS2 NCs was ~0.9 eV. These results show that the metallic FeS and Fe3S4 phase act as intermediate phases to form a semiconducting FeS2 phase. Furthermore, highly conductive impurity phases, such as FeS and Fe3S4, can be removed efficiently by controlling the growth time and adding excess sulfur. Acknowledgments This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20123010010160) funded by the Korea government Ministry of Trade, Industry and Energy. References [1] Y. Bi, Y. Yuan, C.L. Exstrom, S.A. Darveau, I. Huang, Nano Lett. 11 (2012) 4953e4957. €nenkamp, H.J. Wetzel, Mater. Res. 5 [2] R. Schieck, A. Hartmann, S. Fiechter, R. Ke (7) (1990) 1567e1572. [3] H.A. Macpherson, C.R. Stoldt, ACS Nano 6 (10) (2012) 8940e8949. n-Acevedo, M.S. Faber, Y. Tan, R.J. Hamers, S. Jin, Nano Lett. 12 (2012) [4] M. Caba 1977e1982. [5] V. Fthenakis, Renew. Sustain. Energy Rev. 13 (2009) 2746e2750. [6] M. Bazilian, I. Onyeji, M. Liebreich, I. MacGill, J. Chase, J. Shah, D. Gielen, D. Arent, D. Landfear, S. Zhengrong, Renew. Energy 53 (2013) 329e338. [7] A. Luque, S. Hegedus, Handbook of Photovoltaic Science and Engineering, Wiley, Chichester, 2002. [8] D.H. Kim, H.T. Kim, C. Park, Mol. Cryst. Liq. Cryst. 565 (2012) 32e36. [9] P.P. Altermatt, T. Kiesewetter, K. Ellmer, H. Tributsch, Sol. Energy Mater. Sol. Cells 71 (2) (2002) 181e195. [10] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonsovante, K. Buker, M. Bronold, C. Hopfner, H. Tributsch, Sol. Energy Mater. Sol. Cells 29 (4) (1993) 289e370. [11] K. Büker, N. Alonso-Vnate, H. Tributsch, J. Appl. Phys. 72 (12) (1992) 5721e5728. [12] R. Murphy, D.R. Strongin, Sur. Sci. Rep. 64 (2009) 1e45. [13] T.P. Dhakal, L.K. Ganta, D. Vanhart, C.R. Westgate, PVSC 38th IEEE (2012) 170e173. [14] S. Hunger, L.G. Benning, Geochem. Trans. (2007), http://dx.doi.org/10.1186/ 1467-4866-8-1. [15] R.T. Wilkin, H.L. Barnes, Geochimica Cosmochimica Acta 60 (1996) 4167e4179. [16] S. Ilican, M. Caglar, Y. Caglar, Mater. Pol. 25 (3) (2007) 709e718. [17] J. Robertson, Mater. Sci. Eng. R. 37 (2002) 129e281.
Please cite this article in press as: H.T. Kim, et al., Formation mechanisms of pyrite (FeS2) nano-crystals synthesized by colloidal route in sulfur abundant environment, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.09.024