Universal Synthesis of Single-Phase Pyrite FeS2 Nanoparticles

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Universal Synthesis of Single-Phase Pyrite FeS2 Nanoparticles, Nanowires, and Nanosheets Yongxiao Bai,†,‡ Jihyeon Yeom,∥ Ming Yang,† Sang-Ho Cha,† Kai Sun,§ and Nicholas A. Kotov*,†,§,∥,⊥ †

Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States Department of Materials Science and Engineering, Lanzhou University, Lanzhou 730000, China § Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States ∥ Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States ⊥ Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States ‡

S Supporting Information *

ABSTRACT: Nanoscale pyrite FeS2 is considered to be one of few potentially transformative materials for photovoltaics capable of bridging the cost/performance gap of solar batteries. It also holds promise for energy storage applications as the material for high-performance cathodes. Despite prospects, the synthesis of FeS2 nanostructures and diversity of their geometries has been hardly studied. Moreover, the state-ofthe-art aqueous dispersions of nanoscale pyrite, which have special significance for solar energetics, are particularly disappointing due to low quality. There are no known methods to produce well-crystallized nanoparticles and other geometries of nanoscale pyrite in water or mixed aqueous solvents. Here, we describe a successful synthesis of single-phase pyrite nanoparticles with a diameter of 2−5 nm in polar solvent and aqueous dispersions. The particles display high uniformity and crystallographic purity. Moreover, the synthetic approach developed for nanoparticles was proven to be quite universal and can be modified to produce both nanowires and nanosheets, which also display high crystallinity. The diameter of the pyrite nanowires was 80−120 nm with the length exceeding 5 μm. The nanosheets displayed lateral dimensions of 100−200 nm with the thickness of 2 nm. Availability of single-phase FeS2 nanoscale aqueous dispersions is expected to stimulate further studies of these materials in green energy conversion technologies and drug delivery applications.

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INTRODUCTION

Since 1836 when Wöhler made pyrite by reaction of Fe2O3 with liquid sulfur and NHCl4,7 various synthetic methods for making bulk FeS2 have been developed: reactive sputtering, electrodeposition, chemical vapor transposition, spray pyrolysis, thermal sulfidation, and several hydrothermal/solvothermal routes.8−15 It was noted in many of these studies that the variability in grain size and crystallinity, spatial gradient of impurities, and poorly controlled stoichiometric ratio for FexSy family of materials greatly limit optical and electrical properties of FeS2 and, consequently, its performance in PV devices.1,16 Also note the actual synthetic protocols for production of controllable nanoscale versions of pyrite are scarce and include only high boiling point organic solvents with also functioning as long chain surfactants.2c While being fundamentally interesting, such solvents lead to formation of thick electrically insulating shell around NPs impeding efficient charge transport. Since the cost of solar cells represents an important variable in the cost-

Iron(II) sulfide (FeS2), i.e., pyrite or Fool’s Gold, is one of the most interesting materials for energy conversion1,2 and in particular for photovoltaics (PV).3 The bulk form of pyrite FeS2 has a band gap energy of 0.95 eV and high optical absorption coefficient of 5 × 105 cm−1. These two parameters combined with the possibility to vary the band gap using nanoscale confinement, different geometries of nanoscale forms of FeS2, and/or different intercalation chemistries2c make FeS2 an almost ideal semiconductor for harvesting broad spectrum of light with wavelengths as long as 750 nm.4 Besides PV applications, pyrite can also be a promising material for lithium batteries and demonstrated remarkable performance and long shelf life as the high capacity cathode5 and is currently used in Energizer batteries. These unique properties coupled with potentially low costs, environmental friendliness, and high abundance give pyrite a great advantage compared to many other materials including graphene and necessitate further studies of nanoscale chemistry of FeS2. It is our opinion that it contains a lot of blank spots as well as the potential for scientific discoveries.6 © 2013 American Chemical Society

Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: November 9, 2012 Revised: January 4, 2013 Published: January 18, 2013 2567

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Synthesis. In a typical synthesis, 0.259 g (1.30 mmol) of FeCl2·4H2O was dissolved in 90 mL of dimethyl sulfoxide (DMSO) containing the appropriate amount of stabilizer, namely, thioglycolic acid (TGA). The solution of the stabilizer was placed in a three-necked flask fitted with a valve and was deaerated by bubbling of 99.99% nitrogen for 30 min. A solution of Na2S2O3·5H2O obtained by the dissolution of 1.45 g (5.85 mmol) of Na2S2O3·5H2O (Aldrich) in 10 mL of 18 MΩ deionized water under N2 atmosphere was dropwise added into the solution while stirring and continuously purging the reaction media with nitrogen for 30 min. Initial FeS2 nanocolloids formed after the solution was brought to boiling temperature, which was accompanied by the change of the solution color from brown to dark black. The nanoscolloids were allowed to grow and crystallize under conditions of continuous reflux at 139 °C for 2−12 h. As was expected, the time of reflux determined the diameter of the resulting NPs. The FeS2 NPs were synthesized when the molar ratio of [Fe2+]/[ S2O32−]/[TGA] was 1:4.5:4. NRs and NWs could be obtained when the molar ratios of [Fe2+]/[TGA] were 1:3 and 1:2, respectively. They were obtained by partial replacement of DMSO with ethylenediamine (EDA) serving as a cosolvent and costabilizer. The molar ratio of [EDA]/[TGA] was 2:1. EDA was added immediately after all the reactants were dissolved in DMSO but before raising the reaction temperature. Upon completion, the NPs and other nanocolloids were separated from the reaction media by centrifugation. The precipitate was washed several times with ethanol and deionized water. The final products were dried in vacuum at 60 °C for 6 h. Characterization. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed using a JEOL JEM3011 microscope with accelerating voltage of 200 kV. X-ray diffraction (XRD) studies of FeS2 pyrite NPs were carried out at room temperature on a Rigaku Rotating Anode X-ray Diffractometer using Cu Kα radiation (1.54 Å). X-ray photoelectron (XPS) measurements were performed on an AXIS ULTRA X-ray photoelectron spectroscope, using monochromatised Al Kα radiation with an anode voltage of 15 kV and emission current of 3 mA.

efficiency equation for PV, organic solvents represent a technologically unfavorable choice. In addition to the lower cost, aqueous solvents and water are strongly preferred for the synthesis of PV-relevant materials for better environmental compliance. In perspective, this can lead to truly green technological cycle with negative overall balance of carbon emissions. Such need can be contrasted by the very limited knowledge about the synthesis nanoscale pyrite in aqueous solvents. Apart of some examples of low quality nanoscale materials with wide size distribution, variance of particle composition, and poor crystallinity,1e methods of preparation for single-phase pyrite nanoparticles (NPs) in aqueous solvents remain unknown. Developing a reliable method to synthesize single-phase stoichiometric NPs and other nanoscale forms of pyrite with high degree of control is much needed to realize its potential in energy conversion and other applications. One of the significant problems of the current methods of preparation of pyrite nanostructures is the difficulty of controlling the crystallinity of FeS2. Besides the actual pyrite, multiple other isomorphs of FeS2 and iron−sulfur compounds with slightly different stoichiometries are often present and create multiple interfaces, charge carrier traps, and intra bandgap levels. Their presence diminishes open circuit photovoltage. This is the problem that plagued FeS2 photovoltaics for a long time.1,12 Moreover, despite excellent results obtained for the preparation of pyrite NPs in organic solvents, there are no methods reported of producing high quality pyrite NWs, NRs, and other morphologies/dimensionalities of pyrite nanostructures, which could be actually preferred for PV as well as for Li+ storage devices. We also would like to point out that having a fairly unified method to produce them in hydrophilic solvents (preferentially, of course, in water) presents considerable advantages for scaled-up versions of the synthesis and environmental perspective in general. To address this challenge, we decided to look into methods of FeS2 preparation based on our previous experience with CdTe preparation17 and similar synthetic processes developed later for In2S3.18 Here, we report a method for facile chemical synthesis of pyrite NPs with size control, competitive monodispersity, colloidal stability, and high crystalinity. Surprisingly, modification of this synthetic process for NPs allows one to prepare different variation of the nanoscale morphology of FeS2 colloids. We also demonstrate that a wide variety of FeS2 nanostructures from NPs to nanorods (NRs), to nanowires (NWs) and nanosheets (NSs) can be obtained using this synthetic approach by changing the molar ratio of reactants and/or reaction times. Notably, the precursors are inexpensive, and all these nanoscale varieties of FeS2 are single-phase pyrite without the addition of other phases. Such synthetic universality appears to be unique among semiconductor nanostructures. To the best of our knowledge, it was not observed for any other semiconductor material including widely investigated II−VI, III−V, and I−VII and should be attributed to versatility of the cubic lattice where metal atoms are in minority.

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RESULTS AND DISCUSSION FeS2 Nanoparticles. Inspired by the successful preparation of CdTe NPs by the Rogach−Weller method,19 we decided to investigate the possibility of its potential adaptation to the preparation of pyrite nanostructures. As molecular precursors, we decided to use FeCl2·4H2O and Na2S2O3·5H2O complemented by TGA as the stabilizer. The precursors were dissolved in a mixture of DMSO and water and refluxed under N2 atmosphere at 139 °C for a required period of time (2−12 h). Transmission electron microscopy (TEM) images of the dispersions made for reactants with a ratio of [Fe2+]/ [S2O32−]/[TGA] of 1:4.5:3 indicate formation of fairly monodisperse FeS2 NPs with standard size distribution of ca. 16%. Considerably improved monodispersity of the NP dispersions described here is attributed to the use of a more efficient stabilizer, TGA, as compared to the previous studies and possibly stricter control of anaerobic conditions. Fine tuning of temperature, concentration, and reaction media also contributed to the improvement of the quality of the nanoscale dispersions. As one could see the products were very sensitive to these parameters.

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EXPERIMENTAL SECTION Materials. Iron(II) chloride tetrahydrate (FeCl2·4H2O, 99%), sodium thiosulfate pentahydrate (Na2S2O3·5H2O, 99.5%), thioglycolic acid (TGA, 99%), and anhydrous ethanol were purchased from Sigma-Aldrich. E-pure deionized water (18.2 MΩ·cm) was obtained from a Millipore Milli-Q system. All chemicals were used as received without any further purification. 2568

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Figure 1. TEM images of FeS2 NPs obtained for different reaction times of (A) 2 h, (B) 4 h, and (C) 12 h. In the insets: (A,B) HRTEM image of 3 nm and 5 nm, and (C) SAED pattern of 10 nm NPs.

The NP size could be easily varied from 3 nm to 5 nm and 10 nm by extending the reaction time from 2 h to 4 and 12 h, respectively. The crystallinity of the NPs was studied by the high-resolution transmission electron microscopic (HRTEM) and selected area electron diffraction (SAED). The inset HRTEM images and the SAED pattern of 10 nm NPs of single FeS2 NPs indicated that they were in pyrite form (Figure 1) without any traceable presence of other crystalline phases (Figure 1). As could be seen in HRTEM images in Figures

Figure 2. Atomic model of the pyrite FeS2 NPs. The green and yellow spheres denote Fe and S atoms, respectively. Figure 3. (A) XRD pattern and (B,C) XPS spectra of FeS2 NPs. The XRD spectrum was obtained from a sample dispersed in ethanol.

1A,B, the distance between atomic planes was 5.420 Ǻ , which matched well the literature value for pyrite, 5.418 Ǻ .20 There were no diffraction rings from other forms of FexSy. X-ray diffraction (XRD) data of NP aggregates (100−150 nm) (Figure 3A) also indicated that all reflection peaks could be indexed to cubic phase of FeS2 (JCPDS 00-042-1340). No other peaks from impurities such as marcasite, pyrrhotite, or troilite could be detected. The main diffraction peaks of marcasite (PDF 02-1342) should have been observed at 33.5, 39 47.8, and 52°, while peaks of pyrrhotite (PDF 17-0201) could be expected at 30, 34 43.5, 53.2, 65, and 71°. Troilite (PDF 01-1247) should have displayed XRD peaks at 30, 34, 44, 54, 57, 65, and 51.5°. X-ray photoelectron spectroscopy (XPS) (Figure 3B,C) and the iron peaks in the XPS spectrum match energy position expected for FeS2. From the broad scan, we saw that they indeed consisted mainly of iron and sulfur; signals corresponding to oxygen and carbon originate from the stabilizer, i.e., thioglycolic acid (Figure S1, Supporting Information). The sulfur to iron atomic ratio was ∼2 and consistent with pyrite. The Fe 2p3/2 binding energy of 707.5 eV (Figure 3 B) was characteristic of pyrite, while for marcasite, it would have been observed at 707.7. The S 2p3/2 and S 2p1/2 peaks observed at 162.5 and 163.7 eV, respectively (Figure 3C), were also

consistent with the sulfur binding energy in pyrite.16 For marcasite, the sulfur peaks S 2p3/2 and S 2p1/2 would have been observed at 163.1 and 164.1 eV. The as-made FeS2 NPs could be redispersed in polar different solvents. This fact is relevant for further studies of their optical properties as well as applications. The NPs were separated from the reaction mixture by centrifugation. The precipitate can be subsequently redispersed in pure ethanol, methanol, or deionized water (Figure 4). The latter dispersion was used to investigate optical properties of the NPs. As expected from a semiconductor with an indirect band gap, the UV−vis adsorption spectrum displayed a broad featureless band with a maximum centered approximately at 400−600 nm depending on the NP diameter. The room-temperature fluorescence spectrum displayed a maximum at 400−450 nm (Figures 4C and S3, Supporting Information) increasing in intensity for NPs of larger diameters. The position of the PL maxima for 3 nm FeS2 was located at 360 and 460 nm, whereas 5 and 10 nm FeS2 NPs had dominant peaks at 400 and 450 nm, respectively. FeS2 Nanowires. Similarly to the well-studied case of CdSe,21 shape control for nanoscale FeS2 can be a synthetic challenge. It is also one of the key function-determining 2569

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Figure 4. (A,B) Optical photographs of FeS2 NP dispersions in different solvents and (C) fluorescence spectrum of FeS2 NPs with the diameter of 10 nm dispersed in deionized water (20 °C). Additional UV−vis and fluorescence spectra are given in Figure S3, Supporting Information.

Figure 5. Electron microscopy images of FeS2 pyrite NWs. (A,B) SEM images of pyrite NWs and (inset B) XRD pattern. (C,D) Representative TEM images of FeS2 NWs. In the insets: (C) the enlarged part of the NW in panel C and (D) HRTEM image of the single FeS2 NW in panel D.

using organic solvents did not allow for these types of products.22 NWs were synthesized using the same precursors as the case of NPs, namely, FeCl2·4H2O, Na2S2O3·5H2O, and TGA. The difference was in the addition of ethylenediamine (EDA) as a costabilizer and cosolvent. The choice of EDA was governed by the considerations that it has lower energy of binding to Fe atoms than TGA. So, it should be less restrictive for the growth of nanocrystals while still preventing uncontrolled aggregation. Both scanning electron microscopy (SEM) and TEM images

parameters for PV and battery applications. It would be very useful to demonstrate in addition to NPs, which are typically described as zero-dimensional nanoscale semiconductor, onedimensional nanoscale varieties of FeS2 such as NRs and NWs. Two-dimensional nanoscale structures, such as NSs, gain even greater importance after many trials and tribulations related to integration of graphene and related materials in electronic and energy storage devices. Having all of the mentioned morphologies in the toolbox of materials scientists and device engineers will be essential. The previous synthetic approaches 2570

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Figure 6. TEM images of pyrite NWs with different aspect ratios: (A) 35, (B) 55, (C) 60, (D) 70, (E) 80, and (F) >80. The inset in panels E and F is the EDX spectrum of the NW in panel E.

Figure 7. TEM images of FeS2 pyrite NSs. (A,D) TEM images of iron pyrite NSs obtained for reflux time of 20 and 25 h, respectively. (B,C) HRTEM images of different parts of a NS in panel A. (E) XRD pattern of NSs.

10 h to 12 and 14 h, the aspect ratio of the NWs, increased from 35 to 55 and 60, respectively (Figure 6). The highest aspect ratio was obtained for a composition of [Fe2+]/ [S2O32−]/[TGA] = 1:4.5:2. Under these conditions, the reaction times of 20, 26, and 32 h gave NWs with aspect ratios of 70, 80, and 85, respectively. The diameter of NWs varied little over their entire length. For instance, for NWs 2−5 μm in length, the diameter varied between 50 and 75 nm. The XRD patterns of the NWs were the as same as for NPs (Figure 3A and inset in Figure 5B). The electron diffraction X-ray

(Figure 5) indicated that FeS2 NWs were indeed an exclusive product of synthesis in the presence of EDA. FeS2 NWs were between 50 and 200 nm in diameter. The surface of the NWs was smooth and minimal variation over the length of the NWs were observed (Figure 5C). The aspect ratio and growth rate of the pyrite NRs and NWs was systematically controlled by adjusting the ratio of TGA to the precursor and varying the reaction time. Shorter FeS2 NWs were obtained with the molar ratio of [Fe2+]/[S2O32−]/[TGA] of 1:4.5:2.5 (Figure 6). As we increased the reaction time from 2571

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ACKNOWLEDGMENTS We greatly acknowledge financial assistance by China Scholarship Council and National Natural Science Foundation of China (NSFC) #50703017 as well as the National Research Foundation of Korea Grant funded by the Korean Government [NRF-2009-352-D00078] for this work. The work was also supported by EFRI-BSBA 0938019; CBET 0933384; and CBET 0932823 whose funding was used to cover the salaries. This material is based upon work partially supported by the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number #DE-SC0000957 (reactants for NPs). We thank the University of Michigan’s EMAL for its assistance with electron microscopy and for the NSF grant #DMR9871177 funding the JEOL 2010F analytical electron microscope used in this work.

(EDX) spectrum (inset in Figure 6E,F) taken from both ends and in the middle of the NWs in Figure 6E were nearly identical. Elemental analysis by EDX confirmed the FeS2 composition of the NWs with weaker peaks attributed to TGA. FeS2 Nanosheets. FeS2 nanosheets (NSs) were obtained with a reactant ratio of [Fe2+]/[S2O32‑]/[TGA] of 1:4.5:1 and the reaction time exceeding 20 h using DMSO/EDA media as for the synthesis of NWs. The NSs could be obtained from NPs after sufficiently long reflux time at a relatively low TGA concentration. Figure 7A,D displays representative images of FeS2 NSs synthesized with the refluxing times of 20 and 25 h, respectively. The mechanism of morphological control of nanocolloids of FeS2 in our experiments still needs to be investigated. At this point, we can speculate that the differences in reaction conditions described above, TGA concentration, and the presence of EDA as a cosolvent are likely to originate from selective adsorption of these molecules on different crystal phases of FeS2 and related alteration of crystal growth rates along different crystalline directions. Alternatively, the same conditions may also affect the processes of self-assembly of NPs into NWs, NRs, and NSs. Reduction of TGA density on some facets is expected to stimulate the assembly of NPs following specific patterns determined by their geometrical anisometry and anisotropy of the force fields around them. The assembly into the supraparticle structures can be followed by their recrystallization.17a The indication that self-assembly processes rather than continuous crystal growth could be involved in these reactions can be seen in HRTEM images on the NWs in Figure 5D where individual NPs could be discerned.

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CONCLUSIONS We have demonstrated a simple synthetic route for single-phase pyrite nanostructures using common iron salt and stabilizer widely used in CdTe nanochemistry. Various shapes of FeS2 like NPs, NRs, NWs, and NSs were observed under different synthetic conditions. We expect that methodology reported in this communication will stimulate the studies of pyrite preparation in nanoscale form as means to rectify the low energy conversion parameters for photovoltaics (PV)23 observed for FeS2 materials before. The use of polar solvents and the possibility to produce aqueous dispersions will also be helpful in reducing the environmental load of NP synthesis and can serve as an impetus to a variety of biomedical applications of FeS2 nanostructures, for instance, for drug delivery. ASSOCIATED CONTENT

S Supporting Information *

XPS characterization; additional TEM images of FeS2 NPs; absorption and photoluminescence spectra of FeS2 NPs; additional TEM images of FeS2 NRs, NWs, and NSs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2572

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