Synthesis and Characterization of Bournonite PbCuSbS3 Nanocrystals

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Crystal Growth & Design

Synthesis and Characterization of Bournonite PbCuSbS3 Nanocrystals

Kaya Wei1, Joshua Martin2, James R. Salvador3, and George S. Nolas1,* 1

Department of Physics, University of South Florida, Tampa, FL 33620

2

Material Measurement Laboratory, National Institute of Standards and Technology,

Gaithersburg, MD 20899 3

Chemical and Materials Systems Laboratory, GM R&D Center, Warren, MI 48090

PbCuSbS3 nanocrystals were synthesized by a colloidal synthesis route and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction spectroscopy (EDS). PbCuSbS3 particle sizes of 5 nm, 150 nm, and 200 nm were obtained by adjusting the reaction time. The optical band-gap, thermal stability, heat capacity, and magnetic susceptibility of PbCuSbS3 were also investigated. PbCuSbS3 is diamagnetic with an optical band-gap of 1.31 eV and a Debye temperature of 157 K. This study aims to expand and broaden our knowledge of PbCuSbS3 in light of the recent interest in multinary chalcogenides for energy-related applications.

*George S. Nolas, Ph.D. Professor of Physics, ISA 2019 University of South Florida 4202 East Fowler Avenue Tampa, FL 33620 Phone:(813) 974-2233 FAX: (813) 974-5813 E-mail:[email protected] Web: http://shell.cas.usf.edu/gnolas/ 1 ACS Paragon Plus Environment


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2

Material Measurement Laboratory, National Institute of Standards and Technology,



Abstract

PbCuSbS3 nanocrystals were synthesized by a colloidal synthesis route and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction spectroscopy (EDS). PbCuSbS3 particle sizes of 5 nm, 150 nm, and 200 nm were obtained by adjusting the reaction time. The optical band-gap, thermal stability, heat capacity, and magnetic susceptibility of PbCuSbS3 were also investigated. PbCuSbS3 is diamagnetic with an optical band-gap of 1.31 eV and a Debye temperature of 157 K. This study aims to expand and broaden our knowledge of PbCuSbS3 in light of the recent interest in multinary chalcogenides for energy-related applications.

_____________________________________________________________________________________ * E-mail: [email protected] 2 ACS Paragon Plus Environment

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Introduction Quaternary chalcogenides have promising properties for energy-related applications,[1-4] particularly certain metal-chalcogenides compositions with complex crystal structures that allow for band structure modifications.[5-8] It has also been demonstrated that nano-scale effects can directly influence the transport properties of these materials resulting in improved thermoelectric performance.[9-13] In addition, it is of technological importance to develop simple synthetic approaches to these new nanocrystalline phases in order to develop cost-effective processing techniques to incorporate nano-scale features within a bulk material for potential thermoelectric and other energy-related applications.[9-11] Investigating new compositions and structure types in nanocrystalline form is therefore of interest.[9,10,14] PbCuSbS3, a semiconductor with the bournonite crystal structure (orthorhombic space group Pmn21), [15] is one such material.

Using a colloidal synthesis approach, we have synthesized phase pure PbCuSbS3 nanocrystals and sub-micron crystals. Average particle sizes of 5 nm, 150 nm, and 200 nm were obtained by adjusting the reaction time. The resultant materials were characterized using a variety of techniques including low temperature calorimetry to assess specific heat (CP), magnetic susceptibility, transition electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis spectroscopy, and high temperature differential thermal analysis (DTA) and thermal gravimetric analysis (TGA). We find no prior reports describing the low temperature heat capacity nor are magnetic properties for bulk or nanocrystalline PbCuSbS3 reported therefore the analyses reported here are the first. The Debye model was used to investigate the temperature dependent Cp data.

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Experimental Section The chemicals used in this study were lead (IV) acetate [Pb(CH3CO2)4] (95%, Sigma-Aldrich), copper (II) acetate monnohydrate [Cu(OOCCH3)·H2O] (98%, Alfa Aesar), antimony (III) chloride [SbCl3] (99.99%, Sigma-Aldrich), sulfur powder (99.5%, Alfa Aesar) and Oleylamine (80%, Acros Organics).1 All chemicals were used as received without further purification. In a typical synthesis, 1 mmol of Pb(CH3CO2)4, 1 mmol of Cu(OOCCH3)·H2O, 1 mmol of SbCl3, 3 mmol S powder and 40 mL Oleylamine were loaded into a three-neck flask on a Schlenk line. The mixture was kept at room temperature under a N2 flow for 10 min followed by degassing under vacuum for 100 min. A dark brown color was observed during degassing. The solution was then heated to 110 °C for 30 min to dissolve all constituent reactants followed by heating to 280 °C. Three separate syntheses were carried out and differ in the dwell time at 280 oC with lengths of 5 min, 20 min, and 60 min. The dwell time was varied as a means of controlling the particle size of the final product, as will be discussed below. The flask was then rapidly cooled to room temperature in an ice water bath. A typical synthesis yields approximately 0.3 g (40 % of the starting chemical weight) powder specimen. Scaling up to larger batch sizes is possible by proportionally increasing all chemical sources, however much larger batch sizes will presumably necessitate modifications and further optimization of this process. A typical ethanol/chloroform mixture was used to wash the products three times before the product was isolated by

1

Certain commercial equipment, instrumentation, or materials are identified in this document. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose. 4 ACS Paragon Plus Environment

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centrifugation at 9000 rpm for 3 min. The final products were transferred to a vacuum oven for drying prior to structural and chemical analysis. The as-synthesized products were characterized for phase identity, microstructure and composition by powder XRD (Bruker AXS D8), TEM (JEOL 2010F transmission electron microscope operated at 200 kV with Cs = 1.0 mm) and electron diffraction spectroscopy (EDS), respectively. TEM images and electron diffraction patterns were acquired with a Gatan Orius SC200B digital camera while EDS data was collected using an Oxford Instruments INCA EDS system. Magnetic susceptibility measurements were performed on 120 mg of 200 nm PbCuSbS3 (corresponding to a reaction time of 60 min) using a Quantum Design Magnetic Property Measurement System. The samples were cooled to 2 K in zero field and an applied field of 500 Oe was used to measure the sample’s moment. Optical absorption spectra were obtained using a UV-Vis spectrometer (Jasco, V-670 Spectrophotometer) on the powder form specimen. DTA and TGA (TA Instruments Q600) were used to investigate the thermal stability and determine the decomposition temperature of the crystals. Heat capacity was measured using a commercial Quantum Design Physical Property Measurement System in the temperature range of 2 K < T < 390 K. Thermal coupling between the powder specimen and the specimen platform was achieved via Apiezon® N grease. All data were obtained under standard thermal relaxation methods in a zero magnetic field and under vacuum (1.3 x 10-5 Pa). Prior to each specimen measurement, the specimen puck, the specimen platform, and the Apiezon® N grease specifically used in the measurement were thoroughly characterized in a separate addenda measurement. The uncertainty in the measurement was ±7 % throughout the measured temperature range.



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Results and Discussion PbCuSbS3 crystallizes in the orthorhombic space group Pmn21, as shown in Figure 1. The crystal structure of bournonite can be derived from the antimonite stibnite (Sb2S3) by a mixed substitution of the Sb (2a) with Pb and Sb[16] and populating the tetrahedral voids of the Sb2S3 lattice (4b) with Cu atoms.[15] Both effects lower the symmetry from Pnma (spacegroup number 62) of stibnite to the non-centrosymmetric bournonite structure (Pmn21 spacegroup number 31). As in stibnite Sb2S3, the Sb atoms in PbCuSbS3 are in two crystallography nonequivalent positions. [15]

Figure 2 shows the indexed XRD pattern of 200 nm PbCuSbS3 after synthesis. In the first stage of the synthesis, metal acetates and metal chloride dissolve in Oleylamine at an intermediate temperature, 110 °C, to form metal-oleylamine complexes that serve as secondary complex precursors.[17] Later, while the solution is heated at 280 °C, PbCuSbS3 crystals begin to form. Oleylamine acts as the surfactant based capping agent, solvent and reducing agent.[18] Figure 3 shows high resolution TEM (HRTEM) and electron diffraction images of 200 nm PbCuSbS3 crystals. TEM analyses indicate a plate-like morphology, HRTEM analyses indicate lateral facets corresponding to (030) planes (0.289 nm d-spacing),[15] and the uniform electron diffraction patterns indicate a high degree of crystallinity.

Figure 4 shows a room temperature UV-Vis spectrum of 200 nm PbCuSbS3. The data suggest an optical band-gap of 1.3 eV, a value similar to that reported for mineral bournonite (1.24 eV),[19]


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polycrystalline PbCuSbS3 (1.27 eV),[19] and PbCuSbS3 single crystals prepared by a modified zone-melting method (1.20 eV).[20] Figure 5 shows DTA measurements and indicates a decomposition temperature of 633 K. TGA data, also shown in Figure 5, indicates a weight loss at 633 K due to decomposition of PbCuSbS3 to stibnite and oxidation compounds of the sulfides, as suggested by the XRD results after TGA. The main exothermic peak also appears at this temperature, corresponding to the decomposition temperature of PbCuSbS3. This decomposition temperature for our PbCuSbS3 nanocrystals is lower than that reported for large-boule bulk materials,[21] presumably due to the increased surface-to-volume ratio of our much small-sized crystals.

Calorimetric measurements to obtain Cp from 2 K to 350 K for 1.4 mg of 200 nm PbCuSbS3 are shown in Figure 6. The inset in Figure 6 shows Cp/T versus T2 data below 10 K. The solid line in the inset is a fit to the data of the form Cp/T = γ+βT2, where γ is the Sommerfeld coefficient of the electronic contribution to the specific heat and β is the coefficient of the lattice contribution.[22,23] From this fit we obtain γ = 10.9 mJ mol-1 K-2 and β = 3.0 mJ mol-1 K-4, values that are relatively high compared with that of metals and lower than that of oxides and other wide band gap semiconductors.[24-26]

From the low temperature Cp data an estimation of the Density of States (DOS) at the Fermi Level, N(EF), can be obtained by using the relation[22,27]

γ=

π 2 k B2 3

N ( E F )(1 + λe− ph ) ,


(1)


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where λe-ph is the electron-phonon coupling constant. Setting λe-ph to zero as a first approximation N(EF) = 4.6 states eV-1 per formula unit. This value is small compared with typical thermoelectric materials, such as YbFe4Sb12 (N(EF) = 60.1 states eV-1 per formula unit) [28] for example, indicating the DOS near the Fermi level is low with few channels for the charge carriers to flow, consistent with a very low electrical conductivity.[29] Using the relation 13

 12π 4 Rna  θD =    5β 

,

(2)

where R is the molar gas constant and na is the number of atoms per formula unit (na=6 for PbCuSbS3), we estimate a θD of 157 K. This value is smaller than that of the quaternary chalcogenide Cu2ZnSnS4 (θD =297 K) and sulfide bornite Cu5FeS4 (θD =251 K), both of interest as thermoelectric materials.[14,30] In the Debye model, θD is linearly proportional to the Debye cutoff frequency ωD, θD =ωD*ħ/kB , ωD being proportional to the average group velocity Vg (ωD≈ Vg*KD with KD being the maximum allowable wave vector).[27] A relatively small Vg can be an indication of low hardness, and possible “loose” bonding,[31] suggesting that PbCuSbS3 should possess low thermal conductivity.

Figure 7 shows the temperature dependent magnetic susceptibility for 200 nm PbCuSbS3 particles which displays diamagnetic behavior. The relatively strong temperature dependence at low temperatures can be attributed to a trace amount of a paramagnetic impurity that cannot be detected by XRD. Considering the charge balancing scheme Pb2+Cu1+Sb3+(S2-)3, all constituent ions are expected to be closed shell therefore exhibiting diamagnetic susceptibility, consistent with our findings.[32]


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Figure 8 illustrates our investigation of particle size with reaction time. As the reaction time was increased from 5 min to 60 min, the size of the particles increased from 5 ± 2 nm to 200 ± 38 nm, all other synthesis parameters being equal. XRD results show the same crystallinity for all three specimens. The average particle size of the specimen, in the case of the 5 nm particle size, was confirmed by employing the Scherrer equation to the XRD data. From these results it is clear that the reaction time is very effective in controlling the particle size up to 200 nm,[33] although longer reaction times did not result in further increase of the particle size presumably due to the fact that the capping agent, Oleylamine, was used in our synthetic approach.[34,35]

Conclusions Phase pure, highly crystalline PbCuSbS3 crystals were synthesized, for the first time, by a colloidal synthesis route. The room temperature UV-Vis spectrum indicates an optical band-gap of 1.3 eV and DTA data indicates a decomposition temperature of 633 K. The fit to the low temperature Cp data implies N(EF) = 4.6 states eV-1 per formula unit and θD of 157 K while magnetic susceptibility measurements indicate PbCuSbS3 to be diamagnetic. We were able to change the particle size of PbCuSbS3 by varying the reaction time. The results presented describe a simple synthetic approach to obtaining nano and microcrystalline PbCuSbS3 and are intended to contribute to a broader understanding of the physical properties of PbCuSbS3.

Acknowledgements This work was supported, in part, by the National Science Foundation Grant No. DMR-1400957. KW acknowledges support from the II-VI Foundation Block-Gift Program. Financial support



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also comes from GM and the Department of Energy under corporate agreement DEEEE0005432. We thank Dr. Shengqian Ma of USF for the use of the UV-Vis instrument.

References (1) Oudah, M.; Kleinke, K. M.; Kleinke, H. Inorganic Chemistry 2015, 54, 845-849. (2) Yamini, S. A.; Wang, H.; Gibbs, Z. M.; Pei, Y.; Mitchell, D. R. G.; Dou, S. X.; Snyder, G. J. Acta Materialia 2014, 80, 365-372. (3) Dong, Y.; Wang, H.; Nolas, G. S. Phys. Status Solidi RRL 2014, 8, 61-64. (4) Guo, Q.; Ford, G.M.; Yang, W.C.; Walker, B.C.; Stach, E.A.; Hillhouse, H.W.; Agrawal, R. J. Am. Chem. Soc. 2010, 132, 17384-17386. (5) Zou, D.; Nie, G.; Li, Y.; Xu, Y.; Lin, J.; Zheng, H.; Li, J. RSC Advances 2015, DOI: 10.1039/C5RA00477B. (6) Chetty, R.; Dadda, J.; de Boor, J.; Muller, E.; Mallik, R. C. Intermetallics 2015, 57, 156-162. (7) Yamini, S. A.; Wang, H.; Gibbs, Z. M.; Pei, Y.; Dou, S. X.; Snyder, G. J. Physical Chemistry Chemical Physics 2014, 16, 1835-1840. (8) Zhang, S. R.; Zhu, S. F.; Hou, H. J.; Xie, L. H.; Xiang, S. H.; Song, K. H. Chalcogenide Letters 2014, 11, 257-263. (9) Snyder, G. J.; Toberer, E. S. Nature Mater 2008, 7, 105-114. (10) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem Int Ed Engl 2009, 48, 86168639.


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(11) Datta, A.; Popescu, A.; Woods, L.; Nolas, G.S. In CRC Handbook on Thermoelectrics and Its Energy Harvesting on Materials, Preparation and Characterization; Rowe, D. M. Ed.; Taylor & Francis, 2012; Chapter 14. (12) Ibanez, M.; Cadavid, D.; Zamani, R.; Garcia-Costello, N.; Izkuierdo-Roca, V.; Li, W.; Fairbrother, A.; Prades, J.D.; Shavel, A.; Arbiol, J.; Perez-Rodriguez, A.; Morante, J.R.; Cabot, A. Chem. Mater. 2012, 24, 562-570. (13) Popescu, A.; Woods, L. M.; Martin, J.; Nolas, G. S. Phys. Rev. B 2009, 79, 205302/1205302/7. (14) Qiu, P.; Zhang, T.; Qiu, Y.; Shi, X.; Chen, L. Energy Environ. Sci., 2014, 7, 4000–4006. (15) Edenharter, A.; Nowacki, W.; Takeuchi, Y. Z. Kristallogr., Kristallgeometrie, Kristallphys., Kristallchem. 1970, 131, 397-417. (16) Walsh, Aron.; Chen, S.; Wei, S.; Gong, X. Adv. Energy Mater. 2012, 2, 400-409. (17) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100-11105. (18) Mourdikoudis, S.; Liz-Marzan, L. M. Chem. Mater. 2013, 25, 1465-1476. (19) Cody, J. A.; Lafond, A.; Moelo, Y. 231st ACS National Meeting, 2006, 2006, INOR-483. (20) Frumar, M.; Kala, T.; Horák, J. Journal of Crystal Growth 1973, 20, 239-244. (21) Barton, P. B.; Skinner, B. J. In Geochemistry of Hydrothermal Ore Deposits; Barnes, H. L. Eds; John Wiley, New York, 1979; Chapter 7, pp 334. (22) Aydemir, U.; Candolfi, C.; Bormann, H.; Baitinger, M.; Ormeci, A.; Carillo-Cabrera, W.; Chubilleau, C.; Lenoir, B.; Dauscher, A.; Oeschler, N.; Steiglich, F.; Grin, Yu. Dalton Trans. 2010, 39, 1078-1088.



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Figure captions Figure 1. Crystal structure of PbCuSbS3. Figure 2. Indexed XRD pattern of 200 nm PbCuSbS3. Figure 3. (a) TEM, (b) HRTEM, and (c) electron diffraction images of 200 nm PbCuSbS3. Figure 4. Room temperature UV-Vis spectra of 200 nm PbCuSbS3. Figure 5. TGA (blue) and DTA (red dashed) data of 200 nm PbCuSbS3. Figure 6. Temperature dependent Cp of 200 nm PbCuSbS3. The inset shows Cp/T versus T2 data below T=10K. The solid line through the data in the inset is a fit by the expression Cp/T = γ+βT2. Figure 7. Temperature dependent magnetic susceptibility of 200 nm PbCuSbS3. Figure 8. TEM images and corresponding estimated particle size distributions of PbCuSbS3 at (a) 5 min, (b) 20 min, and (c) 60 min reaction times.



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Figures

Figure 1


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Figure 3


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Figure 5


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Figure 7


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Table of Contents Use Only




2

Materials Measurement Science Division, National Institute of Standards and Technology,



PbCuSbS3 nanocrystals and microcrystals were synthesized for the first time by a colloidal synthesis method. The particle size of PbCuSbS3 can be varied from 5 nm to 200 nm by simply adjusting the reaction time. The optical band-gap, thermal stability, heat capacity, and magnetic susceptibility of PbCuSbS3 were investigated.

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Synthesis and Characterization of Bournonite PbCuSbS3 Nanocrystals

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