Solution-based synthesis of chalcostibite (CuSbS₂) nanobricks for solar energy conversion

Abstract

Chalcostibite brick-like nanoparticles have been synthesized using hot-injection method in coordinating solvents. The CuSbS₂ nanobricks possess a band gap of 1.40 eV and the corresponding nanobrick-electrode shows an IPCE of 5%–15% in the visible region. Our work demonstrates CuSbS₂ nanobricks have potential in the field of solar energy conversion.

---

Ternary chalcostibite CuSbS₂ is an emerging semiconductor that is considered as a potential low-cost light-absorbing material for thin film photovoltaics. CuSbS₂ possesses a near-optimal direct band gap in the range of 1.38–1.52 eV, a strong light absorption (α >10⁴ cm⁻¹ for hv >1.3 eV) and a suitable electrical property, which are essential for solar cell application.¹ Also, this material is indicated as a promising alternative to CuIn_xGa_1−xSe₂ (CIGS) and CdTe, because it has relatively non-toxic compositions and the price of antimony is much lower than that of indium.^1b,2 So, it has received increasing amounts of attention and various film-fabricating approaches for CuSbS₂ have been reported such as spray pyrolysis,^2a,3 direct evaporation,⁴ chemical bath deposition,⁵ sulfurization of the electrodeposited Cu-Sb precursor,⁶ and thermal evaporated Sb₂S₃/Cu layers.^1b A preliminary photovoltaic cell has been demonstrated as SnO₂:F-(n)CdS:In-(i)Sb₂S₃-(p)CuSbS₂-Ag, presenting a Voc of 345 mV and Jsc of 0.2 mA cm⁻² without any optimization of the thickness and electrical property of the absorber layer.⁷

Synthesis of CuSbS₂ nanoparticles (including round, triangular nanocrystals, nanorods, nanobricks etc.) is attractive and significant owing to its perspective to conveniently produce large-area solar cells at low cost by the roll-to-roll method utilizing corresponding nanoparticle-ink or paint. Compared with traditional physical vapor deposition (PVD) method, the nanoparticle-ink painting approach possesses the merits of low cost and excellent fabrication scalability.⁸ Therefore, syntheses of photovoltaic nanoparticles arouse great interest not only for conventional industrialized photovoltaic compounds (CdTe,⁹ CIGS¹⁰), but also for potential and/or new emerging semiconductor compounds (CZTS(Se),¹¹ FeS₂,¹² SnS(Se),¹³ CTSe,¹⁴ CFTS¹⁵) that can be utilized in solar cells applications. Recently Embden and Tachibana reported the synthesis of Famatinite (Cu₃SbS₄) nanocrystals.¹⁶ CuSbS₂ particles in millimeter scale and CuSbS₂ quasi-nanorod were synthesized by solvothermal and hydrothermal methods, respectively.^1e,17 Here we present the colloidal route to synthesize high-quality CuSbS₂ nanobricks in oleylamine (OLA). The obtained CuSbS₂ nanobricks can be dispersed in toluene, forming an ink. The chalcostibite film electrodes were prepared by drop-casting the ink onto an ITO substrate. A photoelectrochemical cell is established to evaluate the potential viability of the CuSbS₂ nanobricks for photovoltaics. A notable photocurrent and an incident photo to current efficiency (IPCE) ranging from 5% to 15% in the visible region are obtained using the chalcostibite electrode, demonstrating the promising prospects of CuSbS₂ nanobricks in the application of solar energy conversion.

The syntheses of CuSbS₂ nanobricks were carried out utilizing hot-injection method on a standard air-inert Schlenk line. Briefly, Sulfur was dissolved in OLA completely using ultrasonic method,^11c gaining a S/OLA solution in red-orange color. Then the S/OLA solution was quickly injected to the hot solution of OLA containing stoichiometric amount of copper(II) acetylacetonate and antimony(III) acetate [atom ratio Cu : Sb = 1] at 230 °C for 1 h under an Ar atmosphere. Full experimental details can be found in the Supporting Information (ESI†). Fig. 1 demonstrates basic structural characterization of as-synthesized nanobricks. They are cuboid shaped (∼50–120 nm in length, ∼20–40 nm in width and ∼6–9 nm in thickness) as indicated in the low-magnification TEM images (Fig. 1a and 1b, additional large scale TEM image is included in ESI†). A high-resolution TEM (HRTEM) image (Fig. 1c) displays a clear crystalline surface with the interplanar spacing of 3.2 and 2.1 Å corresponding to the (111) and (2–13) plane of CuSbS₂, respectively. Additionally, an angle of approximately 89.7° between the two planes is carefully measured, matching very well with the corresponding angle (89.6°) of previous structural data reported for bulk chalcostibite,¹⁸ giving nano-scale evidence that the synthesized nanobricks possess the structure of chalcostibite. Fast Fourier Transform (FFT) pattern of the HRTEM (Fig. 1d) matches well with simulated electron diffraction using previously reported CuSbS₂ structure data¹⁸ by Crystalmaker software (simulated patterns and detailed discussion is given in ESI†). The lattice data calculated from selected area electron diffraction (SAED) pattern of randomly chosen region of CuSbS₂ nanobricks corresponds to the lattice parameters for CuSbS₂ (See ESI†, Fig. S3). The XRD pattern of the as-prepared nanobricks reveals that they crystallized in orthorhombic phase (Pnma) and correspond well to chalcostibite structure (JCPDS 88-0822). The elemental composition of as-synthesized CuSbS₂ nanobricks determined by Energy Dispersive X-ray spectroscopy (EDX) is Cu_0.86Sb_1.05S₂ (See ESI†, Fig. S4), which is copper poor. The UV-VIS-NIR spectrum (Fig. 1f) of nanobricks shows a slow absorption rise with a platform ranged from 400–600 nm. A band gap of CuSbS₂ nanobricks is estimated to be 1.40 eV via extrapolating the linear region of the plot of (αhv)²versus photon energy (hv) as illustrated in the inset of Fig. 1f. The band gap value is close to the previous literature for bulk CuSbS₂.

Fig. 1 (a) and (b) are low-magnitude TEM images of as-synthesized nanobricks. (c) High resolution TEM image of a certain nanobrick and (d) the corresponding FFT pattern. The nanobrick is imaged down the [−4 1 3] crystallographic zone axis. (e) XRD pattern of the prepared nanobricks. As a reference, the diffraction pattern of chalcostibite pattern (JCPDS no. 88-0822) is shown. (f) UV-VIS-NIR spectroscopic characterization of the CuSbS₂ nanobricks.

To explore the possible pathway for the formation of CuSbS₂ nanobricks, the phase data for the reaction products at different times were investigated by XRD (See Fig. S5†). CuSbS₂ phase is produced in the initial stage, however, both CuS (Covellite) and Sb₂S₃ (stibnite) are detected. As the reaction goes on, peaks assigned to CuS and Sb₂S₃ gradually disappear. The remaining peaks are attributed to CuSbS₂, suggesting that CuS and Sb₂S₃ would be the intermediates involved in the formation of CuSbS₂. Copper chalcogenide phase seems to be a regular intermediate for the synthesis of copper-containing ternary or quaternary chalcogenide in OLA,^15a,19 which may be due to Cu chalcogenide being kinetically more favorable than the ternary or quaternary target products. In addition, experiments at different reaction (injection) temperatures have been trialled: the products at low synthetic temperature (180 °C) are mainly binary phases (CuS); High temperature (280 °C) injection and reaction lead to uncontrolled growth of CuSbS₂ and severe aggregation of the products.

Photovoltaic performance of new light-absorber materials can be readily tested in a photoelectrochemical (PEC) cell. This method is popular because the liquid electrolyte provides nearly ideal contact to the half cell, avoiding problems such as interface contact with the full device architecture.²⁰ Also, it allows rapid characterization of the absorber layer. Therefore, in order to assess the photoactivity and PV-potential of CuSbS₂ thin films fabricated by nanoink painting route, the photoelectrochemical characterization was carried out in an aqueous PEC cell containing 0.5 M H₂SO₄. The ink containing CuSbS₂ bricks was directly drop-casted on the ITO electrode and then annealed in a flowing Ar atmosphere at 350 °C for 30 min to remove the insulating ligands as well as enhance the electrical conductivity of the obtained films.^11d The composition and crystallographics do not change much with the annealing treatment (the SEM images, XRD, FTIR data and electrical properties for the films before and after annealing are given in the ESI†). Fig. 2(a) displays the current density versus voltage plots for the prepared CuSbS₂ films utilizing chopping method (10 s light on, 10 s light off). The film demonstrates a gradual photo-enhancement effect in the negative potential direction upon illumination, exhibiting a characteristic of a semiconductor with p-type conductivity.²⁰ As for p-type semiconductor materials, electrons are transferred from the conduction band to the oxidant in solution, and then from the back contact (here ITO) into the semiconductor, leading to the observed reductive current.^14,20a The photocurrent density reaches a saturated value of about 0.09 mA cm⁻² at −0.2 V vs. SCE. The transient photocurrent at this voltage is obtained, showing good photostability of nanoink-casting CuSbS₂ film over many cycles (24 cycles demonstrated here, see Fig. S9†). The magnitude of the gained photoresponse is high enough to acquire the IPCE spectrum, as shown in Fig. 2(b). Approximately 5%–15% of the incident photons can be readily converted to electron and hole pairs in the wavelength region of 400–800 nm that covers most of the visible light. The IPCE value reaches zero when the photon energy goes below 1.48 eV, indicating its band gap value is around 1.48 eV, which corresponds to the UV-VIS-NIR data.

Fig. 2 (a) Photocurrent density and potential spectrum of the CuSbS₂ films prepared via nanobricks-ink painting route. The photocurrent was obtained in 0.5 M H₂SO₄ solution using 100 mW cm⁻² chopped illumination. All potential values are given vs. SCE. (b) Incident photo to current efficiency (IPCE) spectrum of prepared CuSbS₂ film.

In summary, a facile colloidal synthesis of CuSbS₂ nanobricks by a simple hot-injection method is described. The structure of prepared CuSbS₂ nanobricks is confirmed by XRD, SAED pattern, lattice-resolution TEM image, and the corresponding FFT pattern. A band gap of 1.40 eV is obtained from the UV-VIS-NIR data. Possible intermediates, CuS and Sb₂S₃, are found during the synthesis process of CuSbS₂ in OLA. The film prepared by casting the ink containing CuSbS₂ nanobricks demonstrates a notable and stable photoresponse. About 5%–15% of the visible light can be converted to electron and hole pairs in the PEC cell using the CuSbS₂ electrode. This work shows that CuSbS₂ nanobricks have potential in the field of solar energy conversion (e.g., water splitting or solar cell). Future work will focus on the utilization of CuSbS₂ nanobricks for photovoltaic devices.

Acknowledgements

We acknowledge Dr Guo Qijie of Purdue University for his advice and help in hot-injection experiments. This work was supported by the National High Technology Research and Development Program of China (863 Program, Grant No. 2012AA050703), the Fundamental Research Funds for the Central Universities (Grant No. 201021100029 and 2012QNZT022) and the National Natural Science Foundation of China (Grant No. 51272292).

References

1. (a) A. Rabhi, M. Kanzari and B. Rezig, Mater. Lett., 2008, 62, 3576; (b) C. Garza, S. Shaji, A. Arato, E. Perez Tijerina, G. Alan Castillo, T. K. Das Roy and B. Krishnan, Sol. Energy Mater. Sol. Cells, 2011, 95, 2001; (c) D. Colombara, L. M. Peter, K. D. Rogers, J. D. Painter and S. Roncallo, Thin Solid Films, 2011, 519, 7438; (d) Y. Rodriguez Lazcano, M. T. S. Nair and P. K. Nair, J Cryst Growth, 2001, 223, 399; (e) J. Zhou, G. Q. Bian, Q. Y. Zhu, Y. Zhang, C. Y. Li and J. Dai, J Solid State Chem, 2009, 182, 259; (f) A. Rabhi and M. Kanzari, Chalcogenide Lett., 2011, 8, 383; (g) J. T. R. Dufton, A. Walsh, P. M. Panchmatia, L. M. Peter, D. Colombara and M. Saiful Islam, Phys. Chem. Chem. Phys., 2012, 14, 7229.
2. (a) S. A. Manolache, L. Andronic, A. Duta and A. Enesca, J Optoelectron Adv M, 2007, 9, 1269; (b) Price information for Metals in Shang Hai Metals Market. URL: http://www.smm.cn/newsinfo/2012-02-10/<?ccdc_no 3164988?><?ccdc_no 3164988?>3164988<?ccdc END?><?ccdc END?>.html..
3. (a) S. A. Manolache, A. Duta and A. Enesca, J Optoelectron Adv M, 2007, 9, 3219; (b) S. Manolache, A. Duta, L. Isac, M. Nanu, A. Goossens and J. Schoonman, Thin Solid Films, 2007, 515, 5957.
4. A. Rabhi, M. Kanzari and B. Rezig, Thin Solid Films, 2009, 517, 2477.
5. S. C. Ezugwu, F. I. Ezema and P. U. Asogwa, Chalcogenide Lett., 2010, 7, 341.
6. D. Colombara, L. M. Peter, K. D. Rogers and K. Hutchings, J Solid State Chem., 2012, 186, 36.
7. Y. Rodriguez-Lazcano, M. T. S. Nair and P. K. Nair, J. Electrochem. Soc., 2005, 152, G635.
8. S. E. Habas, H. A. S. Platt, M. F. A. M. Hest and D. S. Ginley, Chem. Rev., 2010, 110, 6571.
9. I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462.
10. (a) Q. Guo, S. J. Kim, M. Kar, W. N. Shafarman, R. W. Birkmire, E. A. Stach, R. Agrawal and H. W. Hillhouse, Nano Lett., 2008, 8, 2982; (b) Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett., 2009, 9, 3060; (c) M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770; (d) J. Tang, S. Hinds, S. O. Kelley and E. H. Sargent, Chem. Mater., 2008, 20, 6906; (e) Q. J. Guo, G. M. Ford, R. Agrawal and H. W. Hillhouse, Prog. Photovolt: Res. Appl., 201210.1002/pip.2200; (f) V. A. Akhavan, B. W. Goodfellow, M. G. Panthani, D. K. Reid, D. J. Hellebusch, T. Adachi and B. A. Korgel, Energy Environ. Sci., 2010, 3, 1600; (g) S. Jeong, B-S. Lee, S. J. Ahn, K. H. Yoon, Y-H. Seo, Y. Choi and B-H. Ryu, Energy Environ. Sci., 2012, 5, 7539.
11. (a) Q. Guo, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2009, 131, 11672; (b) C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Koo and B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554; (c) S. C. Riha, B. A. Parkinson and A. L.Prieto, J. Am. Chem. Soc., 2009, 131, 12054; (d) X. T. Lu, Z. B. Zhuang, Q. Peng and Y. D. Li, Chem. Commun., 2011, 47, 3141; (e) S. C. Riha, B. A. Parkinson and A. L. Prieto, J. Am. Chem. Soc., 2011, 133, 15272; (f) A. Shavel, J. Arbiol and A. Cabot, J. Am. Chem. Soc., 2010, 132, 4514.
12. (a) J. Puthussery, S. Seefeld, N. Berry, M. Gibbs and M. Law, J. Am. Chem. Soc., 2011, 133, 716; (b) Y. Bi, Y. B. Yuan, C. L. Exstrom, S. A. Darveau and J. S. Huang, Nano Lett., 2011, 11, 4953.
13. (a) S. G. Hickey, C. Waurisch, B. Rellinghaus and A. Eychmuüller, J. Am. Chem. Soc., 2008, 130, 14978; (b) M. A. Franzman, C. W. Schlenker, M. E. Thompson and R. L. Brutchey, J. Am. Chem. Soc., 2010, 132, 4060; (c) W. J. Baumgardner, J. J. Choi, Y.-F. Lim and T. Hanrath, J. Am. Chem. Soc., 2010, 132, 9519; (d) P. D. Antunez, J. J. Buckley and R. L. Brutchey, Nanoscale, 2011, 3, 2399.
14. J. Puthussery, S. Seefeld, N. Berry, M. Gibbs and M. Law, J. Am. Chem. Soc., 2012, 134, 23.
15. (a) C. Yan, C. Huang, J. Yang, F. Liu, J. Liu, Y. Lai, J. Li and Y. Liu, Chem. Commun., 2012, 48, 2603; (b) X. Y. Zhang, N. Z. Bao, K. Ramasamy, Y. H. A. Wang, Y. F. Wang, B. P. Lin and A. Gupta, Chem. Commun., 2012, 48, 4956; (c) L. Li, X. Y. Liu, J. Huang, M. Cao, S. Y. Chen, Y. Shen and L. J. Wang, Mater. Chem. Phys., 2012, 133, 688.
16. J. V. Embden and Y. Tachibana, J. Mater. Chem., 2012, 22, 11466.
17. C. H. An, Q. C. Liu, K. B. Tang, Q. Yang, X. Y. Chen, J. W. Liu and Y. T. Qian, J. Crys. Growth., 2003, 256, 128.
18. A. Kyono and M. Kimata, Am Mineral., 2005, 90, 162.
19. (a) M. Kar, R. Agrawal and H. W. Hillhouse, J. Am. Chem. Soc., 2011, 133, 17239; (b) A. Shavel, D. Cadavid, M. Ibanez, A. Carrete and A. Cabot, J. Am. Chem. Soc., 2012, 134, 1438.
20. (a) H. Ye, H. S. Park, V. A. Akhavan, B. W. Goodfellow, M. G. Panthani, B. A. Korgel and A. J. Bard, J. Phys. Chem. C, 2011, 115, 234; (b) V. A. Akhavan, B. W. Goodfellow, M. G. Panthani, C. Steinhagen, T. B. Harvey, C. J. Stolle and B. A. Korgel, J Solid State Chem, 2012, 189, 2.

---

Footnote

† Electronic Supplementary Information (ESI) available: Experimental details. Materials characterization. Additional large scale TEM image, Fast Fourier Transform (FFT) pattern of as-synthesized CuSbS₂. Simulated CuSbS₂ single crystal electron diffraction data. SAED and EDX data of as-synthesized CuSbS₂ nanobricks. SEM images, XRD, IR data and electrical properties for the CuSbS₂ films before and after annealing. Transient photocurrent spectrum for the prepared CuSbS₂ films. See DOI: 10.1039/c2ra21554c

---