Selective Nanocrystal Synthesis and Calculated Electronic Structure of

Apr 4, 2014 - Center for Materials for Information Technology, The University of. Alabama ... sulfide system consists of four major phases, namely, Cu...
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Selective Nanocrystal Synthesis and Calculated Electronic Structure of All Four Phases of Copper−Antimony−Sulfide Karthik Ramasamy,*,†,§,∥ Hunter Sims,‡,§,⊥ William H. Butler,‡,§ and Arunava Gupta*,†,§ †

Department of Chemistry, ‡Department of Physics, and §Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487, United States S Supporting Information *

ABSTRACT: A wide variety of copper-based semiconducting chalcogenides have been investigated in recent years to address the need for sustainable solar cell materials. An attractive class of materials consisting of nontoxic and earth abundant elements is the copper−antimony−sulfides. The copper−antimony− sulfide system consists of four major phases, namely, CuSbS2 (Chalcostibite), Cu12Sb4S13 (Tetrahedrite), Cu3SbS3 (Skinnerite), and Cu3SbS4 (Fematinite). All four phases are p-type semiconductors having energy band gaps between 0.5 and 2 eV, with reported large absorption coefficient values over 105 cm−1. We have for the first time developed facile colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases. Cu12Sb4S13 and Cu3SbS3 are found to have direct band gaps (1.6 and 1.4 eV, respectively), while the other two phases display indirect band gaps (1.1 and 1.2 eV for CuSbS2 and Cu3SbS4, respectively). The synthesis methods yield nanocrystals with distinct morphology for the different phases. CuSbS2 is synthesized as nanoplates, and Cu12Sb4S13 is isolated as hollow structures, while uniform spherical Cu3SbS3 and oblate spheroid nanocrystals of Cu3SbS4 are obtained. In order to understand the optical and electrical properties, we have calculated the electronic structures of all four phases using the hybrid functional method (HSE 06) and PBE generalized gradient approximation to density functional theory. Consistent with experimental results, the calculations indicate that CuSbS2 and Cu3SbS4 are indirect band gap materials but with somewhat higher band gap values of 1.6 and 2.5 eV, respectively. Similarly, Cu3SbS3 is determined to be a direct band gap material with a gap of 1.5 eV. Interestingly, both PBE and HSE06 methods predict metallic behavior in fully stoichiometric Cu12Sb4S13 phase, with opening up of bands leading to semiconducting or insulating behavior for off-stoichiometric compositions with a varying number of valence electrons. The absorption coefficient values at visible wavelengths for all the phases are estimated to range between 104 and 105 cm−1, confirming their potential for solar energy conversion applications.

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INTRODUCTION

In recent years, different phases of the copper−antimony− sulfide system, composed of sustainable and nontoxic elements, have been proposed as alternatives for CIGS in thin film solar cells.11−13 These materials have a large absorption coefficient of over 105 cm−1 at visible wavelengths, which is comparable or higher than those for the CZTS and CIGS systems.14 Four major phases are known to exist in the Cu−Sb−S system, namely, CuSbS2 (Chalcostibite), Cu12Sb4S13 (Tetrahedrite), Cu3SbS3 (Skinnerite), and Cu3SbS4 (Fematinite).15,16 All four phases are p-type semiconductors with band gap values between 0.5 and 2 eV.17,14,12,18 Of these phases, CuSbS2, has been investigated in some detail in view of its potential as a solar energy absorber for thin film solar cells because of its optimal band gap and large absorption coefficient.19,13 However, a recent theoretical study predicts that CuSbS2 has an indirect band gap with low hole mobility, suggesting that it is unlikely to display the desired transport properties for use in

Energy generation from nonfossil fuels has been accelerating over the past decade to meet the ever growing global energy demand.1 Solar-based technologies are one of the major contributors toward meeting the energy needs.2 As compared to conventional silicon-based solar cells, thin film solar panels are lightweight and flexible and are thus preferable for a variety of applications.3−5 In particular, thin films solar cells fabricated using CdTe and CuInGaSe2 (CIGS) are widely commercially available.3,4 Despite the success, these materials are composed of toxic and less-abundant elements such as tellurium, indium, and gallium.5 An attractive alternative material being extensively investigated is Cu2ZnSnS4 (CZTS), consisting of nontoxic and relatively abundant elements, with an optimum band gap and a large absorption coefficient of over ∼104 cm−1 for solar radiation, a prerequisite for use in solar cells.6,7 Prompt recognition of this material has helped improve solar cell efficiency of CZTS to over 10%.8,9 Nevertheless, it still lags in efficiency, open circuit voltage, and fill factor values relative to CIGS.10 © 2014 American Chemical Society

Received: February 16, 2014 Revised: March 30, 2014 Published: April 4, 2014 2891

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solar cells.11 Despite ruling out the use of CuSbS2 for solar cells, the same authors have suggested that the related material Cu3SbS3 may be preferable provided it exhibits the requisite optical and transport characteristics.11 Thus, it is important to study all four phases of Cu−Sb−S in order to understand their suitability for solar cell applications. The use of nanocrystalline materials as solar inks for the fabrication of thin film solar cells has been proposed as being more cost-effective than traditional vacuum-based fabrication processes.20,21 Solar energy conversion efficiencies over 8% have been achieved from cells fabricated using nanocrystals of CZTS.22 The synthesis of solar cell materials in the form of nanocrystals additionally offers flexibility to tune the optical and electrical properties.23 Accordingly, a number of methods have been developed for the size, shape, and phase-controlled synthesis of various solar cell materials, including CuInS2, CuInGaS2, Cu2ZnSnS4, and Cu2FeSnS4.24−27 The synthesis of Cu−Sb−S nanocrystals has not been extensively explored, albeit a few methods have been reported thus far. The methods include synthesis of nanobricks of CuSbS2, micrometer-size rods of Cu3SbS3, and spherical nanoparticles of Cu12SbS13 and Cu3SbS4.12,14,28,18,29 However, their detailed optical and electrical studies have not been reported. In conventional solid state synthesis these phases usually coexist because of very narrow thermodynamic stability windows. Thus, the selective synthesis of phase-pure nanocrystals of all four phases of Cu− Sb−S is challenging. By careful selection of metal and sulfide precursors and the experimental conditions, herein, we report on solution-based hot-injection methods for phase-pure synthesis of nanocrystals of all four phasesCuSbS2 , Cu12Sb4S13, Cu3SbS3, and Cu3SbS4. Additionally, we have carried out detailed electronic structure calculations of these phases to assess their suitability for solar energy applications. To the best our knowledge this is the first report of selective syntheses of four different phases of any material system using solution-based methods.

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The solution was quickly injected into the metal source mixture held at the target synthesis temperature, with continued stirring of the resulting mix for 10−30 min. After cooling down to room temperature, a mixture of hexane (15 mL) and ethanol (15 mL) was added to precipitate the product. The black precipitate was then isolated via centrifugation (4000 rpm/5 min). The washing process was repeated three times to ensure removal of the excess capping agent. CuSbS2 nanocrystals were also obtained by injecting a mixture of 1dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) Synthesis of Cu3SbS4 Nanocrystals. The above-mentioned experimental procedure was also followed for the synthesis of Cu3SbS3 but using 0.5 mmol of Cu(acac)2, 0.16 mmol of SbCl3· 6H2O, and 10 mL of oleylamine (OLA) at 190 °C. Synthesis of Cu12Sb4S13 Nanocrystals. The above-mentioned experimental procedure was followed with a 1:3 ratio of Cu(acac)2 and SbCl3·6H2O, using S/OLA as sulfur source at 250 and 280 °C or using a mixture of 1-dodecanethiol (1-DDT) and tert-dodecanethiol (tDDT) at 280 °C. Synthesis of Cu3SbS3 Nanocrystals. The above-mentioned experimental procedure with 1:3 ratio of Cu(acac)2 and SbCl3·6H2O was followed but using a mixture of 1-dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) in oleylamine instead of sulfur at 220 °C. Measurements. Transmission electron microscopy (TEM) analysis was performed using a FEI-Tecnai, 200 kV transmission electron microscope equipped with a CCD camera for STEM, HAADF detector, and EDX. TEM image nonlinear processing was carried out using a Gatan digital micrograph version 3.4. Powder XRD patterns were recorded on a Bruker D8 instrument equipped with a Cu Kα radiation source operated as a rotating anode at 40 kV and 20 mA. Scanning electron microscope (SEM) analysis was carried out using a JEOL 7000 FE SEM equipped with energy dispersive X-ray spectroscopy (EDX), wavelength dispersive X-ray spectroscopy (WDS), electron backscatter diffraction (EBSD), secondary electron (SE), backscattered electron (BE), and transmission electron (TE) detectors. Optical measurements were carried out using a Varian-Cary single beam spectrometer.

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RESULTS AND DISCUSSIONS All the experiments were carried out in a fume-hood under N2 atmosphere using standard Schlenk technique. For the syntheses of all four Cu−Sb−S nanocrystal phases, we have chosen copper acetylacetonate [Cu(acac)2] as a copper source based on our previous experience in synthesizing copper-based ternary and quaternary nanocrystals,26,27 SbCl3 as antimony source, given its easy complexation with sulfonating agents, and oleylamine as a capping agent. We have used two different sulfonating agents, namely, a mixture of 1-dodecanethiol and tdodecanethiol, and elemental sulfur dissolved in oleylamine. In brief, the synthesis of CuSbS2 nanocrystals involves heating an equimolar concentration of Cu(acac)2 and SbCl3 in 10 mL of oleylamine to temperatures between 190 and 280 °C and injection of a mixture of 1-dodecanethiol and t-dodecanethiol or elemental sulfur dissolved in oleylamine. For Cu12Sb4S13 nanocrystals, a 3:1 ratio of Cu(acac)2 and SbCl3 was heated to 250 or 280 °C in oleylamine, and 1 mL of sulfur dissolved in oleylamine was injected. A similar experiment was carried out for the synthesis of Cu3SbS4 nanocrystals, but the sulfur solution was injected at 190 or 220 °C. Nanocrystals of Cu3SbS3 were obtained when the reaction was carried out at 220 °C using a mixture of 1-dodecanethiol and t-dodecanethiols as the sulfur source. The reaction conditions to obtain all four phases are presented in Scheme 1, and the detailed synthesis procedures are provided in the Methods section. We determined that the nature of the sulfonating agent(s) and the reaction temperatures are crucial factors for the phasepure isolation of Cu−Sb−S nanocrystals. In particular, Cu3SbS3

METHODS

Materials. All chemicals were used as received, and the solvents were dried in molecular sieves and purged with high purity argon for 30 min before use. 1-Dodecanethiol (1-DDT, 98.0%), t-dodecanethiol (t-DDT, 98.0%), and antimony chloride (SbCl3.6H2O, 99.5%) were obtained from Alfa Aesar; copper acetylacetonate (Cu(acac)2, ≥99.0%) and oleylamine (OLA, ≥80−90.0%) were obtained from Acros Organics and Pfaltz and Bauer. Analytical grade hexane and ethanol were obtained from Aldrich Chemical Co. Computational. All calculations were performed using the Vienna ab initio Simulation Package (VASP).30−32 The calculations using density-functional theory (DFT) were performed within the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE).33,34 We also employed the screened hybrid-functional method of Heyd, Scuseria, and Ernzerhof (HSE06 parameters) as a compromise between accuracy and computational efficiency.35,36 We used the projected augmented-wave (PAW) pseudopotentials of Kresse and Joubert.37,38 Absorption curves were computed within HSE06 from the complex dielectric constant, which is computed via a sum over empty states (for the imaginary part) and a Kramers−Kronig relation (for the real part). Synthesis of CuSbS2 Nanocrystals. In a typical synthesis of CuSbS2, 0.5 mmol of Cu(acac)2, 0.50 mmol of SbCl3·6H2O, and 10 mL of oleylamine (OLA) were degassed at room temperature for 15 min and then backfilled with nitrogen for 15 min. The mixture was subsequently heated to reaction temperatures between 170 and 280 °C under N2 atmosphere. In a separate vessel, elemental sulfur (1.3 mmol) was mixed with 1 mL of oleylamine and degassed three times. 2892

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Figure 2a shows well-defined peaks, which is in agreement with the standard ICDD pattern (red lines; ICDD: 044-1417) of orthorhombic CuSbS2 (Chalcostibite) with space group Pnma. The major diffraction peaks are indexed as the (111), (410), (020), (301), (501), (321), (131), and (212) planes of CuSbS2. The lattice parameters for the CuSbS2 nanoplates synthesized at 280 °C are determined to be 6.002 Å (a), 3.793 Å (b), and 14.477 Å (c), which are comparable with the values reported for the bulk.40 The XRD pattern of Cu12Sb4S13 nanocrystals synthesized using S/OLA at 280 °C exhibit sharp peaks that match with cubic Cu12Sb4S13 (ICDD: 042-0560). The most intense peaks are indexed as the (222), (400), (440), and (622) planes of cubic Cu12Sb4S13 (Figure 2b) with calculated lattice parameter (a) of 10.333 Å (10.323 Å reported for bulk Cu12Sb4S13). Figure 2c displays the X-ray diffraction pattern of Cu3SbS4 nanocrystals, with the peaks indexed as (112), (200), (204), and (312) reflections of tetragonal Cu3SbS4 (ICDD: 035-0581) with estimated lattice parameter values of 5.378 Å (a) and 10.729 Å (c). The nanocrystal phases synthesized at 220 °C using a mixture of thiols is identified as Cu3SbS3 (Figure 3d). The peaks can be matched with the standard diffraction pattern for monoclinic Cu3SbS3 (ICDD: 024-1289). The diffraction pattern exhibits sharp peaks with the highest intensity for the (321) reflection. From the X-ray diffraction analysis it can be noted that all the synthesized nanocrystals are phase pure and free from any impurities. Additional XRD patterns of nanocrystals synthesized using S/OLA, and with Cu(acac)2:SbCl3 ratio of 3:1 at reaction temperatures ranging from 190 to 280 °C, are provided in Supporting Information Figure S1. It is noted that the diffraction pattern gradually evolves from that of pure tetragonal Cu3SbS4 to cubic Cu12Sb4S13 phase as the reaction temperature increases, with the latter phase being predominant at 250 and 280 °C. A similar temperature-dependent phase transformation, but from monoclinic Cu3SbS3 to cubic Cu12Sb4S13, is observed when the reaction is carried out using a mixture of thiols as the sulfur source. These transformations are tracked by the appearance of diffraction peaks at 29.87°, 49.94°, and 59.44° corresponding to (222), (440), and (622) planes of cubic Cu12Sb4S13. The results suggest that Cu12Sb4S13 phase nanocrystals are obtainable only at temperatures above 250 °C. Further, we have also observed tetragonal Cu3SbS4 to cubic Cu12Sb4S13 phase evolution when the reaction is carried out at different time intervals using S/ OLA at 250 °C (Supporting Information Figure S2 and S3). The crystal structures of CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 phases (data adopted from refs 39−42) are shown as insets to Figure 2a−d, which provide insight regarding the coordination around the metals and sulfide ions in the different phases (for detailed structural information please see the cited references). It is important to note that copper exists both in Cu(I) and Cu(II) oxidation states in C12Sb4S13, and depending on the ratio between Cu(I) and Cu(II), the electrical transport of C12Sb4S13 varies from semiconducting to metallic behavior as discussed later.14 Morphological Observations. The morphology and size of the nanocrystals have been investigated from transmission electron microscope (TEM) images. The images in Figure 3 show TEM micrographs of CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 nanocrystals. Monodisperse nanoplates with an average lateral dimension of 325 ± 25 nm are observed for CuSbS2 nanocrystals synthesized at 190 °C using S/OLA as the sulfur source (Figure 3a). The as-synthesized nanoplates have uniform thickness of 19 ± 1 nm, accounting for about 26-

Scheme 1. Syntheses of CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 Nanocrystalsa

a

OLA, oleylamine; DDT, dodecanethiol.

nanocrystals were obtained only with the use of a mixture of thiols at 220 °C, while the Cu3SbS4 phase was obtained when the sulfur source was changed from a mixture of thiols to sulfur in oleylamine at 190 °C. Similarly, keeping the precursors unchanged with reaction temperatures between 250 and 280 °C produced the Cu12Sb4S13 phase. We have noted that the use of a mixture of 1-dodecanethiol and t-dodecanethiol provides better phase control. Further, reactions using trioctylphosphine sulfide (TOP-S) did not yield any of the Cu−Sb−S phases. Reactions carried out below 190 °C predominantly produced copper sulfide and antimony sulfide impurity phases. The products obtained at different temperatures using the two different sulfur sources are shown in Figure 1.

Figure 1. Phases of Cu−Sb−S nanocrystals obtained under different reaction conditions.

Structural Analysis. The crystallinity and phase purity of the nanocrystals synthesized under different reaction conditions have been confirmed by powder X-ray diffraction (XRD). The XRD patterns of all four phases (CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3) are shown in Figure 2. The diffraction pattern of CuSbS2 nanocrystals synthesized using S/OLA at 280 °C in 2893

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Figure 2. Powder X-ray diffraction patterns of (a) CuSbS2, (b) Cu12Sb4S13, (c) Cu3SbS4, and (d) Cu3SbS3 nanocrystals. Insets show the corresponding crystal structures.

CuSbS2 monolayers along the c-axis (Figure 3b). The nanoplate morphology is dictated by the layered structure of CuSbS2. TEM images of Cu12Sb4S13 nanocrystals show irregular to cubeshaped hollow nanostructures with an average size of 100 ± 30 nm (Figure 3c). Hollow structures are usually formed through the Kirkendall effect, which in this case is likely a consequence of the difference in diffusion rates of Cu, Sb, and S in the nanocrystals. As mentioned earlier, Cu12Sb4S13 phase nanocrystals are obtained only at reaction temperatures higher than 250 °C. At these temperatures, it is expected that metal ions with different ionic radii would diffuse at different rates.43 Moreover, reactions at the high temperature are expected to yield larger nanocrystals. The image in Figure 3d displays oblate Cu3SbS4 nanocrystals with an average size of 23 ± 4 nm. On the other hand, the Cu3SbS3 nanocrystals have nearly spherical morphology with an average diameter of 30 ± 5 nm (Figure 3e). In order to further understand the nanocrystal growth, we have performed HRTEM analysis of nanocrystals of all four phases. The image in Figure 3f shows the HRTEM image of CuSbS2 nanoplates, which reveals clear lattice fringes with an average interplanar distance of 0.302 nm, corresponding to (200) planes of orthorhombic CuSbS2. The fast Fourier transform (FFT) image in Figure 3k of the HRTEM image in Figure 3f exhibits bright spots that correspond to the (200), (013), (114), (311), (223), (400) and (220) reflections of orthorhombic CuSbS2. Lattice fringes from edges of the plates in Figure 3g have an average distance of 0.724 ± 0.002 nm, corresponding to the (002) plane along the [001] growth direction that is further supported by the FFT pattern in Figure 3l. Lattice distances in Cu12Sb4S13 phase nanocrystals are observed to be 0.300 and 0.364 nm, corresponding to the (222) and (022) planes. This can also be discerned from the FFT pattern. The HRTEM images of Cu3SbS4 and Cu3SbS3 phases show interlattice distances of 0.31 and 0.278 nm, respectively.

These distances can be correlated with the (112) and (024) planes of the tetragonal Cu3SbS4 and monoclinic Cu3SbS3 phases, respectively (Figure 3i,j and their FFT images in Figure 3n,o). Elemental Composition Determination. The average elemental composition of all four phases of Cu−Sb−S nanocrystals has been determined using energy dispersive Xray spectroscopy (EDX). It is important to acquire reliable and reproducible EDX data for these nanocrystals, since copper, antimony, and sulfur in these compounds exist over a very narrow composition window. For this purpose, we have carried out quantitative EDX analysis at different locations containing a large number of particles from different experiments. The determined compositions are found to be close to the expected ones for all four phases. Using the elemental composition data obtained from EDX, we have constructed a ternary phase diagram for the four phases (Figure 4). The diagram exhibits the composition of copper, antimony, and sulfur for the different phases, which despite being quite close together are clearly distinguishable. From the S/(Cu + Sb) ratio for these nanocrystals, one can infer that all four phases have a slight excess of sulfur. This is not unexpected since the nanocrystals are synthesized using an excess of sulfonating agents. The analysis results and representative SEM-EDX spectra are provided in Supporting Information Table S1 and Figure S4. Elemental composition data of CuSbS2 nanocrystals synthesized at different temperatures using S/OLA are given in Supporting Information Table S2, which shows the sulfur to metal ratio to be slightly higher than expected for the CuSbS2 nanocrystals synthesized at 190 °C. However, the ratio is close to 1 for the nanocrystals grown at 250 °C and 0.89 at 280 °C. The latter is because of sulfur loss at higher reaction temperatures, with both S/OLA and a mixture of thiols as sulfonating agents. 2894

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Figure 4. (a) Ternary phase diagram showing compositions of copper, antimony, and sulfur for the four phases based on elemental compositions determined using FESEM-EDX.

Figure 3. (a and b) Top and side view TEM images of CuSbS2 nanoplates. (c−e) TEM images of Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 nanocrystals (inset to (c) shows hollow structure morphology of Cu12Sb4S13. (f−j) HRTEM images of CuSbS2 (top view (f); side view (g)), Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 nanocrystals. (k−n) Fast Fourier transformation (FFT) images of HRTEM images shown in panels (f−i) (green lines indicate line profile). (o) Autocorrelated image of HRTEM image in (j). Inset shows profile of the line drawn in image (o).

Optical Properties. The as-synthesized nanocrystals of all four phases are readily dispersible in toluene and exhibit dark red to black color, indicating strong absorption in the visible region of the solar spectrum (Figure 5a). The optical properties of the nanocrystals dispersed in toluene have been investigated using UV−vis spectroscopy. Figure 5b shows UV−vis spectra of the four phases synthesized under different reaction conditions. The CuSbS2 spectrum exhibits absorption over a wide wavelength range, with broad peaks around 541 nm (2.29 eV) and 744 nm (1.66 eV) and a drop off in the intensity at longer wavelengths. The absorption spectrum of Cu12Sb4S13 nanocrystals displays a somewhat similar pattern with a peak at 612 nm (2.02 eV) and a shorter wavelength shoulder at 470 nm (2.64 eV). The Cu3SbS3 phase nanocrystals show an absorption peak at 642 nm (1.93 eV), while Cu3SbS4 nanocrystals have a very distinct absorption behavior with broad shoulders having

Figure 5. (a) Photographs of toluene dispersions of CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 nanocrystals. (b) UV−vis absorption spectra of CuSbS2, Cu12Sb4S13, Cu3SbS4, and Cu3SbS3 nanocrystal suspension in toluene.

maxima at 410 nm (3.02 eV), 547 nm (2.26 eV), and 677 nm (1.83 eV). The absorption features observed for Cu3SbS4 nanocrystals are consistent with an earlier report.18 Band gap values of nanocrystals of all four phases have been obtained by plotting (Ahν)2 versus hν for direct band gap and (Ahν)1/2 versus hν for indirect band gap (A = absorbance, h = Planck’s constant, and ν = frequency) and extrapolating the linear portion of the spectrum in the band edge region (Figure 6). The estimated indirect band gap of CuSbS2 nanoplates is 1.1 eV, which is consistent with hybrid functional theory 2895

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reported for bulk (0.47 eV)47,48 and nanocrystals (0.9 eV),18 suggesting large quantum confinement effect because of the small size. The estimated direct band gap of Cu 3SbS 3 nanocrystals is 1.4 eV. The band gap values and direct absorption nature of Cu3SbS3 and Cu12SbS13 phases make these attractive candidates for solar energy conversion and optoelectronic applications. Band Structure Calculations. Considering now the electronic structures of these four Cu−Sb−S phases, it is clear that they share a “family resemblance” due to their similar components, particularly in the densities of states (DOS). Although assigning electronic states to only one type of orbital is not a well-defined task, we can reason from the electronic DOS and electron counting (via the integrated DOS) to cautiously assign orbital character to the states. Each system shows a strong Cu d peak about 2−3 eV below the valence band maximum, which has strong S p character, and each exhibits a conduction band composed almost exclusively of Sb p bands (see Figure 7). This allows us to suggest that the bandFigure 6. Extrapolation of the absorption spectra in the band edge region for the determination of band gaps for (a) CuSbS2, (b) Cu3SbS3, (c) Cu3SbS4, and (d) Cu12Sb4S13 nanocrystals.

predictions.13 Earlier studies have shown that the tetrahedrite phase of Cu−Sb−S exists over a fairly large composition window, ranging from Cu12Sb4S13 (∼204 valence electrons) to Cu14Sb4S13 (∼208 valence electrons) with the presence of mixed Cu(I)/Cu(II) ions (Cu10(I)Cu2(II)Sb4S13) to only Cu(I) ions (Cu14(I)Sb4S13).44 Correspondingly, the electrical characteristics have been found to vary from metallic (∼204 valence electrons) to semiconducting (∼208 valence electrons).44 Interestingly, in addition to size-dependent quantum confinement effect, this provides composition-dependent band gap tunability in nanocrystals of this phase of Cu−Sb−S. The extrapolation of the linear section of the (Ahν)2 versus hν plot for Cu12Sb4S13 nanocrystals reveals a direct band gap with the band gap value estimated to be 1.6 eV, which is somewhat smaller than the value (1.72 eV) reported for naturally occurring tetrahedrite with 208 valence electrons.45 In nanodimensional materials, the surface atomic species are known to exist in higher oxidation states than in the core.46 The fraction of atoms (ions) in the nanocrystals with higher oxidation state can be controlled by use of strong surface passivating ligands and by synthesizing nanocrystals with a larger number of core atoms, which will lead to a smaller fraction of surface atoms (ions). It is evident from the TEM images in Figure 3 that nanocrystals of the tetrahedrite phase are relatively large. The larger size is expected to minimize the number of oxidized copper ions on the surface. The capping agents (oleylamine and mixture of 1-dodecanethiol and tdodecanethiol) used in the synthesis also contribute to minimizing the fraction of oxidized copper ions on the surface of the nanocrystals. Recently, it has been reported that the absorption spectra of spherical Cu12Sb4S13 nanocrystals has a broad band covering the visible and extending to the near IR region, attributed to midband gap states. This is a consequence of the presence of a large fraction of Cu(II) ions on the nanocrystal surface.14 However, we do not observe this absorption feature in our nanocrystal sample, indicating that their surface consists of a much lower fraction of Cu(II) ions. The band gap of Cu3SbS4 nanocrystals is determined to be 1.2 eV (indirect), which is significantly higher than the values

Figure 7. Total densities of states for the four Cu−Sb−S phases. With the exception of the Cu3SbS4 phase all gaps are in good agreement with experiment. Ind. and Dir. denote indirect and direct fundamental gaps, respectively.

edge and higher-energy transitions are mostly between S p and Sb p states, although both of these sets of states are hybridized with Cu d states and with each other. In addition to the electronic band structure, we have computed the complex dielectric constant ϵ(ω) = ϵ1(ω) + iϵ2(ω) within the HSE06 method (Figure 8, dotted red curve) and used it to calculate the absorption coefficient α (solid black curve) via the relation 4πκ α(λ ) = λ where κ=

ϵ12 + ϵ2 2 − ϵ1 2

is the imaginary part of the index of refraction and λ is the wavelength of incident light. It should be noted that only vertical transitions contribute to the computed curves, while the 2896

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pictured) shows that this featureless curve is part of a broad peak with a maximum at around 5 eV. The DOS suggest that this could arise from Cu d−Sb p transitions at energies greater than the fundamental band gap. Like CuSbS2, Cu3SbS4 has a layered structure (although the layers in the latter are simple atomic planes in contrast to the more complicated structure and van der Waals gaps of CuSbS2), and this similarity is apparent in the electronic density of states (Figure 7c), which in both cases is separated into discrete regions due to the nearly two-dimensional states. Unfortunately, our Cu3SbS4 results stand in contrast to the reasonably good agreement seen between the computed and measured band gaps in the other systems. We find a fundamental gap of about 2.5 eV (recall that our measurements indicate a gap of about 1.2 eV and previous reports give gaps of less than 1 eV).18 This larger gap leads to a much smaller calculated absorption in the visible region (Figure 8c), sitting below 104 cm−1 over much of the range. In fact, the curve in this region is mostly due to a small peak centered at around 3.5 eV, and above this feature the absorption increases dramatically toward 105 cm−1, which is consistent with ref 14. If the band gap in this calculation is rigidly or nearly rigidly overestimated, this would indicate that the actual absorption coefficient in the 1−3 eV range should be closer to this latter value. In the above phases, both PBE and HSE06 find band gaps, although the latter are of course larger. However, Cu12Sb4S13 is metallic within both methods (see Figure 7d for HSE06 density of states). As mentioned above, this is because the fully stoichiometric Cu12Sb4S13 phase has an odd number of valence electrons per spin channel (depending on which electrons are counted as valence; in any case the bands are partially filled), and the onsite Coulomb repulsions are not sufficient to lead to a Mott insulating state. However, off-stoichiometric phases can add enough valence electrons to fill the remaining states and yield a band insulator.45,50 These cells would be exceedingly expensive to calculate within the already computationally intensive HSE06 method (the stoichiometric unit cell already contains 58 atoms), but to a reasonable approximation, adding these extra electrons would simply shift the partially filled bands below the Fermi energy. Our computed band gap is 1.7 eV and indirect. This value is in good agreement with other reports45 but a bit larger than our measurement. In contrast with our measurements, we do not find a direct band gap in Cu12Sb4S13, but it is possible that the introduction of the additional electrons in the semiconducting off-stoichiometric structure would yield some differences in the band edges not apparent in the DOS. We do not present the absorption curve for metallic Cu12Sb4S13. The shape, size, and optical properties of the different phase nanocrystals are given in Table 1.

Figure 8. Absorption curves (black solid) and imaginary dielectric constants (red dashed) for semiconducting CuSbS2, Cu3SbS3, and Cu3SbS4 phases, computed from HSE06. Note the differing scales in the bottom figure.

indirect transitions will be important in the low-energy behavior of most of the experimental curves. We now take a closer look at each phase. We note that the electronic structure of bulk CuSbS2 has been studied previously using similar methods13,16 and that the present results, i.e., our calculated densities of states and indirect band gaps within PBE and HSE06 (Eg ∼0.85 and ∼1.6 eV, respectively), agree with these past findings. Although the fundamental band gap is indirect, we find that there are additional direct gaps (for example, at the Γ point) that are only about a tenth of an eV higher in energy. The average ϵ2 (Figure 8a) shows several features at low energy including a slight shoulder at around the band gap energy and two larger features at around 2.4 and 2.7 eV. These likely correspond to direct transitions into the nexthighest conduction bands or other direct transitions between the highly dispersive valence and conduction band edges. Additionally, we find that α is between 104 and 105 cm−1 in the energy range 1−3 eV. In contrast, Cu3SbS3 exhibits a direct gap at the Γ point, which we calculate to be about 1.5 eV within HSE06 (Figure 7b). Taken together with the absorption coefficient (see below), these properties may suggest that Cu3SbS3 has greater potential for use as a solar cell absorber than the other phases we have studied. This computed gap agrees well with that reported by Kehoe et al.49 and with our measurements. We calculate that the absorption coefficient is indeed about 104 cm−1 in the visible range (Figure 8b), somewhat smaller than in CuSbS2. The Cu3SbS3 absorption curve shows no obvious lowenergy features, although looking at higher energies (not

Table 1. Shape, Size, and Experimental and Computed Optical Properties of Different Cu−Sb−S Phase Nanocrystals optical properties (experimental)

theory

compound

shape

size (nm)

band gap (eV)

transition

band gap (eV)

transition

CuSbS2 Cu3SbS3 Cu3SbS4 Cu12Sb4S13

plates spheres oblates hollow structures

325 ± 25 (l) 19 ± 1 (d) 30 ± 5 23 ± 4 100 ± 30

1.1 1.4 1.2 1.6

indirect direct indirect direct

1.6 1.5 2.5 metallic

indirect direct indirect −

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(5) Scheer, R.; Guillemoles, J.-F.; Tiwari, A. N.; Niki, S.; Ferekides, C. S.; Abou-Ras, D.; Katagiri, H.; Jimbo, K.; Maw, W. S.; Oishi, K.; Yamazaki, M.; Araki, H.; Takeuchi, A. Thin Solid Films 2009, 517, 2455−2460. (6) Ramasamy, K.; Malik, M. A.; O’Brien, P. Chem. Commun. 2012, 48, 5703−5714. (7) Ramasamy, K.; Malik, M. A.; O’Brien, P. Chem. Sci. 2011, 2, 1170. (8) Barkhouse, D. A. R.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Mitzi, D. B. Prog. Photovoltaics Res. Appl. 2012, 20, 6−11. (9) Todorov, T. K.; Tang, J.; Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Mitzi, D. B. Adv. Energy Mater. 2013, 3, 34−38. (10) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Barkhouse, D. A. R. Philos. Trans. R. Soc., A 2013, 371, 20110432. (11) Temple, D. J.; Kehoe, A. B.; Allen, J. P.; Watson, G. W.; Scanlon, D. O. J. Phys. Chem. C 2012, 116, 7334−7340. (12) Yan, C.; Su, Z.; Gu, E.; Cao, T.; Yang, J.; Liu, J.; Liu, F.; Lai, Y.; Li, J.; Liu, Y. RSC Adv. 2012, 2, 10481. (13) Dufton, J. T. R.; Walsh, A.; Panchmatia, P. M.; Peter, L. M.; Colombara, D.; Islam, M. S. Phys. Chem. Chem. Phys. 2012, 14, 7229− 7233. (14) Van Embden, J.; Latham, K.; Duffy, N. W.; Tachibana, Y. J. Am. Chem. Soc. 2013, 135, 11562−11571. (15) Skinner, B. J.; Luce, F. D.; Makovicky, E. Econ. Geol. 1972, 67, 924−938. (16) Ramasamy, K.; Sims, H.; Butler, W. H.; Gupta, A. J. Am. Chem. Soc. 2014, 136, 1587. (17) Maiello, P.; Zoppi, G.; Miles, R. W.; Pearsall, N.; Forbes, I. Sol. Energy Mater. Sol. Cells 2013, 113, 186−194. (18) Van Embden, J.; Tachibana, Y. J. Mater. Chem. 2012, 22, 11466. (19) Kumar, M.; Persson, C. J. Renewable Sustainable Energy 2013, 5, 031616. (20) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462−465. (21) Ramasamy, K.; Malik, M. A.; Revaprasadu, N.; O’Brien, P. Chem. Mater. 2013, 25, 3551−3569. (22) Cao, Y.; Denny, M. S.; Caspar, J. V.; Farneth, W. E.; Guo, Q.; Ionkin, A. S.; Johnson, L. K.; Lu, M.; Malajovich, I.; Radu, D.; Rosenfeld, H. D.; Choudhury, K. R.; Wu, W. J. Am. Chem. Soc. 2012, 134, 15644−15647. (23) Ford, G. M.; Guo, Q.; Agrawal, R.; Hillhouse, H. W. Chem. Mater. 2011, 23, 2626−2629. (24) Kruszynska, M.; Borchert, H.; Parisi, J.; Kolny-Olesiak, J. J. Am. Chem. Soc. 2010, 132, 15976−15986. (25) Wang, Y.-H. A.; Zhang, X.; Bao, N.; Lin, B.; Gupta, A. J. Am. Chem. Soc. 2011, 133, 11072−11075. (26) Ramasamy, K.; Zhang, X.; Bennett, R. D.; Gupta, A. RSC Adv. 2013, 1186−1193. (27) Zhang, X.; Bao, N.; Ramasamy, K.; Wang, Y.-H. A.; Wang, Y.; Lin, B.; Gupta, A. Chem. Commun. 2012, 48, 4956−4958. (28) Jiasong, Z.; Weidong, X.; Huaidong, J.; Wen, C.; Lijun, L.; Xinyu, Y.; Xiaojuan, L.; Haitao, L. Mater. Lett. 2010, 64, 1499−1502. (29) Xu, D.; Shen, S.; Zhang, Y.; Gu, H.; Wang, Q. Inorg. Chem. 2013, 52, 12958−12962. (30) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. (31) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251. (32) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (33) Kohn, W.; Sham, L. J. Phys. Rev. 1965, A140, 1133. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (35) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. J. Chem. Phys. 2003, 118, 8207. (36) Hyed, J.; Scuseria, G. E.; Ernzerhof, M. J. Chem. Phys. 2006, 124, 219906. (37) Blochl, P. E. Phys. Rev. B 1994, 50, 17953. (38) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (39) Garin, J.; Parthe, E. Acta Crystallogr., Sect. B 1972, 28, 3672− 3674. (40) Makovicky, E. M. I. L.; Voldgade, Q.; Skinner, B. J. Can. Mineral. 1979, 17, 619.

CONCLUSION In summary, we have developed simple solution-based methods for the selective synthesis of nanocrystals of all four phases of copper−antimony−sulfide. The syntheses methods involve hot injection of sulfonating agents such as sulfur in oleylamine or a mixture of 1-dodecanethiol and t-dodecanethiol into a Cu(acac)2 and SbCl3 in oleylamine. The syntheses have resulted in phase-pure nanocrystals with uniform size and shapes. The morphologies of nanocrystals include nanoplates of CuSbS2, hollow structures of Cu12Sb4S13, spherical nanocrystals of Cu3SbS3, and oblate spheroids of Cu3SbS4. Optical measurements of the nanocrystals suggest that Cu12Sb4S13 and Cu3SbS3 possess direct band gaps, which render them as more promising candidates for use in solar cells over CuSbS2 and Cu3SbS4 that have indirect band gaps. Our HSE06 electronic structure calculations give compatible results for the size and nature of the fundamental gaps for CuSbS2 and Cu3SbS3. The calculation for Cu12Sb4S13 is complicated by the presence of off-stoichiometric phases. Nevertheless our computed gap is in reasonable agreement with other findings and our report. The lone exception is Cu3SbS4, for which HSE06 greatly overestimates the gap, perhaps overemphasizing the localization due to the quasi-two-dimensional layered structure. We report computed optical absorption curves for the semiconducting phases as well, finding that CuSbS2 and Cu3SbS3 possess desirable values of α between 104 and 105 cm−l. The fabrication of thin film solar cells using these nanocrystals is being carried out, and the results will be reported elsewhere.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, EDX data, UV−vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.G.). *E-mail: [email protected] (K.R.). Present Addresses ∥

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Albuquerque, New Mexico 87185, United States (K.R.). ⊥ German Research School for Simulation Science, Jülich, Germany 52425 (H.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-FG02-8ER46537.

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REFERENCES

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