Article pubs.acs.org/IC
Facile Preparation of Wurtzite CuInE2 (E = S, Se) Nanoparticles Under Solvothermal Conditions Xuzhao Zhao,† Yining Huang,*,†,‡ and John F. Corrigan*,†,‡ †
Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 Canada Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, Ontario, N6A 3K7 Canada
‡
S Supporting Information *
ABSTRACT: In this work, the synthesis of nanoscale CuInS2 and CuInSe2 was developed using molecular precursors of the type [(Ph3P)2CuIn(ER)4] (E = S, Se) and solvothermal reactions. Various conditions were investigated including the use of different precursors, reaction temperatures, reaction times and the addition of a secondary chalcogen source to mixtures. After optimizing conditions, nanoparticles of CuInS2 and CuInSe2 were isolated with controlled sizes in the range of 2−5 nm (wurtzite structure), which ultimately tuned the band gap energies of the materials. Characterization methods including powder X-ray diffraction, electron microscopy, and optical spectroscopy were used to investigate their structures and photophysical properties.
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INTRODUCTION Nanoscale semiconductors have emerged as viable candidates for use as the absorber layer in photovoltaic devices.1−4 Group I−III−VI materials have received much of this attention because of their direct band gap and high absorption coefficients. However, it is often challenging to prepare smallsized nanoparticles with uniform size and shape, especially for large-scale production.5 Compared to binary chalcogenide semiconductors (e.g., ZnS and CdSe), ternary I−III−VI semiconductors such as CuInS2 and CuInSe2 offer advantages for incorporation into photovoltaic devices, including the direct nature of their band gap, energies suitable for a variety of applications (1.0−1.5 eV), high conversion efficiency (∼105 cm−1), and relatively low toxicity.5,6−9To date, CuInS2 and CuInSe2 either in bulk form or as thin films have been studied in the most detail, and more recently, attention has shifted toward these materials prepared on the nanometer scale10−13 including recent reports of solvothermal preparation.14,13 Compared to the bulk, solids at the nanoscale can display features that distinguish them from the intrinsic properties of the material: due to quantum confinement effects, the band gap energy of the particle is inversely proportional to its size.15,16 By varying the particle size, it is possible to obtain energy-tunable absorption energies over the entire range of the visible spectrum and corresponding emissions from the visible to near-infrared (NIR) regions of the photoluminescence spectrum.17−19 Xie et al. demonstrated a red shift in the absorption spectra of CuInS2 nanoparticles when increasing particle sizes from 2 to 16 nm.19 Similarly, the emission spectra of CuInS2/ZnS quantum dots, in which ZnS is coated on the surface of a CuInS2 core to form a core/shell structure, were tuned to cover the range from the visible to the infrared.19 © XXXX American Chemical Society
These are important parameters since they can contribute to increased efficiency of solar energy conversion. The morphologies of the particles are also important: for instance, CuInS2 nanoparticles adopting the wurtzite structure are believed to be the most efficient polymorph for device fabrication due to their flexible Cu/In stoichiometric ratio, which provides the ability to tune the Fermi energy over a wider range.10 Both CuInS2 and CuInSe2 are ternary derivatives of the binary ZnS material; thus, like the crystal structures of ZnS, they have cubic and hexagonal polymorphs.20 By substituting copper and indium into the cation positions that would be filled by zinc in ZnS, the structures may exhibit three different types of structural polymorphs: (a) chalcopyrite structure;21 (b) zinc-blende structure;20 and (c) wurtzite structure.13 Recently, Zhou et al. synthesized a new type of CuInS2 crystal type with a spinel structure.22 To date, there are several different methods reported for preparing (crystallographically characterized) nanoclusters and nanoparticles in a manner that allows their sizes and shapes and even the surface chemistry to be manipulated. For example, metal-chalcogenide nanoclusters can be prepared by metathesis reactions involving salts or silane elimination.23−25 More recently, it was found that, by utilizing small clusters as building blocks, clusters can be rearranged to form larger structures with controllable size and shape through photolytic decomposition and solvothermal synthesis.26,27 In this paper, a simplified route combining a mixed-metal precursor and mild solvothermal methods is introduced to form size-controlled CuInS2 and CuInSe2 nanoparticles that adopt the wurtzite structure. The solvothermal method results in conversion of the Received: September 12, 2016
A
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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160 to 180 to 200 °C, respectively. Most of the products exhibited a uniformly colored solid except for 1C, which required a further adjustment of the amount of S8 to yield a more monodisperse product. All products were collected by centrifugation at 4000 rpm for 15 min, washed 10 times with THF and, finally, 5 times with methanol. Samples were then dried under vacuum for 4−6 h. All the reaction conditions and sample colors are listed in Table S1. [(CuInSe2)x] Nanoparticles. [(CuInSe2)x] nanoparticles were prepared by following the same procedure described above, but without the addition of an additional chalcogen source. Precursor 2 (1.33g, 1 mmol) was dissolved in THF (15 mL) in a Teflon-capped glass vial and sealed in an autoclave. After being heated in an oven at a selected temperature (100, 120, 140, 160 °C), the autoclave was cooled to room temperature naturally. All of the products were obtained initially as homogeneous solutions displaying tunable colors without the formation of any precipitate. The trend of color change was yellow (2A) to orange (2B) to orange-red (2C) to dark red (2D) when samples were heated at 100−160 °C (20 °C increments), respectively. All products were precipitated by adding excess methanol (4 × volume of THF) to reaction solutions and collected by centrifugation at 4000 rpm for 30 min. After washing with THF (5×) and methanol (5×), products were dried under vacuum for 4−6 h and stored under an inert atmosphere. Various reaction conditions and sample characteristics are listed in Table S1. Characterization. Solution NMR spectra were obtained on a Varian Mercury 400 spectrometer using standard settings. 1H and 13 C{1H} NMR spectra were referenced internally to SiMe4 using the residual proton and carbon signal of deuterated solvents at operating frequencies of 400.08 and 100.61 MHz. 31P{1H} chemical shifts are referenced to 85% H3PO4. PXRD patterns were obtained using a Rigaku Rotaflex RU-200 BVH rotating-anode X-ray diffractometer with a Co Kα radiation source (λ = 1.79926 Å). The samples were placed on a standard glass holder and measured with a sampling interval of 0.02° and a scan speed of 10°/min (equivalent to 0.5° 2θ on conventional diffractometers) with 2θ values ranging from 2° to 82°. Patterns are compared to those of CuInS2 and CuInSe2 retrieved from the International Centre for Diffraction Data (www.icdd.com). UV−vis absorption spectra of suspensions of samples in THF were acquired using a Varian Cary 300 BioUV−vis spectrometer at 25 °C in 1.00 cm quartz cells and corrected for scattering. Spectra of the pure solvent were subtracted. For diffuse reflectance spectra, all samples were diluted to 5% in mass percentage by mixing with BaSO4 thoroughly and using a Shimadzu 3101PC UV/vis-NIR spectrometer with integrating spheres. Photoluminescence (PL) spectra were taken from a PTI QuantaMaster 300 fluorimeter. Fluorescence quantum yields were estimated by the comparison of the fluorescence intensity with standard dye solutions with the same optical intensity at the excitation wavelength and similar fluorescence wavelength.27 SEM images were obtained by using a LEO (Zeiss) 1540XB FIB/SEM instrument with a field emission gun of 3 kV. For elemental analysis, the same SEM instrument was used, but with a field emission gun operated at 10 kV. Low-resolution TEM images were obtained from a Philips CM10 transmission microscope with an acceleration voltage of 100 kV. Carbon-coated copper grids were dipped in isopropanol to deposit nanoparticles onto the film. HR-TEM images were taken on a JEOL 2010F TEM/STEM instrument with an acceleration voltage of 200 kV. Selected area diffraction (SAD) patterns were also obtained along with HR-TEM measurements. From each sample, 50−100 particles were chosen, and an average diameter of particles was determined. TGA analysis was performed on a TA SDT Q600 device. The heating rate was controlled at a rate of 10 °C/min between 20 and 600 °C under a nitrogen flow.
precursor under modest temperatures and increased pressures, yielding well-defined CuInE2 nanoparticles.
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EXPERIMENTAL SECTION
Materials. Sodium methoxide, sodium metal, thiophenol, tertbutylthiol, ethanethiol, diphenyl diselenide, indium(III) chloride, copper(I) chloride, naphthalene, and sulfur powder were purchased from Aldrich or VWR and used as received. Triphenylphosphine was purchased from Sigma and vacuum-dried for 4 h at room temperature, then stored under an inert atmosphere. Solvents such as tetrahydrofuran (THF) were dried by passage through packed columns of activated alumina using a commercially available MBraun MB-SP Series solvent purification system. Methanol and CH2Cl2 were distilled and dried over CaH2 and P2O5, respectively. The solvent for 1 H, 13C, and 31P NMR spectroscopy was chloroform-d, which was purchased from Aldrich and distilled over P2O5, while DMSO-d6 was purchased from Aldrich and used directly without further purification. All synthetic and handling procedures were carried out under an atmosphere of high purity dried nitrogen using standard double manifold Schlenk line techniques and an MBraun Labmaster 130 glovebox. For the solvothermal synthesis, stainless steel autoclaves and disposable glass bottles (30 mL) with Teflon caps were used. Preparation of Precursors. [(Ph3P)2CuIn(SEt)4] 1. 1 was synthesized by slight modification of the published procedures.26 NaSEt was prepared by adding ethanethiol (0.32 mL, 4.33 mmol) into a methanol solution (15 mL) of NaOCH3 (0.242 g, 4.48 mmol) and stirring for 30 min. InCl3 (0.230 g, 1.04 mmol) dissolved in 5 mL of methanol was added, stirring for another 1.5 h to form Na[In(SEt)4]. PPh3 (0.574 g, 2.19 mmol) and CuCl (0.114 g, 1.15 mmol) were then combined in 15 mL of CH2Cl2, and the mixture was added dropwise to Na[In(SPh)4]. The reaction mixture was stirred overnight, affording a slight yellow solution with a white precipitate. The solvent volume was reduced under vacuum to half, and 20 mL of methanol was added to fully precipitate the product rather than NaCl (byproduct). The white precipitate was collected and redissolved in 20 mL of CH2Cl2 and filtered through Celite to remove residual NaCl and washed with methanol twice. Then, the solid was dried for 2 h under vacuum, affording 1 as a white solid. Yield, 0.63g, 66%; 1H NMR (400 MHz; CDCl3, 23 °C); δ 1.24 (t, J = 7.4 Hz, 12H, CH3); δ 2.67 (q, J = 7.4 Hz, 8H, SCH2); δ 7.33 (m, 30H, P(C6H5)3); 31P NMR CDCl3; δ −4.4. Data match those from a previously published report.26 [(Ph3P)2CuIn(SePh)4] 2. 2 was prepared following a similar procedure as for the synthesis of 1. Na[SePh] (0.710 g, 3.96 mmol) was dissolved in 15 mL of methanol. After stirring for 30 min, InCl3 (0.230 g, 1.04 mmol) dissolved in 5 mL of methanol was added, and reacted for an additional 1.5 h to form Na[In(SePh)4]. PPh3 (0.574 g, 2.19 mmol) and CuCl (0.114 g, 1.15 mmol) were then combined in 15 mL of CH2Cl2, and the mixture was added dropwise to Na[In(SePh)4]. The reaction mixture was kept stirring overnight, affording a light yellow solution with a white precipitate. The white precipitate was collected by filtration and dried for 2 h, affording 2 as a white solid. Yield, 1.06g, 81%; 1H NMR (400 MHz; CDCl3, 23 °C): δ 6.85 (s, 8H); δ 7.00 (d, 4H); δ 7.23 (br d, 10H); δ 7.35 (m, 8H). The data are consistent with reported values.28 Synthesis of Nanoparticles. [(CuInS 2 ) x ] Nanoparticles. [(CuInS2)x] nanoparticles were prepared in a sealed reactor at a controlled temperature and increased pressure using polar organic solvents (solvothermal conditions). A typical procedure for the preparation of [(CuInS2)x] nanoparticles was as follows: An appropriate amount of 1 (0.97 g, 1 mmol) was combined with THF (15 mL) in a 30 mL glass bottle with a Teflon cap. An additional source of sulfur, S8, (0.125−0.25 mmol) was also added and mixed thoroughly for ∼15 min until a clear solution was obtained. The bottle was capped, placed in a stainless steel autoclave, and heated in an oven for a specific period of time at a set temperature. The autoclave was subsequently cooled to room temperature naturally, and precipitates with tunable colors were obtained. With a set reaction time of 24 h, the color of the precipitates generated changed from yellow (1A), red (1B) to brown (1C) to black (1D) at reaction temperatures of 140 to
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RESULTS AND DISCUSSION The conversion of molecular metal-chalcogenolate complexes and small clusters has proven to be a highly effective method for the formation of metal-chalcogenide extended solids,28−30 thin films,31 as well as nanoscale materials.26,32,33 This includes works by Williams et al.26 (photolysis) and Castro et al.32 and B
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. PXRD patterns of (a) [(CuInS2)x]: 1A, 1B, 1C, and 1D, the products of precursor 1 prepared at 140, 160, 180, and 200 °C, respectively. ICCD 01-77-9459 (bottom) and ICDD 04-013-0169 (top) show the standard diffraction patterns of wurtzite and tetragonal forms of CuInS2, respectively. (b) [(CuInSe2)x]: 2A, 2B, 2C, and 2D, the products of precursor 2 prepared at 100, 120, 140, and 160 °C, respectively. ICCD 01-785190 (bottom) and ICD 01-070-3084 (top) show the standard diffraction patterns of the wurtzite and tetragonal forms of CuInSe2, respectively.
Figure 2. Absorption spectra of (a) [(CuInS2)x] products: 1A, 1B, and 1C, the products of precursor 1 by suspending in dry THF. (b) [(CuInSe2)x] products: 2A, 2B, 2C, and 2D, the products of precursor 2 by suspending in dry THF. Calculated band gap curves: (c) from [(CuInS2)x] absorption spectra in (a); (d) [(CuInSe2)x] absorption spectra in (b).
Banger et al.28 (thermolysis), who have demonstrated that the mixed-metal [(Ph3P)2CuIn(ER)4] (E = S, Se) can be used as molecular precursors to the related ternary nanomaterials CuInE2, taking advantage of the well-defined Cu:In:S ratio. Indeed, Castro and co-workers investigated in detail their decomposition to tetragonal CuInS2 in noncoordinating, high boiling solvents where size selective preparation was achieved. We have recently developed a procedure for the preparation of monodisperse CdS nanoparticles via the straightforward
solvothermal conversion of the mononuclear precursor [Me4N]2[Cd(SPh)4],34 building on previous, related reports using this method.35 The simplicity and fine control of particle size offered with CdS via this method prompted us to investigate its utility for the preparation of nanoscopic CuInE2 from [(Ph3P)2CuIn(SEt)4] 1 and [(Ph3P)2CuIn(SePh)4] 2. In this vein, THF solutions of 1 and 2 were heated at varying temperatures (1: 140−200 °C together with S8; 2: 100−200 °C), which resulted in the formation of sizeC
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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[(CuInS2)x] and [(CuInSe2)x] obtained from the absorption data, respectively. The band gap energies obtained from the double derivative of the absorption profiles were used to calculate particle sizes. These illustrate that the band gap decreased from 2.7 eV (1A) to 2.6 eV (1C) for nanoparticles of [(CuInS2)x]. For [(CuInSe2)x], these values were varied from 2.5 eV (2A) to 2.3 eV (2D). Compared to the band gap energy of the bulk materials (Egap(CuInS2) = 1.48 eV; Egap(CuInSe2) = 1.04 eV), the significant shifts are consistent with the manipulation of particle size. From the Brus equation,16 estimates of the particle size can be obtained for these samples: 2.8 nm (1A), 3.0 nm (1B), 3.4 nm (1C), and 3.1 nm (2A), 3.8 nm (2B), 4.0 nm (2C), and 4.1 nm (2D), respectively (listed in Table 1), each smaller than the Bohr radius of CuInS2 and CuInSe2.40,17
controlled [CuInE2] nanoparticles, isolated as highly colored solids (Table S1). The addition of sulfur for the preparation of [CuInS2] allows the reactions to be completed at temperatures low enough that particle size control is possible. Christou used this strategy in the formation of FeS clusters from ironphenylthiolate precursors: the introduction of S8 leads to the low temperature formation and elimination of PhSSPh and the concomitant formation of sulfide ligands.36 The lower Se−C bond dissociation energy (versus S−C)37 is such that the addition of elemental selenium is not required for the formation of nanoscopic [CuInSe2] under reaction conditions used.
The PXRD patterns of the different [(CuInS2)x] and [(CuInSe2)x] particles prepared are plotted in Figure 1 and compared to the standard CuInS2/CuInSe2 structures; both samples display patterns consistent with the wurtzite form of CuInE2. Materials prepared at higher temperatures display sharper reflections with a good match to the reference patterns. Expectedly, samples obtained at lower temperatures display broadened reflections consistent with smaller particle sizes (vide inf ra). When reaction temperatures are 200 °C for 1 and 160 °C for 2, all reflections are well-resolved. This transition is also consistent with the change in color observed for both sets of products (Table S1). Although we also completed similar investigations by using other −SR precursors such as [(Ph 3 P) 2 CuIn(SPh) 4 ] and [(Ph 3 P) 2 CuIn(S t Bu) 4 ] 28 for [(CuInS2)x] preparation, limited particle size control under similar conditions resulted in our focusing exclusively on the ethylthiolate containing precursor 1. The UV−vis absorption spectra (Figure 2) of samples in suspensions as well as the diffuse reflectance spectra of solid samples (Supporting Information, Figure S1) for all samples were acquired. Spectra for both sets of nanomaterials display a broad, yet resolved, peak at different wavelengths for each set, with the exception of the [CuInS2] material prepared at 200 °C (1D) for which no discrete maximum was observed. For [(CuInS2)x] samples 1A−1C, the absorption maximum (Figure 2a) shifted from 457 nm (1A) to 485 nm (1B; see also Figure S2) when increasing the reaction temperature from 140 to 180 °C. This trend is consistent with the phenomenon of a red shift in the excitonic absorption band with increasing particle size.33 Additionally, it is worth noting that the mother liquor from these reactions also displayed a similar color as the corresponding precipitate; at higher reaction temperatures, reaction solutions were colorless. This would be consistent with the idea that relatively small particles can be dissolved in the polar organic solvent, whereas larger particles aggregate and precipitate. The absorption spectra of the selenide samples [(CuInSe2)x] 2A−2D also display a red shift with increased preparation temperature: as shown in Figure 2b, the absorption maximum shifted from 493 to 540 nm. Diffuse reflectance spectra exhibited similar red shifts for both materials with increasing reaction temperatures. Generally, the reflectance percentage (R) was collected, but via the Kubelka−Munk equation: A = −log (R), the absorption spectrum is plotted. All spectra are shown in Figure S1. Together with the absorption spectra, an approximation of the band gap energy calculated from a Tauc Plot of the data is illustrated: 38,39 Figure 2c,d displays these curves for
Table 1. Selected Data for CuInS2 (1A−1C) and CuInSe2 (2A, 2B, 2D) Nanoparticles Prepared at Different Temperatures (T) 1
2
sample
T (°C)
1A 1B 1C 2A 2B 2D
140 160 180 100 120 160
d (nm)a
d (nm)b
Cu:In:S
± ± ± ± ± ±
2.8 3.0 3.4 3.1 4.0 4.2
1.0:0.8:1.2 1.0:0.9:2.4 1.0:1.1:2.0 1.0:1.1:2.0 1.0:1.2:2.0 1.0:1.1:1.8
18 26 28 34 45 60
2 4 5 8 3 11
a Diameters of aggregated particles, as determined by SEM. bDiameters calculated from optical absorption data (double derivative).
Electron microscopy measurements were obtained together with EDX analyses to explore the size, morphology, and composition of the nanoparticles (Figures S1and S2). As shown in Table 1, EDX analysis reveals that Cu:In ratios of both [(CuInS2)x] and [(CuInSe2)x] products are close to an expected, ideal ratio of 1.0. For [(CuInS2)x] samples 1A and 1B, a Cu:S ratio of 1:2.5 is observed. This is consistent with the formation of “CuInS2−X(SEt)2X” at the lower reaction temperatures, where a fraction of the organic sulfur (thiolate) components are still retained on the surfaces of smaller particles. This is supported by thermogravimetric analysis of these samples, where a weight loss of 24.1% and 20.6% is observed upon subsequent heating for 1A and 1B, respectively (Figure S4). The calculated percentages of residue for samples 1A and 1B are 75.4% and 78.5%, respectively. For 1C and all [(CuInSe2)x] samples, particles show ∼1:1 Cu:In and ∼1:2 Cu:E ratios. In the SEM, samples are imaged as spherical particles with a size range of ∼10−60 nm. However, at higher resolution, it is observed clearly that much smaller nanoparticles (2−5 nm) are the components that make up the larger species, even though they are highly aggregated (Figure 3). Observed particle sizes for the system [CuInS2], 1A (2.4 ± 0.5 nm), 1B (3.3 ± 0.3 nm), and 1C (4.4 ± 0.3 nm), are all in reasonable agreement with sizes calculated from spectroscopic features (vide supra). We note, however, that the aggregated nature of the sample renders the statistical analysis more difficult (50−100 particles were measured, per sample) and that, overall, the system is somewhat polydisperse, consistent with the broad absorption profiles in the UV−vis spectra. HR-TEM data illustrate clearly lattice fringes with spacings of 3.4 Å, which is consistent with the distance between the (100) planes of the standard wurtzite CuInS2 (ICCD 01-77-9459). Also, for 1C, a secondary lattice D
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. HR-TEM images and FFT images for samples 1A−1C. Insets: (a) magnified view of highlighted area R1; (b) FFT of area R1; (c) SAD of 0.25 μm2 area. The figure in the bottom right is a TEM image of the small amount of “single crystal” CuInS2 present in samples of 1C (see text for details; inset (a): magnified view; inset (b): FFT image).
images, the amount of this component is less than 5% of the overall sample. Individual nanoparticles of [(CuInSe2)x] were clearly observed in TEM images for the samples 2A, 2B, and 2D selected for additional analysis via electron microscopy (Figure S5). The diameters of the particles, as determined from the TEM images for 2A, 2B, and 2D, are 2.4 ± 0.4 nm, 3.4 ± 0.2 nm, and 3.9 ± 0.2 nm, respectively. These are in reasonable agreement with values calculated from band gap energy, as determined from absorption spectra, although we note again that the aggregated nature of the sample renders the statistical analysis more difficult (50−100 particles were measured, per sample). A HR-TEM image for all samples (inset b) is also illustrated. Sample 2A displays clear lattice fringes with a dspacing of 3.5 Å, which corresponds to the distance of the (100) plane of the standard wurtzite CuInSe2 (ICCD 01-0785190). At the same time, the spaces between lattice fringes of both samples 2B and 3D are 3.3 Å, which is consistent with the distances of the (002) plane. The fast Fourier transition (FFT) images of each sample are also illustrated.
facet was observed due to the increased crystallinity and size of these samples: a spacing of 3.0 Å together with an angle of 69° from the 100 plane is assigned to the (101) planes. The fast Fourier transition (FFT) image (inset b, Figure 3) illustrates a highly crystalline material with resolved reflections that correspond to related planes labeled in the images. From the FFT images, it is also obvious that samples prepared at higher temperatures resulted in the resolution of more reflections. SAED patterns were also measured to obtain an overview within a relatively large area for samples 1A and 1B displaying but faint rings (inset c, Figure 3). This is a sign that the nanoparticles are not arranged into a higher order structure, as was observed for CdS (superlattice).35 When the data are compared to those of the standard wurtzite CuInS2 lattice (ICCD 01-77-9459), all rings can be indexed. Interestingly, a small amount of a second, much larger crystalline sample of CuInS2 was also observed in 1C, as shown in Figure 3. The FFT image inset provides evidence of a highly crystalline material with strong reflections; all reflections are assigned to all the main planes of wurtzite CuInS2 including the (100), (002), (101), (102), and (110) planes. From the observed TEM E
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Top: emission spectra of samples prepared at different temperatures (24 h reaction time): (a) [(CuInS2)x]: 1A, 1B, and 1C; (b) [(CuInSe2)x]: 2A, 2B, 2C, and 2D, by suspending in dry THF. Bottom: emission spectra of samples prepared at different reaction times: (c) [(CuInS2)x] 160 °C for 12 h, 24 h, 36 h; (d) [(CuInSe2)x] 120 °C for 12 h, 24 h, 36 h, 48 h.
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Emission Spectra and Quantum Yield. Two sets of emission spectra were obtained to investigate the influence of reaction temperatures and reactions times on quantum yields. The shifts observed in the emission maxima (Figure 4) are comparable to the trend observed for the absorption spectra. For [(CuInS2)x], the wavelength of the emission maximum shifted from 650 nm (sample 1A, 140 °C) to 740 nm (sample 1C, 180 °C), consistent with the red shift observed in the absorption spectra. Both samples were prepared with a reaction time of 24 h. A similar trend was observed in [(CuInSe2)x] when varying reaction temperatures from 100 to 160 °C (Figure 4b). Here, a red shift of ∼135 nm is observed for [(CuInSe2)x] with increasing reaction temperatures/particle size. The emission maximum shifts from 690 nm (sample 2A) to 825 nm (sample 2D). All peaks are rather broad (W1/2 ≈ 160−200 nm) and red-shifted from the absorption maxima. This can be quite common for semiconductor nanomaterials, especially for I−III−VI systems. According to previous studies, the broadened emission spectra are not necessarily/simply related to size inhomogeneity but to a distribution of surface (defect) states.12 The “tunability” of emission energies has also led to the suggestion that they involve transitions from a donor state to quantized-valence band, from quantized-conduction band to acceptor-level state, or from the inherent band structure. 41−43 Longer reaction times for samples 1B ([(CuInS2)x]; 160 °C) and 2B ([CuInSe2]x; 120 °C) resulted in slight increases in emission intensity, together with a modest red shift (Figure 4c,d). Quantum yields improved from 6.4% (sample 1B-12h) to 11.5% (sample 1B-36h). For [(CuInSe2)x] 2B, the quantum yield reached 19.0% when increasing the reaction time to 48 h.
CONCLUSIONS A low temperature synthetic route that is facile and efficient was developed for the preparation of wurtzite based ternary semiconductor nanoparticles with controllable sizes. By using solvothermal methods and simple precursors, two series of ternary chalcogenide nanoparticles (CuInS2 and CuInSe2) were prepared. It was found that the choice of mixed-metal precursor was crucial to control the size of nanoparticles synthesized. Because of the suitable S/Se−C bond enthalpy, [(Ph3P)2CuIn(SEt)4] 1 and [(Ph3P)2CuIn(SePh)4] 2 offered the best control in size of selectivity versus other thiolate/selenolate complexes. Both series of nanoparticles were isolated with the wurtzite structure; control of nanoparticle size was possible with modulation of reaction temperatures and, to a lesser extent, reaction times.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02177. Table of details of solvothermal synthesis, absorption spectra, SEM images, EDX analysis, TEM images, TGA analysis, and HR-TEM and FFT images (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Y.H.). *E-mail: [email protected] (J.F.C.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F
DOI: 10.1021/acs.inorgchem.6b02177 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Notes
(17) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (18) Torimoto, T.; Kameyama, T.; Kuwabata, S. Photofunctional Materials Fabricated with Chalcopyrite-Type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions. J. Phys. Chem. Lett. 2014, 5, 336−347. (19) Xie, R. G.; Rutherford, M.; Peng, X. G. Formation of HighQuality I-III-VI Semiconductor Nanocrystals by Tuning Relative Reactivity of Cationic Precursors. J. Am. Chem. Soc. 2009, 131, 5691− 5697. (20) Binsma, J. J. M.; Giling, L. J.; Bloem, J. J. Phase Relations in the System Cu2S-In2S3. J. Cryst. Growth 1980, 50, 429−436. (21) Hahn, H.; Frank, G.; Klingler, W.; Meyer, A. D.; Storger, G. Untersuchungen Uber Ternare Chalkogenide 0.5. Uber Einige Ternare Chalkogenide Mit Chalkopyritstruktur. Z. Anorg. Allg. Chem. 1953, 271, 153−170. (22) Lei, S. J.; Wang, C. Y.; Liu, L.; Guo, D. H.; Wang, C. N.; Tang, Q. L.; Cheng, B. C.; Xiao, Y. H.; Zhou, L. Spinel Indium Sulfide Precursor for the Phase-Selective Synthesis of Cu-In-S Nanocrystals with Zinc-Blende, Wurtzite, and Spinel Structures. Chem. Mater. 2013, 25, 2991−2997. (23) Ahlrichs, R.; Crawford, N. R. M.; Eichhoefer, A.; Fenske, D.; Hampe, O.; Kappes, M. M.; Olkowska-Oetzel, J. Synthesis and Structure of Two Ionic Copper Indium Selenolate Cluster Complexes [As(C6H5)(4)](2)[Cu6In4(SeC6H5)(16)Cl-4] and [As(C6H5)(4)][Cu7In4(SeC6H5)(20)]. Eur. J. Inorg. Chem. 2006, 2006, 345−350. (24) Eichhoefer, A.; Fenske, D. Syntheses and Structures of New Copper(I)-Indium(III)-Selenide Clusters. J. Chem. Soc., Dalton Trans. 2000, 941−944. (25) Lee, G. S. H.; Craig, D. C.; Ma, I.; Scudder, M. L.; Bailey, T. D.; Dance, I. G. [S4Cd17(SPh)28]2-, the 1st Member of A 3rd Series of Tetrahedral [SwMx(SR)y]z- Clusters. J. Am. Chem. Soc. 1988, 110, 4863−4864. (26) Williams, M.; Okasha, R. M.; Nairn, J.; Twamley, B.; Afifi, T. H.; Shapiro, P. J. A Photochemical Route to Discrete, Ternary Metal Chalcogenide Clusters. Chem. Commun. 2007, 30, 3177−3179. (27) Zheng, N. F.; Bu, X. H.; Lauda, J.; Feng, P. Y. Zero- and Twodimensional Organization of Tetrahedral Cadmium Chalcogenide Clusters with Bifunctional Covalent Linkers. Chem. Mater. 2006, 18, 4307−4311. (28) Banger, K. K.; Jin, M. H. C.; Harris, J. D.; Fanwick, P. E.; Hepp, A. F. A New Facile Route for the Preparation of Single-source Precursors for Bulk, Thin-film, and Nanocrystallite I-III-VI Semiconductors. Inorg. Chem. 2003, 42, 7713−7715. (29) Courtel, F. M.; Paynter, R. W.; Marsan, B.; Morin, M. Synthesis, Characterization and Growth Mechanism of n-Type CuInS2 Colloidal Particles. Chem. Mater. 2009, 21, 3752−3762. (30) Wang, X.; Zhang, Y.; Deng, Z.; Wang, Y.; Wang, Z.; Shih, I. Preparation and Performance Research of CuInSe2Materials Applied in Solar Cell. J. Cryst. Process Technol. 2012, 2, 142−145. (31) Banger, K. K.; Cowen, J.; Hepp, A. F. Synthesis and Characterization of the First Liquid Single-source Precursors for the Deposition of Ternary Chalcopyrite (CuInS2) Thin Film Materials. Chem. Mater. 2001, 13, 3827−3828. (32) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Nanocrystalline Chalcopyrite Materials (CuInS2 and CuInSe2) via Low-temperature Pyrolysis of Molecular Single-source Precursors. Chem. Mater. 2003, 15, 3142−3147. (33) Xiao, J. P.; Xie, Y.; Tang, R.; Qian, Y. T. Synthesis and Characterization of Ternary CuInS2 Nanorods via A Hydrothermal Route. J. Solid State Chem. 2001, 161, 179−183. (34) Levchenko, T. I.; Kübel, C.; Wang, D.; Khalili Najafabadi, B.; Huang, Y.; Corrigan, J. F. Controlled Solvothermal Routes to Hierarchical 3D Superparticles of Nanoscopic CdS. Chem. Mater. 2015, 27, 3666−3682. (35) Levchenko, T. I.; Kübel, C.; Huang, Y.; Corrigan, J. F. From Molecule to Materials: Crystalline Superlattices of Nanoscopic CdS Clusters. Chem. - Eur. J. 2011, 17, 14394−14398.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful for the financial support of the Natural Sciences and Engineering Research Council (Canada) Discovery Grants and Equipment Grants Programs (J.F.C. and Y.H.). The authors gratefully acknowledge the Nanofabrication Facility at Western University for SEM imaging. They would like to express their gratitude to Dr. Mathew Willans (Western) for his assistance with the NMR instruments, to Dr. Tim Goldhawk (Western) for his help with the SEM measurements, and Dr. Richard Gardiner and Ms. Karen Nygard (Western) for their assistance with the TEM measurements in the Biotron facility. Thanks are also extended to Dr. Carmen M. Andrei (McMaster University) for insightful input with the HR-TEM analysis. Finally, we thank the anonymous reviewers for helpful suggestions in improving the manuscript.
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REFERENCES
(1) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (2) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-stable All-inorganic Nanocrystal Solar Cells Processed From Solution. Science 2005, 310, 462−465. (3) Raffaelle, R. P.; Castro, S. L.; Hepp, A. F.; Bailey, S. G. Quantum Dot Solar Cells. Prog. Photovoltaics 2002, 10, 433−439. (4) Smith, A. M.; Nie, S. M. Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering. Acc. Chem. Res. 2010, 43, 190− 200. (5) Zhong, H. Z.; Bai, Z. L.; Zou, B. S. Tuning the Luminescence Properties of Colloidal I-III-VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167−3175. (6) Hahn, H. Uber Die Strukturen Der Chalkogenide Des Galliums, Indiums Und Thalliums. Angew. Chem. 1953, 65, 538−538. (7) Neumann, H.; Tomlinson, R. D. Band-Gap Narrowing in Normal-Type CuInSe2 Single-Crystals. Solid State Commun. 1986, 57, 591−594. (8) Shay, J. L.; Tell, B. Energy-Band Structure of I-III-VI2 Semiconductors. Surf. Sci. 1973, 37, 748−762. (9) Tell, B.; Shay, J. L.; Kasper, H. M. Electrical Properties, Optical Properties and Band Structure of CuGaS2 and CuInS2. Phys. Rev. B 1971, 4, 2463−2471. (10) Connor, S. T.; Hsu, C. M.; Weil, B. D.; Aloni, S.; Cui, Y. Phase Transformation of Biphasic Cu2S-CuInS2 to Monophasic CuInS2 Nanorods. J. Am. Chem. Soc. 2009, 131, 4962−4966. (11) Kazmerski, L. L.; Sanborn, G. A.; White, F. R.; Merrill, A. J. Thin-Film CuInSe-2-Cds Heterojunctions. Bull. Am. Phys. Soc. 1976, 21, 430−430. (12) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (13) Pan, D. C.; An, L. J.; Sun, Z. M.; Hou, W.; Yang, Y.; Yang, Z. Z.; Lu, Y. F. Synthesis of Cu-In-S Ternary Nanocrystals with Tunable Structure and Composition. J. Am. Chem. Soc. 2008, 130, 5620−5621. (14) Huang, W.-C.; Tseng, C.-H.; Chang, S.-H.; Tuan, H.-Y.; Chiang, C.-C.; Lyu, L.-M.; Huang, M. H. Solvothermal Synthesis of Zincblende and Wurtzite CuInS2 Nanocrystals and Their Photovoltaic Application. Langmuir 2012, 28, 8496−8501. (15) Alivisatos, A. P. Perspectives On the Physical Chemistry of Semiconductor Nanocrystals. J. Phys. Chem. 1996, 100, 13226−13239. (16) Steigerwald, M. L.; Brus, L. E. Semiconductor Crystallites - A Class of Large Molecules. Acc. Chem. Res. 1990, 23, 183−188. G
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Inorganic Chemistry (36) Christou, G.; Garner, C. D. A Convenient Synthesis of Tetrakis[thiolato-[small micro]3-sulphido-iron](2-)clusters. J. Chem. Soc., Dalton Trans. 1979, 1093−1094. (37) Johnson, D. A. Some Thermodynamic Aspects of Inorganic Chemistry, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1982. (38) Stenzel, O. The Physics of Thin Film Optical Spectra: An Introduction; Springer: New York, 2005. (39) Tauc, J. Optical Properties and Electronic Structure of Amorphous Ge and Si. Mater. Res. Bull. 1968, 3, 37−46. (40) Du, C.-F.; You, T.; Jiang, L.; Yang, S.-Q.; Zou, K.; Han, K.-L.; Deng, W.-Q. Controllable Synthesis of Ultrasmall CuInSe2 Quantum Dots for Photovoltaic Application. RSC Adv. 2014, 4, 33855−33860. (41) Omata, T.; Nose, K.; Kurimoto, K.; Kita, M. Electronic Transition Responsible for Size-dependent Photoluminescence of Colloidal CuInS2 Quantum Dots. J. Mater. Chem. C 2014, 2, 6867− 6872. (42) So, D.; Konstantatos, G. Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as Optoelectronic Quantum Dot Solids. Chem. Mater. 2015, 27, 8424−8432. (43) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. SizeDependent Photovoltaic Performance of CuInS2 Quantum DotSensitized Solar Cells. Chem. Mater. 2014, 26, 7221−7228.
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Facile Preparation of Wurtzite CuInE2 (E = S, Se) Nanoparticles Under Solvothermal Conditions Xuzhao Zhao,† Yining Huang,*,†,‡ and John F. Corrigan*,†,‡ †
Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7 Canada Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, Ontario, N6A 3K7 Canada
‡
S Supporting Information *
ABSTRACT: In this work, the synthesis of nanoscale CuInS2 and CuInSe2 was developed using molecular precursors of the type [(Ph3P)2CuIn(ER)4] (E = S, Se) and solvothermal reactions. Various conditions were investigated including the use of different precursors, reaction temperatures, reaction times and the addition of a secondary chalcogen source to mixtures. After optimizing conditions, nanoparticles of CuInS2 and CuInSe2 were isolated with controlled sizes in the range of 2−5 nm (wurtzite structure), which ultimately tuned the band gap energies of the materials. Characterization methods including powder X-ray diffraction, electron microscopy, and optical spectroscopy were used to investigate their structures and photophysical properties.
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INTRODUCTION Nanoscale semiconductors have emerged as viable candidates for use as the absorber layer in photovoltaic devices.1−4 Group I−III−VI materials have received much of this attention because of their direct band gap and high absorption coefficients. However, it is often challenging to prepare smallsized nanoparticles with uniform size and shape, especially for large-scale production.5 Compared to binary chalcogenide semiconductors (e.g., ZnS and CdSe), ternary I−III−VI semiconductors such as CuInS2 and CuInSe2 offer advantages for incorporation into photovoltaic devices, including the direct nature of their band gap, energies suitable for a variety of applications (1.0−1.5 eV), high conversion efficiency (∼105 cm−1), and relatively low toxicity.5,6−9To date, CuInS2 and CuInSe2 either in bulk form or as thin films have been studied in the most detail, and more recently, attention has shifted toward these materials prepared on the nanometer scale10−13 including recent reports of solvothermal preparation.14,13 Compared to the bulk, solids at the nanoscale can display features that distinguish them from the intrinsic properties of the material: due to quantum confinement effects, the band gap energy of the particle is inversely proportional to its size.15,16 By varying the particle size, it is possible to obtain energy-tunable absorption energies over the entire range of the visible spectrum and corresponding emissions from the visible to near-infrared (NIR) regions of the photoluminescence spectrum.17−19 Xie et al. demonstrated a red shift in the absorption spectra of CuInS2 nanoparticles when increasing particle sizes from 2 to 16 nm.19 Similarly, the emission spectra of CuInS2/ZnS quantum dots, in which ZnS is coated on the surface of a CuInS2 core to form a core/shell structure, were tuned to cover the range from the visible to the infrared.19 © XXXX American Chemical Society
These are important parameters since they can contribute to increased efficiency of solar energy conversion. The morphologies of the particles are also important: for instance, CuInS2 nanoparticles adopting the wurtzite structure are believed to be the most efficient polymorph for device fabrication due to their flexible Cu/In stoichiometric ratio, which provides the ability to tune the Fermi energy over a wider range.10 Both CuInS2 and CuInSe2 are ternary derivatives of the binary ZnS material; thus, like the crystal structures of ZnS, they have cubic and hexagonal polymorphs.20 By substituting copper and indium into the cation positions that would be filled by zinc in ZnS, the structures may exhibit three different types of structural polymorphs: (a) chalcopyrite structure;21 (b) zinc-blende structure;20 and (c) wurtzite structure.13 Recently, Zhou et al. synthesized a new type of CuInS2 crystal type with a spinel structure.22 To date, there are several different methods reported for preparing (crystallographically characterized) nanoclusters and nanoparticles in a manner that allows their sizes and shapes and even the surface chemistry to be manipulated. For example, metal-chalcogenide nanoclusters can be prepared by metathesis reactions involving salts or silane elimination.23−25 More recently, it was found that, by utilizing small clusters as building blocks, clusters can be rearranged to form larger structures with controllable size and shape through photolytic decomposition and solvothermal synthesis.26,27 In this paper, a simplified route combining a mixed-metal precursor and mild solvothermal methods is introduced to form size-controlled CuInS2 and CuInSe2 nanoparticles that adopt the wurtzite structure. The solvothermal method results in conversion of the Received: September 12, 2016
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160 to 180 to 200 °C, respectively. Most of the products exhibited a uniformly colored solid except for 1C, which required a further adjustment of the amount of S8 to yield a more monodisperse product. All products were collected by centrifugation at 4000 rpm for 15 min, washed 10 times with THF and, finally, 5 times with methanol. Samples were then dried under vacuum for 4−6 h. All the reaction conditions and sample colors are listed in Table S1. [(CuInSe2)x] Nanoparticles. [(CuInSe2)x] nanoparticles were prepared by following the same procedure described above, but without the addition of an additional chalcogen source. Precursor 2 (1.33g, 1 mmol) was dissolved in THF (15 mL) in a Teflon-capped glass vial and sealed in an autoclave. After being heated in an oven at a selected temperature (100, 120, 140, 160 °C), the autoclave was cooled to room temperature naturally. All of the products were obtained initially as homogeneous solutions displaying tunable colors without the formation of any precipitate. The trend of color change was yellow (2A) to orange (2B) to orange-red (2C) to dark red (2D) when samples were heated at 100−160 °C (20 °C increments), respectively. All products were precipitated by adding excess methanol (4 × volume of THF) to reaction solutions and collected by centrifugation at 4000 rpm for 30 min. After washing with THF (5×) and methanol (5×), products were dried under vacuum for 4−6 h and stored under an inert atmosphere. Various reaction conditions and sample characteristics are listed in Table S1. Characterization. Solution NMR spectra were obtained on a Varian Mercury 400 spectrometer using standard settings. 1H and 13 C{1H} NMR spectra were referenced internally to SiMe4 using the residual proton and carbon signal of deuterated solvents at operating frequencies of 400.08 and 100.61 MHz. 31P{1H} chemical shifts are referenced to 85% H3PO4. PXRD patterns were obtained using a Rigaku Rotaflex RU-200 BVH rotating-anode X-ray diffractometer with a Co Kα radiation source (λ = 1.79926 Å). The samples were placed on a standard glass holder and measured with a sampling interval of 0.02° and a scan speed of 10°/min (equivalent to 0.5° 2θ on conventional diffractometers) with 2θ values ranging from 2° to 82°. Patterns are compared to those of CuInS2 and CuInSe2 retrieved from the International Centre for Diffraction Data (www.icdd.com). UV−vis absorption spectra of suspensions of samples in THF were acquired using a Varian Cary 300 BioUV−vis spectrometer at 25 °C in 1.00 cm quartz cells and corrected for scattering. Spectra of the pure solvent were subtracted. For diffuse reflectance spectra, all samples were diluted to 5% in mass percentage by mixing with BaSO4 thoroughly and using a Shimadzu 3101PC UV/vis-NIR spectrometer with integrating spheres. Photoluminescence (PL) spectra were taken from a PTI QuantaMaster 300 fluorimeter. Fluorescence quantum yields were estimated by the comparison of the fluorescence intensity with standard dye solutions with the same optical intensity at the excitation wavelength and similar fluorescence wavelength.27 SEM images were obtained by using a LEO (Zeiss) 1540XB FIB/SEM instrument with a field emission gun of 3 kV. For elemental analysis, the same SEM instrument was used, but with a field emission gun operated at 10 kV. Low-resolution TEM images were obtained from a Philips CM10 transmission microscope with an acceleration voltage of 100 kV. Carbon-coated copper grids were dipped in isopropanol to deposit nanoparticles onto the film. HR-TEM images were taken on a JEOL 2010F TEM/STEM instrument with an acceleration voltage of 200 kV. Selected area diffraction (SAD) patterns were also obtained along with HR-TEM measurements. From each sample, 50−100 particles were chosen, and an average diameter of particles was determined. TGA analysis was performed on a TA SDT Q600 device. The heating rate was controlled at a rate of 10 °C/min between 20 and 600 °C under a nitrogen flow.
precursor under modest temperatures and increased pressures, yielding well-defined CuInE2 nanoparticles.
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EXPERIMENTAL SECTION
Materials. Sodium methoxide, sodium metal, thiophenol, tertbutylthiol, ethanethiol, diphenyl diselenide, indium(III) chloride, copper(I) chloride, naphthalene, and sulfur powder were purchased from Aldrich or VWR and used as received. Triphenylphosphine was purchased from Sigma and vacuum-dried for 4 h at room temperature, then stored under an inert atmosphere. Solvents such as tetrahydrofuran (THF) were dried by passage through packed columns of activated alumina using a commercially available MBraun MB-SP Series solvent purification system. Methanol and CH2Cl2 were distilled and dried over CaH2 and P2O5, respectively. The solvent for 1 H, 13C, and 31P NMR spectroscopy was chloroform-d, which was purchased from Aldrich and distilled over P2O5, while DMSO-d6 was purchased from Aldrich and used directly without further purification. All synthetic and handling procedures were carried out under an atmosphere of high purity dried nitrogen using standard double manifold Schlenk line techniques and an MBraun Labmaster 130 glovebox. For the solvothermal synthesis, stainless steel autoclaves and disposable glass bottles (30 mL) with Teflon caps were used. Preparation of Precursors. [(Ph3P)2CuIn(SEt)4] 1. 1 was synthesized by slight modification of the published procedures.26 NaSEt was prepared by adding ethanethiol (0.32 mL, 4.33 mmol) into a methanol solution (15 mL) of NaOCH3 (0.242 g, 4.48 mmol) and stirring for 30 min. InCl3 (0.230 g, 1.04 mmol) dissolved in 5 mL of methanol was added, stirring for another 1.5 h to form Na[In(SEt)4]. PPh3 (0.574 g, 2.19 mmol) and CuCl (0.114 g, 1.15 mmol) were then combined in 15 mL of CH2Cl2, and the mixture was added dropwise to Na[In(SPh)4]. The reaction mixture was stirred overnight, affording a slight yellow solution with a white precipitate. The solvent volume was reduced under vacuum to half, and 20 mL of methanol was added to fully precipitate the product rather than NaCl (byproduct). The white precipitate was collected and redissolved in 20 mL of CH2Cl2 and filtered through Celite to remove residual NaCl and washed with methanol twice. Then, the solid was dried for 2 h under vacuum, affording 1 as a white solid. Yield, 0.63g, 66%; 1H NMR (400 MHz; CDCl3, 23 °C); δ 1.24 (t, J = 7.4 Hz, 12H, CH3); δ 2.67 (q, J = 7.4 Hz, 8H, SCH2); δ 7.33 (m, 30H, P(C6H5)3); 31P NMR CDCl3; δ −4.4. Data match those from a previously published report.26 [(Ph3P)2CuIn(SePh)4] 2. 2 was prepared following a similar procedure as for the synthesis of 1. Na[SePh] (0.710 g, 3.96 mmol) was dissolved in 15 mL of methanol. After stirring for 30 min, InCl3 (0.230 g, 1.04 mmol) dissolved in 5 mL of methanol was added, and reacted for an additional 1.5 h to form Na[In(SePh)4]. PPh3 (0.574 g, 2.19 mmol) and CuCl (0.114 g, 1.15 mmol) were then combined in 15 mL of CH2Cl2, and the mixture was added dropwise to Na[In(SePh)4]. The reaction mixture was kept stirring overnight, affording a light yellow solution with a white precipitate. The white precipitate was collected by filtration and dried for 2 h, affording 2 as a white solid. Yield, 1.06g, 81%; 1H NMR (400 MHz; CDCl3, 23 °C): δ 6.85 (s, 8H); δ 7.00 (d, 4H); δ 7.23 (br d, 10H); δ 7.35 (m, 8H). The data are consistent with reported values.28 Synthesis of Nanoparticles. [(CuInS 2 ) x ] Nanoparticles. [(CuInS2)x] nanoparticles were prepared in a sealed reactor at a controlled temperature and increased pressure using polar organic solvents (solvothermal conditions). A typical procedure for the preparation of [(CuInS2)x] nanoparticles was as follows: An appropriate amount of 1 (0.97 g, 1 mmol) was combined with THF (15 mL) in a 30 mL glass bottle with a Teflon cap. An additional source of sulfur, S8, (0.125−0.25 mmol) was also added and mixed thoroughly for ∼15 min until a clear solution was obtained. The bottle was capped, placed in a stainless steel autoclave, and heated in an oven for a specific period of time at a set temperature. The autoclave was subsequently cooled to room temperature naturally, and precipitates with tunable colors were obtained. With a set reaction time of 24 h, the color of the precipitates generated changed from yellow (1A), red (1B) to brown (1C) to black (1D) at reaction temperatures of 140 to
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RESULTS AND DISCUSSION The conversion of molecular metal-chalcogenolate complexes and small clusters has proven to be a highly effective method for the formation of metal-chalcogenide extended solids,28−30 thin films,31 as well as nanoscale materials.26,32,33 This includes works by Williams et al.26 (photolysis) and Castro et al.32 and B
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Figure 1. PXRD patterns of (a) [(CuInS2)x]: 1A, 1B, 1C, and 1D, the products of precursor 1 prepared at 140, 160, 180, and 200 °C, respectively. ICCD 01-77-9459 (bottom) and ICDD 04-013-0169 (top) show the standard diffraction patterns of wurtzite and tetragonal forms of CuInS2, respectively. (b) [(CuInSe2)x]: 2A, 2B, 2C, and 2D, the products of precursor 2 prepared at 100, 120, 140, and 160 °C, respectively. ICCD 01-785190 (bottom) and ICD 01-070-3084 (top) show the standard diffraction patterns of the wurtzite and tetragonal forms of CuInSe2, respectively.
Figure 2. Absorption spectra of (a) [(CuInS2)x] products: 1A, 1B, and 1C, the products of precursor 1 by suspending in dry THF. (b) [(CuInSe2)x] products: 2A, 2B, 2C, and 2D, the products of precursor 2 by suspending in dry THF. Calculated band gap curves: (c) from [(CuInS2)x] absorption spectra in (a); (d) [(CuInSe2)x] absorption spectra in (b).
Banger et al.28 (thermolysis), who have demonstrated that the mixed-metal [(Ph3P)2CuIn(ER)4] (E = S, Se) can be used as molecular precursors to the related ternary nanomaterials CuInE2, taking advantage of the well-defined Cu:In:S ratio. Indeed, Castro and co-workers investigated in detail their decomposition to tetragonal CuInS2 in noncoordinating, high boiling solvents where size selective preparation was achieved. We have recently developed a procedure for the preparation of monodisperse CdS nanoparticles via the straightforward
solvothermal conversion of the mononuclear precursor [Me4N]2[Cd(SPh)4],34 building on previous, related reports using this method.35 The simplicity and fine control of particle size offered with CdS via this method prompted us to investigate its utility for the preparation of nanoscopic CuInE2 from [(Ph3P)2CuIn(SEt)4] 1 and [(Ph3P)2CuIn(SePh)4] 2. In this vein, THF solutions of 1 and 2 were heated at varying temperatures (1: 140−200 °C together with S8; 2: 100−200 °C), which resulted in the formation of sizeC
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[(CuInS2)x] and [(CuInSe2)x] obtained from the absorption data, respectively. The band gap energies obtained from the double derivative of the absorption profiles were used to calculate particle sizes. These illustrate that the band gap decreased from 2.7 eV (1A) to 2.6 eV (1C) for nanoparticles of [(CuInS2)x]. For [(CuInSe2)x], these values were varied from 2.5 eV (2A) to 2.3 eV (2D). Compared to the band gap energy of the bulk materials (Egap(CuInS2) = 1.48 eV; Egap(CuInSe2) = 1.04 eV), the significant shifts are consistent with the manipulation of particle size. From the Brus equation,16 estimates of the particle size can be obtained for these samples: 2.8 nm (1A), 3.0 nm (1B), 3.4 nm (1C), and 3.1 nm (2A), 3.8 nm (2B), 4.0 nm (2C), and 4.1 nm (2D), respectively (listed in Table 1), each smaller than the Bohr radius of CuInS2 and CuInSe2.40,17
controlled [CuInE2] nanoparticles, isolated as highly colored solids (Table S1). The addition of sulfur for the preparation of [CuInS2] allows the reactions to be completed at temperatures low enough that particle size control is possible. Christou used this strategy in the formation of FeS clusters from ironphenylthiolate precursors: the introduction of S8 leads to the low temperature formation and elimination of PhSSPh and the concomitant formation of sulfide ligands.36 The lower Se−C bond dissociation energy (versus S−C)37 is such that the addition of elemental selenium is not required for the formation of nanoscopic [CuInSe2] under reaction conditions used.
The PXRD patterns of the different [(CuInS2)x] and [(CuInSe2)x] particles prepared are plotted in Figure 1 and compared to the standard CuInS2/CuInSe2 structures; both samples display patterns consistent with the wurtzite form of CuInE2. Materials prepared at higher temperatures display sharper reflections with a good match to the reference patterns. Expectedly, samples obtained at lower temperatures display broadened reflections consistent with smaller particle sizes (vide inf ra). When reaction temperatures are 200 °C for 1 and 160 °C for 2, all reflections are well-resolved. This transition is also consistent with the change in color observed for both sets of products (Table S1). Although we also completed similar investigations by using other −SR precursors such as [(Ph 3 P) 2 CuIn(SPh) 4 ] and [(Ph 3 P) 2 CuIn(S t Bu) 4 ] 28 for [(CuInS2)x] preparation, limited particle size control under similar conditions resulted in our focusing exclusively on the ethylthiolate containing precursor 1. The UV−vis absorption spectra (Figure 2) of samples in suspensions as well as the diffuse reflectance spectra of solid samples (Supporting Information, Figure S1) for all samples were acquired. Spectra for both sets of nanomaterials display a broad, yet resolved, peak at different wavelengths for each set, with the exception of the [CuInS2] material prepared at 200 °C (1D) for which no discrete maximum was observed. For [(CuInS2)x] samples 1A−1C, the absorption maximum (Figure 2a) shifted from 457 nm (1A) to 485 nm (1B; see also Figure S2) when increasing the reaction temperature from 140 to 180 °C. This trend is consistent with the phenomenon of a red shift in the excitonic absorption band with increasing particle size.33 Additionally, it is worth noting that the mother liquor from these reactions also displayed a similar color as the corresponding precipitate; at higher reaction temperatures, reaction solutions were colorless. This would be consistent with the idea that relatively small particles can be dissolved in the polar organic solvent, whereas larger particles aggregate and precipitate. The absorption spectra of the selenide samples [(CuInSe2)x] 2A−2D also display a red shift with increased preparation temperature: as shown in Figure 2b, the absorption maximum shifted from 493 to 540 nm. Diffuse reflectance spectra exhibited similar red shifts for both materials with increasing reaction temperatures. Generally, the reflectance percentage (R) was collected, but via the Kubelka−Munk equation: A = −log (R), the absorption spectrum is plotted. All spectra are shown in Figure S1. Together with the absorption spectra, an approximation of the band gap energy calculated from a Tauc Plot of the data is illustrated: 38,39 Figure 2c,d displays these curves for
Table 1. Selected Data for CuInS2 (1A−1C) and CuInSe2 (2A, 2B, 2D) Nanoparticles Prepared at Different Temperatures (T) 1
2
sample
T (°C)
1A 1B 1C 2A 2B 2D
140 160 180 100 120 160
d (nm)a
d (nm)b
Cu:In:S
± ± ± ± ± ±
2.8 3.0 3.4 3.1 4.0 4.2
1.0:0.8:1.2 1.0:0.9:2.4 1.0:1.1:2.0 1.0:1.1:2.0 1.0:1.2:2.0 1.0:1.1:1.8
18 26 28 34 45 60
2 4 5 8 3 11
a Diameters of aggregated particles, as determined by SEM. bDiameters calculated from optical absorption data (double derivative).
Electron microscopy measurements were obtained together with EDX analyses to explore the size, morphology, and composition of the nanoparticles (Figures S1and S2). As shown in Table 1, EDX analysis reveals that Cu:In ratios of both [(CuInS2)x] and [(CuInSe2)x] products are close to an expected, ideal ratio of 1.0. For [(CuInS2)x] samples 1A and 1B, a Cu:S ratio of 1:2.5 is observed. This is consistent with the formation of “CuInS2−X(SEt)2X” at the lower reaction temperatures, where a fraction of the organic sulfur (thiolate) components are still retained on the surfaces of smaller particles. This is supported by thermogravimetric analysis of these samples, where a weight loss of 24.1% and 20.6% is observed upon subsequent heating for 1A and 1B, respectively (Figure S4). The calculated percentages of residue for samples 1A and 1B are 75.4% and 78.5%, respectively. For 1C and all [(CuInSe2)x] samples, particles show ∼1:1 Cu:In and ∼1:2 Cu:E ratios. In the SEM, samples are imaged as spherical particles with a size range of ∼10−60 nm. However, at higher resolution, it is observed clearly that much smaller nanoparticles (2−5 nm) are the components that make up the larger species, even though they are highly aggregated (Figure 3). Observed particle sizes for the system [CuInS2], 1A (2.4 ± 0.5 nm), 1B (3.3 ± 0.3 nm), and 1C (4.4 ± 0.3 nm), are all in reasonable agreement with sizes calculated from spectroscopic features (vide supra). We note, however, that the aggregated nature of the sample renders the statistical analysis more difficult (50−100 particles were measured, per sample) and that, overall, the system is somewhat polydisperse, consistent with the broad absorption profiles in the UV−vis spectra. HR-TEM data illustrate clearly lattice fringes with spacings of 3.4 Å, which is consistent with the distance between the (100) planes of the standard wurtzite CuInS2 (ICCD 01-77-9459). Also, for 1C, a secondary lattice D
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Figure 3. HR-TEM images and FFT images for samples 1A−1C. Insets: (a) magnified view of highlighted area R1; (b) FFT of area R1; (c) SAD of 0.25 μm2 area. The figure in the bottom right is a TEM image of the small amount of “single crystal” CuInS2 present in samples of 1C (see text for details; inset (a): magnified view; inset (b): FFT image).
images, the amount of this component is less than 5% of the overall sample. Individual nanoparticles of [(CuInSe2)x] were clearly observed in TEM images for the samples 2A, 2B, and 2D selected for additional analysis via electron microscopy (Figure S5). The diameters of the particles, as determined from the TEM images for 2A, 2B, and 2D, are 2.4 ± 0.4 nm, 3.4 ± 0.2 nm, and 3.9 ± 0.2 nm, respectively. These are in reasonable agreement with values calculated from band gap energy, as determined from absorption spectra, although we note again that the aggregated nature of the sample renders the statistical analysis more difficult (50−100 particles were measured, per sample). A HR-TEM image for all samples (inset b) is also illustrated. Sample 2A displays clear lattice fringes with a dspacing of 3.5 Å, which corresponds to the distance of the (100) plane of the standard wurtzite CuInSe2 (ICCD 01-0785190). At the same time, the spaces between lattice fringes of both samples 2B and 3D are 3.3 Å, which is consistent with the distances of the (002) plane. The fast Fourier transition (FFT) images of each sample are also illustrated.
facet was observed due to the increased crystallinity and size of these samples: a spacing of 3.0 Å together with an angle of 69° from the 100 plane is assigned to the (101) planes. The fast Fourier transition (FFT) image (inset b, Figure 3) illustrates a highly crystalline material with resolved reflections that correspond to related planes labeled in the images. From the FFT images, it is also obvious that samples prepared at higher temperatures resulted in the resolution of more reflections. SAED patterns were also measured to obtain an overview within a relatively large area for samples 1A and 1B displaying but faint rings (inset c, Figure 3). This is a sign that the nanoparticles are not arranged into a higher order structure, as was observed for CdS (superlattice).35 When the data are compared to those of the standard wurtzite CuInS2 lattice (ICCD 01-77-9459), all rings can be indexed. Interestingly, a small amount of a second, much larger crystalline sample of CuInS2 was also observed in 1C, as shown in Figure 3. The FFT image inset provides evidence of a highly crystalline material with strong reflections; all reflections are assigned to all the main planes of wurtzite CuInS2 including the (100), (002), (101), (102), and (110) planes. From the observed TEM E
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Figure 4. Top: emission spectra of samples prepared at different temperatures (24 h reaction time): (a) [(CuInS2)x]: 1A, 1B, and 1C; (b) [(CuInSe2)x]: 2A, 2B, 2C, and 2D, by suspending in dry THF. Bottom: emission spectra of samples prepared at different reaction times: (c) [(CuInS2)x] 160 °C for 12 h, 24 h, 36 h; (d) [(CuInSe2)x] 120 °C for 12 h, 24 h, 36 h, 48 h.
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Emission Spectra and Quantum Yield. Two sets of emission spectra were obtained to investigate the influence of reaction temperatures and reactions times on quantum yields. The shifts observed in the emission maxima (Figure 4) are comparable to the trend observed for the absorption spectra. For [(CuInS2)x], the wavelength of the emission maximum shifted from 650 nm (sample 1A, 140 °C) to 740 nm (sample 1C, 180 °C), consistent with the red shift observed in the absorption spectra. Both samples were prepared with a reaction time of 24 h. A similar trend was observed in [(CuInSe2)x] when varying reaction temperatures from 100 to 160 °C (Figure 4b). Here, a red shift of ∼135 nm is observed for [(CuInSe2)x] with increasing reaction temperatures/particle size. The emission maximum shifts from 690 nm (sample 2A) to 825 nm (sample 2D). All peaks are rather broad (W1/2 ≈ 160−200 nm) and red-shifted from the absorption maxima. This can be quite common for semiconductor nanomaterials, especially for I−III−VI systems. According to previous studies, the broadened emission spectra are not necessarily/simply related to size inhomogeneity but to a distribution of surface (defect) states.12 The “tunability” of emission energies has also led to the suggestion that they involve transitions from a donor state to quantized-valence band, from quantized-conduction band to acceptor-level state, or from the inherent band structure. 41−43 Longer reaction times for samples 1B ([(CuInS2)x]; 160 °C) and 2B ([CuInSe2]x; 120 °C) resulted in slight increases in emission intensity, together with a modest red shift (Figure 4c,d). Quantum yields improved from 6.4% (sample 1B-12h) to 11.5% (sample 1B-36h). For [(CuInSe2)x] 2B, the quantum yield reached 19.0% when increasing the reaction time to 48 h.
CONCLUSIONS A low temperature synthetic route that is facile and efficient was developed for the preparation of wurtzite based ternary semiconductor nanoparticles with controllable sizes. By using solvothermal methods and simple precursors, two series of ternary chalcogenide nanoparticles (CuInS2 and CuInSe2) were prepared. It was found that the choice of mixed-metal precursor was crucial to control the size of nanoparticles synthesized. Because of the suitable S/Se−C bond enthalpy, [(Ph3P)2CuIn(SEt)4] 1 and [(Ph3P)2CuIn(SePh)4] 2 offered the best control in size of selectivity versus other thiolate/selenolate complexes. Both series of nanoparticles were isolated with the wurtzite structure; control of nanoparticle size was possible with modulation of reaction temperatures and, to a lesser extent, reaction times.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02177. Table of details of solvothermal synthesis, absorption spectra, SEM images, EDX analysis, TEM images, TGA analysis, and HR-TEM and FFT images (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Y.H.). *E-mail: [email protected] (J.F.C.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. F
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Inorganic Chemistry Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful for the financial support of the Natural Sciences and Engineering Research Council (Canada) Discovery Grants and Equipment Grants Programs (J.F.C. and Y.H.). The authors gratefully acknowledge the Nanofabrication Facility at Western University for SEM imaging. They would like to express their gratitude to Dr. Mathew Willans (Western) for his assistance with the NMR instruments, to Dr. Tim Goldhawk (Western) for his help with the SEM measurements, and Dr. Richard Gardiner and Ms. Karen Nygard (Western) for their assistance with the TEM measurements in the Biotron facility. Thanks are also extended to Dr. Carmen M. Andrei (McMaster University) for insightful input with the HR-TEM analysis. Finally, we thank the anonymous reviewers for helpful suggestions in improving the manuscript.
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