Article pubs.acs.org/cm
Spinel Indium Sulfide Precursor for the Phase-Selective Synthesis of Cu−In−S Nanocrystals with Zinc-Blende, Wurtzite, and Spinel Structures Shuijin Lei,*,†,‡ Chunying Wang,†,‡ Lei Liu,† Donghai Guo,† Chuanning Wang,† Qingliu Tang,† Baochang Cheng,†,‡ Yanhe Xiao,†,‡ and Lang Zhou†,‡ †
School of Materials Science and Engineering and ‡School of Photovoltaic Engineering, Nanchang University, Nanchang, Jiangxi 330031, People’s Republic of China S Supporting Information *
ABSTRACT: Group I−III−VI ternary chalcogenides have attracted extensive attention as important functional semiconductors. Among them, Cu−In−S compounds have seen strong research interest due to their potential applications in high-efficiency solar cells. However, the controllable synthesis of Cu−In−S nanostructures with different phases is always difficult. In this research, zincblende CuInS2, wurtzite CuInS2, and spinel CuIn5S8 could be selectively synthesized using spinel In3−xS4 as the precursor by a simple solvothermal method. X-ray powder diffraction was used to determine the phase and crystal structure, and transmission electron microscopy was employed to characterize the morphologies of the as-prepared samples. Experiments showed that the acidity−basicity of the reaction system and the coordination and reducibility of the capping ligands were crucial to the final phases of the products. The UV−vis−NIR spectra of the three phases all exhibited a broad-band absorption over the entire visible light and extending into the near-infrared region, and the zinc-blende, wurtzite, and spinel Cu−In−S nanocrystals showed band gaps of 1.55, 1.54, and 1.51 eV, respectively, which indicates their potential applications in thin-film solar cells. KEYWORDS: Cu−In−S, nanocrystals, zinc-blende, wurtzite, spinel
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INTRODUCTION With declining fossil reserves and increasing greenhouse gas emissions, interest in photovoltaics has grown rapidly during the past few decades, which continues to drive the development of more efficient, less costly, and more stable photovoltaic devices.1 As is well-known, the performance of photovoltaic solar cells is intimately related to the properties of the materials. In recent years, inorganic semiconductor nanocrystals (NCs) have been considerably investigated because of their promising applications in solar energy conversion due to their low cost and easy scalability, as well as the convenience of constructing photovoltaic devices through a solution-phase process such as spin-casting, dip-coating, and printing.2 In addition, the photoelectric properties of semiconductor nanocrystals have proved to be strongly size-dependent on the basis of the quantum confinement effect.3 Multinary chalcogenide semiconductors, especially the I− III−VI family, have emerged as a class of very encouraging light-absorbing materials in photoelectric devices because of their versatile optical and electrical characteristics.4 Recently, the efficiency of Cu(In,Ga)Se2 solar cells has reached 20.3% under laboratory conditions.5 Normally, the I−III−VI family compounds can be expressed as a general formula, I−III2n+1−VI3n+2, while I−III−VI2 and I−III5−VI8 are the two most familiar formats. Among them, Cu−In−S ternary © 2013 American Chemical Society
semiconductors have been extensively explored due to their desirable band gaps which match well with the solar spectrum, high optical absorption coefficient, good photostability, and strong radioresistance and are considered as promising candidates in photovoltaic and photoelectrochemical cells.6−8 They also represent some extraordinary advantages, including the absence of toxic constituents (such as Cd and Se), a more optimum band gap being dispensed with the doping of a rare element (such as Ga and Tl), and lower cost production.9−11 CuInS2, one of the most important chalcogenides, exists in three polymorphs at different temperatures: a stable phase with a tetragonal chalcopyrite structure (from room temperature to 1253 K) and metastable phases with a cubic zinc-blende structure (between 1253 and 1318 K) and a hexagonal wurtzite structure (from 1318 K to the melting point).12 In the bulk, chalcopyrite is the most common phase and is thermodynamically favored at room temperature, the Cu and In atoms being ordered in the cation sublattice sites. In contrast, the zincblende and wurtzite modifications are stable only at high temperatures, the Cu and In atoms being randomly distributed over the cation sites of the lattice.13 As a result, compared with Received: March 14, 2013 Revised: July 3, 2013 Published: July 8, 2013 2991
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grade), sodium diethyldithiocarbamate (NaS2CN(CH3CH2)2·3H2O; analytical grade), oleic acid (C17H33COOH; chemical grade), and 1dodecanethiol (CH3(CH2)11SH; chemical grade) were purchased from Shanghai Chemical Reagent Co., Ltd. Hydrated cupric chloride (CuCl2·2H2O; analytical grade), thiourea (NH2CSNH2; analytical grade), benzene (C6H6; analytical grade), n-hexane (CH3(CH2)4CH3; analytical grade), absolute ethanol (CH3CH2OH; analytical grade), and liquid paraffin (chemical grade) were purchased from Tianjin Damao Chemical Reagent Factory. Oleylamine (C18H35NH2; 80− 90%) was purchased from Shanghai Aladdin Chemical Reagent Co., Ltd. Synthesis. Preparation of the Spinel In3−xS4 Precursor. The method for the preparation of the spinel In3−xS4 precursor was based on our previous report with minor modification.48 Briefly, 2 mmol (0.586 g) of InCl3·4H2O and slightly excessive thiourea (0.25 g) were put into a Teflon-lined autoclave and dissolved in 2−3 mL of distilled water to form the clear solution. Then the autoclave was filled with benzene to 80−90% of the total volume, sealed, and maintained at 160 °C for 12 h. After the solution was cooled to room temperature naturally, the precipitates were filtered off, washed with absolute ethanol and deionized water several times, and then dried at 60 °C for 4 h in air. An orange-red powder was obtained. Following intensive grinding in an agate mortar, the final powder was collected for further use. Preparation of Cu(dedc)2 Precursor. For the preparation of Cu(dedc)2, 10 mmol (2.253 g) of sodium diethyldithiocarbamate (dedc) (Na(dedc)) was dissolved in 50 mL of water and 5 mmol (0.852 g) of CuCl2·2H2O was dissolved in 20 mL of water to form two clear solutions. Subsequently, the CuCl2 aqueous solution was added dropwise to the Na(dedc) aqueous solution under magnetic stirring. Brown-black precipitates were formed immediately as the two solutions were mixed. After all of the CuCl2 solution was added , an additional 30 min of stirring was needed to complete the reaction. The whole process was performed at room temperature. The precipitates were then filtered off, washed with absolute ethanol and deionized water several times, and then dried at 60 °C for a few hours in air. The final powder was ground and collected for further use. Synthesis of Cu−In−S Nanocrystals with Various Structures. In a typical synthesis, 1 mmol of Cu(dedc)2 (or Cu(CH3COO)2·H2O) was added to the mixed solvent, including 10 mL of 1-dodecanethiol (DDT) and 30 mL of liquid paraffin. For comparison of the selective syntheses, the ligand DDT was replaced by 10 mL of oleylamine (OLA), 10 mL of oleic acid (OA), or a mixture of 5 mL of OLA and 5 mL of OA. The copper precursor was then homogeneously dispersed in the solvent under ultrasonic agitation. After the following introduction of 0.5 mmol of In3−xS4 precursor, further ultrasonic treatment was performed to improve the homogeneous mixing and dispersion of the mixture on a microscale. Subsequently, the resulting solution was transferred into a stainless steel Teflon-lined autoclave of 60 mL capacity, which was then placed in an oven equipped with an electronic program control system. The reaction temperature was first increased to 100 °C and maintained for 2 h to facilitate the completed dissolution of Cu(dedc)2 or Cu(CH3COO)2. The temperature was then enhanced to 180 °C and maintained for 12 h. After the reaction was finished, the mixture was cooled to room temperature naturally and precipitated by absolute ethanol. The obtained precipitates were centrifuged at 8000 rpm for 3 min, while the supernatant liquid was decanted, and the isolated solid could be easily redispersed in hexane or benzene. To remove the residual organic contaminants, the precipitates were alternately redispersed in hexane and absolute ethanol and centrifuged at 8000 rpm for 10 min. The above centrifugation and isolation procedure was then repeated several times for purification. Finally, the products were redispersed in hexane for reservation or dried under vacuum for further analyses. Characterization. The X-ray powder diffraction (XRD) patterns were recorded on a D8 Focus diffractometer with Cu Kα radiation (λ = 1.5406 Å) (Bruker, Germany). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) measurements were performed using a JEM-2010
the chalcopyrite structure, the metastable modifications exhibit more flexibility of stoichiometry and then a wider range for tuning the Fermi energy, which is an essential requirement for nanodevice fabrication.14 The disordered structure and the potential application in solar cells are of great significance in exploring the controllable synthesis of metastable CuInS2 nanocrystals stably at room temperature by a simple and lowcost method. Since Pan et al.15 reported the hot-injection synthesis of CuInS2 nanocrystals in zinc-blende and wurtzite phases for the first time, a number of studies have been concerned about the synthesis of metastable CuInS2 nanocrystals.16−19 In comparison with zinc-blende CuInS2, wurtzite CuInS2 has attracted relatively broader research by various methods, including solvothermal synthesis,13,20 hot injection,21 thermal precipitation under an inert gas22−25 or atmospheric26−29 conditions, thermal decomposition of the precursor,14,30−33 and so on. It should be noted that CuInS2 can display n-type (Cu/In < 1) and p-type (Cu/In > 1) semiconductor properties depending on the Cu/In ratio. However, most synthesized CuInS2 reported in the literature behaved with p-type semiconductivity. Up to now, n-type CuInS2 chalcopyrite has been relatively difficult to grow, because control of the stoichiometry is still a major difficulty in the synthesis of multinary materials, despite the fact that several groups reported the synthesis of n-type CuInS2 by colloidal methods.34−37 Nevertheless, the other representative phase, CuIn5S8, as an In-rich compound, is typically an n-type semiconductor with direct transition and has a band gap (∼1.5 eV) similar to that of CuInS2. It crystallizes in the cubic spinel structure with the space group Fd3̅m.38 As reported, the efficiency of Cu−In−S solar cells based on p-type chalcopyrite CuInS2 and n-type spinel CuIn5S8 phases, respectively, with a p-type CuI buffer layer was up to 9.1%.39 Actually, CuIn5S8 has been of great technological and scientific interest because of its unique electrophysical property and potential applications in semiconductor devices and optical electronics, such as photovoltaic solar cells, photochemical cells, light-emitting diodes, nonlinear optical devices, and so on.40 However, almost all of the reported methods for the synthesis of CuIn5S8 at present are mainly focused on its thin films, such as thermal evaporation,41−43 sequential deposition,44,45 the modified Bridgman method,46 and metal organic chemical vapor deposition,47 whereas there are few literature reports on the preparation of CuIn5S8 nanocrystals. Our group has successfully prepared CuIn5S8 and AgIn5S8 porous microspheres via a solvothermal route.48 Therefore, the synthesis of n-type CuIn5S8 nanocrystals is still a great challenge. The phase-controllable synthesis of Cu−In−S nanocrystals is a persistent and active research area. However, to the best of our knowledge, this is the first time the selective fabrications of Cu−In−S nanocrystals with zinc-blende, wurtzite, and spinel structures have been realized simultaneously. Herein, we report on a facile synthesis of Cu−In−S nanocrystals using spinel indium sulfide as the precursor without the requirement of an anhydrous non-oxygen environment. The phase structure can be systematically tuned by varying the copper source and organic ligand.
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EXPERIMENTAL SECTION
Materials. All the chemicals were used as received without any further purification. Hydrated indium chloride (InCl3·4H2O; analytical grade), hydrated cupric acetate (Cu(CH3COO)2·H2O; analytical 2992
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appears in the zinc-blende phase.15,16,49 This should be indirect evidence for the zinc-blende phase of CuInS2. At this point, these diffraction peaks can be indexed to zinc-blende CuInS2 with a calculated lattice constant a = 5.530 Å, which is in good agreement with those in the previous literature.15−19 No characteristic peaks due to impurities such as binary sulfides and other phases of CuInS2 can be observed, which indicates that the level of impurity in the sample is lower than the resolution limit of the XRD instrument. The simulated XRD lines using the calculated lattice parameters based on the zincblende structure (space group F4̅3m (216)) give a good match to the experimental diffraction patterns, which further confirms the zinc-blende structure feature of the product. Figure 1b shows the TEM image of the as-obtained zinc-blende CuInS2 sample. Large-scale nanocrystals with a relatively uniform particle size of 5−10 nm can be observed. It is interesting that, as the copper source was changed to Cu(dedc)2, keeping the other conditions the same, another phase was obtained. The corresponding XRD patterns of this sample are shown in Figure 2a. It exhibits a representative
transmission electron microscope (JEOL, Japan) with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) measurements were preformed on a Quanta 200F field-emission environmental scanning electron microscope (FEI, The Netherlands). The room temperature micro-Raman measurement was performed by a LABRAM-HR confocal laser micro-Raman spectrometer (Jobin-Yvon, France) in the backscattering configuration excited with the 514.5 nm line of an Ar+ ion laser. The X-ray photoelectron spectroscopy (XPS) analyses were carried out using an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα monochromatic radiation at a constant pass energy of 1486.6 eV (Thermo-VG Scientific, United States). All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. The ultraviolet−visible−near-infrared (UV−vis−NIR) absorption spectra were obtained on a Lambda 750 UV−vis−NIR spectrophotometer (Perkin-Elmer, United States) at room temperature.
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RESULTS AND DISCUSSION As we know, there have been some reports on the controlled synthesis of zinc-blende and wurtzite CuInS2 phases previously. In this research, however, three phases of Cu−In−S nanocrystals, including zinc-blende, wurtzite, and spinel, could be selectively prepared on the basis of the reaction precursor of spinel indium sulfide. Figure 1a shows the XRD patterns of the
Figure 2. (a) Experimental XRD patterns and simulated XRD lines and (b) TEM image of the as-prepared wurtzite CuInS2 sample using Cu(dedc)2 as the copper source and DDT as the ligand.
Figure 1. (a) Experimental XRD patterns and simulated XRD lines and (b) TEM image of the as-prepared zinc-blende CuInS2 sample using Cu(Ac)2 as the copper source and DDT as the ligand.
hexagonal characteristic and should be tentatively indexed as a wurtzite structure. Then the lattice constants, a = 3.905 Å and c = 6.400 Å, can be obtained by calculation, which are very consistent with the reported values.15−33 Actually, in the 30− 40° range, only one diffraction peak at 38.6° is observed that belongs to neither chalcopyrite nor zinc-blende CuInS2, so this product should be identified as wurtzite structure CuInS2. To further verify the wurtzite phase, a simulated XRD profile is plotted as the reference, which matches well with the experimental XRD patterns. Therefore, it can be concluded that the as-prepared sample should be the wurtzite CuInS2 pure
sample prepared using Cu(Ac)2 as the copper source and DDT as the capping ligand. It reveals that the product possesses a zinc-blende structure rather than chalcopyrite, because the (101), (103), and (211) reflection peaks of chalcopyrite CuInS2 disappear. Although the difference in XRD patterns between the chalcopyrite and zinc-blende phases is very small, it is reported that the CuInS2 nanocrystals with the chalcopyrite phase usually lose the (200) diffraction peak, which always 2993
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presence of Cu, In, and S and give the Cu/In/S molar ratio as 1/4.98/8.57, which gives further evidence that the CuIn5S8 phase was successfully obtained. More details are discussed in the Supporting Information. The typical TEM image of this CuIn5S8 sample in Figure 3b reveals that the product is made up of a large quantity of platelike nanoparticles. It is evident that these nanocrystals have a larger diameter of about 20 nm compared to zinc-blende and wurtzite CuInS2 samples. The possible reason will be discussed later. As is seen from the TEM image, the platelike architectures represent the horizontal nanoparticles, while the rod-shaped architectures should be ascribed to the vertical nanoplates, which reveal a thickness of about 10 nm. Among these nanoparticles, many hexagonal-like structures can be observed, indicating a definite crystal orientation. The EDS spectrum taken from the platelike nanoparticles is displayed in Figure 3c, which shows the presence of Cu, In, S, Al, C, and Cr. The elements Al and C are derived from the aluminum grid and carbon film, respectively, while the element Cr should be originated from the TEM sample holder. The EDS spectrum shows that the Cu/In/S atomic ratio is 1/4.91/7.87, which is close to the stoichiometry of CuIn5S8. Therefore, it also confirms the formation of the CuIn5S8 phase. To further explore the structure of the as-obtained nanoparticles, HRTEM and SAED analyses were undertaken. As shown in Figure 3d, the HRTEM image shows clearly resolved two-dimensional (2D) atomic lattice fringes, suggesting good crystallinity of the nanoparticles. It is obvious that these lattice points are arranged in a hexagonal array. The interplanar spacings measured to be 0.61 and 0.53 nm correspond well to the separations between the {111} and {200} families of planes of cubic spinel CuIn5S8, respectively. Additionally, the three separation angles are 54.7°, 54.7°, and 70.5°, respectively. It is generally known that this image should be typically taken along the [110] zone axis. The corresponding SAED patterns recorded along the [110] zone axis (shown in the inset of Figure 3d) also can be indexed as the cubic spinel structure of the CuIn5S8 phase and support the single-crystalline nature of these platelike nanoparticles. The crystal data and crystal structure models of these three phases of Cu−In−S as well as the spinel In3−xS4 are available in the Supporting Information (Figure S4). It is generally known that the zinc-blende and wurtzite CuInS2 can be simply modeled by replacing Zn2+ sites using Cu+ and In3+ while maintaining S2− sites in zinc-blende and wurtzite ZnS, in which S atoms are cubic and hexagonal close-packed, respectively, while both Cu and In atoms co-occupy half of tetrahedral interstices with the same occupation position and occupation possibility (Figure S4a,b). In the case of the spinel In3−xS4 precursor, the XRD patterns and SEM images are displayed in Figures S5 and S6, respectively. As discussed in the Supporting Information, In3−xS4 can be normalized to the spinel formula (In1−x)t(In2)oS4, in which In ions occupy part of the tetrahedral sites and the full octahedral sites (Figure S4c). The spinel CuIn5S8, i.e., Cu1/2In5/2S4, can also be written by the general spinel formula (Cu1/2In1/2)t(In2)oS4, where one-fifth of the In atoms have tetrahedral coordination and four-fifths of the In atoms have octahedral coordination, while all the Cu atoms have tetrahedral coordination (Figure S4d). It should be mentioned that, when Cu(Ac)2 was used as the copper source and the mixture of OLA and OA continued to be the capping ligand, the zinc-blend phase of CuInS2 was formed again. In addition, to gain more insight into the impacts of the copper source and capping ligand on the phase structure of the
phase. The TEM image presented in Figure 2b implies that the product also consists of nanoparticles which have nearly the same size, in contrast to the zinc-blende CuInS2 sample. However, not only the copper source but also the capping ligand exerts an important effect on the crystalline structure of the product. Keeping Cu(dedc)2 as the copper source, when DDT was replaced by the mixture of OLA and OA, the phase of the product was changed again. As shown in Figure 3a, all
Figure 3. (a) Experimental XRD patterns and standard XRD lines, (b) TEM image, (c) EDS spectrum, and (d) HRTEM image of the asprepared spinel CuIn5S8 sample using Cu(dedc)2 as the copper source and the mixture of OLA and OA as the ligand. The inset of (b) is the close-up view of the horizontal hexagonal-like nanoparticle and the vertical nanoplate. The inset of (d) presents the corresponding SAED patterns.
the diffraction peaks can be indexed to the spinel structure of CuIn5S8 with cell parameter a = 10.684 Å, corresponding to the literature value (JCPDS card file no. 24-0361, a = 10.680 Å). In fact, from the standard XRD patterns, it can be perceived that the diffraction peaks of both CuIn5S8 and In3−xS4 present similar distribution features due to the same crystal structure. That is to say, the ternary Cu−In−S product maintained the crystal structure of the In3−xS4 precursor. In consideration of the similarity in the XRD patterns between CuIn5S8 and In3−xS4, a Rietveld refinement of the CuIn5S8 XRD data was conducted. To improve the XRD intensity, the sample should be annealed in a N2 atmosphere at 400 °C. The corresponding refinement profile and the detailed results are displayed in the Supporting Information (Figure S1 and Table S1). It can be seen that a good fitting was achieved between the experimental pattern and the calculated one. To further confirm the phase composition of the product, Raman spectroscopy and XPS analyses were performed. As shown in Figure S2 (Supporting Information), the Raman spectrum presents an obvious vibration band at 326 cm−1. This dominant Raman peak can be typically assigned to the F32g mode of spinel CuIn5S8.50 All other Raman vibration peaks in Figure S2 are also consistent with the reported values for CuIn5S8.50 The surface composition and electronic states of the sample were further determined by XPS analyses (Figure S3, Supporting Information). The XPS results indicate the 2994
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Table 1. Summary of the Synthesis Conditions and the Influence of the Acidic−Basic System on the Phase Control of the Cu− In−S Nanocrystalsa sample 1 2 3 4 5 6 7 8
Cu source and its acidity−basicity Cu(Ac)2 Cu(Ac)2 Cu(Ac)2 Cu(Ac)2 Cu(dedc)2 Cu(dedc)2 Cu(dedc)2 Cu(dedc)2
basic basic basic basic neutral neutral neutral neutral
ligand and its acidity−basicity DDT OLA + OA OLA OA OLA OA DDT OLA + OA
neutral neutral basic acidic basic acidic neutral neutral
system acidity−basicity
phase
size (nm)
basic basic basic acidic basic acidic neutral neutral
zinc-blende zinc-blende zinc-blende zinc-blende zinc-blende zinc-blende wurtzite spinel
5−10 5−10 5−10 5−10 ∼20 ∼20 5−10 ∼20
a
In all these synthesis experiments, spinel In3−xS4 precursor and liquid paraffin were used as the indium source and noncoordinating solvent, respectively.
be beneficial to preserve the spinel structure of the In3−xS4 precursor. Then, in this case, the spinel CuIn5S8 phase can be obtained (sample 8). Pan et al.15 fabricated the zinc-blende and wurtzite CuInS2 nanocrystals with a smaller size than those of the original CuS, Cu2S, and In2S3 nanocrystal precursors and proposed that the formation of CuInS2 nanocrystals is due to the dissolution of the precursors and their subsequent reactions at high temperatures, rather than due to the direct reactions of the original nanoparticles. However, this explanation would not be suitable for the obtained CuIn5S8 nanoparticles herein. The formation of the OA−OLA complex would certainly cause a decrease of the carboxylic acid group (−COOH) and the amino group (−NH2) in the system. As a result, not only the coordination capability but also the dissolubility of precursors in them would be decreased. Therefore, we would like to believe that the CuIn5S8 nanocrystals were initially formed by the gradual reaction of Cu(dedc)2 and In3−xS4 precursors, maintaining the spinel structure. At high temperature, with decomplexation of OA−OLA, the CuIn5S8 nanocrystals would then be stabilized by OA and OLA in small size. Meanwhile, it was found that the employment of the Cu(dedc)2 complex even caused a larger nanocrystal size (such as in samples 5, 6, and 8), which indirectly implied the decrease of the ligand coordination effect. However, this is exceptional for sample 7 (wurtzite CuInS2), probably because the reduction reaction enhanced the interaction between DDT and cations. The UV−vis−NIR absorption spectra of the as-synthesized Cu−In−S nanocrystals with zinc-blende (sample 1), wurtzite (sample 7), and spinel (sample 8) structures have been examined at room temperature as shown in Figure 4a. All the samples show a strong absorption over the whole visible light and into the near-infrared region of the spectrum, resulting in the black color of these materials. No pronounced excitonic peak but a broad absorption band with a long absorption tail up to 1300 nm can be observed. This broad and strong absorption feature indicates their potential application as absorbers for solar cells. All three optical absorption spectra present a slight slope with similar absorption band features. To measure the optical band gap (Eg) values of the as-prepared Cu−In−S nanocrystals, the plots of (αhυ)2 versus hυ, shown in Figure 4b, were obtained. It is generally acceptable to determine the direct band gap by extrapolating a straight line to the (αhυ)2 = 0 axis and intercepting the hυ axis to give it. In this way, the lowest excited states, thus the optical band gaps of the synthesized zinc-blende, wurtzite, and spinel Cu−In−S nanocrystals, are found to be about 1.55, 1.54, and 1.51 eV, respectively, which are consistent with the bulk energy band gaps of CuInS252 and CuIn5S8.53 Because of the small exciton Bohr radius in Cu−In−
products, OLA or OA was used alone as the ligand. Experiments showed that all four products still had the zincblende structure. All the XRD patterns of the contrast samples mentioned above are displayed in Figure S7 (Supporting Information), and the corresponding TEM images are shown in Figure S8 (Supporting Information). It can be seen that the XRD results of these five samples show a good peak matching with the simulated patterns of zinc-blende CuInS2. However, the TEM images suggest that the nanocrystals prepared using Cu(Ac)2 as the copper source have a smaller size (5−10 nm) while those prepared using Cu(dedc)2 as the copper source have a larger one (∼20 nm). The influence of the synthesis conditions on the phase and size of the Cu−In−S nanocrystals is summarized in Table 1. In general, these eight experiments were designed from four groups of capping ligands (DDT, OLA, OA, the mixture of OLA and OA) and two groups of copper sources (Cu(Ac)2, Cu(dedc)2). An interesting observation is that zinc-blende CuInS2 can be obtained in most cases (samples 1−6). Among them, when Cu(Ac)2 is used as the copper source, zinc-blende CuInS2 can be prepared for all four groups of ligands (samples 1−4). It is thus noteworthy that DDT is a soft base, while OLA and OA are hard bases. For this reason, it should be difficult to interpret the effect of the experimental parameters on the product phases merely by the familiar hard−soft acid−base model. Herein, the acidity−basicity of the reaction system is also noticed as an essential factor for the formation of the zincblende CuInS2 phase. First, note that OA is an organic acid and OLA is an organic base. When OA or OLA is used as the ligand alone, the acidic or basic system may be favorable for the zincblende structure (samples 3−6). Second, the mixture of OA and OLA can result in an acid−base complex (RCOO−− RNH3+) via charge transfer between the carboxylic acid group (−COOH) of OA and the amino group (−NH2) of OLA,51 which can be regarded as neutral. However, if Cu(Ac)2 is used as the copper source, a relatively strong basic environment also can be formed due to acetate ions (sample 2). Last, by analogy, the use of quasi-neutral DDT and basic Cu(Ac)2 should also be propitious to the zinc-blende CuInS2 (sample 1). As for the other copper source, the organometallic complex Cu(dedc)2 should be neutral too. On one hand, as demonstrated by Pan et al.,15 the formation of the intermediate Cu2S derived from the reduction of Cu(dedc)2 by DDT is crucial for the formation of wurtzite structure CuInS2 (sample 7). On the other hand, for the two neutral complexes of Cu(dedc)2 and OA−OLA (RCOO−−RNH3+), due to the existing coordination effect in them, it is proposed that the coordination capability of the ligands with the cations would be greatly decreased, which may 2995
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ASSOCIATED CONTENT
S Supporting Information *
XRD Rietveld refinement, Raman spectrum, and XPS spectra of the CuIn5S8 sample, crystal structures of spinel In3−xS4 and zinc-blende, wurtzite, and spinel Cu−In−S, XRD patterns and SEM images of the In3−xS4 precursor, and XRD patterns and TEM images of the Cu−In−S samples under various contrast conditions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (Grant 21001062), the Research Fund for the Doctoral Program of Higher Education of China (Grant 20093601120010), the Projects of the Jiangxi Science and Technology Pillar Program (Grant 2009BGA00600), and the Natural Science Foundation of Jiangxi Province (Grant 20132BAB216016) are gratefully acknowledged. Y.X. and B.C. thank the Foundation of Jiangxi Educational Committee (Grant GJJ10032) and the National Natural Science Foundation of China (Grants 51002073 and 51162023) for support.
Figure 4. (a) UV−vis−NIR absorption spectra and (b) plots of (αhυ)2 vs hυ of the as-prepared zinc-blende CuInS2, wurtzite CuInS2, and spinel CuIn5S8 nanocrystals at room temperature.
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REFERENCES
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S (∼4 nm for CuInS2), no significant blue shift due to the quantum size effect can be expected. However, this optimal band gap (∼1.5 eV) makes these nanocrystal the potential candidate absorber material for thin-film solar cells.
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CONCLUSION In summary, the semiconductor nanocrystals of metastable CuInS2 with zinc-blende and wurtzite structures and n-type CuIn5S8 with the spinel structure have been selectively synthesized by a facile solvothermal method on the basis of the spinel In3−xS4 precursor. Experiments proved that the acidic or basic conditions were critical for the formation of zincblende CuInS2, while the reducibility of DDT was pivotal for wurtzite CuInS2. The decrease of the ligand coordination ability due to the formation of the OA−OLA complex then played a key role in the synthesis of spinel CuIn5S8 and resulted in a larger nanocrystal size. The UV−vis−NIR spectra of the prepared zinc-blende, wurtzite, and spinel Cu−In−S nanocrystals exhibited a broad-band absorption in the entire visible region and showed band gaps of 1.55, 1.54, and 1.51 eV, respectively, which would make them particularly interesting for potential applications in photovoltaic devices. Further systematic research on the control of the phase, size, and shape of Cu−In−S nanocrystals by adjusting the more influential factors, such as the sulfur source, reaction temperature and time, nature of the solvents, concentration of the metal reactants, amount of capping ligands, and so on, is under way. In the long run, this approach is expected to be extendable to the controlled synthesis of M−In−S series multinary sulfide nanocrystals. 2996
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