J. Phys. Chem. C 2010, 114, 17293–17297
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A General Synthesis of Cu-In-S Based Multicomponent Solid-Solution Nanocrystals with Tunable Band Gap, Size, and Structure Xiaolei Wang,†,‡ Daocheng Pan,† Ding Weng,† Chen-Yian Low,† Lynn Rice,† Jinyu Han,*,‡ and Yunfeng Lu*,† Department of Chemical & Biomolecular Engineering, UniVersity of California, Los Angeles, California 90095, and School of Chemical Engineering & Technology, Tianjin UniVersity, Tianjin, 300072, People’s Republic of China ReceiVed: April 20, 2010; ReVised Manuscript ReceiVed: August 23, 2010
A general approach has been developed to synthesize high-quality Cu-In-S based multicomponent solidsolution nanocrystals (NCs) of Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z at relatively low temperature. This was achieved in a noncoordinating solvent system (toluene) by a simple solvothermal process using metal diethyldithiocarbamate complexes as the precursors. The composition, crystalline structure, size, and bang gap of the NCs could be readily tuned by the precursors used and synthesis conditions. This work provides useful understanding for the synthesis of solid-solution NCs that are of interest for photocatalyst, solar cell, and other applications. Introduction Semiconductor multicomponent nanocrystals (NCs) are of great interest for a wide spectrum of applications, such as photocatalysis,1-3 optoelectronic devices,4,5 and biomedical tags.6,7 To implement their use, a center theme is to synthesize NCs with controllable composition and size, which are key factors that govern their optoelectronic behavior. Recent advances in synthesis have led to the exploration of tuning optical properties in semiconductor solid solution by changing the constituent stoichiometries and structure. For homogeneous multicomponent quantum dots (QDs), both confinement potential and interfacial strain change with their composition; thus, band gap energy can be tuned even at constant QDs size. In addition, by simply tuning band gaps through adjusting QDs size,8,9 both extending the NCs emission range into parts of the spectrum and achieving a wide range of fluorescence colors can be easily realized. Recently, binary semiconductor NCs have been intensively studied both fundamentally and practically; however, multicomponent solid-solution NCs offer more promising applications, such as QD-based light-emitting diodes (LEDs), in-vivo imaging, and photovoltaics. To date, a great variety of multicomponent semiconductor NCs, such as ZnxCd1-xS10 (ZnxCd1-xSe11), CuGaxIn1-xS212 (CuGaxIn1-xSe213,14), (AgIn)xZn2(1-x)S2,15 ZnS-CuInS2-AgInS2,16 and ZnS-In2S3-Ag2S,3 have been intensively investigated. In spite of these efforts, however, precise synthetic control of multicomponent NCs remains challenging because of easy phase separation of the solid-solution constituents, which is mainly caused by reactivity differences of the precursors or by crystalline mismatching of solid-solution components. In general, to synthesize high-quality multicomponent solid-solution NCs, the reactivity of each precursor should be compatible,17 and a small lattice mismatch of each component is required.18,19 * To whom correspondence should be addressed. E-mail:
[email protected] (Y.L.);
[email protected] (J.H.). † University of California. ‡ Tianjin University.
Herein, we report a general approach toward the synthesis of Cu-In-S based multicomponent solid-solution NCs using a simple toluene-thermal process. This synthesis method developed early by O’Brien and colleagues7,20,21 utilizes metaldialkyl-dithiocarbamate complexes as the precursors. Decomposition and subsequent reactions of the precursors in toluene at elevated temperature lead to the formation of NCs with controlled composition, size, and bang gap. In this work, dialkyl dithiocarbamates of Zn, Cu, In, and Cd were used as the precursors to synthesize a series of Cu-In-S based multicomponent solid-solution NCs. The synthesis conditions were also investigated systematically. Moreover, besides the composition and size control, we also demonstrated herein the synthesis of solid-solution NCs with crystal structure control by the capping ligands used. Experimental Section CuCl2 · 2H2O, InCl3 · 4H2O, Cd(NO3)2 · 4H2O, oleic acid (90%), oleylamine (70%), 1-dodecanethiol (DDT, 98%), toluene (90%), sodium diethyl dithiocarbamate (dedc) (NaS2CNEt2) (98%), zinc diethyl dithiocarbamate (98%), and methanol (99.9%) were purchased from Aldrich. Cu(dedc)2 was synthesized by the following procedure: 9.01 g (40 mmol) of NaS2CNEt2 was dissolved in 300 mL water; a 100 mL aqueous solution with 20 mmol of CuCl2 was added drop by drop under magnetic stirring conditions. The black precipitate was washed three times with water and was dried in an oven overnight to remove water. In(dedc)3, Cd(dedc)2, and Ag(dedc) were made using the same procedure. The synthesis of zinc blende ZnCuInCdS4 NCs was conducted by a simple procedure: 9.1 mg (0.025 mmol) of Zn(dedc)2, 9.0 mg (0.025 mmol) of Cu(dedc)2, 14.0 mg (0.025 mmol) of In(dedc)3, 10.2 mg (0.025 mmol) of Cd(dedc)2, 1.0 mL of oleic acid, and 1.0 mL of oleylamine were dissolved to 12 mL toluene under ultrasonic. Then, the transparent solution was put into an autoclave of 20 mL capacity. Thereafter sealed, the autoclave was kept at 180 °C for 90 min and then was cooled to room temperature. The crude solution was precipitated with 30 mL methanol and was further isolated by centrifugation and decantation. The purified NCs were redispersed in toluene for
10.1021/jp103572g 2010 American Chemical Society Published on Web 09/17/2010
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Figure 1. UV/vis absorption spectra and photographs (inset) of the NCs of (A) ZnS, CuInS2, and CdS; (B) Zn2x(CuIn)1-xS2; (C) (CuIn)1-xCd2xS2; and (D) (ZnS)x(CuInS2)y(CdS)z in toluene.
transmission electron microscopy (TEM, FEI Company, CM120 with accelerating voltage of 120 kV), X-ray diffraction (XRD, Panalytical X’Pert Pro X-ray diffractometer), ultravioletvisible (UV-vis) spectrum (Thermo Scientific, Genesys 6), and energy-disperse X-ray spectroscopy (EDS, JEOL JSM-6700F) measurements without any size sorting. Other solid-solution NCs were synthesized using a similar approach while adjusting the stoichiometric ratios of the precursors (Zn(dedc)2, Cu(dedc)2, In(dedc)3, and Cd(dedc)2). Wurtzite (ZnS)x(CuInS2)y(CdS)z NCs were obtained by using 1-dodecanethiol as a capping agent instead of oleic acid. Results and Discussion Figure 1 shows UV-vis absorption spectra of the NCs of ZnS, CuInS2, and CdS and the spectra of the solid-solution NCs of Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z. The band-edge absorptions of Zn2x(CuIn)1-xS2 and (CuIn)1-xCd2xS2 NCs show gradual red shifts with increasing (CuIn) composition indicating a decrease in optical band gap. In the case of the (ZnS)x(CuInS2)y(CdS)z solid solution, band-edge absorptions are all between those of pure CuInS2 and ZnS and shift gradually with different ratios of x, y, and z. These significant red-shifts indicate the formation of Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z solid-solution NCs via alloying the wider band gap ZnS with the narrower band gap CuInS2 and via alloying CuInS2 with CdS, respectively. The optical band gaps of the NCs can be readily estimated from the wavelength of first absorption peaks by measuring the intercept of the tangent of absorption curves.22 The band gaps of pure ZnS, CuInS2, and CdS NCs were estimated as 3.67, 1.48, and 2.38 eV, respectively, which are very close to that of bulk ZnS (3.66 eV),24 CuInS2 (1.50 eV),14 and CdS (2.42 eV).25 Table 1 lists the optical band gaps of the NCs with different
TABLE 1: Optical Band Gaps of the NCs of ZnS, CuInS2, CdS, Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)za component
band gap (eV)
c (Å)
ZnS (ZnS)3.0(CuInS2)0.5 (ZnS)2.0(CuInS2)1.0 (ZnS)1.0(CuInS2)1.5 CuInS2 (CuInS2)1.5(CdS)1.0 (CuInS2)1.0(CdS)2.0 (CuInS2)0.5(CdS)3.0 CdS (ZnS)1.0(CuInS2)0.5(CdS)2.0 (ZnS)1.0(CuInS2)1.0(CdS)1.0 (ZnS)2.0(CuInS2)0.5(CdS)1.0
3.67 (3) 3.06 (5) 2.74 (6) 2.51 (3) 1.48 (2) 1.72 (2) 1.90 (5) 2.08 (7) 2.38 (2) 3.10 (5) 2.37 (3) 2.69 (2)
5.41995 (3) 5.44264 (4) 5.47372 (2) 5.49253 (2) 5.52312 (3) 5.58920 (2) 5.65886 (4) 5.72308 (6) 5.83909 (2)
a The number within the parentheses is the estimated error of the measurements.
compositions. As shown in Table 1, the band gaps of the Zn2x(CuIn)1-xS2 solid-solution NCs could be tuned from 1.48 to 3.67 eV as x is increased from 0 to 1, while those of (CuIn)1-xCd2xS2 NCs could be tuned from 1.48 to 2.38 eV. It was found that the band gaps of the solid-solution NCs fit well by the modified bowing equations illustrated below CuInS2 Eg(Zn2x(CuIn)1-xS2) ) EZnS · (-x) - bx(-x) g · x + Eg (1) CuInS2 Eg((CuIn)1-xCd2xS2) ) ECdS · (1 - x) g · x + Eg bx(1 - x) (2)
Herein, Eg is the band gap, and b is the bowing parameter. Similarly, the band gaps of the multicomponent (ZnS)x-
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Figure 2. EDS spectrum of the NCs of (A) Zn2x(CuIn)1-xS2, (B) (CuIn)1-xCd2xS2, and (C) (ZnS)x(CuInS2)y(CdS)z.
Figure 3. XRD patterns of the NCs of (A) pure ZnS, CuInS2, and CdS; (B) Zn2x(CuIn)1-xS2; (C) (CuIn)1-xCd2xS2; and (D) (ZnS)x(CuInS2)y(CdS)z.
(CuInS2)y(CdS)z solid-solution NCs can also be adjusted between 1.48 and 3.67 eV by changing the molar ration of ZnS, CuInS2, and CdS constituents. Consistent with the UV-vis results, the color of these solid-solution NCs shifts from nearly colorless to red and black and from yellow to brown and black as the molar ratio of (CuIn) in Zn2x(CuIn)1-xS2 and (CuIn)1-xCd2xS2 solid-solution increases, respectively (Figure 1, inset images). The chemical compositions of these NCs were analyzed by energy-disperse X-ray spectroscopy (EDS). Figure 2 shows the EDS spectrum of the NCs of ZnS, CuInS2, CdS Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z. Clearly, zinc, copper, indium, cadmium, and sulfur are present in these NCs. For the Zn2x(CuIn)1-xS2 NCs, the peak intensity of Zn gradually increases with increasing Zn content, while the intensities of Cu and In peaks decrease; for (CuIn)1-xCd2xS2, the peak intensity of Cd gradually increases with increasing Cd content, while the intensities of Cu and In peaks decrease. The presence of oxygen is mainly due to the presence of capping ligand oleic acid. In all cases, element nitrogen is not observed, which means that
oleylamine does not act as a capping agent but as an activation agent. Pan et al.22 found that oleylamine, as an organic base, can expedite the decomposition process of the precursors and can decrease the reaction temperature benefiting the formation of the homogeneous alloy. For the NCs of (ZnS)x(CuInS2)y(CdS)z, all EDS spectra match well with the designed precursor ratios (see Table S1 of the Supporting Information) suggesting the effective control of the composition of the NCs by simply controlling the ratios of the precursors used. The crystal structures of the NCs were analyzed using X-ray powder diffraction (XRD). Figure 3 shows XRD patterns of the NCs of ZnS, CuInS2, CdS, Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z. All NCs show a similar zinc blende cubic structure over all compositions when oleic acid was used as the capping agent. Three prominent peaks are clearly observed and can be indexed as (111), (220), and (311). In the case of Zn2x(CuIn)1-xS2 NCs, the diffraction peaks shift to higher angles gradually as Zn content increases (Figure 3B), whereas the peaks shift to lower angles as the Cd content increases in
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Figure 4. Lattice parameters (c) of the NCs of (A) Zn2x(CuIn)1-xS2 and (B) (CuIn)1-xCd2xS2 as a function of x in the molecular formulas.
Figure 5. TEM images of the NCs of (A) (ZnS)2.0(CuInS2)1.0, (B) (CuInS2)1.0(CdS)2.0, and (C) (ZnS)1.0(CuInS2)1.0(CdS)1.0 and a high-resolution TEM image of (ZnS)1.0(CuInS2)1.0(CdS)1.0 (inset).
(CuIn)1-xCd2xS2 solid-solution NCs (Figure 3C). The continuous peak shift of the NCs rules out phase separation or separated nucleation of ZnS, CuInS2, or CdS NCs. As shown in Figure 4, a nearly linear increase in the lattice parameter c is observed as the CuIn composition increases. This trend is consistent with Vegard’s law and indicates a homogeneous solid-solution structure.26-29 By using 1-dodecanethiol as the capping agent instead of oleic acid, wurtzite (ZnS)x(CuInS2)y(CdS)z NCs, as opposed to cubic crystal structure, were obtained (see Figure S1 of the Supporting Information). This observation is consistent with our previous work in the synthesis of CuInS2 NCs using a hot-injection method22 in which the use of oleic acid and 1-dedcanethiol as the capping agents led to the formation of NCs with cubic and hexagonal structure, respectively.22 The structure and morphology of the NCs were further studied using transmission electron microscopy (TEM). Figure 5 shows TEM images of representative NCs of (A) (ZnS)2.0(CuInS2)1.0, (B) (CuInS2)1.0(CdS)2.0, and (C) (ZnS)1.0(CuInS2)1.0(CdS)1.0 revealing narrow-sized distributions with an average diameter of 9.7 ( 2.8 nm, 10.9 ( 3.3 nm, and 9.1 ( 3.0 nm, respectively. The inset in Figure 5C shows a high-resolution TEM image of the (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs. The well-resolved lattice fringes demonstrate the highly crystalline nature of the NCs. Besides the control of composition and crystalline structure, the size of the NCs can also be tuned by amount of activation agent and capping agent used. Figure 6 shows XRD patterns of the (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs synthesized with increasing the amount of oleylamine (0 to 0.5 mL) while fixing the volume of capping agent oleic acid used (1.0 mL). As mentioned previously, oleylamine serves as the activation agent in the reaction. Clearly, in the absence of oleylamine, the decomposing of the precursors was slow resulting in the formation of small amorphous particles (pattern 1). Addition of 0.02 mL oleylamine promoted the reaction leading to the formation of NCs (pattern
Figure 6. XRD patterns of the (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs synthesized with (1) 0, (2) 0.02, (3) 0.05, (4) 0.20, (5) 0.50, (6) 2.00, and (7) 4.00 mL of oleylamine and a fixed volume of capping agent (1.0 mL).
2). Since the amount of oleylamine added was small, less critical nuclei were formed and the majority of precursors were used for crystal growth resulting in the formation of large size particles and wider size distribution. Consistently, sharp reflection peaks were observed in addition to the broadened ones. By increasing the amount of oleylamine, more nuclei were formed generating NCs with narrowing size distribution. This phenomenon was confirmed by TEM (Figure S2 of the Supporting Information); the NCs prepared using 0.05 and 0.5 mL of oleylamine showed an average size of 10.7 ( 5.8 nm and 8.4 ( 2.5 nm, respectively. Oleylamine may serve as a cocapping agent when its amount is further increased, which
Synthesis of Cu-In-S Solid-Solution Nanocrystals
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17297 Acknowledgment. This work was partially supported by the Center for Molecularly Assembled Material Architectures for Solar Energy Production, Storage and Carbon Capture, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001342.
Figure 7. XRD patterns of (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs synthesized at various temperatures: (1) 80 °C, (2) 100 °C, (3) 150 °C, (4) 180 °C, and (5) 200 °C and with a fixed volume of oleylamine and oleic acid at 1.0 mL.
Supporting Information Available: The chemical composition of ZnS, CuInS2, and CdS NCs; Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z solid-solution NCs; XRD pattern of the Zn-Cu-In-Cd-S solid-solution NCs with wurtzite structure; TEM images of (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs with various amounts of oleylamine and at different temperatures; XRD patterns of the Zn-Cu-In-Cd-S solidsolution NCs synthesized with different amounts of capping agent and for different reaction times; XRD pattern of the Zn-Cu-In-Ag-Cd-S solid-solution NCs. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes
leads to the formation of NCs with increasing size; consistently, the patterns 6 and 7 show reflection peaks with decreasing width. The effect of capping agent on the nanocrystal size is less significant than the effect of activation agent. In general, capping agents may form stable complexes with the precursors or capping layers on the NCs slowing down the nucleation and growth processes. A high oleic acid concentration reduces the number of nuclei and forms larger NCs. Nevertheless, its effect is much less significant than that of oleylamine (Figure S3 of the Supporting Information). Similar to the synthesis of other NCs, reaction time and temperature also significantly affect the crystal size. Figure 7 shows XRD patterns of (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs synthesized from 80 to 200 °C. Clearly, the diffraction peaks sharpen with increasing reaction temperature indicating an increase of NC size and crystallinity. For example, the average size of the NCs synthesized at 100 and 200 °C was 5.1 ( 2.5 and 10.6 ( 2.8 nm, respectively (Figure S4 of the Supporting Information). Compared with reaction temperature, the effect of reaction time is less significant. Figure S5 of the Supporting Information shows XRD patterns of the (ZnS)1.0(CuInS2)1.0(CdS)1.0 NCs synthesized at various reaction times with a fixed amount of oleylamine and oleic acid. It can be observed that the diffraction peaks also sharpen and slightly shift to the high two-theta region with increasing the reaction time. Therefore, increasing reaction time leads to larger NCs with improved crystallinity. Conclusion In summary, multicomponent solid-solution Zn2x(CuIn)1-xS2, (CuIn)1-xCd2xS2, and (ZnS)x(CuInS2)y(CdS)z NCs were synthesized by a toluene-thermal approach. The composition, structure, size, and band gaps could be readily controlled by the precursors used, the amount of activation and capping agents used, and the reaction time and the reaction temperature. This facile synthesizing method was found to be applicable to introducing more elements such as Ag (see Figure S6 of the Supporting Information) and Ga23 to this solid-solution system. This work provides some elicitations to the controllable synthesis of multicomponent solid-solution NCs and a class of materials for solar cell, photocatalysis, and other applications.
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