7364
Langmuir 2006, 22, 7364-7368
Synthesis and Characterization of Sulfide and Selenide Colloidal Semiconductor Nanocrystals Xun Wang,* Jing Zhuang, Qing Peng, and Yadong Li* Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed January 3, 2006. In Final Form: April 15, 2006 In this paper, we demonstrate the synthesis of sulfide and selenide nanocrystals in a water-ethanol mixed solution system. This synthetic way was based on the direct reactions between metal ions and S2-/Se. Linoleic acid was adopted to protect the nanocrystals from agglomeration. Without involving extreme experimental conditions, this less toxic synthetic route can be expected to bring more opportunities to nanocrystal-related research and application fields.
Introduction During the past decades, sulfide and/or selenide band gap semiconductors have been extensively investigated because of their novel optical and electrical properties,1-4 which are of interest for areas including photovoltaic devices and electroluminescence applications. Especially, the interesting sizedependent properties of colloidal semiconductor nanocrystals have made them both an ideal system for investigating the intrinsic size effects in nanofields and potential candidates for new applications in solar cell, bio-label fields, etc.5-16 Lots of synthetic methods for preparing II-VI semiconductors have been reported,17-25 including MBE17 (molecular beam epitaxy), MOCVD18 (metalorganic vapor chemical deposition), OMVPE19 (organometallic vapor phase epitaxy), solvother* To whom correspondence should be addressed. E-mail: wangxun@ mail.tsinghua.edu.cn (X.W.);
[email protected] (Y.L.). (1) Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398-401. (2) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 55005504. (3) Herron, N.; Wang, Y.; Echert, H. J. Am. Chem. Soc. 1990, 112, 13221326. (4) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298-302. (5) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (6) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018. (7) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (8) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61. (9) Motte, L.; Billoudet, F.; Laxaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 138-144. (10) Cizeron, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 8887-8891. (11) Motte, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104-4109. (12) Peng, X. G. Chem. Eur. J. 2002, 8, 335-339. (13) Peng, X. G. AdV. Mater. 2003, 15, 459-463. (14) Yu, W. W.; Peng, X. G. Angew. Chem., Int. Ed. 2001, 41, 2368-2371. (15) Joo, J.; Na, H. B.; Hu, T.; Yu, J. H.; Kim, Y. W.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100-11105. (16) Lim, W. P.; Zhang, Z.; Low, H. Y.; Chin, W. S. Angew. Chem., Int. Ed. 2004, 43, 5685-5689. (17) Zhang, B. P.; Yasuda, T.; Segawa, Y.; Yaguchi, H.; Onabe, K.; Edamatsu, E.; Itoh, T. Appl. Phys. Lett. 1997, 70, 2413-2415. (18) Liao, M. C. H.; Chang, Y. H.; Chen, Y. F.; Hsu, J. W.; Lin, J. M.; Chou, W. C. Appl. Phys. Lett. 1997, 70, 2256-2258. (19) Bourret-Courchesne, E. D. Appl. Phys. Lett. 1996, 68 (17), 2418-2420. (20) Li, Y. D.; Liao, H. W.; Ding, Y.; Qian, Y. T.; Yang, L.; Zhou, G. E. Chem. Mater. 1998, 10, 2301. (21) Li, Y. D.; Liao, H. W.; Ding, Y.; Fan, Y.; Zhang, Y.; Qian, Y. T. Inorg. Chem. 1999, 38, 1382-1387. (22) Peng, Q.; Dong, Y. J.; Deng, Z. X.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2001, 40, 3840. (23) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027. (24) Ge, J. P.; Li, Y. D. Chem. Commun. 2003, 2498-2499. (25) Stuczyski, S. M.; Brennan, J. G.; Steigerwald, K. L. Inorg. Chem. 1989, 28, 4431-4432.
mal,20,21 and hydrothermal methods,22-24 etc. However, only very few of them could be used to prepare uniform colloidal nanocrystals with satisfied sizes and size distributions. At present, sulfide/selenide semiconductors with variable sizes and different shapes have been synthesized by various solution-phase synthetic schemes5-16 including the most popular organometallic synthetic procedure5,8 employing the high temperature thermolysis of precursors. As compared with the toxic and expensive disadvantages of the organometallic precursors, several greener synthetic routes employing stable inorganic precursors (for example, oxides and chloride) have been developed in organic solvents systems12-16 (like noncoordinating solvents14 and oleylamine15) or water-toluene two-phase systems26 to get nanocrystals with comparable quality, which avoided the use of extreme conditions such as glovebox and inert atmospheres. Of all the synthetic ways to obtain semiconductors, aqueousbased routes have the advantages of environment friendly characteristics and low cost; however, they usually suffer from less controllability in size and size distributions. Pan et al. designed a water-toluene two-phase method26 to prepare high quality CdS nanocrystals, in which sulfide anions were supplied from the aqueous phase and Cd2+ supplied from the organic toluene phase. Similar to the traditional organic phases routes, this method requires careful design of Cd precursors. Very recently, we have developed a liquid-solid-solution synthetic strategy to obtain various functional monodisperse nanocrystals.27 In this procedure, water and ethanol have been adopted as the continuous solution, and inorganic precursors such as sulfate, chloride, and nitrate can be used to supply as metal sources. Here we will report the rational synthesis of sulfide and selenide semiconductors via this procedure, including CdS, ZnS, MnS, PbS, Ag2S, CdSe, ZnSe, PbSe, etc. The direct reactions between metal ions and S2-/Se were adopted to get well-dispersed colloidal nanocrystals of sulfide and selenide. Without involving extreme experimental conditions and subsequent size selection process, the current synthetic way represents a facile and less toxic synthetic procedure to get sulfide and selenide nanocrystals. Experimental Section In a typical synthesis, 0.5 g of NaOH was added to the mixture of 4 mL of linoleic acid and 16 mL of ethanol for about 30 min (alternatively, an equal amount of sodium linoleate could be adopted). Then 7 mL of an aqueous solution containing 0.5 g of MCl2 (CdCl2, (26) Pan, D. C.; Jiang, S. C.; An, L. J.; Jiang, B. Z. AdV. Mater. 2004, 16, 982-985. (27) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121-124.
10.1021/la060023c CCC: $33.50 © 2006 American Chemical Society Published on Web 07/20/2006
S2-/Se Colloidal Semiconductor Nanocrystals
Langmuir, Vol. 22, No. 17, 2006 7365
Figure 1. Scheme of the preparation of Sulfide nanocrystals in water-ethanol system by adopting linoleic acid as the protecting reagent. MnCl2, ZnCl2, PbCl2, etc.) 8 mL of an aqueous solution containing an equal amount of Na2S in mole ratio were added in order, and the mixture were transferred into a 40 mL Teflon vessel, which was treated under hydrothermal conditions for about 12 h at a designated temperature. For the synthesis of selenide nanocrystals, mole ratios of metal ions and Se powder are decisive to get nearly monodisperse nanocrystals, and the mole ratio should be kept as M2-:Se ) 3:6. In a typical synthesis, 0.1 g of Se powder was adopted and the reaction temperature was kept 120∼200 °C. The samples were characterized by a Bruker D8 Advance X-ray diffractmeter (XRD) with Cu KR radiation (λ ) 1.5418 Å). The size and morphology of the nanoparticles were obtained by a JEOL JEM1200EX transmission electron microscope (TEM) and a JEOL JEM2010F high-resolution transmission electron microscope (HRTEM). The electronic absorption spectra were obtained on a Hitachi U-3010 UV-visible spectrometer. Fluorescent spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer.
Results and Discussions Synthesis of Sulfide Nanocrystals. We first demonstrate the synthesis of sulfide semiconductors (Figure 1). Our synthesis is based on the coprecipitation reaction between metal ions and S2-
Mn+ + S2- f M2Sn Inorganic metal sulfate and/or chloride and sodium sulfide were suitable precursors. Fatty acids have been widely used as efficient protecting reagents for the growth of nanocrystals in organic-solvent-based synthetic routes; however, they were never adopted in an aqueous synthetic method because of the limit of solubility in water. In our synthesis, ethanol has been adopted as the common solvent for linoleic acid and water so that the complexation process of linoleic acid and metal ions can occur under a relative homogeneous condition. NaOH was adopted to convert the linoleic acid as sodium linoleate which as the main solid phase will undergo an ion exchange process to form metal linoleate. With the addition of S2- ions into the system, the reaction between metal linoleate and S2- generates metal sulfide monomers, which then interact and form nanocrystals. Along with this process, in situ generated linoleic acid will absorb on the surface of the nanocrystals, which will endow the as-obtained nanocrystals with hydrophobic surfaces and lead to the spontaneous separation of the nanocrystals from the bulky solution. During the reaction, a small amount of oleic-ethanol liquid phase will float on the surface of the bulky aqueous solution, which may enter into the water-oleic-ethanol phase along with the reaction to reach equilibrium. Thorough XRD characterization proved the formation of various sulfide semiconductors through this procedure. Figure 2A shows typical XRD patterns of the pure phase of face-centered cubic CdS (space group F43m/(216)) (JCPDS 75-0581) with lattice constants a ) 5.82 Å, face-centered cubic PbS (space group Fm3m(225)) (JCPDS 5-592) with lattice constants a ) 5.9362 Å, hexagonal phase of MnS (JCPDS 40-1289), facecentered cubic phase of ZnS and monoclinic Ag2S (space group P21/n(14)) (JCPDS 14-0072) with lattice constants a ) 4.229
Figure 2. XRD patterns of sulfide and selenide nanocrystals; A, sulfide nanocrystals, Ag2S (120 °C; Ag+:S2- ) 2:1); CdS (Zn2+:S2) 1:1, 120 °C); MnS (Mn2+:S2- ) 1:1, 120 °C); PbS (Pb2+:S2- ) 1:1, 120 °C); B, selenide nanocrystals, CdSe (Cd2+:Se ) 3:1, 180 °C); ZnSe (Zn2+:Se ) 3:1, 180 °C); PbSe (Pb2+:Se ) 3:1, 180 °C); C, ZnS nanocrystals obtained at different temperature conditions: top, room temperature; middle, 120 °C; bottom, 140 °C.
Å, b ) 6.931 Å, c ) 7.862 Å, R ) 90°, β ) 99.61°, γ ) 90°. Due to the nanometer size of the as-obtained nanocrystals, all of the peaks in the XRD patterns are apparently broadened to some extent. By altering the temperature conditions, the sizes of the as-obtained nanocrystals could be rationally tuned and the XRD patterns gradually broadened (Figure 2C). All of these experimental results prove the successful synthesis of sulfide semiconductors via the current approach. TEM analysis has provided us further insight into the microstructures details of the sulfide semiconductors. As shown in Figure 3A-F, all of the sulfide samples are composed of nearly monodsiperse nanocrystals, which usually self-assemble into two-dimensional arrays on the TEM grids. Different sulfide nanocrystals have different shapes; for example, the PbS nanocrystals have a truncated cubic shape (Figure 3H), Ag2S (Figure 3A,B), ZnS, and MnS (Figure 3C, Figure 3F) have a nearly round shape, whereas the CdS nanocrystals (Figure 3G) have an aspect ratio of about 2 with a diameter of ∼2 nm. High
7366 Langmuir, Vol. 22, No. 17, 2006
Wang et al.
Figure 3. TEM images of sulfide and selenide nanocrystals; A, Ag2S nanocrystals (90 °C); B, Ag2S nanocrystals (180 °C); C, ZnS nanocrystals (120 °C); inset, HRTEM images of an individual ZnS nanocrystal; D, ED patterns taken from ZnS nanocrystals in Figure 3C; E, ZnS nanocrystals with diameters of ∼2.5 nm (40 °C); F, MnS nanocrystals with diameters of ∼8 nm (room temperature); G, CdS nanocrystals (120 °C); H, PbS nanocrystals (120 °C); I, PbSe nanocrystals with nearly round shapes; J, PbSe nanocrystals with cubic shapes; K, CdSe nanocrystals (180 °C); L, ZnSe nanocrystals (180 °C).
resolution TEM and electron diffraction taken from these nanocrystals revealed the crystalline nature of these semiconductors. Figure 3D shows the ED patterns taken from ZnS nanocrystals, which can be readily indexed to the (001), (111), and (002) planes of the face-centered cubic ZnS, agreeing well with XRD results (Figure 2C). HRTEM images taken from the ZnS nanocrystals further proved they are highly crystalline, and the lattice spacing can be calculated to be about 0.31 nm, corresponding to the (111) planes of face-centered-cubic ZnS. Since the direct reaction between the metal and sulfide ions can occur at a wide temperature range, the reaction can be controlled at a temperature range of from room temperature to 200 °C. In this procedure, metal ions will undergo an ion exchange process first to form metal linoleate, which then reacts with S2to form metal sulfide. After the reactants were mixed together, amorphous precipitation would result immediately, and the following hydrothermal treatment or aging process would ensure the monodispersity of the as-obtained nanorystals. This is quite different from the reported organic solution synthetic way, which usually require homogeneous precipitation. In this approach, the diameter was mainly determined by the reaction temperature:
an elongated time above 10 h did not have apparent influences on the diameters, and a treatment time of less than 5 h may result in polydisperse nanocrystals. For example, by altering the reaction temperature from 90 to 180 °C, the diameters of the as-obtained Ag2S nanocrystals could be rationally tuned from about 7 nm (Figure 3A) to about 13 nm (Figure 3B). This method has also shown its potential in generating nanocrystals with quantumconfined sizes. For example, when reaction temperature was controlled at room temperature or 40 °C, ZnS nanocrystals with diameters of 2 and 2.5 nm could be rationally obtained (Figure 3E). With the decreasing sizes, the shape of the nanocrystals tended to become irregular. However, it should note that not all of the sulfide semiconductors can be prepared as quantum confined sizes by only lowering the temperatures, for example, the MnS nanocrystals obtained at room temperature would have diameters of ∼8 nm (Figure 3F). Since these nanocrystals were covered with alkyl chains, they can be easily dispersed into nonpolar solvents such as cyclohexane and chloroform to form homogeneous solutions (Figure 4B, inset). UV-vis spectra were taken from the cyclohexane solutions of these samples to detect the band gap information of these sulfide
S2-/Se Colloidal Semiconductor Nanocrystals
Langmuir, Vol. 22, No. 17, 2006 7367
Figure 4. (A) UV-vis spectra of nanocrystals A, Ag2S; B, CdS; C, CdSe, PbS. (B) UV-vis spectra of ZnS nanocrystals of different sizes. (C) UV-vis spectra of ZnS nanocrystals obtained at different temperature conditions. (D) Photoluminescence of ZnS nanocrystals.
semiconductor nanocrystals (Figure 4A). Since the sizes of the nanocrystals were beyond the exciton Bohr radius, no apparent blue shift was observed for Ag2S and PbS nanocrystals. For ZnS nanocrystals with different sizes of 6 nm (180 °C), 5 nm (120 °C), and 4 nm (60 °C), the absorption edges (Figure 4B) were observed at 3.26 eV (380 nm), 3.54 eV (350 nm), and 3.65 eV (340 nm), respectively, showing a small blue shift with the decreasing of diameters. For ZnS nanocrystals with quantumconfined sizes, the UV-vis absorption peak (Figure 4C) further shifts to 300 nm (4.13 eV) and 320 nm (3.87 eV). When excited with 360 nm incident light, PL spectra of the ZnS samples with diameters of ∼6 nm (180 °C) have an emission peak centered at 430 nm (Figure 4D), which may be attributed to the recombination of excitons at the vacancy of S ions. As a comparison, the spectra taken from the 4 nm obtained at a temperature of 60 °C has an emission peak centered at 450 nm (Figure 4D). A 20 nm red-shift was observed from these two samples. Since the emission peak will shift toward blue as a result of quantum effect with decreasing diameters, this emission may be attributed to the surface trapped emissions. Perhaps due to the relatively low crystallinity of the nanocrystals obtained at low-temperature conditions, ZnS nanocrystals with diameters 2∼2.5 did not emit very well. Synthesis of Selenide Nanocrystals. After a modification, this method can be applied to the synthesis of Selenide semicondutors such as PbSe, CdSe, and ZnSe. Because of the volatility and toxicity of the H2Se, the direct reaction of metal ions and Se2- anions have been avoided in this strategy, and the Se2- could be supplied by the reduction of SeO32- by N2H426 or alternatively from the greener Se powders. In the current study, Se instead of toxic SeO32- has been adopted to prepare series of selenide nanocrystals.
Based on our previous studies,22 Se powder will be transformed as Se sol under hydrothermal conditions, which will release Se2and react with the metal ions to form metal selenide. In this study, Se might be reduced by ethanol and release Se2- and then combine with M2+ to form MSe
Se T Se (sol) Se + C2H5OH + 4OH- f Se2- + CH3COOH + 3H2O M2+ + Se2- f MSe Our experiments show that this simple reaction can be adopted in this water-ethanol solution system to prepare uniform welldispersed nanocrystals. NaOH was adopted to convert the linoleic acid to sodium linoleate, and meanwhile, under a basic condition, the reaction between metal ions and Se powders can occur to yield selenide. Different from the synthesis of sulfide semiconductors, the reaction temperature should be controlled at a temperature above 120 °C to ensure the completion of the reaction. As shown in Figure 2B, face-centered cubic ZnSe (JCPDS 37-1463), CdSe (19-0191), and PbSe (78-1902) could be readily obtained under current experimental conditions. TEM characterization shows the effectiveness of this procedure in generating uniform selenide nanocrystals. As shown in Figure 3I-L, the CdSe, ZnSe, and PbSe products have been prepared as welldispersed nanocrystals. It is worth noting that, although the selenide series all possess face-centered cubic crystal structures, the morphologies of the as-obtained nanocrystals are quite different. Under an identical temperature (180 °C), ratio of water and ethanol (16:16, mL:mL), and molar ratio of metal and Se (3:1) conditions, PbSe nanocrystals usually have a nearly round
7368 Langmuir, Vol. 22, No. 17, 2006
shape, ZnSe nanocrystals are composed of partly elongated nanocrystals, and CdSe nanocrystals are characterized to be polygonal shaped nanocrystals. All of these indicated that the formation processes of these nanocrystals were rather complex. Nevertheless, the successful complexation of linoleate on the surface of these nancorystals ensured the formation of quite dispersed nanocrystals. Similar to the sulfide nanocrystals, these selenide nanocrystals can be dispersed into nonpolar solvents. The existence of ethanol has endowed this reaction system with much versatility. All kinds of organic additives can be introduced to modulate the growth of the nanocrystals. For example, this procedure has shown its potential in shapecontrolled synthesis of PbSe nanocrystals by using octadecylamine as shape controlling reagents. Octadecylamine has been used in organic solution methods. The current water-ethanol system successfully overcomes the limit of solubility in water. Nearly round shaped PbSe nanocrystals with diameters of ∼20 nm (Figure 3I) could be obtained when an appropriate amount of NaOH (for example, 0.2 g) was added to the reaction system, whereas cubic PbSe nanocrystals (Figure 3J) can be obtained when 1 g of octadecylamine instead of NaOH was adopted at the same temperature conditions of 180 °C and a mole ratio of Pb2+:Se ) 3:1. A general sequence of surface energies for crystals with face-centered cubic structures is γ{111} < γ{100} < γ{110}. On the basis of the surface free energy minimization
Wang et al.
principle, a nanocube having six relatively high surface energies facets of {100} is a less stable morphology, and the equilibrium morphology of PbSe is expected to be truncated cubic. It seemed that the effective absorption of linoleic acid and octadecylamine molecular on the {100} facets ensured the growth of PbSe nanocubes. Nevertheless, all of these experimental results prove that this water-ethanol reaction system is quite efficient in preparing selenide semiconductor nanocrystals.
Conclusion In this paper, we have reported the synthesis of sulfide and selenide semiconductors in the water-ethanol reaction system. The adopting of linoleic acid and sodium linoleate facilitate the formation of the sulfide and selenide nanocrystals. After further modification, this less toxic procedure can be expected to prepare uniform nanocrystals with controlled shapes and sizes. It is believed that this simple system can bring more opportunities to the current nanocrystal-related research and application fields. Acknowledgment. This work was supported by NSFC (20501013, 50372030, 20131030, and 90406003), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China and the State Key Project of Fundamental Research for Nanomaterials and Nanostructures (2003CB716901). LA060023C
