Nanocrystalline Metal Chalcogenides Obtained Open to Air: Synthesis

Aug 10, 2009 - UniVersity of Science and Technology, Shanghai 200237, China. ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: June 12, 2009...
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Nanocrystalline Metal Chalcogenides Obtained Open to Air: Synthesis, Morphology, Mechanism, and Optical Properties Aiyu Zhang,† Qian Ma,† Mengkai Lu,*,† Guangjun Zhou,† Chunzhong Li,‡ and Zhaoguang Wang State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China, and Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: June 12, 2009

Nanocrystalline metal chalcogenide (selenide/sulfide/telluride) semiconductors make great sense in the photoelectric applications. Since the size and shape of the nanomaterials have a strong influence on their properties, new methods with high feasibility are always expected for the synthesis of chalcogenides with different dimension and morphology. In the present work, a green acetate-paraffin route was introduced that is suitable for the preparation of different chalcogenides, such as PbSe, Cu2Se, ZnSe, CdSe, NiSe2, CoSe, ZnS, Cu7.2S4, and PbTe. All the experiment processes were conducted open to air. These chalcogenides show various morphologies like grains, nanospheres, nanoplates, and so on. A mechanism was proposed to explain the formation of the different chalcogenide nanostructures. The dimension and shape of the nanostructures are dependent both on the crystal structure and the action of oleic acid on the growth rate of crystal faces. The optical properties of the prepared chalcogenides were characterized by the UV-visible diffuse reflectance spectroscopy (UV-DRS) and photoluminescence (PL) spectroscopy. Introduction Metal chalcogenide (selenide, sulfide, and telluride) compounds have a semiconductor nature, and they are of considerable technical interest in the fields of electronics and optical and electrooptical devices,1 They have been considered as a kind of photovoltaic material for decades,2 for their excellent properties such as high absorption coefficients, energy matching between their band gaps and the solar spectrum, and also the adjustable bandgap depending on the particle size. One factor that limits the utilization of the solar energy is the poor overlap of the absorption spectra of semiconductors and the solar spectrum. One solution for this problem is to choose a semiconductor with a narrower band gap, for example, selenides as absorption materials. Metal selenides can be made as thinfilm or quantum dots to form thin-film solar cells3 or quantum dot sensitized solar cells.4 The application of nanotechnology is also proposed as one potential approach to a high-efficiency solar cell,5 since the properties of semiconductors can be highly modified depending on the size, shape, and surface state of the nanoparticles, which in turn are determined by the preparation method and reaction condition. Recently, the metal chalcogenides have been widely researched for synthesis,6 morphology,7 and their applications, such as photoluminescence,8 biological labeling,9 electronics,10 photoelectronics,4,11 and electrochemiluminescence.12 Although the synthesis of chalcogenides has been studied13 for many years and many preparation methods have been reported, such as solid state metathesis,14 CVD,15 the molecule precursor route,16 the hydrothermal method,17 the solvothermal-reduction process,18 the sonochemical method,19 and some organic solution meth* To whom correspondence should be addressed. E-mail: mklu@ icm.sdu.edu.cn. † Shandong University. ‡ East China University of Science and Technology.

ods,20 there still are few methods generally suitable for most of the chalcogenides, and the raw materials are always expensive and poisonous. For example, selenides were usually prepared by a TOP(trioctylphosphine)-based route, in which Se powder is dissolved in TOP or TBP (tributylphosphine) to obtain the Se/TOP (or TBP) solution, and then the Se/TOP solution reacts with a metal-organic precursor, resulting in metal selenides.21 Many papers have been published on these TOP-based routes.22 However, TOP is hazardous, unstable, and very expensive for large-scale production, and most of these papers only concentrated on the synthesis of several selenides (CdSe and PbSe etc.). In the present work, we aimed to synthesize a series of chalcogenides using a green and generally suitable method. Recently, Deng et al.23 have reported a non-TOP-based route for the synthesis of CdSe nanocrystals, by substituting the cheaper and greener paraffin liquid for TOP/TBP. Their method is environmentally friendly but is only suitable for CdSe. We modified the method by changing one raw material (using metal acetates instead of oxide (CdO)), and prepared a series of metal chalcogenides. The whole process was carried out open to air, and the products were stable in atmosphere. The prepared chalcogenides showed various new morphologies and good optical property, and some uncommon phases were obtained. The formation mechanism of the new nanostructures is discussed in detail. Experimental Section Materials. Metal acetates M(OAc)x · nH2O (M ) Pb, Zn, Cd, Cu, Ni, and Co) (AR), selenium (Se) powder (g99.95%), sublimed sulfur (S) (CP), tellurium powder (AR), oleic acid (OA) (CP), and paraffin liquid (PAL) (CP) were used as the starting materials. Synthesis. In a typical experiment for lead selenide (PbSe), 1 mmol of Pb(OAc)2 · 3H2O, 2 mL of OA, and 10 mL of PAL were mixed in a beaker. The mixture was stirred and heated

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Nanocrystalline Metal Chalcogenides until a light yellow solution was obtained. Se (1 mmol) was added into 50 mL of PAL and the mixture was sonicated for 20 min, and then heated at a temperature above 220 °C until an orange red Se/PAL solution was obtained. Then both solutions were injected into a flask equipped with a reflux condenser. The reactants were heated to a boil and refluxed for 30 min. The whole process was operated open to air. The precipitate was separated by centrifugation, further washed with hexane and ethanol several times, and dried in air at room temperature or in a vacuum at 60 °C. The other chalcogenides were synthesized through a similar process, except that the amount of OA and the heating process may be different. During the synthesis of sulfides and tellurides, the S powder was dissolved in PAL just by sonication while the Te/PAL solution can only be obtained by the heating to 240 °C followed by the removal of the indescribable Te. Characterization. The phase compositions of the samples were studied by X-ray diffraction (XRD) on a Germany Bruker Axs D8-Avance X-ray diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å). The morphologies were observed by a transmission electron microscope (TEM, JEM-100CX) and high-resolution TEM (HRTEM, JEM-2100), and the Samples for TEM and HRTEM were prepared by putting a drop of product dispersed in ethanol or hexane onto an amorphous substrate supported on a copper grid and then allowing the solvent to evaporate at room temperature. Selected area electron diffraction (SAED) patterns were obtained on the TEM. UV-visible diffuse reflectance spectroscopy (UV-DRS) was measured on a UV-visible spectrophotometer (Shimadzu, UV2550). The luminescence spectra were obtained on a Hitachi F-4500 fluorescence spectrophotometer. Stoichiometry data were obtained from energy dispersive spectrometry (EDS) (Horiba EMAX Energy, EX-350). X-ray photoelectron spectroscopy (XPS) was collected on a VG MicroTech ESCA 3000 X-ray photoelectron spectroscope, using monochromatic Al KR with a photon energy of 1486.6 eV at a pressure of >1 × 10-9 Torr, a pass energy of 40 eV, an electron takeoff angle of 60°, and an overall resolution of 0.05 eV.

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Figure 1. XRD patterns of the prepared selenides.

Result and Discussion Figure 1 shows X-ray diffraction (XRD) patterns of the prepared selenides: PbSe, ZnSe, CdSe, Cu2Se, NiSe2, and CoSe. All the diffraction peaks can be indexed to the respective JCPDS data (6-354, 37-1463, 19-191, 65-2982, 65-1843, and 71-4777). The broadening of the diffraction peaks is caused by the small size of the crystallines. The transmission electron micrographs (TEM) in Figure 2 reveal the various morphologies of the obtained selenides: uniform spheres for PbSe, plates (mainly with hexagonal shape) for Cu2Se, grains for ZnSe and CdSe, nanocrystalline clusters for NiSe2, and mulberry-like patterns for CoSe. The different morphologies of the different selenides (PbSe, ZnSe, CdSe, Cu2Se, NiSe2, CoSe) are mainly determined by their essential crystal structures and the action of oleic acid (OA) on the crystal faces (this will be described in detail below). The SAED patterns of the prepared selenides are show in Figure 3. Except for the particles of CoSe which are too thick to obtain a clear SAED pattern, the SAED results agree well with the XRD patterns, further confirming the formation of the crystal phases of the selenides. For the preparation of a series of selenides, M(OAc)x · nH2O were used as metal sources. All these acetates can be dissolved by OA to give oleate precursors (M(OA)x) though the temperature and time they need are different. It was proved in our experiment that the existence of crystal water accelerates the

Figure 2. TEM graphs of (a) PbSe, (b) Cu2Se, (c) CdSe, (d) NiSe2, (e) ZnSe, and (f) CoSe.

reaction between acetate and oleic acid. The oleate precursor’s complete miscibility with paraffin liquid (PAL) ensures the distribution of Mx+ in the reaction system. A metastable solution of Se/PAL is obtained after heating at above 220 °C. In the Se/PAL solution, a reversible reaction occurs to generate H2Se, during which Se is reduced to H2Se gas, while the long alkane chain is oxidated to the long alkene chain. This has been approved by Deng et al. in their paper.23 The generated H2Se can react with precursor M(OA)x to obtain MSex/2, which consumes H2Se and drives the reversible reaction to the right

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Figure 4. XRD patterns (a-c) and TEM graphs (d-f) of ZnS, Cu7.2S4, and PbTe.

Figure 3. Selected area electron diffraction (SAED) patterns of (a) PbSe, (b) Cu2Se, (c) CdSe, (d) ZnSe, (e) NiSe2, and (f) CoSe nanocrystals.

SCHEME 1: Formation Process of the Nanocrystalline Selenides (MSey)

for more H2Se. When the M(OA)x precursor and Se/PAL solution mixed under high temperature, these reactions proceed at almost the same time to give a large amount of MSex/2. Because of the small solubility of MSex/2 in PAL, high supersaturation can be reached, and so nucleation takes place in a moment, resulting in nanocrystals with a uniform shape. The whole process is demonstrated in Scheme 1. The valence of some Mx+ may be changed during the forming process of the selenides, and so the product may not be MSex/2, while it is MSey (y * x/2), just like Cu2Se and NiSe2. The final phase of a selenide is determined by the stability of its various phases under the present condition. The preparation of selenides usually requires vacuum or inert atmosphere protection, because of their sensitivity to the oxygen. In the present work, all the selenides were prepared open to air, but no oxide phases were detected in products by the XRD results. The reductive OA and inert PAL were used as solvents.

During the reaction process, the OA molecules are attached to the surface of the selenides, forming a protecting layer. The main solvents PAL also provide a shield for the selenides. Thus the selenides can be prepared in the air without any extra protection. Our experiments have indicated that most of the metal acetates can react with OA to form a M-OA complex, and paraffin always acts as a solution and reductant. This makes the method available for the synthesis of many selenides. Further study proved that sulfur and tellurium powders can also be dissolved in PAL, and metal sulfides and tellurides can also be obtained by this method. Figure 4 shows the XRD and TEM results of the obtained ZnS, Cu7.2S4, and Altaite-PbTe, which show the morphologies of grains, lotus leaf and cuboid-like, respectively. A Cu7.2S4 phase but not the common CuS or Cu2S was obtained for copper sulfide. The formation process of the sulfides and tellurides is similar to that of the selenides. This synthesis route is generally suitable for metal chalcogenides. All the prepared chalcogenides were further characterized by EDAX and XPS, and the details are shown in the Supporting Information. The metal chalcogenides prepared in the work show various new morphologies. To realize the formation mechanism of these different nanostructures, a detailed study was carried out on the morphology of nanocrystals by seting PbSe as an example. Panels a and b of Figure 5 show the high-resolution transmission electron micrograph (HRTEM) of the spherical PbSe nanocrystals. From Figure 5a, it can be found that the PbSe nanocrystals are not exactly spherical, and they can be considered as cuboctahedra (shown in the inset of Figure 5a is a truncated cube) with the edges and corners rounded. Figure 5c is the TEM graph of PbSe nanocubes which were synthesized at 200 °C (keeping the other reaction conditions the same as that of the spherical ones). It is known that the PbSe phase has a face-centered cubic (fcc) structure, and the shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions.24 Perfect cubes bounded by {100} planes will be formed when R ) 0.58, and the truncated nanocubes will result if R is close to 0.7-0.87. OA are expected to preferentially adsorb at the {111} facets of the fcc structured PbSe (Pb2+/Se2-) due to the presence of a greater charge driven by high atomic density. This would leave

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Figure 5. HRTEM images of spherical PbSe (a, b) at different scale and TEM images of cubic PbSe (c).

Figure 6. UV-vis diffuse reflectance (UV-DR) spectra of the prepared chalcogenides.

the low atomic density {100} facets poorly capped or uncapped with surfactant molecules, hence, remaining available for further nucleation. This preferential growth at the {100} facets would become more prominent (or in other words, the value of R would be larger) if the effective OA concentration was lowered. In the present study, when the reaction temperature is 200 °C, the concentration of OA is high enough to cap all the crystal surface, and thus a balanceable growth rate R ) 0.58 can be achieved, which results in the nanocubes. But when the reaction condition is boiling and refluxing, the effectual concentration of OA in the solution will be reduced because the boiling point of PAL is higher than that of the OA. The {100} facets will be poorly capped, and thus R may increase to a value around 0.7-0.87. Consequently, cuboctahedron-shaped PbSe nanocrystals formed when the reactants were heated to boil, while nanocubes were obtained at 200 °C. Furthermore, it is for the purpose of lowering the surface energy that the nanocrystals have rounded edges and corners. The lattice images (Figure 5b) show the interlayer spacing of 0.31 nm, which agrees well with the lattice spacing of the (200) plane of the cubic PbSe. Therefore, the various morphologies obtained in the present study are determined mainly by two factors: the essential crystal structure and the action of OA on the growing crystal faces. Each chalcogenide has its own structure, and the crystal structure influences the final shape of the crystals. On the other hand, OA has a dramatic effect on the size and morphology of the crystallines by the steric hindrance.25 The OA ligands inhibit the growth of crystal facets by adsorbing on them. The status of OA adsorption has a great influence on the growth rate of each facet, and finally results in the shape of nanocrystals. The UV-visible diffuse reflectance (UV-DR) spectra of the prepared chalcogenides are shown in Figure 6. Almost all the chalcogenides except ZnSe and ZnS exhibit high absorption in the whole UV-visible radiation region between 200 and 800

Figure 7. (a) UV-DR (black dot), luminescence emission (solid), and excitation spectra (dashed) of ZnSe nanocrystals; (b) UV-DR (black dot), luminescence emission (solid), and excitation spectra (dashed) of ZnS nanocrystals.

nm, which match well with the application of the solar spectrum. The absorption spectra in the longer wavelength region cannot be detected for the wavelength limitation of the spectrophotometer. Thus the absorption onsets of most of the products cannot be measured. UV-DR spectra of ZnSe and ZnS are shown in Figure 7a,b. The band gaps estimated from the absorption edges are 2.7 and 3.4 eV for ZnSe and ZnS, respectively. Figure 7 also shows the luminescence excitation and emission spectra of the two samples. Two dominant emission bands around 503 and 450 nm have been observed for the prepared ZnSe and ZnS nanocrystals, respectively. The sharp side of the luminescence spectra of ZnSe may be due to the fringe effect. For both ZnSe and ZnS, it is noted that the excitation spectrum is inconsistent with the absorption spectrum. Thus their luminescence may not

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be from the band-to-band transitions. This is reasonable considering the large size of the ZnSe and ZnS nanocrystals. Referring to the literature,26 the luminescence of ZnSe and ZnS nanocrystals may be ascribed to the defect energy levels in the crystals. It has been reported that zinc chalcogenides synthesized under near stoichiometric conditions easily contain Zn vacancy defects,27 which may be in charge of the luminescence of ZnSe and ZnS. Conclusions In summery, we described an acetates-paraffin approach generally suitable for the synthesis of metal chalcogenide nanocrystals. By using metal acetate and Se/S/Te powder as raw materials, oleic acid and paraffin liquid as solvents, a series of selenides (PbSe, Cu2Se, ZnSe, CdSe, NiSe2, CoSe), sulfides (ZnS, Cu7.2S4), and telluride (PbTe) were prepared open to air. The stucture, morphology, and optical properties of the products were characteriazed. The formation process of the chalcogenide phases was deduced, in which the key step is the reversible reaction generating H2Se. The prepared chalcogenides show various morphologies like grains, nanospheres, nanoplates, and so on. The dimension and shape of the nanostructures are dependent on both the crystal structure and the action of oleic acid on the growth rate of the crystal faces. Almost total absorption in the UV-vis region was observed for PbSe, Cu2Se, NiSe2, CoSe, Cu7.2Se4, and PbTe. Two luminescence emissions from the defects were detected at 503 and 450 nm for ZnSe and ZnS, respectively. Acknowledgment. The present work is supported by the Natural Science Foundation of Shandong Province (No. Y2008F32), the Research Fund for the Doctoral Program of Higher Education of China (No. 200804221044), and the Excellent State Key Laboratory Research Program of the National Natural Science Foundation of China (No. 50823009). Supporting Information Available: Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis of the products. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cordova, R.; Lopez, C.; Orellana, M.; Grez, P.; Schrebler, R.; del Rio, R. J. Phys. Chem. B 2005, 109, 3212. (2) (a) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (b) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (c) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (3) Contreras, M. A.; Egaas, B.; Ramanathan, K.; Hiltner, J.; Swartzlander, A.; Hasoon, F.; Noufi, R. Prog. PhotoVoltaics Res. Appl. 1999, 7, 311. (4) (a) Bang, J. H.; Kamat, P. V. ACS Nano. DOI: 10.1021/nn900324q. Published Online: May 12, 2009. (b) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (c) Ma, W.; Luther, J. M.; Zheng, H.; Wu, Y.; Alivisatos, A. P. Nano Lett. 2009, 9, 1699. (5) Nozik, A. J. Phys. E 2002, 14, 115. (6) (a) Sun, M.; Yang, X. J. Phys. Chem. C 2009, 113, 8701. (b) Ji, X.; Copenhaver, D.; Sichmeller, C.; Peng, X. J. Am. Chem. Soc. 2008, 130, 5726. (c) Yu, Q.; Liu, C.; Zhang, Z.; Liu, Y. J. Phys. Chem. C 2008, 112, 2266. (d) Washington, A. L., II; Strouse, G. F. J. Am. Chem. Soc. 2008, 130, 8916. (e) Ouyang, J.; Vincent, M.; Kingston, D.; Descours, P.; Boivineau, T.; Zaman, M. B.; Wu, X.; Yu, K. J. Phys. Chem. C 2009, 113,

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