A Low-Temperature and Mild Solvothermal Route to the Synthesis of

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A Low-Temperature and Mild Solvothermal Route to the Synthesis of Wurtzite-Type ZnS With Single-Crystalline Nanoplate-like Morphology Gen-Tao Zhou,*,† Xinchen Wang,‡ and Jimmy C. Yu*,‡

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1761-1765

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space of Sciences, University of Science and Technology of China, Hefei 230026, Anhui, China, and Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received January 8, 2005;

Revised Manuscript Received May 30, 2005

ABSTRACT: Wurtzite ZnS, a high-temperature polymorph of ZnS, was prepared by a novel low-temperature and mild solvothermal process over a temperature range of 160-200 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high resolution transmission electron microscopy (HRTEM) analytic techniques were applied to characterize the composition and morphology of the products. The effects of solvent properties and reaction temperatures on the product purities and crystal structures were investigated. The solvent not only acts as a thermal conducting medium and extractant or active nucleophilic agent but also stabilizes the metastable wurtzite phase. As a result, the hightemperature wurtzite ZnS polymorph can be formed under low-temperature solvothermal conditions. TEM observations denote that the synthesized wurtzite ZnS consists of quasi-square or rectangle nanoplate-like morphologies with lateral dimensions ranging from 1 to 2 µm. SAED and HRTEM confirm the single-crystalline nature of the nanoplates. A novel solvothermal extraction mechanism is proposed. Introduction In the past decade, much attention has been focused on the preparation of one-dimensional (1D) nanostructures such as nanorods, nanowires, nanobelts, and nanotubes. This interest stems from their peculiar and fascinating properties and applications that are superior to their bulk counterparts.1 A variety of efficient synthetic strategies for 1D nanostructures, such as the vapor-liquid-solid (VLS) method,2 vapor-solid (VS) growth,3 supercritical fluid-liquid-solid (SFLS) process,4 and diverse solution-based methods,5 have been developed. Two comprehensive reviews on the synthesis of 1D nanostructures have been recently contributed from Xia et al.1a and Rao et al.1b However, nanoplates have not been widely studied. The reason is that the fabrication of two-dimensional (2D) nanoplates is not as popular as 1D nanostructured materials. Thus, developing novel routes to their synthesis is a much more urgent task for material scientists and chemists. ZnS is an important wide band gap II-VI group semiconductor material and is commercially used as a phosphor and in thin-film electroluminescent devices, solar cells, and other optical electronic devices.6 The nanostructured semiconductor ZnS has been reported to have some characteristics different from its bulk counterpart,7 which may extend its application range. Therefore, considerable efforts have been made to synthesize ZnS nanoparticles and films and to explore their novel properties.8 Recently, wurtzite ZnS nanosheets have been reported by thermolysis of the complex ZnS(EN)0.5 (EN ) ethylenediamine) in argon gas or in a vacuum above 250 * Corresponding author. E-mail: [email protected]. † University of Science and Technology of China. ‡ The Chinese University of Hong Kong.

°C.9,10 A similar thermolysis process was also reported for the synthesis of Fe7S8 nanowires from a nanowirelike precursor of Fe1-xS(en)0.5.11 However, obvious striated defects in nanosheets were produced by the precursor thermolysis process,9 which may originate from the less controllable and tempestuous nature of solidstate thermolysis. The solvothermal process has been proven to be a useful and mild technique for the preparation of novel materials with unusual structures and properties. Outstanding examples include GaN nanocrystals and a wealth of II-VI or V-VI group semiconductor nanostructured materials.12 We have obtained a complex of Zn-sulfur-ethylenediamine by the solvothermal method. Its special powder X-ray diffraction (PXRD) results showed that this compound possesses a typical layered structure (Figure 1). SEM observations showed that this compound has a nanoplate-like morphology (Figure 2). More recently, its complex structure and composition have been exactly determined to be orthorhombic ZnS(EN)0.5.13 The zinc atoms in the complex are tetrahedrally coordinated by three sulfurs and one nitrogen from the ethylenediamine molecule. The alternating zinc and sulfur atoms form six-member rings that have a boatlike shape. These rings share edges to form layers that are perpendicular to the a-axis. These zinc sulfur layers, which are similar to those in wurtzite ZnS, are connected to each other through the bonding of nitrogen atoms in ethylenediamine. Therefore, it can be anticipated that if one selects a suitable solvent that can extract or destroy ethylenediamine from the complex under mild conditions the zinc sulfur layers in the complex may be retained. In doing so, high-temperature wurtzite-type ZnS can be formed under mild and low-temperature conditions. On the basis of the above strategies, we carried out the study of the solvothermal synthesis

10.1021/cg050007y CCC: $30.25 © 2005 American Chemical Society Published on Web 06/29/2005

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Figure 1. Typical XRD pattern of the synthesized complex ZnS(EN)0.5.

Figure 2. Representative SEM image of the synthesized complex ZnS(EN)0.5.

below 200 °C by the use of ZnS(EN)0.5 as Zn and S sources and carbon bisulfide, absolute ethanol, benzene, or pyridine as solvent media. As a result, wurtzite-type ZnS with perfect crystallinity and quasi-square or rectangle nanoplate-like morphology can be obtained at a temperature as low as 160 °C. Experimental Section All starting materials were of analytical reagent grade. Anhydrous zinc chloride and pyridine were purchased from Riedel-deHae¨n, and sulfur powder was from PRS, Panreac. Ethylenediamine, carbon bisulfide, absolute ethanol, and benzene were from International Laboratory, BDH Chemicals Ltd., Merck, and Lab-Scan, respectively. All chemicals were used directly without further treatment. Complex ZnS(EN)0.5 was prepared as follows: 0.32 g of sulfur powders and 1.36 g of ZnCl2 were loaded into a 100 mL capacity Teflon-lined stainless steel autoclave, and then 80 mL of ethylenediamine was added. The autoclave containing the reactant solution was sealed and placed into a programmed furnace to be kept at 160 °C for 24 h, and then it was naturally cooled to room temperature. The resulting white precipitate was collected and washed with deionized water and absolute ethanol to remove ions possibly remaining in the final product, and finally it was dried at 80 °C overnight in a vacuum oven. Similar procedures for the synthesis of complex ZnS(EN)0.5 were adopted for the wurtzite-type ZnS nanoplates. A total of 0.1 g of ZnS(EN)0.5 complex as Zn and S sources and 70 mL of carbon disulfide (CS2) as the solvent medium were loaded into a 100 mL capacity Teflon-lined stainless steel autoclave. A solvothermal reaction was performed at 180 °C for 24 h in a programmed furnace. After the sample was naturally cooled to room temperature, the resulting precipitate was filtered and washed with deionized water and absolute ethanol to remove

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Figure 3. XRD pattern of the sample prepared in CS2 solvent medium. byproducts possibly remaining in the final product, and finally the sample was dried at 80 °C overnight in a vacuum oven. Several analytical techniques were used to characterize the synthesized products. PXRD patterns were recorded on a Bruker D8 Advance Powder X-ray diffractometer (using Cu KR λ ) 0.15418 nm radiation) operating at 40 kV/40 mA, with a graphite reflected beam monochromator and variable divergence slits. The scanning rate was 0.02°/s. The morphology and size of as-prepared complex were examined by a LEO 1450VP scanning microscope operating at 20 kV. Transmission electron microscopy (TEM) photographs, selected area electron diffraction (SAED) patterns, and high resolution transmission electron microscopy (HRTEM) images for the wurtzite ZnS were recorded on a JEOL JEM-2010F field emission transmission electron microscope at an acceleration voltage of 200 kV. The X-ray photoelectron spectra (XPS) were collected on an ESCALab MKII X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-ray as the excitation source. The binding energy (BE) values obtained in the XPS analysis were corrected by referencing the C 1s peak to 284.60 eV.

Results and Discussion The synthesized complex was identified by PXRD and SEM techniques. Figure 1 shows the typical PXRD pattern of the complex. Compared with the PXRD pattern presented by Ouyang et al.,13 the observed PXRD pattern can be indexed as orthorhombic ZnS(EN)0.5 with unit parameters of a ) 17.263 Å, b ) 6.93 Å, and c ) 6.205 Å and the space group Pbca (Z ) 8). SEM observations further confirm that the complex consists of a platelike morphology, which is in excellent agreement with the reported results in the literature.9,10 Figure 3 depicts the typical PXRD pattern of ZnS nanoplates obtained in CS2 medium. All of the diffraction peaks in Figure 3 can be indexed to hexagonal wurtzite-2H ZnS with the lattice parameters of a ) 3.82098 Å and c ) 6.25730 Å [S. G. P63mc (186)] (JCPDS 36-1450). No diffraction peaks of the complex ZnS(EN)0.5 and the characteristic (200) peak belonging to cubic sphalerite ZnS at 2θ ) 33.1° (JCPDS 05-0566) could be detected, indicating that pure wurtzite ZnS has been obtained. The sharp and strong peaks also confirmed that the product is well crystallized. It is well-known that ZnS possesses cubic sphalerite and hexagonal wurtzite polymorphs, and wurtzite ZnS consists of three structure polytypes of 2H, 4H, and 6H. These wurtzite structure polytypes and the sphalerite structure are formed by changing the stacking sequence of the closed-packed planes of the ZnS crystal. A

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Figure 4. Schematic unit-cell structures and stacking models of wurtzite and sphalerite ZnS (A). TEM micrograph (B), SAED pattern (C), and HRTEM image (D) of the sample described in Figure 3.

stacking of ABAB forms the hexagonal wurtzite, while a stacking of ABCABC forms the cubic sphalerite. The schematic unit-cell structures and stacking models of hexagonal and cubic ZnS are depicted in Figure 4a. Commonly, cubic sphalerite ZnS is the most stable polymorph under ambient conditions, while hexagonal wurtzite ZnS is a thermodynamically metastable polymorph. The hexagonal wurtzite ZnS can convert spontaneously to the cubic structure. The bulk cubic ZnS transforms to the hexagonal polymorph at a high temperature of 1020 °C.14 Even for nanosized cubic ZnS (2.8 nm), the polymorph transition to a hexagonal phase still requires a high temperature of 400 °C.15 So far, there have been only a few cases in which pure wurtzite ZnS nanostructures were directly obtained either with high-temperature9,10,16 or with a solution-based method.17 Among the high-temperature methods, a minimum temperature of 250 °C is required to obtain wurtizte ZnS nanoplates under the vacuum condition.10 Typical examples of solution-based methods are the polyol-assisted synthesis of wurtzite ZnS nanocrystals with an average size of less than 5 nm at 150 °C17a and the coordination polymer route to wurtzite ZnS nanorods at 140 °C.17d In this regard, the current work provided a novel lowtemperature solution pathway to obtain 2D nanoplates of wurtzite ZnS. Figure 4b shows a representative TEM image of the sample synthesized under the same conditions as that in Figure 3. The TEM image indicates that the wurtzite phase ZnS are very thin plates with square- or rectangle-

like morphology. The lateral dimensions are in the range of 1-2 µm. The corresponding SAED pattern in Figure 4c shows that these nanoplates are well-crystallized single crystals, and these diffraction spots can be readily indexed to wurtzite structure ZnS, consistent with the XRD analysis. HRTEM analysis in Figure 4d exhibits well-resolved 2D lattice fringes, further confirming the single-crystal nature of the nanoplates. The plane spacings of 3.11 and 3.31 Å correspond to the lattice planes of (001) and (100) in wurtzite-2H ZnS, respectively, indicating that the 2D nanoplates grow along the directions of [001] and [100]. Compared with the nanoplates obtained by the thermal decomposition of the precursor ZnS(EN)0.5,9 we obtained much more perfect nanocrystals that manifested no striated fringes and well-resolved 2D lattice planes. This can be contributed to the mild nature of the low-temperature solvothermal process. The purity and composition of synthesized ZnS were examined by XPS analyses. The typical XPS results are shown in Figure 5, including (a) the survey spectrum, (b) S 2p, (c) Zn 2p3, and (d) Zn LMM. The peaks of Zn and S, together with those of C and O, can be clearly seen in the survey spectra. The weak peaks of O and C might come from H2O, O2 and CO2 adsorbed on the surface of the ZnS nanoplates in air. From the survey spectrum, we can conclude that the product was pretty pure. The BEs are 162.05 eV for S 2p and 1022.05 for Zn 2p3, and the kinetic energy for Zn LMM is 990.07 eV. These results are consistent with the BE values

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Figure 5. Typical XPS spectra of the synthesized wurtzite ZnS nanoplates: (a) survey spectrum, (b) S 2p region spectrum, (c) Zn 2p3 region spectrum, and (d) Zn LMM region spectrum.

reported by Wagner.18 Quantification of peaks gave a ratio of Zn to S of 1.03:1.00, which is in excellent agreement with the stoichiometry of ZnS. To investigate the effect of solvent medium on the product purity, we used absolute ethanol, pyridine, benzene, and deionized water instead of carbon bisulfide as the reaction medium. Beyond this, the effect of solvothermal temperature on the purity was also investigated. A series of experimental results showed that product purity significantly depends on the reaction temperature and the solvent nature. In the absolute ethanol medium, a 200 °C solvothermal temperature was required to obtain pure wurtzite ZnS, while in the pyridine or benzene solvent medium pure wurtzite ZnS was obtained at 180 °C. Furthermore, when CS2 was used as solvent medium, a 160 °C solvothermal temperature led to pure wurtzite ZnS. TEM analyses confirmed that theses wurtzite ZnS are composed of nanoplates with rectangle laterals (see Supporting Information). The stability of the precursor complex ZnS(EN)0.5 has been investigated using the thermogravimetric (TG) analysis in inert argon, nitrogen, or air atmosphere by Deng et al.,9 Yu et al.,10 and Ouyang et al.,13 respectively. The TG results showed that the complex ZnS(EN)0.5 starts to decompose above 260 °C. This means that the ZnS(EN)0.5 precursor could sustain the temperatures applied in current work. However, our solvothermal experiments led to pure wurtzite ZnS below the decomposition temperature of the complex ZnS(EN)0.5. It appears that the solvent plays a crucial role in the formation of pure wurtizte ZnS. On one hand, the solvents act as not only a thermal conducting medium but also as an extractant or active nucleophilic agent that can destroy the ethylenediamine bridged between zinc sulfur layers in the complex ZnS(EN)0.5, so that a low-temperature solvothermal process results in the formation of wurtzite ZnS. On the other hand, these

organic solvents also stabilize the formed metastable wurtzite phase. The supporting evidence was provided in the water medium experiments (discussed below). Moreover, in CS2 medium, wurtzite ZnS nanoplates could be obtained at a lower temperature of 160 °C. This can be ascribed that CS2 is not only an extractant and a thermal conducting medium but also a nucleophilic agent that can react with ethylenediamine. The nucleophile reactions of ethylenediamine with CS2 can be formulated as19

CS2 + H2NCH2CH2NH2 ) H2NCH2CH2NHCSSH (1) CS2 + H2NCH2CH2NHCSSH ) HSSCHNCH2CH2NHCSSH (2) Thus, a lower solvothermal temperature resulted in the formation of pure wurtzite ZnS. Moreover, deionized water, as a clean, environmentally friendly, and low-cost solvent, also can be used to obtain wurtzite ZnS. PXRD analyses confirmed that a main phase of sphalerite ZnS, together with a small amount of wurtzite ZnS, was always obtained either changing the reaction temperature or prolonging the reaction time (see Supporting Information). Water medium led to the main phase of sphalerite ZnS. This can be ascribed to strong solvation of water relative to those organic solvents. In water medium, metastable wurtzite ZnS might be first formed, and subsequently a polymorph transition process from wurtizte to sphalerite will occur, so that main phase of cubic ZnS was always observed by PXRD analyses. This might be supported by the concurrence of a trace amount of wurtzite with cubic sphalerite. Further work is in progress.

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Conclusions In summary, we demonstrated the formation of wurtzite ZnS nanoplates in a medium of CS2, benzene, pyridine, or absolute ethanol by a mild solvothermal process over a temperature from 160 to 200 °C. The organic solvents not only act as thermal conducting media and extractants or active reactants, but they also stabilize the metastable wurtzite phase, so that hightemperature wurtzite ZnS can be formed under lowtemperature solvothermal conditions. SAED and HRTEM analyses confirm that the nanoplates are singlecrystalline and have preferable growth in the [001] and [100] directions. It can be expected that other chalcogenide 2D nanostructures can be prepared by method similar to that from organic-inorganic hybrid precursors containing 2D II-VI slabs such as ZnSe(en)0.5 and ZnTe(en)0.5.20 Acknowledgment. This work was partially supported from the Natural Science Foundation of Anhui (Project No. 050440701), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CUHK 4027/ 02P). We thank Prof. Ming-Rong Ji for his assistance with XPS measurements.

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Supporting Information Available: TEM images and corresponding SAED patterns of wurtzite ZnS nanoplates (Figure S1); XRD pattern of the sample obtained in water (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. (13)

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