Synthesis and characterization of ZrS3 nanocrystallites

Materials Research Bulletin 39 (2004) 1083–1089

Synthesis and characterization of ZrS3 nanocrystallites Liying Huanga, Kaibin Tanga,*, Qing Yanga, Guozhen Shena, Shaojin Jiab a

Structure Research Laboratory, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received 3 February 2003; received in revised form 1 December 2003; accepted 24 February 2004

Abstract ZrS3 nanocrystallites have been synthesized via a solvothermal route by the reaction between ZrCl4 and thiourea at relatively low temperature. The product was characterized by X-ray diffraction (XRD), transmission electron microscope, X-ray fluorescence, Raman spectrum, and photoluminescence (PL). X-ray diffraction ˚ , b ¼ 3:611 A ˚ , c ¼ 9:012 A ˚, pattern shows the monoclinic cell of ZrS3 with the lattice constants a ¼ 5:128 A  b ¼ 97:13 . The result of X-ray fluorescence gives a Zr(Hf):S mole ratio 1:2.97. The Raman spectrum of the ZrS3 nanocrystallites has a slightly red shift in comparison with that of ZrS3 single crystals. The room temperature photoluminescence of ZrS3 nanocrystallites is also reported. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Chemical synthesis; C. X-ray diffraction

1. Introduction Recently, considerable progress has been made in the synthesis of metal chalcogenide semiconductor nanocrystallites due to their important physical properties and their great potential applications [1–5]. These fascinating systems are expected to exhibit remarkable optical, electrical, and magnetic properties that are quite different from those of their corresponding bulk materials [6–8]. The trichalcogenides (MX3) of the group-IVB transition metals have been intensively studied in the past decades due to their significant quasi one-dimensional anisotropic characters [9–13]. Among them, zirconium trisulfide, ZrS3, with a lamellar structure, and exhibiting chain structure and cleavage properties, has attracted attention due to its optical and electrical properties [14–16]. *

Corresponding author. Fax: þ86-551-3601791. E-mail address: [email protected] (K.B. Tang).

0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.02.019

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In general, ZrS3 is prepared by direct reaction of zirconium and sulfur in a quartz vessel by vapor transport and chemical transport reactions. Because of the difficulty of the crystal growth, the preparation usually needs high temperature (600–1000 8C) and long reaction time (50–600 h) [16–18]. Up to now, the obtained ZrS3 are usually bulk materials. The preparation routes and properties of ZrS3 nanocrystallites are rarely described in the literatures. So it is necessary for investigators to seek mild and convenient conditions to prepare ZrS3 nanocrystallites. Solvothermal synthesis is an effective method for preparing metal chalcogenides under mild conditions, which has attracted more and more attention in recent years. In this paper, ZrS3 nanocrystallites have been synthesized via solvent thermal process at 160–230 8C. To the best of our knowledge, it is the first report about nanostructured ZrS3.

2. Experimental procedure All the manipulation was conducted in a dry glove box with flowing nitrogen gas. In a typical procedure, analytical grade 2.33 g ZrCl4 (Sigma-Aldrich Chemie, GmbH, Germany) and 3.90 g thiourea were put into an autoclave of 50 ml capacity. The autoclave was then filled with n-hexane up to 75% of the total volume. Thereafter, the sealed autoclave was maintained at 230 8C for 48 h, then allowed to cool to room temperature. The precipitate was filtered and washed with CS2, acetone and distilled water respectively to remove sulfur, ammonium chloride, and other impurities. After drying in vacuum at 60 8C for 2 h, orange powders were obtained. ˚ ). X-ray diffraction (XRD) pattern was carried out on a Philips X’ Pert PRO SUPER (l ¼ 1:54178 A Transmission electronic microscopy (TEM) images and electronic diffraction (ED) pattern were taken on a Hitachi model H-800, using an accelerating voltage of 200 kV; the quantitative analysis of the product was investigated by XRF-1800 Sequential X-ray fluorescence spectrometer; Raman spectrum and photoluminescence (PL) were recorded on JY (micro Raman-hr) micro Raman spectrometer with 514.5 nm radiation from an Arþ laser at room temperature.

3. Results and discussion The XRD pattern of the sample is shown in Fig. 1. All the peaks could be indexed to the monoclinic ˚ , b ¼ 3:611 A ˚ , c ¼ 9:012 A ˚ , b ¼ 97:13 , which are ZrS3 phase with lattice parameters a ¼ 5:128 A ˚ , b ¼ 3:624 A ˚, consistent with the reported values (JCPDS Card No. 80-0926, a ¼ 5:124 A ˚ , b ¼ 97:28 ). The quantitative analysis of XRF was as follows: SO3, 65.2799%; ZrO2, c ¼ 8:980 A 32.7334%; HfO2, 1.9867% (mass percent). It is known that element Zr usually coexists with Hf in nature, so there was a small quantity of element Hf in the product. The mole ratio of (Zr, Hf):S was worked out as 1:2.97, which is almost consistent with the stoichiometry of ZrS3. The morphology of the ZrS3 nanocrystallites was examined by using TEM (Fig. 2a–f), which reveals that reaction time and temperature affect the morphologies of the obtained products. Fig. 2a and b show that the morphology of the product prepared at 230 8C for 48 h is sheet-like. The length and width of the products are about 200–800 and 10–30 nm, respectively. The corresponding ED pattern of Fig. 2b is shown in Fig. 2c, which could be indexed to the monoclinic phase of ZrS3. The ED pattern matches the XRD result. When the reaction time is prolonged to 6 days at 230 8C, the products are irregular laminar

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particles with diameter 150–1000 nm (shown in Fig. 2d). While the temperature decreases to 180 8C and the reaction time maintained is 6 days, the products are regular sheets 4.5–6.7 mm in length and 20–100 nm in width (shown in Fig. 2e), and the sheet-like morphology can be clearly seen from a magnified photo shown in Fig. 2f. These results indicated that the longer reaction time would enlarge the particle size and relative low temperature would favor the crystal growth in order. Fig. 3 shows the Raman spectrum of the sample at room temperature. The spectrum shows three strong peaks at 147, 277, and 318 cm1, and two weak peaks at 240 and 524 cm1. It was found that the strong peaks (147, 277, 318, and 524 cm1) corresponded to Ag mode and the weak peak (240 cm1) to the Bg mode of ZrS3 [19]. The broadening Raman line and a slightly downshift of Raman peaks of ZrS3 nanocrystals (147, 240, 277, 318, and 524 cm1 at 300 K) in comparison with that of ZrS3 bulk materials (150, 243, 280, 320, 527 cm1 at 300 K) can be observed [19]. The observed spectrum can be explained by a phonon confinement effect [20–22]. The phonon confinement model is based upon the fact that in an infinite crystal, only phonons near the center of Brillouin zone (q ¼ 0) contribute to the Raman spectrum because of momentum conservation between phonons and incident light, and so Raman peaks are sharp. On the other hand, in a finite crystal, phonons can be confined in space by crystal boundaries or defects. This results in uncertainty in the phonon momentum, allowing phonons with q 6¼ 0 to contribute to the Raman spectrum. This uncertainty is larger for smaller grain sizes, thus the shifting and broadening of the Raman peak increases as the size of crystallite decreases. This situation is similar to the ones recently reported by Balandin et al. [23] for self-assembled CdS quantum dots and Wang et al. [24] for BiI3 nanocrystals. Therefore, it can be deduced that the shifting and broadening of the Raman peaks of ZrS3 nanocrystallites are mainly attributed to the nanosize effect. The room temperature PL spectrum of ZrS3 nanocrystallites was measured. The spectrum (Fig. 4) shows two strong and broad PL peaks mainly located in the red region at 598 (2.074 eV) and 637 nm (1.947 eV) upon excitation at 514.5 nm. In previous work, the photoluminescence of ZrS3 bulk materials

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Fig. 2. (a) TEM images and ED pattern of the obtained sample, and (b) TEM images of the sample prepared at 230 8C for 48 h; (c) ED pattern of (b); (d) TEM image of the sample prepared at 230 8C for 6 days; (e) and (f) TEM image of the sample prepared at 180 8C for 6 days.

was reported at a very low temperature of 4.2 [14] or 6 K [15] and the PL peaks weakened abruptly in a small range of temperature (6–16 K) [15]. Although the detailed PL mechanism for the nano-ZrS3 is unclear, a possible explanation can be attributed to the involvement of the mid-gap trap states, such as surface defects [25–27]. Further tests need to be done to allow us to draw further conclusions. The effects of reaction conditions on the formation of ZrS3 nanocrystallites were studied. When n-hexane, toluene, or cyclohexane, having no coordinating ability, was tried, ZrS3 nanocrystals were obtained. While in pyridine or ethylenediamine system, keeping the other conditions unchanged, no ZrS3 could be obtained. The possible explanation is as follows: in n-hexane, toluene, or cyclohexane system, strong Lewis acid ZrCl4 can form complex with thiourea through the coordination of zirconium(IV) with sulfur atom; the complex subsequently decomposed and then formed target product. The reaction mechanism was similar to the formation of tin sulfides [28] obtained from the

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decomposition of adduct, which was formed by the Lewis acid SnCl4 and thiourea. While in the pyridine/ethylenediamine system, zirconium(IV) formed complex with pyridine or ethylenediamine through the coordination of zirconium with nitrogen atom immediately. The strong coordination effect of pyridine/ethylenydiamine with zirconium prevented zirconium atoms from combining with sulfur atoms because the coordination ability of the nitrogen atom was much stronger than the sulfur atom. So, solvents having no coordinating ability would favor the formation of Zrthiourea complex, which then decomposed into ZrS3.

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In this reaction system, different sulfide sources were also tried. When sulfur and carbon disulfide (CS2) were used to replace thiourea (NH2CSNH2) under other unchanged reaction conditions, no zirconium sulfides (ZrS2 or ZrS3) were obtained. Although the reason of this mechanism is not clear, it is probably due to the weak reactivity between S/CS2 and ZrCl4 under such conditions. It is thought that the temperature below 230 8C is not favorable for S and CS2 to form efficient S radical or S2 in nonpolar solutions (benzene or hexane), and thus a subsequent sulfuration of ZrCl4 cannot be carried out in the route. The influence of the reaction temperature and time on the formation of ZrS3 was investigated. It was found that no product could be obtained if the temperature is below 160 8C. At 180 8C, ZrS3 could be produced, but the crystallinity was poor. If the temperature is higher than 250 8C, the Teflon-line will be distorted. The suitable temperature for crystalline ZrS3 is about 200–230 8C. A reaction time at 200 8C in the range of 26 days did not significantly affect the crystallinity. If the time is shorter than 30 h, the reaction is incomplete. Experiments also showed that excessive NH2CSNH2 plays an important role in the formation of the ZrS3 nanocrystals. At 200–230 8C, when the mole ratio was 1:x (x  3), the yield of the products was low and the products were poorly crystallized. When the mole ratio was 1:x (x  5), the crystallinity of the products was good and the yield was high. Although this technique depends on several factors described above, improvements are still needed in the solvothermal synthesis of ZrS3 nanocrystallites, such as finding better conditions to synthesize uniform ZrS3 nanocrystallites in various sizes.

4. Conclusion In summary, ZrS3 nanocrystallites of different morphologies were prepared via a simple solvothermal synthetic route at low temperature. Raman spectrum shows a slightly red shift in comparison with that of ZrS3 single crystals, which may be due to the nanosize effect of the sample. The room temperature PL spectrum of the prepared ZrS3 nanocrystallites exhibits two wide emission bands at about 598 and 637 nm, which may be attributed to the surface defects of nanocrystallites. Studies show that the coordinating ability of the solvents and sulfide sources are important for the formation of the ZrS3 nanocrystallites. The present route can be readily extended to synthesize other analogous IVB sulfides, such as TiS3 and HfS3.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 20071028).

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