- Email: [email protected]
Facile synthesis of zirconium trisulfide and hafnium trisulfide nanobelts: Growth mechanism and Raman spectroscopy
Solid State Sciences 13 (2011) 1166e1171
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Facile synthesis of zirconium trisulfide and hafnium trisulfide nanobelts: Growth mechanism and Raman spectroscopy Hua Jin, Dan Cheng, Jixue Li, Xuejing Cao, Benxian Li, Xiaofeng Wang, Xiaoyang Liu*, Xudong Zhao** State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2010 Received in revised form 5 December 2010 Accepted 7 December 2010 Available online 13 December 2010
High-purity zirconium trisulfide (ZrS3) and hafnium trisulfide (HfS3) nanobelts with maximal length of 2e5 mm have been successfully synthesized through a chemical vapor transport (CVT) process. XRD results show that ZrS3 and HfS3 crystallize in the monoclinic system. SEM images reveal that ZrS3 and HfS3 nanobelts have thickness ranging from 60 to 120 nm, and HRTEM images and ED patterns confirm that [100] is the dilated direction and [010] is the elongated direction for both the trisulfide nanobelts. The formation of these trisulfide nanobelts is interpreted by a vapor-solid mechanism. The results of Raman scattering show that a slight red shift and peak broadening are observed relative to those of large crystals, which may be induced by the phonon confinement effect. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nanobelt Zirconium Hafnium Sulfide Chemical vapor transport Raman
1. Introduction Low-dimensional nanomaterials, such as nanobelts and nanowires, have attracted much attention in recent years because of their unusual geometry of high aspect ratios and promising physical properties. Transition-metal chalcogenides, due to their potential applications in nanolasers [1,2], solid-state lubricants [3], catalysis [4], hydrogen storage materials [5,6] and field-emitted devices [7], have also become one of the most actively researched fields. During the past two decades, various morphologies of transition-metal chalcogenide nanomaterials, such as NbS2 nanowires [8] and nanotubes [9], TiS2 nanostructures [10,11], WS2 nanotubes [12], TaS2 nanowires [13], and ZnS nanomaterials [14] have been synthesized successfully through vapor transport or solution reactions. However, most of these preparations require additional catalysts, transport media or templates, and the simple synthesis of sulfide nanobelts remains, therefore, poorly studied. Transition-metal trichalcogenide ZrS3 and HfS3 crystallize isomorphously and both grow in a monoclinic structure, where infinite chains of MS6 (M ¼ Zr, Hf) trigonal prisms sharing faces run parallel to the crystallographic b-axis. Neighboring chains are offset by b/2
* Corresponding author. Tel./fax: þ86 431 8516 8316. ** Corresponding author. Tel./fax: þ86 431 8516 8601. E-mail addresses: [email protected] (X. Liu), [email protected] (X. Zhao). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.12.017
and connected laterally into sheets through metalechalcogen bonds. All layers are weakly held together in a two-dimensional structure only by van der Waals’ forces. Both MS3 (M ¼ Zr, Hf) crystal structures have chain-like and layered characters simultaneously, which facilitate the process of intercalation and, therefore, promise potential uses in rechargeable battery cathodes [15]. Their electrical conductivity, Hall coefficient and Seebeck coefficient at 100e500 K, and optical properties suggest that ZrS3 and HfS3 are wide-gap semiconductors [16e18]. Moreover, they are attractive materials for photoelectrochemical cell applications. Although a few synthetic methods for ZrS3 and HfS3 have been developed, their nanoscale synthesis remains a challenge with limited success. ZrS3 nanocrystallites was synthesized via a solvothermal route from reactions between ZrCl4 and thiourea at low temperature (160e230 C) [19]. As a precursor to ZrS2 nanobelts arrays, ZrS3 nanobelts on Zr foils were prepared by Yu-Ling Zhang et al. [20] through a vapor transport approach with sulfur at 650 C for 5 h. However, concomitant ZrS2 reduced the sample’s purity, which was shown in their X-ray diffraction results. HfS3 nanobelts, to the best of our knowledge, have not yet been reported. Chemical vapor transport (CVT), an effective process improved from chemical vapor deposition (CVD), has become popular for growing low-dimensional nanomaterials, especially sulfides. The essential steps of this methods include: (1) a molecular species is grown from a source; (2) at proper temperature, this species is then transported in vapor phase to a location where it deposits; (3) the
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
1167
broadening are observed in comparison with large crystals. Such observation is likely induced by the phonon confinement effect. 2. Experimental section
Fig. 1. XRD patterns of ZreS and HfeS products obtained at 600 C for 15 days. (a) Monoclinic ZrS3 nanobelts. (b) Monoclinic HfS3 nanobelts.
species forms a nanostructure [21]. Therefore, it is extremely important to control the atmosphere throughout the process according to the involved chemical reactions. In order to obtain sulfides in nanostructure, thermodynamic parameters of the reaction should be optimized, such as reaction temperature, duration, temperature gradient, and the rate of temperature increase. Herein, we report the preparation and characterization of high-purity ZrS3 and HfS3 with nanobelt structures through a facile, cost-effective, and green CVT process that yields nanobelts with up to 2 mme5 mm in length. We also discuss the impacts of reaction parameters with an aim of finding out the optimum reaction condition for the CVT process. Raman spectra of the ZrS3 and HfS3 nanobelts at room temperature are also reported, where a slight red shift and peak
Synthesis of ZrS3 and HfS3 nanobelts:ZrS3 and HfS3 nanobelt crystals were prepared from the reaction between the powders of the individual metal and sulfur in evacuated sealed quartz-glass tubes with the diameter of 10 mm and the length of 150 mm. The mixture of the powders of elemental Zr (Beijing Cui Bo Lin, 99.9%) (or Hf (Beijing Cui Bo Lin, 99.9%)) and sulfur sublimate (Alfa Aesar, 99.99%) were ground as reactants with a molar ratio of M:S ¼ 1:3.5. The reactant mixture (∼1 g) was transferred using a long-stemmed funnel in the quartz-glass tubes closed at one end. The charged tubes were evacuated and sealed, then heated in a horizontal tube furnace. The synthesis was achieved by establishing a temperature gradient of 10 K/cm along the tubes with reactants at the constant temperature area of the furnace center (namely the hot end). Then ZrS3 (or HfS3) nanobelts were obtained on the inner surface of the tubes at the cool end when charges were heated at 600 C for 15 days. The products were characterized by X-ray diffraction (XRD, Cu Ka1 radiation, Rigaku D/max2550VB, Japan), scanning electron microscopy (SEM, JEOL Ltd., JSM-6700F, Japan), transmission electron microscopy (TEM, JEOL Ltd., JEM-3010, Japan), and Raman spectroscopy (Renishaw inVia Raman Microscope, UK). Raman spectra were excited by a solid-state laser at 532 nm radiation. The XRD results were analyzed using MDI Jade 6.5. 3. Results and discussion 3.1. Structure and morphology of ZrS3 and HfS3 nanobelts The powder X-ray diffraction (XRD) patterns of the products obtained at 600 C for 15 days are shown in Fig. 1a and b, which
Fig. 2. SEM images of ZrS3 and HfS3 nanobelts synthesized by the CVT process at 600 C (a) and (c) Characteristic ZrS3 and HfS3 nanobelts. (b) and (d) Details of ZrS3 and HfS3 nanobelts with thickness from 60 to 120 nm.
1168
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
Fig. 3. EDS analysis of ZrS3 and HfS3 nanobelts.
correspond to pure ZrS3 (JCPDS No. 65-2346) and HfS3 (JCPDS No. 65-2348). The diffraction peaks in Fig. 1a can be indexed as the monoclinic phase of ZrS3 with unit cell parameters a ¼ 5.116(3) Å, b ¼ 3.635(7) Å, c ¼ 8.965(2) Å, b ¼ 97.46(9) , while those in Fig. 1b as pure HfS3 with unit cell parameters a ¼ 5.086(5) Å, b ¼ 3.592 (2) Å, c ¼ 8.979(9) Å, b ¼ 97.42(9) also in the monoclinic system. The structure calculations were optimized by least-squares refinement. Furthermore, no impurity peaks were observed in the patterns. Observing with unaided eyes, the translucent crystals of ZrS3 were red and HfS3 were orange. The morphologies of the ZrS3 and HfS3 nanobelts were further examined by SEM (Fig. 2). The nanobelts exhibit belt-like morphology with smooth surfaces and the thickness of the highyield nanobelts ranges approximately from 60 to 120 nm. As shown in the SEM images, nanobelts are the main products, which have an aspect ratio of length to width more than 4000:1, presumably because of the inherent preferential growth habit of the ZrS3 and HfS3 crystals. Energy dispersive spectroscopy (EDS) analysis was also performed on the samples during the SEM measurements. The EDS spectra for the nanobelts are presented in Fig. 3, and the results indicate that the product compositions are close to stoichiometric 1:3 for both ZrS3 and HfS3. TEM and electron diffraction (ED) can provide further insights of the structure and orientation of the as-prepared nanobelts. Both techniques were used to the nanobelts (Fig. 4a and b for ZrS3, Fig. 4c and d for HfS3). TEM images at low magnification (Fig. 4a and c) show that the ground crystal products are nanobelts in appearance,
Fig. 4. TEM and ED images of ZrS3 and HfS3 nanobelts synthesized at 600 C showing the details of their geometry. (a) and (b) Images of ZrS3 nanobelts being single crystals and structurally uniform with an interplanar spacing about 0.51 nm (c) and (d) Images of HfS3 nanobelts with an interplanar spacing about 0.50 nm. Corresponding electron diffraction patterns (inset) confirm that the single-crystal nature of the samples can be indexed to a monoclinic unit cell consistent with the XRD results.
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
while high-resolution TEM images (Fig. 4b and d) reveal that the nanobelts are single crystals. As clearly revealed in Fig. 4b, the ZrS3 nanobelts are structurally uniform with an interplanar spacing of 0.51 nm, which corresponds to the interlayer spacing d100 (0.5124 nm) of monoclinic phase ZrS3, whereas in Fig. 4d the interlayer spacing of the HfS3 nanobelts is 0.50 nm, which correlates to the spacing d100 (0.5092 nm) of monoclinic phase HfS3. Therefore, HRTEM analysis suggests that the dilated growth of the nanobelts occurs along the [100] direction with the a-axis parallel to the dilated direction of the nanobelts. The ED analysis (inset) also confirms that the samples are single crystal with the dilated
1169
direction [100] and elongated direction [010], agreeing well with the HRTEM results. 3.2. Mechanism and the effects of the thermodynamic parameters 3.2.1. Mechanism Based on the ZreS binary phase diagram, when the molar ratio of Zr:S is less than 1:3, ZrS3 is the only product at 500e1100 C. However, ZrSx (x 2) appears if sulfur is insufficient. Therefore, ZrS2 impurity is expected in the report by Yu-Ling Zhang et al. [20], and it is likely brought in by the Zr substrate in the bottom end of
Fig. 5. SEM images of ZrS3 and HfS3 nanobelts synthesized at different temperature. (a) and (b) Nanobelts of ZreS and HfeS obtained at 500 C showing a mixture of short nanobelts and residual reactant impurity (in ellipses). (c) and (d) ZrS3 and HfS3 nanobelts with smooth faces synthesized after heated at 600 C (eeh) Large belt-like crystals obtained at 800 C that are facile to split into nanobelts and nanobelt bundles, with (e) and (g) for ZrS3, and (f) and (h) for HfS3.
1170
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
the ZrS3 nanobelts where there could be sulfur insufficiency. In the present system, the formation of the trisulfides nanobelts can be interpreted by vapor-solid (VS) mechanism. At the reaction temperature 600 C, sulfur (bp 444.6 C) is evaporated into gas form, and then quickly reacts with Zr powders to form ZrS3. The product ZrS3 diffuses in vapor phase, and deposits on the inner surface of the quartz tube at the cool end, where it gradually grows into nanobelts. Because of the highly similar properties between Zr and Hf, HfS3 nanobelts grow following the same process. 3.2.2. Effects of the thermodynamic parameters In order to optimize the thermodynamic parameters, a series of experiments with various reaction parameters were designed and conducted. The morphologies of the ZrS3 and HfS3 crystals obtained at 500, 600, and 800 C for 15 days are shown in Fig. 5. The results manifest that short nanobelts (100e500 mm in length) of ZrS3 and HfS3 with impurities of residual reactant powders (Fig. 5a and b) were obtained at 500 C, which implies that the reaction was incomplete at this condition. According to the temperature gradient 10 K/cm, the temperature of large part of the quartz-glass tube (nearly 10 cm) was below the boiling point of the sulfur (bp 444.6 C), which means that it was easy for sulfur to be deposited there and unfavorable for nanobelts growth. However, at or over 600 C, the temperature of whole quartz tube was over 450 C and the nanobelts grew well. Only a small quantity of residual sulfur was deposited at the end of cool end after the reaction, which was clearly separated from the nanobelts. Therefore, the hot end of the quartz tube should be heated at or over 600 C. After increasing the temperature to 600 C, the
sulfide products turned into smooth and pure nanobelts (Fig. 5c and d) with the maximal length of 2e5 mm, and had an aspect ratio of length to width more than 4000:1. At this reaction temperature, the product growth appeared highly anisotropic with preferential growth along the b-axis direction. The products subsequently grew further to 0.50 cm 10e30 mm 1.0 mm for ZrS3 and 1.0 cm 5.0e10 mm 1.0 mm for HfS3 (Fig. 5e and f) with a decreasing aspect ratio less than 2000:1, when the reaction temperature finally achieved 800 C. At this temperature, the growth along the a-axis increased remarkably. Therefore, the reaction temperature is considered a key parameter to control the shape of the nanobelts. Moreover, large crystals with cuboid-like morphology likely split into nanobelts and nanobelt bundles (Fig. 5g and h), because of the easy cleavage in the bonds of metalechalcogen along the a-axis direction and weak van der Waals’ bonds along the c-axis direction. A comparison between the products obtained after heating for 3 days and 15 days demonstrates that the yield and length of nanobelts increased significantly with prolonged reaction time. In addition, the temperature ramping rate has to be set at 1 C/min, because quick temperature increase frequently could lead to tube explosion failures. Hence, the optimum reaction temperature and duration for the ZrS3 and HfS3 nanobelts growth were found to be 600 C and 15 days. 3.3. Raman spectroscopy of the ZrS3 and HfS3 It is well-known that monoclinic ZrS3 and HfS3 structures belong to the C2h space group. For such a structure, the 24 normal modes at the center G of the Brillouin zone can be represented as:
Fig. 6. Room-temperature Raman spectra of ZrS3 and HfS3 nanobelts and large single crystals. (a) and (b) Raman spectra of ZrS3 and HfS3 large crystals, respectively; (c) and (d) Raman spectra of ZrS3 and HfS3 nanobelts, respectively. All spectra were recorded under the same excitation wavelength of 532 nm.
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171 Table 1 Raman peak wavenumbers (cm1) and symmetries for Raman modes in ZrS3 and HfS3. HfS3
ZrS3
Motion type
Large crystal Nanobelt Sym. Large crystal Nanobelt Sym. (cm1) (cm1) (cm1) (cm1) 107 120 150 234 244 281 320 360
107 120 148 232 241 277 317 355
Ag Ag Bg Bg Bg Ag Ag Ag
529
525
Ag
111 124 138 217 246 259 320.5 349 381 525
109 122 138 213 255.5 316.5 345 377 521
Ag Ag Bg Bg Bg Ag Ag Ag Ag Ag
Rigid
MeS intra- and inter- chain
Diatomic SeS
G ¼ 8Ag þ 4Bg þ 4Au þ 8Bu Each mode has been correlated to a set of atomic vibrations, parallel to the chains of MS6 trigonal prisms for Au and Bg and perpendicular to the chains for Ag and Bu. Hereinto, the 12 evenparity modes of 8Ag and 4Bg are Raman active. Raman spectra of trisulfides (ZrS3 and HfS3) crystals and nanobelts were obtained at room temperature with an excitation wavelength of 532 nm (Fig. 6). All Raman data and assignments of the appropriate motion types are summarized in Table 1. The strong Raman peak at 525 cm1 for ZrS3 nanobelts (Fig. 6c) and one at 521 cm1 for HfS3 nanobelts (Fig. 6d) are attributed to the stretching mode of the (SeS)2 group, and the shoulders around 515 cm1 may originate from an isotopic effect of 34S (natural abundance 4.2%) according to previous reports [22,23]. No weak peak at 470 cm1 is observed in all the samples, which otherwise is considered as the combination peak from excessive sulfide. Moreover, the peaks at 377 cm1 in HfS3 and 360 cm1 in ZrS3 are expected to have a second-order scattering [24]. Since Bg modes always have weak intensity, they are easily masked by strong Ag modes. In this regard, one would expect that the disregard of the Bg mode at 246 cm1 should correspond to the strong Ag modes at 255.5 cm1 for HfS3 nanobelts. The Raman spectra also reveal that the peaks of the ZrS3 and HfS3 nanobelts are slightly shifted toward lower frequency and broadened in comparison with that of the large single crystals (Fig. 6a and b) in our experiments at the same excitation wavelength. Such phenomenon in nanobelts is believed to be ascribed to the phonon confinement effect [19,25], which emanates from the decrease in crystal size. In a large crystal with a perfect translational symmetry, the crystal vibration is continuous, and only phonons at the center of the Brillouin zone (q ¼ 0) can be excited due to momentum conservation. It results in sharp Raman peaks. When the crystal size decreases to nanoscale, therefore destroying the translational symmetry, the crystal vibration is confined in smaller space by crystal boundaries or defects. Thus, the phonons with q s 0 are excited, and additional peaks may appear. This gives rise to broadening and red shift of the Raman peaks, which are more apparent with smaller crystal sizes. L. Huang et al. [19] and Y. Xiang et al. [25] also observed similar changes in the Raman spectra of ZrS3 nanocrystals and Ge nanowires. It is noteworthy that a comparison of wavenumbers between low-frequency region 200 cm1 and 200e400 cm1 should provide information about the orientation of the nanobelts. Theoretical calculation [21,22] confirmed that the former low-frequency modes emanate from rigid-chain characters that are only slightly influenced by the crystal size effect.
1171
Meanwhile, as the latter modes attributed to the MeS intra- and inter- chain stretching motions respond strongly to the crystal size along both the c-axis and a-axis directions, it is obvious that the nanocrystallization mainly arises perpendicular to the b-axis direction, which is consistent with the results of HRTEM in Fig. 4. Thus, the red shift and broadening of the Raman peaks of ZrS3 and HfS3 nanobelts are induced by the phonon confinement effect and interaction with the orientation of the nanobelts. 4. Conclusions In this study, ZrS3 and HfS3 nanobelts with maximal length of 2e5 mm were successfully synthesized at 600 C for 15 days, using a simple and green chemical vapor transport (CVT) method without additional reagents. During the synthetic process, reaction temperature and duration was found to be the two most important factors to control the shape of nanobelts and the thermal parameters was optimized. Raman spectra of the nanobelts revealed a slight red shift and peak broadening in comparison with those of large crystals, which could be induced by the phonon confinement effect. Moreover, this method is expected to apply to large-scale, high-purity growth of a wide range of low-dimensional nanostructures owing to its high yield, simple reaction apparatus, and low contamination. Acknowledgment This work was supported by the Natural Sciences Foundation of China (No. 20471022, 40673051 and 20121103) and NCET. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
C. Falcony, M. Garcia, A. Ortiz, J.C. Alonso, J. Appl. Phys. 72 (1992) 1525e1527. T.V. Prevenslik, J. Lumin. 87e89 (2000) 1210e1212. P. Somuri, Z. Jeffrey, Nature 387 (1997) 761e763. J. Chen, S.L. Li, Q. Xu, T. Koji, Chem. Commum. (2002) 1722e1723. J. Chen, K. Nobuhiro, H. Yuan, T.T. Hiroyuki, S. Tetsuo, J. Am. Chem. Soc. 123 (2001) 11813e11814. J. Chen, S.L. Li, Z.L. Tao, Y.T. Shen, C.X. Cui, J. Am. Chem. Soc. 125 (2003) 5284e5285. M. Bredol, J. Merikhi, J. Mater. Sci. 33 (1998) 471e476. Z.J. Yi, K.H. Wen, L.C. Yu, J.C. Li, Q.Z. Yan, B. Kieren, W.K. Harold, R.M.W. David, Angew. Chem. Int. Ed. 43 (2004) 5670e5674. G. Seiferta, H. Terronesb, M. Terronesc, T. Frauenheima, Solid State Commun. 115 (2000) 635e638. Y.R. Tao, X.C. Wu, Y.L. Zhang, L. Dong, J.J. Zhu, Z. Hu, Cryst. Growth Des 8 (8) (2008) 2990e2994. J. Chen, S.L. Li, Z.L. Tao, F. Gao, Chem. Commun. (2003) 980e981. Y.D. Li, X.L. Li, R.R. He, J. Am. Chem. Soc. 124 (2002) 1411e1416. W.D. Charles, K.E. Hannah, D.B. Paul, H.G. Duncan, Angew. Chem. Int. Ed. 45 (2006) 7060e7063. N. Liang, Y. Shimizu, T. Sasaki, H. Umehara, N. Koshizaki, J. Phys. Chem. B. 108 (2004) 9728e9733. P. NovaÂk, R. Imhof, O. Haas, Electrochim. Acta 45 (1999) 351e367. G. Perluzzoa, A.A. Lakhania, S. Jandla, Solid State Commun. 35 (1980) 301e304. A. Ait-Ouali, S. Jandl, Phys. Rev. B. 53 (1996) 9852e9858. W. Schairer, M.W. Shafer, Phys. Stat. Sol. (a) 17 (1973) 181e184. L. Huang, K. Tang, Q. Yang, G. Shen, S. Jia, Mater. Res. Bull. 39 (2004) 1083e1089. Y.L. Zhang, X.C. Wu, Y.R. Tao, C.J. Mao, J.J. Zhu, Chem. Comm. (2008) 2683e2685. D. Moore, Z.L. Wang, J. Mater, Chem 16 (2006) 3898e3905. P. Gard, C. Soueisseau, O. Gorocho, Phys. Stat. Sol. (b) 144 (1987) 885e901. S.P. Gwet, Y. Mathey, C. Sourisse, Phys. Stat. Sol. (b) 123 (1984) 503e517. A. Ait-Quali, S. Jandl, P. Marinier, J.M. Lopez-Castillo, A.M.S. Tremblay, Phys. Rev. B. 46 (1992) 5183e5193. Y. Xiang, I. Zardo, L.Y. Cao, T. Garma, M. Heiß, J.R. Morante, J. Arbiol, M.L. Brongersma, A. Fontcuberta, I. Morral, Nanotechnology 21 (2010) 105703 (6pp).
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Facile synthesis of zirconium trisulfide and hafnium trisulfide nanobelts: Growth mechanism and Raman spectroscopy Hua Jin, Dan Cheng, Jixue Li, Xuejing Cao, Benxian Li, Xiaofeng Wang, Xiaoyang Liu*, Xudong Zhao** State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2010 Received in revised form 5 December 2010 Accepted 7 December 2010 Available online 13 December 2010
High-purity zirconium trisulfide (ZrS3) and hafnium trisulfide (HfS3) nanobelts with maximal length of 2e5 mm have been successfully synthesized through a chemical vapor transport (CVT) process. XRD results show that ZrS3 and HfS3 crystallize in the monoclinic system. SEM images reveal that ZrS3 and HfS3 nanobelts have thickness ranging from 60 to 120 nm, and HRTEM images and ED patterns confirm that [100] is the dilated direction and [010] is the elongated direction for both the trisulfide nanobelts. The formation of these trisulfide nanobelts is interpreted by a vapor-solid mechanism. The results of Raman scattering show that a slight red shift and peak broadening are observed relative to those of large crystals, which may be induced by the phonon confinement effect. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nanobelt Zirconium Hafnium Sulfide Chemical vapor transport Raman
1. Introduction Low-dimensional nanomaterials, such as nanobelts and nanowires, have attracted much attention in recent years because of their unusual geometry of high aspect ratios and promising physical properties. Transition-metal chalcogenides, due to their potential applications in nanolasers [1,2], solid-state lubricants [3], catalysis [4], hydrogen storage materials [5,6] and field-emitted devices [7], have also become one of the most actively researched fields. During the past two decades, various morphologies of transition-metal chalcogenide nanomaterials, such as NbS2 nanowires [8] and nanotubes [9], TiS2 nanostructures [10,11], WS2 nanotubes [12], TaS2 nanowires [13], and ZnS nanomaterials [14] have been synthesized successfully through vapor transport or solution reactions. However, most of these preparations require additional catalysts, transport media or templates, and the simple synthesis of sulfide nanobelts remains, therefore, poorly studied. Transition-metal trichalcogenide ZrS3 and HfS3 crystallize isomorphously and both grow in a monoclinic structure, where infinite chains of MS6 (M ¼ Zr, Hf) trigonal prisms sharing faces run parallel to the crystallographic b-axis. Neighboring chains are offset by b/2
* Corresponding author. Tel./fax: þ86 431 8516 8316. ** Corresponding author. Tel./fax: þ86 431 8516 8601. E-mail addresses: [email protected] (X. Liu), [email protected] (X. Zhao). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.12.017
and connected laterally into sheets through metalechalcogen bonds. All layers are weakly held together in a two-dimensional structure only by van der Waals’ forces. Both MS3 (M ¼ Zr, Hf) crystal structures have chain-like and layered characters simultaneously, which facilitate the process of intercalation and, therefore, promise potential uses in rechargeable battery cathodes [15]. Their electrical conductivity, Hall coefficient and Seebeck coefficient at 100e500 K, and optical properties suggest that ZrS3 and HfS3 are wide-gap semiconductors [16e18]. Moreover, they are attractive materials for photoelectrochemical cell applications. Although a few synthetic methods for ZrS3 and HfS3 have been developed, their nanoscale synthesis remains a challenge with limited success. ZrS3 nanocrystallites was synthesized via a solvothermal route from reactions between ZrCl4 and thiourea at low temperature (160e230 C) [19]. As a precursor to ZrS2 nanobelts arrays, ZrS3 nanobelts on Zr foils were prepared by Yu-Ling Zhang et al. [20] through a vapor transport approach with sulfur at 650 C for 5 h. However, concomitant ZrS2 reduced the sample’s purity, which was shown in their X-ray diffraction results. HfS3 nanobelts, to the best of our knowledge, have not yet been reported. Chemical vapor transport (CVT), an effective process improved from chemical vapor deposition (CVD), has become popular for growing low-dimensional nanomaterials, especially sulfides. The essential steps of this methods include: (1) a molecular species is grown from a source; (2) at proper temperature, this species is then transported in vapor phase to a location where it deposits; (3) the
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
1167
broadening are observed in comparison with large crystals. Such observation is likely induced by the phonon confinement effect. 2. Experimental section
Fig. 1. XRD patterns of ZreS and HfeS products obtained at 600 C for 15 days. (a) Monoclinic ZrS3 nanobelts. (b) Monoclinic HfS3 nanobelts.
species forms a nanostructure [21]. Therefore, it is extremely important to control the atmosphere throughout the process according to the involved chemical reactions. In order to obtain sulfides in nanostructure, thermodynamic parameters of the reaction should be optimized, such as reaction temperature, duration, temperature gradient, and the rate of temperature increase. Herein, we report the preparation and characterization of high-purity ZrS3 and HfS3 with nanobelt structures through a facile, cost-effective, and green CVT process that yields nanobelts with up to 2 mme5 mm in length. We also discuss the impacts of reaction parameters with an aim of finding out the optimum reaction condition for the CVT process. Raman spectra of the ZrS3 and HfS3 nanobelts at room temperature are also reported, where a slight red shift and peak
Synthesis of ZrS3 and HfS3 nanobelts:ZrS3 and HfS3 nanobelt crystals were prepared from the reaction between the powders of the individual metal and sulfur in evacuated sealed quartz-glass tubes with the diameter of 10 mm and the length of 150 mm. The mixture of the powders of elemental Zr (Beijing Cui Bo Lin, 99.9%) (or Hf (Beijing Cui Bo Lin, 99.9%)) and sulfur sublimate (Alfa Aesar, 99.99%) were ground as reactants with a molar ratio of M:S ¼ 1:3.5. The reactant mixture (∼1 g) was transferred using a long-stemmed funnel in the quartz-glass tubes closed at one end. The charged tubes were evacuated and sealed, then heated in a horizontal tube furnace. The synthesis was achieved by establishing a temperature gradient of 10 K/cm along the tubes with reactants at the constant temperature area of the furnace center (namely the hot end). Then ZrS3 (or HfS3) nanobelts were obtained on the inner surface of the tubes at the cool end when charges were heated at 600 C for 15 days. The products were characterized by X-ray diffraction (XRD, Cu Ka1 radiation, Rigaku D/max2550VB, Japan), scanning electron microscopy (SEM, JEOL Ltd., JSM-6700F, Japan), transmission electron microscopy (TEM, JEOL Ltd., JEM-3010, Japan), and Raman spectroscopy (Renishaw inVia Raman Microscope, UK). Raman spectra were excited by a solid-state laser at 532 nm radiation. The XRD results were analyzed using MDI Jade 6.5. 3. Results and discussion 3.1. Structure and morphology of ZrS3 and HfS3 nanobelts The powder X-ray diffraction (XRD) patterns of the products obtained at 600 C for 15 days are shown in Fig. 1a and b, which
Fig. 2. SEM images of ZrS3 and HfS3 nanobelts synthesized by the CVT process at 600 C (a) and (c) Characteristic ZrS3 and HfS3 nanobelts. (b) and (d) Details of ZrS3 and HfS3 nanobelts with thickness from 60 to 120 nm.
1168
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
Fig. 3. EDS analysis of ZrS3 and HfS3 nanobelts.
correspond to pure ZrS3 (JCPDS No. 65-2346) and HfS3 (JCPDS No. 65-2348). The diffraction peaks in Fig. 1a can be indexed as the monoclinic phase of ZrS3 with unit cell parameters a ¼ 5.116(3) Å, b ¼ 3.635(7) Å, c ¼ 8.965(2) Å, b ¼ 97.46(9) , while those in Fig. 1b as pure HfS3 with unit cell parameters a ¼ 5.086(5) Å, b ¼ 3.592 (2) Å, c ¼ 8.979(9) Å, b ¼ 97.42(9) also in the monoclinic system. The structure calculations were optimized by least-squares refinement. Furthermore, no impurity peaks were observed in the patterns. Observing with unaided eyes, the translucent crystals of ZrS3 were red and HfS3 were orange. The morphologies of the ZrS3 and HfS3 nanobelts were further examined by SEM (Fig. 2). The nanobelts exhibit belt-like morphology with smooth surfaces and the thickness of the highyield nanobelts ranges approximately from 60 to 120 nm. As shown in the SEM images, nanobelts are the main products, which have an aspect ratio of length to width more than 4000:1, presumably because of the inherent preferential growth habit of the ZrS3 and HfS3 crystals. Energy dispersive spectroscopy (EDS) analysis was also performed on the samples during the SEM measurements. The EDS spectra for the nanobelts are presented in Fig. 3, and the results indicate that the product compositions are close to stoichiometric 1:3 for both ZrS3 and HfS3. TEM and electron diffraction (ED) can provide further insights of the structure and orientation of the as-prepared nanobelts. Both techniques were used to the nanobelts (Fig. 4a and b for ZrS3, Fig. 4c and d for HfS3). TEM images at low magnification (Fig. 4a and c) show that the ground crystal products are nanobelts in appearance,
Fig. 4. TEM and ED images of ZrS3 and HfS3 nanobelts synthesized at 600 C showing the details of their geometry. (a) and (b) Images of ZrS3 nanobelts being single crystals and structurally uniform with an interplanar spacing about 0.51 nm (c) and (d) Images of HfS3 nanobelts with an interplanar spacing about 0.50 nm. Corresponding electron diffraction patterns (inset) confirm that the single-crystal nature of the samples can be indexed to a monoclinic unit cell consistent with the XRD results.
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
while high-resolution TEM images (Fig. 4b and d) reveal that the nanobelts are single crystals. As clearly revealed in Fig. 4b, the ZrS3 nanobelts are structurally uniform with an interplanar spacing of 0.51 nm, which corresponds to the interlayer spacing d100 (0.5124 nm) of monoclinic phase ZrS3, whereas in Fig. 4d the interlayer spacing of the HfS3 nanobelts is 0.50 nm, which correlates to the spacing d100 (0.5092 nm) of monoclinic phase HfS3. Therefore, HRTEM analysis suggests that the dilated growth of the nanobelts occurs along the [100] direction with the a-axis parallel to the dilated direction of the nanobelts. The ED analysis (inset) also confirms that the samples are single crystal with the dilated
1169
direction [100] and elongated direction [010], agreeing well with the HRTEM results. 3.2. Mechanism and the effects of the thermodynamic parameters 3.2.1. Mechanism Based on the ZreS binary phase diagram, when the molar ratio of Zr:S is less than 1:3, ZrS3 is the only product at 500e1100 C. However, ZrSx (x 2) appears if sulfur is insufficient. Therefore, ZrS2 impurity is expected in the report by Yu-Ling Zhang et al. [20], and it is likely brought in by the Zr substrate in the bottom end of
Fig. 5. SEM images of ZrS3 and HfS3 nanobelts synthesized at different temperature. (a) and (b) Nanobelts of ZreS and HfeS obtained at 500 C showing a mixture of short nanobelts and residual reactant impurity (in ellipses). (c) and (d) ZrS3 and HfS3 nanobelts with smooth faces synthesized after heated at 600 C (eeh) Large belt-like crystals obtained at 800 C that are facile to split into nanobelts and nanobelt bundles, with (e) and (g) for ZrS3, and (f) and (h) for HfS3.
1170
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171
the ZrS3 nanobelts where there could be sulfur insufficiency. In the present system, the formation of the trisulfides nanobelts can be interpreted by vapor-solid (VS) mechanism. At the reaction temperature 600 C, sulfur (bp 444.6 C) is evaporated into gas form, and then quickly reacts with Zr powders to form ZrS3. The product ZrS3 diffuses in vapor phase, and deposits on the inner surface of the quartz tube at the cool end, where it gradually grows into nanobelts. Because of the highly similar properties between Zr and Hf, HfS3 nanobelts grow following the same process. 3.2.2. Effects of the thermodynamic parameters In order to optimize the thermodynamic parameters, a series of experiments with various reaction parameters were designed and conducted. The morphologies of the ZrS3 and HfS3 crystals obtained at 500, 600, and 800 C for 15 days are shown in Fig. 5. The results manifest that short nanobelts (100e500 mm in length) of ZrS3 and HfS3 with impurities of residual reactant powders (Fig. 5a and b) were obtained at 500 C, which implies that the reaction was incomplete at this condition. According to the temperature gradient 10 K/cm, the temperature of large part of the quartz-glass tube (nearly 10 cm) was below the boiling point of the sulfur (bp 444.6 C), which means that it was easy for sulfur to be deposited there and unfavorable for nanobelts growth. However, at or over 600 C, the temperature of whole quartz tube was over 450 C and the nanobelts grew well. Only a small quantity of residual sulfur was deposited at the end of cool end after the reaction, which was clearly separated from the nanobelts. Therefore, the hot end of the quartz tube should be heated at or over 600 C. After increasing the temperature to 600 C, the
sulfide products turned into smooth and pure nanobelts (Fig. 5c and d) with the maximal length of 2e5 mm, and had an aspect ratio of length to width more than 4000:1. At this reaction temperature, the product growth appeared highly anisotropic with preferential growth along the b-axis direction. The products subsequently grew further to 0.50 cm 10e30 mm 1.0 mm for ZrS3 and 1.0 cm 5.0e10 mm 1.0 mm for HfS3 (Fig. 5e and f) with a decreasing aspect ratio less than 2000:1, when the reaction temperature finally achieved 800 C. At this temperature, the growth along the a-axis increased remarkably. Therefore, the reaction temperature is considered a key parameter to control the shape of the nanobelts. Moreover, large crystals with cuboid-like morphology likely split into nanobelts and nanobelt bundles (Fig. 5g and h), because of the easy cleavage in the bonds of metalechalcogen along the a-axis direction and weak van der Waals’ bonds along the c-axis direction. A comparison between the products obtained after heating for 3 days and 15 days demonstrates that the yield and length of nanobelts increased significantly with prolonged reaction time. In addition, the temperature ramping rate has to be set at 1 C/min, because quick temperature increase frequently could lead to tube explosion failures. Hence, the optimum reaction temperature and duration for the ZrS3 and HfS3 nanobelts growth were found to be 600 C and 15 days. 3.3. Raman spectroscopy of the ZrS3 and HfS3 It is well-known that monoclinic ZrS3 and HfS3 structures belong to the C2h space group. For such a structure, the 24 normal modes at the center G of the Brillouin zone can be represented as:
Fig. 6. Room-temperature Raman spectra of ZrS3 and HfS3 nanobelts and large single crystals. (a) and (b) Raman spectra of ZrS3 and HfS3 large crystals, respectively; (c) and (d) Raman spectra of ZrS3 and HfS3 nanobelts, respectively. All spectra were recorded under the same excitation wavelength of 532 nm.
H. Jin et al. / Solid State Sciences 13 (2011) 1166e1171 Table 1 Raman peak wavenumbers (cm1) and symmetries for Raman modes in ZrS3 and HfS3. HfS3
ZrS3
Motion type
Large crystal Nanobelt Sym. Large crystal Nanobelt Sym. (cm1) (cm1) (cm1) (cm1) 107 120 150 234 244 281 320 360
107 120 148 232 241 277 317 355
Ag Ag Bg Bg Bg Ag Ag Ag
529
525
Ag
111 124 138 217 246 259 320.5 349 381 525
109 122 138 213 255.5 316.5 345 377 521
Ag Ag Bg Bg Bg Ag Ag Ag Ag Ag
Rigid
MeS intra- and inter- chain
Diatomic SeS
G ¼ 8Ag þ 4Bg þ 4Au þ 8Bu Each mode has been correlated to a set of atomic vibrations, parallel to the chains of MS6 trigonal prisms for Au and Bg and perpendicular to the chains for Ag and Bu. Hereinto, the 12 evenparity modes of 8Ag and 4Bg are Raman active. Raman spectra of trisulfides (ZrS3 and HfS3) crystals and nanobelts were obtained at room temperature with an excitation wavelength of 532 nm (Fig. 6). All Raman data and assignments of the appropriate motion types are summarized in Table 1. The strong Raman peak at 525 cm1 for ZrS3 nanobelts (Fig. 6c) and one at 521 cm1 for HfS3 nanobelts (Fig. 6d) are attributed to the stretching mode of the (SeS)2 group, and the shoulders around 515 cm1 may originate from an isotopic effect of 34S (natural abundance 4.2%) according to previous reports [22,23]. No weak peak at 470 cm1 is observed in all the samples, which otherwise is considered as the combination peak from excessive sulfide. Moreover, the peaks at 377 cm1 in HfS3 and 360 cm1 in ZrS3 are expected to have a second-order scattering [24]. Since Bg modes always have weak intensity, they are easily masked by strong Ag modes. In this regard, one would expect that the disregard of the Bg mode at 246 cm1 should correspond to the strong Ag modes at 255.5 cm1 for HfS3 nanobelts. The Raman spectra also reveal that the peaks of the ZrS3 and HfS3 nanobelts are slightly shifted toward lower frequency and broadened in comparison with that of the large single crystals (Fig. 6a and b) in our experiments at the same excitation wavelength. Such phenomenon in nanobelts is believed to be ascribed to the phonon confinement effect [19,25], which emanates from the decrease in crystal size. In a large crystal with a perfect translational symmetry, the crystal vibration is continuous, and only phonons at the center of the Brillouin zone (q ¼ 0) can be excited due to momentum conservation. It results in sharp Raman peaks. When the crystal size decreases to nanoscale, therefore destroying the translational symmetry, the crystal vibration is confined in smaller space by crystal boundaries or defects. Thus, the phonons with q s 0 are excited, and additional peaks may appear. This gives rise to broadening and red shift of the Raman peaks, which are more apparent with smaller crystal sizes. L. Huang et al. [19] and Y. Xiang et al. [25] also observed similar changes in the Raman spectra of ZrS3 nanocrystals and Ge nanowires. It is noteworthy that a comparison of wavenumbers between low-frequency region 200 cm1 and 200e400 cm1 should provide information about the orientation of the nanobelts. Theoretical calculation [21,22] confirmed that the former low-frequency modes emanate from rigid-chain characters that are only slightly influenced by the crystal size effect.
1171
Meanwhile, as the latter modes attributed to the MeS intra- and inter- chain stretching motions respond strongly to the crystal size along both the c-axis and a-axis directions, it is obvious that the nanocrystallization mainly arises perpendicular to the b-axis direction, which is consistent with the results of HRTEM in Fig. 4. Thus, the red shift and broadening of the Raman peaks of ZrS3 and HfS3 nanobelts are induced by the phonon confinement effect and interaction with the orientation of the nanobelts. 4. Conclusions In this study, ZrS3 and HfS3 nanobelts with maximal length of 2e5 mm were successfully synthesized at 600 C for 15 days, using a simple and green chemical vapor transport (CVT) method without additional reagents. During the synthetic process, reaction temperature and duration was found to be the two most important factors to control the shape of nanobelts and the thermal parameters was optimized. Raman spectra of the nanobelts revealed a slight red shift and peak broadening in comparison with those of large crystals, which could be induced by the phonon confinement effect. Moreover, this method is expected to apply to large-scale, high-purity growth of a wide range of low-dimensional nanostructures owing to its high yield, simple reaction apparatus, and low contamination. Acknowledgment This work was supported by the Natural Sciences Foundation of China (No. 20471022, 40673051 and 20121103) and NCET. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
C. Falcony, M. Garcia, A. Ortiz, J.C. Alonso, J. Appl. Phys. 72 (1992) 1525e1527. T.V. Prevenslik, J. Lumin. 87e89 (2000) 1210e1212. P. Somuri, Z. Jeffrey, Nature 387 (1997) 761e763. J. Chen, S.L. Li, Q. Xu, T. Koji, Chem. Commum. (2002) 1722e1723. J. Chen, K. Nobuhiro, H. Yuan, T.T. Hiroyuki, S. Tetsuo, J. Am. Chem. Soc. 123 (2001) 11813e11814. J. Chen, S.L. Li, Z.L. Tao, Y.T. Shen, C.X. Cui, J. Am. Chem. Soc. 125 (2003) 5284e5285. M. Bredol, J. Merikhi, J. Mater. Sci. 33 (1998) 471e476. Z.J. Yi, K.H. Wen, L.C. Yu, J.C. Li, Q.Z. Yan, B. Kieren, W.K. Harold, R.M.W. David, Angew. Chem. Int. Ed. 43 (2004) 5670e5674. G. Seiferta, H. Terronesb, M. Terronesc, T. Frauenheima, Solid State Commun. 115 (2000) 635e638. Y.R. Tao, X.C. Wu, Y.L. Zhang, L. Dong, J.J. Zhu, Z. Hu, Cryst. Growth Des 8 (8) (2008) 2990e2994. J. Chen, S.L. Li, Z.L. Tao, F. Gao, Chem. Commun. (2003) 980e981. Y.D. Li, X.L. Li, R.R. He, J. Am. Chem. Soc. 124 (2002) 1411e1416. W.D. Charles, K.E. Hannah, D.B. Paul, H.G. Duncan, Angew. Chem. Int. Ed. 45 (2006) 7060e7063. N. Liang, Y. Shimizu, T. Sasaki, H. Umehara, N. Koshizaki, J. Phys. Chem. B. 108 (2004) 9728e9733. P. NovaÂk, R. Imhof, O. Haas, Electrochim. Acta 45 (1999) 351e367. G. Perluzzoa, A.A. Lakhania, S. Jandla, Solid State Commun. 35 (1980) 301e304. A. Ait-Ouali, S. Jandl, Phys. Rev. B. 53 (1996) 9852e9858. W. Schairer, M.W. Shafer, Phys. Stat. Sol. (a) 17 (1973) 181e184. L. Huang, K. Tang, Q. Yang, G. Shen, S. Jia, Mater. Res. Bull. 39 (2004) 1083e1089. Y.L. Zhang, X.C. Wu, Y.R. Tao, C.J. Mao, J.J. Zhu, Chem. Comm. (2008) 2683e2685. D. Moore, Z.L. Wang, J. Mater, Chem 16 (2006) 3898e3905. P. Gard, C. Soueisseau, O. Gorocho, Phys. Stat. Sol. (b) 144 (1987) 885e901. S.P. Gwet, Y. Mathey, C. Sourisse, Phys. Stat. Sol. (b) 123 (1984) 503e517. A. Ait-Quali, S. Jandl, P. Marinier, J.M. Lopez-Castillo, A.M.S. Tremblay, Phys. Rev. B. 46 (1992) 5183e5193. Y. Xiang, I. Zardo, L.Y. Cao, T. Garma, M. Heiß, J.R. Morante, J. Arbiol, M.L. Brongersma, A. Fontcuberta, I. Morral, Nanotechnology 21 (2010) 105703 (6pp).