The Study of Structural Transition of ZnS Nanorods under High

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J. Phys. Chem. C 2011, 115, 357–361

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The Study of Structural Transition of ZnS Nanorods under High Pressure Zepeng Li, Bingbing Liu,* Shidan Yu, Jinhua Wang, Quanjun Li, Bo Zou, and Tian Cui State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, China

Zhenxian Liu Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015, United States

Zhiqiang Chen GeoScience Department, Stony Brook UniVersity, Stony Brook, New York 11794, United States

Jing Liu Institute of High Energy physics, Chinese Academy of Sciences, Beijing 100023, China ReceiVed: August 3, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010

The study of pressure behavior for ZnS nanorods by X-ray diffraction measurements up to 37.2 GPa is carried out. ZnS nanorods transform from the initial wurtzite phase to rock salt phase at 19.6 GPa without undergoing the transition to zinc blende structure, and this is the first report about the direct phase transition from wurtzite to rock salt phase for ZnS. The longitudinal c-axis of ZnS nanorods exhibits more compressible behavior than that of radial a-axis direction, which is caused by the special rod morphology, and this induces the special direct phase transition to rock salt phase without zinc blende phase in the transition process. The results show the morphology of ZnS nanorods plays a crucial role in the special pressure behavior and also suggests we could explore the various pressure behaviors applying particular shaped nanomaterials under high pressure. Introduction Nanostructured materials have attracted a great deal of attention in the past few years due to their excellent properties that are different from the bulk materials.1-3 Zinc sulfide (ZnS) as an important wide-band gap (3.6 eV) semiconductor has been used as an important material for ultraviolet light-emitting diodes and injection lasers, flat-panel displays, electroluminescent devices, and infrared (IR) windows.4-7 Thus, various ZnS nanomaterials are given special attention, and the studies of the structure and properties of ZnS nanomaterials are of considerable importance, and thus great efforts have been focused on this topic.8-12 As the properties of materials are relative to the structure, it is both interesting and valuable to investigate the structural behavior of materials. In the past few years, the studies on structural behavior of nanomaterials has mainly focused on the nanoparticles, and the reported results mostly involve the sizeinduced influences on pressure behavior for nanoparticle materials.1,13 Recently, researchers began to explore the pressure behavior of nanomaterials with special morphologies.9,14-18 For example, Park et al. and Guo et al. studied the pressure behaviors of Pt nanocubes and TiO2 nanorods.17,18 Guo et al. observed a face-centered cubic to face-centered tetragonal distortion for the first time,18 and Park et al. find the different compressibility among different shaped TiO2 nanopaticles.17 For ZnS nanomaterials, quasi-one-dimensional ZnS nanomaterials such as nanobelts, nanorods, nanoribbons, and nanotubes are of great * To whom correspondence should be addressed. Phone/Fax: 86-43185168256. E-mail: [email protected].

importance due to their unique nanostructure. Up to now, there are few reports on pressure induced behavior of the samples with special morphologies, and there is only one report for ZnS nanobelts by Wang et al.9 In Wang’s study, it was demonstrated that there is a strong relationship between the mechanical stability and morphology, which are also useful to understand the transformation mechanism. ZnS nanorods, as the typical onedimensional nanomaterials, are expected to exhibit special pressure behavior different from those of ZnS bulk sample and nanopaticles, and it is very interesting to explore the pressure behavior of ZnS nanorods caused by the different pressure effects in different directions. In the previous work, we have ever deduced the phase transition of ZnS nanorods by Raman and photoluminescence (PL) spectra;19 however, there is no study about the pressure behavior for ZnS nanorods by directly determinative X-ray diffraction (XRD) method. In the present work, we have applied high pressure XRD measurements to investigate the pressure behavior of ZnS nanorods. The highest pressure is 37.2 GPa. ZnS nanorods transform from the initial wurtzite (WZ) phase to rock salt (RS) phase at 19.6 GPa without undergoing the transition to zinc blende (ZB) structure. The longitudinal c-axis direction of ZnS nanorod exhibits larger compressibility than that of radial a-axis direction, and this induces the transition from the WZ to the RS phase instead of to the ZB structure. The morphology of ZnS nanorods plays a crucial role in the different pressure behaviors from bulk sample.

10.1021/jp107304v  2011 American Chemical Society Published on Web 12/20/2010

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Li et al.

Figure 2. Pressure-dependent XRD spectra of ZnS nanorods (the indication by the red arrows corresponds to the newly observed diffractions of rock salt structure).

Figure 1. The TEM image of ZnS nanorods sample (a) and the typical rods (b) with its corresponding ED pattern indexed to the WZ structure; (c) the schematic diagram for ZnS nanorod with a, b, and c axes in the WZ structure.

Experimental Section ZnS nanorods used in our high-pressure XRD experiments were synthesized by the solvothermal method as described in our previous work.20 The TEM image of ZnS nanorods, the typical rods with its corresponding ED pattern indexed to the WZ structure, and the schematic diagram for ZnS nanorod with a, b, and c axes in the WZ structure are shown in Figure 1. The diamond anvil cell was used with methanol-ethanol mixture (volume ratio of 4:1) as the pressure medium in the high pressure experiments. The pressure was calibrated by the shift of the ruby R1 line. High-pressure X-ray diffraction experiments were carried out with synchrotron X-ray beam at Brookhaven National Laboratory, US, with the incident monochromatic X-ray beam wavelength of 0.4066 Å and have been repeated at 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF).The MAR345 image plate detector was utilized to record the diffracted X-rays, and the two-dimensional diffraction rings on the image plate were integrated with the FIT2D program to produce diffraction patterns of intensity vs degree. Results and Discussion The sample of ZnS nanorods is in the WZ structure at ambient condition with the growth direction of (002) plane as shown in

Figure 1b.20 The calculated lattice constants are a ) 0.380 nm and c ) 0.622 nm, which agree with the reported data (JCPDS file No.75-1534, space group P63mc). Figure 2 gives the angledispersive high pressure XRD spectra of the ZnS nanorods with the highest pressure of 37.2 GPa. From Figure 2, we find that the position of the diffraction peaks of WZ structure for ZnS nanorods shifts toward to small d-spacing value with the increase of pressure. When pressure is increased up to 19.6 GPa, a new diffraction peak not belonging to WZ structure appears, and the new peak is ascribed to (200) diffraction of RS structure of ZnS which indicates that the phase transition from WZ to RS structure occurs. When pressure is up to 22.1 GPa, all diffraction peaks of WZ structure disappear, and the sample transforms into RS structure completely. RS structure exists until the highest pressure of 37.2 GPa, and no other phase transition happens. In the previous reports, the ZnS sample transforms to the ZB phase first and then to the RS phase under high pressure from the initial WZ phase. For example, Desgreniers et al. has studied the ZnS bulk sample initial in pure WZ and mixture of ZB and WZ structures and found that both samples transform to ZB structure completely and then to RS structure after ZB structure.21 And for nanomaterials, Pan et al. and Qadri et al. studied the phase transition of ZnS nanocrystals under high pressure by energy-dispersive X-ray diffraction and found that the nanocrystals undergo the transition of ZB phase between WZ and RS structure.22,23 Wang et al. also observed the phase transition from WZ to ZB phase in their investigation for ZnS nanobelts and nanoparticles under high pressure by XRD.9,16 For the diffraction patterns of ZB and WZ phases, some diffraction peaks have significant overlapping such as WZ(002)/ ZB(111), WZ(110)/ZB(220), and WZ(112)/ZB(311). In the typical phase transition from WZ to ZB structure, the peaks of WZ(100), WZ(101), WZ(102), and WZ(103) decrease significantly in intensity and then disappear finally; On the contrary, the peaks of WZ(002), WZ(110), and WZ(112) are kept and transform to the corresponding diffractions of ZB(111), ZB(220), and ZB(311). From our experimental data, we have not observed any change which could prove the occurrence of ZB structure in the diffraction pattern, and thus we rule out the possibility of the transformation from WZ to ZB phase. Similar phase transition sequence under high pressure can be observed for ZnO, GaN, and CdSe13,24-29 but has not been reported for either ZnS bulk sample or nanomaterial sample before. Further, the reported critical pressure from the initial WZ phase to the RS phase via ZB phase in the middle or from initial ZB structure to RS structure for ZnS bulk sample and nanoparticles is mainly in range of 12-16 GPa,16,21-23,30-33 and

Study of Structural Transition of ZnS Nanorods

Figure 3. (a) Pressure dependence of lattice parameters a and c of ZnS nanorods in the WZ structure. The dashed line is parallel to the linefitting of parameters a. (b) The strain ratios in the direction of a and c axes for ZnS bulk sample and nanorods from theoretical and experimental data, respectively.

therefore the direct transition from the initial WZ structure to the RS structure shows a higher critical pressure than those from the ZB phase to the RS phase, which is consistent with the theoretical results reported by Wilson.34 By refining the different patterns, we obtain the lattice parameters a and c of the WZ structure of ZnS nanorods under high pressure, and Figure 3a shows the pressure dependency of the decreasing a and c parameters. We find that the decrease of a and c is linear with linear coefficients of -9.3(8.8) × 10-3 Å/GPa and -1.4(1.7) × 10-2 Å/GPa, respectively, and the c parameter decreases more rapidly than a parameter with the pressure. For the WZ structure, the c parameter is larger than the a parameter intrinsically; ZnS nanorod samples grow along the c-axis direction, and the a-axis direction is perpendicular to the nanorod as indicated by Figure 1c. It is reasonable that the larger c parameter decreases rapidly than that of the a parameter under high pressure. However, the nanorods in the longitudinal and radial directions show different pressure effects consequentially due to the difference in the size, which will also result in the different pressure behaviors in the two directions. Thus, the observed different decrease rate of a and c parameters is possibly resulted from the contributions by both factors of the intrinsic property of WZ structure and special rod-shape. To attest that the rod shape take effects on the different decrease rate of a and c parameters of ZnS nanorods, we further studied the strain of a and c axes under high pressure. For the hexagonal crystal, there are five independent elastic constants

J. Phys. Chem. C, Vol. 115, No. 2, 2011 359 (C11, C12, C13, C33, and C55), and corresponding elastic compliance constants are S11, S12, S13, S33, and S55. Thus, we can obtain the strain in different directions for hexagonal crystal basing on the elastic compliance constants at different pressures. There is no report about the study on the strain of hexagonal crystals up to now experimentally, and we employ the theoretical data in the literature to calculate the strain in a- and c-axis directions.35 Figure 3b shows the strain ratios in directions of c and a axes of the ZnS bulk sample from theoretical data and nanorods from experimental data respectively, and we find that the strain in the c axis of ZnS nanorods is larger than that of bulk sample which indicates that nanorods show enhanced compressibility in the c-axis direction comparing with ZnS bulk sample. As is known, the sizes of the a- and c-axis directions differ much for nanorods in WZ structure, and therefore, the nanorods in the two directions show different confinement effect. In the a-axis direction, the confinement is larger than that in c-axis direction resulting in the different compressibility, and this induces the different decrease rate of a and c parameters combining with the contribution of the intrinsic property of the WZ structure under high pressure. Shimojo’s study on the transformation paths from WZ to RS phase of CdSe gave at least three transition paths which are characterized by atomic shifts in the (0001) plane of WZ structure.36 Cai et al. proposed two paths called “hexagonal” and “tetragonal” paths by investigating the transition paths from WZ to RS phase for ZnO.37 They think there is a competition between these two paths in ZnO case, and they also suggest that the axial ratio c/a can serve as a good indicator in experiments to identify the transition path. Boulfelfel et al. suggest two intermediate structures of h-MgO and tetragonal phase in the transition pathways for GaN from WZ to RS phase, and finally they ruled out the possible h-MgO type intermediate structure by static calculations.38 For the two structures of the WZ and ZB phases, it is known that both structures are closely related and that the anions adopt a close-packed arrangement in which the cations occupy one-half of the available tetrahedral holes. From this viewpoint, the structures differ only in the stacking sequences adopted by successive ion layers. In the ZB phase, the sublattices adopt a cubic-close-packed ABCABC... sequence, while in the WZ phase, the ions adopt the hexagonal-closepacked ABABAB... arrangement. The transformation from the WZ to the ZB structure can be achieved by the atomic slippage in the interior of the A or B ion layer easily without changes in the other directions. As discussed above, the ZnS nanorods exhibit different confinement effects in the a- and c-axis directions, and the nanorods are more compressible in the c-axis direction than in the a-axis direction. In the pressure-induced transformation, the atomic slippage in the a-axis direction is influenced by the relatively larger confinement effect than in the c-axis direction compared with the ZnS bulk sample and nonorientational nanoparticles, and this weakens the rearrangement chance in the A or B atomic layer interiorly. On the contrary, the relatively weak confinement and the subsequent facile compressibility in the c-axis direction firstly make atom layers close obviously than that in a-axis direction easily and finally the all atoms evolve into the arrangements of RS structure. Thus, it is the different confinement effect resulted from the special rod shape that induces occurrence of the phase transition behavior from the WZ to the RS phase directly without undergoing ZB structure for ZnS nanorods. This indicates that the shape of ZnS nanorods plays the crucial role in the special pressure

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Li et al. Conclusions We present the study on the pressure behavior of the ZnS nanorods by XRD measurements up to 37.2 GPa. ZnS nanorods transform from the initial hexagonal phase to the RS phase at 19.6 GPa without undergoing the transition to the ZB structure, and this is the first report on the direct transition from WZ to the RS phase for ZnS. The longitudinal c-axis of ZnS nanorods exhibits more compressible behavior than that of the radial a-axis direction that is caused by the special rod morphology, and this induces the special phase transition from the WZ to the RS phase directly in the transition process. The results show the shape of ZnS nanorods plays the crucial role in the special pressure behavior and also suggests we could explore the various pressure behaviors applying particular shaped nanomaterials under high pressure.

Figure 4. Equation of state of ZnS nanorods in the WZ and RS phased with the curve fitted by the Birch-Murnaghan equation.

Acknowledgment. This work was supported financially by the NSFC (10979001, 11074090, 51025206, 51032001, 21073071), the National Basic Research Program of China (2011CB808200), the Cheung Kong Scholars Programme of China, and the National Fund for Fostering Talents of Basic Science (J0730311). Portions of this work performed at National Synchrotron Light Source beamlines X17C and U2A are supported by NSF COMPRES EAR 06-49658 and US-DOE (CDAC, Contract No. DE-AC02-98CH10886). References and Notes

Figure 5. The TEM image of ZnS nanorods after being released from 37.2 GPa to ambient pressure.

behavior, and the result also tells us that we could explore the various pressure behaviors different from that of bulk sample by applying particular shaped nanomaterials under high pressure. Here, we emphasis that the suggestion about the transition process is induced by the analysis about the change behavior of lattice parameters and different strains in a- and c-axis directions of the WZ phase before the transformation because we could not easily observe the structural transition process intuitively experimentally. We encourage further computational studies to reveal the rodshape dependent transition mechanism. The bulk modulus B0 is obtained by fitting the measured V/V0 to the Birch-Murnaghan equation of state where V is the volume at pressure P and V0 is the zero-pressure volume as shown in Figure 4. This yields the bulk modulus of B0 ) 80 ( 1.7 GPa for the WZ phase with the fixed B0′ at 4. The obtained bulk modulus of WZ structure is consistent with that (80.1 GPa) of ZnS bulk sample.21 For the RS structure, the bulk modulus we obtained is 108.7 ( 3.2 GPa with the fixed B0′ at 4 and the zero-pressure volume 32.73 ( 0.13 Å3. The value of the bulk modulus is consistent with that of RS phase reported in the literatures.21,31,33 After phase transition from the WZ to the RS structure, the volume collapse measured is 16.7%. The transmission electron microscopy (TEM) image of the decompressed sample is given in Figure 5, showing that shape of ZnS nanorods is kept after the high pressure treatment.

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