A New Type of 2H-WS2 Particle with Discrete-Formed Plate-Texture Structure Xudong Shan,† Liping You,† Jingmin Zhang,† Xinzheng Zhang,† Jifen Wu,‡ Xun Fu,‡ Dapeng Yu,† and Hengqiang Ye*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4420–4423
Electron Microscopy Laboratory, School of Physics, Peking UniVersity, Beijing 100871, P. R. China, School of Chemistry and Molecular Engineering, Qingdao UniVersity of Science and Technology, Qingdao, Shandong 266042, P. R. China ReceiVed January 29, 2008; ReVised Manuscript ReceiVed August 7, 2008
ABSTRACT: The texture structure of 2H-WS2 plate-like particles has been investigated via electron diffraction (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images. It is found that the ED patterns are a series of speckled ellipses consisting of sharp spots, which are dramatically different from the ED patterns of general textured materials. A new type of 2HWS2 particle with a well-formed plate-texture structure is observed. The particles are well stacked with several 2H-WS2 lamellas which are only several nanometers thick and rotate around the c axis with different angles. Introduction Tungsten disulfide (WS2) is well-known for its layer structures similar to graphite. In the well-studied case of two-dimensional (2D) layered materials (graphite, WS2, MoS2, BN, etc.) at the nanoscale, they prefer to form onion-like structures and nanotubes to reduce the free energy of the system.1-3 A diverse variety of quasi-0D, quasi-1D, and 3D morphologies of WS2, suchastexturethinfilms,4,5 nanotubes,6-9 onion-likestructures,10-12 and closed nanostructures,13 have been synthesized by different methods, which are the potential candidates in catalysts, hydrogen storages, photovoltaic devices, and solid lubricants.14-16 Specially, 2D crystals have also been found from some layered materials in the latest studies,17 and the ED patterns of the monolayer are found to exhibit different characteristics with the bulk material.18 Thus, we think that the layered materials might generate a new variety of morphologies and corresponding dramatic ED patterns due to the intrinsic layered structures. In this work, we report a type of novel ellipse-like ED pattern, which reveal a new type of discrete-formed plate-texture structure in WS2 particles with a thickness of only several nanometers to several tens of nanometers. This work may enrich the understanding of the morphologies of layered materials and their corresponding diffraction patterns, especially the texture structure formed at the nanoscale.
Figure 1. XRD spectrum from the specimen.
Experimental Methods The procedure of synthesizing WS2 nanoparticles was described as follows: (NH4)2WS4 crystals 0.870 g, sodium borohydride (NaBH4) 0.467 g, pyridine 35 mL and trioctylamine 0.327 g were blended in an autoclave with a capacity of 50 mL, and then the mixture was kept at 190 °C for 48 h. After that, the sample was naturally cooled to room temperature. The final products were annealed at 850 °C in argon flow for 2 h after washing and drying in vacuum conditions.19 The as-synthesized samples were studied by general X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM), respectively. The TEM experiments were carried out in a Tecnai F30 with an accelerating voltage of 300 kV.
Results and Discussion A typical powder XRD spectrum of the specimen is shown in Figure 1. All main diffraction peaks match well with 2H* To whom correspondence should be addressed. E-mail:
[email protected]. † Peking University. ‡ Qingdao University of Science and Technology.
Figure 2. (a) A typical elliptic ED pattern obtained from a particle; (b) HRTEM image obtained from the same particle. The inset shows the corresponding FFT, which is the same as the ED pattern in (a). (c) Low-magnification TEM image of WS2 particles; (d) and (e) elliptic electron diffraction patterns acquired from the particles in circle in (c).
WS2 (JCPDS card No. 8-237) structures except the weak peaks arising from the WO3 phase. The small portion of WO3 may
10.1021/cg800107d CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2008
New Type of 2H-WS2 Particle
Figure 3. (a) HRTEM image, obtained from one side of a platelet with three lamellae, which shows a lot of stacking faults and distortions as indexed by arrows; (b) the corresponding ED pattern; (c) a clear stacking fault.
be the byproduct of the synthesis. The (002) peak in the image displays a prominent signal indicating the presence of a wellstacked layered structure. Figure 2 shows that the used 2HWS2 specimens are plate-like particles. The diameters of the particles are in the range of several hundreds of nanometers to several micrometers, and the thicknesses are around only several nanometers to several tens of nanometers. It is dramatic that the ED patterns acquired from a single particle are a series of concentric speckled ellipses, as shown in Figure 2a. The semiminor axis of the smallest ellipse has the identical length for different particles, which corresponds to the (100) reflection of 2H-WS2. However, the length and direction of the semimajor axis of the smallest ellipse are different for different particles, as shown in Figure 2d and Figure 2e (only the smallest ellipse
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of the ED pattern is shown in the following figures). In addition, such ellipse patterns are always obtained as the particles being tilted any angle. We can acquire clear HRTEM images from these thin particles; they even show different ellipses in the ED patterns. However, it is hard to identify the regular lattices in these images. The fast Fourier transforms (FFT) for different small areas (around 10 × 10 nm) in an image are all the same, which are also the same as the ED patterns of the whole particle (Figure 2a and the inset of Figure 2b). It indicates that the particle is well crystallized and uniform in the plate plane. Figure 3 shows the ED pattern and HRTEM image acquired from one plate-like particle with the incident direction perpendicular to the normal of plate. The HRTEM image shows that the particle is stacked by three parallel lamellae with a thickness of 10-15 nm. The 0.62 nm lattice fringes correspond to the 2H-WS2 (002) layers. As shown in the ED patterns, the diffraction spots such as 002, 004, and so on, are almost elongated to a line, which may be caused by two reasons: first, in the point view of the shape factor, the diffraction spots will elongate to rods along the c* direction of the thin lamellae; second, a lot of stacking faults and distortions can be observed in the particle, which can cause diffuse scattering in the ED pattern along the c* direction. Therefore, the elongated diffraction spots are nearly connected to form lines. We note that several reciprocal point-lines parallel to the c axis with varied distances appear as indexed by arrows in Figure 3b, which indicates that these lamellae rotate in different angles around the c axis. In order to interpret the formation mechanism of the elliptic ED patterns, we reconstruct the reciprocal structure of these particles by the tilting method. The semimajor axis of ellipselike ED patterns change to shorter or longer when it is tilted around the semiminor axis of ellipse. In this process, the shortest semimajor axis is equal to the semiminor axis, and a circular
Figure 4. A series of ED patterns acquired under different tilt angles of (a) 0°, (b) 8°, (c) 16°, (d) 24°, (e) 32°, and (f) 40°. A hexagon, indexed as ABCA′B′C′, is shown in (a). ABCA′B′C′ changes along with the transfer of tilting angles, as shown in (b), (c), (d), (e), and (f). The red line is the semimajor axis Rl, the green line is the semiminor Rs, and COA’ (as indexed by θ) is one of the central angles.
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Figure 5. (a) Schematic of reciprocal lattices reconstructed from the diffraction patterns. The vertical dotted lines denote the reciprocal lattices of the particles, and the ellipse indicates the intersection between the Ewald sphere and the reciprocal lattices. Plane ABCA′B′C′ is vertical to the incident direction; (b) the schematic of reciprocal lattice, projected along the direction vertical to the rotation axis, shows clearly the relationship between the semimajor axis Rl and β.
Figure 6. Diagrammatic representation of 2H-WS2 particle, consisting of several lamellae (The diameter of a-b plane is much larger than the thickness along the c direction in real particle).
ED pattern can be obtained (Figure 4a). We take this position as the reference position where the tilting angle is 0°. Figure 4 shows a series of patterns acquired at different tilting angles (0°, 8°, 16°, 24°, 32°, and 40°, denoted by β as shown in Figure 5b) with respect to the reference position. A hexagon indexed by ABCA′B′C′ has been schematically drawn, which indicates the reflections of one lamella. In these patterns, OA has been chosen as the semiminor axis Rs, which the specimen was tilted around. The semimajor axis denoted by the red line Rl becomes longer with increasing tilting angle (Figure 4). We found that Rl and β has the following rule:
Rs ⁄ Rl ) cos β
(1)
The length of Rs does not change from Figure 4a to Figure 4f, but the values of Rl are about Rs, 1.01 Rs, 1.04 Rs, 1.10 Rs, 1.18 Rs, and 1.35 Rs, so the corresponding β values are 0°, 8.1°, 15.9°, 24.6°, 32.1°, and 42.2°, which are consistent with the experimental values of β 0°, 8°, 16°, 24°, 32°, and 40°. This rule favors that these ellipses are obtained by a plane cutting the same cylinder with different angles. According to the experimental results described above, the schematic drawing of the tilting method was summarized in Figure 5. In addition, we also studied the change of central angle of ellipses in the process of tilting. As shown in Figure 5a, the central angle R in the projection of the ellipse can be converted to θ in the ellipse with the following expression:
θ ) arctg(tgR ⁄ cos β)
(2)
Here, R ) 60° and β ) 0°, 8°, 16°, 24°, 32°, and 40°, then the calculated values θ ) 60.00°, 60.24°, 60.97°, 62.19°, 63.91° and 66.14°, respectively. In Figure 4a-f, the θ values are measured to be about 60.0°, 60.3°, 61.1°, 62.6°, 64.1°, and 66.4°, which are in good agreement with the calculated values.
Figure 7. (a) Low-magnification TEM image of a particle, and a series of SAED patterns acquired from edge to center in turn as indexed by arrows; (b) HRTEM image obtained from the boxed region in (a); (c) FFT of HRTEM image shown in (b); (d) and (e) digitally processed images using a filter that includes the spots indicated by green and red circles in FFT in (c). The processed lattice image only is presented on the top half of (e), but is presented fully in (d), which indicates that the green spots and red spots are contributed by different lamellae, respectively.
Therefore, based on eqs 1 and 2, we can conclude that these elliptical diffraction patterns are produced by the reflection plane cutting a cylinder-like reciprocal structure. According to above experiments and analysis, we can establish a discrete-formed plate-texture structure model for the
New Type of 2H-WS2 Particle
WS2 particles: the particles are stacked well with several 2HWS2 lamellae, which are several nanometers thick and rotate around the c axis with different angles (Figure 6). The elliptical electron diffraction (ED) patterns and their corresponding reciprocal structure are well interpreted by this model. According to literature results,20,21 in the general plate-texture, the plates rotate at different angles around the texture axis; a corresponding rotation of the reciprocal lattice will generate a ring, and each Laue zone produces a set of rings lying on coaxial cylinders. The axis of the coaxial cylinders is the texture axis. When the diffraction direction is tilted at a certain angle from the texture axis, the Ewald sphere cuts these rings, and produces the oblique texture ED patterns with an elliptical shape. However, the ellipse-like ED patterns in this work exhibit properties dramatically different from the ED patterns of the general plate-texture. First, in general textures the ellipse can be obtained only with certain tilting angles, but in this work, the ideal ellipse-like ED patterns can always be obtained when the incident direction is tilted away from the c axis. The reason is that due to the shape effect of the thickness and the diffuse scattering of staking faults reflection spots are elongated to a line along the c* direction, and generate an ideal coaxial cylinder along the c axis. Second, in general textures the normal of lamellae usually are randomly disordered around the texture axis, which results in the reciprocal lattice rings becoming spherical belts. The intersection between the spherical belts and the Ewald sphere produces arches.20,21 However, in this work, the ellipses consisting of reflection spots are always sharp. Several lamellae stack rotated around the c axis with different discrete angles, and form an ideal plate-texture structure in one particle. In addition, the number of diffraction spots on one ellipse is generally the sextuple of the number of lamellae; the intensity of spots is decided by the thickness of their corresponding lamella and tilting angle. On the basis of the results obtained above, one further step shows the generation of the discrete-formed plate-texture structure and the corresponding ED patterns more clearly. In Figure 7a, a series of select-area electron diffraction (SAED) patterns were acquired from the edge to center of a particle. The reflection spots gradually connected to ellipse during the movement of the aperture from the edge to the center. The profile of the ellipses is fixed, and the number of spots in the ellipses is increasing. These results reveal well that the particles are stacked by several lamellae with different rotation angles around the c axis. Moreover, Figure 7d and Figure 7e are inversed from a set of six-spot reflections as indicated by green and red circles in Figure 7c, which is the FFT image of Figure
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7b. The layer structure of WS2 is clearly seen. Therefore, the HRTEM image in Figure 7b is formed by different overlapped lamellae. Conclusions A new type of discrete-formed (001) plate-texture structure was found in 2H-WS2 particles with a thickness at the nanoscale. The particle is well stacked with a few lamellae consisting of only several layers or tens of layers, and have a lot of stacking faults in the lamellae. Because of the shape effects of thin lamellae and stacking faults, the diffraction spots are elongated to a line, which is parallel to the c* axis and generate an ideal cylinder. Therefore, the ellipse-like ED patterns are obtained when the incident direction is tilted away from the c axis. We view that the perfect elliptic ED patterns may be realized if the lamellae in such a texture are thin enough, such as the texture stacked by the monolayer of layered material.18
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