Fabrication and Microstructure Characterization of Polytypic ZnS

Nov 12, 2009 - Polytypic ZnS nanocables have been fabricated by controlled experimental conditions. High-resolution transmission electron microscopy ...
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Fabrication and Microstructure Characterization of Polytypic ZnS Nanocables: Symmetrical HWZ Twinned Nanobelts Grown on the Surface of CZB Nanowire Jun Zhang,* Feihong Jiang, and Zhenhong Dai Key Laboratory of Optoelectronic Information Techniques, Shandong Institute Optoelectronic Information Science and Techniques, Yantai UniVersity, Yantai 264005, People’s Republic of China ReceiVed: September 26, 2009; ReVised Manuscript ReceiVed: October 25, 2009

Polytypic ZnS nanocables have been fabricated by controlled experimental conditions. High-resolution transmission electron microscopy (HRTEM) has directly given the evidence that the core wire is a cubic zinc blende (CZB) ZnS nanowire. The sheathed layers grown over the surface of the core in the radial direction are the hexagonal wurtzite (HWZ) ZnS twinned-crystal structure. The growth mechanism of the polytypic ZnS nanocables is proposed on the basis of a two-step process: catalyzed and self-catalyzed. The CZB-phase ZnS nanowires are synthesized by vapor-liquid-solid (VLS) deposition techniques at low temperature. Then, the CZB-phase ZnS plays a role in nucleating HWZ-phase ZnS at higher temperature. So the HWZ-phase ZnS sheathed layer is suggested to be the self-catalyzed growth from the surface of the CZB-phase ZnS. A favorable choice of HWZ phase over CZB when forming nanostructures is likely to be a result of surface energy minimization. 1. Introduction Due to their potential applications, quasi-one-dimensional (Q1D) semiconductor nanostructures, such as nanotubes, nanowires (nanorods), and nanobelts, offer a high degree of interest for furthering the current state of nanotechnology research and development.1–10 As an important wide bandgap (3.91 eV)11 II-VI semiconductor material, ZnS has a high index of refraction and a high transmittance in the visible range. Q-1D ZnS nanostructure has received great attention in recent years for its potential application in optoelectronics. Several Q-1D ZnS nanostructures, including nanowires, nanobelts, nanosaws, nanocables, and hierarchical nanostructures have been reported.12–19 Recently, Wang’s research group has high-yielding synthesized hierarchical structured nanohelices of ZnS.20 Meng’s research group also has successfully fabricated periodically twinned ZnS nanowires and asymmetrically polytypic ZnS nanobelts.21 These experimental results enrich the family of ZnS Q-1D nanostructures. ZnS can be cubic zinc blend (CZB) and hexagonal wurtzite (HWZ).22 Previous research has demonstrated that CZB-phase ZnS nanostructure is prepared at low deposition temperature (680-750 °C) and HWZ-phase ZnS nanostructure is synthesized at higher deposition temperature.23 Here we report the polytypic (CZB/HWZ) ZnS nanocables. The polytypic nanostructure is formed by catalyzed and self-catalyzed mechanism. The CZBphase ZnS nanowires (core) are synthesized by vapor-liquidsolid (VLS) growth. The tip of every polytypic ZnS nanocable has a large head, which is identified as Sn metal particle. The nanoparticle served as the catalyst for the CZB-phase ZnS corewire growth at low deposition temperature. Then, the HWZphase ZnS sheathed layer is fabricated by self-catalyzed growth. The formation of the HWZ-phase ZnS twinned sheathed layers is proposed on the basis of the CZB-phase ZnS core. With the increase of temperature, the CZB-phase ZnS plays an important role in nucleating HWZ-phase ZnS. Therefore, the HWZ-phase ZnS sheathed layers are synthesized to be the self-catalyzed * To whom correspondence should be addressed. Tel.: +86-535-6901933. E-mail: [email protected].

growth of the CZB-phase ZnS surface as the nucleus in the initiation of the nanostructure. The core wire is the CZB-phase ZnS single crystal, the growth direction is along the [111], and the [111] growth direction makes an angle of ca. 30° with the long-axis direction. The shell layers grown on the surface of the core wire are the HWZ-phase ZnS twin crystal, and the twin structure of ZnS shows the (101) lattice fringes. The [101] direction makes two angles of ca. (100° and (75° with the long-axis direction of the nanostructure, respectively. 2. Experimental Section Polytypic ZnS nanocables were fabricated through thermal evaporation via VLS mechanism under controlled experimental conditions. The experimental setup used for the synthesis consists of a horizontal tube furnace, an alumina tube, a gas supply, and a control system. A mixture of commercial metal tin powders and high-purity ZnS nanopowders with mole ratio 1:10 was put in an alumina boat; then the boat was inserted into an alumina tube. The alumina tube was placed inside a horizontal electronic resistance furnace with the center of the boat positioned at the center of the furnace. A silica plate as substrate was typically placed at 5-10 cm from the center of the boat, and the substrate was placed downstream of the gas flow. First, the temperature of the furnace was rapidly increased to 720 °C from room temperature and kept at 720 °C for 0.5 h under a constant flow gas of argon. Argon was introduced into the alumina tube through a mass-flow controller at rates of 100 cm3(STP) min-1. Then the temperature of the furnace was increased to 1000 °C and kept at 1000 °C for 1.0 h under the constant flow gas of argon (about 100 cm3(STP) min-1). After the furnace was slowly cooled to room temperature, the Ar flow was turned off. A layer of woollike products was formed on the walls of the boat and the surface of the substrate. The as-prepared products were characterized and analyzed by scanning electron microscopy (SEM; JEOL JSM-5610LV), high-resolution transmission electron microscopy (HRTEM; JEOL 2010, 200 kV), and energy-dispersive X-ray spectrometer

10.1021/jp909263u  2010 American Chemical Society Published on Web 11/12/2009

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Figure 1. (a) Typical SEM image of the as-synthesized products. (b) EDS recording of an individual nanostructure from the as-synthesized products.

(EDS; JEOL JED-2200) attached to the SEM instrument. The specimens for HRTEM were prepared by putting the as-prepared products in ethanol and immersing them in an ultrasonic bath for 5 min and then dropping a few drops of resulting suspension containing the synthesized materials onto a Cu grid coated with a holey carbon film. 3. Results and Discussion The SEM image in Figure 1a shows the morphologies of the ZnS nanostructures recorded from the as-prepared product. The XRD patterns of the product revealed the overall crystal structure.24 The whole spectrum can be indexed in most peak positions to HWZ-phase ZnS (10-0434), minor CZB-phase ZnS (05-0566), and metal Sn (04-0673).25 Because all strong diffracted peaks in both phases are overlapped in the XRD patterns, it is not appropriate to use the XRD pattern to identify the CZB phase. Careful HRTEMs could be used to further justify the existence of the CZB phase and the HWZ phase. EDS measurements shown in Figure 1b made on an individual ZnS nanostructure indicate that the belt is composed of Zn and S. The molecular ratio of Zn/S of the nanostructure calculated from the EDS quantitative analysis data is close to that of a bulk ZnS crystal. Structural information of the polytypic ZnS nanocables was obtained from HRTEM images as shown in Figure 2a-d. Figure 2a shows that the nanostructure has a long nanowire, and the length is up to several micrometers. It can be seen that a long and straight core wire is in the center and shell layers are grown on the surface of the core wire. The diameter of the core wire is 20 nm. The shell layer was coated over the surface of the core wire in the radial direction. The widths of the shell layers

J. Phys. Chem. C, Vol. 114, No. 1, 2010 295 are about 50 nm. A tip of the nanostructure shown in Figure 2b has a large head as the catalyst for the growth. Therefore, the ZnS nanocables are made up of two parts: The center is the core wire, which is long, straight, and uniform; the sheathed layers are coated on the surface of the core wire. The core wire of the ZnS nanocable (circle A in Figure 2b) is clearly revealed by the HRTEM image in Figure 2c. The direct evidence of observation confirms a crystalline CZB-phase core wire with an interplanar spacing of 0.308 nm, which shows the (111) lattice fringes. The [111] growth direction makes an angle of ca. 30° with the long-axis direction. The shell layer of surrounding the core wire (circle B in Figure 2b) shown in Figure 2d is a twinned-crystal HWZ-phase ZnS, which shows the (101) lattice fringes with an interplanar spacing of 0.292 nm. The [101] directions make an angle of ca. (75° with the long-axis direction of the nanostructure. A selective-area electron-diffraction (SAED) pattern from the stem clearly indicates the shell layers of the nanocable are HWZ-phase and twin-crystal structure, and the core wire is a CZB-phase single crystal. The diffraction patterns of the shell layers are composed of two sets of diffraction spots of twinned structure that have a symmetrical geometrical layout. The symmetrical axis of the diffraction spots was rotated 30°. The structural model of the polytypic ZnS core/shell nanocable is shown in Figure 2e. The core is the CZB-phase single crystal, and the sheathed layers are the HWZ structure. The HWZ-phase twin-crystal ZnS nanobelt was shown in Figure 3a. From the image, it can be seen that the nanostructure terminates at a particle located at the tip and the core wire was not found. The diameter of the droplet is about 10 nm. The widths of the twinned crystal of about 50 nm were symmetrically grown. The HRTEM image of the stem shown in Figure 3b further indicates the microstructure information of the HWZphase and twin-crystal structure ZnS. In this image, the spacing between the lattice planes is about 0.292 nm; the adjacent lattice planes (arrow heads) correspond to the distance between two (101) crystal planes, indicating 〈101〉 as the growth direction for the HWZ-phase ZnS nanowires. The [101] direction makes an angle of ca. (75° with the long-axis direction of the nanostructure. For the tip of Sn catalyst, the adjacent lattice planes (arrow heads) corresponds to the distance between two (200) crystal planes as shown in Figure 3b. The SAED pattern as shown in Figure 3c from the interface area A between the nanoparticle and twinned crystal shows the coexistence of two phases. The two sides are twin structure; the catalyst Sn nanoparticle is single crystal. The SAED pattern as shown in Figure 3d from the selected area B clearly indicates that the nanowire is HWZ phase and twin-crystal structured. The diffraction patterns from Figure 3c,d are composed of two sets of diffraction spots that have a symmetrical geometrical layout. The symmetrical axis of the diffraction spots was also rotated 30°. The structural model of the HWZ-phase twin-crystal ZnS nanobelt is shown in Figure 3e. In our experiment, some amazing growth phenomena were also observed. Several types of wire-ribbon-like growth have also been found in the product. Figure 4a shows that a nanobelt was perpendicularly grown on the surface of a long and straight nanowire. Figure 4b shows that the nanostructure has a long nanowire, a sheet, and a metal head. It can be seen that the nanowire is long and straight; a sheet was grown on the tip of the nanowire. A tip of the sheet shown in Figure 4b has a large head as the catalyst for the growth. The half-feather-like nanostructure (wire ribbon) was found in Figure 4c. A nanobelt was parallel grown on the surface of a long and straight

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Figure 2. (a) TEM image of the ZnS nanocables. (b) Magnified TEM image showing that the uniform diameter of the core is about 20 nm and the width of the shell layer is about 50 nm. The tip of the ZnS nanocable has a large head, which is identified as a Sn metal particle. (c) HRTEM image of the core wire (circle A in b). The result confirms a CZB-phase ZnS with an interplanar spacing of 0.308 nm. The growth direction makes an angle of ca. 30° with the long-axis direction. (d) HRTEM image of the shell layer (circle B in b). The result of observation confirms a HWZphase ZnS with an interplanar spacing of 0.292 nm. The [101] direction of the twinned crystals makes an angle of ca. (75° with the long-axis direction. (e) Structural model of the polytypic ZnS core/shell nanocable.

nanowire. The core diameter is about 100 nm, and the width of the nanoribbon is about 200 nm. The HRTEM image was shown in Figure 4d; the clear lattice fringes indicated a single crystal structure of the ribbon. The spacing between the lattice planes is about 0.223 nm, which agrees well with the (102) spacing of wurtzite ZnS. The adjacent lattice planes (arrow heads) correspond to the distance between two (102) crystal planes, indicating 〈102〉 as the growth direction for the ZnS nanowires. The [102] direction makes an angle of ca. 102° with the longaxis direction of the core wire. In addition, another growth model of the polytypic ZnS nanocable was showed in Figure 5a-d. Figure 5a shows a particle located at the tip of the nanocable. The diameter of the particle is about 50 nm, and the width of the interface is about 20 nm. A magnified TEM image shown in Figure 5b provided further details of the ZnS nanocable. It can be seen that a long and straight axial nanowire is in the center, and two nanoribbons are symmetrically grown on the surface of the axial nanowire.

The core wire was a uniform diameter of 8 nm. The widths of the two nanoribbons are about 50 nm, respectively. The core wire of lattice orientation and shell layers of lattice orientation from HRTEM images shown in Figure 5c,d can clearly be seen between the two nanocrystals from area A and area B in Figure 5a, which depict the interface structure between the Sn particle and the polytypic ZnS nanocable. It is noted that the interface between the Sn particle and the ZnS nanocable is not flat. The domain boundary and change of lattice orientation can clearly be seen between the two nanocrystals. The HRTEM image shown in Figure 5c confirms the CZB-phase core wire with an interplanar spacing of 0.308 nm, and the [111] growth direction also makes an angle of ca. 30° with the long-axis direction. The HWZ-phase shell layers surrounding the core wire are also twinned crystal, which shows the (101) lattice fringes as shown in Figure 5d. But the [101] directions make an angle of ca. (100° with the long-axis direction of the nanostructure.

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Figure 3. (a) TEM image of the catalyzed growth of the twin-crystal ZnS nanoribbons. The nanoribbon terminates at a particle located at the tip. The core wire was not observed. (b) HRTEM image of the HWZ twin-crystal ZnS nanoribbon featherlike structure. The [101] direction makes an angle of ca. (75° with the long-axis direction. (c) SAED pattern clearly indicates the coexistence of two structures of the interface between the nanoparticle and featherlike from the boxed area A. The HWZ twinned structure of the nanoribbons is denoted by normal and dotted lines, and the normal line is rotated an angle of 30°. The catalyst nanoparticle Sn is denoted by a dashed-dotted-dotted line. (d) SAED pattern clearly indicating the twinned structures from boxed area B. The obvious HWZ twinning features of the nanoribbons are denoted by normal and dotted lines, and the normal line is rotated an angle of 30°. (e) Structural model of the twin-crystal ZnS nanoribbon.

The above images from HRTEM reveal that the polytypic ZnS nanocables have been synthesized. The direct experimental evidence of the existence of the CZB-phase ZnS core and the HWZ-phase ZnS shell layers has been given by HRTEM. How did these polytypic ZnS nanocables grow? The VLS process26 has been an important approach in growth of Q-1D nanostructures. On the basis of the VLS growth mechanism, a nanosized catalyst is required as the seed for growth, and the catalyst nanoparticle usually catalyzes growth of just one-phase nanostructure. Our HRTEM images show that a metal nanoparticle is located at the tip of every nanocable. However, there are simultaneously two phases (CZB and HWZ) in every nanocable. So the normal VLS mechanism is not applicable to explain the growth of polytypic ZnS nanocables. Previous research results have demonstrated that the CZB phase has been found as the

nucleus in the initiation of the most common semiconductors, such as ZnO, CdS, CdSe, and MnS.27,28 The metastable CZB structure is rather unstable. Once the crystal becomes bigger, the CZB phase quickly transforms into the HWZ phase. From previous reports and our experimental results described above, a reasonable formation mechanism of the polytypic ZnS nanocables is proposed on the basis of a two-step process: the first step is VLS catalyzed growth CZB-phase ZnS core wire, and the second step is a self-catalyzed growth HWZ-phase ZnS shell layer from the surface of the CZB ZnS core wire, as shown in Figure 6a-c. At low growth temperature (720 °C), CZBphase ZnS core wire is synthesized by VLS mechanism, as shown in Figure 6a. The Sn particles are liquid droplets due to their low melting point (232 °C), and serve as the sites for adsorption of ZnS vapor to grow CZB-phase ZnS core wire.

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Figure 4. (a) TEM image of a nanobelt was perpendicularly grown on the surface of a nanowire. (b) TEM image of a nanostructure has a long nanowire, a sheet, and a metal head. A tip of the sheet has a large head as the catalyst for the growth. (c) TEM image of a half-feather-like nanostructure (wire ribbon). The core diameter is about 100 nm, and the width of the nanoribbon is about 200 nm. (d) HRTEM image of the half-feather-like nanostructure. The adjacent lattice planes (arrow heads) correspond to the distance between two (102) crystal planes, indicating 〈102〉 as the growth direction for the ZnS nanoribbon. The [102] direction makes an angle of ca. 102° with the long-axis direction of the core wire.

Figure 5. (a) TEM image of another polytypic ZnS nanocable. A particle located at the tip of the nanocable. The diameter of the particle is about 50 nm. (b) Magnified TEM image showing that a long and straight core wire is in the center, and shell layers are grown on the surface of the wire. The diameter of the wire is about 8 nm. (c) HRTEM image of the CZB-phase ZnS core wire. The growth direction also makes an angle of ca. 30° with the long-axis direction. (d) HRTEM image of the HWZ-phase ZnS shell layer. The [101] direction makes an angle of ca. (100° with the long-axis direction of the nanostructure.

The direct experimental evidence from Figure 2b, Figure 3a, and Figure 5a has demonstrated that the Sn droplets are located at the growth front the wires and act as the catalytic active sites. The growth direction of the CZB-phase ZnS core wire makes an angle of ca. 30° with the long-axis direction of the nanostructure. With the increase of temperature (above 720 °C), the CZB-phase ZnS becomes rather unstable and the large

surface of the CZB-phase ZnS core wire plays an important role in nucleating HWZ-phase ZnS, as shown in Figure 6b. Therefore, the HWZ-phase ZnS sheathed layers are synthesized to be the self-catalyzed growth of the CZB-phase ZnS surface as the nucleus in the initiation of the nanostructure due to the self-catalysis effect of the cation-terminated surfaces.25,26 Our observation also demonstrates that the shell layers are HWZ

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Figure 6. (a) VLS-catalyzed growth CZB-phase ZnS core wire. (b) Self-catalyzed growth HWZ-phase ZnS shell layer. (c) Proposed growth model for polytypic ZnS nanocables.

phase and twinned crystal and grow on the surface of the core wire, as shown in Figure 6c. In addition, statistical results of the HRTEMs indicate that the growth direction of the HWZphase ZnS shell layers have two models. The growth direction of the HWZ-phase ZnS twinned crystal from model I makes an angle of ca. (75° with the long-axis direction. The growth direction of the HWZ-phase ZnS twinned crystal from model II makes an angle of ca. (100° with the long-axis direction of the nanostructure. In the same way, the growth mechanism of the half-feather-like nanostructure is also interpreted through the two-step process: the catalyzed and self-catalyzed. However, we do not know why only a nanobelt is parallelly grown on the one-side surface of the long nanowire (as shown in Figure 4c). Further detail work is needed to clarify the underlying growth mechanism for the novel nanostructure. 4. Conclusions In summary, we directly observed the CZB-phase ZnS core wire and the HWZ-phase ZnS shell layer of the polytypic ZnS nanocables. A reasonable growth model of the polytypic ZnS nanocables is discussed on the basis of the VLS-catalyzed growth mechanism and self-catalyzed growth mechanism. The CZB-phase ZnS nanowires are synthesized by VLS at low temperature. The HWZ-phase ZnS sheathed layer is suggested to be the self-catalyzed growth of the CZB-phase ZnS surface at higher temperature. The CZB-phase core wire is single crystal. The HWZ-phase ZnS shell layers are twinned crystal. Therefore, the unique nanostructures may be ideal objects for the fabrication of nanoscale functional devices. It is anticipated that these novel structures will have some unique applications in nanophotonics. Acknowledgment. This research was sponsored by the National Natural Science Foundation of China (Grant No. 60277023) and Natural Science Funds of Shandong Province for Distinguished Young Scholar (Grant No. JQ200802). References and Notes (1) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435.

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