Controllable, Surfactant-Free Growth of 2D, Scroll-Like Tellurium Nanocrystals via a Modified Polyol Process Wei
Zhu,†,‡
Wenzhong
Wang,*,†
Haolan
Xu,†,‡
Lin
Zhou,†,‡
Lisha
Zhang,†
and Jianlin
Shi†
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China, and Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 12 2804-2808
ReceiVed July 9, 2006; ReVised Manuscript ReceiVed September 1, 2006
ABSTRACT: In this study, we demonstrate that single crystalline tellurium with a 2D, scroll-like structure that deviates from the intrinsic linear crystallographic nature can be controllably synthesized via a modified polyol process by the reduction of tellurium nitrate (Te-N-O) in refluxing ethylene glycol, where no growth modifier such as surfactants or templates is involved. In the preparation, the kinetic control of the 2D growth is primarily manipulated by modulating the Te-N-O concentration in the solution. The morphology depends on the how the reactant is added. Stepwise addition of the reactant Te-N-O is crucial for the formation of the scroll-like morphology. It is demonstrated that 1D tubular products with sharp tips rather than 2D structures are achieved when Te-N-O with equal quantity is injected in one step. Lowering the reaction temperature is favorable to the shape perfection of the nanotubes. On the basis of the characterizations, we propose the formation mechanism of this unique 2D structure to be an asymmetric growth process and subsequent epitaxial evolution. As the first report about 2D tellurium nanostructure, this strategy is attractive for enhancing the comprehension about the growth behavior of tellurium nanocrystals in solution system, which is helpful for the following property investigation and potential applications in nanotechnology. Introduction Shape control of semiconductor nanocrystals (NCs) is of fundamental and technological interest for mapping their shapeand size-dependent properties and consolidating their promising applications in optics, electrics, catalysis, biosensors, and data storage.1-6 Presently, soft chemical strategy has been demonstrated as a versatile pathway toward NCs synthesis with controlled morphology. Modulation of the parameters (e.g., ligands, precursors, and reaction temperature) plays a powerful role in affecting the kinetics and thermodynamics in the nucleation and growth of NCs, which achieves shape control and leads to the morphologic diversity. Unlike the synthesis of zero-dimensional (0D) particles and one-dimensional (1D) nanostructures that have been successful in many cases,6-11 however, the control of NCs growth in two-dimensions (2D) has shown to be much more difficult. Up to now, 2D nanostructures have only been explored and realized in narrow systems. The examples mainly include utilization of small molecules or polymers as crystal plane controllers to produce Au,12,13 Ag,14 Bi,15 Co,16 Pd, and Ni17 nanoplates; decomposition of organometallic precursor in solution to get Gd2O3 and LaF3 nanoplates;18,19 use of molecular assemblies as soft template to confine the growth of CdS nanotriangles20 and ZnS or TiO2 nanosheets;21 applying Langmuir monolayers to induce the nucleation and growth of triangular PbS and PbSe.22,23 For the fabrications of such shape-controlled 2D NCs, the employment of appropriate organic ligand is generally necessary as the shape modifier. So far, the exploration about the direct, surfactant/ template-free growth of 2D nanostructures, especially for the materials with non-lamellar texture, still remains a great challenge. As a narrow band semiconductor, elemental tellurium has been intensively studied owing to the numerous applications * To whom correspondence should be addressed. Phone: 86-21-52415295. Fax: 86-21-5241-3122. E-mail:
[email protected]. † State Key Laboratory of High Performance Ceramics and Superfine Microstructure. ‡ Graduate School of the Chinese Academy of Sciences.
as photoconductivity, nonlinear optical responses, piezoelectricity and thermoelectricity.24,25 A wealth of chemical methods have been developed for the synthesis of tellurium nanostructures that mainly focuses on 1D shapes, such as wires, rods, tubes, and belts. For example, Gautam and Rao presented a controlled synthesis of crystalline tellurium nanorods, nanowires, nanobelts, and related structures by a self-seeding solution process.26 Zhu et al. reported the microwave-assisted synthesis of tellurium nanorods and nanowires in ionized liquids.27 Komarneni and co-workers demonstrated a biomolecule, alginic acid, assisted synthesis of tellurium nanowires.28 Qian et al. presented a controlled hydrothermal synthesis of thin tellurium nanobelts and nanotubes.29 Xia et al. reported a polyol process to the production of tellurium nanotubes with well-controlled structures.30 And Yu et al. used amino acids as crystal growth modifiers for controlled growth of tellurium in a hydrothermal process, through which shuttlelike nanotubes based on a scrolling mechanism were achieved.31 Because of the intrinsic 31 helical-chain structure of elemental tellurium, generally, 2D growth is hard to be achieved in the solution route. As a result of shape-dependent properties, however, the availability of 2D Te nanostructures is anticipated to bring in new types of applications or to enhance the performance of the current devices. Furthermore, anisotropic 2D ellipsoidal tellurium is considered to be a very promising photonic crystal.32 For an understanding of the NC growth behavior and mechanism and the following fundamental research and because of prospective applications related to their morphologies, the exploration of the synthesis of 2D Te nanostructure is thus attractive and in demand, whereas the concerned production has not been reported to date. In this paper, we present a modified polyol process for the controllable growth of 2D, scroll-like tellurium nanostructure. No surfactant or template is involved in the preparation. The kinetic control of the NCs growth is primarily manipulated by modulating the tellurium nitrate (Te-N-O) concentration in the solution. The shaping process is believed to accompany an asymmetric growth and subsequent epitaxial evolution. The
10.1021/cg060439o CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006
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dependence of the product morphology on the way in which reactant is added (stepwise addition or addition in one step) is investigated as the crucial influential factor. Experimental Section Materials. The reagents were purchased from Shanghai Chemistry Co. and used without further purification, including bulk tellurium powders (5N), condensed nitric acid (HNO3, AR), ethylene glycol (EG, AR), and ethanol (CP). The Te source used in the following reaction, tellurium nitrate (Te-N-O) powders, was prepared from the oxidation of bulk gray tellurium in condensed HNO3 at room temperature. Typically, 1 g of bulk tellurium was added to 7 mL of HNO3. Under agitation, a white suspension formed after about 10 h. The suspension was then evaporated at 40 °C in air, resulting in the Te-N-O powder. Subsequently, 0.1 g of Te-N-O was dissolved in 40 mL of EG at 80 °C as a stock solution. Synthesis of Te NCs. The synthesis of 2D, scroll-like tellurium was carried out without any presence of surfactant at a suitable reaction temperature and time, using EG as the solvent and the above stock solution as the Te source. In a typical experiment, 20 mL of EG was added in a three-necked flask equipped with a condenser and refluxed for about 30 min at 190 °C. Nine milliliters of stock solution was then added to the above solution in three doses of 2, 2, and 5 mL. The injection interval was kept at 30 min. After the third injection, the reaction was allowed to proceed for an additional 60 min for the NC growth. The whole growth process lasted 120 min. A grayer suspension was finally obtained. The resulting gray products were then separated by centrifugation using a mixture of water and EG, washed several times with water and ethanol, and dried at 50 °C in a vacuum dryer. To investigate the dependence of the product morphology on the way in which reactant is added, we used the same reaction conditions and scales as detailed above except that the 9 mL stock solution was added in one step into the 20 mL of refluxing EG, and the influence of the reaction temperature on the morphology was examined by performing the experiment at 170 °C instead of 190 °C. Characterizations. The X-ray diffraction (XRD) patterns were recorded with a Japan Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu KR radiation over the range 15° e 2θ e 70°. The scanning electron microscope (SEM) characterizations were performed on a JEOL JSM-6700F field emission scanning electron microscope. The transmission electron microscope (TEM) analyses were performed by a JEOL JEM-2100F field emission electron microscope.
Figure 1. XRD patterns of (a) Te-N-O powder, used as the Te source for the growth of Te nanocrystals, and (b) 2D Te nanocrystals.
Results and Discussion The composition and phase structure of the products are first examined by X-ray diffraction (XRD). Figure 1 shows the XRD patterns of both the precursor Te-N-O powders and the asprepared tellurium NCs. In Figure 1a, all the diffraction peaks are in good agreement with the corresponding literature data of Te-N-O (JCPDS 01-0669). The Te-N-O powders would be used as the Te source for the growth of elemental tellurium in refluxing EG solution. Figure 1b displays a typical XRD pattern of the final products. As represented in the micrograph, the diffraction peaks can be readily indexed to elemental tellurium with trigonal phase (space group: P3121 (No. 152)). The calculated lattice constants a ) 4.45 Å and c ) 5.89 Å are consistent with the standard literature values (JCPDS 36-1452). Figure 2 reveals the electronic microscopic images demon-
Figure 2. (a) Typical TEM image of the scroll-like nanocrystals; (b) an individual nanocrystal; (c) HRTEM image showing lattice spacings of 5.88 and 2.14 Å, respectively; (d) ED pattern taken on the plane of the scroll-like structure; (e) TEM of a representative Te nanoplate; (f) SEM image of the resulting products, in which the thickness of the 2D structures is measured to be around 40 nm.
strating the morphology and microstructure of the resulting tellurium NCs. As shown in the micrographs, not 1D but 2D nanostructures are achieved as the products. The strong contrast
2806 Crystal Growth & Design, Vol. 6, No. 12, 2006
Zhu et al.
Figure 3. EDX spectrum of the Te nanocrystals with 2D, scroll-like morphology.
between one dark edge and the pale plane occurs in each NC. This type of conformation is similar in shape with a halfunwrapping scroll, where the dark edge could be considered as the axis of the scroll. A clear structure is displayed in Figure 2b that illustrates an individual NC with scroll-like morphology. Corresponding to the region circled in Figure 2b, a highresolution TEM image (Figure 2c) shows the regular crystal lattice, implying the crystal perfection of the structure. Detailed analysis on the lattice fringes gives interplanar spacing of 5.88 and 2.14 Å, which match well with (001) and (210) plane separations of the standard bulk trigonal Te, respectively. Figure 2d shows the corresponding electron diffraction (SAED) pattern obtained by focusing the electron beam along the [120] axis. Both the HRTEM and ED analyses demonstrate that the scrolllike NCs are single crystalline, free of dislocation, and have the [001] growth orientation along the longitudinal direction (parallel to the axis of the scroll) and [210] growth toward the transverse direction (perpendicular to the axis). Besides scrolllike NCs, a small quantity of Te nanoplates also exists in the resulting products (Figure 2e). Figure 2f is the typical SEM image that depicts the exterior morphology of the as-obtained NCs. The 2D characteristic can be clearly seen in the micrograph. The thickness of the structure is estimated to be approximately 40 nm. Figure 3 , which illustrates the energy dispersion X-ray (EDX) spectrum, further confirms the composition of the as-obtained scroll-like Te NCs. It reveals that the chemical component of the scroll-like conformation is pure tellurium. The peaks from N and O are not detected in the sample. This result is in good accordance with the XRD analysis mentioned above. Generally, the formations of 2D nanomaterials are related to the lamellar crystal structure or to the utilization of organic ligands as crystal plane controllers. As the subject material, elemental tellurium is characteristically highly anisotropic with covalently bonded atoms forming unique helical chains. These chains are bound through weak van der Waals interactions and produce hexagonal lattices. Because of the intrinsic linear crystallographic structure, 1D Te nanostructures (e.g., rods, wires, and tubes) are of greater preference than 2D structure development when the material grows. Therefore, the direct, surfactant- and template-free formation of the 2D structure in our synthesis, where only EG serves as the solvent and TeN-O as the Te source, is interesting and significant. Unlike
Figure 4. The images of the products obtained at different stages: (a) 30 min after the first 2 mL of stock solution was added to the refluxing EG (right before the second injection); (b) 30 min after the second 2 mL was introduced (right before the third injection); the insets are the corresponding ED pattern and SEM micrograph; (c) 30 min after the final 5 mL was added into the refluxing solution.
the formation mechanism of other Te nanostructures reported before,26-31 we suggest that the scroll-like Te is formed on the basis of an asymmetric growth along the [001] orientation and a following epitaxial evolution, which will be discussed below. Figure 4 shows the TEM images of the stepwise intermediate samples to further reveal the whole evolution process of the scroll-like structure. Because the total 9 mL stock solution was added to the hot EG in three parts (herein, 2-2-5 mL step by step in sequence, the interval time is 30 min), the three samples were taken from the refluxing solution exactly 30 min after each injection. A fawn solution yielded when it lasted 30 min after the first 2 mL of stock solution had been injected into the refluxing EG. The corresponding TEM image shown in Figure 4a clearly indicates the formation of the seeds and the asymmetric growth along their circumferential edges. Immature tubular structures with a short aspect ratio formed that consisted
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Figure 5. Proposed growth mechanism for the formation of 2D, scrolllike Te nanocrystals in the modified polyol process.
of the central seed and incomplete surrounding wall. Figure 4b illustrates the sample that was obtained 30 min after the second 2 mL had been added. Compared with the first sample shown in Figure 4a, the asymmetric characteristic of the half-tubular structure is enlarged as a result of the subsequent growth. Although the length of the half-tubular structure increases, the lateral dimension is essentially retained. The SAED pattern demonstrates that the growth occurs along the [001] direction. Correspondingly, a SEM image of the structure is also inserted. As shown in the micrograph, the half-tubular characteristic is obvious and the central seed can be clearly seen. A grayer suspension was obtained 30 min after the final 5 mL of stock solution had been supplied. Two-dimensional, scroll-like NCs appear in the sample (Figure 4c). Compared with that in Figure 4b, the dark edge in Figure 4c is noticeably similar in shape with the side face of the half-tubular structure, and the protrudent part near the center (as arrowed in Figure 4c) is similar to the partial seed that is not surrounded by the walls. These similarities imply that the scroll-like NCs may be the result of evolution of the half-tubular structures. The exact mechanism for the formation of the scroll-like nanostructures is still under investigation. We believe that the growth of the 2D conformation is a manifestation of selective growth under different Te concentration conditions. This shaping process in the modified polyol reaction is proposed schematically in Figure 5. EG is a solvent possessing reducibility.30,33 Upon addition of the Te source to the refluxing EG, tellurium atoms are produced from the reduction of Te-N-O and singlecrystal seeds form through homogeneous nucleation. Further addition of Te atoms to the seed surface would preferentially occur at the circumferential edges of each seed along [001] orientation because of the relatively higher free energies. With the high growth rate at the given temperature, however, the low concentration of Te atoms (only 2 mL of stock solution) cannot fulfill the entire evolution to complete nanotubes but produce immature half-tubular structures with a short aspect ratio. Once the partial walls are produced, furthermore, the dangling sites on the walls would serve as the new growing sites while they are more exposed and active. After the second 2 mL of stock solution was injected, the following growth is inclined to take place along the existing walls, resulting in the increase in the aspect ratio of the half-tubular conformation. This shaping process continues until the growth equilibrium under the concentration is broken. Subsequently, the injection of the final 5 mL of stocking solution leads to the production of excess Te atoms. The concentration of Te atoms become supersaturated compared with the demand for the growth of the half nanotubes. As a response to the developing length of the immature
Figure 6. TEM images of tellurium nanocrystals obtained in the experiments, in which the whole 9 mL of the stock solution was added in one step into the refluxing EG at different reaction temperature. (a, b) Mixture of nanotubes and incomplete nanotubes with a short aspect ratio synthesized at 190 °C; the inset of b is the representative ED pattern taken on an individual nanotube; (c, d) nanotubes with a large aspect ratio and more evolutional walls obtained at 170 °C.
nanotubes, further adsorption of Te atoms on the backbone leads to growth of an epitaxial layer along the transverse direction from the dangling sites on the walls. The final shaping process under supersaturated Te concentration results in the formation of scroll-like nanostructures. To understand whether the stepwise addition of the reactant Te-N-O into the refluxing EG was necessary for the growth of scroll-like shape, we investigated the sample obtained in the experiment where the 9 mL stock solution was added in one step. Images a and b of Figure 6 reveal the typical TEM images of the product when the reaction proceeds for 120 min. As shown in the micrographs, this sample contains mainly a mixture of nanotubes and incomplete nanotubes. The tubular nanostructures are straight and have open ends. The representative SAED pattern (inset of Figure 6b) shows that the nanotubes are single crystalline with the [001] growth direction, and the length is measured to be less than 5 µm and the diameter around 220 nm. No evident scroll-like structures are observed in the product. It implies that without the control of Te source concentration, the NC growth occurs only along the [001] orientation that is determined by the intrinsic linear crystallographic nature of trigonal tellurium. On the other hand, it is demonstrated that lowering the reaction temperature is beneficial for the shape of well-defined nanotubes. Images c and d of Figure 6 illustrate the TEM images of a sample obtained when the refluxing temperature was lowered to 170 °C. The majority of the sample is nanotubes with sharp tips. Compared with Figure 6a, the asgenerated nanotubes have a higher aspect ratio, more evolutional walls, and a lower proportion of incomplete nanotubes. This should be ascribed to the relatively low reaction kinetics. At 170 °C, both the generation rate of Te atoms and the growth rate of nanotubes are reduced. As a result, the shape of the circumferential walls would be more symmetrical, leading to the production of nanotubes with more evolutional walls.
2808 Crystal Growth & Design, Vol. 6, No. 12, 2006
Conclusion In summary, we demonstrate a surfactant-free polyol approach to the synthesis of 2D, scroll-like tellurium with single crystalline structure. The control of the Te source concentration in the refluxing solution is crucial for the evolution of the anisotropic shape. The dependence of morphology on the way of adding stock solution is investigated. In view of the linear crystallographic structure of tellurium, the current work is significant for enhancing the comprehension about the NC growth in the solution-phase approach. Furthermore, this solution processing strategy is potentially practicable to other inorganic semiconductor containing chainlike building blocks (e.g., selenium). Acknowledgment. Financial support from the Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. References (1) Klabunde, K. J. Nanoscale Materials in Chemistry; VCH: Weinheim. Germany, 2001. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (3) Alivisatos, A. P. Science 1996, 271, 933-937. (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545-610. (5) Jun, Y.; Choi, J.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 34143439. (6) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353-389. (7) Zhou, G.; Lu, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zhou, Y.; Wang, S. J. Phys. Chem. B 2006, 110, 6543-6548. (8) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706. (9) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61.
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