Solvothermal synthesis of polycrystalline tellurium nanoplates and their conversion into single crystalline nanorods

Abstract

Uniform and polycrystalline tellurium nanoplates with a thickness of 100–300 nm can be rapidly synthesized by a simple solvothermal method in mixtures of water and ethanediamine at 180 °C with the assistance of glucose. By adjusting the concentration of glucose, tellurium microrods and nanoplates can be prepared. Based on the electron microscope observations, a possible mechanism involved the self-assembly process and dissolution–recrystallization mechanism is proposed for explaining the formation of polycrystalline tellurium nanoplates and their transformation into single crystalline nanorods. Furthermore, the Raman scattering measurement for the different morphologies is conducted, and the polycrystalline nanoplates present the stronger Raman scattering spectrum.

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Introduction

Shape control of semiconductor nanocrystals has been attracting much interest for mapping their shape- and size-dependent properties and consolidating their promising applications in optics, catalysis, gas sensors, and energy storage.^1–4 Recently, many solution strategies have been introduced for the synthesis of semiconductor NCs with controlled morphology. Modulation of the parameters (e.g., surfactants,^5,6 precursors,⁷ volume ration of solvents,^8,9 and reaction temperature¹⁰) plays a crucial role in affecting the kinetics and thermodynamics in the nucleation and growth of NCs, resulting in the different morphology of materials. Among these parameters, the kind of surfactant is more important. In a previous report, the surfactant could control the growth direction and the aggregation behavior of the materials, which results in the formation of different morphologies.¹¹ For example, hollow peanut-like BiVO4 was synthesized using L-Lysine as surfactant.¹² Flower-like PbTe dendrites have been fabricated assisted by cyclodextrin.⁹ Hierarchical porous nanostructures assembled from ultrathin MnO₂ nanoflakes have been prepared with the presence of triblock copolymer PEO-PPO-PEO (P123).¹³ Nonetheless, the rational design of nanomaterials with controllable morphologies as well as their formation mechanism still remains a challenge.

Trigonal tellurium, a narrow band semiconductor, has been intensively attracted much attention due to its unique properties such as photoconductivity, thermoelectricity, high piezoelectricity, gas sensing, and nonlinear optical properties.^14–16 Therefore, many approaches have been developed for the synthesis of tellurium nanostructures. Single-crystal tellurium nanotubes have been presented through a poly(ethylene glycol) mediated hydrothermal route.¹⁷ Yu's research group reported the microwave-assisted synthesis tellurium nanowires in the presence of polyvinylpyrrolidone (PVP).¹⁸ Tellurium nanorods have been synthesized via the microwave-assisted method in ionized liquids.¹⁹ Qian and co-workers presented a controlled hydrothermal synthesis of thin tellurium nanobelts and nanotubes.²⁰ And the shuttlelike tellurium nanotubes based on a scrolling mechanism were achieved using amino acids as crystal growth modifiers.²¹ Because of the intrinsic 3₁ helical-chain structure of elemental tellurium, generally, two dimensional nanostructures is hard to be achieved in the solution route. Although two dimensional scroll-like tellurium nanocrystals have been synthesized via a modified polyol process in the previous report,²¹ the exploration of the synthesis of two dimensional tellurium nanostructures and their formation mechanism is still attractive and in demand.

Here in this paper, a facile glucose mediated solvothermal route is presented for the controllable synthesis of two dimensional tellurium polycrystalline nanoplates. And the conversion of polycrystalline nanoplates into single crystalline nanorods driven by reaction temperature is investigated systematically. Based on the experimental results, a rational growth mechanism is proposed. Furthermore, the Raman spectrum for the polycrystalline nanoplates and single crystalline nanorods are measured, and the polycrystalline nanoplates display the stronger Raman scattering.

Experimental section

All chemicals were purchased from Aladdin Chemical Reagent Co., Ltd. and used as received without further purification. In a typical synthetic procedure, the solution was prepared by dissolving 0.6 g of glucose and 0.5 mmol of sodium tellurite into 8 mL of distilled water and 20 mL of ethanediamine at room temperature. After the precursor was dissolved completely, the solution was transferred into a 40 mL of Teflon-lined stainless steel autoclave, sealed, and heated in an oven at 180 °C for 1–8 h. After cooling to room temperature naturally, the precipitate was collected, washed with ethanol and distilled water for several times. Then the product was dried in vacuum at 80 °C for 12 h for further characterization.

The crystal phase and composition of the as-prepared samples were detected on the Rigaku D/Max 2550V X-ray diffractometer with high-intensity Cu-K radiation (λ = 1.54178 Å). The morphology and the crystal structure of the products were recorded with a field-emission scanning electron microscope (FESEM, FEI Quanta 200F) transmission electron microscopy (TEM, FEI Tecnai G2 S-Twin operating at 300 kV). Raman scattering spectra were performed on a Renishaw System 2000 spectrometer using the 514 nm line of Ar⁺ for excitation.

Results and discussion

The X-ray diffraction pattern shown in Fig. 1 confirms the samples obtained from reduction of sodium tellurite in the presence of ethanediamine after 1 h solvothermal treatment. All diffraction peaks can be indexed to the hexagonal phase of tellurium with the space group of R3̄m, which are in good agreement with the standard literature data (JCPDS no. 36-1452). And no other characteristic peaks of impurities such as TeO₂ and Na₂TeO₃ are detected. The lower diffraction intensity of the product fabricated at 180 °C for 1 h indicates that the crystallinity is comparatively poor.

Fig. 1 XRD pattern of the polycrystalline tellurium nanoplates synthesized via a solvothermal method in the mixture of water and ethanediamine at 180 °C for 1 h in the presence of glucose.

The morphologies of the samples were determined by field-emission scanning electron microscopy (FESEM). The typical low-magnification SEM image in Fig. 2a shows that the obtained tellurium is composed of numerous uniform platelike structures, and some nanoplates are aggregated together. The magnified SEM images in Fig. 2b and c further confirm the tellurium nanoplates with the thickness of 100–300 nm. Interestingly, the surface of the nanoplates is coarse, indicating the nanoplates are assembled by some nanoparticles. Meanwhile, some nanorods can be occasionally observed. In order to ascertain that the Te nanoplate are composed of nanoparticles, the TEM image is presented in Fig. S1.† The TEM image (Fig. S1a†) and the corresponding HRTEM image (Fig. S1b†) determine that Te nanoplates are nanoparticle aggregations. To further investigate the morphology and crystallographic feature of the tellurium nanoplates, TEM, HRTEM images and the SAED pattern are recorded (Fig. 2d–f). As is depicted in Fig. 2d, all the particles are plate-like morphology, in agreement with the SEM observation. The HRTEM image clearly shows interfaces with d-spacings of 0.324 nm and 0.385 nm, correspond to the (011) and (100) crystallographic planes of hexagonal tellurium (Fig. 2e). And the different growth directions and the obvious interface can be observed, indicating the polycrystalline nature of the single nanoplate. The corresponding SAED pattern also demonstrates the polycrystalline crystal structure of the nanoplates (Fig. 2f). The dark-field TEM image of the single nanoplate further proves that the nanoplates are single crystalline (Fig. S2†).

Fig. 2 FESEM images of the polycrystalline tellurium nanoplates: (a) lower magnification, (b) middle magnification, (c) higher magnification, (d) TEM image of the tellurium nanoplates, (e) HRTEM image taken from the corner of a nanoplate, (f) corresponding SAED pattern of a single nanoplate.

Possible formation mechanism of tellurium nanoplates

Are the tellurium nanoplates stable in the reducing condition? It is important to know this for understanding the solvothermal reaction. In order to display the formation mechanism of tellurium nanoplates, the samples collected in different stages of the solvothermal reaction were investigated by SEM and TEM. In the initial stage (0.5 h), a large amount of nanoparticles can be observed, and some nanoplates can be found (Fig. S3†). After solvothermal treatment for 1 h, the uniform polycrystalline nanoplates can be obtained (Fig. 2b and c). Increasing the reaction time to 2 h, the morphology of the products changes, and some nanorods appear (Fig. 3a and b). When the reaction time is prolonged to 3 h, the ratio of the nanorods increases obviously with the decreasing of nanoplates (Fig. 3c and d). Further increasing the reaction time to 4 h, the nanorods continues increasing and the nanoplates still exist (Fig. 3e and f). In order to further investigate the stability of the tellurium in the solution, an experiment with 8 h solvothermal treatment is performed. As discussed above, the reaction time determines the final morphology and structure without changing the crystal phase, which can be characterized by XRD pattern (Fig. 4a). From the XRD pattern, one can find that the diffraction peaks became higher and sharper, demonstrating the better crystalline of the samples comparing with the nanoplates. Fig. 4b and c presents the representative SEM images of the as-prepared samples with 8 h solvothermal treatment. The low-magnification FESEM image in Fig. 4b reveals that a large amount of rod-like structures are randomly dispersed on the surface of the substrate. And little nanoplates can be observed. A high-magnification FESEM image (Fig. 4c) shows that the length of rods is in the range of 0.5–5 μm, and that their diameter is about 230 nm. Transmission electron microscopy and high-resolution TEM provide further insight into the microstructural details of the rod-like tellurium nanostructures. Fig. 4d depicts a TEM image of the typical tellurium nanorods. A representative HRTEM image of the top area of the tellurium nanorod in Fig. 4e exhibits well-resolved lattice fringes, demonstrating the single-crystal nature of the nanorod. The plane spacings of 0.59 nm correspond to the lattice planes of (001) lattice planes of hexagonal tellurium. The selected-area electron diffraction (SAED) taken from a single nanorod further confirms the well-crystallized single crystal nature of the nanorod (Fig. 4f). And these diffraction spots can readily be indexed to hexagonal tellurium crystal recorded from the [110] zone axis. These results also suggest that this nanorod grow along the [001] direction.

Fig. 3 The FESEM and TEM images of the products collected at different time intervals: (a and b) 2 h, (c and d) 3 h, (e and f) 4 h.

Fig. 4 Characterization of the single crystalline tellurium nanorods obtained at 180 °C for 8 h in the presence of 0.6 g glucose: (a) typical XRD pattern, (b) lower magnification FESEM image, (c) higher magnification FESEM image, (d) TEM image, (e) HRTEM image, and (f) corresponding SAED pattern of a single nanorod.

Based on the time-dependent experimental results, a possible mechanism of the tellurium nanoplates and their transformation into nanorods was proposed, and the formation process is schematically illustrated in Fig. 5. In the initial stage, the sodium tellurite was reduced into the tellurium nuclei with the assistance of ethanediamine (TeO₃²⁻ + C₂H₈N₂ → Te + N₂ + H₂O). The fresh nuclei were thermodynamically metastable due to their high surface energy, so they preferred to assemble into many bigger agglomerates for minimizing the interfacial energy. However, in our case, glucose was introduced and used as a surfactant. When the tellurium nuclei generated, the glucose molecules would be adsorbed on the surface of them for reducing the surface energy.

Fig. 5 Schematic illustration of the proposed growth mechanism of the different morphology of element tellurium.

As a polyalcohol, the hydrogen bond and electrostatic effect may exist between the glucose molecules, which will promote the capped nuclei to assemble into large plate-like structures. Such capped nuclei transformed into larger plate-like structures are similar to those proposed for the formation of crystalline CaCO₃ and Sb₂Te₃.^22,23 It is known trigonal crystal structure of tellurium is highly anisotropic with along c-axis, as can be seen in Fig. 5b. In the structure, there exist helical chains of covalently bound tellurium atoms with three atoms per turn. And these chains are bounded together through weak van der Waals interactions, resulting in a hexagonal lattice. Due to the anisotropic crystal structure of tellurium, trigonal tellurium has a strong tendency to grow along the c-axis direction into one dimensional structure.^16,24 Therefore, one dimensional nanowires, nanorods, and nanotubes are easily fabricated. In this experiment, only polycrystalline tellurium nanoplates can be synthesized at first. This may be attributed to the aggregation effect of glucose. In order to ascertain our hypothesis, the experiment with different amount of glucose was carried out. Fig. 6a presents the FESEM image of the product in the absence of glucose. It can be seen that the product consists of uniform microrods with the average diameter and length of about 400 nm and 1.85 μm, respectively. When 0.3 g of glucose is added into the reaction system, some nanoplates with the thickness of 100 nm except the irregular nanorods can be observed (Fig. 6b). These results suggest that the glucose molecules can promote the aggregation of nanoparticles and lead to the formation of nanoplates. Under the solvothermal reaction system, the polycrystalline tellurium nanoplates are unstable and start to dissolve into the mother solution. At the same time, the new nuclei form onto the protuberances for reducing the total surface energy. As discussed above, the tellurium has the anisotropic crystal nature, which leads to the formation of one dimensional nanorods along the [001] direction. As the mass diffusion and Ostwald ripening process proceeded, the small nanorods continue to grow until the polycrystalline nanoplates are consumed, thereby the uniform nanorods are fabricated.

Fig. 6 The FESEM images of the products prepared in different concentrations of glucose: (a) 0 g, (b) 0.3 g.

Fig. 7 shows the Raman scattering spectrum of the tellurium with different morphologies. Raman spectra of Te nanorods in Fig. 7a shows that the presence of two peaks at about 115.4 and 134.3 cm⁻¹ is observed at room temperature, which are close to those reported previously.²⁵ Interestingly, no shift in peak positions is observed for the tellurium nanorods. While for the polycrystalline nanoplates, a new peak at 262.8 cm⁻¹ is observed except the two 115.4 and 134.3 cm⁻¹ peaks under the same experimental condition. Meanwhile, the intensity of the peaks is higher than those of rod-like morphology, indicating that the morphology have much effect on the Raman spectrum.

Fig. 7 Raman scattering spectra of different morphologies of tellurium: (a) nanorods (blue), (b) nanoplates (green).

Conclusions

In summary, polycrystalline tellurium nanoplates are fabricated by a facile solvothermal route in the mixtures of water and ethanediamine with the assistance of glucose. By simply controlling the reaction time, the polycrystalline tellurium nanoplates can be transformed into single crystal nanorods. Based on the observation of the products in the different growth process, a two-stage growth mechanism of such Te nanostructures is presented which involves an initial self-assembly process followed by an dissolution–recrystallization process. Glucose used as a surfactant, can promote the aggregation of nanoparticles and lead to the formation of nanoplates because of the hydrogen bond and electrostatic effect. Moreover, the morphology of the tellurium has much effect on the Raman scattering spectrum, the polycrystalline tellurium nanoplates present the enhanced Raman scattering spectrum.

Acknowledgements

This work is not financial supported by any funds or government.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra41801d

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