Morphology Control, Crystal Growth, and Growth Mechanism of

Nov 13, 2012 - Department of Biology and Chemistry, Hunan University of Science and Engineering, Yongzhou Hunan 425100, PR China. Cryst. Growth Des...
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Morphology Control, Crystal Growth, and Growth Mechanism of Hierarchical Tellurium (Te) Microstructures Xiaoping Wu,† Yongqiang Wang,† Shaomin Zhou,*,† Xian You Yuan,‡ Tao Gao,† Ke Wang,† Shiyun Lou,† Yubiao Liu,† and Xiaojing Shi† †

Key Lab for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, People’s Republic of China Department of Biology and Chemistry, Hunan University of Science and Engineering, Yongzhou Hunan 425100, PR China

‡

S Supporting Information *

ABSTRACT: Understanding the factors that influence the growth and final shape of semiconductor tellurium microstructures is important for controlling their properties. However, relative to their single-crystalline nanostructures, the growth of complex structures that are ideally composed of nanostructures arranged in a particular way can be difficult to control. Here, we developed a facile solvothermal method and successfully completed the controlled synthesis of Te particles with distinctive morphologies, including flower-like, ballflower, nestlike, and sheetlike structures. These structures, self-assembled from nanorods and nanosheets, are systematically studied by adjusting the reaction parameters, such as the amount of NaOH, the volume ratio of EG/EN, the amount of PVP, and the reaction time. Results reveal that the morphology of Te microstructures can be easily controlled by simply altering the reaction conditions and that NaOH plays a crucial role in the final morphology of Te products. The growth mechanisms and morphology control of hierarchical Te microstructures are proposed and discussed. This is the first time to report the preparation of complex hierarchical Te microstructures through a simple solution route. This simple solution approach to fabricate hierarchical Te superstructures with controllable morphologies can be easily scaled up and potentially extended to the hierarchical assembly of building blocks of other semiconductors.

1. INTRODUCTION In the past few years, a continuing pursuit of materials nanostructures was with an eye to develop materials with their novel properties, as well as their potential applications in many fields (such as catalysis, chemical sensors, (opto)electronic devices, radiative cooling devices, and field-effect devices),1−6 which depends not only on the composition but also on the morphology of the materials.6−8 Recently, however, the synthesis of complex structures that are ideally composed of nanostructures (such as particles, rods, and sheets) arranged in a particular way as well as their growth mechanism have attracted much more interest, because such hierarchical materials not only possess improved properties originating from their building blocks but also find new applications in many fields.9−13 Up to now, many methods have been used for the preparation of hierarchical structures with complex morphologies.11−15 Nevertheless, the self-assembly technique has been recognized as an important strategy for the exquisite fabrication of hierarchical constructions, because it can easily control the shape growth of materials hierarchical structures.16−18 Recently, elemental tellurium has attracted great interest because it exhibits a wealth of outstanding physical and chemical properties with potential applications in various fields, © 2012 American Chemical Society

such as nonlinear optical responses, piezoelectricity, catalytic activity, photoconductivity, and thermoelectric properties.19−24 Trigonal tellurium (t-Te) has a highly anisotropic crystal structure consisting of helical chains of covalently bonded atoms, which are in proper order bound together through van der Waals interactions in a hexagonal lattice, thus resulting in the fact that its crystals always grow along the c axis and have a tendency to form one-dimensional (1D) structures.19,22 Various 1D Te architectures, such as nanowires, nanorods, nanobelts, and nanotubes, have been synthesized by different methods.25−29 For instance, Xia’s group has reported the synthesis of various 1D nanostructures (wires, rods, and tubes) of t-Te through the reduction of orthotelluric acid (or tellurium oxide) in different solvent systems (ethylene glycol, water, and their mixtures) via a refluxing process.25−27 Te nanobelts and nanotubes were synthesized by the in situ disproportionation of Na2TeO3 in an aqueous ammonia system.22 Nevertheless, to the best of our knowledge, the research of complex and novel hierarchical Te structures with different morphologies has been limited. Hence, further research is still highly desired to realize Received: September 5, 2012 Revised: October 26, 2012 Published: November 13, 2012 136

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the advantages of the self-assembly technique for the preparation of well-defined hierarchical Te structures. In prior work, we reported the controlled growth of various 1D nanostructures (wires, rods) of t-Te crystals, by a high-yield and environmental friendly route.30 Here, we extend our approach to the direct synthesis of more complex Te architectures with a simple, lower-cost, and environmentally friendly synthetic method. The route was designed for the large-scale synthesis of adjustable 3D hierarchical Te microstructures, such as flower-like, spherical, ball-flower, nestlike, and sheet structures. Meanwhile, reaction parameters, such as the volume ratio of EG/EN, the quantity of PVP, the amount of NaOH, and the reaction time, in the morphological control of hierarchical Te microstructures were systematically investigated. With the progress of a series of time-dependent morphological evolution studies, the growth processes of hierarchical Te microstructures were perspicuous step by step during our study.

Figure 1. (a−c) SEM images at different magnifications. (d) XRD pattern of as-synthesized Te flowers.

2. EXPERIMENTAL SECTION

well-ordered and oriented to form Te flowers. The XRD pattern of the typical sample prepared at 180 °C for 8 h is shown in Figure 1d. The diffraction peaks of as-synthesized Te can be indexed to the hexagonal Te with lattice constants of a = b = 4.458 Å, c = 5.927 Å, which agrees with the standard XRD databases (JCPDS, Card No. 36-1452; space group: P3121). The major XRD diffraction peaks appeared at 2θ = 23.44, 28.08, 38.69, 40.96, and 43.76°, attributed to (100), (101), (102), (110), and (111) planes, can be seen clearly. No peaks resulting from impurities were observed, indicating a highpurity Te sample, in agreement with the result reported previously.30 To further ascertain the chemical compositions of the flowerlike microstructures, the as-prepared products were examined by using IR spectroscopy and TG-DTA, and the results are shown in Figure 2a,b.31 In the IR spectrum shown in Figure 2a, the two sharp peaks at 3198 and 3117 cm−1 are assigned to the NH2 asymmetric and symmetric stretching vibrations, the bands at 1591 and 1027 cm−1 are attributed to the NH2 scissors and wagging vibrations, the absorption bands at 1353, 2937, and 2871 cm−1 correspond to wagging, asymmetric, and symmetric stretching vibrations of the CH2, respectively, and the absorption band at 1114 cm−1 can be assigned to the C−N stretching vibrations. The NH2 asymmetric and symmetric stretching vibrations shift toward lower frequency, as compared with that of EN, which could suggest that EN has been intercalated into the precursors. In addition, the absorption band at about 3438 cm−1, which corresponds to OH stretching vibration, could be due to the absorption of EG and H2O in the sample. TG-DTA results are shown in Figure 2b. The TG curve shows that the products undergo a single-step weight-loss process. The measured weight loss of the en is 17.8% from 250 to 400 °C, which is close to the expected value of 19.0% calculated for the change of Te(en)0.5 to Te. The DTA curve shows a continuous change from room temperature to 250 °C, indicating the release of freely bound water. The endothermic peak at 286 °C corresponds to the decomposition of Te(en)0.5. These results reveal that the flower-like microstructures obtained at VEG/VEN = 21:4 are a type of Te−en hybrid compound, Te(en)0.5, which is consistent with what our group has observed previously.30 3.2. The Influencing Factors in the Shape-Controlled Synthesis of Hierarchical Te Microstructures. To get a full

2.1. Materials. The materials used include zinc acetate (Zn(CH3COO)2) (A.R., Tianjin Chemical Co.), sodium tellurite (Na2TeO3) (C.P., Shanghai Chemical Co.), ethylene glycol (EG) (A.R., Tianjin Chemical Co.), ethylenediamine (EN or en) (A.R., Tianjin Chemical Co.), polyvinylpyrrolidone (PVP) (A.R., Tianjin Chemical Co.), and NaOH (A.R., Tianjin Chemical Co.). All of the reagents used in the experiment were directly used without further purification. 2.2. Synthesis. In a typical procedure, 0.055 g of Zn(CH3COO)2, 0.056 g of Na2TeO3, and 0.2 g of PVP (molecular weight 30 000) were initially dissolved in 12.5 mL of different solvents, which consist of different volume ratios of ethylene glycol (EG) and ethylenediamine (EN) (21:4, 22:3, 23:2, 24:1), followed by the addition of different amounts (0, 0.2, 0.4, 0.6, 0.8, 1.0 g) of NaOH. The resultant mixture was dispersed by continuously stirring the solution for about 30 min. The mixed homogeneous solution was then transferred into a Teflonlined stainless steel autoclave with a capacity of 25 mL and maintained at 180 °C for 8 h. After the heating treatment, the autoclave was cooled to room temperature naturally. The products were collected by centrifuging and washed several times with ammonium chloride solution, deionized water, and absolute ethanol, and then dried at 60 °C for 5h. The products were heated to 350 °C at 30 °C h−1 for 1 h in N2 in a horizontal furnace, leading to black powders. 2.3. Characterization. The morphology and the size of the asprepared Te products were characterized by SEM (JEOL JSM5600LV) at an acceleration voltage of 20 kV. Phase identification and structure analysis of the sample were carried out by XRD using a Philips X’ Pert Pro MPD X-ray diffractometer with Cu Ka radiation (λ = 0.154056 nm) operated at 40 kV and 40 mA in the 2θ range of 20− 80° with a step size of 0.04° and a sampling time of 0.5 s. The IR spectrum was recorded using an AVATAR360 Fourier transform IR spectrophotometer at room temperature. Thermogravimetric and scalable differential thermal analysis (TG-SDTA) was carried out at a heating rate of 10 °C min−1 in N2 gas at a flowing rate of 50 mL min−1 using a TGA/SDTA851e system.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Flower-Like Te Microstructures. The morphology and size of the as-prepared flower-like Te microstructures are shown in Figure 1a−c. Figure 1a displays a representative overview of the Te microflowers, which shows that the prepared samples are composed of large-scale microflowers with diameters of 35 μm. Figure 1b,c shows high-magnification SEM images of Te flowers from a different angle of view, which vividly demonstrate that the Te microflowers are built of 2D nanoplates. The nanoplates are 137

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Table 1. Effects of the Different Reaction Conditions on the Morphologies of Resulting Te Products influencing factor VEG/EN

NaOH/g

PVP/g

time/h

temp/°C

24:1 23:2 22:3 21:4 0.4 0.6 0.8 1.0 0 0.05 0.1 0.2 0.2 0.25 0.5 2 4 8 110 120 140 160 180 200

morphologies

size

spherelike microspheres and particles broken microspheres and flake nanoflakes nestlike nestlike nestlike and sheetlike sheetlike sheetlike nestlike ball-flowers and nestlike ball-flowers nanorods nanorods and ball-flowers microspheres nestlike and ball-flowers ball-flowers and nestlike ball-flowers nanorods and ball-flowers microspheres nestlike and ball-flowers ball-flowers and nestlike ball-flowers flowers

700 nm 5 μm 500 nm 5 μm 8 μm 9 μm 6 μm 2 μm 6 μm 7 μm 9 μm 100 nm 8 6 7 9

μm μm μm μm

10 μm 5 μm 8 μm 9 μm 5 μm

Figure 2. (a) IR spectrum and (b) TG-DTA curves of Te flowers.

understanding about the shape evolution of hierarchical Te microstructures in a reaction system, we performed a number of experiments under different conditions, including adjusting the volume ratio of EG/EN (24:1−21:4), the amount of NaOH (0−1 g), the amount of PVP (0−0.2 g), the reaction time (0.2−8 h), and the reaction temperature (110−200 °C). The influences of the reaction conditions on the morphology and size of the as-synthesized Te microstructures are summarized in Table 1, and the influence of these parameters on the morphology and microstructure of the products is discussed in detail in the following sections. 3.2.1. Influence of Volume Ratio of Solvents on the Morphology Evolution of Hierarchical Te Microstructures. The content of the mixed solvents in the solvothermal reaction system is found to be an important synthetic parameter to influence the shapes and microstructures. In our case, the morphology of Te samples was regularly changed by increasing the content of EN in the mixed solvents. Hence, the influence of the EG/EN volume ratio on the morphology of hierarchical Te microstructures was first studied. The amount of NaOH, reaction temperature, and reaction time were kept constant at 0.4 g, 180 °C, and 8 h, respectively. The volume ratio of EG/ EN was changed from 24:1 and 23:2 to 21:4, and the total mixed solvent volume was 12.5 mL. The morphology and size of as-prepared Te samples were investigated by the SEM technique. The products prepared by varying the volume ratio of EG/ EN in the reaction system were characterized by SEM, and the results are shown in Figure 3. Figure 3a shows the SEM images of the product obtained in the solvothermal system with the volume ratio of EG/EN of 24:1. It can be seen that the products are composed of spheres with diameters of about 700

Figure 3. SEM images of Te samples obtained at different EG/EN volume ratios: (a) 24:1, (b) 23:2, (c) 22:3, and (d) 21:4. The amount of NaOH, amount of PVP, reaction temperature, and reaction time are kept constant at 0.2 g, 0.2 g, 180 °C, and 8 h, respectively.

nm. These microspheres have a smooth surface and a very narrow size distribution. When the volume ratio of EG/EN is decreased to 23:2, these microspheres have a rough surface and it appear that some of the small particles have an irregular morphology (Figure 3b). As the volume ratio of EG/EN is decreased to 22:3, the products are composed of a small amount of broken microspheres and a large number of flakes rupture into detritus (Figure 3c). If VEG/VEN is decreased to 21:4, the products are composed of flakes with sizes of about 500 nm (Figure 3d). These results reveal that EN plays a significant role in the microstructural evolution of Te and that 138

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role in controlling the different morphologies of Te microstructures. The comprehensive influences on the morphologies are shown in Figure 5. It was found that the increase in the EN volume ratio and the amount of NaOH will lead to multiform morphologies, such as flower-like, spherical, ball-flower, nestlike, and sheetlike microstructures. Of course, other parameters, including the concentration of precursors, also influence the morphologies. However, we do not introduce and discuss these factors here in detail. Our next work is to understand how and why Te samples grow into such multiform structures. 3.2.3. Influence of PVP on the Morphology Evolution of Hierarchical Te Microstructures. To identify the role of PVP in the formation of Te microstructures, the products obtained at VEG/VEN = 23:2 (the total mixed solvent volume is 12.5 mL) in the presence of different amounts of PVP (0, 0.05, 0.1, and 0.2 g) were characterized by SEM, as shown in Figure 6. The amount of NaOH, reaction temperature, and reaction time were kept constant at 0.4 g, 180 °C, and 8 h, respectively. Figure 6a shows a typical SEM image of the products obtained in the absence of PVP. It can be seen that the sample was inclined to form flat nanoplates, although some of them can also stack together. When PVP was introduced, the morphologies of the products were changed. Figure 6b shows the SEM images of the products obtained in the solvothermal system when the amount of PVP was 0.05 g. The SEM results reveal that the products are composed of hierarchical nest structures. As the amount of PVP was increased to 0.1 g, besides the hierarchical nest structures, ball-flower structures were observed (Figure 6c). If the amount of PVP was increased to 0.2 g, the self-assembled architecture became apparently ballflowers with an average diameter of 9 μm, as shown in Figure 6d. However, close examination of the SEM images revealed that the surfaces of the balls were not smooth. They seemed, rather, to be composite structures consisting of small distorted plates, which make the balls look like a fluorescent chrysanthemum. Therefore, the results indicate that the PVP played an important role in the formation and the assembly of Te plates into monodispersed ball-flower microstructures, which prevents the growth of the Te nanocrystallite and agglomeration of the samples. 3.2.4. Influence of Reaction Temperature on the Morphology Evolution of Hierarchical Te Microstructures. To understand the effects of the reaction temperature on the morphology of Te microstructures, the products obtained at VEG/VEN = 23:2 (the total mixed solvent volume is 12.5 mL) under the circumstance of different reaction temperatures (110, 120, 140, 160, 180, 200 °C) were characterized by SEM, as shown in Figure 7. The amount of NaOH, the amount of PVP, and reaction time were kept constant at 0.4 g, 0.2 g, and 8 h, respectively. If the reaction temperature was lower than 100 °C, the reaction could not occur. As shown in Figure 7a, when the reaction temperature was increased to 110 °C, the main products were nanorods and microspheres, and these microspheres were formed by aggregate/self-assembly of nanorods. While the reaction temperature was increased to 120 °C, typical Te microspheres with a diameter of about 10 μm were obtained, and the Te nanorods disappeared entirely (Figure 7b). Hierarchical nestlike and ball-flower structures were synthesized when the reaction temperature reached 140 or 160 °C (Figure 7c,d). When the reaction temperature became 180 °C, pure Te ball-flowers composed of sheets could be successfully synthesized (Figure 7e). However, if the reaction

controlled synthesis of different morphologies of hierarchical Te microstructures can be achieved by adjusting VEG/VEN. 3.2.2. Influence of the Amount of NaOH on the Morphology Evolution of Hierarchical Te Microstructures. The amount of NaOH was also found to be responsible for the shape control of hierarchical Te microstructures. In the above discussions about the influence of the volume ratio of EG/EN, the samples were prepared with the amount of NaOH kept constant at 0.2 g. To further investigate the effect of the amount of NaOH on the formation of Te microstructures, the images of the products obtained from solutions with the amount of NaOH in the range between 0.2 and 1 g were studied. The volume ratio of EG/EN, reaction temperature, and reaction time were kept constant at 24:1, 180 °C, and 8 h, respectively, and the total mixed solvent volume was 12.5 mL. As shown in Figure 4, the amount of NaOH had a great influence on the

Figure 4. SEM images of Te samples obtained at different amounts of NaOH: (a) 0.4, (b) 0.6, (c) 0.8, and (d) 1.0 g. The EG/EN volume ratio, amount of PVP, reaction temperature, and reaction time are kept constant at 24:1, 0.2 g, 180 °C, and 8 h, respectively.

morphology and size of the Te particles. When the amount of NaOH varied from 0.2 to 1 g, the obtained Te particles changed from nestlike particles to flake particles. When the amount of NaOH was 0.2 g, the sample was inclined to form hierarchical nestlike Te structures consisting of sheets. Figure 4a was a typical SEM image of this structure. If the amount of NaOH was increased to 0.4 g, the sample still consists of hierarchical nest structures, but the size of the pores among the nest structures increased, as shown in Figure 4b. Figure 4c reveals that this tendency is further extended at 0.8 g of NaOH. The products are composed of hierarchical nest structures and a small amount of flake structures, and the size of the pores among the nest structures continues to increase. As the amount of NaOH is increased, the size of the pores among the nest structures increases obviously. At the increased amount of NaOH of 1.0 g, the morphology of the final products consists of thin flakes with sizes of about 6 μm, and a small quantity of aggregation, as displayed in Figure 4d. Interestingly, a gradual change of morphology structures from nestlike to flakes was also observed with increasing the amount of NaOH. Hence, NaOH plays a significant role in building many kinds of hierarchical Te microstructures. On the basis of the above experimental results, the volume ratio of EG/EN and the amount of NaOH play an important 139

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Figure 5. Te microstructures with different shapes obtained as a function of the EG/EN volume ratio and the amount of NaOH.

Figure 6. SEM images of Te samples obtained at different amounts of PVP: (a) 0, (b) 0.05, (c) 0.1, and (d) 0.2 g. The EG/EN volume ratio, amount of NaOH, reaction temperature, and reaction time are kept constant at 23:2, 0.4 g, 180 °C, and 8 h, respectively.

temperature was maintained at 200 °C, flower structures about 5 μm in diameter were synthesized (Figure 7f). Many different morphologies appeared when the reaction temperature varied from 110 to 200 °C. Only when the reaction was maintained at 180 °C for 8 h can the uniform and nearly dispersed ballflowers be obtained. Therefore, the reaction temperature of 180 °C is the optimal condition for the formation of pure Te ballflower structures when the reaction time was kept constant at 8 h.

Figure 7. SEM images of Te samples obtained at different temperatures: (a) 110, (b) 120, (c) 140, (d) 160, (e) 180, and (f) 200 °C. The EG/EN volume ratio, amount of NaOH, amount of PVP, and reaction time are kept constant at 23:2, 0.4 g, 0.2 g, and 8 h, respectively.

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3.3. Role of Zn(CH3COO)2 in the Synthesis of Hierarchical Te Microstructures. To reveal the role of Zn(CH3COO)2 in the synthesis of hierarchical Te microstructures in more detail, controlled experiments were performed, and the corresponding products were examined by SEM and XRD, as shown in Figures S1 and S2 (Supporting Information). All products were obtained in the same conditions as above. If CH3COONa, CH3COONH4, or Mg(CH3COO)2 was used instead of the Zn(CH3COO)2, and keeping other conditions constant, the reaction could not occur. When Zn(NO3)2, ZnCl2, or ZnSO4 was used instead of the Zn(CH3COO)2, the synthesized hierarchical flower microstructures are about 3−5 μm in diameter (Figure S1a−c, Supporting Information). When Cd(CH3COO)2 or Pd(CH3COO)2 was used instead of the Zn(CH3COO)2, the dominant products were particle morphologies with a diameter of about 100−300 nm (Figure S1e,f, Supporting Information). The corresponding XRD patterns are shown in Figure S2 (Supporting Information), which indicate that the hierarchical flower microstructures are hexagonal Te (Figure S2, patterns a−d, Supporting Information). As Cd(CH3COO)2 or Pd(CH3COO)2 was used instead of the Zn(CH3COO)2, the products were CdTe or PbTe, respectively (Figure S2, patterns e and f, Supporting Information). On the basis of the above results, we reasoned that the Zn(CH3COO)2 plays an important role in the formation of hierarchical Te microstructures. The influences of the precursors on the morphology and the composition of the as-synthesized products are listed in detail in Table S1 (Supporting Information). 3.4. Growth Mechanism of Hierarchical Te Microstructures. To further obtain a complete view of the hierarchical Te microstructure formation process and its growth mechanism, the detailed time-dependent evolution of the morphology was evaluated by SEM (Figure 8). As shown in Figure 8a, when the solvothermal reaction proceeded for 12 min, the products are mainly composed of Te nanorods with a diameter of about 100 nm. If the reaction time is increased to 15 min, these Te nanorods aggregate/self-assemble to form microsphere structures, and some of the Te nanorods structures still exist (Figure 8b). When the reaction time was increased to 30 min, the Te nanorods grew into sheets, further forming typical microspheres with a diameter of about 8 μm, and the Te nanorods disappeared entirely (Figure 8c). If the reaction time reaches 2 h, the thickness of the sheets increases via the Ostwald ripening mechanism to form hierarchical nestlike structures (diameter of about 6 μm), and a small amount of microspheres still exists (Figure 8d). As the reaction time was extended to 4 h, the thickness of the sheets further increases and the size of the pores among the nest structures decreases to form hierarchical ball-flower structures. The dominant products are hierarchical ball-flower structures with a diameter of about 7 μm and a small amount of nest structures (Figure 8e). After 8 h, ball-flower microstructures about 9 μm in diameter were synthesized (Figure 8f); meanwhile, the nest structures disappeared almost entirely. We believe that the formation process of the hierarchical Te microsphere structures could be rationally expressed into three sequential steps (Scheme 1): (1) the formation of Te nanorods and their growth into microsphere structures (a smaller roughness) consisting of thin sheets at an early stage; (2) the thickness of the sheets increased via the Ostwald ripening mechanism to form hierarchical nestlike structures; and (3) the thickness of the sheets further increases, and the size of the pores among the

Figure 8. SEM images of Te samples obtained at different times: (a) 12 min, (b) 15 min, (c) 0.5 h, (d) 2 h, (e) 4 h, and (f) 8 h. The EG/ EN volume ratio, amount of NaOH, amount of PVP, and reaction temperature are kept constant at 23:2, 0.4 g, 0.2 g, and 180 °C, respectively.

nest structures decreases, to form hierarchical spherical structures through a dissolution−recrystallization process.9 On the basis of the experimental results and discussion, and according to previous reports,31,32 a possible mechanism for the formation of hierarchical Te microstructures may be explained by the following reactions: 2HOCH 2CH 2OH = 2CH3CHO + 2H 2O

(1)

4CH3CHO + 4OH− + 2Zn 2 + + 2TeO32 − + en = 2Te(en)0.5 + 2ZnO + 4CH3COO− + 4H 2O 2Te(en)0.5 → 2Te+en

(2) (3)

4. CONCLUSION In summary, a variety of hierarchical Te nanostructures with controlled morphologies were prepared by a facile one-pot solvothermal synthesis approach. The reaction parameters, such as the volume ratio of EG/EN, the amount of NaOH, the amount of PVP, and the reaction time, play critical roles in determining the final morphologies of microstructures. On the basis of these results, a possible growth mechanism of the hierarchical Te microstructures has been proposed. This simple, mild solution approach to fabricate hierarchical Te superstructures with controllable morphologies and crystalline phases can be easily scaled up and potentially extended to the hierarchical assembly of nanoscale building blocks of other semiconductors. Moreover, these hierarchical Te microstructures are very competitive candidates due to their lowcost synthesis and easily controlled morphologies for practical applications. 141

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Scheme 1. Schematic Illustration of the Formation Process of the Ball-Flower Te Microstructures

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ASSOCIATED CONTENT

S Supporting Information *

SEM images and XRD patterns of samples obtained with different precursors and effects of the precursors on the morphology and composition of the as-synthesized products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-378-2868833-3712. Fax: +86-378-3881358. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2008 HASTIT002) and by the Natural Science Foundation of China under Grant Nos. 20971036 and 51102077, Changjiang Scholars and Innovative Research Team in University, No. PCS IRT1126, and the construct program of the key discipline in Hunan province (No. 201176).

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dx.doi.org/10.1021/cg301286w | Cryst. Growth Des. 2013, 13, 136−142