The Fabrication and Characterization of Single-Crystalline Selenium

Jun 9, 2006 - A solution-phase approach has been demonstrated for large-scale synthesis of selenium nanoneedles with the stem diameter ranging from 10...
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The Fabrication and Characterization of Single-Crystalline Selenium Nanoneedles Xiong,‡

Xi,‡

Shenglin Baojuan Weizhi Hongyang Zhou,‡ and Yitai Qian*,†,‡

Wang,‡

Chengming

Wang,†

Linfeng

Fei,†

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1711-1716

Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, China ReceiVed January 6, 2006; ReVised Manuscript ReceiVed May 9, 2006

ABSTRACT: A solution-phase approach has been demonstrated for large-scale synthesis of selenium nanoneedles with the stem diameter ranging from 100 to 500 nm and lengths up to tens of micrometers, gradually becoming thinner to form a sharp tip, which were fabricated by reduction of Na2SeO3 with poly(vinyl alcohol) (PVA). An interesting feature of the nanoneedles is their tendency to form branches and junctions. The morphology, microstructure, and chemical compositions of the nanoneedles were characterized using various methods (XRD, XSP, FESEM, TEM, and high-resolution TEM), indicating that the nanoneedles were single crystalline with high purity, structurally uniform, and dislocation-free. On the basis of a series of TEM observations, the nucleation and growth process of selenium nanoneedles could be interpreted by a solid-solution-solid process and PVA-assisted formation mechanism: trigonal (t)-selenium sphere-like nanoparticles initially formed due to the incessant dissolution of the sphere-like amorphous (a)selenium nanoparticles in the current hydrothermal process; then the wire-like nanostructures were gradually created from the t-selenium particles owing to the intrinsically anisotropic structure of t-selenium. The needle-like end shape was attributed to the higher growth rate on the tip than on the stem of wires since PVA can kinetically control the anisotropic growth via selective absorption on different crystalline faces. 1. Introduction In recent years, the fabrication of one-dimensional (1D) semiconductor nanostructures, such as nanowires, nanoribbons, and nanotubes, has been attracting considerable attention.1-4 In comparison with nanowires, nanotubes, and nanobelts, semiconductor nanoneedles are of particular interest because their tips exhibit a sharp curvature, offering potential applications as probing tips with high spatial resolution in both vertical and horizontal dimensions or field-emission tips due to the increased field-enhancement factor.5-7 Unlike the nanowires, it is more difficult to prepare the nanoneedles because at present few methods are designed to successfully prepare the nanoneedles with sharp tips. Until now, to our knowledge, there are only a few reports about the preparation of semiconductor nanoneedles.8 As important members of the VI elemental semiconductor group, selenium nanomaterials with 1D structure are one of the key materials in virtue of the broad applications in optoelectronics devices such as photovoltaic cells, rectifiers, photographic exposure meters, and xerography due to a large number of versatile properties, such as a relatively low melting point (∼217 °C), a high photoconductivity (∼8 × 104 S cm-1), catalytic activity toward organic hydration and oxidation reactions, intrinsic chirality, high refraction coefficient in devices and large birefringence, and relatively large piezoelectric, thermoelectric, and nonlinear optical responses.9,10 In addition, selenium also holds a high reactivity toward large quantities of chemicals that can be exploited to convert selenium into other 1D functional materials, such as CdSe, ZnSe, and Ag2Se etc.11 Moreover, the availability of 1D selenium nanostructures is expected to introduce some new types of applications or enhance * To whom correspondence should be addressed. E-mail: ytqian@ ustc.edu.cn. Tel: 86-551-3603204. Fax: 86-551-3607402. ‡ Department of Chemistry. † Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry.

the performance of the currently existing devices as a result of quantum-size effects.12 Prompted by these merits, well-defined one-dimensional (1D) selenium nanostructures with different morphologies such as nanowires, nanorods, nanobelts, and nanotubes have been fabricated.13-22 It is worth pointing out that Mayers et al. demonstrated the synthesis of tapered selenium nanowires through a sonication-induced SSS process.23 However, as we know, single-crystalline Se nanoneedles have not yet been reported in any literature via conventionally hydrothermal process until now. Herein, we have designed a facile and controllable method to produce uniform nanoneedles of trigonal selenium (t-Se) via one-step in situ reduction of Na2SeO3 by poly(vinyl alcohol) (PVA) under hydrothermal conditions. To our excitement, we found an interesting feature of the nanoneedles that they tend to form branches and junctions. In this synthesis process, poly(vinyl alcohol) (PVA) could serve as both a reductant and a polymer-structure-directing reagent. To further understand the growth mechanism of selenium nanoneedles, we have systematically investigated the growth process of selenium nanoneedles by analyzing the samples at different growth stages. On the basis of obtained results, a solid-solution-solid process and PVAassisted growth mechanism have been proposed to explore the formation of selenium nanoneedles. 2. Experimental Section Synthesis. In a typical experimental procedure, analytically pure Na2SeO3 (2 mmol) and 5 mL of PVA (5 wt %) solution were put into 40 mL of distilled water at room temperature to form a clear solution, which was then stirred strongly for about 0.5 h and transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 140 °C for 48 h and then air-cooled to room temperature naturally. The resulting precipitate collected from the bottom of the container was rinsed with distilled water and absolute alcohol several times. After drying in a vacuum at 40 °C for 4 h, the red-brown powders were collected for characterization. Characterization. The X-ray powder diffraction (XRD) analysis was performed with a Japanese Rigaku D/max-γA rotating anode X-ray

10.1021/cg060005t CCC: $33.50 © 2006 American Chemical Society Published on Web 06/09/2006

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Figure 1. (a) XRD pattern of the selenium nanoneedles obtained at 140 °C for 36 h and (b) XPS analysis of the sample of the selenium nanoneedles. diffractometer equipped with the monochromatic high-intensity Cu ΚR radiation (λ ) 1.541 78). The X-ray photoelectron spectra (XPS) were collected on an ESCALab MKII X-ray photoelectron spectrometer using nonmonochromatized Mg ΚR X-ray as the excitation source. Field emission scanning electron microscope (FESEM) images were taken on a JEOL JSM-6300F SEM. Transmission electron microscopy and electron diffraction (ED) patterns were taken with a Hitachi model H-800 instrument using an accelerating voltage of 200 kV with a tungsten filament. High-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL2010 transmission electron microscope at an acceleration voltage of 200 kV.

3. Results and Discussion 3.1. Composition of the Products. X-ray power diffraction (XRD) analysis was used to examine the phase of the asobtained nanoneedles. Figure 1a shows the XRD pattern of the selenium nanoneedles obtained in the experiments. All the reflection peaks could be readily indexed to a single phase of trigonal-structured selenium with calculated lattice parameters a ) 4.362 Å and c ) 4.952 Å, which agree well with the reported data (JCPDS 06-0362, a ) 4.366 Å, c ) 4.953 Å). No other impurity peaks were detected. Compared with the standard pattern of selenium (t-Se), an exceptionally strong (100) reflection peak was detected in the XRD pattern, which suggests that the as-synthesized selenium crystals might have a preferential growth orientation of [001]. This is demonstrated by the

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following SAED and HRTEM examination in more detail. This XRD pattern indicates that the reduction of Na2SeO3 is complete under present synthetic conditions. Further evidence for the quality and surface composition was obtained by the X-ray photoelectron spectroscopy (XPS) of the as-synthesized products. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C1s to 284.60 eV. Figure 1b indicates XPS spectra taken from the Se nanoneedles. The binding energy at 54.8 eV, corresponding to Se3d, is the characteristic peak for elemental selenium.24 No obvious peaks for other elements or impurities were observed. Both XRD and XPS reveal that t-Se of pure phase is successfully fabricated via the current one-step synthetic route. 3.2. Morphologies and Structures. The morphology and structure of the powders were observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). These images (Figure 2a,b) clearly indicate that the obtained sample with high aspect ratio consisted of a great many nanoneedles. A low-magnification FESEM image is shown in Figure 2a, revealing that as-fabricated Se nanostructure has a needle-like morphology with lengths of up to tens of micrometers. The more highly magnified FESEM image (Figure 2b) gives more details on the morphology of the Se nanoneedles. It can be seen that these nanoneedles have a moderate size distribution, with the stem diameter in the range of 100-500 nm and gradually becoming thinner to form a sharp tip. The inset shows a FESEM image at higher magnification that clearly displays the sharpness of the tip. According to more details of TEM observations shown in Figure 3a-d, an interesting feature of the nanoneedles is found to be their tendency to form branches and junctions. Figure 3a shows the TEM image of V-junctions of nanoneedles, and the selected area electron diffraction (SAED) patterns were recorded from areas 1, 2, and 3 in Figure 3a, revealing their uniform [001] orientation on a flat carbon substrate. The structure and morphology of the nanoneedles were further examined by high-resolution TEM (HRTEM). A typical TEM image of a single nanoneedle is shown in Figure 4a, from which it can be seen that the tip of the nanoneedle exhibits the extremely sharp shape. The selected-area ED pattern related to the nanoneedle can be indexed to the [11h0] zone axis of t-Se, suggesting that the nanoneedle is single crystalline growing along the [001] direction. High-resolution TEM (HRTEM) image of the nanoneedle measured at the tip is shown in Figure 4b, which proves the tip to be only several nanometers in width. The regular spacing of the adjacent lattice fringes was ca. 0.5 nm, which is consistent with the separation of the (001) planes. It also showed that the nanoneedle was single crystalline, structurally uniform, and dislocation-free. In addition, HRTEM images measured along the nanoneedle stem also indicate the highly ordered lattice images in Figure 4c. From the TEM images and SAED pattern of more individual nanoneedles investigated in this study, the results demonstrate the singlecrystal nature of the nanoneedles, which have a preferential [001] growth direction along their long axes. 3.3. The Possible Mechanism for the Formation of Selenium Nanoneedles. To investigate the nucleation and growth process of selenium nanoneedles, a series of experiments was carried out at 140 °C for different periods of time. The samples obtained at different stages of reaction time were examined by using transmission electron microscopy (TEM) techniques. Figure 5 shows TEM images of the samples that were fabricated after hydrothermal reaction was performed for

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Figure 2. Low (a) and high magnification (b) FESEM images showing the general morphology of the Se nanoneedles that were synthesized by the hydrothermal process. The inset highlights the smallness and sharpness of the nanoneedle tips.

Figure 3. Nanoneedles of t-Se containing V-junctions and similar structures obtained by hydrothermal synthesis. Panels a, b, c, and d show TEM images of such structures. The corresponding ED recorded from the areas as indicated by a sequent number from panel a show the unanimous [001] direction.

4, 12, 36, and 48 h. These images clearly reveal the evolution of selenium nanostructures from sphere-like nanoparticles to nanoneedles over time at 140 °C. A small amount of the precipitate was formed when the reaction was 4 h. Figure 5a shows the stage product mainly consisted of colloidal nanoparticles with sizes of 300-500 nm. The SAED pattern shown in the inset of Figure 5a further confirmed that these colloidal nanoparticles were all in the amorphous phase. It confirms that amorphous selenium is formed at first. When the reaction time was increased to 12 or 36 h, the products consisted of three different forms of selenium nanostructures, including needles, wires, and some sphere-like nanoparticles (see Figure 5b,c),

Figure 4. (a) TEM image of a single nanoneedle with an inset showing the corresponding ED pattern, (b) HRTEM image of the sharp tip of the nanoneedle (see the boxed area 1 in panel a), and (c) HRTEM image taken from one side of the nanoneedle (see the boxed area 2 in panel a), both revealing the single crystalline nature and the c-axis growth direction.

indicating that many nanopaticles had been developing into 1D nanostructures of selenium and nanoparticles were gradually decreasing from panel b to panel c. Figure 5b,c shows a view of the roots of several nanoneedles (indicated by the arrows), and these images clearly demonstrate that the selenium nanoneedles were grown out from the surfaces of seeding nanocrystals, and at the same time, some V-junctions were also formed (shown by the arrow in Figure 5b). Further extending the reaction time led to the formation of uniform nanoneedles. Figure 5d is a typical TEM image of a sample prepared after heating for 48 h, showing that the as-obtained products are dominated by the needle-like nanostructures. No selenium nanoparticles were observed. Our experimental results revealed that Na2SeO3 can be reduced to t-Se at 140-180 °C in this system. The residual

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Figure 5. TEM images of three samples, showing different stages of growth for selenium nanoneedles. These samples were taken after the reaction had been carried out for (a) 4, (b) 12, (c) 36, and (d) 48 h. Inset in panel a is the corresponding ED pattern indicating that a-Se existed in the initial products. Inset in panel b is the corresponding ED pattern showing that t-Se existed in this period of products.

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Figure 6. FTIR spectra: (a) the residual solution after reaction at 140 °C for 48 h; (b) pure PVA.

solution after reaction was further examined through the IR spectrum. The result is shown in Figure 6a. Several absorption peaks locate at about 3413.5, 2937.5, and 1432.2 cm-1 in Figure 6a that correspond to the O-H stretching vibration [ν(O-H)], C-H stretching vibration [ν(C-H)], and C-H bending vibration [δ(C-H)], respectively, which is similar to the infrared spectrum of pure PVA (see Figure 6b), while there is a wide absorption peak at about 1084.2 cm-1, which could be assigned as the absorption peak of o´ as(C-O-C). In addition, it is worth mentioning that the pH value remains almost unchanged from 9.80 for the starting initial solution to 9.48 for the residual solution after reaction (see Figure 7), suggesting that the solution after reaction is still basic. Based on the above results, the reaction that occurred under the current hydrothermal conditions could be described as follows: Figure 7. The variation of pH value during the hydrothermal reaction for the formation of nanoneedles.

In our present study, it is noteworthy that the final morphology

of selenium nanostructures synthesized by the current hydrothermal process was highly dependent on the polymer structuredirecting reagent added to the reaction solution. To understand the formation of the selenium nanocrystals, a series of experiments have been carried out. In the formation process of selenium nanoneedles, we find that PVA is necessary. When other reducing reagents, such as biomolecules or other inorganic

Single-Crystalline Selenium Nanoneedles

Figure 8. FESEM images of selenium wire-like nanocrystals with a small fraction of defect nanospheres: (a) 160 °C for 48 h; (b) 180 °C for 48 h.

salts, are used instead of PVA, only 1D structures (nanorods, nanowires, or nanobelts) were obtained in final products.17 But if the concentration of PVA was higher, we could not obtain the needle-like products that were expected. As a result, an appropriate amount of PVA is very crucial for the formation of selenium nanoneedles. On the basis of our experimental results, it is believed that polymer PVA may have at least two major effects on the growth of the selenium nanoneedles. First, as a reductant in our reaction system, PVA can reduce Na2SeO3 to amorphous selenium (a-Se) nanoparticles (or colloids) in the initial stage. Second, as a polymer structure-directing reagent, PVA can kinetically control growth rates of certain crystallographic facets of selenium through selectively adsorbing on these facets during the transition process of particles into 1D structure and in turn tailor the crystal morphology.25,26 At the same time, the reaction may proceed more slowly due to the absorption effect of PVA. The relatively slow reaction favors separating the growth from the nucleation step, which, in turn, can provide enough time for the anisotropic growth of the product. In addition to PVA, we found that the reaction temperature is also a key factor in the morphology-controlling formation of selenium nanocrystals. When the temperature was increased to 160 and 180 °C, the morphology of the final products all changed into wire-like t-Se with a small fraction of defect nanospheres, and no nanoneedles could be observed (Figure 8a,b). Although the exact formation mechanism of needle-like structures was under investigation, the directing role of PVA is assuredly crucial. As mentioned above, when other reducing reagents were substituted for PVA, no needle-like product was obtained. In our experiment, we indeed prepared needle-like structures in the presence of PVA as both reductant and structure-directing reagent. Based on the results, we speculate that the selenium nanoneedles are grown through a solidsolution-solid process and PVA-assisted growth mechanism, described as follows. First, amorphous selenium (a-Se) nano-

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particles (or colloids) are slowly formed through the reduction of Na2SeO3 by PVA at 140 °C. During the subsequent steps, a small amount of a-Se particles first converted into t-Se, due to higher surface free energies of a-Se compared to the seeds of t-Se; then under the current hydrothermal condition, these t-Se sphere-like particles grown at the expense of the gradually dissolved a-Se nanoparticles may then serve as initial seed for the growth of wire-like nanostructures owing to the intrinsically anisotropic structure of t-Se (see Figure 5b,c). Through the change from panel c to panel d of Figure 5, we can clearly see that the tips of the wires began to taper, which may be attributed to the higher growth rate on nanoneedle tips than on stems since PVA can kinetically control the anisotropy growth of the nanocrystals by means of selective absorption on different crystalline faces. What is more, when other reagents instead of PVA were introduced in our experiment system, we could not obtain needle-like structured product. The experimental results can well prove that PVA is an indispensable factor in the formation of the needle-like structure. As the reaction time was extended to 48 h, the sharp end was completely shaped (see Figure 5d). Here, the sharp end of the nanowires in the presence of PVA may be similar to that obtained in ethylene glycol media by a polyol process reported by Xia et al.27 Such similarity suggests that the hydroxyl groups of PVA play a critical role in the formation of nanowires with a sharp end. Of course, our present understanding of forming of the mechanism of the needle-like selenium nanostructures is still limited, and more in-depth studies are in progress. However, this work presents a very convenient, simple, and template-free method to fabricate semiconductor selenium materials with tailored nanostructures. 4. Conclusion In conclusion, large-scale controlled synthesis of single-crystal selenium nanoneedles has been successfully realized via onestep in situ reduction of Na2SeO3 by poly(vinyl alcohol) (PVA) through a solid-solution-solid process and PVA-assisted growth mechanism. In the process of formation of selenium nanoneedles, sphere-like t-selenium nanoparticles initially formed at the expense of the gradual dissolution of a-Se nanoparticles, and then the wire-like nanostructures gradually formed through the incessant dissolution of the sphere-like t-selenium nanoparticles due to the intrinsically anisotropic structure of t-Se. The needle-like structure on the end of stem started to shape due to the higher growth rate on the tip than on the stem, since PVA can selectively absorb on different crystalline faces and in turn kinetically control the anisotropic growth of nanocrystals. FESEM and TEM analysis showed that the nanoneedles with sharp tips had relatively uniform distribution in their diameters and lengths. Furthermore, the as-synthesized selenium nanoneedles are single crystalline, structurally uniform, and dislocation free. Both XRD and HRTEM measurements exhibited the c-axis oriented growth of nanoneedles. It should be pointed out that our present understanding of forming of the mechanism of the needle-like selenium nanostructures is still limited, and more in-depth studies are in progress. Acknowledgment. The financial support of this work, by National Natural Science Foundation of China and the 973 Project of China (No. 2005CB623601), is gratefully acknowledged. References (1) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471.

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