Large-Scale Synthesis and Growth Mechanism of Single-Crystal Se Nanobelts Qin Xie, Zhou Dai, Wanwan Huang, Wu Zhang, Dekun Ma, Xiaokai Hu, and Yitai Qian* Hefei National Laboratory for Physical Science at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1514-1517
ReceiVed September 24, 2005; ReVised Manuscript ReceiVed April 12, 2006
ABSTRACT: Single-crystal trigonal (t) Se nanobelts have been synthesized on a large scale by reducing SeO2 with glucose at 160 °C. Electron microscopy images show that the nanobelts are ∼80 nm in diameter, ∼25 nm in thickness, and up to several hundreds of micrometers in length. HRTEM images prove that the nanobelts are single crystals and preferentially grow along the [001] direction. The time-dependent TEM images revealed that the formation and growth of t-Se nanobelts were governed by a solidsolution-solid growth mechanism. The redox reaction directly produced amorphous (R) Se nanoparticles under hydrothermal conditions. t-Se nanobelts were formed by dissolution and recrystallization of the initial R-Se nanoparticles under the functional capping of poly(vinylpyrrolidone) (PVP). The nanobelts obtained exhibit a quantum size effect in optical properties, showing a blue shift of the band gap and direct transitions relative to the values of bulk t-Se. 1. Introduction Since the discovery of novel oxide semiconductor nanobelts, one-dimensional nanostructures with a rectangle cross-section have attracted extraordinary attention, because these distinctive geometric structures make the belts an ideal system not only for fully understanding dimensionally confined transport phenomena on the nanometer scale but also for building functional devices along individual nanobelts.1 Recently, considerable effort has been devoted to the fabrication of beltlike nanostructures. A class of oxides (including ZnO, SnO2, In2O3, CdO, Ga2O3, and PbO2)2-4 and sulfide (such as ZnS)5,6 was successfully synthesized by a simple thermal evaporation process. In addition, wet chemical methods, which are very promising due to their low cost and potential for scale-up, have been developed to prepare beltlike nanostructures. Ni,7 TiO2,8 MoO3,9 and Te10 nanobelts (nanoribbons) have been prepared in aqueous solution. Helical titanium dioxide nanoribbons were prepared using an organogel template.11 BaCrO4 nanobelt bundles were obtained in catanionic reverse micelles.12 Owing to the distinctive geometric structure and their potential application, it is highly desirable to explore a simple method to prepare nanobelts of functional materials. Elemental selenium is a narrow band gap semiconductor, and it has been extensively studied and widely used in the fields of solar cellars, xerography, and rectifiers due to its useful properties,13 such as high photoconductivity (about 8 × 104 S cm-1), a relatively low melting point (about 217 °C), large piezoelectricity and thermoelectricity, and a high reactivity toward a wealth of functional materials (e.g. Ag2Se, Bi2Se3 Se@Ag2Se, and Se@CdSe).14-17 The monodisperse spherical colloid selenium was considered to be a very promising photonic crystal.18 In the recent fabrication of 1D Se nanorods and nanowires, the so-called solution-phase chemical reduction approach has been proven to be a promising method with hydrazine,19 NaBH4,20,21 (NH4)2S2O3,22 and ascorbic acid23 as reducing agent with or without the assistance of surfactant. Under ultrasonic conditions, Se nanotubes (diameter 80-300 nm) and nanobelts (diameter 80-300 nm) have also been synthesized in micellar solutions of the surfactant poly(oxyeth* To whom correspondence should be addressed. E-mail: ytqian@ ustc.edu.cn. Fax: 86-551-3607402.
ylene) dodecyl ether.24,25 Recently, Cao et al. reported a physical vapor deposition of Se powder at 300 °C to prepare ultrathin Se nanoribbons with zigzag edges.26 However, there has still been little work reported on the preparation of Se nanobelts to date.25-27 In this work, we selected glucose as a mild reducing agent to prepare single-crystal t-Se nanobelts on a large scale via a hydrothermal route. 2. Experimental Section All of the chemical reagents used in our experiments were of analytical grade. In a typical procedure for the synthesis of selenium nanobelts, 2 mmol of SeO2, 5 mmol of glucose, and poly(vinylpyrrolidone) (PVP, polymerization degree 360, 0.2 g) were dissolved in 40 mL of a mixture of water and ethanol (1/1 v/v). With constant stirring, 5 mL of ammonia was added to the clear solution. Then the mixed solution was transferred into a Teflon-lined autoclave of 50 mL capacity and heated to 160 °C for 20 h. After the reaction, suspended wine-colored products were obtained and washed several times with distilled water and absolute ethanol by centrifugation and finally dried in a desiccator at room temperature. The X-ray diffraction (XRD) pattern was recorded using a Rigaku (Japan) D/max-rA X-ray diffractometer equipped with graphitemonochromated Cu KR radiation (λ ) 0.154 187 nm). The Raman scattering spectrum was performed at ambient temperature on a Spex 1403 Raman spectrometer with an argon ion laser at an excitation wavelength of 514.5 nm. The SEM images were obtained using a field emission scanning electron microscope (FESEM, JEOL-6300F). The high-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area electron diffraction (SAED) patterns were obtained with a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. Absorption spectra were collected on a UV-vis spectrophotometer (Shimadzu UV-2401) in the wavelength range of 190-900 nm.
3. Results and Discussion Structure and Morphology Characterization. The X-ray diffraction powder pattern and Raman scattering spectrum were performed to characterize the composition and structures of wirelike nanostructures. Figure 1a shows the X-ray powder diffraction pattern of the sample obtained. All the peaks can be indexed as trigonal selenium with lattice constants of a ) 4.3668 Å and c ) 4.9578 Å, consistent with the standard values for bulk Se of a ) 4.3662 Å and c ) 4.9536 Å (JCPDS File No. 73-0465). In comparison to the standard pattern (shown in Figure
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Figure 2. (a, b) SEM images of the t-Se nanobelts synthesized. (c-e) TEM images of the t-Se nanobelts synthesized (parts d and e show the twist shapes, a characteristic feature of belts).
Figure 1. (a) XRD pattern of the product obtained. (b) Standard XRD pattern of t-Se. (c) Raman scattering spectrum of the product after hydrothermal treating for 20 h.
1b), the intensities of the (100) and (110) diffraction peaks are enhanced, which indicates that these nanostructures tend to preferentially grow in the [001] direction. The strong Raman resonance peak (Figure 1c) at ∼235.4 cm-1 can be attributed to the vibration of the Se helical chain and the peak at ∼438 cm-1 attributed to a second-order resonance,28 further confirming the trigonal phase of Se nanobelts due to its characteristic stretching mode at ∼235 cm-1. In comparison, the peaks of amorphous selenium and monoclinic selenium are centered at ∼264 and ∼256 cm-1, respectively.28 Both the XRD and Raman scattering results confirmed that well-crystallized trigonal selenium could be obtained by the reduction of glucose under the present hydrothermal conditions. Figure 2 shows SEM images of the sample obtained, which displays a large quantity of wirelike Se nanostructures. The lengths are up to several hundreds of micrometers and some are even on the order of millimeters (Figure 2a). The yield of selenium product was more than 95%. High magnification shows that the product exhibits bundled and bending wirelike structures (Figure 2b). Further structural and morphological details were observed by transmission electron microscopy (TEM). Parts c-e of Figure 2 show TEM images of the product, revealing the beltlike morphology features. A ripplelike contrast appeared in the TEM image due to strain resulting from the bending of the belt.1 Twisted shapes, characteristic features of a belt, were clearly seen. Each nanobelt has a uniform width along its entire length, and the average diameter is ∼80 nm. The thickness of the belts is ∼25 nm. The structural orientation of Se nanobelts was investigated by analyzing nanobelts with high-resolution TEM (HRTEM). Figure 3a is the TEM image of an individual t-Se nanobelt (diameter 80 nm, thickness 25 nm). The HRTEM image in Figure 3b shows that the fringe spacings of the crystal planes
Figure 3. (a) TEM image of the individual nanobelt. (b) HRTEM image of the nanobelt shown in Figure 3a and the SAED of this nanobelt (shown in the inset).
perpendicular and parallel to the long axis of the nanobelt are 0.50 and 0.22 nm, corresponding to (001) and (110) planes of t-Se, respectively. This image also reveals that the belt is made up of single crystals and is free from dislocation. The inset in Figure 3b is a typical selected-area electron diffraction (SAED) pattern that was recorded from this nanobelt. These pattern spots demonstrate the single-crystal nature of t-Se nanobelts. The HRTEM and SAED pattern analyses demonstrate that the nanobelts grow along the preferred direction of [001] with side surfaces of ((110) planes, in agreement with the XRD results. Also, the growth direction is consistent with the inherent helical chain of t-Se. Effect of Experiment Conditions on the Growth of t-Se Nanobelts. To substantially understand the effect of PVP on the growth of t-Se nanobelts, a contrasting experiment without PVP was carried out. In the absence of PVP, the reaction rapidly occurred under hydrothermal conditions and produced a mixture of microparticles and nanorods (Figure 4a). Even when the aging time was prolonged, the morphology of products was still preserved. When an appropriate amount (0.2-0.5 g) of PVP was introduced into the reaction system, the obtained product consisted of uniform nanobelts. Moreover, when the usage of PVP was reduced to 0.05 g, the sample was essentially composed of Se nanowires with a diameter of 150-200 nm (Figure 4b), which was wider than the diameter of the nanobelts. From these experimental results, it was observed that the presence of an appropriate amount of polymer PVP played a crucial role in the growth of Se nanobelts.
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Figure 4. SEM images of products synthesized (a) without the assistance of PVP and (b) with 0.05 g of PVP.
Recently, the surfactant PVP has been successfully employed as a stabilizing agent as well as a structure-directing agent for the preparation of Ag, Sb, and Te nanowires.29-31 It is generally believed that a surfactant reagent kinetically controls the growth rates of various faces through selective adsorption and desorption on these surfaces.28 In the present case, PVP is present as the surface stabilizer, and its long polymeric chain would be selectively absorbed on the surfaces of the nanoparticles. On the basis of experimental results, we believe that PVP has two major effects on the growth of Se nanostructures. On the one hand, the presence of PVP prevents the aggregation of nanoparticles into larger particles. On the other hand, PVP could kinetically control the growth rates of various faces by selective adsorption and desorption. Such a behavior significantly affected the diffusion rate of selenium atoms on the surfaces of t-Se nanocrystallites and their epitaxial growth. The different diffusion rates on the growing surfaces determined the growth rates of various planes and finally resulted in the formation of beltlike nanostructures. In addition, reaction temperatures, glucose concentration, and ethanol also played important roles in the growth of selenium nanobelts. To synthesize pure nanocrystalline selenium, the optimal temperature is 140-160 °C and the suitable concentration of glucose is in the range of ∼0.11-0.22 M. A reduction in temperature and the use of glucose could result in the formation of nanorods. When the reaction was carried out at a relatively higher temperature (180 °C) or the concentration of glucose was more than 0.22 M, amorphous carbon hollow spheres were produced due to the decomposition of glucose. Furthermore, in the case of pure water, the product was a mixture of microparticles and wirelike nanostructures. This might be due to the greater solubility of selenium in ethanol than in water. Such a behavior favors the slow diffusion rate of free Se atoms and the formation of nanobelts. Growth Progress and Possible Mechanism. To understand the growth mechanism of the nanobelts, we investigated the evolutional morphology of the intermediates obtained after different reaction times. Parts a-d of Figure 5 shows the TEM images of the products obtained after reaction for 1, 1.5, 3, and
Xie et al.
Figure 5. TEM images of four samples prepared after hydrothermal reaction for (a) 1 h, (b) 1.5 h, (c) 3 h, and (d) 12 h, showing the morphological evolution of the selenium nanobelts.
12 h. After reaction for 1 h, the TEM image and electron diffraction pattern (Figure 5a) reveals the formation of amorphous selenium (R-Se)19 nanoparticles with diameters of 50100 nm. With an extension of the reaction time to 1.5 h, linear nanostructures were formed. As shown in Figure 5b, a great number of nanoparticles were attached on the circumference of linear structures, which suggested that the formation and growth of the linear nanostructures depended on the attached amorphous particles. A beltlike nanostructure was quickly formed, even after aging for 3 h (Figure 5c). When the time was prolonged to 12 h, most of the nanoparticles gradually disappeared (Figure 5d). The nanobelts became longer along their longitudinal axis, and their surfaces became smoother by continuously consuming the attached nanoparticles. Eventually, the product evolved into pure nanobelts with uniform diameters and length up to hundreds of micrometers (Figure 2). The growth process has been revealed by a series of timedependent TEM observations. At the beginning, the redox reaction directly produced amorphous Se nanoparticles. Under the present hydrothermal conditions, amorphous nanoparticles partially dissolve in the solution and generate free selenium atoms. Also, when the concentration of selenium atoms was high enough, selenium atoms recrystallized and formed crystalline t-Se nuclei. The continuous feeding of selenium atoms onto the crystalline seeds led to the formation of linear nanostructures due to the anisotropic crystal structure.19 Additionally, the dissolution and sequential recrystallization carried out a transformation from R-Se to t-Se. Such a growth process should be consistent with a solid-solution-solid transformation mechanism. Also, there are some similarities between the growth of our nanobelts and that of t-Se nanowires reported by the Xia group.19 However, the fact that not nanowires but nanobelts formed could be attributed to the functional capping effect of PVP. PVP controlled the diffusion rate of selenium atoms on the surfaces of the growing crystals and subsequently dominated the growth rate of various planes by selective adsorption and desorption capping functions. Our understanding of the PVP-
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tion band gap and band energy of Se nanobelts exhibits a marked blue shift compared to the bulk material due to quantum size effects. These novel nanobelts can be potentially used as basic building blocks to prepare important complex nanostructures which themselves could exhibit interesting applications in photosensitive and photovoltaic nanodevices in the future. The present synthetic method is easily controllable and can be expected to be useful in the synthesis of other element semiconductor nanobelts by a choice of appropriate reaction parameters. Acknowledgment. Support from the National Natural Science Foundation of China and the 973 Project of China is greatly appreciated. References
Figure 6. UV-vis spectra of the Se nanobelts prepared.
assisted growth of Se nanobelts is still limited, and further investigation is in progress. Optical Properties of t-Se Nanobelts. The optical properties of metallic selenium are of interest due to its photoconductive, photovoltaic, and rectifying response on illumination with visible light. It is generally believed that trigonal selenium is a p-type, extrinsic semiconductor with an indirect band gap of 1.6 eV.19 We performed optical studies on it to evaluate the energy band structure and other potentially optical properties of the nanobelts prepared. Figure 6a shows the UV-vis absorption spectrum of selenium nanobelts synthesized as ethanol dispersions. The main absorption peaks are centered at 589 nm (2.07 eV), 450 nm (2.76 eV), and 360 nm (3.44 eV). The lowest energy came from the intermolecular transition between the unique spiraling chains of trigonal selenium. The higher energy could be attributed to direct transitions having greater blue shifts relative to the reported values by Joannopoulos (∼2.6 and 3.2 eV).32 Figure 6b reveals that the band gap of the Se nanobelts prepared is 1.79 eV, larger than the value (1.6 eV) for the bulk material. The blue shift of the band gap and direct transitions indicate quantum confinement effects in the nanostructures obtained. 4. Conclusions In summary, single-crystal Se nanobelts with diameters of ∼80 nm and lengths up to a few hundred micrometers have been synthesized on a large scale under hydrothermal conditions. A TEM investigation revealed that the entire growth process consisted of the initial formation of R-Se nanoparticles, then the dissolution of partially amorphous nanoparticles and recrystallization as crystalline t-Se nuclei, and finally the growth of nanobelts on the seeds at expense of R-Se nanoparticles attached on the circumferential edges under the functional capping of PVP. This growth could be ascribed to a solidsolution-solid transformation mechanism. The optical absorp-
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