Single Crystalline Trigonal Selenium Nanotubes and Nanowires

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Single Crystalline Trigonal Selenium Nanotubes and Nanowires Synthesized by Sonochemical Process Li,†

Li,*,†

Xuemei Yan Shanqing Irene L. Li,‡ and Zikang Tang‡

Li,†

Weiwei

Zhou,†

Haibin

Chu,†

Wei

Chen,†

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 911-916

Key Laboratory for the Physics and Chemistry of Nanodevices, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China, and Department of Physics and Institute of Nano Science and Technology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received September 21, 2004

ABSTRACT: One-dimensional (1D) nanostructures of trigonal selenium (t-Se) were synthesized by the reduction of H2SeO3 in different solvents with a sonochemical method. The 1D structure of t-Se was formed by the anisotropic growth of selenium crystalline, and the morphology of the products highly depends on the reaction conditions including ultrasonic mode (e.g., frequency, power, and time), aging time, and solvent. Single crystalline trigonal selenium nanotubes with diameters of less than 200 nm and nanowires with diameters of 20-50 nm have been synthesized. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive spectrometry (EDS), and Raman spectra were used to characterize the products. The formation process of t-Se nanotubes and nanowires were investigated. A sonication-induced directional growth mechanism was proposed for the formation of nanotubes. The further aging of tubes in solution leads to the collapse of the tubular structure and the formation of nanowires. Introduction In recent years, controlled synthesis of nanocrystals with well-defined morphology and uniform size became more and more important. Nanocrystals with high aspect ratio such as nanowires, rods, and tubes have received increasing attention because of their unusual properties (e.g., electronic, optical, chemical, and thermal properties) and their potential application as active components or interconnects in fabricating nanoscale electronic, optical, piezoelectrical, and electrochemical devices.1 Many methods have been adopted to synthesize these materials, including a colloidal chemical method,2 a vapor-liquid-solid (VLS) method,1b a solvothermal process,3 and so on. Among all these methods, the sonochemical process is an effective method to produce nanomaterials.4 The chemical effects of ultrasonic irradiation arise from acoustic cavitation: the formation, growth, and collapse of bubbles in a liquid. Selenium is an important semiconductor with band gaps of 1.85 (indirect) and 1.95 eV (direct). It has a relatively low melting point (∼490 K), a photovoltaic effect, and a high photoconductivity (8 × 104 S cm-1).5 Selenium has wide applications in various fields, such as solar cells, rectifiers, photographic exposure meters, xerography, pigments, glasses, and steel.5 Trigonal selenium (t-Se) contains helically arranged Sen chains, that is, infinite linear trigonal spirals, along the c-axis. 1D selenium is of great interest when it is used as a photoconductor. Selenium has a high reactivity toward a variety of chemicals; thus selenium with certain morphologies can be used as a chemical and physical template to produce other functional materials, for example, chalcogenides.6 * To whom correspondence should be addressed. Tel: +86-1062756773. E-mail: [email protected]. † Peking University. ‡ Hong Kong University of Science and Technology.

Several strategies have been applied to prepare selenium nanowires.7 Monoclinic Se nanowires were prepared by a reduction route using cytochrome c37a and via the decomposition of selenodiglutathiones.7b Trigonal selenium nanowires and nanorods have been successfully fabricated by self-seeding solution-phase processes,7c,d a sonochemical process,7e,f a solvothermal method,7g,h and a microwave-assisted route.7i t-Se microspheres and polycrystalline selenium powder had been used as the starting materials for t-Se nanowires.7j-l Polymers7m and surfactants7n were used as the protectors and synthesis templates. All above processes were performed in solution systems. Tang and co-workers obtained a Se helical chain structure along the AlPO4-5 channel direction by vapor phase defusion.7o,p Xie and co-workers fabricated nanowire networks of t-Se also by physical vapor deposition.7q Compared to nanowires, the reports of selenium nanotubes are much fewer.8 Xie and co-workers have obtained selenium nanotubes in various solvents under solvothermal conditions.8a Li and co-workers reported tubular selenium synthesized by reducing seleniuous acid with ascorbic acid in the presence of primary amines at room temperature.8b Rao and co-workers obtained t-Se with scrolled morphology under hydrothermal conditions.8c More recently, Yang’s group obtained selenium nanotubes and nanowires using a sonication treatment following a hydrothermal process, and the size of the t-Se particles obtained from the hydrothermal process is a critical factor in the formation of nanotubes and nanowires.8d To our knowledge, up to now only the above four reports are about the synthesis of crystalline selenium nanotubes, and the formation mechanism is not very clear. Thus, in this paper, single crystalline t-Se nanotubes and nanowires have been synthesized by the sonochemical method, and the possible formation mechanism of t-Se nanotubes and nanowires has been

10.1021/cg049681q CCC: $30.25 © 2005 American Chemical Society Published on Web 03/09/2005

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Figure 1. SEM (A) and TEM (B) images, XRD pattern (C), and Raman spectrum (D) of selenium nanotubes prepared in ethylene glycol by Bransonic B5-E ultrasonic cleaner. Table 1. Typical Conditions Adopted in Sonochemical Synthesis of Selenium Nanomaterials sequence

ultrasonic mode (frequency (kHz), power (W‚cm-2))

solvent

reactants

ultrasonic time (min)

A B C

B5-E (55 ((6%), 21) JY92-2D (20, 100) KQ218 (40, 100)

ethylene glycol (20 mL) water (40 mL) water (40 mL)

H2SeO3 (0.26 g) + 5% N2H4‚H2O (0.16 g) H2SeO3 (0.26 g) + 5% N2H4‚H2O (0.16 g) H2SeO3 (0.26 g) + 5% N2H4‚H2O (0.16 g)

60 45 30

proposed. Different from Xia’s work,7e,f here a longer sonication time is used; different from Yang’s work,8d the sonication is used directly to produce Se nanotubes. In their processes, R-Se was first generated and then converted into one-dimensional t-Se. But in our process, the two stages cannot be separated obviously.

FEG-TEM operated at 200 kV (FEI, Hillsboro, OR). The scanning electron microscopy (SEM) images were obtained using a Dual Beam-235 focused ion beam (FIB) system, operated with an acceleration voltage of 10 kV. The Raman spectra were obtained on a Renishaw System 1000 Raman imaging system (Renishaw plc, U.K.) equipped with a 632.8 nm, 25 mW He-Ne laser (Spectra Physics, Mountain View, CA) and a BH-2 microscope (Olympus, Japan).

Experimental Section Reagents and Apparatus. Selenious acid (H2SeO3, A.R.) and hydrazine hydrate (N2H4‚H2O, 85%) were used without additional purification. Ultrasonic irradiation was accomplished using three sonicators with different frequency and power. One is a B5-E ultrasonic cleaner (55 ((6%) kHz, 21 W‚cm-2, Bransonic, Danbury, CT), another is a JY92-2D Sonifier with a high-intensity ultrasonic probe (10 mm diameter, Ti horn, 20 kHz, 0-1000 W‚cm-2, Xinzhi Co., China), and the third one is a Shumei KQ218 ultrasonic cleaner (40 kHz, 100 W‚cm-2, Kunshan Ultrasonic Instruments Co. LTD, China). Synthesis. Hydrazine hydrate was dissolved in ethylene glycol, water, etc. to form solutions. The solution was added dropwise to the corresponding selenious acid solution. At the same time, ultrasound was preceded to the solution, and the ultrasonic time is 30-60 min. Then the solution was kept still for aging. The several typical preparation conditions are listed in Table 1. Characterization. The X-ray diffraction (XRD) analysis was performed using a Rigaku D/max diffractometer with Cu KR irradiation (λ ) 0.154 18 nm). The transmission electron microscopy (TEM) images, selected-area electron diffraction patterns, and energy spectra were taken using a Hitachi-9000 electron microscope operated at 100 kV and equipped with an energy dispersive spectrometry system (Hitachi, Japan). The high-resolution TEM images were obtained using a Tecnai F30

Results and Discussion According to the literature,7c-f the reaction used in the synthesis was as follows:

H2SeO3 + N2H4 f SeV + N2v + 3H2O Xia’s group has synthesized crystalline selenium nanowires in solutions and on solid supports using a sonochemical approach. In their work, the sonication time is short (15-25 s), and the ultrasonic frequency is certain (42 kHz). Different from Xia’s work, longer ultrasonic time and various ultrasonic modes were used in this paper. We first tried the synthesis in ethylene glycol by sonication with a Bransonic B5-E ultrasonic cleaner under the typical condition A shown in Table 1. After 24 h aging, the product was checked under SEM and TEM. Tubes with the diameter of about 200 nm were found in SEM and TEM images (Figure 1A,B). The yield of the tubular selenium was ca. 80%. It was worth noting that many selenium crystallites have a scrolllike and semi-closed tubular morphology (Figure 1B). The energy-dispersive spectrometry (EDS) result (see

Single Crystalline t-Se Nanotubes and Nanowires

Figure 2. TEM, SAED, and HRTEM images of selenium nanotubes prepared at typical condition A: (A) TEM image and SAED of a selenium nanotube; (B) TEM image of a selenium nanotube after electron beam irradiation for a short time; (C) HRTEM image of the middle part of the nanotube in panel B; (D) FFT of panel C. Panels E, F, and G are the magnifications from panelC corresponding to the areas I, II, and III, respectively.

Supporting Information) shows that the nanotubes are composed of selenium. Figure 1C shows the XRD pattern of the selenium nanotubes. All the peaks can be readily indexed to crystalline trigonal selenium (tSe) (JCPDS 6-362). Figure 1D is the Raman scattering spectrum taken from the as-prepared nanotubes. The resonance peak at 233 cm-1 is a characteristic stretching mode of a chainlike structure that only exists in the trigonal selenium,7c which is assigned to the A1 mode. The resonance peak of one E mode is also at about 233 cm-1, which may be overlapped by the strong peak of A1. The peak of 145 cm-1 is the transverse optical phonon mode (E bond bending mode). The peaks at 436 and 460 cm-1 can be attributed to the second-order spectra of trigonal selenium.9 In comparison, the characteristic Raman peaks for monoclinic selenium and R-Se are at ∼256 and ∼264 cm-1,7d which do not appear in Figure 1D. The structure information for the selenium nanotubes mainly came from selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM), combined with a fast Fourier transform (FFT) technology (Figure 2). Figure 2A is the TEM image and the ED pattern (inset in Figure 2A) of an individual nanotube. The ED results indicate that the growth of the nanotube is along the [001] direction. Figure 2B is a semiclosed nanotube of selenium after the electron beam radiation, and the

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Figure 3. A set of TEM images corresponding to samples obtained at different growth stages of nanotubes (typical condition A in Table 1; scale bars 500 nm): (A) ultrasonic time 4 min; (B) ultrasonic time 15 min; (C) ultrasonic time 30 min; (D) ultrasonic time 60 min; (E) sample from panel D after 24 h aging; (F) sample from panel D after 72 h aging.

tip of the nanotube is not smooth as before, which may be caused by the lower melting point of selenium. Figure 2C shows the HRTEM images of the selenium nanotube. The three framed areas in Figure 2C, which were selected from the central part and the two sides of the nanotube, were enlarged into Figure 2, panels E, F, and G, respectively. They show that the selenium nanotube is a single crystal. Along the growth direction, the lattice spacing is 0.5 nm, which agrees with the space distance between the (001) planes of t-Se. The reciprocal lattice peaks (Figure 2D), which were obtained by twodimensional Fourier transform of the lattice resolved images, also indicate that the nanotube grows along the [001] direction, which was possibly determined by the anisotropic nature of the trigonal selenium with helical chains. To study the formation mechanism of t-Se nanotubes, various growth stages of t-Se nanotubes were investigated. Figure 3 shows a set of TEM images corresponding to samples obtained at different growth stages of nanotubes. After 4 min sonication, spherical R-Se were obtained (Figure 3A), and no ED pattern was detected; when the solution was sonicated for 15 min, sphericallike t-Se particles were formed from spherical R-Se (see

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the inserted ED pattern in Figure 3B), and some of the spheres were broken. The initial stage of Se nanotubes formed from the seeds of spherical-like t-Se was observed when the sonication time was 30 min (Figure 3C). The Se nanotubes grow along with the sonication time prolonged to 60 min (Figure 3D). After 24 h aging, the t-Se nanotubes grow into a well-organized shape (Figure 3E), but after further aging, the nanotubes tend to change into t-Se nanowires and nanorods (Figure 3F). Known as the Stranski rule, in the case of a crystal that may exhibit various allotropes, the least stable and most soluble phase, which is favored kinetically, usually precipitates first, and then this metastable phase is transformed during aging into a more thermodynamically stable phase.10 According to this rule and the experimental results shown in Figure 3, the formation process of t-Se nanotubes and nanowires can be described as follows. First, the unstable spherical R-Se particles were formed. Second, R-Se transformed into more stable t-Se. Third, under ultrasonic inducement, the tubular Se began to growth at the surface of the spherical-like t-Se; then the Se nanotubes were formed after continuous growth, and at last, the nanotubes transformed into nanowires via Ostwald ripening or other processes. Yang et al.8d claimed that the aggregated growth along the circumferential edge of the broken ball gap resulting from the sonication lead to the formation of nanotubes. It seems that this is almost the case here in this study. The long-term sonication somehow breaks the selenium ball and forms a crater on the ball. The annular edge of the crater acts as the initiation spot, and further directional growth of selenium crystal along this annulation gives out the tubular product. The transition of nanotubes into nanowires indicates that the wires are thermodynamically stable. But this transformation only happens in the reaction solution. The selenium nanotubes are stable when they were separated from solvents. We found that the dry selenium nanotubes make no change at all after they were kept in atmosphere over one year. It seems that the annular initiation spot brought by the sonication is essential for the formation of selenium nanotubes. Then, if the sonication power is strong enough or the sonication period lasts long enough, the tubes may be obtained. Therefore, we tried the synthesis using a JY92-2D sonifier with a high-intensity ultrasonic probe and a different ultrasonic frequency. The reaction condition is shown in Table 1B. This sonifier can provide more intensive ultrasonic sound waves to the solution, so the sonication time can be shorter. Figure 4 shows the SEM images of the selenium nanotubes obtained in water under the sonication power of 100 W with 45 min of sonication (condition B in Table 1). XRD and Raman results (Figure 4C,D) show that the products are t-Se. When we tried to use higher power, we could not get tubular products, and the aspect ratio of the product decreases with the increase of sonication power (see supporting materials). When the ultrasonic power is 200 W, the products contain nanowire, nanotube, and particles; when the ultrasonic power increased to more than 300 W, only particles with very small aspect ratio were obtained. This may be due to the low melting point of selenium. When the sonication power is too high, the circumferential edge of the

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Figure 4. SEM (A, B) images, XRD patterns (C), and Raman spectra (D) of selenium nanotubes synthesized in aqueous solution by a JY92-2D sonifier with a high-intensity ultrasonic probe (typical condition B in Table 1).

broken ball cannot form because selenium tends to melt. Thus, no tubes can form in the system. And the aggregation of the small particles of selenium while melting leads to larger particles. In the previous work,7e,f,8d 40 kHz was the most frequently used ultrasonic frequency to prepare selenium nanostructures. We also tried this frequency with a KQ218 ultrasonic cleaner (40 kHz, 100 W‚cm-2). The sonication condition was shown in Table 1C. The SEM and TEM images shown in Figure 5 give a general view of the morphology of the product. XRD and Raman results show that they both are t-Se. As can be seen from the SEM and TEM images (Figure 5A-D), the samples consist of a large quantity of one-dimensional t-Se with average diameters of 20-50 nm. It is very interesting that some selenium nanotubes (Figure 5B) exist in the products when the aging time is 8 h. However, when the aging time was prolonged to 24 h, the tubular selenium disappeared (Figure 5C,D), and only nanowires were found. This is in agreement with the phenomenon that happened in the system of selenium nanotubes by Bransonic B5-E ultrasonic cleaner. But the nanotubes here are much thinner and therefore more unstable. Therefore, they transformed into nanowires at much shorter aging time. The structure and oriented growth of selenium nanowires was investigated with SAED and HRTEM shown in Figure 6. The TEM image of an individual nanowire with diameter of ca. 30 nm is shown in Figure 6A. The inset in Figure 6A shows the corresponding ED pattern of the nanowire, which was obtained by focusing the electron beam along the [010] zone axis. The occurrence of the (002) diffraction may be attributed to the double diffraction of the incident electron in Se crystal.7j The ED results indicate consistently that the nanowires grow predominantly along the [001] direction. An HRTEM image (Figure 6B) taken from an individual nanowire shows three sets of distinct lattice spacings

Single Crystalline t-Se Nanotubes and Nanowires

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dimensional characteristics of the helical chains in the trigonal phase, solvent, and the ultrasonic mode play crucial roles in the formation of selenium nanotubes and nanowires. During the formation of selenium nanotubes and nanowires, several stages were involved and the schematic illustration of the formation process is shown as follows. ultrasonic

ultrasonic

H2SeO3 + N2H4 98 spherical R-Se 98 ultrasonic

aging

spherical-like t-Se 9 8 t-Se nanotubes 98 aging t-Se nanowires Conclusion In summary, single crystalline t-Se nanotubes and nanowires have been synthesized via a sonochemical process. The ultrasonic mode, solvent, and anisotropic properties of selenium crystalline play important roles in controlling the morphologies of final products. The growth direction of nanotubes and nanowires is along the c-axis of t-Se. In the formation process of nanotubes and nanowires, several stages were involved. First, the unstable spherical R-Se particles were formed. Second, R-Se transformed into the more stable t-Se. Third, the tubular Se began to grow at the surface of the sphericallike t-Se induced by sonication; then the Se nanotubes were formed after continuous growth, and at last, the nanotubes transformed into nanowires.

Figure 5. SEM (A, B, C) and TEM (D) images, XRD patterns (E), and Raman spectra (F) of selenium t-Se nanowires and nanotubes obtained in aqueous solution by KQ218 ultrasonic cleaner (typical condition C in Table 1): (A, B) SEM images of selenium nanowires and nanotubes obtained at aging time of 8 h; (C, D) SEM and TEM images of selenium nanowires obtained at aging time of 24 h.

Figure 6. SAED of a selenium nanowire obtained at aging time of 8 h (A) and HRTEM images and corresponding FFT of a selenium nanowire obtained at aging time of 24 h (B).

of ca. 0.5, 0.37, and 0.29 nm that correspond to the (001), (100), and (101) planes of t-Se, respectively. It confirmed that the selenium nanowires are single crystals and grow along the [001] direction. From the analysis in Figures 2 and 6, we found that the selenium had a strong tendency to growth along the [001] direction. This may be caused by its highly anisotropic structure. Although the exact formation and growth mechanisms of selenium nanotubes still need further investigation, our experiment results indicated that the one-

Acknowledgment. This research was supported by the National Natural Science Foundation of China and The Ministry of Science and Technology of China (Project 2001CB610501). Supporting Information Available: The EDS spectrum of the product and the effect of the ultrasonic power on the morphology of the products. This material is available free of charge via the Internet at http://pubs.acs.org.

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