J. Phys. Chem. B 2005, 109, 10653-10657
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Carbothermal Chemical Vapor Deposition Route to Se One-Dimensional Nanostructures and Their Optical Properties Hua Zhang, Ming Zuo, Shun Tan, Gongpu Li, Shuyuan Zhang,* and Jianguo Hou Structure Research Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: December 23, 2004; In Final Form: March 14, 2005
A simple and practical carbothermal chemical vapor deposition route has been developed for the growth of trigonal phase selenium nanowires and nanoribbons. In detail, the mixture of active carbon and selenium was heated for the chemical reaction to occur, followed by thermal evaporation and decomposition into elemental selenium. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, high-resolution electron microscopy, UV-vis absorption, and photoluminescence. The results show that trigonal Se nanowires have uniform diameters ranging from 20 to 60 nm and grow along the [001] direction, with the same growth direction found for nanoribbons. Spectral measurements suggest a large blue shift and two types of electron transition activity. The influences of experimental conditions on morphologies and growth processes are also discussed. This synthetic method should be able to be extended to grow other one-dimensional chalcogens and chalcogenides nanostructures.
Introduction One-dimensional (1D) nanostructures, such as nanowires, nanotubes, and nanoribbons, have been the subject of intensive research on the synthesis, properties, characterization, and applications due to their potential use as building blocks in fabricating nanoscale electric or other devices.1-3 A variety of methods have been demonstrated to synthesize 1D nanostructures from elements to compounds and from inorganic to organic matter.4-7 Among these products, semiconductors8,9 represent a class of important solids that can be hopefully used in many areas. Hexagonal selenium, usually called trigonal Se (t-Se), has an interesting crystal structure containing extended spiral chains of covalently bound Se atoms parallel to its c-axis. So, Se is an ideal candidate for generating 1D nanostructures driven by its inherent anisotropic growth without needing templates or surfactants to direct the orientation. Selenium has many excellent physical and chemical properties. It is well-known as an extrinsic semiconductor with an indirect band gap of ∼1.6 eV, which was applied before germanium and silicon.10 Due to its high photoconductivity, it has been exploited for xerography, rectification, mechanical sensors, and photocells.11-13 In addition, the relatively low melting point, high piezoelectric, catalytic activity, and indispensability in the human body14 have made Se an important material. Recently, the photocatalytic behavior of Se nanoparticles in dye decolorization15 has been reported. Hopefully, 1D Se nanostructures will be exploited for new types of applications, or the currently found properties will be improved upon. Several synthesis methods for 1D Se nanostructures have been reported in the literature. For example, Gates has fabricated t-Se nanowires by precipitating a-Se, which dispersed in colloids16 through the reaction between selenious acid and excess hydrazine. Cheng has fabricated single-crystal Se nanowires by the direct conversion of polycrystalline selenium powder via a * Author to whom correspondence should be addressed. E-mail: zhangsy@ ustc.edu.cn.
hydrothermal process.17 Gautam and co-workers18 have prepared t-Se nanorods and nanowires based on the solution method via the reaction between Se and NaBH4 or thermal decomposition of [(CH3)4N]4Ge4Se10. A sonochemical approach19,20 has also been used to obtain Se nanowires. Most of these reported methods are solution-based routes. Actually, thermal evaporation is also a good way to obtain 1D chalcogenide21-24 nanostructures with a high crystallinity and a narrow size distribution. However, it is still a challenge to fabricate 1D Se nanostructures by this method, especially through the carbothermal route and to study the resulting excellent properties. The only reported Se nanoribbons25 were fabricated by thermal evaporation of Se powders at 600 °C with copper plates as substrates. Here, we report for the first time a simple and practical carbothermal chemical vapor deposition (CTCVD) route to t-Se nanowires and nanoribbons via evaporation of an active carbon (C) and selenium powder mixture, in which precursor Se is reduced by C followed by evaporation and decomposition to obtain the final products. The as-prepared 1D Se nanostructures exhibit novel optical properties and large blue shifts. Experimental Section All of the employed reactants from commercial sources were analytical grade and were used as received without further purification. The equipment was a common horizontal furnace with a quartz tube (diameter of ∼50 mm and length of ∼1 m) and a temperature controller. The thoroughly blended powder mixture of Se and active carbon (C) in different molar ratios were put into a porcelain boat that was placed in the bottom of a short close-ended quartz tube with a diameter of ∼25 mm and a length of ∼10 cm. The short tube with the open end downstream was placed in the center of the long tube followed by sealing and introducing highly pure nitrogen for 2 h to purge the air from the system. After being heated to 400-500 °C for some time, the tube furnace was cooled to room temperature naturally with the constant nitrogen flow. Gray samples were collected from the inner wall of the long tube.
10.1021/jp044152i CCC: $30.25 © 2005 American Chemical Society Published on Web 05/03/2005
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Figure 1. XRD pattern of the as-prepared Se nanocrystallites, which can be indexed as the trigonal phase.
The purity and phase structure of the products were obtained by X-ray powder diffraction (XRD) analysis, which was performed on a D/max-γA X-ray diffractometer (λ ) 0.15405 nm). The morphologies and microstructure were observed and analyzed through transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM), which were performed with a JEOL-2010 transmission electron microscope using an acceleration voltage of 200 kV. The UV-vis absorption and photoluminescence (PL) measurements were carried out on an UV-vis spectrophotometer (UV-240) and a Fluorolog-3Tau steady-state lifetime spectrophotometer from a Xe lamp at room temperature. Results and Discussion When the powder mixture with a selenium to carbon ratio of 4 was heated to 500 °C for 2 h, the typical XRD pattern of the obtained sample is shown in Figure 1, suggesting the high crystallinity. All of the peaks could be indexed as the t-Se according to JCPDS Card No. 6-362 with the lattice parameters of a ) 0.4366 nm and c ) 0.4953 nm. No impurities can be found from the pattern. Figure 2a displays the low-magnification TEM image, showing that the products consist of nanowires with uniform diameters ranging from 20 to 60 nm and with the length extending to several microns. Some nanowires grow out from a large particle several nanometers in size as shown in Figure 2b. The morphology is similar to that prepared by Gates20 but is different from that growing through a vapor-liquid-solid (VLS) mechanism, for the case in which a single wire has a particle on the tip. Figure 2c shows the assembly consisting of Se nanowires parallel to each other. From the inset electron diffraction (ED) pattern, the one and only set of diffraction spots shows that nanowires constituting the array grow along the same orientation. Figures 3a and 3b show the individual Se nanowires with two different ED patterns. A common characteristic of the two patterns is that (001) appears in all of them, because the incident electron beam ([010] and [110], respectively) is perpendicular to the nanowire. The conjectured growth direction is [001], i.e., c-axis, which can be confirmed by the HREM images shown in Figures 3c and 3d. Fringe spacing of ∼0.5 and ∼0.378 nm corresponding to (001) and (100) of t-Se can be seen from Figure 3c, indicating that the nanowire grows along the [001] direction. Fringe spacing of ∼0.5 and ∼0.218 nm corresponding to (001) and (110) of t-Se can be seen from Figure 3d, indicating that the nanowire also grows along the [001] direction. This preferential crystal growth direction is consistent with the helical chain in the t-Se crystal structure and has been usually reported in the literature. Furthermore, the HREM
Figure 2. (a) Low-magnification TEM image of Se nanowires. (b) TEM image of Se nanowires growing out from a large particle with diameters of several microns. (c) TEM image of several Se nanowires assembled parallel to each other. The inset is the corresponding ED pattern with only one set of diffraction spots, which shows the same growth orientation of the nanowires.
images indicate that the present 1D Se nanostructures are perfect without any defects in the crystal lattice. Upon further observation, some nanoribbons were found to be coexistent in the sample with the TEM image shown in Figure 4. The width is as large as ∼150 nm, and the thickness is not very symmetrical throughout the length. However, it can be clearly concluded from the contrast and the successfully obtained of HREM image that the width to thickness ratio is larger. The ED pattern (inset) is the same as the one from the nanowires, exhibiting the single-crystal nature. The preferential growth along [001] can also be confirmed from the HREM image shown in Figure 4b. To understand the growth mechanism of nanowires and nanoribbons as well as the controlled growth, a series of experiments has been carried out upon some changes in the experimental conditions. When the active carbon was absent without other changes in the conditions, polycrystalline film consisting of nanoparticles was obtained as shown in Figure 5a. When the reaction temperature was only decreased to 400 °C for 2 h in the presence of carbon, fewer samples could be found on the tube wall, and large particles accompanied by amorphous belts were obtained (Figures 5b and 5c). When we changed the Se to C ratio to 2 under 500 °C for 2 h, the obtained sample was shown in Figure 5d. Some wires have a tendency to grow out from the large particle. When the reaction time was decreased to 1 h, the morphologies such as those shown in Figure 5e are dominant. Some wires have grown out from the large particle but are not very long. The wire-free particle is also present. Figure 5f shows the monodispersed nanowires in the sample, which have diameters ranging from 20 to 50 nm. It is interesting that there is nearly no difference in diameter between them and those prepared under 2 h reaction time; maybe
CTCVD Route to Se 1D Nanostructures
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Figure 4. (a) TEM image of a single Se nanoribbon and the ED pattern (inset). (b) HREM image of a Se nanoribbon, showing the [001] growth direction.
Figure 3. (a and b) TEM images of individual Se nanowires with two different ED patterns. (c and d) HREM images of Se nanowires corresponding to the different ED patterns shown in parts a and b, respectively, indicating the same [001] growth direction and no lattice defects
the size is determined by the original growth as the sizes of the alloy droplets determine the final sizes of the nanowires via the VLS mechanism. From the experimental results, one can see that active carbon plays an important role in the growth of selenium 1D nanostructures. When it was absent, source selenium was directly evaporated and then solidified on the tube wall in a cooler area. Furthermore, the nanostructures grow as a function of temperature, selenium to carbon ratio, and reaction time. When active carbon was present, the growth process could be reasonably conjectured as the following, which is schematically explained in Figure 6. First, active carbon reacted with selenium26 because of the high temperature and the high chemical reactivity of selenium to form CSe2 or other nonstoichiometric compounds, followed by evaporation, to which increasing temperature is helpful. The experimental conditions are more suitable for the produced compounds to decompose into elemental selenium instead of solidification into compounds.
Figure 5. TEM images of Se prepared under different experimental conditions: (a) absence of active carbon, (b and c) 400 °C, (d) the molar ratio of Se to C is 2, and (e and f) 1h reaction time.
Then the selenium vapor was carried downstream and changed into liquids because of the low melting point. With the further decrease in temperature, nucleation and solidification began. During the solidification, crystallization and preferential growth tended to occur along the [001] direction because of the highly anisotropic trigonal lattice structure. To hasten the covalent bonds in helical chains parallel to the c-axis, the growth rate along the [001] direction should be faster than that along the [100] and [110] directions. In addition, the excess of selenium precursor is necessary in the beginning of preferential growth. This synthesis method can be understood in terms of CTCVD. In previous work, the carbothermal reduction method was usually employed to fabricate oxides,27 carbides,28 and nitrides,29 in which active carbon or graphite generally reacted with source chemicals containing O or N to generate medial compounds. Here, we first introduce the simple and practical CTCVD route to successfully fabricate chalcogen 1D nanostructures, which are expected to be used to synthesize other chalcogen and chalcogenide nanowires and nanoribbons. It is reasonable that
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Figure 8. Room-temperature PL spectra of Se nanocrystallites with different excitation wavelength of 270 and 350 nm. Figure 6. Schematic illustration of a plausible growth mechanism for 1D Se nanostructures through the CTCVD route. (A) Active carbon and excess selenium are reacted to generate CSe2 or other nonstoichiometric compounds followed by evaporation. (B) The experimental conditions were suitable for the produced compounds to decompose into elemental selenium instead of solidification into compounds; maybe the selenium was liquid because of its low melting point. (C) With the decrease in temperature, nucleation and solidification began, and then (D) the preferential growth occurred because of the inherent extended helical chains in the t-Se structure. (E and F) The growth rate along the [001] direction should be faster than that along the [100] and [110] directions. After a period of time, 1D Se nanostructures are formed.
eV), maybe due to the smaller diameters and larger aspect ratio of the nanowires. Figure 8 shows the room-temperature PL spectra of Se nanowires with excitation wavelengths of 270 and 350 nm. When 270 nm was used, the curve exhibits two peaks centered at 341.5 nm (3.63 eV) and 451 nm (2.75 eV), respectively. In comparison with the reported Se nanoparticles dispersed in polymer with a PL peak located at 2.297 eV34 excited at 514.5 nm at room temperature, the present strong and high-energy band should be attributed to direct interband radiative recombination and has a blue shift of ∼1.33 eV. The weak and lowenergy band should come from indirect interband radiative recombination and has a blue shift of ∼1.12 eV compared with that of Se nanoparticles35 prepared by sublimation and measured at 11 K. When 350 nm was used, the curve exhibits only one peak centered at 428.5 nm (2.89 eV), having a blue shift of ∼0.593 eV according to ref 34. We can conclude that the luminescence resulting from direct interband radiative recombination is predominant under higher-energy excitation. Conclusions
Figure 7. Room-temperature UV-vis absorption spectrum of Se nanocrystallites.
the well-known VLS mechanism, with which the growth of 1D nanomaterials through thermal evaporation is often explained, could explain the present growth, although there is a morphological difference as mentioned above. It is generally accepted that t-Se is a p-type, indirect band gap semiconductor, and UV-vis absorption spectra and PL spectra are two of the basic and common optical methods to study the energy band structure and other properties, although few works about Se nanowires have been reported. Here, we exhibit these optical data and hope that they could be helpful to those who are interested in studying the band structure and other properties. Figure 7 shows the UV-vis spectrum of Se nanowires dispersed in ethanol. Two clear peaks centered at ∼280 nm (4.43 eV) and 623 nm (1.99 eV) can be seen. The lower energy corresponds to indirect interband transitions30 and has a small blue shift relative to bulk Se. This effect because of the small size is similar to that reported by Johnson working on Se nanoparticles31 with a median diameter of 5 nm. The higher energy, matching well to the reported nanowires32 with 20-40 nm diameters, could be attributed to direct transitions. It should be noted that there is a large blue shift relative to the two direct transition energies reported by Joannopoulos33 (∼2.6 and 3.2
We have demonstrated a simple and practical CTCVD route to trigonal phase single-crystal one-dimensional Se nanostructures, in which active carbon plays an important role and three steps of chemical reaction, evaporation, and decomposition are included in the growth process under high temperature. In the meantime, some experimental conditions, such as temperature, selenium to carbon ratio, and reaction time, determine the morphologies of Se nanocrystallites. Se nanowires have uniform diameters ranging from 20 to 60 nm, and some nanoribbons have also been found to be coexistent in the sample. These nanostructures are perfect without any crystal defects and grow along the [001] direction, which matches well with its inherent crystal structure with extended helical chains. UV-vis and PL spectral measurements suggest a blue shift compared to the bulk material as well as other reported Se nanoparticles and two types of transition activities in absorption and emission. Due to a number of advantages, such as simple, practical, common, and versatile, the synthetic approach described here may be extended to fabricate other 1D chalcogen and chalcogenide nanomaterials. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. 50132030). References and Notes (1) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353.
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