CRYSTAL GROWTH & DESIGN
Direct Synthesis of Indium Nanotubes from Indium Metal Source
2008 VOL. 8, NO. 1 344–346
Soumitra Kar,*,† Swadeshmukul Santra,†,‡,§ and Subhadra Chaudhuri| NanoScience Technology Center, Department of Chemistry, and Biomolecular Science Center, UniVersity of Central Florida, Orlando, Florida 32826, and Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700032, India ReceiVed December 28, 2006; ReVised Manuscript ReceiVed July 14, 2007
ABSTRACT: Metallic indium (In) nanotubes were prepared by direct thermal evaporation of an In metal source. In metal was heated in an Ar atmosphere at a high temperature, and after certain period of heating the system was allowed to cool to room temperature in normal atmosphere during which the nanotubes were formed. Growth of the nanotubes was initiated by the catalytic effect of Au, whereas the low melting point of the In metal was found to be responsible for the formation of the hollow core of the nanotubes. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive analysis of X-ray. Introduction The discovery of carbon nanotubes has initiated fundamental research in the area of one-dimensional (1-D) inorganic nanomaterials. Bacause of the nanoscale dimension, it is expected that 1-D inorganic nanomaterials (e.g., nanotubes, nanowires, nanorods, etc.) will possess unique physical, chemical, optical, electrical, and magnetic properties, which will be distinct from their bulk counterparts. Again, one should be able to manipulate the nanoscale properties of such 1-D nanostructures to reap their potential applications in nanoscale devices.1–8 In this context, development of 1-D metal nanostructures is quite appealing as the confinement of the metals to 1-D (or quasi 1-D) nanostructures could reveal unique nanoscale properties. To date, a variety of 1-D metal nanostructures such as Au,9,10 Ag,11,12 Cu,13 Fe,14 Zn,15–20 etc. have been reported in the literature, demonstrating their unique nanoscale properties. In a similar context, studies of nanostructures of low melting metals have been minimally investigated. For example, Talukdar et al.21 have recently reported the synthesis of indium (In) metal nanowires by an electrochemical approach. Here we report for the first time the synthesis and characterization of In nanotubes by direct thermal evaporation of In metal bar. The growth mechanism of the nanotubes has been investigated through scanning electron microscopic (SEM) observations. Experimental Details Sample Preparation. Synthesis of the In metal nanotubes was carried out in a horizontal tube furnace. The synthesis set up consisted of a one end closed quartz tube with an inner diameter of 25 mm. The open end of the tube was fitted with a rotary vacuum pump and Ar gas cylinders via vacuum flanges. Small pieces of In metal chunks scooped from a metal bar were placed in a 3-cm-long quartz boat. Cleaned Si wafers were sputter coated with a thin layer (∼25 Å) gold (Au) layer before being used as the substrates. The Au-coated substrates were mounted on the In loaded quartz boat with the Au-coated surface facing the source. The quartz boat was then placed near the closed end of the quartz tube, and the synthesis set up was evacuated through the rotary pump for 30 min and Ar gas was purged through it several times. Finally, the Ar gas was flown through the synthesis set up at a flow * Corresponding author. Telephone: 1-407-882-2848. Fax: 1-407-882-2819. E-mail:
[email protected]. † NanoScience Technology Center, University of Central Florida. ‡ Department of Chemistry, University of Central Florida. § Biomolecular Science Center, University of Central Florida. | Indian Association for the Cultivation of Science.
rate of 50 cm3/min. The gas outlet was passed through an oil chamber. The complete synthesis set up was then introduced inside the preheated furnace and after certain period of time. After the desired time interval, the whole synthesis set up was taken out of the furnace to allow rapid cooling. Synthesis temperature was varied between 900 and 1100 °C. A thin grayish white product was found deposited on the Si wafer and was characterized. Characterization. The products were characterized using X-ray diffractometer (XRD, Seifert 3000P) with Cu KR radiation. Microstructures of the products were obtained by scanning electron microscopy (SEM, Hitachi S-2300), and the compositional analysis was done by energy dispersive analysis of X-ray (EDAX, Kevex, Delta Class I). Transmission electron microscopic (TEM) images of the nanostructures were recorded with JEOL HRTEM [JEM 2100].
Results In our previous works,22–24 we produced In2O3 1-D nanostructures by using either a very high Ar flow rate or introducing oxygen along with Ar gas. In the present work, an attempt to produce similar In2O3 1-D structures in an oxygen deprived environment (i.e., at a lower Ar flow rate of 50 mL/min) resulted in formation of pure In metal nanotubes. The initial synthesis was carried out at 1000 °C for 1 h. Instead of the light yellow color of In2O3 formation, a grayish white layer was observed on the Si wafer. The X-ray diffraction (XRD) patterns of the products were different from that of In2O3. As shown in Figure 1, the XRD pattern of the products was identical to the indexed database for the XRD pattern of In metal [JCPDS card No. 050642], confirming the formation of pure In metal. No other peaks corresponding to In2O3 were observed. The morphology of the product was investigated through the SEM, and one representative image is shown in Figure 2a. The SEM image revealed the formation of mushroom-like nanostructures. The mushroom cap diameter was in the range 50–300 nm as estimated from the magnified view (Figure 2b). The diameter of the mushroom cap was approximately twice the stem diameter. Interestingly, the morphology of the products remained almost identical when the time of deposition was reduced to 20 min. We attempted to manipulate the experimental conditions to engineer a 1-D crystal growth process. The synthesis was carried out under similar experimental conditions except that Ar flow was stopped after 20 min, just before taking out the synthesis set up from the furnace. The XRD studies showed a pattern almost identical to that seen earlier in Figure 1. SEM images (Figure 3) revealed the formation of tadpole-like nanostructures with a round shaped head and a long tail. The diameter of the head portion (100–300 nm) was approximately three times larger
10.1021/cg060953u CCC: $40.75 2008 American Chemical Society Published on Web 12/11/2007
Indium Nanotubes from Indium Metal Source
Crystal Growth & Design, Vol. 8, No. 1, 2008 345
Figure 1. Representative XRD pattern of the In nanotubes.
Figure 3. SEM images of the In nanotubes: (a) general top view, (b, c) magnified image of selected hollow nanostructures.
Figure 2. (a, b) SEM images of the mushroom-like In nanostructures.
than the tail diameter. The diameter of the tail portion remained almost uniform throughout the nanostructure. Magnified view of the SEM images as shown in Figure 3b,c revealed the hollow nature of the stem portion. In another synthesis condition, the temperature was increased to 1100 °C, and like the previous case Ar flow was discontinued before taking out the synthesis set up from the furnace. The XRD pattern of the products again showed similar patterns depicted in Figure 1. Morphological studies through SEM (Figure 4a) however revealed the formation of In nanotubes in relatively high density. Lengths of the nanotubes were also found to increase considerably at high temperature. The high magnification SEM image (Figure 4b) of the broken end of a nanotube shows the presence of a hollow core confirming their nanotube morphology. The hollow nature of the In nanotubes was further confirmed from the TEM studies. Figure 4c depicts the TEM image of a In nanotube that ensures the hollow nature of the products. Synthesis carried out at 900 °C resulted in the formation of a very few nanotubes. No nanotubes were formed when the experiment was conducted at
1000 °C with a bare Si wafer (i.e., without using Au). Instead, spherical sub-millimeter size droplets were only visualized on the wafer. This indicated that nanosize Au droplets must be deposited on the Si wafer substrate for nucleation of the In nanotubes. The chemical composition of the nanotubes was investigated by using EDAX spectroscopy. EDAX spectra were recorded separately on the head and tail part of the nanotubes obtained at 1000 °C. The presence of Au along with In was noticed in the EDAX measurements when total area scan was performed. The signal of Si also appeared from the uncovered portion of the substrate. When the EDAX spectra was recorded from the stem portion of the nanotubes, the presence of only elemental In was revealed (the signature of Si being originated from the substrate was not considered). As expected, the EDAX pattern recorded on the round head portion of the nanostructure revealed the presence of Au along with In and Si. None of the EDAX studies showed the presence of elemental oxygen. Discussion The growth mechanism of the metal nanotubes can be well understood on the basis of two factors viz. (i) the catalytic activity of Au and (ii) the low melting point of In metal. The
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believe that the metallic In nanowires originated from in Au droplets only during the cooling process. As the temperatures dropped, the outer surface of the nanoforms first cooled down and crystallized but the interior part still remained in the liquid state. The solidification of the elongated nanowire part exerted a great deal of pressure inside the tube squeezing out the internal liquid In to form the hollow nanotubes. In the first case (i.e., when Ar gas flow was continued even after taking out the synthesis set up from the furnace), a portion of the In vapor was released out of the system resulting in the formation of short structures. However, the continued evaporation of the interior liquid resulted in the formation of the mushroom-like structures. On the other hand, an increase of synthesis temperature enhanced the local In vapor concentration considerably surrounding the substrate, resulting in the formation of a dense bush of In nanotubes. Conclusion In summary, we have produced for the first time In metal nanotubes from an In metal source by a thermal evaporation process. We believe that the growth of the 1-D nanostructures was initiated by the catalytic effect of Au. The low melting point of the In metal resulted in the difference in the cooling rate of the surface and core region of the nanostructures. A subsequent evaporation of the liquid In from the core region of the newly formed wire-like portion of the metal nanostructures forced the formation of nanotubes.
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Figure 4. SEM image of the In nanotubes: (a) large area view (b) magnified cross-sectional view of one In nanotube, and (c) TEM image of a nanotube ensuring its hollow nature.
growth of the In nanotubes was initiated by the Au-catalyzed vapor–liquid–solid process. This is similar to what we have observed earlier22–24 during the formation of 1-D In2O3 nanostructures. In the present study, the deposition system contained either no or a very low concentration of oxygen as impurity, which was not sufficient to oxidize the products. At high temperature, the Au thin film broke up to form liquid Au nanodroplets followed by the absorption of pure In metal vapors to form Au-In alloy droplets. Upon continuous absorption of In vapor, solid In 1-D nanostructures stem out of the supersaturated alloy droplets. Since the melting point of In (156 °C) is much lower than the synthesis temperatures (900–1100 °C), we believe that the 1-D nanostructures were formed during the cooling process. This was also supported by the fact that keeping the synthesis parameters unchanged an increase in the deposition time did not show any change in nanostructure morphology. Had the nanostructures formed during the heating process as reported in the case of In2O3 nanostructures,23 the dimension would have been changed over the deposition time. We strongly
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