Synthesis of coaxial nanotubes: Titanium oxide sheathed with silicon oxide
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The TiO2–SiO2 composite nanotubes with coaxial structure synthesized by sol-gel template method were presented. This process included 2 steps: first, grow SiO2 nanotubes in the pores of anodic alumina with a diameter of 200–250 nm; second, form TiO2 nanotubes within the SiO2 nanotubes. The TiO2 nanotubes with intact SiO2 sheaths were more uniform than those with sheaths partly removed. The existence of Ti–O–Si bonds and the amorphous sheaths is supposed to take an important role in the formation of composite nanotubes.
I. INTRODUCTION
Nanomaterials find technological applications in many different areas such as catalysis, magnetic recording, high-performance ceramics, and microelectronic components. The discovery of pure carbon nanotubes has resulted in extensive investigations to realize nanotubes or nanowires comprising multiple compositions in radial direction (composite nanotubes or nanowires). By successfully synthesizing composite nanotubes or nanowires, nanoscaled electronic or catalytic devices with a variety of functions may be accomplished. Recently, by using carbon nanotubes as templates, many kinds of nanocomposite structures have been made. Various techniques, including chemical insertion, physical insertion, arc encapsulation, and encapsulation via catalytic growth from the solid phase have been used to encapsulate metal and oxide inside the carbon nanotubes.1–7 Han et al.8 used the carbon nanotubes as templates to synthesize the (BN)xCy and BN nanotubes filled with Boron carbide nanowires by the substitution reaction. Recently, multielement nanocables comprising multiple phases synthesized successfully by means of laser ablation also have attracted more interest. Zhang and co-workers9 reported the synthesis of (BN)xCy nanotubes filled inside with a coaxial SiC and SiO2 core by a laser ablation method. With the laser ablation method, Si nanowires with SiOx sheaths have also been prepared.10,11 Meanwhile, by carbothermal reduction of sol-gel-derived silica xerogels containing carbon nanoparticles, –SiC nanorods with amorphous SiO2 wrapping layers have been achieved.12 Now it is very necessary to develop a feasible method to
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J. Mater. Res., Vol. 16, No. 5, May 2001
fabricate oxide nanocomposite nanotubes, which would open up a new application in the semiconductor and catalytic industries. The template method can provide an effective way for making oxidized metal macroporous structures. To date, the oxide nanotubes or nanowires, such as TiO2, MnO2, Co3O4, WO3, and ZnO have been formed, involving synthesizing the desired materials within the pores of nanoporous membranes.13–15 In our previous articles,16,17 we reported the remarkable SiO2 nanotubes generated by the means of the sol-gel template method. The macroscopic and aligned SiO2 nanotubes with a uniform diameter of approximately 30 nm and different wall sizes have been grown from the ordered nanochannel-array pores in anodic alumina; and it has been proved that the viscosity of SiO2 sol has
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