Well-aligned and suspended polyvinyl pyrrolidone (PVP) nanofibers with 8 mm in length were obtained by electrospinning. Using the aligned suspended PVP nanofibers array as template, aligned ultra-long silicon oxide (SiOx) nanotubes with very high aspect ratios have been prepared by plasma-enhanced chemical vapor deposition (PECVD) process. The inner diameter (20–200 nm) and wall thickness (12–90 nm) of tubes were controlled, respectively, by baking the electrospun nanofibers and by coating time without sacrificing the orientation degree and the length of arrays. The micro-PL spectrum of SiOx nanotubes shows a strong blue–green emission with a peak at about 514 nm accompanied by two shoulders around 415 and 624 nm. The blue–green emission is caused by the defects in the nanotubes.
KeywordsElectrospinning PECVD SiOx nanotubes TUFT process
Since the discovery of carbon nanotubes in 1991 , much effort has been focused on the synthesis of other inorganic tubular nanomaterials, such as MoS2, BN, TiO2, VOX and GaN [2–6]. Nowadays, various inorganic nanotubes have attracted more and more interests in the nanomaterial research [7, 8]. Nanotubes of inorganic materials like silica, which do not have sp2 bonding that favors tube formation, were generally prepared using porous materials [9, 10] or wire-shaped materials as templates . However, once these templates were removed, the tubes would generally bundle up and become less oriented, even be damaged. Considerable efforts have also been made to prepare aligned silica nanotube arrays to improve their functionality in advanced thin film devices. Fan et al.  have developed a process to transformed silicon nanowire arrays into silica nanotube arrays through a thermal oxidation-etching approach. Li et al.  have synthesized ultra-long and well-aligned silica nanotubes by the VLS (In as catalyst) mechanism lately. These SiO2 nanotubes are of special interest because of their potential applications in bioanalysis, bioseparation, optical device and catalysis. Compared with the insulating SiO2 nanotubes, the silicon monoxide (SiO) nanotubes are predicted to be semiconducting and proposed to have prospective applications in the semiconductor and catalysis industries [14, 15]. Although the studied SiO nanotubes are very thin and only of triangular, tetragonal, pentagonal and hexagonal cross-sections considered, the study suggested a possible route to tailor the electronic structures of silicon oxide (SiOx) nanotubes. Meanwhile, the investigation of PL mechanism of SiOx nanotubes have important significance because the room temperature PL of porous Si [16, 17] and Si-ion-implanted SiO2 (SiO2:Si+) [18, 19] has stimulated comprehensive studies on light-emitting devices made from Si-based materials. So far, reports of producing SiOx nanotubes are still very much lacking .
Electrospinning is a simple and highly efficient technique to produce long and extremely fine polymer fiber using an electrostatically repulsive force and an electric field between two electrodes to apply a high voltage to a polymer solution or melt [21, 22]. Meanwhile, different from other nano fiber fabrication processes, electrospinning has the ability to form various fiber assemblies [23, 24]. So the approach of using electrospun polymer fibers as templates [25–27] provides great versatility for the design of tubular materials with controlled dimensions. In this work, the preparation of aligned ultra-long and the synthesis of diameter-controlled SiOx nanotubes array by plasma-enhanced chemical vapor deposition (PECVD) process using electrospun-suspended polymer fiber array as template are reported. The morphology and chemical compositions of SiOx nanotubes were characterized by scanning electron microscope, transmission electron microscope equipped with energy-dispersive X-ray, X-ray photoelectron spectroscopy and micro-Raman. The micro-photoluminescence spectrum was also measured to investigate the luminescence mechanism of SiOx nanotubes.
Poly(vinyl pyrrolidone) (PVP, 0.18 g,Mw ≈ 1 300 000, Sigma–Aldrich) was dissolved in ethanol (3 ml) to form a 7 wt% solution, then loaded to a glass syringe equipped with a stainless steel needle with an inner diameter of 0.34 mm. The needle was connected to a high-voltage supply capable of generating DC voltage up to 60 kV. The voltage for electrospinning was kept at 18 kV. Two pieces of stainless steel stripes with an air gap of 8 mm were placed 18 cm below the tip of the needle . Assisted by electrostatic interactions, the nanofibers were stretched across the gap to form a parallel array. A stainless steel U-shaped frame with a distance of 4 mm between two branches was used to transfer the aligned nanofibers by vertically moving through the gap. The U-shaped frame with suspended nanofiber array span across its two branches was left in dry oven with temperature ranging from 80 to 150°C for 8–10 h to make the PVP template fibers thinner. And then it was transferred to the reaction chamber. The PECVD system is capacitively coupled using a radio frequency (13.56 MHz). After the chamber was pumped down to 3.0 × 10−3 Pa, the pre-treatment of template fibers for surface activation was conducted by the H2 gas and Ar gas injected into the chamber for 10 min. The applied radio frequency power was 60 W. Then, silane gas with the concentration of 2% flowed into the chamber for the coating. The deposition pressure was 130 Pa. After coating, the aligned core–shell nanofibers were transferred to the surface of silicon wafer by vertically moving silicon wafer through the gap of U-shape frames. Finally, the aligned core–shell nanofibers array was heated at 800°C for 2 h in high-purity argon gas (99.999%) to remove the PVP core, which led to nanotubes array.
The morphology of aligned nanotubes was observed by field emission scanning electron microscope (FE-SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEM-2010, 200 kV). Chemical compositions of the nanotubes were detected using an energy-dispersive spectrometer (EDS) attached to the TEM, X-ray photoelectron spectroscopy (XPS, VG ESCALAB 210) using Mg K a radiation and micro-Raman (JY-HR800) with a yttrium aluminum garnet (YAG) laser (532 nm). Furthermore, the micro-photoluminescence (PL) spectrum was measured with a He–Cd laser (325 nm) at room temperature.
Results and Discussion
In summary, it has been shown that aligned ultra-long SiOx nanotubes can be prepared by PECVD system using electrospun aligned PVP template fiber array. The inner diameter and wall thickness of nanotubes were controlled,respectively, by baking the electrospun PVP nanofibers and by coating time without sacrificing the orientation degree and the length of arrays. The PL spectrum of SiOx nanotubes shows a blue–green emission with a peak at about 514 nm accompanied by two shoulders around 415 and 624 nm, which is caused by the defects in the nanotubes. Our method shows a great improvement on the basis of tubes by fiber templates (TUFT) process  and is a straightforward and easy process for preparing aligned ultra-long SiOx nanotubes with very high aspect ratios. These aligned and diameter-controlled SiOx nanotubes obtained by us are of great potential for use in nanoscale fluidic bioseparation, sensing, catalysis and nanodevices. Moreover, this method can be used for preparation of aligned hybrid tubes and nesting structure of nanoparticle/nanofiber/nanotube in tube.
This work was financially supported by the Program for New Century Excellent Talents in University of China (Grant No: NCET-04-0975).