Facile Synthesis of Template-Induced SnO 2 Nanotubes

Large scale SnO 2 nanotubes are successfully obtained by a facile hydrothermal method at a mild temperature. The morphologies and the microstructures of the as-synthesized SnO 2 products are characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The average diameter of the nanotubes is about 100 nm. The phase and composition of the as-obtained products are investigated by X-ray diffraction (XRD). A series of comparison experiments were conducted by varying the experimental parameters, such as temperature, time, and the amount of the alkali, to study the formation mechanism of SnO 2 nanotubes.

SnO 2 is a very important functional metal oxide material with the direct band gap of 3.6 eV at room temperature.Owing to the large specific surface area and aspect ratio, onedimensional SnO 2 nanostructures have wide applications in optical, electronic, and photocatalytic fields [22][23][24][25].Among them, tubular structure is especially interesting due to their hollow characteristics.Jia et al. reported a kind of hydrothermal method to synthesize the SnO 2 nanotubes [26]; Salehi synthesized SnO 2 nanotubes using the vapor thermal evaporation method at 900 ∘ C [27]; Kim and coworkers prepared SnO 2 nanotubes through a penetration method [28]; Lai et al.
synthesized SnO 2 nanotubes through a size-controlled electrochemical way at room temperature [29]; Wang et al. prepared SnO 2 nanotubes by the molten salt assisted, deposited SnO 2 on the a-MoO 3 nanorods [30].Herein, we reported a simple template-induced hydrothermal method to synthesize high yield SnO 2 nanotubes.The growth mechanism of the assynthesized SnO 2 nanotubes was also proposed.

Experimental Details
2.1.Purification of Carbon Nanotubes.All reagents were of an analytical grade and used without further purification.In a typical synthesis procedure, 1 g of MWCNTs was dispersed into 30 mL of thick HNO 3 and heated to boiling reflux for 12 h.After the acidification treatment, the MWCNTs were washed several times with deionized water to neutral.Finally, the product was dried in a chamber at 130 ∘ C for 12 h and treated at nitrogen atmosphere at 600 ∘ C for 3 h.for 14 h.Then the mixture solution was washed several times with ethanol and deionized water, respectively.Finally, the solid black product was dried in a chamber furnace at 80 ∘ C for 10 h and annealed in a muffle kiln at 600 ∘ C for 2 h.The phase and composition of the as-obtained product were characterized by X-ray diffraction (XRD, Rigaku Dmax-rB, Cu K radiation,  = 0.1542 nm 40 KV, 100 mA).The morphology and microstructure of the samples were studied by scanning electron microscope (SEM, Hitachi-4800) and transmission electron microscope (TEM, JEOL 2010EX).

Results and Discussion
The XRD pattern shown in Figure 1 showed that all the diffraction peaks can be indexed to the rutile tetragonal SnO 2 in accordance with the standard PDF card (number 41-1445).No obvious diffraction peaks from other impurities were detected, indicating high purity of the as-synthesized product.Carbon diffraction peaks were not found, revealing oxidation and decomposition of carbon nanotubes during calcination.
Morphology characterization was conducted using SEM and TEM.A typical low magnification SEM image was shown in Figure 2(a), finding large quantities of wire-like structures that were formed.Figure 2(b) shows high magnification SEM image, where there are some particles dispersed on the surface of the wire-like structure.The length of the wire-like structure is several micrometers, and the average diameter is about 50 nm.TEM was used to further characterize the morphology and microstructure of the product.Typical low magnification TEM image of the as-synthesized single SnO 2 structure was shown in Figure 2(c).One finds that the wirelike structure is hollow, named as nanotube.HRTEM image in Figure 2(d) indicated that the as-prepared products are highly crystalline.The measured lattice distance is 0.33 nm, corresponding to the rutile SnO 2 (110) plane.
In order to study the formation mechanism of SnO 2 nanotubes, a series of comparison experiments were conducted by varying the experimental parameters.First, we discuss the effect of the amount of alkali on the morphology of the as-synthesized product.Figure 3 showed SEM images of the as-synthesized products at 160 ∘ C for 14 h with different NaOH concentrations.When the concentration of NaOH was 0.29 mol/L, small irregular SnO 2 nanoparticles could be found, as shown in Figure 3(a).Increasing NaOH amount to 0.57 mol/L, many uniform nanotubes with rough surface could be observed (Figure 3(b)); the outer diameter of the nanotubes is about 100 nm, and inner diameter is about 50 nm.Then when increasing NaOH concentration to 0.86 mol/L, still some nanotubes were formed, but the surface of the product becomes smooth (Figure 3(c)).When the amount of NaOH is reaching 1.1 mol/L, no nanotubes appeared.This shows that the amount of alkali played a key role in the formation of the SnO 2 nanotubes.
Temperature-dependent experiments were also conducted.With the other growth parameters kept constant, when reaction temperatures are 90 ∘ C and 160 ∘ C, respectively, the diameter of nanotubes is about 100 nm and the thickness is about 50 nm, but the wall of the nanotube is thinner at low temperature (shown in Figures 4(a) and 4(b)).Further increasing the temperature to 200 ∘ C, the products are nanospheres with rough surface whose diameter is about 100 nm (Figure 4(c)).Not any product appeared when annealing at room temperature.
Finally, we discuss the effect of reaction time on the morphology of the as-synthesized products.When reaction time reaches 6 h, some irregular nanoparticles appear, as shown in Figure 5(a).With the reaction time elevated to 10 h, 14 h, and 18 h, the morphology of the products is SnO 2 nanotubes, and the diameter of the nanotubes is about 200 nm (Figures 5(b)-5(d)).This revealed that the reaction time has a certain effect on the morphology of the products.The formation of nanoparticles might be due to short reaction time, and there is not enough time for the nanoparticles to be adsorbed to carbon nanotubes.After annealed at 600 ∘ C, carbon nanotubes are removed, only some SnO 2 nanoparticles are left.When increasing the reaction time to 10 h, the SnO 2 nanoparticles adsorbed on carbon nanotubes are relatively less, and when the reaction time becomes longer, more SnO 2 nanoparticles are adsorbed to the outer wall of carbon nanotubes, and SnO 2 nanotubes were formed after carbon nanotubes were calcined.
Based on the above mentioned results, a possible growth mechanism of SnO 2 nanotubes can be initially proposed as follows.First, Sn 4+ reacts with OH − and forms Sn(OH) 4 + particles in alkali solution as a nucleus, and the formed small particles are further converted to SnO 2 particles.Meanwhile, SnO 2 crystal nucleus is immediately gathered in the outer surface of the carbon nanotubes.When annealed in the air at 600 ∘ C, internal carbon nanotubes react with oxygen and form carbon dioxide, and SnO 2 nanotubes were formed with the escape of carbon dioxide.Due to high temperature and high pressure and alkali, C-C covalent bond in carbon nanotubes broke, resulting in the fracture or defects appearing on the wall, and SnO 2 nanospheres were formed with SnO 2 nuclei attached to these breakage carbon nanotubes.Figure 6 shows the growth schematic diagram of the SnO2 nanotubes.

Conclusion
We report a facile and effective template-induced hydrothermal approach to synthesize the SnO 2 nanotubes at a low temperature.The results indicated that each nanotube is 100 nm in diameter and several micrometers in length and owns a rough surface.Formation mechanism for nanotubes is elucidated based on experimental results.

Figure 2 :Figure 3 :Figure 4 :
Figure 2: (a) and (b) are SEM images of the as-synthesized SnO 2 nanotubes at different magnification.(c) and (d) are TEM and HRTEM images of the as-synthesized SnO 2 nanotube.