Synthesis and Characterization of Single-Crystalline SnO2 Nanowires

nanowires have a single crystal tetragonal structure. Scanning electron microscopy observation demonstrates that SnO 2 nanowires are 30–200 nm in diameter and several tens of micrometers in length.The surface vibrationmode resulting from the nanosize effect at 565.1 cm was found from the Fourier transform infrared spectrum. The formation of SnO 2 nanowires follows a vapour-solid (VS) growth mechanism. The gas sensing measurements indicate that SnO 2 nanowire gas sensor obtains peak sensitivity at a low operating temperature of 150C and shows reversible response to H 2 gas (100–1000 ppm) at an operating temperature of RT-300C.


Introduction
In recent years, much more attention has been focused on the research field of quasi-one-dimensional nanostructural materials due to their importance for understanding the fundamental properties of low dimensionality and the wide range of their potential applications in nanodevices [1,2].SnO 2 , as an n-type wide band gap semiconductor material (  = 3.6 eV at 300 K), is a key functional material that has been used extensively for transparent conductors [3,4], gas sensors [5,6], transistors [7], solar cells [8], and optoelectronic devices [9,10].So far, considerable efforts have been devoted to the research on the synthesis and characterization of SnO 2 nanomaterials such as nanowires [11,12], nanotubes [13], nanorods [14], and nanobelts [15,16] by using various synthetic methods.Among these methods, thermal evaporation is widely used because of its simple operation, low cost in preparation, and large-scale production.
SnO 2 nanomaterials are very promising for gas sensors due to their advantageous characteristics of large surface-tovolume ratio and great surface activity.Gas sensors based on SnO 2 nanomaterials have shown higher sensitivity, faster response, and enhanced capability to detect low concentration gases compared with the corresponding thin film materials [16,17].However, most of them are effective at high temperature above 200 ∘ C, which results in high power consumption and complexities in integration.Therefore, there is a need to develop SnO 2 nanomaterials for gas sensors that have good performance at a low temperature.In addition, much more efforts have been devoted to study the CO, NO 2 , or ethanol gas sensing properties of SnO 2 nanomaterials [15][16][17][18][19][20], and only a few reports were related to H 2 gas sensing properties [21][22][23].Deshpande et al. [21] reported that Ptcatalyst SnO 2 nanowires could detect 500 ppm H 2 gas with response time as low as 10 sec at RT. Wang et al. [22] reported that the response of SnO 2 naonwires to 500 ppm H 2 gas could be repeated without observing major changes in the signal at RT. Kolmakov et al. [23] reported that individual Pd-functionalized SnO 2 naonwires and nanobelts exhibited a dramatic improvement in the sensitivity toward oxygen and hydrogen at 200 and 270 ∘ C.However, in their works, the responses of SnO 2 naonwires were investigated only under the fixed temperature and H 2 concentration.
In this study, single-crystalline SnO 2 naonwires were synthesized by thermal evaporation of tin grains at 900 ∘ C in an Ar flow at ambient pressure.Structural characterization of the obtained SnO 2 naonwires was investigated by using X-ray diffraction, scanning electron microscopy, and    transmission electron microscopy.The Fourier transform infrared spectrum and growth mechanism of the nanowires were also investigated and discussed.The measurements of H 2 gas sensing properties demonstrated that SnO 2 nanowire gas sensor obtained peak sensitivity at a low operating temperature of 150 ∘ C and showed reversible response to H 2 gas (100-1000 ppm) at an operating temperature of RT-300 ∘ C.

Experimental
SnO 2 nanowires were synthesized on oxidized Si substrates by thermal evaporation of tin grains.Tin grains with high purity of 99.9% were placed on oxidized Si substrates in an alumina boat.No catalysts or impurities were introduced.The alumina boat was introduced at the center of a quartz tube that was inserted in a horizontal tube furnace.Ar gas with a flow rate of 50 mL min −1 was introduced into the quartz tube at ambient pressure.Then, the furnace was heated to 900 ∘ C and maintained at this temperature for 1 h.After the furnace was cooled down to room temperature naturally, a layer of wire-shaped products was obtained on the substrates around the tin grains, as shown in Figure 1(a).
SnO 2 nanowire gas sensor was fabricated by pouring a few drops of nanowire-suspended ethanol onto oxidized Si substrate with a pair of interdigitated Pt electrodes.The schematic illustration of a SnO 2 nanowire gas sensor device is shown in Figure 1(b).H 2 gas sensing properties were measured in a quartz tube inserted in an electric furnace at the operating temperatures ranging from room temperature (RT ≈ 25 ∘ C) to 300 ∘ C. As shown in Figure 2, dry synthetic air mixed with the desired concentration of H 2 gas flowed through the quartz tube at a rate of 200 mL min −1 .The electrical measurement was carried out by a volt-amperometric method at a constant bias of 10 V, and a multimeter (Agilent 34970A) was used to monitor the change of electrical resistance upon turning H 2 gas on and off.In this study, the sensor sensitivity was defined as  = (  −   )/  , where   and   were the electrical resistances before and after the introduction of H 2 gas, respectively.single crystal structure.The interplanar spacing of 0.34 nm corresponds to the (110) plane in a tetragonal SnO 2 structure.The corresponding selected area electron diffraction (SAED) pattern shown in Figure 5(c) also supports the formation of single crystal tetragonal SnO 2 nanowires.The growth direction of SnO 2 nanowires is found to be [301], which is consistent with previous reports [24,25].To demonstrate the chemical composition of the nanowire, EDX analysis was performed, and the EDX spectrum is illustrated in Figure 5(d).We can see that, except for the Cu element from the copper grid, only peaks of Sn and O elements are observed.

The Growth Mechanism.
Two models have been proposed to describe the growth mechanism of SnO 2 nanowires: the catalyst-assisted vapour-liquid solid (VLS) and the vapour-solid (VS) growth mechanisms [2].The VLS mechanism was first proposed by Wagner and Ellis in 1964 [26].The important feature of the VLS growth process is the existence of metal nanoparticles.The nanoparticles serve as catalysts between the vapour feed and the solid product and usually locate at the ends of the produced nanowires [27].In this study, no metal catalysts were employed, and no metal nanoparticles were observed at any ends of SnO 2 nanowires.Therefore, the growth process might be dominated by the VS growth mechanism.At a high temperature of 900 ∘ C, tin grains are vaporized and then directly condensed on the substrates.Once the condensation process happens, the initial condensed molecules form seed crystals serving as the nucleation sites.As a result, they facilitate directional growth to minimize the surface energy [2].Therefore, SnO 2 nanowires tend to be produced by continuous aggregation of more molecular SnO 2 on the growth front of the initial SnO 2 nuclei via the VS growth mechanism [28].

FTIR Spectrum.
The FTIR spectrum of SnO 2 nanowires is shown in Figure 6.Compared with the data published in the literatures [29][30][31], the peaks observed can be determined.
The peaks located at 603.7 and 692.4 cm −1 can be attributed to the Sn-O stretching vibration in SnO 2 .The peaks at 665.4 and 705.9 cm −1 can be assigned to O-Sn-O bending vibration.The peak at 565.1 cm −1 is noticeable because it cannot be found in the FTIR spectrum of bulk SnO 2 and is similar to the surface vibration mode of the peak at 564 cm −1 in SnO 2 nanopowders resulting from the nanosize effect [32].This surface vibration mode is related to the change of the surface structure.When the influence of the volume atoms on the lattice energy of surface atoms decreases, the surface tensile stress decreases, leading to the renormalization of the surface atoms and forming the new vibration mode [31,32].
3.4.H 2 Gas Sensing Properties.SnO 2 nanowires are very promising due to their large surface-to-volume ratio and great surface activity, which make them ideal candidates for gas sensing materials.Figure 7 shows the sensitivity of SnO 2 nanowire gas sensor as a function of the operating temperature upon exposure to 1000 ppm H 2 gas.It is found that the sensitivity increases with increasing operating temperature below 150 ∘ C, but reverse tendency is observed after 150 ∘ C. At this optimum temperature of 150 ∘ C, the highest sensitivity of 5.54 is obtained.It should be noted that the value of this peak sensitivity is larger than that of a porous SnO 2 sputtered thin film deposited at 24 Pa and RT upon exposure to 1000 ppm H 2 gas [33].In addition, the sensitivity value is comparable to the one found in [22] and is even higher than the value reported in [21,23] for SnO 2 nanowires, showing that the obtained SnO 2 nanowires are good for gas sensing application.
Figure 8 shows the dynamic responses of SnO 2 nanowire gas sensor to H 2 gas with various concentrations at 150 ∘ C. One can see that the resistance decreases upon exposure to H 2 gas and the resistance further decreases with increasing H 2 concentration.In addition, the resistance can recover to its initial value after removing H 2 gas, indicating a good reversibility of SnO 2 nanowire gas sensor.The resistance changes of 0.86, 1.68, 2.98, 3.61, 4.19, and 5.54 times with respect to the baseline are observed towards 100, 200, 400, 600, 800, and 1000 ppm H 2 gas, respectively.The sensor also shows reversible response to H 2 gas with different concentrations at an operating temperature of RT-300 ∘ C, although the corresponding data are not shown in this figure .The relationship between the sensor sensitivity and H 2 concentration at the operating temperature of 150 ∘ C is shown in Figure 9.It is found that the sensitivity increases as H 2 concentration increases.Such a variation implies that the sensitivity  can be described as a function of gas concentration  by an empirical model of  =   , where  and  are constants for a given gas.In this study, "" and "" were found to be 0.065 ± 0.034 and 0.629 ± 0.079, respectively.We see that the experimental data and the theoretical curve obtained from the empirical model show good agreement.

Conclusions
Single-crystalline SnO 2 nanowires were synthesized on oxidized silicon substrates by thermal evaporation of tin grains at 900 ∘ C. The structural characteristics, growth mechanism, and H 2 gas sensing properties of SnO 2 nanowires were investigated.SnO 2 nanowires with a tetragonal structure are 30-200 nm in diameter and several tens of micrometers in length.The nanosize effect induced FTIR surface vibration mode with peak at 565.1 cm −1 was observed.The formation of SnO 2 nanowires follows a vapour-solid (VS) growth mechanism.SnO 2 nanowire gas sensor shows reversible response to H 2 gas at an operating temperature of RT-300 ∘ C. The peak sensitivity is found at a low operating temperature of 150 ∘ C. The sensor sensitivity increases empirically with an increase of H 2 gas concentration.The results demonstrate the potential of SnO 2 nanowires for gas sensor applications.

Figure 2 :
Figure 2: Apparatus used for the measurement of H 2 gas sensing properties.

Figure 5 :
Figure 5: (a) TEM image of a single SnO 2 nanowire with a diameter of 96 nm.(b) HRTEM image of this nanowire.(c) SAED pattern of this nanowire.(d) EDX analysis of this nanowire.

3. 1 .
Structure and Morphology.The XRD pattern of the obtained SnO 2 nanowires is shown in Figure3.All the diffraction peaks can be indexed to the tetragonal SnO 2 structure with lattice constants of  =  = 0.4738 nm and  = 0.3187 nm according to JCPDS card no.41-1445.Moreover, no other crystalline forms such as Sn or SnO are detected, indicating that single phase SnO 2 nanowires are obtained.

Figure 4 (
a) is a typical FESEM image of SnO 2 nanowires, showing that a large quantity of wire-shaped nanostructures can be produced.It is found that SnO 2 nanowires have diameters ranging from 30 to 200 nm and have lengths of several tens of micrometers.A high magnification FESEM image shown in Figure 4(b) indicates that SnO 2 nanowires have smooth sidewalls.

Figure 7 :
Figure 7: The sensitivity of SnO 2 nanowire gas sensor as a function of the operating temperature upon exposure to 1000 ppm H 2 gas.

Figure 8 : 7 H 2 Figure 9 :
Figure 8: Dynamic response of SnO 2 nanowire gas sensor to H 2 gas with various concentrations at 150 ∘ C.