Abstract
Single-capillary electrospinning has been exhibited to be a simple and scalable method for fabricating nanofibers. Construction of graphene/inorganic fibers with the core-shell hollow structure using graphene as skeleton has been rarely reported. Here, we show a facile approach to prepare electrospun reduced graphene oxide@SnO2 composite nanofibers with the hollow structure. The hollow core@shell structure is formed in a single-capillary electrospinning process including sintering, which is promising for the preparation of graphene/inorganic composite nanofibers. The reduction of as-synthesized graphene is realized by stannous ion. Resulting hollow and core-shell structure enables the reduced graphene oxide@SnO2 composite nanofibers to adsorb and desorb the target gas more easily, which is promising for future applications as fast NO2 gas sensor.
1. Introduction
Due to its large specific surface area, high charge transfer rate, and chemical stability, graphene has attracted considerable attention for various applications. In particular, graphene/inorganic composites have exhibited enhanced electric, photo, and catalytic properties for applications as photocatalysts, gas sensors, and energy storage devices [1–5]. Recently, synthesis of graphene/SnO2 composites has received a lot of interest because of their excellent performance as lithium ion battery (LIB) electrodes, photocatalysts, and gas sensors [6–9]. Gas sensor based on SnO2 composites especially the PN junction-type composite with core-shell structure has exhibited enhanced excellent gas-sensing performance [10–12]. According to previous reports, the addition of graphene or graphene oxide can effectively improve the gas-sensing performance of graphene-metal oxide composite due to the unique charge transport features in hetero-interfaces [13–15]. The general route for the preparation of graphene/SnO2 composite involves the reduction of graphene oxide and subsequent in-situ growth of inorganic component on graphene sheets [16, 17]. Graphene/inorganic composites with disordered orientation of nanoparticle aggregates cannot benefit from the mechanical properties of graphene. One-dimensional composites with a graphene sketch structure are promising to overcome this issue. In particular, construction of graphene core nanofibers with hollow structure can allow the internal surface to facilitate the direct contact between gas/electrolyte and graphene to enhance the performance [7, 18–20].
Electrospinning is a simple and scalable technique that has been employed to prepare highly uniform one-dimensional nanofibers [21–23]. Coaxial electrospinning method is effective to prepare core-shell structure. For example, SnO2/TiO2, SnO2/SiO2, and Fe3O4/SnO2 core-shell nanofibers have been fabricated using coaxial electrospinning method [24–27]. However, single-capillary electrospinning is more simple and scalable than coaxial electrospinning method is. The preparation of core-shell hollow structured nanofibers with graphene as skeleton is still a challenge using single-capillary electrospinning method.
Herein, we report on electrospun reduced graphene oxide@SnO2 (rGO@SnO2) composite nanofibers with the hollow structure by using a single-capillary electrospinning process. The formation mechanism is discussed. The hollow and core-shell structure shows fast adsorb and desorb of the target gas, which is demonstrating as a fast NO2 gas sensor.
2. Experimental
2.1. Synthesis of rGO@SnO2 Composite Nanofibers
All chemical reagents were used as received.
Graphite oxide (GO) was synthesized from natural graphite flakes by a modified Hummers method. After a freeze-drying process, 5 mg GO was dispersed in 5 ml deionized water under high-intensity ultrasonication. Then, 4.5 mmol SnCl2 was dissolved in 5 ml ethanol. Subsequently, the GO dispersion was mixed with SnCl2 solution under vigorous stirring. Then, 2.304 g of PVP dissolved in 10 ml DMF was added into the above solution to make the precursor solution. The precursor solution was loaded into a plastic syringe with a 5 μL pipette tip. The syringe was set up vertically in place with a Cu wire as electrode. The electrode was connected to a high-voltage power supply. The applied voltage was 19 kV, and the distance between tip and aluminum collector was 10 cm. The precursor solution was electrospun into nanofibers. The as-prepared film of nanofibers was sintered at 450°C for 2 h under N2 atmosphere with heating rate of 1°C/min. For comparison, pure GO nanofibers were synthesized using the same method as used for reduced graphene oxide@SnO2, but without the addition of SnCl2.
2.2. Characterizations
Crystal structure and morphology of the samples were characterized by powder X-ray diffraction (XRD, Rigaku D/MAX 2600/PC diffractometer with a Cu Kα radiation, Japan) and field-emission scanning electron microscopy (FE-SEM, SU70, Hitachi, Japan). Raman spectra of the products were obtained using a micro-Raman spectrometer (J-Y; HR800, France) under excitation wavelength of 488 nm. Detailed morphology and crystal structure were analyzed using a FEI Tecnai F20 microscope operated at 200 kV for transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED).
2.3. Gas-Sensing Performance
The as-obtained samples were mixed with ethanol and ground in an agate mortar to form a paste. The paste was coated on an alumina tube-like substrate (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter) on which a pair of gold electrodes were printed. The gas-sensing performance was measured using a static gas sensor test system. Specific amounts of target gas were injected into a 10 L closed glass chamber to achieve controlled concentrations. Change in resistance was measured using a computer-interfaced Fluke 8846A RMS digital multimeter. The response of the sensor is defined as Ra/Rg, where Ra and Rg are the resistances of the sensor in air and target gas with air background, respectively. The relative humidity of the testing system is 20%.
3. Result and Discussion
The scanning electron microscopy (SEM) images of the as-prepared sample are shown in Figure 1. The uniform morphology of the as-prepared fibers is clearly observed in Figure 1(a). The average diameter of the fibers is about 200 nm, and the length is several microns. Nanoparticles with irregular shape are grown on the surface of nanofibers. Figure 1(b) shows the segment SEM image of a single nanofiber at high magnification. The particles grown on the fiber surface present plenty of facets with irregular shapes. Some broken fibers with open tips are observed, as shown in Figure 1(c). The hollow structure of as-prepared fibers can be clearly observed. The inner diameter of the hollow structure is about 100 nm. The XRD pattern shown in Figure 1(d) is used to confirm the fiber composition. The pattern has two clear features between 10 and 70 degrees. All the sharp peaks in Figure 1(d) can be matched with tetragonal SnO2 (JCPDS card no. 41-1445). The broadened peak at about 23 degrees is characteristic of graphene and is similar to that of other reports. There are no other peaks in the XRD pattern of an as-prepared fiber, which indicates high purity of the product.
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To further confirm the structure of rGO@SnO2 composite, Raman spectra and FTIR spectra were obtained. Raman spectra of an as-prepared sample after sintering and reactant GO are shown in Figure 2(a). The peaks at 629 cm−1, 688 cm−1, and 769 cm−1 correspond to A1g, A2u, and B2g vibration modes of SnO2, respectively. The two strong peaks in both reactant GO and an as-prepared sample correspond to D-band (ca. 1361 cm−1) and G-band (ca. 1578 cm−1). The D-band is caused by edges or structural defects that can break the selection rule and symmetry, while the G-band can be attributed to the first-order scattering of sp2 carbon domains. The ratio for an as-prepared sample after sintering is larger than that for the reactant GO due to the formation of sp2 domains, which indicates that the GO is reduced to rGO during the reaction.
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As shown in Figure 2(b), the characteristic features of a composite fiber after sintering are the IR absorption bands corresponding to Sn-O stretching in the range of 540 to 750 cm-1, C=O stretching at 1623 cm-1, C=C stretching at 1564 cm-1, C-N stretching at 1402 cm-1, and -OH stretching at 3430 cm-1. These peaks are similar to the results of other inorganic/graphene composites. The decreased C=O stretching absorption indicates the existence of reduced graphene oxide. FTIR spectrum of the fibers without sintering (red line) shows typical PVP features and is similar to the results reported for Pt/PVP composite. It should be noted that the characteristic absorption bands of PVP at 1294 cm-1 and 2932 cm-1 disappear after sintering at 450°C (black line), which indicates the complete decomposition of PVP [28]. It has been previously reported that PVP decomposes above 400°C under inert atmosphere [29]. The C=O stretching vibration at 1623 cm-1 is also weakened due to decomposition of PVP and reduction of GO. Furthermore, the enhanced C-N stretching vibration demonstrates that N doping of graphene accompanies the decomposition of PVP under N2 atmosphere.
Transmission electron microscopy (TEM) images reveal detailed microstructures of the rGO@SnO2 composite fibers. The hollow structure of a rGO@SnO2 fiber is clearly observed in the TEM image of a single fiber segment shown in Figure 3(a). The TEM image indicates that the fiber consists of relatively large nanoparticles on the shell with size of 40 nm and small nanoparticles with size less than 10 nm. The large nanoparticles have obvious crystal facets, which are randomly oriented. The TEM image of a broken fiber with an open tip clearly displays the core-shell hollow structure. As shown in Figure 3(b), graphene layer is found on the edge of the tip, which indicates that the core (inner tube) is graphene. XRD and FTIR analyses also confirm the presence of graphene. The high-resolution TEM image (Figure 3(c)) of the relatively large particles reveals the presence of tetragonal SnO2 phase. The lattice fringes show interplanar distances of about 0.33 nm and 0.26 nm, which match well with the spacing of (110) and (101) planes of tetragonal SnO2, respectively. The small nanoparticles shown in Figure 3(d) also have the same features, which indicates that the phase of the small particles is also tetragonal SnO2. The average size of particles is about 7 nm.
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Based on the above results, the formation mechanism of rGO@SnO2 with hollow structure can be described as shown in Figure 4. The dispersed GO nanosheets, PVP, and Sn2+ constitute the electrospinning solution used to fabricate the precursor fibers. PVP is adsorbed on graphene oxide nanosheets by strong π-π interaction between benzene and pyrrole ring. When the fiber is heated slowly, PVP migrates to the inner space between graphene oxide nanosheets. Moreover, stannous ion reduces GO to form graphene-covered small SnO2 nanoparticles, which build a rGO@SnO2 core-shell structure. Small particles grow into larger particles, and PVP decomposes above 400°C. The decomposition of inner PVP leaves a hollow structure in core-shell composite fibers.
The gas-sensing performance for NO2 was studied at room temperature, and the results are presented in Figure 5. The response-recovery properties of rGO@SnO2 with core-shell structure (black line) and graphene (red line) nanofibers are shown in Figure 5(a). The response time of rGO@SnO2 is 50 s, and recovery time is 90 s. The resistance can return to the initial value. In comparison, graphene fibers show response time of 50 s and recovery time of 120 s. Figure 5(b) shows the sensitivity curves of rGO@SnO2 nanofibers for different concentrations of NO2. It is observed that the sensitivity of rGO@SnO2 increases from 1.5 to 14.9 as a NO2 concentration increases from 10 ppm to 100 ppm. In contrast, the sensitivity of graphene varies from 1.1 to 3.2. The sensitivity of rGO@SnO2 increases linearly without a saturation phenomenon as a gas concentration increases. These results suggest that the inclusion of SnO2 leads to an increased concentration range of the gas sensor. It is possible that SnO2 provides higher carrier density and reduces the width of surface depletion layer, which provides more electrons for the adsorption of oxidation gases. Therefore, the sensor shows high sensitivity towards a large concentration of NO2. Pure graphene cannot be used to detect large concentrations due to limitation of carrier density. The selectivity results of rGO@SnO2 nanofibers are shown in Figure 5(c). It is observed that the sensitivity of rGO@SnO2 for NO2 is much higher than that for other interfering gases such as SO2 and Cl2, which indicates good selectivity of the sensor.
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To further study the mechanism, the content of rGO was first verified. According to the ratio described in Experimental, the concentration of rGO in SnO2 should be 0.7%. Figure 6(a) shows the TG-DTA curves of rGO@SnO2 nanofibers in air. A significant endothermic peak appears at 465°C, and the corresponding mass loss is about 0.64%, which is consistent with the experiment. Based on the results, we summarize the sensing mechanisms similar to those described in previous works, especially the surface depletion layer and PN junction. The gas-sensing process is the redox process between the surface of the sensing material and the target gas. Gas molecules adsorb on the surface to form a depletion layer. The loading of p-type graphene could form a PN heterojunction with n-type SnO2, forming a depletion layer at the interface, increasing the possibility of carrier diffusion from bulk to surface, and resulting in higher sensitivity. Besides, the surface of the porous hollow fiber structure facilitates the rapid adsorption and desorption of the target gas. Especially after the loading of graphene, the tiny graphene particles are highly dispersed in the fiber, blocking the link of SnO2 particles. To further explain the relationship of NO2 sensing performance and the graphene loading, we regulate the mass ratio of rGO to SnO2 in composites. As shown in Figure 6(b), the sensitivity first increases and then decreases as the amount of added rGO increases. rGO contacts with SnO2 to form a PN heterojunction, which results in the formation of a carrier-rich region in the interface, increasing the adsorption possibility of the oxidizing gas NO2. The heterojunctions increases, and the sensitivity gradually increases as the amount of rGO increases. However, when the loading exceeds 0.7%, a large amount of rGO wraps the SnO2 particles, resulting in a decrease in the contact interface, and finally the sensitivity is lowered. We attribute the good gas sensitivity of rGO@SnO2 nanofibers to the optimal ratio of PN heterojunction and specific porous hollow core-shell structure.
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4. Conclusions
In conclusion, we have designed and fabricated rGO@SnO2 composite hollow nanofibers using a single-capillary electrospinning process with heating. PVP allows the graphene oxide nanosheets to migrate into the inner space of the fiber, and then PVP decomposes under inert atmosphere, which constructs a rGO@SnO2 core-shell hollow structure. The graphene core of hollow nanofibers forms a channel to rapidly adsorb and desorb NO2. This hollow and core-shell structure enables rGO@SnO2 composite nanofibers to adsorb and desorb the target gas more easily and shows enhanced gas-sensing performance for NO2 at room temperature.
Data Availability
The experiments and results data used to support the findings of this study are included within the article.
Conflicts of Interest
There is no conflict of interest in the manuscript.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant no. 61603086), and the Science and Technology Development Fund of Jilin Province, China (20160414011GH), and Jilin City Science and Technology Innovation Development Plan (201750212).
Supplementary Materials
The reduced graphene oxide@SnO2 composite nanofibers with hollow structure were prepared using a facile electrospinning method. The hollow core@shell structure is fabricated using a single-capillary electrospinning process with heating method, which is suitable for the preparation of graphene/inorganic composite nanofibers. The reduction of as-synthesized graphene is realized by stannous ion. The resulting hollow and core-shell structure makes reduced graphene oxide@SnO2 composite nanofibers adsorb and desorb the target gas more easily, which is promising for fast NO2 gas sensor. (Supplementary Materials)