Morphology adjustment of SnO₂ and SnO₂/CeO₂ one dimensional nanostructures towards applications in gas sensing and CO oxidation

Abstract

SnO₂ and SnO₂/CeO₂ one dimensional (1D) nanostructures with various morphologies including solid nanofibers (NFs), nanobelts (NBs), nanotubes (NTs), and wire-in-tubes (WTs) were successfully prepared via a single-spinneret electrospinning process and a subsequent heat-treatment by adjusting the heating rate and the amount of CeO₂. The interesting morphology evolution of samples from NTs to NBs was reported with increasing amounts of CeO₂. In addition, SnO₂ particles existed on the surface of the CeO₂ matrix to form diverse structures in the 1D nanofibers. The band gap of the composite oxides decreased with the addition of CeO₂. Compared with other nanostructures, SnO₂/CeO₂ NTs exhibited superior gas sensing properties, such as the highest response to ethanol. Due to the existence of Ce³⁺ and oxygen vacancies and the hollow structure, SnO₂/CeO₂ NTs revealed great CO oxidation performance, indicating the enhanced interactions between the catalyst and the target gas. These excellent properties were attributed to the prominent 1D hollow morphology, the good dispersion of SnO₂ nanoparticles on the surface of the CeO₂ matrix, the ideal crystallinity, and the composite interactions between the two oxides. In addition, the current method could be utilizable to fabricate other metal oxides with various morphologies for property controlling and important applications.

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Introduction

As a special morphology, one-dimensional (1D) nanostructures, including solid nanofibers (NFs), nanobelts (NBs), nanotubes (NTs), and wire-in-tubes (WTs), are of great interest in a wealth of applications, such as energy conversion,¹ tissue engineering scaffolds,² gas sensing,³ catalysis,⁴ and so on. To date, a variety of methods have been invented to fabricate 1D nanostructures, for example, template-assisted synthesis,⁵ solvothermal route,⁶ organic chemical vapor deposition,⁷ structure-selective synthesis.⁸ Notably, electrospinning is a simple, versatile, and cost-effective technique for preparing 1D nanostructures with controllable compositions, diameters, and morphologies for a wealth of applications. This strategy links the dimension and morphology of nano- and macroscale materials to properties in a flexible manner. Many methods including polymer template method,⁹ coaxial electrospinning technique,¹⁰ and phase separation co-electrospinning process,¹¹ have been reported for fabrication of 1D NTs. However, these procedures often suffer from tedious processes, templates and polymers. In addition, to create other 1D morphologies, we have to find other more effective method.

With the development of science and technology, scientists pay much more attention to the environmental problems and sustainable development, such as gas sensing and CO oxidation. Ethanol sensor is one of the most popular gas sensors due to broad applications in medical process, breathalyzer, and food industries. The studies of the catalytic properties of CO oxidation using 1D oxide catalysts are very important in chemical engineering, such as air purification and automotive exhaust cleaning field. Besides a reasonable structure, a suitable composition of the functional materials is also very important. Among those 1D nanocomposites, semiconductor-based complex oxides, such as ZnO/SnO₂,¹² TiO₂/CuO,¹³ In₂O₃/CeO₂,¹⁴ and ZnO/TiO₂,¹⁵ have attracted extensive attentions. SnO₂, as an excellent n-type IV–VI semiconductor, with a wide band gap of ∼3.6 eV, has been proved to be an important gas sensing material with applications in detecting both reducing and oxidizing gases. As a well-known rare earth oxide, CeO₂ exhibits promising properties in catalytic and gas sensing area due to its particular characteristics arising from the 4f electronic shells, rich oxygen vacancies and low redox potential between Ce³⁺ and Ce⁴⁺. It is expected that the composition of CeO₂ and SnO₂ could remarkably enhance the performance due to the strong interactions between two individual components.

Best to our knowledge, little attention has been paid to the control of the morphology of nanostructures fabricated by single-spinneret electrospinning. In this paper, for the first time, we observed a morphology evolution of SnO₂/CeO₂ composites from NTs to NBs. The formation mechanism of the nanostructures was studied. In addition, their bifunctional gas sensing properties and CO oxidation performance based on different mechanisms are also systematically studied. The fibers with various morphologies display different ethanol sensing properties and catalytic performances due to the special feature of the structure and the oxygen vacancies.

Experimental

Materials

N,N-Dimethylformamide (DMF) was taken from Tianjin Chemical Reagent Institute. Poly(vinylpyrrolidone) (PVP; M_w ≈ 1 300 000) was purchased from Aladdin Reagent Company. Ce(NO₃)₃·6H₂O and SnCl₂·2H₂O were purchased from Sinopharm Chemical Reagent Company. All chemicals were analytical grade reagents and they were used directly without any further purification.

Preparation of 1D SnO₂ and SnO₂/CeO₂ nanostructures

In a typical synthesis, different amount of SnCl₂·2H₂O and Ce(NO₃)₃·6H₂O were added to 12 mL of DMF with vigorous stirring for 1 h. Then, 2 g of PVP was added to the above solution and the mixtures were allowed to stir for 24 h. After that, viscous mixed solutions of inorganic salts and PVP were obtained. The detailed preparation conditions and properties of the samples were listed in Table 1.

Table 1 Preparation conditions and properties of samples

Sample	Molar ratio^a	Heating rate (°C min^−1)	Morphology	Average diameter (nm)	Band gap (eV)
a Sn/Ce molar ratio in precursors.
Sn-NFs	N/A	1	Nanofibers	170	3.74
Sn-NTs-1	N/A	5	Nanotubes	600	3.73
Sn-NTs-2	N/A	5	Nanotubes	330	3.76
Sn–Ce-WTs	20 : 1	5	Wire-in-tubes	200	3.53
Sn–Ce-NTs	10 : 1	5	Nanotubes	200	3.3
Sn–Ce-NFs	5 : 1	5	Nanofibers	225	3.07
Sn–Ce-NBs	1 : 1	5	Nanobelts	500	2.91

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The as-spun fibers were prepared via a typical single-spinneret electrospinning method.¹⁶ Each solution was loaded in a 5 mL syringe fitted with a stainless steel needle with ∼0.8 mm inner diameter. The syringe was fixed horizontally on the pump and the optimal feed rate was chosen at 7 μL min⁻¹. An electrospinning process was performed by applying a DC voltage of 15 kV to the needle and the tip-to-collector was 15 cm. As the solvent evaporated, the as-spun fibers were obtained on the collector. After that, these samples were calcined at 550 °C for 3 h with different heating rates. After the fibers were naturally cooled down to room temperature, the target samples were obtained.

Characterization

The morphology observation of samples was carried out using a field-emission scanning electron microscopy (FESEM, QUANTA 250 FEG, FEI, America). The transmission electron microscopy (TEM) images were recorded on a JEM-2010 transmission electron microscope and a transmission/scanning transmission electron microscope (TEM/STEM, Tecnai F20, FEI). The average diameter of samples was calculated based on randomly selected fibers. The crystal structures and phase composition of samples were observed using an X-ray diffraction (XRD) meter (Bruker D8-Advance, Germany). X-ray photoelectron spectroscopy (XPS) experiments were investigated using an X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromatic Al KR source. All the binding energies were referenced to the C 1s peak at 284.7 eV of the surface adventitious carbon. The fitted peaks in the XPS spectra were analyzed using the Avantage 5.52 software. The UV-vis absorption spectra were measured on a Hitachi U-4100 spectrophotometer. The band gaps of samples were estimated from the adsorption spectra as listed in Table 1.

Gas sensing measurement

Gas sensing performance was evaluated using a WS-30A system (Weisheng Instruments Co., Zhengzhou, China). The as-prepared sample was mixed with ethanol to form a paste. Then, the paste was coated on a ceramic tube, on which a pair of gold electrodes had been previously printed. A Cr–Ni heating wire was inserted into the ceramic tube as a resistor. Before measurement, the sensors were dried and aged for about 5 days. When testing, the saturated target gas was injected into the test chamber (about 1 L) by a disposable syringe. After fully mixed with ambient air, the sensor was put into the chamber. The sensor was taken out to recover in ambient air after the response reached a constant value. The gas sensor resistance and response values were obtained by the analysis system automatically. In the experimental process, a working voltage of 5 V was applied on the sensor. The response of the sensor in air or in a target gas could be measured by monitoring the voltage across the reference resistor. The gas sensor response value was defined as below:

Response = R_gas/R_air (1)

where R_air is the electrical resistance of the sensor in air and R_gas is the resistance of sensor in the presence of the target gas. The response time in the case of adsorption or the recovery time in the case of desorption was defined as the time taken by the sensor to achieve 90% of the total resistance change.

CO catalytic oxidation

CO oxidation properties were measured on a fixed-bed flow reactor with 50 mg of samples under atmospheric pressure. The reaction vessel was a quartz tube with an inside diameter of 6 mm. A gas mixture with a total flow rate of 50 mL min⁻¹ flowed through the quartz tube and the feed gas was 1% CO, 10% O₂, and 89% N₂ in volume ratio. The programmed-temperature rate was 2 °C min⁻¹ which was controlled in an electrical furnace. A thermocouple placed in the middle of the reactor was used to monitor the reaction temperature. The catalyst was directly exposed to reaction gas without any pretreatment. The composition analysis of the effluent gas was carried out by a gas analyzer. The CO conversion ratio was observed on the basis of the CO consumption and CO₂ formation.

Results and discussion

Fig. 1a to c show the typical SEM images of samples Sn-NFs, Sn-NTs-1, and Sn-NTs-2. These randomly oriented 1D nanostructures were all composed of lots of uniform nanoparticles. The image in Fig. 1a indicates that sample Sn-NFs revealed a solid fibrous morphology with a diameter of ∼170 nm due to the low heating rate during calcination process. As increasing the heating rate to 5 °C min⁻¹, we obtained well-defined SnO₂ NTs with a diameter of 600 nm, as shown in Fig. 1b. The wall of the tube is composed of a layer of SnO₂ nanoparticles, suggesting the wall thickness is ∼70 nm. As increasing the amount of SnCl₂ in spinning precursor solution, SnO₂ tubes with thick walls are obtained. The inset in Fig. 1d depicts the TEM image of sample Sn-NTs-1, clearly indicating a large quantity of tube-like structures composed of numerous SnO₂ nanoparticles. More detailed information of the SnO₂ NTs and the crystal structures can be observed by high-magnification TEM, as shown in Fig. 1d. The lattice fringes with a spacing of 0.33 nm can be observed, which is in agreement with the spacing of (110) planes of SnO₂. Furthermore, the clear crystal lattice shows sample Sn-NTs-1 with good crystallinity. The various morphologies indicate that both of the content in spinning solution and the heating rate during the calcination process plays an important role in preparing 1D structures via electrospinning.

Fig. 1 (a, b, and c) SEM images of (a) sample Sn-NFs, (b) sample Sn-NTs-1, and (c) sample Sn-NTs-2. (d) HRTEM image of sample Sn-NTs-1. Inset in (d) shows a TEM image with low magnification.

To investigate the effect of Ce amounts on the morphologies of SnO₂/CeO₂ composite structures, SEM images are given in Fig. 2. As we added the CeO₂ component, we observed the interesting morphology evolution of the samples from NTs to NBs. The diameters of them are ∼500, ∼225, ∼200, and ∼200 nm for sample Sn–Ce-NBs, sample Sn–Ce-NFs, sample Sn–Ce-NTs, and sample Sn–Ce-WTs, respectively. It can be observed that all the 1D structures are long, continuous, and randomly oriented. When the molar ratio of Sn and Ce is 1 : 1, NBs with uniform nanoparticles on their surface are observed. SnO₂/CeO₂ NFs, NTs, and WTs were prepared with the further addition of Ce(NO₃)₃ in the spinning solution, as shown in Fig. 2b, c, and d, respectively. As can be investigated from the funny morphology evolution, the hollow structures gradually decrease and the morphologies of the samples are more likely to form ribbon-like structure with the increase of Ce amount. Therefore, the morphologies of SnO₂/CeO₂ samples are greatly influenced by the amounts of CeO₂. These results are consistent with our previous report.¹⁶ However, we also found that the particles with size ranging from 30 to 70 nm were exposed on the surface of 1D structures. Compared with the CeO₂ 1D structures that we prepared before, it was found that the outer particles on the surface of composite structures are SnO₂ particles and the inner component is CeO₂. In other words, the SnO₂ component is more likely to move to the surface of the fibers during calcination. As can be seen from the samples, the SnO₂ particles always exist on the surface of the CeO₂ matrix to form various structures.

Fig. 2 SEM images of (a) sample Sn–Ce-NBs, (b) sample Sn–Ce-NFs, (c) sample Sn–Ce-NTs, and (d) sample Sn–Ce-WTs. The insets in (a) and (b) show the images with high magnification.

XRD measurement was purposely carried out to provide further insight into the crystallinity of the samples. All samples are well-crystallized and impurity peaks are not observed in Fig. 3, indicating the high purity of the products. As can be seen, the prominent peaks correspond to (110), (101), (211) crystal lattice planes and all other smaller peaks coincide with the corresponding peaks of the tetragonal rutile structure of SnO₂ (JCPDS no. 41-1445). As the addition of Ce, the samples display some additional unconspicuous diffraction peaks which can be correspond to CeO₂ (JCPDS no. 65-2975), as marked by stars. As can be seen from the diffraction patterns of sample Sn–Ce-WTs, the CeO₂ peaks are weaker than those of SnO₂, in accordance with the presence of small quantities of CeO₂. When the amounts of Ce is increasing, the diffraction peaks which belong to CeO₂ become more and more apparent, indicating the formation of the pure cubic CeO₂ phase. For SnO₂/CeO₂ samples, there are no obvious peak shifts or any trace of other phases besides rutile SnO₂ and cubic CeO₂, indicating the well-crystallized composite structures.

Fig. 3 XRD patterns of SnO₂ and SnO₂/CeO₂ composite samples.

In order to determine the optical band gap and associated properties, the optical absorbance measurements were carried out at room temperature using an ultraviolet-visible diffused reflectance spectrometer. Furthermore, the band gap energies of the samples are investigated from a plot of (αhν)² vs. energy (hν), as shown in Fig. 4. For a crystalline semiconductor, the optical absorption near the band edge follows eqn (2)^17,18

αhν = A(hν − E_g)^n/2 (2)

where α, ν, A, and E_g are absorption coefficient, light frequency, a constant, and band gap, respectively. According to the above equation, the value of n is 1. As is shown in Table 1, the band gap of Sn-NTs-1 and Sn-NTs-2 located at 3.73 and 3.76 eV, respectively. Due to large surface-to-volume ratio provided by tubular structure and porous nature, the observed values are higher than that of bulk SnO₂ (3.62 eV).^19,20 In detail, as the amount of Ce increased, the band gaps of SnO₂/CeO₂ samples are 3.53, 3.30, 3.07, and 2.91 eV, respectively. As can be seen from the inset of Fig. 4, the band gaps of the mixed oxide gradually shifted to the low energy side with an increase of CeO₂ amounts. This phenomenon can be attributed to the interfacial effects between SnO₂ and CeO₂.²¹

Fig. 4 Plots of (αhν)² vs. energy (hν) for the band gap energies. Inset is the corresponding band gaps of each sample.

Based on the above, in order to further confirm the effect of the SnO₂ structure and CeO₂ distribution, sample Sn–Ce-NBs and Sn–Ce-NTs are proposed and the details are shown in Fig. 5. The inset of each image shows the TEM image, indicating the typical morphologies of NBs and NTs, which is in agreement with the SEM images. It could be easily confirmed that both of SnO₂/CeO₂ NBs and NTs were highly crystalline in the HRTEM images. The interplanar spacing of the outer particles of the samples was measured to be 0.237 and 0.334 nm, which correspond to the d spacing of the (200) and (110) planes of the rutile structure of SnO₂, respectively. In addition, the interplanar spacing of the inner particles of the samples was measured to be 0.270 and 0.312 nm, which correspond to the d spacing of the (200) and (111) planes of the cubic structure of CeO₂. It is also easy to find that the SnO₂ and CeO₂ grains are connected to each other by grain boundaries, suggesting the good crystallinity of the samples. As is depicted, the obvious distribution of outer SnO₂ grains and inner CeO₂ grains is consistent with our previous idea in the analysis of SEM images, indicating the significant effect of the Ce amounts on the morphology. Therefore, the SnO₂ particles always exist on the surface of the CeO₂ matrix to form various structures. These special structures are similar to core–shell structures, which may contribute significantly to the electrical conduction and gas-sensing properties of the material.

Fig. 5 HRTEM images of (a) sample Sn–Ce-NBs and (b) sample Sn–Ce-NTs. Inset shows the corresponding TEM images of each sample.

On the basis of the experimental observations, a possible formation mechanism of SnO₂ nanostructures and SnO₂/CeO₂ composite nanostructures is proposed and the overall morphology evolution of the nanostructures is displayed in Scheme 1. First of all, due to the high voltage applied on the precursor solution, PVP molecular chains were positively charged during the electrospinning process. Under appropriate conditions, the liquid jets of polymer solution emitted and the as-spun fibers were collected on the aluminum foil. Then various nanostructures were obtained after calcination process. For SnO₂ nanostructures, we successfully prepared SnO₂ solid fibers with a heating rate of 1 °C min⁻¹. PVP were slowly decomposed at this slow heating rate and the gases were released slowly during the calcination process, resulting in the formation of solid structure. As increasing the heating rate, the decomposition of PVP became faster. In addition, the release rate of gases inside the fiber is larger than the diffusion rate of gases through the fiber surface, leading to the increase of the pressure inside of the fiber. Then hollow structures were formed.²² With increasing the amount of SnCl₂ in spinning precursor solution, the results suggested that the products were remained with tubular structure with thicker walls. Therefore, for pure SnO₂, fast heating rates are required to facilitate hollow fiber formation.

Scheme 1 Proposed mechanism of the formation of SnO₂ nanostructures and SnO₂/CeO₂ composite nanostructures prepared via single-spinneret electrospinning.

Various SnO₂/CeO₂ nanostructures were obtained via this single-spinneret electrospinning method. As increasing the amount of Ce, we found an interesting morphology evolution from NTs to NBs. For sample Sn–Ce-WTs and Sn–Ce NTs, we observed wire-in-tube and tubular structure with a high heating rate due to the addition of effect of gas expansion to the Kirkendall effect.²³ Then, SnO₂/CeO₂ NFs and NBs were prepared via adjusting the amounts of Ce during electrospinning. Hence, the amounts of Ce played a more important role in adjusting morphology of SnO₂/CeO₂ composite structures. In addition, SnO₂ particles were more likely to form on the surface of nanostructures regardless of the morphologies, as can be previously proved by SEM and TEM images. As is known in calcination process, the mechanism is Kirkendall effect, where the diffusion of precursor toward the surface and away from the center of the fibers due to the concentration gradient. This results in the closer formation of particles to the fiber surface. After decomposition of PVP, larger particles remain self-supported on the exterior, forming the shell of the fiber, whereas particles below 30 nm in size collapse to the center of the fiber.²⁴ During this diffusion process, the SnO₂ particles remained on the exterior of the fiber due to their larger size than that of CeO₂ particles. Because of this possible mechanism, these special structures prepared via a single-spinneret electrospinning method are similar to core–shell structures, which may be used as a model to fabricate more other metal oxides with various morphologies.

In Fig. 6, to further investigate the chemical state and surface composition of the elements, XPS spectra of sample Sn–Ce-NTs were studied. The survey spectra of the sample confirm the presence of Sn, O, Ce, and C, as shown in Fig. 6a. In Fig. 6b, due to the asymmetry of the O 1s profile of sample Sn–Ce-NTs, the peak could be dissolved into three symmetrical peaks. The three peaks are locating at 528.8, 530.1, and 531.5 eV, respectively, suggesting three different kinds of O species in the sample. The peak at 531.5 eV corresponds to chemisorbed oxygen or hydroxyl ions such as OH⁻, O⁻, and O²⁻ at the surface of SnO₂.^25–27 The peaks at 528.5 and 530.1 eV are assigned to core levels of oxygen coordinated to Sn²⁺ and Sn⁴⁺ in the O 1s, respectively. As previous work reported, the O 1s core level of Ce₂O₃ showed a higher binding energy than that of CeO₂.²⁸ It is difficult to clearly distinguish the O 1s peaks of Sn²⁺, Sn⁴⁺, Ce³⁺, and Ce⁴⁺ due to close peak position. However, after introducing Ce to the sample, the O 1s peaks are gradually broadened at low energy, indicating the decrease of the binding energy of the oxygen and the formation of surface oxygen vacancies.

Fig. 6 (a) Survey, (b) O 1s, (c) Sn 3d, and (d) Ce 3d high resolution XPS spectra of sample Sn–Ce-NTs.

Fig. 6c shows characteristic spin–orbit for Sn 3d_5/2 and Sn 3d_3/2 of sample Sn–Ce-NTs. The Sn 3d_5/2 can be resolved into two peaks, locating at 486.2 and 486.9 eV, respectively. The former peak is assigned to Sn²⁺, coming from SnO₂, while the latter peak corresponds to Sn⁴⁺, resulting from SnO₂. Due to the addition of Ce, the formation of Sn²⁺ may increase the surface deficiency of SnO₂ crystallites, resulting in active sites for reactions. The Ce 3d XPS spectrum of sample Sn–Ce-NTs is shown in Fig. 6d. It seems more complex due to the multiplicity of final states reached during the Ce 3d photoionization process.^29,30 Based on the previous reported work, the Ce 3d level can be dissolved into two series of peaks.^31,32 The peaks labeled u₀, u, u₁, u₂, and u₃ are the result of the 3d_3/2 spin–orbit states, and those labeled v₀, v, v₁, v₂, and v₃ are assigned to the corresponding 3d_5/2 states. Among these peaks, six typical peaks marked by u, u₂, u₃, v, v₂, and v₃ are the characteristic peaks of Ce⁴⁺, and the rest of them labeled as u₀, u₁, v₀, and v₁ are in agreement with the previous report about Ce³⁺. In other words, sample Sn–Ce-NTs has a mixture of Ce³⁺/Ce⁴⁺ oxidation states, indicating that the large amount of oxygen vacancies exist on the surface of the sample.

It is significant to investigate the influence of different morphologies, special structure, and large amount of oxygen vacancies on their application performance. The response of SnO₂/CeO₂ nanostructures gas sensors to 100 ppm ethanol as a function of operating temperature is shown in Fig. 7a. As is known to all, the operating temperature has a significant effect on the response of a semiconductor base gas sensor. All the sensors have the same changing trend that the responses first increase with temperature, up to 320 °C, and then gradually decrease. It can be seen that the optimum working temperature for ethanol gas detection is about 320 °C and sample Sn–Ce-NTs gas sensor has the largest response (∼49.1) at this temperature, which is four times higher than that of pure SnO₂ NTs gas sensor. Also, the result is better than the response (∼31) of SnO₂/CeO₂ sensor in previous report.³³ This is ascribed to the addition of Ce component to the composite, providing more absorption site and vacancies in the gas sensing test. It is noted that the responses of all SnO₂/CeO₂ composite nanostructures gas sensors to ethanol have remarkable improvement except sample Sn–Ce-NBs. The reasonable explanation is that the ribbon-like structure cannot provide large amount of active sites like hollow structure. In addition, the introducing of excessive Ce has some negative effect on the response to ethanol, resulting in the decrease of response of sample Sn–Ce-NBs.

Fig. 7 (a) Relationships of the operating temperature versus the response of the samples based sensor toward 100 ppm ethanol, respectively; (b) response and recovery characteristic curves of SnO₂ nanostructures based sensor to 50–1000 ppm ethanol, respectively; (c) the cross-response of the sensors to 100 ppm ethanol, ammonia, benzene, toluene, chloroform, and hexane. (d) Response and recovery characteristic curves of SnO₂/CeO₂ composite nanostructures based sensor to 50–1000 ppm ethanol, respectively.

To further investigate the gas sensing behavior, the dynamic response of pure SnO₂ nanostructures and SnO₂/CeO₂ composite nanostructures are observed. As shown in Fig. 7b, the response of sample Sn-NTs-1 is higher than that of sample Sn-NTs-2, indicating the effect of wall thickness of NTs on gas sensing behavior. Fig. 7d shows that sample Sn–Ce-NTs exhibits the excellent response to ethanol among these samples because of the hollow structure and oxygen vacancies, which are consistent with our previous analysis. Based on the dynamics, the response and recovery time of different samples were obtained and compared. As shown in Table 2, the response of sample Sn–Ce-WTs to 100 ppm ethanol is about 33.8, which is similar to that of sample Sn–Ce-NFs (35.4). This phenomenon is caused by the solid structure, resulting in the less active sites and the decrease of contact area for target gas.

Table 2 Gas sensing and catalytic properties of the samples

Sample	Response^a	Response time^a (s)	Recovery time^a (s)	T10 (°C)	T100 (°C)
a The values are obtained at 100 ppm.
Sn-NTs-1	14.9	14	16	N/A	N/A
Sn-NTs-2	12.1	13	12	250	422
Sn–Ce-WTs	33.8	8	6	222	331
Sn–Ce-NTs	49.1	11	10	203	317
Sn–Ce-NFs	35.4	8	8	238	387
Sn–Ce-NBs	10.6	12	17	222	361

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The sensing mechanism for SnO₂ can be mainly attributed to the adsorption and desorption of the target gas molecules on the surface of the samples. This process would cause the change in resistance in air and in target gas. It is worth mentioning that the adsorption mechanism was greatly influenced by the oxygen vacancies on the surface. Compared with pure SnO₂ samples, SnO₂/CeO₂ composite samples exhibited enhanced ethanol sensitivity, due to the additional redox couple of Ce³⁺ and Ce⁴⁺ as proved by XPS. However, because of the relatively poor conductivity, the excessive addition of Ce could make more CeO₂ nanoparticles formed in composite nanostructures which hinder the electron transfer in gas sensing process. Thus, the relatively high gas sensing property of sample Sn–Ce-NTs could be attributed to the appropriate introduction of Ce and the hollow structure.

CO catalytic oxidation was carried out to investigate the catalytic performance of pure SnO₂ NTs and SnO₂/CeO₂ composite nanostructures, as shown in Fig. 8. The details are shown in Table 2. Due to the continuous oxidation of CO, the CO conversion ratios were gradually increased with temperature. The CO catalytic properties of all the SnO₂/CeO₂ composite nanostructures are better than that of pure SnO₂ NTs, indicating that the introducing of Ce has positive effect on the catalytic behavior. Especially, the onset oxidation temperature (T₁₀) for Sn–Ce-NTs is 203 °C, while the complete CO oxidation temperature (T₁₀₀) is 317 °C. Both of the two temperatures are remarkably below the rest samples, indicating the excellent catalytic performance for CO oxidation. It is well known, both of SnO₂ and CeO₂ can promote the CO oxidation reaction due to their large number of oxygen vacancies form the existence of Sn²⁺/Sn⁴⁺ and Ce³⁺/Ce⁴⁺. In addition, the structure of the sample also played an important role in catalysis. The CO oxidation properties of sample Sn–Ce-NTs and Sn–Ce-WTs are better than that of sample Sn–Ce-NFs with a solid structure, suggesting that the hollow structure may provide more active sites and contact area during catalysis. However, sample Sn–Ce-NBs exhibited low completed CO oxidation temperature compared with sample Sn–Ce-WTs. This is ascribed to the different amounts of Ce in composite samples. Therefore, the introduction of Ce to the samples could significantly enhance the catalytic performance for CO oxidation. Furthermore, it should be mentioned that both the ratios of Ce and the structure of samples could influence the CO oxidation properties of SnO₂/CeO₂ composites.

Fig. 8 CO oxidation of samples Sn-NTs-2, Sn–Ce-NBs, Sn–Ce-NFs, Sn–Ce-NTs, and Sn–Ce-WTs.

Conclusions

1D SnO₂ and SnO₂/CeO₂ fibers with diverse morphologies were fabricated via a single-spinneret electrospinning by adjusting the heating rate and the amounts of Ce. Their applications in gas sensing and CO oxidation were investigated. It can be concluded that the nanostructures were uniformly composed of nanoparticles and as increasing the amounts of Ce, the morphology evolution from NTs to NBs was observed. The band gap of the composite oxides decreases with the addition of CeO₂. The XPS analysis showed the existence of Sn²⁺ and Ce³⁺, indicating the improvement of surface oxygen vacancies. Sample Sn–Ce-NTs had the high response to ethanol and excellent catalytic performance for CO oxidation, which can be attributed to the hollow structure, good crystallinity, and the interactions between SnO₂ and CeO₂. The present work suggests that many other functional metal oxides with diverse morphologies could be prepared via this single-spinneret electrospinning and their potential applications can benefit from their special structures.

Acknowledgements

This work was supported in part by the project from National Basic Research Program of China (973 Program, 2013CB632401), the program for Taishan Scholars, the projects from National Natural Science Foundation of China (51202090, 51302106, 51402123, and 51402124).

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