Enhanced methanol sensing properties of SnO₂ microspheres in a composite with Pt nanoparticles

Abstract

SnO₂ microspheres in a composite with Pt nanoparticles (0, 0.5, 1.5, 2.5, 5.0 mol% Pt loading) were synthesized by a solvothermal method. The crystal structure, morphology, and specific surface area were thoroughly characterized. It is found that the Pt–SnO₂ nanocomposites consist of a large amount of small spheres with average diameters up to hundreds of nanometers, and every small sphere is composed of numerous primary nanocrystallites with an average size of about 8 nm. Compared with the pristine SnO₂, the presence of Pt nanoparticles has no influence on the growth behavior of the SnO₂ microspheres. The gas sensors based SnO₂ microspheres in a composite with Pt nanoparticles not only show a lower operating temperature and immensely enhanced responses, but also exhibit a faster response and recovery speeds and remarkable stability to methanol, especially the 5.0 mol% Pt–SnO₂ nanocomposite. The gas sensor based on the 5.0 mol% Pt–SnO₂ nanocomposite exhibits a response value of 190.88 to 100 ppm methanol at a low operating temperature of 80 °C, while the gas sensor based on pristine SnO₂ only displayed a response value of 19.38 at an operating temperature of 200 °C. The reasonable explanation of the gas-sensing performance enhancement for the gas sensors based on Pt–SnO₂ nanocomposites is attributed to the strong spillover effect of the Pt nanoparticles and the electronic interaction between Pt nanoparticles and SnO₂ microspheres, both of which promoted the low temperature gas-sensing performance.

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1. Introduction

Methanol (CH₃OH) is an important raw material with wide spread applications in automotives and manufacturing, such as dyes, drugs, and so on.¹ Meanwhile, methanol is the most abundant non-methane volatile organic pollutant (VOC) in the atmosphere, which is the important precursor of CO, HCHO, and O₃.² It is highly toxic and often fatal to human beings. Especially, the methanol can affect body's blood system and nervous system, and cause headaches, metabolic acidosis, blindness and even death.³ Therefore, it is very important to detect methanol. On the other hand, methanol is a less studied organic compound compared to ethanol, formaldehyde, toluene, xylene, and so on. So, the development of a fast, convenient, reliable and steady methanol sensor has become imperative. To the best of our knowledge, few reports have focused on the research of methanol gas-sensing performance compared to other volatile organic pollutants. Thus it is necessary to develop a convenient and reliable detecting technique.

Compared with other detecting techniques (such as high-performance liquid chromatography,⁴ enzyme electrodes,⁵ etc.), the gas sensors have many superiorities, including low cost, high-efficiency, and suitable for real-time monitoring. Nowadays, among different types of sensors, metal oxide semiconductor gas sensors are the most popular one. Gas sensors based on metal oxide semiconductor, such as SnO₂,⁶ ZnO,⁷ TiO₂,⁸ In₂O₃,⁹ and Fe₂O₃ ¹⁰ have been attracting the strong interests due to their simple preparation methods, low power consumption, and wide detection range. Among them, tin oxide (SnO₂), as a typical n-type semiconducting metal oxide, is the greatest potential used gas-sensing material due to its good chemical stability. Many recent efforts have been made to synthesize diverse nanostructures of SnO₂ and utilize them as gas sensors with excellent sensing performance to detect ethanol,¹¹ formaldehyde,¹² isopropanol,¹³ acetone,¹⁴ etc. In addition, the gas-sensing performances of SnO₂ based gas sensors have greatly improved by doping or compositing of noble metals, such as Pt,¹⁵ Pd,¹⁶ Ag,¹⁷ Au,¹⁸ etc. Especially, noble metals are widely used to improve the sensing performance of oxide semiconductor for gas detection, which is well established in chemical sensor research. However, to date, the noble metals doped or composited SnO₂ materials have no been used for detecting methanol. Besides, to the best of our knowledge, only a little effort has been made for detecting methanol. Wang's research group synthesized inverse opal SnO₂ with sol–gel method. The response of sensor was 95 for 500 ppm methanol at the operating temperature of 312 °C.¹⁹ Ding et al. also successfully prepared Zn-doped SnO₂ nanorods clusters. The gas-sensing properties of the obtained Zn-doped SnO₂ samples were used to test for methanol. And the response was 59 for 50 ppm methanol at the operating temperature of 270 °C.²⁰ Tang et al. prepared hollow hierarchical SnO₂–ZnO composite nanofibers based on electrospinning method for detecting methanol, and the response was 8.5 for 10 ppm methanol at an operating temperature of 350 °C.²¹ However, the higher operating temperatures of these materials need to consume more energy. Moreover, the responses of these materials are too poor to meet requirements of the practical application to detect for low methanol gas concentration. Therefore, the sensors with excellent gas-sensing performance to methanol have aroused tremendous interest due to their highlighted roles in the areas of public safety and environmental monitoring.

In this article, we report a facile solvothermal method to synthesize SnO₂ microspheres composited by Pt nanoparticles with Pt contents of 0, 0.5, 1.5, 2.5, 5.0 mol%, respectively. The globular structure of Pt–SnO₂ nanocomposite was formed using N,N-dimethylformamide (DMF) as a solvent. Firstly, the crystal structure, morphology, chemistry, and specific surface area of as-synthesized samples have been thoroughly investigated. To demonstrate samples the potential applications, the gas sensors based on as-synthesized samples were fabricated and comparably investigated their gas-sensing properties using methanol as a target gas. The experimental results indicate that the gas sensors based on the Pt–SnO₂ nanocomposites, especially 5.0 mol% Pt–SnO₂ nanocomposite, exhibit an obvious enhancement in response and lower operating temperature than that of pristine SnO₂. It is believed that the 5.0 mol% Pt–SnO₂ nanocomposite could further provide potential for application as a highly effective methanol gas sensor.

2. Experimental

2.1 Materials

All chemical reagents including tin(IV) chloride pentahydrate (SnCl₄·5H₂O, ≥99.0%), (hydro)chloroplatinic acid (H₂PtCl₆·6H₂O, ≥99.9%), N,N-dimethylformamide (DMF, ≥99.5%), sodium hydroxide (NaOH, ≥96%) and methanol solution (CH₃OH, ≥99.5%) were purchased from commercial sources. The analytical grade reagents were used without any further purification.

2.2 Preparation of SnO₂ microspheres composited with Pt nanoparticles

Pristine SnO₂ and SnO₂ microspheres composited with Pt nanoparticles were synthesized by a solvothermal method, respectively. In a typical synthesis procedure, 4.00 mmol SnCl₄·5H₂O was first dissolved in 10 mL distilled water. Afterwards, 12 mL DMF was completely dissolved into the above solution. Then 20 mL of 2.00 M NaOH solution was dropwise added into the above solution under vigorous magnetic stirring for 30 min. Subsequently, more distilled water was added into the above solution to make the whole volume of the solution reach 60 mL (water : DMF = 4 : 1, v/v). This resultant homogenous solution was transferred into a 90 mL Teflon-lined stainless steel autoclave, sealed and maintained at 160 °C for 15 h at an oven. After all the solvothermal reaction processes, the heated autoclave was transformed from the oven and cooled down to room temperature, and the products were collected by repeated centrifugation and washed with ethanol and distilled water, respectively. The obtained samples were dried at 60 °C for 24 h and further annealed at 400 °C in air for 1 h. In the case of SnO₂ microspheres composited with Pt nanoparticles, the solution preparation and experimental procedure were similar except that a certain amount of H₂PtCl₆·6H₂O (0.5, 1.5, 2.5, 5.0 mol% Pt, respectively) was added after SnCl₄·5H₂O. Finally, the as-prepared powders were collected for gas sensor fabrication.

2.3 Characterization of as-prepared powders

The crystal structure and the phase were identified by X-ray powder diffraction (XRD, Rigaku TTRIII) with Cu K_α radiation (λ = 1.54056 Å) operated at 40 kV – 200 mA. Data were collected in steps of 0.02° (2θ) from 10° to 90°. The scanning electron microscopy (SEM) images were performed on a FEI QUANTA 200 with microscope operating at 30 kV. Before SEM imaging, the samples were sputtered with thin layers of gold. The transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were examined on a JEOL JEM-2100 with an acceleration voltage of 200 kV. The energy-dispersive X-ray spectrometer (EDX) was employed to research the chemical composition of the samples. The samples were uniformly dispersed in ethanol and dropped on copper grids for measurement. X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemistries of the samples in the ESCALAB system with Al K_α X-ray radiation at 15 kV. The nitrogen adsorption isotherm was measured at 77.3 K with a Micromeritics ASAP 2010 automated sorption analyzer. Pore size distribution was estimated using the Barrett–Joyner–Halenda (BJH) method from the desorption isotherm. Prior to the measurement, the samples were degassed at 300 °C for 3 h under vacuum.

2.4 Fabrication and measurement of gas sensors

The gas sensors were fabricated with as-synthesized SnO₂ microspheres composited with Pt nanoparticles with different Pt contents (0, 0.5, 1.5, 2.5 and 5.0 mol%) as sensing materials. Firstly, the synthesized products were mixed with a certain amount of deionized water to form a homogeneous paste. Afterwards, the paste was coated onto the outside of an alumina tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter) by a paint pen. The alumina tube with a pair of gold electrodes and the corresponding platinum wires installed at each end.²² The coating should be uniform, and the thickness of the coating was about 0.7 mm. Subsequently, the tube with coated gas sensing materials was dried at 120 °C for 1 h and calcined at 400 °C for 1 h in air. After cooling down, the alumina ceramic tube was connected to a Bakelite base through platinum lead wires to conduct electrical measurement, a small Ni–Cr alloy coil (23 Ω) was inserted in the tube to control the operating temperature by a heating voltage. In order to enhance the stability and repeatability, the gas sensors were aged at 350 °C for 120 h in dry air. Finally, the gas sensing performances were measured on a WS-30A commercial gas sensing measurement system (Weisheng Instruments Co., Zhengzhou, China). The circuit voltage (V_c) was set at 5 V, and the output voltage (V_out) was set as the terminal voltage of the load resistor (R_L).²² The measurement was conducted by a static process: first, the sensors were placed into an 18 L chamber filled with into fresh air. Then the desired amounts of target solution were injected into the chamber by using a micro-syringe, and the solution was evaporated by an evaporator and mixed with air immediately by two fans installed in the chamber. When the resistance of the sensors reached a constant value, the upper cover of the chamber was removed and the sensors began to recover in fresh air. These procedures were repeated for the gas sensing tests under different conditions. Here, the response value (β) of the sensors was defined as β = R_a/R_g, where R_a and R_g are the resistances of the sensors in air and target gas, respectively. In addition, the response time (τ_res) and recovery time (τ_recov) were defined as the time taken by the sensor to achieve 90% of the final equilibrium resistance in the case of adsorption and desorption, respectively.

3. Results and discussion

To inspect the influence of SnO₂ microspheres composited with Pt nanoparticles, XRD analysis was implemented, and the results are shown in Fig. 1. The crystal structure information is obtained by comparing the measured diffraction patterns with standard cards of JCPDS: 04-0802 (Pt phase) and JCPDS: 41-1445 (SnO₂ phase). The main diffraction peaks of pristine SnO₂ and SnO₂ microspheres composited with Pt nanoparticles can match well the standard card of the cassiterite SnO₂ phase, and the phase does not change after compositing with Pt nanoparticles. At the same time, no Pt related peaks are found in 0.5 mol% and 1.5 mol% Pt–SnO₂ due to its small content. Comparing patterns in Fig. 1(f) and (g) with (c)–(e), for higher proportion (2.5 mol% and 5.0 mol%) three new peaks locate at 39.76°, 46.24° and 67.45°, and fit well with (111), (200) and (220) reflections of Pt, respectively. More specifically, the Pt relating diffraction peaks heighten with the increase of Pt contents from 2.5 mol% to 5.0 mol%, suggesting that crystalline Pt nanoparticles are independent of SnO₂. In addition, no other characteristic peaks were observed such as PtO or PtO₂ in the XRD patterns of SnO₂ microspheres composited with Pt nanoparticles, demonstrating the successful of SnO₂ microspheres composited with Pt nanoparticles via a solvothermal method. All of the diffraction peaks are broad, indicating the small grain sizes of as-synthesized samples. The average grain sizes are estimated to be about 8 nm regardless of the Pt loading contents using the Scherrer equation derived from the XRD results. However, it is hard to calculate the grain size of Pt use the same method due to the relatively weak diffraction peaks.

Fig. 1 (a) JCPDS: 41-1445 of the SnO₂ phase, (b) JCPDS: 04-0802 of the Pt phase, and XRD patterns of different contents of Pt–SnO₂: (c) 0 mol%, (d) 0.5 mol%, (e) 1.5 mol%, (f) 2.5 mol%, and (g) 5.0 mol%, respectively.

The morphology of 5.0 mol% Pt–SnO₂ was investigated by SEM, as shown in Fig. 2. The inset shows the SEM micrograph of pristine SnO₂, from which we can observe the nano-architectures of pristine SnO₂ are mostly spherical with bad-dispersed. And the pristine SnO₂ consists of a large amount of small spheres with an average diameter up to hundreds of nanometers. Compare the 5.0 mol% Pt–SnO₂ with the pristine SnO₂, there were no significant difference, suggesting that Pt is not a significant factor for SnO₂ growth in solvothermal synthesis process. In addition, the microspheres tend to be strongly agglomerated, and the Pt nanoparticles can not be found in all images.

Fig. 2 SEM images of 5.0 mol% Pt–SnO₂ at (a) low magnification (the inset shows the SEM micrograph of pristine SnO₂) and (b) high magnification, respectively.

Further detailed morphology and structure of 5.0 mol% Pt–SnO₂ were examined under TEM and HRTEM, and displayed in Fig. 3. The inset of Fig. 3(a) shows the TEM image of pristine SnO₂, it can be seen that the pristine SnO₂ consist of a large number of microspheres, and these microspheres are superimposed together. They are irregular in size, up to hundreds of nanometers in diameter, which is in good agreement with the above estimated size from SEM results. Fig. S1† shows the TEM images of 0.5 mol%, 1.5 mol%, and 2.5 mol%. As can be seen, there are no significant differences in morphology. All of them are composed of abundant microspheres. The TEM image of 5.0 mol% Pt–SnO₂ presents similar features for other Pt contents, as shown in Fig. 3(a). Look carefully, it can be observed that the microspheres are composed of numerous primary nanocrystallites with average sizes of about just a few nanometers. The HRTEM image of 5.0 mol% Pt–SnO₂ is shown in Fig. 3(b). The clear well-developed lattice fringes imply a high crystallinity and random orientation of 5.0 mol% Pt–SnO₂. The high crystallinity is beneficial for the long-term stability of the sensors.¹⁵ At the same time, it can be seen that the size of primary nanocrystallites about 8 nm, which matches well with the results from the XRD studies. With regard to Pt nanoparticles, they are difficult to find due to they disperse among SnO₂ microspheres. The HRTEM micrographs of the special parts marked by the red dashed line in Fig. 3(b) were shown in Fig. 3(c) and (d), respectively. Fig. 3(c) shows lattice fringes with an interplanar spacing of 0.335 nm, which corresponds to the (110) plane of SnO₂. Fig. 3(d) shows lattice fringes with an interplanar spacing of 0.227 nm, which is close to the value of the (111) lattice planes of Pt. At the same time, no signs of PtO or PtO₂ phases are observed, which effectively supports the above XRD measurements again. Composition analysis was examined using EDX as for 5.0 mol% Pt–SnO₂, as indicated in Fig. 3(e). Clearly, Sn, O, Pt, and Cu elements were detected. It is worthwhile to note that the Cu element is also found by the content of 28.10 at%. The Cu element cones from the copper grid were used as a support of the sample, suggesting the SnO₂ microspheres are surely composited with Pt nanoparticles. Meanwhile, the molar ratio of Pt and Sn is 1.3%, which is not consistent with the experimental value due to the Pt nanoparticles are pocketed among SnO₂ microspheres.

Fig. 3 (a) TEM (the inset shows the TEM image of pristine SnO₂), (b) magnified TEM images of 5.0 mol% Pt–SnO₂, (c–d) the corresponding HRTEM images with obvious SnO₂ and Pt lattice fringes, and (e) EDX pattern of 5.0 mol% Pt–SnO₂.

To further analyze and illuminate the composition and chemical state of every element, the surface states of 5.0 mol% Pt–SnO₂ were examined by XPS. The obtained spectra surveys were divided into Gaussian components with a Shirley background by the software XPSPEAK. Fig. 4 shows the XPS survey spectrum and the high-resolution XPS survey spectra of O 1s, Sn 3d, and Pt 4f in 5.0 mol% Pt–SnO₂, respectively. Fig. 4(a) shows XPS survey scan for 5.0 mol% Pt–SnO₂. Apart from the peak of C 1s at 285 eV, as expected the spectrum was observed by the lines of O, Sn and Pt, indicating a perfect purity of the as-synthesized 5.0 mol% Pt–SnO₂ nanocomposite. The O 1s spectra are shown in Fig. 4(b), which can be properly separated into two peaks. The main peak located at 530.95 eV is attributed to oxygen ions in the crystal lattice (O_lat), while the other one at 532.00 eV can be ascribed to the absorbed O_x⁻ (O_2ads⁻, O_ads⁻ or O_ads²⁻) ions.²³ The lattice oxygen could not be interacted with target gas, which is unable to impact the gas-sensing performance. However, the absorbed O_x⁻ could react with gas and then enhance the holes concentration.²⁴ Through calculating the ratio of peak areas, the concentration of absorbed O_x⁻ is estimated to be 38.7% for 5.0 mol% Pt–SnO₂ nanocomposite, while the absorbed O_x⁻ of pristine SnO₂ is calculated to be 29.2% (Fig. S2(a)†). This observation suggests that the Pt nanoparticles supported on SnO₂ microspheres could promote the adsorption and dissociation of oxygen species, which is very beneficial for oxygen spillover. In Fig. 4(c), Sn 3d spectra of 5.0 mol% Pt–SnO₂ are divided into two peaks of Sn 3d_3/2 and Sn 3d_5/2 at 495.53 eV and 487.10 eV, respectively. The binding energy difference of these peaks is 8.43 eV, which is in good agreement with reported values.¹⁵ In Fig. S2(b),† the binding energy peaks of Sn (pristine SnO₂) shows slightly lower values of 495.10 eV and 486.65 eV correspond to Sn 3d_3/2 and Sn 3d_5/2 respectively. The occurrence of change values indicates the possible interaction between Pt nanoparticles and SnO₂ microspheres. Most importantly, many works have claimed that the strong interaction between noble metal and metal oxide plays a crucial role in enhancing the material properties.

Fig. 4 (a) XPS survey spectrum and the high-resolution XPS survey spectra of (b) O 1s, (c) Sn 3d, and (d) Pt 4f for 5.0 mol% Pt–SnO₂.

Especially, as shown in Fig. 4(d), the state of Pt 4f contains spin-orbital of Pt 4f_5/2 and Pt 4f_7/2 at 74.46 eV and 71.11 eV, respectively, which is consistent with reported values and confirms the existence of Pt nanoparticles in 5.0 mol% Pt–SnO₂.²⁵ The result also supports the above XRD and TEM findings that no PtO and PtO₂ phases could be found. After Pt nanoparticles compositing, the interaction between Pt and SnO₂ surface would happen, resulting in the change of the work function of SnO₂, which benefits the gas-sensing performance.¹⁵

The specific surface area of pristine SnO₂ and 5.0 mol% Pt–SnO₂ are investigated by the N₂ adsorption–desorption isotherms and the corresponding BJH pore-size distribution curves, and as shown in Fig. 5. All two samples show obvious hysteresis loop, indicating the existence of pores. As observed in inset of Fig. 5, the pore size distribution shows that the broad peaks appeared in pore size region of 4–16 nm. The main peaks are positioned at 6.196 nm and 9.037 nm have been investigated for pristine SnO₂ and 5.0 mol% Pt–SnO₂, respectively, which suggested that both samples possessed mesoporous structure with the existence of mesopores (the clearances between particles) in the materials. Pores of various sizes were observed in the TEM image. The large pore diameter may be indicative of the presence of disordered domains resulting from the collapse of structures. Moreover, the specific surface area was calculated as high as 64.361 m² g⁻¹ for pristine SnO₂, while it decreases to 49.952 m² g⁻¹ after the SnO₂ microspheres composited with Pt nanoparticles (5.0 mol% Pt–SnO₂). The abrupt decrease also suggests the effect of Pt nanoparticles. And the peak shift of pore size confirms further the particle aggregation in SnO₂ microspheres composited with Pt nanoparticles.²⁶

Fig. 5 N₂ adsorption–desorption isotherms of (a) pristine SnO₂ and (b) 5.0 mol% Pt–SnO₂. The insets are the BJH pore-size distribution curves.

In order to explore the potential application of gas sensor for methanol, gas sensors based on the SnO₂ microspheres composited with Pt nanoparticles were fabricated and their gas sensing performances were investigated. As well known, the operating temperature has a great influence on the response for gas sensor.²²

Fig. 6 shows the temperature-dependence behavior and the optimal operating temperature of the gas sensors based on different Pt–SnO₂ nanocomposite to 100 ppm of methanol. As can be seen, as the temperature is increased, the responses initially increase and then decrease with further increase of the operating temperature. And the sensor based on pristine SnO₂ shows relatively low response at operating temperature in the range from 20 °C to 300 °C, with the maximum response of 19.38 at 200 °C. For gas sensors based on the 0.5 mol% and 1.5 mol% Pt–SnO₂, the optimal operating temperatures decreased to 160 °C and 100 °C, respectively. And the corresponding responses to 100 ppm of methanol are 45.79 and 47.65, respectively. In addition, the response to 100 ppm of methanol was greatly increased by increasing the Pt composited level with the highest responses of 121.83 and 190.88 for gas sensors based on 2.5 and 5.0 mol% Pt–SnO₂ at 80 °C, respectively. These findings including the higher response and low optimal operating temperature are deemed to be the typical characteristics of Pt activation in gas sensing properties, and are also verified the promoted influence of Pt functionalization.^25,27 Hence, the operating temperature of 80 °C was chosen as the optimal operating temperature to proceed with the subsequent detection due to the highest response at that temperature.

Fig. 6 Methanol gas response of as-synthesized SnO₂ microspheres composited with the different contents of Pt nanoparticles towards a gas concentration of 100 ppm at the different operating temperatures.

To further understand the effect of SnO₂ microspheres composited with Pt nanoparticles, the representative dynamic response–recovery characteristics toward methanol of gas sensor based on 5.0 mol% Pt–SnO₂ was measured. Fig. 7(a) shows the typical response and recovery characteristics of 5.0 mol% Pt–SnO₂ toward different methanol concentrations from 5 to 500 ppm at operating temperature of 80 °C. It can be observed that the gas responses increase with the increase of methanol concentrations.

Fig. 7 The gas sensing properties of the sensor based on 5.0 mol% Pt–SnO₂ at an operating temperature of 80 °C. (a) The response–recovery curves toward different methanol concentrations from 5 to 500 ppm and (b) the corresponding exponential relationship between gas response and gas concentration.

Evidently, the gas responses toward 5, 20, 50, 80, 100, 200, and 500 ppm of methanol are 24.85, 52.16, 84.86, 121.94, 190.88, 256.12, and 386.91, respectively. Importantly, the curves obviously shows that the gas sensor exhibit a low detection limit of 5 ppm to methanol. Therefore, the fabricated gas sensor based on SnO₂ microspheres composited with Pt nanoparticles is quite high response to methanol. And its gas response and operating temperature well satisfies the detection needs. The linear dependence of the gas response on the gas concentration was also investigated as shown in Fig. 7(b). It can be found that the sensor response increases nearly exponential relationship with the rising of methanol gas concentration. The responses in the range from 5 to 500 ppm were depicted as follows:

β = −418.26 × exp(−C/222.37) + 430.85 (1)

where β is the gas response and C is the concentration of the target gas. The correlative coefficient R² is 0.98185, indicating the relatively satisfied exponential relationship. Therefore, such a good exponential relationship between the gas response and gas concentration provides potential application in quantitative measurement of methanol concentration.

To verify the reliable reproducibility of the fabricated sensor, the sensor based on 5.0 mol% Pt–SnO₂ was exposed to 100 ppm methanol for six successive cycles at operating temperature of 80 °C, and results are shown in Fig. 8. As evident from the observed results, the gas sensor exhibits a stable and repeatable response with an average response of 190.88, and the gas responses with small fluctuations which are calculated to be 1.08%. In addition, the response and recovery time does not show a distinct difference. These results suggest that the sensor has an excellent reversibility and repeatability.

Fig. 8 The response of 5.0 mol% Pt–SnO₂ to 100 ppm methanol at operating temperature of 80 °C under six cycles.

Response time (τ_res) and recovery time (τ_recov) are also significant parameters of sensor. The real-time detection usually needs fast response. Fig. 9 shows the response–recovery curve of sensor based on 5.0 mol% Pt–SnO₂ to 100 ppm methanol at operating temperature of 80 °C. The sensor can quickly respond to 100 ppm methanol with response time of 10 s, and also exhibits shorter recovery time with of 10 s. It is worth noting that the response and recovery times are much shorter than that of other reports.^21,28 These results indicate that the sensor has superior sensing properties. The fast response–recovery time may be attributed to the effects of the Pt nanoparticles. The catalytic activity of Pt nanoparticles accelerates the dissociation of oxygen molecules, and further facilitates the methanol gas adsorption and desorption.²⁹

Fig. 9 The response time and recovery time of sensor based on 5.0 mol% Pt–SnO₂ to 100 ppm methanol at operating temperature of 80 °C.

For an efficient gas sensor, the selectivity is also a very important factor. In order to further verify the sensing properties of gas sensor based on 5.0 mol% Pt–SnO₂, a selectivity test was carried out at the optimal operating temperature of 80 °C. Fig. 10 shows the response of gas sensor based on 5.0 mol% Pt–SnO₂ to 100 ppm of various gases (VOCs), including ethanol, isopropanol, methanol, formaldehyde, acetone, xylene, n-butanol, toluene, and ammonia, respectively. It can be clearly observed that the gas sensor well responses to methanol compared with the other gases. Detailedly, the response toward methanol is probably 2.90, 7.02, 3.87, 7.13, 14.91, 2.04, 23.03, and 27.08 times higher than that toward ethanol, isopropanol, formaldehyde, acetone, xylene, n-butanol, toluene, and ammonia, respectively, which confirms quite excellent selectivity.

Fig. 10 The selectivity of gas sensor based on 5.0 mol% Pt–SnO₂ to various gases (100 ppm), including ethanol, isopropanol, methanol, formaldehyde, acetone, xylene, n-butanol, toluene, and ammonia at operating temperature of 80 °C, respectively.

For almost all practical applications, the reliability and service life of a gas sensor is very critical. To verify the long-term stability of gas sensor based on 5.0 mol% Pt–SnO₂, the gas response evolutions toward 100 ppm of methanol in about one month were tested, and as shown in Fig. 11. It can be seen that the sensor exhibits a nearly constant response signal to 100 ppm methanol. The average response and standard deviation were estimated to be 191.01 and 0.81%, respectively. There is no doubt that the gas sensor based on 5.0 mol% Pt–SnO₂ has a high stability which may be reused continuously.

Fig. 11 Gas response of gas sensor based on 5.0 mol% Pt–SnO₂ to 100 ppm methanol gas tested once a day for up 30 days at operating temperature of 80 °C.

A gas sensing performance comparison between the SnO₂ microspheres composited with Pt nanoparticles (5.0 mol% Pt–SnO₂) based sensor and literature reports is summarized in Table 1. In view of the excellent methanol gas sensing performances, the SnO₂ microspheres composited with Pt nanoparticles present more obvious advantages compared with previous reports. As has been shown, there are various materials were used to detect methanol, including SnO₂,¹⁹ In₂O₃,³⁰ WO₃,³¹ Fe₂O₃,³² ZnO,³³ and so on. Remarkably, the gas sensor based on 5.0 mol% Pt–SnO₂ has higher response and lower operating temperature than previously reported methanol sensors, such as SnO₂–ZnO nanofibers,²¹ ZnO/SnO₂ nanostructures,³⁴ Zn-doped SnO₂ nanorods clusters,²⁰ Ce-doped In₂O₃ porous nanospheres,³⁵ α-Fe₂O₃ polyhedral crystals,³⁶ Pt-functionalized NiO,²⁴ Pt clusters supported on WO₃,²⁷ Pt-decorated SnO₂ nanocomposites,³⁷ Pt decorated faceted octahedral SnO₂ nanocrystals,³⁸ and so on. Therefore, the gas sensor based on 5.0 mol% Pt–SnO₂ is a promising candidate for the practical detection of methanol gas.

Table 1 Comparison of sensing performances of various metal oxide based sensors toward methanol^a

Materials	C (ppm)	OT (°C)	Response	Ref.
a OT: operating temperature, C: methanol concentration.
SnO2 inverse opal	500	312	95	19
SnO2–ZnO nanofibers	10	350	8.5	21
ZnO/SnO2 nanostructures	100	300	9.6	34
Zn-doped SnO2 nanorods clusters	50	270	59	20
Porous In2O3 nanobelts	20	370	9.5	30
WO3 particles	100	260	24	31
α-Fe2O3 discoid crystals	100	250	6.4	32
Ce-doped In2O3 porous nanospheres	100	320	35.2	35
α-Fe2O3 polyhedral crystals	50	340	2.5	36
Au-decorated ZnO	50	300	7	33
Pt-functionalized NiO	200	500	3.7	24
Pt clusters supported on WO3	140	5	4.7	27
Pt-decorated SnO2 nanocomposites	100	50	2.7	37
Pt decorated faceted octahedral SnO2 nanocrystals	350	200	24	38
SnO2 microspheres composited with Pt nanoparticles (5.0 mol% Pt–SnO2)	5	80	24.85	This work
	100		190.88

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The detection mechanism of methanol for the as-fabricated sensors are based on the change in the electrical resistance, which can be described by Wolkentein's model,³⁹ and as presented in Fig. 12. When the sensors are exposed to air (Fig. 12(a)), oxygen species from air ambient will be absorbed on the surface of semiconducting metal oxide, and then be ionized into chemisorbed oxygen ions O_x⁻ (O_2ads⁻, O_ads⁻ or O_ads²⁻) via attracting electrons from the metal oxide, so a depletion layer would be generated on the surface of the metal oxide and results in an increased resistance, as the following reactions happened:^13,40

O₂(gas) → O₂(ads) (2)

O₂(ads) + e⁻ → O₂(ads)⁻ (3)

O₂(ads)⁻ + e⁻ → 2O(ads)⁻ (4)

O(ads)⁻ + e⁻ → O(ads)²⁻ (5)

Fig. 12 Schematic diagram of the proposed reaction mechanism of the SnO₂ microspheres composited with Pt nanoparticles based sensor in (a) air and (b) methanol.

When the sensors are exposed to methanol gas, methanol reacts with the chemisorbed oxygen ions on the metal oxide surface, resulting in the liberation of the electrons back to the conduction band, then causing an increasing of carrier concentration and a consequent decreasing of the resistance of the sensors, as the following reactions happened:⁴¹

CH₃OH(gas) → CH₃OH(ads) (6)

CH₃OH(ads) + 2O(ads)²⁻ → CO₂ + H₂O(g) + 4e⁻ (7)

On the basis of above theories, the possible contribution of the interaction between SnO₂ microspheres and Pt nanoparticles can be discussed in two sections to explain the enhanced gas-sensing performance. The first one is related with the chemical effect of catalytic noble metal, which is known as the “spillover effect”,^15,37,38 which is illustrated in Fig. 13. For the pristine SnO₂ microspheres (Fig. 13(a)), less amounts of the oxygen molecules adsorbed on the surface of SnO₂ microspheres will attract electrons slowly from the pristine SnO₂ to form the narrow electron depletion layer and less chemisorbed oxygen ions. While for the Pt–SnO₂ nanocomposite (Fig. 13(b)), a large number of the oxygen molecules will adsorb on the surface of Pt nanoparticles at the same time. Afterwards, the spillover effect of Pt nanoparticles will catalyze the dissociation of oxygen molecules and attract electrons quickly from the Pt–SnO₂ nanocomposite. The process increases the quantity of chemisorbed oxygen ions and creates additional active sites with wider electron depletion layer than that the pristine SnO₂. In addition, the methanol molecules can be efficiently adsorbed on the surface of Pt nanoparticles and easily transported to the surface of adjacent SnO₂, thus the reactions will take place easily. Finally, the spillover effect of Pt nanoparticles will likely dissociate methanol molecules into CO₂ and H₂O. So the gas-sensing performance of Pt–SnO₂ nanocomposite based sensor can be greatly enhanced. The second is related with the electronic interaction between Pt nanoparticles and SnO₂ microspheres.⁴² Due to the different work functions of SnO₂ and Pt nanoparticles, the electrons are extracted from the SnO₂ to the Pt nanoparticles. These regions near the interface of SnO₂ and Pt nanoparticles are very active in sensing detect, and the sensor will become more sensitive. Therefore, the 5.0 mol% Pt–SnO₂ nanocomposite can display the excellent sensing performances, such as lower operating temperature, higher response, faster response and recovery characteristic, reliable reproducibility, good selectivity and excellent stability.

Fig. 13 Schematic diagram of the spillover effect in the sensing reactions of the (a) SnO₂ microspheres and (b) SnO₂ microspheres composited with Pt nanoparticles for detecting methanol.

4. Conclusions

In conclusion, the pristine SnO₂ and SnO₂ microspheres composited with Pt nanoparticles were synthesized via a simple solvothermal method. The obtained Pt–SnO₂ nanocomposite is comprised of a large number of small spheres, and every small sphere consists of numerous primary nanocrystallites. Compared with the pristine SnO₂, the gas sensors based on Pt–SnO₂ nanocomposite show considerably lower operating temperature and enhanced gas-sensing performance to methanol with increasing Pt contents. The gas response of the 5.0 mol% Pt–SnO₂ nanocomposite based gas sensor to 100 ppm methanol was 190.88, almost 10 times higher than that of the pristine SnO₂ microspheres. The enhancement of gas-sensing performance is attributed to the catalytic oxidation of Pt nanoparticles and the electronic sensitization between Pt nanoparticles and SnO₂. We believe that these results will not only put forward a simple route to synthesis SnO₂ microspheres, but also offer Pt–SnO₂ nanocomposite materials for gas sensors for methanol detection with excellent gas-sensing performance, making them to be promising candidates for practical detectors for methanol.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 51262029) and Program for Excellent Young Talents, Yunnan University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16636a

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