Synthesis of Cr2O3 Nanoparticle-Coated SnO2 Nanofibers and C2H2 Sensing Properties

In this work, Cr₂O₃ nanoparticles, and SnO₂ nanofibers were fabricated by a sol–gel process and an electrospinning method, respectively. Gas sensitive materials with high sensitivity to C₂H₂ gas were obtained by coating Cr₂O₃ nanoparticles on SnO₂ nanofibers. The prepared Cr₂O₃ nanoparticle-coated SnO₂ nanofibers (Cr₂O₃ NPs. coated SnO₂ NFs.) were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and the gas sensing behaviors to C₂H₂ were studied. The Cr₂O₃ NPs. coated SnO₂ NFs. exhibited low optimal operating temperature, high sensing response, excellent response-recovery time, and long-term stability to C₂H₂. The optimal operating temperature of the measured material to 20 ppm C₂H₂ was about 220°C and the C₂H₂ concentration had a good linear relationship with the response value when the concentration was 60 ppm. In addition, a reasonable gas sensing mechanism was proposed which may enhance the gas sensing performances for the Cr₂O₃ NPs. coated SnO₂ NFs. to C₂H₂.

Introduction

Metal oxide semiconductors have important practical significance in gas sensing, mainly because of good chemical reliability, real-time monitoring, and easy fabrication (Zhou et al., 2018b; Wei et al., 2019b). The basis of functional materials, including ZnO (Wang et al., 2019; Yoo et al., 2019), TiO₂ (Crişan et al., 2018; Meng et al., 2019), In₂O₃ (Liu et al., 2018; Inyawilert et al., 2019), SnO₂ (Zhang et al., 2018; Zhou et al., 2018c; Li et al., 2019a), have been applied in gas sensing in the past for a long time. Among them, SnO₂ is one of the earliest metal oxides, due to its wide band gap, excellent physicochemical properties, and low-price for gas sensing (Uddin and Chung, 2015; Thanihaichelvan et al., 2019). It can easily generate oxygen vacancies on the surface, leading to high specific surface area, and excellent sensing properties (Zhang et al., 2017; Ren et al., 2019). For SnO₂, various morphologies of nanostructures have been reported, for instance, zero dimensional nanoparticles (Ahmed et al., 2019), one dimensional nanorods (Zhou et al., 2016), nanowires (Tonezzer, 2019), and nanofibers (Mudra et al., 2019), and two dimensional nanosheets (Chang et al., 2019). Among them, one dimensional nanostructures have been rapidly developed due to their surprising properties such as easy control dimension, large surface area to volume ratio, and excellent mechanical performance (Wang et al., 2017b; Bai et al., 2018).

In order to improve the sensitivity of pure SnO₂ nanofibers for gas detection more effectively, several approaches have been studied such as the addition of catalysts, doping metals, and metal oxides (Lu et al., 2018; Zhou et al., 2018d; Zheng et al., 2019). Various studies on gas sensing properties of SnO₂ nanofibers doped or coated with metal or metal oxide have been reported so far. Qi et al. reported Sm₂O₃-doped SnO₂ showed high sensitivity under various humidity conditions to C₂H₂ (Qi et al., 2008). Li et al. confirmed the sensor made of La³⁺ doped SnO₂ nanofibers could rapidly correspond to hydrogen, with good selectivity, and long-range linear response (Li et al., 2019b). Chromium is a very interesting dopant because of its unique catalytic properties and several stable valence states. In addition, it can be used to adjust the surface states, energy band gap and carrier transport characteristics of semiconductors. Gönüllü et al. investigated Cr-doped TiO₂ as a high-temperature NO₂ gas sensor (Gönüllü et al., 2015). Wang et al. demonstrated that the response value of Cr₂O₃/ZnO nanofibers was 3.6 to 1 ppm ethanol vapor at 300°C (Wang et al., 2010). Park et al. synthesized In₂O₃ nanorods decorated with Cr₂O₃-nanoparticles which showed excellent sensing properties to ethanol than other gases (Park et al., 2015). However, there are few reports on Synthesis of Cr₂O₃ nanoparticle-coated SnO₂ nanofibers and its C₂H₂ sensing properties.

In the paper, we reported the synthesis of Cr₂O₃ nanoparticle-coated SnO₂ nanofibers (Cr₂O₃ NPs. coated SnO₂ NFs.) by a sol–gel method and electrospinning technique. The prepared Cr₂O₃ NPs. coated SnO₂ NFs. were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and their C₂H₂ sensing properties were studied. Benefiting from the coating of Cr₂O₃, the SnO₂ nanofibers exhibited good sensitivity to C₂H₂ gas. In addition, a plausible gas sensing mechanism was proposed which may enhance the gas sensing performances of C₂H₂.

Experimental Design, Materials, and Methods

Sample Synthesis

Cr₂O₃ nanoparticles (NPs.) were prepared by a sol-gel method (Puerari et al., 2016). Firstly, 5.0g Cr(NO₃)₃.9H₂O and 1.5 g NaOH were added to 100 ml distilled water for 30 min with stirring to obtain the precursor Cr(OH)₃. The Cr(OH)₃ was centrifuged at 5,000 rpm and washed several times with distilled water. It was then dried in an oven at 90C for 24 h. 0.05 g of Cr(OH)₃ powders were mixed with 10 g of deionized water and heated to 60°C and then stirred for 2 h to obtain a homogeneous sol solution.

SnO₂ nanofibers (NFs.) were synthesized by electrospinning method (Yang et al., 2018). Firstly, 6 mL N, N-dimethylformamide (DMF) and 1.2 g SnC${\text{l}}_2^{.}$2H₂O were put into beaker1 and stirred for 2 h with a magnetic stirrer until it became a clarifying solution and set aside. 1 g PVP and 6 mL absolute ethanol were put in beaker2 and stirred with magnetic stirrer at uniform speed until PVP dissolved completely. The solution in beaker2 was added to the solution in beaker1 drop by drop with dropper, and the mixture was obtained as the precursor solution by stirring for 3 h at room temperature. The obtained spinning solution was conveyed to a hypodermic syringe at a constant flow rate, and then 20 kV voltage was applied to electrospinning at the electrode distance of 25 cm. A piece of aluminum foil was used as the cathode, and several sensor substrates were placed on it. Sensor substrates were prepared on SiO₂/Si chips by radio frequency sputtering platinum arrays as signal electrodes (Qi et al., 2014). The thickness of the SiO₂ layer and the platinum array are about 300 and 100 nm, respectively. The precursor solution was directly electrospun on sensor substrates with arrays of interdigitated platinum electrodes. After 2 h of electrospinning, the substrates were calcined in air at 500°C for 2 h, and then impregnated with the sol solution. The substrates were dried on a gently heated hot plate before the next step after each step. Subsequently, the sensors were calcined in air at 600°C for 1 h, and then annealed in hydrogen atmosphere at 300°C for 10 min. Finally, the Cr₂O₃ NPs. coated SnO₂ NFs. sensors were obtained.

Sample Characterization

In this paper, The X-ray diffractometer (D/Max-1200, Rigaku, Japan) was used to recorded XRD patterns at room temperature. The SEM and EDS images were gained by using Field Emission Electron Microscope (Hitachi S-4800, Marco Polo Shanghai Yongming Automation Equipment Co., Ltd., Shanghai, China) and Oxford INCA 250 EDS detector (JSM-6700F, Japan), respectively. The XPS spectra were determined on an X-ray photoelectron spectrometer (KRATOS XSAM800, Kratos, Kingdom).

The gas sensing properties of the obtained Cr₂O₃ NPs. coated SnO₂ NFs. were performed with the CGS-1TP intelligent gas sensitive analysis system (Beijing Elite Tech Co., Ltd, Beijing, China). The structure of planar gas sensor was shown in Figure 1. The gas sensing experiments were tested under laboratory conditions at the room temperature of 25°C and relative humidity of 50%. The gas response of the sensor (R) is defined as R = R_g/R_a (Du et al., 2018), where R_g and R_a are the resistance values of the sensor in the air and the gas to be measured, respectively. The time required for the sensor resistance to change from R_a to R_a-90% × (R_a-R_g) is defined as the response time when the target gas is introduced into the sensor, and the time from R_g to R_g+90% × (R_a-R_g) is defined as recovery time when the target gas is replaced by air (Choi et al., 2019).

FIGURE 1

Figure 1. The structure of the planar gas sensor.

Results

Materials Characterization

Figure 2 demonstrated the XRD patterns of the prepared SnO₂ nanofiber sample with Cr₂O₃. The prominent peaks corresponding to (110), (101), and (211) crystal lattice planes and other smaller peaks showed no difference from the corresponding peaks of the SnO₂ rutile structure given in the standard data file (JCPDS File no. 41-1445). The diffraction peaks were observed at 34.7°, 37.8°, 54.2°, where the inconspicuous Cr₂O₃ peaks were observed, indicating that Cr₂O₃ successfully coated the SnO₂ sample.

FIGURE 2

Figure 2. The XRD spectrum of Cr₂O₃ NPs. coated SnO₂ NFs.

The morphologies of the pure SnO₂ and Cr₂O₃ NPs. coated SnO₂ NFs. were examined by SEM and the representative images are shown in Figure 3. The prepared samples were composed of a plurality of SnO₂ nanofibers as shown in Figure 3A. The SnO₂ nanofibers were uniform in size and irregular arranged. After the synthesis process, the Cr₂O₃ nanoparticles were tightly coated on the surface of SnO₂ nanofibers. The surface of the nanofibers was uneven, long and continuous, without adhesion, intertwined into a network. Furthermore, it was observed from the photomicrograph that the prepared fibrous SnO₂ samples had a porous structure which was of benefit to subsequent gas sensitivity testing.

FIGURE 3

Figure 3. The SEM images of (A) pure SnO₂ NFs. and (B) Cr₂O₃ NPs. coated SnO₂ NFs.

XPS is a kind of useful technique for studying the chemical state of the elements and the surface composition in the sample. Figure 4A showed the XPS spectrum of the Cr₂O₃ NPs. coated SnO₂ NFs. gas sensing material, and the chemical states of various elements in the sample were obtained. The C, Sn, Cr, and O elements appeared in the broad spectrum of the sample. The characteristic energy spectrum reflected that the Cr element had been successfully coated on the surface of SnO₂, which was consistent with the XRD pattern. At the same time, there was no impurity doping into SnO₂. Figure 4B showed the XPS spectrum of Cr 2p. The doublet peaks located at binding energies of 577.5 and 588.4 eV, which is close to the trivalent Cr ion in the standard XPS data, indicating Cr element was in the state of trivalent Cr ions.

FIGURE 4

Figure 4. (A) The XPS spectrum of the Cr₂O₃ NPs. coated SnO₂ NFs. sample, and (B) XPS spectrum of Cr 2p.

An EDS measurement was performed to study the component of the sample. Corresponding EDS spectra from the prepared Cr₂O₃ NPs. coated SnO₂ NFs. sample is shown in Figure 5, which confirmed the presence of Sn, Cr, O, in the sample. So, it showed that the sample was composed of Cr₂O₃ and SnO₂ clearly.

FIGURE 5

Figure 5. The EDS spectrum of Cr₂O₃ NPs. coated SnO₂ NFs.

Sensing Performances

In order to find out whether the coating of Cr₂O₃ has a positive effect on the detection of acetylene gas by pure SnO₂, the gas sensing properties of pure SnO₂ and Cr₂O₃ NPs. coated SnO₂ NFs. for C₂H₂ gas were tested. As we know, the operating temperature is one of important factors that determine the gas sensitivity of materials. Figure 6A shows the response of pure SnO₂ and Cr₂O₃ NPs. coated SnO₂ NFs. to 20 ppm C₂H₂ at different temperatures to explore the relationship between temperature and gas response as well as the optimal operating temperature in the range of 60–440°C. It found that the response of the samples showed the trend of increasing first and then decreasing. The response increased in the range of 60–250°C and reached the highest point at 250°C then decreased in the range of 250–440°C for pure SnO₂. The response increased in the range of 60–220°C and reached the highest point at 220°C then decreased in the range of 220–440°C for Cr₂O₃ NPs. coated SnO₂ NFs. The response values of gas sensor based on pure SnO₂ and Cr₂O₃ NPs. coated SnO₂ NFs. for 20 ppm C₂H₂ gas at optimum operating temperature were 17.12 and 48.54, respectively. Obviously, Cr₂O₃ NPs. coated SnO₂ NFs. exhibited excellent temperature characteristics with the lower optimal operating temperature and higher response value, indicating that the coating of Cr₂O₃ had a positive effect on the measurement of C₂H₂, and the optimum operating temperature was effectively reduced.

FIGURE 6

Figure 6. (A) The gas response of the pure SnO₂ and Cr₂O₃ NPs. coated SnO₂ NFs. toward 20 ppm of C₂H₂ at different working temperatures (60–440°C); (B) the gas response of the Cr₂O₃ NPs. coated SnO₂ NFs. to different concentrations (0–1,000 ppm) C₂H₂ at 220°C and (C) the gas response of the Cr₂O₃ NPs. coated SnO₂ NFs. in the range from 0 to 80 ppm C₂H₂ at 220°C.

Figure 6B revealed the response of Cr₂O₃ NPs. coated SnO₂ NFs. to the concentrations of C₂H₂ in the range of 0 to 1,000 ppm at 220°C. The measured results in Figure 6C showed that the gas responses of Cr₂O₃ NPs. coated SnO₂ NFs. increased in a good linear relationship with the concentrations of C₂H₂ in the range from 0 to 60 ppm. Moreover, when the gas concentrations exceeded 200 ppm, the response increased slowly, indicating that the response gradually became saturated.

Quick response and recovery plays an important role for a gas sensing material. The dynamic response-recovery curve of the Cr₂O₃ NPs. coated SnO₂ NFs. sensors for 1, 5, 10, 15, and 20 ppm C₂H₂ gas at an optimum operating temperature of 220°C was tested and shown in Figure 7A. When the C₂H₂ concentration was increased from 1 to 20 ppm, the response time values of the prepared gas sensor were 9, 12, 15, 17, and 20 s, and the recovery time values were 11, 14, 18, 19, and 23 s. The Cr₂O₃ NPs. coated SnO₂ NFs. responded rapidly and could recover to its initial value when they were exposed to the air again. It showed that the sensors had good response recovery characteristics for different concentrations of C₂H₂ gas, indicating an excellent persistence and stability of C₂H₂ gas.

FIGURE 7

Figure 7. (A) The dynamic response-recovery characteristic of Cr₂O₃ NPs. coated SnO₂ NFs. sample at different concentrations (1, 5, 10, 15, 20 ppm) at optimal operating temperature and (B) the long-term stability of Cr₂O₃ NPs. coated SnO₂ NFs to 5 and 10 ppm C₂H₂ at 220°C.

For the long-term perspective of practicality, in order to ensure the correctness of the test results, gas sensing materials should keep good stability. Therefore, the long-term stability of Cr₂O₃ NPs. coated SnO₂ NFs. to 5 and 10 ppm C₂H₂ were tested at 220°C during 30 days to ensure the reliability, as shown in Figure 7B. Even if the response values changed every day, when the gas concentrations were 5 and 10 ppm, the response values only just fluctuated around 14.5 and 26.5, respectively. So, the Cr₂O₃ NPs. coated SnO₂ NFs. had prominent stability.

The gas-sensing characteristics of C₂H₂ based sensors have been discussed and compared with other metal oxides material gas sensors shown in Table 1. From these reports, gas sensors based on Cr₂O₃ NPs. coated SnO₂ NFs. exhibit lower working temperatures and higher response values compared with most other sensors for C₂H₂ gas. The obtained results indicate that the Cr₂O₃ NPs. coated SnO₂ NFs sensor is promising for C₂H₂ gas sensing.

TABLE 1

Table 1. The gas-sensing characteristics of C₂H₂ sensors based on different metal oxides synthesized by various methods.

SnO₂ is a wide-bandgap semiconductor whose gas-sensing properties is the change in resistance caused by the adsorption and desorption of surface electrons and gas molecules. The gas sensing mechanism of pure SnO₂ NFs. and Cr₂O₃ NPs. coated SnO₂ NFs. was shown in Figure 8. When the sample exposed to the air, the resistant of the sensor was decided by the quantity of chemisorbed oxygen species. In the air, the oxygen would be chemically adsorbed on the surface of SnO₂ and electrons were obtained from conduction band of SnO₂, leading to the formation of ionic species such as O⁻, O²⁻, and ${\text{O}}_2^{-}$. When the SnO₂ NFs. was exposed to the atmosphere of C₂H₂, C₂H₂ gas had many opportunities for absorption and desorption on the surface. Reactions could be induced after coming into contact with acetylene molecules and then being oxidized with ionic oxygen species to produce H₂O and CO₂. Thus, the desorbed oxygen species would set free electrons return into the conduction band of SnO₂, causing a receded depletion zone, decreasing the resistance, and increasing conductivity shown in Figure 8A.

FIGURE 8

Figure 8. The mechanism diagram of C₂H₂ sensing process of (A) pure SnO₂ and (B) Cr₂O₃ nanoparticle-coated SnO₂ nanofibers.

It is well-known that the ability of chemisorbed oxygen is decided by the specific surface area of the materials and the operating temperature. The surface area of Cr₂O₃ NPs. coated SnO₂ NFs. was large, as shown in Figure 3, which indicated that the adsorption capability of Cr₂O₃ NPs. coated SnO₂ NFs. had been enormously enhanced. Cr₂O₃ coating provided an effective means with improving the electronic and catalytic properties for gas interaction at the interface. The concentration of oxygen vacancies was greatly increased. The oxygen vacancies could capture ion-adsorbed oxygen in the atmosphere, which facilitated gas sensing reaction. Meanwhile, Cr₂O₃ coating increased the specific surface area, provided more gas and oxygen adsorption sites, improved the conductivity of SnO₂, and contributed to oxygen adsorption (Li et al., 2018; Zhou et al., 2018a) as shown in Figure 8B. Accordingly, the speed at which the reaction occured was accelerated. The adsorption rate of C₂H₂ to SnO₂ increased with Cr₂O₃ coating, which indicated that Cr₂O₃ could improve the sensitivity of SnO₂ to C₂H₂ gas.

Conclusions

The Cr₂O₃ NPs. coated SnO₂ NFs. were successfully prepared via a sol-gel process and electrospinning method. The gas detection results showed that the prepared sensor was sensitive to C₂H₂, and the optimum temperature was about 220°C which was lower than the optimum temperature of pure SnO₂ nanofibers. C₂H₂ gas had a higher response value, and the C₂H₂ concentration had a good linear relationship with the response value when the concentration was <60 ppm. Moreover, the Cr₂O₃ NPs. coated SnO₂ NFs. had good repeatability and long-term stability. The excellent gas sensing performance of Cr₂O₃ NPs. coated SnO₂ NFs. could be owing to the increase of oxygen vacancies of SnO₂ nanofibers by Cr₂O₃ coating and the large specific surface of the sample. The results certify that the Cr₂O₃ NPs. coated SnO₂ NFs. material is a potential candidate for the detection of acetylene.

Data Availability

All datasets generated for this study are included in the manuscript/supplementary files.

Author Contributions

XG and ZL performed the experiments and analyzed the data with the help from LX and QiZ. XG, QuZ, and WZ wrote and revised the manuscript with input from all authors. All authors read and approved the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This work has been supported in part by the National Natural Science Foundation of China (No. 51507144), Fundamental Research Funds for the Central Universities (No. XDJK2019B021), the artificial intelligence key project of Chongqing (No. cstc2017rgzn-zdyfX0030), the Chongqing Science and Technology Commission (CSTC) (No. cstc2016jcyjA0400), and the project of China Scholarship Council (CSC).

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