MoO2 Nanospheres Synthesized by Microwave-Assisted Solvothermal Method for the Detection of H2S in Wide Concentration Range at Low Temperature

It is crucial to develop highly energy-efficient and selective sensors for wide concentration range of H2S, a common toxic gas that widely exists in petrochemical industries. In this work, MoO2 nanospheres were rapidly synthesized by microwave-assisted solvothermal method, and were subsequently fabricated into H2S gas sensor. The MoO2 nanospheres-based sensor exhibited excellent response toward H2S with good linearity in a wide concentration range (10–240 ppm). Besides, this sensor presented low working temperature, good repeatability, and selectivity against CH4, H2, and CO. The outstanding sensing performance results from the reaction between H2S and abundant chemisorbed oxygen introduced by oxygen vacancies of MoO2. This result indicates that MoO2 nanosphere synthesized by microwave-assisted solvothermal method is a promising sensing material for H2S detection.


INTRODUCTION
H 2 S, a common gas in petroleum refining and storage, would cause serious pollution to air and great damage to human body once leaked (Hu et al., 2018). Therefore, the detection and monitoring of H 2 S are vital for both environmental conservation and human health. In recent years, different kinds of H 2 S sensors have been developed, such as electrochemical sensors, surface acoustic wave sensors and resistive sensors (Mirzaei et al., 2018;Zhao et al., 2018;Khan et al., 2019;Tang et al., 2019). Among them, resistive sensors based on metal oxide nanoparticles have attracted great attention due to the high sensitivity and short recovery time. The metal oxide nanoparticles applied for resistive sensors can be classified into two categories: n-type (ZnO, SnO 2 , Fe 2 O 3 , and MoO 3 ) and p-type (CuO, Cr 2 O 3 , and Co 3 O 4 ) semiconductors (Fine et al., 2010;Walker et al., 2019). However, both of them need high operation temperature to achieve good sensing performance, which results in energy consumption issues and gas explosions risks (Gupta Chatterjee et al., 2015). Besides, the detection range of H 2 S for current nanoparticle based resistive sensors is mainly around the low end (<50 ppm), leading to inaccurate measurement of high concentration H 2 S (Guo Y. et al., 2016;Sukunta et al., 2017;Tian et al., 2017).
MoO 2 , a n-type semiconductor, has been applied as catalysts, photochromic, and electrochromic materials, due to good electronic conductivity and ion transport property (Ni et al., 2015;Jin et al., 2016;Zhang B. et al., 2017;Xia et al., 2018). However, there have been few reports on H 2 S sensors fabricated with MoO 2 . The preparation methodology of MoO 2 needs to be improved as well-MoO 2 is usually synthesized by the reduction of MoO 3 with H 2 or CO at ultrahigh temperature, which exhibits enormous risk of explosion Prabhakar et al., 2018); conventional solvothermal/hydrothermal methods are milder ways to prepare MoO 2 , however, the long processing time, additional surfactants and low yield restricts its application (Xiang et al., 2015;Wang et al., 2016;Zhang et al., 2019). Microwave-assisted solvothermal method is a promising alternative method for the preparation of MoO 2 . Compared to traditional heat source, microwave irradiation generates a rapid heating to attain the desired temperature, due to the direct heating to polar molecules and conducting ions (Zhu and Chen, 2014). In contrast to the conventional solvothermal/hydrothermal methods, which suffer from large thermal gradients between the inner and outer media, the direct heating provides negligible thermal gradients through the reaction system (Mirzaei and Neri, 2016). The uniform heat distribution is beneficial for preparing regular products. Although MoO 2 nanoparticles prepared with microwave-assisted hydrothermal method has been reported, which still need additional carbon or graphene, the resultant MoO 2 nanoparticles shows irregular morphology (Palanisamy et al., 2015;Fattakhova and Zakharova, 2020). There are few works about MoO 2 nanospheres prepared with microwave-assisted solvothermal method without additional surfactants.
In this report, a new method to synthesize MoO 2 nanospheres without surfactant template by the microwave-assisted solvothermal method was presented. The morphology, crystalline, chemical state and stability of samples were investigated by SEM, XRD, XPS, and TGA. The working temperature, response, repeatability, and selectivity of the gas sensors based on MoO 2 nanospheres were further studied in a gas sensing measurement system. Finally, the gas sensing mechanism of MoO 2 nanospheres was discussed.

Materials
MoCl 5 was purchased from Sigma-Aldrich (China), absolute ethanol was purchased from Sinopharm (China). All reagents were of analytical grade without further purification, and the deionized water was used in all experiments.

Fabrication of MoO 2 Nanospheres
MoO 2 nanospheres were synthesized by microwave-assisted solvothermal method. In a typical synthesis procedure, 0.57 g of MoCl 5 was dissolved in 240 ml absolute ethanol with vigorous stirring for 30 min. The MoCl 5 solution was transferred into autoclaves and heated at 200 • C for 3 h in a microwave oven (Multiwave PRO, Anton Paar). After cooled to room temperature, the resulting precipitate was collected and washed by centrifuging in deionized water and absolute ethanol, followed by freeze-drying under vacuum for 2 days. The resultant MoO 2 nanospheres were named as MMOs. MMO-180 and MMO-160 were prepared at 180 • C and 160 • C for 3 h, respectively. For comparison, MoO 2 nanospheres were also synthesized by conventionally solvothermal method, in which the MoCl 5 solution was transferred into autoclaves and heated at 200 • C for 24 h in an oven. The resultant MoO 2 nanospheres were named as CMOs.

Characterization
A scanning electron microscope (SEM, JEOL JSM-7610F) was used to observe the morphologies of MoO 2 . X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance Xray diffractometer with a Cu Kα radiation of 0.154 nm at a generator voltage of 40 kV. The chemical compositions of MoO 2 were measured using Thermo Fisher ESCALAB 250 XI X-ray photoelectron spectroscopy (XPS). Thermogravimetric analysis (TGA) was performed in air atmosphere with a heating rate of 10 • C/min by using a Shimadzu DTG-60 A thermogravimetric analyzer.

Fabrication and Test of Gas Sensors
The MoO 2 powder was ground and mixed with terpineol at the mass ratio of 1:1 to form a paste. The paste was uniformly coated on the surface of alumina ceramic tube attached with a pair of gold electrodes, which were connected by Pt wires. A Ni-Cr heating wire was inserted into the tube to heat the gas sensor. Before the tests, the sensors were aged at 100 • C for 5 days to improve stability. Gas sensing tests were performed on a commercial CGS-8 Gas Sensing Measurement System (Beijing Elite Tech Company Limited) with a test chamber (500 mL in volume). After the sensors' resistance was stabilized at the target temperature, a calculated volume of gas was injected into the chamber. All tests were conducted at a room temperature of 25 ± 5 • C and at 40 ± 5% relative humidity.
The gas response is defined as (R air -R gas )/R air (R air and R gas are the sensors' resistance in air and target gas, respectively). The response time and recovery time is defined as the time taken for the response to reach 90% of total change after testing atmosphere changed.

RESULTS AND DISCUSSION
Morphology and Structure Figure 1 shows the morphology of MoO 2 nanospheres prepared from microwave-assisted and conventional solvothermal method. The diameter of MMOs is in the range of 400-1,000 nm and the average diameter is about 740 nm. In contrast, CMOs own broader distribution of diameter and larger particle size, which affects the homogeneity and sensitivity of gas sensors. Besides, the process of microwave-assisted solvothermal method takes much less time than conventionally solvothermal method, because of the rapid microwave heating (Wang B. et al., 2017). The heating temperature is vital for the regular    morphology of MoO 2 nanospheres during microwave-assisted solvothermal method. As shown in Supplementary Figure 1, prepared at lower temperature, exhibit irregular morphology, which may affect their sensing properties (Cai et al., 2015). Therefore, MMO is chosen to do further characterization and gas tests.
The crystal structure and chemical composition of MMOs were inspected by XRD and XPS. As shown in Figure 2A, MMO has distinct diffraction peaks at 2θ = 26.03 • , 36.852 • , 53.512 • , and 66.456 • , which could be indexed to (−1 1 1), (1 1 1), (−3 1 2), and (2 0 2) planes of monoclinic MoO 2 phase according to the JCPDS 32-0671 (Kim et al., 2009). This suggests MoO 2 was successfully synthesized by microwave-assisted solvothermal method. On the contrary, MMO-180, MMO-160, and CMO have broader and weaker diffraction peaks, applying to the incomplete crystalline phase, which is consisted with the SEM images. To identify the valence of Mo and the chemisorption of O, we characterized the MMOs by XPS. As shown in Figure 2B, XPS spectra of Mo consists of three peaks: two peaks at 231.7 and 235.6 eV present the Mo 3d 5/2 and Mo 3d 3/2 spin-obit components of Mo 6+ , respectively; the peak at 233.1 eV is assigned to Mo 3d 5/2 of Mo 4+ (Choi and Thompson, 1996). The appearance of Mo 6+ indicates the slightly oxidation at the surface of MoO 2 by the exposure to air at room temperature, considering no distinguishing peaks of MoO 3 observed at XRD patterns as shown in Figure 2A. Figure 2C shows the XPS spectra of O 1 s, consisted of two peaks at 531 and 531.9 eV, corresponding to lattice and chemisorbed oxygen, respectively. The appearance of chemisorbed oxygen results from the coordination unsaturation of Mo, implying the presence of oxygen vacancy . The abundant chemisorbed oxygen is beneficial for the sensitivity of MoO 2 , since the resistance change is mainly occurred by the reaction between chemisorbed oxygen and target gas (Jian et al., 2020). TGA curves of MMO ( Figure 2D) shows a decrease of mass before 300 • C, due to the loss of adsorbed water. During this temperature range, there is no obvious increase of mass, which implies MMOs are relative stable at low temperature. The stability of MMOs at low temperature is crucial for the repeatability of gas sensors. At higher temperature, a slight increase of mass occurred, corresponding to the oxidation of MoO 2 .

Gas Sensing Properties
The response to H 2 S depends on the physical and chemical absorption of gas, which is strongly affected by the working temperature . Thus, we investigated the optimal working temperature of MMO gas sensor. As shown in Figure 3A, the response of MMO gas sensors to 10 ppm H 2 S increased first and then decreased as the working temperature rising. The optimal working temperature is 100 • C, which is much lower than that of other metal oxide gas sensors and beneficial for energy saving (Guo W. et al., 2016;Wang et al., 2019;Nguyen et al., 2020). The low working temperature may come from the abundant chemisorbed oxygen and oxygen vacancy in MMO . Therefore, further tests of sensing properties are all completed at 100 • C. Figure 3B presents the response of MMO to H 2 S at different concentrations (1-240 ppm). It can be seen the response increases significantly with increasing concentration of H 2 S, and there is good linear relationship (R 2 = 0.996) between response and the concentration of H 2 S in the whole range. Unlike other sensors' narrow range of linear relationship, sensors of MMO with good linear relationship in a broad range are suitable for detection of H 2 S with large change of concentration (Na et al., 2019;Teng et al., 2020). The response and recovery curve of MMO to 40 ppm H 2 S at 100 • C is shown in Figure 3C with a response time of ∼6 min and recovery time of ∼1 min. The repeatability presented in Figure 3D is also important for gas sensors and other devices (Kong et al., 2021a,b). The curves of response show negligible difference after repeating five cycles of tests to 40 ppm H 2 S, which implies good repeatability and stability of MMO. To investigate the selectivity of MMO sensor, it was exposed to various gases, including CH 4 , H 2 , and CO. As shown in Figure 4A, the sensor exhibits higher response to H 2 S than other gases, which could greatly weaken the interference of nontarget gases. The response of MMO, and CMO are shown in Figure 4B, in which MMO has the highest response to H 2 S. Table 1 summarizes the sensing performance of different metal oxide to H 2 S. Compared to other metal oxide in early work, MMO sensor exhibits lower working temperature and wider concentration range to detect H 2 S. Besides, the good repeatability and selectivity makes MMO sensor suitable for detection of H 2 S leakage in chemical petrochemical companies.

Gas Sensing Mechanism
As a typical n-type semiconductor, the sensing performance of MMO strongly depends on the free electron density (Figure 5). According to the density functional theory (DFT), the adsorption and dissociation of O 2 on MoO 2 surface could occur rapidly at room temperature, due to the high adsorption energy and low dissociation barrier (Zhang Q. et al., 2017). Therefore, when MMO exposed to air, oxygen molecules adsorb onto the surface of MMO and take free electrons from MMO, forming chemisorbed oxygen (O 2 − ) and resistant electron-depletion layer (EDL) as the working temperature below 150 • C (Franke et al., 2006). This leads to decreased free electron density and increased resistance (Mirzaei et al., 2018). After H 2 S was injected into the chamber, H 2 S molecules react with O 2 − to form SO 2 and water vapor. In this process, free electrons trapped by O 2 − come back to the MMO, causing the increased free electron density and decreased resistance (Katoch et al., 2015). After exposed to air again, the oxygen molecules will be re-adsorbed and reconstruct the EDL. During the tests, H 2 O also participated in the reaction via reacting with hole (h + ) to render the radical hydroxyl(•OH), which justifies the optimal working temperature is 100 • C.
The whole reaction is described below: As discussed in XPS characterization before, there is abundant chemisorbed oxygen on the surface of MMO, which could react with a large of H 2 S molecules without saturation. This causes the good linear relationship in a broad range of MMO sensors to H 2 S.

CONCLUSION
MoO 2 nanospheres was rapidly synthesized by microwaveassisted solvothermal method at 200 • C for 3 h. The resultant MMO exhibit more regular dimension than CMON prepared by conventionally solvothermal method. At an optical working temperature of 100 • C, the MMO-based sensors exhibit excellent response, linear relationship, repeatability and selectivity toward a broad concentration range of H 2 S (10-240 ppm). The oxygen vacancies on the surface of MMO results in abundant chemisorbed oxygen which could react with H 2 S, causing outstanding sensing performance of MMO sensors. In a word, MoO 2 nanosphere with abundant chemisorbed oxygen is a promising sensing material for detection of H 2 S leakage in chemical companies.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
FA, SM, BS, and SZ contributed to conception and design of the study. WX organized the database. NL performed the statistical analysis. HW wrote the first draft of the manuscript. SW, CZ, JF, and LW wrote sections of the manuscript. All authors contributed to manuscript revision, read, and approved the submitted version.

FUNDING
Financial support from the National Natural Science Foundation of China (52003297) is gratefully acknowledged.