A fast response cataluminescence ether gas sensor based on GO/Mo2TiC2Tx at low working temperature

A cataluminescence (CTL) ether gas sensor based on GO/Mo2TiC2Tx composite was developed. The sensor has high selectivity and sensitivity to ether with the response and recovery times of 2 and 8 s, respectively. The optimal operating temperature (155 °C) is low compare with common sensors. Under optimal conditions, the linear range of the concentrations of ether is 9.5–950 ppm; CTL signal intensity and ether concentration show a good linear relationship (r = 0.9952); and the detection limit is 0.64 ppm. Furthermore, no response to anything other than acetone after repeatedly tested 10 kinds of common volatile organic compounds, which shows that the sensor has a good selectivity. In addition, the developed sensor has a long life.


Introduction
Most of the volatile organic compounds (VOCs) have a pungent smell, which can cause people's sensory discomfort, and do harm to human health and the ecological environment. Ether, as one of the VOCs, can cause re and explosion accidents. In addition, ether is able to pass through the respiratory tract and be absorbed quickly aer entering the alveoli. It can also be absorbed through the skin and quickly enter the brain and adipose tissue through the blood. Long-term exposure to ether is very harmful to the human body. It has harmful effects on the central nervous system, liver and many other organs. Hence, it is necessary to detect ether quickly and accurately. Currently, the most common method for detecting ether is gas chromatography. 1,2 Although this method has high sensitivity and selectivity, the instrument is large in size, complex in operation and difficult to achieve realtime monitor. Semiconductor metal oxide sensor or surface acoustic wave quartz crystal sensor can also be used to detect ether, which have a good sensitivity but poor selectivity. 3,4 Cataluminescence (CTL) is the phenomenon of chemiluminescence produced by the catalytic reaction of gas on the surface of solid materials. [5][6][7][8][9][10] In 1976, it was rst discovered when Breysse et al. studied the catalytic oxidation of CO on the surface of ThO 2 , and dened it as "cataluminescence". 11 CTL gas sensor has the characteristics of superior selectivity, wide linear range, rapid response and high signal-to-noise ratio, which can monitor gas quickly and accurately. Compared with traditional gas chromatography, it has the characteristics of small size and simple operation. Compared with colorimetry and spectrophotometry, it possesses a continuous monitoring characteristic.
In the 21st century, with the emergence of nanomaterials, the development of CTL has been promoted. Tang et al. prepared an acetone highly sensitive and selective sensor based on nano-La 2 O 3 surface CTL and successfully applied it to the determination of acetone in air samples. 12 Zhen et al. used a-MoO 3 as a gas-sensitive material to prepare a CTL sensor that can detect ether gas at 120 C. 13 Shi et al. used aluminum/iron oxide composite to develop a CTL gas sensor for detecting harmful gases such as ether and n-hexane, in which the response to ether was observed at 180 C. 14 Many nanomaterials-based CTL gas sensors have been reported, such as Sm 2 O 3 , Ag 2 Se, V 2 O 5 , Ti 3 SnLa 2 O 11 . [15][16][17][18] Developing optimal materials with high sensitivity, good selectivity, and mild reaction conditions (at low temperatures) has been an important direction. Two-dimensional materials are benecial for gas sensing applications, and a larger specic surface area will promote surface reactions. Graphene oxide (GO), as a frequently studied two-dimensional material, has been considered as a potential material for a wide range of applications. [19][20][21] Graphene has a stable structure, strong corrosion resistance, and large specic surface area, which is conducive to the composite with other carriers. Two-dimensional transition metal carbides and nitrides, as MXenes, have similar conductivity, adjustable structure and hydrophilicity. 22,23 The chemical formula of this kind of two-dimensional materials is generally expressed as M n+1 X n T x , where M is an early transition metal (e.g., Ti, V, Cr, Nb, Mo, etc.), X is carbon or nitrogen, and T x represents a terminal functional group, such as -O, -OH or -F. 24 It was discovered in 2011, and has attracted attention due to its unique physical and chemical properties, but it is currently less used in the eld of CTL. Herein, we develop a GO/Mo 2 TiC 2 T x composite which is used for gas sensor to detect of ether. 24,25 Experimental Preparation of experimental materials All chemicals used in the experiment are of analytical grade and can be used directly without further purication. Concentrated sulfuric acid, graphite powder, sodium nitrate, potassium permanganate, hydrogen peroxide, hydrochloric acid, hydro-uoric acid, absolute ethanol, ether, acetone, carbon tetrachloride, formaldehyde, chloroform, xylene, acetonitrile, ethyl acetate, ammonia and cyclohexane were purchased from Shanghai Group Chemical Reagent Co., Ltd. Mo 2 TiAlC 2 was purchased from Beijing Beike New Material Technology Co., Ltd.

Preparation of GO
GO was synthesized by a Hummers' method. 26 The typical process is as follows: appropriate amount of concentrated sulfuric acid was added in a 250 mL reaction ask in an ice water bath. Under stirring, the solid mixture of 2 g graphite powder and 1 g sodium nitrate were added, and then 6 g potassium permanganate was added. The reaction temperature was controlled below 20 C, and the solution was stirred. The temperature was increased to about 35 C and continue stirring for 30 min. Then slowly add a certain amount of deionized water, and raise the temperature to 98 C. Aer heating and stirring for 20 min, an appropriate amount of hydrogen peroxide was added to reduce the remaining oxidant and the solution becomes bright yellow. It was washed with 5% HCl solution and deionized water until no sulfate was detected in the ltrate. Finally, the lter cake was fully dried in a vacuum drying oven at 60 C and stored for further use.
Preparation of Mo 2 TiC 2 T x 1.0 g Mo 2 TiAlC 2 was added to 30 mL hydrouoric acid (HF) solution. It was stirred continuously for 72 h under the reaction condition of 55 C. Then the product was washed repeatedly with deionized water, and the centrifuge speed was 4000 rpm, until the pH value of the supernatant was greater than 6. The obtained material was ultrasonically treated for one hour in an argon atmosphere, and ltered and dried to obtain Mo 2 TiC 2 T x .
Preparation of GO/Mo 2 TiC 2 T x 50 mg GO and 50 mg Mo 2 TiC 2 T x were dispersed in 50 mL deionized water. The hydrothermal conditions were controlled at 60 C and stirred continuously for 6 h. Then the material was freeze-dried at À60 C for 12 h aer centrifugation with alcohol and deionized water for several times.

Characterization
Scanning electron microscopy (SEM) were conducted on a Zeiss Auriga instrument operating at 10 kV. Hitachi H-7650 instrument for transmission electron microscope (TEM) observation at 200 kV, energy dispersive spectrometer (EDS) and X-ray diffraction (XRD, X-Pert powder, Cu Ka) were also used to characterized the samples.

Apparatus of gas sensor
Ultra-weak chemiluminescence analyzer (BPCL-1-TIC, Guangzhou Microlight Technology Co., Ltd) was used to detect the CTL signals, the schematic diagram of cataluminescence sensing is shown in the Fig. 1. The whole CTL experimental device is composed of three parts: (1) reaction chamber: it is composed of ceramic heating rod and quartz tube (containing gas inlet and outlet). The gas to be measured ows into the quartz tube from the inlet and fully contacts and reacts with nanomaterials; (2) temperature control system and carrier gas ow rate control system; (3) photoelectric detection and data processing system: BPCL instrument is used to detect, collect and process photoelectric signals, convert weak light signals into electrical signals, and nally transmit the electrical signals to the computer for storage.

Results and discussions
Characterization of GO/Mo 2 TiC 2 T x SEM and TEM images of GO/Mo 2 TiC 2 T x catalyst are shown in Fig. 2. Fig. 2(a) and (b) show SEM images. It's very clear from the images that the crumpled, monolithic, aky structure is graphene oxide. It has a large surface area. The Mo 2 TiC 2 T x material also presents a sheet-like structure with a small size, forming sh scales shaped that are evenly wrapped on the graphene surface. Fig. 2(b) displays an enlarged view of GO/Mo 2 TiC 2 T x composite. Large surface area is conducive to the gas to be tested, and enhances the CTL performance of the sensor. In the EDS spectrum (Fig. 2(d)), we can see that the composite is mainly composed of Ti, O, C, and Mo elements. At the same time, we conducted EDS mapping detection on the material, as shown in Fig. 2(e)-(i). EDS mapping shows the characteristic elements Mo and Ti of Mo 2 TiC 2 T x , the C and O elements of Mo 2 TiC 2 T x and GO. It can be observed that Mo and Ti are uniformly distributed, indicating that Mo 2 TiC 2 T x is uniformly attached to the GO. The (002) peak of Mo 2 TiC 2 T x powders le shis to 6.9 from 10.9 of Mo 2 TiAlC 2 T x as shown in XRD patterns (Fig. 3), indicating the larger d-spacing of Mo 2 TiC 2 T x Fig. 1 Schematic diagram of the CTL-based sensor system. than that of Mo 2 TiAlC 2 T x because of introduced groups during the etching process. The surface of MXene prepared by solution etching is generally accompanied by functional groups. T represents a terminal functional group, such as -O, -OH or -F, which indicate the T of Mo 2 TiC 2 T x in our study would be -O, -OH or -F. 24 The characteristic peak of GO was observed at 2q ¼ 15 . The elemental composition of GO/Mo 2 TiC 2 T x are listed in Table 1.

Sensing performance towards ether
The catalytic luminescence properties of GO, Mo 2 TiC 2 T x and GO/Mo 2 TiC 2 T x were measured, respectively. The results are shown in Fig. 4. Compared with the Mo 2 TiC 2 T x , the hybrid structure shows signicantly enhanced sensing performance. It indicates that the addition of GO can effectively enhance the signal intensity of GO/Mo 2 TiC 2 T x -based sensor for detecting aether. The improved sensing performance is attributed to the effective combination of GO and Mo 2 TiC 2 T x . In our study, the GO/Mo 2 TiC 2 T x composite was investigated as follows. Fig. 5 shows the response curve towards ether on the sensor surface under optimal conditions. The response and recovery times are within 2 and 8 s, respectively; and the signal intensity is high. It can be seen that the sensor has the advantages of high sensitivity and fast response to ether. In    vapor measured four times is 2.1%. The results show that the CTL signal intensity of ether on the material surface is stable, and the reproducibility is good. The excellent sensing performance can be attributed to its two-dimensional structure and large surface area, which is favourable to transfer of electrons, and improve adsorption and desorption of O 2 .

Effect of working temperature
Temperature plays an important role in the CTL process.
Because the catalyst has a lower catalytic activity at lower temperature, external conditions are required to increase the temperature. As shown in Fig. 6, in order to exert the best performance of the material, the relationship between the CTL intensity of ether on the surface of the composite material and the temperature under the optimal conditions of ow rate and concentration. In the range from 100 C to 155 C, the CTL signal strength increases with the increase of temperature. The signal value reaches the maximum at 155 C, and then the temperature rises, which has little effect on the signal strength, the intensity drops slightly. The noise is also not conducive to our detection. We can observe that, with the increase of temperature, not only the CTL signal strength increases, but also the noise value increases, which causes the S/N to increase rst and then decrease with the increasing of temperature. The signal-to-noise ratio (S/N) reaches its maximum at 155 C, which indicates it is the best working temperature. We can see from the Fig. 6 that ether reacts at a relatively lower temperature, as low as 100 C. Low-temperature CTL has always been a thorny problem in this eld. At present, many CTL sensors need to work at high temperatures, and researchers have been exploring low-temperature materials. Compared with previous reports, the sensor based on GO/ Mo 2 TiC 2 T x has a low working temperature and high signal intensity (Table 2). 12,[14][15][16][17][18] Effect of air ow The ow rate of carrier gas also has an effect on the CTL intensity. The relationship between carrier gas velocity and CTL strength was investigated when the temperature was 155 C and the concentration of ether gas was certain, in the ow rate range of 50-600 mL min À1 . It can be seen from Fig. 7 that the CTL response has the highest intensity when the carrier gas ow rate is 400 mL min À1 . When the ow rate is less than 400 mL min À1 , the carrier gas ow rate increases, and the ether gas in contact with the material surface per unit time increases with the increase of the ow rate. Therefore, the CTL signal shows an upward trend, indicating that the reaction rate is limited by the rate when the ether gas transfers to the catalyst surface. With the further increase of the ow rate, when it exceeds 400 mL min À1 , part of the ether gas has been taken away from the reaction chamber before it contacts the surface of the material, resulting in a low signal. Combined with the value of signal-to-  noise ratio and comprehensive consideration, it is considered that 400 mL min À1 would be the best ow rate.

Analysis characteristic
In order to further study the CTL properties under the optimal conditions, different concentrations of ether were injected into the reaction chamber. The linear relationship between CTL strength and the concentration of ether was established, as shown in Fig. 8. In the concentration range of 9.5-950 ppm, we can observe that the CTL intensity is proportional to the concentration of ether. The linear equation is y ¼ 1675.4x + 44 167 (R 2 ¼ 0.9904, n ¼ 6), where x represents the concentration of ether, y represents the intensity of CTL signal, and R is the regression coefficient. The limit of detection (LOD) to ether is 0.64 ppm (S/N ¼ 3). The GO/Mo 2 TiC 2 T x composite developed here has a lower LOD for ether, which is lower than some other sensors, such as ZnWO 4 , SiO 2 /Fe 3 O 4 , a-MoO 3 , Mg-Al LDO 5,6,13,27,28 (Table 3).

Selectivity and life of materials
In order to study the selectivity of the GO/Mo 2 TiC 2 T x composites. Under optimal conditions, we tested 10 common volatile organic compounds that may coexist with ether, including acetone, carbon tetrachloride, ethanol, formaldehyde, chloroform, xylene, acetonitrile, ethyl acetate, ammonia, and cyclohexane. In Fig. 9, none of these gases react on the surface of the material except acetone. Although acetone does react, the signal strength is negligible compared to ether, because the interference of acetone is less than 1%. It indicates a high selectivity to ether. Within 30 days, the sensor was tested once a week. Under the best conditions, 950 ppm of ether was injected into the reaction chamber. By analysing each test, it is found that the GO/Mo 2 -TiC 2 T x composite hardly changes the CTL signal intensity towards ether. The results shows that the gas sensor has a good stability for long-term use.

Possible sensing mechanism
At present, there are few studies on the mechanism of CTL, mainly due to the complex reaction process of CTL reaction. Thorough research on the mechanism of CTL is of great signicance for the controllable synthesis of catalysts with high sensitivity and strong selectivity. A CTL mechanism that has been accepted by many researchers, which is the formation of highly active endoperoxides during the reaction. [29][30][31][32][33][34][35][36][37][38][39] It is considered that GO/Mo 2 TiC 2 T x has sensitive properties to aether because of the reaction of aether with the catalyst to form highly reactive intermediates. Although other gases can also be catalyzed, the amount of CTL intermediates generated by ether is much greater than others, resulting in a good sensitivity to ether. According to Zhang's report, ether is able to be oxidized to excite the species acetaldehyde and CO 2 molecules. 40 The possible CTL mechanism of ether is as follows: under the catalyst conditions, O 2 is adsorbed on the surface of GO/Mo 2 -TiC 2 T x composite, which captures electrons to generate

Conclusions
A new CTL sensing material is developed by combining twodimensional graphene oxide and Mo 2 TiC 2 T x . The ether sensor based on the composite can not only work at a low temperature, but also has high sensitivity and selectivity with the fast response during detection. The results show that GO/Mo 2 TiC 2 T x is a potential candidate with low operating cost and good stability, which can be applied to the real detection towards ether gas.

Conflicts of interest
There are no conicts to declare.