β12 Phase Borophene Enhanced PANI Gas Sensor for CO and NH3 Detection

In cases where the carbon monoxide concentration reaches to 50 ppm, and ammonia concentration reaches to 25 ppm in indoor ambient, the symptoms such as lung failure, heart failure, brain damage are present, and urgent medical attention is required. Therefore, the development of a rapid and sensitive sensor in order to detect the level of CO and NH 3 gases is the critical issue for health and the environment. In this study, borophene nanosheets with the β 12 phased crystalline structure are produced by ultrasonic sonication. Using borophene nanosheets, Borophene and PANI: Borophene sensors are fabricated to investigate the CO and NH 3 gas detection at room temperature. It has been observed that borophene enhances the CO and NH 3 gas detection performance of PANI. The results reveal that borophene sensor detected 6 ppm CO gas with 30 s response time and 40 s recovery time, and 50 ppb NH 3 gas with 40 s response time and 60 s recovery time at room temperature. On the other hand, PANI: Borophene sensor detected 6 ppm CO gas with 300 s response time and 320 s recovery time and 50 ppb NH 3 gas with 100 s response time and 120 s recovery time at room temperature.


Introduction
Toxic gases such as carbon monoxide (CO) and Ammonia (NH 3 ) seriously threaten human health. When CO concentration reaches to 50 ppm, and NH 3 concentration reaches to 25 ppm in indoor ambient, the symptoms such as lung failure, heart failure, brain damage are present and the urgent medical attention is needed. Therefore, early detection of low-level CO and NH 3 gases is a critical issue for human health [1][2][3][4][5][6][7].
In recent years, the detection of toxic gases has been the major focus of sensor research. The solid state gas sensor, one of the solid state electronic devices, has attacted attention due to its high sensitivity, high selectivity, low-cost, and small size. Although metal oxide semiconductors gas sensors, which are commonly used in the detection of CO and NH 3 gases, bene t from high metarial sensitivity and quick response time, they generally operate at high temperatures (200-300 0 C) [8][9][10][11][12][13][14][15]. Recently, researchers have focused on improving the operation conditions of gas sensors in terms of low-level CO detection at room temperature [16][17][18][19][20].
In the literature, various PANI: nanomaterial-based sensor studies for the detection of CO and NH 3 gases at room temperature were reported. Among them, PANI: Zeolite sensor detected 16-1000 ppm CO gas [43].
Using the sensor based on PANI: CNT, 100-1000 ppm CO gas detection with 36 s response time and 330 s recovery time was reported [44]. PANI: SnO 2 based sensor reached the detection of 25-200 ppm CO gas [45]. Furthermore, PANI: Co 3 O 4 based sensor detected 5075 ppm CO gas with 40 s response time and 90 s recovery time [46] and 200-6000 ppm CO gas detection with 180 s response time and 200 s recovery time with PANI: Au nanoparticles-based sensor was reported [47].
On the other hand, Kumara et al. [48] reported 25 ppm NH 3 gas detection with 213 s response time at room temperature by PANI sensors. For detecting NH 3 gas, PANI/TiO 2 nanocomposite-based sensors have been prepared by several research group [49][50][51][52][53][54]. The produced sensors generally detected NH 3 gas in the range of 20 ppm − 45 ppb above the room temperature in 1-2 min. Wu et al. [55] investigated the effect of graphene dispersion on NH 3 gas sensing characteristics of polyaniline gas sensor. The sensor Although there have been inspired theoritical studies about the adsorption of CO molecules to the borophene surface [58][59][60] in the last two years, there are no experiment reports. Also, there have been only a few reports on the detection of NH 3 gas using borophene based sensors over the last four years [61][62][63][64][65][66]. As far as there are no experimental reports available based on borophene nanosheet as an ammonia sensor, theoretical reports revealed that borophene nanosheet is a prominent material for detecting NH 3 molecules. In the present work, inspired from the theoretical studies, borophene and PANI: Borophene sensors have been produced to investigate the CO and NH 3 gas detection at room temperature.

Experimental Details
For use in the fabrication of Borophene and PANI: Borophene sensors, the borophene nanosheets were produced by ultrasonic sonication. 100 mg boron with a particle size of 1.5 µm, purchased from Nanogra , was mixed with 100 mL of Dimethylformamide (DMF) purchased from Merck and then sonicated at 200 W for 3 h in a cabin with controlled Nitrogen (N 2 ) ow. Ambient controlled process is extremely critical to suppress oxidization and contamination. In order to obtain well-ordered borophene nanosheets, rst exfoliated borophene was obtained after centrifugations for 15 min. at 5000 rpm and 15 min. at 12000 rpm. Then, borophene was carefully collected and dryed in a vacuum ambient for 4 h at 50 0 C to obtain borophene powder. The structural, chemical, and morphological analysis of borophene were performed by High Resolution Transmission Electron Microscopy (HRTEM), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FTIR) Spectroscopy.
As a next step, 1 mg of borophene powder was dispersed in 1 mL DMF. At the same time, 1 mg of borophene powder and 1 mg of PANI powder were mixed very slowly in 2 ml N-Methylpyrrolidone (NMP) on the magnetic stirrer for 15 min at room temperature. PANI and NMP were purchased from Sigma-Aldrich. In order to fabricate the sensors, Borophene and PANI: Borophene were spin coated on Interdigital Transdusers (IDTs) at 500 rpm. The produced sensors were xed to the sensor test system, and then the surfaces of the sensors were subjected to a ow of dry to prevent the ambient humidity from affecting the sensor performance. The total ow rate was xed at 200 sscm to reach a constant baseline. The realtime resistances of conductivity type sensors were measured using the Keithley 2700 Data Acquisition System at room temperature and recorded by a computer with corresponding data acquisition hardware and software. The sensor measurements were carried out in the gas concentration range of 6-30 ppm for CO and 50 ppb-1.5 ppm for NH 3 . The CO and NH 3 gas concentrations were controlled by mass ow controllers (MFCs).   [69]. According to the FTIR results, it was con rmed that some peaks characteristic of the electrostatic interaction, along with other peaks, suggesting the partial decreasing the intensity of the peak of C = O and O-H groups.

Results And Discussion
When C and O atoms are adsorbed by borophene nanosheets, a charge transfer mechanism between CO and borophene occurs, C and O atoms gain electrons and therefore borophene nanosheets lose electrons. Therefore, CO gas is a charge acceptor when faced with borophene [58][59][60]. In this regard, adsorption of CO molecules on borophene has increased the electrical resistance of borophene. Figure 3 depicts CO gas detection results and sensitivity plots of the produced borophene, PANI, and PANI: Borophene based sensors. As shown in Fig. 3-a, the borophene-based sensor has exhibited a linear increase in the resistance change with the increase in CO gas in the 6-30 ppm concentration range. The borophene sensor detected 6 ppm CO gas with 30 s response time, and 40 s recovery time at room temperature.
When the Borophene nanosheets were added to PANI, Borophene nanosheets were decorated around the surface of PANI, providing PANI with higher surface area, and therefore more active sites for the adsorption of the CO gas molecules. PANI: Borophene sensor detected CO gas at room temperature as a result of the adsorption of CO molecules by the surface of the borophene nanosheets, charge transfer mechanism occured between CO and PANI: Borophene, positive charges transfered to PANI chain. In this regard, adsorption of CO molecules on PANI: Borophene increased the electrical resistance of PANI: Borophene. PANI: Borophene sensor has also showed a linear increase in the resistance change with increase in 6-30 ppm concentration of CO. PANI: Borophene based sensor detected 6 ppm CO gas with 300 s response time, and 320 s recovery time at room temperature. As seen in the graph of response to gas concentration gven in Fig. 3-b, PANI: Borophene sensor has higher sensitivity for CO detection than PANI and borophene sensors since borophene nanosheets enhance the CO detection performance of PANI. Moreover, compared to the previously reported CO sensors, both the borophene and PANI: Borophene sensors produced in this study detected low-level of CO gas with short response/recovery times at room temperature [43][44][45][46][47].
When NH 3 molecules adsorbed by borophene nanosheets, a charge transfer mechanism between NH 3 molecules and borophene occurs, NH 3 molecules lose electrons and therefore borophene nanosheets gain electrons. Therefore, NH 3 gas behaves as a charge donor when it meets with borophene [60,61]. ppm. The responses of the PANI sensor and PANI: Borophene sensor to the changes in the NH 3 concentrations are represented by the increase in the output resistance ( Fig. 4-b). Likewise, when the borophene nanosheets were added to PANI, a higher surface area and hence more active adsorption sites were provided to PANI for detection of NH 3 gas molecules. While NH 3 molecules adsorbing on the PANI: Borophene, positive charges transfered to PANI, and N + H bonds are formed. Therefore, electrical conductivity of the borophene nanosheets decreased. PANI: Borophene based sensor detected 50 ppb NH 3 gas with 100 s response time, and 120 s recovery time at room temperature. As shown in Fig. 4-c and d, PANI: Borophene sensor has higher sensitivity than PANI and borophene sensors since borophene improves the NH 3 detection performance of PANI. In comparison with the previous reported NH 3 sensors, the prepared Borophene sensor detected trace-level NH 3 gas at room temperature [48][49][50][51][52][53][54][55][56][57].

Conclusion
Detection of low-level CO and NH 3 gases is a critical issue for health and the environment. According to the results, borophene sensor detected 6 ppm CO gas with 30 s response time and 40 s recovery time, and 50 ppb NH 3 gas with 40 s response time and 60 s recovery time at room temperature. On the other hand, PANI: Borophene sensor detected 6 ppm CO gas with 300 s response time and 320 s recovery time and 50 ppb NH 3 gas with 100 s response time and 120 s recovery time at room temperature. Borophene improved CO and NH 3 gas detection performance of PANI as the nanostructures provide high surface area in the sensing material, allowing gas molecules to be adsorbed from much more adsorption sites on the sensing material. In the view of the results, it can be concluded that PANI: Borophene sensor has higher sensitivity than PANI and borophene sensors for both CO and NH 3 gas detection and thus it has a great potential for future applications in high-performance CO and NH 3 gas sensors.