Hierarchical Cobalt Fluoride Hydroxide Nanomaterials for Highly Sensitive Acetone Gas Sensor

Abstract

Introduction The needs of simple diagnosis on diabetes from exhaled human breath alarms us to develop highly sensitive low concentration acetone gas sensor [1]. with Co-based materials have been recognized as promising candidates for catalysts [2]. However, the usage of Co-based materials was mostly limited to cobalt oxides for application in gas sensor [3-4]. In this study, for the first time, we examined the suitability of novel 2-d hierarchical cobalt fluoride hydroxide (Co(OH)F) nanomaterial for gas sensor application. The fluorine in Co(OH)F as an additional catalyst could enable to enhance sensing ability. Moreover, we used SiO2 sphere template to fabricate nanosheets-based unique morphology which has high surface-to-volume ratio, resulting in improvement of sensing ability. The sensor-based on the Co(OH)F nanomaterials exhibited good sensitivity and selectivity toward acetone gas. We also suggested mechanisms behind the good sensing performances. Method Silicon dioxide (SiO2) spheres were used as templates for synthesizing Co(OH)F on surfaces of them. Cobalt(II) chloride (CoCl2), ammonium fluoride (NH4F), and urea(CO(NH2)2) were added as precursors to form Co(OH)F nanostructures in hydrothermal reactor and heated at 120 °C for 10 h to get rid of SiO2 sphere template. To fabricate interdigitated electrodes, platinum was sputtered and patterned using photolithography method on Si/SiO2 substrate with 10 μm distance. Dispersed Co(OH)F powders in water were drop-coated on the interdigitated electrodes and dried at 100 °C. Additional post-annealing at 350 °C for 2 h could transform the Co(OH)F to Co3O4 with some amount of fluorine inside. We tested sensing performances in our homemade sensing system equipped with mass flow controllers (MFCs), cylinders of various gases, a multimeter, and a power supply. Responses were determined by Rg/Ra, where Rg is resistance with gas input, and Ra is resistance in air, respectively. Results and Conclusions Figure 1 (a) demonstrates a SEM image of the hierarchical 2-d Co(OH)F nanostructures with super thin sheets (~10 nm thick). With the high surface-to-volume ratio owing to the thickness of sheets and abundant voids inside, and abundant fluorine acting as a catalyst, a sensor based on this material could show high response of 3.13 toward 1.6 ppm of acetone gas as shown in Figure 1 (b). We also tested the gas responses as functions of operating temperature in the range of 25 °C through 250 °C to acetone gas concentration of 13 ppb to 1.6 ppm, gas species such as acetone, toluene, carbon monoxide, hydrogen, and nitrogen dioxide to figure out the selectivity, and time elapse for around a month. The fabricated gas sensor exhibited good sensitivity and selectivity. We also could obtain fluorine-doped cobalt oxide-based gas sensor with good long-term stability through post-annealing process. In addition, investigating the effects of fluorine amounts on sensor performances, we could figure out the reason of high response of the sensors. We could expect this pioneering work on this adoption of novel 2-d hierarchical Co(OH)F nanostructures for the acetone gas sensor could offer more chances for developing high performance exhaled gas sensor applications for the near future. Acknowledgement J.-S. Park was partially supported by the Project (No. P0006858) of KIAT and MOTIE in Korea.