Abstract
The application of nanomaterials in various scientific sectors is becoming more recognizable with increased research work. The recognition of these nanomaterials is seen in the application of solid-state gas sensors which are portable, efficient, fast, and sensitive. These characteristics permit them to be more advantageous compared to the time- and energy-consuming conventional methods such as flame ionization detectors, mass-selective detectors, and gas chromatography (GC). Solid-state gas sensors have been fabricated using semiconducting metal oxides (SMOs) as sensing layers, however, SMO-based gas sensors are not stable operate at room temperatures, therefore, they are mostly suitable for high-temperature performance. This poses a need for room temperature solid state gas sensors which were successfully fabricated in this study using carbonaceous nanomaterials, SMOs and polymer-based composites. To further the study, the sensing mechanism of room temperature based solid-state gas sensors was extensively investigated. Candle soot CNPs, SMOs [(manganese dioxide (MnO2), titanium dioxide (TiO2), tin dioxide (SnO2), and zinc oxide (ZnO)] and commercial polymers [(polyvinylpyrrolidone (PVP), cellulose acetate (CA), and polyethylene oxide (PEO)] were utilized to prepare various nanocomposites. The solid-state gas sensor based on CNPs: TiO2: PVP (2:1:3) was used for the detection of diethylamine, butyraldehyde, and isobutyrophenone. Various techniques (e.g Transmission Electron Microscopy, Powder X-ray diffraction and others) were used to identify the spherical natures of the CNPs and TiO2, and the anatase phase of the TiO2 was also identified. This sensor exhibited the highest resistive response of 0.07 Ω ppm-1 towards diethylamine and the highest impedance response of 0.14 Ω ppm-1 towards butyraldehyde. Furthermore, the sensor exhibited the fastest response time of 145 s for butyraldehyde and the fastest recovery time of 130 s for diethylamine.
A comparative study between MnO2 nanorods and SnO2 nanoparticles was carried out where two sensors; A- CNPs: MnO2: PVP (1:2:3) and B- CNPs: SnO2: PVP (2:1:3) were successfully fabricated to detect diethylamine, butyraldehyde, and isobutyrophenone. Sensor A exhibited the best resistive and impedance response of 0.045 Ω ppm-1 and 0.027 Ω ppm-1, respectively. Sensor A further exhibited the fastest impedance and resistive response time of 148 s and 85 s, respectively for diethylamine with exceptional results for repeatability test towards the same analyte. Sensor A was further investigated to detect 1,2 dichlorobenzene and dichloromethane where it exhibited resistive and impedance responses of 0.007 Ω ppm-1 and 0.004 Ω ppm-1,
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respectively for 1.2 dichlorobenzene. Due to sensor As’ good performance, it was then used to investigate the sensing mechanisms towards diethylamine, butyraldehyde, and isobutyrophenone in a subsequent study.
The mechanisms were investigated with the use of in situ FTIR-online combined with an LCR meter and Gas chromatography-HRTOF-MS. The study showed that during the exposure of the sensing layer to the gas analytes, a decomposition of the gas analytes took place due to dehydrogenation thus forming CO2 as a by-product. The formation of CO2 was confirmed by FTIR analysis and there were increasing IR bands at 668 cm-1 for each analyte. Although that was the case, butyraldehyde exhibited significantly low-intensity IR bands compared to diethylamine and isobutyrophenone which both exhibited high-intensity IR bands. The chromatograms of diethylamine and isobutyrophenone showed that the analytes gases exhibited fragment ions confirming that they went through a stable decomposition process after the FTIR analysis. This investigation assisted to also discover that the gas analytes undergone double decomposition process where intermediate molecules were produced thus revealing a two-step mechanism process. Diethylamine chromatograms, however, did not exhibit any evidence of expected fragmentation nor intermediate fragments after the FTIR analysis. A subsequent short study was done to investigate the effects of varying concentrations of ZnO on a sensor using sensor A - CNPs: ZnO: PEO (1:1:2), sensor B- CNPs: ZnO: PEO (1:2:2), and sensor C - CNPs: ZnO: PEO (1:3:2) for cycloheptylamine and acetone. The study showed that sensor B responded to both analytes while sensor A and C did not respond. This discovery showed that varying concentrations of SMO in the nanocomposite can either activate or deactivate the sensor. Furthermore, the ZnO on sensor B was replaced with nickel oxide (NiO) and the NiO based sensor was unresponsive thus further proving the efficiency of ZnO.
The collective studies of this research shows that candle soot CNPs, SMOs and polymers are compatible for fabricating high performing room-temperature gas sensors which are potentially marketable and applicable in various applications such as indoor air quality control, monitoring of gas leakages in buildings and medical diagnosis purposes amongst many. The study further poses successful fabrication of portable, low energy consuming, sensitive, and reproducible solid-state gas sensors as well as a novel sensing mechanism study.
Keywords: semiconducting metal oxides, volatile organic compounds, solid-state gas sensor, polymers, candle soot carbon nanoparticles, impedance, resistance.