Photoactivated materials and sensors for NO₂ monitoring

Abstract

This review presents the fundamentals of photoactivated gas sensing, materials, and enabling technologies for sensing ppb concentrations of NO2. Literature photoactivated gas sensors show competitive detection limits and form factors.

Contact resistances between ZnO and Ti, Al, Zn, and Bi: correlation with the density of states at Fermi energies

“…Zinc oxide, due to its direct-and wide-bandgap, relatively high electron mobility, and stable physicochemical nature, is the most widely used metal oxide semiconductor in electronics. 1,2 It has been widely utilized for the fabrication of optoelectronic devices, [3][4][5] varieties of sensors, 6,7 gas sensors, 8,9 memristors, 10 power electronic devices, 11 and thermoelectric segments. [12][13][14] Good quality metallic contacts are needed to connect such devices to the outside circuitry, while achieving low resistance electrical contacts with the thin film semiconductors is still a major challenge.…”

Decoupling the Conflicting Roles of Photoactivation and Boosting Chemiresistor Response by Pulsed UV Light Modulation

“…More importantly, the R a of the chemiresistor can be elevated due to the consumption of (abundant) photoexcited e−h pairs, which also greatly facilitates a higher electrical response measured at pulsed UV-off stage. 7,13 Based on the aforementioned analysis, herein, using the widely adopted ZnO as the photoactivated sensing material, 4,5,22 we have compared the NO 2 response characteristics under both CU and PULM modes. As present ZnO did not undergo sophisticated morphology/defect/heterostructure engineering, 23−26 or noble metal nanoparticle decoration, 27 the response to 20 ppb NO 2 at RT (25 °C) under the optimized CU mode (365 nm UV light, 1.8 mW/cm 2 ) is only 1.9, while the response of the same device could be drastically boosted to 131.3 under PULM mode (UV-off envelope), which is comparable to or better than the state-of-the-art ZnO chemiresistors prepared via morphology/defect engineering or heterostructure coupling.…”

“…Semiconductor chemiresistors are promising in the construction of Internet-of-Things (IoT)-based sensor grids for prompt and precise indoor/outdoor air pollution monitoring − because of their high response, compact size, low cost, and good chip compatibility. Until now, various kinds of semiconductor nanomaterials, including metal oxides semiconductors (MOSs), two-dimensional (2D) graphene, phosphorene, and transition metal dichalcogenide (TMD) layered materials, have been tailored for detecting hazardous gases. − To trigger the gas/semiconductor interfacial charge interaction, and/or accelerate the response and recovery rate, (micro)­heating or UV photoactivation (with photon energy larger than the band gap of the semiconductor) is typically required. − Though UV photoactivation outperforms heating in terms of low power consumption, simple device structure (two terminal devices without integrating a microheater), and excellent compatibility with both rigid and flexible (polymer or paper) platforms, highly sensitive detection of trace (ppb-level) gases in the air background at room temperature (RT), even under the optimized photoactivation condition (wavelength and intensity), remains a big challenge. ,,− …”

“…Based on the aforementioned analysis, herein, using the widely adopted ZnO as the photoactivated sensing material, ,, we have compared the NO 2 response characteristics under both CU and PULM modes. As present ZnO did not undergo sophisticated morphology/defect/heterostructure engineering, − or noble metal nanoparticle decoration, the response to 20 ppb NO 2 at RT (25 °C) under the optimized CU mode (365 nm UV light, 1.8 mW/cm 2 ) is only 1.9, while the response of the same device could be drastically boosted to 131.3 under PULM mode (UV-off envelope), which is comparable to or better than the state-of-the-art ZnO chemiresistors prepared via morphology/defect engineering or heterostructure coupling. ,− The limit of detection (LoD) of the chemiresistor could be noticeably reduced from 2.6 ppb (CU mode) to 0.8 ppb (PULM mode), which is much lower than the annual average standard of 53 ppb recommended by the US Environmental Protection Agency (EPA) and raises the hope of a real application of semiconductor chemiresistor in ambient air quality monitoring, which has strict requirements on high response for the trace target gas .…”

Photoactivated Processes on the Surface of Metal Oxides and Gas Sensitivity to Oxygen

“…The disadvantages of semiconductor sensors can include the high power consumption associated with the need to heat the sensitive metal oxide layer to a temperature of 150–500 °C and the drift of the sensor parameters due to the influence of diffusion and recrystallization processes during operation at high temperatures [ 5 , 6 ]. A promising approach to lowering the operating temperature of semiconductor sensors, which was developed significantly in the last decade, is using UV or visible radiation for the activation of and enhancing the gas sensitivity [ 7 , 8 , 9 ].…”