Low-temperature operating ZnO-based NO2 sensors: a review

Abstract

Owing to its excellent physical and chemical properties, ZnO has been considered to be a promising material for development of NO₂ sensors with high sensitivity, and fast response and recovery. However, due to the low activity of ZnO at low temperature, most of the current work is focused on detecting NO₂ at high operating temperatures (200-500 °C), which will inevitably increase energy consumption and shorten the lifetime of sensors. In order to overcome these problems and improve the practicality of ZnO-based NO₂ sensors, it is necessary to systematically understand the effective strategies and mechanisms of low-temperature NO₂ detection of ZnO sensors. This paper reviews the latest research progress of low-temperature ZnO nanomaterial-based NO₂ gas sensors. Several efficient strategies to achieve low-temperature NO₂ detection (such as morphology modification, noble metal decoration, additive doping, heterostructure sensitization, two-dimensional material composites, and light activation) and corresponding sensing mechanisms (such as depletion layer theory, grain boundary barrier theory, spill-over effects) are also introduced. Finally, the challenges and future development directions of low-temperature ZnO-based NO₂ sensors are outlined.

Fig. 1. Schematic diagram of the sensing mechanism of the ZNW arrays to NO₂. This figure has been reproduced with permission from ref. 1, Elsevier, Copyright 2018 (License Number: 4923970909803).

Fig. 2. (a) Dynamic response curves and responses (b) of the sensors based on the ZnO-100, ZnO-150 and ZnO-300 samples to 1–50 ppm of NO₂ at room temperature; (c) fitting curves of response vs. NO₂ concentration in the range of 1–10 ppm of NO₂. (d) Schematic illustration of the changes in the conduction channel of the mesoporous ZnO sheets assembled by large and small nanoparticles upon exposure to NO₂. This figure has been reproduced with permission from ref. 74, Elsevier, Copyright 2018 (License Number: 4923980499171).

Fig. 3. Dynamic responses of the sensors based on pure (@150 °C) and Pd-ZNWs (@100 °C) upon exposure to NO₂ gas with various concentrations at (a) 30% RH and (b) 60% RH. The corresponding sensor responses as a function of NO₂ concentration at (c) 30% RH and (d) 60% RH. (e) Sensing mechanisms of Pd-ZNWs towards NO₂ and reducing gas. This figure has been reproduced with permission from ref. 91, Elsevier, Copyright 2019 (License Number: 4923990843354).

Fig. 4. (a–c) TEM and (d) HRTEM images of UL ZnO@Au heterojunction NRs, (inset showing the HRTEM image of Au). This figure has been reproduced with permission from ref. 93, Elsevier, Copyright 2017 (License Number: 4924050884483).

Fig. 5. Surface and cross-sectional FESEM images of undoped and Cu doped ZnO thin films, (a) undoped ZnO, (b) CZO-1, (c) CZO-2 and (d) CZO-3. This figure has been reproduced with permission from ref. 90, Elsevier, Copyright 2017 (License Number: 4924050884483). This figure has been reproduced with permission from ref. 95, Elsevier, Copyright 2019 (License Number: 4924060417377).

Fig. 6. (a) PL spectra of (i) undoped and Ce-doped ZnO photocatalysts prepared with various Ce precursor concentrations: (ii) 1, (iii) 2, and (iv) 3 mM. The inset is the enlarged PL spectra within the visible light range. XPS O 1s spectra of (b) undoped ZnO and (c) Ce-doped ZnO nanorods with Ce precursor concentration of 2 mM and rod growth time of 2 h. This figure has been reproduced with permission from ref. 96, Elsevier, Copyright 2014 (License Number: 4924070322626).

Fig. 7. Schematic diagram of the proposed mechanism of NO₂ sensing of ZnO/PANI heterojunctions. This figure has been reproduced with permission from ref. 99, RSC, Copyright 2016.

Fig. 8. (a) Real-time response of the B-SnO₂@ZnO HNSs sensor to NO₂ from 5 ppb to 10 ppm. (b) The linear relationship of the B-SnO₂@ZnO HNSs sensor to NO₂ in the range of 5 ppb to 800 ppb and 1 ppm to 10 ppm. (c) Selectivity comparison of pristine ZnO, SnO₂, and B-SnO₂@ZnO HNSs sensors for different target gases at 1 ppm. This figure has been reproduced with permission from ref. 115, Elsevier, Copyright 2018 (License Number: 4924071319628).

Fig. 9. Schematic illustration of the sensing mechanisms. This figure has been reproduced with permission from ref. 115, Elsevier, Copyright 2018 (License Number: 4924071319628).

Fig. 10. (a and b) Band diagram of hierarchical structure of gas adsorption and change of depletion layer for ZnO/CdO/rGO composite in air and NO₂. This figure has been reproduced with permission from ref. 127, Elsevier, Copyright 2020 (License Number: 4924090847198).

Fig. 11. Dynamic resistance changes of the sensors based on ZnO NW and V_O–ZnO NW to 1000 ppb of NO₂ in dark (a) and under UV illumination (b) and (c) summarized sensing response of the sensors; (d) influence of light intensity on response curves and responses (inset) of the V_O–ZnO NW-based sensor to 1000 ppb of NO₂. (e) Schematic illustration of the mechanism for synergistic effects of UV activation and surface V_O on the room-temperature NO₂ gas sensing performance of ZnO nanowires. This figure has been reproduced with permission from ref. 130, Elsevier, Copyright 2019 (License Number: 4924100231565).

Fig. 12. (a) Responses of the sensor based on the composite materials with the various molar ratio of ZnO to SnO₂ to 500 ppb NO₂ at room temperature with and without UV light irradiation. (b) Response of the ZS3 sensor vs. NO₂ concentration at room temperature with and without UV light irradiation. (c) Schematic diagram: the carriers transport with UV light stimulated, energy band structure and electron–hole pair separation in the ZnO/SnO₂ heterostructure in the area marked with a dashed circle. This figure has been reproduced with permission from ref. 131, Elsevier, Copyright 2012 (License Number: 4924100838478).

Fig. 13. (a) The SEM image of PG (b) the Raman spectra of PG (c) the SEM image and the inset of EDS of ZnO/PG hybrids (d) TEM image of ZnO/rGO hybrids. This figure has been reproduced with permission from ref. 132, Elsevier, Copyright 2018 (License Number: 4924110156230).

Fig. 14. (a) UV-vis spectra of pure ZnO and ZnO/g-C₃N₄ composites. (b) The responses of as-prepared samples to 7 ppm NO₂ under different wavelength light illumination. (c–e) The dynamic resistance curves of ZnO/g-C₃N₄-5 wt%, ZnO/g-C₃N₄-10 wt%, and ZnO/g-C₃N₄-15 wt% to different concentrations of NO₂ under 460 nm light illumination, respectively; (f) linearity of response curves of ZnO/g-C₃N₄ composites. This figure has been reproduced with permission from ref. 136, Elsevier, Copyright 2020 (License Number: 4924111472674).