In2O3–ZnO nanotubes for the sensitive and selective detection of ppb-level NO2 under UV irradiation at room temperature

MoS 2 –ZnO p–n heterojunctions are employed to sensitively detect trace NO 2 gas under UV illumination at room temperature. Nitrogen dioxide (NO 2 ) is a hazardous gas species that could impose a great threat on environmental protection and human health even at very low doses. Thus, it is of great importance to selectively detect trace NO 2 gas below the ppm level. This has been a serious challenge so far, especially in the presence of other interfering gases. Herein, we report ultrasensitive, room-temperature and UV light-assisted NO 2 gas sensing based on few-layer MoS 2 nanosheet/ZnO nanowire composites serving as the sensing layer. A series of characterization techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), Raman, energy-dispersive X-ray spectroscopy (EDS), UV-Visual and ultraviolet photoelectron spectroscopy (UPS), were employed to explore the componential and structural properties of the obtained sensitive materials. Initially, the as-prepared MoS 2 /ZnO composites showed a tiny response (40%) and an incomplete recovery to 10 ppm NO 2 gas in dark condition. While under UV illumination, the sensing response attained 8.4 and 188, toward 50 ppb and 200 ppb NO 2 with a full recovery. Meanwhile, a sensitivity of 0.93 ppb −1 , a detection limit of 50 ppq (10 −15 ), and excellent repeatability and selectivity were also achieved. The experimental results were better than most previous work on NO 2 detection to the best of our knowledge. Two main aspects are responsible for the outstanding performance. One is that a mass of photo-excited electron–hole pairs participated in the reaction with NO 2 molecules under UV illumination. The other lies in numerous p–n MoS 2 /ZnO nanojunctions, favorable for the extension of the depletion region and the separation of the charge carrier. Additionally, long-term stability, as well as the effect of film thickness and carrier gas species on sensor performance, were simply investigated. We combined p–n nanojunctions with the UV illumination method, providing an alternative strategy to realize room-temperature operation and high sensitivity in the field of gas sensors.

Summary Background A link between exposure to the air pollutant nitrogen dioxide (NO2) and respiratory disease has been suggested. Viral infections are the major cause of asthma exacerbations. We aimed to assess whether there is a relation between NO2 exposure and the severity of asthma exacerbations caused by proven respiratory viral infections in children. Methods A cohort of 114 asthmatic children aged between 8 and 11 years recorded daily upper and lower respiratory-tract symptoms, peak expiratory flow (PEF), and measured personal NO2 exposures every week for up to 13 months. We took nasal aspirates during reported episodes of upper respiratory-tract illness and tested for infection by common respiratory viruses and atypical bacteria with RT-PCR assays. We used generalised estimating equations to assess the relation between low ( 14 μg/m3) tertiles of NO2 exposure in the week before or after upper respiratory-tract infection and the severity of asthma exacerbation in the week after the start of an infection. Findings One or more viruses were detected in 78% of reported infection episodes, and the medians of NO2 exposure were 5 (IQR 3·6–6·3), 10 (8·7–12·0), and 21 μg/m3 (16·8–42·9) for low, medium, and high tertiles, respectively. There were significant increases in the severity of lower respiratory-tract symptom scores across the three tertiles (0·6 for all viruses [p=0·05] and >2 for respiratory syncytial virus [p=0·01]) and a reduction in PEF of more than 12 L/min for picornavirus (p=0·04) for high compared with low NO2 exposure before the start of the virus-induced exacerbation. Interpretation High exposure to NO2 in the week before the start of a respiratory viral infection, and at levels within current air quality standards, is associated with an increase in the severity of a resulting asthma exacerbation.