With the rapid development of global industry, air pollution is now a major problem. Ammonia (NH3) is a common toxic and harmful gas that is used in industrial production. Excessive intake of NH3 by the human body can lead to lung swelling and even death. Currently, NH3 detection is mainly conducted using electrochemical and optical methods. However, traditional electrochemical detection methods cannot realize real-time online monitoring. Optical detection can be classified into three types: absorption-spectrum, evanescent-wave, and refractive index variation. The absorption-spectrum type is easily affected by other gases in the same absorption band, whereas the evanescent-wave type has high requirements for achieving a precise tapering process. The refractive index variation type is based on variations in the refractive index of the sensitive film derived from NH3. In recent years, sensing technology that combines optical fiber sensors and functional films for realizing specific gas detection has become a research hotspot. Because zinc oxide (ZnO) has strong adsorption characteristics for NH3, the refractive index of ZnO varies. A no-core fiber is an optical waveguide with a unique structure, and its mode field can directly perceive changes in the external environment. Therefore, a new fiber gas sensor that combines a no-core fiber and ZnO film is studied in depth, which significantly promotes the rapid and accurate measurement of NH3 in atmospheric pollutants.
Based on mode transmission theory, the mode characteristics of a ZnO-coated no-core fiber and the sensing characteristics of a singlemode-no-core-singlemode (SNS) structure were analyzed using MATLAB software. First, the mode field distribution in the no-core fiber coated with ZnO was studied, and variations of the refractive index of the ZnO film with mode excitation coefficient and self-image length were discussed. Second, the relationship between the refractive index of the ZnO film and resonant wavelength of the interference spectrum was established. The NH3 sensing system mainly includes a light source, glass chamber, and spectrometer. Finally, in an experiment, different volume fractions of NH3 were passed into the glass chamber to ensure the sensor made full contact with NH3. When ZnO absorbs NH3, its refractive index changes, and thus the resonant wavelength shifts. In our study, NH3 volume fraction was detected by establishing the relationship between the NH3 volume fraction and the shift in resonant wavelength. The detection limit of the sensor was calculated using the Hubaux-Vos method, and the response time and recovery time were tested. In addition, the ambient temperature was controlled using a temperature-control box to study the effect of temperature on NH3 sensing.
Changes in the refractive index of a ZnO film affect to some degree the mode excitation coefficient and self-image length in a no-core fiber coated with a ZnO film layer. The sensitivity of the sensor increases with increasing film thickness. In our study, the sensitivities of the SNS sensor with thicknesses of 60 nm and 130 nm are 11.8 and 28.6 nm/RIU, respectively (Fig.6). The prepared ZnO film was characterized using scanning electron microscopy (SEM), and the results show good compactness and uniformity (Fig.7). At 16.5 ℃, the resonant wavelength blue-shifts with an increase in NH3 volume fraction. The sensitivities of the SNS sensor at ZnO film thicknesses of 60 and 130 nm are 17.96×106 and 17.86×106 pm, respectively, which is mainly caused by the effects of ZnO on NH3 adsorption saturation (Fig.9). The average sensitivity of the SNS sensor under NH3 detection coated with a ZnO film at a thickness of 60 nm is 16.87×106 pm, and the detection limit is 6.6×10-6 (Fig.10). The effect of time on the detection limit is 0.026×10-6 d-1, and the response and recovery time are 70 and 90 s, respectively (Fig.11). The sensitivities of the sensor under NH3 detection are 17.96×106, 15.38×106, 14.07×106, 9.62×106, and 7.57×106 pm at 16.5, 25, 30, 45.2, and 56 ℃, respectively (Fig.12). The effect of temperature on the detection limit is 0.0237×10-6 ℃-1. The detection sensitivity of NH3 decreases with an increase in temperature, which derives from the fact that the increase in the surface potential barrier of the ZnO film hinders the flow of electrons. In practical sensing applications, measuring the temperature of the environment is first required, and then the volume fraction of NH3 can be calculated based on the data interpolation.
A high-sensitivity NH3 sensor based on a ZnO-coated SNS structure was proposed in this study. SNS sensors with different ZnO film thicknesses were simulated theoretically. The results show that the mode field distribution trend of the no-core fiber coated with ZnO is consistent. When the refractive index of the ZnO film changes from 1.929 to 1.889, it has little effect on the excitation coefficient and self-image length. In general, the sensitivity of an SNS sensor increases with an increase in film thickness. In this study, an SNS sensor with ZnO film thicknesses of 60 and 130 nm was prepared by atomic layer deposition (ALD), with results showing sensitivities of 11.8 and 28.6 nm/RIU, respectively. The results show that the sensitivities to NH3 are essentially the same under the two film thicknesses (17.96×106 and 17.86×106 pm, respectively), which mainly derives from the effects of ZnO on NH3 adsorption saturation. The average sensitivity, detection limit, and response and recovery time are 16.87×106 pm, 6.6×10-6, and 70 and 90 s, respectively. Finally, an in-depth analysis of the temperature characteristics of the sensitivity of the sensor demonstrated that NH3 detection sensitivity decreases with an increase in temperature. At 56 ℃, the sensitivity decreases to 7.57×106 pm. The effects of temperature and time on the detection limit of NH3 are 0.0237×10-6 ℃-1 and 0.026×10-6 d-1, respectively.
