Heavily Doped Zinc Oxide Thin Films for Nitrogen Dioxide Optical Gas Sensing

Abstract

Introduction In this work heavily n-doped ZnO doped with gallium, germanium, aluminum and silicon are studied for NO2 detection, using the sensibility of localized surface plasmonic resonance in the near infrared to the change of environmental conditions. Trivalent cations (Al3+ and Ga3+) or tetravalent cations (Si4+ and Ge4+) are used as dopant agents with nominal concentration of 20%, following a non-aqueous heat up colloidal synthesis [1]. No presence of other phases like metal, dopant oxides, and binary dopant-ZnO oxides is detected by XRD analysis. It is proved by spectrophotometric analysis that introduction of doping agent causes the formation of a surface plasmon resonance in the near infrared, tunable changing dopant nature. We have demonstrated that strong sensitivity of the plasmon resonance to chemical variations at the surface of the nanocrystal make this category of materials very suitable for detection of 1000 ppm NO2 balance in air, finding the best compromise between sensitivity, signal to noise ratio and stability of the sensor in 20% germanium doped ZnO. Materials and Methods Synthesis: Doped ZnO NPs were synthetized through a previously reported non-aqueous heat up colloidal method [2]. Film preparation: Doped zinc oxide thin films were prepared by spin coating technique on fused silica substrates. The films were thermal treated in a tube furnace filled with a 5% H2−95% Ar gas mixture at 450°C for 2h [3]. The final thickness of the films were of 500-600 nm. Film characterization: Nanoparticles morphology and dimension were observed by transmission electron microscope (TEM). Morphology of the films and an estimation of doping efficiency were investigated using a scanning electron microscope (SEM). Crystalline structure of thin films was determined by grazing incidence X-ray diffraction (XRD). Gas sensing measurement were performed in a Harrick gas flow cell connected to a gas handling system via flow meters and coupled with a Jasco V-570 spectrophotometer. Optical absorption was collected using the Jasco V-570 spectrophotometer in the 350-2500 nm wavelength range at operative temperature (OT) set at 150°C in 0.4 L/min flow of synthetic air or of target gas. In this work, 1000 ppm of NO2 in dry air was used as target gas. Optical Absorption Change (OAC = Absgas - Absair) was evaluated to identify the suitable wavelength to perform the dynamic sensing. Results and Conclusions In general, at equilibrium in dry air condition, atmospheric molecular oxygen is adsorbed onto sensor’s surface, due to the interaction with oxygen vacancies, and ionizing. In this work sensing test are performed at 150°C, at this temperature O2¯ is considered stable. If nitrogen dioxide is introduced, equilibrium changes due to the gas’ high oxidizing attitude, which increase the concentration of adsorbed molecules and cause an increment of electrons’ extraction. Therefore, the thickness of Electron Depletion Layer as well as the resistance of material increase. Due to this phenomenon, sensor response can be defined in several different ways as function of differences in measurement criteria. In particular, changing in resistance is most commonly monitoring parameter used for chemoresistive gas sensors, but besides the obvious electrical effects, it can be observed from formula (1) that decreasing in carrier density the angular frequency of LSPR maximum peak’s position decrease and, as consequence, its wavelength increase, obtaining a red-shift of the plasmon position.In Figure 1 are presented the OAC of the various thin films tested and two cycles of Air/NO2 exposition. We found that all the prepared samples are able to detect 1000 ppm of NO2 in dry air but with different dynamic characteristic. In particular, a trade-off between sensibility and signal to noise ratio versus stability was highlighted, which depend from the amount of free charge generated by the dopant agent. In fact, GZO thin film exhibit higher sensitivity and signal to noise ratio, but lower recovering of the baseline due to progressive irreversible degradation of the sensor. On the contrary, SZO has approximately total recovering of baseline but the signal difference between air and gas exposition is lower. GeZO exhibit intermediate proprieties with both good sensitivity and baseline recovering, while AZO present the worst sensing behaviors. The use of colloidal approach to the synthesis overcome some limitations of the sol-gel method, making this process competitive by an industrial point of view compared to other common investigated deposition technique, like magnetron sputtering. In the same time allow to obtain nanoparticles easily dispersible and depositable at room temperature and in ambient condition.Future works will investigate cross-sensitivity and detection limit of so obtained NO2 sensors. Analyzed sensing proprieties coupled with optical transparence in visible light, paving the way to the fabrication of new invisible and highly efficient sensors for detection of hazardous gases.