Etude et caracterisation d’un capteur chimique a base de couches minces

Abstract

"Volatile organic compounds (VOCs) are found in ambient air due to human activities and natural sources. High concentration exposure to some VOCs over a short or long term may cause diseases or serious irreversible effect. VOCs do not only harm the human and survival health but also pollute the environment due to their contribution to stratospheric ozone depletion, tropospheric photochemical ozone formation and enhancement of the global green house effect. The identification and monitoring of VOCs have become serious tasks in many countries of the world and are important for the early control of environmental pollution. The analysis of gases represents one of the main objectives of current research in the sensor field. In this context, one widely investigated device is the quartz crystal microbalance (QCM), in which an increase of mass into the crystal electrode due to sorption of the analyte molecules can generates a shift of the resonance frequency (Δf) according to the well known Sauerbrey equation.. In this work, Hexamethyldisiloxane (HMDSO) thin films coated quartz crystal microbalance (QCM) electrodes have been characterized for the detection of volatile organic compounds (VOCs). The sensitive coatings were plasma polymerized in pure vapor of HMDSO and HMDSO/O2 mixture. The sensor sensitivity was evaluated by monitoring the frequency shift (Δf) of the coated QCM electrode exposed to different concentrations of VOCs vapors, such as ethanol, methanol, benzene and chloroform. The isotherm response characteristics showed good reproducibility and reversibility. For all types of analyte, Δf were found to be linearly correlated with the concentration of VOCs vapor. It was shown that it is possible to tune the chemical affinity of the sensor by changing the oxygen ratio in the deposition gas mixture. Contact angle measurements (CA), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to study surface wettability, chemical composition and surface morphology of the coated QCM electrodes. ATR-FTIR analysis showed the presence of methyl groups around 840 cm−1 due to Si–(CH3)3 rocking vibration making the elaborated sensor surface hydrophobic. When the coating is performed in HMDSO/O2 mixture, AFM and SEM images showed an increase in the effective specific surface area of the sensor due to the increase in surface roughness. Surface morphology combined with chemical composition significantly affects the sensitivity of the QCM-based sensor"