| Quartz crystal microbalance(QCM), as a kind of mass-sensitive electrochemical sensor with a detection limit in nanogram levels, offers the advantages of label-free and real-time for monitoring the mass change in the surface of the electrode. It has been successfully demonstrated the applications of the QCM in electrochemistry, solution chemistry and biosensors. The key sensing part of QCM is an AT-cut piezoelectric quartz crystal wafer. Generally, two “key-shaped” Au or Ag film electrodes are deposited on the surface of the quartz crystal wafer. High-frequency electronic field is applied to oscillate QCM at its fundamental frequency. The frequency stability and lifetime of a QCM depend greatly on the chemical stability of the film electrodes. If the electrode is eroded in a strong corrosive condition, the baseline of the QCM will be drifted seriously, resulting in poor reliability in mass change for process investigation. On the other hand, the Sauerbrey equation is only valid for a rigid film. If the rigidity of the film is reduced, there is a large error for the calculation in mass change.In this thesis, an electrodeless quartz crystal microbalance(EL-QCM) in gaseous phase is designed and applied in monitoring the adsorption process in a strong corrosive condition. As the excitation electrodes is insulate from the corrosive gaseous phase, the influence of electrode corrosion on the estimation of mass change is eliminated efficiently. By combined with absorbance technique, the concentration of the adsorbate and the amount adsorbed are measured in real time simultaneously. By using an impedance analysis method, the responses of QCM in overtone models were measured to estimate the amount adsorbed and the rigidity of the film. Details are listed below. 1. An EL-QCM combinated with absorbance spectroscopy in monitoring the adsorption of I2 on metal-organic frameworks filmsMetal-organic frameworks(MOFs), a particular class of ordered porous solids, hold promise to solve many key challenging societal needs, including hydrogen storage, carbon dioxide capture, renewable catalysts, controlled drug delivery, and so on. Usually, MOFs are excellent sorbents due to their suitable pore aperture size, large specific surface area, and high chemical and thermal stability.Iodine is a solid with obvious sublimation behavior and can be adsorbed on MOFs materials. However, iodine has strong corrosive to the electrode on QCM. A normal QCM is unsituable to investigate the adsorption of I2 by the limition in measurement error and lifetime of the sensor. On the other hand, the concentration of I2 vapor is needed for the investigation. To study the kinetical processes of I2 onto MOFs films, the combination of an EL-QCM with absorbance technique was desighned. Three MOFs films, including MIL-101 and ZIF-67 and ZIF-8, were chosen as the models. The amount of I2 adsorbed and the concentration of I2 vapor were monitored in real time by the frequency shifts of EL-QCM and the change in absorbance, respectively. It was shown that ZIF-67 and ZIF-8 films have high adsorption capacity of 4.41 and 4.81 g/g to I2, which is obviously higher than that on MIL-101(0.63 g/g) and activated carbon(1.04 g/g). On the other hand, 2-methylimidazole(mIM), the organic ligand in the structural unit in ZIF-8 and ZIF-67, exhibits a adsorption capacity of 3.25 g/g, playing an important role for I2 adsorption on ZIF-67 and ZIF-8 films. For adsorption on MIL-101, the correltion between the amount of I2 adsorbed and the concentration of I2 vapor at equilibrium obeys the Henry's law. And most of the adsorption is in a reversible state. The amounts of I2 adsorbed on ZIF-6 and ZIF-8 increase with increasing concentration of I2 vapor at accelerated slopes. The amount of I2 adsorbed increases obviously when the concentration of I2 approaches its saturation vapor, suggesting the coagulation of I2 vapor onto ZIF films. Combining the concentration and amount adsorbed curves, adsorption kinetic parameters can be estimated. This method offers the advantage to measure simultaneously the changes in both of the amount adsorbed and the vapor concentration of a corrosive adsorbate. 2. Overtone response of an EL-QCM and monitor the adsorption of bromine vapor on ZIF-8 filmIn this work, the overtone response of an EL-QCM in gaseous phase was investigated by an impedance analysis method. It was shown that the influence of the electrode gap on the resonant frequency reduces with increasing overtone number. Although reduced with increasing overtone, the effective quality factor is still high enough even at 7th overtone(3.8?104) for the stable resonance of the EL-QCM. The EL-QCM was applied to monitor the adsorption processes related to corrosive gas, modeling by bromine vapor onto ZIF-8 film. It was shown that the 2-methylimidazole unit in ZIF-8 plays important role for Br2 adsorption, preponderating in chemical adsorption. The adsorption kinetic data are in agreement with the pseudo-second-order model. The adsorption isotherms are well fitted by the Langmuir model with the saturation adsorption capacity of 3241 mg/g on ZIF-8 with thickness of 0.699 mm(25?). The correlations of the frequency shifts at the overtone(n=1?3?5) can be employed to judge the rigidity of the film in an adsorption process. 3.Overtone response of QCM in adsorption of VOCs on ZIF-8 filmMOFs have been attracted increasing interest in material science. They are promising materials in removal and recovery volatile organic compounds(VOCs). ZIF-8, as a prototypical MOFs, is an excellent sorbent for VOCs because of its suitable pore aperture size, large specific surface area, and high chemical and thermal stability. By using an impedance analysis method, the overtone responses(n=1,3,5,7)of QCM during the adsorption process of VOCs on ZIF-8 film were investigated. It was shown that the rigidity of ZIF-8 film is nearly unchanged in the intial adsorption stage, indicating by the slight change in motional resistance of the QCM. Hence, the amount adsorbed can be calculated reliably according to the Sauerbrey equation. In the late adsorption stage, with increasing amount of VOCs adsorbed, the the rigidity of loading film is reduced and results in an error in mass estimation by Sauerbrey equation. |
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