Investigation Of Adsorption In The Surface Of Metal Organic Frameworks Films By A Piezoelectric Sensor

Quartz crystal microbalance (QCM) is a mass sensitive device being widely usedto monitor the interface processes in electrochemistry, solution chemistry andbiosensors. In this thesis，the kinetic processes of volatile organic compounds inmetal–organic frameworks (MOFs) film and solvent evaporation on different surfacesare investigated by QCM technique. Details are listed below.1. Monitoring the growth of zeolitic imidazolate frameworks film andadsorption kinetic processesMetal–organic frameworks (MOFs), a new class of hybrid porous solids, haveattracted increasing interest in material science. Among the reported MOFs,zeolitic-imidazolate frameworks (ZIFs) are a sub-class of MOFs, comprising hybridorganic–inorganic moieties and exhibiting regular crystalline lattices withwell-defined pore structures. They have permanent porosity and relatively highthermal and chemical stability, which make them attractive candidates for manyindustrial potential applications, including gas storage, catalyst, biomedicineadsorption and separation.The characterization of gas adsorption in porous materials is often performed bygravimetric or volumetric equipment. Their capital and operating costs are generallyhigh. Furthermore, they require relatively large amount of sample (typically>100mg)to obtain accurate data and also cannot measure gas adsorption in thin films orcoatings. Quartz crystal microbalance (QCM) sensor is sensitive to mass change at thesensing surface. QCM experiments are typically performed with small samplequantities of just a few micrograms, allowing heat and mass transfer issues that occurwith larger sample sizes to be avoided. ZIF-8, as a prototypical ZIF, is formed by combining a Zn2+source and2-methylimidazole in a suitable solvent and crystallizes in a sodalite-type structure. Inthe present study, QCM sensors were employed to investigate the kinetics of ZIF-8film. An impedance analysis method was employed to correct the influence of thenon-mass effect on the measurement of deposition mass of ZIF-8film. The influenceof solvents, temperature, concentration, and growth way on the growth rate andadsorption capacity was tested. Acetone, acetonitrile and cyclohexane were chosen asthe model volatile organic compounds. The influence of thickness of film andtemperature on adsorption isotherms and kinetic parameters was investigated.It was shown that ZIF-8film is grown on QCM surface from the mixture of Zn2+and2-methylimidazole in three solvents (water, ethanol and methanol). Under thesame concentration and temperature, the assembly rate of ZIF-8film in the solventis in the order of methanol> ethanol>water. The growth kinetics consists of a rapidinitial growth and a later slow rearrangement stages. The mass of ZIF-8film grown inthe second growth cycle is1.53,2.31and1.25times of that in the first cycle insolvents of water, ethanol and methanol, respectively, and approaches to stable levelsafter the third cycle. In a continuous mixing cell, the mass of ZIF-8film grownincreases near-linearly with the time. In the layer-by-layer assembly method, theratio of amounts adsorbed of Zn2+to2-methylimidazole is different to the massratio in ZIF-8crystal. Under the experimental conditions, ZIF-8film shows higheradsorption capacity to acetone and acetonitrile. The adsorption of cyclohexane inZIF-8film is much less. The saturation adsorption capacities of359mg/g and73.5mg/g are estimated for acetone and cyclohexane, respectively.2. Response of piezoelectric sensor during solvent evaporation and data analysisSolvent evaporation is a common natural phenomenon that play important role innature, industry and agriculture. Solvent evaporation is also the important process inchemical engineering, food production and surface treatment. In this work, theevaporation processes of organic solvents were monitored by QCM technique.Deviant responses of QCM in both resonant frequency and motional resistance were observed. To understand the deviant responses of QCM, the evaporation of solventunder different conditions were investigated. The influence of solvent, volume,surface property, humidity, and heat exchange on the response of QCM was tested.It was shown that the solvent evaporation process is rather complicated. Thetemperature of QCM surface is decreased, which result in the condensation of watervapor from atmosphere to QCM surface. On the other hand, the volume of solvent isreduced, causing the decrease in contact area. The longitudinal wave effect is also afactor for the deviant responses of QCM. The increase in mass and viscosity of theliquid film is the origin of the decrease in resonant frequency and increase in motionalresistance of the QCM during evaporation process. The deviant response of QCM isobvious in volatile hydrophilic solvent like acetone. With increasing boiling point, thedeviant response of QCM is weakened. In the situation with constant temperature andlow humidity, deviant response of QCM is not observed. A linear correlation ofmotional resistance of QCM and the contact area was obtained. The evaporation ofsolvents on the surfaces of gold, silver, poly-dopamine, polystyrene supports theconclusion that the condensation of water vapor from atmosphere to QCM surface isthe main mechanism for the deviant responses. By using QCM, the mass and viscosityof the liquid film are monitored.