Adsorption of VOC vapors on ZnPc: sensing and kinetic studies

Thin film of sensing unit, 2,3,9,10,16,17-hexakis(3-dietylaminophenoxy)-23-etynylphenyl-(2-ferrocenyl-o-carborane)phthalocyaninato zinc(II) (ZnPc), have been deposited by spray pyrolysis technique on the quartz crystal surface with a fundamental resonance frequency of 10 MHz. The surface morphology of the film has been studied using the contact mode atomic force microscope (AFM) technique and the average surface roughness was determined to be 36.4 nm. Then, this film was exposed to the vapors of methanol, ethanol, 2-propanol, and 1-butanol with various concentrations varying between 50 and 400 ppm. In addition, studies on response time and repeatability of the sensors have also been carried out. Films exhibit maximum sensing response to methanol vapor while the lowest sensitivity of the film towards 1-butanol has been observed. After an exposure time of 10 min, the frequency shift of 415 Hz was recorded for 400 ppm methanol vapor concentration, while a frequency shift of 215 Hz was observed for the same concentration of 1-butanol. The sensor was 1.30 times more sensitive to methanol vapor than ethanol and 2.00 times more sensitive to methanol than to 1-butanol. Variations in the sensitivity of the sensor have been correlated with the number of carbon groups in analyte vapors. The effect of the number of carbon groups in analyte molecule on adsorption kinetics of the film has also been investigated and compared. Adsorption isotherms of four volatile organic compounds on ZnPc were investigated at room temperature and the experimental data obtained were correlated with different existing adsorption isotherm models such as the Langmuir, the Freundlich, and Temkin model. An overall evaluation of the obtained results showed that the sensing performance, adsorption kinetic, and adsorption isotherms are dependent on the molecular size of the analyte molecules.