Metal phthalocyanine based gas sensor
Chia, Sharon Liping
Date of Issue2018-12-27
School of Materials Science and Engineering
Metal phthalocyanine (MPc), a class of organometallic materials, allows not just substituents on its phthalocyanine macrocycle, but also metal center variation. Such flexibility in molecular structure, as well as its semiconducting properties have thus promoted the use of MPc for numerous applications. One of these applications is on the sensing of gas analytes. Despite hundreds of publication on MPc as gas sensors, there is a lack of understanding on how the different substitutions will affect the gas sensitivity or to leverage on MPc properties to improve the selectivity or e-nose capabilities. On top of that, there is a general insufficiency on systematic studies done to investigate the properties of conductivity-based gas sensors based on organic materials. To elaborate, several properties that may be crucial, such as the thickness effect or sensing layer/electrode interface, have not been examined. This thesis attempts to bridge the knowledge gap by addressing three main topics; the thickness effect and the electrode/sensing layer interface effect on gas sensing, as well as giving a first insight on the gas sensing mechanism. In addition, this thesis will mainly focus on the gas sensitivity of MPc to nitrogen dioxide (NO2). By using CuPc chemiresistors, the thickness of the CuPc sensing layer demonstrated an inverse relationship to NO2 sensitivity. The reaction rate of the chemiresistors was found to follow the first order kinetics but that was inadequate to describe the difference in sensitivity. The device response to gas and the sensing layer thickness was then found to correlate to Knudsen diffusion, which aptly describe the inverse relationship of sensitivity and sensing layer thickness due to a decrease in the proportion of gas diffused into thicker sensing layers. The experiment was repeated with some variation in the chemiresistor structure and the same relationship between gas sensitivity and sensing layer thickness was observed. The electrode/sensing layer interface was also investigated in this thesis. CuPc sensing layers on Au or Pt electrodes were found to display ohmic properties while blocking contacts were formed with Al electrodes, in agreement with literature. NO2 sensitivity enhancement was demonstrated in CuPc chemiresistors with Al electrodes as compared to chemiresistors with Au or Pt electrodes due to the modulation in Schottky barrier upon exposure to NO2. As such, it is shown that similar phenomenon in metal oxide can also be expected in MPc sensing layers. Further on, preliminary study on the effect of substituent group or metal center change on NO2 sensitivity was conducted based on 5 different MPc materials. Results showed that the oxidation potential of the materials may be used as an indicator of the NO2 sensitivity. In particular, ZnPc with tertbutyl substituent group (electron donating group) demonstrated the highest sensitivity and may be correlated to the highest electron density in the Pc macrocycle, which is involved in the electron transfer process to NO2. Multiple measurements with new samples were utilized to affirm the results obtained. The interaction site was also examined in this thesis. In comparison to reported methods used to examine the interaction, such as Raman spectroscopy, x-ray absorption spectroscopy (XAS) was used with in-situ gas exposure for the first time on gas interaction characterization. Analysis of the XAS spectra suggests a lack of axial interaction of CuPc with NH3 or NO2 and revealed a compression of the Cu-N bond between the metal center and macrocycle upon exposure to NO2. The absence of a lengthening of this Cu-N also implies that the gas interaction does not occur at the metal center. Repeated measurements were found to display comparable results.