Physicochemical Characterization and Gas Sensing Studies of Cr1-xFexNbO4 and Application of Principal Component Analysis

Abstract

Monitoring the working environment of laboratories and industries for pollutants is of primary concern to ensure the healthiness of working personnel. Semiconducting metal oxides (SMOs) are sensitive to the gas ambience and can be tuned for sensing purpose. As SMOs are not selective, an array of sensors with differential selectivity may resolve to great extent. The objective of the thesis is to understand the physicochemical properties and gas sensing characteristics of Cr1-xFexNbO4. Applying principal component analysis to the sensor response data either for selection of features or for differentiation of analysts is also of concern.

Preparation of Cr1-xFexNbO4, phase characterization, lattice parameters estimation, morphological and micro chemical analysis (SEM & EDX), electrical characterization by direct current (DC & AC) in the temperature range of 423 K to 573 K, weighted magnetic moment of iron and chromium deduced from susceptibility measurements, spin nature of iron and surface compositions of different valences of chromium and iron deduced from X-ray photoelectron spectroscopy of are presented. The wide dynamic range hydrogen sensing characteristics of CrNbO4 bulk pellets at different temperatures along with the cross-sensitivity towards NH3, NOx(NO+NO2) and PG (petroleum gas) are investigated. The preparation of Cr1-xFexNbO4 thick and thin films by screen-printing and PLD are also presented. The thick films are tested at different temperatures towards hydrogen. The n-type or p-type nature of thick films towards hydrogen with varying iron concentration in Cr1-xFexNbO4 is reported. The thin films are characterized for phase formation, morphology by XRD, SEM and AFM. XPS performed surface characterization. Electrical resistance measurements at different temperatures and preliminary experiments on hydrogen sensing are presented. The probable hydrogen sensing mechanism of CrNbO4 was revealed by X-ray photoelectron spectroscopy. The experimentally observed reduction in metal ion oxidation states upon interacting with hydrogen is best illustrated by Kröger Vink notation.

Principal component analysis was applied for three different types of studies: i) The fit parameters of the transient response of CrNbO4 thick films towards hydrogen are analyzed for finding out the better feature for calibration, ii) Different thick films of CrNbO4, Cr0.5Fe0.5NbO4 and FeNbO4 operated at various temperatures for testing H2 and VOCs are analyzed for redundancy in sensor behaviour and iii) Cr0.8Fe0.2NbO4 thick films are studied for sensing H2, NH3 and their mixtures and usefulness of PCA in resolving them in PC-space. In addition, H2 and VOCs are tested at different temperatures and redundancy in temperature is deduced to construct a sensor array with a minimum number of sensors. Finally, a sensor array consisting of Cr0.8Fe0.2NbO4 thick films, operating at different temperatures is built, and qualitative discrimination of analysts in PC-space is demonstrated. Finally, the major findings of the present investigations and suggestions for future aspects of experimentation are provided