Gas sensing properties of nanostructured vanadium oxide semiconductors by chemoresistive and optical methods

Abstract

The aims of this research thesis are to synthesise VO2, V2O5 and V6O13 nanostructures and apply the materials on sensor electrodes for gas and humidity sensing. These materials were synthesised and optimised using chemical vapour deposition (CVD), microwave assisted and pulse laser deposition (PLD) techniques. Analyses with thermogravimetric (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), Raman and Fourier transform infrared (FTIR) spectroscopy showed VOx phases order as NH4VO3 ? VO2 + V2O5 (150 – 200 °C) ? V6O13 (300 °C) ? V2O5 (above 350 °C). This is when the precursor NH4VO3 was annealed in CVD between 100 – 350 °C in H2 atmosphere for 2 hrs. Adsorption analysis of VOx nanostructures showed a profile of Brunauer-Emmett-Teller (BET) surface areas which increased with the annealing temperature until 300 °C after which the transition occurred. Humidity (%) sensing response of VOx showed high response for V6O13 and V2O5 phase whereas, the Langmuir isotherm plot in the form of the response per BET surface area with respect to different levels of relative humidity showed high response for VO2. Phase evolution diagram based on these properties has been proposed. Thermal CVD annealing of NH4VO3 at 500 °C in N2 atmosphere for 2, 12 and 24 hours produced monoclinic V6O13 (at 2 hrs) and ?-orthorhombic V2O5 (at 12 and 24 hrs) nanorod structures using the above characterization techniques. Gas sensing application of these structures revealed that the H2S gas is selective in adsorption to V6O13 phase with 132 % response magnitude at 350 °C and 60 ppm, this response is 647.2 % higher than that of NH3, CH4, NO2, H2 and CO. The response and recovery times are 32 and 129 s respectively which is remarkably short compared with the data in literatures. This V6O13 sensor was ranked with its V2O5 counterpart and still found to be 238.5 % higher for H2S gas. Density functional theory (DFT) through ab initio molecular dynamics of (110) facet of monoclinic V6O13 and ?- orthorhombic V2O5 also showed high H2S adsorption energy for V6O13 than V2O5 with a profile which simulate the experimental findings. Low temperature microwave assisted synthesis of VOx from NH4VO3 without post-annealing treatment demonstrated small size homogeneous crystallite with high BET surface area and high adsorption and desorption pores. These properties translated to sub-ppm room temperature sensing of the flammable CH4 and odorant NH3 and toxic NO2 with high sensitivity. The VO2 (B) phase produced via the same microwave process applied for humidity sensing in the lateral gate metal oxide semiconductor field effect transistor (MOSFET) configuration for 0, 5, 8, 10, 12 and 15 V gate voltages. An optimum percentage humidity response observed at 5 V showed response and recovery times in the order of 60 – 70 s which is remarkably shorter than the ? 300 s response of the non-gated VO2 humidity sensor reported in this thesis. Statistical information extracted from the non-linear S-curve Hill Dose rate showed that the VO2 (B) sensor is very resilient to relative humidity by showing the humidity level of more than 100% where the response of the sensor could be reduced to 50%. In-situ Raman spectroscopy sensing of NH3 gas at the surface of PLD deposited V2O5 thin film was presented. The film crystal structure, depth profile and oxidation state was studied by cross section scanning electron microscope (SEM), time of flight secondary ion mass spectroscopy (TOF-SIMS), XPS and group theoretical analysis. Recoverable red shift of 194 cm-1 and blue shift of 996 cm-1phonons upon the interaction with the NH3 gas at 25 and 100 °C was observed. Decrease in the Raman scattered photons of the 145 cm-1 phonon was also observed for different levels of NH3 exposure. The responses of these phonon properties in NH3 environment compared to the chemoresistive sensing of the film at 40 ppm showed that the in-situ Raman spectroscopy techniques is not only more sensitive but also demonstrated possibility for selective gas detection via blue and red shift of phonon frequencies.