Synthesis and characterisation of metal sulphides and metal oxides for gas sensing applications

Abstract

The thesis is composed of six chapters which is a brief report on the synthesis and coating of metal sulphides and metal oxides using various coating techniques like chemical bath deposition, dip-coating and electrospinning-coating. These materials are coated over a small region of an optical fiber with the cladding removed in that coating area. Then these optical fibers are subjected to various concentrations of test gases in steps of 100 ppm and the output response is recorded. Now these results are compared and analysed in each chapter, separately. Chapter 1 gives a brief view on various synthesis techniques and coating methods which are used in this thesis. Chapter 2 discusses about the various analysis techniques which are employed in the upcoming chapters of the thesis. These include X-Ray diffraction analysis, UV-visible absorption analysis, UV-visible Reflectance mode analysis, Photoluminescence analysis, Scanning Electron Microscopy, Energy Dispersive X-ray analysis and finally Gas-sensing analysis using a modified-cladding optical fiber.Chapter 3 begins with the synthesis of cadmium sulphide (CdS) to be used for gas-sensing application. In this investigation, a clad-modified fiber optic sensor is reported for NO2 gas detection. CdS is coated over cladding-removed region by Chemical Bath Deposition (CBD) which acts as the gas-sensing medium. CdS is coated over four fibers each with a different coating duration of 10, 20, 30 and 40 minutes. X-ray Diffraction (XRD) is performed to study the crystalline structure of synthesised CdS and Hexagonal structure of CdS was confirmed from the XRD pattern. Characterisation of CdS is done by Scanning Electron Microscope (SEM) to determine the surface morphology of the coatings. Mesh-like structures were formed in CdS-10 and CdS-20 and agglomerated structures were formed in CdS-30 and CdS-40. These were confirmed by SEM analysis images. Photoluminescence study (PL) and UV-visible absorption spectroscopy are taken to analyse the optical properties of CdS. The gas-sensing spectral response for different concentrations of NO2, CO2 and SO2 gases are studied on the four fibers separately, and the results are analysed. It is observed that the fiber coated for 20 min (CdS-20) exhibited the best response for NO2 and CdS-10 exhibited the next to best response compared to CdS-30 and CdS-40 and also when compared to the other two gases. The surface area of the mesh structures of CdS in CdS-20 played an important role in the gas sensitivity. newline