NANOSTRUCTURED METAL OXIDES FOR HYDROGEN SENSING APPLICATIONS

Abstract

unique physicochemical properties render it unfeasible to detect hydrogen (H2) gas using human sensory organs and its flammable nature elevates the need for fabrication of highly sensitive, selective and stable sensors, which can sense wide range concentrations of H2 at room temperature. However, a sensor should be facile, economical, compact and easily integrable with existing silicon electronics, in order to be employed at industrial level for H2 sensing applications. Chapter 1 discuss the structure and mechanism of various available H2 sensors and outline their traits for sensing applications. Electrochemical and chemiresistive type sensors are found to be best candidate for commercial H2 sensing applications compared to other existing sensors. In comparison to exhaustive list of sensing materials, physicochemical and electronic properties of metal oxide (MOX) semiconductors bestow them with favourable characteristics of good sensing material for H2 sensing application. However, high desorption activation energy of pristine MOX contributes in elevation of operating temperature which makes them unfavourable for H2 sensing application. Synthesis of nanostructures of MOX, heterostructures with other catalytic active materials and plasmon driven catalysis by radiation exposure, decrease the operating temperature while increasing their sensitivity simultaneously. Based on literature review, the reported work on palladium oxide (PdO) nanostructures till beginning of this thesis research work were limited and they demonstrated good sensitivity for H2 at high temperature. These results became the scientific motivation behind our research work where we synthesized and examine PdO nanomaterials for H2 sensing application at room temperature. Chapter 2 explains various approaches for synthesis of MOX nanostructures and wet chemical synthesis procedure based on bottom up approach is found to be suitable for large scale synthesis of nanomaterials. Sol-gel and hydrothermal synthesis procedures are the most facile and economical methods to obtain homogeneous morphology of nanomaterials comparatively. Setup and working mechanism of x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), ultraviolet-visible (UV- Vis) spectroscopy and Fourier transmission spectroscopy instruments, employed in the following chapters for analysis of crystallographic property, size, shape of nanostructures, thin film surface morphology, optical band gap and nature of chemical bonds are demonstrated extensively. Chapter 3 study the changes in crystallographic properties of PdO nanoparticles are investigated using H2 in-situ XRD analysis at different low gas pressure (25, 50, 75 and 100 mbar) of H2 at room temperature. Complete transformation of t-PdO into face centered cubic (fcc) crystal structure of palladium hydride (PdHx) is observed as pressure rises from 25 to 100 mbar. Also, expansion in lattice parameters corresponding to phase transition takes place as pressure reaches 100 mbar. The results of this study show that pristine PdO nanoparticles are suitable for sensing and storage of H2 at room temperature. Chapter 4 includes the research work on amperometric sensing of H2 gas using PdO nanoparticles thin film as working electrode (WE) and sulphuric acid (H2SO4) as proton conducting electrolyte in three electrodes electrochemical cell to detect wide range concentration (10 to 70%) H2 gas at room temperature. The proposed sensor shows good sensitivity of 0.222 μA/%H2 with fast response time of 60 seconds and low limit of detection (LOD) as compared to other similar reported amperometric H2 sensors. Chapter 5 contains the study on amperometric sensing of H2 gas using heterostructure of PdO nanoparticles and two dimensional (2D) nanosheets of reduced graphene oxide (rGO) based WE and H2SO4 as proton conducting electrolyte in three electrodes electrochemical cell to detect wide range concentration (10 to 80%) H2 gas at room temperature. The proposed sensor demonstrates sensitivity of 0.462 μA/%H2 i.e. two folds increase as compared to PdO/ITO WE, with fast response time of 60 seconds and low limit of detection (LOD) as compared to other similar reported amperometric H2 sensors. Chapter 6 consists of study on chemiresisitve sensing of H2 gas using heterostructures of PdO with polyaniline (PANI) conducting polymer matrix as solid state sensing element at room temperature. PdO-PANI nanocomposites based sensing assembly exhibited two folds increase in sensitivity as compared to pristine PANI. Moreover, the magnitude of sensitivity also increases with the percentage concentration of PdO, but the increase is not comprehensive comparatively. The proposed sensor demonstrates enhanced sensitivity, quick response/recovery time of 3 and 4 seconds respectively on comparison with other similar PANI and its nanocomposites based sensors reported in literature. Chapter 7 consists of elucidated outline of complete research work presented from chapter 1 to chapter 6 correspondingly. It also describes the different course of work which will be approached in future to further enhance the sensitivity of pristine PdO nanomaterials towards H2 gas at room temperature.