Synthesis, characterizations and applications of nanomaterials’ based electrodes

Abstract

In the modern science and technology, a new field, nanoscience, has played an important role in the overall development towards addressing several challenging problems related to society. In recent times, a rise in research areas with societal impact has been observed, with nanoscience and nanotechnology being one such area especially where nanodevices are in focus. It has been emerging rapidly and coming up with new solutions to long researched problems in the areas of energy, electronics, etc. When it comes to electronics, the expectations from nanoscience and nanotechnology is increased because of its reputation in terms of deliverables in applied research involving sensing, energy storage, solar cells, memory, electrochromic devices, etc. Looking at a broader picture, finding new materials and reengineering the existing ones are the need of the moment. With the advent of nanotechnology in material science, various new properties can be observed from same compound by simply controlling the size when miniaturized to the nanoscale. At the same time, other properties get improved due to subtle physics taking place at the nanoscales. In this context it is important to underline importance of various materials like semiconducting oxide, transition metal oxides, complex/hybrid compound, layer double hydroxide etc. for their functional use in variety of applications. Therefore, various inorganic and organic materials and their possible combination in a designed paradigm found to make an improved and power efficient device. Looking at various perspectives of material and their applications, my research work has been summarized in this thesis with primarily focuses on fabricating the inorganic transition metal oxide nanostructures’ based electrodes, e.g. NiO, Co3O4, TiO2, PB etc., followed by their application in device engineering. Various inorganic materials for making improved electrochromism in pre-designed display like structure have been explored after the replacement of synthesis process. Therefore, a dense nanofilm of Co3O4 is synthesized using constant current electrodeposition on a conducting transparent electrode that works not only as a counter electrode but also shows electrochromic (EC) properties on its own. The isolated active nano-Co3O4 electrode shows reduction in the redox potential and good color contrast between its yellowish transparent and dark states at different bias conditions. The electrode shows improved color contrast, stability, and cycle life. In situ spectroelectrochemical studies of the ‘nano’ electrode reveals that the bias induced redox activity of the metal oxide leads to the color change between yellowish and opaque states. The bias-induced color change makes it an active EC counter-ion electrode for appropriate solid-state EC devices. A solid state electrochromic devices fabricated in two paradigms namely, inorganic/organic (“hybrid” in combination of viologen and Co3O4) and inorganic/inorganic (“all-inorganic”, in a combination of Prussian blue and Co3O4) in which the fabricated Co3O4 electrode is a common electrode and improves the performance. The performance of the final device with similar or dissimilar materials’ combination has been checked through switching time, switching speed, color contrast and coloration efficiency. In-situ spectroscopy techniques like UV-Visible and Raman have an extraordinary advantage to understand and to explain the mechanism for coloring and bleaching state of electrochromic active materials. A good color contrast of hybrid device appears between its yellow and blue states under different bias conditions have been observed, which leads to overall performance enhancement. The solid state device shows an improved efficiency of as high as 360 cm2 /C and a switching time of as low as 500 ms. All inorganic solid-state device performance between multiple colors with an applied bias occurs less than a couple of volts. A moderate color contrast of ~40% with 1.5 s switching time has been observed with showing stability for more than 900 s of continuous switching. A redox driven electrochromic behavior of individual electrodes makes it possible for the solid-state device to show beautiful colors with a small applied bias with showing the coloration efficiency of 250cm2 /C. On the whole an “all-inorganic” and “hybrid” solid state electrochromic device have been successfully fabricated which shows performance for real applications. To exhibit the multifunctional role of metal oxides, this thesis work also gives some glimpses over the bifunctional role of material toward electrochromic and glucose sensing application. After the characterization of metal oxides followed by electrochemistry, uniform nanoneedles of binary oxide (Ni and Co) were synthesized on appropriate conducting substrates [fluorine-doped tin oxide (FTO) coated glass and carbon cloth (CC)] for the investigation of dual application. A low-operating-voltage (+2 V) color modulation with 50% contrast between the whitish translucent and dark-brown colors was achieved from the nanoneedle grown on a transparent FTO substrate. Furthermore, additionally NiCo2O4 nanoneedles grown on a CC substrate, with an enhanced exposed surface area, showed selective glucose-sensing properties with a very high sensitivity of 3000 μA/mM/cm2 , as revealed using detailed electrochemical and impedance spectroscopic measurements. Beyond the electrochemistry, electron emission property exploited in field emission display, of metal oxide nanostructures which are known electrochromic materials used in electrochromic displays. Field emission properties of NiO nanopetals and TiO2 nanorods have been studied and analyzed within the frame work of Fowler-Nordheim formulation. An alternative formulation for the interpretation of field emission properties has been developed and validated using theoretical fitting of the experimentally observed data. The proposed model addresses the little incompleteness presence in the traditionally used Fowler-Nordheim model. Therefore, this thesis work provides a new pathway on searching those materials which can show its diversity in multifunctional applications.