Novel Ni-based Nanostructures: Synthesis and Application in Sensing and Energy Storage

Nanotechnology is a field related to materials and devices with characteristic dimensions in the nanometer scale where unique size, shape and structure-dependent physical and chemical properties emerge, opening new opportunities in science and technology. Among the various nanostructured materials, metal oxide nanostructures and, specifically, nickel hydroxide [Ni(OH)2] and nickel oxide (NiO) nanostructures have recently attracted increasing attentions due to their excellent performances in many applications, such as electrochromic smart windows, electrochemical (bio)sensing, gas sensing, energy storage and water splitting. Ni(OH)2 and NiO nanostructures can be prepared by various methods. However, some of them require expensive or complex experimental setup which can not be used for large scale production. Therefore, in the last decades researchers have devoted many efforts in developing simple, low-cost and scalable synthesis methods of Ni(OH)2 and NiO nanostructures. Among them, the chemical bath deposition (CBD) is considered the most advantageous one. In fact, CBD involves the simple immersion of substrates into an aqueous solution of Ni(OH)2 precursors which allows to deposit nanostructured Ni-based thin films, named “nanowalls”, with high surface-to-volume ratio and porous structure. Since the pioneering NiO CBD by Pramanik and Battacharya in 1990, CBD has been widely used to prepare Ni-based nanowalls. Still, as a solution-based method, CBD lacks of a good control and reproducibility. To overcome these limitations, CBD needs to be fully investigated and controlled. In this thesis, a careful analysis of the most important CBD parameters (reagents, deposition time and temperature) is conducted, leading to a better control and reproducibility of the method. Moreover, a growth model for Ni-based nanowalls is proposed. The full comprehension of Ni-based nanowalls growth allows to enhance their electrochemical properties. To further improve the electrical and electrochemical properties of Ni-based nanowalls for specific applications, several strategies are presented, including thermal annealing and/or electrochemical processes, or decoration with metals by electroless deposition method. In this way, novel Ni-based nanostructures, such as Ni nanofoam, Au decorated NiO nanowalls, nanoporous NiO film and Ni(OH)2@Ni core-shell nanochains, are obtained, showing excellent performances for various applications, including non-enzymatic glucose sensing, PCR-free DNA sensing, acetone and NO2 sensing, and energy storage. The thesis is organized as follows: -item the first chapter is an introduction to nickel hydroxide and oxide nanostructures. The fundamental properties and fabrication methods of Ni(OH)2 and NiO nanostructures are first presented. Then, a literature overview of Ni(OH)2 and NiO thin films grown by low-cost CBD is reported, focusing on the proposed models for film formation and growth; - the second chapter concerns the careful investigation of the most important CBD parameters, which leads to a new model for film formation and growth. The morphological, structural and electrochemical properties of Ni-based nanowalls are also presented. The optimized Ni-based nanowalls were transformed into the novel Ni nanofoam by a thermal annealing process in reducing atmosphere and applied for non-enzymatic glucose sensing, showing remarkable performances in terms of high sensitivity, selectivity, stability and low limit of detection (LoD); - the third chapter reports an introduction to Contagious Agalactia (CA), an infectious disease which reduces the milk production in goats and sheep, caused by the bacterium Mycoplasma agalactiae (Ma). To provide a cost and time-saving diagnosis method for CA other than conventional PCR-based approach, a novel impedimetric Ma DNA sensor is presented. The sensor consists of Au decorated NiO nanowalls obtained by gold electroless deposition onto NiO nanowalls, which were functionalised with Ma probe ssDNA and used to sense the hybridization with Ma complementary ssDNA; - the fourth chapter introduces the urgent need for low-cost sensors of acetone and NO2. Then, the acetone sensing performances of NiO nanowalls are presented. Moreover, a novel nanoporous NiO film with impressive sensitivity, selectivity, stability and low LoD to NO2 at room temperature is presented. A model for the NO2-NiO interaction mechanism is also proposed; - the fifth chapter reports an introduction to nanostructures for hybrid supercapacitors. Then, novel Ni(OH2@Ni core-shell nanochains, prepared by CBD, thermal annealing and electrochemical oxidation, are presented. Due to this unique core-shell architecture, high-rate energy storage performances in terms of superior specific capacity, rate capability and stability are obtained.