Evaporation and Magnetic Field Induced Organization of Nanoparticles for Wetting and Catalytic Applications

Abstract

In recent years, most of the electronic and several other devices are getting smaller day by day because of technological advancements; essentially, these devices required small chips, electronic circuits, microprocessors, sensors, etc. As the dimension of these devices decreases to micro or nano-level, it is almost impossible to fabricate those parts with the help of any precise machine. However, the fabrication of these devices at small-scale has been more accessible and inexpensive by the use of a relatively novel technique such as “self-assembly” of nanoparticles (NPs). Self-assembly (SA) refers to a bottom-up process by which any kind of colloidal or nanoparticles, macromolecules are organized into a specific arrangement driven by some favorable intermolecular (capillary forces, van der Waals interaction, hydrogen bond) or externally induced forces (evaporative flux, electrical, and magnetic forces). Among all available techniques of self-assembly; drying-mediated or evaporation-induced assembly (EISA) of nanoparticles is the simplest method of assembling particles on surfaces. Similarly, NPs can also be organized in the bulk phase/ solution phase to get various types of 1D, 2D, or 3D nanostructures (such as nanowire, nanochains, nanoplates, nanocubes). In this case, molecules or particles form a nucleus first and then grow into a larger organized structure because of the favorable interaction with the neighboring building blocks. These types of assemblies are assisted by different interactions, external electrical, or magnetic fields. In recent years, assembled nanostructures are preferred for numerous applications such as wetting, catalysis, sensing, biological and electronic fields over the unorganized one because of unique and enhanced collective properties.
In this thesis, two types of patterns, fractal, and chain-like patterns were developed using evaporation induced and magnetic field-assisted bulk phase assembly, respectively. In the first part of the work (Chapter-3), fractal patterns formation of sodium carboxymethyl cellulose (CMCNa) on various surfaces were investigated during the evaporation of sessile drop in the presence of oxalic acid. These patterns are branched type and typically referred as ‘‘Fractal trees’’. The produced sodium oxalate salt from the reaction of CMCNa and oxalic acid is mainly responsible for the pattern formation due to the dendritic crystallization. These patterns formed under the influence of inner coffee ring deposits and different intermolecular forces. The mechanism of fractal pattern formation, the effects of various parameters, such as the effect of salt, temperature, and reaction components were discussed in this work. Later in continuation, these patterns were used as a removable template for the organization of other NPs (SiO2, TiO2, and PTFE) into fractal patterns on glass and metallic surfaces (Chapter-4). First of all, metallic oxides SiO2 and TiO2 were organized into fractal patterns on the glass surface. The pure NPs suspension showed “coffee ring effect” and did not form an organized pattern on the glass surface after drying. Later these NPs were organized into fractal patterns using an easily removable template consist of CMCNa and oxalic acid mixture in the presence of a cationic surfactant (CTAB). The obtained fractal patterns of SiO2 and TiO2 coated glass surfaces showed superhydrophilic nature with the average water contact angle of ~ 6 and ~8 respectively after calcination, whereas, coating of only NPs without pattern could not achieve such low average contact angle. These coated surfaces showed the antifogging property. Similarly, polytetrafluoroethylene (PTFE) NPs were organized on flat surfaces (glass and steel) and stainless steel mesh using CMCNa template, which in turn leads to the superhydrophobic surface after sintering at 250 ºC (Chapter-5). These PTFE coated surfaces showed the average water contact angle ~152º for glass and flat stainless steel surfaces and 154º for stainless steel mesh with excellent self-cleaning property. The superhydrophobic mesh also showed good efficiency for oil-water separation. In the next part of the thesis, Palladium NPs were synthesized using clove and acacia extract and organized into flower-like fractal morphology on glass and silicon surfaces via evaporation induced assembly (Chapter-6). These organized patterns surface showed enhanced catalytic activity for the reduction of 4 nitrophenol (4- NPh) to 4-Aminophenol (4-APh) and also suitable anode material for direct borohydride fuel cell than the unorganized NPs. The last part of work is related to the organization of Ni NPs in the bulk phase (Chapter-7). Herein, Ni NPs were organized into nanochains in the presence of an external magnetic field in bulk media. The length of as-synthesized chains was 10±2 µm, and 85 nm diameter. These chains showed good magnetic properties and the application of these nanochains showed excellent catalytic activity for the reduction 4 NPh to 4-APh in comparison with only NPs with easy recovery with the help of a magnet. These nanochains also showed very good electro-catalytic activity for ethanol oxidation. The Ni nanochains showed 5.4 times higher current density compared to NPs due to low charge transfer resistance and surface area. The current decay was also less over a period for Ni nanochains, indicating the higher stability of chains than NPs. The thesis provides some important insights, such as cheap and robust strategies for the assembly of particles flat surfaces for several practical applications with improved wetting, catalytic, and electrochemical properties.