Plasma Synthesis and Self-Assembly of Magnetic Nanoparticles

Abstract

ii Cover image: Fe nanoparticles collected into nanotrusses on top, and around the edges, of a Si wafer. Abstract Nanomaterials are important tools for enabling technological progress as they can provide dramatically different properties as compared to the bulk counterparts. The field of nanoparticles is one of the most investigated within nanomaterials, thanks to the existing, relatively simple, means of manufacturing. In this thesis, high-power pulsed hollow cathode sputtering is used to nucleate and grow magnetic nanoparticles in a plasma. This sputtering technique provides a high degree of ionization of the sputtered material, which has previously been shown to aid in the growth of the nanoparticles. The magnetic properties of the particles are utilized and makes it possible for the grown particles to act as building blocks for selfassembly into more sophisticated nanostructures, particularly when an external magnetic field is applied. These structures created are termed "nanowires" or "nanotrusses", depending on the level of branching and inter-linking that occurs.Several different elements have been investigated in this thesis. In a novel approach, it is shown how nanoparticles with more advanced structures, and containing material from two hollow cathodes, can be fabricated using high-power pulses. The dual-element particles are achieved by using two distinct and individual elemental cathodes, and a pulse process that allows tuning of individual pulses separately to them. Nanoparticles grown and investigated are Fe, Ni, Pt, Fe-Ni and Ni-Pt. Alternatively, the addition of oxygen to the process allows the formation of oxide or hybrid metal oxidemetal particles. For all nanoparticles containing several elements, it is demonstrated that the stoichiometry can be easily varied, either by the amount of reactive gas let into the process or by tuning the amount of sputtered material through adjusting the electric power supplied to the different cathodes.One aim of the presented work is to find a suitable material for the use as a catalyst in the production of H2 gas through the process of water splitting. H2 is a good candidate to replace fossil fuels as an energy carrier. However, rare elements (such as Ir or Pt) needs to be used as the catalyst, otherwise a high overpotential is required for the splitting to occur, leading to a low efficiency. This work demonstrates a possible route to avoid this, by using nanomaterials to increase the surface-to-volume ratio, as well as optimizing the elemental ratio between different materials to lower the amount of noble elements required.iv v

Magnetically Collected Platinum/Nickel Alloy Nanoparticles as Catalysts for Hydrogen Evolution

“…The unit is set to deliver 80 μs long pulses, in sequence to the two cathodes, at a total frequency of 1200 Hz, giving an effective pulsing frequency of 600 Hz to each cathode. This pulse pattern is referred to as continuous pulsing in earlier work . The power to the individual cathodes is adjusted, by tweaking the pulse voltage, to tune the amount of sputtered material from each cathode.…”

“…All of these parameters are presented in the process conditions table (Supporting Information, Table S1). The substrates are placed on top of a magnet in order to collect the nanoparticles magnetically, as explained in an earlier work. − However, unlike the previous works, the magnet used here is a permanent cube magnet made from NbFeB with a 7 mm side. This makes the magnetic flux density at the center of the magnet significantly stronger compared to that in the earlier work. − This results in a more efficient collection of nanoparticles.…”

“…This pulse pattern is referred to as continuous pulsing in earlier work. 34 The power to the individual cathodes is adjusted, by tweaking the pulse voltage, to tune the amount of sputtered material from each cathode. For most process conditions, the sum of the average power, from the two cathodes, is maintained at 60 W. This is done to avoid overheating of cathodes.…”