| Summary: | High entropy alloys (HEAs) have been proven to exhibit outstanding catalytic properties, as their electronic structures can be tuned by adjusting the compositions. Owing to the unique high entropy effect, HEAs are thermodynamically stable, which enable them to work in harsh environment without being corroded. Moreover, the highly disordered atomic arrangement of HEA leads to various active sites on the catalyst surface, enhancing the interaction between the surface and absorbents. However, challenges still remain, including the aggregation of HEA nanoparticles, the difficulty in synthesizing homogeneous multi-element alloys, and unknown relationship between the composition and catalytic properties of HEA, etc.
In this thesis, a novel method combining atomic layer deposition and electrical Joule heating (ALD-EJH) is developed to synthesize noble metal high entropy alloy (HEA) thin film as a highly efficient catalyst towards hydrogen evolution reaction (HER). To achieve this goal, several major challenges are to be overcome, including the development of reliable noble metal ALD processes, verifying the feasibility of EJH process to multi-layer coatings, and modifying the catalytic performance of HEA thin films.
Therefore, the scope of this thesis includes both the development of synthesis techniques and the investigation into the materials. For ALD process development, safe and reproducible ALD processes for Pt, Pd, Ir, Rh, and Ru coatings have been created and modified. The deposition of the selected noble metals is examined to obtained uniform surface and precisely controlled thickness. The mechanism behind the reaction is revealed and discussed based on experimental and simulation results. The concerned properties of the deposited thin films, such as growth rate, microstructure, and surface evolution are characterized. For the first time, EJH process has been applied to the multiple layers deposited by ALD method to accomplish the alloying. The ultrafast ramping and cooling rate provided by EJH treatment facilitate the interdiffusion within the multi-element system and prevent the phase separation, resulting in homogeneous HEA thin film, regardless of the large immiscible gaps between selected noble metals. Finally, RhRuPtPdIr HEA nanofilm is fabricated on glassy carbon electrode, and exhibits extraordinary catalytic activity and stability for HER in harsh environment. The mechanism of the performance was analyzed by simulation works, in which the adjustment of electronic structure of d-band of HEA accounts for the outstanding catalytic properties. By combining the experimental and simulation data, the relationship between the surface microstructure of HEA and its electronic structure is established.
In terms of practical advances, this thesis validating the great potential of noble metal HEA nanofilm as an exceptional catalyst. Plus, ALD-EJH is introduced as a universal method to synthesize various nanomaterials, significantly expanding the range of materials that can be prepared using ALD.
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