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Title: Nanostructured molybdenum disulfide thin film based electrocatalysts for hydrogen evolution reaction
Author: Li, Sha
ISNI:       0000 0004 7966 1505
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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2H-Molybdenum disulfide (2H-MoS2) with catalytically active edges has emerged as a promising catalyst for the hydrogen evolution reaction (HER), 2H+ + 2e → H2. Towards designing highly active MoS2 catalyst with maximized edge density and activated basal surfaces, this DPhil project focused on the synthesis of nanostructured MoS2 thin film based HER electrocatalysts, using chemical vapor deposition (CVD), gas phase treatments and thermal annealing (TA) methods, followed by characterisation and electrochemical testing of their structural and catalytic properties. Firstly, a reproducible and scalable synthesis method for high-quality 3D edge rich (3D-ER) MoS2 nanoplatelets utilizing single-step CVD approach was developed. Tailoring CVD parameters resulted in the optimized structure for HER that consists of larger MoS2 platelets with smaller layered MoS2 sheets growing perpendicularly, which increases the total number of edge sites within a given geometric area. By systematically controlling the growth temperature, tunable film thickness and surface morphology were achieved, enabling the optimized combination of edge density, inter-planar charge transport and sufficient binding with the substrate as well as fast removal of gas bubbles via a superaerophobic surface. Subsequently, gas phase treatments for post-growth modification was applied to activate the catalytically inactive basal plane sites of 3D-ER-MoS2. Hydrogen (H2) annealing and oxygen (O2) plasma etching were utilized to generate defects (cracks, pits, holes, and vacancies) that can catalyze HER in the basal surfaces. By controlling O2 plasma exposure time and H2 annealing temperature, tunable defect density, surface area, and catalytic activity enhancement can be obtained. Optimum treatment conditions led to dramatic improvements in electrocatalytic activity. Lastly, a hybrid electrocatalyst was prepared by growing Pt nanocrystals on the surface of 3D-ER-MoS2 via TA of Pt precursor. By varying synthesis conditions - precursor loading and TA temperature - average size and specific surface area of Pt nanoparticles can be precisely controlled. Results show that higher Pt loading yields better HER performance despite a smaller specific surface area; higher TA temperature delivers a larger average particle size of the Pt crystals and lowers HER activity. Larger average size led to fast sintering and thus poor durability of the catalyst. The outstanding HER activity of Pt/MoS2 is attributed to highly-dispersed Pt nanoparticles grown on MoS2 basal surfaces, large MoS2 edge density and Pt - S bonding effect induced activity improvement of MoS2 as well as 3D porous network assisted superaerophobic surface.
Supervisor: Warner, Jamie ; Tsang, S. C. Edman Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Nanostructured materials ; Catalysis