Plasma Functionalization of MoSe2 Nanosheets for Enhancement of the Hydrogen Evolution Reaction Study

等離子體功能化硒化鉏納米片増強氫演化反應研究

Student thesis: Doctoral Thesis

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Award date6 Jul 2021

Abstract

Hydrogen energy featuring zero carbon emission and recycling of water is a potential candidate to replace traditional fossil fuels and hydrogen can be produced by different techniques and one of the attractive means is electrochemical water splitting. Noble metals such as platinum are popular catalysts for the hydrogen evolution reaction (HER) in water splitting but their scarcity and high cost hinder the commercial application. Alternative catalysts such as two-dimensional (2D) transition metal dichalcogenides (TMDs e.g. MoS2 and MoSe2) possess tunable crystalline and electronic structures as well as plenty of active sites and optimizations of catalyst such as heterojunctions and defect engineering show enhanced HER performance. Plasma surface functionalization is an effective means to perform defect engineering, alter the surface morphology, and dope catalysts. However, a lack of knowledge on the interaction between plasma and catalyst severely hinders the application of plasma technology in electrochemistry.

Our research goals are to propose mechanisms of interactions between plasma and catalyst to promote a better understanding of the various plasma processes such as electron impact and radical collisions as well as properties such as radical species and ion energy which is crucial to the design and preparation of catalysts by optimizing the effects of etching, doping, and defect generation under different plasma conditions.

MoSe2 nanosheets are hydrothermally fabricated and conductivity is initially optimized by inserting high-conductive graphene layers (GL) into the nanosheet, which demonstrates an enhanced HER performance owing to that the intercalated GL decreases the Gibbs free energy for H+ absorption accelerating H2 generation and enhances the electric field distribution around the catalyst facilitating better charge migration between the MoSe2/GL and electrolyte. The higher conductivity and more effective use of the basal plane lead to more accessible active sites to promote the catalytic efficacy.

Besides the conductivity promotion, improvement of active sites is achieved by plasma functionalization by adjusting plasma parameters on MoSe2 nanosheets largely fabricated on carbon felt which is similar to graphene layers keeping comparatively high conductivity for better charge transfer.

A low-pressure capacitively-coupled plasma (CCP) with argon discharge is employed to study the effects of the ion energy and flux on the interactions between the plasma and catalyst by tuning the power of the radio frequency (13.56 MHz). After argon plasma functionalization, irregular etched through-holes are randomly distributed on the MoSe2 nanosheets and the size and number vary with the ion energy and density. The channel formed in the through-hole exposes more surface and increases the active sites, conversely, over-etching removes too much catalyst leading to a decrease of surface area. Besides surface exposure, the electric field on the edge of the through-hole is enhanced benefiting the stronger adsorption of H+ and faster electron transfer, and plasma-induced damage (PID) with Se vacancies on catalyst surface renders more Mo atoms as donors to supply electrons for H+ transformation. By optimizing the ion energy, flux, and angular distribution based on plasma simulation, balanced etching and PID can be accomplished for the best HER performance.

Compared to the argon plasma interactions which produce only the physical effect of sputtering, the oxygen plasma also introduces chemical modification and creates an oxygen in-depth distribution. Owing to the complicated effects of physical collisions, electron transfer, and ion-ion recombination in the oxygen discharge, the oxygen plasma has a smaller ion density and flux than the argon discharge and consequently, weak etching is observed relative to argon plasma. The increased polarity after oxygen incorporation improves the surface hydrophilicity of the materials and the synergistic effects rendered by the oxygen dopant and vacancies induced by the ion bombardment facilitate faster charge transfer between the catalyst and electrolyte and promote the conductivity and amount of active sites. However, over-doping is counter-productive because of vacancies reoccupation and excessive replacement of Se sites by oxygen atoms. To achieve the best improvement, the balanced substitution of Se by oxygen atoms and vacancies reservation is essential to the optimal control of the active sites, which can be accomplished by tuning the plasma conditions.