In Situ Nanoultrasonic Imaging of Anodic Oxidation during Photoelectrochemical Water Splitting
Date Issued
2015
Date
2015
Author(s)
Weng, Meng-Yu
Abstract
Phenomenon at solid/liquid interface is interesting and plays a key role in many natural processes. Examples include chemical reactions at electrode surfaces in electrolyte. Photoelectrochemical (PEC) water splitting, one of the methods for hydrogen generation, takes the advantage of using solar energy to split water is regarded as a solution to the energy issue. Including PEC, the chemical reaction of energy transfer happening at solid/liquid interface faces the problem of stability and efficiency. To study and monitor the stability and efficiency problems of an electrode, a technique with capability to in situ image the structure of an electrode at solid/liquid interface is needed. Among all current techniques capable to in situ monitor the structure change of a chemical reaction at solid/liquid interface, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) all require a vacuum environment for operation. Atomic force microscopy (AFM) and scanning tunneling microscope (STM) lack the sub-surface and element analysis information. Taking the development of ultrashort optical pulses, applying sonar technique to thin films and nanostructure has now become possible. Nanoultrasonics with THz coherent acoustic phonons has the capability to image the sub-surface nanostrucutres without the need of vacuum environment. In this thesis, the capability of nanoultrsonics to in situ monitor a chemical reaction at solid/liquid interface is investigated. The surface of n-GaN used as photoelectrode in PEC water splitting is taken as an imaged model for nanoultrasonics technique. In this thesis, we in situ monitor a growth of oxide film process at n-GaN/water interface in PEC water splitting for hydrogen generation. The in situ real time ultrasound image can show the thickness of etched n-GaN cap layer and the thickness of growth Ga2O3 thin film with atomic resolution. The performances of spatial resolution and imaging rate of the nanoultrasonics and current techniques (TEM, SEM, XPS, AFM and STM) with capability monitoring the structure change of a chemical reaction at solid/liquid interface are discussed. The nanoultrasonics gives a way to image a dynamic change of structure at solid/liquid interface with atomic spatial resolution under atmospheric conditions.
Subjects
Nanoultrasonics
Photoelectrochemical water splitting
SDGs
Type
thesis
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