Probing the Photochemical Dynamics of Water Oxidation on n-srTiO3 Through Time-Resolved Optical Spectroscopy

Singh, Suryansh

Graduate Thesis Or Dissertation

Probing the Photochemical Dynamics of Water Oxidation on n-srTiO3 Through Time-Resolved Optical Spectroscopy Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/cr56n268k

Abstract

Photo-electro catalytic water splitting is a sustainable way to produce hydrogen, which is a clean and energy dense fuel. However, high overpotential and poor catalyst life hold back the water splitting process from being used on the scale needed to combat the energy crisis. The bottleneck is the Oxygen evolution reaction [OER], the oxidative half of the overall water splitting process. OER is a complex multi-step process involving 4 distinct electron transfers and several reactive oxygen intermediates (O* intermediates). Theory suggests that the catalytic activity of any material surface is determined by the interaction of the O* intermediates with the surface in any given catalytic environment. Therefore, the key to gaining a better mechanistic understanding of the OER is to isolate and directly observe the O* intermediates on a model surface in different environmental conditions. Analyzing how carrying out OER affects the catalytic surface, in tandem, is also important as it sheds light on problems such as durability of the catalyst.

In this dissertation, I conduct and study photo-electrocatalytic OER holistically on a n-doped Strontium titanate (SrTiO₃ or STO) semiconductor. I primarily use time resolved pump probe spectroscopy along with in-situ electrochemistry to isolate the O* intermediates and provide insight into the mechanistic details of OER. Using these techniques in tandem allows for tuning of the reaction conditions and monitoring the subsequent effects on the time evolution of the intermediates. Moreover, coherent acoustic interferometry is used to study the strain induced in the catalytic surface when carrying out OER. Several imaging techniques, SEM/TEM/EDAX/AFM/XAS, are also used to characterize the changes that the catalytic surface accrues after OER is conducted.

Using time resolved pump probe spectroscopy, I observed that the populations representing the O* intermediates double as a function of pH (pH ranging between 7 to 13) of the solution which links the O* populations to thermodynamic quantities such as the free energy of formation of the first metastable O* intermediate on STO. The O* intermediate (polaron) is stabilized by deformation of the lattice which launches a strain wave that is detected using coherent acoustic interferometry as coherent oscillations in the data. This methodology allows for the detection of spatial extent, magnitude, and generation time of the intermediate induced interfacial strain through the phase and magnitude of the coherent oscillations. OER on any catalytic surface causes restructuring of the surface which is linked to a degradation of catalytic activity. Several imaging techniques were used in tandem with elemental analysis and X-ray spectroscopy to characterize and quantify the changes in the STO surface caused by OER.

Currently I am investigating the conversion of O* intermediates into molecular oxygen and the influence of tuning reaction conditions using a sub nano-second pump, allowing us to access nano-second to millisecond timescales. I have also built an experimental setup capable of performing FSRS (Femto-second stimulated Raman spectroscopy) experiments in reflectance and absorption geometries to study O* intermediates vibrationally on STO surface.

Going forward, FSRS will be used to resonate on the broadband optical transitions of the O* intermediates and differentiate between them using their vibrational signatures. Additionally, the experimental methodology and analysis techniques developed in this work will be used to explore other OER catalysts, such as TiO₂ and RuO₂ in an effort to generalize the mechanistic understanding of OER to a variety of electrode surfaces.

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