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Understanding Pristine Hematite Photoanodes and Their Modification with Cocatalysts for Water Oxydation


Li, Jingguo. Understanding Pristine Hematite Photoanodes and Their Modification with Cocatalysts for Water Oxydation. 2021, University of Zurich, Faculty of Science.

Abstract

This thesis presents some fundamental studies of hematite photoanodes which are important for understanding the interfacial catalytic processes for water oxidation. In the first chapter, we briefly introduce the research background of photoelectrochemical techniques and the importance of the water oxidation reaction. Next, in the second introductory chapter we review the research advance of hematite photoanodes and focus on the up-to-date understanding of charge carriers at different stages. Specifically, it begins with the electronic structure and absorption features of this model material, highlighting the controversial origin of the band edge absorption. Next, the dynamics of charge carriers are discussed both on the ultrafast and on the surface reaction timescales, with special emphasis on the assignment of photogenerated holes and their characteristics. Afterward, the debate of charge transfer via surface states or valence band is reviewed, and the surface states mediated pathway is carefully addressed. Further, the chemical signature of some reaction intermediates is summarized, which is essential to understand the surface state chemistry on hematite photoanodes. In addition, the research on the function of photoanodes modified with cocatalysts is updated, with the aim to exemplify the complexity of charge carrier dynamics on the solid-liquid interface. We aim to provide comprehensive understanding for the development of an efficient hematite photoanode that will also inspire the investigation of many other n-type photoanode designs.
In the third chapter, we present the preparation processes of pristine hematite photoanodes and their modification with molecular cocatalysts. Further, these photoanodes are characterized first with some basic structural and spectroscopic analyses. More importantly, some key analytical (photo)electrochemical techniques are introduced in detail. In the fourth chapter, we identify two different types of surface states on model hematite photoanode surfaces at pH 8 by photo-electrochemical impedance analysis. We show that the first surface states with higher oxidative energies undergo distinct 1st to 3rd order water oxidation transition as the illumination intensity is increased. In contrast, the second type of surface states located at lower potentials exhibits an unexpected 0th to 3rd order reaction kinetics transition under similar light modulation. We explain this result by a dynamic interplay model of both surface states where two reaction intermediates, iron-oxo and iron-peroxo species, are proposed, respectively. Furthermore, we employ fast-cathodic cyclic voltammetry to estimate the lifetimes of both surface states, where an exceptionally long-lived signal (>180 s) is assigned to the second surface states. More importantly, it is observed that the distributions and reaction kinetics of both surface states undergo significant changes upon the modulation of surface protonation state (by changing the pH of electrolyte). All these results highlight the importance of operational parameters (illumination intensity, electrolyte pH, and external bias) on the dynamic interaction both surface states. We propose clear chemical signatures for both surface states which helps to understand the reaction mechanism at the semiconductor-electrolyte interface.
In the next chapter, we employ new hybrid molecular photoanodes (cobalt-based tetranuclear cubanes as cocatalysts on hematite) as model systems. An interesting functionality transition of these molecular cubanes upon modulation of applied bias was revealed with in-house photoelectrochemical and kinetic rate law analyses. Specifically, molecular cocatalysts work predominantly as hole reservoirs at moderate potentials and change to catalytic centres at higher potentials. Detailed kinetic analyses further indicate that this function transition arises from the change of surface equilibrium of photo-generated charge carriers. More importantly, we reproduce this dynamic function of the cocatalysts with several cobalt-based molecular and heterogeneous catalysts. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems which will help to design tailor-made hybrid devices for a wide range of catalytic applications. Finally, we summarize major contributions of this thesis and present follow-up investigations which are important for in-depth understanding the hematite photoanodes.

Abstract

This thesis presents some fundamental studies of hematite photoanodes which are important for understanding the interfacial catalytic processes for water oxidation. In the first chapter, we briefly introduce the research background of photoelectrochemical techniques and the importance of the water oxidation reaction. Next, in the second introductory chapter we review the research advance of hematite photoanodes and focus on the up-to-date understanding of charge carriers at different stages. Specifically, it begins with the electronic structure and absorption features of this model material, highlighting the controversial origin of the band edge absorption. Next, the dynamics of charge carriers are discussed both on the ultrafast and on the surface reaction timescales, with special emphasis on the assignment of photogenerated holes and their characteristics. Afterward, the debate of charge transfer via surface states or valence band is reviewed, and the surface states mediated pathway is carefully addressed. Further, the chemical signature of some reaction intermediates is summarized, which is essential to understand the surface state chemistry on hematite photoanodes. In addition, the research on the function of photoanodes modified with cocatalysts is updated, with the aim to exemplify the complexity of charge carrier dynamics on the solid-liquid interface. We aim to provide comprehensive understanding for the development of an efficient hematite photoanode that will also inspire the investigation of many other n-type photoanode designs.
In the third chapter, we present the preparation processes of pristine hematite photoanodes and their modification with molecular cocatalysts. Further, these photoanodes are characterized first with some basic structural and spectroscopic analyses. More importantly, some key analytical (photo)electrochemical techniques are introduced in detail. In the fourth chapter, we identify two different types of surface states on model hematite photoanode surfaces at pH 8 by photo-electrochemical impedance analysis. We show that the first surface states with higher oxidative energies undergo distinct 1st to 3rd order water oxidation transition as the illumination intensity is increased. In contrast, the second type of surface states located at lower potentials exhibits an unexpected 0th to 3rd order reaction kinetics transition under similar light modulation. We explain this result by a dynamic interplay model of both surface states where two reaction intermediates, iron-oxo and iron-peroxo species, are proposed, respectively. Furthermore, we employ fast-cathodic cyclic voltammetry to estimate the lifetimes of both surface states, where an exceptionally long-lived signal (>180 s) is assigned to the second surface states. More importantly, it is observed that the distributions and reaction kinetics of both surface states undergo significant changes upon the modulation of surface protonation state (by changing the pH of electrolyte). All these results highlight the importance of operational parameters (illumination intensity, electrolyte pH, and external bias) on the dynamic interaction both surface states. We propose clear chemical signatures for both surface states which helps to understand the reaction mechanism at the semiconductor-electrolyte interface.
In the next chapter, we employ new hybrid molecular photoanodes (cobalt-based tetranuclear cubanes as cocatalysts on hematite) as model systems. An interesting functionality transition of these molecular cubanes upon modulation of applied bias was revealed with in-house photoelectrochemical and kinetic rate law analyses. Specifically, molecular cocatalysts work predominantly as hole reservoirs at moderate potentials and change to catalytic centres at higher potentials. Detailed kinetic analyses further indicate that this function transition arises from the change of surface equilibrium of photo-generated charge carriers. More importantly, we reproduce this dynamic function of the cocatalysts with several cobalt-based molecular and heterogeneous catalysts. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems which will help to design tailor-made hybrid devices for a wide range of catalytic applications. Finally, we summarize major contributions of this thesis and present follow-up investigations which are important for in-depth understanding the hematite photoanodes.

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Additional indexing

Item Type:Dissertation (cumulative)
Referees:Patzke Greta R, Tilley David, Osterwalder Jürg
Communities & Collections:07 Faculty of Science > Department of Chemistry
08 Research Priority Programs > Solar Light to Chemical Energy Conversion
UZH Dissertations
Dewey Decimal Classification:540 Chemistry
Language:English
Place of Publication:Zurich
Date:2021
Deposited On:15 Feb 2022 09:44
Last Modified:19 Dec 2023 16:18
OA Status:Closed