Understanding the Surface Chemistry of Porous Silicon for Applications in Hybrid Photoelectrocatalysis

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Mayer, James

Abstract

Increased grid reliance on solar and wind will require novel energy storage technologies. Converting electrical energy to chemical energy using a catalyst is one such technology that has multiple benefits. Liquid fuels are energy dense, chemical products that could be used in a closed loop fuel cell, and catalysts could be designed to avoid critical metals. The Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE) aims to produce hybrid catalysts that can use light and potential to drive the reduction of CO2 to more useful chemicals. These hybrid catalysts are constructed from molecular catalysts attached to a semiconducting support.Porous silicon has proven to be a robust semiconducting platform for covalently attached molecular catalysts. Fabricating and characterizing these hybrid photoelectrodes is nontrivial, but the benefits in terms of product selection and stability during catalysis are significant. The surface chemistry of porous silicon is a crucial factor in determining hybrid photoelectrode performance. After fabrication through an electrochemical etching method, the surface of porous silicon is covered in a monolayer of hydrides. Under catalytic conditions, H2 is produced from these Si-H bonds and the silicon oxidizes even under reducing conditions. Describing and controlling these effects are of prime importance for the design of improved hybrid photoelectrodes. Beyond applications within catalysis, there is broad appeal in studying the fundamental surface chemistry of porous silicon. Porous silicon is used in a range of fields including drug delivery, energetic materials, and chemical sensors. All applications of porous silicon depend on the chemical environment on the surface, and often the modification of the Si-H bonds native to the fresh etched surface. The overarching goal of the work presented herein is to gain insight into the surface chemistry of porous silicon. The conditions under which this surface chemistry is examined are related to catalysis, and indeed two examples of functional hybrid photoelectrodes are presented here. Chapter 2 of this thesis presents a chemist’s perspective on porous silicon. Existing resources describing how to make and characterize porous silicon are often written for an audience with a materials background, or with some existing familiarity. This chapter offers an accessible starting point to learning about porous silicon with particular emphasis on the chemistry of etching and the characterization of porous silicon. Chapter 3 describes the first example of a molecular catalyst attached to H-terminated porous silicon. This hybrid photoelectrode reduces CO2 to CO with high selectivity. The attachment, characterization, and catalytic behavior of this system are all discussed. Chapter 4 contains another example of a functional hybrid photoelectrode. This chapter examines the first reported example of a formate-producing molecular catalyst attached to oxidized porous silicon. Similar in structure to Chapter 3, this work additionally explores how additives control product selectivity during catalysis and compares different catalyst linkers. Chapter 4 is in some ways an extension of Chapter 2. The conditions under which hydrogen is produced in Chapter 2 are analyzed and through isotopic analysis, the various pathways through which porous silicon can make H2 are defined. Chapter 5 is focused on the fundamental reactivity of Si-H bonds and presents a suggested method for quantifying Si-H bonds after etching in order to derive balanced reactions. Also in this chapter is an attempt to understand how applied potential affects the reactivity of Si-H bonds. The work presented in this thesis, and indeed all work within CHASE, is highly collaborative. The specific contributions from each scientist are acknowledged at the beginning of each chapter, and Appendix F contains a summary of the data and goals of ongoing CHASE collaborations involving porous silicon

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