Tracking the evolution of photoexcitations in strongly light absorbing systems

Mohan, Varun

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Description

Title

Tracking the evolution of photoexcitations in strongly light absorbing systems

Author(s)

Mohan, Varun

Issue Date

2020-12-04

Director of Research (if dissertation) or Advisor (if thesis)

Jain, Prashant K

Doctoral Committee Chair(s)

Shim, Moonsub

Committee Member(s)

• Braun, Paul V
• Chen, Qian

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Date of Ingest

2021-03-05T21:47:26Z

Keyword(s)

• Photocatalysis
• Light-Matter interactions
• Natural Gas Upgradation
• Nanoparticles

Abstract

This dissertation consists of the work done towards a Ph.D. degree in the research group of Professor Prashant K. Jain at the University of Illinois at Urbana-Champaign. Here, I describe the study of the conversion of light energy using hybrid perovskite and noble metal-based semiconductor nanoparticles. The large surface-area-to-volume ratios and superlative ability to absorb visible light make these materials worthy candidates for solar energy harvesting. The primary questions I have asked in my research are: what is the fate of photoexcitation in a nanostructured material and how can we channel such photo-excitations in an efficient and selective manner? Chapter 1 of this dissertation introduces some of the theoretical backdrops to my studies of strongly light-absorbing plasmonic nanoparticulate systems. This chapter elucidates what follows and introduces the concepts and terms used. Chapter 2 presents my investigation of hybrid organic-inorganic perovskite materials for potential uses towards light trapping and emission. We discovered that commonly observed luminescence from microcrystals of these materials showed a spectrum that varied with sample morphology and location on the sample. The origin of this spectral heterogeneity was then traced to the phenomenon of luminescence self-absorption, which is prevalent due to the overlapping absorption and emission, i.e., small Stokes-shift, in these materials. Then we explored light-to-chemical-energy conversion in perovskite materials, but they proved to be photochemically unstable; so, we turned our attention to noble metal nanoparticles, which have strong plasmon resonance absorption and high photostability. Chapter 3 describes the investigation of light-to-chemical energy conversion in colloidal gold (Au) nanoparticles In particular, we studied the effect of visible-light excitation of Au nanoparticles in the presence of an electron acceptor (HAuCl4) and a hole acceptor (short-chain alcohol). This led to the discovery of a hitherto unknown photoreaction, which involves the splitting and chlorination of the alcohol generating a chloroalkane and an aldehyde. This reaction was found to take place with several alcohols, which led us to a general reaction mechanism that is catalyzed synergistically by the photoexcited nanoparticle and the Lewis acidic HAuCl4. In the specific case of 2-butanol as the hole acceptor, we found a substantial difference between the product distributions of the light-driven reaction as compared to a thermal reaction. This finding represents an example of light-driven-control of catalytic selectivity. Finally, as presented in Chapter 4, the insights gained from the study described in Chapter 3 led me to a new, simple chemical process for low-temperature chlorination of methane in a non-corrosive aqueous environment. The kinetics and mechanism of this reaction were studied. Methane chlorination is at the heart of natural-gas upgradation, so this new finding represents an ideal culmination of my dissertation. An outlook and potential future directions are presented in Chapter 5.

Graduation Semester

2020-12

Type of Resource

Thesis

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© 2020 VARUN MOHAN

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