Photochemical Sciences Ph.D. Dissertations

Single-molecule interfacial electron transfer dynamics in solar energy conversion

Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

H. Peter Lu (Advisor)

Second Advisor

Yu Zhou (Other)

Third Advisor

Ksenija D. Glusac (Committee Member)

Fourth Advisor

Alexey T. Zayak (Committee Member)

Abstract

This dissertation work investigated the parameters affecting the interfacial electron transfer (ET) dynamics in dye–semiconductor nanoparticles (NPs) system by using single-molecule fluorescence spectroscopy and imaging combined with electrochemistry.

The influence of the molecule–substrate electronic coupling, the molecular structure, binding geometry on the surface and the molecule-attachment surface chemistry on interfacial charge transfer processes was studied on zinc porphyrin–TiO2 NP systems. The fluorescence blinking measurement on TiO2 NP demonstrated that electronic coupling regulates dynamics of charge transfer processes at the interface depending on the conformation of molecule on the surface. Moreover, semiconductor surface charge induced electronic coupling of molecule which is electrostatically adsorbed on the semiconductor surface also predominantly alters the ET dynamics.

Furthermore, interfacial electric field and electron accepting state density dependent ET dynamics has been dissected in zinc porphyrin–TiO2 NP system by observing the single-molecule fluorescence blinking dynamics and fluorescence lifetime with and without applied bias. The significant difference in fluorescence fluctuation and lifetime suggested the modulation of charge transfer dynamics at the interface with external electric field perturbation. Quasi-continuous distribution of fluorescence intensity with applied negative potential was attributed to the faster charge recombination due to reduced density of electron accepting states.

The driving force and electron accepting state density ET dependent dynamics has also been probed in zinc porphyrin–TiO2 NP and zinc porphyrin–indium tin oxide (ITO) systems. Study of a molecule adsorbed on two different semiconductors (ITO and TiO2), with large difference in electron densities and distinct driving forces, allows us to observe the changes in rates of back electron transfer process reflected by the suppressed fluorescence blinking of molecule on ITO surface.

Finally, the electric field effect on the interface properties has been probed by using surface-enhanced Raman spectroscopy and supported by density functional theory calculations in alizarin–TiO2 system. The perturbation, created by the external potential, has been observed to cause a shift and/or splitting interfacial bond vibrational mode, typical indicator of the coupling energy changes between alizarin and TiO2. Such splitting provides evidence for electric field-dependent electronic coupling changes that have a significant impact on the interfacial electron transfer dynamics.

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