Li Zhang

Advisor: Prof. Losego

will defend a doctoral thesis entitled,

Vapor Phase Infiltration of Metal Halides into the Conjugated Polymer P3HT for Doping and Photocatalytic Applications

On

Thursday, September 18 at 10:00 a.m.
Love 311

and

 Virtually via MS Teams 

Committee

            Prof. Mark Losego – School of Materials Science & Engineering (advisor)

            Prof. Juan-Pablo Correa-Baena – School of Materials Science & Engineering
            Prof. Guoxiang Hu– School of Materials Science & Engineering
            Prof. Antonio Facchetti – School of Materials Science & Engineering
            Prof. Marta Hatzell – George W. Woodruff School of Mechanical Engineering

Abstract

Vapor phase infiltration (VPI) is a technique used to hybridize the bulk of a polymer by using a vapor phase precursor. The precursor adsorbs onto the surface, diffuses into the bulk and eventually reacts with the polymer. VPI has found various applications in membranes, energy storage, catalysis, etc. In this thesis I will show the application of VPI for the doping of conjugated polymers and creating hybrid conjugated polymer-metal oxide dye-sensitized photocatalysts. Conjugated polymers have a continuous chain of conjugation along the backbone giving them their sought after semiconducting properties. When these polymers are oxidized through a redox reaction, polaronic charge carriers are created increasing the overall conductivity.

Solution doping of conjugated polymers is well-documented and highly effective, but the organic solvents necessary to swell the polymer can also destroy any patterning or microstructure. The vapor phase precursors TiCl4 and VOCl3 are shown to effectively dope P3HT with an initially increasing conductivity with increasing exposure times that eventually begins to drop down. This same trend has been noted in literature. The initial rise in conductivity is confirmed to be an increase in number of charge carriers. However, the UV-Vis measurements both in situ and ex situ show that the decrease in conductivity is due to both scattering sites being generated and the polymer dedoping, a phenomenon that has not been reported previously. XPS analysis and depth profile data show how limiting the diffusion of the VOCl3 doping agent allows for the dedoping rate to exceed the doping rate beginning at the underside of the film.

Infiltrated MOx materials made by VPI have been previously shown as nothing but scattering agents decreasing the conductivity. Herein, I show that infiltrated TiOx clusters can be used as catalyst centers with the P3HT acting as a dye-sensitizing material. Photoluminescence spectra show that the visible light absorbed by the P3HT excites an electron which is transferred to the TiOx to perform photocatalysis. Furthermore, VPI is shown to make a superior architecture where the TiOx is concentrated towards the surface, as opposed to burying the MOx under the conjugated polymer as is common in literature.