Date of Award

Spring 2023

Project Type

Dissertation

Program or Major

Materials Science

Degree Name

Doctor of Philosophy

First Advisor

Gonghu Li

Second Advisor

Gonghu Li

Third Advisor

James Krzanowski

Abstract

Semiconductor photocatalysis is a viable solution to tackle the increasing global demand for energy and the need for the recovery of wastewater in situ, without the need for fossil fuel dependence. Semiconductor materials active in the visible or near visible regions of light have been explored as suitable candidates for photocatalytic environmental remediation. Magnesium tantalates (MgTa2O6-xNx) were initially explored as promising candidates because their bandgaps can be brought well into the visible region by the introduction of nitrogen atoms as dopants. The synthesis of a highly ordered mesoporous nitrogen doped magnesium tantalate was achieved and the bandgap of magnesium tantalate was successfully shifted from ~4 eV to ~2.1 eV upon nitrogen doping, with a pore size of ~ 3.2 nm and a surface area of ~ 50 m2/g. While these properties suggested that the material would be well suited for CO2 reduction, it showed poor activity for the production of CO from CO2under visible light irradiation, likely due to the lack of good catalytic sites. Graphitic carbon nitrides (CNs), a class of carbon nitride materials which have recently gained popularity because of their favorable properties and ease of synthesis was subsequently explored for modifications that would engineer defects to optimize their properties and minimize their rate of charge carrier recombination. Preliminary studies showed that the optimal synthesis dwelling time of urea-based CN materials is 4 hrs, that the property of the CN materials is reliant on the nature of the starting precursor. The CN materials are capable of producing hydrogen peroxide which, when combined with iron, was successful in the degradation of textile wastewater. The milling of two precursors realized a CN material that showed a favorable response for dye degradation after ammonia exfoliation, and in contrast to the observations noticed from similarly exfoliated counterparts made from single precursors. Modification strategies such as exfoliation with urea showed an alternation between exfoliation and structure refinement, with the structurally refined samples performing better for the degradation of rhodamine B. The use of potassium hydroxide as an alkali modifier caused complete destruction of the CN network for CN to Al ratios exceeding 10 : 3. At a low concentration however, it enhanced the photoactivity of the CN material for the degradation of rhodamine B and showed the highest activity in CO2 reduction when the material was simply heat treated and washed. An attempt to mechanochemically synthesize CN by milling pure precursors and aluminum powder was unsuccessful, but when pre-synthesized CN was used, nitrogen deficiencies were successfully introduced at low milling times, and a C ≡ C defect which is not commonly reported for CN materials was noticed after prolonged milling. These CN materials showed selectivity for CO generation in CO2 reduction, presenting a novel and green route to the denitrification of CN materials for photocatalysis. Catalase was used as a scavenging agent for the removal of CN generated hydrogen peroxide during the degradation of rhodamine B. Such studies indicated that the presence of hydrogen peroxide in photocatalytic degradation of rhodamine B over CN led to a 2.5-fold enhancement in activity. A possible mechanism for this enhancement was proposed as occurring in one of two possible pathways. The prevention of charge carrier recombination which increases the number of holes available for the oxidation of organic matter, or the use of electrons to facilitate the decomposition of hydrogen peroxide and the subsequent generation of hydroxyl radicals which subsequently further the oxidation of organic matter.

Download

Share

COinS