Structural Design for Promoting the Activity of Graphene Oxide Dots in Photocatalytic Hydrogen Generation from Water

Abstract

Graphene-based materials consisting of C H O and N elements are highly potential materials for solving energy and environmental problems by utilizing the clean solar energy One of the features of graphene is the tunable electronic structure and the band gap can be tailored by the chemical modification Converting graphene into graphene oxide dots (GODs) can increase the surface area and the hydrophilic functional groups make GODs well disperse in water The novel physical chemistry properties make GODs interesting candidates for photo-energy conversion applications in photocatalysis This dissertation includes two parts: 1 Incorporating nitrogen-doped graphene oxide dots with graphene oxide sheets for stable and effective hydrogen production through photocatalytic water decomposition 2 Architecting nitrogen functionalities on graphene oxide photocatalysts for boosting hydrogen production in water decomposition process In the first part we incorporate nitrogen-doped graphene oxide dots (NGODs) with graphene oxide (GO) sheets to form a stable and effective NGOD:GO composite for photocatalytic H2 production through water splitting under visible light illumination Although Pt-deposited NGOD catalysts were active in the photocatalytic H2 production reaction they were only moderately stable Introducing GO sheets in light-absorbing NGODs effectively mediated the transfer of photogenerated electrons from the NGODs to the GO sheets This vectorial electron transfer confirmed by a photoluminescence spectroscopy analysis led to the relocation of the reaction sites from the NGODs to the GO sheets protecting the NGODs from attack by reaction intermediates Moreover the GO sheets acted as an electron sink facilitating charge separation in the NGODs When 3 wt% Pt was deposited on the developed NGOD:GO catalyst the catalyst steadily catalyzed H2 production from a 10 vol% aqueous solution of triethanolamine under visible light illumination for 96 h unlike a NGOD catalyst that exhibited an activity decay of 50% within 96 h The apparent quantum yield of H2 under 420-nm light irradiation was 10 0% demonstrating the high activity of the NGOD:GO catalyst In the second part we elucidate how nitrogen functionalities influence the transition and transfer of photogenerated electrons in graphene-based materials Graphene oxide dots (GODs) and N-doped GODs (NGODs) are synthesized by thermally treating graphene oxide (GO) sheets in argon and ammonia respectively and then ultrasonically exfoliating the sheets in nitric acid The nitrogen functionalities of NGODs are mainly quaternary/pyridinic/pyrrolic and the nitrogen atoms in these functionalities are planar to the GO sheets and repair the vacancy defects on the sheets Hydrothermal treatment of NGODs in ammonia yields ammonia-treated NGODs (A-NGODs) with some pyridinic/pyrrolic groups being converted to amino/amide groups The nitrogen atoms in the amino/amide groups are not planar to the GO sheets and are prone to donate their lone pair electrons to resonantly conjugate with the aromatic ? electrons The promoted conjugation facilitates the relaxation of photogenerated electrons to the triplet states and prolongs the electron lifetime When deposited with Pt as the co-catalyst the samples catalyze H2 production from an aqueous triethanolamine solution under 420-nm monochromatic irradiation at quantum yields of 7 3% (GODs) 9 7% (NGODs) and 21% (A-NGODs) The high activity of A-NGODs demonstrates that architecting nitrogen functionalities effectively mediate charge motion in carbon-based materials for application to photoenergy conversion

Date of Award	2016 Jul 19
Original language	English
Supervisor	Hsisheng Teng (Supervisor)