Functionalized-graphene composites for enhancing charge transport, reactivity and selectivity

Abstract

The thesis is concerned with the design of functional graphene analogues (i.e., graphene oxide and reduced graphene oxide) for critical applications concerning the Energy and the Environment. While capitalizing on the exceptional electric conductive property of graphene, the thesis further explores ways of imparting new chemical functions to the graphene as part of the wider smart architecture of high performance device and composites. The work begins with the functionalization of graphene oxide (GO) onto L-cysteine modified Au electrode for the ultrasensitive detection of heavy metal ions (Pb2+, Cu2+ and Hg2+) in drinking water. The GO was attached via amidation using the facile EDC/NHS route. The GO-modified Au electrode exhibits improved detection limits by two orders of magnitudes (reaching sub-ppb levels) relative to the control electrode. This is attributed to the extended sites for the adsorption of heavy metal ions. Compared to reduced graphene oxide (rGO), the GO exhibits higher adsorption capacity of heavy metal ions due to the complexation with the rich oxygenated groups on the basal plane of the graphene sheets. In the second part of the thesis, spatially-controlled deposition of rGO and Pt on TiO2 were fabricated and assessed for the photocatalytic degradation of a range of organic compounds. By selectively depositing Pt onto rGO followed by attachment on TiO2 (Pt-rGO/TiO2), or by exclusively depositing Pt onto TiO2 followed by the attachment of rGO (rGO/Pt-TiO2), and by comparing with Pt/TiO2 and rGO/TiO2, we established the actual roles of Pt and rGO in photocatalysis. The deposition of Pt cocatalyst on TiO2, which formed the Schottky barrier, was found to be more efficient in enhancing photocharge separation than the direct attachment with rGO that merely extracts the photoelectrons interfacially. For the same Pt and rGO loading, the most efficient configuration was achieved by rGO/Pt-TiO2. The relationship between the physicochemical properties of the composites and the photocatalytic efficiencies of direct hole oxidation (oxalic acid), indirect oxidation by hydroxyl radicals (isopropanol and tert-butanol), and de-aromatization and dechlorination (2,4-dichlorophenoxy acetic acid) was established. From there, further enhancement in photocatalytic activity was achieved by optimizing the additional loading of Pt onto the rGO component of the rGO/Pt-TiO2 sample. As a result, new photocatalytic efficiency that is 3-fold that of the optimum Pt/TiO2 was produced. The last part of the work focused on the development of a new Bi2O3-based cocatalyst for the selective CO2 reduction. The cocatalyst consisted of Bi2O3 eposited on Pt/rGO, and which can be conveniently attached onto the BaLa4Ti4O15 (BLTO) and Ag/AgCl photocatalysts. The presence of Bi2O3 on rGO (where photoelectrons are interfacially extracted from the photocatalysts) captures the CO2 in the form of carbonate and allowing it to be photoreduced selectively to hydrocarbons with minimal H2 evolution. The major product from the CO2 reduction was identified as formaldehyde for BLTO and CO for Ag/AgCl, consistent with their conduction band edge potentials. The general cocatalyst developed in this work can be applied to various other photocatalysts, as long as their conduction band edge

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wey Yang TEOH (Supervisor)