Preparation and characterization of surfaces of graphene and graphite and study of reactions of SO₂ and NH₃ with the epoxy groups on the surface of graphite oxide powder

Abstract

Graphene, a single-layer of carbon atoms, has been widely used in various applications due to its unique physical properties and two-dimensional structure. Graphite, which is made of stacked carbon layers, exhibits good electrical and thermal conductivities. Highly oriented pyrolytic graphite (HOPG) is a highly ordered form of synthetic graphite and has a smooth and almost defect-free surface. In this thesis, we used X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to characterize the surface chemical compositions of graphene and graphite annealed at different temperatures and after different surface modifications. First, the surface of graphene after washing with acetone and annealing at different temperatures was monitored by XPS and ToF-SIMS. The concentrations of residual poly(methyl methacrylate) (PMMA) and hydrocarbon contaminants decreased as the annealing temperature increased. A clean graphene surface can be obtained by annealing at 500 °C in an ultra-high vacuum chamber without creating any additional defects. In the XPS C1s spectrum of graphene, besides an asymmetric sp² carbon peak and a π-π* shake-up peak appeared, an additional sp³ carbon peak representing sp³ defects was also present. In the ToF-SIMS positive ion spectrum of clean graphene, a series of C_xH₂^+• (where x=1, 2, 3…) ions originated from the defects of graphene was found. To confirm this finding, defects were created by ion bombardment of a HOPG surface. A detailed examination of the positive ion spectra of ion-bombarded HOPG surfaces reveals the presence of the C_xH₂^+• ions, confirming that these C_xH₂^+• ions, which came from the defects created on the sputtered HOPG surfaces, are similar to the defects present on a graphene surface. A sp³ carbon peak representing sp³ defects appeared in the XPS spectra of sputtered HOPG samples, confirming that the surface of the sputtered HOPG is similar to that of graphene. The sputtered HOPG was then annealed at different temperatures under Ar flow. The XPS and ToF-SIMS spectra of the sputtered HOPG after annealing at 800 °C were observed to be similar to the spectra of the fresh HOPG. The sp³ carbon peak had disappeared from the C1s spectrum and the normalized intensities of the C_xH^- and C_xH₂^+• ions had decreased. These results indicate that defects created by sputtering on the surface of HOPG can be repaired by high temperature annealing. Second, hydrogenated graphite powder was obtained through Birch reduction of graphite powder and characterized by XPS and ToF-SIMS at 500 °C. The formation of sp³ carbons at the edges of the surface of the hydrogenated graphite powder contributes to further analyzing of sp³ carbons. The formed sp³ carbons on the hydrogenated graphite powder surface exhibited a sp³ carbon peak in the XPS C1s spectrum. Two sets of peaks, the C_x^- and C_xH^- ion series, were identified in the ToF-SIMS spectra of both the graphite and hydrogenated graphite powders. The difference between these two spectra represented an increase in the normalized intensities of the H^- and C_xH^- ions in the spectrum of the hydrogenated graphite powder, indicating the formation of more sp³ carbons on the surface. Third, the surface of graphite powder was further modified and used for environmental applications. Graphite oxide powder was obtained using the modified Hummers’ method and characterized using XPS and ToF-SIMS. The XPS results indicate that the epoxy groups are the main functional groups on the graphite oxide powder surface. The graphite oxide powder was then reacted with sulfur dioxide (SO₂) and ammonia (NH₃) gases, respectively, at 25 °C. The XPS and ToF-SIMS analyses of the surface of the reacted graphite oxide powder showed that bisulfate and amine groups were formed on the surface of the graphite oxide powder after the reactions between the graphite oxide powder and SO₂ and NH₃ gases.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology