Soft X-ray Absorption Spectroscopy Studies of the Electronic Structure Recovery of Graphene Oxide upon Chemical Defunctionalization

Abstract

A comparative evaluation of the efficacy of different reducing agents in defunctionalizing graphene oxide is performed using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at carbon and oxygen K-edges. The relative intensities of π* and σ* resonances in carbon K-edge NEXAFS spectra suggest relatively high extents of recoveries of π-conjugation upon reduction using vapor-phase phenylhydrazine or hydrazine as well as neat phenylhydrazine. The use of these reducing agents in combination with modest thermal annealing appears to be particularly effective for deepoxidation and decarboxylation of graphene oxide. Polarized carbon K-edge NEXAFS spectroscopy measurements allow evaluation of the extent of warping or planarity induced upon chemical reduction. Treatment with vapor phenylhydrazine and subsequent thermal annealing entirely disrupts the alignment of reduced graphene oxide while facilitating substantial recovery of π conjugation. In contrast, annealing subsequent to treatment with vapor-phase hydrazine yields high dichroic ratio values that are substantially increased as compared to unreduced graphene oxide, indicating a greater extent of substrate alignment. Restoration of excitonic confinement of the σ* resonance is seen to be most pronounced for reduction of graphene oxide by NaBH 4 and vapor-phase phenylhydrazine after annealing, suggesting relatively large conjugated domains in these materials after chemical reduction. ■ INTRODUCTIONThe peculiarities in the electronic structure of graphene, a monolayer of sp 2 -hybridized carbon atoms arrayed in a hexagonal periodic lattice, give rise to remarkable electrical transport phenomena including the half-integer quantum Hall effect, massless Dirac Fermion behavior of charge carriers, and ballistic conduction. 1−3 Given the exceptional promise of graphene for device integration as the active component of sensing, high-frequency electronics, interconnect, and actuator architectures, 4−6 much effort has been directed toward developing scalable routes for the fabrication and precise positioning of graphene with high crystal quality and controlled dimensions. Chemical vapor deposition 7,8 and solution-phase exfoliation routes 9−11 have emerged as the mainstays of these nascent nanomanufacturing efforts that have been imperative for spurring the commercialization of graphene-based products at an unprecedentedly rapid rate for a new material.As a structurally related material, graphite oxide has also been enjoying a renaissance in recent years. 12−16 Although the edgeand basal-plane functionalities of graphite oxide serve as scattering sites, limiting the obtained carrier mobilities and conductivities, the functional groups also endow the material with beneficial characteristics such as solution processability, chemical tunability, and matrix compatibility. The latter bears much significance for applications of exfoliated graphene oxide (GO) as a filler material within matrices wherein it provides improved electrical conductivity, thermal conductance, and...

Near-edge x-ray absorption fine structure spectroscopy study of nitrogen incorporation in chemically reduced graphene oxide

“…It is noted that upon a posthydrazine treatment annealing step to 150 C there is not a significant reduction in the oxygen edge-jump (concentration) due to the fact that the functional groups that could possibly be removed at this relatively low temperature have for the most part already been removed by the hydrazine reduction process. 11,20,22 This is further evidenced by the spectrum for the sample annealed to a temperature of 600 C [ Fig. 7(b)], which indicates a further diminution in the edge jump suggested further defunctionalization.…”

“…The relative intensity of the p* resonance (I p* /I r* ) increases upon reduction of GO with hydrazine, suggesting substantial restoration of the p-conjugated network. [20][21][22] Another indication of the establishment of extended conjugated domains is the appearance of a characteristic doublepeaked structure for the r* excitonic resonance most clearly visible for the GO sample reduced by hydrazine followed by annealing to 150 C. 51,52 The appearance of a sharp r* excitonic resonance has been correlated to good short range order in solid-state carbons and suggests the stabilization of well-defined aromatic regions in RGO. 53 A pronounced broadening of the p* resonance is observed for the RGO samples.…”

“…17 The bond connectivities and specific spatial distributions of different functional groups in GO remain somewhat contested although a tentative consensus is emerging around a modified Lerf-Klinowski model that places epoxide and hydroxyl groups on basal planes with carbonyl, quinone, carboxylic acid, and lactone moieties residing on edges and decorating holes within the graphene lattice. [18][19][20] Relatively less is known about the bond configurations and remnant functional groups in RGO prepared by reacting exfoliated GO with different reducing agents or through thermal annealing. 18,[20][21][22] The incorporation of oxygen and nitrogen atoms within extended conjugated domains of RGO through constitution of heterocycles is expected to substantially alter the transport properties of graphene including the energy of the Dirac point and the work function while also providing sites that can be selectively functionalized.…”

“…[18][19][20] Relatively less is known about the bond configurations and remnant functional groups in RGO prepared by reacting exfoliated GO with different reducing agents or through thermal annealing. 18,[20][21][22] The incorporation of oxygen and nitrogen atoms within extended conjugated domains of RGO through constitution of heterocycles is expected to substantially alter the transport properties of graphene including the energy of the Dirac point and the work function while also providing sites that can be selectively functionalized.…”

“…12,28 As an element-specific probe of the unoccupied density of states, NEXAFS spectroscopy represents a powerful tool for evaluation of local structure and surface chemistry of both molecules and extended solids alike, and has been previously employed to examine nitrogen doping within graphene as well as to probe restoration of the p-conjugated electronic structure of GO upon thermal or chemical defunctionalization. 12,[20][21][22]24,[29][30][31] In addition, NEXAFS spectroscopy has enabled examination of chemical bonding and electronic structure at graphene/metal and graphene/dielectric interfaces. [32][33][34] In contrast to XPS measurements wherein broad lineshapes render assignments to specific bonding configurations rather ambiguous, the local character of x-ray absorption spectroscopy enables substantial insight to be derived from the peak positions and lineshapes of NEXAFS spectra (which in this instance represent a superposition of transitions from locally modified C-C, C-O, and C-N core levels to unoccupied states above the Fermi level).…”