Abstract
Nitrogen-doped graphene oxides (GO:Nx) were synthesized by a partial reduction of graphene oxide (GO) using urea [CO(NH2)2]. Their electronic/bonding structures were investigated using X-ray absorption near-edge structure (XANES), valence-band photoemission spectroscopy (VB-PES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). During GO:Nx synthesis, different nitrogen-bonding species, such as pyrrolic/graphitic-nitrogen, were formed by replacing of oxygen-containing functional groups. At lower N-content (2.7 at%), pyrrolic-N, owing to surface and subsurface diffusion of C, N and NH is deduced from various X-ray spectroscopies. In contrast, at higher N-content (5.0 at%) graphitic nitrogen was formed in which each N-atom trigonally bonds to three distinct sp2-hybridized carbons with substitution of the N-atoms for C atoms in the graphite layer. Upon nitrogen substitution, the total density of state close to Fermi level is increased to raise the valence-band maximum, as revealed by VB-PES spectra, indicating an electron donation from nitrogen, molecular bonding C/N/O coordination or/and lattice structure reorganization in GO:Nx. The well-ordered chemical environments induced by nitrogen dopant are revealed by XANES and RIXS measurements.
Highlights
The changing gapless property of graphene from dopant-induced perturbation and defect/vacancy-induced rearrangement need to be understood in order to develop energy storage and environmental applications[16,17]
Li et al.[25] developed a chemical method to obtain N-doped graphene oxides (GO) through thermal annealing of GO in ammonia at various temperatures; where they found that the content of doped nitrogen achieved by the replacement of oxygen-containing functional groups is proportional to the increase in temperature
The transmission electron microscopic (TEM)-energy dispersive spectroscopy (EDS) study revealed that the oxygen content gradually decreases in the order from 32.5 (GO) → 26.2 (GO:N2.7) → 19.3 at% (GO:N5.0) as the carbon content increases in the order from 67.5(0) → 71.1(2.7) → 75.7(5.0) at% in GO and GO:Nx
Summary
Cheng-Hao Chuang[1], Sekhar C. GO was used as a starting material to elucidate the mechanism of the nitrogen substitution by replacing the oxygen-related bonds, and to verify the modification of electronic/bonding structures and changes in the binding energy of GO:Nx by the partial reduction of GO with urea using household microwave This process is simple and differs from several other reported approaches[19,25,26,32,33]. The chemical environments in GO:Nx are monitored by studing the C, N, and O at various concentrations of nitrogen; the effect on band gap opening (or energy separation) of the activation of nitrogen and oxygen atoms in the graphene-basal plane network is examined For this purpose, various forms of spectroscopy, including Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), valence-band photoemission spectroscopy (VB-PES), and combined XANES/XES and RIXS techniques were used
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