Twist-boat conformation in graphene oxides

Alternatives for Epoxides in Graphene Oxide

The structural mystery of the long-known graphene oxide (GO) unfolds as one of the most abstract conceptual problems among nanomaterials. Generally construed as the oxidized form of graphene, it is proposed to host a variety of functional groups with oxygen and hydrogen. Theoretical studies are abundant on the highly strained epoxides, while larger cyclic ethers having one or more oxygen atoms and vinylogous carbonyls are paid scant attention even though they are predicted by several structural models. The nature of the geometric and electronic structures of these alternative functional groups, the preferred distribution on the graphene lattice, comparative stability, etc., remain unexplored. Our systematic inquiry into the impact of hexagonal and periodic constraints on these mystic functional groups unveils several surprises. Among those that retain the hexagonal carbon backbone, epoxides are surprisingly more stable than larger ethers despite the excessive strain associated with their acute triangular geometry. Epoxidation conserves the planarity of the carbon backbone, which allows their optimal distribution on the lattice by reducing the repulsive interactions from oxygen lone pairs and π-electrons. These findings categorically rule out the possibility of 1,3-ethers in GO and settle the longstanding debate on its existence. They face severe steric repulsion even in low-dimensional systems that tear the σ-skeleton of graphene completely apart, reducing its dimensionality. We show that selective breaking of the σ-bonds is preferred over that of epoxides if backed by cyclic delocalization of electrons. Particularly, 1,6-diones in trans orientation are thermodynamically favored and justify the large holes experimentally observed through microscopic imaging.

Enhancement of CO2 adsorption on oxygen-functionalized epitaxial graphene surface under near-ambient conditions

The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult under ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied under near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.

Tuning the activity of the inert MoS2 surface via graphene oxide support doping towards chemical functionalization and hydrogen evolution: a density functional study

The basal plane of MoS₂ provides a promising platform for chemical functionalization and the hydrogen evolution reaction (HER); however, its practical utilization remains challenging due to the lack of active sites and its low conductivity. Herein, using first principles simulations, we first proposed a novel and effective strategy for significantly enhancing the activity of the inert MoS₂ surface using a graphene oxide (GO) support (MoS₂/GOs). The chemical bonding of the functional groups (CH₃ and NH₂) on the MoS₂-GO hybrid is stronger than that in freestanding MoS₂ or MoS₂-graphene. Upon increasing the oxygen group concentration or introducing N heteroatoms into the GO support, the stability of the chemically functionalized MoS₂ is improved. Furthermore, use of GOs to support pristine and defective MoS₂ with a S vacancy (S-MoS₂) can greatly promote the HER activity of the basal plane. The catalytic activity of S-MoS₂ is further enhanced by doping N into GOs; this results in a hydrogen adsorption free energy of almost zero (ΔG_H = ∼-0.014 eV). The coupling interaction with the GO substrate reduces the p-type Schottky barrier heights (SBH) of S-MoS₂ and modifies its electronic properties, which facilitate charge transfer between them. Our calculated results are consistent with the experimental observations. Thus, the present results open new avenues for the chemical functionalization of MoS₂-based nanosheets and HER catalysts.

Two-Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications

Fluorinated graphene, an up-rising member of the graphene family, combines a two-dimensional layer-structure, a wide bandgap, and high stability and attracts significant attention because of its unique nanostructure and carbon-fluorine bonds. Here, we give an extensive review of recent progress on synthetic methods and C-F bonding; additionally, we present the optical, electrical and electronic properties of fluorinated graphene and its electrochemical/biological applications. Fluorinated graphene exhibits various types of C-F bonds (covalent, semi-ionic, and ionic bonds), tunable F/C ratios, and different configurations controlled by synthetic methods including direct fluorination and exfoliation methods. The relationship between the types/amounts of C-F bonds and specific properties, such as opened bandgap, high thermal and chemical stability, dispersibility, semiconducting/insulating nature, magnetic, self-lubricating and mechanical properties and thermal conductivity, is discussed comprehensively. By optimizing the C-F bonding character and F/C ratios, fluorinated graphene can be utilized for energy conversion and storage devices, bioapplications, electrochemical sensors and amphiphobicity. Based on current progress, we propose potential problems of fluorinated graphene as well as the future challenge on the synthetic methods and C-F bonding character. This review will provide guidance for controlling C-F bonds, developing fluorine-related effects and promoting the application of fluorinated graphene.

Antibacterial activities and mechanisms of fluorinated graphene and guanidine-modified graphene

The antibacterial properties and mechanism of three types of graphene derivatives, graphene oxide (GO), fluorinated graphene (FG), and guanidine-modified graphene (PHGH-G), were comparatively studied.

Filled and Empty Orbital Interactions in a Planar Covalent Organic Framework on Graphene

The electronic characteristics of a planar covalent organic framework (COF) on graphene are investigated by means of dispersion-corrected density functional theory. The aromatic central molecule of the COF acts as an electron donor to graphene, while the linker of the COF acts as an electron acceptor. The concerted interaction between the filled orbitals of the central molecule and empty orbitals of the linker promotes the formation of planar COF networks on graphene. The calculation results are in very good agreement with experimental findings of an ordered hexagonal and square COF planar on graphene, which sheds light on the supermolecular assembly mechanism.

Π-Bond maximization of graphene in hydrogen addition reactions

Thermodynamic stability of graphene hydrides increases in an approximately linear way with the numbers of π-bonds they contain. Thus, π-bond maximization is the primary driving force for hydrogen addition reactions of graphene. The previously reported thermal preference of sp(2)/sp(3)-phase separation of graphene hydrides is a straightforward effect of π-bond maximization. Although not well applicable to hydroxylation and epoxidation, the π-bond maximization principle also holds approximately for the fluorination reactions of graphene. The findings can be used to help locate the lowest-energy structures for graphene hydrides and to estimate the hydrogenation energy without first-principles calculations.

Band engineering of oxygen doped single-walled carbon nanotubes

We have studied the electronic characteristics of chemically modified single-walled carbon nanotubes by oxygen doping using first-principles density-functional calculations. The oxygen doping, a controlled [2 + 1] cycloaddition scheme, is shown to modify the π-conjugation and impact on the near-infrared band gaps. The implications of tailoring the electronic structure of oxygen doped carbon nanomaterials for future device applications are discussed.

Structural and electronic properties of fluorographene

The structural and electronic characteristics of fluorinated graphene are investigated based on first-principles density-functional calculations. A detailed analysis of the energy order for stoichiometric fluorographene membranes indicates that there exists prominent chair and stirrup conformations, which correlate with the experimentally observed in-plane lattice expansion contrary to a contraction in graphane. The optical response of fluorographene is investigated using the GW-Bethe-Salpeter equation approach. The results are in good conformity with the experimentally observed optical gap and reveal predominant charge-transfer excitations arising from strong electron-hole interactions. The appearance of bounded excitons in the ultraviolet region can result in an excitonic Bose-Einstein condensate in fluorographene.