Valence Band Structure Engineering in Graphene Derivatives.

Engineering of the 2D materials' electronic structure is at the forefront of nanomaterials research nowadays, giving an advance in the development of next-generation photonic devices, e-sensing technologies, and smart materials. Herein, employing core-level spectroscopy methods combined with density functional theory (DFT) modeling, the modification of the graphenes' valence band (VB) upon its derivatization by carboxyls and ketones is revealed. The appearance of a set of localized states in the VB of graphene related to molecular orbitals of the introduced functionalities is signified both experimentally and theoretically. Applying the DFT calculations of the density of states projected on the functional groups, their contributions to the VB structure are decomposed. An empirical approach, allowing one to analyze and predict the impact of a certain functional group on the graphenes' electronic structure in terms of examination of the model molecules, mimicking the introduced functionality, is proposed and validated. The interpretation of the arising states origin is made and their designation, pointing out their symmetry type, is proposed. Taken together, these results guide the band structure engineering of graphene derivatives and give a hint on the mechanisms underlying the alteration of the VB structure of 2D materials upon their derivatization.

Manifesting Epoxide and Hydroxyl Groups in XPS Spectra and Valence Band of Graphene Derivatives.

The derivatization of graphene to engineer its band structure is a subject of significant attention nowadays, extending the frames of graphene material applications in the fields of catalysis, sensing, and energy harvesting. Yet, the accurate identification of a certain group and its effect on graphene's electronic structure is an intricate question. Herein, we propose the advanced fingerprinting of the epoxide and hydroxyl groups on the graphene layers via core-level methods and reveal the modification of their valence band (VB) upon the introduction of these oxygen functionalities. The distinctive contribution of epoxide and hydroxyl groups to the C 1s X-ray photoelectron spectra was indicated experimentally, allowing the quantitative characterization of each group, not just their sum. The appearance of a set of localized states in graphene's VB related to the molecular orbitals of the introduced functionalities was signified both experimentally and theoretically. Applying the density functional theory calculations, the impact of the localized states corresponding to the molecular orbitals of the hydroxyl and epoxide groups was decomposed. Altogether, these findings unveiled the particular contribution of the epoxide and hydroxyl groups to the core-level spectra and band structure of graphene derivatives, advancing graphene functionalization as a tool to engineer its physical properties.

From graphene oxide towards aminated graphene: facile synthesis, its structure and electronic properties.

In this paper we present a facile method for the synthesis of aminated graphene derivative through simultaneous reduction and amination of graphene oxide via two-step liquid phase treatment with hydrobromic acid and ammonia solution in mild conditions. The amination degree of the obtained aminated reduced graphene oxide is of about 4 at.%, whereas C/O ratio is up to 8.8 as determined by means of X-ray photoelectron spectroscopy. The chemical reactivity of the introduced amine groups is further verified by successful test covalent bonding of the obtained aminated graphene with 3-Chlorobenzoyl chloride. The morphological features and electronic properties, namely conductivity, valence band structure and work function are studied as well, illustrating the influence of amine groups on graphene structure and physical properties. Particularly, the increase of the electrical conductivity, reduction of the work function value and tendency to form wrinkled and corrugated graphene layers are observed in the aminated graphene derivative compared to the pristine reduced graphene oxide. As obtained aminated graphene could be used for photovoltaic, biosensing and catalysis application as well as a starting material for further chemical modifications.

Facile reduction of graphene oxide suspensions and films using glass wafers.

This paper reports a facile and green method for conversion of graphene oxide (GO) into graphene by low-temperature heating (80 °C) in the presence of a glass wafer. Compared to conventional GO chemical reduction methods, the presented approach is easy-scalable, operationally simple, and based on the use of a non-toxic recyclable deoxygenation agent. The efficiency of the proposed method is further expanded by the fact that it can be applied for reducing both GO suspensions and large-scale thin films formed on various substrates prior to the reduction process. The quality of the obtained reduced graphene oxide (rGO) strongly depends on the type of the used glass wafer, and, particularly, magnesium silicate glass can provide rGO with the C/O ratio of 7.4 and conductivity of up to 33000 S*cm-1. Based on the data obtained, we have suggested a mechanism of the observed reduction process in terms of the hydrolysis of the glass wafer with subsequent interaction of the leached alkali and alkali earth cations and silicate anions with graphene oxide, resulting in elimination of the oxygen-containing groups from the latter one. The proposed approach can be efficiently used for low-cost bulk-quantity production of graphene and graphene-based materials for a wide field of applications.

Controllable spherical aggregation of monodisperse carbon nanodots.

Monodisperse carbon nanodots (MCNDs) having an identical composition, structure, shape and size possess identical chemical and physical properties, making them highly promising for various technical and medical applications. Herein, we report a facile and effective route to obtain monodisperse carbon nanodots 3.5 ± 0.9 nm in size by thermal decomposition of organosilane within the pores of monodisperse mesoporous silica particles with subsequent removal of the silica template. Structural studies demonstrated that the MCNDs we synthesized consist of ∼7-10 defective graphene layers that are misoriented with respect to each other and contain various oxygen-containing functional groups. It was demonstrated that, owing to their identical size and chemical composition, the MCNDs are formed via coagulation primary aggregates ∼10-30 nm in size, which are, in turn, combined into secondary porous spherical aggregates ∼100-200 nm in diameter. The processes of coagulation of MCNDs and peptization of their hierarchical aggregates are fully reversible and can be controlled by varying the MCND concentration or the pH value of the hydrosols. Submicrometer spherical aggregates of MCNDs are not disintegrated as the hydrosol is dried. The thus obtained porous spherical aggregates of MCNDs are promising for drug delivery as a self-disassembling container for medicinal preparations.

Hydrogenation, purification, and unzipping of carbon nanotubes by reaction with molecular hydrogen: road to graphane nanoribbons.

Reaction of single-walled carbon nanotubes (SWNTs) with hydrogen gas was studied in a temperature interval of 400-550 °C and at hydrogen pressure of 50 bar. Hydrogenation of nanotubes was observed for samples treated at 400-450 °C with about 1/3 of carbon atoms forming covalent C-H bonds, whereas hydrogen treatment at higher temperatures (550 °C) occurs as an etching. Unzipping of some SWNTs into graphene nanoribbons is observed as a result of hydrogenation at 400-550 °C. Annealing in hydrogen gas at elevated conditions for prolonged periods of time (72 h) is demonstrated to result also in nanotube opening, purification of nanotubes from amorphous carbon, and removal of carbon coatings from Fe catalyst particles, which allows their complete elimination by acid treatment.