Harshal P. Mungse

Dispersion of alkylated graphene in organic solvents and its potential for lubrication applications

We report on the preparation of alkylated graphenes on a large scale followed by their dispersion in organic solvents. The alkylated graphenes with variable alkyl chain lengths (Cn = 8, 12, 18) are prepared by coupling of alkylamine with carboxylic groups of graphene oxide (GrO). The FTIR, UV-Visible, and TGA results reveal that, during alkylation, the oxygen functionalities of GrO are reduced significantly and the average size of the sp2 carbon domain increased, which is further supported by Raman characteristics. It is observed that the dispersibility of alkylated graphene in hydrocarbon solvents increases on increasing the chain length of (a) hydrocarbon solvents used for the dispersion and (b) alkyl groups attached to the graphene. The van der Waals interaction between methylene units associated with alkylated graphenes and hydrocarbon solvents plays a crucial role in determining their dispersion characteristics, and such an interaction increases with increasing methylene units. Octadecylamine functionalized graphene (ODA-Gr) dispersion in hexadecane is found to have long-term dispersion stability due to its high degree of cohesive interaction. The lubrication characteristics of hexadecane containing ODA-Gr were probed by evaluating its friction and wear properties. The results reveal that hexadecane doped with an optimized dose of 0.06 mg mL−1 ODA-Gr reduced friction and wear by 26% and 9%, respectively, compared to hexadecane. The lubricity enhancement could be attributed to uninterrupted supplies of graphene nanosheets under the rubbing surfaces, where these nanosheets prevent direct contact between the rubbing surfaces, providing low resistance to shear.

Grafting of oxo-vanadium Schiff base on graphene nanosheets and its catalytic activity for the oxidation of alcohols

Graphene oxide was found to be a convenient and efficient supporting material for grafting of oxo-vanadium Schiff basevia covalent attachment. The low dimensionality and rich surface chemistry of graphene oxide play critical roles in order to achieve a good degree of such grafting. Catalytic potential of the so prepared graphene-bound oxo-vanadium Schiff base and comparison with its homogeneous analogue was studied for the oxidation of various alcohols to carbonyl compounds using tert-butylhydroperoxide as oxidant. The structural and chemical nature of the catalyst was characterized by a variety of techniques including XRD, FTIR, TGA, TEM, and ICP-AES. The immobilized complex was found to be highly efficient and showed comparable catalytic reactivity as its homogenous analogue with the added benefits of facile recovery and recycling of the heterogeneous catalyst. The graphene-bound oxo-vanadium Schiff base was successfully reused for several runs without significant loss in its catalytic activity.

Alkyl-chain-grafted hexagonal boron nitride nanoplatelets as oil-dispersible additives for friction and wear reduction.

Hexagonal boron nitride (h-BN), an isoelectric analogous to graphene multilayer, can easily shear at the contact interfaces and exhibits excellent mechanical strength, higher thermal stability, and resistance toward oxidation, which makes it a promising material for potential lubricant applications. However, the poor dispersibility of h-BN in lube base oil has been a major obstacle. Herein, h-BN powder was exfoliated into h-BN nanoplatelets (h-BNNPs), and then long alkyl chains were chemically grafted, targeting the basal plane defect and edge sites of h-BNNPs. The chemical and structural features of octadecyltriethoxysilane-functionalized h-BNNPs (h-BNNPs-ODTES) were studied by FTIR, XPS, XRD, HRTEM, and TGA analyses. The h-BNNPs-ODTES exhibit long-term dispersion stability in synthetic polyol ester lube base oil because of van der Waals interaction between the octadecyl chains of h-BNNPs-ODTES and alkyl functionalities of polyol ester. Micro- and macrotribology results showed that h-BNNPs-ODTES, as an additive to synthetic polyol ester, significantly reduced both the friction and wear of steel disks. Elemental mapping of the worn area explicitly demonstrates the transfer of h-BNNPs-ODTES on the contact interfaces. Furthermore, insight into the lubrication mechanism for reduction in both friction and wear is deduced based on the experimental results.

Covalently attached graphene–ionic liquid hybrid nanomaterials: synthesis, characterization and tribological application

Hybrid nanomaterials offer task-specific functional properties besides the individual properties of their constituent materials/elements. Herein, graphene–ionic liquid (Gr–IL) hybrid nanomaterials are synthesized to integrate the friction-reducing properties of both ionic liquids and graphene nanosheets. Importantly, the remarkable mechanical strength of graphene improves the anti-wear performance, whereas covalently grafted ionic liquids facilitate the dispersion of the Gr–IL in the polyethylene glycol (PEG 200) synthetic lube base oil. Graphene, prepared by a controlled chemical approach, is used for the covalent grafting of an imidazolium ring. Three variable Gr–IL hybrid nanomaterials, containing bis(salicylato)borate (BScB), oleate (OL), and hexafluorophosphate (PF6) anions, are synthesized to control their surface properties. Detailed chemical and microstructural features of the Gr–IL hybrid nanomaterials are studied using FTIR, XPS, Raman, XRD, FESEM, and HRTEM analyses. The Gr–IL hybrid nanomaterials as lubricant additives for PEG 200 significantly reduced the friction and the wear. Further, the Gr–IL hybrid nanomaterials offer remarkably improved anti-wear properties (55–78%) compared to that of the corresponding ionic liquid (7–39%) blends in PEG 200 and this was attributed to the high mechanical strength of graphene, which protects the contact interfaces against material loss. The elemental and micro-Raman results of the worn surfaces suggested the formation of a tribo-chemical thin film composed of Gr–IL and its tribo-chemical products with steel interfaces. The BScB anion constituted Gr–BScB showed the maximum reduction in friction, whereas the OL analogue exhibited the smallest wear. This study reveals the potential of task-specific Gr–IL hybrid nanomaterials as novel additives for diversified tribological applications.

Synthesis, dispersion and lubrication potential of basal plane functionalized alkylated graphene nanosheets

A single step facile approach for grafting of long alkyl chains in the basal plane of graphene oxide and simultaneous reduction of oxygen functionalities to restore the graphitic characteristics, is reported. Chemical and structural features of the synthesized dual-layer alkylated graphene are elucidated by infrared, 13C solid state nuclear magnetic resonance, X-ray diffraction, and high-resolution transmission electron microscopy analyses. The van der Waals interaction between the octadecyl chains grafted on graphene and the alkyl chains of lube oils provided long-term dispersion stability to the alkylated graphene. The 0.02 mg mL−1 alkylated graphene as an optimized concentration in the lube oil, decreased both friction and wear significantly under the sliding contacts between steel tribo-pairs. Micro-Raman results demonstrate the deposition of graphene nanosheets on the tribo-interfaces under the sheared contact, and reduced the friction and protects the surfaces against undesirable wear.

Hydrothermal deoxygenation of graphene oxide: chemical and structural evolution.

A green and facile approach for the partial deoxygenation of graphene oxide (GO) at moderate temperature (100 °C) and under atmospheric pressure, catalyzed by acidic conditions in water is reported. The chemical and structural changes in GO as a function of hydrothermal time were probed to understand the deoxygenation events. The brown GO dispersion in water was found to gradually turn black over the hydrothermal-treatment time on account of the increasing graphitic content. FTIR, thermogravimetric (TG), Raman, and XRD analyses revealed that the labile oxygen functionalities are progressively eliminated, thereby partially restoring the π-conjugated network. This was further corroborated by X-ray photoelectron spectroscopy (XPS) studies based on quantitative analysis of each carbon component associated with the different chemical functionalities. Carbonyl, carboxyl, ether, and phenolic groups were found to be thermally stable, which hinders complete deoxygenation of GO and makes their dispersion in water stable, as monitored by the ζ potential. It is worth noting that deoxygenation events are expedited under acid-catalyzed hydrothermal treatment relative to thermal deoxygenation in air.

Nitrogen-doped graphene-supported copper complex: a novel photocatalyst for CO2 reduction under visible light irradiation

A copper(II) complex grafted to nitrogen-doped graphene (GrN700–CuC) was synthesized and then demonstrated as an efficient photocatalyst for CO2 reduction into methanol under visible light irradiation using a DMF/water mixture. The chemical and microstructural features of GrN700–CuC nanosheets were studied by FTIR, XPS, XRD and HRTEM analyses. Owing to its truly heterogeneous nature, GrN700–CuC could be easily recovered after the photocatalytic reaction and showed efficient recyclability for subsequent runs.

Metal-free one-pot synthesis of amides using graphene oxide as an efficient catalyst

Graphene oxide (GO), exhibiting a high degree of oxygen functionality and various structural defects, was found to be a highly efficient and cost effective carbocatalyst for the one-pot base-free synthesis of amides from aromatic aldehydes and secondary amine. The chemical and structural features of GO, as probed by FTIR, Raman, XRD and HRTEM analyses, were discussed to understand the catalytic mechanism for the synthesis of amides. The present method obviates the use of transition metal catalysts and needs shorter reaction time.

Hydrothermal deoxygenation of graphene oxide in sub- and supercritical water

Graphene oxide (GO), an oxidized form of graphene, exhibits immense potential for a wide range of applications owing to its rich chemistry. This work reports the controlled deoxygenation of GO under sub- and supercritical hydrothermal conditions, which are considered to be foremost green, environmentally-friendly and economically viable. The remarkable thermo-physical and chemical properties of water, monitored by temperature (373–653 K) and pressure (0.04–22.75 MPa), facilitate the deoxygenation of GO. The gradual chemical and structural changes in GO in hydrothermal reactions, over a wide range of temperature and pressure are elucidated using XPS, FTIR, Raman, XRD, and HRTEM analyses. Plausible deoxygenation mechanisms, particularly elimination of hydroxyl, epoxide, carboxyl, and carbonyl groups and repairing of the π-conjugated network are discussed on the basis of spectroscopic analyses. The addressed hydrothermal route not only avoids the use of toxic and hazardous chemicals as reducing agents but also regulates the deoxygenation events.