Om P. Khatri

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.

Self-assembly of ionic liquid (BMI-PF6)-stabilized gold nanoparticles on a silicon surface: chemical and structural aspects.

Ultrafine monodisperse gold nanoparticles (AuNPs) were synthesized by an elegant sputtering of gold onto 1- n-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF(6)) ionic liquid. It was found that the BMI-PF(6) supramolecular aggregates were loosely coordinated to the gold nanoparticles and were replaceable with thiol molecules. The self-assembly of BMI-PF(6)-stabilized AuNPs onto a (3-mercaptopropyl)trimethoxysilane (MPS)-functionalized silicon surface in 2D arrays, followed by dodecanethiol (DDT) treatment, have been demonstrated using X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and contact angle measurements. DDT treatment of tethered AuNPs revealed two types of interactions between AuNPs and the MPS-functionalized surface: (a) AuNPs anchor through Au-S chemisorption linkage resulting in strong immobilization and (b) some of the AuNPs are supported by physisorption, driven by BMI-PF(6). The attachment of these particles remains unchanged with sonication. The replacement of BMI-PF(6) aggregates from physisorbed AuNPs with DDT molecules advances the dilution of their interaction with the MPS-functionalized surface, and they subsequently detach from the silicon surface. The present finding is promising for the immobilization of ionic liquid-stabilized nanoparticles, which is very desirable for electronic and catalytic device fabrication. Additionally, these environmentally friendly AuNPs are expected to replace conventional citrate-stabilized AuNPs.

Halogen-Free Bis(imidazolium)/Bis(ammonium)-Di[bis(salicylato)borate] Ionic Liquids As Energy-Efficient and Environmentally Friendly Lubricant Additives

Bis(imidazolium)- and bis(ammonium)-di[bis(salicylato)borate] ionic liquids with variable alkyl chain and cyclic ring structures, were synthesized and then evaluated them as potential lubricant additives. The copper strip test results revealed noncorrosive properties of these ionic liquids. Introduction of halogen content in bis(imidazolium) ionic liquid by replacement of bis(salicylato)borate (BScB) anion with hexafluorophosphate (PF6(-)), severely corroded the copper strip. Thermogravimetric results showed that bis(imidazolium) ionic liquids exhibited higher thermal stability than bis(ammonium) ionic liquids owing to compact structure provided by imidazolium rings, higher intermolecular interactions, smaller free volume and low steric hindrance. The lubrication properties of these ionic liquids as additives to synthetic lubricant poly(ethylene) glycol (PEG 200) were evaluated for steel balls. Results showed that bis(ammonium)- and bis(imidazolium)-(BScB)2 ionic liquids as additives significantly reduced both friction coefficient and wear of PEG 200. The structure of cations, particularly the variation in substituted alkyl chain length monitored the degree of reduction in friction and wear. The excellent lubrication properties were attributed to the formation of adsorbed tribo-thin film and tribochemical product during the tribo-contact. Being halogen-, phosphorus-, and sulfur-free, these ionic liquids (a) protects contact surfaces from tribo-corrosive events, (b) reduces the friction and wear, and (c) keep environment green and clean.

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.

Halogen-free imidazolium/ammonium-bis(salicylato)borate ionic liquids as high performance lubricant additives

Halogen-free bis(salicylato)borate anion based ionic liquids having imidazolium and ammonium cations, were designed, synthesized, characterized and then evaluated as potential lubricant additives. Owing to the high polarity, high London dispersive forces and high rigidity associated with the bulk size and ring structure of the bis(salicylato)borate anion, these ionic liquids possess very high viscosities. The interaction of these ionic liquids with contact surfaces has been found to be strongly influenced by the van der Waals interactions in the ionic liquids, which increases with an increase of the alkyl chain length, particularly, for the imidazolium cations. As lubricant additives, both imidazolium and ammonium ionic liquids markedly improve the friction-reducing and anti-wear properties of PEG 200 lube base. Microstructural images and element mapping of the worn surfaces of steel balls reveal that these ionic liquids form tribo-films, which reduces both the friction and wear. Being halogen-, phosphorus-, and sulfur-free, bis(salicylato)borate ionic liquids not only protect contact surfaces from tribo-corrosive events but also keep the environment green and clean. These ionic liquids offer an environmentally friendly alternative to conventional halogenated ionic liquids being currently developed for lubricant applications.

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.

Reduced graphene oxide as an effective adsorbent for removal of malachite green dye: Plausible adsorption pathways

Efficient removal of malachite green (MG) dye from simulated wastewater is demonstrated using high surface area reduced graphene oxide (rGO). The plausible interaction pathways between MG dye and rGO are deduced from nanostructural features (HRTEM) of rGO and spectroscopic analyses (FTIR and Raman). The high surface area (931m2⋅gm-1) of rGO, π-π interaction between the aromatic rings of MG dye and graphitic skeleton, and electrostatic interaction of cationic centre of MG dye with π-electron clouds and negatively charged residual oxygen functionalities of rGO collectively facilitate the adsorption of MG dye on the rGO. The rGO displays adsorption capacity as high as 476.2mg⋅g-1 for MG dye. The thermodynamic parameters calculated from the temperature dependent isotherms suggested that the adsorption was a spontaneous and endothermic process. These results promise the potential of high surface area rGO for efficient removal of cationic dyes for wastewater treatment.

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.

Ultrasound assisted shape regulation of CuO nanorods in ionic liquids and their use as energy efficient lubricant additives

Copper oxide nanorods (CuONR1) having a diameter of about 20–30 nm and a length of about 30–70 nm were prepared by simple chemical processing of copper salt. CuONR1 were morphologically reshaped, by sonication with 1-hexyl-3-methylimidazolium acetate and tetrabutylammonium acetate ionic liquids, to CuONR2 and CuONR3, respectively. Both CuONR2 and CuONR3, with a diameter of about 8–18 nm and a length of about 60–200 nm, exhibit high aspect ratios (length-to-breadth) compared to CuONR1. The XRD pattern reveals the monoclinic crystalline structure of CuONR1, which remains unchanged during their ultrasound treatment with ionic liquids. Dispersion stability and aspect ratio of CuO nanorods are found to play an important role in monitoring the lubrication characteristics. The results show that CuONR2 and CuONR3, stabilized by ionic liquids, exhibit excellent friction-reduction (15–43%) and improved anti-wear properties (26–43%) compared to the PEG 200 and 10W-40 engine oil. Lubricity enhancement is attributed to the synergistic effect of uninterrupted supplies of CuO nanorods under the contact surfaces and their rolling effect mechanism, which eventually leads to energy saving and prevents material loss.