RESUMEN
n-Alkylamines were grafted on the basal plane oxygen functionalities of graphene oxide (GO), yielding molecular pillar supported graphene oxide frameworks (GOFs) with tunable interlayer spacing. A major fraction of n-alkylamines was found to covalently interact with the basal plane epoxy groups via nucleophilic substitution reactions. The d-spacing in GOFs could be tailored between 10.5 and 28.9 Å by varying the chain length of the n-alkylamines. (13)C SSNMR explicitly showed the coexistence of both trans and gauche conformation modes. The relative populations of these modes control the conformational heterogeneity and orientation of n-alkylamines in the GOFs. A plausible bilayer structural model of the GOFs was demonstrated. The terminal methyl and methylene units of the n-alkylamines grafted on the GO basal plane were interdigitated with the counter layer and afforded a double-layer structure of alkyl chain supported GOFs.
RESUMEN
Graphene, the most promising material of the decade, has attracted immense interest in a diversified range of applications. The weak van der Waals interaction between adjacent atomic-thick lamellae, excellent mechanical strength, remarkable thermal conductivity, and high surface area, make graphene a potential candidate for tribological applications. However, the use of graphene as an additive to liquid lubricants has been a major challenge because of poor dispersibility. Herein, a thorough review is presented on preparation, structural models, chemical functionalization, and dispersibility of graphene, graphene oxide, chemically-functionalized graphene, and graphene-derived nanocomposites. The graphene-based materials as additives to water and lubricating oils improved the lubrication properties by reducing the friction, protecting the contact interfaces against the wear, dissipating the heat from tribo-interfaces, and mitigating the corrosion by forming the protecting thin film. The dispersion stability, structural features, and dosage of graphene-based dispersoids, along with contact geometry, play important roles and govern the tribological properties. The chemistry of lubricated surfaces is critically reviewed by emphasizing the graphene-based thin film formation under the tribo-stress, which minimizes the wear. The comprehensive review provides variable approaches for the development of high-performance lubricant systems and accentuates the lubrication mechanisms by highlighting the role of graphene-based materials for enhancement of tribological properties.
RESUMEN
Alkylated graphene oxide (GO)/reduced graphene oxide (rGO) are prepared by covalent interaction with octadecyltrichlorosilane (OTCS) and octadecyltriethoxysilane (OTES). The variable oxygen functionalities in the GO/rGO and hydrolysis rate of octadecylsilanes having different leaving groups viz. trichloro and triethoxy found to govern the grafting density of octadecyl chains on the GO and rGO. FTIR, XPS, and TGA results revealed a higher grafting of octadecyl chains in the GO-OTCS, whereas the rGO-OTES exhibited minimum grafting. The van der Waals interaction between the octadecyl chain of alkylated GO/rGO and octadecenyl chains of polyol ester makes alkylated GO/rGO dispersible in the polyol lube base oil. The dispersion stability is collectively driven by grafting density of octadecyl chains and presence of oxygen functionalities in the GO/rGO. Tribological properties in terms of the coefficient of friction and wear scar diameter revealed a good correlation with the structure of alkylated GO/rGO and their dispersion stability in the polyol lube base oil. Raman analysis of the worn surface revealed the sheared-induced deposition of a graphene-based tribo-thin film, which reduced the friction and protected the tribo-interfaces against the wear.
RESUMEN
Shear-induced transfer of graphene on the contact interfaces was studied by microscopic and spectroscopic analyses of steel balls lubricated with chemically functionalized graphene-based mineral lube base oil (SN-150). The 3,5-di-tert-butyl-4-hydroxybenzaldehyde (DtBHBA) grafted-graphene (Gr-DtBHBA) was prepared by two-steps approach using graphene oxide as a precursor. Chemical and structural features of Gr-DtBHBA are probed by FTIR, XPS, Raman, TGA, and HRTEM analyses. The van der Waals interaction between the tertiary-butyl group in the Gr-DtBHBA and hydrocarbon chains of mineral lube base oil facilitates the dispersion of Gr-DtBHBA in the SN-150 lube base oil, which is very important for the optimized performance of Gr-DtBHBA as a lubricant additive. The minute dosing (0.2-0.8â¯mgâ¯mL-1) of the Gr-DtBHBA in the SN-150 lube base oil showed the significant reduction in the coefficient of friction (40%) and wear scar diameter (17%) under the rolling contact between steel balls. The microscopic and EDX analysis of the worn area suggested the role of Gr-DtBHBA nanosheets for enhanced tribo-performance of the SN-150 lube base oil. A detailed Raman study of the worn area of steel ball revealed the deposition of a graphene-based tribo thin film in the forms of irregular patches. The shear-induced deposition of graphene thin film on the contact interfaces reduced the friction and protected the tribo-surfaces against the wear.
RESUMEN
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.
RESUMEN
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.
RESUMEN
Graphene oxide was found to be a highly efficient, reusable and cost-effective organocatalyst for the aza-Michael addition of amines to activated alkenes to furnish corresponding ß-amino compounds in excellent yields.