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Nadeem, Irfan, et al. Carbon 226 (2024): 119226.
Carboxylated graphene quantum dots (CGQDs) have demonstrated remarkable potential in realizing macroscale superlubricity for engineering steel systems when used as additives in aqueous glycerol lubricants. In this study, CGQDs were chemically adsorbed onto worn steel surfaces, where they underwent tribo-induced degradation and transformed into layered graphitic structures. This dynamic transformation generated a robust, low-shear boundary film that significantly reduced friction and wear.
Frictional tests revealed that pure water and glycerol alone were insufficient to sustain superlubricity, exhibiting coefficients of friction (COF) of ~0.3 and ~0.028, respectively. In contrast, CGQD-enriched aqueous glycerol rapidly achieved superlubricity with a stable COF as low as ~0.006, representing a 72-78% friction reduction compared to base glycerol. Furthermore, the addition of CGQDs reduced wear by up to 69% and enabled superlubricity to be maintained under high contact pressure conditions (123 MPa), surpassing previous limits for aqueous-lubricated steel systems.
Importantly, CGQDs retained long-term dispersion stability, maintaining superlubricity performance even after six months of storage. This highlights their suitability for real-world tribological applications. The success of CGQDs in achieving and sustaining robust superlubricity offers a transformative step toward the development of eco-friendly, energy-efficient lubrication technologies for industrial-scale engineering systems.
Dinh, Le NM, et al. Polymer Chemistry 11.18 (2020): 3217-3224.
Carboxylated graphene quantum dots (cGQDs), synthesized via hydrothermal carbonization of dextrose, have been successfully employed as the sole surfactant in the miniemulsion polymerization of styrene. This novel application highlights the multifunctional capability of cGQDs-combining surface activity, nanoscale stabilization, and functional group reactivity-without the need for conventional surfactants.
In this study, the cGQDs, with an average lateral size of 2.6 nm and abundant surface carboxyl groups, were dispersed in water via ultrasonication. Upon mixing with an organic phase containing styrene (St), hexadecane (HD), and AIBN initiator, stable miniemulsions were formed through further ultrasonication. Polymerization was conducted at 70 °C for 24 hours. The resulting hybrid latex nanoparticles averaged ~200 nm in diameter and exhibited increasingly negative zeta potentials (-23.1 to -44.7 mV) proportional to the cGQD content, confirming effective surface coverage and colloidal stabilization.
The effectiveness of cGQDs in this context stems from their amphiphilic nature and strong interfacial activity, enabling them to replace traditional surfactants while also imparting potential fluorescent functionality. This approach not only reduces the reliance on synthetic surfactants but also opens new avenues for fabricating fluorescent polymer nanocomposites with controlled morphology and enhanced surface functionality. This study demonstrates the promise of cGQDs as green, multifunctional nanomaterials for advanced polymer synthesis applications.
Dinh, Le NM, et al. Polymer Chemistry 11.36 (2020): 5790-5799.
Carboxylated graphene quantum dots (cGQDs), synthesized via hydrothermal carbonization of dextrose, have demonstrated exceptional versatility as sole surfactants in the miniemulsion polymerization of a broad spectrum of vinyl monomers. These include styrene, acrylates, and methacrylates-key building blocks in functional polymer nanomaterials.
In this study, cGQDs effectively stabilized monomer-water miniemulsions without requiring conventional surfactants. Ultrasonicated aqueous cGQD dispersions were mixed with organic phases containing various vinyl monomers, hexadecane, and either a water-soluble (VA-044) or oil-soluble (AIBN) initiator. After emulsification and degassing, polymerizations were carried out at 55 °C for VA-044 and 70 °C for AIBN.
The results revealed that cGQDs provide stable emulsions suitable for polymerization across diverse monomer types. Notably, the water-soluble initiator VA-044 led to significantly higher monomer conversion rates than AIBN, underscoring the importance of aqueous-phase initiation when using cGQDs. Zeta potential measurements confirmed the electrostatic stabilization provided by dissociated carboxyl groups on the GQD surface.
This approach not only eliminates the need for synthetic surfactants but also introduces the potential for fluorescence and additional surface functionality in the resulting nanocomposites. The findings highlight the promise of cGQDs as green, multifunctional surfactants for the synthesis of advanced hybrid polymeric materials via miniemulsion polymerization.
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