Minimization of Ion-Solvent Clusters in Gel Electrolytes Containing Graphene Oxide Quantum Dots for Lithium-Ion Batteries.

This study uses graphene oxide quantum dots (GOQDs) to enhance the Li+ -ion mobility of a gel polymer electrolyte (GPE) for lithium-ion batteries (LIBs). The GPE comprises a framework of poly(acrylonitrile-co-vinylacetate) blended with poly(methyl methacrylate) and a salt LiPF6 solvated in carbonate solvents. The GOQDs, which function as acceptors, are small (3-11 nm) and well dispersed in the polymer framework. The GOQDs suppress the formation of ion-solvent clusters and immobilize PF6- anions, affording the GPE a high ionic conductivity and a high Li+ -ion transference number (0.77). When assembled into Li|electrolyte|LiFePO4 batteries, the GPEs containing GOQDs preserve the battery capacity at high rates (up to 20 C) and exhibit 100% capacity retention after 500 charge-discharge cycles. Smaller GOQDs are more effective in GPE performance enhancement because of the higher dispersion of QDs. The minimization of both the ion-solvent clusters and degree of Li+ -ion solvation in the GPEs with GOQDs results in even plating and stripping of the Li-metal anode; therefore, Li dendrite formation is suppressed during battery operation. This study demonstrates a strategy of using small GOQDs with tunable properties to effectively modulate ion-solvent coordination in GPEs and thus improve the performance and lifespan of LIBs.

Three-dimensional patterned graphene oxide-quantum dot microstructures via two-photon crosslinking.

The two-photon crosslinking of graphene oxide-quantum dots (GOQDs) adopts rose Bengal as the photoactivator to induce the GOQD assembly process. Based on the Förster resonance energy transfer mechanism with oxygen as the crosslinking medium, three-dimensional patterned GOQD microstructures with near diffraction-limit spatial resolution have been fabricated and analyzed by a multiphoton excited fabrication instrument/microscope.

Graphene oxide-based micropatterns via high-throughput multiphoton-induced reduction and ablation.

In this study, a developed temporal focusing-based femtosecond laser system provides high-throughput multiphoton-induced reduction and ablation of graphene oxide (GO) films. Integrated with a digital micromirror device to locally control the laser pulse numbers, GO-based micropatterns can be quickly achieved instantly. Furthermore, the degree of reduction and ablation can be precisely adjusted via controlling the laser wavelength, power, and pulse number. Compared to point-by-point scanning laser direct writing, this approach offers a high-throughput and multiple-function approach to accomplish a large area of micro-scale patterns on GO films. The high-throughput micropatterning of GO via the temporal focusing-based femtosecond laser system fulfills the requirement of mass production for GO-based applications in microelectronic devices.

Roles of nitrogen functionalities in enhancing the excitation-independent green-color photoluminescence of graphene oxide dots.

Fluorescent graphene oxide dots (GODs) are environmentally friendly and biocompatible materials for photoluminescence (PL) applications. In this study, we employed annealing and hydrothermal ammonia treatments at 500 and 140 °C, respectively, to introduce nitrogen functionalities into GODs for enhancing their green-color PL emissions. The hydrothermal treatment preferentially produces pyridinic and amino groups, whereas the annealing treatment produces pyrrolic and amide groups. The hydrothermally treated GODs (A-GODs) present a high conjugation of the nonbonding electrons of nitrogen in pyridinic and amino groups with the aromatic π orbital. This conjugation introduces a nitrogen nonbonding (nN 2p) state 0.3 eV above the oxygen nonbonding state (nO 2p state; the valence band maximum of the GODs). The GODs exhibit excitation-independent green-PL emissions at 530 nm with a maximum quantum yield (QY) of 12% at 470 nm excitation, whereas the A-GODs exhibit a maximum QY of 63%. The transformation of the solvent relaxation-governed π* → nO 2p transition in the GODs to the direct π* → nN 2p transition in the A-GODs possibly accounts for the substantial QY enhancement in the PL emissions. This study elucidates the role of nitrogen functionalities in the PL emissions of graphitic materials and proposes a strategy for designing the electronic structure to promote the PL performance.

Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence.

Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp(2) domains, which leads to a change in the π*-π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology.

Nitrogen-doped graphene oxide quantum dots as photocatalysts for overall water-splitting under visible light illumination.

Nitrogen-doped graphene oxide quantum dots exhibit both p- and n-type conductivities and catalyze overall water-splitting under visible-light irradiation. The quantum dots contain p-n type photochemical diodes, in which the carbon sp(2) clusters serve as the interfacial junction. The active sites for H2 and O2 evolution are the p- and n-domains, respectively, and the reaction mimics biological photosynthesis.