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.

Immobilization of Anions on Polymer Matrices for Gel Electrolytes with High Conductivity and Stability in Lithium Ion Batteries.

This study reports on a high ionic-conductivity gel polymer electrolyte (GPE), which is supported by a TiO2 nanoparticle-decorated polymer framework comprising poly(acrylonitrile-co-vinyl acetate) blended with poly(methyl methacrylate), i.e. , PAVM: TiO2. High conductivity GPE-PAVM: TiO2 is achieved by causing the PAVM:TiO2 polymer framework to swell in 1 M LiPF6 in carbonate solvent. Raman analysis results demonstrate that the poly(acrylonitrile) (PAN) segments and TiO2 nanoparticles strongly adsorb PF6(-) anions, thereby generating 3D percolative space-charge pathways surrounding the polymer framework for Li(+)-ion transport. The ionic conductivity of GPE-PAVM: TiO2 is nearly 1 order of magnitude higher than that of commercial separator-supported liquid electrolyte (SLE). GPE-PAVM: TiO2 has a high Li(+) transference number (0.7), indicating that most of the PF6(-) anions are stationary, which suppresses PF6(-) decomposition and substantially enlarges the voltage that can be applied to GPE-PAVM: TiO2 (to 6.5 V vs Li/Li(+)). Immobilization of PF6(-) anions also leads to the formation of stable solid-electrolyte interface (SEI) layers in a full-cell graphite|electrolyte|LiFePO4 battery, which exhibits low SEI and overall resistances. The graphite|electrolyte|LiFePO4 battery delivers high capacity of 84 mAh g(-1) even at 20 C and presents 90% and 71% capacity retention after 100 and 1000 charge-discharge cycles, respectively. This study demonstrates a GPE architecture comprising 3D space charge pathways for Li(+) ions and suppresses anion decomposition to improve the stability and lifespan of the resulting LIBs.

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.