Luminescent nanomaterials, with wide applications in biosensing, bioimaging, illumination and display techniques, have been consistently garnering enormous research attention. In particular, those with wavelength-controllable emissions could be highly beneficial. Carbon nanostructures, including graphene quantum dots (GQDs) and other graphene oxide derivates (GODs), with excitation-dependent photoluminescence (PL), which means their fluorescence color could be tuned simply by changing the excitation wavelength, have attracted lots of interest. However the intrinsic mechanism for the excitation-dependent PL is still obscure and fiercely debated presently. In this review, we attempt to summarize the latest efforts to explore the mechanism, including the quantum confinement effect, surface traps model, giant red-edge effect, edge states model and electronegativity of heteroatom model, as well as the newly developed synergistic model, to seek some clues to unravel the mechanism. Meanwhile the controversial difficulties for each model are further discussed. Besides this, the challenges and potential influences of the synthetic methodology and development of the materials are illustrated extensively to elicit more thought and constructive attempts toward their application.
Photocatalysts possessing high efficiency in degrading aquatic organic pollutants are highly desirable. Although graphene-based nanocomposites exhibit excellent photocatalytic properties, the role of graphene has been largely underestimated. Herein, the photothermal effect of graphene-based nanocomposites is demonstrated to play an important role in the enhanced photocatalytic performance, which has not been considered previously. In our study on degradation of organic pollutants (methylene blue), the contribution of the photothermal effect caused by a nanocomposite consisting of P25 and reduced graphene oxide can be as high as ∼38% in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal characteristic of graphene-based nanocomposite is vital to photocatalysis. It implies that designing graphene-based nanocomposites with the improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity.
The low quantum yield (∼10−5) has restricted practical use of photoluminescence (PL) from MoS2 composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS2 QDs is also found to be sensitive to aggregation due to the size effect.
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