English abstract
Environmental-friendly and biocompatible fluorescent carbon quantum dots (CQDs)
show great potential for various applications. The functional groups on the surface of
CQDs could dramatically influence the light absorption and emission characteristics
of CQDs. A systematic investigation has been carried out by quantifying the oxygen
based function groups present on the surface of activated CQDs (AC), dehydrated
CQDS, and CQDs prepared by microwave synthesis to understand their role in the
fluorescent emission characteristics. The samples were subjected to further
treatments to achieve different levels of surface functionality with oxygen-containing
functional groups such as carbonyl, carboxyl, phenol, and lactone. Various emissive
pathways were identified by comparing the results of surface functionality and the
deconvoluted emission spectra. Our results suggest that the energy gap of isolated
sp 2 clusters on the surface of CQDs is the highest among the gaps related to various
functional groups. The non-radiative transitions from this level can populate the mid
level emissive states. As proof for this model, the integrated intensity of the high
energy peak was found to decrease, and corresponding enhancements of low-energy
peaks were observed as the surface functionality of carboxyl groups increased. Our
results suggest that selective surface functionalization could control various emissive
pathways in CQDs. These results are quite substantial for understanding the emission
mechanism of CQDs. We have also studied the adsorption mechanism of methylene
blue (MB) dye on the surface of CQDs at different contact times, pH, concentrations,
and temperatures. In this work, we used Langmuir, Freundlich, pseudo-Ξrst, and
second-order adsorption models for the analysis. Our results suggest the
physisorption of MB on CQDs. Various kinetic and thermal parameters associated
with MB adsorption were compared between CQDs prepared by different methods
to identify the best CQDs for applications. The results are essential for applying CQDs,
such as wastewater treatment. Further, a preliminary investigation of the electrical
conductivity of the CQD-PANI polymer composite was done. Our results suggest that
the increase in conductivity in the CQD-PANI composite is associated with electron
transfer from PANI to CQDs. Our studies give important information to increase
CQD's potential for applications.
