Processes of interaction between carbon quantum dots (CQDs) and solutions of Cu(II) Ni(II) and Fe(III) chlorides in the surface layer have been investigated by electron and IR spectroscopy.
When hydrochloric acid is added to the aqueous suspension of CQDs, there is a significant batochromic shift of the average absorption band (AB) by 1285 cm-1 with a decrease in its intensity to ε = 23.39. The presence of copper in the suspension of CQDs at room temperature leads to a decrease in the intensity of this AB (ε = 21.80), which indicates the interaction of CQDs with metal ions.
After heating the suspension for 1 and 3 hours, the gypsochromic shift of this ABs (by 335 cm-1) to 27790 cm-1 with a decrease in intensity depending on the heating time was recorded. Such changes in the UV–Vis Spectrum are due to the redistribution of the electron density of electron transitions n → π *
due to the coordination of functional groups with metal ions and the appearance of transitions with charge transfer from ligand to metal (CQD→Cu2+).
When heating the suspensions significantly increases the absorption intensity of the AB at 22070 cm-1: from ε = 4.59 to ε = 6.75, which indicates the formation of transitions with charge transfer from ligand to metal (ChTLM) due to the coordination of copper ions with CQD.
In the absorption spectra of CQD suspensions with NiCl2 before heating, a hypsochromic shift of AB at 27305 cm-1 by 150 cm-1 and an increase in the intensity of its to ε = 4.95 were registered. That is, Ni(II) ions also form coordination bonds with functional groups on the periphery of the CQD.
After heating hydrochloric acid suspensions of CQD with FeCl3, in contrast to the chlorides of previous metals, in the UV-region registered shoulder-shaped AB at 31545 cm-1, the intensity of which increases with heating time (from ε = 9.59 to ε = 12.10), and in the visible region, a weakly intense shoulder-shaped AB at 19345 cm-1 (ε = 3.71 and 4.58), associated with the presence of dd-electron transitions in the metal ion.
Such changes in the absorption spectra are explained by the fact that iron may interact with CQD in different ways (in addition to coordination with functional donor groups, the formation of coordination bonds with the π-electron system of conjugated CQDs bonds), which leads to additional weak shoulder-like AB at 31545 cm-1.
The IR-spectra data of CQDs showed the presence of a number of characteristic ABs for functionalized CQDs: ν(N–H) at 3260 сm1, (C=O) at 1830, 1840 and 1850 сm1, –С=O(NH) at 1770 сm1, ν(C=N) at 1680 and δ(N–H) at 1640 сm1 and 320-360 см-1 СП ν(Cu–Cl, Ni–Cl, Fe–Cl), which confirms the coordination of metals on the surface of CQDs.
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