Thioglycolic Acid FTIR Spectra on Ag2S Quantum Dots Interfaces

Materials (Basel). 2020 Feb 18;13(4):909. doi: 10.3390/ma13040909.


The mechanism features of colloidal quantum dots (QDs) passivation with thioglycolic acid molecules (TGA) for cases of different luminescent properties is considered using FTIR. This problem is considered based on FTIR spectra analysis for various ionic forms of TGA. Experimental TGA molecules FTIR spectra is interpreted, basing on the data on modeling of TGA vibrational modes, realized in the framework of density functional method (DFT /B3LYP/6-31+G(d)) taking into account the vibrations anharmonicity of every functional group. This approach provides a significant improvement in the agreement between the experimental and calculated data. FTIR spectra of Ag 2 S/TGA QDs with exciton and recombination luminescence are differ from each other and B "freeB" TGA molecules. The ν ( S - H ) TGA peak (2559 cm - 1 ) disappears in FTIR spectra of Ag 2 S/TGA QD samples. This fact indicates the interactions between TGA thiol group and dangling bonds of Ag 2 S nanocrystals. Ag 2 S QDs passivation with TGA molecules leads to emergence ν a s (COO - ) (1584 cm - 1 ) and ν s (COO - ) (1387 cm - 1 ) peaks. It indicates TGA adsorption in ionic form. For Ag 2 S/TGA QDs with exciton luminescence we observed (a) significant low-frequency shift of ν s (COO - ) peak from 1388 cm - 1 to 1359 cm - 1 and high-frequency shift of ν a s (COO - ) peak from 1567 cm - 1 to 1581 cm - 1 ; (b) change in the ratio of intensities of ν a s (COO - ) and ν s (COO - ) vibrations. This feature is caused by the change in the symmetry of TGA molecules due to passivation of Ag 2 S quantum dots.For Ag 2 S/TGA QDs with recombination luminescence, the insignificant high-frequency shift of 7-10 cm - 1 for ν a s (COO - ) at 1567 cm - 1 and low-frequency shift of 3-5 cm - 1 for ν s (COO - ) at 1388 cm - 1 , probably caused by the interaction of thiol with Ag 2 S surface is observed. Using FTIR spectra, it was found that IR luminescence photodegradation is also accompanied by changes in the thioglycolic acid molecules, which capped Ag 2 S QDs. In the case of Ag 2 S QDs with exciton luminescence, the degradation process is non-reversible. It is accompanied by TGA photodegradation with the formation of α -thiol-substituted acyl radical (S-CH 2 -CO • ) TGA.

Keywords: Ag2S quantum dots; FTIR spectra; dimer; ionic form; luminescence; photodegradation; thioglycolic acid (TGA).