Effect of pH on optical and physicochemical properties of green synthesized carbon quantum dots (CQDs) for dye-sensitized solar cell

The influence of pH on the optical and physicochemical properties of carbon quantum dots (CQDs) has been extensively studied. However, the effect of pH conditions during the synthesis of CQDs, particularly when utilizing natural precursors, has received limited attention. In this study, CQDs were synthesized from watermelon peels via a carbonization method under varying pH conditions. CQDs dispersed in methanol and ultrapure water were prepared at different pHs in the range of 3 to 12. The samples were characterized using UV-visible spectroscopy, photoluminescence (PL), dynamic light scattering (DLS), zeta potential and drop-casting test. In exploring different pH conditions, consistent pH-independent PL peak emissions were observed, yet the intensities displayed a pH-dependent trend, amplifying from acidic to neutral and diminishing from neutral to alkaline conditions. Moreover, in addressing drop-casting concerns, the study revealed the occurrence of the coffee-ring effect at pH 2.5, while uniform deposition was achieved at pH 7.5. This phenomenon was discussed based on Derjaguin, Landau, Verwey, Overbeek (DVLO) theory. For the testing of CQDs in dye-sensitized solar cells (DSSC), CQDs in methanol demonstrated the highest efficiency of 1.23%, surpassing CQDs in ultrapure water (0.038%) and the control sample N719 dye (0.34%).

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