Solvent-Mediated Surface Modulation of o -Phenylenediamine-Derived Carbon Quantum Dots for Hydrovoltaic Power Generation

Abstract Though carbon quantum dots (CQDs) are a very interesting topic of current research, most studies have focused on photophysical properties and high quantum yields, while a proper explanation of the underlying sources and mechanisms of fluorescence have not been explored. A stringent identification of compositional and structural parameters of CQDs is therefore required in order to explain their emissive nature. The controversy on whether the fluorescence of CQDs is a result of surface functional groups or from the carbogenic cores has gained momentum in the scientific world in recent times. The present work focuses on the synthesis of o-phenylenediamine and phthalic anhydride-based CQDs in dry ethanol via a solvothermal process. The addition of water along with dry ethanol tunes the optical properties of CQDs from excitation-independent emission to excitation-dependent emission. This approach produced CQDs from the same precursors and synthetic conditions but helped to modulate their properties extensively. In this investigation, we correlate the photophysical properties of the developed CQDs, highlighting the potential to control CQD properties influenced by oxidized o-phenylenediamine, and further uncover the emergence of fluorescence in the CQDs, which was later supported by the metal ions interaction properties in the nanomolar range. As-synthesized CQDs possess interesting surface chemistry, which strengthens water–solid interfacial interactions, facilitating electric double-layer formation and promoting evaporation-driven ion migration, leading to efficient hydrovoltaic power generation with different output parameters. Thus, they are used for charging capacitors of various capacitances. This study demonstrates that rational control of CQD surface chemistry through solvent engineering provides an effective strategy for developing efficient carbon-based hydrovoltaic energy harvesters.