Solvatochromic study and electrochemical performance of green synthesized rGO decorated metal oxide CQDs from Dillenia indica peel extract in nonionic surfactant polymer matrix

The green synthesis of nanoparticles (NPs) and carbon quantum dots (CQDs) offers an eco-friendly and sustainable approach, increasingly gaining attention for advanced energy storage applications. This study explores the green synthesis of NPs and CQDs using peel extract of Dillenia indica (reducing and stabilizing agent) via a hydrothermal method. The peel extract was divided into two parts: the liquied portion was used to synthesize nitrogen- doped Dillenia indica-based n nanoparticles (N-DIBNPs) for photophysical studies, including absorption, emission, and solvatochromic behavior. The residue part was used to synthesize two different CQD-based composites for electrochemical analysis to evaluate their potential in supercapacitor applications. The first composite consisted of nitrogen-doped CQDs with vanadium pentaoxide (V 2 O 5 ) and polyethylene glycol (PEG-400) (N-DIAVPG-CQDs), while the second included V 2 O 5 , titanium dioxide (TiO 2 ), and PEG-400 (N-DIAVTPG-CQDs). Solvatochromic analysis confirmed solute-solvent interactions with correlation coefficients exceeding 0.93, along with the investigation of Stokes shifts. XRD characterization confirmed the crystalline nature of the materials, with crystallite sizes of 28.69 nm (N-DIANPs), 1.73 nm (N-DIAVPG-CQDs), and 2.60 nm (N-DIAVTPG-CQDs). FTIR analysis revealed functional groups essential for stabilization and electrochemical performance. SEM provided insights into morphological features, while EDX confirmend elemental compositions. Electrochemical analysis using cyclic voltammetry (CV) showed specific capacitances of 2253.61 F/g for N-DIAVPG-CQDs and 1755.99 F/g for N-DIAVTPG-CQDs at a scan rate of 10 mV/s. Galvanostatic charge-discharge (GCD) measurements revealed specific capacitances of 1736.28 F/g for N-DIAVPG-CQDs and 733.70 F/g for N-DIAVTPG-CQDs at a current density of 2 A/g. The maximum energy densities were 964.60 Wh/kg for N-DIAVPG-CQDs and 407.61 Wh/kg for N-DIAVTPG-CQDs at a power density of 2000 W/kg. Excellet cyclic stability was observed, with retention value of 108 % (N-DIAVPG-CQDs) and 106 % (N-DAVTPG-CQDs) after 5000 cycles. These green-synthesized, rGO-decorated metal oxide CQDs composites exhibit outstanding electrochemical performance, assessed through simple, cost-effective techniques- highlighting their scalability without requiring sophisticated instrumentation. The results underscore the high specific capacitance, current density, power density, energy density, and cyclic stability of these composities, demonstrating the potential of plant-derived waste materials in high-performance supercapacitor applications. This research emphasized the dual advantage of waste valorization and the development of eco-friendly energy storage materials.