Large-scale fabrication of green carbon quantum dots and its nanocomposite with TiO₂ for enhanced photocatalytic oxidation of organic pollutants

We have successfully synthesized green carbon nanodots (CDs) on a large scale through catalytic hydrothermal carbonization, using apple and grape pomace as the primary carbon sources and KOH as a catalyst. The resulting nanodots are named ACD and GCD, respectively. We then created a nanocomposite by combining CDs and commercially available rutile TiO₂ to enhance its photocatalytic and optical characteristics. Several analytical techniques were applied in the characterization of CDs and/or TiO₂/CDs nanocomposites such as XRD, FTIR, IR, TEM, HRTEM, thermal analysis, spectrophotometric absorbance, and photoluminescence (PL). CDs exhibited a graphite structure with conjugated sp² domains and surface functional groups of carboxylate and hydroxyl, showing an average particle size of approximately 4 nm. When CDs were incorporated into TiO₂, they acted as a peptizing agent and led to a reduction in size, decreasing aggregations, and increasing stability. This resulted in a synergistic effect that enhanced the material’s electronic and photophysical properties. TEM images and XRD analysis indicate that the TiO₂/CDs nanocomposites exhibit considerably reduced particle dimensions in the nanoscale, in contrast to the original micrometer-sized TiO₂. To investigate the TiO₂/CDs nanocomposites’ photocatalytic efficiency, we used malachite green (MG) as a pollutant model. The photocatalytic performance of rutile TiO₂ increased by about 21% after synergistic action with CDs for MG photodegradation during 45 min of reaction under UV illumination. This enhancement of photocatalytic oxidation increased in a basic medium. The addition of CDs to TiO₂ increased its absorption in the UV-visible light region. Superoxide radicals (O₂^•−) and positive holes (h⁺) were identified as the primary active species accountable for the effective photocatalytic degradation of MG by the TiO₂/CDs nanocomposites.