Sustainable TiO2 photocatalysts modified with hollyhock-derived carbon dots and natural dye for enhanced visible-light degradation of Congo red: a comparative study

Abstract

This study introduces a sustainable method for valorizing Alcea rosea (hollyhock) floral waste by developing two novel TiO₂-based photocatalysts modified with biomass-derived materials: a natural dye (HH dye) and carbon dots (HHCDs). The HHCDs were synthesized via a one-pot hydrothermal process at 180 °C, yielding oxygen-rich, amorphous carbon dots. TiO₂ nanoparticles were prepared by a sol–gel method and subsequently modified with either HH dye or HHCDs through environmentally benign procedures. Comprehensive characterization (FTIR, XRD, UV-vis, and FE-SEM) confirmed the successful incorporation of both modifiers and their interaction with the TiO₂ surface. Optical analysis indicated a significant reduction in the bandgap for both composites, with HH dye@TiO₂ (∼2.67 eV) exhibiting a lower bandgap than HHCDs@TiO₂ (∼2.89 eV). Electrochemical measurements revealed that HHCDs@TiO₂ facilitated more effective charge carrier separation, whereas HH dye@TiO₂ demonstrated superior light-harvesting capabilities due to its anthocyanin content. In photocatalytic degradation experiments under visible light, HHCDs@TiO₂ demonstrated superior performance, achieving 97.1% degradation of Congo red dye within 80 minutes, compared to 96.8% in 120 minutes for HH dye@TiO₂. Both composites exhibited remarkable long-term stability, retaining over 95% of their efficiency after 180 days of storage. Optimal degradation conditions were identified at mildly acidic to neutral pH using 0.04 g of HHCDs@TiO₂ and 0.06 g of HH dye@TiO₂. This work presents a novel, dual-approach strategy for fabricating efficient and eco-friendly photocatalysts, highlighting their significant potential for solar-driven water purification and environmental remediation.