In Situ Synthesized Free-Standing Ti3C2Tx MXene/PDAAQ Nanocomposite Films for Adsorption-Assisted Photocatalytic Removal of Methylene Blue and Malachite Green Dyes

Abstract

The increasing contamination of water resources by organic pollutants has become a critical environmental concern due to their persistence, toxicity, and resistance to conventional treatment methods. Photocatalysis has emerged as a promising approach for wastewater remediation; however, conventional powder-based photocatalysts suffer from significant limitations, especially the difficulty of post-treatment separation from the reaction medium. This not only complicates the recovery process but also increases operational costs and limits their practical applicability. To address these issues, a freestanding Ti3C2Tx (MXene)/PDAAQ nanocomposite film was successfully synthesized via in situ oxidative polymerization of poly(1,5-diaminoanthraquinone) (PDAAQ) within MXene layers. The integration of PDAAQ effectively expanded the interlayer spacing from 1.23 to 1.51 nm, suppressing restacking and facilitating improved charge separation. Optical studies revealed band-gap energies of 2.63 and 1.85 eV for MXene and the composite, respectively. Electrochemical impedance spectroscopy revealed that the charge-transfer resistance decreased from 3 ohm for pristine MXene to 1.6 ohm in the dark and further to 0.5 ohm under illumination for the composite. As a result, the MXene/PDAAQ nanocomposite exhibited superior photocatalytic performance, achieving 100% degradation of methylene blue (MB) within 20 min at pH 11 and complete degradation of malachite green (MG) within 35 min at pH 7. Kinetic analysis based on the pseudo-first-order model revealed higher apparent rate constants for the composite. The equilibrium adsorption behavior was well fitted by the Freundlich isotherm model (R2 = 0.99), with a maximum adsorption capacity of 12.375 mg g-1. The freestanding photocatalyst retained approximately 90% of its initial degradation efficiency after five consecutive cycles. TOC and phytotoxicity analyses further confirmed substantial mineralization and significant toxicity reduction following photocatalytic treatment.