Fabrication of black TiO2 through microwave heating for visible light-driven photocatalytic degradation of rhodamine 6G

Abstract

This study aims to prepare titanium dioxide (TiO₂) with a narrower band gap, namely black TiO₂, using sodium tetrahydroborate (NaBH₄) as a reducing material with different mixing ratios and microwave heating, which is a faster, greener, and simpler method than the existing method using furnace heating. Scanning electron microscopy (SEM) inspections indicate that incremental changes of agglomeration are observed upon increasing the NaBH₄ mixing ratio, with a moderate 2-fold increase in the particle size (up to 49.9 ± 3.0 nm). The X-ray diffraction (XRD) patterns and Raman spectroscopy confirm that TiO₂ is fully converted to the anatase phase after microwave-assisted synthesis. The gradual shift in intense E_g phonon vibration mode at 141 cm⁻¹ to a longer Raman wavelength infers simultaneous defect formations on both pristine and reduced TiO₂ surfaces. Furthermore, high-resolution X-ray photoelectron spectroscopy (XPS) measurements confirmed the formation of Ti³⁺ and O_v. The photodegradation results showed that after visible light irradiation for 4 hours, the T-50 sample exhibited R6G degradation of 49.2 ± 2.0%, outperforming the pristine P25. Moreover, bandgap reduction is successfully achieved from 3.20 eV (P25) to 1.50 eV (T-50) from diffuse reflectance UV-vis (DRUV) spectroscopy measurements. Photoluminescence (PL) spectroscopy found that the energy transfer efficiency of the T-50 sample was 30.6 ± 4.6% during the decomposition of R6G. This combined effort promotes the use of potent black TiO₂ through photocatalysis towards fabrication of highly efficient remediation materials in the future.