P-doped TiO2-MWCNTs nanocomposite thin films with enhanced photocatalytic activity under visible light exposure

From a sustainability standpoint, TiO2 semiconductor photocatalyst has a lot of potential in water purification. However, due to TiO2's high band gap and rapid recombination rate of electron/hole (e(-)/h(+)) pairs, visible light exposure is not practicable. To overcome these limitations, this study investigated the effects of phosphorous (P) doping and multi-walled carbon nanotubes (MWCNTs) insertion in the TiO2 matrix. The thin films form of the photocatalyst was focused on to avoid the challenges associated with the usage of powder form photocatalyst. Thin films of TiO2, TiO2/MWCNTs and phosphorous (P)-doped TiO2/MWCNTs composites have been satisfactorily prepared on glass substrates by a newly developed facile sol-gel drop coating (SGDC) method. Characterization of the prepared samples for morphology, structure, and other properties was done by using Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), and Ultraviolet-Visible (UV-Vis) absorption spectroscopy. The XRD pattern of all thin films exhibited an anatase phase of the TiO2 crystal lattice, and calculation of lattice parameters revealed a fair match of lattice values of P-doped TiO2/MWCNTs thin films with the standard value of the same. The photocatalytic activity was estimated by the degradation of methylene blue (MB) dye in water under visible light exposure. The efficiency percentage of the as synthesized P-doped TiO2/MWCNTs thin films was more than doubled when compared to pure TiO2 and MWCNTs-TiO2. The enhancement of photocatalytic activity of P-doped TiO2/MWCNTs thin films is due to the photo-induced electron absorption effect by MWCNTs and the electron trap effect by P, a non-metal within the TiO2 matrix. Additionally, P doping within TiO2 also lowers the band gap, which in turn helps to exhibit pronounced photo-catalytic activity in the visible light region. A first-order kinetics model fitted the experimental photodegradation data well. The findings of the study suggest that P-doped TiO2/MWCNTs composite thin films prepared by the SGDC method can be widely applied in waste water treatment using visible light exposure.