Transition Metal Ions Doped TiO2 Nanoparticles and Effect of External Magnetic Field on Photocatalytic Performance

TiO2-based photocatalysis technology, as an advanced wastewater treatment technology, has significant potential due to its environmental friendliness, energy efficiency and non-polluting nature. This work focusses on improving the photocatalytic performance of TiO2-based nano-particles. The research contains four main parts including, (i) the preparation of transition metal doped TiO2 nanoparticles to be used as photocatalysts, (ii) the characterisation of the synthesised TiO2 and its variants, (iii) the investigation of the coupling effect between photocatalysis and an external magnetic field on the photo-degradation performance of the photocatalysts, and (iv) the study of the catalysts’ properties in the presence of an external magnetic field using the first-principle simulations.

In the study of the effect of the transition metal dopants on the photocatalytic performance of TiO2 nanomaterials, pristine TiO2, and TiO2 doped with transition metal ions at 0.5 at% doping concentration, including Mn2+/TiO2, Fe3+/TiO2, Cu2+/TiO2, Co2+/TiO2 and Cr3+/TiO2, were synthesised using the sol-gel method. The prepared catalysts were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The characterisation results indicate that the metal elements were doped into TiO2 lattice successfully, and some Ti4+ was replaced by the transition metal ions. It has also been confirmed that the concentrations of the doped transition metals were all around 0.5 at%. The crystal phase of these as-prepared photocatalysts was anatase and the particle size was in the range of 10-20nm. The edges of the light absorption spectra for the doped catalysts have an apparent red-shift. The photocatalytic degradation of methylene blue assisted by the synthesised nanomaterials was conducted using an ultraviolet(UV) light source with a wavelength centred around 365 nm. The results of the degradation experiments indicate that the photocatalytic performance of TiO2 varies when it is doped with different transition metal ions. The difference in the photocatalytic performance is considered to be influenced by a combination of several factors, including the inherent characteristics of the doped elements, the particle size, and the adsorption of the solute on the particle surface.

By comparing these degradation results with the ones obtained in the presence of the external magnetic field of 0.05T and 0.1T, the effect of the magnetic field on photocatalysis was examined. The photocatalytic performance of all prepared photocatalysts was not improved in the presence of the magnetic field of 0.05T. It’s considered that the magnetic field at this level is too weak and does not meet a threshold. When subjected to an external magnetic field of 0.1 T, Fe3+/TiO2 exhibited a significant enhancement in photocatalytic performance. Compared with the absence of a magnetic field, Fe3+/TiO2 showed a notable increase in methylene blue (MB) degradation efficiency, reaching 99.3% degradation after 60 min of irradiation under the 0.1T external magnetic field. Similarly, Cu2+/TiO2, Mn2+/TiO2, and pristine TiO2 also exhibited enhanced photocatalytic performance to some extent under the applied magnetic field. The degradation efficiencies of methylene blue after 60 min of irradiation were improved by approximated 7% compared with those without a magnetic field. In contrast, Cr3+/TiO2 and Co2+/TiO2 did not exhibit any improvement or even experienced a decrease in the photocatalytic performance in the presence of the external magnetic field of 0.1T. To evaluate the performance of the aforementioned photocatalysts on the degradation of different pollutants, additional experiments were conducted on coumarin and paracetamol. Compared to methylene blue, both coumarin and paracetamol exhibited significantly lower adsorption on the catalysts’ surface. The experiments on coumarin showed that the degradation of coumarin in the first 60 minutes is lower in comparison to the case of methylene blue, for all photocatalysts. Fe3+/TiO2 still exhibited the highest coumarin removal among all catalysts, achieving approximately 76% degradation after 60 min. Cu2+/TiO2 and Mn2+/TiO2 showed no significant improvement compared to pristine TiO2, which courmain removal percentage are all around 72%, and Co2+/TiO2 and Cr3+/TiO2 showed a noticeable decline in the photocatalytic performance. With the presence of the external magnetic field of 0.1T, the degradation of coumarin by Fe3+/TiO2, Cu2+/TiO2 and Mn2+/TiO2 and pristine TiO2 was improved. However, it remain unchanged for Co2+/TiO2 and Cr3+/TiO2, a trend similar to that observed of methylene blue.

In the experiments on paracetamol, Fe3+/TiO2, Co2+/TiO2 and pristine TiO2 have been tested. The degradation of paracetamol by all three catalysts are 14%, and it was significantly lower than the removal of both coumarin and methylene blue. Additionally, the application of the external magnetic field of 0.1T has not enhanced the photo-degradation performance at all. It is thought that this behaviour is related to two factors: the reduced adsorption of coumarin and paracetamol on the catalysts’ surface, and the inherent structural and chemical properties of the pollutants.

To better understand the underlying mechanisms of the different behaviours, numerical modelling using the VASP code has been carried out to enable the simulation of various photocatalytic properties of the materials of concern, including the band structure, the density of states, the spin and charge density, their optical properties, and the effect of external magnetic field on them. The DFT+U correction method and the PBE XC functional have been employed in the models. The density of state patterns show noticeable spin polarization when the transition metals are doped into TiO2 lattice. Various degrees of increase or decrease in the band gap, and the introduction of new bands within the band gap region have also been observed. The charge density patterns show that the charge densities of Co, Cu, Fe and Mn atoms are significantly higher, compared to Ti and Cr atoms, indicating that the transition metal dopants can form new recombination centres to inhibit the electron and hole recombination. In the simulation of the optical properties, the photocatalytic performance of the namomaterials was estimated from the dielectric function. Cu2+/TiO2 stands out among the six photocatalysts with the highest dielectric constant, suggesting its superior potential for light absorption and the reduction of electron-hole recombination. From the spin density simulations, the spin polarization has been identified to be primarily on the doped transition metal atoms and their first neighbouring oxygen atoms. According to the spin density results, Fe3+/TiO2, Cu2+/TiO2, and Mn2+/TiO2 showed the highest potential of being affected by an external magnetic field, consistent with the experimental findings of the enhanced photo-degradation of the pollutants tested in the presence of an external magnetic field. This part of the study elucidates the mechanisms of the effect of the external magnetic field on the photocatalytic activities at the atomic level.