Photocatalytic activity enhancement of iron-loaded cerium dioxide nanoparticle and its core-shell composites

Abstract

Unloaded CeO2 and nominal 0.50, 1.00, 1.50, 2.00, and 5.00 mol% Fe-loaded CeO2 nanoparticles were synthesized by flame spray pyrolysis (FSP). X-ray diffraction (XRD) results indicated that phase structures of Fe-loaded CeO2 nanoparticles were the mixture of CeO2 and Fe2O3 phases at high iron loading concentrations. High-resolution transmission electron microscopy (HRTEM) images showed the significant change in morphology from cubic to spherical shape at high iron loading concentrations. Increased specific surface area with increasing iron content was also observed. The results from diffuse reflectance UV-visible spectrophotometry (DRS UV-visible) spectra clearly showed the shift of absorption edge towards longer visible light region upon loading CeO2 with iron. Photocatalytic studies showed that Fe-loaded CeO2 sample exhibited higher activity than unloaded CeO2 for the degradation of formic and oxalic acids under visible light irradiation, with optimal 2.00 mol% of iron loading concentration being the most active catalyst. Furthermore, unloaded CeO2 and 0.50, 1.00, 1.50, 2.00, and 5.00 mol% Fe-loaded CeO2 nanoparticles were prepared by the combination of homogeneous precipitation and impregnation methods, and applied as photocatalyst films prepared by a doctor blade technique. The superior photocatalytic performances of the Fe-loaded CeO2 films compared with unloaded CeO2 film were ascribed mainly to a decrease in band gap energy and an increase in specific surface area of the material. The presence of Fe3+ as found from XPS analysis may act as electron acceptor and/or hole donor to facilitate charge carrier localization and support the charge carrier transfer to the CeO2 catalyst surface. Moreover, the effect of the solution pH on dye adsorption on catalyst surface as well as, the kinetics studies of the effect of Fe3+ loading on photocatalytic degradation of MO over CeO2 films was investigated in this study with the best efficiency obtained from a 1.50 mol% iron loading. In order to estimate the long-term photostability of the 1.50 mol% Fe-loaded CeO2 film, the experiments were carried out for 10 cycles. The synthesized material displayed good stability with regards to photocatalytic performance with less than a 10% decrease from its initial activity. Forming the core-shell magnetic photocatalyst is considered as one of the efficient ways in photocatalytic application. The first attempt in producing Fe3O4/CeO2 magnetic photocatalyst involved the direct deposition of CeO2 onto the surface of magnetic Fe3O4 particles. However a direct contact of CeO2 onto the surface of magnetic Fe3O4 particles presented unfavorable heterojunction, thus the SiO2 barrier layer between magnetic Fe3O4 and CeO2 was prepared as a core-shell structure to reduce the negative effect by combining three steps of the hydrothermal, sonochemical and homogeneous precipitation methods. HRTEM images confirmed that Fe3O4/SiO2/CeO2 exhibited core-shell structure including a magnetic Fe3O4 core, a SiO2 middle layer and CeO2 particle coating as expected. In this part, CeO2 without modification was applied as photocatalyst with a band gap of energy ~3.2 eV, thus the unmodified CeO2 required UV light for activation. The results of the photocatalytic activity revealed that the pseudo-first order rate constants for formic and oxalic acids degradation was increased in the following orders: Fe3O4/SiO2/CeO2 core-shell magnetic nanoparticles > single-phase CeO2 > Fe3O4/CeO2 nanoparticles. The possible mechanism of the photoexcited electron-hole separation and transport processes was proposed based on the obtained morphology via HRTEM morphology and UV-visible DRS results. Fe3O4/SiO2/CeO2 core-shell magnetic nanoparticles was compared and recovered in the consecutive cycles of use by external magnetic field. The material showed good stability with regards to photocatalytic performance for three cycles of use.