Kele T.G. Carvalho

Synthesis of BiVO4via oxidant peroxo-method: insights into the photocatalytic performance and degradation mechanism of pollutants

This paper reports the synthesis of monoclinic bismuth vanadate (BiVO4) by the oxidant peroxide method with crystallization under hydrothermal conditions, and its catalytic performance on the photodegradation of pollutants under visible light. The as-synthesized BiVO4 materials were characterized by means of X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) and scanning electron microscopy (SEM). Hydrothermal treatment above 80 °C was required to obtain pure monoclinic BiVO4 phase by releasing V5+ ions from vanadium peroxo complexes. With the increase in hydrothermal reaction temperature, the particle size decreased. All BiVO4 samples presented large size and shape distribution and band gap of approximately 2.40 eV. The as-prepared BiVO4 catalysts showed high photoactivity for decomposition of model pollutants, methylene blue (MB) and rhodamine B (RhB) dyes, under exposure to visible light. The photodegradation mechanism was evaluated by adding scavengers, DMSO and KBrO3, which were used to probe ˙OH radical and conduction band (CB) electrons, respectively. It was observed that photodegradation of MB and RhB dyes is caused by the action of ˙OH radicals, and that BiVO4 CB electrons do not have reduction potential sufficiently high to reduce dissolved oxygen to O2−˙. It was proven that the indirect mechanism, i.e. ˙OH radical formation, plays the major role on the BiVO4-assisted photodegradation process.

An Understanding of the Photocatalytic Properties and Pollutant Degradation Mechanism of SrTiO3 Nanoparticles

Strontium titanate nanoparticles have attracted much attention due to their physical and chemical properties, especially as photocatalysts under ultraviolet irradiation. In this paper, we analyze the effect of heating rate during the crystallization process of SrTiO3 nanoparticles in the degradation of organic pollutants. The relationship between structural, morphological and photocatalytic properties of the SrTiO3 nanoparticles was investigated using different techniques. Transmission electron microscopy and N2 adsorption results show that particle size and surface properties are tuned by the heating rate of the SrTiO3 crystallization process. The SrTiO3 nanoparticles showed good photoactivity for the degradation of methylene blue, rhodamine B and methyl orange dyes, driven by a nonselective process. The SrTiO3 sample with the largest particle size exhibited higher photoactivity per unit area, independent of the molecule to be degraded. The results pointed out that the photodegradation of methylene blue dye catalyzed by SrTiO3 is caused by the action of valence band holes (direct pathway), and the indirect mechanism has a negligible effect, i.e. degradation by O2−• and •OH radicals attack.

Synthesis of ZnO nanoparticles assisted by N-sources and their application in the photodegradation of organic contaminants

A modified polymeric precursor method assisted by N sources (urea or melamine) was used to obtain anion‐doped ZnO nanoparticles. The influence of these molecules on the physical‐chemical and photocatalytic properties of the as‐synthesized samples was investigated. The ZnO nanoparticles exhibited a hexagonal wurtzite phase and crystallite sizes of approximately 20 nm. The addition of urea or melamine to the Zn2+ precursor solution improved the surface properties of the materials and resulted in controlled growth of the N‐doped ZnO nanoparticles, with urea showing superior performance for this purpose. These changes led to improved photocatalytic performance in the degradation of methylene blue (MB) dye and ethionamide antibiotic under UVC irradiation. It was observed that the indirect mechanism involving .OH radical attack played the main role in both photodegradation reactions catalyzed by the as‐synthesized ZnO samples, whereas the photosensitization mechanism had a negligible influence. The use of ESI‐MS analyses showed that the MB dye molecules were broken up by the action of the ZnO photocatalyst, indicating the occurrence of a mineralization process.

Insights into the photocatalytic performance of Bi2O2CO3/BiVO4 heterostructures prepared by one-step hydrothermal method

This paper describes the synthesis of Bi2O2CO3/BiVO4 heterostructures through a one-step method based on the difference in solubility between two semiconductors that possess a metal in common. The as-synthesized Bi2O2CO3/BiVO4 heterostructures were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 physisorption, X-ray photoelectron spectroscopy (XPS) and time resolved photoluminescence spectroscopy (TRPL). The role of the heterojunction formed was evaluated by methylene blue (MB) dye and amiloride photodegradation. The formation of the heterostructure was observed indirectly by the great increase in the thermal stability of the Bi2O2CO3 phase when compared to its pure phase. The amount of heterojunctions formed between the Bi2O2CO3 and BiVO4 was tuned by vanadium precursor concentration. The proposed strategy was efficient for obtaining Bi2O2CO3/BiVO4 heterostructures with enhanced photocatalytic performance when compared to their isolated phases, MB and amiloride photodegradation occurred mainly by the action of ˙OH radicals, i.e. by an indirect mechanism. Based on TRPL spectroscopy and VB-XPS results, an enhancement of photoactivity related to an increase in the spatial separation of photo-generated electron/hole pairs was observed due to the formation of a type-II heterostructure.