Osmando F. Lopes

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.

ÓXIDOS DE NIÓBIO: UMA VISÃO SOBRE A SÍNTESE DO Nb2O5 E SUA APLICAÇÃO EM FOTOCATÁLISE HETEROGÊNEA

Despite the fact that Brazil is the world’s largest niobium mineral producer, governmental interest in exploration of the mineral leading to more valuable derived materials is scarce, which has reduced the country’s knowledge about a wider range of technological applications for this metal. Niobium pentoxide stands out due its remarkable electronic, structural, and textural properties. Therefore, this review aims to highlight its main properties, synthetic methods, and applications, with a particular focus on photocatalysts based on Nb2O5. This review will highlight the potential of Nb2O5 and encourage the study of niobium and its compounds in technological and environmental applications.

Hierarchical growth of ZnO nanorods over SnO2 seed layer: insights into electronic properties from photocatalytic activity

The use of nanostructured heterojunctions has been a promising option for hindering the charge recombination and thus enhancing the photocatalytic performance of catalysts. Here we present a simple strategy to hierarchically grow heterostructures using a hydrothermal treatment route. A buffer SnO2 film was produced by a sol–gel derived method, resulting in a film of approximately 100 nm composed of 5–10 nm nanoparticles. X-ray diffraction and scanning electron microscopy revealed preferential growth of the nanorod-like structures along the c-axis perpendicular to the SnO2 film, with an average nanorod diameter and length of approximately 160 nm and 1.5 μm, respectively. The photoluminescence spectra of ZnO–SnO2 revealed a reduction in UV emission compared to individual ZnO nanorods, indicating that the recombination of the photogenerated carriers was inhibited in the heterojunction. This behavior was confirmed by evaluating the photocatalytic performance of such films against methylene blue degradation, showing that the as-prepared ZnO–SnO2 heterojunction was superior to the individual semiconductors, ZnO and SnO2.

Vanadium pentoxide 1-D nanostructures applied to dye removal from aqueous systems by coupling adsorption and visible-light photodegradation†

This paper evaluates the photocatalytic and dye adsorption properties of highly crystalline orthorhombic vanadium pentoxide (V2O5) one-dimensional (1D) nanostructures, such as nanowires and nanorods, synthesized by a hydrothermal method. The as-synthesized samples were characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), scanning and transmission electron microscopy (SEM and TEM), thermogravimetry (TGA), zeta potential, Fourier transform infrared spectrometry (FTIR), N2 adsorption isotherms and high-resolution 13C nuclear magnetic resonance (NMR). The dye adsorption capability and photocatalytic properties under visible light were mainly studied by the removal of the methylene blue dye (MB). Despite their low specific surface area (approximately 35 m2 g−1), the nanostructures showed high MB adsorption capabilities of greater than 400 mg g−1. Additionally, due to the band-gap values (approximately 2.6 eV), the nanostructures could be successfully applied to photodegradation under visible light, showing higher photoactivity than commercial V2O5. The MB adsorption mechanism onto V2O5 1D-nanostructures surface can be explained via NMe2+ interaction with the negatively charged surface of the studied samples. The observed combination of adsorbent and photocatalytic properties makes the V2O5 1D-nanostructures a promising material for organic pollutant decontamination, which appears more efficient for cationic species.

Zinc hydroxide/oxide and zinc hydroxy stannate photocatalysts as potential scaffolds for environmental remediation

This work describes the facile aqueous solution based syntheses and detailed characterization of zinc based hydroxide/oxide and zinc hydroxy stannate materials and their efficient utilization as catalyst for the photodegradation of harmful organic dye, i.e. Rhodamine B. The detailed studies revealed that the reaction time and temperature are important parameters by which the crystal phases and morphology of prepared materials can be controlled. Therefore, it was observed that with increasing the reaction time and temperature, a mixed phase of Zn(OH)2 and ZnO and pure ZnO can be prepared. The detailed characterizations showed that zinc hydroxy stannate cubes grow in high density and possess cube shaped morphologies. The detailed photocatalytic experiments revealed that as-synthesized ZnSn(OH)6 cubes possess higher photoactivity compared to the zinc based hydroxide/oxide materials. The stability results indicated no significant deactivation even after four successive re-uses in RhB photodegradation. Furthermore, the rate of ˙OH radical formation during UV irradiation was also tracked to investigate the mechanism of RhB photodegradation. The same trend was also observed on comparing the photoactivity and rate of ˙OH radical formation. Thus, it can be proposed that the mechanism of RhB degradation catalyzed by the as-synthesized materials followed an indirect oxidation of the dye by ˙OH radicals formed over the photocatalyst during UV irradiation, instead of direct electron transfer between dye and photocatalyst because there was no remarkable adsorption of the dye onto the photocatalyst surface.

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 20u2005nm. 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.