RESUMO
In this paper, we report the synthesis of silver selenite (Ag2SeO3) by different methods [sonochemistry, ultrasonic probe, coprecipitation, and microwave-assisted hydrothermal methods]. These microcrystals presented a structural long-range order as confirmed by X-ray diffraction (XRD) and Rietveld refinements and a structural short-range order as confirmed by Fourier transform infrared (FTIR) and Raman spectroscopies. X-ray photoelectron spectroscopy (XPS) provided information about the surface of the samples indicating that they were pure. The microcrystals presented different morphologies and sizes due to the synthesis method as observed by field emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were evaluated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. Thermal analysis confirmed the temperature stability of the as-synthetized samples. Further trapping experiments prove that the holes and hydroxyl radicals, to a minor extent, are responsible for the photocatalytic reactions. The experimental results are sustained by first-principles calculations, at the density functional theory (DFT) level, to decipher the structural parameters, electronic properties of the bulk, and surfaces of Ag2SeO3. By matching the experimental FE-SEM images and theoretical morphologies, we are capable of finding a correlation between the morphology and photocatalytic activity, along with photodegradation of the Rhodamine B dye under UV light, based on the different numbers of unsaturated superficial Ag and Se cations (local coordination, i.e., clusters) of each surface.
RESUMO
Present theoretical and experimental work provides an in-depth understanding of the morphological, structural, electronic, and optical properties of hexagonal and monoclinic polymorphs of bismuth phosphate (BiPO4). Herein, we demonstrate how microwave irradiation induces the transformation of a hexagonal phase to a monoclinic phase in a short period of time and, thus, the photocatalytic performance of BiPO4. To complement and rationalize the experimental results, first-principles calculations have been performed within the framework of density functional theory. This was aimed at obtaining the geometric, energetic, and structural parameters as well as vibrational frequencies; further, the electronic properties (band structure diagram and density of states) of the bulk and corresponding surfaces of both the hexagonal and monoclinic phases of BiPO4 were also acquired. A detailed characterization of the low vibrational modes of both the hexagonal and monoclinic polymorphs is key to explaining the irreversible phase transformation from hexagonal to monoclinic. On the basis of the calculated values of the surface energies, a map of the available morphologies of both phases was obtained by using Wulff construction and compared to the observed scanning electron microscopy images. The BiPO4 crystals obtained after 16-32 min of microwave irradiation provided excellent photodegradation of Rhodamine B under visible-light irradiation. This enhancement was found to be related to the surface energy and the types of clusters formed on the exposed surfaces of the morphology. These findings provide details of the hexagonal-to-monoclinic phase transition in BiPO4 during microwave irradiation; further, the results will assist in the design of electronic devices with higher efficiency and reliability.
RESUMO
A systematic theoretical and experimental study was carried out to find a relationship between photoluminescence emissions and photocatalytic activity of Ag2SeO4 obtained by different synthesis methods (sonochemistry, ultrasonic probe, coprecipitation and microwave assisted hydrothermal synthesis). Experimental characterization techniques (XRD with Rietveld refinement, Raman, FTIR, UV-vis, XPS and photoluminescence spectroscopy) were used to elucidate its structural order at short, medium, and long ranges. Morphological analysis performed by FE-SEM showed distinct morphologies due to the different methods of synthesis. Based on density functional theory (DFT) calculations, it was possible to study in detail the Ag2SeO4 surface properties, including its surface energy, geometry, and electronic structure for the (100), (010), (001), (101), (011), (110), (111), (021), (012) and (121) surfaces. The equilibrium morphology of Ag2SeO4 was predicted as a truncated octahedron with exposed (111), (001), (010) and (011) surfaces. Photoluminescence emissions showed a band covering the visible spectrum, and the Ag2SeO4 obtained by the coprecipitation method presented the most intense band with a maximum in the red region. Photocatalytic results confirmed that Ag2SeO4 synthesized by the sonochemistry method is the best photocatalyst for rhodamine B degradation under UV light irradiation.