Facet-dependent photocatalytic and antibacterial properties of a-Ag2WO4 crystals: combining experimental data and theoretical insights

In this paper, we have combined the various experimental results and first-principles calculations with a new and interesting discussion to explain the photocatalytic and antibacterial activities of α-Ag2WO4 crystals, which were obtained using the microwave-hydrothermal (MH) method with anionic surfactants. The advantages of the insights gained through the present work are two-fold. First, the mechanism and origin of the photocatalytic and antibacterial activities can be rationalized. Second, this facile and controllable synthetic method is expected to encourage the synthesis of complex metal oxides with specific active facets, and these insights can contribute to the rational design of new materials for multifunctional applications. X-ray diffraction and Rietveld refinement analysis confirmed that all the crystals have an orthorhombic structure without deleterious phases. Ultraviolet–visible diffuse reflectance spectroscopy indicated the presence of intermediary energy levels and a variation in the optical band gap values (3.09–3.14 eV) with the crystal growth process. The geometry, electronic properties of the bulk, and surface energies of these crystals were evaluated using first-principles quantum mechanical calculations based on the density functional theory. The crystal shapes was experimentally and theoretically modeled based on Rietveld refinement data, emission scanning electron microscopy images, and Wulff construction. To obtain a wide variety of crystal shapes, the morphologies were gradually varied by tuning the surface chemistry, i.e., the relative stability of the faceted crystals. The growth mechanisms of different α-Ag2WO4 crystals and their facet-dependent photocatalytic and antibacterial performances were explored in details. The combination of experimental and theoretical data revealed the presence of (110) and (011) planes with high surface energies together with the disappearance of faces related to the (010)/(0[1 with combining macron]0) planes in α-Ag2WO4 crystals are key factors that can rationalize both the photocatalytic and antibacterial activities. The different activities may be attributed to the different number of unsaturated superficial Ag and W atoms capable of forming the main active adsorption sites. Finally, we discuss how knowledge of surface-specific properties can be utilized to design a number of crystal morphologies that may offer improved performance in various applications.The authors acknowledge the financial support of the following Brazilian research funding institutions: the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP; 2012/14004-5 and 2013/07296-2), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; 479644/2012-8 and 304531/2013-8) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). J.A. and L.G. are grateful to Prometeo/2009/053 (GeneralitatValenciana), Ministerio de Economia y Competitividad (Spain; CTQ2012-36253-C03-02), and the Spanish Brazilian program (PHB2009-0065-PC). We also acknowledge the Servei Informática, Universitat Jaume I for the generous allotment of computer time

Study on the optical and morphological properties of SrMoO4 powders processed in hydrothermal systems

In the last years, the luminescent emissions of SrMoO4 have been investigated for technological applications in solid-state lasers, light emission diodes, cell phone displays and lamps. In terms of synthesis routes, the hydrothermal systems have received special attention of the scientific community because of the versatility in the formation and crystallization processes of ceramic oxides at low temperature conditions (from 100 °C to 250 °C). Besides the reduced costs and environmentally friendlies, these methods are able to allow a control on the morphologies and particle sizes. Hence, in this research, SrMoO4 powders were synthesized by the co-precipitation reaction and processed in conventional and microwave-hydrothermal systems at 140 °C for 2 h. Thus, it was analyzed the influence of the hydrothermal conditions and the different strontium precursors on the optical and morphological properties of these powders. As experimental results, the X-ray diffraction patterns and the Raman-active vibration modes proved that all powders have a scheelite-type tetragonal structure. Diffraction peaks associated to the secondary phases were only identified in the samples prepared with strontium carbonate. The Rietveld refinements suggested the existence of distortions on both [SrO8] and [MoO4] clusters. The typical bending symmetric and stretching anti-symmetric modes between the O-Mo-O bonds were detected by the infrared spectroscopy. The field-emission gun scanning electron microscopy images revealed that the distinct strontium precursors as well as the hydrothermal conditions were able to change the microcrystal shapes. The adsorption/desorption isotherms showed profiles of type IV with hysteresis curves similar to the H3, suggesting the predominance of slit-like vacant spaces between the particles. The ultravioletvisible absorption spectra indicated the presence of intermediary energy levels within the band gap, which are arising from structural defects in the materials. xvi When excited with 350 nm wavelengths, all samples exhibited broad band photoluminescence emissions. Moreover, the profiles of these spectra were influenced by the precursors and/or hydrothermal treatments.Universidade Federal de Sao CarlosNos últimos anos, as emissões luminescentes do SrMoO4 têm sido investigadas para aplicações tecnológicas em lasers de estado sólido, diodos emissores de luz, displays de telefones celulares e lâmpadas. Em termos de rotas de síntese, os sistemas hidrotérmicos têm recebido especial atenção da comunidade científica por causa da versatilidade nos processos de formação e cristalização de óxidos cerâmicos em baixas temperaturas (de 100 °C a 250 °C). Além dos custos reduzidos e ambientalmente corretos, esses métodos são capazes de permitir um controle sobre as morfologias e os tamanhos de partícula. Por isso, nesta pesquisa, pós de SrMoO4 foram sintetizados pela reação de co-precipitação e processados no sistema hidrotérmico convencional e de microondas em 140 °C por 2 h. Deste modo, foram analisados as influências das condições hidrotérmicas e dos diferentes precursores de estrôncio nas propriedades ópticas e morfológicas destes pós. Como resultados experimentais, os padrões de difração de raios X e os modos vibracionais Raman ativos provaram que todos os pós têm uma estrutura tetragonal do tipo scheelita. Picos de difração associados a fases secundárias foram somente identificados nas amostras preparadas com carbonato de estrôncio. Os refinamentos de Rietveld sugeriram a existência de distorções sobre ambos os clusters de [SrO8] e [MoO4]. Os modos típicos de flexão simétricos e de estiramento anti-simétricos entre as ligações O-Mo-O foram detectados pela espectroscopia de absorção na região do infravermelho. As imagens de microscopia eletrônica de varredura com canhão de elétrons por emissão de campo revelaram que os distintos precursores de estrôncio, bem como as condições hidrotérmicas, foram capazes de modificar a forma dos microcristais. As isotermas de adsorção/dessorção mostraram perfis do tipo IV com curvas de histerese similares a H3, sugerindo a predominância de espaços vazios como fendas entre as partículas. Os xiv espectros de absorção nas regiões do ultravioleta e visível indicaram a presença de níveis de energia intermediários dentro do band gap, os quais são provenientes de defeitos estruturais nos materiais. Quando excitadas com comprimentos de onda de 350 nm, todas as amostras exibiram emissões fotoluminescentes de banda larga. Além disso, os perfis destes espectros foram influenciados tanto pelos precursores quanto pelos tratamentos hidrotérmicos

Structural properties and self-activated photoluminescence emissions in hydroxyapatite with distinct particle shapes

The understanding on defect-related photoluminescence (PL) properties of hydroxyapatite (HA) particles has a fundamental importance in the technological field for the development of new non-toxic biomedical and optical devices. However, the mechanisms responsible for this intrinsic PL in HA are not completely elucidated in the literature yet. In the present paper, stoichiometric and calcium-deficient HA nano- and micro-particles were synthesized by chemical precipitation. The influence of structural and morphological features on the intrinsic PL and electronic structure of this material were investigated by varying the addition rate of the phosphate precursor (0.15, 7.00 or 600.00 mL/min) and pH (4.5–5.0 or 9.5–10.0) value adopted in the precipitation. The results indicated that the structural order at long- and short-range varied with the synthesis conditions and particle shapes (rods, needles, plates, and rices). The blue and green PL emissions were attributed to defects (bulk, surface and interface) in the samples. These defects promoted the formation of additional energy levels within the band gap, as revealed by using two distinct excitation wavelengths for photoemission measurements. The energies of these wavelengths (~ 3.54 and ~ 2.98 eV at 350 and 415 nm, respectively) were lower than the band gap energies of HA samples (from 5.59 to 5.72 eV). A general model was proposed to explain the occurrence of self-activated PL in HA structure

Investigation of electronic structure, morphological features, optical, colorimetric, and supercapacitor electrode properties of CoWO4 crystals

Cobalt tungstate (CoWO4) crystals were synthesized by the co-precipitation (CP) and polymeric precursor (PP) methods with posterior heat treatment at 800 °C for 4 h. The electronic structure, morphological features, optical, colorimetric, and supercapacitive properties were investigated in detail. X-ray diffraction, Rietveld refinement data, micro-Raman spectra, and Fourier transform-infrared spectra proved the crystallization of both CoWO4 materials with a wolframite-type monoclinic structure. Rietveld refinement data were employed as input data to simulate all clusters found in this crystalline structure as well as electron density maps. These results indicated the existence of distortions in both octahedral [CoO6] and [WO6] clusters, yielding an inhomogeneous charge distribution in the monoclinic lattice. Field emission scanning electron microscopy and transmission electron microscopy techniques show the presence of asymmetrical CoWO4 crystals. The ultraviolet–visible diffuse reflectance spectroscopy revealed optical band energy values of 2.84 and 2.89 eV for CoWO4 crystals prepared by the CP and PP methods, respectively. Colorimetric results indicated that the CoWO4 crystals have a desirable feature for the development of blue inorganic pigments. The experimental specific capacitance measurements of CoWO4 crystals as an electrode (CP and PP) were 192.5 Fg−1 and 249.1 Fg−1 at 40 mV s−1 and 5 mV s−1 in an electrode with 0.4 mg and 0.8 mg of electroactive materials in 1 M Na2SO4 solution, respectively

A novel approach to obtain highly intense self-activated photoluminescence emissions in hydroxyapatite nanoparticles

Defect-related photoluminescence (PL) in materials have attracted interest for applications including near ultraviolet (NUV) excitable light-emitting diodes and in biomedical field. In this paper, hydroxyapatite [Ca10(PO4)6(OH)2] nanorods with intense PL bands (bluish- and yellowish-white emissions) were obtained when excited under NUV radiation at room temperature. These nanoparticles were synthesized via chemical precipitation at 90 °C followed by distinct heat treatments temperatures (200–800 °C). Intense and broad emission profiles were achieved at 350 °C (380–750 nm) and 400 °C (380–800 nm). UV–Vis spectroscopy revealed band gap energies (5.58–5.78 eV) higher than the excitation energies (~3.54 and ~2.98 eV at 350 and 415 nm, respectively), confirming the contribution of defect energy levels within the forbidden zone for PL emissions. The structural features were characterized by X-ray diffraction, Rietveld refinement, thermogravimetric analysis, and Fourier transform infrared spectroscopy. By means of these techniques, the relation between structural order-disorder induced by defects, chemical reactions at both lattice and surface of the materials as well as the PL, without activator centers, was discussed in details.The authors are grateful to the FAPESP (#2013/11144-3), FAPESP/CEPID (#2013/07296-2), CNPq (#573636/2008-7), CAPES/PNPD (#20131475), Universitat Jaume I (P1 1B2013-65), and Ministerio de Economia y Competitividad (Salvador Madariaga program, PRX155/00261) for the financial support. Special thanks to Mr. R. Camargo for FE-SEM and TEM images

Structural investigation and sonophotocatalytic properties of the solid solutions Sr(Mo1−xWx)O4 crystals synthesized by the sonochemical method

In this paper, the solid solutions of strontium molybdate-tungstate [Sr(Mo1−xWx)O4] crystals with (x = 0, 0.25, 0.50, 0.75, and 1) were synthesized by the sonochemical method. Their structure, morphology, optical, and sonophotocatalytic properties were performed in function of the replacement of Mo6+ by W6+ cations into the lattice. Their structure and elemental composition were characterized using X-ray diffraction, Rietveld refinement, micro-Raman, energy-dispersive X-ray, and Fourier-transform infrared spectroscopies proving that all samples are monophasic, crystalline, and exhibit a scheelite-type tetragonal structure. Field-emission scanning electron microscopy images revealed the octahedral and dumbbell-like morphologies for SrMoO4 and SrWO4 crystals. Moreover, it is noted to pass through spindle-like morphology for the microcrystals containing both Mo6+ and W6+ cations (x = 0.25, 0.50, and 0.75). Ultraviolet-visible diffuse reflectance spectroscopy showed a directly proportional increase in the optical band gap (Egap) values from 4.27 to 5.01 eV. These data indicate an increase in the intermediary electronic levels between valance and conduction bands with the increase in the concentration of W6+ cations in the lattice. Finally, we have obtained good sonophotocatalytic performances for SrMoO4 crystals (90%), and mainly to Sr(Mo0.25W0.75)O4 crystals (98%) in the degradation of Rhodamine B dye until 240 min under UV-C light