Toward an Understanding of the Growth of Ag Filaments on α-Ag2WO4 and Their Photoluminescent Properties: A Combined Experimental and Theoretical Study

A combined experimental and theoretical study was conducted on the structure and electronic properties of α-Ag2WO4 to clarify the nucleation and growth processes of Ag filaments on α-Ag2WO4 crystals induced by electron beam irradiation under electron microscopy. X-ray diffraction with Rietveld analysis, micro-Raman and Fourier-transform infrared spectroscopy were used to analyze the structural order/disorder of α-Ag2WO4 crystals. These complementary techniques indicated that the microwave-assisted hydrothermal method employed in the synthesis of α-Ag2WO4 crystals leads to the freezing of distorted [WO6] and [AgOy] (y = 2, 4, 6 and 7) clusters as the constituent polyhedra of α-Ag2WO4. On the basis of the theoretical and experimental results, we provide a complete assignment of the structure of α-Ag2WO4 and describe the relationship among the disorder, nucleation growth, rate of Ag formation, and photoluminescence behavior before and after the irradiation of the accelerated electron beam. Density functional theory (DFT) studies indicated significant changes in the order–disorder of the initial α-Ag2WO4electronic structure, with a decrease in the band gap value from 3.55 to 2.72 eV. The first stages of the electron irradiation on α-Ag2WO4 crystal were investigated by DFT calculations, and we have derived a mechanism to describe the formation and growth of Ag filaments during the electronic excitation of the [AgO2] cluster.This work is financially supported by the National Council for Scientific and Technological Development (CNPq), São Paulo Research Foundation (FAPESP), Prometeo/2009/053 (Generalitat Valenciana) and Ministerio de Economía y Competitividad (Spain), CTQ2012-36253-C03-02, and the Spanish–Brazilian program (PHB2009-0065-PC) for their financial support. TEM facilities were provided by LME-IQ-UNESP

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