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-Ag 2 WO 4 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-Ag 2 WO 4 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 IJ010)/IJ010) planes in α-Ag 2 WO 4 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.

In the present work, the morphology (hexagonal rod-like vs cuboid-like) of an α-Ag 2 WO 4 solid-state material is manipulated by a simple controlled-precipitation method, with and without the presence of the anionic surfactant sodium dodecyl sulfate (SDS), respectively, over short reaction times. Characterization techniques, such as X-ray diffraction analysis, Rietveld refinement analysis, Fourier-transform (FT) infrared spectroscopy, FT Raman spectroscopy, UV−vis spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction, energy-dispersive X-ray spectroscopy, field emission-scanning electron microscopy (FE-SEM), and photoluminescence emission, are employed to disclose the structural and electronic properties of the α-Ag 2 WO 4 material. First-principles calculations were performed to (i) obtain the relative stability of the six low-index surfaces of α-Ag 2 WO 4 ; (ii) rationalize the crystal morphologies observed in FE-SEM images (using the Wulff construction); and (iii) determine the energy profiles associated with the transformation process between both morphologies induced by the presence of SDS. Finally, we demonstrate a relationship between morphology and photocatalytic activity, evaluated by photodegradation of Rhodamine B dye under UV light, based on the different numbers of unsaturated superficial Ag and W cations (local coordination, i.e., clusters) of each surface.

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In nanotechnology research, significant effort is devoted to fabricating patterns of metallic nanoparticles on the surfaces of different semiconductors to find innovative materials with favorable characteristics, such as antimicrobial and photocatalytic properties, for novel applications. We present experimental and computational progress, involving a combined approach, on the antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) of as-synthesized α-Ag2WO4 samples and Ag nanoparticle composites (Ag NPs)/α-Ag2WO4. The former included two morphologies: hexagonal rod-like (α-Ag2WO4-R) and cuboid-like (α-Ag2WO4-C), and the latter included composites formed under electron beam, Ag NPs/α-Ag2WO4-RE and Ag NPs/α-Ag2WO4-CE, and femtosecond (fs) laser irradiation, Ag NPs/α-Ag2WO4-RL and Ag NPs/α-Ag2WO4-CL. Direct observations of the arrangement of Ag NPs on the Ag NPs/α-Ag2WO4 composites irradiated with an electron beam and laser, through transmission electron microscopy (TEM), high-resolution TEM, energy-dispersive X-ray spectroscopy, and field-emission scanning electron microscopy, allow us to investigate the surface morphology, chemical composition, homogeneity, and crystallinity. Therefore, these experimental factors, and in particular, the facet-dependent response of Ag NPs/α-Ag2WO4 composites were discussed and analyzed from the perspective provided by the results obtained by first-principles calculations. On this basis, α-Ag2WO4-R material proved to be a better bactericidal agent than α-Ag2WO4-C with minimum bactericidal concentration (MBC) values of 128 and 256 μg/mL, respectively. However, the Ag NPs/α-Ag2WO4-CL composite is the most efficient bactericidal agent of all tested samples (MBC = 4 μg/mL). This superior performance can be attributed to the cooperative effects of crystal facets and defect engineering. These results on the synthesis and stability of the Ag NPs/α-Ag2WO4 composites can be used for the development of highly efficient bactericidal agents for use in environmental remediation and the potential extension of methods to produce materials with catalytic applications.

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