Selective Synthesis of α-, ß-, and γ-Ag2WO4 Polymorphs: Promising Platforms for Photocatalytic and Antibacterial Materials.

Silver tungstate (Ag2WO4) shows structural polymorphism with different crystalline phases, namely, orthorhombic, hexagonal, and cubic structures that are commonly known as α, ß, and γ, respectively. In this work, these Ag2WO4 polymorphs were selectively and successfully synthesized through a simple precipitation route at ambient temperature. The polymorph-controlled synthesis was conducted by means of the volumetric ratios of the silver nitrate/tungstate sodium dehydrate precursors in solution. The structural and electronic properties of the as-synthesized Ag2WO4 polymorphs were investigated by using a combination of X-ray diffraction and Rietveld refinements, X-ray absorption spectroscopy, X-ray absorption near-edge structure spectroscopy, field-emission scanning electron microscopy images, and photoluminescence. To complement and rationalize the experimental results, first-principles calculations, at the density functional theory level, were carried out, leading to an unprecedented glimpse into the atomic-level properties of the morphology and the exposed surfaces of Ag2WO4 polymorphs. Following the analysis of the local coordination of Ag and W cations (clusters) at each exposed surface of the three polymorphs, the structure-property relationship between the morphology and the photocatalytic and antibacterial activities against amiloride degradation under ultraviolet light irradiation and methicillin-resistant Staphylococcus aureus, respectively, was investigated. A possible mechanism of the photocatalytic and antibacterial activity as well the formation process and growth of the polymorphs is also explored and proposed.

Computational Chemistry Meets Experiments for Explaining the Geometry, Electronic Structure, and Optical Properties of Ca10V6O25.

In this paper, we present a combined experimental and theoretical study to disclose, for the first time, the structural, electronic, and optical properties of Ca10V6O25 crystals. The microwave-assisted hydrothermal (MAH) method has been employed to synthesize these crystals with different morphologies, within a short reaction time at 120 °C. First-principle quantum mechanical calculations have been performed at the density functional theory level to obtain the geometry and electronic properties of Ca10V6O25 crystal in the fundamental and excited electronic states (singlet and triplet). These results, combined with the measurements of X-ray diffraction (XRD) and Rietveld refinements, confirm that the building blocks lattice of the Ca10V6O25 crystals consist of three types of distorted 6-fold coordination [CaO6] clusters: octahedral, prism and pentagonal pyramidal, and distorted tetrahedral [VO4] clusters. Theoretical and experimental results on the structure and vibrational frequencies are in agreement. Thus, it was possible to assign the Raman modes for the Ca10V6O25 superstructure, which will allow us to show the structure of the unit cell of the material, as well as the coordination of the Ca and V atoms. This also allowed us to understand the charge transfer process that happens in the singlet state (s) and the excited states, singlet (s*) and triplet (t*), generating the photoluminescence emissions of the Ca10V6O25 crystals.

Formation of Ag Nanoparticles on ß-Ag2WO4 through Electron Beam Irradiation: A Synergetic Computational and Experimental Study.

In the present work, a combined theoretical and experimental study was performed on the structure, optical properties, and growth of Ag nanoparticles in metastable ß-Ag2WO4 microcrystals. This material was synthesized using the precipitation method without the presence of surfactants. The structural behavior was analyzed using X-ray diffraction and Raman and infrared spectroscopy. Field-emission scanning electron microscopy revealed the presence of irregular spherical-like Ag nanoparticles on the ß-Ag2WO4 microcrystals, which were induced by electron beam irradiation under high vacuum conditions. A detailed analysis of the optimized ß-Ag2WO4 geometry and theoretical results enabled interpretation of both the Raman and infrared spectra and provided deeper insight into rationalizing the observed morphology. In addition, first-principles calculations, within the quantum theory of atoms in molecules framework, provided an in-depth understanding of the nucleation and early evolution of Ag nanoparticles. The Ag nucleation and formation is the result of structural and electronic changes of the [AgO6] and [AgO5] clusters as a constituent building block of ß-Ag2WO4, which is consistent with Ag metallic formation.

Theoretical and Experimental Insight on Ag2CrO4 Microcrystals: Synthesis, Characterization, and Photoluminescence Properties.

Ag2CrO4 microcrystals were synthesized by means of the coprecipitation method without the use of a surfactant under three different conditions. On the basis of the theoretical and experimental results, we describe the relationship among the structural order/disorder effects, morphology, and photoluminescence of the Ag2CrO4 microcrystals. The experimental results were correlated with the theoretical findings for a deeper understanding of the relationship between the electronic structure, morphology, and photoluminescence properties. First-principles computational studies were used to calculate the geometries of bulk Ag2CrO4 and its low-index (001), (011), (110), (010), (111), and (100) facets based on a slab model. A good agreement between the experimental and the theoretical morphologies was found by varying the ratio of the superficial energy values.

In situ growth of Ag nanoparticles on α-Ag2WO4 under electron irradiation: probing the physical principles.

Exploiting the plasmonic behavior of Ag nanoparticles grown on α-Ag2WO4 is a widely employed strategy to produce efficient photocatalysts, ozone sensors, and bactericides. However, a description of the atomic and electronic structure of the semiconductor sites irradiated by electrons is still not available. Such a description is of great importance to understand the mechanisms underlying these physical processes and to improve the design of silver nanoparticles to enhance their activities. Motivated by this, we studied the growth of silver nanoparticles to investigate this novel class of phenomena using both transmission electron microscopy and field emission scanning electron microscopy. A theoretical framework based on density functional theory calculations (DFT), together with experimental analysis and measurements, were developed to examine the changes in the local geometrical and electronic structure of the materials. The physical principles for the formation of Ag nanoparticles on α-Ag2WO4 by electron beam irradiation are described. Quantum mechanical calculations based on DFT show that the (001) of α-Ag2WO4 displays Ag atoms with different coordination numbers. Some of them are able to diffuse out of the surface with a very low energy barrier (less than 0.1 eV), thus, initiating the growth of metallic Ag nanostructures and leaving Ag vacancies in the bulk material. These processes increase the structural disorder of α-Ag2WO4 as well as its electrical resistance as observed in the experimental measurements.

Effects of chemical substitution on the structural and optical properties of α-Ag2-2xNixWO4 (0 ≤ x ≤ 0.08) solid solutions.

In this work, we investigated the effects of chemical substitution on the structural, electronic, and optical properties of α-Ag2-2xNixWO4 (0 ≤ x ≤ 0.08) solid solutions prepared by a facile microwave-assisted hydrothermal method. The results showed that the increase of Ni concentration in α-Ag2WO4 microcrystals as a host matrix caused a morphological transformation and a shift of the electronic and optical properties. Based on first principles calculations and using Wulff's construction, particle shapes and their transformations in α-Ag2WO4 and α-Ag2-2xNixWO4 can be affected by controlling the ratios of surface energy values between the different facets. In addition, theoretical calculations revealed that Ni substitution in α-Ag2WO4 is more favorable in the Ag2 and Ag4 positions, in which the local coordination of Ag atoms corresponds to clusters with coordination numbers of seven and four, respectively. This behavior could be related to the degree of medium-range structural disorder in α-Ag2-2xNixWO4 crystals. The experimental results were correlated with theoretical simulations to achieve a deeper understanding of the relationship between morphology and properties. These results provide the basis for a rational design for the compositional modulation of structural and optical properties.

Effects of surface stability on the morphological transformation of metals and metal oxides as investigated by first-principles calculations.

Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure-reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and α-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies.

Elucidating the real-time Ag nanoparticle growth on α-Ag2WO4 during electron beam irradiation: experimental evidence and theoretical insights.

Why and how Ag is formed when electron beam irradiation takes place on α-Ag2WO4 in a vacuum transmission electron microscopy chamber? To find an answer, the atomic-scale mechanisms underlying the formation and growth of Ag on α-Ag2WO4 have been investigated by detailed in situ transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) studies, density functional theory based calculations and ab initio molecular dynamics simulations. The growth process at different times, chemical composition, size distribution and element distribution were analyzed in depth at the nanoscale level using FE-SEM, operated at different voltages (5, 10, 15, and 20 kV), and TEM with energy dispersive spectroscopy (EDS) characterization. The size of Ag nanoparticles covers a wide range of values. Most of the Ag particles are in the 20-40 nm range. The nucleation and formation of Ag on α-Ag2WO4 is a result of structural and electronic changes in the AgOx (x = 2,4, 6, and 7) clusters used as constituent building blocks of this material, consistent with metallic Ag formation. First principle calculations point out that Ag-3 and Ag-4-fold coordinated centers, located in the sub-surface of the (100) surface, are the most energetically favorable to undergo the diffusion process to form metallic Ag. Ab initio molecular dynamics simulations and the nudged elastic band (NEB) method were used to investigate the minimum energy pathways of these Ag atoms from positions in the first slab layer to outward sites on the (100) surface of α-Ag2WO4. The results point out that the injection of electrons decreases the activation barrier for this diffusion step and this unusual behavior results from the presence of a lower energy barrier process.

Identifying and rationalizing the morphological, structural, and optical properties of [Formula: see text]-Ag2MoO4 microcrystals, and the formation process of Ag nanoparticles on their surfaces: combining experimental data and first-principles calculations.

We present a combined theoretical and experimental study on the morphological, structural, and optical properties of ß-Ag2MoO4 microcrystals. ß-Ag2MoO4 samples were prepared by a co-precipitation method. The nucleation and formation of Ag nanoparticles on ß-Ag2MoO4 during electron beam irradiation were also analyzed as a function of electron beam dose. These events were directly monitored in real-time using in situ field emission scanning electron microscopy (FE-SEM). The thermodynamic equilibrium shape of the ß-Ag2MoO4 crystals was built with low-index surfaces (001), (011), and (111) through a Wulff construction. This shape suggests that the (011) face is the dominating surface in the ideal morphology. A significant increase in the values of the surface energy for the (011) face versus those of the other surfaces was observed, which allowed us to find agreement between the experimental and theoretical morphologies. Our investigation of the different morphologies and structures of the ß-Ag2MoO4 crystals provided insight into how the crystal morphology can be controlled so that the surface chemistry of ß-Ag2MoO4 can be tuned for specific applications. The presence of structural disorder in the tetrahedral [MoO4] and octahedral [AgO6] clusters, the building blocks of ß-Ag2MoO4, was used to explain the experimentally measured optical properties.

Prediction of dopant atom distribution on nanocrystals using thermodynamic arguments.

A theoretical approach aiming at the prediction of segregation of dopant atoms on nanocrystalline systems is discussed here. It considers the free energy minimization argument in order to provide the most likely dopant distribution as a function of the total doping level. For this, it requires as input (i) a fixed polyhedral geometry with defined facets, and (ii) a set of functions that describe the surface energy as a function of dopant content for different crystallographic planes. Two Sb-doped SnO2 nanocrystalline systems with different morphology and dopant content were selected as a case study, and the calculation of the dopant distributions expected for them is presented in detail. The obtained results were compared to previously reported characterization of this system by a combination of HRTEM and surface energy calculations, and both methods are shown to be equivalent. Considering its application pre-requisites, the present theoretical approach can provide a first estimation of doping atom distribution for a wide range of nanocrystalline systems. We expect that its use will support the reduction of experimental effort for the characterization of doped nanocrystals, and also provide a solution to the characterization of systems where even state-of-art analytical techniques are limited.

CaSO4 and its pressure-induced phase transitions. A density functional theory study.

Theoretical investigations concerning possible calcium sulfate, CaSO(4), high-pressure polymorphs have been carried out. Total-energy calculations and geometry optimizations have been performed by using density functional theory at the B3LYP level for all crystal structures considered. The following sequence of pressure-driven structural transitions has been found: anhydrite, Cmcm (in parentheses the transition pressure) → monazite-type, P2(1)/n (5 GPa) → barite-type, Pnma (8 GPa), and scheelite-type, I4(1)/a (8 GPa). The equation of state of the different polymorphs is determined, while their corresponding vibrational properties have been calculated and compared with previous theoretical results and experimental data.

A Chemical-Pressure-Induced Phase Transition Controlled by Lone Electron Pair Activity.

The chemical pressure approach offers a new paradigm for property control in functional materials. In this work, we disclose a correlation between the ß â†’ α pressure-induced phase transition in SnMoO4 and the substitution process of Mo6+ by W6+ in SnMo1-xWxO4 solid solutions (x = 0-1). Special attention is paid to discriminating the role of the lone pair Sn2+ cation from the structural distortive effect along the Mo/W substitution process, which is crucial to disentangle the driven force of the transition phase. Furthermore, the reverse α → ß transition observed at high temperature in SnWO4 is rationalized on the same basis as a negative pressure effect associated with a decreasing of W6+ percentage in the solid solution. This work opens a versatile chemical approach in which the types of interactions along the formation of solid solutions are clearly differentiated and can also be used to tune their properties, providing opportunities for the development of new materials.

Density-functional study of pressure-induced phase transitions and electronic properties of Zn2V2O7.

We report a study of the high-pressure behavior of the structural and electronic properties of Zn2V2O7 by means of first-principle calculations using the CRYSTAL code. Three different approaches have been used, finding that the Becke-Lee-Yang-Parr functional is the one that best describes Zn2V2O7. The reported calculations contribute to the understanding of previous published experiments. They support the existence of three phase transitions for pressures smaller than 6 GPa. The crystal structure of the different high-pressure phases is reported. We have also made a systematic study of the electronic band-structure, determining the band-gap and its pressure dependence for the different polymorphs. The reported results are compared to previous experimental studies. All the polymorphs of Zn2V2O7 have been found to have a wide band gap, with band-gap energies in the near-ultraviolet region of the electromagnetic spectrum.

Identifying and explaining vibrational modes of sanbornite (low-BaSi2O5) and Ba5Si8O21: A joint experimental and theoretical study.

We report here the analysis of vibrational properties of the sanbornite (low-BaSi2O5) and Ba5Si8O21 using theoretical and experimental approaches, as well as results of high temperature experiments up to 1100-1150 °C. The crystal parameters derived from Rietveld refinement and calculations show excellent agreement, within 4%, while the absolute mean difference between the theoretical and experimental results for the IR and Raman vibrational frequencies was <6 cm-1. The temperature-dependent Raman study renders that both sanbornite and Ba5Si8O21 display specific Ba and Si sites and their BaO and SiO bonds. In the case of the stretching modes assigned to specific Si sites, the frequency dependence on the SiO bond length exhibited very strong correlations. Both phases showed that for a change of 0.01 Å, the vibrational mode shifted 10 ± 2 cm-1. These results are promising for using Raman spectroscopy to track in situ reactions under a wide variety of conditions, especially during crystallization.

Unvealing the role of ß-Ag2MoO4 microcrystals to the improvement of antibacterial activity.

Crystal morphology with different surfaces is important for improving the antibacterial activity of materials. In this experimental and theoretical study, the antibacterial activity of ß-Ag2MoO4 microcrystals against the Gram-positive bacteria, namely, methicillin-resistant Staphylococcus aureus (MRSA), and the Gram-negative bacteria, namely, Escherichia coli (E. coli), was investigated. In this study, ß-Ag2MoO4 crystals with different morphologies were synthetized by a simple co-precipitation method using three different solvents. The antimicrobial efficacy of the obtained microcrystals against both bacteria increased according to the solvent used in the following order: water < ammonia < ethanol. Supported by experimental evidence, a correlation between morphology, surface energy, and antibacterial performance was established. By using the theoretical Wulff construction, which was obtained by means of density functional calculations, the morphologies with large exposition of the (001) surface exhibited superior antibacterial activity. This study provides a low cost route for synthesizing ß-Ag2MoO4 crystals and a guideline for enhancing the biological effect of biocides on pathogenic bacteria by the morphological modulation.

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase.

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states (1A1/3A' ') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ (1Sigma/3Sigma); formation of propynaldehyde and the moiety V-(OH2)+; and two elimination processes of water molecule to yield cationic products, Prod-fc+ and Prod-dc+ where the vanadium atom adopts a four- and di-coordinate structure, respectively.

Mechanism of Antibacterial Activity via Morphology Change of α-AgVO3: Theoretical and Experimental Insights.

The electronic configuration, morphology, optical features, and antibacterial activity of metastable α-AgVO3 crystals have been discussed by a conciliation and association of the results acquired by experimental procedures and first-principles calculations. The α-AgVO3 powders were synthesized using a coprecipitation method at 10, 20, and 30 °C. By using a Wulff construction for all relevant low-index surfaces [(100), (010), (001), (110), (011), (101), and (111)], the fine-tuning of the desired morphologies can be achieved by controlling the values of the surface energies, thereby lending a microscopic understanding to the experimental results. The as-synthesized α-AgVO3 crystals display a high antibacterial activity against methicillin-resistant Staphylococcus aureus. The results obtained from the experimental and theoretical techniques allow us to propose a mechanism for understanding the relationship between the morphological changes and antimicrobial performance of α-AgVO3.

In situ Transmission Electron Microscopy observation of Ag nanocrystal evolution by surfactant free electron-driven synthesis.

The study of the interaction of electron irradiation with matter and the response of the material to the passage of electrons is a very challenging problem. However, the growth mechanism observed during nanostructural evolution appears to be a broad and promising scientific field in nanotechnology. We report the in situ TEM study of nanostructural evolution of electron-driven silver (Ag) nanocrystals through an additive-free synthetic procedure. Observations revealed the direct effect of the electron beam on the morphological evolution of Ag nanocrystals through different mechanisms, such as mass transport, site-selective coalescence, and an appropriate structural configuration after coalescence leading to a more stable configuration. A fundamental understanding of the growth and formation mechanisms of Ag nanocrystals, which interact with the electron beam, is essential to improve the nanocrystal shape-control mechanisms as well as the future design and study of nanomaterials.

Photoluminescence and Photocatalytic Properties of Ag3 PO4 Microcrystals: An Experimental and Theoretical Investigation.

The structural, morphological, and optical properties of Ag3 PO4 microcrystals were systematically characterized by using a combination of theoretical calculations and experimental techniques. These microcrystals were synthesized by the microwave-assisted hydrothermal (MAH) method. XRD, Rietveld refinements, and FTIR spectroscopy were employed to carry out a structural analysis; the morphologies of the microcrystals were examined by FEG-SEM. First-principles computational studies were used to calculate the geometries of bulk Ag3 PO4 and its (010), (100), (001), (110), (101), (011), and (111) surfaces. A continuous decrease in the energy of the (100) surface led to a good agreement between the experimental and theoretical morphologies. Optical properties were investigated by UV/Vis spectroscopy and photoluminescence (PL) measurements, which revealed a maximum PL emission at λ=444 nm. The MAH-synthesized sample exhibited good activity for the photocatalytic degradation of methyl orange dye under visible irradiation. The photocatalytic activity and PL behavior were correlated with the observed morphology.

Synthesis, antifungal evaluation and optical properties of silver molybdate microcrystals in different solvents: a combined experimental and theoretical study.

In this study, we investigate the structure, antifungal activity, and optical properties of ß-Ag2MoO4 using experimental and theoretical approaches. ß-Ag2MoO4 samples were prepared by a co-precipitation method using different solvents (water, ethanol and ammonia), and their antifungal activity against Candida albicans was investigated. The samples were characterized by X-ray diffraction, micro-Raman spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy with energy dispersive spectroscopy. The optical properties were investigated by UV-Vis spectroscopy and photoluminescence measurements at room temperature. The thermodynamic equilibrium shape of the ß-Ag2MoO4 crystals was determined based on the surface energies calculated using Wulff construction. The (011) orientation was the predominant surface in the morphology. The experimental morphology was obtained by varying the surface energy ratio for each facet. A large decrease in surface energy for the (111) surface provided the experimental morphology for crystals synthesized using water and ethanol as solvents; when the surface energies for both (011) and (001) surfaces increased, the crystal morphology obtained using ammonia as a solvent was reproduced. A correlation between the exposed surfaces and antifungal activity was revealed, and an explanation to this behavior that arises from different morphologies and structural data was provided. Theoretical calculations confirm the rationality of the experimental scheme and elucidate the underlying reason for the fungistatic and fungicidal activity against Candida albicans.