Selective Synthesis of Α‑, Β‑, and Γ‑Ag2wo4 Polymorphs
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pubs.acs.org/IC Article ‑ ‑ ‑ Selective Synthesis of α , β , and γ Ag2WO4 Polymorphs: Promising Platforms for Photocatalytic and Antibacterial Materials Roman Alvarez-Roca, Amanda Fernandes Gouveia, Camila Cristina de Foggi, Pablo Santana Lemos, Lourdes Gracia, Luís Fernando da Silva, Carlos Eduardo Vergani, Miguel San-Miguel, Elson Longo, and Juan Andreś * Cite This: Inorg. Chem. 2021, 60, 1062−1079 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Silver tungstate (Ag2WO4) shows structural poly- morphism with different crystalline phases, namely, orthorhombic, hexagonal, and cubic structures that are commonly known as α, β, and γ, respectively. In this work, these Ag2WO4 polymorphs were selectively and successfully synthesized through a simple precip- itation 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 spectros- copy, 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. ■ INTRODUCTION its control, polymorphism has often been considered to be a Polymorphism is the ability of a chemical system to adopt drawback in technological applications, limiting the develop- Downloaded via Roman Alvarez Roca on January 26, 2021 at 13:48:46 (UTC). different crystallized structures with identical chemical ment of various multifunctional materials. Therefore, the understanding of the relationship between the crystal structure composition but different stabilities. As the crystal structure and its functional properties is essential to the rational design determines the properties of a material, polymorphism can 24 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. of the synthesis, property, and function of polymorphism. have a direct impact on its functional properties and thus on its Recently, Gentili et al.10 published a review highlighting technological applications. Different polymorphs can have representative progress in the control and applications of dramatically different chemical, physical, and biological polymorphism as an additional functionality of materials for properties. Often these changes in their crystal structure can technological applications. lead to novel properties and the opening up of potential Solid-state synthesis methods typically rely on equilibrium applications in several research areas, such as photocatalysts, − routes in which the most stable polymorph is obtained, sensing gases, and ion conductors among others.1 10 whereas the metastable solid polymorphs tend to form via Polymorphs of binary or complex metal oxides are well nonequilibrium or metastable states,25,24,26 which may be known in several oxides, namely, TiO2, CaCO3, ZrO2, and 11−16 attained upon quenching an intermediate of the reaction or by WO3 as well as zeolites and SiO2. These polymorphs have different stabilities and may transform, under particular conditions, from an unstable form (metastable phase) to a Received: October 27, 2020 more stable polymorph. Metastable polymorphs are relevant Published: December 29, 2020 for science and technology, offering more promising new design opportunities (both experimental and theoretical) to obtain superior properties for some applications other than − their corresponding stable phase.17 23 Because of difficulties in © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.inorgchem.0c03186 1062 Inorg. Chem. 2021, 60, 1062−1079 Inorganic Chemistry pubs.acs.org/IC Article different synthesis methods, such as the gas-phase, hydro- common precursors thanks to careful control of the synthesis thermal, mechanochemical, or template synthesis.27 The conditions, which revealed an alternative way to control ’ polymorph-controlled synthesis of inorganic compounds, Ag2WO4 s polymorphism. Establishing the atomic-scale though crucially important, still remains a great challenge, structure of the surfaces is a key step to modeling this kind offering a remarkable opportunity for discovering multifunc- of material. Guided by the density functional theory (DFT) tional materials with unusual properties.28 While it is well findings, we will disclose the delicate morphology and defect established that the kinetics required to form different control of its antibacterial and photocatalytic mechanisms to polymorphs from a specific precursor can often be manipulated comprehend, predict, and explain how these emerge. Finally, a by changing their synthesis conditions to obtain a metastable number of insights into the role of the local coordination of the phase, the relative stability of kinetically accessible phases often Ag and W cations in the three polymorphs, together with the falls within a fairly narrow range of energies. However, a crystallization process as a function of solution chemistry, will fundamental understanding of which metastable phases can be be provided, allowing us to propose a mechanism for the synthesized and whether synthesizability is related to the nucleation and growth processes by using the concept of excess enthalpy of a metastable phase above its thermodynamic clusters, namely [AgOx] and [WOy], as building blocks of the ground state is still lacking.19,21 Answering this would enable a different α-, β-, and γ-phases. more rational approach in order to design and synthesize The structures and electronic properties of the as- inorganic metastable phases and desirably assist in their synthesized samples were studied through X-ray diffraction experimental realization.29,30 (XRD) with Rietveld refinements, X-ray photoelectron spec- Silver tungstate (Ag2WO4) has received considerable troscopy (XPS), and X-ray absorption near-edge spectroscopy attention in recent years due to its attractive characteristics, (XANES). Their optical properties were studied by using PL applicable over a wide range of practical scenarios. Ag2WO4 is emission. Field-emission scanning electron microscopy (FE- a wide-band-gap multifunctional semiconductor material that SEM) images were employed to analyze their morphological presents structural polymorphism and can crystallize in three evolution. The performance of the as-synthesized polymorphs different structures. Among them, the α phase is, thermody- as antibacterial agents against methicillin-resistant Staph- − namically, the most stable one.31 33 In addition to the α- ylococcus aureus (MRSA) and their photocatalytic degradation polymorph, with orthorhombic structure and space group of amiloride (AML) under ultraviolet (UV) light irradiation (Pn2n), the β- and γ-metastable phases, with hexagonal were also studied. First-principles calculations, based on DFT, structure and space group (P63/m) and cubic structure and were performed to calculate the relative energy between space group (Fd3̅m), respectively, have also been described. α- different phases, their electronic structures, and their surface 34 Ag2WO4 presents unique structural properties with a wide energies. We applied a joint experimental and theoretical range of electrical and optical properties that can offer strategy, which we developed ourselves, to rationalize the widespread applications including photocatalysis, gas sensors, experimental morphologies of the α-, β-, and γ-Ag2WO4 antibacterial and antifungal agents, and photoluminescence polymorphs. − (PL) emission, among others.32,34 49 The article is organized as follows. In Section 2, the Ag WO polymorphs have been synthesized by precipita- experimental methods are presented. This section comprises 2 4− tion,31,37,50 53 a conventional hydrothermal method,37,54 a four subsections: the synthesis procedure, the characterization − microwave-assisted hydrothermal method,34,37,38,55 57 a sono- techniques, the antibacterial and photocatalytic activities, and chemical method,37,41,58,59 and simple ion exchange.60 the computational methods and model systems. Section 3 Recently, controlled precipitation was reported in the presents the results and the discussion. The main conclusions 52 preparation of metastable β-Ag2WO4, where the addition of are summarized in the last section, Section 4. Eu ions was used as a stabilizing agent of the metastable β- phase. Similarly, through a facile precipitation process in the ■ EXPERIMENTAL SECTION presence of surfactants acting as a phase stabilizer, other Synthesis Procedure. The three different polymorphs of authors have also reported the obtainment