Molecular adsorption on coinage metal subnanoclusters: A DFT+D3 investigation.

Gold and silver subnanoclusters with few atoms are prominent candidates for catalysis-related applications, primarily because of the large fraction of lower-coordinated atoms exposed and ready to interact with external chemical species. However, an in-depth energetic analysis is necessary to characterize the relevant terms within the molecular adsorption process that can frame the interactions within the Sabatier principle. Herein, we investigate the interaction between Agn and Aun subnanoclusters (clu, n = 2-7) and N2 , NO, CO, and O2 molecules, using scalar-relativistic density functional theory calculations within van der Waals D3 corrections. The onefold top site is preferred for all chemisorption cases, with a predominance of linear (≈180°) and bent (≈120°) molecular geometries. A larger magnitude of adsorption energy is correlated with smaller distances between molecules and clusters and with the weakening of the adsorbates bond strength represented by the increase of the equilibrium distances and decrease of molecular stretching frequencies. From the energetic decomposition, the interaction energy term was established as an excellent descriptor to classify subnanoclusters in the adsorption/desorption process concomitant with the Sabatier principle. The limiting cases: (i) weak molecular adsorption on the subnanoclusters, which may compromise the reaction activation, where an interaction energy magnitude close to 0 eV is observed (e.g., physisorption in N2 /Ag6 ); and (ii) strong molecular interactions with the subnanoclusters, given the interaction energy magnitude is larger than at least one of the individual fragment binding energies (e.g., strong chemisorption in CO/Au4 and NO/Au4 ), conferring a decrease in the desorption rate and an increase in the possible poisoning rate. However, the intermediate cases are promising by involving interaction energy magnitudes between zero and fragment binding energies. Following the molecular closed-shell (open-shell) electronic configuration, we find a predominant electrostatic (covalent) nature of the physical interactions for N2 ⋯clu and CO ⋯clu (O2 ⋯clu and NO⋯clu), except in the physisorption case (N2 /Ag6 ) where dispersive interaction is dominant. Our results clarify questions about the molecular adsorption on subnanoclusters as a relevant mechanistic step present in nanocatalytic reactions.

Quantitative evaluation of the surface stability and morphological changes of Cu2O particles.

Cu2O low-index surfaces periodic models have been simulated based on density functional theory. The calculated surfaces energies allowed estimating the morphology by means of the Wulff theorem as well as the investigation of possible paths of morphological changes. Therefore, systematic morphology diagrams and change paths according to the energy modulation in relation to the surfaces stabilizations were elaborated. The applicability of this strategy was exemplified by comparing the obtained results with experimental available data from the literature. The morphology diagrams with the quantitative energetic point of view can be used as a guide to support experimental works in order to understand the relation between surface interactions and crystal growth.

α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties.

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag2WO4. Theses α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag2-2xZnxWO4 models. The electronic properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were linked to distortion effects and oxygen vacancies (VOx) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were explained by means of distortional effects and oxygen vacancies (VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag+ cations was four.

Europium doped zinc sulfide: a correlation between experimental and theoretical calculations.

This paper presents the correlation among electronic and optical property effects induced by the addition of different concentrations of europium (Eu3+) in zinc sulfide (ZnS) by microwave-assisted solvothermal (MAS) method. A shift of the photoluminescence (PL) emission was observed with the increase of Eu3+. The periodic DFT calculations with the B3LYP hybrid functional were performed using the CRYSTAL computer code. The UV-vis spectra and theoretical results indicate a decrease in behavior of the energy gap as a function of dopant concentration. Therefore, new localized states are generated in the forbidden band gap region, the new states increase the probability of less energy transitions which may be responsible for a red shift in the PL bands spectrum.

Potentiated electron transference in α-Ag2WO4 microcrystals with Ag nanofilaments as microbial agent.

This study is a framework proposal for understanding the antimicrobacterial effect of both α-Ag2WO4 microcrystals (AWO) synthesized using a microwave hydrothermal (MH) method and α-Ag2WO4 microcrystals with Ag metallic nanofilaments (AWO:Ag) obtained by irradiation employing an electron beam to combat against planktonic cells of methicillin-resistant Staphylococcus aureus (MRSA). These samples were characterized by X-ray diffraction (XRD), FT-Raman spectroscopy, ultraviolet visible (UV-vis) measurements, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM). The results reveal that both AWO and AWO:Ag solutions have bacteriostatic and bactericidal effects, but the irradiated sample is more efficient; i.e., a 4-fold of the MRSA planktonic cells as compared to the nonirradiated sample was observed. In addition, first principles calculations were performed to obtain structural and electronic properties of AWO and metallic Ag, which provides strong quantitative support for an antimicrobacterial mechanism based on the enhancement of electron transfer processes between α-Ag2WO4 and Ag nanoparticles.

Photocatalytic activity of semiconductor sulfide heterostructures.

This paper reports a theoretical and experimental study of the heterostructure photocatalytic activity in a CdS or ZnS and CdS@ZnS decorated system prepared by a microwave assisted solvothermal (MAS) method. A theoretical model of the decorated system was created in order to analyze the electronic transition mainly in their interface. The results show that CdS and ZnS interfaces produce an electron charge transfer from the CdS electron-populated clusters to the ZnS hole-populated clusters which helps to enhance the photocatalytic activity of the CdS@ZnS decorated system.