Hexagonal Ag Research Articles

A comparative analysis of capping and reducing agents of microbial cell Aspergillus niger and Bacillus subtilis for biosynthesis of Ag doped ZnO nanoparticles

In this study, green synthesis of Ag doped ZnO (AZNPR) has been carried out using the sol-gel method at pH 8.0, using two types of microbial cells, namely Aspergillus niger and Bacillus subtilis as capping and reducing agents. This work centred on the phytomediated biosynthesis and characterisation of AZNPR and its biological utility. Based on infrared analysis, phytochemicals present in microbial cells are extracellular enzymes namely proteolytic, amylolytic, lipolytic, cellulolytic, and pectinase, which can be extracted from A. niger and B. subtilis and are responsible for controlling the growth of AZNPR. The formation of AZNPR was characterised using different analyses. The FTIR shows absorption band located at ∼864 cm−1 that shows the interaction between Ag and ZnO. The XRD results showed the presence of crystalline AZNPR in the forms of hexagonal wurtzite ZnO and Ag. Microstructural and size examination showed that AZNPR was in the form of nanorods with homogeneous distribution with a size of rods in A. niger media 137.0 nm, while in B. subtilis media 197.4 nm. The Ag–ZnO nanorods revealed the ability to inhibit Candida albicans species, a fungus causing infectious disease, with an inhibition zone measuring 2.1–3.2 mm at 24 h.

Preferential Crystallographic Orientation and Optical Properties in (Ag,Zn)‐Based Oxide Nanoparticle Thin Film Depending on Spark Voltage

The effect of spark voltage on nanoparticle (NP) formation, crystal structure, and optical properties of Zn and Ag‐containing metal oxides produced using the spark‐discharge (SD) technique was investigated. Quasispherical‐shaped nanocrystals were observed in ZnO, Ag:ZnO, and AgO NP samples prepared at different spark voltages. The NP sizes in ZnO and Ag:ZnO samples were close and decreased as the spark voltage increased. It was observed that when AgO was prepared with only low‐spark voltage (4.3 kV), tetrahedron/octahedron‐shaped NPs were also formed. 4.3 kV spark voltage increased the crystal growth rate in the (100)‐plane and 7 kV spark voltage also increased the crystal growth in the (002)‐plane in hexagonal wurtzite ZnO and Ag:ZnO samples. Therefore, the a‐axial preferred orientation of these samples at low‐spark voltage had switched to the c‐axial preferred orientation at high‐spark voltage. For AgO, the increase in spark voltage revealed the monoclinic structure more strongly. The film thicknesses of AgO samples were close to each other with both spark voltages. Although, it was determined that the AgO prepared with 7 kV was optically more transparent and its optical band gap was significantly widened. This case may be due to the more presence of dodecahedral‐shaped particles in the morphology of the AgO sample prepared. Therefore, the wider optical band gap of metal oxides decorated with such nanoparticles could indicate facet dependency. In addition, the film thickness and optical band gap of both ZnO and Ag:ZnO samples increased significantly depending on the increase of spark voltage. The optical band gap narrowed (redshifted) due to the effect of Ag ions entering the ZnO structure. As the spark voltage increased, the difference between the optical band gaps of ZnO and Ag:ZnO also increased. With the spark‐voltage effect, the change in preferential crystallographic orientations of ZnO and Ag:ZnO samples and the change in optical band gap depending on the NP shape of the AgO sample can offer significant advantages in terms of optoelectronics.

ZnO/Ag multilayer for enhancing the catalytic activity against 4-nitrophenol

A multilayer thin film from zinc oxide and silver (ZnO/Ag multilayer thin film) has been synthesized via the technique of pulsed infrared laser deposition at 600 °C to be used as a portable catalyst for the degradation of 4-nitrophenol. The multilayer thin film was formed by two steps; the first one was making a thin film with a 300 nm thickness, followed by a 100 nm thin film from Ag, which was characterized by different characterization techniques. The XRD data demonstrated the presence of hexagonal Ag on ZnO in the formation of ZnO thin films and ZnO/Ag thin films. Also, it showed that crystallite size is decreasing as the Ag concentration rises because of the difference in atomic radius between Zn and Ag atoms in the crystal structure formation of ZnO coated with Ag. The crystal quality of ZnO and Ag was evaluated using photoluminescence (PL). The optical investigation showed the decrease in transmittance after coating ZnO film with Ag was due to ZnO films including more voids than ZnO/Ag films. From SEM images, the morphology of ZnO films was modified by the appearance of brilliant spots, which was related to the coating of the Ag layer as confirmed by elemental analysis. Then, based on the findings of catalytic experiments against 4-nitrophenol, the ZnO/Ag multilayer thin film shows an exceptional potential enhancement in compared to that of ZnO thin film.

Hexagonal sheet Ag and Ag nanoparticle inside 3D-TiO2 NTAs decorated Ti gauze by electrodeposition

Noble metal nanoparticles have attracted extensive attention by many researchers because of their optical, electrical and magnetic advantages. Half hexagon sheet Ag and Ag nanoparticles inside 3D-TiO2 nanotube arrays (NTAs) on Ti gauze was firstly synthesized by electrodeposition. The addition has a great influence on the morphology of Ag. When Ag hexagonal side length reached a maximum of 2 μm for 20 pluses, hexagonal Ag would transform oval Sphere with depositional pulses continuing increases, Ag nanoparticle inside 3D-TiO2 NTAs also were found, and the Ag modified NTAs for 20 pulses with the best photo-catalytic activity degraded methyl orange (MO) under solar light irradiation. Due to 3D-TiO2 NTAs supporting Ag catalyst, which is a benefit for recycling after the catalyst. This work is expected to provide a fundamental guide for the design of novel morphology catalysts on Ti gauze, which is conducive to recycling.

Solvent‐Controlled Condensation of [Mo2O5(PTC4A)2]6− Metalloligand in Stepwise Assembly of Hexagonal and Rectangular Ag18 Nanoclusters

AbstractStepwise assembly starting from a preassembled metalloligand is a promising approach to obtain otherwise unattainable silver nanoclusters, but hard to be intrinsically identified due to the lack of convincing evidence to justify such a process. Herein, hexagonal and rectangular Ag18 nanoclusters are constructed from the [Mo2O5(PTC4A)2]6− (H4PTC4A=p‐phenyl‐thiacalix[4]arene) metalloligand through stepwise assembly. The formation of the metalloligand is confirmed by electrospray ionization mass spectrometry, then assembled with silver ions to form two geometrically different Ag18 nanoclusters in different solvents. The cyclization from the metalloligand to [(Mo2O5PTC4A)6]12− can be realized without alcohols and otherwise blocked by them. The installation of this metalloligand not only provides comprehensive understanding of how the solvents regulate the silver nanocluster structures, but also brings new insights for the controllable ligand metallization and subsequent condensation.

Optimized colloidal growth of hexagonal close-packed Ag microparticles and their stability under catalytic conditions

The colloidal synthesis of hcp silver microparticles is optimized by tuning the chemical reduction kinetics and the surface stabilization during synthesis.

Development of Ag and Ag alloys-precipitated Ag2O-TeO2 glass and Ag2O-TeO2 glass/stainless steel reference electrodes for pH sensors

An Ag/AgCl reference electrode that is built into commercially available pH electrodes leaks its internal KCl solution via a liquid junction. This leakage is disadvantageous because the test solution is contaminated by a trace amount of the KCl solution. The applicability of Ag2O-TeO2 glasses to reference electrodes for pH sensors was investigated in this study. The effects of the Ag2O content and heat treatment on the pH sensitivity of these glasses and the pH sensitivity of the Ag2O-TeO2 glass/stainless steel electrodes were examined. The pH sensitivity of Ag2O-TeO2 glasses increased as the Ag2O content increased, but it was less than 40 % and was the lowest among the glasses that have been investigated to date. The heat-treated 25Ag2O･75TeO2 glass and 25Ag2O･75TeO2 glass/stainless steel electrodes precipitated cubic Ag and hexagonal Ag and showed relatively low pH sensitivities of 26 % and 5 %, respectively. Thus, the hexagonal Ag and Ag alloys-precipitated samples were considered candidate reference electrodes for pH sensors.

Синтез и исследование структуры и свойств фотоактивных ZnO-SnO_2-Ag(AgCl) наноматериалов для медицины и экологических приложений

Photoactive ZnO-SnO2-Ag(AgCl) nanomaterials have been prepared by polymer-salt method. Prepared materials able to generate singlet oxygen under UV and blue light irradiation. Materials structure and properties have been studied by optical and luminescence spectroscopy, XRD and SEM analysis. It was found that the structure of ZnO-SnO2-Ag(AgCl) materials consists of hexagonal ZnO, tetragonal SnO2, Ag and AgCl crystals having size 50÷60 nm. Obtained materials able to generate chemically active singlet oxygen under UV irradiation and have high bactericidal properties against both gram-positive and gram-negative bacteria. The increase of Ag content enhances the bactericidal properties of prepared materials.

Enhanced LSPR performance of graphene nanoribbons-silver nanoparticles hybrid as a colorimetric sensor for sequential detection of dopamine and glutathione

In the present study, a novel plasmonic sensing platform was proposed for sequential colorimetric detection of dopamine (DA) and glutathione (GSH) in human serum sample by taking advantage of plasmon hybridization in graphene nanoribbons/sliver nanoparticles (GNR/Ag NPs) hybrid. DA was detected based on etching strategy and morphology transition of label-free Ag NPs hybridized with GNR. As a result of the etching process, hexagonal Ag NPs were changed to smaller corner-truncated nanoparticles and a blue shift was observed in its plasmonic band, accompanied by the color change from green to red. Sequentially, GSH induced aggregation of Ag NPs which resulted in a decrease in absorption intensity of Ag NPs plasmonic band and a color change from red to gray. By employing GNR/Ag NPs hybrid as a sensitive colorimetric sensor, DA and GSH were successfully detected in low concentrations of 0.04 μM and 0.23 μM, respectively. The same experiment was carried out in the absence of GNR and the detection limits were obtained 0.46 and 1.2 μM for DA and GSH, respectively. These results confirmed the effective role of GNR on the sensitivity improvement of GNR/Ag NPs hybrid. The proposed simple and sensitive sensing approach offered a beneficial and promising platform for sequential detection of DA and GSH in the biological samples.

Ag–Ag2S hybrid nanoplates with unique heterostructures: Facile synthesis and photocatalytic application

Ag–Ag2S hybrid nanostructures are of particular interest because of their novel properties and applications; however, the development of a facile method to synthesize Ag–Ag2S hybrid nanostructures with well-defined heterostructures has remained a great challenge. In the current work, a simple and rapid synthetic approach based on the wet-chemical sulfidation of silver (Ag) nanocrystals was developed to prepare Ag–Ag2S hybrid nanoplates with new heterostructures. The key to achieving the unusual hybrid nanostructures was the use of hexagonal Ag nanoplates with anisotropic shapes, and hence spatially varying chemical reactivity, as seed materials. By adjusting the experimental parameters during the sulfidation reaction, Ag–Ag2S hybrid nanoplates with various compositions and morphologies could be obtained, and the localized surface plasmon resonance properties of the hybrid nanoplates could be tuned. The as-prepared Ag–Ag2S hybrid nanoplates were used as photocatalysts for the decomposition of methylene blue under solar irradiation, and their photocatalytic activities and stabilities were investigated. Finally, a facile chemical treatment strategy involving a galvanic replacement reaction was also developed to enhance the photocatalytic activity of the hybrid nanostructures.

Synthesis and in-situ noble metal modification of WO3·0.33H2O nanorods from a tungsten-containing mineral for enhancing NH3 sensing performance

Ag- and Pt-doped WO3·0.33H2O nanorods with high response and selectivity to NH3 were synthesized from a tungsten-containing mineral of scheelite concentrate by a simple combined process, namely by a high pressure leaching method to obtain tungstate ions-containing leaching solution and followed by a hydrothermal method to prepare corresponding nanorods. The microstructure and NH3 sensing performance of the final products were investigated systematically. The microstructure characterization showed that the as-prepared WO3·0.33H2O nanorods had a hexagonal crystal structure, and Ag and Pt nanoparticles were uniformly distributed in the WO3·0.33H2O nanorods. Gas sensing measurements indicated that Ag and Pt nanoparticles not only could obviously enhance NH3 sensing properties in terms of response, selectivity as well as response/recovery time, but also could reduce the optimal operating temperature at which the highest response was achieved. The highest responses of 22.4 and 47.6 for Ag- and Pt-doped WO3·0.33H2O nanorods to 1000 ppm NH3 were obtained at 225 and 175°C, respectively, which were about four and eight folds higher than that of pure one at 250°C. The superior NH3 sensing properties are mainly ascribed to the catalytic activities of noble metals and the different work functions between noble metals and WO3·0.33H2O.

Synthesis of uniform hexagonal Ag nanoprisms with controlled thickness and tunable surface plasmon bands

In this work, we synthesized monodispersed hexagonal Ag nanoprisms in high yields in a system of poly(vinylpyrrolidone) (PVP) in N-methylpyrrolidone (NMP). A blue shift occurred and was strongly dependent on the thickness of the uniform Ag nanoprisms, which had almost the same radial area. When the Ag nanoprisms grew thicker, their in-plane dipole resonance peaks markedly shifted toward shorter wavelengths (i.e., blue shift). PVP played a critical role of favoring vertical growth of the Ag nanoplates, preventing aggregation, and inducing the formation of Ag hexagonal nanoprisms (HNPs) through the transformation from thin Ag triangular nanoprisms (TNPs). Compared with similar previous research, the present study provides quite uniform Ag hexagonal nanoplates, which makes the blue shift related more solely and distinctly to the thickness of the Ag nanoprisms. The findings of this work provide a new perspective toward understanding the unique optical characteristics of Ag HNPs with different aspect ratios.

Dipole Radiation-Induced Extraordinary Optical Transmission for Silver Nanorod-Covered Silver Nanohole Arrays

A new extraordinary optical transmission (EOT) mode is discovered when a hexagonal Ag nanohole array is covered by a tilted Ag nanorod array. The resonant wavelength of this EOT mode red-shifts with respect to the normal EOT mode predicted by plasmon-grating coupling theory or dynamic scattering theory and increases linearly with the nanorod length. The structures also show strong polarization dependence; i.e., when the electric field (E-field) direction of the incident light is perpendicular to the long axis of the nanorods, only normal EOT spectrum is visible, but when the E-field is parallel to nanorod axis, the new mode appears. Our finite-difference-time-domain calculations confirm this finding and show that this new mode is due to the dipole radiation of the nanorods on top of the nanoholes. It is expected that other complicated unit cells of nanohole arrays could generate other new EOT modes and can be used to design new optical devices.

An ab initio study for the formation of two-dimensional III-nitride compound ultrathin films: Effects of Ag(1 1 1) substrate

The formation of two-dimensional (2D) ultrathin films in III-nitride compound on Ag(1 1 1) substrate is investigated using density functional calculations. The calculations demonstrate that there is faint charge distribution between III-nitrides with 2D hexagonal structure and Ag(1 1 1) substrate while metallic bonds are formed at the interface for III-nitrides with wurtzite structure. This indicates that the lattice constraint of Ag(1 1 1) substrate is negligible for III-nitride compound ultrathin films with hexagonal structure. Furthermore, the hexagonal structure is more stable than wurtzite phase for the films up to 15 monolayers due to interlayer electrostatic interactions. The results suggests that various 2D III-nitride compound ultrathin films can be formed on Ag(1 1 1) substrate.

Site-selective growth of Ag nanoparticles controlled by localized surface plasmon resonance of nanobowl arrays.

Hexagonal Ag nanoparticle arrays are exclusively grown on top of the interstices of Au nanobowl arrays. The photoinduced effect of the enhanced electromagnetic field between Au nanobowls accelerated the chemical reaction and is responsible for Ag growth in defined local positions. The enhanced electric field of the Au nanobowl array induced a photoreaction, which resulted in Ag growth in the hot area. Interestingly, the sizes and positions of the Ag nanoparticles distributed in the strong electric field of the Au nanobowl array are easily controlled. A six-axis symmetric pattern of Ag nanoparticle growth is realized based on the use of vertically incident circularly polarized light. Furthermore, a three-axis symmetric nanoperiodic structure is obtained through the use of linearly polarized oblique waves with specific incidence angles. This research shows that an electric field can be used to control a chemical reaction at the nanometer level, enabling the control and design of a wide variety of nanoperiodic structures.

Construction of Long Narrow Gaps in Ag Nanoplates.

Hexagonal Ag nanoplates with long and ultranarrow gaps (about 90 nm in length, 2 nm in width) are synthesized via seed-mediated growth method. By growing around the polymer shell on the seed, the Ag domain cannot merge at the meet-up point, leaving a long narrow gap in the resulting plate. These gapped nanoplates exhibit high sensitivity in SERS detection, with limitation of 10-9 M for 2-naphthalenethiol.

Tailoring the light absorption of Ag-PZT thin films by controlling the growth of hexagonal- and cubic-phase Ag nanoparticles

PbZr0.52Ti0.48O3 thin films containing hexagonal and cubic Ag nanoparticles (Ag NPs) of various sizes were prepared using the sol–gel technique. During the aging process, Ag ions were photo-reduced to form hexagonal Ag NPs. These NPs were uniform in size, and their uniformity was maintained in the thin films during the heat treatment process. Both the total volume and average size of the hexagonal Ag NPs increased with an increasing Ag ion concentration from 0.02 to 0.08 mol l−1. Meanwhile, the remaining Ag ions were reduced to form unstable Ag–Pb alloy particles with Pb ions during the early heating stage. During subsequent heat treatment, these alloys decomposed to form cubic Ag NPs in the thin films. The absorption range of the thin films, quantified as the full width at half maximum in the ultraviolet–visible absorption spectrum, expanded from 6.3 × 1013 Hz (390–425 nm) to 8.4 × 1013 Hz (383–429 nm) as the Ag NPs/PZT ratio increased from 0.2 to 0.8. This work provides an effective way to broaden the absorption range and enhance the optical properties of such films.

Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal Structures and Their Catalytic Applications.

Noble metal nanocrystals own high chemical stability, unique plasmonic and distinctive catalytic properties, making them outstanding in many applications. However, their practical applications are limited by their high cost and scarcity on the earth. One promising strategy to solve these problems is to boost their catalytic performance in order to reduce their usage amount. To realize this target, great research efforts have been devoted to the size-, composition-, shape- and/or architecture-controlled syntheses of noble metal nanocrystals during the past two decades. Impressively, recent experimental studies have revealed that the crystal structure of noble metal nanocrystals can also significantly affect their physicochemical properties, such as optical, magnetic, catalytic, mechanical, electrical and electronic properties. Therefore, besides the well-established size, composition, shape, and architecture control, the rise of crystal structure-controlled synthesis of noble metal nanocrystals will open up new opportunities to further improve their functional properties, and thus promote their potential applications in energy conversion, catalysis, biosensing, information storage, surface enhanced Raman scattering, waveguide, near-infrared photothermal therapy, controlled release, bioimaging, biomedicine, and so on. In this Account, we review the recent research progress on the crystal structure control of noble metal nanocrystals with a template synthetic approach and their crystal structure-dependent catalytic properties. We first describe the template synthetic methods, such as epitaxial growth and galvanic replacement reaction methods, in which a presynthesized noble metal nanocrystal with either new or common crystal structure is used as the template to direct the growth of unusual crystal structures of other noble metals. Significantly, the template synthetic strategy described here provides an efficient, simple and straightforward way to synthesize unusual crystal structures of noble metal nanocrystals, which might not be easily synthesized by commonly used chemical synthesis. To be specific, by using the epitaxial growth method, a series of noble metal nanocrystals with unusual crystal structures has been obtained, such as hexagonal close-packed Ag, 4H Ag, Pd, Pt, Ir, Rh, Os, and Ru, and face-centered cubic Ru nanostructures. Meanwhile, the galvanic replacement reaction method offers an efficient way to synthesize noble metal alloy nanocrystals with unusual crystal structures, such as 4H PdAg, PtAg, and PtPdAg nanostructures. We then briefly introduce the stability of noble metal nanocrystals with unusual crystal structures. After that, we demonstrate the catalytic applications of the resultant noble metal nanocrystals with unusual crystal structures toward different chemical reactions like hydrogen evolution reaction, hydrogen oxidation reaction and organic reactions. The relationship between crystal structures of noble metal nanocrystals and their catalytic performances is discussed. Finally, we summarize the whole paper, and address the current challenges and future opportunities for the template synthesis of noble metal nanocrystals with unusual crystal structures. We expect that this Account will promote the crystal structure-controlled synthesis of noble metal nanocrystals, which can provide a new way to further improve their advanced functional properties toward their practical applications.

Size-induced crossover from itinerant to localized magnetism observed for isolated Fe impurities embedded in different structural polymorphs of silver

Measurements of the local susceptibility and spin relaxation rate for isolated Fe atoms implanted in the nanocrystalline hexagonal phase of Ag (4H-Ag) show a large orbital moment, in stark contrast with the spin-dominated itinerant magnetism in conventional face centered cubic Ag. The orbital moment observed for Fe in hexagonal Ag $(\ensuremath{\approx}1\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B})$ is the largest among $d$-block hosts, and resembles $f$-electron magnetism, suggesting Hund's rule type local moment with strong spin-orbit coupling. Calculations based on density functional theory suggest that the crossover from itinerant to local moment behavior results from localization of Fe-$3d$ states due to size-induced changes in unit cell volume and symmetry.

Highly Active Carbon Supported Pd-Ag Nanofacets Catalysts for Hydrogen Production from HCOOH.

Hydrogen is regarded as a future sustainable and clean energy carrier. Formic acid is a safe and sustainable hydrogen storage medium with many advantages, including high hydrogen content, nontoxicity, and low cost. In this work, a series of highly active catalysts for hydrogen production from formic acid are successfully synthesized by controllably depositing Pd onto Ag nanoplates with different Ag nanofacets, such as Ag{111}, Ag{100}, and the nanofacet on hexagonal close packing Ag crystal (Ag{hcp}). Then, the Pd-Ag nanoplate catalysts are supported on Vulcan XC-72 carbon black to prevent the aggregation of the catalysts. The research reveals that the high activity is attributed to the formation of Pd-Ag alloy nanofacets, such as Pd-Ag{111}, Pd-Ag{100}, and Pd-Ag{hcp}. The activity order of these Pd-decorated Ag nanofacets is Pd-Ag{hcp} > Pd-Ag{111} > Pd-Ag{100}. Particularly, the activity of Pd-Ag{hcp} is up to an extremely high value, i.e., TOF{hcp} = 19 000 ± 1630 h(-1) at 90 °C (lower limit value), which is more than 800 times higher than our previous quasi-spherical Pd-Ag alloy nanocatalyst. The initial activity of Pd-Ag{hcp} even reaches (3.13 ± 0.19) × 10(6) h(-1) at 90 °C. This research not only presents highly active catalysts for hydrogen generation but also shows that the facet on the hcp Ag crystal can act as a potentially highly active catalyst.