Ag SiO2 Research Articles

Modified in situ and self-catalytic growth method for fabrication of Ag-coated nanocomposites with tailorable optical properties

Inorganic or organic solid supported metal nanostructures have been attracted intensive interests in recent years. In this study, a novel and versatile approach for the preparation of solid supported silver nanocomposites is presented for the first time. In this method, the modified aldehyde groups on solid supporters are employed to reduce Ag[(NH3)2]+ ions. The in situ reduced silver nucleus are directly coated on the surfaces of the supports and then used as both seeds and catalysts for the self-catalytic growth of Ag NPs. In this reaction, no additional reduction and protective agents are needed. Moreover, the size of the Ag NPs can be tuned by varying the concentration of the Ag[(NH3)2]+ ions and the reaction time, which leads to the interesting change of the localized surface plasmon resonance absorption of the NPs. Such a synthesis method may realize the separation of nucleation and growth stages for the formation of Ag NPs and is proved to be a versatile method for the preparation of solid supported Ag nanocomposites (PS–Ag, Fe3O4–Ag, and SiO2–Ag), which would benefit the preparations and further applications of functional materials in near future.

Silver containing sol–gel derived silica thin films: effect of aluminum incorporation on optical, microstructural and bactericidal properties

Silver containing silica (Ag–SiO2) thin films with and without aluminum (Al) were prepared on soda-lime-silica glass by spin coating of aqueous sols. The coating sol was formed through mixing tetraethyl orthosilicate [Si(OC2H5)4]/ethanol solution with aqueous silver nitrate (AgNO3) and aluminum nitrate nonahydrate [(AlNO3)3·9H2O] solutions. The deposited films were calcined in air at 100, 300 and 500 °C for 2 h and characterized using x-ray diffraction, UV-visible and x-ray photoelectron spectroscopy. The effect of Al incorporation and calcination treatment on microstructure and durability of the films, and chemical/physical state of silver in the silica thin film have been reported. The bactericidal activity of the films was also determined against Staphylococcus aureus via disk diffusion assay studies before and after chemical durability tests. The investigations revealed that the optical, bactericidal properties and chemical durability of Ag–SiO2 films can be improved by Al addition. The Al-modified Ag–SiO2 thin films do not exhibit any coloring after calcination in the range of 100–500 °C, illustrating that silver is incorporated within the silica gel network in ionic form (Ag+). Al incorporation also improved the overall durability and antibacterial endurance of Ag–SiO2 thin films.

The edges and terrace effect of Ag particles on optical resonance absorption property

Alternative Ag and SiO2 multilayers are prepared by using radio frequency magnetron sputtering. The Ag particles are found to diffuse toward and mostly accumulate near the surface of the Ag—SiO2 composite film via a rapid thermal treatment. Different shapes of the Ag particles are obtained by changing the thickness of each Ag and SiO2 layer. The response absorption property of the Ag composite film is also investigated. We relate the resonance absorption to the surface level and the Fermi level. To induce the obvious resonance absorption in an Ag composite film, it is necessary to maintain special shapes with sharp edges and wide terraces and to maintain the particle sizes ranging from 0 nm to 100 nm.

Surface plasmon-enhanced light-emitting diodes with silver nanoparticles and SiO2 nano-disks embedded in p-GaN

We report the fabrication and characterization of surface plasmon (SP)-enhanced blue light-emitting diodes (LEDs) with Ag nanoparticles and SiO2 nano-disks embedded in the p-GaN layer. Compared with LEDs without Ag nanoparticles, the optical output power increases for the SP-enhanced LEDs with and without SiO2 nano-disks are 72% and 49%, respectively. The greater increase in optical output power for the SP-enhanced LEDs with SiO2 nano-disks compared with SP-enhanced LEDs without SiO2 nano-disks can be attributed to the SiO2 nano-disks in the p-GaN layer, which reduce the Ag nanoparticle-induced defects and enhance the light extraction efficiency of the LEDs.

Surface/Interface Synergetic Effect-Induced Great Optical Spectral Change with Ag Content for Ag Nanoparticles Dispersed in Mesoporous SiO<sub>2</sub>

The evolution of optical absorption and photoluminescence (PL) spectra with different Ag content (from 0.14 wt % to 2.80 wt %) has been investigated for Ag/mesoporous SiO2 when heating in H2 and air. It has been found that, in accordance with alternating heat treatment of the sample with fairly low Ag content in H2 and air, the surface plasmon resonance (SPR) absorption of Ag nanoparticles alternately and reversibly appears and disappears, whereas its PL emission alternately disappears and appears. However, such reversible change can not happen in the sample with high Ag content. With gradual increase of Ag content, the two kinds of spectra take great and regular change. The results have firstly been discussed based on two contrary mechanisms coexisting in air at high temperature (i.e., oxidation reaction and thermal reduction reaction). In particular, the abnormal oxidation and the great optical change with Ag content is closely relative to surface/interface synergetic effect between Ag, O2 and SiO2 matrix despite SiO2 usually being regarded as a chemical inert medium to noble metals. The study would provide valuable reference for optical switch, gas sensor, nano optical modulation of SiO2-based optical devices, and give valuable information for surface/interface chemistry and catalysis in Ag nanoparticles and SiO2 assemble system.

Optical studies on sputter-deposited Ag–SiO 2 nanoparticle composites

Silicon dioxide with nano-size Ag (Au or Cu) particles embedded on its surface has unique optical properties due to the phenomenon known as surface plasmon resonance (SPR). SPR is the resonance of conduction band electrons which generate electromagnetic waves propagating along the dielectric/metal interface. In this study, Ag/SiO 2 films with different Ag layers (3/5/7/10 nm) were deposited on glass in sequence by magnetron sputtering. Various silver particle sizes were then created by the subsequent rapid thermal annealing (RTA). The RTA was carried out at elevated temperatures from 450 to 600 °C over different periods. All samples were examined by field-emission scanning electron microscope (FESEM) for their surface morphology. For optical properties, the films' transmittance and reflectance were measured with wavelengths ranging from 300 to 900 nm by a UV–VIS–NIR spectrophotometer. Experimental results show that for longer annealing time (15 min) and thicker deposited Ag film, larger Ag particles can be produced. These larger Ag particles on SiO 2 surface serve as the resonators of electrons to result in a broader absorption under the visible light irradiation. Following the experimental results, the Mie theory for a small particle in a dielectric medium can be used to estimate the spectrum of absorption. However, such estimate needs an equivalent dielectric constant of medium (SiO 2 + air) as input a priori. A newly proposed fitting scheme based on the Mie solution was tested. This scheme yielded a good fit to the experimental absorbance and simultaneously gave a reasonable estimate on the effective dielectric constant.

Plasmonic coupling of SiO2–Ag “post-cap” nanostructures and silver film for surface enhanced Raman scattering

We demonstrate a surface enhanced Raman scattering (SERS) substrate consisting of SiO2–Ag “post-cap” nanostructures with an underlying silver film fabricated by the glancing angle deposition technique. Electromagnetic simulations predict that SERS enhancement is strongly polarization-dependent, consistent with experimental measurements. Optimized coupling between Ag cap nanoparticles and the underlying silver film can be achieved by controlling the thickness of SiO2 post sandwiched between them to significantly enhance local electric-field intensity and to increase the density of electromagnetic hot spots. A maximum SERS enhancement factor of 2.38×109 within the hot spot region is demonstrated, providing sufficient sensitivity for many important applications.

Preparation of Ag@SiO2 Dispersion in Different Solvents and Investigation of its Optical Properties

In this article, Ag-SiO2 composite nanoparticles with a core-shell structure were prepared by a very simple and rapid wet chemical method in different solvents such as methanol, ethanol and isopropanol, respectively, transmittance electron microscopy (TEM) images show that regular SiO2-covered silver nanoparticles (briefly-marked as Ag@SiO2) can be obtained in ethanol and isopropanol, but not in methanol. The x-ray diffraction (XRD) measurement indicates that the silver nanoparticles are face-centered cubic (fcc) structure with high crystallinity. X-ray photoelectron spectroscopy (XPS) measurement shows that the binding energy of O1 s in Si-O-Ag is lower than that in Si-O-Si, which means that the complex bonding is established between Ag core and SiO2 shell. Ultraviolet-visible (UV-vis) spectroscopy records that the plasmon resonance peak of silver nanoparticles is shifted after the coverage of SiO2. As capping effect, the fluorescence intensity related to surface plasmon resonance (SPR) of Ag core is reduced by SiO2 shell.

Enhancement of photoluminescence in Er-doped Ag–SiO 2 nanocomposite thin films: A post annealing study

Silver–silica (with Ag 2.8 at.% and 8.7 at.%) nanocomposite (NC) thin films doped with Er +3 (0.1–0.9 at.%) were synthesized by atom beam co-sputtering using 1.5 keV Ar atoms. Optical absorption and photoluminescence (PL) studies of pristine and annealed films were performed, together with Rutherford backscattering and secondary ion mass spectroscopy studies for elemental characterization of the NC films. Optical absorption results of pristine and annealed NC film (with Ag ∼8.7 at.%) confirmed the formation of Ag nanoparticles evidenced by the appearance of characteristic surface plasmon resonance absorption features. Photoluminescence (PL) studies, carried out using Ar Laser pumping at 0.488 μm, the wavelength that the Er ions can absorb resonantly, indicated the presence of PL emission around 1.54 μm in the case of all the as-synthesized samples. The observed PL peak corresponds to the atomic transitions of Er as reported in literature. A relative enhancement in the intensity of PL peak has been observed after annealing the NC films. In the case of NC film with 0.9 at.% Er and 8.7 at.% Ag, the enhancement in PL intensity is almost twice, with respect to the as-deposited sample, for a heat treatment of about 1 h at 600 °C in a nitrogen atmosphere. However for a NC film with Er 0.1 at.% + Ag 2.8 at.%, the PL intensity is enhanced by approximately 3.7 times after annealing at 400 °C for 1 h in nitrogen atmosphere. Since the samples with surface plasmon resonance (SPR) did not show the PL enhancement, the role of SPR in the enhancement of the PL is ruled out. The enhanced PL emission from Er and Ag codoped silica indicates that the Er photo-stimulation is mediated by the energy transfer from Ag nanostructures or ions to Er. Lifetimes of PL peaks for the pristine and annealed samples were also studied. The observed lifetime ∼10 ms is a good indication of excellent PL efficiency.

Optoelectronic properties of sputter-deposited Ag–SiO 2 nanoparticle films by rapid thermal annealing

Silicon dioxide with embedded nano-size Ag particles has unique optical absorption due to the phenomenon known as surface plasmon resonance (SPR). SPR is the local resonance of conduction band electrons when excited by external optical fields. In this study, Ag/SiO 2 films with different Ag layers (3/5/7/10 nm) were deposited on glass sequentially by magnetron sputtering. The films then underwent the rapid thermal annealing (RTA) to create various sizes of silver particles. The RTA was carried out at elevated temperatures from 450 to 600 °C over different periods. All samples were examined by field-emission scanning electron microscope (FESEM) and surface profiler for their surface morphology. For optical properties, the films’ transmittance and reflectance were measured with wavelengths ranging from 300 to 900 nm by a UV–VIS–NIR spectrophotometer. Experimental results show that for longer annealing time (15 min) and thicker deposited Ag film, larger Ag particles can be produced. These larger Ag particles on SiO 2 surface serve as the resonators of electrons to result a broader absorption under the visible light irradiation. Following the experimental results, a newly proposed fitting scheme based on the Mie solution was tested and good fits to the experimental absorbance of different films were attained.

Ultrathin, ultrasmooth, and low-loss silver films via wetting and annealing

We have demonstrated that a thermal annealing treatment can reduce the optical losses in ultrathin, ultrasmooth, silver films deposited on a Ge wetting layer to values as low as the bulk material value and at the same time maintain an ultrasmooth surface. The annealing effect is sensitive to the annealing temperature and time, both of which should be carefully controlled. This annealing treatment is also effective for Ag–SiO2 multilayer composite films.

Polyhedral to nearly spherical morphology transformation of silver microcrystals grown from vapor phase

Silver microcrystals of various shapes were grown from co-evaporation of Ag 2O and SiO powders in the nominal evaporating temperature range (heat source temperature) from 800 to 1200 °C under a reductive atmosphere. Pathways for the polyhedral to nearly spherical morphology transformation were established. At evaporating temperatures below 900 °C, the polyhedral crystals are generally bounded by {1 1 1}-facets and {1 0 0}-facets. With increase in evaporation temperature facets of the {1 1 0}-, {2 1 0}- and {3 1 0}-families come forth successively, that make the Ag polyhedron microcrystals transform into nearly spherical microcrystals. A mechanism based on the recession of {1 0 0} or {1 1 0} steps induced by the solvent SiO x ( x∼2.0), which modifies the growth kinetics, is proposed to explain the emergence of {2 1 0}- and {3 1 0}-facets at high temperatures. By evaporating at 1200 °C, smooth-looking spherical Ag crystals with 6 nm multi-layered steps on the {1 1 1} facets were obtained. These results are also of significant industry relevance when preparation of noble metal particles with high-index facets is of concern.

Effective optical constants and effective optical properties of ultrathin trilayer structures

This work presents an extension of the characteristic effective medium approximation (CEMA) to ultrathin trilayer systems. The extension has been carried out analytically and is supported by corresponding calculations of the effective optical constants of Cu–Au–Cu and Ag–SiO–Ag trilayer systems using the CEMA approximation. This work is in essence a generalization of the characteristic effective medium approximation introduced earlier for ultrathin bilayer structures. This method is used to derive the effective optical constants of a trilayer system, consisting of three thin layers with each constituent layer of thickness much less than the wavelength of the incident radiation. Within this regime a trilayer system is viewed as one effective layer referred to as an effective stack (ES) with well defined effective optical constants, which can be used to calculate the optical properties of the trilayer stack within a specified wavelength range. The CEMA based calculations of the effective optical constants are applied to two trilayer systems with a total of five stacks. Three are Cu–Au–Cu and two are Ag–SiO–Ag stacks. The thicknesses of the parent layers in the Cu–Au–Cu stack range from 3 to 30 nm for Cu and 4 to 40 nm for Au; in the Ag–SiO–Ag stack the constituent layers are 6 nm for Ag, but range from 5 to 10 nm for SiO. This study is for normal or near normal incidence spectroscopy in a wavelength range that extends from visible to near infrared. The agreement between CEMA based ES stack results and those of the standard CMT technique is very satisfactory.

Trapping of BTX compounds by SiO 2, Ag–SiO 2, Cu–SiO 2, and Fe–SiO 2 porous substrates

Adsorption isotherms of BTX aromatic hydrocarbons (benzene, toluene, and p-xylene) on pristine (SiO 2) and metal-doped (Ag–SiO 2, Cu–SiO 2 and Fe–SiO 2) mesoporous and microporous substrates were measured and interpreted. These adsorbents were synthesized by the sol–gel procedure and their BTX sorption isotherms were obtained by the gas chromatographic technique (GC) at several temperatures in the range 423–593 K. The uptake amount of these hydrocarbon adsorptives on SiO 2, Ag–SiO 2, Cu–SiO 2 and Fe–SiO 2 mesoporous and microporous substrates was temperature-dependent. Additionally, the interaction of BTX molecules with the pore walls was evaluated by means of the corresponding isosteric heat of adsorption ( q st), which was found to follow the next increasing sequence: q st (benzene) < q st (toluene) < q st ( p-xylene). In general, the isosteric heat of adsorption of aromatic BTX compounds on microporous silica depicted an increasing tendency when the amount adsorbed was raised. This was a consequence of the existence of cohesive interactions (adsorbate–adsorbate) besides of the adhesive ones (adsorbate–adsorbent). The inclusion of silver or iron atoms within the SiO 2 structure leads to an increased adsorbed amount of BTX molecules on the solid surface if compared with the Cu–SiO 2 adsorbent. The adsorption of benzene, but not of toluene and p-xylene, molecules on pristine SiO 2 is facilitated by the pore size of this substrate since this is the highest of all materials.

One-pot synthesis of Ag@SiO2@Ag sandwich nanostructures

Ag@SiO2@Ag sandwich nanostructures were prepared by a facile one-pot synthesis method. The Ag core,SiO2 shell and Ag nanoparticle shell were all synthesized with polyvinylpyrrolidone, catalysed by ammonia,in the one-pot reaction. The polyvinylpyrrolidone, acting as a smart reducing agent, reduced theAg + to Ag cores and Ag shells separately. Furthermore, the polyvinylpyrrolidoneserved as a protective agent to prevent the silver cores from aggregating. TheSiO2 shell and outer layer Ag nanoparticles were obtained when tetraethyl orthosilicateand ammonia were added to the silver core solution. Ammonia, acting asthe catalyst, accelerated the hydrolysis of the tetraethyl orthosilicate toSiO2, which coated the silvercores. Furthermore, Ag(NH3)2 + ions were formed when aqueous ammonia was added to the solution, whichincreased the reduction capability. Then the polyvinylpyrrolidone reduced theAg(NH3)2 + ions to small Ag nanoparticles on the surface of theAg@SiO2 andformed Ag@SiO2@Ag sandwich structures with a standard deviation of less than 4%. This structure effectivelyprevented the Ag nanoparticles on the silica surface from aggregating. Furthermore, theAg@SiO2@Ag sandwich structures showed good catalysis properties due to the large surfacearea/volume value and activity of surface atoms of Ag particles.

Effect of ZnO on the performance of Ag/SiO 2 catalyst for the vapor-phase synthesis of 3-methylindole

The vapor-phase synthesis of 3-methylindole over Ag/SiO 2 doped with ZnO was investigated. The catalysts were characterized by XRD, H 2-TPR, NH 3-TPD and TG techniques. The results indicated that ZnO promoter greatly enhanced the initial activity of the catalyst but disfavored its stability. H 2-TPR and XRD results showed that the reduction peak of Ag 2O shifted to higher temperature and the intensity of silver diffraction peaks was much weaker after the addition of ZnO promoter to Ag/SiO 2. This indicated that there existed the interaction between Ag 2O and SiO 2-ZnO which promoted the silver particles dispersing on the support and inhibited the sintering of silver during the reaction. NH 3-TPD and TG results revealed that the acid amounts of the catalyst and coking increased after adding ZnO to Ag/SiO 2, which resulted in the deactivation of Ag/SiO 2-ZnO catalyst rapidly.

Smart and Reversible Surface Plasmon Resonance Responses to Various Atmospheres for Silver Nanoparticles Loaded in Mesoporous SiO2

The surface plasmon resonance (SPR) optical absorption of Ag nanoparticles loaded in mesoporous SiO2 (with fairly low Ag content, about 0.14 wt %) have been investigated after the nanoparticles were heated in or exposed to various atmospheres (i.e., air, H2, water vapor, Ar, and H2S). Unexpectedly, the SPR optical absorption can completely disappear in quite a short time or intensively emerge or drastically change into semiconductor optical absorption edge. Surprisingly, the SPR switches are well reversible and very rapid when alternating treatment of samples in any two atmospheres mentioned above. In addition, those novel and distinctive SPR changes reflect a series of abnormal chemical redox behaviors in various atmospheres. The mechanisms have been discussed based on modeling multi-interactions between the mesoporous SiO2 matrix, Ag nanoparticles, and atmospheres, revealing a special surface/interface synergetic effect that induces many novel phenomena despite SiO2 usually being regarded as a chemical ...

Modeling of two-dimensional nanoscale Y-bent plasmonic waveguides with cavities for demultiplexing of the telecommunication wavelengths

Numerical simulations, based on a finite-difference-time-domain (FDTD) method, of infrared light propagation for add/drop filtering in two-dimensional (2D) metal–insulator–metal (Ag–SiO2–Ag) resonators are reported to design 2D Y-bent plasmonic waveguides with possible applications in telecommunication wavelength demultiplexing (WDM). First, we study optical transmission and reflection of a nanoscale SiO2 waveguide coupled to a nanocavity of the same insulator located either inside or on the side of a linear waveguide sandwiched between Ag. According to the inside or outside positioning of the nanocavity with respect to the waveguide, the transmission spectrum displays peaks or dips, respectively, which occur at the same central frequency. A fundamental study of the possible cavity modes in the near-infrared frequency band is also given. These filtering properties are then exploited to propose a nanoscale demultiplexer based on a Y-shaped plasmonic waveguide for separation of two different wavelengths, in selection or rejection, from an input broadband signal around 1550 nm. We detail coupling of the 2D add/drop Y connector to two cavities inserted on each of its branches. Selection or rejection of a pair of different wavelengths depends on the inside or outside locations (respectively) of each cavity in the Y plasmonic device.

Selective liquid-phase oxidation of alcohols catalyzed by a silver-based catalyst promoted by the presence of ceria

A number of silver catalysts supported on SiO 2, Al 2O 3, Celite, CeO 2, kaolin, MgO, and activated carbon were screened for their catalytic activity in the selective liquid-phase oxidation of benzyl alcohol using a special screening approach. For this purpose 5–6 catalyst samples were mixed and tested simultaneously. When a high catalytic conversion (>30% over 2 h) was found the number of catalyst components was reduced in the following tests. Thereby, a collaborative effect between a physical mixture of ceria nanoparticles and silver-impregnated silica (10 wt.% Ag–SiO 2) was found. The catalytic activity was highly dependent on the silver loading, the amount of ceria, and especially the calcination procedure. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XANES, EXAFS) demonstrate that silver is mainly present as metallic particles. This is also supported by in situ XAS experiments. Oxygen species incorporated in the silver lattice appear to be important for the catalytic oxidation of the alcohol for which a preliminary mechanism is presented. The application of the catalyst was extended to the oxidation of a wide range of primary and secondary alcohols. Compared to palladium and gold catalysts, the new silver catalyst performed similarly or even superior in the presence of CeO 2. In addition, the presence of ceria increased the catalytic activity of all investigated catalysts.

Influence of the deposition conditions on the optical absorption of Ag–SiO 2 nanocermet thin films

We report a study of the influence of the deposition conditions on the morphological parameters (shapes, size distribution and ordering) of Ag nanoclusters embedded in SiO2, deposited by different techniques of co-sputtering or multilayer deposition. The influence of the substrate temperature on the nucleation and growth process is also studied. The different cluster morphologies determined by various complementary techniques (transmission electron microscopy, scanning electron microscopy, atomic force microscopy) are then correlated with the optical absorption, showing that the fabrication process is the key factor to control the position, shape and width of the surface plasmon resonance peaks and to achieve the desired optical properties. Films deposited by co-sputtering from 2 targets on a substrate heated at 440 °C show a very narrow absorption peak at 370 nm corresponding to the plasma resonance in near spherical silver particles. In multilayered films, the absorption peak is shifted from 340 to 480 nm with a larger half-band width. In cermet films co-sputtered from composite target on a rotating substrate, two absorption peaks are observed due to the more elongated shape of the metal nanoparticles. The experimental results are analysed in the frame of the Maxwell Garnett theory.