Molar Ratio Of Ag Research Articles

Comparative Performance of Citrate, Borohydride, Hydroxylamine and β-Cyclodextrin Silver Sols for Detecting Ibuprofen and Caffeine Pollutants by Means of Surface-Enhanced Raman Spectroscopy.

The detection of emerging contaminants in the aquatic environment, such as ibuprofen and caffeine, was studied by means of surface-enhanced Raman spectroscopy (SERS) using Ag nanoparticles (AgNPs) synthesized with β-cyclodextrin (βCD) as a reducing agent. The effect on the SERS signal of different molar ratios of Ag+/βCD in the synthesis route and the aging process of AgNPs were investigated by using trans-cinnamic as a test molecule. The SERS effectiveness of these β-cyclodextrin colloids (Ag@βCD) was also checked and compared with that of other silver sols usually employed in SERS synthesized by using other reducing agents such as citrate, borohydride and hydroxylamine. All the synthesized SERS substrates were characterized by different techniques. The experimental results indicate that Ag@βCD with the more diluted Ag+/βCD molar ratio showed the best SERS signal, enabling detection at trace concentrations of 0.5 µM in the case of trans-cinnamic acid. The Ag@βCD sols also showed the best sensitivity for detecting ibuprofen and caffeine, reaching the lowest limit of detection (0.1 mM). The proposed synthetic route for Ag@βCD sols provides an improved SERS substrate for detecting organic pollutants with better performance than other standard silver sols.

Effect of composition and calcination on the enzymeless glucose detection of Cu-Ag bimetallic nanocomposites

Development of novel materials that are capable of sensing blood glucose in absence of enzymes could be highly beneficial for affordable diabetic care. The present work discusses the effect of composition and heat treatment on the glucose sensing ability of bimetallic Cu-Ag nanocomposites. Polyol reduction method was employed to prepare nanocomposites with five different Cu:Ag molar ratios viz. 1:1, 2:1, 3:1, 1:2 and 1:3. The nanocomposites were thoroughly characterized by XRD, XPS, EDS, and TEM imaging. The nanocomposite having equimolar ratio of Cu and Ag was the best glucose sensing material among the prepared samples having a high sensitivity of 1340 μA mM−1 cm-2 with a detection limit of 0.6 μM at S/N = 3. The nanocomposite exhibited exhibited noticeable linear response for glucose concentration ranging from 0.01–30 mM, with R2 value of 0.9926. It also showed very high selectivity, stability and good reproducibility with a very low sensitivity loss of 4.1 % after 20 days. A good correlation was obtained with the pathology laboratory data for real blood samples. Moreover, a three-fold increase in sensitivity was observed on calcining the nanocomposite at 600 °C making this sample one of the best sensor materials having high sensitivity, low detection limit with vast linear response range.

Sulfonated hydroxyaluminum phthalocyanine-biogenic Au/Ag alloy nanoparticles mixtures for effective photo-eradication of Candida albicans.

In response to the increasingly widespread resistance of fungi to traditional treatment, we have reported successful photodynamic inactivation of Candida albicans planktonic cells using di-(AlPcS2) and trisulfonated (AlPcS3) hydroxyaluminum phthalocyanines in combination with Au/Ag alloy nanoparticles synthesized by the cell-free filtrate of Trichoderma koningii. These nanostructures with Au:Ag molar ratios 2:1, 1:1 and 1:2 have individual plasmonic band at 513-515 nm, 505-509 nm and 486-489 nm, respectively. XPS analysis of the ratio of gold to silver on the surface of these alloys indicated that Au and Ag formed a bimetallic system, wherein Au was coated with Ag. The XRD pattern revealed the angles at 38.2, 44.5, 64.9 and 78.0°. TEM analysis indicated that the average diameter of the synthesized alloys was 9 ± 3 nm, 8 ± 3 nm and 16 ± 3 nm for structures with Au:Ag molar ratios 1:1, 1:2 and 2:1, respectively. The FTIR band absorption, SEM-EDS analysis and basic elemental composition obtained by XPS confirmed that these nanostructures are stabilized by protein(s). Diode laser with the peak-power wavelength ʎ = 650 nm (output power of 40 mW; power density of 105 mW cm-2) was used as a light source. The mixture of AlPcS2+Au/Ag-NPs (Au:Ag = 2:1) can be considered as an effective photosensitizer, because eradication of C. albicans, as required by the American Society of Microbiology (99.9 %), was achieved at a low dose of light of 31.5 J cm-2. It was postulated that this low dose of light applied to the photo-induced fungicidal effect may be painless for potential patients.

High roughness gold nanoparticles/silver nanowires composites: Fabrication, characterization and ultrasensitive SERS detection towards Rhodamine B

A novel centimeter-scale gold nanoparticles (AuNPs)/silver nanowires (AgNWs) composites surface-enhanced Raman scattering (SERS) substrates prepared by vacuum thermal evaporation and solid-state ionics method at 9 μA direct-current electric field (DCEF) with fast ionic conductor KAg4I5 films was reported. The surface morphologies, microstructures and chemical compositions of the AuNPs/AgNWs composites were characterized. The fractal dimensions of centimeter-scale AuNPs/AgNWs composites was calculated by grid covering method and the SERS performance was detected by Rhodamine B (RB) aqueous solution as probe molecules. The results show that the macroscopic dendritic AuNPs/AgNWs composites with the longest length of 5 cm were prepared, the diameters of AuNPs/AgNWs composites changed from 50 nm to 100 nm and many 20–50 nm AuNPs existed on the prepared nanocomposites, which lead to high nanoscale roughness AuNPs/AgNWs composites. The molar ratio of Ag:Au is 28.67:1 and the fractal dimensions of AuNPs/AgNWs composites was 1.67 because of more dentritic nanocomposites. The limiting concentration of RB for the prepared SERS substrates is 10−15 mol/L, which is far below the strictest safety limits required of RB in China. These results show that the prepared AuNPs/AgNWs composites with high nanoscale roughness, facile material synthesis, low detection concentration and simple detection procedure has strong application prospect in food safety.

Morphology, structure, and composition of sulfidized silver nanoparticles and their aggregation dynamics in river water

The sulfidized form represents an environmentally relevant transformation state of silver nanoparticles (Ag-NPs) released into natural systems via wastewater route. However, the detailed characterization of sulfidized silver nanoparticles (S-Ag-NPs) is missing and their colloidal stability in aquatic systems is only insufficiently studied. The aim of this study was to systematically evaluate the surface properties, morphology, structure, composition, as well as aggregation dynamics of S-Ag-NPs in synthetic and natural river water. The S-Ag-NPs were prepared by sulfidation of citrate-coated silver nanoparticles (Cit-Ag-NPs). The sulfidation of Ag-NPs was accompanied by the formation of fiber-like Ag2S nano-bridges, Ag0-Ag2S core-shell structures, and hollow regions. In contrast to the published literature, the nano-bridges were thinner (2–9 nm) and longer (up to 60 nm), they formed at higher S2−/Ag molar ratio (2.041), and the formation of the core-shell structures was observed even in the absence of natural organic matter (NOM). Furthermore, we observed selective sulfidation of nanoparticles which can induce the hot spots for the release of toxic Ag+ ions. The critical coagulation concentration (CCC) of Ca2+ determined for S-Ag-NPs in reconstituted river water was 2.47 ± 0.23 mmol/L and thus higher than the CCC obtained for Cit-Ag-NPs in our earlier study revealing higher colloidal stability of S-Ag-NPs. In natural river water, S-Ag-NPs were also colloidally more stable compared to the Cit-Ag-NPs. Furthermore, the stabilizing effect of NOM was much higher for S-Ag-NPs than for Cit-Ag-NPs. For S-Ag-NPs stabilized by a low amount of citrate, we expect longer residence times in the water phase of rivers and thus higher risk for aquatic organisms. In contrast to this, the pristine Cit-Ag-NPs are expected to be accumulated faster in the sediments representing higher risk for benthic organisms. This study contributes to better understanding of environmental fate and effects of Ag-NPs released via wastewater route.

Liquid-phase acetylene hydrogenation over Ag-modified Pd/Sibunit catalysts: Effect of Pd to Ag molar ratio

A series of bimetallic Pd–Ag catalysts supported on the carbon material Sibunit was synthesized for selective liquid-phase acetylene hydrogenation. Using XRD and EXAFS it was shown that palladium and silver in the 0.5% Pd–Ag/Sibunit can form bimetallic PdxAg(1−x) nanoparticles with the composition depending on the Pd:Ag ratio. The active component of Pd–Ag(1:0.25)/Sibunit, Pd–Ag(1:1)/Sibunit and Pd–Ag(1:4)/Sibunit is represented by the particles of Pd0.74Ag0.26, Pd0.60Ag0.40 and Pd0.41Ag0.59 nanoalloys, respectively. An increase of the quantity of silver decreases the catalyst activity. The XPS study revealed that this phenomenon is caused by a decrease in the surface concentration of palladium (geometrical effect) and an increase in the degree of palladium electronic modification with silver (electronic effect). The latter exerts a beneficial effect on the reaction selectivity: as the Pd:Ag molar ratio is raised to 1:2, selectivity to ethylene increases to 81% (X > 97%), which exceeds 2.7-fold the selectivity of Pd/Sibunit.

Iodide-doped precious metal nanoparticles: measuring oxidative stress in vivo via photoacoustic imaging.

Accumulation of reactive oxygen and nitrogen species (RONS) can induce cell damage and even cell death. RONS are short-lived species, which makes direct, precise, and real-time measurement difficult. Biologically-relevant RONS levels are in the nM-μM scale; hence, there is a need for highly sensitive RONS probes. We previously used hybrid gold-core silver-shell nanoparticles with mM sensitivity to H2O2. These particles reported the presence of RONS via spectral shifts which could easily be quantified via photoacoustic imaging. Here, we used halide doping to tune the electrochemical properties of these materials to better match the oxidation potential of RONS. This work describes the synthesis, characterization, and application of these AgI-coated gold nanorods (AgI/AuNR). The I : Ag molar ratio, pH, and initial Ag shell thickness were optimized for good RONS detection limits. Halide doping lowers the reduction potential of Ag from to resulting in a 1000-fold increase in H2O2 and 100 000-fold increase in ONOO- sensitivity. The AgI/AuNR system also etches 45-times faster than undoped Ag/AuNR. The AgI/AuNR easily reported the endogenously produced RONS in established cells lines as well as murine models.

Enhanced photocatalytic CO2 reduction activity of silvered titania prepared by wet deposition method

Titania (TiO2) was successfully modified with Ag nanoparticles by wet deposition method. The established structures adopt the anatase crystal structure and contain a mixture of Ag° and Ag1+ species. The Ag loading triggers extended absorption in the visible light region. Furthermore, the Ag loading improves the photocatalytic CO2 reduction efficiency compared to the pure TiO2. The optimal Ag loading is 2% molar ratio of Ag versus TiO2 and the photocatalytic activity will decrease with higher Ag loading. This improved efficiency may be attributed to the synergistic effect between electron trapping and Surface Plasmon Resonance (SPR) phenomenon of Ag nanoparticle on TiO2 surface.

Silver nanoparticles deposited on calcium hydrogenphosphate – silver phosphate matrix; biological activity of the composite

Abstract The composite containing nanosilver uniformly deposited on matrix composed of CaHPO4 x 2H2O (brushite, ca 89 mass %), CaHPO4 (monteonite, ca 9.5 mass%), and Ag3PO4 (0.5 mas%) was obtained by addition of calcium nitrate and silver nitrate aqueous solution at 30:1 Ca:Ag molar ratio into excess of (NH4)2PO4 solution at pH 5.0 – 5.5. The isolated solid was characterized by STEM, XRD, and LDI mass spectrometry. It has been found that nanosilver was uniformly distributed within composite as <10 nm diameter sized nanoparticles. Determination of silver by AAS showed that 60% of silver is present as Ag(0) nanoparticles, the present as matrix Ag3PO4 as identified by XRD method. The composite showed strong growth inhibition in E. coli and P. aeruginosa strains, and moderate towards S. aureus. The C. albicans cells were the most resistant to the tested material, although still composite was moderately cytostatic for the yeast.

AgI loading BiOI composites with enhanced photodegradation efficiency for bisphenol A under simulated solar light

Bismuth oxyiodide (BiOI) is a narrow band gap semiconductor which can be driven by visible irradiation. In order to efficiently separate photo-generated carriers and utilization of visible light, a facile solvothermal approach was used to synthesize a novel AgI loading BiOI 3D hierarchical composite (AgI-BiOI). The AgI-BiOI with Ag and Bi molar ratio of 1:8 (AgI-BiOI (1–8)) showed great enhancement for photocatalytic degradation of bisphenol A (BPA) with pseudo-first degradation rate constant about 3.7 or 14.5 times than that of pristine BiOI or AgI under simulated solar light. This synergistic enhancement for BPA degradation on AgI-BiOI(1–8) is mainly ascribed to enhancing the light absorption intensity and accelerating photo-generated carriers separation due to the formation of AgI-BiOI heterojunction. Free radical quenching experiments proved that positive holes (h+) and superoxide (O2•−) radicals were dominantly responsible for the degradation of BPA rather than singlet oxygen (1O2) or hydroxyl radicals (•OH). The AgI-BiOI(1–8) hardly showed any ecotoxicity to C. elegans through lethal experiments. The luminance bacteria acute toxicity of degradation intermediates of BPA increased before 30 min then reduced significantly with reaction. The good durability and environmental-friendly characteristics make AgI-BiOI(1–8) catalyst to be a good solar light-driven candidate.

Structural characteristics of a Ni–Ag magnetic catalyst and its properties in soybean oil hydrogenation

Ni–Agx/PVP-DB-171/SiO2/Fe3O4 magnetic nanoparticle (NP) catalysts were prepared using PVP-DB-171/SiO2/Fe3O4 magnetic NPs as a support. The prepared catalysts were characterized via X-ray powder diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy, vibrating sample magnetometer, inductively coupled plasma-optical emission spectroscopy and N2 adsorption–desorption isotherm. The results indicate that the dispersion of nickel on the PVP-DB-171/SiO2/Fe3O4NP support was improved by incorporating Ag as a promoter. The optimal Ni–Ag molar ratio was 0.15. Soybean oil hydrogenation was carried out with Raney nickel, Ni/PVP-DB-171/SiO2/Fe3O4 and Ni–Ag0.15/PVP-DB-171/SiO2/Fe3O4 catalysts. The results show that the Ni–Ag0.15/PVP-DB-171/SiO2/Fe3O4 magnetic NP catalyst has the highest activity and stability. The use efficiency of the catalyst decreased to 60% after 8 cycles. The content of trans fatty acids in hydrogenated soybean oil decreased to 10.4% after 90min of hydrogenation. Compared with filtration or centrifugation, the magnetic NP catalyst can be quickly and cost-efficiently separated by applying an external magnetic field.

Differential behaviors of silver nanoparticles and silver ions towards cysteine: Bioremediation and toxicity to Phanerochaete chrysosporium

Potential transformations of silver nanoparticles (AgNPs) upon interaction with naturally ubiquitous organic ligands in aquatic environments influence their transport, persistence, bioavailability, and subsequent toxicity to organisms. In this study, differential behaviors of AgNPs and silver ions (Ag+) towards cysteine (Cys), an amino acid representative of thiol ligands that easily coordinate to Ag+ and graft to nanoparticle surfaces, were investigated in the aspects of bioremediation and their toxicity to Phanerochaete chrysosporium. Total Ag removal, 2,4-dichlorophenol (2,4-DCP) degradation, extracellular protein secretion, and cellular viability were enhanced to some extent after supplement of various concentrations of cysteine under stress of AgNPs and Ag+. However, an obvious decrease in total Ag uptake was observed after 5–50 μM cysteine addition in the groups treated with 10 μM AgNPs and 1 μM Ag+, especially at a Cys:Ag molar ratio of 5. More stabilization in uptake pattern at this ratio was detected under Ag+ exposure than that under AgNP exposure. Furthermore, in the absence of cysteine, all Ag+ treatments stimulated the generation of reactive oxygen species (ROS) more significantly than high-dose AgNPs did. However, cysteine supply under AgNP/Ag+ stress aggravated ROS levels, albeit alleviated at 100 μM Ag+, indicating that the toxicity profiles of AgNPs and Ag+ to P. chrysosporium could be exacerbated or marginally mitigated by cysteine. The results obtained were possibly associated with the lability and bioavailability of AgNP/Ag+-cysteine complexes.

Polarization-dependent fluorescence of CdSe/ZnS quantum dots coupling to a single gold-silver alloy nanotube

The fluorescence intensities of CdSe/ZnS quantum dots on a silver nanowire and a gold-silver alloy nanotube with different molar ratios of gold and silver are investigated under excitation with different polarization angles. The ratio of the fluorescence intensity of perpendicular and parallel polarization excitations is measured to be 4.05 for a silver nanowire and decreases to 1.30, 1.10 and 1.07 for nanotube with Au:Ag molar ratios of 1:4.0, 1:2.5 and 1:1.5, respectively. The numerical simulation results show that the aforementioned experimental phenomena can be attributed to the variation in the distribution of the surface plasmon electromagnetic field under perpendicular polarized light excitation. The electromagnetic field is primarily distributed on the outer surface of the silver nanowire, while the field partially penetrates the interior of the silver nanotube and becomes completely concentrated at the interior of the gold nanotube. This research also found that the productivity and robustness of the chemical synthesis of the alloy nanotubes can be greatly improved by using a microwave reactor.

The Stepwise Reaction of Rhodium and Iridium Complexes of Formula [MCl2 (κ4 C,N,N',P-L)] with Silver Cations: A Case of trans-Influence and Chiral Self-Recognition.

Acetonitrile suspensions of the dichlorido complexes [MCl2 (κ4 C,N,N',P-L)] [M=Rh (1), Ir (2)] react with AgSbF6 in a 1:2 molar ratio affording the bis-acetonitrile complexes [M(κ4 C,N,N',P-L)(NCMe)2 ][SbF6 ]2 (3 and 4). The reaction takes place in a sequential manner and the intermediates can be isolated varying the M:Ag molar ratio. In a 2:1 molar ratio, it affords the dimetallic monochlorido-bridged compounds [{MCl(κ4 C,N,N',P-L)}2 (μ-Cl)][SbF6 ] (5 and 6). In a 1:1 molar ratio, the monosubstituted solvato-complexes [MCl(κ4 C,N,N',P-L)(Solv)][SbF6 ] (Solv=H2 O, MeCN, 7-10) were obtained. Finally, in a 2:3 molar ratio, it gives complexes 11 and 12 of formula [{M(κ4 C,N,N',P-L)(NCMe)(μ-Cl)}2 Ag][SbF6 ]3 in which a silver cation joints two cationic monosubstituted acetonitrile-complexes [MCl(κ4 C,N,N',P-L)(NCMe)]+ through the remaining chlorido ligands and two Ag⋅⋅⋅C interactions with one of the phenyl rings of each PPh2 group. In all the complexes, the aminic nitrogen and the central metal atom are stereogenic centers. In the trimetallic complexes 11 and 12, the silver atom is also a stereogenic center. The formation of the cation of the dimetallic complexes 5 and 6, as well as that of the trimetallic complexes 11 and 12, takes place with chiral molecular self-recognition. Experimental data and DFT calculations provide plausible explanations for the observed molecular recognition. The new complexes have been characterized by analytical, spectroscopic means and by X-ray diffraction methods.

Synthesis and characterization of Ca0.9Mg0.1TiO3:Pr3+,Ag+ phosphor

Ca0.9Mg0.1TiO3:Pr3+,Ag+ phosphors were synthesized by solid-state reaction technique. The crystalline phase and luminescence performances of Ca0.9Mg0.1TiO3:Pr3+,Ag+ were observed by X-ray powder diffractometer (XRD), transmission electron microscope (TEM), photoluminescence spectrometer and brightness meter, respectively. The addition of Ag+ can diminish in the crystal particle sizes of Ca0.9Mg0.1TiO3:Pr3+. Because Ag+ can reduce the concentration of the undesirable defects in the phosphor, luminescence intensity of Ca0.9Mg0.1TiO3:Pr3+,Ag+ is 2.3 times as high as that of Ca0.9Mg0.1TiO3:Pr3+ at the same preparation condition. The effect of Ag+ on the persistent afterglow properties is to deepen the energy storage traps and enhance the energy transfer efficiency from Ca0.9Mg0.1TiO3 to Pr3+. The persistent afterglow properties of Ca0.9Mg0.1TiO3:Pr3+,Ag+ are better than those of Ca0.9Mg0.1TiO3:Pr3+ at the same preparation condition. In conclusion, Ca0.9Mg0.1TiO3:Pr3+,Ag+ phosphor with molar ratio of Ag+ to Pr3+ 3:1 obtained at 900 °C for 4 h exhibits the optimal photoluminescence performances.

Highly efficient catalysts of Mn1 − xAgxCo2O4 spinel oxide for soot combustion

A series of Mn1−xAgxCo2O4 spinel-type catalysts that can promote soot removal were prepared by a sol-gel method along with the characterization of physical and chemical properties. The results show that the selected catalysts feature a spinel-type structure, and Ag phase for Ag-substituted Mn1−xAgxCo2O4 with Ag molar ratio higher than 0.1, and the introduced Ag+ can also tailor the distribution of oxygen species, as such, enhance the formation of the adsorbed oxygen species. Mn0.6Ag0.4Co2O4 demonstrates a superior activity with a T50 of 325°C, with a reduction of 122°C compared to MnCo2O4, and CO2 selectivity of 97%, which is strongly relevant to the substituted Ag+ ions.

Fabrication and surface enhanced Raman scattering effect of centimeter level AgCuAu composite nanowires

Centimeter level AgCuAu composite nanowires were prepared by a solid-state ionics method under a direct current electric field (DCEF) using fast ionic conductor RbAg4I5 films and vacuum thermal evaporation method. The surface morphology and chemical composition of the AuAgCu composite nanowires were characterized by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), respectively. Raman enhancement performance of the AgCuAu composite nanowires substrates was detected by Rhodamine 6G (R6G) aqueous solutions as probe molecules. Long-range order and short-range order AgCuAu composite nanowires with the length of 1 cm were prepared. The nanowires were bamboo-shaped with high surface roughness and the diameters of nanowires ranged from 60 to 100 nm. The molar ratio of Ag:Cu:Au in composite nanowires is 15:2:1. The intrinsic Raman peaks of 10−16 mol/L R6G at 612, 773, 1125, 1182, 1307, 1361, 1418, 1506, 1545, 1575, 1597, 1650 cm−1 are all present when AgCuAu composite nanowires were used as the SERS substrates.

One-pot Sonochemical Synthesis of Hg–Ag Alloy Microspheres from Liquid Mercury

Metallic mercury has always attracted much attention in various fields because of its unique characteristic of forming amalgams. Here, different phases of pure crystalline Hg–Ag amalgam microspheres are synthesized by ultrasonically reacting liquid mercury with an aqueous solution of silver nitrate. Sonicating different molar ratios of liquid metallic Hg with AgNO3 results in the formation of pure crystalline phases of solid silver amalgams with uniform morphology. The resulting Hg–Ag amalgams from various compositions after sonication are physically characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Differential Scanning Calorimetry (DSC). The XRD of the amalgams obtained from the molar ratios of Hg:Ag (1:1.5) and Hg:Ag (1.5:1 and 2:1) match the Schachnerite and Moschellandbergite phases, respectively, whereas the Hg–Ag amalgam prepared from a 1:1Hg:Ag molar ratio results in a mixture of the Schachnerite and Moschellandbergite phases. The obtained amalgam microspheres are between 6 and 10µm in size. The detailed thermal and chemical behaviour of the Ag–Hg systems is also investigated.

Enriching Silver Nanocrystals with a Second Noble Metal

Noble-metal nanocrystals have received considerable interests owing to their fascinating properties and promising applications in areas including plasmonics, catalysis, sensing, imaging, and medicine. As demonstrated by ample examples, the performance of nanocrystals in these and related applications can be augmented by switching from monometallic to bimetallic systems. The inclusion of a second metal can enhance the properties and greatly expand the application landscape by bringing in new capabilities. Seeded growth offers a powerful route to bimetallic nanocrystals. This approach is built upon the concept that preformed nanocrystals with uniform, well-controlled size, shape, and structure can serve as seeds to template and direct the deposition of metal atoms. Seeded growth is, however, limited by galvanic replacement when the deposited metal is less reactive than the seed. The involvement of galvanic replacement not only makes it difficult to control the outcome of seeded growth but also causes degradation to some properties. We have successfully addressed this issue by reducing the salt precursor(s) into atoms with essentially no galvanic replacement. In the absence of self-nucleation, the atoms are preferentially deposited onto the seeds to generate bimetallic nanocrystals with controlled structures. In this Account, we use Ag nanocubes as an example to demonstrate the fabrication of Ag@M and Ag@Ag-M (M = Au, Pd, or Pt) nanocubes with a core-frame or core-shell structure by controlling the deposition of M atoms. A typical synthesis involves the titration of Mn+ (a precursor to M) ions into an aqueous suspension containing Ag nanocubes, ascorbic acid, and poly(vinylpyrrolidone) under ambient conditions. In one approach, aqueous sodium hydroxide is introduced to increase the initial pH of the reaction system. At pH = 11.9, ascorbic acid is dominated by ascorbate monoanion, a much stronger reductant, to suppress the galvanic replacement between Mn+ and Ag. In this case, the M atoms derived from the reduction by ascorbate monoanion are sequentially deposited on the edges, corners, and side faces to generate Ag@M core-frame and then core-shell nanocubes. The other approach involves the use of ascorbic acid as a relatively weak reductant while Mn+ is cotitrated with Ag+ ions in the absence of sodium hydroxide. At pH = 3.2, when the molar ratio of Ag+ to Mn+ is sufficiently high, the added Ag+ ions can effectively push the galvanic reaction backward and thus inhibit it. As a result, coreduction of the two precursors by ascorbic acid produces Ag and M atoms for the generation of Ag@Ag-M core-frame nanocubes with increasingly thicker ridges. The Ag@Ag-Pd core-frame nanocubes can serve as a dual catalyst to promote the stepwise reduction of nitroaromatics to aminoaromatics and then oxidation to azo compounds. The consecutive reactions can be monitored using surface-enhanced Raman scattering (SERS). The Ag@Au core-shell nanocubes with Au shells of three or six atomic layers exhibit plasmonic peaks almost identical to those of the Ag nanocubes while the chemical stability and SERS activity are substantially augmented. For both types of bimetallic nanocubes, the Ag cores can be selectively removed to generate nanoframes and nanoboxes.

Insights into post-annealing and silver doping effects on the internal microstructure of ZnO nanoparticles through X-ray diffraction probe

ZnO nanoparticles were synthesized via Pechini method at various post-annealing temperatures (400°, 500°, and 600 °C) and silver doping concentrations (Zn:Ag molar ratios of 30, 20, and 10). Multifarious microstructural features including crystallite size, size-strain based broadening, residual stress, preferential orientation, crystallinity degree, lattice parameters, unit cell variation, and stacking fault probability were surveyed through phase analysis, Williamson-Hall plot, texture coefficient and unit cell calculations. X-ray probing verified good crystallinity with a hexagonal close pack Wurtzite morphology. Williamson-Hall analysis exhibited distributions of crystallite size and microstrain as well as their contributions on the line broadening of the host ZnO and guest Ag phases upon annealing-doping treatments. Textural analysis revealed the alteration in anisotropic crystallinity of the host phase and transformation of the preferred directions, (100) and (101), as function of annealing-doping processes. Besides, while guest Ag phase was shown to be polycrystalline with randomly orientated crystals at moderate concentration with respect to thermal treatment, preferential orientation went through a major change, (220) to (111), with increment in Ag loadings. Under identical synthetic conditions, the distinction in the lattice constants and unit cell variation between pure and doped ZnO nanoparticles was enforced and results verified major impressionability via annealing and doping factors.