Formation of Ag2S Cages from Polyhedral Ag2O Nanocrystals and their Electrochemical Properties

Growth of Size-Tunable Ag2O Polyhedra and Revelation of Their Bulk and Surface Lattices

By primarily adjusting the reagent amounts, particularly the volume of AgNO3 solution introduced, Ag2O cubes with decreasing sizes from 440 to 79 nm, octahedra from 714 to 106 nm, and rhombic dodecahedra from 644 to 168 nm are synthesized. 733 nm cuboctahedra are also prepared for structural analysis. With in-house X-ray diffraction (XRD) peak calibration, shape-related peak shifts are recognizable. Synchrotron XRD measurements at 100 K reveal the presence of bulk and surface layer lattices. Bulk cell constants also deviate slightly. They show a negative thermal expansion behavior with shrinking cell constants at higher temperatures. The Ag2O crystals exhibit size- and facet-dependent optical properties. Bandgaps red-shift continuously with increasing particle sizes. Optical facet effect is also observable. Moreover, synchrotron XRD peaks of a mixture of Cu2O rhombicuboctahedra and edge- and corner-truncated cubes exposing all three crystal faces can be deconvoluted into three components with the bulk and the [111] microstrain phase as the major component. Interestingly, while the unheated Cu2O sample shows clear diffraction peak asymmetry, annealing the sample to 450 K yields nearly symmetric peaks even when returning the sample to room temperature, meaning even moderately high temperatures can permanently change the crystal lattice.

Nonstoichiometry, structure and properties of nanocrystalline oxides, carbides and sulfides

The results of recent experimental studies of the influence of the size of oxide, carbide and sulfide nanoparticles on changes in their nonstoichiometry are analyzed and generalized. In relation to intrinsically nonstoichiometric titanium oxides and niobium and vanadium carbides, on the one hand, and stoichiometric silver sulfides, on the other hand, it is shown that a decrease in the particle size to the nano scale has a strong impact on the chemical composition of nanocrystalline compounds, viz., it leads to the occurrence of or increase in nonstoichiometry. A conclusion is drawn that interplay between the particle size and nonstoichiometry is common to all solid nanocrystalline substances and should be taken into account in studies on the synthesis, structure and properties of these systems.

The bibliography includes 260 references.

Facile in situ synthesis of silver nanocomposites based on cellulosic paper for photocatalytic applications

In this work, various photocatalysts were synthesized with an impregnation-precipitation process to in situ decorate Ag-based nanoparticles (NPs, including Ag₃PO₄, AgCl, Ag₂O, and Ag₂CO₃) on the cellulosic paper. The structure and properties of the Ag-based composites were characterized by scanning electron microscopy, X-ray diffraction, transmitting electron microscopy, UV-vis diffuse reflectance spectra, and photocatalysis testing. The results showed that cellulosic paper is an efficient carrier which is feasible to grasp NPs due to the cellulosic nanofiber-network microstructure. Among the obtained samples, Ag₂CO₃ and AgCl NPs on cellulosic paper displayed high photocatalytic activity for the degradation of methyl orange under ultraviolet and visible light. However, photo lability of Ag₂CO₃ limits its recyclable. AgCl showed a better reutilization with the assistance of a surface plasmon resonance effect by Ag NPs that were grown in situ on the AgCl NPs, which formed Ag@AgCl nanocomposite structure. The photocatalytic activity of the AgCl/cellulosic paper decreased only slightly after three runs of photodegradation of methyl orange. The possible mechanism for photocatalysis was proposed. This work may provide a new method for the design of silver-based NPs/cellulosic paper nanocomposite photoreactors with favorable photocatalytic activities for industrial applications.

Dip-coating decoration of Ag2O nanoparticles on SnO2 nanowires for high-performance H2S gas sensors

SnO₂ nanowires (NWs) are used in gas sensors, but their response to highly toxic gas H₂S is low. Thus, their performance toward the effective detection of low-level H₂S in air should be improved for environmental-pollution control and monitoring. Herein, Ag₂O nanoparticle decorated SnO₂ NWs were prepared by a simple on-chip growth and subsequent dip-coating method. The amount of decorated Ag₂O nanoparticles on the surface of SnO₂ NWs was modified by changing the concentration of AgNO₃ solution and/or dipping times. Gas-sensing measurements were conducted at various working temperatures (200–400 °C) toward different H₂S concentrations ranging within 0.1–1 ppm. The selectivity of Ag₂O-decorated SnO₂ NW sensors for ammonia and hydrogen gases was tested. Results confirmed that the Ag₂O-decorated SnO₂ NW sensors had excellent response, selectivity, and reproducibility. The gas-sensing mechanism was interpreted under the light of energy-band bending by sulfurization, which converted the p–n junction into n–n, thereby significantly enhancing the sensing performance.

Ag₂O nanoparticles decorated on the surface of on-chip growth SnO₂ nanowires by a dip-coating method possessed excellent sensing performance for H₂S gas.

Formation of Silver Rhombic Dodecahedra, Octahedra, and Cubes through Pseudomorphic Conversion of Ag2O Crystals with Nitroarene Reduction Activity

Using ethanol as a co-solvent, relatively small-sized Ag₂O octahedra (∼645 nm in opposite corner distance) and rhombic dodecahedra (∼540 and 655 nm in opposite face distance) were synthesized in aqueous solutions. Ag₂O cubes synthesized in an aqueous solution have an edge length of ∼425 nm. Band gaps of these crystals have been obtained, revealing the presence of facet-dependent light absorption properties. The Ag₂O rhombic dodecahedra, octahedra, and cubes were treated with ammonia borane in ethanol at 50 °C for just 10 min to pseudomorphically convert to Ag polyhedra of the corresponding morphologies. Transmission electron microscopy characterization confirms that the Ag cubes, octahedra, and rhombic dodecahedra are bound by the {100}, {111}, and {110} faces, respectively. The Ag rhombic dodecahedra, available for the first time, showed more superior catalytic activity toward 4-nitroaniline reduction at 50 °C than Ag octahedra and cubes, and gave 100% product yield after 1 h of reaction. This work demonstrates the value of forming Ag rhombic dodecahedra, exposing {110} surfaces that may be useful in other organic transformations.

Facet-Dependent Optical Properties of Semiconductor Nanocrystals

Recent observations of facet-dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet-dependent surface plasmon resonance absorption of metal-Cu₂ O core-shell nanocrystals with tunable sizes and shapes is discussed. The Cu₂ O shells also exhibit a facet-specific optical absorption feature. The facet-dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu₂ O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu₂ O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu₂ O can be constructed to incorporate these optical effects. Literature also provides examples of facet-dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet-dependent. Some applications of semiconductor optical size and facet effects are considered.

Nanostructured silver sulfide: synthesis of various forms and their application

The results of experimental studies on nanostructured silver sulfide are analyzed and generalized. The influence of small particle size on nonstoichiometry of silver sulfide is discussed. Methods for the synthesis of various forms of nanostructured Ag2S including nanopowders, stable colloidal solutions, quantum dots, core – shell nanoparticles and heteronanostructures are described. The advantages and drawbacks of different synthetic procedures are analyzed. Main fields of application of nanostructured silver sulfide are considered.

The bibliography includes 184 references.

Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent Photocatalytic Activity Examination

Sub- to micrometer-sized Ag2O cubes, great rhombicuboctahedra, cuboctahedra, corner-truncated octahedra, octahedra, and rhombic dodecahedra have been synthesized at room temperature using simple molar ratios of NH4NO3, NaOH, and AgNO3 solutions with a short reaction time. In addition, tuning the concentration of NH3 in the solution can provide more particle morphologies including edge- and corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedra to enrich Ag2O shape diversity. X-ray photoelectron spectroscopy (XPS) spectra indicate surface composition of various crystals as pure Ag2O. Diffuse reflectance spectra have been used to determine the band gap of Ag2O cubes. Ag2O cubes, octahedra, and rhombic dodecahedra having the same total particle surface area were used for facet-dependent photocatalytic activity comparison in the degradation of methyl orange. Cubes are comparably highly active for this reaction, while octahedra and rhombic dodecahedra give moderate and low catalytic activities, respectively. Electron paramagnetic resonance (EPR) measurements confirm this reactivity order. Although all Ag2O samples show significant etching during photocatalysis, metallic silver is not produced.

Direct synthesis of size-tunable PbS nanocubes and octahedra and the pH effect on crystal shape control

Small PbS nanocrystals were synthesized by a one-pot procedure, but large octahedra were produced by changing the reagent introduction sequence.