The synthesis of hollow Ag nanoshells (NSs) with tunable plasmon bands in the visible spectrum and their oxidative-assembly into high-surface-area, mesoporous, transparent, and opaque Ag gel frameworks is reported. Thiolate-coated Ag NSs with varying size and shell thickness were prepared by fast chemical reduction of preformed Ag2O nanoparticles (NPs). These NSs were assembled into monolithic Ag hydrogels via oxidative removal of the surface thiolates, followed by CO2 supercritical drying to produce metallic Ag aerogels. The gelation kinetics have been controlled by tuning the oxidant/thiolate molar ratio (X) that governs the rate of NP condensation, which in turn determines the morphology, optical transparency, opacity, surface area, and porosity of the resultant gel frameworks. The monolithic Ag hydrogels prepared using high concentration of oxidant (X > 7.7) leads to oxidative etching of precursor colloids into significantly smaller NPs (3.2-7.6 nm), which appeared to eliminate the visible light scattering yielding transparent gel materials. In contrast, the opaque Ag aerogels composed entirely of hollow NSs exhibit enormously high surface areas (45-160 m(2)/g), interconnected meso-to-macro-pore network that can be tuned by varying the inner cavity of Ag colloids, and accessibility of chemical species to both inner and outer surface of the hollows, offering perspectives for a number of new technologies. An advantage of current synthesis is the ability to transform Ag NSs into monolithic hydrogels within 4-12 h, which otherwise is reported to require weeks to months for the oxidation-induced metallic gel synthesis reported to date.

Ge1–x Sn x alloys are among a small class of benign semiconductors with composition tunable bandgaps in the near-infrared (NIR) spectrum. As the amount of Sn is increased, the band energy decreases and a transition from indirect to direct band structure occurs. Hence, they are prime candidates for fabrication of Si-compatible electronic and photonic devices, field effect transistors, and novel charge storage device applications. Success has been achieved with the growth of Ge1–x Sn x thin film alloys with Sn compositions up to 34%. However, the synthesis of nanocrystalline alloys has proven difficult, because of larger discrepancies (∼14%) in lattice constants. Moreover, little is known about the chemical factors that govern the growth of Ge1–x Sn x nanoalloys and the effects of quantum confinement on structure and optical properties. Herein, we report the synthesis of phase pure Ge1–x Sn x nanoalloys with sizes in the range of 15–23 and 3.4–4.6 nm and Sn compositions from x = 0.000–0.279, including the factors that have led to the elimination of undesired metallic impurities. The compositional dependence on lattice parameters has been studied using powder X-ray diffraction and Raman spectroscopy, which indicates a nonlinear expansion of the cubic Ge lattice with increasing Sn composition. Furthermore, the quantum size effects have resulted in bandgaps significantly blue-shifted from bulk Ge, for smaller Ge1–x Sn x nanoalloys (3.4–4.6 nm) with indirect energy gaps from 1.31 eV to 0.75 eV and direct energy gaps from 1.47 eV to 0.95 eV for x = 0.000–0.116 compositions. Remarkably, as-synthesized Ge1–x Sn x nanoalloys exhibit high thermal stability and moderate resistance against sintering up to 400–500 °C and are devoid of crystalline and amorphous Sn impurities.

The synthesis of size and shape controlled Au/Ag/Pd alloy nanoparticles (NPs) and their self-supported assembly into monolithic aerogels for electro-oxidation of ethanol is reported. Two distinct morphologies of ultrasmall (3–5 nm) Au/Ag/Pd alloy NPs were produced via stepwise galvanic replacement of thiol-coated Ag NPs. The resultant nanoalloys were self-assembled into large, free-standing, aerogel superstructures that exhibit direct NP connectivity, high surface area (269 ± 18.1–76 ± 6.4 m2/g) and mesoporosity (2–50 nm), and high electrocatalytic activity via controlled oxidation of the surfactant ligands. The gelation kinetics have been tuned by varying the oxidant/surfactant molar ratio that governs the acidity of sol–gel reaction and consequently the extent of Ag dealloying with in situ generated HNO3. As-synthesized Au/Ag/Pd aerogels exhibit polymeric or colloidal gel morphology that can be manipulated by varying the shape and composition of precursor NPs. The electrocatalytic activity of ternary alloy aerogels for oxidation of ethanol was investigated using cyclic voltammetry and chronoamperometry. The monolithic aerogels exhibit high catalytic activity and durability, which is ∼20–30 times greater than those of the discrete Au/Ag/Pd alloy NPs. The polymeric morphology of high Pd-containing alloy aerogels resulted in ∼1 order of magnitude higher current density and mass activity in comparison to low Pd-containing colloidal aerogels. The synergistic effect of trimetallic alloy mitigates the catalyst poisoning effects and increases the stability and durability while the self-supported superstructure with direct NP connectivity, high surface area, and mesoporosity offers a facile conduit for both molecular and electron transport, enabling Au/Ag/Pd aerogel as a high-efficiency electrocatalyst.

GeSn alloy quantum dots (QDs) were synthesized with sizes ranging from 1-3 nm exhibiting visible orange-red photoluminescence. Composition dependent optical properties were characterized and supported by theoretical calculations. Structural analysis suggests the QDs are diamond cubic phase, characteristic of GeSn thin films and nanocrystals (NCs) reported to date.

Hybrid nanomaterials composed of metal-semiconductor components exhibit unique properties in comparison to their individual counterparts, making them of great interest for optoelectronic applications. Theoretical and experimental studies suggest that interfacial interactions of individual components are of paramount importance to produce hybrid electronic states. The direct cross-linking of nanoparticles (NPs) via controlled removal of the surfactant ligands provides a route to tune interfacial interactions in a manner that has not been thoroughly investigated. Herein, we report the synthesis of CdSe/Ag heteronanostructures (aerogels) via oxidation induced self-assembly of thiol-coated NPs and the evolution of optical properties as a function of composition. Three hybrid systems were investigated, where the first and second excitonic energies of CdSe were matched with plasmonic energy of Au or Ag NPs and Ag hollow NPs. Physical characterization of the aerogels suggests the presence of an interconnected network of hexagonal CdSe and cubic Ag NPs. The optical properties of hybrids were systematically examined through UV-vis, photoluminescence (PL), and time-resolved (TR) PL spectroscopic studies that indicate the generation of alternate radiative decay pathways. A new emission (640 nm) from CdSe/Ag aerogels emerged at Ag loading as low as 0.27%, whereas absorption band tailing and PL quenching effects were observed at higher Ag and Au loading, respectively. The TRPL decay time of the new emission (∼600 ns) is markedly different from those of the band-edge (1.83 ± 0.03 ns) and trap-state (1190 ± 120 ns) emission maxima of phase pure CdSe, supporting the existence of alternate radiative relaxation pathways in sol-gel derived CdSe/Ag hybrids.

The direct cross-linking of Au/Ag alloy nanoparticles (NPs) into high surface area, mesoporous Au/Ag aerogels via chemical oxidation of the surface ligands is reported. The precursor alloy NPs with composition-tunable morphologies were produced by galvanic replacement of the preformed Ag hollow NPs. The effect of Au:Ag molar ratio on the NP morphology and surface plasmon resonance has been thoroughly investigated and resulted in smaller Au/Ag alloy NPs (4-8 nm), larger Au/Ag alloy hollow NPs (40-45 nm), and Au/Ag alloy hollow particles decorated with smaller Au NPs (2-5 nm). The oxidative removal of surfactant ligands, followed by supercritical drying, is utilized to construct large (centimeter to millimeter) self-supported Au/Ag alloy aerogels. The resultant assemblies exhibit high surface areas (67-73 m(2)/g), extremely low densities (0.051-0.055 g/cm(3)), and interconnected mesoporous (2-50 nm) networks, making them of great interest for a number of new technologies. The influence of mesoporous gel morphology on surface-enhanced Raman scattering (SERS) has been studied using Rhodamine 101 (Rd 101) as the probe molecule. The alloy aerogels exhibit SERS signal intensities that are 10-42 times higher than those achieved from the precursor Au/Ag alloy NPs. The Au/Ag alloy aerogel III exhibits SERS sensing capability down to 1 nM level. The increased signal intensities attained for alloy aerogels are attributed to highly porous gel morphology and enhanced surface roughness that can potentially generate a large number of plasmonic hot spots, creating efficient SERS substrates for future applications.

No abstract

Tin phosphides make up a class of materials that have received a noteworthy amount of interest in photocatalysis, charge storage, and thermoelectric devices. Dual stable oxidation states of tin (Sn2+ and Sn4+) allow tin phosphides to exhibit different stoichiometries and crystal phases. However, the synthesis of such nanostructures with control over morphology and crystal structure has proven to be a challenging task. Herein, we report the first colloidal synthesis of size-, shape-, and phase-controlled, narrowly disperse rhombohedral Sn4P3, hexagonal SnP, and trigonal Sn3P4 nanoparticles (NPs) displaying tunable morphologies and size-dependent physical properties. The control over NP morphology and crystal phase was achieved by tuning the nucleation/growth temperature, Sn/P molar ratio, and incorporation of additional coordinating solvents (alkylphosphines). The absorption spectra of Sn3P4 NPs (3.0 ± 0.4 to 8.6 ± 1.8 nm) exhibit size-dependent blue shifts in energy gaps (1.38–0.88 eV) compared to the theoretical value of bulk Sn3P4 (0.83 eV), consistent with quantum confinement effects. The trigonal Sn3P4 NPs adopt rhombohedral Sn4P3 and hexagonal SnP crystal structures at 180 and 250 °C, respectively. Structural and surface analysis indicates consistent bond energies for phosphorus across different crystal phases, whereas the rhombohedral Sn4P3 NPs demonstrate Sn oxidation states distinctive from those of the hexagonal and trigonal phases because of the complex chemical structure. All phases exhibit N­(1s) and ν(N–H) energies suggestive of alkylamine surface functionalization and are devoid of tetragonal Sn impurities.