17 Search Results

Robust synthesis of ultrafine metal nanoparticles (ufMNPs) below 5 nm with clean surfaces and strong optical absorption in the visible spectral range is challenging due to their instability originating from large surface-to-volume ratios. Here, this work reports a general strategy involving two sequential steps: i) loading metal precursor ions onto the surface of silica nanospheres (SiO_x NSs) by forming a uniform coating of metal oxyhydroxide [MO_y(OH)_z] through preferred surface acid–base reactions and ii) thermally reducing MO_y(OH)_z in forming gas at elevated temperatures to form ufMNPs evenly dispersed on the surface of SiO_x NSs. The capability of this synthesis strategy is verifiedmore » by loading ufMNPs of various transition metals and bimetallic combinations onto the SiO_x NSs. The ufMNPs exhibit strong optical absorption enhanced by the optical scattering resonances in the SiO_x NSs, which generate intense electric fields near the surface of the SiO_x NSs. The SiO_x NSs also support stabilizing the ufMNPs, which do not need additional organic capping reagents. The successful synthesis of SiO_x-NS-supported ufMNPs with clean surfaces and enhanced optical absorption is promising for exploring the photocatalytic properties of ufMNPs.« less

Photolysis reaction pathways of [Au(III)Cl₄]^- in aqueous solution have been investigated by time-resolved X-ray absorption spectroscopy. Ultraviolet excitation directly breaks the Au-Cl bond in [Au(III)Cl₄]^- to form [Au(II)Cl₃]^- that becomes highly reactive within 79 ps. Disproportionation of [Au(II)Cl₃]^- generates [Au(I)Cl₂]^-, which is stable for ≤ 10 mu s. In contrast, intense near-infrared lasers photolyze water to generate hydrated electrons, which then reduce [Au(III)Cl₄]^- to [Au(II)Cl₃]^- at 5 ns. Hydrated electrons further induce a chain reaction from [Au(II)Cl₃]^- to [Au(0)Cl]^- by successively removing one Cl^-. The zero-valency Au anions quickly polymerize and condense to form Au nanoparticles, which become the dominatingmore » product after 400 s. Here our results reveal that the condensation of zero-valency Au starts with dimerization of gold clusters coordinated with chloride ions rather than direct condensation of pristine Au atoms.« less

Robust synthesis of small colloidal silica (SiO₂) nanoparticles with narrow size distributions (<5%) is challenging because the possible involvement of continuous nucleation and growth of SiO₂ in the widely used Stöber process usually results in silica nanoparticles with large sizes and broad size distributions. Promoting heterogeneous nucleation and growth of silica is promising to tackle the challenges by in-situ forming transient colloidal seeds that provide nucleation surfaces for silica condensate. Here in this work, poly(acrylic acid) (PAA) is introduced to the Stöber synthesis solutions, which are usually composed of ethanol (containing a trace amount of water), tetraethyl orthosilicate (TEOS), andmore » ammonia, to mediate the nucleation of TEOS hydrolysates and growth of SiO₂ nanoparticles. PAA reacts with ammonia to form soft PAA-NH₃ complex colloids that serve as transient seeds to facilitate heterogeneous nucleation and growth of SiO₂ nanoparticles after adding TEOS to the synthesis solution. The number of PAA-NH₃ complex colloidal particles, which strongly depends on the molar ratio of PAA and ammonia, determines the number of the resulting SiO₂ nanoparticles and thus the size of the SiO₂ nanoparticles correspondingly according to the supply of TEOS. Highly uniform SiO₂ nanoparticles with finely tuned diameters in the sub-100 region have been successfully synthesized, and the synthesis protocol is feasible to be scaled up while maintaining the high quality of SiO₂ nanoparticles.« less

In this work, metal nanoparticles of multi-principal element alloys (MPEA) with a single crystalline phase have been synthesized by flash heating/cooling of nanosized metals encapsulated in micelle vesicles dispersed in an oil phase (e.g., cyclohexane). Flash heating is realized by selective absorption of a microwave pulse in metals to rapidly heat metals into uniform melts. The oil phase barely absorbs microwave and maintains the low temperature, which can rapidly quench the high-temperature metal melts to enable the flash cooling process. The precursor ions of four metals, including Au, Pt, Pd, and Cu, can be simultaneously reduced by hydrazine in themore » aqueous solution encapsulated in the micelle vesicles. The resulting metals efficiently absorb microwave energy to locally reach a temperature high enough to melt themselves into a uniform mixture. The duration of microwave pulse is crucial to ensure the reduced metals mix uniformly, while the temperature of oil phase is still low to rapidly quench the metals and freeze the single-phase crystalline lattices in alloy nanoparticles. The microwave-enabled flash heating/cooling provides a new method to synthesize single-phase MPEA nanoparticles of many metal combinations when the appropriate water-in-oil micelle systems and the appropriate reduction reactions of metal precursors are available.« less

VO ₂ particles are promising materials for thermochromic smart windows that reduce building energy loss. Continuous-flow hydrothermal processes showcase advantages for synthesizing VO ₂ particles compared with traditional batch reaction systems. Mixers play a crucial role in particle fabrication in continuous-flow systems. In this study, a Center T-Mixer and a Collision Cross-Mixer are developed and implemented in a hot water fluidized suspension reaction (HWFSR) system. The influence of the resident time on the particle phase and size was evaluated, and properties of particles derived from systems equipped with differing mixers were compared. The resulting particles were characterized using techniques ofmore » X-ray powder diffraction (XRD) analysis, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). When compared with the Center T-Mixer, results indicate that the Collision Cross-Mixer has better control regarding the morphology and size distribution of resulting particles while improving the transition temperatures of the as-synthesized materials. HWFSR systems containing novel mixer designs are capable of producing pure M-phase VO ₂ particles in a single step contrary to the current reactor design that use a second postheat treatment step, and they are capable of synthesizing many other nanoparticle species, especially those requiring high temperature and pressure reaction conditions.« less

Abstract Heating and cooling represent a significant portion of overall energy consumption of our society. Due to the diffusive nature of thermal energy, thermal insulation is critical for energy management to reduce energy waste and improve energy efficiency. Thermal insulation relies on the reduction of thermal conductivity of appropriate materials that are engineerable in compositions and structures. Hollow‐structured materials (HSMs) show a great promise in thermal insulation since the existence of high‐density gaseous voids breaks the continuity of heat‐transport pathways in the HSMs to lower their thermal conductivities efficiently. Herein, a timely overview of the recent progress in developing HSMsmore » for thermal insulation is presented, with the focus on summarizing the strategies for creating gaseous voids in solid materials and thus synthesizing various HSMs. Systematic analysis of the documented results reveals the relationship of thermal conductivities of the HSMs and the size and density of voids, i.e., reducing the void size below ≈350 nm is more favorable to decrease the thermal conductivity of the HSMs because of the possible confinement effect originated from the nanometer‐sized voids. The challenges and promises of the HSMs faced in future research are also discussed.« less

Hollow nanostructures have witnessed an increasing interest in emerging fields, including energy storage electrodes, nanocatalysis, photonics, and biomedical carriers, etc. The fundamental mechanisms for hollowing conversion chemistry of complex nanoalloys is indispensable to grow such nanostructures with controlled compositions and functionalities. In this contribution, the Kirkendall effect on magnetic Fe-Co bimetallic alloys with the formation of hollow nanostructures for exchange interactions was studied. The rich diffusion kinetics of binary Fe-Co alloys accompanied with its conversion from solid, core-shell to hollow nanostructures were been captured through X-ray spectroscopy and electron microscopy. We further report the cation exchange improved magnetic performance ofmore » hollow nanostructures with increasing the crystallinity.« less

Interlayer-expanded MoS ₂ nanosheets directionally assembled on multi-walled carbon nanotubes represent a new class of anode materials for lithium ion batteries with superior high rate capacity.

Vertically aligned interlayer expanded (IE) MoS ₂ on a Ti ₃ C ₂ T _x (MXene) support as an efficient electrocatalytic HER catalyst.