Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of Fe0.28Co0.21Ni0.20Cu0.08Pt0.23HEA nanoparticles (NPs) in an atmospheric pressure dry air environment by in situ gas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner’s theory. Further, the HEA NPs are found to oxidize at a significantly slower rate compared to monometallic NPs. The outward diffusion of transition metals and formation of disordered oxide layer are observed in real time and confirmed through analytical energy dispersive spectroscopy, and electron energy loss spectroscopy characterizations. Localized ordered lattices are identified in the oxide, suggesting the formation of Fe2O3, CoO, NiO, and CuO crystallites in an overall disordered matrix. Hybrid Monte Carlo and molecular dynamics simulations based on first-principles energies and forces support these findings and show that the oxidation drives surface segregation of Fe, Co, Ni, and Cu, while Pt stays in the core region. The present work offers key insights into how HEA NPs behave under high-temperature oxidizing environment and sheds light on future design of highly stable alloys under complex service conditions.

Magnesium nanoparticles (NPs) offer the potential of high-performance reactive materials from both thermodynamic and kinetic perspectives. However, the fundamental energy release mechanisms and kinetics have not been explored due to the lack of facile synthetic routes to high-purity Mg NPs. Here, a vapor-phase route to surface-pure, core−shell nanoscale Mg particles is presented, whereby controlled evaporation and growth are utilized to tune particle sizes (40−500 nm), and their size-dependent reactivity and energetic characteristics are evaluated. Extensive in situ characterizations shed light on the fundamental reaction mechanisms governing the energy release of Mg NP-based energetic composites across particle sizes and oxidizer chemistries. Direct observations from in situ transmission electron microscopy and high-speed temperature-jump/time-of-flight mass spectrometry coupled with ignition characterization reveal that the remarkably high reactivity of Mg NPs is a direct consequence of enhanced vaporization and Mg release from their high-energy surfaces that result in the accelerated energy release kinetics from their composites. Mg NP composites also demonstrate mitigated agglomeration and sintering during reaction due to rapid gasification, enabling complete energy extraction from their oxidation. This work expands the compositional possibilities of nanoscale solid fuels by highlighting the critical relationships between metal volatilization and oxidative energy release from Mg NPs, thus opening new opportunities for strategic design of functional Mg-based nanoenergetic materials for tunable energy release.

Ammonia borane (NH3BH3, AB) represents a promising energy-dense material for hydrogen storage and propulsion; however, its energy release mechanisms on oxidation by solid-state oxidizers are not well understood. In this study, through in situ time-of-flight mass spectrometry supported by attenuated total reflection-Fourier transform infrared spectroscopy and density functional theory calculations, we investigate the fundamental reaction mechanisms involved in the energy release from solid-state AB with different chemical oxidizers. We show that the reaction of AB with oxidizers like KClO4 is mediated by [NH3BH2NH3]+[BH4]− (DADB) formation, resulting in its kinetic entrapment into low-energy BNH x clusters that are resistant to further oxidation, thus limiting complete energy extraction. In contrast, with an ammonium-based oxidizer such as NH4ClO4, the presence of NH4 + ions enables AB to follow an alternative reaction pathway forming [NH3BH2NH3]+[ClO4]− rather than DADB, thus inhibiting the formation of BNH x species and facilitating its complete oxidation. This alternative reaction route causes the AB/NH4ClO4 system to exhibit remarkably higher energy release rates over that of AB/KClO4 (∼27x) and the standard Al/NH4ClO4 propellant (∼7x).

Biocidal nanothermite composites show great potential in combating biological warfare threats because of their high-energy-release rates and rapid biocidal agent release. Despite their high reactivity and combustion performance, these composites suffer from low-energy density because of the voids formed due to inefficient packing of fuel and oxidizer particles. In this study, we explore the potential of plasma-synthesized ultrafine Si nanoparticles (nSi, ∼5 nm) as an energetic filler fuel to increase the energy density of Al/Ca­(IO3)2 energetic-biocidal composites by filling in the voids in the microstructure. Microscopic and elemental analyses show the partial filling of mesoparticle voids by nSi, resulting in an estimated energy density enhancement of ∼21%. In addition, constant-volume combustion cell results show that nSi addition leads to a ∼2–3-fold increase in reactivity and combustion performance, as compared to Al/Ca­(IO3)2 mesoparticles. Oxidation timescale analyses suggest that nSi addition can promote initiation due to faster oxygen transport through the oxide shell of Si nanoparticles. At nSi loadings higher than ∼8%, however, slower burning characteristics of nSi and sintering effects lead to an overall degradation of combustion behavior of the composites.

Thin films of polymer−metal nanocomposites can serve as efficient surface-enhanced Raman scattering (SERS) substrates; it would be highly advantageous if it can be fabricated by a cost-effective and facile protocol, and high enhancement factors (EF) can be realized for a range of excitation wavelengths, enabling sensitive detection of different analytes. Poly(vinyl alcohol) (PVA) thin films with embedded Cu−Ag nanoparticles are fabricated through a simple in situ procedure via stepwise formation of CuO−PVA and Cu−PVA; the fabrication is monitored through the different stages, by spectroscopy, microscopy, electron diffraction, and elemental analysis. Choice of the Cu−Ag combination is dictated by its broad plasmonic extinction, favorable cost factor, and chemical stability of the nanocomposite; the hydrogel character of PVA facilitates analyte absorption and contact with the nanoparticles, leading to efficient SERS. Cu−Ag−PVA thin film with an optimal composition is shown to display plasmonic extinction over an appreciable part of the visible range and to provide EF of the order of 10 7 −10 8 for the analytes, rhodamine 6G and methylene blue, at three different excitation wavelengths; the EF values are significantly higher than those reported earlier, with most of the Cu−Ag based SERS substrates. The novel polymer−bimetal nanocomposite thin film is an efficient SERS substrate providing subpicomolar limits of detection.

This paper presents a fast CO2 laser synthesis and writing technique – laser photothermal synthesis and writing (LPSW) – to generate and write a high concentration of unaggregated, spherical sub-10 nm metal nanoparticles (sMNPs).

Interactions between energetic material relevant nanoscale metal oxides (SiO 2 , TiO 2 , MgO, Al 2 O 3 , CuO, Bi 2 O 3 ) and poly(vinylidene fluoride) (PVDF) at high temperature were investigated by temperature-jump/time-of-flight mass spectrometry (T-jump/TOFMS) and thermogravimetric-differential scanning calorimetry (TGA-DSC). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology of the compositions, while X-ray diffraction (XRD) was utilized to analyze the condensed phase crystalline species at temperatures of interest. The exergonicity and exothermicity of HF gas with hydroxyl-terminated metal oxide surfaces make HF the likely fluorine-bearing moiety and primary mode of the fluorinating reactions, where terminal OH − configurations are replaced by F − in the formation of a stronger metal−fluorine bond. However, not all compositions produce corresponding stable metal fluoride. The results show that while some of the investigated metal oxide−PVDF compositions enhance PVDF decomposition and release HF in larger quantities than PVDF, others release HF in smaller quantities than PVDF and even retard PVDF decomposition. The former compositions demonstrate exothermic, multistep mass loss modes prior to neat PVDF mass loss, and the relative intensity of HF gas increases as the temperature of the release point decreases, implying a correlation between HF release and the progression of exothermic behavior. An interplay dynamic where surface interactions both lower the onset of HF release and engage exothermically with HF gas subsequently is proposed.

Matrix-supported metal nanoclusters (1−10 nm) with unique size-and shape-dependent properties have drawn attention for their potential applications in electronics, catalysis, energy storage, and sensors. However, synthesis of matrixsupported ultrasmall nanoclusters at high concentration and in an unaggregated state is challenging. Here we demonstrate a rapid laser pulse technique to in situ fabricate ultrasmall metal nanoclusters supported on a carbon nanofiber (CNF) matrix with kinetically controllable size and surface density. A rapid laser pulse heating on the metal precursor incorporated CNF matrix triggers the fast nucleation and growth of metal nanoclusters, and a subsequent ultrafast quenching freezes them onto the CNF structure. We find that a shorter laser pulse enables the formation of metal nanoclusters with higher number densities and smaller sizes while longer laser pulse leads to the further growth of metal nanoclusters and the achievement of their equilibrium shape. A characteristic time analysis suggests that the growth of metal nanoclusters is dominated by surface diffusion and sintering, and Ostwald ripening is mainly involved at the early stage of nanocluster formation. We also demonstrate that the catalytic performance of CNF matrix supported metal nanoclusters toward electrocatalytic hydrogen evolution is enhanced for metal nanoclusters with a smaller size and higher number density. This work provides a promising approach for rapid and scalable fabrication of ultrasmall, high-density metal nanoclusters, and nanoclusterbased devices.