Exothermic Self-Sustained Waves with Amorphous Nickel

The synthesis of amorphous Ni (a-Ni) using a liquid-phase chemical reduction approach is reported. Detailed structural analysis indicates that this method allows for efficient fabrication of high surface area (210 m²/g) amorphous Ni nanopowder with low impurity content. We investigated the self-propagating exothermic waves associated with crystallization of Ni from the amorphous precursor. Time-resolved X-ray diffraction indicates that amorphous nickel crystallizes in the temperature range 445–480 K. High-speed infrared imaging reveals that local preheating of compressed a-Ni nanopowder triggers a self-sustaining crystallization wave that propagates with velocity ∼0.3 mm/s. The maximum temperature of crystallization wave depends on the sample density and can be as high as 600 K. The Kissinger approach is used to determine the apparent activation energy (55.4 ± 4 kJ/mol) of crystallization. The self-diffusion activation energy of Ni atoms in a-Ni is ∼60 kJ/mol, determined through molecular dynamics (MD) simulations. This agreement of experimentally derived and theoretically calculated activation energies allows us to conclude that self-diffusion of Ni atoms is the rate-limiting stage for crystallization. Furthermore, utilization of amorphous metal as a reactant significantly increases the rate of solid-state reactions. For example, in reactive intermetallic forming systems, such as Ni + Al, the self-sustaining reaction propagation velocity with a-Ni is twice higher than with crystalline Ni of the same morphology. Additionally, using a-Ni increases the maximum reaction temperature in the Ni + Al system by 300 K

Nickel Oxide Reduction by Hydrogen: Kinetics and Structural Transformations

We studied the reduction kinetics of bulk NiO crystals by hydrogen and the corresponding structural transformations in the temperature range of 543–1593 K. A new experimental approach allows us to arrest and quench the reaction at different stages with millisecond time resolution. Two distinctive temperature intervals are found where the reaction kinetics and product microstructures are different. At relatively low temperatures, 543–773 K, the kinetic curves have a sigmoidal shape with long induction times (up to 2000 s) and result in incomplete conversion. Low-temperature reduction forms a complex polycrystalline Ni/NiO porous structure with characteristic pore size on the order of 100 nm. No induction period was observed for the high-temperature conditions (1173–1593 K), and full reduction of NiO to Ni is achieved within seconds. An extremely fine porous metal structure, with pore size under 10 nm, forms during high-temperature reduction by a novel crystal growth mechanism. This consists of the epitaxial-like transformation of micrometer-sized NiO single crystals into single-crystalline Ni without any crystallographic changes, including shape, size, or crystal orientation. The Avrami nucleation model accurately describes the reaction kinetics in both temperature regimes. However, the structural transformations during reduction in both nanolevel and atomic level are very complex, and the mechanism relies on both nucleation and the critical diffusion length for outward diffusion of water molecules

Origin of the Size-Dependent Stokes Shift in CsPbBr₃ Perovskite Nanocrystals

The origin of the size-dependent Stokes shift in CsPbBr₃ nanocrystals (NCs) is explained for the first time. Stokes shifts range from 82 to 20 meV for NCs with effective edge lengths varying from ∼4 to 13 nm. We show that the Stokes shift is intrinsic to the NC electronic structure and does not arise from extrinsic effects such as residual ensemble size distributions, impurities, or solvent-related effects. The origin of the Stokes shift is elucidated via first-principles calculations. Corresponding theoretical modeling of the CsPbBr₃ NC density of states and band structure reveals the existence of an intrinsic confined hole state 260 to 70 meV above the valence band edge state for NCs with edge lengths from ∼2 to 5 nm. A size-dependent Stokes shift is therefore predicted and is in quantitative agreement with the experimental data. Comparison between bulk and NC calculations shows that the confined hole state is exclusive to NCs. At a broader level, the distinction between absorbing and emitting states in CsPbBr₃ is likely a general feature of other halide perovskite NCs and can be tuned via NC size to enhance applications involving these materials

Transforming Layered to Nonlayered Two-Dimensional Materials: Cation Exchange of SnS₂ to Cu₂SnS₃

We demonstrate the chemical transformation of layered, two-dimensional (2D) SnS₂ to nonlayered Cu₂SnS₃ via cation exchange. Resulting Cu₂SnS₃ nanosheets (NSs) retain the overall starting morphology of their parent, few-layer SnS₂ templates. Specifically, they possess micrometer-sized dimensions and have controlled thicknesses dictated by the number of initial SnS₂ layers. Our demonstration shows that existing layered compounds can serve as templates for difficult-to-synthesize nonlayered 2D specimens with cation exchange providing a bridge between families of layered and nonlayered materials. New 2D systems are therefore accessible, opening the door to future explorations of low-dimensional nanostructure anisotropic optical and electrical properties

Ultrasmall α‑Fe₂O₃ Superparamagnetic Nanoparticles with High Magnetization Prepared by Template-Assisted Combustion Process

A template-assisted combustion-based method is developed to synthesize the ultrasmall (below 5 nm) α-Fe₂O₃ nanoparticles. The iron and ammonium nitrate are used as oxidizers, glycine as a “fuel” and mesoporous silica (SBA-15) as a template. Because of the ultralow sizes and high crystallinity, the combustion-derived α-Fe₂O₃ nanoparticles exhibit superparamagnetism in the temperature range of 70–300 K. The high specific surface area (132 m²/g) of α-Fe₂O₃ indicates the important role of surface magnetic spins resulting in remarkably high magnetization (21 emu/g) at 300 K