Mechanistic Investigation of the Formation of Nickel Nanocrystallites Embedded in Amorphous Silicon Nitride Nanocomposites

Encapsulating Nickel-Iron Alloy Nanoparticles in a Polysilazane-Derived Microporous Si-C-O-N-Based Support to Stimulate Superior OER Activity

The in situ confinement of nickel (Ni)-iron (Fe) nanoparticles (NPs) in a polymer-derived microporous silicon carboxynitride (Si-C-O-N)-based support is investigated to stimulate superior oxygen evolution reaction (OER) activity in an alkaline media. Firstly, we consider a commercial polysilazane (PSZ) and Ni and Fe chlorides to be mixed in N,N-dimethylformamide (DMF) and deliver after overnight solvent reflux a series of Ni-Fe : organosilicon coordination polymers. The latter are then heat-treated at 500 °C in flowing argon to form the title compounds. By considering a Ni : Fe ratio of 1.5, face centred cubic (fcc) NixFey alloy NPs with a size of 15-30 nm are in situ generated in a porous Si-C-O-N-based matrix displaying a specific surface area (SSA) as high as 237 m2 ⋅ g-1. Hence, encapsulated NPs are rendered accessible to promote electrocatalytic water oxidation. An OER overpotential as low as 315 mV at 10 mA ⋅ cm-2 is measured. This high catalytic performance (considering that the metal mass loading is as low as 0.24 mg cm-2) is rather stable as observed after an activation step; thus, validating our synthesis approach. This is clearly attributed to both the strong NP-matrix interaction and the confinement effect of the matrix as highlighted through post mortem microscopy observations.

In Situ Single-phase Formation of LaCl(CN2) Mixed-Anion Compound Via Controlled Pyrolysis of La3+ Modified Melamine Precursor

Nitride (N3-) or cyanamide (CN22-) based mixed-anion compounds stand as attractive materials due to their unique properties derived from the binary or multiple anions, although their synthesis remains challenging in incorporating the N3- or CN22- anions safely. This work highlights the first demonstration of in situ single phase formation of a LaCl(CN2) mixed-anion compound from a stable single source precursor, melamine modified with LaCl3 preparable under aqueous conditions. The in situ formation of LaCl(CN2) involves the chemical modification of melamine with LaCl3 to form a complex. Upon heating of the precursor under N2 flowing, this complex generates cyanamide species around 400 °C, which react with LaCl3 and nonsublimated melamine to afford a binary LaCl(CN2)/g-C3N4 composite. Further pyrolysis at 800 °C decomposes the g-C3N4 counterpart, resulting in the LaCl(CN2) single-phase formation. The electronic properties of the precursor-derived single phase LaCl(CN2) were studied by the density functional theory calculation and UV-vis spectroscopy combined with X-ray photoelectron spectroscopy analyses and characterized by measuring 4.7, 1.8, and -2.9 eV for the band gap energy, the valence band maximum, and the conduction band minimum relative to Fermi energy, respectively. This study paves the way for exploring various cyanamide-based mixed anion compounds, advancing their potential applications in various fields.

Downshift of the Ni d band center over Ni nanoparticles in situ confined within an amorphous silicon nitride matrix

Herein, nanocomposites made of Ni nanoparticles in situ distributed in an amorphous silicon nitride (Ni/a-Si3N4) matrix, on the one hand, and within an amorphous silicon dioxide (Ni/a-SiO2) matrix, on the other hand, were synthesized from the same Ni-modified polysilazane precursor. In both compounds, the Ni/Si atomic ratio (0.06-0.07), average Ni nanocrystallite size (7.0-7.6 nm) and micro/mesoporosity of the matrix were rigorously fixed. Hydrogen (H2)-temperature-programmed desorption (TPD) profile analysis revealed that the activation energy for H2 desorption at about 100-130 °C evaluated for the Ni/a-Si3N4 sample (47.4 kJ mol-1) was lower than that for the Ni/a-SiO2 sample (68.0 kJ mol-1). Mechanistic study with X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations revealed that, at Ni nanoparticle/matrix heterointerfaces, Ni becomes more covalently bonded to N atoms in the a-Si3N4 matrix compared to O atoms in the a-SiO2 matrix. Therefore, based on experimental and theoretical studies, we elucidated that nickel-nitrogen (Ni-N) interactions at the heterointerface lead to remarkable Ni d band broadening and downshifting of the d band center relative to those generated by Ni-oxygen (Ni-O) interactions at the heterointerface. This facilitates H2 desorption, as experimentally observed in the Ni/a-Si3N4 sample.