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

Abstract

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.