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

Herein, nanocomposites made of Ni nanoparticles distributed in an amorphous silicon nitride (Ni/a-SiN) matrix, on the one hand, and within an amorphous silicon dioxide (Ni/a-SiO) 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 (H)-temperature-programmed desorption (TPD) profile analysis revealed that the activation energy for H desorption at about 100-130 °C evaluated for the Ni/a-SiN sample (47.4 kJ mol) was lower than that for the Ni/a-SiO sample (68.0 kJ mol). 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-SiN matrix compared to O atoms in the a-SiO 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 H desorption, as experimentally observed in the Ni/a-SiN sample.