Controllable Compositions and Structures of FeO@SiO₂@C-Ni Hybrids with a Silica Layer as a Mineral Redox Buffer

Mineral redox buffer is a vital concept in geology that can be applied to modulate hybrid compositions and generate nanostructures with expected morphology. Here, via combining a dual coating of an inorganic silica and organic resorcinol-formaldehyde-Ni²⁺ (RF-Ni²⁺) layer on α-Fe₂O₃ spindles with a subsequent calcination process, core-shell FeO@SiO₂@C-Ni composites with multicompositional structures were fabricated as efficient catalysts for 4-nitrophenol (4-NP) reduction. Notably, the silica layer as a redox buffer between hematite cores and the RF-Ni²⁺ shell played a crucial role in modulating the compositions and structures of the FeO@SiO₂@C-Ni. Without the silica layer, Fe₃O₄-Ni/C composites with Ni nanoparticles trapped into the Fe₃O₄ cores were generated. Moreover, a significant impact of the calcination temperature on morphologies and compositions of the FeO@SiO₂@C-Ni catalysts along with their catalytic performances has been verified. As a result, the catalyst annealed at 500 °C exhibited a high magnetic property and optimized morphology with high-density small nickel nanoparticles (∼11.6 nm), showing remarkably enhanced catalytic activity compared to the Fe₃O₄-Ni/C composites and excellent recyclability with a high conservation of about 92%. Furthermore, this synthetic strategy shows significant potential to modulate the nanostructures and phases of other multivalent metal oxide nanocomposites.