Mapping Nickel Nanoparticle Size and Surface Chemistry for Catalytic Applications

Nickel nanoparticles catalyze industrially relevant reactions such as hydrogenation, CO2 conversion, ammonia decomposition, and hydrogen evolution [1-3]. This study presents a detailed exploration of the synthesis, morphological tuning, and physicochemical characterization of nickel nanoparticles (NiNPs) engineered for catalytic applications in various media. Using a modified polyol method, NiNPs were synthesized under controlled thermal and chemical conditions with nickel chloride and sodium borohydride in ethylene glycol according to a modified polyol method [4]. Variations in reaction temperature (120° C, 130° C, 150° C, and 170° C) were found to significantly reduce the particle size from 23 ± 7 nm to 13 ± 4, while increasing surface oxidation and interparticle aggregation. Complementary bulk X-ray photoelectron spectroscopy (XPS), high-angle annular dark field (HAADF) in the scanning transmission electron microscope (STEM) (Fig. 1) and electron energy loss spectroscopy (EELS) mappings (Fig. 2) at the Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory, demonstrated how an increase in synthesis temperature led to smaller nanoparticle sizes while increasing surface oxidation and interparticle aggregation. XPS was used to probe the elemental composition, chemical state (e.g., NiO or Ni(OH)2), oxidation level, and overall electronic structure of NPs [5-7]. We will discuss how surface stabilization approaches using non-ionic steric stabilizers such as poly(vinylpyrrolidone) (PVP) or functionalization with 3-(aminopropyl)triethoxysilane (APTES) influence particle dispersion, agglomeration behavior, and chemical accessibility. The chemical state, oxidation level, and overall electronic structure of nickel nanoparticles are critical for tuning the reactivity and magnetic properties of colloidal nickel. Magnetic hysteresis loops were measured via alternating gradient magnetometry (AGM) at 60 Hz. It was found that the NiNPs’ saturation magnetization and magnetic susceptibility scale with the reaction temperature. Nickel nanoparticles have proved to be catalytically active in two scenarios, i) NiNP-assisted CO2 capture in freshwater and saltwater media, and ii) the electrocatalysis of the hydrogen evolution reaction (HER). These findings provide a systematic foundation for tailoring the nanoparticle surface chemistry for long-term catalyst performance. NiNP size, dispersion, and stability are critical factors for applications in electronics, catalysis, and biomedicine [9]. HR-TEM imaging reveals lattice fringes in NiNP cluster synthesized at 130° C. HR-TEM imaging and EELS mappings of NiNPs synthesized at 120° C. DualEELS and multiple linear least squares (MLLS) fitting revealed a distinct core–shell structure, with metallic Ni0 concentrated in the core (yellow) and NiO (cyan) forming outer shells.