A versatile method to produce metallic nickel nanoparticles is demonstrated. Metallic Ni nanoparticles have been synthesized from aqueous solution of NiCl2 using gamma-radiation induced reduction. To prevent Ni re-oxidation, post-irradiation treatment was elaborated. Structural and compositional analyses were executed using X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. These studies reveal that the synthesized material consists of fcc Ni particles having size of 3.47 +/- 0.71 nm. The nanoparticles have a tendency to agglomerate to the larger clusters. The latter are partially oxidized to form thin amorphous/poor-crystalline Ni(OH)(2)/NiO layers at the surface. Magnetization measurements demonstrate that the nanomaterial exhibit ferromagnetic-like behaviour with magnetization 30% lower than that in bulk Ni. The large active surface area (ECSA, 39.2 m(2) g(-1)) and good electrochemical reversibility, confirmed by the electrochemical studies, make the synthesized material a potential candidate as an active component for energy storage devices.

To reach commercial viability for fuel cells, one needs to develop active and robust Pt-free electrocatalysts. Silver has great potential to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in alkaline media due to its low cost and superior stability. However, its catalytic activity needs to be improved. One possible solution is to fabricate bimetallic nanostructures, which demonstrate a bifunctional enhancement in the electrochemical performance. Here, two types of bimetallic silver-nickel nanocatalysts, core-shells (Ag@NiO) and heterostructures (Ag/Ni), are fabricated using γ-radiation induced synthesis. The Ag@NiO nanoparticles consist of an amorphous, NiO layer as a shell and a facetted crystalline Ag particle as a core. Meanwhile, the Ag/Ni heterostructures comprise Ag particles decorated with Ni/Ni(oxy-hydro)-oxide clusters. Both materials demonstrate similar and increased alkaline ORR activity as compared to monometallic catalysts. It was revealed that the enhanced catalytic activity of the core-shells is mainly attributed to the electronic ligand effect. While in the Ag/Ni heterostructures, a lattice mismatch between the Ni-based clusters and Ag implies a significant lattice strain, which, in turn, is responsible for the increased activity of the catalyst. Also, the Ag/Ni samples exhibit good stability under operating conditions due to the existence of stable Ni3+ compounds on the surface.

The development of Pt-free nanocatalysts with enhanced electrocatalytic activity and durability is essential for the commercialization of fuel cells. Herein, we report a new class of active and durable PdNi nanocatalysts for the oxygen reduction reaction (ORR) in an alkaline medium. PdNi nanoframework (PdNi-NF), consisting of interconnected ultrathin ridges was synthesized through a galvanic replacement reduction (GRR) strategy using Ni nanoparticles, obtained by radiolytic reduction, as seeds. PdNi nanochain (PdNi-NC), composed of aggregated nanoparticles, was produced by one-pot γ-radiation induced reduction. It was found that the carbon-supported PdNi-NF catalysts exhibit higher ORR activity and enhanced durability compared to PdNi-NC/C (PdNi-NC on carbon), Pd-NP/C (Pd nanoparticle on carbon), and commercial Pt/C catalysts. The superior activity of PdNi-NF/C catalyst may originate from the nanoframework morphology that facilitates the O2 diffusion and the electronic effect induced by the sub-surface PdNi alloy; the improved durability can be ascribed to the stable Pd-enriched surface layer. 

Transition metal-based catalysts show great potential to replace Pt-based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here we demonstrate that homogeneous alloy nanoparticles with a varying Ni-to-Co ratio and controlled structure can be synthesized from aqueous Ni (Co) acetate solutions using a facile γ-radiolytic reduction method. The obtained samples are found to possess stacking fault defects that are ordered to form polytype structures. The concentration of these defects depends on the Ni-to-Co ratio, as supported by the results of ab initio calculations. Structural defects may play a significant role in the enhanced activity of catalysts toward oxygen evolution and oxygen reduction reactions. At the same time, the activity of Ni-Co catalysts in the hydrogen evolution reaction is found to be affected by the formation of Ni-OH bonds on the surface rather than by the presence of structural defects. Our study demonstrates that the composition of Ni-Co nanoparticles is an essential factor having an impact on their structure and that both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions. 

A γ-radiation induced synthesis method is used to fabricate manganese oxide catalysts through both reduction and oxidation routes. It is shown that the morphology, composition and electrochemical performance of the produced manganese oxide particles can be tuned by altering the redox conditions. The catalysts prepared via radiolytic oxidation have a hollow spherical morphology, possess γ-MnO2 structure and show high catalytic activity for the complete four-electron reaction pathway of the oxygen reduction reaction (ORR) in alkaline electrolyte. Meanwhile, the catalysts synthesized via radiolytic reduction possess a rod-like morphology with a Mn3O4 bulk structure and favour the incomplete two-electron reaction pathway for ORR. The high catalytic activity of the manganese oxide synthesized via the oxidation route can be attributed to high electrochemical surface area and increased amount of Mn3+ on the surface as compared to those in the sample obtained via the reduction route.

The role of intermediate phases in CeO2 mesocrystal formation from aqueous CeIII solutions subjected to γ-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble CeIII into less soluble CeIV. Transmission electron microscopy (TEM) and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to CeIV: (1) formation of hydrated CeIV hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into “pre-mesocrystals” and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of “mineral bridges”. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures. 

A perfect PtCu nanocube with partial hollow structure was prepared by hydrothermal reaction and its electrocatalytic methanol oxidation reaction (MOR) was studied. The appropriate concentration of shape-control additives KI and triblock pluronic copolymers, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO19-PPO69-PEO19) (P123) play crucial roles in the final product morphology. The PtCu nanocubes can be perfectly in situ immobilizedonto graphene under the action of P123 while the structure and cubic morphologyremain unchanged. The electrochemical tests suggest that the obtained PtCu nanocube (PtCu-NCb) exhibits better MOR activity and stability than PtCu hexagon nanosheet (PtCu-NSt), PtCu nanoellipsoid (PtCu-NEs) and commercial Pt/C in alkaline medium. When in situ immobilized onto graphene, the MOR catalytic activity and stability of PtCu cubes are further improved. The markedly enhanced electrocatalytic activity and durability maybe attributed to the special cubic morphology with partial hollow structure enclosed by highly efficient facet and the probably the synergistic effect of PtCu and intermediate state CuI decorated on the surface and graphene.