Nanocomposites and nanomaterials

Synthesis of thermally stable ultrasmall metallic Ni nanoparticles confined in microporous carbon

Boychuk V.M.1, Kotsyubynsky V.O.1, Rachiy B.I.1, Solonets D.M1,
Zapukhlyak R.M.1, Turovska L.V.2

1Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Str., Ivano-Frankivsk, 76018, Ukraine.
E-mail: kotsuybynsky@gmail.com
2Ivano-Frankivsk National Medical University, 2 Halytska Str., 
Ivano-Frankivsk, 76018, Ukraine

The search for highly efficient, inexpensive, and durable non-noble-metal electrocatalysts for hydrogen evolution reaction in an acidic environment is crucially important for the further progress of a clean sustainable economy. The present study reports on the synthesis and characterization of hybrid materials formed by ultrafine Ni nanoparticles and microporous carbon. A hemp fiber derived carbon template was prepared using the stages of carbonization (800 °С) and chemical activation (KOH as an agent) (labeled as AC materials). Additionally, AC was treated with HNO3 as a source of nitrogen at 70 °С in an N2 atmosphere to form N-containing surface functional groups and increase wettability with an aqueous electrolyte (labeled as ACN materials). The carbon templates were degassed in a vacuum (200 oC, 12 h) and transferred at low pressure to a quartz chamber with vapors of sublimated nickelocene (Ni(Cp)2). After adsorption of Ni(Cp)2, for its dissociation, the obtained materials were irradiated with UV light (27 W, main wavelength of 365 nm) with continuous stirring for 5 days (labeled as Ni:AC and Ni:ACN materials, respectively). According to XRF data (Exper-3L analyzer), Ni load was 4-5 wt %. Thermal treatment of both AC and ACN was carried out in an inert atmosphere (argon flow) at a temperature of 400 °C for 1 hour. XRD patterns (XRD-7000, Shimadzu) of AC and AСN templates show broad peaks (002) and (10l) indicating turbostratic ordering of carbon matrices. There are no changes in the XRD patterns of as-synthesized and annealed at T  400 oC Ni:AC and Ni:AСN samples. A distinct presence of the Ni metal phase was observed for Ni:AC-700 and Ni:AСN-600 materials. Nitrogen adsorption–desorption isotherms (Autosorb Nova 2200e, Quantachrome) of all Ni:AC and Ni:AСN samples are a mixture of types II and IV. As-prepared and annealed samples had micropores (NLDFT model) with a size of 1.35 nm. The presence of Ni particles leads to the filling of some of these micropores and a decrease in the BET surface area from 1200 to 1000 m2/g. Annealing leads to the growth of micropore-confined Ni particles and causes its migration to the surface of carbon grains, which manifests itself in the gradual restoration of the initial pore size distribution and BET surface area.