Effect of Gd-doping in Ni/NiO core/shell magnetic nanoparticles (MNPs) on structural, magnetic, and hydrogen...

In this study, Gd_x-doped Ni/NiO MNPs (x: 0.0%, 2.5%, 5.0%, and 10.0%) with a protective polyvinylpyrrolidone (PVP) layer have been synthesized via a polyol reduction process. The x-ray diffraction patterns revealed that samples have a cubic structure with Fm3̄m space group and no change in the crystallite structure was observed with doping Gd³⁺ ions. The crystallite size (D_c) decreases from 2.70 to 1.27 nm when Gd is doped into Ni/NiO MNPs. Transmission electron microscopy analysis revealed that the Ni/NiO MNPs with Gd(5%) concentration are formed as spherical multicore-like shape core/shell MNPs with a protective PVP layer. The magnetic hysteresis measurements taken at 10 and 300 K show that the saturation magnetization (M_s) decreases with increasing Gd³⁺ ions in the structure. The highest effective magnetic moment (μ_eff) was obtained as 10.34 μ_B in the NG-2 sample. We ascribe that the high μ_eff value in this sample is due to the increase in d-f exchange interaction between Ni(3d⁷) and Gd(4f⁷) and the contribution of the dipole moment of PVP molecules. The electrochemical measurements showed that the current density values were 0.294 and 0.319 mA/cm² at-1.3 V in the absence of Gd (NG-0) and Gd(5%) doped (NG-2) samples, respectively. βc was 159 and 132 mV/dec for NG-0 and NG-2 samples, respectively. The diminishing of βc and the charge resistance (Rct) proved that the Gd doped catalyst enhanced the hydrogen evolution activity and the Gd(5%) doped sample exhibited the highest catalyst performance.