| Summary: | The work reports a facile synthesis of high thermally stable nanocrystalline anatase TiO<sub>2</sub> nanoparticles (NPs) doped with different atomic concentrations (0.5, 1.0, 3.0, and 6.0%) of Gd<sup>3+</sup> and Nd<sup>3+</sup> ions by a template-free and one-step solvothermal process, using titanium(IV) butoxide as a titanium precursor and dimethyl sulfoxide (DMSO) as a solvent. The structure and morphology of the Gd<sup>3+</sup>, Nd<sup>3+</sup>, and 0.5%Gd<sup>3+</sup>-0.5%Nd<sup>3+</sup>/doped TiO<sub>2</sub> NPs have been characterized by using various analytical techniques. The Gd<sup>3+</sup>/ and Nd<sup>3+</sup>/TiO<sub>2</sub> molar ratios were found to have a pronounced impact on the crystalline structure, size, and morphology of TiO<sub>2</sub> NPs. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies revealed the proper substitution of Ti<sup>4+</sup> by Gd<sup>3+</sup> and Nd<sup>3+</sup> ions in the TiO<sub>2</sub> host lattice. The as-prepared Gd<sub>x</sub>/TiO<sub>2</sub>, Nd<sub>x</sub>/TiO<sub>2</sub>, and Gd<sub>1.0</sub>/Nd<sub>x</sub>/TiO<sub>2</sub> bimetallic NPs, x = 0.5, 1.0, 3.0, and 6%, have been investigated as electrocatalysts for hydrogen evolution reaction (HER) in 1.0 M KOH solution using a variety of electrochemical techniques. At any doping percentage, the Gd<sub>1.0</sub>/Nd<sub>x</sub>/TiO<sub>2</sub> bimetallic NPs showed higher HER catalytic performance than their corresponding counterparts, i.e., Gd<sub>x</sub>/TiO<sub>2</sub> and Nd<sub>x</sub>/TiO<sub>2</sub>. Upon increasing the Nd content from 0.5 to 6.0%, the HER catalytic performance of the Gd<sub>1.0</sub>/Nd<sub>x</sub>/TiO<sub>2</sub> bimetallic NPs was generally enhanced. Among the studied materials, the bimetallic Gd<sub>1.0</sub>/Nd<sub>6.0</sub>/TiO<sub>2</sub> NPs emerged as the most promising catalyst with an onset potential of −22 mV vs. RHE, a Tafel slope of 109 mV dec<sup>−1</sup>, and an exchange current density of 0.72 mA cm<sup>−2</sup>. Such HER electrochemical kinetic parameters are close to those recorded by the commercial Pt/C (onset potential: −15 mV, Tafel slope: 106 mV dec<sup>−1</sup>, and exchange current density: 0.80 mA cm<sup>−2</sup>), and also comparable with those measured by the most active electrocatalysts reported in the literature. The synergistic interaction of Gd and Nd is thought to be the major cause of the bimetallic catalyst’s activity.
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