Designing Metallic MoO₂ Nanostructures on Rigid Substrates for Electrochemical Water Activation

In situ growth of metallic MoO₂ films on fluorine-doped tin oxide (FTO) and MoO₂ powder in solution was achieved simultaneously by a simple hydrothermal process employing citric acid as the surfactant. The growth mechanism of MoO₂ nanostructures (NSs) at the heterogeneous interface and in homogeneous medium proceeds in a different manner in which seeds grow in a preferred orientation on FTO, whereas they propagate in all directions in solution. The high lattice matching of FTO and MoO₂ favours the film growth which could not be obtained on other conventional substrates. The disc morphology of MoO₂ nanostructures was changed to other diverse morphology by varying the synthesis conditions, particularly by the addition of nitric acid. A competitive effect of nitric acid and citric acid on the structure direction produced various shapes. The electrochemical water activation studies show that hydrogen-annealed MoO₂ is an excellent hydrogen evolution reaction (HER) catalyst with good stability. H-MoO₂ film/FTO displays a low onset overpotential of72 mV with a Tafel slope of 84.1 mV dec⁻¹, whereas the powder form exhibits an onset overpotential of 46 mV with a Tafel slope of 71.6 mV dec⁻¹. The large active surface area, exposure of fringe facets of (110) and the lesser electrochemical charge-transfer resistance offered by the hydrogen-annealed MoO₂ NSs play a major role in the enhanced HER activity.