Designing Metallic MoO2 Nanostructures on Rigid Substrates for Electrochemical Water Activation.

Abstract

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.