Sauna-like process prepared periodic molybdenum metal catalytic electrodes and their applications in water reduction

To verify that the molybdenum metals exhibit similar catalysis characteristics as the related molybdenum compounds, i.e. molybdenum selenide (MoSe2) and molybdenum sulfide (MoS2) which have been well known as the high-performing catalysts for hydrogen evolution reactions, we may thus seek a low-cost, process-simplified, scalable, and highly-catalytic counterpart. We have grown periodic molybdenum (Mo) metal catalytic electrodes by employing self-assembled polystyrene (PS) spheres prepared by a sauna-like method as templates, followed by a reactive ion etching (RIE) process with oxygen gas and a double-layer deposition by low-temperature magnetron sputtering. By controlling the etching time of oxygen gas on PS spheres during the RIE process, the lateral and vertical feature sizes of Mo catalytic electrodes can be efficiently controlled, thereby having various surface area ratios. According to surface morphologies from atomic force microscopy, electrochemical linear sweep voltammetry, Tafel, and impendency measurements, we have found that the surface roughness and surface area ratios of Mo metal catalytic electrodes can be enhanced by prolonging the etching times of PS spheres, thereby reducing the charge transfer resistances and Tafel slopes, and then improving the hydrogen evolution reactions at the catalysts/electrolyte interfaces. We attribute this improvement to the fact that the Mo metal catalytic electrodes can efficiently form beneficial Schottky junctions with the electrolyte to enhance the carrier transportation, and the increased surface area ratios can improve the effective area of the Schottky junctions, thereby enhancing the carrier transportation at the catalysts/electrolyte interfaces. Tafel slope of the periodic molybdenum (Mo) metal catalytic electrodes in our work is as low as about 53.9 mV/dec, equivalent to highly catalytic materials MoS2 (55 mV/dec) and MoSe2 (105-120 mV/dec). The proposed periodic Mo catalytic electrodes, which combine a simple sauna-like self-assembly process with a double-layer Mo architecture is scalable and simple; and the surface area of periodic molybdenum (Mo) metal catalytic electrodes can also be flexibly controlled, so that the low-temperature magnetron sputtered Mo metal catalytic electrodes are cost-effective and highly compatible with various photovoltaic devices, highlighting the great potential to form high efficient monolithic solar-water-splitting devices.