Oxygen-Vacancy-Rich HfO_2-x Nanoparticles Supported on Reduced Graphene Oxide for Electrocatalytic Hydrogen Evolution Reactions

Designing nanostructured materials with modified surface structures, which exhibit interesting properties and applications, is of great importance. The ability to modify the physical and chemical properties of hafnium oxide not only alters the optical properties but also makes it a suitable candidate for electrocatalytic systems. In this work, we report a simple, cost-effective method to fabricate oxygen-vacancy-enriched HfO2-x supported reduced graphene oxide (HfO2-x/rGO) using the microwave-assisted method and investigate their optical and electrocatalytic properties. The synthesized HfO2-x/rGO with particle size down to similar to 2.2 nm exhibits a characteristic photoluminescence peak at 434 nm (2.85 eV) indicative of surface defects related to oxygen vacancies as corroborated by electron-spin resonance and X-ray photoelectron spectroscopy that reveals similar to 61% of HfO2-x are in the reduced state of Hf3+. Owing to its high surface defects related to oxygen vacancies and improved conductivity, its electrocatalytic applications toward hydrogen evolution reaction in acidic media exhibited a low overpotential of -0.32 V-RHE at 10 mA cm(-2) compared to their bulk and fully oxidized counterparts. This simple experimental strategy for designing highly surface defect-rich transition metal oxides to make them electrocatalytically active for energy applications is indeed interesting.