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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 ∼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 ∼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 VRHE 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. [ABSTRACT FROM AUTHOR]

Nanomaterials integrated surface acoustic wave (SAW) gas sensing technology has emerged as a promising candidate for realtime toxic gas sensing applications for environmental and human health safety. However, the development of novel chemical interface based on two-dimensional (2D) sensing materials for SAW sensors for the rapid and sensitive detection of NH3 gas at room temperature (RT) still remains challenging. Herein, we report a highly selective RT NH3 gas sensor based on sulfur-doped graphitic carbon nitride quantum dots (S@g-C3N4 QD) coated langasite (LGS) SAW sensor with enhanced sensitivity and recovery rate under ultraviolet (UV) illumination. Fascinatingly, the sensitivity of the S@g-C3N4 QD/LGS SAW sensor to NH3 (500 ppb) at RT is dramatically enhanced by ∼ 4.5-fold with a low detection limit (∼ 85 ppb), high selectivity, excellent reproducibility, and fast response/recovery time (70 s/79 s) under UV activation (365 nm) as compared to dark condition. Additionally, the proposed sensor exhibited augmented NH3 detection capability across the broad range of relative humidity (20%–80%). Such remarkable gas sensing performances of the as-prepared sensor to NH3 are attributed to the high surface area, enhanced functional groups, sulfur defects, UV photogenerated charge carriers, and facile charge transfer in the S@g-C3N4 QD sensing layer, which further helps to improve the gas molecules adsorption that causes the increase in conductivity, resulting in larger frequency responses. The gas sensing mechanism of S@g-C3N4 QD/LGS SAW sensor is ascribed to the enhanced electroacoustic effect, which is supported by the correlation of resistive type and COMSOL Multiphysics simulation studies. We envisage that the present work paves a promising strategy to develop the next generation 2D g-C3N4 based high responsive RT SAW gas sensors. [ABSTRACT FROM AUTHOR]