Hydrogen nanosensors based on core/shell ZnO/Al2O3 and ZnO/ZnAl2O4 single nanowires

Abstract

With the increase in cost of natural gas as well as its environmental impact, an alternative energy source like hydrogen is a promising way to lower costs and saturate the growing demand for green energy. Unfortunately, leaks of hydrogen gas are difficult to detect because of its intrinsic properties, meaning that new solid-state portable devices that reliably detect hydrogen gas in short time are needed. In this study we report on the morphological, structural, chemical, and sensor properties of nanostructures and nanodevices subjected to hydrogen gas and other volatile compounds based on core/shell ZnO/Al₂O₃ and ZnO/ZnAl₂O₄ nanowires in dependence of annealing temperature and shell thickness. At an annealing temperature of 975 °C crystallization of the alumina shell forming the ternary ZnAl₂O₄ spinel-type phase was confirmed by TEM, HRTEM and XRD studies. The spinel phase provides high thermal, chemical and structural stability to the nanosensor. Core/shell ZnO/Al₂O₃ or ZnO/ZnAl₂O₄ nanowires were integrated into devices for gas sensing using a FIB/SEM system. Nanosensors based on single ZnO/ZnAl₂O₄ nanowire with a shell thickness of 5 nm showed the most promising results to the detection of hydrogen gas with an operating temperature down to room temperature, obtaining a response value of about 5 and a response value of ∼2411 at an operating temperature of 125 °C. The sensors maintained high response values and selectivity to H₂ at all investigated operating temperatures even after 2 years of storage.

The mechanism of hydrogen sensing of the core/shell ZnO/Al₂O₃ or ZnO/ZnAl₂O₄ nanowire-based sensors was proposed to be electron transport, which is controlled by the depletion region at the interface between the core and the shell. Devices based on ZnO/Al₂O₃ and ZnO/ZnAl₂O₄ nanowire show promising results for future hydrogen gas sensing applications in industrial or biomedical fields. Further optimization of hydrogen nanosensors, utilizing core/shell geometries fabricated using the methods and materials presented here is envisioned building on the insights gained.