Enhanced sensitivity of a GHz surface acoustic wave humidity sensor based on Ni(SO4)0.3(OH)1.4 nanobelts and NiO nanoparticles

Ni(SO4)(0.3)(OH)(1.4) nanobelts (NSOH NBs) and NiO nanoparticles (NPs) were used as the sensitive layers for a surface acoustic wave (SAW) humidity sensor. A high frequency SAW resonator up to 1.54 GHz was fabricated on a lithium niobate substrate as the sensing platform. The SAW sensors based on the NSOH NBs and the NiO NPs exhibited significantly decreased resonant frequencies with enhanced relative humidity (RH) from 11% to 85%, and the frequency response was found to be (sic)2.95 MHz and (sic)5.81 MHz, respectively. Both sensors showed rapid and reversible responses with excellent repeatability and long-term stability. The response and recovery times for NiO NPs were 23 s and 4 s, respectively. The NiO NP based sensor also showed negligible interference with various gases such as hydrogen and methane, demonstrating good cross-selectivity even while working in the GHz range. The high performance could be ascribed to the enhancement of the resonant frequency and the use of hydrophilic nanomaterials with a high surface-to-volume ratio.