Enhanced NO₂ sensitivity of SnO₂ SAW gas sensors by facet engineering

Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to the inherent advantages of passive and wireless operation. This inevitably requires the sensor to improve gas sensing performances at room temperature. The facet engineering of metal oxide (MOS) provided an effective way to obtain MOS gas-sensitive materials with superior performance by the facet-dependent properties. Here, we developed a novel strategy to prepare SnO₂ quantum wires with different (110) facets ratio by adjusting the synthesis time. The as-prepared SnO₂ quantum wires were integrated into SAW devices to optimize the performance of the gas sensor. When the synthesis time was 8 h, the response of the SAW gas sensor was enhanced with the frequency change of 17 kHz, the response and recovery times of 45 s and 96 s, respectively. Our experimental results revealed that the effect of mass loading was the responsible underlying mechanism for the superior NO₂ gas sensing performances. In addition, the adsorption and charge transfer properties between the SnO₂ surface and NO₂ molecules were further discussed by QCM and resistance measurement, respectively. The calculation of density functional theory (DFT) also proves the adsorption energy of NO₂ on the SnO₂ (110) facet is larger than the (101) facet and (211) facet, indicating that the (110) facet is more beneficial to the adsorption of NO₂ molecules. This indicated that the facet engineering of MOS by facet-dependent properties to enhance gas sensitivity may open new opportunities for the design of SAW sensor.