Facile ultrasonic-assisted synthesis of SiO2/ZnO core/shell nanostructures: A selective ethanol sensor at low temperatures with enhanced recovery

Abstract

SiO2/ZnO core/shell and composite sensors with 22–97 wt% ZnO content were synthesized by a facile ultrasonic-assisted deposition-precipitation method. As for the core, uniform silica spherical particles were synthesized by a microemulsion technique. The prepared samples were characterized by XRD, EDS, SEM, DLS, and TEM analyses. The results confirm the formation of ZnO layers on silica core nanoparticles. Then, the sensors were applied for sensing 300 ppm of ethanol, carbon monoxide, toluene, trichloroethylene (TCE), or propane and 1% methane in air at 270–420 ˚C. Core/shell sensors containing more than 8 wt% silica showed a reduction in the maximum sensing temperature of at least 50 ˚C, compared to that of the pure ZnO sensor. At maximum sensing temperature, i.e., 270 °C, the selectivity (i.e., the ratio of sensor response to target to that of interfering gas) of core/shell sensors to ethanol, relative to the interfering gases, varied in the range of 1.8 × 103 to 3.24 × 105, compared to 61.1–5.4 × 103 for the pure ZnO sensor. This reveals a considerable enhancement in the selectivity of core/shell sensors towards ethanol. Compared to pure ZnO, the core/shell sensors containing more than 8 wt% silica show highly stable and reversible response-recovery properties with much lower drifts in the dynamic response to ethanol in 4 consecutive cycles. The best response-recovery property is for the SiO2/ZnO-74 wt% core/shell sensor with nearly no drift in response. The final steady response of the pure ZnO sensor, which happens after four consecutive cycles at 270 °C, is 50 times lower than the initial response. The effect of humidity on gas sensing responses is investigated for the pure ZnO and core/shell sensors. Under humid air, SiO2/ZnO sensors containing 74 wt% and 97 wt% ZnO have further shown increased durability of the responses, compared to the pure ZnO sensor. The finding here reveals the importance of silica addition as the core material to ZnO to increase the durability and reproducibility of the sensor under challenging operating conditions.