Adsorption-Mediated n-Type ZnO Surface Reconstruction for Optically Enhanced Volatile Organic Compound Sensing

Abstract

Specific volatile organic compounds (VOCs), abundant in indoor air, go unestimated due to the unavailability of specific detectors. Here, we have demonstrated n-type ZnO nanoforest thin films as highly sensitive toluene and 2-propanol sensors at reduced temperatures (∼150 °C) under optical irradiation. The sensors demonstrated a maximum response of 2273% for 2-propanol and 1579% for toluene at 190 ppm each, with the response and recovery times of 46 and 36 s for toluene and 42 and 37 s for 2-propanol, respectively, which were significantly high in comparison to those in the dark (312 and 438% for toluene and 2-propanol, respectively, at 190 ppm at 300 °C) with excellent reproducibility and device stability (tested for 70 days). The optical enhancement was due to a significant number of photogenerated carriers in the conduction band contributing to the sensing. The sensor was highly specific toward toluene and 2-propanol compared to other vapors. Principal component analysis was performed on the acquired transient responses to determine a clear specificity among the two overlapping VOCs. The device degraded under humidity, as explained by the competitive binding interaction of water with analytes. The marginally higher response of 2-propanol compared to toluene was due to a higher binding coefficient for 2-propanol. To demonstrate the sensing performance, an adsorption-mediated surface reconstruction mechanism was suggested, which showed a higher degree of dissociation for 2-propanol onto the ZnO(101) surface, thereby creating more dissociative adsorption states favorable for its adsorption (evident from the reduced adsorption energy). This resulted in enhanced sensing of 2-propanol over toluene, thereby supporting the experimental sensing variation. Thus, unlike its commercial competitors, the fabricated sensor can selectively detect individual VOCs.

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