ZnO/carbon black nano-heterostructures for ultrafast ethanol detection with n to p-type switching at room temperature

Designing gas sensors functioning at room temperature with ultra-fast response and fast recovery is a great challenge. Metal oxide semiconductors (MOS) have been the primary choice in designing sensitive, and stable gas sensors due to their inherent physical and chemical characteristics. The performance of the MOS sensors has been enhanced by nanostructures and heterostructures formed between different MOS, MOS/metals, and MOS/carbon allotropes. We have studied n-type ZnO nano heterostructures formed with conductive carbon-black (C65). Pristine ZnO and ZnO/C65 heterostructure thin films are prepared by sol–gel method with varying C65. Films are deposited on the glass slides and n-type silicon substrates by drop cast method followed by annealing. The addition of C65 to ZnO altered the orientational growth and morphology of ZnO nanostructures as evidenced in X-ray diffraction and scanning electron microscope analyses. Moreover, ZnO/C65 heterostructures are studied for the detection of volatile organic compounds (VOCs), e.g. ethanol at room temperature (28 °C). It is observed that the resistance of the samples dropped down for pristine ZnO films on exposure to the reducing gas (i.e., ethanol), whereas for ZnO/C65 heterostructure films, the resistance increased. It indicates the change in the sensing mechanism from n-to p-type for ZnO/C65 heterostructures. Moreover, samples have exhibited ultrafast ethanol sensing response and recovery (2 s) at certain C65 contents. Observed results and enhanced performance are ascribed to the formation of nanostructures, large surface area, high oxygen vacancies, conducting carbon network and heterostructure mechanism. Switching of n- to p-type response was first noticed in Cr₂O₃ films upon exposure to ethanol vapors and later observed in other MOS. It is believed that, n to p-type switching is induced due to space charge effects. ZnO/C65 heterostructures have shown enhanced sensitivity, ultrafast response and recovery, besides interesting n to p-type switching behaviour.