ZnO Nanostructures for Gas Sensing Applications: From Tetrapods-Based Chemoresistive Devices to Carbon Fiber Integration

Zinc oxide (ZnO) nanostructures can be grown in different morphologies by means of a wide range of techniques. Although the strong evidence of their gas sensing capabilities has been reported in several papers, not all of them are suitable for a large-scale or industrial scale production of gas sensor devices. Among the several ZnO nanostructures that have been grown so far at IMEM-CNR Institute, we focused our attention on tetrapods (TPs), because a vapour phase growth process that can produce grams of these nanostructures have been designed and optimized in our laboratories. This quantity of nanostructures, produced in a small lab-scale reactor, is enough to prepare several thousands of gas sensing devices. Moreover, the produced ZnO-TPs are free-standing and not constrained to a growth substrate, and can be easily suspended in a liquid media. The highly-porous entangled network of ZnO-TPs, which can be obtained by direct deposition on sensor substrate, demonstrated to efficiently work as chemoresistor, while its sensing properties (sensitivity, selectivity, ...) can be tuned or modified through the functionalization of TPs surface with other materials (noble metals, inorganic semiconductor nanoparticles, organic semiconductor layers). At the same time, ZnO nanostructures and their multifunctional properties can be used to functionalize the surface of other materials and, eventually, add to them sensing capabilities. As a meaningful example, we can illustrate the adding of ZnO nanorods (ZnO-NRs) to the surface of carbon fibers with a wet chemical process that is easily scalable to a larger scale. It has been recently demonstrated that two crossing ZnO-functionalized carbon fibers can be used to sense and transduce a piezoelectric signal or UV light, as well as a chemoresistive information.