ZnO@ZIF-8: Gas sensitive core-shell hetero-structures show reduced cross-sensitivity to humidity

Abstract

A 'lawn-like' distribution of interconnected zinc oxide nanorods, coated with a metalorganic compound based on zeolitic imidazolate frameworks-ZIF-8 was prepared on microstructured thin-film interdigitated Pt-electrodes forming ZnO@ZIF-8 core-shell heterostructures and investigated as gas sensor material in relation to the identical, but uncovered pure ZnO-layer. This composite combines the gas sensing properties of the metal oxide ZnO with the specific properties of the metal organic framework material which result in a distinct change of the conditions of gas sensing at the ZnO/ZIF-8-interface. Herein, for the first time it is reported that as prepared ZnO@ZIF-8 composite material is an attractive choice to reduce the cross-sensitivity to water vapour (humidity) in the gas sensing response towards propene and ethene. The observed change of sensitivity in relation to uncovered ZnO is discussed to be due to (i) the specific interaction of the ZIF-8 at the interface with the ZnO taking influence on the gas reaction processes, (ii) the diffusivity of ZIF-8 for the different gas components, and (iii) the sorption behaviour of the used gases at the ZnO interface and inside the ZIF-8 material.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

“…Additionally, when exposed to a reduction or oxidation, the target gas adsorbs on the surface of the SMOx and reacts with oxygen on the charged surface and the conductivity of the SMOx is changed by varying the carrier concentration in the space charge layer. The response is reflected according to the change in conductivity before and after the exposure of the target gas [78].…”

“…(iii) Although MOX gas sensors are sensitive to some inorganic gases and volatile organic compounds (VOCs), water vapor is usually present in the air and is one of the main combustion products of combustible gases. Water molecules will interact with oxygen adsorbed on the surface of the oxide by forming hydroxyl groups, which due to the sensing principle, will change the carrier concentration and thus the conductivity as well as creating cross-sensitivity [78]. (iv) Since low temperatures are not sufficient to overcome thermal barriers and trigger gas induction reactions, the operating temperatures required are generally high (200-600 • C), indicating that high energy and high power are required for practical life production applications [14].…”

“…Remaining challenges include the ability to further enhance selectivity for molecules with a similar sensing activity and dealing with interference with water vapor. This has inspired the idea of improving gas sensing performance by developing composite materials, i.e., using gas-sensitive metal oxides combined with other materials to improve the limitations of a single material [78].…”

“…In fact, the diffusion of water in ZIF materials cannot be completely inhibited. Only the absorption of water is negligible compared to the absorption of other gases [78]. This advantage of ZIF can be used to improve the resistance of SMOx to humidity.…”

Stability of Monolithic MOF Thin Films in Acidic and Alkaline Aqueous Media

“…MOFs show a large variety of topologies, window apertures, pore sizes, and functionalities. Given such versatility, the application of MOFs has been explored in multiple fields including gas storage, gas- and liquid-phase separation, sensor technologies, electrochemistry, among others [ 5 , 6 , 7 , 8 , 9 ]. Likewise, the study and production of high-quality thin MOF films supported on flat substrates has been evolving quickly and continues to be an important point of research with great areas of opportunity [ 10 , 11 , 12 ].…”