Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

Abstract

In this work, the exceptionally improved sensing capability of highly porous 3-D hybrid ceramic networks with respect to reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on metal oxides Me= Fe, Cu, Al) doped and alloyed zinc oxide tetrapods (ZnO-T) forming numerous heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO tetrapods grown by flame transport synthesis (FTS) approach with different weight ratios (ZnO:Me, e.g., 20:1) and followed by subsequent thermal annealing them in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of the added metal oxide in 3-D ZnO-T hybrid networks.While pristine ZnO-T networks showed a good response to H2 gas, a change/tune in selectivity to ethanol vapour with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In case of hybridization with ZnAl2O4 an improvement of H2 gas response (to ≈ 7.5) was reached at lower doping concentrations (20:1), whereas the increasing in concentration of ZnAl2O4 (10:1), the selectivity changes to methane CH4 gas (response ≈ 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the sensitivity improvement is mainly associated with additional modulation of resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules supporting the experimentally observed results. The studied materials and sensor structures would provide particular advantages in the field of fundamental research, industrial and ecological applications.

ZnAl2O4‐Functionalized Zinc Oxide Microstructures for Highly Selective Hydrogen Gas Sensing Applications

“…The diameter of both structures (ZnO:Al MW and ZnAl 2 O 4 /ZnO MW) is ≈400 nm. In the case of the ZnO:Al MW a layered morphology is observed, which was demonstrated to be very efficient for the enhancement of the surface‐to‐volume ratio of MWs . This leads to a higher gas response due to a higher amount of adsorbed oxygen species .…”

“…The basic gas sensing mechanism of individual structures can be rationalized in terms of the existing models of ionosorption . At temperatures lower than 150–200 °C on the surface of metal oxides (the temperature range used for the study in this work) mainly molecular species of oxygen ( O 2 ), which capture a free electron at a surface state are adsorbed . In the case of individual structures this will lead to the formation of an electron depleted region at the surface, while the internal conduction channel will be narrowed.…”

“…However, pristine ZnO has a low selectivity to specific gaseous species such as hydrogen gas . Therefore, different methods to improve the selectivity were reported, such as doping or functionalization with Pd, the formation of different types of heterojunctions, etc. However, the surface functionalization of ZnO nano‐ and micro‐structures with oxide spinel compounds to enhance the sensing properties was less reported to date and needs further exploration.…”

“…Herein, we present a simple method for the growth of small and homogenously distributed ZnAl 2 O 4 crystals with a diameter of 50–100 nm on the surface of ZnO nano‐ and micro‐structures using aluminum acetate basic hydrate. In this work, the gas sensing properties such as selectivity to hydrogen gas and the operating temperature of this approach are discussed and compared to sensors based on ZnAl 2 O 4 polycrystalline clusters on ZnO tetrapod networks, reported in our previous works …”

“…The functionalization of ZnO‐T networks with ZnAl 2 O 4 demonstrated an improved H­­ 2 gas sensing response by a factor of 5 . However, the diameter of the ZnAl 2 O 4 polycrystalline clusters was in the micrometric range (up to 10 μm) which results in a low number of heterojunctions.…”