Abstract
ZnO nanotetrapods have recently been exploited for the realization of high-sensitivity gas sensors, but they are affected by the typical drawbacks of metal-oxides, i.e., poor selectivity and a relatively high working temperature. On the other hand, it has been also demonstrated that the combined use of nanostructured metal oxides and organic molecules can improve the gas sensing performance sensitivity or selectivity, even at lower temperatures. A gas sensor device, based on films of interconnected ZnO nanotetrapods properly functionalized by titanyl phthalocyanine (TiOPc), has been realized in order to combine the high surface to volume ratio and structural stability of the crystalline ZnO nanostructures with the enhanced sensitivity of the semiconducting TiOPc molecule, especially at low temperature. The electronic properties of the resulting nanohybrid material are different from those of each single component. The response of the hybrid nanostructure towards different gases has been compared with that of ZnO nanotetrapod without functionalization in order to highlight the peculiar properties of the hybrid interaction(s). The dynamic response in time has been studied for different gases and temperatures; in particular, an increase in the response to NO2 has been observed, even at room temperature. The formation of localized p-n heterojunctions and the possibility of exchanging charge carriers at the hybrid interface is shown to be crucial for the sensing mechanism.
Highlights
Zinc oxide (ZnO) is one of the most used metal oxides for gas sensing applications, together with tin oxide (SnO2), titanium oxide (TiO2), tungsten oxide (WO3), etc
“evaporation” zone by an argon flow (100 sccm) to the “nucleation” region, where direct reaction with oxygen promotes the formation of a large amount of solid ZnO nuclei. These nuclei floating in the carrier gas towards the “growth” zone develop in the form of tetrapod-shaped nanocrystals thanks to the high supersaturation conditions up to when they reach the coldest part at the end of the furnace, where they deposit
0.2–2 μm are functionalized in a non-continuous way by TiOPc spots, leaving part of the ZnO surface directly accessible to the gases
Summary
Zinc oxide (ZnO) is one of the most used metal oxides for gas sensing applications, together with tin oxide (SnO2), titanium oxide (TiO2), tungsten oxide (WO3), etc. Nanotetrapod (TP) is one of its typical crystalline morphologies, consisting in four needle-shaped “legs” that are connected at one common end and arranged as axes of a tetrahedron, which forms highly porous films that have been recently exploited for the realization of high-sensitivity gas sensors [5,6]. These sensors are affected by some of the typical drawbacks of metal-oxides, i.e., a poor selectivity and a relatively high working temperature (often above 200 °C) [7]. The dynamic response vs. time, temperature and relative humidity (RH) have been studied and, in particular, a very high response towards NO2 has been observed, even at room temperature
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