Plasma-Assisted Synthesis of MnO2-Based Nanoarchitectures As Gas Sensors for Safety and Food Quality Monitoring
Authors: Bigiani Lorenzo , Zappa Dario , Comini Elisabetta , Maccato Chiara , Gasparotto Alberto , Barreca Davide , Gri Filippo 
Published: May 2020

The efficient detection of chemical warfare agents (CWAs) for homeland defense and people safety has received a growing attention, due to their high toxicity and dangerous effects on human health even at low concentration levels. In this regard, sensing of acetonitrile, a simulant of cyanide CWAs known to be itself a poisonous gas at low levels in air, is of utmost importance to prevent health hazards. In recent years, efforts have also been devoted to monitor food quality and, in particular, fruit/vegetable ripening/degradation in horticultural industry, for which ethylene is an important marker. Indeed, this analyte is a colorless, odorless and flammable gas whose production by climacteric fruits/vegetables, even at concentrations as low as some ppm, is a tell-tale of their ripeness stage. As a consequence, its efficient monitoring in warehouses is of key importance to extend fruits/vegetables life cycle and suppress detrimental losses of these agricultural products.
In this context, the development of low-cost and reliable gas sensing materials capable of detecting the above gases in their early stages is a key issue to be still properly addressed. Among the various devices for acetonitrile and ethylene detection, chemoresistive gas sensors based on metal oxides with tailored morphology offer manifold advantages, including robustness, stability, low cost, and ease of operation. Among metal oxides, MnO2, an n-type semiconductor, exists in various polymorphs (α-, β-, γ-, δ- and ε-types) among which rutile-type β-MnO2 is the most stable one. This structural flexibility has stimulated efforts toward MnO2 use for various applications, among which volt-amperometric sensors. In this context, an interesting option to improve functional performances is the introduction of F as a dopant into the metal oxide matrix, as previously demonstrated for F-doped Co3O4 and Fe2O3 nanosystems [1,2]. On this basis, in this contribution [3] we report for the first time on acetonitrile and ethylene gas sensing performances of supported β-MnO2-based nanosystems synthesized by plasma enhanced-chemical vapor deposition (PE-CVD). In particular, the attention has been devoted to: (i) the modulation of the system morphology and defectivity, taking advantage of the versatility and inherent benefits offered by the above technique for low-temperature processing of inorganic nanomaterials; (ii) the use of Mn(tfa)2•TMEDA (Htfa = 1,1,1-trifluoro-2,4-pentanedione; TMEDA = N,N,N′,N′-tetramethylethylenediamine) as a single-source molecular precursor for both manganese and fluorine, allowing an in-site material doping during growth process.
After a thorough characterization of MnO2 systems grown at different oxygen partial pressures, the system sensing performances were tested in the detection of acetonitrile and ethylene gases, investigating the interplay between the system characteristics and gas sensing performances.
The results highlight favorable responses already at low operating temperatures with an enhanced detection efficiency toward the target analytes, that could provide a baseline data set for the development of MnO2 sensors for safety and food industry applications outperforming the actual ones.
Afterwards, our attention has been focused on a proof-of-concept on the possibility of enhancing performances in ethylene sensing by MnO2 systems basing on the nanoarchitectonics concept [4]. More specifically, we have exploited the system sensitization with noble metals (M = Ag, Au) to boost the resulting functional behavior thanks to the guest catalytic activity at the nanoscale and to the formation of metal-oxide interfaces, yielding an enhanced charge carrier separation. The target materials have been fabricated by an original two-step plasma-assisted route, consisting in the initial PE-CVD of MnO2 nanoarchitectures, followed by the introduction of silver and gold nanoaggregates by mild sputtering processes. Tailored low-dimensionality nanoarchitectures with controllable features and an even distribution of noble metal particles have been successfully prepared taking advantage of the unique versatility offered by plasma processing, resulting in material properties hardly attainable by means of conventional synthetic routes. Tests aimed at ethylene sensing have revealed enhanced responses and sensitivities at low temperatures, with performances directly dependent on the functionalizing guest agent, as discussed in the framework of a comparative material chemico-physical characterization.

References

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[4] L. Bigiani et al. J. Nanosci. Nanotechnol. 2020, 20, 3025