Noble Metal/Mn3O4 Nanocomposite Systems:from Design to Applications As Ultra-Sensitive Gas Sensors for Nitrogen Mustard Simulant

Abstract

The efficient detection of chemical warfare agents (CWAs) is of paramount importance in the development of reliable sensing devices for safety applications. In particular, the increased threats of chemical attacks by terrorist organizations have stimulated a significant interest in the early detection of CWAs. Due to the high toxicity and necessity of special infrastructures for storage/manipulation of CWAs, the activities aimed at their detection are carried out on mimicking compounds compatible with lab test security level. In this context, the recognition of di(propylene glycol) monomethyl ether (DPGME), a simulant of the vesicant nitrogen mustard, is of key importance. Among the various devices adopted for CWA sensing, chemoresistive gas sensors based on nanostructured metal oxides offer various advantages, encompassing low cost, limited power consumption, and good stability/sensitivity. To date, DPGME detection has been performed only in a few cases by the use of SnO2-based systems, and the development of highly efficient sensors for its selective monitoring is of primary importance for next-generation devices.Among metal oxides, p-type Mn3O4, a mixed-valence and environmentally friendly multifunctional system, is an attractive candidate for CWAs detection thanks to the tunable Mn redox chemistry and the inherent catalytic properties. On the other hand, gas sensors based on pure p-type materials like Mn3O4 typically feature responses lower than n-type ones, as well as modest sensitivity/selectivity. To circumvent these drawbacks, a viable alternative is the functionalization with noble metals particles, like Ag and Au, that can act as catalytic promoters of the involved chemical reactions at the nanoscale and/or favor the formation of metal/oxide Schottky junctions, improving charge carrier separation.Based on preliminary results on the development of pure Mn3O4 nanomaterials as gas sensors [1,2], we have reported for the first time on the gas-sensing performances of nanocomposite systems based on Mn3O4-M (M = Ag, Au) in DPGME detection [3]. The target materials were obtained by the initial chemical vapor deposition (CVD) of Mn3O4 on polycrystalline Al2O3 substrates, followed by silver or gold radio frequency (RF) sputtering, with the aim of obtaining a high dispersion of metal nanoparticles.The results yielded by a multi-technique characterization highlighted the formation of phase-pure Mn3O4 nanostructures eventually functionalized with Ag or Au and an intimate contact between metal oxide matrix and nanoparticles. These features enabled us to enhance the system sensitivity in the detection not only of standard volatile organic compounds (acetone and ethanol), but also of DPGME, down to the ppb scale. Moreover, the possibility of discriminating between DPGME, on one side, and acetonitrile (CH3CN) or dimethyl methyl phosphonate (DMMP), simulants of HCN (blood agent gas) and Sarin nerve gas, respectively, demonstrates the system selectivity toward the target warfare gas simulant. To shed light on the origin of enhanced selectivity due to functionalization, density functional theory (DFT) based calculations were performed. The results, providing a molecular insight into the interactions between the target systems and DPGME, enabled us to elucidate a dual-site activation mechanism based on the binding of DPGME to both manganese oxide and gold nanoparticles, accounting for the higher functional performances registered for Au-Mn3O4 sensors.As a whole, this study represents an important step forward in the fabrication of highly efficient sensors based on dual metal oxide-metal NP composites. The obtained results pave the way to mastering next-generation sensing devices for real world applications, which will be pursued by additional investigation of the system long-term stability and humidity influence on the resulting functional performances.