Toward the Detection of Poisonous Chemicals and Warfare Agents by Functional Mn₃O₄ Nanosystems

The detection of poisonous chemicals and warfare agents, such as acetonitrile and dimethyl methylphosphonate, is of utmost importance for environmental/health protection and public security. In this regard, supported Mn₃O₄ nanosystems were fabricated by vapor deposition on Al₂O₃ substrates, and their structure/morphology were characterized as a function of the used growth atmosphere (dry vs. wet O₂). Thanks to the high surface and peculiar nano-organization, the target systems displayed attractive functional properties, unprecedented for similar p-type systems, in the detection of the above chemical species. Their good responses, selectivity, and sensitivity pave the way to the fabrication of low-cost and secure sensors for different harmful analytes

Tailoring Vapor-Phase Fabrication of Mn₃O₄ Nanosystems: From Synthesis to Gas-Sensing Applications

Supported p-type α-Mn₃O₄ nanosystems were fabricated by means of chemical vapor deposition (CVD) on polycrystalline alumina substrates at temperatures of 400 and 500 °C, using Mn­(hfa)₂·TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine) as precursor compound. The structure, chemical composition, and morphology of the obtained deposits were characterized in detail, devoting particular attention to the influence of the used reaction atmosphere (dry O₂ vs O₂ + H₂O) on the system characteristics. For the first time, the gas-sensing performances of the obtained CVD Mn₃O₄ nanomaterials were investigated toward ethanol and acetone vapors, with concentrations ranging from 10 to 50 and from 25 to 100 ppm, respectively. The developed systems showed the best activity ever reported in the literature for Mn₃O₄ chemoresistive sensors in the detection of the target gases, a result that, along with their low detection limits and good selectivity, is an appealing starting point for eventual technological applications

Tin Oxide Nanowires Decorated with Ag Nanoparticles for Visible Light-Enhanced Hydrogen Sensing at Room Temperature: Bridging Conductometric Gas Sensing and Plasmon-Driven Catalysis

We demonstrate that conductometric gas sensing at room temperature with SnO₂ nanowires (NWs) is enhanced by visible and supraband gap UV irradiation when and only when the metal oxide NWs are decorated with Ag nanoparticles (NPs) (<i>diameter</i> < 20 nm); no enhancement is observed for the bare SnO₂ case. We combine the spectroscopic techniques with conductometric gas sensing to study the wavelength dependency of the sensors’ response, showing a strict correlation between the Ag-loaded SnO₂ optical absorption and its gas response as a function of irradiation wavelength. Our results lead to the hypothesis that the enhanced gas response under UV–vis light is the effect of plasmonic hot electrons populating the Ag NPs surface. Finally, we discuss the chemiresistive properties of Ag-loaded SnO₂ sensor in parallel with the theory of plasmon-driven catalysis, to propose an interpretative framework that is coherent with the established paradigma of these two separated fields of study

Au/ε-Fe₂O₃ Nanocomposites as Selective NO₂ Gas Sensors

A combined chemical vapor deposition (CVD)/radio frequency (rf) sputtering approach to Au/Fe₂O₃ nanocomposites based on the scarcely investigated ε-iron­(III) oxide polymorph is reported. The developed materials, analyzed by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS), consisted of iron oxide nanorods decorated by gold nanoparticles (NPs), whose content and distribution could be tailored as a function of sputtering time. Interestingly, the intimate Au/ε-Fe₂O₃ interfacial contact along with iron oxide one-dimensional (1D) morphology resulted in promising performances for the selective detection of gaseous NO₂ at moderate working temperatures. At variance with the other iron­(III) oxide polymorphs (α-, β-, and γ-Fe₂O₃), that display an <i>n</i>-type semiconducting behavior, ε-Fe₂O₃ exhibited a <i>p</i>-type response, clearly enhanced by Au introduction. As a whole, the obtained results indicate that the sensitization of <i>p</i>-type materials with metal NPs could be a valuable tool for the fabrication of advanced sensing devices

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO₃ Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO₃ quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV–vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO₃, surface covered by unstable W­(V) species, slowly oxidized upon standing in room conditions. The WO₃ nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics

Surface Modification of TiO₂ Nanocrystals by WO_<i>x</i> Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface Chemistry

TiO₂ anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (<i>R</i>_w) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For <i>R</i>_0.16, the as-prepared materials were constituted by an anatase core surface-modified by WO_<i>x</i> monolayers. This structure persisted up to 500 °C, without any WO₃ phase segregation. For <i>R</i>_w up to <i>R</i>_0.64, the anatase core was initially wrapped by an amorphous WO_<i>x</i> gel. Upon heat treatment, the WO_<i>x</i> phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO₃ nanocrystals dispersed into the TiO₂ host after heating at 500 °C, when part of tungsten also migrated into the TiO₂ structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO₂ and WO₃ analogously prepared. They showed that even the simple surface modification of the TiO₂ host resulted in a 3 orders of magnitude response improvement with respect to pure TiO₂

Colloidal Counterpart of the TiO₂‑Supported V₂O₅ System: A Case Study of Oxide-on-Oxide Deposition by Wet Chemical Techniques. Synthesis, Vanadium Speciation, and Gas-Sensing Enhancement

TiO₂ anatase nanocrystals were surface modified by deposition of V­(V) species. The starting amorphous TiO₂ nanoparticles were prepared by hydrolytic processing of TiCl₄-derived solutions. A V-containing solution, prepared from methanolysis of VCl₄, was added to the TiO₂ suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state ⁵¹V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti–O–V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V–O–V network. After heating at 500 °C, V₂O₅ phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of about 5 nm, proving the successful synthesis of the colloidal counterpart of the well-known TiO₂–V₂O₅ catalytic system. Hence, and also due to the complete elimination of organic residuals, this sample was used for processing chemoresistive devices. Ethanol was used as a test gas, and the results showed the beneficial effect of the V surface modification of anatase, with a response improvement up to almost 2 orders of magnitude with respect to pure TiO₂. Moreover, simple comparison of the temperature dependence of the response clearly evidenced the catalytic effect of V addition