In the domain of gas sensing, metal oxide nanostructures have been demonstrated to have very attractive properties due to their large surface-over-volume ratio, combined with the possibility to use multiple materials and multi-functional properties. Here, we review the basic physical principles underlying the transducer function of metal oxide nanostructures, from single nanostructures to nanostructure networks. These principles have been adapted to describe the response of more complex nanostructures, such as heterostructures, combining two different metal oxide materials, or a metal with a metal oxide, in order to further enhance the sensitivity and selectivity of such devices. We finally present the activation of nanostructures by light exposure as a promising alternative to the standard method based on high temperature activation, which is earning increasing consensus in the perspective of low-power Internet of Things applications. Graphical abstract

Cluster-assembled nanostructured films of refractory metal oxides were produced by supersonic cluster beam deposition (SCBD) using a pulsed microplasma cluster source (PMCS). The growth of nanostructured films takes place at room temperature on substrates exposed to nanoparticles beam. Soft landing and limited diffusion are peculiar characteristics of the deposition process, causing the film to grow according to a highly porous structure. We produced nanostructured Mo, W, and Nb oxide films and we characterized their crystal structure and film morphology at the nanoscale together with their gas sensing performances. The effect of thermal treatments on grain growth and morphology modification was particularly addressed in view of the use of the nanostructured layers in gas sensing applications on microfabricated platforms.

Combinatorial approach has been widely recognized as a powerful strategy to develop new-higher performance materials and shed the light on the stoichiometry-dependent properties of known systems. Herein, we take advantage of the unique features of chemical beam vapor deposition to fabricate compositionally graded Na_1+xTaO_3±δ thin films with −0.6 < x < 0.5. Such a varied composition was enabled by the ability of the employed technique to deliver and combine an extensive range of precursors flows over the same deposition area. The film growth occurred in a complex process, where precursor absolute flows, flow ratios, and substrate temperature played a role. The deviation of the measured Na/Ta ratios from those predicted by flow simulations suggests that a chemical-reaction limited regime underlies the growth mechanism and highlights the importance of the Ta precursor in assisting the decomposition of the Na one. The crystallinity was observed to be strongly dependent on its stoichiometry. High under-stoichiometries (e.g., Na_0.5TaO_3−δ) compared to NaTaO₃ were detrimental for the formation of a perovskite framework, owing to the excessive amount of sodium vacancies and oxygen vacancies. Conversely, a well-crystallized orthorhombic perovskite structure peculiar of NaTaO₃ was observed from mildly under-stoichiometric (e.g., Na_0.9TaO_3−δ) to highly over-stoichiometric (e.g., Na_1.5TaO_3+δ) compositions.