Mauro Epifani

Solar light active photocatalyst was prepared as silver phosphate (Ag₃PO₄) coating on titania–silica (TiO₂–SiO₂) microspheres. Titania–silica microsphere was obtained by spray drying TiO₂–SiO₂ colloidal solutions, whereas Ag₃PO₄ was applied by wet impregnation. XRD on the granules and SEM analysis show that the silver phosphate particles cover the surface of the titania–silica microspheres, and UV-visible diffuse reflectance analysis highlights that Ag₃PO₄/TiO₂–SiO₂ composites can absorb the entire visible light spectrum. BET measurements show higher specific surface area of the composite samples compared to bare Ag₃PO₄. Photocatalytic activity was evaluated by dye degradation tests under solar light irradiation. The prepared catalysts follow a pseudo-first-order rate law for dye degradation tests under solar light irradiation. The composite catalysts with an Ag₃PO₄/TiO₂–SiO₂ ratio of 1:1.6 wt% show better catalytic activity towards both rhodamine B and methylene blue degradation and compared with the results with uncoated TiO₂–SiO₂ microspheres and the benchmark commercial TiO₂ (Evonik-P25) as a reference. The composite photocatalyst showed exceptional efficiency compared to its pristine counterparts and reference material. This is explained as having a higher surface area with optimum light absorption capacity.

Electronic nose (EN) is the most advanced technology for the classification and identification of complex gaseous mixtures. The idea is to use a small set of low-selective sensors instead of a huge number of specific ones. The response of such an array is a chemical image (CI) which is a mathematical (or graphic) representation of an analyte. The ability of an EN system to distinguish different mixtures is defined by its ability to produce a unique CI. The latter is defined and limited, first of all, by the sensor’s adsorption properties. We propose the approach to increase the versatility of low-selective sensor arrays by using virtual sensors. By “virtual sensor”, in this case we mean a sensor able to change its adsorption properties in conditions of illumination. Within this scenario, we were able to successfully distinguish between homologous alcohols (methyl, ethyl, and isopropyl) using organic–inorganic nanostructured sensitive layers based on ZnO nanoparticles and phthalocyanines (Pc). The performance of quartz crystal microbalance sensors with hybrid nanostructured organic–inorganic layers is increased in comparison with single-Pc ones. Using illumination allowed us to obtain additional responses which could be considered as virtual sensors. Both factors played the decisive role in the successful discrimination of the alcohols.

Tungsten trioxide (WO₃) is taking on an increasing level of importance as an active material for chemoresistive sensors. However, many different issues have to be considered when trying to understand the sensing properties of WO₃ in order to rationally design sensing devices. In this review, several key points are critically summarized. After a quick review of the sensing results, showing the most timely trends, the complex system of crystallographic WO₃ phase transitions is considered, with reference to the phases possibly involved in gas sensing. Appropriate attention is given to related investigations of first principles, since they have been shown to be a solid support for understanding the physical properties of crucially important systems. Then, the surface properties of WO₃ are considered from both an experimental and first principles point of view, with reference to the paramount importance of oxygen vacancies. Finally, the few investigations of the sensing mechanisms of WO₃ are discussed, showing a promising convergence between the proposed hypotheses and several experimental and theoretical studies presented in the previous sections.

The connection between heterogeneous catalysis and chemoresistive sensors is emerging more and more clearly, as concerns the well-known case of supported noble metals nanoparticles. On the other hand, it appears that a clear connection has not been set up yet for metal oxide catalysts. In particular, the catalytic properties of several different oxides hold the promise for specifically designed gas sensors in terms of selectivity towards given classes of analytes. In this review, several well-known metal oxide catalysts will be considered by first exposing solidly established catalytic properties that emerge from related literature perusal. On this basis, existing gas-sensing applications will be discussed and related, when possible, with the obtained catalysis results. Then, further potential sensing applications will be proposed based on the affinity of the catalytic pathways and possible sensing pathways. It will appear that dialogue with heterogeneous catalysis may help workers in chemoresistive sensors to design new systems and to gain remarkable insight into the existing sensing properties, in particular by applying the approaches and techniques typical of catalysis. However, several divergence points will appear between metal oxide catalysis and gas-sensing. Nevertheless, it will be pointed out how such divergences just push to a closer exchange between the two fields by using the catalysis knowledge as a toolbox for investigating the sensing mechanisms.

Semiconducting metal oxide based gas sensors are an attractive option for a wide array of applications. In particular sensors based on WO3 are promising for applications varying from indoor air quality to breath analysis. There is a great breadth of literature which examines how the sensing characteristics of WO3 can be tuned via changes in e.g. morphology or surface additives. Due to variations in measurement conditions, however, it is difficult to identify inherent qualities of WO3 from these reports. Here, the sensing behavior of five different WO3 samples is examined. The samples show good complementarity to SnO2 (the most commonly used material) based sensors. A surprising homogeneity, despite variation in morphology and preparation method is found. Using operando diffuse reflectance infrared Fourier transform spectroscopy, it is found that the oxygen vacancies are the dominant reaction partner of WO3 with the analyte gas. This surface chemistry is offered as an explanation for the homogeneity of the WO3 based sensors