Application of valence-variable transition-metal-oxide-based nanomaterials in electrochemical analysis: A review

The construction of materials with rapid electron transfer is considered an effective method for enhancing electrochemical activity in electroanalysis. It has been widely demonstrated that valence changes in transition metal ions can promote electron transfer and thus increase electrochemical activity. Recently, valence-variable transition metal oxides (TMOs) have shown popular application in electrochemical analysis by using their abundant valence state changes to accelerate electron transfer during electrochemical detection. In this review, we summarize recent research advances in valence changes of TMOs and their application in electrochemical analysis. This includes the definition and mechanism of valence change, the association of valence changes with electronic structure, and their applications in electrochemical detection, along with the use of density functional theory (DFT) to simulate the process of electron transfer during valence changes. Finally, the challenges and opportunities for developing and applying valence changes in electrochemical analysis are also identified.

Morphological Effects in SnO2 Chemiresistors for Ethanol Detection: A Review in Terms of Central Performances and Outliers

SnO₂ is one of the most studied materials in gas sensing and is often used as a benchmark for other metal oxide-based gas sensors. To optimize its structural and functional features, the fine tuning of the morphology in nanoparticles, nanowires, nanosheets and their eventual hierarchical organization has become an active field of research. In this paper, the different SnO₂ morphologies reported in literature in the last five years are systematically compared in terms of response amplitude through a statistical approach. To have a dataset as homogeneous as possible, which is necessary for a reliable comparison, the analysis is carried out on sensors based on pure SnO₂, focusing on ethanol detection in a dry air background as case study. Concerning the central performances of each morphology, results indicate that none clearly outperform the others, while a few individual materials emerge as remarkable outliers with respect to the whole dataset. The observed central performances and outliers may represent a suitable reference for future research activities in the field.

Preparation of porous Co3O4 and its response to ethanol with low energy consumption

Co₃O₄ is a promising p-type semiconductor for ethanol detection. In this work, ethanol detection sensors were fabricated with nanostructured Co₃O₄, which exhibited higher selectivity and lower operating temperature. The Co₃O₄ was synthesised using ZIF-67 as a sacrificial precursor. The T400-Co₃O₄ that was obtained by calcining ZIF-67 at 400 °C showed the best sensing performance. Its response to 100 ppm ethanol vapor was 221.99 at a low optimal operating temperature (200 °C). Moreover, T400-Co₃O₄ achieved a low detection limit (1 ppm), remarkable repeatability, and higher selectivity compared to ammonia, carbon monoxide, acetone, hydrogen, methane, methanol, and nitrogen dioxide. The enhanced sensing performance was mainly attributed to three factors: (1) the adsorption/desorption of active adsorbed oxygen molecules (e.g. O^- and O^2-) and abundant oxygen vacancies, which increased the number of active sites; (2) the catalytic activity of Co³⁺, which greatly increased the reaction route and decreased the activation energy; and (3) the effective diffusion of gas molecules, which increased the effect of collisions between gas molecules and the material surface. This work provides an effective means to fabricate sensitive ethanol gas sensors with low energy consumption.