Dependence of n-Type Metal-Oxide Gas Sensor Response on the Pressure of Oxygen and Reducing Gases.

The adsorption of oxygen and its reaction with target gases are the basis of the gas detection mechanism by using metal oxides. Here, we present a theoretical analysis of the sensor response, within the ionosorption model, for an n-type polycrystalline semiconductor. Our goal of our work is to reveal the mechanisms of gas sensing from a fundamental point of view. We revisit the existing models in which the sensor response presents a power-law behavior with a reducing gas partial pressure. Then, we show, based on the Wolkenstein theory of chemisorption, that the sensor response depends not only on the reducing gas partial pressure but also on the oxygen partial pressure. We also find that the obtained sensor response does not explicitly depend on the grain size, and if it does, it is exclusively through the rate constants related to the involved reactions.

Comment on "Understanding the Increasing Trend of Sensor Signal with Decreasing Oxygen Partial Pressure by a Sensing-Reaction Model Based on O2-Species".

In the original paper, Zhao and co-workers describe a sensing model to explain the response to a reducing gas as a function of the oxygen partial pressure. In this Comment we specifically analyze the basic gas detection mechanisms proposed in such a work and expressed through surface reactions (eqs 1-5). The consequence of these presented mechanisms lead to inconsistencies that are also regularly found in the literature.

Optimizing graphene production in ultrasonic devices.

In this work, we investigate the efficiency of graphene production from graphite by ultrasonic-induced exfoliation under different operational conditions. An ultrasonic bath and an ultrasonic horn tip were used, to show how the graphene production efficiency depends on the acoustic intensity and cavitation mechanism using graphite crystal with different initial sizes and characteristic lengths. Operational variables such as geometry, location and type of vessel containing the liquid phase, were also explored. All of them have effect on the cavitation process and directly influence the amount of graphene obtained. Remarkably, we found that the initial size of the graphite crystals has a marked impact in exfoliation process. Based on the above arguments we propose a combined protocol with a specific sequence to maximize the amount of graphene obtained.