Percolation transition in the gas-induced conductance of nanograin metal oxide films with defects

We use Monte-Carlo Simulations to study the conductance switching generated by gas-induced electron trapping/-releasing in films of sintered metal oxide nanoparticles by using a site-bond percolation model. We explore the possibilities of gas sensors based on these mechanisms. In our study, we model films of different thicknesses where the conductance values of the grains (sites) and of the contacts (bonds) between these grains depend on the surface density Nr of adsorbed gas molecules from the ambient atmosphere. Below a critical density Nr=Nr,c , the system is insulating due to the interruption of current flow, either through the connecting bonds or through the grain interior. This leads to two competing critical gas covering thresholds N(bond)r,c and N(site)r,c , respectively, that separate the insulating from the conducting phase. For N(site)r,c>N(bond)r,c , the characteristic curve of monodisperse sensors shows a noticeable jump from zero to a finite conductance at Nr=N(site)r,c , while for polydisperse sensors site percolation effects modify the jump into a steep increase of the characteristic curve and thus lead to an enhanced sensitivity. For N(site)r,c<N(bond)r,c , both mono- and polydisperse systems follow the same curves that show a smoother characteristic increase ∝(Nr−N(bond)r,c)2 which reveals that, despite the occurrence of an inherent bond percolation effect close to Nr,c , the increase of the bonds is the dominating effect

A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica

Capacitive sensors are the most commonly used devices for the detection of humidity because they are inexpensive and the detection mechanism is very specific for humidity. However, especially for industrial processes, there is a lack of dielectrics that are stable at high temperature (>200 °C) and under harsh conditions. We present a capacitive sensor based on mesoporous silica as the dielectric in a simple sensor design based on pressed silica pellets. Investigation of the structural stability of the porous silica under simulated operating conditions as well as the influence of the pellet production will be shown. Impedance measurements demonstrate the utility of the sensor at both low (90 °C) and high (up to 210 °C) operating temperatures

UV light-enhanced NO2 Sensing by Mesoporous In2O3: Interpretation of Results by a new Sensing Model

The light-enhanced NO2 sensing behavior of mesoporous In2O3 is measured and interpreted by means of a new sensing model. The model aims at explaining (i) the drop in electronic resistance of n-type semiconducting In2O3 under UV light exposure, (ii) the light-enhanced reaction to oxidizing gases, and (iii) the faster reaction and regeneration in mesoporous In2O3 as compared to non-porous material. Contrary to the conventional double Schottky model the dominating factor for the change in resistance is a change of oxygen vacancy donor states (0.18 eV below the conduction band) in the bulk phase due to photoreduction, instead of chemisorption. For the faster reaction and regeneration we propose an explanation based on enhanced oxygen diffusion in the In2O3 crystal lattice, specifically dominant in the mesoporous structure. The response of ordered mesoporous In2O3 to NO2 is stronger than in case of unstructured bulk material (with an average grain size of ca. 40 nm). The reaction is significantly accelerated by illuminating the samples with UV light. However, the response of the mesoporous material is weaker in the illuminated case

Mechanistic Model for UV light-enhanced NO2 Sensing utilizing Ordered Mesoporous In2O3

Results on light-enhanced NO2 sensing utilizing ordered mesoporous In2O3 are presented and interpreted by means of a new sensing model for ordered mesoporous indium oxide (In2O3). This model aims to explain the drop in electronic resistance of n-type semiconducting In2O3 under UV light exposure as well as the light-enhanced sensing properties to oxidizing gases. Compared to the conventional double Schottky model the dominating factor for the resistance change is a change of oxygen vacancy donor states in the bulk phase due to photoreduction. Comparison of conductivity measurements with varying oxygen partial pressure for ordered mesoporous and non-structured material shows an accumulative behavior in the case of the mesoporous material which can be related to faster photo reduction caused by the nanostructure. IR measurements reveal a donor level of 0.18 eV below the conduction band attributed to oxygen vacancies. The unique properties resulting from the structure allow low-temperature sensing of NO2; especially the recovery times are significantly shorter for the mesoporous material

Photoreduction of Mesoporous In2O3: Mechanistic Model and Utility in Gas Sensing

Abstract: A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In2O3) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In2O3 to oxidizing gases. Essential features of the model are photoreduction and a ratelimiting oxygen-diffusion step. Ordered, mesoporous In2O3 with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N2) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band

NO2 Sensors with Reduced Power Consumption Based on Mesoporous Indium Oxide

We report on sensing properties of ordered mesoporous nanostructures of In2O3 synthesized by nanocasting procedure towards NO2. The nanostructured material shows improved recover times and higher responses compared to non nanostructured material at low operating temperatures (100–150°C) thus allowing the use for low power NO2 sensors. These properties may be related to fast oxygen in and out propagation facilitated by an enhanced surface accessibility of the nanostructure