Sol-gel Synthesis of TiO2 With p-Type Response to Hydrogen Gas at Elevated Temperature

Titanium dioxide is considered as one of the potential candidates for high-temperature gas sensing applications due to its excellent sensitivity and stability. However, its practical use as a gas sensor under elevated conditions is limited on account of its selectivity and insufficient understanding of response conversion from n- to p-type. To this context, the present work is intended to prepare and understand the p-type response of anatase TiO2 toward H2 gas (20–1,000 ppm) at elevated temperature (500°C). Sol-gel route is adopted to facilely synthesize powders containing pure and chromium (1–10 at.%) doped TiO2 nanoparticles, which are then brushed onto substrates with already patterned inter-digitated platinum electrodes. In this work, even, the undoped TiO2 samples showed p-type gas sensing response, which then decreased with Cr doping. However, in comparison to previously reported work, the sensing characteristics of all sensors is improved. For instance, 5 at.% Cr-TiO2 showed high response (147), fast response and recovery (142/123s) time, and good selectivity to hydrogen against monoxide and methane. Despite better response values, the TiO2 based samples show instability and drift in baseline resistance; such issues were not observed for Cr-doped TiO2 samples (≥3 at.%). The powders were further analyzed by XRD, SEM, TEM, and XPS to understand the basic characteristics, p-type response and stability. Further, a plausible sensing mechanism is discussed on basis of results obtained from aforementioned techniques

Adsorption Kinetics of NO2 Gas on Pt/Cr-TiO2/Pt-Based Sensors.

Metal oxides are excellent candidates for the detection of various gases; however, the issues such as the limited operating temperature and selectivity are the most important ones requiring the comprehensive understanding of gas adsorption kinetics on the sensing layer surfaces. To this context, the present study focuses mainly on the fabrication of a Pt/Cr-TiO2/Pt type sensor structure that is highly suitable in reducing the operating temperature (from 400 to 200 °C), extending the lower limit NO2 gas concentration (below 10 ppm) with fast response (37 s) and recovery (24 s) times. This illustrates that the sensor performance is not only solely dependent on the nature of sensing material, but also, it is significantly enhanced by using such a new kind of electrode geometry. Moreover, Cr doping into TiO2 culminates in altering the sensor response from n- to p-type and thus contributes to sensor performance enhancement by detecting low NO2 concentrations selectively at reduced operating temperatures. In addition, the NO2 surface adsorption kinetics are studied by fitting the obtained sensor response curves with Elovich, inter-particle diffusion, and pseudo first-order and pseudo second-order adsorption models. It is found that a pseudo first-order reaction model describes the best NO2 adsorption kinetics toward 7–170 ppm NO2 gas at 200 °C. Finally, the sensing mechanism is discussed on the basis of the obtained results

Low temperature gas sensing with novel top-bottom electrode configuration

Low temperature gas sensing properties (24°-300°C) of TiO2 and TiO2:Cr thin films have been measured. Both layers (2 um thick) were sandwiched between 200 nm thick Pt top and bottom electrodes. The crystal structure, microstructure and chemical composition of the sensors were studied by X-ray diffraction technique (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. This novel electrode configuration showed promising gas sensing behaviour towards both reducing and oxidizing gases

Low Temperature Gas Sensing with Novel Top-bottom Electrode Configuration

Gas sensors based on undoped and Cr doped TiO2 layers with novel top and bottom electrode (TBE) configuration have been employed for NO2 and H2 gas sensing. The sensing layers of about 2 micro-meter thickness were sandwiched between 200 nm thick Pt top and bottom electrodes. These sensors with TBE configuration show promising gas sensing behaviour towards both oxidizing and reducing gases. The crystal structure, microstructure and chemical composition studies of the sensors were performed by X-ray diffraction technique (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. The comparative structural, morphological and compositional studies enable us to understand the effect of Cr3+ (0.755 Å) substitution within Ti4+ (0.745 Å) lattice on gas sensing properties (24°-200°C). The Cr doped TiO2 sensors were able to detect various concentrations of NO2 at 200 °C and H2 at room temperature. Response/recovery times for 90% change in sensor resistance are in the order of tens of seconds for NO2 at 200 °C and a few minutes for H2 at room temperature

Improved response and sensitivity with low power consumption gas sensors based on Pt/semiconducting oxide/Pt configuration

For instance the response of TBE-TiO2:Cr sensors towards 50 ppm NO2 at 200 °C was 212 with fast response/recovery time [1]. This was ten times higher than the sensitivity measured with interdigitated sensor configuration. Through the impedance equivalent circuit modelling, the contribution of sensor components and interfaces on sensing has been identified. It is found from the gas sensing tests of TBE based sensors with covered surface that the gas reaction beneath Pt-TE is dominating. This work aims at low temperature operation of H2 and NO2 sensors based on undoped and doped TiO2 layers with top-bottom electrode (TBE) configuration. For comparison purposes, the same sensing layers were also coated on classical interdigitated electrodes (IDEs)