High Performance of SnO2-Based Gas Sensor by Introducing Perovskite-Type Oxides
Authors: Shimanoe Kengo , Ma Nan , Oyama Tokiharu , Nishibori Maiko , Watanabe Ken 
Published: February 2020

SnO2-bassed gas sensors are widely used for various gases detection such as inflammable gases, toxic gases, bad smell and so on. For materials design of such semiconductor gas sensors, we have reported important three factors i.e. receptor function, transducer function and utility factor. Receptor function in connection with this presentation, concerns the ability of the oxide surface to interact with the target gas. If the sensor is made of a neat oxide, the surface oxygen, especially adsorbed oxygen, on the oxide acts as a receptor. It is well known that the sensors of this type are promoted in sensitivity as the constituent oxides are made smaller in size. This issue was solved completely by developing a theory on the receptor function of small sized oxides [1-3]. The small semiconductors are depleted of electrons in two steps with a progress of oxygen ionosorption (O- and O2-), resulting in the appearance of regional depletion and volume depletion in succession. In the stage of volume depletion, grain size effect is observed. In addition, when the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen. Especially Pd-loaded SnO2 nanoparticles with Pd size of 2.6 nm exhibited significantly high sensitivity to reducing gases [4, 5]. In the above receptor function, there is another method to enhance the sensitivity more. The electric resistance of sensor device is inversely proportional to the surface density of electrons. Therefore the sensor response enhances with increasing oxygen partial pressure. When the special feature of oxygen ionosorption by pulse heating is introduced, the gas sensing properties may be improved by increasing the adsorbed oxygen species on the SnO2 surface. In this presentation, we report the possibility of perovskite-type oxide as a material of oxygen feed.

SnO2 nano-particles were synthesized by hydrothermal method using SnCl4·5H2O and NH4HCO3 solution [4]. The obtained SnO2 sol was dried at 120 °C and calcined at 600 °C for 3 h in oxygen atmosphere. The particle size of SnO2 was about 14 nm by TEM image. 0.7 mol.% Pd was loaded on the SnO2 surface by impregnating Pd(NH3)2(NO2)2 solution into aqueous solution of SnO2, followed by ammonia solution treatment and filtration process, then dried at 120 °C and heat-treated at 500 °C in air. The particles size of Pd was about 2.6 nm by CO pulse adsorption method. Perovskite-type oxide, La0.1Sr0.9Co0.4Fe0.6O3-

δ

(LSCF), was synthesized by AMP method. The powder was grinded for 30 min, followed by further heating at 1050 °C for 5 h. Finally, the LSCF/Pd-SnO2 powders were prepared by directly mixing 15 wt.% LSCF with 0.7%Pd-SnO2. The MEMS device was used for the gas sensor device. The Pt electrodes and heaters were embedded on a silicon-based support membrane (area: 100 μm ×100 μm). To make MEMS type gas sensor, a uniform paste of was firstly prepared by dispersing LSCF/Pd-SnO2 powders into glycerin, and then it was deposited on the membrane by a micro injector. An MEMS sensor coated with Pd-SnO2 nanoparticles was also prepared and tested for comparison. The MEMS sensors were tested in a conventional gas flow apparatus at 250-400 °C by applying appropriate voltage to heaters in pulse mode. The gas sensing performance of MEMS sensors based on Pd-SnO2 and LSCF/Pd-SnO2 was investigated by using 200 ppm target gas such as H2, CO and C7H8. The sensor response (S = Ra/Rg) was defined as the ratio of electric resistance in synthetic air (Ra) and target gas (Rg).

The LSCF/Pd-SnO2 MEMS sensor showed fast response, high sensitivity (2 times or more) and stability to reducing gases (H2 and CO) and VOCs (C7H8) at at 250 °C, as compared with Pd-SnO2 MEMS sensor, by pulse heating with extremely low power consumption. It is considered that the excellent gas sensing properties of LSCF/Pd-SnO2 MEMS sensor are attributed to the effective oxgen transport from LSCF to Pd-SnO2 surface by pulse heating. Such pulse-driven MEMS sensor with high performance holds a great promise for practically application.

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