Study of Polyoxometalates Based Films Formation with High Gas Sensitivity
Authors: Ivanova Olga , Krutovertsev Sergey , Krutovertseva Larisa 
Published: May 2020

Introduction

Inorganic oxide films activated with different additives are the unique sensitive layers for working at room temperature sensors. The sensor behavior of nanostructured activated silica films produced by sol-gel route was examined for environment control. Polyoxometalates (POMs) of Dawson type were used as additives in the silica matrix for gas sensitivity. POMs have good electronic conductivity and chemical stability. These compounds can act as reversible reagents in the reactions of ligand exchange and redox transformations, accompanied by changes in current carrier concentration or catalyze of such transformations that can be used in chemical sensors. Their redox properties can be varied within broad limits due to changing the heteropolyanion composition [1-4]. Porous silica is a unique matrix for chemical sensors, which has a lot of excellent physical and chemical properties [5].

Method

The sensor’s interdigital structure was made by microelectronic planar technology as shown in figure 1. The active area of the sensor was 4 x 4 mm. The thickness of the electrodes was 0,2 mm. The width of finger pairs of electrodes and distance between them were 20 μm. The method of hydrolythic polycondensation from tetraethyl orthosilicate solutions was used for forming complex silica films in this work. Films were deposited by a centrifugation from solutions based on tetraethyl orthosilicate and polyoxometalate. POMs with various compositions have been incorporated into the initial solution. The developed technology of multistep temperature treatment up to 413 K allowed film properties stabilizing.
The conditions of film formation varied during the tests: solution composition, temperature, and application modes. Morphology and infrared spectra of the layers were studied using Solver-P47 scanning probe microscope (NT-MDT, Russia) and spectrometer Bruker EQUINOX 55 (Germany). Standard gas mixtures (MGPZ, Russia) and Environics – 4000 gas-mixture generator (Environics, USA) were used to measure the properties of sensors. Electrical parameters of the sensors have been measured in real-time mode. Environment influence on sol preparation and films forming was investigated.

Results and Conclusions

Films had thickness of 200-300 nm and nanoscale pores. The metrological parameters of sensors depend mainly on different parameters: synthesis conditions and chemical content of sensitive film, temperature of the environment, annealing temperature, storage conditions and duration after heat treatment. Temperature treatment and decreasing of environment humidity lead to acceleration of hydrolythic polycondensation of solutions and stabilization of sensitive films at right conditions as demonstrated in figure 2. Introduction of heteropolyanion into film resulted in film’s mass stabilization. The temperature treatment don’t disturb POM molecule in the film. The polyoxometalates didn’t display their catalytic activity at weight content less than 10 % in the film. The increase in conductivity is observed due to an increase in the concentration of charge carriers on the surface of the film, donors of which are introduced into the film POM molecules. And gas sensitivity maximum decreases at weight content more than 50-60 % in matrix what is explained apparently by the formation of crystalline aggregates POM in the volume of the film. Besides the humidity adsorption on the film’s surface contribute to the sensor response. Adding of Mn3P2Mo18O62 into silica film resulted in high sensitivity to ethanolamine (up to 10 times) more than to ammonia as shown in figure 3. Obviously, this is explained by the reaction of ethanolamine complexation with Mn ions, among the possible valence states of which the strongest complex with ethanolamine molecules is formed by Mn3+. An equilibrium shift towards Mn3+ and a decrease in the redox potential of the Mn3+/Mn2+ pair leads to an intramolecular reduction reaction of the heteropolyanion. This interaction is quick (less than 30 s) and reversible at room temperature.

References

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Figure 1