Porogen induced formation of mesoporous zinc ferrite thin films and their chemiresistive properties
Introduction
Metal oxides are the most widely used materials in chemiresistive sensing. The chemiresistors change their resistance in response to chemical species bound to them [1]. Various thin and thick films of semiconducting metal oxides (SMOs) were reported as trustworthy candidates in chemiresistive sensing. The most common chemiresistive materials are ZnO [2], [3], Co3O4 [4], [5], In2O3 [6], TiO2 [7], SnO2 [8]. The chemiresistive performance of these materials is largely affected by composition, morphology, crystallite size. The porous surface morphology significantly enhances chemiresistive response [8] because the presence of pores on the surface of the material increases the specific surface area which can readily adsorb more analyte species. The change in resistance of the material depends on the chemical nature analyte. The reducing analyte reduces the resistance of n-type SMOs and adds the resistance in p-type SMOs [9], [10]. Exactly opposite phenomenon occurs in the case of the oxidizing analyte [11].
The spinel ferrite family of general chemical formula MFe2O4 (M: divalent metal viz. Cu, Ni, Zn, Co, Mn, etc.) also fall in the same category of chemiresistors. The candidate zinc iron oxide, commonly known as zinc ferrite (ZnFe2O4) shows chemiresistive response mainly for volatile organic gases (VOCs) including highly flammable acetone, methanol, ethanol, etc.[12], [13]. It is an n-type SMO having the indirect bandgap of nearly 1.9 eV [14]. A lot of work on the synthesis of this material has been reported by many research groups. This material can be synthesized using various methods like combustion [15], [16], sol-gel [17], coprecipitation [18], hydrothermal [19], spray pyrolysis [20] etc. Among these synthesis methods, the spray pyrolysis technique is a simple, cost-effective, and direct method to deposit thin film onto the substrate. In addition to these advantages, the thickness can also be controlled by deposition parameters.
The present work is focused to create in-situ pores on the surface of nanocrystalline ZnFe2O4 thin films. An inorganic salt; ammonium chloride was used as the porogen and it has been added in the mixture of nitrate precursors. The deposited films were checked for their chemiresistive properties to sense SO2 gas and liquid glucose. The use of zinc ferrite as a chemiresistive gas sensor [21] and electrochemical glucose sensor [22], [23] were earlier reported by some research groups; but to our best knowledge, this is the first time ZnFe2O4 was used as a chemiresistive glucose sensor.
Section snippets
Materials
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (98.5%) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) (99%) were obtained from Thomas Baker Chemicals, Mumbai (India). Ammonium chloride (NH4Cl) (99%) and conducting silver paste were obtained from Alfa aeser, Mumbai (India). All chemicals were used without further purification. Double distilled water was used to make solutions throughout the procedure. The calibrated test gas cylinders were purchased from Spacecryo Pvt. Ltd. Mumbai.
Deposition of zinc ferrite (ZnFe2O4) thin films
A stoichiometric
Reaction mechanism
The nitrate salts of zinc and iron dissociate in aqueous solution as:
Spraying the stoichiometric mixture on the hot substrate absorbs heat and forms precipitates of zinc hydroxide and iron hydroxide. The nitrate cations decompose as NOx gases.
Further, the hydroxides absorb heat and decompose to form the ZnFe2O4 layer on the substrate above 434 °C temperature [24], [25]
Conclusion
The nanocrystalline zinc ferrite thin films can be prepared by the spray pyrolysis technique. The FESEM images showed the added porogen made voids in intergranular sites. The XRD revealed the formation of nanocrystalline cubic spinel zinc ferrite. The absence of extra peaks in the XRD pattern of ZP series films confirmed the decomposition of the added porogen. The film ZP375 was found better in chemiresistive applications. It showed about 25% response to 100 ppm SO2 gas at optimized 150 °C
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
All authors are thankful to the University Grant Commission (UGC) for the DSA-SAP Phase-II programme and the Department of Science and Technology (DST), Government of India for the PURSE Phase-II programme through which research facilities were made available in our department.
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