Enhanced VOCs Detection By the Co3O4/ZnO Nanocomposites, Obtained By Single Step Flame Spray Pyrolysis

Abstract

Introduction The detection of volatile organic compounds (VOCs), such as acetone, ethanol or formaldehyde is a relevant practical task [1]. Enhanced sensitivity towards such gases has been reported periously for ZnO based nanomaterials, modified with Co3O4 phase [1-3]. However, the reported techniques of such nanocomposites fabrication are usually involve several stages, are time and labor consuming and barely scalable [1-5].In this work we demonstrate facile single step synthesis and improved sensitivity towards wide range of gases in dry and humid air conditions of ZnO-Co3O4 nanocomposites. Long term stability of obtained nanoparticle materials gas sensor response is demonstrated. Materials synthesis, characterization and sensor measurements A series of ZnO based materials were obtained by flame spray pyrolysis technique, described elsewhere [6]. Zn (II) and Co (II) 2-ethylhexanoates were taken as a precursors, which were diluted in toluene in 1:4 ratio by volume. Toluene also served as a fuel. Co (II) 2-ethylhexanoate has been taken in a calculated amount to form 1, 5 and 20% mol Co containing nanocomposite based on ZnO. Mixture of precursors and fuel was fed with 3 ml/min pace to a spray nozzle, where it was dispersed with 3 ml/min oxygen flow at a pressure drop of 3 bar. The nano-powders, forming during synthesis, were collected on a glass fiber filter 75 cm above the nozzle with the use of vacuum pump. During separate synthetic procedure the obtained materials were directly deposited on ceramic alumina plates, fixed at 19 cm above the nozzle on water-cooled surface. Plates were equipped with platinum contacts on one side and platinum heating element on the other, used by us in the previous work [6]. Micro-hotplates with the deposited sensitive element were welded in TO-8 case and used in further gas sensor measurements as is. One sample of alumina plate with deposit for each material were passed through 24h annealing at 500 oC and then used in sensor measurements alongside the sensors with as prepared sensitive films. Powders of synthesized materials were investigated by XRD, BET, TEM with EDX mapping, EPR techniques. Real chemical composition of obtained samples was investigated by ICP-MS method. The interaction of obtained nanoparticulate matter with VOC molecules was studied by in situ DRIFTs and Raman spectroscopy, as well as thermo-programmed desorption, coupled with mass-spectrometry. Results and Conclusions Increase in Co content leads to increase in sensor response towards VOCs, particularly oxygen-containing acetone and methanol (Fig. 1). At the same time this effect is observed only for the material with lowest Co content in case of hydrogen detection. Moreover, the sensor response towards methanol and propane decreases once sensor working temperature exceeds 400 oC in case of nanocomposite with highest Co content. TEM micrograph with EDX mapping shows uniform distribution of Co over ZnO matrix. XRD analysis indicates formation of separate Co3O4 phase in case of material with 20 % mol Co content, while EPR results give a cue for formation of both Co (II) substitutional defect in ZnO matrix as well as Co3O4 phase formation. This allows to conclude that introduction of Co in ZnO based nanocomposite has both electrical and chemical beneficial effects on metal oxide gas sensor performance, especially towards oxygen-containing VOCs. Thermal annealing at 500 oC prior to gas sensor measurements leads to response decrease of thus prepared sensitive layers, however, the resulting response is preserved over long-term operation even in high humidity conditions. Gas sensor, fabricated on the basis of sensitive layers without pre-annealing step, demonstrate slow drift of response value during long-term operation.