Modification Of Zinc Cobaltate Materials And Their Formaldehyde Gas-sensitive Properties

Formaldehyde is also known as formalin and is a colourless,volatile,toxic gas with a strong sexual odour.Formaldehyde,a Group I carcinogen,is harmful to humans in the long term.This is why testing for formaldehyde is essential.Metal oxide semiconductor gas sensors are commonly used as gas-sensitive elements.The spinel-type metal oxide Zn Co₂O₄is a p-type semiconductor material with a spin-type structure.It has good electrochemical activity and conductivity and has a good future in gas sensors.In this paper,Zn Co₂O₄is used as the main object of study.The morphology of Zn Co₂O₄is modified by different fabrication methods and the composites are prepared with modifications to improve the gas-sensitive properties of the material.The main contents of the thesis include:In this paper,the metal oxide semiconductor Zn Co₂O₄is used as the main object of research and on this basis,peptides and precious metals are combined to synthesize composite materials to improve the gas-sensitive properties.The thesis is as follows1.Zn Co₂O₄is prepared by hydrothermal method.Because the preparation of Zn Co₂O₄requires a weakly alkaline environment,Zn Co₂O₄is prepared using urea and sodium citrate,respectively.The characterisation of the samples revealed that the two sets of samples have different morphologies.The Zn Co₂O₄material prepared using sodium citrate is smaller in size,reacts more fully with the gas and is more sensitive to formaldehyde.The Zn Co₂O₄gas sensitised with 1.25 mmol of sodium citrate is the most effective,with a response value of 9.32 to 20 ppm formaldehyde at 200°C,good selectivity and excellent linearity.2.We used sodium citrate to provide an alkaline environment for the Zn Co₂O₄preparation and explored further in the preparation method.The Zn Co₂O₄material is prepared using a hydrothermal method by adding peptides for modulation in the first experiment.The Zn Co₂O₄with 2 ml of peptide solution showed the best gas sensitivity to formaldehyde,with a gas response value of 11.6 to 20 ppm formaldehyde at 200°C.The improved sensitivity and reaction rate of this sample is mainly due to the peptide-rich moieties and binding sites that play a facilitating role in metal oxide synthesis.The second experiment involved further adjustment of the sodium citrate content to prepare a Zn O-Zn Co₂O₄composite.Zn O-Zn Co₂O₄has a response value of 17 for 20 ppm formaldehyde at 200°C.The improvement in the effectiveness of the gas-sensitive properties is associated with an increase in specific surface area and the formation of n-p heterojunctions.3.The precious metals（Au and Ag）are modified onto the surface of Zn Co₂O₄materials prepared with polypeptides using hydrothermal and precipitation methods.The noble metal doping not only reduces the activation energy required for the sensitive material to react with formaldehyde,but also speeds up the rate of electron migration and improves the gas response.The best results in the Au doping experiments are obtained with the addition of 0.25 wt%of the fraction,which have a response value of 14.2 to 20 ppm formaldehyde at 170°C.Ag doping is more effective than Au doping,the best being the addition of 3wt%of the component,which have a response value of 23.5 to 20 ppm formaldehyde at 175°C.