Fabrication And Properties Of Light Assisted HoFeO₃ Formaldehyde Gas Sensors

Formaldehyde（HCHO）is a volatile organic compound（VOC）gas with highly toxic,which seriously endangers people’s lives and health.Therefore,it is of great significance to develop a high-performance formaldehyde gas sensor for indoor environmental monitoring.However,the high operating temperature required by traditional gas sensors is a great problem that needs to be solved urgently,and light-assisted gas sensor is a new strategy that can be used to replace heating.In this paper,taking Ho Fe O₃as the basic sensing material,the correlation mechanism between the light illumination and the gas-sensing properties of the material is studied through the light field adjustment means of different wavelengths and light intensities.The main work of this paper is divided into the following three parts:1. Ho Fe O₃nanoparticles are prepared by sol-gel method and the crystal structure,micromorphology and composition of the materials are analyzed by a series of characterizations.Ho Fe O₃nanoparticles are made into the side heating gas sensors and their gas sensing properties toward HCHO are tested in heating conditions.The result shows that the optimal working temperature of the Ho Fe O₃nanoparticles gas sensor for HCHO is 160℃,and the response value to 100 ppm HCHO is 160,moreover,the adsorption/desorption time are 114 s and 23 s,respectively.Additionally,the lowest detection limit is 1 ppm HCHO.However,Ho Fe O₃nanoparticles show poor selectivity for HCHO under 160℃.2. In order to overcome the defects of conventional gas sensors,such as low selectivity,weak detection limit and the need of heating,light-assisted Ho Fe O₃nanoparticles gas sensor is designed by introducing light fields with different wavelengths（λ=365-660 nm）.Through experimental comparison,it is found that with the assistance of red light at 660 nm,the Ho Fe O₃nanoparticles gas sensor has the best detection performance for HCHO.Compared with the test in dark and heating conditions,the lowest detection limit of HCHO can be as low as 80 ppb under 660 nm light illumination,which meets the international detection standard of HCHO,and it has good anti-interference ability to environmental humidity and good selectivity characteristics.The low detection limit and good anti-interference against humidity are ascribe to the increase of the activation energy on the surface of Ho Fe O₃nanoparticles and suppression of adsorbed H₂O molecules,which are provided by the 660 nm light illumination.This work provides an experimental and theoretical basis for using light field instead of heating to improve the gas sensing properties of gas sensors.3. A plane type light-assisted gas sensor based on Ho Fe O₃films are fabricated by spin-coating method.Comparing their gas sensing properties under different light intensities,it is found that the introduction of red light（630 nm）can expand the detection limit of the Ho Fe O₃thin film gas sensors,which from 100 ppm to 1 ppm.The red lights with different intensities have a great effect on the performance of the Ho Fe O₃thin film gas sensor,that is,it has the best sensitivity under 1 W light illuminaiton.The response speed of the Ho Fe O₃thin film gas sensor is 12 s,and the recovery speed is only 7 s under the illumination of red light with a power of 3 W.These phenomena are attributed to the increase of light intensity which will promote the adsorption and desorption of gases.This work provides experimental guidance for making a fast-response mini formaldehyde gas sensor.