Gas Sensing Properties Of Transition Metal Oxides ZnO And NiFe₂O₄

The current environmental pollution problem is becoming more and more serious.Some of the gases produced therein seriously threaten people's health.Formaldehyde and acetone are two representative gases.Sensors used to detect gases have attracted widespread attention.This topic is based on this background.This article specifically includes two aspects:gas sensitivity calculation of transition metal oxide ZnO?first-principles calculations?,transition metal oxide NiFe₂O₄ gas sensitivity of trace acetone research.In the former,we used the CASTEP software based on density functional theory to calculate the adsorption of formaldehyde on the surface of transition metal oxide zinc oxide,fully considering the oxygen and humidity conditions.According to the calculation results,the best adsorption site of HCHO on ZnO?001?surface under vacuum conditions is vacancy of ZnO?001?.The population analysis showed that formaldehyde molecules existed as a donor;In the presence of oxygen,the adsorption energy of formaldehyde on ZnO?001?surface was smaller than that without oxygen,and the net charge transfer amount increased.There was competition between formaldehyde and oxygen.The direction of charge transfer was from formaldehyde molecules to the surface of zinc oxide.It is the charge transfer that leads to changes in the resistance of the sensor during the experiment;Finally,we considered the effects of humidity.By analyzing the properties of adsorption energy and charge transfer,we found that compared with the vacuum condition,the adsorption energy decreases,and the amount of charge transfer changes most significantly,and the direction of charge transfer changes.From this we conclude that humidity has an effect on the adsorption of formaldehyde.The latter,we use the metal-organic framework?MOFs?route to prepare NiFe₂O₄.This process involves two steps including the synthesis of Ni₃[Fe?CN?₆]₂·xH₂O?NFCN?nanocubes precursor and the subsequent transformation to porous NiFe₂O₄ crystalline nanocubes by thermal annealing in air.The prepared sample was characterized to ensure that it could be used for subsequent gas sensitivity studies of acetone.After characterization by XRD,SEM,TEM and BET,NiFe₂O₄ is a nanocube material with a porous particle size of 80-100 nm and a high specific surface area.Through the UV-visible DRS technology to detect the optical properties of NiFe₂O₄,we know that the optical band gap of NiFe₂O₄ is 2.26 eV.According to the PL spectrum analysis results,NiFe₂O₄ has a photoluminescent property.From XPS test results,there are three kinds of oxygen species which are lattice oxygen,adsorbed oxygen and oxygen in carbonate.The relative content of the adsorbed oxygen in the total surface oxygen is as high as 47.14%.When acetone is adsorbed,the adsorbed oxygen reacts with acetone molecules,and the generated electrons return to the surface of the material,resulting in a change in resistance,and thus a gas sensitive response.The p-type semiconducting NiFe₂O₄nanocubes based sensor can validly sense acetone at a relatively low working temperature?160°C?with rapid response ability,high sensitivity,low detection limit,excellent cyclic and long-term stability for really real-time diabetes diagnosis.Our results are helpful for the gas sensitivity experiments and their specific applications.