Preparation And Performance Enhancement Study Of ZnO-Based Gas Sensors

The development trend of human society is gradually shifting from high speed to high quality,and gas pollution from industrial production and human activities is seriously affecting human health and hindering the realization of this process.Therefore,it is important to develop high-performance gas detection devices to provide real-time and effective feedback on target gas information.Currently,gas sensors are widely used in various fields such as environmental monitoring,health care,and gas emission control.Among the many types of semiconductor gas sensors,semiconductor metal oxide gas sensors are portable,inexpensive,and highly sensitive,so they have been widely studied by many researchers.This paper focuses on the fabrication of semiconductor metal oxide ZnO-based gas sensors,discussing the main factors affecting the device performance,and introducing rare earth elements and transition metals to enhance the responsiveness and selectivity of pure ZnO-based sensors.In addition,based on density functional theory(DFT),the adsorption process of ethanol and acetone gas on the ZnO surface is simulated.The main research contents of the paper are as follows:1.Granular,spherical,and flaky ZnO-based gas-sensitive materials were prepared by water bath,solvent heat,and hydrothermal methods,respectively.X-ray diffraction(XRD)and scanning electron microscopy(TEM)were used to characterize the sample’s morphology and crystal structure.They were prepared as indirectly-heated type sensors and then the performance was systematically tested.The results showed that the ethanol gas sensor made of sheet ZnO gas-sensitive material exhibited higher responsiveness and good response/recovery time(4 s/25 s).Different rare earth elements Yb,Ce,Eu,and Nd were introduced to modify the sheet ZnO gas-sensitive materials,which were prepared into indirectly-heated devices and then tested.The results showed that the NdZnO-based sensor obtained a responsiveness of 144.8 at an atmosphere of 100 ppm ethanol,which was 4.1 times higher than that of the pure phase sheet ZnO-based sensor,while the selectivity to ethanol was also substantially improved.The introduction of rare earth elements provides a new idea to improve the response characteristics and selectivity of the gas sensors.2.Nd-ZnO composite gas-sensitive materials with four different concentration ratios(Nd: Zn = 0.5%,1%,2%,3%)were prepared by hydrothermal method.Different characterization methods were used to analyze the composition and morphology of the four groups of samples.We made it into an indirectly-heated device,and the device performance was tested to determine the optimal introduction ratio of the Nd elements.The results showed that the introduction of the Nd elements can be a good way to enhance and improve the response and selectivity characteristics of the devices,where1% Nd-ZnO devices show the highest responsiveness and selectivity to ethanol.In order to prove that the introduction ratio of the Nd elements has a certain regularity,a planar gas sensor was also developed for performance testing.The performance of the planar-type sensor remains optimal when the introduced concentration of the Nd element is 1%.At a concentration of 100 ppm of ethanol gas,its responsiveness is 91.7.In addition,we investigated the reason for the sensitization of the gas sensor after the introduction of the Nd element.3.Pure phase nanosheet structured ZnO and 0.5%,1%,and 3% Fe-ZnO composite gas-sensitive materials were synthesized by hydrothermal method and used for the preparation of the high-performance acetone gas sensors.The crystal structures,morphological features,and elemental compositions were identified by XRD,SEM,transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS)test.The 0.5% Fe-ZnO sensor can achieve a maximum response of 105.7 to 100 ppm acetone gas at an optimal operating temperature of 365 °C,which was 3.68 times higher than that of the pure ZnO device.It also showed a responsiveness of 2.3 at a low detection limit of 1 ppm,indicating that it still had good responsiveness at low gas concentrations.The gas sensor with the introduction of the Fe element showed excellent selectivity for acetone gas and had good long-term stability.The study of the sensitization mechanism revealed that the improvement of the gas-sensitive performance was related to the reduction of band gap and the increase of the oxygen vacancies after doping,and the introduction of the Fe element brought about a larger specific surface area.All the above results and analyses indicated that Fe-ZnO gassensitive materials had good potential for the preparation of high-performance acetone sensors.4.The basic properties of bulk ZnO,including the energy band structure and density of states,were investigated based on DFT,and the ZnO(0001)surface was selected as the adsorption surface and then cell expansion was carried out.The adsorption energy of the gas molecules on each adsorption site of the ZnO(0001)surface was calculated to determine the optimal adsorption sites,and the charge transfer between each gas molecule and ZnO(0001)surface during the adsorption process was also calculated.The results of the above calculations show that ZnO has a certain adsorption effect on both ethanol and acetone gases when the exposed surface is the(0001)surface.The calculation results provide a theoretical basis for the development of a high-performance ZnO-based gas sensor.