Preparation and Sensing Properties of ZnO Heterostructure Nano Materials

Author:Tian Hai Lin

Supervisor:fan hui qing


Degree Year:2017





The surface and interface effect is significant for the performance of nanomaterials.Therefore,it is very important to improve and expand the property of heterostructures with modifying the surface and interface.Though ZnO nanomaterials are stable in nature,easily synthetic and the electron mobility is high,the performance of ZnO nanomaterials is still enhanced in applications of photoelectric devices,catalyst and sensors.In this paper,the surface and interface modification of ZnO heterostructures had been obtained by various methods,new gas sensing materials possessed of the promoted sensitivity and selectivity,the lower operating temperature and the overall properties,respectively.At the same time,the ZnO heterostructures were applied for the H2O2 electrochemical sensor,and the application of sensors was enlarged in ZnO nanomaterials.The main studies in this paper can be seen as follow:The surface of ZnO nanomaterials was modified by the chemical coupling,which used the APTES and glucose as the coupling agent and carbon sources,to obtain ZnO/C heterostructures after the high temperature carbonization.Compared to ZnO nanomaterials(16.2 cm2/g),the specific surface area of ZnO/C heterostructures was increased to 33.8 cm2/g,because of abundant active sites on the surface and strong gas adsorption ability in ZnO/C heterostructures.In the gas sensing measurement,the sensitivity of ZnO/C heterojunction nanostructures was obviously enhanced for the reductive gases.Zinc oxide(ZnO)and zeolitic imidazolate framework-8(ZIF-8)core-shell heterostructures were obtained by using the self-template strategy where ZnO nanorods not only acted as the template,but also provided Zn2+ions for the formation of ZIF-8 shell.The as-prepared ZnO/ZIF-8 core-shell heterostructures revealed the distinct gas response toward different VOC molecules due to the size-selective effect of the ZIF-8 shell.For example,the kinetic diameter of formaldehyde is 2.43?,which is much smaller than the kinetic diameter of methanol(3.63?),ethanol(4.53?)and acetone(4.60?),indicating that formaldehyde could easily pass the pore of ZIF-8 shell to arrive the surface of ZnO nanorods and other VOCs molecules could not go through those apertures and were blocked by the ZIF-8 shell.In the gas sensing measurement,the selectivity of ZnO/ZIF-8 core-shell heterostructures was improved for formaldehyde molecules.NiO/ZnO heterostructures were successfully synthesized by using a hydrothermal synthesis followed by the calcination.The structural features of NiO/ZnO p-n heterostructures were characterized in detail by XRD,SEM,and TEM,and the asymmetrical I-V curve was indirect evidence to demonstrate the formation of p-n heterojunctions between NiO and ZnO.The change of energy band positions and carrier concentrations was formed in NiO/ZnO p-n heterostructures,and the oxygen species on the surface of the material was significantly transformed due to p-n heterojunctions.In the gas sensing measurement,NiO/ZnO heterostructures exhibited good gas response,fast response/recovery times to ethanol vapor even at 200°C,which was much lower than 350°C of pure ZnO.And the operating temperature of NiO/ZnO p-n heterostructures was effectively reduced.Pt/ZnO/g-C3N4 3D nanostructures were successfully prepared by constructing well-aligned ZnO nanorods on the substrate of exfoliated g-C3N4 nanosheets with microwave-assisted hydrothermal synthesis,followed with the chemical reduction method for the deposition of Pt nanoparticles.In the gas sensing measurement,Pt/ZnO/g-C3N4 3D nanostructures exhibited the remarkable sensitivity,selectivity,and low operating temperature for air pollutants of ethanol and NO2.The application of Pt/ZnO/g-C3N4 3D nanostructures could be used as a dual-functional gas sensing material between 150°C and 250°C through the controlled operating temperature.Additionally,the synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C3N4 3D nanostructures were verified for the gas sensing mechanism.And it revealed the excellent charge carriers transport properties,such as the longer electron lifetime(τn),bigger electron diffusion coefficient(Dn)and effective diffusion length(Ln),which also played an important role for Pt/ZnO/g-C3N4 3D nanostructures with striking gas sensing activities.The exfoliated g-C3N4 nanosheets were primally prepared and then ZnO nanosheets were grown on g-C3N4 nanosheets by microwave-assisted hydrothermal synthesis to obtain ZnO/g-C3N4 hierarchical nanosheets.The g-C3N4 nanosheets combined with ZnO nanosheets could greatly increase the active sites of the material and carrier concentrations.The as-prepared ZnO/g-C3N4 hierarchical nanosheets revealed a sensitivity 540.8μA/mM/cm2 for H2O2,which even was superior to some noble metal-decorated H2O2 sensors.And the broad detection ranges,reliable stability with good anti-interfering ability were also obtained in ZnO/g-C3N4 hierarchical nanosheets.As non-enzyme and noble metal-free H2O2 electrochemical sensor,ZnO/g-C3N4 hierarchical nanosheets exhibited the high efficiency,stability and low cost feature for the detection of H2O2,which could expand the sensing applications of ZnO nanomaterials.