The Preparation And Gas Sensing Properties Of Cu²⁺ Modified And ZnO Composited Porous Silicon Nanowires

Among all NO2 gas sensing materials,porous silicon nanowires?PSiNWs?with the advantages of large surface area,low working temperature,abundant raw materials and easy fabrication process,has been considered as one of the promising candidate to make gas sensors.However,the gas sensitivity and lowest detectable concentration of PSiNWs to NO2 can be further improved.Therefore,compositing with other gas sensing materials and surface modification were used in this work to improve the gas sensing performance of PSiNWs and enhance its gas sensing response to NO2 selectively.With proper synthesis parameters?silvering time and etching time?,n-type PSiNWs fabricated by Ag-assisted chemical etching performed gas sensing response to NO2 at ppm level,and the highest response to 50 ppm NO2 was 8.9%.In this work,ZnO nanorods were grown onto the surface of PSiNWs to form there different types of ZnO nanorods/PSiNWs hybrid structures with different morphologies,which can perform gas sensing response to NO2 with concentration as low as 5 ppm at room temperature,and the largest response to 50 ppm NO2 was also improved to 35.1%.The ZnO nanorods/PSiNWs hybrid structures not only increased the surface area to interact with target gas molecules,but also facilitated the movement of electrons towards the surface owing to the energy band bending phenomenon.As a result,the charge carriers were easier to interact with gas molecules,and the NO2 gas sensing properties of the hybrid structures were improved.In the meantime,the oxygen vacancy concentration of ZnO nanorods increased,which can function as the preferential adsorption sites to NO2 gas molecules as well as catalysis the surface gas sensing reactions,and therefore contributed to the improvement of the NO2 gas sensing performance of ZnO nanorods/PSiNWs hybrid structures in another way.In order to detect NO2 with lower concentrations,PSiNWs were immersed into Cu²⁺solution to get surface modification,and proper amount of Cu nanoparticles were deposited onto the surface of PSiNWs during the process.The gas sensing performance of PSiNWs changed from n-type to p-type after surface modification,and the distinct gas sensing response to NO2 with the concentration of 40 ppb indicated that the NO2 gas sensing properties of PSiNWs was improved to ppb level.Meanwhile,the largest response of PSiNWs to 50 ppm NO2 was promoted to 50.8%.In this experiment,the redox reaction between Cu²⁺and the surface of PSiNWs?large amounts of Si—H bonds and others?changed the major charge carrier type of PSiNWs from electrons to holes.At the same time,Cu nanoparticles acted as the medium during the charge exchange procedure between NO2 molecules and PSiNWs,as well as facilitated the gas sensing interactions.Consequently,the NO2 gas sensing properties of PSiNWs was highly promoted.