Synthesis Of ZnO Nanowire Arrays By Chemical Vapor Deposition And Their Applications

Since 2001,One dimensional(ID)ZnO nanostructures have attracted intense research due to their wide direct bandgap?large exciton bingding energy,and have promising application in many fields,such as nano-elecronics,photonics,light-emitting diodes,microwaves,nano-generators and photovoltaics.However,the controlled growth of the 1D ZnO nanostructures is still a challenge;meanwhile,the effect of their morphologies on the properties and application remain an open theme.In this work,we focused on the growth of 1D ZnO nanostures with different mophologies and their applications in the fields,including emitting,photocatlytic degradation of toxic dyes,and perovskite solar cells.The main results are as follows:(1)Growth of 1D ZnO nanostructures:We designed and grew 1D ZnO nanostructures with different morphologies,including nanowires,nanopencils,nanotowers,and nanoneedles,through chemical vapor deposition method.The results showed that the transition from nanowires to nanopencils occurred when the substrate tempearate decreased.At the higher substrate temperature(520?),we obtained the single diameter ZnO nanowires;while the dual diameter ZnO nanopencils were achieved at the lower substrate temperature(480 C).In addition,we found that the morphologies can be further controlled by the cooling rate.In the faster cooling rate,the ZnO nanotowes were realized;while in the slower cooling rate,we obtained the ZnO nanoneedles.Meanwhile,ZnO nanocones was fabricated thorough controlling the growth time at the higher temperature and cooling growth rate.These results were explained by the morphology dependent surface free energy.(2)The field emission performance of ZnO nanostructures:we studied the field emission performance of four kinds of ZnO nanostructures,i.e.nanowires,nanopencils,nanotowers,and nanoneedles.The results demonstrated that the properties were strongly affected by nanowire morphology,and the nanotower-like ZnO sample exhibited the best performance with a turn-on field of 4.36 V/?m.The classical Fowler-Nordheim theory was employed to analyze the enhancement mechanism.It was found that only the optimized combination of field enhancement factors ? and geometrical factors ? could assure the excellent field emission performance.Finally,the simulation of surface electric field confirmed that the nanotower-like structure possessed the high field enhancement ability and the large emission area due to the reduced screen effect,resulting in the enhanced emission performance.The work could be valuable for the design of field-emission-based devices using ZnO nanowire arrays.(3)Photocatalytic degradation performance of ZnO/ZnSe coaxial nanowires:we studied the photocatalytic degradation performance of single diameter and dual diameter ZnO/ZnSe coaxial nanowires.It is found that compared with the single-diameter coaxial nanowires,the dual-diameter coaxial nanowires bear the larger tensile strain and possess the higher ratio of coherent layer due to the thinner ZnO core;meanwhile,they have the stronger light absorption in the whole visible region because of the light trapping effect and the interfacial transition.Finally,a 1.3-fold enhancement of photocatalytic performance was achieved for the dual-diameter coaxial nanowires,and the possible enhanced mechanism was discussed.This work provides an efficient strategy to improve photocatalytic performance of nanomaterials through interfacial strain modification.(4)ZnO-based perovskite solar cells:we fabricated the perovskite solar cells by using ZnO nanowires and ZnO nanocone as electron transport layers(ETLs).Compared with ZnO nanowires,ZnO nanocone has the higher transmittance rate and the better wetting properites,but the corresponding cells have the lower power conversion efficiency(PCE)of 2.7%.This is because that ZnO nanocone has the higher chemical activity,resuling in the fast degradation of perovskite material and the low PCE.In our work,the maximal PCE of 10.2%was achieved in the perovskite solar cell with ZnO nanowire as the ETL.It is still a big challenge to avoid the degradation of perovskite material in order to improve PCE.