Effect of synthesis temperature and N2/O2 flow on morphology and field emission property of SnO2 nanowires

Abstract

A large amount of tin oxide (SnO2) nanowire arrays were synthesized on the flexible conductive carbon fiber substrate by thermal evaporation of tin powders in a tube furnace. The temperature, as well as the flow rate of the carrier N2 gas and the reaction O2 gas, plays an important role in defining the morphology of the SnO2 nanowires. Morphology and structure of the as-grown SnO2 samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that all the samples possess a typical rutile structure, and no other impurity phases are observed. The morphology changes from rod to wire with the increase of reaction temperature. Ratio of length to diameter of the nanowires increases first and then decreases with the flow ratio of N2/O2 gas. The optimum synthesis conditions of SnO2 nanowire are: reaction temperature 780 ℃, N2 and O2 flow rates being 300 sccm and 3 sccm respectively. In our growth process, the nanowire grows mainly due to the vapor-liquid-solid (VLS) growth process, but both the VLS process and surface diffusion combined with a preferential growth mechanism play the important role in morphology evolution of the SnO2.Field emission measurements for Samples 1-6 are carried out in a vacuum chamber and a diode plate configuration is used. Relationship between the growth orientation, aspect ratio, density and uniformity of the arrays and field emission performances will be investigated first. Results reveal that the field emission performance of SnO2 nanostructures depends on their morphologies and array density. The turn-on electric field (at the current density of 10 upA/cm2) decreases and the emission site density increases with tin oxide array density, and the turn-on electric field of Sample 5 (synthesized at 780 ℃, nitrogen and oxygen flow rates being 300 sccm and 3 sccm respectively) is about 1.03 V/m at a working distance of 500 m. By comparison, for the turn-on electric fields of the not well-aligned SnO2 nanowire arrays we have 1.58, 2.13, 2.42, 1.82, and 1.97 V/m at 500 m. These behaviors indicate that such an ultralow turn-on field emission and marked enhancement in (～ 4670) can be attributed to the better orientation, the good electric contact with the conducting fiber substrate where they grow, and the weaker field-screening effect. Our results demonstrate that well-aligned nanowire arrays, with excellent field-emission performance, grown on fiber substrate can provide the possibility of application in flexible vacuum electron sources.