1-D ZnO nanowire arrays with adsorbed Au nanoparticles through a novel sodium citrate reduction method for field-emission applications under UV light illumination

This paper explores the field-emission (FE) characteristics of devices prepared from one-dimensional (1-D) pure zinc oxide nanowire (ZnO NW) arrays with and without decorated gold nanoparticles (Au NPs) in high vacuum environments. 1-D ZnO nanostructures were firstly grown on indium-tin-oxide (ITO) glass substrates coated with ZnO seed layers by using a simple hydrothermal solution route at temperature of 95 degrees C for 3 h. Au NPs adsorbed on the surfaces of the NWs were reduced with the novel sodium citrate (Na3C6H5O7) reduction method using 0.5 mM Na3C6H5O7 and 0.5 mM tetrachloroauric acid (HAuCl4) and annealed at 400 degrees C for 5 min. Next, material property analyses (e.g. surface structure, crystallinity characteristic, optical performance, and elemental content) were conducted through field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), photoluminescence (PL), UV-visible absorbance, and X-ray photoelectron spectroscopy (XPS). Results showed that all NWs had a hexagonal wurtzite phase and the (002) plane was the dominant c-axis orientation. In addition, the element content of Au NP was approximately 1.09 at%. All FE devices revealed two optical emissions, including UV and green emission regions. The prepared samples were measured under a high vacuum machine with a vacuum value of 5 x 10-6 Torr. As a result, the turn-on electric field decreased from 5.94 (V/mu m) to 4.56 (V/mu m) compared with that in pure ZnO NWs, and the field enhancement factor ((3) increased from 4730 to 5984 after decorating Au NPs. Besides, UV light treatment also improved FE performance. The turn-on electric field and effective field enhancement factor ((3) of the 1-D ZnO FE devices with adsorbed Au NPs were 3.25 V/mu m and 9786, respectively, under UV light condition because of the large number of electrons.