Research On The Properties Of Sol-gel Electrochromic Nickel Oxide Thin Films

Electrochromic (EC) materials belong to the family of chromogenics materialsand are optically active as their transmittance,absorption and reflectance can becontrolled by applying an external field．They are one of functional materials，whichhave very good application fucture．It can be used widely in large-area display，glare-free mirror，deposited devices and smart windows．The representative ofanticathode materials is nickel oxide which is employed as an anodic electrode inconjunction with electrochromic tungsten oxide as colouring electrode．The nickeloxide films can be uses as electrochromic layer and ion storage layer ofelectrochromic device．In this paper，nickel oxide thin films and Al doped NiO films were suecessfullyprepared via sol-gel process and dip-coating methods on ITO glass substrates. Theeffects of Al dopants on the structure and properties of NiO thin films wereinvestigated to optimize the electrochromic performance of the coatings. Thecomposition，microstructure，morphology，transmittance spectra and electrochemicalproperties of NiO thin films were characterized by means of Uv-Vis-near infraredspectra，X-ray diffraction，scanning electron microscopy，FT-IR spectroscopy andcyclic voltammetry and chronoam perometry. The electrochromic mechanism wasdiscussed．This dissertation mainly consists of the following several conclusions：1. The better heat-treatment temperature is300℃for NiO thin films．When heattreatment at low temperature, the nickel oxide exists in the form of amorphous. NiOfilms get loose and polyporous, which lead to the adhesion of the films with thesubstrates is poor. The thin films are easy to fall off. With the increase ofheat-treatment temperature, the adhesion of the films with the substrates turns better.The thin films have large grain siz and poor electrochromic performance. When heattreatment at high temperature, NiO films will lose the electrochromic performance.2. During cyclic voltammetry, the NiO films have an“activation process”.Electrochromic NiO films are among the least understood material and theelectrochemical mechanisms that lead to electrochromism are still controversial. It ishowever accepted that the coloration is due to the oxidation of Ni(II) into Ni(III) involving the transformation of Ni(OH)2into NiOOH.3. In Al doped NiO films,the α-Ni(OH)2phase become stable in strong alkalisolution and the phase has a higher oxidation state of nickel in the charge state．So A1doping make the better transmittance of the bleached state and better electrochromicproperties of NiO films．The increasing thickness of Al doped NiO films also canimprove the spectrum modulation of films.4. Under different heat-treatment temperature, the bleaching time of all Al dopedNiO films is short (within2~4s), the colouration time is long (more than10s). Thebleaching time is smaller than the colouration time. With Al doping and the increaseof heat-treatment temperature, the surface structure of NiO films tend to be dense andthe migration of ions and electrons is blocked, which make the colouration andbleaching time of these films have been extended.These films have a certain memorystorage capacity.