Structure-property correlation of nanostructured WO3 thin films produced by elec

2020-02-15 01:50:48

film WO3 films deposited deg

责任者: Deepa, M.;Srivastava, A.K.;Singh, S.;Agnihotry, S.A. 单位: Nat. Phys. Lab., New Delhi, India 来源出处: Journal of Materials Research(J. Mater. Res. (USA)),Sept. 2004,19(9):2576-85 摘要: Nanocrystalline tungsten oxide (WO3) films were electrodeposited potentiostatically at room temperature on transparent conducting substrates from an ethanolic solution of acetylated peroxotungstic acid prepared from a wet chemistry process. The changes that occur in the microstructure and the grain size of the as-deposited WO3 films as a function of annealing temperature are simultaneously accompanied by a continually varying electrochromic performance. X-ray diffraction studies revealed the transformation of a nanocrystalline as-deposited WO3 film into a highly crystalline triclinic WO3 as the annealing temperature was raised from room temperature to 500°C. The microstructural evolution with the increasing annealing temperature of the as-deposited film was further exemplified by transmission electron microscopy (TEM) studies. While the as-deposited film was composed of uniformly distributed ultra fine nanograins, the most noticeable feature seen in these films annealed at 250°C was the presence of open channels which are believed to promote lithium ion motion. Films annealed at 400°C exhibited coarse grains with prominent grain boundaries that hinder lithium ion movement, which in turn reduces the films ion insertion capacity. In concordance with the TEM results, the 250°C film had the highest ion storage capacity as it exhibited a charge density of 67.4 mC cm-2 μm-1. The effect of microstructure was also reflected in the high transmission modulation (64%) and coloration efficiency (118 cm2 C-1) of the 250°C film at 632.8 nm. Contrary to the superior electrochromic performance of the 250°C film, the optical switching speeds between the colored and bleached states of the as-deposited WO3 film declined considerably as a function of annealing temperature. The diffusion coefficient for lithium ions was also greater by at least an order of magnitude for the as-deposited film as compared to the 250 and 500°C films. In this report, the influence of microstructural changes that are brought about by the annealing of the as-deposited WO3 films on their coloration-bleaching dynamics is evaluated in terms of their structural, electrochromic, and electrochemical properties 关键词: annealing;electrochromism;electrodeposition;grain boundaries;grain boundary diffusion;grain size;nanostructured materials;nanotechnology;solid-state phase transformations;thin films;transmission electron microscopy;tungsten compounds;voltammetry (chemical analysis);X-ray diffraction;structure-property correlation;nanostructured WO3 thin films;electrodeposition;nanocrystalline tungsten oxide films;room temperature;transparent conducting substrates;acetylated peroxotungstic acid;wet chemistry;microstructure;grain size;annealing;electrochromic properties;X-ray diffraction;transmission electron microscopy;TEM;ultra fine nanograins;lithium ion motion;coarse grains;grain boundaries;ion insertion capacity;charge density;high transmission modulation;coloration efficiency;optical switching;diffusion coefficient;coloration-bleaching dynamics;electrochemical properties;crystalline triclinic WO3 phase;phase transformation;20 to 500 degC;632.8 nm;WO3