Low temperature tungsten trioxide nano/micro-systems for applications in gas sensing and electrochromism

Abstract

In this work we primarily set out to investigate the technique of Aqueous Chemical Growth as a means of producing WO3 thin films that find applications in gas sensing and electrochromism. For the first time we demonstrated in this work, the heterogenous nucleation and growth of WO3 thin films on plain glass substrates and F-doped SnO2-glass substrates. This was achieved without the use of surfactants and template directing methods, using as a precursor solution Peroxotungstic Acid generated from the action of 30% H2O2 on pure W powder. The substrates used needed no surface-modification. On the plain glass substrates (soda lime silicates) a variety of micronanostructures could be observed prime of which were nanoplatelets that acted as a basic building block for the self-assembly of more hierarchical 3-d microspheres and thin films. On FTO a wide variety of micro-/nanostructures were observed dominant amongst which were urchin-like microspheres. The dominant crystallographic structure observed (through X-ray diffraction analysis, SAED, HRTEM) for the WO3 thin films on both substrate types post-annealing at 500 ˚C for a period of 1 - 2 h, was hexagonal-WO3. Next was monoclinic WO3. On rarer occasions the formation of triclinic and cubic WO3 was observed. The thin films produced showed a fairly high degree of porosity and had thicknesses in the range of 900 nm - 3.5 μm. I-V characterisation measurements using a 4-point collinear probe Keithley source alongside photoluminescence was used to establish the insulating nature of some of the films as well as their sub-stoichiometric nature. X-ray Photoelectron Spectroscopy was used to confirm the substoichiometric nature of some of the films.