Study of nanostructured metal oxides for electrochromic windows

Abstract

Electrochromic materials have attracted the interest of researchers over the last several decades. They are able to change their color upon the application of a voltage. Tungsten oxide (WO3) and titanium oxide (TiO2) are the conventional electrochromic materials and have been studied most, due to their good electrochromic properties and electrochemical stability. Hence the research in this thesis is mainly based on tungsten oxide and titanium oxide electrochromic materials. By adjusting the precursor, nanostructured electrochromic materials were prepared on the substrates with improved electrochromic properties.

A thin film of hexagonal WO3 nanowires was grown directly onto a bare fluorine-doped tin oxide glass (FTO) without the assistance of a WO3 seed layer via a facile and low-cost solvothermal method. Compared with the WO3 film synthesized with a seed layer, a faster switching time, higher coloration efficiency and more stable cycling were achieved due to direct nanowire contact with the substrate. In order to improve the contact between the WO3 and FTO, and therefore increase the stability of the electrode, it was annealed under N2 and the cycling life increased by almost a factor of 10.

Hierarchical WO3 nanotree-like structures were grown from nanowires by a two-step non-seeded solvothermal technique on FTO. A study of the growth process revealed that the nanotrunks formed before the nanobranches grew, with the trunks orientated along the (002) plane of the hexagonal phase WO3, while the branches were orientated along the (100) and (200) planes. An electrochromic film prepared with WO3 nanotrees are expected to have a large active surface area, which enabled a large optical modulation of 74.7% at 630 nm at the low potential of -0.2 V, fast switching speeds of 2.64 s and 7.28 s for bleaching and coloration, respectively, and a high coloration efficiency of 75.35 cm2 C-1.

A nanocomposite TiO2 film was prepared by decorating a rutile TiO2 nanorod (TiO2 NR) array with anatase TiO2 nanopyramids (TiO2 NPyr) via a facile two-step solvothermal method. The addition of NPyr to the TiO2 NR film is expected to increase the total amount of material, hence providing a greater number of sites for lithium ion insertion, and the NPyr are expected to increase the active surface area. As a result, the TiO2 NR–NPyr film displayed higher optical contrast and coloration efficiency compared with NR films, demonstrating that enhanced electrochromic properties could be improved by using the NR–NPyr composite structure.

Anatase TiO2 nanowires were grown directly onto FTO by a solvothermal method. Different concentrations of precursor can easily control the length of the nanowires. The TiO2 nanowire films showed antireflective properties, increasing the transmittance of bare FTO by 7.2% at 550 nm. The electrochromic properties of the TiO2 nanowire sample was investigated in KCl electrolyte, giving a large optical modulation of 61.4% at 550 nm, a high coloration efficiency of 23.89 cm2 C-1 and good stability.

The TiO2 nanowire film was modified by chemisorption of a monolayer of the redox chromophore 1,4-bis[((N-phosphono-2-ethyl)-4,4’-bipyridinium)-methyl]-benzene tetrachloride (viologen). The best viologen modified TiO2 nanowire film showed an optical modulation of 55.5% in 550 nm, coloration efficiency of 244.5 cm2 C−1 and good stability. Compared with the pure TiO2 nanowire thin film, the coloration efficiency of the viologen modified TiO2 nanowire thin film increased more than 10 times, showing its potential in smart window applications.

Prussian blue was electrodeposited on bare FTO and a TiO2 nanowire thin film. The structure of the Prussian blue on FTO was a compact layer, whereas a core-shell (Prussian blue coating the TiO2) nanostructure was achieved on the TiO2 nanowire film. The core-shell nanostructure showed promising electrochromic properties with an optical modulation of 64% at 550 nm, a high coloration efficiency of 86.7 cm2 C-1 and a superior stability of 1000 cycles without significant decrease in optical modulation. The improvement in electrochromic performance was attributed to the support template of TiO2 nanowires increasing the Prussian blue contact with the electrolyte. Furthermore the presence of the TiO2 nanowires also lowered the current density, which improved the coloration efficiency and the stability.

The electrochromic properties of the TiO2 nanowire thin films in different electrolytes were studied. The nanowire film in electrolyte containing multivalent ions, such as Al3+, showed a large optical modulation of 60.03% at 550 nm, fast insertion kinetics and good stability over 400 cycles. The electrochromic properties of a cycled TiO2 nanowire thin film in a LiClO4 propylene carbonate electrolyte can be recovered by water washing. This is an interesting concept and possible reasons for this behavior were discussed, however further work is required in this area.

A range of nanostructured WO3 and TiO2 thin films have been prepared for this thesis with different morphologies and tested in electrochromic smart window application showing promising potential.