Nanostructured mesoporous tungsten oxide films with fast kinetics for electrochromic smart windows.

A potential driven self-assembly of sodium dodecyl sulfate/tungsten oxide aggregates at the electrolyte-electrode interface followed by template extraction and annealing yielded mesoporous thin films of electrochromic tungsten oxide (WO(3)). Electron microscopy images revealed that the films are characterized by a hitherto unreported hybrid structure comprising nanoparticles and nanorods with a tetragonal crystalline phase of WO(3) with the measured lattice parameters: a = 0.53 nm and c = 0.37 nm. In addition to pentagonal voids characteristic of the tetragonal WO(3) phase at the lattice scale, open channels and pores of 5-10 nm in diameter lie between the nanoparticles, which cumulatively promote rapid charge transport through the film. This resulted in colouration efficiency (η(max)∼90 cm(2) C(-1) at λ = 900 nm) and switching kinetics (colouration time =  3 s and bleaching time =  2 s for a 50% change in transmittance) higher and faster than previously reported values for mesoporous WO(3) films. Repetitive cycling between the clear and blue states has no deleterious effect on the electrochromic performance of the film, which is suggestive of its potential as a cathode in practical electrochromic windows.