Abstract
Thin, porous films of WO3 were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB50-b-PEO45), or a combination of PEG and PIB50-b-PEO45 as structure-directing agents. The influence of the polymers on the composition and porosity of WO3 was investigated by microwave plasma atomic emission spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, and gas sorption analysis. The electrochromic performance of the WO3 thin films was characterized with LiClO4 in propylene carbonate as electrolyte. To analyze the intercalation of the Li+ ions, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy were performed on films in a pristine or reduced state. The use of PEG led to networks of micropores allowing fast reversible electrochromic switching with a high modulation of the optical transmittance and a high coloration efficiency. The use of PIB50-b-PEO45 provided isolated spherical mesopores leading to an electrochromic performance similar to compact WO3, only. Optimum characteristics were obtained in films which had been prepared in the presence of both, PEG and PIB50-b-PEO45, since WO3 films with mesopores were obtained that were interconnected by a microporous network and showed a clear progress in electrochromic switching beyond compact or microporous WO3.
Highlights
Thin, porous films of WO3 were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB50-b-PEO45), or a combination of PEG and PIB50-b-PEO45 as structure-directing agents
Thin films of tungsten oxide with significantly increased internal surface area were prepared by spin-coating using different polymers as structure-directing agents to influence the internal film structure and, the electrochromic performance
The combination of PIB50-b-PEO45, a block copolymer that is known to form micelles under the present preparation conditions, and the homopolymer polyethylene glycol PEG 600 as templates led to WO3|μmp films with interconnected spherical mesopores desirable for the intercalation and deintercalation of charge-balancing Li+ ions allowing high transmittance modulations between the bleached and the colored states of the films, short response times and high effective diffusion coefficients
Summary
Windows are an essential component in almost every building. Switchable glass, known as a smart window, provides tunable shading that can improve the indoor occupant comfort and reduce the energy consumption caused by heating and cooling of a building [1,2]. Further previous studies addressed the impact of different nanoscaled porosity on the electrochromic performance Block copolymers such as polystyrene-block-polyethylene oxide (PS-b-PEO) [31] or Pluronic P123 [32] that are forming micelles allow the formation of mesoporous films with 4–30 nm large pores providing enhanced electrochromic characteristics such as short switching time, high coloration efficiency and large transmittance modulation [31,32]. The largest transmittance modulations, highest coloration efficiencies and fastest diffusion of ions along with highly reversible and stable switching processes could be obtained for WO3 films prepared with PIB50-b-PEO45 combined with PEG 600 as additives, leading to films with interconnected mesopores. Thin WO3|μmp films exhibiting a combination of interconnected micropores and mesopores were prepared by adding around 75–150 μL PEG 600 and 42 mg PIB50-b-PEO45 into 1 mL of the peroxotungstic acid precursor solution. KG (Plauen, Germany) were used, a type that had been used in commercially available electrochromic smart windows
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