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Composite Film of Vanadium Dioxide Nanoparticles and Ionic Liquid-nickelchlorine Complexes with Excellent Visible Thermochromic Performance Jingting Zhu, Aibin Huang, Haibin Ma, Yining Ma, Kun Tong, Shidong Ji, Shanhu Bao, Xun Cao, and Ping Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11202 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016
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ACS Applied Materials & Interfaces
Composite Film of Vanadium Dioxide Nanoparticles and Ionic Liquid-nickel-chlorine Complexes with Excellent Visible Thermochromic Performance Jingting Zhu1,2, Aibin Huang1,2, Haibin Ma1,2, Yining Ma1,2, Kun Tong1,2, Shidong Ji1, Shanhu Bao1, Xun Cao1*, Ping Jin1,3*
1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295, Changning, Shanghai, 200050, China
2 University of Chinese Academy of Sciences, Yuquan 19, Shijingshan, Beijing, 100049, China 3 Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), Moriyama, Nagoya 463-8560, Japan * Author for correspondence. Email:
[email protected], Tel/Fax: +86-21-6990-6213
[email protected], Tel:+86-21-6990-6202
Keywords: vanadium dioxide; ionic liquid-nickel-chlorine complexes; visible thermochromism; excellent performance; color change
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Abstract Vanadium dioxide (VO2), as a typical thermochromic material used in smart windows, is always limited by its weaker solar regulation efficiency (∆Tsol) and lower luminous transmittance (Tlum). Except for common approaches such as doping, coating and special structure, compositing is another effective method. The macroscopic thermochromic (from colorless to blue) ionic liquid-nickel-chlorine (IL-Ni-Cl) complexes are selected in this paper to be combined with VO2 nanoparticles forming a composite film. This novel scheme demonstrates outstanding optical properties: ∆Tsol=26.45% and Tlum,l=66.44%, Tlum,h=43.93%. Besides, the addition of the IL-Ni-Cl complexes endows the film an obvious color change from light brown to dark green as temperature rises. This splendid visible thermochromic performance makes the composite film superior in function exhibiting and application of smart windows.
Introduction Energy conservation has been a matter of immediate importance to security of sustainable development. It is estimated that building energy consumption accounts for 30% - 40% of total social energy consumption in most countries.1-2 Windows, which are major channels for energy exchange between buildings and ambient by means of conduction, convection and radiation, primarily take charge of building energy consumption.2 To reduce this part of consumption, the progress in development of energy-saving windows has led to extensive studies of smart glass with special functional coatings which can modulate solar energy in response to an external stimulus, such as light (photochromism), heat (thermochromism) or electricity (electrochromism).3-7 In this regard, thermochromic smart windows, typically based on a vanadium dioxide (VO2) layer, have attracted particular attention. Because they can respond to environment temperature, making reversible structural change from a monoclinic semiconducting state to a tetragonal metallic state with optical change from infrared-transparent to infrared-translucent partially blocking light while remaining relative transparent.8-10 However, there exists a tough practical challenge for VO2 that how to attain both large solar regulation efficiency (∆Tsol) and high luminous transmittance (Tlum) simultaneously. Numerous methods have been adopted to enhance the optical performance (∆Tsol and Tlum) of VO2. The first is doping. For example, doping VO2 with F or Mg can widen the band gap of VO2, leading to an increase of the Tlum but a deterioration of the ∆Tsol;11-13 doping VO2 with Ti or Eu can improve both ∆Tsol and Tlum although the change range is small.14-16 The second is coating. Coating VO2 nanoparticles (NPs) with SiO2 shells or TiO2 shells for instance can effectively enhance the Tlum and maintain a nearly constant ∆Tsol.17-20 The third is special structure. Such as depositing a single/multi-layered antireflection film,21-24 preparing nanoporous structured film25-26 or biomimetic nanostructures like moth-eye27-28 which can also optimize the optical performance. And the last is compositing. Preparing VO2-Sb:SnO229 or VO2-SiO230 composite film for instance can simultaneously enhance ∆Tsol and Tlum. However, the huge gap between thermochromic performance and demanding practical application remain unfilled. Recently, a great progress was made on optical properties by integrating VO2 with another thermochromic material named poly(N-isopropylacrylamide) (PNIPAm) hydrogel, obtaining ultra-high solar modulation and luminous transmittance.31-32 But the large ∆Tsol is based on the sacrifice of the Tlum (∆Tlum=38.9%) from PNIPAm and the hybrid film turns from transparent to misty at only 35 oC.
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On the premise of large ∆Tsol for energy saving, higher Tlum is needed for actual usage. To address this critical issue, some transition metal complexes (TMCs) which exhibit thermochromism with little Tlum loss at high temperature were chose to be hybridized with VO2 NPs reported by our group.33 The TMCs we used can partly absorb visible light and change color as a response to temperature change upon interaction with an appropriate donor solvent, and effectively avoid severe damage to Tlum at high temperature because the absorption peaks are staggered with the extremum of the light-adapted eye sensitivity function. Through combination of the TMCs and VO2 NPs, outstanding optical performance (∆Tsol =20.82% and Tl,lum =62.73%, Th,lum =55.77% for VO2/CoII-Br-TMP film) can be obtained compared with other VO2-based materials. However, a very common drawback of many TMCs is their relatively low stability, as a result of inevitable solvent evaporation during prolonged heating-cooling cycles.34-35 Such problems can be effectively overcame by the introduction of thermochromic TMCs in a donor-containing ionic liquid (IL) which is nonvolatile in nature which has been reported by Wei et al.36-37 This manuscript describes the successful combination of such thermochromic IL and VO2 NPs, and characteristics of the composite film. The thermochromic IL used in this work is composed of di-(1-butyl-3-methyl-imidazolium) [bmim]2NiCl4 dissolved in an IL, 1-(3-hydroxyethyl)-3-methylimidazolium tetrafluoroboride (C2OHmimBF4). Briefly, when heating from room temperature, the ionic liquid-nickel-chlorine (IL-Ni-Cl) complexes absorb increasing visible light around 656 nm and 705 nm and gradually change color from colorless to blue. The hydroxyl group in the C2OHmim+ is the donor group responsible for the coordination reaction given below,36-37 + − 2 ⇋ + (1) (octahedral, colourless, cold) (tetrahedral, blue, hot) where 0
