Rapid communicationTiO2(R)/VO2(M)/TiO2(A) multilayer film as smart window: Combination of energy-saving, antifogging and self-cleaning functions
Introduction
Windows capable of regulating solar/heat transmission for energy efficiency and comfort are called smart. [1], [2] Smart windows have the potential to moderate the energy consumption of buildings, which is almost the equivalent of 30−40% of the primary energy used in worldwide. [3], [4] Among the various types of chromogenic materials, thermochromic materials have attracted the attention of researchers and been the subject of extensive studies for applications to smart windows. [5], [6], [7] The most widely studied thermochromic material as smart window is VO2 with the monoclinic (M) phase (shown as VO2(M)), which is able to execute a reversible transition at a phase-transition temperature (Tc≈68 °C): below Tc the material is monoclinic, insulating and quite infrared transparent (VO2(M)), and above Tc it is tetragonal, metallic and infrared reflecting (VO2(R)). [8] The idea of using VO2(M) for a smart window can be realized because ~40 °C of Tc has been attained by various dopings, in which the replacement of V4+ by a small amount of penta- or hexavalent ions seems to be the most effective. [9], [10] Currently, to fabricate VO2(M) based films, several methods have been applied, such as wet chemical approaches [11], [12], chemical vapor deposition (CVD) [13], [14], [15], pulsed laser deposition (PLD) [16], [17] and magnetron sputtering (MS) [18], [19], [20].
As is well known, the windows of the buildings and vehicles always need to be washed by the cleaners using detergents. With respect to these cleaning processes, there are two disadvantageous factors: (1) producing additional pollutants from the use of the detergents and (2) wasting a mass of labour. In addition, the common windows are in favor of fogging leading to low visibility. Thus, owing to photocatalytic and photoinduced hydrophilic properties, semiconductor photocatalysts are widely and frequently employed to treat pollutants and withstand fogs. For these photocatalysts, titania (TiO2) is one of the most active and stable, arousing tremendous interests of many researchers. [21], [22], [23], [24] Usually, crystalline TiO2 exists in three different polymorphs: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). [25], [26] Rutile TiO2 (TiO2(R)) is a thermodynamically stable phase at all temperatures and the most common natural form of TiO2. Due to similar lattice parameters, TiO2(R) films are acted as buffer layer and growth template of VO2(M) films. [17] Nevertheless, TiO2(R) films are less efficient photocatalysts than anatase TiO2 (TiO2(A)) films, which occupy an important position in the studies of photocatalytic active.
As mentioned above, it is reasonably hypothesized that the materials consisting of VO2 and TiO2 by the composites of compositions and the multilayer structure would simultaneously present antifogging, self-cleaning and energy-saving effects. This proposed material is a thrilling concept and of great value for applications in buildings. Frankly, a VO2/TiO2 composite film has been prepared by atmospheric pressure CVD and introduced both photocatalytic and thermochromic properties. [27] However, the composite film reveals the graded distribution of surface composition from rich TiO2 on the left to rich VO2 on the right, which results in an unsatisfied combinatorial property. In addition, in 2003, to improve luminous transmittance of the VO2 film, Jin et al. [1] have designed a TiO2(R)/VO2(M)/TiO2(R) multilayer film, where both TiO2 layers are only applied its antireflection effect. Subsequently, the work reported by Evans et al. [28] has dedicated to the fabrication of TiO2(A)/VO2(M) multilayer film and to the exploration of photocatalytic and thermochromic properties. But this film shows a low solar/heat control and a high contact angle (CA) with water. Therefore, to meet the requirements of application, some new innovations over the TiO2/VO2-based multilayer film are required.
Based on the above introduction, we propose a concept for constructing a TiO2(R)/VO2(M)/TiO2(A) multilayer film with antifogging, self-cleaning and energy-saving effects. As the buffer layer and the growth template of VO2(M) layer, bottom TiO2(R) layer with (1 1 0) orientation is a tetragonal form for VO2(M) layer with the less lattice mismatch (3.6%) [29] and restrains the Ca2+ and Na+ of flat glass diffusing into VO2(M) layer during the deposition process. Besides, TiO2(R) layer can increase the luminous transmittance (Tlum) as antireflection layer because of its high transmittance. Then we successfully obtained localized epitaxial growth of VO2(M) layer on TiO2(R) layer at 300 °C of deposition temperature. Following top TiO2(A) layer with a graded structure was deposited on the VO2(M) layer. The graded structure from amorphous to crystalline can prevent the epitaxial growth of top TiO2 layer on VO2(M) layer. It is pointed out by other researchers that the optimal photocatalytic performances are reached in a TiO2 film with a mixed phase, in which anatase phase is predominant. [30] Thus a suitable amount of rutile TiO2 is incorporated into the top TiO2(A) layer to improve the photocatalytic and photoinduced hydrophilic properties. In addition, to measure the multilayer film for application in smart window, we used a large-size MS apparatus with medium frequency pulsed power to deposit large-scale TiO2(R)/VO2(M)/TiO2(A) multilayer film. MS method can afford large areas for industrial purposes due to the advantages below: good adhesion of deposited films on substrates, excellent thickness uniformity, precise compositional control and long-term stability of the deposition process [31].
In this article, we report for the first time preparation of large-scale (400×400 mm2 area) TiO2(R)/VO2(M)/TiO2(A) multilayer film at glass substrate as smart window for building applications. This film performed triple functions: thermochromism from the middle VO2(M) layer for solar energy modulation, photocatalytic and photoinduced hydrophilic properties from the top TiO2(A) layer for antifogging and self-cleaning effects. Furthermore, it was worth noting that the TiO2 layers with a proper thickness could increase the Tlum of the film and enhance the regulation ability of the film for solar energy.
Section snippets
Sample preparation
The n-type Si (1 0 0) and the flat glasses with 400×400 mm2 area were applied as the substrates. TiO2(R)/VO2(M)/TiO2(A) multilayer film was deposited using medium frequency reactive MS (MFRMS) system to sputter planar rectangular metal targets (purity>99..5 wt.%, 610×85 mm2 of area) in Ar (99.99%) and O2 (99.99%) mixed atmosphere. Using a load-lock system, the base pressure of the deposition chamber was kept at 5.0×10–4 Pa. Thereafter, the power was charged on the metal target with injection of mixed
Microstructure
TiO2(R)/VO2(M)/TiO2(A) multilayer film was prepared on a large-scale glass with area of 400×400 mm2 using magnetron sputtering method, as shown in Fig. 1a. The multilayer film is homogeneous and brown-yellow in color. Fig. 1b displays XRD patterns of monolayer (TiO2(R), VO2(M) and TiO2(A)), bilayer (TiO2(R)/VO2(M)) and multilayer (TiO2(R)/VO2(M)/TiO2(A)) films, respectively. The deposition conditions of the monolayer and bilayer films were in line with that of the multilayer film. By comparing
Conclusion
A large-scale multifunctional TiO2(R)/VO2(M)/TiO2(A) multilayer film was deposited by MFRMS under a moderate deposition temperature. The bottom TiO2(R) layer, which only showed rutile phase with (1 1 0) orientation, played as the buffer layer and growth template of VO2(M) layer. The middle VO2(M) layer owned a excellent thermochromic property, and the top TiO2(A) layer displayed a eminent photoinduced hydrophilicity and a considerable photocatalytic activity. In short, the multilayer film
Acknowledgements
The authors are grateful to the high-tech project of MOST (2014AA032802), the national sci-tech support plan, the National Natural Science Foundation of China (NSFC, No.: 51032008, 51102270, 51272271).
Jianyun Zheng received his master׳s degree in Material Engineering from the Lanzhou Institute of Chemical Physics, Chinese Academy of Science, PR China in 2012. He is a Ph.D. candidate at the Shanghai Institute of Ceramics, Chinese Academy of Science, PR China. His research focuses on the energy saving materials, transition metal oxide films and devices for photocatalysts, spin tronics and chromogenic materials.
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Jianyun Zheng received his master׳s degree in Material Engineering from the Lanzhou Institute of Chemical Physics, Chinese Academy of Science, PR China in 2012. He is a Ph.D. candidate at the Shanghai Institute of Ceramics, Chinese Academy of Science, PR China. His research focuses on the energy saving materials, transition metal oxide films and devices for photocatalysts, spin tronics and chromogenic materials.
Shanhu Bao received his Ph.D. Sci. degree in Graduate School of Science and Technology in Shinshu University. He is currently a professor/researcher at Shanghai Institute of Ceramics, Chinese Academy of Sciences. His research focuses on the energy saving materials, oxide-based electrochromic and thermochromic materials and devices, magnesium based thin film switchable mirror and hydrogen sensors.
Jin Ping received his Ph.D. degree from department of Electrical and Computer Engineering in Nagoya Institute of Technology, Japan in 1992. He is currently the director of the Research Center for Industrial Ceramics and the full professor of Shanghai Institute of Ceramics, Chinese Academy of Sciences. His research focuses on the energy saving materials, transition metal oxide films and devices for photoca talysts, optical and chromogenic materials.