Biologically Inspired Nanostructures for Smart Windows with Antireflection and Self-Cleaning Properties

随着环境温度的变化，智能热致变色窗的保温性能得到了广泛的研究，以提高商业和住宅建筑的能源效率。这些窗口通常涂有热致变色材料，这些材料在半导体和金属相之间表现出完全可逆的、温度依赖的转变。在炎热的天气里，智能窗户会通过全部或部分可见辐射，并阻挡大部分太阳的近红外辐射；这样就减少了对空调的需求。在较冷的天气里，可见光和红外（IR）辐射都被充分传输，限制了内部加热的需要。这种智能涂层的流行材料是二氧化钒（VO2），因为i)在红外区域表现出辐射阻带，ii)它可以很容易地应用于大型衬底的优势，以及iii)通过掺杂金属化合物（最常见的是钨）来降低其相变温度的能力。计算表明，在意大利和埃及等气候炎热的国家，VO2涂层可以使建筑物的能耗减少30%。尽管如此，VO2涂层的优点在较冷的气候条件下迅速减弱，在赫尔辛基或莫斯科等地，它们实际上提供了负能量平衡。这种性能逆转的一个非常重要的因素是VO2在其冷透明相中表现出的高折射率，这导致大部分入射光被反射——在玻璃上50 nm厚的VO2薄膜上，30%-35%的可见光被反射。相比之下，传统玻璃窗的反射率<4%，这意味着在冬季，热致变色窗比普通玻璃更暗、更冷，这反过来又意味着建筑照明和供暖所需的能量实际增加。此外，污垢和污渍会进一步降低智能窗的传输性能。为了克服上述限制，首次提出了具有宽带和广角抗反射特性的蛾眼型结构，以大幅改善目前热致变色智能窗口较差的传输性能，并为该技术的商业化铺平道路。与现有的热致变色窗口相比，我们的纳米图案窗口可能具有72%的高透射率，此外，它们同时具有自清洁特性，无需额外的处理。这个具有挑战性的、概念验证的、为期24个月的研究项目专注于蛾眼纳米结构增强智能窗户的制造和表征，并分为两个研究流：A)直接在玻璃上制造和表征抗反射和自清洁蛾眼纳米结构，适用于新的高端窗户产品。B)开发潜在的低成本热致变色聚合物薄膜来改造现有的非智能窗户。

Smart thermochromic windows whose insulation properties are tuned by the ambient temperature have been investigated extensively over recent years to improve energy efficiency of commercial and residential buildings. These windows are typically coated with thermochromic materials that exhibit a fully reversible, temperature dependent transition between semiconductor and metallic phases. During hot weather, a smart window passes all or part of the visible radiation incident and rejects the majority of the Sun's near-infrared radiation; thus the need for air conditioning is reduced. During cooler weather, both visible and infrared (IR) radiation is fully transmitted, limiting the need for internal heating. A popular material for such intelligent coatings is Vanadium dioxide (VO2) due to i) the radiation stop-band manifesting in the IR region, ii) the advantage that it can easily be applied to large substrates and iii) the ability to lower its phase transition temperature by doping it with metal compounds, most commonly tungsten. Calculations have shown that a VO2 coating can deliver a 30% reduction in energy consumption of buildings in countries with hot climates such as Italy and Egypt. Nonetheless, the merits of VO2 coatings quickly diminish in colder climates and in places like Helsinki or Moscow they, in fact, deliver a negative energy balance. One very important factor for this performance reversal is the high refractive index that VO2 exhibits in its cold-transparent phase, which results in a large portion of the incident light being reflected - 30%-35% in the visible for a 50 nm thick VO2 film on glass. This figure compares with <4% reflectivity in conventional glass windows, meaning that a thermochromic window is much darker and colder than its plain glass counterpart in the winter, which in turn translates to an actual increase in the energy required for lighting and heating a building. In addition, dirt and stains further degrade the transmission properties of a smart window. In order to overcome the above limitations, moth-eye type structures engineered to exhibit broadband and wide-angle antireflection properties are proposed, for the first time, to substantially improve the currently poor transmission properties of thermochromic smart windows and to pave the way for the commercialization of this technology. Our nanopatterned windows potentially have 72% higher transmission compared to existing thermochromic windows and in addition, they exhibit simultaneous self-cleaning properties without additional processing. This challenging, proof-of-concept, 24-month research project focuses on the fabrication and characterization of smart windows enhanced with moth-eye nanostructures and is divided into two research streams: A) Fabrication and characterization of antireflection and self-cleaning moth-eye nanostructures directly onto glass, appropriate for new high-end window products. B) Development of potentially low-cost thermochromic polymer thin-film to retrofit existing non-smart windows.