Thermochromic Smart Windows Assisted by Photothermal Nanomaterials

Abstract

Thermochromic smart windows are optical devices that can regulate their optical properties actively in response to external temperature changes. Due to their simple structures and as they do not require other additional energy supply devices, they have great potential in building energy-saving. However, conventional thermochromic smart windows generally have problems with high response temperatures and low response rates. Owing to their great effect in photothermal conversion, photothermal materials are often used in smart windows to assist phase transition so that they can quickly achieve the dual regulation of light and heat at room temperature. Based on this, research progress on the phase transition of photothermal material-assisted thermochromic smart windows is summarized. In this paper, the phase transition mechanisms of several thermochromic materials (VO₂, liquid crystals, and hydrogels) commonly used in the field of smart windows are introduced. Additionally, the applications of carbon-based nanomaterials, noble metal nanoparticles, and semiconductor (metal oxygen/sulfide) nanomaterials in thermochromic smart windows are summarized. The current challenges and solutions are further indicated and future research directions are also proposed.

Keywords: phase change; photothermal materials; smart window; thermochromic.

Figure 1

Types of thermochromic smart windows. (a) Metal-insulator phase transition (MIT) of vanadium dioxide (VO₂). (b) Liquid crystal changes transmittance by adjusting molecular orientation in response to temperature. (c) Thermochromic hydrogels change transparency reversibly with temperature.

Figure 2

(a) Temperature change of pure water and 2.5 mg/mL GO aqueous solution under visible light irradiation [49]. (b) Photo of the transparency transition of PNIPAMm/GO hydrogel window (left) and PNIPAm hydrogel window (right) under sunlight [41]. (c) Photo of the PNDV/GO hydrogel window transmittance gradient change with light intensity [43].

Figure 3

(a) Temperature rise curves of PVA/dye films with and without AuNRs [56]. (b) Digital photographs and infrared photographs of PVA/dye/AuNRs films and PVA/dye films before and after laser irradiation [56]. (c) Schematic representation of the structural shrinkage and deformation of PNIPAm acrylic/AgNRs when the temperature is higher than LCST [57]. (d) SEM images of PNIPAm-acrylic acid/AgNRs hydrogels at different temperatures [57]. (e) Color patterns constructed from arrays of Ag nanodiscs on silica films at different temperatures [58].

Figure 4

(a) Schematic diagram of the chemical structure and working principle of the photochromic supramolecular hydrogel smart window [73]. (b) Photographs of ATO composite hydrogel films with EGP5 contents of 2, 5, and 8 mol%, respectively, before and after irradiation at 100 mW/cm² for 10 min [73]. (c) Transmittance variation of Cs_xWO₃/PAM-PNIPAM film with temperature at 550 nm wavelength [74]. (d) Transparency changes of Cs_xWO₃/PAM-PNIPAM films with different PNIPAM concentrations at 22 °C (above) and 36 °C (below) [74].

Figure 5

(a) Schematic diagram of a VO₂/TiN smart window [86]. (b) UV-Vis-NIR absorption and transmission spectra of PNIPAm hydrogels containing different levels of PDAPs at 20 °C and 40 °C [87].