Designed VSP hydrogel and TCM mechanism model. Fig. 1a schematically illustrates the switchable thermochromic behavior of the designed VSP hydrogel. At lower temperature (e.g., 20 ºC), the window with VSP hydrogel is transparent to let in the solar transmittance; when heated from 20 ºC to 40 ºC, the VSP hydrogel blocks sunlight automatically to cut off solar gain. Moreover, VSP maintains the excellent reversibility of the temperature-dependent spectra (Tsol,20 °C = 78.82%, Tsol,40 °C = 1.62%). The newly developed VSP experienced a hydrophilic to hydrophobic transition at a low LCST. Below the LCST, the water molecules are kept in the interior of the PNIPAm macromolecules, which assures high transparency due to the high solar transmittance of the hydrogel. Once be heated above the LCST, the water molecules will be released from the PNIPAm, and the shrinkage particles will cause strong scattering of the light. The V0.8W0.2O2@SiO2 NPs in the VSP hydrogel plays an important role for enhancing the transmittance of sunlight and solar modulating ability. Particularly, the content of inorganic NPs used in this work was extremely lower than that in other similar reports1, 22. The involved mechanism for the performance improvement will be discussed later.
Importantly, our as-prepared thermo-responsive VSP hydrogel is liquid in the form of gels and thus can be easily laminated in-between two layers of glasses to form a Glass-VSP-Glass type "sandwich" structure, which facilitates to regulate the transmittance of sunlight8, 29 and its large-scale application for a smart window. Owing to the free-flowing feature, the liquid VSP has no any constraint of window shape. A thermo-responsive smart window using the liquid VSP was developed. Fig. 1b presents an illustration of the "sandwich" structure in which VSP hydrogel layer locates in-between two transparent glasses. When a building top is installed with suchlike large-area smart window for solar modulating, a high outdoor temperature drop will be realized and thus a comfortable building environment of "warm winter and cool summer" will be achievable. Interestingly, the trend of the transition state of the fabricated smart window with VSP shows strong time-dependence when illuminated under a 0.1 W/cm2 infrared lamp. Fig. 1c shows the of the window at 505 nm after illumination for different times. Being irradiated for one minute, the window still maintained a high transmittance with the corresponding Tlum value of 82.7%. After four minutes, the window exhibited an opaque state with a very low value reaching 0.17%. Obviously, the luminous transmittance Tlum of the window gradually decreased with increasing the irradiation time. Fig. 1d presents the corresponding photographs showing the time-dependent transmittance evolution of a VSP-based smart window device with an area of 10 cm × 10 cm. Within only ~3 minutes, it can be observed that the smart window changes from high transparency to opaqueness.
Construction of VSP. The synthesis process of VSP is illustrated in Fig. 2a. First, silica coated V0.8W0.2O2 NPs were synthesized. Subsequently, the V0.8W0.2O2@SiO2 NPs suspension in sodium dodecyl sulfate solution was added during the emulsion polymerization of PNIPAm, resulting in formation the VSP. Here the coating of silica enhances the stability of NPs and also the interaction between the groups on SiO2 coating and PNIPAm makes them to form a stable and uniform suspension. As shown in the scanning electron microscopy (SEM) images (Fig. S1a), the V0.8W0.2O2 NPs have sizes of 20~50 nm, which is consistent with that obtained from a magnified transmittance electron microscopy (TEM) image (Fig. S1b). After SiO2 coating, the resulting V0.8W0.2O2@SiO2 NPs with a core-shell structure exhibit clear outline and a uniform shell and their diameters increase to ~70 nm. The thickness of SiO2 shells is thus about 14 nm (Fig. S1d-e). The core-shell structure was further confirmed by energy dispersive spectrometer (EDS) mappings (Fig. S1c, S1f). The distributions of two main elements (V and W) in the VSP are also revealed by the EDS mapping (Fig. S4). Fig. 2b-d show TEM images at different maginfications of the resulting VSP, from which it can be seen that the NPs tend to be randomly distributed in the organic matrix. The V0.8W0.2O2@SiO2 NPs are embedded in the 3D netwok of PNIPAm, which can be confirded by the EDS analysis.
XRD patterns of VO2, V0.8W0.2O2 and V0.8W0.2O2@SiO2 are shown in Fig. 2f, where all the diffraction peaks are assigned to VO2 (M) with a monoclinic lattice symmetry and space group of P21/c (JCPDS card No.43-1051). No other detectable impurity phases are observable in the pattern. To investigate the effects of tungsten ion (W6+) doping and SiO2 on the Tc of VO2, differential scanning calorimetry (DSC) analysis was performed. As shown in Fig. 2g, the Tc of V0.8W0.2O2 NPs (27 ºC) is much lower than that of VO2 NPs (64.5 ºC), which is because the W6+ doping changes the position of V4+ and thus affects electronic states 30. Meanwhile, the nano-effect makes that the Tc of VO2 NPs is below the normal transition temperature of bulk VO2 (68 ºC) 31, 32. After the SiO2 coating, the Tc of V0.8W0.2O2@SiO2 NPs (27.7 ºC) is slightly higher (only 0.7 ºC increase) than that of V0.8W0.2O2 (before coating), which is mainly attributed to the thermal insulation role of SiO2 layer 14, 33. The composition transition from VO2 to V0.8W0.2O2 and to V0.8W0.2O2@SiO2 can be further verified by Fourier Transform Infrared Spectroscopy (FT-IR) analysis (Fig. S2). Also, the interactions between the groups on the SiO2 coated NPs and PNIPAm are demonstrated by the FT-IR analysis.
The thermochromic features are highly related to the particle size because the polymer affects the propagation path of light, leading to different visible light transmittance 34. PNIPAm possesses both hydrophilic amide groups and hydrophobic isopropyl groups, which make it temperature-responsive. The main driving forces of phase transformation are hydrogen bonds and hydrophobicity. The hydrogen bonding force between the polymer chains and water molecules changes with temperature. The vector property of the hydrogen bond causes the polymer chains to stretch (T < LCST) or shrink (T > LCST) while leading to particle size change. The regularity of hydrogen bond is susceptible to temperature, and the hydrophilicity of the polymer chains are also changed, which results in a macroscopic change of VSP particle size distribution. Interestingly, the particle sizes of the VSP are much reduced comparing with the PNIPAm, while the LCST did not change. Addition of V0.8W0.2O2@SiO2 has a great influence on the polymerization process of PNIPAm, which results in an obvious decrease of particle size. However, the LCST is mainly related to the hydrogen bonding of PNIPAm. A decrease of the original density of PNIPAm network causes it fragmented into smaller particles under mechanical agitation. That is to say, the VSP induced size reduction of PNIPAm particles and microstructure change of 3D network of PNIPAm. The W-doping decreases the Tc of VO2 from 68 °C to ~30 °C leading to an unprecedented infrared transmittance modulation range. The strong synergy between PNIPAm and NPs makes the smart window more sensitive to external temperature change.
Optical performance and theoretical calculations. Fig. 4a shows the transmittance spectra recorded at 20 ºC and 40 ºC of VSP containing different contents of V0.8W0.2O2@SiO2 NPs. At 20 ºC, as the V0.8W0.2O2@SiO2 content increases, the ∆Tsol of VSP shows a trend of gradual increase in the wavelength of 200~2500 nm, especially in the visible region (380~780 nm). Based on the curves, the characteristic values (as listed in Table 1) are calculated by using the equations (1-2). The contents of V0.8W0.2O2@SiO2 in the VSP samples increases from zero to 1.0 wt‰. The ∆Tsol and Tlum,20 °C as the function of the solid contents are plotted in Fig. 4b. For the same VSP sample, the ∆Tsol at 40 ºC decreased significantly compared with that of 20 ºC. When the content of V0.8W0.2O2@SiO2 are 0.4 wt‰, 0.6 wt‰ and 0.8 wt‰, the corresponding Tlum,20 °C values are 81.56%, 88.10% and 91.43%, and the ∆Tsol are 73.10%, 77.20% and 61.11%, respectively. Furthermore, increasing the solid content of V0.8W0.2O2@SiO2 from 0 to 1.0 wt‰ results in that the Tlum,20 °C increases proportionally. Notably, the ∆Tsol of the samples increases first and then declines, as shown in Fig. 4b. Once the solid content increases to higher than 0.6 wt‰, the ∆Tsol will be stable with no obvious change. It is worthwhile mentioning that the VSP exhibited drastically enhanced thermochromic properties as compared with that of the reported VO2-based thermochromic works (Table S1), as shown in Fig. 4c. In order to characterize the haze coefficient, the sample was also measured on the haze meter at 20 ºC and 40 ºC, respectively. The results show that the haze of VSP IV was 2.53% (20 ºC) and 91.82% (40 ºC), consistent with the UVPC analysis results. This indicates that the optical properties of VSP are temperature sensitive.