Materials Physics
Accelerating the Bleaching Rate of Photochromic WO3 Composite Films for Smart-Window Applications by Adding Low-Molecular-Weight PEG
2024 Volume 65 Issue 6 Pages 603-607
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2024 Volume 65 Issue 6 Pages 603-607
WO3-based composite films synthesized using peroxo iso-polytungstic acid (W-IPA) and a transparent urethane resin exhibit reversible photochromic properties when irradiated with light. In this study, low-molecular-weight polyethylene glycols (PEGs, average molecular weights: 200–1000) were added to WO3-based composite films and their photochromic properties were evaluated. The WO3 particles in the composite films did not vary significantly in size, irrespective of the molecular weight of the added PEG. The addition of low-molecular-weight PEGs led to composite films with higher coloring and bleaching rates, with a remarkably high bleaching rate observed for the composite film containing PEG with a molecular weight of 400. The improved bleaching properties of the prepared films are mainly attributable to electron transfer associated with the presence of low-molecular-weight PEGs in the composite films.
Fig. 7 Transmittance spectra of the S2 (PEG200) composite film before and after exposure to sunlight.
Tungsten trioxide WO3 is known to change color, from white to blue, upon electrochemical or photochemical reduction [1–8], which is attributable to the reduction of tungsten in its +6 valence state to its +5 valency [4–7]. Colored WO3 is bleached to the original white color via electrochemical or photochemical oxidation. The reversible coloring and bleaching properties of WO3 can be used in smart windows that are capable of automatically controlling the incident sunlight in a room [6, 9, 10].
Previously, we synthesized photochromic composite films by mixing a light-curable urethane resin with WO3 nanoparticles using water-soluble peroxo iso-polytungstic acid (W-IPA) as the raw material [5, 6, 9, 11–13], itself synthesized by the direct reaction of metallic tungsten powder with hydrogen peroxide. The photochromic properties of WO3 composite films are improved by adding other elements [5, 11–13], such as phosphorus [5] or copper [11], to increase the coloration rate. On the other hand, polyethylene glycol (PEG) is known to exhibit significant photoexcited electron transfer upon irradiation with light [3, 5, 6, 14]. We previously reported that the addition of PEG with a molecular weight of 1000 (as a sensitizer) significantly increases the coloration rate of a WO3 composite film [6]. Lower-molecular-weight PEGs may even exhibit greater electron transfer [14] and improved photochromic properties; however, such PEGs are liquids and are difficult to include in a composite that forms a typical WO3 film. In contrast, we showed that liquid PEGs can be complexed to form membranes in our previous study into composite membranes, which means that lower-molecular-weight PEGs can be added to a composite membrane.
In the present study, we fabricated WO3-based photochromic composite films by adding low-molecular-weight PEGs (molecular weights: 200–1000) using the abovementioned W-IPA and a transparent urethane resin. We quantitatively evaluated the effect of the low-molecular-weight PEG by examining the photochromic (coloration and bleaching) properties of the resulting WO3 composite films.
Metallic tungsten powder (5.5 g, Mitsuwa Chemicals Co., Ltd., Japan) was mixed and reacted with 15% H2O2 solution (35 mL) while being cooled. The excess H2O2 was decomposed using a platinum-black-supported platinum net at end of the reaction, after which the solvent was removed by distillation under reduced pressure (1 kPa) to obtain W-IPA as a pale-yellow powder. The synthesized W-IPA was dissolved in methanol to a concentration of 0.05 M, after the precursor solution was prepared by adding PEG (molecular weight: 200, 400, or 1000; Fujifilm Wako Pure Chemicals Co., Ltd., Japan) to a concentration of 50 mass%. The precursor solution (0.5 mL) was mixed with photosensitive urethane liquid resin (3.0 g, 3.3 cm3, Asahi Photo Microware Co., Ltd., Japan) and stirred for 10 min to ensure that the solution was uniformly dispersed in the resin. The mixed slurry was degassed at 20 kPa or less to remove air and excess methanol. The degassed slurry was molded using soda-lime glass plates and cured by irradiation with UV light from a high-pressure 100-W mercury lamp for 5 min. The resulting WO3 composite films were stored in the dark for 2 d to completely bleach them, after which they were used as samples for evaluating microstructural and photochromic properties.
The photochromic properties of the resulting films were investigated at room temperature using a UV-visible spectrophotometer (UV-1600, Shimadzu Corporation, Japan) fitted with the above-mentioned high-pressure mercury lamp. The thicknesses and microstructures of the resulting films were determined using scanning electron microscopy (SEM, JCM-6100 Plus, JEOL Ltd., Japan), with the sizes of the WO3 particles in the composite films evaluated using transmission electron microscopy (TEM, EM-002B, Topcon Corp, Japan), with samples prepared by first grinding each fabricated composite film into a powder that was then placed on a copper grid.
In this study, PEGs with average molecular weights of 200, 400 and 1000 Da were added to WO3 composite films. We also synthesized composite films using ethylene glycol or PEG with an average molecular weight of 2000 in preliminary experiments. A white precipitate formed when ethylene glycol was mixed with W-IPA, whereas a non-uniform sample formed when PEG with a molecular weight of 2000 was used (a precipitate formed) along with a prolonged mixing time. Therefore, in this study, PEGs with average molecular weights of 200, 400, and 1000 were used to synthesize films whose photochromic properties were subsequently evaluated.
Figure 1 shows surface and cross-sectional SEM images of a PEG (molecular weight: 1000)/WO3 composite film (50 mass% PEG was added, as detailed in the Experimental Section) as a representative sample. The composite film is dense, has a flat surface, and is devoid of bubbles. The other samples have similar microstructures. The composite films fabricated in this study hardly scatter incident light owing to grain boundaries in the films or rough film surfaces; hence, we assume that the photochromic properties are ascribable to the composite films and their additives. We evaluated the photochromic properties of the various WO3-based composites; the film devoid of PEG is referred to as S1, while those containing 50 mass% PEGs with molecular weights of 200, 400, and 1000 are referred to as S2, S3, and S4, respectively.
SEM images of the PEG (M.W. 1000)/WO3 composite film.
Figure 2 shows photographic images and transmittance spectra of the composite films before and after UV irradiation (10 min). All composite films turned blue when irradiated with UV light, with broad absorption peaks observed at 650 and 900 nm, which is ascribable to the reduction of the tungsten in the WO3 particles in the film from the +6 to the +5 valence state [5, 6, 8]. The addition of PEG led to slightly enhanced absorption following coloration. Broad absorption, with peaks at 650 and 900 nm, has been previously reported [5, 6], and the absorption properties observed in this study are similar to those previously reported. Therefore, we evaluated the coloring and bleaching rates of the resulting films at a wavelength of 650 nm.
Transmittance spectra of composite films S1 (without PEG), S2 (PEG200), S3 (PEG400), and S4 (PEG1000). The insets show overviews of the composite films before and after UV irradiation.
Figure 3 shows transmittances of the composite films at 650 nm as functions of time during coloration and bleaching; slightly higher coloring rates were observed after the addition of PEG, while the bleaching rate was observed to increase significantly with decreasing PEG molecular weight, which results could be confirmed from the Fig. 4 and Table 2. The coloration and bleaching rates of the composite films were quantitatively evaluated using Arrhenius plots of transmittance vs. time. Figure 4 shows plots for the various composite films acquired during coloration and bleaching, respectively. The coloration and bleaching reaction rate constants were calculated from eq. (1) using a previously report method [5, 6]:
| \begin{equation} - \ln (1 - \text{A}/\text{A}_{\max }) = \text{k}t \end{equation} | (1) |
where A is the absorbance, k is the reaction rate constant, and t is time. The coloring curves are linear; consequently coloration is a first-order reaction process. In contrast, bleaching appears to be a more-than-second-order process; consequently, the bleaching properties of the films were evaluated in two steps according to previous reports [5, 6]. The k1 is the rate constant for the “fast” bleaching that occurs after the coloration. The k2 is the rate constant for the “slow” bleaching, which bleaching is due to the presence of oxygen. The calculated coloring and bleaching rate constants for S1, S2, S3, and S4 are listed in Tables 1 and 2. The addition of low-molecular-weight PEG increases both the coloring and bleaching rate constants of the composite films, with the bleaching rate increasing with increasing PEG molecular weight (see the below of Fig. 4). Indeed, the PEG400-added composite film (i.e., S3) exhibited a bleaching rate 1.8-times that of the PEG-free film in the first bleaching stage. The colored PEG200 and 400 composite films required approximately 500 min to recover approximately 90% of their initial transmittances. The enhanced bleaching rates of the low-molecular-weight-PEG-containing composite films are rationalized in the following way. Firstly, a PEG with a smaller molecular weight increases the hydroxyl-group value [15] and promotes charge transfer between W6+ and W5+ in the WO3 particles. Secondly, more small-molecular-weight PEG chains surround the WO3 particles at the same PEG addition quantity (Fig. 5) owing to the shorter PEG chains, which promotes charge transfer between the WO3 particles and the oxygen atoms and OH groups in the PEG. No significant changes in the coloring and bleaching rates of the composite films were observed when PEG200 or 400 were added, with a slightly higher bleaching rate recorded for the PEG400-containing film. This improved performance is attributable to interactions that include electron transfer between the PEG and the urethane matrix, as well as the optimum molecular weight and amount of PEG added to ensure that the composite film has a higher fading rate.
Composite-film coloring and bleaching as functions of time.
Composite-film absorbances as functions of time.
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Schematic depicting PEG/WO3-particle interactions in the composite film.
Figure 6 shows TEM images of the various samples. Composite film S1 (without any added PEG) is composed of WO3 particles that are 34.0 nm in size, on average, while the analogous values for S2, S3, and S4 are 35.2, 33.3, and 32.7 nm, respectively. The sizes of the particles in the samples with and without PEG are almost the same (all particle sizes are within 10% of the mean). Coloring and bleaching rates have been reported to increase with decreasing WO3 particle size [16]; however, no significant differences in composite-film particle size were observed in the present study, irrespective of the addition or not of PEGs with different molecular weights. In other words, the observed increases in the coloring and bleaching rates are not attributable to the WO3-particle-size distribution; rather, they are presumed to be due to interactions between the added PEG and the WO3 particles in the film.
TEM images of composite films.
S2, as a representative sample, was irradiated with sunlight (6/23/2023, summer solstice, temperature: 24.6°C at 10:00, at Shimane, Japan), and its coloring properties were evaluated. Figure 7 presents an overview of the transmittance spectrum of the fabricated composite S2 film before and after exposure to sunlight for 10 min; it exhibited significant absorption over a wide visible-light wavelength range after exposure to sunlight for 10 min, as was also observed under UV light. All samples exhibited similar coloration and bleaching properties to those of S2 when exposed to sunlight. As mentioned above (As explained in relation to the Table 1 and 2), the colored film took approximately 6 h to recover 90% if its transmittance, which suggests that the films fabricated in this study have photochromic properties even when exposed to sunlight and can possibly be used in smart-window applications.
Transmittance spectra of the S2 (PEG200) composite film before and after exposure to sunlight.
In this study, we fabricated WO3-based photochromic composite films by adding low-molecular-weight polyethylene glycol PEGs (molecular weights: 200–1000) in order to improve their photochromic properties. Although the addition of a low-molecular-weight PEG is generally considered difficult owing to its liquid nature, we showed that PEG can be synthesized and embedded in a urethane resin matrix. All composite films exhibited photochromic properties with broad absorption peaks at 650 and 900 nm when irradiated with UV light, with the PEG-added films showing higher coloration and bleaching rates than the PEG-free film. In particular, PEG was found to significantly affect bleaching, with a 1.8-fold-higher initial bleaching rate constant (k1) recorded after the addition of PEG with a molecular weight of 400, which indicates that the bleaching rate of a WO3-based composite film can be controlled by adding a low-molecular-weight PEG. The fabricated composite films also exhibited photochromic properties when exposed to sunlight, which demonstrates that the films fabricated in this study can potentially be used in smart-window applications.
This study was supported by a grant from the “International Polyurethane Technology Foundation”.
