Effect of Dispersants on Photochromic Behavior of Tungsten Oxide

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Functional Nanostructured Materials (including low-D carbon)

Effect of Dispersants on Photochromic Behavior of Tungsten Oxide Nanoparticles in Methylcellulose Suzuko Yamazaki, Dai Shimizu, Seiji Tani, Kensuke Honda, Michinori Sumimoto, and Kenji Komaguchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04875 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Effect of Dispersants on Photochromic Behavior of Tungsten Oxide Nanoparticles in Methylcellulose Suzuko Yamazaki1*, Dai Shimizu1, Seiji Tani1, Kensuke Honda1, Michinori Sumimoto2, and Kenji Komaguchi3

1

Division of Earth Science, Biology, and Chemistry, Graduate School of Sciences and

Technology for Innovation, Yamaguchi University, Yoshida, Yamaguchi 753-8512, Japan 2

Division of Applied Chemistry, Graduate School of Sciences and Technology for

Innovation, Yamaguchi University, Tokiwadai, Ube 755-8611, Japan 3

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

1-4-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan

*Corresponding author. Tel. & Fax: +81-83-933-5763 E-mail: [email protected]

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Abstract Tungsten oxide-based photochromic films which change reversibly in air between colorless-transparent in the dark and dark blue under UV irradiation were prepared by using methylcellulose as a film matrix and various dispersants. Alpha-hydroxyl acid like glycolic acid (GA) or glyceric acid (GlyA) is the best dispersant because it can make the film transparent by adding a small quantity much less than that of 3-hydroxypropionic acid or ethylene glycol. Fourier-transform infrared spectra and Raman spectra indicate that a strong interaction exists between WO3 and GA or GlyA. The coloration and bleaching processes of the prepared films were investigated to clarify the effect of the dispersants and the moisture contents. The bleaching rate remarkably decreased in the films containing GA or GlyA but accelerated by increasing the contact with O2. Measurements of electron spin resonance reveals that GA and GlyA as dispersants stabilize the W5+ state. This paper shows that the coloring rate and the period for keeping the blue-colored state are tunable by changing the dispersants. The photochromic films containing α-hydroxyl acid as the dispersant have potential for application as rewritable film on which information displayed with blue-colored state can be clearly readable for longer times compared with other dispersants.

KEYWORD: photochromism, tungsten oxide, methylcellulose, dispersant, α-hydroxylic acid


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INTRODUCTION Tungsten oxide has attracted much attention due to its electrochromic or photochromic properties for a variety of applications such as information storage media and smart windows.1-5 Reversible switching between colorless and blue-colored states of WO3 can be achieved when an external voltage is applied or just under UV irradiation. Compared with electrochromism, photochromism is more advantageous because it needs only light irradiation and complex device configuration including electrodes is not necessary. In recent years, significant efforts have been devoted to improve the WO3 photochromic responses. Wei et al. reported WO3-based photochromic materials as nano-inks and fabricated flexible and photowritable membrane by electrospinning.6,7 Jiang et al. reported superhydrophobic WO3 coatings with photochromism.8 We have reported WO3-based transparent films which can control the light transmission properties of ordinary glass windows.9 For practical application, fast response time, high visualization contrast before and after light irradiation, and stable repeatability of the photochromism are required. For that purpose, sufficient fundamental understandings of the photochromism of WO3 is needed. It is generally recognized that under light irradiation, electron and hole are generated in the conduction band and the valence band of WO3, respectively, and the photogenerated electron reduces W6+ to W5+ while the photogenerated hole reacts with the adsorbed water on the WO3 surface to form H+ which diffuses into WO3 for charge compensation. The blue color is attributable to the intervalence charge transfer (IVCT) of electrons between the neighboring W5+ and W6+.10,11 WO3 + hν → WO3* ( e- + h+)

(1)

W6+O3 + xe- + xH+ → HxW6+1-xW5+xO3

(2)



Oxygen oxidizes W5+ to W6+, resulting in the bleaching process.12 Efficient proton intercalation into WO3 is believed to stabilize the blue-colored state. Indeed, the photochromism of WO3 is enhanced in polymer matrices such as poly(vinyl alcohol) (PVA)13-15, polyethylene glycol (PEG)16 and polyvinyl pyrrolidone (PVP)17 which act as good proton donors and provide fast diffusion of the liberated proton via hydrogen bonding. We have used methylcellulose (MC) as the film matrix because it contains abundant hydrogen bonding to facilitate proton transfer, it has low oxygen permeability and it is biodegradable.9 Besides, MC can be mixed with aqueous WO3 sol uniformly. Many studies on the WO3-based photochromism have indicated the reversibility between transparent state and blue-colored state by a small absorbance change. In order to obtain the deep blue-colored state, high concentration of WO3 in the polymer matrix is necessary, which causes lack of the transparency due to the agglomeration of WO3 particles. The transmittance of the WO3/MC film decreased with an increase in the amount of WO3 in the film because of the formation of large aggregates with the size of 1 – 1.5 µm. However, by the addition of polyols such as ethylene glycol (EG), propylene glycol and glycerin as dispersants, we succeeded the fabrication of the photochromic film with high visualization contrast between transparent and deep blue-colored state.9 The coloring rate increased with an increase in the water content in the WO3/polyols/MC film. By using our prepared film, we can study spectral change systematically in the wide range of absorbance from zero (completely transparent) to 3.0 or 4.0 reversibly, which provides insight into understanding the photochromic behavior of WO3 in polymer matrix. In this paper, we use organic compounds containing carboxyl group as the dispersants and show a new finding about the strong interaction between WO3 and α-hydroxyl acid to stabilize the colored W5+ states.


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Scheme 1. Chemical structures of dispersants, (a) acetic acid, (b) oxalic acid, (c) glycolic acid, (d) glyceric acid, (e) 3-hydroxypropionic acid, (f) glycine and (g) ethylene glycol. (a) AA

(b) OA

(e) 3-HPA

(c) GA

(f) Gly

(d) GlyA

(g) EG

EXPERIMENTAL SECTION Preparation of aqueous WO3 sol was reported previously.9 MC (methoxy group: 26-33%) as film matrix, acetic acid (AA), oxalic acid (OA), glycolic acid (GA), glyceric acid (GlyA), 3-hydroxypropionic acid (3-HPA), glycine (Gly) and EG as organic additives were purchased from Wako Pure Chemical or Tokyo Kasei Kogyo and used without further purification. Chemical structures of the organic additives are shown in Scheme 1. Under vigorous stirring, 0.4 gram of MC was dissolved in 20 ml of the aqueous WO3 sol containing the organic additives by keeping at 55oC for 10 min and then cooled in an ice bath for 10 min. The obtained transparent colloidal solution was placed in a petri dish (4.5 cm in radius) and dried at 30oC for 48 h. In order to peel off the film smoothly, the petri dish was subjected to hydrophobic treatment with toluene containing three drops of dichlorodimethylsilane using Pasteur pipette and washed with water thoroughly. The dried film (thickness: ca. 60 ± 5 µm) was cut into small pieces of 3.5 x 1.0 cm2 and fixed in a quartz optical cell. The spectral change was measured by UV/vis spectrophotometer 5 ACS Paragon Plus Environment


(Shimadzu, UV-1800) under irradiation of 250 W extra-high pressure mercury lamp (Ushio USH-250SC2) through a band pass filter (Hoya U-330). The light intensity (λmax = 365 nm) on the surface of the cell was measured to be 95 mW cm-2 by using UV radiometer (UVR-400, Probe 365 nm, Iuchi). The structural characterization of the film was studied by transmission electron microscopy (TEM, Nippon Denshi, JEM-2100) operating at 200 kV accelerating voltage. Samples for TEM analysis were prepared by placing a drop of the colloidal solution which was used to fabricate the films on a carbon-coated Cu grid (200 mesh). Fourier transform infrared spectroscopy (FTIR) measurements of the prepared films were performed by an attenuated total reflectance (ATR) method using Ge prism (Shimadzu FTIR-8400s). The spectrum was taken by adding 256 scans at a resolution of 4 cm-1. In the case of powder samples, KBr pellets were prepared and the FTIR measurements were carried out in transmission mode. Raman spectra of the prepared films were obtained using spectrophotometer (Jasco, NRS-3100) equipped with a 532 nm CW diode laser. Water contents in the films were estimated by the weight loss by heating at 120oC to get a constant weight. The films containing various water contents were prepared as the following method: the films were preheated at 120oC for 30 min until these weight remained unchanged (defined as water content of 0%) and then the water contents in the films were varied by changing time for passing the humid air at the flowrate of 100 ml min-1 through the optical cell in which the films were fixed. For the measurements of electron spin resonance (ESR), the dried film was cut into the size of 6 x 6 cm2 and irradiated with UV as described above. After the irradiation of 10 min, the obtained blue film was cut into small pieces of 6 x 0.2 cm2 and packed into an ESR sample tube (JEOL 6 ACS Paragon Plus Environment

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DATUM). The ESR spectra (Bruker Biospin Corporation ELEXSYS E500) were measured at 77 K with a modulation amplitude of 1 G and microwave frequency of approximately 9.4 GHz. RESULTS AND DISCUSSION The WO3/MC composite film was opaque white and absorbed the light below 400 nm due to the band gap transition of WO3. Figure 1 shows effects of the amount of organic additives on transmittance at 640 nm of the film. The transmittance of WO3/3-HPA/MC film increases with an increase in the molar ratio (γ) of 3-HPA/WO3 and reaches 90% at γ = 1.6.

Figure 1. Effect of the amount of organic additives on transmittance of the WO3/MC composite films before UV irradiation.

This behavior is almost coincident with that of the WO3/EG/MC film. The film with the transmittance of 90% was completely clear because the transmittance of the film in the optical quartz cell was measured against air, i.e. ca. 10% of the light was scattered on the surface of the quartz glass. In the presence of GA and GlyA, the composite films are completely transparent even at γ = 0.1 and their transmittance decreases at γ > 1.0 and γ > 7 ACS Paragon Plus Environment


0.6, respectively. On the other hand, no films were transparent in the presence of AA, OA and Gly. These results suggest GA and GlyA are good dispersants to make the film transparent by adding a small quantity. Figure 2 shows TEM image of the WO3/GA/MC film with γ = 0.6, indicating WO3 particles of ca. 50-100 nm in diameter.

500 nm

Figure 2. TEM image of WO3/GA/MC composite film (WO3 content：3.8×10-5 mol/cm2, GA/WO3 = 0.6).

Figure 3. FTIR spectra. (a) WO3, (b) OA, (c) GA, (d) GlyA, (e) 3-HPA, (f) Gly. On the left, (g) WO3/OA, (h) WO3/GA, (i) WO3/GlyA, (j) WO3/3-HPA and (k) WO3/Gly powders. On the right, (g) MC, (h) WO3/MC, (i) WO3/OA/MC, (j) WO3/GA/MC, (k) WO3/GlyA/MC, (l) WO3/3-HPA/MC and (m) WO3/Gly/MC films. Figure 3 depicts the FTIR spectra of the composite films and their components where WO3 8 ACS Paragon Plus Environment

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or WO3/(organic additives) stands for the powders obtained by drying the pure WO3 sol or the WO3 sol mixed with the organic additives, respectively, and all the notations with MC mean the film. For comparison, these samples were prepared with γ = 1.0. An absorption peak at 1626 cm-1 in WO3 is attributable to H-O-H bending for adsorbed water.18 Comparison of the FTIR spectra of pure WO3, organic additives and WO3/(organic additives) hybrids shows that all peaks observed with pure WO3 and the organic additives appear in the hybrids’ spectra except for slight band shifts. A new peak is observed at 1365 or 1351 cm-1 for WO3/GA or WO3/GlyA, respectively. Goulden et al. described that the O-H bending of glycolate ion at 1240 cm-1 was shifted to 1390 cm-1 upon coordination to Zn(II) ion in aqueous solutions because the metal cation was bounded to both the carboxyl group and the oxygen atom of the hydroxyl group.19 It is well-known that α-hydroxyl acid like GlyA as well as GA can act as bidentate ligands which coordinate to a metal atom through both carboxyl and hydroxyl oxygens. Similar behaviors observed in Figure 3 suggest that W(VI) in WO3 interacts with GA and GlyA just like bidentate ligands. On the contrary, in the case of 3-HPA which is β-hydroxyl acid, the O-H bending at 1261 cm-1 is slightly shifted to 1263 cm-1 but any new peak is not observed in WO3/3-HPA as shown in Figure 3. This finding suggests the interaction of 3-HPA with WO3 only via OH moiety. In the WO3/MC film, the peak at 1626 cm-1 due to H-O-H bending observed in WO3 is shifted to 1637 cm-1, suggesting a formation of hydrogen bonding with the OH moiety in MC. Rangelova et al. reported that the H-O-H bending peak at 1647 cm-1 on SiO2 shifted by ca. 10 cm-1 in MC/SiO2 hybrid due to intermolecular hydrogen bonding.20 Similarly, the peak appeared at 1647 or 1646 cm-1 in the WO3/GA/MC or WO3/GlyA/MC films, respectively, is assignable to the intermolecular hydrogen bonding. Besides, new peaks



appeared at 1375 cm-1 similarly as WO3/GA and WO3/GlyA, suggesting that the interaction like bidentate ligands remains in the composites with MC.


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Figure 4. Raman spectra of (a)WO3 pellets, (b)WO3/MC, (c)WO3/AA/MC, (d)WO3/EG/MC, (e)WO3/3-HPA/MC, (f)WO3/GA/MC, (g)WO3/GlyA/MC and (h)MC films and peak deconvolution of (a) – (g). Dashed lines of red, green and blue are the separation peaks and orange line indicates the sum of these three peaks. 11 ACS Paragon Plus Environment


Raman spectra of WO3 as shown in Figure 4 indicates peaks at 648.5 and 978 cm-1 which are assigned to stretching modes of the bridging oxygens (O-W-O) and the stretching mode of the terminal W=O bond, respectively.18, 21-24 The MC film shows the characteristic cellulose peaks in the range of 500 – 600 cm-1 and 1000 – 1200 cm-1.20 In the opaque WO3/MC or WO3/AA/MC film, the peaks related to O-W-O and W=O bonds are observed but the characteristic MC peaks are not observed. On the other hand, four transparent films containing dispersants, WO3/EG/MC, WO3/3-HPA/MC, WO3/GA/MC, and WO3/GlyA/MC films, show peaks characteristic of MC and WO3. These findings suggest that the characteristic MC peaks are hindered by the large WO3 aggregates formed in WO3/MC and WO3/AA/MC films. Daniel et al. reported the existence of four distinct terminal W=O bonds in the WO3•2H2O compound, which were not vibrationally independent and would be dependent on the geometrical disposition.18 In Figure 4, deconvolution of the Raman spectra assigned to the terminal W=O stretching mode are also shown. The band at 978 cm-1 in WO3 can be deconvoluted into three Raman peaks at 981, 965, and 940 cm-1. Hereafter, these peaks are denoted by peak I, II and III from the longer wavenumber. Similar three peaks are obtained by deconvolution of the Raman spectra for all the composite films with MC and the organic additives. These three peaks were shifted to lower wavenumbers in WO3/MC, WO3/AA/MC, WO3/EG/MC and WO3/3-HPA/MC compared with those in WO3, suggesting that the W=O bonds become weak by the interaction via hydrogen bonding. It is noted that the intensities of peak I and peak II are almost the same for these five samples. On the other hand, in WO3/GA/MC and WO3/GlyA/MC, the position of peak I is coincident with that in WO3 and the relative intensity of peak II to peak I decreases 12 ACS Paragon Plus Environment

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significantly. Such a different behavior also suggests that the interaction of α-hydroxyl acids with WO3 is different from that of other dispersants. Figure 5 (a) - (d) show the spectral changes of the as-prepared transparent WO3/MC films containing GA, GlyA, 3-HPA and EG under UV irradiation for 5 min. Figure 5 (e) indicates the coloration rate to blue is the following order: GlyA > EG > 3-HPA > GA. Figure 5 (f) shows the change of the absorbance in the dark just after UV irradiation for 5 min, indicating that the bleaching of the blue color of the WO3/MC films containing GlyA or GA is much slower than that with EG or 3-HPA. Since these dispersants are hydrophilic, those films contain moisture. We reported that as the moisture increased, the coloration rate of the WO3/EG/MC film was accelerated significantly.9 The moisture contents in the films used in Figure 5 were estimated to be ca. 10% for GA, GlyA and 3-HPA and ca. 12% for EG. Figure 6 (a) – (d) show the spectral changes of these films with the moisture content of 17% under UV irradiation for 3 min. Figure 6 (e) exhibits that the blue coloration rate is the highest for the WO3/GlyA/MC films even under the same moisture contents. Comparison of Figure 5 (a, b) and Figure 6 (a, b) reveals that the shape of the absorption spectra of the WO3/GA/MC and WO3/GlyA/MC change drastically, i.e. the peak in the infrared region increases in the presence of more moisture. In order to clarify the effect of the moisture contents, we prepared the WO3/GlyA/MC films with γ = 0.8 containing the moisture of 0, 7.78, 17.4, and 30.7%. Figure 7 shows that as the moisture increases, the absorption peak at the infrared region (ca. 960 nm) increases significantly, which accompany an increase in the absorption around 640 nm.



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(f)

(e)

Figure 5. Spectral change of the WO3/MC composite films with the molar ratio of (a) GA/WO3 = 0.8, (b) GlyA/WO3 = 0.8, (c) 3-HPA/WO3 = 1.6 and (d) EG/WO3 = 3.0 under UV irradiation. Time profiles of absorbance at the peak wavelength (e) under UV irradiation or (f) in the dark.


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(e)

Figure 6. Spectral change of the WO3/MC composite films with molar ratio of (a) GA/WO3 = 0.8, (b) GlyA/WO3 = 0.8, (c) 3-HPA/WO3 = 1.6 and (d) EG/WO3 = 3.0 under UV irradiation. (e) Time profiles of absorbance at the peak wavelength. Moisture content of all films was 17 %.



Figure 7. Effect of moisture content on the spectral change of the WO3/GlyA/MC composite film during UV irradiation. All films were fabricated with the molar ratio (GlyA/WO3) of 0.8. The absorption spectra were recorded at 1 min intervals. Moisture content: (a) 0, (b) 7.78, (c) 17.4 and (d) 30.7 %.

Figure 8 (a) shows time course of the absorbance at ca. 640 nm, indicating that the blue coloration rate increases with an increase in the moisture, similarly as observed in the WO3/EG/MC film.9 All absorption spectra in Figure 7 were successfully fit by four Lorentz functions and these absorption bands had peaks at 480.5, 639 – 642 (band 1), 774 - 776 (band 2) and 949 – 962 nm (band 3), respectively, as listed in Table 1. Ai et al. reported for phosphotungstic acid imbedded in polyvinyl alcohol (PVA) that d-d transition of W5+ 16 ACS Paragon Plus Environment

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(b)

(a) (a)

Figure 8. (a) Time profiles of absorbance at ca. 640 nm of the WO3/GlyA/MC composite films with various moisture contents. (b) Effect of moisture content on absorbance at ca. 960 nm, 775 nm, 640 nm and 480 nm at the UV irradiation for 3 min. All films were fabricated with the molar ratio (GlyA/WO3) of 0.8. Table 1. Optical parameters obtained by four-peak Lorentz fitting for the WO3/GlyA/MC composite films at UV irradiation for 3 min. Moisture content (%)

0

7.78

17.36

30.69

Wavelength (nm)

Absorbance

480.5

0.366

639.3

0.672

775.6

0.298

961.8

0.856

480.5

0.263

642.5

0.931

775.6

0.299

951.1

1.356

480.5

0.302

642.5

1.217

774.4

0.226

951.1

2.993

480.5

0.302

642.5

1.114

775.6

0.233

949.3

3.436

appeared at about 500 nm and the IVCT transition from W5+ to W6+ was observed at 625 – 800 nm.13 For polytungstic acid (WO3 • n H2O) incorporated in PVA film, Yumashev et al. assigned absorption peaks at 925, 620 and 776 nm to the IVCT transition in edge-sharing, 17 ACS Paragon Plus Environment


corner-sharing octahedral and tritungstic groups of WO6 clusters which have been often considered as building blocks for the formation of WO3.15 Figure 8 (b) shows that the band 3 at ca. 960 nm increases remarkably with increasing the moisture whereas the band 2 at ca. 775 nm remains constant. Previously, we reported that all absorption spectra observed in the WO3/EG/MC film were fit by three bands with peaks at 631-653, ca. 775, and 976-1050 nm.9 These peaks are almost coincident with the bands 1 – 3 in the WO3/GlyA/MC films. However, as the moisture increased from 0 to 41.3% in the WO3/EG/MC film, the absorption peak at ca. 775 nm significantly increased.9 Aqueous WO3 sol became blue in nitrogen atmosphere under UV irradiation and its absorption spectra showed an intense peak at 775 nm.25 Comparison of these data indicates that WO3 in the WO3/EG/MC film containing more moisture is similar to that in aqueous WO3 sol but that in the WO3/GlyA/MC is completely different. The former suggests the hydrogen bonding interaction between WO3 and EG and the latter is explainable in terms of stronger bidentate-like interaction of GlyA. This study also presents a new finding that the WO3/GlyA/MC or the WO3/GA/MC film containing moisture absorb the infrared light more effectively than the visible light and thus it can be used for smart windows to prevent the inside of a building from being heated by the incident sunlight.26-28 The effect of the amount of GlyA (γ = 0.2 – 0.8) on the photochromic behavior of the WO3/GlyA/MC film was examined in the absence of moisture after drying. No appreciable difference was observed in the absorption spectra. As shown in Figure 9, the coloration rate is not dependent on the γ values while the bleaching is accelerated at lower γ values. Figure 10 shows time course of the absorbance of the WO3/GlyA/MC and WO3/GA/MC films in the dark just after UV irradiation to get the coloration with the absorbance of 1.5. In both 18 ACS Paragon Plus Environment

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films, the bleaching rate is accelerated in the following order: Ar < Air < O2 < O2 (at 50 oC). This finding suggests that the bleaching of the blued WO3/GlyA/MC or WO3/GA/MC film can be easily accelerated by making contact with more O2 molecules or enhancing the diffusion of O2 into the film at the elevated temperature.

Figure 9. Time profiles of absorbance at the peak in the visible region of the WO3/GlyA/MC composite films.

Figure 10. Time profiles of absorbance at the peak in the visible region of the WO3/GA/MC (left) and WO3/GlyA/MC (right) composite films.



Figure 11. ESR spectra of the composite films. (a) WO3/MC before UV irradiation and (b) WO3/MC, (c) WO3/EG/MC, (d) WO3/3-HPA/MC, (e) WO3/GA/MC, and (f) WO3/GlyA/MC after UV irradiation. The broken lines in (e) and (f) show simulation.

Figure 11 indicates ESR spectra of WO3/MC, WO3/EG/MC, WO3/3-HPA/MC, WO3/GA/MC and WO3/GlyA/MC films at 77 K. All the films exhibited no significant ESR signals without UV irradiation. After UV irradiation, the WO3/MC, WO3/EG/MC and WO3/3-HPA/MC films show an isotropic signal with g = 1.837, which is coincident with that assigned to W5+ in PVA or poly(ethylenimine) film containing polytungstate.13,29 In these polymers, the colored W5+ state was stabilized through hydrogen bonding between WO6 unit and polymer matrix. On the other hand, a new broad signal appears at g ≈ 1.75 for the WO3/GA/MC and WO3/GlyA/MC films, which is characterized by the anisotropy of 20 ACS Paragon Plus Environment

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the axially symmetric g factor. These findings suggest that the ESR spectra of the WO3/GA/MC and WO3/GlyA/MC film consist of two spectral components for the colored W5+ state, i.e. one is affected by the hydrogen bonding (component A) and another is by stronger interaction, probably due to the coordination with the bidentate GA or GlyA (component B). Anisotropic ESR signal of g = 1.7 was observed for W5+ in tungsten phosphate glasses melted at 1400oC30 and isolated complexes of W5+ in catalytic olefin metathesis.31 The relative amounts of W5+ was evaluated from double integrals of these two component signals. As Table 2 shows, the W5+ in component B is one order of magnitude more than in component A. The broken lines in Figure 11 (e) and (f) indicate simulated profiles using the ESR parameters listed in Table 2. As mentioned above, the bleaching rate of the colored WO3/GA/MC and WO3/GlyA/MC film was much slower than the WO3/EG/MC and WO3/3-HPA/MC films. This fact indicates that GA and GlyA as dispersants stabilize the W5+ state by the strong coordination, resulting in the suppression of the oxidation by O2 in air. Table 2. ESR parameters of W5+ evaluated from simulation for the WO3/GA/MC and WO3/GlyA/MC composite films. WO3/GA/MC

WO3/GlyA/MC

Component A

Component B

Component A

Component B

Relative ratio

1

15.2

1

17.4

g//

1.8120

1.7331

1.8060

1.7331

g⊥

1.8417

1.7871

1.8397

1.7861

giso

1.8318

1.7691

1.8285

1.7684

Line width (G)

37

77

37

82



The photochromic behavior was examined by repeating five coloration-bleaching cycles on the WO3/GA/MC film with γ = 0.8. The interval of each cycle was a week to get full transparency before the next run. Any significant change was not observed in the shape of the absorption spectra. After the UV irradiation for 5 min, the absorbance at 660 nm was 1.76, 1.38, 1.47, 1.26 and 1.23 at the 1-5 cycles. Except for the 1st cycle, the coloring rate and the bleaching rate did not change significantly in each cycle.

CONCLUSION In summary, we have fabricated photochromic films containing MC, WO3 nanoparticles and various dispersing agents. The dispersants play an important role not only in the suppression of the aggregation of WO3 but also in the photochromic response of WO3 to UV irradiation. In our previous report, we described the potential use of the WO3/EG/MC films as detachable films for windows to control the transmission of sunlight because these films exhibited dark blue with absorbance of 3.5 at 960 nm by being exposed to sunlight in the summer and returned to transparent during the night. In this paper, we have demonstrated that the use of α-hydroxyl acid like GA and GlyA as the dispersants stabilize the W5+ state by the strong interaction with two functional groups such as OH and COOH and suppress the bleaching of the colored films. Indeed, it took 3 days to bleach the blued WO3/GA/MC films (absorbance of 1.5 at 640 nm) to transparent. The WO3/GA/MC and WO3/GlyA/MC films have more advantages over the WO3/EG/MC films for the application of rewritable display medium. In the nitrogen atmosphere, the intense blue color of the WO3/GA/MC film remained for 4 weeks. Good feature of our films is that we can 22 ACS Paragon Plus Environment

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control the time for keeping blue color by changing the dispersants from polyols to α-hydroxyl acid and by controlling the contact with O2 in air. Besides, our film gives no disposal problem since MC is environmental benign.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS This work was partially supported by YU Project for Formation of the Core Research Center. We thank Yamaguchi University Science Research Center and Innovation Center for the ESR measurements and TEM observation. REFERENCES (1) Wang, S.; Fan, W.; Liu, Z.; Yu, A; Jiang, X. Advances on Tungsten Oxide Based Photochromic Materials: Strategies to Improve Their Photochromic Properties. J. Mater. Chem. C 2018, 6, 191-212. (2) Wang, Y.; Kim, J.; Gao, Z.; Zandi, O.; Heo, S.; Banerjee, P.; Milliron, D.J. Disentangling Photochromism and Electrochromism by Blocking Hole Transfer at the Electrolyte Interface, Chem. Mater. 2016, 28, 7198-7202. (3) Malara, F.; Cannavale, A.; Carallo, S.; Gigli, G. Smart Windows for Building Integration: A New Architecture for Photovoltachromic Devices. ACS Appl. Mater. Interfaces 2014, 6, 9290-9297. 23 ACS Paragon Plus Environment


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Effect of Dispersants on Photochromic Behavior of Tungsten Oxide

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