Methylcellulose

Research Article www.acsami.org

Photochromic Properties of Tungsten Oxide/Methylcellulose Composite Film Containing Dispersing Agents Suzuko Yamazaki,* Hiroki Ishida, Dai Shimizu, and Kenta Adachi Division of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan S Supporting Information *

ABSTRACT: Tungsten oxide-based photochromic films which changed 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 polyols such as ethylene glycol (EG), propylene glycol (PG), and glycerin (Gly) as dispersing agents. Influence of the dispersing agents and water in the films on the photochromic behavior was systematically studied. Under UV irradiation, absorption bands around 640 and 980 nm increased and the coloring rate was the following order: Gly > EG > PG. An increase in the amounts of dispersing agents or water accelerated the coloring rate. By increasing the water content of the film, a new absorption peak appeared at ca. 775 nm and the Raman spectra indicated a shift of W−O−W stretching vibration to lower wavenumber which was due to the formation of hydrogen bonding. All absorption spectra were fit by three Lorentz functions, whose bands were ascribed to various packing of WO6 octahedra. After the light was turned off, the formation of W5+ was stopped and bleaching occurred by the reaction with O2 in air to recover its original transparent state. We anticipate that the biodegradable photochromic films developed in this study can be applied in recyclable display medium and especially in detachable films for glass windows whose light transmission properties are changed by sunlight, i.e., for usage as an alternative of smart windows without applying voltage. KEYWORDS: photochromism, tungsten oxide, methylcellulose, dispersing agents, transparent films

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INTRODUCTION Photochromic materials have attracted much attention for a variety of applications such as information storage media, display devices, chemical sensors, and smart windows.1,2 Inorganic photochromic materials have better thermal and chemical stability than organic counterparts.1,3 Tungsten oxide is a wide band gap semiconductor and one of the most wellknown photochromic materials.4−6 However, the slight color change, slow response, and poor irreversibility of WO3 under UV irradiation hinder its practical application. In order to improve the photochromic performance, the effect of morphologies3,7,8composites with other metal oxides,9,10 and fabrication of the ultrafine quantum dots11,12 have been widely investigated. In general, the coloration of WO3 in blue occurs by the injection of the photogenerated electron and protons. Under UV irradiation, the photogenerated electrons reduce W6+ to W5+ while the photogenerated holes oxidize the adsorbed molecules on the WO3 surface to liberate protons. The protons are injected in the WO3 structure to give electrical neutrality, forming blue hydrogen−tungsten bronze, HxWVxWVI1−xO3.13−15 The blue color is attributed to the intervalence charge transfer of electrons between the neighboring W5+ and W6+. The double injection of electrons and protons produces the huge amount of color centers.16,17 © 2015 American Chemical Society

Thus, the photochromism is enhanced by hydrogen-containing molecules such as alcohols which act as good proton donors and provide fast diffusion of the liberated proton via hydrogen bonding.18,19 Oxygen oxidizes W5+ to W6+, resulting in the bleaching process. In a colloidal state, the photochromism (blue coloration) of aqueous WO3 sol was observed only under deaerated conditions and the blue color disappeared by exposure to air.20,21 A sputtered crystalline WO3 thin film showed photochromic coloration in the presence of HCOOH vapor under nitrogen atmosphere.13 Integrating photochromic molecules into a polymer matrix is one method of preparing materials with controllable photochemical properties.22 Several research groups studied the photochromism of the WO3/ poly(vinyl alcohol) (PVA) film under air atmosphere, which was obtained by drying PVA solution containing WO3 colloid particles and which exhibited opaque or yellowish transparency before UV irradiation.23−26 Yumashev et al. described that PVA prevented the reoxidation of the photochemically generated W5+ in the film because of the low efficiency of O2 diffusion in the PVA.23 Dejournett and Spicer reported that hexafluoropropylene-co-tetrafluoroethylene polymer films containing Received: October 1, 2015 Accepted: November 9, 2015 Published: November 9, 2015 26326

DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332

Research Article

ACS Applied Materials & Interfaces WO3, which were prepared by chemical vapor deposition, were visually homogeneous blue due to oxygen deficiency and became deep blue with UV exposure.27 However, high contrast of the color of the films before and after UV irradiation, i.e., colorless−transparent and dark blue, is preferred for practical applications of the photochromism. Cellulose derivatives are biodegradable natural polymers and have a good film-forming property.28−30 Previously we prepared colorless−transparent films by casting trifluoroacetic acid solutions uniformly dissolved with microcrystalline cellulose (CE) and then dipping them in WO3 nanocolloid solution to fabricate the WO3/CE film. The interfacial interaction between hydroxyl groups on the surface of the CE substrate and WO3 nanoparticles via hydrogen bonding played a major role in the enhancement of the photochromism.31 However, the color of the WO3/CE film under UV irradiation was not deep blue since the saturated surface concentration of WO3 on the CE film was limited to be as low as (4.6 ± 0.4) × 10−7 mol dm−2. In this work, we have succeeded with the fabrication of the WO3 photochromic films which change reversibly between colorless−transparent in the dark and dark blue under UV irradiation. Methylcellulose (MC) is used as the film matrix because it is water soluble and can be mixed with aqueous WO3 sol uniformly. By casting the mixed solution, the WO3 nanoparticles exist not only on the surface but also in the bulk of the MC films. Furthermore, the addition of polyols as dispersing agents suppresses the agglomeration of the WO3 particles and makes the film colorless−transparent over its wide concentration range, which enables systematic studies on the photochromic behavior of the film. As far as we know, this is the first report on the effect of different parameters such as the amounts of OH moiety, the WO3 particles, and moisture on the enhancement or peak position of the absorption bands in the WO3 photochromic film.

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Scheme 1. Chemical Structures of MC and Various Dispersing Agents

the spectral change was measured by UV/vis spectrophotometer (Shimadzu, UV-1800) under the irradiation of four 4 W black light bulbs (Toshiba FL4BLB; λmax = 352 nm). The intensity of the irradiated light on the surface of the cell was measured to be 4.3 mW/ cm2 by using a UV radiometer (UVR-400, Probe 365 nm, Iuchi). An extrahigh-pressure Hg lamp (250 W) equipped with a U-330 filter (intensity, 95 mW/cm2) was used to examine the reversibility of the coloration−bleaching process. 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). Water contents in the films were estimated by the weight loss by heating at 120 °C for 10 min. The WO3/EG/MC films containing various water contents were prepared by the following method: all of the films were preheated at 120 °C for 10 min to get the water content of 0%, and then the films containing 16.4, 29.0, and 41.3% H2O were fabricated by passing humid air over them at the flow rate of 100 mL min−1 through the optical cell for 30, 90, and 600 min, respectively. Gas permeation experiments were carried out using a vacuum time-lag method to measure the gas permeability coefficients.32

EXPERIMENTAL SECTION

Preparation of WO3 Sol. Aqueous WO3 sol was prepared from Na2WO4·2H2O and HCl. Hydrochloric acid (7.0 mol dm−3, 9.5 mL) was added to an aqueous Na2WO4 solution (0.48 mol dm−3, 90 mL) dropwise under vigorous stirring. The obtained transparent sol was dialyzed in a molecularly porous dialysis tube (molecular weight cutoff (MWCO), 3500) in 1 L of water. The dialysis was continued for 8 h by changing the water every 1 h until the Cl− ion in the water was not detected by ion chromatography (Shimadzu, PIA-1000). The concentration of WO3 in the finally obtained sol was determined to be 0.12 mol dm−3 by inductively coupled plasma spectroscopy (Varian, ICP-AES Liberty Series II). X-ray diffraction (Rikagaku RINT-2500) analysis of the WO3 powders obtained by drying the sol indicated that all of the peaks except for a peak at 2θ = 17.00° corresponded to WO3· 2H2O (JCPDS 18-1419) as shown in Supporting Information Figure S1. Preparation of WO3/MC Composite Film. Methylcellulose (MC; methoxy group, 26−33%) and ethylene glycol (EG), propylene glycol (PG), glycerin (Gly), trimethylene glycol (triMG), tetramethylene glycol (tetraMG), or pentamethylene glycol (pentaMG) as dispersing agents were purchased from Wako Pure Chemical Industries and used without further purification. These chemical structures were shown in Scheme 1. A 1.2 g amount of MC was dissolved in 60 mL of the aqueous WO3 sol by using a magnetic stirrer. The dispersing agents were added to the obtained transparent colloidal solution, stirred for 2 h, and placed in a glass vessel (18 × 18 cm2) to dry at 30 °C for 24 h. Unless otherwise stated, the film contains 7.2 × 10−3 mol of WO3. The WO3 sol (0.12 mol dm−3) was diluted with water to fabricate the films with WO3 less than 7.2 × 10−3 mol. The dried film (thickness, ca. 60 μm) was peeled off and cut into small pieces of 4.5 × 1.0 cm2. This small film was fixed in a quartz cell, and

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RESULTS AND DISCUSSION The WO3/MC composite film had no absorption in the visible region and absorbed the light below 400 nm due to the band gap transition of WO3. Figure 1 shows that the transmittance at

Figure 1. Effect of the amount of WO3 on transmittance at 640 nm of the WO3/MC film before UV irradiation. 26327

DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332

Research Article

ACS Applied Materials & Interfaces

triMG, tetraMG, and pentaMG, 100% transmittance was not obtained even at the molar ratio of 6. The distance between OH groups in the dispersing agents might affect the suppression of the aggregation of the WO3 particles. Hereafter, all of the photochromic experiments were performed with the completely transparent film, i.e., the films containing Gly, EG, or PG with the molar ratios of 1.0, 3.0, or 5.0, respectively. Figure 4 shows the spectral changes of the film containing Gly, EG, or PG every 3 min under UV irradiation. It took 18,

640 nm decreases with an increase in the amount of WO3 in the films. Such a decrease in the transmittance was not due to absorption but scattering of the light because the film became opaque white. The transmittance of the film containing 7.2 × 10−4 mol of WO3 was 95.9% while the film containing 7.2 × 10−3 mol was opaque and its transmittance was only 9.6%. In the former case, the WO3 particles of ca. 100−200 nm in diameter were present in the film as shown in Figure 2a. The

Figure 4. Spectral change of the film containing Gly, EG, or PG under UV irradiation for 18, 24, or 36 min, respectively. Absorption spectra were recorded every 3 min.

24, or 36 min, respectively, in order to reach absorbance of 2. Thus, the effect on the coloring rate of the film is the following order: Gly > EG > PG. We have found that these films contain different water contents (Supporting Information Figure S2). The water content of the MC/WO3 film was ca. 13% and increased with an increase in the amount of EG or Gly in the film. With comparison of the water content at the molar ratio of 3, the WO3/Gly/MC film had 23% H2O which was more than that of WO3/EG/MC film (15%). To clarify the effect of the dispersing agents on the coloring rate, the WO3/EG/MC or WO3/Gly/MC film was preheated at 120 °C for 10 min to remove H2O and then the absorption spectrum under UV irradiation was measured in dry air which passed through a column packed with CaCl2. Figure 5 shows spectral changes every 3 min under UV irradiation for 30 min. These figures clearly indicate that EG and Gly accelerates the coloration of the films and its enhancement by Gly is more effectively than EG.

Figure 2. TEM images of the WO3/MC film containing (a) 7.2 × 10−4 or (b) 7.2 × 10−3 mol of WO3 and (c) the WO3/EG/MC film containing 7.2 × 10−3 mol of WO3 and 2.2 × 10−2 mol of EG.

TEM image indicated that the increase in the WO3 amount to 7.2 × 10−3 mol led to the formation of large aggregates of 1− 1.5 μm (Figure 2b). However, the film containing 7.2 × 10−3 mol of WO3 became transparent with an addition of 2.2 × 10−2 mol of EG, whose TEM image indicated the presence of WO3 particles less than 200 nm (Figure 2c). Figure 3 indicates the effect of the amounts of various dispersing agents on the transmittance of the film. In order to get 100% transmittance, molar ratios of the dispersing agents to WO3 should be greater than 1.0, 3.0, and 5.0 for Gly, EG, and PG, respectively. This suggests that the OH moiety inhibits the aggregation of the WO3 particles. However, in the case of

Figure 3. Effect of the amounts of dispersing agents (DA) on transmittance at 640 nm of the WO3/MC film. The amount of WO3 was 7.2 × 10−3 mol. DA: Gly (black open circles), EG (blue filled circles), PG (red filled triangles), pentaMG (green filled squares), tetraMG (red open diamonds), triMG (blue filled triangles).

Figure 5. Effect of the amount of DA on coloration. Molar ratio: EG/ WO3 = 3.0 (a), 4.0 (b), 5.0 (c), and 6.0 (d); Gly/WO3 = 1.0 (e) and 3.0 (f). Each film was heated at 120 °C before UV irradiation, and the photochromic experiments were conducted under dry air. Absorption spectra were recorded every 3 min. 26328

DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332

Research Article

ACS Applied Materials & Interfaces Figure 6 indicates the effect of the water content on the coloring rate of the WO3/EG/MC film. It should be noted that

Figure 8. Dependence of the coloring rate on the H2O contents in the film. The values of ΔAbs were estimated from the increase in absorbance of absorption band 1 (black open circles), band 2 (blue filled triangles), and band 3 (red open squares) under irradiation of 30 min.

Figure 6. Effect of the water content in the coloring rate of the film containing EG. Water contents: (a) 0%, (b) 16.4%, (c) 29.0%, and (d) 41.3%. Absorption spectra were recorded every 3 min.

accelerates the coloration of the film although band 3, having the absorption peak in the infrared region, contributes slightly the blue coloration of the film. In Raman spectra of WO3, the WO3/MC film, and the WO3/ EG/MC film, a sharp signal at ca. 975 cm−1 corresponds to the stretching mode of the terminal WO bond which exists at the surface of the WO clusters, while a broad signal at ca. 650 cm−1 corresponds to the W−O−W stretching vibration of bridging oxygen bonds. These two vibrations are commonly observed in all types of WO3·nH2O.33,34 Raman shifts in these two peaks were not affected by the presence of MC and EG in the films (Supporting Information Figure S3). Figure 9

the WO3/Gly/MC film is not appropriate to examine the effect of the water content because that film with much H2O was very sticky to glassware. By increasing the water content of the WO3/EG/MC film, the coloration was accelerated and a new peak appeared at ca. 775 nm. We have previously reported that aqueous WO3 sol becomes blue in nitrogen atmosphere under UV irradiation and its absorption spectrum has an intense peak at 775 nm.21 Although the origin of the absorption peak at 775 nm could not be specified, the environment around the WO3 particles in the WO3/EG/MC film containing 41.3% water could be similar to that in aqueous WO3 sol. All absorption spectra in Figure 6 were successfully fit by three Lorentz functions. Figure 7 indicates examples of the fits for the

Figure 9. Effect of the H2O contents on relative peak shift at WO (red filled squares) or W−O (blue filled circles) stretching vibration in Raman spectra of the WO3/EG/MC film.

represents the effect of the water contents on the relative peak shift of these two vibrations, suggesting that the WO bond on the surface is not affected but the W−O−W stretching vibration mode shifts to a lower wavenumber as the water contents increase. This finding indicates the W−O−W bond becomes weaker by the addition of water, which is attributable to the formation of hydrogen bonding in the films. Concerning the mechanism, the photogenerated electron is trapped at W6+ sites, followed by the injection of protons from the dispersing agents or water adsorbed on the WO3 surface to form HxWVxWVI1−xO3,13−15 and then an IVCT transition occurs, making the film a blue color. This means the dispersing agents or water molecules adsorbed on the surface are decomposed by the photogenerated holes to liberate protons. In the photochromism on a sputtered crystalline WO3 thin film or PVA film containing WO3 or phosphotungstic acid, the

Figure 7. Deconvolution of absorption spectra of the film containing various H2O contents under the irradiation for 30 min. Water contents: (a) 0%, (b) 16.4%, (c) 29.0%, and (d) 41.3%. Values in each graph indicated the peak position of three Lorentz functions.

absorption spectra after UV irradiation for 30 min as shown in Figure 6. Three absorption bands have peaks at 631−653 (band 1), ca. 775 (band 2), and 976−1050 nm (band 3), respectively. Figure 8 shows the effect of the water contents on the increase in absorbance (ΔAbs) under UV irradiation for 30 min. The behaviors of band 1 and band 3 are similar: the ΔAbs values increase gradually to reach the maximum at 29.0% and then decrease with an increase in the water content. On the other hand, the ΔAbs values in band 2 increase remarkably from the water contents more than 16.4%. Figure 8 suggests that H2O 26329

DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332

Research Article

ACS Applied Materials & Interfaces

presence of excess water to the IVCT in the tritungstic groups while band 1 and band 3 are in the corner-sharing and the edgesharing octahedra, respectively. The IVCT transition in the edge-sharing octahedra would occur at a lower energy than that in corner-sharing octahedra since the W−W distance in the former is shorter than in the latter.23 We also examined the bleaching process under dark condition. After the films (EG/WO3 = 3.0) containing various H2O contents were UV-irradiated for 30 min, the decrease in absorbance was measured. Under air atmosphere, all of the films return to transparent. However, under argon atmosphere, the bleaching was not observed (Supporting Information Figure S4). Therefore, in the bleaching process, the trapped electron or W5+ is oxidized by O2. This means, during UV light irradiation in air, the coloration and the bleaching processes in the films occur simultaneously. After the light is turned off, the formation of W5+ is stopped and only bleaching is occurred by the reaction with O2 in air to recover its original transparent state. Various cellulose films have been studied as O2 barrier materials.44 The permeability coefficients of the MC and the WO3/EG/MC films were evaluated to be 6.6 × 10−11 and 1.2 × 10−11 cm3(STP) cm cm−2 s−1 cmHg−1, respectively. These values are lower than the permeability coefficients for O2 reported in the membrane of polystyrene or cellulose acetate.45 This low permeability for O2 is responsible for the fact that the WO3/MC film containing dispersing agents shows excellent photochromic properties in air.

oxidation of HCOOH, H2O, or PVA by the photogenerated hole has been observed.13,19,35 By repeating the coloration− bleaching cycle on the WO3/EG/MC film, the coloration rates gradually decreased whereas the bleaching rates in the dark were hardly affected. Figure 10 shows the reversibility of the

Figure 10. Absorbance change at 640 nm of the WO3/EG/MC film during repeated coloration−bleaching steps.

coloration−bleaching cycle of the WO3/EG/MC film. The maximal values in absorbance were the data obtained under UV irradiation for 5 min. It can be seen that the reversibility decreased gradually with increasing the cycle. It might suggest the decomposition of EG, MC, or the adsorbed water although any significant change was not detected in the ATR-FTIR spectra after 10 cycles. Further studies are needed to clarify the species which reacts with the photogenerated holes in the films. Despite extensive studies of various kinds of photochromic WO3 films, the packing of WO6 octahedra in the films is still not elucidated. In photochromism of WO3 dispersed in PVA matrix, Yumashev et al. described that two distinct peaks were observed at 620 and 925 nm and a hidden peak at ca. 776 nm was present.23 The positions of these three peaks are close to those shown in Figure 7. They assigned absorption peaks at 925, 620, and 776 nm to the IVCT transfer in edge-sharing, corner-sharing octahedra and tritungstic groups, respectively, as shown in Scheme 2a−c. These clusters have been often

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CONCLUSION In summary, we have fabricated photochromic films by incorporating WO3 particles in methylcellulose matrix with dispersing agents. These films are biodegradable and easily prepared by solvent-casting method without any expensive equipment. Under UV irradiation, the colorless−transparent films become dark blue in air and the bleaching reaction occurs in the dark. The coloration rate of the films depends on the structure and the amounts of the dispersing agents. More water in the films accelerates the coloring rate accompanying change in the absorption spectra. The contrast of the color change before and after UV irradiation is recognized clearly. These reversible photochromic films show great promise in the application of rewritable display medium by using light. Furthermore, the colored films obtained under UV irradiation can absorb effectively the light in the infrared region. This fact suggests their potential use as detachable films for glass windows whose light transmission properties are changed by sunlight. Indeed, the WO3/EG/MC films exhibited dark blue with the absorbance of 2.5 at 960 nm by being exposed to sunlight in the summer and returned to transparent during the night. Therefore, the developed films in this study can be utilized as detachable films which give functions of smart windows.

Scheme 2. Various Basic Units for WO6 Octahedra Groups

considered as building blocks for the formation of WO3.36−41 Saha et al. revealed the conversion of the edge-sharing into corner-sharing octahedra during the formation of the WO3 solid from the precursor solution by in situ total X-ray scattering study.42 The W−W distances in the edge-sharing and the corner-sharing octahedra were estimated to be 3.282 and 3.684 Å, respectively. Chemseddine and Bloeck reported transformation of the edge-sharing octahedra consisting of three octahedral units to the tritungstic groups by hydrolysis.43 The latter structure is considered to be a basic unit which leads to the formation of hexagonal WO3 crystal by condensation. Therefore, we assign band 2 which was clearly observed in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09310. Figure S1 showing XRD pattern of the WO3 powder obtained by drying aqueous sol, Figure S2 showinig water contents in the film containing no dispersing agent, EG, or Gly with various contents, Figure S3 showing Raman spectra of WO3, MC, WO3/MC, and WO3/EG/ 26330

DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332

Research Article

ACS Applied Materials & Interfaces

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MC, and Figure S4 showing a decrease in absorbance during the bleaching process of the WO3/EG/MC film in air and argon atmosphere (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel. and Fax: +81-83-933-5763. E-mail: yamazaki@ yamaguchi-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Hidetoshi Kita and Associate Prof. Kazuhiro Tanaka at the department of engineering in Yamaguchi University for measuring the permeability coefficients of the films.

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REFERENCES

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DOI: 10.1021/acsami.5b09310 ACS Appl. Mater. Interfaces 2015, 7, 26326−26332