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Min-Sung Kim, Jun-Ho Yoon, Hong-Mo Kim, Dong-Jun Lee, Tamaki Hirose, [Yoshihiko Takeda](https://orcid.org/0000-0003-4961-3687), Jae-Pil Kim

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[Amplifying Photochromic Response in Tungsten Oxide Films with Titanium Oxide and Polyvinylpyrrolidone](https://mdr.nims.go.jp/datasets/2b8ea36b-ceb0-4a9c-8f84-603804bd1c8d)

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Amplifying Photochromic Response in Tungsten Oxide Films with Titanium Oxide and PolyvinylpyrrolidoneCitation: Kim, M.-S.; Yoon, J.-H.; Kim,H.-M.; Lee, D.-J.; Hirose, T.; Takeda, Y.;Kim, J.-P. Amplifying PhotochromicResponse in Tungsten Oxide Filmswith Titanium Oxide andPolyvinylpyrrolidone. Nanomaterials2024, 14, 1121. https://doi.org/10.3390/nano14131121Academic Editor: José M. Doña-RodríguezReceived: 28 May 2024Revised: 26 June 2024Accepted: 27 June 2024Published: 29 June 2024Copyright: © 2024 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).nanomaterialsArticleAmplifying Photochromic Response in Tungsten Oxide Filmswith Titanium Oxide and PolyvinylpyrrolidoneMin-Sung Kim 1,† , Jun-Ho Yoon 1,†, Hong-Mo Kim 2, Dong-Jun Lee 1, Tamaki Hirose 3 , Yoshihiko Takeda 3,*and Jae-Pil Kim 1,*1 Lab of Organic Photo-Functional Materials, Department of Materials Science and Engineering, Seoul NationalUniversity, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea; kms619@snu.ac.kr (M.-S.K.);junho0905@snu.ac.kr (J.-H.Y.); dongjunl@snu.ac.kr (D.-J.L.)2 Semiconductor Analysis Team, Advanced Institute of Convergence Technology, 145 Gwanggyo-ro,Yeongtong-gu, Suwon-si 16229, Republic of Korea; hmkim0118@snu.ac.kr3 Hydrogen Related Materials Group, Research Center for Energy and Environmental Materials, NationalInstitute for Materials Science (NIMS), Tsukuba 305-0003, Japan; hirose.tamaki.gp@alumni.tsukuba.ac.jp* Correspondence: takeda.yoshihiko@nims.go.jp (Y.T.); jaepil@snu.ac.kr (J.-P.K.)† These authors contributed equally to this work.Abstract: Tungsten oxide (WO3) is known for its photochromic properties, making it useful forsmart windows, displays, and sensors. However, its small bandgap leads to rapid recombinationof electron–hole pairs, resulting in poor photochromic performance. This study aims to enhancethe photochromic properties of WO3 by synthesizing hexagonal tungsten oxide via hydrothermalsynthesis, which increases surface area and internal hydrates. Titanium oxide (TiO2) was adsorbedonto the tungsten oxide to inject additional charges and reduce electron–hole recombination. Addi-tionally, polyvinylpyrrolidone (PVP) was used to improve dispersion in organic solvents, allowingfor the fabrication of high-quality films using the doctor blade method. Characterization confirmedthe enhanced surface area, crystal structure, and dispersion stability. Reflectance and transmittancemeasurements demonstrated significant improvements in photochromic properties due to the com-posite structure. These findings suggest that the introduction of TiO2 and PVP to tungsten oxideeffectively enhances its photochromic performance, broadening its applicability in various advancedphotochromic applications.Keywords: tungsten oxide nanoparticles; hybrid composite; dispersibility; photochromic property1. IntroductionTungsten oxide (WO3) is an inorganic material with photochromic properties, charac-terized by high photostability and thermal stability, low toxicity, and ease of synthesis [1,2].Due to these properties, it holds the potential for applications in various fields such assmart windows, displays, optical devices, and sensors [3,4]. Specifically, its photochromicproperties, which cause a color change when exposed to light, offer advantages over ther-mochromic or electrochromic materials as it does not require additional energy sources orcomplex structures [4–6]. This makes it environmentally friendly and highly applicable,leading to extensive research and diverse exploration of its applications [2].However, tungsten oxide has a smaller bandgap compared to other photochromicinorganic materials like ZnO, MoO3, and AgCl [7,8]. This small bandgap allows the easyrecombination of electron–hole pairs generated by light, resulting in poor photochromicproperties and limiting its applicability [2,4]. To overcome these drawbacks, research hasfocused on synthesizing tungsten oxide as quantum dots through self-assembly to increasethe bandgap or modify the surface structure to enhance reactivity [5,7]. Additionally,methods such as doping with various metal particles to generate free electrons or combiningNanomaterials 2024, 14, 1121. https://doi.org/10.3390/nano14131121 https://www.mdpi.com/journal/nanomaterialshttps://doi.org/10.3390/nano14131121https://doi.org/10.3390/nano14131121https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.comhttps://orcid.org/0009-0005-3707-2512https://orcid.org/0009-0007-0680-3647https://orcid.org/0000-0003-4961-3687https://doi.org/10.3390/nano14131121https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.com/article/10.3390/nano14131121?type=check_update&version=1Nanomaterials 2024, 14, 1121 2 of 15with organic materials to enhance photochromic properties by introducing additionalelements like electrons or protons have been studied [9–11].The widely accepted photo-induced coloration mechanism of tungsten oxide can beexplained as follows [12,13]:WO3 + hν → WO∗3 + e−+h+ (1)2h+ + H2O → 2H+ + O (2)WO3 + xH+ + xe− → HxW6+1−xW5+xO3 (3)When UV light irradiates WO3, the electrons generated by the light move to theconduction band, while holes are created in the valence band (reaction (1)). The holes inthe valence band react with the -OH groups of water molecules to produce protons andoxygen radicals (reaction (2)). Finally, the electrons that moved to the conduction bandcombine with W6+ to reduce it to W5+, and these electrons, along with the previouslygenerated protons, form hydrogen tungsten bronze (HxW6+1−xW5+xO3) which has a bluecolor (reaction (3)). This mechanism demonstrates that the presence of e–h pairs andprotons can significantly enhance the photochromic properties [14,15].Enhancing photochromic properties within the photochromic mechanism can beachieved by generating more electron–hole pairs induced by light or producing moreprotons. In this study, we synthesized tungsten oxide with a hexagonal structure con-taining hydrate through hydrothermal synthesis. This hexagonal structure, with bothhexagonal and trigonal channels, has a larger surface area compared to the monoclinicstructure of tungsten oxide, which only has trigonal channels [16–18]. This increasedsurface area enhances reactivity, resulting in improved photochromic properties [18–20].Additionally, the internal structure containing hydrate can supply more protons, furtherenhancing photochromic properties due to the combined effect of increased surface areaand proton availability.We then introduced a structure where titanium oxide is adsorbed onto the synthesizedtungsten oxide to inject more charges. Titanium oxide is highly reactive to light, stable,and possesses high optical transparency, making it useful in batteries, solar cells, andphotocatalysts [21,22]. When titanium oxide is introduced to tungsten oxide, it allowsthe absorption of a wider range of light wavelengths. The large bandgap of titaniumoxide (approximately 3.2 eV for the anatase phase) plays a crucial role in reducing therecombination of electron–hole pairs when incorporated with tungsten oxide. When TiO2is exposed to light, it generates electron–hole pairs. The photo-induced electrons cantransfer from TiO2 to WO3, reducing the recombination rate by spatially separating thecharge carriers. This separation enhances the photochromic properties of the compositematerial [23,24]. This process ensures that more photogenerated electrons participate inthe photochromic reaction, ultimately enhancing the overall photochromic properties. Byincorporating TiO2 with WO3, the composite material exhibits an improved photochromicresponse, demonstrating the effectiveness of this approach.We inject electrons into tungsten oxide, which is a critical element of this photochromicmechanism, and choose the doctor blade method (solution-based process) to producefilms with excellent photochromic properties without the need for vacuum equipment,unlike conventional methods such as CVD, ALD, and sputtering. To achieve this, tungstenoxide particles need to be well dispersed in the solvent. Typically, metal oxide particlesdo not disperse well in organic solvents, leading to particle aggregation or precipitation,which degrades the quality of solution-based films. To overcome this issue, organic ligandsthat act as dispersants can be adsorbed onto the particle surface to form an ion layer thatincreases repulsion between particles, or hydrophilic and lipophilic ligands can be used toimprove mixing with the solvent [11,16,20].Finally, to overcome the aforementioned drawbacks of tungsten oxide, titanium oxidewas introduced to inject more electrons into tungsten oxide and enhance its photochromicproperties. Additionally, to improve dispersion in the solvent for solution-based filmNanomaterials 2024, 14, 1121 3 of 15fabrication, polyvinylpyrrolidone (PVP) was introduced into the particles. This structureallows more electrons generated by light to be injected into tungsten oxide through thejunction with titanium oxide and, the introduction of PVP increases repulsion betweenparticles, minimizing aggregation [25].2. Materials and Methods2.1. MaterialsAmmonium tungstate pentahydrate (ATP, 5(NH4)2O·12WO3·5H2O), polyvinylpyrroli-done (PVP, (C6H9NO)n), and anhydrous oxalic acid (98%) were sourced from Alfa Aesar.PGME (extra pure, 1-Methoxy-2-propanol, 98.5%) and TiO2 (P25) were obtained fromSAMCHUN Pure Chemical Co., Ltd. (Daegu, Republic of Korea). All chemicals wereutilized directly without any additional purification.2.2. Preparation of WO3 NanoparticlesWO3 nanoparticles were synthesized through a hydrothermal process. The steps wereas follows: In a 150 mL reactor, 15.6 g of ATP was dissolved in 70 mL of distilled waterin a beaker. To this, 20 mL of oxalic acid solution (prepared by dissolving 10 g of oxalicacid in 100 mL of deionized water) was added while stirring, and the pH was adjusted to1 using 2 M HCl. The mixture was stirred for 4 h before being transferred to a reactor andheated at 120 ◦C for 12 h. Post-reaction, the resulting tungsten oxide was separated viacentrifugation and the precipitate was washed twice with ethanol.2.3. Synthesis of the CompositeFollowing the synthesis of the nanoparticle composite, it underwent two ethanol anddeionized water washes each. TiO2 was then added in molar ratios of 3%, 5%, and 10%,respectively, and stirred for 3 h. PVP was incorporated into the tungsten oxide–ethanolmixture at 50 wt%, and this mixture was stirred for 1 h, then ultrasonicated for 30 min.The process concluded with a 12 h stirring period. The final product was centrifuged andwashed with PGME, then mixed with 20 wt% PGME solvent to achieve a dispersed solution.2.4. Fabrication of Photochromic FilmA glass slide was cleaned using nitrogen gas. A solution containing 30 wt% tungstenoxide in 1-methoxy-2-propanol (PGME) was combined with 20 wt% acrylate binder (3:2ratio) and stirred for 24 h at room temperature. This solution was then drop-casted ontothe glass slide and coated using a doctor blade technique. The sample was heated at 100 ◦Cfor 2 min to remove any residual solvent.2.5. Characterization and MeasurementUV-Vis reflectance spectra were recorded with a JASCO V-670 and V-770 spectropho-tometer (Tokyo, Japan). Fourier-transform infrared (FT-IR) spectroscopy was performedusing a Bruker TENSOR27 spectrometer (Billerica, MA, USA). UV-Vis transmittance spec-tra were obtained with a Perkin Elmer Lambda 1050 spectrophotometer (Waltham, MA,USA). Field emission scanning electron microscopy (FE-SEM) images were taken with aJEOL JSM-7800F Prime instrument (Tokyo, Japan), with Pt coating using a Zeiss MER-LIN Compact (Oberkochen, Germany). High-resolution X-ray diffraction (HRXRD) of thecoated film was examined using a SmartLab instrument (Tokyo, Japan) with Cu–Kα X-rays(λ = 0.154 nm). Field emission transmission electron microscopy (FE-TEM) images werecaptured using a JEOL JEM-F200 instrument (Tokyo, Japan). Dynamic light scattering (DLS)measurements were conducted with a DLS-8000HAL, and zeta potential analysis with anELSZ-1000, both from Photal Otsuka Electronics (Osaka, Japan), with 1 min measurementsand auto-fitting of the correlation function using Anton Paar’s Kalliope software (304813).Thermogravimetric analysis (TGA) was performed at a heating rate of 10 ◦C/min usinga TA Instruments SDT Q600 (New Castle, DE, USA). X-ray photoelectron spectroscopyNanomaterials 2024, 14, 1121 4 of 15(XPS) was conducted with a Thermo Fisher Scientific Sigma Probe for electron spectroscopychemical analysis (Waltham, MA, USA).3. Results and Discussion3.1. Characterization of the Synthesized Tungsten OxideFollowing the outlined synthesis procedure utilized in our previous study, we suc-cessfully prepared hexagonal WO3 particles exhibiting photochromic properties througha hydrothermal method [25]. The hexagonal crystalline structure of tungsten oxide, asillustrated in Figure 1a, is integral to its photochromic behavior. Unlike the commerciallyavailable monoclinic form of tungsten oxide, which contains only trigonal channels, thehexagonal variant features both trigonal and hexagonal channels. This dual-channel struc-ture results in an increased surface area, enhancing reactivity and facilitating electronmobility, thus significantly improving the photochromic properties.Nanomaterials 2024, 14, x FOR PEER REVIEW 4 of 17   8000HAL, and zeta potential analysis with an ELSZ-1000, both from Photal Otsuka Elec-tronics (Osaka, Japan), with 1 min measurements and auto-fitting of the correlation func-tion using Anton Paar’s Kalliope software (304813). Thermogravimetric analysis (TGA) was performed at a heating rate of 10 °C/min using a TA Instruments SDT Q600 (New Castle, Delaware, USA). X-ray photoelectron spectroscopy (XPS) was conducted with a Thermo Fisher Scientific Sigma Probe for electron spectroscopy chemical analysis (Wal-tham, MA, USA). 3. Results and Discussion 3.1. Characterization of the Synthesized Tungsten Oxide Following the outlined synthesis procedure utilized in our previous study, we suc-cessfully prepared hexagonal WO3 particles exhibiting photochromic properties through a hydrothermal method [25]. The hexagonal crystalline structure of tungsten oxide, as il-lustrated in Figure 1a, is integral to its photochromic behavior. Unlike the commercially available monoclinic form of tungsten oxide, which contains only trigonal channels, the hexagonal variant features both trigonal and hexagonal channels. This dual-channel struc-ture results in an increased surface area, enhancing reactivity and facilitating electron mo-bility, thus significantly improving the photochromic properties.  Figure 1. (a) Monoclinic and hexagonal structure of tungsten oxide, (b) X-ray diffraction of synthe-sized tungsten oxide, (c) FT-IR analysis, and (d) TGA analysis. To examine the crystal structure of the synthesized tungsten oxide, we performed XRD measurements. As depicted in Figure 1b, the principal peak corresponds to the hex-agonal structure (JCPDS card no. 33-1387) [14,26–28]. Hexagonal tungsten oxide crystals feature a three-dimensional framework of corner-sharing octahedral interconnected by oxygen atoms. The bonding characteristics of the synthesized tungsten oxide were vali-dated using Fourier-transform infrared (FT-IR) spectroscopy. In Figure 1c, a vibration peak observed around 700 cm⁻¹ is indicative of the bridging vibration of corner-sharing octahedra (W–Ointer–W) in the WO3 structure. This mode demonstrates the oxygen atoms being shared between neighboring tungsten atoms. Additionally, peaks detected around 1600 cm−1 suggest the presence of intercalated water molecules, which are involved in W–O∙∙∙H2O interactions. Peaks near 3450 cm−1 are associated with the in-plane bending vibra-tions of W–OH, implying that oxygen vacancies or single bonds on the particle surface are Figure 1. (a) Monoclinic and hexagonal structure of tungsten oxide, (b) X-ray diffraction of synthe-sized tungsten oxide, (c) FT-IR analysis, and (d) TGA analysis.To examine the crystal structure of the synthesized tungsten oxide, we performed XRDmeasurements. As depicted in Figure 1b, the principal peak corresponds to the hexagonalstructure (JCPDS card no. 33-1387) [14,26–28]. Hexagonal tungsten oxide crystals featurea three-dimensional framework of corner-sharing octahedral interconnected by oxygenatoms. The bonding characteristics of the synthesized tungsten oxide were validatedusing Fourier-transform infrared (FT-IR) spectroscopy. In Figure 1c, a vibration peak ob-served around 700 cm−1 is indicative of the bridging vibration of corner-sharing octahedra(W–Ointer–W) in the WO3 structure. This mode demonstrates the oxygen atoms be-ing shared between neighboring tungsten atoms. Additionally, peaks detected around1600 cm−1 suggest the presence of intercalated water molecules, which are involved inW–O···H2O interactions. Peaks near 3450 cm−1 are associated with the in-plane bendingvibrations of W–OH, implying that oxygen vacancies or single bonds on the particle sur-face are bonded as hydroxyl groups (OH). The sharp peak at approximately 1400 cm−1can be attributed to the absorption of NH4+ ions from the precursor material [28]. Thesefindings indicate that the synthesized tungsten oxide predominantly consists of W–Obonds, which indirectly confirm the octahedral structure facilitated by oxygen sharing.Moreover, the presence of OH groups as single bonds or oxygen vacancies on the surface isclearly identified.Nanomaterials 2024, 14, 1121 5 of 15Figure 1d illustrates the TGA analysis of the synthesized tungsten oxide, revealingits hydration level. Organic materials adsorbed on the surface begin to decompose attemperatures up to approximately 100 ◦C, and a 7.5% hydrate decomposition occursbetween approximately 150 ◦C and 250 ◦C. These findings confirm the octahedral structureof the synthesized tungsten oxide and indicate the presence of water molecules withinthe framework.Furthermore, by examining the XPS in Figure 2, it can be observed that the tungstenoxide synthesized via hydrothermal synthesis primarily forms a 6+ oxidation state andexhibits a structure with oxygen vacancies. The main peaks in the XPS spectrum correspondto W 4f at 35.7 eV, W 4d at 280.5 eV, C 1s at 285.2 eV, and O 1s at 532.2 eV. The remainingresiduals from the synthesis of carbon and nitrogen can be identified, and slight peaks arestill present even after rinsing. High-resolution XPS peaks reveal the existence of 4f5/2 at38.5 eV and 4f7/2 at 35.5 eV. Upon examining the split peaks, W6+ 4f7/2 at 35.3 eV and W6+4f5/2 at 37.5 eV can be identified, while weak binding energy peaks correspond to W5+ at34.5 eV and W5+ at 36.5 eV.Nanomaterials 2024, 14, x FOR PEER REVIEW 5 of 17   bonded as hydroxyl groups (OH). The sharp peak at approximately 1400 cm−1 can be at-tributed to the absorption of NH4+ ions from the precursor material [28]. These findings indicate that the synthesized tungsten oxide predominantly consists of W–O bonds, which indirectly confirm the octahedral structure facilitated by oxygen sharing. Moreover, the presence of OH groups as single bonds or oxygen vacancies on the surface is clearly iden-tified. Figure 1d illustrates the TGA analysis of the synthesized tungsten oxide, revealing its hydration level. Organic materials adsorbed on the surface begin to decompose at tem-peratures up to approximately 100 °C, and a 7.5% hydrate decomposition occurs between approximately 150 °C and 250 °C. These findings confirm the octahedral structure of the synthesized tungsten oxide and indicate the presence of water molecules within the framework. Furthermore, by examining the XPS in Figure 2, it can be observed that the tungsten oxide synthesized via hydrothermal synthesis primarily forms a 6+ oxidation state and exhibits a structure with oxygen vacancies. The main peaks in the XPS spectrum corre-spond to W 4f at 35.7 eV, W 4d at 280.5 eV, C 1s at 285.2 eV, and O 1s at 532.2 eV. The remaining residuals from the synthesis of carbon and nitrogen can be identified, and slight peaks are still present even after rinsing. High-resolution XPS peaks reveal the existence of 4f5/2 at 38.5 eV and 4f7/2 at 35.5 eV. Upon examining the split peaks, W6+ 4f7/2 at 35.3 eV and W6+ 4f5/2 at 37.5 eV can be identified, while weak binding energy peaks correspond to W5+ at 34.5 eV and W5+ at 36.5 eV.  Figure 2. (a) Survey XPS spectra of synthesized tungsten oxide and a (b) high-resolution W 4f spec-trum of the same sample. To examine the morphology and microstructure of the synthesized tungsten oxide particles, SEM and TEM analyses were conducted. In Figure 3a,b, SEM images reveal that the synthesized tungsten oxide particles exhibit agglomeration ranging in size from ap-proximately 100 nm to several micrometers. TEM images in Figure 3c,d show plate-like shapes with dimensions of approximately 40 nm. High-resolution TEM images indicate a lattice parameter of 0.35 nm for the (002) lattice and 0.67 nm for the (100) lattice [29,30]. Figure 2. (a) Survey XPS spectra of synthesized tungsten oxide and a (b) high-resolution W 4fspectrum of the same sample.To examine the morphology and microstructure of the synthesized tungsten oxideparticles, SEM and TEM analyses were conducted. In Figure 3a,b, SEM images revealthat the synthesized tungsten oxide particles exhibit agglomeration ranging in size fromapproximately 100 nm to several micrometers. TEM images in Figure 3c,d show plate-likeshapes with dimensions of approximately 40 nm. High-resolution TEM images indicate alattice parameter of 0.35 nm for the (002) lattice and 0.67 nm for the (100) lattice [29,30].Nanomaterials 2024, 14, x FOR PEER REVIEW 6 of 17    Figure 3. (a,b) SEM image of synthesized WO3 and (c,d) HRTEM image. The inset of (d) shows lattice parameters of 0.35 nm (002) and plate-like WO3 morphology. 3.2. Characterization of Tungsten Oxide with Titanium Oxide and Polyvinylpyrrolidone The synthesized tungsten oxide possesses a hexagonal structure, which provides a wide surface area and contains internal hydrates that enhance its photochromic proper-ties. However, compared to other photochromic inorganic materials, tungsten oxide faces the challenge of rapid recombination of photo-induced electron–hole pairs, which leads to lower photochromic performance. To address these drawbacks, we adsorbed titanium oxide onto the tungsten oxide. This approach injects additional charges, thereby reducing electron–hole recombination and enhancing the overall photochromic properties of the material. To confirm the adsorption of titanium oxide onto tungsten oxide, XRD and TEM anal-yses were conducted. In Figure 4, the 2θ values of 25.31 for the anatase (101) peak and 27.4 for the rutile (110) peak of P25 titanium oxide can be observed, indicating the presence of adsorbed material of tungsten oxide and titanium oxide in the XRD peak of the blue line [31,32]. Figure 3. (a,b) SEM image of synthesized WO3 and (c,d) HRTEM image. The inset of (d) shows latticeparameters of 0.35 nm (002) and plate-like WO3 morphology.Nanomaterials 2024, 14, 1121 6 of 153.2. Characterization of Tungsten Oxide with Titanium Oxide and PolyvinylpyrrolidoneThe synthesized tungsten oxide possesses a hexagonal structure, which provides awide surface area and contains internal hydrates that enhance its photochromic properties.However, compared to other photochromic inorganic materials, tungsten oxide faces thechallenge of rapid recombination of photo-induced electron–hole pairs, which leads tolower photochromic performance. To address these drawbacks, we adsorbed titanium oxideonto the tungsten oxide. This approach injects additional charges, thereby reducing electron–hole recombination and enhancing the overall photochromic properties of the material.To confirm the adsorption of titanium oxide onto tungsten oxide, XRD and TEManalyses were conducted. In Figure 4, the 2θ values of 25.31 for the anatase (101) peak and27.4 for the rutile (110) peak of P25 titanium oxide can be observed, indicating the presenceof adsorbed material of tungsten oxide and titanium oxide in the XRD peak of the blueline [31,32].Nanomaterials 2024, 14, x FOR PEER REVIEW 7 of 17    Figure 4. X-ray diffraction of synthesized tungsten oxide and titanium oxide. The orange line is tungsten oxide, the violet line is titanium oxide, and the blue line is tungsten oxide with titanium oxide. (The red dotted line indicates the TiO₂ confirmation line) TEM analysis was conducted to investigate the surface structure of tungsten oxide with adsorbed titanium oxide particles. Typically, the conventional method for creating a junction between tungsten oxide and titanium oxide involves co-synthesizing them as precursors. However, it was observed that co-synthesis led to the formation of many re-sidual materials and aggregated particles after synthesis, resulting in the generation of numerous particles on the film surface when films were produced (Figure S1 of Supple-mentary Materials). Therefore, we synthesized tungsten oxide first and then adsorbed TiO2 particles (P25) onto it. Figure 5 shows the structure according to the mole fraction of titanium oxide introduced into tungsten oxide. First, in Figure 5a, it can be observed that the hexagonal structure of tungsten oxide synthesized through hydrothermal synthesis has a plate-like structure with dimensions of approximately 10 nm in width and 30 nm in length. Figure 5b–d demonstrate the adsorption patterns according to the mole fraction of titanium oxide introduced. Particles with a mole fraction of 3% titanium oxide exhibit an appropriate adsorption ratio between tungsten oxide and titanium oxide. However, par-ticles with 5% and 10% mole fractions show excessive coverage of tungsten oxide by tita-nium oxide particles. Therefore, exceeding the optimal input amount leads to a decrease in the amount of light reaching tungsten oxide, resulting in a reduction in photochromic properties and aggregation of particles on the film surface, ultimately degrading the prop-erties of the photochromic film (Figure S3 of Supplementary Material) [32–34]. These find-ings are consistent with subsequent verification of reflectance analysis. Figure 4. X-ray diffraction of synthesized tungsten oxide and titanium oxide. The orange line istungsten oxide, the violet line is titanium oxide, and the blue line is tungsten oxide with titaniumoxide. (The red dotted line indicates the TiO2 confirmation line).TEM analysis was conducted to investigate the surface structure of tungsten oxide withadsorbed titanium oxide particles. Typically, the conventional method for creating a junc-tion between tungsten oxide and titanium oxide involves co-synthesizing them as precur-sors. However, it was observed that co-synthesis led to the formation of many residual mate-rials and aggregated particles after synthesis, resulting in the generation of numerous parti-cles on the film surface when films were produced (Figure S1 of Supplementary Materials).Therefore, we synthesized tungsten oxide first and then adsorbed TiO2 particles (P25) ontoit. Figure 5 shows the structure according to the mole fraction of titanium oxide introducedinto tungsten oxide. First, in Figure 5a, it can be observed that the hexagonal structure oftungsten oxide synthesized through hydrothermal synthesis has a plate-like structure withdimensions of approximately 10 nm in width and 30 nm in length. Figure 5b–d demon-strate the adsorption patterns according to the mole fraction of titanium oxide introduced.Particles with a mole fraction of 3% titanium oxide exhibit an appropriate adsorption ratioNanomaterials 2024, 14, 1121 7 of 15between tungsten oxide and titanium oxide. However, particles with 5% and 10% molefractions show excessive coverage of tungsten oxide by titanium oxide particles. Therefore,exceeding the optimal input amount leads to a decrease in the amount of light reachingtungsten oxide, resulting in a reduction in photochromic properties and aggregation ofparticles on the film surface, ultimately degrading the properties of the photochromicfilm (Figure S3 of Supplementary Material) [32–34]. These findings are consistent withsubsequent verification of reflectance analysis.Nanomaterials 2024, 14, x FOR PEER REVIEW 8 of 17    Figure 5. HRTEM image of tungsten oxide with titanium oxide. (a) Only WO3, (b) addition of 3% TiO2, (c) 5% TiO2, and (d) 10% TiO2 mole fraction. For TiO2, it has a spherical square shape with dimensions of approximately 20 nm. WO3 has a plate-like shape with dimensions of approximately 10 nm in width and 30 nm in length. To produce films using the doctor blade method with tungsten oxide particles ad-sorbed with titanium oxide, it is essential to enhance the dispersibility in the solvent. Typ-ically, metal oxide particles poorly disperse and tend to aggregate or precipitate in organic solvents, resulting in large particles on the film surface during film production [11,35,36]. To address this issue, polyvinylpyrrolidone was introduced as a dispersant. FT-IR analy-sis was conducted to confirm this combination. In Figure 6, when polyvinylpyrrolidone is adsorbed onto synthesized tungsten oxide, shifted peaks are observed. Specifically, when polyvinylpyrrolidone is bound to tungsten oxide or tungsten oxide/titanium oxide parti-cles, the main peak of polyvinylpyrrolidone, the C=O stretching vibration, shifts to 1649 cm−1 and 1651 cm−1, and the C–N vibrations peak shifts to 1290 cm−1 and 1288 cm−1. These shifts suggest that polyvinylpyrrolidone is adsorbed onto and interacts with the surface of tungsten oxide, possibly onto hydrogen or oxygen vacancies, rather than forming a new bonding structure chemically [37,38]. Figure 5. HRTEM image of tungsten oxide with titanium oxide. (a) Only WO3, (b) addition of 3%TiO2, (c) 5% TiO2, and (d) 10% TiO2 mole fraction. For TiO2, it has a spherical square shape withdimensions of approximately 20 nm. WO3 has a plate-like shape with dimensions of approximately10 nm in width and 30 nm in length.To produce films using the doctor blade method with tungsten oxide particles adsorbedwith titanium oxide, it is essential to enhance the dispersibility in the solvent. Typically,metal oxide particles poorly disperse and tend to aggregate or precipitate in organicsolvents, resulting in large particles on the film surface during film production [11,35,36].To address this issue, polyvinylpyrrolidone was introduced as a dispersant. FT-IR analysiswas conducted to confirm this combination. In Figure 6, when polyvinylpyrrolidoneis adsorbed onto synthesized tungsten oxide, shifted peaks are observed. Specifically,when polyvinylpyrrolidone is bound to tungsten oxide or tungsten oxide/titanium oxideparticles, the main peak of polyvinylpyrrolidone, the C=O stretching vibration, shifts to1649 cm−1 and 1651 cm−1, and the C–N vibrations peak shifts to 1290 cm−1 and 1288 cm−1.These shifts suggest that polyvinylpyrrolidone is adsorbed onto and interacts with thesurface of tungsten oxide, possibly onto hydrogen or oxygen vacancies, rather than forminga new bonding structure chemically [37,38].Nanomaterials 2024, 14, 1121 8 of 15Nanomaterials 2024, 14, x FOR PEER REVIEW 9 of 17    Figure 6. FT-IR analysis of tungsten oxide composites. The left image is the overall range of each material and the right image is an enlarged comparison of the range from 1250 to 2000 cm−1. 3.3. Dispersibility Analysis in Organic Solvents for Solution-Based Film Fabrication To assess the dispersion of the synthesized tungsten oxide composite, zeta potential analysis was conducted. Zeta potential analysis is a method to evaluate the dispersion state and stability of particles by measuring the potential of the charged layers on the par-ticle surface. Typically, metal oxide particles are considered to be stably dispersed in a solvent when the zeta potential is approximately 30 mV [39,40]. Values below 30 mV in-dicate insufficient repulsion among particles, leading to aggregation and unstable disper-sion, while values above 30 mV indicate sufficient repulsion and stable particle distribu-tion. Samples were dispersed in solvent through sonication for approximately 10 min be-fore measurement. Upon examination of Figure 7 and Table 1, it was observed that tung-sten oxide particles without the dispersing agent, PVP, and tungsten oxide/titanium oxide composite particles exhibited zeta potentials of 28.8 mV and 28.4 mV, respectively, indi-cating relatively unstable states [41,42]. However, with the introduction of PVP, the zeta potentials increased to 70.2 mV and 68.3 mV, respectively, demonstrating stable disper-sion. Figure 6. FT-IR analysis of tungsten oxide composites. The left image is the overall range of eachmaterial and the right image is an enlarged comparison of the range from 1250 to 2000 cm−1.3.3. Dispersibility Analysis in Organic Solvents for Solution-Based Film FabricationTo assess the dispersion of the synthesized tungsten oxide composite, zeta potentialanalysis was conducted. Zeta potential analysis is a method to evaluate the dispersion stateand stability of particles by measuring the potential of the charged layers on the particlesurface. Typically, metal oxide particles are considered to be stably dispersed in a solventwhen the zeta potential is approximately 30 mV [39,40]. Values below 30 mV indicateinsufficient repulsion among particles, leading to aggregation and unstable dispersion,while values above 30 mV indicate sufficient repulsion and stable particle distribution.Samples were dispersed in solvent through sonication for approximately 10 minbefore measurement. Upon examination of Figure 7 and Table 1, it was observed thattungsten oxide particles without the dispersing agent, PVP, and tungsten oxide/titaniumoxide composite particles exhibited zeta potentials of 28.8 mV and 28.4 mV, respectively,indicating relatively unstable states [41,42]. However, with the introduction of PVP, the zetapotentials increased to 70.2 mV and 68.3 mV, respectively, demonstrating stable dispersion.Nanomaterials 2024, 14, x FOR PEER REVIEW 10 of 17    Figure 7. Zeta potential analysis of WO3 and composites. (a) Only WO3, (b) WO3 with polyvinylpyr-rolidone, (c) WO3 with TiO2, and (d) final composite (WO3 with TiO2 capsulated by polyvinylpyr-rolidone). Table 1. Zeta potential analysis of the tungsten oxide composite. Each material was measured in five repeat measurements, with three times readings per measurement, to calculate the average value. Material WO3 WO3@PVP WO3/TiO2 WO3/TiO2@PVP Average zeta potential (mV) 28.8 70.2 28.4 68.3 Additionally, dynamic light scattering (DLS) analysis was conducted using the same dispersion solution to further evaluate dispersibility [43,44]. As depicted in Figure 8 and Table 2, without PVP, the size distribution showed significant heterogeneity, with particle sizes ranging from 2800 nm to 1284 nm. Conversely, with the addition of PVP, the particle sizes narrowed to 800 nm and 900 nm, indicating improved dispersion. The distribution histogram confirmed that both tungsten oxide and titanium oxide composite particles ex-hibited narrower dispersion widths when PVP was introduced (Figure S4 of Supplemen-tary Materials). Figure 7. Zeta potential analysis of WO3 and composites. (a) Only WO3, (b) WO3 withpolyvinylpyrrolidone, (c) WO3 with TiO2, and (d) final composite (WO3 with TiO2 capsulatedby polyvinylpyrrolidone).Nanomaterials 2024, 14, 1121 9 of 15Table 1. Zeta potential analysis of the tungsten oxide composite. Each material was measured in fiverepeat measurements, with three times readings per measurement, to calculate the average value.Material WO3 WO3@PVP WO3/TiO2 WO3/TiO2@PVPAverage zetapotential (mV) 28.8 70.2 28.4 68.3Additionally, dynamic light scattering (DLS) analysis was conducted using the samedispersion solution to further evaluate dispersibility [43,44]. As depicted in Figure 8 and Table 2,without PVP, the size distribution showed significant heterogeneity, with particle sizes rangingfrom 2800 nm to 1284 nm. Conversely, with the addition of PVP, the particle sizes narrowed to800 nm and 900 nm, indicating improved dispersion. The distribution histogram confirmedthat both tungsten oxide and titanium oxide composite particles exhibited narrower dispersionwidths when PVP was introduced (Figure S4 of Supplementary Materials).Nanomaterials 2024, 14, x FOR PEER REVIEW 11 of 17    Figure 8. DLS analysis of WO3 and composites. (a) Only WO3, (b) WO3 with PVP, (c) WO3 with TiO2, and (d) final composite. Table 2. DLS data were collected through 10 measurements. Average particle size decreased when PVP was introduced and the dispersion showed a narrower distribution. Material WO3 WO3/PVP WO3/TiO2 WO3/TiO2/PVP Distribution range (nm) 48.8~2849 614.7~1417 361.3~1645.3 81.9~982.1 Size variation (nm) 2800.2 802.3 1284 900.2 These results suggest that PVP adsorbs onto the surface of tungsten oxide particles, imparting a negative charge, increasing interparticle repulsion, preventing aggregation, and enhancing dispersion in the solvent [45–47]. 3.4. Reflectance for Confirming Enhanced Photochromic Properties To quantitatively assess the alterations in photochromic properties of the synthesized tungsten oxide composite, reflectance spectroscopy was conducted on the powdered form. Using UV radiation (VL-6.LC, 365 nm tube, 50/60 Hz), each material was subjected to irradiation for 1, 3, 5, and 10 min durations while monitoring the reflectance changes. The resultant data, as illustrated in Figure 9 and summarized in Table 3, delineate the spectral changes observed at 700 nm following 1 min UV irradiation. Figure 8. DLS analysis of WO3 and composites. (a) Only WO3, (b) WO3 with PVP, (c) WO3 withTiO2, and (d) final composite.Table 2. DLS data were collected through 10 measurements. Average particle size decreased whenPVP was introduced and the dispersion showed a narrower distribution.Material WO3 WO3/PVP WO3/TiO2 WO3/TiO2/PVPDistribution range (nm) 48.8~2849 614.7~1417 361.3~1645.3 81.9~982.1Size variation (nm) 2800.2 802.3 1284 900.2These results suggest that PVP adsorbs onto the surface of tungsten oxide particles,imparting a negative charge, increasing interparticle repulsion, preventing aggregation,and enhancing dispersion in the solvent [45–47].Nanomaterials 2024, 14, 1121 10 of 153.4. Reflectance for Confirming Enhanced Photochromic PropertiesTo quantitatively assess the alterations in photochromic properties of the synthesizedtungsten oxide composite, reflectance spectroscopy was conducted on the powdered form.Using UV radiation (VL-6.LC, 365 nm tube, 50/60 Hz), each material was subjected toirradiation for 1, 3, 5, and 10 min durations while monitoring the reflectance changes. Theresultant data, as illustrated in Figure 9 and summarized in Table 3, delineate the spectralchanges observed at 700 nm following 1 min UV irradiation.Nanomaterials 2024, 14, x FOR PEER REVIEW 12 of 17    Figure 9. Reflectance of tungsten oxide composites (The red line indicates the total change observed over 10 minutes). (a) Only WO3, (b) WO3@PVP, (c) WO3/TiO2, and (d) final composite (WO3/TiO2@PVP). (The y-axis is the reflectance value of 0 to 90%.) Table 3. Reflectance was measured in powder form by introducing titanium oxide and polyvi-nylpyrrolidone to WO3. The reflectance variation is at a maximum value and 700 nm by UV radiation for 1 min. Sample Hexagonal WO3 h-WO3/PVP h-WO3/TiO2 h-WO3/TiO2/PVP Reflectance %R (max) %R (700 nm) %R (max) %R (700 nm) %R (max) %R (700 nm) %R (max) %R (700 nm) Initial state 68.0 62.0 75.6 69.7 76.2 70.7 84.1 79.2 UV 1 min 53.9 41.9 53.6 41.8 50.7 39.9 53.0 37.0 ΔR (%) 14.1 20.1 22.0 27.9 25.5 30.8 31.1 42.2 Monoclinic tungsten oxide exhibited a reflectance change of 20.1%, whereas the tung-sten oxide/PVP composite demonstrated 27.9% variation. Conversely, tungsten oxide adorned with titanium oxide showcased a 30.8% shift, while the final tungsten oxide/tita-nium oxide/PVP composite displayed a substantial change of 42.2%. The alterations reveal a significant enhancement in the photochromic properties due to the combined effects of titanium oxide (TiO₂) and polyvinylpyrrolidone (PVP) on tung-sten oxide (WO₃). The incorporation of TiO₂ introduces additional electron pairs into WO₃, thereby augmenting its photochromic attributes. This enhancement is particularly notable due to the narrow bandgap of WO₃, which typically facilitates electron–hole pair recom-bination, thus reducing its photochromic efficiency. By increasing the electron population, which is crucial for the photochromic mechanism, the overall photochromic properties are markedly improved. Moreover, the high electron mobility inherent in TiO₂ facilitates the migration of elec-tron–hole pairs to the WO₃ matrix. When TiO₂ adsorbs onto the WO₃ surface, it results in an expansion of the surface area, providing a greater number of reactive sites for photon-Figure 9. Reflectance of tungsten oxide composites (The red line indicates the total change observedover 10 min). (a) Only WO3, (b) WO3@PVP, (c) WO3/TiO2, and (d) final composite (WO3/TiO2@PVP).(The y-axis is the reflectance value of 0 to 90%.)Table 3. Reflectance was measured in powder form by introducing titanium oxide andpolyvinylpyrrolidone to WO3. The reflectance variation is at a maximum value and 700 nm byUV radiation for 1 min.Sample Hexagonal WO3 h-WO3/PVP h-WO3/TiO2 h-WO3/TiO2/PVPReflectance %R (max) %R (700 nm) %R (max) %R (700 nm) %R (max) %R (700 nm) %R (max) %R (700 nm)Initial state 68.0 62.0 75.6 69.7 76.2 70.7 84.1 79.2UV 1 min 53.9 41.9 53.6 41.8 50.7 39.9 53.0 37.0∆R (%) 14.1 20.1 22.0 27.9 25.5 30.8 31.1 42.2Monoclinic tungsten oxide exhibited a reflectance change of 20.1%, whereas thetungsten oxide/PVP composite demonstrated 27.9% variation. Conversely, tungstenoxide adorned with titanium oxide showcased a 30.8% shift, while the final tungstenoxide/titanium oxide/PVP composite displayed a substantial change of 42.2%.The alterations reveal a significant enhancement in the photochromic properties due tothe combined effects of titanium oxide (TiO2) and polyvinylpyrrolidone (PVP) on tungstenNanomaterials 2024, 14, 1121 11 of 15oxide (WO3). The incorporation of TiO2 introduces additional electron pairs into WO3,thereby augmenting its photochromic attributes. This enhancement is particularly notabledue to the narrow bandgap of WO3, which typically facilitates electron–hole pair recombi-nation, thus reducing its photochromic efficiency. By increasing the electron population,which is crucial for the photochromic mechanism, the overall photochromic properties aremarkedly improved.Moreover, the high electron mobility inherent in TiO2 facilitates the migration ofelectron–hole pairs to the WO3 matrix. When TiO2 adsorbs onto the WO3 surface, it resultsin an expansion of the surface area, providing a greater number of reactive sites for photon-induced reactions. This process not only catalyzes the photochromic mechanism withinWO3 but also causes substantial chromatic changes due to enhanced hydrogen interactions.Furthermore, the addition of PVP plays a dual role by enhancing the dispersibility ofWO3 particles in the solvent and inducing the ligand-to-metal charge transfer (LMCT) effect,which amplifies the photochromic properties. (Figure S2 of Supplementary Materials) [48].The LMCT effect involves the transfer of charge from the ligand species adsorbed onthe particle surface to the metal oxide. For PVP, this charge transfer occurs as the non-bonding electron pairs from nitrogen atoms transition to WO3, strengthening the pho-tochromic mechanism.Additionally, the presence of hydrogen in PVP facilitates the generation of protons,thereby improving the overall electron–proton injection balance required for robust pho-tochromic mechanisms. This improvement is evident from reflectance measurements,which show significant alterations following the introduction of TiO2 and PVP. Thesecombined effects lead to a substantial enhancement in the photochromic properties of WO3particles, thus significantly improving photochromic efficiency.3.5. Photochromic Properties in FilmFinally, the synthesized tungsten oxide composite was dispersed in PGME solvent at20 wt%, mixed with an acrylate binder in a ratio of 3:2 (wt%), and stirred for approximately3 h to ensure thorough blending. Subsequently, the film was fabricated onto a glasssubstrate using the blade coating method. The coated film was baked at approximately100 ◦C for 2 min. The film thickness, 4 to 5 µm, was confirmed through cross-sectionalSEM measurements (Figure S5 of Supplementary Materials). Utilizing the same UV lampspecifications employed in the reflectance analysis, measurements were conducted atvarious time intervals, up to a maximum duration of 20 min (Figure 10). When comparedat a wavelength of 700 nm, the film exhibited an initial transmittance of 85.2%, whichdecreased to 44.7% at the maximum value of 20 min, indicating a variation of approximately40.5% (Table 4). Compared to the WO3@PVP composite film without TiO2, we confirmedthat the introduction of TiO2 resulted in more than double the photochromic performance(Figure S6 of Supplementary Materials). This enhanced photochromic property aligns withthe improved reflectance results observed earlier.Table 4. Transmittance of the photochromic film over time (at 700 nm wavelength).Time 0 min 1 min 3 min 5 min 10 min 20 minTransmittance (%) at 700 nm 85.2 78.7 70.0 63.4 52.6 44.7∆T (%) 85.2 ∆T = 40.5 44.7In summary, we synthesized hexagonal structured tungsten oxide via hydrothermalsynthesis, confirming its potential for enhanced photochromic properties owing to its largersurface area compared to conventional monoclinic structures and the presence of hydrates.Furthermore, by introducing titanium oxide to alleviate the drawback of low photochromicproperties attributed to the recombination of electron–hole pairs due to a small bandgap,and incorporating PVP to serve both as a dispersant and as a means to inject electrons andprotons into the tungsten oxide, we successfully synthesized a tungsten composite filmNanomaterials 2024, 14, 1121 12 of 15with excellent photochromic properties. Ultimately, the synergistic effects of titanium oxideand PVP resulted in the fabrication of an outstanding photochromic film.Nanomaterials 2024, 14, x FOR PEER REVIEW 14 of 17    Figure 10. Transmittance of the photochromic film over time. Maximum coloration time is 20 min. Table 4. Transmittance of the photochromic film over time (at 700 nm wavelength). Time 0 min 1 min 3 min 5 min 10 min 20 min Transmittance (%) at 700 nm 85.2 78.7 70.0 63.4 52.6 44.7 ΔT (%) 85.2 ΔT = 40.5 44.7 4. Conclusions In this study, we synthesized a tungsten oxide composite with exceptional photo-chromic properties and dispersibility by introducing titanium oxide and polyvinylpyrrol-idone (PVP). Notably, the tungsten oxide material obtained through hydrothermal syn-thesis exhibited a hexagonal structure and included hydrate, further enhancing its photo-chromic properties. Utilizing a solution-based doctor blade method, we successfully fab-ricated superior photochromic films without the need for complex processing steps. Through dynamic light scattering (DLS) and zeta potential analysis, we ensured excellent dispersion of tungsten oxide/titanium oxide particles in organic solvents. Additionally, reflectance measurements of the powder state confirmed enhanced photochromic proper-ties through electron injection and proton donation. When fabricating the films, the intro-duction of titanium oxide for electron injection and PVP for ligand-to-metal charge trans-fer (LMCT) effects and proton donation resulted in more than a twofold enhancement in photochromic properties compared to before the introduction of these materials. These findings emphasize the crucial role of PVP ligand interactions in enhancing the photo-chromic properties of tungsten oxide composites. In conclusion, the synthesized tungsten oxide hybrid composite, featuring a hexagonal structure and hydrate inclusion, represents a significant advancement in photochromic material research. These results offer promis-ing prospects for the environmentally friendly and practical application of photochromic Figure 10. Transmittance of the photochromic film over time. Maximum coloration time is 20 min.4. ConclusionsIn this study, we synthesized a tungsten oxide composite with exceptional pho-tochromic properties and dispersibility by introducing titanium oxide and polyvinylpyrroli-done (PVP). Notably, the tungsten oxide material obtained through hydrothermal synthesisexhibited a hexagonal structure and included hydrate, further enhancing its photochromicproperties. Utilizing a solution-based doctor blade method, we successfully fabricatedsuperior photochromic films without the need for complex processing steps. Throughdynamic light scattering (DLS) and zeta potential analysis, we ensured excellent dispersionof tungsten oxide/titanium oxide particles in organic solvents. Additionally, reflectancemeasurements of the powder state confirmed enhanced photochromic properties throughelectron injection and proton donation. When fabricating the films, the introduction oftitanium oxide for electron injection and PVP for ligand-to-metal charge transfer (LMCT)effects and proton donation resulted in more than a twofold enhancement in photochromicproperties compared to before the introduction of these materials. These findings empha-size the crucial role of PVP ligand interactions in enhancing the photochromic propertiesof tungsten oxide composites. In conclusion, the synthesized tungsten oxide hybrid com-posite, featuring a hexagonal structure and hydrate inclusion, represents a significantadvancement in photochromic material research. These results offer promising prospectsfor the environmentally friendly and practical application of photochromic films, markinga departure from complex processing steps and high-cost film fabrication methods.Nanomaterials 2024, 14, 1121 13 of 15Supplementary Materials: The following supporting information can be downloaded athttps://www.mdpi.com/article/10.3390/nano14131121/s1: Scheme S1: The band gap potentialdiagram of the WO3/TiO2 composite; Figure S1: SEM images of the film surfaces by differentmethods for tungsten oxide and titanium oxide; Figure S2: The scheme of ligand to metal chargetransfer (LMCT) effect; Figure S3: Color change depends on the amount of titanium oxide; Figure S4:HRTEM of tungsten oxide hybrid composite; Figure S5: The cross-sectional SEM image of thephotochromic film; Figure S6: Transmittance of photochomic films. Reference [49] is cited in theSupplementary Materials.Author Contributions: Conceptualization, Y.T. and J.-P.K.; Methodology, M.-S.K., J.-H.Y. and H.-M.K.;Formal analysis, M.-S.K., J.-H.Y., D.-J.L. and T.H.; Investigation, M.-S.K., J.-H.Y., H.-M.K., D.-J.L.and T.H.; Data curation, M.-S.K., J.-H.Y., H.-M.K., D.-J.L., T.H. and J.-P.K.; Writing—original draft,M.-S.K.; Writing—review & editing, Y.T. and J.-P.K.; Visualization, M.-S.K.; Supervision, Y.T.; Projectadministration, J.-P.K.; Funding acquisition, Y.T. and J.-P.K. All authors have read and agreed to thepublished version of the manuscript.Funding: This research was funded by Ministry of Trade, Industry and Energy, grant number 20010838.Data Availability Statement: Data are contained within the article and Supplementary Materials.Conflicts of Interest: The authors declare no conflict of interest.References1. Deb, S.K. Optical and photoelectric properties and colour centres in thin films of tungsten oxide. Philos. 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MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1016/j.jphotobiol.2017.11.021https://doi.org/10.1016/j.msec.2019.110494https://doi.org/10.1007/s10570-019-02716-2https://doi.org/10.1016/j.apcatb.2017.12.049 Introduction  Materials and Methods  Materials  Preparation of WO3 Nanoparticles  Synthesis of the Composite  Fabrication of Photochromic Film  Characterization and Measurement  Results and Discussion  Characterization of the Synthesized Tungsten Oxide  Characterization of Tungsten Oxide with Titanium Oxide and Polyvinylpyrrolidone  Dispersibility Analysis in Organic Solvents for Solution-Based Film Fabrication  Reflectance for Confirming Enhanced Photochromic Properties  Photochromic Properties in Film  Conclusions  References