Enhanced Transmittance Modulation of Sio2-Doped Crystalline WO3 Films Prepared from a Polyethylene Oxide (PEO) Template

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Article Enhanced Transmittance Modulation of SiO2-Doped Crystalline WO3 Films Prepared from a Polyethylene Oxide (PEO) Template

Guanguang Zhang ID , Kuankuan Lu, Xiaochen Zhang, Weijian Yuan, Honglong Ning * ID , Ruiqiang Tao, Xianzhe Liu, Rihui Yao * ID and Junbiao Peng

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; [email protected] (G.Z.); [email protected] (K.L.); [email protected] (X.Z.); [email protected] (W.Y.); [email protected] (R.T.); [email protected] (X.L.); [email protected] (J.P.) * Correspondence: [email protected] (H.N.); [email protected] (R.Y.); Tel.: +86-20-8711-4525 (H.N.)

Received: 26 May 2018; Accepted: 23 June 2018; Published: 26 June 2018

Abstract: Polyethylene oxide (PEO)-modified silicon dioxide (SiO2)-doped crystalline tungsten trioxide (WO3) films for use as electrochromic layers were prepared on indium tin oxide (ITO) glass by the sol–gel spin coating technique. The effects of the PEO template and SiO2 on the electrochromic transmittance modulation ability of crystalline WO3 films were investigated. Fourier transform infrared spectroscopy (FT-IR) spectra analysis indicated that PEO was decomposed after ◦ annealing at 500 C for 3 h. X-ray diffraction (XRD) pattern analysis showed that both SiO2 and PEO helped reduce the crystalline grain size of the WO3 films. Atomic force microscope (AFM) images showed that the combined action of SiO2 and PEO was helpful for achieving high surface roughness and a macroporous structure. An electrochromic test indicated that PEO-modified SiO2-doped 2 crystalline WO3 films intercalated more charges (0.0165 C/cm ) than pure WO3 crystalline films (0.0095 C/cm2). The above effects resulted in a good transmittance modulation ability (63.2% at 628 nm) of PEO-modified SiO2-doped crystalline WO3 films, which was higher than that of pure WO3 crystalline films (9.4% at 628 nm).

Keywords: tungsten trioxide; crystalline ﬁlm; doping; morphology; electrochromism; transmittance modulation

1. Introduction

Tungsten trioxide (WO3) is an important functional material, widely studied for applications in gas sensors [1], photochromism [2], photocatalysis [3], and electrochromism [4]. In recent years, the energy consumption of buildings for indoor temperature regulation has been increasing. Smart windows can reduce energy consumption by modulating light and heat inputs [5]. WO3-based electrochromic layers have attracted increasing attention for smart windows applications. There are many methods for preparing WO3 films, such as physical vapor deposition (PVD) [6], chemical vapor deposition (CVD) [7], sol–gel method [8], etc. However, both PVD and CVD have some characteristic drawbacks, such as high cost of equipment, high energy consumption, and vacuum dependency [9]. The sol–gel method can form large-area films in the atmosphere with less energy consumption and low equipment cost [10]. It is easy to realize doping uniformly at the molecular level by the sol–gel method. In addition, the pores formed by solvent evaporation during annealing can enhance the electrochromic properties of the films [11].

Coatings 2018, 8, 228; doi:10.3390/coatings8070228 www.mdpi.com/journal/coatings Coatings 2018,, 8,, 228x FOR PEER REVIEW 2 of 1211

The electrochromic performance of WO3 has been researched for several decades. As we know, The electrochromic performance of WO has been researched for several decades. As we know, the electrochromic performance is closely related3 to the morphology, structure, and crystallinity of the electrochromic performance is closely related to the morphology, structure, and crystallinity of the WO3 films, and one effective method for enhancing the films’ electrochromic performance is to the WO3 films, and one effective method for enhancing the films’ electrochromic performance is to incorporate dopants such as TiO2 [12], Fe2O3 [13], carbon [14], etc. It was found that crystalline WO3 incorporate dopants such as TiO [12], Fe O [13], carbon [14], etc. It was found that crystalline WO films have good redox stability 2[15], good2 3durability [16], and good infrared-reflecting properties3 films have good redox stability [15], good durability [16], and good infrared-reflecting properties (good (good insulation) [17]. However, compared with amorphous WO3 films, a drawback of crystalline insulation) [17]. However, compared with amorphous WO3 films, a drawback of crystalline WO3 films WO3 films is their low transmittance modulation ability [18,19], which limits their application. is their low transmittance modulation ability [18,19], which limits their application. To enhance the electrochromic performance, especially the transmittance modulation ability of To enhance the electrochromic performance, especially the transmittance modulation ability of crystalline WO3 films, a feasible scheme is to develop large-surface area and macroporous crystalline crystalline WO3 films, a feasible scheme is to develop large-surface area and macroporous crystalline WO3 films. In previous studies, polyethylene oxide (PEO) was often used as a template for pore WO3 films. In previous studies, polyethylene oxide (PEO) was often used as a template for pore materials [20]. SiO2 has been confirmed to help form pores [21] and reduce the crystalline grain size materials [20]. SiO2 has been confirmed to help form pores [21] and reduce the crystalline grain size [22]. [22]. In our work, we tried to improve the transmittance modulation ability of crystalline WO3 films In our work, we tried to improve the transmittance modulation ability of crystalline WO3 films by by combining the advantages of PEO and SiO2. As a result, we successfully prepared crystalline pure combining the advantages of PEO and SiO2. As a result, we successfully prepared crystalline pure WO3 films, SiO2-doped crystalline WO3 films, PEO-modified crystalline WO3 films, and PEO-modified WO3 films, SiO2-doped crystalline WO3 films, PEO-modified crystalline WO3 films, and PEO-modified SiO2-doped crystalline films by the sol–gel method. Besides, the FT-IR spectra, crystallization, surface SiO -doped crystalline films by the sol–gel method. Besides, the FT-IR spectra, crystallization, surface morphology2 , and electrochromic properties of these films were investigated. morphology, and electrochromic properties of these films were investigated.

2. Materials and Methods TungstenTungsten powder (purity: 99.5%) 99.5%) and and hydrogen hydrogen peroxide peroxide (mass (mass fraction: fraction: 30%) 30%) were were mixed mixed in ina beaker (weight: 4 g, H2O2: 20 mL) to prepare a tungstate solution. When the tungsten powder a beaker (weight: 4 g, H2O2: 20 mL) to prepare a tungstate solution. When the tungsten powder completely reacted,reacted, the tungstate solution was condensed into 10 mL by heating,heating, in order to removeremove superfluous H2O2. Next, four kinds of solution were prepared, as shown in Table 1. Solution EW superfluous H2O2. Next, four kinds of solution were prepared, as shown in Table1. Solution EW (pure tungstate solution) was a control solution, and solutions EWS (with SiO2), EWP (with PEO), (pure tungstate solution) was a control solution, and solutions EWS (with SiO2), EWP (with PEO), EWPS (with SiO2 and PEO) were experimental solutions. The tungstate solution concentration was EWPS (with SiO2 and PEO) were experimental solutions. The tungstate solution concentration was around 1 mol/L.mol/L. According According to to a a previous previous study study [22], [22], a a c concentrationoncentration of of tetraethyl tetraethyl o orthosilicaterthosilicate ( (TEOS)TEOS) of 0.2 mol/Lmol/L ha hadd a a good good inhibitory inhibitory effect effect.. PEO PEO amounts amounts ha hadd a a significant significant effect on the structure and quality of the WO3 films. In this experiment, the amounts of the above reactants were optimized and quality of the WO3 films. In this experiment, the amounts of the above reactants were optimized and controlledcontrolled for the control experiment.experiment. After stirring and heating heating,, the four kinds of of sol sol were were obtained, as shown shown in in Figure Figure 1.1 .All All of ofthem them were were transparent transparent,, colloid colloid (Tyndall (Tyndall effect) effect),, and orange and orange-yellow,-yellow, which whichindicated indicated that PEO that and PEO TEOS and were TEOS uniformly were uniformly dissolved dissolved in the sol. in the sol.

Table 1.1. The parameters of four kinds of solution. EW, EW, control solution solution,, pure tungstate solution solution;; EWS, EWP, EWPS, experimental solutions (with SiO2, PEO, SiO2 and PEO, respectively). EWP, EWPS, experimental solutions (with SiO2, PEO, SiO2 and PEO, respectively). Tetraethyl Absolute Tungstate Tetraethyl PEO polyethylene Stirring Heating Absolute Tungstate PEO polyethylene Stirring Heating LabelLabel OrthosilicateOrthosilicate Ethanol/mLEthanol/mL Solution/mLSolution/mL OxideOxide (M :(M 600,000)/gw: 600000)/g Time/hTime/h Temperature/Temperature/◦C °C (TEOS)/g(TEOS)/g w EW EW2.5 2.52.5 2.5 00 0 0 EWSEWS 2.5 2.52.5 2.5 0.210.21 0 0 3 70 EWP 2.5 2.5 0 0.1 3 70 EWP 2.5 2.5 0 0.1 EWPS 2.5 2.5 0.21 0.1 EWPS 2.5 2.5 0.21 0.1

Figure 1. Images of sols of EW, EWS, EWP, and EWPS. All of them are transparent, colloid (Tyndall Figure 1. Images of sols of EW, EWS, EWP, and EWPS. All of them are transparent, colloid (Tyndall effect), and orange-yellow, which indicates that PEO and TEOS were uniformly dissolved in the sol. effect), and orange-yellow, which indicates that PEO and TEOS were uniformly dissolved in the sol.

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FinallyFinally,, all thethe filmsfilms werewere prepared prepared on on ITO ITO glass glass by by spin spin coating coating (4000 (4000 revolutions revolutions per per minute minute for 2 for30 s).30 Thes). The area area of the of electrochromicthe electrochromic layer layer was aroundwas around 2 cm 2. Thecm2. pureThe WOpure3 WOfilms,3 films, SiO2-doped SiO2-doped WO3 ◦ WOfilms,3 films, PEO-modified PEO-modified WO3 films,WO3 films and PEO-modified, and PEO-modified SiO2-doped SiO2-doped films annealedfilms annealed at 500 atC 500 were °C named were namedEW500, EW500, EWS500, EWS500, EWP500, EWP500 and EWPS500,, and EWPS500 respectively., respectively Similarly,. the Similarly, PEO-modified the PEO SiO-modified2-doped SiO films2- ◦ doannealedped films at 100annealed and 300 at 100C were and 300 named °C were EWPS100 named and EWPS100 EWPS300, and respectively. EWPS300, respectively. TheThe thicknessthickness of of all all films films was w measuredas measured by a by probe a probe surface surface profiler profiler (Veeco Dektak(Veeco 150,Dektak Veeco, 150 NY,, Veeco USA,) NandY, wasUSA about) and 200was± about10 nm. 200 FT-IR ± 10 spectranm. FT of-IR EWPS100, spectra of EWPS300, EWPS100, and EWPS300 EWPS500, and were EWPS500 recorded were by a recordedFT-IR spectrophotometer by a FT-IR spectrophotometer (SHIMADZU IR (SHIMADZU Prestige-21, SHIMADZU, IR Prestige-21 Tokyo,, SHIMADZU Japan) with, Tokyo an ITO, Japan glass) withacting an as ITO a blank. glass Theacting crystalline as a blank structure. The crystalli of thesene films stru wascture determined of these film bys X-raywas determined Diffraction by using X-ray Cu DiffractionKα radiation using (XRD, Cu PANalytical Kα radiation Empyrean (XRD, DY1577, PANalytical PANalytical, Empyrean Almelo, DY1577, Netherlands). PANalytical The, Almelo surface, Netherlands).morphology was The measured surface morphology by atomic force was measured microscopy by (AFM, atomic Being force Nano-Instruments microscopy (AFM, BY3000, Being NanoBeing-Instruments Nano-Instruments, BY3000 Beijing,, Being China). Nano-Instruments An amount, ofBeijing 0.8 mol/L, China of). lithium An amount perchlorate/propylene of 0.8 mol/L of lithiumcarbonate perchlorate (LiClO4/PC)/propylene electrolyte carbonate was (LiClO used4 for/PC the) electrolyte electrochromic was used test, for the and electrochromic the electrode test gap, andwas the 0.5 electrode cm. The voltagegap was supply0.5 cm. andThe currentvoltage measurementsupply and current were measurement supported by were an electrochemical supported by anworkstation electrochemical (CH Instruments workstation CHI600E, (CH Instruments CH Instruments, CHI600E, Shanghai, CH Instruments China). The, Shanghai transmittance, China of). these The transmittancefilms at the initial of these state, films colored at the state, initial and state, bleached colored state state was, and measured bleached by state a spectrometer was measured (Morpho by a spectrometerPG2000, Morpho, (Morpho Shanghai, PG2000, China) Morpho with, anShanghai ITO glass, China) acting with as blank.an ITO glass acting as blank. 3. Results and Discussion 3. Results and Discussion Figure2 shows the changes of the appearance of the EW500, EWS500, EWP500, and EWPS500 Figure 2 shows the changes of the appearance of the EW500, EWS500, EWP500, and EWPS500 films as the annealing time increased. Because of the carbonization of PEO at high temperature, films as the annealing time increased. Because of the carbonization of PEO at high temperature, it it indicates that the color of samples EWP500 and EWPS500 (PEO-doped) changed from transparent to indicates that the color of samples EWP500 and EWPS500 (PEO-doped) changed from transparent to grey after annealing for 15 min, when compared with EW500 and EWS500 (not PEO-doped), but such grey after annealing for 15 min, when compared with EW500 and EWS500 (not PEO-doped), but such changes did not occur in samples EW500 and EWS500, because of the rapid evaporation of the organic changes did not occur in samples EW500 and EWS500, because of the rapid evaporation of the organic solvents. Finally, the color of samples EWP500 and EWPS500 turned transparent again as the annealing solvents. Finally, the color of samples EWP500 and EWPS500 turned transparent again as the time increased. In the process of PEO pyrolysis, CO2 and water vapor, which acted on the film annealing time increased. In the process of PEO pyrolysis, CO2 and water vapor, which acted on the structure, escaped from the WO3 film. film structure, escaped from the WO3 film.

Figure 2. Changes of the appearance of the EW500, EWS500, EWP500, and EWPS500 films (annealed Figure 2. Changes of the appearance of the EW500, EWS500, EWP500, and EWPS500 ﬁlms (annealed at at 500 °C ) as the annealing time increased. The color changed from transparent to grey and finally 500 ◦C) as the annealing time increased. The color changed from transparent to grey and ﬁnally turned turned transparent again only in the samples EWP and EWPS (PEO-doped), which indicated the transparent again only in the samples EWP and EWPS (PEO-doped), which indicated the carbonization carbonization of PEO at high temperature. of PEO at high temperature. Figure 3 shows the FT-IR spectra of EWPS100, EWPS300, and EWPS500, which were annealed at differentFigure 3temperature shows the FT-IRs. The spectraspectra of (a EWPS100,), (b), and ( EWPS300,c) of Figure and 3 show EWPS500, broad which bands were located annealed at 3500 at– −1 3400different cm temperatures.−1, which we There attributedspectra (a), (b), to and asymmetric (c) of Figure and3 show symmetric broad bands –OH located stretching at 3500–3400 vibrations. cm , Inwhich addition, were attributedanother sharp to asymmetric band around and 1625 symmetric cm−1 wa –OHs attributed stretching to vibrations. –OH bending In addition, vibration another [23]. The –OH was mainly from physisorbed water and surface OH groups.

Coatings 2018, 8, 228 4 of 12 sharp band around 1625 cm−1 was attributed to –OH bending vibration [23]. The –OH was mainly Coatingsfrom physisorbed 2018, 8, x FOR waterPEER REVIEW and surface OH groups. 4 of 11

(c)

711

965

617

1614

1094

3537 (b)

615

(a) 730

1605

3434

1423

2973

891

3072

3473

2976

Transmittance (arb.units) (a) EWPS100 1626

572

(b) EWPS300 894

1074 (c) EWPS500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm-1)

FigureFigure 3 3.. FourierFourier transform transform infrared infrared ( (FT-IR)FT-IR) spectra of EWPS EWPS films: ﬁlms: ( (aa)) EWPS100 EWPS100 ( (annealedannealed at at 1 10000 °C◦C);); ((b)) EWPS300 EWPS300 (annealed (annealed at 300 °C◦C);); ( (cc)) EWPS500. EWPS500. The The a annealingnnealing process process at 500 °C◦C led led to to PEO PEO pyrolysis pyrolysis andand formation formation of of W W–O–W–O–W and Si Si–O–Si–O–Si structures.structures.

As spectra (a) and (b) of Figure 3 show, the bands located around 2900–3000 cm−1 correspond to As spectra (a) and (b) of Figure3 show, the bands located around 2900–3000 cm −1 correspond to the symmetric and antisymmetric –C–H stretching vibration of aliphatic CH3 and CH2 from ethanol the symmetric and antisymmetric –C–H stretching vibration of aliphatic CH and CH from ethanol and PEO [24]. The bands located around 3000–3100 cm−1 correspond to symmetric3 and antisymmetric2 and PEO [24]. The bands located around 3000–3100 cm−1 correspond to symmetric and antisymmetric =C–H stretching vibration, which was attributed to incomplete pyrolysis of PEO [25]. The bands =C–H stretching−1 vibration, which was attributed to incomplete pyrolysis of PEO [25]. The bands around around 1423−1 cm could be assigned to asymmetric –C–H bending vibration [26]. In spectrum (a), the 1423 cm could be assigned−1 to asymmetric –C–H bending vibration [26]. In spectrum (a), the sharp sharp band around 1074−1 cm could be assigned to the asymmetric C–O–C stretching vibration from band around 1074 cm could be assigned to the asymmetric C–O–C−1 stretching vibration from PEO [26]. PEO [26]. In spectrum (a), the bands at − 8941 and 572 cm were attributed to symmetric and In spectrum (a), the bands at 894 and 572 cm were attributed to symmetric and antisymmetric W(O)2 antisymmetric W(O)2 stretching vibration and O–O stretching vibration, respectively [27]. In spectra stretching vibration and O–O stretching vibration, respectively [27]. In spectra (b) and (c), the bands (b) and (c), the bands around 965 cm−1 were attributed to symmetric W=O stretching vibration, and around 965 cm−1 were attributed to symmetric W=O stretching vibration, and the bands at 615 and 730 the bands at 615 and 730 cm−1 were attributed to asymmetric W–O–W stretching vibration [27,28]. cm−1 were attributed to asymmetric W–O–W stretching vibration [27,28]. The bands around 1094 cm−1 The bands around 1094 cm−1 were attributed to asymmetric Si–O–Si stretching vibration in spectrum were attributed to asymmetric Si–O–Si stretching vibration in spectrum (c) [29]. (c) [29]. Therefore, the above FT-IR spectra analysis indicated that the organic components, including PEO Therefore, the above FT-IR spectra analysis indicated that the organic components, including and ethanol, were strongly reduced after film annealing at 500 ◦C for 3 h. PEO was decomposed into PEO and ethanol, were strongly reduced after film annealing at 500 °C for 3 h. PEO was decomposed CO2 and H2O which play a significant role in the formation of large surface area and macroporous into CO2 and H2O which play a significant role in the formation of large surface area and structures. Besides, the annealing process led to the formation of W–O–W and Si–O–Si structures. macroporous structures. Besides, the annealing process led to the formation of W–O–W and Si–O–Si Figure4 shows the XRD patterns of EW500, EWS500, EWP500, EWPS500, and ITO glass, structures. which were analyzed using the software Jade 6.0 and PDF #43-1035. As Figure4 shows, the diffraction Figure 4 shows the XRD patterns of EW500, EWS500, EWP500, EWPS500, and ITO glass, which peaks of WO3 could be observed in the patterns (a), (b), (c), and (d), which indicated that all these were analyzed using the software Jade 6.0 and PDF #43-1035. As Figure◦ 4 shows, the diffraction peaks WO3 films were crystallized. The main peak of WO3 at 2θ ≈ 24.1 corresponds to the (200) planes, of WO3 could be observed in the patterns (a), (b), (c), and (d), which indicated that all these WO3 films and its crystalline grain size was calculated using the Scherrer formula [30]: were crystallized. The main peak of WO3 at 2θ ≈ 24.1° corresponds to the (200) planes, and its crystalline grain size was calculated using the Scherrer0.89γ formula [30]: D = (1) 0.89βγcos θ D  (1) where D is the average size of the grain onβ thecos (200)θ plane, γ is the X-ray wavelength, which is where0.154056 D nm,is theβ isaverage the full size width of at the half grain maximum on the (FWHM)(200) plane of, the γ is diffraction the X-ray peak, wavelength,θ is the diffraction which is 0.154056angle. The nm grain, β is sizesthe full of width these filmsat half are maximum shown in (FWHM) Figure5 ;of the the smallest diffraction grain peak size, θ was is the found diffraction in the angle.sample The EWPS500. grain sizes The of results these indicatedfilms are thatshown both in theFigure PEO 5 and; the SiO smallest2 dopants grain are size helpful was found for reducing in the samplethe grain EWPS500 size; a smaller. The result grains indicate size indicatesd that both that therethe PEO are and more SiO grain2 dopant boundariess are helpful and defects for reducing in the thefilms. grain Further, size; a the smaller XRD patternsgrain size in indicates Figure4 alsothat indicatedthere are more that the grain diffraction boundaries peak and of SiOdefects2 resulted in the films.from theFurther, ITO glassthe XRD rather patterns than in the Figure SiO2 dopant.4 also indicate SiO2 dis that amorphous the diffraction and is peak present of SiO at2 the result grained from the ITO glass rather than the SiO2 dopant. SiO2 is amorphous and is present at the grain boundaries [31,32], inhibiting the continuous growth of the grains; therefore, the grain sizes of samples EWPS500 and EWS500 were smaller than that of sample EW500.

Coatings 2018, 8, 228 5 of 12 boundariesCoatings 2018, [831, x, FOR32], PEER inhibiting REVIEW the continuous growth of the grains; therefore, the grain sizes of samples5 of 11 Coatings 2018, 8, x FOR PEER REVIEW 5 of 11 EWPS500 and EWS500 were smaller than that of sample EW500.

 ITO SiO  WO3 2  ITO SiO  WO3 2 *    * * * * * (a) EW500 * **    * * * * * (a) EW500 (b) EWS500 (b) EWS500

Intensity (a.u.) (d) EWP500

(e) ITO glass (e) ITO glass 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 2 (°) 2 (°) Figure 4. X-ray diffraction (XRD) patterns of different films: (a) EW500, (b) EWS500, (c) EWP500, (d) FigureFigure 4. 4. X-rayX-ray diffraction ( (XRD)XRD) patterns patterns of of different different films: ﬁlms: (a ()a EW500) EW500;, (b ()b EWS500) EWS500;, (c) (EWP500c) EWP500;, (d) EWPS500, (e) indium tin oxide (ITO) glass. All of them were crystallized by annealing at 500 °C.◦ (dEWPS500) EWPS500;, (e) ( eindium) indium tin tin oxide oxide (ITO (ITO)) glass. glass. All All of ofthem them we werere crystallized crystallized by by annealing annealing at at500 500 °C.C.

16 15.8 Grain size 16 15.8 Grain size

14 14 14 14

12.1 12 12.1 Grain size (nm) 12 11.2 Grain size (nm) 11.2

10 10 EW500 EWP500 EWS500 EWPS500 EW500 EWP500 EWS500 EWPS500 Samples Samples Figure 5. Crystalline grain sizes of different WO3 films, calculated using the WO3 main peaks (2θ ≈ Figure 5. Crystalline grain sizes of different WO3 films, calculated using the WO3 main peaks (2θ ≈ Figure24.1°) in 5. theCrystalline XRD pattern grains. sizes of different WO films, calculated using the WO main peaks 24.1°) in the XRD patterns. 3 3 (2θ ≈ 24.1◦) in the XRD patterns. Figures 6 and 7 show the AFM 3D images (10,000 nm × 10,000 nm) of four kinds of samples and Figures 6 and 7 show the AFM 3D images (10,000 nm × 10,000 nm) of four kinds of samples and the surface roughness and increased surface area (∆S, compared with their projected area: 10,000 nm the surfaceFigures roughness6 and7 show and the increased AFM 3D surface images area (10,000 (∆S, compared nm × 10,000 with nm)theirof projected four kinds area of: 10 samples,000 nm × 10,000 nm) of these samples, respectively. In Figure 6, it indicates that in samples EWP500 and and× 10 the,000surface nm) of roughnessthese samples and, respectively increased surface. In Figure area 6 (,∆ itS ,indicates compared that with in sample their projecteds EWP500 area: and EWPS500 (PEO-doped), the macroporous structure is large compared with samples EW500 and 10,000EWPS500 nm × (PEO10,000-doped) nm) of, the these macroporous samples, respectively. structure Inis Figure large 6compared, it indicates with that sample in sampless EW500 EWP500 and EWS500 (not PEO-doped), which indicated that the PEO template plays an important role in the andEWS500 EWPS500 (not (PEO-doped),PEO-doped), whichthe macroporous indicated that structure the PEO is large template compared plays with an important samples EW500 role in and the formation of a rough surface and a macroporous structure, because PEO was decomposed into CO2 EWS500formation (not of PEO-doped),a rough surface which and a indicated macroporous that thestructure PEO template, because playsPEO wa ans importantdecomposed role into in theCO2 and H2O when annealing was carried out at 500 °C for 3 h. Figure 7 indicates that the SiO2 dopant formationand H2O when of a rough anneal surfaceing was and carried a macroporous out at 500 structure, °C for 3 h because. Figure PEO7 indicates was decomposed that the SiO into2 dopant CO2 also helped to increase the roughness compared with◦ the control film EW500. The surface area of all andalso H help2O whened to increase annealing the was roughness carried compared out at 500 withC for the 3 h.control Figure film7 indicates EW500. thatThe thesurface SiO area2 dopant of all the experimental films was larger than that of the control film, and sample EWPS500 showed the best alsothe experimental helped to increase films wa thes roughnesslarger than comparedthat of the withcontrol the film, control and filmsample EW500. EWPS500 The show surfaceed areathe best of result. In this material system, the SiO2 dopant cooperated with PEO to inhibit grain growth and allresult. the experimental In this material films system, was larger the SiO than2 dopant that of thecooperate controld film, with and PEO sample to inhibit EWPS500 grain showedgrowth theand increase surface roughness and surface area. bestincrease result. surface In this roughness material system, and surface the SiO area2 dopant. cooperated with PEO to inhibit grain growth and Figure 8 shows the current curves of different WO3 films in a cycle of electrochromic test under increaseFigure surface 8 shows roughness the current and surface curves area. of different WO3 films in a cycle of electrochromic test under ±2 V voltages. The injecting and extracting charges per unit area (∆Q) were calculated on the basis of ±2 VFigure voltages8 shows. The injecting the current and curves extracting of different charges WO per3 unitfilms area in a(∆ cycleQ) we ofre electrochromic calculated on the test basis under of the current of a cycle of electrochromic test. As we know, the electrochromic mechanism refers to a ±the2 V current voltages. of Thea cycle injecting of electrochromic and extracting test charges. As we per know, unit the area electrochromic (∆Q) were calculated mechanism on the refers basis to of a process of Li+ and electron migration [33]: theprocess current of Li of+ aand cycle electron of electrochromic migration [33 test.]: As we know, the electrochromic mechanism refers to a + -+ (2) process of Li andWO electron33 (colorless)+ migration [33xx]: e-+ + Li Lix WO (Blue) (2) WO33 (colorless)+xx e + Li Lix WO (Blue) + − + Li is the main chargeWO carrier3(colorless in the )LiClO+ xe 4/PC+ x electrolyteLi → Lix.WO The3 (aboveBlue) process finishes when the(2) Li+ is the main charge carrier in the LiClO4/PC electrolyte. The above process finishes when the currents tend to 0 mA. Among the four samples, the ∆Q of sample EW500 were the least, which currents tend to 0 mA. Among the four samples, the ∆Q of sample EW500 were the least, which indicated that there was a low amount of LixWO3 generated in sample EW500. The time indicated that there was a low amount of LixWO3 generated in sample EW500. The time corresponding to 90% of the total current change is defined as the electrochromic response time. In corresponding to 90% of the total current change is defined as the electrochromic response time. In

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+ Li is the main charge carrier in the LiClO4/PC electrolyte. The above process finishes when the currents tend to 0 mA. Among the four samples, the ∆Q of sample EW500 were the least, which indicated that there was a low amount of LixWO3 generated in sample EW500. The time correspondingCoatings 2018 to, 8, x 90% FOR ofPEER the REVIEW total current change is defined as the electrochromic response6 time.of 11 In FigureCoatings8, all 2018 films, 8, x showFOR PEER a short REVIEW electrochromic response time (around 7 s), which is acceptable6 of 11 and Figure 8, all films show a short electrochromic response time (around 7 s), which is acceptable and closed to a normal level [34]. Figureclosed 8 to, all a normalfilms show level a [ short34]. electrochromic response time (around 7 s), which is acceptable and closed to a normal level [34].

Figure 6. Atomic force microscope (AFM) 3D images (10,000 nm × 10,000 nm) of different WO3 films: Figure(a 6.) EW500Atomic, (b force) EWS500 microscope,(c) EWP500 (AFM), (d) EWPS500 3D images. (10,000 nm × 10,000 nm) of different WO3 ﬁlms: Figure 6. Atomic force microscope (AFM) 3D images (10,000 nm × 10,000 nm) of different WO3 films: (a) EW500, (b) EWS500,(c) EWP500, (d) EWPS500. (a) EW500, (b) EWS500,(c) EWP500, (d) EWPS500. 12 11.4 25 Surface roughness 23.1 S 1211 25 Surface roughness 11.4 20 23.1 S 11 10 ) 20 2 15

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10 5 8.7 9 2

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10 5 8.7 9 8

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 10 5 8 3 6.6 6.7 7.6 7 S (

Surface roughness (nm) roughness Surface

 5 0 3 6.6 7 6 Surface roughness (nm) roughness Surface EW500 EWS500 EWP500 EWPS500 0 6 EW500 EWS500 EWP500 EWPS500 Samples Figure 7. The surface roughness and increasedSamples surface area (∆S, compared with their projected area: Figure10,000 7 n. mThe × 10surface,000 n m)roughness of EW500, and EWS500, increased EWP500 surface, and area EWPS500, (∆S, compared which with were their determined projected from area: the Figure 7. The surface roughness and increased surface area (∆S, compared with their projected area: 10corresponding,000 nm × 10,000 AFM nm) images of EW500, by the EWS500, AFM s EWP500upport software, and EWPS500,. which were determined from the 10,000 nm × 10,000 nm) of EW500, EWS500, EWP500, and EWPS500, which were determined from the corresponding AFM images by the AFM support software. corresponding AFM images by the AFM support software.

Coatings 2018, 8, 228 7 of 12 CoatingsCoatings 2018 2018, 8, ,8 x, xFOR FOR PEER PEER REVIEW REVIEW 7 7of of 11 11

3030 2020 (a) (a) EWPS500 EWPS500 1010 ColoredColored BleachedBleached

0 -10 2 -10 QQ =0.0165 =0.0165 C/cm C/cm 2 -20-20

-3030-3030 2020 (b) (b) EWP500 EWP500 1010 ColoredColored BleachedBleached

0 -10-10 2 2 -20-20 QQ =0.0145 =0.0145 C/cm C/cm -3030-3030

2020 (c) (c) EWS500 EWS500 1010 ColoredColored BleachedBleached

Current (mA) Current (mA) -10-10 2 2 -20-20 QQ =0.0155 =0.0155 C/cm C/cm -3030-3030 2020 (d) (d) EW500 EW500 10 10 Colored Colored BleachedBleached

0 -10-10 2 QQ =0.0095 =0.0095 C/cm C/cm 2 -20-20 -30-30 0 0 5 5 1010 1515 2020 2525 3030 3535 4040 Time (s) Time (s)

FigureFigure 8 .8 The. The current current curve curves sof of different different WO WO3 films3 films in in a acycle cycle of of electrochromic electrochromic test test under under ±2V ±2V voltages: voltages: Figure 8. The current curves of different WO3 ﬁlms in a cycle of electrochromic test under ±2V voltages: (a()a )EWPS500 EWPS500, (, b(b) )EWP500 EWP500, (, c()c )EWS500 EWS500, (, d(d) )EW500. EW500. (a) EWPS500; (b) EWP500; (c) EWS500; (d) EW500.

BothBoth t hethe images images and and the the transmittance transmittance of of the the initial initial state, state, colored colored state state, ,and and bleached bleached state state of of thesetheseBoth samples samples the images are are shown shown and the in in transmittanceFigure Figure 9 9. .The The oftransmittance transmittance the initial state, modulation modulation colored state, abilit abilit andyy (∆ bleached(∆TT) )is is defined defined stateof by by these the the followingsamplesfollowing are formula formula shown at inat wavelength Figurewavelength9. The 628 628 transmittance nm nm: : modulation ability ( ∆T) is deﬁned by the following formula at wavelength 628 nm: (3)(3) TTTTTT=∆=T | |=cbcb|Tc − |T | b| (3) In this formula, T and T are the transmittance of the colored state and bleached state at InIn this this formula, formula, T Tc cand and T Tbb bare are the the transmittance transmittance of of the the colored colored state state and and bleached bleached state state at at wavelengthwavelength 628 628 nmnm, nm, ,respectively. respectively. ∆ ∆∆TTT directly directlydirectly shows shows shows the the the optical optical optical contrasts contrasts contrasts in in in a a a certain certain certain percentage. percentage. percentage. InIn this this this experiment, experiment, experiment, the the the ITO ITO ITO glass glass glass (substrate) (substrate) (substrate) was used wawass as used used a blank, as as a whosea blankblank transmittance, , whosewhose transmittancetransmittance was designated wawas s designatedasdesignated 100% in as the as 100% 100% wavelength in in the the wavelength wavelength range of 400–900 range range of nm.of 400 400 According––900900 nm nm. .According According to Figure 9to ,to theFigure Figure∆T 9 of9, ,the thesethe ∆ ∆TT samplesof of these these weresamplessamples calculated, we werere calculated calculated and the, ,and results and the the wereresults results 9.4% we were (EW500),re 9.4% 9.4% (EW500) (EW500) 27.8%,(EWS500), ,27.8% 27.8% (EWS500), (EWS500), 35.5% (EWP500),35.5% 35.5% (EWP500) (EWP500) and 63.2%, ,and and 63.2%(EWPS500).63.2% (EWPS500). (EWPS500). It indicates It It indicates indicates that the that that∆T the theof ∆ sample ∆TT of of sample sample EWPS500, EWPS500 EWPS500 whose, whose, whose color wascolor color deep wa was s bluedeep deep atblue blue the at colorat the the color state, color wasstatestate the, ,wa wa highests sthe the highest highest among among among these samples.these these samples. samples. This indicates This This indicates indicates that sample that that sample EWPS500sample EWPS500 EWPS500 could effectively could could effectively effectively reduce reducethereduce passing the the passing ofpassing light. of of light. light.

100100 100100

9090 EWS500 9090 EWS500 8080 Initial Initial Colored Colored 80 EW500EW500 T =79.0% 70 80 cTc=79.0% 70 Bleached Bleached Initial Initial T =88.4% bTb=88.4% T =64.1%

T =64.1% Colored Colored 6060 c c T =91.9% 70 Bleached Bleached Tb b=91.9%

Transmittance (%) 70

Transmittance (%)

Transmittance (%) Transmittance (%) 5050

6060 4040 400400 500500 600600 700700 800800 900900 400400 500500 600600 700700 800800 900900 Wavelength (nm) Wavelength (nm) WavelengthWavelength (nm) (nm) (a(a) ) (b(b) )

Figure 9. Cont.

Coatings 2018,, 8,, 228x FOR PEER REVIEW 8 of 1211 Coatings 2018, 8, x FOR PEER REVIEW 8 of 11 100 100 100 100 90 90 90 90 80 80 80 70 EWPS500 80 70 EWPS500 60 Initial 70 60 Initial 70 50 Colored 50 Colored 60 40 Bleached

EWP500 T =51.7% 60 c 40 Bleached

EWP500 T =51.7% 50 Initial T c=87.2% 30 T =18.6% b 30 c 50 Initial T =87.2% Tc =18.6%

b Transmittance (%) Transmittance (%) Colored 20

Transmittance (%) T =81.8% Transmittance (%) 40 Colored 20 b 40 Bleached 10 Tb =81.8% Bleached 10 30 0 30400 500 600 700 800 900 4000 500 600 700 800 900 400 500 600 700 800 900 400 500 600 700 800 900 Wavelength (nm) Wavelength (nm) Wavelength (nm) Wavelength (nm) (c) (d) (c) (d)

Figure 9. The transmittance and images of different WO3 films at the initial state, colored state, and FigureFigure 9.9. TheThe transmittance transmittance and and images images of of different different WO WO3 filmsfilms at the at theinitial initial state, state, colored colored state state,, and bleached state: (a) EW500, (b) EWS500, (c) EWP500, (d) EWPS5003 . Tc and Tb are the transmittance of bleached state: (a) EW500, (b) EWS500, (c) EWP500, (d) EWPS500. Tc and Tb are the transmittance of andthe colored bleached state state: and (a )bleached EW500; (stateb) EWS500; at wavelength (c) EWP500; 628 nm (d), EWPS500.respectively.Tc and Tb are the transmittance ofthe the colored colored state state and and bleached bleached state state at atwavelength wavelength 628 628 nm nm,, respectively. respectively. In addition, coloration efficiency (CE) is another important parameter for evaluating the In addition, coloration efficiency (CE) is another important parameter for evaluating the electrochromicIn addition, performance, coloration defined efficiency by the (CE) following is another formu importantla [35,36]: parameter for evaluating the electrochromic performance, defined by the following formula [35,36]: electrochromic performance, defined by the following formula [35,36]: Tb ln Tb ln ln Tb OD∆OD Tc Tc (4) CE=CE=OD  =Tc (4)(4) CE= ∆Q ∆Q QQQQ

In FormulaFormula (4), ∆∆ODOD is the opticaloptical density;density; ∆∆Q,, TTcc, and Tb were defined defined above. Figure 10 shows In Formula (4), ∆OD is the optical density; ∆Q, Tc, and Tb were defined above. Figure 10 shows the different CE of thethe fourfour samples.samples. T Therehere waswas anan increase in thethe CECE ofof thethe WOWO33 crystalline filmsfilms the different CE of the four samples. There was an increase in the CE of the WO3 crystalline films when PEO and SiOSiO22 werewere used as the dopants.dopants. BothBoth PEO and SiO2 wewerere efficientefficient in enhancing the when PEO and SiO2 were used as the dopants. Both PEO and SiO2 were efficient in enhancing the electrochromic performance,performance, but the combined use of SiO2 and PEO could further increase the CE electrochromic2 performance, but the combined2 use of SiO2 and PEO2 could further increase the CE from 23.23 cm2/C/C (EWS500) (EWS500) or or 35.86 35.86 cm cm2/C/C (EWP500) (EWP500) to to89.69 89.69 cm cm2/C /C(EWPS500). (EWPS500). Figure Figure 11 shows 11 shows the from 23.23 cm2/C (EWS500) or 35.86 cm2/C (EWP500) to 89.69 cm2/C (EWPS500). Figure 11 shows the thedifferent different ∆T ∆ofT theof thefirst first and and the the 500th 500th cycle. cycle. It Itindicates indicates that that the the transmittance transmittance modulation modulation change changedd different ∆T of the first and the 500th cycle. It indicates that the transmittance modulation changed little (within (within 5%). 5%). Therefore, Therefore, all fourall four samples samples showed show stableed cyclestable characteristics, cycle characteristics, which is an which advantage is an little (within 5%). Therefore, all four samples showed stable cycle characteristics, which is an ofadvantage the crystalline of the WOcrystalline3 films [WO15,163 film]. s [15,16]. advantage of the crystalline WO3 films [15,16]. It is worth comparing the electroc electrochromichromic transmittance modulation ability in our work with the It is worth comparing the electrochromic transmittance modulation ability in our work with the past reported data.data. TableTable2 2shows shows these these comparisons. comparisons The. The results results showed showed that that the the modulation modulation ability ability of past reported data. Table 2 shows these comparisons. The results showed that the modulation ability theof the crystalline crystalline WO 3 WOfilm3 wasfilm enhancedwas enhanced successfully successfully by PEO by modification PEO modifi andcation SiO2 dopingand SiO compared2 doping of the crystalline WO3 film was enhanced successfully by PEO modification and SiO2 doping withcompared other crystalline with other WO crystalline3 films. Further, WO3 thefilm transmittances. Further, the modulation transmittance ability modulation of EWPS500 ability appeared of compared with other crystalline WO3 films. Further, the transmittance modulation ability of asEWPS500 good as appeared that of the as amorphous good as that WO of 3thefilm. amorphous WO3 film. EWPS500 appeared as good as that of the amorphous WO3 film. 100 100 OD 1.5 89.7 1.5  1.5 89.7 1.5 OD CE 80 CE 80 1.2 1.2 60 60 0.9 /C)

0.9 /C)

 40 OD 35.9

 0.6 35.9 40

0.5

0.6 CE (cm 0.4 23.2 0.5 0.4 23.2 20 CE (cm 0.3 11.6 20 0.3 0.1 11.6 0.1 0.0 0 0.0 EW500 EWS500 EWP500 EWPS500 0 EW500 EWS500 EWP500 EWPS500 Samples Samples

Figure 10. The optical density (∆OD) and coloration efficiency (CE) of samples EW500, EWS500, FigureFigure 10.10. The opticaloptical density density ((∆∆OD)OD) and colorationcoloration efﬁciency efficiency (CE)(CE) ofof samples samples EW500, EW500, EWS500, EWS500, EWP500 and EWPS500. EWP500EWP500 andand EWPS500.EWPS500.

Coatings 2018, 8, 228 9 of 12 Coatings 2018, 8, x FOR PEER REVIEW 9 of 11 80 1st 70 500th cycle 63.2 60.5 60 50 40 35.5 32.18

T (%)

27.8

 30 26.44 20 10 9.4 8.7 0 EW5000 EWS500 EWP500 EWPS500

Sample Figure 11. The transmittance modulation ability (∆T at 628 nm) with ±2 V voltages at the 1st and 500th Figure 11. The transmittance modulation ability (∆T at 628 nm) with ±2 V voltages at the 1st and 500th cycle. The ∆T of these crystalline WO3 films changed within 5%. cycle. The ∆T of these crystalline WO3 ﬁlms changed within 5%.

Table 2. Comparisons with the past reported data: Electrochromic film, synthetic method, crystallinity, and ∆T at 628 nm. Table 2. Comparisons with the past reported data: Electrochromic ﬁlm, synthetic method, crystallinity, andEle∆Tctrochromicat 628 nm. Film Synthetic Method Crystallinity ∆T at 628 nm Reference EWPS500 Sol–gel method Crystalline 63.2% This work Electrochromic Film Synthetic Method Crystallinity ∆T at 628 nm Reference PEG400 doped WO3 Sol–gel method Crystalline ~14% [19]

EWPS500WO3 Thermal Sol–gel methodevaporation Crystalline ~32% 63.2% [36 This] work

PEG400 dopedWO WO3 3 ThermalSol–gel methodevaporation A Crystallinemorphous ~66% ~14% [36] [19] 2%V2O5 doped WO3 RF magnetron sputtering Amorphous ~49% [37] WO3 Thermal evaporation Crystalline ~32% [36]

As mentionWO3 ed above, theThermal electrochromic evaporation performance Amorphous is closely related ~66% to the morphology, [36]

structure2%V2,O and5 doped crystallinity WO3 ofRF the magnetron WO3 films. sputtering The PEO Amorphoustemplate was decomposed ~49% into CO [237 and] H2O after annealing at 500 °C , which resulted in a high surface roughness and a macroporous structure. The highAs mentioned surface roughness above, the indicate electrochromicd that there performance was a large is closely surface related area for to the reaction. morphology, Meanwhile, structure, the pores on the structure provide channels for LiClO4/PC electrolyte, so Li+ can easily enter the internal and crystallinity of the WO3 films. The PEO template was decomposed into CO2 and H2O after annealingstructure. at Therefore, 500 ◦C, which both resulted EWP500 in a and high EWPS500 surface roughness showed andbetter a macroporous electrochromic structure. transmittance The high surfacemodulation roughness ability indicated than EW500 that and there EWS500. was a large In addition surface, areathe SiO for2 reaction.dopant could Meanwhile, further theincrease pores the on surface roughness, so the electrochromic transmittance modulation+ ability of EWPS500 was enhanced the structure provide channels for LiClO4/PC electrolyte, so Li can easily enter the internal structure. Therefore,to the highest both levels EWP500, compared and EWPS500 with that showed of other better samples electrochromic. However, transmittance the influence modulation of crystallinity ability cannot be ignored. As we know, the component of the grain boundaries is amorphous WO3, which than EW500 and EWS500. In addition, the SiO2 dopant could further increase the surface roughness, + socon thetains electrochromic multiple defects. transmittance Li is easily modulation intercalated ability into of EWPS500these defects was, enhancedgeneratin tog blue the highest LixWO levels,3. The comparedXRD analysis with indicate that ofd other that samples.SiO2 could However, inhibit the the growth influence of the of crystallinitygrains effectively cannot, and be ignored.the most Asdirect we proof of this phenomenon was the decrease of the grain size, which indicated that the grain know, the component of the grain boundaries is amorphous WO3, which contains multiple defects. boundaries+ were larger in samples EWS50, EWP500, and EWPS500. Sample EWPS500 had the above Li is easily intercalated into these defects, generating blue LixWO3. The XRD analysis indicated that characteristics, so its transmittance modulation ability was the most outstanding among these SiO2 could inhibit the growth of the grains effectively, and the most direct proof of this phenomenon samples. was the decrease of the grain size, which indicated that the grain boundaries were larger in samples EWS50, EWP500, and EWPS500. Sample EWPS500 had the above characteristics, so its transmittance 4. Conclusions modulation ability was the most outstanding among these samples. PEO-modified and SiO2-doped crystalline WO3 films were prepared by the sol–gel spin coating 4.technique. Conclusions The combined action of PEO and SiO2 increased the surface roughness and formed a macroporous structure on the films. In addition, both PEO and SiO2 could reduce the crystalline grain PEO-modified and SiO2-doped crystalline WO3 films were prepared by the sol–gel spin coating size effectively. The above characteristics of morphology, structure, and crystallinity resulted in a technique. The combined action of PEO and SiO2 increased the surface roughness and formed a good electrochromic transmittance modulation of the crystalline WO3 films. The PEO-modified and macroporous structure on the films. In addition, both PEO and SiO2 could reduce the crystalline grain sizeSiO2- effectively.doped crystalline The above WO3 characteristics films could intercalate of morphology, more charges structure, (0.0165 and C/cm crystallinity2) than the resulted pure WO in a3 films (0.0095 C/cm2). Moreover, the transmittance modulation ability (∆T = 63.2% at 628 nm) of the good electrochromic transmittance modulation of the crystalline WO3 films. The PEO-modified and former films was better than that of the latter films (∆T = 9.4% at 628 nm). 2 SiO2-doped crystalline WO3 films could intercalate more charges (0.0165 C/cm ) than the pure WO3

Coatings 2018, 8, 228 10 of 12

films (0.0095 C/cm2). Moreover, the transmittance modulation ability (∆T = 63.2% at 628 nm) of the former films was better than that of the latter films (∆T = 9.4% at 628 nm).

Author Contributions: Conceptualization, G.Z.; Formal analysis, G.Z.; Funding acquisition, H.N., R.Y., and J.P.; Investigation, G.Z., X.Z., and W.Y.; Methodology, G.Z.; Project administration, H.N., R.Y., and J.P.; Supervision, H.N.; Writing—original draft, G.Z.; Writing—review & editing, K.L., H.N., R.T., X.L., R.Y., and J.P. Funding: This work was supported by the National Key R&D Program of China (2016YFB0401504), National Natural Science Foundation of China (51771074, 51521002 and U1601651), National Key Basic Research and Development Program of China (973 program, 2015CB655004) Founded by MOST, Guangdong Natural Science Foundation (2016A030313459 and 2017A030310028), Guangdong Science and Technology Project (2016B090907001, 2016A040403037, 2016B090906002, 2017B090907016 and 2017A050503002), Guangzhou Science and Technology Project (201804020033). Acknowledgments: We thank Jingxin Qin for her linguistic assistance during the preparation of this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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