Enhancing the Spectroelectrochemical Performance of WO₃ Films by Use of Structure-Directing Agents during Film Growth

Abstract

Thin, porous films of WO₃ were fabricated by solution-based synthesis via spin-coating using polyethylene glycol (PEG), a block copolymer (PIB₅₀-b-PEO₄₅), or a combination of PEG and PIB₅₀-b-PEO₄₅ as structure-directing agents. The influence of the polymers on the composition and porosity of WO₃ was investigated by microwave plasma atomic emission spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, X-ray diffraction, and gas sorption analysis. The electrochromic performance of the WO₃ thin films was characterized with LiClO₄ in propylene carbonate as electrolyte. To analyze the intercalation of the Li⁺ ions, time-of-flight secondary ion mass spectrometry, and X-ray photoelectron spectroscopy were performed on films in a pristine or reduced state. The use of PEG led to networks of micropores allowing fast reversible electrochromic switching with a high modulation of the optical transmittance and a high coloration efficiency. The use of PIB₅₀-b-PEO₄₅ provided isolated spherical mesopores leading to an electrochromic performance similar to compact WO₃, only. Optimum characteristics were obtained in films which had been prepared in the presence of both, PEG and PIB₅₀-b-PEO₄₅, since WO₃ films with mesopores were obtained that were interconnected by a microporous network and showed a clear progress in electrochromic switching beyond compact or microporous WO₃.

1. Introduction

Windows are an essential component in almost every building. Switchable glass, also known as a smart window, provides tunable shading that can improve the indoor occupant comfort and reduce the energy consumption caused by heating and cooling of a building [1,2]. For smart windows, electrochromic materials are of high interest [3,4]. Along with an electrolyte containing small ions, electrochromic thin films can offer a reversible coloration under an applied voltage [1,5]. Tungsten oxide (WO₃) is a widely used electrochromic material [6,7] that can change color from colorless transparent to dark blue with good switching characteristics upon reduction and intercalation of small charge-balancing countercations such as H⁺, Li⁺, Na⁺, or K⁺ ions [7].

Thin films of WO₃ can be fabricated by various deposition techniques [8] such as sputtering [9,10], thermal vapor deposition [11,12], or sol–gel processing [13,14] from precursor solutions by spin-coating [14], dip-coating [15] or electrodeposition [16] which provide good options of device production at low cost and low environmental footprint [8]. Precursor solutions based on tungsten hexachloride [17,18], peroxotungstic acid [14,19], sodium tungstate [16,20] or ammonium metatungstate [21] have been used successfully for the preparation of tungsten oxide thin films. Further, the internal structure of WO₃ can be tuned in a wide range using such solution-based approach [22,23].

The electrochromic properties of tungsten oxide and, in particular, the charge transport in the films are highly influenced by the structure (amorphous vs. crystalline) and the porosity of the films [2,15]. Compact WO₃ thin films with improved electrochromic characteristics could be obtained from a precursor solution containing peroxotungstic acid upon annealing of the samples at 250 °C, and it was observed that amorphous WO₃ thin films showed a larger change in coloration between the colored and bleached state, a faster diffusion of ions and a higher coloration efficiency compared to crystalline films [14].

The addition of polymers or surfactants as structure-directing agents into the precursor solution allows a facile modification of the porosity and atomic arrangement (amorphous vs. crystalline) of the WO₃ films [22,23]. Polyethylene glycol (PEG) of different molecular weight has been largely used in sol–gel processes to inhibit crystallization of WO₃ [22,24]. Djaoued et al. [25] and Cremonesi et al. [26] added PEG 600 to a tungsten hexachloride-based precursor solution and prepared WO₃ thin films by dip-coating followed by heat treatment at different temperatures. The films prepared in the presence of PEG 600 and annealed at 300 °C showed a higher coloration efficiency compared to the films prepared without PEG 600 [25,26]. Lu et al. [27] used PEG 400 as additive in a peroxotungstic acid-based precursor solution in a volume ratio of 1:10. Thin films were obtained by dip-coating and annealing at 300 °C. These films exhibited better electrochromic characteristics than the films prepared without polymers [27]. Hence, the addition of PEG 400 or PEG 600 consistently provided WO₃ films with a high active surface area leading to good electrochromic properties [25,26,27]. The proper amount of polymer in the precursor solution is crucial for adjusting the structure and porosity of the films [22]. WO₃ thin films which were electrodeposited by Deepa et al. [28] showed that the amount of PEG 400 in the peroxotungstic acid deposition solution significantly influenced the morphology of the films and, thus, their electrochromic characteristics. WO₃ thin films with improved electrochromic properties could also be obtained by addition of polyethylene glycol of a higher molecular weight such as PEG 2000 [29] or PEG 20,000 [30] with different precursor solutions. The improvement in electrochromic parameters such as the coloration efficiency by nanoscaled porosity is supposed to be attributable to enhanced diffusion of Li⁺, which is a relevant issue in regard to the desired fast switching times [2]. Hence, further previous studies addressed the impact of different nanoscaled porosity on the electrochromic performance. Block copolymers such as polystyrene-block-polyethylene oxide (PS-b-PEO) [31] or Pluronic P123 [32] that are forming micelles allow the formation of mesoporous films with 4–30 nm large pores providing enhanced electrochromic characteristics such as short switching time, high coloration efficiency and large transmittance modulation [31,32]. Note that in diverse previous studies films with spherical mesopores were investigated which inherently suffer from restricted connection between the spherical pores, impeding Li⁺ diffusion [33,34]. In view of such experience, a combination of block copolymers and long-chained additives might be promising to prepare WO₃ films with both, spherical mesopores as well as interconnected micropores. Such strategy already was successful in the preparation of titanium oxide thin films using polyisobutylene-block-polyethylene oxide (PIB₅₀-b-PEO₄₅) as block copolymer and 1-hexadecyl-3-methylimidazolium chloride (C₁₆mimCl) as surfactant-like ionic liquid in different ratios to tune the porosity and permeability of titanium oxide thin films by creating a film structure with mesopores (caused by PIB₅₀-b-PEO₄₅) connected by worm-like pores (caused by C₁₆mimCl) [35].

In the present work, thin films of WO₃ were prepared via spin-coating with similar polymers as structure-directing agents to influence the internal film structure and porosity and to improve the electrochromic switching characteristics. The precursor solution based on peroxotungstic acid showing a high stability can be easily and inexpensively prepared by dissolution of tungsten in hydrogen peroxide with no need of anhydrous conditions [8,14,36]. It is desirable to use PEG of low molecular weight to create a network of interconnected micropores in the films since such PEG can be easily removed after annealing for a shorter time compared to PEG of high molecular weight [27,30]. Therefore, a peroxotungstic acid-based precursor solution was used with addition of PEG 400, PEG 600 (for a formation of micropores [37]) or the diblock copolymer PIB₅₀-b-PEO₄₅ that had been successfully used for the preparation of mesoporous titanium oxide [35,38]. To obtain WO₃ thin films with interconnected mesopores, a mixture of PEG 600 and PIB₅₀-b-PEO₄₅ was established. The structural, optical, electrochemical and spectroelectrochemical properties of the WO₃ thin films using these approaches were characterized and compared to compact films prepared without additives. The addition of PEG 400 or PEG 600 into the precursor solution created a network of interconnected micropores within the films. An enhancement of the electrochromic switching properties compared to compact WO₃ films was observed. The largest transmittance modulations, highest coloration efficiencies and fastest diffusion of ions along with highly reversible and stable switching processes could be obtained for WO₃ films prepared with PIB₅₀-b-PEO₄₅ combined with PEG 600 as additives, leading to films with interconnected mesopores. The great potential of solution-based processing for WO₃ suitable for the application in smart windows is thereby demonstrated.

2. Experimental

2.1. Preparation of WO₃ Thin Films

Silicon (111) wafers (WACKER, Munich, Germany) or fluorine doped tin oxide (FTO) coated glass substrates (Kaivo, Zhuhai, China, <15 Ohm sq⁻¹, cut into 25 mm × 35 mm pieces) were cleaned for 15 min in RBS solution (Carl Roth, Karlsruhe, Germany), acetone (Carl Roth, Karlsruhe, Germany, ≥99.5%) and isopropanol (Carl Roth, Karlsruhe, Germany, ≥99.8%), subsequently using an ultrasonic bath at room temperature and dried with N₂ gas followed by UV-ozone treatment for 15 min. In case of the FTO-coated glass substrates, a 5 mm wide stripe was masked with a piece of adhesive tape (TESA, Norderstedt, Germany) for later electrical contacting purposes.

The precursor solution containing peroxotungstic acid (PTA) was synthesized as described in Ref. [14] from 3 g tungsten powder (Alfa Aesar, Kandel, Germany, ≥99.9%) by slowly adding 10 mL 30% hydrogen peroxide solution (Sigma Aldrich, Steinheim, Germany) into an Erlenmeyer flask. Afterwards, 3 mL glacial acetic acid (Alfa Aesar, Kandel, Germany, ≥99.7%) and 11 mL ethanol (Carl Roth, Karlsruhe, Germany, ≥99.8%) were added and the mixture was stirred for 2 h while cooling in an ice bath at 0–10 °C. The mixture was left at room temperature for 70 h and subsequently filtered three times to remove excess tungsten powder. This precursor solution was spin-coated onto the substrates at 3500 rpm for 30 s. Spin-coating was repeated three times to ensure a homogeneously coated substrate. These as-deposited WO_3|comp thin films were dried at 60 °C for about 2 h and then baked at 250 °C, if not mentioned otherwise for about 1 h (

Table 1. Preparation conditions (utilized structure-directing agent, calcination temperature T and duration of calcination t) of different tungsten oxide thin films used for the spectroelectrochemical measurements.

Film Type	Polymer	T/°C	t/h
WO3|comp	None	250 (350; 450; 550)	1
WO3|µp400	PEG 400	250	1
WO3|µp600	PEG 600	250 (350; 450; 550)	1
WO3|mp	PIB50-b-PEO45	300	12
WO3|µmp	PIB50-b-PEO45 + PEG 600	300	12

). In order to achieve interconnected micropores, polyethylene glycol (PEG 400 (Sigma Aldrich, Steinheim, Germany) or PEG 600 (Sigma Aldrich, Steinheim, Germany)) was added as structure-directing agent into the precursor solution in different volume ratio (1.0:10, 1.5:10, 3:10 and 5:10) and the WO_3|µp400 and WO_3|µp600 films were prepared following the same procedure as the WO_3|comp films. About 37 mg of polyisobutylene-block-polyethylene oxide (PIB₅₀-b-PEO₄₅) (BASF, Ludwigshafen, Germany) were added into 1 mL of the peroxotungstic acid precursor solution to obtain mesoporous WO_3|mp films with large spherical mesopores. Thin WO_3|µmp films exhibiting a combination of interconnected micropores and mesopores were prepared by adding around 75–150 µL PEG 600 and 42 mg PIB₅₀-b-PEO₄₅ into 1 mL of the peroxotungstic acid precursor solution. Both precursor solutions containing PIB₅₀-b-PEO₄₅ were additionally treated in the ultrasonic bath for at least 30 min. After complete dissolution of PIB₅₀-b-PEO₄₅, agglomerates were removed using a 0.2 µm syringe filter. After spin-coating, these as-deposited films were first dried at 60 °C for about 2 h and then annealed at 300 °C for around 12 h (Table 1). For benchmarking of the present films, samples of a sputter-deposited 650 nm thick commWO₃ film from EControl-Glas GmbH & Co. KG (Plauen, Germany) were used, a type that had been used in commercially available electrochromic smart windows.

2.2. Thin Film Characterization

The surface morphology and the cross-sections were analyzed by scanning electron microscopy (SEM) in a Zeiss MERLIN (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany) at an emission current of 78–100 pA and an acceleration voltage of 2–5 kV. The elemental composition of the films was examined in the Zeiss MERLIN at an emission current of 2000 pA and an acceleration voltage of 9 kV by energy-dispersive X-ray spectroscopy (EDX) using a X-Max 50 mm² EDX detector (Oxford Instruments, Abingdon, UK). Quantification of the elements W, O, Sn and C was carried out with the software Aztec 4.3 (Oxford Instruments, Abingdon, UK).

The crystallinity of the WO₃ thin films was investigated by grazing incidence X-ray diffractometry (GIXRD) using a PANanalytical X’Pert Pro MRD instrument (Malvern Panalytical B.V., Almelo, Netherlands) with Cu-Kα-radiation. In situ GIXRD was performed during step-wise heat treatment of a film from 30–700 °C to determine the crystallization temperature. The diffractograms were analyzed using the software HighScore Plus 3.0e.

The thickness d of the films was measured with an Alpha-Step profilometer from Tencor Instruments (Milpitas, CA, USA) as an average over at least 10 samples at different sites and verified by the analysis of the cross-section of the samples by SEM. Using the Alpha-Step profilometer the surface roughness was also determined. Ellipsometry of the films was measured on polished silicon wafers by means of a variable angle spectroscopic ellipsometer (VASE) from Woollam (Lincoln, NE, USA) and the data were fitted with the WVase32 software. The thickness d_E and the porosity P_E of the films was determined using the Bruggeman effective medium approximation and the compact WO₃ film as reference sample.

Krypton physisorption measurements were performed in an automated gas sorption station (Autosorb iQ2, Quantachrome Corporation, Boynton Beach, FL, USA) at 77 K by using a cryostat (CryoSync, Quantachrome Corporation, Boynton Beach, FL) in a relative pressure range of p/p₀ = 0.05–0.30 to obtain the surface areas of the films. Thin films prepared on silicon wafers were cut and filled into glass tubes with a cylindrical end. Surface areas were determined by applying the Brunauer–Emmett–Teller (BET) model supported by the software ASiQwin 4.0. The mass of tungsten in the different films m(W) was determined by microwave plasma atomic emission spectroscopy (MP-AES) with a 4210 MP-AES from Agilent Technologies (Santa Clara, CA, USA) using a nitrogen plasma. The films were dissolved in a defined volume of 5% NH₃ solution overnight. Sodium tungstate dissolved in NH₃ solution was used for calibration purposes.

The mass of tungsten oxide m(WO₃) in the different thin films was calculated using the content of tungsten m(W) measured by MP-AES, the molar mass of W, and that of WO₃. m(WO₃) was then used to calculate the density ρ of the films based on the volume V of the films as the product of the geometric film area A and the thickness d of the films plus a film volume existing between FTO crystallites. Material that was spin-coated onto the edges of the substrates, which would lead to higher ρ than the true value was considered in another correction. 1 cm² was cut from the central part of some samples, analyzed as described and on average led to values of ρ equaling 78% of those determined after analysis of entire samples prepared in the same way. This factor of 0.78 was used to correctly determine ρ for all samples. Based on ρ, the porosity P_chem = 1 − ρ (WO₃ with additives)/ρ(WO_3|comp) was calculated.

Thermogravimetry (TG) was performed using a Setsys Evolution 16/18 (Setaram, Lyon, France) on the precursor solutions with and without the addition of polymers after they were dried at 60 °C overnight. Raman spectroscopy was performed using a Renishaw InVia Raman microscope system (Renishaw plc., New Mills, UK) equipped with a helium–neon laser with a wavelength of 633 nm that was focused onto the sample using an objective with 50× magnification. With the Software Origin (OriginLab), a fast Fourier transformation (FFT) was performed to increase the resolution of the measured data.

Spectroelectrochemical analysis of the WO₃ thin films was carried out in a cell (Zahner, Kronach, Germany) with a closed environment in order to avoid the presence of oxygen during the measurements while allowing to carry out electrochemical characterization and to measure the optical properties in operando. The respective thin film was mounted as working electrode in the cell with a platinum wire counter electrode (Goodfellow, 99.995%) and a Ag/AgCl leak-free reference electrode (Innovative Instruments, Inc., Tampa, FL, USA) with 100 mg LiClO₄ (Sigma Aldrich, Steinheim, Germany, 99.99%) in 1 L propylene carbonate (PC) (Sigma Aldrich, Steinheim, Germany, ≥99.7%) as electrolyte (0.94 M). Cyclic voltammetry (CV) was carried out between −1.0 V and 1.0 V at different scan rates employing an IviumStat potentiostat/galvanostat (Ivium Technologies B.V., Eindhoven, Netherlands). Chronoamperometry (CA) was performed for up to 50 cycles by switching between −1.0 V and 1.0 V at a time interval of 300 s each or for 60 s at −1.0 V and 100 s at 1.0 V. To analyze the distribution of the Li⁺ ions within the films, a bias potential of −1.0 V was applied for 1 s to 300 s. The optical spectra of the films were simultaneously measured with a tec5 (Steinbach, Germany) UV/Vis spectrometer. At least two specimens of each kind were analyzed to ensure reproducibility. The amount of intercalated Li⁺ ions x was calculated from the charge density obtained from the CV and CA analysis [39].

Electrochemical impedance spectroscopy (EIS) of the films was performed at different potentials (−0.25 V, −0.50 V, −0.75 V, −1.0 V) with an AC amplitude of 10 mV and a frequency of 5 MHz to 100 mHz using a Zahner IM6 potentiostat (Kronach, Germany). The same cell as for the spectroelectrochemical analysis and a low-leak reference electrode (Innovative Instruments, Inc., Tampa, FL, USA) was chosen for these measurements. Before measuring the impedance spectra, the desired potential was applied for 15 min to reach a quasi-steady-state. Between the different impedance measurements, a potential of 1.0 V was applied for 15 min to deintercalate the ions and reoxidize the films. The evaluation of the impedance spectra was carried out with RelaxIS 3.

X-ray photoelectron spectroscopy (XPS) was performed on the films before and after intercalation of Li⁺ ions with a PHI Versaprobe II spectrometer (Physical Electronics, Inc., Chanhassen, MN, USA) using monochromatized Al-Kα (1486.6 eV) radiation allowing excitation at 45° to the surface normal. Survey spectra of the samples were measured at a pass energy of 93.5 eV. Detailed spectra of the W 4f, Li 1s, O 1s, C 1s and Cl 2p core levels were taken at a pass energy of 23.5 eV. Spectra were recorded before and after etching for 120 s, 240 s and 360 s by an Ar⁺ gun and an e⁻ gun to provide charge neutralization. The obtained spectra were fitted using the software CasaXPS (version 2.3.18, Casa Software Ltd., Teignmouth, UK). Energy calibration of the spectra was carried out using the C 1s signal at a binding energy of 284.8 eV. A Shirley background was used and all the spectra were fitted with a Gaussian/Lorentzian line shape. The depth profile of the films before and after the intercalation with Li⁺ ions was also investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) using a ToF.SIMS 5 (IONTOF GmbH, Münster, Germany) equipped with a Bi⁺ primary ion gun (25 keV, 1.2 pA, 100 µm × 100 µm) for analysis and a Cs⁺ gun (1 keV, 120 nA, 200 µm × 200 µm) for depth profiling. All measurements were performed in the negative ion mode with a cycle time of 100 µs. Depth profiles were acquired in spectrometry mode including a noninterlaced sputtering mode. Between two sputter frames, the analysis was carried out after a pause time of 2 s in random raster mode collecting two frames and 1 shot/pixel. Data evaluation was performed using the software SurfaceLab 7.0 (IONTOF GmbH, Münster, Germany). Aside from the Li⁻ signals, signals of WO₃⁻ and SnO⁻ were detected with increasing sputter time to determine the position of the interface between WO₃ and the substrate which is defined by the drop or rise to 50% [40] of the intensity of the WO₃⁻ or SnO⁻ signals, respectively.

3. Results and Discussion

3.1. Morphology and Structure of the Films

From the cross-section of the pure tungsten oxide WO_3|comp thin films prepared from the precursor solution without additives (Figure S1c), a full coverage of the substrate by WO₃ with a film thickness of around 100 nm can be observed, thinner than the reported 250 nm in [14]. This could be caused by differences in the details of experimental conditions during spin-coating. The samples reveal a smooth and compact film structure with less than 2 nm RMS surface roughness (WO_3|comp) on the crystalline FTO substrate as also observed in [14]. While the addition of PEG 400 or PEG 600 in the volume ratio of 3:10 or 5:10 into the precursor solution led to inhomogeneous films, the addition of PEG in the volume ratio of up to 1.5:10 provided films of WO_3|µp400 and WO_3|µp600 with a smooth coverage of the substrate (Figure 1a and Figure S1b) with RMS surface roughness similar to the WO_3|comp films (Figure S1a). The cross-section of these films indicates a grainy film structure and film thicknesses of about 130–150 nm (Figure 1d and Figure S1d). Such a grainy film structure has been also observed for WO₃ thin films prepared via spin-coating from a peroxotungstic acid precursor solution with PEG 20,000 as structure-directing agent and annealed at 300 °C [30]. Using PIB₅₀-b-PEO₄₅ as structure-directing agent results in around 200–230 nm thick WO_3|mp films (Figure 1e) with isolated spherical mesopores of around 10–20 nm diameter (Figure 1b,e). These pore sizes are comparable to the ones obtained for mesoporous titanium oxide using a similar block copolymer [35,38]. Films prepared with PIB₅₀-b-PEO₄₅ as additive were light-brownish in color, indicating some remaining carbon within the films. The combination of PEG 600 and PIB₅₀-b-PEO₄₅ as structure-directing agents led to 230–250 nm thick WO_3|µmp films with interconnected mesopores (Figure 1c,f).

The amorphous character of the films which were annealed at temperatures T ≤ 300 °C is revealed by GIXRD at room temperature (30 °C) as depicted in Figure 2 for a WO_3|µp600 film. No reflexes between 20–60° are observable which would be characteristic of crystalline WO₃. All other reflexes correspond to FTO as confirmed by a GIXRD measurement of a bare FTO glass substrate. Annealing the film during the GIXRD analysis showed the transition of an amorphous film into a crystalline film. At 400 °C, reflexes assigned to triclinic WO₃ (JCPDS card 83-0947) start to appear and become more obvious from 450 °C on. For WO_3|comp as well as for the WO_3|µp400, WO_3|mp and WO_3|µmp films, similar series were obtained, indicating the presence of triclinic WO₃ only after annealing the samples at 450 °C (Figure S2). The crystallite size of the films annealed at 450 °C was estimated from the reflexes at 2Θ around 23° and 42° using the Scherrer equation [41,42,43]. An average crystallite size of around 21 nm was obtained which is well in line with the crystallite size reported in [44] for WO₃ films prepared by dip-coating and annealed at 500 °C. Zhao et al. [14] and Wu et al. [30] have also obtained triclinic WO₃ from peroxotungstic acid solutions. At 600 °C and 650 °C, the relative intensity of the substrate signals increased, presumably caused by thermal expansion of the sample leading to a higher contribution from the substrate. At 700 °C, the glass of the substrate started to melt leading to a decrease of the intensity of all reflexes.

The amorphous character of the WO₃ thin films annealed at temperatures T ≤ 300 °C was confirmed by Raman spectroscopy. In the Raman spectra (Figure S3) measured for the films prepared without and with PEG at T ≤ 300 °C a broad peak at 770 cm⁻¹ was observed caused by vibrations of W⁶⁺-O bonds [12,45] while a relatively sharp peak at 950 cm⁻¹ reflects the stretching of W⁶⁺=O bonds characteristic for amorphous WO₃ [12,45,46]. For the WO_3|µp600 thin film, another broad peak at 220 cm⁻¹ was noticed which might correspond to the bridging of O-W⁴⁺-O bonds and indicates the presence of W⁴⁺ ions and oxygen vacancies within the films [12,45].

Thermogravimetry (TG) of the dried precursor, resembling films following spin-coating, was performed to monitor the decomposition of the polymers during heat treatment and to detect possible phase transformations of WO₃. In Figure 3a, the TG curves for the dried peroxotungstic acid precursor solutions with and without PEG are shown. Starting from room temperature up to a temperature of 100 °C, a small loss in mass of around 1% followed by a larger loss from 100 °C to 200 °C was observed caused by the desorption of water and organic solvents as well as the transformation of peroxotungstic acid to WO₃ [14,27,29]. For the precursors with PEG, mainly its decomposition contributed to this mass loss [27]. As expected, the drop in mass from room temperature to 250 °C is larger for the precursor containing PEG 600 since a larger amount of polymer is present. Within the three hours of treatment at 250 °C, the mass of the precursor without additives decayed by 2% whereas the precursors containing PEG 400 or PEG 600 reduced their mass by only 1% or 0.4%. All three precursors quickly stabilized indicating that keeping them for about 1 h at 250 °C is sufficient for the preparation of amorphous WO₃ thin films. The subsequent small loss of mass (2%) upon further heating to 700 °C can be assigned to the phase transition of amorphous to crystalline WO₃ [14,27,29], which was independently proven by XRD (Figure 2 and Figure S2). The TG curves for the precursors containing the block copolymer PIB₅₀-b-PEO₄₅ or PIB₅₀-b-PEO₄₅ in combination with PEG 600 (Figure 3b) follow the same trend upon heating to 300 °C. At this temperature, however, a mass loss of about 3% was observed over 12 h, indicating continued decomposition of the polymer. Upon subsequent heating to 800 °C, a significant mass loss of another 3% (WO_3|mp) or 7% (WO_3|µmp) was observed consisting of both, crystallization of WO₃ [27] and completion of polymer decomposition in the case of WO_3|µmp. For WO_3|mp, however, the mass loss of the precursor is similar to the one of the precursor without any additives confirming that residuals of the polymer were still present in the films. Since amorphous WO₃ thin films were aimed at, a temperature not higher than 300 °C (Figure 2 and Figure S2) had to be chosen for the heat treatment of the films and it cannot be excluded that the WO_3|mp thin films might still contain some polymer fragments. Comparing the initial loss of mass for the different precursors reveal that the precursor without any additives and the one containing PIB₅₀-b-PEO₄₅ have lost about the same amount of mass (15–18%) up to a temperature of 250 °C while the precursors with PEG 600 have lost more than half of their original mass. The reason for the similar decrease in mass for the pure precursor and the one with PIB₅₀-b-PEO₄₅ can be explained by the small polymer content and its only partial decomposition. While for the precursors with PIB₅₀-b-PEO₄₅, around 37 mg of PIB₅₀-b-PEO₄₅ was added in 1 mL precursor solution, the precursor solution with PEG 600 in a volume ratio of 1.5:10 contained 165.5 mg PEG 600 in 1 mL precursor solution. The observed larger initial loss of mass is explained by almost complete decomposition of PEG at 250 °C.

3.2. Elemental Composition, Density, Surface Area and Porosity

To verify that WO₃ and not any other related compound was obtained when using the precursor solutions containing different polymers as additives, the O:W ratio of the films was determined by EDX (

Table 2. Average elemental composition (and standard deviation) of the different tungsten oxide thin films obtained from EDX and the calculated (O:W)_EDX ratios.

Film Type	W/At%	O/At%	Sn/At%	C/At%	(O:W)EDX
WO3|comp	10.0 ± 0.5	66.6 ± 0.2	17.5 ± 0.6	5.9 ± 0.2	3.16 ± 0.06
WO3|µp400	12.9 ± 0.9	66.0 ± 0.3	14.0 ± 1.6	7.1 ± 0.7	2.94 ± 0.05
WO3|µp600	12.7 ± 1.0	66.1 ± 0.2	14.7 ± 1.3	6.6 ± 0.2	2.90 ± 0.04
WO3|mp	14.0 ± 0.9	63.8 ± 1.4	10.3 ± 2.0	11.9 ± 2.5	3.08 ± 0.08
WO3|µmp	16.8 ± 1.3	65.8 ± 1.4	9.9 ± 1.0	7.5 ± 1.7	2.82 ± 0.20

, (O:W)_EDX). Since the tungsten oxide films are thinner than 300 nm and the penetration depth of EDX into the sample is in the micrometer range [47] emission from the substrate has to be accounted for. The spectra revealed the presence of tin in addition to tungsten and oxygen as well as some carbon contamination. Assuming a ratio of O:Sn = 2 for the FTO substrate, meaningful values of (O:W)_EDX in the films can still be calculated. At first glance, (O:W)_EDX was found to be around 3 for all the films (Table 2) which can be considered a confirmation that WO₃ was obtained from the different precursor solutions. However, (O:W)_EDX decreases for the films prepared with PEG indicating the presence of a higher concentration of oxygen vacancies. It has been reported before that PEG can inhibit the crystallization of the films and might lead to formation of oxygen-deficient WO₃ [48]. For the WO_3|µmp films, (O:W)_EDX deviates in a larger range compared to the other samples. Thus, the presence of mesopores interconnected by micropores (Figure 1c,f) might lead to locally differing transmission of X-rays emitted from Sn in FTO within a given sample area and, therefore, to larger variations in (O:W)_EDX. Further, for the WO_3|mp films, the amount of carbon detected in the films was higher directly indicating that polymer fragments were still present in the films.

X-ray photoelectron spectroscopy (XPS) was performed to complement the elemental analysis by EDX and, in particular, to obtain direct insight into the W oxidation state in pristine films. The overview spectra, which are quite similar for all the films (Figure 4a and Figures S4a–S7a), only show the expected elements such as tungsten and oxygen as well as some carbon impurities, to similar extent in all samples. The W 4f spectra (Figure 4b and Figures S4b–S7b) exhibit a doublet at binding energies around 38 eV and 36 eV, which corresponds to W⁶⁺ 4f_5/2 and W⁶⁺ 4f_7/2 states [14,24,49,50]. The small shoulders at lower binding energies of about 36.4 eV and 34.2 eV point out the presence of W⁵⁺ 4f_5/2 and W⁵⁺ 4f_7/2 states assigned to defects in the film surface [24,49]. The positions of these peaks are in good accordance with the ones reported for WO₃ thin films prepared via sol–gel processes [14,24] or sputter deposition [50]. The O 1s spectra (Figure 4c and Figures S4c–S7c) show a main peak around 530.8 eV which can be assigned to the W=O bonds [24]. The shoulder with less intensity at higher binding energy of about 532.0 eV can be assigned to W–OH groups [24,51].

From the peak areas of the O 1s spectra and the W 4f spectra, the O:W ratio at the surface (

Table 3. Elemental composition of the different tungsten oxide thin films obtained from XPS, and the calculated (O:W)_XPS ratios.

Film Type	W 4f/%	-OH/%	O=W/%	C/%	(O:W)XPS
WO3|comp	15.72	9.46	44.23	29.79	3.42
WO3|µp400	16.37	9.19	43.27	30.48	3.20
WO3|µp600	15.42	8.50	41.03	34.34	3.21
WO3|mp	16.80	12.60	43.59	26.23	3.34
WO3|µmp	18.09	6.79	52.40	21.86	3.27

, (O:W)_XPS) was determined [52]. The values of (O:W)_XPS are higher than the (O:W)_EDX values (Table 2). The reason for this lies in the surface sensitivity of XPS as opposed to EDX [47] leading to a higher contribution of hydroxyl groups at the surface and therefore to higher values for (O:W)_XPS. However, the trend of a decreasing (O:W)_EDX with PEG as precursor additive was confirmed by (O:W)_XPS (Table 3). In the films prepared with any of the additives but in particular for WO_3|µp400 and WO_3|µp600, a higher concentration of W⁵⁺ was seen when compared to WO_3|comp (

Table 4. Concentration of W⁵⁺ in pristine (p) and intercalated (i) WO₃ thin films obtained from XPS and the degree of reduction for these experiments expressed as Li⁺ content x in Li_xWO₃.

Film Type	W5+(p)/%	W5+(i)/%	x at −1.0 V
WO3|comp	0.26	32.30	0.66
WO3|µp400	1.42	37.50	0.62
WO3|µp600	2.76	31.71	0.62
WO3|mp	0.94	16.43	0.27
WO3|µmp	0.68	19.36	0.42

). Hence, as reported in [48], PEG can lead to porous films with more defects and, in particular, more oxygen vacancies, represented by the existence of W⁵⁺ states.

The C 1s signal seems to be dominated by adventitious carbon and no conclusion can be drawn about any residual polymer fragments since even for the WO_3|comp film prepared without any additives a larger amount of carbon was detected than for the WO_3|mp film (Table 3), for which residual carbon was apparent in the color and in EDX. Therefore, the amount of carbon detected by XPS cannot give any evidence for polymer fragments remaining in the films.

During the stepwise etching with Ar⁺, the amount of W⁶⁺ states for all the films decreased while the amount of W⁵⁺ states increased and subsequently even W⁴⁺, and tungsten in even lower ionization states (W^x+) and metallic W⁰ states arose indicating the stepwise reduction of WO₃ under Ar⁺-bombardment even up to metallic tungsten as shown in Figure S8. These observations are well in line with the results obtained for Ar⁺ bombardment of WO₃ films prepared via thermal evaporation [49].

Film characteristics derived from wet chemical analysis and profilometry compared to those independently obtained from the analysis of ellipsometry are summarized in

Table 5. Average thickness d obtained from profilometry and d_E obtained from ellipsometry, density ρ and values of the porosity P_chem and P_E obtained from wet chemical analysis and ellipsometry, respectively.

Film Type	d/nm	dE/nm	ρ/g cm−3	Pchem/%	PE/%
WO3|comp	98	95	7.15	-	-
WO3|µp400	138	163	6.31	11.8	12.3
WO3|µp600	145	161	5.88	17.9	14.4
WO3|mp	215	297 *	4.84	32.3	19.2
WO3|µmp	230	349 *	4.21	41.2	29.9

* not to be compared to d since different set of thicker films was analyzed.

. ρ of the WO_3|comp films was found similar to that reported for bulk WO₃ (${\rho }_{W{O}_3}$ = 7.2 g cm⁻³ [53]). By addition of PEG or PIB₅₀-b-PEO₄₅ as structure-directing agents, ρ of the prepared films decreased, according to the strategy. The porosity P_chem derived from microwave plasma atomic emission spectroscopy of the films dissolved in NH₃ solution increased according to the decreasing ρ of the films (Table 5). The porosity was independently calculated from the ellipsometry data, yielding P_E, which directly confirmed the range of porosity of the samples, but with a trend to slightly lower values than P_chem (Table 5). For the ellipsometric analysis, the films were prepared on smooth silicon wafers as opposed to rough FTO substrates. Such difference and the fundamentally different method of analysis can well explain the small differences between d and d_E and the small systematic difference between P_chem and P_E. Both methods, however, independently yield the same trend of increasing porosity when using the polymers as structure-directing agents with P(WO_3|µp400) < P(WO_3|µp600) << P(WO_3|mp) << P(WO_3|µmp). The combination of PIB₅₀-b-PEO₄₅ and PEG 600 as additives clearly provided the highest porosity as already revealed by SEM (Figure 1c).

The specific surface area S_BET of the as-prepared films was analyzed from the BET plots obtained from Kr physisorption (77 K) shown in Figure S9. The WO_3|mp sample was annealed at 550 °C for 1 h before the BET analysis to remove any residual polymer and obtain a measure for the intrinsic surface area of WO₃, albeit transformation to crystalline WO₃ under such conditions. For the WO_3|comp films prepared without additives, no significant S_BET could be determined, as expected for a compact thin film. For the WO_3|µp600 films, a significant surface area of around 3 m²/g was obtained, comparable to the surface area reported for WO₃ powder (~2 m²/g) [54]. Such surface area is compatible with the presence of micropores between grains of the films (Figure 1d). The values of S_BET of the WO_3|mp films and WO_3|µmp films turned out in the same range as reported for WO₃ nanoflakes (~10 m²/g [55]) and mesoporous WO₃ (~32 m²/g [31]). S_BET ≅ 10 m²/g determined for the WO_3|mp films thereby confirmed the presence of mesopores within the films but the considerably higher S_BET ≅ 20 m²/g of the WO_3|µmp films shows the significantly increased accessibility of the mesopores by the simultaneous presence of micropores serving as interconnecting channels. These S_BET values appear quite small, compared to the surface area and pore volume usually measured for templated metal oxides. Note that the BET surface area is referred to the mass and that WO₃ has a quite high density (ca. 4.2–7.2 g cm⁻³), thus resulting in lower S_BET values compared to a material with identical porosity, but smaller skeleton density, e.g., SiO₂ (ρ ≅ 2.2 g·cm⁻³ [53]). For instance, a value of S_BET ≅ 20 m²/g (WO_3|µmp), assuming an identical pore space, translates into a value of S_BET ≅ 65 m²/g for a corresponding SiO₂ material, which is in accordance with a network of connected spherical mesopores of ca. 20 nm in diameter.

3.3. Redox Characteristics and Intercalation of Li⁺

In order to analyze the electrochromic reduction reaction and the diffusion of Li⁺ ions in the films, cyclic voltammetry (CV) was performed at different scan rates as shown for a WO_3|µmp film in Figure 5 and for all other types of films in Figure S10. Following a CV curve with a just slightly different shape during a first conditioning cycle (Figure 6a), reproducible CV curves were obtained at each scan rate representing a reversible intercalation and deintercalation of Li⁺ upon reduction and reoxidation as expected for amorphous WO₃ films [7].

The reversibility K of the bleaching and coloration process can be determined from the ratio q_out/q_in of the extracted and inserted charge densities [56]. During the first conditioning cycle (

Table 6. Reversibility of the electrochromic switching determined from the first (K₁) and second (K₂) cycle of the CV measurements at a scan rate of 5 mVs⁻¹ measured for at least 2 specimens of a given kind (commWO₃ only measured once).

Film Type	K1/%	K2/%
WO3|comp	66–84	92–100
WO3|µp400	92–96	94–97
WO3|µp600	87–98	91–99
WO3|mp	69–77	95–96
WO3|µmp	93–96	95–97
commWO3	96	97

) and especially for the WO_3|comp and WO_3|mp films, the reversibility is rather low indicating that not all inserted ions can be deintercalated but are trapped within the films. Slow or irreversible movement in compact WO₃ can be expected. For the porous WO_3|mp films, however, this originally came unexpected and points at partial pore filling by residual polymer or polymer fragments, as also concluded from a brownish color after annealing during film preparation and relevant mass losses at higher temperature during TG analysis. For the WO_3|µp400, WO_3|µp600 and WO_3|µmp films, the reversibility is close to 100% confirming that only a few ions are trapped within the films [39,57]. Hence, diffusion in the microporous network and in the absence of residual polymer fragments assures complete extraction of Li⁺ even during the first bleaching process. However, from the second cycle on, all the films showed a high reversibility which means that less ions were trapped inside the films and that the sites where ions might be trapped were already occupied during the first coloration. In this respect, the films are similar to the commercially sputtered commWO₃ film.

The anodic peak current densities j_p,_a and the cathodic current densities at negative scan reversal j_r,_c (Figure 5 and Figure S10) were plotted against the square root of the scan rate v^1/2 following the Randles Ševčík equation at room temperature

j = 2.69 × 10⁵ · n^3/2 · D^1/2 · c · v^1/2

(1)

where n is the number of electrons transferred in the reaction (1 in the present case [14]) and c is the concentration of Li⁺ in the bulk of the electrolyte [14,56] to determine two approximations of the effective diffusion coefficient of Li⁺ and electrons in the films, D = D_a,CV in the anodic branch and D = D_c,CV in the cathodic branch. Diffusion of Li⁺ in the electrolyte can be considered much faster than that in the films, and it has not to be considered separately. The average values of the effective diffusion coefficients determined for a given type of sample are shown in

Table 7. Effective diffusion coefficients D_a,CV and D_c,CV determined from CV.

Film Type	Da,CV/cm2 s−1	Dc,CV/cm2 s−1
WO3|comp	6.0 × 10−11	3.4 × 10−10
WO3|µp400	7.7 × 10−11	3.5 × 10−10
WO3|µp600	1.1 × 10−10	4.7 × 10−10
WO3|mp	6.0 × 10−11	2.5 × 10−10
WO3|µmp	6.2 × 10−10	1.1 × 10−9
commWO3	1.4 × 10−9	2.8 × 10−9

. The values of the effective diffusion coefficient are in the range of 10⁻¹¹–10⁻⁹ cm²·s⁻¹, similar to the values reported for sol–gel derived [14,29,58] or sputter-deposited [59] WO₃ thin films. Slightly larger effective diffusion coefficients were found for WO_3|µp400 and WO_3|µp600 than for WO_3|comp. Obviously, the ions can diffuse more easily through the film network along the microporous pathways available within the films prepared with PEG 400 or PEG 600. For WO_3|mp, on the other hand, an effective diffusion coefficient even smaller than for WO_3|comp was obtained. Despite the mesopores detected in SEM, the remaining polymer fragments of PIB₅₀-b-PEO₄₅ within the films seem to significantly attenuate the diffusion of Li⁺. An effective diffusion coefficient about 6 times higher than that for WO_3|comp, WO_3|µp400 or WO_3|µp600 was observed in WO_3|µmp using a combination of both, PEG and PIB₅₀-b-PEO₄₅ as structure-directing agents. This combination provided films with similarly fast diffusion of Li⁺ as obtained for commWO₃ (Table 7).

3.4. Electrochromic Switching Characteristics

To investigate the switching processes of the films, their optical transmittance T_λ was measured in operando during CV analysis. As expected, the reduction of WO₃ and the intercalation of Li⁺ led to a decrease of T_λ (Figure 6b) and deeply blue-colored films as shown by the transmittance spectra at a potential of −1.0 V (Figure 7 and Figure S11) and the color impressions, which were calculated as reported in [60].

By reoxidation of WO₃ and deintercalation of the ions, T_λ reversibly increased [1,6]. For the WO_3|comp and the WO_3|mp films (Figure S11b,c), T_λ in the bleached state was found lower compared to the initial state indicating partial irreversibility and the presence of trapped ions [39,57] within the films. This is in accordance with their slightly lower reversibility K₁ (Table 6). The WO_3|µp400, WO_3|µp600 and WO_3|µmp films (Figure 7 and Figure S11a) could be fully bleached reaching transmittance characteristics close to the initial state. Facile deintercalation rather than trapping of ions as already revealed by the high reversibility (Table 6) is thereby confirmed.

Comparing the change in transmittance at a wavelength of 710 nm upon reduction and reoxidation of the films (

Table 8. Mean values and standard deviation of the transmittance modulation ∆T_710nm, the Li⁺ content x in Li_xWO₃ and the coloration efficiency CE_710nm obtained from CV at a scan rate of 5 mV s⁻¹ or from CA measurements.

Film Type	∆T710nm/%		x		CE710nm/cm2 C−1
	CV	CA	CV	CA	CV	CA
WO3|comp	28.8 ± 8.4	25.4 ± 2.7	0.37 ± 0.07	0.38 ± 0.05	53.2 ± 12.6	34.5 ± 5.9
WO3|µp400	68.2 ± 3.5	63.5 ± 1.7	0.43 ± 0.04	0.53 ± 0.08	75.4 ± 3.9	58.6 ± 5.6
WO3|µp600	70.0 ± 4.3	64.2 ± 6.6	0.41 ± 0.05	0.57 ± 0.05	74.5 ± 7.9	54.3 ± 8.8
WO3|mp	31.5 ± 1.8	40.8 ± 3.3	0.31 ± 0.06	0.45 ± 0.05	67.7 ± 4.9	40.7 ± 1.6
WO3|µmp	75.3 ± 2.0	75.7 ± 0.2	0.44 ± 0.10	0.68 ± 0.04	85.2 ± 5.2	65.0 ± 4.2
commWO3	59.6	88.5	0.32	0.45	68.3	67.1

) reveals that for WO_3|µp400, WO_3|µp600 and WO_3|µmp films the transmittance modulation ∆T_710nm is larger than for the WO_3|comp or WO_3|mp films. These observations correlate well with the values of the effective diffusion coefficient (Table 7). Residual polymer fragments in the WO_3|mp films might possibly hinder the diffusion of the ions within the mesoporous film structure leading to a change in transmittance comparable to compact films. Facile diffusion of ions supported by micropores present in the films prepared in the presence of PEG allows a high transmittance modulation, which has been also observed earlier for WO₃ prepared by dip-coating from similar precursor solutions [27]. The combination of PEG 600 with PIB₅₀-b-PEO₄₅ led to films reaching even higher values for the transmittance modulation compared to the films prepared with PEG only (Table 8) indicating facilitated diffusion of ions through the film network composed of interconnected mesopores and pore clusters already revealed by SEM (Figure 1f). For the WO_3|comp and WO_3|mp films, the transmittance started to change during the intercalation step at a more negative potential of around −0.25 V compared to films of WO_3|µp400, WO_3|µp600 and WO_3|µmp (Figure 6b). Such a delayed change in transmittance indicates a slower initial intercalation of the ions within the WO_3|comp and WO_3|mp films. For both films, the CV curves at the first cycle (Figure 6a) also stay at smaller current and show a small peak at around −0.4 V and −0.7 V, respectively. Such characteristics are expected for a hindered reaction, typically observed for crystalline WO₃ films [33] but observed here for the amorphous WO_3|comp and WO_3|mp films, indicative of an initial barrier for ion intercalation in line with the compact film structure or the presence of remaining polymer fragments. Nevertheless, from −0.25 V to −1.0 V the slope of the transmittance is comparable for all the films. During the bleaching process, however, a significantly steeper slope of the transmittance–potential curves can be observed for films of WO_3|µp400, WO_3|µp600 and WO_3|µmp suggesting a significantly more facile deintercalation of Li⁺ through the porous film network.

Another important parameter for the comparison of switching characteristics of the electrochromic films is provided by the coloration efficiency CE. It is defined as the change in optical density ∆OD with the intercalated charge density q (see Equation (2)) [2,7]. CE_710nm of the films at 710 nm, a wavelength for which the change was most significant, was determined from a linear fit in a plot of ∆OD = logT_bleach − logT_col over q, the charge inserted during the switching process.

$$CE=\frac{∆OD}{q}$$

(2)

CE_710nm for the WO_3|µp400, WO_3|µp600 and WO_3|µmp films was found considerably higher than for WO_3|comp or WO_3|mp (Table 8). Less charge is needed to allow a high transmittance modulation for the films with interconnected micropores compared to the compact or just mesoporous films. Such enhancement of CE by using appropriate structure-directing agents is in good accordance with earlier reports [27,31]. The highest obtained values for ∆T_710nm and CE_710nm for the porous films were reached for WO_3|µmp, similar or even better than those measured for commWO₃, the commercial benchmark sample, speaking in favor of the present films with a combination of mesopores interconnected by micropores. Interestingly, the CE values reported here are significantly higher than those previously reported for WO₃ films with ordered spherical mesopores caused by a similar block copolymer, and also similar thickness [34]. The observed higher CE in the present study is indicative of differences in the pore space: the mesoporous films reported in Sallard et al. [34] were prepared without additional PEG and, thus, without the improved connectivity between the spherical mesopores in WO_3|µmp. Hence, this comparison clearly supports the need for connecting micropores to maximize the electrochromic response.

CV measurements were also performed for WO_3|comp and WO_3|µp600 following their annealing at temperatures higher than 250 °C (Table S1). For the films calcined at 350 °C, D_a,CV was found in the same order of magnitude as for WO_3|comp calcined at 250 °C (Table 7) and also the values of ∆T_710nm and ∆CE_710nm lie in the range of the upper limit of the obtained values for the films annealed at 250 °C (Table 8). These observations confirm facile diffusion of Li⁺ in amorphous WO₃ that by GIXRD was observed to persist up to 350 °C (Figure 2 and Figure S2). After calcination at T ≥ 450 °C, the values of ∆T_710nm and CE_710nm decreased and were found close to the lower limit of the values for the films calcined at 250 °C and a drastic change can be clearly seen for the effective diffusion coefficients, which are up to two orders of magnitude smaller than D_a,CV determined after annealing at 250 °C. Slow diffusion in crystalline films formed after calcination at T ≥ 400 °C (GIXRD, Figure 2 and Figure S2) confirmed the disadvantage of a crystalline film structure for electrochromic performance as observed earlier [14,61,62]. However, the direct comparison of WO_3|comp films fabricated without additives with WO_3|µp600 prepared with PEG 600 as additive (Table S1) shows a benefit of a microporous starting structure even for a crystalline film resulting in a more facile diffusion of ions.

As shown in Figure 1, the WO_3|µp400, WO_3|µp600 and WO_3|µmp films were thicker than the films prepared without additives. Therefore, to check if the observed improvement of the transmittance modulation over WO_3|comp was provided by such increased thickness of the films and to directly compare the performance to commWO₃, thicker films of WO_3|comp, WO_3|µp600 and WO_3|µmp with similar thickness were prepared by subsequent preparation of several films onto each other. The thicker films (Figure 8) changed into an almost as deep blue as commWO₃ at, however, even slightly improved transparency in the bleached state (Figure S11d). Despite deep initial coloration, a reversible ∆T_710nm (Figure 8) of just 0.35% was measured for a 646 nm thick WO_3|comp film allowing a reversible change between 0.76% and 0.41% only, indicating heavily trapped ions after the initial coloration step. In contrast, for a 702 nm thick WO_3|µp600 film impressive ∆T_710nm = 65.9% and for a 620 nm thick WO_3|µmp film ∆T_710nm = 84.8% were achieved, values even slightly larger than those obtained for the 230–250 nm thick WO_3|µmp films (Table 8). It is thereby directly confirmed that the observed improvements of the switching characteristics are a direct consequence of a fast intercalation and deintercalation of Li⁺ ions in the films facilitated by their porous morphology.

3.5. Long-Term Stability of Switching Characteristics

Response times t_col and t_bleach (

Table 9. Response times t_col and t_bleach of the different films obtained from CA measurements after prior opposite polarization for either 60 s, 100 s, or 300 s.

Film Type	tcol/s (300 s)	tbleach/s (300 s)	tcol/s (60 s)	tbleach/s (100 s)
WO3|comp	4.5	129.0	5.1	48.8
WO3|µp400	5.2	10.4	9.1	23.2
WO3|µp600	4.8	14.1	5.5	17.1
WO3|mp	5.1	160.6	4.6	43.6
WO3|µmp	1.7	1.7	1.7	1.5
commWO3	4.1	148.4	-	-

) corresponding to the time needed to color or bleach, respectively, the films to 90% of ∆T_710nm [24,28] were calculated from the transmittance curves (Figure 9, Figure 10, Figures S12 and S13). t_col shorter than 6 s were consistently achieved for all films speaking in favor of a generally fast intercalation. However, rather long t_bleach > 40 s was determined for WO_3|comp, WO_3|mp and commWO₃ whereas the WO_3|µp400, WO_3|µp600 and WO_3|µmp were bleached within t_bleach < 25 s with short response times of just 2–3 s obtained for some samples. This tendency is again in conformity with the study of Sallard et al. [34], where response times of t_col ca. 20–30 s and t_bleach ca. 70 s were observed for samples similar to WO_3|mp. Despite higher ∆T_710nm for the commercial WO₃ film (Figure S12d), the short bleaching times of the films with micropores confirmed a facile switching of these films (Figure 9, Figure 10, Figures S12 and S13). The trend of t_bleach > t_col for all films fits to the slower transmittance change during positive scans in CV (Figure 6b) and D_c,CV > D_a,CV (Table 7). While the intercalation of Li⁺ into all presently studied films is fast, their extraction is slow for compact films (WO_3|comp and commWO₃) and for WO_3|mp with remaining polymer fragments. By using PEG 400, PEG 600 or both, PIB₅₀-b-PEO₄₅ and PEG 600 as additives, micropores were created that provide pathways for fast deintercalation of the ions.

A tendency of smaller CE_710nm was revealed in CA compared to CV (Table 8). The direct fast application of −1.0 V leads to higher current densities and chances of local overcharging at the beginning of the coloration process as opposed to a slow, well-defined change of the potential during CV. Overcharging or undesired side reactions can result in lower CE_710nm. Comparing the different kinds of films, however, CE_710nm follows the same trend as observed before during CV. From CV, CE_710nm even higher for WO_3|µp400, WO_3|µp600 and WO_3|µmp compared to commWO₃ were reached as opposed to similar CE_710nm for WO_3|comp, WO_3|mp and commWO₃.

Reversibility of the switching process deserves further attention. Films of WO_3|comp showed a monotonous upward shift of T_λ when the films were switched back and forth between −1.0 V and 1.0 V for 300 s each over three cycles (Figure 9a). This trend continued when the films were studied over 50 more cycles (Figure 10a). The compact film obviously contained a considerable concentration of trapped ions [57]. It has been found earlier [63,64] that the concentration of intercalated ions x in Li_xWO₃ in the films, clearly influenced the equilibria of W⁶⁺ ⇌ W⁵⁺ and W⁵⁺ ⇌ W⁴⁺ leading to the observed changes in the optical absorption of WO₃. It was observed that up to x = 0.3–0.5, the transmittance first decreased and then shifted back to higher transmittance for larger x whereby after exceeding x ≈ 0.7 the films became optically irreversible showing a light-brownish color. The range of x < 0.3–0.7 in the present experiments was chosen to provide a good compromise between intense coloration and high reversibility. However, in Figure S14a it can be seen that after the first intercalation of WO_3|comp for 300 s at −1.0 V (1. Cycle) ions were trapped within the films leading to a bluish film even after reversal to 1.0 V (Figure S14c). The color impressions of the film after 53 cycles indicate that the concentration of trapped ions must have increased subsequently until the films became light-brownish with T_λ for both −1.0 V and 1.0 V shifted to higher values (Figure S14a,c). For WO_3|mp, despite mesoporous morphology, the remaining polymer in the film also led to trapped ions and, thus, a similar trend towards increasing T_λ (Figure S13a) as obtained for WO_3|comp. Hence, long coloration times are disadvantageous for such films and lead to the observed irreversibilities. For the WO_3|µp400, WO_3|µp600 and WO_3|µmp films, however, T_λ in the bleached as well as in the colored states remained widely constant at 1.0 V and −1.0 V, respectively (Figure 10b, Figures S13b,c and S14b,c). These films point out the possibility to endure multiple reversible switching at a high stability of the films. The porous network in these films without significant amounts of residual polymer fragments enables a facile intercalation and deintercalation of the Li⁺ ions allowing the observed reversible switching of these films.

The reversibility dependent on the applied negative potential is compared for WO_3|comp and WO_3|µp600 in Figure S15. It could be observed that independent of the negative potential, WO_3|µp600 could be fully bleached again at 1.0 V reaching a transmittance equal to the one in the initial state. For WO_3|comp, however, the transmittance was lower than in the initial state already after prior polarization at −0.25 V only, indicating trapped ions already for weakly reduced WO_3|comp. For both films it can be observed that the spectral shape also changed with increasing negative potential. The transmittance at longer wavelengths decreased less than expected when compared to the change at short wavelengths to a degree that around 600–900 nm the transmittance even increased from −0.75 V to −1.0 V, most clearly seen for WO_3|comp (Figure S15a). This trend can be explained by a Li⁺ concentration increased to x ≈ 0.6 at −1.0 V, a value higher than the limit of x ≥ 0.5, for which such spectral changes were already reported [63,64].

3.6. Chemical Analysis of Reduced Films

The spectroelectrochemical analysis has shown that PEG 600 in combination with PIB₅₀-b-PEO₄₅ as structure-directing agents led to WO₃ films with optimum electrochromic switching characteristics (Table 8, Figure 7b, Figure 9b and Figure S13c). For films prepared with just PEG 600 or PEG 400 as additive, switching characteristics almost as good (Table 8, Figure 7a, Figure 10b, Figures S11a, S12a,b and S13b) were achieved. From a technological point of view, application of PEG only provides other advantages over using both, PEG and PIB₅₀-b-PEO₄₅. The preparation of the films is not only saving the additional demanding precursor but also saves time and energy since a heat treatment of the films is only needed at 250 °C for one hour as opposed to 300 °C for 12 h. Therefore, and in order to allow further optimization of such films in the future, details of the intercalation reactions and of the charge transport were studied for WO_3|µp400 and WO_3|µp600 and compared to WO_3|comp.

The presence of reduced states of WO₃ was directly proven by XPS analysis (Figure S16 vs. Figure 4 and Figures S4–S7) after applying −1.0 V for 300 s in the electrochemical cell and subsequent transfer to vacuum. All films showed an increase in the concentration of W⁵⁺ from around 0.3–2.8% in the pristine state to 16–38% in the reduced state (Figure S8b, Table 4), comparable to the values of 30–35% reported for colored WO₃ films prepared by a cathodic arc plasma [65]. A trend of higher concentrations of W⁵⁺ (at least at the surface) in WO_3|µp400 and WO_3|µp600 and to lower concentrations in WO_3|mp and WO_3|µmp, also when compared to WO_3|comp was observed. Efficient reduction of microporous WO₃ was thereby confirmed. Despite high coloration of WO_3|µmp and low coloration of WO_3|mp, both these mesoporous films showed a similarly low concentration of W⁵⁺ and degree of reduction x, in line with a high CE_710nm observed for WO_3|µmp, but rather low CE_710nm for WO_3|mp (Table 8). Ar⁺-sputtering of the reduced films with intercalated Li⁺ allowed to discuss the depth profile of the redox reactions. The concentration of W⁵⁺ for the reduced films decreased and then saturated with proceeding etching time, i.e., deeper in the films (Figure S8). This is consistent with a reduction reaction proceeding from the surface towards the inner volume of the films as would be expected for a movement of e⁻ still faster also in porous WO₃ than Li⁺, which can be expected to be fast within the pores but slow within solid, albeit porous WO₃, as confirmed by increased but still quite moderate D_CV (Table 7). The subsequent reduction to W⁴⁺, W^x+ and W⁰ occurred faster than in the pristine films, reasonable in view of a start already in a reduced state of WO₃ (Figure S16).

To directly prove the impact of the microporous network and to complement this analysis, the distribution of Li⁺ in the reduced films was also analyzed by ToF-SIMS (Figure 11 and Figure S17). The WO₃⁻ and the SnO⁻ signals (Figure 11) are used for correlating the sputter times and the etched thickness of the films. For the WO_3|µp400 and WO_3|µp600 samples, the decrease in intensity of the WO₃⁻ signal and the increase in intensity of the SnO⁻ signal occurred at longer sputter times compared to the WO_3|comp films, well in line with a higher film thickness for WO_3|µp400 and WO_3|µp600 (Table 5). A rather constant sputter rate of about 0.3 nm/s was obtained for all films. All depth profiles (Figure S17) confirm that an intercalation for around 60 s is sufficient to completely charge the WO₃ film since the depth profiles of the films intercalated for 60 s or 300 s are quite similar, as also revealed by transmittance spectra measured for the films after 60 s and 300 s of intercalation (Figure S18). For WO_3|comp films, an accumulation of Li⁺ at the film surface up to a depth (sputter time) of around 20 nm (60 s) can be clearly observed (Figure 11 and Figure S17a) after reduction for t ≥ 60 s, confirming a compact film structure and, thus, a low accessibility of the inner volume of the film for Li⁺. In strong contrast, reduced WO_3|µp400 and WO_3|µp600 did not show any Li⁺ accumulation at the surface, speaking in favor of a more homogeneous reduction of the films enabled by a good accessibility of the inner parts of the films for Li⁺. An increase of the Li⁻ signal at increased depths and for increased intercalation times observed for all films (Figure S17) points at intercalation of Li⁺ also in the inner parts of the films, but, since unrealistically high Li⁻ concentrations were detected close to the interface with FTO, seems to be superimposed by artefacts of the measurements caused by changing ionization probabilities (matrix effects) and by different sputter probabilities of the elements [66,67], which hinders a more detailed analysis of the depth profiles.

3.7. Charge Transport within Microporous vs. Compact Tungsten Oxide

The direct influence of the microporous network on electrode kinetics of the electrochromic reaction was independently analyzed by impedance spectroscopy in different redox states of WO_3|comp and WO_3|µp600 films. The resulting Nyquist plots (Figure 12) can be fitted well with the equivalent circuit shown in Figure 12a. This circuit is typically used for the description of intercalation processes in electrochromic films [68,69]. R₁ corresponds to a series resistance caused by the electrolyte and the substrate [69,70,71,72]. R_ct is assigned to the resistance of the charge transfer from the WO₃ surface to the electrolyte and describes the intercalation of the ions into the WO₃ film as well as the interfacial redox processes [68,69,71,72,73]. The electrochemical double layer consisting of the electrons at the electrode surface and ions on the electrolyte side can be modelled by a constant phase element or a capacitance [68,69,71,72,73]. A more accurate fit of the present data was obtained by using a constant phase element CPE_dl. A semi-infinite type Warburg impedance Z_w represents the diffusion of Li⁺ ions in the WO₃ films [68,69,71,73] but had to be replaced by a finite-length-type Warburg element Z_w,s [72,73] for the impedance spectra measured at a potential of −1.0 V, which led to more reliable fits of the data. C_L refers to a limiting capacitance caused by the finite length of the WO₃ film and considers a diffusion limited by charge accumulation in the film at low frequencies [68,69,74].

As expected, the series resistance R₁ in the high frequency range of the Nyquist plot (Figure 12) was found quite constant around 30–35 Ω for compact or microporous WO₃ and at different applied potentials. Similarly, Q_CPE and α representing the double layer capacitance and C_L representing the limiting capacitance of WO₃ showed no significant changes either with potential or among the films speaking in favor of a rather constant arrangement of ions and electrons at the electrode surface and of a rather constant maximum charge uptake. The resistance R_ct, however, for WO_3|comp decreased from about 51 Ω at −0.25 V to 9 Ω at −1.0 V, a trend directly seen by a decreasing radius of the semicircle in the moderate frequency range (Figure 12a). At negative potentials, a driving force is established to insert Li⁺ into the films and the barrier to enter is decreased. The higher value of R_ct at −0.25 V compared to the value at −1.0 V points out that a higher barrier has still to be overcome to insert Li⁺ at −0.25 V, possibly caused by slow transport of ions and/or electrons within the WO_3|comp structure. For WO_3|µp600, R_ct of about 248 Ω at −0.25 V was found considerably higher than for WO_3|comp. Since ion transport should not be hindered in a microporous compared to a compact material, the reason should be sought in an attenuated electron transport. The porous morphology and a thicker film can both lead to a larger number of grain boundaries posing additional barriers for electron transport. However, R_ct of the microporous film decreased to around 38 Ω at a potential of −0.75 V and then to about 11 Ω at −1.0 V. Hence, the values of R_ct at −1.0 V are quite similar for both types of films indicating a similar facile electron transport within the films at this potential and allowing fast intercalation of Li⁺. For both types of films, the impedance values at 1.0 V (Figure 12c,d) are much higher than at −1.0 V representing a high barrier for intercalation of the ions in the bleached state as already observed in earlier reports [75,76].

The Warburg coefficient A_w obtained from fits of the impedance spectra allows to obtain an independent measure of Li⁺ diffusion by the ion diffusion coefficient D_I in the films with the active surface area A, the molar gas constant R and the temperature T [77]:

$$D_{\mathrm{I}}=\frac{R^2{T}^2}{2A^2{n}^4{F}^4{c}^2{A}_{\mathrm{w}}^2}$$

(3)

The WO_3|comp film (Table S2) provided generally smaller D_I compared to the WO_3|µp600 film, which is well in line with the tendency observed from the effective diffusion coefficients obtained from the CV measurements. However, D_I in the range of 10⁻¹³–10⁻¹¹ cm² s⁻¹ were found around two orders of magnitude smaller than D_c,CV and D_a,CV. This is a consequence of the fact that D_CV are effective diffusion coefficients describing the combined diffusion of electrons and ions while D_I is characteristic for the diffusion of ions only. Since CV was performed across the whole range of potentials and since the fits of the impedance spectra at −1.0 V needed a dedicated Warburg element, values at intermediate potentials (−0.25 V to −0.75 V) are used for comparison purposes. Throughout this range, D_I was found significantly larger for WO_3|µp600 than for WO_3|comp, well in line with larger D_CV for WO_3|µp600, confirming faster ion transport in WO_3|µp600. The difference in D_CV, however, was found smaller than that in D_I presumably caused by slower electron transport in WO_3|µp600 which also is considered by the effective diffusion coefficient D_CV.

4. Conclusions

Thin films of tungsten oxide with significantly increased internal surface area were prepared by spin-coating using different polymers as structure-directing agents to influence the internal film structure and, thus, the electrochromic performance. The combination of PIB₅₀-b-PEO₄₅, a block copolymer that is known to form micelles under the present preparation conditions, and the homopolymer polyethylene glycol PEG 600 as templates led to WO_3|µmp films with interconnected spherical mesopores desirable for the intercalation and deintercalation of charge-balancing Li⁺ ions allowing high transmittance modulations between the bleached and the colored states of the films, short response times and high effective diffusion coefficients. The use of just PIB₅₀-b-PEO₄₅ provided mesoporous WO_3|mp films but residual polymer fragments or the presence of widely isolated pores separated by rather thick walls of compact WO₃ leading to trapping of ions similar to the compact WO_3|comp films prepared without any additives. Thin films homogeneously containing interconnected micropores were obtained with PEG 400 (WO_3|µp400) or PEG 600 (WO_3|µp600) as additives, showing electrochromic characteristics almost as good as the WO_3|µmp films. Reversible and stable switching processes as well as a homogeneous distribution of the intercalated ions within the films were obtained for all three types of films that contained the interconnected micropores, WO_3|µp400, WO_3|µp600 and WO_3|µmp. Furthermore, the use of PEG only leading to purely microporous films, i.e., to forego additional mesopores, allows a fabrication of tungsten oxide thin films at presumably lower cost and in an environmentally even more benign way favored by a considerably shorter heat treatment at even lower temperature. Nevertheless, the presence of both, micropores as well as interconnected mesopores of defined shape could still provide significant advantages in transmittance modulation, response times and coloration efficiency over films modified by just PEG as additive (WO_3|µp400, WO_3|µp600 as well as films reported in the literature [24,27,28]). These results are in excellent agreement with the enhanced Li⁺ permeation found in corresponding TiO₂ films, also containing spherical mesopores (10–15 nm) connected by small mesopores (ca. 3 nm in diameter) [33]. In conclusion, films using a combination of 10–20 nm mesopores connected through micropores or small mesopores (i.e., <3 nm in diameter) can offer an attractive pathway towards facile diffusion of ions throughout the film network in particular for thicker films of even more intense coloration as long as the mesopores are well accessible for ions. The present study thus confirms that suitable porosity can indeed substantially improve Li⁺-based electrochromic properties.