Sol–gel fabrication of WO₃/RGO nanocomposite film with enhanced electrochromic performance

Abstract

Tungsten trioxide/reduced graphene oxide (WO₃/RGO) nanocomposite film was fabricated by the sol–gel spin-coating technique, using a mixed colloidal dispersion of WO₃ precursor and GO. The structure, surface morphology and optical properties of WO₃ and WO₃/RGO films were investigated by means of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and UV-visible spectrophotometry. The electrochemical and electrochromic performances of the WO₃ and WO₃/RGO films were studied by chronoamperometry, in situ optical transmittance and cyclic voltammogram analysis. In the WO₃/RGO nanocomposite film, monoclinic WO₃ nanoparticles were grafted onto the surface of RGO sheets via C–W and C–O–W chemical bonds. The results showed that the WO₃/RGO nanocomposite film exhibited shorter coloration–bleaching times (t_c = 9.5 s and t_b = 7.6 s), higher coloration efficiency (75.3 cm² C⁻¹ at 633 nm), larger optical modulatory range (59.6% at 633 nm) and better cyclic stability compared with WO₃ films, which are attributed to faster Li⁺ ion diffusion and electron transfer rate. The methodology provides a facile and industrially scalable approach to obtain fast switching electrochromic materials.

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1. Introduction

Electrochromism describes a reversible phenomenon of a material's optical properties (transmittance, absorbance and reflectance) produced by oxidation–reduction reactions under an applied electric field.¹ Since it was discovered by Deb in 1969,² considerable progress has been achieved in the study of electrochromic materials and their applications, such as energy-saving smart windows,³ intelligent optical displays,⁴ antiglare rearview mirrors,⁵ and active camouflages.⁶ Recently, electrochromic smart windows have attracted tremendous attention because the energy crisis requires a substantial energy-saving solution to combat conventional energy source consumption.^7,8 As an important kind of inorganic electrochromic material, tungsten oxide (WO₃) has been extensively studied, due to its relatively fast response time and high coloration efficiency as compared with other electrochromic materials.^9–11 In order to improve the electrochemical stability and switching rate of crystalline structure WO₃ films, many kinds of WO₃ nanocomposites, such as WO₃/carbon nanotubes,¹² WO₃/Ti,¹³ WO₃/V₂O₅,¹⁴ and WO₃/TiO₂,¹⁵ have been studied, and exhibited enhanced electrochromic properties.

The excellent characteristics of graphene, such as high electrical conductivity, large surface area, high optical transmittance and high chemical stability, endow its composites with many enhanced properties.^16,17 There have been several reports of metal oxide/graphene composites for electrochromic application. For example, Fu et al.¹⁸ prepared WO₃/RGO composites by a one-step facile electrochemical deposition method which showed faster switching time, longer cycle life and larger coloration efficiency due to the synergistic effect of incorporation of RGO into WO₃. A WO₃ nanowires/RGO composite was prepared by the Chang group, and the composite exhibited high-quality electrochromic performance with fast color-switching speed, good cyclic stability, and high coloration efficiency.¹⁹ Tu et al.²⁰ also prepared a porous nickel oxide (NiO)/RGO hybrid film with improved electrochromic properties by the combination of electrophoretic deposition and chemical-bath deposition methods, and the porous hybrid film exhibited high coloration efficiency, fast switching speed and better cycling performance compared with a porous NiO thin film. Zhang et al.²¹ synthesized vanadium pentoxide (V₂O₅)/graphene nanocomposite films by a direct intercalation method using V₂O₅ sol and graphene. The intercalation of graphene improves the stability, reversibility, response rate and optical modulatory range the V₂O₅ xerogel films. However, there have been very few studies on the effect of structure and morphology on electrochromic performance of metal oxide electrochromic materials via the incorporation of RGO sheets.

Many methods have been adopted to fabricate WO₃ electrochromic films, including electrochemical deposition,²² spray deposition,²³ electrophoretic deposition,²⁴ chemical vapor deposition,²⁵ sputter deposition,²⁶ and the sol–gel technique.²⁷ Among them, the sol–gel technique is very attractive because of its cost effectiveness and ease of depositing uniform large-area films for window applications. Meanwhile, it is efficient for tailoring the microstructure of films by the introduction of chemical dopants to the reactant sols.²⁸ Herein, we introduce a facile and industrially scalable sol–gel approach to fabricate WO₃/RGO nanocomposite film. The surface morphology, microstructure and electrochromic properties of WO₃ and WO₃/RGO films are studied. We envisage that the unique morphology and high electrical conductivity of RGO sheets could enhance the electrochromic performance by overcoming the intrinsic drawback of the crystalline structure of WO₃.

2. Experimental

2.1 Materials

Natural graphite powder (particle sizes ≤ 500 mesh; purity ≥ 99.9%) was purchased from Qingdao Xingyuan Graphite Co. Ltd, China. Potassium permanganate, sulfuric acid, hydrogen peroxide, sodium nitrate and anhydrous ethanol were purchased from Chengdu Kelong Chemical Reagent Factory, China. Tungsten hexachloride (WCl₆, 99.5%) was purchased from Aladdin Chemistry Co. Ltd. All reagents were of analytical reagent grade and used without further purification.

2.2 WO₃/RGO nanocomposite film fabrication

GO powder was prepared by the modified Hummers method using graphite powder,²⁹ washed with deionized water, and dried in a vacuum freeze drier. The GO powder was dispersed in anhydrous ethanol by sonication to fabricate a stable GO dispersion. 10 g of WCl₆ was dissolved in 100 mL of anhydrous ethanol and stirred for 20 min to produce tungsten alkoxide precursor under nitrogen atmosphere. The tungsten alkoxide precursor solution was aged by heating at 50 °C for 24 hours and kept for another 3 days at room temperature, resulting in the formation of a homogeneous WO₃ precursor solution. GO suspension was added slowly to the WO₃ precursor solution and dispersed by sonication for 2 hours to obtain the mixed colloidal dispersions. The ratio of GO to WO₃ was 3 wt%. Indium tin oxide (ITO, R_s = 5–15 Ω, Zhuhai Kaivo Optoelectronic Technology Co. Ltd, China) glass substrates were sequentially rinsed with 1 mol L⁻¹ NaOH/H₂O₂ mixed solution, acetone, anhydrous ethanol and deionized water. The mixed colloidal dispersions were deposited onto the ITO glass substrates by the spin-coating procedure to fabricate the WO₃/RGO nanocomposite film at a controlled speed of 2000 rpm. Subsequently, the film was annealed in air at 100 °C for 1 h and heated to 500 °C at a rate of 5 °C min⁻¹ and kept for 2 hours. The WO₃ film was prepared using the same conditions but without the addition of GO.

2.3 Characterization techniques

The structure, surface morphology and optical properties of the WO₃ and WO₃/RGO films were investigated by X-ray diffraction (XRD, X'Pert Pro MPD, Cu radiation, λ = 1.5418 Å), Raman spectroscopy (LabRAM HR, laser excitation wavelength of 632.8 nm), X-ray photoelectron spectroscopy (XPS, Kratos XSAM800), scanning electron microscopy (SEM, JEOL JSM-7500F), high-resolution transmission electron microscopy (HRTEM, Philips Tecnai G2 F20) and UV-visible spectrophotometry (Analytik Jena AG, Specord 200). Electrochemical and electrochromic properties were determined in a conventional three-electrode electrochemical cell with an electrochemical workstation (Shanghai CH Instruments Co., CHI660E), where platinum sheet electrode and saturated calomel electrode were used as counter and reference electrodes, respectively. Chronoamperometry (CA) and cyclic voltammetry (CV) measurements were performed in a 1 mol L⁻¹ lithium perchlorate/propylene carbonate (LiClO₄/PC) electrolyte. The optical transmittance of the WO₃ and WO₃/RGO films were measured in situ by a UV-visible spectrophotometer (Analytik Jena AG, Specord 200). Coloration and bleaching times were calculated as the time required for achieving 90% of the total transmission change. The potential range of CV measurements for the films was from 1.5 V to −1.5 V at a scan rate of 50 mV s⁻¹.

3. Results and discussion

3.1 Structural analysis

Fig. 1 shows the XRD spectra of the spin-coated WO₃ and WO₃/RGO films. Four main peaks corresponding to the ITO glass substrate were identified. The XRD patterns of the WO₃ and WO₃/RGO films show WO₃ monoclinic structure (JCPDS no. 43-1035). Susanti et al.³⁰ also found similar results for their WO₃ nanomaterial which was synthesized by a sol–gel method using WCl₆ and C₂H₅OH as precursors followed by annealing at 500 °C. The XRD pattern for the WO₃ film shows three dominant crystalline plane orientations: (002) at 2θ = 23.2°, (020) at 2θ = 23.7° and (200) at 2θ = 24.3°. However, the diffraction pattern of the WO₃/RGO nanocomposite film was found to be shifted towards lower diffraction angles mainly due to the interaction between the oxygen functional groups in RGO and the outermost oxygen in WO₃.³¹ The XRD pattern for the WO₃/RGO nanocomposite film also shows three dominant crystalline plane orientations: (002) at 2θ = 23.1°, (020) at 2θ = 23.5° and (200) at 2θ = 24.1°. The estimate of average crystallite sizes (D, nm) in the materials was calculated by the Scherrer formula (1):³²where λ is the wavelength of the X-ray radiation (nm), B is the full width at half maximum (radian), and θ is Bragg's angle (degree). The mean WO₃ crystallite sizes estimated from (002) peaks of the WO₃ and WO₃/RGO films are 21 nm and 17 nm respectively, in agreement with the following SEM results. Meanwhile, the result shows that the incorporation of RGO sheets has a certain restraining effect on grain growth of WO₃. The diffraction peak of RGO was not observed in the pattern of the WO₃/RGO film probably due to too low a content, but the existence of RGO was confirmed by Raman spectra.

Fig. 1 XRD spectra of ITO glass, WO₃ and WO₃/RGO films.

The Raman spectra of GO powder and WO₃ and WO₃/RGO films are shown in Fig. 2. Raman analysis on WO₃ film shows two peaks centered at 713 cm⁻¹ and 807 cm⁻¹, characteristic features of monoclinic WO₃, that can be ascribed to O–W–O stretching vibrations of the bridging oxygen of WO₆ octahedra.³³ The Raman spectrum of GO powder shows D-band (1331 cm⁻¹) and G-band (1589 cm⁻¹) peaks. The G-band represents the in-plane bond-stretching motion of the pairs of C sp² atoms (the E_2g phonons), whereas the D-band corresponds to the breathing modes of rings or κ-point phonons of A_1g symmetry.³⁴ In addition, the observation of D-band (1326 cm⁻¹) and G-band (1590 cm⁻¹) peaks confirms the existence of RGO in WO₃/RGO nanocomposite film. As for GO and RGO of the WO₃/RGO composite film, the D/G ratios were calculated as 1.23 and 1.38, respectively. The D/G intensity ratio of RGO is larger than that of GO, indicating a decrease in the average size of the sp² domains and formation of a high quantity of defects upon heat reduction of GO, which is in agreement with the literature.^35,36 Compared with that of the WO₃ film, the band at 713 cm⁻¹ was broadened and shifted to 701 cm⁻¹ in the spectrum of the WO₃/RGO composite film, probably because the formation of C–O–W bonds makes the initial W=O bond weaker and a similar phenomenon has been reported.^37,38 It means that WO₃ was grafted onto the surface of RGO sheets via C–O–W bonds rather than physically adsorbed on the surfaces of RGO sheets. This kind of structure is desirable for electron transfer in the WO₃/RGO nanocomposite film.³⁹

Fig. 2 Raman spectra of GO powder, and WO₃ and WO₃/RGO films.

XPS was performed to determine the surface element composition of samples and the valence states of various species. From the full spectrum of WO₃/RGO nanocomposite film in Fig. 3(a), it can be seen that the sample consists of W, O, and C elements without impurities. Fig. 3(b) depicts the W 4f XPS spectra of the WO₃ and WO₃/RGO films. The main peaks of W 4f_7/2 and W 4f_5/2 are located at 35.5 eV and 37.5 eV for the WO₃ film. While the main peaks of W 4f_7/2 and W 4f_5/2 are slightly shifted to 35.6 eV and 37.7 eV for the WO₃/RGO nanocomposite film, associated with interaction between WO₃ and RGO. Furthermore, these are consistent with the typical binding energies of W⁶⁺ oxidation states in stoichiometric WO₃ materials.⁴⁰ Fig. 3(c) shows the C 1s XPS spectrum of the WO₃/RGO nanocomposite film. Peaks at 284.6, 286.5, 287.6 and 288.8 eV are observed, which can be assigned to the C–C, C–O, C=O and O–C=O species, respectively.⁴¹ Based on other research reports about GO and RGO,^34,41,42 it is concluded that most oxygen functional groups are successfully removed during thermal treatment. Moreover, the peaks at about 280.6 eV indicated the formation of C–W chemical bond.

Fig. 3 (a) XPS full spectrum of WO₃/RGO nanocomposite film, (b) W 4f XPS spectra of the WO₃ and WO₃/RGO films, and (c) C 1s XPS spectrum of WO₃/RGO nanocomposite film.

3.2 Morphological analysis

The morphology of the WO₃ and WO₃/RGO films were characterized using SEM. Fig. 4 shows the SEM images with low and high magnifications of WO₃ and WO₃/RGO films. The WO₃ film in Fig. 4(a) demonstrates a smooth and compact surface with a few cracks which are probably attributed to the release of internal stresses during the solvent evaporation.⁴³ Fig. 4(b) shows that the WO₃ film consists of uniform spherical nanoparticles with an average size of around 20 nm. However, the WO₃/RGO nanocomposite film displays a coarse and loose surface with more microcracks (Fig. 4(c)). From the high magnification image of the WO₃/RGO nanocomposite film (Fig. 4(d)), it can be observed that the surfaces and edges of the RGO sheets are covered by WO₃ nanoparticles with smaller size (around 15 nm). Therefore, the WO₃/RGO nanocomposite film has more space to facilitate more ions of the electrolyte to penetrate through the film with shorter diffusion paths.

Fig. 4 SEM images of the WO₃ film (a and b) and WO₃/RGO nanocomposite film (c and d).

The morphology and structural characteristics of the WO₃/RGO nanocomposite were also analyzed by HRTEM. The low magnification image shown in Fig. 5(a) demonstrates that there are good interfacial contacts formed between WO₃ nanoparticles and wrinkled RGO sheets. The lattice distance of WO₃ nanoparticles measured in the HRTEM image (Fig. 5(b)) equals 0.387 nm, which can be indexed to the (002) crystal plane of monoclinic WO₃, and it can be concluded that the WO₃ nanoparticles are attached to the RGO sheets from the HRTEM image.

Fig. 5 (a) TEM and (b) HRTEM images of WO₃/RGO nanocomposite film.

3.3 Optical properties and bandgap calculations

Fig. 6 compares the absorption spectra and optical bandgap (E_g) of WO₃ and WO₃/RGO films prepared by the spin-coating technique. The optical absorption edge of the WO₃/RGO nanocomposite film shows a red shift compared with the WO₃ film owing to the incorporation of RGO sheets. The E_g of a semiconductor material may be estimated from absorption spectra using the following eqn (2):^44,45

αhν = A(hν − E_g)ⁿ (2)

where α, hν, A and E_g are the absorption coefficient, the photon energy, a constant of proportionality, and optical bandgap energy. The exponent n depends on the type of optical transition of a semiconductor material (n = 1/2 for allowed direct transition and n = 2 for allowed indirect transition). WO₃ has an indirect band transition between the 2p electrons from the valence band of the oxygen and the 5d conduction band of tungsten.⁴⁶ WO₃ is an indirect bandgap semiconductor,⁴⁷ and the optical bandgap energy of WO₃ varies from about 2.6 to 3.0 eV,⁴⁸ so the value of n is taken as 2 for WO₃. Thus, the E_g of the WO₃ and WO₃/RGO films could be estimated from Tauc plots of (αhν)^1/2 versus (hν), and the values of E_g were calculated by extrapolating the linear part to zero absorption coefficient α = 0. The E_g for the WO₃ film is calculated as about 3.01 eV, greater than that of bulk WO₃ (2.62 eV) due to quantum size effects of the small crystallite size.⁴⁹ And E_g of WO₃/RGO nanocomposite film is decreased slightly to 2.87 eV. The incorporation of RGO could decrease the bond length of W=O or O–W–O, which is possibly ascribed to the interaction between WO₃ and GO, such as C–O–W and C–W bonds, and as a result the E_g decreases.

Fig. 6 UV-visible optical absorption spectra of WO₃ and WO₃/RGO films. Inset: Tauc plots of (αhν)^1/2 versus (hν).

3.4 Electrochemical and electrochromic performance

The assessment of the coloration–bleaching kinetics is of great importance to determine the practical applications of electrochromic films. Therefore, the switching characteristics of the WO₃ and WO₃/RGO films were measured by CA and the corresponding in situ optical transmittance at an optical wavelength of 633 nm, as shown in Fig. 7(a) and (b). The switching times for the WO₃ film are calculated to be 14.3 s for coloration and 10.8 s for bleaching. As expected, for the WO₃/RGO nanocomposite film, the switching times were calculated to be 9.5 s for coloration and 7.6 s for bleaching. Coloration efficiency (CE) is a crucial characteristic parameter for evaluating the electrochromic performance of materials, which is defined as the change in optical density (ΔOD) per unit of charge density (Q) inserted into (or extracted from) the electrochromic films.⁵⁰ It can be calculated according to the following formulas (3) and (4):⁵¹where T_b and T_c denote the transmittances of the film in bleached and colored states, respectively. Fig. 7(c) shows the plots of ΔOD at a wavelength of 633 nm versus the inserted charge density at an applied potential of −1.5 V. The CE value is calculated to be 46.7 and 75.3 cm² C⁻¹ at 633 nm for the WO₃ and WO₃/RGO films, respectively. Combining the results of coloration–bleaching switching times and coloration efficiency of the WO₃ and WO₃/RGO films, it can be concluded that the WO₃/RGO nanocomposite film has shorter coloration–bleaching times and greater coloration efficiency than the WO₃ film, and can be explained from two facets. Firstly, there are plentiful microcracks and holes in the WO₃/RGO nanocomposite film, facilitating intercalation and de-intercalation of Li⁺ ions at the interface between the WO₃/RGO film and electrolyte. Secondly, it is known that RGO sheets have a high electrical conductivity and specific surface area.^15,16 Massive WO₃ nanoparticles are grafted onto the surface of RGO sheets via chemical bonds, such as C–O–W and C–W bonds. The chemical bonds between WO₃ and RGO sheets can effectively reduce the interface defects and contact resistance, and therefore the RGO sheets promote the electron transfer rate in the WO₃/RGO nanocomposite film by providing low-resistance conduction pathways.³⁷

Fig. 7 (a) Chronoamperometry measurements and (b) corresponding in situ optical transmittance curves for WO₃ and WO₃/RGO films at a fixed wavelength of 633 nm under potentials of −1.5 V (30 s) and 1.5 V (30 s), and (c) variation of the in situ optical density (ΔOD) versus charge density (Q) for WO₃ and WO₃/RGO films at a fixed wavelength of 633 nm under a coloration potential of −1.5 V.

Fig. 8 shows the in situ optical transmittance spectra of both the WO₃ and WO₃/RGO films measured at 1.5 V (bleached state, 30 s) and −1.5 V (colored state, 30 s). As can be seen, both films display low transmittance at −1.5 V and high transmittance at 1.5 V. It is observed that the optical modulation of the WO₃/RGO nanocomposite film is around 59.6% at a wavelength of 633 nm, slightly higher than that of the WO₃ film (53.8%). This proves that the incorporation of RGO has increased the optical modulatory range, in agreement with the literature.¹⁷ Photographs of the WO₃ and WO₃/RGO films in the colored and bleached states are shown in the insets in Fig. 8. It can be seen that the color changes are reversible and homogeneous with varying potential. Briefly, when the applied potential was −1.5 V, the color of the WO₃ and WO₃/RGO films was dark blue (colored states). As the potential increased from −1.5 to 1.5 V, the color of the WO₃ and WO₃/RGO films gradually turned powder blue (bleached states).

Fig. 8 In situ optical transmittance spectra of both WO₃ and WO₃/RGO films measured at 1.5 V (bleached state, 30 s) and −1.5 V (colored state, 30 s). The inset shows photographs of the colored and the bleached states for the WO₃ and WO₃/RGO films.

Fig. 9(a) and (b) shows the cyclic voltammograms of the WO₃ and WO₃/RGO films. During Li⁺ ion intercalation/de-intercalation, the maximum cathodic and anodic peak current densities for WO₃ and WO₃/RGO films are −2.18/1.65 mA cm⁻² and −1.27/0.98 mA cm⁻², respectively. The maximum cathodic and anodic peak currents for the WO₃/RGO nanocomposite film at the 150th cycle are −2.16/1.36 mA cm⁻². But, for the WO₃ film, the maximum cathodic and anodic peak current densities at the 150th cycle drastically decreased to −0.78/0.59 mA cm⁻². The WO₃/RGO nanocomposite film demonstrates much higher current density than the WO₃ film, meaning that more ions and electrons are involved at the interface between the WO₃/RGO nanocomposite film and the electrolyte. The result indicates that the incorporation of RGO can enhance the cyclic stability and the intercalation properties of Li⁺ ions.

Fig. 9 Cyclic voltammograms of WO₃ film (a) and WO₃/RGO nanocomposite film (b).

4. Conclusions

In summary, WO₃/RGO nanocomposite film has been fabricated by a facile and industrially scalable sol–gel spin-coating approach. In the WO₃/RGO nanocomposite film, the incorporation of RGO brings about more microcracks and holes, and massive WO₃ nanoparticles grafted onto the surface of RGO sheets via chemical bonds, facilitating Li⁺ ion diffusion and accelerating the electron transfer rate. As a result, the synergistic effect between the WO₃ nanoparticles and the RGO sheets delivers shorter coloration–bleaching times, higher coloration efficiency and better cyclic stability in comparison with the WO₃ film. The results suggest that the sol–gel spin-coating approach is an effective way to prepare WO₃/RGO nanocomposite electrochromic film for electrochromic devices and energy-saving smart windows.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (no. 51473104 and 61271075).

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