Structural, optical, and electrochromic properties of rare earth material (CeO2)/transitional metal oxide (WO3) thin film composite structure for electrochromic applications

In the present work, cerium oxide nanorods were grown on fluorine-doped tin oxide substrates by the hydrothermal process with growth fluid concentrations from 0.06 to 0.09 M and maintaining the urea content constant at 0.5 M. Optimized tungsten oxide thin films were deposited on these hydrothermally grown cerium oxide nanorods by using DC sputtering process. The developed tungsten oxide-cerium oxide nanostructured hybrid films were characterized for their structural, morphological, optical, and electrochromic (EC) properties, by using various analytical techniques. It was observed that with the increase of growth fluid concentration, the cerium oxide nanorods (CeO2 NRs) become thinner and longer and decrement in transmittance. The highest diffusion coefficient (8.07 ×10−14 cm2/s) in the hybrid films formed with 0.08 M, and the highest coloration efficiency (13.88 cm2/C) in 0.06 M growth fluid concentrations was observed. The influence of CeO2 NRs on WO3 electrochemical performance observed in this study definitely helps in the selection of proper doping components and concentrations for power-saving optoelectronic devices.


Introduction
There has been an increase in research on the electrochromic (EC) phenomena, which has the potential to use less energy, as people's knowledge of global warming has grown [1][2][3][4][5]. ECDs (electrochromic devices) are founded on permanent and reverse optical characteristics that change whenever a voltage is applied across them and are used in green technology applications (i.e., energy-saving sunscreens) [6][7][8][9][10][11]. Smart windows are essential in buildings and cars since people spend more than 80% of their everyday lives in them, which is critical [12,13]. During peak or summer hours, these devices' energy usage, indoor temperature, and optical transmittance were reduced because they absorb more visible wavelengths when operating in colored mode. On the other hand, these devices improve optical transmittance during the bleaching process by reducing the amount of absorption. Electrochromic metal oxides observed either anodic coloration by oxidation or cathodic coloring by reduction. Few materials can alternate between two optical states in response to an applied voltage: bleaching and coloring. The most well-known inorganic EC materials given a lot of attention from reviewers are transition metal oxides, which include cerium, manganese, tungsten, nickel, cobalt, palladium, iridium, titanium, rhodium, molybdenum, and ruthenium oxides [14][15][16][17]. A more significant inorganic EC material commonly used in sunscreens was tungsten oxide (WO 3 ) [18][19][20][21][22][23][24]. As shown in Eq. 1, during the electrochromic process [25,26], WO 3 films in a bleached state (clear W 5+ in WO 3 ) can be regenerated to a colored state (dark blue for W 6+ in WO 3 ): The insertion and exertion of ions from the electrolyte layer next to the EC layer that penetrates the structure of the host EC material network are what cause the variations in optical behavior. The structural and optical properties of the oxide host material are potentially susceptible to change when a small amount of dopant-related oxide is present. Transition metals like Ru, Mo, Ti, Fe, Nb, V, and Ni were used as doping elements for the WO 3 [27][28][29][30][31][32][33][34][35].
Rare earth elements have recently shown many of promise as dopants for improving their ability in photocatalytic activity, photoluminescence, photoelectrochemical, microwave absorption [36][37][38][39][40][41][42][43][44][45], and rare earth catalytic application [46,47]. However, very few works only reported on the EC characteristics of WO 3 using rare earth metal oxide nanostructures. Among them, the inclusion of CeVO 4 in thin films with a channel structure improved the charge insertion rate, and this effect is amplified when thin films are deposited at high RF power [48]. Due to their high specific capacitance and low charge transfer resistance, low Gd-doped WO 3 films have superior electrochromic properties that lead to quick reaction kinetics [49].
Cerium dioxide (CeO 2 ) is another promising rare earth metal oxide used for counter electrodes with ECD because of its optical passivity in both the charged and discharged states [50]. CeO 2 as counter electrodes for ECD has the disadvantage of delayed reaction kinetics and poor charge capacity [51]. In ECD, various mixed oxides, including Ce-TiO 2 [52,53], CeO 2 /WO 3 [54], and CeO 2 -TiO 2 -ZrO 2 [55], have also been employed as counter electrodes to diminish the shortcomings of CeO 2 and enhance their characteristic.
In this work, CeO 2 NRs were grown, using a hydrothermal method, on FTO substrates. CeO 2 NRs grew in diverse concentrations and morphologies, and most of the grown CeO 2 NRs are in crystalline form. The WO 3 thin films were deposited at oxygen partial pressures (ppO 2 ) of 8×10 −4 mbar on CeO 2 NR films grown on FTO. XRD, SEM, UV-vis spectroscopy, and an electrochemical analyzer were used to analyze structural, morphological, optical, and electrochemical studies. (1)

Materials
To carry out the present work, the following materials were used and purchased from Sigma-Aldrich

Preparation of CeO 2 nanostructured films
FTO-coated glasses were used as a substrate for growing CeO 2 NRs. The substrates were cleaned by a proper procedure using an ultrasonicator, soap solution, deionized water, and acetone, and also, nitrogen was used to dry the substrates. A total of 0.5 M of urea and 0.06 M of Ce(NO 3 ) 3 6H 2 O were prepared using deionized water in a separate beaker. The solutions were mixed by stirring for about 20 min. Equation 2 represents the formation of CeO 2 from cerium nitrate hexahydrate and urea solutions: As indicated in Fig. 1, the mixture was then put into a 150-mL Teflon container inside an autoclave. The FTO-coated substrate is held up against (side facing) the container wall. To create a strong binding between the nanorods and the substrate, for 12 h at 100 °C, the autoclave and container are held in a muffle furnace. The substrates were cleaned in DI water and dried with nitrogen gas. The grown samples were annealed for 3 h. The details of parameters applied for growing of CeO 2 nanorods and preparation of hybrid films are shown in Table 1.

Deposition of WO 3 thin films
The deposition of WO 3 thin films on glass substrates and CeO 2 NRs was carried out by using DC magnetron sputtering. During the sputtering process, a 3-in diameter tungsten target was (2) 6CeO NO 3 3 Fig. 1 Autoclave of hydrothermal processes utilized in an environment consisting of oxygen and argon. At room temperature, the substrate was placed 90 mm away from the tungsten target during the deposition in the vacuum chamber, and the vacuum was maintained at 1.0 ×10 −6 mbar. A 1000 LPM rotary blade pump and a 1000 LPS TM pump were used to create the vacuum in the chamber. Even before each deposition, the target is pre-sputtered in an argon environment for 15 min to ensure purity. The tungsten oxide layers were deposited at partial oxygen pressure (ppO 2 ) of 8×10 −4 mbar for 17 min, and the thickness of the formed films was about 425 nm.

Characterization
Structural studies of prepared films were verified by using X-ray diffraction (CuKα radiation, λ = 0.1542 nm) and FTIR spectrometer in the range of 400-4000 cm −1 . SEM (Nano-Instruments, TESCAN-VEGA3-LMU) was used to analyze the surface morphology of films. A SPECORD ultravioletvisible spectrometer with FTO glass as a baseline was used to evaluate the discrete amount of UV and visible wavelengths absorbed. Electrochemical analysis was performed on an SP 300 electrochemical workstation. A three-electrode setup was used in an electrochemical cell to analyze the electrochromism of pure WO 3 and WO 3 /CeO 2 nanostructured hybrid films. In the cell structure, the reference electrode was made of Hg/HgCl 2 , a platinum wire used as a counter electrode, and WO 3 and WO 3 /CeO 2 hybrid films as working electrodes. For all CV tests, sweep voltage was kept between −0.7 and +1 V in an aqueous solution of 0.5 M H 2 SO 4 electrolyte with a scanning rate of 10 mV/s. The same potential range (−0.7 to +1.0 V) is used for all-optical transmittance measurements to compare the bleached and colored states.

XRD analysis
The XRD patterns of CeO 2 NRs synthesized by the hydrothermal process are shown in Fig. 2. The thermally converted NRs had a lattice constant of 4.235 nm with patterns resembling face-centered cubic CeO 2 [56]. The heat treatment patterns of the as-synthesized NRs were comparable to the reference. Peaks were observed at 2θ =26.68°, 33.90°, 37.96°, 51.64°, 54.81°, 61.59°, and 65.53°, in reflections from the (111), (200), (220), (422), (311), (222), and (400) planes, which match to the face-centered cubic phase of CeO 2 (ICDD card no. 75-0076) [57]. The calculated values of FWHM, crystal size, dislocation density, and strain ε of the grown films are listed in Table 2. The dislocation density (δ) attributes to better crystallization of the film, and it was determined by the length of the dislocation line per unit volume of the crystal. The low dislocation density (δ) 0.00018 nm −2 for 0.09 M CeO 2 /WO 3 compared to the other films revealed the high crystallinity [58,59]. Any film process annealing CeO 2 nanorods causes an alteration in the film crystalline phase, which is investigated with XRD. Figure 3 shows the X-ray diffraction spectra of sputtered pure WO 3 film and WO 3 /CeO 2 hybrid films. The XRD pattern of pure WO 3 film sputtered at ambient temperature and did not show any usual diffraction peaks, indicating that the films are amorphous. The XRD spectra of CeO 2 /WO 3 hybrid films reveal that the structure is crystalline. The reflections from the crystalline planes (111), (200), (220), (422), (311), (222), and (400) of CeO 2 nanorods coated with WO 3 films are displayed at 2θ = 26.68°, 33.90°, 37.96°, 51.64°, 54.81°, 61.59°, and 65.53° (ICDD card no. 75-0076). In addition to being necessary for electrochromic devices, this crystalline CeO 2 /WO 3 nanocomposite also allows for recombining photogenerated electrons and holes [9].

SEM analysis
The SEM images (Fig. 4) reveal that CeO 2 NRs have been synthesized effectively on FTO substrates. None of the CeO 2 NRs, grown at various concentrations of growth fluid, had identical NRs generated flower-like complex forms of bunches of many NRs with nanoscale widths. Due to a high concentration of cerium nitrate hexahydrate in the highly saturated precursor environment, and faster nucleation speed than the crystal growth rate, this results in the production of many microscopic particles [57]. As a result of this process, NRs get longer, narrower, and denser. Although longer nanorods are produced in saturated environments, the loss in NR diameter exceeded the gain in NR length. The SEM images of as-sputtered WO 3 films and WO 3 /CeO 2 nanostructured hybrid films are shown in Fig. 5. The as-sputtered pure WO 3 film's SEM image indicates a nonporous morphological structure (Fig. 5a). The CeO 2  concentration impacts how the film's surface is formed. As the concentration of CeO 2 was raised, surface roughness increased. The grown NRs connected to CeO 2 concentration may be clearly seen on the surfaces of the concentration (0.06 to 0.09 M) of WO 3 /CeO 2 NR film ( Fig. 5b-d).

EDX analysis
The EDS spectra of CeO 2 NRs within an energy range of 0 to 15.3 KeV are shown in Fig. 6. There are peaks for the Ce and O atoms, but no other identifying peaks are evident [60]. This means that no other contaminants were developed in the grown NRs and represent the superiority of CeO 2 nanorods prepared by the hydrothermal method. As seen in Table 3, when the concentrations of CeO 2 are raised, the atomic percentage of cerium increases and oxygen decreases. The results showed that the cerium and oxygen stoichiometric ratio (1:2) was not a perfect match. The EDX spectrum of pure WO 3 film, 0.07 M WO 3 / CeO 2 , and 0.09 M WO 3 /CeO 2 hybrid films is shown in Fig. 7, and corresponding atomic concentrations are shown in Table 4.

FTIR spectroscopy
The chemical interaction of oxygen and cerium ions in CeO 2 and tungsten, cerium, and oxygen ions in WO 3 /CeO 2 hybrid films was analyzed by using the FTIR spectrum at 4000-400 cm −1 . The leading molecular vibrational bands were detected in the spectra at 688 cm −1 , 1334 cm −1 , and 1508 cm −1 (refer to Fig. 8a). The presence of O-H, O=C=O, on the surface, is responsible for observing an absorption band at 2367 cm −1 [61]. The vibrational band at 688 cm −1 is produced by stretching vibration (Ce-O). The primary molecular vibrational bands at 672 cm −1 and 1527 cm −1 in the FTIR spectra of the hybrid film are observed in

Optical properties analysis
The transmittance spectra of WO 3 /CeO 2 hybrid films are shown in Fig. 9. The transmittances of the CeO 2 /WO 3 films varied with respect to the concentration of cerium oxide raised, as reported in Table 5. Figure 9 shows the way the proportion of transmittance varied from 0.06 to 0.09 M CeO 2 /WO 3 films. The optical haze is caused by the increasing surface irregularity as the molar highest concentration gradually increases [9]. The increased surface irregularity of the WO 3 /CeO 2 NR films was thought to be the cause of the increased scattered light drops.

Electrochromic properties
CV technique was used to analyze chemical processes in thin nanohybrid film samples by measuring current while being affected by a sweep potential. Figure 10 shows the CV graphs of pure WO 3 , pure CeO 2 , and WO 3 /CeO 2 hybrid films at various growth fluid concentrations in 0.5 M H 2 SO 4 electrolyte, between 1 and −0.7 V at a scan rate of 10 mV/s, when a voltage of −1 V is applied.

µm
The outcome was a dark blue layer that darkened over time. Changing the polarity of the applied voltage, on the other hand, turned the WO 3 /CeO 2 films colorless. H + ion insertion and extraction into WO 3 /CeO 2 NR films are illustrated by the enormous featureless peaks in the CV measurements (Fig. 10). As illustrated by the Randles-Sevcik equation [55], a cyclic voltammogram (CV) has been used to study a number of variables, including the reduction and oxidation peak currents, coloration efficiency, and diffusion coefficients. The Randles-Sevcik equation is shown in Eq.  The cathodic peak current is used to estimate the pace at which H + ions were injected, with a small amount of current indicating a faster rate of insertion. The cathodic peak currents and diffusion coefficients of pure tungsten oxide film and WO 3 /CeO 2 hybrid films are shown in Table 6. The highest value for cathodic peak current and diffusion coefficient was reported for 0.08 M WO 3 / CeO 2 hybrid film. Hsu et al. claimed that the compressed film structure stopped H + ions from intercalating in the sol-gel-generated MoO 3 electrochromic films, by lowering the diffusion coefficient [61]. For all hybrid films, the diffusion coefficient is more when compared with pure WO 3 film; this reveals that electrons and H + ions diffuse more easily, which is the needed quality for EC films. The charge storage capacity of CeO 2 /WO 3 films can be determined by calculating the area that lies beneath the respective CV curves. Consequently, larger areas are seen to be better suited for EC use. The CV plot (Fig. 10) reveals the highest area for 0.08 M hybrid film. The results support that the key electrochromic features of thin films created are enhanced.
Optical modulation (ΔOD) is the most important feature of EC films for device applications; it is given by the difference in transmittance of bleached state (T b ) and colored states (T c ). EC devices should have high transparency in the bleached state, while in the colored state, they should have a strong opaque. Figure 11 shows the optical transmission spectra of WO 3 /CeO 2  nanostructured hybrid films and pure WO 3 film in bleached and colored states and in the wavelength range of 320-900 nm.
The optical modulation of prepared pure WO 3 film and hybrid films at wavelength 700 nm was tabulated in Table 7, and wavelengths 500 nm, 550 nm, 600 nm, and 650 nm are shown in Fig. 12a. The highest value for optical modulation was reported in for pure WO 3 film (42), and in the case of hybrid films, the highest value of 0.06 M WO 3 /CeO 2 was reported for the nanostructured film (29); furthermore, with an increase of growth fluid concentration, the optical modulation decreases (Table 7). Coloration efficiency (CE) is an important parameter to evaluate the performance of EC materials; it is defined as the ratio of changes in optical density to interpenetrating charges per unit area which is known as coloration efficiency (CE).
The equation [62] used to determine it is where Q in denotes the injected charge density; an electrochromic material with a high CE value has a large optical modulation and lower intercalated (or extracted) charge density. The CE values for developed films at wavelength 700 nm are shown in Table 7 and at wavelengths 500 nm, 550 nm, 600 nm, and 650 nm are shown in Fig. 12b. The CE of hybrid films was higher in all the cases except for 0.09 M WO 3 /CeO 2 hybrid film when compared with pure WO 3 film at wavelength 700 nm, and the highest value (13.88) was   Figure 13 shows experimental images of as-deposited, colored, and bleached states of 0.06 M CeO 2 /WO 3 films.

Conclusion
The present research work aimed to develop efficient electrochromic tungsten oxide-rare-earth metal oxide hybrid films. In this context, cerium oxide nanorods were grown on FTO-doped glass substrates at different growth fluid concentrations varying from 0.06 to 0.09 M by using a hydrothermal process and characterized by adopting various analytical techniques. The XRD studies revealed that the structure of CeO 2 NR was fcc crystalline structure, and SEM results showed the effective synthetization of NRs on FTO substrates, and with the increase of growth fluid concentration, the NRs became thinner and longer. The hybrid  films were formed by sputtering tungsten oxide on hydrothermally grown CeO 2 NRs at PaO 2 of 8×10 −4 mbar, and these films were analyzed by using characterization techniques XRD, SEM, EDX, FTIR, UV-visible spectrometry, and CV. XRD analysis showed an amorphous structure for pure WO 3 film and crystalline for WO 3 / CeO 2 nanostructured hybrid films. The SEM images showed that the sputtered pure WO 3 film has smooth and nonporous morphology, hybrid films have rough surfaces, and surface roughness increases with the increase of growth fluid concentration. The EDX results revealed that the stoichiometric ratio of Ce-W-O was not followed completely. The CV studies of pure WO 3 films and WO 3 /CeO 2 hybrid films showed the highest optical modulation (43) for pure WO 3 and the next highest value (29) for 0.06 M WO 3 /CeO 2 hybrid film; after that, with the increase of CeO 2 NR growth fluid concentration, there was a fall in optical modulation observed. Cathodic peak current is one of the important parameters in electrochromism; a higher cathodic peak current means more probability for intercalation/deintercalation of electrons and H + ions. This value reported in hybrid films for all concentrations of CeO 2 NR growth fluid was higher than the pure WO 3 films, and the highest value (−8.52 mA/cm 2 ) was observed for 0.08 M WO 3 /CeO 2 hybrid film. Coloration efficiency was the important parameter to analyze the performance of EC devices, and in the present study, the highest value (13.88 cm 2 /C) was observed for 0.06 M WO 3 /CeO 2 hybrid film. The above results support the enhancement of EC properties in the case of WO 3 /CeO 2 hybrid films when compared with pure WO 3 films. The data provided in this article is definitely useful and serves as a pathway for researchers working in the area of developing EC devices.