Titanium Dioxide Thin Films Produced on FTO Substrate Using the Sol–Gel Process: The Effect of the Dispersant on Optical, Surface and Electrochemical Features

In this study, TiO2 thin films formed by dip-coating on an FTO substrate were obtained and characterized using surface, optical and electrochemical techniques. The impact of the dispersant (polyethylene glycol-PEG) on the surface (morphology, wettability, surface energy), optical (band gap and Urbach energy) and electrochemical (charge-transfer resistance, flat band potential) properties were investigated. When PEG was added to the sol–gel solution, the optical gap energy of the resultant films was reduced from 3.25 to 3.12 eV, and the Urbach energy increased from 646 to 709 meV. The dispersant addition in the sol–gel process influences surface features, as evidenced by lower contact-angle values and higher surface energy achieved for a compact film with a homogenous nanoparticle structure and larger crystallinity size. Electrochemical measurements (cycle voltammetry, electrochemical impedance spectroscopy and the Mott–Schottky technique) revealed improved catalytic properties of the TiO2 film, due to a higher insertion/extraction rate of protons into the TiO2 nanostructure, as well as a decrease in charge-transfer resistance from 418 k to 23.4 k and a decrease in flat band potential from 0.055 eV to −0.019 eV. The obtained TiO2 films are a promising alternative for technological applications, due to their advantageous surface, optical and electrochemical features.


Introduction
Titanium dioxide (TiO 2 ) thin films with high specific surface area and narrow poresize distribution have piqued the interest of industry professionals due to their excellent optical, electrical, and photoelectrochemical properties [1]. TiO 2 is a nontoxic material with wide band gap in its three crystalline forms: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic) [2]. Among these, the anatase phase of TiO 2 has sparked interest among scientists in a variety of fields, due to its potential use in catalysis and photo-catalysis [3,4] for air and water clearing and also in water splitting [5] and in organic pollutant degradation [6], energy storage and solar energy conversion [7,8], electrochromic and self-cleaning smart windows [9,10] and in biomedical applications [11,12]. The synthesis method, experimental settings, sol composition [13] and polymorph structure are just a few of the variables that affect the properties of TiO 2 nanoparticles [14,15]. Chemical vapor deposition (CVD) [16], radio frequency (RF) magnetron sputtering (SPS) [17], nebulized spray deposition [9] and the sol-gel procedure [18] are only some of the more common methods currently used to create anatase TiO 2 thin films. The sol-gel approach is the most appealing of the mentioned methods, due to the simplicity of the required equipment, low processing temperature, good homogeneity, and the ability to achieve multilayer deposition [19] on different substrates, including metals [20], indium-doped tin oxide (ITO) [21] or fluorine-doped tin oxide (FTO) glass [9], depending on the application area.
Thus far, the impact of different conditions on the structural and optical features of TiO 2 thin films has been the subject of numerous investigations. As an example, F. Zeribi that were applied to a glass substrate using the sol-gel spin coating process [18]. N. Barati et al. demonstrated that the withdrawal speed used for the dip-coating process influences the coating thickness, and a drying method for coatings is required to achieve a uniform and nanocrystalline TiO 2 coating on 316L stainless steel [20]. Other studies investigated the effect of the additive and annealing temperatures on the surface morphology of spin-coated TiO 2 thin films [22][23][24]. To change the morphology and crystallinity, some complex agents (such as acetylacetone, diethanolamine, polyethylene glycol, Lauryl lactyl lactate) [25,26] or surfactants (such as cationic-sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide (CTAB), and non-ionic (TritonX-100)) [27] were added. As a result, particle agglomeration was reduced and TiO 2 coating stability and photocatalytic efficiency were improved. Moreover, when two or more distinct surfactants were introduced to the mixture simultaneously, the morphology of TiO 2 changed substantially, adopting spherical and three-dimensional forms because of surfactant coordination [27].
Efforts are still being made to obtain adherent and continuous TiO 2 films on FTO or ITO glass electrodes with improved optical properties in the visible range, as well as improved electrochemical and electrochromic properties, with the goal of using these electrodes in cutting-edge applications, such as smart windows, which are of great interest today, given global warming and the energy crisis.
This study determines the optimal number of dip-coating layers and the effect of the dispersant (PEG) added to the sol-gel precursor solution on the optical (as reduced band gap and increased Urbach energies), electrochemical (as high insertion/extraction rate of protons, decrease in charge-transfer resistance, and flat band potential) and surface (nanomorphology, crystallinity, decreased wettability and high surface energy) properties of titanium dioxide thin films obtained on FTO.
The results of this research will help to develop a practical procedure to create pseudocapacitive electrodes for technological applications by correlating the surface morphology, vibrational, and optical properties of synthesized TiO 2 thin films on FTO substrates with electrochemical properties.

Preparation of TiO 2 Thin Films on FTO Substrate by Dip-Coating Method
The sol-gel precursor solution (Solution 1) was created by mixing together 8 mL Ti(OBu) 4 , (as alkoxide), 80 mL ethanol (as solvent), 8 mL acetylacetone (as catalyst), 8 mL CH 3 COOH (as pH adjuster, to 6) and 8 mL of water [18,28]. A clear solution was obtained after 3 h of stirring the mixture at 50 • C, which was further aged for 24 h at room temperature. The FTO substrates were cleaned with ethanol, acetone, and distilled water for ten minutes each, then dried in air.
The precursor solution was set up by dip-coating onto the surfaces after they had been cleaned using a KSV NIMA Dip-Coater (Biolin Scientific, Västra Frölunda, Sweden) using the following parameters: immersion speed 3 cm/min, immersion time 30 s and lifting speed 10 cm/min. The resulting film was dried in an oven at 200 • C for 10 min after each deposition cycle for the organic solvent evaporation. To obtain films with different thicknesses, the deposition procedure was repeated 2, 4, 6 and 8 times. The names of the samples obtained by this method are listed as "P number layer ". Then, the obtained films were investigated to determine the ideal number of layers needed to generate a TiO 2 film with a lower band gap energy and higher Urbach energy.
After determining the optimal number of TiO 2 layers, we investigated how the presence of a dispersant in the precursor solution influences the nanomorphology of the material, electrochemical and optical properties of the resulting TiO 2 film. In order to accomplish this, a second solution identified as Solution 2 was made by continuously stirring Solution 1 for a period of two hours, until a specific amount of PEG was dissolved [29]. PEG promotes nanopore formation and reduces TiO 2 nanoparticle aggregation [30]. The names of the samples that were obtained using this method are identified as follows: "P number layer_PEG ".
All films obtained from both solutions were annealed in an electric furnace for two hours at 450 • C.
The stages involved in TiO 2 thin film production are depicted in Figure 1.
The precursor solution was set up by dip-coating onto the surfaces after they had been cleaned using a KSV NIMA Dip-Coater (Biolin Scientific, Västra Frölunda, Sweden) using the following parameters: immersion speed 3 cm/min, immersion time 30 s and lifting speed 10 cm/min. The resulting film was dried in an oven at 200 °C for 10 min after each deposition cycle for the organic solvent evaporation. To obtain films with different thicknesses, the deposition procedure was repeated 2, 4, 6 and 8 times. The names of the samples obtained by this method are listed as "Pnumber layer". Then, the obtained films were investigated to determine the ideal number of layers needed to generate a TiO2 film with a lower band gap energy and higher Urbach energy.
After determining the optimal number of TiO2 layers, we investigated how the presence of a dispersant in the precursor solution influences the nanomorphology of the material, electrochemical and optical properties of the resulting TiO2 film. In order to accomplish this, a second solution identified as Solution 2 was made by continuously stirring Solution 1 for a period of two hours, until a specific amount of PEG was dissolved. [29]. PEG promotes nanopore formation and reduces TiO2 nanoparticle aggregation [30]. The names of the samples that were obtained using this method are identified as follows: "Pnumber layer_PEG".
All films obtained from both solutions were annealed in an electric furnace for two hours at 450 °C.
The stages involved in TiO2 thin film production are depicted in Figure 1.

Figure 1.
A simplified schematic of the procedure followed to obtain the TiO2 thin films on FTO substrate by dip-coating method.

The Optical Properties
The band gap energy, transmittance and Urbach energy of TiO2 films obtained on FTO from Solution 1 and Solution 2, were calculated taking UV-Vis light transmittance measurements in the wavelength range of 300-800 nm, using a Perkin Elmer Lambda (Waltham, MA, USA) 650/850/950 UV/Vis spectrophotometer. These studies were carried out three times each, highlighting a minimum statistical and standard deviation in the data.

Physicochemical Characterization
The crystal structure of the TiO2 films was examined with the Panalytical (Malvern, UK) X'Pert Pro MPD X-ray diffractometer, using a CuK α X-ray beam (wavelength 1.54065 Figure 1. A simplified schematic of the procedure followed to obtain the TiO 2 thin films on FTO substrate by dip-coating method.

The Optical Properties
The band gap energy, transmittance and Urbach energy of TiO 2 films obtained on FTO from Solution 1 and Solution 2, were calculated taking UV-Vis light transmittance measurements in the wavelength range of 300-800 nm, using a Perkin Elmer Lambda (Waltham, MA, USA) 650/850/950 UV/Vis spectrophotometer. These studies were carried out three times each, highlighting a minimum statistical and standard deviation in the data.

Physicochemical Characterization
The crystal structure of the TiO 2 films was examined with the Panalytical (Malvern, UK) X'Pert Pro MPD X-ray diffractometer, using a CuK α X-ray beam (wavelength 1.54065 Å) and Bragg-Brentano geometry. The effects of the dispersant present in sol-gel dip-coating solution on the microstructure and morphology of the TiO 2 thin films were investigated using the QUANTA INSPECT F50 (FEI Company, Eindhoven, The Netherlands) scanning electron microscope equipped with a field emission gun (FEG) with a resolution of 1.2 nm and an energy dispersive X-ray spectrometer (EDS) with MnK resolution of 133 eV. The size of the particles were determined using Image J software; at least 10 measurements were performed on SEM images and then the average and standard deviation were calculated. The surface analysis and roughness evaluation were performed using an atomic force microscope (AFM) (APE Research, Trieste, Italy), in contact mode. Structural and chemical bonding of TiO 2 films were studied via infrared spectroscopy using a Perkin Elmer (Waltham, MA, USA) Spectrum 100 FT-IR spectrometer in the attenuated total reflection mode (ATR). ATR/FT-IR spectra were recorded in the 4000 cm −1 -600 cm −1 domain, representing an average of 4 scans collected at a resolution factor of 4 cm −1 . The contact angle measurements completed the surface characterization, quantifying the wettability of a solid surface by a liquid and calculating the surface free energy. Thus, the sessile drop technique was used with three different solvents: distilled water, Ethylene Glycol (EG), and dimethyl sulfoxide (DMSO) to determine the contact angle using an optical contact angle measuring instrument (Contact Angle Meter-KSV Instruments CAM 100 equipment) (Biolin Scientific, Västra Frölunda, Sweden).

Testing of Electrochemical Properties
Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Mott-Schottky measurements were carried out from −0.6 V to +0.2 V domain range in 0.5 M H 2 SO 4 aqueous solution, to investigate the electrochemical stability and the interfacial charge-separation efficiencies of the TiO 2 film as a function of the dispersant presence in sol-gel dip-coating solution. The procedures below were carried out using a three-electrode electrochemical cell: the obtained electrodes were the working electrodes Ag/AgCl, KCl sat. and Pt mesh as reference and counter electrodes, respectively. An Autolab (PGSTAT 204) potentiostat/galvanostat from Metrohm (Herisau, Switserland) system was used to control the parameters as well as collect data.

Determining the Optimum Number of Dip-Coatings Required to Deposit the TiO 2 Film on the FTO Substrate with the Lowest Band-Gap and Highest Urbach-Energy Values
In the research carried out on the mechanism of thin film growth, three distinct phases of film creation are typically identified and discussed: (1) nucleation, (2) cross linking and (3) vertical growth [18]. The substrate temperature determines the onset and rate of the first two phases, whereas the amount of layer deposition, or film thickness, governs the vertical growth phase. Hence, the source of surface roughness and agglomeration and clusters on the top surface may be traced back to the vertical growth, also known as columnar growth. Thus, the number of layers used for TiO 2 dip-coating deposition onto the FTO substrate influences surface properties such as roughness and optical features.
According to the AFM image in Figure 2a, the TiO 2 film produced by two cycles of dip-coating exhibited both tubular and palletized morphologies. The tube dimensions are around 3-5 µm in length and 0.65 µm in width and the pallet constructions have side lengths of 1.4 µm and 0.7 µm. The structures' heights range from 100 to 200 nm.
The topography of the formed film by four dip-coating cycles on the FTO substrate ( Figure 2b) shows that the FTO substrate is completely covered by intricate TiO 2 structures. The TiO 2 structures are irregular clusters of around 1-2 µm in length and 200-400 nm in height. The TiO 2 films formed by six and eight cycles of dip-coating deposition on FTO (Figure 2c,d) completely cover the substrate with irregular TiO 2 dome-like structures of 3 µm in width and heights ranging from 400 to 700 nm. The topography of the FTO substrate on which the TiO 2 films were deposited by dip-coating ( Figure 2e) reveals that the FTO (fluorine-doped tin oxide) particles are granular in appearance and uniformly cover the glass substrate, with grain diameters ranging from 10 to 50 nm and heights from 10 to 25 nm.
The thickness and roughness of the TiO 2 films obtained by various dip-coating deposition cycles on the FTO substrate are shown in Table 1. The thickness and roughness of the TiO2 films obtained by various dip-coating deposition cycles on the FTO substrate are shown in Table 1. The TiO2 film deposition on the FTO substrates from Solution 1 results in thick layers, with thicknesses ranging from 0.674 µm for two layers to 1.945 µm for eight layers. Once many layers are deposited onto the substrate, the thickness increases as reported in the literature [18,31,32], and microcracks begin to emerge on the surface (as seen in AFM pictures corresponding to six and eight layers). Findings from other studies suggest that internal stresses are generated as the density of nucleation centers decreases with layer thickness [33,34], leading to increased surface microcracks [35]. We expect all these factors to affect the films' optical characteristics of samples with the most layers deposited.  The TiO 2 film deposition on the FTO substrates from Solution 1 results in thick layers, with thicknesses ranging from 0.674 µm for two layers to 1.945 µm for eight layers. Once many layers are deposited onto the substrate, the thickness increases as reported in the literature [18,31,32], and microcracks begin to emerge on the surface (as seen in AFM pictures corresponding to six and eight layers). Findings from other studies suggest that internal stresses are generated as the density of nucleation centers decreases with layer thickness [33,34], leading to increased surface microcracks [35]. We expect all these factors to affect the films' optical characteristics of samples with the most layers deposited.
The optical transmittance spectra of TiO 2 thin films deposited from Solution 1 on the FTO substrate in the wavelength range of 300-800 nm are shown in Figure 3a as a function of the number of dip-coating layers.
The inset of Figure 3a shows that once the number of layers increases, the edge of absorption shifts to longer wavelengths as time passes, indicating that the band energy of the films formed on the FTO substrate is reducing. Furthermore, increasing the number of layers of the film makes it thicker and reduces its transmittance. The electronic transition between the TiO 2 valence band and conduction band induces fundamental light absorption in the TiO 2 thin films, as shown in the transmittance spectrum ( Figure 3a) at 407 nm [36]. In addition, for all obtained films, a high transmission region (over 60%) was found in the visible zone (500-800 nm), indicating anti-reflection and UV-protective properties [37]. The optical transmittance spectra of TiO2 thin films deposited from Solution 1 on the FTO substrate in the wavelength range of 300-800 nm are shown in Figure 3a as a function of the number of dip-coating layers. The inset of Figure 3a shows that once the number of layers increases, the edge of absorption shifts to longer wavelengths as time passes, indicating that the band energy of the films formed on the FTO substrate is reducing. Furthermore, increasing the number of layers of the film makes it thicker and reduces its transmittance. The electronic transition between the TiO2 valence band and conduction band induces fundamental light absorption in the TiO2 thin films, as shown in the transmittance spectrum ( Figure 3a) at 407 nm [36]. In addition, for all obtained films, a high transmission region (over 60%) was found in the visible zone (500-800 nm), indicating anti-reflection and UV-protective properties [37].
Plotting curves between (αhν) 2 and (hν) yields the optical energy of the band gap of the TiO2 film produced on the FTO substrates after two, four, six, and eight dip-coating cycles (Figure 3b). The band gap energy is determined using Tauc's equation with the following relationship [38]: where Eg is the bandgap energy, A is a constant, hν is the energy of the incident photon and α is the absorption coefficient [39]. The band gap value decreases with the increasing number of dip-coating deposition cycles, remaining nearly constant after four deposition cycles, at around 3.26 eV ± 0.2 eV.
Defects in the film lattice in the optical band gap region are represented by the Urbach energy (band tail width), (Eu), in Figure 3c. The Urbach tail, or absorption tail, can be traced back to flaws in the forbidden band region. The following formulas were used to calculate the Urbach energy from the absorption spectrum [40]: where α0 is a constant and Eu is the Urbach energy. The Urbach energy is estimated from the graph ln(α) = f (photon energy) (Figure 3c): Eu = 1/Slope. Table 2 displays the computed band gap energy and Urbach energy of the TiO2 thin films after two, four, six, and eight dip-coating cycles. Plotting curves between (αhν) 2 and (hν) yields the optical energy of the band gap of the TiO 2 film produced on the FTO substrates after two, four, six, and eight dip-coating cycles ( Figure 3b). The band gap energy is determined using Tauc's equation with the following relationship [38]: where E g is the bandgap energy, A is a constant, hν is the energy of the incident photon and α is the absorption coefficient [39]. The band gap value decreases with the increasing number of dip-coating deposition cycles, remaining nearly constant after four deposition cycles, at around 3.26 eV ± 0.2 eV.
Defects in the film lattice in the optical band gap region are represented by the Urbach energy (band tail width), (E u ), in Figure 3c. The Urbach tail, or absorption tail, can be traced back to flaws in the forbidden band region. The following formulas were used to calculate the Urbach energy from the absorption spectrum [40]: where α 0 is a constant and E u is the Urbach energy. The Urbach energy is estimated from the graph ln(α) = f (photon energy) ( Figure 3c): E u = 1/Slope. Table 2 displays the computed band gap energy and Urbach energy of the TiO 2 thin films after two, four, six, and eight dip-coating cycles. The band gap energy (E g ) is found to decrease from 3.52 eV to 3.25 eV as the number of layers increases, while the Urbach energy increases from 310 meV to 646 meV. Based on the AFM results, it can be observed that, due to the appearance of many microcracks in P 6 and P 8 , the band gap slowly increased and Urbach energy decreased, compared with sample P 4 , which presented a uniform compact film.
These findings are consistent with previous research [41][42][43]. Band gap energy values of TiO 2 thin layers formed on the glass substrate by sol-gel spin coating were calculated to be between 3.67 and 3.52 eV in earlier research [18]; however, the obtained band gap energy values range from 3.52 to 3.25 eV of the TiO 2 coated by sol-gel dip-coating on the FTO substrate.
Furthermore, cyclic voltammetry (CV) was performed to examine the relationship between the number of layers and the adherence of the obtained TiO 2 layers on the FTO substrate ( Figure 4). Thus, 100 CV cycles were recorded for all samples, to determine the electrochemical stability of the films on the FTO substrate. Results are illustrated in Figure 4. As the number of CV cycles grew, the corresponding constant current values for P 4 indicated (Figure 4b) a strong bond on the substrate and good electrochemical stability [44].  The greater the film's electrochemical stability, the better it adheres to the substrate. Samples of both types, P2 and P4, are stable, with P4 showing a more capacitive behavior. In the samples of P6 and P8, the results changed substantially after almost 20 CV cycles. Microcracks in samples P6 and P8 cause an unstable CV signal and a higher optical transmittance compared to sample P4, as shown by a correlation between the CV test results, the AFM results, and the optical properties.
This phenomenon keeps these films unstable even as they increase in thickness and develop microcracks. Electrolytes infiltrate through microcracks between the films and the substrate, causing the current to fluctuate as the CV cycles increase.
Based on the current experimental results, it has been concluded that four-cycle dipcoating is the ideal approach for depositing TiO2 multilayers onto FTO substrates, because it has the best optical properties and because it presents electrochemical stability.  The greater the film's electrochemical stability, the better it adheres to the substrate. Samples of both types, P 2 and P 4 , are stable, with P 4 showing a more capacitive behavior. In the samples of P 6 and P 8 , the results changed substantially after almost 20 CV cycles. Microcracks in samples P 6 and P 8 cause an unstable CV signal and a higher optical transmittance compared to sample P 4 , as shown by a correlation between the CV test results, the AFM results, and the optical properties.
This phenomenon keeps these films unstable even as they increase in thickness and develop microcracks. Electrolytes infiltrate through microcracks between the films and the substrate, causing the current to fluctuate as the CV cycles increase.
Based on the current experimental results, it has been concluded that four-cycle dipcoating is the ideal approach for depositing TiO 2 multilayers onto FTO substrates, because it has the best optical properties and because it presents electrochemical stability. Conversely, the morphology of the P4_PEG films (Figure 5b) shows the FTO substrate covered by a continuous TiO2 film with a homogeneous distribution of nanoparticles with an average size of around 100 nm (±6.85). As in the literature [30], the addition of PEG in the coating solution promoted the growth of a nanostructured film and enhanced the production of nanoparticles.
These SEM results demonstrate that the dispersant can modify the size and shape o TiO2 nanoparticles deposited on the FTO substrate by the dip-coating method.
To confirm the presence of both Ti and O in the films obtained from both solutions the EDX spectra (Figure 5c,d) was performed and recorded. Oxygen is present in nearly equal amounts in both films, while Ti is more abundant in the P4_PEG film.
The crystalline phase and grain size of the films dip-coated on FTO were investigated using X-ray diffraction patterns ( Figure 6). Conversely, the morphology of the P 4_PEG films (Figure 5b) shows the FTO substrate covered by a continuous TiO 2 film with a homogeneous distribution of nanoparticles with an average size of around 100 nm (±6.85). As in the literature [30], the addition of PEG in the coating solution promoted the growth of a nanostructured film and enhanced the production of nanoparticles.
These SEM results demonstrate that the dispersant can modify the size and shape of TiO 2 nanoparticles deposited on the FTO substrate by the dip-coating method.
To confirm the presence of both Ti and O in the films obtained from both solutions, the EDX spectra (Figure 5c,d) was performed and recorded. Oxygen is present in nearly equal amounts in both films, while Ti is more abundant in the P 4_PEG film. The crystalline phase and grain size of the films dip-coated on FTO were investigated using X-ray diffraction patterns ( Figure 6).
where θ is the Bragg diffraction angle, λ is the light wavelength utilized for diffraction, equal to 1.54065 Å, and β is the full width at half maxima (FWHM) of the diffraction peak, in radians.
The following relationships were also used to calculate the lattice strain (ε) [47] and dislocation density (δ) [48]: ε = 4 tan( ) and δ = 1 In Table 3, the variation of crystallite size, the lattice strain and dislocation density are presented.  204) and (224) planes of TiO 2 anatase, respectively, were observed in both the P 4 and P 4_PEG thin films [45,46]. All the samples exhibited a majority crystallite structure of the TiO 2 anatase phase, and the rutile phase was observed at 53.72 • and 51.56 • , corresponding to the (105) plane, and at 55.10 • and 54.54 • , corresponding to the (211) plane, for both obtained films.
The average crystallite size (D) of the samples was calculated using the Debye-Scherrer equation [18]: where θ is the Bragg diffraction angle, λ is the light wavelength utilized for diffraction, equal to 1.54065 Å, and β is the full width at half maxima (FWHM) of the diffraction peak, in radians.
The following relationships were also used to calculate the lattice strain (ε) [47] and dislocation density (δ) [48]: In Table 3, the variation of crystallite size, the lattice strain and dislocation density are presented. PEG in solution 2 increases the crystallite size while decreasing the strain, and it is expected to influence optical properties in relation to the band gap's low energy [18]. This could be due to the relatively easy decomposition of PEG 8000's high molecular chains during thermal treatment at 450 • C, resulting in the collective fusion of small crystallites into nanoparticle aggregates [15].
Furthermore, the ATR/FT-IR spectra of the TiO 2 thin films prepared from solution 1 without a dispersant and solution 2 with PEG were examined using infrared spectroscopy to determine the type of structural and chemical bonding of TiO 2 (Figure 7). In both spectra, peaks in the 2920 cm −1 and 3025 cm −1 regions [18,49] are observed and are associated with the stretching vibration of hydroxyl groups. The absorption peaks at 2952 cm −1 , 2871 cm −1 , and 1757 cm −1 correspond to the stretching vibrations of the CH 3 , CH 2 , and C=O groups, respectively, of the PEG from the P 4_PEG film [50]. The Ti-O-Ti bonds in the TiO 2 nanoparticles' stretching vibration mode are responsible for the absorption bands at 1075 cm −1 and 874 cm −1 , which correspond to P 4 [18,51]. The film with PEG presents the same Ti-O-Ti stretching vibration mode, but it is shifted to 1189 cm −1 and 925 cm −1 for the P 4_PEG film. The stretching vibration of the Ti-O groups in the anatase phase [18,52] was found at 741 cm −1 and 662 cm −1 for P 4 and at 808 cm −1 and 601 cm −1 for P 4_PEG [30].  PEG in solution 2 increases the crystallite size while decreasing the strain, and it is expected to influence optical properties in relation to the band gap's low energy [18]. This could be due to the relatively easy decomposition of PEG 8000's high molecular chains during thermal treatment at 450 °C, resulting in the collective fusion of small crystallites into nanoparticle aggregates [15].
Furthermore, the ATR/FT-IR spectra of the TiO2 thin films prepared from solution 1 without a dispersant and solution 2 with PEG were examined using infrared spectroscopy to determine the type of structural and chemical bonding of TiO2 (Figure 7). In both spectra, peaks in the 2920 cm −1 and 3025 cm −1 regions [18,49] are observed and are associated with the stretching vibration of hydroxyl groups. The absorption peaks at 2952 cm −1 , 2871 cm −1 , and 1757 cm −1 correspond to the stretching vibrations of the CH3, CH2, and C=O groups, respectively, of the PEG from the P4_PEG film [50]. The Ti-O-Ti bonds in the TiO2 nanoparticles' stretching vibration mode are responsible for the absorption bands at 1075 cm −1 and 874 cm −1 , which correspond to P4 [18,51]. The film with PEG presents the same Ti-O-Ti stretching vibration mode, but it is shifted to 1189 cm −1 and 925 cm −1 for the P4_PEG film. The stretching vibration of the Ti-O groups in the anatase phase [18,52] was found at 741 cm −1 and 662 cm −1 for P4 and at 808 cm −1 and 601 cm −1 for P4_PEG [30].  As expected, results from Table 4 indicate that thin TiO 2 films displayed low contact angle values due to the hydrophilic nature of the mixture components coated on the substrate. The PEG presence in the sol-gel solution influences the wettability of the obtained TiO 2 thin film; the contact angle value (CA = 16 • ) of the film with a dispersant is lower than the one corresponding to the film without a dispersant (CA = 26 • ). Surface free energy, which is a result of film surface defects [53], is important for the pseudo-capacitive processes that occur in many applications such as smart windows. The results of this study indicate that the TiO 2 films' surface free energy varies with the addition of the dispersant. The Owens-Wendt-Rabel-Kaelble (OWRK) model was successfully applied to calculate the surface free energy [54]. In terms of contact angles, the lower the surface energy, the greater the contact angles.

Electrochemical Characterization
The electrochemical behavior of TiO 2 films in 0.5 M H 2 SO 4 solution was investigated during the cycling of applied potentials ranging from −0.6 to +0.2 V at different scan rates of 25, 50, 100, 150, 200, and 300 mV s −1 . Both obtained films exhibited an oxidation peak near to the Fermi level of anatase TiO 2 at −0.5 V (Figure 8) [9]. At rates greater than 50 mV/s, the oxidation peak shifts to less-negative potential values of −0.4 V, due to the oxidation of Ti 3+ to Ti 4+ , because of the deintercalation of H + ions and electrons from TiO 2 nanostructures via the reaction outlined below [55]: TiO 2 + xe − + xH + → TiOOH A shift in peak potentials toward higher positive values indicates a desirable behavior for the utilization of these electrodes in technological applications with catalytic behavior, such as smart windows. As expected, results from Table 4 indicate that thin TiO2 films displayed low contact angle values due to the hydrophilic nature of the mixture components coated on the substrate. The PEG presence in the sol-gel solution influences the wettability of the obtained TiO2 thin film; the contact angle value (CA = 16°) of the film with a dispersant is lower than the one corresponding to the film without a dispersant (CA = 26°).
Surface free energy, which is a result of film surface defects [53], is important for the pseudo-capacitive processes that occur in many applications such as smart windows. The results of this study indicate that the TiO2 films' surface free energy varies with the addition of the dispersant. The Owens-Wendt-Rabel-Kaelble (OWRK) model was successfully applied to calculate the surface free energy [54]. In terms of contact angles, the lower the surface energy, the greater the contact angles.

.2. Electrochemical Characterization
The electrochemical behavior of TiO2 films in 0.5 M H2SO4 solution was investigated during the cycling of applied potentials ranging from −0.6 to +0.2 V at different scan rates of 25, 50, 100, 150, 200, and 300 mV s −1 . Both obtained films exhibited an oxidation peak near to the Fermi level of anatase TiO 2 at −0.5 V (Figure 8) [9]. At rates greater than 50 mV/s, the oxidation peak shifts to less-negative potential values of −0.4 V, due to the oxidation of Ti 3+ to Ti 4+ , because of the deintercalation of H + ions and electrons from TiO2 nanostructures via the reaction outlined below [55]: TiO2 + xe − + xH + → TiOOH A shift in peak potentials toward higher positive values indicates a desirable behavior for the utilization of these electrodes in technological applications with catalytic behavior, such as smart windows.  Moreover, the capacitive behavior of the TiO 2 films obtained in the presence of the dispersant is more prominent, and the current density increases, indicating a more conductive character than the film obtained without PEG. Straight lines are obtained when the peak value of the oxidation current is plotted against the square root of the scan speed for both samples (inset Figures), indicating that the insertion and extraction of H + ions is rapid and that proton diffusion in the TiO 2 nanostructure controls the rate-determining step of the redox reaction. Due to the nanocrystalline form of the grains, according to the SEM image (Figure 5b), the diffusion and charge-transfer process of H + ions in the TiO 2 thin film may be facilitated, which may explain why greater charge densities were reported in the P 4_PEG thin film [9].
To investigate the charge-transfer kinetics and mass transport behavior of the films developed on the FTO substrate, electrochemical impedance spectroscopy (EIS) analysis was performed. The EIS measurements were made in a frequency range of 10 mHz to 100 kHz. Figure 9 shows the Nyquist plots of the P 4 and P 4_PEG thin films measured at free potential voltage in 0.5 M H 2 SO 4 solution. Moreover, the capacitive behavior of the TiO2 films obtained in the presence of the dispersant is more prominent, and the current density increases, indicating a more conductive character than the film obtained without PEG. Straight lines are obtained when the peak value of the oxidation current is plotted against the square root of the scan speed for both samples (inset Figures), indicating that the insertion and extraction of H + ions is rapid and that proton diffusion in the TiO2 nanostructure controls the rate-determining step of the redox reaction. Due to the nanocrystalline form of the grains, according to the SEM image (Figure 5b), the diffusion and charge-transfer process of H + ions in the TiO thin film may be facilitated, which may explain why greater charge densities were reported in the P4_PEG thin film [9].
To investigate the charge-transfer kinetics and mass transport behavior of the films developed on the FTO substrate, electrochemical impedance spectroscopy (EIS) analysis was performed. The EIS measurements were made in a frequency range of 10 mHz to 100 kHz. Figure 9 shows the Nyquist plots of the P4 and P4_PEG thin films measured at free potential voltage in 0.5 M H2SO4 solution. To fit the data, the Randles equivalent circuit model was used, as can be seen in the inset of Figure 9. The equivalent circuit consists of solution resistance (Rs), charge-transfer resistance (Rct) across the developed electrodes/H2SO4 electrolyte interface, and a constan phase element (CPE) which represent a double-layer capacitance at the electrode surface Table 5 shows the electric parameters from the EIS experimental data fitted with the proposed equivalent circuit. To fit the data, the Randles equivalent circuit model was used, as can be seen in the inset of Figure 9. The equivalent circuit consists of solution resistance (R s ), chargetransfer resistance (R ct ) across the developed electrodes/H 2 SO 4 electrolyte interface, and a constant phase element (CPE) which represent a double-layer capacitance at the electrode surface. Table 5 shows the electric parameters from the EIS experimental data fitted with the proposed equivalent circuit. The charge-transfer resistance (R ct ) of the P 4 and P 4_PEG films deposited on the FTO by dip-coating is 418 kΩ and 23.4 kΩ, respectively. As previously noted, the higher surface free energy, as well as the smaller band gap and higher Urbach energy of the film, produce low charge-transfer resistance, which provides less resistance for ion transport and the slowest recombination rate. When TiO 2 was doped with PEG, the density of the nanoparticles in the film increased, leading to higher conductivities. PEG is a well-known long-chain surfactant that helps prevent TiO 2 agglomeration during the dip-coating deposition process [56]. The higher value obtained for the P 4 film, indicating poor charge-transfer characteristics, is improved by adding PEG to the sol-gel precursor solution. The double-layer capacity of the P 4_PEG film is three orders of magnitude greater than that of the TiO 2 film prepared from solution 1. Both films exhibited pseudo-capacitive behavior as a result of n values ranging from 0.8 to 1 [57].
The Mott-Schottky (M-S) Equation (5) was used for further analysis of the impedance data and is defined as follows: where C is the capacity of the space charge layer, q is the elementary charge, ε 0 is the vacuum permittivity, ε is the dielectric constant, N D is the concentration of donors, E is the applied external bias, E fb is the flat band potential, k is Boltzmann's constant, and T is the absolute temperature. Flat band potential (E fb ) is determined by extrapolating to 1/C 2 = 0, which is dependent on the recombination process and interface charge-transfer [58], whereas the slope of the Mott-Schottky plot indicates the doping level (N D ). Indirect electron tunnelling through a semiconductor is a well-known result of a band gap with a large concentration of numerous donor levels.
Each P 4 and P 4_PEG film displayed a positive slope on the Mott-Schottky graphs, as is typical for n-type semiconductors [59], (Figure 10).  The charge-transfer resistance (Rct) of the P4 and P4_PEG films deposited on the FTO by dip-coating is 418 kΩ and 23.4 kΩ, respectively. As previously noted, the higher surface free energy, as well as the smaller band gap and higher Urbach energy of the film, produce low charge-transfer resistance, which provides less resistance for ion transport and the slowest recombination rate. When TiO2 was doped with PEG, the density of the nanoparticles in the film increased, leading to higher conductivities. PEG is a well-known long-chain surfactant that helps prevent TiO2 agglomeration during the dip-coating deposition process [56]. The higher value obtained for the P4 film, indicating poor chargetransfer characteristics, is improved by adding PEG to the sol-gel precursor solution. The double-layer capacity of the P4_PEG film is three orders of magnitude greater than that of the TiO2 film prepared from solution 1. Both films exhibited pseudo-capacitive behavior as a result of n values ranging from 0.8 to 1 [57].
The Mott-Schottky (M-S) Equation (5) was used for further analysis of the impedance data and is defined as follows: where C is the capacity of the space charge layer, q is the elementary charge, ε0 is the vacuum permittivity, ε is the dielectric constant, ND is the concentration of donors, E is the applied external bias, Efb is the flat band potential, k is Boltzmann's constant, and T is the absolute temperature. Flat band potential (Efb) is determined by extrapolating to 1/C 2 = 0, which is dependent on the recombination process and interface charge-transfer [58], whereas the slope of the Mott-Schottky plot indicates the doping level (ND). Indirect electron tunnelling through a semiconductor is a well-known result of a band gap with a large concentration of numerous donor levels.
Each P4 and P4_PEG film displayed a positive slope on the Mott-Schottky graphs, as is typical for n-type semiconductors [59], (Figure 10).  E fb is typically regarded as the conduction band potential (CB) for n-type semiconductors [59]. The calculated value of E fb vs. NHnE (normal hydrogen electrode) for P 4 and P 4_PEG is 0.055 eV and −0.019 eV, respectively. These results imply a slower recombination rate for film formed in the presence of a dispersant, which agrees with the reduction in charge-transfer resistance measured by EIS. The negative shift in the flat-band potential for P 4_PEG suggests that the energy barrier for interfacial electron transport is reduced, resulting in a lower charge-transfer resistance [60]. Moreover, the doping level (N D ) for P 4 and P 4_PEG are 6.5 × 10 19 cm −3 and 2.01 × 10 20 cm −3 , respectively, correlating with the variation in Urbach energy values. We may conclude that adding PEG to the sol-gel precursor solution induces a shift in the flat band potential of TiO 2 to negative values, indicating improved catalytic capabilities and a higher doping level.

Optical Properties-Bandgap and Urbach Energies
Using Tauc's equation (Equation (1)) and based on UV-VIS data, the band gap energy was determined.
The band gap (E g ) values for P 4 were reduced from 3.25 eV to 3.12 eV for P 4_PEG , indicating that the films are more conductive (Figure 11). In addition, for thin films made with a dispersant, the Urbach energy increased from 0.645 eV to 0.709 eV, with these values correlating with the increased surface free energy (Table 4). More structural defects, as shown by the Urbach energy value, can be induced by the delocalization of molecular orbitals caused by the simple breakdown of long interweaving PEG molecule chains, resulting in greater absorption at longer wavelengths and a decrease in band gap value [53,61]. Efb is typically regarded as the conduction band potential (CB) for n-type semiconductors [59]. The calculated value of Efb vs. NHnE (normal hydrogen electrode) for P4 and P4_PEG is 0.055 eV and −0.019 eV, respectively. These results imply a slower recombination rate for film formed in the presence of a dispersant, which agrees with the reduction in charge-transfer resistance measured by EIS. The negative shift in the flatband potential for P4_PEG suggests that the energy barrier for interfacial electron transport is reduced, resulting in a lower charge-transfer resistance [60]. Moreover, the doping level (ND) for P4 and P4_PEG are 6.5 × 10 19 cm −3 and 2.01 × 10 20 cm −3 , respectively, correlating with the variation in Urbach energy values. We may conclude that adding PEG to the sol-gel precursor solution induces a shift in the flat band potential of TiO2 to negative values, indicating improved catalytic capabilities and a higher doping level.

Optical Properties-Bandgap and Urbach Energies
Using Tauc's equation (Equation (1)) and based on UV-VIS data, the band gap energy was determined.
The band gap (Eg) values for P4 were reduced from 3.25 eV to 3.12 eV for P4_PEG, indicating that the films are more conductive (Figure 11). In addition, for thin films made with a dispersant, the Urbach energy increased from 0.645 eV to 0.709 eV, with these values correlating with the increased surface free energy (Table 4). More structural defects, as shown by the Urbach energy value, can be induced by the delocalization of molecular orbitals caused by the simple breakdown of long interweaving PEG molecule chains, resulting in greater absorption at longer wavelengths and a decrease in band gap value [53,61]. The structure diagrams for the obtained films based on the measured M-S flat band potential and band gap from the UV-VIS data are shown in Figure 12. Based on the measured CB and Eg, the valence band (VB) energy of the films deposited on the FTO was computed. The structure diagrams for the obtained films based on the measured M-S flat band potential and band gap from the UV-VIS data are shown in Figure 12. Based on the measured CB and E g , the valence band (VB) energy of the films deposited on the FTO was computed.
As can be seen, P 4 VB is 3.3 eV, while P 4_PEG VB is 3.1 eV. When compared to P 4 and P 4_PEG , the VB is reduced by 0.2 eV. This suggested that the presence of PEG in solution 2 generates electrons on the surface of the film coated on the FTO substrate and improves conductivity, due to the well-controlled grain of the TiO 2 (as depicted in Figure 5b), with greater crystallinity, as observed in the XRD data ( Figure 6). As can be seen, P4 VB is 3.3 eV, while P4_PEG VB is 3.1 eV. When compared to P4 and P4_PEG, the VB is reduced by 0.2 eV. This suggested that the presence of PEG in solution 2 generates electrons on the surface of the film coated on the FTO substrate and improves conductivity, due to the well-controlled grain of the TiO2 (as depicted in Figure 5b), with greater crystallinity, as observed in the XRD data ( Figure 6).
Based on these findings, the P4_PEG/FTO electrode can be employed in a variety of applications, including smart windows and photocatalysts for solar cells or water splitting, where pseudo-capacitive behavior is desired.

Conclusions and Perspectives
The surface characteristics of the TiO2 film deposited on the FTO substrate are enhanced by the incorporation of PEG into the sol-gel precursor solution used for the dipcoated deposition process. This results in a TiO2 film with increased free energy, as well as a more compact structure and nanoparticles with a higher degree of crystallization. The optical and electrochemical properties are affected by these surface features. With a higher insertion/extraction rate of protons into the P4_PEG nanostructure, the band gap is decreased from 3.25 to 3.12 eV, and the Urbach energy increases from 646 to 709 meV. The charge-transfer resistance is decreased from 418 kΩ to 23.4 kΩ, and the flat band potential is decreased from 0.44 V to −0.019 V, indicating improved catalytic properties of the final TiO2 film. These features make P4_PEG film a competitive option for technical uses such as smart windows, solar cells, photocatalysts and gas sensors.
Our long-term goal is to produce a pseudo-capacitive electrochromic electrode using this developed P4_PEG film as a template, which will be employed in the production of smart windows.  Based on these findings, the P 4_PEG /FTO electrode can be employed in a variety of applications, including smart windows and photocatalysts for solar cells or water splitting, where pseudo-capacitive behavior is desired.

Conclusions and Perspectives
The surface characteristics of the TiO 2 film deposited on the FTO substrate are enhanced by the incorporation of PEG into the sol-gel precursor solution used for the dipcoated deposition process. This results in a TiO 2 film with increased free energy, as well as a more compact structure and nanoparticles with a higher degree of crystallization. The optical and electrochemical properties are affected by these surface features. With a higher insertion/extraction rate of protons into the P 4_PEG nanostructure, the band gap is decreased from 3.25 to 3.12 eV, and the Urbach energy increases from 646 to 709 meV. The charge-transfer resistance is decreased from 418 kΩ to 23.4 kΩ, and the flat band potential is decreased from 0.44 V to −0.019 V, indicating improved catalytic properties of the final TiO 2 film. These features make P 4_PEG film a competitive option for technical uses such as smart windows, solar cells, photocatalysts and gas sensors.
Our long-term goal is to produce a pseudo-capacitive electrochromic electrode using this developed P 4_PEG film as a template, which will be employed in the production of smart windows.

Data Availability Statement:
The raw/processed data generated in this work are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare that they do not have any known competing financial interests or personal connections that could appear to have influenced the work presented in this study.