Comparison of Photocatalytic Activity: Impact of Hydrophilic Properties on TiO₂ and ZrO₂ Thin Films

Abstract

Thin films (TFs) of TiO₂ and ZrO₂ were prepared and characterized to evaluate their structural and optical (SO) properties and, later, to test their efficiency for the photocatalytic degradation (PD) of methylene blue (MB) in aqueous solution. The X-ray diffraction patterns showed that the TiO₂ TFs had an anatase crystalline structure, unlike the ZrO₂ TFs, which showed a tetragonal crystalline structure that was verified by Raman spectroscopy. The band gap (BG) energies, as calculated from UV-Vis spectroscopy and diffuse reflectance spectroscopy, corresponded to 3.2 and 3.7 eV for the TiO₂ and ZrO₂ TFs, respectively. SEM examination of the obtained materials was also carried out to assess the surface morphology and topography. The comparative study of the FTIR spectra of the TiO₂ and ZrO₂ TFs successfully confirmed the composition of the two-metal oxide TFs. The electrical properties of the films were studied by conductivity measurements. The two films also showed a similar thickness of about 200 nm and a substantially different photocatalytic performance for the discoloration of MB in aqueous solution. The corresponding rate constants, as obtained from a pseudo-first-order kinetic model, revealed that TiO₂ films promote color removal of the model dye solution almost 20 times faster than the rate observed for ZrO₂ modified glass substrates. We suggest that this difference may be related to the hydrophilic character of the two films under study, which may affect the charge carrier injection process and, therefore, the overall photocatalytic performance.

1. Introduction

The escalating global population and the corresponding surge in energy and water demands from various industries, such as petrochemicals, pharmaceuticals, textiles, agrochemicals, fuels, and plastics, have resulted in a significant environmental challenge [1,2,3,4]. This situation is even more complicated when the widespread contamination of water sources is considered, jeopardizing both wildlife and human health and disrupting fragile ecosystems [5]. Addressing this problem entails ensuring universal access to safe drinking water, a goal advocated by organizations such as the Organization for Economic Co-operation and Development (OECD), which stresses the importance of coordinated government efforts to manage water resources effectively [6]. Over recent decades, several remediation techniques have emerged, encompassing physical, chemical, and biological approaches [7]. Notably, metal oxides exhibit multifunctionality as both absorbents and antimicrobial agents, while semiconductors offer potential for environmental regulation and contaminant mitigation due to their light absorption properties, variable oxidation states and large surface areas [8]. Moreover, the application potential of nanocomposite configurations, combining several unit components with various chemical and physical characteristics, has garnered significant interest in research groups [9,10,11,12]. The formation of nanocomposite materials and the examination of their physical characteristics have been the subject of numerous investigations, showing their adaptability in a range of industries, including photocatalysis, fuel cells, optoelectronics, microelectromechanical systems, and sensors [13,14,15,16,17].

Titanium dioxide (TiO₂), a semiconductor known for its chemical stability and low cost, has received a great deal of attention from academic and technological circles due to its wide-ranging applications in various fields [18]. Of particular interest are its different crystalline forms, including anatase (A) and rutile (R), each characterized by unique atomic structures and properties [19]. While anatase TiO₂ displays an indirect BG, the rutile phase predominantly exhibits direct optical processes [20,21]. Typically, the optical absorption edge of TiO₂ is observed around 400 nm, although exact values may vary based on factors such as sample morphology and measurement conditions [22]. It is worth noting, however, that TiO₂ polymorphs can undergo phase transitions under specific experimental conditions, with the permanent conversion of anatase to rutile taking place at temperatures that exceed 850 K [23]. Despite these complexities, both anatase and rutile TiO₂ have been pivotal in numerous successful applications, spanning from photo-driven processes to environmental sensing and energy storage [24,25,26,27,28]. Moreover, the SO characteristics of TiO₂ significantly influence its performance across a range of uses, for instance, for dye-sensitized solar cells, where the existence of rutile TiO₂ enhances photocurrent and electron generation or compensates for variations in photo-electrode areas [29,30,31]. Therefore, further advancements in TiO₂-based technologies hinge upon a thorough understanding and precise characterization of the properties and behavior of its polymorphs [32,33].

Zirconia (ZrO₂), on the other hand, is a versatile material renowned for its wide BG (ranging from 3 to 5.2 eV) [34,35]. It boasts exceptional stability across chemical, electrical, and thermal domains, and it is highly coveted in various fields, including biomedical, sensing, and catalysis [36,37,38]. Moreover, its tunable conductivity and high refractive index make it valuable for emerging technologies [39]. ZrO₂ has been synthesized using diverse techniques, with both vacuum and solution processing playing crucial roles [40,41]. Tailoring the properties of ZrO₂ TFs involves meticulous consideration of factors such as crystallinity and post-deposition heat treatment [42,43]. These variables affect mechanical strength, ionic conductivity, and dielectric characteristics [44]. The material’s polymorphic phases—monoclinic, tetragonal, and cubic—exhibit distinct characteristics, with their stability being influenced by synthesis temperatures [37,45,46]. Tetragonal and cubic phases, while less stable at room temperature (RT), offer unique advantages for various applications [47,48]. For instance, tetragonal ZrO₂ shows promise in reinforcing materials and increasing fracture toughness [49]. TFs of ZrO₂ find extensive utility in optical storage elements, lasers, and anti-corrosion coatings [50,51]. Furthermore, ZrO₂ displays photocatalytic properties, albeit with limited solar light absorption because of its broad BG [52]. Notably, the choice of calcination temperatures significantly influences the crystal phases and, consequently, the material’s performance in various applications [53,54].

To manufacture TiO₂ and ZrO₂ TFs, a variety of deposition processes have been used, such as pulsed laser deposition [55,56], atomic layer deposition [57,58], chemical vapor deposition (CVD) [39,59] and sol–gel [60,61]. Compared to alternative surface modification methods, the sol–gel method offers several benefits, including (i) easy processing and application at lower temperatures, (ii) flexibility in the substrate’s shape and kind, and (iii) the ability to synthesize high-purity films that are both chemically and physically uniform at small and large scales [62]. Additional benefits of this approach include high purity and great chemical homogeneity, as well as precise control over composition and microstructure. The wet approach (sol–gel), in contrast to traditional synthesis procedures, allows the material to form nanocrystals and nanopores. Utilizing wet synthesis techniques in conjunction with appropriate heat treatment, and dip-coating (DC) deposition, enables the customization of TFs with a precise and repeatable thickness and microstructure. Controlling these attributes is not simple, though, as a variety of factors can alter the TF’s characteristics, many of which are reliant on the sol’s synthesis and the thermal treatments applied after deposition.

In this context, this work’s primary objective is to compare photocatalytic activity, where the importance of hydrophilicity in the different thin films is demonstrated. In addition, the optical and structural characteristics are compared and evaluated. Since DC is a low-cost and frequently employed TF deposition technique, particularly on large-area glass substrates where thickness homogeneity is crucial, it was selected as the deposition method.

2. Results and Discussion

2.1. X-Ray Diffraction

Figure 1 shows the X-ray diffractograms (in the 2θ range 20–80°) of the TiO₂ and ZrO₂ TFs that were calcined at 500 °C. While the diffraction peaks observed for the TiO₂ films correspond to the anatase phases (see Figure 1a), Figure 1b reveals that the ZrO₂ films are characterized by a tetragonal phase (TP) (in both images, the corresponding diffraction planes have been added in parenthesis). Inspection of Figure 1a shows that the main diffraction peaks of the TiO₂ TFs are located at 2θ values of 25.24°, 37.81°, 48.08°, 53.89°, 55.15°, 62.22°, and 62.80°, which correspond to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 1 3), and (2 0 4) planes of anatase TiO₂ (JCPDS No. 21–1272) [63]. Figure 1b, on the other hand, shows that the diffraction peaks of the ZrO₂ TFs are positioned at 30.27°, 35.24°, 50.38°, 50.75°, 60.23°, and 63.01°, which can be indexed to the (0 1 1), (1 1 0), (1 1 2), (0 2 0), (1 2 1), and (2 0 2) planes of tetragonal ZrO₂ (JCPDS No. 10-5089) [64]. In Figure 1a,b, it is possible to see that there are overlapped signals that make the identification of planes a relatively difficult task. See, for example, the insets that show the overlapping of the (213) and (204) planes for TiO₂ and the (112) and (020) planes for ZrO₂. Recently, Wang et al. obtained similar results using both hydrothermal and solvothermal methods to synthesize uniform ZrO₂ nanoparticles [64].

In the following stage of the characterization study, the average grain sizes were determined using Scherrer’s formula [65]. The crystallite size was calculated using the FWHM value of the highest peak. The typical crystallite sizes determined by the TiO₂ and ZrO₂ TFs corresponded to 16, and 41 nm, respectively, at 500 °C. This finding showed that as the annealing temperature was raised, the nanocrystals’ grain sizes grew.

2.2. Raman Spectroscopy

Since the XRD reflection peaks of the TFs under study may overlap with each other, Raman spectroscopy was employed to further investigate the structure of the TiO₂ and ZrO₂ modified surfaces. As expected, the DC deposited TiO₂ TFs showed the anatase phase. As can be observed in Figure 2, six Raman active modes are seen in the anatase phase, with a high signal peak at 143 cm⁻¹ and low intensity peaks at 197, 396, 515, 522, and 640 cm⁻¹. Only one band is visible in our experiment, and the bands at 515 and 522 cm⁻¹ overlap. However, an enlargement of Figure 2 was carried out in order to visualize the two signals. No signals of the rutile or brookite phase were present, which means that the pure anatase phase was obtained. Also, we were able to identify the two materials’ vibrational modes in Figure 2 by comparing their spectra for anatase and the TP [66,67]. Specifically, the Raman peaks were due to the vibrational signatures of the tetragonal ZrO₂, being made abundantly evident by the existence of the 144 cm⁻¹ and 454 cm⁻¹ resonance, which was allocated to TP. No other bands were detected [68]. When compared to ZrO₂ TFs, the TiO₂ spectra exhibit stronger intensity.

2.3. Scanning Electron Microscopy (SEM)

To examine the surface morphologies of the samples under study, SEM measurements were carried out. Figure 3a,d show SEM images taken at 5 k magnification for TiO₂ and ZrO₂ TFs coated on glass substrates. The top aspect of a columnar growth resembles the surface structure of TFs obtained by sputtering at RT [69]. Figure 3b, showing an SEM image of TiO₂ nanocrystals, clearly reveals their spherical shape with very little aggregation. Prucnal et al. reported similar SEM images for TiO₂ films [70]. Figure 3d, on the other hand, shows that ZrO₂ TFs have an excellent uniform aggregation. The films appear free of microcracks and defects, and they are homogeneous. Figure 3e shows a representative image of ZrO₂ TFs, which are dense, highly homogeneous, and have a surface with no imperfections. Additionally, it was noted that all the crystals were in the nanoscale range. Figure 3c,f show the cross-section SEM images of TiO₂ and ZrO₂ TFs, respectively. The thickness of the samples is 352 and 365 nm. A very similar thickness was obtained by Mansilla for ZrO₂ TFs obtained by the sol–gel method [71].

2.4. UV-Vis Analysis

The absorbance, reflectance, and (αhv)ⁿ vs. (hν) spectra of titanium oxide (TiO₂) and zirconium oxide (ZrO₂) TFs that were annealed at 500 °C in the 300–1000 nm wavelength range are shown in Figure 4a–c for TiO₂ and in Figure 4c–e for ZrO₂ TFs. An inspection of Figure 4a,d reveals that the absorbance spectrum can be separated in two distinct regions: The first region is characterized by a high absorption (λ < 385 nm), which is brought on by the light’s fundamental absorption in TFs of TiO₂ and ZrO₂, reflecting the electronic transition of the two materials’ valence and conduction bands. In the visible range (400–800 nm), there is a second significant absorbance zone where the value is low. These materials can be helpful for optical coating applications such as UV-protecting films for optoelectronic devices, anti-reflective films, and wavelength-selective films due to their exceptional optical absorbance.

Plotting a curve of (αhv)ⁿ vs. (hν) yields the optical BG energy of TiO₂ and ZrO₂ TFs, as illustrated in Figure 4b and Figure 4e, respectively. The corresponding BG energies were calculated using Tauc’s equation:

$${\left(\mathsf{\alpha }\mathrm{h}\mathrm{v}\right)}^{\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathrm{v}-{\mathrm{E}}_{\mathrm{g}}\right)$$

(1)

where $\mathrm{h}\mathrm{v}$ is the photon energy, $\mathsf{\alpha }$ is the absorption coefficient, $\mathrm{A}$ is a constant, ${\mathrm{E}}_{\mathrm{g}}$ is the optical BG energy, and $\mathrm{n}$ varies according to the kind of optical transition (direct or indirect) taking place. This absorption coefficient ($\mathsf{\alpha }$) might be calculated using the following equation:

$$\mathsf{\alpha }={\displaystyle \frac{1}{\mathrm{d}}}\ln \left({\displaystyle \frac{1}{\mathrm{T}}}\right)$$

(2)

where $\mathrm{d}$ represents the sample’s thickness. In this way, the TiO₂ indirect optical transition corresponds to $\mathrm{n}$ = 1/2 [72]. Plots of (αhv)^1/2 vs. (hv) are shown in Figure 4b, where ${\mathrm{E}}_{\mathrm{g}}$ is determined by intercepting the photon energy axis and the linear part of the basic absorption edge. The obtained BG energy for TiO₂ was 3.49 eV. Khetta et al. deposited TiO₂ TFs by the ultrasonic spray process and obtained a BG of 3.47 eV, a value that agrees well with our results [73]. For ZrO₂ TFs, a plot of (αhv)² versus photon energy (hν) and projection of the linear portion of the curve to the energy axis, yielded the ($n$ = 2) direct transition [74] $E_g$ values that can be observed in the inset of Figure 4e. The ZrO₂ TFs therefore have a BG of 4.17 eV. Nirun and Surasing obtained ZrO₂ TFs deposited on glass, and the energy BG of the as-deposited film was approximately 4.17 eV [75]. These results are similar to those obtained in this work.

The diffuse reflectance spectral (DRS) study is a method for investigating a semiconductor’s optical characteristics. This characteristic is dependent on the BG, oxygen vacancies, impurity centers, and surface morphology. A material’s optical sensitivity can be described by a combination of assessing the BG and the relative locations of the valence and conduction bands [76]. The Kubelka–Munk (K-M) approach, which is predicated on the idea that the DRS may be converted into the appropriate absorption spectra, was used to estimate the optical BG of the TiO₂ and ZrO₂ TFs [77]:

$$F\left(R_{\propto }\right)={\displaystyle \frac{{\left(1-R_{\propto }\right)}^2}{2R_{\propto }}}$$

(3)

where $R_{\propto }$ is the diffuse reflectance and $F\left(R_{\propto }\right)$ is the K-M absorption function. In Tauc’s technique, the energy-dependent absorption coefficient is substituted with the K-M function:

$${\left(F\left(R\right)hv\right)}^n=A\left(hv-E_g\right)$$

(4)

Assuming the indirect transition for TiO₂ (n = 1/2) and the direct transition for ZrO₂ ($n$ = 2) TFs, it is possible to extrapolate the linear region of the plot of ${\left(F\left(R\right)hv\right)}^n$ vs. ($hv$) at ${\left(F\left(R\right)hv\right)}^n$ = 0 to obtain the corresponding BG energy ($E_g$) [78] (see Figure 4c,f). The obtained values for the BGs corresponded to 3.48 eV and 4.17 eV for TiO₂ and ZrO₂ TFs, respectively.

2.5. Photoluminescence (PL)

The rate of recombination of e-/h+ pairs was analyzed using photoluminescence (PL) spectra, in which electrons were excited using a laser source (325 nm wavelength). As can be observed in Figure 5, there is a clear photoluminescence peak positioned at 440 nm. This signal is most likely produced by oxygen vacancies (defects) related to trap-assisted recombination in TiO₂. U. Chacon-Argaez et al., in their report on Photocatalytic Activity and Biocide Properties of Ag-TiO₂ Composites on Cotton Fabrics, point out that the band at 441 nm is attributed to excitons resulting from vacancies and oxygen defects on the surface [79]. PL emission spectra of the TFs under study exhibited a broad emission peak around 534 nm. Nagaveni et al. reported one peak at about 530 nm which was attributed to energy gap conversion [80]. According to the PL and PL–excitation experiments reported by Pallotti et al., the green luminescence (500 to 550 nm) in anatase TiO₂ is due to the radiative recombination of photogenerated conduction band electrons with self-trapped holes [81]. The type of point flaws in zirconia crystals determines their luminescence. Interstitial/vacancy zirconia and interstitial/vacancy oxygen are the causes of these problems. Deep defect level emission and near band edge emission can result from the defects’ creation of new energy levels in the BG area. According to these reports, the TP can stabilize as a result of increased oxygen vacancies in zirconia nanocrystals. Figure 5b displays the photoluminescence spectra of the samples under study with an excitation wavelength of 320 nm and a range of 350 to 600 nm. The ZrO₂ TF sample exhibits a broad emission peak at 433 nm and emission peaks at 532 nm. The singly ionized oxygen vacancies in the zirconia nanocrystals are responsible for the near emission in the visible range at 433 nm. Prakashbabu et al. point out that the narrow band at 430 nm could originate from the (F-F)⁺ centers which are singly ionized oxygen vacancy defects [82]. Ashraf et al., on the other hand, show that in a shorter wavelength region, a shoulder near 433 nm was also observed and associated with a transition to singly ionized associated oxygen vacancy defects [83]. Because an electron occupies the oxygen vacancy in a photogenerated holes by radiative recombination, the green emission at 532 nm is known as a deep-level or trap-state emission. Gnanamoorthi et al. observed a green emission peak at ~535 nm, which is often attributed to the radiative recombination of photogenerated holes with electrons occupying the singly ionized oxygen vacancy [84].

2.6. Contact Angle Measurements

In order to assess the wettability of a solid surface, the contact angle is often measured using a liquid droplet placed on the surface. Surface wettability is determined by the surface’s excess energy and roughness. In this way, the relative molecular interaction strengths between vapor, liquid, and solid are reflected by the equilibrium contact angle. The wettability of the TiO₂ and ZrO₂ TFs was determined from the water contact angle values over a period of time, as shown in Figure 6. All angles were taken at different times from 0 to 180 s with an increment of 20 s. Then, one minute was allowed to elapse and the last reading was taken. The final contact angle values were 78° and 81.5° for TiO₂ and ZrO₂ TFs, respectively. From these data, TiO₂ TFs showed a lower contact angle value than ZrO₂ TFs, indicating that the surface of TiO₂ TFs is characterized by a stronger hydrophilic character than ZrO₂ TFs. In addition, it should be noted that water droplets spread out better over time on TiO₂ TFs, indicating proper surface wetting [85]. Good surface wetting is a desired characteristic in self-cleaning surfaces in which the PD of organic contaminants takes place. In this context, Al-Shomar et al. carried out a study of Ellipsometric and ultrasonic titanium dioxide. They found a contact angle of 93° in their TiO₂ TFs, a value which is similar to that obtained in this study [86]. In their experiment, Purcar et al. showed that the contact angle of water on coated glass surfaces covered with TiO₂ films was 93 ± 2°, suggesting that the surface properties of the coatings can be classified as partially hydrophobic [87].

2.7. Photocatalytic Decolorization of MB

The photocatalytic performance of TiO₂ and ZrO₂ TFs was followed by means of the discoloration kinetics of an aqueous MB solution. The measured concentration of MB dye exhibited no significant changes upon UV light radiation. The concentration of MB dye, on the other hand, was rapidly reduced under UV lighting using TiO₂ TFs. Using ZrO₂ TFs, however, the degradation was less efficient, as can be seen in Figure 7a. In this way, while TiO₂ TFs reduced the absorption of MB in 70% after 300 min, ZrO₂ TFs showed a discoloration extent of 22%. Consistent with these results, Bathula et al. reported an efficient photodegradation of MB by TiO₂ films achieving discoloration values close to 50% in 180 min [88].

The PD efficiency was computed using the following equation:

$$\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)=\left({\displaystyle \frac{{\mathrm{C}}_0-\mathrm{C}}{{\mathrm{C}}_0}}\right)\times 100$$

(5)

where $\mathrm{C}$ is the dye concentration at different irradiation times and ${\mathrm{C}}_0$ is the starting concentration. Using this equation, Bathula et al. reported similar degradation close to 50% in 180 min [88]. Recently, João et al. also reported a novel apparatus for automated sol–gel solution deposition through a cost-effective spray coating technique, resulting in TiO₂ films with an anatase-like crystalline structure and uniform surface morphology that reach a degradation of almost 75% in five hours [89]. Carlos et al., in their experiment for the removal and PD of MB on ZrO₂ TFs, reported 14% discoloration after 100 min [90]; a value that is similar to that obtained in this work.

The kinetics of the dye degradation photocatalytic process were examined using a pseudo-first-order kinetic model. For this purpose, the following equation was used to fit the experimental data, and from the linearized equation, the pseudo-first-order reaction constant, k, was obtained

$$-\ln \left({\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_0}}\right)=-\mathrm{k}\mathrm{t}$$

(6)

In this equation, k corresponds to the kinetic discoloration rate constant, and t is time. Figure 7b shows the experimental and fitted data that result in the pseudo-first-order kinetic constant values of 0.0856 min⁻¹ and 0.00465 min⁻¹ for TiO₂ and ZrO₂ TFs, respectively.

Inspection of Figure 7 shows not only a reasonably good fitting of the data to a pseudo-first-order kinetic model but also an important difference in the performance of the two TFs under study. As expected, and in terms of the computed rate constants, the TiO₂ value is about 20 times larger than that exhibited by the ZrO₂ film. This difference is obviously related to a variety of semiconductor features, among which the chemical composition, purity, BG energy, surface trap density and dye adsorption properties are commonly identified. From our contact angle measurements and consistent with the fact that more hydrophobic surfaces should make the adsorption of water molecules on the TF surface more thermodynamically difficult, it is possible to suggest that photogenerated hole injection is less efficient for ZrO₂ when compared to TiO₂ films, and therefore, recombination of charge carriers and less efficient photocatalytic performance should be expected for ZrO₂ TFs.

3. Experimental Section

3.1. Substrate Cleaning

The substrates were cleaned following the method described by Jothibas et al. [91]. Initially, glass substrates (Corning 2947) underwent an extensive cleaning procedure involving ultrasonic treatment in a deionized solution for several minutes. They were then sequentially immersed in a chromic acid (H₂CrO₄) solution for three hours, followed by an additional hour in a nitric acid solution at 100 °C. After cleaning, TFs were deposited on the substrates using the DC technique, with a different dipping rate for each material.

3.2. TiO₂ TFs

For the sol–gel synthesis of TiO₂, 1.5 mL of hydrochloric acid (HCl, J.T. Baker, Phillipsburg, NJ, USA, 36.7%) was slowly added (dropwise, 30 min) to 55 mL of isopropyl alcohol ((CH₃)₂CHOH, JT Baker 99.5%) under stirring conditions. Then, 4.5 mL of Titanium(IV) isopropoxide (Ti[OCH(CH₃)₂]₄, Sigma-Aldrich, St. Louis, MI, USA, 97%) was added, followed by drop-wise addition under stirring (60 min) of 0.1 mL of distilled water (H₂O) at RT.

3.3. ZrO₂ TFs

To prepare the precursor solution necessary for the film deposition process, 1.5 mL of nitric acid (HNO₃) was poured into a 100 mL beaker, followed by the addition of 40 mL of ethanol while magnetically stirring for 15 min. To obtain a translucent yellowish solution, 9 mL of zirconium propoxide was then added to the mixture and stirred for an additional hour. The precursor solution was then aged for one day. During the coating process, the substrates were withdrawn at a rate of 2 cm/min. ZrO₂ films were deposited using five coating layers, with each layer being dried in an open-environment oven for three minutes. After applying the required number of coatings, the films were dried at 250 °C for 30 min and subsequently annealed at 500 °C.

3.4. Structural, Optical, and Morphological Characterization of the Films

UV-Vis measurements were carried out using an Evolution 220 UV-Vis Spectrophotometer. X-ray diffraction experiments, on the other hand, were performed using a Philips X-ray diffractometer (PANalytical’s X’pert PRO X-ray diffractometer), that employed a Cu-Kα radiation with a wavelength of 0.154 nm in the 20° ≤ 2θ ≤ 80° range. The voltage and current settings were 30 kV and 40 mA, respectively. The samples were continuously scanned with a step size of 0.02° (2θ) and a count time of 1 s per step. Structural properties were also studied using Raman spectroscopy, which collected data using a Labram Dilor Raman spectrometer equipped with a He-Ne laser exciting source operating between the wavelengths of 150 and 800 nm at ambient temperature (AT). Using a scanning electron microscope (SEM, JEOL JSM-6300), surface images were acquired. The UV–visible transmittance spectrum (Thermo Fisher Scientific, Billerica, MA, USA) of TFs was recorded using a Thermo Scientific™ (Waltham, MA, USA) Evolution 220 UV-Vis Spectrophotometer. Photoluminescence (PL) data were obtained at RT by optical excitation with a He-Cd laser, and luminescence was registered employing a Horiba/Spex 1404 0.85 m Double Spectrometer. The hydrophilic properties of the TFs were evaluated by a contact angle measuring instrument (Dataphysics, model OCA 50, Philips, Warsaw, Poland) on which a 5 µL droplet of deionized water was carefully positioned on the surface of the TFs at RT.

3.5. Photocatalytic Activity Evaluation

The kinetics of MB discoloration in aqueous solutions under UV light were utilized to measure the photocatalytic activity (PA) of the films under study at RT. A standard quartz cell was filled with 3 mL of MB solution, into which a sample TF with an area of 2 cm² was introduced. Five replicates were prepared for each TF being studied. The irradiation light was provided by a 15 W lamp with a wavelength of 254 nm (using a G15T8 germicidal lamp (Philips, Warsaw, Poland) as the excitation source). The distance between the quartz cells and the lamp was maintained at 5 cm. Every hour, the lamp was turned off, a quartz cell was taken, and subsequently the lamp was turned on; the TF was removed from the quartz cell and the dye concentration in the quartz cells without the TFs was analyzed by UV-Vis. This process was repeated 4 more times.

4. Conclusions

In conclusion, TiO₂ and ZrO₂ TFs were prepared and thoroughly characterized using spectroscopy, microscopy and hydrophobicity measurements. The results revealed that, as expected, both films are characterized by a similar energy BG, porosity, thickness and homogeneity. The two films under study are also characterized by substantially different photocatalytic activities for the discoloration of a model dye aqueous solution. The observed difference is consistent with PD experiments reported by other authors and is probably due to differences in the e⁻/h⁺ generation and recombination rates, as well as in the efficiency of the TF materials to produce OH radicals that result from hole injection to water adsorbed molecules. In this regard, the differences observed in the surface hydrophilicity of the TiO₂ and ZrO₂ TFs under study are probably related to the hole injection kinetic performances of the two materials and, therefore, to the different photocatalytic activity that was found in the dye discoloration experiments.