Effect of Iron-Doping on the Structure and Photocatalytic Activity of TiO2 Nanoparticles

Abstract

This research reports a simple, innovative, and low-cost doping method of TiO₂ nanoparticles presenting the effects of calcination and the weight ratio of TiO₂:FeCl₃ (1:0.33–1:4.5). The photocatalytic activity of the nanomaterials was investigated by decolorizing Rhodamine B (RhB) dye in an aqueous solution. The main results showed that there is anatase-to-rutile transformation after the calcination process. The Fe-doped process modified the TiO₂ spectrum and showed a connection in the Ti–O–Fe vibration. The particle size is within the nanometer range, between 20–51 nm, except for calcined TiO₂. The inclusion of Fe in TiO₂ decreased the band gap energy from 3.16 (reference) up to 2.06 eV (1:3). Additionally, after the calcination, there was a decrease in this value from 3.03 eV (reference) up to 1.95 eV (1:1.6). The TiO₂, with a ratio of (1:1.6), showed the highest activity in the photocatalytic degradation of RhB with an efficiency of 93.8% after 3 h of irradiation.

1. Introduction

Titanium dioxide (TiO₂) is a widely studied semiconductor because of its optical and electronic properties, good chemical stability, availability, and low-cost [1,2,3,4,5,6,7]. These characteristics cause TiO₂ to be one of the most promising photocatalysts for treating various environmental pollutants. In large-scale and practical applications, the use limitations of TiO₂ as a material for the photocatalytic process [5] are mainly 3: TiO₂ absorbs only a small part of sunlight in the visible region, making photocatalytic activity at wavelengths less than 400 nm [5,8], the low quantum yield is due to the easy matching of photogenerated electron pairs, and its band gap energy (E_g) range of 3.0–3.2 eV, which is quite large [9,10,11].

TiO₂ is an extrinsic n-type semiconductor in three crystallographic structures: anatase, rutile, and brookite. The anatase and rutile phases are the most frequently used in photocatalytic processes. The low recombination rate of its photogenerated electrons and holes on the TiO₂ surface causes the anatase phase to show better photoactivity [12,13,14].

An acceleration of the chemical reaction is required to occur photocatalysis. Their shortened band gap energy and distinct electronic structure cause semiconductor materials such as TiO₂, ZnO, CeO₂, Cds, ZnS, Fe₂O₃, and others to be chosen as photocatalysts. When these semiconductors are stimulated by solar irradiation, energy levels higher than their band gap energy must be reached [6,15].

Doping in TiO₂ nanoparticles changes the electronic structure of TiO₂ by modifying their chemical composition and optical properties [16]. To decrease the band gap energy down to wavelengths of visible light, doping with metal ions, such as iron, nickel, chromium, zinc, platinum, and manganese, among others, has been studied. Research aimed at decreasing the recombination rate of the e⁻/h⁺ pair and thus increasing the yield of the photocatalytic process by modifying the the semiconductor surfaces with the addition of metal ions [5,9,14,16,17,18,19,20,21,22]. Doping with metal ions increases the recombination time of the e⁻/h⁺ pair making the holes accessible for forming •OH radicals [15].

Sunlight comprises visible and infrared photons, with only 3–5% of the ultraviolet (UV) range. More research is underway on doping TiO₂ nanoparticles to increase photocatalytic efficiency [4,5,6,23,24] which will explain how the physical and chemical properties of TiO₂ determine the photocatalytic activity and develop affordable and appropriate synthesis methods to obtain TiO₂ with superior photocatalytic efficiency [5,14,17,25].

The inclusion of iron (Fe³⁺) allows a broad absorption of the solar spectrum in the visible region. Consequently, the potential for photocatalytic activity is increased since the radius of Fe³⁺ (0.64° A) and Ti⁴⁺ (0.68° A) are very close in terms of size [8,16,26]. Thus, doped-TiO₂ is essential for outdoor applications to maximize photocatalytic efficiency [4,27,28]. For synthesizing Fe-doped TiO₂, different methods are currently used, such as sol-gel [29,30], ultrasonic radiation-assisted hydrothermal [31], molten salt [6], wet-chemical synthesis [32], and co-precipitation [18], among others. For example, Ghorbanpour et al. (2019) synthesized and characterized Fe-doped TiO₂ nanoparticles prepared by the molten salt method and calcination process. The band gap energy of the Fe-doped TiO₂ samples decreased with increasing Fe concentration from 3.1 eV for pure TiO₂ to 3.02–2.80 eV for Fe-doped TiO₂. Thus, the photocatalytic activity was higher for the Fe-doping content of 0.5 wt.% than for pure TiO₂ [6]. Another process was addressed by Carneiro et al. (2014), who investigated the incorporation of Fe into TiO₂ nanoparticles through a ball milling process using stainless steel balls. They concluded that there are improvements in photocatalytic activity (over 60% at about 120 min) and higher rotational speeds [5].

Moreover, Ganesh et al. (2012) studied different amounts of Fe-doped TiO₂ by the co-precipitation method. They found that small amounts of Fe (0.1 wt.%) are sufficient to transform TiO₂ from the rutile phase to the full anatase phase and achieve higher photocatalytic activity [18]. From another approach, Adamek et al. (2012) compared the results of photocatalytic degradation with the presence of TiO₂ or Fe (III) salts as well as a mixture of TiO₂ with Fe (III), more properly with FeCl_3. They concluded that the photodegradation rate constant (k) of TiO₂/FeCl₃ (pH = 3) is approximately four times the rate of the undoped TiO₂. Consequently, adding FeCl₃ to TiO₂ nanoparticles significantly increases the photocatalytic activity by UVA irradiation in the 400–320 nm range after 120 min of stirring [8].

This paper investigated the process of TiO₂ doping with different concentrations of iron chloride (FeCl₃) via the co-precipitation method. This study is relevant as it aims to improve on and innovate the photocatalysis processes since different percentages of FeCl₃ in TiO₂ are studied using a simple method. Additionally, this method is low-cost in terms of doped TiO₂ production that will used as an efficient photocatalyst when activated under UV radiation and visible light, and will therefore be used to, for example, capture pollutants. Hereafter, the optical, structural, chemical, morphological, and photocatalytic properties of Fe-doped TiO₂ are presented and explored before and after calcination.

2. Results and Discussion

2.1. Fourier-Transform Infrared Spectroscopy (FTIR)

The FTIR spectra of the undoped TiO₂ (reference), FeCl₃ (reference), and TiO₂ doped with various concentrations of FeCl₃ before and after the calcination process are shown in Figure 1. The H–O–H vibrational bonds are related to the peaks at 3803, 3382, and 3439 cm⁻¹, and the H₂O vibrational bonds are indexed to the peaks at 1626 cm⁻¹ [33]. These indicate the presence of absorbed hydroxyl groups in the samples [34]. In the photocatalytic process, all oxygen-containing groups play an influential role in the photocatalytic activity and are also capable of generating more hydroxyl radicals [35]. The Ti–O–Fe vibration is present at the peak of 2285 cm⁻¹, which is a bond from the doping process [36]. The peaks observed in the range 932–826 cm⁻¹ are due to the Ti–O group [37].

Concerning the TiO₂ reference spectrum, the Ti–O–Ti bond and the Ti–O stretching vibration correspond to the 800–1200 cm⁻¹ peaks. Regarding the Fe-doped TiO₂ peaks, the O–Ti–O vibration and the Fe–O–Fe symmetric stretching vibration are at 1389 cm⁻¹. The peak observed at 662 cm⁻¹ proves the existence of iron oxyhydroxide Fe–O–OH. The similarity between the peak values of the chemical bonds below 1000 cm⁻¹ means that the Fe-O and Ti–O bending frequency overlap may occur.

Thus, for the different TiO₂:FeCl₃ ratios, there are no significant differences in the material chemical bonds either before or after the calcination process. Nevertheless, the Ti–O–Fe vibration and the hydroxyl groups decrease with the calcination process.

2.2. X-ray Diffraction (XRD)

Figure 2 shows the XRD patterns of the doped TiO₂ nanoparticles before and after calcination. For each of the samples synthesized, there are no significant differences between the XRD patterns and TiO₂ nanoparticles (reference). This means that the fraction of the anatase phase in the doped TiO₂ nanoparticles has the same magnitude as TiO₂ (reference). Therefore, the doping process has no relevant influence on anatase-to-rutile transformation (ART), but after calcination, there is an ART.

It is generally observed that the diffraction peaks become slightly broader after the calcination of Fe-doped TiO₂. Their relative intensities decrease for the anatase phase and increase for the rutile phase. The XRD spectrum peaks of the anatase phase, crystal planes (101), (105), and (200), and the diffraction planes (110) and (101) of the rutile phase were chosen to determine the average crystallite size, lattice parameters and weight fraction of anatase phase in the doped and calcined powders.

After the calcination process, the new peak in the XRD spectrum at 2θ = 32.56° refers to FeTiO₃ (ilmenite). The intensity of this peak is the same for the increasing concentration of FeCl₃ in TiO_2, as observed by Ganesh et al. (2007). These researchers prepared Fe-doped TiO₂ powders by the co-precipitation method and cited the appearance of small amounts of secondary phases α-Fe₂O₃ and FeTiO₃ in their doped samples [18]. In the present study, only FeTiO₃ appeared in the calcination process after TiO₂ doping. On the contrary, Ghorbanpour et al. (2019) studied iron-doped TiO₂ at various concentrations with a calcination process. They reported that no iron-related peaks appeared, i.e., there was no solid iron-titanium solution [6]. The reason for this behavior and the appearance of these new peaks are yet to be fully understood.

As shown in Table 1, before the calcination process, relative to the anatase phase, the crystallite size of TiO₂ (reference) nanoparticles is about 18.49 nm. The Fe-doped TiO₂ nanoparticles′ sizes were 19.40 nm, 19.85 nm, and 19.68 nm, i.e., for the ratios of TiO₂-FeCl₃ at (1:1), (1:1.6), and (1:3), respectively. For the rutile phase, the crystallite size of TiO₂ nanoparticles (reference) is about 29.75 nm. The increasing Fe-doped concentration ranges from 22.05 nm to 25.74 nm, respectively. After the calcination process, the crystallite size of the calcined TiO₂ nanoparticles was about 18.53 nm, i.e., there was no significant difference between the TiO₂ (reference) and the calcined TiO₂.

With the addition of Fe-doped, the crystallite size increased with increasing concentration, from 19.40 nm to 29.93 nm. Thus, for the anatase phase, there is a significant difference between the crystallite size for the doped material with the material after calcination. However, for the rutile phase, crystallite size increases compared to the doped material with the calcined one. In the rutile phase, the TiO₂ doped nanoparticles after the calcination process were about 32.28 nm. With the increase of FeCl₃ concentration, the crystallite size changed between 26.36 nm to 27.02 nm. Compared to the calcined TiO₂, the crystallite size is smaller. Thus, there was an anatase-to-rutile transformation (ART) for the calcination process. Since the ionic radii of titanium and iron are close (Ti⁴⁺ (0.68 Å) and Fe³⁺ (0.64 Å)), the incorporation of the iron ion into the crystal structure of TiO₂ in the anatase phase may occur [6,18].

The lattice parameters were calculated and compared with the crystallographic data of reference TiO₂ collected from the JCPDS International Tables for Crystallographers No. 21-1272 for the anatase phase and No. 21-1276 for the rutile phase. Thus, for the anatase phase, parameter a is 3.7852 Å, and parameter c is 9.5139 Å; for the rutile phase, the parameter a is 4.5933 Å, and the parameter c is 2.9592 Å.

For the set of samples studied, doped and after calcination, the values obtained for parameter a in the anatase phase range from 3.7424 Å to 3.7907 Å, and for the lattice parameter c, range from 9.3839 Å to 9.7596 Å. These parameters differ slightly from the theoretical values. The lattice parameter a ranges from a maximum decrease of 1.13% to an increase of 0.15%. The lattice parameter c ranges from a maximum decrease of 1.37% to a maximum increase of 2.58%. Regarding the rutile phase, parameter a ranges from 4.5717 Å to 4.5946 Å, and parameter c from 2.9288 Å to 2.9644 Å. Similarly, the lattice parameter a range from a maximum decrease of 0.47% to a maximum increase of 0.03%. The parameter c ranges from a maximum reduction of 1.03% to a maximum increase of 0.07%. This may refer to the displacement of atoms from their ideal locations [38]. For each sample, there is no significant difference in the lattice parameters, anatase and rutile phases, and unit cell volume (ų).

Finally, regarding the fraction of the anatase phase before the calcination process, TiO₂ (reference) showed a fraction of 80.82%. In the Fe-doped synthesized samples, the ratios (1:1), (1:1.16), and (1:3) showed a fraction of 87.95%, 92.90%, and 60.27%, respectively. After the calcination process, there is a marked decrease in calcined TiO₂ (37.31%), compared to TiO₂ (reference), and for the different ratios of (1:1), (1.16) and, i.e., 37.16%,42.30%, and 41.69%.

Before the calcination process, the different TiO₂:FeCl₃ ratios show similar behavior for the crystallite size in the anatase phase and a slight difference for the rutile phase. Additionally, they show no significant differences in the lattice parameters and unit cell volume compared to TiO₂ (reference). After the calcination process, the different ratios TiO₂:FeCl₃ show an increase in crystallite size, a decrease in unit cell volume, and a sharp decrease for the anatase phase fraction, with X_A of about 37–41%.

2.3. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

SEM was employed to investigate the homogenization and dispersion of the samples. EDS was performed to analyze their chemical characterization, i.e., elemental analysis of each material. For the analysis, the following samples were selected: (a) TiO₂ (reference), (b) calcined TiO_2, and in the different ratios of TiO₂:FeCl₃ doped (c) (1.16), (d) calcined (1:1.6), and (e) (1:3). Figure 3 presents the SEM microscopy representations and the EDS spectrum.

For the reference TiO₂ (Figure 3a) it is possible to verify the nanometer scale size nanoparticles of TiO₂, ranging from 20 to 30 nanometers; for the reference TiO₂, after the calcination process, the size of the particles is approximately 50 nm (Figure 3b), indicating that the particles have practically doubled their diameter. Additionally, the surface of the particles became smoother and more regular before the calcination process.

Concerning the TiO₂ doped with FeCl₃ at the ratio of (1:1.6), it can be inferred that iron is present in the sample, and the positions of the spectrum’s peaks confirm it. Furthermore, the particle size remains similar to the reference TiO₂ (Figure 3c). Since the different particles of TiO₂ and Fe cannot be observed separately, there are indications of a chemical reaction between them in the doping process; they become single particles, as indicated. Iron is not distributed heterogeneously into the TiO₂ nanoparticles. It is also noticed that there are Cl residues in the spectrum of this sample. After the calcination process (Figure 3d) the same magnitude of particle size follows as the calcined TiO₂. A change in surface appearance to smoother and more regular than before the calcination process was observed. The disappearance of residual Cl occurred, which may be caused by the washing step of the doping process.

Finally, the (1:3) ratio of Fe-doped TiO₂ was analyzed before the calcination process (Figure 3e). It can be observed that the particle dimensions remain with the same size magnitude as the non-calcined reference TiO₂ particles, and there is a residual amount of Cl remaining from the washing step of the doping process. In this case, there are also indications of the chemical reaction between TiO₂ and Fe.

To summarize the results, Table 2 presents each sample’s chemical composition and particle size. The particle sizes show some variation, as observed in the XRD. The TiO₂ (reference) has a particle size between 20–30 nm and is composed of 65.49% titanium and 34.51% oxygen. With respect to calcined TiO₂, the size increases, i.e., ranging between 50–108 nm, but the chemical composition is similar to TiO₂ (reference). Regarding the different TiO₂:FeCl₃ ratios, the particle size varies between 23–27 nm, and 27–38 nm, for ratios (1:1.16) and (1:3), respectively. After the calcination process at the ratio (1:1.6), the size also increased (49–51 nm). In the different proportions, there is the appearance of iron (Fe) with 14.00 to 17.95% and chlorine (Cl) with 1.85 to 2.48%. In the proportion (1:1.6) after the calcination process, chlorine is no longer present in the chemical composition.

Thus, before the calcination process, all TiO₂:FeCl₃ ratios show a similar particle size. After the calcination process, an increase in particle size occurs that is caused by the anatase-to-rutile transformation (ART), as previously discussed in XRD test results section.

2.4. Diffuse Reflectance Spectroscopy (DRS)

To study the effectiveness of TiO₂ doping with FeCl₃, the band gap of the semiconductor nanoparticles was analyzed using diffuse reflectance spectroscopy (DRS) and the Kubelka–Munk transform. The measurement of the light absorption of each material doped with different concentrations of FeCl₃ and TiO₂ (reference) before and after the calcination process (Figure 4) was obtained using the Kubelka–Munk function. The results show that both materials, calcined or uncalcined reference TiO₂, depicts strong photoabsorption at wavelengths below 400 nm, i.e., in the ultraviolet (UV) and visible regions. For all concentrations, before and after calcination, the doped TiO₂ shows a shift of the absorption edge that is caused by the effect of iron (i.e., dashed curves). However, as intended, changes in the optical properties of the material occur in the process of doping with Fe.

These shifts of the absorption edge to a longer wavelength indicate the band gap energy (E_g) of TiO₂ and an increase of the absorption into the visible light region, which is less energetic than the UV region. The values of the Kubelka–Munk transformation versus Energy (eV) can be seen in Figure 5. A tangent line to the inflection point was drawn on the curve to obtain the value of E_g.

It can be observed that the E_g values for the undoped TiO₂ (reference) non-calcined (3.16 eV) and calcined (3.03 eV) are higher if compared to the doped TiO₂ with FeCl₃ in different concentrations. For both calcined or not calcined situations, the E_g values were always the lowest for the concentrations (1:1), (1:1.6), and (1:3). After the calcination process, the E_g values tend to decrease, except for the situation (3:1). Before calcination, the concentrations (1:1), (1:1.6), and (1:3) presented E_g values of 2.08, 2.21, and 2.06 eV, respectively. After calcination, the E_g of these situations became 2.10, 2.11, and 2.03 eV, respectively. After the calcination process, there is a strong tendency for E_g to decrease, which may be related to the ART, i.e., E_g (rutile) < E_g (anatase), to the formation of a dopant energy level (Fe⁴⁺/Fe³⁺), which comes in the excitation of Fe³⁺ and also due to the increase in particle size.

Therefore, the incorporation of Fe³⁺ into the TiO₂ lattice causes a decrease in the energy band gaps. This effect is observed comparing the reference TiO₂ with the TiO₂-FeCl₃ ratio (1:1). Its bang gap energy decreases from 3.16 eV to 2.08 eV.

2.5. Evaluation of the Photocatalytic Activity of the Doped Material

Figure 6 presents the results of photocatalytic efficiency before and after the calcination process. After 3 h of irradiation, the samples after calcination showed a photocatalytic efficiency of less than 70%, except for the calcined TiO₂ (74.0%). Regarding the samples synthesized after the calcination process, the different ratios of TiO₂:FeCl₃ (3:1), (1:1.1), (1:1.16), (1:3), and (1:4.5) showed lower photocatalytic efficiency, i.e., 31.1%, 36.7%, 45.5%, 36.3%, 32.6%, respectively. In contrast, all the samples synthesized before the calcination process show a photocatalytic efficiency higher than 70%. The synthesized sample with the best photocatalytic efficiency was TiO₂:FeCl₃ (1:16), with a percentage of 93.8%. The reference TiO₂ showed a similar photocatalytic efficiency, i.e., 92.43%. The remaining samples, TiO₂:FeCl₃ (3:1), (1:1), (1:3), and (1:4.5), showed 88.7%, 83.2%, 74.5%, and 83.6%, respectively.

The subsequent use of photocatalysts to determine photocatalytic efficiency is a performance measure. For this purpose, the TiO₂:FeCl₃ (1:1.16), as a better catalyst, and the TiO₂ (reference) samples were submitted to four recycling cycles. As shown in Figure 7, the RhB degradation occurred in all cycles, indicating that both samples were still active during the final cycle. About 10%, 15%, and 20% of the photocatalyst were lost during the filtration and cleaning processes after cycles 1, 2, and 3, respectively. Thus, the loss of the photocatalyst may have contributed to the degradation rate decrease, from 93.8% to 88.1% and from 92.4% to 87.3% for TiO₂:FeCl₃ (1:1.16) and TiO₂ (reference), respectively.

Through XRD it was observed that there is an ART after the calcination process, which may influence the photocatalytic efficiency. In SEM analyses, a decrease in the surface area was observed, which also influences the photocatalytic efficiency. According to Kim et al. (2014), this occurs because the diffusion radius of the •OH radical is generated in the anatase phase and not in the rutile phase. Consequently, photocatalytic oxidation in the rutile phase is restricted since the reaction zone of the •OH radical is restricted to the surface in the rutile phase, and in the anatase phase, the diffusion radius of the •OH radical is larger. Thus, the anatase phase has a higher photocatalytic activity than the rutile phase [39].

Although some samples showed similar photocatalytic efficiency during the 3 h period, their absorption spectra are presented in Figure 8. In fact, the TiO₂ (reference) and doped TiO₂-FeCl₃ (1:1.16) ratio material showed a photocatalytic efficiency of 92.4% and 93.8%, respectively. Through the absorption curves, it can be seen that the RhB molecules are likely to have undergone a more advanced degradation process for the TiO₂-FeCl₃ (1:1.16) sample than for TiO₂ (reference). Thus, it can be concluded that at the ratio (1:1.16), a higher and better photocatalytic efficiency occurs.

3. Materials and Methods

The materials used were TiO₂ semiconductor nanoparticles (Aeroxide TiO₂ P25) purchased from Quimidroga (Barcelona, Spain), and their main properties were a purity >99.5%. The Iron (III) chloride (FeCl₃) and Rhodamine B were purchased from Sigma-Aldrich (Lisboa, Portugal). Distilled water was used as a solvent.

An iron (Fe³⁺) doping process was carried out using semiconductor nanoparticles. First, solutions containing a suspension of TiO₂ in distilled water (0.01 g/L) were prepared, and aqueous solutions containing different concentrations of FeCl₃ were based on the concentration of TiO₂. Subsequently, both solutions were mixed and stirred at 70 °C for 150 min, and then they were filtered and washed with distilled water. Finally, the samples were dried at 60 °C to obtain a solid material [10].

The TiO₂:FeCl₃ concentrations selected to reach the lowest band gap energy reduction were (3:1), (1:1), (1:1.6), (1:3), and (1:4.5). For example, the ratio (1:3) means that the sample was made with a concentration of 0.01 g/mL of TiO₂ and 0.03 g/mL of FeCl₃. The particle identification (TiO₂:FeCl₃) was added to their concentration ratio to identify the testing samples. Subsequently, the powders (doped and undoped TiO₂ nanoparticles) were subjected to a calcination process (3 h at 700 °C) [6].

The band gap of the semiconductor nanoparticles was initially analyzed using diffuse reflectance spectroscopy (DRS) and Kubelka–Munk transform to study the effectiveness of doping and calcination. Subsequently, x-ray diffraction (XRD) was used to identify the crystalline phase of each material, as well as lattice parameters, crystal cell volume, and anatase phase fraction. Fourier transform infrared spectroscopy (FTIR) was performed to analyze the chemical composition of the doped materials. Scanning electron microscopy (SEM) was performed to analyze the homogenization and particle size of the samples. Energy dispersive spectroscopy (EDS) was performed to obtain the chemical composition of the doped materials. The doped semiconductor nanoparticles were subsequently immersed in RhB solution to analyze the photocatalytic activity under a sunlight simulator.

3.1. Diffuse Reflectance Spectroscopy (DRS) and Kubelka–Munk Transform

In the first stage, to determine the band gap energy of the reference concentration of TiO₂ (undoped) and the various concentrations of doped TiO₂-FeCl₃ before and after the calcination process, Ultraviolet-Visible diffuse reflectance spectroscopy absorption measurements were performed. This analytical method is similar to the usual UV-vis spectroscopy, except that it is reflected into an integrating sphere and collected instead of transmitting light. Parallel to transmittance for liquids, reflectance, R, is advantageous for quantifying the amount of light reflected on solid surfaces.

The parameter called Kubelka–Munk function, F(R), was established to determine the light absorption [40]. Afterward, using the Kubelka–Munk transform, it was possible to calculate the band gap energy (E_g) [41,42].

3.2. X-ray Diffraction (XRD)

XRD was used to determine the crystallite phase of TiO₂-doped materials using x-ray diffraction with a CuKα source from a Philips PW 1710 X-ray diffractometer (Billerica, MA, USA).

The Debye–Scherrer equation was employed to calculate the crystallite size of the doped particles before and after the calcination process [43]. The value of the lattice spacing for each of the selected peaks in the XRD spectrum was estimated from Bragg’s law [5]. To determine the fraction of the anatase phase in TiO₂ doped powders, the intensity of the first and most intense XRD peaks of each sample containing the mixture of the two phases were used according to [44].

3.3. Fourier-Transform Infrared Spectroscopy (FTIR)

To check the chemical composition of the doped materials, i.e., to identify the chemical bonds existing in the doping of TiO₂ with FeCl₃ in the spectral range from 400 cm⁻¹ to 4000 cm⁻¹, Shimadzu IR-Prestige-21 spectrometer (Kyoto, Japan) was used. The most relevant elements, the chemical bonds in undoped TiO₂ (reference) and different concentrations of TiO₂-FeCl₃ after the calcination process, were pointed out.

3.4. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

Scanning electron microscopy (SEM) (Houston, TX, USA) was used to analyze the homogenization and dispersion of the doped materials. Energy dispersive spectroscopy (EDS) (Austin, TX, USA) was used to verify their chemical characterization, i.e., what chemical elements and their concentrations in the sample. These analyzes had the main objective of ascertaining the incorporation of FeCl₃ in TiO₂ nanoparticles in the doping process.

3.5. Photocatalytic Activity

The photocatalytic efficiency of the doped material was analyzed by Rhodamine B (RhB) degradation. First, 50 mg of nanoparticles were immersed in 50 mL of 2 ppm aqueous RhB solution, and then they were placed in a box 25 cm below a sunlight simulation lamp with a power intensity of 11 W/m². The samples were conditioned in the dark for 80 min and then exposed to light for 3 h. Therefore, the initial adsorption and photocatalysis were split, and this phenomenon could be accurately analyzed. To prevent evaporation of the RhB solution, the systems were covered with a transparent cling film with less than 10% absorbance and reflectance (between 292 and 900 nm), allowing the almost total light transmission to the samples [45]. The photocatalytic degradation of the RhB solutions was monitored through their maximum absorbance values obtained at different time intervals [5,46,47]. During the test, aliquots of 5 mL were withdrawn from the systems and then centrifuged at 6000 rpm for 30 min to obtain decantation of the doped material. Subsequently, aliquots of 5 mL were taken from the centrifuged dispersions, and their absorbance was then measured using a spectrophotometer (SanSpecUV-Vis) in a wavelength range of 400 to 800 nm. The samples’ photocatalytic efficiency was calculated over time to assess their performance and select the best concentration of TiO₂-FeCl₃ regarding photodegradation.

To calculate the photocatalytic efficiency, the maximum absorbance (554 nm) of the dye (RhB) was monitored as a function of time (using a Shimadzu 3101 PC) and determined according to [48]. The best samples considering the photocatalytic efficiency will be submitted to four cycling experiments at the same conditions as the initial tests in order to analyze the stability of the prepared photocatalysts.

4. Conclusions

The main objective of this research was to assess doped TiO₂ semiconductor nanoparticles with different contents of FeCl₃, before and after calcination, through their optical, structural, morphological, chemical, and photocatalytic properties. The following conclusions can be drawn from the results obtained:

• The process of doping TiO₂ with FeCl₃ provided changes in the optical properties of the material and a decrease in the E_g of TiO₂, also after the calcination process. The doping concentrations that presented the lowest E_g values were (1:1), (1:1.6), and (1:3).

• Contrarily to the effect after calcination, in the doping process the nanoparticles have no significant influence on the anatase-to-rutile transformation (ART) compared to the reference TiO₂.

• The Fe-doping process modified the reference TiO₂ spectrum with higher intensity of hydroxyl bonds and vibration of the Ti–O–Fe bond. After the calcination process, a drastic reduction of these bonds occurred.

• The particles are within the nanometer scale, and there are indications of the chemical reaction between TiO₂ and Fe. The calcination process causes a relevant increase in particle size and surface smoothing.

• The ratio (1:1.6) of TiO₂:FeCl₃ showed the highest activity in the photocatalytic degradation of RhB with an efficiency of 93.8% after 3 h of irradiation.

In conclusion: doping TiO₂ nanoparticles using metal ions (e.g., Fe³⁺) will improve photocatalytic activity under visible light irradiation, becoming a promising material for low-cost photocatalytic processes. However, further investigations for industrial-scale applications are needed to characterize the material’s durability in photocatalytic processes. The doped TiO₂ will be applied in different materials for self-cleaning processes, for air purification, e.g., in polymeric materials, ceramics, textiles, and also in civil engineering (e.g., in cementitious materials and asphalt mixtures).

Author Contributions

Conceptualization, C.A., O.L.J. and I.R.S.; methodology, I.R.S., S.L.J., E.F., M.P. and J.C.; validation, I.R.S., S.L.J. and M.P.; formal analysis, C.A., O.L.J., N.H. and I.R.S.; investigation, C.A., I.R.S., S.L.J. and J.C.; resources, J.C., M.F.M.C. and E.F.; data curation, S.L.J. and I.R.S.; writing—original draft preparation, C.A., O.L.J., I.R.S. and S.L.J.; writing—review and editing, É.M., N.H., E.F., M.F.M.C. and J.C.; visualization, I.R.S., S.L.J. and O.L.J.; supervision, I.R.S., E.F. and J.C.; project administration, E.F. and J.C.; funding acquisition, E.F., M.F.M.C. and J.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. FTIR spectra for the TiO2 (reference), FeCl3 (reference), and the samples synthesized before calcination (left) and after calcination (right).

Figure 2. X-ray diffraction patterns of doped and undoped TiO2 before and after calcination: (a) 1:3 and 1:1 and (b) 1:1.6 and reference.

Figure 3. SEM micrograph representation (left) and EDS spectrum (right) of: (a) reference TiO2, (b) calcined TiO2, (c) TiO2-FeCl3 (1:1.6), (d) calcined TiO2-FeCl3 (1:1.6) and (e) TiO2-FeCl3 (1:3).

Figure 4. Kubelka–Munk function versus wavelength for the undoped TiO2 (reference) and the samples synthesized before calcination (left) and after calcination (right).

Figure 5. Plot of Kubelka–Munk transform versus the Energy of the light absorbed for the undoped TiO2 (reference) and the samples synthesized before calcination (top) and after calcination (bottom).

Figure 6. Photocatalytic efficiency for TiO2 (reference), calcined TiO2, and the synthesized samples before calcination (left) and after calcination (right).

Figure 7. Recycling of TiO2:FeCl3 (1:1.6) and TiO2 (reference) for degradation of RhB dye.

Figure 8. Absorbance degradation curves of RhB solution after 3 h of exposure to solar irradiation for the different ratios of doped TiO2:FeCl3 and TiO2 (reference). Where RhB_0 is the RhB solution (2 ppm) at time 0 not irradiated.

Table 1

Influence of doped material before and after calcination as to the on average crystallite size, lattice parameters, unit cell volume, and anatase phase fraction for doped TiO₂ nanoparticles.

Composition	Crystallite Size (nm)		Lattice Parameters (Å)				Unit Cell Volume (Å3)		XA (%)
	Anatase	Rutile	Anatase		Rutile		Anatase	Rutile
			a	c	a	c
TiO2 (reference)	18.4879	29.7536	3.7907	9.5154	4.5946	2.9613	136.7307	62.5141	80.82
Calcined TiO2	18.5277	32.2752	3.7531	9.4134	4.5717	2.9288	132.5949	61.2132	37.31
TiO2-FeCl3 (1:1)	19.4046	22.0470	3.7832	9.4710	4.5913	2.9605	135.5547	62.4074	87.95
Calcined TiO2-FeCl3 (1:1)	28.1559	26.3592	3.7523	9.4259	4.5814	2.9558	132.7144	62.0399	37.16
TiO2-FeCl3 (1:1.6)	19.8477	22.6825	3.7833	9.7596	4.5847	2.9595	139.6927	62.2071	92.90
Calcined TiO2-FeCl3 (1:1.6)	28.8106	27.4820	3.7447	9.4408	4.5864	2.9557	132.3862	62.1733	42.30
TiO2-FeCl3 (1:3)	19.6765	25.7432	3.7855	9.3839	4.5913	2.9644	132.5812	62.4897	60.27
Calcined TiO2-FeCl3 (1:3)	29.9340	27.0145	3.7424	9.4242	4.5814	2.9558	131.9912	62.0399	41.69

Table 2

Particle size variation and characterization of each sample studied.

Samples	Particle Size (nm)	Chemical Composition (%)
		Ti *	O *	Fe *	Cl *
TiO2 (reference)	20–30	65.49	34.51	-	-
Calcined TiO2	50–108	66.79	33.21	-	-
TiO2-FeCl3 (1:1.6)	23–27	44.98	39.14	14.02	1.85
Calcined TiO2-FeCl3 (1:1.6)	49–51	50.69	35.31	14.00	-
TiO2-FeCl3 (1:3)	27–38	37.80	41.77	17.95	2.48

* Ti: Titanium; O: Oxygen; Fe: Iron; Cl: Chlorine.

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