Preparation of Magnetic Photocatalyst Fe₃O₄@SiO₂@Fe-TiO₂ and Photocatalytic Degradation Performance of Methyl Orange in Na₂SO₄ Solution

Abstract

In this study, Fe₃O₄@SiO₂@TiO₂ (FS-FT (0 g)) photocatalysts, featuring a magnetic core–shell structure, and Fe-doped Fe₃O₄@SiO₂@Fe-TiO₂ (FS-FT (x g)) photocatalysts, were fabricated via the sol–gel method. Structural and compositional analyses of the processed samples were systematically conducted through X-ray diffraction (XRD), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), surface-sensitive X-ray photoelectron spectroscopy (XPS), and optical property assessment via UV-Vis diffuse reflectance spectroscopy (UV-DRS). The results show that TiO₂ on the outer layer of FS-FT (0 g) and FS-FT (x g) has an anatase structure, and that Fe is doped into FS-FT (x g). The photodegradation of methyl orange (MO) using FS-FT (0 g) and FS-FT (x g) with various Fe doping levels was evaluated in both pure MO (C₀ = 10 mg/L) and MO-Na₂SO₄-blended solutions. Under irradiation with high-pressure mercury lamps, the removal rates of MO using FS-FT (0 g) and FS-FT (0.36 g) in pure MO solution reached 90.25% and 99% at 25 min, respectively, which indicates that FS-FT (0.36 g) can enhance photocatalytic performance. The removal rates of MO using FS-FT (0 g) and FS-FT (0.36 g) in MO-Na₂SO₄-blended solution (C₀ = 10 mg/L, C_Na2SO4 = 12.5 g/L) reached 92.38% and 97.16% at 25 min, respectively. The removal rate of MO using FS-FT (0.36 g) decreased in MO-Na₂SO₄-blended solution in the previous 25 min, which indicates that Na₂SO₄ can inhibit degradation using FS-FT (0.36 g). The degradation experiments of MO-Na₂SO₄-blended solutions with different concentrations of Na₂SO₄ using FS-FT (0.36 g) showed that as the concentration of Na₂SO₄ increases, the inhibitory effect becomes more pronounced. Recovery and recycling experiments confirmed that the photocatalyst exhibited robust degradation performance over multiple cycles. Kinetic analysis of the photocatalytic data, based on a first-order model, was conducted to explore the underlying degradation principles.

1. Introduction

The fast-paced development of scientific and industrial innovations has led to an increased environmental discharge of wastewater and hazardous materials from industrial facilities [1,2]. Particularly, dye wastewater poses significant treatment challenges due to its persistent chromaticity and complex chemical composition, resulting in elevated treatment costs. This necessitates the development of cost-effective treatment technologies to address these challenges effectively [3,4]. To address these challenges, researchers have proposed strategies encompassing physical, chemical, and semiconductor-based photocatalytic approaches. Semiconductor photocatalytic technology offers distinct advantages over physical and chemical methods, including simple fabrication and the absence of secondary pollutants during the reaction process [5,6]. A. Fujishima and K. Honda initially suggested the concept of photocatalysis. Under illumination, the TiO₂ and Pt electrodes immersed in an aqueous medium induced photocatalytic water splitting, resulting in the successful generation of oxygen and hydrogen gases [7]. Craey et al. demonstrated the photodegradation of polychlorinated biphenyls and haloalkanes using TiO₂ as a photocatalyst [8]. Purden et al. investigated the use of TiO₂ for the removal of trichloroethylene [9]. The proven effectiveness of photocatalytic processes in pollutant degradation establishes this technology as a strategic focus area, with ongoing research being imperative to advance its real-world applicability [10]. Among various semiconductor-based photocatalysts, TiO₂ has emerged as the most extensively studied material, because of its unique combination of advantageous properties, including exceptional structural stability, non-harmfulness, cost-effectiveness, and remarkable photocatalytic activity [11,12]. However, TiO₂ photocatalysis is fundamentally limited by three major technical challenges. The primary limitation stems from its high energy gap (3.2 eV), which restricts photoactivation to ultraviolet wavelengths (λ < 387 nm) [13]. A secondary limitation involves the quick recombination of light-generated electron–hole pairs, resulting in a diminished quantum yield and consequently reduced photocatalytic efficiency [14]. Furthermore, practical applications are hindered by the technical difficulties associated with post-treatment separation and the recovery of TiO₂ particulate matter from aqueous systems [15]. Two predominant approaches for augmenting TiO₂ photocatalysis emerge: lattice modification through controlled dopant incorporation and interfacial engineering by constructing hybrid composites with complementary semiconductor materials. The strategic incorporation of doped metal ions into the TiO₂ crystal structure induces the formation of charge carrier traps, thereby promoting electron mobility. Simultaneously, these dopants create crystalline lattice distortions that generate oxygen vacancy defects, which act as recombination barriers to prolong charge carrier lifetimes. This dual mechanism synergistically increases reactive oxygen species production, ultimately elevating the quantum yield of photocatalytic reactions. To achieve the aim of recovery and prevent secondary pollution, a photocatalyst with magnetic nanoparticles (Fe₃O₄) as the magnetic nucleus, SiO₂ as the protective layer, and the core–shell structure of TiO₂ on the surface can be prepared.

In this study, multilayer core–shell photocatalysts with Fe-doped TiO₂ were designed, and the magnetic FS-FT (0 g) and FS-FT (x g) composites were synthesized and characterized using XRD, TEM, UV-VIS-NIR spectroscopy, DRS, and XPS. Photocatalytic degradation experiments were implemented using pure methyl orange (MO) and MO-Na₂SO₄-blended solutions as model pollutants, and the underlying principles of photodegradation were investigated.

2. Experimental

2.1. Materials and Methods

All chemical reagents, including anhydrous ethanol (AR), FeCl₃·6H₂O (AR), sodium acetate (AR), ammonia (NH₄OH, 25–28%, AR), tetraethyl orthosilicate (AR), Na₂SO₄ (AR), and methyl orange (MO, AR), were sourced from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Tetrabutyl titanate (TBOT, AR) and polyethylene glycol-4000 (AR) were sourced from McLean Biochemical Technology Co., Ltd., Shanghai, China. The high-pressure Hg lamp (GGZ300, Shanghai Jiguang, Shanghai, China) had a power rating of 300 W.

2.2. Preparation of Nanocomposites

Fe₃O₄ was fabricated using a solvothermal approach. First, 2.7 g of FeCl₃·6H₂O was dissolved in 80 mL of ethylene glycol within a beaker, and stirred magnetically until completely dissolved. Subsequently, polyethylene glycol-4000 (2 g) was added to the solution, followed by continuous magnetic stirring until complete dissolution. Finally, sodium acetate (7.2 g) was added to the beaker and magnetically stirred until complete dissolution. Once the solution was completely homogenized, it was transferred into the reaction kettle and maintained at a temperature of 180 °C for 10 h. Once the reaction was complete, the product was cleaned three times using ethanol and subsequently transferred to a vacuum desiccator, where it was maintained at 40 °C until fully dried [16].

Fe₃O₄@SiO₂ was fabricated via the sol–gel method. Fe₃O₄ powder (0.2 g) was dispersed in anhydrous ethanol (80 mL) via 30 min ultrasonication to obtain a homogeneous suspension. Subsequently, 20 mL of ultrapure water was added to the aforementioned suspension, followed by the slow dropwise addition of 5 mL of ammonium hydroxide to the mixture. Ultrasonic treatment and mechanical stirring were performed on the suspension separately for 30 min. Thereafter, 4 mL of tetraethyl orthosilicate was gradually added to the suspension and mechanical stirring of the mixture continued for 6 h. Ultimately, the solution was subjected to magnetic separation using a permanent magnet, the supernatant was discarded, the magnetic solution was cleaned with absolute ethanol three times, and the washed product was transferred to a vacuum desiccator and heated at 40 °C until dry [16].

Fe₃O₄@SiO₂@TiO₂(FS-FT (0 g)) was fabricated using the sol–gel method. TBOT (7 mL) was added dropwise to anhydrous ethanol (120 mL) followed by 10 min ultrasonication to achieve dispersion. Subsequently, Fe₃O₄@SiO₂ (0.2 g) was added to the suspension, followed by 20 min of additional ultrasonication. Then, the beaker containing the suspension was placed into a water bath that had been pre-warmed to 40 °C and the suspension was mixed using mechanical stirring for 8 h. Upon the completion of the reaction, the powder was magnetically separated from the solution using a magnet. The isolated powder was cleaned three times with ethanol and subsequently dried in a vacuum desiccator at 40 °C for 10 h. The dried FS-FT (0 g) was subjected to argon-protected annealing at 450 °C for 3 h in a tube furnace [16].

Fe₃O₄@SiO₂@Fe-TiO₂(FS-FT (0.18–0.45 g)), where ‘0.18–0.45 g’ represents the amount of Fe source FeCl₃·6H₂O, which is 0.18, 0.27, 0.36, and 0.45 g, was fabricated using the sol–gel method. The FS-FT samples with different Fe contents (0.18–0.45 g) can be prepared by simply adjusting the mass of FeCl₃·6H₂O added, and the sample preparation process remains unchanged, taking the doping amount of 0.36 g as an illustration. Firstly, the mixed solution A (TBOT (7 mL) was added dropwise to absolute ethanol (120 mL) and dispersed via ultrasonication for 10 min. Subsequently, Fe₃O₄@SiO₂ (0.27 g) was introduced into the resulting solution and further dispersed via ultrasonication for 15 min, and solution B (FeCl₃·6H₂O (0.36 g) and ultrapure water (2 mL) were incorporated into anhydrous ethanol (20 mL). Solution B was combined drop by drop with solution A. The mixed solution was mechanically stirred for 7 h. Magnetic nanocomposites were separated via magnet-assisted separation. The harvested gray–black pellets were cleaned three times using absolute ethanol and dried for 10 h in a vacuum desiccator at 60 °C. Finally, they were annealed in an annealing furnace with argon as the shielding gas, and at 450 °C for 3 h.

2.3. Characterization

The concentration of methyl orange (MO) solution was measured using TU-1950, Beijing China. The maximum absorption wavelength of MO is at 464 nm. XRD could be used to analyze the crystal structures of the samples (XRD: Empyrean, Panaco, Almelo, The Netherlands). Then, 2θ scans were performed over a range from 20° to 80°, at intervals of 0.06 and a scanning speed of 4°/min. The optical absorption properties were characterized through DRS using a Shimadzu UV-2600, Shimadzu, Japan spectrophotometer fitted with an integrating sphere assembly. Spectral acquisition was performed from 200 to 800 nm under ambient conditions, with the spectral resolution set at 1 nm. All measurements were calibrated against the barium sulfate reference standard, employing Kubelka–Munk transformation for subsequent Tauc plot analysis. The XPS data were analyzed with reference to the C 1s binding energy (284.8 eV) (Thermo SCIENTIFIC ESCALAB 250Xi, Waltham, MA, USA).

2.4. Photodynamic Activity Test

The concentration of MO solution was 10 mg/L (C₀ = 10 mg/L). The concentration of MO in MO-Na₂SO₄-blended solution was 10 mg/L (C₀ = 10 mg/L), and the concentration of Na₂SO₄ in MO-Na₂SO₄-blended solution was 10.0, 12.5, 15.0, and 17.5 g/L, respectively (C_Na2SO4 = 10.0 g/L, C_Na2SO4 = 12.5 g/L, C_Na2SO4 = 15.0 g/L, and C_Na2SO4 = 17.5 g/L). The photocatalyst (20 mg) was dispersed into either MO solution (10 mL) or MO-Na₂SO₄-blended solution (10 mL). The system was then placed in the dark chamber of the photocatalytic reactor for 20 min, in order to build adsorption–desorption equalization for photocatalytic testing. The dye degradation rate η was calculated as follows:

$$\eta \%={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(1)

where C₀ (mg L⁻¹) represents the original concentration and C_t (mg L⁻¹) represents MO concentration at time t (min).

3. Results and Discussion

The XRD patterns of Fe₃O₄, TiO₂, Fe₃O₄@SiO₂, and FS-FT (0 g) are displayed in Figure 1a. The diffraction peak position of TiO₂ on the outer layer of FS-FT (0 g) is the same as that of TiO₂. According to JCPDS 65-3369, the peaks of FS-FT (0 g) are observed at 25.2°, 38.7°, 48°, 54°, and 55.14°, which are attributed to the (101), (004), (200), (105), (211), and (204) crystal planes of TiO₂, respectively. The findings reveal that TiO₂ displays an anatase structure [17]. According to JCPDS 02-0387, the peaks of Fe₃O₄ are observed at 30.1°, 35.5°, 43.1°, 53.4°, 56.9°, and 62.5°. These peaks are attributed to the (220), (311), (400), (422), (511), and (440) planes of the magnetite phase, respectively [18]. The XRD pattern of Fe₃O₄@SiO₂ exhibits diffraction peaks identical to those of Fe₃O₄. In Figure 1a, the peak absorption of Fe₃O₄ in FS-FT (0 g) and Fe₃O₄@SiO₂ is lower than that of Fe₃O₄, due to Fe₃O₄ being coated in the inner layer of FS-FT (0 g) and Fe₃O₄@SiO₂. Figure 1b shows the XRD patterns of FS-FT (0.18–0.45 g). The peaks of TiO₂ appear at 25.3°, 39.2°, 48.5°, and 55.3°, which relate to the (101), (004), (200), and (211) crystal planes, respectively (JCPDS No. 65-3369). In comparison with the peak intensity and the FWHM of FS-FT (0 g), the intensity of FS-FT (0.18–0.45 g) decreases and the FWHM increases; especially at 25.3°, the change is very obvious. The characteristic peaks of the XRD represent the number of crystal faces aligned along the same direction, and Fe atoms enter into the lattice arrangement of TiO₂. The substitution of part of Ti⁴⁺ by Fe³⁺ caused a change in the crystal morphology and resulted in a reduction in the number of crystal planes [19]. It is also apparent that the peaks of TiO₂ (101) move to the right; the observed structural modification originates from the ionic radius discrepancy between Fe³⁺ (0.055 nm) and Ti⁴⁺ (0.061 nm), where smaller Fe³⁺ ions induce lattice contraction through substitutional doping in the TiO₂ matrix [19]. The substitution of Fe³⁺ ions introduces defect states within the bandgap of TiO₂, thus affecting the peak intensity. The characteristic diffraction peaks of the (311), (440), and (533) crystal planes of Fe₃O₄ in the core of Fe₃O₄@SiO₂@Fe-TiO₂ appear, thus indicating that Fe₃O₄@SiO₂@Fe-TiO₂ can provide magnetic solid responsiveness for subsequent cycling experiments [20].

The TEM of FS-FT (0 g) and FS-FT (0.36 g) are shown in Figure 2a,b. It is evident that the samples exhibit a distinct core–shell morphology. As illustrated in Figure 2c, the HRTEM image of FS-FT (0 g) reveals the interplanar distance of 0.355 nm, attributable to the (101) plane of TiO₂, and the distance between the crystal planes is 0.176 nm, related to the (422) plane of Fe₃O₄ [21]. Figure 2d shows the HRTEM image of FS-FT (0.36 g), where the crystal plane distance is 0.354 nm. It can be attributed to the (101) plane of TiO₂, and 0.253 nm can be attributed to the (311) plane of Fe₃O₄. The interplanar distance of the (101) plane of TiO₂ in FS-FT (0.36 g) is slightly smaller than that in FS-FT (0 g); the reason for this may be that the scale of Fe³⁺ in the lattice arrangement is more minor than that of Ti⁴⁺, or it may be caused by a measurement error. Figure 2e,f show the SAED patterns, the (200) distances between the crystal planes of FS-FT (0 g) and FS-FT (0.36 g) are 0.186 nm and 0.179 nm, respectively. The (200) interplanar spacing of FS-FT (0.36 g) is smaller than that of FS-FT (0 g), the reason for this being that the ionic radius of Fe³⁺ within the lattice frame is smaller than that of Ti⁴⁺ [22]. These findings align with the XRD data.

The UV-VIS DRS of FS-FT is depicted in Figure 3a. The samples exhibit high absorption in visible and ultraviolet regions. The optical band gap (E_g) of the samples was derived from the Tauc plot (F(R)hv)² versus hv, as outlined in Equation (2).

$$\alpha \mathrm{h}\mathrm{v}=\mathrm{C}{(\mathrm{h}\nu -{\mathrm{E}}_{\mathrm{g}})}^2$$

(2)

where α represents the absorption coefficient, C is a constant, h denotes the photon energy, and E_g refers to the band gap.

In Figure 3b, the E_g of FS-FT (0 g), FS-FT (0.18 g), FS-FT (0.27 g), FS-FT (0.36 g), and FS-FT (0.45 g) is 2.73, 2.53, 2.39, 2.24, and 2.18 eV, respectively. The data show that the doping of Fe³⁺ can change the energy band gap to varying degrees, and the reason for this may be that the doping of Fe³⁺ leads to hybridization between the d-orbitals of Ti⁴⁺ and Fe³⁺ in the nano-TiO₂, thereby reducing the energy band gap [23,24,25].

In photocatalytic experiments under the same conditions, the photocatalytic performance of the photocatalyst was analyzed by monitoring the degradation of the MO solution. First, 0.02 g of FS-FT with different levels of iron doping was placed in 10 mL of MO solution (C₀ = 10 mg/L) in a dark room until it reached the absorption–desorption equilibrium. The snap values of MO using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g) were 12.12%, 11.81%, 12.12%, 12.65%, and 11.83%, respectively. Figure 4a displays the UV-VIS spectra of MO prior to and after photodegradation in pure MO solution with FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g). The photodegradation rates were 90.25%, 81.37%, 88.67%, 99%, and 79.36%, respectively. The results indicated that FS-FT (0.36 g) exhibited the highest photodegradation efficiency. Figure 4b illustrates the UV-VIS spectra of C₀ = 10 mg/L MO at different irradiation times during the photodegradation of MO using FS-FT (0.36 g). Figure 4c shows the degradation rates of MO solution using FS-FT (0–0.45 g). The data demonstrate that the degradation rate of MO solution using FS-FT (0.36 g) can reach 99% in 25 min. The results indicate that appropriate metal doping can boost the photocatalytic performance of TiO₂. When the concentration of doped Fe³⁺ is too much, it will transform Fe³⁺ from a capturing center into a recombination center, enhancing the re-association of charge carriers and suppressing the photodegradation of acid dyes, leading to a decreased degradation efficiency [17].

Under identical experimental conditions, the recovered FS-FT (0 g, 0.36 g) was utilized for cyclic degradation experiments of the MO solution. Figure 5a shows the photodegradation efficiency of MO using the recovered FS-FT (0 g) for five cycles. The abscissa of the five-cycle data corresponds to the horizontal coordinates of 0–25, 25–50, 50–75, 75–100, and 100–125 min, respectively. The degradation efficiency of FS-FT (0 g) in five cycles was 90.25%, 89.18%, 77.31%, 88.66%, and 85.03%, respectively, and the average degradation rate was 86.08%. In Figure 5b, the degradation rates of MO using FS-FT (0.36 g) were 99%, 97.18%, 95.09%, 96.68%, and 95.21%, respectively, and the average degradation rate was 96.63%. The findings show that the moderate amount of metal doping can improve degradation performance. The cause of the gradual decrease in the degradation efficiency in the first three cycles is that during the degradation process, a water-insoluble intermediate product is adsorbed at the photocatalyst surface. This reduction in the catalyst–dye interfacial contact area limits surface light absorption, consequently lowering the photocatalytic efficiency [17].

Figure 6 illustrates the photodegradation of MO in MO-Na₂SO₄-blended solution (C_MO = 10 mg/L, C_Na2SO4 = 12.5 g/L). First, 0.02 g of FS-FT with different levels of iron doping was placed into 10 mL of MO-Na₂SO₄-blended solution in a dark room until it reached the absorption–desorption equilibrium. The adsorption rates of MO-Na₂SO₄ using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g) were 12.25%, 11.11%, 11.86%, 12.23%, and 10.96%, respectively. Figure 6a illustrates the UV-VIS spectra of MO degradation in MO-Na₂SO₄-blended solution using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g). Figure 6b illustrates the absorption spectra of MO in the UV-VIS range using FS-FT (0.36 g) in MO-Na₂SO₄ at various degradation time intervals. Figure 6c shows the degradation efficiency using FS-FT (0, 0.18, 0.27, 0.36, and 0.45 g) in MO-Na₂SO₄. The photodegradation rates were 92.38%, 80.37%, 85.32%, 97.16%, and 77.54%, respectively. The findings demonstrate that the FS-FT (0.36 g) photocatalyst exhibits optimal degradation efficiency in the MO-Na₂SO₄, but its photocatalytic efficiency is inhibited compared to its performance in degrading pure MO solution. When FS-FT is doped with an appropriate amount of Fe, the addition of Na₂SO₄ suppresses the degradation of MO. The possible explanation for this phenomenon is that SO₄²⁻ competes for the surface-active sites of the photocatalyst, inducing competitive adsorption and thus reducing the photocatalytic performance [24,25]. The relevant photocatalytic efficiency data of the produced materials for MO and MO-Na₂SO₄ degradation are summarized in

Table 1. The photocatalytic effectiveness of the materials.

Material	Initial Concentration of MO (mg/L)	Amount of Catalyst per mL (mg)	(MO) Degradation Time (min) and Degradation Rate	(MO-Na2SO4) Degradation Time (min) and Degradation Rate	Cyclic Average Degradation Rate
ATT-SnO2-TiO2	20	1	30 (99%)	none	none
ZnO/TiO2	16	0.3	100 (98%)	none	none
Pt-TiO2/zeolite	20	2	90 (99%)	none	none
FS-FT (0 g)	10	2	25 (90.25%)	25 (92.38%)	86.08%
FS-FT (0.36 g)	10	2	25 (99%)	25 (97.16%)	96.63%

[26,27,28]. Comparative analysis reveals that the FS-FT (0.36 g) composite exhibits superior photodegradation efficiency. Furthermore, it demonstrates a recyclability rate of up to 90%, highlighting its potential for sustainable applications in photocatalytic systems.

The recovered FS-FT (0 g, 0.36 g) was used to degrade MO in MO-Na₂SO₄-blended solution in the same environment (C_MO = 10 mg/L, C_Na2SO4 = 12.5 g/L). Figure 7a illustrates the photocatalytic performance of the recovered FS-FT (0 g). The degradation efficiency of MO using recovered FS-FT (0 g) for five cycles was 92.38%, 90.41%, 82.61%, 90.48%, and 85.48%, respectively, and the average degradation rate was 88.27%. In Figure 7b, the degradation efficiency of MO using recovered FS-FT (0.36 g) over five cycles was 97.16%, 96.71%, 93.50%, 96.07%, and 94.71%, respectively, and the overall degradation rate was 95.63%. After three cycles of the experiment, the recovered samples underwent a self-cleaning treatment. Subsequently, the photocatalytic efficacy experienced a resurgence in the fourth cycle. It was found that although the addition of Na₂SO₄ inhibited the efficiency of FS-FT (0.36 g) in degrading MO, the inhibitory effect was not very significant; it still had strong photocatalytic activity.

XPS spectra were used to research the photocatalyst electronic characteristics and elemental composition in Figure 8. All core-level XPS spectra were calibrated using the adventitious carbon C 1s peak at 284.8 eV for charge correction.

Ti 2p: Figure 8a,b show the XPS spectra of FS-FT (0 g) and FS-FT (0.36 g) before photodegradation, respectively. Figure 8c,d show the high-resolution XPS spectra of FS-FT (0.36 g, MO) after photodegradation in pure MO solution and FS-FT (0.36 g, MO-Na₂SO₄) after photodegradation in MO-Na₂SO₄-blended solution, respectively. In Figure 8a, the spin–orbit splitting of Ti⁴⁺ results in two peaks: Ti⁴⁺ 2p_3/2 (458.6 eV) and Ti⁴⁺ 2p_1/2 (464.3 eV) [28]. Figure 8b–d show Ti 2p XPS spectra of samples doped with Fe³⁺ with two peaks, respectively. The Ti 2p doublet was observed at binding energies of 458.7 eV (2p_3/2) and 464.4 eV (2p_1/2), exhibiting a spin–orbit splitting of 5.7 eV. This finding aligns with previously reported data in the literature [29]. The Ti 2p peaks exhibited a shift toward a higher binding energy, signifying the successful integration of Fe³⁺ ions into the TiO₂ lattice [29,30].

O 1s: Figure 8e–h show the XPS spectra of O 1s for FS-FT (0 g), FS-FT (0.36 g), FS-FT (0.36 g, MO), and FS-FT (0.36 g, MO-Na₂SO₄), respectively. There are three peaks for both undoped and Fe-doped samples. O1s is in Ti-O at 529.6 eV for FS-FT (0 g) in Figure 8e, 529.8 eV for FS-FT (0.36 g) in Figure 8f, 529.8 eV for FT (0.36 g, MO) in Figure 8g, and 529.8 eV for FS-FT (0.36 g, MO-Na₂SO₄) in Figure 8h, respectively. O1s is in Fe-O at 530.4 eV for FS-FT (0 g) in Figure 8e, 530.64 eV for FS-FT (0.36 g) in Figure 8f, 530.64 eV for FT (0.36 g, MO) in Figure 8g, and 530.64 eV for FS-FT (0.36 g, MO-Na₂SO₄) in Figure 8h, respectively. This is because some Fe³⁺ ions replace Ti⁴⁺ ions, leading to a shift in the peak. Peaks at 532.5 eV (Figure 8e–h) are linked to oxygen atoms bound to O in O-H [29,31].

Fe 2p: In Figure 8i, the XPS spectrum of FS-FT (0 g) before photodegradation is due to the fact that the XPS instrument only scans the surface of the sample, the Fe element is located in the core of the core–shell structure, and the scanning data have poor signal and more noise. Figure 8j shows the XPS spectra of Fe 2p for FS-FT (0.36 g) before photodegradation. The Fe²⁺ 2p spin–orbit doublet was resolved at binding energies of 711.2 eV (2p_3/2) and 724.6 eV (2p_1/2), and the Fe³⁺ 2p spin–orbit doublet was resolved at binding energies of 713.4 eV (2p_3/2) and 727.4 eV (2p_1/2). Figure 8k presents the XPS spectra of Fe 2p for FS-FT (0.36 g, MO) after photodegradation. The Fe²⁺ 2p spin–orbit doublet was resolved at binding energies of 711.0 eV (2p_3/2) and 724.4 eV (2p_1/2), and the Fe³⁺ 2p spin–orbit doublet was resolved at binding energies of 713.3 eV (2p_3/2) and 727.3 eV (2p_1/2). Figure 8l presents the XPS spectra of Fe 2p for FS-FT (0.36 g, MO-Na₂SO₄) after photodegradation. The Fe²⁺ 2p spin–orbit doublet was resolved at binding energies of 711.0 eV (2p_3/2) and 724.4 eV (2p_1/2), and the Fe³⁺ 2p spin–orbit doublet was resolved at binding energies of 713.3 eV (2p_3/2) and 727.3 eV (2p_1/2) [32,33]. The Fe³⁺/Fe²⁺ ratio is 73:100 after iron doping. The reason for the occurrence of the Fe²⁺ peak may be that Fe³⁺ was reduced during the preparation process, or that Fe₃O₄ in the core may be detected. In Figure 8k, the Fe³⁺/Fe²⁺ ratio changes to 63:100 for FS-FT (0.36 g, MO) after degradation in MO solution; it was found that the proportion of Fe³⁺ decreased slightly, which could have led to a decline in photocatalytic performance [34,35]. In Figure 8l, the Fe³⁺/Fe²⁺ ratio is 51:100 for FS-FT (0.36 g, MO-Na₂SO₄) after the degradation of MO-Na₂SO₄-blended solution. The proportion of Fe³⁺ further decreases, indicating that Fe³⁺ was reduced to Fe²⁺ during the reaction process, leading to the decrease in Fe³⁺ and electron concentrations and in the photocatalytic activity [36,37,38].

Since the concentration of Na₂SO₄ used in the printing and dyeing process is approximately 12.5 g/L, the selected concentration range for further studying the impact caused by Na₂SO₄ is 10–17.5 g/L. Figure 9 shows an evaluation of the photodegradation rates of MO using FS-FT (0.36 g) in different MO-Na₂SO₄-blended solutions. The four sets of photocatalytic samples were placed in the dark chamber of the photocatalytic experimental setup for approximately 20 min to attain the adsorption–desorption equilibrium and subjected to irradiation with a HPML. The degradation rates of MO in different MO-Na₂SO₄-blended solutions were 99.21%, 97.16%, 90.12%, and 83.36% for C_Na2SO4 = 10.0, 12.5, 15.0, and 17.5 g/L, respectively. These results indicate a decreasing trend in the degradation efficiency from increasing the concentration of Na₂SO₄ within the range of Na₂SO₄ concentration from 10 to 17.5 g/L. Firstly, protonation of the photocatalyst surface generates positively charged species, establishing a surface charge conducive to photocatalytic reactions, which makes the dye with negative charge compete with SO₄²⁻ in the process of adsorption [39]. This competition reduces the chance of contact between the photocatalyst and dye. Secondly, the results of some researchers showed that in the process of the degradation of dyes using photocatalysts based on titanium dioxide, photogenerated electrons played a major role [24]. Fe³⁺ combines with photogenerated electrons that should have reacted with MO, which inhibits the reaction between photogenerated electrons and MO, and decreases the number of Fe³⁺-capturing electrons, thus reducing the degradation effect [25]. These findings are in agreement with those of XPS in Figure 8j–l. Thirdly, sulfate radicals (SO₄•⁻) are generated via the interaction of photogenerated holes and SO₄²⁻. As the concentration of Na₂SO₄ increases, SO₄²⁻ robs the active sites on the photocatalyst and reacts with h⁺, thus reducing the interaction between the dye and h⁺ and inhibiting photocatalytic efficiency. Although SO₄•⁻ can degrade dyes, photogenerated electrons play a key role [24]. Thus, the rate of photocatalytic removal of MO in MO-Na₂SO₄-blended solution decreases with the boost in the concentration of Na₂SO₄ [25].

Figure 10 illustrates a linear fit of the first-order kinetics. The degradation reaction rate constant, k, is identified using Equation (3):

$$\ln ({\displaystyle \frac{C_0}{C_t}})=kt$$

(3)

In this equation, k signifies the degradation rate constant, t corresponds to the exposure time, C₀ represents the initial concentration of MO, and C_t represents the concentration of MO at time t. The degradation efficiency constant k was calculated in relation to time t. Plotting in relation to time t in terms of ln(C₀/C_t), and thus calculating the incline of the best-fit straight line, yields a rate constant for each sample, k [40]. Figure 10 illustrates that the kinetic curve is nearly linear, with a correlation coefficient (R²) approaching 1. This observation confirms that the photocatalytic process is governed by a first-order kinetic model. The first-order rate constants k for FS-FT (0 g) and FS-FT (0.36 g) in pure MO solution were determined as 0.08181 and 0.12686. A comparison of these k values clearly shows that Fe doping enhances photocatalytic activity. The first-order rate constants k for FS-FT (0 g) and FS-FT (0.36 g) in MO-Na₂SO₄-blended solution were determined as 0.09121 and 0.11097. This suggests that the inclusion of Na₂SO₄ suppresses the photocatalytic efficiency of FS-FT (0.36 g); however, FS-FT (0.36 g) still maintains good photocatalytic performance. According to the literature, the role of Fe³⁺ ions is twofold: firstly, the absorbed wave shifts towards longer wavelengths due to ionic doping; secondly, at appropriate concentrations, Fe³⁺ also serves as a carrier for hole and electron transport [41].

Figure 11 illustrates the photodegradation principle of MO using FS-FT (0.36 g). Photogenerated holes (h⁺) oxidize the adsorbed hydroxyl (OH) and water (H₂O) molecules on the TiO₂ surface, resulting in the production of hydroxyl radicals (•OH) and superoxide anions (O₂₋). Photoinduced electrons (e⁻ react with dissolved oxygen in the aqueous medium via a single-electron transfer process, producing superoxide radical anions (•O₂⁻) as intermediates. The introduction of an appropriate amount of Fe doping as an intermediary for the transport of photogenerated carriers effectively reduces their recombination and thereby enhances the degradation efficiency of the FS-FT (0 g) system in the degradation of MO [40].

Figure 12 illustrates the photodegradation principle of MO-Na₂SO₄-blended solution using FS-FT (0.36 g). The introduction of Na₂SO₄ resulted in a significant increase in SO₄²⁻, which adsorbed onto the photocatalyst surface, functioning as a pore-trapping agent. This interaction facilitated the combination of sulfate ions with photogenerated holes to produce SO₄⁻• [42]. Fe³⁺ ions reacted with free electrons to form Fe²⁺, and Fe³⁺ plays a crucial role in the photodegradation process. However, the decrease in Fe³⁺ and free electron concentration negatively affects the performance of photocatalysis. As the Na₂SO₄ concentration increases, SO₄²⁻ competes for the active locations on the photocatalyst and associates with h⁺, thus reducing the binding of the dye to h⁺ and inhibiting the photocatalytic efficiency. The joint role of both led to a reduced rate of photocatalytic degradation, inhibiting the degradation of the MO-Na₂SO₄-blended solution. These findings were corroborated by XPS analysis of the MO-Na₂SO₄-blended solution and the photocatalytic degradation experiments conducted with varying concentrations of MO-Na₂SO₄ [42].

4. Conclusions

A batch of FS-FT (0 g) catalysts was synthesized using the hot solvent method for the fabrication of anatase-phase TiO₂, utilizing TBOT as the titanium source, along with the sol–gel technique for the fabrication of FS-FT (0 g). The FS-FT (0.18–0.45 g) catalysts were obtained by controlling the amount of FeCl₃·6H₂O added.

The photocatalytic removal of MO was performed using FS-FT catalysts (0–0.45 g) with different iron concentrations. The experimental results indicated that FS-FT (0.36 g) achieved a 99% degradation rate of MO, outperforming other catalysts under identical conditions. In contrast, the FS-FT (0 g) exhibited a degradation rate of 90.25%, suggesting that appropriate iron doping enhances the catalytic performance of FS-FT. The recycling experiment showed that FS-FT (0.36 g) had good recyclability. However, excessive iron doping can transition Fe from a capturing center to a complex center, increasing the recombination rate of photogenerated carriers, thereby inhibiting the photocatalytic degradation of acid dyes and resulting in a decreased degradation rate.

Using a MO-Na₂SO₄-blended solution as a simulated pollutant, the experimental results demonstrated that FS-FT (0.36 g) effectively degraded methyl orange with a degradation rate of 97.16%, while the degradation rate of FS-FT (0 g) was slightly lower at 92.38%. The results demonstrate that FS-FT (0.36 g) still exhibits strong photocatalytic activity even in complex environments. The recycling experiment showed that FS-FT (0.36 g) had good recyclability. The degradation experiments of MO-Na₂SO₄-blended solutions with different concentrations using FS-FT (0.36 g) indicate that the inhibitory effect on FS-FT (0.36 g) becomes stronger as the Na₂SO₄ concentration increases. The observed decrease in photocatalytic degradation efficiency with MO can be attributed to the introduction of a significant amount of sulfate ions (SO₄²⁻) upon the addition of Na₂SO₄. These sulfate ions adsorbed onto the photocatalyst surface and combined with holes, acting as a pore-trapping agents to generate sulfate radicals (SO₄•⁻). Fe³⁺ ions reacted with free electrons to form Fe²⁺, which is crucial in the photocatalytic degradation process. The reduction in the concentrations of Fe³⁺ and free electrons adversely affected the photocatalytic performance, leading to decreased photoelectric conversion efficiency and a lower degradation rate, ultimately inhibiting the degradation of the MO-Na₂SO₄-blended solution.

The results also indicated that the photocatalytic removal of MO using FS-FT (0 g) and FS-FT (0.36 g) followed first-order kinetic equations. The degradation rate of FS-FT (0.36 g) was found to be higher than that of FS-FT (0 g), suggesting that iron doping altered the lattice structure of TiO₂, enhancing its photocatalytic degradation rate.