Synthesis and Characterization of Fe-Doped TiO₂ Microspheres Via New Internal Gelation Method

Abstract

This study focuses on the synthesis and comprehensive characterization of Fe-doped TiO₂ microspheres prepared via a modified internal gelation sol-gel method. This novel synthesis approach enhances environmental sustainability by avoiding toxic organic solvents and employing a single washing step with pressurized water. The synthesized microspheres exhibited a well-defined spherical morphology with diameters ranging from 200 to 400 μm. Structural integrity, notably the absence of cracks, was confirmed through scanning electron microscopy analysis. Fourier-transform infrared spectroscopy identified vibrational modes corresponding to -OH and Ti-OH bonds, which are crucial for surface interactions. X-ray diffraction and transmission electron microscopy confirmed the anatase phase as the predominant crystalline structure. Nitrogen gas adsorption-desorption analysis revealed a larger surface area and pore volume in samples with higher iron content (5wt% Fe). X-ray photoelectron spectroscopy analyses provided insights into the compositional dispersion across the samples and the role of Fe in the surface functionalization of TiO₂.

Keywords:
Microspheres; modified sol-gel synthesis; Fe-doped TiO₂

1. Introduction

The internal gelation method is a versatile sol-gel route for synthesizing microspheres with diameters ranging from approximately 1 to 1000 μm. Although this method was primarily focused on nuclear fuel fabrication, it has proven to be efficient for synthesizing a wide range of stable non-actinide metal oxide microspheres. While nuclear fuel development initially motivated research into uranium, thorium, and plutonium sol-gel synthesis, subsequent investigations have demonstrated the applicability of this method to produce other metal oxides, including those of aluminum, cerium, hafnium, iron, titanium, and zirconium. The applications of metal oxide microspheres are extensive, encompassing, but not limited to, ion exchange materials, catalysts, getters, grinding media, and radiation therapy. Specifically for adsorption and photocatalysis, titanium dioxide (TiO₂) microspheres show promise as a successful alternative to commercial TiO₂ nanoparticles, potentially offering comparable or even superior efficiency in aqueous pollutant remediation. Nanoparticles often exhibit poor hydrodynamic properties, leading to agglomeration and to the formation of large clusters, which consequently reduce their surface area and reactivity. Furthermore, when these powders are pressed into pellets using binding materials such as resins or polymers, the process can reduce the number of available active sites and block pores and passageways within the material structures, thereby affecting their loading capacity and kinetic behavior¹,².

The application of fine powders and nanoparticles presents challenges, primarily due to the inherent difficulty in separating these particles from the reaction medium. For instance, studies indicate that approximately 5% of titanium dioxide (TiO₂) nanoparticles often remain suspended in the medium even after centrifugation, which significantly impedes their recovery and subsequent reuse. Moreover, growing concerns exist regarding the potential health and environmental risks associated with the widespread use of TiO₂ nanoparticles, particularly their possible toxicity to both human health and the environment³-⁵. Microspheres effectively mitigate these disadvantages due to their micrometer size, which not only eliminates toxicity risks but also facilitates their straightforward separation from the reaction medium.

The internal gelation synthesis of TiO₂ microspheres employs two primary precursor solutions: titanium chloride (TiCl₄) and a mixture of urea and hexamethylenetetramine (HMTA). Upon chilling urea complexes with titanium ions, effectively inhibiting premature hydrolysis and precipitation⁶-⁸. Subsequent heating triggers the release of titanium ions, thereby initiating hydrolysis⁶. This hydrolysis reaction generates H⁺ ions, which are subsequently consumed through the protonation and decomposition of HMTA⁹. When optimal reagent ratios are maintained this process results in the formation of a solid gel. Protonated HMTA further reacts with additional H⁺ ions and water, decomposing into ammonium ions and formaldehyde¹⁰. Once the precipitation threshold for hydrous titanium oxide is reached, under acidic pH conditions, the hydrolysis reaction proceeds to completion. Following decomplexation, urea further catalyzes the decomposition of protonated HMTA through reactions involving formaldehyde⁶,⁷,¹¹. Precise control over the hydrochloric acid (HCl) concentration is critical for establishing the target HMTA/H⁺ (0.90–3.5) and HMTA/Ti (1.50–2.75) mole ratios, which are essential for ensuring both broth stability and a controlled pore architecture. All the reactions previously described follow the Equations 1-5 as follows:

$$2CO{\left(N{H}_2\right)}_2+T{i}^{4+}\to Ti{\left[CO{\left(N{H}_2\right)}_2\right]}_2{}^{4+}$$ (1)

$$T{i}^{4+}+x{H}_2\text{O}\to Ti{(OH)}_4\cdot y{H}_2\text{O}+4{\text{H}}^{+}$$ (2)

$$Ti{O}_2+x{H}_2\text{O}\to TiO{(OH)}_2\cdot y{H}_2\text{O}+2{\text{H}}^{+}$$ (3)

$${\left(C{H}_2\right)}_6{\text{N}}_4+{\text{H}}^{+}\to {\left(C{H}_2\right)}_6{\text{N}}_{4}C{H}^{+}$$ (4)

$${\left(C{H}_2\right)}_6{\text{N}}_{4}C{H}^{+}+3{\text{H}}^{+}+6{\text{H}}_2\text{O}\to 4N{H}^{4+}+6C{H}_2\text{O}$$ (5)

Previous studies employing the internal gelation method have focused almost exclusively on single-component metal oxide microspheres (e.g., Al₂O₃, ZrO₂, CeO₂, and undoped TiO₂)¹⁰. In contrast, the present work reports for the first time in the best of our knowledge, the simultaneous dual-precursor incorporation of Fe³⁺ into TiO₂ microspheres through a modified internal gelation process. The key modifications introduced here include the replacement of the static silicone-oil column by a dynamic soybean-oil medium under continuous mechanical agitation, the substitution of toxic silicone oil by food-grade soybean oil removable with non-toxic aqueous reagents; and the adoption of a single pressurized-water washing step in place of sequential organic-solvent washes: collectively distinguish this approach from conventional internal gelation and from alternative Fe-TiO₂ synthesis routes such as hydrothermal, combustion, and spray-drying methods. These modifications simultaneously enhance environmental sustainability, scalability, and product morphological integrity.

Iron doping is anticipated to enhance the adsorption capacity of TiO₂ by introducing additional active sites for pollutant binding, as Fe³⁺ ions facilitate redox reactions and surface complexation¹²-¹⁶. The synthesized microspheres were comprehensively characterized chemically, structurally, and morphologically.

2. Materials and Methods

For the synthesis, the following materials were utilized: titanium tetrachloride (TiCl₄, ≥99% purity, obtained from IPEN/CNEN-SP) as the titanium precursor; urea (99–100.5% purity, Sigma-Aldrich), hydrochloric acid (37% grade, Sigma-Aldrich), and hexamethylenetetramine (HMTA, ≥99.5% purity, Sigma-Aldrich) as gelling agents, and commercial soybean oil as the dispersion medium. The synthesis commenced with the preparation of a homogeneous solution by mixing urea and HMTA at a molar ratio of 1.85 M to Ti (HMTA, Urea/Ti = 1.85). Iron content was incorporated into this solution at various Fe/Ti weight ratios: 0.01 (1% Fe), 0.025 (2.5% Fe), and 0.05 (5% Fe), using Fe(NO₃)₃·9H₂O (≥98% purity, Sigma-Aldrich) as the iron precursor. This solution was then cooled to 5 °C and continuously mixed under these conditions to ensure precise gelation control. Concurrently, the soybean oil was vigorously stirred at approximately 400 rpm/min using a Teflon helix and heated to 85 °C. While maintaining the stirring of the soybean oil, the cooled gel solution was precisely dripped into the oil at a controlled rate of 3.5 mL/min using a pump syringe connected to a tubing with a 0.5 mm nozzle. The nozzle was positioned 10 cm above the soybean oil surface to regulate droplet size and prevent coalescence of the forming microspheres. Upon entering the soybean oil, the droplets underwent rapid gelation due to the reaction between urea and HMTA, resulting in the formation of a solidified gel shell around each droplet. The droplets were allowed to remain in the soybean oil for one hour under continuous stirring. Subsequently, the soybean oil was drained, and the gel microspheres were washed with a 0.1 M reagent-grade NH₄OH (30% NH₃) solution for 24 hours to ensure complete formation of the microsphere gel structure. Following this, the gel microspheres were washed multiple times with distilled water to remove any residual impurities and excess reactants. They were then hydrothermally treated in an autoclave (134 °C for 5 hours) to extract residual organic components, air-dried at room temperature, and further dried in an oven at 100 °C. Finally, the undoped and doped microsphere samples were heat-treated at 550 °C for 1 hour. Figure 1 illustrates the synthesis of microspheres through the modified internal gelation process.

Figure 1
Schematic representation of the process of synthesis for Fe-doped TiO₂ microspheres using the internal gelation method.

The synthesized samples underwent comprehensive characterization to evaluate their structural, chemical, and surface properties. Crystalline phases and structural features were determined by X-ray diffraction (XRD) using a Rigaku SmartLab SE diffractometer, employing Cu Kα radiation (λ = 1.54 Å), a step size of 0.01°, and a 2θ scan range of 5–80°. For Transmission Electron Microscopy (TEM) analysis, the microsphere samples were ground into a fine powder using an agate mortar and pestle. Subsequently, the powder was dispersed in isopropanol and drop-casted onto carbon-coated copper grids (3.05 mm in diameter, 18 μm thick, with a ~5 nm amorphous carbon film). The analyses were carried out using a JEOL JEM 2100 microscope. Morphological features and elements mapping were examined using a Hitachi TM3000 benchtop scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX). Surface topology and microstructures were further characterized with a JEOL JSM-IT700HR SEM equipped with a Schottky field emission gun (FEG), operating at accelerating voltages ranging from 0.5 to 30 kV. The chemical state of the surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) using a Scienta-Omicron ESCA+ spectrometer equipped with a monochromated Al Kα source (hν = 1486.6 eV). Functional groups and chemical bonding were assessed via Fourier-transform infrared spectroscopy (FTIR) using a Thermo Scientific Nicolet NEXUS 670 spectrometer; samples were prepared as KBr pellets and analyzed over the 400–4000 cm⁻¹ range. Nitrogen (N₂) adsorption-desorption analysis was performed to determine the specific surface area, pore size, and pore distribution using a Micromeritics ASAP2020 Plus, configured for mesoporous analysis (standard pressure transducers, without the Micropore Upgrade hardware).

3. Results and Discussion

Figure 2 presents the X-ray diffraction spectra for undoped and Fe-doped TiO₂ microspheres. The samples exhibited characteristics of good crystallinity. Regarding peak indexing, the samples crystallized in the anatase phase of TiO₂, the anatase phase is stable at low calcination temperatures, between 500 and 550 °C¹⁷,¹⁸. A peak around 2θ ≈ 32° indicates the formation of brookite phase¹⁸-²⁰, which gradually diminishes as the iron content raises in the samples. The presence of Fe³⁺ was found to inhibits the crystallite growth, according to the studies of Gervasi et al.²¹ and Soo et al.²². Peaks corresponding to iron in the TiO₂ structure were not identified in the diffraction patterns, suggesting the iron could be effectively incorporated into the TiO₂ crystalline structure or the amount incorporated was too low to be detected by the technique. The broad width of the peaks generally suggests a small particle size, which can be observed on Table 1. Due to the similarity of the Fe³⁺ and Ti⁴⁺ ions’ radii, Fe³⁺ ions might substitute Ti⁴⁺ ions in the TiO₂ lattice. The slightly increase in the crystallite size with Fe content below 1 wt.% is ascribed to the lattice distortion caused by Fe³⁺ substitution whereas the subsequently decrease in the crystallite size and lattice parameters with Fe content above 1 wt% is attributed to an interstitial occupation of Fe⁺³ in the TiO₂ crystal structure¹⁷,¹⁸,²².

Figure 2
X-ray diffraction (XRD) patterns of the undoped TiO₂, and 1%Fe- TiO₂, 2.5% Fe- TiO₂, and 5%Fe- TiO₂.

Table 1
Lattice parameters and crystallite size estimated from XRD data for the synthesized samples.

Sample	Lattice parametersa [Å]	Unit cell volume [Å3]	d-spacing(101)b [nm]	Crystallite sizec [nm]
	a = b c
TiO2	3.8 9.5	135.2	0.350	12
1%Fe	3.8 9.5	135.8	0.351	13
2.5%Fe	3.7 9.3	131.1	0.344	9
5%Fe	3.7 9.3	130.8	0.342	7

• ^a
  Calculated by using the equation, $\frac{1}{d_{\left(hkl\right)}^2}=\frac{h^2+k^2}{a^2}+\frac{l^2}{c^2}$
• ^b
  Calculated from Bragg’s law, $d=\frac{\lambda }{2\sin \theta }$
• ^c
  Calculated from Scherrer equation, $D=\frac{K\lambda }{\beta \cos \theta }$

Figure 3 presents the FTIR curves obtained for the samples and is align with the patterns reported in the literature for TiO₂ nanoparticles²³. Peaks corresponding to the vibrational modes of -OH, Ti-OH, and Ti-O bonds can be observed. The band at 1623.50 cm⁻¹ can be attributed to the vibrational deformation of the Ti-OH stretching mode, while the band at 3404.82 cm⁻¹ corresponds to the symmetric and asymmetric stretching modes of the hydroxyl (-OH) group. These hydroxyl-related bands are particularly significant in the surface analysis of the microspheres which, upon hydrolysis, strongly interact with hydroxyl groups. The spectral region between 400 and 1000 cm⁻¹ exhibits vibrational modes assignable to Ti-O-Ti bonding, specifically at 464 cm^-1, is observed a band in TiO₂ sample which can be attributed to stretching vibrations of Ti-O bonds²⁴-²⁶.

Figure 3
FTIR spectra of TiO₂-based samples calcined at 550°C with different Fe-doping levels (1%, 2.5%, and 5% by weight) compared to undoped TiO₂.

Figure 4a-d show the SEM images of undoped and Fe-doped TiO₂ microspheres with different Fe contents. The microspheres exhibited well-defined spheric shape-format, with diameter sizes ranging from 200 to 400 μm. No cracks were observed in these surfaces which indicates that the synthesis methodology using the modified internal gelation method was successfully applied in this study. A major challenge in the synthesis of ceramic microspheres via internal gelation lies in controlling cracks in the spheres²⁷. The use of organic solvents and multiple washing steps in the original process not only rendered the method highly toxic to the environment but were often ineffective in eliminating cracks²⁷. The modified method introduced in this work eliminates the use of these toxic solvents, and also employs a single washing step with pressurized water to effectively remove excess organics responsible for the stresses that cause cracking during calcination. The absence of cracks in samples, therefore, demonstrates the success of this technique. Figures 4 to 4h show the cross-section of 5% Fe doped-TiO₂ microsphere and the corresponding EDS metal elements mapping, respectively. The sphere displays a smooth internal structure without cracks. The EDS mapping revealed a uniform distribution of the Fe-dopant, suggesting that the methodology was also successful in achieving effective dispersion of Fe.

Figure 4
Micrographs illustrating the synthesizing of ceramic microspheres. Images (a-d) show the surface morphology with sizes ranging from 200 to 400 μm. (a) TiO₂; (b) 1% Fe-TiO₂; (c) 2.5% Fe-TiO₂; (d) 5% Fe-TiO₂; (e,f) cross-sectional view of the 5%Fe-TiO₂; and (g,h) individual elemental distributions of Fe and Ti, respectively.

FEG-SEM micrographs were acquired to characterize the morphology and internal structure of the samples, with results presented in Figure 5. Figure 5a displays an individual undoped microsphere, and Figures 5 and 5c were obtained at greater magnifications via FEG-SEM. It can be seen an internal cavernous and porous architecture of the sphere. The images reveal an interconnected macroporous framework enabling the formation of porous channels, suggesting abundant space for active sites, and consequently enhanced adsorption capacity of the spheres. Additionally, Figure 5c identifies an ordered mesoporous structure within the microspheres with featuring particles presenting approximately 20 nm in diameter.

Figure 5
SEM (a) and FEG-SEM (b and c) of undoped TiO₂ microsphere detailing inner porous structure.

Figure 6 presents the adsorption-desorption isotherms and the pore size distributions of the synthesized microspheres whereas Table 2 shows the values of the specific surface area (SSA; m² g^-1), pore volume (Vp; cm³ g^-1) and the average size of pore diameter (Dp, nm) obtained from N₂ physisorption at -196°C of all synthesized materials.

Figure 6
Adsorption–desorption isotherms of microspheres samples synthesized at 550°C (a) and their respective pore size distributions (b).

Table 2
Textural properties of the prepared samples, calculated from N₂ physisorption at -196°C.

Samples	SSAa	Vpb	Dpc
	[m2 g-1]	[cm3 g-1]	[nm]
TiO2	60.6	0.22	11.6
1%Fe	61.8	0.16	8.1
2.5%Fe	61.8	0.20	10.1
5%Fe	110.6	0.29	8.3

• ^a
  Specific Surface Area (SSA) evaluated according to the Brunauer-Emmett-Teller (BET) method.
• ^b
  Total Pore Volume (Vp) evaluated according to the Barrett-Joyner-Halenda (BJH) method during the adsorption phase.
• ^c
  Average Pore Diameter (Dp) evaluated according to the the Barrett-Joyner-Halenda (BJH) method during the dessorption phase.

All samples display Type IV(a) isotherms with H1-type hysteresis loops, indicative of mesoporous materials composed of well-defined, roughly cylindrical pore channels with a narrow pore size distribution, according to the IUPAC 2015 physisorption classification²⁶. Additionally, the doped samples exhibit a slight increase in uptake at very low relative pressures (p/p₀ < 0.01), which is characteristic of micropore filling, confirming the bimodal porous character of Fe-doped samples. However, due to the instrumental configuration used in this study, such microporosity cannot be reliably quantified. Therefore, the BET values reported here should be interpreted as “apparent specific surface areas”, reflecting essentially the external mesoporous structure of the materials, while the BJH pore size distributions correspond strictly to the mesoporous fraction.

It is well known that textural characteristics of synthesized materials are highly correlated with preparation methods¹⁸. All samples obtained by using this modified internal gelation method exhibited a characteristic hysteresis loop at high relative pressure. Its presence in the curves can be related to the capillary condensation in mesoporous structures²⁸. In the study by Grifasi et al.¹⁸ Fe-doped TiO₂ samples were produced using combustion and hydrothermal routes. The authors compare and highlight the influence of the synthesis methods on the porous structure of the samples and consequently on the surface area values obtained. As in this work, the authors identified type IV through the hysteresis profile in the adsorption isotherm curves, indicating the presence of mesoporous structure in the sample¹⁸. This observation also indicates that the internal gelation sol-gel synthesis method is successful for the synthesis of these structures. Values shown in Table 2 indicate a correlation between SSA and Fe content in samples. The Fe5wt%-doped TiO₂ sample showed the higher specific area and pore volume of the doped samples. This behavior is attributed not to the iron content itself, but to structural effects associated with Fe incorporation during the sol–gel process. Fe doping is known to inhibit crystallite growth in TiO₂, resulting in smaller nanocrystals and a less densely packed oxide network after calcination. This interpretation is consistent with the XRD results, which show a clear reduction in crystallite size for the Fe‑rich sample. The inhibited growth of crystallites leads to a more fragmented structure and contributes to the development and persistence of mesoporosity, ultimately resulting in an increase in mesopore volume and higher specific surface area¹⁸,²⁹-³¹. Finally, by analyzing the pore size distributions, all samples showed a well-defined and narrow pore size distribution of about 8-11 nm.

The 5 wt% Fe sample was selected for further investigation by XPS and TEM techniques as it represents the composition with the most pronounced modifications as smallest crystallite size and highest BET surface area.

Figure 7 displays the Transmission Electron Microscopy (TEM) images of the undoped TiO₂ and 5%Fe-doped TiO₂ samples, revealing nanoparticles with sizes ranging from 10 to 20 nm. The microstructure of these nanoparticles was further investigated by High-resolution transmission electron microscopy (HRTEM). As observed in Figure 7a, the particles exhibit a polycrystalline nature, which is assigned to the anatase phase, as confirmed by the corresponding selected area electron diffraction (SAED) pattern (inset). The measured lattice fringe spacing of 0.350 nm corresponds to the (101) plane of anatase TiO₂³². For the 5%Fe-doped TiO₂ sample (Figure 7b), no segregated iron particles were detected on the surface of the TiO₂ nanoparticles. The observed lattice spacing of 0.332 nm is also attributed to the anatase phase³². This reduction in the lattice spacing suggests the incorporation of iron into the anatase crystal structure accordingly the XRD results, which leads to a decrease in the lattice parameters²¹,²²,³³. This conclusion is further supported by the absence of iron particle segregation. Additionally, Energy-Dispersive X-ray Spectroscopy (EDS) analysis confirmed the presence of iron in the sample.

Figure 7
Transmission Electron Microscopy (TEM) images, EDS and diffraction patterns of the samples: undoped TiO₂ (a) and 5wt%Fe- TiO₂ (b).

Figure 8 displays XPS spectra of bare TiO₂ and 5% Fe-doped TiO₂ microspheres to determine their surface chemical states. All spectra were fitted to the C 1s peak at 284.6 eV (adventitious carbon). A characteristic survey XPS spectrum reveals the presence of oxygen and titanium in both samples, while iron content was observed only at the doped sample, confirming TiO₂ microspheres synthesis and Fe-doping success. Furthermore, high carbon content was found in both samples, which can be residual due to the organic precursors and may also be incorporated into the microspheres. The main components were quantified, and the values are listed in Table 3.

Figure 8
XPS survey spectra of TiO₂ (a) and 5%Fe doped-TiO₂ (b) microspheres.

Table 3
Superficial elemental analysis by XPS of the relative atomic concentrations (at %) of TiO₂ and 5%Fe-doped TiO₂ microspheres.

Sample	Ti	O	C	Fe
	(at %)	(at %)	(at %)	(at %)
TiO2	14.76	50.72	34.52	-
5% Fe-doped TiO2-	16.24	55.23	25.17	3.36

The surface Fe concentration detected by XPS (3.36 at%, Table 3) is somewhat lower than the nominal bulk value (~5.7 at% relative to Ti), which is consistent with a preferential sub-surface Fe distribution and partial signal attenuation by adventitious carbon.

Figures 9 and 9b show the high resolution Ti2p region for undoped and 5%Fe-doped TiO₂, respectively. Doublets of Ti⁺⁴ at 464.6 eV (2p_1/2) and 458.7 eV (2p_3/2) correspond to the anatase phase of TiO₂³⁴, with good agreement to the literature and the previously XRD and SAED reported data. The peaks at 463.3 eV (2p_1/2) and 457.4 eV (2p_3/2) in the undoped sample are related to the presence of titanium in the chemical state Ti3+ ³⁴. The reduction of titanium can be attributed to the presence of carbon, which causes Ti⁴⁺ to be reduced to Ti³⁺ during the heat treatment step³⁵. For the 5%Fe doped-TiO₂ microspheres, peaks were identified at 462.9 eV (2p_1/2) and 458.2 eV (2p_3/2), corresponding to Ti⁺⁴, and 459.6 eV (2p_1/2) and 457.1 eV (2p_3/2), corresponding to Ti³⁺. In Fe-doped sample Ti³⁺ doublets dominate over those of Ti⁴⁺ indicating further surface reactions probably raised by iron incorporation³⁵.

Figure 9
XPS spectra of the TiO₂ (a, c, e) and 5%Fe-doped TiO₂ (b, d, f, g) microspheres.

Figures 9 and 9d display O1s spectra with three components: lattice oxygen (Ti–O–Ti) at 528.5 eV, adsorbed –OH groups at 529.8 eV, and Ti–O–C bonds at 531.5 eV for the undoped sample, and similar values of 528.4 eV, 529.7 eV and 531.4 eV respectively for the doped sample. Peaks around 528.5 eV may be related to lattice oxygen (Ti-O bond) and those around 529.8 eV may be attributed to oxygen adsorbed on the surface of the samples³⁵. Hydroxyl groups showed higher contribution in undoped than in doped TiO₂ .

Figures 9 and 9f correspond to C1s high-resolution XPS spectra. Peaks centered at 283.5–284.7 eV, 284.8–285.8 eV and 287.2–288.1 eV for undoped and 5%Fe-doped TiO₂ are signed to C–C, C–O and C=O bonds, respectively. The presence of the C=O species suggests that a portion of the carbon was incorporated into TiO₂, and promoting the doping of the spheres³⁶.

The high-resolution XPS spectrum of Fe2p is shown in Figure 9g. Doublets at 713.4 eV (2p_1/2) and 723.9 eV (2p_3/2) are signed to Fe³⁺ and those at 722.3 eV (2p_1/2) and 710.3 eV (2p_3/2) are signed to Fe²⁺ in agreement with reference data for iron-doped TiO₂³⁷,³⁸. A satellite peak present at 718.4 eV is characteristic of high-spin Fe³⁺ in octahedral coordination, corroborating incorporation of iron into the anatase lattice rather than its segregation as a secondary iron oxide phase. The peak round 710.3 eV suggests the occurrence of Fe-O bonds³⁸.

The formation of Fe⁺² ions can be attributed to a carbothermal reduction during calcination at temperatures in the order of 550 °C. Residual carbon from the organic precursors (urea, HMTA, soybean oil) consumes lattice oxygen upon combustion, partially reducing Fe³⁺ to Fe²⁺ and Ti⁴⁺ to Ti³⁺ simultaneously³⁵. The notably higher Ti³⁺ fraction in the Fe-doped relative to the undoped sample further indicates that Fe³⁺ incorporation generates additional oxygen vacancies as a charge-compensation mechanism, consistent with theoretical expectations for doping of TiO₂-anatase³³.

Fe³⁺ (ionic radius 0.645 Å) is slightly larger than Ti⁴⁺ (ionic radius 0.605 Å), and its incorporation into the anatase lattice introduces local compressive strain that progressively limits grain growth during calcination, resulting in smaller lattice parameters and crystallite sizes with increasing dopant content (Table 1). The charge imbalance introduced by Fe³⁺ → Ti⁴⁺ substitution is compensated through the generation of oxygen vacancies and Ti³⁺ species, as confirmed by the XPS analyses.

Although XPS analysis identified C=O species in both samples (C 1s, Figure 9e-f), no distinct C=O stretching band was observed in the FTIR spectra in the 1700–1800 cm⁻¹ region. This apparent discrepancy is explained by the complementary nature and differing sensitivities of both techniques: XPS is surface-sensitive (~5–10 nm probing depth) and detects trace C=O concentrations at the microsphere surface, whereas FTIR of bulk KBr pellets integrates signal from the entire sample volume, where surface carbon species are diluted below the FTIR detection threshold. Furthermore, the strong Ti-OH deformation band at 1623.5 cm⁻¹ may mask a weaker, adjacent C=O absorption. Partial incorporation of carbon as Ti-O-C bonding (evidenced by the O 1s peak at ~531.5 eV in XPS) further broadens and red-shifts the effective C=O frequency, rendering it unresolvable from the Ti-OH band³⁶.

4. Conclusions

The synthesis of TiO₂ and Fe-doped TiO₂ microspheres using a novel internal gelation method approach demonstrated well succeed by producing structured ceramic microspheres. FTIR spectra confirmed the presence of -OH groups in the surface and fundamental lattice metal-oxygen bonds (Ti-O), describing the surface interactions and the structural formation of the TiO₂ matrix. XRD analyses revealed that the synthesized samples exhibited the TiO₂-anatase phase as the primary crystalline structure and iron dopant adds local compressive strains into the TiO₂ lattice, restraining crystallite growth and reducing unit cell volume.

SEM analysis confirmed a regular spherical shape of the microspheres with sizes ranging from 200–400 μm and the uniform distribution of Fe dopant across the samples. The methodology effectively minimized structural defects, such as cracks, demonstrating the efficiency of the modified synthesis route.

FEG-SEM images showed a porous-like inner structure of microspheres, with macro and mesopores architecture. N₂ physisorption analysis further confirm mesoporous structure in microspheres and higher specific surface for 5%Fe-doped-TiO₂ sample, suggesting the iron doping content plays a role on specific surface area.

XPS pointed out titania, carbon and iron content. High resolution spectra revealed potential role of carbon and iron on functionalizing microspheres surface by the high content of Ti³⁺ species.

5. Acknowledgments

The authors are grateful to the Brazilian agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES Finance Code 001 and (grants No. 88882.333455/2019-01, 88887.513676/2020 and 88887.803516/2023-0000) and Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq - Proc. No. 420135/2023-5.

6. Data Availability

All data supporting the findings of this study are included in the present paper.

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Edited by

• Associate Editor:
  Celso Santilli.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Publication Dates

• Publication in this collection
  31 July 2026
• Date of issue
  2026

History

• Received
  20 Feb 2026
• Reviewed
  15 May 2026
• Accepted
  28 June 2026