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Article

Synthesis, Optical Properties and Photocatalytic Testing of Sol–Gel TiO2-Fe2O3/PVP Nanopowders

by
Stefani Petrova
1,2,*,
Yoanna Kostova
3,
Martin Tsvetkov
2,
Angelina Stoyanova
4,
Hristina Hitkova
4,
Polya Marinovska
4 and
Albena Bachvarova-Nedelcheva
1,5
1
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 11, 1113 Sofia, Bulgaria
2
Faculty of Chemistry and Pharmacy, Sofia University “St. Kliment Ohridski”, 1164 Sofia, Bulgaria
3
Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre “Acad. A. Balevski”, Bulgarian Academy of Sciences, Shipchenski Prohod Str., 67, 1574 Sofia, Bulgaria
4
Department Chemistry and Biochemistry, Faculty of Pharmacy, Medical University, 1 Kliment Ohridski St., 5800 Pleven, Bulgaria
5
National Centre of Excellence Mechatronics and Clean Technologies, 8 bul., Kl. Ohridski, 1756 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Optics 2025, 6(2), 22; https://doi.org/10.3390/opt6020022
Submission received: 16 April 2025 / Revised: 10 May 2025 / Accepted: 20 May 2025 / Published: 26 May 2025
Browse Figures
Versions Notes

Abstract

:
In this study, TiO2-Fe2O3/polyvinylpyrrolidone (PVP) hybrids were prepared using the sol–gel method. The iron content in the synthesized samples was 10 and 20 wt%. The influence of PVP on the phase transformation, morphology and optical properties of the as-prepared hybrids was characterized by various physicochemical methods—XRD analysis, UV–Vis spectroscopy, IR spectroscopy and SEM. The obtained sol–gel powders were tested for photocatalytic activity against tetracycline hydrochloride in distilled water under ultraviolet and simulated solar light illumination. The obtained results were compared to commercial TiO2 P25 (Evonik). The investigated samples exhibited good photocatalytic efficiency for the degradation of tetracycline hydrochloride; however, better activity was demonstrated by the 90TiO2-10Fe2O3/PVP sample. The latter one displayed weak antibacterial action against E. coli ATCC 25922 in the presence of UVA light.

1. Introduction

Investigations into finding novel photocatalyst materials have been a continuous focus for researchers ever since Honda and Fujishima [1] discovered that TiO2 can be used for water splitting through electrochemical photolysis. Since TiO2 is a wide-band semiconductor, its activation by visible-light irradiation is ineffective. Doping TiO2 with metals and metal oxides is a well-known method for reducing its band gap and increasing its photocatalytic activity [2]. Fe2O3 was found to be an encouraging candidate to form TiO2 composites with improved optical and morphological properties [3]. Several studies have stated that Fe2O3-doped TiO2 materials possess unique potential for catalytic applications [4].
Polymer-based composite nanomaterials are of tremendous scientific and technological interest because of their unique optical, electrical and mechanical capabilities. A wide variety of uses exist for these materials, including optoelectronic components and magneto-optic data storages [5,6]. Numerous attempts have been made to reinforce polymers with metal, semiconductors, carbon nanotubes and magnetic nanoparticles in order to improve the optical properties of polymer nanobased composites [7,8,9,10,11,12,13]. The non-toxic role of Fe2O3 nanoparticles in biological systems makes them desirable for use in biomedical applications. It is known that Fe2O3 nanoparticles are magnetic and can be used as semiconductors, with a band gap of 2.1–2.2 eV [14]. Recently, their effect on biological systems has become a subject of increased scientific interest [15]. Many researchers have reported on various biological uses, mainly focusing on materials containing Fe2O3 [16]. It has been found that these nanoparticles could decrease drug concentrations, toxicity and side effects while boosting the efficacy of treatments. On the other hand, PVP has been chosen as an organic component for hybrid materials because of its unique characteristics. It is highly soluble in polar solvents such as alcohol, and it is preferable to avoid phase separation during reactions. PVP can be thermally crosslinked [10,11,12], resulting in a hybrid material with outstanding thermal stability and high mechanical strength. Furthermore, the amorphous structure of PVP also provides a low scattering loss, which makes it an ideal polymer for hybrid materials with optical applications [17].
According to the literature review, PVP can limit the release of metal oxide nanoparticles and prolong their antimicrobial activity by forming stable complexes [18,19]. It was also found that PVP acts as a capping agent, decreasing the size of prepared particles and causing an increase in the catalytic reduction and antimicrobial efficacy of Fe3O4 [20]. The promising effect of iron oxide nanoparticles on bacterial growth by producing reactive oxygen species (ROS), oxidative stress and cell membrane disruption has also been reported [20]. However, the issue of PVP’s impact on antibacterial properties remains unclear. Numerous studies have been conducted to obtain TiO2–PVP materials for photocatalytic applications [21,22,23]. Previously, TiO2–PVP films have been successfully prepared through the hydrolysis–polycondensation of titanium n-butoxide in PVP acid solution, and they have exhibited promising applications in electronic and optical fields [21]. It has been reported that sol–gel-prepared ternary composites (Fe2O3-TiO2/PVP) are very effective for the post-treatment of greywater [21]. The influence of PVP on the phase transformation of TiO2 was investigated by Zheng et al. [22]. Obviously, the study of various characteristics of Fe-doped TiO2 has not yet been exhausted bearing in mind their significance and future applications. A few research studies have focused on the removal of pharmaceuticals by a TiO2-Fe2O3/PVP photocatalyst and their antibacterial properties, highlighting the novelty of these investigations.
In the present study, TiO2-Fe2O3/PVP hybrids were prepared by a sol–gel method, aiming to study their photocatalytic and antibacterial properties. The influence of PVP on the degradation of tetracycline hydrochloride and the antibacterial activity of the gels was a focus of the paper as well. The antibacterial behavior of the obtained powders against E. coli ATCC 25922 was assessed.

2. Materials and Methods

2.1. Materials

The chemicals Fe(NO3)3·9H2O (p.a., Sigma-Aldrich, Milwaukee, WI, USA), Ti(IV) tetrabutoxide, Ti(OBu)4 (reagent grade, 97%, Sigma-Aldrich, Milwaukee, WI, USA), C2H5OH (96%) and polyvinylpyrrolidone (PVP, Mr 24000, Sigma-Aldrich, Milwaukee, WI, USA) were used in this study. Additionally, tetracycline hydrochloride (Ficher BioReagents, Waltham, MA, USA) was applied as a pharmaceutical.

2.2. Synthesis of TiO2-Fe2O3/PVP Hybrids

Ternary sol–gel composite solutions containing 10 and 20 mol% Fe2O3 were prepared by mixing two types of solutions, A and B. A detailed representation of the experimental procedure is shown in Figure 1. The compositions were denoted as 90TiO2-10Fe2O3/PVP and 80TiO2-20Fe2O3/PVP. The Ti(OBu)4 was mixed with ethanol under vigorous stirring (solution A), while solution B was prepared while keeping the desired ratio of PVP/Fe2O3/H2O as 3:1:1. The precursor solutions were subjected to 5–10 min of intensive stirring at room temperature to achieve complete dissolution. As can be seen from the figure, the resulting solutions were mixed using a magnetic stirrer at 400–500 rpm for 60 min. The gelation for the investigated TiO2-Fe2O3/PVP compositions occurred immediately. To complete the hydrolysis, the aging of gels was performed in air for several days. Aiming to verify the phase and structural transformations, the obtained gels were subjected to heating at 500 °C for one hour of exposure time in air. The temperature was selected bearing in mind our previous investigations [23,24,25,26].

2.3. Experimental Techniques

Powder XRD patterns were recorded at room temperature with a Bruker D8 Advance (Berlin, Germany) X-ray powder diffractometer with Cu Ka radiation (k = 1.54056 Å), with a LynxEye (Berlin, Germany) solid position-sensitive detector and X-ray tube operated at 40 kV and 40 mA. X-ray diffraction patterns were registered in the range of 5.3–80° over 1 h with a step of 0.02° over 2 h. The Rietveld method was used to derive the unit cell characteristics, crystallite size, and phase weight fraction using the MAUD program (version 2.9998) [27]. The infrared spectra were recorded in the range 1600–400 cm−1 using the KBr pellet technique on a Nicolet-320 FTIR spectrometer (Madison, WI, USA) with 64 scans and a resolution of ±1 cm−1. The optical absorption spectra of the powdered samples in the wavelength range of 200–600 nm were recorded using the UV–Vis diffused reflectance spectrophotometer Evolution 300 (Thermo Electron Corporation, Madison, WI, USA) with a magnesium oxide reflectance standard as the baseline. The band gap energy was estimated using Tauc’s equation, αhν = A(hν − Eg)n/2, where A is a constant independent of hν, Eg is the semiconductor band gap, and n depends on the type of transition [28]. The value used for n was 1, reflecting a direct transition. A well-known approach for semiconductor band gap energy determination from the intersection of linear fits of (αhν)1/n versus hv on the x-axis was used, where n can be 1/2 or 2 for direct and indirect band gaps, respectively. A scanning electron microscope (SEM) JSM-5510 (JEOL, Tokyo, Japan), running with an acceleration voltage of 10 kV, was used to image the heated samples and the as-obtained gels. The investigated samples were covered with gold by a JFC-1200 fine coater (JEOL, Tokyo, Japan) before surveillance. The specific surface areas (BETs) of the as-obtained gels were defined by low-temperature (77.4 K) nitrogen adsorption with a NOVA 1200e surface area and pore analyzer (Quantachrome, Boynton Beach, FL, USA) at relative pressures of p/p0 = 0.1–0.3 using the BET equation.

2.4. Photocatalytic Activity Tests

The photocatalytic activity of the as-prepared powdered photocatalysts, which were also heat-treated at 500 °C for 2 h, was tested for the degradation of tetracycline hydrochloride (TCH) under UV light (Sylvania 18W fluorescence blacklight, Wilmington, MA, USA, λmax = 365 nm) and simulated solar light illumination (300 W Ultra Vitalux, Osram, Munich, Germany), both situated 7 cm above the slurry. A model TCH solution in ultra-pure water (18.2 MΩ) with a concentration of 10 ppm and a volume of 200 mL was prepared. The solution was stirred with a magnetic stirrer in a batch slurry reactor (Lenz Laborglas glass, KL-100, Wertheim, Germany) connected to a circulating bath (ArgoLab CB5-10, Arezzo, Italy), keeping a constant temperature of 25 °C ± 0.1 °C. After adding 0.1 g of the catalyst, a 30 min “dark” period followed to establish sorption–desorption equilibrium. The reaction time was 60 min under the irradiation of the light source. Every 10 min, a 3 mL sample of the solution was extracted and filtered with a 0.22 µm membrane syringe filter to remove the catalyst from the solution. All samples taken were then measured for the concentration of TCH by a UV–Vis spectrophotometer (Thermo Scientific, Evolution 300, Madison, WI, USA) with an absorption peak of 355 nm. The results obtained were then compared to those obtained for TiO2 P25 (Evonik, Essen, Germany). A C/C0 vs. time graph was plotted to show the degradation of the pollutant, and a Langmuir–Hinshelwood kinetic model (−ln(C/C0 vs. time plot) was used to determine the rate constant of the reaction (assuming first-order kinetics). The initial concentration, C0, is the concentration after the dark period.

2.5. Antibacterial Assessment

The antibacterial properties of the synthesized TiO2/Fe2O3 composites were tested in the presence of UVA light as a part of the experimental setup previously published [29].

2.5.1. Preparation of Bacterial Suspension

One colony from a fresh culture of E. coli ATCC 25922 was applied in 50 mL of nutrient broth for overnight incubation at 37 °C. The prepared broth culture was separated in 5 sterile tubes. The tubes were centrifuged twice for 10 min at 1000× g, rinsed twice and resuspended with saline. Then, the bacterial solution was standardized to 0.5 McFarland (1.5 × 108 cell/mL) and diluted with saline in a ratio of 1:1000, so a final count of 105 cells/mL was obtained.

2.5.2. Experimental Setup

A sterile flask with a volume of 250 mL was inoculated with 100 mL of bacterial suspension, and 100 mg of TiO2-Fe2O3/PVP powder was added, and as a result, the desired concentration of 1 mg/mL was achieved. The flask with the experimental mixture was put in an electromagnetic stirrer in the presence of light with a 365 nm wavelength (UVA light) produced by an ultraviolet lamp (Sylvania BLB50 Hz 8W T5, Newhaven, UK). The lamp was situated at 10 cm laterally to the flask. The experimental setup continued for up to 90 min, with continuous stirring at 250 rpm to prevent the TiO2/Fe2O3 particles from setting.

2.5.3. Determination of Viable Bacterial Cells by a Spread Plate Method

At 15 min intervals of time, 500 µL of the reaction suspension was taken, and serial dilutions of 10−1 and 10−2 were prepared. Then, 100 µL of the undiluted samples and each dilution was spread onto two plates with Mueller–Hinton agar (MHA), and the plates were cultured overnight at 37 °C. The colonies on the MHA plates were counted and equated to 1 mL. In this manner, the microbial counts during the experiment were determined.

3. Results and Discussion

3.1. XRD and SEM Morphology of As-Prepared Ternary TiO2-Fe2O3/PVP Samples

The purpose of the XRD approach was to gather data on the phase transformations in the samples. The X-ray diffraction patterns of the as-prepared hybrids and species annealed at 500 °C are shown in Figure 2a,b, respectively. They were compared to those of commercial TiO2. The XRD patterns of the gels showed that they were amorphous, while those of the samples heat-treated at 500 °C (Figure 2b) exhibited distinct diffraction peaks, indicating the presence of TiO2 (anatase) (25.28, 37.8, 48.05) (JCPDS 78-2486). Interestingly, no diffraction peaks related to Fe2O3 were found (Figure 2a), but its presence strongly affected the phase composition and microstructural characteristics. The sample containing 10 mol% of Fe2O3 showed the presence of rutile (12.9% ± 0.6), while the sample containing 20 mol% showed only diffraction peaks characteristic of a pure anatase phase. Obviously, the addition of greater amounts of Fe2O3 inhibits the anatase-to-rutile phase transition. No peaks related to Fe2O3 were found (Figure 2a). This phenomenon was not observed in the presence of other additives, such as ZnO, Fe2O3, etc. [17]. As can be seen from the figure, the performed heating was enough to completely remove the organic materials. Furthermore, this is implied by the persistent light-gray color of the powdered samples, ascribed to carbonaceous deposits derived from the decomposition of PVP. The calcination in air of the TiO2-Fe2O3/PVP gels (500 °C, 2 h) removed the organics completely and yielded a mixture of anatase and rutile phases.
Looking at the XRD pattern of sample 80TiO2-20Fe2O3/PVP, it is seen that there are no peaks indicative of rutile, regardless of the amount of Fe dopant. Despite the similarity between the ionic radii of Fe3+ (0.64 Å) and Ti4+ (0.68 Å), the question concerning the influence of iron doping on the anatase-to-rutile phase transformations is very contradictory in the literature. Our results did not confirm a statement made by Ghosh et al. [30], who reported that Fe accelerates the transformation of anatase to rutile. In the catalyst prepared with a higher Fe2O3 content (20 mol%), the absence of a rutile phase indicates that Fe3+ can stabilize the anatase phase of TiO2. Additionally, the Fe dopant did not change the TiO2 (anatase) crystallite size (Table 1). Moreover, the crystallite sizes of pure TiO2 and the investigated samples were very similar, 25 and 28 nm, respectively, which could be the reason for rutile phase retardation [30,31]. There was no indication of a crystalline phase containing Fe, even at 20 mol% Fe, which indicates that the dopant ions were successfully incorporated into the framework of anatase. This result may also be due to the low calcination temperature (500 °C) applied during the catalyst synthesis procedure; the Fe3+ ions did not react with TiO2 to form new crystalline phases such as α-Fe2O3 or Fe2TiO5. Our results coincide well with those obtained by Khan et al. [32]. Obviously, excessive Fe doping (~20 mol%) suppresses the transformation of anatase to rutile.
Additionally, the microstructural characteristics of the samples were also affected by the temperature treatment and the presence of Fe2O3. After annealing at 500 °C, an anatase-to-rutile phase transition was observed in the commercial TiO2, which led to 13.7% rutile in the sample. Most notably, the crystallite size increased from 25.1 nm to 34.7 nm for the anatase phase, while the newly formed polymorph (rutile) had a crystallite size of 43.8 nm. Interestingly, the microstrains of the anatase phase also increased, which, at first glance, is uncharacteristic since the high temperature treatment should lead to the recombination of defects. The increasing microstrains is most likely due to the presence of a secondary phase (rutile), which leads to the deformation of the anatase unit cell. The sample containing 10 mol% of Fe2O3 showed a slightly smaller amount of rutile, which could be attributed to the presence of both Fe2O3 and PVP. Evidence of the inhibited anatase-to-rutile phase transition can also be found through the fact that the crystallite size of the rutile phase in this sample is much smaller compared to the commercial non-modified TiO2: 19.7 nm and 43.8 nm, respectively. Interestingly, the Fe2O3 content does not significantly affect the crystallite size of the anatase phase (in both samples, the crystallite size is ~27.6 nm), but it strongly affects the microstrains (the defective state). Information on the samples’ surfaces was obtained by BET measurements. The investigated gels and annealed samples showed a specific surface value of about 5–7 m2/g, while pure commercial TiO2 exhibited a value of ~80 m2/g.
Additional structural and microstructural information was extracted by the full-profile Rietveld method with the GSAS II crystallography software package [33]. The results are shown on Figure 3 and Table 1.
The surface morphology of the gel samples and the samples annealed at 500 °C with a nominal composition of 90TiO2-10Fe2O3/PVP was examined by SEM, as shown in Figure 4a–d. In addition, a comparison of their morphology with that of commercial TiO2 was made. The SEM images of the samples were taken at the same magnification, with a marker of 20 μm. It was observed that the hybrid was dominated by plate-like particles, with a smooth surface and high agglomeration tendency in the annealed sample. The particles had a size of ~15 μm (Figure 4a,b). Obviously, the heat treatment did not change the surface morphology of the as-prepared powders. The SEM images of the other sample exhibited fine granular nanostructures dispersed homogenously on the surface. High homogeneity in the morphology of the commercial product was observed as well (Figure 4c,d). Our findings are in good agreement with those obtained by other authors [5].

3.2. IR Structural Investigations

The IR spectra of the gels and annealed samples in a wavenumber range from 2000 to 400 cm−1 are shown in Figure 5a,b. The assignment of the bands was carried out in the framework of the local point symmetry approach, and an empirical comparison between the obtained data and well-known spectra for the precursors and crystalline phases existing in the system. Generally, the IR spectra can be divided into two regions—below and above 1000 cm−1. Despite the different Fe2O3 amounts, the IR spectra of both gels are very similar, and well-resolved absorption bands at 800, 720–710, 610–600 and 480 cm−1 are observed. The IR spectrum of TiO2 (anatase) is characterized by bands at 620–610 and 480–470 cm−1 [34,35,36,37,38], connected to the vibrations of TiO6 units. On the other hand, it is known that the characteristic vibrations of FeO4 groups in pure Fe2O3 and ferrite compounds range from 660 to 625 cm−1, while for FeO6, they occur at 580 ± 550 and 470 cm−1 [2,39]. Therefore, it can be concluded that the vibrations observed in our spectral bands are difficult to distinguish due to their overlapping with vibrations characteristic of the TiO6 and FeO4 inorganic structural groups. More interesting is the high-frequency region, where bands in the range 1640–1030 cm−1 are seen. The pure Ti(IV) n-butoxide precursor is characterized by bands located between 1500 and 1300 cm−1 assigned to the bending vibrations of CH3 and CH2 groups [38]. The band at 1120 cm−1 is characteristic of the stretching vibrations of Ti–O–C, while those at 1100 and 1030 cm−1 are assigned to the vibrations of terminal and bridging C–O bonds in butoxy ligands [38]. Broad absorption bands below 1000 cm−1 correspond to C–H, C–O and deformation Ti–O–C vibrations [36,38]. Additionally, three bands at 1540, 1420 and 1320 cm−1 are absent in the IR spectra of the sample containing a higher Fe2O3 content, which could be related to the more complete hydrolysis–condensation processes in this gel. On the other hand, the absorption at 1650–1640 cm−1 in both samples could be ascribed to the C=O groups of PVP [21]. This band is also considered an indication of the interaction between PVP and metal ions [21]. The decreased intensity of this band in the spectra of 80TiO2-20Fe2O3/PVP is additional proof of the completeness of the hydrolysis–condensation processes, which are obviously enhanced by the higher amount of Fe2O3. According to Abbas et al. [2], the band at 1620 cm−1 could be associated with the hydroxyl groups owing to physically adsorbed water. The presence of these radicals could prove the enhanced photocatalytic activity. The presence of such groups on the catalyst’s surface increases the oxidizing power of the photocatalyst system because the hydroxyl group reacts with photo-excited holes during the reaction to form hydroxyl radicals [40,41,42]. An increase in the annealing temperature to 500 °C for a 2 h exposure time leads to the disappearance of the absorption bands of organic groups (Figure 5b). These bands become sharper and more intensive, which indicates the increased crystallinity of the samples. It is obvious that in the IR spectra of both samples, the band at 610–600 cm−1 shifts to lower frequencies that could be ascribed to the distortion of FeO4 tetrahedra [36]. Moreover, the increase in Fe2O3 content led to the disappearance of bands at 450 cm−1 in the IR spectrum of the 80TiO2-20Fe2O3/PVP sample. The other band at 640 cm−1, ascribed to TiOn polyhedra, is not well resolved in the spectrum of the sample containing 10 mol% of Fe2O3. According to Lopez et al. [43], this could be due to the influence of temperature or pH. Furthermore, they claimed that the titanium dioxide behavior corresponds to an ionic solid, and therefore, the vibrations in the low-energy interval are due to a shift in these ions.

3.3. Optical Properties

The optical properties of the gels were studied by UV–Vis diffuse reflectance spectroscopy and compared to those of pure TiO2 (Figure 6). The reflectance spectra were transformed to Kubelka–Munk (KM) coordinates, and the figure shows the KM function plotted against the wavelength. The gels are transparent with a brownish yellow color (Figure 1b), and they both exhibit higher absorbance in the UV region than that of the commercial titania. It is known that the UV–Vis spectra of the gels derived from Ti(IV) butoxide exhibit two maxima at 230–260 nm and 300–325 nm, which could be assigned to the isolated TiO4 and TiO6 units, respectively [24,37]. As shown in the figure, for the commercial TiO2, the intensities of the UV peaks in the spectra at 250 nm and 290 nm are comparable, which is an indication of the similar amounts of TiO4 and TiO6 polyhedra in the gel network. Other investigations have reported that titania showed bands below 250 nm and broad absorption in a wavelength range of 300–350 nm [2,15,44], and the as-prepared samples demonstrated absorption edges in this region depending on the sample composition. Absorption below 400 nm gradually increases with the increase in TiO2 content. Thus, the gel containing a higher amount of TiO2 (90 mol%) possesses higher absorbance compared to that containing 80 mol% of titania. Bearing in mind the assignments made by Izawa et al. [44], the bands at about 330 nm could be ascribed to Fe2+-O charge transfer.
The UV–Vis spectra were also used for the determination of the absorption edge and optical band gap of the gels. As can be seen from the figure, the absorption peaks were found to be in the range of 220–300 nm: 90TiO2–10Fe2O3/PVP (339.7 nm), 80TiO2-20Fe2O3/PVP (350.7 nm) and TiO2 (312.8 nm) (Table 2). According to the literature data, absorption peaks above 400 nm could be related to the presence of Fe2O3 [2]. Looking at our results again, such peaks were not detected. Additionally, it is evident that a higher amount of Fe2O3 leads to the narrowing of the band gap (Eg) due to the red shift in the adsorption edge of the 80TiO2-20Fe2O3/PVP sample. The other gel (with 90 mol% of TiO2) exhibited a shift in the cut-off value toward the lower wavelengths of the spectra (Figure 6). The determined band gaps are as follow: 90TiO2-10Fe2O3/PVP (3.12 eV), 80TiO2-20Fe2O3/PVP (3.12 eV) and TiO2 (3.09 eV) (Table 2). As can be seen, the band gap values of both compositions are higher than those of pure TiO2. It is known that the band gap is a major factor determining the electrical conductivity, and it depends on doping, size, temperature, etc. The higher value of Eg for the samples containing Fe2O3 may be attributed to the smaller particle size of the catalysts, which favor greater surface defects [45]. Another feature was observed in the UV–Vis spectra. The absorption peaks in the spectra of sample 90TiO2-10Fe2O3/PVP are sharper and more intensive compared to those of the other gel. This could potentially be ascribed to the higher amount of Fe2O3 in that sample. Obviously, its presence influences the completeness of the hydrolysis–condensation reactions. From the obtained results, it could be predicted that the investigated samples possess photocatalytic properties.
The refractive index (Table 2) was also determined, and it was calculated using a modification of the Dimitrov–Sakka equation [46]:
n = 3 E g / 20 − 2
where n is the refractive index and Eg is the band gap energy. The refractive index value is inversely proportional to the Eg value, so the greater the band gap energy, the smaller the refractive index.
The potentials of the conduction band (CB) and valence band (VB) were calculated using the equations ECB = X – 4.5 – 0.5Eg and EVB = ECB + Eg, where X denotes the absolute electronegativity of the material, which was determined using Mulliken’s principle. For the composites, the electronegativity was estimated by multiplying the electronegativities of Fe2O3 and TiO2 by their corresponding parts. The energy of the free electrons on the hydrogen scale (E0 = 4.5 eV) was used for the above calculations. There were very small shifts in the CB and VB positions, consistent with a small perturbation from Fe doping or mixing.

3.4. Photocatalytic Properties

The photocatalytic activity of the samples was evaluated through the photodegradation of tetracycline hydrochloride for both TiO2-Fe2O3/PVP gels (Figure 7a–d) and the powders annealed at 500 °C (2 h) (Figure 8a–d). As can be seen, all the obtained materials show photocatalytic activity for the degradation of tetracycline hydrochloride with both light irradiation sources. The as-prepared gels exhibited higher photocatalytic activity for the decomposition of the pharmaceutical (Figure 7a–d) and higher reaction rates, as shown in the kinetic models (Figure 7c,d), than the annealed powders (Figure 8c,d). This could be due to the amorphous state of the materials, which naturally leads to a higher surface area as well as a higher content of -OH groups on the surface of the catalysts, as proven from the IR analysis. The highest catalytic activity under UV irradiation was found for the annealed commercial TiO2 (kapp= 67.9 × 10−3 min−1), followed by the non-annealed commercial TiO2 (kapp = 54.3 × 10−3 min−1) (Table 3). This change in activity is probably due to the formation of a heterojunction between the anatase and rutile after annealing (as mentioned above, the sample before annealing is pure anatase) [47]. Among the samples containing Fe2O3, the one containing 10 mol% of Fe2O3 shows higher photoactivity before and after annealing under UV. Interestingly, the sample containing 20 mol% of Fe2O3 does not exhibit a change in photocatalytic activity before and after annealing. To explore the practical application of our catalyst, experiments under solar light irradiation were performed. It is evident that all the samples are more active under simulated solar light illumination. The photocatalytic activity of the 90TiO2-10Fe2O3/PVP sample before annealing increases by 1.73 times, while after annealing, it increases by 4.74 times, and as a result, both samples have similar apparent rate constants (~15.5 × 10−3 min−1). Therefore, further improvements of the compositions could be made in order to enhance their photocatalytic performance under both UV and solar light.
Bearing in mind the results obtained for the photocatalytic activity of the investigated samples, it is important to note that the activity of the 80TiO2-20Fe2O3/PVP sample is lower than that of the 90TiO2-10Fe2O3/PVP sample (Table 3). Despite the absence of rutile in the XRD pattern for the composition with a higher Fe2O3 content, this could be related to the influence of pH. During the experiments, the measured pH was about 2–3, and it became more acidic with increasing Fe2O3 content. Like many antibiotics, tetracycline hydrochloride (TCH) exhibits multiple ionizable functional groups, including dimethylammonium, tricarbonyl amid, etc., which undergo protonation–deprotonation reactions, forming cation species at low pH values (<3). Hsu et al. found that in such acidic conditions, the electrostatic repulsion between the cationic moieties of TC and the positive charges on the surfaces of the Fe-containing sample lead to a decrease in adsorption [48].
Bearing in mind that the TCH molecules possess negatively charged tricarbonylamide groups, it could be assumed that at low pH the repulsion between the photocatalysts and these molecules hinders their adhesion to the catalyst surface. Consequently, the photodegradation process becomes more challenging as the degradation facilitated by electron–hole pairs is obstructed. In these acidic conditions, the presence of OH− could interrupt the photodegradation process, as it induces the formation of intramolecular hydrogen bonds in electron-donating groups. This confers increased chemical stability to the pharmaceutical, making it more resistant to attacks by hydroxyl radicals. As a result, a decrease in the photocatalytic activity of the 80TiO2-20Fe2O3/PVP sample was observed. On the other hand, it was also found that the acidified solutions led to a coagulation of the photocatalyst, which decreased its activity as well [49].

3.5. Antibacterial Properties

The results of antibacterial activity testing are presented in Figure 9. It shows the bacterial growth of E. coli ATCC 25922 at the beginning of (microbial count 1.2 × 106 CFU/mL) and during the experiment. The number of colonies after 30 min and 90 min of photocatalytic treatment with 1 mg/mL of 90TiO2-10Fe2O3/PVP was 8.2 × 105 CFU/mL and 1.9 × 105 CFU/mL, respectively. Obviously, the powders annealed at 500 °C exhibited weak antibacterial action in the presence of UVA light, and the bacterial colonies slightly reduced from 1.2 × 106 CFU/mL to 1.9 × 105 CFU/mL within 90 min.
A comparison between these results and our previous investigations [50] concerning Fe-containing TiO2 powders revealed that the inactivation of bacteria depends on many factors, such as the method of synthesis and the size and concentration of the dopant, which have an impact on antibacterial action [18]. Evidently, the UVA illumination and the addition of iron did not affect the antibacterial activity of the examined sample. Despite the good photocatalytic properties for the degradation of tetracycline hydrochloride under UV illumination (Figure 8a), a weak antibacterial effect was observed. Thus, the statement that the powders containing Fe2O3 posses moderate or weak bioactivity against Gram-negative bacteria was confirmed [51,52].
Although photocatalytic degradation and antibacterial activity are frequently attributed to the same mechanisms (ROS generation), different trends for photocatalytic and antibacterial activities in our investigation were observed. This phenomenon has also been observed by other researchers [24,53,54]. Moreover, it has already been stated previously that in many applications the removal of both bacterial and chemical contamination may be desirable, while in other applications, it may be preferable to separate these two effects. Usually, the photocatalytic degradation of the pharmaceutical mainly proceeds via direct oxidation by valence-band holes rather than indirect oxidation by generated ROS. This is why the different trends observed could be explained through the involvement of the direct charge transfer of photogenerated carriers for pharmaceutical degradation, while antibacterial activity may occur due to the involvement of the ROS produced [50].

4. Conclusions

By applying a sol–gel method, TiO2-Fe2O3/PVP hybrids were successfully prepared. The structural and microstructural characteristics of the annealed TiO2-Fe2O3/PVP samples (crystallite size, microstrains, and weight fraction of the phases) were obtained through a Rietveld analysis of the XRD data for the composites. The combination of IR and UV–Vis spectroscopies allowed us to reveal the structural peculiarities of the investigated samples. The UV–Vis spectra showed a red shift for the investigated samples. SEM investigations showed the agglomerations in the samples heat-treated at 500 °C. The antibacterial behavior of the investigated samples exhibited a slight reduction in E. coli ATCC 25922 cell colonies under UVA light within 90 min. Both samples exhibited weak photocatalytic efficiency for the degradation of tetracycline hydrochloride, but the 90TiO2-10Fe2O3/PVP sample showed better activity. The improvement of photocatalytic activity will be the subject of our future investigations in search of new compositions with promising environmental applications.

Author Contributions

Conceptualization, A.B.-N.; methodology, A.B.-N., M.T.; investigation, S.P., Y.K.; writing—original draft preparation, A.B.-N., M.T., S.P. and A.S.; writing—A.B.-N., M.T. and A.S.; antibacterial properties, H.H. and P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The author A. Bachvarova-Nedelcheva is thankful for the support provided by the European Regional Development Fund under the “Research Innovation and Digitization for Smart Transformation” program 2021–2027 under the Project BG16RFPR002-1.014-0006 at the National Centre of Excellence for Mechatronics and Clean Technologies. Research equipment from the distributed research infrastructure INFRAMAT, supported by the Bulgarian Ministry of Education and Science, was used.

Conflicts of Interest

The authors declare no conflicts of interest.

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Optics 06 00022 g001
Figure 1. Scheme for the sol–gel synthesis of the ternary TiO2-Fe2O3/PVP hybrids (a) and their visual observations (b).
Figure 1. Scheme for the sol–gel synthesis of the ternary TiO2-Fe2O3/PVP hybrids (a) and their visual observations (b).
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Figure 2. XRD patterns of the TiO2-Fe2O3/PVP samples: (a) hybrids and (b) samples annealed at 500 °C.
Figure 2. XRD patterns of the TiO2-Fe2O3/PVP samples: (a) hybrids and (b) samples annealed at 500 °C.
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Figure 3. Experimentally observed (black dots), Rietveld-calculated (continuous red line), and difference (continuous bottom blue line) profiles obtained after the Rietveld analysis of the XRD data for (a) commercial TiO2 (Evonik), TiO2 annealed at 500 °C (b), 90TiO2-10Fe2O3/PVP annealed at 500 °C (c) and 80TiO2–20Fe2O3/PVP annealed at 500 °C (d). Peak positions are shown at baseline as small markers.
Figure 3. Experimentally observed (black dots), Rietveld-calculated (continuous red line), and difference (continuous bottom blue line) profiles obtained after the Rietveld analysis of the XRD data for (a) commercial TiO2 (Evonik), TiO2 annealed at 500 °C (b), 90TiO2-10Fe2O3/PVP annealed at 500 °C (c) and 80TiO2–20Fe2O3/PVP annealed at 500 °C (d). Peak positions are shown at baseline as small markers.
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Figure 4. SEM images of the 90TiO2-10Fe2O3 /PVP hybrid: non-annealed (a), annealed at 500 °C (b) and non-annealed TiO2 (c), alongside commercial TiO2 (Evonik) annealed at 500 °C (d).
Figure 4. SEM images of the 90TiO2-10Fe2O3 /PVP hybrid: non-annealed (a), annealed at 500 °C (b) and non-annealed TiO2 (c), alongside commercial TiO2 (Evonik) annealed at 500 °C (d).
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Figure 5. IR spectra of the 90TiO2-10Fe2O3/PVP samples—non-annealed (a) and annealed at 500 °C (b).
Figure 5. IR spectra of the 90TiO2-10Fe2O3/PVP samples—non-annealed (a) and annealed at 500 °C (b).
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Figure 6. UV–Vis spectra of the as-prepared gels (a) and energy band gap (Eg) calculated from Tauc’s equation (b).
Figure 6. UV–Vis spectra of the as-prepared gels (a) and energy band gap (Eg) calculated from Tauc’s equation (b).
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Figure 7. Photocatalytic degradation of tetracycline hydrochloride by the as-prepared gels under UV (a) and sunlight illumination (b); Langmuir–Hinshelwood kinetic model for tetracycline degradation under UV light (c) and sunlight (d) illumination.
Figure 7. Photocatalytic degradation of tetracycline hydrochloride by the as-prepared gels under UV (a) and sunlight illumination (b); Langmuir–Hinshelwood kinetic model for tetracycline degradation under UV light (c) and sunlight (d) illumination.
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Figure 8. Photocatalytic degradation of tetracycline hydrochloride by the investigated samples annealed at 500 °C (2 h) under UV (a) and sunlight illumination (b); Langmuir–Hinshelwood kinetic model for tetracycline degradation under UV light (c) and sunlight illumination (d).
Figure 8. Photocatalytic degradation of tetracycline hydrochloride by the investigated samples annealed at 500 °C (2 h) under UV (a) and sunlight illumination (b); Langmuir–Hinshelwood kinetic model for tetracycline degradation under UV light (c) and sunlight illumination (d).
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Figure 9. Growth of E. coli ATCC 25922 on MHA plates with the inoculation of 100 µL of 10−1 and 10−2 dilutions from the initial suspension (0 min) and the suspension treated with 1 mg/mL of TiO2/Fe2O3 and UVA after 30 min and 90 min of exposure (a); kill curves of E. coli ATCC 25922 in the presence of the investigated sample under UVA light (b).
Figure 9. Growth of E. coli ATCC 25922 on MHA plates with the inoculation of 100 µL of 10−1 and 10−2 dilutions from the initial suspension (0 min) and the suspension treated with 1 mg/mL of TiO2/Fe2O3 and UVA after 30 min and 90 min of exposure (a); kill curves of E. coli ATCC 25922 in the presence of the investigated sample under UVA light (b).
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Table 1. Structural and microstructural information extracted by Rietveld method.
Table 1. Structural and microstructural information extracted by Rietveld method.
SamplePhase FractionUnit Cell Parameters, ÅCrystallite Size, nmMicrostrains, ×10−3 a.u. Refinement Details
Evonik100% Anatasea = b = 3.7845 (3)
c = 9.5112 (7)		25.1 (1)2.50 (17)Rwp = 12.73%
χ2 = 1.29
Evonik 500 °C86.3% ± 0.4 Anatasea = b = 3.7861 (3)
c = 9.5079 (7)		34.7 (1)3.80 (13)Rwp = 10.58%
χ2 = 1.18
13.7% ± 0.4 Rutilea = b = 4.5940 (7)
c = 2.9598 (5)		43.8 (4)0.15 (1)
90TiO2–10Fe2O3/
PVP		87.1% ± 0.6 Anatasea = b = 3.7857 (5)
c = 9.5037 (12)		27.6 (10)9.87 (17)Rwp = 12.99%
χ2 = 1.27
12.9% ± 0.6 Rutilea = b = 4.5935 (17)
c = 2.9604 (13)		19.7 (13)0.70 (3)
80TiO2–20Fe2O3/
PVP		100% Anatasea = b = 3.7860 (5)
c = 9.4991 (11)		27.7 (10)10.44 (16)Rwp = 12.97%
χ2 = 1.22
Table 2. Energy of the band gap, potentials of the current band (CB) and valence band (VB), cut-off and refractive index (n).
Table 2. Energy of the band gap, potentials of the current band (CB) and valence band (VB), cut-off and refractive index (n).
SampleEg, eVECB, eVEVB, eVCut-Off, nmRefractive Index (n)
90TiO2-10Fe2O3/PVP (500 °C)3.12−0.2432.877339.72.36
80TiO2-20Fe2O3/PVP (500 °C)3.12−0.2362.884350.72.36
TiO2 (Evonik) (500 °C)3.09−0.2352.855312.82.37
Table 3. Calculated rate constants according to the Langmuir–Hinshelwood kinetic models.
Table 3. Calculated rate constants according to the Langmuir–Hinshelwood kinetic models.
CatalystLight SourceRate Constant ×10−3, min−1Light SourceRate Constant ×10−3, min−1
90TiO2-10Fe2O3/PVP (gel)UV light9.03Sunlight15.66
80TiO2-20Fe2O3/PVP (gel)2.188.92
TiO2 (Evonik) 54.3200.97
90TiO2-10Fe2O3/PVP (500 °C)3.2715.51
80TiO2-20Fe2O3/PVP (500 °C)2.216.75
TiO2 (Evonik) (500 °C)67.9107.05
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Petrova, S.; Kostova, Y.; Tsvetkov, M.; Stoyanova, A.; Hitkova, H.; Marinovska, P.; Bachvarova-Nedelcheva, A. Synthesis, Optical Properties and Photocatalytic Testing of Sol–Gel TiO2-Fe2O3/PVP Nanopowders. Optics 2025, 6, 22. https://doi.org/10.3390/opt6020022

AMA Style

Petrova S, Kostova Y, Tsvetkov M, Stoyanova A, Hitkova H, Marinovska P, Bachvarova-Nedelcheva A. Synthesis, Optical Properties and Photocatalytic Testing of Sol–Gel TiO2-Fe2O3/PVP Nanopowders. Optics. 2025; 6(2):22. https://doi.org/10.3390/opt6020022

Chicago/Turabian Style

Petrova, Stefani, Yoanna Kostova, Martin Tsvetkov, Angelina Stoyanova, Hristina Hitkova, Polya Marinovska, and Albena Bachvarova-Nedelcheva. 2025. "Synthesis, Optical Properties and Photocatalytic Testing of Sol–Gel TiO2-Fe2O3/PVP Nanopowders" Optics 6, no. 2: 22. https://doi.org/10.3390/opt6020022

APA Style

Petrova, S., Kostova, Y., Tsvetkov, M., Stoyanova, A., Hitkova, H., Marinovska, P., & Bachvarova-Nedelcheva, A. (2025). Synthesis, Optical Properties and Photocatalytic Testing of Sol–Gel TiO2-Fe2O3/PVP Nanopowders. Optics, 6(2), 22. https://doi.org/10.3390/opt6020022

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