Characterization of anatase TiO 2 nanoparticle: Comparative synthesis via microwave combustion and sol-gel techniques

Abstract

Using polyacrylic acid (PAA) as the template polymer and titanium tri-chloride as the titanium source, two distinct methods were employed in this study to prepare TiO₂ nanoparticles: a sol-gel method and a microwave combustion method. The methods described above were used to determine the ideal polymer ratio to create three distinct molar proportions of TiCl₃ and PAA (1:1, 1:2, and 1:3) at 300 °C. After calcination at various temperatures (400, 450, 500, and 550 °C), the TiO₂ nanoparticles prepared by the two different methods were characterized by XRD. Infrared (FTIR) spectroscopy was used to determine the chemical structures of the TiO₂ and polymer. The morphology and particle size of TiO₂ at the ideal temperature were evaluated by transmission electron microscopy (TEM). UV-VIS spectroscopy was used to characterize the optical properties of the TiO₂ samples. X-ray diffraction (XRD) analysis determined that a 1:2 ratio of TiCl₃ to PAA (MW2 and SG2) is optimal for producing well-crystallized TiO₂ in the anatase form through a microwave-based method. Calcination at 550 °C further enhanced the crystallization of anatase nanoparticles, mainly when the microwave method was applied. In contrast, the sol-gel method indicated that the powder remained amorphous at 300 °C, with higher calcination temperatures leading to the formation of both anatase and rutile phases. The size of the TiO₂ nanoparticles created by the microwave approach was determined to be 3–9 nm, whereas that of the TiO₂ particles prepared by the sol-gel method was 8–20 nm. MW2 and SG2 had band gaps of 3.17 eV and 3.15 eV. In this study, we successfully synthesized nano-anatase TiO₂ using an efficient and time-saving approach (microwave combustion method) in conjunction with heat treatment. The resulting material has potential applications in various fields, including energy storage, supercapacitor electrodes, photoelectric conversion, optical coatings, beam splitters, and anti-reflection coatings.

Keywords

microwave sol-gel polyacrylic acid nanoparticles anatase rutile cytotoxicity

Introduction

TiO₂ has been considered one of the most investigated metal oxides in recent decades. Its properties include chemical and mechanical stability, corrosion resistance, low cost, non-toxicity, high optical transparency, high refractive index, expansive band gap energy, and photocatalytic activity, making titanium oxides suitable for a wide range of applications such as paints, photochromic, photocatalysis, photovoltaics, electrochromic, and toothpaste.^1–4 In addition, titanium dioxide (TiO₂) is an important industrial material used as a central component of paints, pigments, cosmetics, and catalysts. It has also been used for optical coatings, beam splitters, and anti-reflection coatings owing to its high dielectric constant and refractive index.⁵ Anatase TiO₂ was used by Sankapalet al. in composite form for applications in energy storage supercapacitor electrodes.⁶ Recently, Nakata et al.reviewed TiO₂ applications for photoenergy conversion.⁷ Different methods for preparing nanomaterials help improve the surface nanostructure properties.⁸

The three crystalline forms of titanium dioxide include rutile, brookite, and anatase. Compared to the anatase phase (3.2 eV), rutile is thermodynamically stable with a lower band gap energy (3.0 eV). Numerous TiO₂ preparation techniques have been studied and documented in the literature.^5,9 The initial materials significantly influence the creation of well-defined crystalline TiO₂ nanocrystallites. Titanium alkoxides are typically used in hydrothermal and sol-gel techniques to synthesize nanocrystalline anatase. Recently, citrate gel and digestion processes have been used to prepare ultrafine TiO₂ powders.^5,9 The microwave method is a more recent technology for producing nanocrystalline oxides in a concise amount of time. The microwave technique has three benefits over the traditional sol-gel and hydrothermal methods: (i) rapid crystallization kinetics, (ii) rapid heating to the treatment temperature, and (iii) feasibility. It has been reported that colloidal TiO₂ NPs can be generated in less than an hour with a microwave-assisted hydrothermal process compared to nearly a day with usual hydrothermal and sol-gel methods.^5,9 The speed and effectiveness of microwave heating make it an attractive alternative, as it can generate high reaction rates and maximize production quickly. This contrasts with slower and less productive conventional heating techniques.^10–14 Furthermore, the microwave approach yields a much better product than traditional heating technologies.

Microwave irradiation (MW), in which energy is delivered to reactants through molecular interactions with an electromagnetic field, is a new method that may resolve the problems arising from conventional synthesis methods.¹⁵ Microwave combustion technology is a promising approach for advancing green chemistry. It is a crucial tool for synthesizing a variety of nanostructures with diverse morphologies and uniform particle sizes.^16–23 Single-phase anatase TiO₂ is known to form at low temperatures and high pressure.²⁴ In the microwave-assisted hydrothermal process, methanol can be employed as a solvent to produce a reaction that produces high pressure at lower temperatures locally. Comprehensive characterization (to be described) showed that microwave irradiation for a mere 10 min aided in the anatase phase's nucleation and stability. Anatase TiO₂ nanospheres (ATNS) showed traits pointing to the increased anatase TiO₂ intrinsic photocatalytic activity. For methylene blue Dye,²⁴ the produced ATNS showed 98% photodegradation efficiency.

The photocatalytic activity of titanium dioxide is considered one of its most valuable applications. Among the different polymorphs of TiO₂, the anatase phase exhibits the highest photocatalytic activity.²⁴ This photocatalytic effect depends on generating electron-hole pairs within the titanium dioxide material, which is caused by exposure to ultraviolet (UV) radiation. These generated electron-hole pairs then migrate to the surface of the titanium dioxide, where they participate in chemical reactions that lead to the decomposition of adsorbed substances on the material's surface.²⁵ In addition, the photocatalytic properties of titanium dioxide induce the generation of hydroxyl radicals (OH) and other highly oxidizing species (ROS), and titania is considered a highly effective agent for the inactivation of bacteria.²⁶ Čizmićet al. prepared a nanostructured TiO₂ film for the successful degradation of azithromycin. They evaluated factors such as the pH value, different water matrices, various pollutants, and radiation sources to find the optimum ones.^27,28 The same TiO₂ film was deposited on a glass substrate and used for photocatalytic degradation of lissamine green B dye by Ćurkovićet al..²⁹ Gabelica et al. prepared a ternary core-shell Fe₃O₄/SiO₂/TiO₂ nanocomposite for the degradation of ciprofloxacin, with the prospect of recycling and reuse.³⁰ However, the high band-gap energy limits the photoactivation of TiO₂ to UV light, and it must be modified by doping with various metals, non-metals, and composites to shift its photoactivity to the visible light range.³¹ Sanchez Tobon et al. prepared nitrogen-doped and reduced graphene oxide composite (N/TiO₂/rGO) for degradation of different micropollutants, such as ciprofloxacin, diclofenac, and salicylic acid, under different radiation sources.³² They observed a synergistic effect of adsorption and photocatalysis, whereas the degradation rate was affected by the radiation source, irradiation intensity, and type of pollutant. Malakootian et al. immobilized TiO₂ nanoparticles on a glass plate and investigated the photocatalytic degradation of ciprofloxacin in an aqueous solution.³³

This research study focusing on the preparation of titania nanoparticles (TiO₂) using two different synthesis techniques, sol-gel and microwave combustion utilizing titanium chloride (TiCl₃) as a precursor and polyacrylic acid (PAA) as a template polymer, introduces several novelties. The novelty lies in juxtaposing these methods to analyze the influence on nanoparticle characteristics. This research explores the effect of using the same precursors, titanium chloride and polyacrylic acid polymer, in different synthesis environments (sol-gel vs. microwave) on particle size, crystallinity, phase formation, and Cytotoxicity, which has not been extensively studied. Compared to the more commonly used alkoxides (like titanium isopropoxide), titanium chloride precursors offer a lower cost and more straightforward handling. In addition, the usage of Polyacrylic acid (PAA) as a template polymer during synthesis processes and studying the effect of changing its molar ratio on the prepared titania particles’ properties shows a significant role in controlling the particle growth, preventing aggregation leads to the enhancement of their photocatalytic activity, making them potentially more effective for applications such as photocatalysis and other different applications.⁵

Materials and experimental methods

Starting materials and synthesis methods

TiO₂ nanoparticles were synthesized via two separate methods: microwave combustion and polymeric sol-gel. The first route involved a wet chemical assessment of the microwave combustion technique, using polyacrylic acid as the fuel. The second technique involves a simple reaction with water in the presence of polyacrylic acid as a template polymer to produce the intended TiO₂ nanoparticles. The synthesis process involved mixing titanium trichloride (TiCl₃, 99.9%, purchased from Aldrich) and polyacrylic acid (PAA, purchased from Aldrich) in three different molar ratios (1:1, 1:2, and 1:3) of TiCl₃ and PAA. Sample abbreviations are presented in Table 1. As illustrated in Figure 1, the solution was subjected to mild heating at 50 °C with vigorous stirring for approximately one hour using a magnetic stirrer. The first part of the solution was kept at 70°C for 24 h; this sample was prepared using the polymeric sol-gel method. The gel obtained was calcined at 300°C. The other part of the solution mixture was kept in a microwave oven (900 W EM-D975 W SANYO) for 10 min at 300 °C for each power and frequency of 2.45 GHz; this sample part was prepared by the microwave combustion method. The obtained powder was cooled to the ambient temperature and dried at 100°C. The behavior of the materials obtained using both methods was characterized at 300°C to optimize the amount of PAA. After optimizing the PAA content, other solutions were prepared using the same procedures described above, and the effect of increasing the calcination temperature to 400,450, 500, and 550°C on the crystallization of TiO₂ was investigated.

Figure 1.

Schematic diagram for the two preparation methods: sol-gel and microwave combustion.

Table 1.

Abbreviations of the prepared samples and their compositions.

abbreviation	Synthesis Method	Molar Ratio of TiCl_{3 :} PAA
SG1	Sol-gel method	1:1
SG2	Sol-gel method	1:2
SG3	Sol-gel method	1:3
MW1	Microwave treatment	1:1
MW2	Microwave treatment	1:2
MW3	Microwave treatment	1:3

Powder characterization

Fourier transform infrared spectroscopy (FT-IR, using instruments MB154S and Bomem) was employed to determine the dominant bonds present in the TiO2 powders and assess the impact of the two synthesis methods on the spectral characteristics of the obtained structures. The crystalline phases were detected using an X-ray powder diffractometer (XRD, Bruker D8) with nickel-filtered Cu Kα radiation, following microwave and calcination treatments. A transmission electron microscope (JEOL JEM-1230) equipped with a high-angle angular dark-field detector and an X-ray energy-dispersive spectroscopy system was used to analyze the particle size and degree of crystallinity of the prepared titania nano-powders. Furthermore, the burning out of the template polymer and starting the crystallization process of titania were determined using a differential thermal analysis (DTA: STERAM Labsys TM TG-DSC 1600 °C) apparatus up to 1000 °C at a constant heating rate of 5 °C/min. The microstructures of the titania nano-powders were identified by scanning electron microscopy (SEM, Philips XL 30).

A transmission electron microscope (TEM; JEOL JEM-2100) equipped with a high-angle, angular dark-field detector and an X-ray energy-dispersive spectroscopy system was used to examine the particle size and degree of crystallinity. The UV–visible spectra of the TiO₂ nano-powders were recorded using a Thermo Scientific Evolution 300 UV–visible spectrophotometer to estimate their energy band gaps.

Cytotoxicity test on human fibroblast cell line

Cytotoxicity testing is a valuable tool for the initial screening of materials and substances with potential safety concerns. Cytotoxic effect of 100 μg/ml of prepared sample samples SG2 and MW2 calcined at 550°C were examined on a human regular fibroblast cell line (BJ1) using MTT assay. Cell viability was evaluated by measuring the mitochondrial-dependent reduction of yellow MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) to purple formazan in the Bioassay-Cell Culture Laboratory (National Research Centre, Cairo, Egypt).^34,35 To determine the percentage change in cell viability, the following formula was used.³⁶

((reading of extract / reading of negative control) - 1) \times 100.

The cells were grown in a controlled environment with specific nutrients and growth factors at 37 °C in a 5% CO₂ incubator. After the incubation period, the cells were treated with 100 μg/ml of prepared samples SG2 and MW2 and incubated for 48 h in a 5% CO₂ incubator at 37°C. The cells were washed with SDS. The generated insoluble formazan crystals were dissolved in 100 µL of DMSO solution, and the absorbance of each sample was measured at 595 nm using a microplate reader.^34,35

Results and discussion

XRD investigations of TiO₂ nanoparticles

The X-ray diffraction patterns of TiO₂ nanoparticles synthesized by microwave combustion and polymeric sol-gel methods are presented in Figures 2–5. The X-ray diffraction patterns of the titania prepared using the microwave method (MW2) confirmed the formation of anatase at 300°C. (Anatase XRD card no.:PDF#894921) in all titania samples with different polyacrylic acid concentrations (MW1, MW2, and MW3). The XRD patterns indicate that the intensity of the anatase peaks increased with increasing polymer concentration, as in (MW1 and MW2), followed by a further decrease in the (MW3) sample, as shown in Figure 2. These results can be attributed to the two samples, MW1 and MW2, which have sufficient polymer concentrations to increase the uniform distribution of titanium cations throughout the polymer structure, accelerating the crystallization process. The formation of titania particles lies in the nanosize range, whereas a further increase in the polymer concentration leads to polymer agglomeration, which retards the titania crystallization process. Thus, MW2 was selected to determine the phase composition at different calcination temperatures. The X-ray diffraction pattern of titania prepared by the polymeric sol-gel method (SG) confirmed that TiO₂ was completely amorphous in titania samples calcined at 300°C for all titania samples with different polyacrylic acid concentrations (SG1, SG2, and SG3), as shown in Figure 3. Thus, SG2 was selected to follow the phase composition and conduct a comparative study with the MW2. The earlier formation and extremely rapid crystallization of anatase by microwave combustion is due to the generation of localized high temperatures in the presence of microwaves.⁵

Figure 2.

XRD patterns of titania particles prepared by microwave combustion with different molar ratios of (TiCl₃: PAA); MW1(1:1), MW2(1:2), and MW3(1:3) and calcined at 300°C.

Figure 3.

XRD patterns of titania particles prepared by sol-gel with different molar ratios of (TiCl₃: PAA); SG1(1:1), SG2(1:2), and SG3(1:3) and calcined at 300°C.

Figure 4.

XRD patterns of titania particles prepared by microwave combustion with molar ratios of (TiCl₃: PAA); MW2(1:2) and calcined at different temperatures: 400, 450, 500, and 550°C.

Figure 5.

XRD patterns of titania particles prepared by sol-gel with molar ratios of (TiCl₃: PAA); SG2(1:2) and calcined at different temperatures: 400, 450, 500, and 550°C.

Further calcination of MW2 at 400, 450, 500, and 550 °C (Figure 4) led to an increase in the anatase peak intensity up to 550 °C. The XRD patterns for titania SG2 calcined at 400, 450, 500, and 550 °C are shown (Figure 5). The sample calcined at 400 °C exhibits anatase as the main peak in the rutile phase. Further calcination at temperatures greater than 400 °C increased rutile crystallization and decreased anatase formation. These XRD results indicate that the microwave method is more efficient than the sol-gel method for preparing anatase nano-titania powder. The MW2 and SG2 samples were selected for further characterization.³⁷

Thus, the microwave combustion method obtained a single anatase phase, while the anatase and rutile phases were observed in the sol-gel. These behaviors can be explained as follows: It is well known that anatase and rutile TiO₂ can grow from TiO₆ octahedra and that the phase transition proceeds by rearranging the octahedra. The arrangement of octahedra through face sharing initiates the anatase phase, while edge sharing leads to the rutile phase. In aqueous media, the protonated surfaces of TiO₆ octahedra easily combine with the–OH groups of other TiO₆ octahedra to form Ti–O–Ti oxygen bridge bonds by eliminating water molecules. Protonation followed by the possible face-sharing TiO₆ octahedra will form the anatase phase, while edge sharing leads to the rutile phase. Face sharing is favored when using the microwave combustion method, leading to the anatase phase.⁵

Considering the diffractograms shown in Figures 4 and 5, it can be observed that the titania samples prepared by both synthesis processes obtained at 450 and 500°C present slightly broader peaks, which are also reduced in intensity compared to the samples synthesized at 550°C, revealing that the nanocrystals obtained at lower temperatures have a smaller size. This was evidenced by calculating the average crystallite size using the Scherrer equation. Comparing the samples synthesized at 500 and 550°C temperatures, larger crystallite sizes of SG2 samples of 20.48 nm and 22.92 nm when calcined at 500 and 550°C, respectively, while the MW2 sample showed a smaller crystallite size of 15.91 nm and 22.18 nm at a calcination temperature of 500 and 550°C, respectively. These results can be attributed to the fact that the crystallite sizes increased considerably as the temperature increased, owing to nanocrystal coalescence. Higher calcination temperatures resulted in larger crystallites. Zang (2006) stated that the instability of the anatase phase at high temperatures causes anatase particles to stick together to form larger particles, and the interface of the anatase particles becomes rutile phase nucleation.^38,39

The samples synthesized at 550 °C by microwave combustion presented the smallest crystallite size of 15.91 nm. The sample synthesized by the sol-gel process presented the largest crystallite size of 22.92 nm. The microwave combustion method typically produces smaller crystallites owing to rapid heating and uniform energy distribution, leading to more uniform particle nucleation. Calcination using both methods increased the crystallite size. However, the crystallite size growth of titania particles prepared by the MW method is typically less pronounced than that prepared by the sol-gel method, and this can be attributed to the shorter reaction time and rapid quenching effect.^16–23

Fourier transformation infrared spectroscopy (FTIR) analysis

FTIR spectroscopy was utilized in the 400–4000 cm⁻¹ range to monitor the reaction between polyacrylic acid and titanium tri-chloride to produce TiO₂ nanoparticles for MW2 and SG2 samples calcined at 500 °C, as illustrated in Figure 6. The results showed that the lattice vibrations of TiO₂ were responsible for the substantial absorption observed at 687–719 cm⁻¹. According to Lu et al. (2008), the stretching vibration absorption of the Ti–O and Ti-O-Ti bonds in TiO₂ nanoparticles causes a broad absorption band at 500–1000 cm⁻¹. These bands are critical indicators of the TiO₂ structure. Furthermore, when the TiO₂ particles were prepared by the MW method, the 400–700 cm⁻¹ absorbance bands became much sharper, providing strong evidence that the anatase phase was formed, as Adnen N. and Haider A. J described.^40,41 The sol-gel method typically leaves more hydroxyl groups on the surface, resulting in nanoparticles with hydroxyl (–OH) groups on their surface. This is a more intense band at 3440 cm⁻¹ (O–H stretching) and 1690 cm⁻¹ (O–H bending) associated with surface hydroxyl groups or adsorbed water. These bands are weaker in titania prepared via the microwave method, which tends to produce more crystalline and less hydrated particles.

Figure 6.

FTIR spectra of titania particles prepared by microwave and sol-gel with molar ratios of TiCl₃: PAA (1:2), MW2, and SG2, respectively, and calcined at 500°C.

Figure 6 shows no bands are associated with the organic polymer used during the synthesis process. Therefore, it was confirmed that the PAA polymer was removed. However, a small band at 1319 cm⁻¹ and a band near 1123–1180 and 1570 cm⁻¹ may appear, corresponding to Ti–O–C or residual organic groups from precursors used during synthesis, indicating a small quantity of residual carbon present. These bands are weak with lower intensity in the case of titania particles prepared by the MW method, and this can be attributed to rapid heating in the microwave method, which often leads to better crystallization and fewer organic residues that lead to better crystallization.⁴² Owing to the presence of these organic residue bands, further calcination at 550 °C was performed, as described in the XRD.

The FTIR spectra confirmed that the synthesized TiO₂ nanoparticles prepared by the two different methods showed the same bands of Ti-O-Ti and Ti-O but with lower intensity in the titania-polymer chain as compared to that present in the microwave (TiO₂-MW) pattern owing to the starting crystallization of titania faster than when using the sol-gel method, as indicated in the titania sol-gel (TiO₂-SG) pattern, which is consistent with the X-ray analysis.

Thermal analysis

The thermal stability of the optimum sample (MW2)was analyzed using TGA and DTA to understand the effects of the polymeric structure on TiO₂ at different temperature ranges. Figure 7 displays the DTA and TGA results for the as-prepared MW2 sample, which show three distinct weight loss stages. The initial weight loss occurred between 100 and 210 °C and was likely due to water vapor evaporation, as indicated by the 4.37% weight loss and a faint, broad endothermic peak at 100 °C in the DTA curve.⁴³ The second weight loss of 48.69% between 210 and 650 °C is attributed to the breakdown of the block copolymer and chlorine, as indicated by a sharp endothermic peak at 345.8 °C and a mild endothermic peak at 472 °C on the DTA curve. The crystallization of titania began at 450 °C, which was confirmed by the appearance of exothermic peaks, as shown in Figure 7. This was accompanied by an approximately 6.44% reduction in weight, and it was reported in the literature that the phase transition begins at 350 °C.^43,44 Because endothermic or exothermic processes invariably follow weight losses, the data from the TGA curve correlate directly with the DTA record, as shown by the entire TGA-DTA curve.⁴³

Figure 7.

(A) TGA and (b) DTG thermograms of as-prepared titania particles prepared by microwave with molar ratios of TiCl₃: PAA (1:2), MW2.

SEM and TEM characterization of TiO₂ nanoparticles

The morphology and size of the synthesized TiO₂ nanoparticles for the optimum samples (MW2 and SG2) were examined using SEM and TEM. The SEM and TEM images of the titania powders prepared by the microwave combustion and polymeric sol-gel methods are illustrated in Figures 8 and 9, respectively.

Figure 8.

SEM for titania particles prepared by microwave and sol-gel with molar ratios of TiCl₃: PAA (1:2), MW2, and SG2, respectively, and calcined at 550°C.

Figure 9.

TEM images of (a) titania particles prepared by microwave (MW2) sample; (b) titania particles prepared by sol-gel (SG2) sample calcined at 550 °C; (c) SAED of MW2 sample and (d) SG2 sample; HR-TEM image of (e) MW2 sample and (f) SG2 sample.

Figure 8 (a) shows the SEM image of TiO₂ powder sample MW2 calcined at 550 °C, prepared using microwave combustion. The SEM image shows well-arranged titania nanoneedle-like structure particles, while those prepared using the polymeric sol-gel method SG2 are presented as agglomerations of irregularly designed nanoparticles that hide their texture by the formation of clusters, as demonstrated in Figure 8 (b).³⁷

The TEM images of the titania powder sample MW2 prepared using the microwave combustion method and calcined at 550 °C reveal the explicit morphological nature of the prepared TiO₂nanopowder, as shown in Figure 9. This indicates the accumulation of polygonal nanoparticles with an average size ranging from 3 to 9 nm, which agrees with Ref.⁵ In comparison, the titania powder prepared by the polymeric sol-gel method (SG2) exhibited well-dispersed spherical nanoparticles ranging from 8 to 20 nm, as shown in Figures 9 (a) and (b). The SAED-TEM images of all the titania powders confirm the crystalline structures of the titania prepared by both the microwave and sol-gel methods, as shown in Figures 9(c) and (d). The interplanar space of the titania crystals (d-spacing) was prepared using microwave and sol-gel methods, approximately 3Ao in both methods, as presented in Figure 9(e) and (f).

It is evident from this that the morphology of the materials was significantly influenced by the microwave power used during processing. The fact that the particles produced by the microwave combustion approach are smaller than those produced by the sol-gel method could result from different processing mechanisms. The liquid was heated because of the microwave-induced rotation of the dipoles within it, which forced the polar molecules to align and relax in the field of oscillating electromagnetic radiation. Therefore, unlike other typical methods, the heat generated within the liquid is not transmitted from the vessel. Thus, electromagnetic waves are used in microwave heating, where the material molecules directly absorb them by creating a dipole moment that increases the temperature of the molecules. Owing to their sensitivity, the dipole spins of the molecules align themselves with the external electric field.^45,46 The microwave heating method is not the same as the traditional heating methods. Uniform internal heating is produced during microwave heating through the interaction of the material with electromagnetic radiation. As a result, most TiO₂ began to crystallize in the nano form, some polymers started to break down early at 300°C, and fine nanoparticles were eventually obtained when the treatment was increased to 550°C.On the other hand, in traditional heating methods such as sol-gel, heating starts from the utensil surface and continues by thermal conduction to heat the entire item. The materials are not heated evenly using this heating method, which requires more energy and a longer synthesis time than microwave heating (the temperature of the utensil is greater than that of the material).^45,47 This resulted in the initial components appearing in an amorphous state. The polymer began to decompose after heat treatment from 400 to 550°C, which enhanced grain development as the polymer burned out.

UV-Visible spectroscopy

The TiO₂ nanoparticles were analyzed using UV-Vis. Spectroscopy in the 200–800 nm range and the bandgap energies of the synthesized TiO₂-NPs are shown in Figures 10 (a) and 10 (b). According to equation (1),

(α h υ)^{n} = A (h υ - Eg)

(1)

where A is the absorption coefficient, hυ is the photon energy, and n is equal to 2 or 1/2 for the direct and indirect transitions, respectively. The bandgap energy of the nanoparticles was determined by extrapolating the linear region of the graph of (αhv)² vs. hv (in units of eV). Figure 10 (a)shows that the bandgap energy of the TiO₂ nanoparticles is 3.17 eV for the microwave-synthesized sample (MW2) and 3.15 eV for the sol-gel sample (SG2), as shown in Figure 10 (b), confirming that TiO₂ is a direct semiconductor. The bandgap energy of the TiO₂ nanoparticles increased as the particle size decreased, and the bandgap of the anatase phase was higher than that of the rutile phase, as reported in the literature.⁴⁸ This can be explained by the holes in the valence band and the electrons in the conduction band being confined by the potential barriers of the surface or potential well of the quantum box. Because of the confinement of the electrons and holes, the band gap energy increases between the valence band and the conduction band while decreasing the particle size.⁴⁹

Figure 10.

Direct bandgap of titania particles prepared by microwave and sol-gel with molar ratios of TiCl₃: PAA (1:2), (a) MW2, and (b) SG2 samples calcined at 550 °C.

Cytotoxicity test

The cytotoxicity of the optimum synthesized TiO₂-nanoparticles samples (MW2 and SG2) calcined at 550 °C was assessed against a regular human epithelial cell line at 100 ppm. According to the findings, the cytotoxicities of MW2 and SG2 were approximately 38% and 35%, respectively, after 48 h of incubation. The cytotoxicity of MW2 was higher than that of SG2 because of its smaller size and higher surface area, which can decrease the survival rate of cells. The prepared samples had moderate toxicity at 100 ppm and were safer to use at lower concentrations.^50–52

When the particle sizes of TiO₂ are reduced to the nanometer scale (generally in the range of 1–100 nm), the surface characteristics and surface areas of TiO₂ change dramatically. New or enhanced physical and chemical properties of nanostructured TiO₂ have emerged. In this research, titanium dioxide prepared by the MW and SG methods introduces titanium oxide with particle sizes ranging from 3 to 9 and 8 to 20 nm, respectively, as described in the TEM results. Titanium dioxide (TiO₂) nanoparticles (NPs) are essential nanoscale components of different composites. Their potential for drug delivery and sunscreen design, which has sparked significant interest, is a testament to the exciting possibilities of TiO₂ in various applications.⁵³

Cytotoxicity tests, which assess the potential harm to living cells, are critical when nanoparticles are used because their small size may allow them to penetrate biological membranes more easily, posing health risks. The cytotoxicity should be measured to ensure that prepared titania is non-toxic and safe, helps broaden its application range, and supports its commercial viability.⁵⁴

However, the adverse effects of TiO₂ nanomaterials have not been well studied. Here, we investigated the cytotoxicity of the optimum synthesized TiO₂-nanoparticles samples (MW2 and SG2) calcined at 550 °C and assessed them against the regular human epithelial cell line at 100 ppm. According to the findings, the cytotoxicities of MW2 and SG2 were approximately 38% and 35%, respectively, after 48 h of incubation. The cytotoxicity of MW2 was higher than that of SG2 because of its smaller size and higher surface area, which can decrease the survival rate of cells. Notably, the prepared samples had moderate toxicity at 100 ppm. Still, they were safer to use at lower concentrations, providing a strong reassurance about the safety of TiO₂ at lower doses and instilling a sense of security and confidence in its use.^{50–52,55–57}

Conclusion

This study compares sol-gel and microwave combustion methods for synthesizing titania nanoparticles via hydrolysis of TiCl₃ in an aqueous solution with polyacrylic acid as a template polymer to control TiO₂ particle shape and size. The particle sizes were influenced by the preparation method, precursor molar ratio, and calcination temperature. The microwave combustion method enhanced TiO₂ crystallization with increasing polyacrylic acid concentration, peaking at a 1:2 molar ratio (TiO₂: PAA). FTIR spectra showed that both methods produced TiO₂ nanoparticles with the same peaks, but the microwave method resulted in lower intensity due to faster crystallization. Increasing the calcination temperature to 550 °C improved anatase crystallization using the microwave method. Microwave combustion produced anatase TiO₂, while the sol-gel method produced a mix of anatase and rutile, with rutile crystallization increasing at higher temperatures. Nano TiO₂ particles from the microwave method ranged from 3–9 nm, compared to 8–20 nm from the sol-gel method. Cytotoxicity tests recommended using titania particles (MW2 and SG2) at concentrations below 100 ppm to avoid toxicity. The microwave method was preferred over sol-gel due to faster reaction times, higher energy efficiency, better particle size control, and minimized undesired by-products, resulting in purer nanoparticle phases suitable for various applications. Future work will use titania particles prepared via sol-gel and microwave combustion synthesis as coatings on porous alumina ceramics to improve corrosion resistance and extend lifespan in harsh environments. This study will extend prior research, evaluating how different preparation methods affect adhesion, uniformity, and protective performance of the titania layers, aiming to optimize effectiveness for industrial applications where corrosion is critical.^58,59

Footnotes

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

Seery

George

Floris

, et al. Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis. J Photochem Photobiol A: Chem 2007; 189: 258–263.

Linsebigler

Yates

. Photocatalysis on TiO2 surfaces: principles, mechanisms and selected results. Chem Rev 1995; 95: 735–758.

Ryu

Kim

, et al. Effect of calcination on the structural and optical properties of M/TiO2 thin films by RF magnetron co-sputtering. Mater Lett 2004; 58: 582–587.

Burda

Chen

Narayanan

, et al. Chemistry and properties of nanocrystals of different shapes. Chem Rev 2005; 105: 1025–1102.

Murugan

Samuel

Ravi

. Synthesis of nanocrystalline anatase TiO2 by microwave hydrothermal method. Mater Lett 2006; 60: 479–480.

Sankapal

Gajare

Dubal

, et al. Presenting highest supercapacitance for TiO2/MWNTs nanocomposites: novel method. Chem Eng J 2014; 247: 103–110.

Nakata

Fujishima

. Tio2 photocatalysis: design and applications. J Photochem Photobiol C Photochem Rev 2012; 13: 169–189.

Manea

Gaikwadc

, et al. Synthesis of TiO2 nanoparticles by microwave-assisted Sol-Gel method. 2017.

Alsharaeh

Bora

Soliman

, et al. Sol-Gel-Assisted microwave-derived synthesis of anatase Ag/TiO2/GO nanohybrids toward efficient visible light phenol degradation. Catalysts 2017; 7: 133–144. doi:https://doi.org/10.3390/catal7050133

10.

Fang

Zhang

. One-dimensional (1D) ZnS nanomaterials and nanostructures. J Mater Sci Technol 2006; 22: 721–736.

11.

Cui

Meng

Huang

, et al. Preparation and characterization of MgO nanorods. Mater Res Bulletin 2000; 35: 1653–1659.

12.

Yin

Zhang

Xia

. Synthesis and characterization of MgO nanowires through a vapor-phase precursor method. Adv Func Mater 2002; 12(2): 93–298.

13.

Zhang

Peng

, et al. Fabrication of MgO nanobelts using a halide source and their structural characterization. Appl Phys A Mater Sci Processing 2001; 73: 773–775.

14.

Alhadrami

Al-Hazmi

. Antibacterial activities of titanium oxide nanoparticles. J Bioelectron Nanotechnol 2017; 2: 1–5.

15.

Liuxue

Xiulian

Peng

, et al. Photocatalytic activity of anatase thin films coated cotton fibers prepared via a microwave-assisted liquid phase deposition process. Surf Coatings Technol 2007; 201: 7607–7614.

16.

Al-Hazmi

Umar

Dar

, et al. Microwave assisted rapid growth of Mg(OH)2 nanosheet networks for ethanol chemical sensor application. J Alloys Compd 2012; 519: –8.

17.

Umar

Al-Hazmi

Dar

, et al. Ultra-sensitive ethanol sensor based on rapidly synthesized Mg(OH)2 hexagonal nanodisks. Sensors Actuators B Chem 2012; 166-167: 97–102.

18.

Beall

Duraia

E-SM

El-Tantawy

, et al. Rapid fabrication of nanostructured magnesium hydroxide and hydromagnesite via microwave-assisted technique. Powder Technol 2013; 234: 26–31.

19.

Al-Ghamdi

Al-Hazmi

Alnowaiser

, et al. A new facile synthesis of ultra fine magnesium oxide nanowires and optical properties. J Electroceram 2012; 29: 198–203.

20.

Aal

Al-Hazmi

Al-Ghamdi

, et al. Novel rapid synthesis of zinc oxide nanotubes via hydrothermal technique and antibacterial properties. Spectrochim Acta A Mol Biomol Spectrosc 2015; 135: 871–877.

21.

Al-Ghamdi

Al-Hazmi

Al-Hartomy

, et al. A novel synthesis and optical properties of cuprous oxide nano octahedrons via microwave hydrothermal route. J Sol-Gel Sci Technol 2012; 63: 187–193.

22.

Al-Hazmi

Alnowaiser

Al-Ghamdi

, et al. A new large - scale synthesis of magnesium oxide nanowires: structural and antibacterial properties. Superlattices Microstruct 2012; 52: 200–209.

23.

Al-Tuwirqi

Al-Ghamdi

Al-Hazmi

, et al. Synthesis and physical properties of mixed Co3O4/CoO nanorods by microwave hydrothermal technique. Superlattices Microstruct 2011; 50: 437–448.

24.

Lavudya

Pant

Srikanth

VVSS

, et al. Mesoporous and phase pure anatase TiO2 nanospheres for enhanced photocatalysis. Inorgan Chem Commun 2023; 152: 110699.

25.

Michalcik

Horakova

Spatenka

, et al. Photocatalytic activity of nanostructured titanium dioxide thin films. Int J Photo Energy 2012; 2012: 689154.

26.

Moline-Reyes

Romero-Moran

Uribe-Vargas

, et al. Study on the photocatalytic activity of titanium dioxide nanostructures: Nanoparticles, nanotubes, and ultra-thin films. Catal Today 2018; 33: 2–12.

27.

Cizmic

Ljubas

Rozman

, et al. Photocatalytic Degradation of Azithromycin by Nanostructured TiO(2) Film: Kinetics, Degradation Products, and Toxicity. Materials (Basel) 2019; 12: 873–888. doi:https://doi.org/10.3390/ma12060873

28.

Brisevac

Gabelica

Ljubas

, et al. Effects of TiO(2) Nanoparticles Synthesized via Microwave Assistance on Adsorption and Photocatalytic Degradation of Ciprofloxacin. Molecules 2024; 29: 2935–2948. doi:https://doi.org/10.3390/molecules29122935

29.

Ćurković

Ljubas

Šegota

, et al. Photocatalytic degradation of Lissamine Green B dye by using nanostructured sol-gel TiO2 films. J Alloys Compd 2014; 604: 309–316.

30.

Gabelica

Ćurković

Mandić

, et al. Rapid microwave-assisted synthesis of Fe3O4/SiO2/TiO2 Core-2-layer-shell nanocomposite for photocatalytic degradation of ciprofloxacin. Catalysts 2021; 11: 1136–1154. doi:https://doi.org/10.3390/catal11101136

31.

Kutuzova

Dontsova

Kwapinski

. Application of TiO2-based photocatalysts to antibiotics degradation: cases of sulfamethoxazole, trimethoprim and ciprofloxacin. Catalysts 2021; 11: 728–771. doi:https://doi.org/10.3390/catal11060728

32.

Sanchez Tobon

Panzic

Bafti

, et al. Rapid microwave-assisted synthesis of N/TiO(2)/rGO nanoparticles for the photocatalytic degradation of pharmaceuticals. Nanomaterials (Basel) 2022; 12: 3975–3996. doi:https://doi.org/10.3390/nano12223975

33.

Malakootian

Nasiri

Gharaghani

. Photocatalytic degradation of ciprofloxacin antibiotic by TiO2 nanoparticles immobilized on a glass plate. Chem Eng Commun 2020; 207: 56–72.

34.

Mosmann

. Rapid colorimetric assays for cellular growth and survival: application to proliferation and cytotoxicity assays. J Immunol Methods 1983; 65: 55–63.

35.

Gamal

Abbas

Abdelwahed

NAM

, et al. Optimization strategy of Bacillus subtilis MT453867 levansucrase and evaluation of levan role in pancreatic cancer treatment. Int J Biol Macromol 2021; 182: 1590–1601.

36.

Korowash

Abo-Almaged

Ibrahim

. Comparative study of certain substituted ions in hydroxyapatite: characterization and effect on cell viability. Adv Nat Sci Nanosci Nanotechnol 2022; 13. doi:https://doi.org/10.1088/2043-6262/ac70d6

37.

Luo

Zhu

Wang

, et al. Preparation of TiO2/polyacrylic acid and application in photocatalytic degradation of methyl orange. Polym Polym Compos 2009; 17: 195–203.

38.

Aristanti

Supriyatna

Masduki

, et al. Effect of calcination temperature on the characteristics of TiO2 synthesized from ilmenite and its applications for photocatalysis. In Proceedings of the 2nd mineral processing and technology international conference, 2019.

39.

Zhang

Feng

, et al. UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk. J Phys Chem B 2006; 110: 927–935.

40.

Adnen

Abdullah

Nor

. Synthesis and characterization of B-S Co-doped TiO2 photocatalyst with variable boron concentration. IOP Conf Ser Mater Sci Eng 2020; 440: 1–8.

41.

Haider

Jameel

Taha

. Synthesis and characterization of TiO2 nanoparticles via SolGel method by pulse laser ablation. J Eng Tech 2015; 33: 761–771.

42.

Papatryfonos

Theocharis

. Using swellable polymers as structure directing scaffolds in mesoporous titania synthesis. Mater Sci Appl 2018; 09: 211–227.

43.

Yuliarto

Septina

Fuadi

, et al. Synthesis of nanoporous TiO2 and its potential applicability for dye-sensitized solar cell using antocyanine black rice. Adv Mater Sci Eng 2010; 2010: –6.

44.

Crepaldi

Soler-Illia

Grosso

, et al. Controlled formation of highly organized mesoporous titania thin films: from mesostructured hybrids to mesoporous nanoanatase TiO2. J Am Chem Soc 2003; 125: 9770–9786.

45.

Jalawkhan

RSM

. Synthesis and characterization of TiO2 nanoparticles by microwave and its application toward water treatment. Iraq: University of Kerbala, 2021.

46.

Azad Kumar*

. Different methods used for the synthesis of TiO2 based nanomaterials: a review. Am J Nano Res Appl 2018; 6: 1–10. doi:https://doi.org/10.11648/j.nano.20180601.11

47.

Farmer

. Comparing conventional heating techniques and microwave irradiation for the desorption of lithium borohydride and lithium borohydride composites. England: The University of Birmingham, 2009.

48.

Shah

Nazir

Mazhar

, et al. PEGylated doped- and undoped-TiO2 nanoparticles for photodynamic therapy of cancers. Photodiagnosis Photodyn Ther 2019; 27: 173–183.

49.

Singh

Goyal

Devlal

. Size and shape effects on the band gap of semiconductor compound nanomaterials. J Taibah Univ Sci 2018; 12: 470–475.

50.

Sabouri

Rangrazi

Amiri

, et al. Green synthesis of nickel oxide nanoparticles using Salvia hispanica L. (chia) seeds extract and studies of their photocatalytic activity and cytotoxicity effects. Bioprocess Biosyst Eng 2021; 44: 2407–2415.

51.

Mathur

Raghavan

Chaudhury

, et al. Cytotoxicity of titania nanoparticles towards wastewater isolate Exiguobacterium acetylicum under UVA, visible light and dark conditions. J Environ Chem Eng 2015; 3: 1837–1846.

52.

Mohl

Dombovari

Tuchina

, et al. Titania nanofibers in gypsum composites: an antibacterial and cytotoxicology study. J Mater Chem B 2014; 2: 1307–1316.

53.

Xiaoping

. Applications of titanium dioxide materials. In: Hafiz Muhammad

(eds) Titanium dioxide. Rijeka: IntechOpen, 2021, pp.Ch. 9.

54.

Sha

Gao

Wang

, et al. Cytotoxicity of titanium dioxide nanoparticles differs in four liver cells from human and rat. Compos Part B Eng 2011; 42: 2136–2144.

55.

Mansoori

Mohazzabi

McCormack

, et al. Nanotechnology in cancer prevention, detection and treatment: bright future lies ahead. World Rev Sci Technol Sustain Dev 2007; 4: 226–257. doi:https://doi.org/10.1504/wrstsd.2007.013584

56.

Dondi

Albini

Serpone

. Interactions between different solar UVB/UVA filters contained in commercial sunscreens and consequent loss of UV protection. Photochem Photobiol Sci 2006; 5: 835–843.

57.

Morsella

d’Alessandro

Lanterna

, et al. Improving the sunscreen properties of TiO2 through an understanding of its catalytic properties. ACS Omega 2016; 1: 464–469.

58.

Gaber

Abd El-Hamid

Ngida

REA

, et al. Synthesis, characterization and corrosive resistance of ZnO and ZrO2 coated TiO2 substrate prepared via polymeric method and microwave combustion. Ceram Int 2024; 50: 38917–38932.

59.

El-Hamid

HKA

Gaber

Ngida

REA

, et al. Study of microstructure and corrosion behavior of nano-Al2O3 coating layers on TiO2 substrate via polymeric method and microwave combustion. Sci Rep 2024; 14: 18417.

Characterization of anatase TiO 2 nanoparticle: Comparative synthesis via microwave combustion and sol-gel techniques

Abstract

Keywords

Introduction

Materials and experimental methods

Starting materials and synthesis methods

Powder characterization

Cytotoxicity test on human fibroblast cell line

Results and discussion

XRD investigations of TiO2 nanoparticles

Fourier transformation infrared spectroscopy (FTIR) analysis

Thermal analysis

SEM and TEM characterization of TiO2 nanoparticles

UV-Visible spectroscopy

Cytotoxicity test

Conclusion

Footnotes

Funding

Declaration of conflicting interests

References

XRD investigations of TiO₂ nanoparticles

SEM and TEM characterization of TiO₂ nanoparticles