Sunlight-Activated Photocatalysis of Organic Dyes Using TiO₂ Nanoparticles Prepared by Co-Precipitation
AdilHamid1
MohitSahni1✉Emailmohit.sahni@sharda.ac.in
1Department of Physics, Sharda School of Engineering and ScienceSharda UniversityGreater NoidaUttar PradeshIndia
Adil Hamid1, Mohit Sahni1*
1 Department of Physics, Sharda School of Engineering and Science, Sharda University, Greater Noida, Uttar Pradesh, India
* Corresponding author:
Abstract
TiO₂ nanoparticles were successfully synthesized via a simple co-precipitation method at pH 11, 70°C for 2 hours, followed by calcination at 500°C for 2 hours. The crystalline structure and phase purity were confirmed by X-ray diffraction (XRD) and Raman spectroscopy, revealing a pure anatase phase. High-resolution transmission electron microscopy (HRTEM) showed well-dispersed spherical nanoparticles with an average size of 25.44 nm (SD = 6.7 nm). The photocatalytic activity of the synthesized TiO₂ nanoparticles was evaluated for the degradation of Methylene Blue (MB), Rhodamine B (RhB), and Crystal Violet (CV) dyes under direct sunlight. High degradation efficiencies were observed: ~99% for MB in 100 minutes (k = 0.05147 min⁻¹), ~ 98% for RhB in 140 minutes (k = 0.02743 min⁻¹), and ~ 98% for CV in 160 minutes (k = 0.02409 min⁻¹), despite the relatively large band gap of TiO₂ (3.2 eV). The differences in degradation rates are attributed to the molecular structures of the dyes and their susceptibility to reactive oxygen species generated by photoexcited TiO₂, with MB degrading the fastest and CV degrading the slowest. These results demonstrate the excellent potential of TiO₂ nanoparticles for sustainable, sunlight-driven degradation of organic dyes, highlighting their practical application in wastewater treatment and environmental remediation.
Keywords:
Photocatalytic
nanoparticles
dye degradation
wastewater treatment
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Introduction
Water pollution caused by domestic sewage, industrial effluents, fertilizers, and particularly organic dyes has become a significant global environmental concern [1]. Industries such as textiles, leather processing, cosmetics, and paper manufacturing contribute heavily to this issue. Globally, dye production is estimated at nearly one million tons per year [2]. A large portion of the resulting wastewater is often discharged untreated into rivers and lakes. These dye-laden effluents are not only visually disruptive but also toxic, non-biodegradable, and potentially carcinogenic—even at very low concentrations. Therefore, developing effective and sustainable strategies to treat dye-contaminated wastewater is of critical importance.
Conventional methods for dye removal, including adsorption [3], and coagulation [4], are widely employed. However, these approaches typically do not destroy the pollutants completely; instead, they convert them into solid residues or sludge, which require further handling and disposal.
In recent years, photocatalytic oxidation has emerged as a highly promising technique for water purification. This process relies on light-activated catalysts that degrade pollutants at room temperature, often mineralizing them into carbon dioxide and water without generating secondary toxic byproducts. Metal oxide semiconductors such as titanium dioxide (TiO₂), zinc oxide, silicon dioxide, and cerium dioxide have been widely used as photocatalysts [5]. Among these, TiO₂ is particularly favored due to its high stability, low cost, abundance, strong photoactivity, and environmental safety [6].
The photocatalytic activity of semiconductors depends on their electronic structure, which consists of a valence band (VB) and a conduction band (CB) separated by a bandgap. When the semiconductor is irradiated with photons of energy equal to or greater than the bandgap, electrons (e⁻) are excited from the VB to the CB, leaving behind positively charged holes (h⁺) in the VB. These charge carriers act as strong redox agents: electrons reduce oxygen molecules to form superoxide radicals (O₂•⁻), while holes oxidize water molecules to generate hydroxyl radicals (•OH). These reactive species attack and break down organic dye molecules, ultimately converting them into harmless carbon dioxide and water.
Recent studies (2023–2025) have reported several advancements in photocatalytic materials. Bio-derived carbon–TiO₂ composites have shown sustainable and economically viable approaches for dye degradation [7]. Doping TiO₂ with vanadium has enhanced the degradation efficiency of dyes by increasing surface adsorption sites [8]. Other approaches, including nanomaterial hybrids, metal–organic frameworks (MOFs), non-metal doping, and nanofiber morphologies, have also demonstrated improved photocatalytic performance under visible and sunlight conditions (9, 10).
Although TiO₂ is a widely studied photocatalyst, its relatively large bandgap (~ 3.2 eV) limits the excitation of electrons from the valence band to the conduction band under sunlight, making efficient degradation challenging. In this study, pure TiO₂ nanoparticles were synthesized via the co-precipitation method and tested for the degradation of common organic dyes under direct sunlight. Remarkably, despite the large bandgap, the synthesized TiO₂ nanoparticles achieved up to 99% degradation of dyes, demonstrating their high photocatalytic activity and stability. This work highlights a simple, cost-effective, and environmentally friendly approach for sunlight-driven water purification, providing insights into the practical application of TiO₂ nanoparticles in treating dye-contaminated wastewater.
Materials and methods
Titanium isopropoxide (Ti [OCH(CH₃)₂]₄, 97%) and absolute ethanol were purchased from Sigma-Aldrich and used as received without additional purification. Ammonium hydroxide (NH₄OH, 25%) was obtained from Sigma-Aldrich.
Synthesis of TiO₂ nanoparticles using the Co-Precipitation Method
For the synthesis of TiO₂ nanoparticles, 20 mL of titanium isopropoxide was dissolved in 80 mL of absolute ethanol under magnetic stirring at room temperature for 20 minutes to ensure complete homogenization. The mixture was then heated to 70°C and maintained under continuous magnetic stirring for 1–2 hours. After this stage, ammonium hydroxide (NH₄OH) was added gradually until the pH exceeded 11, prompting the formation of a visible precipitate. The solution was then left to stir for an additional 1–2 hours to ensure complete precipitation. The resulting suspension was centrifuged at 9000 rpm for 5 minutes, and the collected precipitate was washed thoroughly five times- twice with distilled water, once with ethanol, and twice more with distilled water to remove impurities and unreacted residues as shown in Fig. 1.
Fig. 1
Synthesis of TiO2 Nanoparticles Using the Co-Precipitation Method
The purified precipitate was then dried in an oven at 90°C for 12 hours, followed by calcination at 600°C for 2 hours to improve crystallinity and phase formation. Finally, the calcined material was finely ground using a mortar and pestle to obtain a homogeneous powder of TiO₂ nanoparticles.
Results and discussions
XRD Analysis
The crystalline structure and average crystallite size of the synthesized TiO₂ nanoparticles were analyzed using X-ray diffraction (XRD). The diffraction peaks located at 2θ = 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, 68.8°, 70.3°, 75.2°, and 76.1° were assigned to the (101), (004), (200), (105), (211), (204), (116), (220), (215), and (301) planes of anatase TiO₂ (JCPDS card No. 21-1272), shown in Fig. 2, confirming the formation of pure anatase phase without detectable traces of rutile or brookite impurities [11].
Fig. 2
X-ray diffraction (XRD) pattern of TiO₂ nanoparticles synthesized via the co-precipitation method, exhibiting characteristic peaks of the anatase phase (JCPDS card no. 21-1272). The average crystallite size was calculated to be ~ 22 nm using the Scherrer equation
The average crystallite size (D) was calculated from the most intense (101) reflection at 2θ = 25.3° using the Debye–Scherrer equation:
where K is the shape factor (taken as 0.9), λ is the X-ray wavelength of Cu Kα radiation (0.15406 nm), β is the full width at half maximum (FWHM) of the diffraction peak (in radians), and θ is the Bragg angle. Using the measured FWHM of 0.37176° (converted to 0.00649 rad), the crystallite size was estimated to be ~ 22 nm.
This nanocrystalline size falls within the reported optimal range for anatase TiO₂ (10–30 nm), which is known to enhance photocatalytic efficiency due to its high surface-to-volume ratio and effective charge separation [12]. The relatively sharp and intense diffraction peaks also indicate good crystallinity of the synthesized nanoparticles, which plays a crucial role in improving sunlight-driven photocatalytic dye degradation (13, 14).
Raman Analysis
Raman spectroscopy was employed further to confirm the crystalline phase of the synthesized TiO₂ nanoparticles. As shown in Fig. 3, five prominent Raman-active modes are observed at approximately 145, 196, 398, 515, and 639 cm⁻¹. These peaks correspond to the Eg (144 cm⁻¹), Eg (196 cm⁻¹), B1g (399 cm⁻¹), A1g (519 cm⁻¹), and Eg (639 cm⁻¹) vibration modes of anatase TiO₂, respectively.
Fig. 3
Raman spectrum of TiO₂ nanoparticles prepared by the co-precipitation method. The spectrum exhibits the characteristic anatase phase modes at ~ 145 (Eg), 196 (Eg), 398 (B1g), 515 (A1g/B1g), and 639 cm⁻¹ (Eg), confirming the formation of single-phase anatase TiO₂ in agreement with XRD results
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Fig. 4
High-resolution transmission electron microscopy (HRTEM) image of TiO₂ nanoparticles prepared by the co-precipitation method, showing well-dispersed, nearly spherical particles with an average size of ~ 25.44 nm (a). The corresponding selected area electron diffraction (SAED) pattern exhibits distinct concentric rings (d), consistent with the polycrystalline anatase phase observed in the XRD and Raman analyses.
The most intense peak at 145 cm⁻¹ (Eg) is the characteristic signature of anatase, associated with the symmetric stretching of O–Ti–O bonds in the lattice (15, 16). The presence of sharp and well-defined Raman peaks suggests high crystallinity of the synthesized nanoparticles. The absence of additional peaks related to rutile (e.g., 447 cm⁻¹, 612 cm⁻¹) or brookite phases confirms that the sample predominantly crystallizes in the anatase phase [17].
It is also notable that the observed slight peak broadening of the Eg mode at 145 cm⁻¹ compared to bulk TiO₂ indicates the nanocrystalline nature of the sample, which is commonly attributed to phonon confinement effects in nanoparticles [18]. This observation is consistent with the XRD results, further validating the nanoscale crystallinity of the material.
Thus, the Raman analysis supports that the synthesized TiO₂ nanoparticles exhibit a pure anatase phase with high crystallinity, making them suitable for photocatalytic applications due to the anatase structure’s superior charge separation ability [19].
HR-TEM analysis
The HRTEM characterization reveals well-crystallized TiO₂ nanoparticles with an average size of 25.44 nm and a narrow size distribution (σ = 6.7 nm), as shown in Fig. 5a, which is optimal for photocatalytic applications due to the balance between high surface area and crystalline integrity. The narrow size distribution is crucial for applications requiring uniform optical and catalytic properties, as particle size directly influences the bandgap and surface reactivity [20]. The selected area electron diffraction (SAED) pattern confirms the anatase crystal structure with indexed diffraction rings corresponding to (101), (112), (200), (211), (204), and (220) planes shown in Fig. 5d. The anatase phase is particularly significant as it demonstrates superior photocatalytic activity compared to rutile and brookite phases, making it the preferred polymorph for environmental and energy applications [21].
Fig. 5
Photocatalytic degradation of methylene blue (MB) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
The high-resolution lattice imaging demonstrates excellent crystallinity with clear lattice fringes consistent with anatase TiO₂ d-spacings, particularly the prominent (101) planes at ~ 3.52 Å. The crystalline integration of anatase TiO₂ nanocrystals has been shown to significantly influence their optical and electronic properties [22]. While some aggregation is observed, typical for high-surface-area nanomaterials, the individual particles maintain their structural integrity within aggregates. TiO₂ nanoparticles with these characteristics possess distinctive properties, including high surface area, stability, UV protection, and photocatalytic activity [23], making them highly suitable for photocatalytic, solar energy, and environmental remediation applications where the combination of appropriate size, high crystallinity, and pure anatase phase provides enhanced performance characteristics.
Photocatalytic analysis
The photocatalytic degradation of Methylene Blue (MB), Rhodamine B (RhB), and Crystal Violet (CV) dyes using TiO₂ nanoparticles was systematically investigated under direct sunlight. The progress of degradation was monitored using UV–Vis spectroscopy, normalized concentration ratios (C/C₀), and degradation percentages, with kinetics analyzed via ln(C₀/C) versus time.
Methylene Blue (MB):
The characteristic absorption peak at ~ 664 nm (Fig. 6a) decreased progressively with irradiation time, indicating effective photocatalytic breakdown. The normalized concentration ratio C/C₀ declined sharply (Fig. 6b), and the degradation percentage increased steadily, reaching ~ 99% within 100 minutes (Fig. 6c). Kinetic analysis yielded a pseudo-first-order rate constant k = 0.05147 min⁻¹ with R² ≈ 0.99, confirming pseudo-first-order behavior shown in Fig. 6d. [24].
Fig. 6
Photocatalytic degradation of Rhodamine Bar (RhB) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
Fig. 6
Photocatalytic degradation of Crystal Violet (CV) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
Rhodamine B (RhB):
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The λmax at ~ 550 nm decreased gradually over 140 minutes (Fig. 7a), demonstrating progressive dye decomposition. The C/C₀ ratio showed continuous decline (Fig. 7b), while the degradation percentage reached ~ 98% (Fig. 7c). Pseudo-first-order kinetics gave k = 0.02743 min⁻¹ with R² = 0.934, as shown in Fig. 7d. indicating slower but efficient degradation compared to Methylene Blue [25].Crystal Violet (CV):
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The absorption peak at 590–600 nm decreased steadily under sunlight (Fig. 8a). The C/C₀ ratio declined over time (Fig. 8b), and the degradation percentage reached ~ 98% after 160 minutes (Fig. 8c). The pseudo-first-order rate constant was k = 0.02409 min⁻¹ with R² ≈ 0.99, reflecting a slower degradation rate due to the higher stability of CV molecules as shown in Fig. 8d. [26].Comparison:
MB degraded fastest, followed by RhB and CV, correlating with their molecular stability and structural differences. The variation in rate constants and degradation times highlights the influence of molecular structure on susceptibility to TiO₂-generated reactive oxygen species. As shown in Table 1
Dye | λ max (nm) | Time for Complete Degradation | Degradation Efficiency (%) | Pseudo-First-Order Rate Constant (k, min⁻¹) |
|---|---|---|---|---|
Methylene Blue (MB) | ~ 664 | 100 min | 99% | 0.05147 |
Rhodamine B (RhB) | ~ 550 | 140 min | 98% | 0.02743 |
Crystal Violet (CV) | ~ 590–600 | 160 min | 98% | 0.02409 |
The observed photocatalytic degradation rates of MB, RhB, and CV reflect differences in molecular structure and susceptibility to reactive oxygen species (ROS) generated on TiO₂ under sunlight. Methylene Blue (MB) exhibits the fastest degradation due to its relatively simpler thiazine ring structure, which is more accessible to hydroxyl (•OH) and superoxide (•O₂⁻) radicals [27]. Rhodamine B (RhB), containing a xanthene core with ethyl substituents, undergoes stepwise N-de-ethylation during degradation, making it moderately stable and slightly slower to decompose [28]. Crystal Violet (CV), with a highly conjugated triphenylmethane framework, is more structurally stable and sterically hindered, resulting in the slowest degradation among the three dyes. These differences highlight that the efficiency of TiO₂ photocatalysis depends not only on light absorption but also on molecular accessibility and stability of the chromophore towards ROS attack [29].
Conclusion
TiO₂ nanoparticles were successfully synthesized via a simple co-precipitation method at pH 11, 70°C for 2 hours, followed by calcination at 500°C for 2 hours. XRD and Raman analyses confirmed pure anatase phase, with an average crystallite size of 22 nm, while HRTEM revealed spherical nanoparticles with an average size of 25.44 nm (SD = 6.7 nm). The photocatalytic degradation of Methylene Blue (MB), Rhodamine B (RhB), and Crystal Violet (CV) dyes using TiO₂ nanoparticles under direct sunlight. The degradation efficiencies were found to be ~ 99% for MB in 100 minutes (k = 0.05147 min⁻¹), ~ 98% for RhB in 140 minutes (k = 0.02743 min⁻¹), and ~ 98% for CV in 160 minutes (k = 0.02409 min⁻¹), confirming the high photocatalytic performance of TiO₂ despite its relatively large band gap of 3.2 eV.
The differences in degradation rates can be attributed to the molecular structures of the dyes and their susceptibility to reactive oxygen species generated by photoexcited TiO₂. MB, with a simpler thiazine structure, degraded fastest, whereas CV, with a more stable triphenylmethane framework, showed the slowest degradation. These results highlight the effectiveness of TiO₂ nanoparticles under natural sunlight, making them a promising material for environmental remediation, particularly for wastewater treatment of organic pollutants.
Given the excellent photocatalytic performance under sunlight, future work can focus on enhancing the visible-light absorption of TiO₂ through doping or composite formation, exploring real wastewater systems with multiple contaminants, and investigating the reusability and stability of TiO₂ nanoparticles to further advance their practical application in sustainable water purification technologies.
Figures and Tables
Figure 1. Synthesis of TiO2 Nanoparticles Using the Co-Precipitation Method
Figure 2. X-ray diffraction (XRD) pattern of TiO₂ nanoparticles synthesized via the co-precipitation method, exhibiting characteristic peaks of the anatase phase (JCPDS card no. 21-1272). The average crystallite size was calculated to be ~ 22 nm using the Scherrer equation
Figure 3. Raman spectrum of TiO₂ nanoparticles prepared by the co-precipitation method. The spectrum exhibits the characteristic anatase phase modes at ~ 145 (Eg), 196 (Eg), 398 (B1g), 515 (A1g/B1g), and 639 cm⁻¹ (Eg), confirming the formation of single-phase anatase TiO₂ in agreement with XRD results
Figure 4. High-resolution transmission electron microscopy (HRTEM) image of TiO₂ nanoparticles prepared by the co-precipitation method, showing well-dispersed, nearly spherical particles with an average size of ~ 25.44 nm (a). The corresponding selected area electron diffraction (SAED) pattern exhibits distinct concentric rings (d), consistent with the polycrystalline anatase phase observed in the XRD and Raman analyses.
Figure 5. Photocatalytic degradation of methylene blue (MB) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
Figure 6. Photocatalytic degradation of Rhodamine Bar (RhB) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
Figure 6. Photocatalytic degradation of Crystal Violet (CV) under direct sunlight using TiO₂ nanoparticles synthesized via the co-precipitation method. (a) UV–Vis absorbance spectra of MB, as a function of irradiation time; (b) corresponding C/C₀ vs. time plots; (c) degradation efficiency of the three dyes with time; and (d) pseudo-first-order kinetic plots (ln C/C₀ vs. time).
Table 1: Photocatalytic degradation of MB, RhB, and CV using TiO₂ nanoparticles under sunlight
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Author Contribution
Adil Hamid carried out material synthesis, characterization, photocatalytic experiments, data interpretation, and drafted the manuscript. Mohit Sahni provided project supervision, guidance in experimental planning, interpretation of results, and critical revision of the manuscript. Both authors read and approved the final version of the manuscript.
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