This paper is devoted to the synthesis of TiO₂ nanoparticles with N, C and Fe co-doping by sol-gel method and photocatalytic activity of the obtained material for MB and phenol degradation under visible and UV light. The synthesized photocatalysts were then characterized for the crystalline structure of the prepared photocatalysts, shape of the particles and the optical properties of the samples by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Vis diffuse reflectance spectroscopy (UV-DRS) respectively. The parameters that have been used in the study include; dopant concentration, catalyst loading, initial concentration of the pollutant, and the pH of the solution on the photocatalytic degradation efficiency. Apart from reusability and stability, the rate constants of the photocatalytic degradation were also established for the anodic titania nanoparticles.

Some works have also been carried out to assess the photocatalytic performance of the synthesized TiO₂ nanoparticles with dopant to eliminate organic compounds in water. For example, Peng et al. (2012) prepared N-doped TiO₂ nanoparticles by hydrothermal method and the photocatalytic activity for the decolorization of RhB under visible light was evaluated. Thus, the research revealed that the N doped TiO₂ photocatalyst was more effective than the undoped TiO₂ and was able to mineralize RhB dye within sixty minutes. In a similar work, Li et al. (2016) prepared Fe incorporated TiO₂ nanoparticles through sol-gel route and the photocatalytic efficiency of the synthesized material was evaluated for the degradation of MO dye under the UV light source. Here in, Fe doping improved the photocatalytic activity of TiO₂; the degradation of methylene blue was about 95% within 2 hours. Cai et al. (2015) reviewed several parameters which affected the photocatalytic activity of doped TiO₂ nanoparticles and these include; concentration of the dopant, the amount of the photocatalyst employed the concentration of the pollutant, and the pH level. Therefore, it is relevant to study the influence of these parameters on the photocatalytic activity of doped TiO₂ nanoparticles concerning the removal of organic pollutants in water.

1.1. Materials

Titanium(IV)isopropoxide (TTIP, 97%), isopropanol (99.5%), nitric acid (HNO₃, 70%), urea (99%), glucose (99.5%), iron(III)nitratenonahydrate (Fe(NO₃)₃.9H₂O, 98%), Methylene Blue (MB, 99%), and phenol (99%) were purchased from Sigma-Aldrich Co. All solvents and other chemicals used in the synthesis were of analytical grade and were used without any further purification. All the experiments were carried out with de-ionized water.

1.2. Synthesis of Doped TiO₂ Nanoparticles

To prepare undoped and doped TiO₂ nanoparticles, sol-gel method was employed as reported by Kant et al. (2014). For undoped TiO₂, 10 mL of TTIP was added drop wise to 50 mL of isopropanol while stirring the solution. Next, 1 mL of HNO₃ (0.1 M) was added into the solution as the catalyst. It was stirred for 2 h at room temperature and then left for aging for 24 h. The obtained gel was then dried at 80°C for 12 h and calcined at 500°C for 3 h to yield TiO₂ nanoparticles.

The same procedure was used for N-doped TiO₂ (N-TiO₂) but urea was used as the nitrogen source. To prepare the solution, urea at 0.5, 1, and 2 wt% based on TTIP was first dissolved in isopropanol and then TTIP was added. C doped TiO₂ (C-TiO₂) was prepared in a similar way with glucose used as the carbon source. In the case of Fe-doped TiO₂ (Fe-TiO₂), Fe(NO₃)₃.9H₂O was introduced into isopropanol at different concentrations (0.5, 1, and 2 wt% with respect to TTIP) followed by the addition of TTIP.

1.3. Characterization

The synthesized TiO₂ nanoparticles were characterized for their crystalline structure using an X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) of Rigaku MiniFlex 600. Morphology and size of the synthesized nanoparticles were characterized by scanning electron microscopy (SEM) with Hitachi S-4800. The UV-visible diffuse reflectance spectra (DRS) were obtained using UV-Visible spectrophotometer from Shimadzu UV-3600 model using BaSO₄ as the reference.

1.4. Photocatalytic Degradation Experiments

The photocatalytic activity of the prepared TiO₂ nanoparticles was evaluated by monitoring the degradation of MB and phenol under UV and visible light irradiation. A 100 mL aqueous solution containing the pollutant (10 mg/L) and the photocatalyst (1 g/L) was placed in a quartz reactor. The suspension was stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. Then, the reactor was irradiated with a 150 W UV lamp (λ = 365 nm) or a 300 W Xe lamp with a 420 nm cutoff filter for visible light experiments. At predetermined time intervals, aliquots were collected, centrifuged, and analyzed using a UV-visible spectrophotometer.

The effects of dopant concentration (0.5, 1, and 2 wt%), catalyst dosage (0.5, 1, and 2 g/L), pollutant concentration (5, 10, and 20 mg/L), and pH (3, 7, and 11) on the photocatalytic degradation were investigated. The pH was adjusted using HCl or NaOH solutions. The reusability of the photocatalysts was assessed by collecting the used nanoparticles, washing them with water and ethanol, and drying at 80°C before the next cycle. The stability of the photocatalysts was evaluated by comparing the XRD patterns and photocatalytic activity before and after multiple cycles.

In this study, doped TiO₂ nanoparticles were successfully synthesized using the sol-gel method and investigated for the photocatalytic degradation of organic pollutants in wastewater. N-TiO₂ exhibited the highest photocatalytic activity for MB degradation under visible light, while Fe-TiO₂ showed the best performance for phenol degradation under UV light. The optimal dopant concentration was found to be 1 wt% for both N and Fe doping. The photocatalytic degradation followed pseudo-first-order kinetics, and the apparent rate constants increased with higher catalyst dosage and lower pollutant concentration. The prepared doped TiO₂ nanoparticles demonstrated excellent reusability and stability over multiple cycles.

The influence of different factors including dopant concentration, catalyst dosage, pollutants’ concentration, and solution pH on the photocatalytic degradation has been discussed elaborately. The research findings indicated that the photocatalytic activity enhanced with the concentration of the dopant up to 1 wt %, after which there was a decline in the activity. The degradation rate increased with the increase in the amount of the catalyst to 1 g/L and thereafter decreased with the increase in the initial concentration of the pollutant. The photocatalytic degradation efficiency was found to maximum at acidic solution (pH 3) and the efficiency decreases with rise in pH.

From the results of this study, it can be concluded that doped TiO₂ nanoparticles can photocatalytically degrade organic pollutants in wastewater. The improved photocatalytic activity of N-TiO₂ under visible light and Fe-TiO₂ under UV light can be argued with regard to the following factors: A smaller bandgap energy, better charge separation, and more energy levels within the bandgap also. The systematic analysis of the influence of different factors on the photocatalytic degradation efficiency is helpful for the identification of possible ways of the photocatalytic process improvement. According to the outcomes of this study, doped TiO₂ nanoparticles could be suggested as efficient photocatalysts for the decomposition of organic pollutants in water. Therefore, the findings of the study can benefit other research in refining wastewater treatment techniques involving modified TiO₂ nanoparticles.

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