TiO2 is one of the most promising photocatalysts to treat various environmental pollutants due to its high photocatalytic activity, high thermal and chemical stability and non-toxicity [1,2]. Many organic pollutants such as dyes can be degraded by TiO2 nanoparticles [3-5]. To use TiO2 nanoparticles as a photocatalyst, it is necessary to induce them by light irradiation with energy levels higher than their band gap energy. Unfortunately, this induction cannot be done with solar light, which restricts their commercial potential . By doping different elements to the structure of TiO2, nanoparticles can overcome this restriction. In fact, doping decreases the band gap energy of nanoparticles and suppresses (e−/h+) pair recombination by electron/hole trapping . Among the various, the applied dopants, doping with iron results in introducing higher oxygen vacancies in the crystal structure and surface of TiO2, which improves water adsorption, forms surface -OH groups and promotes photocatalytic activity . Adsorbed iron on the surface of TiO2 nanoparticles can be served as an electron or hole trapper that enhances the separation of free carriers .
The synthesis technique plays an important role in the photocatalytic activity of the TiO2 nanoparticles [9-11]. Nowadays, different methods are used for synthesizing Fe doped TiO2 such as sol-gel [11-14], hydrothermal [15-17], wet chemical synthesis , thermal hydrolysis , Solvothermal  and calcination of FexTiS2 . Preparation method and characterization of some Fe-doped TiO2 photocatalysts are summarized in Table 1. An example is Sood et al. who prepared Fe-doped TiO2 nanoparticles by an ultrasonic assisted hydrothermal method followed by calcination. According to their results, the prepared nanoparticles possess small size, high visible light response, enhanced photochemical efficiency and thereby excellent photocatalytic activity for the degradation of para-nitrophenol and methylene blue dye under visible irradiation . Recently, the solid-state molten salt method is developed to prepare different types of nanoparticles such as ZnO, CuO, TiO2 and Ag [9,22-24]. The solid-state molten salt method is an easy, fast and inexpensive approach to provide pure materials. This article introduced a novel, one-step and easy approach for the synthesis of Fe-doped TiO2 nanoparticles. This method involved the formation of Fe-doped TiO2 nanoparticles with anatase phase by calcination of a solid mixture of FeCl3 and TiO2 powder at 700° C for 1 h.
As far as this study is concerned, the synthesis of Fe-doped TiO2 via molten salt method has never been reported in the literature. This study is focused on the synthesis of Fe-doped TiO2 nanoparticles by the molten salt method. Various concentrations of Fe ions were implied. Characterization of prepared nanoparticles was done by UV-Vis diffusion reflection spectroscopy (DRS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDX). The photocatalytic activity of the prepared nanoparticles was also checked against methyl orange under visible light irradiation.
FeCl3 and titanium dioxide powder were purchased from Merck Company. Various iron contents were considered for the synthesis of Fe-doped titanium dioxide nanoparticles. First, 0, 0.005, 0.01 and 0.03 g equivalent to 3, 1, 0 and 0.5 wt% of iron chloride and 1 to 2 drops of deionized water were added to crucible to dissolve iron chloride. Then 1 g of titanium dioxide was added to it. After grinding and stirring so that iron chloride would uniformly spread in the mixture, the sample was placed in a furnace at a constant temperature of 700 ± 5 °C in ambient atmosphere. After 60 minutes, the sample was removed from the furnace and after cooling, it was washed with ultrasound until the electrical conductivity of the water remained constant before and after washing. Then it was dried at ambient temperature after filtration. The schematic view of the preparation process is presented in Fig. 1.
The morphology and chemical states of iron-doped anatase TiO2 were analyzed with scanning electron microscopy (SEM) equipped with Energy-dispersive X-ray spectroscopy (LEO 1430VP, Germany). UV-Vis diffuse reflectance spectroscopy (DRS) was adopted using a spectrophotometer (Scinco S4100, S. Korea). X-ray diffraction )XRD) analysis was done using a PW 1050 diffractometer (Philips, The Netherlands). Scherer’s equation was applied to calculate the average crystalline sizes of the particles as equation (1):
Where is the wavelength of X-ray used, k is a constant (k = 0.9), θ is the Bragg diffraction angle and β is the full-width at the half-maximum (FWHM).
The photocatalytic activity of the prepared Fe-doped TiO2 nanoparticles was measured by the photodegradation of methyl orange (100 mL, 25 ppm and at pH 7) under 250 W visible light radiation (Osram-Germany) in a photoreactor. The rectangular photoreactor (50 cm long, 50 cm wide and 50 cm depth) was made of MDF. The photocatalyst (0.1 g catalyst/100 mL solution) was first dispersed in 50 mL of methyl orange solution and stirred magnetically in the dark for 15 min to achieve adsorption and desorption equilibrium. Then, the solution was irradiated with a source of visible light placed at 5 cm away from the top surface of the glass container. During the experiments, the water jacket circulation system was used to keep the temperature of the solutions at 298 K. Then, the absorbance of the centrifuged solution (10000 rpm for 20 min) was measured to calculate the photocatalytic activity using the UV–visible spectrophotometer. The photocatalytic activity was calculated by the following equation (2):
Degradation (%) = () × 100 (2)
Where A0 represents the initial absorbance of dyes solution and A represents the absorbance of the dye at time t.
RESULTS AND DISCUSSIONS
The XRD pattern of pure and Fe-doped TiO2 nanoparticles was shown in Fig. 2. Interestingly, X-ray structural analysis of these samples only showed the typical peaks of the anatase structure without any detectable peaks related to dopant or other structures. So far, because of the high photocatalytic activity of anatase structure, studies on TiO2 catalysis are often focused on this structure . These results are inconsistent with studies in which the diffraction peaks correspond to the rutile phase . On the other hand, the XRD pattern of prepared samples, even heavily doped sample i.e. TiO2 nanoparticles doped with 3 % Fe, did not show any diffraction peaks of iron or iron-related compounds, which indicated the formation of an iron-titanium solid solution. This is due to similar ionic radii of titanium and iron (Ti (0.68 Å) and Fe (0.64 Å)), which results in incorporation of the iron ion into the TiO2 crystal structure . These effects are slightly different from those provided by other reports. For example, Ganesh et al. (2007) prepared Fe-doped TiO2 powders by co-precipitation method. They reported small quantities of secondary -Fe2O3 and FeTiO3 phases in their doped samples . In another study, Zhu et al. (2006) identified an -Fe2O3 phase in Fe-doped TiO2 powders prepared by sol-gel .
The average crystalline size of the Fe-doped TiO2 powder was calculated using the Scherrer equation (Table 3). According to this table, crystalline size of the pure TiO2 nanoparticles was about 53 nm and that of the Fe-doped TiO2 nanoparticles was in the range of 43.8 to 45.2 nm in diameter. Thus, a major effect of iron doping was the decrease in the anatase-TiO2 crystalline phase with increasing iron concentration. This result was consistent with the previous studies [11,27]. Therefore, the molten salt method produced Fe-doped TiO2 powder nanoparticles with dimensions below 50 nm successfully. However, according to previous studies the size of the prepared nanoparticles with other methods was less than 50 nm [27,28]. Nevertheless, it should be noted that the method used in this research was much simpler.
SEM images of the pure and Fe-doped TiO2 nanoparticles were presented in Fig. 3. This image indicated the spherical shape of pure and Fe-doped TiO2 particles. Obviously, particle agglomeration happened during synthesizing and drying steps, but redistribution could occur in an ultrasonic bath.
The EDX analysis of pure and Fe-doped TiO2 nanoparticles was conducted to reveal the presence of iron ions in the structure of the prepared samples (Table 3). Fe ions were detected in 1% and a higher percentage of Fe doped TiO2 particles. The Fe ions could not be detected in 0.5% Fe doped TiO2 due to the detection limit of the instrument. According to Table 2, the weight ratio of O to Ti in samples increases with increasing the Fe dopant concentration (0.55 for pure TiO2 to 0.77 for 3% Fe doped). This variation is because of introducing more oxygen vacancies in the structure of TiO2 and doping of Fe ions. This observation is consistent with previous studies .
UV–Vis spectra of the samples are exhibited in Fig. 4. In the reflection spectra of all doped samples, a shoulder appeared at around 450 nm. The intensity of this shoulder also increased with increasing dopant concentration. This is due to d–d transition of Fe3+ in the TiO2 structure . Increasing absorption at a wavelength of less than 380 nm (about 3.1 eV) is associated with the intrinsic band gap absorption of pure anatase TiO2. Doping iron results in absorption in the visible region. These absorptions increase with increasing the iron content due to enhanced visible light absorption. This explains changes in the sample’s color from white for pure nanoparticles to yellow or light brown for doped samples. On the other hand, enhancement of absorption in the visible region is related to the excitation of the 3d electrons of Fe3+ ions to the conduction band of TiO2 [15,30].
Fig. 4b showed the Kubelka Munk extrapolation plot i.e. hν versus (hνα) 2, which was used to calculate band gap degrees of purity and Fe-doped TiO2 nanoparticles. Where, ν, h and α represent light frequency, Planck constant and absorption coefficient, respectively. The resultant band gap was summarized in Table 3. According to these results, the band gap energy of the doped TiO2 samples decreases with increasing Fe concentration from 3.1 eV for pure TiO2 to 3.0, 2.92 and 2.80 eV for 0.5, 1 and 3% Fe doped TiO2, respectively. Formation of additional electronic states originating from 3d electrons of Fe3+ and the interaction between valence band electrons and localized d electrons of Fe3+ is the main reason for lowing the band-gap values of doped samples . Accordingly, the photocatalytic activity of TiO2 is influenced by doping of Fe. In similar studies, Fe-doped TiO2 particles have band gap energy in the range of 2.45–3.11 eV with a redshift that leads to successful utilization of visible light or solar energy [15,29,30]. For example, the band gap of prepared Fe-doped TiO2 by Sood et al. was 2.9 eV .
The visible light induced the photocatalytic activity of pure and Fe-doped TiO2 was studied using photocatalytic degradation of methyl orange. The results were presented in Fig. 5. In all of the prepared samples, the concentration of methyl orange decreased rapidly in the first 10 (dark step) min and then decreased slowly (irradiated step). According to Fig. 5, the Fe-doped samples have higher photocatalytic activity than pure TiO2. The maximum degradation activity of 69 % at 2 h was observed in Fe doped sample with 0.5 wt% Fe. This enhancement of photocatalytic activity caused by doping was in agreement with previous results in the literature . However, in some exceptional cases, the authors did not observe any obvious change after iron doping . It seems that this enhancement arises from the smaller crystalline size and higher light absorption capacity of doped samples than pure nanoparticles (Table 3). Furthermore, the adsorbed Fe on the surface of doped TiO2 nanoparticles could serve as electrons or holes trapper and enhance the separation of free carriers .
According to Fig. 5, if the Fe content is increased further, the photocatalytic behavior of the catalyst will decrease. As discussed earlier, Fe3+ ions could serve as electrons and holes trapper. This is due to the reduction of Fe3+ to Fe2+ through photoelectron capture and oxidation of Fe2+ to Fe3+ by the O2 molecules in the media. Therefore, in the presence of small amounts of Fe3+, the electron/hole separation and consequently the photocatalytic activity improved. Nevertheless, when a large amount of Fe3+ was doped, the distance between trappers would sharply decline, and Fe3+ would turn into recombination centers .
Table 1 provides a comparison between the photocatalytic activity of Fe-doped TiO2 presented by various researchers. By comparing the results of this research with these references, it is observed that the results of the present study are even more acceptable in some cases. It should be noted that the initial dye concentration in the present study is 25 ppm, which is twice more than the value used in the studies presented in Table 1.
TiO2 photocatalysts doped with a different concentration of Fe were synthesized by the molten salt method. Characterization of the prepared samples indicates indicated the formation of spherical pure anatase crystalline structure. The EDX results confirmed the presence of Fe in all of the samples. According to DRS results, the band gap energy of doped TiO2 samples decreased with increasing Fe concentration from 3.1 eV for pure TiO2 to 3.02- 2.80 eV for Fe doped TiO2. Finally, the photocatalytic activity of Fe doped nanoparticles was higher than pure TiO2. The maximum degradation activity was achieved at the Fe doping content of 0.5 wt%.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.