Research Article | Open Access
Fe-TiO2 Nanoparticles Synthesized by Green Chemistry for Potential Application in Waste Water Photocatalytic Treatment
Anatase TiO2 nanoparticles doped with iron ions have been synthesized via the green chemistry method using aqueous extract of lemongrass (Cymbopogon citratus) obtained from Soxhlet extraction and doped by wet impregnation. The TiO2 anatase phase has been doped with Fe3+ (0.05, 0.075, and 0.1 Fe3+ : Ti molar ratio) at 550°C and 350°C, respectively. The scanning electron microscopy with energy-dispersive X-ray (SEM-EDS) shows nanoparticle clusters and efficiencies of impregnations between 66.5 and 58.4% depending on the theoretical dopant amount. The electron transmission microscopy (TEM) reveals final particle sizes ranging between 7 and 26 nm depending on the presence or not of the dopant. The cathodoluminescence (CL) and photoluminescence (PL) studies of the doped and undoped nanoparticles show a luminescence signal attributed to surface oxygen vacancies (visible CL emission 380–700 nm and PL emission 350–800 nm); additionally, a decrease in emission intensity is observed due the inhibition of the recombination of the photogenerated electron-holes pairs; moreover, nanopowders were analyzed by UV-Vis spectrophotometry of diffuse reflectance, and the absorption edge of the Fe-TiO2 in comparison to undoped TiO2 is extended greatly toward the visible light. The six bands (A1g + 2B1g + 3Eg) found by Raman spectroscopy and the x-ray diffraction pattern (XRD) confirm that synthesized TiO2 is only anatase phase, which is commonly used as a catalyst in waste water treatment, specifically in heterogeneous photocatalytic processes.
Solar energy is a renewable resource, and its efficient utilization in controlling environmental pollutions by using photocatalytic materials is one of the main goals of modern science and engineering [1–4]. Owing to exceptional and appropriate properties including nontoxic, excellent chemical stability, strong ultraviolet (UV) absorption, and wide band gap energy (anatase 3.2 eV and rutile 3.4 eV), titanium dioxide (TiO2) is a recognized metal oxide semiconductor which could be extensively applied to diverse applications. Hence, the utilization of sustainable luminous energy like visible light or sunlight is limited, because TiO2 can only be excited by ultraviolet light (wavelength lower than 387 nm), which occupies only 3–5% of the entire solar spectrum [5–11]. It is also known that high recombination of activated electron-hole pair of TiO2 is still its major drawback that causes the restriction of practical applications, in particular, solar harvesting aspect [10, 12, 13]. Thus, many approaches have been devoted to improve the light response of TiO2 in the visible light region. For instance, nonmetal doping (B, C, N, S, and F), metal doping (Fe, Co, and La), and codoping (Fe and N, B and C) [14–25].
There are current research studies focusing on the use of plant extracts for the green synthesis of different types of nanomaterials because the phytochemicals present in these natural extracts participate as capping agents or templates for the stabilization of crystalline phases and size control of the nanoparticles produced [26–32]. Recently, a mixture of rutile with impurities for anatase phase has been prepared via green synthesis using M. citrifolia leaves extract by the hydrothermal method with calcination temperature 400°C. The green synthesized compound has a tetragonal structure when characterized by XRD, and the evaluated average crystalline size was 10 nm; also, the surface morphology of TiO2 nanoparticles obtained via green synthesis was observed as high uniform quasi-spherical shape morphology . Furthermore, Rajiv et al.  biosynthesized TiO2 nanoparticles (NPs) using the Momordica charantia leaf aqueous extract as a reducing and stabilizing agent, and these nanoparticles were applied in the in vitro antimalarial activity against Plasmodium falciparum, and the results suggest that the synthesized TiO2 NPs may be employed to develop newer and safer agents for malaria control. In a similar study, Goutam et al.  prepared green titanium dioxide NPs using leaf extract of the biodiesel plant, Jatropha curcas L., and evaluated its performance for the photocatalytic treatment of TWW after the secondary (biological) treatment process, achieving 82.26% and 76.48% of COD and Cr removal, respectively, in a parabolic trough reactor (PTR). Also, Jegadeeswaran et al.  proposed a novel green synthesis of Ag/TiO2 nanocomposites of anatase phase and average particles size of 25.74 nm from Padina tetrastromatica extract.
Accordingly, the present work is focused on the study of the synthesis and structural characterization of the Fe-doped TiO2 nanoparticles prepared by green synthesis. The main aim is to determine the effect of the introduction of the doping elements in the optical, physicochemical, structural, and morphological properties. Nanostructured TiO2 anatase doped with different Fe : Ti molar ratios were successfully synthesized. The prepared samples were characterized by photoluminescence, cathodoluminescence, UV-Vis spectrophotometry of diffuse reflectance, Raman spectroscopy, XRD, SEM-EDS, and TEM-SAED.
2. Materials and Methods
Fresh lemongrass plant leaves (Cymbopogon citratus) were collected from Cartagena, Colombia. Titanium(IV) isopropoxide (C12H28O4Ti) solution (95%) and iron(III) chloride hexahydrate (98% purity) were purchased from Alfa Aesar® and Panreac, respectively. All reactions were carried out using ACS Reagent chemicals.
2.2. Lemongrass Leaf Extract Preparation
The lemongrass leaves were washed with griffin water. Then, they were dried for six hours in an air circulation oven at 60°C (Esco Isotherm®, OFA 32-8) and crushed using a manual mill. The dried and crushed biomass (100 g) was placed in cloth bags and subjected to a solvent extraction process using a Soxhlet extractor for 6 h in approximately 500 mL of distilled water [37, 38]. The extract was stored in the refrigerator at 4°C. Further, this extract was used to synthesize TiO2 nanoparticles . The extraction-concentration was performed using the technique solid-phase microextraction (SPME), monitoring in vapor phase (HS), using a fiber of fused silica coated with PDMS/DVB of thickness 65 micrometers (PDMS/DVB-65 pm). The chromatographic analysis was carried out in an AT 6890 Series Plus gas chromatograph (Agilent Technologies, Palo Alto, California, USA), coupled to a selective mass detector (Agilent Technologies, MSD 5973), operated in the scanning complete radio frequency mode (full scan). The column used in the analysis was DB-5MS (5% phenyl-poly (methylsiloxane), 60 m × 0.25 mm × 0.25 m). The injection was done in the split mode (30 : 1) with the SPME device.
2.3. Green Synthesis of Fe-TiO2 Nanoparticles
In a typical experiment, the reaction was carried out in a 250 mL beaker, which was introduced in an ultrasound processor (ultrasonic processor, WiseClean WUC-A06H, 60 Hz). Twenty milliliters of the precursor agent (titanium isopropoxide) was added to 100 mL of the aqueous extract of lemongrass contained in a burette, at a rate of 1 mL·s−1; additionally, the reaction lasted approximately 30 minutes with constant agitation, making use of a stirring rod [18, 33]. The nanoparticles were washed with 70% vol. ethanol and finally with distilled water, using separation by centrifugation (universal centrifuge PLC-012E) for 15 min at 5000 rpm. The synthesized titanium dioxide nanoparticles were calcined at 550°C for 3 hours in a Thermo Scientific FB1415M-1450 W-50/60 Hz muffle .
CL measurements were performed in Hitachi S2500 SEM at room temperatures using a Hamamatsu photonic multichannel analyzer PMA-12 with excitation voltage of 25 kV. HORIBA Jobin Yvon LabRAM HR800 UV-Vis used the Olympus BX41 confocal microscope was used in photoluminescence (PL) and Raman measurements. PL spectra have been acquired with a 325 nm He-Cd laser using the lowest laser intensity (0.1 I0) to avoid the anatase to rutile phase transformation during the PL acquisition; morever a filter D1, hole 850 μm, spectrometer 327,199 nm, and ×40 objective were used for the photoluminescence study; while µ-Raman spectroscopy has been performed using a red He-Ne laser of 633 nm wavelength. The UV-Vis absorption studies of the photocatalysts were conducted on UV-Vis diffuse reflectance spectrophotometer (Thermo Scientific EVOLUTION-600), with BaSO4 as reference. The samples were studied by X-ray diffraction (XRD) using a Panalytical X’Pert Pro Alpha1 instrument, which is equipped with a primary fast X’Celerator detector operating at 45 kV and 40 mA and fitted with a primary curved Ge 111 monochromator to obtain CuKα1 radiation (λ = 1.5406 Å). Data were collected at 2θ between 10° and 90°, with a step size of 0.04°·s−1. SEM micrographs and energy-dispersive X-ray spectroscopy (EDS) maps were obtained in a Leica 440 SEM coupled to Bruker AXS XFlash Detector 4010. Investigation of particle morphology of Fe3+-doped TiO2 samples was performed on a JEM 2100HT JEOL transmission electron microscope (TEM) provided with 200 kV emission gun, beam current of 108.6 μΑ, and equipped with diffraction mode (camera length of 300 mm) for selected area electron diffraction (SAED). The samples were prepared by ultrasonic dispersing of the powders as slurry in 2-propanol and deposited in TEM grids.
3. Results and Discussion
3.1. Lemongrass Leaf Composition
The lemongrass plant is formed by a variety of chemical components with higher predominance terpenes and terpenoids associated with aldehydes, alcohols, and ketones . To guarantee the formation of the titanium dioxide nanoparticles, it is necessary that the extract of the lemongrass plant has phytochemical components that guarantee the reduction of the titanium (IV) isopropoxide precursor . Figure 1 shows the chromatogram obtained for the lemongrass sample. The presumptive identification of the compounds registered in solid sample was their mass spectra (EI, 70 eV), using the Adams and Wiley databases. In Table 1, appear the presumptive identification and relative quantity (%) of the components present in lemongrass solid sample analyzed by GC-MS operated in the full-scan mode of radio frequency. In order to guarantee the formation of TiO2 nanoparticles with sizes of less than 20 nm, it is necessary that the extract of the lemongrass leaves has phytochemical components that act as surfactants and stabilizers that prevent excessive agglomeration of the nanoparticulate material formed during the synthesis [33, 35, 41–47], as proposed in the mechanism shown in Figure 2. However, it is possible to synthesize TiO2 powders through the complete hydrolysis of the used precursor (titanium isopropoxide) through the direct contact of the titanium source with water [45, 48], with the disadvantage that it does not present alternatives to control the stages of nucleation and growth that are carried out in the process of production of nanoparticles in solution .
The XRD pattern synthesized TiO2 nanoparticles using the leaf extract of the lemongrass plant (Cymbopogon citratus) is depicted in Figure 3. The ten distinct peaks at 2θ = 25.61°, 37.8°, 38.10°, 48.47°, 54.24°, 55.36°, 62.99°, 69.20°, 70.59°, and 75.47° in the XRD pattern of Fe3+-doped TiO2 and undoped nanopowders are consistent with anatase (101), (103), (004), (200), (105), (211), (204), (116), (220), and (107) lattice planes (JCPDS No. 21-1272). The diffraction peaks corresponding to rutile phase only appeared in Degussa (Evonik) P-25 at 2θ = 27.58°, 36.30°, and 54.42° can be attributed to (110), (101), and (211) planes of rutile TiO2 (JCPDS Card No. 21-1276). The sharpness of peaks and the absence of unidentified peaks confirmed the crystalline and high purity of nanoparticles prepared. The average crystalline size of TiO2 nanoparticles was calculated using Debye–Scherrer’s equation :where D is the average crystal size in Å, λ is the wavelength of the X-ray radiation (1.5406 Å), K is the dimensionless shape factor (0.9), β is the line width at half-maximum intensity (FWHM) in radians, and θ is Bragg’s angle in degrees .
The average crystalline size was estimated from the FWHM of the TiO2 main peak (2θ = 25.61°) of XRD patterns, which corresponds to the plane (101). The average crystalline size was around 10 nm for the prepared nanomaterials. Iron (Fe+3) can conveniently integrate into the matrix of TiO2 owing its atomic radius of 0.69 Å which is almost equal to Ti+4 atomic radius of 0.745 Å . However, no reduction in crystalline size is observed, which is attributed to nanoparticles growth in the calcination process of the wet impregnation at 350°C for 3 h. Table 2 summarizes the results obtained for the diameter by Debye–Scherrer’s equation.
3.3. Raman Spectroscopy
Micro-Raman spectroscopy has been carried out for further analysis of the structural phases of pure and Fe-doped TiO2 NPs, as shown in Figure 4. Anatase phase of titanium dioxide has six Raman active modes A1g + 2B1g + 3Eg at 144 cm−1 (Eg), 197 cm−1 (Eg), 399 cm−1 (B1g), 516 cm−1 (A1g + B1g), and 639 cm−1 (Eg), which were identified on the Raman spectra of all the samples under discussion . It has been shown that any peak associated with iron oxide are not observed, even with a highly doped sample. This means that Raman spectral observations are in good agreement with the XRD results. Moreover, Fe-doped TiO2 NPs retained the anatase structure, which indicates that the Fe+3 dopants are successfully incorporated into the TiO2 framework, replacing Ti+4 cations. However, it has been observed that the Raman band at 144 cm−1 (inset in Figure 4) tends to shift to a higher Raman intensity as the amount of the Fe dopant increases. Generally, it has been accepted that shifts in the Raman peak occur because of changes in the structure, particle size, the nature of defects, and so on .
3.4. Cathodoluminescence (CL) and Photoluminescence (PL)
The spectra of the cathodoluminescence detector and photoluminescence obtained for all prepared materials are shown in Figures 5 and 6, respectively. For both cases in the emission (visible spectrum, 380–700 nm) of all samples studied clearly identified the presence of a broadband around the 500–550 nm, which is due to the existence of surface states and the self-trapped excites in the anatase phase.
Also, it is evident the existence of a shoulder 600–650 nm related to the oxygen vacancies in the involved phase, similar to what was reported by several authors [53, 54]. On the other hand, it is noticeable that there is difference in the intensity of the luminescence, which indicates inhibition of the recombination of the electron/hole pairs photogenerated by the ions of Fe3+. The intensity of Fe-TiO2 is lower than that of pure TiO2, which suggests that electron-hollow recombination is much lower, and the effectiveness of separation is much higher. The decrease in the rate of recombination implies that a large number of electrons and photogenerated gaps are involved in the photochemical transformation, which coincides with the reported by other researchers [9, 22, 39, 52, 55].
The surface morphology and particle size (size distribution) of the pure (Fe : Ti = 0) and Fe-doped TiO2 (Fe : Ti = 0.1) photocatalyst were further analyzed by TEM, as shown in Figures 7(a) and 7(b), respectively.
It is evident from these images that the synthesized nanoparticles were agglomerated, and their shapes were quasi-nanospheres; this corresponds to the results reported by several authors in research related to the green synthesis of TiO2 by the use of aqueous extracts of leaves [33–35]. Also, it can be seen that particles have a small size but are perfectly crystalline in nature. The TEM results are in close agreement with the average crystallite size obtained from the XRD pattern. The average particle size increases for Fe : Ti = 0.1 as compared to pure TiO2, which is attributed to nanoparticles growth in the calcination process of the wet impregnation at 350°C for 3 h. The structural information obtained from SAED (Figure 8) pattern shows the polycrystalline nature of Fe-TiO2 nanoparticles (Fe : Ti = 0.1), which is indicated by the (101), (004), (200), and (105) planes of the anatase phase, similar to that reported by Ali et al. . The interplanar space was determined through Equation (2), where, h, k, and l are the Miller indexes, a and c are the network parameters for a tetragonal structure (anatase), and and is the interplanar distance value in Å [9, 17].
The SEM micrographs and EDS analysis of the pure (P-25 and Fe : Ti = 0) and Fe-doped TiO2 (Fe : Ti = 0.05, Fe : Ti = 0.075, and Fe : Ti = 0.1) photocatalyst are depicted in Figures 9(a)–9(e), respectively. Pure and Fe3+-doped TiO2 are ultrafine, so they are coupled and agglomerate due to the high surface energy . This means that the green synthesis method can lead to the creation of the product with agglomerates in the micrometric scale, for which irregular forms are observed, similar to what was reported by Arabi et al.  for the green synthesis of TiO2 using the extract of Moose and Thyme.
The compositional analysis shows the separate peak of titanium (Ti, 4.508 keV), iron (Fe, 6.398 keV), oxygen (O, 0.523 keV), and other elements as Na, Ca, Si, Cl, and K, which come from the water used in the synthesis process . Table 3 shows the atomic percentages obtained for each element by EDS, in addition to the impregnation efficiencies achieved for all prepared samples.
3.7. UV-Vis DRS
The UV-Vis diffuse reflectance spectra of the pure TiO2 and Fe-TiO2 samples are shown in Figure 10. The absorption is enhanced in the visible light region when Fe is doped into TiO2. The Fe-TiO2 samples exhibit absorption in both the UV and visible light regions. Obviously, the diffuse reflectance spectra of all the Fe-TiO2 nanostructures exhibit increased absorption in the visible light range; also, this increase is more evident by increasing the relationship molar Fe : Ti, which is induced by the electron transition from Fe 3d orbitals to TiO2 conduction band (CB) from the O 2p TiO2 valence band (VB), generating a considerable decrease in the band gap energy of TiO2 . In summary, doping Fe3+ causes structural defects of crystal lattice to introduce impurity or defect energy level and induces the local states below the conduction band edge and then results in this redshift and narrows the band gap . Although doping of the Fe ions in the TiO2 does not modify the position of the valence band edge of the TiO2, it introduces new energy levels (Fe3+/Fe4+) of the transition Fe ions into the band gap of the TiO2.
The direct band gap energy (Eg) was calculated using the following Tauc plot as given in the following equation, which is derived assuming a direct transition between the edge of the valence band and conduction:where hν is the photon energy, a is the absorption coefficient, and A is an energy-dependent constant and known as the band tailing parameter. Another constant is n, which is known as power factor of the transition mode of the materials.
The values of n for direct, indirect, direct forbidden, and indirect forbidden transitions are 1/2, 2, 3/2, and 3, respectively. The pure anatase and Degussa P-25 TiO2 used in this research are considered as direct band gap materials [57, 58]. Therefore, the value of n was taken 1/2 to plot the graph (αhν)2 versus hν as shown in Figure 11. It is observed a reduction of 1 eV for the material Fe : Ti = 0.1 in comparison with the pure TiO2 (Fe : Ti = 0) synthesized by the methodology of green chemistry assisted by ultrasound, which would allow to use more efficiently the solar radiation in heterogeneous photocatalytic processes for the degradation of organic pollutants. These results relate to several recent research focused on the doping of titanium dioxide with Fe3+ ions [56, 59–61]. Moradi et al.  observed the decrease of the band gap with the increase of the relationship molar Fe : Ti in catalysts of TiO2 doped with Fe3+ using the technique sol-gel.
In summary, TiO2 (anatase phase only) doped with a different molar ratio Fe3+ : Ti was successfully prepared via green synthesis (using the leaves extract of lemongrass plant leaves) assisted ultrasound method followed by calcination. The average crystalline size calculated by XRD pattern was between 9.37 to 10.33 for the TiO2 (Fe : Ti = 0) and TiO2 (Fe : Ti = 0.05) samples, respectively. The scanning electron microscopy with energy-dispersive X-ray (SEM-EDS) shows nanoparticles clusters and efficiencies of impregnations between 66.5 and 58.4% depending on the theoretical dopant amount. From PL and CL studies, it was confirmed that doping of Fe (III) ions into TiO2 matrix leads to the inhibition of recombination of charge carriers, thereby enhancing photochemical quantum efficiency. From UV-Vis DRS analysis, a reduction of 1 eV was observed for the material Fe : Ti = 0.1 in comparison with the pure TiO2 (Fe : Ti = 0) synthesized by the methodology of green chemistry assisted by ultrasound, which would allow to use more efficiently the solar radiation in heterogeneous photocatalytic processes for the degradation of organic pollutants.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors greatly acknowledge the financial support from University de Cartagena (International Internship 2017, 01735) and the Colombian Administrative Department of Science, Technology and Innovation (Colciencias Young Researchers Program 2017). The authors would also like to thank the Electronic Nanomaterials Physics Research group of Universidad Complutense de Madrid for providing the facility of all the equipment used in this research.
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