Enhanced Visible Light Photocatalytic Degradation of Fe-Doped ZnO Nanoparticles For Organic Dyes

Fe x Zn 1-x O (x = 0, 0.05, 0.075, 0.1 M) nanoparticles based photocatalysts were synthesized by a chemical precipitation method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray emission (EDX) and UV–Vis spectroscopy. The photocatalytic eciency of Fe x Zn 1-x O catalysts was assessed under visible light irradiation using the degradation of methylene blue (MB) and methyl orange (MO) dye in aqueous solution. The present investigation shows that the effect of optimized parameters (pH, catalyst dosage and initial dye concentration) and doping concentrations plays signicant role in photocatalytic activity. The detailed photocatalytic mechanism for the enhancement of photocatalytic activity has also been proposed.


Introduction
In recent years, lot of hazardous organic pollutants such as toxic dyes and organic residuals are released into the environment from various industries [1,2]. The degradation and complete mineralization are di cult because complex structure of the organic dyes. Among these organic dyes, methylene blue (MB:C 16 H 18 N 3 SCl) and methyl orange (MO: C 14 H 14 N 3 NaO 3 S) are highly harmful to the environment, then poses a threat to the health of humans and animals [3]. Industrial wastewater treatment and recycling are fundamental objectives to secure the worldwide biological system and improve the quality of the environment. Various techniques have been extensively utilized for the degradation of pollutants from contaminated water sources [4,5]. Among them, photocatalysis have emerged to be promising way for controlling the current environmental pollution at the massive scale. Especially, photocatalysis using semiconducting nanoparticles has attracted a great deal of attention in photocatalytic reaction because of its unique physical and chemical properties [6][7][8].
Many researchers recently developed a photocatalysis using metal oxide semiconductor nanoparticles, including Bi 2 O 3 , TiO 2 , ZnO, and WO 3 . Among these oxides of transition metal, zinc oxide (ZnO) is found very sensible in photocatalytic method due to their wide-bandgap, non-toxicity and high photosensitivity [9][10][11]. ZnO is a wide-bandgap semiconductor with a direct energy bandgap of (E g ≈ 3.37 eV) is comparable to TiO 2 [12,13]. ZnO nanoparticles are especially attractive for many interesting nanotechnology applications such as transparent conductive coatings [14], electrode materials for dyesensitized solar cells (DSSCs) [15], gas sensors [16] and electro-photo luminescent materials [17].
However, previous studies proved that the ZnO is more e cient catalyst for waste water treatment.
Unfortunately, ZnO can only absorb UV light [18] and photocatalytic e ciency was also restricted by the electron-hole recombination, low adsorption ability and restricted reusability. To overcome this problem numerous reports focused on doping ZnO with transition metal (Fe, Co, Mn) ions [19], non-metal (N, C, S) ions [20] and noble metals loading (Ag, Au, Pd) [21] have been carried out. Combination of two semiconductors with various bandgap incredibly enhance the photocatalytic activity [22][23][24]. Compared with other methods co-precipitation [25] has several advantages such as low-temperature preparation, easy to handle and low-cost.
A substantial number of studies reported that the doping ZnO with transition metal ions has been widely studied for visible light photocatalysts [26,27]. It has been discovered that 2% Fe doped ZnO degraded methyl orange up to 80.8% in 210 mins of sun-light irradiation. The Fe doped ZnO degrades MB in 4 h in sun light [28]. Zhang et al. investigated that the utilization of Fe/ZnO nanowires is obviously superior to that of P25 against MO [29]. The synthesized Fe-doped ZnO nanoparticle via sol-gel technique and degraded 2-chlorophenol in aqueous solution under solar irradiation by Abbad et al. [30]. The optimized dopant centralization of 0.5 wt% Fe, the greatest photocatalytic action was accomplished owing to the fact of small crystallite size and low band gap with a low oxidation-reduction potential [30]. In the present work, we present a different noble Fe-doped ZnO photocatalysts with doping levels in the range 0-0.1 mol% via co-precipitation method. The synthesized photocatalysts were characterized by XRD, SEM-EDX, FTIR, IV and UV-Vis spectrophotometer. Also, we analyze the photocatalytic activity of Fe-

Characterization Techniques
UV-Vis absorbance spectra measurements were performed with a SYSTRONICS: AU-2707 double beam spectrophotometer to detect absorption over the range 190-900 nm. XRD patterns were recorded using a mini desktop X'PERT PRO X-ray diffractometer operated with CuKα 1 radiation of wavelength 1.5406 Å with scanning rate of 2º 2θ min -1 (from 2θ = 10 to 80 ºC). Surface morphology and size of the samples were obtained using scanning electron microscope JEOL JS-6390 instrument attached with an energy dispersive X-ray spectroscopy (EDX). Fourier transform-infrared (FTIR) spectra of prepared photocatalysts were recorded by using a JASCO FTIR-410 spectrophotometer wavenumber in the range 4000-400 cm -1 with a resolution of 2 cm -1 delivered with computer software. The pH of the solution was noted by using ELICO (LI-10T model) digital pH meter. The current-voltage (I-V) characteristic curves were recorded using Keithley electrometer 2400 model.

Evaluation of photocatalytic degradation
The visible light photocatalytic activities of the Fe x Zn 1−x O, various mol% (x = 0, 0.05, 0.075 and 0.1%) nanoparticles under UV light (8 Watts Philips UV lamp) were evaluated by the degradation of MB and MO. The photocatalytic tests were done in a submersion type photoreactor made of wooden chamber. The catalyst powders like undoped ZnO and Fe (0.05, 0.075 and 0.1%) doped ZnO nanoparticles at various pH (2, 4 and 6). Each sample were taken 10 mg was dissolved in 20 ml of 100 μm aqueous solution of MB and MO dye. Before starting the irradiation process, the above prepared solution was stirred in the dark for 1 hour, to establish the adsorption/desorption equilibrium. All samples solution was transferred into the inside of photoreactor for UV exposure. Every 30 mins, 2 ml of dye solution collected from samples for photocatalytic degradation test. The dye solutions with the undoped ZnO and Fe doped ZnO nanoparticles were exposed to UV light from 0 to 180 mins at room temperature. By using a UV visible spectrophotometer at λ max of 664 and 464 nm, the effective degradation of MB and MO was noticed. The degradation e ciency was calculated using the following formula (Eq. 1).  112) and (201). There are no characteristic peaks of Fe phases or its oxides were not detected in any of the samples. Fig. 1 002) and (101) peaks [34].
Where k is the shape of factor of the particles (k= 0.9), β is the full width half maximum of the peak (FWHM), θ and λ are the incident of diffraction angle and wavelength of the X-rays (1.5406 Å), respectively. The lattice constants calculated from XRD are presented in respectively. This evolution can be due to the distortion creates in the crystal lattice by the incorporation of the Fe ions. When increasing Fe doping level, the FWHM decreases suggesting an increase of the average crystallite size. The same results were reported by Jayachitra et al. [35] in Fe doped ZnO nanoparticles, Srinivasan et al. [36] in Mn doped ZnO and Nahm et al. [37] in V 2 O 5 doped ZnO ceramics.   Fig. 2(c-d)]. From the outcomes, it should be noticed that the samples were agglomerated and consists of small irregular pores. The images clearly exposed that undoped ZnO was prepared well with nanometer size and the surface morphology changed with the extension of Fe concentration. Compared with undoped ZnO, 0.1 mol.% of Fe-doped ZnO showed more information on the surface. It can also be noticed that the size and morphology of Fe x Zn 1-x O nanoparticles enhanced with the dopant concentration.

EDX Analysis
EDX analysis (Fig. 3a-d) was carried out to investigate the elemental composition of Fe x Zn 1-x O (x≈0, 0.05, 0.075 and 0.1 mol.%) nanoparticles. Fig. 3(a) reports the spectrum of undoped ZnO, showing the characteristic peaks associated with O and Zn elements. The measured atomic percentage of these elements is 49.5 and 50.5 at. %, respectively. Fig. 3(b-d)
The absorption edge is shifted towards higher wavelength region which means that the band gap decreases. The red-shift is due to the increase of crystallite size and it was con rmed from the XRD results.

FTIR Analysis
The FTIR spectra of Fe x Zn 1-x O (x≈0 and 0.1 mol.%) nanoparticles in the range 4000-400 cm -1 to determine the presence of various functional groups shown in the Fig. 5 (a, b). The broad band located around 3452-3446 cm -1 corresponds to O-H stretching vibration of adsorbed water molecules, while the band located around 1625 and 1591 is due to O-H bending vibration of the same atmospheric water [42].
The bands with lower intensity absorbed at around 2380 cm -1 , and attributed to symmetric and asymmetric C-H bonds, respectively, appears due to an environmental contamination. The strong absorption band at 428 cm -1 is due to Zn-O stretching mode for undoped ZnO nanoparticles; similarly, for 0.1 mol.% Fe doped ZnO samples this is pointed out from absorption peaks in the range of 449 cm -1 , respectively. The small stretch observed at 601 cm -1 only found in the 0.1% Fe-doped ZnO sample, is characteristic of a Fe-O stretch as reported by Liu et al. [43]. Therefore, it might be due to Fe ions substituted by Zn ions and incorporated into the crystal lattice of ZnO. New absorption peaks at 1120, 1122 and 800 cm −1 appear on both samples was ascribed to the sulphate group [43].

I-V Characteristics
The I-V characteristics studies of Fe x Zn 1-x O (x≈0 and 0.1 mol.%) nanoparticles have been carried out using silver paste to make good electrical contact shown in the Fig. 6 (a, b). From these curves, DC  [44]. The remarkable increase in the conductivity by these samples may results in higher advantage for electro-optic device fabrication.

Effect of pH
The adsorption of MB and MO dye molecules on undoped ZnO nanoparticles strongly depends on the pH of the solution are displayed in Fig. 7(a). The effect of pH on the photodegradation of MB and MO dye was studied by varying the pH of the solution from 2 to 6. The outcome shows that photodegradation is high at base medium and afterward decreases when the pH of the solution decreases. The degradation arrives at most extreme at pH = 6 and afterward decreases sensibly up to pH = 2. Henceforth, the pH = 6 was accepted as ideal pH and utilized for additional investigation.

Effect of catalyst concentration
The effect of catalyst concentration on the photodegradation of MB and MO dye was tested using ZnO catalyst concentrations from 5 to 15 mg/50 ml in 10 ppm MB and MO dye solution at pH=6 are displayed in Fig. 7(b). The variation in photodegradation can be explained by the availability of number of surfaceactive sites and the immersion of UV light towards the dye solution. The results are displayed in Fig. 6; the gure visibly shows that the photodegradation reaches maximum at 10 mg/50 ml and higher catalyst concentration more than 15 mg/50 ml results in the decrease in photodegradation. The reduced photodegradation at higher catalyst concentration (15 mg/50 ml) may be due to the aggregation of ZnO nanoparticles. The aggregation of nanoparticles leads to the increase in scattering effect and, thereby decreases the active sites [45]. Consequently, 10 mg/50 ml ZnO photocatalyst is expected as ideal catalyst load.

Effect of UV irradiation time of MB
Photocatalytic experiments were carried out with an initial MB concentration of 2.0 mM, catalyst concentration of 10 mg, pH = 6 and irradiation time upto 180 min. The photocatalytic activity of MB dye solution (10 PPM) was also studied for reference. Fig. 8(a-d) shows the change in absorption spectra of MB exposed to UV light for various irradiation times (0, 30, 60, 90, 120 and 150 min) in the presence of  Fig. 9(e). The results revealed that the Fe doped ZnO showed higher photocatalytic activity than that of undoped ZnO. Fe (0.075%) doped ZnO showed enhanced photocatalytic activity with a degradation e ciency of 55% for MO (pH = 6) dye at 150 min [49].
In this process narrow semiconductor act as sensitizer to enhance the photodegradation of the organic dye and it can produce more photons to excite the electrons from the valence to the conduction band. When the photocatalyst was illuminated with higher energy photons, it allws the oxidation of the dye molecule [45]. We have proposed a mechanism for the enhanced photocatalytic activity of Fe doped ZnO nanoparticles. It can be described as follows: ZnO + hγ ZnO (h + VB + e − CB ) These super oxide anion (O 2°− ) and hydroxyl radicals (OH°) are powerful oxidizing species and it will degrade of MB and MO dye molecule (Eq. 7) [49]. From these results we can conclude that the surface of Fe doped ZnO nanoparticles has played a primary role in degradation of organic dyes.

Conclusion
In this paper, Fe x Zn 1-x O (x≈0, 0.05, 0.075, 0.1 mol.%) nanoparticles were synthesized by co-precipitation method. The average crystallite size of Fe x Zn 1-x O nanoparticles was found to be decreases when the doping concentration of Fe increases, which is con rmed by the XRD analysis. Doping with Fe modi ed the morphology of ZnO nanoparticles obtained. The particle size of the synthesized nanoparticles was