Efficient photocatalytic degradation of organic dyes using Fe x Zn 1-x O nanoparticles

Fe x Zn 1-x O (x = 0, 0.05, 0.075, and 0.1 M) nanoparticles based photocatalysts are successfully synthesized by co-precipitation method. The synthesized nanoparticles are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), and UV–visible double beam spectroscopy (UV–Vis) techniques. The prepared catalysts and its photocatalytic activity was evaluated by methylene blue (MB) and methyl orange (MO) dye under visible light irradiation. The effect of various photocatalyst parameters such as pH, catalyst dosage, and initial dye concentration on the photodegradation was examined in detail.


INTRODUCTION
In recent years, many hazardous organic pollutants, such as toxic dyes and organic residuals released into the atmosphere from several industries [1,2]. The degradation and fullmineralization process are vicious, because of merged structure of organic dyes. Among these organic dyes, methylene blue (MB:C16H18N3SCl) and methyl orange (MO: C14H14N3NaO3S) are highly harmful to the atmosphere, which 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 environment's quality. Various techniques have been extensively utilized to suppress pollutants from contaminated water sources [4,5]. Among them, photocatalysis has emerged to be a promising way to control the massive scale's current environmental pollution. The photocatalysts with semiconducting nanostructures have concerned much more attention due to their exceptional physico-chemical properties in the photocatalytic reaction [6][7][8].
Many researchers recently developed many photocatalysts using metal oxide semiconductor nanoparticles, including Bi2O3, TiO2, ZnO, and WO3. Among these transition metal oxides, zinc oxide (ZnO) is found very sensible in the 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 (Eg≈3.37 eV) [12,13]. ZnO nanoparticles are especially attractive for many interesting nanotechnology applications such as transparent conductive coatings [14], photoanodes for dye-sensitized solar cells (DSSCs) [15], gas sensors [16], and electro-photo luminescent materials [17]. Unfortunately, ZnO can only absorb UV light [18], and photocatalytic degradation efficiency was confined by the electron-hole charge carriers, low-adsorption, and low-reusability. Rectify 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. Various techniques for the synthesis ZnO and Fe-doped ZnO nanoparticles are reported in the literature: hydrothermal method [22], combustion [23], and sol-gel method [24]. Among these synthesis methods, co-precipitation [25] is a flexible method for synthesizing the ZnO nanoparticles due to its low-cost, and easy to operate.
Several studies have been reported that the doping of ZnO with transition metal ions for visible light photocatalysts [26,27]. It has been discovered that 2% Fe-doped ZnO nanoparticles degraded the methyl orange dye up to 80.8% within 210 mins under sunlight irradiation. The Fe doped ZnO degrades MB in 4h in sunlight [28]. Zhang et al. investigated that Fe/ZnO nanowires' utilization is superior to P25 against MO [29]. Abbad et al. synthesized a Fe-doped ZnO nanoparticle and degraded 2-chlorophenol under solar irradiation. The most significant photocatalytic action was accomplished with the optimized dopant centralization of 0.5 wt% Fe because of the small crystallite size and low bandgap with a low oxidation-reduction potential [30].
In the present work, we reported the various properties of an efficient Fe-doped ZnO nanoparticles for photocatalytic reaction. The effect various dopant concentration on structural, morphological, optical, and electrical properties of ZnO nanoparticles was investigated.
Furthermore, we investigated the photocatalytic degradation of MB and MO dye under UV light irradiation.

Characterization Techniques
XRD patterns of the samples were recorded using a mini desktop X-ray diffractometer

Photocatalytic degradation test
The photocatalytic activity of the samples was evaluated by the degradation of MB and MO under UV light irradiation (8 W Philips). 20 mg of FexZn1−xO photocatalysts were dissolved in 20 mL aqueous MB and MO dye solution at various pH (2, 4, and 6). The dye solutions with FexZn1−xO photocatalysts were exposed to UV light from 0 to 180 min. at room temperature.
Every 30 minutes, sampling out 2 ml of dye solution collected from samples for photocatalytic degradation test. The photocatalytic degradation of MB and MO was observed λmax at ~664 and 464 nm respectively using a UV-visible spectrophotometer in the wavelength range 200-800 nm. This result suggests that the Fe ions substitute into the ZnO lattice [33]. The average crystallite sizes (D) of the FexZn1-xO was determined from the Debye-Scherrer equation (Eq. 1) [34].

X-ray diffraction (XRD)
(1) Where λ is the incident of diffraction angle, β is the full width half maximum of the peak (FWHM), θ is the wavelength of the X-rays (1.5406 Å), respectively. The lattice parameters and average crystalline size of the samples are listed in Table 1. The average crystallite size (D) of FexZn1-xO calculated from XRD data are 23.5, 21.6, 16.2 and 12 nm, respectively. Furthermore, the increasing Fe content reduces the lattice parameters and average crystallite size. Previous reports 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 V2O5 doped ZnO ceramics are also similar to the obtained results.

SEM Analysis
The surface morphologies of FexZn1-xO (x≈0, 0.05, 0.075, and 0.1 mol.%) nanoparticles are shown in Fig. 2(a-d).   (Fig. 3a). The atomic percentage of these elements found to be 49.5 and 50.5%, respectively. Fig. 3(b-d) Fig. 4 (a-d). The absorption edge is shifted towards a higher wavelength region, which means that the band gap decreases. The red-shift is due to the increase of crystallite size, and it was confirmed from the XRD results.

FTIR Analysis
FTIR spectra of FexZn1-xO (x≈0 and 0.1 mol.%) nanoparticles in the range 4000 -400 cm -1 are presented in Fig. 5 (a, b). The broad absorption band appearing in the range 3452-3446 cm -1 corresponds to -OH stretching vibration, while the two peaks located at 1625 and 1591 cm -1 is due to -OH bending vibration of the adsorbed H2O molecules [42]. The band lower intensity absorbed around 2380 cm -1 which corresponds to the symmetric and asymmetric C-H bond. Two weak absorption peaks at 428 and 449 cm -1 for undoped and 0.1 mol.% Fe-doped ZnO sample may corresponds to Zn-O stretching mode. The absorption peaks located at 1120, 1122, and 800 cm −1 attributed to the sulfate group, respectively [43]. Also, FTIR spectra of 0.1 mol.% Fe-doped ZnO shows that the small stretch observed at 601 cm -1 corresponds to Fe-O stretch, as reported by Liu et al. [43]. Therefore, it might be due to Fe 3+ ions substituted in Zn.

I-V Characteristics
The I-V characteristics of FexZn1-xO (x≈0 and 0.1 mol.%) nanoparticles have been determined by using Ag-paste for better electrical contact and the result is shown in Fig. 6 (a, b).  [44]. The remarkable increase in these samples conductivity may result in a higher advantage for optical device fabrication.

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

Influence of catalyst concentration
The influence of catalyst concentration on the photodegradation of MB and MO dye was verified using ZnO catalyst concentrations from 5 to 15 mg/30 ml in 10 ppm MB, and MO dye solution at pH=6 and it was displayed in Fig. 7(b). The variation in photodegradation can be clarified by accessibility of several surface-active sites and UV light radiation into dye solution.
The photodegradation reaches maximum at 10 mg/30 ml. The reduced photodegradation at higher catalyst concentration (15 mg/30 ml) may be due to ZnO nanoparticles aggregation increases the scattering effect [45]. Consequently, 10 mg/30 ml ZnO photocatalyst was expected as an ideal catalyst weight.

Influence of UV irradiation time of MB
The photocatalytic activity was carried out with MB concentration of 2.0 mM, catalyst concentration of 10 mg, pH = 6 and irradiation time up to 150 min. Fig. 8(a-d) Fig. 8(e). The result reveals that the Fedoped ZnO shows higher photocatalytic activity than that of undoped ZnO. Fe (0.075%) doped ZnO shows enhanced photocatalytic activity with a degradation efficiency of 68% for MB (pH = 6) dye at 150 min. Fe(0.075%)-doped ZnO nanoparticles less time to degrade the MB dye compared to other concentration of Fe. The reduction of photocatalytic activity at higher concentration Fe (0.1%) doped ZnO may be due to photons' reduced path length [45]. Similar results are also observed by Suganthi et al. [46]. Another reason for the increase in the photocatalytic activity of Fe-doped ZnO nanoparticles, Fe ions substituted into ZnO surface may suppress the electron-hole pairs recombination and enhance the dye degradation efficiency [47,48].

Influence of UV irradiation time of MO
The photocatalytic activity was carried out with MO concentration of 3.    In this process, narrow semiconductors act as a sensitizer to improve the dye's photodegradation based on their electronic band structure. When the photocatalyst was illuminated with higher energy photons, it only allows the dye molecule oxidation [45]. We proposed a mechanism of FexZn1-xO nanoparticles for efficient photocatalytic activity. It can be described as follows: ZnO + hγ ZnO (h + VB + e − CB) These superoxide anions (O2°−) and hydroxyl radicals (OH°) are strong oxidizing species, and it will degrade of MB and MO dye molecule (Eq. 7) [49]. From these results, novel Fedoped ZnO nanoparticles have played a primary role in the degradation of organic dyes from wastewater.

Conclusion
In this paper, we reported the synthesis of