3.1 X‑ray diffraction (XRD)
Fig. 1(a) shows the powder XRD characteristic patterns for FexZn1-xO (x≈0, 0.05, 0.075, 0.1%) nanoparticles obtained by co-precipitation method. All the diffraction peaks could be indexed to hexagonal structure of crystalline ZnO, which further confirmed from the JCPDS No. 36-1451 (a = 0.325 nm and c = 0.5207 nm) [31]. Wurtzite phase of ZnO [32] were obtained for all samples, with the following diffraction peaks (100), (002), (101), (102), (110), (103), (200), (112) and (201). There are no characteristic peaks of Fe phases or its oxides were not detected in any of the samples. Fig. 1(b) shows the slightly shifted to higher angle (right) of peaks observed in the 30–38° interval with the increasing of Fe doping content, indicating overlapping of peaks. The magnification of the primary peaks at 31.7, 34.4 and 36.3° were slightly shifted to lower angle (left) occurs in the Fe0.075Zn0.925O sample. The ionic radii of Fe3+ and Zn2+ are different (Fe3+ = 0.64 Å and Zn2+ = 0.74 Å). The result revealed that Fe ions substituting into the ZnO [33]. The average crystalline sizes of the FexZn1–xO nanoparticles were estimated by applying the Debye–Scherrer equation (Eq. 2) to the half intensity width of the (100), (002) and (101) peaks [34].
D = kλ/ β cosθ (2)
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 Table 1. The average crystalline sizes are decreases as 23, 21, 16 and 12 nm for ZnO, Fe0.05Zn0.95O, Fe0.075Zn0.925O and Fe0.1Zn0.9O, 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 V2O5 doped ZnO ceramics.
Table 1. The lattice parameters of FexZn1-xO nanoparticles annealed at 450 °C.
Fe Doping
Concentration (mol.%)
|
hkl
|
2θ
(degree)
|
a[Å]
|
c[Å]
|
Crystallite size (nm)
|
ZnO
|
(100)
(002)
(101)
|
31.77
34.40
36.30
|
3.24
|
5.20
|
23.53
|
Fe0.05Zn0.95O
|
(100)
(002)
(101)
|
31.81
34.23
36.34
|
3.25
|
5.21
|
21.66
|
Fe0.075Zn0.925O
|
(100)
(002)
(101)
|
31.71
34.43
36.37
|
3.25
|
5.20
|
16.23
|
Fe0.1Zn0.9O
|
(100)
(002)
(101)
|
31.81
34.23
36.33
|
3.24
|
5.21
|
12.03
|
3.2 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. 2(a) shows the surface morphology of undoped ZnO nanoparticles. It has an irregular structure with crystalline size of ~42-68 nm and having a better crystalline quality. The doping of 0.05 mol.% of Fe, ZnO nanoparticles are turned into irregular pores like structure [Fig. 2(b)]. On further doping of Fe from 0.075 to 0.1 mol.% in the Fe doped ZnO lattice resulted in the mixture of needle and spherical like structure [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 FexZn1-xO nanoparticles enhanced with the dopant concentration.
3.3 EDX Analysis
EDX analysis (Fig. 3a–d) was carried out to investigate the elemental composition of FexZn1-xO (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) shows the EDX analysis of Fe doped ZnO nanoparticles. The inset of EDX image FexZn1-xO nanoparticles data were given in table format. The measured Fe concentration of 0.05, 0.075 and 0.1 mol.%, are about 2.71, 14.94 and 18.21 at. % respectively. The EDX results confirm the presence of Fe, Zn and O are clearly observed at their normal energy. The observed atomic percentage values almost match well with the samples.
3.4 UV-Vis spectral studies
UV-absorption spectra of FexZn1-xO (x≈0, 0.05, 0.075 and 0.1 mol.%) nanoparticles shown in Fig. 4 (a-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 confirmed from the XRD results. The optical bandgap (Eg) of ZnO and Fe doped ZnO nanoparticles were calculated by using the formula Eg = hc/λ [38-40]. The calculated optical band gap Eg values are 3.90, 3.89, 3.86 and 3.82 eV which corresponds to undoped ZnO and 0.05, 0.075 & 0.1% Fe doped ZnO nanoparticles for the increasing doping level. Hassan et al. revealed that the absorbance also depends upon some factors like crystalline size, oxygen deficiency, defects in grain structure, lattice strain, thickness etc [41]. The replacement of Fe3+ with Zn2+ causes the creation of oxygen vacancies and additional energy levels, which reduces the band gap with the dopants.
3.5 FTIR Analysis
The FTIR spectra of FexZn1-xO (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].
3.5 I-V Characteristics
The I-V characteristics studies of FexZn1-xO (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 electrical conductivity measurement of FexZn1-xO (x≈0 and 0.1 mol.%) nanoparticles taken under varying temperatures of 30, 50, 70, 90, 110 and 130 °C (Increase of 20 °C). Thus, increases the conductivity of 0.1% Fe doped ZnO nanoparticles due to increase the mobility of charge carriers [44]. The remarkable increase in the conductivity by these samples may results in higher advantage for electro-optic device fabrication.
3.6 Photodegradation of MB and MO
3.6.1 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.
3.6.2 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 surface-active sites and the immersion of UV light towards the dye solution. The results are displayed in Fig. 6; the figure 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.
3.6.3 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 FexZn1-xO (x≈0, 0.05, 0.075, 0.1 mol.%) nanoparticles. The absorption maxima at 664 nm decreased gradually with extension of irradiation time.
The MB dye was degraded under UV light almost completely within 150 min. in the presence of FexZn1-xO (x≈0, 0.05, 0.075, 0.1 mol.%) nanoparticles shown in Fig. 8(e). The results revealed that the Fe doped ZnO showed higher photocatalytic activity than that of undoped ZnO. Fe (0.075 mol.%) doped ZnO showed enhanced photocatalytic activity with a degradation efficiency of 68% for MB (pH = 6) dye at 150 min. Where, Fe (0.075%) doped ZnO nanoparticles takes short period of time to degrade the MB dye compared to other concentration of the Fe doped ZnO nanoparticles. The reduction of photocatalytic activity at higher concentration Fe (0.1%) doped ZnO may be due to the reduced path length of photons [45]. The obtained results are similar to the results reported by Suganthi et al. for the photodegradation of malachite green (MG) [46]. Another reason for increase in photocatalytic activity of Fe doped ZnO nanoparticles is due to the fact that dopant incorporated on to the surface of the ZnO can suppress the recombination of electron–hole pairs due to enhance their efficiency [47, 48].
3.6.4 Effect of UV irradiation time of MO
Photocatalytic experiments were carried out with an initial MO concentration of 3.0 mM, catalyst concentration of 10 mg, pH = 6 and irradiation time upto 180 min. The photocatalytic activity of MO dye solution (10 PPM) was also studied for reference. The variation in absorption spectra of MO showing to UV light for various irradiation times (0, 30, 60, 90, 120 and 150 min) in the presence of FexZn1-xO (x≈0, 0.05, 0.075, 0.1 mol.%) nanoparticles. The absorption maxima at 454 nm decreased gradually with extension of irradiation time shown in Fig. 9(a-d).
MO dye was degraded under UV light almost completely within 150 min. in the presence of FexZn1-xO (x≈0, 0.05, 0.075, 0.1 mol.%) nanoparticles shown in 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 efficiency 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) (3)
(h+ VB) + dye dye+ VB oxidation of dye molecule (4)
When ZnO is irradiated by visible light, holes (h+ VB) and electrons(e−CB) are generated in its valence band and conduction band respectively. The photogenerated electrons and holes combine with Fe3+ ions respectively.
Fe3+ + O2 Fe5+ + O2°− (electron release) (5)
Fe3+ + OH− Fe2+ + OH° (hole release) (6)
OH° + dye (MB & MO) oxidation of dye molecule (7)
These super oxide anion (O2°−) 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.