Among the nanoparticles with commercial applications, iron oxide nanoparticles have unique properties with abundant applications. Thus, in this study, maghemite nanoparticles (γ-Fe2O3 NPs) were synthesized using aqueous solution of Z. jujube through simple and fast route with high efficiency.
PXRD analysis
PXRD is one of the efficient devices for materials analysis. The nature and size of particles were characterized using PXRD technique and the resulted patterns [9]. PXRD spectra of the γ-Fe2O3 NPs at 300, 400, and 500 ˚C are demonstrated in Fig. 2. In PXRD spectra, sharp peaks indicated the stability of nanoparticles. The high intensity of the peaks indicated strong scattering centers of X-ray in the crystalline phase. Generally, broadening of peaks in PXRD patterns of solid material can be attributed to particle size; wider PXRD peak indicated smaller particle sizes. Additionally, the sharpness of peaks implied the crystallinity of synthesized samples. According to the results obtained from PXRD, Miller indices of (220), (311), (400), (422), (511), (440) and (533) implied that synthesized samples are maghemite nanoparticles (γ-Fe2O3 NPs) with cubic structure (JCPDS: 39-1346) [22]. The crystallite size of these nanoparticles was calculated using the Debye-Scherrer equation (Equation 2).
D = kλ / β cosθ Eq. 2
Where, D is particle size on nanometer, k is a fixed factor that is usually k=1, λ is wavelength of X-ray (3.54 Å), θ is radiation angle of X-ray (in degrees), and β is peak width in half height of the peak that be expressed in units of length [23]. The crystallite size of synthesized γ-Fe2O3 nanoparticles at 300, 400, and 500 ˚C were estimated at 28.39, 36.81, and 45.72 nm, respectively.
FESEM and TEM analysis
Analysis of FE-SEM for synthesized γ-Fe2O3 nanoparticles exhibited the round-shaped morphology of nanoparticles. The size ranges for synthesized γ-Fe2O3 NPs at 300, 400, 500 ˚C were 20-25, ~35, and ~45 nm, respectively (Fig. 3), which were well-surrounded by the respective green coating. The images confirmed that these natural product-based nanoparticles had spherical shapes. The TEM image of synthesized γ-Fe2O3 NPs at 400 ˚C showed in Fig. 3, which this figure has been depicted the particles about 40-50 nm sizes with spherical and almost uniform shapes.
FT-IR analysis
The FT-IR spectra of synthesized γ-Fe2O3 NPs at 300, 400, and 500 ˚C are shown in Fig. 4. In the FT-IR spectra, the absorption band in range 3414 cm-1 caused strong stretching vibration by created hydrogen bonding with OH groups, which absorbed by sample from medium. The absorption band in the range of 1624 cm-1 belongs to the adsorbed H2O groups on the nanoparticle surface. The presence of these peaks indicated that iron oxide nanoparticles are capable of absorbing large amounts of OH groups of H2O on their outer surfaces. The absorption band in the range of 619-500 cm-1 corresponds to the vibration bands O-Fe-O and Fe-O [22,24].
Raman analysis
The Raman spectrum of the synthesized maghemite nanoparticles using aqueous extract of Z. jujube at 400 ˚C are shown in Fig. 5. The Raman modes of cubic spinal structures had five active modes including A1 + E + 3T1. In the region of 286, 687 and 716 cm-1, which are according to A1, E and T1 modes, respectively. Raman spectrum confirmed the cubic structure of γ-Fe2O3 NPs [25].
VSM analysis
The magnetic properties of the synthesized maghemite nanoparticles at 400 ˚C were investigated using the VSM technique (Fig. 6). Iron nanoparticles had no hysteresis ring and were superparamagnetic at room temperature. The saturation magnetization (Ms) at 400 ˚C was 65 emu/g, which was less than the magnetic state of its bulk (Ms = 76 emu/g) [26]. This decrease in saturation might be due to the effect of the reduced particle size.
The survey of ability of lead removes
Studies demonstrated that iron oxide nanoparticles were able to remove contaminants. Thus, in this study, the ability of nanoparticles to remove lead was investigated. To evaluate the effect of pH on nanoparticle synthesis, three pHs including 4, 6, and 8 were considered. Based on the results, the pH optimum exhibited that alkaline ambiance was the best condition for the removal of lead. Considering that, the surface charge of γ-Fe2O3 NPs can play the most important role in this phenomenon; alkaline ambiance improves the surface charge of nanoparticles [7]. Therefore, the adsorption of lead ions in alkaline ambiance was more than in neutral and acidic ambiance.
In this test, other variables such as contact time and concentration of the synthesized γ-Fe2O3 NPs (at 400 ˚C) were considered constantly, and the ability of nanoparticles to remove lead was investigated at 7.90 to 500 mg/L concentrations of pollutants. Lead solution without nanoparticles was considered as a control. The adsorption efficiency was calculated using equation 3.
R = (Ci –Ce/Ci) × 100 Eq. 3
Where, Ci is the initial concentration of lead in solution (mg/L), Ce is equilibrium concentration of lead in solution after adsorption process (mg/L), and R is absorption efficiency. Residual concentrations of lead in solution after adsorption process were calculated by the obtained line equation in Fig. 7A; and by using the obtained results, the adsorption efficiency of lead via the synthesized γ-Fe2O3 nanoparticles at 400 ˚C was calculated (Fig. 7B).
Fig. 7B shows that by decreasing the concentrations of lead in the reaction ambiance, the lead removal by nanoparticles was successfully completed. The ratio of 1:1 between the concentration of nanoparticles and lead showed the highest absorption efficiency. As a result, γ-Fe2O3 NPs were able to remove lead.
Photocatalytic activity evaluations
Methylene blue degradation
Fig. 8A shows the adsorption diagram of methylene blue dye by the synthetized nanoparticles at 300, 400, and 500 ˚C calcination temperatures. In order to investigate the photocatalytic activity of obtained samples, the applied test condition included pH= 7, 1 g/L of catalyst and 20 mg/L of initial dye concentration. The adsorption of organic dyes using photocatalysts method is directly related to the specific surface area of the photocatalysts. In Fig. 8A, the absorption rate of γ-Fe2O3 NPs at 300, 400, and 500 ˚C were 18.2, 20.3, and 16.3%, respectively, which demonstrated that the adsorption rate synthesized sample at 400 ˚C was higher than other samples. Fig. 8B demonstrates that the degradation rate for γ-Fe2O3 NPs at 300, 400, and 500 ˚C were 84.8%, 92.8%, and 70.2%, respectively. According to XRD and FESEM results, by increasing the calcination temperature from 300 ˚C to 500 ˚C, the crystal size was also increased. In photocatalytic activity, ratio of surface-to-volume have role importance than to crystallite size of particles. So, the ratio of surface-to-volume of synthesized nanoparticles at 400 ˚C is higher than to samples of 300 ˚C [27,28], in results, the synthesized nanoparticles at 400 ˚C were exhibited better photocatalytic performances. In sample of 500 ˚C, degradation activity was reduced due to saturation of active sites.
Influence of pH
pH is one of the most effective operational parameters in the photocatalytic process. Given that pH of solution changes the surface charge of nanocatalyst, it has a great influence between the dye molecules and electric charge of photocatalytic. Thus, finding the right amount of pH parameter is essential to increase the removal efficiency [29]. In order to find the optimal pH, the photocatalytic analyses were performed in three different values of 3, 7 and 11. Fig. 9 demonstrates that the removal efficiency of MB dye at pH ranges of 3, 7 and 10 were about 44.9%, 89.1% and 95.8%, respectively. The results exhibited that γ-Fe2O3 NPs (at 400 ˚C) had good photocatalytic activity at basic and natural media.
Reusability study
Reusability of photocatalytic is a very important and decisive factor for practical applications. To investigate the stability of synthesized γ-Fe2O3 NPs at 400 ˚C, they were exposed to solar light at initial concentrations of 20 mg/L of methylene blue, 1 g/L of catalyst loading and pH = 7 for 160 minutes. After each photocatalytic test, γ-Fe2O3 NPs was separated, washed and dried in an oven at 110 ˚C for about 2 hours. The results showed that after four cycles of using the γ-Fe2O3-300, the obtained degradation of methylene blue was about 87.1% (Fig. 10). It appears that γ-Fe2O3 NPs can be considered as reusable, functional and active of nanophotocatalysts in environmental applications.