One-step preparation of RGO/Fe3O4–FeVO4 nanocomposites as highly effective photocatalysts under natural sunlight illumination

The study used a one-step hydrothermal method to prepare Fe3O4–FeVO4 and xRGO/Fe3O4–FeVO4 nanocomposites. XRD, TEM, EDS, XPS, DRS, and PL techniques were used to examine the structurally and morphologically properties of the prepared samples. The XRD results appeared that the Fe3O4–FeVO4 has a triclinic crystal structure. Under hydrothermal treatment, (GO) was effectively reduced to (RGO) as illustrated by XRD and XPS results. UV–Vis analysis revealed that the addition of RGO enhanced the absorption in the visible region and narrowed the band gap energy. The photoactivities of the prepared samples were evaluated by degrading methylene blue (MB), phenol and brilliant green under sunlight illumination. As indicated by all the nanocomposites, photocatalytic activity was higher than the pure Fe3O4–FeVO4 photocatalyst, and the highest photodegradation efficiency of MB and phenol was shown by the 10%RGO/Fe3O4–FeVO4. In addition, the study examined the mineralization (TOC), photodegradation process, and photocatalytic reaction kinetics of MB and phenol.

Characterization methods. Crystal structure of the prepared samples was characterized by X-ray diffraction (XRD, Bruker-D8-AXS diffractometer, Germany) with Cu-Kα radiation at a setting of 40 kV and 150 mA. The images were captured using a transmission electron microscope (TEM) with a Jeol JEM-1230 apparatus operating at 120 kV. With the same EDAX detector, an energy-dispersive X-ray (EDx) study was performed (SEM, Hitachi S-4200). The chemical compositions (Axis Ultra DLD, Kratos) were performed using X-ray photoelectron spectroscopy (XPS) with a 325 nm excitation wavelength. At room temperature, UV-visible absorption studies were conducted by the UV-Vis 2450 (Shimadzu) spectrophotometer to record diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were carried out with a fluorescent spectrophotometer (HORIBA-Jobin-Yvon).
Photocatalytic activity study. Photodegradation studies. The photocatalytic activity of the produced catalysts was assessed for MB, phenol and BG photodegradation. Natural sunshine provided irradiation, and the reactor was encased in a water-cooling system. The solution was transported to the photoreactor after 0.05 g of sample powder was added to 50 ml of pollutant (Co = 10 MB/L). The degradation of MB, phenol and BG in solar light was performed on sunny days between 11.00 a.m. and 2.00 p.m. with a maximum temperature of 35 °C. The intensity solar light was measured every 30 min over LT Lutron LX-10/A digital Lux meter and the average light intensity was nearly constant during the experiments. The mixture was first agitated in the dark for 30 min to achieve the adsorption-desorption equilibrium. The mixture was then stirred on a magnetically under direct sunshine lighting. The degradation of MB, phenol and BG was calculated using the Eq. (1) 28 , and the change in pollutant concentrations was measured using a Shimadzu, MPC-2200 UV-Vis spectrophotometer.
where C o and C t represent the concentrations dyes before and after irradiation, respectively (t). The reactive radicals that might be formed in photocatalytic processes were also investigated employing several scavengers at concentrations of 1 mM, including benzoquinone (BQ), isopropanol (IPA), and Na 2 EDTA as · O 2 − , · OH, and h + scavengers, respectively [29][30][31] . The total organic carbon (TOC) was measured using a Shimadzu 5000 TOC Analyzer which applied to investigate the mineralization of MB, phenol and BG. After photodegradation, the %TOC of MB, phenol and BG was estimated using the following equation: Results and discussion XRD analysis. The 34 .
The XRD patterns of xRGO/Fe 3 O 4 -FeVO 4 showed similar peaks as of Fe 3 O 4 -FeVO 4 . Moreover, the sample with 15 wt.% of RGO displayed a small peak appeared at 2θ = 26.05° indicating the existence of RGO 21,35 . As seen in Fig. 1S displays positional shift of the peaks after the addition of RGO. This resulted from effect the introduction of RGO which led to changes in the lattice parameters of Fe 3 O 4 -FeVO 4 (d-spacing changed from 3.17 to 3.19 Å at 2θ = 27.9°). In addition, Fig. 1 illustrates that the intensity of characteristic peaks increased with the percentage of RGO increased.      23,40 . The formation of V 4+ and V 3+ in the sample indicates to existence of oxygen vacancies (Vo) in their crystal structure 23 .  where υ is the wavenumber, h is Planck constant, α is absorption coefficient, E g is the energy band gap and A is a constant 9,29 . From the plot of (αhυ) 1/2 versus photon energy (eV) as shown in Fig. 3S, the band gap energies (E g ) of the prepared samples were calculated and the resulted values are listed in Table 1. As shown in Table 1 Fig. 6S. The apparent values of k and correlation coefficient (R 2 ) were calculated and listed in Table 2. The values of R 2 indicated that the degradation of MB, phenol and BG follows pseudo 1st order kinetics. Also, Table 2 shows the values of k increased with increasing the RGO content and the sample with 10 wt.% of RGO displayed the highest photodegradation rate compared with other tested photocatalysts. Figure 7S illustrates the values of %TOC of MB, phenol and BG degradation over 10%RGO/Fe 3 O 4 -FeVO 4 . The results illustrated that the mineralization of MB, phenol and BG after 180 min was 92.8%, 85.3% and 99.9%, respectively. Comparing these results with the photodegradation results we found the values of %TOC were lower than the photodegradation values. This indicates to existence of some un-degraded intermediates (colorless). However, with increasing the irradiation time to 360 min, the %TOC values increased sharply to achieve 100% of mineralization of both MB, phenol and BG. Table 1S displays comparison between the photocatalytic activity of our samples that obtained in this work and that found in other literatures 17,19,20,22,23,25,27,[54][55][56][57][58] . As shown in Table 1S the 10%RGO/Fe 3 O 4 -FeVO 4 showed the highest photoactivity for photodegradation of organic pollutants under solar light. Also, our samples showed high visible light absorption, photogenerated charges separation and reusability. Moreover, in this work, the reduction of graphene oxide was performed effectively without using chemical reduction agents by green reduction method and during short time (2 h) comparing to other literatures.
Photocatalytic mechanism. To understand the photodegradation mechanism of MB, the radical scavengers were added during the photodegradation of MB. Figure 8S shows effect the addition of scavengers on the degradation of MB over 10%RGO/Fe 3 O 4 -FeVO 4 sample. As shown in Fig. 8S, the addition of BQ ( · O 2 − radical) was accompanied with sharply reduction in the degradation of MB while the addition of Na 2 EDTA (h + radical) and IPA ( · OH radical) accompanied with high suppression for Na 2 EDTA and venial reduction for IPA. These results imply that the · O 2 − played the main role in degradation of MB while the · OH played the minor role. This indicates that the · O 2 − is the more active radical contributed in the degradation of MB. The photoinduced eon the CB of Fe 3 O 4 can transfer to the VB of FeVO 4 , and then quickly recombine with the h + of FeVO 4 were achieved, which is favorable for higher separation rate of e --h + pairs of the single photocatalyst and higher redox potential 59,60 . In this case, the eaccumulated in CB of FeVO 4 are taken by the RGO and then reduce O 2 into · O 2-[Eqs. (5), (6)], while the h + in VB of Fe 3 O 4 VB are more positive potential and has sufficient oxidation capacity to oxidize OHinto · OH or react with pollutants molecules directly [Eqs. (7-9)] 59-63 . These results imply the contribution of generated radicals in the degradation of MB and phenol to gives CO 2 Figure 9S displays the degradation of MB, phenol and BG after four cycles (runs) under the same conditions. The catalyst powder was separated from the reaction mixture after each run and soaked in ethanol for 1 h. Then, the powder was washed with water and finally dried at 100 °C for 8 h 30 . The obtain results illustrated no significant decline in photodegradation activity of 10%RGO/Fe 3 O 4 -FeVO 4 was observed after five runs. To investigate the effect of reused times on the structural properties of catalyst, the 10%RGO/Fe 3 O 4 -FeVO 4 was investigated by the XRD and TEM techniques before and after reuse as shown in Fig. 9 and the results showed no changes were observed in the structural properties of 10%RGO/Fe 3 O 4 -FeVO 4 indicating the excellent reusability and sustainability of the prepared photocatalysts. The kinetic studies results illustrated that the degradation of MB and phenol follows the pseudo 1st order kinetics. The prepared samples showed excellent reusability after five runs without significant reduction in the photoaactivity. Based on these results, we can be concluded that the xRGO/Fe 3 O 4 -FeVO 4 are suitable and convenient material for treatment of organic pollutants and industrial effluent.