3.1. Structural properties
The XRD spectra of the crystalline orientation of the RGO, Fe2(MoO4) and Fe2(MoO4)3/RGO composites are shown as shown in Fig.1. It can be seen that GO shows an extreme diffraction peak at 2θ of 10.75° attributed to (002) the GO plane, which emerges from graphite via the Hummers process. Well crystalline with sharp diffraction peaks were found in the pure Fe2(MoO4)3, the peaks are correlate with monoclinic phase of monoclinic crystal phase of Fe2(MoO4)3 (JCPDS 31-0642). Both GO and Fe2(MoO4)3 diffractions were could be found in all the composite samples. Moreover, when the content of RGO increases the diffraction intensity of Fe2(MoO4)3 was sharpened while the intensity of RGO peaks could be decreased. The results suggest that the incorporated RGO significantly improve the crystalline nature of Fe2(MoO4)3. The structural details of the samples were further examined by Raman analysis and the correlated spectrum is shown in Figure 2. As seen in the figure 2, the Raman peaks at 980, 812 correspond to M=O streak (Ag, ʋs), M=O streak (Ag, ʋas), each. The values at 1585 and 1345 cm−1 refer to the G-band (E2g symmetry, in-plane bond-stretching motion of pairs of sp2 C atoms) and the D-band (defect-related) of the graphene sheet . The typical characteristic peaks of Fe2(MoO4)3 and RGO occur simultaneously, leading to the formation of Fe2(MoO4)3/RGO nanocomposite.
3.2. Morphological analysis
To evaluate the responsibility of GO scaffold in Fe2(MoO4)3/RGO composites, SEM and TEM images were taken. Figure 3 (a-c) shows the SEM images of bare RGO, Fe2(MoO4)3 and Fe2(MoO4)3/RGO (FMG5) samples, respectively. SEM image of bare RGO (Fig. 3a) shows decreased thickness with the surface scrunched up, which is advantageous to the deformation of other metal oxides. Perfectly obvious with integrated spherical shaped nanoparticles are found in pristine Fe2(MoO4)3 nanoparticles (Fig. 3a), which are consistently covered on the RGO sheet surface (Fig. 3c). The distribution of Fe2(MoO4)3 NPs over all the surface of the RGO was further inveterate via high magnification TEM picture, as shown in Fig 3 (d-f). The images reveal that neither of them surfaced NPs or isolated NPs outside of the RGO nanosheets have been found. Implied that the rapid expansion of ultrafine Fe2(MoO4)3 NPs existed only on the GO surface. To confirm the stoichiometry of these samples, we mapped the elemental distribution of Fe, Mo, C and O on the surface of samples and the relevant images are shown in Fig. 3 (g-j). The elemental mapping clearly shows that the RGO was successfully incorporated in Fe2(MoO4)3.
3.3. Optical properties
UV-Vis DRS was further explored to investigate the optical characteristics of the photoanode materials. Figure 4a) shows the absorption spectra of all the photoanode samples. The absorption of pure sample lies in the visible light region (~485 nm). Further the extended absorption in the red shift wavelength was noticed after loading the RGO content. The K-M plot [28-30] is used to estimate the optical band gap. The band gap energy of pure Fe2(MoO4)3, FMG1, FMG2 and FMG5 are 2.55, 2.47, 2.35 and 2.25 eV, respectively (Fig. 4b). With the aim of studying the coupling photo-induced electron-hole pairing mechanisms, Photoluminescence (PL) spectral analysis is often used and the resultant spectrum is shown in Figure 5. A clear emission band appeared around at480 nm for all the photoanodes. Remarkably, the intensity emission was gradually decreased when the loading amount of RGO is increases, which suggesting that photo-generated electron-hole recombination system was suppressed due to the solid electron transfers potential of RGO.
3.4. Textural and elemental composition analysis
Brunauer–Emmett–Teller adsorption/desorption of the liquid nitrogen technique is used to explore the surface area and size of pore values. The isotherm was taken for Fe2(MoO4) and Fe2(MoO4)3/RGO (FMG5) samples and the pertinent plot is shown in Figure 6 a). Both the samples exhibit the IV type adsorption isotherm, complemented by H3 hysteresis. This characteristics nature resembles that mesoporous of the materials [31-33]. Due to the synergic effect between the Fe2(MoO4)3 and RGO the light absorption property is significantly improved as well the high surface area (112.5 m2/g) and pore size (38.7 nm) was achieved than compared with bare Fe2(MoO4)3 (88.5 m2/g and 17.8 nm). Chemical state of the samples was further elucidating by XPS analysis. Figure 7a) shows the XPS survey spectrum of Fe2(MoO4)3/RGO (FMG5) samples, which is rendering the key elements of Fe 2p, Mo 3d, O1s and C1s. The Fe 2p1/2 and 2p3/2 states were presented at 724.1 and 711.1 eV, respectively (Fig. 7b). While the two binding energies were positioned at 235.5 and 232.4 eV, corresponding to the Mo 3d3/2 and Mo 3d5/2, accordingly (Fig. 7c). The O 1s state of the composite was found with equivalent binding energy of 530.1 eV (Fig. 7d). Core scale of binding C 1s energies of the spectrum shown in Figure 7(e) could be the visible peaks at 284.5eV and 288.6 eV, respectively. This could be due to the characteristic sp2 hybridized graphite-like aromatic rings of RGO and atoms exist.
3.5. Photovoltaic studies
DSSC performance was evaluated for the constructed sandwich type device, which is shown in Figure 8a). The detailed setup with description was also mentioned in detail for our previous published work . The current (J) – voltage (V) characteristics of the DSSC was monitored under sun illumination (1.5 AM) conditions for each of these electrodes (Fig. 8b) and the results are summarized in Table 1. The results reveal that Fe2(MoO4)3/RGO (FMG5) photoanode show high PCE of 9.65% due to the higher current density (20.98). While the PCE value of pure Pt, Fe2(MoO4)3, FMG1 and FMG3 are 6.17, 4.33, 6.89 and 7.35%. Due to the high reduction of triiodide and electrical conductivity by RGO, the PCE is significantly improved for the composite electrode. Incident photon-to-current conversion efficiency (IPCE) was further carried out to know the role of RGO on the performance of the DSSC and the relevant plot is shown in 8c). The IPCE values are clearly observed in the wavelength between 400-700 nm, which is characteristics behavior of N17 dye. The FMG3 photoanode shows high IPCE value of 87% than compared with other photoanode materials such as Fe2(MoO4)3,(41%) FMG1 (54%) and FMG3 (67%). The high stability of the device was further confirmed through multiple experiments of the J-V plot. The device was evaluated over 60 day with regular interval of 10 day. The device loss only 2.3% of PCE value from its initial value. In addition, electrochemical impedance spectra (EIS) were tested to know the catalytic skill of the electrode samples. Figure 9a) shows the Nquist plot of the electrodes with equivalent circuit (inset). The semicircle evidently illustrate that smaller Rs (5.4 Ω) and Rct (12.5 Ω) value of Fe2(MoO4)3/RGO (FMG5) photoanode than other samples. The factors obtained from EIS was also depicts in Table 2. Moreover, the FMG photoanode has high electron life time (78 ns), which is caused by the good electrocatalytic activity to reduction in the triiodide. The photovoltaic mechanism of the proposed work is shown in Figure 9b). The Fe2(MoO4)3 nanoparticles are anchored to the RGO, which results in excited electrons are generated without impediment. The electrons obtained can be exported from Fe2(MoO4)3. Hence, the conductive layer was effectively prevented by RGO bridges and therefore the recombination of electron-hole pair is effectively prevented.