3.1. Powder X-ray diffraction (XRD) analysis
The powder XRD sequence of composites of RGO, WO3, and WO3/ZnO as seen in Fig. 1. A sharp crest at 2 θ = 26.0 ° referring to the crystalline plane (002) belonging to graphene oxide is in bare RGO. The pure WO3 XRD pattern adopted the regular XRD pattern of m-WO3 well (PDF NO: 01-083-0951). The sharp peaks displayed the strong crystallinity and no other peak was discovered indicating the high WO3 purity obtained. The XRD patterns were very similar to WO3 for RGO-WO3 composites, but in the XRD pattern of WGO1, a large peak centred at around 25–30 was observed. In agreement with the (002) planes occurring from the graphic graphene sheet, this broad peak was. More importantly, compared to the XRD pattern of GO, the distinctive peak of GO at around 10o was not noted in the three RGO-WO3 composites, authenticating that GO had been significantly lowered to RGO.
3.2. Morphological analysis
SEM images of RGO, WO3, and RGO/WO3 composite samples are shown in Fig. 2. The bare sheet-exposed RGO (Fig. 2a) and WO3 show great, precise morphology with certain exterior conglomerations (Fig. 2). In the composite sample, the WO3 nanoparticles are coated on the RGO surface (Fig. 2c). In the TEM image, the freckle type with cleverer framework sheet of RGO was obviously found (Fig. 2d). The less coagulated specific WO3 nanoparticles (Fig. 2e) are found and distributed evenly on the exterior of the RGO sheets (Fig. 2f). To find out the RGO in WO3, the elemental mapping of the WGO1 sample was evaluated and the associated photographs are shown in Fig. 2. (g-i).
3.3. Raman spectra analysis
As shown in Fig. 3, the Raman spectral analysis was performed to examine the ability to interact among both RGO and WO3. There were two peaks in the Raman spectra of bare RGO: the D-band peak at 1387 cm− 1 and the G-band peak at 1556 cm− 1 wavenumber. The peaks acquired in the Raman spectrum confirm the WO3 crystallographic phase and are well suited to literature reports . At 715 and 826 cm− 1, the W-O-W bending mode was ascertained in the WO3 system. In the RGO/WO3 matrix, the WO3 peak magnitudes are reduced, that may be due to a reduction in the shape of WO3 particles on RGO. It is important to note that the G band was rise from 1556 to 1570 cm− 1 contrasted to RGO in the RGO-WO3 nanostructures, confirming the synthetic doping of carbon materials in WO3. Rather than mixing equal RGO and WO3, this chemical doping indicates the creation of an authentic composite.
3.4. Optical studies
The UV-vis DRS spectrum of WO3 and three composite materials of RGO/WO3 are shown in Fig. 4 (a). Meanwhile, Bare WO3 showed light absorption in the visible range with the on uptake at 460 nm. By contrast, as reported earlier in other graphene-based composite materials, the RGO/WO3 composite exhibited largely increased absorption in the visible light range (480–550 nm) resulting from the emergence of RGO. Two possible reasons might be attributed to this improved visible light response: (1) RGO's background light absorption in the visible light region, and (2) WO3's improved ground electrical potential regarding the possible electronic transition between both the n orbit of the reactive oxygen π → π * of RGO and n → π*. The band gaps were computed using the modified function of Kubelka-Munk as a graph in the indented Fig. 4 4 (b). For WO3, WGO0.25, WGO0.5 and WGO1, respectively, the estimated band gap energies are 2.58, 2.49, 2.31 and 2.25 eV. The room temperature PL of the samples with an excitation wavelength of 325 nm is shown in Fig. 5. In the visible light region (460–550 nm), all the samples show a broad emission that is in excellent accordance with the UV results. The RGO/WO3 nanocomposites can effectively affect electron-hole pair replication and wholeheartedly support the flow of energy from the WO3 band gap to the rGO electron density. Reduced PL emissions could considerably enhance the solar performance of the device by preventing the electron-hole charge recombination.
3.5. Textural and elemental composition analysis
Figure 6 (a&b) shows the BET surface area of the pure WO3 and RGO/WO3 (WGO1) nanocomposites and the corresponding BJH pore size. Strangely, both samples showed isotherm models of type IV, attributed to the formation of mesoporous materials with a pressure range of 0.1 to 0.9 [22, 23]. 39.96 and 78.67 m2/g were found to be specific surface areas of pure WO3 and RGO/WO3 (WGO1), respectively. While, 12.53 and 21.45 nm are the corresponding pore sizes, respectively. The increased efficiency of the RGO/WO3 (WGO1) nanocomposite is confirmed by this outcome and the small rise in pore size may be attributed to the addition of a larger number of layers of graphene.
3.6. XPS analysis
The survey spectrum with W 4f, O 1s and C 1s core level spectra are shown in Fig. 7(a–d). The W 4f core level corresponds to binding energies 35.1 and 37.9 eV for W 4f + and W 4f5/2 respectively. The O 1s spectrum (Fig. 7(c)) shows two peaks positioned at 530.4 and 532.5 eV. The former peak with maximum intensity is due to the W = O bonding modes of WO3 corresponding to oxygen atoms O2 in the lattice. Further, the C 1s shows two peaks at 284.1 and 287.5 eV as seen in Fig. 7(d), which correspond to the binding states of the C = C and C–O–W bonds in the nanocomposite samples.
3.7. Photovoltaic studies
Figure 8a) displays the schematic description of the fabricated DSSC. The utility of DSSCs from various photoanodes is seen in Fig. 8b). Dye sensitized solar cells prepared using RGO/WO3 (WGO1) nanocomposites as photoanode shows the power conversion efficiency (ꞃ) of 7.9 %, which is huge than compared to DSSC with WO3 (4.1%). With more surface area for adsorption capacity, the WO3 nanostructures result in an improvement in electron-hole pair production and thus short-circuit density. The photon-to-current conversion efficiency (IPCE) incident of dye sensitised solar cells equipped using all the photoanodes is shown in Fig. 8c. The finding provides evidence that dye sensitised solar cells designed using RGO/WO3 (WGO1) have more red-shifted effective photon-to-current reactions compared to bare WO3, which means that the spectral absorption spectrum is essentially expanded. In comparison, RGO/WO3 (WGO1) has a strong IPCE of 71 % relative to the bare WO3 (32 %). In order to understand the use of functional system implementations, stability checks were also carried out regularly over a 60 day duration (Fig. 8d). At the end of the 60-day test, a marginal decline in PCE was observed. This is primarily due to dye molecule inactivation and degradation. In attempt to comprehend the charge kinetic mechanism, EIS was done. The Nyquist plot of the electrode samples as seen in Fig. 9a.) The charge transfer resistance (Rct) was measured (Table 2) as per the configured corresponding circuit (inset of Fig. 9(a)) from the Nyquist plot of the electrodes. The Rct value of the RGO/WO3 (WGO1) photoanode indicates a lower value of 18.5 Ω cm2 relative to the bare WO3 (112.5 Ω cm2), which is responsible for the increase of the captured solar light in charge transport. The big decrease in the Rct value of photoanode composites within RGO/WO3 (WGO1) is due to the effective isolation of photo - generated electron-hole pairs and the fast transition of interaction charges. Figure 9b) displays the potential DSSCs' photovoltaic process. The RGO/WO3 composite's high solar efficiency is due to the improved ion transport and light diffusion characteristics that can constrain the light beam to the electrolyte/electrode and reduce the intercellular resistance, thus enhancing the productivity of the cell. The RGO integration will have the high surface area and conducting nature can enhance the cells' light harvesting efficiency by eliminating the recombination rate and the performance of DSSCs photo-conversion.