## 3 Results and Discussion  
### Characterization of rGOP  
Free-standing rGOP is fabricated through vacuum filtration, followed by chemical reduction in HI aqueous solution. It is a facile process to control the thickness of free-standing rGOP by adding different volumes of GO aqueous suspension, as shown in Figure 2. Chemical reduction by HI could effectively remove most oxygen-functional groups and the covalent bond structure in GOP. Comparedto GOP, rGOP has an improved electron transfer ability, is silvery in color and shows metallic gloss on both sides. The as-prepared rGOP is highly flexible and can be randomly folded; one of them was formed into a paper crane (Figure 2A). The morphology of rGOP is characterized by SEM with high magnification, as shown in Figure 2B,C. The images reveal that rGOP is constructed layer-by-layer by parallel rGOP nanosheets, with uniform thickness throughout its cross-section. The rGOP is intensely crumpled, and the typical wrinkled nanostructure is caused by the overlap of graphene nanosheets after chemical reduction (Figure 2D). To examine the surface chemical composition and electronic states of the elements, X-ray photoelectron spectroscopy (XPS) was used. The results are presented in Figure 3 for GOP and rGOP, with or without PtNPs. Figure 3A shows that there are big differences in C 1s peaks between GOP and rGOP in XPS spectra. The two peaks with a binding energy of 284.6 eV (C=C/C-C) and 286.8 eV (C-O) are clearly exhibited in the GOP spectrum. After reduction, the main peak in rGOP greatly decreases to around 284.8 eV, which is attributed to C-O. This clearly demonstrates that GOP was successfully reduced to rGOP by HI reduction method. As Figure 3B shows, the peaks with a binding energy of 74.58 eV and 71.18 eV are related to Pt 4f\({}_{5/2}\) and Pt 4f\({}_{7/2}\), respectively, indicating that the Pt is mainly metallic in Pt/rGOP [42]. Raman spectroscopy is an important experimental method in graphene research, which provides information about the structural defects and graphite structure of GOP and rGOP. As illustrated in Figure 4, the Raman spectra of GOP and rGOP have two obvious peaks. The peak at 1344 cm\({}^{-1}\) is named the D band, which is related to the degree of defect in the lattice of graphene structure. The peak at 1586 cm\({}^{-1}\) is named G band, which is assigned to the first-order scattering of the E\({}_{2g}\) mode of GO and corresponds to the in-plane vibration of sp\({}^{2}\)-bonded carbon atoms in a 2D hexagonal lattice [43]. The intensity ratio of G band over D band, I\({}_{\text{G}}\)/I\({}_{\text{D}}\) ratio, is widely used to estimate the density of defects in graphene structures: the smaller the value of I\({}_{\text{G}}\)/I\({}_{\text{D}}\) ratio, the higher the defect density [44]. Figure 2: (**A**) Digital photo of rGOP. SEM images of rGOP, with (**B**) and (**C**) cross-section view and (**D**) top view. The value of \(\rm I_{G}/I_{D}\) decreased from 0.916 to 0.696 once the GOP reduced to rGOP. The decrement of \(\rm I_{G}/I_{D}\) indicates that rGOP has more \(\rm sp^{3}\) electronic configurations and defects in the carbon atoms lattice structure when GOP is reduced to rGOP. After electrodeposition of PtNPs, the value of \(\rm I_{G}/I_{D}\) increased from 0.696 to 0.812, which suggests that PtNPs, electrodeposited between rGOP layers, may reduce the defect density. Pt nanoparticles were electrodeposited by cyclic voltammetry with different cycles (5, 10, 15, 20, 25 cycles) in a solution of 1 mmol/L \(\rm H_{2}PtCl_{6}\) and 0.5 mol/L \(\rm H_{2}SO_{4}\). The same magnification was selected to determine the morphologies and distribution of Pt nanoparticles on rGOP, as the FESEM images show in Figure 5A-E. Figure 5A shows that Pt nanoparticles were electrodeposited by 5 cycles on rGOP. SEM images (Figure 5B-D) of Pt nanoparticles, obtained by different electrodeposition cycles, indicate that the diameter of Pt nanoparticles is less than 30 nm, and that they are homogeneously distributed on the rGOP without obvious agglomeration.