## 3 Results and Discussion  
### Characterization of GO and RGO  
The surface morphology of GO and RGO was investigated by SEM characterization. As seen from the results (Figure 1), the samples were composed of corrugated ripple-like sheets, which showed the substantive characteristics of graphene. The surface oxygen functional groups of GO made it seemed as if the crumpled sheets were stuck together. After the GO was reduced, the topographic characteristics of RGO evolved into an analogous mass of dark clouds, and the wrinkled sheets were increased and randomly aggregated to a certain degree, which led to the formation of well-developed mesopores.  
In order to realize the surface functional group information of GO and RGO, the XPS spectra of the samples were analysed and are shown in Figure 2. The C1s XPS spectra of GO was decomposed to five peaks of C=C/C-C (284.4 eV), C-OH (285.7 eV), C-O-C (286.7 eV), C=O and COOH (288 and 289.1 eV) [27]. After the GO was reduced, the deconvolution of the C1s peak (RGO) showed distinguishably only a main sp\({}^{2}\) C=C peak and several small oxygen-bound C peaks. After a comparison, it was found that the reduction process resulted in the peak intensity of the oxygen functional groups becoming distinctly weakened. This confirmed that GO contained more O-containing functional groups than RGO.  
Figure S1 compares the FT-IR and Raman spectra of GO and RGO samples. The FT-IR spectra for GO exhibited the peaks of O-H and/or water molecule vibration (3412 cm\({}^{-1}\)), C=O (COOH) stretching vibration (1726 cm\({}^{-1}\)), C=C (C-C) stretching vibration (1605 cm\({}^{-1}\)), O-H deformation vibration (1400 cm\({}^{-1}\)), C-O (epoxy, 1221 cm\({}^{-1}\)) and C-O (alkoxy, 1054 cm\({}^{-1}\)) stretching vibration, respectively [28; 29]. After reduction treatment, the peaks decreased significantly or even disappeared in intensity, further suggesting the removal of oxygen functional groups. In the Raman spectrum, both GO and RGO samples displayed two characteristic peaks at around 1350 and 1603 cm\({}^{-1}\), corresponding to D band and G band, respectively [29; 30]. A more meticulous comparison found that the I\({}_{\rm D}\)/I\({}_{\rm G}\) value of GO (0.99) was smaller than that of RGO (1.43), probably due to a decrease in the average size of the sp\({}^{2}\) domains upon the reduction of GO [30].  
The magnitude of the adsorption driving force of GO and RGO was studied by comparing the UV adsorption spectra of RGO (7.5 mg L\({}^{-1}\)), naphthalene (1 mg L\({}^{-1}\)) and their mixture in aqueous suspensions (Figure 3). The characteristic absorption occurs at 220 nm. If there is no remarkable interaction between RGO and naphthalene, the mixture absorbance should be equal to the sum of their  
Figure 1: SEM images of (**a**) graphene oxide (GO) and (**b**) reduced graphene oxide (RGO).  
Figure 2: Deconvolution of the C1s peak of (**a**) GO and (**b**) RGO.  
individual ones. However, the mixture adsorption spectrum was far below that calculated by summing the spectra of naphthalene and RGO individuals. This indicates that there is an interaction between naphthalene and RGO, which is attributed to the \(\pi\)-\(\pi\) interaction. We may estimate the strength of the \(\pi\)-\(\pi\) interaction by considering that a stronger \(\pi\)-\(\pi\) interaction will result in a greater decrease in the absorbance of the mixture at 220 nm in comparison with the sum of their individual suspensions. Therefore, it was obvious that the \(\pi\)-\(\pi\) interaction of GO for aromatics was weaker than that of RGO.