# Arginine-Mediated Self-Assembly of Porphyrin on Graphene: A Photocatalyst for Degradation of Dyes
## 3 Results
These significant decreases in the emission peaks are likely due to the coupling from the spatial packing of the TCPP porphyrins [26] and the quenching effect of graphene, suggesting an efficient photoinduced process of electron injection from TCPP to GNPs [44]. Figure 5 exhibits XRD patterns of monomeric porphyrin molecules and GNPs@porphyrin nanofibers. There are no peaks observed in the XRD pattern of monomeric TCPP (black line), indicating the non-crystalline nature of the TCPP monomers. However, the XRD pattern of GNPs@TCPP nanofibers (red line) shows several characteristic diffraction peaks of both graphene and TCPP aggregates, while a strong peak at around 26\({}^{\circ}\) and a weak peak at 55\({}^{\circ}\) are assigned to the graphitic nature of graphene, and other weak diffractions peaks are indexed for TCPP nanofibers. These results indicate that assembled TCPP aggregates are crystalline in nature, which may be due to aromatic \(\pi\)-\(\pi\) stacking between the porphyrin molecules [25]. Fourier transform infrared (FTIR) and Raman spectra of monomeric TCPP and GNPs@TCPP nanofibers also reveal characteristic bands for both GNPs and porphyrin components (Figures S3 and S4) [45]. Figure 4: (**A**) and inset of (**A**) UV-vis absorption spectra and (**B**) photoluminescence (PL) spectra of monomeric TCPP molecules (black line), free-standing TCPP nanobelts (red line) and graphene nanoplates (GNP)s@TCPP fibers (blue line). TCPP porphyrin aggregates have been demonstrated to have photocatalytic properties in many biological energy transduction processes in plants and algae [46; 47]. It has also been confirmed that TCPP aggregates only exhibit photocatalytic properties under visible light [25; 27; 48]. Thus, in this work, we assess the photocatalytic degradation of Rhodamine B (RhB) and methyl orange (MO) dyes by using GNPs@TCPP nanofibers as a photocatalyst in comparison with GNPs and free-standing TCPP aggregates. The decreases in the absorption peak at 553 and 464 nm of RhB and MO, respectively, as a function of time are monitored to evaluate the photocatalytic performance. Figure 6A,C display the C/C\({}_{\text{o}}\) versus time plot of RhB and MO under different photocatalytic reaction conditions, where C\({}_{\text{o}}\) is the initial concentration of dyes and C is the concentration at time t. It is clear in both cases of RhB and MO that with the blank experiment (no photocatalysts), negligible dye degradation could be observed, suggesting that the self-sensitized photodegradation of dyes does not occur under these conditions. When GNPs were used as photocatalyst, while a decrease in RhB concentration of approximately 25% was observed, there is an insignificant decrease in the concentration of MO. This is likely due to good adsorption affinity of graphene toward RhB rather than MO [37]. When using free-standing TCPP aggregate photocatalysts, the concentration of RhB and MO are decreased by 90% and 30% respectively after 180 min of irradiation time under simulated sunlight. This result demonstrates that the free-standing TCPP aggregates display photocatalytic activity toward RhB and MO under visible-light irradiation [23]. Interestingly, the photocatalytic performance increases significantly when GNPs@TCPP nanofibers are used. While RhB is completely degraded after 150 min, MO degradation also reaches 80% after 180 min. These results indicate that GNPs@TCPP nanofibers exhibit enhanced photocatalytic activity in comparison with GNPs and free-standing TCPP aggregates. The photocatalytic activity of GNPs@TCPP nanofibers toward dyes in the dark condition was also tested (Figure S5), which show virtually no photocatalytic activity in this condition, the small decrease in dyes concentration is ascribed to adsorption of dyes by GNPs. The kinetics of the photocatalytic reaction of RhB and MO by GNPs@TCPP nanofibers are also determined through the plot of In (\(A_{t}\)/\(A_{o}\)) vs. time, where \(A_{t}\) is the peak intensity at time t, and \(A_{o}\) is the intensity at time zero (Figure 6B,D).