## 3 Results and Discussion
### Photocatalytic Performance  
The photocatalytic performances of the synthesized SnO\({}_{2}\)/CC, Ag\({}_{3}\)PO\({}_{4}\)/CC, and Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC samples were evaluated by photodegradation of the RhB and MB aqueous solution under visible light irradiation at room temperature. The RhB and MB are the popular probe molecule in heterogeneous catalytic reactions, showing the absorption peak at around 554 nm and 663 nm in the absorption spectra, respectively. The adsorption activities of the as-synthesized samples were evaluated prior to irradiation, and the results are shown in the Supplementary Materials Figure S1. After treatment for 60 min, all the samples reached adsorption saturation. Figure 7a shows the relationship between the concentration of RhB solution (C/C\({}_{0}\)) and visible light irradiation time for the synthesized samples, where C\({}_{0}\) represents the initial concentration of RhB after adsorption equilibrium, and C represents the corresponding concentration at a certain time interval. It was clearly observed that the RhB solution, without the photocatalyst, shows only a little degradation under visible light irradiation. This reveals that the RhB solution, without the photocatalyst, is very stable under visible light irradiation. As can be seen in Figure 7b, the degradation rate of the synthesized Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC sample at 70 min is about 95.9%, which is much higher than that of the synthesized Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\) (79.2%), Ag\({}_{3}\)PO\({}_{4}\)/CC (54.3%) and SnO\({}_{2}\)/CC (7.7%) samples. The results show that the RhB solution can be almost completely degraded by the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC sample under visible light irradiation. However, only a certain percentage of the RhB solution can be degraded by the Ag\({}_{3}\)PO\({}_{4}\)/CC and SnO\({}_{2}\)/CC samples under the same conditions. Figure 7c shows the plot of the photodegradation rate constant of RhB versus degradation time. This reaction follows the pseudo-first-order reaction kinetic, which can be expressed as ln(C/C\({}_{0}\)) = -kt, where k and t represent the reaction rate constant and the reaction time, respectively. As illustrated in Figure 7d, the calculated k for the degradation of RhB over Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC is 0.04716 min\({}^{-1}\), which is higher than those calculated for the reactions over Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\) (0.02283 min\({}^{-1}\)), Ag\({}_{3}\)PO\({}_{4}\)/CC (0.0113 min\({}^{-1}\)) and SnO\({}_{2}\)/CC (0.00117 min\({}^{-1}\)). As shown in Figure 8a, the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC heterojunction exhibited the highest degradation rate of MB (96.6%) under visible light irradiation within 70 min, which is far more than that of other catalysts. The reaction kinetics of MB photodegradation by various photocatalysts is also performed (Figure 8b). The value of k is determined to be 0.04916 min\({}^{-1}\), 0.0263 min\({}^{-1}\), 0.01347 min\({}^{-1}\), 0.00401 min\({}^{-1}\), and 0.00306 min\({}^{-1}\) for the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC, Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\), Ag\({}_{3}\)PO\({}_{4}\) /CC, SnO\({}_{2}\)/CC, and MB without a catalyst, respectively. The above results indicate that the photocatalytic efficiency of the sample significantly improved, when Ag\({}_{3}\)PO\({}_{4}\) particles are deposited on the surface of SnO\({}_{2}\)/CC. Figure 7: Time-course variation of C/C\({}_{0}\) of RhB over various catalysts (**a**). The degradation rate of various catalysts (**b**). The plots of ln(C\({}_{0}\)/C) versus time of various catalysts (**c**). The calculated k for the degradation of RhB over various catalysts (**d**). The stability and recycling of the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC heterojunction were further analyzed and are shown in Figure 9a. After three cycling runs, the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC photocatalysts still possess a high photocatalytic efficiency, up to 79.7%.