## 3 Results and Discussion  
### Microstructure and Morphology  
The crystal structure of the synthesized samples is confirmed by XRD characterization, shown in Figure 1. For the pure CC sample, the diffraction peaks at around 26\({}^{\circ}\) and 44\({}^{\circ}\) correspond with the carbon cloth (JCPDS card No. 65-6212) [46]. All the diffraction peaks of the Ag3 PO4/SnO2/CC sample can be indexed to the tetragonal rutile phase of SnO2 (JCPDS card No. 41-1445), Ag3 PO4 (JCPDS card No. 06-0505) and CC [48,49]. No diffraction peaks belonging to the impurity phases were detected. As illustrated in Figure 2a,b, SnO2 nanorods with a diameter of about 150-300 nm grow evenly on the surface of the carbon cloth. In contrast, it can be clearly seen from the photographs in Figure 2c that only a few Ag3 PO4 particles are sparsely deposited on the carbon cloth. The enlarged SEM image shows that Ag3 PO4 consists of spherical particles and some particles with irregular shapes. The SEM images of the Ag3 PO4/SnO2/CC sample are shown in Figure 2e,f. When compared with the Ag3 PO4/CC sample, many more Ag3 PO4 particles with irregular shapes are deposited on the carbon cloth, which should be attributed to the manufacturing process. It can clearly be seen from the SEM results that the surface of the carbon cloth is smooth, while the SnO2/CC sample shows a rougher surface. When the SnO2/CC was put into the AgNO3 solution, many more Ag\({}^{+}\) ions were easily adsorbed onto the surface  
Figure 1: XRD patterns of pure CC, Ag3 PO4/CC, SnO2/CC and Ag3 PO4/SnO2/CC. of SnO\({}_{2}\)/CC. They then reacted with Na\({}_{2}\)HPO\({}_{4}\) to generate Ag\({}_{3}\)PO\({}_{4}\), which was more difficult to shed from the surface of SnO\({}_{2}\)/CC, compared to that on the carbon cloth. This is also evidenced by the weight of the synthesized samples. Furthermore, the elemental mapping analysis of the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC sample for the detected elements Sn, O, Ag, P and C, is shown in Figure 3a-g. All five elements are uniformly distributed throughout the whole carbon fiber. In addition, Figure 3 clearly shows the distribution of Sn, O, Ag and P within the carbon fiber, which further confirms the formation of the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC heterojunction structure [50]. The TEM image of the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC is shown in Figure 4a, which clearly indicates that the Ag\({}_{3}\)PO\({}_{4}\) particles are uniformly distributed on SnO\({}_{2}\) nanorods. The HRTEM image of Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC is shown in Figure 4b, which clearly uncovers a set of fringes with an interplanar spacing of 0.246 nm ascribed to the (211) plane of Ag\({}_{3}\)PO\({}_{4}\), and the other lattice fringe of 0.336 nm consistent with the (110) plane of SnO\({}_{2}\)[48; 51]. The results above confirm the formation of the Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\) heterojunction. XPS was utilized to further analyze the elemental compositions of the as-prepared samples (Figure 5). In the wide-scan, sharp peaks of Sn, Ag, P, O and C are detected in Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC (Figure 5a). Analysis of the XPS spectrum for Sn 3d, Ag 3d, and P 2p is shown in Figure 5b-d. The peaks located at 486.5 and 494.8 eV (Figure 5b), can be indexed to Sn 3d\({}_{5/2}\) and Sn 3d\({}_{3/2}\) in SnO\({}_{2}\)[52]. Meanwhile, two sharp peaks at 367.7 and 373.8 eV (Figure 5c), can be ascribed to Ag 3d\({}_{3/2}\) and Ag 3d\({}_{5/2}\) of Ag\({}^{+}\) ions, respectively [16]. The characteristic peak (P 2p) at 132.8 eV (Figure 5d), corresponds to P\({}^{5+}\),  
Figure 3: SEM image of Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC (**a**) and the elemental mapping images of C (**b**), Sn (**c**), O (**d**), Ag (**e**), P (**f**) and Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC (**g**). Figure 2: SEM images of the SnO\({}_{2}\)/CC (**a**,**b**), Ag\({}_{3}\)PO\({}_{4}\)/CC (**c**,**d**), Ag\({}_{3}\)PO\({}_{4}\)/SnO\({}_{2}\)/CC (**e**,**f**). according to previous literature [18].