# ZnO@MoS2 Core-Shell Heterostructures Enabling Improved Photocatalytic Performance
## 3 Results and Discussion  
XRD was applied to determine the crystalline structure of the three samples, and the results are shown in Figure 1. The XRD pattern of a blank ITO substrate was also taken for comparison. From Figure 1, one can see clearly that, apart from the ITO peaks, the patterns for the ZnO@MoS\({}_{2}\) core-shell heterostructures samples have some peaks that can be well-indexed to ZnO (JCPDS:36-1451). Strangely, no independent peaks are assigned to MoS\({}_{2}\). However, when Mo\({}_{2}\)S\({}_{3}\) (JCPDS:40-0972) or Mo\({}_{15}\)S\({}_{19}\) (JCPDS:40-0936) are taken as a reference, three or four peaks appear in the same positions as those of ZnO or ITO. This result indicates that MoS\({}_{2}\) with rich sulfur vacancies is possibly fabricated on ZnO. Further analysis should be carried out to confirm the fabrication of MoS\({}_{2}\). Furthermore, the element mapping images from SEM were deployed to confirm the fabrication of MoS\({}_{2}\) directly. Figure 2a is an SEM image of the ZnO@MoS\({}_{2}\) core-shell heterostructure with the MoS\({}_{2}\) shell grown for 1 h, and Figure 2b-e are the element mapping images corresponding to O, S, Zn, and Mo elements, respectively. The O signal is from the ZnO microrod and ITO substrates, while the Zn signal comes from the ZnO microrod due to the high working voltage of 15 kV. Accordingly, as can be seen, the shapes of S, Zn, and Mo are in the same position as the ZnO microrod of Figure 2a, indicating that MoS\({}_{2}\) has been achieved after hydrothermal growth, and ZnO micro/nanorods have been almost decorated with the MoS\({}_{2}\) shell layer. At the same time, in Figure 2f, the corresponding energy dispersive X-ray spectrometry (EDX) pattern of Figure 2a also verifies the existences of S and Mo. Therefore, the ZnO@MoS\({}_{2}\) core-shell heterostructures have been fabricated by the two-step hydrothermal method. Except the element mapping images, the Raman spectrum is another technology used to confirm the fabrication of MoS\({}_{2}\) directly. As shown in Figure 3, when the MoS\({}_{2}\) growth time is 6 h, the intense peak of ZnO micro/nanorods at -437 cm\({}^{-1}\) is sharply decreased, and a very weak peak at about 406.4 cm\({}^{-1}\) turns up. As the MoS\({}_{2}\) growth time is increased to 8 h, the peak at 406 cm\({}^{-1}\) becomes more prominent, and a new and weak peak at 377.3 cm\({}^{-1}\) appears. When the MoS\({}_{2}\) growth time is up to 10 h, the peaks at 406.4 cm\({}^{-1}\) and 377.3 cm\({}^{-1}\) can be seen distinctly. Meanwhile, the peak at 325 cm\({}^{-1}\) gradually becomes weaker and disappears as the growth time of MoS\({}_{2}\) is from 6 h to 10 h. According to previous publications [16; 17; 18; 19], the peaks located at 377.3 cm\({}^{-1}\) and 406.4 cm\({}^{-1}\) can be assigned to the A\({}_{1g}\) mode of MoS\({}_{2}\) and to E\({}_{2g}\) mode of the Mo-S bond, respectively. These results are well-matched with those of the MoS\({}_{2}\) fabricated by the hydrothermal method [7]. Thus, the ZnO@MoS\({}_{2}\) core-shell heterostructures have been successfully achieved by two-step hydrothermal growth. Figure 1: XRD patterns of the ITO substrate and the as-received ZnO@MoS\({}_{2}\) core–shell heterostructures under the MoS\({}_{2}\) growth time of 6, 8, and 10 h.  
Figure 3: Raman spectra of the as–received ZnO micro/nanorods and the as-received ZnO@MoS\({}_{2}\) core–shell heterostructures under the MoS\({}_{2}\) growth time of 6, 8, and 10 h.  
Figure 2: (**a**) SEM image of the as–received ZnO@MoS\({}_{2}\) core–shell heterostructure with the MoS\({}_{2}\) growth time of 1 h and its element mapping of (**b**) O, (**c**) S, (**d**) Zn, and (**e**) Mo. (**f**) Its corresponding EDX pattern. It should be noted that the intensities of the peaks at 406.4 cm\({}^{-1}\) and 377.3 cm\({}^{-1}\) are about one-twentieth of that of the ZnO peak at \(\sim\)437 cm\({}^{-1}\). The intensity of the Raman peak is dependent on the quality and thickness of the materials based on our previous experiments.