# ZnO@MoS2 Core-Shell Heterostructures Enabling Improved Photocatalytic Performance
## 3 Results and Discussion
UV-visible spectra are employed to characterize the light absorption of ITO substrate, ZnO micro/nanorods, and ZnO@MoS\({}_{2}\) core-shell heterostructures with the MoS\({}_{2}\) growth time of 6 h, as illustrated in Figure 7. Compared with the blank ITO substrate, the ZnO micro/nanorods reveal a slightly higher light absorption in the range from 290 to 350 nm. The ZnO@MoS\({}_{2}\) core-shell heterostructures show a much higher light absorption in the range from 290 to 600 nm in the three samples. Accordingly, a higher light absorption usually results in a higher photocatalytic efficiency. It also can be confirmed by the color change of the methylene blue after photocatalytic degradation, as shown in the insert picture of Figure 7. The reason for the higher photocatalytic efficiency of the ZnO@MoS  
Figure 5: SEM images of the as-received ZnO micro/nanorods and the as-received ZnO@MoS\({}_{2}\) core-shell micro/nanorods under the MoS\({}_{2}\) growth time of 6, 8, and 10 h: (**a,c,e**) correspond to MoS\({}_{2}\) (the bottom); (**b,d,f**) correspond to the as-received ZnO@MoS\({}_{2}\) core–shell heterostructures. core-shell heterostructures may be due to three aspects. The first is the rich sulfur vacancies in the MoS\({}_{2}\) nanosheets shell layer [11; 12]. The sulfur vacancies on the surface of the MoS\({}_{2}\) shell layer have been demonstrated to be the active sites of the catalytic reactions. The more sulfur vacancies there are, the higher the photocatalytic efficiency is. The second is attributed to the heterojunction of MoS\({}_{2}\)/ZnO [2; 15; 25]. It has been demonstrated that the MoS\({}_{2}\) chemically bonded to ZnO under high-pressure growth conditions to form the MoS\({}_{2}\)/ZnO heterojunction [2]. It should be noted that a strong internal field is set up by the MoS\({}_{2}\)/ZnO heterojunction, which leads to a significant promotion in the spatial separation of the photogenerated carriers [2]. The photogenerated carriers are believed to be the power source of the photocatalytic action. This indicates that the enhanced internal field created by the MoS\({}_{2}\)/ZnO heterojunction favors the photocatalytic efficiency. The third is the specific surface area enhanced by the MoS\({}_{2}\) nanosheets. It is widely accepted that the MoS\({}_{2}\) nanosheet's shell layer owns a more extensive specific surface area than that of the ZnO micro/nanorods [11]. A bigger and rougher surface can absorb more organic dyes, suggesting a better photocatalytic efficiency [11]. However, as the growth time exceeds 6 h, the photocatalytic efficiency of the ZnO@MoS\({}_{2}\) core-shell heterostructures is reduced and kept down as the growth time is increased. As mentioned above, the thickness of the MoS\({}_{2}\) nanosheet's shell layer is thicker as the growth  
Figure 6: Photocatalytic ability of the as-received ZnO micro/nanorods and the as-received ZnO@MoS\({}_{2}\) core–shell heterostructures grown for 6, 8, and 10 h on methylene blue with a photocatalytic time of 3 h: (**a**) absorption spectra and (**b**) photocatalytic efficiency. Figure 7: Absorption spectra of ITO substrate (black line), the as-received ZnO micro/nanorods (red line), and the as-received ZnO@MoS\({}_{2}\) core–shell heterostructures (blue line) grown for 6 h and the insert picture is of the methylene blue solution after a 3 h photocatalytic reaction: (**a**) without any photocatalyst, (**b**) ZnO micro/nanorods, and (**c**) ZnO@MoS\({}_{2}\) core–shell heterostructures grown for 6 h.  
time is increased. On the one hand, increasing the MoS2 nanosheet's shell layer thickness will weaken the light absorption of the MoS2/ZnO heterojunction or ZnO micro/nanorods and further cut down on the spatial separation of the internal field [10]. Accordingly, this leads to a reduction in the photocatalytic efficiency.