# ZnO@MoS2 Core-Shell Heterostructures Enabling Improved Photocatalytic Performance
## 1 Introduction  
Nowadays, widely applied, organic reagents cause a large amount of organic pollutants [1], which are considered a major threat to human health and survival. Therefore, the harmless treatment of organic pollutants has become a hot topic in past decades [2, 3]. One of the green approaches is employing semiconductor materials to degrade the organic pollutants into non-toxic compounds under sunlight [4, 5]. A great work has been carried out to pursue highly efficient, semiconductor photocatalysts, such as a single crystal TiO2, zinc oxide (ZnO) CdS, etc. [6, 7, 8]. Among the highlighted, semiconductor photocatalysts, ZnO has attracted particular attention and has been studied numerous times due to its outstanding properties [3]. First, ZnO has versatility, availability, and low toxicity [9], which means low cost and environmental protection. Significantly, ZnO has a 3.37 eV band gap and high photo-responsivity (60 meV), which make it suitable for oxidation and reduction reactions by absorbing visible light for significant solar energy harvesting [10]. However, ZnO still suffers from some deficiency in photocatalysis. On the one hand, the photocatalytic reaction of ZnO is limited by the low, solar-energy conversion efficiency caused by the intrinsic, wide-band gap; high recombination rate of photo-generated, charge carriers; and possible photo-corrosion. On the other hand, the chemical stability of ZnO should not be ignored. In our previous study, we demonstrated that ZnO thin film can be easily etched by a 0.001 mol/L hydrochloric acid solution. In practice, organic-dye sewage is most likely to be acidic, which might allow it to cut down on ZnO materials.  
One way to solve the first issue is by fabricating ZnO micro/nanomaterials. ZnO micro/nanomaterials possess large specific surface areas, quantum effects, and piezoelectric photoelectron effects. These perfect effects can greatly improve the solar-energy conversionefficiency. Another effective method is to bring in co-catalysts, such as metal nanoparticles (Pt, Ag, Co, etc.) with graphene, graphene oxide, and transition metal dichalcogenides (i.e., MoS\({}_{2}\), WS\({}_{2}\), etc.) [6; 9; 11]. Among the above co-catalysts, MoS\({}_{2}\) exhibits nice chemical stability [10], indicating that the ZnO@MoS\({}_{2}\) core-shell nano-structure should be an outstanding photocatalyst. In this regard, MoS\({}_{2}\) nanosheets have been widely studied [9; 10]. In numerous previous successful studies, the growth time of the MoS\({}_{2}\) nanomaterials with a hydrothermal method was over 10 h [12; 13; 14; 15]. On the premise of ensuring efficient catalysis, shortening the growth time of the MoS\({}_{2}\) nanosheets can bring in two benefits. One is that saving time means saving source materials and cost. The other is that saving time results in improving production efficiency. As the famous quote says, \"time is money; time is efficiency\". Therefore, cutting down on growth time is meaningful for the ZnO@MoS\({}_{2}\) core-shell nano-structure photocatalyst on the premise of ensuring efficient catalysis.  
In this work, we explored ZnO@MoS\({}_{2}\) core-shell heterostructures that benefited from the excellent chemical stability and synergetic photocatalytic performance of the MoS\({}_{2}\) nanosheets. The MoS\({}_{2}\) nanosheets' growth time was also investigated for effectively saving growth time on the premise of ensuring efficient catalysis.