# Designing Dual-Function Nanostructures for Water Purification in Sunlight
## 3 Results and Discussion
### Water Purification by Dual Process: Adsorption and Photocatalysis
The enhanced photocatalytic activity of Zn-Al-GO1 can be explained by the following four reasons: (i) improving the adsorption capacity of Zn-Al-GO1 because of new production of micro- and meso-porous structure; (ii) the enhanced optical properties under irradiation with visible light were due to the decreased band gap energies of the Zn-Al-GO1; (iii) the Zn-Al-GO1 exhibited more effective interfacial charge transfer and more efficient separation of the photogenerated electron-hole pairs because of the excellent conductivity of graphene oxide; and (iv) the active sites on the Zn-AlGO1 composites were more fully exposed because these composites had relatively larger specific surface areas. However, the photocatalytic degradation of ZnAl-GO2 and Zn-Al-GO3 decreased as the graphene oxide content increased to 2.3% and 3.5%, which indicates that excess graphene oxide loadings detrimentally affected the photocatalytic performance of the Zn-Al-GO2 and Zn-Al-GO3. The further increasing of graphene oxide content had three disadvantage effects. The first disadvantage is the excess graphene oxide that prevents the incident light from directly reaching the catalyst's surface, thereby decreasing the production of photo-excited electron-hole pairs. The second one is blocking of the micro- and meso-pores because of the excess graphene oxide. The third problem is that the excess graphene oxide could provide recombination centers to accelerate the recombination of photogenerated electron-hole pairs. In order to characterize the photocatalytic behavior of the Zn-Al LDH and their composites with graphene oxide during the degradation of Acid green 1, the experimental data were fitted with the first order kinetic equation:  
\[\ln\left(\text{A}_{\text{o}}/\text{A}_{\text{e}}\right)=\text{kt}\]  
where k represents the rate constant (h\({}^{-1}\)). A\({}_{\text{e}}\) is the absorption of acid green 1 at different irradiation time and A\({}_{\text{o}}\) is the initial absorption of the dye [45]. The first-order kinetics for green dyes photocatalytic degradation with the samples Zn-Al LDHs, Zn-Al-GO1, Zn-Al-GO2, and Zn-Al-GO3 are shown in Figure 15. The good linear relationships, which obtained between -ln (A\({}_{\text{e}}\)/A\({}_{\text{o}}\)) and the irradiation time in all cases, confirmed that the photocatalytic degradation processes of green dyes with Zn-Al LDHs, Zn-Al-GO1, Zn-Al-GO2, and Zn-Al-GO3 followed pseudo-first order kinetics. In addition, the k values were calculated for the different samples according to this model as shown in the Figure 15. The  
Figure 14: Photocatalytic activity of Zn-Al LDHs before and after doping with graphene oxide at different times. photocatalytic degradation rate constant for pure Zn-Al LDHs was determined as 0.06 h\({}^{-1}\) in presence sun light. By combining 1.3% of graphene oxide with ZnAl LDHs, the rate constant of the process accelerated to be 0.45 h\({}^{-1}\). It means that the 1.3% of graphene oxide accelerated the photocatalytic degradation of dyes in sun light 7 times higher than that of the pure the Zn-Al LDHs. With increasing the percentage of graphene oxide to be 2.3%, the rate constant increased from 0.06 h\({}^{-1}\) to 0.09 h\({}^{-1}\). By further increasing with graphene oxide to become 3.5%, the rate constant decreased from 0.06 h\({}^{-1}\) to 0.03 h\({}^{-1}\). The results are clearly demonstrated that the Zn-Al-GO1 with 1.3% of graphene oxide had the highest photocatalytic activity in the degradation of acid green 1 under sun light comparing with the other samples. These results clearly indicate that Zn-Al LDHs converted to be effective photocatalyst in sun light by combining with 1.3% of graphene oxide.