# Designing Dual-Function Nanostructures for Water Purification in Sunlight
## 3 Results and Discussion
### Optical Properties  
A detailed study of UV-vis absorption was used to indicate the effect of graphene oxide on the optical properties of Zn-Al LDHs. Figure 10 shows the UV-vis absorption spectra for Zn-Al LDHs and its composites Zn-Al-GO1, Zn-Al-GO2 and Zn-Al-GO3. The sharp rise in absorbance is due to the excitonic transition. The direct electronic transitions from the valence band to the conduction band leads to the cut-off behavior at the blue end of the spectrum. It can be seen that absorption edge of Zn-Al LDH is shifted towards visible region by combining with graphene oxides to produce effective photocatalysts in sunlight as shown in Figure 10. However, the main characteristic absorption band of GO is centered at 232 nm as reported by [PERSON], et al. [45]. This red shift became clearer through calculating the band gap energy. In order to determine the band gap energy of Zn-Al LDHs and their composites, we have made complete calculations of the band gap energy by following the Tauc equation [46];  
\[\rm{(Abs\cdot E)^{n}=A(E-E_{g})}\] \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **Graphene Oxide** & **S\({}_{\rm{SET}}\)** & **S\({}_{\rm{ext}}\)** & **S\({}_{\rm{int}}\)** & **S\({}_{\rm{BJH}}\)** & **V\({}_{\rm{pH}}\)** & **V\({}_{\rm{MP}}\)** & **R\({}_{\rm{pH}}\)** \\  & **(Wt.\%)** & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & **(cm\({}^{3}\)/g)** & **(nm)** \\ \hline Zn-Al & 0 & 5.6 & 5.6 & - & 3.3 & 0.012 & - & 1.8 \\ LDHs & 1.3 & 60.3 & 54.7 & 5.6 & 57 & 0.29 & 0.0019 & 3.5 \\ Zn-Al-GO2 & 2.3 & 61.5 & 64.0 & - & 75.3 & 0.13 & - & 3.2 \\ Zn-Al-GO3 & 3.5 & 33.4 & 36.7 & - & 42 & 0.08 & - & 3.5 \\ \hline \hline \end{tabular}  
* S\({}_{\rm{BHL}}\), V\({}_{\rm{pBH}}\) and R\({}_{\rm{pBHL}}\) specific surface area, total pore volume and pore radius calculated by BJH method, respectively; S\({}_{\rm{Int}}\) and V\({}_{\rm{MP}}\), internal surface area (micropore surface area) and micro pore volume calculated by the de Boer method; S\({}_{\rm{Ext}}\), external surface area calculated by the de Boer method; S\({}_{\rm{BET}}\), specific surface area calculated by BET method.  
\end{table} 
Table 1: Adsorption data derived from adsorption–desorption isotherms for Zn-Al LDHs before and after combining with graphene oxide using Brunauer–Emmett–Teller (BET), de Boer and Barrett–Jømer–Halenda (BJH) methods. Figure 10: Absorbance spectra of **(a)** Zn-Al LDH, **(b)** Zn-Al-GO1, **(c)** Zn-Al-GO2, and **(d)** Zn-Al-GO3. The energy of photon (E) = hv, the absorption coefficient is the value of Abs, and the energy band gap is E\({}_{\text{g}}\). The term (n), which denotes the nature of the transitions, = 2 for direct transition. By plotting (Abs\(\cdot\)E)\({}^{2}\) versus energy of photon (E), the band gap energy is determined through the tangent drawn to the curve intersect the energy axis at 0. Figures 11 and 12 show the Tauc plots of Zn-Al LDH, Zn-Al-GO1, Zn-Al-GO2 and Zn-Al-GO3 for direct transition. In Figure 11a, the estimated direct band gap energy of Zn-Al LDHs was found to be 5.5 eV. By combining graphene oxide with Zn-Al LDHs, the band gap energy decreased to lower values. The band gap energy became 2.5 eV after doping with the lower percentage of graphene oxide as shown in Figure 11b. It means that Zn-Al-GO1 is a suitable material to act in sun light. By increasing the content of graphene oxide, the band gap energy decreased from 5.5 eV to be 4.0 eV as shown in Figure 12. These results indicated that narrowing of band gap energy of Zn-Al LDHs (red shift) was observed through doping with graphene oxide. In addition, it concluded that 1.3% of graphene oxide was the optimum percentage for converting Zn-Al LDHs to become photo-active in the visible region. The high narrowing of the band gap of Zn-Al-GO1 may be explained according to many body effects of graphene nanosheets on the conduction and the valence bands.