# Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanolay Materials
## 4 Discussion  
Br-Hec and I-Hec have been successfully synthesized and, to the best of the authors' knowledge, their structural and spectroscopic features are here reported for the first time. They present the same hexagonal crystal structure as nanocrystalline fluorohectorite without impurity phases detectable with XPD. Both materials' crystallites are approximately 3 nm along the 001 direction and have a d\({}_{001}\) spacing of 14.30 A, which is suitable for intercalation of two monolayers of water in their interlayers at room temperature. The XRF spectra confirmed the chemical composition expected for the materials, showing the signals related to the Mg K\({}_{\alpha 1}\) and Si K\({}_{\alpha 1}\) emissions for both and the respective halogen emission of Br L\({}_{\alpha 1,2}\) or I L\({}_{\alpha 1}\), I L\({}_{\beta 1}\) and I L\({}_{\beta 2}\). Additionally, the spectra of both materials indicate the presence of Ti as an impurity, seen mainly through the weak line of its related K\({}_{\beta 1}\) emission at 4.9 keV. The FTIR spectra of Br-Hec and I-Hec indicate the total replacement of the hydroxyl groups of hectorite by the respective halides, since bands related to the vibrations of hydroxyl groups in the octahedral layers of clay minerals were not observed. The \({}^{29}\)Si NMR spectra of both materials show the same chemical environment predicted for the silicon tetrahedral layers of synthetic hectorite, giving the Q\({}^{3}\) (ca. \(-\)97 ppm) and Q\({}^{2}\) (\(-\)87) signals of Si(OMg)(OSi)\({}_{3}\) and Si(OMg)(OSi)\({}_{2}\)X (X = Br\({}^{-}\) or I\({}^{-}\)) sites, respectively. The Q\({}^{2}\)/Q\({}^{3}\) ratio indicates a higher degree of branching for the polymeric sheets of the I-Hec (0.41) than for the Br-Hec (0.37) materials. The nitrogen sorption isotherms of Br-Hec and I-Hec indicated that both are mesoporous materials with plate-like packing. Br-Hec has a surface area of 385 m\({}^{2}\) g\({}^{-1}\), while I-Hec shows an area of 363 m\({}^{2}\) g\({}^{-1}\), considering the model of a multilayer coverage. The materials have a mesopore volume of ca. 0.50 cm\({}^{3}\) g\({}^{-1}\), constant for the adsorption and desorption curves, suggesting a potential feature for catalyst application. Both materials show thermal stability up to 800 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{l c c c c c c} \hline
**Parameters** & **\(\mathbf{\tau_{1}/s}\)** & **A\({}_{1}\)/\(\mathbf{\
u_{\%}}\)** & **\(\mathbf{\tau_{2}/s}\)** & **A\({}_{2}\)/\(\mathbf{\
u_{\%}}\)** & **\(\mathbf{\tau_{3}/s}\)** & **A\({}_{3}\)/\(\mathbf{\
u_{\%}}\)** \\ \hline Bromhoetorite & 0.04(6) & 61 & 0.20(4) & 29 & 0.87(11) & 10 \\ Iodohectorite & 0.06(6) & 53 & 0.30(3) & 35 & 1.07(8) & 12 \\ \hline \end{tabular}
\end{table} 
Table 1: Emission lifetimes calculated for bromhoetorite and iodohectorite by fitting the luminescence intensity (I) to the function: \(I=I_{0}+A_{1}e^{-t/\tau 1}+A_{2}e^{-t/\tau 2}+A_{3}e^{-t/\tau 3}\). Here, A = amplitude, t = time and \(\tau\) = lifetime. The values in parentheses show the SD (standard deviation) calculated. Figure 9: The luminescence decay (\(\lambda_{\text{exc}}\): 255 nm) of bromhoetorite (\(\lambda_{\text{em}}\): 470 nm) and iodohectorite (\(\lambda_{\text{em}}\): 490 nm). Br-Hec and I-Hec have blue-green emission under UV radiation. The luminescence features are attributed to a Ti\({}^{3+}\) impurity acting as the luminescent center in these materials. When irradiated at 255 nm, Br-Hec has this main emission band at 470 nm, while observed at 490 nm for the I-Hec. This red shift was explained using the MO theory to occur due to the differing \(\pi\)-donor strengths of the halogens. The center of the main emission band varies with the excitation wavelength, indicating that the activator center is occupying multiple sites in the host lattice, which was also observed from the emission band widths.