# Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanolay Materials
## 3 Results
### Luminescence Properties  
The Br-Hec and I-Hec materials display a yellowish body color under white light; however, when the materials are exposed to UV radiation, they show blue emission (Figure 7). The emission spectra (\(\lambda_{\text{exc}}\): 255 nm) show a main broad blue-green emission band for both materials (Figure 7, upper). This main emission is centered at 470 nm for the Br-Hec material, while the I-Hec material has the center of the band red shifted to 490 nm. Additionally, both materials have a weak red emission at ca. 710 nm, which is more prominent for Br-Hec but probably overlapping with the main emission for the I-Hec material, where it is seen as a weak shoulder. When the materials are excited at 365 nm, their main emission band appears as a broad green-yellow emission (Figure 7, lower), having its barycenter at ca. 530 and 562 nm for Br-Hec and I-Hec, respectively, while the weak red emission remains at ca. 710 nm. When the materials are excited at different wavelengths, there is a tendency that the main emission band is shifted towards red with increasing excitation wavelength (Figures A4 and A5). This change in the barycenter of the emission bands with the variation of the excitation wavelength is partially due to the cutoff caused by the filters used to eliminate the excitation signal from the emission spectrum. However, the shift may indicate either that the activator center is occupying multiple sites with different crystal field strengths in the host lattice, or the presence of defects in the structure of the X-Hec materials. The multisite origin for emission is also witnessed in the decrease of emission band width [42] with increasing excitation wavelength. Figure 7: The emission spectra of the bromohectorite and iodohectorite under 255 nm (**top**) and 365 nm (**bottom**) excitation. The inserted photos illustrate the visual aspect of the materials under white light and when irradiated with 255 nm or 365 nm. Similar blue-green emission has previously been observed for other silicates such as fluorohectorite [23], chlorohectorite [24], montmorillonite [43], kaolinite [44], pyrophyllite [45], topaz (Al\({}_{2}\)SiO\({}_{4}\)(F,OH)\({}_{2}\)) [46], synthetic hackmanite (Na\({}_{8}\)Al\({}_{6}\)Si\({}_{6}\)O\({}_{24}\)(Cl,S)\({}_{2}\)) [47; 48], benitoite (BaTiSi\({}_{3}\)O\({}_{9}\)) [49], and SiO\({}_{2}\)[50; 51]. The literature presents mainly two different explanations for the origin of this blue-green emission. One proposition categorizes this emission as photoelectric response as well as inner center and recombination type of luminescence [52], attributing the blue emission to triplet-singlet transition in twofold coordinated Si centers [53], due to oxygen deficit and irradiation effects [50]. The second proposition associates the emission with luminescent centers containing trivalent titanium paired with an oxide vacancy, which may exist either as matrix element or as an impurity [23; 24; 46; 47; 48; 49]. It was shown previously for synthetic chlorohectorite [24] that the intensity of the blue emission band increases if titanium is deliberately doped to the material. Therefore, the band was assigned to Ti\({}^{3+}\). It has also been verified that although natural hectorite presents a similar blue emission often attributed to [SiO\({}_{2}\)]\({}^{2-}\) centers, it has different luminescence features than have been observed for halogen-hectorites [24]. Also, previous reports have shown that Ti species can occur as impurities in silicates, generating similar emission features as reported for silicate glasses [54], fluorohectorites [23] and chlorohectorites [24]. The materials studied in the current work have low contents of Ti, confirmed from their respective XRF spectra (Figure 2). It has been proposed previously that, in the case of synthetic halogen-hectorites, the titanium impurities originate from the SiO\({}_{2}\) used as a precursor to prepare the materials, where Si\({}^{\rm IV}\) has been replaced by Ti\({}^{\rm IV}\).