# Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanolay Materials
## 3 Results
### Luminescence Properties
400 and 410 nm for Br-Hec and I-Hec, respectively, are assigned to electronic transitions from the t\({}_{2g}\) to e\({}_{g}\) energy levels of Ti\({}^{3+}\), which agrees with previous reports for Ti\({}^{3+}\) in fluorohectorite (380 nm) [23], chlorohectorite (370 nm) [24] and monoclinic ZrO\({}_{2}\) (314 nm) [58], considering that d-d transitions are sensitive to the crystal field strength. The wide bands at ca. 300 nm are related to the e\({}^{-}\)(O\({}^{2-}\)(2p))\(\rightarrow\)Ti\({}^{\text{IV}}\) charge transfer, which can occur through two possible molecular transitions: t\({}_{1u}\)(\(\pi^{\text{b}}\))\(\rightarrow\)t\({}_{2g}\)(\(\pi^{\text{e}}\)) or t\({}_{2u}\)(\(\pi\))\(\rightarrow\)t\({}_{2g}\)(\(\pi^{\text{e}}\)), being typically observed at ca. 280 nm [23; 24; 42; 48; 59]. Lastly, the energy gaps between the valence and the conduction bands are identified at ca. 256 nm (4.84 eV) and 248 nm (5.00 eV) for Br-Hec and I-Hec, respectively, which agree with the characteristic band gap energy 4.7 eV (260 nm) of hectorites [60]. Since the excitation spectra were recorded by monitoring the blue emission, the observation of the valence to conduction band transition confirms that titanium is in the hectorite structure and not present as an isolated impurity phase. When the excitation spectra are recorded at different emission wavelengths (Figures 10 and 11), these three bands remain mainly in the same spectral position, which proves that the same activator center (Ti\({}^{3+}\)) is responsible for the observed blue-green emission. When the excitation spectrum of Br-Hec is recorded at 710 nm emission (Figure 10), it shows two bands at ca. 400 and 577 nm. In minerals, the \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{4}\)T\({}_{1}\) and \({}^{4}\)A\({}_{2}\)\(\rightarrow\)\({}^{4}\)T\({}_{2}\) transitions that excite the red/NIR emission are commonly observed at 410 and 550 nm for Cr\({}^{3+}\) as well as 420 and 530 nm for Mn\({}^{4+}\)[46]. This confirms that one or both of these ions is the origin of this line emission at 710 nm. The luminescence decay curves of Br-Hec and I-Hec (Figure 9) indicated that the materials present a short persistent luminescence, with a duration of ca. 5 s, at room temperature. Both materials have at least three lifetimes obtained from a multiexponential fitting function. However, even with three components, only a part of each decay curve could be fitted reasonably. This indicates that several contributing processes are involved in the overall decay. It has been reported that Ti\({}^{3+}\)-doped materials have a typical emission lifetime lasting microseconds [61], but halogen-hectorites have shown much longer lifetimes, on the order of hundreds of milliseconds to seconds [23; 24], which agrees with the lifetimes obtained from the fits to the decay curves of Br-Hec and I-Hec in this work (Table 1). This short persistent luminescence is explained through the existence of defects in the layered structure of these materials acting as shallow electron traps for short-time energy storage. Even though the promo- and  
Figure 8: The excitation spectra of the bromohectorite (\(\lambda_{\text{em}}\): 470 nm) and iodohectorite (\(\lambda_{\text{em}}\): 490 nm). iodohetorites can store optical energy, they possess shallow traps that do not allow for any significant persistent luminescence at room temperature.