2.1.1. X-ray Diffraction (XRD)

Figure 1 shows the XRD patterns of hectorite and TiO2/hectorite (TH-1, TH-2, TH-3, TH-4, and TH-5). As for the hectorite, characteristic diffraction peaks of 2θ approximately appear at 19.6, 28.0, 35.1, 53.3, 61.0, and 72.3◦ [20,21]. It can be observed that hectorite has low crystallinity and small particle size from the widened peaks. Some weak peaks of 2θ appear at 29.0, 32.0, 38.7, 48.9, and 54.7◦, which may be attributed to the residual starting materials and some intermediate products, such as lithium fluoride, lithium, and silicate [10].

**Figure 1.** XRD patterns of hectorite: TH-1, TH-2, TH-3, TH-4, and TH-5.

TiO2 shows the characteristic diffraction peaks of the anatase phase (Joint Committee on Powder Diffraction Standards Card NO. 21-1272) at 25.3, 37.8, 48.06, 54.0, 55.07, and 62.8◦, which correspond to the diffraction planes of (101), (004), (200), (105), (211), and (204) [10]. The same diffraction peaks are found for the TiO2/hectorite photocatalysts with different molar ratios of Li, Mg, and Si. However, the characteristic peaks of hectorite become slightly weaker in comparison to the synthesized TiO2/hectorite, indicating that the layered structure of hectorite is partially destroyed, but its skeleton structure is still maintained [22]. The crystallite size of TiO2 can be calculated according to the Debye– Scherrer equation (Equation (1)) [23]:

*D* = *Kλ*/*βcosθ*, (1)

where the *K* constant is the shape factor (value as 0.89), *λ* is the wavelength, and *β* is the half-peak width of the signal. The crystallite sizes of TiO2 in TH-1, TH-2, TH-3, TH-4, and TH-5 are 11.3, 10.8, 11.2, 11.4, and 11.9 nm, respectively, according to the (101) peak by Equation (1), which are all smaller than that of P25 (21 nm) (Table 1). The appropriate particle size of TiO2 is beneficial to increase the surface area, providing more photocatalytic active sites to improve the photocatalytic efficiency.

**Table 1.** Specific surface area (SBET), pore size, pore volume, and crystal size of samples.


It has been found that for hectorite, magnesium from the magnesium oxide octahedron is more easily replaced by lithium ions, while silicon from silica the tetrahedron is difficult [20]. The number of lithium ions and magnesium ions have some effect on TiO2/hectorite in the hydrothermal synthesis. With the increase in Li+, the crystal size of TiO2 decreases first and then increases gradually. Li+ can accelerate the conversion of lithium silicate into hectorite, as well as increase the number of Li+ replacing Mg2+ on the lamella, enhancing the layer charge, so that more titanium ions have more chances to intercalate the interlayer to balance the negative charge. As for the samples, TiO2 in TH-2 shows the best crystallinity. With the increase in the amount of lithium, diffraction peaks of TH-4 and TH-5 appear at 38.70, 44.99, and 65.48◦, corresponding to the diffraction of (111), (200), and (220) crystal planes of the cubic LiF crystal, suggesting the increase in residual LiF. In addition, the peaks at 48.9 and 54.7◦ are caused by the intermediate product lithium silicate.

### 2.1.2. Fourier Transform Infrared (FTIR) Spectroscopy

Figure 2 shows the FTIR spectra of the samples in the range of 4000 to 400 cm<sup>−</sup>1. It can be obviously observed that there are similar peaks between hectorite and TiO2/hectorite, indicating that the basic skeleton structure of hectorite is not damaged during the introduction of TiO2. The absorption bands such as -OH stretching vibrations of crystalline water at 3620 cm−<sup>l</sup> , Si–O stretching at around 1030 cm−<sup>l</sup> , Mg–O at 650–670 cm−<sup>l</sup> , and the vibration peak of Si–O–Si bonds at 470 cm−<sup>l</sup> can be observed, which are characteristics of hectorite [24,25]. The peaks at 1640 and 3460 cm−<sup>l</sup> are attributed to the bound water and the vibration of structural hydroxyl groups in the samples [26,27]. The bands of all the composite photocatalysts centered on 640 and 605 cm−<sup>1</sup> are typical for TiO2 (anatase phase), corresponding to the stretching modes of Ti–OH and Ti–O bonds, respectively [28]. The bands between 700 and 500 cm−<sup>1</sup> are usually assigned to the stretching vibration modes characteristic of Ti–OH and Ti–O–Ti bonds. All samples have these peaks and thus all have TiO2.

**Figure 2.** FTIR spectra for hectorite, TH-1, TH-2, TH-3, TH-4, and TH-5.
