*3.4. Photocatalytic Degradation*

The photocatalysis of TiO2/hectorite was studied by the degradation of MB in a photocatalytic reactor [10]. The photocatalytic degradation was performed under a 125 W high-pressure mercury lamp. The Mercury lamp was preheated for 30 min before reaction to ensure stable luminescence. The lamp was kept approximately 30 cm away from the solution.

Firstly, 3 mg of the TiO2/hectorite catalyst was dispersed in 100 mL of 10 ppm MB solution. Then, the mixture was stirred magnetically for 30 min in the dark to achieve adsorption-desorption equilibrium. After that, with underexposure of UV light, 3 mL of suspension was taken every 10 min and filtered by a 0.45 μm membrane to remove photocatalyst. The absorbance of the filtrate was determined by a WF Z UV-2800H UV-vis spectrophotometer (Unico, Suite E Dayton, NJ, USA) at 664 nm to evaluate the photocatalytic efficiency. The removal rate of MB was calculated by Equation (5):

$$R = \frac{\mathcal{C}\_0 - \mathcal{C}\_t}{\mathcal{C}\_0} \times 100 \,\text{\AA} \tag{5}$$

where *C*<sup>0</sup> (mg/L) is the initial concentration of MB and *Ct* (mg/L) corresponds to its concentration at time *t*.

After the photocatalytic degradation, the MB adsorbed on the material was completely degraded by irradiation under UV light. Then, the MB solution and photocatalytic materials were separated by centrifuge and dried at 120 ◦C for 2 h for recycling.

### **4. Conclusions**

TiO2 was introduced into the interlayer of hectorite by the one-pot hydrothermal method and the synthesized TiO2/hectorite composites exhibited a higher UV photocatalytic activity than commercial P25. The number of titanium ions entering into the hectorite layer was changed by adjusting the molar ratio of lithium and magnesium in the raw material. The materials synthesized in this study showed anatase phase TiO2, and the appropriate amount of lithium ion was beneficial to improve the crystallinity of the products according to XRD and FTIR analysis. SME and TEM studies indicated that TiO2/hectorite showed a looser structure after being pillared by TiO2 in comparison with hectorite, and the layer spacing of TiO2 was 0.353 nm. In the N2 adsorption-desorption analysis, TH-2 presented the largest specific surface area, the strongest adsorption capacity, and the best photocatalytic effect. UV-Vis DRS studies showed that the absorption band edge of TiO2/hectorite was redshifted and its UV absorption capacity became stronger after TiO2 incorporation, indicating the electron Ti–O transformation of TiO2. XPS analysis indicated that Li or Na ions of hectorite were easily replaced by titanium ions or hydrogen ions during the preparation process, which can promote the separation of e<sup>−</sup>-h+ pairs.

The results showed that a suitable ratio of lithium to magnesium is beneficial to the improvement of the photocatalytic effect. When the molar ratio of lithium, magnesium, and silicon was TH-2 of 1.32:5.34:8, the TiO2/hectorite photocatalyst had the highest removal rate of MB dye (97.8%). In addition, the TH-2 sample could be easily recycled and the removal rate of the MB still achieved 87.9% after five cycles, indicating good reusability. High specific surface area, strong light capture ability, and great e<sup>−</sup>-h+ separation efficiency are favorable for the promotion of photoactivity. The formation of oxygen vacancies and Ti3+ can also promote the separation of photogenic e<sup>−</sup> and h+. Therefore, TiO2/hectorite has good photoactivity and reusability as a UV photocatalyst to be used in the field of photodegradation of organic pollutants.

**Author Contributions:** Literature search and study design, D.Y. and J.C. (Jinyang Chen); investigation and data collection, D.Y., X.H. and J.C. (Jingying Cui); artwork and figures, D.Y.; writing—original draft preparation, D.Y.; writing—review and editing, J.C. (Jinyang Chen); data curation, D.Y. and L.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the Program for Innovative Research Team in University (No. IRT13078).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for the SEM and TEM tests.

**Conflicts of Interest:** The authors declare no conflict of interest.
