**4. Conclusions**

In this work, g-C3N4 nanosheets were exfoliated from bulk-g-C3N4 by thermal exfoliation in the presence of HNO3. The exfoliated g-C3N4 resulted in nanosheets with a large specific surface area and N vacancy defects, which can be prepared and further used in the fabrication of TiO2 base composites having superior photocatalytic activity, under UV-Vis light irradiation, to bulk the g-C3N4 and g-C3N4/TiO2 composites. Results show that the g-C3N4/TiO2 photocatalyst exhibited higher removal efficiency for IMI than g-C3N4 and TiO2, indicating that a synergistic effect exists between Ti3+-TiO2 and g-C3N4. With the increase of g-C3N4 loading, the photocatalytic activity of g-C3N4/TiO2 composites may also decrease the photocatalytic activity of g-C3N4. The sample of 0.5CNS/TiO2 showed the highest photocatalytic activity with 93% removal efficiency within 150 min. The enhanced photocatalytic performance of the g-C3N4/TiO2 composites could be due to the generation of reactive oxidation species induced by photogenerated electrons and the effective suppression of the recombination of the charge carriers. In addition, the g-C3N4/TiO2 photocatalyst showed good stability for multiple recycling. Thus, the g-C3N4/TiO2 could be effectively used as material for the photodegradation of imidacloprid pesticide in wastewater. Comprehensive photoelectrochemical analysis of the g-C3N4/TiO2 materials should be further studied to obtain the photogenerated charge recombination rates in detail.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal12020120/s1, Figure S1: Powder XRD patterns of 1CNS/TiO2, 4CNS/TiO2, 10CNS/TiO2, and 15CNS/TiO2, Figure S2: Photolysis of imidacloprid.

**Author Contributions:** Conceptualization, methodology, formal analysis, investigation, visualization, T.K. and S.M.S.; writing—original draft preparation, T.K. and J.T.; writing— review and editing, S.M.S.; supervision, T.A. and S.M.S.; writing—review and editing, S.M.S.; resources, P.K. and S.M.S.; funding acquisition, P.K. and S.M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** National Research Council of Thailand (Grant No. IRN62W0005).

**Data Availability Statement:** The data presented in this study are openly available in Mendeley repository at doi:10.17632/3v2mgmgzpz.1.

**Acknowledgments:** This work was partially supported by the National Research Council Thailand (Grant No. IRN62W0005). We thank Mahidol University Frontier Research Facility (MU-FRF) for instrument support and the MU-FRF scientists, Nawapol Udpuay and Suwilai Chaveanghong, for their kind assistance in Raman and FE-SEM analyses.

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