**4. Conclusions**

In this review, information on the main mechanisms of water splitting and decomposition of organic compounds by TiO2-based photocatalysts was collected and analyzed. The e fficiency of TiO2 doping by various components, which significantly increases the photocatalytic activity, is shown. In addition, other factors, such as the bandgap, charge carrier recombination, the use of sacrificial agents, the size and type of photocatalyst structures, surface morphology, photocatalyst concentration in solution, radiation source power, etc., which can a ffect the photocatalysis were analyzed. Despite the reports of scientists on the development of e ffective photocatalysts capable of operating under sunlight, their use is still not economically viable. Therefore, due to the growth of environmental problems and the limited availability of hydrocarbon fuels, investigations in the field of hydrogen production by "artificial photosynthesis" will undoubtedly increase.

**Author Contributions:** Conceptualization, B.B. and C.D.; writing—original draft preparation, F.S., R.B., A.B., I.C. and B.B.; writing—review and editing, A.U. and A.M.; project administration, R.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by Ministry of Education and Science of the Republic of Kazakhstan № AP05135273. The name of the focal area of the development of science, on which the application is submitted: Energy and Machine industry. The name of the specialized research area, on which the application is filed, type of study: Alternative energy and technology: renewable energy sources, nuclear and hydrogen energy, other energy sources, applied research.

**Acknowledgments:** Askhat Bekbaev is grateful to al-Farabi Kazakh National University for financial support in the frame of the postdoctoral fellowship program.

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