The pressing need to address the consequences of global warming has led to an increased focus on sustainable energy solutions. Solar energy, when harnessed in conjunction with carefully selected semiconductors, offers a promising avenue for photoelectrocatalytic water splitting, a process yielding hydrogen gas—heralded as the fuel of the future. The demand for semiconductors with photocatalytic and photoelectrocatalytic properties under solar illumination has surged, prompting numerous scientific endeavors to modify existing semiconductors by tuning their bandgap. Notably, photointercalation, a phenomenon exerting a significant influence on the photoelectrochemical and photocatalytic properties of layered materials, remains relatively unexplored. This project's primary goal is to investigate the impact of photointercalation on optical, structural, and, crucially, photoelectrochemical properties in layered materials, encompassing transition metal oxides and sulfides (WS2, WSe2, MoS2, MoSe2). The researcher has observed the adverse effects of photointercalation of alkali metal cations on the photoelectrochemical properties of MoO3 and has proposed a solution for mitigating the negative influence on WO3, documented in a reputable journal (Applied Catalysis B: Environmental, IF = 22.1). However, a comprehensive exploration across a broader spectrum of layered materials is essential to validate these proposed solutions. This project is not only dedicated to investigating the effects of photointercalation on a diverse range of layered materials but also aims to identify methods to alleviate its negative impact on photoelectrochemical properties. The research emphasizes two critical challenges: confirming the hypothesis that the photo-intercalation process influences the photoelectrochemical properties of layered materials and discovering methods to overcome the negative intercalation process during irradiation. Successful verification and modification of these methods will not only deepen our understanding of photointercalation's impact on layered semiconductors but also broaden the range of materials applicable in photoelectrochemical processes. This expansion holds great potential for advancements in optoelectronics, photocatalysis, photonics, and related fields. Indirect measures for GUT: 222 076 PLN