Photocatalysis is one of the most desired approaches for several energy production-related and sustainable applications, i.e., hydrogen production from water splitting or wastewater treatment. Nevertheless, it is crucial to use appropriate photocatalytic materials, endowed with specific features such as low charge carriers recombination, stability and activity in the visible range. The latter represents the most required property considering that 40% of sunlight is composed by visible light and the target of any photocatalytic application is the use of Sun as energetic source. Yet, the use of visible light-active materials is a countertrend compared to the current state of the art, which has seen TiO₂ as the most used photocatalyst. Even though it has high activity for many photocatalytic applications,^[1] TiO₂ suffers from a wide bandgap (3.0 eV)^[2] and consequently it is not active in visible light. Hence, our materials are in line with the tendency of research for new photocatalytic materials.

We chose transition metal hydroxides as target materials due to their interesting, layered morphology, which allows tunability of their bandgap up to the visible-light range.^[3] These materials suffer from radiation damage and high carriers recombination,^[4] so they need to be modified to be efficiently applied in photocatalysis. We synthesized different materials based on Co(OH)₂, i.e., Co₃O₄@Co(OH)₂ and Eu@Co(OH)₂ and tested them in photocatalytic hydrogen production from water splitting and degradation of microplastics and methylene blue. Comparing the results with the bare Co(OH)₂, we could appreciate the improvements obtained by modifying the material. Indeed, the stability under light and the catalytic activity were enhanced and the charge carriers recombination was diminished. Moreover, we studied the synthesis of a novel Ni carbonate hydroxide. In this case we could obtain for the first time a hierarchical structure of Ni₂(CO₃)(OH)₂·4H₂O with high potential for photocatalytic hydrogen evolution from water splitting. The materials presented were synthesized using a low-temperature approach and they were fully characterized to confirm our findings. Indeed, the UV-Visible spectra of the materials confirmed their low bandgap (2.0-2.4 eV) and the use of photocurrent and electron impedance spectroscopy showed the lower charge recombination compared to the bare hydroxides.