Synthesis and photocatalytic evaluation of first nickel dioxo-haloselenate complexes immobilized on mesoporous TiO2 for enhanced solar-driven hydrogen production

Abstract

[NiBr(SeO₂Br)(bipy)₂] (1), [NiBr(SeO₂Br)(phen)₂] (2), and [NiCl(SeO₂Cl)(bipy)₂] (3), represent the first nickel complexes incorporating dioxo-haloselenate anions. These complexes were selected due to their unique structural features, and light absorption properties, which make them promising candidates for photocatalytic applications. The compounds were immobilized onto mesoporous TiO₂ (m-TiO₂), resulting in photocatalysts 1@m-TiO₂, 2@m-TiO₂, and 3@m-TiO₂, which were evaluated for solar-driven water splitting to produce hydrogen. Electrochemical analyses revealed suitable HOMO–LUMO energy levels for visible-light activation, and X-ray photoelectron spectroscopy confirmed changes in surface composition upon immobilization. Among the photocatalysts, 2@m-TiO₂ exhibited the highest H₂ production, achieving 927 μmol g⁻¹ in 6 hours, outperforming both 1@m-TiO₂ (803 μmol g⁻¹) and 3@m-TiO₂ (810 μmol g⁻¹), as well as reference materials m-TiO₂ and Degussa P25. Moreover, complex 1 demonstrated intrinsic electrochemical activity for H₂ evolution in acidic media. Recycling experiments showed the stability of 2@m-TiO₂ over three cycles, confirming its robustness. These findings not only demonstrate the effective cocatalytic role of complexes 1–3 in enhancing H₂ generation but also shed light on interfacial photo-induced processes via HOMO–LUMO evaluation and XPS analysis. This study introduces the first example of nickel-based dioxo-haloselenate complexes as cocatalysts, opening new avenues for the development of sustainable hydrogen-evolving materials.