Enhancement of Photocatalytic Activity for Hydrogen Production of Nano-TiO2 Using Ru(II)-Phenantroline Derivatives

Abstract

Two novel Ru(II)-phenanthroline derivatives complexes, Ru-1 and Ru-2, were synthesized and characterized. The key distinction between Ru-1 and Ru-2 lies in their ligands: L1 (2-hydroxy-5-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl) benzoic acid) and L2 (2-hydroxy-3-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid). In L1, the –OH group is located in the para-position, while in L2, it resides in the ortho-position. Subsequently, Pt/TiO₂ and Ru-1/Pt/TiO₂ (and Ru-2/Pt/TiO₂) composites were prepared using photo-deposition and impregnation methods, respectively. The Ru-1/Pt/TiO₂ and Ru-2/Pt/TiO₂ composites were thoroughly characterized using various techniques, including ultraviolet-visible spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), fluorescence spectroscopy (FL), cyclic voltammetry (CV) experiments, and other relevant techniques. Photocatalytic hydrogen production systems were established by employing Ru-1/Pt-TiO₂ and Ru-2/Pt-TiO₂ as photocatalysts and ascorbic acid (H₂A) as a sacrificial reagent. The results demonstrated that the maximum hydrogen production reached 1461 μmol (Ru-1/Pt/TiO₂) and 843 μmol (Ru-2/Pt/TiO₂) under optimized conditions with 20 mg of composite photocatalyst, 0.3 mol L^–1 of H₂A, and pH 4, within 4 h of irradiation (λ > 420 nm). Correspondingly, the photocatalytic hydrogen production rates were 18 267 and 10 523 μmol g^–1 h^–1, respectively. Mechanism studies revealed that electrons flow from the highest occupied molecular orbital (HOMO) of Ru-1 to the conduction band (CB) of TiO₂, subsequently combining with H⁺ on the surface of the Pt metal nanoparticles to generate hydrogen gas. The holes on the lowest unoccupied molecular orbital (LUMO) of the photosensitizer are oxidized by H₂A, thereby regenerating the activity of the composite catalyst by restoring the photosensitizer.