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

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.