Activation of a Pt-loaded Pb₂Ti₂O_5.4F_1.2 photocatalyst by alkaline chloride treatment for improved H₂ evolution under visible light

Pb₂Ti₂O_5.4F_1.2 is a new photocatalyst that has recently been reported to function under visible light with suitable stability, albeit with insufficient photocatalytic activity. The present work demonstrates that the photocatalytic activity of Pb₂Ti₂O_5.4F_1.2 during visible-light-driven H₂ evolution is improved following post-treatment with alkaline chlorides. The photocatalytic activity of the post-treated Pb₂Ti₂O_5.4F_1.2 was found to be greatly affected by both the type of alkaline chloride and the treatment temperature. Among the alkaline chlorides examined, a eutectic NaCl-CsCl mixture (35 : 65 molar ratio) was found to be the most effective. The optimized material, further modified with a Pt cocatalyst using an in situ photodeposition method with a H₂PtCl₆ precursor, showed approximately 10 times higher activity than that of an untreated analogue. The optimally modified Pb₂Ti₂O_5.4F_1.2 was also found to outperform Ru-loaded, Rh-doped SrTiO₃, which is one of the most active metal oxide photocatalysts workable under visible light. Transient absorption spectroscopy indicated that the reactivity of photogenerated free and/or shallowly trapped electrons in the Pt-loaded, NaCl-CsCl-treated Pb₂Ti₂O_5.4F_1.2 with H₂O was higher than that in an analogue not treated with the NaCl-CsCl mixture. Based on various physicochemical analyses, it is evident that the NaCl-CsCl treatment creates ion-exchangeable alkaline-titanate species on the catalyst surface that undergo proton-exchange during the photodeposition of Pt, leading to improved affinity for H₂O. This effect explains the high activity of the NaCl-CsCl-treated Pb₂Ti₂O_5.4F_1.2 during H₂ evolution under visible light.