Role of phosphate in ruthenium-complex-sensitized TiO2 system for hydrogen production: Mechanism and kinetics

Most Ru(II) complex-sensitized TiO2 systems for hydrogen (H-2) production suffer from instability of the photosensitized system because the anchoring groups of Ru(II) dyes, which are required for their adsorption on TiO2, are intrinsically vulnerable to chemical and photochemical cleavage. In this study, a new method that enables the use of a Ru(II) dye without any anchoring groups (Ru(bpy)(3)(2+)) was developed and investigated. The stable photocatalytic efficiency in repeated H-2 production cycles under visible-light irradiation indicates that the Ru(II) dye without anchoring groups is highly stable during dye-sensitized H-2 production. The dye-sensitized H-2 production in the Ru(bpy)(3)(2+)-sensitized TiO2 system comprising Ru(bpy)(3)(2+) as a photosensitizer, platinized TiO2 (Pt-TiO2) as a cocatalyst-electron mediator, and ethylenediaminetetraacetic acid as an electron donor was negligible. However, the addition of phosphate (PO43-) to the Ru(bpy)(3)(2+)-sensitized TiO2 system enabled the production of H-2 via dye sensitization in the absence of any anchoring groups on the dye. The adsorption of PO(4)(3-)changed the surface charge of Pt-TiO2 from positive to negative under acidic conditions, thereby inducing adsorption of cationic Ru(bpy)(3)(2+) on the surface of Pt-TiO2 and facilitating electron transfer from excited Ru (bpy)(3)(2+) to the conduction band of TiO2. The PO(4)(3-)adsorption-induced change in the surface charge and the subsequent adsorption of Ru(bpy)(3)(2+) on the surface of PO43--adsorbed Pt-TiO2 were confirmed by zeta potential measurements and Fourier transform infrared spectroscopy, respectively. In contrast with H-2 production, the presence of PO(4)(3-)had little effect on the kinetics of anionic chromate (CrO42-) reduction in the Ru(bpy)(3)(2+)sensitized TiO2 system. This result indicates that electron transfer from Pt to the electron acceptor on PO(4)(3-)adsorbed Pt-TiO2 is highly dependent on the charge character of the electron acceptor (i.e., electron transfer to the cationic electron acceptor is more favored). The negative charge on the surface of Pt-TiO2 induced by the adsorption of PO(4)(3-)attracts the positively charged protons to the surface, which kinetically enhanced electron transfer from Pt to the protons. The (photo) electrochemical data demonstrate that PO(4)(3-)adsorbed on Pt-TiO2 facilitates the interfacial electron transfer processes by enhancing the adsorption of Ru(bpy)(3)(2+) and attracting protons to the surface. The positive effect of PO(4)(3-)on H-2 production increased with increasing PO(4)(3-)concentration and decreasing pH, where the conditions are more favorable for PO(4)(3-)and proton adsorption on the surface of Pt-TiO2. Among the five anions evaluated in this study (PO43-, AsO43-, F-, NO3-, and SO42-), PO(4)(3-)was most efficient and facilitated stable H-2 production.