Dye mixing effects on photocatalytic water splitting activity on dye-modified KTaO₃ catalyst

Dye mixing effects on water splitting activity of KTaO₃ photocatalyst modified by various porphyrinoids was investigated. Photocatalytic activity of dye-modified KTaO₃ catalyst is greatly improved by mixing various porphyrinoids, in which transition metals such as Cr, Fe, and Co are used as central metal. Pentametylene bis[4-(10, 15, 20-triphenyl porphine-5-yl) benzoate]-dizinc(II) (Zn-TPP dimer) showed positive effect on photocatalytic activity, and H₂ and O₂ formation rates are 365 μmol g^-1_-cat. h^-1 and 152 μmol gg^-1_-cat. h^-1, respectively. On the other hand, the combination of chloroprotoporphyrinato iron (III) (chlorohemin) with Cr-phthalocyanine (CrPc) is the most effective for increasing water splitting activity, and the formation rates of H₂ and O₂ are 2.12 mmol gg^-1_-cat. h^-1 and 1.11 mmol gg ^-1_-cat.h^-1, respectively. Energy conversion efficiency of this photocatalyst is estimated to be approximately 0.05%. From X-ray photoelectron spectroscopy measurement and cyclic voltammetry of organic dyes, it is suggested that the electronic state of the dye mixture is modified compared with that of a single dye. Energy transfer between mixing dyes is observed in the fluorescence spectra of dye-modified KTaO₃ photocatalysts. Recombination of photoexited charge in KTaO₃ is clearly suppressed by dye mixing, thus improvement of photocatalytic activity is attributed to the lengthening of excitation lifetime. This study reveals that mixing porphyrinoid dyes is an effective method for increasing water splitting activity of dyemodified KTaO₃ photocatalysts.