Modification effects of porphyrin dyes on the photocatalytic activity of tantalum (oxy)nitride (TaON) were investigated. The nitrogen content in tantalum (oxy)nitride was increased by increasing the heat treatment period. The optimized nitridation conditions were found to be calcination at 800 °C for 14 h under a NH₃ gas flow (25 mL min⁻¹). Among the porphyrin dyes examined, pentamethylene bis[4-(10,15,20-triphenylporphine-5-yl) benzoate]-dizinc (II) (Zn-TPPD) showed the most positive effect on the photocatalytic activity of TaON for H₂ production from Na₂S aqueous solution. From the results of the photocatalytic reaction using various combinations of catalyst components, it was found that the modification dye and PtO_x co-catalysts were necessary to achieving photocatalytic H₂ formation. In the PtO_x/Zn-TPPD/TaON photocatalyst, the expected charge transfer mechanism was a two-step excitation of both TaON and Zn-TPPD, and the oxidation and reduction sites were TaON and PtO_x co-catalyst, respectively. These results indicate that dye modification has the potential to improve the photocatalytic activity of various (oxy)nitride photocatalysts.

Sr_2−xBa_xTa₃O_10−yN_z (x = 0.0, 0.5, 1.0) nanosheets were prepared by exfoliating layered perovskite compounds (CsSr_2−xBa_xTa₃O_10−yN_z). The Sr_1.5Ba_0.5Ta₃O_9.7N_0.2 nanosheet showed the highest photocatalytic activity for H₂ production from the water/methanol system among the Sr_2−xBa_xTa₃O_9.7N_0.2 nanosheets prepared. In addition, Rh-loaded Sr_1.5Ba_0.5Ta₃O_9.6N_0.3 nanosheet showed the photocatalytic activity for oxygen and hydrogen production from water. The ratio of hydrogen to oxygen evolved was around two. These results indicate that the Rh-loaded Sr_1.5Ba_0.5Ta₃O_9.6N_0.3 nanosheet is a potential catalyst for photocatalytic water splitting.

Artificial photosynthesis is one of the chemists’ dreams and the separation of charges with long lives is fundamental for achieving artificial photosynthesis. In actual photosynthesis, Z-scheme excitation separates electronic charge with high efficiency using solar light. Here we report that photo-excitation in Cr-tetraphenylporphyrin chroride (Cr-TPPCl)/Zr doped KTaO₃ (KTa(Zr)O₃) is analogous to the Z-scheme in photosynthesis, and that we achieve complete charge separation at room temperature. Photovoltaic decay and transient fluorescence spectra measurements showed that the photo-excited charge in KTa(Zr)O₃ transferred to the HOMO of Cr-TPPCl within a few hundred pico-seconds on charge transfer. In contrast, the reduced state of the Cr-TPPCl species that was formed by the electronic injection from KTa(Zr)O₃ was observed for more than 0.5 s at room temperature in the transient decay of the absorption spectra change after the initial excitation of KTa(Zr)O₃. The formed reduced state of Cr-TPPCl was highly stable and was detected by static ESR measurements.

Photocatalytic water splitting is one of the ideal methods for solving the global energy crisis and its associated environmental problems. In this study, the effect of altering the molecular structure of porphyrins was investigated to improve the water splitting activity of Zr-doped KTaO₃ (KTa(Zr)O₃) modified with porphyrin dyes. UV-vis spectra indicated that porphyrins with long alkoxy chains tended to form well-developed H-aggregates on the KTa(Zr)O₃ surface. The photocatalytic activity of Pt-loaded KTa(Zr)O₃ was improved by using porphyrins with longer alkoxy chains because of the improvement in the charge migration between porphyrin dye molecules. While the charge transfer between the inorganic semiconductor and porphyrin dye interface is important, it was found that the formation of H-aggregation was more effective in improving the water splitting activity of the porphyrin-modified photocatalysts.