| Summary: | In this study, three types of Nasicon-type materials, Co<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-CO<sub>2</sub>P<sub>2</sub>O<sub>7</sub>, Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Ni<sub>2</sub>P<sub>2</sub>O<sub>7</sub>, and Cu<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Cu<sub>2</sub>P<sub>2</sub>O<sub>7</sub>, were synthesized as mixed-phase catalysts (MPCs) for evaluating their potential as new photocatalytic candidates (called Co<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-CO<sub>2</sub>P<sub>2</sub>O<sub>7</sub>mpc, Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Ni<sub>2</sub>P<sub>2</sub>O<sub>7</sub>mpc, and Cu<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Cu<sub>2</sub>P<sub>2</sub>O<sub>7</sub>mpc herein). Based on various physical properties, it was confirmed that there are two phases, M<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> and M<sub>2</sub>P<sub>2</sub>O<sub>7</sub>, in which a similar phase equilibrium energy coexists. These colored powders showed UV and visible light responses suitable to our aim of developing 365-nm light-response photocatalysts for overall water-splitting. The photocatalytic performance of Ni<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>-Ni<sub>2</sub>P<sub>2</sub>O<sub>7</sub> MPC showed negligible or no activity toward H<sub>2</sub> evolution. However, Co<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>-Co<sub>2</sub>P<sub>2</sub>O<sub>7</sub> MPC and Cu<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Cu<sub>2</sub>P<sub>2</sub>O<sub>7</sub> MPC were determined as interesting materials because of their ability to absorb visible light within a suitable band. Moreover, an internal interface charge transfer was suggested to occur that would lower the recombination rate of electrons and holes. For Cu<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-Cu<sub>2</sub>P<sub>2</sub>O<sub>7</sub> MPC, the charge separation between the electron and hole was advantageously achieved, a water-splitting reaction was promoted, and hydrogen generation was considerably increased. The performance of a catalyst depended on the nature of the active metal added. In addition, the performance of the catalyst was improved when electrons migrated between the inter-phases despite the lack of a heterojunction with other crystals.
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