Ionothermal　synthesis　of　amorphous　melon　in　a　ternary　salt　melt　(ZnCl2-LiCl-KCl)　with　lower　melting　points　could　promote　the　polycondensation　process　and　improve　the　degree　of　crystallinity.　As　Zn2+　ions　tend　to　incorporate　with　edge　nitrogen　of　carbon　nitride　via　Lewis　acid-base　interactions,　a　series　of　crystalline　hybrid　poly　heptazine　imides　(PHI),　namely,　Zn-PHI/PHI,　integrated　by　van　der　Waals　interactions,　were　generated　as　main　products.　Interestingly,　the　contents　of　doped　Zn2+　ions　of　the　as-synthesized　Zn-PHI/PHI　gradually　decrease　from　the　bulk　interior　to　surfaces,　due　to　the　evaporation　of　ZnCl2　during　thermal　heating　processes.　Accordingly,　a　built-in　electric　field　would　be　created　in　the　catalyst,　which　largely　accelerates　a　separation　of　the　photogenerated　charge　carriers　at　the　interfaces.　Owing　to　the　fast　charge　carrier　separation　and　transfer,　this　hybrid　polymer　is　active　for　photocatalytic　water　oxidation　with　an　apparent　quantum　yield　of　up　to　3.6%　at　420　nm.　This　work　offers　a　strategy　for　the　rational　design　and　synthesis　of　polymeric　photocatalysts　for　water　oxidation　reaction.　©　2021　American　Chemical　Society