Poly　(triazine　imide)　(PTI/Li+Cl−),　one　of　the　crystalline　versions　of　polymeric　carbon　nitrides,　holds　great　promise　for　photocatalytic　overall　water　splitting.　In　principle,　the　photocatalytic　activity　of　PTI/Li+Cl−　is　closely　related　to　the　morphology,　which　could　be　reasonably　tailored　by　the　modulation　of　the　polycondensation　process.　Herein,　we　demonstrate　that　the　hexagonal　prisms　of　PTI/Li+Cl−　could　be　converted　to　hexagonal　nanosheets　by　adjusting　the　binary　eutectic　salts　from　LiCl/KCl　or　NaCl/LiCl　to　ternary　LiCl/KCl/NaCl.　Results　reveal　that　the　extension　of　in-plane　conjugation　is　preferred,　when　the　polymerisation　was　performed　in　the　presence　of　ternary　eutectic　salts.　The　hexagonal　nanosheets　bears　longer　lifetimes　of　charge　carriers　than　that　of　hexagonal　prisms　due　to　lower　intensity　of　structure　defects　and　shorter　hopping　distance　of　charge　carriers　along　the　stacking　direction　of　triazine　nanosheets.　The　optimized　hexagonal　nanosheets　exhibits　a　record　apparent　quantum　yield　value　of　25　%　(λ=365　nm)　for　solar　hydrogen　production　by　one-step　excitation　overall　water　splitting.　©　2023　Wiley-VCH　GmbH.