Photocatalytic　water　splitting　represents　a　promising　approach　for　solar　energy　conversion　and　hydrogen　production.　Polymeric　carbon　nitride　(PCN)　exhibits　potential　but　suffers　from　charge　recombination　and　low　specific　surface　area.　This　study　introduces　a　novel　LaOBr@PCN　one-photon　excited　heterojunction　designed　to　mitigate　these　challenges.　By　anchoring　Melon,　a　PCN　precursor,　onto　LaOBr　through　acid-base　coordination　followed　by　in-situ　thermal　polymerization,　three-dimensional　hierarchical　PCN　nanosheets　are　fabricated　on　LaOBr　microplates.　This　structure　increases　the　specific　surface　area,　exposes　more　active　sites,　enhances　light-harvesting　ability,　and　improves　charge　carrier　separation.　Consequently,　the　optimized　LaOBr@PCN　achieves　hydrogen　and　oxygen　production　rates　of　69.9　μmol/h　and　34.7　μmol/h,　respectively.　This　work　provides　a　viable　strategy　for　developing　advanced　PCN-based　composite　photocatalysts.　©　2025　Elsevier　Inc.