Polymeric　carbon　nitride　has　been　widely　developed　as　a　promising　photocatalyst　for　solar　hydrogen　production　via　photocatalytic　water　splitting.　However,　pristine　carbon　nitride　prepared　by　traditional　solid-state　polymerization　usually　encounters　issues　such　as　rapid　carrier　recombination　and　insufficient　absorption　of　visible　light　below　460　nm.　Herein,　poly(heptazine　imide)　with　a　distinctive　nanoplate　structure　was　synthesized　in　a　binary　molten　salt　of　NaCl–CaCl2.　The　salt　template　allows　the　formation　of　the　thin　nanoplate　structure,　which　promotes　the　charge　separation　and　migration.　Besides,　the　intercalation　of　Ca2+　ions　between　the　conjugated　layers　endows　the　activation　of　n–π*　electron　transition　due　to　the　distortion　of　in-plane　heptazine　layers.　Accordingly,　the　optimized　poly(heptazine　imide)　nanoplates　achieve　an　apparent　quantum　efficiency　of　up　to　17.3%　at　500　nm　for　photocatalytic　hydrogen　production　from　water.　This　work　shares　new　idea　for　rational　control　of　the　optical　absorption　and　charge　carrier　dynamics　of　poly(heptazine　imide).　©　Science　China　Press　2024.