Photocatalytic　N2　fixation　is　a　green　process　for　ammonia　synthesis　by　converting　N2　and　H2O　to　NH3　and　O2　directly　using　solar　energy.　Its　efficiency　is　largely　limited　by　the　chemisorption　and　activation　of　N2.　This　work　adopts　defect　engineering　strategy　to　develop　a　series　of　MIL-68(Fe)　MOFs　with　varying　concentrations　of　defects　by　doping　Cu2+　on　the　metal　nodes　of　MOFs.　Upon　Cu2+　doping,　a　larger　number　of　oxygen　vacancy　defects　are　created　due　to　the　induced　crystal　distortion　to　form　coordinatively　unsaturated　Fe2+　sites,　which　can　serve　as　the　active　sites　for　promoting　the　chemisorption　and　activation　of　N2.　The　photogenerated　holes　oxidize　H2O　to　O2　and　H+,　and　the　photogenerated　electrons　combine　with　formed　H+　to　reduce　the　activated　N2　to　NH3.　It　was　found　that　the　sample　with　10　mol%　Cu2+　doping　shows　the　highest　NH3　production　rate　(21.0　μmol·g−1·h−1),　which　is　8.4　times　higher　than　that　(2.50　μmol·g−1·h−1)　of　the　pristine　MIL-68(Fe).　The　excellent　performance　is　ascribed　to　the　adequate　Fe2+　active　sites　to　chemisorb　and　activate　N2　and　the　optimal　mobility　of　photogenerated　charges.　Finally,　a　mechanism　is　proposed　to　illustrate　how　the　number　of　defects　affects　the　photocatalytic　N2　fixation　performance　at　the　molecular　level.　©　2024　Elsevier　Inc.