The　solar-to-hydrogen　conversion　efficiency　in　photocatalytic　water　splitting　heavily　depends　on　the　accumulation　of　multiple　electrons　at　the　catalytically　active　sites　and　rapid　charge　transport/separation.　Herein,　we　demonstrate　the　construction　of　0D-2D　nickel-doped　and　Ti3C2TX　MXene　(MN)-encapsulated　transition　metal　chalcogenide　quantum　dot　(TMC　QD:Ni)/Ti3C2TX　MN　heterostructures　via　an　elaborate　electrostatic　self-assembly　strategy.　The　mechanistic　studies　revealed　that　the　defects　induced　by　atomic-level　foreign　metal　ion　doping　create　a　mid-bandgap　state,　which　broadens　the　optical　absorption　range　and　extends　the　photo-excited　carrier　lifetime　of　the　TMC　QDs.　The　density　functional　calculation　results　verified　that　Ni2+　ion　doping　introduces　a　donor　impurity　level　and　increases　the　density　of　state　at　the　valence　band　maximum,　leading　to　a　significant　increase　in　the　number　of　active　sites　and　lower　energy　barrier　for　photocatalytic　hydrogen　evolution.　The　subsequent　self-assembly　of　TMC　QDs:Ni　on　the　Ti3C2TX　MN　framework　further　accelerates　the　charge　separation　and　transfer　due　to　the　formation　of　an　ideal　unidirectional　electron　migration　pathway　by　Ti3C2TX　MN,　which　functions　as　an　electron-withdrawing　mediator.　The　synergistic　effect　of　Ni2+　ion　doping　and　Ti3C2TX　MN　decoration　significantly　decreases　the　charge　transfer　resistance　at　the　photosensitizer　(TMC　QD)/co-catalyst　(Ti3C2TX　MN)　interface　and　promotes　the　chemisorption　of　protons　on　the　catalyst　surface,　resulting　in　an　excellent　solar-to-hydrogen　conversion　efficiency.　Our　work　provides　valuable　guidance　for　the　rational　design　of　high-efficiency　photocatalysts　via　precise　atomic-level　metal　ion　doping　and　co-catalyst　modulation　towards　emerging　artificial　photosynthesis.　©　2022　The　Royal　Society　of　Chemistry.