Hematite　is　a　promising　candidate　as　photoanode　for　solar-driven　water　splitting,　with　a　theoretically　predicted　maximum　solar-to-hydrogen　conversion　efficiency　of　∼16%.　However,　the　interfacial　charge　transfer　and　recombination　greatly　limits　its　activity　for　photoelectrochemical　water　splitting.　Carbon　dots　exhibit　great　potential　in　photoelectrochemical　water　splitting　for　solar　to　hydrogen　conversion　as　photosensitisers　and　co-catalysts.　Here　we　developed　a　novel　carbon　underlayer　from　low-cost　and　environmental-friendly　carbon　dots　through　a　facile　hydrothermal　process,　introduced　between　the　fluorine-doped　tin　oxide　conducting　substrate　and　hematite　photoanodes.　This　led　to　a　remarkable　enhancement　in　the　photocurrent　density.　Owing　to　the　triple　functional　role　of　carbon　dots　underlayer　in　improving　the　interfacial　properties　of　FTO/hematite　and　providing　carbon　source　for　the　overlayer　as　well　as　the　change　in　the　iron　oxidation　state,　the　bulk　and　interfacial　charge　transfer　dynamics　of　hematite　are　significantly　enhanced,　and　consequently　led　to　a　remarkable　enhancement　in　the　photocurrent　density.　The　results　revealed　a　substantial　improvement　in　the　charge　transfer　rate,　yielding　a　charge　transfer　efficiency　of　up　to　80%　at　1.25　V　vs.　RHE.　In　addition,　a　significant　enhancement　in　the　lifetime　of　photogenerated　electrons　and　an　increased　carrier　density　were　observed　for　the　hematite　photoanodes　modified　with　a　carbon　underlayer,　confirming　that　the　use　of　sustainable　carbon　nanomaterials　is　an　effective　strategy　to　boost　the　photoelectrochemical　performance　of　semiconductors　for　energy　conversion.　©　2020　The　Royal　Society　of　Chemistry.