Photocatalytic　water　splitting　is　considered　as　one　of　the　promising　ways　to　provide　clean　fuels.　Extensive　efforts　have　been　made　in　the　past　to　develop　various　inorganic　and　organic　materials　systems　as　photocatalysts　for　water　splitting　by　using　visible　light.　Among　these　photocatalysts,　it　was　recently　demonstrated　that　incorporation　of　carbon　dots　(CDs)　into　graphitic　carbon　nitride　(g-C3N4)　results　in　new　metal-free　composites　(g-C3N4-C)　with　excellent　stability　and　impressive　performance　for　photocatalytic　water　splitting.　However,　fundamental　questions　still　remain　to　be　addressed　such　as　how　added　CDs　influence　the　photocatalytic　reaction　through　the　bandgap　tunability,　charge　transfer　route　and　efficiency,　as　well　as　the　specific　function　of　CDs　in　the　photocatalytic　process.　Understanding　the　chemical　and　physical　behaviors　of　added　CDs　for　the　control　of　g-C3N4-C　architecture　is　a　critical　need　for　its　emergence　from　fundamental　design　of　more　efficient　photocatalysts　to　practical　applications.　In　this　article,　we　report　new　materials　with　well-controlled　architecture　allowing　for　fine-tuning　band　gaps　for　broader　visible　light　absorption　and　controlled　understanding　of　the　photocatalytic　process.　The　well-defined　model　materials　allow　us　to　address　the　fundamental　question　regarding　chemically　bonding　of　CDs　and　how　the　chemical　bonded　CDs　promote　charge　separation　and　transfer　for　highly　efficient　generation　of　H-2.