We　report　the　successful　growth　of　high　quality　carbon　nitride　based　copolymer　(CNBC)　films　on　various　substrates　(glass,　FTO,　and　TiO2)　through　a　facile　solvothermal　method　with　post-annealing.　The　structural　characterization　of　the　films　reveals　the　polymeric　carbon　nitride　network　consisting　of　tri-s-triazines　and　s-triazines　bridged　by　different　functional　groups.　The　strong　bonding　between　the　CNBC　films　and　the　substrates　via　covalent　linkage　ensures　intimate　contact　and　smooth　interfacial　charge　transport.　As　such,　the　CNBC　films　exhibit　excellent　photoelectrochemical　(PEC)　performance,　which　is　attributed　to　stronger　visible　light　harvesting,　better　conductivity,　and　more　efficient　charge　separation　and　transport,　as　compared　to　a　reference　electrode　made　by　pasting　bulk　g-C3N4　powder.　Furthermore,　we　demonstrate　that　a　similar　solvothermal　approach　can　be　also　used　to　create　CNBC　films　on　a　TiO2　surface,　and　the　resulting　CNBC/TiO2　junction　allows　for　vectorial　charge　transfer,　giving　greatly　enhanced　photocurrent　responses.　Importantly,　this　work　provides　a　general　guide　to　solvothermal　methods　for　in　situ　growing　metal-free　films　on　various　substrates　for　efficient　PEC　water　splitting.　©　2017　The　Royal　Society　of　Chemistry.