Compact carbon nitride based copolymer films with controllable thickness for photoelectrochemical water splittingElectronic supplementary information (ESI) available. See DOI: 10.1039/c7ta06081e

We report the successful growth of high quality carbon nitride based copolymer (CNBC) films on various substrates (glass, FTO, and TiO 2 ) 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-C 3 N 4 powder. Furthermore, we demonstrate that a similar solvothermal approach can be also used to create CNBC films on a TiO 2 surface, and the resulting CNBC/TiO 2 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. Carbon nitride based copolymer films consisting of tri- s -triazines and s -triazines are in situ grown on various substrates and exhibit excellent photoelectrochemical performance, which is attributed to strong visible light harvesting transport.