Synthesis of a carbon nitride structure for visible-light catalysis by copolymerization.

and nonmetallic elements (N, C, B) creates localized/ delocalized states in the band gap and thus extends its optical absorption to the visible region, but doping usually comes with accelerated charge recombination and lower stability of the doped materials. Meanwhile, various other inorganic, non-TiO2-based, visible-light catalysts have been developed (e.g., metal oxides, nitrides, sulfides, phosphides, and their mixed solid solutions), whereby Ga, Ge, In, Ta, Nb, and W are the main metal constituents. However, sustained utilization of solar energy calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far. Recently, a polymeric semiconductor on the basis of a defecteous graphitic carbon nitride (g-C3N4), was introduced as a metal-free photocatalyst which fulfills the basic requirements for a water-splitting catalyst, including being abundant, stable, and responsive to visible light. In the following, we use the notation “g-C3N4” to describe this class of materials rather than the idealized structure. The most active system is in fact presumably an N-bridged “poly(tri-s-triazine)”, already described by Liebig as “melon”. A semiconductor structure with band edges straddling the water redox potential was revealed for melon by DFT calculations, albeit electrochemical analysis is still awaited. g-C3N4 is considered to be the most stable phase of covalent carbon nitride, and facile synthesis of the melon substructure from simple liquid precursors and monomers allows easy engineering of carbon nitride materials to achieve the desired nanostructures via soft-chemical processing routes and methods. For instance, a high surface area (67–400 mg ) can be imparted on g-C3N4 materials by polymerization of cyanamide on a silica template, which results in photocatalytically more active g-C3N4 nanostructures. [8] Metal-doped gC3N4 can also be conveniently obtained by polymerization of dicyandiamine in the presence of metal salts, and thus multifunctionalization of such materials for a variety of applications can be achieved. Most importantly, the electronic and optical properties of carbon nitride, regarded as a polymer semiconductor, are in principle adjustable by organic protocols. Such organic protocols have been widely used to control the performance of traditional p-conjugated polymers, for example, to improve solar-cell efficiencies by constructing copolymerized donor–acceptor structures, or to modify electronic properties by co-blending with p/n-type organic dopants. Our aim was to use such organic modifications to extend the insufficient light absorption of g-C3N4 (a result of its large band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum. Here we demonstrate that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA). The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for hydrogen production from water containing an appropriate sacrificial reagent with visible light. In principle, BA can be directly incorporated into the classical carbon nitride condensation scheme (Scheme 1). New carbon nitride structures were therefore synthesized by dissolving dicyandiamide with different amounts of BA in water, followed by thermally induced copolymerization at 823 K. For simplicity, the resulting samples are denoted CNBx, where x (0.05, 0.1, 0.2, 0.5, 1, 2) refers to the weighedin amount of BA. The structure, texture, and electrochemical properties of these materials were characterized, and their photochemical performance analyzed. Their XRD patterns (Figure S1, Supporting Information) are dominated by the characteristic (002) peak at 27.48 of a graphitic, layered structure with an interlayer distance of d = [*] J. Zhang, X. Chen , Prof. X. Fu, Prof. X. Wang State Key Laboratory Breeding Base of Photocatalysis Fuzhou University, Fuzhou 350002 (China) E-mail: xcwang@fzu.edu. cn