A new dimension: The doping of amorphous poly(triazine imide) (PTI) through ionothermal copolymerization of dicyandiamide with 4-amino-2,6-dihydroxypyrimidine (4AP) results in triazine-based carbon nitrides with increased photoactivity for water splitting compared to crystalline poly(triazine imide) (PTI/Li(+)Cl(-), see picture) and melon-type carbon nitrides. This family of carbon nitride semiconductors has potential as low-cost, environmentally clean photocatalysts for solar fuel production.

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li(+)Cl(-)) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li(+)Cl(-) resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li(+)Cl(-) a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ(2)) of 5.035 and wRp=0.05937. PTI/Li(+)Cl(-) (P6(3)cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li(+) and Cl(-) ions. The presence of salt ions in the nanocrystallites as well as the existence of sp(2)-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using (15)N-labeled PTI/Li(+)Cl(-) proved the absence of heptazine building blocks and NH(2) groups and corroborated the highly condensed, triazine-based structure model.

Until recently, melam, [C(3)N(3)(NH(2))(2)](2)NH, has been regarded as a short-lived intermediate in the condensation process of melamine that is only detectable under special reaction conditions owing to its high reactivity. A new synthetic approach has allowed a closer look at the formation and condensation behavior of melam by using elevated ammonia pressure in autoclaves. Whereas the thermal treatment of dicyandiamide at 450 °C and 0.2 MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9 days) led to the formation of a melam-melem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 °C (24 h) yields melam hydrate, [C(3)N(3)(NH(2))(2)](2)NH⋅2 H(2)O (space group P2(1)/c; a=676.84(2), b=1220.28(4), c=1394.24(4) pm; β=98.372(2)°; V=1139.28(6)×10(6)  pm(3); Z=4), which crystallizes as a layered structure that is composed of almost-planar melam molecules, thereby forming ellipsoidal rosette-like motifs. The resulting voids are filled with four water molecules, thus forming a dense network of hydrogen bonds.

We have used a combination of X-ray and electron energy-loss spectroscopies along with DFT calculations to investigate the electronic structure of PTI/LiCl, a graphitic carbon nitride-type material with LiCl intercalation shown to photocatalyze the water-splitting reaction. The material is shown to have a band gap that is tunable with LiCl loading, with a minimum gap of 2.2 eV when fully loaded. This suggests that PTI/LiCl may be further optimized through control of their LiCl loading and shows that graphitic carbon nitride-type materials can be chemically tuned to improve their photocatalytic activity.

The development of catalysts that enable the direct conversion of solar energy into chemical energy has been defined as one of the major challenges of modern materials chemistry. Hydrogen generated by photochemical water splitting has been identified as a promising energy carrier that offers a high energy density while being environmentally clean.[1] Nevertheless, to realize a light-driven hydrogen-based economy, the exploration of new materials for highly efficient, stable, economically viable, and environmentally friendly photocatalysts is required.To date, numerous inorganic semiconductors have been developed for water splitting, most of them being transition metal compounds containing heavy metals such as La, Bi, Ta, or Nb, which impede scalability, increase cost, and add complexity.[2] Recently, attention has been attracted to a new class of metal-free photocatalysts, comprising polymeric melon-type carbon nitrides (CNs) based on imide-bridged heptazine units (see Figure 1 a). [3] CNs are readily accessible, lightweight, stable, and low-cost compounds that offer an attractive alternative to metal-rich catalysts while still maintaining efficient photoactivity.[4] Thermal condensation of CNs forms a wide variety of chemical species that differ substantially with respect to their degree of condensation, hydrogen content, crystallinity, and morphology. [5,6] The chemical modification of CNs by molecular "dopants" has resulted in a number of CN materials with improved photocatalytic activity. [7] Although the evidence is largely empirical, the property enhancement presumably originates from subtle modifications of the parent structures by incorporation of heteroatoms as well as structural defects, to give rise to enhanced absorption in the visible light range and a more complete exploitation of the solar energy spectrum.In contrast to all known CN photocatalysts, which are composed of heptazine building blocks, poly(triazine imide) (PTI/Li + Cl À ) is the only structurally characterized 2D CN network featuring imide-linked triazine units (see Figure 1 b). [8,9] Owing to its high level of crystallinity, PTI/ Li + Cl À lends itself as an excellent model system to study photocatalytic activity towards water splitting as a function of the number of building blocks, the composition, and the degree of structural perfection of the system. Herein, we present a new generation of CN photocatalysts based on triazine building blocks and demonstrate their enhanced photocatalytic activity in comparison to heptazine-based CNs. Moreover, we show that their performance can be amplified by small-molecule doping, thus rendering them the most active nonmetal photocatalysts for the hydrogen evolution reaction that have been reported to date.As a starting point, we synthesized crystalline PTI/Li + Cl À as a model structure for triazine-based CNs in a two-step ionothermal synthesis according to the procedure of Wirnhier et al. [8,9] To study the effect of crystallinity on the photocatalytic activity, we also synthesized an amorphous va...