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.

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...

Abstract. Tuning the chemical composition and structure for targeted functionality in two-dimensional (2D) nanosheets has become a major objective in the rapidly growing area of 2D materials. In the context of photocatalysis, both miniaturization and extending the light absorption of UV active photocatalysts are major assets. Here, we investigate the solid solution between two photocatalytic systems known from literature to evolve H 2 from water/methanol under UV -RbCa 2 Nb 3 O 10 (E g = 3.7 eV) -and visible light irradiation -RbPb 2 Nb 3 O 10 (E g = 3.0 eV) -by synthesizing hypothetical RbCa 2-x Pb x Nb 3 O 10 . While the calcium niobate can easily be exfoliated into individual nanosheets via cation-proton exchange and subsequent treatment with tetra-n-butyl-