Structural Insights into Poly(Heptazine Imides): A Light-Storing Carbon Nitride Material for Dark Photocatalysis

Abstract

Solving the structure of carbon nitrides has been a long-standing challenge due to the low crystallinity and complex structures observed within this class of earth-abundant photocatalysts. Herein, we report on two-dimensional layered potassium poly(heptazine imide) (K-PHI) and its proton-exchanged counterpart (H-PHI), obtained by ionothermal synthesis using a molecular precursor route. We present a comprehensive analysis of the in-plane and three-dimensional structure of PHI. Transmission electron microscopy and solid-state NMR spectroscopy, supported by quantum-chemical calculations, suggest a planar, imide-bridged heptazine backbone with trigonal symmetry in both K-PHI and H-PHI, whereas pair distribution function analyses and X-ray powder diffraction using recursive-like simulations of planar defects point to a structure-directing function of the pore content. While the out-of-plane structure of K-PHI exhibits a unidirectional layer offset, mediated by hydrated potassium ions, H-PHI is characterized by a high degree of stacking faults due to the weaker structure directing influence of pore water. Structure–property relationships in PHI reveal that a loss of in-plane coherence, materializing in smaller lateral platelet dimensions and increased terminal cyanamide groups, correlates with improved photocatalytic performance. Size-optimized H-PHI is highly active toward photocatalytic hydrogen evolution, with a rate of 3363 μmol/gh H2 placing it on par with the most active carbon nitrides. K- and H-PHI adopt a uniquely long-lived photoreduced polaronic state in which light-induced electrons are stored for more than 6 h in the dark and released upon addition of a Pt cocatalyst. This work highlights the importance of structure–property relationships in carbon nitrides for the rational design of highly active hydrogen evolution photocatalysts.

Green radicals of potassium poly(heptazine imide) using light and benzylamine

“…S2a †). 20,21 The valence band maximum (VBM) in K-PHI determined by ultraviolet photoelectron spectroscopy (UPS) is located at $2.5 V ( Fig. S2e †).…”

Cobalt‐Exchanged Poly(Heptazine Imides) as Transition Metal–Nx Electrocatalysts for the Oxygen Evolution Reaction

“…While in the former heptazine units are connected by amine bridges, in PHI the heptazine units are linked by deprotonated imides, whose charge is counterbalanced by metal cations introduced by the salt template . The negatively charged nitrogen sites not only remarkably enhance the conductivity of the material, but also efficiently host various metals as countercations, yielding an excellent dispersion and periodicity within the structure . Furthermore, PHI can host different metal ions in its structure, introduced during PHI formation or via post modification, i.e., ion exchange, and is therefore a promising material for developing novel transition metal–N x catalysts.…”

Ionic Carbon Nitrides in Solar Hydrogen Production and Organic Synthesis: Exciting Chemistry and Economic Advantages

“…acid treated K‐PHI, H‐PHI) on the other hand showed ca. 38 times higher activity in HER compared to K‐PHI …”

“…Lotsch et al. solved the structure of K‐PHI using transmission electron microscopy, while 13 C‐ 15 N 2D HETCOR spectrum confirmed that negative charge is localized at the deprotonated imide bridge between neighboring heptazine units . Furthermore, X‐Ray photoelectron spectroscopy (XPS) can be used to confirm the presence of deprotonated nitrogen atoms in the structure of K‐PHI as a signal at 396.6 eV in N 1s spectrum .…”

“…Therefore, we do not deal with a random elemental doping of the semiconductor in the ppm range, but with the formation of a new phase . K + cations in the K‐PHI matrix do not only compensate negative charge of the poly(anion), but they also mediate interactions to further distant layers and therefore contribute to the formation of more ordered structure compared to the protonated PHI …”