Aleksandr Savateev

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity

Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π-π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Advancing the n → π* electron transition of carbon nitride nanotubes for H2 photosynthesis

Melon-based carbon nitride (g-C3N4) is a promising metal-free and sustainable material for photocatalytic water splitting. In principle, pristine carbon nitride only exhibits moderate activity due to insufficient visible light absorption and fast charge recombination. Enhancement of the solar-to-energy conversion efficiency of g-C3N4 depends on the rational design of its morphology and electronic structure. Herein, we report the self-assembly of g-C3N4 nanotubes by co-polycondensation of urea and oxamide with their similar structure and reactivity to optimize the textural and electronic properties. Unlike pristine g-C3N4, the obtained copolymers exhibit clear optical absorption above 465 nm, which is ascribed to the n → π* electron transition involving lone pairs of the edge nitrogen atoms of the heptazine units. Besides, the charge carrier mobility was also optimized in the spatially separated nanotube structure, which contributes to the generation of more hot electrons. The optimized copolymers show dramatically enhanced H2 evolution activities especially with green light. The achieved apparent quantum yield (AQY) of optimal CN-OA-0.05 for H2 evolution with a green LED (λ = 525 nm) reaches 1.3%, which is about 10 times higher than that of pure CN with state-of-the-art activity in this wavelength region.

Potassium Poly(heptazine imides) from Aminotetrazoles: Shifting Band Gaps of Carbon Nitride‐like Materials by 0.7 V for More Efficient Solar Hydrogen and Oxygen Evolution

Potassium poly(heptazine imide) (PHI) is a photocatalytically active carbon nitride material that was recently prepared from substituted 1,2,4‐triazoles. Here, we show that the more acidic precursors, such as commercially available 5‐aminotetrazole, upon pyrolysis in LiCl/KCl salt melt yield PHI with the greatly improved structural order and thermodynamic stability. Tetrazole‐derived PHIs feature long‐range crystallinities and unconventionally small layer stacking distances, leading to the altered electronic band structures as shown by Mott–Schottky analyses. Under the optimized synthesis conditions, visible‐light driven hydrogen evolution rates reach twice the rate provided by the previous gold standard, mesoporous graphitic carbon nitride, which has a much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared with ordinary carbon nitrides makes it an efficient water oxidation photocatalyst, which works even in the absence of any metal‐based co‐catalysts under visible light. To our knowledge, this is the first case of metal‐free oxygen liberation from water.

“The Easier the Better” Preparation of Efficient Photocatalysts—Metastable Poly(heptazine imide) Salts

Cost-efficient, visible-light-driven hydrogen production from water is an attractive potential source of clean, sustainable fuel. Here, it is shown that thermal solid state reactions of traditional carbon nitride precursors (cyanamide, melamine) with NaCl, KCl, or CsCl are a cheap and straightforward way to prepare poly(heptazine imide) alkali metal salts, whose thermodynamic stability decreases upon the increase of the metal atom size. The chemical structure of the prepared salts is confirmed by the results of X-ray photoelectron and infrared spectroscopies, powder X-ray diffraction and electron microscopy studies, and, in the case of sodium poly(heptazine imide), additionally by atomic pair distribution function analysis and 2D powder X-ray diffraction pattern simulations. In contrast, reactions with LiCl yield thermodynamically stable poly(triazine imides). Owing to the metastability and high structural order, the obtained heptazine imide salts are found to be highly active photocatalysts in Rhodamine B and 4-chlorophenol degradation, and Pt-assisted sacrificial water reduction reactions under visible light irradiation. The measured hydrogen evolution rates are up to four times higher than those provided by a benchmark photocatalyst, mesoporous graphitic carbon nitride. Moreover, the products are able to photocatalytically reduce water with considerable reaction rates, even when glycerol is used as a sacrificial hole scavenger.

Synthesis of an electronically modified carbon nitride from a processable semiconductor, 3-amino-1,2,4-triazole oligomer, via a topotactic-like phase transition

A thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the chemical structures of two C,N,H-containing materials. Namely, the oligomer of 3-amino-1,2,4-triazole (OATA) is transformed into an electronically modified graphitic carbon nitride (OATA-CN) upon heating at 550 °C. During the transition, the flat band potential of the organic semiconductor is only slightly shifted from −0.11 eV to −0.06 eV, while the optical band gap is significantly expanded from 1.8 eV to 2.2 eV. The advantage of the suggested approach is the processability of the starting semiconductor combined with minor morphology changes during the heat-treatment that enable preservation of the original oligomer micro- and macrostructures in the resulting carbon nitrides. As an illustration, different OATA morphologies, including spherical nanoparticles, nanobarrels, nanowires and self-assembled macrospheres and composite sheets are synthesized and then transformed into OATA-CN with the retention of morphology. The surface area of the final carbon nitrides reaches 66 m2 g−1, without using any template, auxiliary reagent or post treatment. As a consequence, the photocatalytic activity of the obtained carbon nitrides in visible light driven hydrogen evolution is up to 5 times higher than that measured for the reference bulk carbon nitride prepared by pyrolysis of melamine.

Towards Organic Zeolites and Inclusion Catalysts: Heptazine Imide Salts Can Exchange Metal Cations in the Solid State

Highly crystalline potassium (heptazine imides) were prepared by the thermal condensation of substituted 1,2,4-triazoles in eutectic salt melts. These semiconducting salts are already known to be highly active photocatalysts, for example, for the visible-light-driven generation of hydrogen from water. Herein, we show that within the solid-state structure, potassium ions can be exchanged to other metal ions while the crystal habitus is essentially preserved.

Carbon nitride creates thioamides in high yields by the photocatalytic Kindler reaction

Potassium poly(heptazine imide), a carbon nitride based photocatalyst, effectively promotes the Kindler reaction of thioamide bond formation using amines and elemental sulfur as building blocks under visible light irradiation. The feasibility of the developed methodology was confirmed using 14 different primary and secondary amines, including substituted benzylamines and heterocyclic and aliphatic methylamines, which were successfully converted into thioamides with 68–92% isolated yields.

Baking ‘crumbly’ carbon nitrides with improved photocatalytic properties using ammonium chloride

Ammonium chloride can serve as a green, unreactive and reusable template to prepare heptazine-based graphitic carbon nitrides with surface areas up to 30 m2 g−1 and up to 6 times higher photocatalytic activity, by a simplified procedure that comprises only pyrolysis of the reaction mixture containing precursor and salt.

Ionothermal Synthesis of Triazine–Heptazine‐Based Copolymers with Apparent Quantum Yields of 60 % at 420 nm for Solar Hydrogen Production from “Sea Water”

Polymeric carbon nitride (PCN), in either triazine or heptazine form, has been regarded as a promising metal-free, environmentally benign, and sustainable photocatalyst for solar hydrogen production. However, PCN in most cases only exhibits moderate activity owing to its inherent properties, such as rapid charge carrier recombination. Herein we present a triazine-heptazine copolymer synthesized by simple post-calcination of PCN in eutectic salts, that is, NaCl/KCl, to modulate the polymerization process and optimize the structure. The construction of an internal triazine-heptazine donor-acceptor (D-A) heterostructure was affirmed to significantly accelerate interface charge transfer (CT) and thus boost the photocatalytic activity (AQY=60 % at 420 nm). This study highlights the construction of intermolecular D-A copolymers in NaCl/KCl molten salts with higher melting points but in the absence of lithium to modulate the chemical structure and properties of PCN.

Facile Synthesis of Potassium Poly(heptazine imide) (PHIK)/Ti-Based Metal–Organic Framework (MIL-125-NH2) Composites for Photocatalytic Applications

Photocatalytically active composites comprising potassium poly(heptazine imide) (PHIK) and a Ti-based metal-organic framework (MOF, MIL-125-NH2) are prepared in situ by simply dispersing both materials in water. The driving forces of composite formation are the electrostatic interactions between the solids and the diffusion of potassium ions from PHIK to MIL-125-NH2. This mechanism implies that other composites of poly(heptazine imide) salts and different MOFs bearing positive surface charge can potentially be obtained in a similar fashion. The suggested strategy thus opens a new avenue for the facile synthesis of such materials. The composites are shown to have a superior photocatalytic activity in Rhodamine B degradation under blue light irradiation. The reaction rate is doubled compared to that of pure MOF compound and is 7 times higher than the activity of the pristine PHIK. The results of the electron paramagnetic resonance (EPR) investigations and the analysis of the electronic structures of the solids suggest the electron transfer from MIL-125-NH2 to PHIK in the composite. The possible pathways for the dye degradation and the rationalization of the increased activity of the composites are elaborated.