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

Abstract

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 golden standard, mesoporous graphitic carbon nitride having much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared to the 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 a metal free oxygen liberation from water as such

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

“…[20] In the last decade,b reakthroughs have also been made in the ionothermal synthesis of covalent materials. [21][22][23][24][25][26][27][28] Recently,w ew ere able to show that by postcondensation of melon in LiCl/KCl salt melts,t he resulting PCN showed an improved degree of condensation, high crystallinity,a nd fewer defects. [21][22][23] Furthermore, through solvation and coordination effects,salt melts change the route of carbon nitride polymerization significantly,a nd make poly(triazine imide) (PTI) the preferred species (see Figures S1 and S2 in the Supporting Information) in the presence of lithium, [23,24] which has up to now shown even much lower activity for photocatalytic water splitting.…”

“…[14][15][16] In the past few years, however, great improvements have been made by rational modulation of the structure and optical and electronic properties of pristine PCN.F or example,e ither by doping or copolymerization, the visible-light absorption edge of PCN could be extended to 650 nm. [21][22][23][24][25][26][27][28] Recently,w ew ere able to show that by postcondensation of melon in LiCl/KCl salt melts,t he resulting PCN showed an improved degree of condensation, high crystallinity,a nd fewer defects. [17] Theuse of molten salts (MSs) as the solvent and template to accelerate the polymerization process and further tailor the structure and properties of the polymer has been widely regarded as afeasible way to synthesize well-defined materials,especially for metallic and ionic compounds.…”

“…[27] It was very interesting to observe that after calcination in molten salts the structure changed significantly.T he first peak of the resulting polymers moved to alower diffraction degree of 88 8 (Figure 1a). It is most likely that the cations (e.g.,K + or Na + ) with alarger atomic size than Cand Nincorporated between the triazine-heptazine units (Scheme 1) are able to enlarge the packing distance within the layer planes, [28] as verified by the symmetric (987 cm À1 )and asymmetric (1065 cm À1 )vibrations of NC 2 bonds (Figure 1b)ofmetal-NC 2 groups.Aseries of weak diffraction peaks belonging to typical PTI appeared in both CN-LiK and CN-LiNa, thus indicating the formation of PTI in the presence of LiCl owing to strong solvation and coordination effects.T his PTI structure is believed to be thermodynamically unstable and less active than the normal triazine/heptazine counterparts. [23] In contrast, only broad and weak peaks centered at 8a nd 27.48 8 were revealed for CN-NaK, thus implying the absence of lithium, and the higher melting point of NaCl/KCl would avoid the formation of PTI and most likely lead to the partial structural transformation from heptazine to triazine to form well-designed triazineheptazine copolymers (see below).…”

Highly Crystalline K‐Intercalated Polymeric Carbon Nitride for Visible‐Light Photocatalytic Alkenes and Alkynes Deuterations

“…The traditional direct thermal polymerization method usually leads to low crystalline PCN due to the predominantly kinetic hindrance. [11] Therefore, how to design and construct a well-defined and highly condensed crystalline PCN to improve its photocatalytic efficiency is significative but challenging. [10] A well-defined structure with an ordered molecular arrangement and a controllable morphology not only enhances electron and mass transfer but also facilitates the separation of electron-hole pairs to promote redox reactions.…”

Semi‐heterogene duale Nickel‐/Photokatalyse mit Kohlenstoffnitriden: Veresterung von Carbonsäuren mit Arylhalogeniden

“…Photokatalytisch aktive Komplexe häufig auf der Erde vorkommender Metalle [6] oder organische Farbstoffe [7] sind weniger effektiv oder zersetzen sich unter diesen Reaktionsbedingungen. Mesoporçses graphitisches Kohlenstoffnitrid (mpg-CN), [10] ein modifiziertes Kohlenstoffnitrid auf Basis eines Cyanursäure/Melamid/Barbitursäure-Komplexes (CMB 0.05 -CN), [11] ein mit Schwefel versetztes Material (CNS 600 ) [12] und das stark oxidierende Kaliumpoly(heptazinimid) (K-PHI) [13] lieferten jeweils 12-15 %d es Esters 1.E ine Kohlenstoffnitridvariante,d ie durch Cokondensation von Harnstoff und Oxamid mit nachfolgender Kalzinierung in einer Salzschmelze (CN-OA-m) angefertigt wird, [14] zeigte die hçchste Aktivitäti ne rsten Versuchen. [8] Graphitische Kohlenstoffnitride (g-CN) sind eine Klasse von metallfreien Polymeren und die wahrscheinlich bestgeeigneten Materialien fürden Einsatz als heterogene Photokatalysatoren.…”