Triazine‐based Carbon Nitrides for Visible‐Light‐Driven Hydrogen Evolution

Abstract

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...

Lithium Charge Storage Mechanisms of Cross-Linked Triazine Networks and Their Porous Carbon Derivatives

“…The PTI materials studied here were characterized as described in a previous publication, and the characterization is con--sistent with the idealized structure of triazine units bridged by imides. 25 The presence of well--defined triazine moieties in these materials despite the high synthesis temperatures is due to the ionothermal synthesis condi--tions which stabilize the formation of triazine units. De--tails of the characterization of the materials are presented in the supplementary information.…”

“…Doping the amorphous materials by copolymerizing with 4--amino--2,6--dihydroxy pyrimidine (4AP) to obtain O and OH moieties as well as extra carbon results in no signifi--cant change in the electrochemical performance ( Figure 8c and 8d), although the doped materials work better as photocatalysts. 25 To determine if electrical contact to the active material is the cause of poor capacity, electrode composites were also prepared by ball milling to reduce . The prefix of the name indicates the long--range order (c -crystalline, a -amorphous) and "d" indi--cates the material has been copolymerized with 4AP to obtain O and OH moieties as well as extra carbon resulting in a "doped" material.…”

“…The extremely low capacity of the PTI materials, particularly in the range of cathode materials, suggests that the cell is storing charge by virtue of polarizing the electrodes and double layer storage on the high surface area Super P®. The ordering of the c--PTI--600 network produces channels that can be occupied by interstitial ions, 25 so we expect that these channels also store Li + . This mechanism could explain the slightly higher capacity and the appearance of broad plateaus in the discharge and charge profiles of the c--PTI--600.…”

Surface modification of g-C3N4 by hydrazine: Simple way for noble-metal free hydrogen evolution catalysts

“…Due to the strong C-N good chemical stability and thermal stability (up to 600 o C). All these merits make it to be an idea material for the application in hydrogen evolution 19,[21][22][23][24][25][26][27][28][29][30] as well as environmental pollutant degradation. [31][32][33][34][35][36][37][38][39][40] g-C 3 N 4 is a semiconductor with a band gap of 2.7 eV, with a VB level suitable for hydrogen and oxygen evolution both.…”

Visible Light–Driven Hydrogen Production by Carbon based Polymeric Materials

“…Ham of sacrificial agents and co-catalyst (Pt, 5 wt.%), whereas it showed low activity in comparison to that of Pt/g-C 3 N 4 which may be assigned to the weak optical absorption in the visible light region [30]. Schwinghammer et al modify the PTI/Li + Cl − by using 4-amino-2, 6-dihydroxypyrimidine (4AP) as the dopant [31]. This modified PTI showed red-shifted absorption spectrum compared to the crystalline PTI and more similar to the melon structure, which improves the visible light absorption.…”