Developing
metal-free electrocatalysts for direct nitrate-to-ammonia
reduction is promising to remediate wastewater yet challenged by the
poor ammonia selectivity. Amorphization has become an emerging strategy
to afford conventional materials with exotic physical, chemical, and
electronic properties. Transient laser heating of polymers produces
graphene with an unusual polycrystalline lattice, yet the control
of graphene amorphicity is difficult due to the extreme conditions
and fast kinetics of the lasing process. Here, we report the synthesis
of amorphous graphene with a tailorable heterophase, topologically
disparate from crystalline graphene and amorphous carbon. Atomic-resolution
imaging reveals the intermediate crystallinity comprising both six-membered
rings and polygons, the ratio of which directly correlates with the
aromatic structures of the precursors. These amorphous graphenes,
as metal-free catalysts, show high performance in direct nitrate-to-ammonia
electroreduction. The performance is associated with the amorphicity
of graphene and reaches a maximum ammonia Faradaic efficiency of 83.7%
at −0.94 V vs reversible hydrogen electrode.
X-ray pair distribution functions and paramagnetism disclose the elongated
carbon–carbon bonds and rich unpaired electrons in amorphous
graphene, which exhibit more favorable adsorption of nitrate as suggested
by theoretical calculations. Our findings shed light on the controllable
synthesis of graphene with unusual topologies that could find broad
applications in electronics, catalysis, and sensors.
Ammonia is an indispensable commodity in the agricultural and pharmaceutical industries. Direct nitrate‐to‐ammonia electroreduction is a decentralized route yet challenged by competing side reactions. Most catalysts are metal‐based, and metal‐free catalysts with high nitrate‐to‐ammonia conversion activity are rarely reported. Herein, it is shown that amorphous graphene synthesized by laser induction and comprising strained and disordered pentagons, hexagons, and heptagons can electrocatalyze the eight‐electron reduction of NO3− to NH3 with a Faradaic efficiency of ≈100% and an ammonia production rate of 2859 µg cm−2 h−1 at −0.93 V versus reversible hydrogen electrode. X‐ray pair‐distribution function analysis and electron microscopy reveal the unique molecular features of amorphous graphene that facilitate NO3− reduction. In situ Fourier transform infrared spectroscopy and theoretical calculations establish the critical role of these features in stabilizing the reaction intermediates via structural relaxation. The enhanced catalytic activity enables the implementation of flow electrolysis for the on‐demand synthesis and release of ammonia with >70% selectivity, resulting in significantly increased yields and survival rates when applied to plant cultivation. The results of this study show significant promise for remediating nitrate‐polluted water and completing the NOx cycle.
A series of tetracoordinate boron-doped polycyclic aromatic hydrocarbons have been synthesized under mild conditions, featuring delayed fluorescence and aggregation-induced emission.
The incorporation of heteroatoms and/or heptagons as the defects into helicenes expands the variety of chiroptical materials with novel properties. However, it is still challenging to construct novel boron‐doped heptagon‐containing helicenes with high photoluminescence quantum yields (PLQYs) and narrow full‐width‐at‐half‐maximum (FWHM) values. We report an efficient and scalable synthesis of a quadruple helicene 4Cz‐NBN with two nitrogen‐boron‐nitrogen (NBN) units and a double helicene 4Cz‐NBN‐P1 bearing two NBN‐doped heptagons, the latter could be formed via a two‐fold Scholl reaction of the former. The helicenes 4Cz‐NBN and 4Cz‐NBN‐P1 exhibit excellent PLQYs up to 99 % and 65 % with narrow FWHM of 24 nm and 22 nm, respectively. The emission wavelengths are tunable via stepwise titration experiments of 4Cz‐NBN‐P1 toward fluoride, enabling distinguished circularly polarized luminescence (CPL) from green, orange (4Cz‐NBN‐P1‐F1) to yellow (trans/cis‐4Cz‐NBN‐P1‐F2) with near‐unity PLQYs and broader circular dichroism (CD) ranges. The five structures of the aforementioned four helicenes were confirmed by single crystal X‐ray diffraction analysis. This work provides a novel design strategy for construction of non‐benzenoid multiple helicenes exhibiting narrow emissions with superior PLQYs.
Modification of π-conjugated systems using a boron atom as the dopant has become a powerful approach to create new structures and new properties. Herein, we report a facile synthesis of replacing the carbon edges of [4]triangulene by three oxygen-boron-oxygen (OBO) units. The triangulenes are structurally similar to [4]triangulene and isoelectronic to the trianion of [4]triangulene. The structure of triangulene is confirmed by single-crystal X-ray diffraction analysis, revealing an off-plane core with three edgemodified OBO units. These triangulenes exhibit excellent thermal stability. These compounds have phosphorescence with lifetime longer than 1 s at 77 K. Both theoretical calculations and photophysical investigation of triangulenes indicate that this kind of molecules display a rare anti-Kasha fluorescence and phosphorescence emissions from multiple higher excited states.
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