C2N: A Class of Covalent Frameworks with Unique Properties

Abstract

C₂N is a unique member of the C_nN_m family (carbon nitrides), i.e., having a covalent structure that is ideally composed of carbon and nitrogen with only 33 mol% of nitrogen. C₂N, with a stable composition, can easily be prepared using a number of precursors. Moreover, it is currently gaining extensive interest owing to its high polarity and good thermal and chemical stability, complementing carbon as well as classical carbon nitride (C₃N₄) in various applications, such as catalysis, environmental science, energy storage, and biotechnology. In this review, a comprehensive overview on C₂N is provided; starting with its preparation methods, followed by a fundamental understanding of structure-property relationships, and finally introducing its application in gas sorption and separation technologies, as supercapacitor and battery electrodes, and in catalytic and biological processes. The review with an outlook on current research questions and future possibilities and extensions based on these material concepts is ended.

Keywords: C2N; applications; carbon materials; heteroatoms; regular pores.

Figure 1

Schematic illustration of structure, unique features, and wide applications of C₂N materials.

Figure 2

Preparation and structure of C₂N‐h2D crystal. a) Schematic representation of the reaction between hexaaminobenzene (HAB) trihydrochloride and hexaketocyclohexane (HKH) octahydrate to produce the C₂N‐h2D crystal; the inset in the image of HAB is a polarized optical microscopic image of a single HAB crystal. Digital photographs: b) as‐prepared C₂N‐h2D crystal; c) solution‐cast C₂N‐h2D crystal on SiO₂surface after heat‐treatment at 700 °C; d) a C₂N‐h2D crystal film (thickness: ≈330 nm) transferred onto a PET substrate. The shiny metallic reflection of the sample indicates that it is highly conductive. Reproduced with permission.^{[ 2 ]} Copyright 2015, Springer Nature.

Figure 3

Schematic representation of synthesis based on cyclohexanehexone and urea reactants to a cross‐linked intermediate material (top), which systematically converts to a disordered C₂N at temperatures greater than 500 °C (T ₁<T ₂), then to a slightly nitrogen‐depleted C₂N containing primarily pyridinic and pyrazinic nitrogen (bottom). Reproduced with permission.^{[ 3 ]} Copyright 2016, Wiley‐VCH.

Figure 4

Two reaction paths of carbonization towards noble carbons (starting from barely oxidizable precursors) and non‐noble carbons (starting from easily oxidizable precursors) displayed on an electrochemical scale. Elimination of the same classical fragment drives the reactions to two different sides of the “nobility equator” (red line); either non‐noble or noble. Reproduced with permission.^{[ 41 ]} Copyright 2018, Wiley‐VCH.

Figure 5

a) Idealized model for the formation of C₂N structure by condensation of HAT‐CN precursor, b) argon physisorption isotherms (87 K), and c) water vapor physisorption isotherms (298 K) of C‐HAT materials. Reproduced with permission.^{[ 38 ]} Copyright 2018, Wiley‐VCH.

Figure 6

a) Idealized structure of C₂N_xO_{1‐ x}. b) Water vapor physisorption isotherms at 298 K. Reproduced with permission.^{[ 37 ]} Copyright 2018,The Royal Society of Chemistry.

Figure 7

Scheme of the synthesis and lithiation–delithiation mechanism of M@C₂N (M = Ru, Pd, and Co) and C₂N. Reproduced with permission.^{[ 110 ]} Copyright 2018, Elsevier.

Figure 8

a) Preparation of porous HAT‐CNF using electrospinning followed by condensation. Reproduced with permission.^{[ 45 ]} Copyright 2019, Elsevier. b) Preparation of HAT550@ZTC. Reproduced with permission.^{[ 46 ]} Copyright 2020, Elsevier.

Figure 9

a) Schematic illustration of the synthesis and structure of Ru@C₂N electrocatalyst. Hexaaminobenzene and hexaketocyclohexane in N‐methyl‐2‐pyrrolidone (NMP) react to form the C₂N framework, while RuCl₃ and NaBH ₄serve as the Ru precursor and reducing agent. b) TEM image of Ru@C₂N. Inset: corresponding particle size distribution of the Ru nanoparticles. Scale bar: 20 nm. Atomic‐resolution TEM image of Ru@C₂ Nc) and corresponding fast‐Fourier transform (FFT) pattern d) Scale bar: 1 nm. e) STEM image and STEM‐EDS element mapping of Ru@C₂N. Scale bar: 20 nm. Reproduced with permission.^{[ 39 ]} Copyright 2017, Springer Nature.

Figure 10

Schematic representation of the structural evolution of Fe@C₂N catalyst, showing an in situ sandwiching of Fe³⁺in C₂N layers (Fe³⁺@C₂N) in NMP, reduction of Fe³⁺@C₂N into Fe_xO_y@C₂N by sodium borohydride, and subsequent annealing of Fe_xO_y@C₂N into the Fe@C₂N catalyst at 800 °C. The structure of the Fe@C₂N catalyst consists of Fe nanoparticle cores encased in well‐ordered nitrogenated graphitic shells (Fe@C₂N nanoparticles), which are uniformly distributed on the C₂N matrix. Reproduced with permission.^{[ 49 ]} Copyright 2018, Elsevier.

Figure 11

a) Optical microscopy and b) SEM images of red blood cells (RBCs) treated by rGO and C₂N at 200 µg mL⁻¹. Diluted RBCs incubated in 1 × PBS were regarded as negative control. The black arrows in panel A indicate lysed RBCs. Reproduced with permission.^{[ 113 ]} Copyright 2018, Wiley‐VCH.