# A simple and efficient process for the synthesis of 2D carbon nitrides and related materials.

Vincent Huc, Cora Moreira Da Silva, Maxime Vallet, Romuald Saint-Martin, Jocelyne Leroy, Clément Semion, Diana Dragoe, François Brisset, Thomas Blin, Regis Guillot, et al.

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Vincent Huc, Cora Moreira Da Silva, Maxime Vallet, Romuald Saint-Martin, Jocelyne Leroy, et al. A simple and efficient process for the synthesis of 2D carbon nitrides and related materials. 2022. cea-03874452

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# A simple and efficient process for the synthesis of 2D carbon nitrides and related materials.

**Vincent Huc** (vincent.huc@universite-paris-saclay.fr)

University of Paris-Sud

**Cora Moreira Da Silva**

University of Paris-Saclay

**Maxime Vallet**

Université Paris-Saclay, Ecole Centrale Sup'Élec

**Romuald Saint-Martin**

University of Paris-Saclay

**Jocelyne Leroy**

Paris-Saclay University, CEA Saclay

**Diana Dragoé**;

University of Paris-Saclay

**Clément Semion**

ONERA and CNRS

**François Brisset**

University of Paris-Saclay

**Thomas Blin**

University of Paris-Saclay

**Regis Guillot**

University of Paris-Saclay

**Cynthia Gillet**

Institute of Integrative Biology of the Cell

## Article

### Keywords:

**Posted Date:** August 8th, 2022

**DOI:** https://doi.org/10.21203/rs.3.rs-1901298/v1

**License:** This work is licensed under a Creative Commons Attribution 4.0 International License.
**Read Full License**
<figure><img src="image_2.png" /></figure>
## A simple and efficient process for the synthesis of 2D carbon nitrides and related materials.

Cora Moreira Da Silva ¹, Maxime Vallet ², Clément Semion ⁵, Thomas Blin ¹, Romuald Saint-Martin ¹, Jocelyne Leroy ³, Diana Dragoé ¹, François Brisset ¹, Cynthia Gilet ⁴, Régis Guillot ¹, Vincent Huc¹*.

1. Université Paris-Saclay, Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO), rue du doyen Georges Poitou, FR91405 Orsay, France.

2. Universite Paris-Saclay, Ecole Centrale Sup'Elec, France

3. Commissariat à l'Energie Atomique (CEA), site de Saclay, laboratoire NIMBE, France

4. CNRS-Institut de Biologie Intégrative de la Cellule (I2BC), France

5. Laboratoire d'Etude des Microstructures (LEM, ONERA), Châtillon, France.

### Abstract.

This manuscript describes a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such as C2N and CN carbon nitrides. The structure/quality of these materials is demonstrated by extensive cross-characterisations at different scales. The availability of such very large, high quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...). We thus consider that this work could be of interest to a broad readership, considering that this topic is among the current hottest ones.

### Introduction.

The first preparation of graphene by Geim and Novoselov in 2004 [1,2] sparkled an ever increasing interest in 2D materials. Indeed, along with graphene, dichalcogenides such as $MoS_2$ or $WO_2$ [3, 4], silicene [5, 6], phosphorene [7, 8, 9], hexagonal boron nitride [10, 11] etc... are also being investigated all over the world, due to their fascinating physical [12, 13, 14, 15] and chemical properties [16, 17, 18]. More recently, 2D COFs (Covalent Organic Frameworks), a family of 2D materials obtained by covalent assembly of organic building units have received a growing interest [19, 20, 21, 22, 23, 24].

These compounds are often obtained by surface-confined reaction: a surface acts as a template to promote the adsorption of the reactants, allowing them to organise and condense by means of different bond-forming reactions [25, 26, 27]. A variant of this approach uses the surface both as a template and as a reactant. Many examples of this later approach are to be found in literature, for example by using reactive Cu surfaces in combination with halogenated aromatics [26]. In most cases, the presence of the associated by product (usually a copper halide) is considered as detrimental, as it may form aggregates on the surface (if not sublimed under UHV conditions).

We wish to propose here a new variation of the reactive surface strategy.

We consider here the synthesis of 2D materials from the assembly of soluble, organic building units at the surface of a reactive metal. The solvent used is chosen so as to make the metallic halide by-product insoluble. We reasoned that in such a situation, the presence of this halide by-product would be far from detrimental. Indeed, it could offer a supplementary confinement effect, acting cooperatively with the one associated with the metallic surface. By the same time, the solvent would then provides a continuous supply of reagents to the metal/inorganic salt interface where the reaction takes place. An illustration is shown on Figure 1.
This « double confinement strategy » could improve the quality of synthetic graphene-like 2D materials.

<figure><img src="image_3.png" /><figcaption>**Figure 1:** doubly confined growth of 2D materials</figcaption></figure>

## Results

Since the pioneering work of Chuanbao et al. [28, 29, 30], the synthesis of 2D materials by alkali metal-promoted reduction of halogenated precursors was left largely unexplored. Seeking for simple synthesis, that could be performed using common lab equipment, we designed a protocol that runs under close to ambient conditions. To favor a 2D-confined reaction, the solvent used is chosen so as to dissolve the reactant (i.e. allows for the diffusion of the halogenated species to the metallic surface), but neither the final 2D material, nor the inorganic by-products (or any other metal-containing species), to reinforce the confinement effect.

### A) synthesis of CN carbon nitride.

We decided to perform a first experiment targeting a carbon nitride with the general formula CN (Figure 2) [28, 29, 30, 31 32]. Indeed, carbon nitrides of various stoichiometries are promising for a plethora of applications [33 34 35]. Moreover, the envisioned starting product, cyanuric chloride, is a cheap, commercially available one.
<figure><img src="image_4.png" /><figcaption>**Figure 2: synthesis of CN carbon nitride.**</figcaption></figure>

Last, potassium was chosen as the reactive metal, because the targeted carbon nitrides do contain coordinating cavities with 6 inward-pointing nitrogen ligands [³⁶]. Indeed, it was shown that K⁺ ions shows a high affinity for such 6-membered cavities, in comparition with other ones with 5 or 8 coordinating atoms (Figure 3) [³⁷]. It can thus be guessed that this cation will template the selective formation of six-membered cavities, thus further reinforcing the selectivity towards the formation of the targeted carbon nitrides.

<figure><img src="image_5.png" /><figcaption>**Figure 3: selectivity of the K⁺ ion for 6 fold coordinating cavities.**</figcaption></figure>

# Results.

The details of the synthetic process are to be found in the Supporting Informations (S1), along with a sequence showing the evolution of the reaction media as the reaction goes by (S1A→E).

## Crude material analysis

First, an analysis of the crude reaction media (supernatant and solid separately) was performed to qualitatively assess the yield of the reaction.

The amount of remaining organics in the supernatant was found negligible (less than 100 mg). The presence of left residual starting product and/or partial reaction products can thus be considered as rather low.
Then, a sample of the solid obtained after the filtration step (i.e. the removal of soluble residual organics) was directly analysed by XPS spectroscopy (S1H). The Cl 2p spectra can be deconvoluted in two main components, at 197.74 and 199.26 eV. The first one is attributed to KCl (inorganic chloride), and the second one to organic chloride, i.e. to C-bonded chlorine. This later component may be attributed to incompletely reduced species.

The relative surfaces of these two components (along with the very low amount of recovered soluble species) drives to the conclusion that the yield of the reaction is over 80%.

The deconvolution of the N 1s signals shows two components, at 399.21 and 400.57 eV. The 399.21 eV component is attributed to triazinic nitrogens [³⁸, ³⁹, ⁴⁰, ⁴¹]. The higher energy component is attributed to nitrogens bonded to included species, such as metallic cations or H-bonded species. In accordance with the presence of inorganic chloride, strong signals associated with K⁺ ions are also observed

## Water-washed sample analysis

A final washing with water is necessary, to remove the water-soluble KCl by-product (see SI). CN is obtained as a light brown solid (Figure 4A), easily redispersed in water (Figure 4B).

An optical microscopy analysis of a water suspension shows this material as made of thin, very large flakes (some of them exceeding 500 µm (Figure 4C)). Characteristic features of 2D objects (such as foldings) are clearly observed (Figure 4D). More images are shown in the SI (Figures S1F→G).

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** A) the crude CN material; B) water suspended CN, showing the Tyndall effect; C and D) optical microscopy analysis.</figcaption></figure>
Some hollow spheres are also sometimes observed, probably the result of a molding of melted potassium droplets (Figure S1G).

These optical microscopy analyses were also performed using polarised light (Figure 5), with flakes suspended in dispersion oil. Under these conditions, some flakes appear alternatively bright and dark as a function of the relative angles between the polarizer and the sample. This shows that these flakes are crystalline. The fact that different domains appear inside the flake is attributed to foldings (Figures 5 D and E). Other flakes appear as uniformly bright, regardless of the angle. This could be attributed to the fact that they are constituted of several stacked layers, without orientational correlations between them. The presence of extensive crumpling may also be a plausible explanation, as these foldings may also change the polarisation angle of the transmitted light. A handling experiment of this flake is shown in the SI (Figure S1L) along with one more example of polarised light analysis of a CN flake (S1M).

<figure><img src="image_10.png" /><figcaption>**Figure 5:** polarized optical microscopy analysis of a CN flake (yellow circle). A) normal light; B) and C) polarised light. The brightness of the flake changes as a function of the angle, evidencing its crystallinity. D) and E): a simple model showing how foldings may give rise to different domains inside a single crystal flake.</figcaption></figure>

SEM

A SEM microscopy analysis of CN is shown on Figure 6.
Larges flakes (Figure 6A), that tend to roll (Figure 6B) are observed. These features confirm the
formation of a layered material.

An EDS analysis (Figure 7) confirms the presence of the expected elements, along with residual KCl.
Oxygen is also observed, probably as trapped/adsorbed species.
<figure><img src="image_11.png" /><figcaption>**Figure 6.** SEM analysis of CN.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 7.** EDS analysis of CN.</figcaption></figure>

## XPS

An XPS analysis of a washed sample of CN is shown on Figure 8.

Analysis of the Cl 2p core-level spectrum revealed the presence of two spin-orbit doublets whose Cl 2p$_{3/2}$ components are located at 201.3 eV and 198.2 eV. The higher binding energy component is attributed to traces of residual C-Cl bonds while the lower BE component may be related to chloride ions as remaining KCl and CaCl$_2$. Indeed, EDS analyses also confirms the presence of both potassium and calcium (see below, EDS analyses and S1i→J). The very low intensities of the C-Cl signal confirms the low amount of defaults in the material. The deconvolution of C 1s signals shows three main components. The first one at 285eV corresponds to carbon contamination, ubiquitously encountered in such analyses. The two other main components can be attributed to triazinic carbons, from free or N-bonded ones (metallic cations or H-bonded, adsorbed species).

Regarding the N 1s signal, two main components are observed at 399.91 and 401.35 eV [38, 39, 40, 41]. The high energy component, has strongly decreased compared with the crude sample of CN. This confirms the attribution of this high energy peak to nitrogens interacting with included species, such as metallic cations. Oxygen is still observed in the material, showing the presence of a large amount of included, oxygen-containing species. This is coherent with our interpretation of the N 1s signals. Such a tendency to absorb invited species is not surprising, considering the highly porous nature of this material. Similar effects have already been observed with related carbon nitrides [43, 46, 52]. The C/N intensity ratio between normalized, 1s peaks from the relevant carbon signals and nitrogen is 1.2, close to the expected value of 1.
<figure><img src="image_13.png" /><figcaption>**Figure 8.** XPS analysis of CN.</figcaption></figure>

## XRD

A compared XRD analysis of a crude and washed CN sample was performed. The results are shown on Figure 9.

The crude sample (Figure 9A, red) only shows signals from the by-product of the reaction, i.e. KCl. No other peaks are visible.
<figure><img src="image_14.png" /><figcaption>**Figure 9:** A) compared powder X-ray diffractograms of a crude (red) and washed (blue) CN samples (same batch). Insert (green): zoom over the blue curve. B) Powder X-ray diffractogram of a washed CN sample Blue stars : signals from the aluminum sample holder ; red stars : signals from KCl by-product.</figcaption></figure>

Interestingly, strong evolutions of the same sample are observed upon washing (Figure 9A, blue). Along with the disappearance of the KCl-related signals, a broad peak, centered at 26° appears (Figure 9A, green insert), that was absent from the crude sample. As this peak is characteristic of the interplane spacing of graphenic materials, it can thus be concluded that the flakes mostly exist as individual/few layers objects in the crude material. These flakes are probably kept apart inside a KCl matrix. Upon washing and removal of KCl, these flakes then get aggregated, resulting in the appearance of the 26° peak. Another conclusion that is to be drawn from this comparative study is that the 2D layers are likely to grow as single/few layers aggregates during the synthesis.

TEM

The resulting analyses are shown on Figure 10. The images are shown at increasingly high magnifications, to assess the quality of the flakes over different scales.

The material appears as constituted of very large flakes, in accordance with SEM and optical microscopy images. These flakes are very homogeneous and thin, most of them seemingly constituted of bilayers (Figure 10C). They appear quite smooth, with very few visible defects (Figure 10D→F). The well-known affinity of planar, hexacoordinated ligands for uranyl ions was used to load CN with this ion, in order to increase contrasts (Figure S1K-A). TEM analysis of an isolated flake (Figure S1K-B→E) shows the presence of Moiré effects on top of superimposed layers, thus evidencing their crystallinity.
<figure><img src="image_15.png" /><figcaption>**Figure 10.** TEM images of CN flakes.</figcaption></figure>
## EDS

EDS analyses were also performed on this material. Figure 11 shows a detailed analysis at the N-K edge.

<figure><img src="image_16.png" /><figcaption>Figure 11. EDS map of CN at the N-K edge.</figcaption></figure>

The HAADF micrograph shows the presence of very thin flakes (Figure 11A). A comparition between this image (Figures 11A and 11C) and the corresponding N maping (Figures 11B and 11D) shows a correlation between the shape of the flake and the intensity of the N signal. Moreover, the intensity of this signal increases where thicker flakes are observed (foldings), as evidenced on Figures 11E and 11F (yellow circles). These observations are showing that the flakes are made of a nitrogen-rich material, as expected. This EDS analysis also shows the presence of K and Ca (Figure S1i→J), in accordance with EDS, XPS and EELS (see below) analyses.
EELS
---

An EELS analysis of this material (Figure 12) shows the presence of the expected elements C and N. Ca is also observed as an impurity from the starting metallic potassium. The structure of the C 1s and N 1s edges shows a fine structure that is characteristic of extended π-delocalisation [⁴²], in accordance with the expected structure of CN.

<figure><img src="image_17.png" /><figcaption>**Figure 12.** EELS analysis of CN.</figcaption></figure>

All these analyses do show the presence of residual metals (K and Ca) in the material, most probably as cations included inside the coordinating cavities of CN. This may explain the presence of the different signals observed for N 1s by XPS analysis. Indeed, the direct coordination of the N to the positively charged cations probably gives rise to the observed higher energy component.
## B) : synthesis of C₂N carbon nitride.

These very encouraging results prompted us to generalise our synthetic strategy. As a second example, we have chosen to synthesise a well documented 2D material, the « C₂N » carbon nitride (Figure 13). Since its discovery in 2015 [⁴³], this material is under the spotlight, due to its tremendous properties in many different fields [⁴⁴, ⁴⁵, ⁴⁶, ⁴⁷, ⁴⁸, ⁴⁹, ⁵⁰]. Surprisingly, only a handful of synthetic approaches are described so far, all relying on the condensation of complementary carbonyl and amine containing reagents under different conditions [⁴⁵, ⁵¹], or high temperature pyrolysis of organic precursors [⁵²]. In our work, the targeted carbon nitride is obtained by reduction of tetrachloropyrazine using potassium.

<figure><img src="image_18.png" /><figcaption>**Figure 13.** Synthesis of C₂N.</figcaption></figure>

The detailed synthetic procedure is to be found in the Supporting Information (S2).

### Crude material analysis

We first performed an analysis of the crude material, to qualitatively estimate the yield of the synthesis. The reaction media was filtered, and the resulting black solid washed with acetone (see S2A→E). All the organic phases were combined and evaporated to dryness on a vacuum line, leaving a very small amount (50mg) of a dark brown residue. A sample of the solid black material left after filtration was then analysed. An XPS analysis of this crude sample is shown on Figure S2F. It shows the presence of strong potassium signals (Figure S2F-D) as cationic species. Analysis of the corresponding Cl 2p signals (Figure S2F-B) shows the presence of two different kinds of chlorine atoms. The one with the lowest binding energy is characteristic of anionic chlorine, and is thus ascribed to KCl. This is confirmed by the measured 1/1 K/Cl ratio. A second Cl component with a higher binding energy is attributed to organic chlorine (i.e. bonded to carbon atoms). A comparision of the relative areas of these two species (along with the very small amount of soluble material (50mg), see previous paragraph), shows that the yield of the reaction is above 90%.

An analysis of the N 1s signals (Figure S2F-C) reveals two components attributed to pyridinic-type nitrogens [³⁸, ³⁹, ⁴⁰, ⁴¹]. The higher energy one probably belongs to metallic cations-bonded nitrogens. As already mentioned, strong signals of metallic potassium (along with a few percent of calcium) are also observed.

### Washed sample analysis

The purified C₂N material is obtained as a fluffy, black powder, (Figures 14A and B).
<figure><img src="image_19.png" /><figcaption>**Figure 14:** washed C₂N. Optical microscopy (A); (B) same material as a compressed pellet; C), D): optical microscopy of an HCl dispersed C₂N sample; E) orange-colored HCl suspension of C₂N showing the Tyndall effect.</figcaption></figure>

An observation of the same material as an acidic (1M HCl) water suspension shows it to be constituted of large, variously aggregated and folded flakes, with sizes ranging from a few tens to 300 µm (Figures 14C and 14D). The color of the thinnest flakes appears as deep orange, rapidly turning to black with increasing thickness (S2j). The bright orange color of acidic C₂N suspensions confirms that a significant amount of the material is made of single/few layers flakes (Figure 14E). In some instances, tubular (S2H) or shell-shaped objects (S2i) were also observed. In the latter case, this is likely to be the result of a molding of melted potassium droplets, getting encapsulated during the growth of C₂N around them.

Interestingly, we also found that some flakes (thin enough and not too folded/crumpled) do rotate the polarisation plane of incident polarised light, as expected for single crystals. One example is shown on Figure 15. Other examples are provided in the SI (Figures S2K→S2M). The flakes (after washings) were dried on a glass plate overnight and diluted in dispersion oil to ease their observation and handling. These extremely thin flakes are nearly invisible (even with the microscope). They can only be detected due to their changing brightness under polarized light. As the flakes gets thicker, the effect disappears, probably due to: i) the absence of correlation of the orientation of the layers inside thick flakes, ii)
their increasing absorbance and iii) the presence of foldings, that tend to average the effect on polarised light. Note that the observation of single layer graphene using polarised light has already been reported [⁵³, ⁵⁴].

<figure><img src="image_20.png" /><figcaption>**Figure 15:** polarised light study of thin C₂N flakes (yellow circles). A): normal light; B): same flakes under cross polarized light and C) same sample upon rotation. The appearance of the flakes alternatively goes brighter and darker as a function of the angle. D) and E): a simple model accounting for the observation of different domains inside a single flake.</figcaption></figure>

### XRD.

A compared XRD analysis of a crude and washed C₂N sample was performed. The results are shown on Figure 16.
<figure><img src="image_21.png" /></figure>

**Figure 16: A)** compared powder X-ray diffractograms of a crude (red) and washed (blue) C₂N samples (same batch). Insert (green): zoom over the blue curve. Blue stars : signals from the aluminum sample holder ; red stars : signals from KCl by-product. **B)** : Powder x-ray diffractogram of the washed C₂N sample. Black stars : signals from the aluminum sample holder.

The same trends as the ones observed with CN are evidenced. The crude sample (Figure 16A, red) only shows signals from the KCl by-product. However, the same sample shows a broad peak, centered at 27° (Figure 16A, insert) after washings. This signal was absent from the crude sample. Once again, it can thus be concluded that i) the flakes mostly exist as individual/few layers objects in the crude material, getting aggregated during washings, and ii) they are thus likely to grow individually during the synthesis.

Figure 16B shows the powder X-ray analysis of the washed sample. The presence of the peak around 2θ=27° shows that the material has a layered structure. The broadness of the peak evidences an irregular spacing between the sheets. Indeed, as the sheets get aggregated during washing, some foldings and/or inclusion of residual solvents occurs between the layers, resulting in a broad distribution of interplane spacings. A shoulder around 2θ=40 degrees corresponds to the in-plane signal [⁴⁶,⁵²].

## XPS

The composition of washed samples was assessed by XPS analysis (Figure 17). A survey shows the presence of the expected elements, C and N. A prominent peak of oxygen is also observed. It may be due to the presence of trapped oxygen-containing species, a commonly observed feature with this type of material [⁴³, ⁴⁶, ⁵¹, ⁵²].
<figure><img src="image_22.png" /><figcaption>**Figure 17** : XPS analysis of a washed C₂N sample.</figcaption></figure>

The deconvolution of the C 1s peak shows two main components, at 285,9 eV (C=N) and 284.9 eV (C-C). This later peak, ubiquitous, may originate from contamination. The former one is associated with the C=N bonds of the 2D framework. Minor peaks at higher binding energies are from C=O containing adsorbed contamination, commonly encountered in XPS analyses. These may also originate from the interaction between N and included species (protons, as the material was washed with HCl, or remaining metallic cations), in line with the observed features of the N 1s signals. Indeed, the
corresponding electron-withdrawing influence may affect as well the adjacent N-bonded carbon atoms
(see below).

Regarding the N 1s signal, it may be deconvoluted in two components, at 399.12 and 400.46 eV.

The first one is attributed to pyridinic-type N, commonly observed between 398 and 399 eV, as discussed in the previous paragraph. [³⁸, ³⁹, ⁴⁰, ⁴¹]. Indeed, our synthesis does not involve any kind of NH₂ -type species, thus ruling out this possible attribution. Moreover, the dark black appearance of our material excludes any kind of reduction reaction, that would break the conjugation of the network, rendering the material colorless. Moreover, tetrachloropyrazine was introduced in close to stoechiometric amount vs. potassium. It thus excludes a possible overreduction of the C₂N backbone to N-H containing species, as this would need significant amounts of excess metal. Moreover, as the post-synthesis workup was done under ambient conditions, such adventitious reduced species would have been re-oxidized to the main C=N ones. Last, it was shown that the exact value of the position of this peak is strongly dependent on the environment of the corresponding nitrogens [³⁸, ³⁹, ⁴⁰, ⁴¹].

The High energy component (400 eV) may be attributed at first sight to residual tetrachloropyrazine.
However, this later interpretation is ruled out, as the amount of remaining C-Cl in the product is very
low (Figure 17, survey). The first interpretation also seems unlikely, as these by-products are recovered
in the supernatant and the corresponding amount of materials is very low.

This high energy component is attributed to the interaction of pyridinic nitrogens with included,
remaining metallic cations or hydrogen-bonded impurities that appeared during post-synthesis
workup. This last interpretation is in line with the presence of oxygenated species (Figure 17, O1s
analysis) and remaining potassium and calcium. The C/N ratio is 3, higher than the expected value of
2. This may be ascribed to commonly observed carbon contamination of the sample.

As said before, traces of chlorine are also observed (Figure 17), with binding energies associated with C-bonded chlorine. This confirms the very small amount of defaults lefts in the flakes, as only a very small amount of unreacted pyrazinic C-Cl bonds are present in the material.

SEM

<figure><img src="image_23.png" /><figcaption>Figure 18 : SEM imaging of a C₂N sample (after washings).</figcaption></figure>

SEM observation of an as-prepared sample (Figure 18) shows the presence of large flakes (>10µm).
Their layered structure after washing and drying is in accordance with the expected one for C₂N [⁴³].
## EDS

Along with C, N and O, EDS analysis (Figure 19) shows the presence of small amount of chlorine, in accordance with XPS analyses.

<figure><img src="image_24.png" /><figcaption>Figure 19: EDS analysis of a washed C₂N sample (star: signal from the substrate).</figcaption></figure>

## TEM

Extensive TEM analyses were performed with washed samples. Different solvents were tested to optimise the dispersions (see supporting informations, Figure S2G). Figure 20A shows a typical image of a C₂N sample, washed according to the following sequence: 1) acetone, 2) water, 3) HCl (see S2). HCl was shown to be the best choice for the redispersion of the different materials, in accordance with previous observations [⁵⁵]. The images on Figure 20 are shown at increasingly high magnifications, to assess the quality of the flakes over different scales. Very large and smooth flakes are observed, with sizes exceeding 100 µm (Figures 20A and B, others micrographs of HCl-dispersed samples are provided in S2G-c). Some flakes are so large that they were first misinterpreted as broken carbon films from the TEM grid. However, a detailed analysis of the micrographs showed that: i) some typical features of carbon grids next to the flakes are still observed, and ii) the contrasts next to different flakes on different squares were found similar, thus proving that the carbon film was still present. The flakes are rather thin, being quite transparent to the e-beam. A detailed observation of the edges of these flakes shows that they're made of few layers. (Figures 20E and F (yellow circle). These flakes appear as defect-free over very large areas (even at high magnifications, (Figures 20D→F)), assessing their structural quality. In some cases, these flakes were found to roll, forming tubular structures reminiscent of carbon nanotubes (Figure S2H).
<figure><img src="image_25.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>E</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>F</figcaption></figure>

<figure><img src="image_30.png" /></figure>
<caption>**Figure 20 : TEM analysis of a C₂N sample.**</caption>

The obtained C₂N material thus exhibits similar characteristics compared with the ones of the previously described CN.

A detailed observation of the edges of the flakes (Figure 21A, yellow circle) shows a regular pattern (Figure 21B, zoom Figure 21C), exhibiting a characteristic distance around 0.5 nm (Figure 21C). Note that this pattern is observed all the edge of the flake long.

<figure><img src="image_31.png" /><figcaption>**Figure 21 : TEM analysis of a C₂N flake showing a moiré effect.** A) the observed flake; B) zoom over the yellow circle figure A; C) zoom over the yellow square Figure B) (insert: FFT of the image); D) two stacked C₂N layers (blue and red, only nitrogen atoms shown for clarity, unit cells shown as rhombus) exhibiting the observed 0.5 nm distance.</figcaption></figure>
This pattern is attributed to a moiré effect between few superimposed layers. One possibility is exemplified on Figure 21D, with two superimposed layers following an AB type stacking. This moiré effect evidences the crystallinity of the whole flakes, as there's no reason for them to be more crystalline at their edges. Note that the material degraded very rapidly under the influence of the e-beam. Such an instability of 2D materials under TEM analyses is well documented in literature [⁵⁶].

We were not able to get convincing diffractions patterns from this material. This may be due to the previously mentioned instability of the material under electron irradiation. In some instances, clear diffraction patterns were observed (one example Figure S2N). However, these were finally considered as artifacts as i) the characteristic distance associated with the [100] plane was not observed, and ii) the brightness of the spots was too high to be associated with few layers flake. These artifacts may be due to remaining inorganics trapped inside the material. Great care should thus be taken while analysing the SAED of layered 2D materials.

## EELS

An EELS analysis of a C₂N thin flake is shown on Figure 22. The presence of nitrogen is only observed on the flake (Figure 22B), not on the carbon grid (Figure 22A). C₂N appears as oxygen-free (Figure 22C). This confirms that the oxygen signals observed by XPS analysis are probably due to contamination, or to adventitious oxygen-containing species trapped between aggregated flakes. The near edge structures observed for the C and N signals confirms that these elements are sp² hybridized in the material [⁵⁷].

<figure><img src="image_32.png" /><figcaption>**Figure 22** : EELS analysis of a single C₂N flake suspended on holey carbon grid. A): carbon grid analysis, B) and C): C₂N sample. The regular, circular holes observed on the sample are due to electron-beam-induced damages.</figcaption></figure>
### C) : synthesis of a triazine-based COF

The potassium-promoted reduction of polyhalogenated aromatic building blocks thus appears as a very efficient tool for 2D materials synthesis. We then decided to further expand the scope of this process by throwing a third example, sketched on Figure 23. This material belongs to the so-called Covalent Triazine Frameworks (CTF) family. These materials are indeed receiving an ever increasing interest, due to their interesting properties for example in the field of photocatalysis [58 59 60].

<figure><img src="image_33.png" /><figcaption>**Figure 23** : Synthesis of 2D material by reduction of a polyhalogenated triazine derivative.</figcaption></figure>

The starting tris(bromophenyl) triazine B was synthesised by acid-promoted trimerisation of 4-(bromo)benzonitrile A [61]. The synthesis/purification of CTF-2D material is detailed in the Supporting Informations (S3).

Interestingly, a detailed observation of the surface of the solid material during the reaction (S3A→E) shows that it grows in a bark-tree like fashion (S3F, orange circle), along with KBr scales (S3F, yellow circles), both visible with the naked eye. The concomitant formation of layered KBr / CTF (see below) strongly supports our initial assumption that the inorganic by-product (KBr in this case) is likely to favor the confined growth of 2D materials.

### Optical microscopy

The CTF material is obtained as a green-brown powder after drying, washing with dichloromethane and water (Figure 24A).
An observation of this material (as a water suspension) by optical microscopy shows it to be made of very large, thin flakes, with sizes ranging from a few µm to 500 µm (Figure 24B). More images are provided in the Supporting Informations (S3G→i). The characteristic features of 2D materials (such as foldings) are easily observed (Figure 24C, center image). The flakes appear as nearly transparent, evidencing their small thickness and large bandgap. Their yellow-green color is quite similar to the one observed using a different synthesis of this material [58]. Needle-like crystals of the starting material are sometimes observed (Figure 24C, yellow circle), as this starting tris(bromophenyl)triazine is highly insoluble and difficult to remove completely.

<figure><img src="image_34.png" /><figcaption>**Figure 24 :** A) crude CTF 2D material ; B) and C) optical microscopy of CTF 2D material, yellow: starting product.</figcaption></figure>

As for the previously described materials, we also found that some flakes (thin enough and not too crumpled), suspended in dispersion oil do rotate the polarisation plane of incident polarised light, as expected for single crystals. One example is shown on Figure 25. Another example is shown on S3K. These extremely thin flakes are nearly invisible (even with the microscope). They can only be detected due to their changing brightness under polarized light.

Note that the observation of other 2D materials such as single layer graphene using polarised light has already been reported [53, 54].
<figure><img src="image_35.png" /><figcaption>**Figure 25:** polarised light study of a thin CTF 2D flake (yellow circle). A): normal light; B): the same flake under cross polarized light and C) same sample upon rotation (yellow arrows on B and C as a guide to the eye). The appearance of the flake alternatively goes brighter and darker as a function of the angle. Bright crystals of the starting product are also observed.</figcaption></figure>

## SEM

Figure 26 shows an SEM image of CTF-2D material. The presence of large, folded flakes is clearly evidenced. An EDS analysis of such a flake shows the presence of C, N and oxygen-containing contamination (Figure 26 B). Note that the N signal is absent from the carbon substrate used for this experiment (Figure 26 A). The low relative intensity of the N signal is ascribed to i) the low intrinsic relative amount of carbon in the material (C/N<sub>theo</sub>=7), and ii) the presence of the underlying carbon substrate.

In the same way, an EDS analysis of the needle-shaped objects sometimes observed with the CTF flakes shows the presence of bromine along with C, N and O. This confirms the attribution of these needle-shaped objects to the starting product (S3i). A comparison between the EDS analyses of CTF-2D material and the starting product shows that the relative C/N ratio is close in both cases, as expected.
<figure><img src="image_36.png" /><figcaption>**Figure 26:** SEM/EDS analysis of CTF-2D material.</figcaption></figure>

<figure><img src="image_37.png" /></figure>

## TEM

Figure 27 shows a TEM analysis of the CTF 2D material. As for the two previous materials, the images are shown at increasingly high magnifications, to assess the quality of the flakes over different scales. It confirms CTF material to be made of very thin and large flakes, with length sometimes exceeding 100 µm (Figures 27A and B). The flakes appear as very homogeneous and seemingly defect-free, even at large magnifications (Figures 27C and D). In a few places, some contamination by what seems to be needle-shaped crystals of the starting product are however observed (Figures 27E and F), in accordance with optical (Figure 24C) and EDS (S3i) observations.
<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>
**Figure 27 :** TEM analysis of CTF-2D material.

Some rare, highly folded CTF flakes were sometimes observed. A careful examination of the corresponding high resolution TEM micrograph (one example Figure S3j) over foldings allows to get a rough estimate of the thickness of the flakes. The obtained values fall between 1.5 and 3 nm, corresponding to 5 to 10 2D layers.

Some rare tube-shaped object were also observed (Figure S3k), reminiscent of a bamboo-like growth of carbon nanotubes [⁶²]. This may be due to a molding of potassium droplets during the growth

Figure 28B shows the SAED pattern obtained from the flake shown on Figure 28A. This pattern shows the characteristic distance of 2.2 nm expected for the [100] plane of TPTZ. In the same way, a distance of 6 angströms along the z axis is observed (corresponding to the thickness of two conjugated layers) indicative of an AB type stacking.

<figure><img src="image_44.png" /><figcaption>**Figure 28:** A) TEM analysis of CTF-2D material. Yellow circle: SAED analysed area. Insert: model of the analysed area; B) corresponding SAED analysis.</figcaption></figure>

Such a pattern is observed during SAED analyses of 2D materials where several crystalline, stacked layers are parallel to the incident electron beam (Figure 28A, insert) [⁶³]. It was not possible to observe diffraction on flat-lying sheets, presumably due to their small thickness and the very porous structure of CTF-2D layers. It is likely that in the present case, the observed pattern Figure 28B originates from the folded sides of the studied area (Figure 28A, insert). This experiment confirms that the CTF flakes are crystalline.

Note that (as for the two previous 2D materials) artifacts were commonly observed during SAED experiments (some examples Figure S3l). They were finally discarded after careful analyses, as the characteristic distances expected for the [100] plane were never observed. These artifacts may originate from trapped inorganic salt, or remaining starting product.

## XPS

Figure 29 shows an XPS analysis of CTF-2D material. It shows the presence of strong signals from oxygen and silicon (with a relative O/Si intensity=2), from the silicon wafer used as the substrate for
the analysis. All the expected elements are observed, essentially carbon and nitrogen, with a relative C/N intensity of 12, the expected value being 7. This excess carbon is likely to be due to commonly encountered contamination. Only traces of bromine are observed, from remaining, unreacted starting product.

<figure><img src="image_45.png" /><figcaption>**Figure 29** : XPS analysis of CTF-2D material.</figcaption></figure>

D) Synthesis of C₂N using mercury/potassium amalgam

In line with the « double confinement » strategy, the synthesis of the C₂N carbon nitride (Figure 13) was also realised using another kind of reactive metal interface, the mercury/potassium amalgam. Indeed, the propensity of mercury to form liquid alloys with many metals, including alkaline ones is well known. We reasoned that under these conditions, the metallic interface should be more stable, as mercury itself is not reactive towards tetrachloropyrazine. This could in turn favor the formation of even higher quality 2D layers. Moreover, the liquid nature of the mercury/potassium amalgam should allow for a continuous supply of metallic potassium to the organic solvent/metal interface thus compensating for its consumption during the formation of the C₂N 2D network. Last, the inorganic by-product (KCl) is completely insoluble in the solvent used for the reaction (toluene), thus increasing the confinement effect. The details of the process are to be found in the Supporting Informations (S4A).

The initially shiny mercury interface rapidly gets covered with an orange-greenish deposit (S4A). The amalgam is removed using a pipette, and the supernating solid is washed with ethanol and water. The
material is obtained as a light brown/orange powder, with many orange scales easily visible with the naked eye (Figure 30, yellow circles).

<figure><img src="image_46.png" /><figcaption>**Figure 30 :** Hg/K-based synthesis of C₂N material.</figcaption></figure>

**Optical microscopy analysis**

Figure 31 shows an optical microscopy analysis of this material (as an EtOH/HCl suspension). Others images are provided in the SI (S4B).
<figure><img src="image_47.png" /><figcaption>**Figure 31:** optical microscopy analysis of C₂N obtained using Hg/K amalgam. The flakes were suspended in a 1 : 2 / EtOH : HCl 2M (v/v) solution.</figcaption></figure>

It shows the presence of very thin (transparent), very large flakes (often mm sized, Figure 31A), all exhibiting the characteristic orange color shades previously observed for thin C₂N flakes. This confirms that the mercury amalgam-based synthesis indeed mostly results in single/few layers flakes. Only a small amount of black materials (strongly aggregated/folded flakes) is observed.

A polarised light analysis of these flakes shows them to be quite crystalline, as they alternatively appear bright/dark as a function of the angle between the polarizers (Figure 32). One more example is shown in S4C.
<figure><img src="image_48.png" /><figcaption>**Figure 32** : polarised optical microscopy analysis of a broken, tube-shaped C₂N flake. The flakes were suspended in a 1 : 2 / EtOH : HCl 2M (v/v) solution.</figcaption></figure>

If kept in suspension for several days, the flakes tend to fold and roll, but are still crystalline (S4D).

TEM

Figure 33 shows a TEM analysis of C₂N obtained using the K/Hg amalgam. As for the previous materials, the best suspensions were obtained in acidic media. In line with the previous C₂N synthesis, very large and smooth flakes are observed, with few visible defects even at high magnifications (Figure 33E→F). An examination of the edges of these flakes clearly shows their layered structure (Figure 33E→F, yellow circles).
<figure><img src="image_49.png" /></figure>

A

<figure><img src="image_50.png" /></figure>

B

<figure><img src="image_51.png" /></figure>

C

<figure><img src="image_52.png" /></figure>

D

<figure><img src="image_53.png" /></figure>

E

<figure><img src="image_54.png" /></figure>

F
**Figure 33:** TEM analysis of K/Hg amalgam-promoted C₂N synthesis. The flakes were suspended in a 1 : 2 / EtOH : HCl 2M (v/v) solution.

In order to insure that the observed objects are not artifacts (broken carbon film from the grid), a TEM
analysis of the same sample was performed using a holey carbon grid (Figure S4E). It indeed confirms
the presence of very large flakes. Some expands over several squares on the TEM grid (Figure S4E-
A→E). All are showing the characteristic features of sheet-like materials (folds, overlaps, etc...).

Figure S4F shows a TEM micrograph of a folded, thicker flake, clearly evidencing its layered structure.

Some rare, highly folded C₂N flakes were sometimes observed. A careful examination of the corresponding high resolution TEM micrograph (two examples Figure S4G) over foldings allows to get a rough estimate of the thickness of the flakes. The obtained values fall between 4 and 9 nm, corresponding to 10 to 20 2D layers.

XPS

Figure 34 shows the XPS analysis of one single C₂N flake (Figure 34A) deposited onto a silicon wafer. It first shows the presence of strong signals from oxygen and silicon (Figure 34B) (with an O/Si atomic ratio of 2), from the native SiO₂ layer of the silicon wafer. Carbon and nitrogen are observed (Figures 34C and 34D) as expected. Regarding the N 1s signal, only one single peak is observed, with a binding energy characteristic of pyridinic-type nitrogen atoms [³⁸, ³⁹, ⁴⁰, ⁴¹]. The observed relative C/N intensity is 3, higher than the expected value of 2. This is likely to be due to commonly encountered carbon contamination. This is confirmed by the presence of a strong signal around 285 eV on the C 1s spectra (Figure 34D). Note that this C/N ratio is the same as the one measured for the previously described synthesis of C₂N (using potassium only as a reducing agent). Remainings of mercury (1 atomic %, Figure 34E) and potassium (3 atomic %, Figure 34D) are also observed, due to left starting products/by products. Chlorine is also observed as chloride ions, in a close to 1-to-1 atomic ratio vs. potassium, indicative of the presence of remaining KCl. This confirms the absence of significant amounts of left C-Cl bonds, and thus the formation of a continuous, defaults-free C₂N network. The signal of mercury steadily decreases during vacuuming (Figure 34E), confirming its presence as a metal.
<figure><img src="image_55.png" /><figcaption>**Figure 34:** XPS analysis of K/Hg amalgam-promoted C₂N synthesis. A): the analysed flake; B): survey and C)→D): high resolution analyses. The flakes were suspended in a 1 : 2 / EtOH : HCl 2M (v/v) solution. E): evolution of the signal of Hg at the same place as a function of time.</figcaption></figure>

**Discussion.**

The structure and composition of the 2D materials described in this work are assessed by
extensive cross-analyses (optical microscopy/XPS/XRD/SEM/TEM/EDS). Regarding compositions,
they're are very similar to literature-reported ones, thus assessing the succes of our « double
confinement » synthetic strategy. The first example we describe (CN carbon nitride) has already been
synthesised using a related protocol (reduction of cyanuric chloride with sodium under high pressure
conditions in the presence of nickel as a catalyst [28, 29, 30]). However, the morphology of the material
we obtained appears as very different. In our case, very large, well defined and seemingly defect-free
2D layers are obtained. These differences might be related to the difference in reactions conditions,
and/or to the smaller ionic radii of Na⁺ (Na beeing the reducing agent used in the previously cited
studies) vs. K⁺. Interestingly, our process lead to a material that easily redisperses in water.

The same features were also observed for the two other COFs described in the present work, i.e. very
large, and defect-free flakes with good composition analyses. Accordingly, a comparition between the
EDS and XPS analyses of CN, C₂N, and CTF shows a continuous decrease of the relative amount of
nitrogen from CN to CTF, in accordance with the corresponding respective theoretical nitrogen
contents in their structures.

In the case of CN and C2N, DRX shows the presence of the characteristic peak associated with
interplane spacing [43, 46, 51, 52]. However, this peak exhibits some specific features. First, it only appears
after washing the samples. Second it is quite broadened. As TEM demonstrates the very high quality
of the flakes, we consider that the broadness of the peak is likely to be due to a broad distribution of
interplane spacings. This originates from the fact that the layers grow individually, getting stacked
during the washing steps. This may induce an irregular spacing after drying. This may also eases the
inclusion of metallic cations inside the coordinating holes. Indeed, a few percents of remaining K and
Ca (a commonly observed impurity in commercial-grade potassium) are present in our CN and C₂N samples.

For the first time, the flakes described in our work are sufficently large and defect-free to behave as single-crystals, as evidenced by polarised light microscopy analyses.

TEM confirms the exceptional quality/crystallinity of the 2D layers of all three COFs. The best dispersions (thinnest and largest flakes) were observed upon treatment of the as prepared materials with HCl, in comparison with the other dispersant that we used. Aqueous HCl is thus the best choice for the suspension/processing of these materials, in line with previous observations [⁵⁵]. This may be due to partial protonation of embedded nitrogen atoms, inducing electrostatic repulsions between flakes. This may also explain the presence of higher binding energy components in the N 1s XPS analyses.

Interestingly, thin flakes of the three COFs that we synthesised exhibit different intrinsic colors, brown for CN, greenish-brown for CTF, whereas C₂N exhibits a deep orange one. This evidences the smallest bandgap of the later.

It should be underlined here that for all the COFs that we synthesised, the whole materials are made of flakes such as the ones shown on the different figures (both on the paper and in the SI). Indeed, all the observed areas on several TEM grids from different syntheses do show similar objects.

Improving the stability of the metallic interface by using potassium-mercury amalgam considerably improves the quality of the C₂N flakes, as expected for a surface-confined growth process. For the first time, millimeter-scale flakes of C₂N were obtained, an exceptional feature in the field of COFs.

The successful synthesis of three different 2D networks, using very different building blocks shows that our metallic potassium based process appears as quite general. This opens the door towards the synthesis of COFs with more complex structures.

## Conclusion.

A simple protocol for the synthesis of graphene-related COFs was designed, and exemplified on three different examples.

The exceptional quality of the resulting materials (in terms of flakes sizes and defect-free nature) shows that the proposed « double confinement strategy » approach appears interesting for the synthesis of high quality 2D materials. This high quality, along with the soft conditions used and common laboratory equiments makes our process a significant improvement in the field.

The fact that the characteristic X-ray diffraction peak around 26° is only observed on washed samples tend to indicate that the growth of these 2D materials occurs as single/few layers, a new feature for wet syntheses of 2D materials.

These materials hold great promises in the field on nanoelectronics (high band gap support for graphene, transport studies, etc...). They're also interesting for photophysics, due to: i) the large size of the crystalline domains, ii) their low defects content and iii) their direct synthesis as highly exfoliated sheets. These studies are underway, and will be reported in due course.

## Acknowledgements

We gratefully acknowledge Dr. Frederic Fossard (LEM, ONERA) for some of the elemental analyses shown in the paper.
The present work has benefited from Imagerie-Gif core facility supported by l'Agence Nationale de la Recherche (ANR-11-EQPX-0029/Morphoscope, ANR-10-INBS-04/FranceBioImaging ; ANR-11-IDEX-0003-02/ Saclay Plant Sciences).

**Data availability:** The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
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