A brand-new two-dimensional (2D) carbon crystal, different from graphene, has actually been prepared from 1,3,5-trihydroxybenzene, consists of 4-carbon and 6-carbon rings in 1:1 ratio, named 4–6 carbophene by authors, in which all carbon atom possess sp2 hybrid orbitals with some distortion, forming considerable conjugated π-bonding planar structure. The angles between the 3 σ-bonds that the carbon sp2 orbitals are about 120°, 90°, and also 150°. Every of the three non-adjacent sides of a 6C-ring is common with a 4C-ring; and also each the the two opposite political parties of a 4C-ring is mutual with a 6C-ring. Dodecagonal holes through a diameter of almost right 5.8 Å are regularly situated throughout the 2D carbon crystal. Also though the link energies in 4–6 carbophene room weaker than those in the graphene, the brand-new planar decision is fairly stable in ambient conditions. The 4–6 carbophene have the right to be synthetized from 1,3,5-trihydroxybenzene or other benzene derivatives with dehydration and also polymerization reactions, and also may possess several feasible patterns that kind a family members of 2D carbon crystals. A possible side reaction involving 1,3,5-trihydroxybenzene is also discussed, i m sorry may produce a carbon-oxygen two dimensional crystal.

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Graphene1,2 is an allotrope of element carbon in the form of one atomic-scale, two-dimensional, honey-comb lattice, in which all carbon atoms are in sp2 electron configuration, resulting in an extensive conjugated π-system. Graphene is the simple structural element of several other carbon allotropes, consisting of graphite3,4, carbon nanotubes5 and also fullerenes (C60)6. Graphene is an excellent material possessing plenty of outstanding physical and also chemical properties, such as high electrical and thermal conductivity7, huge certain surface area8, good optical transparency2, and also excellent mechanical strength9,10. Therefore, graphene is supposed to have many applications in wide areas11,12.

From material science and structural chemistry viewpoints, graphene is a two-dimensional (2D) crystal and a 2D carbon material13. Because the an initial 2D carbon decision graphene was discovered in 20041,2, several various other 2D crystals and materials the carbon have actually emerged14,15. In recent years the 2D crystals and also materials come to be a booming research field. A an extensive review top top this ar can be uncovered in a review article by Peng et al.16. Chemically, a sheet of graphene is a big aromatic molecule consisting of hexagonal planar structure units17,18 the are comparable to anthracene and also phenanthrene. In graphene, every carbon atoms are in traditional sp2 digital configuration so the the angles between the three σ-bonds the carbon sp2 orbitals space 120°, 120°, and also 120°, simply as they are in the carbon atom in benzene and also other aromatic molecules19,20.

Actually, the orbitals in carbon atoms can likewise be in a distorted sp2 hybrid form, resulting in planar structure units other than hexagons21,22,23,24. This opportunity raises an interesting question: “Are there various other two-dimensional carbon crystals that are different from graphene?” We have actually explored this question from quantum chemistry theory and also synthetic chemistry.

In this paper, us report top top a new 2D carbon decision that has been ready from 1,3,5-trihydroxybenzene with dehydration and also polymerization reactions. The new kind of 2D carbon crystals may exist in several possible forms, resulting in a 2D family of closely related crystals.


The carbon atom of benzene have actually standard sp2 hybrid orbitals, in which the three σ-bonds form three equal angle 120°, 120°, and also 120°, as displayed in Fig. 1(a). In contrast, in cyclobutadiene, the carbon atoms have distorted sp2 hybrid orbitals, so the the 3 angles in between the three σ-bonds space 90°, 135°, and also 135°, shown in Fig. 1(b). Biphenylene is a well-studied planar molecule, quite stable both experimentally and thermodynamically25,26, which is a polycyclic hydrocarbon, written of two benzene ring joined with each other by a cyclobutadiene, thus developing a 6-4-6 arene system. Every of the 4 carbon atoms in the square ring cyclobutadiene moiety of biphenylene has distorted sp2 hybrid orbitals and associated σ-bond angles of 120°, 90°, and also 150°, as presented in Fig. 1(c). A number of higher polycyclics include the biphenylene nuclei were prepared and studied in 1970 s and 1980s27,28,29,30,31,32. This more complicated molecules, containing numerous biphenylene structures, room actually little pieces the 2D carbon crystals. Therefore, we thought that it should be feasible to construct brand-new 2D carbon crystals, various from graphene, utilizing the carbon atoms through these distorted sp2 electron orbitals.


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(a) In benzene the carbon atoms space in traditional sp2 hybrid orbital, in which the three angles between the 3 σ-bonds are 120°, 120°, and 120°. (b) In cyclobutadiene the carbon atoms space in distorted sp2 hybrid orbital, in i m sorry the three angles in between the 3 σ-bonds are 90°, 135°, and also 135°. (c) Biphenylene consists of 2 benzene rings joined with each other by a cyclobutadiene, thus forming a 6-4-6 arene system. In biphenylene the four carbon atoms in the square ring space in distorted sp2 hybrid orbital, in i beg your pardon the three angles in between the three σ-bonds are 90°, 120°, and also 150°.


The in its entirety chemical reactions in the synthesis of brand-new 2D carbon crystal room outlined in Fig. 2. The polymerization reaction might happen through intra-molecular dehydration the 1,3,5-trihydroxybenzene, displayed in Fig. 2(a). After 3 water molecules are stripped indigenous a 1,3,5-trihydroxybenzene molecule by dehydrant aluminum oxide (γ-Al2O3), the bare 6 C rings (benzynes) combine with every other, forming a little fragment that the 2D carbon crystal. The polymerization reaction additionally could happen through inter-molecular dehydration in between 1,3,5-trihydroxybenzene molecules, as shown in Fig. 2(b). With the authorized of much more 1,3,5-trihydroxybenzene molecule the pieces of 2D carbon crystal grow easily (Fig. 2c).


Figure 2: Illustration of chemistry reaction device from 1,3,5-trihydroxybenzene to 4–6 carbophene 2D crystal.

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(a) The polymerization reaction might happen with intra-molecular dehydration of 1,3,5-trihydroxybenzene. (b) The polymerization reaction also could happen through inter-molecular dehydration in between 1,3,5-trihydroxybenzene molecules. (c) v the involvement of more 1,3,5-trihydroxybenzene molecule the pieces of 2D carbon crystal grows increase quickly.


Figure 2 is just a straightforward illustration of the all at once chemical reaction of 1,3,5-trihydroxybenzene that have the right to lead come a 4–6 carbophene 2D crystal. The really reaction mechanism may be much an ext complex. The dehydration reaction of 1,3,5-trihydroxybenzene may take place in number of steps, shedding one, two, and also three water molecule step-by-step, forming various intermediate arynes (or benzynes) in the process33,34. In addition, the dehydration and also polymerization reaction of 1,3,5-trihydroxybenzene might be add by various side reactions, yielding spin-offs under certain conditions. Among such side reaction is learned in next section, i beg your pardon produces a benzene-ether 2D crystal, a by-product.

In this study, the synthetic of the new crystals were performed in a quartz glass tube heating system in one argon atmosphere at a temperature of 350–380 °C utilizing γ-aluminum oxide (γ-Al2O3) as the dehydrant. Comprehensive experimental operations and conditions are included in the Supplementary details (SI-1).


QM calculations that reaction Gibbs totally free energies

The inside energies (ΔU°R), enthalpies (ΔH°R), and also Gibbs free energies (ΔG°R) the the synthesis reactions for the brand-new 2D carbon crystal have been calculated using DFT an approach B3LYP/6-311 + G(d,p)31,32,33,34 for two possible reaction pathways. In the an initial pathway, 2 water molecules are removed from 2 1,3,5-trihydroxybenzene molecules, developing a biphenylene molecule.


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Based on QM calculations, the Gibbs cost-free energy of this reaction at 350 °C (T = 623 K) is positive; i.e., ΔG°R1_1 = 28.84 kCal/mol. In the following step, the water molecules are took in by the dehydrant, γ-aluminum oxide:


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The Gibbs cost-free energy because that the adsorption of one mole water molecule is calculate to it is in negative; i.e., ΔG°R1_2 = −66.02 kCal/mol. Therefore, the complete Gibbs totally free energy that the entire very first reaction pathway is ΔG°R1 = ΔG°R1_1 + ΔG°R1_2 = 28.84 + −66.02 × 2 = −103.19 kCal/mol; i.e., significantly an adverse and thermodynamically spontaneous.

In the second reaction pathway, one water molecule is eliminated from two 1,3,5-trihydroxybenzene molecules, creating a benzene-ether molecule:


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In this first step of the reaction, the Gibbs free energy is positive, ΔG°R2_1 = 7.49 kCal/mol. In the 2nd step, once one mole that water is took in by aluminum oxide (Al2O3), the reaction Gibbs complimentary energy ΔG°R2_2 = −66.02 kCal/mol. As a result, the complete Gibbs free energy that the 2nd pathway is ΔG°R2 = ΔG°R2_1 + ΔG°R2_2 = 7.49–66.02 = −58.53 kCal/mol.

Therefore, the total Gibbs totally free energy is an ext favourable for the very first reaction pathway (ΔG°R1 = −103.19 kCal/mol) than for the 2nd pathway (ΔG°R2 = −58.53 kCal/mol). However, the previous pathway needs a greater reaction temperature than the latter, since of the higher reaction enthalpy in the an initial step (ΔH°R1_1 = 46.52 kCal/mol, compared to ΔH°R2_1 = 11.40 kCal/mol).

The QM calculation outcomes are summary in Table 1, and the in-depth QM calculate results have the right to be uncovered in Supplementary info (SI-2 and also SI-3).


Table 1 The inside energies, enthalpy, and totally free energies that two possible reaction courses from 1,3,5-trihydroxybenzene (B3LYP/6-311 + G(d,p), 350 °C).

Crystal structure of the 4–6 carbophene

The membrane of 2D carbon crystal, developed on copper foil and also on a quartz glass sheet, are displayed in Fig. 3(a) and (b). As soon as the 4–6 movies are removed from the glass sheet utilizing N,N-dimethyl formamide, transparent carbon films are obtained. The SEM (scanning electron microscopy) images35 that the films are displayed in Fig. 3(c) and also (d). ~ the copper foils are liquified in FeCl3 + HCl equipment completely, the movies of the 4–6 carbophene were separated, as displayed in Fig. 3(e) and (f).


(a) comparison of copper foil before (left) and also after (right) the 4–6 carbophene film was coated. (b) to compare of quartz glass sheet prior to (left) and also after (right) the 4–6 carbophene film was coated. (c) The SEM (scanning electron microscope) image of the 2D carbon crystal film separated indigenous copper foil. (d) The SEM image of the 2D carbon decision film separated indigenous quartz glass sheet. After the carbon movies of the 2D crystal space separated native the copper foil and quartz glass sheet, transparent carbon films of 4–6 carbophene are obtained. (e,f) The 4–6 carbophene films separated native copper foil.


The numbered red squares ~ above the 4–6 carbophene movie separated indigenous the quatz glass paper (Fig. 3(d)) suggest the areas where the element analyses to be performed using the EDS (energy dispersive spectrometry)36 listed by the SEM tool (IXRF, 550i). The facet components in the 4–6 carbophene are detailed in Table 2, showing that carbon is the dominant aspect in the films. Other elements (oxygen and also silicon) exist only in map quantities, which might originate native the quartz glass. These data confirm that the 2D carbon crystal is a carbon allotrope.


The 2D carbon crystal consists of four-carbon rings and also six-carbon rings, in i beg your pardon the 3 non-bordering sides of a six-carbon ring are common by three four-carbon rings; and the 2 opposite sides of a four-carbon ring are mutual by two six-carbon rings. The framework of the 2D carbon crystal is presented in Fig. 4. The next lengths the the six-carbon rings are approximately 1.42~1.46 Å, and also the two bonds, joining the 2 six-carbon rings, are roughly 1.50~1.52 Å. In the 2D carbon crystal dodecagon holes through diameter approximately 5.8 Å are consistently located. The 2D crystal of 4–6 carbophene has actually two species of edge structure patterns. In one edge sample the grooves room separated by 6C-ring spur; and also in the other pattern the grooves are separated through spur the (6C-ring)-(4C-ring)-(6C-ring).


The 2D carbon crystal consists of 4-carbon rings and also 6-carbon rings in ratio 1:1. The next lengths the the six-carbon rings are about 1.42~1.46 Å, and also the 2 bonds, authorized the two six-carbon rings, are approximately 1.50~1.52 Å. In the 2D carbon crystal dodecagon holes v diameter approximately 5.8 Å are consistently located. The 2D carbon crystal has two species of edge structural patterns. In one edge pattern the grooves space separated through six-ring spur; and also in the various other pattern the grooves space separated through spur that (6C-ring)-(4C-ring)-(6C-ring).


XRD sample of 4–6 carbophene crystal

The X-ray diffraction (XRD) pattern of the new carbon decision (Fig. 5a) has only one height that is not an extremely sharp, comparable to the XRD pattern of diminished graphene oxide (rGO)37,38, presented in Fig. 5(b), denote a few-layer structure. However, the position of diffraction angle (2θ) in the 4–6 carbophene 2D decision is roughly 2θ = ~23°, no the 2θ = ~26° it was observed in the XRD of graphene39. In graphene, the interlayer street (d002) is 0.343 nm for the AB-stack40. Based upon the XRD equation


(a) The XRD sample of 4–6 carbophene has only one peak, describe a few-layer structure. The place of diffraction angle in the 4–6 carbophene 2D carbon decision is around 2θ = ~23°. (b) The XRD pattern of reduced graphene oxide (rGO). The position of diffraction angle of rGO is at 2θ = ~26°. In graphene the interlayer distance (d002) is 0.343 nm for the AB-stack. The estimated interlayer street in the brand-new 2D carbon crystal is 0.387 nm. The smaller 2θ and larger d002 might be resulted in by the distorted sp2 hybrid orbitals of carbon atom in the 4–6 carbophene crystal.


estimated interlayer distance in the 4–6 carbophene is 0.387 nm. The bigger d002 and also smaller 2θ may be caused by the weaker van der Waals interaction in between the carbon class that outcomes from the distorted sp2 hybrid orbitals that carbon atoms in the 4–6 carbophene.

Raman spectrum of 4–6 carbophene 2D crystal

The 2D carbon crystal has actually a lot more complex symmetrical framework than does graphene. Graphene just possesses the hexagonal symmetry. In comparison in the 4–6 carbophene there are 4-carbon rings, 6-carbon rings, huge dodecagonal holes, and two species of edges. The Raman spectrum the 4–6 carbophene is displayed in Fig. 6, i beg your pardon was derived using a laser beam with a 785 nm wavelength. Whenever the strength of the laser beam to be stronger, the 4–6 carbophene movie broke, denote a weaker bond energy in the brand-new material 보다 in graphene.


(a) The brand-new 2D carbon crystal has actually much more complicated symmetrical structure than the of graphere. Graphene only possesses hexagonal symmetry. In comparison in the 4–6 carbophene there are 4-carbon rings, 6-carbon rings, huge dodecagon holes, and also two varieties of edges. The form of Raman spectrum that the 4–6 carbophene is like a rock v a steep left side, a large and cragged top, and also a slope ideal side. (b) Raman peaks connected in the Raman spectrum of new 2D carbon crystal. Simulation and also fitting calculations reveal that the Raman spectrum of the 4–6 carbonphene crystal is composed of 6 gauss peaks.


The shape of Raman spectrum that the 4–6 carbophene is very different from the of graphene41,42,43. The Raman spectrum of the 4–6 carbophene has a steep left side, a large and cragged top, and also a sloping appropriate side, as presented in Fig. 6(a). Simulation and fitting calculations expose that the Raman spectrum the the 4–6 carbophene crystal is composed of 6 Gaussian peaks, as presented in Fig. 6(b). The wavenumbers and intensities of the 6 peaks are provided in Table 3. The peaks in the Raman spectrum of 4–6 carbophene have to be identified by experts.


Table 3 The wavenumbers and intensities that the six gauss peaks in the Raman spectrum that the 4–6 carbonphene crystal.

XPS spectra and also analysis

The chemical composition the the 4–6 carbophene samples were additional studied by XPS analysis44,45,46,47. Figure 7(a) is the completely XPS spectrum of the 2D decision sample coated top top copper foil, which shows peaks matching to facets C, Cu, Al, and O. The obvious resource of the Cu peak is the copper foil, if the Al peak likely originated from the aluminum oxide dehydrant. However, the resource of the oxygen is not clear. As pointed out above, two feasible products can be produced from the reactant 1,3,5-trihydroxybenzene; one is 4–6 carbophene (containing C and H) and also the various other is the benzene-ether 2D decision (containing, C, H, and O). The high resolution XPS spectrum the carbon is displayed in Fig. 7(b), in which the asymmetrical peak at 284.0 eV suggests that the carbon atoms room in sp2 electron configuration and result in a π-π structure46. Figure 7(c) is the high resolution XPS spectrum of the oxygen signal. Generally the binding energies that oxygen atoms in metal oxides room in the selection 529~530 eV, and the oxygen atoms in essential compounds are in the 531.5~532 eV range42. In Fig. 7(c) the optimal position that oxygen is in ~ 530.0 eV, indicating the the oxygen atoms are most likely from the oxides that aluminium and/or copper; i.e., not from organic compounds.


(a) fully scanned survey of the 4–6 carbophene sample coated ~ above copper foil. (b) High resolution XPS spectra that carbon (C1s) in 4–6 carbophene sample. The asymmetrical top at 284.0 eV indicates that the carbon atoms are in sp2 electron configuration and compose π-π structure. (c) High resolution XPS spectra of oxygen (O1s) in 4–6 carbophene sample. The optimal position the oxygen is at 530.0 eV, indicating the oxygen atoms space from oxides that aluminium and (or) copper, not from essential compounds.


Based on quantum chemical considerations, the 2D planar crystals the carbon have the right to only be created of carbon atoms with sp2 electron configuration. In addition to the traditional sp2 hybrid orbital of carbon atoms, which compose the hexagonal framework units, carbon atoms can likewise have distorted sp2 hybrid orbitals, such together those that have actually been report in square48,49 or octagonal50,51 structure units, such as in cyclobutadiene, biphenylene48,49, and planar cyclooctatetraene50,51,52,53,54,55,56,57. In short, 2D carbon crystals, in addition to graphene, might be fabricated indigenous carbon atoms with the distorted sp2 hybrid orbital.

During the synthesis, the 4–6 carbophene samples deserve to be conveniently contaminated with the dehydrant aluminum oxide if vacuum pumping and also purging through argon. Therefore, it to be not feasible to attain AFM (atomic force microscopy) and STM (scanning tunneling microscope) pictures at atomic level. In order to attain the high purity samples and high resolution AFM images that room needed, the synthetic methods and equipment will have to be improved.

However, despite the absence of such observations, we space confident the 4–6 carbophene to be formed since the calculation Gibbs free energy that the reaction is quite an adverse (ΔG°R1 = −103.20 kCal/mol), indicating the its development is thermodynamically favored. In addition, the XRD and XPS experiments carry out indirect evidence of the structure and also composition the the 4–6 carbophene.

The 2D decision of 4–6 carbophene is less stable than graphene due to the fact that of the distorted sp2 hybrid orbitals. However, it deserve to exist in ambient conditions and also may possess chemical and also physical properties various from those of graphene. Because that example, the planer crystal may have a larger particular surface area and also lower thickness than that of graphene because of the large dodecagonal holes in the 4–6 carbophene structure. Further, we expect that 4–6 carbophene may have actually several different 2D crystal patterns. For example, Fig. 8(a) mirrors a possibility of a larger hexagonal symmetrical structure resulting from combinations of quick (6C-ring)-(4C-ring)-(6C-ring) segments (Fig. 8(b)). In addition, within each hexagonal hole there are 6 vertex carbon atoms, to which a solitary valence atom or atomic group might be bonded (Fig. 8(c)).


(a) The 2D carbon crystal is composed of involvement hexagonal units. (b) every side the the hexagonal unit is linearly ranged (6C-ring)-(4C-ring)-(6C-ring). (c) In every hexagonal hole over there are six carbon vertexes, on i beg your pardon a solitary valence atom or atomic team is bonded.


In conclusion, the significant findings native this study have the right to be summarized as follows. (1) A new form of carbon 2D crystals has been built using planar 4-carbon rings and 6-carbon rings, i m sorry we have actually named “4–6 carbophenes”. (2) every carbon atom in the 4–6 carbophenes room in sp2 electron configuration through some distortion, causing a huge planar conjugated π-system. (3) The 4–6 carbophene may include several different 2D crystal patterns, creating a family members of such crystals. (4) The 4–6 carbophenes have the right to be synthetized through dehydration and also polymerization reaction of 1,3,5-trihydroxybenzene or other benzene derivatives. (5) The 4–6 carbophenes are less stable 보다 graphene due to the fact that of the distorted sp2 orbitals; however, they deserve to exist in ambient conditions and may own chemical and physical properties the are various from those the graphene.


How to mention this article: Du, Q.-S. Et al. A new form of two-dimensional carbon crystal ready from 1,3,5-trihydroxybenzene. Sci. Rep. 7, 40796; doi: 10.1038/srep40796 (2017).

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Affiliations

State vital Laboratory of Bioenergy Enzyme Technology, National engineering Research center for Non-food Biorefinery, Guangxi Academy that Sciences, Nanning, 530007, Guangxi, China

Qi-Shi Du, Pei-Duo Tang, Hua-Lin Huang, Fang-Li Du, Kai Huang, Neng-Zhong Xie, Si-Yu Long, Yan-Ming Li, Jie-Shan Qiu & Ri-Bo Huang

Gordon Life scientific research Institute, 53 southern Cottage Road, Belmont, 02478, MA, USA

Qi-Shi Du

Institute that Carbon Materials, Dalian college of Technology, No.2 Linggong Road, Ganjingzi District, Dalian, 116024, Liaoning, China

Jie-Shan Qiu


Contributions

D.Q.S., Q.J.S. And also H.R.B. Conceived the concept and method and design the experiments; T.P.D., H.H.L. And also H.K. Carried out the experiments. D.F.L. And also X.N.Z. Analyzed the samples. L.S.Y. And also L.Y.M. Prepared the materials and instruments. D.Q.S. And also H.R.B. Created the article. Every authors disputed the results and also commented on the manuscript.