A planar-sheet nongraphitic zero-bandgap sp² carbon phase made by the low-temperature reaction of γ-graphyne

Significance

A nongraphitic planar-sheet sp² carbon phase having a zero bandgap is made by low-temperature intrasheet thermal reaction that eliminates most of the acetylene groups in γ-graphyne without breaking graphyne bonds. Its formation and properties are experimentally and theoretically analyzed. While the normally predicted intrasheet acetylene reaction to form polyene chains is also considered as a reaction possibility, it is excluded by experimental results.

Abstract

The highest sheet symmetry form of graphyne, with one triple bond between each neighboring hexagon in graphene, irreversibly transforms exothermically at ambient pressure and low temperatures into a nongraphitic, planar-sheet, zero-bandgap phase consisting of intrasheet-bonded sp² carbons. The synthesis of this sp² carbon phase is demonstrated, and other carbon phases are described for possible future synthesis from graphyne without breaking graphyne bonds. While measurements and theory indicate that the reacting graphyne becomes nonplanar because of sheet wrinkling produced by dimensional mismatch between reacted and nonreacted sheet regions, sheet planarity is regained when the reaction is complete. Although the observed elimination of triple bonds to make parallel planar sp² carbon sheets likely requires ordered transformation within each sheet, diffraction data for reacted multisheet stacks indicate that the relative lateral positions of neighboring sheets are disordered, as predicted, since no crystalline diffraction peak (other than for the intersheet spacing) is observed.

The structure and properties of periodic carbon phases containing aromatic rings separated by triple bonds were predicted in 1987 by Baughman, Eckhardt, and Kertesz (1), and oligomers were later made by Haley (2, 3), Diederich (4, 5), and collaborators. The two most symmetric such carbon sheet structures were synthesized, which contain the aromatic rings in graphene separated from neighboring aromatic rings by solely only one acetylene or one diacetylene group. For simplicity, we here call these structures graphyne and graphdiyne, rather than the more precise γ-graphyne and γ-graphdiyne. Zhu’s group (6) first synthesized graphdiyne. There have been numerous efforts to synthesize graphyne (7), and the method we will use is the irreversible Sonogashira coupling of Rodionov’s teams (8).

We here make an sp² carbon phase by heating the previously described graphyne (8) at ambient pressure from above ~160 to ~310 °C. Various sp² carbon phases have been proposed by Hoffman et al. (9) and by Belenkov and Ali-Pasha (10), including all-sp² carbon phases theoretically derived by complete intersheet reaction of the triple bonds in graphyne and graphdiyne to form polyacetylene chains. Possible high-pressure transformation products were proposed for graphyne (11, 12) and graphdiyne (13) that additionally involve benzene ring coupling into a lonsdaleite-diamond (11) or helical configuration (12). However, the sole experimental evidence for a phase transformation was from Raman spectroscopy during diamond-pressure-cell graphdiyne compression, where a phase transition started at ~5.2 GPa (14), and the thermal or laser-produced heating of graphyne (8). This pressure-induced graphdiyne transformation reportedly results from irreversible intersheet sp bond to sp² bond conversion, which ends by ~13 GPa (14).

Little was known about graphyne’s ambient-pressure thermal transformation (8), except that it irreversibly decreases the quantity of sp carbons. New measurements are described for this transformation and compared with new predictions for low-energy sp-free and low-sp content phases that might result. Two transformation types are considered. The first is polyacetylene chain formation by intersheet triple bond reaction, and the second is an intrasheet reaction that converts all sp carbons to sp² ring carbons. We show that this low-temperature reaction at ambient pressure is intrasheet. The product is a planar-sheet zero-bandgap sp² carbon phase that was neither experimentally nor theoretically known.

Results

Synthesis and Characterization of ¹³C-Labeled Graphyne.

The synthesis and characterization of the precursors used to make graphyne with ¹³C-labeled acetylene groups are described in SI Appendix and in associated SI Appendix, Figs. S1–S7. Nuclear magnetic ¹³C spectroscopy results for graphyne containing the ¹³C-labeled acetylene groups before and after thermal annealing are in SI Appendix, Figs. S8 and S9, respectively. Characterization of impurities in graphyne is in SI Appendix, Figs. S10–S15.

Measurements for Graphyne’s Transition.

Differential scanning calorimetry (DSC) shows that the investigated graphyne has a large exothermic transformation (Top panel of Fig. 1A). This transformation varies in temperature, likely depending on graphyne perfection, but is typically detected between 160 and 310 °C, with a heat-release peak between 250 and 280 °C. The transition heat varied from ~0.45 to ~0.55 kJ/g, depending upon the graphyne synthesis run. Heating graphyne at 5 °C/min from RT to 305 °C produces ~3% weight loss and further heating to 600 °C provides 43% additional weight loss (SI Appendix, Fig. S16). Measurements between 150 and 350 °C provided evidence of C₆H₅Br emission, by likely decomposition of groups on graphyne sheet edges (SI Appendix, Figs. S10–S14).

Fig. 1.

DSC, spectroscopic, and electrochemical indication of the thermal transformation of graphyne. (A) The DSC-measured temperature dependence of exothermic heat flow above nearby pretransition and posttransition heat flows, using 2 °C/min heating in N₂ (Top panel) and the annealing temperature dependence of 1) the FT-IR absorption for internal and terminal triple bonds (second panel); 2) the Raman triple bond intensity using low-power 532 nm excitation (third panel); and 3) the average band gaps obtained using a Tauc plot (15) of absorbed energy (brown curve) and obtained electrochemically (black curve), as described in the text (fourth panel). The second average bandgap estimate in this fourth panel was obtained from SI Appendix, Fig. S21, which is from potential scans like in SI Appendix, Fig. S20. The wavenumbers of internal and terminal triple bonds were 2,193 cm⁻¹ and 2,110 cm⁻¹, respectively. The FT-IR absorption (B) and Raman spectra (C) used to obtain the results in the second and third panels of (A), respectively, are shifted vertically for clarity. (D) The pictured Tauc plot method was used to obtain the results in the fourth panel of (A) from the absorbance spectra obtained after progressively higher temperature anneals (SI Appendix, Fig. S18). The results in (B–D) and SI Appendix, Fig. S18 were conducted at RT after step-by-step heating the initially unreacted graphyne in 0.1 mTorr vacuum for 1 h at the indicated progressively higher temperatures.

The lower Fig. 1A panels are RT measurements for gradually annealed samples. The top three Fig. 1A panels and Fig. 1 B and C show that the exothermic DSC transition occurs at temperatures where decreases occur in infrared absorbance due to internal (2,193 cm⁻¹) and terminal triple bonds (2,110 cm⁻¹) and in the Raman peak intensity from internal triple bonds. Since the Raman scattering requires strong laser excitation, that might overheat the sample, all measurements were conducted at a low beam power of 0.2 mW (SI Appendix, Fig. S17), which did not affect the intensity of triple-bond peak during 120-min excitation (SI Appendix, Fig. S17 and S18). SI Appendix, Fig. S19 shows the room temperature IR absorbance spectra for graphyne that was annealed for an hour at progressively increased higher temperatures. The red shift of the plasmonic edge shown by the dashed arrow line in SI Appendix, Fig. S19 is responsible for the derived band gap in Fig. 1A (Bottom panel).

Evidence for complete conversion of sp carbons to sp² carbons by heating to 325 °C was obtained by ¹³C NMR measurements (SI Appendix, Figs. S8 and S9) on thermally annealed graphyne in which all sp carbons were ¹³C labeled (SI Appendix, Figs. S1–S9). A significant decrease in the T₁ relaxation time for the labeled nuclei was observed, suggesting that either the material’s surface became more accessible to ambient oxygen, the material’s crystallinity decreased, or both factors were at play.

The bottom Fig. 1A panel alternatively uses the Tauc (15) absorption plots of Fig. 1D or an electrochemical method (SI Appendix, Figs. S20 and S21) to obtain the average bandgap. The latter annealing temperature dependence of average bandgap resulted from normalizing the ratio of redox peak height to CV capacitance, so that the redox-peak-separation bandgap of nonannealed graphyne was obtained at 25 °C. This capacitance normalization is needed because the diffraction-confirmed transitory sheet wrinkling during transformation (Fig. 2) increases the available area for double layer and Faradaic processes. Both bottom panel bandgaps completely vanish during annealing to ~350 °C. Our previous redox-peak-separation and Tauc plot bandgaps for graphyne were 0.47 eV and 0.48 eV (8), respectively, which are close to those for the presently used graphyne sample (0.52 eV and 0.56 eV). Due to the small graphyne amount made in each synthesis run (300 mg or smaller), each sample investigated in Fig. 1A was from a different batch, and we know this can influence the phase transition.

Fig. 2.

Changes in intersheet diffraction lines measured in situ in a synchrotron, using 0.590 Å radiation, during thermal annealing of graphyne from 100 to 320 °C using 2 °C/min heating. (A) The intersheet d-spacing, its Full Width at Half Maximum (FWHM), and its integrated peak intensity as a function of the maximum temperature of thermal exposure during the temperature scan, when measured for graphyne powder on a nonrotated flat-substrate. The time for diffraction data collection during the thermal scan was ~2 min during which the sample temperature changed only about 4 °C. (B) The observed X-ray diffraction spectra for different peak annealing temperatures, in the d-spacing range where graphyne-related intersheet diffraction is observed. Little or no interplanar line intensity is seen between ~172 and ~256 °C, likely because sheet wrinkling, caused by intrasheet strain mismatch between reacted and nonreacted sheet areas, decreases the intensity of the interplanar spacing to near zero. When the transformation is sufficiently high that some planar reacted sheet regions can stack parallel because of increased unwrinkling, diffraction due to the reacted product phase increasingly appears. Since intersheet reaction by formation of polyene chains would decrease the interplanar spacing, this observed increase in d-spacing indicates that this thermal transformation is intrasheet.

DSC measurements revealed surprising transformation kinetics. A graphyne sample was first heated in the DSC from 25 °C to a low temperature (150 °C) within the exotherm range, annealed for an hour at this temperature, and then naturally cooled back to RT, before this whole process was conducted during the next scans from 25 °C to successively higher annealing temperatures (SI Appendix, Fig. S22). Little or no exotherm occurred until reaching the previous annealing temperature, and then, it largely stopped when the targeted new constant annealing temperature was reached. This behavior suggests that regions of reacting graphyne transform when their local free-energy change becomes near zero, like for the free-energy-driven melting of a mixture of non-melt-miscible particles. SI Appendix, Fig. S22 shows the close correlation between these results and the heat flow during uninterrupted heating of this graphyne sample at 2 °C/min.

Transmission electron microscope images (SI Appendix, Fig. S23A) indicate that the lateral size of individual graphyne sheet stacks is not much larger than about 300 nm. SI Appendix, Fig. S23B shows the electron diffraction pattern for these individual sheet stacks, which is the same as previously reported in our publication on the synthesis of the presently used graphyne (8). SI Appendix, Fig. S23C shows a SEM microscope image of a typical large graphyne particle, which is a poorly orientationally ordered aggregate of an enormous number of individual thin graphyne plate stacks.

Synchrotron diffraction measurements were conducted in situ versus temperature on graphyne deposited on a stationary flat plate, as well as at RT for a continuously rotated capillary-packed sample. The latter agrees with our previous rotating capillary results for graphyne (8) and with the theoretically predicted cell parameters (SI Appendix, Table S1). Because of stacking faults, only (h,k,0) reflections and a strong intersheet reflection were observed. Due to the preferred orientation and the high intensity of this reflection (observed in both flat-plate substrate and rotating capillary powder measurements), only the intersheet reflection was seen in nonrotating flat-plate-substrate powder diffraction.

The diffraction intensity of graphyne’s interlayer spacing progressively disappeared during heating to 172 °C, while its d-spacing little changed (Fig. 2). A new peak from the interlayer spacing of transformed graphyne begins to significantly appear at 264 °C and then dramatically increase intensity, decrease line-width, and slightly increase interlayer spacing during heating to 320 °C, where its d-spacing increased to 3.55 Å. This new diffraction peak was maintained and provided a small contraction to 3.51 Å during cooling to 50 °C.

The disappearance of the intersheet diffraction peak for reacting graphyne at between 165 and 172 °C and the progressive appearance of the interlayer peak for the thermal reaction product at above 264 °C is explained by large strains due to mismatch of the lateral sheet dimensions of reacted and unreacted sheet areas (SI Appendix, Table S5). These strains cause wrinkling of the initially planar sheet, which displaces sheet atoms out-of-plane, thereby dramatically decreasing the intersheet diffraction intensity. As the transformation progresses, the gradual elimination of this wrinkling enables the appearance and strengthening of the interplanar diffraction for the reacted graphyne. This diffraction intensity decreases very early in the transformation, before it is noticeable in DSC measurements, which is likely because only small reaction in each graphyne sheet is needed to disturb sheet planarity and since the heat of reaction per reacted sp carbon is initially reduced by associated intersheet and intrasheet strains.

Our diffraction measurements on the presently synthesized multisheet stacks of reacted graphyne do not show diffraction peaks other than the intense interlayer separation diffraction peak, likely because of the below discussed laterally disordered intersheet stacking. Note that the distorting effect on the interplane diffraction linewidth due to stacked fully reacted sheets by the wrinkling of partially reacted sheets in their proximity is suggested in Fig. 2A by the major decrease in the FWHM linewidth of the interplanar d-spacing diffraction peak as the maximum thermal annealing goes from 255 °C (where it is about 0.25 Å) to 320 °C (where it is about 0.12 Å). However, some of this linewidth decrease can result from the diffraction linewidth consequence of the increase in the number of fully ringized adjacent layers that are stacked together, rather than from their small distortion by nearby buckled sheets.

Possible Low-Energy Topochemical Graphyne Transformation Products and Their Comparison with Experiments.

Since graphyne reacts at low temperatures (without external mechanical, chemical, or radiation-induced initiation), we consider lower energy product structures that could form without rupturing any periodic bonds in graphyne. Transformations involving interlayer coupling of benzene rings by sp³ carbons are not considered since the above experiments indicate that the transformed graphyne contains mostly sp² carbons.

Two graphyne transformation types are considered: intersheet acetylene group reaction and triple bond elimination by intraplane reaction. The products are called polyeneized graphyne and ringized graphyne, since the first results in interlayer polyene chains and the second produces new sp² intralayer carbon rings.

We first consider transformations to polyeneized graphyne, a possibility conceived when graphynes were first proposed (1). However, polyene chain formation from all graphyne triple bonds can occur only for graphyne having eclipsed sheets, which is not the lowest energy predicted stacking (SI Appendix, Fig. S24) or one observed for graphyne. We do not consider transformations having intersheet stresses so high that eclipsed phenyl rings covalently couple by forming sp³ carbons. They are excluded since there is no sp³ peak in our solid-state ¹³C NMR spectra of reacted graphyne (SI Appendix, Fig. S9). Also, the Raman G band from aromatic groups initially increases three-fold during thermal transformation, rather than decreasing (SI Appendix, Figs. S17 and S18).

Intersheet transformations by triple bond reaction to form polyene chains must be topochemical, like for diacetylene solid-state polymerization (16). The only sheet stacking that provides both a lower predicted energy than the eclipsed structure (SI Appendix, Table S3) and the topochemical ability to enable 1,2-addition acetylene reaction is the AB1 stacking of SI Appendix, Fig. S24. Our earlier molecular dynamics (MD) calculations using ReaxFF force field (8) and present density functional theory (DFT) calculations at 0 K (SI Appendix) agree with previous predictions (17) that this structure has a lower energy than the eclipsed structure. However, even when graphyne is perfectly stacked as AB1 (SI Appendix, Fig. S24), only 2/3 of the acetylene groups can potentially react since 1/3 of the acetylene groups are between aromatic groups of neighboring sheets. The chain fraction whose propagation by interlayer reaction is blocked would be greatly increased when such intersheet interactions are only between nearest-neighbor sheets, and the next intersheet interactions are of the same type, but nonperiodic.

Intersheet graphyne polymerization is energetically most likely by reaction of acetylene group arrays to form up to three linked cis or trans polyacetylene chains per aromatic carbon ring, when double counting due to chain sharing between rings is avoided by assigning ½ to each connection. Hence, we call these polyeneized graphyne structures with favorable formation energies for low-temperature thermal generation 2-cis, 1-sp polygraphyne; 2-trans, 1-sp polygraphyne; 3-cis polygraphyne; 1-cis, 2-trans polygraphyne; and 3-trans polygraphyne, where a sp index of 1 indicates acetylenes with two connections to rings are unpolymerized. Fig. 3 B and C show that the polyacetylene chains in 2-cis, 1-sp polygraphyne and in 2-trans, 1-sp polygraphyne are far from planar. Hence, the chain is called cis or trans depending on whether the bonds formed by reaction are in cis or trans positions in the reacting acetylene group array.

Fig. 3.

The bond arrangements in nonreacted graphyne and graphyne intersheet reacted by formation of cis or trans polyacetylene chains. (A) a graphyne sheet; (B) 2-cis, 1-sp polygraphyne; (C) 2-trans, 1-sp polygraphyne; (D) 3-cis polygraphyne; (E) 1-cis, 2-trans polygraphyne; and (F) 3-trans polygraphyne. These views are orthogonal to the graphyne sheet plane and in approximately the same direction for the reacted graphyne. The bonds that connected atoms in successive sheets in the parent graphyne are colored blue, red, or gray, respectively, as the graphyne sheets recede into the distance, and the bonds formed by intersheet reaction are black. The enthalpy changes are from DFT calculations at 0 K (SI Appendix, Table S4). SI Appendix, Table S6 provides the calculated structural parameters for these phases.

DFT and MD calculations (18, 19) were conducted for all investigated polygraphyne and ringized graphyne structures. SI Appendix, Table S4 provides the predicted changes of phase enthalpies and the percentage changes in dimensions during the transition from graphyne to each polygraphyne in Fig. 3. The polygraphynes without completely polymerized acetylene groups are 2-cis, 1-sp polygraphyne and 2-trans, 1-sp polygraphyne, which are for the AB1 stacking mode that blocks the reaction of 1/3 of the acetylene groups. Diffraction results (8) show that the presently synthesized graphyne lacks long-range periodicity in intersheet stacking, so the extent of sp bond elimination by forming interlayer polyene chains is further restricted to be far below 2/3.

We next predict transformed structures for graphyne that is ringized by possibly a Bergman-like reaction (20). Various planar ringized nongraphitic sp² carbon phases have been theoretically described, like the pentagon and heptagon sheets of Crespi et al. (21), the pentagon, hexagon, and heptagon sheets of Terrones et al. (22), and the graphene allotropes of Enyaskin and Ivanovski (23). Ma et al. considered transformations of graphyne and graphdiyne at extreme temperatures to produce disordered carbon sheets by bond breaking (24). In contrast, our presented transformations provide all sp² carbon sheets without breaking bonds in the periodic graphyne structure. The only previously described (1, 25, 26) planar all-sp² carbon sheet that could potentially result from an in-plane reaction without bond rupture in graphyne is the biphenylene carbon of SI Appendix, Fig. S25E. However, we exclude this structure since group-increment-based results (1) and quantum chemical calculations (SI Appendix, Table S5) predict that its single sheet formation energy is higher than graphyne.

We found four ringene all sp² carbon phases with lower enthalpies than graphyne, which could theoretically form by in-plane reaction of graphyne without graphyne bond breaking. These favorable formation enthalpy reactions result from the transformation of sp carbons into sp² carbons within larger rings, rather than being exclusively in 4-carbon rings. According to the number of carbons in the rings formed during intrasheet reaction, the ringized sheet structures (Fig. 4) are called 5,6,9 ringene; 4-9 ringene; 4-9 double ringene; and 4-8 ringene. While we have found many other in-plane reacted structures that could conceivably form by eliminating all of graphyne’s triple bonds without breaking any of its periodic covalent bonds, the only structures we found that can do this while providing the experimentally observed planar fully reacted state are the above (except 4-9 ringene) and the below mentioned simple variants. This figure shows that the 5,6,9 ringene structure, which contains no 4-member rings, has the lowest enthalpy of formation from graphyne at 0 K (−1.84 kJ/g). However, this most favorable formation enthalpy does not necessarily imply that this structure is the reaction product, since the reaction product might be kinetically derived, rather than thermodynamically determined.

Fig. 4.

The bond connections between parent sp carbons potentially made during in-plane reaction of graphyne, without bond breaking, and the resulting all-sp² carbon structures. These transformations are for (A) 5,6,9 ringene, (B) 4-9 ringene, (C) 4-9 double ringene, and (D) 4-8 ringene. The red arrows in each part indicate the direction of the irreversible thermal transformation. The number of carbon atoms in each ring of the reacted structure is indicated at ring center, the unit cells of the products are shown, and a side view of each reacted sheet is provided below the orthogonal view of each reacted sheet. Differently colored areas usually indicate rings and ring reactions that result in different carbon-number rings, but 6-carbon rings in the original graphyne are red and those formed by reaction are light blue. The provided ΔH for each investigated reacted graphyne structure corresponds to the MD ReaxFF calculated difference in the theoretically calculated enthalpy of the in-plane reacted graphyne phase and unreacted graphyne at 0 K (SI Appendix, Table S5). The calculated structural parameters for these phases are in SI Appendix, Table S7.

Discussion

We could experimentally detect only sp² carbons in the transformed graphyne. If graphyne’s thermal transformation exclusively occurred by intersheet acetylene polymerization, only a fraction of the acetylene groups could polymerize unless graphyne sheets predominately stack eclipsed, which is neither energetically favored nor observed. Complete intersheet acetylene reaction providing polyene chains is further excluded since it provides large predicted percent contractions in intersheet spacing (−21.6 to −26.1% and −22.6 to −24.2% from 0 K DFT and MD calculations, respectively, in SI Appendix, Table S4). If complete reaction occurred by combined intrasheet ring formation and intersheet polyene formation (SI Appendix, Fig. S26), large contractions in the intersheet spacing are still predicted (−15.3 and −19.1% for the cis-chain and trans-chain structures, respectively), although only 20% of the triple bonds react intersheet. These results disagree in sign and size with the small diffraction-measured interplanar spacing expansion produced by complete reaction, where the spacing of the reacted graphyne is 2.93% larger than for the nonreacted graphyne at 100 °C. This small observed increase in interplanar spacing during in-plane reaction is reasonable because of the increase in areal sheet density and the likely non-energy-minimized relationship between nearest-neighbor sheets produced by in-plane reaction.

However, we have not yet found a way to fully exfoliate either graphyne or thermally transformed graphyne into single sheets. This poses a problem since we consequently do not have the present ability to experimentally prove by electron diffraction measurements that individual reacted sheets are highly ordered. Disorder in the lateral orientation of neighboring sheets arises because a) the reaction direction in these sheets differs since it depends on the location of reaction initiation and b) additional lateral sheet disorder in intersheet stacking is expected because of sheet wrinkling and dewrinkling during reaction. Equilibration to obtain ordered intersheet stacking cannot be obtained at lower annealing temperatures than the ringene degradation temperature because of the structural complexity of ringene phases and the resulting small energy decrease driving ordered stacking. This lateral disorder in sheet stacking means that only the intense (001) intersheet diffraction peak will appear. Until the graphyne synthesis method is dramatically improved, so that impurities deposited on the graphyne surface are eliminated or methods to remove these contaminants are developed, powerful nanoscale imaging methods cannot be usefully deployed for experimental confirmation of the reacted graphyne’s sheet structure.

These results indicate that the large DSC-observed exotherm arises from intrasheet transformation of sp carbons of graphyne to sp² ring carbons. Fig. 1 shows that the large DSC-measured temperature range of the exothermic transformation coincides with the temperature range where a large fraction of the FT-IR triple-bond absorbance disappears and essentially all of the Raman triple-bond intensity disappears. Also, the absorption-based and electrochemical-based average bandgap changes occur in the exotherm’s approximate temperature range.

Of the periodic in-plane-reacted reactions in Fig. 4, the most likely is 5,6,9 ringene, which has the most favorable 0 K-calculated enthalpy decrease compared to graphyne (−1.84 kJ/g). It is predicted to have the observed planar sheets (Fig. 2) and a zero bandgap (SI Appendix, Table S8), which agrees with experiments (Fig. 1A). The 4-9 ringene is excluded as the sole product because its calculated structure is nonplanar. The 4-9 double ringene and 4-8 ringene are excluded as the main product because their calculated bandgaps are 0.23 eV and 0.17 eV, respectively.

A quite similar 0 K enthalpy difference between 5,6,9 ringene and unreacted graphyne was obtained using MD (ReaxFF potential), DFT, and Quantum Espresso (QE) calculations (−1.84, −2.21, and −2.17 kJ/g, respectively). The QE-calculated heat capacities for these phases, which are valid only for temperatures below where reaction or later volatilization starts, are in SI Appendix, Fig. S27, as are the thereby calculated small temperature dependence of the enthalpy difference due to the integrated heat capacity difference. These results show that the calculated enthalpy differences between these phases at 0 K (from −1.84 to −2.21 kJ/g) should approximately equal the measured enthalpy change during the transition. However, the measured transition exotherm (~0.5 kJ/g) is smaller than calculated for two possible reasons: the weight percent of graphyne in the synthesized samples is unknown, and likely low (no higher than 66.4% based on EDX element analysis of SI Appendix, Fig. S15 and Table S2), and the enthalpy of the synthesized ringene will be slightly increased by the below discussed intrasheet disorder. It is also possible that the reacted graphyne’s structure is kinetically determined, rather than thermodynamically determined, so the heat of reaction might be much lower than for the most enthalpically favored structure.

Note that the labeled G and D bands in the Raman spectra of graphyne look nearly identical to peaks in the same spectral region for largely acetylene-free thermally annealed graphyne (SI Appendix, Fig. S17A), despite the complex structure changes needed to eliminate graphyne’s triple bonds by in-plane reaction. In order to show that a planar sp² hydrocarbon containing 5 and 6 carbon rings can provide similar Raman vibrations to those observed for reacted graphyne, we compare the observed Raman peaks of reacted graphyne at RT with those for solid fluoranthene (which contains only 5 and 6 carbon rings and is resonance stabilized like our in-plane-reacted graphyne). Although fluoranthene (after thermal annealing to remove impurities) has much narrower Raman peak widths than observed for reacted graphyne, it provides RT peaks at 1,612 and 1,455 cm⁻¹ (27). These results are reasonably consistent with observations for thermally annealed graphyne, which has Raman vibration peaks at 1,582 and 1,360 cm⁻¹ (SI Appendix, Fig. S17A). Moreover, the extremely broad vibration at 1,360 cm⁻¹ in reacted graphyne might arise from the superposition of a sharp vibration at about 1,455 cm⁻¹ (like in fluoranthene) and a possibly much stronger broad D band derived from disordered groups at sheet edges in nonreacted graphyne.

While the two-dimensionally periodic models of Fig. 4 show how periodic in-plane reaction can occur to eliminate essentially all triple bonds, transmission electron diffraction measurements on multilayer fully reacted graphyne plates show only diffuse scattering. This lack of (h,k,0) reflections in the transformed multilayer graphyne is not surprising, even when it is recognized that topochemical in-plane reaction must be occurring to produce each of the observed planar sp² sheets. The key point is that reaction in an individual sheet is equally likely to occur in any of the three crystallographically equivalent in-plane graphyne directions. Since the energy driving intersheet equilibration in lateral positioning is small, the structures equilibrated are long and quite complex, and the annealing temperatures that can be applied are low, the equilibration needed to provide intersheet order in lateral sheet position cannot be obtained. Also, note that the two most enthalpically favorable products are structurally related in the in-sheet-plane reaction direction. Fig. 4 shows that 5,6,9 ringene reaction becomes the slightly less enthalpically favorable 4-9 ringene reaction when formation of a single 4-member carbon ring occurs for one-sixth of the phenyl rings in the original graphyne plane. Hence, intrasheet disorder would be introduced if these 4-member rings are introduced in the reaction direction less periodically than in 4-9 ringene. Nevertheless, the 5,6,9 ringene zigzag chains of graphyne-derived phenyl fused on opposite sides by two fused five-member rings are expected for the Fig. 4 models and for these less periodic variants.

Despite this absence of structural periodicity in sheet stacking, the experimentally observed planarity and zero bandgap of the graphyne reaction product is noteworthy for applications, especially if graphyne synthesis can be inexpensively upscaled. While Toh et al. recently reported (28) the exciting laser-assisted synthesis of freestanding sp² carbon monolayers containing five-to-eight member rings, they were amorphous and the center-to-center separation between neighboring stacked sheets was about 6 Å. There are clearly important properties and applications worlds to be explored for the present graphyne reaction product and its graphyne precursor, especially for their doped compositions. It will also be interesting to explore ways the present structural transformation of graphyne can be usefully modified by applied stresses and other means to provide other new phases, whose structures and energies are calculated here. Also, it will be interesting to explore various modifications of our synthesis method that might increase order in intersheet stacking, such as initiating in-plane graphyne reaction in one direction for all sheets by using thermal, radiation, or chemical initiation perpendicular to the desired reaction propagation direction.

Materials and Methods

Materials.

All reagents and solvents were acquired from commercial suppliers (Acros Organics, Sigma-Adrich, TCI Chemicals, Fisher Scientific, Oakwood Chemical, and VWR International) and used without further purification, unless otherwise noted. Tetrahydrofuran (THF) was distilled over Na/benzophenone. Triethylamine (TEA) was distilled over CaH₂. Anhydrous pyridine (Py) was purchased from Acros in AcroSeal packaging and used without further purification.

Synthetic Methods.

Reactions were monitored by thin-layer chromatography (TLC) carried out on 0.25 mm MilliporeSigma aluminum-backed silica gel plates (60F-254). Plates were visualized using 254 nm UV light and basic potassium permanganate stain (1.5 g KMnO₄, 0.5 g NaOH, and 10 g K₂CO₃ in 150 mL water; terminal alkynes stain yellow). Flash chromatography was performed on Luknova SuperSepTM (230 to 400 mesh) silica gel. Reactions requiring anhydrous or air-free conditions were performed under positive Ar pressure using standard Schlenk line techniques.

NMR Spectrometry.

Routine NMR spectra were recorded on a Bruker Avance III HD 500 spectrometer operating at 500.24 (¹H), 125.79 (¹³C) MHz and equipped with a Bruker Ascend 500 MHz US Narrow Bore Magnet and Broadband Prodigy TCI CryoProbe. NMR chemical shifts are reported in parts per million (ppm) and are calibrated against residual solvent signals of CDCl₃ (¹H δ 7.26, ¹³C δ 77.16) and DMSO-d6 (¹H δ 2.5, ¹³C δ 39.52). The following abbreviations are used to describe signal multiplicities: s—singlet, d—doublet, t—triplet, dd—doublet of doublets. The values of the spin–spin interaction constants (J) are provided in hertz (Hz).

GC–MS and EI–MS.

GC–MS (gas chromatography–mass spectrometry) and EI–MS (electron ionization mass spectrometry) analyses were performed on an Agilent 7890B/5977B GC/MSD instrument equipped with an Agilent 7890B automatic liquid sampler, Agilent G4381A Thermal Separation Probe (TSP), and a 30 m × 0.25 mm DB-5MS capillary column (with 25-μm film thickness). Liquid samples (typically 1 μL) were introduced to the column via split mode injection with a 50:1 split ratio. The temperatures of the injection port (220 °C), MSD (Mass Selective Detector) transfer line (280 °C), MS source (230 °C), and MS quad (150 °C) were set, and the electron ionization energy used for the MS was 69.9 eV. The oven temperature was set at an initial temperature of 60 °C for 2.25 min, then ramped to 225 °C at 35°C min⁻¹ and held at this final temperature for 3 min. The system deployed He gas flowing at 3.0 mL min⁻¹ as the mobile phase. The method used a 3 min solvent delay. Solid samples were introduced into the instrument using the TSP. After the TSP was preheated to the set temperature, the sample carrier was inserted into the probe, which was connected directly to the MSD transfer line by a deactivated quartz capillary. Data analysis was performed using Agilent MassHunter Qualitative Analysis Navigator.

The graphyne samples were positioned within a preheated Thermal Separation Probe (29), set to temperatures ranging between 150 and 400 °C. The TSP was directly linked to a quadrupole mass spectrometer via a brief deactivated capillary column. No identifiable fragments were observed in the obtained data for set temperatures below 200 °C. However, from 200 °C onward, the predominant species in the mass spectra consistently matched bromobenzene. This identification was confirmed by the presence of molecular ions C₆H₅Br at 156 and 158 m/z, along with the C₆H₅ peak at 77 m/z resulting from the loss of Br. These findings suggest that the transformation of graphyne initiates at sheet edges, involving significant skeletal rearrangement and C–C bond breaking at sheet edges even at relatively mild temperatures. The infrared spectroscopy observations, indicating the conversion of terminal alkynes before affecting internal alkynes, align with the results of the TSP–MS experiment. These results suggest that graphyne with a negligible fraction of triple bonds in terminal positions at internal or external sheet edges might have greatly enhanced thermal stability. In fact, it seems possible that more defect-free graphyne might not display the present structural transformation.

Infrared Spectroscopy.

Routine small-molecule FTIR spectra were collected on an Agilent Cary 630 FTIR instrument equipped with a single-reflection germanium attenuated total reflectance module. The instrument was calibrated before sampling against a newly cleaned (acetone) and dried crystal surface. Solid samples were placed directly on the crystal and secured with a needle press. 512 scans from 4,000 to 600 cm⁻¹ were recorded. A background was collected for each sample. The FT-IR spectra were collected using a PerkinElmer Spotlight 200i FTIR Microscopy System equipped with a Spectrum Two spectrometer capable of both transmittance and reflectance measurements in the mid-IR to near-IR range (600 to 7,800 cm⁻¹).

Thermogravimetric Analysis.

The weight loss for graphyne was measured using a TA SDT Q600 thermal analyzer, which enables simultaneous measurement of weight change (TGA) and heat flow (DSC) from ambient temperature to 1,500 °C. In this study, the weight loss from 0 to 600 °C was measured at a 5 °C /min scan rate for 14.2 mg of graphyne powder filled into a 165.2 mg alumina ceramic pan. The heater chamber was continuously purged during the measurement with ultrapurified (99.99%) nitrogen gas at a flow rate of 50 mL/min.

Theoretical Methods.

DFT and MD simulations were employed to study the formation energies and other properties, including dynamical tests of all polyeneized and ringized structures, as well as pristine graphyne structures and their stacks. Unless otherwise mentioned, the DFT calculations were done with the Vienna Ab-Initio Simulation Package (30), and the MD simulations were performed using the LAMMPS package (18). During DFT calculations, the generalized gradient approximation of the Perdew–Burke–Ernzerhof functional (31) was used for the exchange and correlation interactions of electrons. For higher accuracy, the bandgaps of in-plane reacted structures were calculated at the hybrid HSE06 functional level (32). Considering the dispersion interactions, van der Waals corrections using the DFT-D3 (33) method were adopted in all calculations. An energy cutoff of 520 eV was used, and a k-point mesh with a density of about 50 Å was used for Brillouin zone sampling. All structures were fully relaxed using a conjugate gradient algorithm with a stringent convergence criterion of the force on each atom (10⁻² eV Å⁻¹). For MD simulations, the state-of-the-art ReaxFF reactive force field (19) used two sets of parameters for simulating the interaction between the atoms within structures, one that is able to simulate hydrocarbons (34) and another that was developed for solid carbon (35). In SI Appendix, section 7, we present the formation energies and other properties of the structures considered in the present work.

MD energy minimizations were obtained for all structures with periodic boundary conditions (PBC) imposed along all directions in space (3D for stacks of graphynes and polyeneized structures and 2D for pristine graphyne and ringized structures). The protocols for these calculations combined sets of energy minimization and free evolution dynamics algorithms to determine the lowest-energy structure, as suggested by Sihn et al. (36) and implemented by Kanegae and Fonseca (37) for determining the mechanical properties of graphyne families. Conjugate gradient and volume change methods provided by LAMMPS were used to improve precision in the calculated structural energies. Thermal equilibration of the structures was also performed with PBC. Dynamical tests were performed with a fixed number of particles, by varying temperature and/or volume. Langevin thermostat and Nosé–Hoover thermostat and barostat were used when the structures were simulated with fixed volume and fixed pressure, respectively. In this last case, the volumes of the structures were allowed to change. Temperature damping factors were chosen to be 1.0 fs. Pressure-damping factors were chosen between 100 and 1,000 fs to decrease the oscillations caused by the algorithms of pressure convergence. Timesteps between 0.01 and 0.025 fs were considered.

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.