Carbon materials with zigzag and armchair edges - ACS Applied

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Functional Nanostructured Materials (including low-D carbon)

Carbon materials with zigzag and armchair edges Yasuhiro Yamada, Miki Kawai, Hideki Yorimitsu, Shinya Otsuka, Motoharu Takanashi, and Satoshi Sato ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11022 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Carbon Materials with Zigzag and Armchair Edges Yasuhiro Yamada,*, † Miki Kawai, † Hideki Yorimitsu, ‡ Shinya Otsuka, ‡ Motoharu Takanashi, ∥ Satoshi Sato† † Department

of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan ‡Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo, Kyoto 606-

8502, Japan ∥Instrumental Analysis Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa, 2400067, Japan KEYWORDS. Carbon materials, Zigzag edge, Armchair edge, Raman spectra, IR spectra, Oxidation, Activation energy, Structural control. ABSTRACT: Carbon materials such as graphene and graphene nanoribbon with zigzag and armchair edges have attracted much attention because of various applications such as electronics, batteries, adsorbents, and catalyst supports. Preparation of carbon materials with different edge structure at large scale is essential for the future of carbon materials, but it is generally difficult and expensive because of the necessity of organic synthesis on metal substrates. This work demonstrated a simple preparation method of carbon materials with zigzag and armchair edges with/without non-metallic silica supports from aromatic compounds such as tetracene with zigzag edges and chrysene with armchair edges and also determined the edge structures in detail by three types of analyses such as (1) reactive molecular dynamic simulation (ReaxFF), (2) Raman and infrared (IR) spectra combined with calculation of spectra, and (3) reactivity analyzed by oxidative gasification using thermogravimetric analysis. Two different types of carbon materials with characteristic Raman and IR spectra could be prepared. These carbon materials with different edge structures also showed clear different tendency in oxidative gasification. This work did not only show the simple preparation method of carbon materials with different edge structures, but also contributes to the development of detailed analyses for carbon materials.

1. INTRODUCTION Nano carbon materials such as graphene and graphene nanoribbon with controlled zigzag and armchair edges (Figure 1) have recently attracted much attention.1-3 It is well known that edge structures affect chemical reactivity, electronic, and magnetic properties.4-6 These different edge structures should generate the distinct differences in property of various applications such as sensors, batteries, adsorbent, and catalysts.7-10 Especially, it has been reported that zigzag edges are more reactive than armchair edges and can be utilized for hydrogen storage,11 whereas armchair edges are less reactive, so that armchair edges can be utilized to improve stability against chemicals. Thus, controlling edge structures and analyses of those edges are challenging and meaningful. Graphene with controlled edges has been synthesized using bottom-up methods. For example, single graphene crystal with zigzag edges1 and graphene nanoribbons with zigzag edges12 and armchair edges2 were synthesized by catalytic reaction of aromatic compounds and methane gas on metallic substrates. But the shortcoming of these bottom-up methods is the amount of produced graphene because of the presence of metallic catalyst substrates. The substrate can be removed by etching, but metal residue remains after etching metal substrates. It is essential to develop the methods to synthesize the large amount of carbon materials with controlled edge structures in the absence of metallic substrates

for various general applications such as battery, adsorbent, and catalysts.

Figure 1. Structure of graphene with edges and the basal plane. Our group has reported a simple method to prepare a large amount of carbon materials with either controlled zigzag or armchair edges in the absence of metallic substrate.11 However, the specific surface areas of these carbon materials are low. We have reported nitrogen-containing carbon materials evenly coated on spherical SiO2 with high surface area.13 It is possible to synthesize carbon materials with zigzag and armchair edges at a large scale in the absence of metallic substrates using SiO2. In addition, our above-mentioned method11 for carbon materials with controlled edge structures requires introduction of reactive functional groups such as ethynyl groups, which enhances the cost. In order to develop a novel inexpensive method, understanding the mechanism of carbonization of polycyclic aromatic hydrocarbons (PAHs) with zigzag and armchair edges is essential.

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Carbonization of PAHs with zigzag edges such as anthracene,14 tetracene (C18H12, Figure 2a),15 and pentacene16,17 and PAHs with armchair edges such as phenanthrene14,18 and chrysene (C18H12, Figure 2a)18 have been extensively studied in the past two decades. These works mentioned the presence of zigzag and armchair edges in carbonized structures, but basically only the simplified results were exhibited, and the details are still under debate because of the difficulty to characterize the edge structures of these carbon materials without observing structures at atomic scale under microscopes. It is essential to develop the ubiquitous methods for determination of edge structures of threedimensional carbon materials with different edge structures.

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carbonization process including our previous work,11,20 because MD simulation can estimate carbonization reactions and carbonized products rapidly, unlike transition state calculation, which requires calculation of many reaction routes individually. But other analyses have to be combined with the MD simulation to improve the reliability of estimated structures of carbon materials (Figure 3b and c). (a)

(b)

(c) Figure 2. Flow chart of this work. (a) Preparation methods of carbon materials with zigzag and armchair edges. The numbers in pink on molecular structures of precursors such as tetracene and chrysene are the numbers of positions. The numbers in black are the amount of electron on each carbon atom calculated from the Mulliken charge. The positions with the large number of electron were marked with pink circles. (b) Carbonized materials. Conceptual structures of carbon materials with zigzag and armchair edges are shown at the bottom. (c) Analyses conducted in this work such as ReaxFF, Raman, IR, reactivity, and others. The most difficult problem to analyze carbon materials is so called “inverse problems” (Figure 3a). It is relatively easy to obtain experimental and calculated spectrum from carbon materials. On the other hand, using the spectrum, it is difficult to obtain the structure of carbon materials because of the presence of similar bonding states. Thus, it is essential to obtain various results from different analytical methods such as experimental spectrum, calculated spectrum, and molecular dynamic (MD) simulation to complement the information. The structure of carbon materials that can be explained by all of those different analytical methods including calculation is the actual structure of carbon materials (Figure 3b). MD simulation with a reactive force field (ReaxFF)19 is one of promising methods to estimate the edge structures of threedimensional carbon materials because the formation mechanism of carbon materials can be estimated. MD simulation has been recently often utilized to estimate

Figure 3. Problems of analysis of carbon materials and concept of this work. (a) Inverse problem of analyses of carbon materials. It is easy to obtain experimental and calculated spectra from samples, but it is difficult to obtain exact structures from experimental spectra. (b) Concept of detailed analyses of carbon materials in this work. (c) Flow chart of spectral analyses in this work. Raman3,21-29 and infrared (IR) spectroscopy30-34 are also promising analytical techniques to determine the edge structures of three-dimensional carbon materials. It has been reported that peaks at 1350 (D band)3,21 and 153024 cm-1 of Raman spectra relate to the presence of armchair edges, whereas a peak at 145024 cm-1 relates to the presence of zigzag edges (Figure 4a). These peaks at 1450 and 1530 cm-1 have also been reported in another work of graphene.25 In this manner, Raman spectroscopy is among the most used techniques, but the detailed structure cannot be determined using only Raman spectra. It has been reported that SOLO at 910-860 cm-1 and DUO at 860-800 cm-1 of IR spectra correspond to out-of-plane sp2CH vibration on zigzag edges and armchair edges, respectively (Figure 4b).30-32,34 TRIO at 810-750 cm-1 has also been reported to correspond to out-of-plane sp2C-H bending on


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other edges in such a way that three sp2C-Hs next to each other are present in one benzene ring, and QUATRO at 770-720 cm-1 has also been reported to correspond to out-of-plane sp2C-H bending on other edges in such a way that four sp2C-Hs next to each other can be clarified. In addition, calculated peak positions of in-plane sp2C-H stretching vibration of zigzag edges and armchair edges, which were not adjusted to experimental peak positions, have also been reported as ca. 3160 and 3205 cm-1, respectively.30 These peak positions are useful, but the estimation of detailed structures is difficult using only these references.

Figure 4. Reported peak positions of C=C vibration for Raman spectra and sp2C-H vibration for IR spectra. (a) Peak positions of reported Raman spectra. (ai) Zigzag edges. (aii) Armchair edges. (b) Peak positions of reported IR spectra. (bi) SOLO (zigzag edges). (bii) DUO (armchair edges). (biii) TRIO. (biv) QUATRO. Simulation of spectra is one of the recently developing methods. Applications of the calculation could reveal the structures of carbonized aromatic compounds. Especially, Raman23,26 and IR30,31 spectra of aromatic compounds as well as Raman22,24 and IR spectra33 of carbon materials and large aromatic compounds have been intensively studied computationally. Our group have been studying analyses of carbon materials using simulated Raman spectra35 and simulated infrared spectra36,37 in addition to simulated X-ray photoelectron spectra.36,38-40 Especially, our recently developed method to analyze the edge structure of carbon materials is closely relevant to this work (Figure 4).11,37 These methods are helpful to determine the structures of carbon materials in detail, but calculation of carbon materials has just begun in the last decade and more detailed studies are essential to reveal structures of carbon materials. Oxidation of carbon materials can also be one of the methods to determine edge structures of carbon materials. Oxidation is historically among the most studied reactions for carbon materials, although the reaction routes are still under debate.4,41-44 It is well known that the basal plane of graphite and graphene is less reactive than edges of graphite and graphene.42 But the basal plane of single graphite crystal was reacted with oxygen gas at high temperatures and submicrometer hexagonal pits and holes were introduced in the basal plane of graphite to compare the reactivity of edges.41 The activation energies for gasification reaction of each edge

was 259 kJmol-1 for zigzag edges and 276 kJ mol-1 for armchair edges has been shown using the optical microscope images of graphite oxidized at 1085 K.41 It indicates that zigzag edges tend to be oxidized easily. In addition, the important concept, so called active surface area, was introduced. The amount of reactive edges was estimated by reacting carbon material with oxygen gas at 573 K and the active surface area was obtained.4 The active surface area is a possible method to determine edge structure of carbon materials, because the important edges for most applications are the edges, that can be reached by nitrogen gas or larger molecules, rather than the actual edges existing in carbon materials. However, this method has not been generally applied to determination of the edge structures of carbon materials, probably because oxidation mechanisms of carbon materials are complicated and still unclear. In this work, carbon materials with zigzag and armchair edges were prepared from tetracene and chrysene (Figure 2ab), respectively. The structures of carbon materials were determined in detail and showed the clear difference of edge structures between carbonized tetracene and chrysene using mainly three following methods (Figures 2c). The first method is MD simulation with a reactive force field (ReaxFF), which was used to estimate the structures of carbonized tetracene and chrysene (Figure 3c). Upon carbonization of tetracene and chrysene, many possible structures exist, and it is difficult to simulate all of possible structures for Raman and IR spectra. 1) Thus, firstly ReaxFF is used to narrow down the number of possible structures in a short time. 2) Then, secondly Raman spectroscopy and IR spectroscopy combined with computation were conducted to analyze the structures of carbonized tetracene and chrysene. 3) As a last step, carbonized tetracene and chrysene were oxidized by thermogravimetric analysis (TG) to exhibit the clear difference in edge structures between carbonized tetracene and carbonized chrysene. The difference of edge structures after oxidation was analyzed by ReaxFF and IR spectroscopy combined with simulated IR spectra. 2. EXPERIMENTAL 2.1 Preparation Tetracene and chrysene (both purities > 97%) were purchased from Tokyo Chemical Industry Co., Ltd. 5,12Dihydrotetracene (purity: not available) was purchased from Sigma-Aldrich Corp. These compounds were used without further purification. Carbon materials with zigzag and armchair edges were synthesized by drying tetracene and chrysene at 353 K for 2 h under a reduced pressure in glass/quartz ampoule tube and heating the sealed ampoule tubes at 653-923 K for 1 h. Carbon materials with zigzag and armchair edges were also coated on silica (nanoballoon (XR100), Grandex Co., Ltd.) at 853 and 933 K in ampoule tubes for increasing the surface area of carbon materials. Nanoballoon has a shell structure with ca. 10 nm of shell thickness and ca. 2 nm of pore. Nanoballoon rather than conventional silica supports was selected to show the carbon coating clearly using transmission electron microscope (TEM). 2.2 Analyses 2.2.1 Analyses of carbonized PAHs Carbonized tetracene and chrysene were analyzed by elemental analysis (CE-440F, Elemental Analysis, Inc.) and mass spectrometry (MS) analysis (Exactive, Thermo Fisher Scientific Inc., ionization method: atmospheric pressure chemical ionization (APCI)), and another MS analysis



(microTOF Bruker Corp., ionization method (APCI)). Samples were dissolved in dichlorobenzene and diluted with tetrahydrofuran for MS analysis by Exactive, and deuterochloroform for MS analysis by microTOF. Especially, using the results of MS, the structures of carbonized PAHs were roughly estimated. From the results of these analyses, structures for simulated Raman and IR spectra were constructed. Raman spectroscopy (Renishaw PLC, inVia Reflex, laser wavelength; 532 and 785 nm) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy (FT-IR-4200, JASCO Corp., detector of MCT_M, resolution of 8 cm-1, cumulated number of 64 times) were also conducted and compared with the results of simulated Raman and IR spectra. 1H and 13C nuclear magnetic resonance (NMR) (ECA-600, JEOL) was used to analyze the structure of carbonized tetracene in deuterochloroform. 2.2.2 Analyses of oxidized carbon materials Carbonized tetracene and chrysene without silica support were oxidized at 573 K for 20 and 100 h in an oxygen flow at a rate of 20 cm3 min-1 and analyzed by DRIFT spectroscopy. 1.5 mg of carbon materials on SiO2 support (nanoballoon) prepared from tetracene at 853 K and chrysene at 933 K was placed on the quartz pan in TG (Thermo plus 8120, Rigaku Corp.) and oxidized in an oxygen flow at a rate of 20 cm3 min-1. Tetracene and chrysene heated with SiO2 were observed using TEM (JEOL JEM-2100F). Brunauer, Emmett and Teller (BET) specific surface area was analyzed by nitrogen adsorption (BELSORP-max-N, MicrotracBEL, Osaka, Japan).

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For carbonization reaction, basically 100 molecules of tetracene, chrysene, and related molecules were randomly positioned in 3.5 X 3.5 X 3.5 nm3 using a builder function in ReaxFF (software name: ADF package released in 2016, Dell Optiplex7040 computer with CPU Corei7 and RAM 64 GB) using ReaxFF (Force field: “dispersion/CHONSSi-lg.ff”46 was used for dehydrogenation reaction, and “CHO.ff”47 was used for oxidation reaction. Method: Velocity Verlet + Berendsen.). For oxidation reaction of aromatic compounds, basically 20 molecules of tetracene and chrysene with 1000 molecules of oxygen were randomly positioned in 3.5 X 3.5 X 3.5 nm3. For oxidation reaction of graphene nanoribbon, two graphene nanoribbons with zigzag and armchair edges were randomly positioned in 2.0 X 2.0 X 8.0 nm3. These compounds were heated from 0 to either 2500 and 3000 K at a heating rate of 10 K (dT) at the time step of 0.25 fs for dehydrogenation reaction and also heated at 3000 K at a heating rate of 0.1 K and at the time step of 0.1fs for oxidation reaction and kept at each temperature. Dispersion/CHONSSilg.ff is a force field containing London dispersion correction terms.46 CHO.ff is a force field obtained by calculating hydrocarbon oxidation.47 Table 1. Changes in number of hydrogen of tetracene upon heat treatment analyzed by elemental analysis and their appearance. Temp. /K

2.3 Calculation

Number of hydrog en atoms*

Appearance of samples in glass ampoule tubes

State

2.3.1 Simulation of Raman and IR spectra Density functional theory (DFT) calculation was conducted at either B3LYP/6-31G(d) or M062X/6-31G(d) level with integral=grid=ultrafine using Gaussian 09 (Dell Optiplex9020 computer with CPU Corei7 and RAM 4 GB).45 But B3LYP/6-31G(d) was used because the spectra using B3LYP/631G(d) was close to the experimental spectra compared to M062X/6-31G(d) (Figures S1 and S2). Modeled structures were optimized (Figures S3-S6), and IR and Raman spectra were simulated using the keyword of opt freq=Raman (Figures S7-S58). Full width at half maximum (FWHM) of simulated IR spectra was set as 15 cm-1. For adjusting simulated IR spectra to experimental spectra at 2700-3500 cm-1, a scaling factor of 0.96 was multiplied with simulated IR spectra and 30 cm-1 was further deducted to adjust simulated IR spectra with experimental DRIFT spectra. For adjusting at 1000-1800 cm-1, a scaling factor of 0.96 was multiplied with simulated IR spectra and 6 cm-1 was further added to adjust simulated IR spectra with experimental DRIFT spectra. For adjusting at 7001000 cm-1, a scaling factor of 1.057 was multiplied with simulated IR spectra and 60 cm-1 was further deducted to adjust simulated IR spectra with experimental DRIFT spectra. A scaling factor of 0.96 was multiplied with simulated Raman spectra and 15 cm-1 was further added to adjust simulated Raman spectra with experimental Raman spectra. These scaling factors were obtained by comparing calculated spectra and experimental spectra of several aromatic compounds such as tetracene, chrysene, dihydrotetracene, dihydrophenanthrene, and dihydroanthracene. Charge and spin multiplicity were set as 0 and 1, respectively.

Image

298

11.9

Orange powder

653

12.1

Orange solid + black solid (surface) with orange solid (inside)

693

11.9

Gray (surface) orange (inside)

733

11.4

Black solid

773

10.3

Black solid

813

8.4

Black solid

853

9.2

Black solid

powder with solid

(5.7)** 893

7.6

Black solid

933

6.3

Black solid

* 12 Hydrogen atoms are theoretically present in asreceived tetracene (C18H12). ** After removal of uncarbonized components under reduced pressure.

2.3.2 Molecular dynamic simulation


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3. RESULTS and DISCUSSION 3.1 Appearance, elemental analysis, MS, and NMR analysis of heat-treated tetracene and chrysene Tables 1 and 2 show changes in appearance upon heat treatment and changes in number of hydrogen atoms for tetracene and chrysene upon heat treatment, and the data was plotted in Figure 5 to show the number of eliminated hydrogen upon heat treatment clearly. As temperatures increased from 298 to 733 K, orange powder of tetracene changed into black solid, and the number of hydrogen atoms in tetracene decreased from 11.9 to 11.4 (- 0.5 H). The reactivity and the change in color of tetracene, which is also named as naphthacene, at such low temperatures (653 K) have been reported by Lewis et al.48 and Tamai et al.15 Generally, PAHs turn into black color at 823-923 K, but tetracene changed the color into black at ca. 653 K. At 933 K, the number of hydrogen atoms in tetracene decreased from 11.9 to 6.3 (- 5.6 H). Table 2. Changes in number of hydrogen atoms of chrysene upon heat treatment analyzed by elemental analysis and their appearance. Temp /K

Number of hydrog en atoms*

Appearance of samples in glass ampoule tubes

State

Image

298

11.9

White powder

693

12.0

White solid

733

12.1

White solid

813

12.1

White solid

853

12.0

White solid

893

11.5

White + black solid

933

8.5

Black solid

+

brown

(6.3)** * 12 Hydrogen atoms are theorecially present in asreceived chrysene (C18H12). ** After removal of uncarbonized components under reduced pressure.

Figure 5. The number of eliminated hydrogen calculated using the results obtained by elemental analysis. ●: Tetracene. ◇: Chrysene. Plots in this figure were obtained by averaging multiple data of elemental analysis unlike Tables 1 and 2. White powder of chrysene, on the other hand, did not show change in color up to 853 K. At 893 K, the number of hydrogen atoms in chrysene decreased from 11.9 to 11.5. Chrysene is known as a stable aromatic compound48 and it has been carbonized above 1073 K.49 This work also showed that chrysene was carbonized above 893 K, which is much higher than tetracene. At 933 K, the number of hydrogen atoms in chrysene decreased from 11.9 to 8.5 (- 3.4 H). These results indicate that tetracene tends to dehydrogenate much more easily than chrysene. Figure 5 shows the number of eliminated hydrogen from one precursor molecule. Clearly, the dehydrogenation of tetracene proceeded at much lower temperature than that of chrysene. It has been reported that pentacene which is a linear molecule and which has a similar structure to tetracene showed low carbonization temperature compared to other aromatic compounds such as coronene and perylene.50 It shows that the high reactivity of zigzag edges in linear molecules such as tetracene and pentacene. Heat-treated tetracene and chrysene were further analyzed by MS. Tetracene heated at 693 K for 30 min were a mixture of yellow and black solid and contained monomers (C18H12), hydrogenated monomers (C18H13), dimers formed by dehydrogenation (mainly C36H22 and C36H20), trimers formed by dehydrogenation (mainly C54H32 and C54H34) and tetramers formed by dehydrogenation (mainly C72H44) (Figure S59b and c). The amount of the dimers was more than those of monomers, trimers, and tetramers. But the order of the amount does not show the actual degree of dehydrogenation, because only the dissolved molecules with smaller molecular weight can be analyzed using MS. Tetracene heated at 713 K was partially dissolved in deuterochloroform, and 90% of dissolved compounds was 5,12-dihydrotetracene and 10% of them was 1,2,3,4-tetrahydrotetracene as analyzed by MS (Figure S60) and NMR (Figures S61 and S62). These results indicate that dehydrogenation and hydrogenation proceeds at the same time between 693 and 713 K, and small molecules such as unreacted monomer, dimers, trimers, and tetramers coexist. Tetracene heated at 853 K for 1 h contained dimer (mainly C36H20 (-4H)) the most, similar to 693 K. Trimers (mainly C54H30 (-6H) and C54H36) and tetramers (C72H42 (-6H)) were also obtained. The reason, why the amount of dimer was more than trimer as well as tetramer, was partly because of the low solubility of dehydrogenated tetracene with high molecular weight in a solvent (dichlorobenzene). Trimers with C54H36 has same molecular weight before and after heat treatment (C18H12 * 3 = C54H36), possibly because three tetracene molecules dehydrogenated and further hydrogenated. As results of MS analysis of chrysene heated at 773 K for 110 min, a mixture of white and black solid in the heat-treated chrysene contained dimers (mainly C36H20 (-4H) and some C36H18 (-6H)), trimers (mainly C54H30 (-6H) and some C54H28 (8H)), and tetramers (mainly C72H40 (-8H)) were detected (Figure S63b and c). These structures were also observed as results of MD simulation. Thus, Raman and IR spectra of many dimers, trimers, and tetramers of tetracene and chrysene were simulated in supporting information (Figures S7-S58). Among all of the calculated structures, only the selected structures,



which showed similar simulated spectra compared to the experimental spectra, were shown in the following section. 3.2 MD simulation 3.2.1 Carbonization of tetracene and chrysene Various force fields such as CHO.ff, dispersion/CHONSSilg.ff, and others were compared. CHO.ff was used in our previous work,26 but precursors tended to decompose easily without dehydrogenation among precursors. On the other hand, especially dispersion/CHONSSi-lg.ff tended to promote dehydrogenation without decomposition (Results are not shown). Thus, dispersion/CHONSSi-lg.ff was selected as force field in the following calculation. Figures 6a and 7a show one hundred tetracene and chrysene molecules heated at 3000 K in ReaxFF using the force field of dispersion/CHONSSi-lg.ff. Conversion of tetracene, which is a percentage of reacted tetracene, reached 100 % after reaction for 289 ps, whereas that of chrysene reached 100% after reaction for 352 ps. It indicates that tetracene is more reactive than chrysene. Tetracene and chrysene dehydrogenated and bitetracene and bichrysene formed in addition to the formation of a small number of hydrogenated tetracene (up to 7 molecules) and hydrogenated chrysene (up to 3 molecules). It indicates that hydrogenation of tetracene easily proceeds compared to that of chrysene, as studied by our previous work.26 (a) Beginning of reactions from tetracene to other molecules

(b) Intramolecular dehydrogenation of bitetracene


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(c) Further dehydrogenation dehydrogenated bitetracene


and

carbonization

of

Figure 6. Representative structures of tetracene before and after heat treatment at 3000 K in ReaxFF. (a) Beginning of reactions from tetracene to other molecules. A snapshot above



the first arrow contains 100 tetracene molecules before heat treatment. Structures below the first arrow are the representative structures at the beginning of reactions. Gray sphere: carbon atom. White sphere: hydrogen atom. Numbers below each structure are dehydrogenated positions. Numbers in parentheses indicate minimum reaction times to form the structures. Percentages in parentheses indicate selectivities of dehydrogenated tetracene and percentages of remaining zigzag edges calculated using eq. 1. (b) Intramolecular reactions from bitetracene to dehydrogenated bitetracene. 50 molecules of bitetracene were heated at 3000 K. (c) Further dehydrogenation and carbonization of dehydrogenated bitetracene. 20 dehydrogenated bitetracene molecules and 60 tetracene molecules were heated at 3000 K. The number of zigzag edges and the percentage of zigzag edges, which is equal to 100 times ”The number of zigzag edges after dehydrogenation”/“The total number of zigzag edges of tetracene molecule(s)”, were shown next to each dehydrogenated structure in (b,c).

of zigzag edges after dehydrogenation, whereas only one intermolecular dehydrogenation such as 2,2’ did not influence the number of zigzag edges. Thus, more zigzag edges would be present after carbonization, if 2,2’ were dehydrogenated selectively at the beginning of reactions. (a) Beginning of reactions from chrysene to other molecules

Percentages in parentheses under dehydrogenated positions (Figures 6a and 7a) indicate the percentage of products among dehydrogenated products, which is generally called as selectivity, in addition to the reaction time to form the product. These percentages were obtained by calculating more than three times of heat treatment of 100 tetracene and chrysene molecules because the types of products vary depending on the initial positions of molecules. In order to show the difference of remaining zigzag and armchair edges depending on the reaction process, the percentage of remaining edges, named as structural control by our group, was calculated using eq. 1.11 For example, the number of zigzag edges in tetracene was counted as 4, because 4 edges of SOLO and 2 edges of QUATRO are present. Similarly, the number of remained edges for tetracene dehydrogenated at 2,5’ was counted as 6, which corresponds to 75% of remained zigzag edges using eq. 1 (Figure 6a). The number of remained edges for tetracene dehydrogenated at 2,5’:1,6’ was 5, which corresponds to 63% using eq. 1 (Figure 6b). For armchair edges of carbonized chrysene, armchair edges formed by dehydrogenation between two chrysene molecules without formation of hexagon, such as bonding at positions of 3 and 6’ without formation of hexagon (Figure 6a) and bonding at positions 2,7’ and 3,6’ with formation of pentagon (Figure 6b), were not counted as armchair edges. For better control of edge structures of carbon materials, we have recently reported that elimination of hydrogen generated from precursors upon heat treatment and introduction of reactive functional groups near the edges were essential.11 Remaining edges [%] = 100 × 𝑁 (𝑍𝑖𝑔𝑧𝑎𝑔 𝑜𝑟 𝑎𝑟𝑚𝑐ℎ𝑎𝑖𝑟 𝑖𝑛 𝑑𝑒ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛𝑎𝑡𝑒𝑑 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒𝑠) eq. 1 𝑁 (𝑍𝑖𝑔𝑧𝑎𝑔 𝑜𝑟 𝑎𝑟𝑚𝑐ℎ𝑎𝑖𝑟 𝑖𝑛 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟𝑠) By heat treatment of tetracene, 2,5’-, 5,5’- (or 5,6’-), 1,2’-, 1,5’-, 2,2’-, 1,1’-, as well as 2,5’:3,6’-bitetracene formed (Figure 6a). Carbon atoms at positions except for 5,5’ and 1,5’, tended to bond easily because of the small steric hindrance. Carbon atoms at positions 5,5’ and 1,5’ also bonded each other because of the high electron density of zigzag edges (cf. Mulliken charge of tetracene in Figure 2a) despite high formation energy due to steric hindrance. Most of intermolecular dehydrogenations such as positions 2,5’, 5,5’, 1,5’, 1,2’, and 1,1’ reduce the number

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(b) Intramolecular dehydrogenation of bichrysene


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ACS Applied Materials & Interfaces (c) Further dehydrogenation dehydrogenated bichrysene


and

carbonization

of


Figure 7. Representative structures of chrysene before and after heat treatment at 3000 K in ReaxFF. (a) Beginning of reactions from chrysene to other molecules. A snapshot above the first arrow contains 100 chrysene molecules before heat treatment. Structures below the first arrow are the representative structures at the beginning of reactions. Gray sphere: carbon atom. White sphere: hydrogen atom. Numbers below each structure are dehydrogenated positions. Numbers in parentheses indicate minimum reaction times to form the structures. Percentages in parentheses indicate selectivity of dehydrogenated chrysene and percentages of remaining armchair edges calculated using eq. 1. (b) Intramolecular reaction from bichrysene to dehydrogenated bichrysene. 50 molecules of bichrysene were heated at 3000 K. (c) Further dehydrogenation and carbonization of dehydrogenated bichrysene. 20 dehydrogenated bichrysene molecules and 60 chrysene molecules were heated at 3000 K. The number of armchair edges and the percentage of armchair edges, which is equal to 100 times “the number of armchair edges after dehydrogenation”/ “the total number of armchair edges of chrysene molecule(s)”, were shown next to each dehydrogenated structure in (b,c). For hydrogenation of tetracene, a carbon atom at position 5 was the most hydrogenated compared to other positions (Figure 6a), whose tendency was similar to the dehydrogenation. sp3C-H formed mainly because hydrogen transferred from one tetracene to another tetracene molecule. Hydrogenated tetracene molecules were thermally unstable at 3000 K and most hydrogenated tetracene molecules tended to change back to tetracene. A few hydrogenated tetracene molecules tended to decompose by sp3C-sp3C scission. By heat treatment of bitetracene, intramolecular dehydrogenation proceeded and pentagons and hexagons formed (Figure 6b). The time required to form pentagons was basically shorter than that to form hexagons, implying that the pentagon formed at lower carbonization temperature than hexagon. Among the intramolecular dehydrogenation, a formation of 2,5’:1,6’-bitetracene was the fastest (57 ps). Once pentagons formed, those pentagons were stable and did not transform into other ring structures such as hexagons and heptagons. Formation of pentagons was not clear in the recently published papers regarding carbonization of tetracene-like compounds such as pentacene.16 On the other hand, pioneering researches have mentioned the presence of pentagon in dehydrogenated naphthalene.51 In this work, some of intramolecular dehydrogenations such as 2,5’:3,4’ and 1,2':12,3’ maintained the number of zigzag edges as 6 (75% compared to the edge of precursor). The other of intramolecular dehydrogenation such as 4,6’:5,5’:6,4’ decreased the zigzag edges into 4 (50%). By heat treatment of dehydrogenated bitetracene together with tetracene, intermolecular dehydrogenation proceeded rather than intramolecular dehydrogenation (Figure 6c). Thus, it is estimated that cross-linking among tetracene molecules proceeds and mesh-like structures form at the beginning of reactions. Then, further dehydrogenation proceeds and carbonized tetracene forms. By heat treatment of chrysene molecules, 3,6’-, 2,3’-, 2,5’-, 1,5’-, 3,4’-, 1,6’-, 2,6’-, 4,4’-, 2,5’:3,4’-, and 4,7’:5,6’-bitetracene formed (Figure 7a). Carbon atoms at positions except for 2,5’, 1,5’, 3,4’, and 4,4’ tended to be bonded easily because of the small steric hindrance. Carbon atoms at positions 2,5’, 1,5’, 3,4’, and 4,4’ also bonded each other because of the higher electron

Page 10 of 35

density of carbon atoms on armchair edges (cf. Mulliken charge of chrysene in Figure 2) than other edges in chrysene despite high formation energy due to steric hindrance. Some of intermolecular dehydrogenations such as positions 2,5’, 1,5’, and 3,4’, and 4,4’ reduced the number of armchair edges from 4 (the number of armchair edges on chrysene) to either 3 (75%) or 2 (50%), whereas the other intermolecular dehydrogenations such as 3,6’, 2,3’, 1,6’, and 2,6’ maintained the number of armchair edges as 4 (100%). For hydrogenation of chrysene, a carbon atom at position 3 was the most hydrogenated (Figure 7a). Hydrogenated chrysene molecules are thermally unstable at 3000 K and most hydrogenated chrysene molecules tend to change back to chrysene in a similar way to dehydrogenated tetracene. A few hydrogenated chrysene molecules tend to decompose because of the low thermal stability of sp3C-sp3C bonding. By heat treatment of bichrysene, intramolecular dehydrogenation proceeded and pentagons and hexagons formed (Figure 7b). Unlike tetracene, the reaction times required to form hexagon at e.g. 2,5’:3,4’ and 1,5’:2,4’ by dehydrogenation of bichrysene were shorter than those to form pentagon, but formation time of pentagon and hexagon was close other than 2,5’:3,4’ (90 ps) and 1,5’:2,4’ (49 ps), implying that both pentagon and hexagon can form at similar temperatures. In an analogous way to the case of tetracene, once pentagons were formed, those pentagons were stable and did not transform into other structures. Formation of pentagon by carbonization of phenanthrene has been estimated by calculation.52 Thus, it is expected that the formation of pentagon proceeded by carbonization of chrysene. Unlike tetracene, some of intramolecular dehydrogenations such as the formation of 2,5’:3,4’ and 6,7’:7,6’ increased the number of armchair edges up to 5-6 and decrease QUATRO edges. Increment of armchair edges from 4 to 6 indicates that 150% of armchair edges is present compared to chrysene as the precursor. Some of intramolecular dehydrogenations such as 1,5’:2,4’ and 3,5’:4,5’:5,3’ maintained the number of armchair edges as 4 (100%). The only one type of intramolecular dehydrogenation such as 3,4’:4,5’:5,6’ decreased the armchair edges into 3 (75%). Thus, more armchair edges would be present at the early stage of carbonization of chrysene, especially if 2,5’:3,4’ and 6,7’:7,6’ formed selectively from chrysene. 3.2.2 Oxidative gasification of tetracene and chrysene before and after carbonization Figures 8a and 9a show tetracene and chrysene oxidized at 3000 K using ReaxFF, respectively. Oxygen molecules chemisorbed mostly at position 5 of tetracene was more stable than those at positions 1 and 2. A few oxygen molecules chemisorped at positions 1 and 2, which caused C-C scission. On the other hand, oxygen molecules chemisorbed on chrysene was unstable despite the positions. Oxidation at positions 5 and 12 of tetracene and positions 9 and 10 of anthracene and formation of para quinone have been reported.53 2-Hydroxy-1,4-anthraquinone has also been detected at slow reaction rate and further oxidation of this product formed phthalic anhydride, CO2, and H2O.53 Our calculated results and these references indicate that oxidized zigzag edges (or SOLO) tend to be stable and remained, causing increment of weight, whereas other edges such as QUATRO are unstable after oxidation and then gasification proceeds easily. Increment of weight was observed as results of TG as explained later.


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(a) Oxidation of tetracene


(b) Oxidation of zigzag edges on graphene nanoribbon

Figure 8. Representative structures of tetracene before and after oxidation and zigzag edges on graphene nanoribbon after oxidation at 3000 K in ReaxFF. (a) Oxidation of tetracene. Structures at the right side of the arrows are structures after oxidation. (b) Oxidation of zigzag edges on graphene nanoribbon. The numbers in front of “ps” above each structure indicate reaction time in pico second. The numbers of C and O in parentheses stand for the increased or decreased numbers of carbon and oxygen atoms after oxidation of zigzag edges. Reaction time of tetracene at conversion of 50% (13 ps) and 100% (55 ps) (Figure 8a) was shorter than that of chrysene at conversion of 50% (17 ps) and 100% (66 ps) (Figure 9a), where the conversion indicates that the



percentage of “the number of reacted precursors” out of “the number of precursors before reaction” (eq. 2). Thus, tetracene was easily more oxidized than chrysene. However, once the oxidation started, chrysene tended to be oxidized further easily and tended to be gasified. For example, most tetracene molecules were oxidized by one oxygen molecule at conversion of 50%, whereas most chrysene molecules were oxidized by three or more oxygen molecules. Conversion [%] = 100 × 𝑁 (𝑅𝑒𝑎𝑐𝑡𝑒𝑑 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟𝑠) 𝑁 (𝑃𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟𝑠 𝑏𝑒𝑓𝑜𝑟𝑒 𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛)

Page 12 of 35

(b) Oxidation of armchair edges on graphene nanoribbon

eq. 2

(a) Oxidation of chrysene

Figure 9. Representative structures of chrysene before and after oxidation and armchair edges on graphene nanoribbon after oxidation at 3000 K in ReaxFF. (a) Oxidation of chrysene. Structures at the right side of the arrows are structures after oxidation. (b) Oxidation of armchair edges on graphene nanoribbon. The numbers in front of “ps” above each structure indicate reaction time in pico second. The numbers of C and O in parentheses stands for the increased or decreased numbers of carbon and oxygen atoms after oxidation of armchair edges. Figures 8b and 9b show graphene nanoribbon with zigzag edges and armchair edges oxidized at 3000 K using ReaxFF, respectively. Graphene nanoribbons were calculated as alternative structures instead of carbonized tetracene and


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chrysene because carbonized tetracene and chrysene contain various structures and difficult to clarify the difference. Graphene nanoribbon with zigzag and armchair edges were simultaneously heated for fair comparison in one box. Thus, the snapshots at the top of Figure 8b and 9b are the same image and include two graphene nanoribbons with either zigzag or armchair edges in addition to oxygen molecules. Left graphene nanoribbon in the top image in Figures 8b and 9b is graphene nanoribbon with armchair edges and right side is graphene nanoribbon with zigzag edges. The numbers of carbon atoms and oxygen atoms changed before and after heat treatment at zigzag and armchair edges is shown in parentheses.

　　　　　 Figure 10. Experimental Raman spectra of as-received and heat-treated tetracene. Excitation wavelength of laser was 532 nm, except for as-received and tetracene heated at 653 and 693 K. 785 nm was used for 653 and 693 K to avoid fluorescence background. Colors of samples are written in parentheses. Arm, Tetra Zig, Graph Zig, Penta, DeH Tetra, Quad, and Quad-like, stands for armchair edges, tetracene-like zigzag edges, graphene-like zigzag edges, structure containing pentagon, dehydrogenated tetracene, quadrant ring stretch and quadrant-like ring stretch, respectively. The vibrations above this Figure are quadrant stretch, semicircle stretch, and inplane sp2C-H bending.

Figure 11. Experimental Raman spectra of tetracene and heattreated tetracene and calculated Raman spectra of tetracene and possible structures of heat-treated tetracene. Three spectra at the bottom are experimental spectra shown in Figure 10. (a-i) Calculated spectra of tetracene and possible structures of heat-treated tetracene. (j) Averaged spectrum of (d-i). Arm, Tetra Zig, Graph Zig, Penta, DeH Tetra, Quad, and Quad like, stands for armchair edges, tetracene-like zigzag edges, graphene-like zigzag edges, structure containing pentagon, dehydrogenated tetracene, quadrant ring stretch and quadrant-like ring stretch, respectively. The vibrations above this Figure are quadrant stretch, semicircle stretch, and in-plane sp2C-H bending. After oxidation of graphene nanoribbon with zigzag edges for 5 ps (Figure 8b), zigzag edges were oxidized and quinone/oxygen radical and hydroxy groups formed. After 15



ps, the number of oxygen increased to 19 with formation of neither CO nor CO2 gases. After 25 and 40 ps, two and three carbon atoms were removed from zigzag edges. Contrary to the stable oxygen-containing functional groups on zigzag edges of graphene nanoribbon, oxygencontaining functional groups on armchair edges of graphene nanoribbon were unstable in a similar way to the case of oxidation of tetracene and chrysene. After oxidation of armchair edges for 5 ps, the edges were not oxidized, indicating that armchair edges are less reactive. However, once the armchair edges were oxidized, carbon atoms on armchair edges were removed by formation of gasses (CO and CO2) and lactone, cyclic ether, and carboxylic group formed. 3.3 Raman spectra 3.3.1 Raman spectra of as-received and heat-treated tetracene Figure 10 shows experimental Raman spectra of asreceived and heat-treated tetracene, as assigned by the calculated results (Figures 11 and S7-S13 and Table 3). Although the assignments of Raman spectrum for tetracene have been explained by Alajtal et al.,27 more detail was obtained in this study. As-received tetracene showed peaks at 1620, 1606, 1544, 1384, 1198, and 1162 cm-1. After heat treatment at 653 K, peaks at 1618, 1450, and 1180 cm-1 either increased or newly formed. The peak at 1450 cm-1 corresponds to the dehydrogenated products between two tetracene molecules (Figure S8). After heat treatment at 693 K, the sharp peaks of as-received tetracene became unclear and a weak peak at 1336 cm-1 with the broad convex curve of a baseline appeared (Figure 10). The broad convex curve of the baseline is generally caused by the interference of background with fluorescence emission.27 After heat treatment above 733 K, clear peaks appeared at 1162, 1270, 1336, 1437, 1580, and 1606 cm-1. Figures S7-S13 and 11 show calculated Raman spectra of tetracene and possible carbonized tetracene compared with experimental Raman spectra in Figure 10. Only the selected structures are shown in Figure 11. These possible structures were selected as results of ReaxFF and the shape of calculated IR and Raman spectra compared with experimental IR and Raman spectra. The peak positions of calculated Raman spectrum of tetracene were almost the same as those of experimental Raman spectrum of as-received tetracene, although the intensities of the peaks, for example, at 1554 cm-1 for experimental spectrum was much higher than that for calculated spectrum (Figure 11). The experimental peak at 1620 cm-1 corresponds to quadrant stretch of C=C in tetracene mainly at positions 1, 2, 3, 4, 7, 8, 9, and 10 of tetracene. Peaks around the region have been called as D’ band among researchers of carbon materials, which related to the type of defects such as sp3C, vacancy, and grain boundary containing pentagons and heptagons in carbon materials.54 Above 733 K, the relative intensity of this peak at 1620 cm-1 became small compared to other peaks, and new peaks at 1606 and 1580 cm-1 appeared (Figure 10). This negative shift from 1620 to 1606 and 1580 cm-1 can be explained by the formation of dehydrogenated structure and graphene-like structure (G band). From the point of view of PAHs, peaks at the region of G and D’ band are known as quadrant stretch rather than G and D’ band.55,56 This negative shift of the peak of quadrant stretch upon increment of the conjugated system has been reported from the point of view of polyacetylene.57 Raman spectra of most of all possible

Page 14 of 35

dehydrogenated structures of tetracene were calculated, and the selected spectra are shown in Figure S8. Formation of one hydrogen molecule from two tetracene molecules via dehydrogenation shifted the peak top of quadrant stretch from 1620 to 1606 cm-1 depending on the position of dehydrogenation. Further dehydrogenation formed graphenelike structures and showed the calculated peak at ca. 1580 cm1 (Figure 11f, h, and i). Similar negative shifts upon carbonization of aromatic compounds have been observed for the nitrogen-containing carbon materials by our group.35 Table 3. Peak positions of experimental Raman spectra for asreceived and heat-treated tetracene. Types of vibration were determined by calculated Raman spectra in Figures 11 and S7S13. Sample name

Assignment*

Raman shift / cm-1

Tetracene

Tetracene heated at 733 and 933 K

Quadrant stretch of C=C mainly at positions 1-4 and 7-10 (D’ band)

1620

Quadrant stretch mainly at positions 5, 6, 11, and 12 (D’ band, Related to zigzag edge)

1606

Quadrant stretch-like in the 2-3 and 8-9 directions (Quad-like)

1544

Semicircle stretch of C=C mainly at 5, 6, 11, and 12 (Tetracene-like zigzag edge)

1384

In-plane sp2C-H bending/ C=C stretch mainly at 5, 6, 11, 12 (Tetracene-like zigzag edge)

1198

In-plane sp2C-H bending at all positions

1162

Quadrant stretch mainly at 5, 6, 11, and 12 (D’ band, Related to zigzag edge)

1606

Quadrant stretch-like positions (G band)

all

1580

Semicircle stretch-like at all positions (Graphene-like zigzag edge) + pinch mode (pentagonal) + out-of-plane wagging of sp3C-H of hydrogenated tetracene (Figure 11b)

1437

Semicircle stretch-like at positions, breathing mode band)

13361350

at

all (D

In-plane sp2C-H bending (Armchair edge) + C=C stretch

1270

In-plane sp2C-H bending at selected positions (positional dependence)

11981232

In-plane sp2C-H bending graphene-like structure

1164

in

* Peaks were assigned based on the calculated results in this work. The experimental peak at 1450 cm-1 has been reported to correspond to the characteristic peak for zigzag edges,24


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semicircle ring stretch vibration of PAHs,56 C-H rock vibration of five-membered ring heterocyclic compounds,56,58 and pentagons in fullerene and other aromatic compounds.58,59 Thus, the peak at 1437 cm-1 observed in this work would correspond to these peaks including pinch mode in the presence of pentagons (Figures 11d, e, and S9) and the presence of zigzag edges, although the peak intensity of zigzag edges was weak (Figures S10 and S11). In addition, the peak at 1437 cm-1 was related to sp3C-H (Figures 11b, 11c, and S7) and dehydrogenated structure between tetracene molecules (Figures S8). This peak remained even after heat treatment at 933 K. The presence of sp3C-H was observed as results of DRIFT spectra of tetracene heated below 853 K, but not above 893 K. Thus, the peak of experimental Raman spectra at 1437 cm-1 would include sp3C-H at 773 K, but the peak of experimental Raman spectra at 1437 cm-1 does not include sp3C-H. Thus, the peak of experimental spectra of tetracene heated at 933 K at 1437 cm-1 can be assigned as mainly tetracene-like zigzag edges with pentagon and dehydrogenated tetracene.

In-plane sp2C-H bending (armchair edge) + C=C stretch

1240

In-plane sp2C-H bending

1050

* Peaks were assigned based on the calculated results in this work.

Table 4. Peak positions of experimental Raman spectra for asreceived and heat-treated chrysene. Types of vibration were determined by calculated Raman spectra in Figures 13 and S14-S19. Sample name

Assignment*

Raman shift / cm-1

Chrysene

Chrysene at 893 K

Chrysene at 933 K

Quadrant stretch

16001628

Quadrant stretch-like

1574

Semicircle stretch-like

1434, 1380

In-plane sp2C-H bending at 1, 4, 7, 10 (armchair edge)

1240

In-plane sp2C-H bending at all positions

1160

C=C stretch (D’/ influence of pentagon)

1685

Quadrant stretch-like pentagonal)

1655

(D’/

Quadrant stretch-like (D’ band)

1625

Quadrant stretch-like (G band, hexagonal)

1574

C=C stretch

1540

Semicircle stretch-like (D band)

1340

In-plane sp2C-H bending (TRIO) + C=C stretch

1295

In-plane sp2C-H bending (armchair edge) + C=C stretch

1240


1160, 1050

Quadrant stretch-like pentagonal)

(D’/

1655

Quadrant stretch-like (D’ band, G band)

16001602

Quadrant stretch-like (G band)

1574

Semicircle stretch-like (D band)

1340

Figure 12. Experimental Raman spectra of as-received and heat-treated chrysene. Excitation wavelength of laser was 785 nm. Colors of samples are written in parentheses. “Arm”, “Trio sp2C-H”, “Chrysene Semi”, and “Penta” stand for armchair edges, a Trio structure, semicircle ring stretch of chrysene, and pentagon, respectively. A peak at around 1350 cm-1 is well known to be so called D band (A1g), which originates from ring-breathing mode/inphase ring stretch and intervalley scattering.28,29 But this D band generally includes other mode such as semicircle stretch at 1336-1350 cm-1 in addition to in-plane C-H bending between 1100-1300 cm-1 because of the overlapping of broad peaks around D band region and difficulty to separate peaks for general carbon materials. The D band formed as temperature reached above 693 K. Peaks from 1019 to 1247 cm-1 have been reported as inplane sp2C-H bending and/or sp2C-H rocking of aromatic



compounds.60 A peak at 1198 cm-1 corresponds to in-plane sp2C-H bending and C=C stretch mainly at 5, 6, 11, and 12, and a peak at 1162 cm-1 corresponds to in-plane sp2C-H bending at all sp2C-H positions of tetracene as results of calculation in this work. Relative intensities of these peaks became small, but still remained even at 933 K.

Page 16 of 35

Table 5. Peak positions of experimental DRIFT spectra for asreceived and heat-treated tetracene. Types of vibration were determined by calculated DRIFT spectra in Figures 15, 17, 19, and S20-S40. Sample name

Assignments

Wavenu mber / cm-1

Asreceived tetracen e

sp2C-H stretching (Others)

3045 (m)

sp2C-H stretching (Zigzag)

3015 (w)

Quadrant vibration

stretching

1630 (m)

In-plane sp2C-H bending mainly at 5, 6, 11, and 12 (Zigzag)

1294 (m)

C=C

Out-of-plane positions

Tetracen e heated between 693 and 813 K

Figure 13. Experimental Raman spectra of chrysene and heattreated chrysene and calculated Raman spectra of chrysene and possible structures of heat-treated chrysene. Three spectra at the bottom are experimental spectra. (a-j) Calculated spectra of chrysene and possible structures of heat-treated chrysene. (k) Averaged spectrum of (c-j). “Arm”, “Trio sp2C-H”, “Chrysene Semi”, and “Penta” stand for armchair edges, a trio structure, semicircle ring stretches of chrysene, and pentagon, respectively.

Tetracen e heated

sp2C-H

all

957 (w)

Tetracene-like SOLO (Fig. 19a), out-of-plane sp2C-H mainly at 5, 6, 11, and 12 (SOLO) (Zigzag)

903 (s)

Out-of-plane sp2C-H at 1, 2, 3, 4, 7, 8, 9, and 10 (QUATRO)

740 (m)

sp2C-H stretching influenced by sp3C-H

3057, 3029 (m)

sp2C-H stretching (Zigzag)

3015 (w)

sp3C-H

stretching (hydrogenated tetracene)

2960 (w)

sp3C-H dimer)

(tetracene

2930 (w)

sp3C-H stretching

2802 (w)

Quadrant C=C stretching vibration of hydrogenated and dimerized tetracene

1602 (m)

In-plane Semicircle vibration

bending, stretching

1486 (m), 1459 (m), 1419 (m)

Semicircle C=C stretching of hydrogenated tetracene and dimerized tetracene (Fig. 17b and c)

1346 (w)

In-plane sp2C-H bending, C=C stretching vibration

1270 (w)


1255 (s)

Tetracene-like SOLO influenced by sp3C-H (Fig. 19b and c), sp3CH rocking vibration and out-ofplane sp2C-H (Quaternary and SOLO)

918 (w)

Out-of-plane sp2C-H vibration (Tetracene-like SOLO influenced by sp3C-H (Fig. 19b and c), Graphene-like SOLO (Fig. 19f-i)

867 (w)

Out-of-plane sp2C-H vibration (Graphene-like DUO+TRIO (Fig. 19f)

810 (w)

stretching

sp2C-H C=C

at

sp2C-H stretching (Armchair)


3075 (w)73

Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

between 853 and 933 K

ACS Applied Materials & Interfaces sp2C-H like)

stretching

Quadrant vibration tetracene In-plane semicircle vibration

of

(Armchair-

C=C stretching dehydrogenated

sp2C-H C=C

3066 (w)73 1602 (m)

bending, stretching

1500 (m)

In-plane sp2C-H bending, semicircle C=C stretching vibration (Zigzag), pentagon pinch mode (Fig. 17d and e)

1440 (s)

In-plane sp2C-H semicircle C=C

1346 (m)

bending,


1153 (s)

C=C stretching vibration

945 (w)

Graphene-like SOLO (Fig. 19f-i)

880 (s)

Graphene-like Bay

835 (w)

Graphene-like DUO+TRIO (Fig. 19f-i)

795 (w)

Out-of-plane sp2C-H (TRIO)

760 (w)

Among calculated Raman spectra of selected structures in Figure 11, spectra in Figure 11d-i fitted well with experimental Raman spectra of tetracene heated above 733 K. For example, peaks at 1437 cm-1 calculated Raman spectra of Figure 11d and e fitted well with those of the experimental Raman spectra. Structures in Figure 11d-i have dehydrogenated positions at 5, 6, 11, and 12 in common because the number of electrons on carbon atoms at positions 5, 6, 11, and 12, as calculated from Mulliken charge, was the largest (6.27) among all the positions (Figure 1). Sasaki et al. have reported that electron-rich carbon atoms in anthracene reacted easily.14 Thus, the hydrogen atoms at positions 5, 6, 7, 11, and 12 in tetracene are expected to be dehydrogenated easily. Most works, which studied structural change upon polymerization of acene such as anthracene and tetracene, exhibited the formation of hexagon by dehydrogenation (Figure 11f)16,17,52 and hydrogenated and dimerized structures (Figure 11b and c).17,61 However, this work showed that pentagons (Figure 11d and e) also formed as indicated by Sasaki et al. who studied carbonization of anthracene.14 Some of the calculated Raman spectra for representative structures after heat treatment of tetracene such as Figure 11d-i were summed and Raman spectrum of tetracene heated above 733 K was simulated (Figure 11j). Except for the strong intensity of sp2C-H between 1150-1250 cm-1, the peak positions and intensities of calculated Raman spectra were close to those of experimental Raman spectra of tetracene heated above 733 K. 3.3.2 Raman spectra of as-received and heat-treated chrysene Figure 12 shows experimental Raman spectra of asreceived chrysene and chrysene heated at 693-933 K, as assigned by the calculated results (Figures 13 and S14-S19 and Table 4). As explained in the prior work, peaks of chrysene at 1601, 1574, 1432, 1380, and 1363 cm-1 correspond to C=C stretching, ring stretching, stretching vibrations, skeletal ring vibration, and C=C stretching vibration, respectively.27 At temperatures between 773 and 813 K and between 893 and 933 K, the shape of spectra drastically changed. The changes in

shape of spectra above 813 and 933 K were correlated with that in color of the heated samples (Table 2). Spectra of brown samples at 853 and 893 K showed quite different shape compared with that of the black sample at 933 K. These peaks are experimental peaks, but not an error of a device of Raman spectroscopy, because we could obtain the similar peaks at different temperatures between 813 and 893 K (Figure 12). It has been reported that pentagon can be formed by dehydrogenation between two phenanthrene molecules.52 In this work, formation of pentagon was also observed in IR spectra, as explained later. The thermal stability of pentagon has been reported as 753 K62 and 623-773 K.63 Thus, it is no wonder that pentagon formed from chrysene in this work changed into other structure between 893 and 933 K. One of the possible explanations for diminishing pentagons are hydrogenation of pentagons by hydrogen formed via dehydrogenation between chrysene molecules in ampoule tubes. Figure 13 shows calculated Raman spectra of chrysene and possible structures of carbonized chrysene which fitted well with experimental Raman spectra of as-received chrysene and chrysene heated at 893 and 933 K. These possible structures were selected as results of ReaxFF and the shape of calculated IR and Raman spectra compared with experimental IR and Raman spectra. Peaks at 1685, 1655, 1625, 1602, 1574, 1540, 1380, 1340, 1295, 1240, 1160, and 1050 cm-1 were observed at these temperatures. Peaks at 1685 and 1655 cm-1 correspond to quadrant stretch-like vibration of C=C for the structure in the presence of pentagon of dehydrogenated chrysene (e.g. Figure 13b). Peaks between 1602 and 1574 cm-1 correspond to quadrant stretch of C=C in dehydrogenated chrysene (Figure 13b-d). Peaks between 1295 and 1050 cm-1 correspond to in-plane sp2C-H bending. Especially, a characteristic peak at 1240 cm-1 corresponds to in-plane sp2CH bending mainly for armchair edges. This peak was not observed for heat-treated tetracene. The selected structures of Figure 13b-d have bonding at positions 4, 5, and 6. The positions of bonding accord well with the results reported by Sasaki et al.14 They explained that phenanthrene molecules, which contain three aromatic rings, reacted each other at electron rich positions. The numbers of electrons at positions 4, 5, and 6 (or 10, 11, and 12) were higher than those at positions 2 and 3 (or 8 and 9) in this work (Figure 2a). However, those positions showed difficulty to react as results of ReaxFF because of the steric hinderance (Figure 7a). Thus, in addition to the factor of electron density, the steric hinderance has to be considered. The structure in Figure 13b has bonding at position 3 in addition to the bonding at positions 4, 5, and 6. This structure in Figure 13b is expected to be one of the possible structures of chrysene heated at 893 K, because the experimental peak observed at 1655 cm-1 was not present in the calculated peak of dehydrogenated structures without pentagon. It is also because the steric hindrance is present between hydrogen atoms of the structure in Figure 13c and easily dehydrogenated. The peak at 1655 cm-1 of chrysene heated at 893 K in Figure 13b was mostly eliminated at 933 K. In this work, carbon materials were prepared in sealed ampoule tubes. Thus, hydrogen is produced by dehydrogenation inside the ampoule tubes, and the hydrogen may have attacked pentagons and removed pentagons. It has been reported that peaks at 13503,21 and 153024 cm-1 relate to the presence of armchair edges. In this work, peaks at ca. 1340 cm-1 were observed in calculated and experimental



Raman spectra of carbonized tetracene in addition to carbonized chrysene because of the semicircle stretch of aromatic rings. But the normalized intensity (I/IMax) of carbonized chrysene at ca. 1340 cm-1 was much higher than that of carbonized tetracene at ca. 1340 cm-1 by comparing with the intensity of G band. The intensity of peaks at around 1540 cm-1 was observed at 893 K in this work (Figures 12 and 13). This peak may correspond to the reported peak of armchair edges at 1530 cm-1.24 The normalized intensity of the peak reduced at 933 K, but it still remained. The experimental peak at 1574 cm-1 obtained in chrysene carbonized at 933 K was closed to the calculated peak of dehydrogenated chrysene (Figure 13b and d-j). Thus, armchair edges should be present even after being carbonized at 933 K, which well accords with results of ReaxFF (Figure 7).

Page 18 of 35

peak of sp3C-H was eliminated and a peak corresponding to sp2C-H stretching vibration at 3045 cm-1 remained above 823 K. We also examined the elution analysis of heat-treated tetracene by dissolving in deuterochloroform as a solvent. As it is mentioned above, tetracene heated at 713 K was dissolved in the solution and 90% in the dissolved compounds was 5,12dihydrotetracene, and the rest of the dissolved compound was 1,2,3,4-tetrahydrotetracene as results of NMR analysis (Figures S61 and S62).

3.3.3 Comparison of Raman spectra between heat-treated tetracene and chrysene. By comparing experimental Raman spectra of heattreated tetracene and chrysene (Figures 10-13), the intensity ratio of ID/IG band as well as the shape and peak tops of the spectra are quite different. For example, the ID/IG ratio of tetracene heated at 933 K was 0.58, whereas that of chrysene heated at 933 K was 1.20. This clearly shows that the heattreated chrysene includes much more armchair edges than the heat-treated tetracene, and the presence of armchair edges relates to the D band, as reported.3,21 The peak top of G band of heat-treated tetracene was 1606 cm-1, whereas that of heattreated chrysene was 1574 cm-1. This difference of the peak top of G band was also observed for the calculated spectra. In terms of the characteristic peak of zigzag edges, a peak of tetracenelike zigzag edges at 1384 cm-1 was shifted to 1437 cm-1 by the formation of graphene-like zigzag edges and pentagon above 773 K (Figure 10). On the other hand, a peak of chrysene-like armchair edges at 1240 cm-1 was almost absent, but the normalized intensity of the peak of armchair edges at 1240 cm1 became high by heat treatment above 893 K (Figure 12). These results clearly show that different types of carbon materials with either zigzag or armchair edges could be prepared by simply heat treatment of different precursors. 3.4. IR spectroscopy 3.4.1 IR spectra of as-received and heat-treated tetracene Figure 14 shows experimental DRIFT spectra of asreceived and heat-treated tetracene between 2700 and 3200 cm-1, as assigned by the calculated results (Figure 15 and Table 5). IR spectra of aromatic compounds with zigzag edges such as anthracene and tetracene have been reported61 including our recent work,11,37 but the detailed analyses of carbonized tetracene and chrysene by IR spectra have not been conducted. Above 693 K, the color of tetracene turned from orange color into black color (Table 1 and Figure 14) and peaks between 2802 and 2960 cm-1 as well as two peaks at 3029 and 3057 cm-1 appeared. Peaks at 2960 and 2930 cm-1 of tetracene heated at 693 to 853 K correspond to sp3C-H vibration, because of the presence of 5,12-dihydrotetracene (the second spectrum from the bottom of Figure 15 and Figure 15b), 1,2,3,4tetrahydrotetracene (Figure S20c), and dimerized tetracene (Figures 15c and S20g, h). Tamai et al. have reported that the increment of a peak at 2800-3000 cm-1 for tetracene heated above 673 K is caused partly by the formation of sp3C-H in 5,12dihydrotetracene as mentioned in the reported work.15 The

Figure 14. Experimental DRIFT spectra of as-received and heat-treated tetracene in the region between 2700 and 3200 cm-1. Colors of samples are written in parentheses. “Dimer”, “+H2”, “Zig”, and “Zig-like” stand for dimerized tetracene, hydrogenated tetracene, zigzag edges, and zigzag-like edges, respectively. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in Figure 18. In addition to the hydrogenation of tetracene, a peak at 2930 cm-1 is relevant to dimer formation of tetracene (Figures 14 and 15). It is well known that aromatic compounds with


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zigzag edge such as anthracene and tetracene forms dimer by cycloaddition under light.61,64 Especially, peaks at 3030, 2941, and 2924 cm-1 of the photodimer of anthracene have been assigned as aromatic C-H stretching mode, aliphatic C-H stretching mode, and C-H stretching mode in hydrogenated PAHs hot band of the aromatic C-H stretch, respectively.64 This work also calculated activation energy for the formation of dimer via either the formation of C=C bonding between tetracene molecules without dehydrogenation such as Figure 15c or dehydrogenated structures between two tetracene molecules using transition state calculation using Gaussian09 (b3lyp/6-31G(d)), and indeed the formation of dimer via the formation of C=C bonding between tetracene molecules without dehydrogenation was much lower (not shown), which is similar to the reported work.57

stand for dimerized tetracene, hydrogenated tetracene, zigzag edges, and zigzag-like edges, respectively. 　　　　

　　　 Figure 16. Experimental DRIFT spectra of as-received and heat-treated tetracene in the region between 1000 and 1800 cm-1. Colors of samples are written in parentheses. “Quad”, “Glike”, “D’-like”, “Semi”, “Pentagon + Zig”, “H Tetra/dimer”, “Tetra”, and “(+ C=C)”, stand for “quadrant ring stretch”, “Gband-like”, “D’-band-like”, “semicircle ring stretch”, “pentagon and zigzag edges”, “semicircle ring stretch influenced by the presence of hydrogenated tetracene and dimer of tetracene”, “sp2C-H bending of tetracene”, and “C=C stretching vibration”. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in Figure 18.

Figure 15. Experimental and calculated DRIFT spectra of tetracene and possible structures of heat-treated tetracene in the region between 2700 and 3200 cm-1. Four spectra at the bottom are experimental spectra. (a-i) Calculated spectra of tetracene and possible structures of heat-treated tetracene. (j) Averaged spectrum of (d-i). “Dimer”, “+H2”, “Zig”, and “Zig-like”

Compared with as-received tetracene, tetracene heated above 893 K showed high intensities of peaks at 3029 and 3057 cm-1 relative to the intensity of the peak at 3045 cm-1. These peaks are assigned as sp2C-H influenced by the presence of sp3C-H (the second spectrum from the bottom in Figure 15 (5,12-dihydrotetracene) and Figure 15b and c). Above 893 K, peaks originated from sp3C disappeared, and only sp2C-H remained (Figure 14). The spectra of tetracene heated above 893 K showed a broader peak than that of asreceived tetracene because of the presence of various bonding states of sp2C-H on carbon materials.30 The peaks at 3015, 3029,



3066, and 3075 cm-1 above 893 K are assigned as zigzag edges, zigzag-like edges without influence of sp3C-H, armchair-like edges, and armchair edges base on our recently reported work37 because sp3C-H was absent above 893 K, respectively.

Page 20 of 35

Some of the calculated IR spectra for representative structures after heat treatment of tetracene such as Figure 15d-i were averaged and IR spectrum of tetracene heated at 933 K was simulated (Figure 15j). Calculated peak positions as well as intensities were close to the experimental DRIFT spectra of tetracene heated at 933 K. Figure 16 shows experimental DRIFT spectra of asreceived and heat-treated tetracene between 1000 and 1800 cm-1, as assigned by the calculated results (Figure 17 and Table 5). A peak at 1630 cm-1 of as-received tetracene corresponds to quadrant stretching vibration, which is similar to D’ band in Raman spectra. A peak at 1294 cm-1 of as-received tetracene corresponds to in-plane sp2C-H vibration at positions 5, 6, 11, and 12. A peak at 1153 cm-1 corresponds to in-plane sp2C-H bending mainly at positions 2, 3, 8, and 9. At temperatures between 693 and 813 K, peaks at 1419, 1459, and 1486 cm-1 appeared (Figure 16). These peaks originate from semicircle stretch of C=C and in-plane sp2C-H bending influenced by the formation of sp3C-H as results of experimental DRIFT spectra of 5,12-dihydrotetracene and calculated IR spectra of 5,12-dihydrotetracene (Figure 17b) and tetracene dimer (Figure 17c). Especially, the peak positions and the calculated spectral shape of tetracene dimer (Figure 17c) were close to those of the experimental spectrum of tetracene heated at 773 K. Thus, either this or similar structure with sp3C-H must be present at such low temperatures. At temperatures above 853 K, peaks at 1602, 1500, 1440, and 1346 cm-1 appeared (Figure 16). A peak at 1602 cm-1 corresponds to quadrant stretching vibration of C=C at all positions. The peak shift from 1630 (tetracene) to 1602 cm-1 (heated tetracene above 853 K) relates to the increment of conjugated system, which is similar to the negative shift of Raman spectra35,56 Formation of pentagon by dehydrogenation between two tetracene molecules generated similar spectral shape to the experimental DRIFT spectra (Figure 17d, e). Those structures with pentagons are not main structures above 853 K, but the pentagon should be present. The peak at ca. 1440 cm-1 has been reported as the peak originating from either pentagon in fullerene65 or zigzag edges.24 As results of calculation of IR spectra in this work, both pentagon and zigzag edges are possibly present above 853 K, because calculated IR spectra of the structure with pentagon (Figure 17d) were similar to the experimental DRIFT spectra of tetracene heated above 853 K. The possible formation of pentagons and aliphatic products from anthracene and chrysene and elimination of pentagons at higher temperatures has been reported.14,18 Thus, the possibility of the presence of pentagons is high in tetracene heated above 853 K.

Figure 17. Experimental and calculated DRIFT spectra of tetracene and possible structures of heat-treated tetracene in the region of 1000-1800 cm-1. Four spectra at the bottom are experimental spectra. (a-i) Calculated spectra of tetracene and possible structures of heat-treated tetracene. (j) Averaged spectrum of (d-i). “Quad”, “G-like”, “D’-like”, “Semi”, “Pentagon + Zig”, “H Tetra/dimer”, “Tetra”, and “(+ C=C)”, stand for “quadrant ring stretch”, “G-band-like”, “D’-band-like”, “semicircle ring stretch”, “pentagon and zigzag edges”, “semicircle ring stretch influenced by the presence of hydrogenated tetracene and dimer of tetracene”, “sp2C-H bending of tetracene”, and “C=C stretching vibration”.

Some of the calculated IR spectra for representative structures after heat treatment of tetracene such as Figure 17d-i were averaged and IR spectrum of tetracene heated at 933 K was simulated (Figure 17j). Averaged calculated peak positions as well as normalized intensities of possible structures were close to the experimental DRIFT spectra of tetracene heated at 933 K. Thus, these structures are possible structures. Figure 18 shows experimental DRIFT spectra of asreceived and heat-treated tetracene in the region between 700 and 1000 cm-1, as assigned by the calculated results (Figures 19 and S35-S40 and Table 5). Tetracene photodimer has been studied at the region between 700 and 1000 cm-1,61 but the detailed assignment has not been reported. As results of


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calculation of tetracene (Figure 19a), a weak peak at 957 cm-1 of tetracene originates from C=C stretching at positions 1, 2, 3, 4, 7, 8, 9, and 10 and out-of-plane sp2C-H bending at all positions. A strong peak at 903 cm-1 of as-received tetracene corresponds to out-of-plane sp2C-H vibration mainly at 5, 6, 11, and 12 (SOLO position). The presence of SOLO at 903 cm-1 indicates the presence of tetracene-like zigzag edge. A strong peak at 748 cm-1 of as-received tetracene corresponds to outof-plane sp2C-H bending at positions 1, 2, 3, 4, 7, 8, 9, and 10 (QUATRO positions).

　 Figure 18. Experimental DRIFT spectra of as-received and heat-treated tetracene in the region between 700 and 1000 cm1. Colors of samples are written in parentheses. Graphene-like SOLO, tetracene-like SOLO, tetracene-like SOLO with sp3C-H, bay structure, armchair-like structure, DUO and DUO-like, Trio, and Quatro are written as “Graph-like SOLO”, “Tetra-like SOLO”, “Tetra-like SOLO with sp3C-H”, “Bay”, “Arm-like”, “DUO”, “TRIO”, and “QUAT”, respectively. Intensity of peaks were normalized at the maximum peak top between 700 and 1000 cm-1.

Figure 19. Experimental and calculated DRIFT spectra of tetracene and possible structures of heat-treated tetracene in the region between 700 and 1000 cm-1. Four spectra at the bottom are experimental spectra. (a-i) Calculated spectra of tetracene and possible structures of heat-treated tetracene. (j) Averaged spectrum of (d-i). Graphene-like SOLO, tetracene-like SOLO, tetracene-like SOLO with sp3C-H, bay structure, armchair-like structure, DUO and DUO-like, Trio, and QUATRO are written as “Graph-like SOLO”, “Tetra-like SOLO”, “Tetra-like SOLO with sp3C-H”, “Bay”, “Arm-like”, “DUO”, “TRIO”, and “QUAT”, respectively.



Page 22 of 35

At temperatures between 693 and 813 K, a peak at 918 cm1 appeared (Figures 18 and 19). This peak corresponds to out-

of-plane sp2C-H vibration influenced by the presence of hydrogenated tetracene, as results of experimental and calculated IR spectra of 5,12-dihydrotetracene (Figure 19b). In addition, a peak at 867 cm-1 also appeared. This peak indicates the presence of hydrogenated tetracene, dimerized tetracene, and graphene-like SOLO (Figure 19b, c, g, and h). A peak at 880 cm-1 corresponds to graphene-like SOLO (Figure 19f-i). It indicates that hydrogenation, dimerization, and carbonization proceeded between 693 and 813 K.

　　 Figure 20. Experimental DRIFT spectra of as-received and heat-treated chrysene in the region between 2700 and 3500 cm-1. Colors of samples are written in parentheses. The peak at 3200 cm-1 is OH group and water. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in Figure 24.

Figure 21. Calculated DRIFT spectra of as-received chrysene and possible structures of heat-treated chrysene compared with experimental DRIFT spectra of as-received and heattreated chrysene in the region between 2700 and 3500 cm-1. Three spectra at the bottom are experimental spectra. (a-j) Calculated spectra of chrysene and possible structures of heattreated chrysene. (k) Averaged spectrum of (c-j).


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Table 6. Assignments of peaks for as-received and heat-treated chrysene using DRIFT. Types of vibration were determined by calculated IR spectra in Figures 21, 23, 25, and S41-S58. Sample name

Assignments

As-received chrysene

In-plane sp2C-H stretching (Armchair)

3085 (m)

In-plane sp2C-H stretching at all positions

3052 (s)

In-plane sp2C-H stretching (other than Armchair)

3021(m)

Overtone of a peak (867 (C=C stretch) and 817 (DUO) cm-1)

1833-1689 (vw)

Quadrant stretching of C=C

1594, 1514 (m)

Semicircle stretch of C=C/ sp2C-H in-plane bending

1485 (m), 1430 (s)

Semicircle stretch of C=C

1357 (w)

In-plane sp2C-H bending (mainly at Armchair)

1263 (s)

In-plane sp2C-H bending (mainly other than Armchair)

1150 (w)

C=C stretching at positions 2-3 and 8-9

1027 (m)

Out-of-plane sp2C-H at all positions

945 (w)

Out-of-plane sp2C-H bending at most positions except for 6 and 12

867 (m)

Out-of-plane sp2C-H at all positions except for positions 2 and 8 (Duo) (Armchair)

817 (s)

Out-of-plane sp2C-H at positions 1, 2, 3, 4, 7, 8, 9, and 10 (Quatro)

758 (vs)

Semicircle stretch in the presence of pentagon

1400 (w)

In-plane C=C bending

886 (w)

Out-of-plane sp2C-H bending (DUO influenced by the presence of pentagon)

867 (w)

Out-of-plane sp2C-H bending (Graphene-like DUO)

840 (w)

Out-of-plane sp2C-H bending (DUO influenced by the presence of pentagon)

800 (w)

Chrysene heated at 933 K

Wavenumber / cm-1

the dehydrogenation among tetracene molecules (Figure 19di). The peaks at 795 and 810 cm-1 are located at positions of DUO and TRIO. Increment of the peaks at 867 and 880 cm-1 above 853 K is caused by the formation of graphene-like SOLO. From these results above 853 K, it can be explained that carbonization further proceeded. Some of the calculated IR spectra for representative structures after heat treatment of tetracene such as Figure 19d-i were averaged and IR spectrum of tetracene heated at 933 K was simulated (Figure 19j). Calculated peak positions as well as normalized intensities in Figure 19j were close to the experimental DRIFT spectra of tetracene heated at 933 K. Thus, these structures were confirmed to be possible structures of carbonized tetracene.

　　 Figure 22. Experimental DRIFT spectra of as-received and heat-treated chrysene in the region between 1000 and 1800 cm-1. Colors of samples are written in parentheses. “Arm”, “Semicircle”, and “Quad” stand for armchair edges, semicircle ring stretch, and quadrant ring stretch, respectively. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in Figure 24.

At temperatures above 853 K, intensities of peaks at 760, 795, 810, 867, and 880 cm-1 increased (Figure 18). The peak at 760 cm-1 corresponds to TRIO. This peak increased because of



Page 24 of 35

of sp2C-H, which were different from the bonding state of sp2CH in as-received chrysene, were possibly formed. It is because calculated IR spectra of dehydrogenated chrysene in Figure 21b-j showed similar spectra to chrysene, but the peaks at slightly different positions were observed. As results of calculation, peaks at 3085 cm-1 relate to sp2C-H stretching vibration of armchair edges, as indicated by our recent work.37 Peaks at 3052 cm-1 originate from sp2C-H stretching vibration at all positions. Peaks at 3021 cm-1 relate to sp2C-H stretching at all positions other than armchair edges.

Figure 23. Calculated DRIFT spectra of as-received chrysene and possible structures of heat-treated chrysene compared with experimental DRIFT spectra of as-received and heattreated chrysene in the region between 1000 and 1800 cm-1. Three spectra at the bottom are experimental spectra. (a-j) Calculated spectra of chrysene and possible structures of heattreated chrysene. (k) Averaged spectrum of (c-j). “Arm”, “Semicircle”, “Quad”, and “G-like” stand for armchair edges, semicircle ring stretch, quadrant ring stretch, and G-band-like stretch, respectively.

　　 Figure 24. Experimental DRIFT spectra of as-received and heat-treated chrysene in the region between 700 and 1000 cm1. Colors of samples are written in parentheses. Duo influenced by the presence of pentagon, graphene-like Duo, chrysene-like Duo, armchair-like influenced by either steric hinderance or non-hexagonal rings, and quadrant influenced by either steric hinderance or non-hexagonal rings are written as “Duo (Penta)”, “Grap-Duo”, “Chrys-Duo”, “Arm-like (Steric)”, and “Quat (Steric)”, respectively. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in this figure.

3.4.2 IR spectra of as-received and heat-treated chrysene Figure 20 shows experimental DRIFT spectra of chrysene and chrysene heated between 693 and 933 K in the region between 2700 and 3500 cm-1, as assigned by the calculated results (Figure 21 and Table 6). The shapes of spectra are similar to each other, but the normalized intensity of peaks at 3021 and 3085 cm-1 decreased. It indicates that bonding states

Figure 22 shows experimental DRIFT spectra of chrysene and chrysene heated from 693 and 933 K in the region between 1000 and 1800 cm-1, as assigned by the calculated results (Figures 23 and S47-S52 and Table 6). Above 893 K, a peak at 1400 cm-1 appeared. This peak relates to the pentagon formation during dehydrogenation among chrysene molecules (Figure 23b). Except for the peak at 1400 cm-1, positions of


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other peaks are similar. Although the color of chrysene became black, the shapes of spectra were similar to that of as-received chrysene in the region between 1000 and 1800 cm-1. A peak at 1263 cm-1 in experimental DRIFT spectra originates from inplane sp2C-H bending mainly at armchair edges as results of calculation (Figure 23a-d). This peak was also observed in Raman spectra at 1240 cm-1 (Figures 12 and 13). Thus, the presence of armchair edges can be determined from the presence of the peak at 1263 cm-1 of DRIFT spectra.

chrysene-like DUO, and Quatro influenced by either steric hinderance or non-hexagonal rings are written as “Duo (Penta)”, “Grap-Duo”, “Chrys-Duo”, and “Quat (Steric)”, respectively. Intensities of spectra were normalized at the maximum peak top between 700 and 1000 cm-1 in Figure 24.

H

H

H

693 813 K

H

H

Route 1

H C

HH C

H

H

H H

H

-H2

H

H

H

H

H H

H

H

H

H

H

H H

H

H H

Route 2

HC

H H

H

H

H

-H2

H

H H

C

H

H

H H

H H

H H

H H

H

H

H H H H

H C

Route 3

H C

H

H H

H

H

H H

H

H

-H2

H

H

H H H H

H

H

H

H H

H

H

Route 4

H

H

HH C H

CH

-H2 H

HH

H

H H

H H

H H

H H

H

H

H

H

Side reactions (Routes for hydrogenation, causing decomposition.)

H

H

H

H

H

H

H

H

H

H

H

H

C H

H H

H

H

H

H

+H

H

C H

H

693 773 K

H

H

H

H

+3H

H

H

H

H

H

H H H

Figure 26. Plausible reaction routes of carbonization of tetracene below 813 K.

Figure 25. Calculated DRIFT spectra of as-received chrysene and possible structures of heat-treated chrysene compared with experimental DRIFT spectra of as-received and heattreated chrysene in the region between 700 and 1000 cm-1. Three spectra at the bottom are experimental spectra. (a-j) Calculated spectrum of chrysene and possible structures of heat-treated chrysene. (k) Averaged spectrum of (c-j). DUO influenced by the presence of pentagon, graphene-like DUO,

Figure 24 shows experimental DRIFT spectra of chrysene in the region between 700 and 1000 cm-1, as assigned by the calculated results (Figure 25 and Table 6). A peak at 945 cm-1 corresponds to out-of-plane sp2C-H vibration mainly at all positions 1, 2, 3, 4, 7, 8, 9, and 10. A peak at 870 cm-1 corresponds to stretching C=C vibration. A peak at 817 cm-1 corresponds to out-of-plane sp2C-H bending vibration (DUO) at all positions except for 2 and 8. A peak at 758 cm-1 corresponds to out-of-plane sp2C-H bending (QUATRO) mainly at positions 1, 2, 3, 4, 9, 10, 11, and 12. A peak at 680 cm-1 corresponds to C=C stretching vibration mainly at all positions. Figure 24 also shows experimental DRIFT spectra of heat-treated chrysene in the region between 700 and 1000 cm-1. The shape of the peak of chrysene heated even at 933 K was similar to that of asreceived chrysene. But the intensities of peaks at 800, 840, and 886 cm-1 increased above 853 K. Peaks at 800 and 840 cm-1 correspond to out-of-plane sp2C-H bending (Graphene-like



Page 26 of 35

DUO). A peak at 886 cm-1 corresponds to in-plane C=C bending. Most peaks are similar to those of as-received chrysene even after carbonization. It indicates that in addition to the structures in Figure 25b-j, other structures, whose structures are close to chrysene, are also present.

853 933 K

3.5 Plausible reaction routes for carbonization

Route 2 -2H2

Route 1 -3H2

-4H2

-3H2

3.5.1 Carbonization of tetracene Figures 26-28 show plausible reaction routes for carbonization of tetracene below 813 K and above 853 K, respectively. These reaction routes were estimated as results of elemental analysis, MS, Raman spectra, DRIFT spectra in addition to the simulated results of Raman, IR spectra, and ReaxFF. Reaction mechanisms of dimerization of acenes60 and carbonization of anthracene,14 and pentacene17 by dehydrogenation through radical reactions have been proposed,17 but those structures are still under debate because of the difficulty to analyze carbon materials with different edges.

Route 3 -6H2

Route 4 -6H2

773 933 K Route 3 -H2 H

H

H

H

813 893 K -H2

Side reactions (Routes for dehydrogenation)

773 933 K

-2H2 H

H

H

H

H

H H

Route 4 -H2

H

Figure 28. Plausible reaction routes of carbonization of tetracene between 813 and 933 K. Figure 27. Plausible reaction routes of carbonization of tetracene between 733 and 933 K.

Carbonization routes of tetracene can be estimated to proceed through the following five routes (Figures 26-28). The first route (Route 1) is dimerization between two tetracene molecules, forming sp3C-H. The second route (Route 2) is intermolecular dehydrogenation between two tetracene molecules. The third route (Route 3) is hydrogenation of tetracene. These three routes begin to proceed between 653 and 693 K. The fourth route (Route 4) is formation of pentagon via further dehydrogenation above 773 K, as determined by DRIFT spectra at the region between 700 and 1000 cm-1. The fifth route (Route 5) is elimination of sp3C-H as well as further dehydrogenation above 853 K, forming small graphene-like structure. 3.5.2 Carbonization of chrysene Figures 29 shows plausible reaction routes for carbonization of chrysene. In a similar way to carbonization of aromatic compounds with zigzag edges, reaction mechanisms of carbonization of aromatic compounds with armchair edges such as phenanthrene14 and chrysene18 by dehydrogenation have been proposed, but those structures are also still under debate because of the difficulty to analyze carbon materials with different edges.


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813893 K -H2 , -2H2

853 893 K -xH2

Figure 29. Plausible reaction routes of carbonization of chrysene above 813 K. Carbonization routes of chrysene can be estimated to proceed through the following three routes. The first route (Route 1) is intermolecular dehydrogenation between two chrysene molecules. The second route (Route 2) is formation of the small amount of pentagon via dehydrogenation between 813 and 893 K, as determined by Raman and DRIFT spectra in addition to ReaxFF. The third route (Route 3) is further dehydrogenation and formation of hexagon above 853 K, forming small graphene-like structure. Unlike tetracene, chrysene was carbonized without formation of sp3C-H.

3.6 Oxidation reaction 3.6.1 Analyses of carbon materials coated on SiO2 by TEM and BET. Figure 30 shows TEM image of nanoballoon and nanoballoon heated with either tetracene at 853 K or chrysene at 923 K. Carbonized tetracene and chrysene could not be clearly observed on nanoballoon, while the result of TG in Figure 31 showed the presence of carbonaceous compound, indicating that tetracene and chrysene were evenly coated as carbon materials on nanoballoon. Indeed, BET specific surface area of nanoballoon itself was 213 m2g-1 (Table 7), while either tetracene or chrysene covered on nanoballoon by heat treatment at 973 K showed 61 and 86 m2g-1, respectively. The specific surface area did not decrease significantly even after heat treatment at 973 K. The specific surface area became close to half or one third of the surface area of original nanoballoon, indicating that one or two layers of graphene-like carbon materials were deposited on nanoballoon.

Figure 30. TEM image of SiO2 (nanoballoon), nanoballoon with either tetracene heated at 853 K or chrysene heated at 923 K. (a)　nanoballoon at low magnification. (b) nanoballoon at high magnification. (c) nanoballoon with tetracene heated at 853 K. (d) nanoballoon with chrysene heated at 923 K. 3.6.2 Analysis of oxidation of carbon materials coated on SiO2 by TG Figure 31 shows TG curves of samples heated in oxygen gas. Tetracene carbonized with nanoballoon at 853 K and chrysene carbonized with nanoballoon at 973 K were used as samples and heated in oxygen gas in isothermal TG. Carbonized tetracene increased its weight once and decreased after ca. 80 min., whereas carbonized chrysene showed little increment of weight and decreased. It indicates that oxidized zigzag edges are more stable than oxidized armchair edges, which was also observed as results of ReaxFF (Figures 8 and 9). The slope of decrement of tetracene was steeper than that of chrysene, indicating that the decomposition of oxygen-containing functional groups on carbonized tetracene can be easily proceed compared to that on carbonized chrysene. Thus, zigzag edges can be oxidized and decompose easily compared to armchair edges. Table 7. BET specific surface area of nanoballoon and nanoballoon with either tetracene or chrysene heated at 973 K. Sample name

BET specific surface area/ m2g-1

Nanoballoon (SiO2)

213

Tetracene/Nanoballoon 973 K

61

Chrysene/Nanoballoon 973 K

86

Table 8. Activation energy of oxidation of tetracene and chrysene carbonized on SiO2. Activation energies of gasification by oxidation were calculated from Figure 32. Sample name

Activation energy of gasification by oxidation / kJ mol-1

Tetracene/Nanoballoon 853 K

104

Chrysene/Nanoballoon 933 K

123



Page 28 of 35

(a)

(b) (b)

Figure 31. Isothermal oxidation either tetracene with nanoballoon heated at 853 K or chrysene with nanoballoon heated at 933 K using TG. Samples were oxidized at 588, 593, 603, and 613 K under oxygen. The weight of silica was subtracted. (a) Tetracene. (b) Chrysene. Black line: 588 K. Blue line: 593 K. Orange line: 603 K. Red line: 613 K. Figure 32 shows Arrhenius plots using isothermal heat treatment by TG (Figure 31), which can provide activation energies of gasification upon oxidation of carbonized tetracene on SiO2 at 853 K for 1 h and carbonized chrysene at 933 K for 1 h. The activation energy for gasification for carbonized tetracene was 104 kJ mol-1, whereas that for carbonized chrysene was 123 kJ mol-1 (Table 8). Sendt et al. have calculated that the energy required for desorption of CO from armchair edges is higher than that from zigzag edges.66,67 Our results have a good agreement with the results of Sendt et al. Also, as written in the introduction, the basal plane of single graphite crystal was reacted with oxygen gas at 1085 K and activation energies for each edge such as 259 kJmol-1 for zigzag edges and 276 kJ mol-1 for armchair edges has been obtained by monitoring etching speed of each edge using optical microscope.41 The value of activation energies are different between this work and the reported work probably because the reported work used the high temperature at 1085 K, but the tendency, which zigzag edges were more reactive than armchair edges, was the same. (a)

Figure 32. Arrhenius plot of gasification upon oxidation of carbonized tetracene (a) and chrysene (b) on nanoballoon at a weight loss of 10, 20, and 30 wt.% using isothermal TG. Tetracene heated at 853 K for 1 h and chrysene heated at 933 K for 1 h were used for this experiment. The weight of silica was subtracted.

3.6.3 Analysis of oxidized carbon materials by DRIFT and elemental analysis Figures 33 and 34 show DRIFT spectra of tetracene and chrysene heated at 853 and 933 K and further heated in oxygen gas at 573 K for different heating time, respectively. The distinct difference in oxidation between carbonized tetracene and chrysene was peak intensities at each peak, but the types of functional groups were basically similar. A peak at 3460 cm1 is either OH groups or adsorbed water influenced by hydrogen bonding. After oxidation at 573 K for 20-100h, peaks corresponding to C=O in acid anhydride and/or 5-memberedring lactone68 or aggregated lactone36 at 1840 cm-1, C=O in acid anhydride and 6-membered-ring lactone without hydrogen bonding and C=O in 5-membered-ring lactone with hydrogen bonding at 1740 and 1760 cm-1,68 C=O in cyclic ketone at 1680 cm-1,36,68 C=C in quinone and C-O-C in ether/lactone at 1268 cm-1, out-of-plane sp2C-H bending at 1170-1180 cm-1, and SOLO influenced by oxidation at 910 and 920 cm-1 were observed for both carbonized and oxidized tetracene and chrysene. Influence of introduced functional groups on the peak position for out-of-plane sp2C-H vibration has been


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reported.55 Peaks are known to shift to higher wavenumber in the presence of C=O-containing electron-withdrawing functional groups. Thus, the presence of lactone and quinone found in the sample in this work is considered to be the reason for the peak shift to higher wavenumber.

(c)

The peak at 1739 cm-1 can be assigned as the peak of aldehyde, but no peak was observed at 2700-2800 cm-1.69 Thus, aldehyde does not exist in the oxidized sample. The presence of 5-membered-ring lactone was observed in oxidized chrysene and oxidized graphene nanoribbon as results of ReaxFF (Figure 9a, b) in this work. Thus, 5-membered-ring lactone is one of the most probable functional groups at 1840 cm-1. The assignment of C=C in p-quinone and cyclic ether at ca. 1268 cm1 was obtained as results of our calculated results (not shown). Radovic et al. and Orrego et al. have that reported oxidized zigzag edges such as semiquinone form lactone via migration of epoxy near the edges43 and also reported that pentagon is formed by further heat treatment and gasification of CO2.44 Sendt et al. have reported that oxidation of armchair edges forms o-quinone and further heat treatment possibly forms cyclopentadienone and lactone.67 (a)

Figure 33. DRIFT spectra of carbonized tetracene before and after oxidization at 573 K for 20 and 100 h. Tetracene heated at 853 K and further heat treatment under reduced pressure was used as carbonized tetracene. (a) 700-1000 cm-1 (b) 10002000 cm-1 (c) 2800-3800 cm-1. The intensity of the peak top between 700 and 1000 cm-1 was adjusted at 1 and the same number was multiplied with other regions. As results of Tables 9 and 10, increment of oxygen was much higher than decrement of hydrogen after oxidation for 100 h, indicating that ether, lactone, and/or acid anhydride can be present. The difference in composition between carbonized tetracene and chrysene after oxidation was small, whereas the difference in the ratio of peak intensities between carbonized tetracene and chrysene after oxidation was clear. It indicates that carbon materials with different types of edge structures were prepared.

(b)

Intensities of peaks other than 920 cm-1 decreased, indicating that edges such as QUATRO and TRIO tend to react rather than SOLO or Armchair-like edges with pentagon. This result accords well with the results of ReaxFF obtained from oxidation of tetracene, chrysene, and graphene nanoribbon with zigzag and armchair edges (Figures 8 and 9). Figure 35 shows summary of the analyzed edge structures in this work. Characteristic peaks of D band of Raman spectra were 1437 cm-1 for zigzag edges and 1240 and 1340 cm-1 for armchair edges in this work, while those of G band were overlapping each other for zigzag and armchair edges (Figure 35(1)). Characteristic peaks of IR spectra were 880-867 cm-1 for SOLO, 3015 cm-1 for zigzag edges, 835 cm-1 for DUO and 3085 cm-1 for armchair edges (Figure 35(2)). These peak positions were well correlated with our recently studied peak positions, which were obtained by analyzing various aromatic compounds as reference compounds.37 In addition, carbonized tetracene with zigzag edges and carbonized chrysene with armchair edges showed clear difference in reactivity with oxygen (Figure 35(3)).



(a)

Page 30 of 35

Figure 34. DRIFT spectra of carbonized chrysene before and after oxidization at 573 K for 20 and 100 h. Chrysene heated at 933 K and further heat treatment under reduced pressure was used as carbonized chrysene. (a) 700-1000 cm-1 (b) 1000-2000 cm-1 (c) 2800-3800 cm-1. The intensity of the peak top between 700 and 1000 cm-1 was adjusted at 1 and the same number was multiplied with other regions. Table 9. Molecular formula of tetracene after carbonization at 853 K and oxidation at 573 K for 0-100h. Oxidati on

Molecular formula

Ratio

time/ h

C*

H

O

O/C

O/H

0

18

5.7**

0.5

0.03

0.09

20

18

4.3

3.1

0.18

0.72

100

18

4.0

5.5

0.31

1.38

* The

(b)

number of C was adjusted as 18, which is the number of carbon atoms in as-received tetracene, to clarify the change of H and O after oxidation of carbonized tetracene. ** Same

value as Table 1.

Table 10. Molecular formula of chrysene after carbonization at 933 K and oxidation at 573 K for 0-100h. Oxidati on

Molecular formula

Ratio

time/ h

C*

H

O

O/C

O/H

0

18

6.3**

0.1

0.01

0.02

20

18

4.8

3.4

0.19

0.71

100

18

4.4

6.3

0.35

1.43

* The

number of C was adjusted as 18, which is the number of carbon atoms in as-received chrysene, to clarify the change of H and O after oxidation of carbonized chrysene. ** Same

value as Table 2.

(c)


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Figure 35. Summary of peak positions and activation energies of zigzag and armchair edges analyzed in this work using three analytical methods. Some of structures in this figure are same as those in Figure 4, but the peak positions are different. (1) Determination of edge structures by characteristic peaks of Raman spectra. (1a) Structure of zigzag edges and corresponding peak positions obtained in this work. (1b) Structure of armchair edges and corresponding peak positions obtained in this work. (2) Determination of edge structures by characteristic peaks of IR spectra. (2a) Structure of SOLO/zigzag edges and corresponding peak positions obtained in this work. (2b) Structure of DUO/armchair edges and corresponding peak positions obtained in this work. (3) Determination of edge structures by activation energies required for gasification by oxidation. (3a) Representative estimated structures of zigzag edges before and after oxidation. (3b) Representative estimated structures of armchair edges before and after oxidation. Probable reaction routes were estimated in this work (Figures 8 and 9).

4. Conclusion Carbon materials with zigzag and armchair edges were prepared by simple thermal treatment of tetracene and chrysene. The difference in structures of edges between carbon materials prepared from tetracene and chrysene was determined in detail by three simple methods in detail such as (1) Raman spectra combined with calculation and (2) IR spectra combined with calculation, and (3) oxidation reaction of carbon materials by TG. From these results, it was revealed that armchair edges are easier to retain their edge structures than zigzag edges after carbonization. In addition, the detailed

carbonization mechanisms could be revealed by experimental and calculated Raman and IR spectra combined with ReaxFF. Tetracene was dehydrogenated via formation of sp3C at 693 K and carbonized via elimination of sp3C at 853 K. On the other hand, chrysene was dehydrogenated without formation of sp3C at 853 K and carbonized at 893 K, which is much higher than the carbonization temperature of tetracene. Especially, for tetracene with zigzag edges, this molecule has a linear structure with sp2C-H on zigzag edges as well as sp2C-H at the para positions. The presence of sp2C-H at the para positions significantly changed the reactivity between tetracene and chrysene, as it has been observed for hydrogenation, formation of sp3C-H, on zigzag edges of tetracene. Types of dehydrogenation such as intermolecular and intramolecular dehydrogenation, positions of carbon atom on edges, and steric hindrance influenced structures of edges on carbon materials. At the beginning, intermolecular dehydrogenation mainly proceeded because of the presence of highly reactive sites and steric hinderance. Then, intramolecular dehydrogenation proceeded despite the existence of steric hinderance, generating pentagons and the other non-hexagonal rings. These edge structures could be determined by comparing simulated and experimental Raman and IR spectra. The tendency of oxidation reaction on carbonized tetracene with zigzag edges and carbonized chrysene with armchair edges were clearly different. Zigzag edges tended to be oxidized and increased their weight, whereas armchair edges tended to decompose without increasing their weight. The difference in oxidative gasification on different edge structures was also explained from the difference in activation energy. These results clearly showed that carbon materials with different edge structures could be successfully prepared from two different precursors and demonstrated the analytical methods of carbon materials with different edge structures in detail. This type of detailed research of carbonization of aromatic compounds have not been reported elsewhere. The result of this work helps synthesizing carbon materials with different edge structure and understanding the complicated structure of general carbon materials in the future.

ABBREVIATIONS DRIFT, diffuse reflectance infrared Fourier transform; IR, infrared; TEM, transmission electron microscopy; BET, Brunauer, Emmett and Teller; TG, thermogravimetric analysis; MS, mass spectrometry; Zig, zigzag edge; Arm, armchair edge; Graph-like, Graphene-like; Tetra, tetracene; DeH, dehydrogenated; dimer, dimerized tetracene; +H2, hydrogenated tetracene; Chrys, chrysene; Penta, pentagon; Hexa, hexagon; Hepta, heptagon; Steric, sterically-hindered; QUAT, Quatro; Quad, quadrant; Semi, semicircle; DFT, Density functional theory.

ASSOCIATED CONTENT Supporting Information Comparison of functionals used for simulating Raman spectra using Gaussian 09, calculated structures, calculated Raman, calculated IR spectra, MS spectra, and NMR spectra. These materials are available free of charge via the Internet at http://pubs.acs.org.



Page 32 of 35

(13) Tanabe, T.; Yamada, Y.; Kim, J.; Koinuma, M.; Kubo, S.; Shimano, N.; Sato, S. Knoevenagel Condensation using Nitrogen-doped Carbon Catalysts. Carbon 2016, 109, 208-220.

AUTHOR INFORMATION Corresponding Author

CONFLICT OF INTEREST

(14) Sasaki, T.; Jenkins, R. G.; Eser, S.; Schobert, H. H. Carbonization of Anthracene and Phenanthrene. 2. Spectroscopy and Mechanisms. Energ. Fuel. 1993, 7, 10471053.

Funding Source. This work was supported by JSPS KAKENHI Grant Number JP18K04833.

(15) Tamai, K.; Nakamizo, M. Carbonization Processes of Some Aromatic Hydrocarbons -Infrared Spectroscopic Studies-. Tanso 1984, 1984, 30-34.

ACKNOWLEDGMENTS

(16) Ishii, Y.; Song, H.; Kato, H.; Takatori, M.; Kawasaki, S. Facile Bottom-up Synthesis of Graphene Nanofragments and Nanoribbons by Thermal Polymerization of Pentacenes. Nanoscale 2012, 4, 6553-6561.

Acknowledgments are made to Grandex Co., Ltd. for providing nanoballoon. Dr. Ryohei Kishi in Osaka University in Japan provided precious comments on this research. This work was supported by JSPS KAKENHI Grant Number JP18K04833.

(17) Northrop, B. H.; Norton, J. E.; Houk, K. N. On the Mechanism of Peripentacene Formation from Pentacene: Computational Studies of a Prototype for Graphene Formation from Smaller Acenes. J. Am. Chem. Soc. 2007, 129, 6536-6546.

* E-mail

address: [email protected] (Y. Yamada).

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Carbon materials with zigzag and armchair edges - ACS Applied

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