Redox Route from Inorganic Precursor Li2C2 to Nanopatterned

Redox Route from Inorganic Precursor Li2C2 to Nanopatterned Carbon Paul Simon,* Xian-Juan Feng, Matej Bobnar, Peter Höhn, Ulrich Schwarz, Wilder Carrillo-Cabrera, Michael Baitinger, and Yuri Grin Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187 Dresden, Germany S Supporting Information *

ABSTRACT: We present the synthesis route to carbon with hierarchical morphology on the nanoscale. The structures are generated using crystalline orthorhombic lithium carbide (Li2C2) as precursor with nanolamellar organization. Careful treatment by SnI4 oxidizes carbon at the fairly low temperature of 80 °C to the elemental state and keeps intact the initial crystallite shape, the internal lamellar texture of particles, and the lamellae stacking. The reaction product is amorphous but displays in the microstructure parallel band-like arrangements with diameters in the range of 200−500 nm. These bands exhibit internal fine structure made up by thin strips of about 60 nm width running inclined with respect to the long axis of the band. The stripes of neighboring columns sometimes meet and give rise to arrowlike arrangements in the microstructure. This is an alternative preparation method of nanostructured carbon from an inorganic precursor by a chemical redox route without applying physical methods such as ion implantation, printing, or ablation. The polymerization reaction of the triple bond of acetylide anions gives rise to a network of carbon sp2 species with statistically sized and distributed pores with diameters between 2 and 6 Å resembling zeolite structures. The pores show partially paracrystal-like ordering and may indicate the possible formation of carbon species derived from graphitic foams. KEYWORDS: amorphous carbon, nanopatterning, lithium carbide, redox reaction, TEM, pores

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retention of the original shape and volume of the precursor. In the case of chemically stable carbide precursors high temperatures between 200 and 1200 °C and/or reactive compounds such as HCl or chlorine are used and needed in order to remove the metal atoms.22 If the remaining reaction products (e.g., metal chlorides) are trapped in the pores, they can be removed by subsequent treatment such as hydrogenation or vacuum annealing. Mostly, CDCs are formed from carbide precursors by chemical extraction of the metal atoms by halogenation, hydrothermal treatment, or vacuum decomposition. Among these methods chlorination was found to be the most economic and scalable method for CDC synthesis. Additionally, acid etching or reactions with inorganic salts are another possibility to remove the excess material. The products are often porous and show a high bulk porosity larger than 50 vol % with a high specific surface area of more than 2000 m2 g−1, which is characteristic of CDCs obtained via halogenation.24,25 The bulk porosity of CDCs is largely determined by the carbide structure. When metal atoms are extracted from the carbide lattice, the remaining carbon forms microporous CDC

tructures at the nanoscale possess great potential concerning applications in electric or magnetic devices and as well as chemical or optical sensors.1−7 In general, carbon-based compounds display strong mechanical, thermal, and chemical stability.8,9 By introducing nanopatterning, this kind of material should achieve a high specific surface, which may serve as a support for catalyst10,11 or as separation medium.12,13 Mesoporous carbon can be used for energy storage in lithium−sulfur batteries14 or in supercapacitors.15 Carbonaceous materials that are synthesized by carbide precursors are called carbide-derived carbons (CDCs). CDCs have been produced from many binary and ternary carbide precursors such as Al4C3, B4C, BaC2, CaC2, Cr3C2, Fe3C, Mo2C, MoC, Nb2C, NbC, SiC, SrC2, Ta2C, TaC, Ti2AlC, Ti3AlC2, Ti3SiC2, TiC, VC, W2C, WC, and ZrC using a variety of treatment conditions, which lead to a wide range of carbon materials such as graphene,16 nanotubes,17 nanodiamond,18,19 hollow and solid spheres,20 onion-like graphite,21 or amorphous carbon.22 However, there are quite a few studies on similar oxidations with alkaline or earth alkaline metal carbides as precursor such as CaC2 or Li2C2, assumably due to their low stability against moisture in air.20,23 In the course of the CDC synthesis carbon is formed by extraction of the metal or metalloid atoms, transforming the carbide structure into pure carbon. In this way, the carbon layer is formed, usually with the © 2017 American Chemical Society

Received: October 6, 2016 Accepted: February 10, 2017 Published: February 10, 2017 1455

DOI: 10.1021/acsnano.6b06721 ACS Nano 2017, 11, 1455−1465

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Li2C2 using SnI4 in toluene as oxidizing agent. The product turns out to be amorphous, but it preserves the nanopatterned morphology of the precursor.

structures. There exists also a large difference in the resulting ordering and pore size distribution with a narrow distribution of pore sizes, e.g., below 1 nm for β-SiC and a broader range of pore sizes of between 0.5 and 4 nm for Ti3SiC2.26 The important milestones in CDC development were the observation of CNT formation17 and graphene growth27 during thermal decomposition of silicon carbide, as well as the discovery of the tunability of the pore size.28 The ability to control the pore size and pore size distribution by varying chlorination temperature and choice of carbide precursors led to the development of a large family of porous carbons.29−31 The synthesis of patterned carbonaceous compounds is often performed by using a prestructured porous membrane such as mesoporous silica or by direct carbonization of metal−organic frameworks.15,32 Another versatile method is based on the selfassembly of block copolymers where immiscible polymer/ oligomer sequences are chemically fixed. The mutually insoluble components are separated on the meso/nanoscale, leading to a meso/nanostructuring. At high temperature, the block copolymer precursor is carbonized, keeping the precursor prestructuring after heat treatment.8,14,33,34 The electronic and atomic structure of amorphous carbon was the topic of calculations on a number of model structures containing different arrangements of sp2 and sp3 sites. The most stable one of the sp2 sites was predicted as a compact cluster of fused 6fold rings, i.e., graphitic layers of about 15 Å in diameter, bound by sp3 sites. It is argued that amorphous carbon forms such finite clusters in order to relieve strain.35 Beyond pure amorphous carbon, there exist a variety of hydrogenated amorphous carbons, ranging from polymeric to tetrahedral diamond-like depending on hydrogen content.36,37 Our approach for fabricating nanopatterned amorphous carbon is based on lithium carbide (Li2C2) as educt. The synthesis of Li2C2 was described first by Juza et al.38 The crystal structure of the low-temperature modification was determined as orthorhombic (space group Immm), whereas the hightemperature phase was found to be cubic (space group Fm3̅m). In the orthorhombic structure, an ordered alignment of the acetylide dumbbells parallel to the [010] axis is observed, whereby in the cubic high-temperature modification, the carbon dumbbells are rotationally disordered. This causes a strong distortion of the originally cubic lattice in the orthorhombic low-temperature modification. The lattice parameters a = 3.65 Å and b = 4.83 Å of the low-temperature Li2C2 strongly deviate from a/√2 = 4.22 Å of the high-temperature modification. Furthermore, the lattice parameter c = 5.43 Å of lowtemperature Li2C2 is compressed with respect to a = 5.97 Å of the high-temperature phase. During the formation of the low-temperature L2C2, twins and trillings may occur.39 The phase transition also has a drastic effect on the cell volume. In the high-temperature modification, the volume per formula unit Li2C2 is ∼11% larger than in the low-temperature one, which points to a free rotation of the acetylide dumbbells in the hightemperature modification.39 Nesper et al. decomposed Li2C2 thermally. The metathesis reaction with selected transitionmetal halogenides as catalyst precursors in the temperature range of 550−1000 °C leads to a large variety of carbonaceous nanomaterials. Optimizations for carbon nanotube and nanofiber formation have been pursued, with the best yields resulting from the use of lithium carbide in the low-temperature region.23 This phenomenon leads to modified crystal structures and carbon materials. We studied the redox behavior of carbon in

RESULTS The precursor of Li2C2 was synthesized by a solid-state reaction, which is described in the Experimental Methods. For a more detailed description of the synthesis reaction encompassing XRD of the raw product together with TEM characterization of byproducts and further information on the washing procedure, see the Supporting Information. The X-ray powder diffraction of the reaction product (Figure 1, red curve) shows the expected reflections of Li2C2 (Figure 1, black curve, together with some less intense reflections of minority phase(s)).

Figure 1. X-ray powder diffraction patterns of the precursor Li2C2 (red) and corresponding calculated reflection positions and intensities (black). The oxidized product is amorphous (blue).

A scanning-electron microscopic (SEM) study of the polycrystalline lithium carbide shows particles with sizes between 5 and 100 μm (Figure 2a). Inspection of a singular particle reveals a surface patterning in the regions marked with red and blue rectangles in Figure 2b. At larger magnification, fine striation of the particles is observed 200−500 nm in size in the region marked by the red frame (Figure 2c). In the area marked by the blue rectangle even finer layer stacking is evidenced with a size of 100−400 nm (Figure 2d). The stratification pattern characteristic for the twinning in the material is found at every particle of Li2C2 (Figure 2e,f). However, in thin layers the stacking layers are not fixed but slightly shifted with respect to each other as proved by the TEM micrograph in Figure 3a. The stacked fragments sometimes show a substructuring on the scale of about 50 nm (Figure 3b). The high-resolution TEM imaging was proved to be difficult due to the high beam sensitivity of the material. Most probably, this sensitivity appears because the lithium sublimes because of the energy pumped into the system by irradiation with electrons at an acceleration voltage of 300 kV. Nevertheless, by applying low energy by spreading the beam, it was possible to image lattice planes of the Li2C2 crystal (Figure 3c,d). The Fourier-filtered representation (Figure 3e) shows lattice planes corresponding to the interplane distance with d(200) = 2.68 Å, as read out from the fast Fourier transform (FFT, Figure 3f). After treatment with SnI4 the final solid product is amorphous and displays no reflections at all in the X-ray diffraction pattern (Figure 1, blue curve). The 7Li NMR spectrum measured at the magic spinning angle (MAS) reveals 1456


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Figure 2. Morphology of the Li2C2 particles as revealed by SEM. (a) Typical particle size amounts to 50−100 μm. (b) Single-particle image with areas of interest marked by red (see enlarged micrograph in (c)) and blue (see enlarged micrograph in (d)) rectangles, respectively. (e) The parallel layers through the whole particle are strictly following the topology of the crystal. (f) Central part of (e) displaying stratification.

Figure 3. Morphology and substructure of Li2C2 particles as revealed by TEM: (a) Stacking of Li2C2 crystal fragments; the packing is not fixed, the lamellae slide easily over each other, giving rise to overlapped and rotated layers. (b) Fine structures of parallel lamellas (∼50 nm) inside the layers. (c) High-resolution TEM image of Li2C2: crystal grain with region of interest marked by a frame. (d) High-resolution image showing horizontally oriented fine stripes corresponding to the [010] direction and lattice spacing of 2.68 Å. (e) Fourier-filtered image of (d). (f) Fast-Fourier transform of (d).

that only traces of lithium remain in the sample after performing the redox reaction and washing procedure (Figure 4a). The EDXS analysis of the solid product shows carbon (∼94 at. %) and a low amount of oxygen (∼6 at. %, Figure 4b). Thus, the possible chemical reaction by this treatment is

The Raman spectrum (Figure 4d) shows the expected graphitic C−C peak around 1576 cm−1 corresponding to stretching modes of the C−C bond and the disorder peak at 1339 cm−1 assigned to the breathing of the sp2 atoms in the rings. The values given in the literature are around 1560 and 1360 cm−1, respectively, and are assigned to the sp2 sites.40 The intensity ratio between the two peaks is regarded as the measure for graphitic ordering.20,36,40 The intensity ratio of the disordered to the graphitic peak in the carbon product amounts to about 0.69 and thus indicates a hydrogen amount of about 15%. This value is larger than the amount of about 10%, which is derived from the graphitic peak position.40 The calculated sp3/sp2 ratio considers only the presence of sp3 bonds due to hydrogen bonds. The sp3 bonds due to diamond-like order can be measured by the so-called T-peak, which is sensitive only for UV-Raman. Moreover, the spectrum shows more peaks than expected for amorphous carbon, especially in the region from 500 to 1200 cm−1. Even an additional peak at 1428 cm−1 just between the graphitic and the disorder peak appears, which

(Li1 +)2 (C2 2 −) + Sn 4 +(I1 −)4 → 2Li1 +I1 − + Sn 2 +(I1 −)2 + 2C0

The absence of tin and iodine in the EDXS spectrum of the solid product proves that the washing procedure removed the oxidizing agent SnI4 and reaction products completely. The IR spectrum shows sp3 C−H valence stretch modes at 2922 and 2853 cm−1 and CC valence modes at 1573 and 1451 cm−1. In the fingerprint regime, around 1150 cm−1 a broad peak is observed, which is assigned mainly to C−C valence modes and may also indicate the presence of secondary alcohols. This peak overlaps with C−H out-of-plane modes at 990−840 cm−1. Thus, we can conclude that the product consists of mainly sp3and sp2-coordinated carbon containing some carboxyl and hydroxyl groups (Figure 4c). 1457


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Figure 4. (a) NMR 7Li spectrum of the amorphous carbon reveals only trace amounts of Li in the reaction product. For the total spectrum, 8.000 and 128.000 scans for MAS@12 kHz and static spectrum, respectively, were measured. A lithium-containing compound with about 1− 10 at. % Li would give a similar signal-to-noise ratio after 1−100 scans. (b) EDX spectrum of the reaction product yields 94 at. % of carbon and about 6 at. % of oxygen; the copper signal stems from the TEM grid. (c) IR spectrum of the amorphous carbon. The C−C and additionally C−OH vibrations (broad band around 1150 cm−1) are observed besides the C−H valence modes at 2922 and 2853 cm−1. (d) Raman spectrum of amorphous carbon measured using 514 nm laser excitation. The graphitic sp2 C−C stretching and the disordered sp2 C− C breathing peaks are marked by arrows.

may be assigned to CH2 deformational vibrations. The strong peak at 1171 cm−1 is typical for branched alkanes. The sharp peak at 2309 cm−1 is attributed to double-substituted acetylides, which normally appears in the range of 2.325 to 2.290 cm−1. Possible byproducts such as SnO, SnO2, Li2O, and LiOH·H2O appear below 840 cm−1. TEM imaging revealed that the final product was indeed amorphous. However, at a suitable magnification a nanopatterning of the amorphous carbon material was visualized (Figure 5a). At higher magnification, parallel aligned columnlike bands were observed with a size in the range of 200−500 nm (Figure 5b). Inside the bands, tilted nanostripes of about 50−100 nm were resolved. The nanostripes in the vicinal bands are oriented opposite to each other, thus producing a zigzag patterning, which may stem from twinning of the precursor crystal lamellae. Tilting experiments on the thin film show the herringbone structure is even more pronounced at higher tilting angles, giving rise to a zigzag formation normal to the band long axis (Figure 5c). Tilting to the reverse direction reveals a corrugated and bent surface bearing a strong resemblance to a thin-film replica (Figure 5d). The tilting series from 0° to +50° and 0° to −50° are available in the Supporting Information (movies 1 and 2, respectively). At higher tilting angle, the edges of the thin plate become observable as fine dark region marked by arrows; see Figure 5e. The thickness of 15 nm corresponds to the width of this dark stripe. Figure 5f displays the model of micro twinned bands and nanotwinning inside based on Figure 5b.

The size of the bands considerably varies depending on the region of interest. At certain regions, large bands with diameters of even 500−1000 nm are detected (Figure 6a). Interestingly, the fine structure inside the bands can show a continuous increase in stripe width starting from about 50 nm and ending up at about 200 nm (Figure 6b, top). The tilting angle of the stripes within the bands amounts to 37° with respect to the band walls. Hereafter this “tilting” will be called “shadow twinning”, i.e., twinning without crystallographic basis and diffraction proof in this carbonaceous material. The pronounced alternation of white/black stripes in Figure 6c is lacking; nevertheless the edges give rise to a regular periodicity of about 59 nm stemming from the right column as proved by the FFT of the small-angle region corresponding to large-area structures (Figure 6c, inset top right). The left band show a less pronounced periodicity of ∼56 nm. Again the tilting angle of 37° within the column is observed. In Figure 6d besides the 37° tilting also stripes with tilting angles of 90° occur. Thus, the process depends on local concentration of the oxidizing agent and surface topology. The oxidation agent SnI4 may act locally differently depending on crystal lattice defects in the material. Very thin replicas of about 10 nm thick show sometimes a pronounced arrow-tip-like pattern (Figure 7a,b). These tips represent the tips from the zigzag patterns (Figure 7c). Additionally, porous spherical structures with 200 nm diameter appear as shown in Figure 7d. The nanostructuring of the samples may be generated by two martensitic transitions from cubic to an unknown tetragonal structure and from tetragonal to an orthorhombic structure in the precursor Li2C2 during 1458


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Figure 5. (a) TEM image of amorphous carbon plate. (b) Enlarged region of interest marked with a red rectangle in (a) shows bands of 200− 500 nm in size with nanopatterning. The fine lines inside the bands are tilted with respect to the long axis of the bands and give the impression of a helical winding. Image (b) corresponds to 0° tilt (starting angle) for the tilting series. (c) TEM image of the thin plate at a tilting angle of −30° reveals the herringbone arrangement of twins running through the bands. (d) TEM image at a tilting angle of +40° demonstrates the corrugated morphology of the thin film replica. (e) Zoomed area of (d) marked with a red rectangle shows the edge of the sample. The thickness of about 15 nm of the thin carbon film corresponds to the width of the dark stripe (film edge). (f) Model of bands with fine striation pattern within the bands taken from (b).

rotation of the C2 dumbbells stops in one direction, leading to the squeezing of the Li8 cube along one axis toward a tetragonal prism (Figure 8 bottom, middle panel). The regions with different orientation of the tetragonal axis (wide bands) are separated in Figure 8 (top panels) by dashed lines. The Li8 prisms undergo in the next step another deformation along [110] and fix the C2 dumbbells in one orientation. The regions with different orientations of the dumbbell axes are shown in Figure 8 by band-like patterns within the wide bands. After the oxidation, the twinned micro- and nanostructuring of the precursor in the carbon product is still conserved. This is valid at least for very thin (about 10 nm) layered particles of the

cooling the sample. The tetragonal phase was not observed in the Li2C2 system and may exist only in a narrow temperature range, as it occurs in the Na2C2 system.41,42 The scheme of the possible changes in the microstructure of the precursor caused by double twinning during the two possible transitions is shown in Figure 8. Obviously, the submicrometer-sized twinning stems from the Li2C2 precursor phase (Figure 8, top left; see also Figure 2c−f), as evidenced by SEM images, and is kept in the carbonaceous oxidation product (Figure 8, top right). The phase transition from the cubic phase may lead first to the tetragonal structure with the space group I4/mmm and then to the orthorhombic one (space group Immm, Figure 8, bottom).53 During the first transition, the 1459


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Figure 6. (a) TEM image of amorphous nanopatterned carbon plate. The thickness of the plate of about 20 nm was assessed by the edge; see dark area marked by arrows. The thickness is in the range of the thin film replica shown in Figure 5. (b) Large bands of 0.5−1 μm in size. The nanostripes inside have a certain tilt angle of 37° with respect to each other at the neighboring columns. (c) The white/black contrast between vicinal stripes vanished after a long reaction time; however, the edges remain visible, giving rise to a regular periodicity of about 59 nm as revealed by the FFT (inset, top right). (d) 90° (2 × 45°) tilting angle besides “regular” tilting of 74° (2 × 37°) of neighboring Li2C2 grains oriented differently.

precursor produced during grinding or during the oxidation process. The final microstructure of the precursor can also be understood by the static disorder in orientation of the dumbbell axes. During the first transition, the ordering of the axes happens in one direction (perpendicular to the tetragonal axis of the Li8 prisms). Within the second transition, the dumbbells order completely. Within recent publications, predictions for a static “ribboning” in Li2C2 were made evoked by polymeric anionic carbon chains, occurring at elevated pressures.43,44 In their calculations, the authors find different widths of carbon nanoribbons coexisting in the metastable phases Imm2 and Cmcm of Li2C2. The carbon nanoribbons can be formed by one, two, or three zigzag carbon chains and, thus, have different widths. Carbon ribbons should be obtained by compressing Li2C2 to larger than 6.2 GPa, and if chemical disproportionation occurs, the pressure needed for structure transformations of Li2C2 would be even much lower. In addition, polyanionic carbides of Li2C2 are predicted to be either semimetals or metals where the metallic forms were predicted to be superconductors with Tc = 14.2 K.45 The Raman spectrum (Figure 4d) of the carbonaceous product shows the presence of double-substituted acetylide (sharp peak at 2309 cm−1). This may correspond to oligomeric or even polymeric acetylide chains with alternating triple and single bonds along the chain. Thus, it could be assumed that the oxidation of the acetylide dumbbells may lead to polymeric chains and thus to a similar nanopattern as proposed for the Imm2 and Cmcm phases.

The oxidized carbon product is X-ray amorphous (blue curve, Figure 1). Nevertheless, using high-resolution TEM, randomly distributed subnanometer-sized cavities were observed at the edge of the sample and loops, which may indicate initial states for formation of the pores. Within the amorphous carbon, graphitic fragments are found with a next-nearestneighbor distance of 2.5 Å typical for graphite (Figure 9a). The FFT of the high-resolution image of Figure 9a showed no preferred orientation but just an amorphous halo, indicating a disordered structure (Figure 9b). The pore diameters are mainly in the range 2−6 Å, as shown in Figure 9c. At certain regions, however, the pores are fused and showed a rough local order (Figure 9d). At higher magnification, even a paracrystallike ordered pattern of the pores became evident in the Fourierfiltered images (Figure 9e). Larger pores of 5.5−4.5 Å in diameter dominate, giving rise to necklace-like strings besides the presence of smaller pores with sizes between 3.5 and 2.5 Å. The corresponding FFT reveals diffuse “reflections” at 9.4 and 5.4 Å in the small-angle region (Figure 9f). This kind of ordering may correspond to a graphitic foam structure, since the Raman spectrum indicates the highly graphitic nature of the sample. Carbon foams are hypothetical carbon allotropes that contain graphite-like sp2 carbon segments, connected by sp3 carbon atoms, resulting in porous structures. These carbon species or allotropes were proposed already by Kuc and Seifert, however, on the mesoscale.46 They predicted different possible structures where the aromatic phenyl groups serve as aromatic building blocks that 1460


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Figure 7. (a) Thin film (highly tilted) showing tringle-shaped or arrowhead-shaped tips. At the high tilt angle the thin edge of the film becomes observable; see arrow. The thickness of the film amounts to 15−10 nm. (b) Arrowhead-like pattern displayed at higher magnification. (c) Area showing relationship of twinned bands and arrow tips. When the film starts to bend, the zigzag pattern (at the left) turns upright and solely the tips of the zigzag remain visible (at the right). (d) Nanopore pattern is observed in the amorphous carbon films. Diameter of the pores amounts to about 200 nm and may result from an acetylene release.

are interconnected by sp3 bonds. Thus, the foam forms a network where the number of phenyl groups between the connecting points determines the size of the meshes. In Figure 9g a hypothetic 3 × 3 network is shown, whereas Figure 9h represents the smallest possible sized 1 × 1 mesh foam. For further characterization electron energy loss spectroscopy (EELS) will be a powerful method as a complement to the Raman findings to unveil the detailed structures of amorphous and nanocrystalline carbon films.47−52 Raman provides averaged information over a beam spot area of about 20 μm in diameter as used in our experimental setup; however, for nanosized crystal grains, methods with high spatial resolution such as EELS are needed. Generally, the high-loss region of the carbon K edge in the carbon film EELS spectrum is usually used to yield quantitative information on the sp3 and sp2 contents.51,52 For example, EELS proved to be useful in the unambiguous identification of diamond and graphite.48−50,53,54 For diamond in the higher energy loss region, only the K edge peak at around 290 eV due to the excitation of σ* states of sp3 appears, but the peak at 285 eV due to excitation to π* states cannot be observed. Both the 285 and 290 eV peaks will appear in graphite EELS. The latter peak is due to the σ* states of sp2. In amorphous carbon films or in nanocrystalline carbon films, both peaks will appear because the film structures are usually composed of either sp2 or sp3. For the nanocrystal in Figure 9e we assume a graphitic foam structure based on the Raman findings and based on the observed large lattice spacings. The measured lattice values do

not correspond to known existing carbon allotropes; thus we exclude diamond, graphite, or C60 by high-resolution TEM. In order to rule out that the investigated crystal grain (embedded in the amorphous carbon matrix) is, for example, another hypothetic sp3 carbon allotrope such as clathrate II55 (which shows similar d-spacings from the [110] zone), EELS should be performed at the local (nanometer scale) level.

CONCLUSION In summary, we describe the redox preparation of nanopatterned amorphous carbon via a route based on naturally prestructured inorganic precursor−orthorhombic Li2C2, at low temperature of 80 °C. A hierarchical patterning of the carbon product is observed where bands on the submicrometer scale are decorated with internal nanostripes showing a tilted orientation with respect to the band axis. This hierarchical stratification structure stems from the twinned microstructure of the low-temperature modification of Li2C2.39 During the redox process a polymerization of the triple-bonded acetylide anions takes place, yielding the sp2 carbon species and giving rise to pores with diameters between 2 and 6 Å, being similar to zeolite structures. The pores show partially paracrystal-like ordering with distinct reflections in the corresponding Fourier transform and may indicate the possible formation of carbon species derived from graphitic foams. EXPERIMENTAL METHODS Synthesis of Precursor Li2C2. Single-phase Li2C2 was synthesized by solid-state reaction of metallic lithium with graphite: 0.088 g of 1461


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Figure 8. Structural relationship between cubic, hypothetic tetragonal, and orthorhombic Li2C2 phases. (Top) The submicrometer-sized twinning originates from the Li2C2 precursor crystal. SEM image of Li2C2 (left) is compared with TEM image (right), showing typical band pattern of the carbonaceous product. (Bottom) The transition from the cubic to the orthorhombic phase may go through the tetragonal phase with the space group I4/mmm. The changes in a Li8 cube during these transformations are indicated by dashed lines. lithium (99.9%, Alfa Aesar) was cut into small bars (about 0.2 × 0.2 × 1 cm3) after cleaning the surface and mixed with 0.152 g of graphite powder (99.99%, Sigma-Aldrich) in a tantalum tube, which was then arc-sealed in the glovebox. The ampule was heated to 800 °C for 2 h and then cooled to room temperature. A white powder of Li2C2 was obtained by opening the tantalum crucible. The powder X-ray diffraction pattern showed an amorphous product, whereas the lowtemperature-phase Li2C2 revealed the expected reflections (see Figure 1). Synthesis of Nanopatterned Carbon. Nanopatterned carbon was synthesized by oxidation of Li2C2 with SnI4 in toluene at 80 °C under argon. The reaction was controlled using a two-neck boiling flask. For preparation, 0.038 g of Li2C2 powder was put into the bottom of the flask and 0.470 g of SnI4 powder was put into a funnel that was connected to one of the necks. Both necks were closed before transferring from a glovebox into the hood. The flask was connected to a water cooling condenser via the other neck, while the funnel was connected to a vacuum/Ar system. Below the flask, a heating plate and magnetic stirrer were used to control the temperature and homogeneity of the reaction mixture. After several vacuum/Ar filling cycles, 10 and 20 mL of toluene were added into the flask and funnel separately, by opening the condenser and funnel under an argon flow. By stirring, Li2C2 was suspended into toluene. SnI4 was dissolved in

toluene, immediately yielding a clear orange solution. Then the SnI4 solution was added slowly into the Li2C2 suspension, by opening the valve of the neck. The whole flask was heated to 80 °C for 72 h and stirred at the same time. After 72 h, the color of the solution in the reactor did not change anymore, indicating that the reaction stopped. It may be shorter, due to the inaccurate judgment based on the visual color perception. At the end, as the reaction completes, SnI4 decreases and the amount of SnI2 increases. This is why the color of the solution turned from orange to yellow. The originally white color of the solid residue (Li2C2) became black due to the formation of amorphous carbon, which is the final product. The solid product was washed by using toluene and water three times, respectively, and then dried under vacuum for 12 h. SEM and TEM. SEM investigations were performed by means of an ESEM FEI Quanta 200 FEGi (FEI Company, Eindhoven, NL) system operated in high-vacuum mode (2 × 10−4 Pa) at an acceleration voltage of 15 kV. Secondary electron images were recorded. The sample was dispersed in methanol for TEM investigations. Several drops of the solutions were loaded on a carbon-coated copper grid and transferred to the microscope after complete dryness. The Quantifoil S7/2 (100-mesh hexagonal) copper grids were covered with a 2 nm thin carbon film (Quantifoil Micro Tools, Jena, Germany). TEM analyses of the sample were performed using an FEI Tecnai 10 1462


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Å, and the information limit amounted to about 1.2 Å. The microscope was equipped with a wide-angle slow-scan CCD camera (MultiScan, 2k × 2k pixels; Gatan Inc., Pleasanton, CA, USA). The analysis of the TEM images was made by means of the Digital Micrograph software (Gatan, USA). High-resolution TEM imaging of the sample in Figure 9 was performed by a JEM-ARM300F (JEOL, Tokyo) at an acceleration voltage of 80 kV. The spherical aberration of the objective lens is corrected by dodecapole correctors in the image-forming system. The TEM resolution is 0.5−0.7 Å depending on the resolution criterion applied. TEM images were recorded on a 4k × 4k pixel CCD array (Gatan US4000). IR Spectroscopy. Infrared spectra of the powder samples were recorded in attenuated total reflectance (ATR) mode with a PerkinElmer UATR-2 FT-IR spectrometer inside the glovebox. No water in the samples has been detected. Raman Spectroscopy. The measurements were performed with a LabRAM System 010 (Jobin Yvon) in backscattering mode on the powder sample. The setup, equipped with a microscope (objective 100×) and additional filters for low-frequency performance, used the Ar ion laser 514 nm line with 15 mW as excitation source. NMR Spectroscopy. The static and MAS NMR investigations were carried out on a Bruker AVANCE spectrometer with a magnetic field of B0 = 11.74 T, using a standard Bruker MAS probe. The 7Li signals were referenced to a saturated water solution of LiCl, having a Larmor frequency of 194.3657 MHz. The sample was located in a ZrO2 rotor of 4 mm in diameter and in the case of MAS measurements spun at 12 kHz. The signals were obtained by an echo sequence using two equally long pulses of 1.6 μs and a recovery time of 2 s.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06721. Description of the synthesis and characterization of initial reaction products (PDF) Movie 1: Tilt series of thin carbon replica from 0° to +50° (AVI) Movie 2: Tilt series of thin carbon replica from 0° to −50° (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +49-351-4646 4227. Fax: +49-351-4646 4002.

Figure 9. High-resolution images of carbon obtained by oxidation of Li2C2. (a) The carbon product appears as a disordered network of differently sized pores. At the edge of the sample loops of different sizes are observed, indicating the formation of the pores (see arrows). (b) The FFT of image (a) shows no distinct reflections but only an amorphous halo. (c) Pore sizes are in the range 2−6 Å. (d) In regions with high pore density they are fused and assembled (see arrows). (e) Paracrystal-like ordering observed in certain regions (Fourier-filtered image). (d) The FFT of image (e) displays several diffuse diffraction spots with distances of 9.4 and 5.4 Å in the small-angle region. (g) Model of graphitic foam with 3 × 3 phenyl groups as a building block. (h) Model of the smallest possible 1 × 1 variant.46

ORCID

Paul Simon: 0000-0003-1115-4024 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge H. Borrmann and S. Hückmann for support with powder XRD measurements and B. Böhme and I. Baburin for fruitful discussions. We like to thank R. Ramlau and U. Köhler for their help with the TEM experiments. This work is supported by the Deutsche Forschungsgemeinschaft (SPP 1708, Materialsynthese nahe Raumtemperatur).

(FEI Company) with a LaB6 source at an acceleration voltage of 100 kV. Micrographs were recorded with a slow-scan CCD camera (TemCam-F224HD, 2k × 2k pixels; Tietz Video and Image Processing Systems GmbH, Gauting, Germany). Tilting experiments were performed by tilting the sample around the TEM holder axis in 2° steps from −50° to +50°. High-resolution TEM (HRTEM) imaging of the sample was performed by an FEI Tecnai F30 with a field-emission gun at an acceleration voltage of 300 kV. The point resolution amounted to 1.9

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Redox Route from Inorganic Precursor Li2C2 to Nanopatterned

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