Domino games: Controlling structure and patterns of carbon nanomaterials in 2D & 3D

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Domino games: Controlling structure and patterns of carbon nanomaterials in 2D & 3D Nina Fechler, Markus Antonietti ∗ Max Planck Institute of Coloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, Am Mühlenberg 1, D-14424 Potsdam, Germany Received 28 April 2015; received in revised form 24 July 2015; accepted 27 July 2015

KEYWORDS Carbon nanostructures; Patched carbon; Ionothermal synthesis; Donor acceptor structures; Electrocatalysis

Summary This perspective article reviews and conceptualizes some paths towards structured nanoobjects made of carbon. We focus on possibilities where controlled patterns within 2D carbon planes or in 3D volume objects are important as they are able to enhanced or even introduce new properties. For instance, one can pattern electron poor and electron rich domains of carbon, resulting in donor—acceptor based charge separation and transport. A novel chemistry towards such designer carbons, however, can only be made accessible if control over (high temperature) reactions can be introduced and improved, i.e. product geometry and properties have been determined throughout the condensation reaction as early as possible. The task is to transcribe dynamic but at the same time very ordered monomers and monomer patterns into functional carbons, while keeping the targeted structure as much as possible. Here, precursors with ‘‘encoded information’’ and the utilization of (natural) assembly and bonding schemes such as strong H-bridges, polymeric character, Coulomb forces or metal coordination are promising tools and presented. © 2015 Elsevier Ltd. All rights reserved.

Carbon is the element of choice in nature as it is provides the chemical properties to create complexity in the broadest possible way, from solid state structures like diamond to carbon-based metabolic chains in living organisms. But why is that? This question has been discussed intensively, with many good answers. For us as materials chemists, one

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answer is the trilogy of properties which does not only define versatility, but is also relevant for the understanding of the resulting complex functionality of carbon nanostructures:

(1) Carbon’s electronic configuration and hybridization states allow for at least three stable bonding schemes (i.e. sp3, sp2 and sp). Furthermore, these are easily convertible into each other via temperature and pressure or ordinary chemical reactions. This is the inherent property/prerequisite for dynamic optimization processes which finally enable the formation of

http://dx.doi.org/10.1016/j.nantod.2015.07.003 1748-0132/© 2015 Elsevier Ltd. All rights reserved.

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functional carbonaceous material structures. Additionally, the hybridization also defines geometry and the flexibility of shapes, and this makes carbon the element of ‘‘high chemical resolution’’, i.e. where the finest structural details down to the Angstrom level can be designed into. (2) The carbon atom has the ability to adopt with easily all oxidation states ranging from a formal (−IV) (e.g. in methane) to (+IV) (in CO2 ). It will be shown that the electron density of carbon within a structure defines the HOMO levels of the resulting patterns and thereby their oxidation stability (‘‘nobility’’). This ‘‘synthesis design rule’’ is much underestimated, however, there are many consequences. For instance, the convenient synthesis of pure carbons can only be achieved by mixtures which exhibit an average oxidation state (±0). (3) The carbon atom possesses a ‘‘chemical multiculturality’’, as it is able to form covalent bonds with several elements ranging from H, O, N, B, etc. to metals, here as carbides. This allows not only for a multiplicity of precursor structures for carbon nanostructure synthesis, but also convenient functionalization and doping by heteroatoms. We call these points the ‘‘three design principles’’ for carbon nanofabrication and will explain them in detail along the given examples. Commonly, carbon structures can be obtained by several methods including classical carbonization, i.e. the heating of organic matter to high temperatures, but also by hydrothermal treatment of carbon precursors such as sugars. Here, the carbon formation is mainly driven by the choice of starting materials with good leaving groups such as water, ammonia, CO or H2 S, giving a gain of free energy as well as a kinetic population of the available reaction paths [1,2]. This is in many cases referring to the second design principle, as for instance sugars (which can be effectively carbonized even in a household oven or by sulfuric acid in the test tube with high yields) have an average carbon oxidation number of zero. Additionally, they can form simple and stable leaving groups, and thereby do not rely on more complicated, consecutive conversions of the carbon oxidation state. Analyzing these previous model cases and spending some more thoughts on structural aspects, we can also see that carbon allows nano-(shaping, carving, casting, templating) with the highest possible precision of all ordinary material choices. Other materials have — by their local bonding schemes — a structural resolution which is defined by the elementary unit (i.e. the tetrahedrons or octahedrons of metal oxides) and the binding angle between these units. This minimal volume resolution, the ‘‘chemical voxel’’, can be rather big for polymers, but even for oxides and nitrides, and the voxel problem restricts specific surface areas, curvatures, or precision of ‘‘molecular’’ imprinting. As in carbon materials the carbon atom is the smallest possible entity while binding angles can be rather flexible — within but also between the different states of hybridization — the ‘‘resolution’’ is very high. This enables for the formation of a variety of shapes in a flexible manner, below the 1 nm scale. That is essentially a reiteration of the first design principle. Traditionally, high surface areas and fine structural details in carbon materials are addressed by thermal cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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processes and serendipity, called thermal carbonization and activation. Already these notations indicate the low level of control by means of chemistry. Addition of appropriate catalysts leads in many cases to rather organized species. Here, graphene, nanotubes, and fullerenes are relatively easy to make, but they represent homogeneous, averaged structures, that is practically all carbon atoms are similar, which is not the topic of the present perspective article. On the contrary, we want to report on rational approaches to control the carbon structure by organic and physical chemistry. This enables for the encoding of as much information of the final carbon material as possible already in the precursor structures and for placing different carbon atoms beside each other, i.e. for the control of structure and chemical functionality with the smallest voxel size possible. This is of course tightly related to the final point of introduction, the wish to alter the electronic landscape of carbons, which provides a platform for the construction of very diverse functional materials properties. Carbon materials can be isolators, semi-conductors or conductors even though constituted of the same atoms, as seen by comparing the well-known diamond, graphite and amorphous carbon. Apart from the obvious differences of diamond and graphite, also fullerenes and carbon nanotubes became famous examples, and nitrogen-doped carbons just revolutionized the field of electrocatalysis. Initial progress to control and improve electronic properties was based on physical methods (re-condensation, exfoliation, etc.), however, during the last years also chemistry takes part and contributes new ideas and methods. We can only point the interested reader to the available excellent reviews on those issues [3,4]. In this article, we rather want to illustrate one of the next steps where controlled patterns within 2D carbon planes or in 3D volume objects are important as they are able to enhance or even introduce new properties. This follows essentially the example of (1D) conducting polymers where the introduction of sequenced donor—acceptor structures within the chain really made a game-change in terms of stability, charge separation, and the coupled photovoltaic performance. Of course, such a patterning is also possible in 2D and 3D structures, and this will significantly extend the possibilities. Following the second design rule, we want to pattern electron poor (formally high oxidation states of carbon, ‘‘black’’) and electron rich (formally low oxidation states of carbon, ‘‘white’’) domains of carbon. For illustration, these electron densities are depicted here as black and white sections. As 2D-patterns are not unknown to human technology, we picked some illustrations from everyday life, here strict mosaics in 2D (Scheme 1) where the single tiles represent the already discussed structural voxel. A novel chemistry towards such designer carbons, however, can only be made accessible if control over (high temperature) reactions can be introduced and improved, i.e. determination of product geometry and properties in the condensation reaction as early as possible. This can be synthetically promoted by choosing ‘‘easy carbonization reactions’’, such as cyclization, Diels-Alder reactions or clearly defined elimination schemes. However, in addition and architecture-wise, the building blocks to be linked to the final carbon have, as with the tiling of a floor, to Fechler,

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Scheme 1 Conceptional visualization of mosaic-like patterns in 2D-carbons: (a) local spots of deviating electron density, (b) waves and mesophases. Black and white indicates carbon-structures of different properties, e.g. electron density or varying HOMO levels.

be placed at the correct place before the structure solidifies. Here, we enter the field of supramolecular chemistry, the reversible and self-optimizing organization of molecules into superstructures governed by the chemistry of intermolecular forces which was pioneered by Lehn [5,6]. These concepts launched inspiration already for quite some time and offers possibilities towards the dream of ‘‘designer materials’’ [7]. The term ‘‘supramolecular synthon’’ has been introduced by Desiraju in 1995 [8] which was a revolution in the organic crystal chemistry/crystal engineering as it was redefining the definition of crystal nature. From these times on, crystals were no longer considered as static solids but rather as supramolecular dynamic tectonic objects. The task is now to transcribe dynamic but at the same time very ordered monomers and monomer patterns into functional carbons, while keeping the targeted structure as much as possible. Here, precursors with ‘‘encoded information’’ and the utilization of (natural) assembly and bonding schemes such as strong H-bridges, polymeric character, Coulomb forces or metal coordination are considered as promising tools [9]. Especially hydrogen bonding is a well elaborated case, as it is strong but at the same time reversible and generates inherently directional structures. Such precursors or ‘‘supramolecular synthons’’ only have to follow a few rules defining geometry and interactions, and the simpler these rules are, the higher the chances for success to end up with the correct structure (a usually overseen manifestation of the 2nd law of thermodynamics). As a conceptional inspiration, Scheme 2 gives an overview of some fictive structure motives. ‘‘Poker chips’’ represent discotic structures which tend to undergo columnar or nematic liquid crystalline order, where the mutual alignment of the chips within the planes can be adjusted by reciprocal supramolecular recognition motifs. ‘‘Domino games’’ are definitely one order of complexity higher, as mutual alignment is directed by shape and encoded information of each building block. This adds sequence, direction, potential defects, but also mesoscale effects to the self-organization scheme. This review will first collect some representative works from the literature which aim towards the synthesis of cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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‘‘mosaic carbons’’ and their improved performance compared to traditional systems. The following sections will be organized by the choice of precursor, i.e. ‘‘(1) Functional molecules and additives’’, ‘‘(2) Polymer and larger starting molecules’’, ‘‘(3) Dynamic processes and in situ nanocoating’’ and ‘‘(4) Liquid crystalline precursors’’, ‘‘(5) Coordination Complexes’’ and ‘‘(6) D—A structures and charge transfer in carbon systems: the role of patches’’. At the end we give a short perspective on forthcoming challenges but also possibilities which are in reach of the diverse working groups of this field.

Functional molecules and additives The preparation of carbons may sound easy as it seems to be a kind of ‘‘burning’’ action. However, the indeed tedious quest for suitable precursors and methods becomes clear rather fast. This is, besides others, not at least due to the still relatively unpredictable processes occurring at higher temperatures as well as the lack of char-forming materials: most molecules simply ‘‘vanish’’ in form of volatile fragments of high stability. And, as simple as it sounds: a high carbonization yield is simply the first precondition for the preservation of any structure, as this has to be composed of as much original matter as possible. We therefore do not want to dream about structural preservation without sufficient carbonization yields. Recently, ionic liquids (IL) have been discovered as highly suitable carbon precursors [10,11]. One important character is their fluid nature which facilitates entirely new possibilities in terms of processing such as hard templating [12]. Furthermore they possess low vapor pressure and high thermal stability, and they allow for the in-situ introduction of high amounts of heteroatoms which is one of the to-date best known methods to alter properties of otherwise pure carbon materials [13]. Especially nitrogen is highly attractive as doping atom as it has different properties compared to carbon while still fitting into a carbon lattice. Thus, not only the chemical and thermal stability is changed, but it also alters the electronic landscape of carbons, e.g. a change in HOMO and LUMO positions, eventually leading to Fechler,

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Scheme 2 Pattern concepts with self-organization (a) in-plane, self-aligned discotic frameworks including porosity, (b) stacking effects of planar patterns to create donor—acceptor effects in the third dimension, say for charge separation, conductance, or even chirality. Black and white again indicates carbon-structures of different properties, e.g. electron density or varying HOMO levels. (c) An illustration of more complex patterns with ‘‘encoded’’ building blocks.

increased (electro)chemical activity [14]. In order to gain control over this powerful phenomenon the search for the exact chemical structure of the active site or active motives has started [15]. One way to enter such catalytic motifs, beyond the single nitrogen atom, is the dissolution of thermally rather stable, functional molecules which were believed to keep certain atom neighborhood relations even after thermal cocondensation with the ionic liquids. A promising example was the choice of nucleic acids (Fig. 1) [16]. Compared to the pure ionic liquid-derived carbon, the addition of minute amounts of nucleic acid led to their successful incorporation into the carbon framework after high temperature treatment and changed the properties of the material drastically, more than only by the amount of added nitrogen atoms. Such a tiling is of course still disordered with respect to the motifs, but a first step towards presumable control nitrogen—nitrogen neighbor relations. To access such patches for electrochemistry, high surface area carbons are required. As classical ‘‘activation’’ is an etching technique which essentially destroys all previous trials to organize carbonaceous matter, salt templating or salt flux synthesis is a powerful choice [17]. Here, ordinary porogen salts serve as inert space holder and high temperature solvent during the cross-linking of the precursor mixtures, leading to more dynamic reactions and controlled phase separation by the liquid nature of the medium. It was shown that very high surface areas of up to 2000 m2 g−1 and adjustable pores in the micro to meso region could be made where the properties can be tuned via the amount and nature of

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the template salt used (Fig. 2a). Resulting carbon aerogellike materials were then shown to possess superior activity in the oxygen reduction reaction, even when compared to ordinary N-doped carbons (Fig. 2b—e). This is suspected to result from a synergistic combination of pore structure, pronounced exposure of correct patterns of heteroatoms and a good electronic connectivity [18]. The high costs of commercial IL were recently overcome by the successful utilization of ‘‘bio-imidazolium zwitterions’’, imidazolium compound which are derived from natural amino acids and dioxo-derivatives [19]. Also these precursors can be converted into carbons with outstandingly high surface areas of 2658 m2 g−1 , containing nitrogen from rationally designed and substituted imidazoles. Therefore, this platform offers a convenient and sustainable approach towards noble, electrochemically active carbons from biomass with custom-made functionality. The idea to keep as much of the original chemical patterns and environments of well-chosen precursor molecules as possible in the target carbon material can indeed be illustrated by the utilization of such (and similar) chiral ionic liquids, derived from inherently chiral natural amino acids. By following the salt templated synthesis of porous carbons from ionic liquids as described above together with carefully chosen temperature programs and the additional stabilization of the structure in the salt melt, it was indeed possible to keep at least parts of the chiral character of the precursors in the final carbon. Here, the chiral ionic liquids (CIL) were synthesized from natural amino acids (CILs) [20]. A series of CILs was prepared, and the chiral

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Fig. 1 (a) Precursors and heating procedure for N-doped carbon materials. The materials have been synthesized in nitrogen and ambient pressure atmosphere with a heating rate of 10 K min−1 . (b) Polarization curves on a glassy carbon rotating disk electrode for N-doped carbons, as compared with 20 wt% Pt/C in O2 -saturated 0.1 M KOH at a scan rate of 10 mV s−1 and rotation rate of 1600 rpm. (c) Voltamperograms for oxygen reduction on a meso-EmG electrode in O2 -saturated 0.1 M KOH at various rotation speeds; scan rates, 10 mV s−1 . (d) Koutecky—Levich plot of meso-EmU at −0.35 V. Parameters used for the calculation are as follows: concentration of O2 , 1.2 × 10−3 mol L−1 ; diffusion coefficient of O2 , 1.9 × 10−5 cm2 s−1 ; kinematic viscosity of the electrolyte solution, 0.01 cm2 s−1 . (e) Current—time (i—t) response of meso-EmG and 20 wt% Pt/carbon at −0.26 V in 0.1 M KOH saturated with N2 (0—1000s) or O2 (1000—2000s) and in O2 -saturated 3 M CH3 OH (2000—3000s) [16]. Printed with permission from ACS.

character also of the final carbon was studied representatively for L- and D- chiral ionic liquids of phenylalanine as an example of the general process. The preservation of chirality in the carbons was proven by circular dichroism (CD) spectroscopy by quantifying the selective chiral adsorption of D- and L-phenylalanine enantiomers onto the mesoporous carbon (Fig. 3a and b). It was found that the carbon synthesized from D-enantiomer favored the adsorption of the

L-phenylalanine while the opposite case was observed for the carbon derived from the L-enantiomer (Fig. 3c and d). Here, the rate of charge transfer is higher when electrode and electrolyte possess the same handedness. The electrically conductive nature of the carbons further allowed for the chronoamperometric evaluation of diverse chiral marker molecules. Using the chiral carbon and a chiral electrolyte (D- or L-tartaric acid) in water indeed

Fig. 2 (a) Nitrogen sorption isotherms of Bmp-dca-derived carbons templated with LiZ, SZ and PZ at equal mass ratios [17]. (b—e) Electrocatalytic activity of NC—NZ catalysts: (b) LSV polarization curves in O2 saturated 0.05 M H2 SO4 ; (c) LSV polarization curves in O2 saturated 0.1 M KOH. LSV curves were recorded at scan rate 10 mV s−1 and at rotation speed 1600 rpm; (d) and (e) Hydrogen peroxide yield plots obtained in O2 saturated 0.05 M H2 SO4 and 0.1 M KOH, respectively. Insets—–electron transferred number of NCNZ-13 at different potentials determined from the corresponding RRDE data. For all the RRDE measurements, the catalyst loading is around 0.21 mg cm−2 (including Pt/C catalyst) [18]. Printed with permission from Wiley (a) and RSC (b—e).

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Fig. 3 CD spectra and adsorption ratio for both types of chiral carbon: (a) D-CIL-C with absorbance of L- and D-phenylalanine solution (L-Phe in blue, D-Phe in red, dashed line is the mother solution signal at 5 mM), (b) bar chart of the adsorption ratio for both CIL-Cs. D-CIL carbons favor the absorbance of L-Phe, while the opposite picture is observed for the L-CIL carbon (c and d) Chronoamperometry measurements of the D- (c) and L-chiral (d) mesoporous carbons in D- and L-tartaric acid. (red line indicated the use of D-tartaric acid as electrolyte, as blue for L-tartaric acid) [20]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

revealed significant kinetic differences of the electrochemical double layer processes. Here, D-carbon ‘‘recognized’’ D-electrolytes faster than the L-electrolyte, and vice versa.

Polymers and larger starting molecules Another way to avoid volatility and to end up with higher carbon yields and thereby potentially also higher structural preservation of precursor structures is the employment of stable larger molecules and polymers. This is why nowadays more and more also organic and polymer chemistry contribute to the field of carbon materials. Recently, Sakamoto and co-workers presented the synthesis of a periodic two-dimensional polymer network by organic synthesis reminiscent to graphene [21]. Here, the well-chosen photoreactive monomers first form crystals of layered structure which can then be photo-polymerized (Fig. 4). Afterwards, free-standing micron-sized layers formed by 2D-polymers can be exfoliated using a solvent. The intermediate step of crystal formation effectively forces the precursors into a planar setting, eventually cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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reducing molecular motion and contributing to the high ordering of the final structure. Another way to guide the reaction towards a wanted direction is the utilization of precursors with inherent structural connectivity information such as hyperbranched polyphenylene [22,23]. After impregnation into a commercial aluminum oxide membrane with nano channels followed by thermal annealing, this molecule was shown to form flexible 1D nanocarbons which possessed good properties for electrode applications. Another type of ordered carbons are graphene nanoribbons (GNR) which are essentially 2D bands of graphene. Besides others, due to the high aspect ratio, materials properties strongly depend on molecular orientation and ribbon dimensions. Recently, ribbons with thirteen repetition units and atomically precise graphitization could be synthesized from covalently pre-organized molecules on a gold surface (Fig. 5) [24]. Also along this direction but with the possibility to further proceed towards functional carbon nano membranes (CNMs), Turchanin and co-workers recently employed targeted condensation reactions of various crosslinked polyaromatic molecules [25]. Using this technique, CNMs with a thickness between 0.5 and 3 nm could be synthesized where Fechler,

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Fig. 4 Two-dimensional pre-organization in the monomer crystal. (a—c) Chemical structure of the monomer, 1, (a) and optical microscopy images of its single crystals obtained as plates (b) and rods (c) from a mixed organic solvent comprising TCE and THF. (d) Lamellar crystal structure obtained by XRD with all anthracenes and alkynes displayed as space filling and all other parts as stick models. The monomers oriented up and down are shown in red and grey, respectively. (e and f) Laterally hexagonally packed monomers in each layer (e) result in the C9,10 anthracene positions of one monomer being opposed to the alkynes of the adjacent ˚ , respectively (f). Note that incorporated solvent molecules are not displayed. Scale bars: monomers at distances of 4.4 and 3.6 A 100 mm [21]. Printed with Permission from Nature Publishing Group.

the thickness, porosity and surface functionality could be adjusted by the choice of precursor molecules. The role and importance of structural perfection in materials can be and is indeed discussed in a controversial fashion. For example, catalytic processes often even only take place on defect sites so that highly homogeneous, structurally and chemically ‘‘flat’’ materials only show modest activities. Furthermore, especially when it comes to largescale applications, the introduction of a defined structure becomes more difficult, time consuming and costly if surface templates are used. Here, in the best case one seeks for template-free methods, however, this is a rather challenging task as this requires the self-promotion of structural features ‘‘in distinct directions’’, i.e. one structure motif has to be highly favored by the system in order to obtain homogeneous properties. One example in the literature is given with the dynamic imine exchange reaction which ends up in structured polyazomethines [26]. By varying the ratio of reactants, here 1,4-terephthalaldehyde (TPA), aminopyridine (AP) and 1,4-phenylenediamine (PPDA), the morphology could be tuned from spheres over elliptic to disc-like structures with up to 5 ␮m in diameter (Fig. 6). Furthermore, also the dynamic reaction character could be used for control. For proof of principle, in the constitutional dynamic cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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chemistry controlled reaction induced crystallization, PPDA was exchanged by 3,5-diamino-1,2,4-triazole, and indeed, a distinct morphology of spike-decorated spheres could be obtained (Fig. 6a). All the resulting polymers exhibited high thermal stability and could therefore easily be transformed into the respective carbons in high yield and under preservation of the microstructure (Fig. 6b and c). On another length scale, the work of Matyjaszewski et al. offers extension to use polymer chemistry and softtemplating towards nitrogen-doped carbons employing the atom transfer radical polymerization (ATRP) [27]. The strategy utilized in this work is based on the idea to integrate the structure constituting sacrificial agent already into the carbon-precursor polymer chain so that secondary selfassembly schemes can be circumvented. For that, hairy nanoparticles as all-organic porogenic precursors were synthesized from poly(methyl methacrylate) (PMMA) cores as sacrificial template from which surface polyacrylonitrile (PAN) shells were grafted (Fig. 7). A pre-oxidation step in air guaranteed chemical stabilization by intra- and intermolecular cross-linking reactions of PAN for an improved structure preservation during carbonization. After heat-treatment in inert atmosphere, indeed mesoporous carbons with defined spherical mesopores were Fechler,

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Fig. 5 Synthesis of 13-AGNRs. (a) Schematic representation of the synthesis of 13-AGNRs from molecular building block 1. The precursor molecules 1 colligate to form polymers (poly-1) following the homolytic cleavage of the labile CBr bonds at 200 ◦ C on Au(1 1 1). At 400 ◦ C a cyclization/dehydrogenation sequence converts the polymers to 13-AGNRs. (b) STM image of the polymer (poly-1) formed after the deposition of 1 onto a Au(1 1 1) surface held at 200 ◦ C (Vs = 0.50 V, It = 3 pA). (c) High resolution STM image ˚ (Vs = 0.30 V, It = 33 pA). (d) STM image of 13of the polymer poly-1. The polymers are nonplanar with an apparent height of 3.5 A AGNRs formed after annealing poly-1 at 400 ◦ C (Vs = 0.50 V, It = 12 pA). (e) Close-up STM image of a 13-AGNR (Vs = 0.70 V, It = 7.02 nA; a higher tunneling current was used here to obtain higher spatial resolution). A structural model of a 13-AGNR has been overlaid onto the STM image. Poly-1 tends to align with the Au(1 1 1) herringbone reconstruction, while 13-AGNRs do not exhibit a preferred orientation [24]. Printed with Permission from ACS.

obtained. The final materials possessed micro- as well as mesopores and a nitrogen content of 20.7 and 12.2 wt% after carbonization at 500 and 800 ◦ C, respectively. Due to the special porous nature and the structural nitrogen, these carbons revealed good performances as electrodes for supercapacitors (261 F g−1 ) and for CO2 adsorption (2.4 and 3.2 mmol g−1 ).

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Dynamic processes and in situ nanocoating Wang and co-workers presented a nice example where the combination of an easily carbonizeable carbohydrate and the structured crystal of the functional comonomer underwent a controlled self-transformation process towards a structured nitrogen-doped carbon (Fig. 8) [28]. Fechler,

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Fig. 6 (a) Representative SEM images and corresponding schematic figures of TPA-PPDA and TPA-xAP PPDA (x 1/4 1, 2, 3, 4) (from left to right). (b and c) Polyazomethines and carbon with controlled surface roughness, (b) before, and (c) after carbonization at 900 ◦ C for 4 h [26]. Printed with permission from Elsevier.

Here, micelles (used as soft templates) were coassembled on the surface of melamine sulfate crystals, which during hydrothermal carbonization (HTC) of fructose (as a precursor with very high carbonization yield, following the second design rule), eventually guided the carbon morphology towards a very complex, flower-like superstructure with highly organized surfactant based mesopores. These experiments demonstrate, besides beauty, the superiority of

self-controlled materials via non-covalent interactions also for organic materials. Using also a dynamic self-organization process to create an in-situ template, which is then turned into a co-monomer for pathing, Li et el. presented a simple one-pot approach towards n-doped, strictly 2D, patched graphenes from glucose and cyanamide [29]. Here, glucose serves as an effective carbon precursor (following design rule 2), while

Fig. 7 (a) Templated synthesis of nitrogen-enriched nanoporous carbons using all-organic hairy nanoparticles as building blocks. (b) TEM image of NNC-800 [27]. Printed with permission from Wiley.

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Fig. 8 (a) Mechanism illustration of the carbonaceous products with flower-like structure and ordered mesopores. (b and c) SEM images of sample OMCC-3-1 prepared at a D-fructose/melamine molar ratio of 3 and a sulfuric acid/melamine molar ratio of 1. (d) The photo of the flower named hydrangea. (e) The magnified SEM image of the flower-like carbons [28]. Printed with permission from ACS.

cyanamide turns in-situ into layered carbon nitride which served as sacrificial lamination agent to finally lead to thin carbon sheets (Fig. 9a). This doped, geometrically patched graphene is constituted of irregular nano-crystalline domains of 2—15 nm which brings along very special properties (Fig. 9b). As shown earlier with graphene patches made from electron irradiation and thermal annealing of aromatic selfassembled monolayers (SAMs), the patching occurs along with an insulator-to metal transition, thus revealing a method to render the electronic properties via temperature and sheet stacking thickness [30]. This is further supported by state-of-the-art density functional calculations with allcarbon-hybrids revealing a significant charge transfer from graphene to carbon nitride [31]. Another, potentially related example for well-defined dynamically evolving, self-organized carbon structures, is found in the salt flux synthesis of functional graphenelike materials [32]. Here, the heteroatom-doping as well as creation of patches and patterns originate, again by a selfprocess, from the addition of functional salts, e.g. sulfur is taken from sulfite anions (Fig. 10). Here, a ‘‘black powder’’ chemistry was utilized in order to perform controlled

oxidation reactions of the as-formed carbon materials, enabling the isolation of only the most stable structures while amorphous side products were etched away. The diverse functional salt mixtures further allowed for the positioning of nitrogen, sulfur and phosphorous atoms in unusual stacking motifs within the very high surface area carbon materials [33]. These carbons revealed high electrocatalytical activities, excellent energy storage capacities as well as good hydrophobic absorption properties. Strange enough and apparently in contrast to their superhigh specific surface area, the higher magnification structures rather look like perfect oligostacks or graphenes. However, as found by ACHRTEM and EELS, all characteristics can be, ascribed to a special morphology of highly porous, woven ‘‘ramen noodle’’ sheets, where the heteroatoms seem to systematically line the primary carbon nanoribbons [34].

Liquid crystalline precursors A most nearby way to pre-organize and assemble the precursor units is by using liquid crystallinity. Maybe slightly faded

Fig. 9 (a) Proposed synthetic protocol for free-standing N-doped, patched graphenes. Bottom: Repetition motifs of an ideal g-C3 N4 plane (middle) and of graphene (right); Carbon black or gray, Nitrogen blue. (b) HRTEM image of a similar area showing the patchted, quilt-like structure of the as prepared sheets. Black arrow indicates single layer patch with a grain boundary built from pentagon—heptagon pairs; this is similar to what is found in CVD graphene samples. The irradiation of the electron beam may generate new holes (marked within white arrow) in the plane. Note that the whole process is scalable, as indicated by the macroscopic ‘‘carbon cupcake’’ made as such [26]. Printed with permission from Wiley. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 10 Schematic illustration for the formation of (a) N- and (b) S-doped porous carbon sheets through the reaction of glucose and nitrate (NO3− )or sulfate (SO4 2− ) anions in eutectic molten LiCl/KCl. Graphene synthesized from glucose in MS at 800 ◦ C (glucose/salt = 1:100) as observed by (c) SEM, (d) TEM (the inset is the electron diffraction pattern), and (e) HRTEM. (f) Typical AFM image and the corresponding height measurements [34]. Printed with permission from Wiley.

into obscurity, but one of the first high quality carbon fiber was spun from so-called ‘‘mesophase pitch’’, a tar product constituted from larger aromatic discotic entities which selfassembled into liquid crystalline nematic phases at elevated temperatures [35]. In terms of the discussed patterning of electronic properties within a material, a nematic order mainly influences the alignment between the single disks or layers, that is electronic properties in the third dimension. Recently, Müllen et al. successful combined theory and actual synthesis on the way to come up with fundamental design prerequisites for efficient charge-carrier motilities in discotic large graphene subunits [36]. Based on Marcus theory which can be used to describe charge carrier transport in discotic liquid crystals, they calculated the transfer integral for some symmetric polyaromatic hydrocarbon cores in dependence on the rotation angle (Fig. 11). These calculations reveal that the most favorable relative rotation angle of the molecules is either 0◦ (co-facial) or 60◦ (except triphenylene). Additionally, charge transfers increase with larger conjugated cores and smaller intermolecular distances. The molecule shown in Fig. 11b was chosen for practical studies and indeed showed experimentally a high electron mobility of 0.2 cm2 V−1 s−1 in the stacking direction. However, structural imperfections are still present, as theoretical calculations suggest much higher values. Because all these calculation are made with rather homogeneous in-plane structures, we of course expect patterned structures, using donor—acceptor effects between the planes, to contribute to both better stacking and values of the transfer integral.

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When talking about discotic molecules, the possibility for chirality naturally comes into mind (see also Scheme 2). This is one of the most important and powerful structural motifs and is ubiquitous in nature. Especially for carbon materials which are usually produced at higher temperatures introduction or preservation of chiral structures is rather difficult. However, recently some successful cases have been reported, with the case of the condensation of chiral ionic liquids already being discussed above. Coming from the more sustainable side and talking about larger objects, natural polysaccharides (and their nanocrystals) such as cellulose and chitin are commonly known to possess high degree of chirality while being broadly available. Here, White et al. presented a very sustainable and elegant approach using prawn shells as precursor for highly textured carbon materials with high surface areas and pore volume [37]. These prawn shells are composed of marine chitin, a natural nitrogen-containing polysaccharide also containing inorganic components such as CaCO3. Via the process of hydrothermal carbonization (HTC) the organic part can be efficiently converted into nitrogen-containing carbonaceous materials while the inorganic part serves as in-situ structure directing agent. The latter can then be removed by simple acid treatment. The special textural properties of the initial precursor material can be indeed found also in the final carbon material as clearly visible from TEM and SEM images (Fig. 12). It has been described that the ordering of chitin is of a chiral nematic nature. Apparently, this is structurally preserved also in the carbon species which however remained to be investigated.

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Fig. 11 (a) Absolute value of the transfer integral J as a function of the azimuthal rotation angle for several symmetric polyaromatic hydrocarbon cores. The separation was fixed to 0.36 nm. The insets illustrate face-to-face and staggered stacking of two typical disc-shaped molecules: triphenylene (staggered twisting angle is 60◦ ) and HBC (staggered twisting angle is 30◦ ). Note that even though the maxima of the overlap integral decrease with the increase of the core size, the overall hopping rate ω increases owing to the simultaneous decrease of the reorganization energy . (b) Structures of the compounds studied [36]. Printed with permission from Nature Publishing Group.

Following the same idea but taking an artificial route, McLachlan and coworkers succeeded in the synthesis of chiral carbon materials from the lyotropic liquid crystalline phases of nanocrystalline cellulose [38]. The trick to avoid destruction of the chiral organization throughout the high temperature conversion process was the structural fixation of the nematic phase formed by cellulose crystals in

water via petrification with a silica matrix for embedding (Fig. 13). Indeed, already optical microscopy revealed the undisturbed formation of a chiral nematic phase in the cellulose—silica composite film which after drying appeared as colorful free-standing flakes (Fig. 13d). After carbonization at 900 ◦ C in inert atmosphere followed by removal of

Fig. 12 SEM (a and b) and TEM (c and d) images of prawn shell-derived carbon materials at 750 ◦ C after acid processing [34]. Printed with permission from RSC.

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Fig. 13 Synthesis of chiral nematic mesoporous carbon. (a) NCC prepared by hydrolysis with sulfuric acid is mixed with TMOS and slowly evaporated to form chiral nematic NCC—–silica composite films. (b) NCC—–silica composite films are pyrolyzed in an inert atmosphere at 900 ◦ C to generate carbon—silica composite films. (c) Silica is removed from the carbon—silica composite films using 2m NaOH to generate chiral nematic mesoporous carbon. (d) Polarized optical microscopy of the composite films shows strong birefringence and a texture consistent with a poly-domain chiral nematic phase (scale bar = 200 ␮m), (e) photograph of the carbon sample before and (f) after silica removal, scale bar 2 cm and (g) TEM image, scale bar 200 nm [38]. Printed with permission from Wiley.

the silica phase, silver shiny carbon flakes were obtained (Fig. 13f). Also on the nanoscale, SEM and TEM revealed the preservation of alignment and the inverted replication of this carbon with silica indeed results in highly chiral silica confirming the successful synthesis of chiral carbons. Also along the scheme of liquid crystalline phases, however, now only constituted of organic compounds, we recently reported the synthesis of highly nitrogen-containing carbon materials starting from structured deep eutectic mixtures [39]. It was shown that first different phenols/ketones and urea form eutectic mixtures with inherent ordering in form of liquid crystals as can be seen, besides others, from optical microscopy images (Fig. 14). When these liquids were heat-treated in inert atmosphere, carbon materials with unusually high nitrogen contents of up to 35 wt% were obtained (Fig. 14c). Due to the liquid nature, the precursors could be shaped via hardtemplating or the application of salt melt synthesis. These materials revealed an inherent lamellar structuring which is attributed to the special nature of the ordered ketone—urea mixture which leads rather to a condensation process than common carbonization (Fig. 14d and e). This potentially offers more control of the final carbon material as the initial cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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precursors can be ‘‘encoded’’ through the choice of ketone. Additionally, all the data of the carbon material derived from this process hint to a carbon species which can be vied as a disordered version of ‘‘C2N’’ where the nitrogen is predominantly incorporated in pyrazinic position. As there exists an entire world of phenol/ketone molecules not at least also directly found in nature which can then be combined with a set of urea-derivatives, the opened up library of functional materials nicely fits with the new ‘‘nanoarchitectonics’’ concept which is the general theme of this issue, here especially directed towards rational design of carbon frameworks by a click-like fashion where the final properties can already be determined through the choice of precursor.

Coordination complexes Macrocycles and chelating ligands Another way to lower volatility of a carbon precursor while bringing in local organization which might partly be preserved in the final carbon is by metal coordination and/or salt formation. This is a very classical approach, maybe the Fechler,

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Fig. 14 Photographs of the synthesis process: (a) powder of the cyclohexanehexone:urea mixture at room temperature, (b) Optical microscopy of the liquid crystalline eutectic mixture in the supercooled liquidus, partial polarization mode (inset: macroscopic view), (c) cohesive, silvery reflecting monolithic carbon foam after condensation at 500 ◦ C. (d) Transmission electron microscopy of morphosynthesis experiments with supramolecular carbonaceous materials based on cyclohexanehexone/urea to illustrate the structuring of the preorganized monomer mixtures: templated with ZnCl2 salt melt for highly-accessible micropores. (d) Fourierfiltered image [39]. Printed with permission from Wiley.

oldest known to us to result also in functionally patterned carbons, and for a long time, it is known that carbonization of porphyrines or phtalocyanines gives n-doped carbons with largely increase electrocatalytic activity, e.g. in the oxygen reduction reaction (ORR), one of the main reactions occurring in fuel cells [40]. Here, the oxygen bond was believed to be weakened by the metal center. However, these compounds were not stable under fuel cell conditions and it took few more decades until this concept became applicable. As a recent example, Dodelet et al. prepared ORR catalysts using porous carbons in combination with condensing phenanthrolines, pre-coordinated with the iron species (Fig. 15) [41,42]. As shown in Fig. 15b, this non-precious metal carbon system possessed a very favorable performance in the oxygen reduction reaction, the more complicated reaction taking place in fuel cells. The superior performance of this catalyst system was attributed to iron species which are coordinated to pyridinic sites on the carbon. However, the activity decreased with too thick electrodes and stability of the catalyst decreased during cycling, which means that the whole carbon system structure was those days still not optimized cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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with respect to reactant transport and electronic conductivity. Many similar attempts have been made towards porous, metal-containing carbon catalysts, and in spite of partly high success in the reactions under consideration, the rather uncontrolled ‘‘heat-and-beat’’-approaches during high-temperature treatment result in carbon structures and compositions with only limited control. For the field of carbon supported nanocatalysts, there are also other, very comprehensive reviews available which we point onto [43]. An alternative approach which certainly suits our discussion of controlled deposition of functional patterns within a carbon nanostructure utilizes a cobalt porphyrine-based conjugated mesoporous polymer (Fig. 16a and b) [44]. Due to the inherent properties of this system, i.e. the well-defined positioning of a metal, pores, the presence of pyrrolic nitrogen and a high carbon content, the template-free temperature treatment resulted in the controlled integration of metal nano particles into a functional porous nitrogendoped carbon (Fig. 16c and d). This material showed also a high activity for the oxygen reduction reaction with high stability. Fechler,

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Fig. 15 (a) Schematic representation of catalytic site formation in the micropores of the carbon material (a) Simplified 3D view of a slit pore between two adjacent graphitic crystallites in the carbon support. (b) Plan view of an empty slit pore between two crystallites. (c) Plan view of a slit pore filled with pore filler and iron precursor after planetary ball-milling. (d) Plan view of the presumed catalytic site (incomplete) and graphitic sheet growth (shaded aromatic cycles) between two crystallites after pyrolysis. (b) Comparison of the best NPMC in this work with a Pt-based catalyst. Polarization curves from H2 —O2 fuel cell testing (PO2 = PH2 = 1.5 bar, 80 ◦ C, 100% RH) are shown for cathodes made with the best NPMC of this work, one with a loading of 1 mg cm−2 (circles) and another with a loading of 5.3 mg cm−2 (stars). Also shown is a ready-to-use Gore PRIMEA 5510 membrane electrode assembly (MEA; W. L. Gore & Associates) with ∼0.4 mg Pt cm−2 at cathode and anode (black line). The actual Fe content in our catalyst is 1.7 wt%, resulting in a Fe loading of 17 mg cm−2 for a catalyst loading of 1 mg cm−2 . Open circuit voltages are 1.03, 1.03, and 1.01 V for the MEAs using 17 and 90 mg Fe cm−2 and 400 mg Pt cm−2 at the cathode, respectively [41]. Printed with permission from Science Publishing Group.

The above examples just indicate the potential of pre-organized systems using non-covalent metal—ligand interactions. With this, the design repertoire offers many possibilities. A very versatile extension of common macrocycles has been shown by Mirkin et al. which leads to ‘‘designer macrocycles’’ with switchable coordination [45]. This so-called weak-link approach (WLA) to supramolecular functional structures is based on the utilization of wellchosen multiple metals and ligands. Key of this strategy is the rational placement of strong and weak ligand—metal coordinations. The weaker metal—ligand bonds can be further coordinated to additional/stronger linkers without destroying the initial supramolecular structure thus creating flexible and switchable macrocyclic structures. In light of the work on porous carbons and composites from

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macrocycles, this method could provide further design elements also for high performance electrode applications. Speaking about ordering, one also easily ends up with salts as all-ionic species that can build highly ordered structures in form of crystals. If the crystals additionally contain organic compounds, there exists the possibility to transform the ordered nature of a crystal also to a carbon material built from the organic part. One example is given by the utilization of task-specific semi-organic crystals which upon a one-step thermal decomposition can be converted into hierarchically structured heteroatom containing carbons [46]. Here, e.g. sodium monochloroaetate or sodium-2-chrloropyridine-3-carboxylate were used as precursors. The in-situ precipitation of sodium crystals guides the decomposition of the organic part towards the formation

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Fig. 16 (a) Schematic representation of the chemical synthesis of metalloporphyrin-based conjugated mesoporous polymer frameworks (M = Co, H2 ). (b) Nitrogen adsorption and desorption isotherm plot. (c) Barrett—Joyner—Halenda (BJH) pore size distribution of the CoP—CMP framework, indicating the unusually high control of pore size via the choice of precursor [44]. Printed with permission from Wiley.

of monolithic carbons with cube-shaped porosity which can be isolated easily by simple washing with water afterwards (Fig. 17). This shows that the NaCl crystals act as hard endotemplates as otherwise the intermediate formation of a liquid salt melt would create percolation networks instead of defined edge-structures. In general the formed structures are very light-weight monolith which is generated by the in situ release of CO2 acting as foaming agent. This nicely demonstrates the translation of defined structures not only to metal containing systems but also for all-organic functional carbons.

Metal organic frameworks Metal organic framework (MOF) are probably the most famous coordination-based materials. Via the directed and reversible linkage between metal clusters and organic bridging ligands, highly porous frameworks with super-high surface areas are obtainable. Through the choice of metal, cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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ligand and reaction conditions, properties such as pore sizes and functionality are precisely adjustable [47]. MOFs offered fundamental insights into artificial coordination systems ranging from synthesis strategies to applications, and we point to excellent other reviews [48,49]. Initiation and development of MOFs created a highly diverse platform of different material/linkage classes and the creation of synthesis approaches. Xu et al. recently presented an electroactive covalent metal—organic framework (CMOF) based on electroactive metal—dithiolene complexes (Fig. 18) [50]. Due to the platinum—dithiolene-based structure motif, this anionic framework further revealed semiconductivity and high robustness. Already the similarity of those structures with the other discussed cases of this perspective indicates that we expect high carbonization yields and a high structural preservation/definition also for nanocarbons made from such systems. Standard MOFs based on carboxylates have—–in the context of carbon conversion — conceptually the weak link that all organic acids decarboxylate, i.e. metal center and Fechler,

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Fig. 17 SEM images of thermally decomposed (a) NaAc—Cl, and (b) NaPyr—Cl. The inset in (a) shows a macroscopic foam synthesized from 20 g NaAc—Cl [46]. Printed with permission from RCS.

carbonizeable framework get separated rather early in the process. The recent (re-)focus on phenolic systems as scaffolding entities is therefore rather exciting [51]. Phenols are in addition redox active (2. design rule), naturally abundant (tannins in red wine or tee) and thus potentially suitable for large scale production. Due to the inherent flexibility, processing steps can be additionally applied. Using the coordination between tannic acid and different metals, Caruso et al. recently presented the successful synthesis of metal—phenolic films and capsules [52,53]. Here, the films are formed simply by mixing the two components in the presence of a substrate. It was found that the mode of metal—ligand linkage (in solution as well as in the film) is influenced by the metal concentrations and the preparation series (one-step vs. multi-step). If a particulate template is introduced, these films can be coated to form metal—phenolic capsules. As the natural polyphenol tannic acid is capable to coordinate with a variety of metals, functionality and film/capsule properties are easily tunable even introducing catalytic activities e.g. for the hydrogenation of quinoline. Along the same idea of metal—phenol coordination systems, Dai et al. succeeded to prepare soluble porous polymers using a solvent-based and mechanochemistry approach

[54]. Inspired by the principles for polymers of intrinsic microporosity (PIMs) where rigid and void-forming units are used, a contorted tetraphenol ligand was employed in combination with zinc or iron ions (Fig. 19). Within a solution-based method, aggregated nanoparticles were formed where the void space causes porosity and a surface area of 190 m2 g−1 (Zn-species) to 300 m2 g−1 (Fe-species). For the mechanochemistry approach, the ligand and respective metal source were mechanically milled together. Here, the presence of a stoichiometric amount of base was needed in order to obtain sufficient interaction. Due to the 1D polymer structure, it was then possible to dissolve the formed polymers in acetic acid which after film casting and solvent evaporation resulted in a transparent film under reconstruction of the polymeric backbone (Fig. 19b). These films, however, could not be dissolved without structural decomposition, thus, a hybrid system was synthesized by the introduction of tetrafluoroterephthalonitrile (TFTPN). This indeed enhanced the film solubility making them processable and further increased the surface area with increasing amount of TFTPN up to 428 m2 g−1 (Fig. 19c). Furthermore it could be demonstrated that well-defined mesoporous polymers could be also derived from cheap and ubiquitous polyphenol biomass, here mimosa tannin. Due to

Fig. 18 A schematic drawing of the honeycomb net of HTT-Pt (panel a), a single net from a crystal structure model based on standard bonding geometries (b), and two neighboring sheets showing the staggered alignment thereof (c). The Na+ ions are not included in the model [50]. Printed with permission from RSC.

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Fig. 19 (a) A suggested reaction route for SPCP Zn; (b) a film cast on a glass dish by evaporating acetic acid from the SPCP Zn solution. (c) N2 sorption isotherms of hybrid SPCPs and ‘‘x/y’’ corresponding to the molar ratio of TFTPN and Zn(OAc) 2. For clarity, the isotherm curve is offset by 100 cm3 g−1 for (x:y = 3:1) [54]. Printed with permission from Wiley.

the diverse chelating sites and rigid backbone this molecule can easily coordinate with zinc ions. It is expected, that these polymers are highly suitable precursors for the transformation into the respective carbons.

Carbon nitride So called graphitic carbon nitride is an organic semiconductor and has be investigated extensively during the last years revealing, besides others, high (photo)catalytic activity [55]. Concerning our carbon design rules, it could be understood as a material where every carbon atom has the formal oxidation state (+4), i.e. it is ideal for very noble patches attracting all electrons, and when stacked in the third dimension with (also graphitic) carbon layers, very strong donor—acceptor effects, Mott Schottky heterojunctions [56] and electron transfer between the layers has to be expected. Very interestingly, in the context of the present article, the previous thermal synthesis of carbon nitride was extended to supramolecular starting complexes of e.g. melamine and cyanuric acid with unique superstructures which are then heat-treated. It became possible to govern shape and introduce processability via self-organization and the employment of well-chosen hydrogen-bonding motifs [57,58] (Fig. 20). The pre-orientation and fixation of the molecules structurally directs the condensation process, thus leading to more ordered structures in the final product.

D—A structures and charge transfer in carbon systems: the role of patches Switching gears towards patches and superstructures, attractive candidates are the combination of semiconducting (noble) carbon nitride and semimetallic conductive carbons such as graphene. In this regard, Qiao et al. recently presented the combination of two layered carbons, i.e. carbon nitride as semiconductor and nitrogen-doped graphene as conductor [59]. Here, the carbon nitride was directly grown on graphene oxide films which eventually reduced the latter cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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to nitrogen-doped graphene. The Moiré pattern of the final composite reveal the stacked structure of carbon nitride on nitrogen-doped graphene film (Fig. 21). Electron energy loss spectroscopy (EELS) shows the homogeneous presence of sp2 carbon while defect sp3 carbons are additionally found in the regions of carbon nitride coverage. This finding supports the formation of covalent bonds between the two sheet materials. Experiments and DFT calculations revealed that the intrinsic chemical and electronic coupling of this system favors the adsorption of protons as well as reduction kinetics which finally gives an explanation for the very high activity of the composite for the hydrogen evolution reaction (HER) even outperforming current metal based systems. In another but similar example, Qiao et al. used graphitic carbon nitride nanosheets in combination with carbon nanotubes. These composites were shown to be highly efficient as Oxygen evolution reaction catalysts [60]. Taken together, these examples not only present real metal-free systems with sufficient performance but now indeed combine all three design rules introduced initially finally giving us new tools for advanced carbon design. Such approaches are nicely backed by state-of-theart density functional calculations with all-carbon-hybrids revealing a significant charge transfer from graphene to carbon nitride [31]. In general, carbon allotropes such as graphene and carbon nanotubes (CNTs) represent at least useful model systems as they possess defined structure—property-relations. However, major drawbacks are their low solubility or dispersability in common solvents and their inert surfaces. The latter can be easily understood for graphene as it is only a planar homogeneous, i.e. ‘‘flat’’, framework where the reactivity differs only at the edges. For CNTs reactivity correlates to the tube diameter, thus the degree of bending should result in different reactivities as it alters the electron density of sp2-carbons. The influence of curvature was indeed shown by Guldi and co-workers who chemically attached extended tetrathiafulvalene (exTTF) to different nano carbon modifications introducing an electro active moiety as well as strain. The more functionalized material possessed a photoexcitation induced electron transfer from Fechler,

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Fig. 20 (a) Hydrogen-bonded supramolecular C—M—Mp complex and the proposed carbon nitride structure after calcination. (b) SEM images of the C1—M0.5—Mp0.5 and C1—M0−Mp1 complexes. (c) XRD patterns. (d) FT-IR spectra of the C1−M0−Mp1 complex and raw materials [58]. Printed with permission from ACS.

TTF to SWCNT which revealed good electronic communication within this system essentially based on the alteration of the sp2 carbon pool, i.e. the ‘‘patching’’ successfully modified the whole system [61]. With respect to ‘‘patching’’, they also synthesized an electron-donor—acceptor nanohybrid via non-covalent interactions [62]. Here, p-type SWCNT were combined with the n-type molecule 11,11,12,12tetracyano-9,10-anthraquinodimethane (TCAQ). These very stable n-/p-type hybrid aqueous dispersions reveal efficient charge separation properties. Another example for electronic patching is given by Yuan and co-workes who applied an ‘‘all-carbon’’ approach to increase the dispersability of carbon allotropes [63]. In this work, nitrogen-doped carbon hollow spheres, named carbon nanobubbels (CNBs), were used in order to disperse carbon nanotubes via pi—pi interaction between the relatively electron-poor CNBs and the electron-rich cite this article in press as: Please http://dx.doi.org/10.1016/j.nantod.2015.07.003

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CNTs. The CNBs were derived from the carbonization of polyionic-liquids (PILs) in the presence of spherical silica templates. These CNBs were shown to possess a superior dispersability in neutral water. This was attributed to spontaneous charge transfer and hydroxylate transfer. As can be seen from TEM images, the CNBs specifically bind to CNTs resembling a typical surfactant-introduced stabilization phenomenon eventually leading to an aqueous dispersion as such (Fig. 22). The potential of this distinctive interaction could be furthermore demonstrated by the fabrication of a conductive membrane from an ordinary cellulose filter paper and the CNBs. Here, the filterpaper which is normally an insulator was infiltrated with the CNB solution which causes the decoration of cellulose with the CNBs after drying. These membranes were then shown to be thereby converted into an electron conductor as they could act to close the circuit Fechler,

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Fig. 21 Electron microscopy characterization of C3 N4 @NG nanosheets. (a) Aberration-corrected and monochromated highresolution transmission electron microscopy image of freshly prepared C3 N4 @NG hybrid. (b) High-resolution transmission electron microscopy image of the same area as (a) after removal of g-C3 N4 layer by electron beam irradiation (under a prolonged exposure of B20 s). Scale bar, 2 nm (a and b) [49]. Printed with permission from Nature Publishing Group.

of an LED lamp (figure). With this, it is not only possible to retain the original flexibility of the membrane but also easy patterning via utilization of masks or printing can be imagined. This approach therefore stimulates many more

potential processing ways and applications including the design of p—n junctions, again all made possible simply by the spontaneous charge transfer from electron-rich to -poor carbon materials.

Fig. 22 TEM images of the aqueous—dispersion samples (a) CNB-90 and (b) CNB-90 (0.1 mg mL−1 ) with SWCNTs (0.03 mg mL−1 ). Photographs of (c) pristine cellulose filter membrane, (d) conductive membrane formed by depositing the CNT/CNB network onto the filter paper and (e) filter paper with a conductive pattern. [63]. Printed with permission from Wiley.

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Summary and future challenges Exemplified with a variety of cases from the literature including heteroatom-doped carbons, coordination materials and polymers, strategies towards more controlled ‘‘designer’’ carbons were systematized. Three principles for carbon design have been introduced in the beginning which are fundamental to consider when a special effect is to be planned. In general, carbon chemistry is more than ‘‘burning your cake’’ and should be more operated as a controlled condensation with a significant preservation of positions and functionalities, instead of carbonization. Here, special choices of carbon precursors allows for plenty of options, as for instance bonding schemes to be reflected in the carbon can be at least partly pre-adjusted by dynamic supramolecular self-assembly schemes. The new generation of functional carbons can be envisioned to be created from ‘‘encoded’’ tectons in a domino-like fashion, with functionality and local bonding patterns being positioned by self-recognition principles, for instance at the surface of a pore with controlled geometry. The translation of knowledge gained from electronic 1D structures, i.e. donor—acceptor-polymers, also to 2D and 3D carbon objects will reveal new options of nano-design, including chirality, controlled molecular heterojunctions or ‘‘diades’’, special stacking motives to direct charge flow, but of course also band position and band gap engineering. In order to progress along that way, precursor systems with defined shape (1. Principle), oxidation state and composition (2. Principle) and ‘‘interaction tags’’ (3. Principle) have to be selected while synthesis condensation conditions have to become milder. With this, we invite you to play some carbon domino.

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from ionic liquids for energy-related applications and contributed to the establishment of salts as versatile porogens. Afterwards, she continued as a postdoctoral fellow at the same institute (2013) where she worked on supercapacitors and the extension of the salt approach also to other material classes. In this period, she also stayed at the University of California Santa Barbara in the group of Brad Chmelka for solid state NMR investigations of carbon materials. Since 2014, she is research coordinator and group leader at the MPI of Colloids and Interfaces. Her research interest focuses on the sustainable synthesis and processing of carbon nanomaterials for energy storage and conversion using supramolecular self-assembly approaches. Besides science, she has passion for photography, motorbikes and sports. Markus Antonietti is director of the Max Planck Institute of Colloids and Interfaces. Starting from polymer science, he now drives modern materials chemistry, where sustainable processes and materials are a central theme. Carbon Materials indeed exert a special fascination to him. He published around 650 Papers and issued 90 patents, his H-index is 124. Besides being a devoted scientist and a higher academic teacher, he also is a passionate chef specialized in fusion cuisine and plays in a Rock ‘n’ Roll band.

Nina Fechler studied nanostructure and molecular sciences at the University of Kassel and the Fraunhofer Institute for Applied Polymer Research in Potsdam/Golm (2005—2010) where she was working on thermoresponsive ¸ois Lutz. polymers with Dr. Habil. Jean-Franc She obtained her Ph.D. degree from the Max Planck Institute of Colloids and Interfaces in Potsdam/Golm (2010—2013) with Professor Markus Antonietti. During this time, she focused on porous carbon-based materials

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