Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Porous sulfur-doped carbon was synthesised by using a thienyl-based polymer network as a precursor. The sulfur amount varies from 5–23 m% while the materials show microporosity with BET surface areas of up to 711 m2 g−1.DFG, EXC 314, Unifying Concepts in Catalysi

Paraknowitsch, Jens Peter

Schmidt, Johannes

Thomas, Arne

Chemical Communications

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8283–8285 8283Cite this: Chem. Commun., 2011, 47, 8283–8285Microporous sulfur-doped carbon from thienyl-based polymer networkprecursorswJens Peter Paraknowitsch, Arne Thomas and Johannes Schmidt*Received 19th April 2011, Accepted 27th May 2011DOI: 10.1039/c1cc12272jPorous sulfur-doped carbon was synthesised by using a thienyl-based polymer network as a precursor. The sulfur amount variesfrom 5–23 m% while the materials show microporosity withBET surface areas of up to 711 m2 g1.Heteroatom doping of carbon materials is a major tool fortuning their properties, especially with nitrogen as a dopant.1–15The doped materials have altered properties, giving rise to variousapplications, e.g. as electrodes in supercapacitors8,10,12,13,16,17or as electrocatalysts for the oxygen reduction reaction (ORR)in fuel cells.9,11,18–23 Besides nitrogen, boron is another dopantof current interest, e.g. in carbon nanotube or grapheneresearch.24–26 More recently materials scientists have startedto focus on sulfur doping. This is yielding interesting andbeneﬁcial inﬂuences on the properties of the envisaged carbonmaterials: graphite–sulfur-composites that exhibit super-conductive properties have already been presented.27,28 Goingfrom composites to sulfur-doped carbons, the band structuresof the materials are modiﬁed and can be tuned depending onthe amount of sulfur atoms, e.g. in sulfur-doped graphene.29,30These sulfur-doped graphenes can be synthesised by adsorption ofthiol groups at defect sites,29,31 and they are discussed aspotential sensors for polluting gases, such as NOx.32 Furthermore,S-doped carbon nanotubes with altered electronic properties,33 oreven S-doped graphitic carbon nitride materials for photo-catalysis34 are discussed. These versatile works represent thegrowing interest in sulfur-doped carbon materials. Our interestwas to synthesise sulfur-doped carbon based on sulfur richprecursors via thermal treatment in a one-step process. Thisconcept of thermolysis of heteroatom rich precursors for thesynthesis of doped carbonaceous materials has been appliedfrequently, e.g. by using ionic liquids with dicyanamide anionsand nitrogen-rich cations in the synthesis of N-doped carbon.1–4Furthermore microporous covalent triazine networks havebeen shown to form porous nitrogen-rich carbon materialswith a remarkably high active surface area upon annealing.35,36Transferring this concept to sulfur-doping, a microporouspoly(1,3,5-tris(thienyl)benzene) (PTTB) network37 has beenselected as a precursor (compare Fig. 1). Besides its highamount of sulfur atoms due to the thienyl units, the micro-porosity represents a further advantage as it is most likely toinduce intrinsic porosity in the carbonaceous materials. It ismost interesting to see that the crosslinked structure of theprecursor network is the crucial key to the formation of theS-doped carbon material: comparing the thermogravimetricresults for the PTTB-network and its monomer (Fig. 2), themonomeric structure appears to evaporate completely uponthermal treatment in an inert gas atmosphere (boiling point:310 1C),38 while the crosslinked microporous network yieldshigh residual masses of B68% at a temperature as highas 900 1C.We thermally treated the thienyl-based network in an argonatmosphere at temperatures of 600 1C, 700 1C, 800 1C, 900 1CFig. 1 (A) 1,3,5-tris(thienyl)benzene and (B) poly(1,3,5-tris(thienyl)-benzene).Fig. 2 Thermogravimetric analysis for a polymer network precursorand the respective monomer, measured under inert gas.Technical University of Berlin, Institute of Chemistry,Functional Materials, Englische Straße 20, 10587 Berlin, Germany.E-mail: johannes.schmidt@tu-berlin.de; Fax: +49 (0)30 31429271;Tel: +49 (0)30 31425241w Electronic supplementary information (ESI) available: Experimentaldetails and additional ﬁgures. See DOI: 10.1039/c1cc12272jChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONPublished on 14 June 2011. Downloaded by TU Berlin - Universitaetsbibl on 01/04/2016 07:01:19. View Article Online / Journal Homepage / Table of Contents for this issue8284 Chem. Commun., 2011, 47, 8283–8285 This journal is c The Royal Society of Chemistry 2011and 1000 1C. The synthesis procedures yield homogeneouslyblack solids with varying sulfur contents according to combustionanalysis results (see Table 1). The crosslinked structure ofthe precursor inhibits a complete decomposition, while theelimination of sulfur atoms is partially suppressed due to thelack of available protons neighbouring the sulfur atoms, as thethienyl units are known to decompose via the elimination ofH2S.39 Thus the sulfur content of the targeted materials can befacilely tuned by simply adjusting the reaction temperature.It can be expected that the chemical environment of thesulfur and its incorporation into the carbon forming networkis altered the more the reaction temperature is increased.This can be further followed by FT-IR spectroscopy (seeFig. S1, ESIw): upon increasing the temperature of synthesis,carbonisation is proceeding, yielding a lack of IR-activefunctional groups, while the sulfur atoms are directly incorporatedinto the forming carbon backbone. For gaining more insightinto this molecular structure, X-ray photoelectron spectroscopy(XPS) has been performed on samples synthesised at 1000 1C.Detailed scans for the C1s and the S2p orbitals including theirdeconvolutions are shown in Fig. S2 (ESIw) and Fig. 3. TheC1s-scan reveals a dominant contribution of graphite-likecarbon in an aromatic environment (284.5 eV).40–43 Furtherthere are minor contributions to the XPS signal at bindingenergies of 286.0 eV and 288.8 eV, respectively. Those verybroadened contributions can be assigned to sulfur-boundcarbon atoms,44,45 but oxidative contaminations cannot beexcluded, either.43,46,47 The according spectrum of the S-2porbital (see Fig. 3) exhibits a higher complexity, amongother things due to spin–orbit coupling phenomena. A clearassignation is diﬃcult, especially being based on these dataobtained by deconvolution. Nevertheless the values coincidewith data from the literature. There are minor contributions(165.6 eV, 166.7 eV, and 168.8 eV) of oxidised sulfur species,such as sulfonate and sulfate groups. Values of 168.7 eV aree.g. known to refer to sulfur atoms in sulfonated aromaticsystems, which ﬁts the carbon material under discussion.48Nevertheless, sulfur is dominantly found in other valencestates, as the spectrum reveals more pronounced contributionsat binding energy values of 163.9 eV, 164.7 eV and 165.2 eV.They all refer to carbon-bound sulfur atoms: 163.8 eV is abinding energy known from S–S bonds between two aromaticrings, 163.9 eV can be assigned to other S–S bonds neighbouringcarbon atoms, and to CQS double bonds. Sulfur atoms inmonomeric thiophenes exhibit energy values for the 2p orbitalof 164.5 eV, a structural motif that is most likely to be found inour materials, according to the molecular structure of theprecursor.48 Nevertheless this is not the most prominent peakin the spectrum, which shows that structural rearrangementsare taking place upon carbonisation of the network towardsother C/S binding motifs. It can thus be concluded apredominance of sulfur atoms that are directly incorporatedinto the carbon backbone of the material. This is not only inagreement with the XPS spectrum exhibiting only littlecontributions that can be assigned to oxidised sulfur species.Furthermore, the structure of the precursor is most unlikely toinduce the mere formation of surface functionalities.Quantitative evaluations of the XPS measurements revealedan atomic sulfur content of 2.6 mol% which refers toB7 m%of sulfur bound in the material. This value is in agreement withthe data obtained from elemental combustion analysis andonly slightly exceeds these values (compare Table 1).Nitrogen sorption isotherms that have been measured for allsulfur-doped carbonaceous materials synthesised throughoutthis work reveal their microporosity and high active surfaceareas. Fig. 4 represents the respective isotherm for the materialsynthesised at 1000 1C, in direct comparison to the precursornetwork. From the shape of the isotherm and its pronouncedslope at very low values of relative pressure, it is obvious thatthe microporous morphology of the precursor network ismaintained in the sulfur-doped carbon. The uptake of nitrogenat relative pressures close to unity indicates also external surfacearea which results from interspaces between agglomeratedparticles (see SEM micrographs in Fig. S3/S4, ESIw). Identicalobservations can be made for all other synthesis temperaturesthat have been applied, as it can be observed in Fig. S5–S8(ESIw). BET evaluations reveal active surface areas that areslightly reduced with respect to the precursor polymer network(see Table 2).Table 1 Sulfur contents of doped carbon materials synthesised atdiﬀerent temperatures600 1C 700 1C 800 1C 900 1C 1000 1CS/m% 23.2 18.0 13.5 9.3 5.6Fig. 3 Deconvoluted XPS spectrum of the S-2p-orbital, measured onthe sample synthesis at 1000 1C.Fig. 4 Nitrogen sorption isotherms of precursors polymer networkand carbonised sulfur doped material.Published on 14 June 2011. Downloaded by TU Berlin - Universitaetsbibl on 01/04/2016 07:01:19. View Article OnlineThis journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 8283–8285 8285Interestingly, the surface area drops from 1060 m2 g1 to599 m2 g1 when the precursor is thermally treated at 600 1C.When increasing the temperature of synthesis stepwise to1000 1C the surface area is growing again. Similar eﬀectsare observed for the micropore volume at higher reactiontemperatures, reaching a value of 0.24 cm3 g1 at 1000 1C(compare Table 2). It seems that ﬁrst the porous structure ofthe precursor is at least partially collapsing throughout thethermal treatment process. Nevertheless due to the crosslinkedstructure the preconditions for microporosity are maintainedso that upon further elimination of side products, such ascarbon and sulfur residues, additional pores are created andthe surface area increases again.In summary, we used a thienyl-based microporous polymernetwork as a precursor for the synthesis of sulfur dopedcarbon. The precursor provides a high content of sulfurthat is maintained upon thermal treatment in an inert gasatmosphere. Varying the synthesis temperature enables thetuning of the sulfur content between 5 m% and 23 m%. As themicroporosity of the precursor is also kept, the material hasto be considered as a promising support material for theimmobilisation of metal nanoparticles which is a subject ofcurrent studies.Financial support from the Cluster of Excellence ‘‘UnifyingConcepts in Catalysis’’ (supported by the Deutsche Forschungs-gemeinschaft and administered by the TU Berlin) and the DFGproject ‘‘Nanocarbon’’ (10033287) is gratefully acknowledged.We thank Dr Anna Fischer for SEM measurements and DrYuanjian Zhang (NIMS) for XPS measurements.Notes and references1 J. S. Lee, X. Q. Wang, H. M. Luo, G. A. Baker and S. Dai, J. Am.Chem. Soc., 2009, 131, 4596–4597.2 J. S. Lee, X. Q. Wang, H. M. Luo and S. Dai, Adv. Mater., 2010,22, 1004–1007.3 J. P. Paraknowitsch, A. Thomas and M. Antonietti, J. Mater.Chem., 2010, 20, 6746–6758.4 J. P. Paraknowitsch, J. Zhang, D. S. Su, A. Thomas andM. Antonietti, Adv. Mater., 2010, 22, 87–92.5 L. Zhao, N. Baccile, S. Gross, Y. J. Zhang, W. Wei, Y. H. Sun,M. 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Microporous sulfur-doped carbon from thienyl-based polymer network precursors

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Microporous sulfur-doped carbon from thienyl-based polymer network precursors

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