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.1-Butyl-3-methyl-pyridinium-dicyanamide (BMP-dca) is carbonised with tetra-alkyl-phosphonium-bromide additives yielding nitrogen- and phosphorus-co-doped carbons with enhanced BET surface areas promoted by the additives.DFG, EXC 314, Unifying Concepts in Catalysi

Paraknowitsch, Jens Peter

Thomas, Arne

Wienert, Björn

Zhang, Yuanjian

Chemical Communications

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1208 Chem. Commun., 2013, 49, 1208--1210 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 1208Nitrogen- and phosphorus-co-doped carbons withtunable enhanced surface areas promoted by thedoping additives†Jens Peter Paraknowitsch,*a Yuanjian Zhang,b Bjo¨rn Wienerta and Arne Thomasa1-Butyl-3-methyl-pyridinium-dicyanamide (BMP-dca) is carbonisedwith tetra-alkyl-phosphonium-bromide additives yielding nitrogen-and phosphorus-co-doped carbons with enhanced BET surface areaspromoted by the additives.Heteroatom doping of carbon materials is an emerging fieldof materials science. Nitrogen is by far the most prominentdoping candidate and numerous examples with versatileapplications, e.g. in electrochemistry, are known.1–22 Althoughother heteroatoms are of thriving interest, synthetic pathwaystowards boron-,23–26 sulphur-27–35 and phosphorus-doped36–38carbons are still rare. As especially bigger atoms like S or P havemost preferable eﬀects on the polarisability of the carbonmaterials and thus on their performance in electrochemicalapplications,31–36 new synthesis routes are inevitable. IL basedsyntheses of carbon materials have been established in mani-fold ways in recent years of research.1–7,21–23,39–48 Especiallyliquid salts with dicyanamide anions are a powerful precursorfor N-doped carbons,1–7 and for advanced co-doping with S.27Herein we present a modified IL route using phosphoniumadditives, paving the way towards P- and N-co-doped carbons –a class of materials only rarely reported in the literature so far.36BMP-dca with 40 mol% of tetrabutyl-phosphonium-bromide(TBuPBr) as a phosphorus source was therefore carbonised at1000 1C in a constant flow of inert gas (see ESI† for details). Theblack solid product is a carbonaceous material exhibiting alocal graphitic order that is limited in its extension. This can bederived from the powder X-ray diﬀraction (PXRD) patternsshowing broadened, but intense, (002) and (100) reflexes typicalfor a stacking motif in turbostratic-like carbons (compareFig. S1, ESI†). The black product was also characterised bymeans of elemental combustion analysis (EA) and inductivelycoupled plasma optical emission spectrometry (ICP-OES) todetermine the degree of heteroatom-doping: 4.1 wt% ofN-doping (EA) and 5.7 wt% of P-doping (ICP-OES) are revealed.The material is thus successfully dually doped; especially theremarkably high degree of P-doping compared to otherapproaches36–38 is noteworthy and points out the advantagesof the method. To also prove the homogeneity of the material,elemental mapping based on energy dispersive X-ray spectro-scopy (EDX) and wavelength dispersive X-ray spectroscopy(WDX) was performed. The data are provided in the ESI†(Fig. S2–S5, ESI†), showing clearly that the dopants are spreadhomogeneously throughout the material. Significant localconcentration maxima cannot be found.The binding environments of both N and P could beelucidated by X-ray photoelectron spectroscopy (XPS), and thedetailed N1s and P2p scans are depicted in Fig. 1. The resultsfor nitrogen are as expected according to our previous stu-dies:1,2,27,46 three deconvolved contributions appear at 398.13eV, 400.88 eV and 402.80 eV that can be assigned to pyridinicand pyrollic nitrogen species, quaternary graphitic nitrogenand minor oxidised nitrogen binding motifs (due to oxidativeprocesses on the surface of the materials), respectively.16,34,49,50The N doping thus occurs with N atoms firmly bound tothe carbon backbone. To clearly assign the P2p XPS scanto certain chemical environments is however more diﬃcult.Fig. 1 Deconvolved XPS scans of the N1s and P2p orbitals of the material.a Technische Universita¨t Berlin, Institute of Chemistry, Division of FunctionalMaterials, Hardenbergstr. 40, 10623 Berlin, Germany.E-mail: jens.p.paraknowitsch@tu-berlin.de; Fax: +49-(0)30-314-29271;Tel: +49-(0)30-314-25241b Southeast University, School of Chemistry and Chemical Engineering,Nanjing 211189, PR China† Electronic supplementary information (ESI) available: Experimental details andPXRD patterns. See DOI: 10.1039/c2cc37398jReceived 10th October 2012,Accepted 19th December 2012DOI: 10.1039/c2cc37398jwww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 19 December 2012. Downloaded by TU Berlin - Universitaetsbibl on 30/03/2016 12:49:26. View Article OnlineView Journal  | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1208--1210 1209Three deconvolved contributions appear at 129.33 eV, 132.66 eVand 137.15 eV. The value of B137 eV has been observed forphosphorus in high oxidation states,51 thus referring to surfaceoxidised P species. The two more dominant contributions arecontroversially discussed,37,51–53 but the most comprehensiveexplanation is the contribution at 132.66 eV attributed to P–C-binding motifs.37,51 In this range of binding energies, P atomsneighbouring N have also been reported,51 which is alsocomprehensive. The binding energy of 129.33 eV derives fromP–P binding sites.51 The assignment of the P2p XPS scan is thusnot totally clear, but considering both the data reported in theliterature and the chemical circumstances of our syntheticapproach, the explanation of P firmly bound to the carbonbackbone is the most comprehensive. The XPS data also revealthe changes in the chemical environments of the dopantsduring the formation of the material. Nitrogen atoms arealready part of aromatic units in the precursor, or are easilyincorporated by triazine formation. Phosphorus atoms do notappear in any aromatic environment in the precursor system.However, P atoms are nevertheless already bound to carbonatoms in the phosphonium salts used, easing their incorpora-tion into the forming carbon backbone.The nitrogen sorption isotherm of the P/N-doped material(see Fig. 2) surprisingly showed that the co-doped carbon doesexhibit an enhanced BET surface area of B283 m2 g1. Aspurely N-doped carbon derived from BMP-dca without additivesis just a bulk material, this alteration must be due to thestructure directing eﬀect of the phosphorus source. We focusedon this phenomenon more closely by investigating the eﬀect ofdiﬀerent additives. Interestingly, tetra-alkyl-phosphonium-halideswith nitrile groups ((4-cyanobenzyl)-triphenyl-phosphonium-chloride [CN-add-1] and cyanomethyl-triphenyl-phosphonium-chloride [CN-add-2]) used as additives (40 mol% in BMP-dca,1000 1C under inert gas flow) did yield P/N-codoped carbons(N: 5.1 wt% (both additives)/P: 3.5 wt% (CN-add-1) and 6.9 wt%(CN-add-2)), but the materials did not exhibit any enhancementof the BET surface area. The nitrile groups surely promote anintensified chemical reactivity by triazine formation betweenthe BMP-dca and the additive at an earlier stage of the reaction.In contrast, diﬀerent nitrile free phosphonium additives showthe structure directing properties, which we investigatedusing TBuPBr, tetra-ethyl-phosphonium-bromide (TEtPBr) andtetra-phenyl-phosphonium-bromide (TPhPBr) as additives atdiﬀerent concentrations. All nitrogen sorption isotherms areshown in Fig. 2, the respective BET surface areas are summedup in Table 1.The values reveal a clear tendency of tetra-alkyl-phosphoniumadditives to promote the enhancement of the BET surface area inIL-derived carbons. Both the properties of the alkyl chains andthe concentration of the additives determine the achievedsurface areas, indicating a templating eﬀect of the phosphoniumsalts. Both external surfaces and mesoporosity contribute to themeasured surface areas, which can be estimated from the shapesof the measured isotherms. Transmission electron micrographs(TEM) support this idea, showing the sponge-like structures ofthe two materials with the highest surface areas, synthesisedusing TBuPBr- and TPhPBr-concentrations of 40 mol% (seeFig. S6, ESI†).Although the intrinsic increase of the surface area is highlyinteresting, from an application directed point of view it shouldbe further enhanced. We were able to achieve this usingLudoxs silica nanoparticles as a hard template during thesynthetic procedure. Removal of the template yields a P/N-co-doped carbon – P: 4.8 wt% (ICP-OES)/N: 2.8 wt% (EA) – withan open mesoporous structure. The morphology can be seen inthe micrographs in Fig. 3 and the nitrogen sorption isotherm isdepicted in Fig. S7 (ESI†), showing a typical hysteresis formesoporous materials with disordered pore structures andexhibiting a BET surface area of 832 m2 g1.It can be concluded that this work presents a novel pathwaytowards multiply-doped carbons containing phosphorus.Especially the intrinsically increased surface areas promotedby the applied phosphonium additives make the materialhighly interesting, while additional mesoporosity can be easilyinduced by hard templating. Furthermore, the possibility offinely modifying cheap commercial carbons with such anadvanced material – which is envisaged – makes the IL-derivedP/N-co-doped carbon a promising candidate for electrocatalyticor other applications.Fig. 2 Nitrogen sorption isotherms of BMP-dca derived carbon using diﬀerent additives at diﬀerent concentrations.Table 1 BET surface areas of BMP-derived P/N-codoped carbons using diﬀerentphosphorus sources at various concentrationsTEtPBr TBuPBr TPhPBr10 mol% 0.2 m2 g1 2 m2 g1 1.5 m2 g120 mol% 17 m2 g1 41 m2 g1 10 m2 g130 mol% 32 m2 g1 137 m2 g1 146 m2 g140 mol% 80 m2 g1 283 m2 g1 253 m2 g1Communication ChemCommPublished on 19 December 2012. Downloaded by TU Berlin - Universitaetsbibl on 30/03/2016 12:49:26. View Article Online1210 Chem. Commun., 2013, 49, 1208--1210 This journal is c The Royal Society of Chemistry 2013Funding was received from DFG within the framework ofUNICAT, which is gratefully acknowledged. 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Nitrogen- and phosphorus-co-doped carbons with tunable enhanced surface areas promoted by the doping additives

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Nitrogen- and phosphorus-co-doped carbons with tunable enhanced surface areas promoted by the doping additives

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