Direct Synthesis of TiN/Mesoporous Carbon Nanocomposite by Nitridation of a Hybrid Inorganic/Organic Mesostructured Material

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Direct Synthesis of TiN/Mesoporous Carbon Nanocomposite by Nitridation ofa Hybrid Inorganic/Organic Mesostructured MaterialSebastien Schlienger, Ovidiu Ersen, Lucian Roiban, and Julien Parmentier,Institut de Science des Materiaux de Mulhouse, LRC CNRS 7228, Universite de Haute-Alsace, 15 rue Jean Starcky,BP 2488- 68057, Mulhouse cedex, FranceInstitut de Physique et de Chimie des Materiaux de Strasbourg, UMR 7504, Universite de Strasbourg,23 rue du Loess, 67037 Strasbourg cedex, FranceA titanium nitride (TiN)/mesoporous carbon nanocompositewith a high surface area (320 m2/g) has been successfullysynthesized by a direct route based on a concomitant polymeri-zation of organic and inorganic species (phenolformaldehyderesin and TiO2-based oligomers, respectively). These polymersself-assemble in the presence of a pore-structuring agent, anamphiphilic copolymer ((poly(ethylene oxide))106(poly(propyl-ene oxide))70(poly(ethylene oxide))106) during the solventevaporation step and yield to an ordered mesostructured hybridorganic/inorganic materials. Its heat treatment in ammonia ledto the carbonization of the organic part and to the nitridationof the TiO2-based part. A TiN/mesoporous C nanocompositeconstituted of TiN (61 wt%) nanocrystallites (from 10 to50 nm in diameter) embedded into a mesoporous carbon matrixwas obtained.I. IntroductionTRANSITION metal nitrides (TMN) have garnered signi-cant attention among the ceramic community due totheir unique physical and chemical properties. They associatethe extreme hardness and brittleness of covalent solids, highmelting temperature compared to oxide materials, and elec-tronic/magnetic properties similar to transition metals.13 Tra-ditionally, their main uses are related to hard coatingmaterials, but new potential applications are emerging in theelds of catalysis,47 electrocatalysis in fuel cells,8 and energystorage both in Li-based batteries912 and in supercapacitors.2Their performances could then be improved by increasingtheir surface area and therefore decreasing their particle sizeat the nanometer scale. The production of nanostructuredTMN is then a key issue for these emerging applications.Among TMN, preparation of nanostructured titanium nitride(TiN) has been investigated by several authors during this lastdecade. Nanocrystalline and non-porous TiN powders wereobtained by nitridation of TiO2 nanoparticles in NH3.13 Inparallel, TiN nanoparticles14 were formed by a ligand-assistedammonolysis reaction of solid TiCl4 complexes, with a spe-cic surface area around 140 m2/g. The high-surface-areaTiN synthesis was taken a step further with creation of poros-ity within the material. Adding a structuring agent, mesopor-ous TiO2 lms15 or powders16 were prepared with dierentordered pore networks and surface areas. These lms wereonly partly nitrided yielding to titanium oxynitride com-pounds with modied band-gap for photocatalysis applica-tion.17 Other processes based on aerosol synthesis18 or evenby low-pressure, chemical vapor deposition19 were also devel-oped for the synthesis of TiN (and TiOxNy) nanoparticles orlms. Hard templating methods, where TMN nanoparticleshave been synthesized inside the conned media of mesopor-ous materials, were also investigated.20 However, in somecase, performances of TMN (here TiN) nanocrystalline mate-rials could be improved if this active phase is embeddedwithin an addition phase, such as carbon.8 Indeed, this sec-ondary phase brings an additional surface and/or (meso)porosity that increases the catalytic activity by enhancementof reactive-species adsorption close to the active catalyticphase and by coupling the catalytic activity of TMN to therelatively good electronic conductivity of the carbon phase. Itthen promotes a high current density in polymer electrolytefuel cell applications for instance.8 In addition, it could allowa considerable improvement of the durability of electrochemi-cal-energy storage device by limiting the cushion eect due toa large volume variation of the active phase during successiveelectrochemical cycling.9 Finally, synthesis of nanocrystallineTiN with a carbon-based extra-phase could also facilitate theshaping of the materials and then its handling. Nevertheless,syntheses of such TMN/porous carbon nanocomposites arescarcely described in the literature and require time consumingmultistep routes. The purpose of our work was then to syn-thesize TiN/mesoporous C nanocomposites with controlledTiN/C ratio by a fast, reproducible, and direct way via thenitridation of an ordered TiO2/mesoporous carbon nano-composite. The latter materials were prepared according tothe procedure described by Huang et al.21, that is, basedon the evaporation-induced self-assembly (EISA) process.Soluble species, such as a surfactant (a triblock copolymeracting as pore-structuring agent), a phenolic resin precursor(called resol) and titania-based oligomers self-assembletogether under evaporation conditions and yield to anordered mesostructured array of micelles surrounded byinterwoven titania- and phenolic resin-based polymers. Heattreatment of this hybrid organic/inorganic material leads tothe pyrolysis of the surfactant (by freeing thus an orderedmesoporosity), to the carbonization of the phenol resin,and the completion of the polymerization/crystallization ofthe TiO2-based oligomers. The resulting TiO2/C-orderednanocomposite could then be heat-treated in inert atmo-sphere up to 950C to give TiC/C nanocomposite22 or nitri-dated under NH3 to give TiN/C nanocomposites (presentedin this work). The dierent degrees of structuration of theTiN/C materials were investigated by X-ray diraction(XRD), high resolution transmission electron microscopyK. Itatanicontributing editorManuscript No. 29915. Received June 24, 2011; approved September 14, 2011.Author to whom correspondence should be addressed. e-mail: julien.parmentier@univ-mulhouse.fr4142J. Am. Ceram. Soc., 94 [12] 41424145 (2011)DOI: 10.1111/j.1551-2916.2011.04893.x 2011 The American Ceramic SocietyJournal(HRTEM), nitrogen physisorption at 77 K, and elementalanalysis.II. Experimental Procedure(1) Synthesis of TiO2/Mesoporous C NanocompositesThe preparation of TiO2/C nanocomposites was inspired bythe procedure developed by Huang et al.21 and is based onthe evaporation-induced self-assembly process (EISA). First,the carbon precursor solution named resol was prepared bythe polycondensation of phenol (0.61 g) with formaldehydesolution (1.06 g of 36.5 wt% solution) in basic media(0.13 g of NaOH aqueous solution at 20 wt%). After reux-ing this solution at 65C for 60 min, the resol was dried ina vacuum rotary evaporator with a maximum temperatureof 48C. A quantity of 1 g of resol was then dissolved in4 g of ethanol. In parallel, the titania-precursor solutionwas prepared by adding, drop-by-drop under vigorous stir-ring, 4 g of TiCl4 to a mixture containing 8 g of distilledwater and 8 g of absolute ethanol. The TiCl4 must be addedvery carefully with protection equipments and under a fume-hood due to its violent reaction with water associatedwith a HCl production and a heat release. The solutionwas stirred for 1 h. Second, 1.5 g of triblock copolymerpoly(ethylene oxide)106poly(propylene oxide)70poly(ethyleneoxide)106, named Pluronic F127, was dissolved in 10 g ofethanol absolute. This copolymer served both as pore-structuring and porogene agents. A quantity of 5.55 g ofprehydrolized TiCl4 solution was then introduced in 2.95 gof resol solution. The mixture was stirred for 10 min beforeto be added drop-by-drop in the F127 solution. The com-plete solution was stirred for 1 h before to be deposed inthin lms in Petri dishes and heated at 40C during 24 hand for an additional 24 h at 100C. The resulting hybridorganic/inorganic mesostructured material was nallycrushed into powders and was separated into two parts.One part was heat-treated in inert atmosphere up to 600Cfor 3 h to carbonize the phenolic resin into carbon, to pyro-lyze the surfactant to free the porosity, and to complete thecondensation of the titania-based polymer. The resultingmaterial was denoted as TiO2/C. The second part was sub-mitted to a nitridation procedure.(2) Nitridation of TiO2/C in TiN/CThe organic/inorganic mesostructure was calcined underammonia atmosphere at 700C or 800C for 3 h (heatingrate of 1C/min; ow rate of 10 L/h). During this heattreatment, the TiO2-precursor was converted into TiN via aTiO2 phase. The resulting nanocomposite was referred asTiN/C.(3) Characterization of Nanocomposites TiN/Mesoporous CThe TiN/C nanocomposite was characterized by XRD, N2physisorption, and (HR)TEM to get structural insight andporosity information on this material at dierent scale. X-raydiractograms were collected with an Xpert Pro PANalyticalwith CuKa1 radiation (k = 0.15406 nm). The cubic TiN cellparameter was determined with the software Eracel taking intoaccount the zero angle shift.23 Nanocomposites were observedby HRTEM using a JEOL 2100F microscope which operatesat 200 kV and has a point-to-point resolution of 0.21 nm. TheN2 physisorption at 77 K was done on the ASAP 2420 of Mi-cromeritics. The specic surface area, the pore volumes, andthe pore size distribution were calculated with the SoftwareData Master provided with the apparatus.The chemical analyses of the TiN/C materials were per-formed by elemental analysis, whereas the carbon content wasdetermined by thermogravimetric analysis (TGA). First, theas-made TiO2/resin material was heated in inert atmosphere(N2) until 700C. The observed weight loss was attributed tothe surfactant pyrolysis, the carbonization of the resin, andthe dehydration of TiO2-based inorganic polymer. When theweight was constant, the gas was switched to air to oxidizethe carbon. The corresponding weight loss at this temperaturehas been attributed to the decrease of the carbon content.III. Results and Discussion(1) Characterization of the TiO2/Mesoporous C MaterialObtained from the Hybrid Organic/Inorganic MesostructuredMaterialsThe intermediate TiO2/C material was characterized by TEMafter being calcined in an inert atmosphere at 600C. Regu-larly spaced fringes characteristic of an ordered mesostruc-ture were observed (with a 2-D periodicity of about 8.5 nm,Fig. 1). It corresponds to nanometric domains (size below10 nm) constituted of TiO2 and C, and separated by a regu-lar array of mesopores.(2) Synthesis and Characterization of TiN/CNanocomposite(A) Nitridation of TiO2/Mesoporous C Material: TheTiO2/C material can be described as TiO2 nanoparticlesembedded into an ordered mesoporous carbon framework.21As described below, this composite is a highly reactive sys-tem for TiO2 nitridation due to the nanocrystalline size ofTiO2, the presence of C that could promote the partial reduc-tion of titania in presence of NH3 according to reaction (1)24and a highly mesoporous structure for promoting diusionof reactive species [NH3 or its decomposition products(H2 and N2)]. Therefore, nitridation could be achieved invery mild conditions (3 h at 700C) as evidenced by XRD(see hereafter), where no traces of TiO2 or intermediate TiOxproducts (e.g.: Ti5O9, Ti4O7, Ti3O5, Ti2O3) could be detected.Fig 1. TEM micrographs of TiO2/C nanocomposite calcined in aninert atmosphere at 600CDecember 2011 Rapid Communications of the American Ceramic Society 41434TiO2 CH2 ! 2Ti2O3 H2O CO 1The TG analysis performed on the as-made TiO2/resincomposite with the procedure described in the experimentalpart has shown that carbonization is nearly complete at 700Cfor 3 h and that the TiO2 content in the TiO2/C intermediatematerial is around 53 wt%. This composition is in relativeagreement with the 50 wt% composition expected. At thisstep, the Ti/C atomic ratio is around 0.17. If we assume thatnitridation process converts all TiO2 into TiN, the totalweight loss measured during the nitridation step (80%)allows to calculate, by dierence, the expected remaining car-bon content (30 wt%). The calculated Ti/C ratio is thenaround 0.42. This value is in close agreement with the experi-mental ratio determined by elemental analysis (0.41). Theincrease of the Ti/C ratio during the nitridation process cor-responds to a carbon weight loss of 50 wt%. It implies a sig-nicant consumption of the carbon in the nitridationprocess, probably by reduction reactions of type (1).The chemical analysis of the nanocomposite calcined at800C shows atomic percentages for C, H, N, O, and Ti of41.8, 13.4, 17.5, 9.4, and 17.4, respectively. The amount ofhydrogen is typical of a pure mesoporous carbon matrix pre-pared by the EISA process with a phenolic resin and calcinedat the same temperature. The resulting Ti/N atomic ratiodetermined by elemental analysis is around 1.0 suggesting theformation of a stoichiometric TiN phase. Nevertheless, as thecarbon matrix can also contain heteroatoms, such as nitro-gen (with oxygen and hydrogen), and due to the existence ofsolid solutions between TiNTiC and TiO24, stoichiometricTiN composition cannot be assumed. Further characteriza-tions were then performed on the TiN/C nanocomposite toclarify this point.(B) Structural Characterization by XRD and (HR)TEM: The XRD patterns were recorded for the TiN/C com-posites nitrided at 700C and 800C for 3 h (Fig. 2). They show3 and 5 peaks for 2h going from 5 to 902h indexed in the spacegroup Fm3m with calculated cubic-cell parameters of 0.41896and 0.42243 nm (SD 0.00006 nm) for the samples nitrided at700C and 800C for 3 h, respectively. These values standbetween the cell parameter value of TiN (0.42417 nm fromICDD Card no. 00-038-1420) and the one of TiO (0.41770 nmfrom ICDD Card no. 00-008-0117, Fm3m), both isomorphicphases. If we consider that the TiOTiN solid solution followsthe Vegards law, their calculated compositions correspond toTiN0.19O0.81 and TiN0.73O0.27, respectively. The eect of thenitridation temperature is clearly evidenced by the reductionof the oxygen content within these phases. As their surfacesprobably also reoxidize upon storing in air, the obtainedphases are not a truly pure TiN but a titanium oxynitride witha face-centered cubic arrangement of atoms (NaCl structuretype) similar to TiN structure and a composition close to TiNat higher temperature. For reasons of convenience, these mate-rials are still called TiN. Even if the incorporation of carboninto the TiN-based materials cannot be completely excluded,its presence was not considered herein, as it should be small.Indeed, it is well known that TiN formation is thermodynami-cally promoted compared to TiC (ICDD Card no. 00-002-1179) and that the tendency to form a titanium carbonitridephase is signicantly reduced when H2 gas (due to NH3decomposition) is present during the heat treatment.25 The rel-atively important XRD-peak widths suggest that TiN particleshave small sizes as conrmed by TEM analysis.(3) Evidence of the Nanostructuration of TiN/CNanocomposite by TEM and HRTEMThe TEM and HRTEM images of the TiN sample nitridedat 800C for 3 h are reported in Fig. 3. We can see a hetero-geneous particles size with TiN nanoparticles from 10 to50 nm [Figs. 3(a) and (b)]. In some parts, not representativeof the materials, the initial hexagonal mesostructure of thehybrid material was preserved (data not reported). Highresolution technique [Fig. 3(c)] gives a lattice parameter of0.4224 nm in very good agreement with the value determinedby XRD analysis (0.42243 nm).(4) Porosity Analysis by Nitrogen Physisorption at 77 KThis technique is dedicated for the analysis of micro andmesoporous materials (pore size below 2 nm and between 2Fig. 2. XRD patterns (CuKa radiation) of TiN/C nanocompositeobtained at 700C and 800C (3 h) in NH3 atmosphere.Fig. 3. TEM images of TiN/mesoporous C nanocomposite (a) and (b); (c) HRTEM image of the selected area of TiN nanocrystal representedon the image (b)4144 Rapid Communications of the American Ceramic Society Vol. 94, No. 12and 50 nm, respectively). Nitrogen physisorption isothermsof TiN/C composite heat-treated at 800C for 3 h are shownin Fig. 4(a). Its treatment leads to a specic surface area of320 m2/g with a total pore volume of 0.29 cm3/g (at P/P0 = 0.95), where P and P0 are, respectively, the pressureand the saturation pressure of nitrogen at 77 K. The poresize distribution obtained by NLDFT method26 with slit-shaped pores is reported in Fig. 4(b) with its correspondingcumulative pore volume. It appears that beside the micropo-rosity (pore volume 0.08 cm3/g), the mesoporosity, with apore volume around 0.21 cm3/g, is broadly distributed withpore sizes between 2 and 20 nm.IV. ConclusionWe have reported herein the synthesis of TiN/mesoporous Cnanocomposite by a fast and facile route. This direct synthesisbased on the self-assembly of a surfactant, a phenolformalde-hyde resin and a TiO2 precursor has led to a nanostructuredTiO2/C materials which was further nitrided into TiN/meso-porous C nanocomposite. The high versatility of the synthesis(nature of the oxide, oxide/C ratio, annealing temperature,and gaseous atmosphere) allows to obtain nanoparticles ofthe active phase [e.g., TiO2 and TiN (this study) or TiC22]dispersed in a mesoporous carbon matrix. The latter couldpromote the pre-adsorption of reactive species for catalyticapplication or/and could limit the grain growth during eitherconversion reaction in new generation of Li-based batteries orheat treatments.AcknowledgmentsWe are grateful to the Agence Nationale de la Recherche (ANR) for support-ing this research program through the ANR-08-BLAN-0189-018 contract. Wealso thank the Region Alsace through the Pole Materiaux et NanosciencesAlsace for its nancial support No 964-10 C1.References1W. Lengauer, Handbook of Ceramic Hard Materials. Vol. 1, Wiley-VCH,Weinheim, Chapter 7, 2000.2D. Choi, G. E. Blomgren, and P. N. Kumta, Fast and Reversible SurfaceRedox Reaction in Nanocrystalline Vanadium Nitride Supercapacitors, Adv.Mater., 18, 117882 (2006).3L. E. Toth, Transition Metal Carbides and Nitrides. Academic Press, NewYork, 1971.4J. G. Chen, Carbide and Nitride Overlayers on Early Transition MetalSurfaces: Preparation, Characterization, and Reactivities, Chem. 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