High-Surface-Area Nanoporous Boron Carbon Nitrides for Hydrogen Storage

By David Portehault,*Cristina Giordano,Christel Gervais,Irena Senkovska,

Stefan Kaskel,Cle

′ment Sanchez,and Markus Antonietti 1.Introduction Due to their hardness,thermal stability,and chemical inertness,boron nitrides are strategic materials for use as protective coatings and reinforcing structures.As a bulk material,hexagonal boron nitride (h -BN),with a graphitelike structure,is also known for its strong luminescence in the UV range,which makes it a potential material for UV lasing.[1]

Bulk properties of h -BN are preserved in nanoscale materials and a number of groups now deal with the preparation of nanostructured h -BN for chemically and thermally resistant catalyst supports.[2–4]

Novel properties are also expected to emerge for h -BN nanostructures,including gas sorption and electron-?eld emission.[5–9]Experimental

and theoretical studies have recently de-monstrated that nanostructured boron

nitrides exhibit high hydrogen uptake

because of strong interactions with the H 2molecule due to the dipole moment of B àN bonds and the local curvature of the

surface.[5,10–12]Furthermore,it is believed that the control of the composition of boron carbon nitrides in the B àC àN solid

solution system could provide an additional

way to control storage properties,as this allows modifying the nature and the energy

of the bond between hydrogen molecules and the surface.[11,13]Despite promising theoretical works,

experimental studies on sorption properties

and storage performances of boron carbon

nitrides B x C y N z are still in their infancy

because of the dif?culty to control the texture.Up to now,many

synthetic procedures have been proposed in order to modulate the

shape of such materials.For example,boron carbon nitride

nanotubes can be synthesized using arc discharge,[6]chemical vapor deposition,[14–16]substitution reactions,[17]and chemical synthesis.[18–20]In parallel,solvothermal syntheses are now emerging for boron nitride nanoparticles.[21–23]While these processes involve signi?cantly lower temperatures (300

6008C)than usual solid-state chemistry (T !16008C),they also require further washing steps and are still limited to the synthesis of aggregated isotropic BN nanoparticles with low surface area.

For applications where high surface area and mechanical strength are required,mesoporous boron carbon nitrides could provide

more ef?cient materials.Few studies report high-surface-area xerogels,[24]while the highest speci?c surface area was obtained for ordered mesoporous structures from organized block-copolymer precursors,with a value of 950m 2g à1.[25]Mesoporous hard templates are commonly employed,such as silica [26,27]or carbon,[28–30]even if they suffer from severe

drawbacks,such as inef?cient ?lling of the mesopores by the molecular precursor and removal of the template by harmful etching reagents (hydrogen ?uoride or oxygen peroxide).To our knowledge,the highest speci?c surface area for BCN materials obtained by hard templating was reported by Vinu et al.with 740m 2g à1using a carbon matrix as the template.[29]The study presented herein reports on a novel hard-templating approach towards mesoporous boron carbon nitrides.A poly-meric,mesoporous graphitic carbon nitride (mpg-C 3N 4)is used as FULL

PAPER

[*]Dr.D.Portehault,Dr.C.Giordano,Pr.M.Antonietti

Max-Planck-Institute of Colloids and Interfaces Department of Colloid Chemistry Research Campus Golm,14424Potsdam (Germany)E-mail:David.Portehault@mpikg.mpg.de

Dr.C.Gervais,Pr.C.Sanchez

UPMC Univ Paris 06,CNRS,UMR 7574

Chimie de la Matie

`re Condense ′e de Paris,Colle `ge de France 11place Marcelin Berthelot

75231Paris Cedex 05(France)Dr.I.Senkovska,S.Kaskel Department of Inorganic Chemistry Dresden University of Technology

Mommsenstrasse 6,01069Dresden (Germany)

DOI:10.1002/adfm.201000281

Nano-and mesoporous boron carbon nitrides with very high surface areas up

to 1560m 2g à1are obtained by pyrolysis of a graphitic carbon nitride

mpg-C 3N 4in?ltrated with a borane complex.This reactive hard-templating approach provides easy composition and texture tuning by temperature adjustment between 800and 1400-C.The process yields B x C y N z O v H w materials as direct copies of the initial template with controlled compositions

of 0.15￡x ￡0.36,0.10￡y ￡0.12,0.14￡z ￡0.32,and 0.11￡v ￡0.28.The nano

and mesoporosities can also be tuned in order to provide hierarchical

materials with speci?c surface areas ranging from 610to 1560m 2g à1.Such high values,coupled with resistance against air oxidation up to 700-C,suggest potential materials for gas storage and as catalyst supports.Indeed,it

is demonstrated that these compounds exhibit high and tunable H 2uptakes

from 0.55to 1.07wt.%at 77K and 1bar,thus guiding further search of

materials for hydrogen storage.

F U L L P A P E R

a template.This matrix completely reacts upon heating,so that no isolation step is required,providing an easy route towards these materials without a puri?cation process.Moreover,the carbon nitride matrix acts as an ef?cient nitrogen donor and supports carbon substitution by boron,yielding a direct copy of the template,rather than the inverse replica usually observed for common hard templating.Another advantage of reactive hard templating is that the number of reactants that have to ef?ciently ?ll the template can be reduced,ideally to one.Thirdly,easy tuning of the composition as well as of the speci?c surface area is achieved by temperature control.We report values of up to 1560m 2g à1.We also highlight the ?rst experimental evidence of the in?uence of the composition and the textural features of boron carbon nitrides on the hydrogen sorption properties.

2.Results and Discussion

Pyrolysis of impregnated mpg-C 3N 4was used as a simple method for the synthesis of boron carbon nitrides,as previously described for metal nitrides.[31,32]Impregnation was performed with BH 3-tert -butylamine (see Experimental Section),then pyrolysis was carried out under nitrogen.Elemental analysis of mpg-C 3N 4yields a composition of C 0.27N 0.36O 0.13H 0.23,with a C:N ratio equal to the stoichiometric value of 3:4.Then,oxygen and hydrogen originate mainly from adsorbed water.Except for this adsorbed amount,the synthesis procedure is oxygen free.The pore size of the mesoporous carbon nitride was 12nm according to the Barrett-Joyner àHalenda (BJH)method applied to N 2sorption.mpg-C 3N 4synthesis involves silica nanoparticles as templates.The pore size of mpg-C 3N 4re?ects the initial silica particle diameter and is easily adjusted.

Thermogravimetric analysis (TGA)under N 2?ow (Fig.S1of the Supporting Information)of the initial mpg-C 3N 4matrix indicates that decomposition starts at 5008C,with a mass loss of $75%between 600and 7008C and a negligible remaining mass at 10008C,thus highlighting complete decomposition.X-ray diffraction (XRD)patterns of samples synthesized at different temperatures from the in?ltrated template were recorded in order to investigate the reaction pathway (Fig.1).mpg-C 3N 4exhibits a strong re?ection at 27.38(2u ),which accounts for the lamellar stacking of g-C 3N 4layers in a graphitic structure.[33]The intensity of this re?ection decreases at 5508C,indicating that the matrix started to react.For T >5508C,two broad peaks are observed at 25.08and 43.38,which are commonly observed in layered boron nitrides obtained at temperatures below 16008C.[1,22,30,34]These peaks correspond respectively to (002)interlayer and (101),(100)in-plane re?ections in the graphitic turbostratic structure of boron carbon nitride.No additional peak that could originate from boron containing crystalline species was identi?ed during heat-treat-ment experiments.Interestingly,increasing the temperature at 14008C leads to a narrower peak at 43.38,indicating an increase of the in-plane order,while the (002)re?ection is unaffected.The stacked layers are then still misoriented.Fourier transform infrared (FT-IR)spectroscopy con?rms the disordered graphitic structure of boron carbon nitride at 14008C (Fig.S2)with bands at 1400and 800cm à1,[1,35]characteristic of sp 2-bonded BN in-plane vibrations and out-of-plane BNB vibrations,respectively.For samples obtained at T 10008C,these bands are enlarged and

shifted at 1355and 780cm à1,indicating a turbostratic structure and/or a small size of the nanodomains.A wide band at 3250cm à1(not shown)is characteristic of N àH and O àH bonds,showing the presence of adsorbed water molecules as well as potential B àOH groups.[1,35]

Elemental analysis (Fig.2and summary in Table 1)indicates that heat treatment at T 10008Ccauses a decrease of the nitrogen and carbon contents while the boron content increases.Such variations are most likely due to the evolution of C 2N x and C 3N x gases.[36]The content of oxygen ($27%)for boron carbon nitrides samples obtained at T 10008C is surprisingly high since the synthesis procedure is mainly oxygen free.This high oxygen amount cannot be reasonably explained by water or O 2adsorption and originates from spontaneous oxidation upon air exposure of the ?nal product after cooling.At 14008C,the oxygen and hydrogen contents dramatically decrease,indicating that the sample becomes more stable against oxidation.Accordingly,TGA under air (Fig.S3)of the boron carbon nitrides indicates a weight loss at 6008C,corresponding to the carbon loss,and a mass increase due to oxidation of unstable boron moieties at 775,775,and 8258C for samples obtained at 800,1000,and 14008C,respectively.Thus,higher-temperature compounds exhibit higher oxidation inertness.By adjusting the processing temperature in the range 800–14008C,the composition and the resistance against oxidation of boron carbon nitrides can then be easily tuned.In order to get a closer insight into the local structure of these boron nitride-based materials,11B solid-state NMR,analyzed by simulation of the spectra,was used to investigate chemical bonding (Fig.3).A signal at d iso ?29.5ppm (C Q ?2.9MHz,h ?0.2)indicates the presence of B àN bonds as observed in planar BN 3groups within BN graphitic layers.[37]Involvement of oxygen within trigonal BO 3sp 2(d iso ?20.5ppm,C Q ?2.65MHz,h ?0.2)and tetragonal BO 4(d iso ?0.7ppm,no quadrupolar shape)groups [38–40]shows that the reactivity towards oxygen indeed originates from boron centers.The relative intensity of O-based groups in comparison to BN 3decreases upon heating,

with

Figure 1.XRD patterns of mesoporous C 3N 4impregnated with a solution of BH 3-BuNH 2and pyrolysed at various temperatures under N 2.An enlargement of the pattern at 10008C is presented in the inset.The indexation is performed according to hexagonal boron nitride.

FULL PAPER

evaluated ratio between BO x (x ?3or 4)units and BN 3groups of 0.89,0.43,and 0.005for 800,1000,and 14008C,respectively.The carbon environment has been assessed through 1H-13C CPMAS NMR (Fig.SI-4).A ?rst characteristic peak of sp 2C àN bonds is observed at 157ppm,which could account for C 3N 3rings.These groups could originate from BN domains where boron is partly substituted by carbon,which retains a carbon nitride-type environment.The second peak at 125ppm highlights C àC bonds

within amorphous graphitic carbon.Neither B àB nor B àC bonds were identi?ed,as already observed by Komatsu et al.,for bulk B àC àN compounds.[41]Therefore,BN-rich domains and amor-phous-carbon-rich areas are present but separated within the compounds.

Scanning electron microscopy (SEM)(Fig.4a–c)indicates that the samples are homogeneous and morphologically similar to the initial mpg-C 3N 4.Coarsening seems to occur for the sample obtained at 14008C,with an apparent grain size of 40nm.Transmission electron microscopy (TEM)investigations (Fig.4d,e)of a sample heated at 10008C highlight a mesoporous structure with pores of diameter ranging between 5and 20nm,which are not ordered.Selected-area electron diffraction (SAED)patterns exhibit two diffuse rings at 0.12and 0.21nm,which account for in-plane re?ections within the turbostratic structure.High-resolution (HR)TEM studies (Fig.4f)reveal small crystalline domains with (002)interlayer fringes.

Textural properties were investigated by N 2sorption and are summarized in Table 1.The sample obtained at 5508C exhibits

Figure 2.a)Molar composition and b)molar contents relative to boron of samples obtained at different temperatures.

Table 1.Heat treatment,composition,and textural features of mpg-C 3N 4and resulting boron carbon nitrides.

Temperature treatment [8C]

Composition

Speci?c surface area [m 2ág à1][a]

Microporous volume [cm 3ág à1][b]

Mesoporous volume [cm 3ág à1][c]

BJH mesopore diameter [nm][c]

DFT mesopore diameter [nm][d]

Room temperature C 0.27N 0.36O 0.13H 0.23(mpg-C 3N 4)2000.100.7311.99.1550B 0.08C 0.15N 0.25O 0.19H 0.3336–[e]–[e]–[e]–[e]800B 0.15C 0.10N 0.17O 0.27H 0.316100.29 1.227.6 6.91000B 0.18C 0.10N 0.14O 0.28H 0.306100.30 1.54 6.4 6.91400B 0.36C 0.12N 0.32O 0.11H 0.09

1560

0.64

2.23

7.6

7.1

[a]Evaluated from the BET method applied to N 2sorption.[b]Evaluated from the Dubinin–Astakhov equation applied to N 2sorption.[c]Evaluated from the BJH method applied to N 2sorption.[d]Evaluated from the NLDFT method applied to the desorption branch.[e]No relevant value could be obtained in relation with the low porosity because of ?lled

pores.

Figure 3.11B MAS NMR spectra of samples obtained at different tem-peratures:experimental spectra (solid),simulated spectra (long dash),BN 3(short dash),BO 3(dot),and BO 4(dash-dot).

F U L L P A P E R

low surface area (36m 2g à1

),indicative of pores that are still

blocked by boron species,in relation to the XRD pattern.Isotherms

(Fig.5)of samples obtained at higher temperature are of type IV,

typical for mesoporous compounds.The increase of adsorbed

volume (at P /P 0?0.95)is typical of interparticle voids including

macroporosity,in relation with SEM pictures.Upon temperature

increase,the speci?c surface area increases from 200for initial

mpg-C 3N 4to 1560m 2g à1

for the sample treated at 14008C.While

the pore size of initial mpg-C 3N 4re?ects the diameter of the silica particles used as template,the pore size distribution of the calcined boron carbon nitride is multimodal,with a majority of pores between 6and 8nm.Shrinkage of the pores upon heating is typically observed for mesoporous compounds.However,the sudden rise of surface area for the sample obtained at 14008C is more uncommon and corresponds to the formation of a large number of micropores as seen from the increase of the adsorbed volume at low relative pressure,together with an increase of the microporous and mesoporous volumes (Table 1).The corresponding isotherm re?ects a hierarchical architecture with trimodal pore size distribu-tion corresponding to micropores of $1.6nm,mesopores with diameter below 10nm,and macropores evidenced by the volume increase for P /P o !0.95.Such results unambiguously indicate that boron carbon nitrides are not obtained as inverse replica of mpg-C 3N 4,namely nanoparticles,but as mesoporous direct copies of the initial template.

H 2sorption properties have been investi-gated at 77K up to 1bar (Fig.6a).Sorption isotherms are fully reversible and the steady slope at 1bar indicates that higher uptakes can be expected at higher pressure.[10,12]Signi?cant differences are observed between the uptakes of the various samples.Firstly,the boron carbon nitride obtained at 14008C exhibits the highest

uptake with 1.07wt.%at 77K and 1bar.

Secondly,compounds synthesized at 800and 10008Cadsorb 0.82and 0.55wt.%,respectively,

despite the very similar textural properties.The difference between the sample behaviors becomes ampli?ed when normalizing the H 2uptake versus the microporous volume (Fig.6b).Despite the strong increase of the surface area and the microporous volume upon further heating,the H 2af?nity strongly decreases from 0.028to 0.016g cm à3.This peculiar behavior could originate from different micropore sizes,but also from surface atoms in different chemical environments with different reactivities.[11]In particular,the higher C:B ratio (0.67as compared 0.33)of the sample obtained at 8008C as well as its higher sensitivity towards oxygen is already indicative of the underlying chemical variations.We are now embarking on further work in order to provide a more quantitative description of the surface reactivity.

Combined results from SEM,TEM and N 2sorption studies indicate that direct mesoporous copies of the con?ning media are obtained (Scheme 1).The use of mpg-C 3N 4as a reactive hard template has been reported where metal nitride nanoparticles were obtained as a negative replica of the initial template.[31]More recently,TiN/carbon composites were fabricated by coating of macroporous g-C 3N 4with a titania precursor,followed by decomposition of the g-C 3N 4template,yielding TiN hollow spheres coated with carbon.[32]These previous reports were based on the evolution of ammonia and related gases from pyrolyzed g-C 3N 4to provide nitrogen donor species that could react with

the

Figure 4.SEM images of a)initial mpg-C 3N 4and boron carbon nitrides obtained after treatment

at b)10008C and c)14008C.d,e)TEM,f)HRTEM,and corresponding SAED pattern of a boron

carbon nitride obtained at 10008

Figure 5.N 2sorption isotherms and pore size distribution from the BJH

method (inset)for mpg-C 3N 4(solid)and boron carbon nitrides obtained

after treatment at 8008C (dash),10008C (dot),and 14008C (dash-dot).

FULL PAPER

metal precursor.As a consequence,inverse replica or related nanostructures were obtained.The results presented herein rely on a different pathway since a direct copy of the template is formed.The different behavior in respect to the boron precursor could originate from the close properties of boron and carbon.Indeed,substitution of carbon by boron has been proved at high temperature by Han et al.on carbon nanotubes,[17]and is reasoned by the solid solution behavior of the B àC àN ternary system.Such a reaction pathway is also made easier by the lamellar structure of g-C 3N 4and the small thickness of the template walls,which enhance diffusion of boron into the structure.The maintaining of the initial template morphology indicates that boron starts to diffuse and to substitute carbon in mpg-C 3N 4before decomposi-tion at 6008C.

Heating at 14008C leads to a decrease of the carbon content together with the opening of micropores,while solid-state NMR indicates BN 3,CN x ,and amorphous-carbon-rich domains.Therefore,a structure of the boron carbon nitrides can be proposed that comprises graphitic BN sheets,where boron is partly substituted by carbon,together with amorphous-carbon-rich nanodomains.These carbon-based domains act as ?nal micropore templates and are eliminated through further heat treatment at 14008C (Scheme 1).Further NMR characterizations are underway to provide a more precise representation of the local structure.Solid-state NMR shows that oxidation sensitivity for samples obtained below 10008C originates from dangling bonds or edge reactivity within the weakly ordered BN layers,leading to unsatis?ed boron valence and electrophilic reactivity towards nucleophilic species such as oxygen.At 14008C,XRD indicates higher ordering within the graphitic layers,which is correlated with a decrease of the oxygen content observed by elemental analysis and 11B NMR.A temperature of 14008C is required to incorporate the vast majority of boron atoms in BN 3groups suf?ciently stable to avoid oxygen sensitivity upon air exposure.It is worth noting that the high-temperature compound is strikingly resistant to oxidation up to 8258C.Moreover,the boron carbon nitride obtained at 14008C exhibits a hierarchical porous structure with a trimodal pore size distribution at the nano-,meso-,and macroscale,as well as a speci?c surface area of 1560m 2g à1,which is the highest reported for boron nitride based compounds.[25]Such textural properties pave the way towards the use of these matrixes as oxidation resistant catalyst supports as well as gas-storage systems.

H 2sorption measurements provide valuable information about the storage potential of such compounds.Indeed,while high surface area enhances hydrogen uptake,a speci?c,less-ordered boron environment seems to be a key feature for uptake enhancement with given textural properties.Further experiments are underway in order to characterize the micropore system and the surface reactivity of these materials.

3.Conclusions

Reactive hard templating from a mesoporous graphitic carbon nitride matrix provides a simple way towards direct copies of mesoporous boron carbon nitrides with the highest surface areas reported up to now.A new substitution reactivity of carbon nitride with boron species is evidenced that paves the way toward the development of a novel class of materials,providing composi-tional,textural tailoring,and production of hierarchical porous structures through simple adjustment of the heat

treatment.

Figure 6.H 2sorption isotherms of boron carbon nitrides obtained after treatment at 8008C (squares),10008C (circles),and 14008C (triangles).a)H 2weight uptake,b):normalization versus the microporous

volume.

Scheme 1.Scheme of the reaction pathway towards boron carbon nitrides through reactive hard templating.

F U L L P A P E R

Strong modi?cations of the hydrogen uptake are observed depending on the heat treatment and we are now embarking on an in-depth study of the surface reactivity.This work highlights new opportunities on the course of ef?cient storage materials as well as potential catalyst supports.

4.Experimental

Mesoporous Graphitic C 3N 4(mpg-C 3N 4):mpg-C 3N 4was synthesized according to previous reports [31].Cyanamide (5g)and Ludox-HS40silica dispersion are mixed with a cyanamide:silica ratio 1:1(5g of pure silica,so 12.5g of Ludox dispersion at 40wt.%)until complete dissolution of cyanamide.The mixture is heated in an oil bath at 1008C upon stirring for $3h until removal of water and formation of a white solid.The powder is then ground in a mortar,transfered into a crucible,and heated under air at 2.38C min à1up to 5508C and then treated at 5508C for 4h.The as-obtained yellow powder is ground in a mortar and then treated under stirring for 2d in a NH 4HF 24mol L à1solution (40mL per gram of initial cyanamide).The dispersion is then ?ltered,the precipitate is washed with water two times,then dispersed in water (40mL per gram of initial cyanamide)and stirred for 2d.The dispersion is then ?ltered,the precipitate is washed with water one time,ethanol two times,and then dispersed in ethanol (40mL per gram of initial cyanamide)under stirring for 1d.After ?ltering,the yellow compound is dried under vacuum at 608C.Boron Carbon Nitrides :The impregnation was carried out in inert atmosphere using degassed solids and argon-saturated liquids in an argon-?lled glovebox and with classical Schlenk techniques.Mesoporous g-C 3N 4(0.5g)is added to a solution of BH 3-NH 2t Bu (4.65g,Alfa Aesar)in tert -butylamine (15mL).The mixture is stirred for 2h at 508C,then the powder is recovered by centrifugation and washed thrice with tert -butylamine (3?10mL).The powder is dried under N 2?ow,transferred in a crucible,and pyrolyzed under N 2with a 2.78C min à1rate and a 6h stage.The mass yield in respect to initial mpg-C 3N 4is about 20%.

XRD :XRD measurements were performed on a D8Bruker apparatus operating at the Cu-K a 1radiation.

Elemental Analysis :Carbon,hydrogen,and nitrogen contents were measured using a Vario EL Elementar instrument.B was titrated at the

Fraunhofer Institute fu

¨r Angewandte Polymerforschung,Golm using ICP-OES.The O content was evaluated as the completing value of the above mentioned contents.

FT-IR Spectroscopy :Infrared spectra were recording on KBr pellets by using a Varian 1000FT-IR apparatus.

Thermogravimetric Analysis (TGA):TGA was recording on a TG 209F1,Netzsch,apparatus under nitrogen or synthetic air ?ow of 15mL min à1,with a 108C min à1heating rate.

Solid-state NMR Spectroscopy :11B MAS NMR spectra were recorded at 11.75T on a Bruker Avance500wide-bore spectrometer operating at 128.28MHz,using a Bruker 4-mm probe and a rotor spinning frequency of 14kHz.The spectra were acquired using a spin-echo u –t –2u pulse sequence with u ?908to overcome problems of probe signal.The t delay was synchronized with the spinning frequency and recycle delay of 1s was used.Chemical shifts were referenced to BF 3(OEt)2(d ?0ppm).Spectra were ?tted using the DMFIT program [42].

Field-Emission SEM :Observations were performed on a LEO 1550-Gemini instrument.The samples were loaded on carbon coated stubs and coated by sputtering an Au/Pd alloy prior to imaging.

TEM :Studies were carried out by using a Philips CM 200LaB 6apparatus.Samples were prepared by evaporating a drop of aqueous diluted suspension on a carbon-coated copper grid.

N 2sorption :Samples were degassed under vacuum at 1508C for 20h before measuring on a Quadrasorb apparatus at 78K.For comparison,the values of the mesopore volume and diameter obtained by the common BJH method were compared with those obtained by ?tting the desorption branches by using the non-linear density functional theory (NLDFT)according to an equilibrium model where the substrate is treated as carbon with cylindrical pores.The ?tting error was about 0.4%.

H 2sorption :The hydrogen sorption isotherms at 77K up to 1bar were measured using a Quantachrome Autosorb-1C apparatus.Hydrogen with 99.999%purity was used for the measurements.Prior to the measure-ments,the samples were evacuated at 2008C for 48h.

Acknowledgements

The authors acknowledge ?nancial funding from the Max Planck Institute –CNRS Post-Doctoral Program for Nanomaterials and thank the Fritz Haber Institute,Berlin for TEM facilities.Supporting Information is available online from Wiley InterScience or from the author.

Received:February 10,2010Revised:March 19,2010Published online:May 11,2010

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