Controlled synthesis of NiCo nanoalloys embedded in ordered porous carbon by a novel soft-template strategy

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Controlled synthesis of NiCo nanoalloys embedded in ordered porous carbon by a novel soft-template strategy Camelia Matei Ghimbeu a,*, Jean-Marc Le Meins a, Claudia Zlotea b, Loı¨c Vidal a, Gautier Schrodj a, Michel Latroche b, Cathie Vix-Guterl a a b

Institut de Science des Mate´riaux de Mulhouse, CNRS UMR 7361, 15 rue Jean Starcky, 68057 Mulhouse, France Institut de Chimie et des Mate´riaux Paris-Est UMR 7182, CMTR-UPEC, 2-8, rue Henri Dunant, 94320 Thiais, France

A R T I C L E I N F O

A B S T R A C T

Article history:

A novel synthesis pathway is proposed in this work to prepare NiCo nanoalloy embedded in

Received 19 July 2013

highly ordered porous carbons via a soft-template approach. This involves the multicompo-

Accepted 30 September 2013

nent co-assembly of environmental friendly carbon precursors (phloroglucinol and gly-

Available online 8 October 2013

oxal), amphiphilic copolymer Pluronic F127 as structure directing agent followed by metal precursor salts incorporation. This synthesis affords a good control of the alloy quantity, composition and particle size in the carbon matrix. The influence of the metal loading in the NiCo alloy and of two chelating agents, i.e., citric acid and oxalic acid on the physical–chemical characteristics of carbon@NiCo materials was investigated. In the absence of chelating agents, the carbon@NiCo hybrid materials have a 2-D hexagonally arranged pore structure, high surface area and porous volume. The NiCo nanoparticles are homogeneously distributed in the carbon network and they exhibit sizes between 30 and 40 nm depending on the Ni/Co ratio. The use of chelating agents allows downsizing the particle size to 15 nm and the modification of the hexagonally carbon structure into a worm-like one. The advantage of the present synthesized hybrids lays in the use of less expensive 3d transition metals than noble elements that may provide comparable effect on hydrogen sorption properties. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Owing to their advantages i.e., high surface area, controlled pore size and geometry, ordered structure, and tunable surface chemistry, the carbon templates proved to be over the last decade valuable materials which exhibit improved performances in many kinds of application (supercapacitors, batteries, catalysis, sensors. . .) compared to conventional carbon materials [1–3]. As many time highlighted in the literature, they could be employed as model materials for the understanding of the fundamental physical/chemical phenomena

of molecules/ions interactions occurring during application processes. Moreover, they have been used as ideal scaffolds for embedding nanoparticle species. Mainly, single metal nanoparticles have been inserted into various carbons with different pore sizes and geometries in order to improve the carbon performances for applications in separation, catalysis, sensors and clean energy applications [1,4–7]. In many cases, bimetallic nanoparticles can exhibit great enhancement on specific physical and chemical properties due to synergistic effects [8,9]. However, only few papers report on the synthesis of hybrid materials formed of bimetallic nanoparticles [10,11]

* Corresponding author. E-mail address: [email protected] (C. Matei Ghimbeu). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.09.089

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embedded in ordered nanoporous hosts and their potential use in the clean energy field is largely unexplored. In this context, the organization of different metallic nanoparticles with tunable chemical compositions, sizes and morphologies integrated into highly porous scaffolds is critical to the development of novel hybrids that display high performances in several types of applications. Generally, the synthesis method to obtain these hybrid materials is based on the impregnation of the carbon host with a metal salt solution followed by a reduction step [7,12,13]. Previously, we have inserted Pd–Ni nanoalloys into an ordered carbon host and their structural, magnetic and hydrogen sorption properties have been studied [14,15]. More recently, a tuneable synthetic route was developed for Mg–Ni nanoparticles confined into a mesoporous carbon template and either Mg2Ni or Mg2NiH4 nanocompounds (4 nm particle size) could be directly formed starting from separate Ni and MgH2 nanospecies. These hybrid materials show extremely good stability against coalescence during hydrogen sorption cycling and prolonged exposure to high temperature [16]. However, it should be underlined the important drawbacks of carbon template synthesis mainly related to their low yield, the high production cost and the time consuming due to the multiple step synthesis. Besides, the expensive template is sacrificed using chemical etching with dangerous reagents such as fluorhydric acid. Therefore, to overcome these inconvenients a much simple alternative method for the synthesis of ordered carbons was recently developed, i.e., the so called soft-template route [17,18]. This process involves the selfassembly of amphiphilic block copolymers as organic templates in the presence of carbon precursors (phenolic resins), in which the molecules are organized in nanospaces through hydrogen bonding, hydrophilic/hydrophobic interactions, ion pairing and dative interactions [17–19]. This type of synthesis becomes very attractive compared to the hard-template method due to reduced number of involved steps, low cost, good reproducibility and suitability for large-scale industrial applications [20,21]. However, the main inconvenient is related to the carcinogenic nature of the carbon precursors (phenol, formaldehyde) usually employed in such synthesis [21–24]. Several papers report on the synthesis of metal/carbon composites such as Co, Ni, Fe/C [22,23] but few papers describe nanoalloys inserted in carbon [25] for adsorption, separation and catalysis applications. In the present work, more environmentally friendly carbon precursors such as phloroglucinol/glyoxal are used to synthesize carbon hybrid materials with confined NiCo nanoparticles. Phloroglucinol/glyoxal couple was recently proposed as efficient for the synthesis of mesoporous carbon materials [26] but not yet employed to our knowledge to synthesize carbon/metal hybrid materials [21]. In this type of synthesis the metal is incorporated into the carbon matrix by addition of a soluble metal salt to the initial mixture containing the carbon source and the triblock polymer. The reaction is conducted in basic or acid conditions and a phase separation containing a polymer-rich phase and the organic solvent takes place. Further separation of these two phases and subsequent carbonization of the recovered polymer gel

261

allows to obtain dispersed metal particles into the carbon matrix. Nonetheless, when the resorcinol and/or phloroglucinol are employed with formaldehyde it has been underlined that the metal quantity incorporated into the carbon is much lower reported to the calculated one, hence an important limitation for preparing hybrid materials with these precursors [24,27]. A new and convenient approach is proposed in this work to encompass this major inconvenient. It consists by adding the precursor salt after the gel formation and separation, followed by carbonization. Hence, we report here on the synthesis of NiCo nanoalloys confined in ordered micro/mesoporous carbons by a simple and environmentally friendly pathway via selfassembly of carbon precursors, metal salts and a structuredirecting agent. The influence of Ni and Co amounts and of two chelating agents (citric acid and oxalic acid) on the physical–chemical properties (texture, structure, particle size) of the as-synthesized materials and also on their hydrogen adsorption properties are investigated by different analysis techniques.

2.

Experimental

2.1.

Chemicals

Triblock copolymer Pluronic F127 (poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly-(ethylene oxide, PEO106PPO70 PEO106, Mw = 12,600 Da), phloroglucinol (1,3,5-benzentriol, C6H6O3), glyoxal aqueous solution (40%, C2H2O2), ethanol absolute (C2H6O), citric acid (C6H8O7ÆH2O), oxalic acid (H2C2O4Æ2H2O), nickel(II) nitrate hexahydrate [Ni(NO3)2Æ6H2O], cobalt(II) nitrate hexahydrate [Co(NO3)2Æ6H2O], HCl (37%) were purchased from Sigma–Aldrich. Millipore water was used to prepare the metallic salt solutions. The chemicals were used as received without any further purification.

2.2.

Synthesis

Mesoporous carbon/metal nanoparticles or nanoalloys (Ni, Co and NiCo) composites were synthesized by co-assembly of block copolymer (Pluronic F127), phenolic resin (phloroglucinol–glyoxal) and metal nitrates under acidic conditions via the soft template method. In a typical synthesis, 3.3 g of phloroglucinol and 6.5 g of F127 were dissolved in 162 mL of ethanol and 1.2 mL of HCl aqueous solution (37%). After complete dissolution at room temperature, 3.25 mL of glyoxal aqueous solution was added. The reaction mixture was stirred for 1 h and then was aged for three days when a phase separation is observed. The supernatant was discarded and the remaining pale yellow polymer-rich gel was mixed vigorously with 40 mL of ethanol. To this gel, 4 mL of nickel or cobalt metallic salt solution (20% in water) was incorporated drop by drop under stirring in order to ensure homogeneous distribution of the metal solution in the gel. In the case of NiCo alloys preparation, the volume of nickel and cobalt precursor solutions were varied in such way to keep constant the total volume of metallic precursor solution (4 mL) but in the same time to allow to

262

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obtained different NixCoy compositions in the carbon (where x and y denotes the atomic content of Ni and Co, respectively, in the alloy). After complete ethanol evaporation, the gel is thermo-cured at 80 °C for one night and subsequently, carbonization, salt decomposition and reduction was carried out under Ar by heating at 600 °C for 2 h with a heating rate of 2 °C min1. The materials were grinded until homogeneous powders were obtained. For comparison reasons a composite was prepared by the classical method, i.e., by adding the metal precursor in the same time as the carbon precursors and not after the polymer gel formation (see Supporting Informations). The influence of two chelating agents (oxalic acid and citric acid) on the materials characteristics was investigated by adding them in the salt solution mixture (50 wt.% reported to the metal salt quantity, i.e., 0.4 g). The selected quantity corresponds to the required amount to obtain metal citrate or oxalate complexes. As the metal citrate and oxalate posses higher decomposition temperature, the growth of the metal particles can be delayed. Lower quantities than the required ones induce the formation of high particles size while higher quantities will not further modify the system as has been emphasized in the literature for Fe2O3 particles obtained using another chelating agent i.e., acetylacetone [23]. The as-obtained hybrid materials are denoted C@NixCoy. A carbon material was prepared in the same conditions in the absence of metal salts which is denoted, C.

2.3.

Characterization

The quantity of metal and the composition of each metal in the alloys were determined with an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 7600 Thermo ScientificTM). The textural properties of the materials were investigated with a Micromeritics ASAP 2020 instrument using N2 adsorbate at 196 °C. Prior to the analyse, the samples were outgassed overnight in vacuum at 200 °C on the degassing port followed by 4 h out-gassing on the analyse port in order to eliminate the backfill gas which could possibly be adsorbed in the micropores during the backfilling step of the analyse tube. This preliminary step is of great importance for assessing very low relative pressures (P/P0 < 106) and correct pore size distribution in the micropores range. The BET (Brunauer–Emmett–Teller) surface area (SBET) was calculated from the linear plot in the relative pressure range of 0.05–0.3 while the micropore volume (Vmicro) was determined using the Dubinin–Radushkevich (DR) equation. The mesopore volume (Vmeso) was obtained by subtracting the micropore volume from the total pore volume of N2 adsorbed at relative pressure P/P0 of 0.95. The pore size distribution was evaluated with the BJH (Barrett–Joyner–Halenda) model for the desorption branch. Thermo-gravimetric analysis (TGA) was used to study the mass loss during gel carbonization by heating with 2 °C min1 up to 600 °C under nitrogen (METTLER-TOLEDO TGA 851e). Information about the species evolved during the thermal treatment were assessed by temperature programmed desorption coupled with mass spectrometry technique (TPD-MS). In a typical measurement, a small quantity of gel

(1.5 mg) is placed in a quartz tube in a furnace and heattreated in vacuum between of 25 °C and 600 °C with a linear heating rate of 2 °C min1. The sample morphologies were evaluated by transmission electron microscopy (TEM) with a Philips CM200 instrument working at 200 kV. The 3-D reconstructed TEM movies were obtained by mathematical image treatment of hundreds TEM photos obtained with a Jeol JEM-2100F instrument using a tilting angular range between 65° and +67°. The metal particle size distribution was determined from TEM images using the ImageJ software [28]. X-ray powder diffraction (XRD) data were collected with a Philips X’Pert MPD diffractometer with Cu Ka1,2 doublet and a flat-plate Bragg–Brentano theta–theta geometry. The wide angle diffraction data were analyzed using the Le Bail method [29,30] as implemented in the Fullprof program [31]. Peak shape was described by a Thompson-Cox-Hastings modified Pseudo-Voigt function [32,33] with additional asymmetric parameters for low-angle domain peaks (below 2h = 55°) [34], while the background level was fitted with a linear interpolation between a set of 40 to 65 given points with refinable heights. An excluded zone is applied for data before 28°2h. More details about this part can be found in the Supporting Information. The long-range ordering of the materials was studied by Small Angle X-ray Scattering (SAXS) analysis using a Rigaku SMax 3000 equipped with a rotating Cu anode Micromax007HF (40 kV, 30 mA) and OSMIC CMF optics. The detector is a 2D multiwires Gabriel chamber with 120 mm active diameter. The collected q domain is ranging from 0.07 to 1.60 nm1. We use the Open Source ImageJ [28] software for the SAXS treatment of different images from our X-rays 2D detector. Different plugins developed at the CEA (French Alternative Energies and Atomic Energy Commission) are available to open image and make several treatments like radial averaging via a user friendly toolbar. The unit cell parameter, a0 was calp culated using the formula 2Æd100/ 3 [35].

2.4.

Hydrogen sorption measurements

Hydrogen excess sorption properties were determined by measuring the pressure-composition isotherms (PCI) at 298 K up to 9 MPa hydrogen pressure. The PCI curves were recorded using a manual volumetric device (Sievert’s method) equipped with calibrated and thermostated volumes and pressure gauges. The samples were enclosed in a stainless steel sample holder closed with a metal seal. Before any sorption measurements, the samples were outgassed under secondary vacuum at 423 K for 12 h. The sample holder was immersed in a thermostated water bath maintained at 298 K, and high purity hydrogen (6 N) was introduced step by step up to 9 MPa. The pressure variations due to both gas expansion and hydrogen sorption are measured after reaching thermodynamic equilibrium, usually in the range of minutes. The real equation of state for hydrogen gas was used from the program GASPAK V3.32.15. Sample volume correction is derived from density measurement obtained with a helium UltraPyc 1200 Quantachrom pycnometer. The measured densities of materials are in the range 1.5–1.8 (±0.1) g cm3. All capacities reported here in

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wt.% are determined with respect to the sample weight after outgassing.

3.

Results and discussion

Porous carbon materials with embedded Ni, Co and NiCo nanoparticles in their frameworks were synthesized by selfassembly of glyoxal–pholoroglucinol, Pluronic F127 and metal nitrates in ethanol medium under acidic conditions. The classical synthesis involved the mixture of carbon, metal salt precursors and surfactant, separation of the polymer-rich phase from the solution and the curing and carbonization of the recovered gel. When performing the synthesis in this way, the pH of the initial solution containing the carbon precursors and the surfactant is strongly acid (0.5) and slightly increases to 0.9 when the metal precursor solution (having pH= 6) is added. The pH modification is accompanied also by the modification of the solution color from colorless to deep blue or green–yellow in the case of Co and Ni, respectively. As the initial color of the Co and Ni salt aqueous solution was pink or green respectively, hence corresponding to [Co(H2O)6]2+ and [Ni(H2O)6]2+ complexes, the color modification is related to the replacement of H2O groups with Cl following the next Eqs. (1) and (2): ½CoðH2 OÞ6 2þ þ4HCl ) ½CoCl4  ½NiðH2 OÞ6  green

2

þ2H2 O þ 4H3 Oþ

ð1Þ

þ2H2 O þ 4H3 Oþ

ð2Þ

deep blue

Pink 2þ

þ4HCl ) ½NiCl4 

2

greenyellow

The present form of Ni and Co complexes in the solution is not favorable for creating specific interactions by hydrogen bonding with the ethylene oxide (EO) units of F127. For this reason, the major part of the metal precursor remains in the supernatant solution (which is further discarded) and not bonded to the polymer-gel. Consequently, the main encountered problem was related to the quantity of metal introduced in the carbon which is significantly lower than the theoretical expected one, as clearly shown by TGA measurements performed in air (Fig. S1a, Supporting Information). This problem was already mentioned in the literature [24,27,36] for the synthesis conducted in acid conditions contrary to the basic conditions (formaldehyde based systems) where interactions occur between the hydroxo metal complexes and the surfactant and high metal loading can be achieved [23,37]. To overcome this inconvenient we propose a modified synthesis pathway where the metal precursor salt is incorporated after the gel formation, by stirring the mixture until complete evaporation of the solvent. This approach allows complete incorporation of the desired quantity of metal in the carbon matrix (Fig. S1b, Supporting Information). Besides this great advantage, the morphology of the carbon changes from worm-like structure to an ordered carbon morphology by introducing the precursor after gel formation, as revealed by TEM pictures (Fig. S2, Supporting Information). To explain the observed modifications several factors were taken into account. It is known the pH is an important factor which determines the polymerization rate of the phenolic resin and induce the formation of ordered mesostructures through hydrogen bonding or Columbic interactions between the

263

phenolic resins, surfactant, metal precursors salt and catalyst (usually HCl) [19,37]. Therefore, the pH of the solutions at different states of the reaction was measured. After phase separation, the supernatant solution having a pH of about 0.5 was discarded and the remaining gel was diluted in ethanol. The pH in this case is slightly increased to 0.7 and further to 1.7 by the introduction of the salt solution (pH = 6). The color of the solution is pink and green, respectively for Co and Ni systems implying the formation of [CoCl(H2O)5]+ and [NiCl(H2O)5]+ complexes. Hence, the higher pH, and the presence of metals in complexes positively charged favors the interaction by hydrogen bonding with the surfactant and induces the self-assembly of components into ordered hexagonal structures. The Ni particles are more visible in this case and they show a good dispersion (Fig. S3, Supporting Information). The long range ordered structure and the formation of Ni metallic particles is confirmed by SAXS and XRD analysis, respectively (Fig. S3, Supporting Information). Another modification induced by this synthesis route is related to the textural properties. The surface area and the porous volume are rather similar, slightly higher for the composites obtained by introducing the salt after gel formation but significant difference is noticed in the mean pore size which decreases from 9 nm to 6 nm. Taking into account the advantages of the as-proposed synthesis pathway, all the hybrid materials further discussed in the work are prepared accordingly. The carbonization of the as-prepared Ni0.50Co0.50 and Ni0.50Co0.50 (citric acid) gels was investigated in situ by TGA (Fig. 1 left) and TPD-MS (Fig. 1 right) analysis. A total mass loss of about 80 wt.% is observed by TGA, slightly higher when citric acid is used during the synthesis. For both samples, this is superior to carbon gel (75 wt.%, data not shown), and can be related in the case of [email protected] to the consumption of carbon to reduce the NixCoyOz to NixCoy and in addition for [email protected] (citric acid) probably due to the decomposition of citrate, respectively. Two main weight loss steps are noticed between 80–150 °C and 300–400 °C and a small weight loss around 200–250 °C. Further details about the species evolved in these temperature ranges are provided by TPD-MS measurements (Fig. 1 right). The same main peaks are detected by this technique in good agreement with the TGA, though the temperature corresponding to the main desorption is shifted toward lower temperature due to the fact that the TPD-MS desorption curves are recorded under vacuum contrary to TGA in inert atmosphere. In the first step (<100 °C), several species such as H2 (m/z = 2), H2O (m/z = 18) and ethanol (m/z = 31) and fragments of hydrocarbons, i.e., methyl ion, CH3-(m/z = 15), ethyl ion, C2H5-(m/z = 29) are desorbed. For Ni0.50Co0.50 (citric acid) the water desorption is much important than for Ni0.50Co0.50 which can be probably related to the to the polyesterification reaction between the carboxyl group from citric acid and the hydroxyl groups of triblock copolymer. This step represents the curing step usually made before the thermal treatment, which allow the polymer framework to reinforce and for the metal precursor to be well fixed in the polymer matrix. In a much smaller extent, the desorbed species found around 100 °C are also

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1.0x10

Desorption rate (mol g-1s-1)

(a)

100 Weigth loss (wt.%)

6 7 ( 2 0 1 4 ) 2 6 0 –2 7 2

80 60 40 20

Ni0.50Co0.50 Ni0.50Co0.50-citric acid

0

(c)

m/z=2 m/z=15 m/z=18 m/z=28 m/z=31 m/z=43

-9

8.0x10

-9

6.0x10

-9

4.0x10

-9

2.0x10

0.0

0

100

200

300

400

500

600

0

100

1.6x10

231°C 106°C 1/C

-0.004 93°C 363°C

-0.008 Ni0.50Co0.50

Ni0.50Co0.50-citric acid 360°C

-0.012 0

100

200 300 400 Temperature (°C)

500

600

300

400

(d)

-1 -1

Desorption rate (mol g s )

(b)

0.000

200

500

600

Temperature (°C)

-8

m/z= 2 m/z=15 m/z=18 m/z=28 m/z=31 m/z=43

-8

1.2x10

-9

8.0x10

-9

4.0x10

0.0 0

100

200

300

400

500

600

Temperature (°C)

Fig. 1 – TGA weight loss (a) and derivative weight loss (b) curves and TPD-MS desorption profile of Ni0.50Co0.50 (c) and Ni0.50Co0.50-citric acid (d) gels (A colour version of this figure can be viewed online.)

visible at higher temperature (200 °C) and could correspond to the hydrogen bonding braking of the precursors. At higher temperature (350 °C) an important gas desorption is remarked, the main species evolved being the water, CO and/or NO (m/z = 28), ethyl ion, propyl ion (m/z = 43) and methyl ion. These species result of the thermal decomposition of the triblock polymer Pluronic 127. When the temperature is superior to 400 °C desorption of H2, H2O and CO is noticed suggesting the organization of the carbon. Hence, similar species are evolved from the two gels but their quantities are rather different. These differences between the gels prepared with or in the absence of citric acid can induce distinct final characteristics of these hybrids. In a first step, the Ni and Co content in the alloy was varied and the properties of the obtained carbon hybrid materials were evaluated. The Ni and Co content in the hybrid was determined by elemental analysis and is 4.68 wt.% and 4.29 wt.%, respectively, for the single Ni or Co hybrid material. For the NiCo alloys, the following compositions are obtained: Ni0.26Co0.74, Ni0.50Co0.50 and Ni0.74Co0.26. Adding the chelating agents during the synthesis does not modify the metal alloy content. It is important to emphasize that these quantities are very close to the calculated ones, hence the proposed synthesis pathway allow to introduce the desired quantity of metal without any losses, which represent a great advantage in controlling the composition of the hybrid material. The hybrid materials are denoted C@NixCoy where x and y stands for the atomic composition of Ni and Co respectively in the alloy (see Table 1).

The wide-angle XRD patterns of C, C@Ni, C@Co and C@NixCoy hybrid materials are depicted in Fig. 2. An identification search match via Diffrac + EVA [38] applied to the C@Co materials lead to two allotropic phases: hexagonal beta-cobalt (ICDD PDF N°01-089-4308) and cubic cobalt (ICDD PDF N°00-015-0806). The later phase is usually observed for high temperature treatments. Regarding the C@Ni materials, the identification process leads to a single inorganic Ni phase (ICDD PDF N°00-004-0850), which is isotypic with cubic cobalt. In the case of C@NixCoy composites the XRD patterns are mainly characterized by 3 strong peaks as well as for the C@Ni materials and, following the Ni/Co ratio or chelating agent, very weak peaks from hexagonal Co phase. To have a closer view on the peak positions, a zoom in the XRD patterns between 50° and 54° 2h on (2 0 0) reflection is presented in Fig. 2b. As it can be noticed the peaks of C@NixCoy materials are shifted compared to that of pure C@Co (cubic) and C@Ni materials. Increasing the Ni content in the alloy gradually shifts the peak position towards closer values of pure C@Ni materials. This indicates the successful formation of Ni–Co  ˚ ) and solid solutions. The nickel phase (Fm 3 m, a = 3.5238 A  ˚ the Co one (Fm 3 m, a = 3.5447 A) form a solid solution by substitution (atomic radii very similar). The lattice parameter values strongly depend on the initial composition of the metal mixture: as it is richer in Ni, the lattice parameter of the solid solution gets closer to that of Ni. The Le Bail method [29,30], i.e.: profile refinement without structure constraint was used to get precise unit cell parameters and information about microstructural aspect. In the initial stage of the refinement process, when C@NixCoy compounds including both cubic Co

SBET is the total surface area determined by the BET method. VT, Vmeso and Vmicro are respectively, the total pore volume, mesopore volume and micropore volume. DP is the mesopore diameter p calculated by means of the BJH method from the desorption branch of N2 isotherm; a0 is the unit cell parameter calculated by SAXS, being a0 = 2Æd100/ 3 (where the d100 is the d-spacing of the (1 0 0) reflexion) and the wall thickness is calculated using the equation twall = a0  DP for hexagonal p6mm structures. All the textural properties values are expressed by gram of composite.

9.2 7.3 7.4 7.1 7.5 7.4 9.3 5.8 18.1 13.1 12.5 12.5 12.7 13.0 15.9 10.6 15.71 11.31 10.87 10.87 10.99 11.23 13.78 9.22 8.9 5.7 5.1 5.5 5.2 5.6 6.6 4.8 0.38 0.38 0.35 0.40 0.37 0.34 0.34 0.34 0.86 0.50 0.41 0.43 0.43 0.47 0.49 0.44 1.24 0.88 0.76 0.83 0.80 0.81 0.83 0.78 745 788 747 772 780 724 704 747 – 4.68 – 1.32 2.35 3.09 2.36 2.51 C C@Ni C@Co C@ Ni0.26 Co0.74 C@ Ni0.50Co0.50 C@ Ni0.74Co0.26 [email protected] (citric acid) [email protected] (oxalic acid)

– – 4.29 3.68 2.36 1.07 2.42 2.56

twall nm a0 nm d100 nm Dp Nm Vmicro cm3 g1 Vmeso cm3 g1 VT cm3 g1 SBET m2 g1 Co wt.% Ni wt.% Sample name

Table 1 – Composition, textural and structural properties of carbon@nanoparticles nickel–cobalt hybrid materials, determined by elemental analysis, nitrogen adsorption and SAXS analysis, respectively.

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265

and Ni phases were studied, two independent Co and Ni cubic phases were introduced. The corresponding unit cell parameters were refined independently. Systematically, both the unit cell parameters always converge toward a similar value. This behavior is in accordance with a simple observation of the powder diffraction data: a non negligible microstructural effect involves a noticeable peak broadening and it is not possible to separate cubic Co reflexion from Ni ones. For example, the (2 0 0) reflexions for Ni and Co cubic phases are only separated from 0.33°2h (CuKa1). The experimental FWHM (Full Width at Half Maximum) observed for the (2 0 0) reflexion in the C@NixCoy materials is ranging from 0.35(1) to 0.62(1)°2h. Thus it is not possible to split both cubic Ni and Co phases, hence, all further refinements were studied with one Ni–Co cubic solid solution instead of two independent Ni and Co cubic phases. This gives a quick idea about the peak broadening due to microstructural effects (Fig. S6, Supporting Information). The chemical composition heterogeneities may also partially explain the broadening of XRD peaks. EDX measurements (not shown) realized on [email protected] individual particles show that the Ni–Co alloys chemical composition is not homogeneous throughout the nanoparticles even if the entire sample composition is confirmed at atomic level.Beside the metal peaks contribution, a broad peak around 23° corresponding to the (0 0 2) diffraction for graphite-like carbon is observed along with another peak around 43° corresponding to the (0 0 4) reflection of typical disordered carbon. This result indicates that the metal particles are well crystallized in a poorly graphitized carbon matrix. No other peaks corresponding to impurities or metal oxides or carbides are detected. This suggests the decomposition of the metal precursor salt into its oxide and its further reduction to metallic state took place during the thermal treatment performed at 600 °C. The approach used for the Le Bail refinement is detailed in the Supporting information along with an example for one C@NixCoy hybrid material (Fig. S7). This highlight that a too simple approach with application of the Scherrer formula to extract particle size information may be misused here. Main refinement results are listed in Table 2, gathering solid solution unit cell parameters, particles apparent size and reliability factors. The determined apparent particle size for Ni, Co and NiCo alloys is comprised between 21.5 and 25.5 nm. When the synthesis is performed in the presence of chelating agents (Fig. 1c and d), the metal alloy diffraction peaks become larger. This have a positive effect on the apparent particle size which significantly decreases (12–13 nm) in the presence of citric acid and oxalic acid. This behavior can be related to the complexation between the metal cation (Ni2+,Co2+) with citric acid and/or oxalic acid which forms metal citrates and oxalates that are characterized by higher decomposition temperature (250–370 °C) compared to metal nitrate (56 °C). During the carbonization step the delay in the metal salt decomposition favors the inhibition of particle growth. Similar behavior was recently reported for Fe2O3 nanoparticles embedded in mesoporous carbon synthesized with acetyl acetone as chelating agent [23]. The long range ordered structure of the materials was investigated by SAXS analysis (Fig. 2). All materials containing Ni, Co and NiCo are characterized by three well-resolved diffraction peaks indexed as 10, 11 and 20 of p6mm symmetry

266

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#

(a)

(b)

C@Ni

#

Intensity (a.u.)

Intensity (a.u.)

#

[email protected] [email protected] [email protected] C@Co

*

°* *

[email protected] C@Ni 0.50Co0.50 C@Ni 0.74Co0.26

°*

°

C@Ni

* C@Co

C

C 10

20

30

40

50

60

70

80

90

50

51

(220)

[email protected] -citric acid-

Intensity (a.u.)

(200)

Intensity (a.u.)

[email protected] -citric acid-

[email protected] -oxalic acid-

[email protected] -oxalic acid-

[email protected]

20

40

53

(111)

(d)

(111)

(c)

52

2θ (°)

2θ (°)

60

[email protected]

80

2θ (°)

43

44

45

46

2θ (°) Fig. 2 – Wide-angle XRD patterns of C@NixCoy hybrid materials with (a and b) different Ni and Co amounts and (c and d) same Ni and Co amount but synthesized in the presence of chelating agents. For b and d figures, the peak positions are corrected from all positions aberration (i.e., sample displacement, sample transparency and the zero of the diffractometer) (A colour version of this figure can be viewed online.)

indicating the ordered 2-D hexagonal structure. The as-made carbon in the absence of metal salt precursor exhibits only one poor resolved peak. The unit cell parameters (a0) were calculated and the values are reported in Table 1. For the C@Co and the C@Ni composites, a0 are 12.5 nm and 13.1 nm respectively. As for the C@NixCoy nanoalloy hybrid materials the a0 values are 12.5, 12.7 and 13 nm for [email protected], [email protected] and [email protected], respectively. These values increase with the amount of Ni in the alloy and are getting closer to that of the C@Ni materials. Hence, the metal (cation in the precursor) can influence slightly the carbon long range structure. Contrary, the anion can have a much greater influence on the structure. As seen in Fig. S5 (Supporting

Information), when the acetate anion is used instead of nitrate, the carbon becomes less organized and the morphology changes as well. The lattice parameter increases from 12.5 nm (C@Ni) to 13.4 nm for C@Co materials synthesized using cobalt nitrate and cobalt acetate, respectively. When chelating agents are introduced during the synthesis with the aim of downsizing the nanoalloy particles, the SAXS pattern shows a decrease in peak intensity when oxalic acid is used and almost peak disappearance when citric acid is employed (Fig. 3b). This suggests a partial to total degradation of the ordered structure. Moreover, their scattering peak positions shift toward higher and lower q value position for the oxalic and citric acid synthesized materials respectively.

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6 7 (2 0 1 4) 2 6 0–27 2

Table 2 – Main results from XRD powder patterns profile refinement. Hybrid material

Nb. of phases

C@Ni C@Co

1 2

[email protected]

2

[email protected] [email protected] [email protected] Citric acid

1 1 2

[email protected] Oxalic acid

1

a b c

Apparent crystallite size nm

25.5 CoHex:24.3 CoCub:22.9 CoHex:6.1 SSol:15.5 21.5 24.0 CoHex:6.3 SSol: 13.1 12.0

Reliability factorsa,b

˚ Cell parameters A

3.5241(1) a = 2.5062(1), c = 4.0741(4) 3.5439(3) a = 2.5054, c = 4.0893c 3.5382(3) 3.5268(1) 3.5265(1) a = 2.5054, c = 4.0893c 3.5296(1) 3.5329(1)

Rp %

Rxp %

v2

16.2 23.3

8.56 15.3

1.15 1.61

28.0

16.1

1.41

14.6 23.5 19.3

8.99 12.0 11.9

1.32 1.15 1.22

18.4

11.3

1.31

Conventional Rietveld R-factors for points with Bragg contribution. Definitions following [39]. Cell parameters not refined and fixed to PDF N°01-089-4308 for hexagonal b-Co SSol for Ni–Co solid solution.

(a)

[email protected]

(b)

[email protected]

[email protected] Co 0.50 Ni50Co50@C -oxalic acid-

[email protected]

C@Co

Intensity (a.u.)

Intensity (a.u.)

- oxalic acid -

[email protected] Co 0.50 Ni50Co50@C -citric acid- citric acid -

C@Ni

C

C@Ni Ni50Co50@C 0.50 Co 0.50 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-1

q (nm )

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-1

q (nm )

Fig. 3 – SAXS patterns of C@NixCoy materials with (a) different Ni and Co amounts and (b) same Ni and Co amount but synthesized in the presence of chelating agents (A colour version of this figure can be viewed online.)

The calculated cell parameter, a0 (Table 1) are 10.6 nm and 15.9 nm corresponding to the contraction, respectively, expansion of the cell. Therefore the variation is of about 11.7% and 21% reported to the reference material [email protected]. The TEM images of several C@NixCoy hybrid materials along with their particle size distribution curves are presented in Fig. 4(a–d). In the absence of chelating agents (Fig. 4a–c), regardless the metal used, all materials exhibit stripe-like and hexagonal pore morphology. The distance between the parallel channels of carbon is ranging between 10.8 nm and 11.6 nm. These observations are in good agreement with the SAXS results showing an ordered 2-D hexagonal mesostructure (Fig. 3 and Table 1). The metal particles can be observed as black quasi-spheres shapes having a homogeneously distribution in the carbon matrix. The Co particles exhibit the highest particle size (mean size diameter centered around 42 nm) while the Ni particles the smallest ones (mean size diameter centered around 32 nm). When

Ni–Co solid solution is formed, they reveal intermediate sizes between Ni and Co single phases. Clearly, the metal precursor salt type has an influence on the particle size. It should also be noticed that the particle size is significant higher than the carbon pore size (in-set Fig. 5). Insights about this aspect were assessed by TEM tomography performed on C@Co and C@Ni composites (Fig. S8). The 3-D TEM reconstructed movie showing the penetration in the carbon structure depth (by steps of few nanometers) (Fig. S8a) proves the presence of the particles inside the carbon porosity. Interestingly, it can be clearly observed (Fig. S8b) that the carbon pores are interconnected which allows the growth of the particles over several pores explaining why the particle size exceed the pore size. When chelating agents are used (Fig. 4d) the particle size are considerably downsized and their size is less homogeneously as emphasized by the particles size histogram. The introduction of the citric and oxalic acid during the synthesis triggers a disorganization of carbon structure regularity, a

268

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6 7 ( 2 0 1 4 ) 2 6 0 –2 7 2

Mean size: 42 nm

Frequency

C@Co

0

40

80

120

160

200

Particle size (nm)

C@Ni

Frequency

Mean size: 32 nm

10

20

30 40 Particle size (nm)

50

60

Mean size: 45 nm [email protected]

Frequency

Mean size: 28 nm

20

30

40 50 60 70 Particle size (nm)

Frequency

Mean size: 14 nm

80

90

[email protected] citric acid

Mean size: 28 nm

0

10

20 30 40 Particle size (nm)

50

Fig. 4 – TEM images and the particle size distribution of selected carbon/metal hybrid materials (a) C@Co (b) C@Ni (c) [email protected] (d) [email protected] acid. White circles figured here are apparent crystallites sizes obtained from XRD powder pattern profile fitting (A colour version of this figure can be viewed online.)

worm-like structure being formed instead of hexagonal structure. The observed morphology cannot be explained by the solution pH modification triggered by chelating agents addition since they have low pH (1.5 and 0.5 for citric and oxalic acid, respectively). However, both contains carboxylic acid groups which can play a role of catalyze in the polymerization process [40] but also form complexes with the metal precursors and in this case their interaction with the surfactant facilitate rather the formation of phloroglucinol/glyoxal polymer gel which is randomly distributed around the F127 template micelles through hydrogen bonding instead of assembling with the PEO segments to give ordered structures. The material particle sizes determined by TEM are compared with the crystallite size obtained from XRD powder pattern profile refinements which is represented on the TEM

pictures as white circles. It is important to keep in mind the following remarks in order to avoid any confusion in the results interpretation. The powder pattern profile refinement allows determination of crystallite sizes (and/or micro strain). Size analysis using profile widths gives the crystallite or domain apparent size, which is not directly comparable with results from other techniques. Within the particle (sometimes named ‘‘grains’’) domains may exist which are separated by domain walls, disturbing the coherent scattering of the X-rays. These domain walls are not commonly visible by TEM. The apparent crystallite size obtained from XRD is the size of the domain over which the diffraction is coherent. This is why often and in the present case, the size obtained from TEM analysis is larger than the size resulting from X-ray powder data profile analysis.

6 7 (2 0 1 4) 2 6 0–27 2

(a)

0.8 0.6

3

dV/dD (cm / g nm)

800

3

Adsorbed N2 Volume (cm /g)

CARBON

600

0.4 0.2 0.0

400

2

4

6 8 10 12 14 Pore Diameter (nm)

16

C C@Ni C@Co [email protected] [email protected] [email protected]

200

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

3

400

(b)

0.8 dV/dD (cm / g nm)

3

Adsorbed N2 Volume (cm /g)

600

0.6 0.4 0.2 0.0 2

4

6 8 10 12 14 Pore Diameter (nm)

16

200

[email protected] [email protected] (oxalic acid) [email protected] (citric acid)

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0) Fig. 5 – Nitrogen adsorption/desorption isotherms of C@NixCoy materials with (a) different Ni and Co amounts and (b) same NiCo amount but synthesized in the presence of chelating agents; full marks (adsorption) and open marks (desorption) (A colour version of this figure can be viewed online.)

A second remark should be also made here: even when the crystallite shapes can be considered as approximately constant (i.e.: isotropic, possibly modeled as a sphere), the size can be dispersed according to mono- or multi-modal distributions (as it is the case for the single phased [email protected] hybrid material). If the size distribution is multimodal, a single ‘‘mean size’’ number is of little use, and possibly misleading. The nitrogen adsorption was used to investigate the textural properties of materials. The N2 adsorption/desorption isotherms and their corresponding pore size distribution are shown in Fig. 5. For comparison purposes the carbon isotherm is also presented. All the isotherms are of type IV according to the IUPAC classification [41] and exhibit a H1 hysteresis loop indicating the N2 condensation in the mesopores. The increase in the nitrogen adsorbed amount at low pressures (P/P0 = 0–0.1) indicates the presence of micropores while the well-defined and steep steps visible around P/P0 = 0.4–0.8 describes the nitrogen filling of the mesopores due to the capillary condensation. All isotherms are characterized by similar quantities of adsorbed nitrogen. No

269

significant difference from textural point of view is noticed between the hybrid materials due probably to the same type of precursor used (nitrates) and also to their similar quantities. From the textural properties listed in Table 1 one can observe that the hybrid materials have similar textural properties being characterized by high surface area (750 m2 g-1), high porous volume (up to 1 cm3 g-1) and developed microporous volume (up to 0.4 cm3 g-1). It should mentioned that the microporous volume is one of the highest compared to about 0.15 cm3 g-1 usually reported for carbon hybrid materials synthesized by soft-template method [24,25,27]. Compared to carbon (without particles) the hybrid textural properties values (total porous volume, mesoporous) are much lower. This can be explained in some extent by the presence of metallic particles having higher density which induce the increase of carbon density from 1.5 cm3/g up to 1.8 cm3/g (for C@NixCoy hybrids). The particle localization inside the pores (as shown by TEM tomography) can also explain the lower textural properties. However, if we subtract the contribution of metal and express these values in cm3/g of carbon and not per g of composite (as presented in Table 1) the hybrid textural values become only slightly higher and still remain much lower than those of carbon. Hence, the presence of the metal can only partly explain the observed results. This suggests again that the carbon obtained in the absence of the metal salts is significantly different from textural point of view than that synthesized by incorporation of metal salts. In this case, a strictly influence of the metal on the textural properties of carbon is not possible to be done. The pore size distribution derived from the desorption branches using the BJH model reveals the presence of uniform mesopores. In the case of carbon, the mean pore size is higher (10 nm) compared to that of carbon@metal materials (6 nm) (in-set Fig. 5a). When chelating agents are used, the textural properties remains rather comparable, but the pore size of C@Ni50Co50 shifts to lower (4 nm) and higher values (8 nm), respectively, when oxalic acid and citric acids are used (in-set Fig. 5b). This can be due probably to the size of the introduced molecules and their interactions with the surfactant and phenolic resin. It is well known that the pore size can be modified by tuning the surfactant hydrophobic chain size, or by using swelling agents [21]. Usually, hydrophobic organic hydrocarbons are employed which can be solubilized inside the hydrophobic regions of the surfactant micelles leading to the micelle swelling and pore enlargement. However, in the case of swelling agents such as alcohol having polar group is believed to be located at the hydrophilic–hydrophobic interface and interacts with both PPO and PEO segments. Nevertheless, it should be considered that citric acid and oxalic acid are bonded to the metal precursors thus forming metal complexes. In the case of citric acid, the complex size is larger than for oxalic acid and moreover it posses larger amounts of carboxylic groups which can interact with the surfactant by hydrogen bonding in a differ manner, explaining probably the higher pore size observed. However, the mechanism of pore formation in the presence of metal complexes was never studied and is certainly more complicated and requires further supplementary investigations. The calculated values of pore wall thickness are comprised between 5.5 and 5.9 nm (Table 2), less than the size of

270

CARBON

6 7 ( 2 0 1 4 ) 2 6 0 –2 7 2

H2 excess capacity (wt%)

0.30 C@Ni C@Co [email protected] [email protected] [email protected]

0.25 0.20 0.15 0.10 0.05

(a)

0.00 -1

0

1

2

3

4

5

6

7

8

9

10

11

Pressure (MPa)

H2 excess capacity (wt%)

0.30 [email protected] [email protected] (oxalic acid) [email protected] (citric acid)

0.25 0.20 0.15 0.10 0.05

(b)

0.00 -1

0

1

2

3

4

5

6

7

8

9

10

Pressure (MPa)

Fig. 6 – Hydrogen sorption isotherms at room temperature and high pressure for C@NixCoy materials with (a) different Ni and Co amounts and (b) same Ni and Co amount but different particle sizes (A colour version of this figure can be viewed online.) particles (15–40 nm) suggesting that the particles can penetrate into the mesoporous walls and are partially exposed in the mesopore channels. Fig. 6 shows the hydrogen adsorption/desorption isotherms for all hybrid materials performed at room temperature and high pressure. All hydrogen sorption PCI curves show an almost linear Henry law behavior, which is typical for physisorption of H2 molecules on carbonaceous materials at room temperature. Moreover, the PCI curves exhibit reversible adsorption/desorption. The excess capacity values are in the range 0.2–0.3 wt.% at 9 MPa, in good agreement with previous studies on carbon based materials with similar textural properties [7,42–47]. Materials containing nanoparticles of Co show the largest hydrogen sorption capacity while those having Ni particles while the lowest one. This result is in very good agreement with previously reported increases of both the capacity at room temperature for Co doped ordered mesoporous carbons [42] and the isosteric heat of adsorption for Co doped carbon aerogels [43,45]. Similarly, the presence of Pd nanoparticles embedded into an ordered mesoporous carbon template also raises the enthalpy of H2 physisorption [7]. As for the C@NixCoy composites, the values are in between those of Ni and Co but the two of them with small amount of Co (26–50 wt.%) are very close to pure Co. Probably, small

quantities of Co in the NiCo alloy might induce similar hydrogen storage capacities as pure Co. This can be a positive issue when expensive noble metals are required to be used. Taking into consideration that the composite textural properties (specific surface area and microporous volume) are analogous this might be explained by the quantity of metallic components in the alloy and probably their atomic arrangement. However, more investigations on other metal systems are required to validate these observations. In the case of PCI curves of [email protected] materials with different particle sizes (Fig. 6b), the trend is less obvious. For these materials, the specific surface area and microporous volume are comparable but differences in carbon mesopore sizes and organization exist as already shown in Figs. 5b and 3b, respectively. As the hydrogen adsorption take place mainly into the micropores (<2 m) [48] the mesopore size and ordering cannot explain the higher hydrogen sorbed amounts by the hybrids obtained in the presence of chelating agents. They rather have an important role in the nanoconfinement of the nanoparticles, i.e., in stabilizing the nanoparticles and controlling the particles growth and dispersion thus avoiding the coalescence [7]. It seems however, that smaller particle sizes are slightly more favorable for hydrogen sorption. This result is in good agreement with our previous works realized on C@Pd hybrid materials [5,47]. In the case of very small nanoparticles the thermodynamic and kinetic properties are modified compared to bulk material due to the increase of the ratio between the surface and bulk atoms allowing in this way better performances to be achieved. Therefore, the development of hybrid materials with small confined nanoparticles by employing simple, faster and environmental friendly synthesis pathways is still of great challenge.

4.

Conclusions

NiCo nanoalloy embedded in highly ordered carbons is synthesized in this work by a new synthesis pathway which allows a good control of the alloy quantity, composition and size. This approach involved the self-assembly of environmental friendly carbon precursors with a structure directing agent followed by metal precursor salt addition. Thus, micro/mesoporous carbon/NiCo hybrid materials having a 2-D hexagonally arranged pore structure, uniform pore size, high surface area and porous volume are obtained. The NiCo nanoparticles have sizes comprised between 30 and 40 nm depending on the Ni and Co ratio, which are uniformly distributed in the carbon network. When the synthesis is performed in the presence of citric and oxalic acid, no significant change in the textural properties of as-obtained NiCo/hybrid is observed, except the pore size which modifies considerably. On the contrary, the use of such chelating agents induces a positive effect in downsizing the particle size to 15 nm. The 2-D hexagonally pore structure is only partly or no longer preserved, a worm-like mesoporous structure being obtained instead. The hybrid materials adsorb hydrogen at room temperature and the hydrogen storage capacity is depending on the metal type, alloy composition and particle size. Furthermore, the advantage of the present

CARBON

6 7 (2 0 1 4) 2 6 0–27 2

hybrids lays in the use of less expensive 3d transition metals than noble elements that may bring comparable effect on hydrogen sorption properties. Beyond the clean energy fields, these hybrids may witness a wider range of applications from adsorption, catalysis to biomedicine as nanoalloys generally possess enhanced specific properties due to synergistic effects.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.09.089.

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