RESEARCH PAPER

Facile and rapid one-pot microwave-assisted synthesisof Pd-Ni magnetic nanoalloys confined in mesoporous carbons

Alicia Martínez de Yuso & Jean-Marc Le Meins &

Yassine Oumellal & Valérie Paul-Boncour &

Claudia Zlotea & Camelia Matei Ghimbeu

Received: 1 September 2016 /Accepted: 18 November 2016# Springer Science+Business Media Dordrecht 2016

Abstract An easy and rapid one-pot microwave-assisted soft-template synthesis method for the prepara-tion of Pd-Ni nanoalloys confined in mesoporous carbonis reported. This approach allows the formation of meso-porous carbon and the growth of the particles at the sametime, under short microwave irradiation (4 h) comparedto the several days spent for the classical approach. Inaddition, the synthesis steps are diminished and nothermopolymerization step or reduction treatment beingrequired. The influence of the Pd-Ni composition on theparticle size and on the carbon characteristics was inves-tigated. Pd-Ni solid solutions in the whole compositionrange could be obtained, and the metallic compositionproved to have an important effect on the nanoparticlesize but low influence on carbon textural properties.Small and uniformly distributed nanoparticles were con-fined in mesoporous carbon with uniform pore size dis-tribution, and dependence between the nanoparticle sizeand the nanoalloy composition was observed, i.e., in-crease of the particle size with increasing the Ni content(from 5 to 14 nm). The magnetic properties of the mate-rials showed a strong nanoparticle size and/or composi-tion effect. The blocking temperature of Pd-Ni

nanoalloys increases with the increase of Ni amountand therefore of particle size. The magnetization valuesare smaller than the bulk counterpart particularly for theNi-rich compositions due to the formed graphitic shellssurrounding the particles inducing a dead magnetic layer.

Keywords Pd-Ni nanoalloy . Mesoporous carbon .

One-pot synthesis . Microwave synthesis . Magneticproperties . Soft template . Carbon-metalnanocomposites

Introduction

During the past years, the synthesis of magnetic nano-particles has been intensively developed due to the in-creasing number of applications in biosensing field(Fenzl et al. 2016), medical applications like targeteddrug delivery (Kumar and Mohammad 2011) or contrastagent for magnetic resonance imaging (Ding et al. 2016),environmental applications (Tang and Lo 2013) and ca-talysis (Kharisov et al. 2014). However, common mag-netic nanoparticles, including metals like Fe, Ni, Co, andMn, are unstable towards oxidation and dissolution inacid environment. Therefore, the development of stablemagnetic nanoparticles in all conditions is imperative.

Among all magnetic materials, carbon-supportednanoparticles exhibiting magnetic properties have re-ceived increasing attention in the past decade. The ad-dition of magnetic nanoparticles into a carbon matrixprovides hybrid materials being easily separated andrecovered from reaction medium after their application,

J Nanopart Res (2016) 18:380 DOI 10.1007/s11051-016-3682-9

A. Martínez de Yuso : J.<M. Le Meins :C. Matei Ghimbeu (*)Université de Strasbourg, Université de Haute-Alsace, Institut deScience des Matériaux de Mulhouse, CNRS UMR, 7361-15 rueJean Starcky, 68057 Mulhouse, Francee-mail: camelia.ghimbeu@uha.fr

Y. Oumellal :V. Paul-Boncour :C. ZloteaInstitut de Chimie et des Matériaux Paris Est, UMR 7182,CNRS-UPEC, 2-8, rue Henri Dunant, 94320 Thiais, France

simplifying the separation process and reducing theenergy consumption (Ruan et al. 2016). Mesoporouscarbons decorated with magnetic nanoparticles can alsoact as selective catalysts and adsorbents (Zhai et al.2009). The porous and conductive nature of carbonallows extending the application range of metallic nano-particles by combining advantages of both materials andby synergistic effects. Furthermore, carbon materials arehighly resistant to chemicals and thermal treatmentswhich can provide the stability of the nanoparticles bytheir confinement inside the carbon material, avoidingthe sintering or the oxidation of the nanoparticles duringthe application or the thermal regeneration process ofthe material (Baaziz et al. 2013). Besides, the confine-ment of the nanoparticles in the carbon matrix allowscontrolling the growth of the particles providing a finecontrol of the final nanoparticle size.

Bimetallic catalysts based on Pd (Pd-M) can exhibitenhanced activity and stability compared to Pd catalysts;hence, research efforts have been focused on nanoparti-cles such as Pd-Pt and Pd-Au. Nickel is more abundantin natural sources and therefore cheaper than Pd andother elements frequently combined with Pd. For thisreason, recently, Pd-Ni bimetallic catalysts gain an in-creased interest in several fields of applications.

Pd-Ni nanoparticles have proved to be a good alter-native for Pt and pure Pd nanoparticles in direct meth-anol and formic acid fuel cells (DMFCs and DFAFCs)resulting in a cheaper material, with good catalyticproperties and higher stability and durability (Aminet al. 2014a; Li et al. 2011; Matin et al. 2014). Thesecatalysts have been tested successfully for methanoloxidation reaction (Amin et al. 2014a, 2014b; Kumaret al. 2008; Liu et al. 2009, Shobha et al. 2003), ethanoloxidation (Ding et al. 2013; Shen et al. 2010; Zhanget al. 2011), formic acid electrooxidation (Du et al.2010; Li et al. 2011; Matin et al. 2014), allyl alcoholoxidation (Jin et al. 2012), hydrogen oxidation (Aleskeret al. 2016; Bakos et al. 2015), and oxygen reduction(Ramos-Sánchez et al. 2008). Other applications such asselective hydrogenation (Hou et al. 2015) or hydrogenstorage (Campesi et al. 2009a) have been also testedwith improved performances.

The selection of the support is of great impor-tance since it influences on the final metallic struc-ture and size/dispersion of the nanoparticles (Houet al. 2015). Most reported works preparing Pd-Ninanoparticles use carbon black as support (Aminet al. 2014a, 2014b; Jin et al. 2012; Liu et al.

2009; Matin et al. 2014; Zhang et al. 2011), butalso, multi-wall carbon nanotubes (Ding et al.2013; Li et al. 2011), metal oxides (Hou et al.2015), and ordered mesoporous carbons (Campesiet al. 2009a) have been employed.

Various approaches have been used to prepare Ni-Pd nanoparticles, such as impregnation (Campesiet al. 2009a; Hou et al. 2015), ultrasonic-assistedchemical reduction (Li et al. 2011), chemical reduc-tion (Amin et al. 2014a; Jin et al. 2012; Liu et al.2009; Ramos-Sánchez et al. 2008; Shen et al. 2010;Zhang et al. 2011), electrochemical deposition (Duet al. 2010; Kumar et al. 2008; Qiu et al. 2010;Shobha et al. 2003), pyrolysis in ionic liquid (Dinget al. 2013), sonochemistry (Matin et al. 2014), ormicrowave-assisted chemical reduction (Amin et al.2014b; Shviro and Zitoun 2013). Also, carbon coat-ing has been widely employed due to its thermal andchemical stability, and its application on magneticnanoparticles has been proved under different con-ditions (Qiu et al. 2010; Ruan et al. 2016).

Such approaches involve the use of a synthesizedcarbon or a commercial carbon support followed bythe addition of metallic salts and a reduction step,therefore multi-step synthesis approaches. Amongthese routes, the microwave-assisted synthesis hasbeen used for the preparation of metallic nanoparti-cles supported on carbon providing nanostructureswith small particle size and narrow size distribution(Antonetti et al. 2012; Tsuji et al. 2005). Not manyworks have been reported on the microwave-assistedsynthesis of Pd-Ni nanoparticles, but this methodallows obtaining small nanoparticles (∼3 nm) asreported by Amin et al. (2014b). Microwave ap-proach provides a rapid and homogeneous heatingof the sample enhancing the reaction and facilitatingthe uniform nucleation of the sample as well as ashorter crystallization time, very short induction pe-riod, and moreover, it is energy efficient and envi-ronmental friendly (Tsuji et al. 2005). Regarding tothe synthesis of metallic nanoparticles by micro-wave, the polyol process is the most commonemployed one (Chuang et al. 2011; Harish et al.2012). In this method, a high boiling point liquid,generally ethylene glycol, acts as solvent for themetal precursor and as reducing agent. However,the high boiling points and viscosities of the sol-vents used in polyol synthesis present a disadvan-tage during the isolation of metal nanoparticles.

380 Page 2 of 14 J Nanopart Res (2016) 18:380

In previous works, we prepared bimetallicnanoalloys by impregnation of a carbon host withmetal solutions followed by a reduction step (Zloteaand Latroche 2013), but an ulterior H2 thermal treat-ment was required. An alternative to the abovemulti-step routes for the preparation of carbon/metal nanoparticle materials is the one-pot approach.Recently, we developed one-pot synthesis routes formesoporous carbon containing bimetallic nanoparti-cles Pd-Co (Matei Ghimbeu et al. 2016) and Ni-Co(Matei Ghimbeu et al. 2014) by evaporation inducedself-assembly (EISA) and phase separation ap-proaches. This consists in the use of phenolic resinprecursors, a surfactant template and metallic saltsdissolved together in ethanol followed by casting inpetri dishes (for EISA) of by aging for several days(phase separation). Polymer films or gels are obtain-ed, and subsequent thermal treatment allows thesimultaneous formation of the mesoporous carbona-ceous matrix (OMC) with in situ generated nanopar-ticles with no reducing treatment needed. On thecontrary, the synthesis time is still long, from1 day to several days depending on the method. Inaddition, a thermopolymerization step taking at least12 h previous to the carbonization step is required tocross-link the phenolic resin polymer.

In this work, we report Pd-Ni based nanoalloys con-fined in mesoporous carbon by a green, simple, and fastone-pot microwave-assisted synthesis approach. Themicrowave proposed synthesis allows decreasing thesynthesis time to 4 h compared to the classical approach,and in addition, no thermopolymerization step or reduc-tion treatment is required.

Different Pd-Ni compositions were synthesized(PdxNi100-x, where x = 90, 75, 50, and 25) and onesample containing only Ni to study the effect of theintroduction of different amount of metallic salts in thefinal nanoparticles and carbon properties. Mesoporouscarbon materials with surface area around 800 m2 g−1

decorated by Pd-Ni and Ni nanoparticles were obtained.Pd-Ni alloys were formed in the whole compositionrange for the first time to our knowledge (Ding et al.2013; Jin et al. 2012; Matin et al. 2014). Small andhomogeneous particle sizes (5–14 nm) were found forthe nanoparticles confined in the carbon matrix. Theconfined particles remain in the metallic state beingprevented to be oxidized even at 150 °C. The hybridmaterials present magnetic properties strongly affectedby their size and/or composition.

Methods

Chemicals

Triblock copolymer Pluronic F127 [poly(ethylene ox-ide)-blockpoly(propylene oxide)-block-poly-(ethyleneoxide, PEO106PPO70PEO106, Mw = 12,600 Da)],phloroglucinol (1,3,5-benzentriol, C6H6O3), glyoxylicacid monohydrate (C2H2O3∙H2O), glyoxal aqueous so-lution (40% C2H2O2), citric acid (C6H8O7∙H2O), andethanol absolute (C2H6O) were purchased from Sig-ma–Aldrich. Nickel (II) nitrate hexahydrate[Ni(NO3)2·6H2O] and palladium(II) nitrate hydrate(Pd(NO3)2∙H2O) were purchased from StremChemicals. The chemicals were used as received with-out any further purification.

Material synthesis

Mesoporous C@Pd-Ni hybrids were synthesized byone-pot microwave-assisted method. A mixture of atriblock copolymer (Pluronic F127), phenolic resin(phloroglucinol–glyoxal or phloroglucinol–glyoxylicacid) and metallic salts were dissolved in ethanol/water,and the reaction was induced using a CEM discover SPmicrowave synthesizer with a PC control. The instru-ment consists of a single microwave mode, self tuningcavity, operating with a continuous power generatorcapable of supplying an irradiation power up to 300 Wwith an automatic control of the sample temperature.The reaction was carried out under stirring in an open100-mL flat-bottomed flask.

In a typical synthesis, 0.81 g of block copolymerPluronic F127, 0.41 g of phloroglucinol were dissolvedin 9 mL of ethanol absolute in a flat-bottomed flask andthe solution was stirred at room temperature until com-plete dissolution. The proper amount of aqueous metal-lic salts of Pd(NO3)2∙H2O, and Ni(NO3)2·6H2O solvedin 1 mL of water, was added to this solution drop bydrop to ensure homogeneous mixture. After few mi-nutes of mixing, 0.05 g of citric acid was added andmixed until dissolution followed by the incorporation of0.405 mL of glyoxal (or 0.305 g of glyoxylic acid). Theresulting solution was stirred for 10 min and MW-irradiated up to constant temperature with a maximumirradiation power of 25 W. During the microwave treat-ment, the solvent is evaporated at 40 °C for 3 h (step 1)and a phenolic-resin gel is obtained which is furtherprogressively heated up to 80 °C (steps 2, 3, and 4) in

J Nanopart Res (2016) 18:380 Page 3 of 14 380

order to polymerize the material according to theMW synthesis conditions summarized in Table 1.Finally, the material was thermally treated at 600 °Cfor 2 h under Ar flow with a heating rate of 2 °C/min.The total final metal quantity was set to ∼5 wt.% withrespect to carbon framework (which was ∼0.4 g).Different PdxNi100-x nanoparticle compositions wereprepared: C@Pd90Ni10, C@Pd75Ni10, C@Pd50Ni50,C@Pd25Ni75, and one sample with pure Ni nanoparticles.

For comparison reasons, a sample with Pd50Ni50composition was prepared by the classical EISAmethodwhich consists in the disposal of the same preparedsolution in petri dishes followed by evaporation (12 h),thermopolymerization (80 °C/12 h), and thermal treat-ment (600 °C/2 h). More details about the EISA ap-proach can be found elsewhere (Matei Ghimbeu et al.2014).

Material characterization

Nitrogen adsorption analysis was performed on aMicromeritics ASAP 2420. Prior to any measurement,the samples were degassed overnight at 300 °C in vac-uum on the degassing port followed by 4 h out-gassingon the analyze port to eliminate the backfill gas. TheBrunauer–Emmett–Teller (BET) surface area (SSA)was calculated from the linear BET plot in the relativepressure 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 microporevolume from the total pore volume of N2 adsorbed atrelative pressure P/P0 of 0.95. The average mesoporesize distribution was evaluated with the Barrett–Joyner–Halenda (BJH) model for the adsorptionbranch. X-ray powder diffraction (XRD) data werecollected with a Philips X’Pert MPD difractometerwith Cu Kα1,2 doublet and a flat-plate Bragg–Brentano theta–theta geometry. XRD patterns underoxygen at different temperatures were measured in

Anton Paar HTK-1200 oven, and the spectra werecollected by a PANalytical PW3040 difractometer.The sample morphologies were evaluated bytransmission electron microscopy (TEM) with a JeolARM-200F instrument working at 200 kV. EDXmapping was obtained with a JED 2300 detector.X-ray photoelectron spectroscopy (XPS) wasperformed with a VG Scienta SES 200–2 spectrom-eter equipped with a monochromatized Al Kα X-raysource (1486.6 eV) and a hemispherical analyzer.The pass energy was 100 eV.

Magnetic characterization

The magnetic characterization was performed on thesamples in epoxy resin by the help of Quantum DesignPhysical Properties Measurement System (PPMS) oper-ating up to 9 T. The magnetic susceptibility was mea-sured by cooling the samples in zero field and measur-ing them upon heating with a field of 100 Oe (zero fieldcooled (ZFC)) and then upon cooling in the presence ofthe applied field (field cooled (FC)). The magnetizationcurves versus field were measured at 4.2 and 300 K. Themagnetization values were calculated considering thetotal mass of composites and corrected from the dia-magnetic signal of the carbon support and the emptysample holder. Saturation magnetization (MS) was esti-mated from the extrapolation of the linear part of themagnetization curve at zero field.

Results and discussion

Synthesis condition optimization

Synthesis conditions were optimized by the preparationof different samples with Pd50Ni50 composition with nomodification of the metallic precursor. Two differentformulations were studied using different cross-linkerto form the phenolic resin, glyoxal, or glyoxylic acid.The classical EISA and the microwave approach werecompared as well.

The materials obtained with glyoxylic acid by EISA(Fig. 1a) present smaller particle size, 9.6 nm, than theone prepared by microwave with 14.2 nm (Fig. 1b).However, the replacement of glyoxylic acid by glyoxalin the microwave synthesis allows obtaining significant-ly smaller particle size, 5.8 nm, than the previous twomaterials (Fig. 1c). It should be mentioned the

Table 1 Microwave synthesis conditions

Step Temperature (°C) Time (min)

1 40 180

2 60 15

3 75 5

4 80 30

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advantage of the microwave method which allows re-ducing the synthesis steps and time (4 h) compared toEISA method (more than 1 day), and in addition, small-er and well-dispersed particles can be obtained.

Hence, for investigating the influence of metal com-position on material characteristics, the glyoxal wasselected as cross-linker.

Influence of the alloy composition

XRD patterns of the materials are presented in Fig. 2a.The material containing only Ni (Ni100) shows threemain diffraction peaks at 44.5°, 51.8°, and 76.4° whichcorrespond to the (1 1 1), (2 0 0), and (2 2 0) diffractionplanes of the face-centered-cubic (fcc) crystalline struc-ture, in agreement with pure Ni (Gorria et al. 2006). Forall the Pd-Ni composite materials, the main diffractionpeaks are shifted to lower 2θ angles, due to the Pdincorporation. A closer view of the peak positions isshown in Fig. 2b. The presented zoom between 38° and46° 2θ shows a shift in the peaks to lower 2θ angles withrespect to the reported values for single-phase Ni indi-cating the formation of Pd-Ni alloy (Amin et al. 2014a,2014b; Campesi et al. 2009a; Du et al. 2010; Li et al.2011; Matin et al. 2014; Ramos-Sánchez et al. 2008).Higher amount of Pd induces a higher shift to lowerangles and the peaks become narrower, suggesting anincrease of the crystallite size. The carbon networkpresents a broad peak around 20–25° indicating aturbostatic carbon structure.

The evolution of lattice parameter with the Pd–Nialloy composition is shown in Fig. 2c. The lattice

parameter of the fcc structure of Ni (a = 3.524 Å) andPd (a = 3.8907 Å) are also represented for comparisonpurposes. A linear relationship between the lattice pa-rameter and the Pd-Ni composition is observed; thisvalue increases with the Pd content and is related tothe higher atomic radius of Pd (137 pm) compared to Ni(125 pm). The resulting values of the lattice parameterfor the Pd-Ni nanoalloy samples lie between those of Pdand Ni, confirming the formation of alloys within thewhole composition range, in agreement with thecompletely miscible bulk Pd-Ni phase diagram and theVegard’s law for substitution alloys.

The average crystallites size values calculated byScherrer formula showed small crystallites for the Pd-Ni alloys (Table 2). An increase of the crystallite sizefrom 3.7 to 6.2 nm is induced by increasing the Nicontent from Pd75Ni25 to Pd25Ni75. A higher crystallitesize is obtained for pure Ni particles, around 21 nm.Similar trend was observed for Pd-Ni or Ni nanoparti-cles made by impregnation approach followed by hy-drogen reduction (Oumellal et al. 2016). On the con-trary, decrease of particles size with the increase of Nicontent was reported by ultrasound route followed bychemical reduction (Jin et al. 2012; Li et al. 2011).

Therefore, the size evolution depends on the synthe-sis pathway and the involved parameters (type of carbonused, metal precursors, reduction route, etc.). In the caseof one-pot route, a very important parameter is the typeof precursor used and its properties. Usually, nitratesprovide smaller particle size than chlorides (MateiGhimbeu et al. 2015). When Pd chloride is used tosynthesize Pd-Co alloys, an increase in particle size

(a) (b)

(c)

Fig. 1 TEM images of samplePd50Ni50 prepared with differentcross-linkers and synthesisapproaches: a glyoxylic acid byevaporation (EISA); b glyoxylicacid by microwave; c glyoxal bymicrowave

J Nanopart Res (2016) 18:380 Page 5 of 14 380

was noticed with the increase of Pd chloride amount(Matei Ghimbeu et al. 2016), while the Pd nitrate in-duces lower Pd-Ni sizes in the present work. This be-havior may be related to different interactions of suchmetal salts with the phenolic resin, template, and theirdifferent behavior during the one-pot thermal annealing.

To confirm the Pd-Ni alloy formation, EDXmappingwas performed on the sample Pd50Ni50. The results,compiled in Fig. 3, show the existence of Pd and Ni inthe same particles which confirms the nanoalloy forma-tion. Quantitative EDX analysis revealed a compositionfor the sample Pd50Ni50 of 49.2 at.% of Ni and 50.8 at.%of Pd in atomic concentration (average of 18 nanoparti-cles studied from three different pictures). These valuesfit well with the theoretical Pd50Ni50 composition.

The TEM pictures and the particle size distribution ofthe C@PdNi materials with different compositions aredisplayed in Fig. 4.

Uniform distribution of the Pd-Ni nanoparticles inthe carbon matrix can be observed for all the studiedcompositions. The carbon structure is similar for all thematerials presenting a worm-like morphology with uni-form pore size in accordance with previous works(Matei Ghimbeu et al. 2016), indicating that the intro-duction of different metallic salts does not modify thecarbon network structure. The average particle size ofthe materials decreases with the incorporation of Pd,from 14.3 nm for Ni nanoparticles to 4.8 nm forPd75Ni25. For rich Pd alloys (Pd50Ni50, Pd75Ni25,Pd90Ni10) similar particle size values were found(Table 2) indicating a low influence of the Pdincorporation.

The nitrogen adsorption/desorption isotherms andthe corresponding pore size distribution for the Pd-Ninanoalloy samples and the C@Ni sample are depicted inFig. 5. All the samples present a type IV isothermaccording to IUPAC classification and exhibit a H1hysteresis loop indicating the N2 condensation in themesopores. The increase in the amount of nitrogenadsorbed at low pressures (P/P0 = 0–0.1) indicates thepresence of micropores while the well-defined and steepsteps visible around P/P0 = 0.4–0.6 describes the nitro-gen filling of the mesopores, related to the mesopore

(3

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10 20 30 40 50 60 70 80 90

38 40 42 44 46 0 25 50 75 100

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NiLattice p

aram

eters (Å

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Pd at% in PdNi alloy

(a)

(b) (c)

Fig. 2 aXRD patterns of C@Pd-Ni materials having different Pdand Ni compositions. b Zoom in the XRD pattern between 38° and46° 2θ. c Evolution of fcc lattice parameters with the nanoalloycomposition

Table 2 Textural and structural properties of magnetic mesoporous carbon Pd-Ni hybrids

Materials SSA (m2 g−1) Vt (cm3 g−1) Vmicro (cm

3 g−1) Vmeso (cm3 g−1) TEM particle

size(nm)XRD crystallitesize (nm)

Pd90Ni10 791 0.49 0.36 0.13 4.9 4.8

Pd75Ni25 763 0.47 0.35 0.12 4.8 3.7

Pd50Ni50 780 0.46 0.35 0.11 5.8 4.2

Pd25Ni75 775 0.50 0.35 0.15 6.7 6.2

Ni100 832 0.58 0.38 0.20 14.3 20.9

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formation by removal of the pluronic template. Allisotherms are characterized by rather similar quantitiesof adsorbed nitrogen, and only the sample Ni100 shows aslightly higher amount of adsorbed nitrogen.

The main textural properties of the samples are com-piled in Table 2. No significant differences were foundbetween the textural properties of C@Pd-Ni composi-tions. A surface area around 775 m2 g−1 characterizes allthe materials, a total pore volume between 0.46 and0.58 cm3 g−1, high microporous contribution of approx-imately 0.35 cm3 g−1, and a lower mesoporous contri-bution around 0.13 cm3 g−1. The sample Ni100 presents aslightly higher surface area of 832 m2 g−1 and a higherpore volume, 0.58 cm3 g−1, with a greater contributionof mesoporosity, 0.20 cm3 g−1 and a similar microporecontribution, 0.38 cm3 g−1. According to the adsorptionbranches using the BJHmodel, the pore size distributionreveals uniform mesopores distribution with pore diam-eter from 3 to 4 nm, with a clear increase with the Nicontent and therefore with the average particle size ofthe Pd-Ni nanoalloys (Fig. 5b).

The small difference in the carbon porosity is relatedto the ratio between the Pd and Ni salts in the solution.Both cations and anions may affect the self-assemblyprocess of Pluronic template with the phenolic resin, and

the anions have greatest influence than cations. For bothmetals, nitrates are used; therefore, the observed poros-ity development may be linked to the Ni cation. Thismay suggest that the nickel salt is somehow located inthe vicinity of Pluronic template inducing modificationof the porosity concomitantly to the particle growthduring the thermal treatment. Therefore, increasing theNi content, the particle size increases and the pore sizeas well, in agreement with the trend observed on Pd-Ptalloys confined in mesoporous carbon (Martínez deYuso et al. 2016).

A thermogravimetric analysis under air was per-formed at 2 °C/min up to 800 °C on the samplesNi100, Pd50Ni50, and Pd90Ni10 and the TGA and DTGcurves are shown in Fig. 6. The samples present twowell-defined peaks corresponding to the oxidation of thecarbon in the presence of the nanoalloy particles. Inter-estingly, it seems that each peak correspond to one of themetal, first one to Pd and the second one to Ni, suggest-ing different reactivities with the carbonmatrix vs. metaltype and size. We observe that the first peak is shiftedtowards higher temperatures with increasing the Ni con-tent in the material and it merge with the second peak forpure Ni. The smaller particles are more reactive induc-ing oxidation at lower temperatures, 414 °C for

50Ni50 +Ni(a)

(c) (d)

(b)Pd Pd

Pd Ni

Fig. 3 a Dark field TEM and bEDX mapping of the samplePd50Ni50 where b Pd and Nioverlapped contributions, c Pdand d Ni

J Nanopart Res (2016) 18:380 Page 7 of 14 380

0 5 10 15 20 25

14.3 nm

Fre

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Ni100

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6.7 nm

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Pd90

Ni10

(a)

(b)

(c)

(d)

(e)

Fig. 4 TEM images (left) and theparticle size distribution (right) ofC@Pd-Ni hybrid materials: aNi100, b Pd25Ni75, c Pd50Ni50, dPd75Ni25, and e Pd90Ni10

380 Page 8 of 14 J Nanopart Res (2016) 18:380

Pd90Ni10 with size of 4.8 nm compared to 452 °C for Nihaving 21 nm, respectively. Similar trend has beenhighlighted before for Co nanoparticles confined inmesoporous carbon (Matei Ghimbeu et al. 2015). Thesecond peak does not change its position, but instead, itsintensity is increased with increasing Ni content. Thispeak can be related to the oxidation of more graphiticcarbon formed in the presence of higher Ni content aswill be shown further by TEM analysis.

From the remaining residue after the oxidation, theamount of Ni present in the sample Ni100 resulted in4.81 wt.% of Ni content in the material. This value isclose to the theoretical 5 wt.% Ni content of the sample.

The chemical analysis of the Ni100 sample obtainedby XPS analysis revealed a mass concentration as fol-lows: 86.39 wt.% of C 1s, 10.57 wt.% % of O 1s, and0.97 wt.% of Ni 2p3/2. The Ni sample content (0.97%) islow compared to the TGA determined one. Consideringthe XPS analysis conditions (10-nm depth from theanalyzed surface) and the particles size (14 nm), it canbe assumed that the majority of the particles are con-fined in the carbon network surrounded by a graphiticlayer, and therefore not accessible by XPS.

Figure 7a shows a normal Shirley background appliedto the Ni 2p3/2 core level of the Ni100 sample spectrum aswell as its deconvolution in terms of three differentcontributions, Ni°, NiO, and Ni(OH)2. The Ni° contribu-tion, centered at 852.72 eV, and two contributions at853.69 and 855.0 eV, corresponding to NiO andNi(OH)2, respectively, as well a satellite peak at

860.9 eV, were found in good agreement with literature(Biesinger et al. 2009; Grosvenor et al. 2006; Nesbitt

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Fig. 5 a Nitrogen adsorption/desorption isotherms of C@Pd-Ni samples with different compositions and b the corresponding BJHadsorption pore size distribution

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Pd50

Ni50

Pd90

Ni50

Fig. 6 TGA (a) and DTG (b) curves of Ni100, Pd50Ni50 andPd90Ni10 in air atmosphere at a heating rate of 2 °C/min

J Nanopart Res (2016) 18:380 Page 9 of 14 380

et al. 2000; Prieto et al. 2012). The atomic concentrationof nickel in these three different chemical states is 27.45,31.54, and 41.03 at.% for metallic Ni, NiO, and Ni(OH)2,respectively. Considering that these concentrations areonly related to the outer layers of some Ni nanoparticlesthat are not confined, the results suggest the formation ofa core–shell structure where a pure nickel core issurrounded by a NiO + Ni(OH)2 shell (Prieto et al.2012). We have already shown that Pd particles support-ed on carbon form a layer of palladium oxide in thesurface of Pd which has a thickness dependent on theparticle size (Matei Ghimbeu et al. 2011).

HRTEM (Fig. 7b, c) demonstrates the encapsulationof Ni particles in graphitic layers. Catalytic graphitiza-tion occurs for all the samples, and the nanoparticlescatalyze the conversion of the amorphous carbon sup-port into ribbons of turbostatic graphitic carbon accord-ing to the HRTEM pictures. This encapsulated metallicparticles has been previously observed in the literature(Asokan et al. 2015; Hoekstra et al. 2015; Lamber et al.1988) for carbonization temperatures of 500 °C andhigher. Lamber et al. (1988) and recently Hoekstraet al. (2015) suggest attributing the segregation of thenickel carbide into graphitic carbons on the surface ofthe nickel particles due to its higher thermodynamicstability. This is in line with Schaefer et al. (2010)preparing spherical nanoshells of graphite-like carbonby heating Ni3C nanoparticles just above their

decomposition temperature (420 °C). This confinementof Ni in carbon walls protects the particles towardsfurther oxidation.

To confirm the stability of the nanoparticles to-wards oxidation, temperature dependence X-ray dif-fraction was measured (TD-XRD). The materialswere heat-treated under air up to 150 °C, and theXRD pattern was measured at 25, 50, 100, and150 °C. As example, the XRD patterns for thesample Pd25Ni75 are displayed in Fig. 8. Weremarked no change in the peaks position or widthduring the heating from 25 to 150 °C under air,indicating that the nanoparticles are stable and arenot oxidized even at such high temperature. It iswell known that small nanoparticles are not stable,and they are rapidly oxidized. In the present case,the surrounding carbon wall prevents the oxidationof the nanoparticles. Few works demonstrate thatconfinement concept may be useful to preventoxidation of the nanoparticles. Baaziz et al. (2013)reported a low oxidation of Fe3-xO4 nanoparticlesmonodispersed in carbon nanotubes under air expo-sure by XRD measurements, indicating that the sur-rounding CNT walls prevent the excessive oxidationof the nanoparticles. Similar results were reported byTessonnier et al. (2005) for the Co-Fe alloy nano-particles cast inside the CNT channels, where higheroxidative resistance was observed by XRD measure-ments when the material is heated to 150 °C underair, showing high resistance towards air oxidation asno trace of CoFe2O4 was observed even afterheating up to 200 °C.

(a)

(c)

868 864 860 856 852

240

245

250

255

260

Ni°

NiO

Ni(OH)2

CP

S

Binding energy (eV)

(b)

Fig. 7 a Ni 2p3/2 XPS spectra fitted with Ni°, NiO, and Ni(OH)2multiplet envelopes; b, c HRTEM for Ni100

20 30 40 50 60 70 80 90

(2 2

0)

(2 0

0)

(1 1

1)

Inten

sit

y (

a.u

.)

2 Theta (°)

Pd25Ni

75 25°C

Pd25Ni

75 50°C

Pd25Ni

75 100°C

Pd25Ni

75 150°C

Fig. 8 TD-XRD patterns of the Pd25Ni75 sample under oxygen at25, 50, 100, and 150 °C

380 Page 10 of 14 J Nanopart Res (2016) 18:380

Magnetic properties

The ZFC/FC curves and themagnetization versusmagneticfield of the Pd-Ni samples are shown in Figs. 9 and 10,respectively. The magnetic properties are listed in Table 3.

The ZFC curves of Pd-Ni nanoalloys (Fig. 9)show a maximum at the blocking temperature TBbelow which the magnetic moments are blocked infixed direction. The ZFC and FC curves coincide athigh temperature (TFC-ZFC irr), indicating that thesamples behave as superparamagnets.

In the superparamagnetic theory, the TB isdirectly proportional to the volume of the particlevia the effective anisotropy constant (Knobel et al.2008). The TB of Pd-Ni nanoalloys decreases from111 to 16 K with increasing the Pd content,suggesting a shrink of both the volume and theeffective anisotropy constant of the nanoparticles.This finding is in agreement with the reducedaverage particle size with increasing the Pdcontent, as determined by TEM. The TB for pureNi nanoparticles (∼14 nm) is 96 K, which is closeto the value of 95 K previously reported for Ninanoparticles (between 5 and 20 nm) embeddedinto a porous carbon (Campesi et al. 2009b).However, the TB of the pure Ni nanoparticles isslightly smaller than for the Pd25Ni75 nanoalloy(111 K). The latter has smaller size (∼7 nm)compared to Ni100 (∼14 nm), suggesting that bothsize and chemical composition are influencing theblocking temperature of Pd-Ni nanoalloys.

At low temperature, the magnetization curves do notreach the saturation (Fig. 10), irrespective of the chem-ical composition. On the contrary, at room temperature,the magnetization curves are reaching the saturation atmoderate magnetic field.

The calculated MS over the metal content can becompared to the MS previously reported for bulk Pd-Nialloys at low temperature (Cable and Child 1970;Wollan 1968). The saturation magnetizations of thenanoalloys show lower values than the bulk counter-parts with a difference between bulk and nanoscaledmaterials that increases steadily with Ni content. Theagreement between the nanosized and the bulk alloy israther good only for the richest Pd composition, whereasfor pure Ni nanoparticles,MS is almost two times lowerthan bulk Ni. This suggests a size and/or compositioneffect that strongly alters the magnetic properties. Size-dependent magnetic properties have been already re-ported for Ni nanoparticles (Sun and Dong 2002;Zhang et al. 2006). For example, 10 nm Ni nanocrystalsshow a MS around 25 emu/g at 2 K (Sun and Dong

0 50 100 150 200 250 300

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ni

Pd25

Ni75

Pd50

Ni50

Pd75

Ni25

Pd90

Ni10

M (em

u/g

)

T (K)

Fig. 9 FC and ZFC curves for C@Pd-Ni nanoalloys measuredwith an applied field of 300 Oe

-20000 0 20000 40000 60000 80000 100000

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

T=4.2 K

Ni

Pd25

Ni75

Pd50

Ni50

Pd75

Ni25

Pd90

Ni10

)g/

um

e(

M

H (Oe)

-20000 0 20000 40000 60000 80000 100000

-1.0

-0.5

0.0

0.5

1.0

Ni

Pd25

Ni75

Pd50

Ni50

Pd75

Ni25

Pd90

Ni10

)g/

um

e(

M

H (Oe)

T=300 K

Fig. 10 Magnetization curves at 4.2 (above) and 300 K (below)for the C@Pd-Ni nanoalloys

J Nanopart Res (2016) 18:380 Page 11 of 14 380

2002), which is close with our result for Ni nanoparti-cles with size ∼14 nm (31 emu/g).

Both experimental and theoretical (DFT) studieshave proven that the saturation magnetization of Pd-Ni bulk is dependent on the chemical composition(Cable and Child 1970; Kudrnovský et al. 2008;Wollan 1968). The magnetic moments of bulk alloysslightly decrease up to 50 at.% of Pd and show a strongdrop for higher Pd content (Wollan 1968). In thisbinary system, only Ni is magnetic as pure element.However, the d electronic shell of Pd can be easilypolarized by Ni, and thus, Pd atoms carry nonnegligiblemagnetic moment. However, in the present Pd-Ninanoalloys, the magnetization values are much smallerthan the bulk counterpart especially for the Ni-richcompositions. Since the particle size variation amongthe nanoalloys is rather small (between 5 and 7 nm),this important decrease of cannot be accounted by sizeeffect. Another possible explanation might be the for-mation of dead magnetic layers at the surface of thenanoparticles. In this context, we recently proved thatthe formation of an amorphous oxide layer at the sur-faces of Co nanoparticles prepared by similar one-potmethod is mainly responsible for the decrease of mag-netic moment (Matei Ghimbeu et al. 2015). However,this reason can be excluded presently due to the effec-tive embedding of the nanoparticles into the carbonwalls, since XPS showed only few outside particlesthat protect the nanoparticles from air exposure, asdiscussed above. On the other hand, the formation ofan amorphous carbide layer at the surface was alreadyproposed in the literature to explain the reduction of themagnetic moment of Ni nanoparticles coated with car-bon (Sun and Dong 2002). Thus, we suggest that theobserved decrease in MS can be attributed to the partial

carbon dissolution, with carbides and graphitic struc-ture formation, in the surface layer of the Pd-Ninanoalloys importantly increasing with Ni content ashighlighted by TEM.

Conclusions

Pd-Ni alloy nanoparticles confined in mesoporous carbonswere synthesized by a novel one-pot microwave-assistedmethod. The synthesis time could be reduced to few hourscompared to EISA classical approach, and no supplemen-tary thermopolymerization step was required. Pd-Ninanoalloys are obtained in whole composition range andthey are uniformly distributed in the carbon framework.The average Pd-Ni particle size increases with the Nicontent from 4.9 nm for Pd90Ni10 to 14 nm for pure Ninanoparticles. The textural properties of the materials werenot significantly modified by the addition of nanoparticles;only small differences were found for the material withpure Ni.

The particles are confined in the carbon walls andgraphitic shells are formed around the particles when theNi content is increased in the alloy, affording a highstability towards oxidation up to 150 °C.

The magnetic properties of the materials exhibit astrong size and/or composition effect. The ZFC and FCcurves coincide at high temperature, indicating that thesamples behave as superparamagnets. The blockingtemperature of Pd-Ni nanoalloys increases with the in-crease of Ni content and therefore of the particle size.The magnetization values are much smaller than thebulk counterpart especially for the Ni-rich compositionsand related to the presence of graphite dead magneticlayer surrounding the particles.

Table 3 Magnetic properties of C@Pd-Ni nanoalloys (Ms, TZFC max, and TFC-ZFC irr). For comparison, theMs values for Pd-Ni bulk alloysat low temperature are also listed from Wollan (1968)

Sample MS

emu/g composite

MS

emu/g metal

MS bulkemu/g

TB(K)

TFC-ZFC irr(K)

4.2 K 300 K 4.2 K 2 K

Ni 1.49(1) 1.125(1) 31 59 96 180

Pd25Ni75 1.33(1) 0.919 (2) 31 46 111 179

Pd50Ni50 0.83(1) 0.487(2) 16.6 37 76 100

Pd75Ni25 0.87(1) 0.215(3) 15.2 25 28 95

Pd90Ni10 0.59(2) 0.112(1) 9.6 12 16 25

380 Page 12 of 14 J Nanopart Res (2016) 18:380

Acknowledgements The authors thank Prof. Jean Daou for thehelp provided to initiate the microwave synthesis, Dr. Loïc Vidalfor performing the TEM analysis, Gauthier Schrodj for the TGAanalysis, Philippe Fioux for the XPS analysis and the helpfuldiscussion of the results, and Laure Michelin for the in situ XRDanalysis.

Compliance with ethical standards

Funding We acknowledge financial support from the FrenchNational Research Agency (ANR) through the ANR project GEN-ESIS (ANR-13-BS08-0004-02).

Conflict of interest The authors declare that they have no con-flict of interest.

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