Chemical Synthesis and Self-Assembly of Hollow Ni/Ni2P Hybrid Nanospheres

Irene Zafiropoulou,† Konstantinos Papagelis,‡ Nikos Boukos,† Angeliki Siokou,§

Dimitris Niarchos,† and Vassilios Tzitzios*,†

NCSR “Demokritos”, Institute of Materials Science, Agia ParaskeVi 15310, Athens, Greece, Department ofMaterials Science, UniVersity of Patras, 26504 Rio Patras, Greece, and Foundation of Research andTechnology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes, Stadiou str.Platani, P. O. Box 1414, Patras 26504, Greece

ReceiVed: October 24, 2009; ReVised Manuscript ReceiVed: March 12, 2010

Elevated interest is gathered around hybrid nanostructured materials, due to their combined physical-chemicalproperties. In this article, the synthesis and characterization of nanoparticles consisting of both NixPy andmetallic Ni are reported. The nanoparticles have a core/shell structure, and they most probably combine bothsemiconducting and magnetic properties. To the best of our knowledge, this is the first time hybrid materialwith Ni, as well as 3D self-assembly of such material, is reported.

1. Introduction

The study of matter on the nanoscale size came to boostmaterials science, introducing materials with improved or evennovel physicochemical properties. Over the past few years animportant research direction in nanomaterials synthesis is theexpansion from single-component nanoparticles to hybrid nano-structures, with discrete domains of different materials arrangedin a controlled fashion. Thus, different functionalities can beintegrated, with the dimension and material parameters of theindividual components optimized independently. Transition-metal phosphides are very attractive candidates for catalytic,electronic, and magnetic applications. Especially diluted mag-netic semiconductors, when doped with magnetic ions, are ofexceptional scientific and technological interest and, at the sametime, one of the most difficult kinds of materials to prepare incolloidal form.1 Conventional transition-metal phosphides areprepared via methods which involve high temperature processesfor the reduction of phosphates or solvothermal reactions.2-10

Recently, solution-phase routes have been developed for thesynthesis of transition-metals phosphides, mainly based on theuse of inorganic metal salts, metal alkyls, or metal carbonylsas metal sources and tri-n-octylphosphine (TOP) or tri-n-octylphosphine oxide (TOPO) as phosphorus sources.11,12 Thesemethods are based on the ability of the TOP and TOPOmolecules to act as P atom donors, through thermal decomposi-tion at temperatures above 330 °C. It is worth to mention thatin some casessespecially when using TOP as a phosphorussourceshollow Ni2P particles can be formed.13,14 Very recentlythe use of white phosphorus as a P source for the synthesis ofnickel phosphide nanoparticles has been reported.15 Ni2P is well-known to be one of the most efficient hydrotreating catalysts.16

The synthesis of a hybrid material and even more with hollowmorphology may lead to a new class of catalytic systems withmoderate behavior, let alone the development of novel opticaland optoelectronic technologies, through the hybridization of a

magnetic and a semiconducting material. Herein we report ona moderate chemical method for the synthesis of ultrafine,surface functionalized hollow Ni/Ni2P nanoparticles, whichreadily form 3D superlattices, using triphenylphosphine (TPP)as a phosphorus source. To the best of our knowledge suchhybrid nanostructured particles are synthesized for the first time.

2. Experimental Section

In a typical experimental procedure 0.5 mmol of Ni(acac)2

is dissolved in 20 mL of oleyl amine containing 2 mmol ofTPP at 100 °C. The reaction mixture is then heated at 330 °Cand kept at this temperature for 1 h. The color of the reactionmixture changes from green to dark green and finally black.The nanoparticles are precipitated, after cooling at roomtemperature, by adding ethanol and separated by centrifugation.

The materials were characterized with X-ray diffraction usinga Siemens D500 diffractometer, with Cu KR radiation (λ )1.5418 A), while the magnetic properties were measured at roomtemperature with a VSM PAR Model 155. TEM images werecollected using a Philips CM20 operated at 200 kV microscope.For the preparation of the TEM samples, the materials weredispersed in chloroform and drops of the solution were allowedto dry on a carbon-coated Cu grid. The photoemission measure-ments were carried out at room temperature, in a commercialUHV chamber, with base pressure 5 × 10-10 mbar, equippedwith a hemispherical electron energy analyzer (SPECS LH-10).The unmonochromatized Al KR line at 1486.6 eV and constantanalyzer pass energy of 97 eV, giving a full width at half-maximum (fwhm) of 1.7 eV for the Au4f7/2 peak, were usedin all XPS measurements. The XPS core level spectra wereanalyzed with a fitting routine which decomposes each spectruminto individual mixed Gaussian-Lorentzian peaks after a Shirleybackground subtraction. Regarding the measurement errors, forthe XPS core level peaks, we estimate that for a good signal-to-noise ratio, errors in peak positions can be (0.1 eV.

3. Results and Discussion

In this work the presence of both phosphines and therespective phosphinoxides with Ni(acac)2 in oleylamine wasstudied. Metallic hcp Ni spherical nanoparticles or hcp Ninanorods are synthesized, when TOPO (trioctylphosphine oxide)

* To whom correspondence should be addressed. E-mail: tzitzios@ims.demokritos.gr. Telephone: +30 210 6503321. Fax: +30 210 6519430.

† Institute of Materials Science.‡ University of Patras.§ Institute of Chemical Engineering and High Temperature Chemical

Processes.

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10.1021/jp910160g 2010 American Chemical SocietyPublished on Web 04/08/2010

or TPPO (triphenylphosphine oxide) are used, respectively. TheTEM images and XRD patterns of the hcp Ni nanoparticles andnanoneedles with common starting point are shown in Figure1S (in the Supporting Information). The XRD patterns clearlyindicate the hcp Ni structure without the presence of anyphosphides. Therefore phosphine oxides seem to be inappropri-ate etching agents for the synthesis of phosphides structures, inthe current reaction conditions, contrary to TOP. The latter, avery reactive molecule, leads to the formation of Ni2P hollownanoparticles as revealed from the TEM images (Figure 2S inthe Supporting Information), which is also in agreement withthe literature.13 TOP seriously etched the metallic Ni, whichwas formed at first, probably through the formation of Ni-TOPcomplexes. As a result, the reaction leads to the synthesis ofhollow nickel phosphides particles. TPP, on the other hand, is

quite more stable than TOP, and our first intention was to useit in order to control the rate of the phosphidation reaction andtherefore synthesize the hybrid nanomaterial. After testingseveral experimental parameters, the appropriate synthesisconditions were standardized as described in the ExperimentalSection. The particles form very stable colloidal solutions innonpolar organic solvents, such as toluene, hexane, andchloroform. The materials were characterized by X-ray diffrac-tion (XRD), X-ray photoelectron spectroscopy (XPS), andtransmission electron microscopy (TEM) in order to investigatetheir structural and morphological characteristics. As far as themechanistic path of the reaction is concerned, we found thatmetallic Ni nanoparticles are initially formed, since the Niprecursor (Ni(acac)2) decomposes in oleylamine at relativelylow temperature to give Ni2+ ions, which can be readily reduced.The phosphine also partially decomposes, but at higher tem-perature (therefore after the metallic Ni formation), acting likeP precursor. P reacts with the Ni atoms at the surface of thenanoparticles, and NiP (which is a very stable compound) forms.The XRD pattern and the magnetic hysteresis loop (as inset)of the material after a 10 min reaction are shown in Figure 1,where the formation of metallic fcc Ni particles is clearlyindicated. Ferromagnetic behavior of the particles is verifiedby magnetic measurement (inset in Figure 1) at room temper-ature, which reveals saturation magnetization of 26 emu/g.

The final reaction product (reaction time: 1 h) is hybrid Ni/Ni2P nanoparticles, with hollow morphology as confirmed bythe TEM and HRTEM images shown in Figure 2a-c. Thecoexistence of hexagonal metallic Ni and nickel phosphide, withthe dominant phase being the hexagonal Ni2P, is also clearly

Figure 1. XRD pattern and magnetic hysteresis loop at roomtemperature (inset) of the reaction product after a 10 min reaction.

Figure 2. TEM images (a-c, inset HRTEM image) and XRD pattern (d) of hollow Ni/Ni2P nanoparticles synthesized by the reaction of Ni(acac)2

in oleyl amine, in the presence of TPP. In the XRD pattern the peaks corresponding to hcp metallic Ni are indicated by an asterisk.

Hybrid Self-Assembled Hollow Spheres J. Phys. Chem. C, Vol. 114, No. 17, 2010 7583

shown by the XRD pattern (Figure 2d). The hollow hybrid Ni/Ni2P nanoparticles have spherical shape and about 15 nm meandiameter, with very good uniformity. Furthermore, the func-tionalization of the particles with the organic molecules (oleylamine and TPP) allows them to form organized close packed3D superlattices, just through the slow solvent evaporation ofthe colloidal chloroform solution. Careful analysis leads to theconclusion that these structures are actually three-dimensionalsuperlattices possessing an fcc packing geometry. These 3Dsuperstructures consist of multiple layers (the array is so thickthat only the edges could be clearly imaged with the microscope)and extend to large areas. The excellent self-assembling behaviordemonstrated by this material can be attributed to the presenceof the aromatic phenyl groups of the TPP. The geometry andthe size of these groups cause the nanoparticles to stackhomogeneously in three directions and large areas. This is notthe case for the octyl groups of the TOP molecule, which leadsto the formation of hollow spheres but not to their self-organization. We believe that, by choosing the adequate solutionconcentration and quantity on the TEM grid (which is understudy), fully covered extended areas of self-assembled core/shell nanoparticles can be produced.

Figure 3 shows the XP P2p and Ni2p3/2 spectra of the samplesynthesized in the presence of TPP, as-received and after 10min of mild Ar+ sputtering. The P2p spectrum has been analyzedinto three components, each one of them a P2p3/2/P2p1/2 doubletwith spin-orbit splitting ∆ ) 0.87 eV and intensity ratio 0.5.The binding energy (BE) values mentioned here are referred tothe P2p3/2 component. The peak at BE ) 129.5 is attributed toP atoms in NiP [a], while the peaks at higher BE values areattributed to oxides which are more pronounced in the as-received sample. The Ni2p spectra are analyzed in a similarway to P2p the spin-orbit splitting being ∆E ) 17.4 eV forthe component that corresponds to Ni atoms in NiP or metallicNi (Ni2p3/2, BE ) 852.7 eV) and ∆E ) 18.4 eV for NiOx(Ni2p3/2, BE ) 855-856 eV). The single and wide peak at 861.9eV is a satellite of the Ni2p3/2 component of the oxide. Thefact that it is wide indicates the contribution of other chemicalstates of Ni atoms. After 10 min of mild sputtering, the intensityof the low BE component (NiP or Ni) increases considerablywhile the components attributed to oxides decrease. The satelliteshifts to BE ) 860.2 eV, an energy value that has been by manyauthors assigned to the satellite originating from Ni atoms inNiP.17

Quantitative analysis was performed using the intensities ofthe P2p and Ni2p spectra normalized by their atomic sensitivityfactors.18 It was found that Ni/P before and after sputtering is2. The apparent Ni excess is an indication of the existence ofNi clusters in the NiP particles.

The magnetic hysteresis loop at room temperature of thehybrid nanoparticles is shown in Figure 4. The material showsferromagnetic behavior with 0.1 emu/g saturation magnetizationand 490 Oe coercive field. The low value of the saturationmagnetization, in comparison with that of hcp Ni nanoparticlesin the same size range found in the literature, is due to theconversion of a the large amount of metallic Ni to Ni2P.Nevertheless, the ferromagnetic behavior still remains.

4. Conclusions

In conclusion, a simple synthesis method for metallic Nidoped Ni2P hollow particles is reported. The particles have auniform hollow spherical morphology, demonstrate ferromag-netic behavior, and readily form well ordered extended 3-Dsuperstructures (simply via solvent evaporation). The use ofTPPsmainly through the three phenyl groups of the moleculesnot only allows the control over Ni phosphidation and leads toa hybrid magnetic/semiconducting material but also provides itwith remarkable self-assembling behavior. This methodologyis expected to lead to new opportunities for the synthesis ofnovel hybrid magnetic/semiconducting nanostructures. In fact,we have also synthesized Fe2P and Co2P nanostructures usingthe same procedure. Research is in progress in order to studymore thoroughly the morphological, chemical, and physicalproperties of these materials.

Supporting Information Available: TEM images and XRDpatterns of the hcp Ni nanoparticles and nanorods (Figure 1S);TEM images of hollow Ni2P nanoparticles synthesized in thepresence of TOP (Figure 2S). This material is available free ofcharge via the Internet at http://pubs.acs.org.

References and Notes

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Figure 3. XPS spectra for P2p (a) and Ni2p3/2 (b) of the samplesynthesized in the presence of TPP, as-received (bottom spectra) andafter Ar+ sputtering (top spectra).

Figure 4. Room temperature magnetic hysteresis loop from hollowhybrid Ni/Ni2P nanoparticles synthesized by the reaction of Ni(acac)2

in oleyl amine in the presence of TPP.

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