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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 of Materials Science, UniVersity of Patras, 26504 Rio Patras, Greece, and Foundation of Research and Technology 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-chemical properties. In this article, the synthesis and characterization of nanoparticles consisting of both NixPy and metallic Ni are reported. The nanoparticles have a core/shell structure, and they most probably combine both semiconducting and magnetic properties. To the best of our knowledge, this is the first time hybrid material with Ni, as well as 3D self-assembly of such material, is reported. 1. Introduction The study of matter on the nanoscale size came to boost materials science, introducing materials with improved or even novel physicochemical properties. Over the past few years an important research direction in nanomaterials synthesis is the expansion from single-component nanoparticles to hybrid nanostructures, with discrete domains of different materials arranged in a controlled fashion. Thus, different functionalities can be integrated, with the dimension and material parameters of the individual components optimized independently. Transitionmetal phosphides are very attractive candidates for catalytic, electronic, and magnetic applications. Especially diluted magnetic semiconductors, when doped with magnetic ions, are of exceptional scientific and technological interest and, at the same time, one of the most difficult kinds of materials to prepare in colloidal form.1 Conventional transition-metal phosphides are prepared via methods which involve high temperature processes for the reduction of phosphates or solvothermal reactions.2-10 Recently, solution-phase routes have been developed for the synthesis of transition-metals phosphides, mainly based on the use of inorganic metal salts, metal alkyls, or metal carbonyls as metal sources and tri-n-octylphosphine (TOP) or tri-noctylphosphine oxide (TOPO) as phosphorus sources.11,12 These methods are based on the ability of the TOP and TOPO molecules to act as P atom donors, through thermal decomposition at temperatures above 330 °C. It is worth to mention that in some casessespecially when using TOP as a phosphorus sourceshollow Ni2P particles can be formed.13,14 Very recently the use of white phosphorus as a P source for the synthesis of nickel phosphide nanoparticles has been reported.15 Ni2P is wellknown to be one of the most efficient hydrotreating catalysts.16 The synthesis of a hybrid material and even more with hollow morphology may lead to a new class of catalytic systems with moderate behavior, let alone the development of novel optical and optoelectronic technologies, through the hybridization of a * 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.
magnetic and a semiconducting material. Herein we report on a moderate chemical method for the synthesis of ultrafine, surface functionalized hollow Ni/Ni2P nanoparticles, which readily form 3D superlattices, using triphenylphosphine (TPP) as a phosphorus source. To the best of our knowledge such hybrid 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 of TPP at 100 °C. The reaction mixture is then heated at 330 °C and kept at this temperature for 1 h. The color of the reaction mixture changes from green to dark green and finally black. The nanoparticles are precipitated, after cooling at room temperature, by adding ethanol and separated by centrifugation. The materials were characterized with X-ray diffraction using a Siemens D500 diffractometer, with Cu KR radiation (λ ) 1.5418 A), while the magnetic properties were measured at room temperature with a VSM PAR Model 155. TEM images were collected using a Philips CM20 operated at 200 kV microscope. For the preparation of the TEM samples, the materials were dispersed in chloroform and drops of the solution were allowed to dry on a carbon-coated Cu grid. The photoemission measurements were carried out at room temperature, in a commercial UHV chamber, with base pressure 5 × 10-10 mbar, equipped with a hemispherical electron energy analyzer (SPECS LH-10). The unmonochromatized Al KR line at 1486.6 eV and constant analyzer pass energy of 97 eV, giving a full width at halfmaximum (fwhm) of 1.7 eV for the Au4f7/2 peak, were used in all XPS measurements. The XPS core level spectra were analyzed with a fitting routine which decomposes each spectrum into individual mixed Gaussian-Lorentzian peaks after a Shirley background subtraction. Regarding the measurement errors, for the XPS core level peaks, we estimate that for a good signalto-noise ratio, errors in peak positions can be (0.1 eV. 3. Results and Discussion In this work the presence of both phosphines and the respective phosphinoxides with Ni(acac)2 in oleylamine was studied. Metallic hcp Ni spherical nanoparticles or hcp Ni nanorods are synthesized, when TOPO (trioctylphosphine oxide)
10.1021/jp910160g 2010 American Chemical Society Published on Web 04/08/2010
Hybrid Self-Assembled Hollow Spheres
Figure 1. XRD pattern and magnetic hysteresis loop at room temperature (inset) of the reaction product after a 10 min reaction.
or TPPO (triphenylphosphine oxide) are used, respectively. The TEM images and XRD patterns of the hcp Ni nanoparticles and nanoneedles with common starting point are shown in Figure 1S (in the Supporting Information). The XRD patterns clearly indicate the hcp Ni structure without the presence of any phosphides. Therefore phosphine oxides seem to be inappropriate etching agents for the synthesis of phosphides structures, in the current reaction conditions, contrary to TOP. The latter, a very reactive molecule, leads to the formation of Ni2P hollow nanoparticles as revealed from the TEM images (Figure 2S in the Supporting Information), which is also in agreement with the literature.13 TOP seriously etched the metallic Ni, which was formed at first, probably through the formation of Ni-TOP complexes. As a result, the reaction leads to the synthesis of hollow nickel phosphides particles. TPP, on the other hand, is
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7583 quite more stable than TOP, and our first intention was to use it in order to control the rate of the phosphidation reaction and therefore synthesize the hybrid nanomaterial. After testing several experimental parameters, the appropriate synthesis conditions were standardized as described in the Experimental Section. The particles form very stable colloidal solutions in nonpolar organic solvents, such as toluene, hexane, and chloroform. The materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) in order to investigate their structural and morphological characteristics. As far as the mechanistic path of the reaction is concerned, we found that metallic Ni nanoparticles are initially formed, since the Ni precursor (Ni(acac)2) decomposes in oleylamine at relatively low temperature to give Ni2+ ions, which can be readily reduced. The phosphine also partially decomposes, but at higher temperature (therefore after the metallic Ni formation), acting like P precursor. P reacts with the Ni atoms at the surface of the nanoparticles, 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 clearly indicated. Ferromagnetic behavior of the particles is verified by magnetic measurement (inset in Figure 1) at room temperature, 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 by the TEM and HRTEM images shown in Figure 2a-c. The coexistence of hexagonal metallic Ni and nickel phosphide, with the dominant phase being the hexagonal Ni2P, is also clearly
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.
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Figure 3. XPS spectra for P2p (a) and Ni2p3/2 (b) of the sample synthesized in the presence of TPP, as-received (bottom spectra) and after Ar+ sputtering (top spectra).
shown by the XRD pattern (Figure 2d). The hollow hybrid Ni/ Ni2P nanoparticles have spherical shape and about 15 nm mean diameter, with very good uniformity. Furthermore, the functionalization of the particles with the organic molecules (oleyl amine and TPP) allows them to form organized close packed 3D superlattices, just through the slow solvent evaporation of the colloidal chloroform solution. Careful analysis leads to the conclusion that these structures are actually three-dimensional superlattices possessing an fcc packing geometry. These 3D superstructures consist of multiple layers (the array is so thick that only the edges could be clearly imaged with the microscope) and extend to large areas. The excellent self-assembling behavior demonstrated by this material can be attributed to the presence of the aromatic phenyl groups of the TPP. The geometry and the size of these groups cause the nanoparticles to stack homogeneously in three directions and large areas. This is not the case for the octyl groups of the TOP molecule, which leads to the formation of hollow spheres but not to their selforganization. We believe that, by choosing the adequate solution concentration and quantity on the TEM grid (which is under study), 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 sample synthesized in the presence of TPP, as-received and after 10 min of mild Ar+ sputtering. The P2p spectrum has been analyzed into three components, each one of them a P2p3/2/P2p1/2 doublet with spin-orbit splitting ∆ ) 0.87 eV and intensity ratio 0.5. The binding energy (BE) values mentioned here are referred to the P2p3/2 component. The peak at BE ) 129.5 is attributed to P atoms in NiP [a], while the peaks at higher BE values are attributed to oxides which are more pronounced in the asreceived sample. The Ni2p spectra are analyzed in a similar way to P2p the spin-orbit splitting being ∆E ) 17.4 eV for the component that corresponds to Ni atoms in NiP or metallic Ni (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 861.9 eV is a satellite of the Ni2p3/2 component of the oxide. The fact that it is wide indicates the contribution of other chemical states of Ni atoms. After 10 min of mild sputtering, the intensity of the low BE component (NiP or Ni) increases considerably while the components attributed to oxides decrease. The satellite shifts to BE ) 860.2 eV, an energy value that has been by many authors assigned to the satellite originating from Ni atoms in NiP.17 Quantitative analysis was performed using the intensities of the P2p and Ni2p spectra normalized by their atomic sensitivity factors.18 It was found that Ni/P before and after sputtering is 2. The apparent Ni excess is an indication of the existence of Ni clusters in the NiP particles.
Figure 4. Room temperature magnetic hysteresis loop from hollow hybrid Ni/Ni2P nanoparticles synthesized by the reaction of Ni(acac)2 in oleyl amine in the presence of TPP.
The magnetic hysteresis loop at room temperature of the hybrid nanoparticles is shown in Figure 4. The material shows ferromagnetic behavior with 0.1 emu/g saturation magnetization and 490 Oe coercive field. The low value of the saturation magnetization, in comparison with that of hcp Ni nanoparticles in the same size range found in the literature, is due to the conversion 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 Ni doped Ni2P hollow particles is reported. The particles have a uniform hollow spherical morphology, demonstrate ferromagnetic behavior, and readily form well ordered extended 3-D superstructures (simply via solvent evaporation). The use of TPPsmainly through the three phenyl groups of the molecules not only allows the control over Ni phosphidation and leads to a hybrid magnetic/semiconducting material but also provides it with remarkable self-assembling behavior. This methodology is expected to lead to new opportunities for the synthesis of novel hybrid magnetic/semiconducting nanostructures. In fact, we have also synthesized Fe2P and Co2P nanostructures using the same procedure. Research is in progress in order to study more thoroughly the morphological, chemical, and physical properties of these materials. Supporting Information Available: TEM images and XRD patterns of the hcp Ni nanoparticles and nanorods (Figure 1S); TEM images of hollow Ni2P nanoparticles synthesized in the presence of TOP (Figure 2S). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Koo, K. Y.; Park, S. B. J. Am. Ceram. Soc. 2007, 90, 3767. (2) Gopalakrishnan, J.; Pandey, S.; Rangan, K. K. Chem. Mater. 1997, 9, 2113. (3) Lukehart, C. M.; Milne, S. B.; Stock, S. R. Chem. Mater. 1998, 10, 903. (4) Jarvis, R. F.; Jacubinas, R. M.; Kaner, R. B. Inorg. Chem. 2000, 39, 3243. (5) Stamm, K.; Garno, J. C.; Liu, G. Y.; Brock, S. L. J. Am. Chem. Soc. 2003, 125, 4038. (6) Liu, J.; Chen, X.; Shao, M.; An, C.; Yu, W.; Qian, Y. J. Cryst. Growth 2003, 252, 297. (7) Lu, B.; Bai, Y. J.; Feng, X.; Zhao, Y. R.; Yang, J.; Chi, J. R. J. Cryst. Growth 2004, 260, 115. (8) Luo, F.; Su, H. L.; Song, W.; Wang, Z. M.; Yang, Z. G.; Yan, C. H. J. Mater. Chem. 2004, 14, 111. (9) Wang, J.; Johnston-Peck, A. C.; Tracy, J. B. Chem. Mater. 2009, 21, 4462.
Hybrid Self-Assembled Hollow Spheres (10) Zheng, X.; Yuan, S.; Tian, Z.; Yin, S.; He, J.; Liu, K.; Liu, L. Chem. Mater. 2009, 21, 4839. (11) Qian, C.; Kim, F.; Ma, L.; Tsui, F.; Yang, P.; Liu, J. J. Am. Chem. Soc. 2004, 126, 1195. (12) Park, J.; Koo, B.; Yoon, K. Y.; Hwang, Y.; Kang, M.; Park, J.-G.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 8433. (13) Chiang, R.-K.; Chiang, R.-T. Inorg. Chem. 2007, 46, 369. (14) Henkes, A. E.; Vasquez, Y.; Schaak, R. E. J. Am. Chem. Soc. 2007, 129, 1896.
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7585 (15) Carenco, S.; Resa, I.; Le Goff, X.; Le Floch, P.; Me´zailles, N. Chem. Commun. 2008, 2568. (16) (a) Shu, Y.; Oyama, S. T. Chem. Commun. 2005, 1143. (b) Ochs, D.; Dieckhoff, S.; Cord, B. Surf. Interface Anal. 2000, 30, 12. (17) Elsener, B.; Atzei, D.; Krolikowski, A.; Rossi Albertini, V.; Sadun, C.; Caminiti, R.; Rossi, A. Chem. Mater. 2004, 16, 4216. (18) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; Wiley: New York, 1996; Vol. 1.
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