DOI: 10.1007/s00339-007-4008-7 Appl. Phys. A 88, 277–284 (2007) Materials Science & Processing Applied Physics A p. strunz 1,2 d. mukherji 3, ✉ g. pigozzi 4 r. gilles 5 t. geue 6 k. pranzas 7 Characterization of core-shell nanoparticles by small angle neutron scattering 1 Nuclear Physics Institute (NPI), 25068 ˇ Reˇ z, Czech Republic 2 Research Centre ˇ Reˇ z, 25068 ˇ Reˇ z, Czech Republic 3 TU Braunschweig, IfW, Langer Kamp 8, 38106 Braunschweig, Germany 4 ETH Zürich, Laboratory for Nanometallurgy, 8093 Zürich, Switzerland 5 TU München, ZWE FRM-II, Lichtenbergstr. 1, 85747 Garching, Germany 6 PSI & ETH Zürich, Laboratory for Neutron Scattering, 5232 Villigen PSI, Switzerland 7 GKSS Research Centre, Institute of Materials Research, 21494 Geesthacht, Germany Received: 6 December 2006/Accepted: 8 March 2007 Published online: 23 May 2007 • © Springer-Verlag 2007 ABSTRACT The Ni 3 Si-type nanoparticles dispersed in a mix- ture of H 2 O/D 2 O were characterised by SANS using the con- trast variation method. The existence of a core-shell structure in the nanoparticles with a Ni 3 Si(Al) core and amorphous SiO x shell is confirmed by the SANS measurements. The nanopar- ticles were produced by extracting precipitates from a bulk Ni-13.3Si-2Al ( at. %) alloy using electrochemical phase sep- aration technique and were pre-characterised by X-ray diffrac- tion and transmission electron microscopy. By comparing the precipitate morphology in the Ni-Si-Al alloy with the extracted nanoparticles in the SANS measurements, it is clearly estab- lished that the precipitates shape and size are unaffected by the extraction process and that the amorphous shell forms on top of the particle core. However, the present measurement could not confirm or exclude the presence of H atoms in the shell structure. PACS 61.12.Ex; 61.12.-q; 61.46.Df; 61.82.Rx 1 Introduction Multifunctional nanostructures are becoming in- creasingly important and surface modifications of nanoparti- cles in order to form a core-shell type structure have emerged as an important class of functional nanostructures. Nanoparti- cles having an inorganic core with a different inorganic shell are suitable for potential applications in many diverse fields, especially for optical devices, magnetic storage media and in health sciences. Naturally occurring inert oxide shells on metal nanoparticles are not uncommon. Therefore, we dis- tinguish between shells that are obtained naturally and arti- ficially. This distinction is important because for many ap- plications a shell covering the nanoparticles is deliberately formed either to protect the core or to enhance a specific prop- erty on the surface of the particle. For example, for enhanced specific chemical functions, nanoparticles may be covered by a catalytically active shell material, or magnetic nanoparti- cles may be covered by biocompatible organic shell mate- rial [1]. Amorphous Si and SiO x are suitable shell materials in many of the above mentioned applications [2]. Generally, ✉ Fax: 0049-531-3913058, E-mail: d.mukherji@tu-bs.de the core-shell nanoparticles are made in two steps – by form- ing the core particle first and then coating the shell on top of the core, either through wet chemistry or gas-phase synthe- sis. Recently, we reported an alternative method for producing core-shell type nanoparticles [3]. In this method, Ni 3 Si(Al) intermetallic phase nanoparticle cores covered with amorph- ous SiO x shells are produced in a single step from two-phase metallic alloys. Nano-sized precipitates are extracted from a two-phase bulk metallic alloy by selectively dissolving the matrix phase (electrochemical selective phase dissolution – ESPD) to obtain the nanoparticles. The selection is achieved through the suitable choice of electrolyte and by controlling the ESPD processing parameters [4]. The shell forms in situ during the extraction process through the dealloying of the matrix [3]. Such core-shell nanoparticles are unique and may be useful for many attractive functional applications. ESPD can be generally applied to produce core-shell nanoparticles of other compositions as well. Typical core-shell nanoparti- cles produced by this method are seen in Fig. 1. The core-shell nanoparticles produced by ESPD were characterised using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to determine the structure of the core and shell and, in the case of the Ni 3 Si-type particles, it was found that the structure of the shell is amorphous. It is suggested that a possible shell formation mechanism may in- volve dealloying of the Ni(Si) solid-solution matrix [3] from which the particles are extracted. Energy-dispersive spec- troscopy (EDS) in TEM was used to determine the composi- tion of the shell, but due to the limitations inherent in EDS, quantitative determination of the composition was not pos- sible. In contrast to the shell, the structure and the composition of the core could be determined more precisely, and it was found to be unchanged from that of the precipitates in the bulk alloy, from which they were extracted [3]. Apparently, the nanoparticles retain the shape and size of the precipitates in the bulk alloy, thus, it is difficult to say whether or not the precipitate phase is affected by the selective dissolution of the matrix phase. The TEM measurement was unable to conclu- sively determine if surface dissolution had occurred during extraction. This is an important question regarding the shell formation mechanism. Details of structure and composition at interfaces may be obtained by small-angle neutron scattering (SANS) using contrast variation [5]. This technique has also been used to