Nickel platinum (NixPt1−x) nanoalloy monodisperse particles without the core–shell structure by colloidal synthesis

We report a new and versatile colloidal route towards the synthesis of nanoalloys with controlled size and chemical composition in the solid solution phase (without phase segregation such as core–shell structure or Janus structure) or chemical ordering. The principle of the procedure is based on the correlation between the oxidation–reduction potential of metal cations present in the precursors and the required synthesis temperature to nucleate particles without phase segregation. The procedure is demonstrated via the synthesis of Face Centered Cubic (FCC) NixPt1−x nanoparticles, which was elaborated by the co-reduction of nickel(ii) acetylacetonate and platinum(ii) acetylacetonate with 1,2-hexadecanediol in benzyl ether, using oleylamine and oleic acid as surfactants. The chemical composition and solid solution FCC structure of the nanoalloy are demonstrated by crosslinking imaging and chemical analysis using transmission electron microscopy and X-ray diffraction techniques. Whatever the chemical composition inspected, a systematic expansion of the lattice parameters is measured in relation to the respective bulk counterpart.


Analyses of core-shell Ni 3 Pt nanoparticles
Fig. S1: Left HAADF-STEM image of core-shell Pt@Ni NPs with d = (16 ± 3) nm and the size distribution histogram. Right NPs electronic diffraction clearly shows 111, 200 and 220 peaks related to a FCC lattice parameter a = (0.369 ± 0.007) nm. Other peaks are observable but they are diffuse due to the core-shell structure.
Electronic Supplementary Material (ESI) for Nanoscale Advances. This journal is © The Royal Society of Chemistry 2020 Top HAADF image of the four analyzed NP labelled 1-4. Their core is highlighted in red and this shell in blue and comparison of the 4 spectra recorded on each NP. The EDX spectra comparison of the 4 isolated NPs indicates an identical chemical composition near 87 % in nickel against 13 % in platinum (atomic %). Bottom comparison for each particle of EDX spectra recorded from the core (red curve) and from this shell (blue curve).The chemical composition comparison between the core and the shell in isolated particles (see in Fig. 1) illustrates the Pt-rich core against Ni-rich shell. * The copper peaks at 8.040 keV is due to the TEM grid. Fig. S3: Ni (in red) and Pt (in green) intensity profiles along the NPs shown in Fig. 1 and S2. At the edge of the particle, the amount of Pt varies a little, additionally it is in the same quantity as for Ni. On the contrary, in the NPs center, a strong signal from Pt and a weak signal from Ni are observed.

Analyses of core-shell Ni 3 Pt nanoparticles
Fig. S4: Profile intensity comparison between core-shell and alloyed particles. (a) The intensity profile of the core-shell particle shows a non-continuous signal with an increase in the signal at the center. This increase is not due to the thickness but to the presence of more important Z atoms in the center of the particles (here Pt). (b) The intensity profile of the alloyed particle shows a continuity of the signal due to the thickness of the particle, a homogeneous chemical composition can be deduced within the NP. Bottom comparison for each particle of EDX spectra recorded from the core (red curve) and from their shell (blue curve). The EDX chemical composition comparison between the inner and outer part of isolated particles attests the composition homogeneity within each NP. * The aluminum peaks at 1.486 keV is due to the aluminum TEM grid.

Alloyed Ni x Pt 1-x NPs
Ni 3 Pt sample In order to have a precise measurement of the lattice parameter, the (hkl) indexing and determination of the FCC lattice parameter was performed on 5 diffractions patterns from 5 different zones on the TEM grid

NiPt sample
In this diffraction pattern we note the presence of all (hkl) up to 511, in order : 111,200,220,311,222,400,331,420,422,511 Here again, analysis has been performed on 5 different zones.

NiPt 3 sample
Here to, we note hkl peaks up to 511 but for the sake of understanding the images, not all patterns are indexed and the analysis has been done on 5 different zones.   Table S1: Lattice parameters deduced from peaks indexation of PXRD patterns presented in Fig. 2 (Ni 3 Pt core-shell sample) and Fig. 6 (NiPt alloy sample).

Scherrer formula
With τ corresponding to the mean crystalline domain size, K the shape factor (taken as 0.89), λ the wavelength of the X-ray beam (here λ = 1.541 Å), β the full at half maximum (FWHM) of the diffraction peak and θ the Bragg angle.  For each fringe, we take the maximum of the intensity thanks to the intensity profile obtained on the blue line on electron diffraction pattern. Then, we calculate the lattice parameter for each observable atomic plane. Finally, we average the different "a" obtained and we calculate the standard deviation which is our error bar. Here, a = (0.366 ± 0.003) nm.