Seed-mediated synthesis of monodisperse plasmonic magnesium nanoparticles

We reduce di-n-butylmagnesium with arene (naphthalene, biphenyl, phenanthrene) radical anions and dianions to obtain metallic, plasmonic Mg nanoparticles. Their size and shape depends on the dianion concentration and reduction potential. Based on these results, we demonstrate a seeded growth Mg nanoparticle synthesis and report homogeneous shapes with controllable monodisperse size distributions.


Mg NP synthesis
General effect of arene dianions on Mg synthesis.
To investigate the general effect of the dianion on colloidal Mg synthesis, Mg NPs were synthesised from an organometallic precursor, di-n-butylmagnesium (MgBu 2 ), using the Li 2 Napht dianion or the LiNapht radical anion as the reducing agent. A typical procedure for Li 2 Napht formation was as follows: 0.028 g of Li pellets (4.05 mmol), 0.260 g of dry naphthalene (2.03 mmol), 20 mg PVP (0.18 mmol monomer) and 10.75 mL of degassed anhydrous THF were added to a 25 mL Schlenk flask under Ar atmosphere and sonicated for 45 minutes (Allendale Ultrasonics, 100 W 3 L). LiNapht was prepared following the same protocol using twice the amount of naphthalene (0.530 g, 4.05 mmol).
The effect of the electron carrier was investigated using biphenyl, phenanthrene and anthracene as an alternative to naphthalene. In a typical synthesis of Mg NPs, 1.75 mL of MgBu 2 in heptane (1.0 M) was injected quickly into freshly prepared reducing agent and the reaction was allowed to proceed for 18 hours at room temperature (20 o C) before quenching with 2 mL of anhydrous IPA. The solid product was recovered by centrifugation and residual by-products were removed by centrifugation and redispersion steps in anhydrous IPA twice, anhydrous THF twice and anhydrous IPA twice, before redispersing in anhydrous IPA.
WARNING: MgBu 2 is pyrophoric and should be handled and transferred in air-free conditions.

Effect of Li 2 Napht concentration.
In this series of experiments, Li 2 Napht was prepared as described above, but the sonication time was varied from 15 to 60 minutes to obtain different Li 2 Napht/LiNapht ratios in the resulting reducing agent.
The concentration of Li 2 Napht increased with sonication time and reached its highest level of 75% of the theoretical maximum after 45 minutes, being 30% after 15 minutes (when the purple colour had S2 just started to form), 49% after 30 minutes and 59% after 1 hour ( Figure S4). The longer sonication time leads to a decrease in the dianion concentration apparently due to its reaction with THF, which can be facilitated under sonication and by rising temperatures in the sonication bath.
To elucidate the effect of the Li 2 Napht concentration on the initial stages of Mg NP formation, the reaction between the reducing agent and MgBu 2 was quenched after 5 minutes with 2 mL of anhydrous IPA.

Seed mediated synthesis of Mg NPs
In these experiments, the reduction potential of the reaction mixture was changed in-situ by transforming Li 2 Napht to LiNapht with an excess of naphthalene. Mg NP synthesis was initiated by injecting quickly 1.75 mL of MgBu 2 in heptane (1.0 M) into freshly prepared Li 2 Napht solution followed by addition of 2 mL of naphthalene in THF (1.0 M) after 5 minutes of reaction, to convert all unreacted Li 2 Napht to LiNapht. The resulting mixture was left to react for 60 minutes and then quenched with 2 mL of IPA and cleaned as described above. This reaction is called 2-steps 1xMg 60 min. A further dose of 1.75 mL of MgBu 2 in heptane (1.0 M) was added 30 minutes after the naphthalene addition and left to react either a further 60 minutes (2-steps 2xMg 90 min) or 18 hours (2-steps 2xMg 18h).

Double titration method
The amount of Li 2 Napht dianion in the reduction solution was determined by a double titration method. 27,28 To find the total amount of Li present, a 0.5 mL aliquot of the dianion solution was hydrolysed with 5 mL of deionised water and titrated against standardised 0.1 M hydrochloric acid with phenolphthalein as an indicator. The amount of residual (non-reactive) Li was then found by quenching another 0.5 mL of dianion solution with 1 mL of ethylene bromide and titrating against the hydrochloric acid after dilution with 5 mL water. The amount of Li 2 Napht was calculated based on the difference between the amount of reactive Li determined by double titration and the amount of naphthalene used to prepare the dianion solution.

Characterisation of Mg NPs
Freshly prepared Mg NP samples were drop cast onto Si wafers for scanning electron microscopy (SEM) imaging, performed on a FEI Nova NanoSEM, operated at 5 kV and equipped with an in-lens detector for secondary electron imaging. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images, electron diffraction patterns, STEM energy dispersive X-ray spectroscopy (STEM-EDS) maps and STEM electron energy loss spectroscopy (STEM-EELS) maps S3 of samples drop cast on a Cu-supported lacey ultrathin carbon membrane were acquired at 200 kV on a FEI Osiris STEM with a Bruker Super-X quadruple EDS detector and a Gatan Enfinium ER 977 electron spectrometer, except for the HAADF-STEM, STEM-EDS and STEM-EELS shown in Figures S6-S7 which were acquired on a Thermo Fisher Spectra equipped with a Bruker Super-X quadruple EDS detector and a Gatan Continuum electron spectrometer. STEM-EELS maps of the Mg bulk plasmon (Fig. 1h, S2) were produced by integrating the intensity of the spectrum image from 9.5 to 11.5 eV, and STEM-EDS maps (Fig. 1f) were obtained by integrating the Kα peaks of O and Mg.
NP size distributions were obtained by measuring at least 90 NPs with clearly visible edges from SEM images. The sizes measured were the longest dimension, e.g., the distance between opposite corners for a hexagonal platelet or the longest length visible of thick, block-like NPs. Reported size polydispersities are the standard deviation of all measurements for monomodal samples, and the standard deviation of measurements of each shape type for bimodal samples.
Inductively coupled plasma mass spectrometry (ICP-MS) was performed using a Perkin Elmer Nexion 350D quadrupole-based mass spectrometer. Samples were digested in a 0.2 M nitric acid matrix and diluted to around 10 ppb. Errors on the ICP-MS measurements are reported as the standard deviation of three measurements of the same sample. Extinction spectra were measured using a Thermo Scientific Evolution 220 spectrophotometer.    [Li 2 Napht] produced from a 2:1 Li:Napht mixture in THF as a function of sonication time. The [Li 2 Napht] is reported such that 30% [Li 2 Napht] means that 30% of the total naphthalene is in the dianion form and the remaining 70% is a radical anion.