# Spontaneous Emulsification of Organometallic Complexes Applied to the Synthesis of Nanocapsules Active for H2 Release from Ammonia-Borane

Olivier Gazil, Ludivine Rault, Déborah Iglicki, V. Collière, Gizem

Karacaoglan, Didier Poinsot, Moad Bouzid, Jean-Cyrille Hierso, Myrtil L

Kahn, Nick Virgilio, et al.

► To cite this version:

Olivier Gazil, Ludivine Rault, Déborah Iglicki, V. Collière, Gizem Karacaoglan, et al. Spon- taneous Emulsification of Organometallic Complexes Applied to the Synthesis of Nanocapsules Active for H2 Release from Ammonia-Borane. Langmuir, 2024, 40 (32), pp.16824 - 16832. 10.1021/acs.langmuir.4c01307. hal-04762632v2

HAL Id: hal-04762632

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Submitted on 31 Oct 2024

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# Surfactant-Free Emulsions and Microemulsions of Organometallic Complexes: Application to the synthesis of Metal Nanocapsules and Catalytic Hydrogen Delivery from Ammonia-Borane

Olivier Gazil,ᵃᵇ Ludivine Rault,ᶜ Déborah Iglicki,ᵃ Vincent Collière,ᵈ Gizem Karacaoglan,ᵉ Didier Poinsot,ᵉ Moad Bouzid,ᵉ Jean-Cyrille Hierso,*,ᵉ Myrtil L. Kahn,ᵈ Nick Virgilio,ᵇ and Fabienne Gauffre*ᵃ

a. Univ Rennes, CNRS, ISCR-UMR6226, F-35000 Rennes, France. E-mail: fabienne.gauffre@univ-rennes.fr

b. CREPEC, Department of Chemical Engineering, Polytechnique Montréal, C.P. 6079 Succursale Centre-Ville, Montréal, Québec H3C 3A7, Canada.

c. Univ Rennes, CNRS, ScanMAT UAR 2025, F-35000 Rennes, France.

d. LCC-CNRS, UPR8241, 205 Route de Narbonne, CEDEX 4, 31077 Toulouse, France.

e. Université de Bourgogne, ICMUB-UMR CNRS 6302, 21078 Dijon Cedex, France. E-mail: jecyrille.hierso@u-bourgogne.fr

**Abstract.** In this work, we demonstrate for the first time that spontaneous surfactant-free microemulsions (SFME) and Ouzo emulsions can be formulated using solely organometallic precursors, to ultimately synthesize sub-µm metal nanocapsules after interfacial reduction. Depending on the proportion of the three components, water, solvent, and the precursor, either "surfactant-free microemulsions" (SFME) or "Ouzo emulsions" are formed. We investigated the case of several catalytic transition metals (Au, Pd and Pt), individually or as combinations, stabilized by various ligands. Upon reduction of the precursors, the two different types of emulsions lead to different materials: shells of discrete nanoparticles (NP) or continuous shells, for the SFME and Ouzo emulsions respectively. Particularly interesting is the case of Au-Pd mixed emulsions, which leads to a unique structural morphology: Au-Pd nanoparticles embedded in a continuous submicronic metal shell. In addition, we show that AuNP are available to grow larger particles within the NP-shell, using a seeded-growth approach. These water-stable and surfactant-free nanocapsules are appealing as catalysts and as such, were evaluated for the catalytic hydrogen delivery from ammonia-borane, which is one of the most promising water compatible strategy of hydrogen storage and delivery ammonia-borane (AB). Our work establishes for the first time a genuine activity of gold colloids for this reaction.

**Introduction.** The elaboration of well-organized nanostructures designed for specific functionalities requires precise control of the synthetic steps.<sup>1,2</sup> In this respect, methodology developments can significantly broaden the horizons of material synthesis and of material applications.<sup>3–6</sup> In particular, efforts have been made to create spherical shells of inorganic materials. To date, one of the most widespread strategies consists in assembling preformed nanoparticles (NPs) at the surface of either solid particles or emulsion droplets, forming the so-called "colloidosomes".<sup>7,8</sup> Hollow capsules were either derived from colloidosomes by sacrificing the core particle or from alternative processes such as spray drying.<sup>9</sup> Despite the undeniable benefits of these methods - relatively easy to implement and well suited to produce objects larger than 1 µm - it is still a synthetic challenge to access to submicronic inorganic capsules.<sup>10</sup> The first successful strategies were based on the use of block copolymers, whose amphiphilic nature directs the assembly of nanoparticles.<sup>11,12</sup> However, these polymers maybe quite costly. A few examples also report the formation of hybrid vesicles from the self-assembly of nanoparticles and low-molecular weight surfactants.<sup>13,14</sup> In terms of applications, submicronic metal shells would be desirable in nanomedicine (as radiosensitizers or for thermotherapy), as building blocks for metamaterials, or in catalysis.<sup>15–17</sup> Note that for this specific case, spherical shells containing NPs provide the advantage of ease of separation and the ability to incorporate multiple types of catalytic materials within a single shell,<sup>18,19</sup> but the presence of surface active polymers might be detrimental for surface reactivity. Therefore, there is still a need for innovative strategies to access to submicronic nanospheres with accessible inorganic material at the surface, preferably without the use of surface-active molecules.

In this context, nanoscale emulsions are interesting candidates to form submicronic droplets that can be further used to template spherical NP-shells.<sup>20–22</sup> In particular the spontaneous "Ouzo" emulsification (i.e. the nanoprecipitation of a hydrophobic solute) in appropriate mixtures of water and water-miscible solvents forms droplets with sizes typically in the range 80 nm – 200 nm.<sup>23,24</sup> These droplets can be used as template for capsules.<sup>19,25–27</sup> In particular, stable NP-laden droplets form in the presence of hydrophobic nanoparticles.<sup>28</sup> Their properties such as permeability, elasticity and biocompatibility can be fine-tuned through the proper choice of the NPs.<sup>29,30</sup> Thermodynamically stable "surfactant-free microemulsions" (SFME)
are also found in the vicinity of the Ouzo emulsion domain, and might similarly be used as templates to form NP-shells.³¹ Note that strictly speaking, a microemulsion is made of a solvent, an oil and surfactants.³² The expression « surfactant-free microemulsion » refers to an isotropic and thermodynamically stable system with dispersed domain varying approximately from 1 to 100 nm, without surfactant.³¹ This absence of surfactant is an advantage, since amphiphilic surfactants pose several problems at the industrial level, such as foaming, and ecotoxicity,³³,³⁴ and is in great part responsible for the success of Pickering emulsions (together with the mechanical robustness of their particle-laden interfaces). However, nanoparticles that are involved in the elaboration of submicronic vesicles or capsules via template or self-assembly strategies, are usually below 20 nm and most often coated with alkylamines or alkylacids,²⁸,³⁵ or polymers.²¹,²² Therefore, the synthesis of the NPs directly at the interface of Ouzo emulsions represents a promising alternative for the development of "surfactant-free" submicronic spheres with NP-shells. We previously demonstrated the feasibility of this strategy, using an interfacial reduction with the metallic precursor and the reductant dissolved respectively in the dispersed and continuous phase of an Ouzo emulsion.³⁵

In the present work, we're tackling the challenge of the elaboration of submicronic and surfactant-free metal nanospheres with a metal-shell. To this aim, we achieved the interfacial reduction of metal precursors within Ouzo and SFME emulsions. We investigated the structure and the catalytic efficiency of the produced materials. Several precursors of Au, Pd and Pt were used, individually or as combinations. As a model of catalytic reaction, we selected the hydrogen ($H_2$) production from the hydrolysis of ammonia-borne ($NH_3BH_3$, AB). Indeed, energy storage using $H_2$ vectors is an attractive approach towards a decarbonated industrial processes.³⁷,³⁸ Transition metal-catalyzed hydrolysis of safer solid hydrogen storage materials having a high gravimetric content of $H_2$, like AB, has been intensively studied in the last decades,³⁹ and AB solvolysis is considered a promising route for fast, controlled and secured $H_2$ delivery from an innocuous solvent (water, alcohols). While some dispersed colloids and many heterogeneous catalysts of the platinum group metals have been investigated under many instances,³⁹⁻⁴² the performances of metal-shells from interfacial (micro-)emulsion synthesis, as produced in our present approach, are still to examine.

**Results and discussion.** In the present study, we used organometallic complexes as the "oil" (more generally called "solute" in the following) in mixtures of water and THF, to form SFME and Ouzo emulsions. Ouzo emulsions and SFME are typically found at low solute concentrations (see ternary diagram in **Scheme 1**, which is not to scale for ease of representation). Although both domains are adjacent, the Ouzo emulsion lays in the metastable region (between the binodal and spinodal lines) whereas the SFME domain is located in the thermodynamically stable single phase region.⁴³ It is important to note that the size of the droplets formed in the Ouzo and SFME domain may be of the same order (typically 100 nm) at short time. However, SFME droplets will remain stable whereas Ouzo droplets grow by coalescence if not stabilized by surfactants or particles. In addition, the dispersed phase of the SFME exhibit a solvent-rich composition, in contrast to the solute-rich composition of the dispersed phase in the Ouzo domain.²⁸

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Right: schematic representation of a typical phase diagram showing the Ouzo and SFME domains in ternary systems consisting of water, a water-miscible solvent, and a hydrophobic solute. Left: mechanism of droplets formation from organometallic precursor in Ouzo emulsions.</figcaption></figure>

**Stabilization of Ouzo emulsions by in situ reduction of the precursor.** Since particles are able to prevent the coalescence of droplets, we first checked whether an Ouzo emulsion made from a metal precursor as the solute was stabilized by the *in situ* reduction of the precursor. **Figure 1a** shows the time evolution of the hydrodynamic diameter of an Ouzo emulsion of the metal precursor $Pd(PPh_3)_2Cl_2$, without reducing agent. The sample was prepared by adding water rapidly over a THF solution of the precursor, followed by vortexing for a few seconds. The specific composition water:THF = 75:25 and [Pd] = 0.125 mM was
selected because of its slow evolution. The droplets grow from D~h~ ~ 100 nm a few seconds after mixing, to several microns
within a few tens of minutes. We investigated the effect of adding the reducing agent (NaBH~4~) either 1 minute or 10 minutes
after water addition (time points b and c on **Figure 1a**). **Figure 1b&c** shows TEM micrographs of the samples after reduction (a
droplet of solution is casted on the TEM grid at the mentioned time). Spheres with an average diameter of 180 nm are clearly
observed on **Figure 1b** (time point 1 min), showing that the size of the emulsion is roughly preserved (D~h~ ≈ 200 nm at time point
"b" on fig 1a). DLS measurements ran after 1 month show that the reduced suspension remains stable over this period, with
no sign of aggregation (Figure S1). Sedimentation is also not observed. The spheres look aggregated and partly fused on the
TEM grid, which can be due to concentration during drying. When the reducing agent was added after 10 min, large and uneven
shapes are formed (**Figure 1c**), most probably due to coalescence of the Ouzo emulsion before the reduction takes place. This
suggests the formation of a stabilizing metal shell upon reduction of Pd.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) Monitoring (DLS) of the growth of the Ouzo emulsion using Pd(PPh₃)₂Cl₂ as the solute (without reduction). TEM micrographs after the addition of a reducing agent (NaBH₄) to the growing Ouzo emulsion at 1 min (b) and 10 min (c). [Pd] = 0.125 mM and [NaBH₄] = 7.5 mM.</figcaption></figure>

**Effect of the precursor metal atom.** To investigate the influence of the transition metal, we prepared various compositions using AuPPh₃Cl, Pd(PPh₃)₂Cl₂ or Pt(PPh₃)₂Cl₂, as the hydrophobic solute (**Figure 2a**). For gold, white crystals are observed for the three highest concentrations (see **Figure S2**). Crystallization precludes the formation of Ouzo emulsions because the nanoprecipitate dissolves to feed the crystals which are more thermodynamically stable.³¹ Conversely, all samples prepared from Pd and Pt precursors appear to be homogenous and transparent, or slightly turbid. The transmission of these samples was monitored over time, using a static multiple light scattering device (SMLS, Turbiscan).³¹ Indeed, the time evolution of a sample's optical transmittance is indicative of a metastable Ouzo emulsion. In contrast, a stable transmittance corresponds either to a SFME (if droplets are present) or to a completely homogeneous system. For instance, considering Pd(PPh₃)₂Cl₂ in **Figure 2**, composition ① has a constant turbidity, while composition ② shows a rapid decrease in relative transmission (**Figure 2b**). The presence of droplets in sample ① is assessed by DLS (**Figure 2c**), even though the solution seems transparent. These precursor droplets keep a constant diameter, which is a valuable gauge that the system at this concentration is in the SFME domain. In summary, in water/THF mixtures, both Pd(PPh₃)₂Cl₂ and Pt(PPh₃)₂Cl₂ form a SFME (at lower precursor concentrations) and an Ouzo emulsion (at higher concentrations). In contrast, no Ouzo domain is observed for AuPPh₃Cl. Note that an Ouzo domain may exist at different water/THF ratio.

Compositions ① and ② were then reacted with 7.5 mM reducing agent added initially and the resulting materials were
examined by TEM (**Figure 2d & e**). The difference in morphology is striking: reduction of the SFME yields spheres with a shell
made of discrete NPs (NP-shells), whereas a continuous shell is observed from samples initially in the Ouzo domain. In this
latter case, a core-shell morphology is clearly observed with a less contrasted shell **Figure 2e**. In addition, the size distribution
is much narrower (the diameter drops from 230 nm for ② to 60 nm for ① and the polydispersity index drops respectively
from 0.33 to 0.03) for the SFME sample. These major differences can be explained by the difference in droplets composition
between the two different emulsions: an Ouzo droplet consists of pure or almost pure solute, while a SFME droplet usually
contains a high ratio of solvent (here THF), as **Scheme 1** indicates.³¹ Note that we also achieved the reduction of the
concentrated (0.375 mM) AuPPh₃Cl samples, by adding NaBH₄ prior to the formation of the macroscopic crystals. This yields
randomly dispersed AuNPs as shown in **Figure S3**. Furthermore, the 0.375 mM Pt(PPh₃)₂Cl₂ samples were also reduced in the
same fashion and yielded objects (**Figure S4**) that were similar to their Pd counterpart ②.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Partial phase diagram for ternary mixtures of 25 %v/v THF, 75 %v/v water and different triphenylphosphine-based metal chlorides (without reduction). Green: Ouzo domain; yellow: SFME domain; "X": biphasic. The remaining white squares can be either in the miscible region or SFME since they exhibit no change in their optical transmission. SMLS (b) and DLS (c) analyses for a set of two data points ① (0.0375 mM Pd(PPh₃)₂Cl₂) and ② (0.375 mM Pd(PPh₃)₂Cl₂) alongside their respective TEM micrographs, after reduction with 7.5 mM NaBH₄ (added to the aqueous phase prior to emulsification) (d-e).</figcaption></figure>

Analyses of the continuous shell samples (reduction of sample ②) by SEM, cryo-TEM and TEM tomography were also performed and are presented in **Figure 3**. All three observations confirmed that the surface is smooth. However, both SEM (**Figure 3a**) and TEM tomography (**Figure 3c**) analyses showcased local surface depressions. Such deformations are commonly observed in vesicular structures⁴⁴ and are probably due to analysis artefacts. Indeed, samples submitted to TEM or SEM analysis are first dried and then placed under vacuum in the microscope. These operations can cause buckling or explosion of core-shell particles, if their core is susceptible to evaporate. It therefore confirms that the outer surface of the sphere is continuous. In contrast, cryo-TEM images (**Figure 3b**) do not show these surface defects, consistently with the fact that the sample is not dried, but frozen. In this case, a less contrasted area is visible in the sphere, suggesting a void.³¹ **Figure 3c** evidences a core-shell morphology obtained when using Pd(PCy₃)₂Cl₂ as the precursor, with the core appearing darker than the shell.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Analysis of Pd Ouzo emulsion after reduction: a) SEM micrograph; b) Cryo-TEM micrograph; c) Snapshot of the TEM 3D reconstruction view of a Pd nano-object with shallow depression on the surface (see ESI for full reconstruction). d) TEM micrograph after reduction of a Pd Ouzo emulsion made with Pd(PCy₃)₂Cl₂ – note the difference of contrast between the core and shell. [Pd] = 0.375 mM and [NaBH₄] = 7.5 mM.</figcaption></figure>

X-ray photoelectron spectroscopy (XPS) was used to probe the chemical state of Pd near the surface (XPS has a typical penetration depth of c.a. 5 to 10 nm) of the continuous shell of sample ② (Figure 5). The XPS surface analysis in the binding energy area of palladium Pd3d$_{5/2}$ evidences three chemical palladium states: Pd(0) at 335.2 eV but also higher chemical
palladium states, Pd(I) at 336.5 eV and Pd(II) at 337.8 eV, related to an uncomplete reduction. Therefore, the continuous shell consists in a mixture of various and probably multivalent complexes of Pd. This incomplete reduction could be due to a lack of solubility of NaBH₄ in the precursor droplets or to the formation of a thin impermeable shell upon reduction. Altogether, these results consistently suggest the formation of a continuous shell, which explains why the Ouzo emulsion stops growing by coalescence shortly after adding the reducer.

**Ligand effects on palladium-based complexes.** We next investigated what happened for a given metal (here Pd) in the Ouzo domain, with a variety of different ligands. Four precursors were chosen with increasing solubility in water: Pd(PCy₃)₂Cl₂, Pd(PPh₃)₂Cl₂, Pd(OAc)₂ and PdCl₂. The four precursors were reduced at a concentration of [Pd] = 0.375 mM, where all precursors formed an Ouzo emulsion (confirmed by DLS, see **Figure S6**). The objects were purified by dialysis (see Methodology in ESI) and analyzed by TEM (**Figure 4**). For the two least water-soluble precursors, Pd(PCy₃)₂Cl₂ and Pd(PPh₃)₂Cl₂, spherical nano-objects with continuous shells are observed. Their size is smaller, with a narrower size distribution for Pd(PCy₃)₂Cl₂ (160 ± 40 nm), compared to Pd(PPh₃)₂Cl₂ (230 ± 140 nm). In addition, a few nanoparticles are included within the continuous shells for Pd(PCy₃)₂Cl₂ (**Figure 4a & e**). The formation of these individual nanoparticles is probably favored when the precursor decomposition is fast, allowing high local concentration in metal atom. The samples with Pd(OAc)₂ and PdCl₂ did not form capsules, but instead PdNPs agglomerated in undefined shapes (in **Figure S7**, EDX measurement suggests that the particles are mostly Pd(0) alongside the SAED from **Figure 4g**). This striking contrast with the cases of Pd(PCy₃)₂Cl₂ and Pd(PPh₃)₂Cl₂ can be explained by the water solubility of the precursor's ligand. Indeed, the insoluble ligands (PPh₃) and (PCy₃) should remain in the emulsion core after reduction, whereas the emulsion will rapidly disappear when OAc⁻ and Cl⁻ are released, due to their solubility in water.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** TEM micrographs of various Pd nano-objects synthesized with: a) Pd(PCy₃)₂Cl₂, b) Pd(PPh₃)₂Cl₂, c) Pd(OAc)₂, and d) PdCl₂. e-h) Respective HRTEM of the preceding micrographs. Note that in g), the crystallinity of the palladium nanoparticles is showcased in the inset with a SAED pattern. [Pd] = 0.375 mM and [NaBH₄] = 7.5 mM. i) Relative solubility in water of the ligands used.</figcaption></figure>

**Addressing the issue of crystallization from gold precursors.** As mentioned previously, the Au precursor tends to demix from the solution and crystallize. Following the strategy described in our earlier work,³⁶ we used either PPh₃ or BHT as an additive to form a proper Ouzo emulsion with the gold precursor, avoiding the issue of crystallization. Note that an alternative strategy could be to decrease the gold concentration. BHT (3.63x10⁻⁴ mmol) and AuPPh₃Cl (3.75x10⁻⁴ mmol) were both dissolved in THF (0.25 mL) before the addition of the desired amount of water (0.75 mL). **Figure S8** shows that the size of mixed BHT/AuPPh₃Cl droplets grows slowly and remains much smaller compared to the sizes measured in the absence of BHT. In this case, NaBH₄ was dissolved into the water before being mixed with the BHT/AuPPh₃Cl organic solution. In addition, 1 min after emulsification, 50 µL of a 4.4 mg/mL poly(allylamine hydrochloride) (PAA HCl) aqueous solution was added to freeze the morphology. Indeed, polymers were shown to reinforce the mechanical stability of nanoparticle shells. In contrast with surfactants, polymers are not expected to cover the whole surface and block catalytic activity.⁴⁵ SEM, cryo-TEM and HRTEM
characterizations (Figure 5) shows the formation of nanospheres composed of individual nanoparticles. Interestingly, this result confirms the previous observation, that a dispersed phase with a low concentration of precursor leads to individual nanoparticles in contrast to the continuous shell obtained with a precursor-rich dispersed phase. The condition of low concentration of precursor is achieved either in the SFME domain or in an Ouzo domain, when the metal precursor is mixed with an organic co-solute for instance.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** (a) TEM micrographs of mixed AuPPh₃Cl and PPh₃ emulsions after reduction and stabilization with PAA HCl. (b) Cryo-TEM image of mixed AuPPh₃Cl and BHT emulsions after reduction and stabilization with PAA HCl and (c) HRTEM image of the same sample. [PPh₃] = 0.375 mM, [BHT] ≈ 0.36 mM, [Au] = 0.375 mM, [NaBH₄] = 7.5 mM and [PAA.HCl] = 0.22 mg/mL.</figcaption></figure>

The amount of NaBH₄ was varied to tune the ratio of precursor: reducing agent from 10 : 1 to 1 : 10, without a significant change in the morphology or size of the assemblies.

Next, we investigated the effect of adding the polymer at different time intervals. As can be seen from TEM analyses (Figure 6), the time of addition greatly impacts the diameter of the spheres. From 1 min to 10 and 60 min, the nano-objects get larger as polymer addition is delayed. The diameter increases from 50 ± 10 nm at 1 min, to 270 ± 100 nm at 10 min, and to 300 ± 100 nm at 60 min.

This points to the fact that particles alone do not stabilize the droplets, but they can do it in combination with the polymer. However, if the polymer is added initially at the same time as the reducing agent, the objects are larger in average (see micrograph in Figure S9), compared to when one waits 1 min before adding the polymer. Some interaction between the charged group of the polymer and the reducing agent are suspected. A summary of the different distributions obtained at different times of polymer addition is displayed with boxplots in Figure S10. Clearly, the NP-shells are still evolving after formation of the particles and the polymer needs to be added to stabilize the droplet and “freeze” their morphology. Even though we would expect the particles to have a Pickering effect stabilizing the drops, it seems that the AuNPs are too small to inhibit the growth of the droplets, which reflects in the increased size of the NP-shell.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Influence of delay of polymer addition over morphology of mixed AuPPh₃Cl and BHT emulsions after reduction. PAA HCl is added at 1 min (a), 10 min (b) and 60 min (c) after onset of synthesis. See Figure S9 for the TEM micrograph when the polymer is added with the reducer, and Figure S10 for the statistical distribution of each population. [BHT] ≈ 0.36 mM, [Au] = 0.375 mM, [NaBH₄] = 7.5 mM and [PAA HCl] = 0.22 mg/mL.</figcaption></figure>

To understand how BHT helps the formulation of AuPPh₃Cl droplets, we used a thermodynamic model called Conductor Like Screening Model for Real Solvents (known as COSMO-RS). This model can estimate the solubility of molecules in various phases. It is already known that BHT forms Ouzo droplets in water/THF mixtures at these concentrations.¹³ Also, these drops are more or less swollen with THF. Using COSMO-RS, we calculated that BHT is soluble in THF up to 0.81 g/g. For the water/THF mixture used in this study, the solubility limit of BHT is exceedingly lower (4.9x10⁻⁴ g/g). In the present study we used BHT at mass fraction (8x10⁻⁵ g/g). This is consistent with the formation of BHT-rich droplets on addition of water, as observed in the so called “Ouzo effect”. Now for AuPPh₃Cl, we calculated the following solubilities: 0.03 g/g in THF, 0.067 g/g in THF-BHT
(mass ratio 1:1) solution and 0.089 g/g in pure BHT. Undeniably, AuPPh₃Cl is more soluble when BHT is present, so BHT helps preventing its crystallization. Nevertheless, the concentration of the precursor in BHT/THF droplets remains somewhat low, which might explain the formation of discrete particles.

We also investigated the case of Pd(PPh₃)₂Cl₂ in the presence of BHT, even though this precursor was able to form its own Ouzo emulsion. Interestingly, two types of droplets are present (see **Figure S11**): one that has a minor fraction of Pd(PPh₃)₂Cl₂, most certainly reaching the limit of solubility of Pd(PPh₃)₂Cl₂ in BHT, and the other one that has a major fraction of Pd(PPh₃)₂Cl₂ with possibly the presence of a smaller fraction of BHT. This can be seen in the TEM micrographs obtained after reduction (**Figure S12**), where two types of Pd continuous capsules co-exist: highly contrasted capsules with a higher relative concentration of Pd and lighter contrasted ones that have a smaller relative concentration of Pd (confirmed in EDX when comparing the Pd peak to the C or P peak). In conclusion, depending on the relative solubilities the use of an additional solute during nanoprecipitation may help prevent the crystallization of a poorly soluble compound, (case of AuPPh₃Cl) or make an emulsion with two types of droplets (case of Pd(PPh₃)₂Cl₂).

**Synthesis and characterization of Au-Pd bimetallic capsules.** Our previous work suggested that AuPPh₃Cl can be solubilized into Pd(PPh₃)₂Cl₂-rich droplets.³⁶ This is confirmed here, since AuPPh₃Cl does not precipitate in the presence of the palladium precursor. Furthermore, DLS shows that droplets are much more stable when the two metallic precursors are combined as compared to the Pd precursor alone (**Figure S13**). This surprising result suggests a possible interaction or reaction between the two precursors. After reduction, continuous shells containing highly contrasted inclusions are observed when both precursors are mixed (**Figure 7**). One can see that as the ratio of Au precursors to Pd increases, the morphology changes from a continuous shell (0% Au), to increasing levels of inclusions in continuous shells (10%, 25%, 50% Au), and to NPs stuck together (75% Au). At the other end of the spectrum (100% Au), we would expect to have NP-shells if the precursor would not crystallize on its own. This is quite interesting: we can find a systematic progression between continuous shells for palladium to discrete NP-shells for gold, which means that we truly have bi-metallic Ouzo droplets (as confirmed by EDX).

<figure><img src="image_9.png" /></figure>

**Figure 7.** TEM micrographs of the gold-palladium hybrids highlighting the gradual shift from one morphology (smooth nanosphere) to the second (sphere composed of individual particles). The nominal molar compositions are as follow (Au-Pd): a) 0 - 100, b) 10 - 90, c) 25 - Pd, d) 50 - 50, and e) 75 - 25. [Au + Pd] = 0.25 mM and [NaBH₄] = 7.5 mM.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** a) Dark field ARM of gold-palladium (equimolar composition of Au and Pd) nano-hybrids highlighting the presence of highly refractive inclusions. SAED (b) and EDX (c) patterns with corresponding Miller indices and characteristic peaks for Pd (green) and gold (yellow). d) EDX line spectrum passing through 3 different inclusions. [Au] = 0.125 mM, [Pd] = 0.125 mM and [NaBH₄] = 7.5 mM.</figcaption></figure>

As comparison to the *in situ* synthesis of hybrid capsules, we synthesized gold NPs *ex situ* and incorporated them in an *in situ* Pd(PPh₃)₂Cl₂ synthesis (see **Figure S14**). Interestingly, we were able to replicate the morphology of hybrid capsules having a gold content ≤ 50% molar, where we can see highly contrasted inclusions in a sphere. Thus, we wanted to probe the composition of the shell and inclusions with atomic resolution microscopy (ARM), revealing a quite interesting story. Indeed, in **Figure 8a**, one can see in ARM dark field mode that for an equimolar *in situ* synthesis with both precursors, the capsules display contrasted inclusions. SAED is provided in **Figure 8b**, but since face-centered cubic (fcc) Pd and fcc Au have similar crystalline planes and distances, specific identification is hard to achieve. EDX (**Figure 8c**) reveals the expected equimolar composition of both metals in the whole capsule – a perfect 50% is not obtained due to the high thickness of the capsules. Moreover, given the great resolution accuracy of ARM, it was possible to realize a linear EDX analysis profile (**Figure 8d**) with a level of resolution on par with the inclusions’ size. This provided us with an unexpected result: each time the electron beam reaches an inclusion, both Pd and Au levels rise proportionally. If we were in presence of gold inclusions, gold would be the sole element to increase in intensity, while Pd would stay constant. This suggest that the inclusions are not merely composed of a singular element, as we initially considered, but are rather a solid solution of both metals.²⁹ Since the SAED of Pd continuous nanospheres showed an amorphous halo without any diffraction, the contrast in TEM micrographs and the diffraction pattern (**Figure 8b**) observed in the hybrid nanospheres can then be attributed to the higher crystallinity of these “inclusions”. Indeed, contrast in TEM depends on the interaction of the electron beam with the sample, thus, diffraction linked to crystallinity leads to an increase in contrast. Taking into account that the SAED of Pd continuous nanospheres revealed only an amorphous halo and no diffraction, the contrast in the hybrid nanospheres and the diffraction pattern (**Figure 8b**) would then only be a result of the higher crystallinity of these “inclusions” – in TEM, contrast is a function of the interaction of the electron beam with the sample, so diffraction linked to crystallinity increases contrast. This brings forth an interesting technique to yield dual-metallic
nano-objects, but also morphologies that might display interesting catalytic properties.

**Catalytic hydrolytic dehydrogenation of ammonia-borane.** Since they are fully water-compatible and can form low-concentration colloids, palladium-, platinum- and gold-NP-shells were all tested for the H₂ releasing hydrolysis of ammonia-borane (Equation 1). Note that low metal content catalysts are needed for sustainability reasons (see⁴⁶ and references therein).

In addition, the metal availability of the capsules with discrete particles is significantly greater than that of the capsules with a continuous shell, since a high proportion of metal is stuck in the core of the latters.

$$ \mathrm{NH_3BH_3} + 2\mathrm{H_2O} \rightarrow \mathrm{NH_4^+} + \mathrm{BO_2^-} + 3\mathrm{H_2} \quad (\text{Equation 1}) $$

The hydrolysis reactions of AB were conducted in water by suspending 1% weight of metal to AB (1 mmol). The activity of samples prepared from *in situ* synthesis and *ex situ* syntheses (see Table S1) were compared in Table 1. Both XPS (Figure S15) and ICP studies (Table 1) confirmed the presence of the metal and its content, respectively. TEM micrographs of these samples and their dehydrogenation kinetics can be found respectively in Figure S16 and Figure S17 in the ESI. While nanocatalysts based on platinum and palladium are recognized for their good activity for the hydrolysis and alcoholysis of ammonia-borane,⁴⁷,⁴⁸ to the best of our knowledge only a few systems incorporating nanogold in the framework of multimetallic composites have been reported as active in this reaction.⁴⁹⁻⁵³ The initial exploration of AB hydrolysis from freshly formed AuNP-shells (sample 1) achieved at 25 °C a turnover frequency (TOF – see Equation S1) of 10.5 min⁻¹ in hydrogen delivery (Figure S17). Unfortunately, under our catalytic conditions, this high activity was hardly reproducible. The instability of the colloidal system was visible from its temperature sensitivity since at 50 °C, the activity of sample 1 dropped at 7.1 min⁻¹ (Figure S18). We finally established a reproducible TOF of 1.4 ± 0.5 min⁻¹, achieved at 25 °C from different colloidal solutions at 0.16, 0.17, 0.20 or 0.23 mM Au, including samples aged several months (see Table S2). These results established for the first time the modest, but genuine activity of gold colloidal nanodroplets for AB hydrolytic dehydrogenation.

While the activity of platinum NP-shells, sample 3, was disappointing (TOF = 5.4 min⁻¹ after 1 h), palladium NP-shells sample 2 achieved a TOF of 9.2 min⁻¹ at 25 °C in AB hydrolysis that consistently increased up to 42 min⁻¹ at 50 °C (Figure S18), and therefore did not suffer from catalytic deactivation like nanogold sample 1. None of the bimetallic mixed systems (Au/Pd and Au/Pt samples 4, 5) achieved notable catalytic AB hydrolysis under our standard conditions. It is interesting to note that sample 4 has lower TOFs compared to the corresponding NP-shells in samples 1 and 2, which might be explained by the morphology of the shells (the catalytic surface is harder to reach in continuous shells as opposed to discrete particles).

These preliminary studies indicated that nanodroplet systems synthesized from the Ouzo emulsion approach in water may be active in the hydrolytic dehydrogenation of ammonia-borane, provided stable systems are formed. Complementary studies have shown that the metal surface of the NP-shells is accessible whether a polymer is used or not. Since the presence of the polymer improves the stability of the systems, it is more promising in such catalyst applications.

<table><caption>**Table 1.** Catalytic properties of the NP-shells used in dehydrogenation of ammonia-borane at 25°C.</caption><thead><tr><th>Sample</th><th>Metal/morphology</th><th>ICP Concentration (mM)</th><th>Reaction time (h)</th><th>TOF<sub>1 h</sub> (min<sup>-1</sup>)</th><th>TOF<sub>end</sub> (min<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>AuNPs</td><td>0.23</td><td>2.5</td><td>18.54</td><td>10.51</td></tr><tr><td>2</td><td>PdNPs</td><td>0.26</td><td>1</td><td>9.18</td><td>9.18</td></tr><tr><td>3</td><td>PtNPs</td><td>0.06</td><td>7.5</td><td>5.36</td><td>2.91</td></tr><tr><td rowspan="2">4</td><td rowspan="2">Au – Pd shells and inclusions</td><td>Au: 0.09</td><td rowspan="2">3.75</td><td rowspan="2">1.38</td><td rowspan="2">6.90</td></tr><tr><td>Pd: 0.06</td></tr><tr><td rowspan="2">5</td><td rowspan="2">Au – Pt NPs</td><td>Au: 0.06</td><td rowspan="2">4.58</td><td rowspan="2">1.33</td><td rowspan="2">0.70</td></tr><tr><td>Pt: 0.05</td></tr></tbody></table>

**Controlled growth of the AuNPs using a seed-mediated approach.**

Since the metal surface was shown to be available (active in catalysis), a seeded growth approach was tested. To this aim, we used the AuNPs shells stabilized with PAA HCl and dialyzed (see Figure 9a). Their initial shell consists of small AuNPs (~2.7 nm, Figure 9a). In the first assay (Figure 9b&c), a gold precursor (HAuCl₄) was added, followed by a reducing agent (ascorbic acid – AA). The individual particles grow in size up to ca 9 nm. Most surprisingly, the NP-shell also grow significantly, suggesting the formation of a thick layer of AuNPs. In the second assay (Figure S19), only the gold precursor was added. The polymer stabilizing the shells (PAA HCl) contains a primary amine group, strong enough to reduce HAuCl₄.⁵⁴ Large anisotropic particles (29 nm) are observed in this case. Still, these assemblies of particles retain their initial droplet morphologies. A color change
is perceived in both cases, especially for the first synthesis with both Au and reducing agent where a stable blue solution is obtained (see **Figure S20**). The color changes are due to plasmonic effects, related both to the growth of the AuNPs and the change in their mean distance.⁵⁵,⁵⁶

<figure><img src="image_11.png" /><figcaption>**Figure 9.** TEM micrographs of AuNP-shells (a) before and (b-c) after seeded-growth in the presence of a mild reducing agent (ascorbic acid).</figcaption></figure>

Finally, to further demonstrate the versatility of the interfacial reduction of metal precursors in Ouzo systems, we modified the reducer and, in a separated experiment, the Ouzo system. Firstly, NaBH₄ was replaced with tetrakis(hydroxymethyl)phosphonium chloride (THPC). Then, the phases in the emulsion were replaced with methanol instead of water, toluene instead of THF and heptane instead of the hydrophobic solute BHT. In both cases, the gold precursor AuPPh₃Cl was used, and AuNP-shells were obtained as previously (see micrographs of **Figure S21**).

**Conclusion**

The interfacial reduction of metallic precursors formulated as SFME and Ouzo emulsions yielded submicronic capsules made of a shell of discrete nanoparticles, continuous shell or even a combination of both, in a scalable one-pot approach. Such oil- and surfactant-free emulsions with a metal precursor core had not yet been reported in the literature. Very interestingly, the reported strategy enables to control the morphology of the capsules shell, by working at selected compositions either in the Ouzo domain or in the SFME region. This was explained by the highly solvated content of the SFME droplets as opposed to the Ouzo droplets, which are more concentrated in the solute. Control over the size of the nanospheres can also be achieved, via balancing reaction time vs emulsion growth time, as the building up of the shell stops the coalescence. Noticeably, nanospheres with a continuous shell containing embedded nanoparticles, were obtained from a mixture of Au and Pd precursors. Advanced TEM and chemical analyses (ARM) suggest that the two metals alloy. It is also important to note, that the metal-shells were synthesized in the absence of surfactants and are stable in water, which is rarely the case for nanoparticles synthesized in solution or in conventional emulsions. This is particularly relevant in the field of catalysis. Indeed, some of the synthesized NP-shells performed the hydrolytic dehydrogenation of AB at room temperature, with reasonable TOFs for H₂ release concerning Pd and PtNPs. We also observed for the first time that gold colloids can be active in this reaction. Importantly, the presented approach to synthesize submicronic shells, is not limited to specific material types but can be extended to a wide array of materials of interest.

**Supporting Information**

Contains additional experimental materials and procedures including a scheme of the amine-borane dehydrogenation setup, discussion about ICP and XPS, supplementary information about characterization of the nanomaterials and catalytic dehydrogenation kinetics.

**Acknowledgements**

This work was supported by the Alexander Graham Bell Canada graduate scholarships program from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fond Européen de Développement Régional and the graduate school EUR-EIPHI (ProdHyg project "Secured Hydrogen Production"), by the French National Agency of Research (ANR-20-CE06-0031-01 OUZOFAN), and from the IRN Nanomatériaux Multifonctionnels Contrôlés (IRN NMC). Prof. Virgilio acknowledges the financial support of NSERC via the Discovery Grants program. F. Gauffre acknowledges the financial support of University of Rennes (AAP 2024: Collaborations Internationales). F. Gauffre & M. Kahn acknowledge the financial support of CNRS. The authors also wish to thank the center Castaing (UAR3623), L. Joanny (CMEBA-ScanMAT UAR 2025) and A. Dupont (Biosit MRic) for electron microscopy.

**CRediT authorship contribution statement**
**Olivier Gazil:** Conceptualization, Methodology, Investigation, Data Curation, Writing – original draft, Writing – Review & Editing, Visualization. **Ludivine Rault:** Investigation. **Déborah Iglicki:** Investigation. **Vincent Collière:** Investigation. **Gizem Karacaoglan:** Investigation, Visualization. **Didier Poinsot:** Investigation. **Moad Bouzid:** Investigation. **Jean-Cyrille Hierso:** Conceptualization, Writing – original draft. **Myrtil L. Kahn:** Conceptualization, Writing – Review & Editing. **Nick Virgilio:** Conceptualization, Funding acquisition, Writing – original draft, Supervision. **Fabienne Gauffre:** Conceptualization, Writing – original draft, Funding acquisition, Resources, Supervision.

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