Andrew D Pendergast

Matthew W Glasscott

Christophe Renault

Jeffrey E Dick
email: jedick@email.unc.edu

One-Step Electrodeposition of PdPt Alloy Nanoparticles from Water Droplets: Controlling Size, Coverage, and Elemental Stoichiometry

Keywords: Alloy, Nanoparticle, Bimetallic, Emulsion Droplet, Electrodeposition

We present a robust and facile method to produce metal nanoparticle (NP) alloys in a one-step synthesis using direct electrodeposition onto highly oriented pyrolytic graphite (HOPG). Precursor salts, H2PtCl6 and Pd(NO3)2, were dissolved in a 1 mM sodium dodecylsulfate (SDS) water droplet with 1X phosphate buffered saline solution and suspended in a dichloroethane (DCE) continuous phase.

Tetrabutylammonium perchlorate was added to the DCE continuous phase to maintain charge balance during electrodeposition. NP fabrication via electrodeposition was driven by droplet collisions onto HOPG, which was biased at a potential where the metal precursor salts would reduce to their respective zerovalent atoms. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) were used to study the size, coverage, and morphology of the NPs as well as the atomic stoichiometry. EDX mapping indicated homogeneous distribution of Pd and Pt at the single NP level. Homogeneously alloyed PdPt NPs were realized from this study with demonstrated control over metal composition, surface coverage, and NP size.

Introduction

Owing to their unique chemical and physical properties that sometimes differ from bulk, polycrystalline metals, metallic nanoparticles (NPs) have received extensive interest in the field of electrochemistry with a particular emphasis on electrocatalysis and sensing applications [START_REF] Rassaei | Nanoparticles in electrochemical sensors for environmental monitoring[END_REF][START_REF] Penner | Electrodeposited nanophotonics[END_REF][START_REF] Bartlett | Nanoelectrode array formation by electrolytic nanoparticle impacts[END_REF][START_REF] Stevenson | A materials driven approach for understanding single entity nano impact electrochemistry[END_REF][START_REF] Allerston | Nanoparticle impacts in innovative electrochemistry[END_REF].

Previous articles have reported metal nanoparticle collision events at ultramicroelectrodes to study fundamental electrochemical processes at the single NP level with applications towards understanding electrocatalytic activity at the nanoscale [START_REF] Bartlett | Nanoelectrode array formation by electrolytic nanoparticle impacts[END_REF][START_REF] Sokolov | Electrode-particle impacts: a users guide[END_REF]. Sometimes, however, NPs made of a single metal do not present desired stability or activity. For electrocatalysis in particular, metal alloy NPs (MANPs) offer tremendous advantages over single metal NPs in their chemical and physical properties [START_REF] Chen | Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces[END_REF][START_REF] Qiao | A general synthesis strategy of multi-metallic nanoparticles within mesoporous titania via in situ photo-deposition[END_REF]. For instance, direct methanol oxidation on platinum is difficult due to the deleterious effects of carbon monoxide poisoning; however, it has been shown that the incorporation of palladium into platinum allows for effective methanol oxidation without the poisoning effects of carbon monoxide [START_REF] He | An efficient reduction route for the production of Pd-Pt nanoparticles anchored on graphene nanosheets for use as durable oxygen reduction electrocatalysts[END_REF][START_REF] Liu | Composition-controlled synthesis of bimetallic PdPt nanoparticles and their electro-oxidation of methanol[END_REF][START_REF] Wang | Strategic Synthesis of trimetallic Au@Pd@Pt core-shell nanoparticles from poly(vinylpyrrolidone)based aqueous solution toward highly active electrocatalysts[END_REF]. Furthermore, the addition of iron oxide into a nickel oxide electrocatalyst has been shown to greatly increase the activity toward the oxygen evolution reaction [START_REF] Barforoush | Insights into the active electrocatalytic areas of layered double hydroxide and amorphous nickel-iron oxide oxygen evolution electrocatalysts[END_REF]. Beyond electrocatalytic applications, bimetallic and higher-order alloyed NPs can offer unique structural, magnetic, and optical properties compared to monometallic and bulk materials [START_REF] Ferrando | Nanoalloys: from theory to applications of alloy clusters and nanoparticles[END_REF]. Importantly, the structure and elemental composition of homogeneous NPs can be modified to optimize catalytic pathways by combining unary elemental properties and revealing novel dynamic properties [START_REF] Kleijn | Electrochemistry of nanoparticles[END_REF][START_REF] Chen | Polyelemental nanoparticle libraries[END_REF]. Thus, the ability to alloy metal NPs and control the stoichiometry is of great importance.

To study the electrochemical properties of MANPs, the general requirement of attaching the MANPs to a substrate must be met for measurements to be made. While success has been made in the homogeneous synthesis of MANPs and subsequent adsorption of MANPs onto conductive surfaces [START_REF] Ferrando | Nanoalloys: from theory to applications of alloy clusters and nanoparticles[END_REF], these MANPs must incorporate a ligand to achieve a stable colloidal suspension.

Previously, Booth and co-workers used an electron-transfer mediator to generate gold NPs at the interface between a trifluorotoluene droplet and water, effectively producing a Pickering emulsion droplet [START_REF] Booth | Electrodeposition of gold nanostructures at the Interface of a Pickering emulsion[END_REF]. From these experiments, two main limitations must be considered: 1.) The use of ligands may block surface sites for electrocatalytic reactions, impeding the study of NP reactivity [START_REF] Xiao | Observing single nanoparticle collisions at an ultramicroelectrode by electrocatalytic amplification[END_REF][START_REF] Fan | Observing single nanoparticle collisions by electrogenerated chemiluminescence amplification[END_REF][START_REF] Zhou | Observation of discrete Au nanoparticle collisions by Eeectrocatalytic amplification using Pt ultramicroelectrode surface modification[END_REF][START_REF] Dick | Toward the digital electrochemical recognition of cobalt, iridium, nickel, and iron ion collisions by catalytic amplification[END_REF][START_REF] Percival | Fast-scan cyclic voltammetry allows determination of electron-transfer kinetic constants in single nanoparticle collision[END_REF] and 2.) drop-casting and adsorption onto a substrate does not guarantee electrical connection of the MANP to the underlying substrate, thereby leading to variability in the total charge transfer observed via electrochemical characterization [START_REF] Wilson | Imaging electrogenerated chemiluminescence at single gold nanowire electrodes[END_REF]. One method of solving these issues is by direct electrodeposition of MANPs onto the surface of an electrode. Direct electrodeposition does not require stabilizing ligands, maximizing the surface area of a MANP available for electrocatalytic studies. Electrodeposition also requires contact to be made to the underlying substrate, such as highly-oriented pyrolytic graphite (HOPG) [START_REF] Zhang | Electrochemistry at highly oriented pyrolytic graphite (HOPG): lower limit for the kinetics of outer-sphere redox processes and general implications for electron transfer models[END_REF], minimizing the fraction of MANPs without electrical contact. Previous efforts have suggested that emulsionbased systems can be used to generate NPs by direct electrodeposition [START_REF] Jeun | Surfactant-free electrochemical synthesis of metallic nanoparticles via stochastic collisions of aqueous nanodroplet reactors[END_REF], however, these techniques demonstrate limited control over NP size, morphology, and coverage.

Classical electrodeposition of MANPs is not well characterized due to difficulties in controlling size, coverage, morphology, and composition of the NPs. We recently addressed the issue of non-homogeneous electrodeposition of metal NPs with a new method. This method is based on the electrodeposition of monometallic NPs from aqueous droplets suspended in a dichloroethane (DCE) phase (i.e. a water-in-oil emulsion). Briefly, a metal precursor confined in a water droplet is reduced to metal when the freely diffusing droplet collides with the surface of a conducting substrate biased at a sufficiently cathodic potential for electrodeposition to occur.

We assume that each droplet collision results in the electrodeposition of a metal NP, which was supported by an analysis of the mass transfer of droplets [START_REF] Glasscott | A universal platform for the electrodeposition of ligand-free metal nanoparticles from a water-in-oil emulsionsSystem[END_REF]. The stochastic nature of the collision generates a random distribution of NPs on the surface. We have demonstrated accurate control over surface coverage and size for Pt NPs deposited on amorphous graphitic substrates [START_REF] Glasscott | A universal platform for the electrodeposition of ligand-free metal nanoparticles from a water-in-oil emulsionsSystem[END_REF][START_REF] Glasscott | Direct Eeectrochemical observation of single platinum cluster electrocatalysis on ultramicroelectrodes[END_REF]. Our approach was also extended to the electrodeposition of Au, Ag, Sn, Ce, Fe, and Cu NPs onto various substrates including boron-doped diamond, gold, silicon, and highly oriented HOPG. Here, we extend this method to the electrosynthesis of PdPt MANPs and demonstrate control over size, coverage on HOPG, and stoichiometric composition of metals. This method represents the first of its kind to robustly control size, coverage, and metal composition during electrodeposition experiments.

Experimental

All chemical reagents were analytic grade and used without further purification. Stock solutions of each metal (100-300 mM) were made in MilliQ water (>18.2 MΩ•cm) and diluted as necessary to be used in the emulsion preparation. It is important to note that metal salt precursor stock solutions can photodegrade under ambient conditions, therefore a fresh stock solution should be made for daily experiments. The electrodeposition experiments were performed using a CHI model 601E potentiostat (CH Instruments, Austin, TX) on the "Amperometric i-t curve" mode. Scanning Electron Microscopy (SEM) images and Energy Dispersive X-Ray Spectroscopy (EDX) spectra were taken using a Helios 600 Nanolab Dual Beam System (FEI, Hillsboro, OR) and INCA PentaFET -x3 (Oxford, Abingdon, United Kingdom) respectively at 30 keV and 0.69 nA. A schematic diagram of the NP electrodeposition procedure is presented in Figure 1.

First, a water-in-DCE emulsion was prepared from a 30 µL 1 mM sodium dodecylsulfate (SDS) aqueous droplet containing 15 mM H2PtCl6 and 15 mM Pd(NO3)2 and 1X phosphate buffered saline solution (PBS), in an organic phase of 5 mL DCE with 0.1 M tetrabutlyammonium perchlorate (TBAClO4). TBAClO4 was used as a non-aqueous supporting electrolyte necessary to maintain charge balance during the electrodeposition by the transfer of TBA+ across the oil|water interface [START_REF] Barker | Measurement of solute partitioning across liquid/liquid interfaces using scanning electrochemical microscopy-double potential step chronoamperometry (SECM-DPSC): principles, theory, and application to ferrocenium ion transfer across the 1,2-dichloroethane/aqueous interface[END_REF]. Three different emulsions were prepared to demonstrate stoichiometric control: Pd0.1Pt0.9, Pd0.5Pt0.5, and Pd0.9Pt0.1. Emulsions were formed by ultrasonication (500 W, amplitude 40%, 1/16 th '' microtip probe, Q500 Ultrasonic Processor, Qsonica, CT) with a pulsed sonication method (5 s on, 5 s off; total 6 cycles). A HOPG substrate with an exposed radius of 2 mm (the working electrode, WE in Figure 1) is biased at -0.2 V vs Ag/AgCl (1M KCl quasi reference electrode, RE in Figure 1) [START_REF] Glasscott | A universal platform for the electrodeposition of ligand-free metal nanoparticles from a water-in-oil emulsionsSystem[END_REF][START_REF] East | Easy-to-make Ag/AgCl reference electrode[END_REF], a sufficiently negative potential to ensure the reduction of both metal salts as depicted in the schematic zoom of a single droplet in Figure 1. We found that -0.2 V was sufficient enough for electrodeposition due to the manifestation of NPs in electron micrographs. The counter electrode (CE in Figure 1) is a Pt wire. The cell is filled with 2.0 mL of emulsion solution, and a chronoamperogram was recorded. After electrodeposition over typically 1000 s, the substrate is rinsed with DCE, 100% ethanol, and MilliQ water for 5 minutes each to remove residual salts and surfactants. A UV-Vis analysis indicated up to 1.6% of metal salt partitioned out of the droplets due to the sonication process (data not shown).

While this small amount of salt had no negative effects on NP electrosynthesis, the effects of metal ion partitioning at the water-DCE interface on NP structure is currently being investigated.

Results and Discussion

Size and Coverage Control

A representative SEM image of a HOPG substrate following PdPt alloy NP electrodeposition is shown in Figure 1c with bright spots corresponding to NPs. Under our experimental conditions, the surface density of NPs is homogeneous as previously shown for monometallic NPs electrodeposited using this method [START_REF] Glasscott | A universal platform for the electrodeposition of ligand-free metal nanoparticles from a water-in-oil emulsionsSystem[END_REF]. The NP density on the underlying substrate can be adjusted by varying electrodeposition time.

Figure 2a evidences that the coverage of NP varies with the duration of electrodeposition, as expected for a system where where C is the concentration of droplets, D is the diffusion coefficient of droplets, t is time, and NA is Avogadro's number. Here, the diffusion coefficient of a droplet is estimated using the Stokes-Einstein approximation. The observed deviation from Cottrellian behavior may be explained by the presence of droplet stabilizing surfactant which can inhibit electron transfer at the droplet-electrode interface coupled with polydispersity in the emulsion droplet size [START_REF] Glasscott | Direct Eeectrochemical observation of single platinum cluster electrocatalysis on ultramicroelectrodes[END_REF].

Understanding the underlying complex NP electrodeposition and precursor ion diffusion mechanisms is a current avenue of study in this lab.

Representative NP micrographs are additionally presented in Figure 1c demonstrating unique morphologies via electrodeposition. The average radius of the NPs is 32 ± 8 nm with a hemispherical morphology with a size distribution as presented in Figure 2c. The size of the NP can be tuned by changing the concentration of metal precursor inside the droplets. Figure 2b shows statistically significant variations in mean NP radius from 20 to 30 nm when changing the total metal precursor concentration from 10 to 25 mM (statistical analysis via One-Way ANOVA with Turkey's t-test for multiple comparisons). The relationship between initial precursor concentration in the droplet and NP radius follows a cubic root law (see blue line in Figure 2b).

The volume, and thus the size, of the resulting NP is found by multiplying the volume of the unit cell by the total number of unit cells, assuming that each molecule of precursor salt reduces to a single metal atom in a fcc lattice.

Stoichiometric Control

We were interested in determining whether or not the electrodeposited NPs are an alloy of Pt and Pd or two separated phases. Representative SEM and EDX images are shown in Figure 3 Pd0.52Pt0.48, and Pd0.18Pt0.82 respectively. The Pd0.89Pt0.11and Pd0.52Pt0.48 molar ratio follow closely the initial ratio of precursors inside the droplet. The minor deviation observed for the Pd0.1Pt0.9 case can possibly be attributed to uncertainties in the quantification. Most importantly, the composition of the alloy can be conveniently tuned by adjusting the initial relative concentration of precursors inside the droplet. The mechanism of NP formation within the droplet is currently under investigation. As elemental stoichiometry has been previously shown to enhance electrocatalytic properties of alloyed materials above monometallic metal systems, NP electrocatalytic characterization via voltammetry is a current investigation of this laboratory.

Conclusion

In conclusion, we demonstrate that electrodeposition of NPs via co-reduction of PtCl6 -2 and Pd(NO3)2 in a water-in-oil emulsion system generates alloyed NPs with tuneable stoichiometry.

Fabrication of these alloy NPs can be controlled with respect to NP size and substrate coverage.

While a number of NP fabrication techniques currently exist, this proposed droplet deposition technique offers a novel method to generate alloy NPs. This approach enables homogeneous functionalization of any conducting substrate with alloy NPs and in principle can be extended to any alloy provided metal precursors are stable in water and can be electrochemically reduced.

The flexibility of our electrodeposition technique coupled with the numerous interesting properties of alloys renders this technique desirable for a wide range of applications.

Figure 1 .

 1 Figure 1. a) A water-in-DCE emulsion was prepared by horn sonication, b) following ultrasonication, droplet-mediated electrodeposition occurs at the droplet-electrode interface, c) Surface coverage and NP size were controlled by electrodeposition time and metal salt precursor concentration respectively. SEM images on the right show a representative HOPG substrate following electrodeposition of Pd0.5Pt0.5 NPs and representative single NP morphologies.

Figure 2 .

 2 Figure 2. a) Substrate coverage by PdPt NPs (30 mM total metal salt precursor) can be controlled by tuning electrodeposition time. b) Pd0.5Pt0.5 NP size can be modulated by varying total metal precursor ion concentration in aqueous droplet. c) 30 mM Pd0.5Pt0.5 NP population radius distribution following 1000s deposition.

Figure 3 .

 3 Figure 3. Representative SEM images, EDX elemental maps, and EDX spectra of varying stoichiometry PdxPty NPs demonstrating consistent elemental homogeneity independent of precursor ion concentration. Representative EDX spectra presented with highlighted characteristic Pd Lα X-ray peak at 2.84 KeV and Pt Mα X-ray peak at 2.05 KeV. EDX elemental map brightness has been normalized according to theoretical stoichiometry.

Acknowledgements

We would like to acknowledge the University of North Carolina at Chapel Hill for start-up funds, which supported this work. This work was performed in part at the Chapel Hill Analytical and Nanofabrication Laboratory, CHANL, a member of the North Carolina Research Triangle Nanotechnology Network, RTNN, which is supported by the National Science Foundation, Grant ECCS-1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI. C.R. acknowledges the funding of the CNRS and the Agence Nationale de la Recherche (ANR-17-CE09-0034-01, "SEE").

Conflict of Interest Statement

Declarations of interest: none.