Supported Silver Nanoparticle and Near-Interface Solution Dynamics in a Deep Eutectic Solvent Joshua A. Hammons,* ,† Jon Ustarroz, ‡ Thibault Muselle, ‡ Angel A. J. Torriero, § Herman Terryn, ‡ Kamlesh Suthar, † and Jan Ilavsky † † X-ray Science Division, Argonne National Laboratory, 9700 South Cass, Argonne, Illinois 60439, United States ‡ Department of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium § Centre for Chemistry and Biotechnology, School of Life and Environmental Sciences, Deakin University, Burwood, Victoria 3125, Australia * S Supporting Information ABSTRACT: Type III deep eutectic solvents (DES) have attracted significant interest as both environmentally friendly and functional solvents that are, in some ways, advantageous to traditional aqueous systems. While these solvents continue to produce remarkable thin films and nanoparticle assemblies, their interactions with metallic surfaces are complex and difficult to manipulate. In this study, the near-surface region (2-600 nm) of a carbon surface is investigated immediately following silver nanoparticle nucleation and growth. This is accomplished, in situ, using a novel grazing transmission small- angle X-ray scattering approach with simultaneous voltamme- try and electrochemical impedance spectroscopy. With this physical and electrochemical approach, the time evolution of three distinct surface interaction phenomena is observed: aggregation and coalescence of Ag nanoparticles, multilayer perturbations induced by nonaggregated Ag nanoparticles, and a stepwise transport of dissolved Ag species from the carbon surface. The multilayer perturbations contain charge-separated regions of positively charged choline-ethylene and negatively charged Ag and Cl species. Both aggregation-coalescence and the stepwise decrease in Ag precursor near the surface are observed to be very slow (∼2 h) processes, as both ion and particle transport are significantly impeded in a DES as compared to aqueous electrolytes. Altogether, this study shows how the unique chemistry of the DES changes near the surface and in the presence of nanoparticles that adsorb the constituent species. ■ INTRODUCTION Type III deep eutectic solvents (DES) have attracted significant interest as green alternatives to aqueous solvents, 1,2 as their components are tunable, readily available, and their use would reduce the strain on naturally occurring water resources. These two-component solvents typically consist of a quaternary ammonium salt and hydrogen-bond donor, at their eutectic composition. 3 Advantages of these solvents include the ability to dissolve a wide range of precursors, tunability, nonvolatility, they are nontoxic (some), biodegradable, and they have slightly wider electrochemical windows than aqueous solutions. 3 Because of their unique physiochemical properties, DESs are being considered for an array of applications, 4 including nanotechnology. 5 These unique solutions have also been shown to facilitate Au, 6 PbS, 7 Pt, 8 Pd, 9,10 and Ni 11 nanoparticle assembly, as well as improved thin film deposition. 12 Altogether, DESs have the potential to provide a cost-effective and environmentally friendly medium to synthesize and assemble various nanomaterials. Silver nanoparticles have a wide variety of applications, ranging from antimicrobial agents, 13,14 sensors, 15,16 catalysis, 17 to photoelectrochemistry. 18 In this study, Ag nanoparticles were electrodeposited onto a carbon surface from the DES solution, which contained dissolved AgCl. This approach was chosen because it can be used as a scalable, bottom-up approach to supported nanoparticle synthesis. 19 To have reliable operation of such electrochemical surfaces, it is important to understand and manipulate the mechanisms that can control the size of the deposited nanomaterial. 20-22 To this end, we consider both the stability of the deposited particles as well as the interfacial chemistry as it affects the growth kinetics of the Ag particles. The nucleation and growth of nanoparticles, during the electrodeposition pulse, is most often considered either instantaneous or progressive. 19 Indeed, the nucleation and growth of surface nanoparticles will affect the size distribu- tion. 19 However, this study focuses on so-called “late-stage” phenomena, which occur after the electrodeposition pulse. For Received: October 8, 2015 Revised: December 9, 2015 Published: December 9, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 1534 DOI: 10.1021/acs.jpcc.5b09836 J. Phys. Chem. C 2016, 120, 1534-1545