ИСТИНА

Polarizable soft interfaces between two immiscible electrolyte solutions (ITIES) have emerged as a new platform at which to study charge transfer reactions that impact energy research, chiefly the hydrogen evolution (HER)[1] and oxygen reduction reaction (ORR) [2]. Soft interfaces also represent an ideal system at which to self-assemble molecules or nanoparticles (NPs) into 2D films that are defect-free, mechanically flexible and exhibit self-recovery [3]. Also, these assemblies of NPs are under vast researcher’s attempts in many areas such as Raman-spectroscopy[4], nano-sized mirrors and filters[5] etc. From electrochemical view point, NPs, especially metallic ones, can be considered as multivalent redox species. Their properties depend on the excess charge present on NPs, which can be tuned either by incorporating NPs with a redox couple in solution or through Galvani potential difference [6]. Here we present interfacial redox catalysis phenomenon and highlight the catalytic properties of a gold nanoparticle film towards heterogeneous electron transfer (HET) reactions. We developed a methodology to gently functionalize soft or “liquid-liquid” interfaces with mirror-like films of gold nanoparticles (AuNPs) using precise interfacial microinjection (Fig.1A-B). Presence of electron-donor in organic phase, such as Fc, immediately leads to charging of AuNPs assemblies and formation of Fc+ in the vicinity of the interface (Fig.1C). Figure 1. A) Schematic of the capillary and syringe-pump setup used to settle AuNPs directly at the ITIES. B) Examples of AuNP films prepared at flat soft water | trifluorotoluene interface using AuNPs with mean diameters of 38 nm. C) Cyclic voltammograms showing formation of Fc+ after charging of AuNPs. Scan rate for CVs is 25 mV/s. The surface coverage of the film was calculated based on UV-Vis spectroscopy in accordance with previously developed procedure [7] and also characterized by ion transfer voltammetry, giving estimation that ca. 30% of the surface is occupied by AuNPs. This is equivalent of roughly ½ monolayer of close packed spheres. These gold nanoparticle films represent an ideal model system for studying both the thermodynamic and kinetic aspects of interfacial redox catalysis. Thus, electron transfer (ET) across the interface was examined between oil-solubilized Fc+/0 and aqueous [Fe(CN)6]3–/4– redox couples, revealed significant differences (Fig.1). In the case of the bare interface (Fig.2A), the process takes place by a combination of (i) slow HET and (ii) ion-transfer mechanism. However, ET appears significantly more reversible under semi-infinite linear diffusion control when the AuNP film is present, providing clear evidence that the AuNP film acts as an efficient redox catalyst for interfacial Fermi level equilibration. Figure 2. Experimental evidence of interfacial redox catalysis in the presence of AuNP films. Cyclic voltammograms (iR compensated) of ET between the oil-solubilized Fc+/0 redox couple and the aqueous [Fe(CN)6]3–/4– redox couple in the absence A) and the presence B) of AuNPs. References [1] I. Hatay, B. Su, F. Li, R. Partovi-Nia, H. Vrubel, X. Hu, M. Ersoz, H. H. Girault, Angew. Chemie 2009, 48, 5139–42. [2] P. Peljo, L. Murtomäki, T. Kallio, H.-J. Xu, M. Meyer, C. P. Gros, J.-M. Barbe, H. H. Girault, K. Laasonen, K. Kontturi, J. Am. Chem. Soc. 2012, 134, 5974–84. [3] J. B. Edel, A. a Kornyshev, M. Urbakh, ACS Nano 2013, 7, 9526–32. [4] M. P. Cecchini, V. A. Turek, J. Paget, A. A. Kornyshev, J. B. Edel, Nat. Mater. 2012, 12, 165–71. [5] M. E. Flatte, A. A. Kornyshev, M. Urbakh, J. Phys. Chem. C 2010, 114, 1735–1747. [6] M. Scanlon, P. Peljo, M. A. Mendez, E. A. Smirnov, H. Girault, Chem. Sci. 2015, DOI 10.1039/C5SC00461F. [7] E. Smirnov, M. D. Scanlon, D. Momotenko, H. Vrubel, M. a Méndez, P.-F. Brevet, H. H. Girault, ACS Nano 2014, 8, 9471–81.