The electrochemical properties of Au electrodes sequentially modified by self-assembled monolayers (SAM) of carboxyl-terminated alkane thiols, ultrathin poly-L-lysine (PLL) film, and diluted monolayers of Au nanoparticles are investigated by electrochemical impedance spectroscopy (EIS). The phenomenological chargetransfer resistance (R ct ) for the hexacyanoferrate redox couple at the equilibrium potential exhibited an exponential increase with increasing methylene units (x) in the SAM. The increase of R ct between x ) 1 and 10 was described by a well-defined decay parameter β ) 1.16 ( 0.04 per methylene unit. This behavior suggests that the kinetics of electron transfer is controlled by coherent electron tunneling across the carboxylterminated SAM. Adsorption of the PLL brings about an average 2.5 times decrease in R ct independent of x. The ultrathin PLL film (thickness less than 1 nm) induces an increase of the surface concentration of the redox couple without affecting the β value observed for the SAM-terminated electrodes. Diluted monolayers of Au nanoparticles with an average 19.2 ( 2.1 nm diameter generate significant changes in the dynamics of electron transfer. In contrast to the behavior in the absence of nanoparticles, a distance-independent R ct was observed for x > 5. Detailed analysis of the electrochemical responses as a function of the particle number density revealed that the rate-determining step is the charging of the nanoparticles by the redox species. It is concluded that the electronic communication between the nanoparticles and the electrode surface over distances as large as 13 Å originates from electron transport through the trapped redox probe. The several orders of magnitude changes of the apparent R ct upon nanoparticle adsorption further suggest that electron transport through the film does not occur via a classical hopping mechanism. A mechanism based on nonthermalized electron transport via the density of the redox probe at the Fermi energy (hot electron transport) is proposed to account for the experimental observations.

The electrochemical behavior of arrays of Au nanoparticles assembled on Au electrodes modified by 11-mercaptoundecanoic acid (MUA) and poly-L-lysine (PLYS) was investigated as a function of the particle number density. The self-assembled MUA and PLYS layers formed compact ultrathin films with a low density of defects as examined by scanning tunneling microscopy. The electrostatic adsorption of Au particles of 19 +/- 3 nm on the PLYS layer resulted in randomly distributed arrays in which the particle number density is controlled by the adsorption time. In the absence of the nanoparticles, the dynamics of electron transfer involving the hexacynoferrate redox couple is strongly hindered by the self-assembled film. This effect is primarily associated with a decrease in the electron tunneling probability as the redox couple cannot permeate through the MUA monolayer at the electrode surface. Adsorption of the Au nanoparticles dramatically affects the electron-transfer dynamics even at low particle number density. Cyclic voltammetry and impedance spectroscopy were interpreted in terms of classical models developed for partially blocked surfaces. The analysis shows that the electron transfer across a single particle exhibits the same phenomenological rate constant of electron transfer as for a clean Au surface. The apparent unhindered electron exchange between the nanoparticles and the electrode surface is discussed in terms of established models for electron tunneling across metal-insulator-metal junctions.

The dynamics of electron transfer across Au electrodes modified by ultrathin polyelectrolyte multilayers (PEM) and a diluted monolayer of Au nanoparticle was investigated as a function of the film thickness. Au electrodes were sequentially modified by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), followed by alternate adsorption of poly-l-lysine (PLL) and poly-l-glutamic acid (PGA) layers. Submonolayer coverage of citrate stabilized 19.2 ± 2.1 nm Au nanoparticles was achieved by electrostatic adsorption on PLL terminated surfaces. In the absence of nanoparticles, cyclic voltammetry and electrochemical impedance spectroscopy of the hexacyanoferrate redox probe showed that the charge-transfer resistance is independent of the number of adsorbed polyelectrolyte layers. These results revealed that the redox species can penetrate the PEM film and the electrochemical responses are controlled by the electron tunneling across the initial monolayer of MUA. The phenomenological charge-transfer resistance decreased by more than 2 orders of magnitude upon adsorption of the Au nanoparticles. Normalization of the electrochemical responses with the number density of particles revealed that the PEM thickness introduces insignificant effects on the charge-transfer resistance. The effective distance independent electron-transfer kinetic was observed for film thickness up to 6.5 nm. Furthermore, in situ atomic force microscopy studies show that the Au nanoparticles do not introduce measurable local deformation (compression) of the PEM films. The unique long-range electronic communication in this system is interpreted in terms of a resonant transport process involving the density of states of trapped redox species at the redox Fermi energy.

The kinetics of charge transfer across a metal−insulator−metal architecture is investigated by electrochemical impedance spectroscopy. The insulating component of the architecture is composed by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), polyelectrolyte multilayers, and a monolayer of 22 nm SiO2 nanoparticles. The charge transfer to the hexacyanoferrate couple is strongly hindered by the MUA monolayer. The blocking properties effectively vanish with the adsorption of a diluted monolayer of Au nanoparticles (19 nm). Atomic force microscopy and scanning electron microscopy analyses demonstrate that the Au nanoparticles are physically separated from the Au surface by the SiO2 monolayer. The strong electronic communication between the metal nanoparticles and the electrode is rationalized by a nonthermalized transport process involving redox species trapped in the multilayer assembly.

The kinetics of the hydrogen evolution reaction (HER) catalyzed by two-dimensional assemblies of 13 nm diameter palladium (Pd) nanoparticles on mica substrates was investigated by scanning electrochemical microscopy (SECM). The assemblies were prepared by electrostatic adsorption of citrate-stabilized Pd nanoparticles on poly-l-lysine treated mica. Atomic force microscopy (AFM) studies of the adsorption process provided information on the nanoparticle number density as a function of the adsorption time. The HER kinetics was determined by examining SECM feedback approach curves, employing the methyl viologen (MV2+/+•) couple as the redox probe. With this configuration, the potential of the Pd nanoparticles is effectively determined by the local concentration ratio of the redox probe. The overpotential for proton reduction can be finely tuned by the concentration of the redox species, the size of the ultramicroelectrode (UME) tip, and the distance between the UME and the nanoparticle assembly. The SECM analysis allowed the mean exchange current density per Pd nanoparticle [j 0(pH = 3) = (1.19 ± 0.08) × 10−6 A cm−2] to be evaluated. Significantly, the SECM methodology described is highly sensitive to the transfer coefficient of the HER, with a value of 0.5 providing the most satisfactory fit in the overpotential range investigated. It is concluded that the reactivity of the Pd nanoparticles is comparable to the bulk metal, which is consistent with the bulk-like electronic structure of Pd clusters of this dimension.

Organic–inorganic perovskite solar cells have recently emerged at the forefront of photovoltaics research.

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