Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters

Abstract

This work examines the temperature dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)18(0/1-), Au25(SR)18(1+/0), and Au144(SR)60(1+/0), where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temperature and for ca. 1:1 mol:mol mixed valencies, the bimolecular ET rate constants (assuming a cubic lattice model) are ~2 × 10(6) M(-1) s(-1) for Au25(SR)18(0/1-), ~3 × 10(5) M(-1) s(-1) for Au25(SR)18(1+/0), and ~1 × 10(8) M(-1) s(-1) for Au144(SR)60(1+/0). Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, respectively. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temperature but, among the different MPCs, fall in the same order of ET rate: Au144(+1/0) > Au25(0/1-) > Au25(1+/0).

Catalytic Reduction by Quasi‐Homogeneous Gold Nanoclusters in the Liquid Phase

“…Finally, Au 25 clusters turned out to be highly efficient ET mediators when the stable redox couples Au 25 0 /Au 25 − and Au 25 + /Au 25 0 were employed. Notably, Murray and co‐workers examined the ET kinetics in the solid‐state films of mixed‐valent states of Au NCs and found that the bimolecular ET rate constants fell in the order of Au 144 + / 0 >Au 25 0 / − >Au 25 + / 0 21. Accordingly, Maran et al.…”

Catalytic Amplification of Au144 Nanocluster Collisions by Hydrogen Evolution Reaction

“…[6,7,[9][10][11] The atomic NCs typically exhibit high monodispersity [12] and well-defined molecular composition. [15,16] Most electrochemical studies of particle collisions employed relatively large (e. g., > 5 nm) NPs. [8,13,14] In the present work, we monitor the collisions of individual Au 144 nanoclusters with a carbon ultramicroelectrode (UME) catalytically amplified by the hydrogen evolution reaction (HER).…”

“…One should notice that the HER kinetics is hard to measure at an NC-modified electrode because of large film resistance. [15,16] Most electrochemical studies of particle collisions employed relatively large (e. g., > 5 nm) NPs. [17][18][19][20] The Bard group reported proton reduction at single Pt NPs.…”

“…The discrepancy of this magnitude is common in NP collision experiments. [13][14][15][16][17][18] 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 The steady-state diffusion-limiting current of protons to a single Au 144 nanocluster in contact with the carbon electrode surface can be evaluated from Eq. (2): [14] i ss ¼ 4pnFDcrðln2Þ ð 2Þ…”

Charge-Transfer Effects in Ligand Exchange Reactions of Au25 Monolayer-Protected Clusters

“…17,18 All 1 H NMR measurements were made using a Bruker 500 mHz spectrometer. About 20 mg of Au 25 NPs was dissolved in ∼1 mL of CD 2 Cl 2 containing a known amount of acetonitrile (serving as an internal standard), and the solution was transferred into an NMR tube.…”