We describe the formation of stable, adherent, mesoporous films of 2 nm diameter IrIVO x nanoparticles on glassy carbon electrodes, by a previously unreported method of controlled potential electro-flocculation from pH 13 nanoparticle solutions. These films initiate O2 evolution from water oxidation and then achieve 100% current efficiency, at overpotentials only ∼0.15 and ∼0.25 V higher, respectively, than the reversible H2O/O2 potential. The overpotentials, measured at ∼0.5 mA/cm2, are independent of pH and are the smallest yet reported for electrochemical water oxidation, a property important in possible uses in electrochemical solar cells. The films appear to be mesoporous and microscopically accessible, since O2 evolution currents increase proportionately to multilayer nanoparticle film coverage but without a concurrent increase in overpotential.

Electrospray ionization triple-quadrupole mass spectrometry of ca. 1.6 nm diameter thiolate-protected gold nanoparticles has been achieved at higher resolution than in previous reports. The results reveal the presence of nanoparticles with formulas Au(144)L(60) and Au(146)L(59), present in the sample as a mixture. The improved resolution is based on lowering m/z by exchanging multiple [-SC(11)H(22)N(CH(2)CH(3))(3)(+)] ligands into the original [-S(CH(2))(5)CH(3)] ligand shell. The nanoparticles are thus intrinsically cationized and appear as a series of 10+ to 15+ mass spectral peaks. The assigned state of charge was confirmed by a collision-induced dissociation measurement.

Multilayers of mixed monolayer thiolate-protected Au nanoparticles with polycationic ligand shells are strongly and persistently adsorbed on Pt electrodes from dilute CH 3 CN/electrolyte solutions of the nanoparticles. The adsorption is so robust that the electrodes can be transferred, with rinsing, to nanoparticle-free CH 3 CN/ electrolyte (and other organic solvents) and their voltammetry observed without significant desorption. The nanoparticles have the average composition [Au 225 (TEA-thiolateThe cation sites are a quaternary ammonium-terminated ligand (TEA-thiolate + ) -S(CH 2 ) 11 N(CH 2 CH 3 ) 3 + ). The ferrocene content of the ligand shell (Y) allows voltammetric detection of the adsorption, and also affects it. This work describes how the extent of nanoparticle adsorption (surface coverage, Γ NP mol/cm 2 ) depends on the manner of exposure of the electrode (with potential scanning, at a fixed potential, or at open circuit) to the nanoparticle solution, and on the nanoparticle concentration, the electrolyte and its concentration, and on the mixed monolayer composition. Increase of the cationic TEA-thiolate + component of the mixed monolayer from X ) 18 to 27 increases, for a given exposure mode, the attained amount of adsorption from < one monolayer to > two monolayers, and broadens the voltammetric wave. The adsorption phenomenon is interpreted as induced by multiple ion-pair bridges between cation sites and electrolyte anions, and as such represents an entropic consequence of multiple interactions between nanoparticles and between nanoparticles and the electrode.

We describe the electrochemistry of 15 nm diameter silica nanoparticles densely functionalized with ferrocene (FcSiO(2)) through siloxane couplings. Each nanoparticle bears approximately 600 Fc sites, as measured by potentiometric titration (590 Fc) and diffusion-controlled voltammetry (585 Fc) and estimated by XPS (630 Fc). The nanoparticle ferrocene coverage amounts to ca. a complete monolayer of ferrocene sites, which react electrochemically without mutual interactions and which are apparently fully accessible for diffusion-controlled electrode reactions. Diffusion-controlled voltammetry of the FcSiO(2) nanoparticles was observed in dilute methanol dispersions and in more concentrated slurry phases formed in methanol/acetonitrile mixtures. Electrochemical studies reveal interesting behavior in the dilute and more concentrated solutions. Because of the large nanoparticle surface area/volume ratio, the ferrocene-coated silica nanoparticles are capable of storing up to 5 x 10(7) C/m(3) of redox charge as dry phases and 6 x 10(5) C/m(3) in the concentrated slurries.

The robust, irreversible adsorption of omega-ferrocene hexanethiolate-protected gold nanoparticles (composition ca. {Au(225)(SC6Fc)(43)}) on electrodes provides an opportunity to investigate their submonolayer and monolayer films in nanoparticle-free solutions. Observations of nanoparticle adsorption on unmodified electrodes are extended here to Au electrodes having more explicitly controlled surfaces, namely self-assembled monolayers (SAMs) of alkanethiolates with omega-sulfonate, carboxylate, and methyl termini, and in different Bu(4)N(+)X(-) electrolyte (X(-) = C(7)H(7)SO(3)(-), ClO(4)(-), CF(3)SO(3)(-), PF(6)(-), NO(3)(-)) solutions in CH(2)Cl(2). The nanoparticle surface coverage (Gamma(NP)) and the stability of the adsorbed nanoparticle film to repeated ferrocene/ferrocenium redox cycling decrease in the order of sulfonate > carboxylate > methyl terminated SAM, with increasing hydrophobicity of X(-) and with increasing alkyl chain length. The results are consistent with the proposal that the strong surface adsorption is jointly associated with the polyfunctional character of the nanoparticles, analogous to entropically driven adsorptions of polymeric ions on charged surfaces, and with lateral, ion-bridged nanoparticle-nanoparticle interactions.