Formation of Iridium(IV) Oxide (IrO_<i>X</i>) Films by Electroflocculation

Films of iridium­(IV) oxide nanoparticles (IrO_<i>X</i> NPs) become deposited on electrodes from nanoparticle solutions when potentials sufficient to initiate water oxidation are applied. Evidence is given that the film-forming mechanism is nanoparticle precipitation. Following an induction period during which a significant amount of charge is passed, the NPs begin to deposit as islands. It appears that the proton release that accompanies nanoparticle oxidation triggers the nanoparticle electroflocculation and subsequent precipitation. Flocculation from nanoparticle solutions can also be induced by the addition of a chemical oxidant (Ce­(IV)). The film formation is followed by cyclic voltammetry (CV), rotated ring disk voltammetry (RRDE), and electrochemical quartz crystal microbalance (eQCM) measurements, supplemented with AFM and SEM microscopies

Electron Transfer Dynamics of Iridium Oxide Nanoparticles Attached to Electrodes by Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of carboxylated alkanethiolates (−S­(CH₂)_<i>n</i>−1CO₂^–) on flat gold electrode surfaces are used to tether small (ca. 2 nm d.) iridium­(IV) oxide nanoparticles (Ir^IVO_X NPs) to the electrode. Peak potential separations in cyclic voltammetry (CV) of the nanoparticle Ir^IV/III wave, in pH 13 aqueous base, increase with <i>n</i>, showing that the Ir^IV/III apparent electron transfer kinetics of metal oxide sites in the nanoparticles respond to the imposed SAM electron transfer tunneling barrier. Estimated apparent electron transfer rate constants (<i>k</i>_app⁰) for <i>n</i> = 12 and 16 are 9.8 and 0.12 s^–1. Owing to uncompensated solution resistance, <i>k</i>_app⁰ for <i>n</i> = 8 was too large to measure in the potential sweep experiment. For the cathodic scans, coulometric charges under the Ir^IV/III voltammetric waves were independent of potential scan rate, suggesting participation of all of the iridium oxide redox sites (ca. 130 per NP) in the NPs. These experiments show that it is possible to control and study electron transfer dynamics of electroactive nanoparticles including, as shown by preliminary experiments, that of the electrocatalysis of water oxidation by iridium oxide nanoparticles

Temperature Dependence of Solid-State Electron Exchanges of Mixed-Valent Ferrocenated Au Monolayer-Protected Clusters

Electron transfers (ETs) in mixed-valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, the presence of ∼1:1 mixed-valent ferrocenated thiolates in the organothiolate ligand shells of <2 nm diameter Au₂₂₅, Au₁₄₄, and Au₂₅ monolayer-protected clusters (MPCs) exerts a retarding effect on ET between them at and below room temperature. Near room temperature, in dry samples, bimolecular rate constants for ET between organothiolate-ligated MPCs are diminished by the addition of ferrocenated ligands to their ligand shells. At lower temperatures (down to ∼77 K), the thermally activated (Arrhenius) ET process dissipates, and the ET rates become temperature-independent. Among the Au₂₂₅, Au₁₄₄, and Au₂₅ MPCs, the temperature-independent ET rates fall in the same order as at ambient temperatures: Au₂₂₅ > Au₁₄₄ > Au₂₅. The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands and are primarily determined by the Au nanoparticle core size

Synthesis and Electrochemistry of 6 nm Ferrocenated Indium–Tin Oxide Nanoparticles

Indium–tin oxide (ITO) nanoparticles, 6.1 ± 0.8 nm in diameter, were synthesized using a hot injection method. After reaction with 3-aminopropyldimethylethoxysilane to replace the initial oleylamine and oleic acid capping ligands, the aminated nanoparticles were rendered electroactive by functionalization with ferrocenoyl chloride. The nanoparticle color changed from blue-green to light brown, and the nanoparticles became more soluble in polar solvents, notably acetonitrile. The nanoparticle diffusion coefficient (<i>D</i> = 1.0 × 10^–6 cm²/s) and effective ferrocene concentration (<i>C</i> = 0.60 mM) in acetonitrile solutions were determined using ratios of <i>DC</i> and <i>D</i>^1/2<i>C</i> data measured by microdisk voltammetry and chronoamperometry. The <i>D</i> result compares favorably to an Einstein–Stokes estimate (2.1 × 10^–6 cm²/s), assuming an 8 nm hydrodynamic diameter in acetonitrile (6 nm for the ITO core plus 2 nm for the ligand shell). The ferrocene concentration result is lower than anticipated (ca. 1.60 mM) based on a potentiometric titration of the ferrocene sites with Cu­(II) in acetonitrile. Cyclic voltammetric data indicate tendency of the ferrocenated nanoparticles to adsorb on the Pt working electrode

Solution Voltammetry of 4 nm Magnetite Iron Oxide Nanoparticles

The voltammetry of solution-dispersed magnetite iron oxide Fe₃O₄ nanoparticles is described. Their currents are controlled by nanoparticle transport rates, as shown with potential step chronoamperometry and rotated disk voltammetry. In pH 2 citrate buffer with added NaClO₄ electrolyte, solution cyclic voltammetry of these nanoparticles (average diameter 4.4 ± 0.9 nm, each containing ca. 30 Fe sites) displays an electrochemically irreversible oxidation with <i>E</i>_PEAK at ca. +0.52 V and an irreversible reduction with <i>E</i>_PEAK at ca. +0.2 V vs Ag/AgCl reference electrode. These processes are presumed to correspond to the formal potentials for one-electron oxidation of Fe­(II) and reduction of Fe­(III) at their different sites in the magnetite nanoparticle structure. The heterogeneous electrode reaction rates of the nanoparticles are very slow, in the 10^–5 cm/s range. The nanoparticles are additionally characterized by a variety of tools, e.g., TEM, UV/vis, and XPS spectroscopies

Synthesis, Electrochemistry, and Excited-State Properties of Three Ru(II) Quaterpyridine Complexes

The complexes [Ru­(qpy)­LL′]²⁺ (qpy = 2,2′:6′,2″:6″,2‴-quaterpyridine), with <b>1</b>: L = acetonitrile, L′= chloride; <b>2</b>: L = L′= acetonitrile; and <b>3</b>: L = L′= vinylpyridine, have been prepared from [Ru­(qpy) (Cl)₂]. Their absorption spectra in CH₃CN exhibit broad metal-to-ligand charge transfer (MLCT) absorptions arising from overlapping ¹A₁ → ¹MLCT transitions. Photoluminescence is not observed at room temperature, but all three are weakly emissive in 4:1 ethanol/methanol glasses at 77 K with broad, featureless emissions observed between 600 and 1000 nm consistent with MLCT phosphorescence. Cyclic voltammograms in CH₃CN reveal the expected Ru^III/II redox couples. In 0.1 M trifluoroacetic acid (TFA), <b>1</b> and <b>2</b> undergo aquation to give [Ru^II(qpy)­(OH₂)₂]²⁺, as evidenced by the appearance of waves for the couples [Ru^III(qpy)­(OH₂)₂]³⁺/[Ru^II(qpy)­(OH₂)₂]²⁺, [Ru^IV(qpy)­(O)­(OH₂)]²⁺/[Ru^III(qpy)­(OH₂)₂]³⁺, and [Ru^VI(qpy)­(O)₂]²⁺/[Ru^IV(qpy)­(O)­(OH₂)]²⁺ in cyclic voltammograms

Visible Photoelectrochemical Water Splitting Based on a Ru(II) Polypyridyl Chromophore and Iridium Oxide Nanoparticle Catalyst

Preparation of Ru­(II) polypyridyl–iridium oxide nanoparticle (IrO_X NP) chromophore–catalyst assemblies on an FTO|<i>nano</i>ITO|TiO₂ core/shell by a layer-by-layer procedure is described for application in dye-sensitized photoelectrosynthesis cells (DSPEC). Significantly enhanced, bias-dependent photocurrents with Lumencor 455 nm 14.5 mW/cm² irradiation are observed for core/shell structures compared to TiO₂ after derivatization with [Ru­(4,4′-PO₃H₂bpy)₂(bpy)]²⁺ (RuP₂) and uncapped IrO_X NPs at pH 5.8 in NaSiF₆ buffer with a Pt cathode. Photocurrents arising from photolysis of the resulting photoanodes, FTO|<i>nano</i>ITO|TiO₂|−RuP₂,IrO₂, are dependent on TiO₂ shell thickness and applied bias, reaching 0.2 mA/cm² at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long-term photolysis in the NaSiF₆ buffer results in a marked decrease in photocurrent over time due to surface hydrolysis and loss of the chromophore from the surface. Long-term stability, with sustained photocurrents, has been obtained by atomic layer deposition (ALD) of overlayers of TiO₂ to stabilize surface binding of −RuP₂ prior to the addition of the IrO_X NPs