7 research outputs found
Formation of Iridium(IV) Oxide (IrO<sub><i>X</i></sub>) Films by Electroflocculation
Films
of iridiumÂ(IV) oxide nanoparticles (IrO<sub><i>X</i></sub> 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 (CeÂ(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
(−SÂ(CH<sub>2</sub>)<sub><i>n</i>−1</sub>CO<sub>2</sub><sup>–</sup>) on flat gold electrode surfaces are used
to tether small (ca. 2 nm d.) iridiumÂ(IV) oxide nanoparticles (Ir<sup>IV</sup>O<sub>X</sub> NPs) to the electrode. Peak potential separations
in cyclic voltammetry (CV) of the nanoparticle Ir<sup>IV/III</sup> wave, in pH 13 aqueous base, increase with <i>n</i>, showing
that the Ir<sup>IV/III</sup> 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><sub>app</sub><sup>0</sup>) for <i>n</i> = 12 and 16 are 9.8 and 0.12 s<sup>–1</sup>. Owing
to uncompensated solution resistance, <i>k</i><sub>app</sub><sup>0</sup> for <i>n</i> = 8 was too large to measure
in the potential sweep experiment. For the cathodic scans, coulometric
charges under the Ir<sup>IV/III</sup> 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 Au<sub>225</sub>, Au<sub>144</sub>, and Au<sub>25</sub> 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 Au<sub>225</sub>, Au<sub>144</sub>, and Au<sub>25</sub> MPCs, the temperature-independent ET rates
fall in the same order as at ambient temperatures: Au<sub>225</sub> > Au<sub>144</sub> > Au<sub>25</sub>. 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<sup>–6</sup> cm<sup>2</sup>/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><sup>1/2</sup><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<sup>–6</sup> cm<sup>2</sup>/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 CuÂ(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 Fe<sub>3</sub>O<sub>4</sub> 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 NaClO<sub>4</sub> 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><sub>PEAK</sub> at ca. +0.52 V and an irreversible
reduction with <i>E</i><sub>PEAK</sub> 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 FeÂ(II) and
reduction of FeÂ(III) at their different sites in the magnetite nanoparticle
structure. The heterogeneous electrode reaction rates of the nanoparticles
are very slow, in the 10<sup>–5</sup> 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 [RuÂ(qpy)ÂLL′]<sup>2+</sup> (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 [RuÂ(qpy) (Cl)<sub>2</sub>].
Their absorption spectra in CH<sub>3</sub>CN exhibit broad metal-to-ligand
charge transfer (MLCT) absorptions arising from overlapping <sup>1</sup>A<sub>1</sub> → <sup>1</sup>MLCT 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 CH<sub>3</sub>CN reveal the expected Ru<sup>III/II</sup> redox couples. In 0.1 M trifluoroacetic acid (TFA), <b>1</b> and <b>2</b> undergo aquation to give [Ru<sup>II</sup>(qpy)Â(OH<sub>2</sub>)<sub>2</sub>]<sup>2+</sup>, as evidenced by
the appearance of waves for the couples [Ru<sup>III</sup>(qpy)Â(OH<sub>2</sub>)<sub>2</sub>]<sup>3+</sup>/[Ru<sup>II</sup>(qpy)Â(OH<sub>2</sub>)<sub>2</sub>]<sup>2+</sup>, [Ru<sup>IV</sup>(qpy)Â(O)Â(OH<sub>2</sub>)]<sup>2+</sup>/[Ru<sup>III</sup>(qpy)Â(OH<sub>2</sub>)<sub>2</sub>]<sup>3+</sup>, and [Ru<sup>VI</sup>(qpy)Â(O)<sub>2</sub>]<sup>2+</sup>/[Ru<sup>IV</sup>(qpy)Â(O)Â(OH<sub>2</sub>)]<sup>2+</sup> in cyclic
voltammograms
Visible Photoelectrochemical Water Splitting Based on a Ru(II) Polypyridyl Chromophore and Iridium Oxide Nanoparticle Catalyst
Preparation
of RuÂ(II) polypyridyl–iridium oxide nanoparticle (IrO<sub>X</sub> NP) chromophore–catalyst assemblies on an FTO|<i>nano</i>ITO|TiO<sub>2</sub> 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/cm<sup>2</sup> irradiation are observed for core/shell
structures compared to TiO<sub>2</sub> after derivatization with [RuÂ(4,4′-PO<sub>3</sub>H<sub>2</sub>bpy)<sub>2</sub>(bpy)]<sup>2+</sup> (RuP<sub>2</sub>) and uncapped IrO<sub>X</sub> NPs at pH 5.8 in NaSiF<sub>6</sub> buffer with a Pt cathode. Photocurrents arising from photolysis
of the resulting photoanodes, FTO|<i>nano</i>ITO|TiO<sub>2</sub>|−RuP<sub>2</sub>,IrO<sub>2</sub>, are dependent on
TiO<sub>2</sub> shell thickness and applied bias, reaching 0.2 mA/cm<sup>2</sup> at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long-term
photolysis in the NaSiF<sub>6</sub> 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 TiO<sub>2</sub> to stabilize surface binding of −RuP<sub>2</sub> prior to the addition of the IrO<sub>X</sub> NPs