A new transparent iridium oxide (IrO
x
) film on fluorine-doped tin oxide (FTO)
electrodes were achieved
from a homogeneous precursor complex solution by employing a facile
spin-coating technique. The composition of the nanostructure and crystallinity
of the IrO
x
film is tunable by a simple
annealing treatment of a compact complex layer, which is responsible
for their significantly different electrocatalytic performances for
water oxidation. Transmission electron microscopy (TEM) observations
showed uniformly dispersed small IrO
x
nanoparticles
of dimensions ca. 2–5 nm for the film annealed at 300 °C,
and the nanoparticles gradually agglomerated to form relatively large
particles at higher temperatures (400 and 500 °C). The IrO
x
films prepared at different annealing temperatures
are characterized by Raman spectroscopic data to reveal intermediate
IrO
x
(OH)
y
nanoparticles
with two oxygen binding motifs: terminal hydroxo and bridging oxo
at 300 and 350 °C annealing, via amorphous IrO
x
at 400 °C, transforming ultimately to crystalline IrO2 nanoparticles at 500 °C. Cyclic voltammetry suggests
that the intrinsic activity of catalytic Ir sites in intermediate
IrO
x
(OH)
y
nanoparticles
formed at 300 °C annealing is higher in comparison with amorphous
and crystalline IrO
x
nanoparticles. Electrochemical
impedance data showed that the charge transfer resistance (R
ct = 232 Ω) for the IrO
x
(OH)
y
film annealed at 300 °C
is lower relative to that of films annealed at higher temperatures.
This is ascribable to the facilitated electron transfer in grain boundaries
between smaller IrO
x
particles to lead
the efficient electron transport in the film. The high intrinsic activity
of catalytic Ir sites and efficient electron transport are responsible
for the high electrocatalytic performance observed for the intermediate
IrO
x
(OH)
y
film
annealed at 300 °C; it provides the lowest overpotential (η)
of 0.24 V and Tafel slope of 42 mV dec–1 for water
oxidation at neutral pH, which are comparable with values for amorphous
IrO
x
·nH2O nanoparticle films (40–50 mV dec–1) reported
as some of the most efficient electrocatalysts so far.
We report the first accessible channel-like open pore architecture of ordered 2D hexagonal mesoporous IrO2 films and its utilization as an efficient anode for electrocatalytic water oxidation. A well-ordered mesostructure of circa 7 nm pores were obtained by a facile one-pot soft-templating strategy, employing a [Ir(OH)6](2-) precursor stabilized by a triblock copolymer "Pluronic F127" as a pore-directing template. A mesoporous IrO2 film calcined at 400 °C (∼70 nm thick) affords a high surface area of 512 m(2) cm(-3) and 2 times higher O2 evolution during the electrocatalytic water oxidation relative to an untemplated IrO2 coating film.
The effects of hydrogenation on the structure, transport, and magnetic properties of TM/RE (TM = Fe, Co; RE = La, Y, Gd) multilayers were investigated. The following effects were found to be common to all TM/RE multilayers: (a) transition of RE layers from metal to semiconductor, (b) expansion of RE layers, and (c) enhancement of saturation magnetization. The hydrogenation technique is practicable for changing the interface state and producing a semiconductor layer inside the multilayer, and thus shows potential for producing new functional magnetic multilayers.
Polychromium‐oxo‐deposited TiO2 (CrIIIxOy/TiO2) electrodes were fabricated by a simple electrochemical technique by using different TiO2 basal electrodes (anatase, rutile, and mixed polymorphic phases P25) as earth‐abundant photoanodes for visible‐light‐driven water oxidation. The high‐resolution transmission electron microscopy (HR‐TEM) observation illustrated that an CrIIIxOy layer with approximately 2–3 nm thickness was formed on the surface of the crystalline TiO2 particles. Upon visible‐light irradiation of the electrodes, the photoanodic current based on water oxidation was generated at the CrIIIxOy/TiO2 electrodes. However, the wavelength (below 620 nm) for photocurrent generation at CrIIIxOy/TiO2‐rutile was longer than that (below 560 nm) at CrIIIxOy/TiO2‐P25 by 60 nm, which is in agreement with the difference (0.2 eV) in the conduction band (CB) edge energy between rutile and anatase TiO2. This gives a quantitative account for the photocurrent generation based on interfacial charge transfer (IFCT) from Cr 3d of the deposited CrIIIxOy layer to the TiO2 CB. The photocurrent generated for CrIIIxOy/TiO2‐rutile was higher than that for CrIIIxOy/TiO2‐anatase, which is ascribed to 1) more effective CrIIIxOy deposition on the rutile particles, 2) a larger electrolyte/CrIIIxOy interface for water oxidation as a result of smaller rutile particles (ca. 30–40 nm) compared with larger P25 particles (ca. 40–80 nm), and 3) more effective use of visible light owing to the low energy IFCT transition of rutile.
We have investigated the change of interlayer exchange coupling in Fe (3.0 nm)/Y (t nm) multilayers upon increasing the thickness of the spacer layer by hydrogenation. The coupling behavior changes from an antiferromagnetic (AFM) state to a non-coupled (NC) state, both before and after hydrogenation. The maximum value of coupling strength (J
1) is found to become much lower after hydrogenation than before hydrogenation. The range of the spacer layer thickness for maintaining AFM coupling is found to be extended from 1.4 nm to 1.8 nm by hydrogenation. Both the difference in the AFM coupling strength and the extension of the spacer layer thickness of AFM coupling should be attributed to the change from a Y layer to a yttrium–hydride (Y–H) layer by hydrogenation.
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