12 research outputs found
Highly Oriented Sulfonic Acid Groups in a Nafion Thin Film on Si Substrate
Solid state ionics is a research field attracting much
current
attentions because of the ideal power sources for use with portable
electronic devices having high power-to-weight ratios. One of the
most urgent subjects in this field is to understand proton transport
properties at the interface between inorganic materials and polymer
electrolyte from the viewpoint of developing much more powerful energy.
In this study, a 150-nm-thick Nafion thin film was prepared by spin-coating
on a silicon (Si) substrate to investigate the proton transport property
at the interface. The infrared (IR) p-polarized multiple-angle incidence
resolution spectrometry (p-MAIRS) technique was applied to investigate
the in-plane (IP) and out-of-plane (OP) spectra to the surface. The
IP spectrum showed a well-known spectrum, but the OP spectrum was
quite different from the IP spectrum. An anomalous IR peak was observed
in the OP spectrum at 1260 cm<sup>–1</sup>. From density functional
theory (DFT) calculations, this peak was attributed to the −SO<sub>3</sub>H vibration modes between two sulfonic acid groups with hydrogen
bonds. These results demonstrate that the Nafion thin film on Si substrate
had a highly oriented structure with the sulfonic acid groups at the
side chain. Impedance measurements of Nafion thin film were conducted
to investigate the proton transport property of the Nafion thin film
on SiO<sub>2</sub> substrate. The proton conductivity of the thin
film exhibited a lower value than that of the commercial Nafion membrane.
The low proton conductivity of the Nafion thin film was related with
these highly oriented structures
Multilayer Growth of Porphyrin-Based Polyurea Thin Film Using Solution-Based Molecular Layer Deposition Technique
Controllable synthesis of organic
thin film materials on solid
surfaces is a challenging issue in the research field of surface science,
as it is affected by several physical parameters. In this work, we
demonstrated a solution-based molecular layer deposition (MLD) approach
to prepare porphyrin-based covalent organic molecular networks on
a 3-aminopropyl trimethoxysilane (APTMS) modified substrate surface
using the urea coupling reaction between 1,4-phenylene diisocyanate
(1,4-PDI) and 5,10,15,20-tetrakis-Â(4-aminophenyl) porphyrin (H<sub>2</sub>TAPP) at room temperature (22 ± 2 °C). Multilayer
growth was investigated under different relative humidity (RH) conditions
of the reaction chamber. Sequential molecular growth at low relative
humidity (≤10% RH) was observed using UV–vis absorption
spectroscopy and atomic force microscopy (AFM). The high-RH condition
shows limited film growth. Infrared spectroscopy (IR) and X-ray photoelectron
spectroscopy (XPS) revealed the polyurea bond formation in sequential
multilayer thin films, demonstrating that stepwise multilayer film
growth was achieved using the urea coupling reaction
Visualization of Ion Conductivity: Vapochromic Luminescence of an Ion-Conductive Ruthenium(II) Metalloligand-Based Porous Coordination Polymer
We
synthesized a new porous coordination polymer {La<sub>1.75</sub>(OH)<sub>1.25</sub>[RuÂ(dcbpy)<sub>3</sub>]·16H<sub>2</sub>O} (<b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub>; H<sub>2</sub>dcbpy = 4,4′-dicarboxy-2,2′-bipyridine)
composed of a luminescent rutheniumÂ(II) metalloligand [RuÂ(4,4′-dcbpy)<sub>3</sub>]<sup>4–</sup> and La<sup>3+</sup> cations. X-ray analysis
for <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> revealed that the La<sup>3+</sup> cations and
[4Ru] metalloligands are crystallized in a molar ratio of 7:4 with
OH<sup>–</sup> counteranions and a void fraction of ∼25.5%.
Interestingly, <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> shows a reversible structural transition
triggered by water ad/desorption, which affects not only the triplet
metal-to-ligand charge-transfer (<sup>3</sup>MLCT) emission energy
but also the ion conductivity in the solid state. This correlation
suggests that <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> is an interesting material that enables
visualization of the ion conductivity via the <sup>3</sup>MLCT emission
energy
Visualization of Ion Conductivity: Vapochromic Luminescence of an Ion-Conductive Ruthenium(II) Metalloligand-Based Porous Coordination Polymer
We
synthesized a new porous coordination polymer {La<sub>1.75</sub>(OH)<sub>1.25</sub>[RuÂ(dcbpy)<sub>3</sub>]·16H<sub>2</sub>O} (<b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub>; H<sub>2</sub>dcbpy = 4,4′-dicarboxy-2,2′-bipyridine)
composed of a luminescent rutheniumÂ(II) metalloligand [RuÂ(4,4′-dcbpy)<sub>3</sub>]<sup>4–</sup> and La<sup>3+</sup> cations. X-ray analysis
for <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> revealed that the La<sup>3+</sup> cations and
[4Ru] metalloligands are crystallized in a molar ratio of 7:4 with
OH<sup>–</sup> counteranions and a void fraction of ∼25.5%.
Interestingly, <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> shows a reversible structural transition
triggered by water ad/desorption, which affects not only the triplet
metal-to-ligand charge-transfer (<sup>3</sup>MLCT) emission energy
but also the ion conductivity in the solid state. This correlation
suggests that <b>La</b><sub><b>7</b></sub><b>-[4Ru]</b><sub><b>4</b></sub> is an interesting material that enables
visualization of the ion conductivity via the <sup>3</sup>MLCT emission
energy
High Proton Conduction of Organized Sulfonated Polyimide Thin Films with Planar and Bent Backbones
Fast proton conduction was achieved
in organized lamellar structures
with in-plane oriented structure parallel to the substrate surface
using a lyotropic liquid-crystalline (LC) property. Alkyl sulfonated
polyimides (ASPIs) with bent main chain structure were newly synthesized
to investigate relations between the higher order structure and proton
transport properties. Proton conductivity of all polyimide thin films
was greater than 10<sup>–2</sup> S/cm. Grazing-incidence small-angle
X-ray scattering (GI-SAXS) revealed that both planar and bent ASPI
thin films exhibited humidity-induced lyotropic lamellar structure.
Infrared p-polarized multiple-angle incidence resolution (pMAIR) studies
revealed that main chain backbones of both planar and bent ASPI thin
films show an in-plane orientation parallel to the substrate surface.
Results demonstrate that sulfonated alkyl side chains contribute strongly
to the lyotropic LC property, which enhances molecular orderings and
proton conductivity by water uptake. This study extends knowledge
of the molecular design for highly proton conductive polymers with
humidity-induced lyotropic LC property
Proton Conductivities of Lamellae-Forming Bioinspired Block Copolymer Thin Films Containing Silver Nanoparticles
Size-controlled metal nanoparticles
(NPs) were spontaneously formed
when the amphiphilic diblock copolymers consisting of polyÂ(vinyl catechol)
and polystyrene (PVCa-<i>b</i>-PSt) were used as reductants
and templates for NPs. In the present study, the proton conductivity
of well-aligned lamellae structured PVCa-<i>b</i>-PSt films
with Ag NPs was evaluated. We found that the proton conductivity of
PVCa-<i>b</i>-PSt film was increased 10-fold by the addition
of Ag NPs into the proton conduction channels filled with catechol
moieties. In addition, the effect of humidity and the origin of proton
conductivity enhancement was investigated
Unusual Redox Behavior of Rh/AlPO<sub>4</sub> and Its Impact on Three-Way Catalysis
The
influence of the redox behavior of Rh/AlPO<sub>4</sub> on automotive
three-way catalysis (TWC) was studied to correlate catalytic activity
with thermal stability and metal–support interactions. Compared
with a reference Rh/Al<sub>2</sub>O<sub>3</sub> catalyst, Rh/AlPO<sub>4</sub> exhibited a much higher stability against thermal aging under
an oxidizing atmosphere; further deactivation was induced by a high-temperature
reduction treatment. In situ X-ray absorption fine structure experiments
revealed a higher reducibility of Rh oxide (RhO<sub><i>x</i></sub>) to Rh, and the metal showed a higher tolerance to reoxidation
when supported on AlPO<sub>4</sub> compared with Al<sub>2</sub>O<sub>3</sub>. This unusual redox behavior is associated with an Rh–O–P
interfacial linkage, which is preserved under oxidizing and reducing
atmospheres. Another effect of the Rh–O–P interfacial
linkage was observed for the metallic Rh with an electron-deficient
character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed
CO or NO, which is a possible reason for the deactivation by high-temperature
reduction treatments. On the other hand, surface acid sites on AlPO<sub>4</sub> promoted oxidative adsorption of C<sub>3</sub>H<sub>6</sub> as aldehyde, which showed a higher reactivity toward O<sub>2</sub>, as well as NO, compared with carboxylate adsorbed on Al<sub>2</sub>O<sub>3</sub>. A precise control of the acid–base character
of the metal phosphate supports is therefore a key to enhance the
catalytic performance of supported Rh catalysts for TWC applications
Rhodium Nanoparticle Anchoring on AlPO<sub>4</sub> for Efficient Catalyst Sintering Suppression
Rhodium
catalysts exhibited higher dispersion with tridymite-type
AlPO<sub>4</sub> supports than with Al<sub>2</sub>O<sub>3</sub> during
thermal aging at 900 °C under an oxidizing atmosphere. The local
structural analysis via X-ray photoelectron spectroscopy, transmission
electron microscopy, X-ray absorption fine structure, and infrared
spectroscopy suggested that the sintering of AlPO<sub>4</sub>-supported
Rh nanoparticles was significantly suppressed because of anchoring
via a Rh–O–P linkage at the interface between the metal
and support. Most of the AlPO<sub>4</sub> surface was terminated by
phosphate P–OH groups, which were converted into a Rh–O–P
linkage when Rh oxide (RhO<sub><i>x</i></sub>) was loaded.
This interaction enables the thin planar RhO<sub><i>x</i></sub> nanoparticles to establish close and stable contact with the
AlPO<sub>4</sub> surface. It differs from Rh–O–Al bonding
in the oxide-supported catalyst Rh/Al<sub>2</sub>O<sub>3</sub>, which
causes undesired solid reactions that yield deactivated phases. The
Rh–O–P interfacial linkage was preserved under oxidizing
and reducing atmospheres, which contrasts with conventional metal
oxide supports that only present the anchoring effect under an oxidizing
atmosphere. These experimental results agree with a density functional
theory optimized coherent interface RhO<sub><i>x</i></sub>/AlPO<sub>4</sub> model
Tuning the Electron Density of Rh Supported on Metal Phosphates for Three-Way Catalysis
The
automotive three-way catalysis (TWC) performance of Rh supported
on alkaline-earth and rare-earth phosphates was studied in comparison
to that of Rh on aluminum phosphate (AlPO<sub>4</sub>). The anchoring
of Rh via interfacial Rh–O–P bonding in Rh/AlPO<sub>4</sub> leads to efficient Rh sintering suppression. However, the
electron-withdrawing nature of the phosphate affords electron-deficient
Rh, which has a negative impact on its catalytic activity under a
reducing atmosphere due to a decrease in back-donation from the Rh <i>d</i>-orbitals to the antibonding π* orbitals of adsorbed
CO and NO molecules. Notably, the extent of this electron deficiency
could successfully be reduced by replacing AlPO<sub>4</sub> with alkaline-earth
or rare-earth phosphates, and the Rh oxide formed on these phosphate
supports was readily reduced to metallic Rh. This behavior is in complete
contrast to that of corresponding metal oxide supports, because the
higher basicity of these supports yields Rh oxides that are more difficult
to reduce. Among the phosphate-supported catalysts investigated in
the present study, Rh/LaPO<sub>4</sub> demonstrated the highest TWC
performance after thermal aging under both oxidizing and reducing
atmospheres. The effect of the higher basicity of LaPO<sub>4</sub> compared to that of AlPO<sub>4</sub> is most obvious in its improved
catalytic activity for elementary CO–O<sub>2</sub>, CO–H<sub>2</sub>O, and CO–NO reactions. Importantly, this improvement
is achieved while maintaining the activity toward C<sub>3</sub>H<sub>6</sub> as an advanced feature of phosphate supports
Enhancement of Proton Transport in an Oriented Polypeptide Thin Film
Proton transport properties of a
partially protonated polyÂ(aspartic
acid)/sodium polyaspartate (P-Asp) were investigated. A remarkable
enhancement of proton conductivity has been achieved in the thin film.
Proton conductivity of 60-nm-thick thin film prepared on MgO(100)
substrate was 3.4 × 10<sup>–3</sup> S cm<sup>–1</sup> at 298 K. The electrical conductivity of the oriented thin film
was 1 order of magnitude higher than the bulk specimen, and the activation
energies for the proton conductivity were 0.34 eV for the oriented
thin film and 0.65 eV for the pelletized sample, respectively. This
enhancement of the proton transport is attributable to the highly
oriented structure on MgO(100) substrate. This result proposes great
potential for a new strategy to produce a highly proton-conductive
material using the concept of an oriented thin film structure without
strong acid groups