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^–1. From density functional theory (DFT) calculations, this peak was attributed to the −SO₃H 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 SiO₂ 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-­(4-aminophenyl) porphyrin (H₂TAPP) 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_1.75(OH)_1.25[Ru­(dcbpy)₃]·16H₂O} (<b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b>; H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) composed of a luminescent ruthenium­(II) metalloligand [Ru­(4,4′-dcbpy)₃]^4– and La³⁺ cations. X-ray analysis for <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> revealed that the La³⁺ cations and [4Ru] metalloligands are crystallized in a molar ratio of 7:4 with OH^– counteranions and a void fraction of ∼25.5%. Interestingly, <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> shows a reversible structural transition triggered by water ad/desorption, which affects not only the triplet metal-to-ligand charge-transfer (³MLCT) emission energy but also the ion conductivity in the solid state. This correlation suggests that <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> is an interesting material that enables visualization of the ion conductivity via the ³MLCT 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_1.75(OH)_1.25[Ru­(dcbpy)₃]·16H₂O} (<b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b>; H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) composed of a luminescent ruthenium­(II) metalloligand [Ru­(4,4′-dcbpy)₃]^4– and La³⁺ cations. X-ray analysis for <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> revealed that the La³⁺ cations and [4Ru] metalloligands are crystallized in a molar ratio of 7:4 with OH^– counteranions and a void fraction of ∼25.5%. Interestingly, <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> shows a reversible structural transition triggered by water ad/desorption, which affects not only the triplet metal-to-ligand charge-transfer (³MLCT) emission energy but also the ion conductivity in the solid state. This correlation suggests that <b>La</b>_<b>7</b><b>-[4Ru]</b>_<b>4</b> is an interesting material that enables visualization of the ion conductivity via the ³MLCT 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^–2 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 poly­(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/AlPO₄ and Its Impact on Three-Way Catalysis

The influence of the redox behavior of Rh/AlPO₄ on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al₂O₃ catalyst, Rh/AlPO₄ 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_<i>x</i>) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO₄ compared with Al₂O₃. 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 AlPO₄ promoted oxidative adsorption of C₃H₆ as aldehyde, which showed a higher reactivity toward O₂, as well as NO, compared with carboxylate adsorbed on Al₂O₃. 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 AlPO₄ for Efficient Catalyst Sintering Suppression

Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO₄ supports than with Al₂O₃ 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 AlPO₄-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 AlPO₄ surface was terminated by phosphate P–OH groups, which were converted into a Rh–O–P linkage when Rh oxide (RhO_<i>x</i>) was loaded. This interaction enables the thin planar RhO_<i>x</i> nanoparticles to establish close and stable contact with the AlPO₄ surface. It differs from Rh–O–Al bonding in the oxide-supported catalyst Rh/Al₂O₃, 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_<i>x</i>/AlPO₄ 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 (AlPO₄). The anchoring of Rh via interfacial Rh–O–P bonding in Rh/AlPO₄ 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 AlPO₄ 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/LaPO₄ demonstrated the highest TWC performance after thermal aging under both oxidizing and reducing atmospheres. The effect of the higher basicity of LaPO₄ compared to that of AlPO₄ is most obvious in its improved catalytic activity for elementary CO–O₂, CO–H₂O, and CO–NO reactions. Importantly, this improvement is achieved while maintaining the activity toward C₃H₆ as an advanced feature of phosphate supports

Enhancement of Proton Transport in an Oriented Polypeptide Thin Film

Proton transport properties of a partially protonated poly­(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^–3 S cm^–1 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