Controlling the Strong Metal-Support Interaction Overlayer Structure in Pt/TiO2 Catalysts Prevents Particle Evaporation

Platinum nanoparticles (NPs) supported by titania exhibit a strong metal-support interaction (SMSI)^[1] that can induce overlayer formation and encapsulation of the NP's with a thin layer of support material. This encapsulation modifies the catalyst's properties, such as increasing its chemoselectivity^[2] and stabilizing it against sintering.^[3] Encapsulation is typically induced during high-temperature reductive activation and can be reversed through oxidative treatments.^[1] However, recent findings indicate that the overlayer can be stable in oxygen.^{[4, 5]} Using in situ transmission electron microscopy, we investigated how the overlayer changes with varying conditions. We found that exposure to oxygen below 400 °C caused disorder and removal of the overlayer upon subsequent hydrogen treatment. In contrast, elevating the temperature to 900 °C while maintaining the oxygen atmosphere preserved the overlayer, preventing platinum evaporation when exposed to oxygen. Our findings demonstrate how different treatments can influence the stability of nanoparticles with or without titania overlayers. expanding the concept of SMSI and enabling noble metal catalysts to operate in harsh environments without evaporation associated losses during burn-off cycling.     

Tracking Ion Transport in Nanochannels via Transient Single-Particle Imaging

The transport behavior of ions in the nanopores has an important impact on the performance of the electrochemical devices. Although the classical Transmission-Line (TL) model has long been used to describe ion transport in pores, the boundary conditions for the applicability of the TL model remain controversial. Here, we investigated the transport kinetics of different ions, within nanochannels of different lengths, by using transient single-particle imaging with temporal resolution up to microseconds. We found that the ion transport kinetics within short nanochannels may deviate significantly from the TL model. The reason is that the ion transport under nanoconfinement is composed of multi basic stages, and the kinetics differ much under different stage domination. With the shortening of nanochannels, the electrical double layer (EDL) formation would become the “rate-determining step” and dominate the apparent ion kinetics. Our results imply that using the TL model directly and treating the in-pore mobility as an unchanged parameter to estimate the ion transport kinetics in short nanopores/nanochannels may lead to orders of magnitude bias. These findings may advance the understanding of the nanoconfined ion transport and promote the related applications.     

Renaissance patinas in Úbeda (Spain): mineralogic,petrographic and spectroscopic study

Different analytical techniques were used for microstructural and compositional analysis of the ochre-coloured patinas that appear on the calcarenite substrate of monuments in the historical settings of Úbeda and Baeza (Spain). Optical microscopy, scanning electron microscopy–energy dispersive x-ray spectrometry (SEM-EDX), x-ray diffraction, Raman spectroscopy and attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR) were employed and a critical comparison of their experimental requirements, strengths and weaknesses is presented. The study focussed on two churches in Úbeda where patinas were widespread in ornamental elements. These films contained calcite as the principal component, and traces of dolomite and feldspars. Clear identification of calcium oxalate, mainly in the form of whewellite, was achieved by infrared and Raman spectroscopic studies. Results from texture, distribution and composition of the patinas in ornamental elements suggest that ancient treatments were applied for protection of Renaissance façades and consolidation of weathered older façades. The patinas were seldom found on supporting elements. Their different composition, apatite was found together with oxalates, and location may suggest a biogenic origin here. Gypsum crusts were sometimes found over the patinas.     

现场显微红外光谱电化学技术及超微铂盘电极上的反射吸收光谱测定

本文阐述了一种新的光谱电化学技术——现场显微红外光谱电化学法的反射式方法的技术特点和优势,报告了一种适于水溶剂和非水溶剂的反射式现场显微红外光谱电化学池的设计,并首次在25um直径的超微铂盘工作电极上,对Fe(CN)_6~(4-)/FE(CN)_6~(3-)进行了现场显微红外光谱电化学的测量。     

Nanostructures of gold coated iron core-shell nanoparticles and the nanobands assembled under magnetic field

Pure metal iron nanoparticles are unstable in the air. By a coating iron on nanoparticle surface with a stable noble metal, these air-stable nanoparticles are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential application in high density memory device by forming self-assembling nanoarrays. Gold-coated iron core-shell structure nanoparticles (Fe/Au) synthesized using reverse micelles were characterized by transmission electron microscopy (TEM). The average nanoparticle size of the core-shell structure is about 8 nm, with about 6 nm diameter core and 1∼2 nm shell. Since the gold shell is not epitaxial growth related to the iron core, the morié pattern can be seen from the overlapping of iron core and gold shell. However, the gold shell lattice can be seen by changing the defocus of TEM. An energy dispersive X-ray spectrum (EDS) also shows the nanoparticles are air-stable. The magnetic measurement of the nanoparticles also proved successful synthesis of gold coated iron core-shell structure. The nanoparticles were then assembled under 0.5 T magnetic field and formed parallel nanobands with about 10 μm long. Assembling two dimensional ordered nanoarrays are still under going. Received 29 November 2000     

In-situ Microscopic FT-IR Spectroelectrochemical Investigation of Polythiophene Film Modified Electrode

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Role of material amorphicity in laser-induced phase transition

The crystallization patterns induced by nano- and picosecond laser pulses within an amorphous matrix, with various degrees of relaxation, present morphological instabilities. We show by TEM observations that the final crystalline structures of the relaxed amorphous state, after nanosecond laser excitation, and the “as-deposited” amorphous state after picosecond excitation, present similar morphology. Structurally, metastable crystalline states have been formed under laser irradiation. A probable process for these instabilities is related to competition between the light-induced electronic excitation and thermal processes during the nucleation stage.     

Solvent- and Temperature-Dependent Assembly in Monolayer Films of a Ferrocene-Naphthyridine Hybrid on HOPG

The formation of a monolayer film of bis-naphthyridyl ferrocene on highly oriented pyrolytic graphite (HOPG) at ambient conditions is demonstrated. The films are prepared by drop casting from different solvents. The microscopic structure of the films is understood using atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). The analysis reveals two different types of Phases (I and II) in the films and the relative percentage of these phases depends on the nature of the solvents used for the preparation and the thermodynamical condition. Solvents like methanol, acetonitrile and DMF exclusively select Phase-I, whereas acetone and ethanol show a mix of both phases at room temperature. The different phases are formed by different conformers of the molecule. We also show that the selectivity of one of the phases over the other is related to the difference in the energetics for the formation of these phases.     

Microscopic Evaluation of Acid-Etched Optical Fibers

Abstract

Optical and scanning electron microscopy were used to examine the changes in the surface morphology of optical fibers as a result of acid etching. The resulting surface modifications are modeled and the resulting structures are considered as alternatives to conventional fibers for chemical sensor development. Hydrofluoric acid (HF) etching has been performed on the tips of flat-end graded index fibers, and spherical-end graded and step index fibers. The acid treatment caused the formation of a cone-shaped hollow in the center of graded index fiber tips. This structure provides a surface area enhancement of up to 5.3-fold over untreated fibers. In addition, this cone-shaped cavity provides a sub-nanoliter reservoir in which reagent can be held at the sensing tip of the fiber. Spherical-end fibers provide surface area increases of up to 35-fold compared to flat-end fibers. With spherical-end step index fibers, HF etches the cladding, but not the core, thereby providing an even greater surface area for reagent immobilization. The potential utility of these acid etched fibers for the development of fiber-optic chemical sensors (FOCS) is discussed.