We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell–Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of ...2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core–Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm2 and 1.83 A/mgPt, respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core–shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by ∼1.5 fold after a simulated catalyst lifetime.
One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be ...advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO3, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm3̅m and the lowest frequency A1 feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red-shifted band gap in nanoscale BiFeO3 compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide-based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.
In this report, we utilize the U-tube double diffusion device as a reliable, environmentally friendly method for the size-controlled synthesis of high-quality, single crystalline Pd nanowires. The ...nanowires grown in 200 and 15 nm polycarbonate template pores maintain diameters of 270 ± 45 nm and 45 ± 9 nm, respectively, and could be isolated either as individual nanowires or as ordered free-standing arrays. The growth mechanism of these nanowires has been extensively explored, and we have carried out characterization of the isolated nanowires, free-standing nanowire arrays, and cross sections of the filled template in order to determine that a unique two-step growth process predominates within the template pores. Moreover, as-prepared submicrometer and nanosized wires were studied by comparison with ultrathin 2 nm Pd nanowires in order to elucidate the size-dependent trend in oxygen reduction reaction (ORR) electrocatalysis. Subsequently, the desired platinum monolayer overcoating was reliably deposited onto the surface of the Pd nanowires by Cu underpotential deposition (UPD) followed by galvanic displacement of the Cu adatoms. The specific and platinum mass activity of the core–shell catalysts was found to increase from 0.40 mA/cm2 and 1.01 A/mg to 0.74 mA/cm2 and 1.74 A/mg as the diameter was decreased from the submicrometer size regime to the ultrathin nanometer range.
One-dimensional (1D) nanostructures, such as nanowires, nanotubes, nanorods, and nanoribbons, have attracted significant attention stemming from the plethora of interesting size-dependent and, more ...importantly, structure-related properties resulting from confinement effects. In particular, the novel properties of 1D nanostructures of metals and metal oxides (binary and ternary) render them as prime candidates for a wide range of applications including the fabrication of nanoscale devices associated with solar cells, energy storage, fuel cells, molecular computing and information storage, medical imaging, diagnosis and detection, drug delivery, sensors and catalysis. Thus, it has been simultaneously necessary and critical to create synthetic protocols for the production of these materials which not only are reliable and reproducible, but also can generate compositionally pure, monodisperse, highly crystalline products of a desired 1D morphology. Solution-based methodologies have demonstrated significant advantages over other approaches, as they are facile, simple, flexible, 'green' by nature, and can be applied to a wide range of nanomaterials with diverse chemical compositions. Moreover, these methods can often be scaled so as to produce large quantities of products which are advantageous from an applications' standpoint. Herein, we present synthetic advances associated with solution-based approaches. Specifically solvothermal/hydrothermal, molten salt, electrospinning, template-directed, solution/one-pot, and sol-gel methodologies are discussed with the primary goal of achieving the reproducible synthesis of 1D motifs of metals, binary metal oxides, and ternary metal oxide systems.
Solution-based methodologies have demonstrated significant advances over other approaches in the synthesis of a wide range of one-dimensional metal-containing nanostructures.
We evaluated the cytotoxicity of various morphological classes of TiO2 nanostructures (including 0-D nanoparticles, 1-D nanorods, and 3-D assemblies) toward living cells. These TiO2 nanostructures ...were modified with fluorescent dye molecules, mediated via a dopamine linkage, in order to facilitate a confocal study of their internalization. Specifically, we noted that both TiO2 1-D nanorods and 0-D nanoparticles could internalize into cells after 24 h of incubation time. However, only incubation with TiO2 1-D nanorods and 3-D micrometer-scale sea urchin-like assemblies at concentrations of up to 125 μg/mL yielded data suggestive of cell viabilities of close to 100%. Moreover, upon irradiation with UV light for periods of a few minutes at energy densities of up to 1 J/cm2, we observed up to 60% mortality rates, indicative of the cytotoxic potential of photoirradiated TiO2 nanostructures due to the generation of reactive oxygen species.
Herein, we report the synthesis of one-dimensional chromium oxide nanostructures, utilizing a modified sol−gel technique combined with a constrained template environment. Using scanning electron ...microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM), we noted that individual nanowires were cylindrical in nature and appeared to be composed of smaller, crystalline, constituent nanoparticles, sintered and aggregated together so as to form a discrete, polycrystalline structure. Spectroscopic and diffraction investigations of our nanostructures confirmed their chemical composition with little if any impurities. Moreover, we further investigated the properties of our nanomaterials using both electrical and magnetic characterization. Interestingly, the magnetic properties of our nanostructures are strongly modified as compared with the bulk, due to the emergence of a net magnetic moment induced by uncompensated surface spins. Catalysis data showed that these nanostructures were active toward the thermal decomposition of KClO4.
A template-directed, sol–gel synthesis is utilized to produce crystalline RuO2 nanowires. Crystalline nanowires with a diameter of 128 ± 15 nm were synthesized after treating the nanowires at 600 °C ...in air. Analysis of these nanowires by X-ray powder diffraction revealed the major crystalline phase to be tetragonal RuO2 with a small quantity of metallic ruthenium present. Further analysis of the nanowire structures by high-resolution transmission electron microscopy reveals that they are polycrystalline and are composed of interconnected, highly crystalline, nanoparticles having an average size of ∼25 nm. Uniform 3 nm Pt nanoparticles were dispersed on the surface of RuO2 nanowires using an ambient, solution-based technique yielding a hybrid catalyst for methanol oxidation. Linear sweep voltammograms (LSVs) and chronoamperometry performed in the presence of methanol in an acidic electrolyte revealed a significant enhancement in the onset potential, mass activity, and long-term stability compared with analogous Pt nanoparticles supported on commercially available Vulcan XC-72R carbon nanoparticles. Formic acid oxidation LSVs and CO stripping voltammetry revealed that the RuO2-supported Pt nanoparticles exhibit significantly higher CO tolerance, which leads to higher catalytic stability over a period of several hours. X-ray photoelectron spectroscopy results suggest that crystalline RuO2 leads to less-significant oxidation of the Pt surface relative to more widely studied hydrous RuO2 supports, thereby increasing catalytic performance.
Cholinesterase inhibitors positively affect cognition in Alzheimer’s disease (AD) and other conditions, but no controlled functional MRI studies have examined where their effects occur in the brain. ...We examined the effects of donepezil hydrochloride (Aricept®) on cognition and brain activity in patients with amnestic mild cognitive impairment (MCI), a diagnosis associated with a high risk of developing AD. Nine older adults with MCI were compared with nine healthy, demographically matched controls. At baseline, patients showed reduced activation of frontoparietal regions relative to controls during a working memory task. After stabilization on donepezil (5.7 ± 1.7 weeks at 10 mg) patients showed increased frontal activity relative to unmedicated controls, which was positively correlated with improvement in task performance (r = 0.49, P = 0.05) as well as baseline hippocampal volume (r = 0.62, P < 0.05). The patients’ overall cognitive function was stable or improved throughout the study. Short‐term treatment with a cholinesterase inhibitor appears to enhance the activity of frontal circuitry in patients with MCI, and this increase appears to be related to improved cognition and to baseline integrity of the hippocampus. These relationships have implications for understanding the mechanisms by which cognition‐enhancing medications exert their effects on brain function and for the use of functional MRI in early detection and treatment monitoring of AD and MCI.
The reproducible gram-scale synthesis of crystalline nanoscale multiferroics is critical for the development of the next generation of commercially relevant electronic devices. Of the subset of ...multiferroic materials, yttrium manganese oxide (YMnO3) is highly attractive, because of its large magneto-electric coupling constants and the recent observation of giant polarization under pressure in these types of rare earth manganites. Utilizing a unique synthetic methodology that combines metal–oleate thermal degradation with the use of a molten salt protocol, we were able to reproducibly generate monodisperse distributions of morphologically distinctive yttrium manganese oxides. Specifically, using a molten NaCl flux, we were able to synthesize phase-pure, single-crystalline hexagonal YMnO3 nanoplates, measuring 441 ± 241 nm in diameter and 46 ± 6 nm in height. Moreover, these nanoplates gave rise to multiferroic behavior, which was confirmed by the observation of a ferroelectric phase from a combination of high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) analysis. Magnetic measurements are consistent with the onset of a spin glass state below 5 K. To highlight the generalizability of the synthetic method we have developed herein, as a demonstration of principle, we have also successfully used the same protocol to produce nanocubes of lanthanum aluminum oxide (LaAlO3).
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