53 research outputs found
Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction
Developing improved catalysts for
the oxygen evolution reaction
(OER) is key to the advancement of a number of renewable energy technologies,
including solar fuels production and metal air batteries. In this
study, we employ electrochemical methods and synchrotron techniques
to systematically investigate interactions between metal oxides and
noble metals that lead to enhanced OER catalysis for water oxidation.
In particular, we synthesize porous MnO<sub><i>x</i></sub> films together with nanoparticles of Au, Pd, Pt, or Ag and observe
significant improvement in activity for the combined catalysts. Soft
X-ray absorption spectroscopy (XAS) shows that increased activity
correlates with increased Mn oxidation states to 4+ under OER conditions
compared to bare MnO<sub><i>x</i></sub>, which exhibits
minimal OER current and remains in a 3+ oxidation state. Thickness
studies of bare MnO<sub><i>x</i></sub> films and of MnO<sub><i>x</i></sub> films deposited on Au nanoparticles reveal
trends suggesting that the enhancement in activity arises from interfacial
sites between Au and MnO<sub><i>x</i></sub>
Co<sub>11</sub>Li[(OH)<sub>5</sub>O][(PO<sub>3</sub>OH)(PO<sub>4</sub>)<sub>5</sub>], a Lithium-Stabilized, Mixed-Valent Cobalt(II,III) Hydroxide Phosphate Framework
A new metastable
phase, featuring a lithium-stabilized mixed-valence cobaltÂ(II,III)
hydroxide phosphate framework, Co<sub>11.0(1)</sub>Li<sub>1.0(2)</sub>[(OH)<sub>5</sub>O]Â[(PO<sub>3</sub>OH)Â(PO<sub>4</sub>)<sub>5</sub>], corresponding to the simplified composition Co<sub>1.84(2)</sub>Li<sub>0.16(3)</sub>Â(OH)ÂPO<sub>4</sub>, is prepared
by hydrothermal synthesis. Because the pH-dependent formation of other
phases such as Co<sub>3</sub>(OH)<sub>2</sub>Â(PO<sub>3</sub>OH)<sub>2</sub> and olivine-type LiCoPO<sub>4</sub> competes in the
process, a pH value of 5.0 is crucial for obtaining a single-phase
material. The crystals with dimensions of 15 μm × 30 μm
exhibit a unique elongated triangular pyramid morphology with a lamellar
fine structure. Powder X-ray diffraction experiments reveal that the
phase is isostructural with the natural phosphate minerals holtedahlite
and satterlyite, and crystallizes in the trigonal space group <i>P</i>31<i>m</i> (<i>a =</i> 11.2533(4) Ã…, <i>c =</i> 4.9940(2) Ã…, <i>V =</i> 547.70(3) Ã…<sup>3</sup>, <i>Z =</i> 1). The three-dimensional network structure
is characterized by partially Li-substituted, octahedral [M<sub>2</sub>O<sub>8</sub>(OH)] (M = Co, Li) dimer units which form double chains
that run along the [001] direction and are connected by [PO<sub>4</sub>] and [PO<sub>3</sub>(OH)] tetrahedra. Because no Li-free <i>P</i>31<i>m</i>-type Co<sub>2</sub>(OH)ÂPO<sub>4</sub> phase could be prepared, it can be assumed that the Li ions are
crucial for the stabilization of the framework. Co L-edge X-ray absorption
spectroscopy demonstrates that the cobalt ions adopt the oxidation
states +2 and +3 and hence provides further evidence for the incorporation
of Li in the charge-balanced framework. The presence of three independent
hydroxyl groups is further confirmed by infrared spectroscopy. Magnetization
measurements imply a paramagnetic to antiferromagnetic transition
at around <i>T</i> = 25 K as well as a second transition
at around 9–12 K with a ferromagnetic component below this
temperature. The metastable character of the phase is demonstrated
by thermogravimetric analysis and differential scanning calorimetry,
which above 558 °C reveal a two-step decomposition to CoO, Co<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>, and olivine-type LiCoPO<sub>4</sub> with release of water and oxygen
Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons
Aromaticity
profoundly affects molecular orbitals in polycyclic
aromatic hydrocarbons. X-ray core electron spectroscopy has observed
that carbon 1s−π* transitions can be broadened or even
split in some polycyclic systems, although the origin of the effect
has remained obscure. The π electrons in polycyclic systems
are typically classified in the Clar model as belonging to either
true aromatic sextets (similar to benzene) or isolated double bonds
(similar to olefins). Here, bulk-sensitive carbon core excitation
spectra are presented for a series of polycyclic systems and show
that the magnitude of the 1s−π* splitting is determined
primarily by the ratio of true aromatic sextets to isolated double
bonds. The observed splitting can be rationalized in terms of ground
state energetics as described by Hückel, driven by the π
electron structure described by Clar. This simple model including
only ground state energetics is shown to explain the basics physics
behind the spectral evolution for a broad set of polycyclic aromatic
hydrocarbons, although some residual deviations between this model
and experiment can likely be improved by including a more detailed
electronic structure and the core hole effect
Scalable Low-Cost Fabrication of Disposable Paper Sensors for DNA Detection
Controlled integration of features
that enhance the analytical performance of a sensor chip is a challenging
task in the development of paper sensors. A critical issue in the
fabrication of low-cost biosensor chips is the activation of the device
surface in a reliable and controllable manner compatible with large-scale
production. Here, we report stable, well-adherent, and repeatable
site-selective deposition of bioreactive amine functionalities and
biorepellant polyethylene glycol-like (PEG) functionalities on paper
sensors by aerosol-assisted, atmospheric-pressure, plasma-enhanced
chemical vapor deposition. This approach requires only 20 s of deposition
time, compared to previous reports on cellulose functionalization,
which takes hours. A detailed analysis of the near-edge X-ray absorption
fine structure (NEXAFS) and its sensitivity to the local electronic
structure of the carbon and nitrogen functionalities. σ*, π*,
and Rydberg transitions in C and N K-edges are presented. Application
of the plasma-processed paper sensors in DNA detection is also demonstrated
Mn<sub>3</sub>O<sub>4</sub> Supported on Glassy Carbon: An Active Non-Precious Metal Catalyst for the Oxygen Reduction Reaction
In this work, we explore the interplay between manganese
oxide
(MnO<sub><i>x</i></sub>) nanomaterials and a glassy carbon
(GC) support in catalyzing the oxygen reduction reaction (ORR) in
an alkaline environment. Initially, we characterize the ORR activity
of bare GC electrodes as a function of heat treatments in air, and
find that ORR activity increases with increasing temperature up to
500 °C. Modification of GC with size-selected 1 nm MnO<sub><i>x</i></sub> nanoparticles prior to the 500 °C heat treatment
yields a highly porous GC (pGC) structure, devoid of MnO<sub><i>x</i></sub>. This pGC sample exhibits the highest ORR performance
of the bare carbon electrodes reaching an onset potential of 0.75
V vs the reversible hydrogen electrode (RHE) and a complete 2-electron
reduction of oxygen to peroxide. Having established ORR activity of
bare GC electrodes, we deposit size-selected 14 nm MnO nanoparticles
onto the GC and pGC electrodes and then incite phase changes in MnO
through heat treatments in air. Electrochemical characterization of
the resulting electrodes reveals that MnO nanoparticles offer no improvement
in the ORR onset potential over bare GC or pGC and only slightly increase
the number of electrons transferred. By contrast, thermal oxidation
of MnO nanoparticles to Mn<sub>3</sub>O<sub>4</sub> at 500 °C,
confirmed by Mn L-edge X-ray absorption spectroscopy, results in an
improved ORR onset potential of 0.80 V and a 4-electron reduction
of oxygen. Thus at low overpotentials, where GC and pGC were inactive
for the ORR, MnO<sub><i>x</i></sub> sites must contribute
to all steps of the reaction. The catalyst’s estimated specific
activity of 3700 μA·cm<sup>–2</sup><sub>cat</sub> at 0.75 V compares favorably with specific activities of Pt/C as
well as the best nonprecious metal catalysts. This establishes Mn<sub>3</sub>O<sub>4</sub> as another MnO<sub><i>x</i></sub> phase
with high activity for the ORR
Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation
Much is known about the rate of photoexcited
charge generation
in at organic donor/acceptor (D/A) heterojunctions overaged over all
relative arrangements. However, there has been very little experimental
work investigating how the photoexcited electron transfer (ET) rate
depends on the precise relative molecular orientation between D and
A in thin solid films. This is the question that we address in this
work. We find that the ET rate depends strongly on the relative molecular
arrangement: The interface where the model donor compound copper phthalocyanine
is oriented face-on with respect to the fullerene C<sub>60</sub> acceptor
yields a rate that is approximately 4 times faster than that of the
edge-on oriented interface. Our results suggest that the D/A electronic
coupling is significantly enhanced in the face-on case, which agrees
well with theoretical predictions, underscoring the importance of
controlling the relative interfacial molecular orientation
Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals
Doped
metal oxide nanocrystals represent an exciting frontier for
colloidal synthesis of plasmonic materials, displaying unique optoelectronic
properties and showing promise for a variety of applications. However,
fundamental questions about the nature of doping in these materials
remain. In this article, the strong influence of radial dopant distribution
on the optoelectronic properties of colloidal indium tin oxide nanocrystals
is reported. Comparing elemental depth-profiling by X-ray photoelectron
spectroscopy (XPS) with detailed modeling and simulation of the optical
extinction of these nanocrystals using the Drude model for free electrons,
a correlation between surface segregation of tin ions and the average
activation of dopants is observed. A strong influence of surface segregation
of tin on the line shape of the localized surface plasmon resonance
(LSPR) is also reported. Samples with tin segregated near the surface
show a symmetric line shape that suggests weak or no damping of the
plasmon by ionized impurities. It is suggested that segregation of
tin near the surface facilitates compensation of the dopant ions by
electronic defects and oxygen interstitials, thus reducing activation.
A core–shell model is proposed to explain the observed differences
in line shape. These results demonstrate the nuanced role of dopant
distribution in determining the optoelectronic properties of semiconductor
nanocrystals and suggest that more detailed study of the distribution
and structure of defects in plasmonic colloidal nanocrystals is warranted
Synergistic Role of Dopants on the Morphology of Alloyed Copper Chalcogenide Nanocrystals
The
presence of antimony, as a dopant in the colloidal growth reaction
for CuIn<sub>1–<i>x</i></sub>Ga<sub><i>x</i></sub>S<sub>2</sub> (CIGS) nanocrystals, causes end-to-end fusion
of nanorod pairs into nanodumbbells at high yield. The influence of
the dopant on shape is indirect; antimony catalyzes the incorporation
of gallium, which is found in high concentration at the junction between
the fused nanorods
Sensitivity of X‑ray Core Spectroscopy to Changes in Metal Ligation: A Systematic Study of Low-Coordinate, High-Spin Ferrous Complexes
In
order to assess the sensitivity and complementarity of X-ray
absorption and emission spectroscopies for determining changes in
the metal ligation sphere, a systematic experimental and theoretical
study of iron model complexes has been carried out. A series of high-spin
ferrous complexes, in which the ligation sphere has been varied from
a three-coordinate complex, [L<sup>tBu</sup>FeÂ(SPh)] (<b>1</b>) (where L<sup>tBu</sup> = bulky β-diketiminate ligand; SPh
= phenyl thiolate) to four-coordinate complexes [L<sup>tBu</sup>FeÂ(SPh)Â(X)]
(where X = CN<sup>t</sup>Bu (<b>2</b>); 1-methylimidazole (<b>3</b>); or <i>N</i>,<i>N</i>-dimethylformamide
(DMF) (<b>4</b>)), has been investigated using a combination
of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES)
spectroscopies. The Fe K XAS pre-edge and edge of all four complexes
are consistent with a high-spin ferrous assignment, with the largest
differences in the pre-edge intensities attributed to changes in covalency
of the fourth coordination site. The X-ray emission spectra show pronounced
changes in the valence to core region (V2C) as the identity of the
coordinated ligand is varied. The experimental results have been correlated
to density functional theory (DFT) calculations, to understand key
molecular orbital contributions to the observed absorption and emission
features. The calculations also have been extended to a series of
hypothetical high-spin iron complexes to understand the sensitivity
of XAS and XES techniques to different ligand protonation states ([L<sup>tBu</sup>Fe<sup>II</sup>(SPh)Â(NH<sub><i>n</i></sub>)]<sup>3–<i>n</i></sup> (<i>n</i> = 3, 2, 1,
0)), metal oxidation states [L<sup>tBu</sup>FeÂ(SPh)Â(N)]<sup><i>n</i>−</sup> (<i>n</i> = 3, 2, 1), and changes
in the ligand identity [L<sup>tBu</sup>Fe<sup>IV</sup>(SPh)Â(X)]<sup><i>n</i>−</sup> (X = C<sup>4–</sup>, N<sup>3–</sup>, O<sup>2–</sup>; <i>n</i> = 2, 1,
0). This study demonstrates that XAS pre-edge data have greater sensitivity
to changes in oxidation state, while valence to core (V2C) XES data
provide a more sensitive probe of ligand identity and protonation
state. The combination of multiple X-ray spectroscopic methods with
DFT results thus has the potential to provide for detailed characterization
of complex inorganic systems in both chemical and biological catalysis
Effect of Backbone Chemistry on the Structure of Polyurea Films Deposited by Molecular Layer Deposition
An experimental investigation
into the growth of polyurea films
by molecular layer deposition was performed by examining trends in
the growth rate, crystallinity, and orientation of chains as a function
of backbone flexibility. Growth curves obtained for films containing
backbones of aliphatic and phenyl groups indicate that an increase
in backbone flexibility leads to a reduction in growth rate from 4
to 1 Ã…/cycle. Crystallinity measurements collected using grazing
incidence X-ray diffraction and Fourier transform infrared spectroscopy
suggest that some chains form paracrystalline, out-of-plane stacks
of polymer segments with packing distances ranging from 4.4 to 3.7
Ã… depending on the monomer size. Diffraction intensity is largely
a function of the homogeneity of the backbone. Near-edge X-ray absorption
fine structure measurements for thin and thick samples show an average
chain orientation of ∼25° relative to the substrate across
all samples, suggesting that changes in growth rate are not caused
by differences in chain angle but instead may be caused by differences
in the frequency of chain terminations. These results suggest a model
of molecular layer deposition-based chain growth in which films consist
of a mixture of upward growing chains and horizontally aligned layers
of paracrystalline polymer segments
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