9 research outputs found
Quasi-in-Situ Single-Grain Photoelectron Microspectroscopy of Co/PPy Nanocomposites under Oxygen Reduction Reaction
This paper reports an investigation
into the aging of pyrolyzed cobalt/polypyrrole (Co/PPy) oxygen reduction
reaction (ORR) electrocatalysts, based on quasi-in-situ photoelectron
microspectroscopy. The catalyst precursor was prepared by potentiostatic
reverse-pulse coelectrodeposition from an acetonitrile solution on
graphite. Accelerated aging was obtained by quasi-in-situ voltammetric
cycling in an acidic electrolyte. Using photoelectron imaging and
microspectroscopy of single Co/PPy grains at a resolution of 100 nm,
we tracked the ORR-induced changes in the morphology and chemical
state of the pristine material, consisting of uniformly distributed ∼20
nm nanoparticles, initially consisting of a mixture of Co(II) and
Co(III) oxidation states in almost equal amounts. The evolution of
the Co 2p, O 1s, and N 1s spectra revealed that the main effects of
aging are a gradual loss of the Co present at the surface and the
reduction of Co(III) to Co(II), accompanied by the emergence and growth
of a N 1s signal, corresponding to electrocatalytically active C–N
sites
Depth-Dependent Scanning Photoelectron Microspectroscopy Unravels the Mechanism of Dynamic Pattern Formation in Alloy Electrodeposition
Fascinating spatiotemporal
patterns forming during the electrodeposition
of some alloys have attracted the interest of the scientific communities
dealing with electrochemical materials science and dynamic processes.
Notwithstanding extensive experimental work and recently achieved
theoretical insights, several aspects of the physical chemistry of
these dynamic structures are still elusive. In particular, the analytical
methods employed so far to characterize these structures invariably
failed to pinpoint any chemical or structural patterns correlated
to those perceived by the naked eye or with a light microscope. In
this work, we have made systematic use of the extreme surface sensitivity
provided by synchrotron-based scanning photoelectron microspectroscopy,
combined with progressive erosion by precisely controlled Ar<sup>+</sup> sputtering, to achieve quantitative 3D understanding of the compositional
and chemical-state distribution of an Ag–In electrodeposited
layer, following the key elements Ag, In, and O. The results revealed
that the pattern is present only in the topmost region (ca. 100 nm)
of the layer and exhibits a regular distribution of the alloying elements
in certain chemical states. Specifically, pattern formation in Ag–In
electrodeposits is crucially controlled by the space distribution
of surface In<sup>3+</sup> oxi-/hydroxides, deriving from reaction-diffusion
processes taking place during alloy growth, and this pattern disappears
in depth because of the delayed reduction of In<sup>3+</sup> present
in this film to elemental In, followed by intermetallic formation
Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)
Submonolayer coverages of V-oxide on Rh(111) condense
during catalytic
methanol oxidation into a pattern of macroscopic stripes or islands.
Under reaction conditions, a phase separation occurs within the VOx islands that has been studied in a pressure
range of 10–6–10–4 mbar
with photoemission electron microscopy (PEEM), low-energy electron
microscopy (LEEM), microspot-low-energy electron diffraction (μLEED),
and microspot-X-ray photoelectron spectroscopy (μXPS). An oxidized
outer ring with a (√7 × √7)R19.1° structure
coexists with an inner (12 × 12) Moiré-type boundary layer
and a reduced core exhibiting a (√3 × √3)R30°
Moiré type pattern. The dependence of the substructure on the
reaction conditions, on V coverage, and on island size was investigated.
With μXPS, the V coverages of the different phases in the VOx islands were determined
Plasma Fluorination of Vertically Aligned Carbon Nanotubes
Functionalization of vertically aligned
multiwalled carbon nanotube
carpets was performed via exposure to CF<sub>4</sub> or Ar:F<sub>2</sub> RF plasmas. Rapid fluorination was observed via X-ray photoelectron
spectroscopy (XPS) with surface fluorine concentration, bonding type,
and patterning dependent on gas mixture and exposure time. Surface
properties of the v-MWCNTs forests have been changed by the introduction
of fluorine-containing groups, as demonstrated via surface wettability
studies, while scanning electron microscopy shows that overall nanotube
alignment and separation is conserved. Scanning X-ray photoelectron
spectromicroscopy (SPEM) shows that the plasma treatment results in
selective functionalization of the surface tips of the nanotubes.
This opens the way to nanotube carpet structures with activated surfaces,
which maintain the desirable conductive properties of the pristine
nanotubes near to the substrate
Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO<sub>2</sub> on Polycrystalline Ti
Mixtures or composites
of titania and carbon have gained considerable
research interest as innovative catalyst supports for low- and intermediate-temperature
proton-exchange membrane fuel cells. For applications in electrocatalysis,
variations in the local physicochemical properties of the employed
materials can have significant effects on their behavior as catalyst
supports. To assess
microscopic heterogeneities in composition, structure, and morphology,
a microscopic multitechnique approach is required. In this work, compact
anodic TiO<sub>2</sub> films on planar polycrystalline Ti substrates
are converted into carbon/titania composites or multiphase titanium
oxycarbides through carbothermal treatment in an acetylene/argon atmosphere
in a flow reactor. The local chemical composition, structure, and
morphology of the converted films are studied with scanning photoelectron
microscopy, micro-Raman spectroscopy, and scanning electron microscopy
and are related with the crystallographic orientations of the Ti substrate
grains by means of electron backscatter diffraction. Different annealing
temperatures, ranging from 550 to 850 °C, are found to yield
different substrate grain-dependent chemical compositions, structures,
and morphologies. The present study reveals individual time scales
for the carbothermal conversion and subsequent surface re-oxidation
on substrate grains of a given orientation. Furthermore, it demonstrates
the power of a microscopic multitechnique approach for studying polycrystalline
heterogeneous materials for electrocatalytic applications
Reversible Compositional Control of Oxide Surfaces by Electrochemical Potentials
Perovskite oxides can exhibit a wide range of interesting
characteristics
such as being catalytically active and electronically/ionically conducting,
and thus, they have been used in a number of solid-state devices such
as solid oxide fuel cells (SOFCs) and sensors. As the surface compositions
of perovskites can greatly influence the catalytic properties, knowing
and controlling their surface compositions is crucial to enhance device
performance. In this study, we demonstrate that the surface strontium
(Sr) and cobalt (Co) concentrations of perovskite-based thin films
can be controlled reversibly at elevated temperatures by applying
small electrical potential biases. The surface compositional changes
of La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3−δ</sub> (LSC<sub>113</sub>), (La<sub>0.5</sub>Sr<sub>0.5</sub>)<sub>2</sub>CoO<sub>4±δ</sub> (LSC<sub>214</sub>), and LSC<sub>214</sub>-decorated
LSC<sub>113</sub> films (LSC<sub>113/214</sub>) were investigated
in situ by utilizing synchrotron-based X-ray photoelectron spectroscopy
(XPS), where the largest changes of surface Sr were found for the
LSC<sub>113/214</sub> surface. These findings offer the potential
of reversibly controlling the surface functionality of perovskites
Carbothermal Transformation of TiO<sub>2</sub> into TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> in UHV: Tracking Intrinsic Chemical Stabilities
The conversion of anodic TiO<sub>2</sub> films into TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> in ultrahigh-vacuum
(UHV) has been traced by photoemission spectroscopy in order to optimize
the process parameters and study the different phase stabilities.
In addition, density functional theory (DFT) calculations have been
performed in order to elucidate the main questions about TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> composition and stability.
The experimental data indicate that the anodic TiO<sub>2</sub> film
is stable both in UHV and ethylene background up to ca. 600 K, and
at this temperature, it starts to reduce leading to suboxide TiO<sub><i>x</i></sub> species. Above ca. 750 K, the formation
of TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> starts,
since the oxygen vacancies begin to be replaced by carbon atoms. A
surface enrichment in TiO<sub>2</sub> and elemental carbon has been
detected on the converted TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> film at room temperature. Real-time measurements
have shown that this phenomenon takes place during the cool down process
and DFT calculations suggest a possible explanation: as the temperature
decreases below ca. 750 K (temperature at which the formation of TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> starts), the
TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> phase
is not thermodynamically stable, and it decomposes into TiO<sub>2</sub> and elemental carbon. The comparison of the experimental valence
band data with DFT results has also allowed to establish that the
film surface is not homogeneous and that segregation of TiO and TiC
systems may take place. On the other hand, the local compositional
study carried out by scanning photoelectron microscopy has shown that
the conversion of the film is not homogeneous but depends on the grain
orientation, in particular crystallites with an orientation close
to <21̅1̅0> and <101̅0> planes show
a higher
grade of conversion. Both experimental and DFT data validate the use
of TiO<sub><i>x</i></sub>C<sub><i>y</i></sub> as
an innovative support for electrocatalysis
Ferromagnetic Layers in a Topological Insulator (Bi,Sb)<sub>2</sub>Te<sub>3</sub> Crystal Doped with Mn
Magnetic topological insulators (MTIs) have recently
become a subject
of poignant interest; among them, Z2 topological insulators
with magnetic moment ordering caused by embedded magnetic atoms attract
special attention. In such systems, the case of magnetic anisotropy
perpendicular to the surface that holds a topologically nontrivial
surface state is the most intriguing one. Such materials demonstrate
the quantum anomalous Hall effect, which manifests itself as chiral
edge conduction channels that can be manipulated by switching the
polarization of magnetic domains. In the present paper, we uncover
the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction
with its electronic and magnetic properties; this material is characterized
by naturally formed ferromagnetic layers inside the insulating matrix,
where the Fermi level is tuned to the bulk band gap. We found that
in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which
is separated by two or three (Bi,Sb)2Te3 quintuple
layers (QLs); such a structure possesses ferromagnetic properties.
The surface obtained by cleavage includes terraces with different
terminations. Manganese atoms preferentially occupy the central positions
in the SLs and in a very small proportion can appear in the QLs, as
indirectly indicated by a reshaped Dirac cone
Laterally Selective Oxidation of Large-Scale Graphene with Atomic Oxygen
Using
X-ray photoemission microscopy, we discovered that oxidation
of commercial large-scale graphene on Cu foil, which typically has
bilayer islands, by atomic oxygen proceeds with the formation of the
specific structures: though relatively mobile epoxy groups are generated
uniformly across the surface of single-layer graphene, their concentration
is significantly lower for bilayer islands. More oxidized species
like carbonyl and lactones are preferably located at the centers of
these bilayer islands. Such structures are randomly distributed over
the surface with a mean density of about 3× 10<sup>6</sup> cm<sup>–2</sup> in our case. Using a set of advanced spectromicroscopy
instruments including Raman microscopy, X-ray photoelectron spectroscopy
(μ-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved
photoelectron spectroscopy (μ-ARPES), we found that the centers
of the bilayer islands where the second layer nucleates have a high
defect concentration and serve as the active sites for deep oxidation.
This information can be potentially useful in developing lateral heterostructures
for electronics and optoelectronics based on graphene/graphene oxide
heterojunction