26 research outputs found
Low-Temperature Dissociation of CO<sub>2</sub> on a Ni/CeO<sub>2</sub>(111)/Ru(0001) Model Catalyst
The
adsorption of CO<sub>2</sub> on CeO<sub>2â<i>x</i></sub>(111) and Ni/CeO<sub>2â<i>x</i></sub>(111)/RuÂ(0001)
surfaces has been studied with reflection absorption infrared spectroscopy
(RAIRS) and X-ray photoelectron spectroscopy (XPS). On the maximal-oxidized
CeO<sub>2</sub>(111) surface physisorbed linear CO<sub>2</sub> and
a CO<sub>2</sub><sup>â</sup> species are identified at 97 K. The reduced CeO<sub>2â<i>x</i></sub>(111) surface exhibits higher reactivity toward adsorbed
CO<sub>2</sub>, which leads to higher coverages of CO<sub>2</sub><sup>â</sup> and promotes
CO<sub>2</sub> dissociating into CO and an active oxygen species at
higher temperature, reoxidizing the reduced CeO<sub>2â<i>x</i></sub>(111) films. Deposition of Ni on the maximal-oxidized
CeO<sub>2</sub> thin films leads to slight reduction of ceria films.
Adsorption of CO<sub>2</sub> on Ni/CeO<sub>2â<i>x</i></sub>(111) films causes dissociation at 97 K and leads to Ni-CO
adsorbates plus partial oxidation of Ni nanoparticles. This process
is inhibited when Ni nanoparticles on CeO<sub>2</sub> are fully oxidized.
In contrast to the results reported for CO<sub>2</sub> adsorption
on Ni single-crystals, where the dissociation temperature was found
to be higher than 240 K, the much lower dissociation temperature (âź97
K) for CO<sub>2</sub> on Ni nanoparticles supported on CeO<sub>2</sub>(111) suggests that the Ni/CeO<sub>2</sub> catalyst exhibits high
activity toward CO<sub>2</sub> activation
Structural and Optical Interplay of Palladium-Modified TiO<sub>2</sub> Nanoheterostructure
The
electronic structure and optical properties of Pd-modified
TiO<sub>2</sub> nanotubes (NTs) with a vertically aligned nanotubular
structure grown by a two-step electrochemical anodization method have
been studied using X-ray spectroscopy. X-ray absorption near-edge
structure (XANES) at the Ti L<sub>3,2</sub>- and O K-edges was used
to investigate the TiO<sub>2</sub> NTs before and after Pd modification.
It was found that Pd nanoparticles (NPs) are uniformly coated on the
NT surface. The Pd L<sub>3</sub>-edge of the deposited Pd NPs shows
a more intense whiteline and a blue shift for the Pd L<sub>3</sub>-edge absorption threshold relative to Pd metal, indicating charge
depletion from the Pd 4d orbital as a result NP formation. The lattice
of Pd is slightly contracted upon NP formation, although it remains
fcc as revealed by extended X-ray absorption fine structure (EXAFS)
analysis at the Pd K-edge. X-ray-excited optical luminescence (XEOL)
together with XANES with element and site specificity was used to
study the optical luminescence from TiO<sub>2</sub> NTs. It was found
that the defect-originated XEOL intensity dropped noticeably in the
Pd NP-coated NTs, suggesting a Pd NPâTiO<sub>2</sub>-interaction-mediated
reduction in the radiative recombination of electrons and holes. Further
evidence is provided by Ti 2p resonant inelastic X-ray scattering
(RIXS), in which no low-energy loss features (dâd transitions)
were observed. The implications of these results are discussed
Traces of Potassium Induce Restructuring of the Anatase TiO<sub>2</sub>(001)-(1Ă4) Surface from a Reactive to an Inert Structure
Reconstruction of solid surfaces is generally accompanied
by changes
in surface activities. Here, via a combined experimental and theoretical
study, we successfully identified that a trace amount of potassium
dopant restructures the mineral anatase TiO2(001) single-crystal
surface from an added molecule (ADM) termination to an added oxygen
(AOM) one without changing the (1Ă4) periodicity. The anatase
TiO2(001)-(1Ă4)-ADM surface terminated with 4-fold
coordinated Ti4c and 2-fold coordinated O2c sites
is (photo)catalytically active, whereas the anatase TiO2(001)-(1Ă4)-AOM surface terminated with O2c and inaccessible
5-fold coordinated Ti5c sites is inert. These results unveiled
a mechanism of dopant-induced transformation from a reactive to an
inert TiO2(001)-(1Ă4) surface, which unifies the existing
arguments about the surface structures and (photo)catalytic activity
of anatase TiO2(001)-(1Ă4)
Tracking the Local Effect of Fluorine Self-Doping in Anodic TiO<sub>2</sub> Nanotubes
We report herein a study in which
we reveal the role of F<sup>â</sup> incorporated in the very
anodic TiO<sub>2</sub> nanotubes prepared
electrochemically from a Ti foil using a fluoride based electrolyte.
X-ray absorption near edge structure (XANES), resonant X-ray emission
spectroscopy (RXES), and X-ray photoelectron spectroscopy (XPS) have
been used to examine the as-prepared and the annealed TiO<sub>2</sub> nanotubes. It is found that the additional electron resulting from
the substitution of O<sup>2â</sup> by self-doped F<sup>â</sup> in the TiO<sub>2</sub> lattice is localized in the t<sub>2g</sub> state. Consequently, a localized Ti<sup>3+</sup> state can be tracked
by a dâd energy loss peak with a constant energy of 1.6 eV
in the RXES, in contrast to TiO<sub>2</sub> nanostructures where this
peak is hardly noticeable when F<sup>â</sup> is driven out
of the lattice upon annealing
Traces of Potassium Induce Restructuring of the Anatase TiO<sub>2</sub>(001)-(1Ă4) Surface from a Reactive to an Inert Structure
Reconstruction of solid surfaces is generally accompanied
by changes
in surface activities. Here, via a combined experimental and theoretical
study, we successfully identified that a trace amount of potassium
dopant restructures the mineral anatase TiO2(001) single-crystal
surface from an added molecule (ADM) termination to an added oxygen
(AOM) one without changing the (1Ă4) periodicity. The anatase
TiO2(001)-(1Ă4)-ADM surface terminated with 4-fold
coordinated Ti4c and 2-fold coordinated O2c sites
is (photo)catalytically active, whereas the anatase TiO2(001)-(1Ă4)-AOM surface terminated with O2c and inaccessible
5-fold coordinated Ti5c sites is inert. These results unveiled
a mechanism of dopant-induced transformation from a reactive to an
inert TiO2(001)-(1Ă4) surface, which unifies the existing
arguments about the surface structures and (photo)catalytic activity
of anatase TiO2(001)-(1Ă4)
Confined Synthesis of Organometallic Chains and Macrocycles by CuâO Surface Templating
The bottom-up construction of low-dimensional
macromolecular nanostructures
directly on a surface is a promising approach for future application
in molecular electronics and integrated circuit production. However,
challenges still remain in controlling the formation of these nanostructures
with predetermined patterns (such as linear or cyclic) or dimensions
(such as the length of one-dimensional (1D) chains). Here, we demonstrate
that a high degree of structural control can be achieved by employing
a Cu(110)-(2Ă1)O nanotemplate for the confined synthesis of organometallic
chains and macrocycles. This template contains ordered arrays of alternating
stripes of CuâO chains and bare Cu, the widths of which are
controllable. Using scanning tunneling microscopy and low-energy electron
diffraction, we show that well-defined, ordered 1D zigzag organometallic
oligomeric chains with uniform lengths can be fabricated on the Cu
stripes (width >5.6 nm) of the Cu(110)-(2Ă1)O surface. In
addition,
the lengths of the <i>meta</i>-terphenyl (MTP)-based chains
can be adjusted by controlling the widths of the Cu stripes within
a certain range. When reducing the widths of Cu stripes to a range
of 2.6 to 5.6 nm, organometallic macrocycles including tetramer (MTP-Cu)<sub>4</sub>, hexamer (MTP-Cu)<sub>6</sub>, and octamer (MTP-Cu)<sub>8</sub> species are formed due to the spatial confinement effect and attraction
to the CuâO chains. An overview of all formed organometallic
macrocycles on the Cu stripes with different widths reveals that the
origin of the formation of these macrocycles is the <i>cis</i>-configured organometallic dimer (MTP)<sub>2</sub>Cu<sub>3</sub>,
which was observed on the extremely narrow Cu stripe with a width
of 1.5 nm
Interaction of Au with Thin ZrO<sub>2</sub> Films: Influence of ZrO<sub>2</sub> Morphology on the Adsorption and Thermal Stability of Au Nanoparticles
The model catalysts of ZrO<sub>2</sub>-supported Au nanoparticles
have been prepared by deposition of Au atoms onto the surfaces of
thin ZrO<sub>2</sub> films with different morphologies. The adsorption
and thermal stability of Au nanoparticles on thin ZrO<sub>2</sub> films
have been investigated using synchrotron radiation photoemission spectroscopy
(SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO<sub>2</sub> films were prepared by two different methods, giving rise
to different morphologies. The first method utilized wet chemical
impregnation to synthesize the thin ZrO<sub>2</sub> film through the
procedure of first spin-coating a zirconium ethoxide (ZrÂ(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub>) precursor onto a SiO<sub>2</sub>/SiÂ(100)
substrate at room temperature followed by calcination at 773 K for
12 h. Scanning electron microscopy (SEM) investigations indicate that
highly porous âsponge-like nanostructuresâ were obtained
in this case. The second method was epitaxial growth of a ZrO<sub>2</sub>(111) film through vacuum evaporation of Zr metal onto Pt(111)
in 1 Ă 10<sup>â6</sup> Torr of oxygen at 550 K followed
by annealing at 1000 K. The structural analysis with low energy electron
diffraction (LEED) of this film exhibits good long-range ordering.
It has been found that Au forms smaller particles on the porous ZrO<sub>2</sub> film as compared to those on the ordered ZrO<sub>2</sub>(111)
film at a given coverage. Thermal annealing experiments demonstrate
that Au particles are more thermally stable on the porous ZrO<sub>2</sub> surface than on the ZrO<sub>2</sub>(111) surface, although
on both surfaces, Au particles experience significant sintering at
elevated temperatures. In addition, by annealing the surfaces to 1100
K, Au particles desorb completely from ZrO<sub>2</sub>(111) but not
from porous ZrO<sub>2</sub>. The enhanced thermal stability for Au
on porous ZrO<sub>2</sub> can be attributed to the stronger interaction
of the adsorbed Au with the defects and the hindered migration or
coalescence resulting from the porous structures
Surface-Assisted Formation, Assembly, and Dynamics of Planar Organometallic Macrocycles and Zigzag Shaped Polymer Chains with CâCuâC Bonds
The formation, structure, and dynamics of planar organometallic macrocycles (<i>meta</i>-terphenyl-Cu)<sub><i>n</i></sub> and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 4,4âł-dibromo-<i>meta</i>-terphenyl (DMTP) onto Cu(111) at 300 K leads to CâBr bond scission and formation of CâCuâC bonds, which connect neighboring <i>meta</i>-terphenyl fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (>200 Ă 200 nm<sup>2</sup>). STM sequences show the formation of ordered structures through reversible scission and reformation of the CâCuâC bonds. The cyclic organometallic species such as the hexamer (<i>meta</i>-terphenyl-Cu)<sub>6</sub> may represent intermediates in the surface-confined Ullmann synthesis of hydrocarbon macrocycles such as the recently discovered hyperbenzene
Unravelling the Mechanism of Glaser Coupling Reaction on Ag(111) and Cu(111) Surfaces: a Case for Halogen Substituted Terminal Alkyne
The mechanisms of
Glaser coupling reaction on metal surfaces have
been poorly understood. Herein, we propose a reaction pathway toward
surface-confined Glaser coupling which is initiated by single-molecule
dehydrogenation of terminal alkyne. This is inspired by our experimental
observations of alkynylâAgâalkynyl and alkynylâCuâalkynyl
type organometallic intermediates in the coupling reaction of 1,1â˛-biphenyl,4-bromo-4â˛-ethynyl
(BPBE) on Ag(111) and Cu(111), respectively. Theoretical calculations
reveal that the dehydrogenation process of terminal ethynyl of BPBE
is most likely catalyzed by a stray H adatom on Ag(111) but by a Cu
adatom on Cu(111), followed by the formation of the organometallic
intermediates. After the release of interstitial metal adatoms, the
final CâC coupling occurs easily on Ag(111) but shows extremely
low efficiency on Cu(111), due to the too strong interaction between
ethynylene and the Cu(111) substrate
Interplay between Interfacial Structures and Device Performance in Organic Solar Cells: A Case Study with the Low Work Function Metal, Calcium
A better understanding of how interfacial
structure affects charge carrier recombination would benefit the development
of highly efficient organic photovoltaic (OPV) devices. In this paper,
transient photovoltage (TPV) and charge extraction (CE) measurements
are used in combination with synchrotron radiation photoemission spectroscopy
(SRPES) to gain insight into the correlation between interfacial properties
and device performance. OPV devices based on PCDTBT/PC<sub>71</sub>BM with a Ca interlayer were studied as a reference system to investigate
the interfacial effects on device performance. Devices with a Ca interlayer
exhibit a lower recombination than devices with only an Al cathode
at a given charge carrier density (<i>n</i>). In addition,
the interfacial band structures indicate that the strong dipole moment
produced by the Ca interlayer can facilitate the extraction of electrons
and drive holes away from the cathode/polymer interface, resulting
in beneficial reduction in interfacial recombination losses. These
results help explain the higher efficiencies of devices made with
Ca interlayers compared to that without the Ca interlayer