22 research outputs found
Noncrystallographic Atomic Arrangement Driven Enhancement of the Catalytic Activity of Au Nanoparticles
Determining the atomic-scale structure of nanosized particles
remains
a challenge and crucial goal for today’s science and technology.
We investigate the atomic-scale structure of 3–8 nm Au particles
obtained by a fast solution reaction and find it to be of a noncrystallographic
icosahedral type, in particular, close to the particles’ surface.
This noncrystallographic structure may well explain the previously
observed but poorly understood enhancement of the particles’
catalytic properties. Our finding demonstrates that together with
size the structure type of nanosized particles can be used as a tunable
parameter for achieving improved functionality
3D Atomic Arrangement at Functional Interfaces Inside Nanoparticles by Resonant High-Energy X‑ray Diffraction
With current science and technology
moving rapidly into smaller scales, nanometer-sized materials, often
referred to as NPs, are produced in increasing numbers and explored
for numerous useful applications. Evidence is mounting, however, that
useful properties of NPs can be improved further and even new NP functionality
achieved by not only controlling the NP size and shape but also interfacing
chemically or structurally distinct entities into single, so-called
“composite” NPs. A typical example is core–shell
NPs wherein the synergy of distinct atoms at the core\shell interface
endows the NPs with otherwise unachievable functionality. However,
though advantageous, the concept of functional interfaces inside NPs
is still pursued largely by trial-and-error. That is because it is
difficut to assess the interfaces precisely at the atomic level using
traditional experimental techniques and, hence, difficult to take
control of. Using the core\shell interface in less than 10 nm in size
Ru core–Pt shells NPs as an example, we demonstrate that precise
knowledge of the 3D atomic arrangement at functional interfaces inside
NPs can be obtained by resonant high-energy X-ray diffraction (XRD)
coupled to element-specific atomic pair distribution function (PDF)
analysis. On the basis of the unique structure knowledge obtained,
we scrutinize the still-debatable influence of core\shell interface
on the catalytic functionality of Ru core–Pt shell NPs, thus
evidencing the usefulness of this nontraditional technique for practical
applications
<i>In Situ</i> Study of Atomic Structure Transformations of Pt–Ni Nanoparticle Catalysts during Electrochemical Potential Cycling
When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core–shell structures. Structural evolution of PtNi<sub>6</sub> and PtNi<sub>3</sub> NP catalysts during their electrochemical activation and catalysis was studied by <i>in situ</i> anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. <i>Ex situ</i> high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au<sub>3</sub>Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi<sub>6</sub>. Pt-enriched core–shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi<sub>3</sub> alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core–shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles
PdCu Nanoalloy Electrocatalysts in Oxygen Reduction Reaction: Role of Composition and Phase State in Catalytic Synergy
The catalytic synergy of nanoalloy
catalysts depends on the nanoscale size, composition, phase state,
and surface properties. This report describes findings of an investigation
of their roles in the enhancement of electrocatalytic activity of
PdCu alloy nanoparticle catalysts for oxygen reduction reaction (ORR).
Pd<sub><i>n</i></sub>Cu<sub>100–<i>n</i></sub> nanoalloys with controlled composition and subtle differences
in size and phase state were synthesized by two different wet chemical
methods. Detailed electrochemical characterization was performed to
determine the surface properties and the catalytic activities. The
atomic-scale structures of these catalysts were also characterized
by high-energy synchrotron X-ray diffraction coupled with atomic pair
distribution function analysis. The electrocatalytic activity and
stability were shown to depend on the size, composition, and phase
structure. With Pd<sub><i>n</i></sub>Cu<sub>100–<i>n</i></sub> catalysts from both methods, a maximum ORR activity
was revealed at Pd/Cu ratio close to 50:50. Structurally, Pd<sub>50</sub>Cu<sub>50</sub> nanoalloys feature a mixed phase consisting of chemically
ordered (body-centered cubic type) and disordered (face-centered cubic
type) domains. The phase-segregated structure is shown to change to
a single phase upon electrochemical potential cycling in ORR condition.
While the surface Cu dissolution occurred in PdCu catalysts from the
two different synthesis methods, the PdCu with a single-phase character
is found to exhibit a tendency of a much greater dissolution than
that with the phase segregation. Analysis of the results, along theoretical
modeling based on density functional theory calculation, has provided
new insights for the correlation between the electrocatalytic activity
and the catalyst structures
Pt–Au Alloying at the Nanoscale
The formation of nanosized alloys between a pair of elements,
which
are largely immiscible in bulk, is examined in the archetypical case
of Pt and Au. Element specific resonant high-energy X-ray diffraction
experiments coupled to atomic pair distribution functions analysis
and computer simulations prove the formation of Pt–Au alloys
in particles less than 10 nm in size. In the alloys, Au–Au
and Pt–Pt bond lengths differing in 0.1 Å are present
leading to extra structural distortions as compared to pure Pt and
Au particles. The alloys are found to be stable over a wide range
of Pt–Au compositions and temperatures contrary to what current
theory predicts. The alloy-type structure of Pt–Au nanoparticles
comes along with a high catalytic activity for electrooxidation of
methanol making an excellent example of the synergistic effect of
alloying at the nanoscale on functional properties
Structure–Properties Correlation in Si Nanoparticles by Total Scattering and Computer Simulations
High-energy synchrotron X-ray diffraction
coupled to atomic pair
distribution function analysis and computer simulations is used to
determine the atomic-scale structure of silicon (Si) nanoparticles
obtained by two different synthetic routes. Results show that Si nanoparticles
may have significant structural differences depending on the synthesis
route and surface chemistry. In this case, one method produced Si
nanoparticles that are highly crystalline but surface oxidized, whereas
a different method yields organic ligand-passivated nanoparticles
without surface oxide but that are structurally distorted at the atomic
scale. Particular structural features of the oxide-free Si nanoparticles
such as average first coordination numbers, length of structural coherence,
and degree of local distortions are compared to their optical properties
such as photoluminescence emission energy, quantum yield, and Raman
spectra. A clear structure–properties correlation is observed
indicating that the former may need to be taken into account when
considering the latter
Composition–Structure–Activity Relationships for Palladium-Alloyed Nanocatalysts in Oxygen Reduction Reaction: An Ex-Situ/In-Situ High Energy X‑ray Diffraction Study
Understanding how the composition
and atomic-scale structure of
a nanocatalyst changes when it is operated under realistic oxygen
reduction reaction (ORR) conditions is essential for enabling the
design and preparation of active and robust catalysts in proton exchange
membrane fuel cells (PEMFCs). This report describes a study of palladium-alloyed
electrocatalysts (PdNi) with different bimetallic compositions, aiming
at establishing the relationship between catalyst’s composition,
atomic structure, and activity for ORR taking place at the cathode
of an operating PEMFC. Ex-situ and in-situ synchrotron high-energy
X-ray diffraction (HE-XRD) coupled to atomic pair distribution function
(PDF) analysis are employed to probe the structural evolution of the
catalysts under PEMFC operation conditions. The study reveals an intriguing
composition–activity synergy manifested by its strong dependence
on
the fuel cell operation induced leaching process of base metals from
the catalysts. In particular, the synergy sustains during electrochemical
potential cycling in the ORR operation potential window. The alloy
with Pd:Ni ratio of 50:50 atomic ratio is shown to exhibit the highest
possible surface Pd–Pd and Pd–Ni coordination numbers,
near which an activity is observed. The analysis of the Ni-leaching
process in terms of atomic-scale structure evolution sheds further
light on the activity–composition–structure correlation.
The results not only show a sustainable alloy characteristic upon
leaching of Ni consistent with catalytic synergy but also reveal a
persistent fluctuation pattern of interatomic distances along with
an atomic-level reconstruction under the ORR and fuel cell operation
conditions. The understanding of this type of interatomic distance
fluctuation in the catalysts in correlation with the base metal leaching
and realloying mechanisms under the electrocatalytic operation conditions
may have important implications in the design and preparation of catalysts
with controlled activity and stability
Surface Atomic Structure and Functionality of Metallic Nanoparticles: A Case Study of Au–Pd Nanoalloy Catalysts
The
surface atomic structure of metallic nanoparticles (NPs) plays
a key role in shaping their physicochemical properties and response
to external stimuli. Not surprisingly, current research increasingly
focuses on exploiting its prime characteristics, including the amount,
location, coordination, and electronic configuration of distinct surface
atomic species, as tunable parameters for improving the functionality
of metallic NPs in practical applications. The effort requires clear
understanding of the extent to which changes in each of these characteristics
would contribute to achieving the targeted functionality. This, in
the first place, requires good knowledge of the actual surface of
metallic NPs at atomic level. Through a case study on Au–Pd
nanoalloy catalysts of industrial and environmental importance, we
demonstrate that the surface atomic structure of metallic NPs can
be determined in good detail by resonant high-energy X-ray diffraction
(HE-XRD). Furthermore, using our experimental surface structure and
CO oxidation activity data, we shed new light on the elusive origin
of the remarkable catalytic synergy between surface Au and Pd atoms
in the nanoalloys. In particular, we show that it arises from the
formation of a specific “skin” on top of the nanoalloys
that involves as many unlike, i.e., Au–Pd and Pd–Au,
atomic pairs as possible given the overall chemical composition of
the NPs. Moreover, unlike atoms from the “skin” interact
strongly, including both changing their size and electronic structure
in inverse proportions. That is, Au atoms shrink and acquire a partial
positive charge of 5d-character whereas Pd atoms expand and become
somewhat 4d-electron deficient. Accordingly, the reactivity of Au
increases whereas Pd atoms become less reactive, as compared to atoms
at the surface of pure Au and Pd NPs, respectively. Ultimately, this
renders Au–Pd alloy NPs superb catalysts for CO oxidation reaction
over a broad range of alloy compositions. Our findings are corroborated
by DFT calculations based on a refined version of d-band center theory
on the catalytic properties of late transition metals and alloys.
We discuss opportunities for improving the accuracy of current theory
on surface-controlled properties of metallic NPs through augmenting
the theory with surface structure data obtained by resonant XRD
Resolving Atomic Ordering Differences in Group 11 Nanosized Metals and Binary Alloy Catalysts by Resonant High-Energy X‑ray Diffraction and Computer Simulations
Resonant high-energy X-ray diffraction
coupled to atomic pair distribution
function analysis and computer simulations is used to study the atomic-scale
structure of group 11 nanosized metals and binary alloy catalysts.
We find that nanosized Cu is quite disordered structurally whereas
nanosized Ag and especially Au exhibit a very good degree of crystallinity.
We resolve Cu–Cu and Ag–Ag atomic correlations from
Au-involving ones in Au–Cu and Au–Ag nanoalloys and
show that depending on the synthetic route group 11 binary alloys
may adopt structural states that obey or markedly violate Vegard’s
law. In the latter case, Cu and Ag atoms undergo substantial size
expansion and contraction by as much as 0.3 and 0.03 Ă…, respectively,
while heavier Au atoms remain practically intact. The size change
of Cu and Ag atoms does not follow Pauling’s rule of electronegativity
predicting charge flow toward the more electronegative Au but occurs
in a way such that Cu/Au and Ag/Au atomic size ratios in the nanoalloys
become closer to one. Atomic size adjusting and the concurrent charge
redistribution result in a synergistic effect of oxygen inactive Au
and oxygen very active Cu and Ag leading to nanoalloys with very good
activity for low-temperature oxidation of CO
Resolving Atomic Ordering Differences in Group 11 Nanosized Metals and Binary Alloy Catalysts by Resonant High-Energy X‑ray Diffraction and Computer Simulations
Resonant high-energy X-ray diffraction
coupled to atomic pair distribution
function analysis and computer simulations is used to study the atomic-scale
structure of group 11 nanosized metals and binary alloy catalysts.
We find that nanosized Cu is quite disordered structurally whereas
nanosized Ag and especially Au exhibit a very good degree of crystallinity.
We resolve Cu–Cu and Ag–Ag atomic correlations from
Au-involving ones in Au–Cu and Au–Ag nanoalloys and
show that depending on the synthetic route group 11 binary alloys
may adopt structural states that obey or markedly violate Vegard’s
law. In the latter case, Cu and Ag atoms undergo substantial size
expansion and contraction by as much as 0.3 and 0.03 Ă…, respectively,
while heavier Au atoms remain practically intact. The size change
of Cu and Ag atoms does not follow Pauling’s rule of electronegativity
predicting charge flow toward the more electronegative Au but occurs
in a way such that Cu/Au and Ag/Au atomic size ratios in the nanoalloys
become closer to one. Atomic size adjusting and the concurrent charge
redistribution result in a synergistic effect of oxygen inactive Au
and oxygen very active Cu and Ag leading to nanoalloys with very good
activity for low-temperature oxidation of CO