14 research outputs found
Atomistic Simulations of Flash Memory Materials Based on Chalcogenide Glasses
In this chapter, by using ab-initio molecular dynamics, we introduce the
latest simulation results on two materials for flash memory devices: Ge2Sb2Te5
and Ge-Se-Cu-Ag. This chapter is a review of our previous work including some
of our published figures and text in Cai et al. (2010) and Prasai & Drabold
(2011) and also includes several new results.Comment: 24 pages, 20 figures. This is a chapter submitted for the book under
the working title "Flash Memory" (to be published by Intech ISBN
978-953-307-272-2
3D Atomic Arrangement at Functional Interfaces Inside Nanoparticles by Resonant High-Energy Xâray 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
âcompositeâ NPs. A typical example is coreâshell
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 coreâPt 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 coreâPt shell NPs, thus
evidencing the usefulness of this nontraditional technique for practical
applications
Atomic-Structural Synergy for Catalytic CO Oxidation over Palladium-Nickel Nanoalloys
DOE-BES [DE-SC0006877]; DOE [AC02-06CH11357]; DOE's Office of Biological and Environmental ResearchAlloying palladium (Pd) with other transition metals at the nanoscale has become an important pathway for preparation of low-cost, highly active and stable catalysts. However, the lack of understanding of how the alloying phase state, chemical composition and atomic-scale structure of the alloys at the nanoscale influence their catalytic activity impedes the rational design of Pd-nanoalloy catalysts. This work addresses this challenge by a novel approach to investigating the catalytic oxidation of carbon monoxide (CO) over palladium nickel (PdNi) nanoalloys with well-defined bimetallic composition, which reveals a remarkable maximal catalytic activity at Pd:Ni ratio of similar to 50:50. Key to understanding the structural-catalytic synergy is the use of high-energy synchrotron X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis to probe the atomic structure of PdNi nanoalloys under controlled thermochemical treatments and CO reaction conditions. Three-dimensional (3D) models of the atomic structure of the nanoalloy particles were generated by reverse Monte Carlo simulations (RMC) guided by the experimental HE-XRD/PDF data. Structural details of the PdNi nanoalloys were extracted from the respective 3D models and compared with the measured catalytic properties. The comparison revealed a strong correlation between the phase state, chemical composition and atomic-scale structure of PdNi nanoalloys and their catalytic activity for CO oxidation. This correlation is further substantiated by analyzing the first atomic neighbor distances and coordination numbers inside the nanoalloy particles and at their surfaces. These findings have provided new insights into the structural synergy of nanoalloy catalysts by controlling the phase state, composition and atomic structure, complementing findings of traditional density functional theory studies
CompositionâStructureâActivity Relationships for Palladium-Alloyed Nanocatalysts in Oxygen Reduction Reaction: An Ex-Situ/In-Situ High Energy Xâray 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 catalystâs 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
compositionâactivity 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 PdâPd and PdâNi 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 activityâcompositionâstructure 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 AuâPd 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 AuâPd
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 âskinâ on top of the nanoalloys
that involves as many unlike, i.e., AuâPd and PdâAu,
atomic pairs as possible given the overall chemical composition of
the NPs. Moreover, unlike atoms from the âskinâ 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 AuâPd 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
CompositionâStructureâActivity Correlation of PlatinumâRuthenium Nanoalloy Catalysts for Ethanol Oxidation Reaction
Understanding
the evolution of the composition and atomic structure of nanoalloy
catalysts in the ethanol oxidation reaction (EOR) is essential for
the design of active and robust catalysts for direct ethanol fuel
cells. This article describes a study of carbon-supported platinumâruthenium
electrocatalysts (PtRu/C) with different bimetallic compositions and
their activities in the EOR, an important anode reaction in direct
ethanol fuel cells (DEFCs). The study focused on establishing the
relationship between the catalystâs composition, atomic structure,
and catalytic activity for the EOR. Ex situ and in situ synchrotron
high-energy X-ray diffraction (HE-XRD) experiments coupled with atomic
pair distribution function (PDF) analysis and in situ energy-dispersive
X-ray (EDX) analysis were employed to probe the composition and structural
evolution of the catalysts during the in situ EOR inside a membrane
electrode assembly (MEA) in the fuel cell. The results revealed an
intriguing compositionâstructureâactivity relationship
for the PtRu electrocatalysts under EOR experimental conditions. In
particular, the alloy with a Pt/Ru ratio of âŒ50:50 was found
to exhibit a maximum EOR activity as a function of the bimetallic
composition. This compositionâactivity relationship coincides
with the relationship between the Pt interatomic distances and coordination
numbers and the bimetallic composition. Notably, the catalytic activities
of the PtRu electrocatalysts showed a significant improvement during
the EOR, which can be related to atomic-level structural changes in
the nanoalloys occurring during the EOR, as indicated by in situ HE-XRD/PDF/EDX
data. The findings shed some new light on the mechanism of the ethanol
oxidation reaction over bimetallic alloy nanocatalysts, which is important
for the rational design and synthesis of active nanoalloy catalysts
for DEFCs