chemnanomat - scartsandsciences.sc.edu/chemgroup/wang/sites/sc.edu... · electrons transferred upon...
TRANSCRIPT
www.chemnanomat.org
REPRINTA Journal of
CHEMNANOMATCHEMISTRY OF NANOMATERIALS FOR ENERGY, BIOLOGY AND MORE
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
Heterogeneous Catalysis
Dealloyed Nanoporous Gold Catalysts: From Macroscopic Foams toNanoparticulate ArchitecturesGuangfang Grace Li[a] and Hui Wang*[a]
897ChemNanoMat 2018, 4, 897–908 T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus ReviewDOI: 10.1002/cnma.201800161
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
Abstract: Dealloyed nanoporous Au membranes and spongy
Au nanoparticles exhibit a set of unique structural features
highly desirable for heterogeneous catalysis and electro-
catalysis. In this Focus Review, we present the state-of-the-art
understanding of the complex mechanisms dictating the
nanoscale porosity evolution during percolation dealloying of
alloys and the structure-composition-performance correla-
tions underpinning the catalytic behaviors of dealloyed
nanoporous Au. We focus on several fundamentally intriguing
but widely debated topics concerning the nature of the
active sites, the dynamic surface reconstruction under
reaction conditions, and the origin of catalytic selectivity
toward certain reactions. We also provide perspectives on
versatile dealloying-based synthetic approaches for precise
architectural tailoring of metallic nanocatalysts as well as
exciting opportunities of harnessing the combined optical
and catalytic properties of dealloyed nanoporous Au to drive
or enhance unconventional interfacial chemical transforma-
tions.
1. Catalysis on Nanoporous Au: A ParadigmShift in Heterogeneous Catalysis
The ever-increasing interest in heterogeneous catalysis by
metallic Au dates back to 1980s, when Haruta and coworkers[1]
discovered that oxide-supported sub-5 nm Au nanoparticles
exhibited surprisingly high catalytic activity toward aerobic CO
oxidation under mild reaction conditions even below 0 8C,whereas Au nanoparticles larger than 5 nm showed diminished
catalytic activity. At first glance, Haruta’s observations appear
counter-intuitive because Au has long been considered an inert
noble metal and is the only one exhibiting an endothermic O2
chemisorption energy among all the late transition metals.[2]
While the detailed mechanisms underpinning the size-depend-
ent activity of Au nanocatalysts have long been a fundamen-
tally intriguing subject under intense debate, consensus has
been reached that the undercoordinated surface atoms, which
become highly abundant when the particle sizes shrink to the
sub-5 nm regime, are indispensable for catalyzing the chemical
transformations.[3] However, the origin of the catalytic activities
of Au nanocatalysts cannot be interpreted solely in the context
of the surface atomic coordinations. To prevent the particle
sintering under reaction conditions, the Au nanoparticles are
typically dispersed on high surface area oxide supports, which
also provide crucial contributions to the overall catalytic
activities due to intricate particle-support and molecule-support
interactions.[4–8] In addition, for sub-5 nm nanoparticles, quan-
tum confinement becomes a predominant effect that modifies
the electronic structures of the materials, which may arguably
influence the catalytic activity of Au as well.[9–10] Therefore,
multiple intertwining effects come into play and synergistically
dictate the size-dependent catalytic behaviors of the interface-
rich, substrate-supported Au nanocatalysts.
The emergence of dealloyed nanoporous Au catalysts, in
the form of either foamy membranes or spongy nanoparticles,
represents a paradigm shift in Au-based heterogeneous
catalysis.[11–12] Bicontinuous nanoporous Au catalysts composed
of interconnected nanoligaments are typically derived from
alloys through nanoporosity-evolving percolation dealloying.[13]
Although the feature sizes of both the ligaments and pores are
typically beyond 5 nm, the dealloyed nanoporous Au exhibits
remarkable catalytic activities comparable to or even surpassing
those of the oxide-supported sub-5 nm Au nanopar-
ticles.[11–12,14–17] Dealloyed nanoporous Au catalysts provide a
unique free-standing materials system that enables detailed
correlation of the intrinsic catalytic activity of Au to the atomic-
level surface structures without complications introduced by
the support materials or quantum confinement effects.
In this Focus Review, we first discuss the complex mecha-
nisms dictating the nanoporosity-evolving percolation deal-
loying of alloys, which serve as the key knowledge foundation
for deliberate structural control of nanoporous Au catalysts.
Then we focus on several fundamentally intriguing but still
controversial issues regarding the structure-property relation-
ships of dealloyed nanoporous Au catalysts, such as the nature
of the catalytically active sites, the dynamic structural remodel-
ing of catalyst surfaces, and the origin of catalytic selectivity.
Through a detailed case study using electrocatalytic alcohol
oxidation as model reactions, we further demonstrate how the
structures and compositions of dealloyed spongy nanoparticles
can be systematically fine-tailored by controlling the dealloying
of Au@Cu alloy nanoparticles, based on which optimal catalytic
performance can be achieved. Finally, we briefly summarize the
state-of-the-art knowledge regarding the catalytic behaviors of
dealloyed nanoporous Au and provide perspectives on new
synthetic approaches that will further enhance our capabilities
to fine-optimize the catalyst structures as well as exciting
opportunities of exploring unconventional interfacial molecular
transformations on the dealloyed nanoporous Au catalysts.
2. Nanoporosity-Evolving PercolationDealloying
Percolation dealloying of alloys involves selective etching of the
less-noble constituents, entangled with the structural rear-
rangement of the nonleachable more-noble components.[18] A
prototypical binary alloy system of particular interest has been
macroscopic Au@Ag alloy membranes, which transform through
[a] G. G. Li, Prof. H. WangDepartment of Chemistry and BiochemistryUniversity of South Carolina631 Sumter Street, Columbia, South Carolina 29208, United StatesE-mail: [email protected]
898ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
percolation dealloying into a unique 3D solid/void bicontinuous
structure consisting of Au-rich nanoligaments that are hierarchi-
cally interconnected (Figure 1A and 1B).[13] The nanoporosity
evolution during percolation dealloying involves a series of
intriguing structure-rearranging processes that dynamically
interplay over multiple length- and time-scales both in the bulk
of alloy matrices and at the evolving solid/electrolyte inter-
faces.[18] According to a surface-diffusion continuum model
(Figure 1C) proposed by Erlebacher and coworkers,[13,18] the
percolation dealloying of an Au@Ag alloy is initiated upon
dissolution of an Ag surface atom at the solid/electrolyte
interface. As the interfacial dissolution of Ag atoms further
proceeds, the undercoordinated Au atoms left behind rapidly
agglomerate into Au-rich patchy islands through surface
migration. Therefore, the alloy surface at the dealloying frontier
is essentially composed of Au-rich surface-passivating domains
and patches of undealloyed material still exposed to the
etchant. Further dealloying results in a branched pore channel
network, which eventually evolves into the bicontinuous
spongy structures through a ligament and pore coarsening
process.
The dealloying behaviors of bimetallic alloys are composi-
tion-dependent. For example, a macroscopic Au@Ag alloy
membrane may selectively undergo either surface atomic
dealloying or nanoporosity-evolving percolation dealloying,
depending on the atomic fraction of Ag in the alloys and the
detailed experimental conditions under which the dealloying
occurs.[13] The onset potential of percolation dealloying is
termed as critical potential, Ec.[19] At potentials below Ec, only
surface dealloying occurs at the top-most atomic layer of the
alloy materials. Therefore, the current remains close to zero as
the potential increases until reaching Ec, above which the
current increases drastically (Figure 1D) due to percolation
dissolution of Ag from the alloy matrices. Ec is essentially a
function of Au/Ag stoichiometric ratio (Figure 1D and 1E). A
bulk binary alloy with a compositional formula of A1-pBp (A and
B represent the nonleachable and leachable constituents,
respectively, and p is atomic fraction of B) has a characteristic Ecexpressed as[20]
EcðpÞ ¼ EeqðpÞ þ 4gB=elecWA
nFx, ð1Þ
where gB/elec is the interfacial free energy of B exposed to the
electrolyte, WA is the molar volume of A, n is the number of
electrons transferred upon oxidation of one B atom, F is the
Faraday constant, and x is the local radius of the surface where
a cylindrical pit is created upon dealloying. E-c is the critical
potential of the bulk alloy. E-eq refers to the onset potential for
surface dealloying, also known as the equilibrium potential.
Under certain dealloying conditions, almost all bimetallic alloys
feature a characteristic threshold p known as the parting limit,
above which percolation dealloying occurs. The parting limits
of Au@Ag and Au@Cu alloys were determined to be ~55–60 atomic% of Ag[13,19,21–22] and ~70 atomic% of Cu,[23–25] respec-
tively, in acidic etching environments at room temperature.
Nanoparticles of alloys exhibit size-dependent critical
potentials that are negatively shifted relative to those of their
bulk analogs with the same compositional stoichiome-
tries.[19,22,26] The critical potential, Ec(p,r), of a spherical A1-pBp alloy
nanoparticle with a radius of r is given by[26]
Ecðp; rÞ ¼ EcðpÞ@ gAlloyðWAÞ þ fAlloyðW_
A @ Wh iÞh i
> 2
nFr
. -, ð2Þ
where gAlloy and fAlloy represent the free energy and the stress at
the alloy/electrolyte interface, respectively. is the partial molar
volume of A in the alloy, and Wh i is the average molar volume
of the alloy. Ec and E-c become virtually equivalent when r is
larger than ~5 nm, because the maximum values of gAlloy and
fAlloy are ~2 and ~6 Jm@2,[26] respectively. Equation (2) essentially
reflects the surface curvature-dependent Gibbs-Thomson ef-
fects, which diminish at a length scale larger than 10 nm.
However, pronounced negative shifts of Ec have been exper-
imentally observed on Au@Ag alloy nanoparticles over a size
regime far broader than that predicted by equation (2), ranging
from tens of millivolts for ~100 nm sized particles up to
hundreds of millivolts for 10–15 nm sized particles, because of
additional complication arising from the microstructural ef-
Guangfang Grace Li received her B.S.and M.S. both in chemistry from Wu-han Institute of Technology in China in2009 and 2012, respectively. Since 2013,she has been a graduate studentmajoring in Physical Chemistry at theUniversity of South Carolina. Her Ph.D.work, supervised by Hui Wang, hasbeen focusing on the structural trans-formations and the electrocatalyticproperties of complex multimetallicnanostructures.
Hui Wang received his B.S. (Chemistry)with honors from Nanjing University inChina in 2001 and Ph.D. (PhysicalChemistry) from Rice University in 2007.His Ph.D. work, supervised by Naomi J.Halas, focused on tunable plasmonicnanostructures and plasmon-enhancedspectroscopies. He did postdoctoralresearch on single-molecule spectro-scopy under the tutelage of Paul F.Barbara at the University of Texas atAustin. He joined the faculty ofChemistry and Biochemistry at theUniversity of South Carolina as atenure-track Assistant Professor in 2010and was promoted to Associate Profes-sor with tenure in 2016. His independ-ent research has been focusing on thestructure-property relationships of com-plex nanostructures and the mecha-nisms of catalytic molecular transfor-mations at the nanoparticle-moleculeinterfaces.
899ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
fects.[22] Alloy nanoparticles may undergo dealloying-driven
structural transformations that are more versatile than those of
their bulk and thin film counterparts displaying a planar surface
to the electrolyte. For example, Au0.23Ag0.77 alloy nanoparticles
smaller than 10 nm transform into core-shell nanoparticles each
of which is composed of an alloy core and an Au shell
(Figure 2A–2C), whereas Au0.23Ag0.77 alloy nanoparticles larger
than 20 nm evolve into spongy nanoparticles (Figure 2D–2F)
under otherwise identical electrochemical dealloying condi-
tions. Alloy nanoparticles substantially larger than ~20 nm
typically undergo nanoporosity-evolving morphological
changes involving both ligament pinch-off and void bubble
formation during percolation dealloying (Figure 2G),[27–28] analo-
gous to their macroscopic bulk counterparts with the same
compositions. The percolation dealloying of alloy nanoparticles
enables controlled introduction of nanoscale porosity to a large
variety of substrate-supported or free-standing Au nanostruc-
tures (Figure 2H–2L).[19,28–32]
3. Structure-Property RelationshipsUnderpinning the Catalytic Behaviors
Dealloyed nanoporous Au possesses large surface-to-volume
ratios, highly abundant undercoordinated surface atoms, con-
ductive skeletal frameworks, and fully accessible open surface
structures, all of which are highly desired for heterogeneous
catalysis and electrocatalysis. The exact nature of the catalyti-
cally active sites, however, still remains elusive because of the
overwhelmingly complicated surface structures of dealloyed
nanoporous Au and lack of a unified mechanism broadly
applicable to a diverse set of reactions. Here we focus on
several intensively debated critical issues regarding the roles of
the undercoordinated Au surface atoms and the residual less
noble elements in dictating the activity, durability, and
selectivity of the dealloyed nanoporous Au catalysts.
3.1. Nature of Active Sites
It has become increasingly evident that the catalytic activity of
dealloyed nanoporous Au toward a variety of important
oxidation and hydrogenation reactions[11–12,14–17] originates from
the highly abundant undercoordinated atoms on the locally
curved ligament surfaces.[12,33–36] As illustrated in Figure 3A, the
surface atoms can be categorized into three types located at
terraces, step edges, and kink sites, respectively, in the order of
decreasing atomic coordination numbers (ACNs). The latter two
represent the undercoordinated surface atoms that are catalyti-
cally active.[12] Density Functional Theory (DFT) calculations
suggest that Au becomes catalytically active for activation of
molecular oxygen only when the surface atoms are under-
coordinated.[37] As revealed by high-resolution transmission
electron microscopy (HRTEM) images, the complex 3D surfaces
of dealloyed Au membranes (Figure 3B–3D)[33] and dealloyed
spongy Au nanoparticles (Figure 3E)[23] are enclosed by high
densities of various types of undercoordinated atoms at steps
and kinks, which may serve as the primary active sites for
catalysis.
Figure 1. (A) Cross-sectional and (B) plan view scanning electron microscopy (SEM) images of nanoporous Au foam made by selective dissolution of Ag fromAg@Au alloys immersed in nitric acid. Reprinted with permission from ref [13]. Copyright 2001, Springer Nature. (C) Kinetic Monte Carlo simulations of atomic-scale structural evolution of an Au@Ag alloy membrane during percolation dealloying: (a) the planar surface prior to dealloying, (b) vacancy island nucleation,(c) surface roughening, (d) formation of a branched pore channel network, and (e) pore and ligament coarsening. Ag and Au atoms are represented by greyand yellow spheres, respectively. Reprinted with permission from ref [18]. Copyright 2018, Cambridge University Press. (D) Composition-dependent current-potential behaviors of Ag@Au alloys dealloyed in 0.1 M HClO4+0.1 M Ag+. The Au atomic% of each Ag@Au alloy sample was labeled in the figure. (E)Comparison of experimental (line) and simulated (triangles) critical potentials. The zero of overpotential has been set equal to the onset of dissolution of puresilver both in simulation and in experiment. Reprinted with permission from ref [13]. Copyright 2001, Springer Nature.
900ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
Recent experimental observations, however, strongly sug-
gest that the undercoordinated Au surface atoms alone are
unlikely to be capable of activating molecular O2 and thus
efficiently catalyzing the CO oxidation.[11–12,38–41] For example,
high-index faceting {211} Au surface constitutes a prototypical
step-terrace structure with high density of steps. Although Au
{221} facet shows strong affinity for CO chemisorption, it
exhibits plain reactivity toward CO oxidation.[42] Friend and
coworkers also observed that dealloyed nanoporous Au
catalysts highly active for selective methanol oxidation were
not necessarily active for CO oxidation,[43–45] in spite of the fact
that both reactions required O2 activation. Another surprising
observation made by Tao and coworkers shows that pretreating
nanoporous Au with ozone, which is usually viewed as an
efficient way to eliminate undercoordinated surface sites,
drastically enhances the activity for cyclohexene oxidation.[46]
All these observations coherently point to a more complicated
underlying structure-property relationship that cannot be fully
elucidated solely based on the surface atomic coordination.
Although compositionally Au-rich, the dealloyed nano-
porous catalysts inevitably contain residual less-noble elements,
most commonly Ag and Cu, that cannot be completely
removed during the nanoporosity-evolving percolation deal-
loying.[11–12,33,38,40,47–50] The retention and distribution of the less
noble elements in the dealloyed nanoporous structures are
influenced by the rates of atomic dissolution and the potential
under which the dealloying occurs.[49–50] The residual Ag may
either remain fully alloyed with Au[33,47] or segregate at the
ligament surfaces to form localized patchy islands (Figure 3F–
3I).[48] The presence of residual Ag remarkably enhances the
catalytic activities of the dealloyed nanoporous Au toward
certain oxidation reactions,[40–41] though the quantitative corre-
lation between the amount of residual Ag and the overall
catalytic activity still remains open to further scrutiny. For
catalytic CO oxidation, it is highly likely that adsorbed oxygen is
Figure 2. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ag0.77Au0.23 nanoparticles(nominally 4 nm in diameter) (A) prior to dealloying and (B) after electrochemical dealloying at 1.4 V (vs. NHE). (C) Elemental map of a 4.8 nm diameter Ag0.77
Au0.23 nanoparticle dealloyed at 1.4 V (vs. NHE). Blue: dark field image intensity; Magenta: Ag signals from electron energy loss spectroscopy (EELS).Representative HAADF-STEM images of ~40 nm diameter Ag0.77Au0.23 nanoparticles (D) before dealloying and after dealloying for 6 h at (E) 0.54 V and (F)0.74 V. Reprinted with permission from ref [19]. Copyright 2014, American Chemical Society. (G) Kinetic Monte Carlo simulations modeling the nanoporosityevolution during electrochemical dealloying of a nanoparticle. At the early stages, the surface becomes roughened and porosity begins to evolve on thesurface. As dealloying time increases (to the right and down), porosity fully penetrates into the particle and the average feature size increases as aconsequence of ligament and pore coarsening. Reprinted with permission from ref [28]. Copyright 2018, Cambridge University Press. (H) Planar view and (I)cross-sectional SEM images of a substrate-supported dealloyed nanoporous Au island. Reprinted with permission from ref [29]. Copyright 2016, AmericanChemical Society. (J) Transmission electron microscopy (TEM) image of spongy Au nanoparticles encapsulated in a SiO2 shell synthesized through dealloying ofSiO2-coated Au@Ag alloy nanoparticles. Reprinted with permission from ref [30]. Copyright 2016, American Chemical Society. (K) TEM image of spongy Aunanoparticles synthesized through dealloying of Au@Cu alloy nanoparticles. Reprinted with permission from ref [31]. Copyright 2016, Wiley-VCH. (L) TEMimage of Au nanotubes with nanoporous walls synthesized through dealloying of Ag@Ag@Au alloy core-shell nanowires. Reprinted with permission from ref[32]. Copyright 2009, Springer Nature.
901ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
associated with Ag or Ag@Au sites rather than undercoordi-
nated Au surface atoms.[12] By introducing Ag atoms to the Au
surfaces, the binding affinity of molecular oxygen to the
catalyst surfaces increases while the energy barrier of O2
dissociation decreases, both of which facilitate O2 activation.
Theoretical analysis reveals that the substitution of surface Au
atoms with Ag on the {321} model surface significantly reduces
the O2 dissociation barrier,[40,51] which is in line with experimen-
tal observations. Residual Cu in dealloyed nanoporous Au also
exhibits similar effects as residual Ag on catalytic enhancement
for aerobic oxidation reactions.[52] The detailed surface atomic
configurations involving the residual Ag or Cu specifically
required for catalytic enhancements, however, still remains
unclear at this point. Interestingly, the roles of residual less
noble elements vary drastically from reaction to reaction. For
chemoselective hydrogenation reactions under low pressures,
the catalytic activity decreases with the increase in the fraction
of residual Ag, which exhibits an opposite trend to the aerobic
oxidation reactions.[53] For electrocatalytic oxidation reactions
that does not involve O2, no clear correlation between the
residual less noble elements and catalytic activity has been
observed so far.[23,54] The exact nature of the active sites on the
dealloyed nanoporous Au for various reactions is still a
fundamentally intriguing topic under intense debate and well-
worthy of further investigations.
3.2. Dynamic Surface Structural Remodeling Under ReactionConditions
The nanoligament surfaces may undergo dynamic structural
rearrangements to evolve into thermodynamically more stable
Figure 3. (A) Schematic illustration of surface atoms at terrace, step edge, and kink sites with various characteristic atomic coordination numbers (ACNs). (B)Bright-field TEM image and (C) 3D tomographic reconstruction by electron tomography of dealloyed nanoporous Au foams. (D) TEM image of a nanopore.The electron diffraction pattern (inset) shows that the incident direction is [01(1]. The labelled squares, b, c, d, and e, indicate the areas imaged by high-resolution HAADF-STEM in panels D-b, D-c, D-d, and D-e, respectively. Reprinted with permission from ref [33]. Copyright 2012, Springer Nature. (E) HAADF-STEM image of an individual nanoporous Au0.97Cu0.03 particle dealloyed from a Au0.19Cu0.81 alloy nanoparticle. High-resolution HAADF-STEM images showingthe atomic-level structures of regions a and b are shown in panels E-a and E-b, respectively. The insets in panels E-a and E-b are the FFT patterns of theregions labeled as i, ii, and iii, respectively. In the high-resolution HAADF-STEM images, the crystalline domains were projected along the [01(1] zone axis.Reprinted with permission from ref [23]. Copyright 2016, American Chemical Society. (F) HAADF-STEM image and (G) elemental distribution of Au (red signal)and Ag (green signal) of dealloyed nanoporous Au with 11 at% residual Ag. (H) HAADF-STEM image and (I) elemental distribution of Au (red signal) and Ag(green signal) of dealloyed nanoporous Au with 8 at% residual Ag. Reprinted with permission from ref [48]. Copyright 2017, Elsevier.
902ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
but catalytically less active surface structures during the
catalytic reactions, resulting in deterioration of catalytic activity.
Direct correlations between the activity deterioration and
ligament coarsening were observed during electrocatalytic
methanol oxidation (Figure 4A and 4B)[54] and catalytic aerobic
CO oxidation (Figure 4C).[47] The coarsening of nanoligaments
during CO oxidation was mainly dictated by rapid layer-by-layer
diffusion of Au atoms at undercoordinated surface sites (Fig-
ure 4D and 4E) driven by the interactions between the reactant
molecules and the active surface atoms.[47] Because CO
oxidation is an exothermic reaction, locally generated heat on
the catalyst surface may serve as the primary driving force for
the surface reconstruction and ligament coarsening. Interest-
ingly, twin boundaries could function as pinning sites to
surface-diffusing atoms at the propagating front and the
surface atomic diffusion could be effectively hindered by
judiciously introducing planar defects onto the nanoligament
surfaces. The dealloyed nanoporous Au also undergoes sub-
stantial surface reconstruction to form thermodynamically more
stable low-index facets on the ligament surfaces upon exposure
to electrochemical potential cycling,[55] resulting in catalytic
activity decay during electro-oxidation of methanol.
The surface reconstruction and ligament coarsening under
reaction conditions involve not only the surface migration of
Au atoms but also the diffusion of residual Ag atoms both on
the surface and in the bulk of the nanoligaments. Ag
segregation at the ligament surfaces was clearly observed
during catalytic CO oxidation.[47–48] It was recently revealed by
ab initio molecular dynamics (AIMD) calculations and exper-
imentally verified by Auger electron spectroscopy that surface-
adsorbed oxygen drove the migration of subsurface Ag atoms
to the O-rich sites on the stepped model surfaces that
mimicked the active sites on the nanoligament surfaces.[56]
Besides the vanishment of undercoordinated surface atoms, the
formation of phase segregated monometallic Ag islands on the
ligament surface may be another key factor causing the
catalytic activity deterioration, though it still remains unclear
how the dimensions and atomic surface configurations of the
segregated Ag domains affect the overall catalytic activity.
The highly reactive and dynamic nature of the active sites
imply that the dealloyed nanoporous Au catalysts may
constantly undergo surface reconstructions even under steady
state reaction conditions. Achieving long-term catalytic durabil-
ity does not always necessarily require the presence of
structurally stable active sites on the nanoligament surfaces.
Alternatively, the catalytic activity can also be preserved if the
active surface sites are continuously replenished through
dynamic structural rearrangements as a result of the constant
formation and depletion of molecular adsorbates in the
catalytic processes. The dynamic surface reconstruction ob-
served by in situ HRTEM, however, may not precisely reflect the
structural dynamics of the complex 3D surfaces under real
reaction conditions,[57] because in situ HRTEM visualizes essen-
tially a 2D projection of the nanoporous structure. The lack of
3D structural confinement and larger exposure surface to the
reactants make a slice of 2D porous structure on the TEM grid
Figure 4. SEM images of a nanoporous Au electrode measured (A) before and (B) after electrocatalytic oxidation of methanol. Insets in two images show thecolor change of the nanoporous Au electrode before and after the electrocatalytic reaction. Reprinted with permission from ref [54]. Copyright 2007, AmericanChemical Society. (C) Catalytic performance of dealloyed nanoporous Au at 30 8C for aerobic CO oxidation. Insets are SEM images of the nanoporous Ausamples after catalytic reactions at initial conversion rate, and 90, 80, and 70% of the initial conversion rate, as indicated by arrows. (D, E) Twin boundary as apinning site at the propagating front of coarsening. Dotted lines in panels D-i and E-i represent twin boundaries, and circles represent a kink or step. Asshown in panels D-ii, D-iii, E-ii, and E-iii, the {111} plane diffuses away after pointed kinks disappear. Reprinted with permission from ref [47]. Copyright 2014,American Chemical Society.
903ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
more vulnerable to the reaction environments than the 3D
bicontinuous structure. In addition, the specimen inside the
in situ HRTEM chamber is located where the gas phase
reactants directly hit the catalyst surfaces, which may boost
both the catalytic reactions and structural rearrangements.
Furthermore, the influence of electron beam irradiation has not
been completely ruled out, though the catalytic reactions were
observed to be the primary driving force for the surface
reconstruction and ligament coarsening.[33,57]
3.3. Origin of Catalytic Selectivity
Metallic Au-based nanocatalysts have unique capabilities to
catalyze chemoselective oxidation reactions at low temper-
atures and pressures, which distinguishes them from their
counterparts made of other transition metals, such as Pd and
Pt.[11–12] Friend, Baumer, and co-workers discovered that meth-
anol could be selectively oxidized into methyl formate (partial
oxidation product) or CO2 (complete combustion product) on
dealloyed nanoporous Au catalysts, depending on the amount
of residual Ag, surface abundance of O2, and reaction temper-
ature.[14] To achieve high selectivity toward partial oxidation, the
partially oxidized products must adsorb to the Au catalyst
surfaces with sufficiently low affinity, such that they can rapidly
desorb from the catalyst surfaces before being further oxidized.
The selectivity of a nanoporous Au catalyst toward catalytic
oxidation reactions essentially originates from the delicate
balance between its capability to activate surface-adsorbed
oxygen and its weak, dynamic interactions with the partial
oxidation products.
As illustrated in Figure 5A, oxidation of methanol by O2 on
Au catalyst surfaces may result in a variety of partially oxidized
products in addition to the thermodynamically favored com-
bustion product, CO2. On fully dealloyed nanoporous Au
catalysts with residual Ag less than 1 atomic%, methyl formate
is exclusively produced at a reactant stoichiometry of 1 volume
% O2+2 volume % CH3OH. The selectivity toward methyl
formate approaches 100% at room temperature, and only
slightly decreases to 97% even at 80 8C (Figure 5B and 5C). An
oxygen-rich reaction condition favors the formation of CO2,
resulting in loss of selectivity toward partial oxidation as the
temperature increases (Figure 5C). Increasing the residual Ag
content also leads to loss of selectivity at reaction temperature
higher than 80 8C, even in an oxygen deficient reactant
atmosphere (2 volume % methanol+1 volume% O2) (Figur-
es 5D and 5E). When the fraction of residual Ag is above 10
atomic%, the catalytic selectivity drastically drops as CO2
becomes the dominate product and methyl formate is no
longer formed in the entire temperature range up to 80 8C.These observations strongly indicate that residual Ag regulates
the abundance of reactive oxygen on the catalyst surfaces and
thus controls the selectivity toward the partial oxidation of
methanol. More recently, it has also been observed that the
residual Ag in dealloyed nanoporous Au plays crucial roles in
chemoselective hydrogenation of C=C, C/C, C=N, and C=O
bonds under mild conditions,[53] though the detailed mecha-
nisms are still open to further investigations.
Methanol oxidation catalyzed by dealloyed nanoporous Au
exemplifies how the strong synergy between the undercoordi-
nated Au surface atoms and residual Ag dictates chemo-
selective catalytic molecular transformations. More quantitative
examination of the structure-composition-performance rela-
tionships will lead to key design principles for achieving
optimal catalytic selectivity on dealloyed nanoporous Au
catalysts.
Figure 5. (A) Proposed mechanism of selective oxidation of methanol on dealloyed nanoporous Au catalysts. Methanol is activated by surface oxygen andbonded at the surface as methoxy. Subsequent deprotonation leads to the aldehyde. Fast reaction of the highly reactive aldehyde with further methoxy leadsto the coupling product methyl formate (HCO2CH3). In the case of excess oxygen, the aldehyde can be further oxidized, resulting in CO2 formation. Thebackground of panel A is an SEM image showing the structure of monolithic nanoporous Au. (B) Conversion % of total oxidation of methanol (to CO2; bluerhombuses) and partial oxidation of methanol (to methyl formate; gray squares) at various reaction temperatures catalyzed by dealloyed nanoporous Au withresidual Ag less than 1 atomic%. (C) Selectivity (fraction of methanol that is converted into methyl formate) at various reaction temperatures on dealloyednanoporous Au with residual Ag less than 1 atomic%. (D) Activity and (E) selectivity of methanol oxidation catalyzed by dealloyed nanoporous Au with2.5 atomic% of residual Ag. Reprinted with permission from ref [14]. Copyright 2010, The American Association for the Advancement of Science.
904ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
4. Dealloying of Alloy Nanoparticles towardElectrocatalysis Optimization: A Case Study ofAlcohol Electro-Oxidation
A general design criterion for structural optimization of
catalysts is to maximize both the surface area-to-volume ratio
and the density of surface active sites. Using the electrocatalytic
methanol oxidation reaction (MOR) as a model reaction, we
recently demonstrated that both the mass-specific surface area
and the density of surface active sites of spongy nanoparticles
could be systematically fine-tuned through kinetically con-
trolled percolation dealloying of Au@Cu alloy nanoparticles.[23,58]
Analogous to that of the macroscopic Au@Ag alloy membranes,
the percolation dealloying of Au@Cu alloy nanoparticles is
essentially dictated by two interplaying structure-transforming
processes, leaching of Cu atoms from the alloy and coarsening
of the nanoligaments. While the leaching of Cu atoms creates
high densities of atomically undercoordinated surface sites and
drives the nanoporosity evolution, ligament coarsening-driven
surface reconstruction causes both the surface area-to-volume
ratio and the fraction of undercoordinated surface atoms to
decrease. On noble metal nanocatalyst surfaces, it is essentially
the undercoordinated surface atoms that serves as the primary
active sites for electrocatalytic oxidation of liquid alco-
hols.[23,54,58–60] Therefore, nanoporous Au catalysts exhibiting the
optimal electrocatalytic performance are expected to possess
thin nanoliamgents enclosed by highly abundant undercoordi-
nated surface atoms, which can be synthetically achieved by
judiciously maneuvering the Cu leaching and ligament coarsen-
ing processes during percolation dealloying of Au@Cu alloy
nanoparticles under kinetically controlled conditions.
Our synthetic approach to spongy Au nanoparticles
involves a stepwise nanoscale alloying-dealloying process as
illustrated in Figure 6A. We started from Au@Cu2O core-shell
nanoparticles whose core and shell dimensions could be
precisely tuned over a broad size range using a seed-mediated
growth method.[61] The Au@Cu2O core-shell nanoparticles first
transformed into Au@Cu bimetallic heteronanostructures
through chemical reduction, and then underwent intraparticle
alloying to form Au@Cu alloy nanoparticles upon thermal
treatment in either a reducing atmosphere,[23] such as H2, or in a
high boiling point-polyol solvent,[62] such as tetraethylene
glycol. The sizes and Cu/Au stoichiometric ratios of the alloy
nanoparticles were essentially predetermined by the core and
shell dimensions of their parental Au@Cu2O core-shell nano-
particles and thereby could be systematically tuned over a
broad range.
The relative rate of Cu leaching with respect to that of the
ligament coarsening can be tuned by choosing different
etchants or varying the etchant concentrations.[23] By kinetically
trapping the partially dealloyed spongy nanoframes (NFs) at
various dealloying stages, both the mass-specific surface area
and the density of surface active sites can be controlled. As the
percolation dealloying proceeds, the electrochemical surface
area (ECSA) first increased due to nanoporosity formation and
then decreased as a consequence of ligament coarsening, while
the specific activity (SA) normalized to the surface area
progressively decreased because of ligament coarsening-driven
surface reconstruction (Figures 5C). The mass activity (MA) of
the dealloyed nanoparticles is essentially determined by both
the ECSA and SA. No clear correlation between the catalytic
activity and amount of residual Cu was observed, possibly
because the electrocatalytic MOR does not involved surface
adsorbed oxygen. Therefore, it is highly likely that the SA is
primarily determined by the density of the undercoordinated
surface atoms rather than the residual Cu on the nanoligament
surfaces. Despite their remarkable initial activities, the dealloyed
NFs underwent activity deterioration over time under the
reaction conditions due to electrochemically induced surface
reconstruction and ligament coarsening (Figure 6D), which
motivated us to further explore new ways to effectively
enhance both the activity and durability of the dealloyed
nanoporous electrocatalysts.
It has been demonstrated by Erlebacher and coworkers[63]
that enhanced catalytic durability of dealloyed nanoporous Au
can be achieved by incorporating Pt into the Au@Ag alloy
precursors such that the undercoordinated surface atoms can
be stabilized upon surface accumulation of Pt around the
atomic step edges and kinks during percolation dealloying.
More recently, we found that residual Ag was also capable of
enhancing the electrocatalytic durability of dealloyed nano-
sponge (NS) particles,[58] though Ag was a leachable less-noble
element whose behaviors appeared fundamentally distinct
from those of the non-leachable Pt. The co-leaching of Ag and
Cu from Au@Ag@Cu ternary alloy nanoparticles not only greatly
accelerated Cu leaching, but also effectively suppressed
ligament coarsening. As shown in Figures 6E and 6F, the fully
dealloyed NS particles obtained from dealloying of Au@Ag@Cuternary alloy nanoparticles (denoted as NS-T) exhibited substan-
tially larger specific surface areas, higher densities of surface
active sites, thinner ligaments, and smaller average pore sizes in
comparison to their Ag-free counterparts derived from Au@Cubinary alloy nanoparticles (denoted as NS-B). As a consequence,
the dealloyed NS-T particles exhibited remarkably enhanced
electrocatalytic activities toward a series of alcohol oxidation
reactions (Figures 6G–6 J), including MOR, ethanol oxidation
reaction (EOR), iso-propanol oxidation reaction (i-POR), and
ethylene glycol oxidation reaction (EGOR), in comparison to
those of the Ag-free NS-B particles. The residual Ag in the
dealloyed NS-T particles greatly suppressed the surface recon-
struction during electrocatalytic reactions, enabling the reten-
tion of the superior catalytic activities over much longer periods
(Figure 6K). The insights gained from this case study shed light
on the crucial roles of residual less noble elements in enhancing
the durability of dealloyed metallic electrocatalysts for fuel cell
applications.
5. Summary and Outlook
Nanoporous Au catalysts derived from percolation dealloying of
Au-containing alloys exhibit remarkable catalytic activities
toward a large variety of reactions ranging from industrially
905ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
important CO and alcohol oxidation to synthetically challenging
C@C ring formation.[11–12] Of particular interest to the catalysis
community is the unique capability of dealloyed nanoporous
Au to efficiently catalyze chemoselective reactions under mild
reaction conditions.[14,53] Instead of providing an exhaustive
survey on all catalytic reactions reported in the literature, this
review focuses on several representative chemical and electro-
chemical oxidation reactions to shed light on the structure-
composition-property correlations of the dealloyed nanoporous
Au catalysts. The nanoporosity evolution during percolation
dealloying involves a series of intriguing structure-rearranging
processes, such as interfacial atomic dissolution, surface
reconstruction, atomic interdiffusion, and ligament coarsening.
Erlebacher’s surface-diffusion continuum model[13,18,28] rigorously
interprets how multiple structural remodeling processes syn-
ergistically modulate the nanoporosity evolution. The success
in structure-controlled synthesis through kinetically maneu-
vered percolation dealloying enables systematic invesitigations
of the structure-composition-property relationships underpin-
ning the intriguing catalytic behaviors of the dealloyed nano-
porous Au catalysts. The undercoordinated surface atoms and
the residual less-noble elements have been identified as two
key factors that dicate the catalytic activity, durability, and
selectivity of the dealloyed nanoporous Au catalysts. In situ
environmental HRTEM provides a powerful characterization tool
capable of resolving not only the complex atomic-level surface
structures but also, more importantly, the dynamic restructuring
of the active sites under reaction conditions.[33,47,57] Many
aspects regarding the detailed mechanisms of nanoporous Au-
based catalysis, however, vary significantly from reaction to
reaction and thus still remain ambiguous. The synergy between
undercoordinated surface atoms and redisual less-noble ele-
Figure 6. (A) Scheme illustrating the transformation of Au@Cu2O core@shell nanoparticles (NPs) into Au@Cu alloy NPs and the percolation dealloying of Au@Cualloy NPs. (B) Cyclic voltammetry curves of MOR on Au0.19Cu0.81 alloy NPs and various dealloyed spongy nanoframes (NFs) in 1.0 M methanol and 0.5 M KOH ata potential sweep rate of 10 mVs@1. The NF samples labeled as NF-i, NF-ii, NF-iii, and NF-iv correspond to the samples obtained through dealloying of Au0.19
Cu0.81 alloy NPs in 0.2, 0.5, 1.0, and 2.0 M Fe(NO3)3 for 2 h, respectively. The sample labeled as NF-v was obtained through dealloying of Au0.19Cu0.81 alloy NPs in2.0 M HNO3 for 1 h. HAADF-STEM images of one representative particle for each NF sample are also shown as the insets. (C) Cu atomic%, mass activities (MAs),electrochemical surface areas (ECSAs), and specific activities (SAs) of Au0.19Cu0.81 alloy NPs and dealloyed NFs. The Cu atomic% was quantified by energydispersive spectroscopy (EDS) and inductively coupled plasma mass spectrometry (ICP-MS). (D) Chronoamperometry curves collected on Au0.19Cu0.81 alloy NPsand dealloyed NFs for MOR at 0.1 V (vs. SCE) and 0.3 V (vs. SCE). Reprinted with permission from ref [23]. Copyright 2016, American Chemical Society. TEMimages of (E) NS-T (dealloyed from Au0.14Ag0.14Cu0.72 ternary alloy NPs) and (F) NS-B (dealloyed from Au0.16Cu0.84 binary alloy NPs). Cyclic voltammetry curves ofNS-T and NS-B in 0.5 M KOH electrolyte solutions containing (G) 1 M methanol, (H) 1 M ethanol, (I) 1 M isopropanol, and (J) 0.25 M ethylene glycol. (K) MAs,SAs, and the ratio of current density after 2 hours to the initial current density (jp,2 h/jp,i) for NS-B and NS-T. Reprinted with permission from ref [58]. Copyright2016, American Chemical Society.
906ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
ments, the driving force for the surface reconstruction under
reaction conditions, and the transition states associated with
the molecular transformations on the active sites are all well-
worthy of further investigations.
We envision that dealloyed spongy nanoparticles will attract
conitnuously increasing attention in the fields of Au-based
heterogeneous catalysis and electrocatalysis because they
exhibit a unique set of advantages over the macroscopic
nanoporous foams in terms of catalytic performance, materials
processablity, and structural tunability. First, the catalytically
active sites on the ligament surfaces are easily accessible by the
reactant molecules when the nanoscale porosity is created
inside a nanoparticle with a finite size, whereas the molecules
must overcome long diffusion distances and convoluted paths
to reach the active sites buried inside the interior of a
macroscopic nanoporous membrane. Second, the pore and
ligament dimensions of a macroscopic nanoporous membrane
vary significantly from location to location, with narrower pore
channels and finer ligaments on the outer surfaces than those
in the interior regions.[64] Such intrinsic structural heterogeneity
significantly complicates the energy landscapes and overall
kinetics of the surface-catalyzed molecular transformations.
Switching from bulk materials to nanoparticulate systems
makes it possible to achieve uniform ligament thickness and
pore sizes. Third, using colloidal nanoparticles as an easily
processable ink allows for straightforward nanoparticle assem-
bly on a large variety of substrates, including the flexible and
microstrucured substrates, for constructing high-performance
but low-cost catalysts for widespread applications. Fourth and
most importantly, alloy nanoparticles exhibit drastically en-
hanced structural diversity and tunability compared to their
bulk counterparts, creating unique opportunities for us to fine-
tailor a series of geometric and compositional parameters.
Taking Au@Cu bimetallic nanoparticles as an example, recent
advances in colloidal syntheses allow one to fine-tailor not only
the size, shape, and composition but also intraparticle atomic
configurations (disordered alloys, ordered intermetallic phases,
intraparticle compositional gradient, and phase segrega-
tion),[61–62,65–67] all of which are crucial factors profoundly
influencing the structural transformations of the nanopartilces
during the percolation dealloying. By coupling the percolation
dealloying with other chemical reactions, such as galvanic
replacement reactions[62] and electrochemical atomic layer
deposition,[34,54,68–69] it becomes possible to incorporate other
catalytically active materials into spongy Au nanoparticles in a
highly controllable manner, enbaling us to further fine-tune the
catalytic properties of the dealloyed nanoparticles at a level of
detail and precision unachievable on those dealloyed bulk
materials.
In addition to their catalytic properties, dealloyed nano-
porous Au also exhibits interesting optical properties that are
dominated by the collective oscillation of free electrons known
as plasmons.[34,70–72] Introduction of nanoporosity to the surface
or the bulk of Au nanoparticles results in greatly enhanced
tunability of the plasmon resonance frequencies over a broad
spectral range and enormous local electric field enhancements
on the nanoligament surfaces exploitable for plasmon-en-
hanced spectroscopies.[29–30,73–75] Benefiting from their unique
combination of catalytic and plasmonic properties, the deal-
loyed spongy Au nanoparticles may serve as a dual-functional
materials platform enabling the use of surface-enhanced Raman
scattering (SERS) as a time-resolving and molecular finger-
printing tool to track detailed interfacial molecular transforma-
tions in real time during catalytic reactions.[76–80] In addition, the
dealloyed spongy Au nanoparticles exhibit interesting photo-
thermal behaviors upon plasmonic excitations.[31] The photo-
thermally generated heat on the local surfaces of the catalysts
can be harnessed to further boost a variety of interfacial
thermal catalytic reactions. Furthmore, it has been recently
observed that the energetic hot electrons generated during
plasmon decay can be harnessed to efficiently drive a series of
interesting photocatalytic reactions along unconventional path-
ways distinct from those involved in the conventional thermal
catalytic reactions and semiconductor-based photocataly-
sis.[81–84] Plasmon-driven photocatalysis is a newly emerging field
full of open questions, challenges, and oppotunities. The
dealloyed spongy Au nanoparticles provide an interesting
materials system that may help us interrogate intriguing but
challenging fundamental questions regarding the detailed
mechanisms of plasmon-mediated chemistry and photochemis-
try.
Acknowledgements
The authors thank the United States National Science Founda-
tion (DMR-1253231 and OIA-1655740), United States Depart-
ment of Energy (DE-SC0016574), and the University of South
Carolina (Startup Funds, ASPIRE@I Track I Award, and SPARC
Award) for funding support.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: gold nanocatalysts · nanoporosity · percolation
dealloying · undercoordinated surface atoms · electrocatalysis
[1] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chem. Lett. 1987, 405–408.[2] B. Hvolbaek, T. V. W. Janssens, B. S. Clausen, H. Falsig, C. H. Christensen,
J. K. Norskov, Nano Today 2007, 2, 14–18.[3] T. V. W. Janssens, B. S. Clausen, B. Hvolbaek, H. Falsig, C. H. Christensen,
T. Bligaard, J. K. Norskov, Top. Catal. 2007, 44, 15–26.[4] G. C. Bond, D. T. Thompson, Catal. Rev. Sci. Eng. 1999, 41, 319–388.[5] A. Abad, P. Concepcion, A. Corma, H. Garcia, Angew. Chem. Int. Ed. 2005,
44, 4066–4069; Angew. Chem. 2005, 117, 4134–4137.[6] S. Carrettin, P. Concepcion, A. Corma, J. M. L. Nieto, V. F. Puntes, Angew.
Chem. Int. Ed. 2004, 43, 2538–2540; Angew. Chem. 2004, 116, 2592–2594.
[7] M. Haruta, M. Date, Appl. Catal. A 2001, 222, 427–437.[8] I. X. Green, W. J. Tang, M. Neurock, J. T. Yates, Science 2011, 333, 736–
739.[9] M. Valden, X. Lai, D. W. Goodman, Science 1998, 281, 1647–1650.
[10] C. Lemire, R. Meyer, S. Shaikhutdinov, H. J. Freund, Angew. Chem. Int. Ed.2004, 43, 118–121; Angew. Chem. 2004, 116, 121–124.
[11] A. Wittstock, M. Baumer, Acc. Chem. Res. 2014, 47, 731–739.
907ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review
123456789
101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657
[12] J. Biener, M. M. Biener, R. J. Madix, C. M. Friend, ACS Catal. 2015, 5,6263–6270.
[13] J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature 2001,410, 450–453.
[14] A. Wittstock, V. Zielasek, J. Biener, C. M. Friend, M. Baumer, Science2010, 327, 319–322.
[15] N. Asao, Y. Ishikawa, N. Hatakeyama, Menggenbateer, Y. Yamamoto,M. W. Chen, W. Zhang, A. Inoue, Angew. Chem. Int. Ed. 2010, 49, 10093–10095; Angew. Chem. 2010, 122, 10291–10293.
[16] V. Zielasek, B. Jurgens, C. Schulz, J. Biener, M. M. Biener, A. V. Hamza, M.Baumer, Angew. Chem. Int. Ed. 2006, 45, 8241–8244; Angew. Chem.2006, 118, 8421–8425.
[17] C. X. Xu, J. X. Su, X. H. Xu, P. P. Liu, H. J. Zhao, F. Tian, Y. Ding, J. Am.Chem. Soc. 2007, 129, 42–43.
[18] J. Weissmuller, K. Sieradzki, MRS Bull. 2018, 43, 14–19.[19] X. Q. Li, Q. Chen, I. McCue, J. Snyder, P. Crozier, J. Erlebacher, K.
Sieradzki, Nano Lett. 2014, 14, 2569–2577.[20] J. Rugolo, J. Erlebacher, K. Sieradzki, Nat. Mater. 2006, 5, 946–949.[21] D. M. Artymowicz, J. Erlebacher, R. C. Newman, Philos. Mag. 2009, 89,
1663–1693.[22] M. Kamundi, L. Bromberg, E. Fey, C. Mitchell, M. Fayette, N. Dimitrov, J.
Phys. Chem. C 2012, 116, 14123–14133.[23] G. G. Li, E. Villarreal, Q. F. Zhang, T. T. Zheng, J. J. Zhu, H. Wang, ACS
Appl. Mater. Interfaces 2016, 8, 23920–23931.[24] J. X. Xia, S. Ambrozik, C. C. Crane, J. Y. Chen, N. Dimitrov, J. Phys. Chem.
C 2016, 120, 2299–2308.[25] A. Chauvin, C. Delacote, L. Molina-Luna, M. Duerrschnabel, M. Boujtita,
D. Thiry, K. Du, J. J. Ding, C. H. Choi, P. Y. Tessier, A. A. El Mel, ACS Appl.Mater. Interfaces 2016, 8, 6611–6620.
[26] R. C. Cammarata, Prog. Surf. Sci. 1994, 46, 1–38.[27] J. Erlebacher, Phys. Rev. Lett. 2011, 106, 225504.[28] I. McCue, A. Karma, J. Erlebacher, MRS Bull. 2018, 43, 27–34.[29] Y. Yan, A. I. Radu, W. Rao, H. Wang, G. Chen, K. Weber, D. Wang, D.
Cialla-May, J. Popp, P. Schaaf, Chem. Mater. 2016, 28, 7673–7682.[30] K. Liu, Y. Bai, L. Zhang, Z. Yang, Q. Fan, H. Zheng, Y. Yin, C. Gao, Nano
Lett. 2016, 16, 3675–3681.[31] T. Zheng, G. G. Li, F. Zhou, R. Wu, J. J. Zhu, H. Wang, Adv. Mater. 2016,
28, 8218–8226.[32] X. Gu, L. Xu, F. Tian, Y. Ding, Nano Res. 2009, 2, 386–393.[33] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga,
S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J.Erlebacher, M. Chen, Nat. Mater. 2012, 11, 775.
[34] Y. Ding, M. Chen, MRS Bull. 2011, 34, 569–576.[35] R. Zeis, T. Lei, K. Sieradzki, J. Snyder, J. Erlebacher, J. Catal. 2008, 253,
132–138.[36] W. L. Yim, T. Nowitzki, M. Necke, H. Schnars, P. Nickut, J. Biener, M. M.
Biener, V. Zielasek, K. Al-Shamery, T. Kluner, M. Baumer, J. Phys. Chem. C2007, 111, 445–451.
[37] H. Falsig, B. Hvolbaek, I. S. Kristensen, T. Jiang, T. Bligaard, C. H.Christensen, J. K. Norskov, Angew. Chem. Int. Ed. 2008, 47, 4835–4839;Angew. Chem. 2008, 120, 4913–4917.
[38] A. Wittstock, B. Neumann, A. Schaefer, K. Dumbuya, C. Kubel, M. M.Biener, V. Zielasek, H. P. Steinruck, J. M. Gottfried, J. Biener, A. Hamza, M.Baumer, J. Phys. Chem. C 2009, 113, 5593–5600.
[39] J. Kim, E. Samano, B. E. Koel, J. Phys. Chem. B 2006, 110, 17512–17517.[40] L. V. Moskaleva, S. Rohe, A. Wittstock, V. Zielasek, T. Kluner, K. M.
Neyman, M. Baumer, Phys. Chem. Chem. Phys. 2011, 13, 4529–4539.[41] J. L. C. Fajin, M. Cordeiro, J. R. B. Gomes, Chem. Commun. 2011, 47,
8403–8405.[42] J. Kim, E. Samano, B. E. Koel, J. Phys. Chem. B 2006, 110, 17512–17517.[43] M. L. Personick, B. Zugic, M. M. Biener, J. Biener, R. J. Madix, C. M. Friend,
ACS Catal. 2015, 5, 4237–4241.[44] B. K. Min, X. Deng, D. Pinnaduwage, R. Schalek, C. M. Friend, Phys. Rev. B
2005, 72, 121410.[45] V. Zielasek, B. Xu, X. Liu, M. B-umer, C. M. Friend, J. Phys. Chem. C 2009,
113, 8924–8929.[46] J. Dou, Y. Tang, L. Nguyen, X. Tong, P. S. Thapa, F. F. Tao, Catal. Lett.
2017, 147, 442–452.[47] T. Fujita, T. Tokunaga, L. Zhang, D. W. Li, L. Y. Chen, S. Arai, Y. Yamamoto,
A. Hirata, N. Tanaka, Y. Ding, M. W. Chen, Nano Lett. 2014, 14, 1172–1177.
[48] C. Mahr, P. Kundu, A. Lackmann, D. Zanaga, K. Thiel, M. Schowalter, M.Schwan, S. Bals, A. Wittstock, A. Rosenauer, J. Catal. 2017, 352, 52–58.
[49] Y. Liu, S. Bliznakov, N. Dimitrov, J. Electrochem. Soc. 2010, 157, K168-K176.
[50] D. Artymowicz, Z. Coull, M. Bryk, R. Newman, ECS Trans. 2011, 33, 1–14.[51] L. V. Moskaleva, T. Weiss, T. Kluner, M. Baumer, J. Phys. Chem. C 2015,
119, 9215–9226.[52] S. Kameoka, A. P. Tsai, Catal. Lett. 2008, 121, 337–341.[53] B. S. Takale, X. J. Feng, Y. Lu, M. Bao, T. A. Jin, T. Minato, Y. Yamamoto, J.
Am. Chem. Soc. 2016, 138, 10356–10364.[54] J. T. Zhang, P. P. Liu, H. Y. Ma, Y. Ding, J. Phys. Chem. C 2007, 111, 10382–
10388.[55] Z. L. Wang, S. C. Ning, P. Liu, Y. Ding, A. Hirata, T. Fujita, M. W. Chen, Adv.
Mater. 2017, 29, 1703601.[56] Y. Li, W. Dononelli, R. Moreira, T. Risse, M. Baumer, T. Kluner, L. V.
Moskaleva, J. Phys. Chem. C 2018, 122, 5349–5357.[57] P. Liu, P. F. Guan, A. Hirata, L. Zhang, L. Y. Chen, Y. R. Wen, Y. Ding, T.
Fujita, J. Erlebacher, M. W. Chen, Adv. Mater. 2016, 28, 1753–1759.[58] G. G. Li, Y. Lin, H. Wang, Nano Lett. 2016, 16, 7248–7253.[59] N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding, Z. L. Wang, Science 2007, 316, 732–
735.[60] Y. H. Song, T. T. Miao, P. N. Zhang, C. X. Bi, H. B. Xia, D. Y. Wang, X. T. Tao,
Nanoscale 2015, 7, 8405–8415.[61] L. Zhang, H. Jing, G. Boisvert, J. Z. He, H. Wang, ACS Nano 2012, 6,
3514–3527.[62] G. G. Li, M. Sun, E. Villarreal, S. Pandey, S. R. Phillpot, H. Wang, Langmuir
2018, 34, 4340–4350.[63] J. Snyder, P. Asanithi, A. B. Dalton, J. Erlebacher, Adv. Mater. 2008, 20,
4883–4886.[64] M. Graf, M. Haensch, J. Carstens, G. Wittstock, J. Weissmuller, Nanoscale
2017, 9, 17839–17848.[65] W. Chen, R. Yu, L. L. Li, A. N. Wang, Q. Peng, Y. D. Li, Angew. Chem. Int.
Ed. 2010, 49, 2917–2921; Angew. Chem. 2010, 122, 2979–2983.[66] R. E. Schaak, A. K. Sra, B. M. Leonard, R. E. Cable, J. C. Bauer, Y. F. Han, J.
Means, W. Teizer, Y. Vasquez, E. S. Funck, J. Am. Chem. Soc. 2005, 127,3506–3515.
[67] S. T. Chen, S. V. Jenkins, J. Tao, Y. M. Zhu, J. Y. Chen, J. Phys. Chem. C2013, 117, 8924–8932.
[68] D. A. McCurry, M. Kamundi, M. Fayette, F. Wafula, N. Dimitrov, ACS Appl.Mater. Interfaces 2011, 3, 4459–4468.
[69] J. Xia, I. Achari, S. Ambrozik, N. Dimitrov, Mater. Res. Bull. 2017, 85, 1–9.[70] J. Biener, G. W. Nyce, A. M. Hodge, M. M. Biener, A. V. Hamza, S. A. Maier,
Adv. Mater. 2008, 20, 1211–1217.[71] M. Bosman, G. R. Anstis, V. J. Keast, J. D. Clarke, M. B. Cortie, ACS Nano
2012, 6, 319–326.[72] F. Yu, S. Ahl, A.-M. Caminade, J.-P. Majoral, W. Knoll, J. Erlebacher, Anal.
Chem. 2006, 78, 7346–7350.[73] J. Qi, P. Motwani, M. Gheewala, C. Brennan, J. C. Wolfe, W. C. Shih,
Nanoscale 2013, 5, 4105–4109.[74] C. Vidal, D. Wang, P. Schaaf, C. Hrelescu, T. A. Klar, ACS Photonics 2015,
2, 1436–1442.[75] Q. F. Zhang, N. Large, P. Nordlander, H. Wang, J. Phys. Chem. Lett. 2014,
5, 370–374.[76] W. Xie, C. Herrmann, K. Kompe, M. Haase, S. Schlucker, J. Am. Chem. Soc.
2011, 133, 19302–19305.[77] J. Huang, Y. Zhu, M. Lin, Q. Wang, L. Zhao, Y. Yang, K. X. Yao, Y. Han, J.
Am. Chem. Soc. 2013, 135, 8552–8561.[78] Q. Zhang, D. A. Blom, H. Wang, Chem. Mater. 2014, 26, 5131–5142.[79] Q. F. Zhang, Y. D. Zhou, E. Villarreal, Y. Lin, S. L. Zou, H. Wang, Nano Lett.
2015, 15, 4161–4169.[80] J. W. Zhang, S. A. Winget, Y. R. Wu, D. Su, X. J. Sun, Z. X. Xie, D. Qin, ACS
Nano 2016, 10, 2607–2616.[81] S. Linic, P. Christopher, H. L. Xin, A. Marimuthu, Acc. Chem. Res. 2013, 46,
1890–1899.[82] M. J. Kale, T. Avanesian, P. Christopher, ACS Catal. 2014, 4, 116–128.[83] M. L. Brongersma, N. J. Halas, P. Nordlander, Nat. Nanotechnol. 2015, 10,
25–34.[84] Q. Zhang, H. Wang, J. Phys. Chem. C 2018, 122, 5686–5697.
Manuscript received: April 10, 2018Version of record online: May 30, 2018
908ChemNanoMat 2018, 4, 897–908 www.chemnanomat.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Focus Review