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To advance the understanding of the degradation of the liquid electrolyte and Si electrode, and their interface, we exploit the latest developments in cryo-atom probe tomography. We evidence Si anode corrosion from the decomposition of the Li salt before charge-discharge cycles even begin. Volume shrinkage during delithiation leads to the development of nanograins from recrystallization in regions left amorphous by the lithiation. The newly created grain boundaries facilitate pulverization of nanoscale Si fragments, and one is found floating in the electrolyte. P is segregated to these grain boundaries, which confirms the decomposition of the electrolyte. As structural defects are bound to assist the nucleation of Li-rich phases in subsequent lithiations and accelerate the electrolyte's decomposition, these insights into the developed nanoscale microstructure interacting with the electrolyte contribute to understanding the self-catalyzed/accelerated degradation Si anodes and can inform new battery designs unaffected by these life-limiting factors.

Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications owing to their enhanced catalytic, optical, and magnetic properties, which are often better than their monometallic counterparts. Most of their properties strongly depend on their chemical composition, crystallographic structure, and phase distribution. However, little is known of how their crystal structure, on the nanoscale, transforms over time at elevated temperatures, even though this knowledge is highly relevant in case nanoparticles are used in, e.g., high-temperature catalysis. Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au. Here, we report on the in situ temporal evolution of the crystalline ordering in Au-Fe nanoparticles, obtained from a modern laser ablation in liquids synthesis. Our in-depth analysis, complemented by dedicated atomistic simulations, includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution. We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions, phase distributions, and size-depended microstrains. The elevated temperature induces a diffusion-controlled recrystallization and phase merging, resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase, which demonstrates the metastability of these structures. Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions. Furthermore, the in situ evaluation of the crystal structure changes upon heating is potentially relevant for high-temperature process utilization of bimetallic nanoparticles, e.g., during catalysis.

Nanoporous gold made by dealloying AgAuPt (NPG-Pt) has been shown to exhibit several interesting catalytic properties, tied to its exceptionally high surface area; however, structural degradation may occur owing to thermal coarsening. To understand the effect of atmosphere chemistry on thermal coarsening and degradation, and means of limiting it, this study focuses on the high-resolution characterization of NPG-Pt layers coarsened in reductive Ar-H2 atmosphere, and in oxidative air. Atom probe tomography (APT) analysis is performed on NPG-Pt, coarsened separately in either Ar-H2 or air, to characterize the atomic-scale chemical changes in the nanoligaments and to develop a mechanistic view of the inherent processes. A tendency of Ag to segregate to the surface during coarsening is found to lead to complete elimination of the nanoligament core-shell structures in both cases. Large Pt segregates form during coarsening in Ar-H2, but under the surface of the ligaments, having relatively little effect on the coarsening rate. The oxygen-induced segregation of Pt was observed to cause the inhibition of thermal coarsening after minor loss in surface area-to-volume ratio. Findings in this paper help in understanding further the thermal coarsening of heterogeneous nanomaterials made by dealloying, and the pertinent factors that come into play in different chemical environments.

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back towards re-oxidation into cubic-Fe1-xO (where x refers to Fe deficiency, space group Fm3-m). This effect helps to understand the sluggish reduction of cubic-Fe1-xO by hydrogen, a key process for future sustainable steelmaking.

Fuel cells recombine water from H-2 and O-2 thereby can power, for example, cars or houses with no direct carbon emission. In anion-exchange membrane fuel cells (AEMFCs), to reach high power densities, operating at high pH is an alternative to using large volumes of noble metals catalysts at the cathode, where the oxygen-reduction reaction occurs. However, the sluggish kinetics of the hydrogen-oxidation reaction (HOR) hinders upscaling despite promising catalysts. Here, the authors observe an unexpected ingress of B into Pd nanocatalysts synthesized by wet-chemistry, gaining control over this B-doping, and report on its influence on the HOR activity in alkaline conditions. They rationalize their findings using ab initio calculations of both H- and OH-adsorption on B-doped Pd. Using this "impurity engineering" approach, they thus design Pt-free catalysts as required in electrochemical energy conversion devices, for example, next generations of AEMFCs, that satisfy the economic and environmental constraints, that is, reasonable operating costs and long-term stability, to enable the "hydrogen economy."

Journal of the American Chemical Society 144(2), 987-994 (2022). doi:10.1021/jacs.1c11680, Published by American Chemical Society, Washington, DC

The worldwide development of electric vehicles as well as large-scale or grid-scale energy storage to compensate for the intermittent nature of renewable energy generation has led to a surge of interest in battery technology. Understanding the factors controlling battery capacity and, critically, their degradation mechanisms to ensure long-term, sustainable and safe operation requires detailed knowledge of their microstructure and chemistry, and their evolution under operating conditions, on the nanoscale. Atom probe tomography (APT) provides compositional mapping of materials in three dimensions with sub-nanometre resolution, and is poised to play a key role in battery research. However, APT is underpinned by an intense electric field that can drive lithium migration, and many battery materials are reactive oxides, requiring careful handling and sample transfer. Here, we report on the analysis of both anode and cathode materials and show that electric-field driven migration can be suppressed by using shielding by embedding powder particles in a metallic matrix or by using a thin conducting surface layer. We demonstrate that for a typical cathode material, cryogenic specimen preparation and transport under ultra-high vacuum leads to major delithiation of the specimen during the analysis. In contrast, the transport of specimens through air enables the analysis of the material. Finally, we discuss the possible physical underpinnings and discuss ways forward to enable shielding from the electric field, which helps address the challenges inherent to the APT analysis of battery materials.

Fluorophores functionalized with heavy elements show enhanced intersystem crossing due to increased spin-orbit coupling, which in turn shortens the fluorescence decay lifetime (tau(PL)). This phenomenon is known as the heavy-atom effect (HAE). Here, we report the observation of increased tau(PL) upon functionalisation of near-infrared photoluminescent gold nanoclusters with iodine. The heavy atom-mediated increase in tau(PL) is in striking contrast with the HAE and referred to as inverse HAE. Femtosecond and nanosecond transient absorption spectroscopy revealed overcompensation of a slight decrease in lifetime of the transition associated with the Au core (ps) by a large increase in the long-lived triplet state lifetime associated with the Au shell, which contributed to the observed inverse HAE. This unique observation of inverse HAE in gold nanoclusters provides the means to enhance the triplet excited state lifetime.

The selective removal of a less noble (more chemically active) metal from a mixture of 2–3 metals can yield a bicontinuous, open-pore, 3D nanoporous metal (NPM), that is rich in the more noble metal(s)1. NPMs have been successfully developed by the intelligent use of the conventionally-undesired dealloying corrosion. The excellent properties of NPMs are attributed to the surface area-to-volume ratio, and high curvature of nanoligament surfaces 2,3 . Experimental work on NPMs revealed the formation of uniformly nanoporous structure by dealloying AgAu alloy in nitric acid, a method still widely used until today for making nanoporous gold (NPG). This dealloying process is complex: the selective dissolution of the less-noble element should lead to the creation of surface vacancies or adatoms, which migrate across the surface to form surface roughening features, and thus assisting the migration of the residual more-noble atoms, leading to island growth4. Ex-situ characterization cannot fully explain some intricate details at the dealloying interface and at the surface of the formed nanoligaments. Gaining insight into initial stages of dealloying, and the inherent competition between surface roughening from the dissolution of silver atoms, and surface smoothening from surface diffusion of gold atoms5, can only be done effectively by monitoring the changes occurring at the surfaces of alloys in-situ. Several notable in-situ methods were used to characterize formation of NPMs, such as TEM 6, often limited by the 2D nature of the analysis. Also, synchrotron-based methods such as X-ray nanotomography 7 and neutron scattering 8 were limited by resolution and lack of compositional contrast. APT is a powerful technique that provides 3D characterization9 and near-atomic-scale compositional analysis of materials, and could complement the abovementioned suite of techniques, yet the analysis of nanoporous structure comes with challenges. Aiming to develop a universal method for probing corrosion systems by APT, the concept of embedding frozen solutions in corroded systems was developed and reported in a recent reporting 10 of analyzing NPMs along with frozen water-based solutions. This involved the joint use of an ensemble of equipment and techniques that connect the frozen liquid to the atom probe, including a plasma-focused ion beam (PFIB) where the preparation of APT specimens uses a cryostage, and transfer of the frozen sample through ultra-high-vacuum suitcase11. This paved the way for many advances in characterization of corrosion processes, as the possibility of freezing corrosion reactions for APT (now known as cryo-APT) arises. Here, we further develop cryo-APT to probe into the mechanisms of dealloying in AgAu and how that leads to the formation of NPG using our in-situ approach. Nanoligament structure will be correlated with dealloying conditions by making observations at the solid-liquid interface in 3D. References Newman, R. C. 2.05 - Dealloying. in Shreir’s Corrosion (eds. Cottis, B. et al.) 801–809 (Elsevier, 2010). Zielasek, V. et al. Gold Catalysts: Nanoporous Gold Foams. Angew. Chemie Int. Ed. 45, 8241–8244 (2006). Xue, Y., Markmann, J., Duan, H., Weissmüller, J. & Huber, P. Switchable imbibition in nanoporous gold. Nat. Commun. 5, (2014). Forty, A. J. & Rowlands, G. A possible model for corrosion pitting and tunneling in noble-metal alloys. Philos. Mag. A Phys. Condens. Matter, Struct. Defects Mech. Prop. (1981) doi:10.1080/01418618108239399. Erlebacher, J., Newman, C. & Sieradzki, K. Fundamental physics and chemistry of nanoporosity evolution during dealloying. RSC Nanosci. Nanotechnol. 11–29 (2012) doi:10.1039/9781849735285-00011. Liu, P. et al. Dealloying Kinetics of AgAu Nanoparticles by In Situ Liquid-Cell Scanning Transmission Electron Microscopy. Nano Lett. 20, 1944–1951 (2020). Chen-Wiegart, Y. C. K. et al. In situ imaging of dealloying during nanoporous gold formation by transmission X-ray microscopy. Acta Mater. 61, 1118–1125 (2013). Corcoran, S. G., Wiesler, D. G. & Sieradzki, K. An in Situ Small Angle Neutron Scattering Investigation of Ag0.7Au0.3 Dealloying Under Potential Control. MRS Proc. 451, 93 (1996). Gault, B., Moody, M. P., Cairney, J. M. & Ringer, imon P. Atom Probe Microscopy. vol. 160 (Springer New York, 2012). El-Zoka, A. A. et al. Enabling near-atomic–scale analysis of frozen water. Sci. Adv. 6, eabd6324 (2020). Stephenson, L. T. et al. The LaplacE project: An integrated suite for preparing and transferring atom probe samples under cryogenic and UHV conditions. PLoS One (2018) doi:10.1371/journal.pone.0209211. Figure 1

Analyzing porous (nano)materials via atom probe tomography has been notoriously difficult. Voids and pores act as concentrators of the electrostatic pressure, which results in premature specimen failure, and the electrostatic field distribution near voids leads to aberrations that are difficult to predict. In this study, we propose a new encapsulating method for porous samples using a low melting point Bi–In–Sn alloy, known as Field's metal. As a model material, we used porous iron made by direct-hydrogen reduction of single-crystalline wüstite. The complete encapsulation was performed using in situ heating on the stage of a scanning electron microscope. No visible corrosion nor dissolution of the sample occurred. Subsequently, specimens were shaped by focused ion-beam milling under cryogenic conditions at −190°C. The proposed approach is versatile and can be applied to provide good quality atom probe datasets from micro/nanoporous materials.

Imaging of liquids and cryogenic biological materials by electron microscopy has been recently enabled by innovative approaches for specimen preparation and the fast development of optimised instruments for cryo-enabled electron microscopy (cryo-EM). Yet, Cryo-EM typically lacks advanced analytical capabilities, in particular for light elements. With the development of protocols for frozen wet specimen preparation, atom probe tomography (APT) could advantageously complement insights gained by cryo-EM. Here, we report on different approaches that have been recently proposed to enable the analysis of relatively large volumes of frozen liquids from either a flat substrate or the fractured surface of a wire. Both allowed for analysing water ice layers which are several microns thick consisting of pure water, pure heavy-water and aqueous solutions. We discuss the merits of both approaches, and prospects for further developments in this area. Preliminary results raise numerous questions, in part concerning the physics underpinning field evaporation. We discuss these aspects and lay out some of the challenges regarding the APT analysis of frozen liquids., Comment: submitted for publication assocaited to the APT&M 2020 conference

Nanoporous gold (NPG), formed by the chemical or electrochemical dealloying of binary or ternary solid solutions, has strong prospects as a catalyst, sensor substrate, membrane, actuator, and other applications. In view of the wide range of thermochemical conditions expected, understanding the evolution of NPG during thermal coarsening is necessary to assess and improve the prospects for its functionality. The addition of Pt to NPG (NPG-Pt) was shown in recent years to have an effect of inhibition of coarsening, both during dealloying and during oxidizing post-treatment. In this study, the direct observation of the coarsening of nanoligaments by simultaneous transmitted electron (TE) and secondary electron (SE) imaging reveals the complex nature of the coarsening process. As a result of adding Pt, alterations in the volume fraction of ligaments are shown to have a strong impact on the operating coarsening mechanism and kinetics in a low-pressure hydrogen environment. Pt was also shown to increase the temperature for the onset of thermal coarsening. The important roles of surface diffusion and the possibility of coalescence-controlled coarsening kinetics are discussed in detail. Reproducible observations of ligament collapse and void annihilation serve to expand the current understanding of thermal coarsening of NPG.

The effect of Pt on the structural evolution of nanoporous gold (NPG) is complex and is controlled by processes including secondary dealloying of Ag, surface diffusion of Au, and Pt segregation. High-resolution studies are carried out to investigate the interplay of the inherent processes. Observations such as Pt segregation at nanoligament surfaces, core-shell structures of nanoligaments, and Ag retention inside individual nanoligaments are seen at unprecedented detail using high resolution characterization techniques, including Atom Probe Tomography (APT) and Electron Energy Loss Spectroscopy (EELS). Quantitative estimates of the surface area-to-volume ratios of nanoporous layers with different Pt content indicate their relative functionalities for surface area driven applications.

With 1.85 billion tons produced per year, steel is the most important material class in terms of volume and environmental impact. While steel is a sustainability enabler, for instance through lightweight design, magnetic devices, and efficient turbines, its primary production is not. For 3000 years, iron has been reduced from ores using carbon. Today 2.1 tons CO2 are produced per ton of steel, causing 30% of the global CO2 emissions in the manufacturing sector, which translates to 6.5% of the global CO2 emissions. These numbers qualify iron- and steel-making as the largest single industrial greenhouse gas emission source. The envisaged future industrial route to mitigate these CO2 emissions targets green hydrogen as a reductant. Although this reaction has been studied for decades, its kinetics is not well understood, particularly during the wustite reduction step which is dramatically slower than the hematite reduction. Many rate-limiting factors of this reaction are set by the micro- and nanostructure as well as the local chemistry of the ores. Their quantification allows knowledge-driven ore preparation and process optimization to make the hydrogen-based reduction of iron ores commercially viable, enabling the required massive CO2 mitigation to ease global warming. Here, we report on a multi-scale structure and composition analysis of iron reduced from hematite with pure H2, reaching down to near-atomic scale.

We propose a protocol for near-atomic–scale analysis of frozen water and embedded nanostructures by cryo–atom probe tomography., Transmission electron microscopy went through a revolution enabling routine cryo-imaging of biological and (bio)chemical systems, in liquid form. Yet, these approaches typically lack advanced analytical capabilities. Here, we used atom probe tomography to analyze frozen liquids in three dimensions with subnanometer resolution. We introduce a specimen preparation strategy using nanoporous gold. We report data on 2- to 3-μm-thick layers of ice formed from both high-purity deuterated water and a solution of 50 mM NaCl in high-purity deuterated water. The analysis of the gold-ice interface reveals a substantial increase in the solute concentrations across the interface. We explore a range of experimental parameters to show that atom probe analyses of bulk aqueous specimens come with their own challenges and discuss physical processes that produce the observed phenomena. Our study demonstrates the viability of using frozen water as a carrier for near-atomic–scale analysis of objects in solution by atom probe tomography.

Steel is the most important material class in terms of volume and environmental impact. While it is a sustainability enabler, for instance through lightweight design, magnetic devices, and efficient turbines, its primary production is not. Iron is reduced from ores by carbon, causing 30% of the global CO2 emissions in manufacturing, qualifying it as the largest single industrial greenhouse gas emission source. Hydrogen is thus attractive as alternative reductant. Although this reaction has been studied for decades, its kinetics is not well understood, particularly during the wustite reduction step which is much slower than hematite reduction. Some rate-limiting factors of this reaction are determined by the microstructure and local chemistry of the ores. Here, we report on a multi-scale structure and composition analysis of iron reduced from hematite with pure H2, reaching down to near-atomic scale. During reduction a complex pore- and microstructure evolves, due to oxygen loss and non-volume conserving phase transformations. The microstructure after reduction is an aggregate of nearly pure iron crystals, containing inherited and acquired pores and cracks. We observe several types of lattice defects that accelerate mass transport as well as several chemical impurities (Na, Mg, Ti, V) within the Fe in the form of oxide islands that were not reduced. With this study, we aim to open the perspective in the field of carbon-neutral iron production from macroscopic processing towards better understanding of the underlying microscopic transport and reduction mechanisms and kinetics.

Correction for ‘Inverse heavy-atom effect in near infrared photoluminescent gold nanoclusters’ by Goutam Pramanik et al., Nanoscale, 2021, DOI: 10.1039/d1nr02440j.

Recent atom-probe tomography (APT) and ATEM studies of dealloyed layers in binary AgAu and ternary AgAuPt alloys showed the various ways in which more-noble elements enrich during dealloying, at the atomic scale (1). We have since extended this research in several directions – towards less expensive alloys, lean in more-noble elements, and towards the potential applications of such materials in gas sensing. In this presentation, we will start with a review of published and unpublished data from the APT study, which was done using alloys with 23 at% (Au + Pt). New information will then be shown for alloys with less than 6 at.% (Au + Pt). The dealloyed layers are very Ag-rich, and very interesting conflicts arise between enrichment of the more-noble element(s) on the ligament surfaces and the ultimate supply of those elements. For gas sensing applications, we take advantage of the effect of adsorption on ligament surfaces on electron scattering during conduction in the confined metal phase. Delicate impedance measurements at the milli-ohm level show that two kinds of environmental effect can be distinguished. Adsorption of water or other gases up to several molecular layers induce changes in the measured resistance of the metal phase, while thicker layers induce a double-layer charge separation leading to capacitive changes.

Dealloying of Ag-Au alloys is known to produce Au-rich, bicontinous, nanoporous layers [1-2]. The high functionality of nanoporous gold, demonstrated in a variety of applications [3-4], invites us to study the fabrication of other nanoporous metals of interest by exploiting corrosion processes. Recent studies have shown how the use of ruthenium, which is the cheapest Pt-group metal, for catalysis could be expanded from the classical example of CO oxidation [5], to other important reactions such as hydrogen evolution [6]. Although several porous/non-porous Ru-containing systems have been studied before [7-8], there remains a knowledge-gap with regards to the design and optimization of Ru-based catalysts. As a first step towards the development of nanoporous ruthenium by metal corrosion, this work shall focus on the design of binary transition metal-Ru systems. High resolution APT (atom probe tomography) studies of Ru-lean alloys (Fe-Ru and Ni-Ru) will assess the bulk microstructure of as-cast alloys and how it is affected by processing conditions [9]. Furthermore, pathways to inducing Ru-surface enrichment in different thermochemical environments (oxidative/inert/reductive) will be investigated by combined APT and XPS (X-ray photoelectron spectroscopy) [10]. The results of this study will provide guidance to making low-cost, high surface area nanoporous ruthenium catalysts by dealloying in acidic media. References [1] H. W. Pickering and P. R. Swann. Electron metallography of chemical attack upon some alloys susceptible to stress corrosion cracking, Corrosion, 1963, 19, 373t. [2] R.C. Newman. Dealloying, Shreir's Corrosion (fourth ed.), 2, Elsevier (2010), 801-809. [3] A.A. Vega, R.C. Newman. Methanol electro-oxidation on nanoporous metals formed by dealloying of Ag–Au–Pt alloys, Journal of Applied Electrochemistry, 2016, 46, 995-1010. [4] Y. Xue, J. Markmann, H. Duan, J. Weissmüller and Patrick Huber. Switchable imbibition in nanoporous gold, Nature Communications, 2014, 5, 4237. [5] N.W. Cant, P.C. Hicks and B.S. Lennon. Steady-state oxidation of carbon monoxide over supported noble metals with particular reference to platinum, Journal of Catalysis, 1978, 54, 372-383. [6] B. Lu, L. Guo, F. Wu, Y. Peng, J.E. Lu, T.J. Smart, N. Wang, Y.Z. Finfrock, D. Morris, P. Zhang, N. Li, P. Gao, Y. Ping and S. Chen. Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media, Nature Communications, 2019, 10, 631. [7] T. Li, P.A.J. Bagot, E.A. Marquis, S.C.E. Tsang and G.D.W. Smith. Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh, Journal of Physical Chemistry C, 2012, 116, 17633–17640. [8] M. Hakamada, J. Motomura, F. Hirashima and M. Mabuchi. Preparation of nanoporous ruthenium catalyst and Its CO oxidation characteristics, Materials Transactions, 53, 2012, 524-530. [9] L. Li, Z. Li, A.K. da Silva, Z. Peng, H. Zhao, B. Gault, D. Raabe. Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy, Acta Materialia, 2019, 178, 1-9. [10] K. Schweinar, O. Kasian, R.L. Nicholls, C.R. Rajamathi, P. Zeller, M. Amati, L. Gregoratti, D. Raabe, M. Greiner, and B. Gault. An integrated workflow to investigate electrocatalytic surfaces by correlative X-ray Photoemission Spectroscopy, Scanning Photoemission Electron Microscopy and Atom Probe Tomography, Microscopy and Microanalysis, 2019, 25, 306–307.

Nanoporous gold (NPG) is usually made by electrochemical dealloying of Ag from binary AgAu alloys. The resulting nanoscale ligaments are not very stable, and tend to coarsen with time by surface self-diffusion, especially in electrolyte, which may lead to inferior electrocatalytic properties. Addition of a small amount of Pt to the precursor alloy is known to refine and stabilize the nanoporous product (NPG-Pt). However, the mechanisms by which Pt serves to refine the microstructure remain poorly understood. The present study aims to expand our knowledge of the role of Pt by examining NPG-Pt at atomic resolution with Atom Probe Tomography (APT), as well as by aberration-corrected Transmission Electron Microscopy. Atomic level observation of Pt enrichment on ligament surfaces sheds light on the underlying mechanisms that give rise to Pt's refining effect. Owing to improved Ag retention with higher Pt content, NPG-Pt1 (made by dealloying Ag77Au22Pt1) was shown to have the highest surface area-to-volume ratio, compared to NPG-Pt3 (made by dealloying Ag77Au20Pt3). Quantitative estimates reveal up to 5-fold enrichment of Pt at nanoligament surfaces, compared to the precursor content, in NPG-Pt. The interface between the dealloyed layer and the substrate was captured by APT, for the first time. The findings of this investigation add insight into the functionality of NPG-Pt and its prospective catalytic performance.

Dealloying of binary AgAu and ternary AgAuPt alloys has been proven to yield nanoporous metals (NPG and NPG-Pt) that are promising candidates as catalysts and smart materials. The alloy is exposed to electrolytic conditions at which it is favorable for one metal to dissolve, leaving behind a nanoporous network that is enriched in the other more noble metal(s) [1-2]. The structural stability, and catalytic activity of nanoporous metals are mainly affected by aspects including structural evolution, surface area stability and nanoscale composition. Achieving a deeper realization of the mechanisms that govern the nanoscale behavior in this material at such small scale is vital for pushing forward this class of materials to industrial applications. Thus, high-resolution characterization could be used to improve the already existing insights into the formation and tunability of nanoporous metals. Atom probe tomography (APT) of as-dealloyed and thermally coarsened NPG/NPG-Pt was enabled by inner-pore potentiostatic deposition of Cu (Fig 1) [3]. The recently demonstrated ability of complete infiltration of NPG/NPG-Pt with Cu brings attention to the peculiar electrochemical properties of NPG that are worth further consideration. A previous investigation by Lee et al. [4] on the electrodeposition of Cu onto nanoporous Pt, made by dealloying of CuPt alloys, revealed a sub-potential deposition regime that lies between the UPD (underpotential deposition) and the equilibrium deposition regimes. The sub-potential deposition regime was attributed to the curvature of the nanoporous Pt surface. We report a deeper understanding of the complete infiltration of NPG and NPG-Pt with potentiostatically electrodeposited Cu from acidic solutions of copper sulfate. The main parameters controlling the extent of filling are shown to include ligament /pore sizes, the concentration of Cu2+ ions in solution and the electrochemical parameters of the deposition. The crystallographic structure of Cu and its relation to that of the NPG substrate is also studied at high resolution. The infiltration of NPG and NPG-Pt at significant depths serves to highlight their interesting electroactive properties, and the great prospects that lie in using NPG as a template for growth of 3D nanostructures. Fig 1. Schematic of the electrochemical process by which NPG samples are formed and prepared for APT sample fabrication. From bulk AgAuPt alloy (a) selective dissolution of Ag creates a dealloyed layer of NPG-Pt (b). Consolidation of this open-cell network for APT sample preparation is then achieved by electrodeposition of Cu (c). References [1] R.C. Newman, “Dealloying,” in Shreir’s Corrosion (4th ed.), Elsevier, vol. 2, pp. 801– 809, 2010. [2] J. Erlebacher, M.J. Aziz, A. Karma, N. Dimitrov, and Karl Sieradzki, “Evolution of nanoporosity in dealloying,” Nat., vol. 410, pp. 450-453, 2001. [3] A.A. El-Zoka, B. Langelier, G.A. Botton, and R.C. Newman, “Enhanced analysis of nanoporous gold by atom probe tomography,” Mater. Charact., vol. 128, pp. 269-277, 2017. [4] L. Lee, D. He, A. G. Carcea, and R. C. Newman, “Exploring the reactivity and nanoscale morphology of de-alloyed layers,” Corros. Sci., vol. 49, pp. 72–80, 2007. Figure 1

Nanoporous gold (NPG) is usually fabricated by the selective electrolytic dissolution (dealloying) of Ag from a binary Ag-Au alloy, or sometimes from a ternary alloy such as Ag-Au-Pt. The resulting ligament-pore structure is bicontinuous and has an extraordinarily high surface area-to-volume ratio, posing it as a promising candidate for catalyst applications [1-2]. The ligament size can be as small as 3-5 nm. While a lot of recent research has been focusing on the structure/property relationships of NPG as a functional material in its own right [2-5], the further use of NPG as a sophisticated template for fabricating complex nanostructured materials is worthy of attention. Figure 1 - SEM image of FIB (focused ion beam) cross-section showing the dealloyed layer after deposition of Cu. [5] Recent work showing atomic-scale characterization of NPG by electrodeposition of a fully compact Cu support opens the door to implementing that newly developed method to the creation of finely tuned nanostructures using NPG as a template [5]. Fabrication of nanostructures/nanosurfaces pertaining to the interests of the catalysis community will be demonstrated through the electrodeposition of metals, such as Cu and Co, and metal oxides, such as Cr2O3 and MoO2, on NPG. Furthermore, notable characteristics of NPG and the electrodeposition methodology that enables such compact deposits have been investigated experimentally and theoretically through reaction-transport modelling. A key aspect is the use of thin nanoporous layers, such that the characteristic diffusion time through the layer is short, in the ms range, but there are other subtleties. Electron diffraction and X-ray diffraction reveal the crystalline structure and orientation of the deposits and the associated growth mechanism. For metal deposition within a nanoporous material, not only ordinary thermodynamics have to be considered, but also the “subpotential” curvature-driven deposition described by Lee et al. [6]. References [1] R C. Newman, “Dealloying,” in Shreir’s Corrosion (4th ed.), Elsevier, vol. 2, pp. 801–809, 2010. [2] A.A. Vega, and R.C. Newman, “Nanoporous metals fabricated through electrochemical dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio,” Journal of the Electrochemical Society, vol. 161, pp. 1-10, 2014. [3] M. Yan, T. Jin, Q. Chen, H. E. Ho, T. Fujita, L. Y. Chen, M. Bao, M. W. Chen, N. Asao, and Y. Yamamoto, “Unsupported nanoporous gold catalyst for highly selective hydrogenation of quinolines,” Organic Letters, vol. 15, pp. 1484–1487, 2013. [4] A.A. El-Zoka, J. Howe, R.C. Newman, S. Dogel, M. Reynolds, H. Hosseinkhannazer, and D.D. Perovic, “Understanding the coarsening behaviors of nanoporous gold via in situ heating,” Microscopy and Microanalysis, vol. 22, pp. 1968-1969, 2016. [5] A.A. El-Zoka, B. Langelier, G.A. Botton, and R.C. Newman, “Enhanced analysis of nanoporous gold by atom probe tomography,” Materials Characterization, In Press, Available online 10 March 2017. [6] L. Lee, D. He, A.G. Carcea, and R.C. Newman, “Exploring the reactivity and nanoscale morphology of de-alloyed layers,” Corrosion Science, vol. 49, pp. 72-80, 2007. Figure 1

The high resolution characterization of nanoporous gold (NPG), a material with promising functional capabilities, facilitates better understanding of its structure-property relationships. An improved characterization of NPG by atom probe tomography (APT) was recently reported, using Cu electroplating to fill the pores [1]. Atomic scale analysis of nanoligaments showed the expected core-shell structure in high detail (Figure 1), confirming and extending previous findings using other characterization methods [2]. Further to those findings, finer, more complex nanoporous materials, formed by dealloying of AgAuPt alloys, have been studied by APT as well as STEM tomography, demonstrating an ever clearer 3D picture of the structure. As the extent of aberrations found in the data is directly related to the relative evaporation fields of the metals in the sample [3], further improvement of this technique prompts the search for alternative metallic fillers. Figure 1- (a) Atom map of a single nanoligament in NPG formed by dealloying of Ag77Au23, (b) Chemical profiling across a single ligament. References [1] Ayman A. El-Zoka., Doug D. Perovic, Roger C. Newman, and Brian Langelier. "High Resolution Studies of Dealloyed Layers." In Meeting Abstracts, The Electrochemical Society, vol. 12, pp. 1259-1259, 2016. [2] 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 and M. Chen, "Atomic origins of the high catalytic activity of nanoporous gold," Nature Materials, vol. 11, pp. 775-780, 2012. [3] T.T Tsong, “Field ion formation,” Surf. Sci., vol. 70, pp. 211-233, 1978. Figure 1

The nanoscale morphology of dealloyed materials has been studied for many years, from Pickering and Swann [1], through A.J. Forty [2], to recent TEM and atom-probe studies [3, 4]. Modern instruments have the ability to reveal key features of dealloying at unprecedented resolution, as well as having facilities for heating and environment control. Our particular interest is the dealloying behaviour of ternary alloys, such as AgAuPt, following the recent work of Vega [5]. Another area of interest is stress corrosion cracking of such materials, which features film-induced brittle events [6, 7]. Progress will be reported in several areas, including – High-resolution ATEM studies of dealloyed binary and ternary alloys , to observe and account for the distributions of Ag, Au and Pt for various dealloying conditions (binary AgPt is included in this part of the study). In situ heating studies of dealloyed binary and ternary alloys , to gain more insight into the observations of Vega [8] regarding oxygen-induced surface segregation of Pt, and other ways to manipulate the surface composition of the ligaments within the nanoporous material. Conventional underpotential deposition and novel “sub”-potential deposition of bulk Cu , as shown by Lee et al. for deposition of Cu into dealloyed CuPt [9]. Pore-filling of dealloyed materials by electrodeposition , often believed to be impossible, but actually a relatively easy method, provided the conditions are controlled very precisely and one is only dealing with a surface layer. Naturally this method is more challenging for ternary than binary alloys, owing to the smaller pore size. Copper is the initial metal of choice for pore filling. Atom-probe tomography studies are in progress, as favourable sites within the sample can be chosen for tip fabrication, even when pore filling is uneven. Initial results show good promise as a way to determine definitively the elemental distributions. References [1] H. W. Pickering and P. R. Swann. Electron metallography of chemical attack upon some alloys susceptible to stress corrosion cracking, Corrosion, 1963, 19, 373t. [2] A. J. Forty. Corrosion micro-morphology of noble-metal alloys and depletion gilding, Nature, 1979, 282, 597. [3] B. Pfeiffer, T. Erichsen, E. Epler, C. A. Volkert, P. Trompenaars, and C. Nowak. Characterization of nanoporous materials with atom probe tomography, Microscopy and Microanalysis, 2015, 21, 557-563. [4] 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 and M. Chen. Atomic origins of the high catalytic activity of nanoporous gold, Nature Materials, 2012, 11, 775–780. [5] A. A. Vega and R. C. Newman. Nanoporous metals fabricated through electrochemical dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio. Journal of the Electrochemical Society, 2014, 161, C1-C10. [6] Andrew Barnes, N.A. Senior and R. C. Newman. Film-induced cleavage of Ag-Au alloys. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 2009, 40, 58-68. [7] S. Sun, X. Chen, N. Badwe and K. Sieradzki. Potential-dependent dynamic fracture of nanoporous gold. Nature Materials, 2015, 14, 894-898. [8] A. A. Vega and R. C. Newman. Beneficial effects of adsorbate-induced surface segregation of Pt in nanoporous metals fabricated by dealloying of Ag-Au-Pt alloys. Journal of the Electrochemical Society, 2014, 161, C11-C19. [9] L. Lee, D. He, A.G. Carcea and R.C. Newman. Exploring the reactivity and nanoscale morphology of de-alloyed layers. Corrosion Science, 2007, 49, 72–80.