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Icosahedral Platinum Alloy Nanocrystals with Enhanced Electrocatalytic Activities
Abstract
This communication describes the synthesis of Pt-M (M = Au, Ni, Pd) icosahedral nanocrystals based on the gas reducing agent in liquid solution method. Both CO gas and organic surface capping agents play critical roles in stabilizing the icosahedral shape with {111} surfaces. Among the Pt-M alloy icosahedral nanocrystals generated, Pt(3)Ni had an impressive ORR specific activity of 1.83 mA/cm(2)(Pt) and 0.62 A/mg(Pt). Our results further show that the area-specific activity of icosahedral Pt(3)Ni catalysts was about 50% higher than that of the octahedral Pt(3)Ni catalysts (1.26 mA/cm(2)(Pt)), even though both shapes are bound by {111} facets. Density functional theory calculations and molecular dynamics simulations indicate that this improvement may arise from strain-induced electronic effects.
... 69 It has been found that the optimum compression is between 2 and 3%. 70,71 Wu et al. 72 found that the specific activity of Pt 3 Ni icosahedra, with a tensile-stressed surface, was 1.5 times higher than that of Pt 3 Ni octahedra of similar size, despite both having a surface covered by {111} facets. Thus, the use of nanocrystals with both {111} facets and twin defects on their surfaces can further enhance the specific activity of an ORR catalyst. ...
The annual increase in demand for renewable energy is driving the development of catalysis-based technologies that generate, store and convert clean energy by splitting and forming chemical bonds. Thanks to efforts over the last two decades, great progress has been made in the use of core-shell nanostructures to improve the performance of metallic catalysts. The successful preparation and application of a large number of bimetallic core-shell nanocrystals demonstrates the wide range of possibilities they offer and suggests further advances in this field. Here, we have reviewed recent advances in the synthesis and study of core-shell nanostructures that are promising for catalysis. Particular attention has been paid to the structural tuning of the catalytic properties of core-shell nanostructures and to theoretical methods capable of describing their catalytic properties in order to efficiently search for new catalysts with desired properties. We have also identified the most promising areas of research in this field, in terms of experimental and theoretical studies, and in terms of promising materials to be studied.
... In response to the growing demand for clean energy in both daily life and industry, signi cant attention has been directed toward proton exchange membrane fuel cells (PEMFCs) characterized by the imperative for secure, reliable, renewable, and sustainable energy source [1][2][3][4][5][6][7][8][9][10] . PEMFCs stand out as a promising alternative energy solution due to their advantageous attributes, including straightforward operation, high e ciency, environmental friendliness, and excellent stability [11][12][13][14][15][16] . Furthermore, the substantial conversion of chemical energy into electrical energy at a notably high e ciency renders PEMFCs suitable for diverse applications such as distributed generation, transportation power, and portable power for numerous electronic devices [17][18][19][20][21][22][23][24] . ...
Maximizing catalyst activity and stability while minimizing costs remains a formidable challenge. In this study, we employed the straightforward and easily executed ethylene glycol reduction method to synthesize highly active and stable Pt-Ni alloy catalysts, utilizing SBA15-modified carbon as the supporting material. Subsequent meticulous examinations delved into their physicochemical properties and electrocatalytic activities.Transmission electron microscopy (TEM) analyses unveiled a uniform distribution of PtNi particles on the support, showcasing a narrow particle size distribution centered around approximately 1.91 nm with minimal aggregation. Electrochemical assessments demonstrated that Pt3Ni/SBA15-C outperforms Pt/C, exhibiting 50 and 150 mV higher half-wave potentials (E 1/2 ) and onset potential (E onse t), respectively. Furthermore, our meticulously prepared Pt3Ni/SBA15-C, featuring a cage structure, displayed remarkable stability while sustaining superior catalytic durability under an applied potential of + 0.7 V. These findings underscore the effectiveness of the cage structure catalyst, comprising porous nanoparticles, in ensuring both catalytic activity and stability. The results collectively contribute to advancing our understanding of catalyst design and performance optimization in electrochemical applications.
... The last decades have witnessed the preparation of PtPd alloy with different shapes such as wires [11][12][13], icosahedral [14], tetrahedral [15], dendrites [16], clusters [17], octahedral [18] and cubes [19]. For example, Zhang and his co-workers prepared PtPd alloys with selective shapes by a one pot solvothermal method [20]. ...
This study demonstrated a rapid method of producing PtPd via chemical reduction by varying metal composition and types of capping agents. The effect of different compositions between the two-metal precursor, Pt and Pd, is invested by tuning at various volume ratios of 3:1, 1:3, and 1:1 and comparing them with their single counterparts. The FESEM results show that all prepared samples exhibit nanosponges structure. PtPd of 1:1 ratio has the highest ECSA of 14.25 m2 /g, while PtPd with PVP exhibits a high ECSA value of 692.7 m2 /g compared to those without capping agent (287.8 m2 /g), indicating large active sites, which increases its catalytic function.
... These results are consistent with the theoretical calculations reported in literature. [34][35][36] The atomic-resolution TEM images of icosahedral and octahedral nanocrystals can be found in several publications, further confirming our statement regarding the strains in Pd nanocrystals. [32,37] The nanocrystals were converted from Pd to PdH x by treatment with N 2 H 4 . ...
We report for the first time that Pd nanocrystals can absorb H via a “single‐phase pathway” when particles with a proper combination of shape and size are used. Specifically, when Pd icosahedral nanocrystals of 7‐ and 12‐nm in size are exposed to H atoms, the H‐saturated twin boundaries can divide each particle into 20 smaller single‐crystal units in which the formation of phase boundaries is no longer favored. As such, absorption of H atoms is dominated by the single‐phase pathway and one can readily obtain PdHx with anyx in the range of 0–0.7. When switched to Pd octahedral nanocrystals, the single‐phase pathway is only observed for particles of 7 nm in size. We also establish that the H‐absorption kinetics will be accelerated if there is a tensile strain in the nanocrystals due to the increase in lattice spacing. Besides the unique H‐absorption behaviors, the PdHx (x=0–0.7) icosahedral nanocrystals show remarkable thermal and catalytic stability toward the formic acid oxidation due tothe decrease in chemical potential for H atoms in a Pd lattice under tensile strain.
... Furthermore, the investigation of the ORR activities of PtM (M ¼ Au, Ni, Pd) alloy icosahedra demonstrated that the ORR activity of Pt 3 Ni icosahedra was 5% higher than that of their octahedra counterparts. 36 Although multiply twinned NCs showed promising performances for specific electrocatalytic reactions due to their modified surface strain, the tensile surface strain in these NCs is intrinsically bound to their crystal structure and cannot be finely tuned. This limitation calls for novel approaches that can enable the adjustment of both tensile and compressive strain on the surface of NCs. ...
Since the clean energy industry emerged, developing efficient nanocrystal catalysts has attracted ever-increasing attention. Recently, the utilization of metal nanocrystals as catalysts for electrochemical reactions is entering a new era with the development of theories and techniques that help incorporate surface chemistry into nanoscale materials. Current approaches in the field of nanocrystal catalysts include detailed analyses and modifications of the surface atoms of nanocrystals, with which optimal structures and compositions for target electrochemical reactions could be realized. This review presents two major strategies to engineer the surface structure of nanocrystals: control over the atomic arrangement and composition of nanocrystal surfaces. The first section mainly covers the modification of surface atom arrangements with various methods, including the induction of various facets, strains, and defects. The generation of anomalous crystal structures of nanocrystals is also discussed. The second section encompasses recent advances in controlling the composition of nanocrystal surfaces by bringing high entropy or periodicity to the metal elements in nanocrystals to attain high electrocatalytic activity and stability.
... Pure Pt nanoparticles are normally stable in the single crystal structure, whereas Pt alloy nanoparticles can exist in multiply-twinned structures such as an icosahedron [15][16][17][18], although the shape of Pt nanoparticles can be controlled by capping materials [19]. It is thus crucial to understand the characteristics of structure motifs, such as icosahedron and octahedron. ...
CoPt nanoparticle catalysts are integral to commercial fuel cells. Such systems are prohibitive to fully characterize with electronic structure calculations. Machine-learned potentials offer a scalable solution; however, such potentials are only reliable if representative training data can be employed, which typically requires large electronic structure calculations. Here, we use the nearsighted-force training approach to make high-fidelity machine-learned predictions on large nanoparticles with $>$5,000 atoms using only systematically generated small structures ranging from 38-168 atoms. The resulting ensemble model shows good accuracy and transferability in describing relative energetics for CoPt nanoparticles with various shapes, sizes and Co compositions. It is found that the fcc(100) surface is more likely to form a L1$_0$ ordered structure than the fcc(111) surface. The energy convex hull of the icosahedron shows the most stable particles have Pt-rich skins and Co-rich underlayers. Although the truncated octahedron is the most stable shape across all sizes of Pt nanoparticles, a crossover to icosahedron exists due to a large downshift of surface energy for CoPt nanoparticle alloys. The downshift can be attributed to strain release on the icosahedron surface due to Co alloying. We introduced a simple empirical model to describe the role of Co alloying in the crossover for CoPt nanoparticles. With Monte-Carlo simulations we additionally searched for the most stable atomic arrangement for a truncated octahedron with equal Pt and Co compositions, and also we studied its order-disorder phase transition. We validated the most stable configurations with a new highly scalable density functional theory code called SPARC. Lastly, the order-disorder phase transition for a CoPt nanoparticle exhibits a lower transition temperature and a smoother transition, compared to the bulk CoPt alloy.
... Through deliberately controlling the experimental parameters, plenty of well-defined Pt-based polyhedrons with improved performance and durability have been prepared successfully. However, most Pt-based polyhedrons have large sizes, sometimes larger than 10 nm [142,150,158,175], resulting in a low Pt utilization efficiency. Synthesizing 2À5 nm well-defined Pt-based polyhedrons is still a challenge. ...
... By changing the surface atomic distances, the catalysts' surface electronic structure can be modulated, resulting in tunable reaction intermediates energetics. [1][2][3][4] The strain engineering approaches have been exploited to metal electrocatalysts for a range of reactions, [5,6] including the hydrogen evolution, [7] oxygen reduction, [8,9] CO 2 reduction [10,11] and other reactions. [12,13] Therefore, understanding the correlation between the surface strain and catalytic reactivity is of fundamental importance for the development of highly efficient catalysts. ...
Tuning the surface strain of heterogeneous catalysts is recognized as a powerful strategy for tailoring their catalytic activity. However, a clear understanding of the strain effect in electrocatalysis at single‐particle resolution is still lacking. Here, we explore the electrochemical hydrogen evolution reaction (HER) of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facet and similar sizes using scanning electrochemical cell microscopy (SECCM). It is revealed that tensilely strained Pd icosahedra display significantly superior HER electrocatalytic activity. The estimated turnover frequency at −0.87 V vs RHE on Pd icosahedra is about two times higher than that on Pd octahedra. Our single‐particle electrochemistry study using SECCM at Pd nanocrystals unambiguously highlights the importance of tensile strain on electrocatalytic activity and may offer new strategy for understanding the fundamental relationship between surface strain and reactivity.
... A simple explanation for these discrepancies is the presence of undercoordinated atoms at the nanocatalyst surface. Indeed, molecular dynamics simulations by Wu et al. showed that undercoordinated atoms are intrinsically stabilized by inward displacement (compression) while atoms located at the facets probably undergo outward displacement (tensile strain) for both icosahedral and cuboctahedral Pt nanostructures 36 . Other BCDI studies on noble metal nanocrystals have shown that compression is mainly localized at vertices and edges, with weaker strain on flat surfaces, while the bulk remains relatively strain free following adsorption [37][38][39] . ...
Surface strain is widely employed in gas phase catalysis and electrocatalysis to control the binding energies of adsorbates on active sites. However, in situ or operando strain measurements are experimentally challenging, especially on nanomaterials. Here we exploit coherent diffraction at the new fourth-generation Extremely Brilliant Source of the European Synchrotron Radiation Facility to map and quantify strain within individual Pt catalyst nanoparticles under electrochemical control. Three-dimensional nanoresolution strain microscopy, together with density functional theory and atomistic simulations, show evidence of heterogeneous and potential-dependent strain distribution between highly coordinated ({100} and {111} facets) and undercoordinated atoms (edges and corners), as well as evidence of strain propagation from the surface to the bulk of the nanoparticle. These dynamic structural relationships directly inform the design of strain-engineered nanocatalysts for energy storage and conversion applications.
... It is generally believed that ∆E O is determined by the d-band center position (calculated with respect to the Fermi level) of Pt/Pd-based ORR catalysts [24,25]. Modulating the position of the d-band center plays a decisive role in improving catalytic properties, and the shift of the d-band center can be achieved by the ligand effect and strain effect [26][27][28]; the former can effectively change the density of states near the Fermi level, while the latter can adjust the energy of the d-band center. The combination of experiment and theory is essential to explain the formation and dissociation of intermediates and the process of the reaction, and to determine the active site and the rate-determining step of the reaction, which is important to optimize the catalyst activity and develop a high-efficiency catalyst. ...
The oxygen reduction reaction (ORR) is one of the key catalytic reactions for hydrogen fuel cells, biofuel cells and metal–air cells. However, due to the complex four-electron catalytic process, the kinetics of the oxygen reduction reaction are sluggish. Platinum group metal (PGM) catalysts represented by platinum and palladium are considered to be the most active ORR catalysts. However, the price and reserves of Pt/Pd are major concerns and issues for their commercial application. Improving the catalytic performance of PGM catalysts can effectively reduce their loading and material cost in a catalytic system, and they will be more economical and practical. In this review, we introduce the kinetics and mechanisms of Pt/Pd-based catalysts for the ORR, summarize the main factors affecting the catalytic performance of PGMs, and discuss the recent progress of Pt/Pd-based catalysts. In addition, the remaining challenges and future prospects in the design and improvement of Pt/Pd-based catalysts of the ORR are also discussed.
... Platinum may be a good electro-catalyst for the electro-oxidation reaction of methanol (MOR); however, its high cost and easy poisoning by reaction intermediates are the key limitations to its application. Platinum-based bimetallic alloys (such as PtAu, PtCo, PtFe, and PtNi) are considered the substitute catalysts of monometallic platinum, which is attributed to their excellent electrocatalytic properties and lower price [18][19][20]. The electrocatalytic performances of Pt-based alloys are related to their inner and surface structures and compositions [21,22]. ...
Sub-nanosized PtAu particles within three-dimensional carbon materials were obtained via a mercaptosilane-assisted preparation method. This strategy can effectively control the size of PtAu particles while avoiding the use of an additional carbon precursor. The as-synthesized three-dimensional carbon (3D carbon) material possesses excellent properties compared to other carbon materials. PtAu particles on three-dimensional carbon (PtAu/3D carbon) exhibited superior activities for methanol oxidation and hydrogen evolution reactions compared to Pt/3D carbon and a commercial Pt/Carbon (Pt/C) catalyst. Specifically, the methanol peak current density on PtAu/3D carbon was almost 2.3 times higher than that of Pt/3D carbon and 1.9 times higher than that of commercial Pt/C. The Tafel slopes of PtAu/3D carbon, Pt/3D carbon, and the commercial Pt/C were approximately 112, 124, and 106 mV dec−1, respectively, demonstrating that electrochemical desorption is the rate-limiting step in the hydrogen evolution reaction of the as-synthesized catalysts.
... 9,11,[21][22][23][24][25][26] This is especially promising given recent advances in core-shell nanoparticle synthesis and nano-heterostructure synthesis through deposition, allowing for strain control to be achieved in high surface area materials systems which are ideal for catalytic applications. [27][28][29][30] By breaking these scaling relations, including strain as a degree of freedom in catalyst design significantly increases the complexity of an already high dimensional space that intrinsically already covers the catalyst structure and composition, the surface facet, the adsorption site, and the adsorbate composition. ...
Modifying the adsorption energies of reaction intermediates on different material surfaces can significantly improve heterogeneous catalysis by reducing energy barriers for intermediate elementary reaction steps. Surface strain can increase or decrease the adsorption energy depending on the surface composition, adsorbate composition, surface facet, and adsorbate site, breaking traditional scaling relationships which inhibit energy barrier alteration in reactions such as ammonia synthesis. We aim to generate a model that maps the adsorption energy response to a given input strain for a range of adsorbates and catalyst structures. After generating a training dataset of strained copper binary alloy catalyst + adsorbate complexes from the Open Catalyst Project and calculating the adsorption energy with first-principles calculations (dataset made available), we train a graph neural network to learn the relationship between catalyst + adsorbate structure, surface strain, and adsorption energy. The model successfully predicts the nature of the adsorption energy response for 85% of surface strains, outperforming simpler model baselines. Using the ammonia synthesis reaction as an example system, we identify Cu-S alloy catalysts as promising candidates for strain engineering since the majority of surface strain patterns raise the adsorption energy of the *NH intermediate. We find that the strain response of similar adsorbates on the same surface can greatly vary due to the competition between surface relaxation under strain and relaxation of the coordination environment. Our presented machine learning approach can be applied to additional datasets to identify target strain patterns that can reduce energy barriers in heterogeneous catalysis.
... However, their low oxygen reduction reaction (ORR) efficiency, limited service time, and high cost are the major challenges that restrict their use in commercial applications [2][3][4][5][6][7]. Current platinum-based catalysts can efficiently catalyze acidic ORR [8], and although it has been demonstrated that platinum-based catalysts are optimal ORR catalysts [9], their high cost and low stability are still of great concern [10]. Many studies have been conducted to address these drawbacks by developing alternative materials ...
Platinum-based catalysts are widely used for efficient catalysis of the acidic oxygen reduction reaction (ORR). However, the agglomeration and leaching of metallic Pt nanoparticles limit the catalytic activity and durability of the catalysts and restrict their large-scale commercialization. Therefore, this study aimed to achieve a uniform distribution and strong anchoring of Pt nanoparticles on a carbon support and improve the ORR activity and durability of proton-exchange membrane fuel cells. Herein, we report on the facile one-pot synthesis of a novel ORR catalyst using metal–nitrogen–carbon (M–N–C) bonding, which is formed in situ during the ion exchange and pyrolysis processes. An ion-exchange resin was used as the carbon source containing R-N+(CH3)3 groups, which coordinate with PtCl62− to form nanosized Pt clusters confined within the macroporous framework. After pyrolysis, strong M-N-C bonds were formed, thereby preventing the leaching and aggregation of Pt nanoparticles. The as-synthesized Pt supported on the N-doped hierarchically porous carbon catalyst (Pt/NHPC-800) showed high specific activity (0.3 mA cm−2) and mass activity (0.165 A mgPt−1), which are approximately 2.7 and 1.5 times higher than those of commercial Pt/C, respectively. The electrochemical surface area of Pt/NHPC-800 remained unchanged (~1% loss) after an accelerated durability test of 10,000 cycles. The mass activity loss after ADT of Pt/NHPC-800 was 18%, which is considerably lower than that of commercial Pt/C (43%). Thus, a novel ORR catalyst with highly accessible and homogeneously dispersed Pt-N-C sites, high activity, and durability was successfully prepared via one-pot synthesis. This facile and scalable synthesis strategy for high-efficiency catalysts guides the further synthesis of commercially available ORR catalysts.
... Atomic Force Microscopy (AFM) images of the defective crystals showed a change in the expression of its crystal planes: an increase in {100}:{110} facet size ratio was observed ( Fig. 4a and Supplementary Figs. 9, 10). This suggested a change in relative plane energies upon defect engineering, which we attribute to strain relaxation of the external surface terminations 45 . These findings are supported by DFT results, which show that an exchange of imidazole linker with pyrrole is thermodynamically most favorable on the high energy cut of the {100} plane (Supplementary Table 3 To inspect whether defect linker incorporation can be linked to surface energies, we measured a 1 × 1 µm 2 hyperspectral image of a defective crystal in N 2 . ...
Many catalytic processes depend on the sorption and conversion of gaseous molecules on the surface of (porous) functional materials. These events often preferentially occur on specific, undercoordinated, external surface sites. Here we show the combination of in situ Photo-induced Force Microscopy (PiFM) with Density Functional Theory (DFT) calculations to study the site-specific sorption and conversion of formaldehyde on the external surfaces of well-defined faceted ZIF-8 microcrystals with nanoscale resolution. We observed preferential adsorption of formaldehyde on high index planes. Moreover, in situ PiFM allowed us to visualize unsaturated nanodomains within extended external crystal planes, showing enhanced sorption behavior on the nanoscale. Additionally, on defective ZIF-8 crystals, structure sensitive conversion of formaldehyde through a methoxy- and a formate mechanism mediated by Lewis acidity was found. Strikingly, sorption and conversion were influenced more by the external surface termination than by the concentration of defects. DFT calculations showed that this is due to the presence of specific atomic arrangements on high-index crystal surfaces. With this research, we showcase the high potential of in situ PiFM for structure sensitivity studies on porous functional materials.
... [76] It has been reported the ORR activity of Pt 3 Ni icosahedral nanocrystals with a large tensile strain induced by multiple twin boundaries is about 50 % higher than that of the Pt 3 Ni octahedral nanocrystals, even though they are both with exposed {111} facets. [77] The IMNCs with similar size, composition, and structure but different exposed crystal facets, for example, intermetallic cubes with exposed {100} facets, and octahedra with exposed {111} facets, still lack successful achievement. To address this problem, seeds with specific morphologies including cubes, octahedra, and icosahedra are highly desirable to be utilized to prepare cubic, octahedral, and icosahedral IMNCs. ...
Throwing the seed: This review comprehensively summarizes the preparation of intermetallic nanocrystals (IMNCs) by three approaches of seed‐mediated synthesis: seed‐mediated growth, seed‐mediated transformation, and seed‐mediated co‐reduction. The precise control over the size, shape, composition, structure and exposed crystal facet of IMNCs by seed‐mediated synthesis and their widespread applications in electrocatalytic reduction reactions are summarized. After that, the current challenges associated with the controllable synthesis of IMNCs and conclude the article with perspectives are addressed (DOI: 10.1002/chem.202202221).
... Strain has been suggested as a promising strategy to break these scaling relationships by changing the surface bonding environment (19,20), and there are multiple experimental observations indicating that strain can effectively manipulate catalystadsorbate interactions and modify catalyst activity across different reactions (9,11,(21)(22)(23)(24)(25)(26). This is especially promising given recent advances in core-shell nanoparticle synthesis and nano-heterostructure synthesis through deposition, allowing for strain control to be achieved in high-surface area systems ideal for catalytic applications (27)(28)(29)(30). By breaking these scaling relations, including strain as a degree of freedom in catalyst design greatly increases the complexity of an already high-dimensional search space that includes the catalyst structure and composition, the surface facet, the adsorption site, and the adsorbate composition. ...
Small-molecule adsorption energies correlate with energy barriers of catalyzed intermediate reaction steps, determining the dominant microkinetic mechanism. Straining the catalyst can alter adsorption energies and break scaling relationships that inhibit reaction engineering, but identifying desirable strain patterns using density functional theory is intractable because of the high-dimensional search space. We train a graph neural network to predict the adsorption energy response of a catalyst/adsorbate system under a proposed surface strain pattern. The training data are generated by randomly straining and relaxing Cu-based binary alloy catalyst complexes taken from the Open Catalyst Project. The trained model successfully predicts the adsorption energy response for 85% of strains in unseen test data, outperforming ensemble linear baselines. Using ammonia synthesis as an example, we identify Cu-S alloy catalysts as promising candidates for strain engineering. Our approach can locate strain patterns that break adsorption energy scaling relations to improve catalyst performance.
... and there have been multiple experimental observations indicating that strain can effectively manipulate catalyst-adsorbate interactions and modify catalyst activity across different reactions. [159,161,[171][172][173][174][175][176] This is especially promising given recent advances in core-shell nanoparticle synthesis and nano-heterostructure synthesis through deposition, allowing for strain control to be achieved in high surface area materials systems which are ideal for catalytic applications.[177][178][179][180] By breaking these scaling relations, including strain as a degree of freedom in catalyst design significantly increases the complexity of an already high dimensional space that intrinsically already covers the catalyst structure and composition, the surface facet, the adsorption site, and the adsorbate composition. ...
Crucial thrusts in modern technology from electronic information processing to engineering cellular systems require manipulation and control of materials on smaller and smaller scales to succeed. A simple and successful way to break conventional material property limitations or design multifunctional devices is to interface two different materials together. At small length scales, the surface to bulk ratio of each component material increases, to the point that the interfacial physics can dominate the properties of the engineered system. Simultaneously, the combinatorial space of possible interfaces between materials and/or molecules is far too vast to explore by trial-and-error experimentation alone. Intuitive theoretical models can greatly improve our ability to navigate such large search spaces by providing insight on how two materials are likely to interact. The goal of this thesis is to develop predictive physical models which explain emergent phenomena at material interfaces across multiple length and time scales. A variety of state-of-the-art tools were applied to realize this goal, including analytical mathematics, quantum mechanical simulations, finite element methods, and deep neural networks. At the electron scale, a continuum model parametrized by first-principles simulations was employed to develop design criteria for confined quantum states in lateral heterostructures of two-dimensional materials. At the atomic scale, a chemo-mechanical model incorporating long-range electrostatics was developed to explain synthesizability trends in composite heterostructures of inorganic perovskites and organic molecules. A machine learning graph neural network model was developed and applied to predict the impact of general surface strains on the adsorption energy of small molecule intermediates on catalyst surfaces. Finally, at the microscale, a nonlinear kinetic model was developed to explain how cells acquire and retain memory of the mechanical properties of their surroundings across multiple timescales, which can lead to irreversible adaptation and differentiation. The methods and results presented in this thesis can improve our understanding of physical phenomena arising at interfaces and provide a blueprint for future applications of multiscale computational modeling to science and engineering problems.
... Interfaces and surfaces play a crucial role in determining many material functionalities and characterizing the strain or structural distortions at high spatial and temporal times is essential for developing a fundamental understanding [2] [3]. For example, surface strain can regulate and control surface diffusion processes and can change the chemical reactivity of a surface, such as, enhancement of oxygen reduction activity [4], [5], catalytic properties (light-off temperature and attainable activity) [6]- [8], adsorption energy at the surface [9] strain-induced corrosion [10] etc. In reducible oxides, point defects such as oxygen vacancies distort/strain the cation sub-lattice and influence surface properties such as reactivity, and structural stability. ...
Surface strain often controls properties of the material including charge transport and chemical reactivity. Localized surface strain is measured with atomic resolution on (111) ceria nanoparticle surfaces using environmental transmission electron microscopy under different redox conditions. Density Functional Theory (DFT) coupled with TEM image simulations have been used for aid in interpreting the experimental data. Oxygen vacancy creation/annihilation introduces strain at surface and near surface regions on cation sublattice. Static and fluxional strainmaps are generated from images at these different conditions and compared. While fluxional strain is highest at locations associated with unstable vacancies at active sites, highly inhomogeneous static strain fields comprising of alternating tensile/compressing strain is seen at surface and subsurfaces linked to the presence of stable oxygen vacancies. Interestingly, both stable and unstable oxygen vacancies are found within a few atomic spacing of each other on the same surface. The static strain pattern depends on the ambient inside TEM. Oxidizing environments tend to lower vacancy concentrations at the surface whereas a highly reducing environment created using high electron dose creates oxygen vacancies everywhere (bulk and surfaces) in the nanoparticle.
... Nanocatalysts with exposed (1 1 1)-degree faces, such as those generated by Yang and colleagues, have been shown to have four times the mass activity of commercial Pt/C and 1.8 times the mass activity of regular Pt 3 Ni octahedrons, respectively [166]. After developing a method for generating uniform icosahedral nanocrys-tals of different PtM alloys (M = Au, Ni, and Pd), the scientists demonstrated that icosahedral Pt3Ni demonstrates increased ORR activity [167]. Because Pt 3 Ni nanocrystals can exist in either an octahedral or an icosahedral structure, one possible explanation for the improvement in ORR performance is the presence of elastic strain. ...
Metal-air batteries (MABs) and fuel cells (FCs) critically rely on electrocatalytic O2 activation, and O2 reduction reaction (ORR), with noble metal-free materials. However, the inception of their synergist reactivity is still unclear due to several electronic and structural limitations. Therefore, the correlation between their science and engineering and their experimental as well as theoretical activity descriptors can pave the way for the development of novel cheap, and efficient catalysts. Moreover, with this framework, several volcanic correlations were established, indicating that catalyst activity increases linearly with increasing binding energy of ORR intermediates up to a certain point, but after that, the activity decreases as binding energy increases. The motivation of this review is to highlight (i) recent designs and developments on non-noble-metal-containing electrocatalysts for ORR, (ii) correlations between science and engineering and existing activity descriptors to improve the electrocatalyst’s ORR performance, and (iii) prospects and challenges with non-noble-metal-based electrocatalysts. The “science and engineering” of the electrode materials discussed in this review will aid researchers in selecting and designing ORR electrocatalysts for energy conversion processes.
Proton exchange membrane fuel cells (PEMFCs) have been identified as a highly promising means of achieving sustainable energy conversion. A crucial factor in enhancing the performance of PEMFCs for further potential energy applications is the advancement in the field of catalyst engineering that has led to remarkable performance enhancement in facilitating the oxygen reduction reaction (ORR). Subsequently, it is important to acknowledge that the techniques used in preparation of membrane electrode assemblies (MEAs), the vital constituents of PEMFCs, also possess direct and critical influence on exhibiting the full catalytic activity of meticulously crafted catalysts. Here, a succinct summary of the most recent advancements in Pt catalysts for ORR was offered and their underly catalytic mechanism were discussed. Then, both laboratory-scale and industrial-scale MEA fabrication techniques of Pt catalysts were summarized. Furthermore, a detailed analysis of the connections between materials, process, and performance in MEA fabrication was presented in order to facilitate the development of optimal catalyst layers.
The atomic structure and chemical ordering of Ni-Pt nanoalloys of different sizes and shapes are studied by numerical simulations using Monte Carlo methods and a realistic interatomic potential. The bulk Ni-Pt ordering tendency remains in fcc nanoparticles but we show some chemical ordering frustrations linked to surface reconstructions depending on the cluster size and shape. A reversed temperature dependence of Pt surface segregation is also established. In the particular case of fivefold symmetry as in icosahedra, ordering is observed in the core and on the facets at low temperatures with segregation of the smaller element (Ni) in the core because of atomic strain. We show that the icosahedral shape favors Pt surface segregation in comparison with octahedral and truncated octahedral structures.
The electrocatalytic conversion of carbon dioxide (CO2) into valuable carbon-based compounds has attracted considerable attention. In the quest for efficient electrocatalysts, strain engineering, characterized by localized relative deformation, emerges as...
Shape-controlled metal-based nanomaterials provide a special platform to regulate the surface physicochemical properties for optimizing the electrocatalytic performance, due to their homogeneously arranged surface atomic structures. Accordingly, owing to the solely {100} facets-ended surfaces, highly uniform Pt nanocubes (NCs) are utilized in this study, to probe the effects of synergizing with Ir species (a very effective metal for various electrocatalysis) on the electrocatalytic performance for the oxygen reduction reaction (ORR). Electrochemical results present that carbon-supported Pt1Ir1 NCs with surface Ir–O species (Pt1Ir1–O NCs/C) have much enhanced electrocatalytic activity and electrochemical stability than both Pt NCs/C and commercial Pt/C. Physicochemical characterization and theoretical calculation analyses further elucidate the mechanism of the improved electrocatalytic performance: the strong electronic interaction between Pt and Ir, the lowered d-band center of the surface metal atoms, and especially the synergistic effect induced by the surface Ir–O species. This study not only develops a highly active and stable ORR electrocatalyst, but also demonstrates an effective and universal research paradigm to explore the surface engineering effects on the electrocatalytic performance, which can be further applied in other crystalline facets for other (electro)catalytic reactions.
Noble metal‐engineered catalysts (NMECs) play an important role in promoting the practical utilization of water‐splitting devices in hydrogen energy systems. While owing to the complicated catalytic centers, diverse support structures, and changing microenvironments, NMECs still face many challenges when it comes to designing and analyzing the precise catalytic sites and activity‐mechanism analyses, which are crucial for their future developments. Here, this cutting‐edge review systematically discusses recent advancements in designing NMECs for water electrolysis, including the structure evolution, microenvironment modulation, structure‐reactivity correlation, and new horizons. First, the fundamental advantages, mechanisms, and evaluation methods of NMECs for water splitting are outlined. Then, the strategies to modulate the catalytic microenvironments of NMECs are thoroughly summarized, such as crystal phase modulation, alloying effects, crystallization degrees, size effects, and substrate effects. In particular, there are valuable perspectives on bond interactions, theoretical calculations, and evaluation methods to disclose the catalytic mechanisms. Thereafter, a special emphasis is given on structure‐reactivity correlation, performances, and water‐splitting devices. Finally, a thorough discussion of the upcoming difficulties and new directions for developing next‐generation NMECs is presented. It is believed that the review will have a significant influence on creating noble metal‐based catalysts in the field of electrolytic water‐splitting.
We report for the first time that Pd nanocrystals can absorb H via a “single‐phase pathway” when particles with a proper combination of shape and size are used. Specifically, when Pd icosahedral nanocrystals of 7‐ and 12‐nm in size are exposed to H atoms, the H‐saturated twin boundaries can divide each particle into 20 smaller single‐crystal units in which the formation of phase boundaries is no longer favored. As such, absorption of H atoms is dominated by the single‐phase pathway and one can readily obtain PdHx with any x in the range of 0−0.7. When switched to Pd octahedral nanocrystals, the single‐phase pathway is only observed for particles of 7 nm in size. We also establish that the H‐absorption kinetics will be accelerated if there is a tensile strain in the nanocrystals due to the increase in lattice spacing. Besides the unique H‐absorption behaviors, the PdHx (x = 0–0.7) icosahedral nanocrystals show remarkable thermal and catalytic stability toward the formic acid oxidation because of the decrease in chemical potential for H atoms in a Pd lattice under tensile strain.
Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochemical energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphology control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addition to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.
The unique catalytic activity of small nanoparticles can be attributed to their distinctive electronic structure and/or their ability to expose sites with a unique geometry. Quantifying and distinguishing the contributions of these effects to catalytic performance presents a challenge, given the complexity arising from multiple influencing factors and the lack of a quantitative structure-activity relationship. Here, we show that the intrinsic activity of platinum atom at the perimeter corner sites is three orders of magnitude higher as a result of an electronic structure effect, with a threshold occurring at an average nanoparticle size of about 1.25 nm. The contributions to the activity of atomically dispersed platinum, large nanoparticles and sodium-induced support modification are minor. This comprehensive and quantitative structure-activity correlation was demonstrated on real-world Pt/CeO 2 catalysts for the water-gas shift reaction by utilizing operando X-ray photoelectron spectroscopy, electron microscopy, theoretical modeling, and kinetic models.
Formate is a promising hydrogen carrier for safe storage and transport and a fuel for direct formate fuel cells. However, the sluggish kinetics of catalysts for formate dehydrogenation (FDH) and oxidation reactions (FORs) significantly limit the potential applications of formate. Strain effects can effectively modulate catalytic properties by altering the electronic structure. Nevertheless, the lack of theoretical principles to quantify atomic strain and its effects on FDH and FOR catalytic activity has made experimental efforts laborious. In this work, we establish a database of atomic strain distributions for AgPd nanoalloys, reveals that the presence of compressive strain at the edges and corners and compressive strain exerted on the surface of Ag@Pd nanoalloys, particularly the one with an icosahedral shape, boost the FDH and FOR catalytic activity by shifting down the d-band center, thus weakening the adsorption of key intermediate Had. This study provides a theoretical perspective on the development and use of formate as a hydrogen carrier and fuel.
Tuning the surface strain of heterogeneous catalysts is recognized as a powerful strategy for tailoring their catalytic activity. However, a clear understanding of the strain effect in electrocatalysis at single‐particle resolution is still lacking. Here, we explore the electrochemical hydrogen evolution reaction (HER) of single Pd octahedra and icosahedra with the same surface bounded {111} crystal facet and similar sizes using scanning electrochemical cell microscopy (SECCM). It is revealed that tensilely strained Pd icosahedra display significantly superior HER electrocatalytic activity. The estimated turnover frequency at −0.87 V vs RHE on Pd icosahedra is about two times higher than that on Pd octahedra. Our single‐particle electrochemistry study using SECCM at Pd nanocrystals unambiguously highlights the importance of tensile strain on electrocatalytic activity and may offer new strategy for understanding the fundamental relationship between surface strain and reactivity.
Pt-based alloy nanoparticles have broad application prospects as cathode catalyst materials for proton-exchange membrane fuel cells (PEMFCs). Optimization of the oxygen adsorption energy is crucial to boost the performance of oxygen reduction catalysis. We successfully synthesized well-dispersed Pt1.2Ni tetrahedra and obtained the Pt1.2Ni/C catalyst adopting the one-pot synthetic protocol, which exhibits superb activity and good long-term stability for oxygen reduction reaction (ORR), achieving a mass activity of 1.53 A/mgPt at 0.90 VRHE, which is 12 times higher than that of commercial Pt/C. On combining X-ray photoelectron spectroscopy and density functional theory calculations, abundant water is adsorbed stably on the Pt1.2Ni alloy surface. We find that the intense interaction between the adsorbed O atom and adsorbed water can weaken the adsorption of oxygen, contributing to the ORR performance.
Catalyst activity affects the reaction rate, and an increasing number of studies have shown that strain can significantly increase the electrocatalytic activity. Catalysts such as alloys and core-shell structures can modulate their properties through strain effects. Reasonable simulation techniques can be used to predict and design the catalytic performance based on understanding the strain action mechanism. Therefore, the methodological flow of theoretical simulations is summarised in this review. The mechanism underlying the strain-adsorption-reaction relationship is discussed using density functional theory (DFT) calculations. An introduction to DFT is given first, followed by a quick rundown of the strain classification and application. Typical electrocatalytic reactions, namely, the hydrogen and oxygen evolution reactions and oxygen reduction reaction, are taken as examples. After briefly explaining these reactions, the relevant studies on simulating the strain to tune the catalyst performance are covered. The simulation methods are summarised and analysed to observe the effects of strain on electrocatalytic properties. Finally, a summary of the issues with simulated strain-assisted design and a discussion on the perspectives and forecasts for the future design of effective catalysts are provided.
Although the unique properties of nanomaterials have endowed enzyme-mimic catalysts with broad applications, the development of catalysts still relies on trial-and-error strategies without predictive indicators. Surface electronic structures have rarely been studied in enzyme-mimic catalysts. Herein, we present a platform for understanding the impact of surface electronic structures on electrocatalysis toward H2O2 decomposition, using the Pd icosahedra (Pd ico), Pd octahedra (Pd oct) and Pd cubic nanocrystals as electrocatalysts. The electronic properties on Pd were modulated with a correlation of surface orientation. We revealed the relationship between the electronic properties and electrocatalytic performance, in which the surface electron accumulation can boost the electrocatalytic activity of the enzyme-mimic catalysts. As a result, the Pd icodimer exhibits the highest electrocatalytic and sensing efficiency. This work offers new perspectives for the investigation of structure-activity relationships and provides an effective knob for utilizing the surface electronic structures to boost the catalytic performance for enzyme-mimics.
The development of ordered Pt-based intermetallic compounds is an effective way to optimize the electronic characteristics of Pt and its disordered alloys, inhibit the loss of transition metal elements, and prepare fuel cell catalysts with high activity and long-term durability for the oxygen reduction reaction (ORR). This paper reviews the structure–activity characteristics, research advances, problems, and improvements in Pt-based intermetallic compound fuel cell catalysts for the ORR. First, the structural characteristics and performance advantages of Pt-based intermetallic compounds are analyzed and explained. Second, starting with 3d transition metals such as Fe, Co, and Ni, whose research achievements are common, the preparation process and properties of Pt-based intermetallic compound catalysts for the ORR are introduced in detail according to element types. Third, in view of preparation problems, improvements in the preparation processes of Pt-based intermetallic compounds are also summarized in regard to four aspects: coating to control the crystal size, doping to promote ordering transformation, constructing a “Pt skin” to improve performance, and anchoring and confinement to enhance the interaction between the crystal and support. Finally, by analyzing the research status of Pt-based intermetallic compound catalysts for the ORR, prospective research directions are suggested.
Currently, the rarity and high cost of platinum (Pt)-based electrocatalysts seriously limit their commercial application in fuel cells cathode. Decorating Pt with atomically dispersed metal-nitrogen sites possibly offers an effective pathway to synergy tailor their catalytic activity and stability. Here active and stable oxygen reduction reaction (ORR) electrocatalysts (Pt3 Ni@Ni-N4 -C) by in situ loading Pt3 Ni nanocages with Pt skin on single-atom nickel-nitrogen (Ni-N4 ) embedded carbon supports are designed and constructed. The Pt3 Ni@Ni-N4 -C exhibits excellent mass activity (MA) of 1.92 A mgPt -1 and specific activity of 2.65 mA cmPt -2 , together with superior durability of 10 mV decay in half-wave potential and only 2.1% loss in MA after 30 000 cycles. Theoretical calculations demonstrate that Ni-N4 sites significant redistribute of electrons and make them transfer from both the adjacent carbon and Pt atoms to the Ni-N4 . The resultant electron accumulation region successfully anchored Pt3 Ni, that not only improves structural stability of the Pt3 Ni, but importantly makes the surface Pt more positive to weaken the adsorption of *OH to enhance ORR activity. This strategy lays the groundwork for the development of super effective and durable Pt-based ORR catalysts.
Designing shape-controlled Pt-based core-shell nanocrystals is a prospective strategy to maximize the utilization of Pt while maintaining high activity for oxygen reduction reaction (ORR). However, the core-shell structures with ultrathin Pt shell exhibit limited electrochemical durability. Therefore, a thicker shell is proposed to successfully improve the durability of the core-shell structures by preventing the core from dissolution. Nevertheless, the deposition of Pt tends to switch to the Stranski-Krastanov (S-K) growth mode with the increase of the number of layer, resulting in the absence of a conformal morphology. Herein, we realize the deposition of three-to-five-layer epitaxial Pt-Co layers on Pd octahedral seeds by introducing tensile strain in the epitaxial layer to impede the S-K growth. The as-obtained Pd@Pt-Co octahedra with four layers exhibit enhanced mass activity (0.69 A/mgPt) and specific activity (1.00 mA/cm2) for ORR, which are 4.93 and 5 times that of the commercial Pt/C, respectively. Furthermore, it shows only 17% decay for specific activity after a 30,000-cycle durability test. This work is expected to enlighten the design and synthesis of related core-shell nanocrystals with facetted multicomponent shells, offering a promising strategy for designing cost-effective and efficient catalysts.
Revealing the nucleation and evolution processes of nanocrystals is of great importance for their application as catalytic materials, the performance of which is closely related to their surface and interface...
Proton exchange membrane fuel cells (PEMFCs) have attracted extensive attention because of their high efficiency, environmental friendliness, and lack of noise pollution. However, PEMFCs still face many difficulties in practical application, such as insufficient power density, high cost, and poor durability. The main reason for these difficulties is the slow oxygen reduction reaction (ORR) on the cathode due to the insufficient stability and catalytic activity of the catalyst. Therefore, it is very important to develop advanced platinum (Pt)-based catalysts to realize low Pt loads and long-term operation of membrane electrode assembly (MEA) modules to improve the performance of PEMFC. At present, the research on PEMFC has mainly been focused on two areas: Pt-based catalysts and the structural design of catalytic layers. This review focused on the latest research progress of the controllable preparation of Pt-based ORR catalysts and structural design of catalytic layers in PEMFC. Firstly, the design principle of advanced Pt-based catalysts was introduced. Secondly, the controllable preparation of catalyst structure, morphology, composition and support, and their influence on catalytic activity of ORR and overall performance of PEMFC, were discussed. Thirdly, the effects of optimizing the structure of the catalytic layer (CL) on the performance of MEA were analyzed. Finally, the challenges and prospects of Pt-based catalysts and catalytic layer design were discussed.
Developing low-loading Pt-based catalysts possessing glorious catalytic performance can accelerate oxygen reduction reaction (ORR) and hence significantly advance the commercialization of proton exchange membrane fuel cells. In this report, we propose a hybrid catalyst that consists of low-loading sub-3 nm PtCo intermetallic nanoparticles carried on Co-N-C (PtCo/Co-N-C) via the microwave-assisted polyol procedure and subsequent heat treatment. Atomically dispersed Co atoms embedded in the Co-N-C carriers diffuse into the lattice of Pt, thus forming ultrasmall PtCo intermetallic nanoparticles. Owing to the dual effect of the enhanced metal-support interaction and alloy effect, as-fabricated PtCo/Co-N-C catalysts deliver an extraordinary performance, achieving a half-wave potential of 0.921 V, a mass activity of 0.700 A mgPt-1@0.9 V, and brilliant durability in the acidic medium. The fuel cell employing PtCo/Co-N-C as the cathode catalyst with an ultralow Pt loading of 0.05 mg cm-2 exhibits an impressive peak power density of 0.700 W cm-2, higher than that of commercial Pt/C under the same condition. Furthermore, the enhanced intrinsic ORR activity and stability are imputed to the downshifted d-band center and the strengthened metal-support interaction, as revealed by density functional theory calculations. This report affords a facile tactic to fabricate Pt-based alloy composite catalysts, which is also applicable to other alloy catalysts.
Morphology-controlled Pt-based catalysts were synthesized by a solvothermal method and were used as oxygen reduction reaction (ORR) catalysts in proton exchange membrane fuel cells (PEMFCs). A mild acid treatment combined with electrochemical de-alloying was used to develop Pt-skinned octahedral PtCu/C-X catalysts. PtCu/C exhibited a mass activity (MA) of 0.694 A mgPt⁻¹@0.9 V and specific activity (SA) of 2.242 mA cm⁻², respectively. After 30000 potential cycles, the MA and SA of this catalyst were enhanced by 22% to 0.849 A mgPt⁻¹ and 110% to 4.697 mA cm⁻², respectively. It is hypothesized that the rearrangement of surface Pt atoms in electrochemical de-alloying effectively filled the defects formed by acid treatment and enhanced the activity of the catalyst accordingly.
Hydrogen peroxide (H2O2) formation rates in a proton exchange membrane fuel cell (PEMFC) anode and cathode were estimated as a function of humidity and temperature by studying the oxygen reduction reaction (ORR) on a rotating ring disk electrode. Fuel cell conditions were replicated by depositing a film of Pt/Vulcan XC-72 catalyst onto the disk and by varying the temperature, dissolved O-2 concentration, and the acidity levels in hydrochloric acid (HClO4). The HClO4 acidity was correlated to ionomer water activity and hence fuel cell humidity. The H2O2 formation rates showed a linear dependence on oxygen concentration and square dependence on water activity. The H2O2 selectivity in ORR was independent of oxygen concentration but increased with the decrease in water activity (i.e., decreased humidity). Potential dependent activation energy for the H2O2 formation reaction was estimated from data obtained at different temperatures. (c) 2007 The Electrochemical Society.
The growth of free silver nanoclusters is investigated by molecular-dynamics simulations up to sizes close to N=600 atoms on realistic time scales, and in a temperature range from 400 to 650 K. At low and intermediate temperatures, we grow mainly noncrystalline structures, as icosahedra and decahedra. In particular, at N>200, we obtain that perfectly ordered metastable icosahedra are very likely grown: either by a shell-by-shell mode on a small-size stable icosahedron, or by a complete structural transformation from a decahedron to a metastable icosahedron. The latter mechanism can explain why large silver icosahedra are more abundant than large decahedra in experiments. At high temperatures, crystalline fcc clusters are very frequently grown.
Although the platinum-gold (Pt-Au) phase diagram shows a wide miscibility gap due to limited mutual solubility of the components, small particles (<3 nm) form homogeneous alloys because all atoms retain their atomic electronic structure, and rehybridisation due to band formation does not take place. Supported Pt-Au catalysts are often superior to those containing Pt alone for low-temperature selective oxidations.
Silver nanocrystals were synthesized by reducing AgNO3 in N,N-dimethylformamide (DMF) solution. Besides decahedral and icosahedral nanoparticles, a series of their intermediate particles, which consist of a combination of two and more tetrahedra, are obtained. It was found that decahedral and icosahedral nanoparticles are not formed through assembling of tetrahedra formed separately but produced through the stepwise growth of tetrahedral units on specific facets in DMF. A simple combination model of tetrahedral units suggested that the growth position of the fourth tetrahedral unit determines whether a decahedron or icosahedron is finally produced. In the formation of icosahedron, the crystal growth occurs inside of decahedral units. No further growth from decahedron to icosahedron was observed, indicating that there is a large energy barrier for the addition of a tetrahedron unit to a decahedron. Our study gives new information on the stepwise growth mechanism of decahedra and icosahedra in DMF solution.
We have successfully controlled the shape of gold nanocrystals through a simple and low-cost hydrothermal method based on a modified polyol process. Well-defined gold nanocrystals of icosahedral shape were synthesized in high yields by the rapid reduction of gold precursors with ethylene glycol (EG) in the presence of poly(vinyl pyrrolidone) (PVP) under hydrothermal conditions for 1 h. Truncated icosahedra (football-shaped) have been prepared for the first time by prolonging the reaction time to 4 h. Both nanocrystal shapes were obtained quantitatively. Addition of citric acid inhibits the shape-change process (from icosahedron to truncated icosahedron) by blocking oxidative etching, while addition of Fe(III) facilitates the shape-change process by enhancing oxidative etching. We propose that growth of truncated icosahedra can be induced and maintained through interplay of the following processes: generation of multiple twinned seeds, shape- and size-focusing by Ostwald ripening, and oxidative etching and preferential growth on the {100} face.
The energetics of nanoclusters is investigated for five different metals Ag, Cu, Au, Pd, and Pt by means of quenched molecular dynamics simulations. Results are obtained for two different semiempirical potentials. Three different structural motifs are considered: icosahedra Ih, decahedra Dh, and truncated octahedra TO. The crossover sizes among structural motifs are directly calculated, considering cluster up to sizes N40 000. For all the systems considered, it is found that icosahedra are favored at small sizes, decahedra at intermediate sizes, and truncated octahedra at large sizes. However, the crossover sizes depend strongly on the metal: in Cu, the icosahedral interval is rather large, and it is followed by a very wide decahedral window; on the contrary, in Au, the icosahedral interval is practically absent, and the decahedral window is narrow. The other metals display intermediate behaviors, Ag being close to Cu, and Pd and Pt being close to Au. A simple criterion, which is based on the ratio between the bulk modulus and the cohesive energy per atom, is developed to account for the differences among the metals. © 2002 American Institute of Physics.
Using high temperature CO oxidation as the example, trends in the reactivity of transition metals are discussed on the basis
of density functional theory (DFT) calculations. Volcano type relations between the catalytic rate and adsorption energies
of important intermediates are introduced and the effect of adsorbate–adsorbate interaction on the trends is discussed. We
find that adsorbate–adsorbate interactions significantly increase the activity of strong binding metals (left side of the
volcano) but the interactions do not change the relative activity of different metals and have a very small influence on the
position of the top of the volcano, that is, on which metal is the best catalyst.
KeywordsHeterogeneous catalysis-Coverage effects-Adsorbate–adsorbate interactions-CO oxidation-Volcano plots
Bimetallic PtAu heteronanostructures have been synthesized from Pt-on-Au nanoparticles, which were made from platinum acetylacetonate
and gold nanoparticles. Using the Pt-on-Au nanoparticles as precursors, Ptsurface rich PtAu bimetallic heteronanostructures
can be produced through controlled thermal treatments, as confirmed by field emission high-resolution transmission electron
microscopy (HR-TEM) and elemental mapping using a high-angle annular dark-field scanning transmission electron microscope
(HAADF-STEM). Oxidation of formic acid was used as a model reaction to demonstrate the effects of varying composition and
surface structure on the catalytic performance of PtAu bimetallic nanostructures. Cyclic voltammetry (CV) showed that these
carbon-supported PtAu heteronanostructures were much more active than platinum in catalyzing the oxidation of formic acid,
judging by the mass current density. The results showed that postsynthesis modification can be a very useful approach to the
control of composition distributions in alloy nanostructures.
KeywordsNanostructure-alloy-platinum-gold-formic acid oxidation-electrocatalyst-polymer electrolyte membrane fuel cell (PEMFC)
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
The structural properties of free nanoclusters are reviewed. Special
attention is paid to the interplay of energetic, thermodynamic, and
kinetic factors in the explanation of cluster structures that are
actually observed in experiments. The review starts with a brief
summary of the experimental methods for the production of free nanoclusters
and then considers theoretical and simulation issues, always discussed
in close connection with the experimental results. The energetic
properties are treated first, along with methods for modeling elementary
constituent interactions and for global optimization on the cluster
potential-energy surface. After that, a section on cluster thermodynamics
follows. The discussion includes the analysis of solid-solid structural
transitions and of melting, with its size dependence. The last section
is devoted to the growth kinetics of free nanoclusters and treats
the growth of isolated clusters and their coalescence. Several specific
systems are analyzed.
The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V.
In many previous studies, nonaqueous synthesis of Pt nanocubes with tunable size has been achieved by the use of metal carbonyls (e.g., Fe(CO)(5), Co(2)(CO)(8), W(CO)(6)). The presence of zero-valent metals in the carbonyls was demonstrated as the key factor to the nanocube formation but the role of CO was entirely ignored. By using CO alone, we have now demonstrated that the favorable growth of Pt nanocubes in the presence of CO is mainly owing to the effect that the Pt (100) surface is stabilized by the co-adsorption of CO and amine.
Electrocatalysis will play a key role in future energy conversion and storage technologies, such as water electrolysers, fuel cells and metal-air batteries. Molecular interactions between chemical reactants and the catalytic surface control the activity and efficiency, and hence need to be optimized; however, generalized experimental strategies to do so are scarce. Here we show how lattice strain can be used experimentally to tune the catalytic activity of dealloyed bimetallic nanoparticles for the oxygen-reduction reaction, a key barrier to the application of fuel cells and metal-air batteries. We demonstrate the core-shell structure of the catalyst and clarify the mechanistic origin of its activity. The platinum-rich shell exhibits compressive strain, which results in a shift of the electronic band structure of platinum and weakening chemisorption of oxygenated species. We combine synthesis, measurements and an understanding of strain from theory to generate a reactivity-strain relationship that provides guidelines for tuning electrocatalytic activity.
We demonstrate that the sequence of reactants plays a key factor in the final size of Au nanocrystals. The growth mechanism markedly differs with seed size and/or internal crystallinity. Conversely to what could be expected when the multiple-twinned particle (MTP) seeds are rather large (3.5 nm) they control the nanocrystal growth. When the single domain seeds are very small (1.5 nm) the majority of nanocrystals produced are large icosahedral (85%) nanoparticles as predicted from a theoretical model.
Controlling the morphology of Pt nanostructures can provide a great opportunity to improve their catalytic properties and
increase their activity on a mass basis. We synthesized Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches
on a Pd core by reducing K2PtCl4 with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution. The Pt branches supported on
faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets toward the oxygen reduction
reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell. The Pd-Pt nanodendrites were two and a
half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/C catalyst and five times
more active than the first-generation supportless Pt-black catalyst.
Low energy ion scattering (IS) was used to locate the oxygen adsorbed on Pt(111) in the “chemisorbed” and in the “oxide” state. The IS spectra show that while the “chemisorbed” oxygen is located in the topmost surface layer, this layer consists only of Pt atoms in the “oxide” case.
The interaction of oxygen with Pd(111) and polycrystalline palladium foil has been studied with ellipsometry, LEED, AES and XPS in the temperature range of 300 to 770 K and pressures up to 1 Pa. Ellipsometry was used to monitor the adsorption of oxygen and gave indication for the formation of a surface oxide at higher temperatures (T ≥ 470 K) and pressures (p ≥ 10−4Pa). The presence of a surface oxide is supported by a complex LEED pattern, ascribed to a square lattice with a = 7.5 ± 0.5Å and domains in six orientations. It was not possible to match this structure with a simple overlayer structure on the (111) plane or with an unreconstructed crystal plane of PdO. XPS measurements on palladium foil, after the same treatment, showed the presence of ≈0.5 ML PdO on the surface. Bulk oxide was not formed. The amount of oxygen on the surface could not be determined with AES because during AES the electron beam easily removed adsorbed oxygen, especially on Pd(111). On palladium foil the oxygen is removed less effectively by the electron beam, which indicates that oxygen is bound more tightly to defects. Bulk palladium oxide, produced by heating the palladium foil in air, was not affected by the electron beam, even at high current densities. The ellipsometric parameters δΔ and δΨ never exceeded 0.40° and 0.08° respectively, both on Pd(111) and palladium foil. This indicates that the diffusion of oxygen is limited to surface layer(s) under the conditions studied. Diffusion to the bulk did not occur.
Recent advances in the design and preparation of platinum-based nanostructures and their applications as electrocatalysts for low-temperature fuel cells are reviewed. Discussions are focused on the fundamental understanding and new experimental designs in the control of shape, composition and nanoscale structure of platinum and its alloy particles in colloidal systems. We explain the formation of various heteronanostructures using the Frank–van der Merwe (FM), Volmer–Weber (VW) and Stranski–Krastanov (SK) growth modes. Phenomena that exist in nanometer-sized regime, such as the disappearance of miscibility gaps for certain platinum alloys are given special attentions. The relationship between electronic structure or surface atomic arrangement and catalytic properties of platinum-based nanostructures is discussed.
A novel molecule formed by laser irradiation of graphite is reported.
This 60-carbon molecule is remarkably stable, with a structure akin to a
truncated icosahedron - a polygon with 60 vertices and 32 faces, 12 of
which are pentagons and 12 hexagons. This share is commonly encountered
as a football, or American soccer ball. The molecule has all valences
satisfied by two single bonds and one double bond, has many resonance
structures, and appears to be aromatic. It might give rise to a
superstable species which might exist in interstellar space and
circumstellar shells.
Using a newly developed gold Embedded Atom Model potential we conducted molecular dynamics simulations of inert gas aggregation type nanoparticle growth to investigate morphology transformations during the growth process. Our simulations reveal a size and temperature dependent mechanism whereby nanoparticles of Marks decahedron (m-D(h)) morphology are transformed to icosahedron (I(h)) nanoparticles via a solid-state process. The findings provide a theoretical explanation for recent experimental population statistics of gold nanoparticles produced by inert gas aggregation methods and may assist in developing methods for producing highly uniform nanoparticle morphologies. (C) 2009 Elsevier B. V. All rights reserved.
Single-crystal cubes and tetrahedrons of silver with truncated corners/edges have been prepared for the first time in high yields by reducing silver nitrate with ethylene glycol heated to 148 °C in the presence of poly(vinyl pyrrolidone) and a trace amount of sodium chloride. These nanoparticles were relatively monodisperse in size and shape, and their dimensions could be readily controlled in the range of 20 to 80 nm by varying the reaction time and other experimental parameters. We propose that the defects inherent in twinned nuclei of silver led to their selective etching and dissolution by chloride and oxygen (from air), leaving only the single crystalline ones to grow into nanoscale cubes and tetrahedrons.
Morphological control over platinum nanoparticles was realized by varying the amount of NaNO3 added to a polyol process, where H2PtCl6 was reduced by ethylene glycol to form PtCl42- and Pt0 at 160 °C. As the molar ratio between NaNO3 and H2PtCl6 was increased from 0 to 11, the morphology of Pt nanoparticles evolved from irregular spheroids with rounded profiles to tetrahedra and octahedra with well-defined facets. Absorption spectroscopy studies suggest that nitrate was reduced to nitrite by PtCl42- in the early stage of the synthesis, and the nitrite could then form stable complexes with both Pt(II) and Pt(IV) species. As a result, the reduction of Pt precursors by ethylene glycol was greatly slowed. This change in reaction kinetics altered the growth rates associated with different crystallographic directions of the Pt nanocrystals and ultimately led to the formation of different morphologies.
Electrification of the automobile provides a means of sustaining personal mobility in the face of petroleum resource limitations and environmental imperatives. Lithium ion (Li ion) batteries and hydrogen fuel cells provide pure-electrification solutions for different mass and usage segments of automotive application. Battery electric vehicles based on current and targeted Li ion battery technology will be limited to small-vehicle low-mileage-per-day applications; this is due to relatively low specific energy (kWh/kg) and long recharge time constraints. We briefly discuss new generations of Li ion positive and negative electrode intercalation compounds that are needed and under development to achieve energy storage density, durability, and cost targets. Lithium−air batteries give promise of extending the range, but scientists and engineers must surmount a plethora of challenges if growing research investments in this area are to prove effective. Hydrogen fuel cell vehicles have demonstrated the required 300 mile range and the ability to operate in all climates, but the cost of Pt-based catalysts, a low efficiency of utilization of presently cost-effective renewable sources of primary energy (e.g., electricity from wind), and the development of hydrogen infrastructure present significant challenges. Dramatic decreases in the amount of Pt used are required and are being brought to fruition along several lines of development that are described in some detail.
The physical and chemical properties of nanophase materials rely on their crystal and surface structures. Transmission electron microscopy (TEM) is a powerful and unique technique for structure characterization. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. This article introduces the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies. By forming a nanometer size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and mapping the valence states are even more attractive. In situ TEM is demonstrated for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
On the basis of a model for size-dependent cohesive energy, the size, shape, and dimensionality effects on melting temperatures of nanocrystals are modeled in a unified form. The model predicts that the melting temperature Tm(D,d,λ) decreases with reducing size D and dimensionality d or increasing shape factor λ. For nanoparticles with the same D values, there is Tm(icosahedron) > Tm(sphere or cube) > Tm(octahedron) > Tm(tetrahedron). Moreover, the ratio of depression of Tm(D,d,λ) is about 1:2λwire:3λparticle for thin films, nanowires, and nanoparticles when D is large enough, for example, 6 nm. The model is found to be in accordance with available experimental, MD simulation, and other theoretical results for Au, Ag, Ni, Ar, Si, Pb, and In nanocrystals.
This paper describes the synthesis of platinum multipods from platinum acetylacetonate in the presence of adamantanecarboxylic acid (ACA), hexadecylamine (HDA), and 1,2-alkanediol. Regular cubes and a range of other shapes can be generated in these reaction mixtures using diphenyl ether as the solvent and at a reaction temperature ranging from 160 to 200 °C. The formation of both planar and three-dimensional multipods of platinum can be attributed to the twin defects in the seed crystals. High-resolution transmission electron microscopy (HR-TEM) and electron diffraction (ED) data of platinum multipods show the stacking fault plays a key role in the reduction of symmetry in face-centered cubic metals such as platinum and enables the formation of mono-, bi-, tri-, and multipods of metal nanocrystals. The final shapes of the nanocrystals depend on both the type and number of defects, which can be changed by varying the reaction conditions such as the ACA/HDA molar ratio, the type of diols, the reaction time, and the temperature. High-aspect-ratio multipods of platinum can be generated by using 1,2-dodecanediol. The mechanisms that govern the formation of platinum multipods should be applicable for making other metal multipods.
The ability to control the composition and phase properties of bimetallic nanoparticles is critical in exploring catalytic properties. In this paper we present results from a study aimed at determining those properties for carbon-supported gold−platinum (AuPt) catalysts with different bimetallic compositions. The bimetallic nanoparticle catalysts are prepared by a two-phase synthesis protocol employing organic monolayer encapsulation on bimetallic AuPt cores (2 nm). The size-controlled nanoparticles are assembled on carbon black support materials with controllable dispersion and metal loading and are further treated by calcination under controlled temperature and atmosphere. The core composition of the bimetallic nanoparticles is determined by direct current plasma-atomic emission spectroscopy. Structural characterization is carried out by X-ray diffraction. The bimetallic nanoparticles were shown to display alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterpart of the bimetallic metals. This finding demonstrates the difference of the physical and chemical properties for nanoscale materials from the bulk crystalline state, revealing important details of the phase properties of the bimetallic nanoparticle catalysts and new information for the correlation between the composition and the phase properties at the nanoscale. Implications of our findings to the design and manipulation of the bimetallic nanoparticles for catalytic applications are also discussed.
In this review, syntheses and structural systematics of a series of vertex-sharing polyicosahedral clusters containing group 11 (Au,Ag,Cu) and group 10 (Pt,Pd,Ni) metals are discussed. This particular series of clusters follows a well-defined growth pathway in which the basic building block is the 13-atom centered icosahedron. The design rule is vertex sharing and the cluster “grows” by successive additions of icosahedral units via sharing of atoms. This cluster of clusters growth mechanism from a single icosahedron (13 atoms) to an icosahedron of icosahedra (127 atoms) parallels the atom-by-atom growth from a single atom to a 13-atom icosahedron and hence may be considered as a manifestation of the spontaneous self-organization and self-similarity aggregation process in the early stages of particle growth. This tendency to form polyicosahedral clusters may be termed polyicosahedricity. Recent developments in synthetic strategies and stereochemical principles of bi- and trimetallic vertex-sharing polyicosahedral clusters are highlighted with emphasis on (1) endo icosahedral chemistry by incorporating group 10 metals in the centers of the icosahedra, (2) exo icosahedral chemistry by capping the icosahedral faces with metal atoms or by “capturing” small molecules in the cluster cavities, and (3) framework icosahedral chemistry by changing the metal combination (group 11 metals) of the cluster architecture. Specifically, a new synthetic strategy based on “preformed clusters”, site preference rules, new concepts such as rotamerism and roulettamerism, and a new intracavity chemistry on a cluster surface resembling Venus flytrap are discussed. It is hoped that basic understanding of the stereochemical and bonding principles governing alloy formation in multimetallic clusters will lead to better electronic and stereochemical controls of their structures and reactivities and, ultimately, give rise to better design and manufacture or fabrication of structurally well-defined and functionally optimized nanoarchitecture, multimetallic catalysts, etc.
This article provides an overview of recent developments regarding synthesis of Pd nanocrystals with well-controlled shapes in aqueous solutions. In a solution-phase synthesis, the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueous system to achieve shape control for Pd nanocrystals is discussed. L-ascorbic acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the reduction kinetics, with citric acid and B-ions used as capping agents to selectively promote the formation of {111} and {100} facets, respectively. The distribution of single-crystal versus multiple-twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapes obtained for the Pd nanocrystals include truncated octahedron, icosahedron,octahedron, decahedron, hexagonal and triangular plates, rectangular bar, and cube. The ability to control the shape of Pd nanocrystals provides a great opportunity to systematically investigate their catalytic, electrical, and plasmonic properties.
Shapes, surface atomic arrangement and structural evolution induced by annealing of monodisperse FePt nanocrystals, synthesized by a solution phase chemical procedure, have been studied by high-resolution transmission electron microscopy. The as-synthesized FePt nanocrystals display dominantly a truncated octahedron shape enclosed by flat {1 0 0}, stepped {1 1 1} and zig-zag {1 1 0} facets. The Marks decahedron FePt nanocrystals and the icosahedron based multiply twined FePt nanocrystals are identified in the as-synthesized particles. An improved structural model has been proposed for the multiply twined nanocrystals. After annealing, the {1 1 0} facets disappear and a regular cuboctahedron becomes the dominant shape. Surfaces of the FePt nanocrystals show no reconstruction but with some atomic steps and kinks.
The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.
Recent experiments on nanostructured materials, such as nanoparticles, nanowires, nanotubes, nanopillars, thin films, and nanocrystals have revealed a host of “ultra-strength” phenomena, defined by stresses in a material component generally rising up to a significant fraction of its ideal strength – the highest achievable stress of a defect-free crystal at zero temperature. While conventional materials deform or fracture at sample-wide stresses far below the ideal strength, rapid development of nanotechnology has brought about a need to understand ultra-strength phenomena, as nanoscale materials apparently have a larger dynamic range of sustainable stress (“strength”) than conventional materials. Ultra-strength phenomena not only have to do with the shape stability and deformation kinetics of a component, but also the tuning of its physical and chemical properties by stress. Reaching ultra-strength enables “elastic strain engineering”, where by controlling the elastic strain field one achieves desired electronic, magnetic, optical, phononic, catalytic, etc. properties in the component, imparting a new meaning to Feynman’s statement “there’s plenty of room at the bottom”. This article presents an overview of the principal deformation mechanisms of ultra-strength materials. The fundamental defect processes that initiate and sustain plastic flow and fracture are described, and the mechanics and physics of both displacive and diffusive mechanisms are reviewed. The effects of temperature, strain rate and sample size are discussed. Important unresolved issues are identified.
Low energy ion scattering (IS) was used to locate the oxygen adsorbed on Pt(111) in the “chemisorbed” and in the “oxide” state. The IS spectra show that while the “chemisorbed” oxygen is located in the topmost surface layer, this layer consists only of Pt atoms in the “oxide” case.
We describe the adaptation of the recently developed thin-film rotating disk electrode method and its application in a rotating ring disk configuration (RRDE) to the investigation of the oxygen reduction reaction (orr) on a supported catalyst powder (Pt/Vulcan XC 72 carbon). This allows the determination of kinetic data, such as reaction orders or apparent activation energies, for the orr directly without mathematical modeling. Collection experiments reveal a potential and rotation rate independent collection efficiency. RRDE measurements allow, for the first time, the direct determination of the fraction of peroxide production during oxygen reduction on supported catalysts. Finally, comparison of measurements in 0.5 M H2SO4 and 0.5 M HClO4, respectively, reveals a significant effect of (bi)sulfate adsorption on the orr activity. On the basis of the present results, predictions are made on the kinetic limit of the orr in polymer electrolyte fuel cells, in the absence of ohmic and mass transport resistances at 100% utilization.
In this study, density function theory calculations are applied to the simulation of CO oxidation reactions over platinum, palladium and rhodium surfaces. On the basis of these calculations alone the detailed reaction scenario together with activation energies, pre-factors and rate constants can be derived. Such studies allow a systematic analysis of trends due to exactly identical conditions. The comparison with observed reaction rates demonstrates that such an approach gives reliable results and provides further insight into the reaction mechanism.
Based on density functional theory calculations of H2 dissociation on Al(111), Cu(111), Pt(111) and Cu3Pt(111) we present a consistent picture of some key physical properties determining the reactivity of metal and alloy surfaces. The four metal surfaces are chosen to represent metals with no t-bands, with filled d-bands and with d-states at the Fermi level. We show that electronic states in the entire valence band of the metal surface are responsible for the reactivity, which consequently cannot be understood solely in terms of the density of states at the Fermi nor in terms d-states above it. Rather we suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present), and we demonstrate that a simple frozen potential based estimate of the hybridization energy correlates well with the calculated variation of the barrier height for the different metal surfaces.
The shape of metal alloy nanocrystals plays an important role in catalytic performances. Many methods developed so far in controlling the morphologies of nanocrystals are however limited by the synthesis that is often material and shape specific. Here we show using a gas reducing agent in liquid solution (GRAILS) method, different Pt alloy (Pt-M, M = Co, Fe, Ni, Pd) nanocrystals with cubic and octahedral morphologies can be prepared under the same kind of reducing reaction condition. A broad range of compositions can also be obtained for these Pt alloy nanocrystals. Thus, this GRAILS method is a general approach to the preparation of uniform shape and composition-controlled Pt alloy nanocrystals. The area-specific oxygen reduction reaction (ORR) activities of Pt(3)Ni catalysts at 0.9 V are 0.85 mA/cm(2)(Pt) for the nanocubes, and 1.26 mA/cm(2)(Pt) for the nanooctahedra. The ORR mass activity of the octahedral Pt(3)Ni catalyst reaches 0.44 A/mg(Pt).
Chikungunya virus (CHIKV) is an emerging mosquito-borne alphavirus that has caused widespread outbreaks of debilitating human disease in the past five years. CHIKV invasion of susceptible cells is mediated by two viral glycoproteins, E1 and E2, which carry the main antigenic determinants and form an icosahedral shell at the virion surface. Glycoprotein E2, derived from furin cleavage of the p62 precursor into E3 and E2, is responsible for receptor binding, and E1 for membrane fusion. In the context of a concerted multidisciplinary effort to understand the biology of CHIKV, here we report the crystal structures of the precursor p62-E1 heterodimer and of the mature E3-E2-E1 glycoprotein complexes. The resulting atomic models allow the synthesis of a wealth of genetic, biochemical, immunological and electron microscopy data accumulated over the years on alphaviruses in general. This combination yields a detailed picture of the functional architecture of the 25 MDa alphavirus surface glycoprotein shell. Together with the accompanying report on the structure of the Sindbis virus E2-E1 heterodimer at acidic pH (ref. 3), this work also provides new insight into the acid-triggered conformational change on the virus particle and its inbuilt inhibition mechanism in the immature complex.
Putting the pedal to the metal: A facile strategy for the synthesis of metal nanocrystals is demonstrated that employs carbon monoxide as a reducing agent. Highly monodisperse platinum nanocubes, spherical palladium nanocrystals, and ultrathin gold nanowires can be produced within 15 minutes.
PtAu nanoparticles (NPs) were shown to strongly enhance the kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Li-O(2) cells. Li-O(2) cells with PtAu/C were found to exhibit the highest round-trip efficiency reported to date. During ORR via xLi(+) + O(2) + xe(-) --> Li(x)O(2), the discharge voltage with PtAu/C was considerably higher than that of pure carbon and comparable to that of Au/C. During OER via Li(x)O(2) --> xLi(+) + O(2) + xe(-), the charge voltages with PtAu/C fell in the range from 3.4 to 3.8 V(Li), which is slightly lower than obtained with Pt. It is hypothesized that PtAu NPs exhibit bifunctional catalytic activity, having surface Au and Pt atoms primarily responsible for ORR and OER kinetics in Li-O(2) cells, respectively.
The facile decomposition of ammonia to produce hydrogen is critical to its use as a hydrogen storage medium in a hydrogen economy, and although ruthenium shows good activity for catalysing this process, its expense and scarcity are prohibitive to large-scale commercialization. The need to develop alternative catalysts has been addressed here, using microkinetic modelling combined with density functional studies to identify suitable monolayer bimetallic (surface or subsurface) catalysts based on nitrogen binding energies. The Ni-Pt-Pt(111) surface, with one monolayer of Ni atoms residing on a Pt(111) substrate, was predicted to be a catalytically active surface. This was verified using temperature-programmed desorption and high-resolution electron energy loss spectroscopy experiments. The results reported here provide a framework for complex catalyst discovery. They also demonstrate the critical importance of combining theoretical and experimental approaches for identifying desirable monolayer bimetallic systems when the surface properties are not a linear function of the parent metals.
This communication describes the preparation of carbon-supported truncated-octahedral Pt(3)Ni nanoparticle catalysts for the oxygen reduction reaction. Besides the composition, size, and shape controls, this work develops a new butylamine-based surface treatment approach for removing the long-alkane-chain capping agents used in the solution-phase synthesis. These Pt(3)Ni catalysts can have an area-specific activity as high as 850 muA/cm(2)(Pt) at 0.9 V, which is approximately 4 times better than the commercial Pt/C catalyst ( approximately 0.2 mA/cm(2)(Pt) at 0.9 V). The mass activity reached 0.53 A/mg(Pt) at 0.9 V, which is close to a factor of 4 increase in mass activity, the threshold value that allows fuel-cell power trains to become cost-competitive with their internal-combustion counterparts. Our results also show that the mass activities of these carbon-supported Pt(3)Ni nanoparticle catalysts strongly depend on the (111) surface fraction, which validates the results of studies based on Pt(3)Ni extended-single-crystal surfaces, suggesting that further development of catalysts with still higher mass activities is highly plausible.
Platinum-based alloys have been extensively shown to be effective catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Most of these catalysts are nanoparticles without shape control. Recently, extended Pt(3)Ni(111) surfaces prepared in ultrahigh vacuum were demonstrated to possess enhanced ORR catalytic activity as compared to the state-of-the-art carbon supported Pt (Pt/C) nanoparticle catalysts. How and whether this promising surface can be transformed into practical nanoscale electrocatalysts used in PEMFCs remain a challenge. We report a new wet-chemical approach of preparing monodisperse Pt(3)Ni nanoctahedra and nanocubes terminated with {111} and {100} facets, respectively. We further show that the ORR activity on the Pt(3)Ni nanoctahedra is approximately 5-fold higher than that of nanocubes with a similar size. Comparison of ORR activity between carbon-supported Pt(3)Ni nanoctahedra and commercial Pt/C reveals that the Pt(3)Ni nanoctahedra are highly active electrocatalysts. This synthetic strategy may be extended to the preparation of other shape-controlled fuel cell electrocatalysts.
The challenge of chemistry in the 21st century is to achieve 100% selectivity of the desired product molecule in multipath reactions ("green chemistry") and develop renewable energy based processes. Surface chemistry and catalysis play key roles in this enterprise. Development of in situ surface techniques such as high-pressure scanning tunneling microscopy, sum frequency generation (SFG) vibrational spectroscopy, time-resolved Fourier transform infrared methods, and ambient pressure X-ray photoelectron spectroscopy enabled the rapid advancement of three fields: nanocatalysts, biointerfaces, and renewable energy conversion chemistry. In materials nanoscience, synthetic methods have been developed to produce monodisperse metal and oxide nanoparticles (NPs) in the 0.8-10 nm range with controlled shape, oxidation states, and composition; these NPs can be used as selective catalysts since chemical selectivity appears to be dependent on all of these experimental parameters. New spectroscopic and microscopic techniques have been developed that operate under reaction conditions and reveal the dynamic change of molecular structure of catalysts and adsorbed molecules as the reactions proceed with changes in reaction intermediates, catalyst composition, and oxidation states. SFG vibrational spectroscopy detects amino acids, peptides, and proteins adsorbed at hydrophobic and hydrophilic interfaces and monitors the change of surface structure and interactions with coadsorbed water. Exothermic reactions and photons generate hot electrons in metal NPs that may be utilized in chemical energy conversion. The photosplitting of water and carbon dioxide, an important research direction in renewable energy conversion, is discussed.
Herein we describe a protocol that generates Au icosahedra in high yields by simply mixing aqueous solutions of HAuCl(4) and N-vinyl pyrrolidone. Our mechanistic study reveals that water plays an important role in this synthesis: as a nucleophile, it attacks the gold-vinyl complex, leading to the production of an alcohol-based Au(I) intermediate. This intermediate then undergoes a redox reaction in which Au(I) is reduced to Au(0), leading to the formation of Au atoms and then Au icosahedra of about 18 nm in size at a yield of 94 %, together with a carboxylic acid in the final product. This new protocol has also been employed to prepare multiply twinned nanoparticles of Ag (15-20 nm in size), spherical aggregates (25-30 nm in size) of Pd nanoparticles, and very small nanoparticles of Pt (2 nm in size). Since no organic solvent, surfactant, or polymer stabilizer is needed for all these syntheses, this protocol may provide a simple, versatile, and environmentally benign route to noble-metal nanoparticles having various compositions and morphologies.
Tailored nanoparticles: Uniform Pd icosahedra (see TEM images) were controllably synthesized in high yield by a facile polyol process. Their size can be readily tailored from 15 to 42 nm by tuning the reaction parameters. The high density of twin boundaries and sharp edges on the surfaces of the Pd icosahedra could make them promising for many applications, for example, catalysis.
This report on the solid-to-liquid transition region of an Ag-Pd bimetallic nanocluster is based on a constant energy microcanonical ensemble molecular dynamics simulation combined with a collision method. By varying the size and composition of an Ag-Pd bimetallic cluster, we obtained a complete solid-solution type of binary phase diagram of the Ag-Pd system. Irrespective of the size and composition of the cluster, the melting temperature of Ag-Pd bimetallic clusters is lower than that of the bulk state and rises as the cluster size and the Pd composition increase. Additionally, the slope of the phase boundaries (even though not exactly linear) is lowered when the cluster size is reduced on account of the complex relations of the surface tension, the bulk melting temperature, and the heat of fusion. The melting of the cluster initially starts at the surface layer. The initiation and propagation of a five-fold icosahedron symmetry is related to the sequential melting of the cluster.
Platinum-on-palladium bimetallic heterogeneous nanostructures were prepared using a sequential synthetic method, in which 3-nm Pt particles grew on the surfaces of 5-nm Pd nanoparticles. Electrochemical study of carbon-supported Pt-on-Pd heteronanostructures shows not only enhancement in electrocatalytic activity for oxygen reduction reaction (ORR) but also much improved stability in comparison to a commercial platinum catalyst (E-TEK, 20 wt % Pt, diameter: 2.5 nm). The greatly suppressed hydroxyl adsorption on active sites by introducing Pd was attributed to the enhanced activity, while the retention of active surface area, morphology, and composition because of the large overall bimetallic particle size and unique architectures could be the key factors for the much improved stability over 30,000 cycles. Our work shows heterogeneous platinum-on-metal bimetallic nanostructures offer new opportunities to the design of hierarchically ordered multifunctional fuel cell catalysts.
Using a genetic algorithm global optimization approach combined with density functional theory calculations, a search has been made for the lowest energies of (AgAu)(m) nanoalloys with 20-150 atoms (diameters of 1.0-2.0 nm). A total of 31 decahedra, 35 icosahedra, and 2 close-packed motifs are identified in two icosahedral windows and one Marks-decahedral window. These structural motifs have twinned, capped, defective, and distorted atomic packing compared to classical clusters, such as the icosahedron. The magic numbers, atomic ordering, electronic structure, and melting behavior are further studied, and a new poly-nanocrystalline decahedral motif, Ag(44)Au(44), is found to have high structural, electronic, and thermal stability. Our results show that alloying can lead to a remarkable stabilization of local order and provide a comprehensive model for the structures and properties of Ag-Au nanoalloys.
Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.