Low Dimensional Platinum-Based Bimetallic Nanostructures for

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Low Dimensional Platinum-Based Bimetallic Nanostructures for Advanced Catalysis Qi Shao, Pengtang Wang, Ting Zhu, and Xiaoqing Huang* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China

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S Supporting Information *

CONSPECTUS: The development of renewable energy storage and conversion has been greatly promoted by the achievements in platinum (Pt)-based catalysts, which possess remarkable catalytic performance. However, the high cost and limited resources of Pt have hindered the practical applications and thus stimulated extensive efforts to achieve maximized catalytic performance with minimized Pt content. Low dimensional Pt-based bimetallic nanomaterials (such as nanoplates and nanowires) hold enormous potential to realize this target owing to their special atomic arrangement and electronic structures. Recent achievements reveal that strain engineering (e.g., the compressive or tensile strain existing on the Pt skin), surface engineering (e.g., high-index facets, Pt-rich surface, and highly open structures), and interface engineering (e.g., compositionsegregated nanostructures) for such nanomaterials can readily lead to electronic modification, more active sites, and strong synergistic effect, thus opening up new avenues toward greatly enhanced catalytic performance. In this Account, we focus on recent advances in low dimensional Pt-based bimetallic nanomaterials as promising catalysts with high activity, long-term stability, and enhanced selectivity for both electrocatalysis and heterogeneous reactions. We begin by illustrating the important role of several strategies on optimizing the catalytic performance: (1) regulated electronic structure by strain effect, (2) increased active sites by surface modification, and (3) the optimized synergistic effect by interfacial engineering. First of all, a difference in atomic bonding strength can result in compressive or tensile force, leading to downshift or upshift of the d-band center. Such effects can be significantly amplified in low-dimensionally confined nanostructures, producing optimized bonding strength for improved catalysis. Furthermore, a high density of high-index facets and a Pt-rich surface in shape-controlled nanostructures based on surface engineering provide further enhancement due to the increased Pt atom utilization and optimal adsorption energy. Finally, interfacial engineering of low dimensional Pt-based bimetallic nanomaterials with high composition-segregation can facilitate the catalytic process due to a strong synergetic effect, which effectively tunes the electronic structure, modifies the coordination environment, and prevents catalysts from serious aggregation. The rational design of low dimensional Pt-based bimetallic nanomaterials with superior catalytic properties based on strain, surface, and interface engineering could help realize enhanced catalysis, gain deep understanding of the structure−performance relationship, and expand access to Pt-based materials for general communities of materials science, chemical engineering, and catalysis in renewable energy research fields.

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INTRODUCTION

Low-dimensional (low-D) nanostructures, such as 2D nanoplates or nanosheets and 1D nanowires (NWs) or nanotubes, play important roles in exploring advanced catalysts due to their large exposed surface area for catalytic reaction, fast electron transfer for high conductivity, and good resistance to dissolution, Ostwald ripening, and aggregation.6 It is noteworthy that some interesting properties become dominant in the low-D nanosystems owing to the dimensional confinement effect. For example, in 2D nanosheet structures, the magnitude of strain is inversely proportional to the slab’s thickness.7 All these important advantages enable low-D Ptbased bimetallic nanomaterials to be promising candidates for achieving highly efficient catalysis.

Platinum (Pt)-based materials are considered as “the Holy Grail” of many catalytic reactions for their excellent performance.1−3 Among various candidates, bimetallic Pt-based materials have received extensive research interest due to the reduced Pt usage.4 More importantly, when alloying Pt with a second metal, the electronic structure of the surface active sites can be effectively manipulated, largely facilitating the catalytic process.5 For example, forming bimetallic Ptbased nanomaterials (such as Pt−Ni, Pt−Co, Pt−Cu, Pt−Y) can significantly enhance the specific activity for the oxygen reduction reaction (ORR) via effectively manipulating the oxygen adsorption energy.5 Hence, it is of utmost importance to develop new methodologies for designing novel Pt-based bimetallic nanostructures to discover new potential applications. © XXXX American Chemical Society

Received: May 19, 2019

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DOI: 10.1021/acs.accounts.9b00262 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

high production is also demonstrated by presenting representative examples. Based on the fundamental understanding of these engineered strategies, the achievements for these catalysts for catalytic reactions are then presented in detail. Finally, a brief conclusion and perspective is proposed, which may shed light on the future development of this research direction.

Currently, although the structure and composition based strategies have been widely used for designing new catalysts, the property enhancement is still far from the urgent requirements for catalysts with practicability and scalability. It is worth mentioning that recent years have witnessed unprecedented improvements in catalyst performance achieved by advanced design strategies, including strain, surface, and interface engineering by regulating electronic structure, modifying active sites, and optimizing synergistic effects.8−10 Benefiting from these advanced design strategies, low-D Pt-based bimetallic materials have achieved significant advances (Scheme 1). First of all, through applying

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ADVANCED REGULATION OF LOW-DIMENSIONAL Pt-BASED BIMETALLIC NANOMATERIALS FOR ENHANCED ELECTROCATALYSIS

Regulating Electronic Structure by Strain Engineering

Scheme 1. Schematic Illustration of (1) Strain, (2) Surface, and (3) Interface Engineering of Low-Dimensional PtBased Bimetallic Nanostructures for Catalytic Reactionsa

To achieve catalysts with enhanced catalytic performance, it is significant to investigate the relationship between advanced engineering strategies and improved electrocatalytic performance. On the close-packed surfaces, the ratio between the intrinsic surface pressure and the corresponding bulk modulus of Pt is up to 8.3%, larger than those of Au (7.8%), Pd (6.5%), Cu (6.1%), Ni (5.8%), Ir (5.8%), and Os (5.6%).7 This fascinating property leads to a blossom of fundamental research on strain-effect-based design of Pt-based bimetallic catalysts. As a result, only 1% strain change can induce ∼0.1 eV shift of the d-band center of Pt, which is sufficient to affect the adsorption strength of reactants on the surface.12−14 More importantly, when the diameter of the nanowires (NWs) is confined to atomic thickness, some interesting properties will emerge, which give additional possibilities for modulating the interaction between the intermediate and the surface active sites. Recent advances demonstrate that strain tuning in 2D materials has unique advantages, opening a new direction for stain engineering. First, the strain effect for a given element increases with decreasing slab thickness. For example, on the fcc Pd(111) surface, the compressive strain is increased from 1.4% to 5.7% when the thickness is decreased from 2 to 0.5 nm.7 The strain effect is very sensitive to the exposed phase. However, the strain effect on the nanoparticles is often obscured due to the large numbers of undercoordinated defects and the reason that the achieved catalytic enhancement cannot be well organized due to the complex surface geometries. Thus, it is highly desirable to build architectures with precisely exposed crystal facets. Two-dimensional nanostructures provide an ideal platform to study facetdependent properties because of their distinctly exposed facets.15 For example, two kinds of facet are clearly presented in the PtPb/Pt nanoplates (Figure 1a−g). One is PtPb(010)//Pt(110) planes between PtPb and edge-Pt layers and the other is PtPb(001)//Pt(110) planes between PtPb and the top (bottom) Pt layer, in which the Pt(110) surface facilitates the ORR reaction for better ORR activity.15 Some unique strain effects along the exposed crystal plane of 2D materials may bring new possibilities for the optimization of ORR performance. In general, only the compressive strain is useful for optimizing the oxygen adsorption energy of Pt for enhanced ORR performance.16 However, such a viewpoint becomes a controversial in the PtPb/Pt nanoplates. In the edge layer of PtPb/Pt nanoplates, the [001] direction of Pt is fully confined within PtPb, leading to a 7.5% tensile strain and limited compressive strain along [110]Pt. In the top Pt layer, it carries out an 11% compressive strain along the [011̅]Pt and a 7.6% tensile strain along

a

Orange sphere is Pt, green sphere is the second metal, and white sphere is O or S.

compressive or tensile force on the Pt surface, the d-band center of catalysts can be effectively modulated (Scheme 1, part 1).11 More importantly, such effect can be greatly amplified in the dimensionally confined nanostructures, contributing to the optimization of bonding strength for boosted catalysis.7 In addition, achieving high density of highindex facets or high degree of porosity based on the surface engineering in the shape-controlled structures can largely increase the number of active sites and improve their intrinsic performance (Scheme 1, part 2).9 Furthermore, interface engineering has realized improved performance via strong synergistic effect, which not only changes the electronic environment of active sites but also improves stability (Scheme 1, part 3).10 In spite of the significant progress achieved for low-D Pt-based bimetals for enhanced catalysis based on strain, surface, and interface engineering, a systematic summary of the achievements and issues in this area is lacking but indispensable. This Account aims to give an overview of the recent achievements in strain, surface, and interface engineering of low-D Pt-based bimetallic materials for enhanced catalysis, including ORR, hydrogen evolution reaction (HER), and heterogeneous reactions. First, the progress in strain, surface, and interface engineering to regulate catalytic reactions is summarized, including (1) regulation of electronic structure by strain engineering, (2) modification of active sites by surface engineering, and (3) optimization of synergistic effects by interface engineering. The versatile synthesis of low-D Ptbased bimetallic catalysts followed by different strategies in B


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Figure 1. (a) Model of one single hexagonal nanoplate. HAADF−STEM images from different directions: (b) in-plate view and (c) out-of-plate view. (d) Schematic atom models of the nanoplate showing the top and side interfaces. (e, f) High-resolution HAADF images from the selected areas in panel b. (g) Schematic atom models of the nanoplate showing the top and side interfaces. Reproduced with permission from ref 15. Copyright 2016 American Association for the Advancement of Science.

Pt−Bi hexagonal nanoplates with an average size of 100 nm are synthesized assisted by CO. The coordination agent CO acts as a key reducing or shape controlling agent due to its preferential adsorption on Pt(100) (Figure 2c,d).25

[100]Pt. Consequently, appreciably large biaxial tensile strain is applied on the most exposed Pt(110) facet, resulting in the ORR enhancement.15 All these advantages highlight the importance of developing strain engineered 2D Pt-based bimetallic catalysts. Although significant progress has been achieved for synthesizing 2D pure metal materials, such as Pd, Au, and Ru,17−20 the synthesis of 2D Pt-based bimetallic materials is highly challenging. To overcome this obstacle, Wang’s group reported a successful synthesis of 2D PtCu nanosheets, where potassium iodide plays a key role in the control of size and thickness.21 Later on, they fabricated sandwich and nanoringlike structures by using PtCu nanosheets as seeds.22 Our group fabricated PtPb/Pt hexagonal nanoplates, using platinum(II) acetylacetonate (Pt(acac)2) and lead(II) acetylacetonate (Pb(acac)2) as metal precursors, L-ascorbic acid (AA) as reducing agent, and 1-octadecene/oleylamine as mixed solvent (Figure 2a,b).15 Detailed characterizations reveal a typical core−shell structure with the covering Pt only ∼1 nm thick. Pt at the edge shows cubic structure, while PtPb in the core belongs to hexagonal phase. It should be noted that the amount of reducing agent is critical for the synthesis of PtPb/Pt nanoplate since a complete morphological transformation from nanoplate to octahedra was found if we reduced the amount of AA.23 Inspired by this work, interesting PtPb/Pt core/shell nanodisks with tunable Pt thickness were then prepared by modulating the amount of Pt precursor.24 We also extended the synthesis of 2D Pt-based bimetallic nanomaterials to other compositions. For example,

Modifying Active Sites by Surface Engineering

Surface provides the active sites for catalytic reaction, therefore making it significantly important to achieve a better modification of the surface environment via the surface engineering strategy. As is well-known, the key point of surface engineering strategy relies on two essential elements, namely, the number and the intrinsic activity of active sites.26 Regarding the first element, constructing highly porous structures or ultrathin structures is highly appreciated and can largely increase the number of active sites for catalytic reaction.27,28 One typical example is to construct nanostructures with high porosity by using highly segregated Pt-based NWs as the template (Figure 3a,b).29 In this regard, a porous structure is achieved by acidic etching of highly segregated PtNi NWs (Figure 3c−e).28 The bright field transmission electron microscope (TEM) image clearly exhibits the porous Pt framework with Ni largely etched. A similar strategy was also reported by Li et al. They prepared jagged Pt NWs with the electrochemical active surface area (ECSA) up to 118 m2/ gPt by electrochemically dealloying PtNi alloyed NWs.30 The ultrathin NW is a potential candidate for catalytic reaction due to the greatly increased number of active sites. However, the synthesis is still a great challenge since metal atoms tend to grow along three dimensions.31 We overcame this problem by C


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Figure 2. Characterizations of (a, b) PtPb nanoplate, (c, d) PtBi nanoplate, and (e−g) STEM, TEM, and HRTEM images of sub-nanometer PtNi nanowires. (h−k) TEM images of sub-nanometer Pt nanowires. (p, q) TEM images of sub-nanometer PtCo and PtNiCo nanowires. Characterizations of (l, m) PtNi nanowires. (n−q) TEM images of PtCo nanowires, PtFe nanowires, PtNiCo nanowires, and PtNiFe nanowires. Panels a and b reproduced with permission from ref 15. Copyright 2016 American Association for the Advancement of Science. Panels e−k reproduced with permission from ref 27. Copyright 2017 American Association for the Advancement of Science. Panels l−q reproduced with permission from ref 29. Copyright 2015 John Wiley & Sons, Inc.

enhancement is restricted when approaching the maximum loading limitation. A catalyst with a smooth surface is not favorable to improve the catalytic performance due to the limited number of active sites and the weak electronic effect.33 Surface engineering strategy opens up a unique avenue to overcome this problem by constructing high-index surfaces for higher energy, leading to higher catalytic performance. Thus, constructing a certain number of high-index facets on the catalyst surface is also an important approach for enhancing catalytic performance.34

introducing two different reducing agents (Mo(CO)6 and glucose) at the same time. Mo(CO)6 was important for diameter control where only the nanoparticles can be obtained without using Mo(CO)6.27 As shown in Figure 2e−k, the diameter of PtNi NWs is down to 0.8 nm and the length is up to several tens of nanometers. In another work, Mao et al. also reported the synthesis of ultrathin Pt−Mo−M (M = Fe, Co, Mn, Ru, etc.) NWs via a H2-assisted solution route.32 Besides increasing the number of active sites, improving the intrinsic activity of active sites is highly required since the D


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Figure 3. Characterizations of secondary treated (a, b) highly segregated PtNi NWs: (c−e) PtNi NWs with highly porous structure, (f−h) nanowires with PtNi/NiSx interface and (i−k) hydroxide-membrane-coated PtNi nanowires. Panels a and b reproduced with permission from ref 29. Copyright 2016 John Wiley & Sons, Inc. Panels c−e reproduced with permission from ref 28. Copyright 2017 John Wiley & Sons, Inc. Panels f−h reproduced with permission from ref 45. Copyright 2017 Nature Publishing Group. Panels i−k reproduced with permission from ref 46. Copyright 2018 John Wiley & Sons, Inc.

an upgraded two-step strategy, that is, the second metal precursor was added into the solution after formation of the Pt NWs.37,38 In a later work, we greatly improved the density of steps, ledges, and kinks on PtCo NWs34 (Figure 4a−c). According to the 3D reconstructed tomograms and atomic resolution HAADF−STEM images, there are abundant {310} high-index facets found on the surface of Pt−Co NWs, which provides an abundance of active sites for catalytic reactions (Figure 4d− h). Encouraged by this work, we also expanded the composition to pure Pt NWs and Pt−Fe NWs with rough surfaces and zigzag borders.39

Based on this assumption, we developed a new class of Ptbased NWs with rough surfaces. Conventional pathways to obtain 1D Pt-based structures have been widely reported, such as template controlled, magnetic field assisted, and catalyst assisted methods.35 However, a main problem with previous studies is the lacking of a universal synthesis method with large-scale production, rough surface, and desired composition. To solve this problem, our group developed a universal method for preparing Pt-based NWs from bimetallic to trimetallic compositions, including PtNi, PtCo, PtFe, PtRh, PtNiFe, and PtNiCo (Figure 2l−q).36 The reducing agent (glucose) and structure-directing agent (hexadecyl trimethylammonium chloride, CTAC) are the essential factors in this method. More importantly, the surface of PtNi NWs is not smooth, where a high density of low-coordinate atomic steps can be observed, providing abundant active sites for reaction. The work based on the detailed characterizations of the intermediates at different reaction periods explains how this synthetic method works. Initially, only ultrathin Pt NWs with a diameter of ∼1 nm are formed. Then the NWs become thicker, and the diameter increases to 10 nm. The energy dispersive X-ray spectroscopy (EDX) results reveal a large portion of Ni, suggesting the deposition of Ni on the ultrathin Pt NWs. Next, a large increase of Pt content can be observed, suggesting that the reduction of Pt plays a dominant role in this step. The diffusion of Pt into the Ni-rich NW is the final step for forming alloyed NWs. Inspired by this work, we demonstrated the fabrication of Pt−Cu and Pt−Pb NWs by

Optimizing Synergistic Effect by Interface Engineering

Apart from the strain and surface engineering strategies, the interface engineering strategy provides another way of obtaining Pt-based bimetallic electrocatalysts with enhanced activity, stability, and selectivity. The interface in the catalyst is the boundary between two domains (named domain A and domain B), where the catalytic reaction takes place. In one case, domain A provides active sites and domain B acts as the support. Based on the rational design and specific synthesis process, the electron transfer between domain A and domain B has an intensive effect on balancing the adsorption and desorption between intermediates and domain A, leading to an enhancement in activity.40 In another case, a complete reaction finishes after two or more than two steps. Each step occurs on domain A and domain B separately. Consequently, E


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Figure 4. (a−c) TEM images of PtCo nanowires. Series of reconstructed 3D tomograms and as-acquired HAADF−STEM images at the viewing angles of (d) −60 °C, (e) 0 °C, and (f) 60 °C. (g) Atomic resolution HAADF−STEM image at the viewing angle of 0°, and (h) indexes of the flat planes of the Pt3Co NWs. Reproduced with permission from ref 34. Copyright 2016 Nature Publishing Group.

the synergistic effect between different steps leads to the optimized reaction pathway, further enhancing the catalytic efficiency.41 In addition, with the aid of the interface, strong bonding between catalyst and support can be generated, largely preventing agglomeration and therefore improving the stability. It can also assist in improving the antipoisoning effect toward the intermediates.42 More importantly, interface effects provide new insight into modulating selectivity. High selectivity suggests that the reaction undergoes a desirable pathway to the target products, instead of byproducts. By utilizing the powerful effect of the interface on the binding strength, high selectivity to the target product can be achieved.43

The essence of interface engineering of low-D Pt-based bimetallic catalysts is that the number of available interfaces in low-D structures is greater than that in other structures, such as nanoparticles. Until now the reported interfaces in low-D Pt-based bimetallic catalysts include metal−metal interfaces,29 metal−organic interfaces,44 metal−oxide interfaces,29 and metal−sulfide interfaces.45 Unique interface structures also can be built on the NWs based on alloyed NWs with high composition segregation, such as a typical candidate with metal−metal interfaces (Figure 3a,b). Via secondary treatment, Pt-based bimetallic derivatives with different kinds of interfaces (PtNi/Ni(OH)2, PtNi/NiSx, and PtNi/NiO interfaces) were systemically built by using Pt−Ni NWs as the precursor (Figures 3f−k, 5, and 6).29,45,46 For example, Pt−Ni F


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Figure 5. Scheme of thermal treatment of (a) Pt3Nix NWs/C in different conditions: (b) in H2/N2 and (c−e) in air. Reproduced with permission from ref 29. Copyright 2016 John Wiley & Sons, Inc.

the stability of other catalysts (Table S1), the PtPb/Pt nanoplate ensures superior stability with negligible activity decay and structural or compositional change after 50000 cycles, mainly due to the Pt shell and intermetallic core. Calculation results indicate that the tensile strain along edgePt and top (bottom)-Pt(110) facets help optimize the Pt−O bond strength (Figure 7f−i). Besides engineering the strain effect in the 2D nanoplate, we also manipulated the tensile strain in the 2D Pd3Pb/Pd tetragonal nanosheet. Pd is a typical element for forming the 2D metallic nanostructure with the help of CO.19 The essence of our work is that both the top-Pd and edge-Pd surface boost ORR. As a result, the Pd3Pb/Pd nanosheets showed better performance than Pd3Pb/Pd nanocrystals and the unshelled counterpart (PdPb nanosheet).48 Besides the positive effect from the tensile strain, Li et al. reported a great enhancement in ORR reaction by manipulating the compressive strain in the jagged Pt nanowires, where the compressive strain originates from the short Pt−Pt distance on the surface.30 Moreover, performing surface engineering strategy on lowD Pt-based material is effective in developing high-performance ORR catalysts. For general ORR studies, when the size of the nanoparticle is less than 2 nm, the ORR activity largely decreases due to the strong oxygen adsorption energy of O on the Pt surface.27 However, such issue would not exist for subnanometer Pt based NWs, where the PtNiCo ultrathin NWs show the highest mass activity of 4.20 A/mg and specific activity of 5.11 mA/cm2. DFT results suggest that active sites

NWs with PtNi/NiO interfaces were obtained by annealing Pt−Ni NWs in air. More importantly, the tunable interface contact can be easily controlled by using Pt−Ni NWs with different Pt/Ni ratios (Figure 5c−e).29

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APPLICATIONS OF LOW-DIMENSIONAL Pt-BASED CATALYSTS FOR ENHANCED ELECTROCATALYSIS

Oxygen Reduction Reaction

Fuel cell, as a typical chemical energy converter, has become a promising candidate for relieving the energy crisis.26 In the cathodic reaction, a complete ORR process undergoes a typical four-electron reaction. However, the sluggish ORR kinetics leads to a large overpotential under the typical operation conditions, thus requiring efficient electrocatalysts to boost the reaction rate and efficiency. Pt is regarded as the best candidate for this reaction while its high cost and poor ORR kinetic leads to further exploration under the typical operation condition.47 Bimetallic Pt-based catalysts have become promising candidates to solve this problem for the reduced Pt usage and modified electronic environment. Under this circumstance, the advanced design of low-D Pt-based bimetallic materials will lead to exceptional development of ORR catalysts for practical applications. Our group successfully introduced biaxial tensile strain on the [110] facets of the PtPb/Pt nanoplates.15 Such 2D nanoplates deliver high specific activity of 7.8 mA cm−2 and mass activity of 4.3 A mg Pt−1 at 0.9 V versus reversible hydrogen electrode (RHE) (Figure 7a−e). Compared with G


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Figure 6. Interface characterizations of (a−d) PtNi−Ni(OH)2, (e) PtNi−NiSx, and (f) PtNi−NiO. Panels a−d reproduced with permission from ref 46. Copyright 2018 John Wiley & Sons, Inc. Panel e reproduced with permission from ref 45. Copyright 2017 Nature Publishing Group. Panel f reproduced with permission from ref 29. Copyright 2016 John Wiley & Sons, Inc.

adsorbed H (Hads).49 However, it is difficult for Pt to dissociate water, as an important step for HER process in basic conditions. As a result, the HER performance on Pt is 2−3 orders of magnitude lower in basic conditions.50 This longstanding issue triggered substantial studies on developing high-performance Pt-based HER catalysts. Interface engineering of Pt-based bimetallic catalysts provides an effective way to address the above issue for their strong synergistic effect. For example, although Pt is good at adsorption and recombination of the absorbed Had*, it is a poor catalyst for water dissociation. On the other hand, metal oxide is well-known for its good ability to cleave the H− OH bond, while it is inefficient for H2 generation via Had.51 Therefore, it may be a good tactic to combine Pt with metal oxide into one interface unit to pursue excellent Pt-based base HER catalysts. Guided by this rule, our group developed highly efficient HER catalysts by using Pt NW derivatives. We begin from Pt−Ni NWs with Pt/NiO interface by air annealing, which shows excellent activity and stability in both acidic and basic conditions.29 After that, we greatly improved the HER performance of Pt by modifying PtNi NWs through a

on (111) facets of sub-nanometer Pt-based NWs result in optimized oxygen adsorption energy (EO), leading to the high ORR performance. This work gives a better understanding of the positive effect of the thin atomic layer on enhancing catalytic performance.27 The surface engineering strategy can also be applied to develop high-performance Pt-based NWs by generating an abundant of high-index facets. In one of our works, Pt−Co NWs with a rough surface exhibit much higher ORR specific and mass activities than Pt/C. HAADF−STEM images show that this nanowire contains many uneven surfaces and highindex facets (Figure 4g). Calculation results reveal that the oxygen adsorption energy (EO) of adsorption sites on {310} surface is closer to zero due to the compressive strain, thus boosting the ORR performance34 In addition, such catalyst shows superior electrochemical stability due to its excellent antiaggregation properties (Table S1). Hydrogen Evolution Reaction

Hydrogen evolution reaction provides an efficient and clean way to produce high-purity H2.26 Pt-based catalysts have achieved excellent performance for HER in acidic conditions since it is good for the adsorption and recombination of the H


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Figure 7. (a−e) Electrocatalytic performance of PtPb nanoplate/C, PtPb nanoparticle/C, and commercial Pt/C catalysts for ORR. (f−i) DFT calculations of oxygen adsorption energy. Reproduced with permission ref 15. Copyright 2016 American Association for the Advancement of Science.

sulfuration process, wherein a PtNi/NiSx interface can be constructed.45 In this system, different components are responsible for different catalytic procedures: NiSx promotes water dissociation, while PtNi enhances Hads adsorption. Taking advantage of the synergy effect between PtNi and NiSx, a current density of 37.2 mA/cm2 at an overpotential of 70 mV was achieved for PtNi/NiSx catalysts (Figure 8). The PtNi/NiSx largely retains its origin performance after longterm stability test due to its 1D structure (Table S2). An additional advance was realized in the ultrathin Pt NWs grown on 2D Ni(OH)2 nanoplates by Tang’s group.52 The coeffect from Pt and Ni(OH)2 contributes to the unprecedented catalytic activity. In addition, the large contact between Pt and Ni(OH)2 alleviates possible deterioration, thus greatly contributing to the stability enhancement.

For example, in the aspect of selective hydrogenation of α,β-unsaturated aldehyde to unsaturated alcohol (UOL) or saturated aldehyde (SA), our group reported that the catalytic performance can be greatly improved by using zigzag Pt−M NWs.39 As shown in Figure 9, the selectivity for UOL is up to 95%, achieved by PtFe zigzag NWs, and the selectivity for SA can reach as high as 94% using PtFeNi zigzag NWs + AlCl3. The high resolution TEM (HRTEM) image shows that the steps and high-index facets ({410}, {310}, {220}, and {210}) on the surface provide abundant active sites to guarantee high activity. The high selectivity for UOL is attributed to the low electron density of surface Pt, and that for SA comes from the assistance of AlCl3 as CO protector (Figure 9f,g). The work highlights the importance of designing 1D Pt-based catalysts for high activity and selectivity. High activity and selectivity of this reaction were also achieved by interface engineered Pt NWs. Briefly, the Pt NWs with Pt/Ni(OH)2 interfaces show high selectivity of 87.9% from cinnamaldehyde (CAL) to hydrocinnamaldehyde (HCAL).46

Heterogeneous Reactions

Besides electrocatalysis, Pt-based catalysts have been widely studied for heterogeneous reactions due to their unique size, shape, and composition.53 Although the obtained activity is quite appealing, the poor selectivity largely restricts practical applications of these catalysts. This is because the conventional method for improving the selectivity is to modify the surface active sites by surface ligands, thiolate, and so on, which largely sacrifices the activity.54 Recent advances show that bimetallic Pt-based materials are promising candidates to improve both activity and selectivity due to synergistic effects and modified d-band locations.55

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CONCLUSIONS AND PERSPECTIVES In this Account, we focused on the most recent progress in strain, surface, and interface engineering strategies for low-D Pt-based bimetallic nanomaterials as promising catalyst candidates. The catalytic performance can be improved by strain effects for their ability to manipulate surface adsorption properties. Moreover, surface engineering provides an effective I


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Figure 8. (a−d) TEM images of PtNi−NiSx nanowires. (e, f) Electrochemical HER performance of different PtNi NW-S/C and (g, h) related DFT simulation of HER. Reproduced with permission from ref 45. Copyright 2017 Nature Publishing Group.

for other catalytic reactions. Therefore, it is necessary to apply the strain engineering strategy for Pt catalysts to other important applications, such as heterogeneous reactions. Third, selectivity is a vital parameter to evaluate the catalytic performance. However, very few works about enhancing selectivity via the interface engineering strategies are reported. For example, building different interfaces in 2D Pt nanoplates and exploring their effects for heterogeneous reactions is rarely reported. Both experimental and theoretical efforts are required to make up the gap in this area. Fourth, there is no conclusion whether the positive effect on the catalyst from surface engineering can be preserved during the long-term catalytic processes. Hence accurately ascertaining the surface structure and element distribution of catalysts under different conditions is highly recommended. Finally, there is a trade-off between activity and stability in the creation of highperformance catalysts. Compared with works on improving activity via advanced strategies, few explorations have been performed on long-term stability. For example, the stability of Pt-based electrocatalysts in alkaline condition is quite

way to optimize catalytic performance via constructing highindex facets, highly porous structures, and ultrathin architectures. In addition, the important role of interface engineering on enhancing catalytic performance is highlighted in strong synergistic effects from different components of the interface. Up-to-date synthesis, simulations, and applications related to the above strategies for low-D Pt based bimetallic catalysts are also presented. Despite the significant progress achieved for low-D Pt-based bimetallic materials, several key problems and obstacles related to activity, stability, and selectivity still remain to be overcome (Scheme 2). First of all, to widen the number of candidates, developing reliable and facile synthesis methods for obtaining high-quality 2D Pt bimetallic nanostructures with new compositions and functionalities is highly required. In this regard, comprehensive characterizations of the intermediates in different growth stages are needed. Second, although the strain engineering strategy of Pt-based catalysts has been intensively explored for enhancing ORR, limited effort has been made and limited success has been achieved J


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Figure 9. (a) Schematic CAL hydrogenation. (b−e) CAL hydrogenation performances of (b) Pt ZNWs, (c) PtFe ZNWs, and (d) PtFeNi ZNWs + AlCl3. (f, g) XPS analysis of Pt, PtFe, and PtFeNi nanowires. Reproduced with permission from ref 39. Copyright 2018 American Chemical Society.

based catalysts will promote the rapid future development of catalysts.

Scheme 2. Perspectives of Strain, Surface and Interface Engineering of Low-Dimensional Pt-Based Bimetallic Nanostructures for Future Catalytic Reactions

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.9b00262.

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Summary of ORR and HER activity and stability of lowdimensional Pt based catalysts (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qi Shao: 0000-0002-9858-0458 Xiaoqing Huang: 0000-0003-3219-4316 Notes

The authors declare no competing financial interest. Biographies

unfavorable. This calls for the collection of more comprehensive in situ characterizations during catalytic processes. We believe that all these advanced developments of low-D Pt-

Qi Shao received her Ph.D. degree in Applied Physics from City University of Hong Kong in 2016. Now she is an assistant professor in Professor Huang’s group. Her current research interests are K


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focused on non-noble metal based catalysts for electrochemical applications. Pengtang Wang received his B.S. degree at Soochow University in 2015. Now he is pursuing his Ph.D. degree in Professor Huang’s group. His current research interests are focused on phase and interface control of metal-based catalysts for electrochemical applications. Ting Zhu received his bachelor degree in Materials Chemistry from East China University of Technology in 2016. He is currently a joint master student under the supervision of Professor Huang. His research focuses on the design of metal-based nanomaterials for electrochemical water splitting. Xiaoqing Huang is currently a Professor at College of Chemistry, Chemical Engineering and Materials Science, Soochow University. He obtained his B.Sc. in chemistry education from Southwest Normal University (2005) and Ph.D. in inorganic chemistry from Xiamen University (2011) under the supervision of Profs. Nanfeng Zheng and Lansun Zheng. Then he joined the group of Profs. Yu Huang and Xiangfeng Duan as a postdoctoral research associate from 2011 to 2014 at University of California, Los Angeles. His current research interests are in the design of nanoscale materials for electrocatalysis and heterogeneous catalysis.

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ACKNOWLEDGMENTS This work was financially supported by Ministry of Science and Technology (Grant Nos. 2016YFA0204100 and 2017YFA0208200), the National Natural Science Foundation of China (Grant 21571135), Young Thousand Talented Program, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Natural Science Foundation of Jiangsu Higher Education Institutions (Grant 17KJB150032), and start-up support from Soochow University.

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