Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size

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Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-dependent Localized Ostwald Ripening Duan Wenchao, Peina Zhang, Yujiao Xiahou, Yahui Song, Cuixia Bi, Jie Zhan, Wei Du, Lihui Huang, Helmuth Möhwald, and Haibing Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04823 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Materials & Interfaces

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-dependent Localized Ostwald Ripening Wenchao Duan,† Peina Zhang,† Yujiao Xiahou,† Yahui Song,† Cuixia Bi,† Jie Zhan,† Wei Du, ‡ Lihui Huang,§ Helmuth Möhwaldǁ and Haibing Xia,*,† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, P. R. China;

‡

School of Environment and Material Engineering, Yantai University, Yantai 264005, Shandong, China; §

School of Environmental Science and Engineering, Shandong University, Jinan, 250100, P. R. China; ǁ

Max Planck Institute of Colloids and Interfaces, Potsdam-Golm Science Park, 14476 Potsdam, Germany. KEYWORDS: aerogel, surface facet, size-dependent, Ostwald ripening, electrocatalysis

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ABSTRACT

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Aum-n aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle-number ratios, where m and n represent the size of Au NPs, respectively. In addition, 3D Aum-n -Pd aerogels were further synthesized on the basis of Aum-n aerogels and also bear controlled surface facets due to the formation of ultrathin Pd layers on Aum-n aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Aum-n and Aum-n-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent nanoparticles. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening, but also will open up a new way to improve electrocatalytic performance of three-dimensional metallic aerogels by surface regulation.


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INTRODUCTION Traditional aerogels have been widely studied due to their ultra-low density, high surface area, and large open interconnected pores.1 More recently, studies on noble-metal aerogels assembled from colloidal nanoparticles (NPs) have become a hot research topic due to their unique physical and chemical properties.1,2 For instance, noble-metal aerogels bear new features (such as good electrical and thermal conductivity, catalytic activity, and ductility/malleability) due to their continuous metal backbone nano-networks, in comparison with traditional aerogels. Therefore, noble-metal aerogels with various chemical compositions, structures and morphologies3,4 have been extensively prepared in the last few years. However, what is lacking are simple and fast methods to fabricate aerogels with controlled facets and hence activity. Current synthetic approaches towards noble-metal aerogels are mainly classified as follows: (i) spontaneous onestep gelation processes5,6 and (ii) two-step gelation processes.2,5 The separation of the gelformation step from particle formation in the latter method can give additional control over the final morphology of noble-metal aerogels by the controllable destabilization of NP sols, which is generally achieved by slight reduction of stabilization or partial removal of the surfactants,7 further NP aggregation and fusion, which is achieved by localized Oswald ripening.8–12 Currently, a series of mono/multi-noble metal aerogels have been prepared from solutions of colloidal NPs via a controllable destabilization approach.1,2,13 However, most metallic aerogels are currently prepared by the use of only a single size of NPs. Accordingly, the Oswald ripening between the adjacent NPs is nearly the same and is also rather slow due to the slight difference in size.11 Thus, the types and area ratios of exposed surface facets (active facets and inactive facets) on their surfaces are fixed and cannot be tuned. In addition, the defect density on their surfaces is also low due to the slower Oswald ripening.11


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It is well known that a colloidal solution consisting of different sizes of NPs is not thermodynamically stable due to the presence of the large interfacial area of smaller particles. Thus, further stabilization may be achieved by the reduction of the interfacial area.14 In the case of metallic NPs, the situation can be described as the growth of larger NPs (with a lower free energy) at the expense of smaller ones (with a higher free energy). This is one of the common behavior of metallic NPs, known as Ostwald ripening.11 More recently, multiple rounds of Ostwald ripening have become a matter-relocation approach to fabricate functional nanostructured materials15,16 in addition to its utilization as a powerful means for synthesis of uniform particulate matter.17,18 Thus, it may be used to assist the preparation of noble-metal aerogels on the basis of the current method.1 Due to the big difference in size of small and large NPs, the dissolution of smaller NPs and the redeposition of the dissolved species on the surfaces of NP aggregates formed by small and large NPs, is expected to occur. In addition, surface reconstruction may also occur, which may further tune the area ratio of exposed facets and increase the defect density on their surfaces due to multiple rounds of Oswald ripening, if the curvature between adjacent NPs is rather large. Hence, the formation of different facets and different area ratios on the surfaces of metallic NP aggregates (metallic aerogels) may be realized by the mismatch of the sizes of NPs used for fabrication of NP aggregates, and this may affect their reactivity and stability. Moreover, core-shell structured metallic aerogels with ultrathin skin of only a few atomic layers may be synthesized by addition of optimal amounts of aqueous precursor solution of the second metal to directly deposit onto the surfaces of the aerogels of the first metal as the core during their formation process, and surface facets of the resulting coreshell structured metallic aerogels may be controlled by our method. Thus, our method will be


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more convenient, in comparison with the just reported sacrificial template method for synthesis of core-shell structured metallic aerogels, not yet providing controlled surface facets.19 The activity and stability of electrocatalysts made up of dispersed NPs are largely dependent on their surface crystalline structure.20–22 However, surface properties (such as defect density, lattice strain, and its correspondence to local surface curvature) of ligaments in noble-metal aerogels towards electrocatalytic performance are less investigated. There is even no work reported on manipulating surface facets of noble-metal aerogels. Therefore, the current investigation on the supposedly excellent performance of noble-metal aerogels as electrocatalysts is still in the early stages and exploits only superficially, although their synthesis has been well developed. For instance, the improvements in electrocatalytic performance of noble-metal aerogels are still achieved by means of the known properties of traditional aerogels (high surface areas and high porosity).2 Only recently, Chen's group reported that the area ratio of {111} and {100} facets on the surfaces of three-dimensional (3D) nanoporous gold (NPG) can be tuned by optimizing the dealloying conditions with the aid of different types of surfactants. Accordingly, the resulting NPG can exhibit significantly enhanced electrocatalytic activities on methanol oxidation and oxygen reduction reactions, respectively, due to facet-dependent performance.23 In addition, it was also found, that there is an antagonism between stability and reactivity24 due to the area ratio of active facets and inactive facets on the exposed surfaces of electrocatalysts. Thus, the fabrication of noble-metal aerogels with varied surface facets and their area ratios should combine the merits (excellent activity and stability) for different catalytic applications by aggregation of two sizes of NPs and further localized Ostwald ripening. Herein, in this work, on the basis of salt-induced assembly of citrate-stabilized Au NPs for preparation of noble-metal aerogels,1,3


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Scheme 1. Schematic representation of formation process of Aum-n aerogels by salt-induced assembly of citrate-stabilized Au NPs of two different sizes：(a) mixing of small NPs and big NPs; (b) aggregation of Au NPs of two different sizes induced by addition of NaCl; (c) formation of Aum-n aerogels by localized Ostwald ripening, and (d) SEM image of Aum-n aerogels by freezing drying. small Au NPs (6 nm in size), which were used as the source of species for dissolution and redeposition, were mixed with large Au NPs (16, 30 or 50 nm in size) to fabricate Aum-n aerogels by addition of a proper amount of NaCl solution to induce the aggregation of Au NPs of two different sizes at controlled particle-number ratios in solution (Scheme 1a and 1b), followed by formation of Aum-n aerogels with different surface facets due to size-dependent localized Ostwald ripening (Scheme 1c) and further quenching the structure by freezing drying (Scheme 1d), where m and n represent the size of Au NPs, respectively. In addition, Aum-ny-Pdx aerogels are further synthesized by addition of optimal amounts of aqueous Na2PdCl4 solution during the formation process of Aum-n aerogels, where x and y represent the molar fractions of Pd in the shells and Au in the cores, respectively. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Aumn

aerogels and Aum-ny-Pdx aerogels as examples, the relationship between the types and area

ratios of exposed facets on their surfaces and their electrocatalytic performance (the activity and stability) of electrocatalysts are established. EXPERIMENTAL Material Hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4·4H2O, 99%), sodium chloride (NaCl, 99.8%) and trisodium citrate dihydrate (Na3C6H5O7, 99%), were purchased from Sinopharm


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Chemical Reagent Co. Ltd (Shanghai, China). Silver nitrate (AgNO3, 99+%) and sodium tetrachloropalladate (II) (Na2PdCl4, 99%) was purchased from Alfa Aesar (Tianjin, China). All the chemicals were used as received. All glasswares were cleaned with aqua regia (3:1 v/v HCl (37%): HNO3 (65%) solutions) and then rinsed thoroughly with Milli-Q water before use. (Caution: aqua regia solutions are dangerous and should be used with extreme care; never store these solutions in closed containers.) The water in all experiments was Milli-Q water (18 MΩ cm, Millipore). The aqueous solution of HAuCl4 (25 mM) was prepared and stored at ca. 4 °C before use. Synthesis of Citrate-Stabilized Au NPs Aqueous suspensions of citrate-stabilized Au NPs with sizes of 6, 16, 30, and 50 nm were prepared according to previous work.25–27 A typical synthetic procedure of 16 nm Au NPs was as follows: 0.50 mL of aqueous HAuCl4 solution (25 mM) was added into 48 mL of boiling water without noticeable bubbling under stirring, followed by further boiling for 10 min. Subsequently, 1.5 mL of aqueous solution of sodium citrate (1 w t %) was rapidly injected into the boiling HAuCl4 solution under vigorous stirring. The reaction solution was further refluxed for 30 min under stirring to warrant the formation of monodisperse, quasi-spherical Au-NPs with sizes of 16 nm. All the recipes for synthesis of Au NPs of various sizes were listed in Table S1. The sizes of as-prepared Au NPs were determined by TEM to be 6 ± 0.5 nm, 16 ± 0.5 nm, 30 ± 5 nm, and 50 ± 3 nm (Figure S1). It was assuming that all added Au3+ ions were reduced to Au0 atoms by citrate in solution and formed spherical Au NPs. The particle number concentration can be calculated by 6 /(ρ π ), where is the mass of Au (which is derived from the amount of HAuCl4 used for the synthesis of Au NPs), ρ is the density of Au, is the diameter


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of the corresponding Au-NPs, and is the volume of solution (in this case, the volume of solution is 50 mL ). Accordingly, the particle number concentration of 6, 16, 30, and 50 nm Au NPs were determined to be 1.76×1013/mL, 1.15×1012/mL, 2.6×1011/mL, and 6.87×1010/mL, respectively. Preparation of Aum-n Aerogels (where m and n represent the size of Au NPs used, respectively). In order to obtain Aum-n aerogels with optimal electrocatalytic performance, two sizes of Au NPs were mixed at different particle-number ratios, followed by formation of Aum-n aerogels with different surface facets and further quenching the structure by freezing drying. A typical synthesis of Au6-16 aerogels with particle-number ratio of 9:1 (6 nm: 16 nm) was as follows: 0.74 ml of 6 nm Au NPs dispersion (1.76×1013/mL) and 1.26 ml of 16 nm Au NPs dispersion (1.15×1012/mL) were added to the vial. The mixture of two-sized NP dispersion was stirred for 30 min at room temperature to ensure the formation of a homogeneous dispersion. Then 0.5 ml of aqueous NaCl solution (0.4 M) was added, followed by slightly shaking for 1 min. After 12 hours, the supernatant of the NP dispersion became transparent, leaving black precipitates at the bottom. Au6-16 aerogels with other particle-number ratios were prepared by the same procedure, in which the particle numbers of 16 nm Au NPs were adjusted and that of 6 nm Au NPs was fixed (Table S2). Similarly, other Au6-n aerogels (n= 30, and 50 nm) with different particle-number ratios were also prepared by the same method. All of the recipes for synthesis of Aum-n aerogels were listed in Table S2.


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Preparation of Aum-ny-Pdx Aerogels (where x and y represent the molar fractions of Pd and Au in the shells and the cores, respectively). To achieve their best catalytic performance toward ethanol electro-oxidation, a series of Aumn

y-Pdx

aerogels with different Pd content were synthesized on the basis of Aum-n aerogels with

optimal electrocatalytic performance, which was determined by comparison of their ECSAs and the corresponding electrocatalytic performance. For instance, Au6-1690.3-Pd9.7 aerogel was based on synthesis of Au6-16 aerogels with particle-number ratio of 9:1 (6 nm: 16 nm). A typical synthesis of Au6-1690.3-Pd9.7 aerogel is as follows: During the formation process of Au6-16 aerogels, 50 µL of aqueous Na2PdCl4 (1 mM) solution was added to the mixture of two-sized NP dispersion after the addition of aqueous NaCl solution (0.5 ml, 0.4 M) was stirred for 30 min. Within 48h, Au6-1690.3-Pd9.7 precipitates were generated at the bottom of the vial. After being washed with water thrice, Au6-1690.3-Pd9.7 aerogel was obtained via freezing drying under 30 mTorr at -50℃ for 2 days using a Pilot 1-2 freeze dryer. A series of other Aum-ny-Pdx aerogels (Au6-30y-Pdx and Au6-50y-Pdx) with different Pd content were synthesized on the basis of corresponding Aum-n aerogels with optimal electrocatalytic performance. The detailed recipes for synthesis of Aum-ny-Pdx Aerogel were listed in Table S3. Characterization Techniques UV–Vis absorption spectra were recorded by using a Cary 50 spectrophotometer. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2100F transmission electron microscope operating at an acceleration voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping images were acquired by a scanning transmission electron microscope— energy dispersive spectrometer (STEM-EDS) using a JEOL-2100F electron microscope


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equipped with a STEM unit. The scanning electron microscope (SEM) images were performed on an S-4800 (Hitachi, Japan) SEM. X-ray photoelectron spectroscopy (XPS) spectra were obtained using an XPS instrument (ESCALAB 250) equipped with a monochromatic Al X-ray source (Alα, 1.4866 keV). The surface area of the sample was determined by Brunauer-EmmettTeller method (BET, ASAP2020, Micromeritics Inc) with nitrogen as adsorption gas. Before the analysis, the sample was firstly degassed in a vacuum (250 mHg). Then, the weight of the dry, outgassed sample was obtained. Lastly, the sample tube containing the sample was transferred to the analysis port to start the analysis. Electrochemical Measurement Cyclic voltammetric (CV) and chronoamperomeric (CA) experiments were performed in a standard three-electrode cell in a CHI660D work station. The working electrodes were prepared by fixing as-prepared Aum-n aerogels or Aum-ny-Pdx aerogels onto glassy carbon electrodes (GCE) (3 mm in diameter) employed as the working electrodes, while an Ag/AgCl electrode and Pt wire were used as the reference electrode and auxiliary electrode, respectively. The bare GCE was polished with 0.30 and 0.05 µm alumina slurry successively, followed by rinsing thoroughly with pure water and drying at room temperature before use. The typical procedure for the preparation of the working electrodes was as follows, in which GCEs was modified by as-prepared Au6-16 aerogels. After post-treatment, the stock solution of Au6-16 aerogels was obtained by multiple synthesis, in which the mass concentration of the stock solution of Au6-16 aerogels is about 1.41 g/ L(Cstock), determined by inductively coupled plasma optical emission spectrometry (ICP-OES). 6 µL of stock solution of Au6-16 aerogels was taken out and drop-coated on the bare GCE, followed by drying in air. Then, 10 µL of the ethanol solution of Nafion (1 wt %) was cast on the surface of the GCE coated by Au6-16 aerogel,


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followed by drying in air for further use. The Au mass of Au6-16 aerogels loaded on the GCE can be calculated accordingly (Aumass = Cstock*6 L), which was about 8.46 g. Similarly, the mass concentration of Au6-30 aerogels and Au6-50 aerogels are about 1.37 and 1.34 g/ L(Cstock), also determined by ICP-OES. The Au mass of Au6-30 aerogels and Au6-50 aerogels loaded on the GCE were about 8.22 g and 8.04 g, respectively. Similarly, the Pd mass concentration of the stock solution of Au6-1690.3-Pd9.7 aerogels, Au630 97.5-Pd2.5

aerogels and Au6-5097.7-Pd2.3 aerogels were 0.0819 gPd/ L, 0.019 gPd/ L and 0.017

gPd/ L, respectively. Accordingly, The Pd mass of Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels loaded on the GCE were about 0.491 gPd (mAu+Pd=8.95

g), 0.114 gPd (mAu+Pd=8.33 g) and 0.102 gPd (mAu+Pd=8.14 g), respectively. An aqueous solution of H2SO4 (0.50 M) or KOH (0.50M) was employed as an electrolyte solution for CV characterization. The voltage scan rate in N2-saturated H2SO4 (0.50 M) and N2saturated KOH solutions (0.50 M) was 50 mV s-1. It is assumed that the charges relevant to the reduction of oxide species are 0.493 and 0.430 mC cm-2 for Au and Pd surface, respectively, the electrochemically active surface areas (ECSA) of Aum-n aerogels and Aum-ny-Pdx aerogels are theoretically calculated from the observed charges in 0.5 M KOH solution at room temperature at a scan rate of 50 mVs-1. For methanol oxidation in alkaline media, CVs were recorded between -0.50 and 0.50 V in N2- saturated 0.50 M KOH and 0.50 M methanol solution, while for ethanol oxidation in alkaline media, CVs were recorded between -0.80 and 0.40 V in N2saturated 0.50 M KOH and 0.50 M ethanol solution. The scan rate in both is 20 mV s-1. Their specific current densities and mass current densities were normalized by their ECSA values and the loaded Au or Pd mass of each catalyst, respectively.


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RESULTS AND DISCUSSION

Figure 1. TEM images of the corresponding intermediates formed at different times after addition of NaCl solution into the mixed aqueous dispersions of Au NPs of 6 nm and 16 nm: (A) 0 min, (B) 1 min, (C) 1 h, (D) 6 h, (E) 12 h. SEM image (F) and photograph (G) of as-prepared Au6-16 aerogels (E). The insets in (A to E) are photographs of corresponding colour of mixed aqueous dispersions of Au NPs in the presence of NaCl at different times. TEM images (Figure 1A-E) show the corresponding intermediates during the formation process of Au6-16 aerogels. After addition of NaCl solution into the mixed aqueous dispersion of Au NPs of 6 nm and 16 nm, the color of the dispersion changed from wine red to bluish gray within a minute (Insets in Figure 1A and 1B) and turned to light grey after about 1 hour (Inset in Figure 1C). Then, black precipitates gradually formed at the bottom of the vial while the color of the aqueous dispersion gradually became clear and displayed pale gray for about 6 hours (Inset in Figure 1D). To guarantee that all Au NPs form Au aerogels, the reaction time was extended to 12 h, at which the color of the aqueous solution was totally transparent (Inset in Figure 1E). Thanks to the unique character of the surface plasmon resonance (SPR) band of the Au NPs, the color changes indicate the variations in aggregation state of Au NPs in solution. The corresponding self-assembled intermediates during the formation process of Au6-16 aerogels were taken out and characterized by TEM images (Figure 1A-E and Figure S2A). The corresponding


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TEM images reveal that well-dispersed Au NPs (Figure 1A) area aggregated into short NP chains (Figure 1B) within 1 minute and then into interconnected NP agglomeration (Figure 1C) after about 1 h, followed by formation of a small NP-based network with multi-branches after about 1 h (Figure 1D) and large-scale interconnected Au6-16 aerogels within 12h (Figure 1E and Figure S2A). Note that in our controlled experiments, CV curves of as-prepared aerogels obtained at 12 h are rather similar to those obtained at one week, indicating that the resulting aerogels are rather close to their equilibrium state. After freeze drying, the same morphology of Au6-16 aerogels is observed in the SEM images (Figure 1F), and their photograph is shown in Figure 1G. Thus, extinction spectra, being sensitive to size and shape of the intermediates in the solutions (samples shown in Figure 1A to 1E), are shown in Figure S3A. Before addition of NaCl solution, the mixed aqueous dispersion of Au NPs of 6 nm and 16 nm shows a strong SPR peak at 519 nm, indicative of well dispersed individual Au NPs. After addition of NaCl solution, the SPR intensity at 519 nm gradually decreases while a new SPR peak appears at about 675 nm, indicating the formation of short chains made up of individual Au NPs.12,28 Subsequently, almost-flat absorbance curves with a broad peak from 600 to 1000 nm are observed in the extinction spectra, indicating the formation of network-like NP aggregation.10 The results are in good agreement with TEM results (Figure 1A to 1C). Compared to Au aerogels prepared in previous works,1,12,29 our Au6-16 aerogels were prepared by two differently-sized Au NPs, especially the introduction of small Au NPs with diameters of 6 nm, which are rather active. However, the final morphology of our Au6-16 aerogels is also rather similar to those reported in literature.1,12,29 Au6-16 aerogels in our case are mainly made up of three different structural features, which are named as bridge ①, ligament ②, and island ③, respectively, highlighted by arrows (Figure 2A).


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Figure 2. Low (A, D and G), and high (B, E and H) magnification TEM images and HRTEM images (C, F and I) of the edge areas of Au6-16 aerogels (A), Au6-30 aerogels (D) and Au6-50 aerogels (G). Table 1. Comparison of average sizes of bridges, ligaments and islands in the Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels. Sample

average size bridges (nm)

of average size of ligaments average size of islands (nm) (nm)

Au6-16 aerogels

10.2±1

24.6±2

62.2±5.3

Au6-30 aerogels

23.3±2.5

39.0±1.5

86.3±3.5

Au6-50 aerogels

35.2±2.5

51.8±3.3

99.5±2.8

For bridge structures, their average sizes are between 16 nm (size of big Au NPs) and 6 nm (size of small Au NPs), indicating that the bridge structure is mainly formed by 6 nm Au NPs and further grows. Accordingly, the average sizes of ligaments in Au6-16 aerogels are larger than


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16 nm, indicating that the ligament structure is mainly formed by 16 nm Au NPs and further grows. As for island structure, their average NP sizes are in the range of 50 to 70 nm, much larger than 16 nm. The formation of island structure may be due to the fusion of several Au NPs and further growth.24 In our case, the formation and further growth of the three structural features mentioned above are due to localized Ostwald ripening at the expense of 6 nm Au NPs,10,30 which are dissolved to provide extra atoms for structure growth by localized Ostwald ripening.8– 12

Note that TEM images of the intermediates taken out within 18 min (Figure S4) clearly show

the presence of many small Au NPs with becoming smaller than 6 nm due to the dissolution. Eventually, these small Au NPs completely disappear after 6 h (Figure 1D). On the basis of our previous work and others, citrate-stabilized Au NPs are polycrystalline and bear inborn twin defects.21,27,31,32 Besides the inborn defects of Au NPs, new defects also form in the sintering regions between two Au NPs during the formation process of Aum-n aerogels, especially at the change of the growth direction24,33 (Figure 2B and Figure S2D). The presence of defects in Aum-n aerogels could enhance their catalytic performance enormously.21 Very similar to those of the freestanding films (FSFs) obtained via interfacial assembly and overgrowth,24 the ligaments of the resulting Au6-16 aerogels are randomly oriented and have a highly curved (concave or convex) morphology (Figure 2A). Original quasi-spherical Au NPs are expected to be covered with low-index crystalline facets, mainly {111} and {100} facets (Figure S5 and Table S4). These low-index facets are still preserved on the resulting gold ligaments (Figure 2C). According to literature,24,34 the intensity variation of the TEM images across the ligaments suggests the presence of high-index crystalline facets with a very high density of atomic steps and kinks at the highly curved surfaces, which are distinct from nanocrystals with well-defined facets.35 To better understand the crystalline facets on the surfaces of the resulting Au6-16


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aerogels, high resolution transmission electron microscopy (HRTEM) images of the edge area were measured (shown in Figure 2C). The lattice fringe spacings were about 0.204 nm and 0.236 nm, which correspond to the spacings of the {100} planes and the {111} planes of face-centered cubic (fcc) gold, respectively. Thus, atomic steps along the edge areas are composed of a series of {100} terraces and {111} steps, which are highlighted by red dashed lines and black dotted lines for guidance. In addition, the fraction of {100} terraces in the edge areas is much higher than that of {111} steps. Similarly, Au6-30 aerogels (Figure 2D and Figure S2B) and Au6-50 aerogels (Figure 2G and Figure S2C) were prepared by the same synthetic procedure except that 16 nm Au NPs were replaced by 30 nm Au NPs and 50 nm Au NPs, respectively. In addition, optimal particlenumber ratios of 30 nm Au NPs and 50 nm Au NPs to 6 nm Au NPs were also adjusted accordingly. Au6-30 aerogels and Au6-50 aerogels both bear similar morphology as Au6-16 aerogels except the difference in average sizes of bridges, ligaments and islands (Table 1). With the increase in size of large Au NPs (from 16 to 30 and 50 nm) mixed with 6 nm Au NPs, average sizes of bridges, ligaments and islands in Aum-n aerogels all increase accordingly. Moreover, Au630

aerogels and Au6-50 aerogels both bear a wealth of defects (Figure 2E, 2H, Figure S2E and

S2F). However, on the basis of HRTEM images (Figure 2F and 2I), the crystalline facets on their surfaces are different from those of Au6-16 aerogels. The lattice fringe spacings of Au6-30 aerogels and Au6-50 aerogels both are about 0.235 nm and 0.144 nm, which is consistent with the spacings between the {111} planes and the {110} planes of fcc Au, respectively. Thus, atomic steps along the edges are composed of a range of {110} terraces and {111} steps, also indicated by red dashed lines and black dotted lines for guidance, which are different from that of Au6-16 aerogels.


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Figure 3. Cyclic voltammograms of Aum-n aerogels on glassy carbon electrodes in 0.5M H2SO4 at a scan rate of 50mV s-1: Au6-16 aerogels (a), Au6-30 aerogels (b) and Au6-50 aerogels (c). Table 2. Peak positions, compositions of atomic steps on the surfaces and their ratios in Aum-n aerogels obtained by CV curves. Sample

peak position compositions of atomic steps and III (V) their ratios

peak position

peak position

I (V)

II (V)

Au6-16 aerogels

——

1.23

——

{100}+{111} and Γ{100} >> Γ{111}

Au6-30 aerogels

1.15

——

1.36

{110}+{111} and Γ{110} > Γ{111}

Au6-50 aerogels

1.15

——

1.36

{110}+{111} and Γ{110} < Γ{111}

Γ represents the percentage of possession of different crystal planes Cyclic voltammetry (CV) curves of gold in concentrated acid solutions can serve as the “fingerprint” of specific lattice planes36,37 and were widely used to characterize specific lattice structures of nanostructured gold.23 Thus, the exposed facets on the resulting Aum-n aerogels were further investigated by electrochemical measurements. The standard potentials (versus Ag/AgCl, 3 M KCl) of the {110}, {100} and {111} planes of gold electrodes in aqueous H2SO4 solution (0.5 M) are around 1.14 V, 1.2 V and 1.4 V, respectively.36,38 Since the line profiles of the CV curves of electrodes modified by our three different aerogels in 0.5M H2SO4 are indeed different


17


Page 18 of 38

from each other, (Figure 3 and Table 2), the exposed facets on their surfaces should be different. For instance, only one pronounced peak at around 1.23 V (versus Ag/AgCl, 3 M KCl) is shown in the CV curve of Au6-16 aerogels, confirming that the atomic steps on the surface are predominantly composed of a {100} subfacet. In the case of Au6-30 and Au6-50 aerogels, there are both two clear oxidation peaks at around 1.15 V and 1.36 V (versus Ag/AgCl, 3 M KCl) shown in their CV curves, confirming that atomic steps on their surface are composed of {110} and {111} subfacets. In addition, the ratio of {110} subfacet to {111} subfacet in the edge areas of Au6-30 aerogels is higher and that of Au6-50 aerogels is lower. This is also in good agreement with TEM results. The underpotential deposition of lead (Pb-upd) in alkaline media was further conducted to determine crystal planes of the corresponding Au aerogels (Figure S6). As shown in Figure S6A, two peaks located at about -0.6 V and -0.47 V were observed, which are attributed to the step and kink sites, and {100} facets of Au6-16 aerogels (Figure S6A),23,39 indicating that Au6-16 aerogels are mainly enclosed by {100} facets. Moreover, two peaks located at about -0.51 V and -0.37 V both are also displayed in Figure S6B and S6C, which are attributed to {111} and {110} facets of Au6-30 aerogels and Au6-50 aerogels, respectively. However, the intensity ratios of the peak at -0.51 V to the peak at -0.37 V in Figure S6B and Figure S6C are different, indicating that the ratio of {110} facets to {111} facets in the edge areas of Au6-30 aerogels is higher and that of Au6-50 aerogels is lower. These results are in good agreement with CV results. To better understand the evolution of surface facets of Au NPs of two different sizes during the formation of Aum-n aerogels, a thermodynamic model is employed for qualitative analysis.40–42 The total Gibbs free energies of Au NPs of two different sizes (6, 16, 30 and 50 nm) and the intermediates of Aum-n aerogels are given by Equation (1) and (2), respectively, in which c, ,


18

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Scheme 2. Schematic representation of the evolution of surface facets of Au NPs of two different sizes during the formation of Aum-n aerogels without (a) and with surface reconstruction (b).

, and are cohesive energy, surface energy, elastic strain energy and twin-boundary energy, respectively; V, S and T are the volume, the total surface area and the twin-boundary area, respectively; , , W and are the cohesive energy per unit volume, the surface energy per unit area, the elastic strain energy density and the twin-boundary energy per unit area, respectively; , and (1--) are the percentages of the {111} facet, {110} facet and {100} facet in the total surface area, respectively. It is assumed that, if particles with different shapes contain the same number of atoms, they contain the same cohesive energy c41, because it depends on the volume.It can be concluded from Equation (1) and (2) that the cohesive energy c remains unchanged before and after the formation of Aum-n aerogels. Therefore, the reduction of the total Gibbs free energies of the final products mainly depends on the co-action of surface energy , newly generated elastic strain energy and newly created twin boundary energy .

!

"

!

(1)

% &''' &''( (1 ) ) )&'((

!

"

!


19


Page 20 of 38

*+,-.

*+,-.

*+,-. *+,-. *+,-. " /0 ! 10 ! "/0 ! 10 ! " (2)

( *+,-. )% & *+,-. ''' & *+,-. ''( (1 ) ) )& *+,-. '(( *+,-. 3 *+,-. 4 *+,-. *+,-. /0 ! 10 ! "/0 ! "10 !

In the solution of citrate-stabilized Au NPs of two different sizes, NaCl was employed as a destabilizing agent to induce their aggregation. During the formation of Aum-n aerogels, the surface energy should decrease due to the decrease in total surface area while the elastic strain energy and the twin boundary energy are increased due to new formation of the connections. Since Aum-n aerogels are formed due to size-dependent localized Ostwald ripening, it is clear that the reduction of the surface energy is greater than the augment of the elastic strain energy and the twin boundary energy sum. To maximally reduce the interfacial energy of Aum-n aerogels formed (the sum of specific interfacial energies for the twin boundary and the free surface43), Au NPs of small size and medium size will preferentially fuse together by their {111} facets,44 this would lead to the decrease of {111} facets and predominant existence of {100} facets in Aum-n aerogels (Scheme 2a) if there is no occurrence of surface reconstruction during the formation of Aum-n aerogels. However, surface reconstruction during the formation of Aum-n aerogels would be unavoidable to reduce the stress and elastic strain energy which would occur due to the large curvature in the fusion region. (Scheme 2b) It is generally thought that quasi-spherical NPs are covered with low-index crystalline facets, mainly {111} and {100} facets; the area ratio of {111} facets to {100} facets (R) on the surfaces increases with the increasing particle size.45,46 Thus, it is reasonable that the area ratio of {111} facets to {100} facets on the surfaces of 6 nm Au NPs is rather close while that of 16 nm Au NPs is a little larger. However, due to the high number ratio of 6 nm Au NPs to 16 nm Au NPs of 9:1, most {111} facets of 6 nm Au NPs and 16 nm Au NPs are fused together (Figure S7A). In


20

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addition, there would be no generation of remarkable strain as the minor difference in size between them would result in a smaller curvature. Accordingly, in our case of Au6-16 aerogels, {100} facets predominantly exist in Aum-n aerogels while most {111} facets disappear due to the fusion between Au NPs of small size and medium size. As for Au6-30 aerogels and Au6-50 aerogels (Figure S7B and S7C), due to relatively high ratio of {111} facets existing on the surfaces of Au NPs of 30 nm and 50 nm, there are still remaining the {111} facets after fusion and the proportion of the remaining {111} facets on Au6-30 aerogels is lower than that on Au6-50 aerogels (Figure 3 and Table 2). However, due to too high differences in size between large Au NPs (30 and 50 nm) and small Au NPs (6 nm), the large curvature in the fusion region would appear during the formation of Au6-30 aerogels and Au6-50 aerogels (Scheme 2b), thus leading to significant stress and elastic strain energy and further driving the surface reconstruction.47,48 Considering the difference in the surface free energies (5) of Au {111}, Au {100} and Au {110} facet, which are 1.52 J m-2, 1.80 J m-2 and 1.94 J m-2, respectively,49 the transformation of {100} to {110} facet during the formation of Aum-n aerogels is highly possible. Thus, in Au6-30 and Au650

aerogels, the original Au {100} facets disappear, and a high ratio of newly formed Au {110}

facet appears instead (Figure 3, S5, Table 2 and Table S4). In addition, the ratio of newly formed Au {110} facets on Au6-30 aerogels is higher than that on Au6-50 aerogels (Figure 3 and Table 2), as the ratio of {100} facet on the surfaces of 30 nm Au NPs is higher than that of 50 nm Au NPs (Figure S5).45,46 In order to obtain Aum-n aerogels with optimal electrocatalytic performance, two sizes of Au NPs were mixed at different particle-number ratios to prepare a series of Aum-n aerogels (Table S2). In our cases, the particle number of 6 nm Au NPs was fixed and those of other Au NPs (16, 30 and 50 nm) are accordingly adjusted. The electrocatalytic activities (such as mass and specific


21


Page 22 of 38

activity) of a series of Aum-n aerogels obtained were investigated and compared to select the corresponding Aum-n aerogels with optimal electrocatalytic performance on methanol oxidation (Figure S8, S9, S10, Table S5, S6 and S7).

Figure 4. CV curves (A to C) and potential cycling stability (D) of GCEs modified by Au6-16 aerogels (a, black curve), Au6-30 aerogels (b, red curve) and Au6-50 aerogels (c, blue curve), respectively, measured in 0.5M KOH solution in the absence (A) and presence (B to D) of 1.0 M methanol. The scan rates in panels (A, D) and (B, C) are 50 and 20 mV s -1, respectively. The current densities are normalized by the Au mass loaded (A and B) and the ECSA values (C), respectively. Table 3. Summarized data of ECSA values, mass activities, specific activities and the percentage of oxidation peak current density after 500 cycles of Aum-n aerogels toward the methanol electrooxidation.

Sample

Au Au Au

6-16 6-30 6-50

Average mass Average specific Percentage of oxidation Average activity ECSA[m2 g- activity peak current density 1 -1 -2 ] after 500 cycles [A g ] [µA cm ]

aerogels

2.3

6.18

268.7

84.4±1.5%

aerogels

2.09

4.32

206.7

88.9±1.8%

aerogels

1.70

3.67

215.9

94.2±1.9%


22

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On the basis of test results, the optimal particle ratios for synthesis of Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels with optimal electrocatalytic performance in each group are 9:1, 12:1 and 20:1, respectively (Figure S8, S9, S10, Table S5, S6 and S7). Moreover, in order to demonstrate the effect of their exposed facets on electrocatalytic performance in methanol oxidation, Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels with optimal electrocatalytic performance in each group were further compared. The ECSA values of Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels are 2.30, 2.09 and 1.70 m2 g-1, respectively (Figure 4A and Table 3). This is reasonable, because the order of average sizes of bridges, ligaments and islands in Aum-n aerogels is Au6-16 aerogels < Au6-30 aerogels < Au6-50 aerogels (Table 1). The BET surface area of our Au6-50 aerogels (about 5.4 m2/g) is much lower than those of Au aerogels3 with ligaments of about 6 nm (about 50.1 m2/g) and is close to those of nanoporous gold of comparable size.50 Thus, the low ECSA values of the resulting Aum-n aerogels may be attributed to their big ligaments in size. Figure 4B depicts the mass activities of Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels. Their order is as follows: Au6-16 aerogels (6.18 A g-1) > Au6-30 aerogels (4.32 A g-1) > Au6-50 aerogels (3.67 A g-1). As the ECSAs of Au6-16 aerogels and Au6-30 aerogels are rather close, the differences in their mass activities may be attributed to the differences in their specific activities, which are affected by facets exposed on their surfaces.51,52 As expected, the specific activity of Au6-16 aerogels (268.7 µA cm-2) is far larger than that of Au6-30 aerogels (206.7 µA cm-2) or Au650

aerogels (215.9 µA cm-2) (Figure 4C and Table 3). It is known that Au NPs of small size can

bear high mass activities and specific activities but bear bad stability, in comparison with those of big size.53 Accordingly, 6 nm citrated-capped Au NPs bear the highest mass activities and specific activities. In addition, the mass activities and specific activities of Au6-16 aerogels, Au6-30


23


Page 24 of 38

aerogels and Au6-50 aerogels are better than those of the corresponding 16, 30 and 50 nm citratedcapped Au NPs, respectively (Figure S11, Table S8). Thus, the Aum-n aerogels can combine the merits of both small (higher mass activities and specific activities) and large NPs (good stability). It has been reported that the electrocatalytic activity of gold in methanol oxidation is favored by the {111} and {100} facets, both of which are more active than the {110} facet.51,52 As mentioned above, the surfaces of Au6-16 aerogels are mainly uncovered by {100} facets while the surfaces of Au6-30 aerogels and Au6-50 aerogels are mainly uncovered by {111} and {110} facets. In addition, the percentage of {110} facets on the surfaces of Au6-30 aerogels is higher than that in Au6-50 aerogels (Table 2). Therefore, the results of their specific activities on methanol oxidation are in good agreement with previous reports51,52 and our TEM results. It is worth noting that Au6-16 aerogels（268.7 µA cm-2）exhibit higher specific activity for MOR under the same experimental conditions, compared with those reported Au catalysts, including spherical Au nanoparticles (65 µA cm-2)24, nanoporous gold film (103 µA cm-2)24, hollow nanoporous gold nanoparticles (117 µA cm-2)54, trisoctahedral gold nanoparticles (178 µA cm-2)35 and pyrogallolmodified nanoporous gold (267.3 µA cm-2).23 Taking the importance of durability of catalysts into account, the cycling stability of Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels on methanol oxidation was also studied by using CV cycling (Figure 4D and Figure S12). After 500 cycling tests, Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels all exhibited excellent stability and their oxidation current densities were reduced to about 84.4%, 88.9% and 94.2%, respectively. It is found that the order of stability of Au6-16 aerogels, Au6-30 aerogels and Au6-50 aerogels on methanol oxidation is highly related to the percentage of exposed {111} facets on their surfaces (Figure 3 and Table 2). The results indicate


24

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that the presence of the most stable {111} facet among the three low-index facets55–57 can improve the stability of electrocatalysts, which is also facet-dependent. Thus, the specific activity and stability of electrocatalysts both are facet-dependent. To further demonstrate facet-dependent performance of electrocatalysts with different exposed facets, Aum-n-Pd aerogels were also prepared on the basis of Aum-n aerogels with optimal electrocatalytic performance as preferential surface facets of Aum-n-Pd aerogels for ethanol oxidation are different from those of Aum-n aerogels for methanol oxidation. It is widely accepted that depositing the second metal with catalytic activity as ultrathin skin of only a few atomic layers on the first metal as core, the catalytic activity of these bimetallic catalysts can be highly enhanced.58–60 Moreover, due to the formation of ultrathin shells on the cores, the resulting coreshell structured bimetallic catalysts can bear the same exposed crystalline facets as the cores. Thus, Aum-n-Pd aerogels with optimal ultrathin Pd shell were synthesized by adjusting the concentration of Pd2+ ions (Figure S13, S14, S15, Table S9, S10 and S11). It is known that citrate cannot reduce Pd2+ ions to Pd atoms at room temperature.61 However, the ligaments of Aum-n aerogels have a highly curved (concave or convex) morphology, associated with the local lattice strain and wrinkling of atomic planes,61,62 during the formation of Au aerogels. Thus, these active sites on the surfaces of Au aerogels may induce the deposition of Pd atoms onto them to form core-shell Au-Pd aerogels in the absence of reducing agents. As shown in TEM images (Figure 5A, 5D, 5G, Figure S16A, S16B and S16C), the resulting Aum-n-Pd aerogels are all made up of interconnected networks. The difference among them is still the difference in their average sizes of bridges, ligaments and islands. In comparison with the corresponding Aum-n aerogels (Figure 2A, Figure 2D, and 2G), as-prepared Aum-n-Pd aerogels show hardly any change


25


Page 26 of 38

in their morphology after Pd growth. The results indicate that the Pd shells on the corresponding Aum-n aerogels are so thin that they are hardly detectable by TEM images.

Figure 5. TEM images (A, D and G), HAADF-STEM-EDS mapping images (B, E and H) and HAADF-STEM images with cross-sectional compositional line profiles (C, F and I) of Au6-1690.3Pd9.7 aerogels (A, B, and C), Au6-3097.5-Pd2.5 aerogels (D, E and F) and Au6-5097.7-Pd2.3 aerogels (G, H and I). However, tiny amounts of Pd in the resulting Aum-n-Pd aerogels were characterized by HAADF-STEM-EDS mapping and cross-sectional compositional line profiles (Figure 5B, 5E, 5H, 5C, 5F, and 5I). The results of HAADF-STEM-EDS mapping demonstrate the formation of core-shell structure (Figure 5B, 5E and 5H). In addition, their cross-sectional compositional line


26

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profiles reveal a tiny amount of elemental Pd on the surfaces of the corresponding Aum-n aerogels, thus confirming the core-shell structure of the resulting Aum-n-Pd aerogels (Figure 5C, 5F, and 5I). The negative BE shifts of Au 4f and positive BE shifts of Pd 3d in their X-ray photoelectron spectroscopy (XPS) spectra (Figure S17, Figure S18 and Table S12) further indicate the formation of core-shell structure, not the formation of AuPd alloy on their surfaces. In the CV curves of Au-Pd aerogels, the Au peaks will appear in the CV curves if there is naked Au on their surface. The Au peaks of the samples, which were taken out at different reaction times, gradually disappear, indicating there is no any Au naked on the surfaces of Au-Pd aerogels and the formation of core-shell Au-Pd aerogels eventually (Figure S19). Taken together, the core-shell structure, entire elemental compositions and surface compositions of the resulting Aum-n-Pd aerogels were determined, which were denoted as Au6-1690.3-Pd9.7 aerogels (Figure 5B), Au6-3097.5-Pd2.5 aerogels (Figure 5E) and Au6-5097.7-Pd2.3 aerogels (Figure 5H), respectively. Due to the growth of ultrathin Pd shells on the corresponding Aum-n aerogels, the types of defects and exposed crystalline facets on the surfaces of the corresponding Aum-n aerogels (Figure 2A, Figure 2D and 2G) are also hardly affected. In addition, it was reported that the crystalline structures of thin Pd epitaxially grown on all three low-index planes of gold can be determined by the gold surface.63,64 Thus, the resulting Aum-n-Pd aerogels should bear the same exposed crystalline facets as the corresponding Aum-n aerogels (Figure S16D, S16E, S16F, S16G, S16H and S16I). Similarly, the surfaces of Au6-1690.3-Pd9.7 aerogels were predominantly covered by Pd {100} facets while Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels were mainly covered by Pd {110} and Pd {111} facets. Moreover, the ratio of {110} facet to {111} facet in Au6-3097.5-Pd2.5 aerogels is higher than that in Au6-5097.7-Pd2.3 aerogels.


27


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To demonstrate facet-dependence of the specific activity and stability of electrocatalysts, the electrocatalytic performance of Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels, Au6-5097.7-Pd2.3 aerogels and commercial Pd/C catalysts on ethanol oxidation were studied in detail and also compared (Figure 6, Figure S20 and Table 4). The specific activities (Figure 6A) of the Au616 90.3-Pd9.7

aerogels, Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels were 12.7 mA cm-2,

10.9 mA cm-2 and 10.0 mA cm-2, respectively. The specific activity of Au6-1690.3-Pd9.7 aerogels was the largest, which was 10.6-fold larger than that of commercial Pd/C catalyst (about 1.2 mA cm-2). In addition, the specific activity of the Au6-1690.3-Pd9.7 aerogels (12.7 mA cm-2) are about 5.50-fold, 4.43-fold, 1.37-fold and 1.02-fold larger than that of core-shell Au-Pd nanodendrites (2.31 mA cm-2),65 Au@Pd core shell nanobricks (2.87 mA cm-2),66 spherical Au@Pd nanoparticles (9.3 mA cm-2),21 and PdAu3/C (12.46 mA cm-2).67

Figure 6. CV curves (A) and potential cycling stability (B) of the GCEs modified by Au6-1690.3Pd9.7 aerogels (a, black curve), Au6-3097.5-Pd2.5 aerogels (b, red curve), Au6-5097.7-Pd2.3 aerogels (c, blue curve) and commercial Pd/C catalyst (d, magenta curve), respectively, measured in 0.5 M KOH solution in the presence of 0.5 M ethanol. The scan rates in panels A and B are 20 and 50 mV s -1, respectively. The current densities are normalized by the ECSA values (A) and the Pd mass loaded (B), respectively. Table 4. Summarized data of the specific activities and the percentage of oxidation peak current density after 500 cycles of Aum-ny-Pdx aerogels and commercial Pd/C catalyst toward the ethanol electro-oxidation, respectively. Sample

Average specific activity

Percentage of oxidation peak

[mA cm-2]

current density after 500 cycles


28

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Au6-1690.3-Pd9.7 aerogels

12.7

76.2±1.8%

Au6-3097.5-Pd2.5 aerogels

10.9

78.2±1.9%

Au6-50 97.7-Pd2.3 aerogels

10.0

85.1±2.1%

Pd/C

1.2

28.1±1.4%

The difference in their specific activities is also highly related to exposed facets on their surfaces. Since the Pd {100} facet is more efficient toward ethanol oxidation than the Pd {110} and Pd {111} facet,68 it is reasonable that Au6-1690.3-Pd9.7 aerogels mainly covered by Pd {100} facets bear the highest specific activity as Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels both are mainly covered by Pd {110} and Pd {111} facets. In addition, the specific activity of Au6-3097.5-Pd2.5 aerogels is a little higher than that of Au6-5097.7-Pd2.3 aerogels due to higher ratio of {110} facets on the surfaces of Au6-3097.5-Pd2.5 aerogels as the Pd {110} facet is more efficient toward ethanol oxidation than the Pd {111} facet. The facet-dependent stability of the aerogels in ethanol oxidation was also conducted by using the cycling test (Figure 6B and Figure S20). After 500 cycling tests, Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels and Au6-50

97.7-Pd2.3

aerogels all exhibited excellent stability and their

oxidation current densities were reduced to about 76.2%, 78.2% and 85.1%, respectively, which were about 2.7 times, 2.8 times and 3.0 times higher than that of the commercial Pd/C catalyst (28.1%). Obviously, the order of stability is as follows: Au6-5097.7-Pd2.3 aerogels > Au6-3097.5-Pd2.5 aerogels > Au6-1690.3-Pd9.7 aerogels. Similarly, the order of stability of the resulting Aum-n-Pd aerogels is strongly related to the percentage of Pd {111} facets on their surfaces. In addition, after 500 cycling test, there are obvious {111} peaks shown in the CV curve of Au6-16 aerogels while there are no apparent changes in the CV curves of Au6-30 aerogels and Au6-50 aerogels (Figure S21), indicating that higher ratio of {111} facets on the surfaces of Au aerogels indeed can be beneficial for their stability as catalysts.


29


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The facet-dependent stability of Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels and Au650 97.7-Pd2.3

aerogels on ethanol oxidation was also investigated by CA experiments at -0.3 V

versus Ag/AgCl (Figure S22). After 7200s test, the current densities of the Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels were reduced to about 0.13 A mg1

, 0.61 A mg-1 and 1.13 A mg-1, respectively, which were about 10.6 times, 50.8 times and 94

times higher than that of commercial Pd/C catalyst (0.012 A mg-1). The result is in good agreement with that of cycling tests. Our results are also in good agreement with a previous report.22 In our case, the Au6-1690.3-Pd9.7 aerogels are mainly covered by Pd {100} facets (active facets). Thus, they display higher specific activity than Pd-based aerogels without facet control reported in literature.69 For instance, their specific activity (12.7 mA cm-2 in Table 4) is about 1.03-fold, 1.39-fold, and 1.92-fold than Pd aerogels (~12.3 mA cm-2),69 Pd68Cu32 (~9.16 mA cm-2)69 and pure palladium aerogels (~6.6 mA cm-2),69 respectively. In addition, the Au6-5097.7-Pd2.3 aerogels bear a higher ratio of Pd {111} facets (inactive facets). Accordingly, they should exhibit better stability than Pd-based aerogels without facet control reported thus far.69,70 As expected, the current density of the Au6-5097.7-Pd2.3 aerogels (2.25 A mgPd-1 after 2000s; 1.13 A mgPd-1 after 7200s in Figure S22) is significantly higher than that of the Pd aerogels (0.05 A mgPd-1 after 2000s),69 Pd68Cu32 (0.3 A mgPd-1 after 2000s)69 and Au/Ag/Pd aerogels (0.8 A mgPd-1 after 7200s).70 Thus, it is desirable to regulate the surface facets of metallic aerogels, which can render them simultaneously bear high activity and excellent stability, especially for different catalytic applications. The excellent performance of the Au6-1690.3-Pd9.7 aerogels, Au6-3097.5-Pd2.5 aerogels and Au6-5097.7-Pd2.3 aerogels on ethanol oxidation can be attributed the following combined features: (1) core-shell structured Au-Pd aerogels with an ultrathin Pd shell, which improves the


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utilization of Pd shell elements;19,21 (2) the Au cores in the Au-Pd aerogels bear abundant defects, which effectively regulate the electronic structure of the Pd shells by impacting their dband center;21 (3) the Pd shell also can retain a large number of active sites from the Au cores due to ultrathin thickness, thereby greatly improving the catalytic performance. CONCLUSIONS In summary, we have shown that varying the mismatch of sizes of fusing NP affects the types and area ratios of facets of the metallic aerogels. Surface facets of the resulting metallic Aum-n aerogels can be regulated by size-dependent localized Ostwald ripening of Au NPs of two different sizes at controlled and optimized particle-number ratios. We thus have successfully prepared Aum-n aerogels by salt-induced assembly of citrate-stabilized Au NPs of two different sizes and Aum-ny-Pdx aerogels with ultrathin Pd layers by addition of Pd precursors during the formation of Aum-n aerogels. In addition, Aum-ny-Pdx aerogels also bear regulated surface facets due to the formation of ultrathin Pd layers on Aum-n aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Aum-n aerogels and Aumn

y-Pdx

aerogels as examples, it is found that surface facets of metallic aerogels with excellent

performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively, due to facet-dependent performance. Moreover, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent nanoparticles. Thus, our study demonstrates that not only the regulation of the surface facets of metallic aerogels can be realized by size-dependent localized Ostwald ripening, but also the electrocatalytic performance of 3D metallic aerogels can be enhanced by surface regulation as a new way.


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ASSOCIATED CONTENT Supporting Information. Supporting information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional TEM images, extinction spectra, XPS spectra, CV curves of Aum-n aerogels and Aum-ny@Pdx aerogels (Figures S1−S22), and summarized data in Tables S1-S12. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H.X.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is financially supported by the Natural Science Foundation of China (21473105, 21773142 and 51372138), Taishan Scholarship in Shandong Province (No. tsqn20161001), Fundamental Research Fund of Shandong University (2016JC003), and Shandong Provincial Natural Science Foundation for Distinguished Young Scientists (JQ201405).


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REFERENCES (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8) (9) (10)

(11) (12)

(13) (14) (15)

(16)

Bigall, N. C.; Herrmann, A.-K.; Vogel, M.; Rose, M.; Simon, P.; Carrillo-Cabrera, W.; Dorfs, D.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Hydrogels and Aerogels from Noble Metal Nanoparticles. Angew. Chem. Int. Ed. 2009, 48 (51), 9731–9734. Herrmann, A.-K.; Formanek, P.; Borchardt, L.; Klose, M.; Giebeler, L.; Eckert, J.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Multimetallic Aerogels by Template-Free Self-Assembly of Au, Ag, Pt, and Pd Nanoparticles. Chem. Mater. 2014, 26 (2), 1074–1083. Wen, D.; Liu, W.; Haubold, D.; Zhu, C.; Oschatz, M.; Holzschuh, M.; Wolf, A.; Simon, F.; Kaskel, S.; Eychmüller, A. Gold Aerogels: Three-Dimensional Assembly of Nanoparticles and Their Use as Electrocatalytic Interfaces. ACS Nano 2016, 10 (2), 2559– 2567. Cai, B.; Dianat, A.; Hübner, R.; Liu, W.; Wen, D.; Benad, A.; Sonntag, L.; Gemming, T.; Cuniberti, G.; Eychmüller, A. Multimetallic Hierarchical Aerogels: Shape Engineering of the Building Blocks for Efficient Electrocatalysis. Adv. Mater. 2017, 29 (11), 1605254. Liu, W.; Herrmann, A.-K.; Geiger, D.; Borchardt, L.; Simon, F.; Kaskel, S.; Gaponik, N.; Eychmüller, A. High-Performance Electrocatalysis on Palladium Aerogels. Angew. Chem. Int. Ed. 2012, 51 (23), 5743–5747. Liu, W.; Herrmann, A.-K.; Bigall, N. C.; Rodriguez, P.; Wen, D.; Oezaslan, M.; Schmidt, T. J.; Gaponik, N.; Eychmüller, A. Noble Metal Aerogels—Synthesis, Characterization, and Application as Electrocatalysts. Acc. Chem. Res. 2015, 48 (2), 154–162. Gaponik, N.; Herrmann, A.-K.; Eychmüller, A. Colloidal Nanocrystal-Based Gels and Aerogels: Material Aspects and Application Perspectives. J. Phys. Chem. Lett. 2012, 3 (1), 8–17. Aerogels Handbook; Aegerter, M. A., Leventis, N., Koebel, M. M., Eds.; Springer New York: New York, NY, 2011. Rechberger, F.; Niederberger, M. Synthesis of Aerogels: From Molecular Routes to 3Dimensional Nanoparticle Assembly. Nanoscale Horiz. 2017, 2 (1), 6–30. Pei, L.; Mori, K.; Adachi, M. Formation Process of Two-Dimensional Networked Gold Nanowires by Citrate Reduction of AuCl 4 - and the Shape Stabilization. Langmuir 2004, 20 (18), 7837–7843. Ostwald, W. Studien Über Die Bildung Und Umwandlung Fester Körper. Z. Für Phys. Chem. 1897, 22 (1), 289–330. Lee, M.-J.; Lim, S.-H.; Ha, J.-M.; Choi, S.-M. Green Synthesis of High-Purity Mesoporous Gold Sponges Using Self-Assembly of Gold Nanoparticles Induced by Thiolated Poly(Ethylene Glycol). Langmuir 2016, 32 (23), 5937–5945. Ziegler, C.; Wolf, A.; Liu, W.; Herrmann, A.-K.; Gaponik, N.; Eychmüller, A. Modern Inorganic Aerogels. Angew. Chem. Int. Ed. 2017, 56 (43), 13200–13221. Kahlweit, M. Ostwald Ripening of Precipitates. Adv. Colloid Interface Sci. 1975, 5 (1), 1– 35. Liu, X.; Stroppa, D. G.; Heggen, M.; Ermolenko, Y.; Offenhäusser, A.; Mourzina, Y. Electrochemically Induced Ostwald Ripening in Au/TiO2 Nanocomposite. J. Phys. Chem. C 2015, 119 (19), 10336–10344. Yec, C. C.; Zeng, H. C. Synthesis of Complex Nanomaterials via Ostwald Ripening. J. Mater. Chem. A 2014, 2 (14), 4843–4851.


33


Page 34 of 38

(17) Rinkel, T.; Nordmann, J.; Raj, A. N.; Haase, M. Ostwald-Ripening and Particle Size Focussing of Sub-10 Nm NaYF4 Upconversion Nanocrystals. Nanoscale 2014, 6 (23), 14523–14530. (18) Zhang, Z.; Wang, Z.; He, S.; Wang, C.; Jin, M.; Yin, Y. Redox Reaction Induced Ostwald Ripening for Size- and Shape-Focusing of Palladium Nanocrystals. Chem. Sci. 2015, 6 (9), 5197–5203. (19) Cai, B.; Hübner, R.; Sasaki, K.; Zhang, Y.; Su, D.; Ziegler, C.; Vukmirovic, M. B.; Rellinghaus, B.; Adzic, R. R.; Eychmüller, A. Core–Shell Structuring of Pure Metallic Aerogels towards Highly Efficient Platinum Utilization for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2018, 57 (11), 2963–2966. (20) Wang, G.; Guan, J.; Xiao, L.; Huang, B.; Wu, N.; Lu, J.; Zhuang, L. Pd Skin on AuCu Intermetallic Nanoparticles: A Highly Active Electrocatalyst for Oxygen Reduction Reaction in Alkaline Media. Nano Energy 2016, 29, 268–274. (21) Zhang, P.; Xiahou, Y.; Wang, J.; Hang, L.; Wang, D.; Xia, H. Revitalizing Spherical Au@Pd Nanoparticles with Controlled Surface-Defect Density as High Performance Electrocatalysts. J. Mater. Chem. A 2017, 5 (15), 6992–7000. (22) Carrera-Cerritos, R.; Fuentes-Ramírez, R.; Cuevas-Muñiz, F. M.; Ledesma-García, J.; Arriaga, L. G. Performance and Stability of Pd Nanostructures in an Alkaline Direct Ethanol Fuel Cell. J. Power Sources 2014, 269, 370–378. (23) Wang, Z.; Liu, P.; Han, J.; Cheng, C.; Ning, S.; Hirata, A.; Fujita, T.; Chen, M. Engineering the Internal Surfaces of Three-Dimensional Nanoporous Catalysts by Surfactant-Modified Dealloying. Nat. Commun. 2017, 8 (1), 1066. (24) Xia, H.; Ran, Y.; Li, H.; Tao, X.; Wang, D. Freestanding Monolayered Nanoporous Gold Films with High Electrocatalytic Activity via Interfacial Self-Assembly and Overgrowth. J. Mater. Chem. A 2013, 1 (15), 4678–4684. (25) Frens G. Controlled Nucleation for the Regulation of the Particles Size in Monodisperse Gold Suspensions. Nat. Phys. Sci. 1973, 241 (105), 20–22. (26) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55–75. (27) Xia, H.; Xiahou, Y.; Zhang, P.; Ding, W.; Wang, D. Revitalizing the Frens Method To Synthesize Uniform, Quasi-Spherical Gold Nanoparticles with Deliberately Regulated Sizes from 2 to 330 Nm. Langmuir 2016, 32 (23), 5870–5880. (28) Xia, H.; Su, G.; Wang, D. Size-Dependent Electrostatic Chain Growth of PH-Sensitive Hairy Nanoparticles. Angew. Chem. Int. Ed. 2013, 52 (13), 3726–3730. (29) Yi, Z.; Luo, J.; Tan, X.; Yi, Y.; Yao, W.; Kang, X.; Ye, X.; Zhu, W.; Duan, T.; Yi, Y.; Tang, Y. Mesoporous Gold Sponges: Electric Charge-Assisted Seed Mediated Synthesis and Application as Surface-Enhanced Raman Scattering Substrates. Sci. Rep. 2015, 5, 16137. (30) Bastús, N. G.; Comenge, J.; Puntes, V. Kinetically Controlled Seeded Growth Synthesis of Citrate-Stabilized Gold Nanoparticles of up to 200 Nm: Size Focusing versus Ostwald Ripening. Langmuir 2011, 27 (17), 11098–11105. (31) Uyeda, N.; Nishino, M.; Suito, E. Nucleus Interaction and Fine Structures of Colloidal Gold Particles. J. Colloid Interface Sci. 1973, 43 (2), 264–276. (32) Xia, H.; Bai, S.; Hartmann, J.; Wang, D. Synthesis of Monodisperse Quasi-Spherical Gold Nanoparticles in Water via Silver(I)-Assisted Citrate Reduction. Langmuir 2010, 26 (5), 3585–3589.


34

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Wang, S.; Kuai, L.; Han, X.; Geng, B. Branched Twinned Au Nanostructures: Facile Hydrothermal Reduction Fabrication, Growth Mechanism and Electrochemical Properties. CrystEngComm 2012, 14 (20), 6581–6585. (34) Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; Ishikawa Y.; Asao N.; Yamamoto Y.; Erlebacher J.; Chen M. Atomic Origins of the High Catalytic Activity of Nanoporous Gold. Nat. Mater. 2012, 11 (9), 775–780. (35) Song, Y.; Miao, T.; Zhang, P.; Bi, C.; Xia, H.; Wang, D.; Tao, X. {331}-Faceted Trisoctahedral Gold Nanocrystals: Synthesis, Superior Electrocatalytic Performance and Highly Efficient SERS Activity. Nanoscale 2015, 7 (18), 8405–8415. (36) Tang, Y.; Cheng, W. Nanoparticle-Modified Electrode with Size- and Shape-Dependent Electrocatalytic Activities. Langmuir 2013, 29 (9), 3125–3132. (37) Hamelin, A. Cyclic Voltammetry at Gold Single-Crystal Surfaces. Part 1. Behaviour at Low-Index Faces. J. Electroanal. Chem. 1996, 407 (1–2), 1–11. (38) Han, D.-D.; Li, S.-S.; Guo, Z.; Chen, X.; Liu, J.-H.; Huang, X.-J. Shape Dependent Stripping Behavior of Au Nanoparticles toward Arsenic Detection: Evidence of Enhanced Sensitivity on the Au (111) Facet. RSC Adv 2016, 6 (36), 30337–30344. (39) Hebié, S.; Cornu, L.; Napporn, T. W.; Rousseau, J.; Kokoh, B. K. Insight on the Surface Structure Effect of Free Gold Nanorods on Glucose Electrooxidation. J. Phys. Chem. C 2013, 117 (19), 9872–9880. (40) Niu, Z.; Peng, Q.; Gong, M.; Rong, H.; Li, Y. Oleylamine-Mediated Shape Evolution of Palladium Nanocrystals. Angew. Chem. Int. Ed. 2011, 50 (28), 6315–6319. (41) Ling, T.; Zhu, J.; Yu, H.; Xie, L. Size Effect on Crystal Morphology of Faceted FaceCentered Cubic Fe Nanoparticles. J. Phys. Chem. C 2009, 113 (22), 9450–9453. (42) Müller, M.; Albe, K. Structural Stability of Multiply Twinned FePt Nanoparticles. Acta Mater. 2007, 55 (19), 6617–6626. (43) Ch’ng, H. N.; Pan, J. Sintering of Particles of Different Sizes. Acta Mater. 2007, 55 (3), 813–824. (44) Halder, A.; Ravishankar, N. Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment. Adv. Mater. 2007, 19 (14), 1854–1858. (45) Tang, J.; Deng, L.; Xiao, S.; Deng, H.; Zhang, X.; Hu, W. Chemical Ordering and Surface Segregation in Cu–Pt Nanoalloys: The Synergetic Roles in the Formation of Multishell Structures. J. Phys. Chem. C 2015, 119 (37), 21515–21527. (46) Wang, Z. L. Structural Analysis of Self-Assembling Nanocrystal Superlattices. Adv. Mater. 1998, 10 (1), 13–30. (47) Cammarata, R. C. Continuum Model for Surface Reconstructions in (111) and (100) Oriented Surfaces of Fcc Metals. Surf. Sci. 1992, 279 (3), 341–348. (48) Fiorentini, V.; Methfessel, M.; Scheffler, M. Reconstruction Mechanism of Fcc Transition Metal (001) Surfaces. Phys. Rev. Lett. 1993, 71 (7), 1051–1054. (49) Jiang, Q.; Lu, H. M.; Zhao, M. Modelling of Surface Energies of Elemental Crystals. J. Phys. Condens. Matter 2004, 16 (4), 521–530. (50) Wittstock, A.; Bäumer, M. Catalysis by Unsupported Skeletal Gold Catalysts. Acc. Chem. Res. 2014, 47 (3), 731–739. (51) Borkowska, Z.; Tymosiak-Zielinska, A.; Shul, G. Electrooxidation of Methanol on Polycrystalline and Single Crystal Gold Electrodes. Electrochimica Acta 2004, 49 (8), 1209–1220.


35


Page 36 of 38

(52) Hernández, J.; Solla-Gullón, J.; Herrero, E.; Aldaz, A.; Feliu, J. M. Methanol Oxidation on Gold Nanoparticles in Alkaline Media: Unusual Electrocatalytic Activity. Electrochimica Acta 2006, 52 (4), 1662–1669. (53) Zhang, Z.; Li, H.; Zhang, F.; Wu, Y.; Guo, Z.; Zhou, L.; Li, J. Investigation of HalideInduced Aggregation of Au Nanoparticles into Spongelike Gold. Langmuir 2014, 30 (10), 2648–2659. (54) Pedireddy, S.; Lee, H. K.; Tjiu, W. W.; Phang, I. Y.; Tan, H. R.; Chua, S. Q.; Troadec, C.; Ling, X. Y. One-Step Synthesis of Zero-Dimensional Hollow Nanoporous Gold Nanoparticles with Enhanced Methanol Electrooxidation Performance. Nat. Commun. 2014, 5, 4947. (55) Kolb, D. M. Reconstruction Phenomena at Metal-Electrolyte Interfaces. Prog. Surf. Sci. 1996, 51 (2), 109–173. (56) Kolb, D. M.; Schneider, J. Surface Reconstruction in Electrochemistry: Au (100-(5× 20), Au (111)-(1× 23) and Au (110)-(1× 2). Electrochimica Acta 1986, 31 (8), 929–936. (57) Pessoa, A. M.; Fajín, J. L. C.; Gomes, J. R. B.; Cordeiro, M. N. D. S. Ionic and Radical Adsorption on the Au(Hkl) Surfaces: A DFT Study. Surf. Sci. 2012, 606 (1–2), 69–77. (58) Xie, S.; Choi, S.-I.; Lu, N.; Roling, L. T.; Herron, J. A.; Zhang, L.; Park, J.; Wang, J.; Kim, M. J.; Xie, Z.; Mavrikakis M.; Xia Y. Atomic Layer-by-Layer Deposition of Pt on Pd Nanocubes for Catalysts with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2014, 14 (6), 3570–3576. (59) Wang, X.; Orikasa, Y.; Takesue, Y.; Inoue, H.; Nakamura, M.; Minato, T.; Hoshi, N.; Uchimoto, Y. Quantitating the Lattice Strain Dependence of Monolayer Pt Shell Activity toward Oxygen Reduction. J. Am. Chem. Soc. 2013, 135 (16), 5938–5941. (60) Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y.; Liu, P.; Zhou, W.-P.; Adzic, R. R. Oxygen Reduction on Well-Defined Core−Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects. J. Am. Chem. Soc. 2009, 131 (47), 17298–17302. (61) Wang, J.; Zhang, P.; Xiahou, Y.; Wang, D.; Xia, H.; Möhwald, H. Simple Synthesis of Au–Pd Alloy Nanowire Networks as Macroscopic, Flexible Electrocatalysts with Excellent Performance. ACS Appl. Mater. Interfaces 2018, 10 (1), 602–613. (62) Roy, A.; Kundu, S.; Müller, K.; Rosenauer, A.; Singh, S.; Pant, P.; Gururajan, M. P.; Kumar, P.; Weissmüller, J.; Singh, A. K.; Ravishankar, N. Wrinkling of Atomic Planes in Ultrathin Au Nanowires. Nano Lett. 2014, 14 (8), 4859–4866. (63) Baldauf, M.; Kolb, D. M. A Hydrogen Adsorption and Absorption Study with Ultrathin Pd Overlayers on Au (111) and Au (100). Electrochimica Acta 1993, 38 (15), 2145–2153. (64) Baldauf, M.; Kolb, D. M. Formic Acid Oxidation on Ultrathin Pd Films on Au (Hkl) and Pt (Hkl) Electrodes. J. Phys. Chem. 1996, 100 (27), 11375–11381. (65) Su, G.; Jiang, H.; Zhu, H.; Lv, J.-J.; Yang, G.; Yan, B.; Zhu, J.-J. Controlled Deposition of Palladium Nanodendrites on the Tips of Gold Nanorods and Their Enhanced Catalytic Activity. Nanoscale 2017, 9 (34), 12494–12502. (66) Wang, W.; Zhang, J.; Yang, S.; Ding, B.; Song, X. Au@Pd Core-Shell Nanobricks with Concave Structures and Their Catalysis of Ethanol Oxidation. ChemSusChem 2013, 6 (10), 1945–1951. (67) Shen, S.; Guo, Y.; Luo, L.; Li, F.; Li, L.; Wei, G.; Yin, J.; Ke, C.; Zhang, J. Comprehensive Analysis on the Highly Active and Stable PdAu/C Electrocatalyst for Ethanol Oxidation Reaction in Alkaline Media. J. Phys. Chem. C 2018, 122 (3), 1604– 1611.


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(68) Wang, E. D.; Xu, J. B.; Zhao, T. S. Density Functional Theory Studies of the Structure Sensitivity of Ethanol Oxidation on Palladium Surfaces. J. Phys. Chem. C 2010, 114 (23), 10489–10497. (69) Zareie Yazdan-Abad, M.; Noroozifar, M.; Modaresi Alam, A. R.; Saravani, H. Palladium Aerogel as a High-Performance Electrocatalyst for Ethanol Electro-Oxidation in Alkaline Media. J. Mater. Chem. A 2017, 5 (21), 10244–10249. (70) Nahar, L.; Farghaly, A. A.; Esteves, R. J. A.; Arachchige, I. U. Shape Controlled Synthesis of Au/Ag/Pd Nanoalloys and Their Oxidation-Induced Self-Assembly into Electrocatalytically Active Aerogel Monoliths. Chem. Mater. 2017, 29 (18), 7704–7715.


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Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size

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