Asymmetric Multimetallic Mesoporous Nanospheres

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Asymmetric Multimetallic Mesoporous Nanospheres Hao Lv, Dongdong Xu, Lizhi Sun, Joel Henzie, Aaron Lopes, Qingyu Gu, Yusuke Yamauchi, and Ben Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01223 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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Asymmetric Multimetallic Mesoporous Nanospheres Hao Lv,1,+ Dongdong Xu,1,+ Lizhi Sun,1 Joel Henzie,2,3 Aaron Lopes,4 Qingyu Gu,1 Yusuke Yamauchi,2,5,6 and Ben Liu1,*

1Jiangsu

Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical

Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China 2Key

Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao

University of Science and Technology, Qingdao 266042, China 3International

Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science

(NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 4Department

of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,

United States 5School

of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The

University of Queensland, Brisbane, QLD 4072, Australia 6Department

of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero,

Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea +H.L.

and D.X. contributed equally to this work.

*E-mail: [email protected]

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Abstract: Mesoporous colloidal nanospheres with tailorable asymmetric nanostructures and multimetallic elemental compositions are building blocks in next-generation heterogeneous catalysts. Introducing structural asymmetry into metallic mesoporous frameworks has never been demonstrated, but it would be beneficial because the asymmetry enables the spatial control of catalytic interfaces, facilitates the electron/mass transfer and assists in the removal of poisonous intermediates. Herein, we describe a simple bottom-up strategy to generate uniform sub-100 nm multimetallic asymmetric bowl-shaped mesoporous nanospheres (BMSs). This method uses a surfactant-directed “dual”-template to control the kinetics of metal reduction on the surface of a vesicle, forming mesoporous metal islands on its surface whose spherical cone angle can be precisely controlled. The symmetric BMS mesostructures with different spherical cone angles (structural asymmetries) and elemental compositions are demonstrated. The high surface area and asymmetric nature of the metal surfaces are shown to enhance catalytic performance in the alcohol oxidation reactions. The findings illustrated here offer the novel and interesting opportunities for rational design and synthesis of hierarchically asymmetric nanostructures with desired functions for a wide range of applications. Keywords: Mesoporous nanospheres; asymmetry; alloy; synergetic effect; electrocatalytic alcohol oxidation


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Asymmetric noble metal nanocrystals (NCs) with hierarchical and/or anisotropic morphologies have widespread applications as functional materials.1-8 Of multiple asymmetric nanostructures available, bowl-shaped nanospheres in particular facilitate the applications in catalytic electrodes, energy generation/storage devices, and biotechnology.9-19 Thus, introducing rationally engineered internal structures into bowl-shaped nanospheres should further enhance their electrochemically active surface area (i.e., increase accessible surface area normalized by mass), facilitate electron/mass transfer and assist in the removal of poisonous catalytic intermediates. As a result, asymmetric bowl-shaped mesoporous nanospheres (BMSs) designed in this manner are expected to outperform their symmetric spherical counterparts in (electro)catalysis and energy storage, as well as drug delivery.20-21 BMS-type structures have been demonstrated in amorphous SiO2 and carbon-based materials.9,20,22 However, current synthesis techniques have never been employed to generate metallic BMS structures, let alone the introduction of the BMSs with structural asymmetries. Developing a facile synthetic approach to fabricate asymmetric noble metal-based BMSs will introduce novel fundamental understanding on the impact of mesoporosity on the properties of shaped structures and extend the applications of noble metal nanocrystals. Here we describe a simple yet general synthetic methodology to craft highly uniform, monodisperse, sub100 nm, and crystalline multimetallic asymmetric BMSs via a dual-template-directed anisotropic island growth approach. Then, we demonstrate their application as an electrode material in electrocatalytic alcohol oxidation reactions (AORs). Amphiphilic dioctadecyldimethylammonium chloride (DODAC) is a surfactant that forms both vesicles and cylindrical micelles in an aqueous solution. Under certain conditions, these two phases can coexist and form a “dual” architecture that serve as the templates to organize metal precursors into hierarchical networks. The metal precursors were subsequently reduced inside the DODAC template, driving asymmetric island growth of multimetallic BMSs outwards (Scheme 1). The tunability of the reaction kinetics allows us to control the spherical cone angle of the structure, enabling us to rationally engineer asymmetry into the structure from whole nanospheres to structures containing 1/8 of the nanosphere. The method is flexible and can be used with various combinations of metal co-precursors. We demonstrated the BMSs composed of bimetallic PdAg, trimetallic PdAgCu and PdPtAg, and tetrametallic PdPtAgCu and PdAgCuFe. The intrinsic features of the ACS Paragon Plus Environment

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nanoparticles include an asymmetric bowl-shaped architecture with cylindrical openings that allow molecules to freely navigate into the continuous mesoporous framework. Various multimetallic elemental compositions are compatible with the BMS synthesis method, expanding the kinds of (electro)catalytic active sites that can be explored.23-24 Structural and material flexibility of this method allow us to explore how electron/mass transfer can be improved, as well as how to facilitate the removal of adsorbed intermediates that poison the catalyst,25-26 and decrease the energy barriers of the chemical reaction.27 In particular, we found that asymmetric PdAgCu BMSs exhibited the best electrocatalytic performances for AORs, compared to the hollow and solid MSs catalysts. Multimetallic BMSs were formed by a room-temperature solution-phase synthesis in a mixed solvent composed of H2O and ethanol. Taking trimetallic PdAgCu BMSs as a typical example, DODAC and the metal precursors (PdCl42-, Ag+ and Cu2+) were first mixed in H2O/ethanol to obtain a homogeneous solution. Subsequently, excess reducing agent (ascorbic acid, AA) was injected under gentle shaking. The color of the solution changed immediately to dark brown, indicating the formation of metallic NCs. After washing with acetic acid and ethanol/H2O to remove the surfactant, highly uniform PdAgCu asymmetric BMSs were obtained accordingly (see experimental details in the Supporting Information (SI)). The reaction conditions, including concentrations of DODAC and metal precursors, pH, and temperatures, were carefully tuned to optimize the structural asymmetries of the BMSs (Scheme 1). To evaluate the structural asymmetries easier, the BMSs with different asymmetric levels were denoted as BMS-x, where x was calculated based on the exposed size of the BMS divided by intact diameter of single sphere (see Figure S1 for details). Asymmetric nanostructures and trimetallic elemental compositions of PdAgCu BMS-4/8 (as the example) were thoroughly investigated by various characterization techniques. Both scanning electron microscopy (SEM) and low-magnification transition electron microscopy (TEM) images show that trimetallic PdAgCu BMSs were highly uniform with a perfectly asymmetric bowl-like and hollow hemispherical architecture (Figures 1a and S2). High-magnification TEM (Figures 1b-d) and high-angle annular dark-field scanning TEM (HAADF-STEM) measurements (Figure 1e) show that asymmetric BMS-4/8 is precisely asymmetric bowl-shaped hemispherical morphology. The average diameter of the BMSs is 90 nm with a hollow cavity of 50 nm and a framework shell of 20 nm (see more TEM images in Figure S3). The mesoporous nanochannels appear to be partially ordered, ACS Paragon Plus Environment

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with 2.5-3.5 nm cylindrical openings. The mesoporous frameworks are continuous (not NP aggregates) with an average thickness of 3.0 nm (Figures 1c-d). Small-angle X-ray diffraction (XRD) exhibited a broad signal at 1.4o. In our case, since the arrangement of pores are not perfectly ordered, this d-spacing means the average pore-topore distance of 6.4 nm (Figure S4). This value is almost matched with the results obtained with TEM. In addition, the BMS-4/8 has a higher surface area than the HMS according to nitrogen adsorption-desorption isotherms (Figure S5). TEM tomography was further used to clearly observe the BMS-4/8 at multiple tilting angles (-60o to 60o). Both individual BMS (Figures 1g and S6) and multiple BMSs (Figure S7) illustrate remarkable asymmetric bowl-shaped architecture and cylindrically opened mesoporous framework by a series of tomographic TEM images with different facing directions (see Movie S1 also). Crystallinity and elemental compositions of trimetallic PdAgCu asymmetric BMS-4/8 were further examined with wide-angle XRD and electron diffraction techniques. Only single set of diffraction peaks are seen in XRD, that roughly match the patterns of pure face centered cubic (fcc) Pd, Ag and Cu (Figure S8). This indicates that the material is PdAgCu nanoalloys with no obvious phase separation. The fcc crystalline structure of the alloyed PdAgCu BMS-4/8 was further characterized by high-resolution TEM and selected-area electron diffraction (SAED) (Inset of Figure 1d). The TEM micrographs show clear lattice fringes with a d-spacing of 0.229 nm that matches the (111) plane (see more TEM images in Figure S9). STEM elemental mapping with energy dispersive x-ray spectroscopy (EDX) shows that the Pd, Ag and Cu are homogeneously distributed throughout the asymmetric BMSs (Figure 1f), further confirming the material is a trimetallic nanoalloy (Figure S10). According to the EDX, the elemental compositions of Pd, Ag and Cu in the BMS-4/8 is 48.3 :36.1 :15.6 (wt. %) (Figure S11). These results closely match the measurements obtained with inductively coupled plasma mass spectroscopy (ICP-MS, 46.7 : 37.4 : 15.9 by wt. %) and X-ray photoelectron spectroscopy (XPS) (Figure S12). The structural asymmetries of PdAgCu BMSs were precisely tailored by changing the synthetic conditions in the mixed solution of H2O/ethanol. Figure 2 displays typical TEM images of PdAgCu BMSs with increased structural asymmetries obtained with different DODAC concentrations (Figure S13 for the diameters along different directions). All the PdAgCu BMSs synthesized under these conditions had similar compositions, ACS Paragon Plus Environment

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indicating the ratio of metal precursors had little impact on the final asymmetric nanostructures (Figure S14). At low DODAC concentrations (< 0.1 mg mL-1), hollow and mesoporous nanospheres with intact and symmetric architectures were obtained (Figure 2a). The hollow MSs (HMSs) are relatively uniform and have the same outer diameter (~ 90 nm) and hollow cavity diameter (~50 nm) of the BMS-4/8 structures, which is slightly smaller than the ones synthesized in the absence of ethanol (~ 115 nm).23 By contrast, increasing the DODAC concentration to 0.3 mg mL-1 caused the HMSs to form the openings on the surface of nanospheres (denoted as BMS-g (Figure S1), see TEM images in Figure S15a-c). The openings of the BMSs increased in size with higher DODAC concentrations. As shown in Figure 2b-h, trimetallic PdAgCu BMSs with increasing asymmetry could be observed from 7/8 (BMS-7/8) to 1/8 (BMS-1/8) by increasing DODAC concentration from 0.6 to 3.0 mg mL-1. All the BMSs appeared to be partially cut off from the intact and symmetric HMSs (see more TEM images in Figure S15). Similar to BMS-4/8, the BMSs with different degrees of asymmetry were well-dispersed and possessed continuous mesopores with cylindrical openings. The hollow cavity of the BMSs with different degrees of asymmetry were identical in size (~50 nm); however, a significant decrease in the size of the BMS from ~110 to ~40 nm was observed. Noticeably, asymmetric BMS-1/8 is nearly two-dimensional (2D), like a nanosheet (Figure 2h), indicating the potential of this method to generate 2D metallic mesoporous nanomaterials.28 Higher and lower DODAC concentrations (< 0.05 or > 4 mg mL-1), however, resulted in symmetric nanostructures or irregular shapes (Figure S16), possibly because the conditions are not able to form the “dual” templates.29,30 Structural asymmetries of PdAgCu BMSs were also tailored by tuning other synthetic parameters (precursor concentration, pH and temperature). Initially we changed the concentration of metal precursors with the same synthetic conditions to the BMS-4/8, and found a similar asymmetric evolution process (Figure S17). At very low metal precursor concentration, the obtained nanostructures were bowl-shaped with a higher asymmetry, with only a ~5-nm-thick mesoporous shell. Increasing precursor concentration gradually resulted in the formation of robust BMSs, but the structural asymmetry slightly decreased. Next, we found that the reduction rates were also tunable for tailoring the structural asymmetries of PdAgCu BMSs. For example, decreasing the pH accelerated the kinetic reduction rate of the metal precursors, facilitating the formation of PdAgCu BMSs with increasing degrees of asymmetry (Figure S18). Similarly, the asymmetry was increased at higher reaction temperatures, ACS Paragon Plus Environment

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although the higher temperatures (> 75 oC) caused the formation of ultrathin nanowires (Figure S19).31 All the synthetic parameters for the formation of the PdAgCu nanostructures have been summarized in Figure S20 and Table S1. This synthetic method can be extended to other multimetallic asymmetric BMSs with different elemental ratios and compositions. First, we tuned the compositional ratios of PdAgCu BMS-4/8 by simply changing the feed ratios of metal precursors (Figure S21). Increasing Ag content in PdAgCu BMSs (from 0 to 64.7 wt. %) did not obviously change the bowl-shaped architectures, although no loading of Ag content resulted in spherical particles (Figure S21a-c). However, the higher Ag content would slightly increase the thickness of mesoporous frameworks.23 Second, the synthetic method was also extended to other multimetallic BMSs with different elements. Metals containing different valence electron levels such as Cu and Fe (3d), Ag (4d) and Pt (5d) were successfully alloyed with Pd to form multimetallic BMSs (Figure 3). The resulting bimetallic PdAg, trimetallic PdPtAg, and tetrametallic PdAgPtCu and PdAgCuFe BMS were highly uniform with bowl-shaped mesoporous structures and multimetallic elemental compositions, further confirming the ability of this method to generate a number of multimetallic BMS compositions. Based on structural and compositional studies above, multimetallic asymmetric BMSs with ordered mesoporous frameworks and controllable structural asymmetries were successfully achieved for the first time. We deduced that anisotropic island growth of mesoporous framework on the interfaces of vesicle templates possibly resulted in the formation of asymmetric BMSs.9,32 As demonstrated previously,23 amphiphilic DODAC bonded with metal precursors to drive the self-assembly into “dual” templates of vesicle and cylinder micelles (Scheme 1b). In this step, vesicles direct to produce hollow nanostructure, while cylindrical micelles on the surface facilitate the formation of mesoporous frameworks (Figure S22 for other PdAgCu nanostructures synthesized with different surfactants). Once the initial nuclei are formed on the surface of the vesicle micelles, further metal deposition and crystallization derived by a reducing agent preferentially proceeds from the initial nuclei, rather than new nucleation at other places in the same vesicle micelles. In order to verify this mechanism, we carefully checked the evolution of the structure over time by imaging TEM at different growth periods (Figure S23). In the very initial stage of the crystalline growth, some small nanoparticles appeared on the surface of the ACS Paragon Plus Environment

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vesicles and gradually grew bigger. Subsequently, a small fragment of the mesoporous islands can be observed, due to the presence of the rod-like micelles. Obviously, mesoporous island crystals grow normal to the tangent of the sphere at any given point and move outward. Then, the mesoporous islands grew bigger and finally evolved into perfectly asymmetrical BMSs under limited metal precursors. Considering the similar evolution processes of the BMSs synthesized with different metal precursors, we concluded that the PdAgCu BMSs form via the nucleation of metal islands and subsequent growth of mesoporous metal frameworks. However, when a higher vesicle concentration or a lower precursor concentration was used, the amount of metal precursors on each vesicle template would be decreased. Insufficient metal precursors inevitably caused the growth of the smaller mesoporous islands with asymmetric bowl-shaped nanostructures (Scheme 1c-d, see Figures S24-26 for more structural characterizations).33 By contrast, both lower pH and higher reaction temperatures accelerated the nucleation of metal NCs, and resulted in the BMSs with increasing structural asymmetries. Several smaller BMS islands with higher asymmetry (partially connected BMS aggregates on one vesicle) were also seen directly under the higher reaction temperature (Figure S19g-i), further bolstering our observations. In addition, reducing agent is also important for the formation of the BMSs (Figures S27-28). A higher or lower amount of AA and other reducing agents resulted in partially disordered BMSs or even totally destroyed asymmetric nanostructures of the BMSs, indicating there is an optimal reduction rate needed to control the formation of asymmetric BMSs. The resulting PdAgCu BMSs (BMS-4/8 as the example) are expected to expose more catalytic active sites, facilitate the electron/mass transfer and limit CO poisoning. Thus, BMSs would boost their electrocatalytic AOR performance accordingly. To highlight uniquely structural and compositional merits of the structures, PdAgCu HMSs (Figure 2a), MSs (Figure S22a-c), and PdAg BMSs (Figure 3a) as well as commercial Pd black (PdB) were compared. Initially we estimated the electrochemical active surface areas (ECSAs) by cyclic voltammograms (CVs) in 1.0 M KOH (Figure 4a). A largest ECSA of 52.3 m2 gPd-1 was achieved for PdAgCu BMSs, ~4.15-fold higher than PdB (12.5 m2 gPd-1) (Figure S29), highlighting its advantages in terms of structure and composition. Meanwhile, CO stripping experiments were performed, in which both the lower onset potential


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and larger active area were seen for PdAgCu BMS, implying dramatically improved CO anti-poisoning ability (Figure 4b and Figure S30). AOR performance of asymmetric PdAgCu BMSs was performed by measuring ethanol electrooxidation as an example. Figure 4c depicts typical CVs in 1.0 M ethanol and 1.0 M KOH at a scan rate of 50 mV s-1. The PdAgCu BMSs showed the highest peak mass density of 6.36 A mgpd-1, which was 1.22, 2.01, 1.24 and 8.83 folds higher than that of PdAgCu HMSs (5.20 A mgPd-1) and MSs (3.09 A mgPd-1), PdAg BMSs (5.12 A mgPd-1) and PdB (0.72 A mgPd-1), respectively (Figure 4d). We also compared the onset potentials and If/Ib (the ratio of forward and backward peak mass current) values of PdAgCu BMS with its counterparts (Figure 4e). The lowest onset potential of -778 mV (Figure S31) and highest If/Ib value of 1.13 for PdAgCu BMSs further indicated the decreased energy barrier and quicker electrocatalytic kinetics during the electrooxidation of ethanol (Table S2).3436

The results closely matched those obtained from ECSAs and CO anti-poisoning experiments. Synergistically

structural and compositional merits also enhanced electrocatalytic stability of PdAgCu BMSs. Current-time (i-t) chronoamperometry tests were performed on all nanocatalysts. The PdAgCu BMSs had the highest mass activity (1.42 A mgPd-1) which was retained after the electrocatalysis for 5000 seconds (Figure S32). More importantly, the enhancement in activity for asymmetric PdAgCu BMSs is not specific to ethanol electrooxidation, and can be extendable to other AORs, including methanol, formic acid, glycerol and glucose electrooxidations (Figure 4f and Figure S33-36). These results further revealed the generality of synergistically structural and compositional advantages of asymmetric PdAgCu BMSs in electrocatalytic AORs. In conclusion, monodispersed and highly uniform sub-100 nm multimetallic asymmetric BMSs were successfully synthesized for the first time by rationally controlling the reduction kinetics of metal NCs via the “dual”-template surfactant-directing method. The structural asymmetries of trimetallic BMSs can be precisely tailored from whole nanosphere to 1/8 of nanosphere, depending on the reaction conditions. Meanwhile, asymmetric BMSs have well-ordered cylindrically opened mesostructures and adjustable elemental compositions. Our solution-phase synthetic method for multimetallic asymmetric BMSs is a one-pot reaction, operates at room temperature, can be synthesized in less than 5 minutes and scaled to the gram level. Moreover, uniform BMSs showed excellent structural and compositional merits to synergistically enhance the catalytic kinetics. As a proofACS Paragon Plus Environment

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of-concept application, PdAgCu BMSs displayed remarkably high AOR performance, with respect to their counterparts and commercial PdB catalysts. We believe that this discovery of multimetallic asymmetric BMSs may facilitate the development of more sophisticated asymmetric nanostructures with spatial control of catalytic interfaces to enable desired catalytic functions in energy-related applications and beyond.

Supporting Information Synthesis and characterization details, more electron microscopy, XRD, XPS, EDX, tomographic movie, and electrocatalytic tests. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Information *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements The authors thank the financial support from Jiangsu Specially Appointed Professor Plan and Natural Science Foundation of Jiangsu Province (No. BK20180723), National Natural Science Foundation of China (No. 21501095). This work is also supported by Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering Research Center of Biomedical Functional Materials.

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Figures

Scheme 1. A scheme illustrating the formation of asymmetric BMSs with controlled structural asymmetries via an anisotropic island growth process. (a) DODAC and metal precursors self-assemble into (b) spherical vesicle and cylinder micelles in a H2O/ethanol mixed solution. Then, (c) anisotropic island growth of mesoporous metal nanocrystals along spherical vesicles result in the formation of PdAgCu BMSs with controlled structural asymmetries. Lastly, (d) mesoporous nanochannels and hollow openings of asymmetric BMSs were released by the removal of DODAC.


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Figure 1. Structural and compositional characterizations of PdAgCu asymmetric BMS-4/8: (a) lowmagnification, (b, c) high-magnification, (d) high-resolution TEM images and corresponding SAED (inset), (e) HAADF-STEM image and (f) corresponding elemental mappings, (g) tomographic TEM images and corresponding schematics.


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Figure 2. Trimetallic PdAgCu BMSs with precisely tailorable asymmetries. TEM images and corresponding structural renderings of (a) symmetric HMSs synthesized with DODAC concentration of 0.1 mg/mL, the BMSs with distinct asymmetries obtained in DODAC concentration of (b) 0.6, (c) 0.9, (d) 1.2, (e) 1.5, (f) 2.0, (g) 2.5, and (h) 3.0 mg/mL, respectively.


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Figure 3. The universality of the synthetic method. (a, c, e, g) TEM images, (b, d, f, h) HAADF-STEM image and corresponding elemental mappings of (a, b) bimetallic PdAg, (c, d) trimetallic PdPtAg, and tetrametallic (e, f) PdPtAgCu and (g, h) PdAgCuFe BMSs.


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Figure 4. Electrocatalytic performances of asymmetric PdAgCu BMSs. (a) CV curves of PdAgCu BMSs, HMSs and MSs, PdAg BMSs and PdB collected in 1.0 M KOH. (b) CO stripping curves of PdAgCu BMSs and MSs, and PdAg BMSs. (c) CV curves, (d) summarized mass activities, and (e) Onset potentials and If/Ib values of PdAgCu BMSs, HMSs and MSs, PdAg BMSs and PdB obtained in 1.0 M KOH and 1.0 M ethanol. (f) Summarized mass activities of PdAgCu BMSs, HMSs and MSs, PdAg BMSs and PdB in methanol, glycerol and formic acid electrooxidations.


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Asymmetric Multimetallic Mesoporous Nanospheres

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