Tubular Monolayer Superlattices of Hollow Mn3O4 Nanocrystals and

Aug 24, 2017 - Self-assembled nanocrystal (NC) superlattices are emerging as an important class of materials with rationally modulated properties. Eng...
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Tubular Monolayer Superlattices of Hollow Mn3O4 Nanocrystals and Their Oxygen Reduction Activity Tongtao Li, Bin Xue, Biwei Wang, Guannan Guo, Dandan Han, Yancui Yan, and Angang Dong* Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China S Supporting Information *

simultaneously facilitating mass transport properties. To this end, a number of assembly techniques have been developed in the past few years, resulting in various low-dimensional NC superstructures, including 1D chains,13,14 2D membranes,7,15,16 and higher-ordered colloidal spheres and vesicles.17 Despite these advancements, the application of NC superlattices in energy-related fields is still largely hindered due to the inherent low electrical conductivity arising from the organic capping ligands attached to the NC surface.9,12 Moreover, most prior superstructures are self-assembled from solid NCs, which may not be optimal for applications such as catalysis owing to the limited the exposure of active sites at the NC surface.18 Herein, we describe our efforts to develop a novel class of lowdimensional NC superlattices, so-called freestanding, carboncoated tubular monolayer superlattices (TMSLs). Our method is based on the epitaxial-growth-like assembly of colloidal NCs within a porous anodized aluminum-oxide (AAO) template followed by in situ ligand carbonization and selective etching. Although this epitaxial assembly strategy is, in principle, applicable to a variety of NCs, we hereafter focus the discussion on Mn3O4 mainly because of its wide applications in electrochemical energy conversion and storage.19−21 Specifically, we design and fabricate hierarchical TMSLs comprising hollow Mn3O4 NCs (h-Mn3O4-TMSLs) by exploiting structural evolution of MnO NCs.22 The h-Mn3O4-TMSLs obtained possess a number of unique structural features that are unavailable in conventional NC superlattices, making them particularly attractive for applications in electrochemical energy conversion. First, h-Mn3O4-TMSLs adopt a well-defined mesoscale tubular geometry, which is postulated advantageous for fast mass transport. Second, the hollow structure of the constituent Mn3O4 NCs combined with the monolayer assembly geometry greatly increases electrochemically active surfaces fully accessible by electrolyte ions. Moreover, the thin carbon coating derived from the organic capping ligands not only facilitates electron transport but also assures structural robustness of the tubular superlattices. As a proof of concept, h-Mn3O4-TMSLs have been employed as electrocatalysts for oxygen reduction, which exhibit comparable catalytic performance to commercial Pt/C catalysts in alkaline media. Scheme 1 illustrates the fabrication process of h-Mn3O4TMSLs. Octahedral MnO NCs (Figure 1a) with an average diameter of 18 nm stabilized with oleic acid (OA) are the starting material for constructing h-Mn3O4-TMSLs, with AAO mem-

ABSTRACT: Self-assembled nanocrystal (NC) superlattices are emerging as an important class of materials with rationally modulated properties. Engineering the nanoscale structure of constituent building blocks as well as the mesoscale morphology of NC superlattices is a crucial step in widening their range of applications. Here, we report a template-assisted epitaxial assembly strategy, enabling growth of freestanding, carbon-coated tubular monolayer superlattices (TMSLs). Specifically, we design and construct TMSLs of hollow Mn3O4 NCs (h-Mn3O4TMSLs) by exploiting structural evolution of MnO NCs. The tubular superlattices obtained possess a number of unique and advantageous structural features unavailable in conventional NC superlattices, rendering them particularly attractive for energy conversion applications. We demonstrate this by employing h-Mn3O4-TMSLs as electrocatalysts for oxygen reduction, the catalytic performance of which is comparable to that of state-of-the-art Pt/C catalysts and superior to that of most manganese oxidebased catalysts reported.

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ontrolled assembly of preformed nanocrystals (NCs) into three-dimensional (3D) superlattices is a promising route toward construction of solid-state materials with tailored structures and potentially programmable properties.1,2 Recent advances in colloidal synthesis and assembly provide access to a diverse set of single- and multicomponent NC superlattices,3,4 which have been pursed for a myriad of applications ranging from thin-film electronic and optoelectronic devices5 to energy conversion and storage.6 Owing to the well-defined geometric arrangement and interparticle spacing, NC superlattices can also serve as a fundamentally important platform for probing physiochemical properties of NC solids, such as electronic coupling,5 spin transport,7,8 and interfacial energy transfer.9 Despite the tremendous prospects, we note the intrinsic closepacked nature of NC superlattices, although beneficial for charge transport and energy transfer, is undesirable for applications demanding fast mass transport and facilitated exposure of the constituent NCs.9−12 In this context, conventional dense NC superlattices are inappropriate structural motifs for catalysis, sensing, and energy conversion and storage due to the impeded molecular accessibility of NCs by foreign species (gas molecules, ions, electrolytes, etc.).12 In pursuit of widening the range of applications of NC superlattices, it is desirable to develop new architectures that enable full access of NC constituents while © 2017 American Chemical Society

Received: June 25, 2017 Published: August 24, 2017 12133

DOI: 10.1021/jacs.7b06587 J. Am. Chem. Soc. 2017, 139, 12133−12136

Communication

Journal of the American Chemical Society

their shape and diameter, are homogeneously coated with a layer of MnO NCs upon solvent evaporation (Figure 1b,c and Figure S2), demonstrating the effectiveness of this epitaxial assembly strategy in achieving high-quality NC monolayers within AAO channels. To obtain h-Mn3O4-TMSLs, the AAO template containing the as-assembled MnO NC monolayers (MnO@AAO) is subjected to a series of treatment processes involving (i) controlled oxidation, (ii) ligand carbonization, and (iii) selective etching (Scheme 1). Mn3O4 is known to be a native oxide of MnO,22 whereas heat treatment at 120 °C in air homogeneously oxidizes the surface of MnO NCs, resulting in MnO@Mn3O4 core−shell NCs while preserving NC ordering (Figure S3). The XRD pattern of this thermally treated sample (MnO@Mn3O4@AAO) contains reflections mainly ascribed to the cubic MnO phase (Figure 1d, red pattern), whereas the emergence of small peaks corresponding to Mn3O4 also indicates the oxidation of MnO. Surface composition analysis with X-ray photoelectron spectroscopy (XPS) verifies the presence of both Mn2+ and Mn3+ in MnO@Mn3O4 NCs (Figure 1e), in accordance with the partial oxidation of MnO NCs. The subsequent calcination at 500 °C in N2 converts the OA capping ligands to a thin carbon shell, resulting in MnO@Mn3O4@C NCs (Scheme 1). Notably, this calcination step simultaneously carbonizes the OA molecules adhering to the channel surface to afford a tubular carbon wall, which is crucial for maintaining the structural integrity of NC monolayers after the removal of AAO. Freestanding h-Mn3O4-TMSLs are obtained by simply soaking the AAO template containing MnO@Mn3O4@C NCs in 6 M KOH for 1 h. The crystallinity of h-Mn3O4-TMSLs is examined by XRD (Figure 1d, black pattern), where all the reflections can be indexed to the tetragonal spinel-type Mn3O4 (JPCDS No. 24-0734). Note the diffraction peaks corresponding to MnO are almost invisible in the XRD pattern, suggesting nearly complete leaching of MnO cores by KOH treatment. The evolution of hollow Mn3O4 NCs is also verified by XPS spectra (Figure 1e), in which the Mn 2p3/2 and Mn 2p1/2 peaks of hMn3O4-TMSLs noticeably shift to the higher binding energy relative to MnO@Mn3O4 NCs. The low-magnification SEM image, given in Figure 2a, shows vertically aligned arrays of freestanding h-Mn3O4-TMSLs, which faithfully replicate the porous structure of the original AAO template over large areas, indicative of their considerable mechanical stability. High-resolution SEM (HRSEM) reveals individual tubes are composed of a monolayer of NCs with good structural ordering inherited from the original MnO NC arrays (Figure 2b). The morphology of h-Mn3O4-TMSLs is examined further by TEM, which reveals the hollow interiors adopted by Mn3O4 NCs (Figure 2c). The carbon wall immobilizing hollow Mn3O4 NCs, derived from the tethered OA molecules, is also observable in TEM (Figure 2c). It is noteworthy 120 °C is an optimal oxidation temperature, enabling the high-yield evolution of hollow Mn3O4 NCs with preserved structural ordering (Figure S4). High-resolution TEM (HRTEM) confirms the constituent hollow Mn3O4 NCs maintain the same octahedral shape with the initial MnO NCs with an average shell thickness of ∼3 nm (Figure 2d). Figure 2e shows the HRTEM image of a single hollow Mn3O4 NC, where the lattice fringes with a spacing of 0.481 nm correspond to the (101) plane of Mn3O4. Dark-field scanning TEM (STEM, Figure 2f,g) and energy dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 2h−j) corroborate the evolution of hollow Mn3O4 NCs, which are packed uniformly within the entire tube. The Mn/O molar ratio

Scheme 1. Schematic Illustration of the Fabrication of hMn3O4-TMSLs

Figure 1. (a) TEM image of octahedral MnO NCs used for constructing h-Mn3O4-TMSLs. Cross section SEM images of MnO NC monolayers self-assembled within the AAO template having (b) circular and (c) hexagonal channels, respectively. (d) XRD patterns of MnO@Mn3O4@ AAO and h-Mn3O4-TMSLs, respectively. The blue asterisks indicate the reflections corresponding to Mn3O4. (e) High-resolution Mn 2p XPS spectra of MnO@Mn3O4 NCs and h-Mn3O4-TMSLs, respectively.

branes having pore sizes of 200−500 nm and lengths of ∼60 μm being used as the template. It is worth mentioning that although the AAO-based method has been widely employed to fabricate nanoparticle tubes,23 tubular superlattices composed of a monolayer of NCs have not been accessible. The key to the formation of TMSLs in our work is the confined assembly of a single layer of colloidal NCs within AAO channels. To achieve this, we functionalize AAO channels with OA, which is expected to bind to the alumina surface through the carboxylate group (Scheme 1). The binding of OA molecules to AAO channels is confirmed by Fourier-transform infrared (FTIR) spectroscopy as indicated by the strong C−H stretching bands near 2900 cm−1 (Figure S1). With this modification, the initial hydrophilic channel surface becomes hydrophobic, which favors spreading of the NC solution (in hexane) over entire channels. Importantly, during the drying of the NC solution, the alkyl chain of the tethered OA molecules ideally matches that of the OA ligands originally attached to the NC surface (Scheme 1), enabling the efficient anchorage of a monolayer of close-packed NCs through a process analogous to epitaxial growth.16 Cross section scanning electron microscopy (SEM) reveals all channels, regardless of 12134

DOI: 10.1021/jacs.7b06587 J. Am. Chem. Soc. 2017, 139, 12133−12136

Communication

Journal of the American Chemical Society

Figure 2. (a) Low-magnification SEM and (b) HRSEM images of hMn3O4-TMSLs. (c) TEM and (d) HRTEM images of h-Mn3O4TMSLs. (e) HRTEM image of a single hollow Mn3O4 NC. (f) STEM image of h-Mn3O4-TMSLs. (g) High-magnification STEM image of the region indicated in panel f and (h−j) the corresponding elemental mapping images.

Figure 3. (a) CV curves of h-Mn3O4-TMSLs in N2- and O2-saturated 0.1 M KOH solution, respectively. (b) Polarization curves of MnO-TMSLs, s-Mn3O4-TMSLs, h-Mn3O4-TMSLs, and Pt/C at a rotation speed of 1600 rpm. (c) Polarization curves of h-Mn3O4-TMSLs at various rotation speeds. (d) K−L plots for MnO-TMSLs, s-Mn3O4-TMSLs, and h-Mn3O4-TMSLs at 0.4 V (vs RHE). (e) Electron transfer number as a functional of potential for MnO-TMSLs, s-Mn3O4-TMSLs, and hMn3O4-TMSLs. (f) Chronoamperometric profiles of h-Mn3O4-TMSLs and Pt/C, respectively.

determined from EDS analysis is ∼1:2.6, consistent with XPS results (Table S1). The excess O content relative to Mn is presumably attributed to the oxygen-containing groups in the OA-derived carbon shells and carbon walls. Manganese oxides including MnO,24 Mn3O4,25 and Mn2O326 are promising nonprecious electrocatalysts for oxygen reduction reaction (ORR), a pivotal cathode reaction for both fuel cells and metal−air batteries.27 Among manganese oxides targeted for ORR, Mn3O4 is of interest because of its rich electrochemical properties afforded by the mixed valence of Mn.28 However, the intrinsic low electrical conductivity of Mn3O4 and its electrochemically poor structural stability significantly hamper its use as high-performance ORR catalysts.19,29 Our h-Mn3O4-TMSLs possess unique hierarchical structures that can not only circumvent the aforementioned obstacles but also enable the access of a large number of active sites, it is therefore of great interest to test their potential as ORR catalysts. To better understand the catalytic performance of h-Mn3O4-TMSLs, two control TMSL samples, composed of MnO NCs (MnO-TMSLs) and solid Mn3O4 NCs (s-Mn3O4-TMSLs), respectively, are also prepared from the same MnO NCs by regulating the oxidation and etching conditions (see Methods in Supporting Information). TEM verifies the tubular geometry adopted by both MnOTMSLs and s-Mn3O4-TMSLs, whereas XRD confirms their respective crystal structure (Figure S5). The ORR measurements are performed in 0.1 M KOH by using rotating disk electrode (RDE) techniques. Figure 3a shows cyclic voltammagrams (CVs) of h-Mn3O4-TMSLs collected at a scan rate of 10 mV s−1. Compared with the CV curve measured in N2-saturated 0.1 M KOH, a more prominent cathodic peak at ∼0.79 V corresponding to the reduction of O2 is observed in O2saturated 0.1 M KOH (Figure 3a), highlighting the effective ORR activity of h-Mn3O4-TMSLs. The RDE polarization curves, presented in Figure 3b, show h-Mn3O4-TMSLs exhibit superior electrocatalytic activity relative to both MnO-TMSLs and sMn3O4-TMSLs, as manifested by the more positive onset potential (∼0.91 V, vs RHE) and the higher diffusion-limiting

current density (∼5.7 mA/cm2), which are comparable to those (∼0.90 V, vs RHE and ∼5.8 mA/cm2, respectively) of Pt/C. The RDE measurement reproducibility is verified by measuring multiple electrodes with varying mass loadings (Figure S6). Particularly, the half-wave potential of h-Mn3O4-TMSLs is located at 0.84 V (vs RHE), which is only ∼10 mV more negative than that of Pt/C (0.85 V, vs RHE) and far surpasses that of MnO-TMSLs (0.71 V, vs RHE) and s-Mn3O4-TMSLs (0.70 V, vs RHE). It is noteworthy that the tubular morphology and the hollow NC structure of h-Mn3O4-TMSLs are largely retained after RDE measurements, despite shortened tube length (Figure S7). The electrocatalytic kinetics of various TMSL catalysts is further examined by collecting the polarization curves at different electrode rotation speeds. In all cases, the current density is increased with increasing rotation speed due to the shortened O2 diffusion distance,24 yet the limiting current density of h-Mn3O4TMSLs is higher than that of MnO-TMSLs and s-Mn3O4TMSLs at any rotation speed (Figure 3c and Figure S8). All three types of TMSL-based catalysts exhibit good linearity and near parallelism in the Koutecky−Levich (K−L) plots (Figure 3d and Figure S9), suggestive of the first-order reaction kinetics toward ORR.30 The electron transfer number (n) can be calculated from the K−L plot slopes and the result is presented in Figure 3e. Apparently, the electron transfer number of h-Mn3O4-TMSLs is much higher than that of MnO-TMSLs and s-Mn3O4-TMSLs at potentials from 0.35 to 0.6 V. The average electron transfer number of h-Mn3O4-TMSLs is 3.91, indicative of a dominant four-electron oxygen reduction pathway.26 The superior ORR performance of h-Mn3O4-TMSLs relative to s-Mn3O4-TMSLs and MnO-TMSLs is also reflected by the smaller Tafel slope of the kinetic current (Figure S10). We also estimate the electrochemically active surface area (ECSA) of various TMSLbased catalysts based on double-layer capacitance measurements 12135

DOI: 10.1021/jacs.7b06587 J. Am. Chem. Soc. 2017, 139, 12133−12136

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(Figure S11). The ECSA of h-Mn3O4-TMSLs is about three and two times higher than that of s-Mn3O4-TMSLs and MnOTMSLs, respectively (Table S2), corroborating the increased available active sites in h-Mn3O4-TMSLs.31 Besides the high activity, h-Mn3O4-TMSLs also manifest excellent durability as indicated by chronoamperometric measurements. As shown in Figure 3f, h-Mn3O4-TMSLs can maintain a high current retention of 91% after 10 h of continuous operation (0.70 V, vs RHE), much higher than that (74%) of Pt/C tested under the identical conditions. Taken together, the overall ORR performance of our h-Mn3O4-TMSLs, comparable to that of Pt/C, is among the best reported for manganese oxide ORR catalysts (Table S3). The superior ORR performance of h-Mn3O4-TMSLs is attributable to the combination of advantageous structural features, including tubular geometry, monolayer superlattice structure, and carbon coating. Specifically, the mesoscale tubular geometry of h-Mn3O4-TMSLs is beneficial for fast mass transport; the monolayer superlattice structure enhances the molecular accessibility of NC constituents; the carbon coating not only facilitates electron transport but also effectively prevents the agglomeration of NCs during the electrocatalytic process. Moreover, the far superior catalytic performance of h-Mn3O4TMSLs relative to s-Mn3O4-TMSLs establishes the hollow NC nature of h-Mn3O4-TMSLs also makes a remarkable contribution to the enhanced ORR activity, presumably benefiting from the significantly increased electroactive surface areas.18 In summary, we have demonstrated fabrication of freestanding, carbon-coated tubular superlattices comprising a monolayer of hollow Mn3O4 NCs by a template-assisted epitaxial assembly strategy. The hierarchical h-Mn3O4-TMSLs obtained synergistically combine the merits of the hollow structure on both nanometer and micrometer scales, enabling efficient utilization of NC constituents while simultaneously promoting mass transport and charge transfer properties. The practical application of h-Mn3O4-TMSLs has been demonstrated by their use as high-efficiency electrocatalysts toward ORR. This confined epitaxial assembly technique is amenable to a variety of colloidal NCs, offering new opportunities in designing NC superstructures with tailored morphologies that are of interest for a broad range of applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06587. Experimental details and supplementary results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Angang Dong: 0000-0002-9677-8778 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.D. acknowledges the financial support from MOST (2017YFA0207303 and 2014CB845602), NSFC (21373052), and Shanghai International Science and Technology Cooperation Project (15520720100). 12136

DOI: 10.1021/jacs.7b06587 J. Am. Chem. Soc. 2017, 139, 12133−12136