Laterally-uniform Mn₃O₄ colloidal nanosheets: oriented growth and size-controlled synthesis

Abstract

In this work, laterally-uniform Mn₃O₄ nanosheets with regular square-like shapes and tunable lateral dimensions are synthesized through an effective one-pot solvothermal chemical reaction. The formation pathway of the Mn₃O₄ nanosheets and sized-controlled methods are also investigated.

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Two-dimensional (2D) structured nanomaterials, known as nanosheets or nanoplatelets, have attracted great interest in recent research due to their intriguing size-dependent properties, which has made them appealing candidates in applications that include batteries, light emitting diodes, catalysts, etc.¹ A recent example is graphene, where the 2D structure directly endows it with unique thermal, optical and electrical properties and a wide range of applications.² To date, to facilitate these thick nanosheets, synthetic methods including exfoliation of layered compounds, epitaxial growth on substrates, and solvothermal chemical synthesis have been well-established.³ Compared to other methods, solvothermal routes can directly generate colloidal nanosheets, which are also proved to be low-cost, convenient, reliable, and the most valuable, advantageous in controlling the edge dimensions, lateral uniformity, and vertical thickness of nanosheets. Such chemical synthesis strategies have been widely exploited in a diverse range of metal oxides and chalcogenide semiconductors, for instance, CeO₂,⁴ WO₃,⁵ PbS,⁶ and SnSe.⁷ However, for many other materials, it still remains a challenge to yield colloidal nanosheets with controllable morphology characteristic features, for instance, regular shapes, tunable dimensions and thicknesses.

Recently, a variety of Mn₃O₄ nanostructures, including nanofibers, nanocubes, and nanosheets, have been fabricated and widely exploited in energy storage, catalysts, soft magnetic materials and so on.⁸ Despite their important applications, there has been few report on the synthesis of Mn₃O₄ nanosheets with regular shapes and controlled-sizes, let alone further investigation of their dimension-dependent uses. Herein, we report a simple one-pot colloidal synthetic pathway of Mn₃O₄ nanosheets and reveal the fundamental insights of their nanostructure formation. The as-synthesized Mn₃O₄ nanosheets adopt a uniform square-like morphology with an average thickness of ∼8 nm, and the lateral dimensions can be tuned from approximately 20 nm × 20 nm to 200 nm × 200 nm by adjusting the feeding concentration of fatty acid reagents in the reaction system.

Typically, a slurry of manganous acetate (0.49 g) and stearic acid (SA, 0.142 g) in 40 mL 1-octadecene (ODE) was reacted at 210 °C for 3 h, and aged at 270 °C for 5 min under argon. The resulting mixture was cooled to room temperature to give a brown suspension. The suspension was precipitated by adding a large amount of ethanol followed by centrifugation at 5000 rpm to remove excess SA and ODE. The brown precipitate could be easily redispersed in various organic solvents such as cyclohexane, hexane, and toluene to form a clear brown solution after sonication for 10 min.

The X-ray photoelectron spectroscopy (XPS) spectrum of the collected sample in Fig. 1(a) shows two peaks at 653.4 and 641.6 eV in the Mn 2p region corresponding to the binding energies of Mn 2p_1/2 and Mn 2p_3/2 respectively. The observed spin–orbit splitting between the two levels is 11.8 eV, which is similar to those of the reported Mn₃O₄ elsewhere.⁹ The binding energy of O 1s is 530.4 eV and attributed to the lattice oxygen of manganese oxide (Fig. S1, ESI†). X-ray diffraction (XRD) pattern of the nanosheets is shown in Fig. 1(b). Diffractions of (101), (112), (200), (103), (211), (004), (220), (105) and (312) crystalline planes are assigned to the standard tetragonal hausmannite Mn₃O₄ structure (a = 5.76 Å and c = 9.47 Å, JCPDS card no.: 24-0734, I/amd space group), with no observable crystalline impurities, in good agreement with the selected-area electron-diffraction pattern (SAED; Fig. S2, ESI†). The relative intensity of (200) plane of nanosheets is significantly enhanced as compared to that of bulk Mn₃O₄ (red bars in Fig. 1(b)), which indicates that the preferable orientation of the nanosheets is along the a-axis direction. The as-obtained XRD, XPS and energy-dispersive spectroscopy (EDS; Fig. S3, ESI†) data confirm the presence of phase pure hausmannite Mn₃O₄.

Fig. 1 (a) XPS spectrum of the Mn 2p region, (b) XRD pattern, (c), (d) TEM, and (e) HRTEM images of the as-synthesized Mn₃O₄ nanosheets. The inset in (e) is the FTT pattern of (e).

Fig. 1(c) displays the representative low-resolution transmission electron microscope (TEM) image of the freestanding Mn₃O₄ nanosheets. It can be clearly observed that highly laterally-uniform sheet-like structures are formed. A corresponding size distribution histogram of the nanosheets represented in Fig. 1(c) (Fig. S4, ESI†) demonstrates that the average largest-edge length is 207 ± 29 nm and the average smallest-edge length is 168 ± 21 nm. Fig. 1(d) shows a stacked structure of nanosheets with an average thickness of approximately 8 nm. Zooming in at the corner of a single flat nanosheet, shown in Fig. 1(e), the high-resolution TEM (HRTEM) image reveals a highly crystalline lamellar structure of Mn₃O₄. Obvious lattice spacings of 0.290 nm and an intersection angle of approximately 90 angles are observed, which matches well with the (200) and (020) set planes of tetragonal hausmanntite Mn₃O₄. Consistent with this, the corners of the Mn₃O₄ nanosheets observed in the TEM image also form 90 angles. The corresponding fast Fourier transform (FFT) pattern is shown in the inset of Fig. 1(e).

The presence of pure Mn₃O₄ phase of the as-prepared nanosheets is very interesting, since the formation of other manganese oxides such as MnO₂, Mn₂O₃, and MnO has been excluded. Although the employed reaction has been conducted with exclusion of air by continuously purging argon, the synthetic environment is found partially oxidative. The oxidation of Mn²⁺ to Mn³⁺ may originate from some trapped air. Considering the low oxygen concentration in the present case, Mn²⁺ should be partially oxidized to Mn³⁺ and yield Mn₃O₄ by reconstructing remaining Mn²⁺, and deeper oxidation to MnO₂ are avoided. Furthermore, it has been reported that Mn₃O₄ has the lowest Gibbs free energies of formation among all the polymorphs of manganese oxides as mentioned before.¹⁰ This also implies that Mn₃O₄ may be the most stable compound to be obtained during the solvothermal synthetic route. Taking all the factors into account, the possible reaction equation is provided as follows:

Mn²⁺ + 2OH⁻ → Mn(OH)₂ (1)

Mn(OH)₂ → MnO + H₂O (2)

6MnO + O₂ → 2Mn₃O₄ (3)

As we know, Mn₃O₄ do not have a layered crystal structure in nature, so 2D anisotropic growth of Mn₃O₄ nanoparticles is lack of intrinsic driving force. The colloidal Mn₃O₄ nanosheets are probably formed by the 2D oriented attachment of preformed small Mn₃O₄ nanocrystalline building blocks, which is similar to other reported 2D nanomaterials.^4,6,11 TEM and XRD analyses of aliquots taken during the course of a reaction are used to understand the formation pathway of Mn₃O₄ nanosheets. TEM snapshots of the nanostructures of Mn₃O₄ are presented in Fig. 2(a)–(c). As can be seen in Fig. 2(a), individual octahedral Mn₃O₄ nanocrystals of approximately 1 to 10 nm in diameter are readily formed in the earliest growing stage, which may be employed as seeds to create heterostructured manganese oxides. With reaction time extended, bigger aggregates that contain dozens of particles are self-assembled as expected in Fig. 2(b). Further growth, these small aggregates appear to coalesce and fuse together into irregularly shaped 2D structures (Fig. 2(c)). A side-on view of some stacked nanosheets reveals that the thicknesses of them are approximately 6 to 8 nm, which is consistent with the ultimate product. It is important to note that most of the aggregates have transformed to irregular square-like nanosheets of varying sizes, but some of them can still be observed as polycrystalline forms (marked by red circles). HRTEM of these polycrystalline structures provide further insights into the growth process (Fig. S5, ESI†). As can be seen in a typical polycrystalline sheet, Mn₃O₄ nanoparticles assemble together and the orientations of the constituent particles vary from random to well-order. The lattice spacings (d = ∼2.9 Å) match well with the (200) and (020) plane in Mn₃O₄ crystal, which is consistent with the [100] orientation presented earlier before. During the crystal morphological evolution, the polycrystalline sheets are certain to crystallize to single crystal 2D nanostructures. Single crystal Mn₃O₄ nanosheets after coalescence stage finally can be extended laterally to obtain uniform square-like nanosheets by continued addition of primary seeds in two dimensions. XRD pattern shown in Fig. 2(d) help further characterize the systematic variation of composition in the nanosheets. In the early stages of growth, preformed Mn₃O₄, MnO, and unconsumed manganous acetate are observed in the ensemble. The crystallinity of samples increases with the evolution of the reaction, and highly-crystallized phase pure Mn₃O₄ is formed once the 2D oriented growth progress is complete. The information provided by the XRD is consistent with the TEM analysis during the formation. TEM and XRD techniques both confirm the 2D oriented attachment formation pathway of Mn₃O₄ nanosheets. A simple possible schematic illustration of the laterally-growth summarized from the previous data is shown in Fig. 2(e).

Fig. 2 (a)–(c) TEM images of aliquots taken in the initial nucleation, agglomeration and coalescence stages during the growth of nanosheets. (d) Corresponding XRD patterns of the aliquots taken of each formation step. (e) Schematic illustration of the 2D oriented growth of Mn₃O₄ nanosheets.

The size and shape of colloidal nanosheets are found to be mostly dependent on the type of precursor, the concentration of surfactant, the chain length of surfactant, and the reaction temperature. Manganous acetate was commonly adopted as a starting precursor to produce manganese oxide nanoparticles via thermal decomposition in organic solvent systems.¹² Precursors such as MnSO₄, Mn(NO₃)₂ and MnCl₂ were used in this study, but neither of them could yield any Mn₃O₄ nanostructures under the employed reaction conditions. Reaction temperature also plays an important role in the nucleation and crystallization of Mn₃O₄ nanostructures. A simple change in the reaction temperatures can result in variation of the size of Mn₃O₄ nanostructures (Fig. S6, ESI†). For example, when the reaction temperature is 180 °C, most of the product observed are Mn₃O₄ nanoparticles of varying sizes (1–10 nm), and only a few nanosheets can be obtained. When the reaction is proceeded at 250 °C, smaller nanosheet fragments are obtained. Therefore, it can be inferred that lower or higher temperature is not available for transition of amorphous nanocrystals to higher 2D nanostructures.

Lateral size control of the Mn₃O₄ nanosheets was achieved by adding concentration of SA in ODE, without any change in precursor, temperature or solvents in the reaction systems. As shown in Fig. 3, the shape of colloidal Mn₃O₄ nanosheets is generally controlled to be square or rectangular when the feeding molar concentration of SA ranges from 25 to 200 mmol L⁻¹. The TEM images and thickness statistical charts of stacked nanosheets synthesized with different SA concentration (Fig. S7, ESI†) shows that the average thicknesses of these nanosheets are all approximately 8 nm, which is also close to the nanosheet in Fig. 1(d). Meanwhile, the colloidal nanosheets with larger lateral dimensions intend to stack on top of each other substantially. On the contrary, the nanosheets with smaller lateral dimensions are less stacked. Statistical chart of their size distribution (Fig. S8, ESI†) indicates that a high concentration of fatty acid ligands will result in large lateral dimensions while other reaction conditions maintain the same. When the concentration of SA is 200 mmol L⁻¹, the nanosheets are observed with average dimensions on the order of 20 nm × 20 nm, which is about 10 times smaller than the Mn₃O₄ nanosheets produced at the concentration of 12.5 mmol L⁻¹. According to Wuff facet theory, crystal growth occurs rapidly on high-free-energy facets. During the 2D growth of colloidal Mn₃O₄ nanosheets, [100] and [010] facets are high-free-energy facets. With the increasing amount of SA, more complete interaction between the Mn₃O₄ nanocrystals and SA molecules could inactivate the [100] and [010] facets, resulting in low-free-energy facets.^10,13 Eventually, smaller energetically favorable square-like nanostructures are formed with assistance of SA capping. Furthermore, smaller 2D sheet-like nanostructure is also thermodynamically favourable in the autonomous coalescence stage. The phenomena are quite similar to what had been observed in a shape-controlled synthesis of Mn₃O₄ nanostructures elsewhere.¹⁰ Our findings help to merge the oriented attachment and surfactant-mediated strategies for nanocrystal synthesis and provide guidelines for controlling the lateral size of 2D metal oxide nanomaterials.

Fig. 3 Control of lateral sizes of Mn₃O₄ nanosheets. TEM images of nanosheets synthesized in presence of different feeding molar concentration of SA: (a) 25 mmol L⁻¹, (b) 50 mmol L⁻¹, (c) 100 mmol L⁻¹, and (d) 200 mmol L⁻¹.

Conclusions

A one-pot template-free solvothermal strategy was developed to synthesis highly crystalline, monodisperse, and colloidal Mn₃O₄ nanosheets. The nanosheets adopted square/rectangle-like morphology with uniform lateral dimensions and similar thicknesses. Further studies of the growth pathway collectively suggested that Mn₃O₄ nucleation occurred initially, followed by nuclei agglomeration and lateral-direction oriented coalescence. Uniform square-like Mn₃O₄ nanosheets of tunable sizes were fabricated by simply adding various amounts of fatty acid into the reaction system, without any changes in precursor, temperature or solvents. This work will offer important enlightenments for many other relevant metal oxides, helping to guide efforts in the fabrication and optimization of novel 2D nanostructures for technological applications.

Acknowledgements

The work was supported by NNSF of China (51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province (no. BA2014100), the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1417) and the Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, EDS spectra, additional XPS, TEM, SAED and size distribution charts. See DOI: 10.1039/c5ra00341e

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