Directed tuning of nanostructure from 1D to 3D by doping diverse valent cations

Abstract

We design an unconventional strategy for controlling the nanostructure evolution of magnesium hydroxide from lamellar to rose-like, then to torispherical species by introducing diverse valent cations (Zn²⁺, Al³⁺, Sn⁴⁺). Lattice distortion induced by invasive cations would be responsible for the morphology transformation of metal hydroxide.

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Seeking ways to control the synthesis of different dimensional nano-sized materials has been one of the most important goals of materials scientists since many applications and new functions of materials depend on their particle shapes.¹ Numerous fabrication and synthesis methods for desired nanostructures, such as discs, rods and spheres, have been extensively reported.^2–6 However, the role of cations on morphology control, commonly interstitial in lattices due to their minor radius,^7,8 has been neglected all the time. Here we demonstrate that doped-cations induce polytypism⁹ or heterostructures¹ for two or more crystal structures in different domains of the same nanoparticle, and can be exploited to controllably tune inorganic nanostructures. For the case of Mg(OH)₂, we design a simple strategy to modify a nanostructure from 2D planar to 3D flower and to 1D sphere by way of doping diverse valent cations: Zn²⁺, Al³⁺ and Sn⁴⁺. We expect that our strategy can be employed as a general method to fabricate other materials with desired nanostructures.

The direct hydration of mixtures of MgO–ZnO, MgO–Al₂O₃ and MgO–SnO₂, yields a series of bimetal hydroxides in all cases through a simple hydrothermal process. In our designed approach, other disturbance of the morphology were taken into account as much as possible, such as the same temperature, time, volume filled in, no surfactant, and of course, only involving OH⁻ anions. Because of the difference in the chemical bond configuration of the cations,¹⁰ we reasoned three typical consequences of the growth of crystals with invasive cations substituting some Mg²⁺ (Fig. 1). (1) The case of isomorphic cations replacing the Mg²⁺ (octahedral configuration) in some positions in brucite could not induce distinct changes in the bonding linkages between cations and hydroxyl groups, therefore retaining the original nanostructures. (2) Remarkably, the tetrahedral chemical bonding configuration of Al³⁺ cations results in lattice distortion, which modifies the growth orientation of crystals along a new plane, which also means that the nanostructure changes from 2D to 3D. (3) Lattice distortion derived from partial Sn⁴⁺ (tetrahedral configuration) substitution would also tune the nanostructure, leading to crystal growth on a different plane.

Fig. 1 Schematic illustration for controlling the nanostructure evolution of magnesium hydroxide from lamellar to rose-like, and then to spherical species by doping diverse cations (Zn²⁺, Al³⁺, Sn⁴⁺).

We first tested the hypothesis by synthesizing nanosheets of Mg(OH)₂ and Zn-doped Mg(OH)₂ lamellar nanostructure, as shown in Fig. 2a, Fig. 2b and 2e, respectively. The X-ray diffraction (XRD) patterns (Fig. 3a and 3b) indicate that Zn-doped Mg(OH)₂ is a nonstoichiometric species and maintains its Mg(OH)₂ nature. As is well-known, Mg(OH)₂ commonly tends to present a layered structure, in which magnesium cations are octahedrally coordinated to hydroxyl groups.¹¹ According to density function theory, isomorphic substitution of Mg²⁺ in the brucite structure by Zn²⁺ would be stable in the molar ratio of 1/4 ∼ 3/4.¹² To provide intuitionistic Zn²⁺ doped cases, we illustrate some examples of intralayer and interlayer admixtures in brucite-like structures, as seen in Fig. S1 (Supporting Information, †). Owing to their similar ionic radius and chemical bonding configuration, it was found that the (100) lattice planes and ED patterns of Zn²⁺ doped Mg(OH)₂ display subtle distortions (Fig. 4a, 4b, 4e–h), which implies that the orientation of crystal growth did not have change. In fact, this might be the intrinsic factor to keep Zn²⁺ doped Mg(OH)₂ in a lamellar morphology.

Fig. 2 SEM images of Mg(OH)₂ and a series of bimetal hydroxides by direct hydration. Evolution images of the nanostructures resulting from partial cation substitution. a, The as-synthesized Mg(OH)₂ displayed nanosheets without surfactant. b, Zn²⁺ doped Mg(OH)₂ conserved the lamellar morphology of pure Mg(OH)₂. c, Particles were modified to fabricate rose-like structures by way of introducing the trivalent cation Al³⁺ into the system. d, Torispherical species emerged as the Sn⁴⁺ cation was introduced. e, f and g, Magnified images corresponding to the respective low magnitude.

Fig. 3 X-ray diffraction of Mg(OH)₂ and bimetal hydroxides produced from the direct hydration of MgO, MgO–ZnO, MgO–Al₂O₃, and MgO–SnO₂. The labels in parentheses correspond to the SEM images of the samples shown in Fig. 2. In b and c, the samples conserve the d values of Mg(OH)₂. But the intensity of d₀₀₁ in b was enhanced. A boehmite phase emerged in c. Comparing d with a, the lattice parameters a and c of the sample were calculated from the d₁₁₀ and d₀₀₁ planes, respectively, in which a = 2 × d₁₁₀ and c = 3 × d₀₀₁. In d, the a parameter increased and the c parameter decreased.

Fig. 4 TEM images of Mg(OH)₂ and bimetal hydroxides produced from the direct hydration of MgO, MgO–ZnO, MgO–Al₂O₃, and MgO–SnO₂. The labels (a, b, c and d) correspond to the SEM images of the samples shown in Fig. 2. High-resolution electron microscopy: e, d = 0.272 nm corresponds to the (100) plane of pure Mg(OH)₂; g and i also display the nature of the (100) plane. Moreover, each HRTEM coincides well with respective their ED in f, h and j. In k, d = 0.274 nm corresponds to the (220) plane (l) of MgSn(OH)₆.

To demonstrate the crucial effect of the trivalent cation on the morphology of the particles, we introduce the Al³⁺ cation into this system through the direct hydration of mixtures of MgO and Al₂O₃ under hydrothermal conditions. The typical rose-like structure was observed in Fig. 2c and 2g. The XRD pattern of the rose-like structure (Fig. 3c) shows that all the peaks are consistent with those of a brucite phase (JCPDS 07-0239) and AlOOH (boehmite) phase (JCPDS 21-1307). Layered doubled hydroxides (LDH) can be obtained by substituting the Mg²⁺ cation with an Al³⁺ cation in the presence of a charge-compensating anion, such as CO₃²⁻. Otherwise phase separation may occur due to the instability of the doubled hydroxide structures without a charge-compensating anion.^13,14 Moreover, the distinction of the ionic radius between Al³⁺ and Mg²⁺ may also cause phase separation. To investigate the distribution mode of AlOOH in the rose-like architecture, high resolution TEM (HRTEM) characterization was carried out. The d-spacing values closely match the (100) planes of Mg(OH)₂ in Fig. 4e and 4i. Fig. 4c, 4i, 4j and Fig. S3 (Supporting Information, †) reveal that the brucite phase is concentrated in the green and blue zone, as illustrated in Fig. S4, ESI.† So we propose that the boehmite phase is distributed in the joint region between the receptacle and the petals (the red zone). If that were true, the rose-like structures would be disintegrated into a number of plates in NaOH solution. As expected, SEM images of the particles treated by NaOH solution gave us the direct evidence as shown in Fig. S5 (Supporting Information, †). In other words, the heterostructure induced by that Al³⁺ substitution results in an over-distortion of the lattice (phase separation) and could modify the morphology from plane to folded rose (2D to 3D).¹

When the Sn⁴⁺ cation was introduced into this system, a torispherical species emerged, as seen in Fig. 2d and 2f. Which is different from Al³⁺ substitution which leads to a heterostructure. The XRD pattern (Fig. 3d) shows that the Sn⁴⁺ substituted species is homogeneous MgSn(OH)₆ (JCPDS 09-0027). In contrast with one well-known example of a spherical SiO₂ structure due to the tetrahedral chemical bond configuration of silicon,¹⁵ it remains a challenge to rationally design a Mg²⁺ system with only hydroxyl groups to fabricate a sphere-like morphology. However, we prepared the sphere-like nanostructures with Sn⁴⁺ substituting some of the Mg²⁺, which suggests that the lamellar nanostructure of the Mg²⁺–hydroxyl system could be tuned by Sn⁴⁺ cation substitution to a torispherical morphology. Based on the facts mentioned above, the different chemical bonding nature between Sn⁴⁺ and Mg²⁺, such as bond length and angle, could lead to sustainable lattice distortion (d-spacing value shifting from 0.272 nm (100) to 0.274 nm (220) as shown in Fig. 4d, 4k and 4l), presenting polytypism but not a macroscopic heterostructure (Fig. 3d).^16,17

In conclusion, we have demonstrated an unconventional method for controlling the nanostructure evolution from lamellar to rose-like, then to torispherical by introducing diverse cations (Zn²⁺, Al³⁺, Sn⁴⁺). It is believed that the effects of different cations on the lattice distortion is hierarchical due to the distinction of their chemical bonding configuration and radius. Distinct lattice distortion induced by invasive cations substitute some Mg²⁺, which easily leads to the polytypism or heterostructure shown in nanostructures, and would be responsible for the morphology transformation of metal hydroxide. Subtle distortions resulting from isomorphic substitution could play a role of conservation rather than modification in the control nanostructures. We are currently investigating whether this strategy can be extended to prepare an even wider range of materials with desired nanostructures.

Acknowledgements

G. L. Ning and other authors thank the NSFC (Grant 21076041) for financial support

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental process, EDX analysis of as-synthesized Zn²⁺-doped, Al³⁺-doped and Sn⁴⁺-doped samples, SEM images of Al³⁺-substitution Mg(OH)₂ dissolved in NaOH solution. See DOI: 10.1039/c1ra00055a

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