Facile controlled synthesis and growth mechanisms of flower-like and tubular MnO₂ nanostructures by microwave-assisted hydrothermal method

Abstract

Birnessite flower-like and α-type tubular MnO₂ nanostructures were selectively synthesized through simple decomposition of KMnO₄ under hydrochloric acid condition by controlling reaction temperature using a microwave-assisted hydrothermal method. The as-prepared samples were characterized in detail by various techniques including X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform-infrared spectroscopy, and Raman scattering spectroscopy. While the growth of flower-like birnessite-MnO₂ might follow a widely accepted Ostwald ripening process, we proposed a formation mechanism of the nanotubular α-MnO₂ based on our evidence, which was assembly of nanorods through an “oriented attachment” process.

Introduction

Synthesis of nanostructured materials with controlled shape, architecture, and size is of importance to ensure their property performance in various device applications, such as optics, magnetism, mechanics, and electronics [1]. Among various synthesis approaches, solution-based wet chemical methods feature the virtue of multiplex routes, and high output, templates, surfactants, and complexing species have been applied to control size and structure of the products [2], [3], [4]. However, these methods usually imply complex and costly procedures, which are not versatile or environmental friendly.

Recently, microwave irradiation has been increasingly applied to replace conventional heating methods in material synthesis and sample digestion since it provides a selective, fast, and homogenous heat that significantly reduces processing time and cost [5], [6], [7]. Combining the microwave irradiation with wet chemical methods such as hydrothermal or solvothermal techniques required temperature and pressure for nanostructure growth can be rapidly achieved, which leads to enhanced kinetics of crystallization and promotes the formation of new phase of product [8]. In this case, nanomaterials with novel structures and architectures are expected to be generated. Up to now, different kinds of nanomaterials have been synthesized via a microwave-assisted route, including NiO [9], ZnO [10], Fe₃O₄ [11], WO₃ [12], Co₃O₄ [13].

Manganese dioxide (MnO₂) nanostructures exhibit distinctive physical and chemical properties and have wide applications in molecular/ion sieves [14], catalysts [15], sensors [16], and energy storage [17]. It is known that the properties of nanostructured MnO₂ highly depend on the crystal structure, dimension, and morphology [18]. Various MnO₂ nanostructures such as nanoparticles [19], nanorods/nanofibers [20], nanowires [21], and nanotubes [22] have been prepared by sol–gel [23], precipitation [24], reflux [25], thermal deposition [26], and hydrothermal [27] techniques. As a direct and one-step wet chemical route, hydrothermal method has been prevalently employed in synthesizing MnO₂ nanostructures. For example, Wang et al. reported a selected-control low-temperature hydrothermal method of synthesizing 1D MnO₂ nanostructures through the oxidation of Mn²⁺ by ${\text{S}}_2{\text{O}}_8^{2-}\text{,}{\text{MnO}}_4^{-}$ or ClO⁻ [28], [29]. Cheng et al. synthesized 2D hexagram-like and dendrite-like hierarchical MnO₂ nanostructures by decomposition of Mn(NO₃)₂ solution with or without nitric acid [17]. Yu et al. obtained 3D urchin shaped and clewlike MnO₂ nanostructures in the presence of Al³⁺ or Fe³⁺ under hydrothermal condition [30]. However, these conventional hydrothermal methods require prolonged reaction time for more than 12 h, even for several days. Recently, microwave-assisted hydrothermal method (MA-HM) was employed to synthesize octahedral molecular sieves (OMS-2) nanomaterials, and the samples possess better catalytic activity for cinnamyl alcohol oxidation than their conventional counterparts [31]. But to the best of our knowledge, synthesis of MnO₂ nanotubes by MA-HM has not been reported previously.

Herein, we demonstrate a facile rapid procedure to fabricate MnO₂ nanostructures through the decomposition of KMnO₄ under hydrochloric acid condition by MA-HM. 3D hierarchical nanostructures and 1D nanotubes of MnO₂ with different crystallographic forms were selectively obtained by controlling the reaction parameters. The growth mechanism was studied and discussed in detail based on detailed observations in different growth stages. This synthetic route requires no templates, catalysts, or organic reagents, which promises large-scale production of nanostructured MnO₂ with controlled structure and size.