In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and rea...

Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures

A low-cost high-performance solid-state flexible asymmetric supercapacitor (ASC) with α-MnO2 nanowires and amorphous Fe2O3 nanotubes grown on flexible carbon fabric is first designed and fabricated. The assembled novel flexible ASC device with an extended operating voltage window of 1.6 V exhibits excellent performance such as a high energy density of 0.55 mWh/cm3 and good rate capability. The ASC devices can find numerous applications as effective power sources, such as powering color-switchable sun glasses and smart windows.

Low-Cost High-Performance Solid-State Asymmetric Supercapacitors Based on MnO2 Nanowires and Fe2O3 Nanotubes

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na0.5 MnO2 nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na0.5 MnO2 nanowall arrays can be extended to 0-1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g-1 . The extended potential window for the Na0.5 MnO2 electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon-coated Fe3 O4 nanorod arrays are synthesized as the anode and can stably work in a negative potential window of -1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na0.5 MnO2 nanowall arrays as the cathode and carbon-coated Fe3 O4 nanorod arrays as the anode. In particular, the 2.6 V Na0.5 MnO2 //Fe3 O4 @C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg-1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO2 -based supercapacitors. This work provides new opportunities for developing high-voltage aqueous asymmetric supercapacitors with further increased energy density.

High-Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

In this work, three types of MnO2 nanostructures, viz., microsphere/nanosheet core−corona hierarchical architectures, one-dimensional (1D) nanorods, and nanotubes, have been synthesized employing a simple hydrothermal process. The formation mechanisms have been rationalized. The materials have been thoroughly characterized by X-ray diffraction, Brunauer−Emmett−Teller spectrometry, field-emission scanning electron miscroscopy, energy dispersive spectroscopy, and transmission electron microscopy. The microsphere/nanosheet core−corona hierarchical structures are found to be the layered birnessite-type MnO2, while 1D nanorods and nanotubes are of the α-MnO2 phase. These MnO2 nanostructures are used as a model system for studying the shape/phase-dependent electrocatalytic properties for the oxygen reduction reaction, which have be investigated by cyclic and linear sweep voltammetry. It is found that α-MnO2 nanorods/tubes possess largely enhanced electrocatalytic activity compared to birnessite-type MnO2 core−c...

Shape-controlled synthesis of MnO2 nanostructures with enhanced electrocatalytic activity for oxygen reduction

MnO2/carbon nanotube [CNT] nanocomposites with a CNT core/porous MnO2 sheath hierarchy architecture are synthesized by a simple hydrothermal treatment. X-ray diffraction and Raman spectroscopy analyses reveal that birnessite-type MnO2 is produced through the hydrothermal synthesis. Morphological characterization reveals that three-dimensional hierarchy architecture is built with a highly porous layer consisting of interconnected MnO2 nanoflakes uniformly coated on the CNT surface. The nanocomposite with a composition of 72 wt.% (K0.2MnO2·0.33 H2O)/28 wt.% CNT has a large specific surface area of 237.8 m2/g. Electrochemical properties of the CNT, the pure MnO2, and the MnO2/CNT nanocomposite electrodes are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The MnO2/CNT nanocomposite electrode exhibits much larger specific capacitance compared with both the CNT electrode and the pure MnO2 electrode and significantly improves rate capability compared to the pure MnO2 electrode. The superior supercapacitive performance of the MnO2/CNT nancomposite electrode is due to its high specific surface area and unique hierarchy architecture which facilitate fast electron and ion transport.

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Hydrothermal synthesis of MnO2/CNT nanocomposite with a CNT core/porous MnO2 sheath hierarchy architecture for supercapacitors

Single-crystal tetragonal α-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in the hydrochloric acid solution. The sample has been characterized by X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy, and orientation dependent Raman spectroscopy, which indicates the nanotubes have high-quality crystalline and shape-dependent optical properties. The morphology evolution of the sample reveals that the nanotubes are formed via the solid nanorod by a chemical etching process.

Synthesis of Single-Crystal Tetragonal α-MnO2 Nanotubes

Birnessite-type MnO2 nanowalls have been fabricated on Si(111) substrate by a facile solution method. The XRD pattern indicates that the sample has the typical feature of turbostratic disorder and prefers to grow in the ab plane. The nanowalls are composed of thin flakes distributing uniformly over the surface of the Si(111) substrate. A magnetic transition temperature of 9.2 K is determined. Prominent magnetic anisotropy with the easy magnetization direction in the ab plane is observed at 5 K.

Birnessite-type MnO2 Nanowalls and Their Magnetic Properties

α-KxMnO2 nanotubes with x = 0.10, 0.12, 0.15, and 0.17 have been synthesized through acid and alkali treatments of the precursor α-MnO2 nanotubes. For the samples with x = 0.10 and 0.12, a large divergence of the ZFC and FC susceptibilities and pronounced hysteresis at low temperature are observed. However, the samples with x = 0.15 and 0.17 are antiferromagnetic with Neel temperatures of 25.4 and 25.3 K, respectively. The reciprocal susceptibility and electrical conductivity indicate that the samples with x = 0.15 and 0.17 undergo a charge separation transition from mixed/averaged valence state manganese ions to Mn3+ and Mn4+ between 250 and 300 K. Below half occupancy of the K+ site (x ≤ 0.125), the strong geometrical frustration due to the triangular lattice configuration and the mixture of Mn3+/Mn4+ result in the spin-glass behavior of the samples with x = 0.10 and 0.12. Above half occupancy of the K+ site, the samples with x = 0.15 and 0.17 exhibit an antiferromagnetic feature due to the charge separ...

G. H. Rao

Papers

Synthesis of Single-Crystal Tetragonal α-MnO2 Nanotubes

Birnessite-type MnO2 Nanowalls and Their Magnetic Properties

Tuning Magnetic Properties of α-MnO2 Nanotubes by K+ Doping