CHAPTER I. INTRODUCTION AND LITERATURE REVIEW
1.5 Initiatives of using 2-D layered -MnO 2 and VO 2 (B) nanosheets as supercapacitor
1.5.2 Commonly used approaches to fabricate 2-D layered -MnO 2 and VO 2 (B) nanosheets . 29
1.5.2 Commonly used approaches to fabricate 2-D layered -MnO2 and VO2(B)
Sasaki et al106 have investigated the exfoliation process of the layered manganese oxide upon contact with aqueous TBA ions. They found that the H0.13MnO2•0.7H2O underwent various reactions including intercalation, osmotic swelling, and delamination into single sheets, which depending on the TBA concentration. Hwang et al98 have successfully prepared the porous assembly via flocculation of the exfoliated MnO2 nanosheets with alkali metal cations. The as-prepared samples exhibit mesoporous structures with large surface area (50-70 m2/g) and good supercapacitor properties. Our group has also synthesized -MnO2 nanosheets via exfoliation with TBAOH under ultrasonication, and flocculation with acid. The observation pf porous structures after reassembly highlights the usefulness of our ultrasonic-assisted exfoliation and flocculation procedures, where the exfoliation rate is greatly enhanced compared with most other reported procedures.25
Except the method mentioned above, other commonly used approaches for making
-MnO2 nanosheets include hydrothermal preparation, electrodeposition, wet chemistry method, etc. For example, Zhang et al110 have synthesized hierarchical porous nanostructures assembled from ultrathin birnessite-type MnO2 nanosheets through a rapid hydrothermal method without using any template and surfactant. The SEM image in Figure 23(a) demonstrates that the products are composed of many ultrathin nanosheets with core-corona structure. Liu et al111 have directly grown MnO2 nanosheet arrays on Ni foam current collectors using one-step electrodeposition method. The obtained samples exhibit well defined mesoporous structures, as shown in Figure 23(b). The as-deposited MnO2 nanosheets exhibit an average thickness of 20-25 nm with a pore size ranging from 2 to 8 nm. Sinha et al112 reported the nucleation and growth of δ-MnO2 nanosheets using a simple wet chemical reaction at low temperature. The obtained samples exhibit typical sheet-like morphology as shown in Figure 23(c), demonstrating the usefulness of the wet chemistry method.
Figure 23. The typical SEM morphologies of -MnO2 nanosheets prepared by (a) hydrothermal method, from Zhang, et al,110 (b) electrodeposition method, from Liu, et
al,111 and (c) wet chemistry method. From Sinha, et al.112
Unlike the versatile preparation methods for -MnO2 nanosheets that have been developed, most of the VO2(B) nanosheets are generally synthesized via hydrothermal reactions according to recently published papers. Through adjusting the hydrothermal reaction conditions, various nanostructures with desired morphologies assembled from VO2(B) nanosheets can be easily obtained. Liang et al113 have prepared ultra-large (over 100 μm) VO2(B) nanosheets with an exceptionally small thickness of only 2–5 nm, as shown in Figure 24(a), through a facile template-free hydrothermal synthesis approach.
Mai et al114 have investigated the influence of PVP and PEG on structures and surface morphologies during hydrothermal reactions, and has successfully prepared pure thin VO2(B) nanosheets as shown in Figure 24(b). Zhang et al115 have successfully prepared flowerlike VO2(B) nanostructures assembled from single-crystalline nanosheets via a facile hydrothermal method using PVP as a capping reagent, with the typical morphology shown in Figure 24(c). They have also investigated the crystal growth mechanism, which was dominated by nucleation and growth, self-assembly, and Ostwald ripening. Several other papers have also reported the successful preparation of VO2(B) nanosheets with desired morphologies, which confirm the effectiveness of the hydrothermal synthesis method.
Figure 24. (a) HRTEM image of the VO2(B) nanosheets prepared by hydrothermal method, from Liang, et al,113 (b) TEM image of pure VO2(B) nanosheets, from Mai, et al,114 and (c) High-magnification SEM image of the flower-like VO2(B) samples. From
Zhang, et al.115
Recently, Liu et al116 have proposed a room-temperature intercalation- deintercalation strategy to obtain ultrathin nanosheets exfoliated from layered materials.
The schematic diagram is shown in Figure 25. The key point of this method is successive insertion of lithium ions and larger molecules, which will efficiently enlarge the interplanar distance and thereby weaken their strong covalent interactions. Specifically speaking, the bulk VO2(B) is prepared by hydrothermal method. Then the bulk product was dispersed in deionized water by ultrasonication and stirred with LiCl for 24 hours.
After lithiation, the solid intermediate was washed with deionized water sufficiently to remove the LiCl adsorbed on the VO2(B) surface. Finally, single layers of VO2(B) nanosheets can be exfoliated from the solid intermediates in DMF/H2O (V/V=1:1) solution under ultrasonication for 5 hours. The TEM image shows a large-area and nearly transparent ultrathin VO2(B) nanosheet with lateral dimensions of ca. 200-500 nm. The AFM results indicate the large 2-D nanosheets exhibit a height of ca. 0.62 nm, which matches well with the theoretical thickness of 0.615 nm for VO2(B) single layers along the [001] direction, indicating the successful exfoliation of single-layer VO2(B) nanosheet. Therefore, this work provides a new scalable strategy of making single-layer VO2(B) nanosheet.
Figure 25. Schematic illustration shows the formation process of VO2(B) ultrathin nanosheets: (a) bulk VO2(B), (b) lithiated VO2(B), (c) hydrated VO2(B), and (d) single
layers VO2(B). From Liu, et al.116
1.6 Motivations of incorporating structural defects into the transition metal oxides as a promising way to improve their charge storage properties
The performance of electrochemical charge storage devices is largely determined by the physicochemical properties of the active electrode materials, such as morphologies, porosity, crystal structures, electrical conductivity, etc. While most works in the past decades have been focused on developing nanomaterials with large surface areas, or mixing the metal oxides with conductive materials to improve the electrical conductivity, a new strategy, which is mainly depends on the intentional introduction of atomic scale structural defects into the host lattice, has emerged in recent years and attracted more and more attention as an effective and alternative way to further improve the charge storage properties.
1.6.1 Commonly used approaches to improve the electrode properties of transition