Siwen Zhang

Phase-controlled synthesis of polymorphic MnO2 structures for electrochemical energy storage

Manganese dioxide (MnO2) with α-, β- and δ-type structures was controllably synthesized by hydrothermal treatment of an acidic solution of KMnO4 containing different concentrations of ions at 160 °C. The effects of metallic cations, H+ and anions on the structures and morphologies of MnO2 were investigated systematically. The experimental results indicated that cations played a significant part in the formation of MnO2 with different structures. When K+ ions were in competition with H+ ions in solution, different MnO2 structures were formed. α-MnO2 was formed when the amount of K+ was higher than the amount of H+, whereas a higher amount of H+ was favorable for the growth of β-MnO2 structures. When the concentration of K+ was much greater than that of H+, δ-MnO2 was obtained. Possible formation mechanisms are proposed based on a series of controlled experiments. The electrochemical properties of supercapacitors based on different types of MnO2 electrodes were investigated in detail. The specific capacitance measured for MnO2 strongly depended on the crystallographic structure and decreased in the order α-MnO2 > δ-MnO2 > β-MnO2 at a current density of 0.5 A g−1. A specific capacitance of 535 F g−1 was obtained for α-MnO2, but was only 155 F g−1 for β-MnO2. α-MnO2 was more suitable for use as the working electrode at high current densities. Both α- and δ-MnO2 had a poor cycling performance after 3000 cycles, whereas β-MnO2 showed good stability, maintaining a cycling efficiency of 80% after under the same conditions.

Facile synthesis of ultralong MnO2 nanowires as high performance supercapacitor electrodes and photocatalysts with enhanced photocatalytic activities

A simple hydrothermal route was developed for the synthesis of uniform α-MnO2 nanowires using a controllable redox reaction in a MnCl2–KMnO4 aqueous solution system. Electrochemical properties of the resulting products as an electrode were investigated by cyclic voltammetry, chronopotentiometry and electrochemical impedance tests. The electrode can provide a specific capacitance of 180 F g−1 at a current density of 1.0 A g−1. After 2000 cycles, the specific capacitance was maintained above 78%. Besides, the as-synthesized products were also used as the photocatalyst for the photocatalytic degradation of several dye molecules. The results show almost complete degradation (~99%) of Congo red dye molecules in 30 min. Such ultralong MnO2 nanowires are expected to have potential applications in wastewater treatment and energy storage.

Enhanced electrochemical performance of hybrid SnO2@MOx (M = Ni, Co, Mn) core–shell nanostructures grown on flexible carbon fibers as the supercapacitor electrode materials

In this study, hierarchical SnO2@MOx (SnO2@NiO, SnO2@Co3O4, and SnO2@MnO2) heterostructures grown on carbon cloth (CC) for high performance supercapacitors were fabricated by a two-step solution-based method involving a hydrothermal process and a chemical bath deposition, which utilizes the better electronic conductivity of SnO2 nanosheets as the supporting backbone to deposit MOx for supercapacitor electrodes. Particularly, the as-formed SnO2@MOx heterostructure electrodes showed better electrochemical performance than bare SnO2 nanosheets. Remarkably, the SnO2@MnO2 heterostructure electrode showed the highest discharge areal capacitance (980 mF cm−2 at 1 mA cm−2), good rate capability (still 767 mF cm−2 at 20 mA cm−2), and excellent cycling stability (∼21.9% loss after 6000 repetitive cycles at a charge–discharge current density of 1 mA cm−2). The enhanced pseudocapacitive performance was mainly attributed to its unique hybrid structure, which provides fast ion and electron transfer, a large number of active sites, and good strain accommodation. The excellent electrochemical performance of the as-obtained heterostructures will undoubtedly make these hybrid structures attractive for high performance supercapacitors with high power and energy densities.

Hierarchical Semiconductor Oxide Photocatalyst: A Case of the SnO2 Microflower

Hierarchically assembled SnO2 microflowers were synthesized by a facile hydrothermal process. Field emission scanning electron microscope results showed these hierarchical nanostructures were built from two dimensional nanosheets with the thicknesses of about 50 nm. Photoluminescence spectrum of the as-obtained products demonstrated a strong visual emission peak at 564 nm. The photochemical measurement results indicated that the as-prepared sample exhibits excellent photocatalytic performance. These three dimensional SnO2 hierarchical nanostructures may have potential applications in waste water purification.

Facile hydrothermal approach to ZnO nanorods at mild temperature

In this work, ZnO nanorods are obtained through a facile hydrothermal route. The structure and morphology of the resultant products are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The experimental results indicated that the as-synthesized ZnO nanorods have an average diameter of approximate 100 nm. A possible growth mechanism for ZnO nanorods was proposed based on the experimental results and found that Zn powder plays a critical role for the morphology of the products. Room temperature photoluminescence property of ZnO nanorods shows an ultraviolet emission peak at 390 nm.

ZnO Film Photocatalysts

We have synthesized high-quality, nanoscale ultrathin ZnO films at relatively low temperature using a facile and effective hydrothermal approach. ZnO films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, photoluminescence spectra (PL), and UV-vis absorption spectroscopy. The products demonstrated 95% photodegradation efficiency with Congo red (CR) after 40 min irradiation. The photocatalytic degradation experiments of methyl orange (MO) and eosin red also were carried out. The results indicate that the as-obtained ZnO films might be promising candidates as the excellent photocatalysts for elimination of waste water.

Nanosheet-Assembled ZnO microflower photocatalysts

Large scale ZnO microflowers assembled by numerous nanosheets are synthesized through a facile and effective hydrothermal route. The structure and morphology of the resultant products are characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Photocatalytic properties of the as-synthesized products are also investigated. The results demonstrate that eosin red aqueous solution can be degraded over 97% after 110 min under UV light irradiation. In addition, methyl orange (MO) and Congo red (CR) aqueous solution degradation experiments also are conducted in the same condition, respectively. It showed that nanosheet-assembled ZnO microflowers represent high photocatalytic activities with a degradation efficiency of 91% for CR with 90 min of irradiation and 90% for MO with 60 min of irradiation. The reported ZnO products may be promising candidates as the photocatalysts in waste water treatment.