Junwu Zhu

Graphene oxide--MnO2 nanocomposites for supercapacitors.

A composite of graphene oxide supported by needle-like MnO(2) nanocrystals (GO-MnO(2) nanocomposites) has been fabricated through a simple soft chemical route in a water-isopropyl alcohol system. The formation mechanism of these intriguing nanocomposites investigated by transmission electron microscopy and Raman and ultraviolet-visible absorption spectroscopy is proposed as intercalation and adsorption of manganese ions onto the GO sheets, followed by the nucleation and growth of the crystal species in a double solvent system via dissolution-crystallization and oriented attachment mechanisms, which in turn results in the exfoliation of GO sheets. Interestingly, it was found that the electrochemical performance of as-prepared nanocomposites could be enhanced by the chemical interaction between GO and MnO(2). This method provides a facile and straightforward approach to deposit MnO(2) nanoparticles onto the graphene oxide sheets (single layer of graphite oxide) and may be readily extended to the preparation of other classes of hybrids based on GO sheets for technological applications.

Bioinspired effective prevention of restacking in multilayered graphene films: towards the next generation of high-performance supercapacitors.

Figure 1 . Structure of the SSG fi lm. a,b) Photographs of the as-formed fl exible SSG fi lms. c) Schematic of the cross-section of the SSG fi lm. d) SEM image of the cross-section of a freeze-dried SSG fi lm. Given that the undried SSG fi lm is approximately 20 times thicker than the dried one, the undried SSG fi lm must be much more porous than shown in this SEM image. e) XRD patterns of as-prepared and freeze-dried SSG fi lms. The broad diffraction peak of the wet SSG fi lm (from 27.9 ° to 44.8 ° ) is likely due to the water confi ned in the CCG network because the corresponding d -spacing is less than the d (002) of graphite. Further experiments are required to understand the structure of the water in this SSG fi lm. A combination of extraordinary electrical, thermal, and mechanical properties makes graphene sheets not only attractive as atom-thick components in nanoelectronic devices, but also excellent molecular building blocks for assembling new macroscopic materials for widespread applications, [ 1 ] particularly as electrodes for energy storage devices. [ 1d , 2 ] Since signifi cant advances have recently been achieved in the cost-effective production of graphene in large amounts, [ 1b − e, 3 ] large-scale application of graphene in a bulk assembly form appears to be close to the market. [ 1d ] However, the exploitation of graphene-based bulk materials faces a key scientifi c and technical challenge. As with other polymeric or molecular materials, the performance of graphene-based materials is strongly affected by the way the individual sheets are arranged. Due to the intersheet van der Waals attractions, aggregation or restacking inevitably occurs in graphene assemblies. [ 2a , 3a , 4 ] Consequently, many of the unique properties that individual sheets possess, such as high specifi c surface area and peculiar electron transport behaviors, are signifi cantly compromised or even unavailable in an assembly. For example, when used as electrodes for electrochemical capacitors or supercapacitors, physically separated graphene sheets that are vertically grown on a metal substrate show an exceptional frequency response. [ 5 ] This result, however, has been considered diffi cult to realize in porous bulk graphene fi lms due to the increased diffi culty of ions diffusion caused by intersheet aggregation. [ 5 ] The performance of bulk graphene fi lms [ 2a − j]

Deposition of Co3O4nanoparticles onto exfoliated graphite oxide sheets

A composite of graphite oxide (GO)-supported Co3O4nanoparticles (NPs) was prepared through a facile chemical route. The lamellar GO sheets in this composite were exfoliated and decorated randomly by Co3O4 particles with an average size of 100 nm. The formation route to anchor Co3O4 NPs onto the exfoliated GO sheets was proposed as the intercalation and adsorption of cobalt ions into the layered GO sheets, followed by the nucleation and growth of Co3O4 crystals, resulting in the exfoliation of GO sheets. The nanocomposite reveals unusual catalytic effect for the decomposition of ammonium perchlorate due to the concerted effects of GO and Co3O4 NPs or their integrated properties. This methodology made the synthesis of GO-NP composites possible and may be further extended to prepare more complicated nanocomposites based on GO sheets for technological applications.

Decorating graphene oxide with CuO nanoparticles in a water–isopropanol system

A facile chemical procedure capable of aligning CuO nanoparticles on graphene oxide (GO) in a water-isopropanol system has been described. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations indicate that the exfoliated GO sheets are decorated randomly by spindly or spherical CuO nanoparticle aggregates, forming well-ordered CuO:GO nanocomposites. A formation mechanism of these interesting nanocomposites is proposed as intercalation and adsorption of Cu2+ ions onto the GO sheets, followed by the nucleation and growth of the CuO crystallites, which in return resulted in the exfoliation of GO sheets. Moreover, the obtained nanocomposites exhibit a high catalytic activity for the thermal decomposition of ammonium perchlorate (AP), due to the concerted effect of CuO and GO.

From graphene to metal oxide nanolamellas: a phenomenon of morphology transmission.

Single-layer-graphene and few-layer-graphene structures have been predicted to have high specific surface area. Recent research has focused largely on utilizing the intriguing morphology of graphene as building blocks or substrates, keeping the structure undisturbed. Relatively little attention has been paid to explore the framework substitution of graphene. Here, we report a procedure for morphology transmission from graphene to metal oxide nanolamellas by in situ replacement with the framework of graphene. Our approach involves using graphene sheets as the starting reagent, thereby transmitting the morphology of layered structure from graphene to as-prepared metal oxides. The heteroconfiguration of as-prepared MnO(2) could play a role in preventing microstructure degradation in the electrochemical cycling process, bestowing MnO(2) nanolamellas an excellent electrochemical stability as a supercapacitor electrode. It is worth mentioning that this methodology is readily adaptable to fabricating MnO(2), Co(3)O(4), and Cr(2)O(3) nanowires from single-walled carbon nanotubes and Co(3)O(4) and Cr(2)O(3) nanolamellas from graphene sheets.

Graphene-based 3D composite hydrogel by anchoring Co3O4 nanoparticles with enhanced electrochemical properties

Three-dimensional (3D) graphene-based composite materials have attracted increasing attention, owing to their specific surface area, high conductivity and electronic interactions. Here, we report a convenient route to fabricate a 3D Co3O4/Graphene Hydrogel (CGH) composite as an electrode material for supercapacitors. Utilizing the gelation of a graphene oxide dispersion enables the anchoring of Co3O4 nanoparticles on the graphene sheet surfaces and formation of the hydrogel simultaneously. Remarkably, the spherical Co3O4 particles can serve as spacers to keep the neighboring graphene sheets separated. The CGH exhibits a high specific capacitance (Cs) of 757.5 F g(-1) at a current density of 0.5 A g(-1), indicating its potential application as an electrode material for supercapacitors.

Fabrication of a low defect density graphene-nickel hydroxide nanosheet hybrid with enhanced electrochemical performance

AbstractThe development of efficient energy storage devices with high capacity and excellent stability is a demanding necessary to satisfy future societal and environmental needs. A hybrid material composed of low defect density graphene-supported Ni(OH)2 sheets has been fabricated via a soft chemistry route and investigated as an advanced electrochemical pseudocapacitor material. The low defect density graphene effectively prevents the restacking of Ni(OH)2 nanosheets as well as boosting the conductivity of the hybrid electrodes, giving a dramatic rise in capacity performance of the overall system. Moreover, graphene simultaneously acts as both nucleation center and template for the in situ growth of smooth and large scale Ni(OH)2 nanosheets. By virtue of the unique two-dimensional nanostructure of graphene, the as-obtained Ni(OH)2 sheets are closely protected by graphene, effectively suppressing their microstructural degradation during the charge and discharge processes, enabling an enhancement in cycling capability. Electrochemical measurements demonstrated that the specific capacitance of the as-obtained composite is high as 1162.7 F/g at a scan rate of 5 mV/s and 1087.9 F/g at a current density of 1.5 A/g. In addition, there was no marked decrease in capacitance at a current density of 10·A/g after 2000 cycles, suggesting excellent long-term cycling stability.

Synthesis and characterization of graphene paper with controllable properties via chemical reduction

The synthesis of graphene paper with controllable properties via chemical reduction of exfoliated graphene oxides was investigated. UV-Vis absorption spectra, elemental analysis, FT-IR spectra, X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), mechanical analysis, electrical conductivity and cyclic voltammetry (CV) measurements were applied to the characterization. It was proven that different reduction levels of graphene oxides can significantly alter the thermal, mechanical and electrical properties of the corresponding chemically reduced graphene (CRG) paper. Moreover, UV-vis spectrometry was found to be a valid and facile method to monitor and control the reduction level of CRG, and furthermore to fine tune these properties, which may pave the way for large scale fabrication of graphene-based nanomaterials.

One-step synthesis of low defect density carbon nanotube-doped Ni(OH)2 nanosheets with improved electrochemical performances

A facile soft chemistry route is described to fabricate a composite of α-Ni(OH)2 nanosheets doped with low defect density carbon nanotubes (CNTs) via one step. As a result of the hydrolysis of nickel nitrate in a water/N-methyl-pyrrolidone (NMP) system, the as-produced H+ and NO3− species can react with carbon atoms of CNTs, yielding a small amount of oxygen-containing functional groups which may serve as the anchor sites. It is interesting to find that CNTs simultaneously act as both nucleation centers and templates, eventually leading to the formation of smooth and micro-scale α-Ni(OH)2 nanosheets–CNTs hybrids. Because of the soft oxidation nature of as-proposed synthetic pathway, the conjugation of carbon atoms in the framework is less disrupted and the CNT backbones in the composite have a characteristically low defect density; thereby, the physical properties of CNTs are well preserved in the composite material. As expected, a high specific capacitance of 1302.5 F g−1 of the composite are obtained in comparison with its individual components (372.1 F g−1 for Ni(OH)2 and 101.4 F g−1 for CNTs), highlighting the importance of rational design and synthesis of hybrid nanostructures for high-performance energy storage applications.

Cobalt Sulfide/Graphene Composite Hydrogel as Electrode for High-Performance Pseudocapacitors.

Graphene and its composite hydrogels with interconnected three-dimensional (3D) structure have raised continuous attention in energy storage. Herein, we describe a simple hydrothermal strategy to synthesize 3D CoS/graphene composite hydrogel (CGH), which contains the reduction of GO sheets and anchoring of CoS nanoparticles on graphene sheets. The formed special 3D structure endows this composite with high electrochemical performance. Remarkably, the obtained 3D CGH exhibits high specific capacitance (Cs) of 564 F g−1 at a current density of 1 A g−1 (about 1.3 times higher than pure CoS), superior rate capability and high stability. It is worth mentioning that this methodology is readily adaptable to decorating CoS nanoparticles onto graphene sheets and may be extended to the preparation of other pseudocapacitive materials based on graphene hydrogels for electrochemical applications.