Minwei Zhang

Graphene papers: smart architecture and specific functionalization for biomimetics, electrocatalytic sensing and energy storage

Paper is an attractively assembled form of materials and has accompanied our daily life almost everywhere. Two-dimensional layered materials, especially graphene, have unique intrinsic structures to be exploited for smart architecture of macroscopic papers that are offering many newly emerging applications. Research advances in graphene based papers in the past few years have created a new category of composite materials. This review aims at offering an up-to-date comprehensive summary of graphene-supported papers, with the emphasis on smart assembly and purpose-driven specific functionalization for their critical applications associated with sensing, environmental and energy technologies. The contents of this review are based on a balance combination of our own studies and selected research studies done by worldwide academic groups. We first give a brief introduction to graphene as a versatile building block and to the current status of research studies on graphene papers. This is followed by addressing some crucial methods of how to prepare graphene papers. We then summarize multiple possibilities of functionalizing graphene papers, membranes or films. Finally, we evaluate some key applications of graphene papers in the areas of chemical/electrochemical sensors, biomimetics and energy storage devices, just before leading to our concluding remarks and perspectives.

Interlocked graphene-Prussian blue hybrid composites enable multifunctional electrochemical applications

There has been increasing interest recently in mixed-valence inorganic nanostructure functionalized graphene composites, represented by Prussian blue, because they can cost-effectively apply to biosensors and energy devices. In this work, we present a one-pot green method to synthesize interlocked graphene-Prussian Blue hybrid composites as high-performance materials for biosensors and supercapacitor electrodes. Given the fact that graphene oxide (GO) can act as an electron acceptor, we used iron(II) and glucose as co-reducing agents to reduce GO under mild reaction conditions without introducing toxic agents. High quality Prussian blue nanocubes with no or little coordinated water were generated simultaneously. Reduced graphene oxide (rGO) was thus functionalized by Prussian blue nanocubes via chemical bonding to form a kind of interlocked microstructure with high stability and good conductivity. The as-synthesized composites were tested for biosensing of hydrogen peroxide (H2O2) and as supercapacitor electrode materials. The specific capacitance of the microcomposite based electrodes can reach 428Fg-1, with good cycling stability. The microcomposite also displays high performance catalysis towards electroreduction of H2O2 with a high sensitivity of 1.5Acm-2M-1.

Free-standing and flexible graphene papers as disposable non-enzymatic electrochemical sensors

We have explored AuNPs (13 nm) both as a catalyst and as a core for synthesizing water-dispersible and highly stable core-shell structural gold@Prussian blue (Au@PB) nanoparticles (NPs). Systematic characterization by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) disclosed AuNPs coated uniformly by a 5 nm thick PB layer. Au@PB NPs were attached to single-layer graphene oxide (GO) to form Au@PB decorated GO sheets. The resulting hybrid material was filtered layer-by-layer into flexible and free-standing GO paper, which was further converted into conductive reduced GO (RGO)/Au@PB paper via hydrazine vapour reduction. High-resolution TEM images suggested that RGO papers are multiply sandwich-like structures functionalized with core-shell NPs. Resulting sandwich functionalized graphene papers have high conductivity, sufficient flexibility, and robust mechanical strength, which can be cut into free-standing electrodes. Such electrodes, used as non-enzymatic electrochemical sensors, were tested systematically for electrocatalytic sensing of hydrogen peroxide. The high performance was indicated by some of the key parameters, for example the linear H2O2 concentration response range (1-30 μM), the detection limit (100 nM), and the high amperometric sensitivity (5 A cm(-2) M(-1)). With the advantages of low cost and scalable production capacity, such graphene supported functional papers are of particular interest in the use as flexible disposable sensors.

Graphene encapsulated Fe3O4 nanorods assembled into a mesoporous hybrid composite used as a high-performance lithium-ion battery anode material

The discovery of new anode materials and engineering their fine structures are the core elements in the development of new-generation lithium ion batteries (LIBs). To this end, we herein report a novel nanostructured composite consisting of approximately 75% Fe3O4 nanorods and 25% reduced graphene oxide (rGO). Microscopy and spectroscopy analyses have identified that the Fe3O4 nanorods are wrapped (or encapsulated) by the rGO nanosheets via covalent bonding, which further self-assemble into a mesoporous hybrid composite networked by the graphene matrix. The composite has an average pore size around 20 nm and exhibits a high surface area of 152 m2 g−1, which is 76 times as high as that of conventional Fe3O4 powder. We have used the composite as an LIB anode material to fabricate coin-type prototype cells with lithium as the cathode. Systematic half-cell testing evaluations show that the electrochemical performance of the present composite material is amongst the best of the transition metal-oxide based LIB anode materials. The performances are characterized by a high reversible capacity of 1053 mA h g−1 subjected to 250 charge–discharge cycles at 500 mA g−1 and an excellent rate capability with the deliverable energy of 788–541 mA h g−1 upon the application of high current densities of 1000–5000 mA g−1. Overall, we have demonstrated that Fe3O4 nanorod–rGO hybrid composite is an interesting and promising material for the fabrication of LIB anodes.

Electroactive and biocompatible functionalization of graphene for the development of biosensing platforms

Design and synthesis of low-cost, highly stable, electroactive and biocompatible material is one of the key steps for the advancement of electrochemical biosensing systems. To this end, we have explored a facile way for the successful synthesis of redox active and bioengineering of reduced graphene oxide (RGO) for the development of versatile biosensing platform. A highly branched polymer (PEI) is used for reduction and simultaneous derivation of graphene oxide (GO) to form a biocompatible polymeric matrix on RGO nanosheet. Ferrocene redox moieties are then wired onto RGO nanosheets through the polymer matrix. The as-prepared functional composite is electrochemically active and enables to accommodate enzymes stably. For proof-of-concept studies, two crucial redox enzymes for biosensors (i.e. cholesterol oxidase and glucose oxidase) are targeted. The enzyme integrated and RGO supported biosensing hybrid systems show high stability, excellent selectivity, good reproducibility and fast sensing response. As measured, the detection limit of the biosensors for glucose and cholesterol is 5µM and 0.5µM (S/N=3), respectively. The linear response range of the biosensor is from 0.1 to 15.5mM for glucose and from 2.5 to 25µM for cholesterol. Furthermore, this biosensing platform shows good anti-interference ability and reasonable stability. The nanohybrid biosensing materials can be combined with screen-printed electrodes, which are successfully used for measuring the glucose and cholesterol level of real human serum samples.

Reagent-Free Synthesis and Plasmonic Antioxidation of Unique Nanostructured Metal-Metal Oxide Core-Shell Microfibers

A photoresponsive inorganic microfiber with a plasmonic core-shell structure responds to visible light to achieve self-protection against oxidation in an open environment. The microfibers are synthesized via a newly developed reagent-free electrolytic method and have unique interfacial structures and high surface activity.

Ultralight, Flexible, and Semi-Transparent Metal Oxide Papers for Photoelectrochemical Water Splitting

Thanks to their versatile functionality, metal oxides (MOs) constitute one of the key family materials in a variety of current demands for sensor, catalysis, energy storage and conversion, optical electronics, and piezoelectric mechanics. Much effort has focused on engineering specific nanostructure and macroscopic morphology of MOs that aims to enhance their performances, but the design and controlled synthesis of ultrafine nanostructured MOs in a cost-effective and facile way remains a challenge. In this work, we have exploited the advantages of intrinsic structures of graphene oxide (GO) papers, serving as a sacrificial template, to design and synthesize two-dimensional (2D) layered and free-standing MO papers with ultrafine nanostructures. Physicochemical characterizations showed that these MO materials are nanostructured, porous, flexible, and ultralight. The as-synthesized materials were tested for their potential application in photoelectrochemical (PEC) energy conversion. In terms of PEC water splitting, copper oxide papers were used as an example and exhibited excellent performances with an extremely high photocurrent-to-weight ratio of 3 A cm-2 g-1. We have also shown that the synthesis method is generally valid for many earth-abundant transition metals including copper, nickel, iron, cobalt, and manganese.

Freestanding and flexible graphene papers as bioelectrochemical cathode for selective and efficient CO2 conversion

During microbial electrosynthesis (MES) driven CO2 reduction, cathode plays a vital role by donating electrons to microbe. Here, we exploited the advantage of reduced graphene oxide (RGO) paper as novel cathode material to enhance electron transfer between the cathode and microbe, which in turn facilitated CO2 reduction. The acetate production rate of Sporomusa ovata-driven MES reactors was 168.5 ± 22.4 mmol m−2 d−1 with RGO paper cathodes poised at −690 mV versus standard hydrogen electrode. This rate was approximately 8 fold faster than for carbon paper electrodes of the same dimension. The current density with RGO paper cathodes of 2580 ± 540 mA m−2 was increased 7 fold compared to carbon paper cathodes. This also corresponded to a better cathodic current response on their cyclic voltammetric curves. The coulombic efficiency for the electrons conversion into acetate was 90.7 ± 9.3% with RGO paper cathodes and 83.8 ± 4.2% with carbon paper cathodes, respectively. Furthermore, more intensive cell attachment was observed on RGO paper electrodes than on carbon paper electrodes with confocal laser scanning microscopy and scanning electron microscopy. These results highlight the potential of RGO paper as a promising cathode for MES from CO2.

Electrocatalytic Applications of Graphene–Metal Oxide Nanohybrid Materials

Development of state-of-the-art electrocatalysts using commercially available precursors with low cost is an essential step in the advancement of next-generation electrochemical energy storage/conversion systems. In this regard, noble metal-free and graphene-sup‐ ported nanocomposites are of particular interest. Graphene-based nanocomposite is an excellent candidate as energy-device and sensor-related electrode materials, largely due to their high electrical conductivity, large specific surface area, high-speed electron/heat mobility, and reasonably good mechanical strength. Among many types of graphenebased composite materials, graphene–metal oxide nanohybrids hold great promise to‐ ward engineering efficient electrocatalysts and have attracted increasing interest in both scientific communities and industrial partners around the world. The goal of this chapter is primarily set on an overview of cutting-edge developments in graphene–metal oxide nanohybrid materials, with the recently reported results from worldwide research groups. This chapter is presented first with an introduction, followed by synthetic meth‐ ods and structural characterization of nanocomposites, an emphasis on their applications in energy and sensor-related fields, and finally completed with brief conclusions and out‐ look.

Design, Assembly, and Fabrication of Two-Dimensional Nanomaterials into Functional Biomimetic Device Systems

Diverse functioning biosystems in nature have inspired us and offered unique opportunities in developing novel concepts as well as new class of materials and devices. The design of bioinspired functional materials with tailored properties for actuation, sensing, electronics, and communication has enabled synthetic devices to mimic natural behavior. Among which, artificial muscle and electronic skin that enable to sense and respond to various environmental stimuli in a human-like way have been widely recognized as a significant step toward robotics applications. Polymer materials have previously been dominant in fabricating such functional biomimetic devices owing to their soft nature. However, lacking multifunctionality, handling difficulty, and other setbacks have limited their practical applications. Recently, versatile and highperformance two-dimensional (2D) materials such as graphene and its derivatives have been studied and proven as promising alternatives in this area. In this chapter, we highlight the recent efforts on fabrication and assembly of 2D nanomaterials into functional biomimetic systems. We discuss the structure-function relationships for the development of 2D materials–based biomimetic devices, their tailoring property features, and their variety of applications. We start with a brief introduction of artificial functional biomimetic materials and devices, then summarize some key 2D materials– based systems, including their fabrication, properties, advantages and demonstra‐ tions, and finally present concluding remarks and outlook.