Novel Graphene, Graphene-like Two-dimensional Layered Nanomaterials And Their Composites: Preparation, Performance Regulation And Electrochemical Applications

As a typical two-dimensional layered nanomaterial, graphene has triggered a wave of research at home and abroad, owing to its unique physical, electronic and chemical properties, and been applied in various applications including electronics, optoelectronics, energy conversion and storage, sensing and so on. With the development of graphene in research and applications, various kinds of graphene-like two-dimensional layered materials, such as transition metal disulfides, oxides and hexagonal boron nitride, have also aroused people’s extensive research interest. Owing to their unique layered crystal structure and high degree of anisotropy, these two-dimensional layered materials exhibit controllable properties, which can be regulated by dimension reduction, intercalation and functionalization. In this thesis, two kinds of typical two-dimensional layered nanomaterials - graphene and molybdenum disulfide（MoS₂） were selected as the research objects. The research focus is on exploring the relationship between structure and property of the materials, improving their electrochemical performances, and expanding the electrochemical applications. The main works are as follows:1. GrapheneEver since the first successful isolation of graphene in 2004, tremendous efforts have been devoted to exploring the electrochemical applications of graphene, and a large number of studies have been reported. The focal point of this thesis is concentrated on chemically modified graphenes and their composites with polyaniline. The brief researching content is the relationship between the structure and the properties of different kinds of chemically modified graphenes, and their influence or enhancement effect on properties of polyaniline composites. The details are as follows:1) Different kinds of chemically modified graphenes, such as graphene oxide and reduced graphene oxide, have different surface chemistry and electrical conductivity, due to their difference in the number of surface functional groups and the density of defects, which will have an impact on the performance of their nanocomposites. For in-depth study, three nanocomposites composed of different chemically modified graphenes and sulfonic acid-doped polyaniline nanofiber（SPAN） were fabricated and comprehensive evaluated. There kinds of chemically modified graphenes（involving graphene oxide, electrochemically reduced graphene oxide and thermally reduced graphene oxide） were chosen to show their effect on the morphologies, electrochemical properties and DNA biosensing performances of the nanocomposites, and the influencing factors has been discussed. The findings in this work will provide useful guides for a better understanding of the differences among the chemically modified graphenes, and the interaction between chemically modified graphenes and SPAN, as well as for development of high-performance functional materials for electrochemical sensing and biosensing. Moreover, a preliminary study toward capacitive characteristics of one of the composites（thermally reduced graphene oxide-SPAN） has been conducted, owing to its typical capacitive behavior exhibited in the electrochemical measurements. The experimental results suggest that the thermally reduced graphene oxide-SPAN nanocomposites not only can be used to construct electrochemical sensing platform, but also has potential application in supercapacitors.2) The result of the above research shows that thermally reduced graphene oxide-polyaniline（PANI） nanocomposite not only can be used to construct electrochemical sensing platform, but also good electrode material for supercapacitors. However, according to the previous literatures, the most studied PANI materials in supercapacitors were simple structured nanowires, tubes and rods, and PANI electrodes with complicated hierarchical structures were seldom studied. Herein, a 1. Graphenethree-dimensional hierarchical micro/nanostructure of PANI（H-PANI） was synthesized through a facile chemical polymerization method without using any templates or organic structure directing reagents. The H-PANI has excellent electrical conductivity（14 S cm-1） and unique porous structure, which could promote rapid charge transfer and ion diffusion. Compared with the simple structured PANI（S-PANI） prepared in this paper, the H-PANI electrode shows a significantly enhanced capacitance performance. Nevertheless, the cycling performance of the H-PANI electrode is still not satisfactory, limited by the charge storage properties of PANI. To improve the cycling stability, the H-PANI was incorporated with the thermally reduced graphene oxide, and a hybrid electrode was prepared. As a result, the hybrid electrode leads to obvious improvements in specific capacitance and cycling stability. This study will provide a new perspective for development of much wider variety of PANI materials for supercapacitor applications, and the role of the thermally reduced graphene oxide in improving the capacitance performance of PANI electrode has been confirmed. 2. MoS₂MoS₂ is a typical representative of transition metal disulfides. As an analogue of graphene, the development of MoS₂ in the field of electrochemical sensing has been encumbered by its poor intrinsic conductivity which is different from graphene. To overcome this limitation, MoS₂ materials with good electrical conductivity and electrochemical activity should be prepared. For this purpose, two strategies have been adopted in this thesis. The first one is exploring and designing efficient preparation methods for MoS₂ with high conductivity and electrochemical activity, and the second is functionalized modification. Details of the studies are as follows:1) A simple ultrasound exfoliation method was used to exfoliate and disperse the bulk MoS₂. The electrochemical properties of MoS₂ can be regulated with the changes of its layers and size in the process of ultrasound exfoliation. The effect of ultrasonic time was investigated, and the thin-layered MoS₂ nanosheets（TLMoS₂） with high conductivity and electrochemical activity were obtained under optimal conditions. Based on the high electrochemical activity and different affinity toward ss DNA versus ds DNA of the TLMoS₂, a label-free and ultrasensitive electrochemical DNA biosensor was constructed, and the tlh gene sequence from Vibrio parahaemolyticus was successfully detected with a detection limit of 1.9 × 10-17 M. Moreover, the proposed sensing platform has good electrocatalytic activity, and can be extended to detect more targets, such as guanine and adenine. Without labelling and the use of amplifiers, the biosensor described here not only offers a viable alternative for DNA analysis, which has the priority in sensitivity, simplicity and costs, but also expands the application of MoS₂ in the field of electrochemical sensing.2) Through a simple one-step electrosynthesis method, the electrochemical reduction of the TLMoS₂ and the in situ polymerization of xanthurenic acid（Xa） on the TLMoS₂ were simultaneously realized, and a reduced TLMoS₂-poly（xanthurenic acid） nanocomposite（r TLMoS₂-PXa） was obtained. It is found that the reduction of the TLMoS₂ and the polymerization of Xa both have the promotional effects on the improvement of the electrical conductivity and the electrochemical activity of the nanocomposite. The nanocomposite modified electrode possesses improved electron transfer capability and exhibits good electrochemical sensing performance towards several heterocyclic and aromatic ring compounds（2′-deoxyguanosine 5′-triphosphate trisodium salt, d GTP, bisphenol A, BPA and 2,4,6-trinitrotoluene, TNT）, which have Ï€-Ï€ interaction with the electroactive PXa and never or rarely been analyzed by MoS₂-based sensing platform. This research provides a new electrochemical sensing platform for simple and sensitive detection of d GTP, BPA and TNT, and further extends the application of MoS₂ in the field of electrochemical sensing.These studies have certain significance for better understanding the properties of graphene and MoS₂, improving their electrochemical performance as well as expanding their electrochemical applications.