Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Xiao, Feng, Hexagonal boron nitride nanosheets synthesis and applications, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2016. https://ro.uow.edu.au/theses/4838
h-BN nanosheet (denoted as h-BNNS) is a layer-structured material that has many outstanding properties, such as high thermal and mechanical performance, superb chemical and thermal stability, excellent lubricity, and strong insulating properties. These remarkable properties enable h-BNNS to be utilized in various applications, such as multi-function composites, military and electronic coatings, and biological materials. In this thesis, I have completed two projects on the synthesis of h-BNNS and its use in different applications. In the first part of the work, upon flowing hot water steam over hexagonal boron nitride (h- BN) bulk powder, efficient exfoliation and hydroxylation of h-BN occur simultaneously. This method features high yield, high purity, low cost, and excellent scalability. For the first time, it has been confirmed that the hydroxyl groups are predominantly along the edges of the BN nanosheets via electron energy loss spectroscopy (EELS) mapping, in contrast to graphene oxide, where functionalization occurs both along the edges and on the basal planes. Although h-BN is known to be very inert and hard to functionalize, which severely limits its application, the hydroxyl (OH) groups can interact with various types of inorganic and organic matter via hydrogen bonding, and thus can help to explore the intrinsic properties of h-BNNS when dispersed in a foreign medium. The excellent dispersibility of OH-BNNS in water and alcohol enables a homogeneous distribution of the nanosheets in poly(Nisopropylacrylamide) (PNIPAM) hydrogel, which shows a 44% thermal enhancement compared with the pure hydrogel. Additionally, as one of the most commonly used temperature sensitive hydrogels, with applications ranging from drug delivery to various types of smart systems, PNIPAM hydrogel, upon mixing with OH-BNNS, shows dramatic improvement in two key aspects: dimensional change and dye desorption upon heating. h-BNNS is nontoxic and chemically inert, and therefore, this finding could be highly valuable for bionic and soft robotic applications of PNIPAM hydrogel.
In the second part of the work, a bottom-up synthesis of h-BNNS with controllable thickness has been developed by using MgB2 as a dynamic template, which naturally has layers of boron atoms sandwiched by magnesium atoms. This method enables the growth of predominantly bi-layered h-BNNS, which gives it an advantage over the commonly used exfoliation, which results in a wide distribution of layer numbers, and is being extended to the synthesis of other few-layered compounds based on existing layered atomic sheets in the starting materials. The layered structure of MgB2 is also conducive to the controllable formation of micropores and mesopores, which are directly affected by the evaporation and decomposition of the Mg-based intermediates that are sandwiched by the h-BN sheets during the synthesis. For the first time, it has been experimentally demonstrated that boron rich h- BNNS can deliver great CO2/N2 adsorption selectivity, high CO2 adsorption capacity, and a surprisingly large heat of adsorption for high CO2 coverage. The strong interaction between CO2 and h-BNNS is related to the boron active sites, which can be effectively controlled. This method offers a practical way to improve the CO2 adsorption capacity of h-BNNS. These studies demonstrate the importance of developing novel methods for synthesis and functionalization, based on which the intrinsic properties of h-BNNS can be effectively explored.