Boron-rich Clusters as Molecular Cross-linkers for Hierarchical Hybrid Materials
Covalent cross-linking plays an important role for materials to form robust networks with enhanced thermal/mechanical properties compared to pristine materials. However, there are a limited number of cross-linkers that can produce rigid 3-dimensional networks, leading to significant modifications of properties in materials. Our group has developed a “molecular cross-linking” approach whereby perhydroxylated dodecaborate clusters ([B12(OH)12]2-) are incorporated in the network of metal oxides to create hierarchical hybrid materials. These clusters, capable of withstanding harsh thermal and oxidizing conditions required for the synthesis of many metal oxides, allow the formation of hybrid metal oxides. We showcase how the robust [B12(OH)12]2- cluster can be successfully cross-linked with TiO2 with dramatically altered photo-physical and electrochemical properties. The comprehensive structural characterization of this material reveals the formation of a hybrid molecular boron oxide material that consists of a cross-linked network of intact boron clusters and TiO2 nanocrystals in the anatase phase. The unique structure of this hybrid metal oxide consequently engenders unprecedentedly superior electro- and photochemical properties than that of pristine TiO2. Furthermore, we expand this molecular cross-linking approach to other metal oxides such as WO3 for energy storage applications.
In addition, we explore the possibility to cross-link [B12(OH)12]2- clusters to organic monomers to create densely cross-linked polymeric materials. We found out the molecular boron-rich cluster can act as an inorganic polyol equivalent in the synthesis of polyurethane-based materials, serving as a molecular cross-linker. We highlight incorporating [B12(OH)12]2- building blocks can effectively improve the thermal stability of the resultant polyurethane materials compared to analogous polymers made from carbon-based polyols. The successful modification of the materials ranging from inorganic metal oxides to organic polymeric materials highlights the value of molecular cross-linking as a noble strategy to alter the properties of materials for diverse applications.