Many metal oxides have a ubiquitous presence in modern life. The most popular examples, silicon dioxide (SiO2), iron oxide (Fe3O4), aluminium oxide (Al2O3) and titanium dioxide (TiO2), are earth-abundant and have a wide range of applications, ranging from energy storage to catalysis. Despite their importance, the capability to tune the properties of these materials through mild and operationally straightforward methods still remains challenging, as there exist a limited number of techniques available to do so. For example, TiO2 has attracted enormous attention in the field of renewable energy due to its potential as a photocatalyst. However, its wide optical bandgap (~3.2 eV) makes it only capture ultraviolet (UV) light, which makes up ~7 % of the solar spectrum and exclude visible light. As a result, a particular area of interest is to increase the range of sunlight that TiO2 can absorb. Most efforts have focused on the use of molecular organic and inorganic dyes, elemental doping with light elements or defect engineering.
In this work, we introduce a new approach, we refer to as “molecular cross-linking”, whereby a hybrid material is formed by cross-linking polyhedral boron cluster precursors, [B12(OH)12]2-, to the network of TiO2 using a simple solution-based synthesis. This new approach is enabled by the inherent robustness of the molecular boron clusters, which is compatible with harsh conditions required for the synthesis of metal oxides. The combined comprehensive structural characterization of this hybrid material from X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provides evidence for the presence of crystalline TiO2 in the anatase phase with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) confirming Ti4+. The intact boron clusters were further probed via solid state NMR spectroscopies and pair distribution function (PDF) analysis. The excellent light absorption property of this cross-linked material renders significantly enhanced photocatalytic activity compared to pristine TiO2. Experimentally this property manifests in much faster photodegradation of organic pollutants under a low power red LED. The electrochemically-active nature and high electrical conductivity of the cross-linked material allowed us to explore it for energy storage. A pouch-cell supercapacitor containing cross-linked TiO2 exhibited superior performances in comparison to both forms of TiO2. The successful modification of metal oxides demonstrates the value of molecular cross-linking as a new and previously unattainable strategy to induce change in the properties of metal oxide materials. The simplicity and generality of the route, where cross-linking molecular clusters to a metal oxide via a facile reaction proceeding at room temperature using readily available precious metal-free precursors, makes this potentially applicable to a wide range of other metals and associated applications.