BaTiO₃ Nanocubes Functionalized by Catechol-Based Organic Molecules via Ligand-Exchange and Chemical Reactions: Implications for Closed Packing of Nanoblocks

We report a strategy for surface functionalization of BaTiO3 nanocubes (BTNcs) with a controlled shape and size using 3,4-dihydroxybenzoic acid (DHBA) as an immobilizing ligand. Since the direct immobilization of DHBA on nanoparticles is significantly difficult due to multiple OH groups in the ligands, we attempted replacing surfactant molecules on BTNcs with a ligand that formed the basic structure of DHBA via a ligand-exchange protocol. Final functionalization was then achieved by only modifying the ions of the end groups through chemical treatment. First, BTNcs modified by oleic acid (OA) were synthesized by the hydrothermal method. Then, OA was replaced with methyl 3,4-dihydroxybenzoate (MDB) through a ligand-exchange reaction, chosen as an alternative ligand to form the base structure of DHBA. Since a COOCH3 end group in MDB acts as a protective group, it directs the COO– group away from the surface. As a result, the MDB-modified BTNcs (BT-MDB) bandgap energy is significantly reduced from the 3.20 eV of pristine BT to 2.05 eV, indicating an interfacial charge transfer (ICT) caused by direct surface bonding between MDB and BTNcs. Subsequently, BT-MDB is dispersed to KOH and HCl aqueous solutions to promote demethylation, representing COO–K+ and COOH as end groups, respectively, indicating the successful formation of DHBA in the final product. This functionalization protocol thus realizes the exact replacement of a targeted functional group or element (CH3, K+, and H) in the end group of immobilized molecules on the surface of BTNcs. As a result, the dispersibility, ICT characteristics, resultant electronic structure, and chemical bonding between nanocrystals and molecules could vary due to the end-group elements’ different electron affinities. This study provides a surface functionalization strategy for developing dielectric oxide nanocubes with tunable properties that can be used to design functional materials and building blocks with potential applications in energy conversion, data storage, and biomedical fields.