Colloidal state of exfoliated oxide nanosheets of layered niobate characterized with a molecular-level spectroscopic technique and macroscopic observations

Inorganic oxide nanosheets, prepared by exfoliation of layered hexaniobate K4Nb6O17, were investigated in a colloidal state with different hierarchies: micrometer to submillimeter level with microscopies and molecular level with visible spectroscopy. A cationic porphyrin 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21 H,23H-porphine (H2TMPyP) was added to the colloid as a spectroscopic probe. The Soret absorption band of porphyrin adsorbed on the niobate nanosheets was split when the colloid was concentrated. The modified spectral profile was retained when the colloid was re-diluted, indicating irreversible alteration that reflected concentration history of the dispersed state of the nanosheets. Fluorescence optical and transmission electron microscopies indicated aggregation and crumpling of the nanosheets in the colloids that were concentrated. The alteration of the colloidal nanosheets upon concentration was ascribed to the enrichment of electrolytes coexisting in the colloid. The results demonstrated that the microscopic status of the colloidally dispersed exfoliated nanosheets is related to the microenvironments formed by functional molecules immobilized on the nanosheets.