Ultralong-Range Periodic Alignment and Structural Coloration of Titanate and Titanoniobate Nanosheets in Aqueous...

Structural coloration has been reported for many types of two-dimensional (2D) materials, including metal oxide nanosheets. In particular, metal oxide nanosheets feature a wealth of composition and structure, but their impact on structural color has not been explored. In the current study, we report the structural coloration observed for aqueous suspensions of three types of transition metal oxide nanosheets (Ti_1.73O₄ ^1.08-, TiNbO₅ ^-, and Ti₂NbO₇ ^-) obtained via the exfoliation of precursory layered compounds with tetramethylammonium hydroxide (TMAOH). It was revealed that a minute amount of TMA⁺ was retained even after deionization via repeated centrifugation, acting as countercations to the negatively charged nanosheets. These residual TMA⁺ ions contributed to stabilizing the dispersion of nanosheets under low-ionic-strength conditions, while the intersheet spacing was expanded to a magnitude comparable to the wavelength of the visible-light range, resulting in structural coloration. The wavelength of structural colors was dependent not only on nanosheet concentration but also on the type of nanosheets, correlating with the charge (ζ-potential) and thickness of the nanosheets, as well as the amount of TMA⁺ dissociated from the nanosheet surfaces. Specifically, TiNbO₅ ^- was featured with the highest amount of TMA⁺ in the solution, which screened the negative charge of the nanosheets and resulted in the smallest intersheet spacing. On the other hand, when the dissociated amount of TMA⁺ was diminished, Ti₂NbO₇ ^-, with a larger thickness, yielded a larger intersheet spacing than that of Ti_1.73O₄ ^1.08-. These findings provide a valuable clue to the rational control of structural colors derived from nanosheets with different compositions and structures for a wide range of applications.