Nanomaterials - 04

Abstract

Recently, there has been significant interest in nanoarchitectonics using nanomaterials as building blocks. Among various nanomaterials, nanosheets—charged colloidal materials with high two-dimensional (2D) anisotropy—have attracted much attention[1]. For example, it has been reported that removing electrolytes from nanosheet suspensions significantly expands the intersheet spacing, forming ultra-long-range periodic structures[2]. As the spacing is comparable to the wavelength of visible light, structural coloration is induced in the dispersed suspensions. This intriguing phenomenon can be elucidated by two key factors: (1) The huge 2D anisotropy of the nanosheets promotes cofacial alignment, leading to periodic arrangements. (2) In a low ionic strength environment, the strong electrostatic repulsion between the nanosheets enables a long-range separation up to several hundred nanometers, which is a requisite for the interference of visible light. Therefore, the colloidal charge and 2D anisotropy of nanosheets are expected to influence the construction and behavior of ultralong periodic structures. However, the impact of both factors on the structural color remains largely unexplored.
 In this study, to investigate the effect of the aspect ratio of nanosheets on structural color, we attempted to control the average lateral size of Ti_0.87O₂ nanosheets by adjusting the duration of ultrasonic treatment. The lateral size progressively decreased over time, with the average size down from approximately 3 µm to 0.6 µm after 4 hours of treatment (Fig. 1), corresponding to a reduction in the aspect ratio from approximately 3000 to 600. Subsequently, electrolytes were mostly removed from suspensions containing the fractured nanosheets to achieve a low ionic strength environment. As a result, despite the reduced aspect ratio, structural coloration was observed in the suspension. Compared to the untreated samples (Fig.2, left), more vivid structural colors were obtained, and the reflection spectra showed a 10% increase in reflection intensity as well as a narrowing trend of the reflection peak width (Fig.2, right). Notably, the reduction in the aspect ratio systematically caused a blue shift in the peak wavelength (λ) of the structural color produced by the same nanosheet concentration (see both samples of 55 mM in Fig. 2), indicating a decrease in intersheet distance.