Nanoscale Cuticle Mass Density Variations in Bio-Inspired Thermochromic Coatings Influenced by Hierarchical Structuring

Abstract

The pursuit of energy-efficient building envelopes has driven significant research into advanced materials. Bio-inspired hierarchical nanostructures, such as those found on flower petals, offer a promising avenue for developing adaptive thermal-responsive coatings. However, the precise relationship between the distribution of thermochromic materials within these complex structures and their overall performance remains poorly understood. This study aims to bridge this gap by investigating the nanoscale mass density variations in a novel bio-inspired thermochromic coating. We developed a thermochromic coating that mimics the hierarchical micro/nanostructure of rose petals, incorporating vanadium dioxide (VO2) nanoparticles as the active thermal-responsive component. We then employed ptychographic X-ray computed tomography (PXCT) to quantitatively determine, at nanoscale resolution, the three-dimensional mass density distribution of the coating with and without the hierarchical structuring. By comparing these two coating types, we investigated how the hierarchical structure influences the distribution of VO2 nanoparticles and the resulting thermochromic properties. Our findings reveal that the hierarchical structure significantly influences the distribution of VO2 nanoparticles, leading to a more heterogeneous mass density distribution compared to the flat coating. The density of the VO2-rich regions is inversely correlated with the underlying hierarchical topography. The structured coating exhibits a more pronounced thermochromic effect, with a larger change in solar transmittance between its low- and high-temperature states. We correlate these macroscopic optical properties with the nanoscale structural features, showing that the hierarchical design enhances the thermochromic performance by optimizing the distribution of the active material. This study provides the first nanoscale 3D characterization of a bio-inspired thermochromic coating, demonstrating the critical role of hierarchical structuring in modulating its performance. Our findings offer a new design paradigm for advanced building materials, where the precise control of nanoscale architecture can be used to create highly efficient and adaptive thermal-responsive surfaces. This work paves the way for the development of next-generation smart building envelopes with significantly improved energy efficiency.