Self‐Assembled Ultra High Strength, Ultra Stiff Mechanical Metamaterials Based on Inverse Opals

Abstract

Inverse opals are most widely used as photonic crystals for ultraviolet, optical, and infrared applications. [1] These highly interconnected porous structures are also attractive for applications such as sensors, fuel cells, filters, and catalysts. [2] At the same time, engineers are aiming for lightweight structures with optimized mechanical strength, often inspired by nature's cellular materials with foam-like structures such as sponges, [3] trabecular bone, [4] or plant parenchyma. [5] The resultant optimized strut-based structures have shown high stiffness-and compressive strength-to-weight ratios, [6][7][8][9] but can suffer from strut buckling and a lack of mass production techniques. Here, we show that mechanical metamaterials based on ceramic inverse opaline structures with densities in the range of 330-910 kg m À3 are not only suitable as photonic crystals but also show better stiffness-and compressive strength-to-weight ratios compared to microfabricated optimized strut-based structures, [6][7][8][9] but lower than carbon nanoframes fabricated by interference lithography. [10] Pure silica inverse opal structures and silica inverse opals whose pores have been internally coated by a thin TiO 2 layer have been fabricated and their structural and mechanical behavior was investigated. Our experimental results, supported by numerical simulations, show that these arch-shaped porous structures outperform both strut-based and honeycomb structures due to their nearly isotropic mechanical response.The silica inverse opal films presented here are fabricated by vertical co-assembly based on the procedure described by Hatton et al. [11] Monodisperse colloidal polystyrene (PS) spheres were mixed in a water-based suspension containing tetraethylorthosilicate (TEOS) solution and vertically assembled on soda-lime silica glass. The resulting FCC opaline structure was calcined at 500°C for 30 min to burn out the PS template leaving an inverse-FCC opaline structure of nanoporous amorphous silica in which the adjacent pores are interconnected by %170 nm diameter holes (Figure 1a). Some samples were subsequently coated with an amorphous layer of TiO 2 by atomic layer deposition (ALD). After full crystallization to anatase, the layer thickness was %28 nm (Figure 1b). The specular reflection in Figure 1c shows the characteristic reflection peaks, which can be tuned by both the pore diameter and/or the TiO 2 coating.The effective density of the silica inverse opals was estimated by four independent methods: gravimetric, pycnometric, and two opticals. All four methods yield a comparable density of 330 kg m À3 AE 10%. The effective density of the TiO 2 ALD-coated silica inverse opals was estimated gravimetrically and optically to be 910 kg m À3 AE 10%. Detailed information can be found in the Supporting Information.Some examples of mechanical properties of opaline structures can be found in the literature. These works investigated polymer, [12,13] metal, [14] and ceramic-based [14][15][16] opals. Toivola et al. [15] investigated s...

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