Inorganic microstructured materials are ubiquitous in nature. However, their formation in artificial self-assembly systems is challenging as it involves a complex interplay of competing forces during and after assembly. For example, colloidal assembly requires fine-tuning of factors such as the size and surface charge of the particles and electrolyte strength of the solvent to enable successful self-assembly and minimize crack formation. Co-assembly of templating colloidal particles together with a sol–gel matrix precursor material helps to release stresses that accumulate during drying and solidification, as previously shown for the formation of high-quality inverse opal (IO) films out of amorphous silica. Expanding this methodology to crystalline materials would result in microscale architectures with enhanced photonic, electronic, and catalytic properties. This work describes tailoring the crystallinity of metal oxide precursors that enable the formation of highly ordered, large-area (mm2) crack-free titania, zirconia, and alumina IO films. The same bioinspired approach can be applied to other crystalline materials as well as structures beyond IOs.
Catalysis is one of the most sophisticated areas of materials research that encompasses a diverse set of materials and phenomena occurring on multiple length and time scales. Designing catalysts that can be broadly applied toward global energy and environmental challenges requires the development of universal frameworks for complex catalytic systems through rational and independent (or quasi-independent) optimization of multiple structural and compositional features. Toward addressing this goal, a modular platform is presented in which sacrificial organic colloids bearing catalytic nanoparticles on their surfaces self-assemble with matrix precursors, simultaneously structuring the resulting porous networks and fine-tuning the locations of catalyst particles. This strategy allows combinatorial variations of the material building blocks and their organization, in turn providing numerous degrees of freedom for optimizing the material’s functional properties, from the nanoscale to the macroscale. The platform enables systematic studies and rational design of efficient and robust systems for a wide range of catalytic and photocatalytic reactions, as well as their integration into industrial and other real-life environments.
Although common in biological systems, synthetic self-assembly routes to complex 3D photonic structures with tailored degrees of disorder remain elusive. Here we show how liquids can be used to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In these optofluidic crystals, dynamically tunable disorder is superimposed onto the periodic optical structure through partial wetting or evaporation. In both cases, macroscopic symmetry breaking is driven by subtle sub-wavelength variations in the pore geometry. These variations direct site-selective infiltration of liquids through capillary interactions. Incorporating cross-linkable resins into our liquids, we developed methods to freeze in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These percolation lithography techniques produced permanent photonic structures with adjustable disorder. By coupling strong changes in optical properties to subtle differences in fluid behavior, optofluidic crystals may also prove useful in rapid analysis of liquids.
We provide an overview of our recent advances in the manipulation of wetting in inverse-opal photonic crystals. Exploiting photonic crystals with spatially patterned surface chemistry to confine the infiltration of fluids to liquidspecific spatial patterns, we developed a highly selective scheme for colorimetry, where organic liquids are distinguished based on wetting. The high selectivity of wetting, upon-which the sensitivity of the response relies, and the bright iridescent color, which disappears when the pores are filled with liquid, are both a result of the highly symmetric pore structure of our inverse-opal films. The application of horizontally or vertically orientated gradients in the surface chemistry allows a unique response to be tailored to specific liquids. While the generic nature of wetting makes our approach to colorimetry suitable for applications in liquid authentication or identification across a broad range of industries, it also ensures chemical non-specificity. However, we show that chemical specificity can be achieved combinatorially using an array of indicators that each exploits different chemical gradients to cover the same dynamic range of response. Finally, incorporating a photo-responsive polyelectrolyte surface layer into the pores, we are able to dynamically and continuously photo-tune the wetting response, even while the film is immersed in liquid. This in situ optical control of liquid percolation in our photonic-crystal films may also provide an error-free means to tailor indicator response, naturally compensating for batch-to-batch variability in the pore geometry.