Person:

Phillips, Katherine

Controlling nano to microscale structuration enables one to alter a material’s optical, wetting, mechanical, and chemical properties. Structuration on this scale can be formed from spherical building blocks; in particular, monodisperse, spherical colloids assemble into crystals that can be used to template an ordered, porous structure known as an inverse opal. The structure’s porosity and periodicity provide control over both light (photonic effects) and fluid flow (wetting effects). Controlling the composition allows chemical functionality to be added to the ordered, porous structure. Inverse opals are widely used in many applications that take advantage of these properties, including optical, wetting, sensing, catalytic, and electrode applications; however, high quality structures are necessary to maintain consistent properties. Many of their properties stem from the structure itself, so controlling inverse opals’ structure (including the local composition) provides the ability to control their properties, with the potential to improve some applications and potentially enable additional ones.

This thesis explores how molecular precursors can be used to control colloidal assembly and therefore alter the optical and wetting properties of high quality inverse opals. Using a bio-inspired approach, highly ordered, crack-free, silica inverse opals can be grown by co-assembling the colloidal template with a sol-gel matrix precursor using evaporation-induced self-assembly. Using sol-gel chemistry, the size, shape, and charge of the precursor can be controlled, which can be used to tune the colloidal assembly process. Here, we use the sol-gel chemistry of the precursors to control both the morphology and composition of these photonic structures.

In particular, temperature-induced condensation of the silica sol-gel matrix alters the shape of an inverse opal’s pores (Chapter 2), and silica and titania precursors can be mixed to make hybrid oxide structures (Chapter 3). Additionally, rationally designed precursors enable the fabrication of crack-free inverse opals in materials beyond silica, which we show for titania as a proof-of-concept (Chapter 4). By controlling the structure and composition with sol-gel chemistry, we can tailor both the optical and wetting properties, as discussed in the second part of each chapter; these properties have important effects for the various applications. In this way, sol-gel chemistry can be used to assemble inverse opals with complex functionality.

Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the realizable structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method to introduce anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol-gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure, while completely avoiding the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy studies clearly identify increasing degrees of sol-gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.

Porous materials display interesting transport phenomena due to the restricted motion of fluids within the nano- to micro-scale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy.

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.