Sol‐Gel Inks for Direct‐Write Assembly of Functional Oxides

The ability to pattern oxide structures at the microscale in both planar and three-dimensional forms is important for a broad range of emerging applications, including sensors, micro-fuel cells and batteries, photocatalysts, solar arrays, and photonic bandgap (PBG) materials. Here, we report the fabrication of micro-periodic oxide structures by direct-write assembly of sol-gel inks. Specifically, we create both planar and three-dimensional (3D) architectures composed of submicron features, which are converted to the desired oxide phase upon calcination. Atomic force microscopy (AFM) and optical reflectivity measurements acquired on these micro-periodic structures reveal their high degree of structural uniformity. Several techniques have recently been introduced for patterning materials, including colloidal self-assembly, holographic lithography, and direct laser and ink writing approaches. Unfortunately, these approaches, apart from two notable exceptions, are confined to polymeric systems that lack the specific functionality required for a given application. As a consequence, the as-patterned structures require additional processing step(s) to produce the desired functional replicas. For example, 3D micro-fuel cells and photonic bandgap materials with inverse face centered cubic (fcc) structures have been templated from colloidal crystals, while silicon photonic crystals in both normal and inverse woodpile architectures have been templated from polymer structures produced by direct laser and ink writing, respectively. To circumvent the need for complicated templating schemes, we are developing a family of sol-gel inks that enable the direct ink writing (DIW) of functional oxides at the microscale. DIW is a layer-by-layer assembly technique, in which materials are fabricated in arbitrary planar and 3D forms with lateral dimensions that are two orders of magnitude lower than those achieved by ink-jet printing. Paramount to our approach is the creation of concentrated inks that can be extruded through fine deposition nozzles as filament(s), which then undergo rapid solidification to maintain their shape even as they span gaps in underlying layer(s). Unlike our prior efforts based on polyelectrolyte inks that require a reservoir-induced coagulation to enable 3D printing, these new inks can be directly printed in air providing exquisite control over the deposition process (e.g., the ink flow can now be started/stopped repeatedly during assembly). We first demonstrate this new ink design by creating a solgel precursor solution based on a chelated titanium alkoxide, titanium diisopropoxide bisacetylacetonate (TIA). TIA has an octahedral coordination of two isopropoxide and two acetylacetone (acac) groups about a central titanium (Ti) atom (Fig. 1a). This molecular structure is ideal, as it leads to the formation of soluble linear chains upon hydrolysis and condensation of the labile isopropoxide groups in the presence of a base catalyst. Subsequent slow hydrolysis of the acac groups leads to further gelation, which when coupled with solvent evaporation, results in the formation of a concentrated ink (Fig. 1b). An organic polymer, polyvinyl-pyrrolidone (PVP), is also incorporated to mitigate stresses that occur during drying and calcination of the as-patterned structures. We tailor the ink viscosity for DIW through micron-sized nozzles by regulating the solids content, defined as Ti + PVP concentration (wt %). The initial precursor solution contains 6.6 wt % solids and possesses a low viscosity ∼ 0.01 Pa·s, as shown in Figure 1c. However, upon concentrating the ink via solvent evaporation, a dramatic rise in viscosity is observed. For example, a nearly four-fold increase in ink concentration leads to a three orders of magnitude rise in its viscosity. Using an approximation of the Hagen–Poiseuille model, we estimate that the optimal ink viscosity ranges from 2.1– 4.2 Pa·s for the DIW conditions employed in this study (i.e., 1 lm deposition nozzle, 400 lm s deposition speed, and an applied pressure of 275–550 kPa). There is good agreement between the predicted values and those deemed optimal experimentally for microscale printing, as highlighted by the shaded region in Figure 1c. These inks not only flow readily, but are concentrated enough to rapidly solidify and maintain their cylindrical shape upon exiting the nozzle. The ink solidification mechanism can be understood by examining its elastic (G′) and viscous (G′′) moduli in the presence and absence of an ethanol solvent trap (Fig. 1d). The asprinted ink exhibits a liquid-like response (i.e., G′′> G′). HowC O M M U N IC A IO N