Targeted Functional Methylcellulose-Nanocomposite Materials via Self-Limiting Electrospray Deposition

Electrospray deposition (ESD) uses strong electric fields to produce generations of monodisperse droplets from solutions/dispersions that are driven towards grounded targets. Self-limiting electrospray deposition (SLED) is a phenomenon recently discovered by our group in which targeted, 3D coatings can be achieved by spraying insulating polymers below their glass transition temperatures, which trap charge and repel incoming spray droplets. Recent work from our group has shown that methylcellulose (MC) exhibits self-limiting behavior when sprayed from water/ethanol solutions and is also capable of forming nanowire morphologies. This unique spray regime is an intermediate between the electrospinning of wire mats and particle sprays. As part of this prior work, it was demonstrated that because MC forms fibril gels, the shear of spray leads to shear thickening and homogenous gelation of the MC that allows for nanowire particles to form, eventually forming a forest as deposition continues. However, when conductive particles are added to SLED sprays, the buildup of charge required to repel incoming material becomes disrupted as particle loading increases. As would be expected, this relationship is not dependent solely on concentration. Here, we are able to show that an interaction exists between particle morphology and the formation of the MC nanowire in composite sprays. While larger spherical particles, 200 nm and above, will disrupt nanowire formation, smaller particles, 50 nm and below, become incorporated in the nanowire. Furthermore, the concentration of smaller particles changes the length of the nanowire. We take advantage of these phenomena to use MC nanowires to prevent and reduce the percolation of conductive particles during spray which allows for highly controlled deposition. For conductive particles, we use 2D Ti₃C₂ MXene of two different sizes as well as spherical ITO which all have similar sheet resistances. To quantify the growth of these materials and examine the dynamic interaction of composition, particle properties, and feature geometry, photolithography patterned chips with gratings of varying widths and spacings were designed. These chips were sprayed with composite materials of varying concentration, and the widths of the features after spray were measured. Scanning electron microscopy shows these deposited materials are able to maintain unique low-density morphologies. Using these findings, it was possible to design highly controlled, optimally loaded, functional targeted sprays of MC containing either MXene or ITO. As a demonstration, MXene containing spray was used to functionalize interdigitated electrodes with features and spacings as low as 50 microns and a substantial increase in capacitance was measured. Upgraded patterns on polyimide substrates demonstrate the compatibility of this process with flexible electronics.