Color Tuning with Metal Nanoparticles Supported on Electrospun Titanium Dioxide for Electrochromic Applications: Chemical Reaction Engineering by Additive Manufacturing of Mesoscale MetaMaterials

Titanium dioxide (TiO2) has attracted considerable attention in the field of (photo)electrocatalysis, optoelectronics, and sensors [1]. Although a great number of strategies for the synthesis of TiO2 structures exist, methods leading to one-dimensional (1D) structures are of great interest due to the benefits of higher surface area and ease of functionalization with external entities [2], like metal nanoparticles. Here electrospinning will be discussed as an attractive and versatile fabrication method to obtain high aspect ratio TiO2 (nano)fibers, which additionally allows the overlaying of several layers of TiO2 fibers. Such structures can form an open porous network, that allows maximizing surface-to-volume ratio and has benefits for light absorption in optoelectronic devices [3]. In this study, suspended nanoparticles of TiO2 in a polyvinylpyrrolidone (PVP)/ethanol solution are used for the formulation of active layers with electrospinning for applications in electrochromic (EC) devices. A coaxial nozzle is utilized to fabricate TiO2 fibers with spatially distributed metal (M) nanoparticles (Cu, Au, and Ag). The synthesized TiO2 fibers have anatase phase with 335±24 nm average diameter. Electrospun TiO2 and TiO2-M fibers on fluorine-doped tin oxide (FTO) glass substrates are tested for EC behavior by applying an external electrical potential. The response time of these fibrous structures is 10 seconds when cycling between ±3V. The color of the additively manufactured fibrous layers can be tuned from green (TiO2-Cu) to blue (TiO2-Au) or brown (TiO2-Ag) while maintaining a stable device structure over 150 cycles. Potentiostatic/galvanostatic measurements in half-cell configuration are carried out to understand the coloration and bleaching mechanism of the electrochromically active layers. It is hypothesized that the meso-porosity of the TiO2 fibers enhances the redox reaction kinetics when compared to that of thin films. Furthermore, attained coloration is attributed to the tailored optical absorption with nanoparticles decorating the fiber structure. This concept can pave the way to a more efficiently colored generation of optical windows.