Electrospun Composite TiO2 Fibers for Color Tuning in Electrochromic Devices

Engineering one-dimensional (1D) nanostructures with multiple components can unlock properties that are not attainable by single elements alone. Herein, we use coaxial electrospinning for the synthesis of non-woven webs nanofibers with different metal(oxide) nanoparticles (NPs), i.e. silver (Ag), gold (Au), and copper oxide (CuO). The approach includes the structuring of nanofibers with an average diameter of 198.7±21.5 nm comprised of mesoporous TiO 2 nanopowders mixed with metal(oxide) NPs to enable color tuning upon the application of an electrical potential. The nanofiber composites are supported on a fluorine-doped tin oxide (FTO) glass substrate and assembled in an electrochromic device in a half-cell configuration. It is observed that the color attained can be altered by changing the NP composition; such that Ag shows a black-brown color, Au yields dark blue, and CuO leads to a dark green color under negative bias. Fast color switching (<10 seconds) is possible per each TiO 2 nanofiber/NP configuration. This is attributed to the faster kinetics of electrospun nanofibers in redox reactions gained by the high surface area to volume ratio. Band-gap deployment of the nanofiber composites is also observed, depending on the synthesized TiO 2 nanofibers the band gap can be modulated between 2.76 to 3.01 eV allowing fine-color tuning upon EC cycles between ±2V. The rational design of the composite nanofibers is confirmed chemically, structurally and optically. The experiments show that material functionalities like color tuning are achievable with the use of metal(oxide) NPs. These findings can pave a new path for the rational design of nanofiber materials with applications in smart colored windows.