Nickel oxide film with tertiary hierarchical porous structure and high electrochromic performance and stability

Abstract

The NiO electrochromic (EC) film with hierarchical porous structure was prepared by hydrothermal method and heat treatment. X-ray photoelectron spectroscope (XPS), scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM) and X-ray diffractometer (XRD) were employed to characterize the morphology, microstructure and component of the obtained films. The EC properties including transmittance, cyclic voltammetry (CV), electrochemical impedance, switch speed and cycle stability were examined. Results show that the NiO film has a tertiary hierarchical porous structure constructed by NiO nanoparticles, nanosheets and laminated thick slices step by step. The mesopores that appear in the nanosheets are formed by the surrounding of nanoparticles. Response times (tc < 3.00 s, and tb is about 0.60 s) are short. The control factors of charge transport changed with EC cycling. The stability of EC properties could be kept from the 1000th cycle to the 25000th cycle. The most noteworthy is that after 40,000 cycles the hierarchical structure did not change significantly. The reason (or mechanism) of the performance was discussed and suggested.