Tungsten Trioxide/Zinc Tungstate Bilayers: Electrochromic Behaviors, Energy Storage and Electron Transfer

Materials with integrated functions of energy storage and electrochromism are highly desirable from the point of view of sustainable development.^{1, 2} Herein, pair-sequentially spin-coated tungsten trioxide (WO₃) and zinc tungstate (ZnWO₄) bilayer films onto indium tin oxide (ITO) coated glass slides have been prepared via sol-gel methods followed by annealing. The bilayers of ZnWO₄/WO₃ (the bilayer film consisting of an inner layer of ZnWO₄ and an outer layer of WO₃ on the ITO glass) and WO₃/ZnWO₄ (the bilayer film consisting of an inner layer of WO₃ and an outer layer of ZnWO₄ on the ITO glass) exhibit integrated functions of electrochromic and energy storage behaviors as indicated by the in situ spectroelectrochemistry and cyclic voltammetry (CV) results. Accordingly, blue color was observed for the bilayer films at -1 V in 0.5 M H₂SO₄ solution. An areal capacitance of 140 and 230 μF/cm² was obtained for the ZnWO₄/WO₃, and WO₃/ZnWO₄ film, respectively, at a scan rate of 0.05 V/s in the CV measurements. The CV results also unveiled the electron transfer behavior between the semiconductor films in the oxidation process, suggesting a sequence-dependent electrochemical response in the bilayer films. Meanwhile, methylene blue (MB) was used as an indicator to study the electron transfer phenomenon during the reduction process at negative potentials of -0.4 and -0.8 V, in 0.5 M Na₂SO₄. The results indicated that the electrons transfer across the bilayers was enhanced at more negative potentials.

References

1.  H. Wei, X. Yan, S. Wu, Z. Luo, S. Wei and Z. Guo, The Journal of Physical Chemistry C, 2012, 116, 25052-25064.

2.  H. Wei, J. Zhu, S. Wu, S. Wei and Z. Guo, Polymer, 2013, 54, 1820-1831.