Photon Factory Activity Report 2022 #40 (2023)

BL-9C/2021G032

In Operando X-ray Absorption Fine Structure on Local Structure andElectronic State of Tungsten Oxide Nanoparticles underElectrochromism
Mari TAKAHASHI,1 Yuki KITAZAKI,1 Masafumi HARADA,2 and Shinya MAENOSONO1,*
1 School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai,Nomi, Ishikawa 923-1292, Japan

2 Department of Computer Science and Clothing Environment, Faculty of Human Life andEnvironment, Nara Women’s University, Nara 630-8506, Japan

1 Introduction
Tungsten oxide (WO3-x) is one of the most wellknown electrochromic (EC) materials. Recently, inorder to make flexible EC film, printing techniques
using nanoparticles (NPs) as an ink has attracted
attention.1 Although it is known that the crystal
structure and morphology of WO3-x NPs influences
EC performance, the underlying mechanisms of thecolor deterioration have not yet been clarified.
Therefore, in this study, we performed in operando
XAFS measurement for EC films composed of two
different crystal structured WO3-x NPs duringcoloration and bleaching cycles.
2 Experiment
Two different WO3-x NPs, namely monoclinic
W18O49 NPs and hexagonal (NH4)0.75W3O9 NPs, weresynthesized by wet chemical synthesis. Those NPs
were then coated onto an ITO-PET film. The film wasset in a tailor-made cell. Using LiPF6 as an electrolyte,+3 V was applied for coloring and –3 V was appliedfor bleaching. During the alternate voltage
applications, XANES measurements at W L3-edge
were conducted in BL-9C and photos were taken of
the film at each step. EXAFS spectra were obtained
before applying the voltage and after the color of thefilm deteriorated. The color of the film was analyzed
based on the CIE1976 L*a*b* color space.
3 Results and Discussion
As synthesized NPs were found to have rod shapebased on TEM images. XRD patterns showed thatthe produced NPs were either monoclinic or
hexagonal phase. Fig. 1a,b show photos ofmonoclinic or hexagonal NP-loaded films (m-Film andh-Film respectively) under repeating voltage
application. The plot of b* value shows the cyclic
dependence of chromaticity (Fig. 1c) where a morenegative value means a darker blue. Colordeterioration was observed after 8 cycles for the mFilm and 7 cycles for the h-Film. Here, one cycle
corresponds to applying +3 V for 52 min followed by–3 V for 52 min.
XANES spectra obtained from the m-Film and theh-Film under 0 V are shown in Fig. 1d. Since WO3-x

has a WO6 octahedron unit in the crystal structure,
the ligand field splits the 5d orbital level into the t2gand eg levels. Based on simultaneous fitting of theXANES spectrum and its second derivative, theenergy levels of t2g and eg were determined as shownin Fig. 1e. A periodic change was clearly observed in
the eg levels until the color deteriorated (8 cycles form-Film and 7 cycles for h-Film). Because the eg level
was formed by the on-axis alignment of the 5d orbitaland ligand field, they were more sensitive to thechange of the ligand field than the t2g. The gradualincrease of the eg level in the m-Film was considered
to be due to the formation of distortions.2 Analyses of
EXAFS spectra revealed that W-O bonds in the mFilm increased after color deterioration while no bondchange was observed in the h-Film, suggesting thatdifferent crystal structures have different coloration
deterioration mechanisms. In the case of the m-Film,the accumulation of distortions stopped the reversible
color change. On the other hand, the reason for thecolor deterioration in the h-Film was explained by thetrapping of Li+ in the optically inactive trigonal cavities
during repeated voltage application. A detailed
discussion was written in the published paper.

Photon Factory Activity Report 2022 #40 (2023)

Fig. 1 Photos of the (a) m-Film and (b) h-Film under either+3 V or –3 V. (c) The plot of the b* values at each step. (d)XANES spectra of W L3-edge. The blue, pink, green andred lines are t2g, eg, background and the sum of thecomponents respectively. (e) Variation of t2g and eg energy
positions with respect to each step.

References[1] P. J. Wojcik et al., Nanoscale 7, 1696 (2015)[2] S. Yamazoe et al., J. Phys. Chem. C 112, 6869
(2008)
Research Achievements
This research was published in J. Phys. Chem. C(2023) DOI: 10.1021/acs.jpcc.3c01449*shinya@jaist.ac.jp