XAS, XRD, AFM and Raman studies of nickel tungstate electrochromic thin films

Article abstract of DOI:10.1016/S0013-4686(01)00365-6

Systematic studies of nanocrystalline nickel tungstate, NiWO₄, thin films were performed by several experimental techniques such as Ni K- and W L_1,3-edges X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy and cyclic voltammetry measurements. We found that the NiWO₄ thin films exhibit electrochromic properties similar to that of amorphous tungsten trioxide films, but show better structural stability upon multiple colouring/bleaching cycling. It was observed that a nanocrystallinity of the thin films results in strong modifications of the Ni-O and W-O interactions, which affect both local atomic and vibrational structures.

Full text of DOI:10.1016/S0013-4686(01)00365-6

Electrochimica Acta 46 (2001) 2233–2236
www.elsevier.nl/locate/electacta
XAS, XRD, AFM and Raman studies of nickel tungstate
electrochromic thin films
a
a,
a
b
b
A. Kuzmin , J. Purans *, R. Kalendarev , D. Pailharey , Y. Mathey
a
Institute of Solid State Physics, Uni6ersity of Lat6ia, Kengaraga street 8, LV-1063 Riga, Lat6ia
b
GPEC-UMR 6631, case 901, Facult e´ des Science de Lumini, 13009 Marseille, France
Received 21 August 2000; received in revised form 9 November 2000
Abstract
Systematic studies of nanocrystalline nickel tungstate, NiWO , thin films were performed by several experimental
4
techniques such as Ni K- and W L1,3-edges X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy,
atomic force microscopy and cyclic voltammetry measurements. We found that the NiWO4 thin films exhibit
electrochromic properties similar to that of amorphous tungsten trioxide films, but show better structural stability
upon multiple colouring/bleaching cycling. It was observed that a nanocrystallinity of the thin films results in strong
modifications of the NiꢀO and WꢀO interactions, which affect both local atomic and vibrational structures. © 2001
Elsevier Science Ltd. All rights reserved.
Keywords: Nickel tungstate thin films; X-ray absorption spectroscopy; X-ray diffraction; Raman spectroscopy; Atomic force
microscopy
were performed to improve the stability of a-WO thin
films [2].
1
. Introduction
3
In this work, we present a systematic study of a new
promising material, nanocrystalline nickel tungstate
NiWO4 thin films, having electrochromic properties
Amorphous tungsten trioxide (a-WO ) thin films are
3
well-known materials for the use in electrochromic
devices [1,2]. They belong to a class of cathodically
colouring substances, which possess a reduced coloured
state, i.e. tungsten ions change their formal valence
state from 6+ to 5+ upon colouration. From the
point of view of commercial applications, long-term
stability and performance of electrochromic devices
similar to a-WO , but better long-lasting structural
3
stability.
2. Experimental
based on a-WO present the main problem. The degra-
3
dation of thin films is related to their transformation
from metastable amorphous state into crystalline state
upon multiple colouring/bleaching cycling [3], accom-
panied by injection/extraction of small cations, such as
Tungsten oxide, WO , nickel oxide, NiO, and NiWO4
3
thin films (t.f.) were produced from metallic tungsten
and nickel targets by separate and simultaneous reac-
tive magnetron sputtering in a plasma-focusing DC
magnetic field. A gas mixture of argon (80%) and
oxygen (20%) was used as the sputter atmosphere. The
sputter deposition was performed during 60–90 min at
a total gas pressure 6–6.7 Pa. Pure glass substrates and
the ones covered with ITO (In O /SnO ) were mounted
+
+
H
and Li and electrons. Therefore, many attempts
*
Corresponding author. Tel.: +371-718-7302; fax: +371-
711-2583.
E-mail address: a.kuzmin@cfi.lu.lv (A. Kuzmin).
2
3
2
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(01)00365-6

2234
A. Kuzmin et al. / Electrochimica Acta 46 (2001) 2233–2236
about 8 cm above the target and were not intentionally
heated. The films had a thickness of 0.5–1.2 mm. The
as-deposited NiWO films were nearly transparent, but
4
the WO films were bluish-grey, and the NiO films were
3
brownish. For comparison, we used a-NiWO4 and
polycrystalline (c-) NiWO powders [4], and commercial
4
polycrystalline CaWO and WO .
4
3
The electrochemical properties of t.f.-NiWO4 were
studied by cyclic voltammetric measurements. They
were performed in 0.1 N H SO4 acid electrolyte at a
2
scan rate of 50 mV/s, and the potential was measured
using the three-electrodes scheme with platinum coun-
ter electrode and Ag/AgCl reference electrode. The
surface topography of all films was examined at
micrometer scale by tapping mode atomic force mi-
croscopy (TM-AFM) [5].
The local environment around nickel and tungsten
ions in thin films and two NiWO4 compounds was
probed by the Ni K-edge and W L1,3-edges X-ray
absorption spectroscopy (XAS). The details of the ex-
periments and data analysis can be found in Ref. [4].
The conventional macro-Raman measurements were
also done in 90°-geometry to investigate the vibrational
dynamics of the tungstenꢀoxygen bonds [4].
Fig. 1. Cyclic voltammograms for NiWO and WO thin films
4
3
taken in 0.1 N H SO acid electrolyte. The film thickness was
2
4
about 0.5 mm. The curves shown were measured after several
tens cycles, when the shape of the voltammogram become
nearly unchanged.
3. Results and discussion
Voltammograms for NiWO and WO thin films are
4
3
shown in Fig. 1 and suggest similar response for both
films on charge insertion/extraction. The surface topol-
ogy of the prepared films can be evidenced from the
AFM images presented in Fig. 2. Both WO and NiO
3
thin films exhibit similar size of grains of the order of
20–60 nm. The grain size increases several times for
t.f.-NiWO , being about 60–120 nm. These results are
4
in good agreement with our XRD studies [4]. The
as-prepared NiWO films have nanocrystalline structure
4
with Ž111 preferential orientation, and the crystallites
(
size is larger than in the other two oxides. No change in
the XRD patterns was observed for tungstate films
after annealing at 450°C, which suggests good struc-
tural stability [4].
Crystalline NiWO4 has wolframite-type structure,
which is built up of hexagonally close-packed oxygens
2
+
6+
with certain octahedral sites filled by Ni
and W
cations in an ordered way [6]. The [NiO ] octahedra
6
possess small tetragonal distortion, whereas [WO ] oc-
6
tahedra are strongly distorted with tungsten ions being
off-centre. The octahedra of the same type are con-
nected via edges, but of the opposite type by corner
2
+
sharing. An interaction between neighbouring Ni
8
Fig. 2. AFM 1×1 mm images of WO , NiWO and NiO thin
[3d ] (S=1) ions is of superexchange-type and is ex-
pected to be strong enough, because NiWO4 orders
antiferromagnetically below 67 K [6].
3
4
films, obtained in tapping mode [5]. Note larger grain size in
NiWO4 compared to WO3 and NiO.

A. Kuzmin et al. / Electrochimica Acta 46 (2001) 2233–2236
2235
2
Fig. 3. Left panel: Fourier transforms (FTs) of the calculated Ni K-edge EXAFS x(k)k signals for NiO Ni (x=0,1,2) clusters. The
6
x
six oxygen atoms are responsible for the main peak at 1.5 A, and one/two nickel atoms contribute at 2.5 A. The peaks due to
, ,
multiple-scattering contribution from the first oxygen shell are also indicated. Right panel: FTs (dashed line — experiment; solid
line — calculation, based on the model from left panel) of the Ni K-edge EXAFS signals in three typical NiWO4 thin films. The
origin of peaks A, B and C is as in left panel.
The local structure around nickel and tungsten ions
was probed directly by XAS. Apart from the grain size
effect [7], it resembles, in general, that of polycrystalline
octahedron, surround the central nickel atom. The re-
maining two nickel ions are connected to the central
nickel via oxygens of the two opposite octahedron
edges. Thus, the in-plane NiꢀOꢀNi angles are equal to
90°. The FTs of the calculated EXAFS signals are
shown in Fig. 3 (left panel). For x=1 (solid line), the
NiWO . A comparison of the intensity of the pre-edge
4
peak at the W L -edge, related to the dipole-forbidden
1
transition 2s(W)“5d(W) [8] in t.f.-NiWO4 with a-
NiWO and WO suggests that a degree of the [WO ]
octahedra distortion in t.f.-NiWO4 and a-NiWO4 is
two peaks at 2.5 and 3.5 A correspond to the MS
,
4
3
6
contributions from the first shell octahedron [10]. For
x=1 (dashed line) and x=2 (dotted line), an addi-
tional contribution from one and two nickel atoms,
similar, being larger than in WO . However, in spite of
3
large [WO ] octahedra distortion, tungsten ions in
6
tungstates are still octahedrally coordinated that is sup-
respectively, appears at 2.5 A. This model is able to fit
,
ported by a comparison with the W L -edge in CaWO4
perfectly the experimental signals (Fig. 3, right panel).
However, more precise analysis shows that six oxygen
atoms surrounding the central nickel ion can be addi-
tionally separated into two groups: four strongly bound
in the plane with square-like arrangement and two
weekly bound at the remaining vertices of an octahe-
dron above and below the plane. The latter two oxy-
gens are strongly bound to tungsten ions. Note also
that no significant NiꢀW contributions are observed in
the Ni K-edge EXAFS signal, which suggests weak
interaction between the tungsten and the nickel
sublattices.
1
[4]. In the latter case, the tetrahedral coordination of
tungsten ions results in the pre-edge peak of larger
intensity. This conclusion also agrees with the results
for the W L -edge. Thus, the coordination of tungsten
3
in t.f.-NiWO4 can be described as [WO O ], meaning
4
2
four short and two long WꢀO bonds [4].
The Ni K-edge spectra allowed us to obtain comple-
mentary information on the NiꢀO bonding. The
Fourier transforms (FTs) of the experimental EXAFS
signals are shown in Fig. 3 (dashed lines in left panel).
They are dominated by peak A, being due to six oxygen
atoms of the first shell, whereas the peaks above 4 A
,
The Ni K-edge XANES signals (not shown) allow to
obtain complementary information on the character of
the NiꢀO bonding by studying the variation of the
are smeared out due to large static disorder. To under-
stand the origin of the two peaks B and C, we per-
8
+i i
formed
the
model
calculations
within
the
pre-edge peak, which corresponds to ꢀ1s3d
L6 “
8
+i i−1
i
multiple-scattering (MS) approach [9] for the small
NiO Nix (x=0,1,2) clusters, which represent a frag-
ment of the local structure around nickel ions in crys-
talline NiWO . Six oxygen atoms, forming a regular
ꢀ16
s
6
3d  (i=1,2) transition [7,11]. Here L de-
L6
notes the oxygen 2p-orbitals having i-holes in the place
of i-electrons back donated to the nickel 3d orbitals,
and 1s stands for a Ni 1s core hole. The change of the
6
4

2236
A. Kuzmin et al. / Electrochimica Acta 46 (2001) 2233–2236
4. Conclusions
In this work, we propose a new electrochromic mate-
rial — nanocrystalline NiWO thin films. A systematic
4
study of the thin film structure and vibrational proper-
ties using X-ray absorption spectroscopy, X-ray diffrac-
tion and Raman spectroscopy suggests that the local
NiꢀO and WꢀO chemical bonding depends strongly on
the size of the NiWO4 grains. The obtained results
show that the interaction between nickel–oxygen and
tungsten–oxygen polyhedra in thin films is weak, and
the local environment around nickel and tungsten ions
can be described as tetrahedral-like [WO ]O or plane-
4
2
square-like [NiO ]O . Note that in this model both
4
2
tungsten and nickel ions are coordinated by six oxygen
atoms but are strongly bound with only four of them.
Such a situation for tungsten ions represents an inter-
mediate case between WO and CaWO compounds.
3
4
Fig. 4. Pre-edge peak intensity Ipre-peak at the Ni K-edge in
several compounds. The frequencies n(WꢀO) of the highest
stretching WꢀO mode, obtained from Raman spectra, are also
given for the tungstates. Note a negative correlation between
Ipre-peak and n(WꢀO).
Acknowledgements
J.P. is indebted to the LURE laboratory for provid-
ing the beam time. He is also grateful to Dr A. Traverse
for assistance during XAS experiments. This work was
supported in part by the Latvian Government Research
Grant No. 96.0670.
pre-edge peak intensity for several compounds is shown
in Fig. 4 (the data for c-NiO and c-MgO/Ni were taken
from Ref. [12] and for t.f.-NiO from Ref. [7]): it is
related to the variation in the covalency of the NiꢀO
bonding or more precisely to the change in the oxygen-
to-nickel back charge transfer [11]. The higher intensity
of the pre-edge peak indicates stronger covalency or
larger charge transfer. One can compare (Fig. 4) the
pre-edge peak intensities with the values of the stretch-
ing mode frequency n(WꢀO), provided by Raman spec-
troscopy. The negative correlation is observed
.
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−
1
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[
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4
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[
WO ] octahedra.
6