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Characterization of evaporated nickel oxide and its application to electrochromic glazing
Solar Energy Materials and Solar Cells 31 (1993) 291-299 North-Holland
Solar Energy Materials and Solar Cells
Characterization of evaporated nickel oxide and its application to electrochromic glazing Junichi N a g a i Advanced Glass R & D Center, Asahi Glass Co., Ltd. 1150 Hazawa-Cho, Kanagawa-Ku, Yokohama 221, Japan Received 25 January 1993; in revised form 17 May 1993 In this work, optical and electrochemical properties of electrochromic NiO x films and their application to electrochromic glazing are studied. Quartz crystal microbalance (QCM) study reveals that O H - insertion/extraction occurs during coloration and bleach reaction of NiO x film in an organic electrolyte. This process is also confirmed by reflectance m e a s u r e m e n t by FTIR. Electrochromic glazing consisting of W O 3 / P E O - b a s e d Li ÷ electrolyte/NiO x system shows promising solar control properties. The possibility towards NIR-reflectance control using W O 3 is exemplified.
I. Introduction
Electrochromic glazings are the state of the art in solar energy materials [1,2]. In contrast to light scattering windows using a liquid crystal, the electrochromics are attractive having translucent and solar gain control properties. Their comparatively slow responses (about 3 minutes for a 30 × 30 cm device) may not cause a practical problem because the intensity of sunlight changes gradually. WO 3 films have been widely used for cathodes for the electrochromic glazings. They have been proved to be stable and thus suitable for this purpose as cathodes. NiO x films have been intensively studied as candidate materials serving as anodes. At 633 nm wavelength, they have better coloration efficiency (28 cm2/C) than IrO 2 films (10 - 15 c m 2 / C ) [3]. However, the electrochemical mechanism that leads to electrochromism in NiO based coatings is still controversial. Summarizing the proposed mechanisms so far for electrochromic reactions of nickel oxide, hydroxides and oxyhydroxides, the following three are typical [4,5]: NiO + O H - ~
NiOOH + e-,
(1)
Ni(OH)2 + O H - . ~ N i O O H + H 2 0 + e - ,
(2)
Ni(OH)2 ~ N i O O H + H + + e - .
(3)
In this paper, the fundamental properties of nickel oxyhydroxides are firstly shown. Also the device properties using W O 3 / P E O - b a s e d Li+-electrolyte/NiOx and other WO3-based devices are analyzed and discussed. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
J. Nagai / Euaporated nickel oxide
292
2. Experimental details 2.1. Fabrication o f WO 3 and NiO x films W O 3 and NiO x films are deposited by electron beam (EB) evaporation of the corresponding W O 3 (99.9%) and NiO (99.9%) on ITO coated glass (10 1 2 / [ ] ) as previously reported. During the evaporations, 0 2 gas is introduced to keep a constant pressure of 1 x 10 - 4 Torr and 8 X 10 -5 Torr for WO 3 and NiO x, respectively, and the substrates are not heated unless otherwise specified. Deposition rates are about 10 , ~ / s for WO 3 and 1-2 , ~ / s for NiO x films, respectively. Prior to the depositions, the vacuum chamber is evacuated to 1 x 10 -5 Torr. According to X R D analysis, the WO 3 film is amorphous and the NiO x shows a preferential orientation of cubic NiO ( l i D .
2.2. Electrochemical measurements
Electrochemical measurements were reported previously [2]. Cyclic voltammograms with a scan rate v = 50 m V / s and AC-impedance spectra were measured by the three-electrode system. To measure the impedance of the electrode, a small AC-signal (5 mVo_ o, f = 1 kHz) was superposed on a triangular wave (v = 10 m V / s ) . Then the AC potential and current signals were analyzed simultaneously by two phase lock-in amplifiers. The complex impedance is expressed in terms of a series connection of a resistor (resistance; R m) and a capacitor (capacitance; Cm). y-butyrolactone with 1 M LiC104 (ca. 1000 ppm H 2 0 ) and aqueous electrolytes with 1 M LiCIO 4 or LiOH are mostly used. Other electrolytes are described in the later sections if necessary. 2.3. Quartz crystal microbalance measurements
The schematic of the quartz crystal microbalance (QCM) method [6,7] is shown in fig. 1. A 10 MHz A T cut quartz is sealed to the bottom of a 6 mm i.d. glass tube.
~ Glass
Coulomb
I-L ~
I COmputer I I
Tub .l I ~-~.pt I I ,7 L' II
Electr°lytel~ E C II I I film I I Seal. kl, ~----~U Ouartz- ~ ]'_ , Au" I
I Frequencyl
Au "~
I
I C°unter I I
I
Oscill't°rl [ Circuit I I
Fig. 1. Diagram of the quartz crystal microbalance (QCM) method.
J. Nagai / Evaporated nickel oxide
293
The electrolyte used was acetone (99.5%) with 0.01 M LiC10 4. This method was unsuccessful for other organic electrolytes such as y-butyrolactone, propylene carbonate and dimethoxyethane with 0.01 M LiC10 4 because no stable oscillation of the quartz was generated. The increase of mass of the film is calculated from the decrease in resonant frequency by A f = - 2( ptx ) - 1 / 2 f 2 A m / A ,
(4)
where A f is the change of resonant frequency (Hz), p the density of quartz (2.648 g / c m 3 ) , / z the shear modulus (2.947 × 10 tt dyne/cmZ), f0 the resonant frequency (Hz), Am the mass increase (g), and A the area of the electrode (cm 2) The precision of the present system was evaluated by analyzing the electrodeposition of silver from a 0.01 M AgCIO 4 aqueous solution onto gold and was found to be + 5% of the molecular weight of silver. 2.4. Optical measurement Optical measurements in the UV to NIR region and the IR were conducted using a spectrophotometer (U-3500, Hitachi) and a F T I R (model 4200, Shimadzu), respectively.
3. E x p e r i m e n t a l
results on NiO x films
3.1. Electrochromic reaction After deposition of the NiO x film onto an ITO coated glass substrate, it is immersed in 1 M LiC1OJy-butyrolactone or 1 M L i C 1 0 4 / H 2 0 . Typical cyclic
I
I
l
I
NiOx MI /i-/
/
j
/
<
EO -I
q
i n lff L i C I O J T -BL i n lff L i C 1 0 J H20
I
I
-1.0
-0.5
I
0 0.5 1.0 E [VvsSCE]
Fig. 2. Cyclic voltammograms of NiO~ film. (Thickness = 2 I~m, scan rate = 50 m V / s ) .
294
J.
E
Nagai / Euaporated nickel oxide
2 inlM-l'iOHoq
I
% E
rr
o eq J LL
8
%
'E t._)
0
i
-1.0
I
0 V vs SeE/volt
1.0
Fig. 3. I m p e d a n c e s p e c t r a o f N i O x film. ( T h i c k n e s s = 2 ~ m , s c a n r a t e = 10 m V / s , a r e a = 0.25 c m e ) .
voltammograms generated for these experiments are shown in fig. 2. The current density becomes larger in the aqueous electrolyte. Fig. 3 shows the potential dependence of R m and 1/C2m for 1 M L i O H / H E O . The resistance decreases as the coloration proceeds while the capacitance increases (i.e. decrease of 1/C2m). This shows that the carrier density in the NiO x increased by coloration. In fig. 4, the linear relationship between increased mass and injected charge during the coloration is shown. Above 2 mC (i.e. 10 mC/cm2), NiOx shows an irreversible reaction, which might cause the scattering of the data in fig. 4.
1.0
.-1
Mass Change=17.66g/F 0.8 0.01MLiClO4/Acetone "0.196¢m2 EB NiOx / /
/
01 O1
:E
O.6
"10 01
El
yo°
0, 0.2
o
0
1
o
2
3
4
Injected Chorge [mC] Fig. 4. Massincrease of colored NiOx.
J. Nagai / Euaporated nickel oxide
0.1
'
I
'
295
. . . .
I
EB Ni0x (0.581J,m) ~3 Colored/(~SrnC/cm 2) ~2 V4 /
V
"i 0.05 t-
00.3
,
I 0.5
,
,
,
,
Wavelength
I 1.0
,
,
,
I
1.5
h [l~m]
Fig. 5. Extinction coefficient of NiO x film.
Using a linear least squares fit, the mass change was determined from fig. 4 and was 17.66 g / F with a correlation coefficient of 0.94, which points at O H - injection during the coloration. Thus eqs. (2) and (3) are ruled out for electrochromic reaction of NiO x film. However in eq. (2), if water formed during coloration is not released from the host but adsorbed, then the following O H - insertion mechanism can be expected: Ni(OH)2 + O H - ~
NiOOH • H20 + e-.
(5)
Although a more elaborate and extensive study is required to identify the species of ions responsible for electrochromism, the present result is sufficient to prove that the mass of the inserted or extracted ion is heavier than proton or lithium. 3.2. Optical properties
In the UV to N I R region, the transmittance and reflectance of NiO films were measured to determine the complex optical constant (n, k). In fig. 5, the obtained extinction coefficients k are plotted with respect to wavelength before and after coloration. The refractive index n is almost constant (n = 2.0) in this region. Small absorption peaks are present in the spectrum. In fig. 6, the absorption spectrum is drawn for an aqueous solution of NiSO 4 which has almost the same absorption peaks as shown in fig. 5. This result shows that the evaporated N i O x film has the character of a hydrated nickel ion, i.e. N i ( H 2 0 ) 2+ [8-10]. In the IR region, the absorption spectra are derived from reflectance measurements where the nickel oxides were deposited on gold coated glasses. Fig. 7 shows the absorption spectra thus obtained for the deposited and colored films using 1 M LiCIOa/3,-butyrolactone. In the colored state a broad absorbance peak between
296
J. Nagai / Evaporated nickel oxide
1 ' I ....
I
. . . .
NiSO 4 lOmM aq. ( d : l c m )
U t-O kO t/I
v3
0
.
5
<
~ V2
00.3
V~
0. 5
1.0
1.5
Wavelength ~, [lain] Fig. 6. Absorption spectrum of nickel sulfate.
3600 and 3650 cm - t (due to free O H ) is increased [11-14]. This evidence also supports that O H - insertion takes place during the coloration. As broad absorption due to O H - is found in the F T I R spectrum, the evaporated nickel oxide is more like NiO x (slightly hydrated) than Ni(OH) 2. Therefore, the most plausible electrochromic reaction of NiO x is expected to be eq. (1). The O H - insertion takes place through defects in the NiOx structure, because a fully crystallized NiO film did not show electrochromism in y-butyrolactone with lithium perchlorate which is very similar to the result of ref. [5].
I
I
r
Cotored (20mC/cm 2) ta"}
0,1
Ii
~o, o5 ~
I
0
As deposited <
0,1 ~05
o r-~, 4000
2000
1400 Wave number [cm -1]
Fig. 7. FFIR spectra of NiOx films.
800
400
J. Nagai / Evaporated nickel oxide
297
4. Evaluation of electrochromic glazings 4.1. WO 3 / P E O - L i C I O 4 / NiO x system Using a liquid electrolyte of P E O (MW 200) with 1 M lithium perchlorate and 1000 p p m water, an electrochromic device was fabricated. Between the W O 3 and NiO x electrodes separated by 5 0 / z m , the liquid electrolyte was back-filled. Fig. 8 shows the spectral transmittance changes. The inset in fig. 8 indicates visible and solar transmittances and reflectances, where the following notations are used: Tv; visible transmittance, TE; solar transmittance, R v ; visible reflectance, RE; solar reflectance. The device shows a greyish neutral coloration and a wide modulation of solar transmittance is advantageous for future energy efficient windows. 4.2. Towards the modulation of N I R reflection The electrochromic glazing based on amorphous W O 3 has a heat absorbing property. Regarding the solar efficiency, a variable heat reflective system is considered to be superior to a heat absorbing one [15]. It is well-known that the amorphous tungsten bronze LixWO 3 shows metallic conduction when x > 0.16. In
60.0
60-0"
# I
o tO
50.040.0
G/ ITO/WO3/PEO-LiCIO4/NiOx/ITO/G
A(a) / k~ _
7o.0
/
Tv(%)
TE(%)
Rv(%) RE(~)
k~
(a)
70.3
50.6
8.1
12.2
\
(b)
3"7.0
24.9
8.9
13.7
(c)
\
6,
"
"~ 3 0 . 0 (-
20.0 I0.0 0.0
200
soo'
t 006 Wavelength
t soo
"'
zooo
[nm]
Fig. 8. Transmittance spectrum of electrochromic device (WO 3 and NiOx based device, (a) completely bleached state, (b) intermediate state, (c) colored state).
298
J. Nagai / Ecaporated nickel oxide
Table 1 Optical properties of WO3 based electrochromic glazing Tv 71.4 54.9 22.4 9.8 7.9
TE 44.3 25.7 8.9 3.8 2.9
Rv 14,8 9,9 4.4 5.9 5.7
RE 18.2 15.5 11.7 18.9 20.1
x in LixWO3 0 0.024 0.106 0.182 0.202
the present work, the relationship between coloration level and optical properties were obtained to examine the possibility of variable heat reflectance. An electrochromic device having the following construction as previously published [16]: glass (2 ram) / I T O / W O 3 / e l e c t r o l y t e / I T O / g l a s s
(2 ram),
was assembled for optical measurements. In this experiment, W O 3 was deposited at a pressure of 8 × 10 -5 Torr 0 2 and the substrate temperature was 330°C to have a dense film. Applying a constant voltage to the device, transmittance and reflectance from W O 3 side were measured at saturation level of the coloration. In table 1 are listed the fabrication condition of W O 3 and optical properties. As the visible transmittance decreases, the solar reflectance decreases due to the heat absorbing nature. However, after the visible transmittance reaches 9.8%, the solar reflectance begins to increase. At this critical transmittance, the value of x in LixWO 3 can be calculated from the following equation: x = Q[WO3]/(FAdp)
= 0.182,
(5)
where Q is the injected charge = AOD/~7 = 0.0216 C / c m 2, A O D the optical density change = 1og~0(71.4/9.8), r/ the coloration e f f i c i e n c y - 40 cm2/C, F the Faraday constant, p the density of W O 3 film = 5.7 g / c m 3, and A the area of W O 3 film with thickness d = 500 nm. It can be verified that a device having a electron density above the percolation threshold shows a heat reflective property. Although a further study is necessary, increase of electron density is the key for variable heat reflection for amorphous W O 3. To achieve this, control of crystallinity and the usage of thinner films will be required.
5. Conclusion
The optical and electrochemical properties of electrochromic NiO x films were analyzed. In the organic electrolyte, the coloration reaction appears to be O H insertion. Electrochromic glazing consisting of W O 3 / P E O - b a s e d Li ÷ electrolyte/ NiO x shows a promising solar control property. Towards the modulation of N I R reflection using the LixWO 3 based device, deep coloration gives an increase in
J. Nagai / Evaporated nickel oxide
299
N I R reflectance. Further study is now in progress to realize a solar energy efficient window.
Acknowledgements The author would like to thank Mr. T. Seike for carrying out the thin film fabrications and G.D. McMeeking for his helpful discussions relevant to this work. He also wishes to thank Dr. C.M. Lampert for his continued encouragement.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
C.M. Lampert and C.G. Granqvist, Eds., Large-Area Cromogenics, SPIE IS-4 (1990). T. Seike and J. Nagai, Sol. Energy Mater. 22 (1991) 107. T. Seike and J. Nagai, Hyomen Kagaku 10 (1989) 314 [in Japanese]. A. Agrawal, H.R. Habibi, R.K. Agrawal, J.P. Cronin, D.M. Roberts, R.C. Popowich and C.M. Lampert, Thin Solid Films 221 (1992) 239. A. Nemetz, A. Temmink, K. Bange, S.C. de Torresi, C. Gabrielli, R. Torresi and A. Hugot-Le Goff, Sol. Energy Mater. Sol. Cells 25 (1992) 93. S.J. Babinec, Sol. Energy Mater. Sol. Cells 25 (1992) 269. S.I. Cordoba-Torresi, C. Gabrielli A. Hugot-LeGoff and R. Torresi, J. Electrochem. Soc., 138 (1991) 1548, 1554. R.Newman and R.M. Chrenko, Phys. Rev. 114 (1959) 1507. R.J. Powell and W.E. Spicer, Phys. Rev. B 2 (1970) 2182. J.S.E.M. Svensson and C.G. Granqvist, Appl. Opt. 26 (1987) 1554. F.P. Kober, J. Electrochem. Soc. 112 (1965) 1064. P. Oliva, J. Leonardi, J.F. Laurent, C. Delmas, J.J. Braconnier, M. Figlarz, F. Fievet and A. de Guibert, J. Power Sources 8 (1982) 229. P.C. Yu and C.M. Lampert, Sol. Energy Mater. 19 (1989) 16. G. Nazri, D.A. Corrigan and S.P. Maheswari, Langmuir 5 (1989) 17. J. Nagai, Proc. SPIE 1016 (1988) 28. J. Nagai, T. Kamimori and M. Mizuhashi, Sol. Energy Mater. 14 (1986) 175.
Solar Energy Materials and Solar Cells
Characterization of evaporated nickel oxide and its application to electrochromic glazing Junichi N a g a i Advanced Glass R & D Center, Asahi Glass Co., Ltd. 1150 Hazawa-Cho, Kanagawa-Ku, Yokohama 221, Japan Received 25 January 1993; in revised form 17 May 1993 In this work, optical and electrochemical properties of electrochromic NiO x films and their application to electrochromic glazing are studied. Quartz crystal microbalance (QCM) study reveals that O H - insertion/extraction occurs during coloration and bleach reaction of NiO x film in an organic electrolyte. This process is also confirmed by reflectance m e a s u r e m e n t by FTIR. Electrochromic glazing consisting of W O 3 / P E O - b a s e d Li ÷ electrolyte/NiO x system shows promising solar control properties. The possibility towards NIR-reflectance control using W O 3 is exemplified.
I. Introduction
Electrochromic glazings are the state of the art in solar energy materials [1,2]. In contrast to light scattering windows using a liquid crystal, the electrochromics are attractive having translucent and solar gain control properties. Their comparatively slow responses (about 3 minutes for a 30 × 30 cm device) may not cause a practical problem because the intensity of sunlight changes gradually. WO 3 films have been widely used for cathodes for the electrochromic glazings. They have been proved to be stable and thus suitable for this purpose as cathodes. NiO x films have been intensively studied as candidate materials serving as anodes. At 633 nm wavelength, they have better coloration efficiency (28 cm2/C) than IrO 2 films (10 - 15 c m 2 / C ) [3]. However, the electrochemical mechanism that leads to electrochromism in NiO based coatings is still controversial. Summarizing the proposed mechanisms so far for electrochromic reactions of nickel oxide, hydroxides and oxyhydroxides, the following three are typical [4,5]: NiO + O H - ~
NiOOH + e-,
(1)
Ni(OH)2 + O H - . ~ N i O O H + H 2 0 + e - ,
(2)
Ni(OH)2 ~ N i O O H + H + + e - .
(3)
In this paper, the fundamental properties of nickel oxyhydroxides are firstly shown. Also the device properties using W O 3 / P E O - b a s e d Li+-electrolyte/NiOx and other WO3-based devices are analyzed and discussed. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
J. Nagai / Euaporated nickel oxide
292
2. Experimental details 2.1. Fabrication o f WO 3 and NiO x films W O 3 and NiO x films are deposited by electron beam (EB) evaporation of the corresponding W O 3 (99.9%) and NiO (99.9%) on ITO coated glass (10 1 2 / [ ] ) as previously reported. During the evaporations, 0 2 gas is introduced to keep a constant pressure of 1 x 10 - 4 Torr and 8 X 10 -5 Torr for WO 3 and NiO x, respectively, and the substrates are not heated unless otherwise specified. Deposition rates are about 10 , ~ / s for WO 3 and 1-2 , ~ / s for NiO x films, respectively. Prior to the depositions, the vacuum chamber is evacuated to 1 x 10 -5 Torr. According to X R D analysis, the WO 3 film is amorphous and the NiO x shows a preferential orientation of cubic NiO ( l i D .
2.2. Electrochemical measurements
Electrochemical measurements were reported previously [2]. Cyclic voltammograms with a scan rate v = 50 m V / s and AC-impedance spectra were measured by the three-electrode system. To measure the impedance of the electrode, a small AC-signal (5 mVo_ o, f = 1 kHz) was superposed on a triangular wave (v = 10 m V / s ) . Then the AC potential and current signals were analyzed simultaneously by two phase lock-in amplifiers. The complex impedance is expressed in terms of a series connection of a resistor (resistance; R m) and a capacitor (capacitance; Cm). y-butyrolactone with 1 M LiC104 (ca. 1000 ppm H 2 0 ) and aqueous electrolytes with 1 M LiCIO 4 or LiOH are mostly used. Other electrolytes are described in the later sections if necessary. 2.3. Quartz crystal microbalance measurements
The schematic of the quartz crystal microbalance (QCM) method [6,7] is shown in fig. 1. A 10 MHz A T cut quartz is sealed to the bottom of a 6 mm i.d. glass tube.
~ Glass
Coulomb
I-L ~
I COmputer I I
Tub .l I ~-~.pt I I ,7 L' II
Electr°lytel~ E C II I I film I I Seal. kl, ~----~U Ouartz- ~ ]'_ , Au" I
I Frequencyl
Au "~
I
I C°unter I I
I
Oscill't°rl [ Circuit I I
Fig. 1. Diagram of the quartz crystal microbalance (QCM) method.
J. Nagai / Evaporated nickel oxide
293
The electrolyte used was acetone (99.5%) with 0.01 M LiC10 4. This method was unsuccessful for other organic electrolytes such as y-butyrolactone, propylene carbonate and dimethoxyethane with 0.01 M LiC10 4 because no stable oscillation of the quartz was generated. The increase of mass of the film is calculated from the decrease in resonant frequency by A f = - 2( ptx ) - 1 / 2 f 2 A m / A ,
(4)
where A f is the change of resonant frequency (Hz), p the density of quartz (2.648 g / c m 3 ) , / z the shear modulus (2.947 × 10 tt dyne/cmZ), f0 the resonant frequency (Hz), Am the mass increase (g), and A the area of the electrode (cm 2) The precision of the present system was evaluated by analyzing the electrodeposition of silver from a 0.01 M AgCIO 4 aqueous solution onto gold and was found to be + 5% of the molecular weight of silver. 2.4. Optical measurement Optical measurements in the UV to NIR region and the IR were conducted using a spectrophotometer (U-3500, Hitachi) and a F T I R (model 4200, Shimadzu), respectively.
3. E x p e r i m e n t a l
results on NiO x films
3.1. Electrochromic reaction After deposition of the NiO x film onto an ITO coated glass substrate, it is immersed in 1 M LiC1OJy-butyrolactone or 1 M L i C 1 0 4 / H 2 0 . Typical cyclic
I
I
l
I
NiOx MI /i-/
/
j
/
<
EO -I
q
i n lff L i C I O J T -BL i n lff L i C 1 0 J H20
I
I
-1.0
-0.5
I
0 0.5 1.0 E [VvsSCE]
Fig. 2. Cyclic voltammograms of NiO~ film. (Thickness = 2 I~m, scan rate = 50 m V / s ) .
294
J.
E
Nagai / Euaporated nickel oxide
2 inlM-l'iOHoq
I
% E
rr
o eq J LL
8
%
'E t._)
0
i
-1.0
I
0 V vs SeE/volt
1.0
Fig. 3. I m p e d a n c e s p e c t r a o f N i O x film. ( T h i c k n e s s = 2 ~ m , s c a n r a t e = 10 m V / s , a r e a = 0.25 c m e ) .
voltammograms generated for these experiments are shown in fig. 2. The current density becomes larger in the aqueous electrolyte. Fig. 3 shows the potential dependence of R m and 1/C2m for 1 M L i O H / H E O . The resistance decreases as the coloration proceeds while the capacitance increases (i.e. decrease of 1/C2m). This shows that the carrier density in the NiO x increased by coloration. In fig. 4, the linear relationship between increased mass and injected charge during the coloration is shown. Above 2 mC (i.e. 10 mC/cm2), NiOx shows an irreversible reaction, which might cause the scattering of the data in fig. 4.
1.0
.-1
Mass Change=17.66g/F 0.8 0.01MLiClO4/Acetone "0.196¢m2 EB NiOx / /
/
01 O1
:E
O.6
"10 01
El
yo°
0, 0.2
o
0
1
o
2
3
4
Injected Chorge [mC] Fig. 4. Massincrease of colored NiOx.
J. Nagai / Euaporated nickel oxide
0.1
'
I
'
295
. . . .
I
EB Ni0x (0.581J,m) ~3 Colored/(~SrnC/cm 2) ~2 V4 /
V
"i 0.05 t-
00.3
,
I 0.5
,
,
,
,
Wavelength
I 1.0
,
,
,
I
1.5
h [l~m]
Fig. 5. Extinction coefficient of NiO x film.
Using a linear least squares fit, the mass change was determined from fig. 4 and was 17.66 g / F with a correlation coefficient of 0.94, which points at O H - injection during the coloration. Thus eqs. (2) and (3) are ruled out for electrochromic reaction of NiO x film. However in eq. (2), if water formed during coloration is not released from the host but adsorbed, then the following O H - insertion mechanism can be expected: Ni(OH)2 + O H - ~
NiOOH • H20 + e-.
(5)
Although a more elaborate and extensive study is required to identify the species of ions responsible for electrochromism, the present result is sufficient to prove that the mass of the inserted or extracted ion is heavier than proton or lithium. 3.2. Optical properties
In the UV to N I R region, the transmittance and reflectance of NiO films were measured to determine the complex optical constant (n, k). In fig. 5, the obtained extinction coefficients k are plotted with respect to wavelength before and after coloration. The refractive index n is almost constant (n = 2.0) in this region. Small absorption peaks are present in the spectrum. In fig. 6, the absorption spectrum is drawn for an aqueous solution of NiSO 4 which has almost the same absorption peaks as shown in fig. 5. This result shows that the evaporated N i O x film has the character of a hydrated nickel ion, i.e. N i ( H 2 0 ) 2+ [8-10]. In the IR region, the absorption spectra are derived from reflectance measurements where the nickel oxides were deposited on gold coated glasses. Fig. 7 shows the absorption spectra thus obtained for the deposited and colored films using 1 M LiCIOa/3,-butyrolactone. In the colored state a broad absorbance peak between
296
J. Nagai / Evaporated nickel oxide
1 ' I ....
I
. . . .
NiSO 4 lOmM aq. ( d : l c m )
U t-O kO t/I
v3
0
.
5
<
~ V2
00.3
V~
0. 5
1.0
1.5
Wavelength ~, [lain] Fig. 6. Absorption spectrum of nickel sulfate.
3600 and 3650 cm - t (due to free O H ) is increased [11-14]. This evidence also supports that O H - insertion takes place during the coloration. As broad absorption due to O H - is found in the F T I R spectrum, the evaporated nickel oxide is more like NiO x (slightly hydrated) than Ni(OH) 2. Therefore, the most plausible electrochromic reaction of NiO x is expected to be eq. (1). The O H - insertion takes place through defects in the NiOx structure, because a fully crystallized NiO film did not show electrochromism in y-butyrolactone with lithium perchlorate which is very similar to the result of ref. [5].
I
I
r
Cotored (20mC/cm 2) ta"}
0,1
Ii
~o, o5 ~
I
0
As deposited <
0,1 ~05
o r-~, 4000
2000
1400 Wave number [cm -1]
Fig. 7. FFIR spectra of NiOx films.
800
400
J. Nagai / Evaporated nickel oxide
297
4. Evaluation of electrochromic glazings 4.1. WO 3 / P E O - L i C I O 4 / NiO x system Using a liquid electrolyte of P E O (MW 200) with 1 M lithium perchlorate and 1000 p p m water, an electrochromic device was fabricated. Between the W O 3 and NiO x electrodes separated by 5 0 / z m , the liquid electrolyte was back-filled. Fig. 8 shows the spectral transmittance changes. The inset in fig. 8 indicates visible and solar transmittances and reflectances, where the following notations are used: Tv; visible transmittance, TE; solar transmittance, R v ; visible reflectance, RE; solar reflectance. The device shows a greyish neutral coloration and a wide modulation of solar transmittance is advantageous for future energy efficient windows. 4.2. Towards the modulation of N I R reflection The electrochromic glazing based on amorphous W O 3 has a heat absorbing property. Regarding the solar efficiency, a variable heat reflective system is considered to be superior to a heat absorbing one [15]. It is well-known that the amorphous tungsten bronze LixWO 3 shows metallic conduction when x > 0.16. In
60.0
60-0"
# I
o tO
50.040.0
G/ ITO/WO3/PEO-LiCIO4/NiOx/ITO/G
A(a) / k~ _
7o.0
/
Tv(%)
TE(%)
Rv(%) RE(~)
k~
(a)
70.3
50.6
8.1
12.2
\
(b)
3"7.0
24.9
8.9
13.7
(c)
\
6,
"
"~ 3 0 . 0 (-
20.0 I0.0 0.0
200
soo'
t 006 Wavelength
t soo
"'
zooo
[nm]
Fig. 8. Transmittance spectrum of electrochromic device (WO 3 and NiOx based device, (a) completely bleached state, (b) intermediate state, (c) colored state).
298
J. Nagai / Ecaporated nickel oxide
Table 1 Optical properties of WO3 based electrochromic glazing Tv 71.4 54.9 22.4 9.8 7.9
TE 44.3 25.7 8.9 3.8 2.9
Rv 14,8 9,9 4.4 5.9 5.7
RE 18.2 15.5 11.7 18.9 20.1
x in LixWO3 0 0.024 0.106 0.182 0.202
the present work, the relationship between coloration level and optical properties were obtained to examine the possibility of variable heat reflectance. An electrochromic device having the following construction as previously published [16]: glass (2 ram) / I T O / W O 3 / e l e c t r o l y t e / I T O / g l a s s
(2 ram),
was assembled for optical measurements. In this experiment, W O 3 was deposited at a pressure of 8 × 10 -5 Torr 0 2 and the substrate temperature was 330°C to have a dense film. Applying a constant voltage to the device, transmittance and reflectance from W O 3 side were measured at saturation level of the coloration. In table 1 are listed the fabrication condition of W O 3 and optical properties. As the visible transmittance decreases, the solar reflectance decreases due to the heat absorbing nature. However, after the visible transmittance reaches 9.8%, the solar reflectance begins to increase. At this critical transmittance, the value of x in LixWO 3 can be calculated from the following equation: x = Q[WO3]/(FAdp)
= 0.182,
(5)
where Q is the injected charge = AOD/~7 = 0.0216 C / c m 2, A O D the optical density change = 1og~0(71.4/9.8), r/ the coloration e f f i c i e n c y - 40 cm2/C, F the Faraday constant, p the density of W O 3 film = 5.7 g / c m 3, and A the area of W O 3 film with thickness d = 500 nm. It can be verified that a device having a electron density above the percolation threshold shows a heat reflective property. Although a further study is necessary, increase of electron density is the key for variable heat reflection for amorphous W O 3. To achieve this, control of crystallinity and the usage of thinner films will be required.
5. Conclusion
The optical and electrochemical properties of electrochromic NiO x films were analyzed. In the organic electrolyte, the coloration reaction appears to be O H insertion. Electrochromic glazing consisting of W O 3 / P E O - b a s e d Li ÷ electrolyte/ NiO x shows a promising solar control property. Towards the modulation of N I R reflection using the LixWO 3 based device, deep coloration gives an increase in
J. Nagai / Evaporated nickel oxide
299
N I R reflectance. Further study is now in progress to realize a solar energy efficient window.
Acknowledgements The author would like to thank Mr. T. Seike for carrying out the thin film fabrications and G.D. McMeeking for his helpful discussions relevant to this work. He also wishes to thank Dr. C.M. Lampert for his continued encouragement.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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