Degradation of the electrochromic nickel oxide film upon redox cycling

ELSEVIER

Thin Solid Films 280 (1996) 233-237

Degradation of the electrochromic nickel oxide film upon redox cycling Yoshiji.ro Ushio, Akira Ishikawa, Tatsuo Niwa Main Research Laboratory, NIKON Corp., 1-6-3 Nishi-Ooi, Shinagawa-ku. Tokyo 140. Japan

Received 26 July 1995; accepted 30 October 1995

Abstract The degradation mechanism of electrochromic nickel oxide films by redox cycles was investigated. Films fabricated by sputtering of an oxide target were driven between the colored (oxidized) and bleached (reduced) states in 1 M KOH solution. With the cycles, the transmittance of the colored state decreased. While the peak potential of the redox became larger at first and then became smaller, the response time became rapid at first and slower afterward. The surface of heavily degraded films became uneven and easy to be removed. A higher oxidation voltage caused faster degradation. Heat treatment during deposition or making a film thicker was shown to affect durability. Keywords: Electrochemistry;Nickel oxide; Optical properties; Sputtering

I. Introduction

The nickel oxide film that is popular as a battery electrode has been also known to function as an electrochromic device. The electrochromic nickel oxide film is transparent for visible light in the reduced state and displays a brown color in the oxidized state. It has attracted attention because of the brown color, the coloration efficiency and the low cost for the application to the display or light controlling device. Various methods such as evaporation [1-5], electrodeposition [6-12], sputtering [ 13-22] or others have been attempted for the preparation, and the electrochromic properties of each film have been investigated. Unlike the tungsten oxide film, the electrochromic nickel oxide film has hardly been applied for practical use. The poor durability of that film is still one of the problems. [ 1,23,24 ] Some films formed by sputtering were reported to have good durability [ 16,17] and the addition of some metal ions was shown to be effective for the improvement of durability. [9,25 ] However, there have been not so many studies about the mechanism of the degradation of electrochromic nickel oxide film by redox (coloration-bleaching) cycles. In this study, the degradation of electrochromic properties of nickel oxide films prepared by sputtering were investigated. The changes of structure, composition, coloration efficiency, and cyclic voltammogram were measured for investigating the mechanism of degradation. 0040-6090/96/$15.00 © 1996Elsevier Science S.A. All rights reserved SSD!0040-6090(95 )08211-5

2. Experimental Nickel oxide fihns were deposited by r.f. magnetron sputtering on glass substrates coated with an ITO electrode film. The ITO film had 0.2 ~m thickness and 10 ~ cm -2 sheet resistivity. The conditions of film preparation are shown in Table 1. Conditions of deposition were determined so that the film had a widely variable range of transmittance as an electrochromic device. The redox drive of films was conducted in 1 M KOH aqueous solution. Pt mesh and SCE were used as the counter electrode and the reference electrode, respectively. The redox was conducted by a square wave of 10 s reduction and 10 s oxidation. The voltage level of a square wave was set at 500 mV, 1 000 mV, and 1 500 mV vs. SCE. The optical transmittances of colored and bleached states were measured by the C standard source (one of the CIE standard light source) Table 1 Sputtering conditions for filmpreparation Target Substrate Thickness Sputtering gas Sputtering pressure R.f. power Rate Substrate temperature

NiO ( sintemd disk of 5 inch ~b) ITO film on glass 0.1 Ixm Ar + O2 (partial pressure 1/ 3) 3 mTon" 400W (3.16 W cm-:) 20 A min40 °C (water cooling)/200 °C

234

Y. Ushio et al. / Thin Solid Films 280 (1996) 233-237

with luminous efficiency filter. The injected charge and cyclic voltammogram (scan rate was 20 mV min - l ) were also measured. The change of composition and structure of films were monitored by scanning electron microscopy (SEM), Auger electron spectroscopy (AMES), and X-ray phcloelectron spectroscopy (XPS) measurements.

3. Results

The film was assured to have a polycrystalline structure from X-ray diffraction. The optical transmittance was about 100

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Colored

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300

400

500

600

700

500

Wavelength(rim) Fig. 1. Spectral transmittance of a NiO film at - 500 mV and + 1 000 mV

40% in the as-deposited film. By applying a cathodic bias once in KOH solution, the film became transparent (T,,- 70%). The spectroscopic transmittance of bleached and colored states of a film are shown in Fig. 1. Fig. 2 shows the change of transmittance of colored and bleached states, injected charge (Q~ and coloration efficiency AOD/Q under cycling between - 500 and + 500 mV. Here, A OD represents the difference of optical density between colored and bleached states, and - means reduction and + means oxidation of the film. Both the transmittance and the injected charge changed little after about 4 000 cycles (cycling for 23 h). The changes under cycling between - 500 and + 1 000 mV are shown in Fig. 3. The decrease of transmittance and the increase of injected charge were observed. (As a result, the coloration efficiency became smaller.) Under cycling between - 500 and + 1 500 mV, the changes of values proceeded more rapidly as shown in Fig. 4. The injected charge showed the decrease after an initial increase. The change of voltage for reduction ( - 500 mV, - 1 000 mV, - 1 500 mV) was proved to little affect the change of the electrochromic properties. The cyclic voltammogram (CV) curve scarcely changed under cycling between - 500 and + 500 mV. The cycling of a higher oxidized voltage caused a change of the CV curve. Fig. 5 shows the change of CV curve under cycling between - 5 0 0 and + 1 500 mV. As the degradation (the change of

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Time(h) Fig. 2. Change of transmittance (a), injected charge, and coloration efficiency (b) under cycling between - 5 0 0 and +500 mY. Transmittance is measured by the C standard light with a luminous efficiency filter.

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Time(h) Fig. 3. Change of transmittance (a), injected charge, and coloration efficiency (b) under cycling between - 5 0 0 and + 1 000 mV. Transmittance is measured by the C standard light with a luminous efficiency filter.

235

Y. Ushio et a l . / Thin Solid Fihns 280 (1996) 233-237 80

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Time(h) Fig. 4. Change of transmittance (a), injected charge, and coloration efficiency (b) under cycling between - 5 0 0 and + 1 500 mV. Transmittance is measured by the C standard light with a luminous efficiency filter.

transmittance) proceeded, the potential position of anodic and cathodic peak shifted positively and negatively, respectively. (2.5 h after) When the cycling was continued further (3.5 h after), both peaks shifted back to reverse directions. These changes of peak positions correlated with the change of the injected charge. Similarly, it was seen that the response time of coloration and bleaching for a step bias became longer at the first phase of degradation and shorter afterward. When the degradation proceeded heavily, the surface of the film became rough. The ~canning micrographs of the surface after redox cycles (cycling between - 5 0 0 and

+ 1 500 mV for 4 h) are shown in Fig. 6. The roughness of the order of several microns appeared and the rough region of surface was very easily removed by rubbing. After this region was removed, the inner part of the film showed the same electrochromic behavior as an as-deposited film. AES measurement indicated the precipitation of K in the surface region of the heavily degraded films. No change of composition of Ni or O was observed. In the XPS measurements, the differences of peak position of nickel (Ni 2p~/2) for as-deposited, colored and bleached states were not observed. However, the degraded films

NI 2p3/2 oxidized

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Fig. 5. Cyclic voltammograms after the cycling between - 500 and + 1 500 mV.

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Fig. 7. Depth profiles of Ni 2p3/2 signals in XPS measurement.

236

Y. Ushio et el. /Thin Solid Fihns 280 (1996) 233-237

showed the distinct different valence conditions of nickel. Fig. 7 shows the profile of the Ni 2p3/2 peak at various times of Ar ion e,tehing. The surface region of a degraded film seems to be in a more reduced condition. The profiles of valence conditions like this were not observed in any unaegraded films. In the film deposited at 200 °C, the injected charge and the range of transmittance variation were smaller than films deposited at lower temperature, as shown in Fig. 8(a). Fig. 8(b) shows that the progress of degradation at the same voltage became slower compared with the film deposited at lower temperature. A higher oxidation voltage caused faster degradation (the change of transmittance) also in this case. Contrary to the case of fihns deposited at lower temperature, the surface of a heated film remained smooth even after heavy degradation. The thick films (about 0.25 la,m thick) prepared with the same conditions of sputtering were also measured. A higher reduction voltage was needed for making a thick film transparent. They showed the variable transmittances of 80% ~ 30% by the drive of - 1 500 mV <-->+ 1 500 mV. The changes of the transmittances were small after about 1 500 cycles (for 8 h) (Fig. 9).

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4. Discussion

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Time(h) Fig. 8. Change of transmittance (a), injected charge, and coloration efficiency (b) of a film deposited at 200 °C under cycling between - 5 0 0 and + ! 000 mV, Transmittance is measured by the C standard light with a luminous efficiency filter.

Seeing from the peak shifts of tile CV curve, progressive degradation accompanied the formation of a phase that could be hardly oxidized or reduced. The growth of this stable phase seemed to occur in the surface region. The observed structural fragility of this layer was perhaps caused by the expansion of films at reduction or the contraction at oxidation. The quantity of injected charge when colored was considered to play an important role in the degradation considering that the higher oxidized voltage led to the rapid degradation. After the degradation proceeded to some extent, the injected charge and the response time began to become small. These are perhaps due to the fact that the effective thickness of the active electrochromic layer became thinner by the complete formation of the passive phase at the surface region. Generally, a nickel oxide film in an aqueous solution is considered to behave as a hydroxide. (Some investigators claimed that an electroehromic nickel oxide film did not react as a hydroxide [26] .) One set of redox reactions accepted is as follows.

Y. Ushio et al. / Thin Solid Films 280 (1996) 233-237

237

charge flow was considered to play an important role in the degradation mechanism. a-Ni(OH)2

~ ~-NiOOH + e" •,¢-,....... ,'~ References

~Ni(OH)2

~

®

~NiOOH + e-

1 and 2 are reactions by electron transfer, and 3 and 4 are chemical reactions [ 25,27 ]. If the reactions above apply to the present case, the passive layer could be estimated as/3Ni(OH)~. or ~/-NiOOH. The CV curve in the present study did not indicate the simple transition from the c~~ 'y cycle to the/3(II) ~ / 3 ( I I I ) cycle. This simple transition was observed in some nickel hydroxide films in previous studies [22,28,291. The XPS results in the present study indicated the reduced phase at surface region of a degraded film. In the case of these transition metals, the surface valence states could be changed easily by the Ar-ion etching. The data could not necessarily reflect the actual valence conditions of films. However, the tendency of reduction seems different from the results of some studies that showed the existence of higher valence states other than Ni 2÷ or Ni a ÷ in the ordinary nickel oxide electrode [ 30,31 ]. The smaller injected cha~rge of a film deposited at 200 °C could be attributed to the better crystalline quality or the small quantity of binding H20 inside the film. The better durability both in transmittance and structural strength of this film show the general tendency that the injection of much charge promoted the progress of degradation. The thick film showed a rather good durability when driven with a large amount of injected charge (per unit volume of film). The larger impedance of the oxidized insulating states of the thick film was confirmed by the shift of peaks of the CV curve. Also the response time was observed to be longer, especially at reduction. Thus, the density of charge flow in a thick film would be smaller than a thin film. These results would indicate that the degradation process depends not on the quantity of total ions injected but on the flux of ions (current density) at redox cycle or the effect of overpotential by the applied voltage.

5. Conclusions The degradation of electrochromic nickel oxide films, the decrease of transmittance and coloration efficiency under redox cycling, was promoted by higher oxidation potential. The degradation occurred from the surface region, and heavily degraded regions became structurally fragile. The injected

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