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Electrochromic NiOx Film by DC Sputtering
Xu Yanzhong, Qiu Muqing, Qiu Sichou, Dai Jing, Hunag Hanrou, Cao Guangjun
To cite this version:
Xu Yanzhong, Qiu Muqing, Qiu Sichou, Dai Jing, Hunag Hanrou, et al.. Electrochromic NiOx Film by DC Sputtering. Journal de Physique III, EDP Sciences, 1995, 5 (10), pp.1491-1499.
�10.1051/jp3:1995206�. �jpa-00249397�
Classification Physics Abstracts 81.15C
Electrochromic Niox Film by DC Sputtering(*)
Xu Yanzhong, Qiu Muqing, Qiu Sichou, Dai Jing, Hunag Hanrou and Cao Guangjun Dept. of Solid State Electronics, Huazhong University of Science Ic Technology, Wuhan, Hubei, P-R-C-, China
(Received it October 1994, revised 3 April 1995, accepted 28 June 1995)
Abstract. DC sputtering technique was applied to prepare NiO~ film, in which nickel hy- droxide was used directly
as the target. XRD and SEM examination show that the film is uniform and full of small granules of almost the same size. Slight difference exists between the
as-deposited film and the colored film. Some weak crystal peaks appear in the latter film. The film has good electrochromic property. The content of oxygen in the vacuum chamber has no obvious effect on the electrochromic property of the film. XPS was used to investigate the nickel atomic state in the film. The result shows that Ni~+ is oxided into Ni~+ with coloring. The
transmittance of the film can vary from 5il to 74il in the visible range.
1. Introduction
In this paper, an electrochromic film grown by using DC sputtering and a novel target of nickel hydroxide is presented. The work was carried out under the support of the Natural
Science Fund committee of China. It was showed by structural, electrochemical and optical investigations that the film has good electrochromic property and relatively long time stability.
The film has good initial electrochromic property without activation. Some small structural
changes were found after several cycles of coloring and bleach. The galvanostatic curves also showed slight differences between as-deposited film and the film after several cycles.
Historically, electrochromic technology has been widely researched for electronic information displays and other small area applications. Recently, a great deal of research and development
have been focused on large area electrochromic windows. Many efforts have been paid on the examination of the dynamics and structures of the electrochromic materials. In the past few years, tungsten oxide [1-5], Prussian Blue [6,7] were widely studied. Because the Ni(OH)2
film has good stability, low cost, much attention has been focused on the material as well.
Among different preparation ways like anodical and electrochemical deposition, electron beam evaporation, sputtering and oxidation of nickel at high temperature, the electrochemical de- position was first applied to prepare the nickel hydroxide electrochromic film, for the method
was one of the simplest among the preparation methods of electrochromic films. The cost of (") Supported by NSFC
© Les Editions de Physique 1995
1492 JOURNAL DE PHYSIQUE III N°10
the method is low, and large area film can be easily produced in this way. References [9-12]
gave details of the method and the properties of the electrochromic materials. Usually, the film by electrochemical method has good electrochromism, it does not need activation after the deposition of film, and the response time is short. This means its switching times both from coloring to bleach and the reverse change are short. This property is vital in information
displays. Unfortunately, the film is not stable enough for practical use, it degrades after bleach and coloring cycles. The adherence of the film is poor, even after treatment under relatively high temperature of 80 degrees, there is not improvement. We found the film irreversibly
lost the electrochromic property after storage for one or two weeks under dry room conditions rather than in aqueous solution. To improve the property of nickel hydroxide film, people turn their attention to vacuum preparation method. In reference [14,15] the mechanism of sput- tered nickel-oxide film by nuclear analysis was investigated, which revealed that coloration of
nickel-oxide occurs upon hydrogen extraction. Metal nickel was used as the reactive sputtering target [16] as well. Both in our past experiments and reference [16], it was found that the variation of optical density was lower than that from the electrochemical deposition. The film needed to be hydrated by a long time treatment in KOH, because the film lacked the initial
electrochromic property. Reference [17] described the e~-beam evaporation of NiO(OH)~ film.
It is usually believed that a redox reaction corresponding to coloring and bleach of the electrochromic film-
NiOOH + H+ + e~ # Ni(OH)2 (1)
Ni(OH)2 is colorless, but NiOOH is dark brown. With the double injection and extraction of protons and electrons, the film reversibly changes its color.
Nevertheless, the actual reaction is quite complex, the nickel oxide and nickel hydroxide have several structures [18]. The film from sputtering is amorphous, and usually non-stoichiometric.
This determines that the coloring and bleach procedures are complicated. Hence any novel solution and study would be worthy its effort toward the practical application of the elec- trochromic materials. To improve the electrochromic property, different methods and target materials indicated above have been applied. However, there is no report about the film from the nickel hydroxide target. We directly use the nickel hydroxide as the sputtering target. The results are satisfactory. The film has the initial electrochromic property without activation.
This feature will make the technology fit quick preparation requirement and we studied the structural change between the as-deposited film and used film. There is not publication dealing
this property of the film yet-
2. Experimental
2.I. PREPARATION oF THE TARGET. AR grade nickel sulphate was dissolved in a beaker with the distilled water. AR grade potassium hydroxide was added into the solution. The mixture was stirred thoroughly in a magnetic stirrer. The light blue nickel hydroxide precipitate would occur according to the next formula.
NiS04 + KOH # K2S04 + Ni(OH)~ i
After thorough wash, the result precipitate was dried by an infrared lamp at the temperature about 120 °C to lso °C- Then the solid nickel hydroxide was crushed and sieved. The target
was preparated by compacting the powder in a mould of 50 mm diameter under the pressure of lso kg/cm~.
2.2. DEPOSITION oF THE NICKEL HYDROXIDE FILM. The substrate was cleaned by a supersonic cleaner in methyl benzene, acetone ethanol and distilled water in sequential, then the substrate glass was dried. The vacuum chamber was first evacuated to 6-8 mPa and kept for 30 min. After that, argon and oxygen gases were introduced into the vacuum chamber.
The pressure in the vacuum chamber was maintained at loo mPa in deposition of the film.
The distance from the target to the substrate was 4-6 cm. The substrate was not heated. The
sputtering voltage was about 700 V. Oxygen flux was adjusted in a wide range. But it was found that the change of oxygen flux had no obvious influence on the electrochromic property of the film. To investigate the structure of the film, two kinds of samples were prepared; one
was the film on the glass, which was used to examine the property of the as deposited film,
the second was the film deposited onto the ITO coated glass, which was used in comparing the difference with the as-deposited film. The sheet resistance of the ITO coated glass was about 20 ohm /square.
2.3. STRUCTURE OF THE NICKEL HYDROXIDE FILM. The morphology of the film was
examined by SEM- The structure of the film was examined by XRD and XPS. The results are shown in Figures 1, 2 and 3- Two kinds of specimen were tested to investigate the structure of the nickel hydroxide film. One is as-deposited film. Another is the film after several bleach
and coloring cycles, and in the colored state for measurement.
2.4. DEVICE STRUCTURE AND ITS ELECTROCHROMISM. The electrochromic device was
incorporated like in Figure 4. The platinum sheet served as the counter electrode. The elec-
trolyte was lM potassium hydroxide. A microcomputer system was developed to examine the electrochromism of the film, and a spectrophotometer was used to measure the optical prop-
erties of the nickel-hydroxide film. The description of the measuring system was reported in reference [13]. In our experiments, the current-voltage relation, current-time relation and the
optical electric charge relation were obtainable. Because the galvanostatic measurement was the typical electrochemical method investigating the electrochemical property of the film, only result of current-voltage relation was presented in this report.
3. Results and Discussion
Figure I shows image of the morphology of the nickel hydroxide film. It is obvious that the film is uniform and full of small granules. The size of the granules is about 0.2 ~m. XRD in
Figure 4a shows that the deposited film on bare glass is amorphous without obvious crystal peaks in the curve. After several bleach and coloring cycles, a colored film on the ITO coated glass substrate was re-examined, the structural changes. It is easily to speculate that there should be no peaks except the ITO peaks in the XRD spectrum. But there are weak crystal peaks relating the nickel appear. They are Ni(OH)2[100][III], NiOOH[l10] [III][210][200], in which NiOOH[all] is a relatively obvious peak. Since NiOOH is dark brown, that NiOOH
peaks are more obvious coincident with the colored state of the film. As there is no other procedure that may change the structure of the film. The crystal peaks occurred in the XRD
can only contribute to the electrochemical cycles of coloring and bleach.
XPS spectra of the as-deposited film and the colored film are shown in Figure 3. It is obvious that there exist three valence states of nickel in both cases, that is metal nickel, Ni~+
state and Ni~~ state, in which Ni~+ corresponds to the colored, while Ni~+ to the transparent.
Comparing Figure 3a and b, we can found that the Ni~+ decreases, while Ni~+ increases when the film is colored. That corresponds to the oxidation of Ni~+ to Ni~+ in the expression (1).
1494 JOURNAL DE PHYSIQUE III N°10
a)
b)
Fig- 1. Morphology of NiO~ film. The film is full of uniform granules about 0-2 pm size.
The results have the same meaning indicated in the XRD spectrum. That is there should be
more NiOOH than Ni(OH)2 in the colored film.
The results of galvanostatic cycle show that there is small difference between the first cycle
and the following cycles. The inner loop is the first cycle, the following loops gradually expand
and approach the out loop after several cycles. The procedure is indicated by arrows in Figure 5.
This may be due to the transformation of the film reflected in the XRD spectrum. Galvanostatic
curve is important in investigating the reversibility in the electrochemical reaction. If there is
no side reaction, the area under positive current will be equal to that under negative current.
In our case, the reversibility is shown to be good. At +1.4V, there is a very strong peak, which
is the upper bound for the film's operation under positive direction. Over this biJund will
directly cause electrolysis of the electrolyte. In fact, we observed obvious small bulbs, when the bias voltage was further increased. The quick strong electrolysis is detrimental both to the
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curves. a) after several cycles of bleach and coloring and in the colored
state. There are some weak crystal peaks appear. b) the as-deposited film. This is a typical amorphous
film's diffraction curve.
film and device, since emerging gases make the encapsulation difficult and severely decrease the adherence of the electrochromic film and ITO films.
The optical transmittances of the film under different voltages are shown in Figure 6. The op- tical density varies from 5i~ to 72$l at 550 nm wavelength. Large magnitude of optical variation is preferable to film's applications on energy efficient windows and non-emissive information
displays. In the measurement, transmission includes not only that from the electrochromic film
1496 JOURNAL DE PHYSIQUE III N°10
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CLASS
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Fig- 4. The structure of the electrochromic devices. EC layer is the electrochromic film layer, while CE is platinum and WE is ITO layers. EL is I M KOH solution.
but also from electrolyte, substrate glass and ITO film. Commercially available ITO coated
glass, which we used in experiments, has 85$l transmission in average. Counting on this base transmission loss, the test results indicate that the film has good electrochromism. Durability
to repeated redox cycles is an important parameter. To investigate this property, the film was set in extended color-bleach cycles to find its durability. There was no obvious degrading up
to lo~ redox cycles. Another interesting phenomenon is that the nickel hydroxide has good
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1498 JOURNAL DE PHYSIQUE III N°10
electrochromism even after the film has been stored under the dry room condition for over six
months, instead of in aqueous solution. Contrasting with the loss of electrochromic property of the film from the electrochemical deposition, that exhibits this film is stable.
4. Conclusion
Nickel hydroxide film have been prepared by DC sputtering. Nickel hydroxide was directly used as target material. Comparing to the films from electrochemical deposition and that from reactive sputtering from the metal nickel target, the film has the following advantages: I) good adherence, it) initial electrochromic property without activation, iii) satisfactory reversibility, iv) relatively larger magnitude of variation in optical transmittance. There is no obvious
degrading up to lo~ times cycles of bleach and coloring. And the film still showed good
electrochromic property after a long time of storage under room dry condition. SEM, XRD results showed the as-deposited film is uniformly amorphous film- But there are some weak
crystal peaks appear in the XRD after cycles of coloring and bleach, which indicates that there is some crystallization. Galvanostatic curve showed the film has satisfactory reversibility, and
the upper bound for positive operation is +1.4V. Above this limit will damage the device and
the film- Some minor differences in the galvanostatic curve have been observed, which may
correspond to the structure change of the film. The transmittances spectrum of the film showed the optical density might change as high as 70$l.
Acknowledgments
The authors are grateful for the help from Prof. Li Naipin, Zhang Xin and Liu Lili. We also like to thank the anonymous reviewers for their suggestion.
References
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