Growth rate enhancement of liquid phase deposited nickel oxide film on ITO/glass under UV photo-irradiation

Article abstract of DOI:10.1002/pssa.201127322

The growth of nickel oxide film grown on indium-tin oxide/glass substrate by liquid phase deposition is enhanced under ultraviolet photo-irradiation was studied. α-Ni(OH)₂ dominates the composition of as-grown NiO film. After thermal treatment

Full text of DOI:10.1002/pssa.201127322

Phys. Status Solidi A 209, No. 4, 702–707 (2012) / DOI 10.1002/pssa.201127322
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applications and materials science
Growth rate enhancement of liquid
phase deposited nickel oxide film on
ITO/glass under UV photo-irradiation
Ming-Kwei Lee* and Cho-Han Fan
Department of Electrical Engineering, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
Received 2 June 2011, revised 19 December 2011, accepted 1 January 2012
Published online 25 January 2012
Keywords electrochromic, liquid phase deposition, nickel oxide, UV photo-irradiation
* Corresponding author: e-mail mklee@mail.ee.nsysu.edu.tw, Phone: 886 7 5252000, ext: 4120, Fax: 886 7 5254199
The growth of nickel oxide film grown on indium–tin oxide/ treated NiO under ultraviolet photo-irradiation, the recrystal-
glass substrate by liquid phase deposition is enhanced under lization and the colored and bleached transmittance after
ultraviolet photo-irradiation was studied. a-Ni(OH)2 dominates 50 times electrochromic test were improved. Both improve-
the composition of as-grown NiO film. After thermal treatment ments come from fluorine passivation.
at 300 8C, a-Ni(OH)2 is transformed into NiO. For thermally
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction In recent years, nickel oxide (NiO) is irradiation can enhance the reactions and hence the growth
an attractive material for its chemical stability, excellent rate. Therefore, the growth temperature can be kept lower to
optical, and electrical properties [1]. NiO thin films have prevent these problems from high growth temperature.
been studied for applications in electrochromic device [2, 3],
photoelectronic device [4], and MOS gas sensor [5].
In this study, the growth and the electrochromic
characteristics of LPD-NiO films on ITO/glass under UV
There are various methods for the growth of NiO films photo-irradiation (UV-NiO) and thermal treatment were
such as evaporation [6], electrodeposition [7], sputtering [8], investigated.
sol–gel [9], chemical vapor deposition (CVD) [10], chemical
bath deposition (CBD) [11], and liquid phase deposition
2 Experimental details The ITO coated glass with a
(LPD) [12]. LPD has many advantages such as low sheet resistance of 20V cm was used as the substrate in this
temperature (room temperature is permissible), high experiment. The growth solution was prepared with saturated
uniformity, good selectivity, large area, and low cost [12]. NiF2 4H2O and 0.6 M boric acid solutions. The saturated NiF2
It is especially suitable for the deposition on large area and solution was prepared by mixing 10.5g of NiF2 4H2O with
low melting point window glass. The growth rate of LPD- 300 ml deionized (DI) water at room temperature. After
NiO film was very low (50–75 nm in 16–72 h) using filtering NiF2 4H2O precipitate, the saturated NiF2 solution
saturated NiF2 4H2O and H3BO3 as the aqueous precursors was obtained. The 0.6 M boric acid solution was prepared by
at 30 8C [12]. The very low growth rate has to be improved mixing 18.9g pure boric acid with 500 ml DI water. The
for practical applications. The growth rate can be enhanced growth temperature of LPD-NiO was kept at 308C. For UV-
to 25.2 nm/h at the higher growth temperature of 40 8C from NiO growth, a 10W mercury lamp of 254 nm wavelength was
the endothermic reaction [13]. However, the growth used to irradiate the growth solution. The distance between
temperature is limited due to the high vaporization of substrate and lamp is 10cm. Scanning electron microscope
aqueous solution. The growth is also unstable due to the (SEM)wasusedtomeasurethefilmthickness.X-raywasused
convention of the aqueous solution. By UV photo-irradia- to examine the crystalline. X-ray photoelectron spectroscopy
tion, the growth rates of CVD, sol–gel process, and LPD- (XPS) was used to examine the chemical structure.
SiO2 were enhanced [14]. The growth rate is mainly
The UV-NiO and LPD-NiO films were performed using
controlled by the formation and dehydration of intermediate a BRUKER 66v/s for the Fourier-transform infrared
species [15]. The high photon energy of UV photo- spectrometer (FTIR) measurements. The spectra were
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Phys. Status Solidi A 209, No. 4 (2012)
703
recorded from 400 to 4000 cmꢀ1 which was using the KBr
Beam splitter and PE Beam splitter and 808 grazing incident
angle reflectance.
For the EC measurement, part of NiO film was removed
for probing the ITO electrode. Propylene carbonate solution
dissolved with LiClO4 and a platinum (Pt) foil were used as
the electrolyte and the counter electrode. The Liþ electrode
potential is ꢀ3.045 V, which is harder reductive than that of
Kþ of ꢀ2.952 V. The electrolyte with Liþ ion has better
stability for electrochromic device. In addition, the Liþ ion
300
250
200
150
100
50
(1) as-grown LPD-NiO
(2) as-grown UV-NiO
(2)
(1)
˚
with smaller ionic radius (0.78 A) has higher mobility in
0
metal oxide film than Kþ (1.33 A). The coloring and
˚
bleaching speeds of the Liþ ion are faster in device. The
LiClO4 in propylene carbonate solution as the electrolyte is
more appropriate than alkaline electrolyte such as KOH. The
colored and bleached states of NiO/ITO/glass films were
characterized by electrochromic test under ꢁ5 V for 30 s.
The transmittance of NiO/ITO/glass films in the bleached
0
10
20
30
40
50
Growth Time (h)
Figure 1 (onlinecolorat:www.pss-a.com)LPD-NiOandUV-NiO
thicknesses as a function of growth time.
and colored states was determined at the wavelength of 30 8C as a function of the growth time. For LPD-NiO film, the
550 nm because it is the strongest intensity of sun radiation. growth rate is 1.9 nm/h. For UV-NiO film, the growth rate is
The film thickness of UV-NiO and LPD-NiO films were kept much enhanced to 5.9 nm/h. A smooth surface of UV-NiO
at 100 nm for electrochromic test. A reflecting spectrograph film with the thickness of 260 nm grown on ITO/glass
was used to measure the transmittance. The transparency substrate is shown in Fig. 2(a), and LPD-NiO film cross-
ratio is derived from DT% ¼ Tb% ꢀ Tc%, where Tb is the section of grown on ITO/glass substrate is shown in Fig. 2(b).
transmittance of bleaching state, and Tc is the transmittance Figure 2(c) and (d) are top views of UV-NiO and LPD-NiO
of coloring state. The optical density change is derived from film, respectively.
DOD ¼ logðTb%=Tc%Þ. The coloration efficiency is derived
The FTIR spectrum for LPD-NiO film is exhibited in
from h ¼ DOD=q, where q is intercalated/deintercalated Fig. 3(1). The peaks at 3375 and 1600 cmꢀ1 have been
charge per unit area. assigned to H–O bond vibrations of physisorbed and
For the cyclic voltammetry (c–v) measurement, a chemisorbed water linked to NiO film, respectively [16].
CHI627C electrochemical gauge was used with a scanning The peaks at 1460 and 1250 cmꢀ1 are assigned to C–H bond
rate of 1 mVsꢀ1 in the range from 0.2 to 1.2 V.
and C–O stretching vibration from surface contamination
[17, 18]. The very weak peaks at around 1000 cmꢀ1 could be
3 Results and discussion Figure 1 shows the growth ascribed to Si–O and Si–OH [19] from the etching of glass
thicknesses of LPD-NiO and UV-NiO films prepared at substrate during the LPD-NiO deposition. The peak at
Figure 2 (a) and (b) for SEM cross-section,
and (c) and (d) for top views of UV-NiO and
LPD-NiO, respectively.
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M.-K. Lee and C.-H. Fan: Growth rate enhancement of liquid phase deposited NiO film
(2)
(1)
ITO
o
(1) as-grown LPD-NiO
(2) as-grown UV-NiO
(1) UV-NiO treated at 300 C
o
(2) LPD-NiO treated at 300 C
H-O (physisorbed)
ITO
NiO(220)
NiO(200)
Ni-O
Ni-OH
C-H
H-O (chemisorbed)
(1)
(2)
Si-OH
Si-O
4000 3500 3000 2500 2000 1500 1000 500
30
40
50
60
Wavenumber (cm-1)
2θ
Figure 3 (online color at: www.pss-a.com) FTIR spectra for LPD-
NiO and UV-NiO films.
Figure 5 (online color at: www.pss-a.com) X-ray diffraction pat-
terns of LPD-NiO and UV-NiO films.
707 cmꢀ1 corresponds to the vibration of Ni2þ–OH from using Ni(NO3) 6H2O as the precursor [20]. The FTIR
a-Ni(OH)2 [19].
spectrum for UV-NiO film shown in Fig. 3(2) is almost the
The peak 480 cmꢀ1 is attributed to the vibration of same as LPD-NiO. But a stronger peak of a-Ni(OH)2 is
Ni2þ–O from b-Ni(OH)2. a-Ni(OH)2 dominates the com- observed. It comes from higher fluorine (F) passivation [21]
position of as-grown LPD-NiO film. It is similar to the result due to higher F incorporation in the film under UV irradiation
(a)
(c)
(1) LPD-NiO
(2) UV-NiO
Ni
O
O
F
C
Si
(2)
(1)
1000
800
600
400
200
0
Binding Energy (eV)
Binding Energy (eV)
(b)
(d)
(1) LPD-NiO film
(2) UV-NiO film
Ni 2p3/2
Ni 2p1/2
(2)
(1)
(2)
(1)
(1) LPD-NiO treated at 300 oC
(2) UV-NiO treated at 300 oC
890
880
870
860
850
840
700
695
690
685
680
Binding Energy (eV)
Binding Energy (eV)
Figure 4 (online color at: www.pss-a.com) X-ray photoelectron spectrum of LPD-NiO and UV-NiO films.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Phys. Status Solidi A 209, No. 4 (2012)
705
[14]. Because a-Ni(OH)2 has a more disorder structure and
larger interlamellar spacing, the peaks of physisorbed water
at 3375 cmꢀ1 and carbon surface contamination at
1460 cmꢀ1 are stronger.
From a previous study, NiO film shows the best
electrochromic characteristics at the annealing temperature
of 300 8C in air [13, 22]. In this study, the thermal treatment
temperature is kept at 300 8C. The full ESCA spectra for
LPD-NiO and UV-NiO films thermally treated at 300 8C in
air are shown in Fig. 4(a). There are Ni, O, F, C, and Si in the
UV-NiO and LPD-NiO films. The C is from the surface
contamination, and Si is from substrate (ITO/glass). The
clear Ni peaks are shown in Fig. 4(b). The peaks at about 857
and 862 eV are from Ni 2p3/2 and the peaks at about 874 and
882 eV are from Ni 2p1/2 belong to NiO [23]. The peaks of
UV-NiO film are stronger and show red shifts compared with
that of LPD-NiO from more F incorporated in UV-NiO film.
The decomposed Ni 2p3/2 shows 854 and 862 eV belongs to
Ni2þ state in NiO and 858 eV to Ni2þ in Ni(OH)2 as shown in
Fig. 4(c). It indicates that the transformation of Ni(OH)2 to
NiO is not complete under 300 8C thermal annealing in air.
The Ni–F peak at about 687 eV for UV-NiO and LPD-NiO
films are shown in Fig. 4(d). The Ni–F peak intensity of UV-
Figure 6 (online color at: www.pss-a.com) Atomic force micro-
scopy 2D and 3D pictures of (a) LPD-NiO and (b) UV-NiO films.
(a)
(a) 100
(1) LPD-NiO (1 st)
(2) LPD-NiO (50 th)
(2)
80
(1)
-4
(1)
(2)
(4)
60
40
20
0
(3)
(1)
0
(1) colored (UV-NiO)
(2) bleached (UV-NiO)
(3) colored (LPD-NiO)
(4) bleached (LPD-NiO)
(2)
0.0
0.4
0.8
1.2
300
400
500
600
700
800
Potential (V vs. Hg/HgO)
Wavelength (nm)
(b)
(b)
100
80
60
40
20
0
(1) UV-NiO (1 st)
(2) UV-NiO (50 th)
-4
(2)
(4)
-2
0
(1)
(2)
(1)
(1)
(3)
(2)
(1) coloerd (UV-NiO)
(2) bleached (UV-NiO)
(3) coloerd (LPD-NiO)
(4) bleached (LPD-NiO)
2
200
400
600
800
0.4
0.8
1.2
Wavelength (nm)
Potential (V vs. Hg/HgO)
Figure 7 (online color at: www.pss-a.com) Transmittance spectra Figure 8 (online color at: www.pss-a.com) Cyclic voltammo-
of (a) 1st and (b) 50th colored and bleached states for thermally grams (c–v) of (a) LPD-NiO and (b) UV-NiO films after 1st and
treated UV-NiO and LPD-NiO films.
50th the cyclic test.
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M.-K. Lee and C.-H. Fan: Growth rate enhancement of liquid phase deposited NiO film
NiO film is higher than that of LPD-NiO film. The higher F and bleached states are shown in Fig. 7(a) and (b). The
concentration has better passivation on defects and traps in colored and bleached transmittance for LPD-NiO is lower
UV-NiO film. The mass fractions of F in LPD-NiO and UV- than that of UV-NiO. It also comes from UV-NiO with fewer
NiO films are 1.93 and 3.42%, respectively.
From X-ray diffraction examination, the structure of as-
traps by F passivation.
Figure 8 shows cyclic voltammograms (c–v) of (a) LPD-
grown LPD-NiO and UV-NiO are amorphous. After thermal NiO and (b) UV-NiO films after 1st and 50th the cyclic test.
treatment, weak (200) and (220) planes were observed for the After 50th cyclic test, the area of c–v curve for LPD-NiO
structure of LPD-NiO and stronger (220) and (200) planes decreases significantly and that for UV-NiO almost keeps the
can be obtained for UV-NiO as shown in Fig. 5. They same. The UV-NiO film shows highly stable is also from
indicate polycrystalline LPD-NiO and UV-NiO are obtained higher F concentration and better passivation.
after thermal treatment. The high crystalline quality of UV-
NiO also comes from higher F concentration and better are 60.5 and 19.07 cm2/C after 50 cyclic test at a wavelength
passivation of defects and traps on UV-NiO film. of 550 nm. The durabilities of LPD-NiO and UV-NiO films
The coloration efficiencies (h) of UV-NiO and LPD-NiO
The surface smoothness of thermally annealed LPD-NiO after 200 times cyclic test are shown in Fig. 9(a) and (b),
and UV-NiO films were characterized by AFM measurement respectively. The durability of UV-NiO is better than that of
as shown in Fig. 6. The root mean squared (rms) roughness of LPD-NiO. Figure 10(a) and (b) shows the transmittance as a
LPD-NiO and UV-NiO films are 52.1 and 3.9 nm, respect- function of the coloring and bleaching time for LPD-NiO
ively. The much smoother UV-NiO film could be from the and UV-NiO films. The coloring and bleaching speeds of
scission of clusters of precursors in the solution by the high UV-NiO film are faster than that of LPD-NiO film from
photon energy under UV photo-irradiation [24].
higher F concentration and better passivation. Compared
The transmittance of thermally treated LPD-NiO/ITO/ with sputtering, CBD and CVD, they show better durability
glass and UV-NiO/ITO/glass are 80 and 86% at the but worse transparency ratio, coloring, and bleaching speed.
wavelength of 550 nm before electrochromic test. The
higher transmittance of UV-NiO/ITO/glass is from the F
passivation of defect and traps. Normally, the lifetime of
electrochromic device is associated with the Liþ ions
residual quantity in NiO films. For the 1st and 50th
electrochromic tests, the transmittance spectra of colored
(a)
LPD-NiO film
UV-NiO film
80
60
40
0
10
20
30
Coloring time (sec)
(b)
LPD-NiO film
UV-NiO film
80
70
60
50
40
0
10
20
30
Bleaching time (sec)
Figure 10 (online color at: www.pss-a.com) Transmittance as a
Figure 9 (onlinecolorat:www.pss-a.com)Curvesof(a)LPD-NiO function of the (a) coloring and (b) bleaching time for LPD-NiO and
and (b) UV-NiO after 200 times cyclic test.
UV-NiO films.
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Phys. Status Solidi A 209, No. 4 (2012)
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The evaporation, electrodeposition, and sol–gel NiO does
not show better electrochromic characteristics.
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4 Conclusions The growth rate of LPD-NiO film on
ITO/glass is improved to 5.9 nm/h by high photon energy
under UV photo-irradation. The thermal treatment can
transform a-Ni(OH)2 into NiO. For thermally treated NiO
under ultraviolet photo-irradiation, the rms value of surface
roughness is improved to 3.9 nm. The transmittance of
thermally treated LPD-NiO/ITO/glass and UV-NiO/ITO/
glass are 80 and 86% at the wavelength of 550 nm before
electrochromic test. After 50 times electrochromic test, the
colored and bleached transmittance for LPD-NiO are lower
than that of UV-NiO. The electrochromic properties of the
UV-NiO film are better than that of LPD-NiO. These
improvements come from UV-NiO with fewer traps by F
passivation.
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Acknowledgements The authors would like to
acknowledge the support of the National Science Council of the
Republic of China under Contract No. NSC97-2221-E-110-073-
MY3.
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