chapter-chapter ---iiiiii nickel oxide thin films...

CHAPTERCHAPTERCHAPTERCHAPTER----IIIIIIIIIIII

NICKEL OXIDE THIN NICKEL OXIDE THIN NICKEL OXIDE THIN NICKEL OXIDE THIN

FILMS PREPARED BY FILMS PREPARED BY FILMS PREPARED BY FILMS PREPARED BY

SOLSOLSOLSOL----GEL ROUTE AND GEL ROUTE AND GEL ROUTE AND GEL ROUTE AND

THEIR THEIR THEIR THEIR

ELECTROCHROMIC ELECTROCHROMIC ELECTROCHROMIC ELECTROCHROMIC

PERFORMANCEPERFORMANCEPERFORMANCEPERFORMANCE

Chapter-III

Nickel Oxide Thin Films Prepared by Sol-Gel Route and their

Electrochromic Performance

Sr.

No.

Title Page

No.

3.1 Outline……………………………………………………………………………………………. 95

3.2 Introduction……………………………………………………………………………………. 96

3.3 Experimental

3.3.1 Preparation of Nickel Oxide Thin Films……………………………………..

3.3.2 Fabrication of Sol-gel Deposited NiO based Electrochromic

Device……………………………………………………………………………………………...

3.3.3 Characterization………………………………………………………………………

97

97

98

3.4 Results and Discussion

3.4.1 X-Ray Diffraction Studies.…………………………………………………………

3.4.2 Fourier Transform Infrared (FT-IR) Spectroscopic Studies………...

3.4.3 X-Ray Photoelectron Spectroscopic (XPS) Studies …………………….

3.4.4 Surface Morphological Studies………………………………………………….

3.4.5 Electrochromic Properties………………………………………………………..

3.4.6 Optical Transmittance Studies…………………………………………………..

3.4.7 In-Situ Transmittance for Response Time Measurement……………

3.4.8 Colorimetric Analysis……………………………………………………………….

99

99

101

102

103

105

106

107

3.5 Conclusions……………………………………………………………………………………... 108

References………………………………………………………………………………………. 109

Nickel Oxide Thin Films Prepared by Sol-Gel Route………….

Chapter-III Page 95

CHAPTER

THREE

Nickel Oxide Thin Films Prepared by Sol-Gel

Route and their Electrochromic Performance

3.1: Outline

The nickel oxide (NiO) thin films have been prepared from nickel acetate

precursor by sol-gel dip coating technique. As deposited films were annealed at 300

oC to get NiO thin films. The NiO thin films were characterized for their structural,

compositional, morphological, electrochromic, optical and colorimetric properties

using X-ray diffraction, X-Ray photoelectron spectroscopy (XPS), Scanning electron

microscopy (SEM), FT-IR spectroscopy, cyclic voltammetry (CV), optical

transmittance and CIE system of colorimetric measurements respectively.

A smart window (device) with the configuration:

glass/ITO/NiO/KOH/ITO/glass was fabricated using the thin film and EC

parameters were evaluated.


Chapter-III Page 96

3.2 Introduction

Nickel oxide (NiO) is an anodically coloring material. However, despite

promising features such as high electrochromic efficiency, good cycling reversibility,

cost effectiveness [1, 2] and gray coloration useful for smart window technology [3],

NiO is the least understood of the electrochromic materials. The electrochromism in

NiO thin films is rather complicated, although it is generally accepted that the

reversible transition between colored and bleached states is related to redox process

between the (Ni2+) and (Ni3+) states [4].

NiO films are commonly prepared by thermal evaporation [5], sputtering [6]

and electrochemical deposition [7]. These energetic methods are not always suitable

for large-scale production. Also fabrication cost is one of the greatest barriers to

development of the large-area smart windows [8, 9].

An alternative method is the low-cost sol-gel combined with dip coating

technique [10, 11]. The sol-gel technique offers a low-temperature (room

temperature) method for synthesizing materials on large area that are either totally

inorganic in nature or both inorganic and organic. The process, which is based on the

hydrolysis and condensation reaction of organometallic compounds in alcoholic

solutions, offers many advantages for the fabrication of coatings, including excellent

control of the stoichiometry, ease of compositional modifications, customizable

microstructure, ease of introducing various functional groups or encapsulating

sensing elements, relatively low annealing temperatures, the possibility of coating

deposition on large-area substrates with a simple and inexpensive equipment. In the

recent years, a number of developments in coating processes and equipment

automation have made the sol-gel technique even more widespread.

Literature survey shows that sol-gel NiO films have been deposited from

solutions of nickel sulphate in alcohol [12] or nickel chloride (NiCl2.6H2O) in alcohol

and ethylene glycol [13]. There are concerns, however, about the presence of chlorine

in the films impairing the reversibility of the cycling. Films have been deposited from

nickel nitrate Ni(NO3)2.6H2O in ethylene glycol [14] or alcohol [15]. The thermal

stability of metal nitrates are likely to restrict its use to small-scale applications.

Recently, NiO films deposited from nickel diacetate precursor in methanol have been

reported [10]. Sol-gel deposition of NiO has been restricted by a lack of suitable

precursors, which have sufficient solubility and stability in alcohol solvents. Most


Chapter-III Page 97

simple nickel alkoxides are polymeric and insoluble in alcohol at room temperature

[16].

The current chapter deals with the preparation of NiO thin films by using simple

and low cost sol-gel dip coating and its electrochromic performance.

3.3: Experimental

3.3.1: Preparation of Nickel Oxide Thin Films

Nickel hydroxide thin films were deposited on Indium doped Tin Oxide (ITO)

(Kintec corp. Ltd, Hong Kong) coated transparent conducting glass having sheet

resistance of 25-30 Ω/cm2, via sol-gel route using 0.5 M nickel diacetate tetrahydrate

(Ni(CH3COO)2·4H2O)), in 50 ml ethanol and 0.5 ml of HCl. The resulting solution was

refluxed for 1 h at 60 oC and allowed to cool at room temperature. The solution was

greenish transparent in nature. Prior to deposition, ITO’s were cleaned with

ultrasonic treatment in acetone and de-ionized water respectively. Thin films were

deposited by dip coating with ten deposition cycles and annealed at 300 °C at 90 min

to get NiO.

2.3.2 Fabrication of Sol-gel Deposited NiO electrochromic Device

The EC device configuration for sol-gel deposited NiO thin film was

Glass/ITO/NiO/KOH/ITO/Glass. The schematic diagram of the sandwich-type EC

device is shown in Fig.3.1.

Figure.3.1. The schematic diagram of the NiO thin film based electrochromic

device.


Chapter-III Page 98

NiO thin films deposited on ITO coated conducting glass substrate acts as a working

electrode and ITO coated conducting glass substrate acts as counter electrode were

assembled together with double sided tesco tape of 1.5 mm thickness to produce a

sandwich-type electrochromic device. The liquid electrolyte (1M KOH) was filled into

the device through a small hole and sealed it with resibond epoxy glue. The EC device

was dried in air for 1 day before studying EC performance.

3.3.3: Characterization

The structural properties of the NiO thin films were studied with X-ray

diffractometer (Philips, PW 3710, Almelo, Holland) operated at 25 kV, 20 mA with Cu

kα radiation (λ=1.54 Å). The infrared (IR) spectrum was recorded using Perkin-Elmer

IR spectrophotometer (model-100) in the spectral range 400-4000 cm-1. The pellets

were prepared by mixing KBr with NiO powder collected by scratching film from glass

substrate in the ratio 300:1 and then pressing the powder between two pieces of

polished steel. The surface morphology of the films was examined by scanning

electron microscopy (SEM) (Model JEOL-JSM-6360, Japan) and a field emission

scanning electron microscope (FESEM, JEOL JSM-6500F). The elemental and

structural information of the NiO thin films were analyzed using X-ray photoelectron

spectrometer (XPS, VG Multilab 2000-Thermo Scientific Inc. UK, K-alpha) with a

microfocus monochromated Al Kα X-ray working with high photonic energies from

0.1 to 3 KeV. During the data processing of the XPS spectra, binding energy values

were calibrated by the C-1s peak (284.6 eV) from the adventitious contamination

layer. A Shirley-type background was subtracted from the signals and the peaks were

deconvoluted by Gaussian-Lorentz fitting using XPSPEAK software. The

electrochromic measurements were performed in an electrolyte of 1 M KOH in a

conventional three-electrode arrangement comprising NiO thin film as a working

electrode, platinum wire as the counter electrode and saturated calomel electrode

(SCE) as the reference electrode using an electrochemical quartz crystal (EQCM)

(model-CHI-400A) by CH Instrument, USA. In-situ transmittance was recorded using a

He-Ne Laser (λ=632.8 nm), a Si photodiode and a storage oscilloscope. To obtain the

L*a*b* and Yxy coordinate values, colorimetric determinations were recorded by

analyzing the transmittance spectra of color/bleach state using Shimadzu color

analysis software. Utilizing the evaluated CIE L*a*b* and Yxy coordinate values, the


Chapter-III Page 99

color in reduced and oxidized state was obtained by 1931 2o observer and D-65

illuminant.

3.4: Results and Discussion

3.4.1: X-Ray Diffraction (XRD) Studies

Fig.3.2 shows the XRD patterns of the films deposited on ITO coated glass

substrate in as deposited and annealed at 300 oC. No characteristic diffraction peaks

of NiO were detected even after annealing implying that NiO film was predominantly

amorphous.

10 20 30 40 50 60 70

Annealed

As-deposited

Inte

nsi

ty (

A.U

)

2θθθθ (Degree)

ITO-JCPDS-39-1050

x

x

x x

x

x

x

x

[c]

[b]

[a]

Figure.3.2: XRD patterns of (a) The peaks characteristic of ITO, NiO thin film in

(b) As deposited and (c) annealed at 300 oC for 90 min.

3.4.2: Fourier Transform Infrared (FT-IR) Spectroscopic Studies

The IR spectroscopy is a fingerprint of chemical structure of the material. It

gives clear evidence of the bonding system in the material. The infrared absorption

modes are related to dipole moments of the functional groups in the material. A


Chapter-III Page 100

comparison between IR spectra of crystalline bulk and that of thin film reveals the

degree of hydroxylation and hydration in the deposited sample. The functional groups

in the material show their characteristic absorption peaks when frequency of IR

radiation is equal to the natural frequency of molecular vibration. Thus an absorption

peak in IR spectrum indicates the presence of functional group in the sample.

Comparison between IR spectra of thin film and crystalline bulk is most ideal.

Fig.3.3 (a, b) shows the IR transmission spectra of the as deposited and

annealed NiO samples recorded over 400-4000 cm-1 range. The broad and intense

band centered at 3421 cm-1 is assigned to the O-H stretching vibration of the

interlayer water molecules and of the H-bound OH group in the as deposited sample.

The peak observed at 1640 cm-1 is assigned to the bending vibration of water

molecules [17]. The peaks corresponding to vibrations of bridge-bonded acetate

groups are observed at 1028 cm-1 and 1415 cm-1 [18]. The band positioned at 673

cm–1 corresponding to in-plane δ(Ni–OH) deformation and the 466 cm−1 band due to

Ni-O stretching vibration υ(Ni-O) [19]. The decrease in intensity of the band centered

at 3421 cm−1 indicates that annealing removes some amount of hydration, leading to

the NiO formation.

4000 3500 3000 2500 2000 1500 1000 500

46

6 c

m-1

[b]

[a]

16

40

cm

-1

14

15

cm

-1

10

28

cm

-1

67

3 c

m-1

61

8 c

m-1

νν νν(C

OO

- )

νν νν(C

OO

- )

δδ δδ(H

OH

)

νν νν(O

H)

νν νν (

Ni-

O)

δδ δδ(N

i-O

H)

Tr

an

smit

tan

ce

(A

.U)

Wavenumber (cm-1

)

Figure.3.3: FT-IR spectra of (a) As deposited and (b) annealed NiO samples

scratched from thin films.



3.4.3: X-Ray Photoelectron Spectroscopic (XPS) Studies

Figure 3.4 (a) shows the survey spectrum of the nickel oxide deposited by sol-

gel route and annealed at 300 oC. The only elements detected on the surface of

annealed film are nickel, oxygen and also some carbon showing photoelectron peaks,

Ni2p for nickel, O-1s for oxygen and C-1s for carbon. Presence of atmospheric carbon

is very likely and, in fact, it is often used to calibrate peak positions. The high

resolution XPS spectrum of Ni (2p) and O (1s) core levels is shown in Fig.3.4 (b, c).

1200 1000 800 600 400 200 0

Ni3

pN

i3s

[a]

C1s

Ni2p O1s

Inte

ns

ity

(A

.U.)

Binding energy (eV)890 885 880 875 870 865 860 855 850

[b]

Inte

nsi

ty (

A.U

.)

Ni(2p)1/2

Ni(2p)3/2

6.01 eV

17.58 eV

5.51 eV

87

9.0

4

87

3.0

3

86

6.6

8

86

0.9

6

85

5.4

5

Binding energy (eV)

536 534 532 530 528 526

Inte

nsi

ty (

A.U

.)

[c]

O1s

53

2.2

3

53

0.9

7

52

9.0

0

Binding energy (eV)

Figure.3.4: The high resolution XPS spectrum (a) Wide scanning XPS spectrum

of NiO, (b) core level spectra of Ni (2p) (c) core level spectra of O (1s) and (d) C

(1s).

The experimental spectra of Ni 2p were deconvoluted by Gaussian curves into

five different peaks. The Ni 2p spectra comprise of two regions representing the Ni

2p3/2 (850-865 eV) and Ni 2p1/2 (870-885 eV) spin-orbit levels. This double peak


Chapter-III 2

feature of Ni (2p), corresponds to the Ni (2p3/2) and Ni (2p1/2) located at a binding

energy of 855.45 and 872.41 eV respectively as seen in Fig. 3.4 (b) is clearly revealed.

The shake-up satellite peaks were observed at ~5.51 eV and ~6.01 eV higher binding

energy than that of Ni (2p3/2) and Ni (2p1/2) peaks, respectively. The peak of Ni (2p3/2)

at a binding energy of 855.45 with their concomitant shake-up satellite peaks at

860.96 indicated the presence of Ni2+ cations and not of Ni3+ cations. The same is

observed for Ni (2p1/2) peak. This observation confirmed the sol-gel deposited NiO

was composed of pure NiO phase [20, 21]. The energy separation of 17.58 eV

observed between Ni (2p3/2) and Ni (2p1/2) peaks assigned to NiO, which is in well

agreement with the earlier reports [22, 23].

The O-1s XPS spectrum of NiO film is showed in Fig. 3.4 (c) after deconvolution

into three peaks. The results showed a fit to three peaks located at 529, 530.97 and

532.23 eV. The high intense peak located at B.E. of 530.97 eV with shoulder peak at

529 eV corresponds to the O-1s core level of the O2- anions in the NiO. The shoulder

peak has been proposed for the defect sites within the oxide crystal [24] adsorbed

oxygen [25] or hydroxide species [26]. The binding energies of the main and satellite

peaks in the O-1s and Ni-2p core levels are consistent with NiO. The lower binding

energy peak corresponds to the O-1s core level of the O2- anions in the NiO [27, 28].

The O-1s peak was related to the Ni-O chemical bonding. The higher binding energy

peak at 532.23 eV was attributed the H-O-H bond for the residual water [29]. This

again confirms that the NiO thin film is composed of pure stoichiometric NiO phase.

3.4.4: Surface Morphological Studies

The Scanning electron micrograph images of NiO thin film prepared by a sol-

gel method are shown in Fig.3.5 at different magnifications. Though the surface seems

to be compact and smooth at lower magnification (5000X), it is a porous network of

micro granules.



Figure.3.5: SEM images of NiO thin film at (a) 5,000 X (b) 10,000 X (c) 15,000 X

and (d) 20,000 X magnification.

3.4.5: Electrochromic Properties

Cyclic Voltammetry (CV) was employed to investigate the electrochromic

(cathodic/anodic) behavior of NiO. Fig.3.6 presents the CV of the sol gel deposited

NiO, which was recorded for first five cycles at a scan rate of 50 mV/sec in 1 M KOH

electrolyte with linear potential sweep between ±1V vs. SCE. The arrows represent

the forward and reverse scan directions. During the anodic (positive potential) scan,

OH- ions are intercalated into the NiO film, the Ni2+ ions get oxidized to Ni3+ and the

transparent NiO film turns to dark-brown (anodically coloring material). During the

cathodic (negative potential) scan, deintercalation of OH- ions from the film takes

place Ni3+ ions are reduced to Ni2+ and the NiO film became transparent again

(bleaching).

[a] [b]

[c] [d]



-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-1

0

1

2

3

)( ) (

)(

brow nblackt transparen

ze N iO O HO HzN iO− −

+ ↔+

Bleached

Colored Ni2+

Ni2+

Ni3+

Ni3+

Cu

rr

en

t D

en

sity

(m

A/

cm

2)

Applied Voltage (V) vs SCE

Figure.3.6: Cyclic voltammogram for sol gel deposited NiO thin film.

The electrochromic behavior of the device made from these films shown in

photograph (Fig3.7) confirms this mechanism.

Figure.3.7: The photographs showing the original, colored and bleached states

of the electrochromic device (of dimensions 3 × 2 cm2) made up of sol-gel

deposited NiO thin film.

A simplified redox scheme indicating the optical switching due to

intercalation/deintercalation of OH- ions and electrons in electrochromic NiO film

represented by following equations [30];

orzebrowndarkNiOOHOHzttransparenNiO−− +↔+ )()()(

( )1322 .)( −−−−−−−−++↔+ −−zeOHNiOOHzOHOHNi

The Diffusion coefficient (D) for the OH- ions for intercalation (Di) and deintercalation

(Dd) was evaluated from the Randles-Sevcik equation [31].

Bleached, -1V 0 V Colored, +1V



(3.2)CA3/2n5102.72

pj1/2D

0

−−−−−−−−−−×××××

=21 /ν

Where n is the number of electrons assumed to be 1, Co is the concentration of active

ions in the electrolyte, ν the scan rate, jp is the anodic or cathodic peak current density

and A is surface area of the film. The values are indicated in Table.3.1.

Table.3.1: Electrochromic parameters for sol gel deposited NiO thin films.

3.4.6: Optical Transmittance Studies

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

70

80

90

100

Colored

Bleached

∆∆∆∆T=41%

Tr

an

sm

itta

nc

e (

%T

)

Wavelength (nm)

Figure.3.8: Optical transmission spectra of a Glass/ITO/NiO/KOH/ITO/Glass EC

device showing colored (+1V) and bleached (-1V) states.

Anodic potential (mV/s) 600 mV

Cathodic potential (mV/s) 100 mV

Anodic peak current (mA/cm2) 1.31 Cathodic Peak current (mA/cm2) 1.20

Transmittance (%T) Tc 82

Tb 41 Transmittance difference (ΔT %) 41

Optical Density change (ΔOD) 0.68

Response Time (sec) tc 5.95

tb 4.50

Diffusion coefficient (cm2/S) Di 7.48x10-11

Dd 6.27x10-11 Coloration efficiency - η (cm2/C) 32



Fig.3.8 shows the optical transmission spectra of colored and bleached states of sol

gel deposited NiO thin film, in the wavelength range 300 to 1100 nm. The optical

transmittance in colored (Tc) and bleached (Tb) states at 630 nm were 42 % and 82 %

exhibiting an optical transmittance modulation (ΔT) of 41 %. Thus the film

morphology provides the suitable conduits for intercalation/deintercalation of OH-

ions.

3.4.7: In-Situ Transmittance for Response Time Measurement

0 5 10 15 20 25 3040

50

60

70

80

80 %

80 %

Tc=5.95sec

Tb=4.50 sec

Time (sec)

Tr

an

sm

itta

nc

e,(

%T

)

Figure.3.9: In-situ transmittance response of the NiO thin film for 15 sec.

In-situ transmittance measurement was employed to study the switching

characteristics of the sol-gel deposited NiO thin films. The single coloration and

bleaching cycle shown in Fig.3.9 illustrates the switching-time response

characteristics. The switching-response times were calculated on the level of a 80 %

transmittance change. For coloration it is denoted by tc and for bleaching by tb. The

switching-times for these NiO thin films were found to be 5.95/4.50 s for coloration

and bleaching which is much lower than those for NiO nanowalls reported by Xia et al.

[32].

The coloration efficiency (CE) η determines the amount of optical density

change (ΔOD) as a function of the injected/ejected electronic charge (Qi) at a specific



wavelength, i.e., the amount of charge required for changing the optical density [33].

It is given by,

).()/ln(

inmi

33

630

−−−−−−−−−−

=

=

=Q

TT

Q

OD cb

λ

η∆

Where Tb is the bleached transmittance, Tc is the colored transmittance.

The coloration efficiency of the sol gel deposited NiO thin films was found to be

32 cm2/C, which is larger than that reported for NiO thick and thin films (28.8 cm2/C),

respectively [34]. The improved CE may be attributed to the larger textural

boundaries and larger active-surface area, where actual coloration/bleaching

processes take place.

3.4.8: Colorimetric Analysis

The calorimetric measurements [35] were used as a quantitative scale to

determine the colors of sol gel deposited NiO thin film. The attributes of color, hue -a

(its position between red and green, where negative values tends towards green and

positive values tends toward red), saturation-b (its position in between blue and

yellow, where negative value tends toward blue and positive value tends toward

yellow) and lightness-L (where 0 is black and 100 is white color) [36], A two-

dimensional x-y representation known as the chromaticity diagram utilized to

determine the colors of NiO thin film are shown in Fig.3.10 (a, b) respectively. The x

and y were calculated from the tristimulus values. The shift in x-y co-ordinates occurs

once the potential was switched from reduction (bleaching) to oxidation (coloration)

states. The shift in x-y co-ordinates illustrates that, the color of the NiO immensely

changes from highly transmittive (bleached-indicated by white point) state to deep

brown absorptive state. To identify the darkness/brightness with respect to applied

potential, the lightness was calculated from L*a*b* coordinates. The lightness

difference (∆L) of 29.41% was observed. These chromaticity parameters were

analyzed from transmittance spectra in the range 380-780 nm of colored and

bleached states of NiO thin film. These parameters are listed in Table.3.2.



Figure.3.10: (a) CIE 1931 Yxy chromaticity diagram for 2 observer and D-65

illuminant. The straight line shows variations in x-y values and Lightness vs applied

potential for sol gel deposited NiO thin-film in its colored and bleached states. Dashed

vertical lines indicate difference of lightness in its colored and bleached state.

Table.3.2: Chromaticity parameters for sol gel deposited NiO thin films.

3.5: Conclusions

Sol-gel deposited NiO films were successfully grown from sols of nickel acetate.

The NiO phase formation is confirmed by XRD and XPS studies. The morphological

studies revealed that the film composed of porous micro granule structure. The

optical transmittance and in-situ transmittance measurement showed that the film

exhibited transmittance difference (ΔT) of 41 % and response time of 5.95/4.43 s for

coloration and bleaching respectively. The chromaticity measurements showed a

lightness difference of about 29.41 %. The coloration efficiency was found to be 32

cm2/C at 630 nm.

Device Y x y L* a* b* ΔL*

NiO-Bleached State 80.38 0.3172 0.3338 91.86 -0.01 2.48 29.41

NiO-Colored State 30.93 0.358 0.3494 62.45 8.58 11.34



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