CHAPTERCHAPTER- ---IVIIVVIV MICROPOROUS NICKEL OXIDE …shodhganga.inflibnet.ac.in/bitstream/10603/9930/9/09_chapter 4.pdf · oxide thin films by chemical bath deposition (CBD) method

CHAPTERCHAPTERCHAPTERCHAPTER----IVIVIVIV

MICROPOROUS MICROPOROUS MICROPOROUS MICROPOROUS

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

FILMS PREPARED FILMS PREPARED FILMS PREPARED FILMS PREPARED

BY CHEMICAL BATH BY CHEMICAL BATH BY CHEMICAL BATH BY CHEMICAL BATH

DEPOSITION AND DEPOSITION AND DEPOSITION AND DEPOSITION AND

THEIR THEIR THEIR THEIR

ELECTROCHROMIC ELECTROCHROMIC ELECTROCHROMIC ELECTROCHROMIC

PERFORMANCEPERFORMANCEPERFORMANCEPERFORMANCE

Chapter-IV

Microporous Nickel Oxide Thin Films Prepared by Chemical

Bath Deposition and their Electrochromic Performance

Sr.

No.

Title Page

No.

4.1 Outline……………………………………………………………………………………………. 111

4.2 Introduction……………………………………………………………………………………. 112

4.3 Experimental

4.3.1 Preparation of Nickel Oxide Thin Film………………………………………

4.3.2 Characterization………………………………………………………………………

113

113

4.4 Results and Discussion

4.4.1Mechanism for Formation of NiO Thin Films………………………………

4.4.2 X-Ray Diffraction (XRD) Studies………………………………………………..

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

4.4.4 X-Ray Photoelectron Spectroscopic Studies……………………………….

4.4.5 Surface Morphological Studies………………………………………………….

4.4.6 Optical Absorption Studies……………………………………………………….

4.4.7 Electrochromic Properties………………………………………………………

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

4.4.9 Optical Transmittance Studies…………………………………………………..

113

113

114

115

117

118

119

123

124

4.5 Conclusions……………………………………………………………………………………... 126

References………………………………………………………………………………………. 128

Microporous Nickel Oxide Thin Films Prepared by chemical bath deposition………

Chapter-IV Page 111

CHAPTER

FOUR

Microporous Nickel Oxide Thin Films Prepared by

Chemical Bath Deposition and their

Electrochromic Performance

4.1: Outline

Electrochromic properties of chemically bath deposited Nickel Oxide thin films

were deposited using Nickel sulphate precursor, aqueous ammonia and potassium

persulphate as complexing and oxidizing agent respectively. As deposited films were

annealed at 300 oC to get NiO thin films. The 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-IV Page 112

4.2: Introduction

Nickel Oxide/hydroxide films have attracted special attention due to their

good dynamic range, cyclic reversibility, durability, gray coloration and low material

cost useful for smart window technology. It exhibit anodic electrochromism due to

intercalation/deintercalation of OH- ions into it. NiO thin films have been widely

investigated due to their potential applications in large scale optical switching glazing,

electronic information display [1], transparent organic light emitting diode (TOLED)

[2] tandem dye-sensitized solar cells [3], lithium ion batteries [4] and supercapacitor

[5].

EC properties of NiO thin films were investigated by different techniques [6-

10] but due to compact nature their coloration efficiency (CE) is rather limited. In

order to override this limitation several novel routes have been adopted. Remarkable

improvements have been reported for nanostructured [11, 12], micro/nanoporous

NiO thin films [13, 14] and combination of NiO with conducting polymers such as:

Poly (3, 4 ethylyenedioxythiophene) (PEDOT), Polyaniline (PANI) and Polypyrrole

(PPy) [15-18].

The designation of a material as porous is of particular significance when it has

a model or tailor-made pore structure. The term porous structure specifies the

structure with cavities or channels that are deeper than their diameter. Porous

network provide enhancement in specific surface area, facilitate the contact between

electrolyte and oxide surface with open space between individual pores and allows

the easier diffusion of ions through them. A representative elementary volume may be

found if the pore structure is relatively homogeneous above a certain length scale and

one of the simplest ways to further improve the EC performance and augment CE is to

increase the volume of active mass deposited without perturbing porous network. An

interesting and facile methodology for augmenting the overall performance of porous

NiO thin films has been reported by Xia et al. [13]. This is the promising for scaling-up

the CE without compromising response times. In this case the pore structure with

deeper channel (film thickness) at an adequate length scale plays an important role.

In this chapter, an emphasis has been given on the deposition of porous nickel

oxide thin films by chemical bath deposition (CBD) method which is simple, low cost

and suitable for mass production. In CBD, the film thickness can be easily controlled,

merely with the help of deposition time. The influence of the film thickness on EC


Chapter-IV Page 113

properties was found to be profound. The structural, morphological, electrical, and

optical properties of the materials are strongly dependent on the type of the method

of thin film synthesis [1, 19].

4.3: Experimental

4.3.1: Preparation of Porous NiO Thin Films

NiO thin films have been deposited using CBD. A precursor solution was

prepared by using 80 ml of 1M NiSO4.6H2O, 60ml of 0.M K2S2O8, and 20 ml of aqueous

ammonia (25-28%) in 0 ml beaker. Indium doped tin oxide (ITO) (Kintec corp. Ltd,

Hong Kong) coated transparent conducting glass was used as substrate with sheet

resistance of 25-30 Ω/cm2. Prior to deposition, ITO’s were cleaned with ultrasonic

treatment in acetone and de-ionized water respectively. Finally the ITO coated glass

substrates were placed vertically in the freshly prepared quiescent solution at room

temperature and extracted with/after a time interval of 10, 20, 30, 40, 50 and 60 min

and are abbreviated as Ni10, Ni20, Ni30, Ni40, Ni50 and Ni60 respectively. The deposited

films were washed with de-ionized water in order to remove loosely bounded

particles and further annealed at 300 °C in air for 90 min.

4.3.2 Characterizations

The structural, compositional, morphological, optical, electrochromic and

colorimetric properties were studied using X-ray diffraction, X-Ray photoelectron

spectroscopy (XPS), Scanning electron microscopy (SEM), FT-IR spectroscopy, UV-Vis

spectroscopy, cyclic voltammetry (CV) and CIE system of colorimetric measurements,

respectively and discussed detail in section 3.3.2 of chapter 3.

4.4: Results and discussion

4.4.1: Mechanism for Formation of NiO Thin Films

After the chemical bath deposition, the as-deposited precursor film is uniform

in appearance and exhibits black blown in color. The chemical reactions for CBD may

occur as follows

42

2

82

2

63822324 26 SOKOSNHNiOSKNHOHNiSO ++↔++⋅−+

)(

3

2

82

22

82

2

63 6NHOSNiOSNHNi ++↔+−+−+

)(

2

2 2 )(OHNiOHNi ↔+ −+


Chapter-IV Page 114

).()()( 1422222

42

822 −−−−−−−−++↔+ +−−HSOOHNiOOSOHNi

After annealing at 300 oC for 90 min, the mixed phase of Ni(OH)2/NiOOH converts into

NiO composed of nano-flakes.

4.4.2: X-Ray Diffraction (XRD) Studies

Fig.4.1 (a-f) shows x-ray diffraction patterns as a function of film thickness for

chemically bath deposited NiO thin films (Ni10-Ni60) annealed at 300 °C for 90 min.

From the patterns (a-f) in Fig.4.1, the annealed films show diffraction peaks at

2θ=37.24°, 43.29° and 62.87°, along (111), (200) and (220) planes, corresponding to a

cubic NiO phase (JCPDS 22-1189), respectively, indicating that polycrystalline NiO

films have formed after heat treatment.

10 20 30 40 50 60 70 80

(22

0)

(20

0)

(11

1)

[NiO-JCPDS-22-1189]

[b]

[e]

[f]

[d]

[c]

[a]

2θθθθ (Degree)

Inte

ns

ity

(A

.U.)

Figure.4.1: XRD patterns of NiO samples (a) Ni10, (b) Ni20, (c) Ni30, (d) Ni40, (e)

Ni50, (f) Ni60 annealed at 300 °C.

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

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


Chapter-IV Page 115

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

ideal.

The IR transmission spectra of the as deposited and annealed NiO samples

collected from films were recorded over 400-4000 cm-1 (shown in Fig.4.2 (a) and (b)).

The spectrum of as-deposited sample exhibit a shoulder at 3580 cm−1 corresponding

to non-hydrogen bonded ν(O-H) group. A broad band centered at 3285 cm-1 is

indicative of hydrogen bonded water (OH) within the film structure and the band at

1630 cm−1 is characteristic of the bending vibration of water. The intense peak

positioned at 1115 cm−1 corresponds to stretching vibrations of free sulfate ions [20].

The bands at 618 cm−1 correspond to δ(OH) and the broad band at 436 cm-1 can be

assigned to Ni-O interaction respectively [21]. From Fig.4.2 (b) it is seen that intensity

of the band centered at 3580 cm−1 and 3285 cm-1 lowers which indicates that thermal

treatment removes some amount of hydration, leading to the hydrated NiO formation.

4000 3500 3000 2500 2000 1500 1000 500

43

6 c

m-1

11

15

cm

-1

Tr

an

sm

itta

nc

e (

A.U

)

Wavenumber (cm-1

)

16

30

cm

-1

SO

42

-

δδ δδ(N

i-O

H)

61

8 c

m-1

δδ δδ(H

OH

)

v(OH)

[a]

[b]

νν νν(N

i-O

)

Figure.4.2: FT-IR spectra of NiO samples (a) as deposited and (b) annealed at

300 °C.

4.4.4: X-Ray Photoelectron Spectroscopic (XPS) Studies

To study the compositions and chemical states of the NiO sample, XPS study

was conducted. Fig.4.3 (a) shows the wide scanning XPS survey spectra of NiO film.

The binding energies of the samples were corrected using a value of 284.6 eV for the

C-1s peak of carbon. It was observed that there is no contaminated element except


Chapter-IV Page 116

Carbon. Fig.4.3 (b, c) shows Ni-2p and O-1s core level signals. The Ni -2p signal could

be deconvoluted into five 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. As shown in Fig.4.3 (b) the higher binding energy of Ni-(2p3/2) film is 855.62

eV corresponds to Ni2+ with a shakeup satellite peak at 5.41 eV above the main peak.

Similar features are observed for the Ni-(2p1/2) region. The binding energy separation

between Ni-(2p3/2) and Ni-(2p1/2) region is 17.62 eV.

1200 1000 800 600 400 200 0

Ni2p3/2

O1s

C1s

Ni3

s

Ni3

p

[a] Survey spectrum

Binding Energy (eV)

Inte

nsi

ty (

A.U

.)

885 880 875 870 865 860 855 850

[b] [Ni2p]

5.94 eV

5.41 eV

17.62 eV

Ni(2p)3/2

Ni(2p)1/2

87

9.1

8

87

3.2

4 86

4.9

5

86

1.0

3

85

5.6

2

Inte

nsi

ty(A

.U)

Binding Energy (eV)

536 534 532 530 528 526

[C] [O1s]

Inte

ns

ity

(A

.U.)

Binding Energy (eV)

53

2.9

0

53

1.1

6

52

9.1

3

Figure.4.3: (a) Wide scanning XPS survey spectra and (b, c) Ni-2p and O-1s core

level signals of NiO film.

The O-(1s) XPS spectrum of NiO is shown in Fig.4.3 (c) after deconvolution into

three peaks. The binding energies of 529.2 eV in O-1s region and 855.62 eV in

Ni(2p3/2) region are consistent with the peaks of NiO. The peak having less intensity

at a binding energy of 529.13 eV with a shoulder at ∼2.04 eV higher binding energy

corresponds to the O-1s peak of NiO. The shoulder peak has been proposed for the


Chapter-IV Page 117

defect sites within the oxide crystal [22] adsorbed oxygen [23] or hydroxide species

[24]. Here, we attribute the peak to the existence of defect sites on the NiO surface.

The lower binding energy peak corresponds to the O-(1s) core level of the O2- anions

in the NiO [, 26]. The higher binding energy peak at 532.90 eV was attributed to the H-

O-H bond for the residual water [27].

4.4.5: Surface Morphological Studies

The SEM micrographs of the films deposited at various time intervals (10 to 60

min respectively) are shown in Fig.4.4 (a-f) and the inset shows the cross section

images of NiO thin films.


Chapter-IV Page 118

It is observed that due to quiescent solution micro-porous framework does not

get disturbed during growth which causes increase in film thickness and pores

structure with deeper channels with increase in deposition time. Large thickness with

open pore structure offer more amount of active mass deposited that allows ions to

diffuse along its length which help augmenting properties. All the films are micro

porous and composed of interconnected network does not change with deposition

time and is similar to that reported by Xia and Wu et al [13, 14]. Such a microporous

interconnecting network facilitates the control over surface area and porosity/open

structure, affecting the ion insertion kinetics (ion diffusion length and time, ionic

mobility, etc) leading to enhanced EC performance. The FE-SEM image shows clear

picture of the microporous morphology as evident in Fig.4.4 (g).

4.4.6: Optical Absorption Studies

Fig.4.5 (a-f) shows plots of (αhν)2 as a function of photon energy (hν) for NiO

thin films deposited onto ITO-coated glass substrates having different film

thicknesses annealed at 300 °C. The optical absorption data were analyzed using the

following classical relation for near edge optical absorption in semiconductor [28]:

( )).( 240 −−−−−−−−−−−

−=

ν

ναα

h

Ehn

g

where Eg is the optical energy gap between bottom of the conduction band and top of

the valence band, hν is the photon energy and n is the constant equal to ½ for direct

transition and 2 for indirect transition [28]. The initial absorption curves were

recorded in the wavelength range 350-1000 nm. The nature of the plots indicates the

existence of direct optical transition. The extrapolation of the straight line to zero

absorption coefficient (∝=0) gives an estimate of the band gap energy (Eg). The inset

of Fig.4.5 shows the variation of band gap energy with film thickness. It is observed

(g) Figure.4.4: SEM images of the NiO

samples: (a) Ni10, (b) Ni20, (c) Ni30, (d)

Ni40, (e) Ni50, (f) Ni60, (g) FE-SEM image of

sample Ni60, annealed at 300 °C.


Chapter-IV Page 119

that, with increase in film thickness there is gradual decrement in the band gap

energy from 3.22 to 2.80 eV. Similar band gap narrowing effect has been reported by

many authors [7, 29, 30].

1.0 1.5 2.0 2.5 3.0 3.50

40

80

120

160

200

240

280

320

( αα ααh

υυ υυ)2

x 1

08 (

eV

/c

m)2

10 20 30 40 50 60

2.8

2.9

3.0

3.1

3.2

3.3

Ban

d g

ap e

ner

gy (

eV)

deposition time (min)

Ni10

Ni20

Ni30

Ni40

Ni50

Ni60

Photon Energy, hνννν (eV)

Figure.4.5: Plots of (αhν)2 as a function of photon energy (hν) for NiO thin films

deposited onto ITO-coated glass substrates at various time intervals (a) Ni10, (b)

Ni20, (c) Ni30, (d) Ni40, (e) Ni50, (f) Ni60, annealed at 300 °C.

4.4.7: Electrochromic Properties

The cyclic voltammograms (CVs) for the films deposited at various time

interval (10 to 60 min resp.) were recorded at the scan rate of 50 mV/sec in 1 M KOH

electrolyte with linear potential sweep between +1.2 V to -1.2 V versus SCE (shown in

Fig.4.6 (a-f)) and photographs of a NiO film with a size of 2.5 X 2.5 cm2 in colored

(+1.2 V) and bleached (-1.2 V) states are shown in Fig.4.7.

The broad peaks are visible in both cathodic (C1 = -0.8V) and anodic (A1 = 0.85)

scans, which are associated with the bleaching and coloring process in NiO. A

simplified redox scheme for representing the gradual optical change that takes place

under intercalation/deintercalation of OH- ions in an electrochromic NiO film is

represented by equation (4.3),

).( 34−−−−−−−−+⇔+ −−eNiOOHOHNiO

Bleached Colored (Brownish gray)


Chapter-IV Page 120

Electrochromism in NiO thin films is related to a charge transfer process

between Ni2+ to Ni3+ [27]. During the cathodic scan, the reduction of Ni3+ to Ni2+ leads

to bleaching of the film. In the reverse anodic scan oxidation of Ni2+ to Ni3+ causes

coloration of the film.

The general features of the CVs are similar to that obtained for NiO thin films

prepared by spray pyrolysis technique and sol-gel method [20, 31]. But this feature

was not observed for Ni10 sample due to less active mass deposited on the substrate.

As the deposition time increases from 10 (Ni10) to 60 (Ni60) min, the magnitudes of

the terminal cathodic and anodic peak current densities increased, which is an

indication of increase in area of CVs. For Ni60 sample the anodic and cathodic peak

current density was found to be 3.34 mA/cm2 and 4.56 mA/cm2 which are higher

than other samples (Ni10, to Ni50). Also large changes in the magnitude of anodic peak

currents (1.26 mA/cm2 for Ni10 and 3.34 mA/cm2 for Ni60) indicate that intercalation

process follows the same behavior with mass deposited. This is probably due to the

increment in depth of diffusion (or diffusion length) with thickness of the film. This

suggests that the amount of charge transferred back and forth upon cycling within a

certain potential range depends on the thicknesses of the films and the electrolyte as

well as surface morphology plays a decisive role in the ionic

intercalation/deintercalation process [32]. Thus microporous and interconnected

porous network with increased effective surface area is beneficial for large amount of

charges to be intercalated/deintercalated. The diffusion coefficient (D) has been

estimated using the formula (4.4)

(4.4)CA

3/2n5102.72

pj

1/2D

0

−−−−−−−−−−×××××

=21 /ν

Here 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 the anodic/cathodic peak current and D the

diffusion coefficient. The value of D for all Ni10-Ni60 is calculated using the relation

(4.3) and listed in Table.4.1. It was found that the diffusion coefficient for Ni60 sample

is higher, which is 4.89 x 10-10 cm2/s for anodic and 9.12 x 10-10 cm2 /s for cathodic

peaks.


Chapter-IV Page 121

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2-6

-4

-2

0

2

4

6

Ni10

Applied Voltage (V) vs SCE

Cu

rre

nt

De

nsi

ty (

mA

/cm

2)

Cu

rr

en

t D

en

sity

(m

A/

cm

2)

Ni20


3.72mA/cm2

Ni30


Cu

rr

en

t D

en

sity

(m

A/

cm

2)

4.18mA/cm2

3.97 mA/cm2

Ni40


Cu

rre

nt

De

nsi

ty (

mA

/cm

2)

Ni50

C

ur

re

nt

De

nsi

ty (

mA

/c

m2)


Ni2+

Ni3+

Ni3+

Ni2+

1.26mA/cm2

3.34mA/cm2

3.47mA/cm2

4.56 mA/cm2

1.90 mA/cm2

Ni50


Cu

rre

nt

De

nsi

ty (

mA

/cm

2)

Figure.4.6: Cyclic voltammograms of the NiO films: (a) Ni10, (b) Ni20, (c) Ni30, d)

Ni40, (e) Ni50 and (f) Ni60, recorded in 1M KOH electrolyte. The potential swept

from +1.2 V to -1.2 V versus SCE at the scan rate of 50 mV/sec.


Chapter-IV Page 122

Figure.4.7: The photographs of a NiO film with a size of 2.5 X 2.5 cm2 in colored

(+1.2 V) and bleached (-1.2 V) states.

Fig.4.8 shows cyclic voltammogram, recorded after 5th-300th colored/bleached

cycles, for Ni60 sample in 1 M KOH electrolyte at a scan rate of 50 mV/sec. Up to 300th

bleach and coloring cycles the dramatic charge fading of 29 % in was observed.

-1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5

-4

-2

0

2

4

6

300th

200th

100th

Cu

rr

en

t D

en

sit

y (

mA

/c

m2) 5

th

100th

200th

300th


5th

Figure.4.8: Overlays of CV (a) for Ni60 sample after 5th and 300th c/b cycle in the

potential range of +1.2 V to -1.2 in 1 M KOH electrolyte, versus SCE at the scan

rate of 50 mV/sec.


Chapter-IV Page 123

The degradation is associated with an increase of the mass of the layer after

each cycle due to the irreversible incorporation of OH- ions as indicated by decrease

in cathodic peak current after successive cycles i.e. the charge intercalated does not

change but that deintercalated goes on decreasing as indicated by non appreciable

change in anodic peak current.

4.4.8: In-Situ Transmittance Response Time Measurement

The switching characteristics of all the films were studied from in-situ

photodiode response at 632.8 nm. The studies were performed by switching the films

from an oxidized state to a reduced state by applying photodiode as a square wave

voltage (+1.2 V and -1.2 V). Fig.4.9 (A) shows the resultant photodiode response for

all the films up to first 10 cycles. NiO exhibits faster response speed with about 2.9 s

for bleaching (reduction) and 3.5 s for coloration (oxidation) kinetics in KOH

electrolyte. Fig.4.9 (B) shows the photodiode response of all the samples recorded for

one cycle.

0 1 2 3 4 5 6 7 8 9 10

[B]

[c]

[b]

[f]

[e]

[a]

[d]

Ph

oto

dio

de

re

spo

nse

Time (sec)

Figure.4.9: In-situ photodiode response for all the films up to first 10 cycles for

NiO films: (A) (a) Ni10, (b) Ni20, (c) Ni30, (d) Ni40, (e) Ni50, (f) Ni60 by applying

alternating square potentials (+1.2 V and -1.2 V) and (B) In-Situ photodiode

response for all the films for one cycle 1M KOH electrolyte .


Chapter-IV Page 124

4.4.9: Optical Transmittance Studies

300 400 500 600 700 800 900 1000 110010

20

30

40

50

60

70

80

90

100

∆∆∆∆T=20%

(a)

Tr

an

smit

tan

ce

(%

T)

Wavelength (nm)

bleached

colored

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

70

80

90

100

∆∆∆∆T=25%

(b)

Wavelength (nm)

Tr

an

sm

itta

nc

e (

%T

)

bleached

colored

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

70

80

90

100

∆∆∆∆T=33%

(c)

Wavelength (nm)

Tr

an

sm

itta

nc

e (

%T

) bleached

colored

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

70

80

90

100

∆∆∆∆T=37%

(d)

Wavelength (nm)

Tra

nsm

itta

nc

e (

%T

) bleached

colored

300 400 500 600 700 800 900 1000 11000

10

20

30

40

50

60

70

80

90

100

∆∆∆∆T=42%

(e)

Wavelength (nm)

Tra

nsm

itta

nce

(%

T)

bleached

colored

300 400 500 600 700 800 900 10000

10

20

30

40

50

60

70

80

90

100

∆∆∆∆T=46%

(f)

Wavelength (nm)

Tr

an

smit

tan

ce

(%

T) bleached

colored

Figure.4.10: Optical transmission spectra of NiO samples: (a) Ni10, (b) Ni20, (c)

Ni30, d) Ni40, (e) Ni50, (f) Ni60 in their colored and bleached states recorded in the

wavelength range of 350-1000 nm in 1 M KOH electrolyte.

Fig.4.10 (a-f) shows the optical transmission spectra for all the NiO samples in

their colored and bleached states recorded in the wavelength range of 350-1000 nm


Chapter-IV Page 125

after electrochemical polarization of +1.2 V to -1.2 V versus SCE in 1M KOH electrolyte.

The sample Ni10 shows relatively less optical modulation due to less active mass

deposited on the substrate. The ΔT of all the samples in their colored and bleached

state at 630 nm was found to be 20 %, 25 %, 33 %, 37 %, 42 %, 46 % which is

increased with increasing deposition time. The highest ΔT of 46 % was observed for

Ni60 sample. This is mainly due to large amount of active mass deposited on the

substrate and interconnected nanoporous network with deeper channels of the film is

favorable for effective electrolyte penetration.

Table.4.1 Parameters obtained from cyclic voltammetry and optical

transmittance studies

Sample

Code

Thickness

(nm)

Transmittance

(Tb) (%) at

630 nm

Transmittance

(Tc) (%) at

630 nm

ΔT

(%)

Optical

Density

(∆OD

Coloration

efficiency

(η)

(cm2 /C)

Diffusion Coefficient

10-10 (cm2 /s)

DCC DCA

N10 860 80.50 60.64 19.86 0.51 29.51 1.58 6.96x10-11

N20 1020 79 54.27 24.73 0.346 20 5.28 2.26

N30 1090 78.13 45. 32.88 0.527 28.57 6.07 2.48

N40 1160 75.40 38.29 37.11 0.61 34.57 6.91 2.67

N50 1230 75.10 33.76 41.37 0.736 40 7.66 2.96

N60 1310 74.64 28.30 46.3 1.14 41.18 9.12 4.89

In order to explore the electrochromic properties in more details the results of

Fig.4.10 (a-f) is quantified in Fig.4.11 (a-b). Fig.4.11 (a) shows the transmittance data

for bleached and colored state as a function of deposition time for 1M KOH electrolyte.

It reveals that maximum bleached and colored transmittance at 630 nm increases

with film thickness and is highest (46 %) for Ni60 sample.

The optical density difference and CE at 630 nm as a function of deposition

time for 1M KOH electrolyte is shown in Fig.4.11 (b) and were calculated by using

relation (4.5) and (4.6), respectively.

).()ln( 54630 −−−−−−−−−−−−=∆ = nm

c

b

T

TOD λ

).()()( 64630 −−−−−−−−−−−−−∆

= = nm

iQ

ODCE λη

As evidence from Fig.4.11 (b), ΔOD gradually increases with increase in deposition

time, which results in the increase in CE. The CE of 42 cm2/C is observed for Ni60

sample in KOH electrolyte which is comparable to those reported for NiO films


Chapter-IV Page 126

prepared by electrodeposition (42 cm2/C) [33], CVD (44 cm2/C) [34], CBD (42 cm2/C)

[13], Sol-gel (41 cm2/C) [35], spray pyrolysis (30 cm2/C) [36].

Figure.4.11: (a) Transmittance for the bleached and colored states as a function

of deposition time and (b) ΔOD and CE as a function of deposition time in 1M

KOH electrolyte, as calculated from data in Fig.4.10 (a-f).

4.5: Conclusions

NiO thin films of different thickness with change in deposition time have been

successfully deposited by a simple and cost effective chemical bath deposition method.

It is observed that the film thickness and microporous structure plays a crucial role in

enhancing the electrochromic properties. The microporous interconnected network

with well defined 3D envelopes facilitates the control over the surface area and

porosity/open structure, affecting the ion insertion kinetics (ion diffusion length and

time, ionic mobility, etc) leading to enhanced EC performance. It was observed that

NiO thin films deposited for 60 min showed maximum transmittance modulation and

coloration efficiency (ΔT = 46 % and CE=42 cm2/C at 630 nm) and exhibits faster

response time (2.9 s for bleaching and 3.5 s for coloration).


Chapter-IV Page 127

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