Electrochromic behavior of NiO film prepared by e beam evaporation 2017 Journal of Science Advanced Materials and Devices

Sahua,b,*, Tzu-Jung Wub, Sheng-Chang Wangc, Jow-Lay Huangb,d,** a Department of Natural and Applied Sciences, Namibia University of Science and Technology, Private Bag 13388, Windhoek, N

Original Article

D.R Sahua,b,*, Tzu-Jung Wub, Sheng-Chang Wangc, Jow-Lay Huangb,d,**

a Department of Natural and Applied Sciences, Namibia University of Science and Technology, Private Bag 13388, Windhoek, Namibia

b Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, ROC

c Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan 710, Taiwan, ROC

d Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 1 December 2016

Received in revised form

4 May 2017

Accepted 5 May 2017

Available online 12 May 2017

Keywords:

E-beam evaporation

Nickel oxide

Electrochromic properties

a b s t r a c t

The NiO thinfilms were prepared by the electron beam evaporation method using synthesized sintered targets As-preparedfilms were characterized using X-ray diffraction, scanning electron microscopy,

UVeVIS spectroscopy and cyclic voltammetry The thicker films were found to exhibit a well-defined structure and a well-developed crystallite size with greater transmittance modulation and durability The as-deposited thinnerfilms of 170 nm showed a faster response time during electrochromic cycles with a coloration efficiency of 53.1 C/cm2than the thicker ones However, the thickerfilms showed no enhanced electrochromic properties such as a larger intercalated charge than the thinner ones The electrochromic properties of the thinnerfilms became deteriorated after 800 cycling tests

© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Nickel oxide (NiO) is used as an efficient electrochromic (EC)

material in EC devices It is an anodic coloring material, which is

optically and electrochemically compatible with the well-known

tungsten trioxides [1e3] The electrochromics is a special

prop-erty of a material to change its color reversibly by the application of

a voltage The electrochromic process is unique and it strongly

depends on the method of preparation of the materials[4e6] To

ensure the sustainable development of its functional properties in

devices, high quality thinfilms can be produced as a coating on the

surface of the devices The properties of the thinfilms are thickness

dependent [6e8] and this makes the film thickness a more

important parameter not only as a geometrical but also as a

func-tional one in the design of materials and devices

Efforts have been explored to understand the thickness

depen-dence of optical properties in nanocrystalline oxidefilms[6e9]and

demonstrated that the optical constants of very thin oxidefilms may

deviate considerably from those of the bulk materials This means

that the respective materials constant is sometimes no longer con-stant in thinfilms As a matter of fact, systematic investigations of thickness-dependent film properties are necessary for device application NiO films have been prepared by chemical, electro-chemical techniques and by vacuum technologies, such as e-beam evaporation and sputtering[1,4,5,10e12] Among these methods, those which employ vacuum technologies provide highly stable EC NiOfilms with good electrochemical efficiency The electrochromic property is also affected by the structure, binding conditions, water contents, stoichiometry and thickness of thefilms[13,14] In this work, nickel oxidefilms with different thickness were prepared by e-beam evaporation methods and investigated Their EC properties have been compared with those of differentfilms with respect to structure, surface morphology and electrochemical behavior for possible device applications

2 Experimental The NiO thinfilms were deposited on Indium tin oxide (ITO) glass substrates in an e-beam evaporation system with electron energy in the order of 4 keV using a NiO sintered target The target was prepared by pressing high purity NiO powder (OSAKA, purity 99.99%,<325 mesh) into a disk type pellet of 11 nm in diameter and

4 mm in thickness followed by 4 h sintering procedure at 1300
C For fabricating the NiOfilms, the electron beam was focused on the sintered target mounted on a water-cooled holder while the sub-strate (of 3 cm 5 cm in size) was placed parallel to the target at a

*Corresponding author Department of Natural and Applied Sciences, Namibia

University of Science and Technology, Private Bag 13388, Windhoek, Namibia.

** Corresponding author Department of Materials Science and Engineering,

National Cheng-Kung University, Tainan 701, Taiwan, ROC Fax: þ886 6 276 3586.

E-mail addresses: dsahu@nust.na (D.R Sahu), scwang@mail.stust.edu.tw

(S.-C Wang), jlh888@mail.ncku.edu.tw (J.-L Huang).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.05.001

2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

distance of 130 mm The e-beam chamber was pumped down to

7 107torr prior to the deposition, whereas deposition of the NiO

films was performed at a pressure of about 1  105torr in the

evaporation chamber Specimens of 170 nm, 270 nm, 380 nm and

540 nm thickness, respectively were fabricated by adjusting the

appropriate depositing time

The thickness of thefilms was measured using a surface profiler

(Alpha-step 500, TENCOR) Conventionalqe2qXRD

characteriza-tion of the as-preparedfilms was carried out in a Rigaku (D/MAX

2500) diffractometer using the CuKaradiation Surface morphology

was observed on afield emission scanning electron microscope

(FE-SEM, XL-40) The XPS analysis was carried out with a VG Scientific

210, UK employing the AlKa line AFM micrographs were taken using an AFM of, Digital Instrument INC, NanoScope, California, USA Optical transmittance was measured in the range of

300e800 nm by a UVeVISeIR spectrophotometer (Hewlett Packard 8452A, Palo Alto CA) The electrochromic properties of thefilms were studied by cyclic voltammetry (CV-Autolab Potentiostat 30) using a three-electrode cell system with an electrolyte solution of lithium perchlorate in propylene carbonate of 0.5 M strength (0.5 M LiClO4-PC) The working electrode was made of NiOfilm on Indium Tin Oxide (ITO) glass substrate The reference and counter elec-trodes were a saturated calomel electrode (SCE) and a platinum grid, respectively

3 Results and discussion

Fig 1 presents the XRD pattern of the NiO films of various thicknesses (170, 270, 380 and 540 nm) as prepared on the ITO glass substrate The XRD patterns reveal the polycrystalline structure for all thefilms under investigation As the thickness of the film in-creases, the diffraction intensity increases due to more crystallites grown in the structures[15] The diffraction pattern of eachfilm is featured by three peaks The peaks located at 37, 43 and 63
are indexed as (111), (200) and (220), respectively, using a cubic NiO structure (Space group: Fm3m) similarly to the analysis in Ref.[16]

It has been reported that allfilms have (111) preferential planes which is different than the preferred (200) growth [9,17] and furthermore, that with the increase of the film, other peaks appear[9,17,18] In our study, however, we have observed all these peaks in allfilms and this may be due to the fact that the different conditions of thefilm growth in our investigations are in overall also similar to those used by other authors[14,19] The crystallite

540 nm

380 nm

270 nm

170 nm

ITO

*

2θ (deg.)

(220) (200)

(111)

*

Fig 1 XRD patterns of various NiO films with different film thicknesses.

films with different thickness: (a) 170, (b) 270, (c) 380 and (d) 540 nm.

D.R Sahu et al / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232 226

sizes calculated using Scherrer formula[20]from the full width and

half maximum of (111) peak show that the crystallite sizes increase

with the increase offilm thickness, which may be interpreted in

terms of a columnar grain growth This is in good consistency with

the discussions made by other authors[18]that, the peak intensity

increase with the increasing film thickness also indicates an

improvement of crystallinity This suggests that e-beam

evapora-tion provides more favorite condievapora-tions than other methods for the

growth of NiO polycrystalline films with high texture planes

[21,22] The SEM images of NiO film with different thickness are

shown inFig 2 Observations on the surface morphology of NiO

films reveal that the grain size increases in the film with greater

film thickness which corroborates the XRD results There is more agglomeration of particles for thickerfilms This also improves the effect of ion insertion/extraction due to the change of surface morphology[2,17]

The composition of the nickel oxidefilms was determined by XPS.Fig 3shows the O1 spectra for three NiOfilms Two binding energy peaks at 529.7 and 531.5 eV are identified The binding energy of 529.7 eV corresponds to the O1s peak of NiO[23]and that

of 531.5 eV corresponds to the O1s peak of Ni2O3 [23,24] This confirms the formation of Ni2þand Ni3þin thefilms There was no change found in the chemical composition and in the orientation of thefilms with different thickness These XPS results are in good agreement with XRD data and the data presented as potential cyclic curves below as well The AFM pictures of thefilms with different thickness (170, 270, 380 and 540 nm) presented inFig 4indicate a roughness of about 2.5 nm for thefilms under investigation which slightly increases in thefilm with greater thickness However, no pronounced (obvious) variation of the roughness with the film thickness could be derived from these results

The transmittance spectra of the as-deposited NiO films pre-sented inFig 5(a) show that the transmittance decreases with in-crease of the thickness of thefilm The transmittance spectra fall rapidly at low wavelength region The transmittance spectra show different modulated pattern due to the interference of reflected light from both faces of the layer There occurs the change of phase

as the thickness of thefilm increased from 170 nm to 270 nm and this could be related to the constructive or destructive interference between rays coming from the top and bottom of thefilm[25] There is also a crossover of maximum transmittance value from

170 nm thickness to 270 nm evaluated at 550 nm In thickerfilms, the onset of absorption edge became less sharp, this is due to the

Ni+3

Binding Energy (eV)

540nm 380nm 270nm 170nm

Ni+2

Fig 3 XPS spectra of NiO films with different film thicknesses.

films with different thickness: (a) 170, (b) 270, (c) 380 and (d) 540 nm.

Fig 5 (a) The transmittance spectra of as-deposited NiO films with different thicknesses (b) The transmittance spectra of as-deposited, bleached and color state of NiO films with different thicknesses (c) The change of the optical density in NiO films with different thicknesses.

D.R Sahu et al / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232 228

presence of bigger crystallites sizes and increased scattering due to

surface roughness[26,27] With the increasingfilm thickness, the

onset of the fundamental absorption is observed to shift towards

the shorter wavelength [28,29] The optical absorption edge is

related to the defect electronic states, which are associated with the

nickel vacancy and interstitial oxygen atoms The transmittance

spectra for the deposited, bleached and colored state of thefilms for

different thicknesses are presented inFig 5(b) It shows that all the

films have less transmittance in the color state than that of as

deposited and bleached state The transmittance spectra for the

deposited, bleached and colored state at 550 nm wavelength also

show that the 270 nm thick film has greater transmittance at

bleached and color state than other films Detailed results are

presented inTable 1 However, theDT (Tbleached Tcolored) increases

in thefilms with increased thickness The transmission modulation

results are comparable to those obtained on other NiOfilms which

generally depend on the growth techniques and the electrolytes

used[30,31] Our results are similar to those other research groups

generally obtained on post depositionfilms[18,21,22]

The change in optical density (DD) of thefilms with wavelength

is presented in Fig 5(c) The change in optical density [9] is

defined as

DOD¼ logðTb=TcÞ;

where Tband Tc refer to the transmittance in the bleached and

colored states, respectively It shows that the thickfilm has higher

optical density than the thinfilm In the visible wavelength region

the change in the optical density is highest compared to the other

regions This indicates that the electrochromic material changes its

optical density upon the inflow and outflow of the electron Further,

there are few maximum in the curve of optical density vs

wave-length in the visible wavewave-length region An electrochromic material

shows an optical density rise when subjected to application of a

voltage to receive or discharge electron and shows an optical

density drop when electron moves in the direction opposite that of

density rise However, the rapid changing in OD means that there is

smoothflow of electrons into the EC materials and smooth outflow

of electrons from the EC materials Here in the thickfilm, the first

average OD is in a wavelength range 350e380 nm, 2nd average OD

in a wavelength range 420e450 nm, third in 450e500 nm, fourth in

500e540 nm and so on The increased maximum in OD also

in-dicates increasing coloration The peak also inin-dicates the change of

color of thefilm due to the degree of crystallinity of the film As the

crystalline size increases with thickness, there is shift of the

ab-sorption band of OD curve towards larger wavelength giving rise to

different color The changes in OD also indicate that coloration is a

result of the creation of induced defect center at these bands

Maximum peak around 400 nm indicates brown color of thefilm

The blue color of thefilm arises in the region of 450e500 nm and

gray color at higher wavelength region[32,33]

The electrochromic behavior of the nickel oxide was tested by

means of the standard electrochemical technique of cyclic

vol-tammogram.Fig 6shows the voltammogram obtained for thefilms

with different thicknesses under investigation From thefigure, it is observed that the applied potential position of the anodic and the cathodic peak shifts towards positively with the positive current and negatively with the negative current, respectively These changes of peak positions correlated with the change of the injected charge There is an increase of reaction charges with the increasedfilm thickness So the polarization of charges increases withfilm thickness The positions of the anodic and cathodic peaks shift to higher and lower potentials, respectively, with increasing thickness with respect to a higher polarization [34] The set of peaks is the signature of the Ni3þ/Ni2þredox process which cor-roborates the XPS results.Fig 7 presents the charge intercalated during the electrochromic cycling for thefilms under investigation

It is seen that charge intercalation increases in the films with increasedfilm thickness The difference in the charge intercalation

Table 1

Comparison on electrochromic properties of NiO film having different thickness at 550 nm wavelength.

Thickness (nm) As deposited

transmittance (T)

Bleach state transmittance (T Bleach )

Color state transmittance (T colored )

Difference (DT)

Response time

CE (C/cm 2 )

Durability

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -0.0012

-0.0009 -0.0006 -0.0003 0.0000 0.0003 0.0006 0.0009 0.0012

Potential (V) (vs SCE)

170nm 270nm 380nm 540nm

Fig 6 Cyclic voltammograms of the NiO films with different thicknesses.

0 200 400 600 800 0.0006

0.0009 0.0012 0.0015 0.0018 0.0021 0.0024 0.0027

Number of cycles

Qin

170 nm

270 nm

380 nm

540 nm

Fig 7 Charge intercalated during electrochromic cycling for different films.

is due to the increase in the depth of diffusion (e.g diffusion length)

with thefilm thickness The amount of charge transferred back and

forth upon cycling within a certain potential range also depends on

the film thicknesses The electrolytes as well as the surface

morphology plays a decisive role in the ionic

intercalation/dein-tercalation processes[21,35] During the electrochromic cycling, it

initially increases up to 150 cycles and then decreases with the

increase of cycles in the case of the thickfilms However, in the case

of thinfilms the charge intercalation increases with the increase of

cycles With the increase offilm thickness, the charge intercalation

increases, so there is an increase of transmittance modulation For

the comparison between the relative capacity of the maximum

amount of charge intercalation for thefilm with different thickness,

the relative capacity as a function of the number of cycles is

pre-sented inFig 8 It is observed that the 540 nm thickfilm has the

maximum amount of charge intercalation at 150 cycles However, it

reaches to 650 and 300 cycles for the 170 nm and 270 nm thinfilms,

respectively The relative capacity of the film decreases with the

increasefilm thickness and that is due to the increase in the amount

of grain boundaries and of the reaction surfaces [36] So it is

obvious that thinnerfilms needed to be cycled with more cycles to

reach the maximum amount of charges The cycling life of the NiO

films is typically based on a three-step process, namely the

acti-vation period, the steady state and the degradation period[34] The

first state during which the capacity increases may last from a few

to hundreds of cycles depending on the reference electrode con-centration or on the scanned potential window In a more impor-tant manner, the activation period, during which thefilm nature is progressively modified, appears longer duration for the thinner and less agglomeratedfilms which is observed for the film with 170 nm thickness Whatever the film thickness will be, coulometric ca-pacity reaches once its maximum in the steady state and then de-creases in the degradation period This degradation period is of much larger intensity in case of the thicker agglomeratedfilms After about 800 cycles, 78% of capacity was found left for the

540 nm thickfilms

The response time of thefilms was tested using potential step measurements.Fig 9shows the results of the potential step mea-surement for thefilms under investigation It indicates that the thickerfilm needs longer time to respond than the thinner film, so the response time is fast in thinfilms The coloration efficiency (CE)

is defined as

50

60

70

80

90

100

Number of cycles

170 nm

270 nm

380 nm

540 nm

Fig 8 Relative capacity as a function of the number of electrochromic cycles for

different films with different thicknesses.

0 0

0

0.000

0.002

0.004

0.006

0.008

0.010

Elapsed time (s)

540 nm

170 nm

Fig 9 Electrochromic response time during coloration of 170 nm and 540 nm thick

films.

cycle 800 times

as-deposited

2θ (deg.)

Fig 10 XRD patterns of 540 nm thick NiO films in as-deposited state and after 800 cyclic tests.

(111)

cycle 800 times

as-deposited

2θ (deg.)

Fig 11 XRD patterns of a 170 nm thick NiO in as-deposited state and after 800 cyclic D.R Sahu et al / Journal of Science: Advanced Materials and Devices 2 (2017) 225e232

230

It is found to decrease with the increasingfilm thickness It is

about 53.1 C/cm2for the 170 nm and 32.4 C/cm2for the 540 nm

film There is an increase of the injected charge and a decrease in

the transmittance As a result the coloration efficiency becomes

smaller for thickerfilms[37] Our result is in contrast with those

reported by other authors that the CE increases with the thickness

of NiOfilms where the value obtained at 630 nm responses to our

results at 550 nm[9,38] Here, the CE decreases rapidly for thinner

films after undergoing electrochromic cycles The thicker films have

longer cycling life than that of the thinnerfilm

The XRD pattern of the 540 nm thickfilm after 800 cycles

pre-sented inFig 10indicates that there is no change in thefilm

struc-ture after 800 cycles of electrochromic cycling However, in the case

of the XRD pattern for the 170 nm thickfilm shown inFig 11, there

appears the (111) peak after 800 cycles of electrochromic cycling

Thefilm structure changes with the electrochromic cycles along

with the changes in the surface morphology Films degrade with

cyclic tests The cyclic property is related to film integrity[39]

Therefore, thinfilms appear to have short response time and high

coloration efficiency and so they are fast responding The thick films

have greater transmittance modulation and durability This suggests

that e-beam evaporation synthesized NiOfilms can well meet the

requirement of application in electrochromic devices

4 Conclusion

NiOfilms with various thicknesses were successfully deposited

on ITO glass using the e-beam evaporation method The relation

between the thickness of NiO films and their electrochromic

properties is investigated and discussed In thicker films,

well-developed crystal structure and crystallite size have been

devel-oped which enable greater transmittance modulation and

dura-bility As-deposited thinnerfilms of 170 nm thickness show fast

response time during electrochromic cycles with coloration ef

fi-ciency in the order of 53.1 C/cm2 The thickerfilms are found to

have larger amounts of intercalated charge than the thinner ones

but the electrochromic properties do not increase proportionally

The thickness variation of electrochromic properties NiOfilms

de-pends on thefilm preparation conditions and methods along with

the electrolytes used The e-beam evaporation deposition method

without using post deposition technique seems to be suitable for

the production of future electrochromic devices

Acknowledgments

Authors are thankful for thefinancial grant received from

Na-tional Science Council, Taiwan under Contract No: NSC

96-2218-E-006-006

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