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CHAPTERCHAPTERCHAPTERCHAPTER----IIII
INTRODUCTION INTRODUCTION INTRODUCTION INTRODUCTION
AND AND AND AND
THEORETICAL THEORETICAL THEORETICAL THEORETICAL
BACKGROUNDBACKGROUNDBACKGROUNDBACKGROUND
CHAPTER – I
Introduction and Theoretical Background
Sr.
No.
Title Page
No.
1.1 Introduction……………………………………………………………………………............
1.1.1 Nanostructured Materials………………………………………………………
1.1.2 Nanoporous Materials……………………………………………………………
1.1.3 Chromogenic Devices……………………………………………………............
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1.2 Electrochromic Devices……………………………………………………………............
1.2.1 History of Electrochromism…………………………………………..............
1.2.2 Structure of the Electrochromic Devices…………………………………
1.2.3 Parameters Involved in Electrochromic Devices……………………..
1.2.3 (A) Electrochromic Contrast…………………………………………
1.2.3 (B) Coloration Efficiency………………………………………………
1.2.3 (C) Switching Speed……………………………………………………..
1.2.3 (D) Reversibility…………………………………………………………..
1.2.3 (E) Stability………………………………………………………………....
1.2.3 (F) Optical Memory……………………………………………..............
1.2.3 (G) Diffusion Coefficient……………………………………………….
1.2.4 Present Status of Electrochromic Devices………………………....…….
1.2.5 Various Applications of Electrochromic Devices (ECDs)…………..
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1.3 Transition Metal Oxide………...…………………………………………………..............
1.3.1 Nickel Oxide (NiO)……..…………………………………………………………….
1.3.2 Nickel Hydroxide Ni(OH)2………………………………………………..........
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1.4 Mechanism of Coloration in NiO and Formation of NiOOH…………............ 18
1.5 Crystal field Theory of NiO……………………………………………………………….. 20
1.6 Literature Survey on NiO…………………………………………………………………. 22
1.7 Purpose of the Dissertation……………………………………………………………… 34
1.8 Plan of Work……………………………………………………………………………………. 36
References………………………………………………………………………………............ 37
Introduction and Theoretical Background…………….
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Introduction and Theoretical Background
1.1 : Introduction
1.1.1 : Nanostructured Materials
The nanostructured materials are those having at least one dimension falling
in nanometer scale and include nanoparticles, nanorods and nanowires, ultra thin
films and bulk materials made of nanoscale building blocks or consisting of nanoscale
structures. What makes nanoscale building blocks interesting is that by controlling
the size in the range of 1-100 nm and the assembly of such constituents, one could
alter and prescribe the properties of the assembled nanostructures. It is possible to
dial-in the properties by judiciously controlling the dimensions and aspect ratio.
Nanostructured materials have been widely investigated in the past two
decades. The impact of these researches on both fundamental science and potential
industrial application has been attracting tremendous interest, investment and effort
in research and development around the world. Nanostructured materials possess
nanoscale crystallites, long-range ordered or disordered structures or pore space.
Nanomaterials can be designed and tailor-made at the molecular level to have desired
functionalities and properties. Manipulating matter at such a small scale with precise
control of its properties is one of the hallmarks of nanotechnology. As Professor Roald
Hoffmann-the Chemistry Nobel Laureate put it, "Nanotechnology is the way of
ingeniously controlling the building of small and large structures, with intricate
properties; it is the way of the future, with incidentally, environmental benignness
built in by design". The research on nanostructured materials is highly
interdisciplinary because of different synthetic methodologies involved, as well as
many different physical characterization techniques used. The success of the
nanostructured material research is increasingly relying upon the collective efforts
from various disciplines [1]. There are many exciting examples of nanostructured
materials in the past decades including colloidal nanocrystals, bucky ball C60, carbon
nanotube, semiconductor nanowire, and nanoporous material.
CHAPTER
FIRST
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1.1.2: Nanoporous Materials
The term nanoporous materials is defined as the materials with cavities or
channels that are deeper than their width. Generally porous materials have porosity
(volume ratio of pore space to the total volume of the material) between 0.2-0.95.
The presence of pores (holes) in a material can render itself all sorts of useful
properties that the corresponding bulk material would not have such as high specific
area, fluid permeability and molecular sieving and shape-selective effects. Different
nanoporous materials with varying pore size, porosity, pore size distribution and
composition have different pore and surface properties that will eventually determine
their potential applications [2].
Nanoporous materials are of scientific and technological importance because
of their vast ability to absorb and interact with atoms, ions and molecules on their
large interior surfaces and in the nanometer sized pore space. Nanoporous materials
have specifically a high surface to volume ratio, with a high surface area and large
porosity, of course, and very ordered, uniform pore structure. A lot of inorganic
nanoporous materials are made of oxides. They are often non-toxic, inert, and
chemically and thermally stable [3].
1.1.3: Chromogenic Devices
The term chromism is a process that induces a reversible change in the optical
properties of compounds [4]. In most cases, chromism is based on a change in the
electronic states of molecules, especially the π- or d-electron state. This phenomenon
is induced by various external stimuli which can alter the electron density of
substances. So, chromism can be classified according to the kinds of used stimuli. The
major kinds of chromism are
� Thermochromic Devices: Thermochromic layers in the windows change
transmission continuously over a range of temperatures so they not only reduce
heat loads (especially at times of peak demand), but they maximize daylighting.
� Photochromism Devices: Photochromic devices change their transparency in
response to light intensity. Photochromic materials have been used in eyeglasses
that change from clear in the dim indoor light to dark in the bright outdoors.
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� Electrochromic Devices: The change in optical properties of materials in
persistent and reversible manner when the potential is applied across it. The
original state retains after reversal of the polarity.
� Gasochromic Devices: The change in optical properties of gasochromic materials
is caused by the chemical reaction between a special layer coated on the glass, and
a gas fed into the cavity between the two glass panes.
� Phase dispersed Liquid Crystal Devices: An optical change in such materials
brought about by a change in the orientation of liquid crystals suspended
between two sheets of transparent conductor coated glass or polyester
electrodes on the application of a voltage.
� Suspended Particle Device (SPD): This electrically controlled film utilizes a thin,
liquid-like layer in which numerous microscopic particles are suspended. In its
unpowered state the particles are randomly oriented and partially block sunlight
transmission and view. Transparent electrical conductors allow an electric field to
be applied to the dispersed particle film, aligning the particles and raising the
transmittance.
We live in a dynamic environment, but until now our building envelopes have
been static in nature-unable to effectively control the flow of the sun’s light and heat
into buildings from hour to hour and from season to season. Global warming is
receiving worldwide attention, and means to improve its harmful consequences are of
the greatest urgency [5]. Major changes in energy technology will be necessary, which
will impact global economy [6]. The changes must account for an increasing
population, whose accumulation in mega-cities leads to “heat islands” which tend to
enhance the warming [7]. The use of fossil fuel must be curtailed, which will influence
the use of energy in industry, for transport, and in buildings. Particular attention on
the built environment is natural considering the fact that this sector uses as much as
30 to 40 % of the primary energy in the world [8]. This energy is used predominantly
for heating, cooling, ventilation, and lighting. In particular, the energy demand for air
conditioning has grown very rapidly-by about 17 % per year-in the EU [9], and
already today electrically driven air conditioning dominates the peak power during
the summer in parts of Europe as well as in the U.S.A.; in more extreme climates, the
electrical peak power may be entirely dominated by air conditioning [10].
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Saving energy in the building sector and automotive industry is a major global
socio-economic target in energy efficiency as well as from environmental viewpoint.
Substantial savings in energy consumption can be realized through an optimal solar
radiations management with the emerging smart photonics in minimizing the usage
of air-conditioning systems. With worldwide ≈2 billion m2 of smart photonics coated
glass windows, energy saving in the two mentioned air-conditioning segments i.e.
buildings and cars, has been estimated to be ∼1 billion GJ and CO2 atmospheric
emissions would be reduced by ∼100 millions of tons [11]. The global production of
glass which could be solar regulated to minimize the air conditioning using emerging
smart nano-photonics, could be a part of 1 billion m2/year with about 25 % for
building and ~11 % for automotive industry [11-12]. To set the scene, one has to note
that heating, cooling, lighting, ventilation and powering of buildings and automotives
account for more than the half of the total energy consumption worldwide and hence
responsible for more energy consumption than any other end-user sector such as
industrial production. The considered electrochromic smart nano-photonics are also
valuable to the Southern African landscapes too. Indeed, the annual global solar
radiation received by South Africa averages 5.5 kWh/m2/day, one of the highest
national levels in the world [13].
Electronically tintable glass provides the means to develop a dynamic facade
with variable visible light transmission and solar heat gain coefficient, which saves
cooling and lighting energy, and solves problems of excessive solar heat gain, glare,
fading and the need for unsightly blinds. In fact, the U.S. Department of Energy (DOE)
states that their goal of a “zero energy building” in 2030 cannot be achieved without
the use of dynamic glazing. Electronically tintable glass is now commercially available
and is being actively specified and installed in building envelopes [14].
Electronically tintable glass is the future of the building envelope. It allows
people to dynamically control the heat and light flow into a building depending on the
needs of the occupant and the changing outside environment. Control systems can be
fully automated, manually controlled or hybridized and they can be integrated into
other building systems, such as lighting or AC, to provide optimized control and
energy efficiency.
The Environmental Protection Agency (EPA) estimates that up to 30 % of
commercial buildings’ energy is used for lighting and as much as 80 % of this lighting
Introduction and Theoretical Background…………….
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energy results in heat, which must be removed by air conditioning. Additionally,
HVAC systems account for more than 35 % of energy use in commercial buildings. In
an assessment conducted by Lawrence Berkeley National Laboratory (LBNL),
electrochromic windows were shown to save up to 60 % of daily lighting energy [15].
The U.S. Department of Energy (DOE) predicts that commercial buildings relying on
electrochromic window systems could save up to 28 % in energy costs when
compared to buildings with static, spectrally selective, low-e windows. The DOE also
reports [16] that electrochromic glass products can help achieve:
� 10-20 % operating cost savings;
� 15-24 % peak demand reduction;
� Up to 25 % decrease in HVAC system size.
In order to achieve the goal of zero energy building in the future, various attempts
have been made to make the glazing tintable with various chromogenic devices
depends on the external stimuli.
1.2 : Electrochromic Devices
The word electrochromism is a combination of two words: electro and
chromism. Electrochromic devices are able to change their optical properties in a
persistent and reversible manner when they are subjected to external applied electric
field. Briefly, an electrochromic device (ECD) consists of five major layers: ion
storage layer, ion conductor or electrolyte and an electrochromic layer on
transparent conducting substrates. When a voltage is applied between two electrodes,
simultaneous injection (or extraction) of ions and charge compensating electrons
reduces (oxidizes) the electrochromic layer to a colored state. On the reversal of
polarity, the electrochromic layer is oxidized back to its transparent state.
Electrochromism is exhibited by almost all transition metal oxides, viologens and
conducting polymers. Depending on their properties of coloration on
intercalation/deintercalation of ions, they are classified as cathodic and anodic
materials. Common inorganic electrochromic materials are tungsten oxide, nickel
oxide, vanadium oxide, niobium oxide etc. Electrochromic materials have an added
advantage of exhibiting memory effect i.e. the device remains in the same state even
after the removal of the voltage for a long time. Only a small voltage (1-5 V) is
required to change the glass from transparent to colored and vice versa. Thus
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power is necessary only when change from one state to another is required. By
controlling the voltage, various stages of coloration can be achieved.
1.2.1: History of Electrochromism
The electrochromism history started in 1704, when Diesbach discovered the
Prussian blue, an excellent dye which had also electrochromic properties. This
material changes its state from dark blue to transparent when a voltage is applied
across it. In 1815 the electrochromism of WO3 was discovered, by Berzelius [17]. In
fact, it was showed that pure WO3 changed color on reduction when warmed under a
flow of dry hydrogen gas. Later in 1824 Wohler effected a similar chemical reduction
with sodium metal. Kobosew and Nekrasso in 1830, recorded that WO3 powders
could acquire the color blue by electrochemical reduction in an acidic solution. The
first step towards an electrochromic device was taken in 1942 by Talmey, in studies
on the coloration associated with electrolytic reduction of artificially produced
particulate molybdenum and tungsten oxide layers. In 1953 Kraus made a very clear
description of electrochromism in tungsten oxide films. As none of these studies
attracted much attention, probably most current investigators attribute the first
widely accepted suggestion of an electrochromic device to Deb, in 1969, with the
tungsten oxide films, and after this point, there was a visible increase of the interest in
electrochromism. In spite of the innovation on Deb’s first electrochromic device it
wasn’t able to keep up with the fast development of liquid crystal devices [17, 18].
In 1971, Blanc and Staebler produced an electrochromic effect superior to
most of the previously published. They applied electrodes to the opposing faces of
doped, crystalline SrTiO3 (Strontium Titanium Trioxide) and observed an
electrochromic color move into the crystal from the two electrodes. In 1972, Beegle
developed a display having identical counter and working electrodes as the one from
Blanc and Staebler, but made of WO3 [17, 18]. Nowadays, Deb’s paper form 1973 is
quoted as the work responsible for the true birth of electrochromic technology.
Faughan et al. [19] in 1975 accomplished a significant progress in developing the
electrochromic display device. This was followed by an increase in electrochromic
devices developed for display applications.
Nevertheless, electrochromism has remained an active area for basic and
applied research, with large possibilities for applications in emerging technologies.
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The interest was boosted in the mid-1980s with the awareness that electrochromism
was of much interest as a mean to achieve energy efficiency in buildings, using smart
windows [20]. The smart windows and other electrochromic systems consist of two
electrodes and an electrolyte. When applied voltage with appropriate polarity, charge
in the cell drives in and out of the electrochromic material and an electrochemical
redox reaction causes a corresponding color change. Therefore electrochromic
materials are currently attracting much interest in industry for their commercial
applications [21].
Possible applications of electrochromic materials include, among others,
electrochromic displays, cathode ray tubes, thermal-exposure indicator for frozen
foodstuffs, electrochromic mirrors and windows. Electrochromic displays are an
application where significant advances were made in the 1970s and 1980s with the
development of watch and clock displays. However some of their biggest limitations
were speed and lifetime. But for this kind of applications these limitations weren’t
critical because these areas do not require very rapid updating of display information
[22].
Electrochromic materials can also be used in cathode ray tubes with variable
transmittance. An electrochromically darkening cathode ray tube screen employing
oxides is an alternative to the common brilliance adjustments of TV tubes, when room
illumination alters. Electrochromic darkening is preferable to direct electrical control
as color values are thereby better preserved [23].
Another application can be a thermal-exposure indicator for frozen foodstuffs.
For the case of the electrochromic mirrors, the most common device is the car rear
view mirror. This mirror changes its color to a dark blue-green color that allows only
the outline of the usual dazzling headlights to appear. Here an optically absorbing
electrochromic color is evoked over the reflecting surface, reducing reflection
intensity and thereby alleviating driver discomfort. The back electrode is a reflective
material allowing customary mirror reflection in the bleached state [23].
1.2.2: Structure of the Electrochromic Devices
There are a number of ways of constructing an electrochromic device, the most
common one being as shown in the Fig.1.1. Two glass or plastic substrates form the
base of the ECDs. These glass or plastic substrates are usually coated with transparent
Introduction and Theoretical Background…………….
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conducting oxides like Indium Tin Oxide (ITO), F-doped tin oxide (FTO), ZnO and
sometimes a thin layer of gold to make them conducting. An electrochromic film,
usually WO3 or conducting polymer is deposited on one of the transparent conducting
oxides. This layer behaves as the working electrode. The second substrate acts as a
counter electrode or an ion storage layer. This layer may be either a plane glass or
plastic coated with TCO (passive electrode) or it may be deposited with another layer
of EC material whose color is in phase with working electrode to make it EC active.
The ion conducting electrolyte lies in between them. The ion conducting layer may be
in the form of a liquid or a solid electrolyte rich in ions with high mobility, mostly H+
or alkali ions with a small radius to facilitate easy ion insertion/extraction. The device
functioning takes place in the following order: Whenever a small voltage of the order
of a few volts is applied, transport of H+ or Li+ ions takes place from the ion storage
layer through the ion conduction layer and the ions are inserted into the
electrochromic layer, at the same time there is a corresponding charge balancing
counter flow of electron takes place from transparent conductor to compensate the
charge. These electrons remain in the electrochromic film as long as ions reside there
and optical properties of the film changes to make film color. Now if the voltage
source is removed, the device exhibits open circuit or non-volatile memory and the
colored state is retained for a long time. This zero current consumption after
coloration, non-volatile memory is often cited as a valuable desired property in large
electrochromic systems [23]. If the polarities of the voltage are reversed, the EC film
returns back to its original transparent state as shown in Fig.1.2. The coloration can
be stopped at any level i.e. different stages of coloration can be achieved depending on
the preference of the user.
It is one of the most important applications, which works as a light and heat
filters for the external glazing of buildings leading to a reduction in fossil fuel
consumption.
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Figure.1.1: A basic electrochromic film device structure.
Figure.1.2: Electrochromic windows (ECWs) with illustration of visible light and
solar heat energy during operation.
The primary advantage of an EC glazing is the capability for a dynamic control
of the transmittance of the glass. In summer it is used to reject the maximum possible
IR radiation to lower the air conditioning cost while in winter it is set to pass as much
as possible the IR to warm the interior place. This has great economical benefits. One
main advantage of EC glazing is the dynamic control of the color state so it can
provide night time’s insulation and privacy and can be also adjusted to reduce glare. A
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number of criteria are necessary for electrochromic smart window applications. They
are summarized as follows [24, 25].
• continuous range in solar and optical transmittance, reflectance and
absorptions between bleached and colored states,
• contrast ratio (CR) of at least 5 : 1,
• coloring and bleaching times (switching speed) of a few minutes,
• operating glass surface temperatures of –20 °C to +80 °C,
• switching with applied voltages of 1 to 5 V,
• open circuit memory of a few hours (maintains a fixed state of transmittance
without corrective voltage pulses),
• acceptable neutral color,
• large area with excellent optical clarity,
• sustained performance over 20–30 yr,
• acceptable cost
1.2.3: Parameters Involved in Electrochromic Devices
1.2.3 (A): Electrochromic Contrast
It is defined as a percent transmittance change (∆T %) between colored and
bleached state at a specified wavelength where the electrochromic material has the
highest optical contrast. Mathematically,
).( 11−−−−−−−−−−−−=∆= cbconst
TTTλ
For some applications, it is more useful to report a contrast over a specified
range rather than a single wavelength. In order to obtain an overall electrochromic
contrast, measuring the relative luminance change provides more realistic contrast
values since it offers a perspective on the transmissivity of a material as it relates to
the human eye perception of transmittance over the entire visible spectrum [26, 27].
In this case chromic contrast is given by the equation.
).( 21−−−−−−−−−−−−=∆ ∗∗coloredbleached LLL
1.2.3 (B): Coloration Efficiency
The amount of optical density change (∆OD) induced as a function of the
injected/ejected electronic charge (Qi), i.e. the amount of charge necessary to produce
the optical change is determined by coloration efficiency. It is given by the equation:
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).(]ln[
31−−−−−−−−−−=∆
ODcT
bT
i
η
where η (cm2/C) is the coloration efficiency at a given λ, and Tb and Tc are the
bleached and colored transmittance values, respectively. An electrochromic device
with high coloration efficiency provides big difference in transmittance with small
amount of electric charge [28].
1.2.3 (C): Switching Speed
Switching speed is often reported as the time required for the
coloring/bleaching process of an EC material. It is important especially for
applications such as dynamic displays and switchable mirrors. The switching speed of
electrochromic materials is dependent on several factors such as ionic conductivity of
the electrolyte, accessibility of the ions to the electroactive sites (ion diffusion in thin
films), magnitude of the applied potential, film thickness, and the morphology of the
thin film.
1.2.3 (D): Reversibility
From the chronocoulometry curves the amount of charges intercalated (Qi)
and deintercalated (Qdi) during redox process are calculated. The electrochemical
reversibility is calculated using the relation,
).(Re 41−−−−−−−−−−−−=i
di
Q
Qyversibilit
Though a reversibility of 1 is ideal, in most of the cases it is less than 1 owing
to the residual charges in the material.
1.2.3 (E): Stability
This term is often used to describe the cycling life of an electrochromic film. It
is defined as the number of cycles an ECD remains stable without any degradation in
the physical structure or its electrochromic performance. Electrochromic stability is
usually associated with electrochemical stability since the degradation of the active
redox couple results in the loss of electrochromic contrast and hence the performance
of the EC material. Common degradation paths include irreversible oxidation or
reduction at extreme potentials, iR loss of the electrode or the electrolyte leading to
internal heating, side reactions due to the presence of water or oxygen in the cell, and
the heat released due to the resistive parts in the system. Although current reports
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include switching stabilities of up to 106 cycles without significance performance loss,
the lack of durability (especially compared to LCDs) is still an important drawback for
commercialization of ECDs. Defect-free processing of thin films, careful charge
balance of the electroactive components, and air-free sealing of devices are important
factors for long-term operation of ECDs.
1.2.3 (F): Optical Memory
It is defined as the time an electrochromic material retains its absorption state
after removal of the electric field. In solution based electrochromic systems such as
viologens, the colored state quickly bleaches upon termination of current due to the
diffusion of soluble electrochromes away from the electrodes and reacting in the
electrolyte (a phenomenon called self-erasing). In solid state ECDs where the
electrochromes are adhered to electrodes, electrochromic memory can be as long as
days or weeks with no further current required. In reality however, ECDs may require
small refreshing charges in order to maintain the charge state because side reactions
or short circuits change the desired color [29].
1.2.3 (G): Diffusion Coefficient
It is the measure of ease with which an ionic species can
intercalate/deintercalate in the host lattice. It is given by Randles-Sevcik equation
[30],
).(vCA3/2n5102.72
pj
1/2D
0
5121
−−−−−−−−−−−−×××××
=/
where, jp = cathodic or anodic peak current,
C = concentration of ionic species, n = ionic charge,
A = number of electrons transferred during redox reaction and
ν = potential scan rate.
1.2.4: Present Status of Electrochromic Devices
The most common applications of EC materials include a variety of displays,
smart windows, optical shutters, and mirror devices. Prototypes of “smart windows”
for buildings based on inorganic metal oxides have been developed, but often suffer
from low cost-effectiveness. Windows allow the Sun's warmth and light to permeate
our living space and overviews of outside. Unfortunately this luxury brings high costs
of heating, cooling and shading. Companies such as the Gentex Corp. and Donnelly
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have commercialized electrochromic mirrors as rearview visors for the automobile
industry.
The International Energy Agency (IEA), Department of Energy (DOE) and
National Renewable Energy Laboratory (NREL), in US, have focused on the
development of a process for testing electrochromic windows, ECWs. DOE is actually
testing the ECWs of the four companies, Donelly Corp., OCLI (Santa Rosa, California),
EIC Labs (Norwood. Miami) and Anderson Windows (St. Paul, Minnesota).
Moreover, St. Paul and SAGE Electrochromics, Inc., Minnesota have received,
from NIST in US, a huge fund, aimed at developing large area electrochromic devices
on plastic substrates. SAGE Electrochromics, Inc. announced more than $100 million
in DOE loan guarantees and government tax credits, spurred on by the Department of
Energy’s Loan Guarantee Program, which was established under the Energy Policy Act
of 2005.
Additionally, researchers from Lawrence Berkeley Laboratories (LBNL) have
installed and tested smart windows for office rooms in Oakland, CA, and the National
Renewable Energy Laboratories (NREL) has ongoing research on developing
prototypes of vertically integrated, photovoltaic powered electrochromic displays. In
a different application, the optical change of chromogenic materials due to proton
intercalation is promising for hydrogen sensor applications and has been
demonstrated by NREL researchers using WO3 as a molecular hydrogen sensor. LBNL
aimed at developing large area electrochromic polymer layers (PEDOT) working with
lithium ions. Pilkington introduced its first commercial electrochromic smart window
product on glass in late 1998 and carried out multi electrochromic glazing under
program of Joule 11. Flachglass (Germany), Davionics AS (Denmark), Oxford Brookes
University, University of Southampton and others, are involved in this project.
Flachglass and Saint-Gobain are producing sunroofs for cars in size of 46 × 78 cm2
which transmit 14 % of the visible light in the colored state. Asahi Glass has steadily
developed ECWs based on LixWO3/inorganic lithium ion conductor/NiOx under
Sunshine Project funded by the Japanese government. Asahi Glass applied about 200
pieces of their prototype products into Seto Bridge Meseum (Kojima, Okayama-Pref.,
Japan).
The Dow Chemical Co.’s COMMOTION technology, based on printed
electrochromic inks, has been specifically developed for use in promotional products
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including smart label and inexpensive display applications. COMMOTION has already
been used by a U.K. retailer, Marks and Spencer, for an animated greeting card
application. NTERA’s NanoChromics technology allows for fabrication of flexible
display devices benefiting from high surface area of nanostructured metal oxides
blended with organic electrochromes. Finally, Cidetec of North Spain has introduced
polymer-based electrochromic false nails, probably the oddest application the field
has encountered.
Owing to the mentioned global activities above, EControl-glass successfully
commercialized smart electrochromic windows for buildings for the first time in
2006; the electrochromic thin films were deposited by sputtering. EControl-glass
established a mass production line with vertical typed sputtering machines in Plauen
(Germany) in 2009. In 2008 SAGE Electrochromics, Inc. started the commercialization
in North America of large area ECWs (100 × 150 cm2) with 3~5 min of switching time
[31]. They used the monolithic technology to construct their Li-based ECWs. Saint-
Gobain is starting the commercialization of proton-based ECWs for buildings with
relatively short switching response time. In 2010, Gesimat produces the world's most
energy-efficient window glass [32].
1.2.5: Various Applications of Electrochromic Devices (ECDs)
In addition to electrochromic windows (ECWs) for buildings, Fig. 4 gathers
other practical applications using electrochromic devices (ECDs) such as smart
sunroofs, filters, rear-view mirrors, smart glass wears, helmets, and displays.
Figure.1.3: Various application electrochromic devices [33].
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The automotive glazings like sunroof and side windows have an advantage
over the architectural windows because of their smaller size and shorter required life
time, although the upper temperature limits are higher (90–100 °C) [34].
1.3: Transition Metal Oxide
Cathodic coloration is found in oxides of Ti, Nb, Mo, Ta and W, according to the
following equation; while anodic coloration is found in oxides of Cr, Mn, Fe, Co,
Ni, Rh and Ir. Electrochromism is based on the formation of colored compound from
oxides of polyvalent metal upon insertion or extraction of small ion like H+ or OH-, Li+,
F- etc. according to the following equation;
For cathodically coloring material (WO3):
MeOn + xA+ + xe- ⇔ AxMcOn ……(1.6)
For anodically coloring material:
MeOn + xA- + xh+ ⇔ AxMcOn ……(1.7)
where, Me is metal atom; A+ is a singly charged small ion like H+ or Li+; A- is a singly
charged small ion like OH- or F-; e- is an electron; n depends on the particular type of
oxide; x is generally 0 < x < 1.
In cathodically coloring EC material (WO3), the coloration takes place upon
intercalation (insertion) H+ or Li+ ions and bleaching takes place upon deintercalation
(extraction) of H+ or Li+ ions. Colored compounds (Bronzes) are partially reduced
oxides of polyvalent metal, which contain the cations of the reducing element (H+ or
Li+) [35].
In anodically coloring EC material (NiO), the coloration takes place upon
intercalation of OH- ions and bleaching takes place upon deintercalation of OH- ions.
For anodic coloring EC materials, reaction can be written in terms of both OH- and H+
ions, which usually require hydrated form of oxides [36]. The position of equilibrium
(equation 1.6 and 1.7) is dependent on the standard free enthalpy of formation and on
the relative concentration or mole fraction of the component [37].
1.3.1: Nickel Oxide (NiO)
Nickel monoxide (NiO) belongs to the 3d transition metal oxides with a NaCl
structure as shown in Fig.1.4, with a lattice parameter of 0.4173 nm and a density of
6.7 g/cm3 [38]. The electronic structure of Ni is 1s2 2s2 2p6 3s2 3p6 3d8 4s2. Its outer
electrons shell has a 4s2 3d8 configuration.
Introduction and Theoretical Background…………….
Chapter-I
Figure.1.4: Crystal structure of NiO
Two transition-metal electrons saturate the O2p shell, leading to O
[He] 2s2 2p6 configuration and TM
Oxide Electronic Configuration
O2− TM2+
NiO [He]2s2 2p6 [Ar]3d
Stoichiometric NiO is an anti
stoichiometric NiO crystals, since they always exhibit defect structure with excess
oxygen and nickel vacancies [39
structure; instead vacancies related to Ni
Ni2+ to Ni3+ around the nickel vacancies which pushes the Fermi level to the top of the
valence band (oxygen 2p band) [
semiconductor [41, 42].
The conduction is explained
upper part of the valence band consists of Ni 3d states starting at ~2 eV below the
Fermi level. In reality, these states overlap with a wide O 2p band at ~4 to 8 eV from
the Fermi level [43]. The Ni 3d and O 2p
There is a splitting between the Ni 3d levels with different spin directions [
The optical band gap is ~4 eV [
minority spin t2g and eg bands [4
Introduction and Theoretical Background…………….
Crystal structure of NiO
metal electrons saturate the O2p shell, leading to O2− ions with the
figuration and TM2+ ions with the [Ar] 3dn configuration (Table
figuration Insulating
gap (eV)
Neel
Temperature (K)
Lattice Constant
(nm)2+
[Ar]3d8 3.1–4.3 523 0.417
Stoichiometric NiO is an anti-ferromagnetic insulator. It is not possible to make
stoichiometric NiO crystals, since they always exhibit defect structure with excess
9]. The extra oxygen cannot be placed inside the NaCl
structure; instead vacancies related to Ni2+ are created. This causes the oxidation of
around the nickel vacancies which pushes the Fermi level to the top of the
valence band (oxygen 2p band) [40] and the monoxide behaves like a p
The conduction is explained as a result of the migration of these vacancies.
upper part of the valence band consists of Ni 3d states starting at ~2 eV below the
Fermi level. In reality, these states overlap with a wide O 2p band at ~4 to 8 eV from
he Ni 3d and O 2p states are almost completely hybridized [
There is a splitting between the Ni 3d levels with different spin directions [
The optical band gap is ~4 eV [47]; it may arise from the separation between the
44]. The conduction band consists of unoccupied Ni 3d
Page 16
ions with the
figuration (Table 1.1).
Lattice Constant
(nm)
0.417
It is not possible to make
stoichiometric NiO crystals, since they always exhibit defect structure with excess
The extra oxygen cannot be placed inside the NaCl
This causes the oxidation of
around the nickel vacancies which pushes the Fermi level to the top of the
] and the monoxide behaves like a p-type
of the migration of these vacancies. The
upper part of the valence band consists of Ni 3d states starting at ~2 eV below the
Fermi level. In reality, these states overlap with a wide O 2p band at ~4 to 8 eV from
states are almost completely hybridized [44].
There is a splitting between the Ni 3d levels with different spin directions [45, 46].
]; it may arise from the separation between the
onduction band consists of unoccupied Ni 3d
Introduction and Theoretical Background…………….
Chapter-I
(eg) states with an admixture of O2p states; 4s and 4p bands are present at higher
energies [41, 48]. A schematic band diagram is depicted in Fig.
Figure.1.5: Schematic band structure of NiO [
well as majority and minority spin states are indicated using standard notation,
together with the Fermi level (E
accommodated in the bands. Filled states are shaded.
1.3.2: Nickel hydroxide Ni(OH)
Nickel hydroxide is an n
while the oxyhydroxide phase is a p
1.8 eV [49]. Additional absorption close to 1.5 eV has also been obs
oxyhydroxide [49]. The Ni(OH)
difference is the quantity of
content of water and the β phase at high content [
β-Ni(OH)2 has a layered structure, each layer consisting of an hexagonal
structure that consist of packing of hydroxyl ions with Ni
of octahedral coordination of oxygen, three oxygen atoms lying above the Ni plane
and three lying below. Protons are located in tetrahedral sites above and below the
oxygen sites. The layers are stacked along the c
Introduction and Theoretical Background…………….
) states with an admixture of O2p states; 4s and 4p bands are present at higher
A schematic band diagram is depicted in Fig. 1.5
Schematic band structure of NiO [35]. Nickel and oxygen orbitals as
well as majority and minority spin states are indicated using standard notation,
together with the Fermi level (Ef). The indicated number of electrons (e
accommodated in the bands. Filled states are shaded.
Nickel hydroxide Ni(OH)2
Nickel hydroxide is an n-type semiconductor with a band gap of ~3.6 to 3.9 eV,
while the oxyhydroxide phase is a p-type semiconductor with a band gap of ~1.7 to
]. Additional absorption close to 1.5 eV has also been observed for the Ni
Ni(OH)2 exists in two modifications, α and
difference is the quantity of water needed for stabilization. A α phase occurs at low
content of water and the β phase at high content [50].
has a layered structure, each layer consisting of an hexagonal
that consist of packing of hydroxyl ions with Ni2+ occupying alternate rows
coordination of oxygen, three oxygen atoms lying above the Ni plane
and three lying below. Protons are located in tetrahedral sites above and below the
oxygen sites. The layers are stacked along the c-axis with the distance between layers
Page 17
) states with an admixture of O2p states; 4s and 4p bands are present at higher
and oxygen orbitals as
well as majority and minority spin states are indicated using standard notation,
). The indicated number of electrons (e-) can be
type semiconductor with a band gap of ~3.6 to 3.9 eV,
type semiconductor with a band gap of ~1.7 to
rved for the Ni
and β. Their only
phase occurs at low
has a layered structure, each layer consisting of an hexagonal
occupying alternate rows
coordination of oxygen, three oxygen atoms lying above the Ni plane
and three lying below. Protons are located in tetrahedral sites above and below the
with the distance between layers
Introduction and Theoretical Background…………….
Chapter-I
of 4.6 Å [51, 52]. The O-H bond is thought to be parallel to c
between the OH- bonds of the different planes [
a= 3.12Å (Ni-Ni distance in the layer) and c= 4.6 Å (inter slab distance) [
α-(NiOH)2 is the hydrated form of the β
intercalated between Ni(OH)2 layers.
impurities such as water, NO3
resulting in higher distance bet
hydroxides can be obtained depending on the degree of hydrated turbostratic
hydroxide. The variety occurs between a highly turbostratic α
stacked β-Ni(OH)2, as proposed by Le Bihan et al.
transparent in their thin film form.
Figure.1.6: Illustration of α-Ni(OH)
1.4: Mechanism of Coloration in NiO and
Nickel oxide has been studied intensively for used as an active material for
anodically coloring electrode in electrochromic devices. NiO is able to change
from a transparent state to a dark brown color
means of extraction of protons or insertion of OH
balancing electrons are simultaneously extracted from the valence band. The films are
probably a mixture of oxide and hydroxide components in the bleached state, since a
reservoir of protons seems to exist in the films
of electrochemistry of Ni-oxides and hydroxides, the oxidation/reduction mechanism
is far not well understood. To throw some light on electrochromic mechanism it may
good to start with the proposed
battery. Most of the studies are based on the old reaction scheme proposed by Bode et
al. for NiO electrodes cycles in KOH [
Introduction and Theoretical Background…………….
H bond is thought to be parallel to c-axis with no interaction
of the different planes [50]. The unit cell of this structure has
Ni distance in the layer) and c= 4.6 Å (inter slab distance) [50].
hydrated form of the β-Ni(OH)2 where water molecules are
layers. As illustrated in the left panel of Fig.1.
3- and CO32- are intercalated in the Ni(OH)
resulting in higher distance between them (c~7.6 Å). A great variety of α
hydroxides can be obtained depending on the degree of hydrated turbostratic
hydroxide. The variety occurs between a highly turbostratic α-Ni(OH)2 and the well
, as proposed by Le Bihan et al. [53]. Both types of Ni(OH)
transparent in their thin film form.
Ni(OH)2 and β-Ni(OH)2 phases [50].
in NiO and Formation of NiOOH
studied intensively for used as an active material for
anodically coloring electrode in electrochromic devices. NiO is able to change
dark brown color upon oxidation of Ni(II) to Ni(III) by
otons or insertion of OH- ions. At the same time c
balancing electrons are simultaneously extracted from the valence band. The films are
probably a mixture of oxide and hydroxide components in the bleached state, since a
exist in the films [54]. Regardless the long investigation
oxides and hydroxides, the oxidation/reduction mechanism
. To throw some light on electrochromic mechanism it may
good to start with the proposed mechanisms for bulk nickel oxide materials used in
battery. Most of the studies are based on the old reaction scheme proposed by Bode et
al. for NiO electrodes cycles in KOH [54].
Page 18
axis with no interaction
]. The unit cell of this structure has
where water molecules are
1.6, some
are intercalated in the Ni(OH)2 slabs
). A great variety of α-type
hydroxides can be obtained depending on the degree of hydrated turbostratic
and the well
oth types of Ni(OH)2 are
studied intensively for used as an active material for
anodically coloring electrode in electrochromic devices. NiO is able to change its color
upon oxidation of Ni(II) to Ni(III) by
At the same time charge-
balancing electrons are simultaneously extracted from the valence band. The films are
probably a mixture of oxide and hydroxide components in the bleached state, since a
Regardless the long investigation
oxides and hydroxides, the oxidation/reduction mechanism
. To throw some light on electrochromic mechanism it may
mechanisms for bulk nickel oxide materials used in
battery. Most of the studies are based on the old reaction scheme proposed by Bode et
Introduction and Theoretical Background…………….
Chapter-I
Figure.1.7: Bode scheme for NiO electrode oxidation reduction reaction
As shown in above Fig.1.7, t
transparent, while those on the right
from one to the other thus gives rise to the characteristic electrochromism of
hydrated Ni-oxide-based materials.
The main feature of the Bode scheme is the transition between Ni(II) and
Ni(III) by dehydration. There are two possible routes for the transition, (1) or (2).
Route (1) implies a β-Ni(OH)
transferred. The electrode can be cycled between these two phases by avoiding
overcharge. In the α-Ni(OH)
transferred due to the higher valence of the nickel in the γ
kinetics of the two reactions, it is reported that α
the oxidation potential of the later is about 50 mV higher than that of the former [
Irreversible β-NiOOH transformation to γ
the electrode or aging in KOH [
volume expansion or swell
intersheet distances in γ-NiOOH compared to β
electrostatic repulsion between the oxygen atoms in the adjacent layers after
removing the protons [55,
correlated to a 44 % increase in volume. There is also a complete family of γ
compounds. γ-NiOOH has different oxidation states ranging from 3 to 3.75 depending
on the preparation route. It has also a layered
ions inserted between the slabs of NiO
Figure.1.8: Illustration of β
Introduction and Theoretical Background…………….
Bode scheme for NiO electrode oxidation reduction reaction
As shown in above Fig.1.7, the compounds on the left-hand side are optically
rent, while those on the right-hand side are absorbing. The transformation
from one to the other thus gives rise to the characteristic electrochromism of
based materials.
The main feature of the Bode scheme is the transition between Ni(II) and
Ni(III) by dehydration. There are two possible routes for the transition, (1) or (2).
Ni(OH)2 to β-NiOOH transition, with only one electron is
transferred. The electrode can be cycled between these two phases by avoiding
Ni(OH)2 to γ-NiOOH transition (route 2) at least 1.3 electrons are
transferred due to the higher valence of the nickel in the γ-phase. Regarding the
kinetics of the two reactions, it is reported that α to γ is faster than β-II
al of the later is about 50 mV higher than that of the former [
NiOOH transformation to γ-NiOOH is possible through overcharge of
the electrode or aging in KOH [50]. The formation of γ-NiOOH is associated with a
volume expansion or swelling of the nickel hydroxide electrode due to the greater
NiOOH compared to β-NiOOH (7 Å versus 4.85 Å) due to the
electrostatic repulsion between the oxygen atoms in the adjacent layers after
, 56]. The phase change from β-NiOOH to γ-NiOOH can be
correlated to a 44 % increase in volume. There is also a complete family of γ
NiOOH has different oxidation states ranging from 3 to 3.75 depending
on the preparation route. It has also a layered structure with water, protons and alkali
ions inserted between the slabs of NiO2 (Fig.1.8).
Illustration of β-NiOOH and γ-NiOOH phases [50].
Page 19
Bode scheme for NiO electrode oxidation reduction reaction.
hand side are optically
hand side are absorbing. The transformation
from one to the other thus gives rise to the characteristic electrochromism of
The main feature of the Bode scheme is the transition between Ni(II) and
Ni(III) by dehydration. There are two possible routes for the transition, (1) or (2).
NiOOH transition, with only one electron is
transferred. The electrode can be cycled between these two phases by avoiding
NiOOH transition (route 2) at least 1.3 electrons are
phase. Regarding the
II to β-III while
al of the later is about 50 mV higher than that of the former [50].
NiOOH is possible through overcharge of
NiOOH is associated with a
ing of the nickel hydroxide electrode due to the greater
NiOOH (7 Å versus 4.85 Å) due to the
electrostatic repulsion between the oxygen atoms in the adjacent layers after
NiOOH can be
correlated to a 44 % increase in volume. There is also a complete family of γ-NiOOH
NiOOH has different oxidation states ranging from 3 to 3.75 depending
structure with water, protons and alkali
Introduction and Theoretical Background…………….
Chapter-I
The electrochromic mechanisms of the nickel oxides thin films based on the
above schemes are still unclear. There are three main reactions based on studies of
NiO thin films in aqueous media. The reactions differ according to the composition of
the layer (NiO or Ni(OH)2) and on the intercalated ions (H
Ni(OH)2 ⇔ NiOOH + H
Ni(OH)2 +OH- ⇔
NiO+ OH- ⇔ NiOOH + e
NiO and Ni(OH)2 are transparent while NiOOH is colored in thin film form.
1.5: Crystal field Theory of NiO
Crystal Field Theory describes how
presence of ligands (molecular compounds or ions that attach to a central transition
metal to form a coordination complex). Based on the strength of the ligands, magnetic
properties and color are changed. Although Cr
strength of the bonding, it does not however describe the actual bonding. This theory
was developed by Hans Bethe and John Hasbrouck van Vleck [
Figure.1.9: Crystal-field splitting ∆
Introduction and Theoretical Background…………….
The electrochromic mechanisms of the nickel oxides thin films based on the
unclear. There are three main reactions based on studies of
NiO thin films in aqueous media. The reactions differ according to the composition of
) and on the intercalated ions (H+ or OH-). They are:
NiOOH + H+ + e- (1.8
⇔ NiOOH + H2O + e- (1.9
NiOOH + e- (1.10
are transparent while NiOOH is colored in thin film form.
NiO
Crystal Field Theory describes how electrons fill out energy levels in the
presence of ligands (molecular compounds or ions that attach to a central transition
metal to form a coordination complex). Based on the strength of the ligands, magnetic
properties and color are changed. Although Crystal Field Theory describes the
strength of the bonding, it does not however describe the actual bonding. This theory
was developed by Hans Bethe and John Hasbrouck van Vleck [57].
field splitting ∆CF and exchange splitting ∆Ex
Page 20
The electrochromic mechanisms of the nickel oxides thin films based on the
unclear. There are three main reactions based on studies of
NiO thin films in aqueous media. The reactions differ according to the composition of
). They are:
1.8)
1.9)
1.10)
electrons fill out energy levels in the
presence of ligands (molecular compounds or ions that attach to a central transition
metal to form a coordination complex). Based on the strength of the ligands, magnetic
ystal Field Theory describes the
strength of the bonding, it does not however describe the actual bonding. This theory
Introduction and Theoretical Background…………….
Chapter-I Page 21
In order to understand Crystal Field Theory, we have to know the description
of the lobes.
dxy: the four lobes lie in-between the x and the y axes.
dxz: the four lobes lie in-between the x and the z axes.
dyz: the four lobes lie in-between the y and the z axes.
d(x2-y2): the four lobes lie on the x and y axes.
dz2: there are two lobes on the z axes and there is a donut shape ring that lies on the xy
plane around the other two lobes.
When a transition metal atom (Ni) or molecule is by itself, all five of the d
orbitals have an equal energy but are at different spacial orientations around the
nucleus. The orbital dz² has lobes along the z axis and circling around the metal ion
parallel to the x and y axes. The orbital dx²-y² has lobes lying on the x and y axes and for
the dxy, dxz, and dyz axes lobes occupy the regions in the planes of the respective axes
rather than lie on the axes as shown in Fig.1.9.
When electrons from a ligand approach a metal ion some follow a more direct
line of opposition from the d orbital electrons than others, depending on the structure
of the molecule. For an octahedral structure, ligands approach the metal ion along the
x, y and z axes. Therefore, the electrons in the dz² and dx²-y² orbitals, which lie along the
z axis and the x and y axes respectively, feel the most repulsion. It takes more energy
to have an electron in these orbitals than it would to put an electron in one of the
other orbitals. This causes a splitting in the energy level of the d orbitals known as
crystal field splitting as shown in Fig.1.9. Crystal field splitting is denoted by ∆CF, with
the subscript CF in this case to show it refers to an octahedral complex. The dz² and
dx²-y² orbitals energies increase from the normal energy of the d orbitals referred to as
eg and the dxy, dxz, and dyz orbitals decrease with respect to this normal energy
referred to as t2g, because the dxy, dxz, and dyz orbitals decrease in energy, they become
more stable.
For a transition metal complex, the d orbitals have this energy distribution
however which electrons get placed into which orbitals can be different. According to
Hund's rule, the electrons are initially placed into the lowest energy orbitals, thus
being the dxy, dxz, and dyz orbitals. The next electron however can be different. If the
dxy, dxz, and dyz orbitals are looked at as their own energy level, according to the
Aufbau Principle it would seem like the next electrons should fill the dxy, dxz, and dyz
Introduction and Theoretical Background…………….
Chapter-I Page 22
orbitals and pair up with the electrons already in these orbitals. This too however
requires energy, known as pairing energy. If the pairing energy is less than the crystal
field splitting energy, ∆₀, then the next electrons go into the dxy, dxz, and dyz orbitals
because greater stability is found in going into the orbital with lower energy. This
situation causes there to be the least amount of unpaired electrons and is known as
low spin. If the pairing energy is greater than ∆CF then the next electrons go into the
dz² and dx²-y² orbitals as unpaired electrons. This situation allows for the most number
of unpaired electrons, which is known as a high spin molecule. Ligands that cause a
transition metal to have a small crystal field splitting will also have a high spin. These
ligands are known as weak-field ligands. Ligands that produce a large crystal field
splitting have a low spin and are called strong field ligands. Ligands can be ordered by
their abilities to cause crystal field splitting and this ordering is known as the
spectrochemical series [58-59].
1.6: Literature Survey on NiO (1979-2012)
Among all cathodically and anodically coloring transition metal oxides, nickel
oxide (NiO) is by far extensively studied electrochromic material for use in smart
window as a anodically coloring material due to its high EC efficiency, large dynamic
range, cyclic reversibility and low material cost. Many techniques are available to
prepare EC NiO thin films. The major methods are categorized into physical, chemical
and electrochemical ones. The detailed survey on electrochromic NiO thin films
prepared by various techniques has been summarized.
Sol-Gel Deposition:
NiO films have been prepared using sols of different precursors. In the
beginning sol-gel routes went into two main ways, using either sulphate or nitrate
precursor but later other precursors like nickel chloride and nickel acetate have been
used. Aqueous sols of Ni(OH)2 have been obtained by precipitating NiSO4 in presence
of LiOH and acetic acid. NiO with grain size of about 2-3 nm was detected by TEM. The
transmittance variation in the range of 40-60 % and 35-40 mC/cm2 charge capacity is
reported. The CE of the films was 35-41 cm2/C depending on the number of cycles
[60, 61, 62]. It is also worthwhile to mention the preparation of Ni(Si)-oxide films
where small NiO nanoparticles (size of 2-5 nm) have been grown at 300 °C. Film with
improved adherence on ITO coated glass were deposited via sol-gel route using a
Introduction and Theoretical Background…………….
Chapter-I Page 23
dip-coating technique from sols containing in addition a nickel sulphate precursor
3-aminopropyltrimelhoxysilane (3-APMS). The electrochromic effect with
coloring/bleaching changes upto 55% was noted in 0.1M LiOH and CE was ∼26 cm2/C
[63].
Hydrated NiO films were prepared using a dip-coating technique from
NiSO4.7H2O precursor in combination with glycerol, formamide and polyvinyl alcohol
revealed that the CE was 23.5 cm2/C. The observed coloring/bleaching transmittance
of a 100 nm thick film changed during potential cycling (20 cycles) by 45 % [64]. Sol-
gel nickel oxide films have been formed by dissolving or reacting nickel nitrates in
alcohol and then using the product as precursors or from nickel 2-methoxyethoxide
based solution [65]. The NiO films obtained by wetting the substrates in a solution of
NiCl2.6H2O in butanol and ethylene glycol showed enhancement in the transmittance
modulation of 38 % for the film heat-treated at 300 oC while it was 16 % for the film
heat-treated at 250 oC [66].
Lithiated nickel oxide (LixNiO2, 0≤x≤1) sintered at high temperature has also
been reported. The films have LiNiO2 layer structure with crystallite size of 12 nm and
CE of 9 cm2/C [67]. Sols of nickel diacetate tetrahydrate (Ni(CH3COO)2.4H2O)
dissolved in pure methanol or in dry dimethylaminoethanol (dmaeth) or of nickel
diacetate dmaeH (Ni(acetate)2(dmaeH)2) in dry dimethylaminoethanol have been
used. The CE of the amorphous films varied between 6.2 and 16.4 cm2/C according to
the concentration of the sol and the thickness of the layer [68]. The NiO films
prepared by dissolving NiCl2·6H2O in butanol to a concentration of 0.4 M showed that
transmittance of the film shift from 90 to 40 % with stability observed up to 100
cycles [69]. However, the films prepared with Ni(CH3COO)2 precursor, optimal
response is achieved when only 25% decomposition occurs and NiO grain size
reaches 3-4 nm. The transmittance variation (ΔT) of 40.6 % and CE of 38 cm2/C was
observed at at 480 nm [70].
All solid state EC devices formed with the structure of
glass/FTO/NiO/ZrO2/FTO/glass showed the change in optical transmission (ΔT) =
33% (at 630 nm) and the photopic contrast ratio (PCR) is 2.43 (at 630 nm) [71].
Nickel oxide/hydroxide thin films were prepared by alternately dipping deposition
from NiSO4 and LiOH solutions followed by heat-treatment. The change in ΔT was low
at the beginning of cycling (13 % at λ = 480 nm), but increased during cycling and
Introduction and Theoretical Background…………….
Chapter-I Page 24
reached 45 % in the 101st cycle with corresponding CE reached up to 37 cm2/C [72].
The effect of boring doping in NiO thin films demonstrated that with the increasing of
the boron content, the variation of transmittance difference is up to 60 % in the range
of 380-500 nm [73]. The bleached state absorbance could be significantly lowered
when the Al added. EC efficiencies measured at λ=500 nm of the films with different
Al doping content reach ∼30 cm2/C, with a change in transmittance up to 70 % [74].
Nanostructured NiO films deposited on FTO glass substrate via sol-gel dip
coating method with a thickness of 306 nm exhibits maximum anodic/cathodic
diffusion coefficient of 11x10-12 cm2/s/6.44x10-12 cm2/s. This study indicates that the
NiO films prepared at 300 oC are optimally suited for electrochromic applications with
a ΔT of 68 %, good reversibility and a coloration/bleaching response time of 5.6/2.3 s
lasting beyond 1000 cycles [75]. Nickel oxide thin films was prepared by mixing 0.5 M
nickel acetate tetrahydrate [Ni(CH3COO)2.4H2O] in absolute ethanol. It was observed
that the highest value of the optical density is 0.334 at 693 K indicating the best
electrochromic performance of the crystalline NiO films, while the lowest value of the
optical density is 0.197 for films prepared at 733 K indicating the poor electrochromic
performance at high temperatures [76].
Park et al [77] studied electrochromic performance of micron scaled NiO thick
and thin films prepared from mixed solvent containing N, N-dimethylaminoethanol
(dmaeH) and distilled water at a 1:1 ratio which exhibits porous structure. The
transmittance difference of 22.8 % and 60.2 % with corresponding CE of 28.8 and
58.4 cm2/C was observed for thin and thick films respectively. The Co doped NiO thin
films exhibit highest optical modulation of 83 % with a photopic contrast ratio of
12.54 and the coloration and bleaching time were 5.9 and 2.4 s respectively [78].
Chemical Bath Deposition:
NiO thin films prepared by this method assigned to crystal structure β-Ni(OH)2
exhibited excellent CE of 56 cm2/C and its electrochromism is based on the reversible
oxidation of β-2NiO2.NiOOH structure [79]. An electrochromic devices (ECD)
fabricated by Restova et al. consisting of SnO2/NiOx/NaOH-H2O/SnO2, showed good
optical modulation and CE of 24.3 cm2/C [80]. Chen et al. synthesized NiO thin films
by chemical deposition followed by oxidization with H2O2 showed transmittance
difference of 53.9 % and fast colored/bleached response time of 1.7 s and 3.3 s
Introduction and Theoretical Background…………….
Chapter-I Page 25
respectively [81]. The highly porous NiO prepared Xia et al showed the CE of 42
cm2/C at 550 nm, with a variation of transmittance up to 82 % and good reaction
kinetics with fast switching speed [82]. The porous structure of the film NiO thin films
facilitates Ni(OH)2 phase formation by hydration in the electrolyte. These two phases
participate in the coloration process by producing Ni2O3 units giving high optical
contrast and good reversibility obtained up to 1000 c/b cycles and with a CE∼17
cm2/C [83].
The effect of annealing on the electrochromic performance of porous Ni(OH)2
was studied by Inamdar et al [84]. The change in the optical density (ΔOD) was
found to be 0.79 for the as-deposited films, whereas it was 0.53 and 0.50 for
the films annealed at 150 oC and 200 oC, respectively. The as deposited film
showed ΔT of 45 %, CE∼40 cm2/C and fast response time (190 ms for bleaching and
290 ms for coloration).
Electrodeposition:
The first attempt to produce EC NiO thin films is in 1979 by anodic oxidation
with electrodeposition [85]. The color transition from colorless to deep bronze was
reported in 1985 by Lampert et al. and it was predicted that the bronze color of the
films in the dark state is due to the presence of nickel metal inside the films [86]. The
cathodically deposited films showed crystal structure corresponds to nickel
hydroxide and showed porous structure with transmittance modulation (ΔT) around
68 % and the CE∼50 cm2/C at 450 nm with an absorption coefficient of 4 x 10-4 cm-1
[87].
The mechanism of coloration was investigated by proton intercalation
followed by Bode scheme [88, 89, 90] for anodically deposited hydrated NiO thin
films. It was claimed that reversible process between bleached and colored states
involves solid-state diffusion of the proton through the film, where the hydroxyl
group plays an important role in the reaction mechanism. The improvement in the
durability and charge capacity after thermal treatment was shown by anodically
deposited Ni(OH)2 thin film with CE of 40 cm2/C [91].
Anodically deposited NiO thin films prepared at 2 V with precursor containing
nickel sulphate and urea showed CE of 42 cm2/C at a charge capacity of Q=4 mC/cm2
[92]. The low angle X-ray diffraction (LAXRD) was performed to explain in detail
Introduction and Theoretical Background…………….
Chapter-I Page 26
mechanism of the EC transformation of the films based on the conversion between
two types of layered structure, α-Ni(OH)2/γ2-2NiO2.NiOOH or β-Ni(OH)2/β-NiOOH
[93]. It was shown that whether the insertion/extraction of cations occurs into the
films during anodic/cathodic process mainly depends on the structure of initial films,
concretely the interlayer distance, and the valence of oxidized nickel in the interlayer.
The Ni(OH)2/Ni/ITO electrode showed improved interfacial properties over
Ni(OH)2/ITO due to the metallic Ni nano-layer in between them. It was suggested that
this approach can be applied to the deposition of electrochromic counter electrode
and used to fabricate an ITO-free electrochromic layer because of improved
interfacial properties via enhanced conductivity or adhesion by the nano-layer of
metallic Ni between ITO and Ni(OH)2 [94].
The potentiostatically deposited NiO thin films on Au, Pt or ITO electrode
showed 33 % decrement in ΔT after 3000 c/b cycles having a CE of 39 cm2/C [95].
The effect of thermal oxidation on as deposited NiS thin films were studied by Uplane
et al. and it was observed that the film oxidized at 425 oC showed CE∼21 cm2/C and
reversibility∼89 % [96].
The effect of deposition potentials on the electrochromic properties of
nanostructured NiO thin films were studied by Wu et al. [97]. It was demonstrated
that pore size decreases with increase in deposition potential. The bigger pore made
up of intercrossing nanoflakes deposited at lower potential (0.9 V) provides much
more paths for electron conduction and the flake like structure shortens proton
diffusion paths within the bulk of solid nickel oxide, thereby enhancing the ΔT of 80
%, provides superior performances in cycle-life stability, high-rate capability.
The galvanostatic electrodeposition route was employed by Lin et al. to
prepare nano-composite NiO (NNO) film. The electrochromic electrodes fabricated
with NNO film is composed of the core-shell structure of NiO/conducting ITO nano-
particles produce high ΔT∼66.2 % at 550 nm, fast switching speed (coloring: 3.5 s;
bleaching: 4 s) and good durability, which were much better than those of one’s made
with the traditional nickel oxide films [98].
The effect of film thickness on its electrochromic performance was
investigated by Sonavane et al. The maximum CE of 107 cm2/C and stability of up to
10,000 c/b cycles were observed for the films deposited for 20 min (film thickness of
104 nm) [99]. The nanostructured NiO thin films prepared with the aid of various
Introduction and Theoretical Background…………….
Chapter-I Page 27
surfactants and their effect on the electrochromic properties studied by Dalavi et al.
and it was observed that SDS mediated NiO produce high ΔT up to 58 % and CE of 54
cm2/C with an excellent reversibility of 97%. [100].
Yuan et al. synthesized highly ordered porous NiO films by self-assembled
monolayer polystyrene sphere template-assisted electrodeposition. It exhibits a
noticeable color changes from transparent to dark brown, and presents quite good
transmittance modulation with ΔT up to 76% at 550 nm, CE of 41 cm2/C, fast
switching speed (3 s and 6 s) and good cycling performance, compared with the dense
NiO film prepared without PS sphere template [101]. The effect of Cu-doping on the
electrochromic properties of cathodically electrodeposited NiO was studied by Zhao
et al. The Cu-doped films showed enhanced transmittance difference (80 %), better
reversibility and faster response time [102].
Yoshino et al. fabricated nanostructured mesoporous lamellar NiO films by
cathodic deposition, using a cylinders and bilayers assembly of SDS as the template.
The mesoporous NiO caused remarkable ΔT of 62.2% as compared to conventional
film (ΔT∼50 %) and quick optical response accompanied by the redox reaction in
aqueous solution of 100 mM LiOH with a charge of 6.58 mC/cm2 [103]. Cathodically
electrodeposited nanostructured Ni(OH)2 thin film synthesized with subsequent
annealing treatment at 250 oC exhibited a considerable ΔT=43.2% at 550 nm without
any post-activation treatment [104].
Spray Pyrolysis:
Effect of substrate temperature on the electrochromic performance of spray
deposited NiO thin films was studied by Arakaki et al. It was found that films obtained
below 300 °C showed amorphous or nanostructure having transmittance modulation
of 35 % with CE of 30 cm2/C. However, the polycrystalline NiOx, obtained at 400 °C,
did not show electrochromism means that a crystallization process decreases the
electrochromic capacity in NiOx [105].
An investigation on electrochromic properties of nickel oxide done by Kadam
et al. showed that no changes in crystal structure were evident after coloration and
bleaching of the film from XRD studies and coloration/bleaching processes believed
to occur at grain boundaries and on grain surface. The coloration efficiency at 630 nm
was calculated to be 37 cm2/C [106].
Introduction and Theoretical Background…………….
Chapter-I Page 28
The influence of substrate temperature as well as film thickness on the
electrochromic (EC) performance was investigated by Kamal et al. The improvement
in EC performance of crystalline NiO by electrochemical cycling is attributed to re-
hydration of the film to nickel hydroxide. The film thickness affects the EC
performance, the results showed that, films of thickness <120 nm posses high
transparency but full coloration could not be achieved, while films of thickness >200
nm showed deeper coloration but less transparent in the bleached state. Films in the
range 140–150 nm give highest optical modulation in both visible (ΔTv=51 %) and
solar spectra (ΔTs=35 %), injected charge=14 mC and coloration efficiency η=44
cm2/C [107].
The NiO thin films were deposited on nanostructured ITO layer and it is
evident that the NiO film covers the surface of the ITO nano-particle layer and forms a
nanostructured NiO (NSNO) film. Due to the porosity of the ITO nano-particle layer,
the NSNO based EC electrodes provided slightly more surface area than that of
conventional NiO (CNO) based ones. Therefore, the switching time and transmittance
contrast (51 %) of the NSNO based EC electrodes were slightly improved than that of
CNO (48 %) and great improvement in the cycling durability (3000 c/b cycles),
compared to the CNO based ones [108].
Layer By Layer Deposition:
The nanostructured Ni(OH)2 thin films with particle size of about 5 nm
prepared by layer by layer deposition. The nanoparticulated Ni(OH)2 shows increased
electrochromic efficiency with 5 bilayers and it was found to be 80.3 cm2/C and
bleaching and coloring times were 300 ms 100 ms [109].
Chemical Vapor Deposition:
The amorphous NiO thin films obtained at a substrate temperature 250 °C
from nickel acetylacetonate precursor showed the reduction and oxidation of the
films in 1 M KOH resulted in bleaching and coloration with CE of 44 cm2/C [110].
RF Magnetron Sputtering:
The electrochromic response of NiO thin film prepared by RF magnetron
sputtering is greatly reduced by modest heat treatment, suggesting that this response
is associated with easily removed excess defects at the grain boundaries. This
Introduction and Theoretical Background…………….
Chapter-I Page 29
hypothesis is supported by XRD and IR absorption measurements on the films which
indicate that there is no change in crystal structure when films are colored or
bleached [111].
The effect of deposition atmosphere or rf power on the electrochromic activity
of lithiated NiO films were studied by Urbano et al. It was observed that the films
deposited under pure Ar atmosphere and high rf power or under Ar +O2 atmosphere
and low power did not present significant electrochromic activity, and the valence
band photoelectron spectrum did not show the presence of the Ni 3d line for the as-
grown samples. In contrast, samples deposited under Ar+O2 atmosphere and high
power, or pure Ar atmosphere and low power showed a 70 % transmittance change,
and a stable voltammetric profile after some cycles. From this it was conclude that
electron population at the Ni 3d levels in the as-grown state seems to be responsible
for the electrochromic ability [112].
The role of defects on the electrochromic response time NiO films was tested
by Ahn et al. where it was found that the excess interstitial oxygen and voids disturb
the proton intercalation/deintercalation process [113]. The electrochromic devices
fabricated with the NiO-Ta2O5 nanocomposite electrodes produces high optical
modulation (41-95%), fast response time (colored: 3-4 s, bleached: 1 s), high CE (30.5
cm2/C) and good durability, which were much better than those of one’s made with
the single NiO electrode [114]. Electrochemical and EC properties of NiO thin films
were studied by Abe et al. in KCl+H2SO4 acidic aqueous solutions. The NiO films
exhibit CE of ∼30 cm2/C and a maximum ΔT of ∼30% [115]. RF sputtered NiOx films
are non-stoichiometric with Ni vacancies and consist of both Ni2+ and Ni3+. The as-
deposited NiOx films show excellent electrochromic properties. With the increase of
annealing temperature, the electrochromic properties of NiOx films are weakened.
The insertion and extraction of the H+ causes the different situation of t2g energy
levels of Ni3+ and Ni2+ ions and the reversible change of Ni3+ and Ni2+ [116].
The NiO thin films prepared from metallic nickel in the presence of Ar, O2 and
H2O. The water vapor had a strong influences on the electrochromism and led to
lower colored-state transmittance, almost unchanged bleached-state transmittance,
increased charge capacity and decreased coloration efficiency (decreased from about
0.07 to 0.06 cm2/mC) [117].
Introduction and Theoretical Background…………….
Chapter-I Page 30
Reactive DC magnetron sputtering:
The effect of substrate temperatures on the microstructure and electrochromic
properties of NiO films were studied by Xuping et al. The deposited layers have a
polycrystalline NiO structure with (111) preferred orientation. The NiO films
deposited at room temperature show a large dynamic range of ΔT=64 %, whereas the
films deposited at 200 oC showed poor transmittance modulation (ΔT=44 %) [118].
Thin films of oxides based on Ni, Ni-V, Ni-Mg, and Ni-V-Mg were studied by
Azen et al. The addition of Al or Mg increased the luminous transmittance
significantly, while the charge capacity was maintained. Al and Mg containing films
were superior to the conventional Ni oxide electrodes [119-122]. Lee et al. showed
the increase of charge transfer density with increasing concentration of tungsten in
Ni-W oxide (atomic ratio of W/Ni = 0.33) films are mainly due to the decrease of
charge transfer resistance and enhancement of lithium diffusion coefficient with CE of
61 cm2/C at 550 nm [123].
A significant decrease of bleached-state absorptance was found at short
wavelengths for additives being Mg, Al, Si, Zr, Nb, and Ta, whereas films containing
vanadium or silver did not show any improvement in their optical properties
compared to those of pure NiO. One could note that metals known to form oxides with
wide band gaps tend to yield low bleached-state absorptance [124].
Hydrated nickel vanadium oxide (Ni1-xVxOy) films exposed to ozone resulted in
dark brown coloration associated with an appearance of Ni3+ ions and accompanied
by a modification of the local electronic and atomic structures of the V and Ni ions
[125]. An investigation was conducted on the electrochromic properties of plasma
sputtered NiVxOy thin films on flexible polyethylene terephthalate (PET)/indium tin
oxide (ITO) substrates. The light modulation with up to 52 %, optical density change
of 0.446 and CE of 63.8 cm2/C at 550 nm was obtained for 200 cycles in Li+ based
electrolyte [126]. However, NiOx thin films sputtered on 40 Ω/□ flexible PET/ITO
substrates showed Light modulation with up to 40%, optical density change of 0.354
and CE of 67 cm2/C at a 550 nm was obtained for 1100 cycles in the same electrolyte
[127].
Sputter Deposition:
Kitao et al. studied the effect of hydrogen content in the sputtering
atmosphere. It was observed that the CE of 36 cm2/C at 8 Pa total pressure with
Introduction and Theoretical Background…………….
Chapter-I Page 31
hydrogen content of 40 %, but decreased steeply at 50 % [128]. Green et al. identified
a suitability of electrolyte, compatible with both NiOz and WOy. The optical
modulation was superior for NiOz cycled in KOH. For the applied potential ranges it
was also observed that KOH only colours the film relative to the state of the as-
deposited film, while Li-PC and propionic acid only bleach the film relative to the as-
deposited case. Even though the optical modulation was largest when using KOH, the
CE was larger for Li-PC and propionic acid, which gives a hint that the optical
modulation would be improved if one could increase the charge capacity of the film in
the latter electrolytes [129].
DC -Reactive Sputtering:
Passerini and Decker et al. described effective lithiation requires an
electrochemical "activation", which in itself leads to some transmittance increase and
bleaching of initially colored films, can take place by Li+ implantation [130, 131]. The
electrochromic performance of NiO in alkaline electrolyte showed optical
transmittance of the bleach state at visible range is as high as over 70 % with no
obvious degradation for cycle’s upto 105 color/bleach cycles [132].
The effect of RF power on the structure of non stoichiometric NiOx films
sputtered from metallic nickel target was investigated by Ferreira et al. and it was
found that the lattice parameter decreases by increasing the rf. power. Oxygen to
nickel ratio higher than one was detected by Rutherford back-scattering analysis with
huge ratio of hydrogen to nickel. The maximum transmittance change (52 %) and CE
of 30.8 cm2/C was detected for rf power of 150 W [133].
RF-Sputtered Deposition:
The new composition Li+0.6NiII0.7O2- was proposed by Campet et al. and thin
films were prepared by room temperature electrochemical insertion of lithium into
amorphous NiO based film. The film undergoes a reversible electrochemical Li+
insertion process which is accompanied by a net electrochromic effect. It is obvious
that lithium insertion-extraction reaction with Li+2x-yNiII1-x-yNiIII02- films are of both
technical and fundamental interest [134]. Thin films of lithium nickel oxide were
deposited from a stoichiometric LiNiO2 target. The films were achieved phototopic
transmittance modulation close 70 % with good reversibility and CE was of about 30
to 40 cm2/C [135].
Introduction and Theoretical Background…………….
Chapter-I Page 32
The nanostructured NiO films with grain size about 5-10 nm possess excellent
electrochromic characteristics and their performances becomes poorer with
increasing the amount of amorphous phase or the size of nanocrystalline grains in the
films. It is believed that the large quantity of grain boundaries existing in the
nanocrystalline films play an important role for the transport of alkali metal ions
during the electrochromic process [136].
Nickel–vanadium mixed oxide thin films were deposited by RF sputtering. The
samples showed interesting electrochemical and optical properties, a good
electrochemical reversibility and ion-storage capacity exceeding 40 mC/cm2, lasting
beyond several hundred cycles at a current of 50 mA/cm2 [137]. The electrochromic
(EC) properties between short-range ordered and polycrystalline nickel oxides grown
by reactive radio frequency (rf) sputtering. The values for ΔT and CE for the
deintercalated charge were 41.3 %, 21.3 cm2/C and 3.43 %, 7.4 cm2/C at 550 nm for
the potential-cycled short-range ordered and polycrystalline NiO [138].
The kinetics of electrochemical lithium insertion inside RF sputtered Ni/V
mixed oxides thin films have been investigated employing different electrochemical
techniques. It was concluded that the diffusion kinetics of lithium in Ni/V mixed oxide
films is about 100 times slower than in WO3 [139]. Electrochromic properties of
NiOOH thin films prepared by reactive sputtering in an H2O atmosphere in various
aqueous electrolytes. A stable EC cycling up to 100 cycles was obtained for the NiOOH
thin films in electrolyte solutions with a wide pH range of 4.1-13.5 [140].
Vacuum Evaporation:
Nickel oxide (NiOx) thin films were prepared by a low vacuum evaporation
(LVE). The coloration and bleaching times were found to be 1.7 and 4.2s and the CE
was 32.4 cm2/C. The NiOx/FTO electrode could be electrochemically switched for 104
cycles without serious deterioration. The variations in the transmittance at 670 nm
due to the charge insertion/extraction of more than 40 % were achieved [141].
Electron beam Evaporation:
The optical transmittance decrease from 79 % to 35 % with a charge density of
10 mC/cm2 and CE of 28 cm2/C was found for the electron beam evaporated NiOx thin
films. It was suggested that hydroxyl sites in the NiOx film might have an important
role in EC reaction [142]. The effect of the deposition pressure on the microstructures
Introduction and Theoretical Background…………….
Chapter-I Page 33
and electrochromic properties of NiO layers was tested by Agrawal et al. and it was
found that with increasing pressure grain size decreases from 6.7 to 4 nm and CE
decreases from 40 to 32.5 cm2/C at 550 nm. [143]. Electrochromic glazing consisting
of WO3/PEO-based Li+ electrolyte/NiOx showed a promising solar control property
towards the modulation of NIR reflection using the LixWO3 based device [144].
Pulsed Laser Deposition:
The Pulsed laser deposition technique was used also to produce
electrochromic lithiated NiO [145] and the optical transmission range is almost 70 %
at 550 nm for 150 nm-thick films. The layers were polycrystalline with the cubic NiO
structure. Cobalt and tantalum have been added to enhance the electrochromic
performances of NiO thin films. Among the two series of metal additions (M ≤ 20%),
the Ni–Co–O (5% Co) and Ni–Ta–O (10 % Ta) thin films showed the highest
electrochemical performances and especially an improved durability [146]. W
addition was tried also by Penin et al. [147] and led to a progressive film
amorphization. An increase in cyclability for Ni-W-O (5 % WO3) electrode, cycled in 1
M KOH electrolyte associated with a limited dissolution of the oxidized phases was
achieved with tungsten addition. HR-TEM investigations of cycled films revealed that
the stabilization is correlated to the existence of an α(II)-Ni(OH)2 phase. Pulsed Laser
deposition gives films with grain size of NiO from 50 to 150 nm with a very low CE∼15
cm2/mC [148].
The NiO films grown at room temperature (RT) under a 10−1 mbar oxygen
pressure and it was suggested that cycling life of PLD-NiO thin films is typically based
on a three-step process, namely the activation period (I-5th to 50th), the steady state
(II-200th to 500th) and the degradation period (III- 500th to 1000th). The activation
period is associated with a sharp decrease of the transmittance in the colored state
(Tc) from 80 to 42 % from the 5th to 200th cycle, respectively. Upon further cycling, Tc
reaches a 45 % average value with a slight increase in the degradation period. The
coloration efficiency (CE) increases from 21 to 39 cm2/C [149].
Introduction and Theoretical Background…………….
Chapter-I Page 34
1.7: Purpose of the Dissertation
Saving energy in the building sector and automotive industry is a major global
socio-economic target in energy efficiency as well as from environmental viewpoint.
Substantial savings in energy consumption can be realized through solar heat control
with an emerging smart window technology in minimizing the usage of air-
conditioning systems heating, cooling, lighting, ventilation and powering of buildings.
With worldwide ≈2 billion m2 of electrochromically active material coated glass
windows, energy saving in the two mentioned air-conditioning segments i.e. buildings
and cars, has been estimated to be 1.1 MGJ (million Giga joules or 26 million toe
(tones of oil equivalent) per year, while the CO2 reduction was estimated at 82
million tons per year. The global production of glass which could be solar regulated to
minimize the air conditioning using emerging smart nano-photonics, could be a part
of 1 billion m2/year with about 25% for building and ~11% for automotive industry.
Examples of these smart photonics include electrochromic tungsten oxide and nickel
oxide based devices. Due to such a significant optical modulation, this later
nanotechnology with a well established scientific platform could play a key role in
energy management in both automotive and architectural sectors as mentioned
previously. An estimate of the expected savings in heating, lighting and air-
conditioning costs is $ 11–20 billion per year. According to the EPA, even a $ 7 billion
saving would equate to a reduction in carbon emissions at power generating plants
equal to taking 3,36,000 cars off the road, while the energy savings would be enough
to light every home in New York City. To set the scene, one has to note that heating,
cooling, lighting, ventilation and powering of buildings and automotives account for
more than the half of the total energy consumption worldwide and hence responsible
for more energy consumption than any other end-user sector such as industrial
production.
Nickel oxide (NiO) is the most exhaustively investigated transition metal oxide
exhibiting semiconducting properties. It offers promising candidature for many
applications such as electrocatalysis or electrosynthesis, positive electrode in
batteries, fuel cell, EC devices with good electrochemical stability and cyclic
durability, solar thermal absorber, catalyst for oxygen evolution and photo
electrolysis. Nickel oxide is an anodically coloring EC material having high changes of
optical density between fully bleached and fully colored states and low materials cost.
Introduction and Theoretical Background…………….
Chapter-I Page 35
It is also considered to be a model semiconductor with p-type conductivity because of
its wide band-gap energy range from 3.6 to 4.0 eV.
Additionally, an anodically colored NiO thin film play important roles as a
complementary counter electrode which color and bleaches in phase cathodically
colored tungsten oxide working electrode during oxidation–reduction cycling for
enhancing the coloration efficiency and contrast ratio. Among the cathodic colored
materials, tungsten oxide, showing a blue color in the reduced state, has been mostly
investigated due to its high coloration efficiency and cycling durability, resulting in
commercialized electrochromic windows (ECWs). However, there has been an
increasing demand of electrochromic devices, based on anodically colored nickel
oxide used as an optically active counter electrode, which show a neutral-grey
coloration. This is due to the natural grey color which results from combining the
brown color of NiO in the oxidized state together with the blue color of the reduced
tungsten trioxide.
Plethora of chemical and physical techniques have been used to deposit NiO
thin films such as chemical bath deposition (CBD), sol-gel technique, spray pyrolysis
(SP), hydrothermal synthesis, electrodeposition (ED), chemical vapor deposition
(CVD), pulse laser deposition (PLD), electron beam evaporation technique, RF
magnetron sputtering, and DC magnetron sputtering. Among all methods mentioned
above, chemical methods such as Sol-Gel, chemical bath deposition and
electrodeposition of nickel oxide thin films are of particular interest due to low-cost,
environmental friendly process and feasibility of room temperature growth on large
area. Additionally various hierarchical nanostructured morphologies can be evaluate
just by adjusting the deposition parameters which augment for EC properties.
The main thrust of this work is the utilization of simple and inexpensive
chemical techniques such as sol-gel route, chemical bath deposition, electrodeposition
and chemical precipitation techniques for the synthesis of nanostructured NiO thin
films with different morphologies for the enhancement of the optical modulation and
electrochromic coloration efficiency, which are the key parameters for the smart
window application. Hence attempts were made to deposit nanostructured NiO thin
films by four methods: 1) Sol-Gel Route, 2) Chemical Bath Deposition 3)
Electrodeposition and 4) Chemical Precipitation. Attempts have been made to
investigate the physics and chemistry of the electrochromic phenomena in them.
Introduction and Theoretical Background…………….
Chapter-I Page 36
1.8: Plan of work
The literature survey and theoretical background reveals that the
nanostructured NiO thin films have an important application as smart windows.
Morphology of nanostructured NiO has a positive impact on electrochromic
performance. Hence different nanostructured morphologies have been deposited
using simple chemical and electrochemical methods, which are cost-effective. This
will result in to the cost-driven smart window technology. As deposited films are
nickel hydroxide which was annealed in an ambient air at 300 oC to obtain NiO thin
films.
Electrochromic properties of these NiO thin films are investigated and the
results obtained for different morphologies have been compared.
Introduction and Theoretical Background…………….
Chapter-I Page 37
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