MASC534term paper-Huijian Tian

MASC 534 term paper
Use XRD to identify the molecular structure of NiO ﬁlms annealed
for different durations or in different temperature
Huijian Tian
USC ID:2386525387
Fall 2014 
MASC 534 HUIJIAN TIAN !1

MASC 534 term paper
Use XRD to identify the molecular structure of NiO ﬁlms annealed
for different durations or in different temperature
Abstract
In this paper, the nickel oxide (NiO) thin films were prepared by sol–gel dip
coating process on indium tin oxide glass which is known as ITO. In this
situation, the nickel oxide samples were prepared in the same atmosphere while
annealed for different durations in the same temperature, or annealed in different
temperature for the same time. XRD was used to analyze the structure of the film
and the composition of the film. Through x-ray diffraction pattern, structures of
these sample films were showed, and the particle size could be calculated by
Scherrer formula. After different annealed durations, nickel oxide films structure
showed to be different. While annealed duration is long as 60 mins, its structure
become a crystalline nature. While annealed durations were shorter than 45
mins, x-ray diffraction patterns showed to be amorphous in nature. In different
temperatures situations, all films showed a crystalline structure. While annealing
temperatures increase, crystalline size of nickel thin film increase too.
Keywords: X-ray diffraction, nickel oxide ﬁlms, annealing temperature, annealing
durations

1. Introduction
X-ray particle can be derived by atomic transitions in the energy gap
difference between the two energy levels generated. It is a kind of
electromagnetic radiation and its wavelength is between ultraviolet and γ-rays. Its
wavelength is very short and is between about 0.01 to 100 angstroms. In 1895, a
genius German physicist WK Roentgen discovered it, so it is called X-rays. “X-
ray crystallography is a tool used for identifying the atomic and molecular
structure of a crystal, in which the crystalline atoms cause a beam of incident X-
rays to diffract into many specific directions.”[‑ ] This paper introduce the principle1
of XRD, and its applications in studying properties and structure of materials.
Electrochromic material is a kind of material whose optical property
(reflectivity, transmissivity, absorptivity) occurs a stable, a reversible color change
under the applied electric field, and it shows a reversible change in color and
transparency in appearance. Material has electrochromic properties is called
electrochromic material. What’s more, electricity electrochromic material made
devices are called electrochromic devices. “Microporous and mesoporous
transition metal oxide films find use in a number of potential applications like
sensors, batteries, electrochromic (EC) devices, and photonic and
electrocatalytic materials”[‑ ]. Oxides of Nb, Mo, W, Ta and Ti exhibit cathodic2
electrochromism. However oxides of Ir, Rb, Ce, Fe, Co, Mn, and Ni exhibit anodic
electrochromism. Except for both cathodic oxides and anodic oxides, V2O5 is

something special and it exhibits both types of electrochromism. “This strongly
indicates that electrochromism has a strong relation with the electronic structure
oft he materials”.[‑ ] Nickel oxide films are this kind of metal oxide, and they have3
many essential advantages. “NiO adopts the NaCl structure, with octahedral
Ni(II) and O2− sites.”[‑ ] “Nickel oxide is a very popular and new electrochromic4
materials, with many advantages such as high electrochromic efficiency, cyclic
reversibility, durability, and grey coloration”.[‑ ] Usually people make5
electrochromic multilayer with nickel oxide and tungsten oxide, but their optical
performance are still not high as much as people expect. With different annealing
durations and different annealing temperatures, NiO films have different
microstructures which show different electrochromic properties.
There are four main techniques to prepare nickel oxide films such as
spraying[‑ ], pulsed laser deposition[‑ ], vacuum deposition[‑ ], sol–gel process[‑ ].6 7 8 9
In this paper, several theory of electrochromism and some application of
electrochromism will be mentioned, and sol–gel dip coating process on indium tin
oxide glass which is known as ITO will be discussed to prepare nickel oxides thin
film. After that we will discuss the principle of x-ray diffraction and state that how
to use XRD method to identify the molecular structure of NiO film, and then we
state the impact of different annealing durations and temperatures on the
electrochromic properties of NiO film.

2. Theory of electrochromism
Inorganic electrochromic material’s electron shell structure is generally
unstable and prone to accept or release the electrons, then the valence turn to
difference, along with color’s change occurred. There are two types of
electrochromic metal oxides. One of them is called cathodic, and another is
called anodic. “Figure 1 shows which metals are capable of forming oxides of
these two varieties and also indicates that oxides based on vanadium can be
viewed as a hybrid”[‑ ]. From this figure, nickel oxide is defined by anodic oxide.10
Actually a standard electrochromic device combines two types of electrochromic
films, and it is easy to derive that to combine one “cathodic’’ oxide and one
“anodic’’ oxide is better than just one kind of oxide. When electrons transfer from
one side to another, both oxides color.
2.1 Electrochromic chemical reaction and optical properties
In this paper, we just discuss about the anodic oxide: nickel oxide. When
anodic coloring materials in its reduced state, it is in achromatic state, losing
electrons to become high valence(oxidation state) and it is in the colored state,
the electrochromic reaction is:
(1)

In this formula, MOy is the electrochromic metal oxides. A is a positive ion, and e
is the electrons.
If this metal oxide is nickel oxide, discoloration of NiO film was mainly due
to the proton and the electron’s injecting and extracting from the film [‑ ], so that11
some Ni(OH)2 convert to NiOOH. The formula has this form:
NiO + OH- == Ni(OOH) + e-
It means that the injection of OH ion makes NiO transfer to NiOOH, and color
shows. Although the mechanism of nickel oxides’ electrochromism still has a lot
of controversy, researchers have done a lot of research under different
experimental conditions, made a lot of different theories, but no matter what
views, during the reaction of NiO film’s electrochromic, the view that nickel ions’
Figure 1: Periodic system of the elements (apart from the lanthanides and actinides). The
differently shaded boxes indicate transition metals whose oxides display clear ‘‘cathodic’’ and
‘‘anodic’’ electrochromism. From Ref. [10]

transferring from the divalent to trivalent caused coloration is accepted by
everyone.
Table 1[‑ ] shows some electrochromic materials’ coloration, and we know12
that NiO is black brown in oxidation state.
2.2 Structure and energy band
Why do these electrochromic oxides show electrochromic properties? It is
fair to argue that crystalline structure play a big role in it. “All of these structures
can be treated with in a frame work of “ubiquitous’’ MeO6 octahedra (with Me
denoting metal) connected by sharing common corners and/or by sharing
common edges”[‑ ]. NiO belongs to the close-packed face-centered cubic13
sodium chloride structure with lattice constants a = 0.418 nm[‑ ]. It is arranged14
by NiO6 octahedral with highly regulated, and the gap between the octahedron
can be used as a channel for H, Li and other small ions’ migration or injection.
Table 1
Classﬁcation Materials oxidation state reduction state
Cathodic oxides
WO3 None Blue
MnO3 None Blue
Nb2O5 None light Blue
TiO2 None light Blue
NiO Black brown None
Anodic oxides Ir2O3 Black blue None
MnO2 Blur purple None

Figure 2 is the lattice structure of NiO; from this pattern, the big shade circle
stands for oxide atom, and the small transparent circle stands for nickel atoms.
The existence of octahedral coordination is very important for the electronic
properties of the EC oxides[‑ ]. As we know, every atom has lots of energy band,15
such as 1s, 2s, 2p, 3s, 3p, 3d, and electrons stay in these energy state following
some certain principles. Outer electrons of nickel atoms are arranged 3d84s2. D
band will split into eg and t2g band. The oxygen 2p band is separated from d
band. Figure 3 illustrates the band levels of Tungsten oxides and nickel
oxides[‑ ]: the left hand panel is for tungsten oxide, while the right hand panel is16
for nickel oxide. It is enough for us to just discuss nickel oxides. In Nickel oxides,
Figure 2

when nickel is trivalent, which means NiOOH, it has an unfilled oxygen band.
Some photons are absorbed by electron transition if , so this material is colored.
While nickel is divalent, which means NiO, t2g state is filled. If only the band gap
is large enough, electrons cannot jump to eg state by absorbing photons, so this
material becomes transparent.
3. X-ray diffraction and its application on
electrochromism
X-ray diffraction techniques get more and more attentions in materials,
chemical, physical, minerals, geology and other disciplines. In addition to study
the microscopic structure of the crystal, it has developed into a practical
application of laser science. X-ray is an analysis of non-destructive testing
methods, and it uses few samples, with good accuracy, no damage to the
sample. But this approach also has its shortcomings. Its equipment is relatively
Figure 3

complex and expensive, and it requires people to maintain a certain expertise,
what’s more it also belong to indirect tests. In actual work, X-ray analysis is
usually used in conjunction with other methods[‑ ].17
3.1 X-ray’s production
The traditional way to produce x-rays is that a high voltage is applied on
two electrodes, in several tens of kV, then electrons will be emitted in high speed
with enough kinetic energy, from the cathode to anode. When electron hits the
metal target, they slow down and all its kinetic energy turn into photons, which
are x-ray. Since there are different kinds of slow down to electrons, the produced
x-rays will have different wavelength.
3.2 X-ray diffraction theoretical basis
X-ray diffraction analysis method is based on the crystalline samples
diffracted x-ray’s characteristic, then people calculate crystalline structure and
lattice parameters. The basic principle of x-ray diffraction can be illustrated by
Bragg’s law:
That is, to a certain wavelength beam, if the angle between the incident direction
and a group of the crystal plane and the interplanar spacing of crystalline plane
both satisfy the equation, the diffraction spots show in a certain direction. One
thing should be pointed is that the interplanar spacing d can be derived by
plane’s miller indices. For example, to cubic system:

If we have an x-ray diffraction pattern, we can trial that which miller indices satisfy
this equation, which means that we can get the structure of crystalline sample.
Figure 4[‑ ] is an example of standard x-ray diffraction pattern. In this pattern, six18
peaks represent different crystal plane.
Figure 4

While in actual samples, a particle of a real crystalline grain sample
generally consists of many very small units called “crystallites”. This fine unit can
be considered as a single crystal[‑ ]. Figure 5 [‑ ] shows this situation. Although19 20
in some situations the grain size is the same as crystallites’s size, they are totally
different physical principles because it is the crystallite to make a diffraction peak
in x-ray diffraction, not the whole grain. It is necessary for us to us x-ray
diffraction to identify a sample material’s crystallite size. The following equation is
called Scherrer’s equation[‑ ]:21
Figure 5

In this equation, Ɵ is Bragg angel. B is the full width at half its maximum intensity
for the corresponding peak, and ƛ is x-ray wavelength. The value of t stands for
the diameter of crystallites, which is related to the corresponding peak. The wider
the corresponding peak is, it means the smaller the crystallite’s size is. “The
dislocation density (ð) is defined as the length of dislocation lines per unit volume
of the crystal”[‑ ]. It is derived by the following equation[‑ ]:22 23
3.3 Use x-ray diffraction to identify the structure of NiO ﬁlm annealed for
different durations
In this work, nickel oxide films were made by using sol-get process,
“3.7329 g of Ni(Ac)2 4H2O was dissolved in 100 ml of 2-methoxyethanol and 2
drops of concentrated HCl was added to the solution. The solution was stirred at
60 centigrade for an hour and then aged for 24 h at room temperature. The NiO
films were coated on FTO(fluorine doped tin oxide) coated glass substrate at a
withdrawal speed of 15cm/min. After each coating, the films were dried in air for 5
min and oxidized at300 1C for 5 min. Totally 8 layers have been
coated(optimized number of layers is 8 and optimized temperatures 300
centigrade).”[‑ ] In the end of this work, what is the most important, the sample24
films were annealed at 300 centigrade for 4 kinds of durations, 15, 30,45 and 60
mins. Then the work is to get x-ray diffraction from prepared nickel oxide films,

and x-ray diffractometer use CuK radiation, which wavelength is 1.54
angstrom[‑ ]. Figure 6 [‑ ] following is the x-ray diffraction pattern annealed for25 26
different durations:
Figure 6 shows the x-ray diffraction pattern. It is obvious that when it
comes to the films annealed for 15 mins, 30 mins, or 45 mins, the structure of
films seems to be amorphous in nature. However, when duration extends to 60
mins, there are diffraction corresponding peaks, which means the structure
shows some crystalline nature.
Figure 6: x-ray diffraction pattern annealed for different durations
in 300 centigrade. (a) curve is 15 mins, (b) is 30 mins, (c) is 45 mins,
(d) is 60 mins

XRD results show that NiO film annealed for 60 mins shows the cubic phase
structure, and characteristic diffraction peaks corresponding to (111),
(200) crystal plane were formed when the values of 2Ɵ were 37.290, 43.270.
Here we use Scherrer’s equation to calculate the crystallite’s size corresponding
to the crystal plane (200), where wavelength is 0.154 nm. From this pattern, we
take the value of B for 0.0243 and Bragg angel for 43.056, then we get the value
of crystallite’s size t is about 6.15 nm. It presents a characteristic of crystalline
particles, and XRD analysis with the sol-gel films showed that with the annealed
duration increases, NiO crystallization degree and the grain size has been greatly
enhanced, resulting a deterioration of uniformity in the film’s surface, which is
bad for its electrochromic properties.
3.4 Use x-ray diffraction to identify the structure of NiO ﬁlm annealed in
different temperature
In this work, nickel oxide films were made by using sol-get process. “0.5 M
nickel acetate tetrahydrate [Ni(CH3COO)2 4H2O (99%)] was dissolved in absolute
ethylalcohol. The solution was stirred in a closed vessel at 313K until a very clear
transparent solution (green color) was obtained. The sol was left to cool firstly in
the ordinary atmosphere, after that, the sol was kept into the refrigerator for 24 h
to allow the gelation process. “[‑ ] Finally, what is the most important, these27
sample films were prepared and annealing in different temperatures for 15 mins,

and these temperatures are 673, 693, 713 and 733 K. After getting sample films,
the work is to get x-ray diffraction pattern and analysis the structure of NiO films,
including calculating the grain size. The X-ray diffractometer use CuK radiation,
which wavelength is 1.54 angstrom[‑ ]. Following figure 7 is x-ray diffraction28
pattern of NiO films at different annealing pattern:
From this pattern, it is obvious that all four kinds of NiO films show a crystal
nature, and there are three main characteristic diffraction peaks, which
corresponding to (111), (200), and (220) crystalline plane. The corresponding
Figure 7: NiO ﬁlms prepared at different annealing
temperature. (a) 673 K; (b) 693 K; (c) 713 K and (d) 733 K

crystalline plane can be derived by Bragg’s law. To a cubic system, the planar
spacing d is given by the following equation:
Then we get the formation of Bragg’s law:
The right side of this equation is always a constant to any X-ray diffraction
pattern. In this pattern, the Bragg angels corresponding to three peaks are 37.1,
43.1, 62.9, so the ratio of square sine Bragg angel value is 0.364:0.467:0.792. If
we turn this ratio to integer, the ratio is approximate 3:4:8. There are only three
possibilities in cubic systems except for diamond lattice, and the following figure
8 shows Miller indices, and we know that NiO films are simple cubic structure. As
the annealing temperature increasing, the intensity of diffraction peaks increases,
indicating the degree of crystallization of NiO increase.
Use Scherrer’s equation to calculate the crystallite’s size corresponding to
(200) crystalline plane, where the wavelength of X-ray is 0.154 nm. Along with
the increasing of B1/2 (the full width at half its maximum intensity), the crystallite’s
size t will decrease, and dislocation density is inversely proportional to the value
of t. The result is that with the annealing temperature’s increase, the dislocation

density decreases, which means a better quality of NiO films formed in a higher
annealing temperature.
Conclusion:
In this paper, electrochromism’s theory basis has been discussed and the
anodic electrochromic material NiO has been analyzed particularly. X-ray
diffraction is introduced to analysis the structure of NiO films. When it comes to
sol-gel process to prepare NiO films, different annealing temperature and
different annealing durations have an impact on NiO films’ structure, and X-ray
diffraction patterns show that with the annealed duration increases, NiO
Figure 8

crystallization degree and the grain size has been greatly enhanced; and with the
annealing temperature’s increase, the dislocation density decreases, which
means a better quality of crystal structure. 

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