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 principle 1

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 cathodic 2

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


http://en.wikipedia.org/wiki/Atom

http://en.wikipedia.org/wiki/X-rays

http://en.wikipedia.org/wiki/Diffraction

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 have 3

many essential advantages. “NiO adopts the NaCl structure, with octahedral

Ni(II) and O2− sites.”[‑ ] “Nickel oxide is a very popular and new electrochromic 4

materials, with many advantages such as high electrochromic efficiency, cyclic

reversibility, durability, and grey coloration”.[‑ ] Usually people make 5

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 that 11

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 know 12

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 cubic 13

sodium chloride structure with lattice constants a = 0.418 nm[‑ ]. It is arranged 14

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 is 16

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, six 18

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. Although 19 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 sample 24

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 for 25 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, these 27

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 diffraction 28

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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