In this work, nickel oxide thin films was fabricated on glass substrate at different temperature by spray
pyrolysis technique. The NiO layers were obtained with different molar concentrations. The NiO thin
films were crystallized with a cubic structure that can be related to obtaining peaks in the XRD diffraction
of NiO thin films. The optical transmission of the deposited films was measured in the range of (300–
900nm) by using an ultraviolet-visible spectrophotometer (LAMBDA 25). The electrical conductivity σ
was measured by four point methods. This study shows that the NiO thin films have a good electrical
conductivity. The NiO thin films have different electrical conductivity values. The pure NiO thin films
prepared with a molar concentration C= 0.1mol/l at a temperature of T=360°C has the best electrical
conductivity of σ= 11.24 (Ω.cm)-1. The NiO:8Li thin films prepared with a molar concentration C=1mol/l
at a temperature of T= 420°C has the best electrical conductivity of σ= 100 (Ω.cm)-1.
In the latest research, the nickel oxide NiO was found in the cubic
structure with a lattice parameter (a= 0.4186 nm) [1]. NiO is forming
of nickel metal and oxygen element, it is a p-type of semiconducting
nature. NiO was used in a variety of technology such as optoelectronic
devices and gas sensing [2,3] due to having a good structure crystallinity,
good electrical conductivity and high transparency in the visible
region. The optical band gap of NiO thin films varied between 3.6
to 4 eV [4]. However, the NiO thin films can be used in various applications
due to the simplicity of synthesis such as solar cells [5], chemical
sensors [6], photo detectors [7], electro chromic minors [8], organic
light-emitting diodes [9], UV detectors [10], vans parent diodes
[11], and defrosting windows [12]. Table 1 shows the physical and
chemical properties of NiO material, it is found that the NiO has a high
solubility in water with a refractive index of 2.18. The NiO as a thin
film was studied on verities of substrates with chemical and physical
methods; it was used to improve the structural optical and electrical
properties. The pulsed laser deposition [13], chemical vapor deposition
[14], electrochemical deposition [15], abeam evaporation [16],
anodic deposition [17], electroless bath deposition [18] sputtering
[19], chemical vapor deposition [20] and spray pyrolysis techniques
[21-24], are used to prepare the NiO thin films, the spray techniques
also were favorites with comparing by others methods due to the simple
deposition and best cost. The mean objective of this research is to
study the electrical and magnetic properties of NiO thin films based
on past research. In this work we have proposed a review of original
research to nanostructured NiO prepared by spray techniques.
Table 1: Summary of the basic physical and chemical properties of NiO material.
In the deposition of NiO thin films by using the spray techniques
with NiO solution, it is prepared by various methods and protocol as
shown in the following steps:
Nio Thin Film Deposition from Nickel Chloride Solutions
The NiO thin films prepared by using chloride nickel can be dissolved
in various solvents such as water, ethanol and methanol solution.
However in the preparation of NiO solution from water solvent,
when dissolving the nickel chloride dehydrate (NiCl2 , 2H2O) in water
( H2O) , before using and deposit NiO thin films into substrates
which heating the final solutions at 50°C than add drops of HCl to the
solution for stabilization. Menaka and Umadevi [25] they discussed
that process decomposition of nickel chloride to nickel oxide in the
presence of water, according to the following equation (Table 2) [26]:
Table 2: Information of Nickel types.
NiO Thin Film Deposition from Nickel Nitrate Solutions
NiO solution can be prepared by using nitrate Nickel with various
solvents such as water, ethanol and methanol. However, in the preparation
of NiO solution from water solvent, when introduce of nickel
nitrate dehydrate (Ni(NO3)2 .2H2O) in a volume equal to wither solvent
(H2O) (Table 2) [27]:
NiO Thin Film Deposition from Nickel Acetate Solutions
NiO solution can be prepared by using acetate Nickel with various
solvents such as water, ethanol and methanol. However, in the
preparation of NiO solution from water solvent, when introduce
nickel acetate dehydrate (Ni(CH3COO)2 − 4H2O) in a volume equal
to wither solvent (Η2O ) .(Ni(CH3COO)2 − 4H2O) are monoclinic
with space group P21/c the unit cell off dimension(see Table 2) [28]:
The electronic band of the nickel oxygen are expressed as:
Ni: 1s22s22p63s23p64s23d8
O: 1s22s22p4
NiO is a native p-type semiconducting material [29].
The electrical conductivity of NiO films has a string depended on
the microstructure defect existing in NiO crystallites, such as nickel
vacancies and interstitial.
Furthermore, the microstructure and composition, as well as the
deposition conditions and environment, are the main factor affecting
the electrical properties of NiO thin films [30].
They found that the electrical conductivity of NiO is strongly related
to the formation of microstructure defect inside the NiO crystallites,
such as nickel vacancies and interstitial oxygen, it was related
the decreasing in carrier concentration and mobility, which as a
p-type semiconductor, in which vacancies occur in caption sites, from
each caption vacancy, two electron holes are formed.
1. The existing in NiO films are electron holes, which are responsible
for the electrical conductivity of the undoped nickel
oxide.
2. The resistivity is inverse proportional to the product of the
carrier concentration with their mobility.
3. The decrease in resistivity can be explained by the improved
stoichiometry of the film.
The four-points probe method was used to determine the electrical
conductivity of Ni1-xZnxO thin films, it is based on measuring the
sheet resistance of the films as expressed by [23,31,32]:
where I is the applied currant = 1 nA and V is the measurement
voltage. However, the electrical conductivity σ is also determined by
the following equation:
Alver, et al. [33], investigated the synthesis and characterization
of born-doped NiO thin films produced by spray pyrolysis. Obtained
an electrical resistivity in the range of 0 to 19 Ω.m, they found the
resistivity of Boron doped NiO films with doping by annealing temperature
is smeller then without, when they introduced Boron atoms
in the ZnO matrix the decrease in resistivity might be mainly due to
the substitution of B3+ with Ni2+ in the lattices, which provides more
free electrons for the conduction mechanism. Similar results are obtained
with [34] (Table 3).
Table 3: Electrical conductivity values of NiO thin films deposited at
different conditions.
The change of crystal structure with temperature actually is associated
with the magnetic properties of nickel oxide. The Neel temperature
(TN) defends as the temperature at which ant ferromagnetism
changes to paramagnetism. Nickel oxide is ant ferromagnetic
at room temperature, and paramagnetic above (TN= 250°C).
Each magnetic unit cell contains four chemical unit cells. Above
the Neel temperature, the spin ordering disappears and spin becomes
random [35-60].
Nickel oxide (NiO) has attracted a great deal of attention due to its
wide direct band gap of (3.6-4.2 eV), which exhibits p-type conductivity.
Stoichio metric NiO is an insulator with a resistivity of the order
of 1013Ω.cm at room temperature. NiO is one of the most important
oxide materials due to its excellent chemical stability and durability,
low toxicity, large span optical density, low cost and good thermal
stabilityand high stability that are similar to ZnO. NiO can be used in
various potential applications such as solar cells due to the p-type
semiconducting, transparent diodes, transparent transistors, displays
and defrosting windows because their transparency can be used for
the UV photo detectors and touch screens due to the good responsively.
NiO can be produced by several techniques such as reactive
evaporation, molecular beam epitaxy (MBE), magnetron sputtering
technique, pulsed laser deposition (PLD), spray pyrolysis, sol‒gel process,
chemical vapor deposition, and electrochemical deposition. This
study shows that the NiO thin films have a good electrical conductivity.
The NiO thin films have different electrical conductivity values. The
pure NiO thin films prepared with a molar concentration C= 0.1mol/l
at a temperature of T=360°C has the best electrical conductivity of σ=
11.24 (Ω.cm)-1. The NiO:8Li thin films prepared with a molar concentration
C=1mol/l at a temperature of T= 420°C has the best electrical
conductivity of σ= 100 (Ω.cm)-1.
A C Nwanya, S U I Offiah, C Amaechi, S Agbo, S C Ezugwu, et al. (2015) Electrochromic and electrochemical supercapacitive properties of room temperature PVP capped Ni (OH) 2/NiO thin films. Electrochimica Acta 171: 128-141.
ISSN: 2574 -1241
Abstract
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