PREPARATION AND OPTICAL PROPERTIES CADIMUM CHLORIDE
THIN FILMS PREPARED BY SOLUTION GROWTH TECHNIQUE.
F. I. EZEMA
fiezema@yahoo.com
DEPARTMENT OF PHYSICS AND ASTRONOMY,
UNIVERSITY OF NIGERIA, NSUKKA, ENUGU, NIGERIA.
ABSTRACT
Cadmium chloride (CdCl2) thin films were
prepared by solution growth technique from chemical baths containing solutions
of cadmium acetate, ammonia and potassium chloride with NH3 as
complexing agent. The effects of varying dip times on the optical properties of
the film and the influence on the growth rate were studied. Our reasonably
inexpensive and simple solution growth technique has been found suitable in
obtaining halide thin films with reasonably satisfactory properties. The film
properties studies are chemical composition and absorbance/transmittance. Other
properties deduced from absorbance/transmittance include refractive index (n),
which has been obtained in the average range between 2.15 and 2.70, extinction
coefficient (k) ranges between 1.40 x 10-2 and 5.50 x 10-2
and optical conductivity (s) ranges between 2.13 x 1013S-1 and 8.07 x 1013
S-1. Thickness in the range between 0.150 and 0.611mm has been
deposited. Some of the films were found to have very high absorbance in UV-
VIS- NIR regions while the films absorbance increases as thickness increases
hence they could find applications in solar radiation absorbers for solar cell
fabrications.
Keywords: solution growth technique, CdCl2, solar cells
INTRODUCTION
The solution growth
technique (SGT) has been effectively used in the deposition of oxide,
chalcogenide and even ternary compound thin films and the chemical reactions
involved well understood (Chopra and Das 1983, Eze and Okeke 1997). The use of
solution growth technique for the production of halide and halcogenate thin
films has not been well documented. The conventional methods for the deposition
of halide and halcogenate thin films have been chemical vapour technique and spray pyrolysis (Chopra and Das
1983). In solution growth technique, ions rather than atoms or molecules are
the building blocks and most first row metallic (alkali) halides such as K and
Na halides which form part of the deposition bath are ionic halides and easily
form ionic (conducting) solutions in water according to the reactions:
KCl + H20 K+ + Cl-
+ KOH
This enhances quick release of the halide ions, which react with the
desired metallic ions without an applied external force. The alkali halides are
abundant, cheap and easily available.
The metallic ions
needed for reaction with halide ions are easily provided from a nitrate or
suphate or higher oxidation state halide salt of desired metal base. There salt
are easily obtained and most of them are soluble in water at ordinary temperature
or upon little heating (Lange 1973) to form acidic solutions and / or a
complex.
The cations can be released upon the addition of a base such as NH4OH.
The cation can be released as a free ion in the presence NH4OH
according to the reaction
The released cations can now react with the halogen ions from alkali
halide salt to form the desired metal halide molecule. This is because the NH4OH
ensures a slow release of the metal ions in the solution. To produce CdCl2
thin films, the reaction bath constitutes cadmium acetate [Cd(CH3OO)2],
Ammonium solution (NH3) as a complexing agent, potassium chloride
(KCl) and distilled water. The film growth takes place through ion – by- ion
condensation of Cd2+ and Cl- ions on the glass substrate
when Cd2+ and Cl- ions exist over the solubility limit
(Choi et al 1998).
The deposition of
cadmium chloride was based the reaction between NH3 as complexing of
Cd2+ and KCl.
The reactions
involved in the deposition process are as follows:
Cd(CH3COO)2
+ NH3 Cd(NH3)4 (CH3COO)2
The right hand side of the above equation is cadmium ions in
solution.
[Cd(NH3)4](CH3COO)2 Cd2+ +4NH3
+ (CH3COO)2
2KCl 2K
+ Cl-
Cd2+
+ Cl- CdCl2
The Cl- ions are released by the hydrolysis of KCl but Cd2+
ions from tetra amine cadmium (II) complex ions by combining with NH3
in alkaline solutions (pH10-11). These Cd(NH3)42+
complexes adsorb on the glass substrate, then a heterogeneous nucleation and
growth takes place by ionic exchange with Cl- ions. This process is
called an ion-by-ion process (Choi et al, 1998) and in this manner CdCl2
is deposited in form of whitish, uniform and adherent films at above 12hours
dip times although deposition of the film started before 1hour dip times.
The optical properties of the film studied include the Absorbance,
Transmittance and Reflectance, which were used to calculate the other
properties such as refractive index, extinction coefficient, dielectric
constant and optical conductivity. The optical properties and band gap of the
film were deduced from equations given in literatures (Pankove 1971, Janai et
al, 1979, Ndukwe, 1996, Ezema and Okeke, 2003) while optical method (Theye,
1985) was used to estimate the thickness of the film.
EXPERIMENTAL DETAILS
The depositions of cadmium chloride films were based on the reaction
between tetra amine cadmium (II) compex and potassium chloride in cold water.
The film growth reaction baths were made up of cadmium acetate Cd(CH3COO)2,
ammonium solution(NH3) as a complexing agent and potassium chloride
(KCl) solutions added into 50ml beakers in that order, which were stired
thoroughly using glass rod at each stage to obtain a homogenous mixtureof the
solutions. Each bath was made up to 40ml with distilled water and allowed to
stay between 1 and 30 hours dip times.
The variation of the bath composition and concentration is shown in
table I below:
Table1: Variation of the Bath Composition and Concentration.
|
Reaction
bath
|
Dip
time (hr)
|
Cd(CH3OO)2
|
NH3
|
KCl
|
H2O
|
pH
|
|
Mol.
(M)
|
Vol.
(ml)
|
Mol.
(M)
|
Vol.
(ml)
|
Mol.
(M)
|
Vol.
(ml)
|
Vol.
(ml)
|
|
a241
|
24
|
1.0
|
10
|
10
|
10
|
1.0
|
10
|
10
|
10.2
|
|
a24
|
24
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a12
|
12
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a8
|
1
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a7
|
2
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a4
|
3
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a3
|
4
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a2
|
7
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
|
a1
|
8
|
1.0
|
5
|
10
|
5
|
1.0
|
5
|
25
|
10.2
|
CdCl2 films were deposited on 26mm x 76mm x 1mm
commercial microscopic glass slides at initial bath solution pH value 10.2. The
glass slides were previously degreased in nitric acid for 48 hours, cleaned
with detergent, rinsed with distilled water and drip dried in air. The glass
substrates were then inserted and suspended vertically from synthetic foam,
which cover the beakers containing the solutions. After the dip times, the
glass substrates were withdrawn rinsed and dip-dried in air. The films were
characterized for the absorbance, transmittance and reflectance characteristics
using PYE UNICAM UV SP8-100 spectrophotometers in the UV- VIS- NIR regions. The
use of single bean Fourier transform spectroscopy involves dissolving the films
in nujol to determine the peaks in the far infrared regions.
RESULTS AND DISCUSSION
Figure 1 shows the combined effect of film – nujol system on
transmittance of infrared for CdCl2 when compared with nujol. The
blank background of infrared spectroscopy for nujol indicates the peaks at 1377cm-1, 146cm-1,
2855cm-1, 2924 cm-1, 2953cm-1, and 3436cm-1
with percentage transmittance between 5 and 57%.
When the film was dissolved in nujol, it showed other peaks at
1156cm-1, 1035cm-1, 859cm-1, and 540cm-1
with percentage transmittance between 54 and 66% which resulted due to the
dissolved film. However dissolving the film in nujol modified the nujol peaks
with modified nujol peaks showing percentage transmittance that ranged between
33 and 53%.
The transmittance of the
film with regard to nujol peaks before dissolving the film and the new peaks
after dissolving the film shows percentage transmittance ranged between 4 and
27%. Comparing the nujol spectra to
that after dissolving of the film in the nujol that is the nujol – film
spectra, the percentage transmittance after dissolving of the film is less than
that before dissolving of the film.
This means that the film suppresses transmittance within the far
infrared regions. The poor transmission
of infrared radiation by this film show that is can be used to screen off the
heating portion of the solar radiation when used as coating on windows. Szatra
et al ( 1991) reported that characteristic absorption bands from 710-400cm-1
are due to bonds of heavier element O-CI; hence 540cm-1 (figure 1)
is due to O-CI. According to Conley
(1966), CIO3- characteristic absorption bands occur from 985-923cm-1 ,
662-646cm-1 and 539-477cm
-1, and CIO4- characteristic absorption bands from
1108-1062cm-1 and 662-646cm-1, which were not
incorporated in the film.NH4+ characteristic
absorption bands occur from ranges between 3323 - 3015cm-1 and 1477- 1400cm-1
(Conley ,1966). The peak 1454cm-1
in the range between 1477 and 1400cm-1 is as a result of nujol peak
modification. The water of crystallization characteristic absorption bands
occur from peaks (Conley, 1966) between ranges 3554 - 3139cm-1 and
1692 - 1600cm-1, therefore 3437cm-1 peak in the range
between 3554 and 3139cm-1 is also as a result of nujol peak
modification, which shows the absence of the water of crystallization. The
spectral absorbance of cadmium chloride grown under varying conditions at 300K
is displayed in figure 2. Samples al and a12 absorbed heavily
throughout the UV-VIS-NIR regions, samples a2 and a3 absorb
moderately throughout UV-VIS-NIR regions and samples a4, a5, and a7 absorb
poorly throughout UV-VIS-NIR regions.
All the samples show absorption peak values between 280 and 380 from
where they decreased to low values towards the NIR regions. The high absorbance in the UV-VIS-NIR
regions exhibited by samples a1 and a12 show they could be
effective for solar radiation absorbers for solar cell
fabrications.
From the spectral absorbance we observe that as the dip times
increases the deposited film absorbs heavily although there is no appreciable
difference in absorption within the first three hours. Figure 3 shows the
spectral transmittance – reflectance of CdCI2.
Samples a4, a7 and a8 show high transmittance in the VU-VIS-NIR
regions ranging between 76 and 86% after the fundamental absorption region
while exhibiting poor reflectance ranging between 8 and 11%. The high transmittance and poor reflectance
exhibited throughout UV-VIS-NIR regions by these samples show that they have
potential applications in the area of antireflection coatings and thermal
control coatings for cold climates. Samples a2 and a3 show moderate
transmittance in the UV-VIS-NIR regions ranging between 51 and 71% while exhibiting
high reflectance throughout UV-VIS-NIR regions, which averages between 14 and
20%.
The films with moderate transmittance in UV-VIS regions could be
used as antidazzling coating for windscreen of a car. Samples a1 and a12 show poor transmittance which range between 14
and 54% throughout within the VIS-NIR while exhibiting high reflectance, which
averages between 18 and 20% within VIS-NIR regions.
Figure 4 gives the variation of a with hn studied for
CdCI2 samples.
There are absorption edges,
which are characteristics of the crystalline state of the film.
It can be seen from Figure 4 that the absorption edge shifted to
shorter or higher wavelength regions depending on the variation in the dip
times.
For examples the dip times of 3 and 4 hours shifted the absorption
edge to the shorter wavelength regions, while the dip times of 12 hours shifted
the absorption edge to the higher wavelength regions.
The region of higher values
of a
that is a > 10cm-1 correspond to transmittance between
extended state in both valence and conduction bands. The region of lower values of a that is a< 10cm-1
is the region where absorption presents roughly exponential behavior (Kotkata
et al, 1994, Fayek et al, 1995).
The usual method
for determining the values of Eg involves plotting of (ahn)n against
hn. If the appropriate value of n is used, the
value of Eg will be given by the 
intercept on the hn axis. These result were
found for n = ½ for direct photon transitions 
as shown in Figure 5.
It can be seen (Figure 5) that the Eg decreased depending on the
variation in the dip times, which also depends on the thickness of the
film. The band gaps were found to range
between 0.5 and 2.45eV.
The values of
refractive index (n) calculated from R and T data are plotted in figure 6 as a
function of photon energy (hn). At the lower energies for
samples a3 and a4, n increases with increasing wavelength to peak values of
2.22 and 2.17 respectively then decreases with increasing wavelength.
Sample a12
decreases from 2.28 at the low energy to a minimum of 0.94 at the high energy
and increases with increasing wavelength at the same region. Sample a1 shows peak at 4.14eV while a2, a3
and a7 show minima at the same point.
The values of
extinction coefficient (k) calculated from R and T data are plotted in figure 7
as a function of photon energy (hn).
All the samples show similar spectral curve except a12, which showed
some irregularities at the low energies.
As the dip time increased the k values increased.
The plots of er against hn are displayed in Figure
8. At the lower energies for samples a3
and a4, er increases with increasing wavelength
to peak values of 4.93 and 4.70 respectively then decreases with increasing
wavelength. Samples a12 decreases from
5.20 at the low energy to a minimum of 0.88 at the high energy and increases
with wavelength at the same region.
Sample a1 shows peak at 4.14eV while a2, a3 and a7 show minima at
the same point. It can be seen that the
shapes of spectral curves for n (figure 6) and that of er (figure 8) are strikingly similar.
The plots of ei against hn are displayed in Figure 9. All the samples show similar spectral
curve except a12, which showed some irregularities at the low energies. However, ei decrease from high values at low energy to low values at the
high-energy regions.
The plot of optical conductivity (so) against hv is shown in figure 10. Samples a1, a3, a4 and a12 show so increases from minimum to maximum values between 3.25 x 10 S-1 and
5.95 x 10 S-1 and then decreased to low values.
CONCLUSION
Thin film of CdCl2 with thickness 0.150-0.611um has been
successfully deposited using solution growth technique. The film was
characterized using FTIR spectroscopy for composition while spectrophotometer
was used to deduce the optical properties such absorbance/transmittance/reflectance
from which other optical properties such refractive index,
extinction coefficient, dielectric constants and optical conductivity were
calculated. Some of the films were found to have high absorbance in the
UV-VIS-NIR regions, hence, they could be effective for solar radiation
absorbers for solar cell fabrications, those which show moderate transmittance
in the UV-VIS regions could be used as antidazzling coating for car windscreen
while those with high transmittance and low reflectance could be used as
antireflection coating as well as thermal control coating for cold climates.
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