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Materials Chemistry and Physics 121 (2010) 555–560
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Effect of complexing agent on growth process and properties of nanostructured
Bi2 S3 thin films deposited by chemical bath deposition method
A.U. Ubale ∗
Thin Film Physics Laboratory, Department of Physics, Govt. Vidarbha Institute of Science and Humanities, VMV Road, Amravati 444604, Maharashtra, India
a r t i c l e
i n f o
Article history:
Received 14 August 2009
Received in revised form 25 January 2010
Accepted 14 February 2010
Keywords:
Bi2 S3
EDTA
Electrical and structural properties
a b s t r a c t
Nanostructured Bi2 S3 thin films have been prepared onto amorphous glass substrates by chemical bath
deposition method at room temperature using bismuth nitrate and sodium thiosulphate as cationic and
anionic precursors with EDTA as complexing agent in aqueous medium. The X-ray diffraction study
reveals that the films deposited without the complexing agent are amorphous in nature and becomes
nanocrystalline in the presence of EDTA. The resistivity for the films prepared from EDTA complexed bath
is decreased due to the improvement in grain structure. The decrease in optical bandgap and activation
energy is observed as the thickness of the film varies from 45 to 211 nm on account of the variation of
the volume of complexing agent in reaction bath. Studies reveal that the growth mechanism of Bi2 S3 gets
affected in the presence of complexing agent EDTA and shows impact on structural, electrical and optical
properties.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Semiconductor nanostructured thin films are always important in materials science due to their outstanding electronic and
optical properties and extensively useful applications in various
optoelectronic devices. To manufacture an electronic device needs
a lot of information about the deposition processes, patterning and
etching of various nanostructured semiconducting materials. There
are number of chemical and physical methods such as chemical
vapour deposition, spray pyrolysis, screen printing, electrodeposition, chemical bath deposition, pulsed laser deposition, sol–gel,
electron beam evaporation, and metal organic chemical vapour
deposition. Every technique has its own advantages and disadvantages. One of the greatest disadvantages is that some of them need
very sophisticated instruments along with vacuum which increases
the production cost of the material. But solution-based deposition
method, i.e. chemical bath deposition of semiconducting materials
offers the possibility of depositing thin films at low temperature
under atmospheric conditions at low fabrication cost. Binary semiconductors of the type AV BVI have been receiving considerable
attention in recent years because of the possibility of their utilization in the fabrication of solar energy converters. Hence, it is
interesting to study in detail the various properties of some of
the members of this family. Bismuth-sulphide, which has a large
bandgap, appears to be a suitable material for solar applications
and has been used in liquid junction solar cells [1–3].
∗ Tel.: +91 0721 2531706; fax: +91 0721 2531705.
E-mail address: [email protected]
0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2010.02.021
Different workers have reported chemical deposition of Bi2 S3 on
different types of substrates with characterization such as chemical
deposition [4], interface gas–solution [5], electrodeposition [6], and
spray pyrolysis [7]. Krishnamurthy and Shivkumar [8] deposited
Bi2 S3 films using the hot wall chemical deposition technique. Benramdane et al. [9] deposited Bi2 S3 films onto glass substrates by
spray pyrolysis method using bismuth chloride and thiourea having
bismuth and sulphur source respectively. Pawar et al. [10,11] prepared amorphous Bi2 S3 and Sb2−x Bix S3 films by the solution–gas
interface method. Electrodeposition method was used by Lokhande
and Bhosale [12] to prepare polycrystalline Bi2 S3 thin films. Krishna
Moorthy [13] has prepared polycrystalline stoichiometric Bi2 S3
films by physical deposition technique. Pramanik and Bhattacharya
[14] have also deposited amorphous Bi2 S3 thin films from an alkaline bath using TEA as a complexing agent. Biswas et al. [15] have
prepared thin films of Bi2 S3 by solution growth technique using triethanolamine (TEA) as the complexing agent. Lokhande et al. [16,4]
have deposited thin films of Bi2 S3 from an alkaline as well as acidic
bath using EDTA complexing agent. Ubale et al. [17] have prepared
Bi2 S3 thin films by modified chemical bath deposition at room temperature and reported their electrical and optical properties. The
non-aqueous chemical deposition of the Bi2 S3 thin films has been
reported by Desai and Lokhande [18] using bismuth nitrate and
thiourea in acetic acid and formaldehyde solvents respectively.
In this paper we report on the growth mechanism of nanostructured Bi2 S3 thin films deposited by chemical bath deposition
method. Any insoluble surface, metallic or nonmetallic of any shape
and size kept in contact with the solution may be deposited. The
uniqueness of chemical bath deposition method is the low deposition temperature which avoids oxidation or corrosion of metallic
556
A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560
substrates. The deposition parameters can be easily controlled in
order to deposit nanostructured material on the substrate. The
main purpose of this work is to describe how the growth process of
Bi2 S3 gets affected in the presence of the complexing agent EDTA
(ethylenediaminetetraacetic acid) and also to study its impact on
the structural, electrical and optical properties.
2. Experimental
It is important to take proper precautions before depositing the films. One of
these is the careful cleaning of the substrates as it helps adhere the films on the
substrates most effectively. Initially, the slides were washed with liquid detergent
and then, boiled in conc. chromic acid (0.5 M) for 2 h and kept in it for 48 h. The
substrates were then washed with double distilled water. Finally the substrates were
dried using AR grade acetone before using it for further procedure. For deposition
of Bi2 S3 thin films, bismuth nitrate was used as Bi3+ and sodium thiosulphate as
S2− ion source in acidic medium. For this, 25 ml solutions of 0.05 M bismuth nitrate,
and 25 ml of 0.05 M sodium thiosulphate were mixed together (pH = 2.5) and kept at
303 K temperatures for further deposition. The chemical deposition was based on the
reaction between the EDTA complex of Bi3+ ions and sodium thiosulphate in acidic
media. If the ionic product (IP) of Bi3+ and S2− exceeds the solubility product (SP)
of Bi2 S3 , the formation of nuclei in the solution takes place. Thus to study the effect
of complexing agent, six different bath compositions were carried out by changing
the volume of EDTA.
The thickness of the film was measured by the gravimetric method. The twopoint dc probe method of dark electrical resistivity was used to study the variation
of resistivity with temperature. The copper block was used as a sample holder and
chromel–alumel thermocouple was used to measure the temperature. The area of
the film (0.5 cm2 ) was defined and silver paste was applied to insure good ohmic
contact to Bi2 S3 films. For the measurement of resistivity, a constant voltage was
applied across the sample and the current was noted using a digital nanometer.
The structural studies were carried out using Philips PW 1710 diffractometer, with
Cu-Ka radiation having wavelength = 1.5405 Å. Using JSM-6360 scanning electron
microscope carried out the microstructural studies. The optical properties of Bi2 S3
were studied by using Systronics-119 spectrophotometer.
3. Results and discussion
3.1. Bi2 S3 growth mechanism
To deposit bismuth-sulphide (Bi2 S3 ) thin film a slow release
of Bi3+ and S2− ions are required in an aqueous medium. These
ions then get condensed on the substrate placed in the solution
when its ionic product (I.P.) exceeds the solubility product (S.P.).
The deposition process follows an ion-by-ion or cluster-by-cluster
condensation on the substrate. In the ion-by-ion growth method for
deposition of thin films, the adsorption of metal ions on the surface
of the substrate is an important step which forms the nucleation
centers. However, in cluster-by-cluster growth process, the relatively large numbers of nuclei are formed in the solution as well
as on the surface of the substrate which produces large number of
small particles. As a result, there are large number of centers are
formed upon which growth process can take place; none of the,
particles grow very large and a colloidal suspension is formed. The
process of precipitation of a substance from the solution onto a
substrate depends mainly on the formation of a nucleus and subsequent growth of a film. The rate of precipitate formation in the
solution depends on the ionic concentration of bismuth, sulphur
and complexing agent and deposition temperature. A schematic
diagram that illustrates the CBD Bi2 S3 growth mechanism based on
our current understanding is given in Fig. 2. As mentioned earlier,
there are two major competing reactions in a CBD Bi2 S3 growth
process: ion-by-ion and cluster-by-cluster precipitate formation.
The bismuth salt (Bi(NO3 )3 ·5H2 O) produces free bismuth ions (Bi3+ )
through a dissociation reaction. Sodium thiosulphate releases free
sulphide ions through an equilibrium hydrolysis reaction. Free bismuth ions then react with free sulphide ions to form Bi2 S3 particles
in the bulk solution. However, in the presence of EDTA, bismuth
ions form a complex, controlling the concentration of free bismuth
ions. When EDTA is not added in the reaction bath, the film growth
rate is very high giving minimum terminal film thickness indicating cluster-by-cluster deposition process. However, addition of
EDTA suppresses the reaction rate indicating ion-by-ion deposition
which gives higher film thickness. Thus, the addition of EDTA in the
reaction bath was increased from 0 to 25 ml. When EDTA content
was small, the main species of bismuth are (Bi3+ ) non-chelated. In
the presence of EDTA the Bi3+ ions form a complex, as trivalent bismuth has high affinity to form complex with organic ligands, which
influences availability of Bi3+ ions in the solution. The complexation of organic ligands may also influence the activity of organic
ligands. Low quantity of EDTA in reaction bath can increase the solubility of many metals, and thereby, may increase their availability.
For bismuth, which has high solubility, the added EDTA does not
Fig. 1. Schematic diagram of CBD Bi2 S3 growth mechanism.
A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560
557
Table 1
Variation of Bi2 S3 film thickness with EDTA volume.
Film
A
B
C
D
E
F
Bath composition (ml)
Thickness (nm)
0.5 M bismuth nitrate
0.2 M EDTA
0.2 M sodium thiosulphate
25
25
25
25
25
25
00
05
10
15
20
25
25
25
25
25
25
25
45
81
111
139
191
201
likely increase the bismuth stimulative effect in the growth process. In the growth process, the availability of Bi3+ ions for S2−
is higher than [Bi(EDTA)]+ . Consequently, bismuth chelated with
EDTA, would not increase its stimulative effect. In this way, EDTA
is an amino derived organic compound known to be a strong hexdentate chelating agent. It forms a complex with metal ions and
dissociates reversibly at low rate. The ion-by-ion and cluster-bycluster growth mechanisms of Bi2 S3 are schematically represented
in Fig. 1. In present process, it was observed that without EDTA, the
rate of dissociation is high which causes fast precipitation in the
bath. However, when EDTA is added, it reduces the rate of dissociation by forming complex. The rate of dissociation decreases with
EDTA volume in reaction bath which gives more terminal thickness.
The variation of film thickness with EDTA volume for 4 h deposition
time is shown in Table 1.
3.2. Structural characterization
X-ray diffraction is a powerful non-destructive method for
material characterization, by which the crystal structure, orientation, and grain size can be determined. Structural identification of
Bi2 S3 films was carried out with X-ray diffraction in the range of
angle 2 between 20 and 60◦ . Fig. 2 shows the XRD patterns of
Bi2 S3 thin films. The observed broad hump in XRD pattern is due to
amorphous glass substrate. The film having thickness 45 and 81 nm
is amorphous in nature and show nanocrystalline nature with
orthorhombic structure for higher thickness. Table 2 summaries
the crystallographic data of these films compared with ASTM data
file (JCPDS 170320) [19]. Well defined (2 2 0), (1 3 0), (2 1 1), (3 1 1)
and (2 3 1) peaks are observed in the XRD pattern. These results are
in good agreement with that obtained by Benramdane et al. [20],
and Mizogushi et al. [21]. The films are oriented in the (1 3 0) direction. The crystallite size of the film was determined from the line
(1 3 0) by using Scherrer formula:
d=
ˇ cos Fig. 2. X-ray diffraction patterns of Bi2 S3 thin film of thickness: (A) 45 nm; (B) 81 nm;
(C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm.
(1)
where is the wavelength used (1.54 Å); ˇ is the angular line width
at half maximum intensity in radians; is the Bragg’s angle.
It was found that crystallite size increases from 11 to 26 nm
as film thickness increases from 111 to 201 nm. This significant
improvement in crystallite size is due to controlled slow release of
Fig. 3. Variation of log vs. 1/T × 103 (K−1 ) for Bi2 S3 films of thickness: (A) 45 nm;
(B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm.
Table 2
Comparison of XRD data for Bi2 S3 thin films with the JCPDS card.
Standard data
JCPDS card 170320
Observed data Bi2 S3 film
Thickness 201 nm
Thickness 191 nm
Thickness 139 nm
Thickness 111 nm
hkl
d (Å)
d (Å)
d (Å)
d (Å)
d (Å)
220
130
211
311
231
3.967
3.569
3.118
2.641
2.305
3.977
3.566
3.112
2.634
2.318
3.965
3.559
3.120
2.648
2.311
–
3.561
3.110
2.639
–
–
3.570
3.116
–
–
558
A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560
Fig. 4. Variation of resistivity of Bi2 S3 film with reciprocal of thickness at temperature 373 K.
bismuth ions from its complex [Bi(EDTA)]+ in the solutions which
give probability of growth of larger particles.
3.3. Electrical resistivity
The dark electrical resistivity of Bi2 S3 films was studied in the
temperature range 333–453 K using dc two-point probe method.
Fig. 3 shows the variation of log of resistivity (log ) with reciprocal
of temperature (1/T) × 103 . It is seen that resistivity decreases with
temperature indicating semiconducting nature of films. The resistivity variation with film thickness at temperature 373 K is given
in Fig. 4. The resistivity of Bi2 S3 is of the order of 104 cm and it
decreases from 24.08 × 104 to 4.33 × 104 cm as the film thickness
increases from 45 to 201 nm. The high value of resistivity is due to
large number of grain boundaries and discontinues in the film. The
thermal activation energy was calculated using the relation:
= 0 exp
E 0
KT
,
(2)
where is resistivity at temperature T, 0 is a constant, K is Boltzmann constant (8.62 × 10−5 eV K−1 ) and E0 is the activation energy
required for conduction.
Fig. 5. Variation of film thickness and activation energy of Bi2 S3 with EDTA volume
in reaction bath.
The activation energy was found to be decreased from 0.85 to
0. 41 nm as film thickness varies from 45 to 201 nm. Fig. 5 shows
variation of activation energy and film thickness with volume of
EDTA in reaction bath. As per reaction mechanism, the addition of
EDTA slows down the growth rate giving more terminal thickness
with better grain growth.
3.4. Surface morphology
The SEM of the Bi2 S3 thin films of thickness 45, 81, 139 and
191 nm deposited on the glass substrate was examined (Fig. 6). The
surface of the film is smooth, well covering the glass substrate. The
nucleation centers formation rate of Bi2 S3 depends on the rates of
release Bi3+ and S2− ions. Hence, only a large number of the ions are
utilized for film formation by cluster-by-cluster, resulting in a lower
final thickness with smaller grains as seen in sample (A). However
in the presence of EDTA, complex formation takes place and due to
slow release of bismuth ions few nucleation centers are formed by
ion-by-ion growth resulting in a higher thickness with larger grains
as seen in sample (B), (D) and (E). The improvement in grain growth
Fig. 6. SEM images of Bi2 S3 film of thickness: (A) 45 nm; (B) 81 nm; (D) 139 nm and (E) 191 nm.
A.U. Ubale / Materials Chemistry and Physics 121 (2010) 555–560
559
Fig. 9. Variation of film thickness and optical bandgap energy of Bi2 S3 with EDTA
volume in reaction bath.
by using the relation:
n
Fig. 7. Variation of optical absorption vs. wavelength of Bi2 S3 films of thickness: (A)
45 nm; (B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm.
is observed with percentage of EDTA in reaction bath. The grain is
quite irregular in shape for the sample deposited without EDTA and
shows somewhat circular in nature for the samples deposited with
EDTA (sample E) which confirms ion-by-ion growth of Bi2 S3 .
3.5. Optical properties
The optical properties of the chemically deposited Bi2 S3 films
were characterized by optical absorption measurements. The
absorption spectra were recorded with a Systronics-119 spectrophotometer typically in the wavelength range of 300–1100 nm
at room temperature (Fig. 7). The nature of transition involved in
semiconductors can be determined on the basis of dependence of
absorption coefficient (˛) of the material on incident photon energy
(h). The nature of the transition (direct or indirect) is determined
˛=
A(hv − Eg)
,
hv
(3)
where h is the photon energy, Eg is the bandgap energy, A and n
are constants. For allowed direct transitions n = 1/2 and for allowed
indirect transitions n = 2. Fig. 8 shows the spectral dependence of ˛,
in the form of (˛h)1/2 against h. The values of the optical bandgap
were determined for Bi2 S3 films of different thicknesses and are
shown in Fig. 9. It is clear from this figure that the energy gap
increases from 1.84 eV to 2.66 eV with increasing thickness of the
films from 201 to 45 nm. The presence of defects in the nanostructured films produces discrete states in the band structure which
is responsible for the high value of the energy gap in the case of
film of thickness 45 nm. However, for higher thickness, the films
are more homogeneous and reduce the number of defects and disorder which decreases the density of localized states in the band
structure and consequently decreases the optical energy gap. Also
the result of the confinement of the exciton, essentially as the particle size is reduced, the hole and the electron are forced closer
together, and the separation between the energy levels changes.
4. Conclusions
In the present investigation, the effect of complexing agent EDTA
on growth process was studied. EDTA is an amino derived organic
compound known to be a strong hexdentate chelating agent. It
forms a complex with metal ions and dissociates reversibly at low
rate. It is also observed that without EDTA the rate of dissociation
is high which causes fast precipitation in the bath giving clusterby-cluster deposition. However when EDTA is added, it reduces
the rate of dissociation by forming a complex. The rate of dissociation decreases with EDTA which causes slow precipitation in
the bath giving ion-by-ion deposition that gives more terminal
thickness. The XRD studies reveal that deposited Bi2 S3 films are
nanocrystalline with orthorhombic structure. The optical bandgap,
electrical resistivity, and activation energy are observed to be thickness dependent.
Acknowledgement
The author is thankful to University Grants Commission, WRO,
Pune (India), for financial support under the project (No: F47258/07).
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Fig. 8. Variation of (˛h)2 vs. h for Bi2 S3 films of different thicknesses: (A) 45 nm;
(B) 81 nm; (C) 111 nm; (D) 139 nm; (E) 191 nm and (F) 201 nm.
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