Materials Sciences and Applications, 2011, 2, 163-168
doi:10.4236/msa.2011.23020 Published Online March 2011 (
Copyright © 2011 SciRes. MSA
Optical and Electrical Properties of Doped and
Undoped Bi2S3–PVA Films Prepared by
Chemical Drop Method
Anayara Begum, Amir Hussain, Atowar Rahman
Department of physics, Gauhati University, Guwahati, India.
Received January 5th, 2011; revised January 24th, 2011; accepted February 2nd, 2011.
Bismuth sulfide was prepared in PVA matrix by chemical method using solutions of Bi(NO3)3 and Na2S. Bi2S3 was
doped during preparation using Br2 vapour and also liquid drops of Br2. Both doped and undoped Bi2S3–PVA films
were characterized by SEM, XRF, optical absorption and electrical conductivity measurements. The undoped films
consisted of particles of sizes 156 nm-184 nm as revealed by SEM micrographs. The films doped with Bromine (Br2)
vapour were found to consist of rods of diameters ranging from 75 nm to 80 nm. The films doped with Br2 liquid drops
showed rods of diameters ranging from 4843 nm to 6930 nm. XRF spectra confirmed the presence of bismuth, sulfur
and bromine in the doped films. The temperature variation of doped and undoped films in the temperature range from
298 K to 383 K shows more or less similar pattern of variation with two regions of conduction. The band gap obtained
from the absorption spectra was found to change from 3.61 eV to 3.78 eV and the absorption edge shifted towards the
lower wavelength with decrease in diameter of the particles or rods.
Keywords: Nanostructures, Doping, Bismuth Sulfide, Semiconducting Materials, Solar Cells
1. Introduction
The members of the V-VI group of compound semicon-
ductors are technically important materials because of
their photosensitivity, photoconductivity and thermoe-
lectric power [1,2]. Bismuth sulfide (Bi2S3) is an impor-
tant member of V-VI group of compound semiconductor
and it is a direct band gap material. The earlier reported
[3] band gap of bulk Bi2S3 is 1.3 eV. The more recent
value of band gap is reported to be in the range 1.3-1.7
eV [4], which lies in the visible solar energy spectrum. It
has large absorption coefficient [5].Thus it is an ideal
candidate for solar cells and photodetectors in the visible
wavelength region. Due to its significant thermoelectric
effect, this material is important in thermoelectric appli-
cations [6]. The excellent electrochemical hydrogen sto-
rage properties of Bi2S3 flower like patterns with well
aligned nanorods and discs like Bi2S3 nanorods networks
have been investigated by Xie and Qi groups respectively
[7,8]. Bi2S3 is a layered semiconductor that crystallizes in
the orthorhombic system and is iso structural to antimony
sulfide (Sb2S3) and selenide (Sb2Se3) [3]. Bi2S3 thin films
have been reported to be prepared by many workers fol-
lowing different chemical routes and using different
complexing agents and sulfide sources [9-11]. Recently
considerable researches have been carried out focusing
on the synthesis of one dimensional (1D) microstructures
of Bi2S3. Depending on growth condition, different 1D
structure such as nanotubes [12], nanowires [13,14], na-
norods, nanoflowers [15] and, nanoribbons of Bi2S3 have
been reported to be synthesized by different research
groups. In addition to 1D, two dimensional (2D), and
three dimensional (3D) nanostructures of Bi2S3 can be
obtained via physical and chemical methods but the con-
trol of size and shape of the structures remain as a diffi-
cult task. The importance of one dimensional material is
that as the diameter of the semiconductor approaches the
exciton Bohr diameter, its electronic properties changes
(Quantum size effect). This can be observed as a blue
shift in the optical band gap or excitation energy.
In the present communication, we have reported the
preparation of Bi2S3 within the self organized pores of
Poly vinyl alcohol (PVA) via chemical drop method.
Thin films of Bi2S3 were then doped with bromine. Both
Optical and Electrical Properties of Doped and Undoped Bi2S3–PVA Films Prepared by Chemical Drop Method
Copyright © 2011 SciRes. MSA
doped and undoped films were characterized for their
structural, optical and electrical properties.
2. Experimental Details
Thin films of Bi2S3–PVA were deposited on glass sub-
strates using PVA as matrix, Bi(NO3)3 and Na2S as Bi3+
and S2– ion source respectively. All chemical products
used in this work were of analytical grade. They were
used without further purification. First the matrix solu-
tion was prepared by mixing 5 wt% of PVA in double
distilled water and stirred in a magnetic stirrer at a con-
stant temperature until a transparent solution was ob-
tained. To this solution, 0.01 M (molarity) of Bi(NO3)3
was added in the ratio of 2:1 and the stirring was contin-
ued at the same temperature for three hours and then
brought down to the room temperature (300 K). To the
transparent solution obtained, 0.03 M of Na2S was added
drop by drop until the solution turned into dark brown.
The resultant solution was kept in dark undisturbed for
12 hours for stabilization. This solution was then cast
over glass substrates drop by drop and dried at room
temperature (300 K). This way undoped Bi2S3–PVA
films were obtained. Doped Bi2S3–PVA were obtained
by passing bromine (Br2) while adding 0.03 M Na2S to
the transparent solution containing Bi(NO3)3 and PVA at
room temperature and stirred for half an hour.
The thicknesses of the films were measured by multi-
ple beam interferometer technique. Surface morphologi-
cal studies of the chemically deposited Bi2S3–PVA thin
films were done using Scanning Electron Microscope
(LEO 1430 VP) operating with an accelerating voltage
15 kV. X-ray fluorescence study (XRFS) was done using
Axios XRF spectrometer for elemental analysis of the as
prepared doped films. Optical absorption studies were
carried out using a UV-visible spectrophotometer (Hi-
tachi U-3210) in the wavelength range 350-750 nm.
The electrical conductivity measurements of the films
were done using two coplanar Aluminium (Al) electrodes
separated by a small gap. These electrodes were vacuum
deposited at the two ends of the rectangular Bi2S3–PVA
strip. The deposition of electrodes was performed at a
reduced pressure of 105 Torr in a vacuum deposition unit
(VICO-12). The conductivity of Bi2S3–PVA was deter-
mined by measuring the resistance of the samples using
an electrometer (Keithley 6514) in the temperature range
298 K - 383 K and the same temperature was varied us-
ing a temperature controller. Temperature of the samples
was measured by means of a thermocouple.
3. Results and Discussion
3.1. SEM Analysis
Figure 1 shows the SEM photograph of a film containing
undoped Bi2S3
embedded in a PVA matrix. From the
photograph it is observed that the distributions of grains
are not uniform throughout the PVA matrix. But the film
is smooth without any visible pores or cracks. The grain
size is found to be in the range 156-183 nm. Figure 2
shows the SEM photograph of Bi2S3 doped with bromine
vapour which shows rod shaped structures of diameters
in the range 78-80 nm (nanorods). The rods are not dis-
tributed uniformly throughout the matrix. Figure 3 shows
SEM photograph of Bi2S3 doped with bromine liquid
drops. The figure reveals rod like structures with diame-
ter in the range 4843-6930 nm (microrods) distributed
unevenly throughout the PVA matrix. The rods doped
with Br2 is dispersed in PVA matrix whose conductivity
is very low and hence concentration could not be meas-
ured by conventional methods. The doping was perfor-
med using MERCK made 99.99% pure Br2. The thickness
of undoped Bi2S3–PVA film was 8 × 105 m, while
doped films consisting of nanorods and microrods were 3
× 105 m and 7 × 105 m thick respectively.
3.2. X-Ray Fluorescence (XRF) Studies
The Figures 4(a) and 4(b) show the XRF spectra of doped
Figure 1. SEM image of undope d Bi 2S3 in PVA matrix.
Figure 2. SEM image of bromine doped Bi2S3 nanorods in
PVA matrix.
Optical and Electrical Properties of Doped and Undoped Bi2S3–PVA Films Prepared by Chemical Drop Method
Copyright © 2011 SciRes. MSA
Figure 3. SEM image of bromine doped Bi2S3 microrods in
PVA matrix.
Figure 4. XRF spectra of doped Bi2S3 nanorods in PVA ma-
Bi2S3–PVA films. The spectrum in Figure 4(a) exhibits
the prominent peaks of BrKβ, BiLβ, BrKα and BiLα1
lines showing the presence of Bi and Br in the prepared
film. The spectrum shown in Figure 4(b) shows the peaks
of BiMβ, BiMα1 and SKα lines along with the lines of
ClKα, RhLα and PKα. Peaks for Cl, Rh and P appear
from the target used in the XRF instrument and from the
possible impurities present in the glass substrates.
3.3. Optical Absorption Studies
The optical (electronic) properties of semiconducting
compounds result from band structures of semiconduct-
ing materials and are very important in a large number of
applications. Figure 5 shows the absorbance spectra of
doped and undoped Bi2S3–PVA films. From the spectra
the absorbance edge for undoped Bi2S3 is found to occur
at 400 nm and the same has been found to occur at 377.7
nm and 422 nm for Bi2S3 doped with Br2 vapour and Br2
liquid respectively. An extra absorption peak at 370 nm
which corresponds to 3.35 eV has been observed in the
film doped by Br2 vapour. The extra peak showed in
nanorods doped by Br2 vapour has been thought to be
due to some defects which were crept in to the films
during preparation. This peak has been found to disap-
pear on aging of the films. The nature of transition (direct
or indirect) involved can be determined considering the
relation of absorption coefficient
where “a” is constant, Eg is the separation between the
conduction band and valence band. The absorption coef-
ficient α is calculated from the relation [16]
where A is the absorbance and t the film thickness. In all
the three samples, the plots of (αhυ)2 vs (hυ) are linear as
shown in Figure 6 indicating that the transition is a di-
rect band gap edge transition. The extrapolation of the
linear portion of such a plot to α = 0 yields the band gap
energy. The band gap so obtained for undoped Bi2S3 par-
ticles is 3.70 eV while the doped Bi2S3 nanorods have a
band gap value 3.78 eV and that of the doped Bi2S3 mi-
crorods is 3.61 eV.
3.4. Electrical Conductivity
The variation of electrical conductivity of doped and
undoped Bi2S3–PVA films measured in the 298 K-383 K
temperature range are shown in Figure 7. The conduc-
tivity was observed to increase rapidly in all the three
samples as the temperature was raised from 298 K at-
taining a maximum around 322 K, after which the con-
ductivity was found to decrease and reached a minimum
around 342 K beyond which the conductivity showed an
Optical and Electrical Properties of Doped and Undoped Bi2S3–PVA Films Prepared by Chemical Drop Method
Copyright © 2011 SciRes. MSA
Figure 5. UV-absorption spectra of doped and undoped
Bi2S3–PVA films.
Figure 6. Energy band determination of doped and un-
doped Bi2S3–PVA films from (αhν)2 vs (hν) plot.
Figure 7. Variation of log (σ) vs 1/T for doped and undoped
Bi2S3–PVA thin films.
increase again. The nature of variation of conductivity of
PVA film alone with temperature is found to be similar
as reported in our earlier paper [17] and by Ahmed and
Abo-Ellil [18]. Thus the nature of variation of Bi2S3
–PVA films is dominated by the conductivity variation of
the PVA matrix. The value of conductivity of undoped
and doped Bi2S3–PVA films are given in Table 1.That
doping occurred in Bi2S3 is indicated by the increase of
conductivity in nanorods and microrods. Such rods like
structures of Bi2S3 have been reported by some other
workers [19-21]. It is not however, understood as how
the rods like structures were initiated in the process of
doping. The probable mechanism for the formation of
rods may be due to preferential growth in certain direc-
tion initiated by dopant material. Growth of rod like
structure has also been reported by Zhou et al. [22] in
undoped Bi2S3.
The lnσ versus 1000/T plots for the doped and un-
doped Bi2S3–PVA films depict two conduction regions.
These regions have different activation energies which
were calculated using the relation
where, Ea is activation energy, σ0 is a constant, k is
Boltzmann’s constant and T is absolute temperature.
Different activation energies indicate different conduc-
tion processes such as electronic and ionic conduction
[23]. Activation energies for doped and undoped Bi2S3
PVA films are tabulated in Table 1.
4. Conclusions
Undoped Bi2S3–PVA films were prepared and found to
consist of grain sizes in the range 158-183 nm. Bi2S3-
PVA films with nanorods and microrods were obtained
on doping with Br2 vapour and Br2 liquid respectively.
The band gap energy was maximum in case of doped
Bi2S3–PVA nanorods and minimum in case of Bi2S3–PVA
microrods. Thus the band gap energy has been found to
Table 1. Electrical conductivity and activation energy val-
ues of Bi2S3–PVA thin films.
Activation energy (eV)
Room temperature
10–5 × (–1·m–1) Region I Region II
Bi2S3–PVA 0.28 0.397 0.999
2.46 0.345 0.746
7.91 0.37 0.539
Optical and Electrical Properties of Doped and Undoped Bi2S3–PVA Films Prepared by Chemical Drop Method
Copyright © 2011 SciRes. MSA
increase with decrease in size showing a blue shift in the
absorption spectra. On doping there was an increase in
the electrical conductivity of Bi2S3–PVA films. However
the conductivity of all the samples was largely influenced
by the conductivity of PVA structure.
5. Acknowledgements
The authors express their gratefulness to the Department
of Chemistry, Gauhati University, Guwahati and IIT Gu-
wahati for providing us the UV-Visible Spectra and SEM
facilities respectively.
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