CHAPTER - III
STUDIES ON SILVER SULFIDE NANOPARTICLES
3.1 INTRODUCTION
Silver sulfide (Ag 2 S) is an important inorganic compound which has been
studied for its numerous applications. Silver sulfide (Ag 2 S) belongs to I-VI
compound semiconductor materials. Silver sulphide appears to be a promising
material for conversion of solar energy into electrical energy as it has absorption
edge in the vicinity of 400 nm [1] and 1000 nm [2]. The semiconductor silver
sulfide has photoelectric and thermoelectric properties. In the present investigation,
silver sulfide nanoparticles were synthesized by the reaction of silver acetate and
thiourea by solvothermal method. This is the first time that solvothermal method is
used for the synthesis of silver sulfide nanoparticles. The as- synthesized
nanoparticles annealed at various temperatures were characterized by
X ray
diffraction, scanning electron microscopy and atomic force microscope to confirm
the crystal structure, morphology and size. The DC electrical resistance was
measured in the temperature range 310K-485K and
from these measurements
band gap energies are calculated and are compared with band gap energies
obtained from UV-VIS spectra. Materials with band gap in the optical region can
be used as photo detectors as they absorb photons in the optical region and give out
electrons that can be used in a conduction process which in turn show the presence
of a radiation.
59
3.2 SYNTHESIS
To prepare Ag 2 S nanoparticles by microwave-assisted solvothermal route,
silver acetate and thiourea were taken as the starting precursor materials in 1:3
molar ratio. The amount of precursor material dissolved in volume V of ethylene
glycol is calculated using the formula
Required substance "
MXV
in gram units !
1000
(3.1)
where M is the molecular weight of the substance, X is the concentration in
molar units and V is the volume of ethylene glycol. In a typical synthesis of Ag 2 S
nanoparticles, the amount of silver acetate and thiourea required to dissolve in 100
ml of ethylene glycol was calculated to be 21.95 g and 22.83 g respectively. The
precursor materials were dissolved in ethylene glycol and stirred well using a
magnetic stirrer. The well-mixed solution was taken in a bowl and it was kept in a
domestic microwave oven (Electrolux 800 W model
provided with six stage
adjustable power/time domain). The apparatus is fully loaded with rotating hard
glass circular plate at the bottom to place the bowl on it. The side wall of the
apparatus is well protected with tin sheet cover. The solution is placed into the
bowl with top cover which is sustainable for the heat produced inside the oven.
The solution/material was subjected to microwave irradiation of 800 W for 20
minutes. The colloidal precipitate obtained after this irradiation was air cooled to
room temperature. This cooled substance was washed several times using doubly
distilled water and then with acetone to remove the organic impurities, if any,
60
present in the sample. The sample was then filtered and dried in atmospheric air
and collected as the yield.
During the process of synthesis, the strong reaction between silver
acetate and thiourea lead to the formation of silver–thiourea complex, which
prevents the production of a large number of free S2- in the solution and will
become favourable for the formation of the products. Then the silver-thiourea
complex undergo thermal decomposition under microwave irradiation to produce
silver sulphide (Ag 2 S). The unannealed sample was finally annealed to get one
sample at 500 C for 1 hour and another at 100 0C for 1 hour, two sets of phase pure
Ag 2 S nanoparticles.
3.3 STRUCTURAL STUDIES
X-ray diffraction (XRD) patterns of the samples were recorded on a
Philips model PW-1830 X- !"#$%&& !'()*+(+ ##,%(-#.)#/0## !$%!(%)1#)&#,!2+3+ngth
1.788A0 in the range of scanning angles 100- 800. Figures 3.1a, 3.1b and 3.1c
show the X-ray diffraction patterns of samples of Ag 2 S
nanoparticles which are
unannealed, annealed at 500C and 1000 C respectively. Comparing to the JCPDS
file no 14-4456#789#:;#(-+#)<=+ 2+$#>+!?=#'!1#<+#%1$+@+$#!=#(-)=+#)&#A#BC 2 S with
monoclinic structure.
61
Fig 3. 1a: XRD pattern of unannealed silver sulfide nanoparticles synthesized
by solvothermal method
Fig 3.1b : XRD pattern of silver sulfide nanoparticles annealed at 50 0c
synthesized by solvothermal method
62
Fig 3. 1c : XRD pattern of silver sulfide nanoparticles annealed at 100 0c
synthesized by solvothermal method
A c++ program using non-linear least square fit was developed (appendix A)
to fit the peaks of the XRD pattern to a Gaussian and from the width of the
Gaussian the crystallite sizes were calculated using the following Debye-Scherrer
equation [5-7] .
DE4FGHIJA')=KL
M-+ +##D#%=#(-+#' "=(!33%(+#=%N+#%1#1*9#H#%=#(-+#,!2+3+1C(-#)&#(-+#O- !"=9#A#%=#(-+#
&P33#,%$(-#!(#-!3&#*!@%*P*#!1$#K#%=#(-+#$%&& !'(%)1#>+!?#!1C3+F#Q%C#8F6#=-),=#(-+#
Gaussian fitted (1 2 0) peak of the unannealed silver sulfide nanoparticles.
63
Fig 3.2 : Gaussian fitted (1 2 0) peak of unannealed silver sulfide
nanoparticles
Crystallite sizes were calculated from different single peaks for the three
samples and are tabulated in table 3.1. It is found that the average size of the Ag 2 S
nanoparticles decreases with increase in annealing temperature. Even though the
peaks are located at the expected angles in the XRD pattern of the three samples
the height of the peaks of the unannealed sample are not in the expected order and
hence it was not used for further studies.
64
Table – 3. 1
Peak
Index
Grain size nm
unannealed
sample
annealed at
annealed at
0
50 C
1000C
(1 2 0)
46
42
29
(-1 1 1)
45
45
34
(-1 1 2)
46
46
29
Average
size nm
46
44
31
Fig 3.3: SEM image of the 50oC annealed silver sulfide nanoparticle
The SEM image of the powdered 500 C annealed sample (fig 3.3) shows
the uniform distribution of Ag 2 S nanoparticles. AFM image of the above sample
(fig 3.4) confirms the nano size of the particles.
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Fig 3.4: AFM image of the 500C annealed silver sulfide nanoparticle
Fig 3.5: XRD pattern of silver sulfide nanoparticles after using it for
resistance measurements.
Figure 3.5 shows the indexed XRD pattern of the powder form of the pellet,
which was used for resistance measurements. The above pellet was heated to
66
2100 C and cooled back to room temperature repeatedly and then annealed at 1200 C
for one hour and again repeatedly used for resistance measurements at various
temperatures. The most intense peak of fig 3.5 agrees with the XRD pattern of the
Ag 2 S collected at 2500 C 0# phase [8]. Assuming the structure to be cubic as
reported and indexing the most intense peak as BCC (2 0 0) the lattice parameter
for the structure after phase transition was calculated. The calculated value 4.8 A0
agrees with the lattice parameter of BCC silver sulfide [9]. So the sample is now a
mixture of monoclinic and BCC phases.
3.4 OPTICAL STUDIES
Fig 3.6a shows optical absorption spectrum of Ag 2 S nanoparticles annealed
at 500C. There is an absorption band with absorption edge above 900nm (<1.4 eV).
Another absorption band with absorption edge around 360 nm (3.5 eV) also exists.
Fig 3.6a: UV-VIS absorption spectrum of silver sulfide nanoparticles
annealed at 500C
67
Fig 3.6b shows optical absorption spectrum of
Ag 2 S
nanoparticles
annealed at 1000C. It looks similar to fig (5a) except that one more absorption band
with absorption edge at 300 nm (4 eV) exists.
Fig 3.6b: UV-VIS absorption spectra of silver sulfide nanoparticles
annealed at 1000C
3.5 ELECTRICAL STUDIES
Assuming that mobilities of holes and electrons are independent of
temperature the electrical conductivity of an intrinsic semiconductor is given
by [10]
68
R#E#R 0 T3/2 exp(-E g /2kT)
S+=%=(%2%("########T#E#T 0 T -3/2 exp(E g /2kT)
Resistivity = Resistance * area / thickness
On using the same pellet area and thickness of the sample are constants.
S+=%=(!1'+JSL####0####S+=%=(%2%("#JTL###
R = R 0 T -3/2 exp(E g /2kT)----------------(1)
E g %=#(-+#%1( %1=%'#<!1$#C!>#+1+ C"F#R 0, T 0 and R 0 are constants . k and T are
Boltzman constant and absolute temperature respectively. As exp(E g /2kT) is more
sensitive to change in temperature than T
-3/2
the temperature dependence of
resistance will be dominated by the exponential dependence [11] . So the plot of
variation of log resistance with reciprocal of temperature will be a straight line for
any intrinsic semiconductor with a band gap energy.
The samples were pelletized by applying high pressure 10 tons over
an area of 1 cm-sq . The resulting pellet is a compressed collection of
nanoparticles. The DC electrical resistance of the pellet form of the samples
annealed at 1000C (sample b) and 500C (sample c) were measured in the
temperature range of 310 K- 485 K by the four probe technique.
69
Fig 3.7: Variation of ln(R) with 1/T of silver sulfide nanoparticles
a, annealed at 1200C b, at 1000C and c, at 500C
The plot of variation of log resistance with reciprocal of temperature of the
Ag 2 S nanoparticles samples “ b” and “c” are shown in fig 3.7 . Plot for samples
“b” and “c” are not straight lines as expected. Plot for samples “b”
and “c”
consists of three regions. Regions 1 and 111 are straight lines with different band
gap energies. Region 11 is a region in the vicinity of phase transformation.
The measured resistances at different temperatures were fitted to
equation (1) using a nonlinear curve fitting c++ program developed (appendix B)
using least square principle. Keeping one of the parameters (R
70
0
or E g ) fixed, the
other parameter is varied. The pair for which sum of the squares of the deviation of
the calculated values from the experimental values is least is selected.
The best fit for region 1 (310 K-380K) for samples “ b” and “ c”
were obtained for the values E g = 1eV and 0.4 eV respectively. The measured
resistances and the curve for
R = 60921 * T -3/2 * exp (1.03/2kT)
seen in region 1 pertaining to the sample “ b”
for various temperatures is
shown in fig 3.8.
Fig 3.8: Variation of resistance with temperature . Experimental points and
fitted curve for region1 (310K-380K) for 1000C annealed sample.
71
The measured resistances and the curve for
R = 1.3 * 107 * T -3/2 exp(.4/2kT)
for region 1 for sample ”c” for various temperatures is shown in fig 3.9.
Fig 3.9: Variation of resistance with temperature . Experimental points and
fitted curve for region1 (310K-390K) for 500C annealed sample.
The best fit for region 111 for sample ” b” (450k-485k) and sample
“c” ( 430K-485K) were obtained
for E g =3.5and 2.7 eV respectively. The
measured resistances and the curve for
R = 2.7 * 10-9 * T-3/2 * exp(3.5/2kT)
seen in region 111 pertaining to the sample “b” for various temperatures is
shown in fig 3.10. The measured resistances and the curve for
R = 8.6 * 10-6 * T-3/2 * exp(2.7/2kT)
for region 111 for sample “c” for various temperatures is shown in fig 3.11.
72
Fig 3.10: Variation of resistance with temperature . Experimental points and
fitted curve for region111 (450K---485K) for 1000 annealed sample
Fig 3.11: Variation of resistance with temperature . Experimental points and
fitted curve for region111 (430K---475K) for 500 annealed sample.
The behaviour of sample “b” in region 1 corresponds to a band gap energy
73
1.0 eV. The next region (region 11) on increasing the temperature has a nonuniform
variation of resistance with temperature, indicating a region in the vicinity of a
phase transformation. Further increase takes the sample to a region 111 where the
band gap energy is 3.5 eV. For sample “ c” initially there is slow variation of
resistance with temperature up to 380K corresponds to a band gap energy 0.4 eV
after which it bends towards higher band gap energy(region 11) . This bend was
also observed by B.R.Sankapal and R.S.Mane [12]. After region 11 there is jump
to region 111 where the band gap energy is 2.7eV at an earlier temperature (430K)
than for sample “b” where it happened at 450K. When the above used pellet was
annealed to 1200 C for one hour and the experiment was repeated plot “a” (fig3.7)
was obtained, where there is a jump in resistance from the order of 107 ohm to 103
ohm around 480K.
3.6 COMPARISON OF ELECTRICAL AND OPTICAL STUDIES
For sample “b” jump from band gap energy of 1 eV to 3.5eV was obtained
from the resistance measurements. The band gap energy 3.5 eV(360nm)
corresponds to the absorption edge of the second absorption band of the UV-VIS
spectrum of sample “ b “(fig 3.6b) and edge of the first absorption band of sample
“c “. Such a high band gap energy was also obtained by Lubna Hasmi and Prabha
Sana for self assembly of Ag 2 S nanoparticles inside chitoson matrix from UV
spectrum [13]. The band gap energy of 1.0eV from the UV-VIS spectrum was
previously reported by B.R Sankapal and R.S.Mane for Ag 2 S thin films[12],
Kensuke and Akamatsu for polymer thin films containing silver sulfide
nanoparticles [14] and others[2,15]. It agrees with the band gap energy of the bulk
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[16]. It also agrees with the absorption edge above 900 nm of both UV.
For sample “c” jump in band gap takes place from 0.4 eV to 2.7 eV
(region I to region II). Band gap energy of 2.8 eV was previously reported by Rui
Chen and Noel T Nuhfer for silver sulfide nanoparticles assembly [1]. The band
gap energies obtained from resistance measurements using equation(1) for regions
1 and 111 for 1000C and 500C annealed silver sulfide nanoparticles are in good
agreement with the existing and previously reported band gap energies obtained
from UV-VIS spectra.
Hence we report here a jump in band gap energy from nearly 1.0eV to
3.5 eV around 450K for 1000C and 0.4 eV to 2.7 eV around 430K for 500C
annealed silver sulfide nanoparticles .This is due to a phase transformation, may be
due to the transformation from monoclinic to BCC form [9].The temperature at
which phase transformation takes place and difference between the band gap
energies before and after jump also increases with increase in annealing
temperature.
If the 500C annealed sample after using for resistance measurement is
annealed to 1200C and again when used for resistance measurement a heavy jump
in resistance from the order of 107 ohm to 103ohm was also observed at 480K. The
resistance remains almost constant before this jump. So it can be used as a
temperature sensor. Since the XRD of the above powdered pellet after cooling to
room temperature (fig 3.5) is a mixture of monoclinic and BCC form it is
confirmed that each time when the temperature is raised above transition
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temperature it transforms from monoclinic to BCC form and when it is cooled it
regains it monoclinic form. On repeated heating and cooling above transition
temperature some of the nanoparticles get frozen in the BCC form not regaining
the monoclinic form. The heavy jump in resistance may be the behaviour of the
sample in the mixed phase.
3.7 CONCLUSION
1.
Silver sulfide nanoparticles were synthesized by solvothermal method.
2.
XRD studies showed that the as- synthesized nanoparticles annealed at 333k
and 373k have monoclinic structure.
3.
The grain size decreases with increase in annealing temperature.
4.
The band gap energies obtained from resistance measurements are in good
agreement with the
existing and previously reported band gap energies
from UV-VIS spectra.
5.
A sudden increase in band gap energy or a jump in electrical band gap energy
was observed around 450K due to a phase transition.
6.
The temperature at which this transition takes place increases with increase in
annealing temperature.
7.
It is also of interest that the pellet which was annealed at 50 degrees but heated
to 2100C and cooled back to room temperature repeatedly and then annealed to
1200C for one hour had a drastic switch from 107 ohm to 103 ohm at 480K.
76
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