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Preparation of silver sulphide from chemically deposited silver films
MATERIALS CHEM;lTR’RY~dD ELSEVIER
Materials Chemistry and Physics 41 ( 1995) 75-78
Material Science Communication
Preparation of silver sulphide from chemically deposited silver films A.B. Kulkarni, M.D. Uplane, C.D. Lokhande Received II July 1994; accepted 13 February 1995
Abstract A simple chemical method has been used for the conversion of thin films of Ag into Ag,S films. Thin, uniform and large-area films of Ag were prepared by chemical deposition using the ‘adsorption-reduction’ method on glass substrates and dipped in Na2S solution to be converted into Ag,S films. Their structural, electrical and optical properties have been studied. Keywords:
Sliver
sulphide films; Chemical deposition; Semiconductors
1. Introduction
Metal chalcogenides have been studied intensively over the past 25 years in view of their actual and potential applications. Silver sulphide is a semiconductor ( Eg = 0.7-0.9 eV) which has been used in IR detectors, photoconductors, photovoltaic cells, electrochemical storage cells, solar selective coatings, etc. Recently an electrochemical photovoltaic storage cell with Ag,S electrodes has been reported [ 11. Ag$, with its relatively small band gap, good chemical stability and easy preparation, is an interesting material for application in many devices. Preparation of Ag,S films has been carried out by a number of chemical methods. Mangalam et al. have deposited Ag,S films from an alkaline medium using thiourea as a sulphide source [ 21, while Dhumure and Lokhande have reported on chemical deposition from an acidic bath using sodium thiosulphate and thioacetamide as sulphide ion releasing source [3-51. Lokhande et al. have reported the conversion of tin disulphide into silver sulphide by a simple chemical method of displacement [ 61. In this paper we report on the conversion of Ag films into Ag,S films by a simple chemical method. The silver films were deposited on amorphous glass substrates using the ‘adsorption-reduction’ method and were converted into Ag,S films by being dipped in sodium sulphide solution. The preparation of Ag,S films by this method is more economical than direct chemical deposition, as no bulk precipitate is formed in the solution. Using this method, large-area, good-quality Ag,S films have been formed on the glass substrates ( = 3 X IO cm’). The method is reproducible, and the film 0254-0584/95/$09.50 0 199.5Elsevier Science S.A. All rights reserved SSDI0254-0584(95)01499-K
properties are comparable with those for films prepared by the direct chemical deposition method. The Ag,S films thus prepared on the glass surface were characterised by X-ray diffraction, optical absorption, scanning electron microscopy and electrical resistivity techniques.
2. Experimental In the present investigation, the cleaning of the glass substrates was carried out using the following procedure: ( 1) the glass substrates were boiled in chromic acid for 5 min and were washed with doubly distilled water; (2) the substrates were dipped in ‘labogent’ detergent and again washed with doubly distilled water; (3) finally, the substrates were cleaned with an ultrasonic cleaner for 5 min. To deposit the silver onto the glass substrates, the glass substrates were first dipped in a warm (80 “C), alkaline, 0.1 M silver nitrate solution. Then these glass substrates were dipped in a warm (80 “C) 37.41% formaldehyde solution so as to form silver films on the glass substrates. For the conversion of Ag films into Ag,S films, a 0.1 M Na,S solution was prepared and the Ag films were dipped into it. The film colour changed from silvery to blackish. The converted films were taken out of the solution, washed thoroughly, dried and preserved. The thickness of the Ag,S films was estimated by the weight difference method using a bulk density of 7.31 g cme3. The Ag,S films were characterised in terms of their optical, structural and electrical properties. X-ray diffraction
76
A.B. Kulkarni et al. /Materials
Chemistry and Physics 41 (1995) 75-78
patterns were taken using a Philips PW- 17 10 diffractometer. The optical absorption studies were carried out using a WVis-NIR photospectrometer. The grain size of the particles was determined using a scanning electron microscope. Electrical resistivity and thermoelectric power measurements were also carried out. The films were annealed under vacuum at 200 “C for 2 h in order to study their electrical properties.
3. Results and discussion 3.1. The jlm formation
mechanism
The solution growth processes, such as chemical conversion, electrochemical conversion and electrodeposition are low cost processes for the preparation of chalcogenide films. Chemical conversion is a process that can be defined by following reaction: Sub/M =
Sub/MX
5
Glass/Ag,S
3.2.3. Microstructure Fig. 4(a) is an SEM micrograph of silver film on the glass substrate; it consists of closely packed ground grains. Using the Contrell method, the grain size was estimated to be 0.1 pm [8]. Fig. 4(b) is an SEM micrograph of Ag,S film Table 1 Comparison
of d values (A) of Ag,S films
(1)
In the present investigation, Ag,S films have been prepared from Ag films by chemical conversion. The proposed reaction is as follows: GlasslAg
3.2.2. Optical absorption The optical absorption of converted Ag,S films was studied in the wavelength range 400-2000 nm. The variation of the absorbance (at) versus wavelength (A) shows a sharp decrease in the absorbance with increasing wavelength, as shown in Fig. 2. The band gap energy Es for Ag,S was determined by plotting (ah v) * versus hv and extrapolating to the energy axis, as shown in Fig. 3. The band gap energy for unannealed and annealed Ag,S films was about 0.88 eV. This value is in good agreement with the values reported for chemically deposited Ag,S films [3-S].
Plane
Observed
Calculated
Standard
l(ll0) 2 (023) 3 (ios)
3.5865 2.0699
3.5799 2.076 I .5397
3.571 2.072 1.540
1.5394
(2)
Ag films with thicknesses between 1000 and 3000 A were deposited on the glass and dipped in a 0.1 M Na$ solution at room temperature for different periods of time ranging from 1 to 30 min. The film colour changed from silvery to blackish. It was observed that a period of 10 min is sufficient for total conversion of Ag into Ag,S films of 3000 A thickness. After 10 min the film surface becomes rough and the films are easily detached from the surface. 3.2. Film characterisation 3.2.1. X-ray diffraction In order to confirm the total conversion of Ag into Ag,S, X-ray diffraction studies were carried out. XRD revealed the total conversion of Ag into Ag,S. The Ag films have ( 111)) (200), (220) and (3 11) planes, whereas converted Ag,S films have ( 1 lo), (023) and (iO5) planes along with a strong S( 110) plane. The observed d values of Ag,S are in good agreement with the standard d values taken from the ASTM diffraction data file [7] _ The d values calculated assuming a monoclinic crystal structure of Ag,S match the standard d values, confirming total conversion of Ag into Ag,S. A comparison of the observed, calculated and standard values is shown in Table 1. Fig. 1 (a) shows the XRD pattern for unannealed Ag,S films. The X-ray diffraction patterns of Ag,S films annealed at 200 “C under vacuum for 2 h showed an increase in the intensity of the peaks of Ag,S. This shows some improvement in the crystallinity after heating, as depicted in Fig. 1 (b).
Fig. 1. XRD patterns of Ag,S films: (a) unannealed;
(b) annealed
0.25
040
@05L-Loo5 LOO
2coo
800
Wovdenglh
Fig. 2. Variation of czt, vs. wavelength annealed.
( A 1,
“m
for Ag,S films: (a) unannealed;
(b)
71
A.B. Kukami et al. /Materials Chemistry and Physics 41 (1995) 75-78
0.8 hv,(eV
Fig. 3. Variation annealed.
of (ahv)’
resistivity exhibits a monotonic decrease with increasing thickness. The change in resistance may be attributed to the change in density of free electrons and the change in the mechanism of scattering at the surface [ 93. For unannealed Ag films, p= 1.12X lop5 ohm cm at t= 1246 A and is reduced to p = 0.56 X 10m5 ohm cm at t = 3260 A, whereas for unannealed Ag,S films, p = 1.38 X 10’ ohm cm at t = 1330 A and is reduced to p= 0.53 X lo* ohm cm at t = 3210 A. Fig. 5 (b) shows the variation of the electrical resistivity with the film thickness for annealed Ag,S films. After annealing, the resistivity of Ag,S films was slightly reduced.
1.0 ).
vs. hv for AgzS films: (a) unannealed;
(b)
3.2.5. Thermoelectric power The therm0 electromotive force (e.m.f.) was measured as a function of temperature for both unannealed and annealed films. The polarity of the thermally generated voltage at the hot end was positive, indicating that the films have n-type electrical conductivity. It was observed that the therm0 e.m.f. increases linearly with increasing temperature. Fig. 6(a) shows the variation of the thermoelectric power S with the film thickness r for unannealed Ag,S films. The thermoelectric power increases monotonically with increasing film thickness, which is attributed to the increase in size effects [ lo]. For Ag films, S was of the order of PV ‘C-l, whereas for Ag,S films S was of the order of mV “C -’
(a)
09
15 ThICkn2dS
(::
Fig. 5. Variation of the electrical resistivity Ag,S films: (a) unannealed; (b) annealed.
X 1033%'
p with the film thickness
r of
(b) Fig. 4. SEM micrographs: A& films (magnification
(a) silver film (magnification 3000 x ).
20 000 X ); (b)
formed by dipping Ag film in a Na,S solution. The film surface has changed to a hairlike structure and has become rough. The film adheres well to the substrates. 3.2.4. Electrical resistivity The electrical resistivity of Ag,S films was studied using Van der Pauw’s method. Fig. 5 (a) shows the variation of the electrical resistivity p with the film thickness t for unannealed Ag,S films. The
09
1.5
2.1
Thickness
2.7
( t )x
33
39
l$A'
Fig. 6. Variation of the thermoelectric power S vs. the film thickness Ag2.S films: (a) unannealed; (b) annealed.
t of
78
A.B. Kulkarni et al. /Materials
Chemiswy and Physics 41 (1995)
(S= 1.0 mV ‘C-’ at t= 1330 A, and S=2.6 mV OC-’ at t=3210A). Fig. 6(b) shows that after annealing the thermoelectric power was reduced slightly, which can be attributed to the fact that the relaxation time changes owing to annealing, to approach that of the bulk material [ 111.
75-78
of a Teacher Fellowship, and also to the General Secretary, Mahatma Phule Shikshan Sanstha, Islampur, Dist. Sangli (Maharashtra) .
References [ 11 S.S. Dhumure and C.D. Lokhande,
4. Conclusions Metallic silver films have been converted into semiconducting silver sulphide films by using a simple chemical conversion method. Ag,S films have an optical band gap energy equal to 0.87 eV, and the electrical resistivity was of the order of lo*-lo3 ohm cm. Annealing the Ag,S films under vacuum improved the electrical properties slightly.
Acknowledgements The authors are thankful to Shri N.S. Yesugade and Shri K.M. Gadave for experimental help. One of the authors (ABK) is indebted to the U.G.C., New Delhi, for the award
(1993)
Sol. Energy Mater. Sol. Cells, 29
188.
[2] M.J. Mangalam,
K.N. Rao, N. Rangarajan and C.V. Suryanarayana, 628. [3] S.S. Dhumure and C.D. Lokhande, Mater. Chem. Phys., 27 (1991) Indian .I. Pure Appl. Phys., 7 (1969) 321. [4] S.S. Dhumure
[5] [6] [7] [8] [9]
[lo]
and CD. Lokhande, Mater. Chem. Phys., 28 (1991) 141. S.S. Dhumure and CD. Lokhande, Bull. Electrochem., accepted for publication, 199 1. C.D. Lokhande, V.V. Bhad and S.S. Dhumure, J. Phys. D: Appl. Phys., 25 (1992) 315. ASTM data file 4-0783. A. Contrell,Introduction to Metallurgy, Arnold, London, 1975, p. 173. G. Wedler and W. Wiebauer, Thin Solid Films, 28 ( 1975) 65. R. Suri, A.P. Thakoor and K.L. Chopra, J. Appl. Phys., 46 (1975) 2574.
[ 1 l] H. Sugawara, T. Nagano and A. Kinbara, 33.
Thin Solid Films, 21 (1974)
Materials Chemistry and Physics 41 ( 1995) 75-78
Material Science Communication
Preparation of silver sulphide from chemically deposited silver films A.B. Kulkarni, M.D. Uplane, C.D. Lokhande Received II July 1994; accepted 13 February 1995
Abstract A simple chemical method has been used for the conversion of thin films of Ag into Ag,S films. Thin, uniform and large-area films of Ag were prepared by chemical deposition using the ‘adsorption-reduction’ method on glass substrates and dipped in Na2S solution to be converted into Ag,S films. Their structural, electrical and optical properties have been studied. Keywords:
Sliver
sulphide films; Chemical deposition; Semiconductors
1. Introduction
Metal chalcogenides have been studied intensively over the past 25 years in view of their actual and potential applications. Silver sulphide is a semiconductor ( Eg = 0.7-0.9 eV) which has been used in IR detectors, photoconductors, photovoltaic cells, electrochemical storage cells, solar selective coatings, etc. Recently an electrochemical photovoltaic storage cell with Ag,S electrodes has been reported [ 11. Ag$, with its relatively small band gap, good chemical stability and easy preparation, is an interesting material for application in many devices. Preparation of Ag,S films has been carried out by a number of chemical methods. Mangalam et al. have deposited Ag,S films from an alkaline medium using thiourea as a sulphide source [ 21, while Dhumure and Lokhande have reported on chemical deposition from an acidic bath using sodium thiosulphate and thioacetamide as sulphide ion releasing source [3-51. Lokhande et al. have reported the conversion of tin disulphide into silver sulphide by a simple chemical method of displacement [ 61. In this paper we report on the conversion of Ag films into Ag,S films by a simple chemical method. The silver films were deposited on amorphous glass substrates using the ‘adsorption-reduction’ method and were converted into Ag,S films by being dipped in sodium sulphide solution. The preparation of Ag,S films by this method is more economical than direct chemical deposition, as no bulk precipitate is formed in the solution. Using this method, large-area, good-quality Ag,S films have been formed on the glass substrates ( = 3 X IO cm’). The method is reproducible, and the film 0254-0584/95/$09.50 0 199.5Elsevier Science S.A. All rights reserved SSDI0254-0584(95)01499-K
properties are comparable with those for films prepared by the direct chemical deposition method. The Ag,S films thus prepared on the glass surface were characterised by X-ray diffraction, optical absorption, scanning electron microscopy and electrical resistivity techniques.
2. Experimental In the present investigation, the cleaning of the glass substrates was carried out using the following procedure: ( 1) the glass substrates were boiled in chromic acid for 5 min and were washed with doubly distilled water; (2) the substrates were dipped in ‘labogent’ detergent and again washed with doubly distilled water; (3) finally, the substrates were cleaned with an ultrasonic cleaner for 5 min. To deposit the silver onto the glass substrates, the glass substrates were first dipped in a warm (80 “C), alkaline, 0.1 M silver nitrate solution. Then these glass substrates were dipped in a warm (80 “C) 37.41% formaldehyde solution so as to form silver films on the glass substrates. For the conversion of Ag films into Ag,S films, a 0.1 M Na,S solution was prepared and the Ag films were dipped into it. The film colour changed from silvery to blackish. The converted films were taken out of the solution, washed thoroughly, dried and preserved. The thickness of the Ag,S films was estimated by the weight difference method using a bulk density of 7.31 g cme3. The Ag,S films were characterised in terms of their optical, structural and electrical properties. X-ray diffraction
76
A.B. Kulkarni et al. /Materials
Chemistry and Physics 41 (1995) 75-78
patterns were taken using a Philips PW- 17 10 diffractometer. The optical absorption studies were carried out using a WVis-NIR photospectrometer. The grain size of the particles was determined using a scanning electron microscope. Electrical resistivity and thermoelectric power measurements were also carried out. The films were annealed under vacuum at 200 “C for 2 h in order to study their electrical properties.
3. Results and discussion 3.1. The jlm formation
mechanism
The solution growth processes, such as chemical conversion, electrochemical conversion and electrodeposition are low cost processes for the preparation of chalcogenide films. Chemical conversion is a process that can be defined by following reaction: Sub/M =
Sub/MX
5
Glass/Ag,S
3.2.3. Microstructure Fig. 4(a) is an SEM micrograph of silver film on the glass substrate; it consists of closely packed ground grains. Using the Contrell method, the grain size was estimated to be 0.1 pm [8]. Fig. 4(b) is an SEM micrograph of Ag,S film Table 1 Comparison
of d values (A) of Ag,S films
(1)
In the present investigation, Ag,S films have been prepared from Ag films by chemical conversion. The proposed reaction is as follows: GlasslAg
3.2.2. Optical absorption The optical absorption of converted Ag,S films was studied in the wavelength range 400-2000 nm. The variation of the absorbance (at) versus wavelength (A) shows a sharp decrease in the absorbance with increasing wavelength, as shown in Fig. 2. The band gap energy Es for Ag,S was determined by plotting (ah v) * versus hv and extrapolating to the energy axis, as shown in Fig. 3. The band gap energy for unannealed and annealed Ag,S films was about 0.88 eV. This value is in good agreement with the values reported for chemically deposited Ag,S films [3-S].
Plane
Observed
Calculated
Standard
l(ll0) 2 (023) 3 (ios)
3.5865 2.0699
3.5799 2.076 I .5397
3.571 2.072 1.540
1.5394
(2)
Ag films with thicknesses between 1000 and 3000 A were deposited on the glass and dipped in a 0.1 M Na$ solution at room temperature for different periods of time ranging from 1 to 30 min. The film colour changed from silvery to blackish. It was observed that a period of 10 min is sufficient for total conversion of Ag into Ag,S films of 3000 A thickness. After 10 min the film surface becomes rough and the films are easily detached from the surface. 3.2. Film characterisation 3.2.1. X-ray diffraction In order to confirm the total conversion of Ag into Ag,S, X-ray diffraction studies were carried out. XRD revealed the total conversion of Ag into Ag,S. The Ag films have ( 111)) (200), (220) and (3 11) planes, whereas converted Ag,S films have ( 1 lo), (023) and (iO5) planes along with a strong S( 110) plane. The observed d values of Ag,S are in good agreement with the standard d values taken from the ASTM diffraction data file [7] _ The d values calculated assuming a monoclinic crystal structure of Ag,S match the standard d values, confirming total conversion of Ag into Ag,S. A comparison of the observed, calculated and standard values is shown in Table 1. Fig. 1 (a) shows the XRD pattern for unannealed Ag,S films. The X-ray diffraction patterns of Ag,S films annealed at 200 “C under vacuum for 2 h showed an increase in the intensity of the peaks of Ag,S. This shows some improvement in the crystallinity after heating, as depicted in Fig. 1 (b).
Fig. 1. XRD patterns of Ag,S films: (a) unannealed;
(b) annealed
0.25
040
@05L-Loo5 LOO
2coo
800
Wovdenglh
Fig. 2. Variation of czt, vs. wavelength annealed.
( A 1,
“m
for Ag,S films: (a) unannealed;
(b)
71
A.B. Kukami et al. /Materials Chemistry and Physics 41 (1995) 75-78
0.8 hv,(eV
Fig. 3. Variation annealed.
of (ahv)’
resistivity exhibits a monotonic decrease with increasing thickness. The change in resistance may be attributed to the change in density of free electrons and the change in the mechanism of scattering at the surface [ 93. For unannealed Ag films, p= 1.12X lop5 ohm cm at t= 1246 A and is reduced to p = 0.56 X 10m5 ohm cm at t = 3260 A, whereas for unannealed Ag,S films, p = 1.38 X 10’ ohm cm at t = 1330 A and is reduced to p= 0.53 X lo* ohm cm at t = 3210 A. Fig. 5 (b) shows the variation of the electrical resistivity with the film thickness for annealed Ag,S films. After annealing, the resistivity of Ag,S films was slightly reduced.
1.0 ).
vs. hv for AgzS films: (a) unannealed;
(b)
3.2.5. Thermoelectric power The therm0 electromotive force (e.m.f.) was measured as a function of temperature for both unannealed and annealed films. The polarity of the thermally generated voltage at the hot end was positive, indicating that the films have n-type electrical conductivity. It was observed that the therm0 e.m.f. increases linearly with increasing temperature. Fig. 6(a) shows the variation of the thermoelectric power S with the film thickness r for unannealed Ag,S films. The thermoelectric power increases monotonically with increasing film thickness, which is attributed to the increase in size effects [ lo]. For Ag films, S was of the order of PV ‘C-l, whereas for Ag,S films S was of the order of mV “C -’
(a)
09
15 ThICkn2dS
(::
Fig. 5. Variation of the electrical resistivity Ag,S films: (a) unannealed; (b) annealed.
X 1033%'
p with the film thickness
r of
(b) Fig. 4. SEM micrographs: A& films (magnification
(a) silver film (magnification 3000 x ).
20 000 X ); (b)
formed by dipping Ag film in a Na,S solution. The film surface has changed to a hairlike structure and has become rough. The film adheres well to the substrates. 3.2.4. Electrical resistivity The electrical resistivity of Ag,S films was studied using Van der Pauw’s method. Fig. 5 (a) shows the variation of the electrical resistivity p with the film thickness t for unannealed Ag,S films. The
09
1.5
2.1
Thickness
2.7
( t )x
33
39
l$A'
Fig. 6. Variation of the thermoelectric power S vs. the film thickness Ag2.S films: (a) unannealed; (b) annealed.
t of
78
A.B. Kulkarni et al. /Materials
Chemiswy and Physics 41 (1995)
(S= 1.0 mV ‘C-’ at t= 1330 A, and S=2.6 mV OC-’ at t=3210A). Fig. 6(b) shows that after annealing the thermoelectric power was reduced slightly, which can be attributed to the fact that the relaxation time changes owing to annealing, to approach that of the bulk material [ 111.
75-78
of a Teacher Fellowship, and also to the General Secretary, Mahatma Phule Shikshan Sanstha, Islampur, Dist. Sangli (Maharashtra) .
References [ 11 S.S. Dhumure and C.D. Lokhande,
4. Conclusions Metallic silver films have been converted into semiconducting silver sulphide films by using a simple chemical conversion method. Ag,S films have an optical band gap energy equal to 0.87 eV, and the electrical resistivity was of the order of lo*-lo3 ohm cm. Annealing the Ag,S films under vacuum improved the electrical properties slightly.
Acknowledgements The authors are thankful to Shri N.S. Yesugade and Shri K.M. Gadave for experimental help. One of the authors (ABK) is indebted to the U.G.C., New Delhi, for the award
(1993)
Sol. Energy Mater. Sol. Cells, 29
188.
[2] M.J. Mangalam,
K.N. Rao, N. Rangarajan and C.V. Suryanarayana, 628. [3] S.S. Dhumure and C.D. Lokhande, Mater. Chem. Phys., 27 (1991) Indian .I. Pure Appl. Phys., 7 (1969) 321. [4] S.S. Dhumure
[5] [6] [7] [8] [9]
[lo]
and CD. Lokhande, Mater. Chem. Phys., 28 (1991) 141. S.S. Dhumure and CD. Lokhande, Bull. Electrochem., accepted for publication, 199 1. C.D. Lokhande, V.V. Bhad and S.S. Dhumure, J. Phys. D: Appl. Phys., 25 (1992) 315. ASTM data file 4-0783. A. Contrell,Introduction to Metallurgy, Arnold, London, 1975, p. 173. G. Wedler and W. Wiebauer, Thin Solid Films, 28 ( 1975) 65. R. Suri, A.P. Thakoor and K.L. Chopra, J. Appl. Phys., 46 (1975) 2574.
[ 1 l] H. Sugawara, T. Nagano and A. Kinbara, 33.
Thin Solid Films, 21 (1974)