structure and photoactivity of electrodeposited tin ... · zulkarnain et al.: structure and...

Malaysian Journal of Analytical Sciences, Vol. 7, No. 1 (2001) 197-202

___________________________________________________________________________________________

Structure and Photoactivity of Electrodeposited Tin Selenide Films on Tin Substrate

Zulkarnain Zainal, Ali Jimale Ali, Anuar Kassim and Mohd Zobir Hussein

Chemistry Department, University Putra Malaysia, 43400 UPM Serdang, Selangor D. E.

(Received 6 September 2000)

Abstract. Tin selenide semiconductor films have been electrodeposited on tin substrate from aqueous solution containing SnCl2 and Na2SeO3. Deposition at various potentials and different electrolyte concentrations was attempted in order to investigate the effect of these parameters on the film properties. The structure, morphology and photoactivity of the films were studied by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and linear sweep photovoltammetry (LSPV) technique respectively. XRD showed the formation of polycrystalline SnSe with three strongest faces of (111), (400) and (311) at potentials negative than -0.550 V vs Ag/AgCl reference electrode. Films prepared by using higher amount of Na2SeO3 gave better photoactivity. SEM showed the relationship between the crystalline size and photosensitivity of the deposits. Abstrak. Filem nipis semikonduktor timah selenida telah dielektroenapkan di atas substrat timah daripada larutan akueus yang mengandungi SnCl2 dan Na2SeO3. Pengenapan pada pelbagai keupayaan dan kepekatan elektrolit telah lakukan bagi mengkaji kesan parameter-parameter ini ke atas ciri-ciri filem nipis. Struktur, morfologi dan fotoaktiviti filem nipis dikaji menggunakan teknik belauan sinar-X (XRD), mikroskopi imbasan elektron (SEM), dan voltammetri pengimbasan linear (LSPV). XRD menunjukkan pembentukan polihablur SnSe dengan tiga fasa terkuat pada (111), (400) dan (311) pada keupayaan negatif melebihi -0.550 V melawan Ag/AgCl elektrod rujukan. Penyediaan filem nipis menggunakan jumlah Na2SeO3 yang banyak memberikan fotoaktiviti yang lebih baik. SEM menunjukan perkaitan diantara saiz hablur dan fotosensitiviti yang dienapkan. ___________________________________________________________________________________________

Introduction For the past few decades, polycrystalline semiconductors were getting steady consideration over their monocrystalline semiconductor counterparts in the photovoltaic application. As they do not require sophisticated and costly processing steps, they can replace the extra-pure monocrystalline materials in the solar energy conversion due to the competitiveness of the production cost [1]. Investigations to find such polycrystalline materials for photoelectrochemical cells (PECs) are thus being seriously done throughout the world. Among the recently identified ones are the chalcogenides of group II elements, specially CdSe and CdS [1-3]. Other promising compounds, which has been less extensively studied, are tin chalcogenides such as SnS and SnSe [4-6]. There are many ways to prepare these materials, but the quality of these materials are sensitive to the method of preparation. Recently, the electrochemical synthesis has been indicated as a promising method for the preparation of the semiconductor films [3-8]. In this paper, we report the electrochemical deposition of SnSe film semiconductor on tin substrate from aqueous solution. The structure, morphology and semiconducting property of the deposit will be discussed.

Experimental Electrochemical Deposition The electrodeposition was carried out potentiostatically in a standard three electrode cell for one hour. Ag/AgCl, KCl (saturated) was used as the reference electrode to which all potentials were quoted. The working (substrate) and counter electrode were made from tin and platinum foils, respectively. The working electrode was polished and all its surface not to be contacted by the electrolyte were sealed using PTFE electrode holder before the insertion into the cell. The electrolyte bath comprised of acidified SnCl2 with EDTA and Na2SeO3 solutions [9]. Deposition at various potentials, -0.55, -0.65, -0.75, -0.85 and -0.95 V versus Ag/AgCl, was attempted in order to determine the optimum deposition potential. The concentrations of the electrolytes were varied, in a way to get a SnCl2:Na2SO3 ratio of 2:1, 1:1, 2:3 and 1:2, to investigate the effect of the electrolytes composition on the deposit quality. This was achieved by keeping the SnCl2 concentration constant at 0.010 M and by varying the concentration of Na2SO3 at 0.005, 0.010, 0.015 and 0.020 M. After the deposition, the films were washed, dried and kept for analysis.

ZULKARNAIN et al.: STRUCTURE AND PHOTOACTIVITY OF ELECTRODEPOSITED TIN SELENIDE

0

200

400

600

2 20 40 60

♦

♦

♦

♦

-0.550 VSn

Sn

Sn Sn

♦♦

♦♦

♦♦

♦♦

♦ SnSe

-0.950 V

-0.850 V

-0.750 V

-0.650 V

2θ/degrees

Inte

nsity

(Arb

itrar

y un

its)

Figure 1 : X-ray diffraction spectra of the samples electrodeposited

in different potentials: -0.55, -0.65, -0.75, -0.85 and -0.95 V

0

200

400

600

2 20 40 60

♦

♦

♦

♦♦♦ Sn

(0.005 M)

(0.01 M)

(0.015 M)

(0.02 M)

2θ/degrees

Sn

♦

♦

♦

♦ SnSe

Inte

nsity

(arb

itrar

y un

its)

Figure 2 : XRD patterns of the samples prepared at different Na2Se03

concentrations: 0.005, 0.010, 0.015 and 0.020 M and constant SnCl2 concentration, 0.010 M

198


Analysis X-ray diffraction and SEM analysis were performed by using Philips PM1730 diffractometer (for 2θ range from 2° to 60° with Cu K∝ radiation) and SEM JSM.6400 JEOL Scanning Microscope, respectively. The photo-activity test was carried out by running linear sweep photovoltammetry (LSPV) technique on the deposit films, deployed as cathode in contact with sodium thiosulfate solution of 0.01 M, and illuminated with chopped white light from tungsten-halogen lamp. An EG&G Princeton Applied Research (PAR) VersaStat, driven by model 270 Electrochemical Analysis System software, was employed to run the LSPV and to monitor the photocurrent.

Results and Discussion After electrodeposition at various potentials and electrolyte concentrations, thin film deposit, grey in colour probably SnSe, was evident on the tin substrate. Previous works attributed this colour to the stoichiometric SnSe [10]. The results of the XRD, LSPV and SEM analysis of the samples are elaborated in the following section. X-ray diffraction Figure 1 shows the XRD patterns of the films deposited in various potentials. There are three strong peaks at 2θ=30.4°, 31.1° and 38.1° for the samples deposited in the potentials more negative than -0.55 V. The corresponding interplanar distances are well in agreement with the JCPDS values of 2.92 Å, 2.85 Å and 2.38 Å for SnSe [4,11,12]. This clearly supports that SnSe thin film can be electrodeposited on tin at these potentials. The peaks at 30.4, 31.1 and 38.1° are indicative that an orthorhombic SnSe structure with (111), (400) and (311) planes have been deposited [4,12].

Figure 2 shows the XRD patterns of the films prepared at different Na2SeO3 concentrations and constant SnCl2 concentration, at 0.85 V vs Ag/AgCl. SnSe characteristic peaks were detected in all samples except for the sample prepared at Na2SeO3 concentration of 0.005 M. This suggest that a bath with equal ratio of electrolytes or higher Na2SeO3 concentration can produce SnSe film. Photoactivity This is an important test as the films are expected to be a semiconductors, thus, should be sensitive to the light, with energy higher than its Eg, by showing a photocurrent in the region corresponding to their minority carriers current flow. The film quality was mainly charged with the photoactivity results. Figure.

3 shows the results of the photoactivity test and compares the white light photoresponse between the samples prepared at various potentials and 0.015 M of Na2SeO3. The photocurrent output in mA magnitude (Figure 3) can be regarded as high values and confirms that the films are good semiconductors. The upper value of the current correspond to the photocurrent when the sample, employed as cathode, was illuminated while the lower value correspond to the darkcurrent when the illumination was interrupted by chopping. The fact that the photocurrent occur on the negative potentials region indicates that electrons are minority carriers of the film and their concentration was then enhanced by the illumination. Thus, the films prepared are p-type semiconductor and can be deployed as photocathode in the PECs application to facilitate a reduction of the electroactive species in the solution. From the graph, it is also evident that the samples prepared at -0.85 and -0.95 V have better photosensitivity. Thus, these are preferable potentials in the synthesis of SnSe semiconductor on tin substrate.

Figure. 4 compares the photoactivity of the samples prepared in various Na2SeO3 concentrations and constant 0.010 M of SnCl2. Samples prepared at different concentrations show different photosensitivity. The sample obtained from the bath of SnCl2:Na2SeO3 of 2:3 has the best photoactivity. The photosensitivity results are in line with the XRD results, but in addition it indicates that a small amount of tin selenide, below the detectability of XRD, was also obtained when the Na2SeO3 concentrations is 0.005 M, Figure. 4(a). Scanning electron microscopy Figure 5 shows the SEM micrograph of the films prepared at different potentials. It is evident that the deposits are crystalline and their shape and grain size varies with the variation of the deposition potential. The samples deposited at more negative potentials -0.85 (c) and -0.95 V (d) that exhibited better photoactivity, have smaller grain size. Thus the photoactivity depends on the grain size as well as the deposition potential. This correlation between the grain size and photoactivity can be explained that when the grain size is smaller we do have more grains in every unit area thus more electron hole pairs (EHP) can be generated in every unit area after the illumination and more interaction between the semiconductor and electrolyte, within the contact, can take place.

The photoactivity of film obtained at -0.95 V is comparable to one prepared at -0.85 V, but as shown Figure. 5 (d) it contain crack, due to hydrogen evolution process which is more likely to occur at the more negative potentials.

199


a) b)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

c) d)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

Figure 3 : Comparison of the photosensitivity of the SnSe films deposited at different potentials (a) -0.65, (b) -0.75, (c) -0.85 and (d) -0.95 V

a) b)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

c) d)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

0

0.2

0.4

0.6

-1.0 -0.8 -0.6 -0.4 -0.2

E (V)

I (m

A)

Figure 4 : Comparison of the photosensitivity of the different SnSe thin film samples prepared through different Na2SeO3 concentrations, (a) 0.005, (b) 0.010, (c) 0.015 and (d) 0.020 M. The concentration

of SnCl2 is kept constant at 0.010 M.

200


(a)

(b)

(c)

(d)

Figure. 5: SEM micrography of SnSe films deposited at different

potentials: (a) -0.65, (b) -0.75, (c) -0.85 and (d) -0.95 V

(a)

(b)

(c)

(d)

Figure 6 : SEM micrographs of SnSe films deposited in various Na2SeO3 concentrations: (a) 0.005, (b) 0.010, (c) 0.015 and (d) 0.020 M. The concentration of SnCl2 is constant at 0.010 M.

201


Figure 6 shows the SEM micrograph of the films prepared in various Na2SeO3 concentrations. It is clear that the films obtained are crystalline in nature, which support the X-ray diffraction results. It is also clear that the crystal size varies with the variation of Na2SeO3 concentration. The higher is Na2SeO3 concentration the smaller the crystal size. The samples with smaller crystal size showed better photosensitivity. This supports the findings for samples obtained at different deposition potentials.

Thus, we can safely assume that better

photosensitivity lies with the smaller crystal size, may be due to the improved crystal density of the deposit. Films with smaller crystal size and better photoactivity have been obtained from the bath with slightly lower SnCl2 concentration than Na2SeO3 (i.e.) at the ratio of 2:3.

Conclusion

SnSe film semiconductor have been successfully electrodeposited on tin substrate from aqueous solution containing SnCl2 and Na2SeO3 in the presence of EDTA. The as-deposited film at various potentials and different electrolytes concentration ratios showed good photoactivity to the white light and exhibited p-type semiconductor behaviour. X-ray diffraction pattern confirmed that a SnSe polycrystalline with major peaks at 2θ=30.4º, 31.1° and 38.1° was obtained. The corresponding d-spacing of 2.92, 2.85 and 2.38 Å are indicative that an orthorhombic SnSe structure with (111), (400) and (311) planes was deposited. SEM images reaffirmed the crystallinity of the deposits whose grain size are highly effected by the deposition potential and electrolytes concentrations.

Films with smaller grain size proved to have better photosensitivity, may be due to the improved crystal density of the deposit. Deposition potential of -0.85 V versus Ag/AgCl and concentration ratio of SnCl2:Na2SeO3 of 2:3 proved to offer a reasonably good SnSe film semiconductor.

Acknowledgement We would like to thank the Government of Malaysia for funding this project under IPRA (Project Number 09-02-04-026) Programme.

References 1. G. C. Morris and Venderveen ,1992. CdS Films

Prepared by Pulsed Electrodeposition, Sol. Energy Mater. Sol. Cells, 27: 305-319

2. A. B. Ellis., W.K. Steven, S. W. Mark,1976. Visible light to electrical energy conversion: Stable CdS and CdSe photoelectrodes. Journal of American Chemical Society. 98(6): 1635-1637

3. K. Mishra, K. Rajeshwar, W. Alex, R.D. Engelken and M. Staylon, 1989. Electrodeposition and characterisation of SnS thin films. J. Electrochem. Soc. 136(7): 1915-1923.

4. R.D. Engelken, A.K. Berry, T.P. Doren, J.L. Boone and A. Shahnazary, 1986. Electrodeposition and analysis of tin selenide films. J. Electrochem. Soc. 133(3): 581-585.

5. Zainal, Z., Hussein, M. Z., and Ghazalli, A., 1996. Cathodic electrodeposition of SnS thin film from aqueous solution. Solar Energy Materials and Solar Cells, 40: 347-357.

6. A. C. Rastogi, K.S. Balakirishnan, A. Garg, 1993. New Electrochemical Selenisation Technique for

Preparation of Metal Selenide Semiconductor Thin Film, J. Electrochem. Soc., 140(8): 2373-2375.

7. K.S. Balakrishnan, 1993. Properties of Cu and In Selenide Semiconductor Thin Films for Solar

Application, Physics of Semiconductor Devices, Lal Ed. New Delhi: 554-556.

8. A. S. Baranski and W.R Fawcett, 1980. The electrodeposition of metal chalcogenides. J.

Electrochem. Soc. 127(3): 766-767. 9. Zainal, Z., Hussein, M. Z., Anuar Kassim and Ghazali,

A.,1997. Cathodic electrodeposition of SnS in the presence of EDTA in aqueous media. Solar Energy Materials and Solar Cells, 55: 237-249.

10. D.R. Lide and H.P. Frederikse, 1994. Handbook of chemistry and physics. (75th ed.). CRC Press:

Boca Rotan 11. M. Sharon, K. Basavasawaran, 1987.

Photoelectrochemical Studies of p-type SnSe Electrodes. Solar Cells, 20: 323-332.

12. JCSP, Powder Diffraction File, 1990. P670.

202

structure and photoactivity of electrodeposited tin ... · zulkarnain et al.: structure and...

Documents