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γ-Irradiation preparation and characterization of nanocrystalline ZnS
Materials Chemistry and Physics 62 (2000) 88±90
Materials Science Communication
g-Irradiation preparation and characterization of nanocrystalline ZnS Zhengping Qiao, Yi Xie*, Yitai Qian, Yingjie Zhu Structure Research Lab and Department of Chemistry, University of Science & Technology of China, Hefei, Anhui 230026, PR China Received 28 April 1999; received in revised form 1 June 1999; accepted 17 June 1999
Abstract The g-irradiation method was used successfully to prepare nanocrystalline ZnS in sphalerite structure at room temperature. Sodium thiosulfate, thiourea and mercaptoethanol were used as sulfur sources. The products obtained from irradiated solutions containing zinc ions and different sulfur sources are all sphalerite phase. The TEM images indicate that the product from mercaptoethanol is in best dispersion, and that from sodium thiosulfate aggregates most heavily. The possible mechanisms of the formation of zinc sul®de in different systems are proposed. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanocrystalline; Semiconductor; Zinc sul®de; g-Irradiation
1. Introduction In recent years, the synthesis and characterization of chalcogenides of different groups have attracted considerable attention due to their important nonlinear properties [1], luminescent properties [2], quantum size effects [3] and other important physical and chemical properties [4]. Among these materials, zinc sul®de is one of the most interesting materials due to its attractive application in infrared window materials and phosphors for cathode ray tubes [5]. For application, it would be desirable to use particles with spherical shape and uniform size. Considerable progress has been made in the synthesis of zinc sul®de powders. Conventionally, zinc sul®de could be prepared by a variety of methods, including gas-phase, solid/ vapor, and aqueous solution reactions [6,7]. Usually, ZnS is prepared by passing H2S through aqueous solution containing Zn2 ions. However, as-prepared ZnS has irregular crystallites with a wide size distribution ranging from 25 to 200 nm. Using organometallic precursors was another route for synthesising ZnS, but most of the organometallic precursors were toxic, easy to hydrolyze, oxidize and hard to deal with. g-Irradiation has been developed to be a new method to prepare nanocrystalline metals, alloys, metal oxide and *
Corresponding author. Tel.: 86-551-360-3987; fax: 86-551-3631760 E-mail address: [email protected] (Y. Xie)
glass-metal, and silver/polyacrylamide nanocomposites [8± 11]. Yin et al. [12] successfully obtained CdS from solutions containing cadmium ions and sodium thiosulfate by g-irradiation. In this paper, we report the successful extension of the method to the preparation of nanocrystalline zinc sul®de. Different sulfur sources such as sodium thiosulfate, thiourea and mercaptoethanol were used successfully to obtain nanocrystalline ZnS. Every process is carried out in aqueous solution at room temperature and ambient pressure. 2. Experimental The solutions were prepared by dissolving an appropriate amount of analytically pure zinc sulfate and various sulfur sources, i.e. sodium thiosulfate, thiourea and mercaptoethanol, in distilled water. Isopropanol was added as a scavenger for hydroxyl radicals. The solutions were irradiated in the ®eld of a 2.59 l015 Bq 60 Co g-ray source with an absorbed dose of 3.0 104 Gy. The irradiated solutions were kept for several hours to precipitate powders. White precipitates were collected, and washed with absolute ethanol and distilled water in sequence to remove by-products. The ®nal products were dried in vacuum for 2 h. The samples were characterized by X-ray powder diffraction (XRD) patterns employing a scanning rate of 0.058 sÿ1 in a 2 range from 20 to 608, using a Japan Rigaku Dmax gA X-ray diffractometer equipped with a graphite monochro-
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 1 - 0
Z. Qiao et al. / Materials Chemistry and Physics 62 (2000) 88±90
89
Ê ). The morphology matized Cu Ka radiation ( 1.54178 A and particle sizes were determined by transmission electron microscopy (TEM). The TEM images were taken with a Hitachi Model H-800 Transmission Electron Microscope with an accelerating voltage of 200 kV. 3. Results and discussion Some compositions of the solutions are shown in Table 1 and the corresponding XRD patterns of the samples are shown in Fig. 1. All the peaks of every pattern could be indexed as the cubic b-ZnS phase with a cell constant Ê . The broadening of the peaks indicates around 5.406 A the nanocrystalline nature of the samples. The crystalline sizes of the samples, which are calculated from the halfwidth of diffraction peaks using the Scherrer formula [13] is ca. 5, 4 and 4 nm for the samples from sodium thiosulfate, thiourea and mercaptoethanol, respectively. No obvious difference could be gauged from the XRD patterns. The particle sizes and shapes were investigated by TEM. The result is given in Fig. 2 from which one can see that the product from mercaptoethanol consists of uniform spherical crystallites in best dispersion and narrowest size distribution. The average size is about 5±6 nm, which is consistent with the XRD results. The aggregation of the products from sodium thiosulfate is obviously heavier than that from mercaptoethanol. And the product from thiourea is somewhere between them. Aggregation is a common phenomenon of nanoparticles due to their extremely small dimensions and high surface energy. However, from Fig. 2(3), one can see that the product from mercaptoethanol is in best dispersion. Based on this TEM image, a histogram
Fig. 1. XRD patterns of nanocrystalline ZnS from different sulfur sources: (1) sodium thiosulfate, (2) thiourea and (3) mercaptoethanol.
(Fig. 3) of the product shows a distribution of particle sizes ranging from 2 to 6 nm. The average particle size calculated was 3.7 nm with a standard deviation of 0.7. In these g-irradiation processes, the mechanisms of the formation of nanocrystalline sul®de are not very clear up to now. Few reports have been given on the transformation of the above sulfur sources under g-irradiation. However, some probable reactions may be going on under g-irradiation. It is clear that, as a result of reduction of zinc ions with hydrated electrons produced in solution by g-irradiation, zinc ions in lower valence are formed [14]: H2 O
irradiation ÿ ! eaq ;
H3 O ; H ; H2 ; OH ; H2 O2
2 eÿ aq Zn !Zn
(1) (2)
Research of g-irradiation on thiourea shows that free sulfur may be liberated in the process [15]. However, we did not
Table 1 Composition of solutions Samples
Metal ions source
Sulfur ions source
Scavenger for hydroxyl radicals
Solvent
1 2 3
zinc sulfate (0.05 M) zinc sulfate (0.05 M) zinc sulfate (0.05 M)
sodium thiosulfate (0.l M) thiourea (0.5 M) mercaptoethanol (0.5 M)
isopropenol (3 M) isopropenol (3 M) isopropenol (3 M)
water water water
Fig. 2. TEM images of nanocrystalline ZnS from difference sources: (1) sodium thiosulfate, (2) thiourea and (3) mercaptoethnol.
90
Z. Qiao et al. / Materials Chemistry and Physics 62 (2000) 88±90
ture. Sodium thiosulfate, thiourea and mercaptoethanol were used as sulfur sources. The TEM images show that the product from mercaptoethanol is in best dispersion, and that from sodium thiosulfate aggregates most heavily. The possible mechanisms of the formation of zinc sul®de in different systems are proposed. This method is expected to prepare other nanocrystalline sul®des at room temperature and ambient pressure.
Acknowledgements Fig. 3. Histogram of particle size in the sample of Fig. 1(3).
observe any sulfur in the products. The precipitate of ZnS probably prevents the formation of sulfur from S radical in the irradiation. In the case of sodium thiosulfate as the sulfur source, research on g-irradiation of sodium thiosulfate shows the following radiolytic reaction [16]:
References
radiation
ÿ ! HSÿ 3SO3 2ÿ 2S2 O2ÿ 3 4OH 2H
3H H2 O
(3)
D. Hayes et al. [17] report that aqueous thiol irradiated by gray releases sulfide ions into the solution. Another sulfur source, mercaptoethanol, which also has an ±SH group, may also release HSÿ ions under g-irradiation. In both cases, HSÿ can be formed in the g-irradiation of the solution. ÿ RSH eÿ aq !R HS
(4)
HSÿ !S2ÿ H
(5)
Thus, ZnS can be formed in the following way: Zn2 S2ÿ !ZnS
Financial support from the Chinese National Foundation of Natural Science Research through No. 29771027 and the National Outstanding Youth Fund is gratefully acknowledged. This work is also supported by the Climbing Plan from the State Science and Technology Commission of China.
(6)
2
The Zn transfer to more stable precipitate ZnS and new equilibrium will probably be established between reactions (5) and (6). 4. Conclusions Nanocrystalline ZnS in sphalerite structure was prepared successfully using g-irradiation method at room tempera-
[1] L.E. Brus, Appl. Phys. A 53 (1991) 465. [2] W. Hoheisel, V.L. Colvin, C.S. Johnson, A.P. Alivisatos, J. Chem. Phys. 101 (1994) 8555. [3] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [4] H. Weller, Angrew. Chem. Chem. Int. Ed. Engl. 32 (1993) 41. [5] A. Celikkaya, M. Akinc, J. Am. Ceram. Soc. 73 (1990) 2360. [6] D.M. Wilhelmy, E. Matijevic, J. Chem. Soc., Faraday Trans. 80 (1984) 563. [7] R. Williams, P.N. Yocom, F.S. Sotofko, J. Colloid Interface Sci. 106 (1985) 388. [8] Y.J. Zhu, Y.T. Qian, M.W. Zhang, J. Mater. Sci. Lett. 13 (1994) 1243. [9] Y.J. Zhu, Y.T. Qian, M.W. Zhang, Mater. Lett. 17 (1993) 314. [10] Y.J. Zhu, Y.T. Qian, M.W. Zhang, Mater. Sci. Eng. B 23 (1994) 116. [11] Y. Zhu, Y. Qian, X. Li, M. Zhang, Chem. Commun. (1997) 1081. [12] Y. Yin, X. Xu, C. Xia, X. Ge, Z. Zhang, Chem. Commun. (1998) 1641. [13] C.N.J. Wagner, E.N. Aqua, Adv. X-Ray Anal. 4 (1964) 46. [14] P.N. Moorthy, J.J. Weiss, Nature 201 (1964) 1317. [15] W.M. Dale, J.V. Davies, Radiation Res. 7 (1957) 35. [16] R.L. Eager, D.S. Mahadevappa, Can. J. Chem. 43 (1965) 614. [17] D. Hayes, O.I. Micic, M.T. Nenadovie, J. Phys. Chem. 93 (1989) 4603.
Materials Science Communication
g-Irradiation preparation and characterization of nanocrystalline ZnS Zhengping Qiao, Yi Xie*, Yitai Qian, Yingjie Zhu Structure Research Lab and Department of Chemistry, University of Science & Technology of China, Hefei, Anhui 230026, PR China Received 28 April 1999; received in revised form 1 June 1999; accepted 17 June 1999
Abstract The g-irradiation method was used successfully to prepare nanocrystalline ZnS in sphalerite structure at room temperature. Sodium thiosulfate, thiourea and mercaptoethanol were used as sulfur sources. The products obtained from irradiated solutions containing zinc ions and different sulfur sources are all sphalerite phase. The TEM images indicate that the product from mercaptoethanol is in best dispersion, and that from sodium thiosulfate aggregates most heavily. The possible mechanisms of the formation of zinc sul®de in different systems are proposed. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Nanocrystalline; Semiconductor; Zinc sul®de; g-Irradiation
1. Introduction In recent years, the synthesis and characterization of chalcogenides of different groups have attracted considerable attention due to their important nonlinear properties [1], luminescent properties [2], quantum size effects [3] and other important physical and chemical properties [4]. Among these materials, zinc sul®de is one of the most interesting materials due to its attractive application in infrared window materials and phosphors for cathode ray tubes [5]. For application, it would be desirable to use particles with spherical shape and uniform size. Considerable progress has been made in the synthesis of zinc sul®de powders. Conventionally, zinc sul®de could be prepared by a variety of methods, including gas-phase, solid/ vapor, and aqueous solution reactions [6,7]. Usually, ZnS is prepared by passing H2S through aqueous solution containing Zn2 ions. However, as-prepared ZnS has irregular crystallites with a wide size distribution ranging from 25 to 200 nm. Using organometallic precursors was another route for synthesising ZnS, but most of the organometallic precursors were toxic, easy to hydrolyze, oxidize and hard to deal with. g-Irradiation has been developed to be a new method to prepare nanocrystalline metals, alloys, metal oxide and *
Corresponding author. Tel.: 86-551-360-3987; fax: 86-551-3631760 E-mail address: [email protected] (Y. Xie)
glass-metal, and silver/polyacrylamide nanocomposites [8± 11]. Yin et al. [12] successfully obtained CdS from solutions containing cadmium ions and sodium thiosulfate by g-irradiation. In this paper, we report the successful extension of the method to the preparation of nanocrystalline zinc sul®de. Different sulfur sources such as sodium thiosulfate, thiourea and mercaptoethanol were used successfully to obtain nanocrystalline ZnS. Every process is carried out in aqueous solution at room temperature and ambient pressure. 2. Experimental The solutions were prepared by dissolving an appropriate amount of analytically pure zinc sulfate and various sulfur sources, i.e. sodium thiosulfate, thiourea and mercaptoethanol, in distilled water. Isopropanol was added as a scavenger for hydroxyl radicals. The solutions were irradiated in the ®eld of a 2.59 l015 Bq 60 Co g-ray source with an absorbed dose of 3.0 104 Gy. The irradiated solutions were kept for several hours to precipitate powders. White precipitates were collected, and washed with absolute ethanol and distilled water in sequence to remove by-products. The ®nal products were dried in vacuum for 2 h. The samples were characterized by X-ray powder diffraction (XRD) patterns employing a scanning rate of 0.058 sÿ1 in a 2 range from 20 to 608, using a Japan Rigaku Dmax gA X-ray diffractometer equipped with a graphite monochro-
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 5 1 - 0
Z. Qiao et al. / Materials Chemistry and Physics 62 (2000) 88±90
89
Ê ). The morphology matized Cu Ka radiation ( 1.54178 A and particle sizes were determined by transmission electron microscopy (TEM). The TEM images were taken with a Hitachi Model H-800 Transmission Electron Microscope with an accelerating voltage of 200 kV. 3. Results and discussion Some compositions of the solutions are shown in Table 1 and the corresponding XRD patterns of the samples are shown in Fig. 1. All the peaks of every pattern could be indexed as the cubic b-ZnS phase with a cell constant Ê . The broadening of the peaks indicates around 5.406 A the nanocrystalline nature of the samples. The crystalline sizes of the samples, which are calculated from the halfwidth of diffraction peaks using the Scherrer formula [13] is ca. 5, 4 and 4 nm for the samples from sodium thiosulfate, thiourea and mercaptoethanol, respectively. No obvious difference could be gauged from the XRD patterns. The particle sizes and shapes were investigated by TEM. The result is given in Fig. 2 from which one can see that the product from mercaptoethanol consists of uniform spherical crystallites in best dispersion and narrowest size distribution. The average size is about 5±6 nm, which is consistent with the XRD results. The aggregation of the products from sodium thiosulfate is obviously heavier than that from mercaptoethanol. And the product from thiourea is somewhere between them. Aggregation is a common phenomenon of nanoparticles due to their extremely small dimensions and high surface energy. However, from Fig. 2(3), one can see that the product from mercaptoethanol is in best dispersion. Based on this TEM image, a histogram
Fig. 1. XRD patterns of nanocrystalline ZnS from different sulfur sources: (1) sodium thiosulfate, (2) thiourea and (3) mercaptoethanol.
(Fig. 3) of the product shows a distribution of particle sizes ranging from 2 to 6 nm. The average particle size calculated was 3.7 nm with a standard deviation of 0.7. In these g-irradiation processes, the mechanisms of the formation of nanocrystalline sul®de are not very clear up to now. Few reports have been given on the transformation of the above sulfur sources under g-irradiation. However, some probable reactions may be going on under g-irradiation. It is clear that, as a result of reduction of zinc ions with hydrated electrons produced in solution by g-irradiation, zinc ions in lower valence are formed [14]: H2 O
irradiation ÿ ! eaq ;
H3 O ; H ; H2 ; OH ; H2 O2
2 eÿ aq Zn !Zn
(1) (2)
Research of g-irradiation on thiourea shows that free sulfur may be liberated in the process [15]. However, we did not
Table 1 Composition of solutions Samples
Metal ions source
Sulfur ions source
Scavenger for hydroxyl radicals
Solvent
1 2 3
zinc sulfate (0.05 M) zinc sulfate (0.05 M) zinc sulfate (0.05 M)
sodium thiosulfate (0.l M) thiourea (0.5 M) mercaptoethanol (0.5 M)
isopropenol (3 M) isopropenol (3 M) isopropenol (3 M)
water water water
Fig. 2. TEM images of nanocrystalline ZnS from difference sources: (1) sodium thiosulfate, (2) thiourea and (3) mercaptoethnol.
90
Z. Qiao et al. / Materials Chemistry and Physics 62 (2000) 88±90
ture. Sodium thiosulfate, thiourea and mercaptoethanol were used as sulfur sources. The TEM images show that the product from mercaptoethanol is in best dispersion, and that from sodium thiosulfate aggregates most heavily. The possible mechanisms of the formation of zinc sul®de in different systems are proposed. This method is expected to prepare other nanocrystalline sul®des at room temperature and ambient pressure.
Acknowledgements Fig. 3. Histogram of particle size in the sample of Fig. 1(3).
observe any sulfur in the products. The precipitate of ZnS probably prevents the formation of sulfur from S radical in the irradiation. In the case of sodium thiosulfate as the sulfur source, research on g-irradiation of sodium thiosulfate shows the following radiolytic reaction [16]:
References
radiation
ÿ ! HSÿ 3SO3 2ÿ 2S2 O2ÿ 3 4OH 2H
3H H2 O
(3)
D. Hayes et al. [17] report that aqueous thiol irradiated by gray releases sulfide ions into the solution. Another sulfur source, mercaptoethanol, which also has an ±SH group, may also release HSÿ ions under g-irradiation. In both cases, HSÿ can be formed in the g-irradiation of the solution. ÿ RSH eÿ aq !R HS
(4)
HSÿ !S2ÿ H
(5)
Thus, ZnS can be formed in the following way: Zn2 S2ÿ !ZnS
Financial support from the Chinese National Foundation of Natural Science Research through No. 29771027 and the National Outstanding Youth Fund is gratefully acknowledged. This work is also supported by the Climbing Plan from the State Science and Technology Commission of China.
(6)
2
The Zn transfer to more stable precipitate ZnS and new equilibrium will probably be established between reactions (5) and (6). 4. Conclusions Nanocrystalline ZnS in sphalerite structure was prepared successfully using g-irradiation method at room tempera-
[1] L.E. Brus, Appl. Phys. A 53 (1991) 465. [2] W. Hoheisel, V.L. Colvin, C.S. Johnson, A.P. Alivisatos, J. Chem. Phys. 101 (1994) 8555. [3] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [4] H. Weller, Angrew. Chem. Chem. Int. Ed. Engl. 32 (1993) 41. [5] A. Celikkaya, M. Akinc, J. Am. Ceram. Soc. 73 (1990) 2360. [6] D.M. Wilhelmy, E. Matijevic, J. Chem. Soc., Faraday Trans. 80 (1984) 563. [7] R. Williams, P.N. Yocom, F.S. Sotofko, J. Colloid Interface Sci. 106 (1985) 388. [8] Y.J. Zhu, Y.T. Qian, M.W. Zhang, J. Mater. Sci. Lett. 13 (1994) 1243. [9] Y.J. Zhu, Y.T. Qian, M.W. Zhang, Mater. Lett. 17 (1993) 314. [10] Y.J. Zhu, Y.T. Qian, M.W. Zhang, Mater. Sci. Eng. B 23 (1994) 116. [11] Y. Zhu, Y. Qian, X. Li, M. Zhang, Chem. Commun. (1997) 1081. [12] Y. Yin, X. Xu, C. Xia, X. Ge, Z. Zhang, Chem. Commun. (1998) 1641. [13] C.N.J. Wagner, E.N. Aqua, Adv. X-Ray Anal. 4 (1964) 46. [14] P.N. Moorthy, J.J. Weiss, Nature 201 (1964) 1317. [15] W.M. Dale, J.V. Davies, Radiation Res. 7 (1957) 35. [16] R.L. Eager, D.S. Mahadevappa, Can. J. Chem. 43 (1965) 614. [17] D. Hayes, O.I. Micic, M.T. Nenadovie, J. Phys. Chem. 93 (1989) 4603.