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A new preparation of zinc sulfide nanoparticles by solid-state method at low temperature
Pergamon
Materials Research Bulletin 35 (2000) 695–701
A new preparation of zinc sulfide nanoparticles by solidstate method at low temperature L.P. Wanga, G.Y. Honga,b,* a
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, People’s Republic of China b Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences, Changchun, Jilin, 130022, People’s Republic of China (Refereed) Received 21 June 1999; accepted 6 July 1999
Abstract A novel solid-state method of the preparation of zinc sulfide nanoparticles is reported. By solid-state reaction of zinc acetate and thioacetamide at low temperature, zinc sulfide nanoparticles of different sizes were prepared. The temperature of preparation varied from room temperature to 300°C. The particles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), and photoluminescence spectrum. X-ray diffraction patterns revealed that the particles exhibited pure zinc-blende crystal structure and that particle size increased with increasing temperature. The TEM micrograph showed that the mean particle size was about 40 nm for the sample heated at 100°C. A blue shift was observed in the photoluminescence emission spectrum. A possible mechanism of the reaction corresponding to our observation is proposed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Semiconductor nanoparticles, which have changed properties resulting from quantum confinement, have drawn considerable interest and are currently being investigated [1,2].
* Corresponding author. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 6 1 - 0
696
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
These nanoparticles may find applications in nonlinear optical devices, photocatalysis, etc. [3–5]. Being a compound semiconductor, zinc sulfide has found many applications in various fields, such as phosphors, solar cells, and IR window [6 – 8]. Although in some cases it is used as thin films, zinc sulfide is often utilized in the form of particles. It has been shown that the particle size and its distribution play an important role in applications. Taking phosphors, for example, it has been shown that the brightness and voltage characteristic depend strongly upon the size of particles [9,10]. Thus, it is an important challenge to synthesize particles with small size and monodistribution, especially in the nanometer range. To obtain nanometer-sized particles, a variety of methods has been proposed, including precipitation in aqueous and organic media [11], thermal decomposition [12], hydrothermal synthesis [13], and microemulsion method [14]. Wilhemly and Matijevic [12] employed thermal decomposition of thioacetamide to prepare micrometer-sized spherical zinc sulfide particles by aging the reaction mixture several hours, using a two-step procedure in solution. The complexity and expansiveness of some of these methods, however, justify the need to develop a simple method. In this paper, a new solid-state method by which zinc sulfide nanoparticles can be obtained easily via solid-state chemical reaction of zinc acetate and thioacetamide at low temperature is described.
2. Experimental Zinc sulfide nanoparticles were prepared as follows: First, zinc acetate [Zn(CH3COO)2] and thioacetamide (TAA) were milled separately. Then, appropriate amounts of the Zn(CH3COO)2 and TAA powders were mixed together and milled thoroughly. Finally, the mixed powder was heated in an oven for 4 h at 80, 100, 150, 200, and 300°C. To determine crystallite sizes and phase purity of the powders, X-ray diffraction spectra were obtained with a Rigaku D/max-rA X-ray diffractometer using Cu K␣ ( ⫽ 1.54056 Å) radiation. Investigations were carried out using a JEOL JEM 2010 transmission electron microscope operating at 200 kV. TEM samples were prepared by dispersing a small amount of powder in acetone. A drop of the dispersion was then transferred onto a carbon-coated grit and dried for observation. Thermal analysis was carried out with a Shimadzu DT-30 thermometer. Photoluminescence spectrum was recorded on a Hitachi MPF-4 fluorescence spectrophotometer.
3. Results and discussions 3.1. X-ray diffraction patterns X-ray diffraction patterns of ZnS nanoparticles prepared at different temperatures are shown in Fig. 1. They reveal that, except for the sample prepared at 80°C, in which only a small amount of reactant existed, all samples exhibited pure zinc-blende crystal structure. The three diffraction peaks correspond to (111), (220), and (311) planes, respectively, of
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
697
Fig. 1. XRD patterns of ZnS nanoparticles prepared at 80, 100, 150, 200, and 300°C, respectively.
cubic ZnS. No diffraction peaks of ZnS appeared for the sample prepared at room temperature. It is known that decrease in particle size results in a broadening of the diffraction peaks. Thus, the broadening of X-ray diffraction peaks in Fig. 1 indicates that particle sizes are in the nanometer range. The average crystallite size can be determined from the half-width of the diffraction peaks using Debeye–Scherrer formula D ⫽ ␣/cos, where D is the mean particle size, ␣ is a geometric factor (equal to 0.94), is the X-ray wavelength (1.54056 Å),  is the half-width of diffraction peak, and is the degree of the diffraction peak. Here, corresponding to (111) plane is selected. Estimating from the formula, the average crystallite sizes of the ZnS nanoparticles prepared at 80, 100, 150, 200, and 300°C are 1.7, 3.2, 3.9, 4.5, and 5.2 nm, respectively. It can be seen that the average size of the crystallites increased as the heating temperature increased. This indicates that the size of the crystallites can be adjusted by controlling the temperature of the reaction. 3.2. TEM observation A transmission electron micrograph and corresponding particle size distribution of the sample prepared at 100°C are shown in Figs. 2 and 3, respectively. As Fig. 2 clearly indicates, the morphology of the particles is roughly spherical and homogeneous. Some of the particles are agglomerates. The average particle size is about 40 nm. From Fig. 3, it can be seen that the particle sizes ranged from 20 to 60 nm, and the largest percentage is in the size range of 30 to 45 nm. This reveals that the size distribution was relatively narrow. The size of particles observed in the TEM micrograph is much larger than that of the crystallites estimated from the Debeye–Sherrer formula. We think that each particle is composed of fine
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L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
Fig. 2. TEM of the sample prepared at 100°C.
Fig. 3. Particle size distribution of ZnS nanoparticles prepared at 100°C.
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
699
Fig. 4. TG/DTA curves of the mixture of milled zinc acetate and thioacetamide.
crystallites, whose sizes were determined by XRD technique. Thus, each particle observed in the TEM micrograph was polycrystallite. 3.3. Thermal analysis To provide more information on the mechanism of the formation of ZnS nanoparticles, we carried out a thermal study of the mixture of milled zinc acetate and thioacetamide. The corresponding TG/DTA curves are shown in Fig. 4. In the TG/DTA curves, there are two small endothermic peaks accompanied by a large weight loss and a small exothermic peak accompanied by a small weight loss. The first endothermic peak at 103°C probably corresponds to the melting of thioacetamide (melting point 107.5°C), and the second endothermic peak at 122°C probably corresponds to the vaporization of acetic acid (boiling point 118.7°C). ZnS nanoparticles formed during these processes. The large weight loss at about 103°C and 122°C in the DTA curve is due to the vaporization of acetic acid and other small volatile components. The two endothermic peaks are very small, indicating that only a little heat is needed in the reaction. That is the reason why no reaction occurs at room temperature, but the reaction can be accomplished at low temperature. As it is mentioned in 3.1, zinc sulfide nanoparticles had already formed when the mixture was heated at 100°C, but no zinc sulfide was found for the sample prepared at room temperature. The results are consistent with those obtained from TG/DTA. As for the small exothermic peak at 588°C accompanied by a small weight loss, the mechanism is not clear. We suspect it may represent the decomposition of byproduct. The reaction may be described as follows: Zn共CH 3COO兲 2 ⫹ CH 3CSNH 2 3 ZnS ⫹ CH 3COOH ⫹ by product
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Fig. 5. Emission spectrum of the sample prepared at 100°C (ex ⫽ 320 nm).
3.4. Photoluminescence property Fig. 5 shows the photoluminescence spectrum of the sample prepared at 100°C. For most semiconductor nanocrystals, two emissions arising from excitonic and trapped luminescence can be observed. The former is sharp and located at the absorption edge, while the latter is broad and stokes-shifted [15]. Only trapped luminescence arising from surface states is observed in Fig. 5. The trapped emission of ZnS nanoparticles shifts to the blue, compared with bulk ZnS. This result is consistent with that observed by Chen et al. [11]. The size effects of the surface states have been discussed by Chestnoy et al. [16]. While the band gap increases as the particle size decreases, the separation between the electron-hole states increases. This results in the blue shift of the luminescence arising from surface states.
4. Conclusion We report a novel solid-state method for preparing nanoparticles. Using this method, cubic zinc sulfide nanoparticles of various sizes were prepared simply via the reaction of zinc acetate and TAA powders at relatively low temperature. The particle size can be adjusted by controlling the reaction temperature. The reaction involves two processes, and zinc sulfide nanoparticles are formed simultaneously during these processes.
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
701
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
L.E. Brus, J Phys Chem 90 (1986) 2555. Y. Wang, N. Herron, J Phys Chem 95 (1991) 525. A.Z. Khosravi, M.Kundu, B.A. Kuruvilla, G.S. Shekhawat, R.P. Gupta, A.K. Sharma, P.D. Vyas, S.K. Kulkarmi, Appl Phys Lett 67 (1995) 2506. L.E. Brus, Acc Chem Res 23 (1990) 183. R.P. Andres, R.S. Averback, W.L. Brown, L.E. Brus, W.A. Goddard, K. Kaldor, S.G. Louie, M. Moscovits, P.S. Peercy, S.J. Riley, R.W. Seigel, F. Spaepen, Y. Wang, J Mater Res 4 (1989) 704. H.W. Leverenz, Luminescence of Solids, Wiley, New York, 1950. A. Clandra, M. Mishra, Energy Conver 25 (1985) 387. A. Celikkaya, M. Akinc, J Am Ceram Soc 73 (1990) 2360. S. Faria, J Electrochem Soc 135 (1988) 2627. W. Lehmann, J Electrochem Soc 107 (1960) 20. W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, Appl Phys Lett 70 (1997) 1465. D.M. Wilhemly, E. Matijevic, J Chem Soc Faraday Trans 80 (1984) 563. Y.T. Qian, Y. Su, Y. Xie, Q.W. Chen, Z.Y. Chen, L. Wang, Mater Res Bull 30 (1995) 601. H. Takaguki, S. Hiroshi, K. Isao, Ind Eng Chem Res 33 (1994) 3262. L. Spanhel, M. Haase, H. Weller, A. Henglein, J Am Chem Soc 109 (1987) 5649. N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J Phys Chem 90 (1986) 3393.
Materials Research Bulletin 35 (2000) 695–701
A new preparation of zinc sulfide nanoparticles by solidstate method at low temperature L.P. Wanga, G.Y. Honga,b,* a
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, People’s Republic of China b Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences, Changchun, Jilin, 130022, People’s Republic of China (Refereed) Received 21 June 1999; accepted 6 July 1999
Abstract A novel solid-state method of the preparation of zinc sulfide nanoparticles is reported. By solid-state reaction of zinc acetate and thioacetamide at low temperature, zinc sulfide nanoparticles of different sizes were prepared. The temperature of preparation varied from room temperature to 300°C. The particles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), and photoluminescence spectrum. X-ray diffraction patterns revealed that the particles exhibited pure zinc-blende crystal structure and that particle size increased with increasing temperature. The TEM micrograph showed that the mean particle size was about 40 nm for the sample heated at 100°C. A blue shift was observed in the photoluminescence emission spectrum. A possible mechanism of the reaction corresponding to our observation is proposed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1. Introduction Semiconductor nanoparticles, which have changed properties resulting from quantum confinement, have drawn considerable interest and are currently being investigated [1,2].
* Corresponding author. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 6 1 - 0
696
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
These nanoparticles may find applications in nonlinear optical devices, photocatalysis, etc. [3–5]. Being a compound semiconductor, zinc sulfide has found many applications in various fields, such as phosphors, solar cells, and IR window [6 – 8]. Although in some cases it is used as thin films, zinc sulfide is often utilized in the form of particles. It has been shown that the particle size and its distribution play an important role in applications. Taking phosphors, for example, it has been shown that the brightness and voltage characteristic depend strongly upon the size of particles [9,10]. Thus, it is an important challenge to synthesize particles with small size and monodistribution, especially in the nanometer range. To obtain nanometer-sized particles, a variety of methods has been proposed, including precipitation in aqueous and organic media [11], thermal decomposition [12], hydrothermal synthesis [13], and microemulsion method [14]. Wilhemly and Matijevic [12] employed thermal decomposition of thioacetamide to prepare micrometer-sized spherical zinc sulfide particles by aging the reaction mixture several hours, using a two-step procedure in solution. The complexity and expansiveness of some of these methods, however, justify the need to develop a simple method. In this paper, a new solid-state method by which zinc sulfide nanoparticles can be obtained easily via solid-state chemical reaction of zinc acetate and thioacetamide at low temperature is described.
2. Experimental Zinc sulfide nanoparticles were prepared as follows: First, zinc acetate [Zn(CH3COO)2] and thioacetamide (TAA) were milled separately. Then, appropriate amounts of the Zn(CH3COO)2 and TAA powders were mixed together and milled thoroughly. Finally, the mixed powder was heated in an oven for 4 h at 80, 100, 150, 200, and 300°C. To determine crystallite sizes and phase purity of the powders, X-ray diffraction spectra were obtained with a Rigaku D/max-rA X-ray diffractometer using Cu K␣ ( ⫽ 1.54056 Å) radiation. Investigations were carried out using a JEOL JEM 2010 transmission electron microscope operating at 200 kV. TEM samples were prepared by dispersing a small amount of powder in acetone. A drop of the dispersion was then transferred onto a carbon-coated grit and dried for observation. Thermal analysis was carried out with a Shimadzu DT-30 thermometer. Photoluminescence spectrum was recorded on a Hitachi MPF-4 fluorescence spectrophotometer.
3. Results and discussions 3.1. X-ray diffraction patterns X-ray diffraction patterns of ZnS nanoparticles prepared at different temperatures are shown in Fig. 1. They reveal that, except for the sample prepared at 80°C, in which only a small amount of reactant existed, all samples exhibited pure zinc-blende crystal structure. The three diffraction peaks correspond to (111), (220), and (311) planes, respectively, of
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
697
Fig. 1. XRD patterns of ZnS nanoparticles prepared at 80, 100, 150, 200, and 300°C, respectively.
cubic ZnS. No diffraction peaks of ZnS appeared for the sample prepared at room temperature. It is known that decrease in particle size results in a broadening of the diffraction peaks. Thus, the broadening of X-ray diffraction peaks in Fig. 1 indicates that particle sizes are in the nanometer range. The average crystallite size can be determined from the half-width of the diffraction peaks using Debeye–Scherrer formula D ⫽ ␣/cos, where D is the mean particle size, ␣ is a geometric factor (equal to 0.94), is the X-ray wavelength (1.54056 Å),  is the half-width of diffraction peak, and is the degree of the diffraction peak. Here, corresponding to (111) plane is selected. Estimating from the formula, the average crystallite sizes of the ZnS nanoparticles prepared at 80, 100, 150, 200, and 300°C are 1.7, 3.2, 3.9, 4.5, and 5.2 nm, respectively. It can be seen that the average size of the crystallites increased as the heating temperature increased. This indicates that the size of the crystallites can be adjusted by controlling the temperature of the reaction. 3.2. TEM observation A transmission electron micrograph and corresponding particle size distribution of the sample prepared at 100°C are shown in Figs. 2 and 3, respectively. As Fig. 2 clearly indicates, the morphology of the particles is roughly spherical and homogeneous. Some of the particles are agglomerates. The average particle size is about 40 nm. From Fig. 3, it can be seen that the particle sizes ranged from 20 to 60 nm, and the largest percentage is in the size range of 30 to 45 nm. This reveals that the size distribution was relatively narrow. The size of particles observed in the TEM micrograph is much larger than that of the crystallites estimated from the Debeye–Sherrer formula. We think that each particle is composed of fine
698
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
Fig. 2. TEM of the sample prepared at 100°C.
Fig. 3. Particle size distribution of ZnS nanoparticles prepared at 100°C.
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
699
Fig. 4. TG/DTA curves of the mixture of milled zinc acetate and thioacetamide.
crystallites, whose sizes were determined by XRD technique. Thus, each particle observed in the TEM micrograph was polycrystallite. 3.3. Thermal analysis To provide more information on the mechanism of the formation of ZnS nanoparticles, we carried out a thermal study of the mixture of milled zinc acetate and thioacetamide. The corresponding TG/DTA curves are shown in Fig. 4. In the TG/DTA curves, there are two small endothermic peaks accompanied by a large weight loss and a small exothermic peak accompanied by a small weight loss. The first endothermic peak at 103°C probably corresponds to the melting of thioacetamide (melting point 107.5°C), and the second endothermic peak at 122°C probably corresponds to the vaporization of acetic acid (boiling point 118.7°C). ZnS nanoparticles formed during these processes. The large weight loss at about 103°C and 122°C in the DTA curve is due to the vaporization of acetic acid and other small volatile components. The two endothermic peaks are very small, indicating that only a little heat is needed in the reaction. That is the reason why no reaction occurs at room temperature, but the reaction can be accomplished at low temperature. As it is mentioned in 3.1, zinc sulfide nanoparticles had already formed when the mixture was heated at 100°C, but no zinc sulfide was found for the sample prepared at room temperature. The results are consistent with those obtained from TG/DTA. As for the small exothermic peak at 588°C accompanied by a small weight loss, the mechanism is not clear. We suspect it may represent the decomposition of byproduct. The reaction may be described as follows: Zn共CH 3COO兲 2 ⫹ CH 3CSNH 2 3 ZnS ⫹ CH 3COOH ⫹ by product
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Fig. 5. Emission spectrum of the sample prepared at 100°C (ex ⫽ 320 nm).
3.4. Photoluminescence property Fig. 5 shows the photoluminescence spectrum of the sample prepared at 100°C. For most semiconductor nanocrystals, two emissions arising from excitonic and trapped luminescence can be observed. The former is sharp and located at the absorption edge, while the latter is broad and stokes-shifted [15]. Only trapped luminescence arising from surface states is observed in Fig. 5. The trapped emission of ZnS nanoparticles shifts to the blue, compared with bulk ZnS. This result is consistent with that observed by Chen et al. [11]. The size effects of the surface states have been discussed by Chestnoy et al. [16]. While the band gap increases as the particle size decreases, the separation between the electron-hole states increases. This results in the blue shift of the luminescence arising from surface states.
4. Conclusion We report a novel solid-state method for preparing nanoparticles. Using this method, cubic zinc sulfide nanoparticles of various sizes were prepared simply via the reaction of zinc acetate and TAA powders at relatively low temperature. The particle size can be adjusted by controlling the reaction temperature. The reaction involves two processes, and zinc sulfide nanoparticles are formed simultaneously during these processes.
L.P. Wang, G.Y. Hong / Materials Research Bulletin 35 (2000) 695–701
701
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
L.E. Brus, J Phys Chem 90 (1986) 2555. Y. Wang, N. Herron, J Phys Chem 95 (1991) 525. A.Z. Khosravi, M.Kundu, B.A. Kuruvilla, G.S. Shekhawat, R.P. Gupta, A.K. Sharma, P.D. Vyas, S.K. Kulkarmi, Appl Phys Lett 67 (1995) 2506. L.E. Brus, Acc Chem Res 23 (1990) 183. R.P. Andres, R.S. Averback, W.L. Brown, L.E. Brus, W.A. Goddard, K. Kaldor, S.G. Louie, M. Moscovits, P.S. Peercy, S.J. Riley, R.W. Seigel, F. Spaepen, Y. Wang, J Mater Res 4 (1989) 704. H.W. Leverenz, Luminescence of Solids, Wiley, New York, 1950. A. Clandra, M. Mishra, Energy Conver 25 (1985) 387. A. Celikkaya, M. Akinc, J Am Ceram Soc 73 (1990) 2360. S. Faria, J Electrochem Soc 135 (1988) 2627. W. Lehmann, J Electrochem Soc 107 (1960) 20. W. Chen, Z.G. Wang, Z.J. Lin, L.Y. Lin, Appl Phys Lett 70 (1997) 1465. D.M. Wilhemly, E. Matijevic, J Chem Soc Faraday Trans 80 (1984) 563. Y.T. Qian, Y. Su, Y. Xie, Q.W. Chen, Z.Y. Chen, L. Wang, Mater Res Bull 30 (1995) 601. H. Takaguki, S. Hiroshi, K. Isao, Ind Eng Chem Res 33 (1994) 3262. L. Spanhel, M. Haase, H. Weller, A. Henglein, J Am Chem Soc 109 (1987) 5649. N. Chestnoy, T.D. Harris, R. Hull, L.E. Brus, J Phys Chem 90 (1986) 3393.