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Mechanochemical synthesis of metal sulphide nanoparticles
NanoSmwxured
Pergamon
Materials,
Vol.
12, pp. 75-78, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-see front matter
PII SO9659773(99)00069-O
MECHANOCHEMICAL
SYNTHESIS OF METAL NANOPARTICLES
SULPHIDE
Takuya Tsuzuki and Paul G. McCormick Special Research Centre for Advanced Mineral and Materials Processing The University of Western Australia, Nedlands, Perth, WA 6907, Australia
Abstract The synthesis of ZnS, CdS ana’ Ce,S3 nanoparticles by mechanochemical reaction has been reviewed. During mechanical milling, solid-state displacement reactions between the respective metal chloride and alkali sulphide or alkaline earth sulphide were induced in a steady-state manner, leading to the formation of metal sulphia!e nanoparticles. A simple washing process to remove the chloride by-product yielded separated particles of ZnS, CdS and Ce,S, of -8 nm, -4 nm and -20 nm, respectively. The resulting particles and crystallite sizes were dependent on the milling conditions, starting materials and the presence of a diluent. Structural change with decreasing particle size was observed for CdS & Ce,S,. 01999 Acta Metallurgica kc.
INTRODUCTION Metal sulphides have been recognised as advanced materials for many applications including opto-electric materials, phosphors, pigments and magnetic materials. Nanoparticles have significant potential for these applications due to their small size, high-surface area and low sintering temperatures. In particular, semiconductor quantum dots have attracted considerable attention in many fields of science (1). In recent years, a number of studies have been carried out to develop methods for synthesising sulphide nanoparticles (l-4). Recently, Ding et al. (5-9) have reported the synthesis of nano-sized particles of a number of transition metals and ceramics including Fe, Cu, Ni, Co, Al,O, and ZrO, by mechanochemical processing. This paper reviews the synthesis of ZnS, CdS and Ce& nanoparticles by mechanochemical reactions between the respective metal chloride and alkali sulphide or alkaline earth sulphide (10,ll).
MECHANOCHEMICAL
PROCESSING
Mechanochemical processing is a novel method involving the mechanical activation of solid-state displacement chemical reactions, either during ball milling or during subsequent heat treatment (12). This process is character&d by the repeated welding and fracture of reacting particles during ball-powder collisions, which continually regenerate reacting interfaces. As a consequence, reactions which would normally require high temperatures to occur, due to 75
76
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
separationof the reacting phasesby the product phases,can occur at low temperaturesin a ball mill. For example, mechanical milling of a mixture of FeCl, and Na powders causes the reaction FeCl, + 3Na + Fe + 3NaCl to occur (5). Careful control of milling conditions to avoid combustion enablesthe reactionsto occur in a steady-statemanner. On completion of the reaction, the as-milled powders usually consist of a nanocompositemixture of the product phases.A simple washing processto remove the by-product phaseyields particles of the desired phase.
SYNTHESIS
OF SULPHIDES
Sulphide nanoparticleswere synthesisedvia the reactions [l-8] listed in Table 1. The starting materials were anhydrous ZnCl,, CdCl,, CeCl, and commercial grade (CG) CaS. Mechanically alloyed (MA) CaS and Na,S, which comprise considerably smaller particles (- 20 run) than CG-CaS (~100 nm), were also used as a starting reactant. CaCl, and NaCl were used asa diluent. The reactantswere sealedin a hardenedsteel vial with steel balls under a high-purity Ar-gas atmosphere.Milling was performed with a Spex 8000 mixer/mill. The powders were milled for the time required to complete the reaction. All the reactions in Table 1 occurred during milling in a steady state manner. The resulting particles and crystallite sizes were dependenton the milling conditions, starting materials and the presenceof a diluent. It was only possible to form separatedsingle crystal nanoparticlesin samplesmilled with a diluent. The powderssynthesisedvia the reactions [l] and [6] consisted of large aggregatesof crystallites, whilst the reactions [2] and [7] resulted in separatedsingle crystal nanoparticles.It is clear that the volume ratio of the minor to major product phasesmust be sufficiently small for separatednanoparticlesof the minor phase to form. The particle size of the sulphide products is related to the particle size of the starting reactant. The reaction using MA-CaS (reaction [3]) resulted in smaller ZnS particle size (crystallite size) than using CG-CaS (reaction [2]). The sametendency was observedfor Ce,S, (reactions [7] and [S]). In a nano-compositeof the reactantsformed during milling, the effective reaction volume is determinedby the crystallite size of the reactants. ZnS particle size is determinedby the effective reaction volume from which ZnS particles am nucleated.Therefore, small particles of ZnS are obtained from smallparticles of the reactant. Comparing the reactions [3] and [4], it is evident that addition of a diluent has little effect on the sulphide particle size. This supportsthe view that the effective reaction volume is mainly associatedwith the size of starting reactants,providing the samemilling conditions am applied. Grinding-ball size is another factor which influences the particle size. For CdS, the particle size decreasedwith decreasing milling-ball size (reaction [5]). CdS is a II-VI semiconductorwith a direct band-gapof 2.4 eV. Since the CdS particles were smaller than the exciton Bohr diameter of 8 nm, a quantum size effect was observedas a blue-shift of the bandgap energy (11,13). Figure 1 showsa TEM image of the CdS quantumdots. While all the ZnS particles had a cubic structure (zinc-blende), a structural transition with decreasingparticle size was observedfor CdS and Ce,S,. The structure of CdS changed from a mixture of wurtzite (hexagonal) and zinc-blende (cubic) to zinc-blende, as the mean
FOURTH
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TABLE 1 XRD Crystallite Size, dcVstir of the Obtained Sulphide Powders as a Function of Grinding-Ball Size, D,,,, and Starting Powders. Rv is the volume ratio of the chloride to the sulphide in the product phase. Product
ZnS
CdS
CeA
Reaction equation
ZnCl, ZnCl, ZnCl, ZnCl, CdCl,
+ CG-CaS + CG-CaS + MA-CaS + MA-CaS + MA-Na,S
+ ZnS + CaCl, + 3.6CaC1, + ZnS + 4.6CaC1, + 3.6CaCl,+ ZnS + 4.6CaC1, + 8.2CaC1, + ZnS + 9.2CaC1, + 16NaClj CdS + 18NaCl
[l] [2] [3] ]4]
[51
Rv
ha,, (mm>
3:l 1O:l 1O:l 2O:l 16:l 16:l 16:l 16:l
12.7 12.7 12.7 12.7 12.7 9.5 6.4 4.8
4rysta , (nm) 12 16 8.0 8.6 8.2 6.5 5.9 4.3
CeCl, + CG-CaS + Ce,S, + CaCl, [6] CeCl, + CG-CaS + llCaC1, + Ce,& + 12CaC1, [7] CeCl, + MA-CaS + llCaC1, + Ce,S, + 12CaC1, [8]
2:l
12.7
32
1O:l 1O:l
12.7 12.7
29 20
30
40
50
60
28 (degrees)
Figure 1. TEM micrograph of the CdS quantum dots synthesised by mechanochemical processing.
Figure 2. X-ray diffraction patterns of milled and washedCe,S, powders via the reactions (a) CeCl, + CG-CaS + Ce,S, + CaCl,, and (b) CeCl, + MA-CaS + 1lCaC1, + Ce,S, + 12CaC1,.
FOURTH INTERNATIONAL CONFERENCE
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MATERIALS
particle size decreased (11). In Ce,S,, the particles with mean diameter of >30 nm had the tetragonal P-Ce,S, structure, whereas particles smaller than 20 nm had the cubic y-Ce,S, structure (Figure 2). It is interesting to note that these cubic structures in CdS and Ce$, ate. thermodynamically metastable at room temperature. Size dependence of the structure in nanoparticles has also been reported for A&O, (8,14) and ZrG, (9,15), and has been explained as an surface-energy effect (14,15).
CONCLUSIONS Mechanochemical processing enables the direct synthesis of sulphide nanoparticles without the need for high temperatures. The average particle size can be controlled by changing milling conditions and starting materials. This novel synthesis method is applicable for the synthesis of a wide range of sulphide and other chalcogenide nanoparticles. Moreover, it has significant potential for large scale production due to high efficiency and low cost process.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Brus, S.E., Journal of Physical Chemistry, 1986, 90, 2555. Henglein, A., Fojtik, A. and Weller, H., Berichte der Bunsengesellschaf f?ir Physikalische Chemie, 1987, 91, 441. Ekimov, A., Journal of Luminescence, 1997, 70, 1. Takada, T., Mackenzie, J.D., Yamate, M., Kang, K., Payghambarian, N., Reeves, R.J., Knobbe, E.T. and Powell, R.C., Journal of Materials Science, 1996, 3 1, 423. Ding, J., Miao, W.F., McCormick, P.G. and Street, R., Applied Physics Letters, 1995, 67, 3804. Ding, J., Tsuzuki, T., McCormick, P.G. and Street, R., Journal of Alloys and Compounds, 1996, 234, Ll. Ding, J., Tsuzuki, T., McCormick, P.G. and Street, R., Journal of Physics D: Applied Physics, 1996, 29, 2365. Ding, J., Tsuzuki, T. and McCormick, P.G., Journal of American Ceramic Society, 1996, 79, 2956. Ding, J., Tsuzuki, T. and McCormick P.G., Nanostructured Materials, 1997, 8, 75. Tsuzuki, T., Ding, J. and McCormick P.G., Physica B, 1997, 239,378. Tsuzuki, T. and McCormick, P.G., Applied Physics A, 1997, 65, 607. McCormick, P.G., Ding, J., Yang, H. and Tsuzuki, T., Materials Research 96, The Institute of Metals and Materials Australia, 1996, ~01.1, p.85. Nakashima, P.N.H., Tsuzuki, T. and Johnson, A.W.S., Journal of Applied Physics, submitted for publication. McHale, J.M., Auroux, A., Perrotta, A.J. and Navrotsky, A., Science, 1997, 277, 788. Winterer, M., Nitsche, R., Redfem, S.A.T., Schmahl, W.W. and Hahn, H., Nanostructured Materials, 1995,5,679.
Pergamon
Materials,
Vol.
12, pp. 75-78, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-see front matter
PII SO9659773(99)00069-O
MECHANOCHEMICAL
SYNTHESIS OF METAL NANOPARTICLES
SULPHIDE
Takuya Tsuzuki and Paul G. McCormick Special Research Centre for Advanced Mineral and Materials Processing The University of Western Australia, Nedlands, Perth, WA 6907, Australia
Abstract The synthesis of ZnS, CdS ana’ Ce,S3 nanoparticles by mechanochemical reaction has been reviewed. During mechanical milling, solid-state displacement reactions between the respective metal chloride and alkali sulphide or alkaline earth sulphide were induced in a steady-state manner, leading to the formation of metal sulphia!e nanoparticles. A simple washing process to remove the chloride by-product yielded separated particles of ZnS, CdS and Ce,S, of -8 nm, -4 nm and -20 nm, respectively. The resulting particles and crystallite sizes were dependent on the milling conditions, starting materials and the presence of a diluent. Structural change with decreasing particle size was observed for CdS & Ce,S,. 01999 Acta Metallurgica kc.
INTRODUCTION Metal sulphides have been recognised as advanced materials for many applications including opto-electric materials, phosphors, pigments and magnetic materials. Nanoparticles have significant potential for these applications due to their small size, high-surface area and low sintering temperatures. In particular, semiconductor quantum dots have attracted considerable attention in many fields of science (1). In recent years, a number of studies have been carried out to develop methods for synthesising sulphide nanoparticles (l-4). Recently, Ding et al. (5-9) have reported the synthesis of nano-sized particles of a number of transition metals and ceramics including Fe, Cu, Ni, Co, Al,O, and ZrO, by mechanochemical processing. This paper reviews the synthesis of ZnS, CdS and Ce& nanoparticles by mechanochemical reactions between the respective metal chloride and alkali sulphide or alkaline earth sulphide (10,ll).
MECHANOCHEMICAL
PROCESSING
Mechanochemical processing is a novel method involving the mechanical activation of solid-state displacement chemical reactions, either during ball milling or during subsequent heat treatment (12). This process is character&d by the repeated welding and fracture of reacting particles during ball-powder collisions, which continually regenerate reacting interfaces. As a consequence, reactions which would normally require high temperatures to occur, due to 75
76
FOURTH INTERNATIONAL
CONFERENCE
ON NANOSTRUCTURED
MATERIALS
separationof the reacting phasesby the product phases,can occur at low temperaturesin a ball mill. For example, mechanical milling of a mixture of FeCl, and Na powders causes the reaction FeCl, + 3Na + Fe + 3NaCl to occur (5). Careful control of milling conditions to avoid combustion enablesthe reactionsto occur in a steady-statemanner. On completion of the reaction, the as-milled powders usually consist of a nanocompositemixture of the product phases.A simple washing processto remove the by-product phaseyields particles of the desired phase.
SYNTHESIS
OF SULPHIDES
Sulphide nanoparticleswere synthesisedvia the reactions [l-8] listed in Table 1. The starting materials were anhydrous ZnCl,, CdCl,, CeCl, and commercial grade (CG) CaS. Mechanically alloyed (MA) CaS and Na,S, which comprise considerably smaller particles (- 20 run) than CG-CaS (~100 nm), were also used as a starting reactant. CaCl, and NaCl were used asa diluent. The reactantswere sealedin a hardenedsteel vial with steel balls under a high-purity Ar-gas atmosphere.Milling was performed with a Spex 8000 mixer/mill. The powders were milled for the time required to complete the reaction. All the reactions in Table 1 occurred during milling in a steady state manner. The resulting particles and crystallite sizes were dependenton the milling conditions, starting materials and the presenceof a diluent. It was only possible to form separatedsingle crystal nanoparticlesin samplesmilled with a diluent. The powderssynthesisedvia the reactions [l] and [6] consisted of large aggregatesof crystallites, whilst the reactions [2] and [7] resulted in separatedsingle crystal nanoparticles.It is clear that the volume ratio of the minor to major product phasesmust be sufficiently small for separatednanoparticlesof the minor phase to form. The particle size of the sulphide products is related to the particle size of the starting reactant. The reaction using MA-CaS (reaction [3]) resulted in smaller ZnS particle size (crystallite size) than using CG-CaS (reaction [2]). The sametendency was observedfor Ce,S, (reactions [7] and [S]). In a nano-compositeof the reactantsformed during milling, the effective reaction volume is determinedby the crystallite size of the reactants. ZnS particle size is determinedby the effective reaction volume from which ZnS particles am nucleated.Therefore, small particles of ZnS are obtained from smallparticles of the reactant. Comparing the reactions [3] and [4], it is evident that addition of a diluent has little effect on the sulphide particle size. This supportsthe view that the effective reaction volume is mainly associatedwith the size of starting reactants,providing the samemilling conditions am applied. Grinding-ball size is another factor which influences the particle size. For CdS, the particle size decreasedwith decreasing milling-ball size (reaction [5]). CdS is a II-VI semiconductorwith a direct band-gapof 2.4 eV. Since the CdS particles were smaller than the exciton Bohr diameter of 8 nm, a quantum size effect was observedas a blue-shift of the bandgap energy (11,13). Figure 1 showsa TEM image of the CdS quantumdots. While all the ZnS particles had a cubic structure (zinc-blende), a structural transition with decreasingparticle size was observedfor CdS and Ce,S,. The structure of CdS changed from a mixture of wurtzite (hexagonal) and zinc-blende (cubic) to zinc-blende, as the mean
FOURTH
INTERNATIONAL
CONFERENCE
ON NAWSTRUCTURED
77
MATERIALS
TABLE 1 XRD Crystallite Size, dcVstir of the Obtained Sulphide Powders as a Function of Grinding-Ball Size, D,,,, and Starting Powders. Rv is the volume ratio of the chloride to the sulphide in the product phase. Product
ZnS
CdS
CeA
Reaction equation
ZnCl, ZnCl, ZnCl, ZnCl, CdCl,
+ CG-CaS + CG-CaS + MA-CaS + MA-CaS + MA-Na,S
+ ZnS + CaCl, + 3.6CaC1, + ZnS + 4.6CaC1, + 3.6CaCl,+ ZnS + 4.6CaC1, + 8.2CaC1, + ZnS + 9.2CaC1, + 16NaClj CdS + 18NaCl
[l] [2] [3] ]4]
[51
Rv
ha,, (mm>
3:l 1O:l 1O:l 2O:l 16:l 16:l 16:l 16:l
12.7 12.7 12.7 12.7 12.7 9.5 6.4 4.8
4rysta , (nm) 12 16 8.0 8.6 8.2 6.5 5.9 4.3
CeCl, + CG-CaS + Ce,S, + CaCl, [6] CeCl, + CG-CaS + llCaC1, + Ce,& + 12CaC1, [7] CeCl, + MA-CaS + llCaC1, + Ce,S, + 12CaC1, [8]
2:l
12.7
32
1O:l 1O:l
12.7 12.7
29 20
30
40
50
60
28 (degrees)
Figure 1. TEM micrograph of the CdS quantum dots synthesised by mechanochemical processing.
Figure 2. X-ray diffraction patterns of milled and washedCe,S, powders via the reactions (a) CeCl, + CG-CaS + Ce,S, + CaCl,, and (b) CeCl, + MA-CaS + 1lCaC1, + Ce,S, + 12CaC1,.
FOURTH INTERNATIONAL CONFERENCE
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particle size decreased (11). In Ce,S,, the particles with mean diameter of >30 nm had the tetragonal P-Ce,S, structure, whereas particles smaller than 20 nm had the cubic y-Ce,S, structure (Figure 2). It is interesting to note that these cubic structures in CdS and Ce$, ate. thermodynamically metastable at room temperature. Size dependence of the structure in nanoparticles has also been reported for A&O, (8,14) and ZrG, (9,15), and has been explained as an surface-energy effect (14,15).
CONCLUSIONS Mechanochemical processing enables the direct synthesis of sulphide nanoparticles without the need for high temperatures. The average particle size can be controlled by changing milling conditions and starting materials. This novel synthesis method is applicable for the synthesis of a wide range of sulphide and other chalcogenide nanoparticles. Moreover, it has significant potential for large scale production due to high efficiency and low cost process.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Brus, S.E., Journal of Physical Chemistry, 1986, 90, 2555. Henglein, A., Fojtik, A. and Weller, H., Berichte der Bunsengesellschaf f?ir Physikalische Chemie, 1987, 91, 441. Ekimov, A., Journal of Luminescence, 1997, 70, 1. Takada, T., Mackenzie, J.D., Yamate, M., Kang, K., Payghambarian, N., Reeves, R.J., Knobbe, E.T. and Powell, R.C., Journal of Materials Science, 1996, 3 1, 423. Ding, J., Miao, W.F., McCormick, P.G. and Street, R., Applied Physics Letters, 1995, 67, 3804. Ding, J., Tsuzuki, T., McCormick, P.G. and Street, R., Journal of Alloys and Compounds, 1996, 234, Ll. Ding, J., Tsuzuki, T., McCormick, P.G. and Street, R., Journal of Physics D: Applied Physics, 1996, 29, 2365. Ding, J., Tsuzuki, T. and McCormick, P.G., Journal of American Ceramic Society, 1996, 79, 2956. Ding, J., Tsuzuki, T. and McCormick P.G., Nanostructured Materials, 1997, 8, 75. Tsuzuki, T., Ding, J. and McCormick P.G., Physica B, 1997, 239,378. Tsuzuki, T. and McCormick, P.G., Applied Physics A, 1997, 65, 607. McCormick, P.G., Ding, J., Yang, H. and Tsuzuki, T., Materials Research 96, The Institute of Metals and Materials Australia, 1996, ~01.1, p.85. Nakashima, P.N.H., Tsuzuki, T. and Johnson, A.W.S., Journal of Applied Physics, submitted for publication. McHale, J.M., Auroux, A., Perrotta, A.J. and Navrotsky, A., Science, 1997, 277, 788. Winterer, M., Nitsche, R., Redfem, S.A.T., Schmahl, W.W. and Hahn, H., Nanostructured Materials, 1995,5,679.