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Synthesis of composite nanoparticle pairs
JAerosolSci.
Vol. 31, Suppl. 1, pp. S1007 S1008, 2000
Pergamon
www.elsevier.com/locate/jaerosci
Session 9E - Charaina and charge effects on nanoparticles (Special Session) SYNTHESIS OF COMPOSITE NANOPARTICLE PAIRS A. Maisels, F.E. Kruis and H. Fissan Process- and Aerosol Measurement Technology, Gerhard-Mercator-Universit~itDuisburg, 47048 Duisburg, Germany
KEYWORDS
Nanoparticles, aggregation, bipolar mixing INTRODUCTION Nanoparticles have a wide application field, including electronics, optics, gas sensors, etc. A special case is formed by composite particles, here defined as aggregates composed of two different particle classes. The technological use of such particles is manifold (Kruis et al., 1998), e.g.: improve the functional properties of semiconductor nanoparticles by adding a second semiconductor particle which provides a charge transfer, thereby improving the quantum yield, improve the functional properties of nanoparticles by adding another nanoparticle which has catalytic properties, such as Pt or Pd which enhances the sensitivity of SnO= nanoparticles used for gas sensors, - improve the superconductive properties of YBCO particles by adding smaller particle which act as flux pinning enhancement (Takao et al., 1997), improve the mixing characteristics when two ultrafine powders have to be mixed: when the powders are mixed in aerosol form, the occurrence of large aggregates of one component, which is detrimental for the mixing, can be suppressed. Here we propose a procedure for synthesis of composite nanoparticles in the gas phase. Two different charged aerosol flows with different nanoparticles were mixed and the ensuing aggregation led to formation of composite nanoparticles. Nanoparticle pairs containing equal amounts of particles of each kind were selected on basis of their electric neutrality. METHOD
In our experiments nanoparticles of two different substances (PbS and Ag) were generated separately in the gas phase and mixed bipolarly. The experimental set up is presented in Fig. I. Ag N2
Tube furnace 1
Neutraliser
N2
DMA
Tube furnace 2
-HV
PbS
Aggregation tube
El. Precipitator
Neutraliser [ DMA
t~
"
Figure 1. Experimental set up for synthesis of composite particle pairs.
S1007
UCPC l
ESP
S1008
Abstracts of the 2000 European Aerosol Conference
Powders of lead sulfide and silver were heated in tube furnaces under nitrogen atmosphere. Evaporation of material in the heated zone with subsequent cooling leads to supersaturation of vapor which results in formation of nanoparticles. Both aerosol flows were charged bipolarly in radioactive neutralizers. The selection of particle size and charge polarity was carried out using differential mobility analyzers (DMA). With the aim of reducing the influence of particle form on the aggregation process, both aerosols were conducted through sinter furnaces. Spherical monodisperse particles obtained were mixed in aggregation tube. The aggregation process is directed to the building of composite particles due to opposite charges of the primary aerosols. Applying an electrostatic precipitator enables to remove all charged particles, see Fig 2. In this case the only electrically neutral particles possible are composite nanoparticle pairs. 1000 °
without el. filtering / - - with el. filtering ]
100 E Z
10
10
100 d m (nm)
Figure 2. Particle size distribution after bipolar mixing of Ag particles (24.5 nm)and PbS particles (31.5 nm). Using electrostatic filtering removes the unaggregated primary particles, leaving only particle pairs. Fig 3a is representative for composite nanoparticles formed. A single composite nanoparticle is shown separately in Fig 3b. Further experiments are conducted in order to obtain more spherical primary nanoparticles by optimizing the sintering temperature. Also the mixing time is being increased to maximize the yield. i :i
a)
b)
Figure3. TEM pictures of Ag-PbS nanoparticle pairs. REFERENCES
Kruis, F.E., Fissan H., and Peled, A. (1998a) J.Aerosol Sci 29 5/6 511. Takao, Y., Awano, M. and Kuwahara, Y. (1997)AIChE J., 43, 2616.
Vol. 31, Suppl. 1, pp. S1007 S1008, 2000
Pergamon
www.elsevier.com/locate/jaerosci
Session 9E - Charaina and charge effects on nanoparticles (Special Session) SYNTHESIS OF COMPOSITE NANOPARTICLE PAIRS A. Maisels, F.E. Kruis and H. Fissan Process- and Aerosol Measurement Technology, Gerhard-Mercator-Universit~itDuisburg, 47048 Duisburg, Germany
KEYWORDS
Nanoparticles, aggregation, bipolar mixing INTRODUCTION Nanoparticles have a wide application field, including electronics, optics, gas sensors, etc. A special case is formed by composite particles, here defined as aggregates composed of two different particle classes. The technological use of such particles is manifold (Kruis et al., 1998), e.g.: improve the functional properties of semiconductor nanoparticles by adding a second semiconductor particle which provides a charge transfer, thereby improving the quantum yield, improve the functional properties of nanoparticles by adding another nanoparticle which has catalytic properties, such as Pt or Pd which enhances the sensitivity of SnO= nanoparticles used for gas sensors, - improve the superconductive properties of YBCO particles by adding smaller particle which act as flux pinning enhancement (Takao et al., 1997), improve the mixing characteristics when two ultrafine powders have to be mixed: when the powders are mixed in aerosol form, the occurrence of large aggregates of one component, which is detrimental for the mixing, can be suppressed. Here we propose a procedure for synthesis of composite nanoparticles in the gas phase. Two different charged aerosol flows with different nanoparticles were mixed and the ensuing aggregation led to formation of composite nanoparticles. Nanoparticle pairs containing equal amounts of particles of each kind were selected on basis of their electric neutrality. METHOD
In our experiments nanoparticles of two different substances (PbS and Ag) were generated separately in the gas phase and mixed bipolarly. The experimental set up is presented in Fig. I. Ag N2
Tube furnace 1
Neutraliser
N2
DMA
Tube furnace 2
-HV
PbS
Aggregation tube
El. Precipitator
Neutraliser [ DMA
t~
"
Figure 1. Experimental set up for synthesis of composite particle pairs.
S1007
UCPC l
ESP
S1008
Abstracts of the 2000 European Aerosol Conference
Powders of lead sulfide and silver were heated in tube furnaces under nitrogen atmosphere. Evaporation of material in the heated zone with subsequent cooling leads to supersaturation of vapor which results in formation of nanoparticles. Both aerosol flows were charged bipolarly in radioactive neutralizers. The selection of particle size and charge polarity was carried out using differential mobility analyzers (DMA). With the aim of reducing the influence of particle form on the aggregation process, both aerosols were conducted through sinter furnaces. Spherical monodisperse particles obtained were mixed in aggregation tube. The aggregation process is directed to the building of composite particles due to opposite charges of the primary aerosols. Applying an electrostatic precipitator enables to remove all charged particles, see Fig 2. In this case the only electrically neutral particles possible are composite nanoparticle pairs. 1000 °
without el. filtering / - - with el. filtering ]
100 E Z
10
10
100 d m (nm)
Figure 2. Particle size distribution after bipolar mixing of Ag particles (24.5 nm)and PbS particles (31.5 nm). Using electrostatic filtering removes the unaggregated primary particles, leaving only particle pairs. Fig 3a is representative for composite nanoparticles formed. A single composite nanoparticle is shown separately in Fig 3b. Further experiments are conducted in order to obtain more spherical primary nanoparticles by optimizing the sintering temperature. Also the mixing time is being increased to maximize the yield. i :i
a)
b)
Figure3. TEM pictures of Ag-PbS nanoparticle pairs. REFERENCES
Kruis, F.E., Fissan H., and Peled, A. (1998a) J.Aerosol Sci 29 5/6 511. Takao, Y., Awano, M. and Kuwahara, Y. (1997)AIChE J., 43, 2616.