화염법에 의한 나노입자 제조 및 성장제어기술

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

최만수1,2  양상선1,2 이지현2

1서울대학교 기계항공공학부

2서울대학교 나노입자제어기술연구단

Nanoparticle Synthesis and Growth Control in Flame

Mansoo Choi1,2, Sangsun Yang1,2 and Ji-Hyun Yi2

1School of Mechanical and Aerospace Engineering, Seoul National University

2National CRI Center for Nano Particle Control, Seoul National University

We present a method of coalescence enhanced synthesis of nanoparticles using laser beam irradiation on aggregates formed in flames. We demonstrate this principle by not only synthesizing smaller and unagglomerated nanoparticles, but also generating high concentrations. We also show that the present method is capable of even controlling the crystalline phase of nanoparticles. In addition to this, we also report that sudden size evolution of iron oxide particles along flame height can occur in a flame.

1. Introduction

The morphology and size of particles are determined by the competition between collision and coalescence of particles. The formation of aggregates is due to faster collision of particles than coalescence. Even though the method of salt addition to coat on particles generated in a flame had been shown to produce unagglomerate nanoparticles, high purity of particles could not be obtained and the subsequent process of washing may pose an additional problem of agglomeration. The electrical field had been applied in a particle generating flame to successfully control the size of primary particles and aggregates however, the formation of aggregates could not be avoided.

We present a method using laser irradiation on aggregates that form in a flame, which is "Coalescence Enhanced Synthesis". We applied this method to both noncrystalline and crystalline particle synthesis to examine the controllability of the crystalline phase of nanoparticles as well as the size and morphology. The validity of this method is quantitatively verified by measuring the changes of number density, volume fraction and size distributions with and without laser irradiation. Our method relies on controlling the characteristic time for coalescence of nanoparticlesl,2 by irradiating CO2 laser beam on the early stage aggregates formed during flame synthesis. Small aggregates that begin to form in a flame are irradiated by a high power CW CO2 laser beam to rapidly coalesce and change into spheres. Since spherical particles have much smaller collision cross-sections than volume equivalent aggregates, much slower growth of nanoparticles can be achieved. Rapid heating of nanoparticles due to the irradiation of laser beam and subsequent cooling after passing the irradiation zone have been utilized to control the crystalline phase of nanocrystalline particles. In addition to the results of SiO2 and TiO2, we present other applications to single component or multicomponent nanoparticles synthesis. The presentation also includes our recent results about the sudden change of sizes of iron oxide nanoparticles in a flame along the flame height.

2. Experimental Methods and Results

A laminar diffusion flame burner composed of six concentric stainless-steel tubes3 was used. SiO2 and TiO2 Particles or others are generated in an oxy-hydrogen diffusion flame by hydrolysis and oxidation of SiCl4, TiCl4 or other chemical precursors, respectively. Laser light scattering experiments are conducted using Ar ion laser beam (514.5 nm in wavelength) and a high power CW CO2 laser (10.6 □m) is employed to rapidly heat aggregates forming in a flame. The experimental apparatus has been described in detail in our previous papers4,5. Note that the CO2 laser irradiation height from the burner surface (hL) is positioned below the height (hp) where Ar ion light scattering measurement and local sampling are done.

Figure 1. TEM photographs showing the change of size and morphology of TiO2 particles with or without CO2 laser irradiation. These reveal that original aggregates are transformed into much smaller spheres ; hL = 13 mm, hp = 16 mm, Bar represents 100 nm. a: 0 W, b: 1665 W.

Figure 1 shows the changes of size and morphology of titania particles captured at 16 mm without or with CO2 laser beam irradiated at 13 mm in a flame. As CO2 laser powers increase, coalescence is clearly shown to be enhanced. The magnified TEM image indicates that seemingly agglomerate particles in Figure 1(b) are just multiple depositions of isolated sphere particles on TEM grid. It is estimated from TEM image analysis6 that the volume equivalent diameters of particles also decrease substantially; 82□3 nm for aggregates in Figure 1(a) and 28□1 nm for spheres in Figure 1(b). CO2 laser beam irradiation near 13 mm enhances rapidly the coalescence of early stage aggregates formed near 13 mm and transforms them into spherical particles that have much smaller collision cross sections than the original (volume equivalent) aggregates. Therefore, the growth due to Brownian collision of particles is retarded to finally produce much smaller sizes at 16 mm.

The controllability of crystalline phase has also been investigated. Figure 2(a) shows X-ray diffraction profiles of titania particles collected at 65 mm for different laser powers when the laser beam is irradiated at 25 mm. A and R denote of anatase and rutile, respectively. It is very interesting that rutile contents decrease monotonically with the increase of laser power. Even for different CO2 laser irradiation positions, the trend of decreasing rutile weight percent is the same. For sufficiently high laser powers, rutile peaks completely disappear. This is a surprising result since rutile phase is well known to be more stable than anatase phase when particle size exceeds 14 nm. The present study reveals that the irradiation of CO2 laser beam can alter the phase from rutile to anatase. For example, 17 % rutile weight percent of particles at 65 mm without CO2 laser irradiation is now changed to 0 %, i.e., pure anatase particles when CO2 laser beam is irradiated at 25 mm with P = 326 W. This could be explained by the possible occurrence of melting of titania particles due to the irradiation of laser beam and subsequent re-crystallization after passing the irradiation zone (beam diameter: 3.3 mm). In summary, we report a coalescence enhanced synthesis to successfully control the size, morphology and crystalline phase of nanoparticles at high concentrations7 and this principle seems to be applicable to the synthesis of other single or multicomponent nanoparticles with controlled structure and may be extended to other processes as well as flame synthesis. We also examined the grain growth of 20 nm spherical titania particles captured at 17 mm with irradiating CO2 laser beam in a flame at 15 mm. Green compact pellet was heated pressurelessly in air up to 800 oC at the rate of 10 oC/min and held at 800 oC for 30 min, which yielded a full dense bulk of 100 % rutile having nearly theoretical density (> 98 %). XRD study and Scherrer equation reveal this fully dense rutile has 60 nm grain size which is only 3 times the starting powder size. Such small grain sizes in fully dense ceramics have not been achieved so far without employing any special sintering techniques8. Nanograins in fully dense TiO2 can be clearly seen from FE-SEM micrograph without showing any apparent pores (Figure 2(b)).

화학공학의 이론과 응용 제8권 제1호 2002년

(a)                                     (b)

Figure 2. (a) Variations of XRD profiles different CO2 laser powers; hL = 25 mm, hp = 65 mm. (b)  Nanostructure of fully dense titania is shown: pressureless sintering in air yielded a full dense bulk titania with density > 98 % of theoretical density8. There are no apparent pores. From FE-SEM micrographs, average grain size is estimated as about 75 nm while XRD data and Scherrer eq. gives 60 nm grains. Such small grain size in full dense ceramics has not been achieved before without employing special sintering techniques.

We now show the results for the growth of iron oxide nanoparticles in a flame. The iron(III) acetylacetonate contained in a stainless steel vessel was heated at 170 oC. Then the carrier gas(N2)  including iron acetylacetonate vapor was fed through the center of the burner. The morphology and the crystalline phase of the particles were determined by TEM, HRTEM, XRD and FTIR. We report that sudden change of size and phase of Fe2O3 along the flame can occur. At hydrogen flow rate of 4.0 l/min, a different phenomenon of size evolution was found. Particles collected at the height of 20 mm have the mean diameter of 17 nm and those collected at 35 mm height have slightly increased one, 20 nm. However, particles collected at 40 mm consist of two kinds of particles: relatively large ones, about 20 nm, and relatively small ones, about 3-5 nm. As the collection position increases, the number of large particles decreases while that of small ones increases. And then from the collection position of 44 mm only small particles can be observed. A fundamental study is now in progress in order to determine the origin of this new phenomenon. The superconducting quantum interference device (SQUID) studies confirmed nanoparticles collected at 60 mm exhibit good superparamagnetic properties.

3. Conclusions

Coalescence enhanced synthesis of nanoparticles using laser beam irradiation on aggregates formed in flames was shown to successfully control the size, morphology, and crystalline phase of high concentration nanoparticles. This method manipulating the characteristic time for coalescence of nanoparticles could be one solution for the aggregation problem that occurs when nanoparticles are generated at high concentrations. This principle seems to be applicable to the synthesis of other single or multicomponent unagglomerated nanoparticles at high concentrations and may be extended to other gas phase methods as well as flame synthesis. Sudden change of sizes of iron oxide nanoparticles along the flame height was also revealed.

Acknowledgements

This work was funded by the Creative Research Initiatives Program supported by the Ministry of Science and Technology.

References

1. B.W. Lee, J.I. Jeong, J.Y. Hwang, M. Choi and S.H. Chung: J. Aerosol Sci., 32, 165-185 (2001).

2. J.I. Jeong and M. Choi: J. Aerosol Sci., 32, 633-650 (2001).

3. J.Y. Hwang, Y.S. Gil, J.I. Kim, M. Choi and S.H. Chung: J. Aerosol Sci., 32, 669-681 (2001).

4. D. Lee and M. Choi: J. Aerosol Sci., 31, 1145-1163 (2000).

5. J. Cho and M. Choi: J. Aerosol Sci., 31(9), 1077-1095 (2000).

6. M. Choi, J. Cho, J. Lee and H. W. Kim: J. Nanoparticle Res., 1,169-183 (1999).

7. D. Lee and M. Choi: J. Aerosol Sci., 33(1), 1-16 (2002).

8. D. Lee, S. Yang and M. Choi: Appl. Phys. Lett., 79(15), 2459-2461 (2001).

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년