Growth Mechanism of Monodisperse Spherical ... - ACS Publications

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Growth Mechanism of Monodisperse Spherical Particles under Nanosecond Pulsed Laser Irradiation Alexander Pyatenko,*,† Hongqiang Wang,*,‡ and Naoto Koshizaki† †

Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8565, Japan ‡ Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany ABSTRACT: It is well-known that the colloidal nanoparticles irradiated by laser can be reduced in size. Recently, we demonstrated that at particular experimental conditions, colloidal nanoparticles of different materials can grow under pulse laser irradiation, finally transforming to the near monodisperse submicron spheres. To date, the detailed mechanism of this process was not revealed. In this work, we explore the mechanism of particle growth by applying the particle heating-melting-evaporation model. Our investigations show that the process of particle growth can be divided into two stages. At the initial stage, particle agglomeration is the dominant factor for the fast particle growth. Then, the process continues as long as the particles can be melted. The final particle diameter is determined by the value of laser fluence, because the laser fluence determines the maximum diameter of the particle which can be melted. Good agreement between the experimental results and results of calculation was obtained.

1. INTRODUCTION

2. EXPERIMENTAL RESULTS

Particle size reduction caused by laser irradiation is a wellknown phenomenon. Two different mechanisms: Coulomb explosion induced by electron ejection and particle evaporation were proposed by Kamat1 and Takami et al.2 for this phenomena. Both mechanisms suggest the particle decomposition into smaller fragment and/or individual atoms and molecules, when the energy absorbed by a particle from the laser pulse is enough for that. Recently, our group observed a new phenomenon: colloidal nanoparticles of many different materials can grow under nanosecond pulse laser irradiation. Moreover, this particle growth results in formation of perfect submicrometer spheres with near monodisperse particle size distribution.3−10 Experimentally, the method appears very simple. First, commercial NPs are ultrasonically dispersed in liquid (e.g., water, ethanol, or acetone). The colloid solution is then irradiated by the unfocused laser beam of a nanosecond Nd:YAG laser. Irradiation time is varied from 10 to 60 min, and laser power is varied as well. Second or third harmonics with wavelengths of 532 and 355 nm are used. The results depend strongly on the experimental parameters. To control particle formation for different materials, we must understand the mechanism of this process. In this work, we will clarify the mechanism of particle growth with formation of perfect spherical shapes for the copper oxide system, where the experimental data were obtained under a wide variety of experimental parameters.4,11

The experimental results for copper oxide particle growth under different experimental conditions are summarized in Figure 1. Several conclusions can be drawn from this figure.

© 2014 American Chemical Society

Figure 1. Average particle size of the products as a function of laser fluence obtained by laser irradiation with the wavelength of 355 nm or 532 nm on Cu nanoparticles dispersed in liquid.4 (1) acetone, λ = 355 nm; (2) acetone, λ = 532 nm; (3) H2O, λ = 355 nm; and (4) H2O, λ = 532 nm. Received: December 6, 2013 Revised: February 5, 2014 Published: February 6, 2014 4495

dx.doi.org/10.1021/jp411958v | J. Phys. Chem. C 2014, 118, 4495−4500

The Journal of Physical Chemistry C

Article

First, the laser fluence threshold for spherical particle formation is observed. Spherical particle growth started only when the laser fluence exceeded threshold. The threshold itself depends on the laser wavelength but not on the liquid media (water or acetone). Above the threshold, the average diameter of spherical particles increased with the laser fluence increase. At the same laser fluence, the average particle size is larger for acetone than for water. When copper oxide particles were irradiated in acetone by the second harmonic at 532 nm, the second process, rapid size reduction, was observed at high (nearly maximum) laser fluences. The time evolution of spherical particle formation was also studied.11 Large particles were produced very rapidly, but most of these particles were not spherical. For example, for acetone, λ = 355 nm, and J = 67 mJ·pulse−1·cm−2, many large particles were observed after 10 s of irradiation. After 60 s of irradiation, most of the particles were large, but their shape was still irregular. The process was practically completed after 5 min, when all the particles became spherical. Basically, the process accelerated when the laser fluence increased. For example, at J = 133 mJ·pulse−1·cm−2, a very short irradiation time (30 s) led to the formation of uniform colloid spheres. As the result of these experiments near monodisperse spherical particles with submicrometer sizes were formed instead of strongly agglomerated initial particles with irregular size and shape, as it is shown in Figure 2.

3. MODEL To understand the mechanism of particle growth, first we have to define the absorption mechanism of laser pulse electromagnetic energy by colloidal particle, and then to consider how this energy is redistributed between a particle and the surrounding media. As shown experimentally12 and theoretically,12,13 it is possible to suppose that the particle heating−melting− evaporation mechanism2,12−14 could be applicable for a nanosecond laser-colloidal nanoparticle interaction process (this subject was discussed in more detail in our previous paper13). The main statement of this model is that all energy absorbed by a particle from the laser pulse is spent for the particle heating−melting−evaporation process. Notably, that this statement is not just postulated. As shown in our previous papers,12,13 the typical times needed for particle cooling and solidification processes are much longer than the duration of one pulse, therefore possible heat losses are negligible during particle heating. However, these particle cooling and solidification times are much shorter than the time between two consecutive laser pulses. This permits us to make all necessary calculations for one individual laser pulse. Therefore, according to the model, the energy absorbed by a particle from the individual laser pulse is spent for particle heating−melting− evaporation process. If the amount of absorbed energy is rather small, then only particle heating can be expected. λ Jσabs = mp

∫T

T

c ps(T ) dT

0

Figure 2. Morphological changes of CuO particles produced by laser irradiation. (a) Raw CuO particles. (b) After 10 min pulse laser irradiation by the second harmonic (532 nm) of an Nd:YAG laser. The inset indicates particle size distribution. Reprinted from ref 4 with the permission of Wiley-VCH.

If the absorbed energy is even higher, then the particle evaporates. λ Jσabs = mp

∫T

Tm

0

c ps(T ) dT + mpΔHm

Tm

c ps(T ) dT + mpΔHm

0

+ mp

∫T

Tb

m

c pl(T ) dT + mpΔHev

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Here, J = E0/S0 is the laser fluence of laser beam with pulse energy, E0, and cross section, S0; σλabs is the particle absorption cross section; csp and clp are the particle heat capacities in solid and liquid states; mp = ρp ((πd3p)/(6)) is the particle mass, T0 is the particle initial temperature, Tm is the melting temperature, Tb is the boiling temperature, ΔHm is the heat of melting, and ΔHev is the heat of evaporation. Using this approach, we can calculate the critical values of laser fluence needed for different phase transitions of a material (e.g., melting and evaporation). Plotting these laser fluences against particle diameter, we produce a new type of phase diagrams. The curves Ji(dp) in such diagrams represent the boundaries between different phases (solid, solid + liquid, liquid, liquid + gas, gas). In other words, these curves represent the start and completion of possible phase transition processes (melting, evaporation).

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With more absorbed energy, melting occurs. λ Jσabs = mp

∫T

(2) 4496



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Now we will demonstrate how these phase diagrams can be very helpful in the analysis of the processes occuring between the colloidal particles and pulse laser irradiation.

4. MECHANISM OF PARTICLE GROWTH The optical and thermodynamic properties of copper oxide are well-known. Table 1 lists the refractive indices and extinction Table 1. Optical Properties of Copper Oxide, CuO (Adopted from Ref 15) n k

λ = 355 nm

λ = 532 nm

λ = 1064 nm

2.2489 1.0069

2.5648 0.6222

2.6217 0.0025

coefficients for three main wavelengths of an Nd:YAG laser (fundamental, second, and third harmonics). These data were taken from Palik’s reference book.15 Using these n,k values, the absorption cross sections or absorption efficiencies can be calculated by Mie theory16−18 as a function of particle diameter for each of the three wavelengths. All thermodynamic data needed for further calculation of phase diagrams can be found in the JANAF Tables19 and listed in Table 2. In contrast with Table 2. Thermodynamic and Chemical Processes in the Cu−O System with CuO as Initial Material ΔH (T) reaction CuO decomposition Cu2O melting Cu2O decomposition and Cu evaporation

process 2CuO(c) = Cu2O(c) +1/2O2 Cu2O(c) = Cu2O(l) Cu2O(l) = 2Cu(g) + 1/2O2

T (K)

kJ/mol

1397

67

1517 2850

64.8 705.5

Figure 3. Phase diagram calculated for cupric oxide CuO at two different wavelengths. J1, CuO decomposition start; J2, CuO decomposition complete; J3, Cu2O melting start; J4, Cu2O melting complete; J5, Cu evaporation start; and J6, Cu evaporation complete.

at 40−45 mJ·pulse−1·cm−2 if we use a third harmonic (λ = 355 nm) and at 120−130 mJ·pulse−1·cm−2 for the second harmonic (λ = 532 nm) (Figure 3). At the same time, the experimental results indicate that spherical particles formed at lower laser fluences: 33 mJ·pulse−1·cm−2 for the third harmonic and 50 mJ· pulse−1·cm−2 for the second harmonic. This difference can be explained by the agglomeration of primary particles. To prove that, we measured the UV−vis absorption spectrum for primary raw particles and recalculated the spectrum of absorption efficiency, Qabs(λ) = ((4σλabs)/(πd2p)), from these experimental results. The result is presented in Figure 4, along with the results of Mie calculations performed for monodisperse spherical CuO particles of different diameters close to the average size of primary particles, d̅p = 34 nm. Apparently, the Qabs(λ) curve (black experimental curve) shown in Figure 4 is red-shifted compared with the theory curves (red, green, blue curves). Due to this shift, the absorption efficiency of agglomerated particles increased; therefore, the critical laser fluences needed for particle melting decreased. The UV−vis absorption spectrum was measured under stationary conditions without stirring. Stirring in real particle formation experiments inhibits particle agglomeration, but cannot completely suppress it. Moreover, due to stirring, the agglomeration has a very dynamic character. Every moment, some agglomerates are destroyed by shear stress produced by stirring. Deagglomerated products (e.g., individual particles and smaller agglomerates), start to agglomerate again, producing new larger agglomerates.

the noble metals, where only two simple phase transitions (melting and evaporation) occur, copper oxide has an additional phase transition, thermal decomposition of copper oxide (II) to copper oxide (I) at 1397 K. As a consequence, the phase diagram for CuO (Figure 3) contains one additional phase, Cu2O, and two additional phase boundary curves, J1 and J2, corresponding to the start and end of decomposition. Also Cu2O evaporates noncongruently in the form of metal Cu at 2850 K, which is very close to the boiling temperature of metal copper. Curves J2 and J3 are very close to each other (Figure 3); thus, the region where the Cu2O(s) phase exists is very narrow, due to the small difference between CuO decomposition temperature (Td(CuO) = 1397 K) and Cu2O melting point (Tm(Cu2O) = 1517 K). Therefore, the energy needed to heat Cu2O(s) from 1397 to 1517 K is much less than all other specific energy values needed for other phase transitions. Next, we attempt to construct a mechanism of large spherical particle formation based on the above results. First, it is rather obvious that the formation of a perfect spherical shape is the result of fast melting and solidification of a particle in a liquid environment where there is no net force acting on it, and therefore the particle shape after melting must be spherical. However, the rapid size reduction of the particles under high laser fluence is the result of particle evaporation. This corresponds well with our previous results.14,20 But it is impossible to explain spherical particle formation just by melting the raw particle. Indeed, according to our calculations, the melting of raw particles with an average size of 34 nm starts 4497



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relatively slow growth and spherical shape formation of those large particles (Scheme 1(b)). The process is complete when none of the particles can be melted by this particular laser fluence. According to our model, the diameter of a spherical particle that can be melted by absorbing the laser pulse increases with an increase in laser pulse fluence (curves J3 and J4 in Figure 3). This qualitatively explains the increase in average particle size with the increase in laser fluence observed in experiments (Figure 1). To quantitatively compare the experimental results with the calculated results, some corrections are necessary. The phase diagram in Figure 3 is absolutely correct when CuO particles were irradiated by one individual laser pulse. As the experimental result shown in Figure 5 was obtained via 10 Figure 4. Absorption efficiency experimentally obtained from UV−vis absorption spectra measured for CuO colloid and efficiencies calculated by Mie theory for CuO spherical particles of different sizes in water.

In each nanosecond pulse, some of these agglomerates melt and become relatively large particles after fast solidification. If the agglomerate absorbs enough heat from a laser beam for complete melting, then the particles become spherical; if not, then their shape is irregular. Thus, at the beginning of the irradiation experiment, we have some relatively large particles in a pool of primary particles. Agglomeration then continues, due to Brownian motion of the particles. The primary particles form new agglomerates and are aggregated with already formed large particles. Thus, agglomeration is responsible for the initial fast formation of relatively large particles (Scheme 1(a)). After this Scheme 1. Illustration of the Process of Large Particle Formation

Figure 5. Boundary curves for start of melting process calculated for CuO and Cu2O particles at the wavelength of (a) 355 nm and (b) 532 nm. The average diameters of spherical particles produced at specific fluences (Experimental Results) are plotted in the same graphs.

min irradiation under strong magnetic stirring, this long time irradiation is sufficient for all particles to be illuminated at the top area of the solution, thus making the size of spherical particles relatively homogeneous by approaching the size that corresponds to input laser fluence (fluence at topmost of the solution). Therefore, we do not consider the influence of the laser fluence distribution along the path of the laser beam on formed spherical particles size when we compared the theory with experiments. In addition, we experimentally found that even if the raw materials have broad size distribution, then the resultant spherical particles will have relatively homogeneous size after long time irradiation. This is because the final particle size is determined by the input laser fluence. Thereby, we do not consider this factor in calculation. For any particular particle size, CuO thermal decomposition, Cu2O melting, and evaporation in the form of Cu occur, depending on laser fluence. As discussed above, the melted particle has enough time to recrystallize before the next laser pulse. However, the chemical composition of the particle (CuO or Cu2O) depends on the oxygen concentration inside the particle. If all oxygen released during decomposition remains

initial period, practically all the primary particles are absorbed by larger particles. Because the total mass or total volume of the particles is constant, the particle number concentration decreases during this period. As the particles become larger and their concentration decreases compared with particles in the initial period, the probability of simultaneous collisions of more than two particles decreases dramatically. If two particles collide at the moment of laser pulse irradiation, then they couple together during melting and form one particle after solidification. Depending on the laser fluence, this particle may or may not be spherical, and subsequent pulse irradiation may be necessary to make it spherical. Therefore, after the initial fast formation of large particles, there is a subsequent period of 4498



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determined by the value of laser fluence used in the experimentthe larger the laser fluence, the larger the maximum diameter of the particle that can be melted. Therefore, the size of spherical particles produced in these pulse laser irradiation experiments is determined by the value of the laser fluence, and can be turnable by varying the laser fluence.

inside the particle, then Cu2O is oxidized to CuO again during particle cooling. Thus, we again have a CuO particle for next laser pulse, and the process is repeated. However, if all the oxygen diffuses from the particle very rapidly before particle cooling, oxidation does not occur, and for the next laser pulse, we have a Cu2O particle, not a CuO particle. A new phase diagram for the Cu2O particle must then be calculated. The diffusion time of oxygen from the particle estimated using the general diffusion equation. tdif =

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Corresponding Authors

d p2 Ddif

AUTHOR INFORMATION

*Tel: +81-298-614411; e-mail: [email protected]. *Tel: +49-331-5679256; e-mail: Hongqiang.Wang@mpikg. mpg.de.

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Using the diffusion coefficient from classic work21 indicates that this diffusion time is one or two orders longer than the particle’s temperature relaxation time (see refs 12 and 13). Thus, most of the oxygen released during the decomposition reaction remains inside the particle’s core and oxidizes Cu2O to CuO. However, the oxygen diffuses completely outward from the particle’s outer layer with a thickness of about 10% of the particle diameter. Therefore, a particle irradiated by one laser pulse has a core−shell structure, with a thick CuO core and a thin Cu2O shell. In the next pulses, the shell layer can grow thicker, and the chemical composition of the particle gradually shifts from CuO to Cu2O. An actual situation looks even more complicated because the oxygen diffused from the particle to liquid can diffuse inward in the particle again and cause oxidation. However, if the process is carried out in acetone (or another organic solvent), then the particle’s outer layer can be reduced by the pyrolyzed products of acetone at high temperatures. The results of our measurements indicate that the finally produced spherical particles have a chemical composition intermediate between CuO and Cu2O for most experimental conditions. On the basis of this fact, we calculate the boundary conditions J3(dp) corresponding to the start of particle melting for both copper oxides, CuO and Cu2O (Figure 5), and compare them with the experimental results indicating the average diameter of the spherical particles produced at different laser fluences. Experimental points were taken from Figure 1 and replotted in Figure 5. Two theoretical curves indicate the boundaries between S and S+L phases for CuO and Cu2O. Several initial experimental points measured when the laser fluences were below the threshold needed for particle growth correspond to the solid phase of Cu2O, when the Cu2O shell had not yet started melting. All other experimental points lay between the two theoretical curves, corresponding to the solid CuO core and the liquid Cu2O shell. The results confirm the growth mechanism of spherical particles discussed above, and therefore the particle heating−melting−evaporation mechanism. Figure 5 illustrates again that the spherical particles produced in acetone are on average larger than those produced in water. More experiments are needed to clarify these results. However, one possible reason is the different abilities for the particle to agglomerate in different liquids.

Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kamat, P.; Flumiani, M.; Hartland, G. Picosecond Dynamics of Silver Nanostructures. Photoejection of Electrons and Fragmentation. J. Phys. Chem. B 1998, 102, 3123−3128. (2) Takami, A.; Kurita, H.; Koda, S. Laser-Induced Size Reduction of Noble Metal Particles. J. Phys. Chem. B 1999, 103, 1226−1232. (3) Ishikawa, Y.; Feng, Q.; Koshizaki, N. Growth Fusion of Submicron Spherical Boron Carbide Particles by Repetitive Pulsed Laser Irradiation in Liquid Media. Appl. Phys. A: Mater. Sci. Process. 2010, 99, 797−803. (4) Wang, H.; Pyatenko, A.; Kawaguchi, K.; Li, X.; SwiatkowskaWarkocka, Z.; Koshizaki, N. Selective Pulsed Heating for the Synthesis of Semiconductor and Metal Submicrometer Spheres. Angew. Chem., Int. Ed. 2010, 49, 6361−6364. (5) Wang, H.; Koshizaki, N.; Li, L.; Jia, L.; Kawaguchi, K.; Li, X.; Pyatenko, A.; Swiatkowska-Warkocka, Z.; Bando, Y.; Golberg, D. SizeTailored ZnO Submicrometer Spheres: Bottom-Up Construction, Size-Related Optical Extinction, and Selective Aniline Trapping. Adv. Mater. 2010, 23, 1865−1870. (6) Li, X.; Pyatenko, A.; Shimizu, Y.; Wang, H.; Koga, K.; Koshizaki, N. Fabrication of Crystalline Silicon Spheres by Selective Laser Heating in Liquid Medium. Langmuir 2011, 27, 5076−5080. (7) Li, X.; Shimizu, Y.; Pyatenko, A.; Wang, H.; Koshizaki, N. Carbon-Assisted Fabrication of Submicrometre Spheres for LowOptical-Absorbance Materials by Selective Laser Heating in Liquid. J. Mater. Chem. 2011, 21, 14406−14409. (8) Wang, H.; Miyauchi, M.; Ishikawa, Y.; Pyatenko, A.; Koshizaki, N.; Li, Y.; Li, L.; Li, X.; Bando, Y.; Golberg, D. Single-Crystalline Rutile TiO2 Hollow Spheres: Room-Temperature Synthesis, Tailored Visible-Light-Extinction, and Effective Scattering Layer for Quantum Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 19102−19109. (9) Swiatkowska-Warkocka, Z.; Kawaguchi, K.; Shimizu, Y.; Pyatenko, A.; Wang, H.; Koshizaki, N. Synthesis of Au-Based Porous Magnetic Spheres by Selective Laser Heating in Liquid. Langmuir 2012, 28, 4903−4907. (10) Swiatkowska-Warkocka, Z.; Koga, K.; Kawaguchi, K.; Wang, H.; Pyatenko, A.; Koshizaki, N. Pulsed Laser Irradiation of Colloidal Nanoparticles: A New Synthesis Route for the Production of NonEquilibrium Bimetallic Alloy Submicrometer Spheres. RSC Adv. 2013, 3, 79−83. (11) Wang, H.; Kawaguchi, K.; Pyatenko, A.; Li, X.; SwiatkowskaWarkocka, Z.; Katou, Y.; Koshizaki, N. General Bottom-Up Construction of Spherical Particles by Pulsed Laser Irradiation of Colloidal Nanoparticles: A Cause Study on CuO. Chem.Eur. J. 2012, 18, 163−169. (12) Pyatenko, A.; Yamaguchi, M.; Suzuki, M. Mechanisms of Size Reduction of Colloidal Silver and Gold Nanoparticles Irradiated by Nd:YAG Laser. J. Phys. Chem. C 2009, 113, 9078−9085. (13) Pyatenko, A.; Wang, H.; Koshizaki, N.; Tsuji, T. Mechanism of Pulse Laser Interaction with Colloidal Nanoparticles. Laser Photonics Rev. 2013, 7, 596−604.

5. CONCLUSIONS In summary, we demonstrated that the growth of nearly monodisperse spherical particles under pulse laser irradiation is the result of repeated particle melting and solidification. Agglomeration of relatively small primary particles is responsible for the fast initial stage of the particle growth. The process of the particle growth then continues as long as the particles can be melted. The particle melting process is, in turn, 4499



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(14) Pyatenko, A.; Yamaguchi, M.; Suzuki, M. Synthesis of Spherical Silver Nanoparticles with Controllable Sizes in Aqueous Solutions. J. Phys. Chem. C 2007, 111, 7910−7917. (15) Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press, Inc.: New York, 1985. (16) Mie, G. Beiträge zur Optik Trüber Medien, speziell Kolloidaler Metallösungen. Ann. Phys. 1908, 330, 377−445. (17) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles, second ed.; Wiley: New York, 1998; p 530 . (18) Kreibig, U.; Volmer, M. Optical Properties of Metal Clusters; Springer Series in Material Science 25; Springer: Berlin, 1995. (19) JANAF Thermochemical Tables, 1974, Supplements. (20) Pyatenko, A.; Yamaguchi, M.; Suzuki, M. Laser Photolysis of Silver Colloid Prepared by Citric Acid Reduction Method. J. Phys. Chem. B 2005, 109, 21608−21611. (21) Moore, W.; Ebisuzaki, Y.; Sluss, J. Exchange and Diffusion of Oxygen in Crystalline Cuprous Oxide. J. Phys. Chem. 1958, 62, 1438− 1441.

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