UV Photoactivation for Size and Shape Controlled Synthesis and

Formation of stable gold nanoparticles of different size and shape has been reported via UV photoactivation of HAuCl4 in variable concentrations of TX...
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Articles UV Photoactivation for Size and Shape Controlled Synthesis and Coalescence of Gold Nanoparticles in Micelles Madhuri Mandal,† Sujit Kumar Ghosh,† Subrata Kundu,† K. Esumi,‡ and Tarasankar Pal*,† Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India, and Department of Applied Chemistry, Science University of Tokyo, Tokyo 162-8601, Japan Received December 14, 2001. In Final Form: July 30, 2002 Formation of stable gold nanoparticles of different size and shape has been reported via UV photoactivation of HAuCl4 in variable concentrations of TX-100. It has been shown that smaller particles of gold preferentially dissolved by cyanide at a faster rate in air in comparison to that for the larger particles because of higher mobility and possibly because of lower redox potentials of the smaller particles. It is observed that smaller gold nanoparticles preferentially coalesce together at higher temperature in micelles. Temperature induced changes in the shape of the micelle provide the required template/capping condition for linear growth of gold nanoparticles.

1. Introduction Nanoparticle research has become the subject of novel interest in science and technology since the 1970s. Particles in the nanoregime are of immense importance due to their potential applications in the fields of physics, chemistry, biology, medicine, and material science and their different interdisciplinary fields. These particles of “negligible dimension” exhibit special properties in many aspects compared to those of their bulk samples, for example, catalysis,1 size and shape dependent optical properties,2 electronic properties,3 medicinal applications,4 applications in optical devices,5 and so forth. Particularly, semiconductor6 and noble metal nanoparticles7,8 have been areas of research interest due to their inherent physical, chemical, and optoelectronic properties. Metallic nanoparticles, especially the coinage metals, have mainly been studied because of their strong optical absorption in the visible region, and they have shown promise in catalysis1 and SERS studies.9 The λmax and width of the plasmon * E-mail: [email protected]. Phone: 91-03222 83320. Fax: 91-03222-755303. † Indian Institute of Technology. ‡ Science University of Tokyo. (1) (a) Pradhan, N.; Pal, A.; Pal, T. Langmuir 2001, 17, 1800. (b) Jana, N. R.; Pal, T. Langmuir 1999, 15, 3458. (c) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103, 115. (2) (a) Robert, H. D.; Rao, P. J. Mater. Res. 1996, 11, 2834. (b) Henglein, A. Isr. J. Chem. 1993, 33, 77. (c) Jana, N. R.; Gearhert, L.; Murphy, C. J. Chem. Commun. 2001, 617. (3) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (4) (a) Falkenhagen, D. Artif. Organs. 1995, 19, 792. (b) Polato, L.; Benedetti, L. M.; Callegaro, L. J. Drug Targeting 1994, 2, 53. (5) Gaponik, N. P.; Talapin, D. V.; Rogach, A. L. J. Mater. Chem. 2000, 10, 2163. (6) Beydonn, D.; Amal, R.; Low, G.; McEvoy, S. J. Nanopart. Res. 1999, 1, 439. (7) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (8) Kreibig, U.; Vollmer, M. Optical properties of metal clusters; Springer: Berlin, 1995. (9) Pal, T.; Jana, N. R.; Pal, A.; Creighton, J. A.; Beezer, A. E. J. Indian Chem. Soc. 2000, 77, 34.

absorption band of the nanoparticles have been shown to depend on the particle size and shape and also on the nature of the adsorbate. Many theories have been put forward to explain the observed phenomenon.8 Several workers have demonstrated a large number of routes for the preparation of metal nanoparticles, for example, chemical reduction,10 photoactivation,11 γ-radiolysis,12 laser pulse methods,13 sonochemical methods,14 and so forth. But to realize the full potential of the metal nanoparticles, the first and foremost necessity is to design synthetic methodologies to control their size and shape, and it has now become a preparative challenge to the material scientists. Templates, matrixes, and LB films can be used to synthesize nanoparticles of various size and shape. Various types of templates, such as porous alumina membranes,15 porous carbonate membranes,16 lithographically processed masks,17 micelles,18 different ligands, organic polymers,19 and so forth, are used to control the size and shape of nanoparticles. However, the (10) Frens, G. Nature 1972, 20, 241. (11) Pal, A. Talanta 1998, 46, 583. (12) (a) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 903. (b) Henglein, A. Chem. Rev. 1989, 89, 1861. (c) Belloni, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 184. (d) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (13) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. 1988, 92, 531. (14) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (15) (a) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (b) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (16) Schonenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kruger, M.; Bachtold, A.; Huber, R.; Birk, H.; Staufer, U. J. Phys. Chem. B 1997, 101, 5497. (17) Gotschy, W.; Vonmetz, K.; Litner, A.; Aussenegg, F. R. Opt. Lett. 1996, 21, 1099. (18) Bonnemann, H.; Binkmann, R.; Neileler, P. Appl. Organomet. Chem. 1994, 8, 361. (19) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157.

10.1021/la0118107 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/19/2002

Gold Nanoparticles in Micelles

nucleation and growth mechanism of particles in different templates is still a matter of dispute. In this article we have addressed successfully the use of micelles as template/capping agents, as they have an advantage due to their stabilizing property as well as the growth controlling capacity for the aggregation of particles.20 Of all the methodologies developed for the preparation of metal nanoparticles in either physical or chemical ways, the photoactivation technique offers a very simple way by which almost monodispersed (tight size distribution) metal nanoparticles can be generated.11 We have exploited this technique to prepare gold nanoparticles of different size and shape in TX-100 [poly(oxyethylene) isooctyl phenyl ether] surfactant by varying its concentration. Here TX-100 serves the purpose of a reducing agent (triggered by UV light) and simultaneously acts as a stabilizer and provides the required template for linear aggregation of gold nanoparticles. This method does not require any harsh reducing agent that might increase the local concentration of any reagent in solution during addition and requires no manipulative skill; hence, it is reproducible,21 unlike the preparation of nanoparticles by the wet chemical method. 2. Experimental Section 2.1. Reagents. All reagents were of AR grade. A stock solution of gold(III) chloride was prepared by dissolving 1 g of chloroauric acid (Johnson Matthey, Royston, Hestfordshire, U.K.) in 500 mL of double-distilled water, and it was standardized by the quinol method.22 A TX-100 solution of 0.1 mol dm-3 was prepared in double-distilled water and diluted to different concentrations as necessary. A sodium cyanide (NaCN) (M & B, India) solution of 10-2 mol dm-3 was used. 2.2. Instruments. All absorbance measurements were carried out in a Shimadzu UV-160 digital spectrophotometer. Photoirradiations were carried out with a photoreactor fitted with ordinary germicidal lamps of wavelength 365 nm (Sankyo, Denki, Japan) of UVC G8 T5. The photoreactor can produce a variable flux from 100 to 850 Lux. The flux was monitored using a Digital Lux Meter (model LX-101), Taiwan. The photoreactor (intensity of the light) was calibrated with an Ophir power meter (NOVA display and 30-A-SH sensor). The number of photons absorbed per unit volume of the sample per second from the photoreactor of 100 Lux is 3.03 × 1015. Photochemical reactions were carried out in a 1 cm, well-stoppered quartz cuvette. The cell was kept in an upright position and 3 cm from the light source for UV irradiation. TEM and EDX analysis were performed with a Hitachi S-4300, using the voltage 100 kV, by evaporating the solution on a carbon coated copper grid. 2.3. Preparation of Metal Nanoparticles of Different Size and Shape. An aliquot of a standard HAuCl4 solution and TX100 solution were mixed together, varying their concentrations. Thus, five sets of mixtures were prepared. In set I, the final concentration of Au(III) ion was 9.5 × 10-4 mol dm-3 and the concentration of TX-100 was 8.1 × 10-2 mol dm-3. In the other four sets (II, III, IV, and V), the concentration of Au(III) ion was 3.9 × 10-5 mol dm-3 and the concentration of TX-100 was varied. The concentration of TX-100 was adjusted to 9.9 × 10-2, 9.9 × 10-3, 1.6 × 10-4, and 7.9 × 10-6 mol dm-3 for sets II, III, IV, and V, respectively. All the solutions were taken separately in a 1 cm quartz cuvette and irradiated for 25 min (under UV light of wavelength ∼ 365 nm) in the photoreactor. After photoirradiation, set I became red, sets II, III, and IV turned pink, and for set V a faint blue solution was produced. The characteristic plasmon absorption band for all the solutions was measured in the spectrophotometer. (20) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (21) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 9266. (22) Vogel, A. I. A Text Book of Quantitative Inorganic Analysis; Longman: London, 1973; p 464.

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Figure 1. Plasmon absorption for gold nanoparticles of different sizes. Conditions: [TX-100] ) 9.9 × 10-2, 9.9 × 10-3, 1.6 × 10-4, and 7.9 × 10-6 mol dm-3 for sets II, III, IV, and V, respectively. [Au] ) 3.9 × 10-5 mol dm-3 for each set. Irradiation time ) 25 min. Reduction is complete.

3. Results and Discussion 3.1. Size and Shape Controlled Synthesis of Gold Nanoparticles. Redox reactions between a Au(III) complex and air saturated aqueous solutions of poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA) have recently been reported,23 where Au(III) is converted to a pink gold sol by PEG and PVA. This occurs at room temperature, but at a slow rate, requiring ∼8 h for PEG and ∼6 days for PVA. The fact that PEG polymers are oxidized to acetoxy compounds has been confirmed from infrared (IR) spectra. Recently, the photochemical reduction of aqueous HAuCl4 in the presence of 2-propanol and acetone has been reported24 using a steady state xenon lamp or repetitive laser pulses to synthesize useful nanoscale gold particles, and sodium polyphosphate was used as a particle stabilizer in this system. We have recently demonstrated the seed-mediated successive growth of different nanoparticles in solution by a wet chemical technique.25 There, for the successive growth of particles, different ratios of seed and corresponding metal ion solutions were taken for the autocatalytic growth. The merit of photoreduction of HAuCl4 has been exploited here in TX-100 micelles. The micelle stabilizes the metal particles in the usual way and offers a control over the size and shape of metal particles during the particle growth. TX-100, having an alcoholic skeleton, reduces Au(III) ions without increasing the local concentration of the added reducing agent.26 Thus, the photoirradiation gives elegant reproducibility with homogeneous flux of the light, unlike the case of the wet chemical technique. The kinetics of the growth of particles in a TX-100 micellar medium have also been studied. It has been shown that the size of the particles could be altered by changing the concentration of TX-100 while keeping the gold concentration fixed. Taking the concentration of TX-100 as 9.9 × 10-2, 9.9 × 10-3, 1.6 × 10-4, and 7.9 × 10-6 mol dm-3 in different sets but maintaining the concentration of HAuCl4 at 3.9 × 10-5 mol dm-3 in each set, we obtained particles of average sizes 3 ( 0.21, 4.5 ( 1.5, 7 ( 1.8, and 12 ( 2 nm, respectively. In Figure 1, the plasmon absorption band is shown for Au(0) sol prepared in different surfactant concentrations. The λmax (23) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (24) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. 1991, 92, 31. (25) Pradhan, N.; Jana, N. R.; Mallick, K.; Pal, T. J. Surf. Sci. Technol. 2000, 16, 188. (26) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 1257.

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Figure 2. Transmission electron micrograph (TEM) and corresponding histogram of varied size gold nanoparticles (A, B, C, and D for sets II, III, IV, and V, respectively). Conditions: [TX-100] ) 9.9 × 10-2, 9.9 × 10-3, 1.6 × 10-4, and 7.9 × 10-6 mol dm-3 for sets II, III, IV, and V, respectively. [Au] ) 3.9 × 10-5 mol dm-3 for each set. Irradiation time ) 25 min.

values have been observed in the range 523-545 nm, indicating the formation of Au(0) nanoparticles. The plasmon band due to gold particles red shifted and became wider with the decrease in concentration of TX-100, similar to what has been reported by Belloni et al.27 From this characteristic plasmon band we presume that particle size decreases with the increase in concentration of TX-100, which has been authenticated from TEM measurements (Figure 2). The size distributions of the particles for different sets are also shown in Figure 2. EDX analysis revealed the presence of gold particles. Keeping the Au(III) ion concentration unaltered, one can have gold particles of larger dimensions with the lowering of surfactant concentration. This clearly tells that the particle growth becomes facile in the presence of dilute surfactant concentrations, that is, under less restricted conditions. This may be due to the easy passage of Au(III) ion by diffusion toward the already produced Au(0) nanoparticles in less concentrated surfactant. If the concentration of surfactant falls below the cmc, polydispersity of particles was always observed. A higher concentration of the surfactant (∼0.1 mol dm-3) acts as a template/capping agent for the preparation of different shaped gold nanoparticles. Using higher concentrations of TX-100 (8.1 × 10-2 mol dm-3) and keeping the concentration of HAuCl4 constant (9.5 × 10-4 mol dm-3) (set I), we obtained gold particles of different shapes, such as rodlike, cubic, and tetragonal, along with spherical particles (Figure 3). This is due to the fact that successive growth of gold nanoparticles has a natural tendency to be haphazard; that is, the growth of the particles takes place in such a fashion that larger particles of various shapes (spherical, triangular, rodlike, cubic) are obtained. This (27) Francois, L.; Mostafavi, M.; Belloni, J. J. Phys. Chem. B 2000, 104, 6133.

Figure 3. Transmission electron micrograph (TEM) of different shaped gold nanoparticles (set I). Conditions: [TX-100] ) 8.1 × 10-2 mol dm-3. [Au] ) 9.5 × 10-4 mol dm-3. Irradiation time ) 25 min.

happens at room temperature when there remains a large concentration of Au(III) ions in the micelle. The problem of TEM of the Au particles at this high concentration of surfactant was noticed and was solved by centrifugation. Repetitive centrifugation resulted in separation of smaller particles from the larger, as has been demonstrated.28 Here, for set I, different types of particles are shown before their separation.

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Figure 5. Plasmon absorption band for gold sol (a) before addition of NaCN, (b) after addition of NaCN and shaking for 10 min, and (c) after shaking for more than 30 min. Conditions: [TX-100] ) 8.9 × 10-3 mol dm-3. [Au] ) 5.7 × 10-4 mol dm-3. [NaCN] ) 4.0 × 10-5 mol dm-3.

Figure 4. Graphical representation of the kinetics of formation of gold sol for (A) set I and (B) set II. Conditions: For set I, [TX-100] ) 8.1 × 10-2 mol dm-3, and [Au] ) 9.5 × 10-4 mol dm-3. For set II, [TX-100] ) 9.9 × 10-2 mol dm-3, and [Au] ) 3.9 × 10-5 mol dm-3. Successive irradiation of 1 min was done for each measurement.

The mechanism of formation of gold particles can be formulated as follows. The trivalent gold from AuIII Cl4is reduced by the hydroxymethyl radical generated by the photolysis of TX-100 [R-0-CH2CH2OH where R ) (CH3)3CCH2C(CH3)2C6H4(OCH2CH2)∼9-] into Au(II). The next step is the fast disproportionation to Au(I) and Au(III). Then, accumulated aurous ions Au(I) are reduced by the hydroxymethyl radical to Au(0), and the primary hydroxyl function of TX-100 is oxidized to the carboxylate group, which has been authenticated from IR spectra.11 The atoms are formed with a homogeneous distribution throughout the solution. The atoms, then, tend to dimerize when encountered in pairs or associated with excess ions. But the surfactant molecules inhibit this association through the capping/template effect and thus act as a particle stabilizer.26 Detailed kinetics for gold particle formation for the representative solutions (sets I and II) were studied with successive irradiations at intervals of 1 min in each case. The mixture was successively irradiated for 1 min in each step, and absorbance was measured as rapidly as possible. The kinetics for set I followed a sigmoidal curve whereas for set II the curve was an exponential one (Figure 4). The sigmoidal nature of the curve speaks for the autocatalytic growth of Au(0) particles, and the rate goes through a maximum that is reflected by the point of inflection on the curve.29 The exposure time for complete reduction of all the Au(III) ions in TX-100 was 25 min for set I and 15 (28) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (29) Schmid, R.; Sapunav, V. N. Nonformal Kinetics; 1981; p 54.

min for set II under 600 Lux. However, a higher irradiation time (∼25 min, say, for set II) did not change the absorption profile. The complete reduction of Au(III) ion in TX-100 solution was confirmed from the constancy of the absorbance values for the exposed solution. However, for higher concentrations of gold, the constancy of the absorbance values could not be recorded in a straightforward way because of the spectrophotometric sensitivity. The constancy in the absorbance was checked after dilution of a fixed amount of an aliquot of the exposed solution. Once the reduction of Au(III) was complete, the particles remained stable in micelles for months. Next, we have studied the formation of gold particles from a well stirred solution containing 3.9 × 10-5 mol dm-3 HAuCl4 in 10-2 mol dm-3 TX-100 and compared the results with that of an unstirred solution. In both the cases there was no change in the absorption spectra, which tells us that stirring has no effect. This might be due to the fact that stirring was not vigorous enough to make a real difference in mass transport. 3.2. Study on the Effect of Cyanide Ion on the Au(0) Sol. To study the effect of cyanide ions on the Au sol, another set, VI, was prepared with a moderately higher concentration of gold. In this set, TX-100 (8.9 × 10-3 mol dm-3) and the Au(III) solution (5.7 × 10-4 mol dm-3) were taken and irradiated for 25 min, and the absorbance was measured. A shoulder in the spectrum was observed (Figure 5a). Then 10 µL of NaCN (final concentration in the mixture 5 × 10-5 mol dm-3) was added to this solution, the mixture was shaken well in air for 10 min, and then the spectrum was measured. The peak height decreased with the shift of the shoulder peak to higher wavelength, and the solution acquired a bluish tinge. This solution on further shaking for another 30 min became blue, giving a larger red shifted absorbance peak with lowering of the absorbance (Figure 5). Under ambient conditions, that is, in air, oxygen helps the dissolution of gold nanoparticles in the presence of cyanide.30,31

4Au(0) + 16NaCN + 3O2 + 6H2O f 4Na[Au(CN)4] + 12NaOH (1) The reaction with NaCN speaks for the dissolution of bulk metal also. Though the reaction in either case remains the same, while it is the case of a gold nanoparticle, the situation becomes interesting if the dissolution is monitored spectrophotometrically. Here cyanide ion affects the (30) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (31) Pal, T.; Ganguly, A.; Maity, D. S. Anal. Chem. 1986, 58, 1564.

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plasmon band in two different ways: first, as a strong nucleophile, it is adsorbed onto the surface of the nanoparticles. This should shift the plasmon peak position to the blue region. Second, NaCN as an added electrolyte should help the aggregation of gold particles and this effect should be reflected by the red shift of the plasmon peak.32 In the absence of air/oxygen, a dilute cyanide ion concentration induces the nucleophilic action (i.e., blue shift) only. On the other hand, with a higher dose of cyanide under ambient conditions, generally the electrolytic effect becomes predominant and a red shift is observed.33 However, the electrolytic effect is not evidenced while the nanoparticles are stabilized in a higher concentration (g10-1 mol dm-3) of TX-100. Control over the dissolution reaction was achieved either by controlling the supply of NaCN solution or by limiting the supply of air by purging N2 through the solution. In both cases, it has been observed that smaller particles of gold have a tendency to be dissolved at the very beginning in comparison to the larger particles. It may be stated that smaller particles are more prone to dissolution because they are large in number and are highly mobile and possibly because of their lower reduction potential.34 The present observation revealed that larger gold particles in a higher surfactant concentration become smaller and smaller before they get finally dissolved by cyanide in air due to surface oxidation. During dissolution, the plasmon band shifts to a longer wavelength region with a broadening of the spectra and then a red shift occurs gradually until it vanishes completely. The usual red shift is due to withdrawal of electron density from the surface, which, in the present case, can be attributed to the removal of electron density from the particle surface. As oxygen provides a suitable condition for dissolution, it is reasonable that oxygen removes the electron density from the particle surface. Hence, there is the possibility of oxidation of the surface of gold atoms. Such oxidation seems feasible from the redox potential point of view. The redox potential of the particle decreases with decrease in particle size and upon adsorption of nucleophile on particle surfaces. The more the electron donation by the nucleophile, the more negative is the shift in the redox potential. Thus, adsorption of cyanide ion makes the nanoparticle susceptible toward oxidation by oxygen.32 3.3. Studies on the Heating Effect of a Au(0) Sol Formed under Variable Flux. Last, we have studied heating effects on gold nanoparticles. Two different sets of solutions were prepared in two different quartz cuvettes taking Au(III) ion and TX-100, and their final concentrations in solution were 3.9 × 10-4 mol dm-3 and 8.9 × 10-3 mol dm-3, respectively. One set was photoirradiated under UV light of 100 Lux while the other set was irradiated with 600 Lux. In each case the irradiation time was kept constant (15 min). The irradiation was carried out at 25 °C. When the Au(III) solution was irradiated with UV light of higher flux (600 Lux), the pink solution that was formed showed a λmax at 523 nm, but for the solution irradiated with lower flux (100 Lux), the result was a blue solution having a λmax at ∼533 nm. When the constant absorbance values were obtained, then both sets were heated on a water bath at ∼60 °C for 0.5 h. Above 60 °C the surfactant at the experimental concentration becomes cloudy (cloud point of the surfactant is 60 °C), and the shape of the micelle deviates from spherocity (rodlike or wormlike) in the temperature range 40-65 °C. The shape (32) Pal, A.; Bandyopadhyay, M. Ind. J. Chem. Technol. 2000, 7, 75. (33) Pal, A. J. Photochem. Photobiol., A 2001, 142, 59. (34) Jana, N. R.; Wang, Z. L.; Pal, T. Langmuir 2000, 16, 2457.

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Figure 6. Transmission electron micrograph (TEM) of linearly aggregated gold nanoparticles. Conditions: [TX-100] ) 8.9 × 10-3 mol dm-3. [Au] ) 3.9 × 10-4 mol dm-3.

of the micelle presumably promotes the linear aggregation of the particles. During crystallization, solute molecules get separated as tiny crystals if the solution is chilled suddenly. Similarly, when the solution is irradiated with a higher flux of light, a large number of nucleation centers are produced, as is the case for the suddenly chilled solution. On the other hand, seeding growth of crystals from solution results in larger particles. Photoactivation under a lower flux (