Article pubs.acs.org/Langmuir
Colloidal Beading: Sonication-Induced Stringing of Selenium Particles Choon Hwee Bernard Ng and Wai Yip Fan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 ABSTRACT: We report the preparation of monodispersed Se colloidal aggregates (dimers and trimers) via sonication-induced aggregation of spherical monomers. Control over the size and morphology of the products was achieved by changing the aging and sonication times, respectively. The possible mechanisms for the formation of colloidal aggregates were discussed. This method can provide a simple and versatile approach to the production of colloidal molecules of particles composed of different materials, which will be useful for fundamental studies related to colloidal systems.
1. INTRODUCTION Assembly of spherical colloids into complex aggregates has generated much interest due to the prospect of original properties.1,2 This outlook has fueled research in developing synthetic strategies to produce colloidal aggregates with good monodispersity and shape uniformity in a reproducible manner.3 Preparation of uniform samples would allow determination of how changes in microstructure influence macroscopic properties such as optical properties.4 Fundamental studies on the assembly of spherical colloids can also shed light on physical phenomena such as particle-particle interactions and phase transitions.5 Several strategies to produce colloidal aggregates in solution with adequate morphology control have been reported. A common approach is to induce aggregation of colloidal particles by raising the ionic strength of the suspension, that is, “salting out”.6 Colloidal particles can be stabilized in dispersions through the adsorption of ions on the surface where repulsion between the electrical double layers serves as a barrier against aggregation. Addition of ions to such stable suspensions, however, can cause ionic shielding of the electrostatic repulsive forces and induce aggregation.7 Micrometer-sized silica dimers have been prepared by destabilizing dispersions of spherical colloids via addition of ammonia.8,9 Velegol, et al. designed a salting out-quenching-fusing (SQF) technique based on this approach to produce polymer colloids and silica colloids, as well as metal−metal dimers.10,11 The addition of NaCl and Fe(NO3)3 has also been reported to yield small aggregates of Ag and Au in the nanometer range.12−14 Another strategy to prepare colloidal aggregates employs linkers to facilitate the assembly of the particles. Well-defined clusters of Au and Ag nanoparticles have been produced using phenylacetylene-based scaffolds where the symmetry of the scaffolds directs the structure of the resulting clusters.15−17 Oligonucleotides have © 2014 American Chemical Society
also been determined to be suitable linkers by exploiting the base pairing interactions between complementary sequences.18 Several groups have successfully utilized DNA as linkers to assemble metal nanoparticles colloids.19−23 Lastly, controlled aggregation via application of heat has been shown to produce ZnS24 and polystyrene dimers.25 In this approach, aggregation is triggered by raising the temperature which increases Brownian motion and is quenched on cooling. In this article, we report the preparation of monodispersed Se colloidal dimers and trimers in the nanometer regime via sonication-induced aggregation of spherical Se particles. We demonstrate control over the size and morphology of the colloidal aggregates by changing the aging and sonication times of the monomers, respectively. Sonication has been reported to cause aggregation of nanoparticles,26,27 but to the best of our knowledge, the application of sonication to produce monodispersed dimers and trimers has yet to be achieved. Selenium is known for its intriguing physical properties such as the thermoconductivity anisotropy, superconductivity, low photomelting temperature, and high piezoelectric, thermoelectric, and nonlinear optical responses.28,29 It is widely used widely used in solar cells, semiconductor rectifiers, photographic exposure meters, and xerography.30−32 Trace Se is also found to be an essential nutrient for human health. It can serve to improve the activity of the selenoenzyme and protect cells against free radical damage.33,34 Showing excellent bioavailability, high biological activity, and low toxicity,35,36 colloidal Se has been applied in medical diagnostics as red labels for antibodies.37,38 Another important motivation for the preparation of Se nanostructures is its known reactivity toward other Received: April 2, 2014 Revised: May 7, 2014 Published: June 10, 2014 7313
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chemicals to produce functional materials such as CdSe, ZnSe, and Bi2Se3.39,40 Hence the ability to prepare Se colloidal aggregates could serve to be a crucial intermediate step to forming the Bi2Se3 counterparts. Research in Bi2Se3, together with Bi2Te3 and Sb2Te3, has gained prominence recently on discovery of their topological insulator properties41 which are attractive for fundamental studies as well as applications in spintronics, quantum information, and low-energy dissipation electronics.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Se Spherical Monomers. The reagent-grade chemicals were obtained from Sigma-Aldrich and used without further purification. Se spherical particles were synthesized using a solution phase procedure. A 20 mL aqueous solution of sodium selenite (Na2SeO3, 0.04 mmol) was mixed with 20 mL of potassium iodide (KI, 0.06 mmol) under stirring in a 100 mL conical flask. Next, 10 mL of ascorbic acid (C6H8O6, 0.06 mmol) was introduced in one addition. A peach-red colloid formed after 3-4 min of stirring and the stirring was stopped after 30 min. To obtain the 75 nm particles, the colloid was immediately centrifuged at 3500 rpm for 30 min. To obtain the 135 nm particles, the colloid was aged for 24 h before carrying out the centrifugation. The red precipitate was washed twice with ethanol and water and the purified product was then redispersed in 5 mL of deionized water for further use. 2.2. Formation of Se Colloidal Aggregates. Spherical Se monomers dispersed in deionized water were contained in a 15 mL centrifuge tube and sonicated in an ultrasonic cleaning bath (S30H Elmasonic, 37 kHz) for different periods of time. The powers entering the system were determined using calorimetry to be 0.24 W cm−3. The sonicated colloids were sampled directly for characterization. 2.3. Instrumentation. Transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the JEOL 3010 transmission electron microscope with the accelerating voltage set at 300 kV. The samples for HRTEM were prepared by drop-casting a dilute aqueous dispersion onto carbonformvar coated copper grids (150 mesh), followed by drying in air. Powder X-ray diffraction (XRD) studies were performed using a Bruker D5005 diffractometer (Cu Kα λ = 0.15418 nm). Concentrated aqueous dispersion of the samples was dropcast onto glass slides and dried under vacuum. Ultraviolet-visible (UV−vis) spectroscopy was performed on the Shimadzu UV2550 spectrometer, using aqueous samples in 1 cm width quartz cuvettes.
Figure 1. Schematic representation of the pathways for the preparation of Se colloidal aggregates.
Figure 2. TEM images of the Se particles at various times of the reduction: (A) 75 nm nanospheres after 30 min, and (B) 135 nm nanospheres after aging for 24 h.
2B, the monomers are stable toward aggregation in the absence of sonication. The monomers were identified to be amorphous Se by EDX, SAED, and XRD characterization (Figure 3). EDX analysis gave peaks due to Se only. The Cu signals observed in the spectrum were due to the Cu grids used for nanoparticle deposition. The amorphous structure was reflected in the diffused SAED rings and lack of obvious peaks in the powder XRD spectrum. Sonication of the colloid containing the 75 nm spherical particles for 10 and 30 s resulted in the formation of dimers and trimers, respectively. As represented in Figure 4A,B, a significant proportion of the particles in the final product existed in the dimeric and trimeric structure. By counting over 100 Se particles from different parts of the grid, we concluded that the yields for the dimers and trimers were ∼65% and ∼60% respectively. As shown in Figure 4C,D, the sonicationinduced aggregation can be extended to the 135 nm Se particles with similar yields. XRD, SAED and EDX analyses showed no change in the composition and crystallinity of the particles on sonication. Close-packed dimers were observed on TEM imaging (Figure 5) which shows the stability against aggregation during deposition on the Cu grid. This confirms that the dimers and trimers were formed in solution rather than on the substrate. From magnified TEM images of the dimers and trimers (Figure 6A−D), we can observe that the colloidal aggregates formed from the smaller monomers had larger dihedral angles (∼145°) than those formed from the larger counterparts (∼110°). Such a pattern was consistent throughout the samples. This observation suggests that the monomers with
3. RESULTS AND DISCUSSION The pathways to prepare Se dimers and trimers are illustrated schematically in Figure 1. Spherical Se particles are first synthesized by reducing Na2SeO3 with ascorbic acid in the presence of KI as a stabilizing agent. The ability of I− to act as a stabilizing agent has been reported and it is believed that the surface adsorbed I− and the counterions (Na+ or K+) forms a double layer, which affords electrostatic stabilization against aggregation.42−44 Monodispersity of the particles was promoted by rapid introduction of the reducing agent to trigger a burst of nucleation. Uniformity of the particle size distribution is commonly achieved through a short nucleation period followed by subsequent growth of the nuclei.45 In view of the potential medical applications of Se nanoparticles, the use of ascorbic acid as a reductant in this method becomes important as both the reagent and its product of oxidation (CO2) are not invasive and poisonous for human cells in a wide range of their concentrations.46,47 3.1. Characterization of Se Colloidal Aggregates. Figure 2A,B shows the TEM images of the monodispersed 75 and 135 nm nanospheres formed in the reaction mixture after 30 min and 24 h of aging, respectively. As shown in Figure 7314
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Figure 5. TEM image showing close packed dimers.
Figure 6. Magnified TEM images of dimers and trimers formed from (A,B) 75 nm spheres and (C,D) 135 nm spheres. Dihedral angles of the dimers are illustrated. Scale bars = 50 nm.
By increasing the sonication times, small aggregates from dimers to hexamers were also observed (Figure 7). However, Figure 3. (A) SAED pattern, (B) powder XRD, and (C) EDX of the nanospheres which identified them to be amorphous Se.
Figure 7. TEM images of colloidal aggregates that were observed in our study: monomers to hexamers.
we were not able to isolate monodispersed aggregates of tetramers and higher order aggregates. The inability to do so is not unexpected considering the greater polydispersity that can occur as the sonication duration increases, as the dimers and trimers can undergo further aggregation, yielding a variety of aggregates. On prolonged sonication (more than 5 min), the 75 nm spheres coalesced to form irregular clusters (Figure 8). For the 135 nm spheres, red solid was observed to precipitate from the
Figure 4. TEM images of dimers and trimers formed from the sonication of (A,B) 75 nm nanospheres and (C,D) 135 nm nanospheres, respectively. The dimers and trimers were formed by sonicating the monomers for 10 and 30 s, respectively.
smaller sizes had coalesced to a greater extent during sonication. 7315
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Another phenomenon associated with cavitation is the generation of intense shock waves, giving rise to microjets that propagate through the liquid. These microjets can cause the Se particles to collide at high velocities, resulting in extreme heating at the point of impact and particle fusion due to effective local melting.52,53 Rapid cooling at the contact point due to particles rebounding immediately after impact is believed to arrest the aggregation process, freezing the particles at the neck formation stage, instead of coalescing completely into one larger particle.52 A point of contention for this mechanism would be that of studies showing that agglomeration does not occur for particles that are too small or too large. We performed similar calculations for the Se particles in our study to obtain the critical velocity (vc) required to melt the particle as well as the actual velocities reached by the particles (v) using eqs 1 and 2 respectively:53
Figure 8. TEM image of the irregular clusters that formed on prolonged sonication of the 75 nm nanospheres.
colloid. TEM images of the precipitate revealed the formation of Se nanowires (Figure 9). SAED and HRTEM imaging confirmed the trigonal phase which is consistent with observations reported by Xia et al.48
νc =
2 C(Tm − Tb) + L
(1) −1
−1
where C is specific heat (321 J kg K for Se), L heat of fusion (6.9 × 104 J kg−1 for Se), Tm melting temperature (490.15 K), and Tb bath temperature (303.15 K). ν≈
Figure 9. (A) TEM image of the nanowires formed on prolonged sonication of the 135 nm nanospheres. (B) SAED patterns of a single nanowire. (C) HRTEM image of nanowire showing visible lattice fringes.
⎛ 9ηΔt ⎞⎤ PR ⎡ ⎢1 − exp⎜ − ⎟⎥ 6η ⎣ ⎝ 2ρR2 ⎠⎦
(2)
where Δt is collision time (Δt ≈ (λ/v1)), λ mean free path (estimated from electron micrographs to be 0.2 and 0.5 μm for the 75 and 135 nm particles respectively), R particle radius, P shock wave pressure (1 MPa as measured for laser-induced cavitation), η average viscosity (8.9 × 10−4 Pa s for water), ρ density (4.79 × 103 kg m−3 for Se), and v1 speed of sound in water (1497 m s−1). The calculations show that the average velocities attained by the 75 and 135 nm particles are 7 m s−1 and 13 m s−1 respectively, which are much less than the critical velocity for particle melting to occur (vc ≈ 508 m s−1). Although the result appears to suggest that the sizes of the particles were too small for collision with sufficient energy to agglomerate, it can still be interpreted to be consistent with our experimental observations. First, one of the approximations made in the formulation of the equations was that complete melting of the particle occurs to form dense agglomerates. Therefore, the small Se particles colliding with lower energies could not agglomerate into larger masses but only fused at the region of contact forming the dimers and trimers observed. Second, the result could indicate that only a small fraction of particles would gain sufficient energy during sonication to fuse upon collision. This condition again corresponds well with the formation of small aggregates (dimers and trimers). Small aggregates are typically observed at the initial stages of aggregation where the sticking probability is low, that is, many collisions must occur before particles combine into larger aggregates.54,55 In the case where the sizes of particles are favorable toward aggregation, the sticking probability would be high and large agglomerates are expected to form in a short time. Moreover, the ability for sonication to fuse sub-100 nm nanoparticles have been reported.26,27 3.3. Optical Properties. UV−vis absorption spectroscopy was used to determine the effect of sonication on the absorption characteristics of the Se colloidal aggregates. It is important to note that the samples need to be purified via washing with water (to remove excess KI) and ethanol (to
3.2. Mechanism for Formation of Dimers and Trimers. The formation of dimers and trimers on sonication can be explained based on the various phenomena associated with cavitation. Cavitation is defined as the formation, growth and subsequent implosion of microbubbles during sonication.49 First, cavitation gives rise to acoustic streaming and turbulent fluid movement in the medium which drives the transport and contact of the monomer particles.50 Upon contact, an energy source must have been present to cause sintering of the particles. We examine the three possible mechanisms based on the three sources of energy present: (1) bulk thermal heating, (2) high local temperatures created in the hot-spot interfacial regions, and (3) high-velocity interparticle collisions. Sonication is known to cause bulk thermal heating50 which in our case caused the temperature of the solution to increase by 3.5−4 °C in 30 seconds. To investigate the effect of bulk thermal heating on the aggregation phenomenon, additional experiments were conducted where the colloid was immersed into an oil bath maintained at 100 °C, which increased its temperature by ∼4 °C in 30 seconds. Analysis of the dispersion using UV spectroscopy and TEM revealed that little or no aggregation had taken place. The outcome suggests that bulk thermal heating during sonication is not primarily responsible for the aggregation process. On implosion, the microbubbles would act as hotspots, generating huge amounts of energy to increase the temperature and pressure up to 5000 °C and 500 atm, respectively.51 Although the high temperatures reached in the interfacial regions of the cavities could be thought to induce the melting of the particles on contact, Suslick, et al. have shown that the particle aggregation due to cavitation is primarily due to high speed interparticle collisions rather than local high temperatures in hot spot regions.52,53 7316
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Figure 10. UV-vis absorption spectra showing the aggregation process of the (A) 75 nm nanospheres and (B) 135 nm nanospheres. The spectra were offset vertically for clarity.
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ACKNOWLEDGMENTS The project was supported by a National University of Singapore (NUS) research grant under Grant No. 143-000553-112. C.H.B.N. thanks NUS for a PGF research scholarship.
remove excess ascorbic acid/ascorbate) before characterization. The tail of the strong characteristic absorptions of iodide (226 nm) and ascorbate (265 nm) can contaminate the spectra and obscure the Se absorptions. Figure 10A,B displays the absorption spectra for the products on sonicating the 75 and 135 nm Se colloids respectively for different durations. The 75 nm spheres exhibited an absorption at ∼300 nm which is in good agreement with theoretical calculations made by Dauchot and Watillon.56 On sonicating of the monomers, the ∼300 nm absorption decreased in intensity and a new absorption was observed around ∼550 nm. An increase of the sonication durations was accompanied by a red shift and broadening of the band. This could be due to increasing extent of aggregation, as well as the increase in polydispersity of the sample. For the 135 nm spheres, the unsonicated sample gave an absorption at ∼590 nm. As the sonication durations were increased, the absorption maxima was again observed to be red-shifted and broadened. After long durations of sonication (more than 5 min), the absorption profile for the sample after 5 min of sonication matched that of the trigonal Se nanowires.57
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4. CONCLUSION A facile method for the preparation of monodispersed Se colloidal aggregates (dimers and trimers) is presented. Control over the size and morphology of the colloidal aggregates was achieved by changing the aging and sonication times, respectively. The formation of dimers and trimers can be best explained based on high-velocity interparticle collisions due to cavitation. Using mild sonication (0.24 W cm−3) under room temperatures and short sonication durations (10−30 s), we were able to isolate the dimers and trimers in good yields. It is envisioned that these colloidal aggregates may be utilized as starting materials to react with other salts (e.g Bi(NO3)3) for the formation of other functional counterparts (e.g., Bi2Se3 which is a topological insulator).
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]; fax: 6567791691. Notes
The authors declare no competing financial interest. 7317
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Colloidal Beading: Sonication-Induced Stringing of Selenium Particles Choon Hwee Bernard Ng and Wai Yip Fan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 ABSTRACT: We report the preparation of monodispersed Se colloidal aggregates (dimers and trimers) via sonication-induced aggregation of spherical monomers. Control over the size and morphology of the products was achieved by changing the aging and sonication times, respectively. The possible mechanisms for the formation of colloidal aggregates were discussed. This method can provide a simple and versatile approach to the production of colloidal molecules of particles composed of different materials, which will be useful for fundamental studies related to colloidal systems.
1. INTRODUCTION Assembly of spherical colloids into complex aggregates has generated much interest due to the prospect of original properties.1,2 This outlook has fueled research in developing synthetic strategies to produce colloidal aggregates with good monodispersity and shape uniformity in a reproducible manner.3 Preparation of uniform samples would allow determination of how changes in microstructure influence macroscopic properties such as optical properties.4 Fundamental studies on the assembly of spherical colloids can also shed light on physical phenomena such as particle-particle interactions and phase transitions.5 Several strategies to produce colloidal aggregates in solution with adequate morphology control have been reported. A common approach is to induce aggregation of colloidal particles by raising the ionic strength of the suspension, that is, “salting out”.6 Colloidal particles can be stabilized in dispersions through the adsorption of ions on the surface where repulsion between the electrical double layers serves as a barrier against aggregation. Addition of ions to such stable suspensions, however, can cause ionic shielding of the electrostatic repulsive forces and induce aggregation.7 Micrometer-sized silica dimers have been prepared by destabilizing dispersions of spherical colloids via addition of ammonia.8,9 Velegol, et al. designed a salting out-quenching-fusing (SQF) technique based on this approach to produce polymer colloids and silica colloids, as well as metal−metal dimers.10,11 The addition of NaCl and Fe(NO3)3 has also been reported to yield small aggregates of Ag and Au in the nanometer range.12−14 Another strategy to prepare colloidal aggregates employs linkers to facilitate the assembly of the particles. Well-defined clusters of Au and Ag nanoparticles have been produced using phenylacetylene-based scaffolds where the symmetry of the scaffolds directs the structure of the resulting clusters.15−17 Oligonucleotides have © 2014 American Chemical Society
also been determined to be suitable linkers by exploiting the base pairing interactions between complementary sequences.18 Several groups have successfully utilized DNA as linkers to assemble metal nanoparticles colloids.19−23 Lastly, controlled aggregation via application of heat has been shown to produce ZnS24 and polystyrene dimers.25 In this approach, aggregation is triggered by raising the temperature which increases Brownian motion and is quenched on cooling. In this article, we report the preparation of monodispersed Se colloidal dimers and trimers in the nanometer regime via sonication-induced aggregation of spherical Se particles. We demonstrate control over the size and morphology of the colloidal aggregates by changing the aging and sonication times of the monomers, respectively. Sonication has been reported to cause aggregation of nanoparticles,26,27 but to the best of our knowledge, the application of sonication to produce monodispersed dimers and trimers has yet to be achieved. Selenium is known for its intriguing physical properties such as the thermoconductivity anisotropy, superconductivity, low photomelting temperature, and high piezoelectric, thermoelectric, and nonlinear optical responses.28,29 It is widely used widely used in solar cells, semiconductor rectifiers, photographic exposure meters, and xerography.30−32 Trace Se is also found to be an essential nutrient for human health. It can serve to improve the activity of the selenoenzyme and protect cells against free radical damage.33,34 Showing excellent bioavailability, high biological activity, and low toxicity,35,36 colloidal Se has been applied in medical diagnostics as red labels for antibodies.37,38 Another important motivation for the preparation of Se nanostructures is its known reactivity toward other Received: April 2, 2014 Revised: May 7, 2014 Published: June 10, 2014 7313
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chemicals to produce functional materials such as CdSe, ZnSe, and Bi2Se3.39,40 Hence the ability to prepare Se colloidal aggregates could serve to be a crucial intermediate step to forming the Bi2Se3 counterparts. Research in Bi2Se3, together with Bi2Te3 and Sb2Te3, has gained prominence recently on discovery of their topological insulator properties41 which are attractive for fundamental studies as well as applications in spintronics, quantum information, and low-energy dissipation electronics.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Se Spherical Monomers. The reagent-grade chemicals were obtained from Sigma-Aldrich and used without further purification. Se spherical particles were synthesized using a solution phase procedure. A 20 mL aqueous solution of sodium selenite (Na2SeO3, 0.04 mmol) was mixed with 20 mL of potassium iodide (KI, 0.06 mmol) under stirring in a 100 mL conical flask. Next, 10 mL of ascorbic acid (C6H8O6, 0.06 mmol) was introduced in one addition. A peach-red colloid formed after 3-4 min of stirring and the stirring was stopped after 30 min. To obtain the 75 nm particles, the colloid was immediately centrifuged at 3500 rpm for 30 min. To obtain the 135 nm particles, the colloid was aged for 24 h before carrying out the centrifugation. The red precipitate was washed twice with ethanol and water and the purified product was then redispersed in 5 mL of deionized water for further use. 2.2. Formation of Se Colloidal Aggregates. Spherical Se monomers dispersed in deionized water were contained in a 15 mL centrifuge tube and sonicated in an ultrasonic cleaning bath (S30H Elmasonic, 37 kHz) for different periods of time. The powers entering the system were determined using calorimetry to be 0.24 W cm−3. The sonicated colloids were sampled directly for characterization. 2.3. Instrumentation. Transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the JEOL 3010 transmission electron microscope with the accelerating voltage set at 300 kV. The samples for HRTEM were prepared by drop-casting a dilute aqueous dispersion onto carbonformvar coated copper grids (150 mesh), followed by drying in air. Powder X-ray diffraction (XRD) studies were performed using a Bruker D5005 diffractometer (Cu Kα λ = 0.15418 nm). Concentrated aqueous dispersion of the samples was dropcast onto glass slides and dried under vacuum. Ultraviolet-visible (UV−vis) spectroscopy was performed on the Shimadzu UV2550 spectrometer, using aqueous samples in 1 cm width quartz cuvettes.
Figure 1. Schematic representation of the pathways for the preparation of Se colloidal aggregates.
Figure 2. TEM images of the Se particles at various times of the reduction: (A) 75 nm nanospheres after 30 min, and (B) 135 nm nanospheres after aging for 24 h.
2B, the monomers are stable toward aggregation in the absence of sonication. The monomers were identified to be amorphous Se by EDX, SAED, and XRD characterization (Figure 3). EDX analysis gave peaks due to Se only. The Cu signals observed in the spectrum were due to the Cu grids used for nanoparticle deposition. The amorphous structure was reflected in the diffused SAED rings and lack of obvious peaks in the powder XRD spectrum. Sonication of the colloid containing the 75 nm spherical particles for 10 and 30 s resulted in the formation of dimers and trimers, respectively. As represented in Figure 4A,B, a significant proportion of the particles in the final product existed in the dimeric and trimeric structure. By counting over 100 Se particles from different parts of the grid, we concluded that the yields for the dimers and trimers were ∼65% and ∼60% respectively. As shown in Figure 4C,D, the sonicationinduced aggregation can be extended to the 135 nm Se particles with similar yields. XRD, SAED and EDX analyses showed no change in the composition and crystallinity of the particles on sonication. Close-packed dimers were observed on TEM imaging (Figure 5) which shows the stability against aggregation during deposition on the Cu grid. This confirms that the dimers and trimers were formed in solution rather than on the substrate. From magnified TEM images of the dimers and trimers (Figure 6A−D), we can observe that the colloidal aggregates formed from the smaller monomers had larger dihedral angles (∼145°) than those formed from the larger counterparts (∼110°). Such a pattern was consistent throughout the samples. This observation suggests that the monomers with
3. RESULTS AND DISCUSSION The pathways to prepare Se dimers and trimers are illustrated schematically in Figure 1. Spherical Se particles are first synthesized by reducing Na2SeO3 with ascorbic acid in the presence of KI as a stabilizing agent. The ability of I− to act as a stabilizing agent has been reported and it is believed that the surface adsorbed I− and the counterions (Na+ or K+) forms a double layer, which affords electrostatic stabilization against aggregation.42−44 Monodispersity of the particles was promoted by rapid introduction of the reducing agent to trigger a burst of nucleation. Uniformity of the particle size distribution is commonly achieved through a short nucleation period followed by subsequent growth of the nuclei.45 In view of the potential medical applications of Se nanoparticles, the use of ascorbic acid as a reductant in this method becomes important as both the reagent and its product of oxidation (CO2) are not invasive and poisonous for human cells in a wide range of their concentrations.46,47 3.1. Characterization of Se Colloidal Aggregates. Figure 2A,B shows the TEM images of the monodispersed 75 and 135 nm nanospheres formed in the reaction mixture after 30 min and 24 h of aging, respectively. As shown in Figure 7314
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Figure 5. TEM image showing close packed dimers.
Figure 6. Magnified TEM images of dimers and trimers formed from (A,B) 75 nm spheres and (C,D) 135 nm spheres. Dihedral angles of the dimers are illustrated. Scale bars = 50 nm.
By increasing the sonication times, small aggregates from dimers to hexamers were also observed (Figure 7). However, Figure 3. (A) SAED pattern, (B) powder XRD, and (C) EDX of the nanospheres which identified them to be amorphous Se.
Figure 7. TEM images of colloidal aggregates that were observed in our study: monomers to hexamers.
we were not able to isolate monodispersed aggregates of tetramers and higher order aggregates. The inability to do so is not unexpected considering the greater polydispersity that can occur as the sonication duration increases, as the dimers and trimers can undergo further aggregation, yielding a variety of aggregates. On prolonged sonication (more than 5 min), the 75 nm spheres coalesced to form irregular clusters (Figure 8). For the 135 nm spheres, red solid was observed to precipitate from the
Figure 4. TEM images of dimers and trimers formed from the sonication of (A,B) 75 nm nanospheres and (C,D) 135 nm nanospheres, respectively. The dimers and trimers were formed by sonicating the monomers for 10 and 30 s, respectively.
smaller sizes had coalesced to a greater extent during sonication. 7315
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Another phenomenon associated with cavitation is the generation of intense shock waves, giving rise to microjets that propagate through the liquid. These microjets can cause the Se particles to collide at high velocities, resulting in extreme heating at the point of impact and particle fusion due to effective local melting.52,53 Rapid cooling at the contact point due to particles rebounding immediately after impact is believed to arrest the aggregation process, freezing the particles at the neck formation stage, instead of coalescing completely into one larger particle.52 A point of contention for this mechanism would be that of studies showing that agglomeration does not occur for particles that are too small or too large. We performed similar calculations for the Se particles in our study to obtain the critical velocity (vc) required to melt the particle as well as the actual velocities reached by the particles (v) using eqs 1 and 2 respectively:53
Figure 8. TEM image of the irregular clusters that formed on prolonged sonication of the 75 nm nanospheres.
colloid. TEM images of the precipitate revealed the formation of Se nanowires (Figure 9). SAED and HRTEM imaging confirmed the trigonal phase which is consistent with observations reported by Xia et al.48
νc =
2 C(Tm − Tb) + L
(1) −1
−1
where C is specific heat (321 J kg K for Se), L heat of fusion (6.9 × 104 J kg−1 for Se), Tm melting temperature (490.15 K), and Tb bath temperature (303.15 K). ν≈
Figure 9. (A) TEM image of the nanowires formed on prolonged sonication of the 135 nm nanospheres. (B) SAED patterns of a single nanowire. (C) HRTEM image of nanowire showing visible lattice fringes.
⎛ 9ηΔt ⎞⎤ PR ⎡ ⎢1 − exp⎜ − ⎟⎥ 6η ⎣ ⎝ 2ρR2 ⎠⎦
(2)
where Δt is collision time (Δt ≈ (λ/v1)), λ mean free path (estimated from electron micrographs to be 0.2 and 0.5 μm for the 75 and 135 nm particles respectively), R particle radius, P shock wave pressure (1 MPa as measured for laser-induced cavitation), η average viscosity (8.9 × 10−4 Pa s for water), ρ density (4.79 × 103 kg m−3 for Se), and v1 speed of sound in water (1497 m s−1). The calculations show that the average velocities attained by the 75 and 135 nm particles are 7 m s−1 and 13 m s−1 respectively, which are much less than the critical velocity for particle melting to occur (vc ≈ 508 m s−1). Although the result appears to suggest that the sizes of the particles were too small for collision with sufficient energy to agglomerate, it can still be interpreted to be consistent with our experimental observations. First, one of the approximations made in the formulation of the equations was that complete melting of the particle occurs to form dense agglomerates. Therefore, the small Se particles colliding with lower energies could not agglomerate into larger masses but only fused at the region of contact forming the dimers and trimers observed. Second, the result could indicate that only a small fraction of particles would gain sufficient energy during sonication to fuse upon collision. This condition again corresponds well with the formation of small aggregates (dimers and trimers). Small aggregates are typically observed at the initial stages of aggregation where the sticking probability is low, that is, many collisions must occur before particles combine into larger aggregates.54,55 In the case where the sizes of particles are favorable toward aggregation, the sticking probability would be high and large agglomerates are expected to form in a short time. Moreover, the ability for sonication to fuse sub-100 nm nanoparticles have been reported.26,27 3.3. Optical Properties. UV−vis absorption spectroscopy was used to determine the effect of sonication on the absorption characteristics of the Se colloidal aggregates. It is important to note that the samples need to be purified via washing with water (to remove excess KI) and ethanol (to
3.2. Mechanism for Formation of Dimers and Trimers. The formation of dimers and trimers on sonication can be explained based on the various phenomena associated with cavitation. Cavitation is defined as the formation, growth and subsequent implosion of microbubbles during sonication.49 First, cavitation gives rise to acoustic streaming and turbulent fluid movement in the medium which drives the transport and contact of the monomer particles.50 Upon contact, an energy source must have been present to cause sintering of the particles. We examine the three possible mechanisms based on the three sources of energy present: (1) bulk thermal heating, (2) high local temperatures created in the hot-spot interfacial regions, and (3) high-velocity interparticle collisions. Sonication is known to cause bulk thermal heating50 which in our case caused the temperature of the solution to increase by 3.5−4 °C in 30 seconds. To investigate the effect of bulk thermal heating on the aggregation phenomenon, additional experiments were conducted where the colloid was immersed into an oil bath maintained at 100 °C, which increased its temperature by ∼4 °C in 30 seconds. Analysis of the dispersion using UV spectroscopy and TEM revealed that little or no aggregation had taken place. The outcome suggests that bulk thermal heating during sonication is not primarily responsible for the aggregation process. On implosion, the microbubbles would act as hotspots, generating huge amounts of energy to increase the temperature and pressure up to 5000 °C and 500 atm, respectively.51 Although the high temperatures reached in the interfacial regions of the cavities could be thought to induce the melting of the particles on contact, Suslick, et al. have shown that the particle aggregation due to cavitation is primarily due to high speed interparticle collisions rather than local high temperatures in hot spot regions.52,53 7316
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Figure 10. UV-vis absorption spectra showing the aggregation process of the (A) 75 nm nanospheres and (B) 135 nm nanospheres. The spectra were offset vertically for clarity.
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ACKNOWLEDGMENTS The project was supported by a National University of Singapore (NUS) research grant under Grant No. 143-000553-112. C.H.B.N. thanks NUS for a PGF research scholarship.
remove excess ascorbic acid/ascorbate) before characterization. The tail of the strong characteristic absorptions of iodide (226 nm) and ascorbate (265 nm) can contaminate the spectra and obscure the Se absorptions. Figure 10A,B displays the absorption spectra for the products on sonicating the 75 and 135 nm Se colloids respectively for different durations. The 75 nm spheres exhibited an absorption at ∼300 nm which is in good agreement with theoretical calculations made by Dauchot and Watillon.56 On sonicating of the monomers, the ∼300 nm absorption decreased in intensity and a new absorption was observed around ∼550 nm. An increase of the sonication durations was accompanied by a red shift and broadening of the band. This could be due to increasing extent of aggregation, as well as the increase in polydispersity of the sample. For the 135 nm spheres, the unsonicated sample gave an absorption at ∼590 nm. As the sonication durations were increased, the absorption maxima was again observed to be red-shifted and broadened. After long durations of sonication (more than 5 min), the absorption profile for the sample after 5 min of sonication matched that of the trigonal Se nanowires.57
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4. CONCLUSION A facile method for the preparation of monodispersed Se colloidal aggregates (dimers and trimers) is presented. Control over the size and morphology of the colloidal aggregates was achieved by changing the aging and sonication times, respectively. The formation of dimers and trimers can be best explained based on high-velocity interparticle collisions due to cavitation. Using mild sonication (0.24 W cm−3) under room temperatures and short sonication durations (10−30 s), we were able to isolate the dimers and trimers in good yields. It is envisioned that these colloidal aggregates may be utilized as starting materials to react with other salts (e.g Bi(NO3)3) for the formation of other functional counterparts (e.g., Bi2Se3 which is a topological insulator).
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]; fax: 6567791691. Notes
The authors declare no competing financial interest. 7317
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