A New Model for the Synthesis of Hollow Particles via the Bubble Templating Method

1、A New Model for the Synthesis of Hollow Particles via the Bubble Templating MethodYongsheng Han,*, Masayoshi Fuji,*, Dmitry Shchukin, Helmuth Mohwald, and Minoru TakahashiCeramics Research Laboratory, Nagoya Institute of Technology, Japan, and Max Planck Institute of Colloids and Interfaces, D-14476

2、 Potsdam/Golm, GermanyReceiVed April 24, 2009; ReVised Manuscript ReceiVed June 5, 2009CRYSTALGROWTH& DESIGN2009VOL. 9, NO. 837713775ABSTRACT: Hollow CaCO3 particles were synthesized by passing CO2/N2 bubbles into a CaCl2 solution. The formation of such hollow particles was investigated via the nucl

3、eation mechanism of calcium carbonate. It was found that homogeneous and heterogeneous nucleation both occurred via a three phase gas-liquid-solid reaction. However, only the latter played a role in the formation of the described hollow particles. Nuclei were initially formed at the bubble surface a

4、nd subsequently aggregated together to form the initial shell. Growth of the initial shell structure proceeded along the bubble surface by the subsequent aggregation of nuclei. Concurrently it was found that the combination of reaction ions, Ca2+ and CO32-, adsorbed on the outer surface of the growi

5、ng shell resulted in the increase of shell thickness in a direction perpendicular to the bubble template surface. Both aggregation of nuclei and crystal growth contributed to the formation of the shell structure, with distinguishable inner and outer layers present. A new model was proposed based on

6、these investigations, in which crystal growth was for the first time highlighted as a key component in the formation of hollow particles by the bubble template route.1. IntroductionIntense research in the preparation of hollow particles, typically those of a spherical geometry, has arisen due to aca

7、demic/scientific interest and potential technological applica-tions. Hollow particles with controlled porosity have found application in catalysis, fillers, separations and biomedical engineering.1-3 Thus far, the most successful application of hollow particles is the encapsulation and controlled re

8、lease of effectors (e.g., drugs, cosmetics, dyes and inks).4-6 Many methods have been developed so far for the synthesis of hollow particles. Most of them can be classified into two categories, namely, the hard template method and the soft template method. The hard template method utilizes templates

9、 including but not limited to silica particles,7 titania particles,8 carbon particles,9 calcium carbonate particles10 and polystyrene (PS) beads,11 while the soft template method has previously utilized emulsion droplets,12 surfactant micelles,13 polymer vesicles14 and bubbles.15 The bubble template

10、 method15-17 is a new method having the advantage of synthesizing hollow particles in a one step approach, negating the template removal step in material preparation. We have previously demonstrated the preparation of hollow calcium carbonate particles by passing CO2/N2 bubbles through a calcium car

11、bonate solution.15 The hollow particles prepared in this manner have a spherical shape with a hollow inner center and an outer solid shell. The formation mechanism for such particle structuring has been proposed previously, but some ambiguous questions still remain, includ-ing, for example, where do

12、es nucleation happen in such a three phase gas-liquid-solid reaction? And what is the driving force behind the attachment of the precipitates at the bubble surface?Previous reports regarding bubble templating have tried to explain the formation of hollow particles in some other groups.* Correspondin

13、g authors: Dr. Yongsheng Han, Interface Department, Max Planck Institute of Colloids and Interfaces, D-14476 Potsdam/Golm, Germany. E-mail: yshanmpikg.mpg.de. Prof. Masayoshi Fuji, Ceramics Research Labora-tory, Nagoya Institute of Technology, Asahigaoka 10-6-29, Tajimi 507-0071, Japan. E-mail: fuji

14、nitech.ac.jp. Nagoya Institute of Technology. Max Planck Institute of Colloids and Interfaces.ZnSe hollow spheres have been synthesized under hydrothermal conditions, with the authors proposing that hollow ZnSe spheres were formed in two steps.16 The first step was proposed to be a homogeneous nucle

15、ation of ZnSe particles in the bulk solution resulting in the formation of precursor nanocrystals. The second step was suggested to proceed via the aggregation of these nanocrystals at the bubble template surface ultimately forming the hollow microsphere structure. However, other literature reports

16、do not entirely concur with this proposed mechanism. Aquilano et al. have prepared hollow cubic calcite by passing CO2 bubbles through a saturated CaCO3 solution at pH 12.18 Aquilano et al. stated that the hollow calcite was formed via the direct nucleation of calcite at the CO2 bubble surface. The

17、formation of tubular structures in Nature has also been attributed to the heterogeneous nucleation at the bubble surface.19 A debate remains as to what role nucleation plays in the formation of the hollow particle architecture. Therefore further investigation is necessary to allow for the developmen

18、t and improved understanding of the underlying formation mechanism of hollow spheres via the bubble templating method. Clarification of the mechanism will also be helpful for the design and controlled preparation of products in a reaction involving gas bubbles.In the study reported herein, we invest

19、igate the formation mechanism of hollow particles prepared via the bubble tem-plating route by focusing on the formation starting point of nucleation. The nucleation processes potentially involved in the synthesis of hollow particles are discussed and their contribution to the formation of hollow pa

20、rticles is described. The hetero-geneous nucleation at the bubble surface is emphasized, and the evolution of nuclei to the formation of shell is presented. Finally, a new model is proposed for the formation of hollow particles via the bubble template method.2. Experimental SectionMaterials. Calcium

21、 chloride and aqueous ammonia-water (25 wt %) were purchased from Wako Pure Chemicals, Japan. CO2 and N2 gas with purity above 99% were provided by Taiyo Nippon Sanso, Japan. All chemicals were used without further purification. The water10.1021/cg900456t CCC: $40.752009 American Chemical SocietyPub

22、lished on Web 06/17/20093772Crystal Growth & Design, Vol. 9, No. 8, 2009Figure 1. Micrographs of the CaCO3 particles prepared by passing CO2/ N2 bubbles into a calcium chloride solution. (A) general SEM image of the prepared sample; (B) TEM image of an unbroken hollow particle;(C) SEM image of a bro

23、ken dense particle.used in all experiments was prepared in a three stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 m cm.Han et al.Preparation of Hollow Calcium Carbonate Particles. Calcium chloride and ammonia-water were mixed with water to form a solution

24、(CCa ) 0.1 M, CNH3H2O ) 0.5 wt %) with a pH 11. A mixed gas flow (33.3 wt % CO2 + 66.7 wt % N2) was bubbled through this solutionresulting immediately in a white precipitate. The reaction was ther-mostatted at 25 C by a heated water bath. The pH of the solution decreased continuously with bubbling.

25、The reaction was considered complete when the pH reached 7.5. Reaction time was approximately 10 min, depending on the concentration of calcium ions and the gas flow rate. The precipitated products were collected by filtering through a membrane filter (0.2 m in diameter) and dried at 120 C.Character

26、ization. XRD (RINT 1100, Rigaku, Japan) measurements were conducted to identify the composition of samples. A JSM-7100F (JEOL, Japan) SEM microscope was used to observe the morphology of samples. TEM was performed using a JSM 2000EXII microscope (JEOL, Japan) at an accelerating voltage of 160 kV. Th

27、e zeta potential of the calcium carbonate particles was measured by a Zetasizer Nano-Z instrument (Malvern, England).3. Results and DiscussionThe as-prepared particles were characterized by SEM and TEM, as shown in Figure 1. Figure 1A shows a general view of particle morphology. Particles were of sp

28、herical shape with the main composition of vateritethe metastable phase of calcium carbonate.20 Particle sizes were found in the diameter range of 1-5 m. Size polydispersion was attributed to asynchronous nucleation, with formation of the smaller particles occurring late in the gas bubbling step, ge

29、nerating small particle size as a result of a short growth time. Hollow particles were found as both small and big particle diameters, indicating that hollow particles can be formed throughout the bubbling time. A broken particle was found in the sample (Figure 1A), clearly demonstrating the hollow

30、interior of the particle. Examples of unbroken particles were examined by TEM. Some of them demonstrate the hollow structure inside (Figure 1B). In addition to the hollow particles, dense particles are also observed in the sample (Figure 1C), presenting similar particle morphology to materials prepa

31、red under supersaturated solution conditions.21 The presence of dense particles indicates the occurrence ofFigure 2. Zeta potential of the prepared CaCO3 nanoparticle and bubble surface (inset).Model for the Synthesis of Hollow ParticlesFigure 3. Micrographs of the prepared hollow CaCO3 particles wi

32、th emphasis on the solid shell. (A) SEM image of a broken hollow particle;(B) SEM image of the inner shell from the hollow particle; (C) SEM image of the magnified shell, which presents the shell having two distinguishable layers, namely, inner layer and outer layer.homogeneous nucleation during the

33、 reaction. When a high calcium ion concentration and high gas flow rate are considered, the occurrence of homogeneous nucleation is not difficult to understand. The ratio of dense particles and hollow particles is dependent on the preparation conditions.22 Under optimized conditions, the ratio of ho

34、llow particles is up to 20% in number. Our interest in this paper was to initially ascertain and describe the formation mechanism of hollow particles and to elaborate the nucleation behavior contributing to their formation. If homogeneous nucleation contributed to the formation of hollow particles,

35、one could expect that homogeneous nucleation first proceeds in the solution and then the resulting nuclei adsorb and aggregate at the bubble surface, generating the hollow particle morphology.For adsorption of nanoparticles at the bubble surface to occur, they must first undergo a sufficiently close

36、 encounter, a process which is controlled by hydrodynamics. When the nanoparticles approach the bubble within the range of surface forces, the adsorption probability is determined by the surface forces between the nanoparticle and the bubble. In aqueous solution one would expect at least three relev

37、ant forces: the electrostatic double layer force, the van der Waals force and the hydrophobic force. Since at neutral and basic conditions (where precipitation happens) both calcium carbonate particles and air bubbles are negatively charged (Figure 2), the electrostatic double layer force will cause

38、 repulsion between nanoparticle and bubble. The van der Waals force between nanoparticle and bubble through water will also be repulsive when their distance is less than dozens of nanometers.23 For hydrophilic calcium carbonate particles it would be expected that only repulsive forces between nano-p

39、article and bubble occur since air bubbles are hydrophobic in character. Therefore, there will be no attractive force to enable the close approach. A high energy barrier has to be overcome to facilitate the insertion of the nanoparticles into the interface of bubbles. A mode of homogeneous nucleatio

40、n followed by the adsorption of the nanoparticles to bubble surface does not adequately explain the formation of the observed hollow particle morphology.Crystal Growth & Design, Vol. 9, No. 8, 20093773Figure 4. The increase of shell thicknesses with bubbling time.In the precipitation reaction involv

41、ing bubbles, it is also possible for heterogeneous nucleation to occur since the bubble surface can efficiently reduce the nucleation activation barrier and hence improve the nucleation on the bubble surface.24 A general equation for the rate of steady state nucleation (I) in condensed systems is as

42、 follows (eq 1):I )Aexp(-G)(1)KTwhere A represents a jump frequency and is weakly temperature dependent. K is the Boltzmann constant, T is temperature (K), is the dynamic viscosity, and G is the thermodynamic energy barrier to form a nucleus of critical size, as related by eq 2:G )163V2(2)3where is

43、the surface free energy of the new interface (bubble surface), V the volume of growth units and the bulk free energy change for crystallization.Owing to the extreme dependence of the nucleation rate (I) on the surface free energy (), small changes in result in a large change in I. For example, a 30%

44、 decrease in results in a 109 increase in the nucleation rate. It is reported that bubbles dissolved in solution usually have a concentrated ion layer on the surface. For example, in the presence of dissolved CO2, HCO3- ions are strongly adsorbed on the surface of bubbles. The concentration of HCO3-

45、 is more than 500 times larger than that of OH- at the bubble/solution interface.25 The concentrated ion layer in effect behaves like a surfactant layer leading to a reduction of the surface free energy, which results in preferential nucleation at the bubble surface. The existence of a concentrated

46、ion layer on the bubble surface has been examined in our previous work.24 A single ammonia bubble was through a silica sol, and it was found that the sol only gelates around the bubble surface due to the highest concentration of ammonia ions there. In the case of dissolved CO2, besides a high concen

47、tration of HCO3-, one can also expect a high concentration of CO32- at the surface of the template bubble. The Ca2+ coming from the bulk can react with CO32- at the bubble surface forming CaCO3 nuclei which are stabilized at the bubble surface via the formation a strong three-phase gas-solid-liquid

48、contact. The detachment of nuclei may happen only if a sufficient kinetic energy is supplied, but the adsorbed nuclei can move around3774Crystal Growth & Design, Vol. 9, No. 8, 2009Han et al.Figure 5. A proposed model for the formation of hollow CaCO3 particles by bubble template method. The nuclei

49、formed at bubble surface aggregate together forming the inner layer of the shell and the crystals growing on the surface of the initial shell forms the outer layer of the shell.the bubble without breaking the three phase contact line. In most cases, the adsorbed nuclei are moved to the rear of the b

50、ubble by the shear force of the fluid.26 The nuclei in the rear of the bubble are relatively stable as a product of the counterbalance of capillary force, gravity, hydrostatic pressure, buoyancy and shear force. The adsorbed nuclei are therefore expected to aggregate with the subsequent approaching

51、nuclei at the bubble surface forming a hollow particle around the bubble.Image magnification of the broken shell structure demon-strates that the shell contains two layers, namely, an inner layer and an outer layer (Figure 3A). Examination of the inner shell indicates it is composed of wormlike part

52、icles (Figure 3B), possessing a diameter of ca. 30 nm and a length in the range of 50-100 nm. The worm particles are formed by the aggregation of nuclei, as a moving nucleus meets another nucleus on the bubble surface, resulting in collision, producing a neck contact of two nuclei followed by the gr

53、owth of the neck.27 Since the nuclei are moving on the bubble surface driven by fluid flow, the nucleus equator which is against the flow has the highest possibility to collide with the approaching nucleus. Therefore the aggregates will preferentially grow in one direction, forming the observed worm

54、like particles. When the growing particle exceeds a critical length to attach the bubble surface, the feeding growth is stopped. With the shrinkage of bubbles due to the dissolution of CO2, the wormlike particles are compacted forming the inner shell. Magnification of the outer layer indicates a dif

55、ferent morphology compared with the inner shell (Figure 3C). The particles in the outer layer are found to be rodlike and assembled in parallel in the shell structure, and perpendicular to the bubble surface. The growth of these outer shell particles cannot be attributed to the aggregation of the ad

56、sorbed nuclei since it would be difficult for the nuclei to first detach from the bubble surface and second make a direct connection on the outer shell. Therefore, there should be other reason for the formation of the observed outer layer. The growth of calcium carbonate particles usually starts fro

57、m the adsorption of ions (Ca2+ and CO32-) on the crystal surface. Then the ions combine and form a new face covering the old surface, resulting in the particle growth. Both Ca2+ and CO32- are present at the outer surface of the growing shell: Ca2+ ions coming from the solution while the CO32- ions o

58、riginate from the dissolved CO2. These two ions adsorb at the outer surface of the shell, inducing the growth of calcium carbonate in the direction perpendicular to the bubble surface and forming rodlike particles. The outer layer is denser than the inner layer, stabilizing the shell structure. The growth of the outer layer not only makes the shell stable but also increases the gravity of the