Article pubs.acs.org/crystal
Hollow Structure Formation Mechanism of Calcium Carbonate Particles Synthesized by the CO2 Bubbling Method Tatsuya Tomioka,* Masayoshi Fuji,* Minoru Takahashi, Chika Takai, and Mitsuo Utsuno Ceramics Research Laboratory, Nagoya Institute of Technology, 3-101-1 Hon-machi, Tajimi 507-0033, Japan ABSTRACT: The hollow structure formation mechanism of calcium carbonate particles synthesized by the CO2 bubbling method has been investigated. Samples were sequentially taken during the bubbling process to be structurally evaluated by X-ray diffraction and scanning electron microscopy. As a result, it has been elucidated that there are three development stages (A, B, and C) in the formation of the hollow structure. In stage A, amorphous CaCO3 precipitates as primary particles with the decreasing pH by CO2 bubbling. The primary particles then aggregate around pH 9.1 to form secondary particles, and turbidity ensues. Furthermore, because of the decreasing pH, the primary particles on the surface of the secondary particles begin to transform into the vaterite phase and form a shell. During stage B, the surface potential decreases along with a steep decrease in the pH, interparticle attraction of the primary particles on the surface of the secondary particles becomes stronger, and a highly compacted shell is formed. At the same time as vaterite precipitation on the surface, dissolution of the amorphous phase on the inside rapidly proceeds to precipitate vaterite particles by adhesion to the inside of the outer shell. At this time, contraction due to the release of water occurs and the hollow structure is formed. Moreover, it has been elucidated that, at pH ≤7, redissolution of the particulates inside the secondary particles occurs and definite shell structures are formed.
1. INTRODUCTION Inorganic hollow particles are attracting attention in many fields such as pharmaceuticals, foods, cosmetics, paints, etc., because of their combinability and light weight.1,2 In particular, calcium carbonate (CaCO3) fills many market needs because it is not toxic to the human body.3,4 To meet such needs, we have been proposing, as a novel process for mass production, a synthetic method of inexpensive hollow particles by bubbling carbon dioxide (CO2) into an aqueous calcium chloride (CaCl2) solution.5−8 This method is different from traditional ones in which particles and the like are used as templates, and therefore, there is no need to eliminate core materials in the postprocess.9−11 Therefore, this method has advantages: no waste disposal is required during removal by dissolution or combustion, and the process itself can be considerably simplified. The previous study has elucidated that hollow particles synthesized by the CO2 bubbling method have the vaterite phase structure, and the pH change during the continuous bubbling of CO2 into an aqueous calcium salt solution plays an important role in hollow particle formation.8 This may show that hollow structure formation of particles synthesized by this process is derived from the kinetic variation in the chemical synthetic environment. As crystalline polymorphs of CaCO3, there are three types of polymorphs, i.e., calcite, aragonite, and vaterite. In addition, its amorphous, monohydrate, and hexahydrate forms have been identified.12−14 The stabilities of each of these phases are significantly affected by supersaturation, temperature, pH, pressure, etc., and known to exist under such conditions in the ideal form based on a thermodynamic equilibrium. Because of this, the formation © 2011 American Chemical Society
mechanism of the hollow particle structure was investigated in this study by focusing on the pH variation and particle morphology variation.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Hollow Particles. Figure 1 shows a schematic diagram of the synthesis equipment for the hollow CaCO3 particles. An aqueous CaCl2 solution was adjusted to 0.1 mol/L using CaCl2 reagent (Wako Pure Chemical Industries, Ltd., special grade) and ionexchanged water. The synthesis was conducted by controlling the solution temperature at 28 °C, and stirring was conducted using a circulation pump at 0.8 MPa and 2 L min−1 (L of solution)−1. The initial pH was adjusted using an ammonia/water mixture (Wako Pure Chemical Industries, Ltd.; 25%) and hydrochloric acid (Kanto Chemical Co, Inc.; 35%) so that the obtained particles to have the vaterite phase. CO2, the flow rate of which was controlled from 0.5 to 1.25 L min−1 (L of solution)−1 using a mass flow controller, was introduced from the bottom of the reaction vessel via an ejection nozzle. The synthesized samples were filtered under pressurization through a membrane filter (Advantech Co.; pore size of 0.1 μm), washed with ion-exchanged water, dried under reduced pressure at 80 °C, and then subjected to evaluation tests for hollow particle formation. The changes in pH and calcium ion concentration during the gas bubbling were continuously measured using a composite glass electrode with a built-in reference electrode (seven multi-pro S40, Mettler-Toledo International Inc.). 2.2. Evaluation of Synthesized Particles. The time-dependent changes caused by CO2 bubbling in the structure and morphology of Received: August 23, 2011 Revised: November 22, 2011 Published: December 6, 2011 771
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be the end point of the carbonization,15 the rate of variation of pH decreases, though the decrease in pH still continues because dissolution of the CO2 into the solution occurs. To examine the behavior when the hollow particles are formed during the pH variation process during CO2 bubbling, samples were collected every 60 s from 120 s after the beginning of the bubbling, and analysis of the particles by SEM and XRD was performed. Figure 3 shows cross-sectional images
Figure 1. Schematic diagram of the experimental setup for producing calcium carbonate hollow particles. the particles were observed by scanning electron microscopy (SEM) (FE-SEM, JSM7000F, JEOL Ltd.), and the crystal structures were confirmed by X-ray diffraction analysis (RINT1000, Rigaku Corp.; Cu Kα, 30 kV, 20 mA, scanning rate of 2°/min). The hollow structure was confirmed by observing a cross-section slice by SEM, i.e., embedding the particles in epoxy resin and then taking a cross section using an ultramicrotome (MU/EMFC6, Leica Microsystems Japan) to produce a slice (1 μm) from the embedded resin.
3. RESULTS AND DISCUSSION 3.1. Changes in the Shape and Structure of Particles during Gas Bubbling. To investigate the formation mechanism of the hollow particles, we synthesized the hollow particles under the conditions in which the hollow structure could be constantly obtained, that is, the initial pH (9.5), the ammonia/water mixture (5 mL/L of solution), and the CO2 flow rate [0.6 L min−1 (L of solution)−1].8 Figure 2 shows the change in the pH of the CaCl2 solution versus the CO2 gas bubbling time. At point A, 25 s after the
Figure 3. SEM images of calcium carbonate particles synthesized at various elapsed times of CO2 gas bubbling. Initial pH of 9.5, NH3 rate of 5 mL/L of solution, and CO2 rate of 0.6 L min−1 (L of solution)−1.
of the samples that were prepared after CO2 bubbling for (a) 120, (b) 180, (c) 240, and (d) 360 s. There was no sign of hollow particle formation of the samples collected after bubbling for 120 s (Figure 3a). Hollow particles were only slightly observed even after bubbling for 180 s (Figure 3b). However, hollow structures were clearly observed at 240 s (Figure 3c) and 360 s (Figure 3d). Figure 4 shows the XRD
Figure 2. Change in pH by bubbling of CO2 gas into a CaCl2 solution. Initial pH of 9.5, NH3 rate of 5 mL/L of solution, and CO2 rate of 0.6 L min−1 (L of solution)−1.
Figure 4. XRD spectrum of calcium carbonate hollow particles synthesized at various elapsed times of CO2 gas bubbling. Initial pH of 9.5, NH3 rate of 5 mL/L of solution, and CO2 rate of 0.6 L min−1 (L of solution)−1.
start of bubbling of CO2 into an aqueous calcium chloride solution, a slight pH change was observed. At this point in time, the solution was still transparent. Moreover, CO2 bubbling advances, and then at point B, 110 s, a slightly larger change occurs and the turbidity due to aggregation of the precipitated particles ensues. At point C, 220 s after the start of the bubbling, the solution becomes completely clouded and the pH starts to rapidly decrease. After point D, which is considered to
patterns of the particles that were prepared after CO2 bubbling for (a) 120, (b) 180, (c) 240, and (d) 360 s, corresponding to Figure 3. Although a calcite peak is observed after gas bubbling for 120 s (Figure 4a), this shows mostly an amorphous phase as previously reported.16−19 After bubbling for 180 s (Figure 4b), the vaterite phase was mostly observed, but the peak intensity is low in comparison to that after bubbling for 240 s (Figure 4c) and 360 s (Figure 4d), when the vaterite phase was mostly 772
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As recognized from the XRD results (Figure 4a), the precipitated particles show the characteristic amorphous patterns 120 s after the start of bubbling. In addition, it was reported that amorphous particles themselves contain crystal water.12,13,18,19 At this stage, the secondary particles show no hollow structure as shown in Figure 3a. As for the reaction solution, its pH is ∼9.1, the potential is still high, and the attractive force between the primary particles is not strong; therefore, the interparticle space is large. Accordingly, some of the aqueous solution may be present between the particles.18,19 Next, the variation in structure from 180 to 240 s after the start of bubbling was considered. At 180 s, the solution pH decreased to pH 8.6 (in Figure 2). From the XRD patterns (Figure 4b), the appearance of the vaterite phase is observed. From the SEM images in Figure 3b, it is understood that the shell structure begins to be formed and hollow structures appear. Accordingly, in the meantime, the following process is thought to occur. One hundred eighty seconds after the start of gas bubbling, as the pH decreases, primary particles on the surface, which compose the secondary particles, transform into vaterite. In addition, a shell structure starts to clearly form because the attractive force between the primary particles becomes stronger because of a decrease in the surface potential. On the other hand, as mentioned in the previous part at “120 s”, amorphous particles still exist in its inside. A portion of the amorphous particles with a high water solubility transforms into vaterite by a “dissolution and reprecipitation process”, and then the precipitates shrink by releasing crystal water and the aqueous solution existing in the particle spaces.22,23 It is considered that the dissolution and reprecipitaion process due to the difference in solubility23 in an aqueous solution as shown in Table1 has an
observed along with a slight calcite peak. Figure 5 shows the results of observing the sample’s cross section by SEM at each
Figure 5. Shapes of calcium carbonate particles at various bubbling times during bubbling of CO2 gas into a 0.1 mol/L CaCl2 solution. Initial pH of 9.5, NH3 rate of 2.5 mL/L of solution, CO2 rate of 0.6 L min−1 (L of solution)−1.
point in time after the beginning of gas bubbling that was repeatedly synthesized under the same conditions as in Figure 2 and then collected. It was observed that good hollow particles concentrated around pH 7 and 6.8 when carbonization by gas bubbling ended and the rate of change in pH became slow, respectively. 3.2. Formation of Hollow Structure Due to Volumetric Shrinkage of the Amorphous Vaterite Core Shell. The precipitation of hollow particles by the CO2 bubbling method is supposed to be caused by the following process. During bubbling of CO2 into an aqueous CaCl2 solution, the CO 2 begins to dissolve into the CaCl2 solution and heterogeneous nucleation begin at the gas−liquid interfaces. The first amorphous particles begin to precipitate 25 s after the beginning of gas bubbling (point A in Figure 2). According to DLVO theory, the force between particles is the sum of the interparticle repulsive force (VR) and the interparticle attractive force (VA) represented by the following equation (eq 1).
V = VR + VA
Table 1. Logarithmic Solubility Products of Polymorphs of Calcium Carbonate15
(1)
Although VR depends on the interfacial potential, VR is approximately the ζ potential. Therefore, aggregation of the particles can be checked by measuring the ζ potential,20 which strongly depends on the pH. The isoelectric point of CaCO3 is pH 7.5 and shows −20 mV at pH 9.5 according to Han et al.21 It seems that particles formed after bubbling for 25 s (termed “primary particles”) exhibit a high negative surface charge, thus resulting in a high VR to repel each other. For this reason, the solution remains transparent because the aggregation force among the primary particles is weak and the particle size has not reached the critical size that allows such primary particles to grow into secondary particles. Gebauer et al. reported that such a primary amorphous phase is the aggregation state of clusters having a “solute character” that is formed in advance.18,19 The turbidity of the CaCl2 solution ensues 120 s after the bubbling starts (point B in Figure 2), because the primary particles aggregate into secondary particles because of a decrease in pH, and a decrease in the surface potential to give VA > VR. At this time, a short-range fluctuation corresponding to the bubble size in the distribution of the precipitates may exist in solution. Therefore, the sizes of the secondary particles are supposed to be affected by bubble size.
pK(25 °C)
log K(T in K)
amorphous
6.28
vaterite
7.913
calcite
8.480
6.1987 + 0.0053369(T − 273) + 0.0001096(T − 273)2 172.1295 + 0.077966T − 3074.688/T − 71.595 log(T) 171.9065 + 0.077993T − 2839.319/T − 71.595 log(T)
important influence on the behavior concerning the dissolution and precipitation exhibited by both phases during the transformation.18,19,22 As the XRD patterns show in Figure 4b, the diffraction peak of the vaterite phase at 180 s is lower than those at 240 and 360 s. This may support the fact that amorphous particles still remain among the secondary particles. Two hundred twenty seconds after the start of bubbling and thereafter, a significant decrease in pH in the reaction solution starts. As for the particles 240 s after the start of gas bubbling, as shown in Figure 4c, a strong diffraction pattern of vaterite is observed. On the other hand, at 240 s, as shown in Figure 2, the pH of the aqueous solution becomes 7.8 (sampling point C in Figure 2), which is almost near the isoelectric point of calcium carbonate, the VR decreases, the aggregation force VA of the primary particles of vaterite on the surface becomes higher, and then a highly compacted shell is formed. As the vaterite phase precipitates, dissolution of the amorphous particles in its inside remaining at the point of 180 s (as discussed above) rapidly advances and fresh vaterite particles precipitate.22−24At this 773
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time, particles precipitate in a manner in which they stick from the inside to outer wall that is highly compacted and strong and grow into a columnar structure. Panels a and b of Figure 6 show an enlarged SEM picture of the shell part of the hollow particles 240 s after the start of
Figure 7. Change of pH by bubbling of CO2 into a CaCl2 solution. Initial pH of 9.5, NH3 rate of 5mL/L of solution, and CO2 rate of 1.25 L min−1 (L of solution)−1.
The pH at the end of bubbling was determined to be 6.8 as shown in Figure 3, at which definite hollow particles were obtained. The SEM images of those hollow particles are shown in Figure 8. The shell thickness of the hollow particles shown in
Figure 6. Enlarged SEM images of a shell of a hollow particle by CO2 gas bubbling after 240 s. Initial pH of 9.5, NH3 rate of 5 mL/L of solution, and CO2 rate of 0.6 L min−1 (L of solution)−1.
bubbling. This picture well shows the situation in which the precipitated vaterite particles stick to the inside of the outer wall and may support the mechanism of formation of the hollow particle structure mentioned above. Consequently, their shrinking further advances. Because of this, on the inside of the secondary particles, a large water phase, which is formed by combining the aqueous solution existing between particles and released crystal water, remains and then a definite shell structure appears. In the following paragraph, the observation of definite hollow particles from pH ∼7.0 to ∼6.8 is discussed. On the other hand, when the rate of pH change is slow, a time lag hardly occurs in the structural change inside and outside of the secondary particles. Because of this, the particles may almost entirely transform from being amorphous to vaterite and homogeneous shrinking may occur; thus, the hollow particles do not appear. 3.3. Influence of the Rate of Change of pH and the pH at the End of Bubbling on the Hollow Particle Structure. On the basis of the results described in the preceding paragraph, the shell thickness, which is an important property of the hollow particles, may be considerably affected by the rate of change in pH during CO2 bubbling. The equilibrium expressions for the synthetic process of calcium carbonate are as follows:
CO2 (gas) + H2O ⇄ H2CO3
(2)
H2CO3 ⇄ HCO3− + H+
(3)
H CO3− ⇄ CO32 + + H+
(4)
Ca 2 + + CO32 − ⇄ CaCO3
(5)
Figure 8. SEM images (Compo) of calcium carbonate hollow particles synthesized at a high rate of bubbling of CO2 gas into a CaCl2 solution. NH3 rate of 5mL/L of solution and CO2 rate of 1.25 L min−1 (L of solution)−1.
Figure 8, in which the rate of change of pH was accelerated (the pH at the end of the bubbling was 6.8 and the time after the start of bubbling 240 s), becomes apparently thinner compared to that for the particles obtained in Figure 3d (the pH at the end of the bubbling was 6.8 and the time after the start of bubbling 360 s). In Figure 7, the change in the concurrently measured calcium ion (Ca2+) concentration is shown. The interesting phenomenon that the Ca2+ concentration increases at pH ≤7.8 was observed. As understood from expressions 4 and 5, as the CO2 bubbling time increases, the level of bicarbonate ion decreases and the level of carbonic acid increases, and it is known that at pH ≤8, this tendency becomes remarkable.12,14,19 These measurements may correspond to this phenomenon. It is noteworthy that for the bubbling conditions of Figure 2, at 240 s (bubbling time) fine particles stick to the inside of a shell, which are observed in Figures 3c and 6a,b; however, in Figure 8, fine particles are hardly observed (Figure 8a,b) and the shell becomes extremely thin and then the shape of the hollow particles becomes remarkably improved. This is because the fine particles, which are transformed into vaterite during the final stage at the end of the bubbling and precipitated inside the particles, redissolve because of the decrease in pH and the appearance of carbonic acid (H2CO3). In Figure 7, the level of Ca2+ increased at a pH value slightly below 7.5. This may show a redissolution of the fine particles inside the particles. This result may support the fact that
From expressions 3 and 4, the rate of change of pH can be enhanced by increasing the CO2 flow rate. Figure 7 shows the pH change curve [initial pH of calcium chloride of 9.5, ammonia/water rate of 5 mL/L of solution, and CO2 flow rate of 1.25 L min−1 (L of solution)−1] for the purpose of accelerating the rate of change of pH. When compared to the case in Figure 2, the pH variation rate is accelerated. 774
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Figure 9. Hollow structure formation process of the CaCO3 hollow particle.
definite hollow particles were obtained and concentrated in the pH range from 7.0 to 6.8 at the end of the gas bubbling in Figure 5.
ACKNOWLEDGMENTS
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REFERENCES
Part of this investigation is a result of R&D “Development of mass production process by bubble template method of hollow particles having nanoparticle structure”, supported by JST Supporting Program for Creating University Ventures (FY2005).
4. CONCLUSION The hollow structure formation mechanism of calcium carbonate particles prepared by a CO2 bubbling method was investigated. From our results, the hollow structure formation mechanism can be summarized as the schematic diagram shown in Figure 9. There are three stages (A, B, and C) that occur in the formation of the hollow structure. In stage A, when CO2 dissolves into a CaCl2 solution along with the beginning of CO2 bubbling at pH 9.5, an amorphous phase precipitates and forms primary particles containing water at pH ∼9.45. The primary particles aggregate at pH ∼9.1 to form secondary particles, and turbidity ensues. In stage B, when the pH was lowered to ≤9.1, the primary particles on the surface of the secondary particle transform into vaterite and a shell is formed. Because of the continuing bubbling of CO2 into the solution, the pH starts to rapidly decrease and a highly compacted shell is formed. Concurrent with the precipitation of vaterite on the surface, by dissolution and reprecipitation, internal amorphous particles rapidly dissolve and then fresh vaterite particles precipitate in a form that adheres to the outer shell and grows into a columnar structure. At this time, the volume contraction due to water release occurs and a hollow structure is formed. In the final stage, at pH ≤6.8, redissolution of particulates inside the secondary particles occurs and definite shell structures are formed.
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