3332
Ind. Eng. Chem. Res. 2006, 45, 3332-3334
CaCO3/Biopolymer Composite Films Prepared Using Supercritical CO2 Hiroaki Wakayama,*,† Simon R. Hall,‡ Yoshiaki Fukushima,† and Stephen Mann‡ Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan, and School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K.
Supercritical CO2 was used to produce CaCO3 thin films on biopolymers such as cellulose and chitosan coated on glass substrates from aqueous solutions of calcium acetate and poly(acrylic acid) (PAA). The scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray (EDX) results suggest that nucleation occurred throughout the biopolymer matrix, followed by crystal growth into vaterite particles. This methodology is a promising process for the fixation of CO2 and utilization of biomass. Introduction At present, the adverse effect on the global environment due to the activities of mankind has become a huge social problem. In the field of materials engineering, it is important to develop environmentally harmonious processes to produce materials with the least energy consumption and minimum exhaust into the environment. In the meantime, organisms synthesize inorganic crystals at normal temperatures and pressures.1-2 Examples of such processes can be found in the formation of bones, teeth, shells, and pearls. It is a great advantage that the waste generated in the environment by such in vivo processes that produce ceramics, the so-called biomineralization processes, is very small. If it would be possible to imitate biomineralization artificially, the ideal environmentally harmonized process would be realized. Biomaterials such as bones, teeth, shells, and pearls are good examples of novel functional materials with energy savings and the lowest environmental loading possible. For example, pearls and shells consist of laminated structures of calcium carbonate and biopolymers. They show a peculiar luster due to the multilayer reflection of the incident light, and their composite construction shows a high mechanical strength. Several attempts to produce elaborate material syntheses by biomineralization have been demonstrated.3-6 It became possible in recent years to mimic such basic biomineralization principles. However, CaCO3 products are limited, in powders, to a size of not more than micrometers in most cases. Although a few reports on the formation of CaCO3 thin films can be found, it takes several days to achieve it.5,6 Much effort is necessary to develop an industrial versatile route for bioinspired materials. We have demonstrated that supercritical CO27,8 can be used to facilitate the mineralization of CaCO3 to produce large area thin films.9 In this paper, the formation mechanism of CaCO3/ biopolymer composite films, derived from the results of scanning electron microscopy (SEM) observation and energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) measurements for the samples prepared in a short period of time of the process, was investigated. Experimental Section Chitosan (low molecular weight), cellulose (powder, -20 µm), acetic acid, copper(II) hydroxide, poly(acrylic acid) (PAA, * To whom correspondence should be addressed. Tel.: +81-56163-4762. Fax: +81-561-63-6137. E-mail:
[email protected]. † Toyota Central R&D Labs., Inc. ‡ University of Bristol.
Figure 1. Powder X-ray diffraction pattern of CaCO3 films on cellulose in the presence of PAA (2.4 × 10-3 wt %): (closed circles) vaterite.
Mw ) 2000), and calcium acetate monohydrate were purchased from Aldrich. Ammonia (28 wt % aqueous solution) was obtained from Lancaster. All reagents were of analytical grade and used without further purification. 1. Preparation of Biopolymer Films. Both 0.2 g of chitosan and 0.198 g of acetic acid were dissolved in 19.602 g of ionexchanged water (relative resistance, 20.0 MΩ‚cm). A 2 g portion of copper(II) hydroxide was dissolved in 20 mL of an ammonia aqueous solution (28 wt %). A 0.1 g portion of cellulose was dissolved in 10 mL of the supernatant liquid of the copper(II) hydroxide ammonia solution. The solid films of cellulose and chitosan were obtained by spin-coating on a glass substrate using a piece of spin-coating equipment (WS-400A6NPP/LITE/HSP, Laurell Technologies Co.). The cellulose films were washed with dilute hydrochloric acid and ion-exchanged water. The chitosan films were washed with a dilute ammonia aqueous solution and ion-exchanged water. 2. Preparation of CaCO3 Films. Poly(acrylic acid) (PAA) was dissolved in ion-exchanged water. Calcium acetate monohydrate was dissolved in the PAA aqueous solution at a concentration of 2.4 × 10-3 to 2.4 × 10-2 wt %. The biopolymer coated glass substrate was immersed in 80 mL of the calcium acetate monohydrate aqueous solution in a stainless steel autoclave (100-120 mL). The autoclave was filled with CO2 and heated in an oil bath at 323 K and 7.5 MPa for 2 h. 3. Characterization. The samples were observed by optical microscopy (Carl Zeiss, SV11). The SEM images and the energy-dispersive X-ray (EDX) spectra were obtained using a JEOL JSM 5600LV and a JEOL JSM 6300F FEGSEM. The X-ray spectra were collected using an X-ray diffractometer (D8 advance) with Cu KR radiation. The Fourier transform infrared (FT-IR) measurements were performed using an FT-IR spectrometer (Perkin-Elmer Spectrum One) using KBr disks.
10.1021/ie050659j CCC: $33.50 © 2006 American Chemical Society Published on Web 11/05/2005
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3333
Figure 2. FT-IR spectrum of CaCO3 films on cellulose in the presence of PAA (2.4 × 10-3 wt %).
Results and Discussion As shown in Figure 1, the XRD spectrum recorded from the samples prepared using cellulose or chitosan in the presence of PAA (2.4 × 10-3 wt %) shows peaks of vaterite (closed circles). For the CaCO3 film samples prepared with cellulose or chitosan without PAA, the result indicated that the films consisted of vaterite with a small amount of calcium acetate. A broad peak corresponding to amorphous CaCO3 can be seen in the sample of the CaCO3 film coated on glass substrates prepared without the PAA and biopolymers. The sizes of the CaCO3 crystallites determined from XRD data by the Scherrer equation were 25 nm (45 min), 43 nm (1 h), and 57 nm (2 h). Absorption bands for the polymers (cellulose and chitosan) and vaterite are observed in the FT-IR spectra of the CaCO3 film samples prepared with cellulose (Figure 2) or chitosan. The result for the sample prepared without polymers (cellulose and chitosan) in the presence of PAA (2.4 × 10-3 wt %) also indicated absorption bands for vaterite. The CO32- absorption band observed in the FT-IR spectrum of the sample prepared without polymers (cellulose and chitosan) and PAA confirms the amorphous CaCO3. The CaCO3 films prepared in the presence of PAA (2.4 × 10-3 wt %) consisted of donutlike vaterite particles with a small size distribution. Figure 3 shows the SEM images for the CaCO3 film sample prepared with cellulose in the presence of PAA (2.4 × 10-3 wt %). As seen in Figure 3a for the sample treated for 45 min, there were round tubercles in the polymer film. The EDX results showed calcium peaks at these tubercles. Calcium was not present in the spot except for these tubercles in the EDX spectra. The EDX spectra for the sample treated for 1 h showed the presence of calcium in the spots on and near the tubercles (Figure 3b). After treatment for 2 h, densely packed particles can be obtained (Figure 3c). The possible particle growth mechanism derived from the XRD, SEM, EDX, and FT-IR results could be elucidated as follows: first, calcium ions were dissolved into biopolymer matrixes and nucleation occurred throughout the biopolymer matrixes, followed by the crystal growth into vaterite particles until they were closed together with neighboring particles. The composite structure suggested is that of CaCO3 particles dispersed in biopolymer matrixes, that is, not a simple bilayer formation of a CaCO3 layer grown on a biopolymer layer. The relationship between the CaCO3 yields and the concentration of PAA is shown in Figure 4. The CaCO3 yields significantly increased at a concentration of 2.4 × 10-3 wt % in the presence and absence of biopolymers. The largest amount of CaCO3 precipitation was obtained on cellulose in the presence of PAA at a concentration of 2.4 × 10-3 wt %. The yields with or without the biopolymer decreased with the increase in the amount of PAA to 2.4 × 10-2 wt %.
Figure 3. SEM images of CaCO3 films on cellulose in the presence of PAA (2.4 × 10-3 wt %) treated for (a) 45 min, (b) 1 h, and (c) 2 h.
Figure 4. Plot of the CaCO3 yield as a function of the concentration of poly(acrylic acid) in calcium acetate aqueous solution.
In the absence of PAA, the weak interaction between the ionic species of Ca2+/CO32- and biopolymers or glass substrates resulted in the small amount of CaCO3 precipitation. According to Figure 4, the concentration of saturated PAA adsorbed on the cellulose film should be between 2.4 × 10-3 and 2.4 × 10-2 wt %. At 2.4 × 10-2 wt %, a large amount of free PAA stabilizes Ca2+ in the solution, resulting in the small amount of CaCO3 precipitation. At a concentration of 2.4 × 10-3 wt %, PAA adsorbed by the biopolymers recognized Ca2+ ions, inducing the crystallization of vaterite. A high calcium con-
3334
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
centration induced the precipitation of CaCO3 in the polymorph of vaterite that is kinetically stable in the accordance with Ostwald’s rule. The pH of the solution of calcium acetate and PAA is higher than the pKa for chitosan and for PAA under all the experimental conditions. The amount of PAA adsorbed to neutralized the amino units of chitosan is small,10 resulting in a small amount of CaCO3 precipitation. Supercritical CO2 facilitates the dissolution of CO2/HCO3-, resulting in the rapid precipitation of CaCO3, as compared to conventional processes using supersaturated CaCO3.8 Conclusion A large area of glossy CaCO3 thin films can be produced on biopolymers using supercritical CO2. The CaCO3/cellulose film consisted of densely packed vaterite of micrometer diameter sized particles. The formation mechanism of the CaCO3/ biopolymer composite film was elucidated that first calcium ions were dissolved into biopolymer matrixes and nucleation occurred throughout the biopolymer matrixes, followed by the crystal growth into vaterite particles until they were closed together with neighboring particles. In comparison with conventional methods using supersaturated CaCO3 that usually take place over a period of several days,5-6 supercritical CO2 facilitates the rapid precipitation of CaCO3 within biopolymer matrixes over 2 h. Thus, from the industrial viewpoint, this method should be a versatile process for the fixation of CO2 and the utilization of biomass to produce CaCO3/biopolymer composite materials with controlled mechanical and optical properties.
Literature Cited (1) Mann, S. Biomineralization. Principle and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: Oxford, 2001. (2) Sarikaya, M.; Aksay, I. A. Biomimetics. Design and Processing of Materials; American Institute of Physics: New York, 1995. (3) Heywood, B. R.; Mann, S. Template-Directed Nucleation and Growth of Inorganic Materials. AdV. Mater. 1994, 6, 9. (4) Colfen, H. Precipitation of Carbonates: Recent Progress in Controlled Production of Complex Shapes. Curr. Opin. Colloid Interface Sci. 2003, 8, 23. (5) Pach, L.; Hrabe, Z.; Komarneni, S.; Roy, R. Controlled Crystallization of Vaterite from Viscous Solutions of Organic Colloids. J. Mater. Res. 1990, 5, 2928. (6) Kato, T. Polymer/Calcium Carbonate Layered Thin-Film Composites. AdV. Mater. 2000, 12, 1543. (7) Jessop, P. G.; Leitner, W. Chemical Synthesis using Supercritical Fluids; Wiley-VCH: New York, 1999. (8) Darr, J. A.; Poliakoff, M. New Directions in Inorganic and MetalOrganic Coordination Chemistry in Supercritical Fluids. Chem. ReV. 1999, 99, 495. (9) Wakayama, H.; Hall, S. R.; Mann, S. Fabrication of CaCO3Biopolymer Thin Films Using Supercritical Carbon Dioxide. J. Mater. Chem. 2005, 15, 1134. (10) Zhang, S.; Gonsalves, K. E. Influence of the Chitosan Surface Profile on the Nucleation and Growth of Calcium Carbonate Films. Langmuir 1998, 14, 6761.
ReceiVed for reView June 8, 2005 ReVised manuscript receiVed September 27, 2005 Accepted September 30, 2005 IE050659J
