PAA Multilayer Film on a Nacre

Nov 15, 2010 - ABSTRACT: We devised a new process for fabricating an artificial nacreous structure on a natural nacre substrate. A film of alternating...
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DOI: 10.1021/cg100709m

Fabrication of a Nacre-Like Aragonite/PAA Multilayer Film on a Nacre Substrate

2010, Vol. 10 5085–5091

Akiko Hayashi, Toshihiro Nakamura, and Tomoaki Watanabe* 1-1-1 Higashimita, Tama-ku Kawasaki, Kanagawa 214-8571, Japan Received May 27, 2010; Revised Manuscript Received October 7, 2010

ABSTRACT: We devised a new process for fabricating an artificial nacreous structure on a natural nacre substrate. A film of alternating layers of natural nacre-like aragonite (CaCO3) and poly(acrylic acid) (PAA) was fabricated directly on an aragonitic nacre substrate from a solution at room temperature. Alternate repetition of aragonite growth in a CaCl2 solution and PAA drop coating gave a multilayer inorganic/organic nanocomposite film of alternating aragonite nanostacks and porous PAA. The microstructure and nanostructure of the film were characterized by field-emission scanning electron microscopy. The degree of orientation of deposited aragonite was determined by examination of fabricated aragonite-type strontianite (SrCO3) nanostacks. Analysis by X-ray diffractometry shows that the crystallographic c axis of the nanostacks is oriented perpendicular to the substrate surface. Analysis by transmission electron microscopy selected-area electron diffractometry shows that the a and b axes of the strontianite nanostacks are oriented perfectly. Orientation habit, crystallite size, and strontianite character can be controlled by the temperature and concentration of the SrCl2 solution.

Introduction Among organic/inorganic nanocomposites with multilayered structures, a particularly important such material is nacre, or mother-of-pearl. Fabrication of artificial nacre is a major goal of biomineralization research and particularly mimetics because nacre is formed by an environmentally friendly process. Nacre formation involves both soluble and insoluble organic molecules.1 Chitin and hydrophobic proteins form layered sheets that eventually inhibit further growth. A new layer can be induced by formation of a mineral bridge through small pores in the layered sheets. Soluble acid proteins consisting of amino acids with carboxy and hydroxy groups strongly interact with carbonate crystals and produce miniaturized nanosize building blocks that have controlled polymorphism and orientation. In addition, organic additives also control nucleation and crystallite size in biomineralization.2 For example, polyaspartic acid,3,4polyglutamic acid,5 and poly(acrylic acid)6-8 induce formation of a CaCO3 thin film on organic substrates such as cellulose, chitosan, and porphyrin. A CaCO3 thin film can also be formed on inorganic substrates such as silanized silicon9 and glass10,11 substrates. During formation of a CaCO3 thin film by interaction of polyaspartic acid and chitosan, the addition of magnesium ions into a CaCl2 solution encourages formation of aragonite and inhibits formation of calcite and vaterite.12 The c axis of the aragonite film radiates from a nucleation point.4 The orientation habit of this nacre-shaped product differs from that of a nacre plate, which grows perpendicular to the (001) plane. A process was recently reported for fabricating c-axisoriented aragonite on a nacre substrate in a CaCl2 solution in a closed system of NH4HCO3.13 We speculated that this same process might allow reproduction of the nacre-like orientation. However, the process is not directly applicable to homogeneous-film fabrication because the nacre surface is

covered with hydrophobic proteins that retard the growth of aragonite.14,15 To encourage homogeneous-film fabrication, we tried pretreating the nacre surface, and we succeeded in fabricating a nacre-like aragonite (CaCO3)/porous poly(acrylic acid) (PAA) multilayer on nacre by alternating aragonite film growth through mineral bridges and drop-coating of PAA film. Thompson et al.16 reported a transition to (001) aragonite growth on a (104) calcite surface in a mixture of EDTA-soluble proteins found in abalone nacre. In contrast, our new and simple fabrication process permits (001) aragonite film growth on nacre substrate without soluble proteins. Based on the multiplier effect of CaCO3 epitaxial growth and the drop-coating of PAA, our reported process is the first to enable fabrication of multilayers that imitate nacre’s nanocrystal structure. Experimental Section

*Corresponding author tel/fax: þ81-45-934-7206. E-mail: tomowata@ meiji.ac.jp.

A nacre plate of a Mytilus edulis shell (10  5 mm2) was washed with a 0.1 M NaOH solution before use as a substrate. CaCO3 (SrCO3) was deposited by slow diffusion of NH4HCO3 (2 g) into cellculture dishes containing a CaCl2 (SrCl2) solution (10 mL) in a closed container (15  20  18 cm3). Layered composites were prepared by alternate deposition of a thin film of aragonite (CaCO3) from the CaCl2 solution and drop-coating of a layer of 0.1 mM PAA/ethanol. To obtain multilayered structures that mimic those of the natural nacre of shells, it is important to control the orientation habit, crystallite size, and film thickness of the composite. Thus, we examined the effects of the SrCl2 solution concentration and temperature on crystallization using 1-100 mM SrCl2 solutions at 4-100 °C. Solution temperature was controlled in a refrigerator (GR-424BK, Toshiba) and an electric drying oven (FS-320, Advantec). The temperature of the refrigerator (4 ( 1 °C) was monitored with a digital thermometer (PC-3300, Sato Keiryoki). The morphology and structure of aragonite and strontianite (SrCO3) depositions were characterized with a field-emission scanning electron microscope (FE-SEM) equipped with an energydispersive spectroscope (EDS; S-5200, Hitachi), X-ray diffractometer (XRD; Rint 2200, Rigaku), and transmission electron microscope (TEM; JEM-2100, JEOL). The silicon (111) plane of Si powder (Wako) was used as the standard of integrated width for determining

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Figure 1. Secondary electron micrographs of treated substrate: (a) the substrate soaked in 2 mM CaCl2 solution for 4 h.; (b) the substrate soaked in 2 mM CaCl2 solution for 4 h after treatment of 0.1 M NaOH solution.

Figure 2. Schematic of CaCO3/PAA multilayer formation by reputations of CaCO3 growth in 2 mM CaCl2 solution-(NH4)2CO3 closed system and drop coating of 0.1 mM PAA/ethanol. crystallite size by the Debye-Scherrer method with sharp constant K = 1 for spherical crystallites.

Results and Discussion Formation of Aragonite Thin Film on Nacre Substrate. A layered composite film was successfully formed on a substrate by 10 repetitions of alternating crystallization of aragonite and drop-coating of PAA. We expected that aragonite would not grow uniformly on the substrate because the substrate surface is covered with hydrophobic proteins that retard its growth,14,15 and so we shifted our attention to the use of a NaOH solution for surface modification. Figure 1 shows that alkaline treatment of the substrate facilitates deposition of aragonite. An untreated substrate supports only partial growth of aragonite stacks, whereas a treated substrate is completely covered with an aragonite film, indicating that alkaline pretreatment of the substrate is important for fabrication of homogeneous aragonite film. Figure 2 shows a schematic image of the formation of an aragonite/PAA multilayer film on a nacre substrate. The substrate is pretreated with a 1 M NaOH solution, which dissolves the organic component of the substrate and enables observation of an arrangement of nanostacks that pile up in

parallel and form a multilayer structure. Aragonite film is deposited by slow diffusion of NH4HCO3 vapor into cellculture dishes containing a CaCl2 solution in a closed container, washed with deionized water, and dried. Finally, a 0.1 mM PAA/ethanol solution is dropped on the film. Figure 3 shows SEM images of the formation process. Figure 3a shows the pretreated substrate surface. Figure 3b shows the first layer of aragonite completely covering the substrate. Figure 3c shows an aragonite layer (coarse light regions) covered with a porous PAA layer (smooth dark regions). Figure 3d shows a cross section of the multilayer film on the substrate. Figure 3e shows 10 layers of aragonite film (lower porous multilayers) deposited on the substrate (upper dense multilayers). Figure 3f shows the top surface of the multilayers. PAA is generally used as a soluble accelerator for CaCO3 crystallization because it induces crystallization on numerous organic templates.6-11,17 However, we expected PAA to function as an interlamellar substrate for the multilayered aragonite film. Figure 3c shows that 0.1 mM PAA/ethanol solution includes a suitable amount of PAA for porous PAA film formation. The tenth layer (Figure 3f ) has the same surface morphology as the first layer (Figure 3b). The original longitudinal structure of the substrate is observed after the organic component of the substrate is dissolved in

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Figure 3. (a) Secondary electron micrographs of the fabrication process of a CaCO3 multilayered film on a substrate treated with 0.1 M NaOH solution; (b) first layer of aragonite; (c) aragonite layer covered with a porous PAA layer; (d) cross section of the multilayer film on the substrate; (e) ten layers of aragonite film on the substrate; (f ) top surface of the multilayers.

Figure 4. EDS spectra of a cross section of aragonitic nacre and a strontianite film fabricated in 5 mM SrCl2 solution for 6 h.

NaOH solution (Figure 2). The cross section of the substrate is composed of nanostacks combined with organic matter. However, because the nanostacks grew in inorganic solution, their cross sections have several openings. Thus, the original tight structure of the substrate clearly differs from the rough structure of the deposited film. Although the prepared multilayer film is identical to natural nacre in crystallite size (30-40 nm) and single-layer thickness (∼1 μm), it is far less dense than natural nacre because our aragonite/PAA multilayer fabrication process differs from the natural biomineralization process that occurs via self-assembly. Formation of a Strontianite Thin Film on a Nacre Substrate. Deposited aragonite crystals and substrate aragonite crystals cannot be differentiated by XRD because XRD provides information along only the thickness direction. Thus, to determine the orientation habit, crystallite size, and production of deposited aragonite crystals, we deposited strontianite on substrate and characterized the resulting strontianite layer by XRD. If the deposited strontianite is identical to substrate aragonite in orientation habit and crystallite size, then we can assume that the deposited aragonite is identical to substrate aragonite with respect to these two characteristics. We also observed a concentration dependence, temperature dependence, and temporal change of the orientation habit and crystallite size of the deposited strontianite. Figure 4 shows FE-SEM images and EDS spectra of substrate aragonite and of strontianite deposited on a substrate

Figure 5. X-ray diffraction pattern of a thin film surface fabricated on a substrate in 5 mM SrCl2 solution for 6 h.

from a 5 mM SrCl2 solution for 6 h. Deposited strontianite consists of bundles of parallel nanostacks covering the aragonite surface entirely. The morphology of the film is similar to that of nacre substrate treated with NaOH solution (Figure 2) and of film deposited on nacre substrate (Figure 3). The cross section of nacre substrate (Figure 4) lacks the longitudinal structure of parallel nanostacks because nanostacks in natural nacre adhere to organic matter. The EDS spectrum of the substrate shows the presence of C (KR), O (KR), and Ca (KR, Kβ), while that of the deposited film shows the presence of C (KR), O (KR), Ca (KR, Kβ), and Sr (KR). A Ca signal is evident on the deposited film because the region of electron diffusion is larger than the beam diameter, and the XRD analysis region is even larger. Moreover, EDS requires a higher acceleration voltage than does SEM to obtain sufficient spectrum intensity. These observations

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Figure 6. TEM-SAED image of a strontianite thin film fabricated on a nacre substrate.

Figure 7. Secondary electron micrographs of strontianite thin film fabricated on a substrate for 6 h in several concentrations of SrCl2 solution.

suggest that the deposited film contains only Sr. Also, Ca cannot be present in the deposited film because it does not diffuse through the solid phase at this temperature. Figure 5 shows an XRD pattern of the deposited film. The pattern suggests that deposited strontianite nanostacks have their c axis oriented perpendicular to the aragonite (001) plane of the substrate, indicating that deposited strontianite crystals and substrate aragonite crystals have the same orientation. Figure 6 shows a TEM image and selected-area electron diffractometry (SAED) patterns of the deposited film. Both show that the strontianite nanostacks are single crystals. The SAED pattern shows that the a, b, and c axes of the crystals in the deposited film are perfectly oriented. Thus, FE-SEM, TEM-SAED, and XRD results all demonstrate that deposited strontianite and substrate aragonite are similar in crystallographic orientation and morphology, suggesting that deposited aragonite is identical to substrate aragonite in terms of orientation habit. The degree of orientation (I(002)/[I(111) þ I(021) þ I(012)]) of film fabricated on a nacre substrate for 6 h was analyzed for different SrCl2 concentrations (5, 10, 20, 50, and 100 mM). (Film fabricated in 1 mM SrCl2 solution for as long as 12 h lacked sufficient XRD intensity to be included in this analysis.) Figure 7 shows secondary electron micrographs of deposited strontianite. The film’s degree of orientation is much smaller for fabrication in 20 mM solution than for fabrication in

Figure 8. Orientation degree analysis of a strontianite thin film fabricated on a nacre surface in 5 mM SrCl2 solution at different reaction times. Data are expressed as the mean ( SD of three individual experiments.

any of the other solutions. In 5 and 100 mM solutions, the nanostacks grew perpendicular to the substrate; in 20 mM

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Figure 9. Crystallite size ratio analysis of a strontianite thin film fabricated in 5 mM SrCl2 solution on nacre and aragonite of nacre at different reaction times. Data are expressed as the mean ( SD of three individual experiments.

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solution, they grew in the direction of the nucleation core and then formed a domelike aggregation. These observations agree with the measured c-axis degree of orientation. In the two highest-concentration (50 and 100 mM) solutions, crystals do not grow because the CO2 vapor-rich liquid surface is the dominant site of crystal growth. Thus, in high-concentration SrCl2 solutions, crystals tend to grow on the liquid surface because CO2 vapor reaches the substrate. In the lowestconcentration (1 mM) solution, sufficiently thick strontianite film fails to grow on the substrate. In 5 mM solution, highly caxis-oriented strontianite grows efficiently on the substrate because there is neither too much nor too little Sr. Strontianite thin film was fabricated on nacre substrate for 6 h at different concentrations of SrCl2 (5, 10, 20, 50, and 100 mM). The XRD intensities of the four strongest diffraction signals (I(002), I(012), I(021), and I(111)) are highest at concentrations of 10 and 20 mM and lower at 5, 50, and 100 mM. Thus, film thickness tends to decrease with increasing SrCl2 concentration. In low-concentration (5-20 mM) solutions, strontianite crystallite size increases with concentration, and in high-concentration (50-100 mM) solutions, it decreases with concentration, in accordance with the SEM images. This is in accordance with the well-known observation that heterogeneous-like crystallization encourages crystal growth

Figure 10. Secondary electron micrographs of aragonite thin film and strontianite thin film fabricated on nacre in 5 mM SrCl2 and CaCl2 solution for 6 h at different temperatures.

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almost the same as the crystallite size of aragonite present in nacre.1,18,19 Figure 11 shows the degree of orientation (I(002)/[I(111) þ I(021) þ I(012)]) of strontianite deposited on a nacre substrate in 5 mM SrCl2 solution at different temperatures (4, 20, 40, 60, and 100 °C). The degree of orientation in the c-axis direction correlates with temperature. At low temperatures, large crystals grow sparsely on the substrate and deposited strontianite grows inclined to the direction of the nucleation core and then forms a domelike aggregation because of the dearth of nucleation points. At room temperature, small crystals grow on the entire substrate. At high temperatures, strontianite deposits predominantly over the liquid surface rather than over the substrate, suggesting that deposition efficiency decreases at high temperature. Thus, reaction at room temperature promotes the deposition of well-orientated strontianite. Conclusions Figure 11. Orientation degree analysis of a strontianite thin film fabricated on a nacre surface in 5 mM SrCl2 solution at different temperatures. Data are expressed as the mean ( SD of three individual experiments.

in random directions and homogeneous-like crystallization encourages crystal growth in the same direction. We already knew that strontianite deposited from a 1 mM SrCl2 solution lacked sufficient diffraction intensity to permit us to determine the degree of orientation. However, we expected that an initial strontianite layer deposited from a 1 mM SrCl2 solution to suppress homogeneous nucleation would improve the degree of orientation of subsequent layers. Therefore, we pretreated the substrate in 1 mM SrCl2 for 6 h, soaked it in 5 mM SrCl2, and measured the thickness, orientation habit, and crystallite size of deposited strontianite at both stages. XRD intensities of strontianite film fabricated in 5 mM SrCl2 solution were analyzed for different reaction times (5 min, 1 h, 2 h, 6 h, and 11 h). The intensities of the four strongest strontianite signals (I(002), I(012), I(021), I(111)) increase with reaction time, suggesting that film thickness increases with reaction time. Figure 8 shows the degree of orientation (I(002)/[I(111) þ I(021) þ I(012)]) of strontianite deposited on nacre substrate in 5 mM SrCl2 solution at different reaction times. The degree of orientation tends to increase with reaction time, and it is four times higher if the substrate has been pretreated in 1 mM SrCl2 for 6 h and then soaked in 5 mM SrCl2 than if it has not. This result suggests that depositing the initial strontianite layer from low-concentration SrCl2 solution improves the degree of orientation of the next strontianite layer. Figure 9 shows the ratio of the crystallite size of deposited strontianite to the crystallite size of substrate aragonite at different reaction times. The size of deposited strontianite (30-40 nm) is half that of the substrate aragonite, and it decreases with reaction time. Figure 10 shows SEM images of the substrate aragonite and deposited strontianite. At 4, 20, and 40 °C, strontianite nanostacks are deposited perpendicular to the substrate, and crystallite sizes are about as large as those of the substrate aragonite. This is consistent with the findings of Hosoda et al.6 and Sugawara et al.,4 who reported that the crystallite size of aragonite deposited on an organic substrate in a mixed solution of aqueous organic matter and calcium is 10-30 nm,

A natural nacre-like aragonite (CaCO3)/poly(acrylic acid) (PAA) multilayer film was fabricated directly on an aragonitic nacre substrate from solution at room temperature. Alternate repetition of aragonite growth by slow diffusion of NH4HCO3 vapor into a 5 mM CaCl2 or SrCl2 solution and drop-coating of 0.1 mM PAA/ethanol solution creates an aragonitenanostacks/porous-PAA multilayer film. The prepared multilayer is identical to natural nacre in crystallite size (30-40 nm) and single-layer thickness (∼1 μm). The optimal concentration of SrCl2 for the growth of highly c-axis-oriented strontianite (SrCO3) is 5 mM. Deposition of the initial strontianite layer from a low-concentration SrCl2 solution improves the degree of orientation of the next strontianite layer. Crystallite size and the orientation of deposited strontianite can be controlled by the temperature and concentration of the SrCl2 solution but not by reaction time. Acknowledgment. We thank H. Yoshimura of Meiji University for helpful technical advice concerning FE-SEM and TEM analyses.

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