Templating Effects of Stearate Monolayer on Formation of Mg−Al

Xiaoxiao Guo , Fazhi Zhang , Sailong Xu , Zhaohui Cui , David G. Evans and Xue ..... Xiaodong Lei , Lan Yang , Fazhi Zhang , David Geraint Evans , Xue...
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Templating Effects of Stearate Monolayer on Formation of Mg-Al-Hydrotalcite Jun Xiang He,† Satoko Yamashita,† William Jones,‡ and Akihiko Yamagishi*,† Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan, and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom Received June 18, 2001. In Final Form: November 13, 2001 The effect of a floating stearate monolayer on the formation of Mg-Al-hydrotalcite (Mg-Al-LDH) has been studied. A subphase was an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) adjusted at pH ) 10.5 by 1.0 M NaOH. Templating effects were studied in the following two ways. First, we spread a chloroform solution of stearic acid onto the above subphase and waited 6 h at zero surface pressure (0.51 nm2 molecule-1). After the surface was compressed to 20 mN m-1 (0.26 nm2 molecule-1), the floating film was deposited onto mica as a Z-type film (method 1). Second, we spread the same chloroform solution onto the subphase and started to compress the surface to the molecular area (0.40-0.20 nm2 molecule-1) after 30 min. The floating film was maintained at the constant surface pressure for 6 h. Thereafter, it was deposited onto mica as a Z-type film (method 2). From the atomic force microscope (AFM) images of the films, it was concluded that the largest thin crystals (ca. 3 × 10 µm) with the thickness of 10 ( 1 nm were obtained according to method 2 when the deposition was done at 0.36 nm2 molecule-1 (0.78 mN m-1). This optimum molecular area was close to the area occupied by one negative charge for 3:1 Mg-Al-LDH (0.34 nm2). From the X-ray diffraction measurements and elemental analyses, the deposited films prepared by both methods were suggested to be Mg-Al-CO32- -LDH. As a comparison, a 0.1 mL portion of the subphase solution, which had been aged for 6 h, was cast onto mica and dried under the air. The AFM image of such a cast sample showed noncrystalline aggregates of small particles with the diameter of 0.2-3 µm. These results indicated that a stearate monolayer acted as a template for the crystallization of Mg-Al-LDH at an air-water interface.

1. Introduction Recently, there has been a lot of interest in applications of ceramic thin films for sensors, electrode modifiers, pyroelectric materials, and nonlinear optical devices.1 To prepare these films, high-temperature or high-pressure syntheses or sol-gel reactions in solutions are applied. In contrast to these traditional methods, attempts have been explored to promote the effect of templates on crystal nucleation and growth.2 For these purposes, a selfassembled film on a solid substrate or a Langmuir monolayer at an air/water interface is promising due to their well-ordered two-dimensional molecular arrangements.3,4 Calcium carbonate, iron oxides, cadmium or zinc sulfide, and more recently semiconducting films and zirconium oxides have been prepared under such amphiphilic monolayers.5-11 * Corresponding author. Tel: +81-11-706-2769. Fax: +81-11706-4909. E-mail: [email protected]. † Hokkaido University. ‡ University of Cambridge. (1) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87 (5), 735. (2) Xu, G.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (3) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353. (4) Copper, S. J.; Sessions, R. B.; Lubetkin, S.; Lubetkin, D. J. Am. Chem. Soc. 1998, 120, 2090. (5) Heywood, B. R.; Mann, S. Chem. Mater. 1994, 6, 311. (6) Lin, H.; Sakamoto, W. S.; Kuwabara, K.; Koumoto, K. J. Cryst. Growth 1998, 192, 250. (7) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (8) Miller, K. T.; Chan, C. J.; Cain, M. G.; Lange, F. F. J. Mater. Res. 1993, 8, 169. (9) Talham, D. R. Langmuir 2000, 16, 7449. (10) Armand, F.; Araspin, O.; Barraud, A.; Cabezo¨n, B.; Torres, T. Synth. Met. 1997, 84, 879.

In the present studies, we have examined such a templating effect of a floating monolayer on the formation of layered double hydroxides (LDHs). LDHs are a layered hydroxide consisting of edge-sharing octahedra of MII(OH)6 and MIII(OH)6 as schematically shown in Figure 1A.12 Usually the ratio of M(III) to M(II) takes a value of [M(III)]/ [M(II)] ) 0-0.33. Due to the substitution of M(II) with M(III), a layer is charged positively so that it possesses anion exchange properties. Exchanged anions are believed to exist in the interlayer spaces for a bulk material. Figure 1B shows the ideal structure of the basal surface of 3:1 Mg-Al-LDH. As for the synthesis of LDHs, they are prepared by coprecipitating M(II) and M(III) at an appropriate pH in the presence of anions. They are also prepared by hydrolyzing mixed oxides of M(II) and M(III) in water.13 Larger crystals are obtained when the pH of a solution is raised slowly for months by thermal decomposition of urea.14 LDHs are mostly used as a bulk material, but some attempts have been reported to exfoliate them infinitely into single layers.15 We have used a stearate monolayer as a negatively charged template for the formation of positively charged layers of MgII-AlIII-LDH. For that purpose, a stearate monolayer was formed onto a subphase containing Mg(NO3)2‚6H2O and Al(NO3)3‚6H2O at pH ) 10.5. The optimum conditions for the charge density of a monolayer and reaction time were sought for the growth of Mg-AlLDH crystals. Our main purpose was to develop a method (11) Li, H. H.; Mao, G. Z.; Simon Ng, K. Y. Thin Solid Films 2000, 358, 62. (12) Miyata, S. Clays Clay Miner. 1983, 31, 305. (13) Qiu, J.; Villemure, G. J. Electroanal. Chem. 1997, 428, 16. (14) Cai, H.; Hillier, A. C.; Franklin, K. R.; Num, C. C.; Ward, M. D. Science 1994, 266, 15551. (15) Adachi-Pagano, M.; Forano, C.; Besse, J. P. J. Chem. Soc., Chem. Commun. 2000, 91.

10.1021/la0109157 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

Templating Effects of a Stearate Monolayer

Figure 1. (A) The layered structure of LDH and (B) the schematic representation of an ideal surface of 3:1 Mg-AlLDH.

of preparing a large, thin LDH crystal with uniform thickness in a shorter time. Such crystals will be applicable as an electrode modifier or a sensor film.13

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Figure 2. The π-A curves when a chloroform solution of stearic acid (8.45 × 10-4 M) was spread onto the following subphases: (A) an aqueous NaOH solution at pH ) 10.5; (B) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) at pH ) 10.5; (C) an aqueous solution of Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5; (D) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5; (E) the same solution as in (D) but aged for 6 h before compression.

Materials. Stearic acid (Wako Pure Chemical Ind., Japan) was recrystallized two times from chloroform. Mg(NO3)2‚6H2O and Al(NO3)3‚6H2O were used as purchased. Water was purified with a Milli-Q SP reagent water system (Millipore) to a specific resistivity of 18.2 MΩ cm. Method and Instruments. The measurements of surface pressure versus molecular area (π-A) isotherms and the Langmuir-Blodgett (LB) film deposition were carried out by using a computer-controlled film balance system FSI-110 (USI System, Japan). A typical initial surface area was 10.0 cm × 13.0 cm. The composition of a subphase will be described in the results section. The temperature of a subphase was kept at 20.0 ( 0.2 °C. The pH of a subphase was adjusted by adding a small amount of an aqueous 1.0 M NaOH solution. The initial and final pHs were measured to make sure the variation of pH was less than 0.2. The rate of compression was 10 cm2 min-1. Brewster angle microscope (BAM) measurements were performed by a BAM instrument BM-1000 (USI System). A He-Ne laser (power 8 mW) was incident with a spot diameter of 3 mm on a subphase surface at the Brewster angle. The reflecting light at the air-water interface was imaged at 100× magnification, using a lens onto the chip of a conventional video. Atomic force microscope (AFM) images were obtained under the air with a Nanoscope III scanning probe microscope (Digital Instruments, Santa Barbara, CA), using Nanoprobe cantilever [Si3N4] integral tips with the spring constant of 0.06 nm-1 (Park Scientific). The measurements were done in the tapping mode with filters off. The “d” scan head was used, which had a maximum scan range of 100 µm × 100 µm × 6 µm. X-ray diffraction (XRD) patterns were measured with an X-ray diffractometer (Rigaku, Japan) on the film samples deposited onto a hydrophilic glass plate. The measurements were performed under the conditions of 40 kV, 30 mA, and Cu KR (λ ) 0.154 nm). The elemental composition of deposited films was analyzed with an ICP instrument (ICAP-575 II, Jarrel Ash). The samples were prepared by dissolving the deposited films into an aqueous hydrochloric acid solution (1 M).

of stearate ions were compared among the following five kinds of subphases: (A) an aqueous NaOH solution at pH ) 10.5, (B) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) at pH ) 10.5, (C) an aqueous solution of Al(NO3)3‚ 6H2O (5.3 × 10-4 M) at pH ) 10.5, (D) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5, and (E) the same solution as in (D) which was aged for 6 h after the spread of stearic acid. Compression started 30 min after spreading a chloroform solution of stearic acid (8.45 × 10-4 M) except for (E). At this pH, stearic acid is deprotonated completely to stearate ion. As shown by curve A in Figure 2, the π-A curve on subphase A shows the rise of the surface pressure around 0.22 nm2 molecule-1, nearly coinciding with the molecular area of the alkyl group of a stearate ion (0.20 nm2 molecule-1). On subphase B, the π-A curve rose at the same molecular area as in (A) but it shows a much steeper slope. The results indicated that the coordination of hydrated Mg2+ ions with stearate ions enhanced the rigidity of a monolayer probably due to the decrease of the electrostatic repulsion among the negatively charged carboxylate groups.17 The π-A curve on subphase C shows the rise of the surface pressure around 0.45 nm2 molecule-1 which is much larger than the molecular area of a stearate ion (curve A). It implied that the attachment of hydrolyzed Al3+ ions or their clusters expanded the stearate monolayer to a remarkable extent. The π-A curve on subphase D shows the rise of the surface pressure around 0.36 nm2 molecule-1 which lies between the lift-off areas on subphases B and C. The results suggest that the coordination of Mg2+ and/or partially hydrolyzed Al3+ ions took place simultaneously to result in the expansion of the monolayer of stearate ions. The inflection point of the curve around 0.26 nm2 molecule-1 indicates that the monolayer collapsed at the surface pressure above 20 mN m-1. The π-A curve on subphase E shows the lift-off area at the same molecular

3. Results Surface Pressure versus Molecular Area Curves. The surface pressure versus molecular area (π-A) curves

(16) Hosoi, H.; Akiyama, H.; Hatta, E.; Ishii, T.; Mukasa, K. J Appl. Phys. 1997, 36, 6927. (17) Kobayashi, K.; Takaoka, K. Thin Solid Films 1988, 159, 267.

2. Experimental Section

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Figure 3. The BAM images of an air-water interface when a trough surface was compressed 30 min after spreading a chloroform solution of stearic acid (8.45 × 10-4 M). (A), (B), and (C) are the surface images on subphase A at the surface pressure of 0.0 mN m-1 (0.31 nm2 molecule-1), 22.3 mN m-1 (0.18 nm2 molecule-1), and 45.0 mN m-1 (0.16 nm2 molecule-1), respectively. (D), (E), and (F) are the surface images on subphase D at the surface pressure of 0.0 mN m-1 (0.51 nm2 molecule-1), 0.8 mN m-1 (0.36 nm2 molecule-1), and 20.0 mN m-1 (0.26 nm2 molecule-1).

area as in (D), but it did not collapse above 40 mN m-1. During 6 h, the mixture of Mg2+ and hydrodyzed Al3+ ions was thought to convert into crystalline LDH under the monolayer of stearate. This resulted in the increase of the rigidity of the monolayer. Brewster Angle Microscopic Observations. The BAM images of an air-water interface were measured on compressing a trough surface 30 min after spreading a chloroform solution of stearic acid (8.45 × 10-4 M) onto the subphase of either (A) an aqueous NaOH solution at pH ) 10.5 or (D) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5. Parts A, B, and C of Figure 3 are the surface images on subphase A at the surface pressure of 0.0 mN m-1 (0.31 nm2 molecule-1), 22.3 mN m-1 (0.18 nm2 molecule-1), and 45.0 (0.16 nm2 molecule-1) mN m-1, respectively. Even at zero surface pressure, there existed small bright regions of less than 1 µm in diameter (Figure 3A). At higher surface pressure, these regions were closely packed and the intensity of these regions increased (Figure 3B,C). No change was observed when the surface was maintained for 2 h at 20 mN m-1. These results indicate that stearate ions formed an aggregate spontaneously at the air-water interface and they gathered on compressing the surface.16 Figure 3D-F are the surface images on subphase D at the surface pressure of 0.0 mN m-1 (0.51 nm2 molecule-1), 0.8 mN m-1 (0.36 nm2 molecule-1), and 20.0 mN m-1 (0.26 nm2 molecule-1). In the last figure (Figure 3F), the image was recorded 2 h after the surface was maintained at that surface pressure. At zero surface pressure, no bright domain was observed on the surface (Figure 3D). Instead, the homogeneously bright regions (ca. 1 mm in diameter) existed. On compressing the surface, there appeared more illuminating regions in the dark regions (Figure 3E). Such regions became clearer with time when the surface was maintained for 2 h at the constant surface pressure (Figure 3F). The results suggest that stearate ions interacted with

Mg(II) and/or hydrolyzed Al(III) ions already at zero surface pressure, inhibiting the formation of spontaneous aggregates as seen on subphase A (Figure 3A). Chemical reactions proceeded in 2 h under the stearate monolayer to result in the appearance of highly illuminating boundary regions (Figure 3F). AFM Observations of Stearate/Inorganic Hydroxide Hybrid Films. A chloroform solution of stearic acid (8.45 × 10-4 M) was spread onto three kinds of subphases: (B) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) at pH ) 10.5, (C) an aqueous solution of Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5, and (D) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5. The deposition of a floating film from each subphase was performed in either of the following two ways. According to the first method, the surface was compressed to 20 mN m-1 (0.26 nm2 molecule-1) after waiting for 6 h at zero surface pressure (0.51 nm2 molecule-1). The compressed film was deposited onto mica or a hydrophilic quartz plate by the vertical dipping method as a Z-type film (method 1). According to the second method, the surface was compressed to the molecular area of 0.40-0.20 nm2 molecule-1 30 min after spreading. The surface was maintained at the constant surface pressure for 6 h. Thereafter, the floating film was deposited onto mica as a Z-type film (method 2) at 20 mN m-1. In both of the methods, the transfer ratios were close to unity. The surface structures of these films were examined with an AFM. Figure 4A shows the AFM image of a film deposited from subphase B according to method 1. There are needlelike crystals observed on a surface. The orientation of crystals is widely distributed on the plane. The thickness of each crystal ranges from 30 to 200 nm. These crystals are considered to be magnesium hydroxides (brucite). Figure 4B shows the AFM image of a film deposited from subphase C according to method 1. Particles with

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Figure 4. The AFM images of the floating films of a stearate monolayer deposited onto mica according to method 1 (see text). The subphases were (A) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) at pH ) 10.5; (B) an aqueous solution of Al(NO3)3‚ 6H2O (5.3 × 10-4 M) at pH ) 10.5; (C) an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5. (D) The AFM image of a cast film of an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5. The solution was aged for 6 h.

the radii of 50-150 nm and the height of 5-15 nm are seen. Some larger aggregated regions are also observed. These are most probably assigned to aluminum hydroxides.

Figure 4C shows the AFM image of a film deposited from subphase D according to method 1. Platelike crystals are seen. The crystals had the size of 2-6 µm and the thickness of 20-50 nm. Some parts of the crystals

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Figure 5. The AFM images of the floating films of a stearate monolayer deposited onto mica according to method 2 (see text). The subphase was an aqueous solution of Mg(NO3)2‚6H2O (1.6 × 10-3 M) and Al(NO3)3‚6H2O (5.3 × 10-4 M) at pH ) 10.5. The monolayer of stearate ions was initially compressed to 0.36 nm2 molecule-1 (0.78 mN m-1). Thereafter, the monolayer was maintained at that surface pressure for 6 h. (A) and (B) are the images of the different parts of the same sample.

consisted of multilayered regions as indicated by the sectional analyses (shown below the figure). The results indicate that the growth of crystals took place in a layerby-layer way. The orientation of crystals is not unique. These crystals are thought to be Mg-Al-LDH. It should be emphasized that no such large crystal was observed when a drop of the subphase (0.1 mL), which had been aged for 6 h, was cast on mica and dried under the air. Figure 4D shows such an AFM image. The aggregates whose sizes ranged from 0.2 to 3 µm are seen. Thus, no crystal growth proceeded in a homogeneous phase during this hours under these conditions. The preparation of thin films of Mg-Al-LDH according to method 2 was attempted on subphase D. A chloroform solution of stearic acid was spread onto subphase D. After 30 min, the surface began to be compressed until the molecular area attained a certain value (0.40-0.20 nm2 molecule-1). After the surface was maintained at the constant surface pressure for 6 h, the floating film was deposited onto mica by the vertical dipping method as a Z-type film at 20.0 mN m-1. Figure 5A shows an example of such experiments, where the molecular area was chosen to be 0.36 nm2 molecule-1 at the surface pressure of 0.78 mN m-1. An extremely large crystal was seen whose size was ca. 3 × 10 µm and thickness was 10 ( 1 nm. Figure 5B shows another part of the same sample, showing that three thin crystals formed a multilayered region with the displacement of ca. 1 µm. The thickness of each layer is estimated to be 10 ( 1 nm. This latter image suggests again that the crystallization occurred in a layer-by-layer way with the unit of this thickness.

Table 1. Element Composition of the Mg-AL-LDH Sample sample LB film colloids

method 1 method 2 method 1 method 2

Mg2+ (mol/L)

Al3+ (mol/L)

Mg2+/Al3+

9.651 × 10-5 3.426 × 10-5 88.50 × 10-5 30.04 × 10-5

5.658 × 10-5 2.050 × 10-5 31.40 × 10-5 10.54 × 10-5

1.71 1.67 2.82 2.85

Similar experiments were performed by maintaining a trough surface for 6 h at the various molecular areas from 0.40 to 0.20 nm2 molecule-1. The growth of thin crystals was seen under these conditions. The largest thin crystals were found to grow when the surface was maintained at 0.36 nm2 molecule-1 as shown in Figure 5A,B. Therefore, this molecular area was considered to be an optimum area for the crystal growth of LDH from the present composition of a subphase. The waiting time was changed after the surface was compressed to the molecular area of 0.36 nm2 molecule-1. It was found that the surface density of thin crystals increased with the increase of waiting time until 6 h. No further change was observed when the waiting time was lengthened to 10 h. Analyses of Mg-Al-LDH Thin Films. The elemental compositions of the film deposited onto a glass plate under the conditions of Figures 4C and 5A were determined as described in the Experimental Section. The results are given in Table 1. The ratio of Mg2+ to Al3+ in the films is obtained to be [Mg]/[Al] ) 1.71 ( 0.2 and 1.67 ( 0.2 for the films prepared by methods 1 and 2, respectively. The values were contrasted with the value of [Mg]/[Al] ) 2.83 ( 0.2 for the Mg-Al-LDH colloids formed in a subphase.

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The latter value is well coincident with that of a subphase composition ([Mg(II)]/[Al(III)] ) 3.0). These results will be discussed in the discussion section. Figure 6 shows the XRD pattern for the film prepared by method 2 under the conditions of Figure 5A. The peaks at 2θ ) 11.0° (0.78 nm) and 22.5° (0.39 nm) correspond to the (001) and (002) basal spacings of Mg-Al-LDH, respectively. The (001) value is close to that of Mg-AlCO32- -LDH (0.78 nm).18 The same results were obtained for the film prepared by method 1 under the conditions of Figure 4C. The intercalated anion, CO32-, might be formed in a subphase by the dissolution of CO2 from the atmosphere. In our experiments, no elimination of CO2 was attempted during the crystal growth. 4. Discussion There have been a number of investigations on the crystallization of inorganic compounds under the floating monoalyers at an air-water interface.1-11 Most of the works, however, have been concerned with simple ionic compounds. Thus, the event occurring on a monolayer surface is merely the combination of cations and anions from a homogeneous subphase. The present work is the initial attempt to study the monolayer effect on the formation of metal hydroxides or Mg-Al-LDH. In these cases, the chemical reaction or hydroxylation should take place to form the bridging of metal ions by hydroxyl groups. In the present attempts, the following facts assist the presence of the templating effect by a stearate monolayer on the crystallization of Mg-Al-LDH: (1) From the X-ray diffraction measurements on the deposited films, the thin crystals were suggested to be Mg-Al-LDH with CO32- as an intercalated anion. (2) The sizes of Mg-Al-LDH crystals which formed under the monolayer are much larger (3-10 µm) than that of Mg-Al-LDH colloids formed homogeneously from a subphase (0.2-3 µm). (3) There exists an optimum condition for the negative charge density of a stearate monolayer for the growth of LDH crystals. In an aqueous solution at pH 10.5 as in subphase D, the formation reactions of Mg-Al-LDH are considered to be the binding of Mg2+ ions with Al(OH)3 to form a unit of Mg3Al(OH)8+ as below:19

Al(OH)3 + 3Mg2+ + 5OH- S Mg3Al(OH)8+

(1)

If we assume the similar reactions as in a homogeneous phase, the following reaction scheme is presented for the formation of 3:1 Mg-Al-LDH crystals: -

OOC- + Mg2+ S Mg2+--OOC-

(2)

3(Mg2+--OOC-) + Al(OH)3 + 5OH- S Mg3Al(OH)8+ -OOC- + 2-OOC- (3) in which -OOC- denotes the deprotonated carboxylic group of the stearate ion. Equation 2 is the binding of Mg2+ to the stearate ion. This reaction gave merely a slight effect on the expansion of the monolayer but increased its rigidity (curve B in Figure 2). Equation 3 is the formation of the LDH unit by the further binding of hydrolyzed Al3+ ions. This is considered to expand the monolayer remarkably (curve D in Figure 2). During this reaction, some of (18) Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. (19) Boclair, J. W.; Braterman, P. S.; Jiang, J.; Lou, S.; Yarberry, F. Chem. Mater. 1999, 11, 303.

Figure 6. The X-ray diffraction patterns of the deposited film prepared under the conditions of Figure 5A according to method 2. Table 2. Area Occupied by One Positive Charge on the Mg-Al-LDH Crystal Mg/Al

area occupied by one positive charge on Mg-Al-LDH crystal

5:1 4:1 3:1 2:1 1:1

0.49 0.42 0.34 0.25 0.17

the OH- groups are coordinated to Mg2+. Crystallization proceeds by way of the connecting of the above LDH units and is completed in 6 h. Two methods have been attempted to examine the templating effects of a stearate monolayer. According to method 1, the stearate ions form a small aggregated domain at zero surface pressure (Figure 3A). Thus, reactions 2 and 3 might proceed beneath such aggregates. The maximum size of the LDH crystals will be limited by the size of stearate aggregates. In fact, the crystal size of the LDH prepared in this method ranged from 1 to 3 µm (Figure 4C). The stearate aggregates might expand or shrink according to the surface charge density of produced LDH crystals in order to neutralize the positive charge of the crystals. According to method 2, reactions 2 and 3 proceeded beneath the monolayer of stearate ions. Under these conditions, there was no size limit of crystal growth. In fact, a large crystal of LDH was formed according to this method (Figure 5A). There might be an optimum condition of the initial charge density of the monolayer so that it coincides with the charge density of the produced LDH crystal. The optimum charge density of the stearate monolayer was found to be one negative charge (-COO-) per 0.36 nm2. On the basis of this, we compare the surface charge density between a stearate monolayer and the Mg-Al LDH crystal at various ratios of Mg/Al (Table 2). One negative charge for a stearate monolayer in the crystalline phase is estimated to occupy 0.22 nm2, while one positive charge on the Mg-Al-LDH crystal occupies 0.17-0.49 nm2 depending on the ratio of Mg(II) to Al(III). To compensate these differences, the stearate monolayer is expected to expand or shrink although the shrinkage of a crystalline monolayer would be difficult. Under the present experimental conditions, the optimum condition of the charge density of a stearate monolayer was found to be one negative charge per 0.36 nm2 molecule, which is close to the positive charge density of 1:3 Mg-Al-LDH (0.34 nm2). According to the elemental analyses, the ratio of Mg to Al in the Mg-Al-LD formed under such a monolayer was

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determined to be 1.67 or the value was rather closer to the 2:1 Mg-Al-LDH than to the 3:1 Mg-Al-LDH. To the contrary, the ratio of Mg to Al of the LDH colloids formed in a subphase was 2.82-2.85, which was close to the 3:1 Mg-Al-LDH. One reason for the higher value of the Al content for the deposited film was the attachment of aluminum hydroxide colloids to the stearate monolayer at the early stage of the reaction. In fact, noncrystalline aggregate is seen in the AFM images of the deposited films (Figures 4C and 5A). If this decreased the apparent ratio of Mg(II) to Al(III), the LDH crystal formed under the stearate monolayer can be 3:1 Mg-Al-LDH, satisfying the conditions of the coincidence of the charge density between the stearate monolayer and the LDH crystal surface. One of the interesting aspects as to the formation mechanism is that the thickness of the crystals is close to 10 nm, which corresponds to ca. 20 layers. Moreover, the multilayered regions of the observed crystals showed the steps of such thickness (Figures 4C and 5B). This is thought to be an indication that the LDH crystal grows

He et al.

by the unit of this crystal thickness. In other words, we derive the conclusion that LDH crystals are stable only when they form a stacking structure of ca. 20 layers. The situations may correspond to the fact that it is difficult to exfoliate LDH crystals to single layers unless anions with long alkyl chains are used as an intercalator.15 In such an attempt, the hydrophobic interactions among the alkyl chains may enhance the stability of a single LDH layer. A large thin crystal of Mg-Al-LDH as prepared according to method 2 (Figure 4A) might be of practical value for the use of sensors and electrode modifiers. In this respect, the present method of applying a negatively charged monolayer as a template opens a new method of preparing a large LDH crystal in a shorter time. Acknowledgment. Thanks are due to the Ministry of Education for the financial support. LA0109157