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Oct 30, 2009 - Scanning electron microscopy (SEM) images showed that the LDH crystallites are almost perpendicular to the substrate and interlaced wit...
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MATERIALS AND INTERFACES Effects of Varying the Preparation Conditions on the Dielectric Constant of Mixed Metal Oxide Films Derived from Layered Double Hydroxide Precursor Films Xiaoxiao Guo, Fazhi Zhang,* Sailong Xu, Zhaohui Cui, David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China

Magnesium/aluminum layered double hydroxide (LDH) films have been prepared in situ by means of urea hydrolysis using aluminum metal as both the substrate and sole source of aluminum. Scanning electron microscopy (SEM) images showed that the LDH crystallites are almost perpendicular to the substrate and interlaced with each other forming a network of vacant pockets. The film showed outstanding mechanical strength and high adhesion with the substrate. After being calcined at 500 °C, MgAl mixed metal oxide (MMO) films were obtained which retained the morphology of the precursor films. The values of dielectric constant (k) of the MgAl-MMO films were measured and showed low values in the range 2.0-4.7 arising from the low density of the interlaced structure. The effect of varying the preparation conditions of the LDH precursor films on the microstructure and k-value of the corresponding MMO films was investigated by varying the metal ion concentration, crystallization time, and crystallization temperature. The MMO films became more compact with increasing metal ion concentration and crystallization time, which resulted in increased k-values. Introduction Layered double hydroxides (LDH) are a class of anionic clays whose structure is based on brucitelike layers in which some of the divalent cations have been replaced by trivalent ions giving positively charged sheets.1-3 This layer charge is balanced by intercalation of anions in the hydrated interlayer regions. LDH can be represented by the general formula [MII1-xMIIIx(OH)2]x+(An-)x/n · yH2O. The identities of the divalent and trivalent cations (MII and MIII, respectively) and the interlayer anion (An-) together with the value of the stoichiometric coefficient (x) may be varied over a wide range, giving rise to a large class of isostructural materials.4 This flexibility in composition allows LDH with a wide variety of properties to be prepared and is one of their most attractive features.5 LDH have a wide variety of applications including as additives in polymers, as precursors to magnetic materials, in biology and medicine, and in catalysis and environmental remediation.6 Recently, ways have been reported to organize LDH microcrystals into large uniform films, which have widened the range of applications of LDH as catalysts,7 metallic anticorrosion coatings,8,9 and in optical, electric, magnetic devices.10-23 The resulting LDH films can be divided into two groups: films with the c-axis perpendicular to the substrate (i.e., the plateletlike LDH crystallites lie face down on the substrate) and films with the c-axis parallel to the substrate (i.e., the plateletlike LDH crystallites lie edge on to the substrate). For the former group, physical deposition methods are usually used, which means that the LDH nanoparticles,13-16 LDH suspension,17-19 or exfoliated nanosheets20-23 are separately prepared and subsequently deposited on a suitable substrate. For the latter group, the electrochemical24 and hydrothermal crystallization methods are * To whom correspondence should be addressed. E-mail: zhangfz@ mail.buct.edu.cn.

mainly used. In these films, the adhesion to the substrate is much stronger than in the former case because of the presence of chemical bonds between the substrate and the crystallites in the film; this is clearly advantageous as far as many applications of such films are concerned. For instance, Chen et al. prepared an NiAl-LDH film on the surface of anodized alumina with the c-axis of the platelets parallel to the substrate by an in situ growth method.25 Further work showed that after calcination, the resulting mixed metal oxide (MMO) film retained the original morphology of the LDH precursor film.26 After modification with laurate anions, the wettability of the LDH precursor film and the MMO film changed from hydrophilic to hydrophobic. On the basis of the above results, Zhang et al. prepared a laurate anion intercalated-ZnAl-LDH film with superhydrophobic properties.8 Investigation of the corrosion resistance of the film showed that the superhydrophobic nature of the intercalated ZnAl-LDH film provides a long-term corrosion protection of the coated aluminum substrate and provides an effective barrier to aggressive species. Liu et al. fabricated LDH films on divalent metal substrates such as Zn which acted as both reactant and support and a calcined ZnO/ ZnAl2O4 film prepared from such an LDH film was shown to be an effective anode material for Li-ion batteries.27 Apparently, these and other applications of LDH films have attracted considerable attention in recent years. In the microelectronic engineering area, the miniaturization of integrated circuits generally results in problems with interconnect resistance-capacitance delays, signal crosstalk, and power consumption. To deal with these problems, a new dielectric material with a very low dielectric constant (k) is required to replace the current interlayer dielectric-silicon dioxide (k ≈ 4).28 It is generally accepted that further reduction in dielectric constant must be achieved by reducing the density of the film, and that incorporating pores in materials is the most

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effective way. Actually, the k-value could be controlled by varying the film density of the films, for instance, Yan et al. prepared a tunable low k-value MFI film by introducing different amounts of γ-cyclodextrin as porogen.37 Theije et al.38 and Lee et al.39 obtained mesoporous organosilica films with different k-values by varying the processing conditions. However, the dielectric property of the LDH films or the mixed metal oxide (MMO) films derived from LDH films has not been reported as far. We have reported an MgAl-LDH film prepared on the surface of an anodized aluminum oxide (AAO) substrate by using the urea hydrolysis method.7 The film is composed of hexagonal platelike LDH crystals which are approximately perpendicular to the substrate and interlaced with each other forming abundant large pores. After calcination and rehydration, the resulting film could retain the morphology of the LDH precursor film. Inspired by the ways to reduce dielectric constant; here, we prepared a similar MgAl-LDH film on the surface of a pure aluminum substrate by using the same method. The anodization of the aluminum was eliminated which simplify the procedure. After calcination, an MgAl-MMO film was obtained. The k-values of the MMO films were measured and the effect of varying the preparation conditions of the precursor LDH films on the k-value is concerned. We believe the film densities of the MMO film could be controlled by varying the preparation conditions of the LDH precursor films and, thus, result in a variation of the k-values of the MMO films. Experimental Section Materials. The pure aluminum substrate (purity >99.99%, thickness 0.1 mm) was purchased from Beijing General Research Institute for Non-Ferrous Metals, and the following analytical grade chemicals were purchased from Beijing Yili Fine Chemical Reagent Co. and used without further purification: ethanol, NaOH, (NH2)2CO, Mg(NO3)2 · 6H2O. Deionized water was used in all the experimental processes. Fabrication of MgAl-LDH Films. The MgAl-LDH films were prepared by in situ crystallization on the pure aluminum metal substrate. The substrate was cleaned with ethanol, 0.5 wt % NaOH solution, and deionized water in sequence before use. In a typical procedure, 0.0075 mol Mg(NO3)2 · 6H2O and 0.06 mol (NH2)2CO were dissolved in deionized water to form a clear solution with a total volume of 100 mL. The Al substrate was placed vertically in a Teflon-lined stainless steel autoclave, which was placed in a conventional oven at 90 °C for 6 h. After completion of the LDH film growth, the substrates were taken out of the autoclave, rinsed with deionized water, and dried at room temperature. The experiments were repeated with different crystallization conditions. Synthesis of MgAl-MMO Films. The as-prepared MgAlLDH films were heated to 500 °C with a heating rate of 10 °C/min, kept at that temperature for 5 h, and then slowly cooled to room temperature to obtain the MMO films. Characterization. X-ray diffraction (XRD) patterns of samples were obtained on a Shimadzu XRD-6000 diffractometer, using Cu KR radiation (λ ) 0.154 nm) at 40 kV, 30 mA, a scanning rate of 10°/min, a step size of 0.02°/s, and a 2θ angle ranging from 3 to 70°. Room temperature Fourier transform IR (FTIR) spectra were recorded in the range 400-4000 cm-1 with a resolution of 2 cm-1 on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique. The morphology and the thickness of the films were investigated by using a scanning electron microscope (SEM, Hitachi S-4700). The accelerating voltage applied was 20 kV. All samples were sputtered with gold. Transmission electron microscopy (TEM)

Figure 1. XRD patterns of the Al substrate (A), the as-prepared MgAlLDH precursor film (B), and the MgAl-MMO powder scraped from the MMO film (C). The * symbol indicates peaks from the Al substrate, and ∆ indicates peaks of MgO.

images were obtained using a JEOL JEM-2100 transmission electron microscope. Elemental analyses for Mg and Al were performed with a Shimadzu ICPS-7500 inductively coupled plasma spectroscopy (ICP) instrument on solutions prepared by dissolving the powders scraped from the films in dilute HNO3 (1:1). The dielectric constant of the film was measured by using an Agilent 4294A Precision Impedance Analyzer at a frequency of 1 MHz as reported elsewhere.40-42 The details are given in the Supporting Information. Results and Discussion The MgAl-LDH film was prepared by in situ crystallization on a pure aluminum metal substrate, and the XRD patterns of the films (see Figure 1B) are similar to the film prepared on AAO substrate.7 Reflections characteristic of LDH can be clearly observed in Figure 1B,43,44 which demonstrates the successful formation of an MgAl-LDH film on the Al substrate. For the MMO powder sample depicted in Figure 1C, the characteristic peaks of a poorly crystalline MgO structure were detected in accordance with previous reports.45 Because tiny amounts of the aluminum substrate were also unavoidably scraped from the surface, the characteristic peaks of aluminum metal were also observed in the XRD pattern of the MMO powder sample. XRD pattern clearly indicates LDH film could be formed on pure aluminum substrate without anodization. The FTIR spectrum of the powder scraped from the films is shown in Figure S1 in the Supporting Information. The morphology of the film samples was characterized by SEM, and the images are shown in Figure 2. We can find that the morphology of the film (Figure 2A and B) we obtained is similar to the reported film on the AAO substrate; uniform hexagonal platelike LDH microcrystals with the ab-face almost perpendicular to the substrate can be clearly observed. However, we also find that the hexagonal platelets are not directly grown on the surface of aluminum, but on a dense base layer which is believed to be aluminum oxide.46 It seems aluminum oxide will first form when aluminum is immersed in a hydrothermal solution. After being calcined, the MMO film remained firmly immobilized on the aluminum substrate, as depicted in Figure 2C and D, and retained the hexagonal platelets of the LDH precursor film. The hexagonal platelets are interlaced with each other which produce a random array of pockets. The structures of the as-prepared MgAl-LDH films were further investigated by TEM. The TEM pictures of the scraped powder are shown in Figure 3 as below.

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Figure 2. SEM images of the MgAl-LDH precursor film (A, B) and the corresponding MMO film (C, D).

Figure 3. TEM images of the MgAl-LDH structure (A) and the MMO structure (B). The inset shows the corresponding selected-area electron diffraction (SAED) patterns.

Figure 3A shows the TEM images of one piece of a MgAlLDH platelet scraped from the LDH precursor film. We can clearly observe the hexagonal grain boundary of the crystal. In

addition, the corresponding SAED pattern exhibits hexagonally arranged bright spots, confirming its single-crystal nature. The LDH nanostructures are very sensitive to the electron-beam illumination. When we tried to obtain a high-resolution TEM image, the structure was rapidly damaged. All the results are in good agreement with the previous report of LDH films fabricated on abivalent metal substrate.27 Figure 3B shows the TEM images of the MMO platelet scraped from the corresponding calcined MMO film. The SAED pattern of the sample is shown in the inset image of Figure 3B which can be indexed as the [011] zone axis of cubic MgO. The spots are not bright which indicates the poor crystallinity of the MgO phase in MMO film which is in agreement with the XRD results. High adhesion strength of the film to the substrate is generally required to enhance the stability of its property. The adhesion of the MMO film to the aluminum substrate was analyzed according to the literature method,46 and the result is shown in Figure S2 in the Supporting Information. There was no significant peeling of the MMO layer after cross-cutting through the film i.e., the adhesion between the substrate and the MMO film is strong. The dielectric constant of a medium is defined as the ratio of the capacitances of a capacitor with and without the dielectric in place. It is the physical property that describes the electric polarizability of a dielectric. There are several microscopic mechanisms of polarization: electronic polarization (Re), distortion polarization (ionic polarization, Rd), and orientation polarization; the dielectric constant is the sum of the contributions from all these mechanisms. However, except for the inherent properties of the dielectric, the number of molecules per unit of volume (density) plays an important role in determining the k-value. In some level, the effect of varying the density on the k-value is even stronger than the effect of molecular polarizability. This explains why reducing the film density by introducing pores into a dielectric has been widely investigated for low-k material.47 The k-value of the MMO film was measured and it results as 2.8, which is in the low range. We suggest that the low k-value of the MMO film can be mostly attributed to the array of voids formed by the hexagonal platelets. In this case, the k-value will be controlled by the microstructure of the film. Therefore, we attempted to alter the microstructure of the film by varying the preparation conditions, such as metal ion concentration and crystallization time, and investigated the effect of this on the k-value. The metal ion concentration was first varied. LDH films were prepared with a fixed hydrothermal temperature of 90 °C, fixed crystallization time of 6 h, and variable metal ion concentration of 0.075, 0.10, and 0.15 mol/L. The morphology of the corresponding MMO films were investigated by SEM, and the results are shown in Figure 4 (Figure 4A-C). The MMO films clearly become more compact with increasing metal ion concentration. In addition, the hexagonal crystallites which make up the film decrease in size. This result is from the higher nucleation rates at high metal ion concentrations. The effect of varying the crystallization time on the microstructure of the film was investigated with a fixed metal ion concentration of 0.05 mol/L, fixed hydrothermal temperature of 90 °C, and variable crystallization time of 3, 6, and 9 h. The morphologies of the corresponding MMO films are compared as Figure 4D-F depicts. The size of the crystallites increases with crystallization time, and the crystallites are more densely packed in the film crystallized for 6 and 9 h than in the film crystallized for 3 h. Besides, in order to obtain quantitative comparison of the film

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Figure 5. Three-dimensional curved surface of the k-value of the MgAlMMO film as a function of the metal ion concentration (c) and the crystallization time (t) employed in the synthesis of the precursor LDH films. Table 2. Comparison of the k-Values Obtained by Measurement and the Simulated Equation Figure 4. SEM images of MMO films obtained by calcination of LDH films prepared with metal ion concentrations of 0.075 (A), 0.1 (B), and 0.15 mol/L (C) heating at 90 °C for 6 h; with synthesis times of 3 (D), 6 (E), and 9 h (F) carried out at 90 °C and metal ion concentration of 0.05 mol/L. Table 1. Effect of Varying Preparation Conditions on the k-Values of MMO Films film temperature/ concentration/ thickness/ Mg/Al °C (mol/L) time/h µm ratio densitya k-value 90 90 90 90 90 90

0.05 0.05 0.05 0.075 0.10 0.15

3 6 9 6 6 6

1.2 3.0 3.4 2.5 2.1 2.0

1.88 1.94 2.05 2.09 3.71 3.94

181508 220787 267887 237471 364627 552943

2.0 2.5 3.2 2.8 3.8 4.7

a The density is obtained by analyzing the area of the particles in the images with Image-Pro Plus 5.0 software.

density, Image-Pro Plus 5.0 software was used to analyze the area of the particles in the pictures which can be rationalized as the film density,48 and the result is shown in Table 1 together with the basic information and the k-values of the MMO film samples. According to Table 1, when the metal ion concentration is fixed, the film thickness, Mg/Al ratio, and the film density increase with the increasing crystallization time. These variations lead to an increment of the k-values. However, when the crystallization time is fixed, with the increasing of metal ion concentration, the film thickness decreases, Mg/Al ratio and the density of the film increase. These variations also lead to the increment of the k-values. Considering all these variations, the film thickness, Mg/Al ratio, and the film density could affect their k-values, and the effects could be very complex. However, in the case of the film density, it is very clear that increasing film density results in increased k-values which is in accordance with the reports.47 Since the k-value of the MMO films is sensitive to the metal ion concentration and crystallization time used in the preparation of the precursor LDH films, we constructed a three-dimensional curved surface equation to simulate the variation in k-value of

film temperature/ concentration/ thickness/ Mg/Al k-value k-value °C (mol/L) time/h µm ratio (measured) (calculated) 90 90

0.05 0.125

10 6

3.4 2.0

2.07 3.79

3.5 4.3

3.5 4.2

the MMO film as a function of the metal ion concentration and crystallization time according to the interpolation theory of algebraic surfaces.49 This is a polynomial interpolation surface of degree two, which can fit the experimental data well. The equation is 2537c2 - 1.428k2 - 94.28ck - 111.4c + 17.14k 16.91 - t ) 0, and the surface is shown in Figure 5, in which c is the metal ion concentration, t is the crystallization time, and k is the k-value. We find that the k-value (k), the metal ion concentration (c), and crystallization time (t) vary on a vertical saddle-shaped surface and the k-value increases with the increase of the metal ion concentration and crystallization time. We tested the agreement of the equation to the experiment by verifying with some other points on the surface and the result is shown in Table 2 as below. We can find that the calculated k-value is basically in accordance with the value measured in the experiment. That indicates that the curved surface we constructed will be helpful in preparing desired k-value dielectric in a low range. Conclusions MgAl-LDH films have been prepared on the surface of a pure aluminum metal substrate by in situ crystallization. LDH crystals interlace with each other with a vertical orientation to produce random voids, which results in the film having a low density. The film has outstanding mechanical strength and adhesion with the substrate. After calcination, the LDH films were transformed into MMO films, which retained the interspaced structure of the precursor films. The low density of the MMO film results in a low dielectric constant. Furthermore, we successfully controlled the microstructure of the MMO films by varying the crystallization conditions (metal ion concentration, and crystallization time) of the

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corresponding precursor LDH films. Relatively low k-values in the range 2.0-4.7 were achieved. Acknowledgment This work was supported by the National Natural Science Foundation of China, the Program for Changjiang Scholars and Innovative Research Teams in Universities (PCSIRT), the 111 Project (No. B07004), 973 Program (No. 2009CB939802), and the Program fro New Century Excellent Talents in Universities (No. NCET-07-0055). Supporting Information Available: Method of measuring the dielectric constants; FTIR spectrum of the powder scratched from the LDH precursor film and the calcined MMO film; SEM image of the MMO film on aluminum after testing for adhesion. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Braterman, P. S.; Xu, Z. P.; Yarberry, F. In Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2004; Chapter 8, pp 373-474. (2) Evans, D. G.; Duan, X. Preparation of Layered Double Hydroxides and Their Applications as Additives in Polymers, as Precursors to Magnetic Materials and in Biology and Medicine. Chem. Commun. 2006, 485. (3) Williams, G. R.; O’Hare, D. Towards Understanding, Control and Application of Layered Double Hydroxide Chemistry. J. Mater. Chem. 2006, 16, 3065. (4) Evans, D. G.; Slade, R. C. T. Structural Aspects of Layered Double Hydroxides. Struct. Bonding (Berlin) 2006, 119, 1. (5) He, J.; Wei, M.; Li, B.; Kang, Y.; Evans, D. G.; Duan, X. Preparation of Layered Double Hydroxides. Struct. Bonding (Berlin) 2006, 119, 89. (6) Li, F.; Duan, X. Applications of Layered Double Hydroxides. Struct. Bonding (Berlin) 2006, 119, 193. (7) Lu¨, Z.; Zhang, F.; Lei, X.; Yang, L.; Xu, S.; Duan, X. In Situ Growth of Layered Double Hydroxide Films on Anodic Aluminum Oxide/Aluminum and Its Catalytic Feature in Aldol Condensation of Acetone. Chem. Eng. Sci. 2008, 63, 4055. (8) Zhang, F. Z.; Zhao, L. L.; Chen, H. Y.; Xu, S. L.; Evans, D. G.; Duan, X. Corrosion Resistance of Superhydrophobic Layered Double Hydroxide Films on Aluminum. Angew. Chem., Int. Ed. 2008, 47, 2466. (9) Zhang, F. Z.; Sun, M.; Xu, S. L.; Zhao, L. L.; Zhang, B. W. Fabrication of Oriented Layered Double Hydroxide Films by Spin Coating and Their Use in Corrosion Protection. Chem. Eng. J. 2008, 141, 362. (10) Chen, X.; Fu, C. L.; Wang, Y.; Yang, W. S.; Evans, D. G. Direct Electrochemistry and Electrocatalysis Based on a Film of Horseradish Peroxidase Intercalated into Ni-Al Layered Double Hydroxide Nanosheets. Biosens. Bioelectron. 2008, 24, 356. (11) Ai, H. H.; Huang, X. T.; Zhu, Z. H.; Liu, J. P.; Chi, Q. B.; Li, Y. Y.; Li, Z. K.; Ji, X. X. A Novel Glucose Sensor Based on Monodispersed Ni/Al Layered Double Hydroxide and Chitosan. Biosens. Bioelectron. 2008, 24, 1048. (12) He, J. X.; Yamashita, S.; Jones, W.; Yamagishi, A. Templating Effects of Stearate Monolayer on Formation of Mg-Al-Hydrotalcite. Langmuir 2002, 18, 1580. (13) Wang, L. Y.; Li, C.; Liu, M.; Evans, D. G.; Duan, X. Large Continuous, Transparent and Oriented Self-supporting Films of Layered Double Hydroxides with Tunable Chemical Composition. Chem. Commun. 2007, 123. (14) Lee, J. H.; Rhee, S. W.; Jung, D. Y. Selective Layer Reaction of Layer-by-Layer Assembled Layered Double-Hydroxide Nanocrystals. J. Am. Chem. Soc. 2007, 129, 3522. (15) Lee, J. H.; Rhee, S. W.; Jung, D. Y. Solvothermal Ion Exchange of Aliphatic Dicarboxylates into the Gallery Space of Layered Double Hydroxides Immobilized on Si Substrates. Chem. Mater. 2004, 16, 3774. (16) Lee, J. H.; Rhee, S. W.; Nam, H. J.; Jung, D. Y. Surface Selective Deposition of PMMA on Layered Double Hydroxide Nanocrystals Immobilized on Solid Substrates. AdV. Mater. 2008, 20, 1. (17) Gursky, J. A.; Blough, S. D.; Luna, C.; Gomez, C.; Luevano, A. N.; Gardner, E. A. Particle-Particle Interactions between Layered Double Hydroxide Nanoparticles. J. Am. Chem. Soc. 2006, 128, 8376.

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ReceiVed for reView April 27, 2009 ReVised manuscript receiVed September 22, 2009 Accepted October 16, 2009 IE900671B