From Layered Double Hydroxides to ZnO-based Mixed Metal Oxides

Jun 3, 2010 - Carla G. Fonseca , Sérgio R. Tavares , Carla V. Soares , Bruno G. daFonseca , Fábio J.F.S. Henrique , Viviane S. Vaiss , Wladmir F. So...
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Chem. Mater. 2010, 22, 3933–3942 3933 DOI:10.1021/cm100383d

From Layered Double Hydroxides to ZnO-based Mixed Metal Oxides by Thermal Decomposition: Transformation Mechanism and UV-Blocking Properties of the Product Xiaofei Zhao, Fazhi Zhang,* Sailong Xu, David G. Evans, and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Received February 6, 2010. Revised Manuscript Received May 19, 2010

Thermal decomposition of layered double hydroxides (LDHs) is a way of fabricating mixed metal oxide (MMO) nanocomposite materials composed of metal oxide and spinel phases. A detailed understanding of the mechanism of the transformation of the LDH precursor to the MMO should allow the properties of the resulting MMO nanocomposites to be tailored to specific applications. Here we report a systematic investigation of the structure, composition, and morphology evolution from ZnAl-LDHs to ZnO-based MMO nanocomposites composed of ZnO and ZnAl2O4 on calcination at different temperatures. The nucleation and oriented growth of ZnO crystallites and the formation of ZnAl2O4 were monitored by high resolution transmission electron microscopy (HRTEM) combined with selected-area electron diffraction (SAED), in situ X-ray diffraction (XRD), solid-state 27Al magic-angle spinning nuclear magnetic resonance (27Al MAS NMR), and thermogravimetric and differential thermal analyses (TG-DTA). The layered structure of the LDH precursor was maintained as the temperature was increased from room temperature to 180 C. Upon further heating from 200 to 400 C, ZnO nuclei doped with Al3þ were first formed as an amorphous phase and then underwent oriented growth along the Æ1010æ direction. The high aspect ratio of the LDH platelets is responsible for the oriented growth of the resulting ZnO crystallites. On further increasing the calcination temperature, Zn2þ ions were continuously released from the amorphous phase resulting in the formation of crystalline ZnO nanoparticles doped with Al3þ, which are homogeneously dispersed throughout the amorphous phase. When the calcination temperature reached 500 C, Al3þ ions were released from the ZnO-like structure resulting in the formation of ZnAl2O4 spinel and the crystallinity of the spinel increased gradually with increasing temperature. Sintering of ZnO and ZnAl2O4, with concomitant loss of the platelet-like morphology, occurred below 800 C. UV-visible spectroscopy showed that the ZnO/ZnAl2O4 nanocomposite prepared by calcination of the ZnAl-LDH precursor at 800 C has superior UV-blocking properties to both commercial ZnO and a physical mixture of ZnO and ZnAl2O4. Introduction Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, are a family of two-dimensional anionic clays with the general chemical composition [MII1-xMIIIx(OH)2]An-x/n 3 mH2O, where MII and MIII are respectively divalent and trivalent cations, the value of the coefficient x is equal to the molar ratio MIII/(MII þ MIII), and Anis an anion with charge n.1 In the LDH structure, the metal octahedra share edges to form two-dimensional infinite sheets with a very similar structure to that of brucite [Mg(OH)2], with the partial substitution of MII cations by MIII cations leading to a positive charge on the brucite-like layers. The layers stack to build a three-dimensional network with the positive charge being balanced by anions arranged in hydrated interlayer galleries. A recent solid-state multinuclear NMR study2 of MgAl-LDHs showed that the metal *Corresponding author. Fax: (þ86) 10-64425385. E-mail: zhangfz@mail. buct.edu.cn.

(1) Cavani, F.; Trifir o, F.; Vaccari, A. Catal. Today 1991, 11, 173. (2) Sideris, P. J.; Nielsen, U. G.; Gan, Z. H.; Grey, C. P. Science 2008, 321, 113. r 2010 American Chemical Society

cations within the layers adopt an ordered arrangement when the Mg2þ/Al3þ ratio is 2:1. A major advantage of LDHs as functional materials or their precursors is that their composition is very flexible: the identity of the divalent and trivalent metal ions, their atomic ratio, and the nature of the interlayer anion can be varied over a wide range without altering the basic structure of the material.3 As a result, LDHs have a found a wide variety of applications in industry, including as additives in polymers, in biology and medicine, in catalysis, and in environmental remediation.4,5 Calcination of MIIMIII-LDHs is an alternative to traditional chemical and physical methods for the fabrication of a wide variety of mixed metal oxide (MMO) nanocomposite materials composed of a metal oxide phase (MIIO) and a spinel-like phase (MIIMIII2O4). The properties of the resulting MMO materials can be superior to those prepared by traditional methods. For instance, Ni-based MMO materials derived from LDHs have been widely used as catalysts (3) Evans, D. G.; Slade, R. C. T. Struct. Bonding (Berlin) 2006, 119, 1. (4) Leroux, F.; Taviot-Gueho, C. J. Mater. Chem. 2006, 15, 3628. (5) Tichit, D.; Coq, B. Cattech 2003, 7, 206.

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in;among other reactions;the steam reforming of methanol, the partial oxidation of methane to synthesis gas, the growth of carbon nanomaterials, and the liquidphase oxidation of benzyl alcohol to benzaldehyde.6,7 It has been found that the catalytic activity and selectivity of such catalysts is very dependent on the calcination conditions. Our group has reported that pure MIIFeIII2O4 spinel ferrites (MII = Mg, Co, and Ni) fabricated by thermal decomposition of the corresponding single MII-FeIIFeIII-LDH precursors show superior magnetic properties to those prepared by conventional ceramic and wet chemical routes.8 A detailed understanding of the mechanism of the transformation of the LDH precursor into the MMO is essential if the properties of the latter are to be successfully tailored for specific target applications. During the past few years, a variety of characterization techniques have been employed to investigate the structural evolution process during thermal decomposition of various LDH species under different conditions. For instance, in situ X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), extended X-ray absorption fine structure (EXAFS), solid-state 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR), and temperature-programmed reduction (TPR) analysis have all been employed in structural investigations of the thermal decomposition of MgAl- and NiFe-LDHs.9,10 Moreover, for LDHs containing oxidizable metal ions, such as CoIIAl-LDH, the impact of different atmospheres on the thermal decomposition mechanism has been investigated.11 Although there are some minor differences during the structural evolution of all the above LDH precursors to the final MMO, they are all believed to involve a topotactic transformation because the MO (such as MgO, NiO, CoO) is cubic with a lattice constant about half of that of the corresponding spinel (MgAl2O4, NiAl2O4, NiFe2O4, CoAl2O4) and has the same face centered cubic type oxygen sublattice as that in cubic spinels.12,13 In the topotactic transformation, oxide nucleation occurs on the (00l) plane of the LDH where the packing of cations is the densest, giving the lowest nucleation energy, resulting in the formation of the (111)-oriented MO and (111)-oriented spinels without rearrangement of the O atoms. The decomposition of ZnAl-LDHs has some unique features because;in contrast to the previously mentioned LDH-MMO systems;the solid-state transformation (6) Qi, C. X.; Amphlett, J. C.; Peppley, B. A. Appl. Catal., A 2006, 302, 237. (7) Morioka, H.; Shimizu, Y.; Sukenobu, M.; Ito, K.; Tanabe, E.; Shishido, T.; Takehira, K. Appl. Catal., A 2001, 215, 11. (8) Li, F.; Liu, J. J.; Evans, D. G.; Duan, X. Chem. Mater. 2004, 16, 1597. (9) (a) Millange, F.; Walton, R. I.; O’Hare, D. J. Mater. Chem. 2000, 10, 1713. (b) Bellotto, M.; Rebours, B.; Clause, O.; Lynch, J. J. Phys. Chem. 1996, 100, 8535. (c) Rocha, J.; del Arco, M.; Rives, V.; Ulibarri, M. A. J. Mater. Chem. 1999, 9, 2499. (10) del Acro, M.; Malet, P.; Trujillano, R.; Rives, V. Chem. Mater. 1999, 11, 624. (11) Perez-Ramı´ rez, J.; Mul, G.; Kapteijn, F.; Moulijn, J. A. J. Mater. Chem. 2001, 11, 821. (12) Fan, H. J.; Yang, Y.; Zacharias, M. J. Mater. Chem. 2009, 19, 885. (13) (a) Markov, L.; Petrov, K.; Lyubchova, A. Solid State Ionics 1990, 39, 187. (b) Li, C.; Wang, L. Y.; Wei, M.; Evans, D. G.; Duan, X. J. Mater. Chem. 2008, 18, 2666.

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from ZnO (wurtzite structure with a = 0.3250 nm and c = 0.5207 nm) into ZnAl2O4 spinel (cubic structure with a=0.880 nm) is not a topotactic transformation process, and the migration of both Zn and O occurs during the transformation of ZnO into the spinel.12 There have been some previous studies of the thermal transformation of ZnAl-LDH precursors into ZnO-based MMO materials. For instance, by means of an integrated X-ray powder diffraction (XPRD)-molecular dynamics (MD) approach, the structural modifications of ZnAl LDHs during thermal treatment in the temperature range in which the layered structure is retained (∼25-200 C) have been clearly delineated. The results showed that the identity of the interlayer anions (such as chloride and carbonate) present in the ZnAl-LDH affects the stability of the dehydrated ZnAlLDH.14 The XRD patterns, along with thermogravimetricdifferential thermal analysis (TG-DTA), FT-IR and N2 physisorption measurements, have also been investigated in order to interpret the solid base catalytic activity of ZnObased MMO materials derived by calcination of ZnAlLDHs at different temperatures.15 Despite these efforts to understand the thermal decomposition process, because of the limitations of the above-mentioned characterization techniques, it has not been possible to obtain much direct information about the transformation mechanism, such as the evolution of particle morphology, phase composition, and structure during the whole thermal decomposition process. High-resolution transmission electron microscopy (HRTEM) can provide real-space images with a resolution of down to 0.1 nm and has been shown to be a powerful tool for materials characterization.16 TEM has been used to monitor;during the process itself;the structural changes, nucleation processes, and phase evolution17 of a variety of reactions including the formation of zeolite A, the fabrication of b-oriented silica zeolite MFI films, the nucleation and growth of ZrO2 nanoparticles, and the formation of onionlike WS2 and MoS2. It should therefore be able to provide much more direct information, such as particle morphology, phase composition, and structure evolution, about the whole thermal decomposition process from the LDH precursor to the corresponding MMO nanocomposites. Moreover, as an example of a II-VI semiconductor, ZnO is widely used in industrial applications;such as nanoelectromechanical systems, gas sensors, photocatalysts, field-emission displays, and field-effect transistors;because of its wide band gap energy (Eg) (3.37 eV) and high exciton binding energy (60 mV).18 Different physicochemical methods have been employed to (14) Lombardo, G. M.; Pappalardo, G. C. Chem. Mater. 2008, 20, 5585. (15) Montanari, T.; Sisani, M.; Nocchetti, M.; Vivani, R.; Delgado, M. C. H.; Ramis, G.; Busca, G.; Costantino, U. Catal. Today 2009No. DOI:101016/j. cattod. 2009. 09.012. (16) (a) Hansen, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Topsøe, H. Science 2002, 295, 2053. (b) Kim, J. S.; Lagrange, T.; Reed, B. W.; Taheri, M. L.; Armstrong, M. R.; King, W. E.; Browning, N. D.; Campbell, G. H. Science 2008, 321, 1472. (17) (a) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958. (b) Li, S; Li, Z. J.; Bozhilov, K. N.; Chen, Z. W.; Yan, Y. S. J. Am. Chem. Soc. 2004, 126, 10732. (c) Zhao, N. N.; Pan, D. C.; Nie, W.; Ji, X. L. J. Am. Chem. Soc. 2006, 128, 10118. (d) Zink, N.; Therese, H. A.; Pansiot, J.; Yella, A.; Banhart, F.; Tremel, W. Chem. Mater. 2008, 20, 65. (18) Wang, Z. L.; Song, J. H. Science 2006, 312, 242.

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synthesize ZnO materials with the aim of improving its properties by controlling crystallinity, crystallographic orientation, crystallite size, and morphology.19-24 We25 and others26 have previously shown that ZnO-based MMO materials with tunable properties can be obtained from Zn-containing LDH precursors after high temperature treatment, but the properties of the materials have not been widely investigated. In this work, we report a systematic investigation of the mechanism of transformation of ZnAl-LDHs into MMOs using HRTEM combined with in situ XRD, MAS NMR, and TG-DTA techniques, and based on the results a mechanism for the decomposition of ZnAl-LDH has been proposed. Moreover, the variation in UV-absorbing as well as semiconductor properties of the ZnO-based MMOs obtained at different thermal decomposition temperatures shows how the properties of the material can be successfully tailored once we have a detailed understanding of the structural evolution during the thermal decomposition process. We believe this work will also provide some useful structural information to underpin the development of applications of such ZnO-based MMO materials in other fields. Experimental Section Preparation of the ZnAl-LDH Precursor and the Corresponding MMO Material. The ZnAl-LDH precursor was synthesized by a method involving separate nucleation and aging steps (the SNAS method) developed in our laboratory:27 An aqueous solution of Zn(NO3)2 3 6H2O and Al(NO3)3 3 9H2O with Zn/Al ratio of 2.0, 3.0, or 4.0 in deionized water ([Zn2þ]þ[Al3þ] = 1.2 M) and an aqueous solution of NaOH (1.92 M) and Na2CO3 (0.8 M) in deionized water were simultaneously added to a colloid mill with a rotor speed of 3000 rpm and mixed for 1 min. The resulting suspension was removed from the colloid mill and aged at 80 C for 48 h. The final suspension was washed several times with water and separated by centrifugation, and finally dried at 60 C for 24 h. Samples of the above-synthesized ZnAl-LDH precursor were calcined in air at temperatures ranging from 200 to 800 C for 4 h with a heating rate of 5 C/min. For comparison, commercial ZnO powder and ZnAl2O4 powder (fabricated by the coprecipitation method described in ref 28) were physically mixed with a Zn2þ:Al3þ molar ratio of 2:1 and denoted ZA-C1. Characterization. TEM and HRTEM images were recorded with a JEOL JEM-2100 transmission electron microscope. The accelerating voltage was 200 kV. The TEM specimens were prepared as follows: the ZnAl-LDH precursor was suspended in

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ethanol and ultrasonicated for 0.5 h. A drop of the resulting suspension was then deposited onto the TEM sample stage coated with a thin Si3N4 film. After TEM showed that the LDH precursor particles were well-separated, the TEM samples were heated to different temperatures under the same conditions as those used for the preparation of the bulk MMO materials. The TEM samples were then put in a vacuum drying oven under 60 C until the TEM characterization began. High-angle annular dark-field (HAADF) imaging was carried out to determine particle distributions for some of the calcined samples. The in situ powder X-ray diffraction (in situ XRD) data were recorded with a Shimadzu XRD-6000 powder diffractometer in the temperature range 200-800 C in air, using Cu KR radiation (λ = 0.154 nm) at 40 kV and 30 mA. Before characterization, the ZnAlLDH precursor was first dispersed in ethanol and then carefully ultrasonicated for some time at room temperature in order to minimize aggregation of the particles. The suspension was deposited on a single-crystal silicon wafer and dried at 40 C. This process was repeated several times until there was sufficient sample on the silicon wafer for the in situ XRD measurements. The samples were step-scanned in steps of 5/min in the 2θ range from 3 to 70 using a count time of 4 s per step. The rate of temperature increase was 10 C/min with a holding time of 5 min before each measurement. Average crystallite sizes were estimated using the Scherrer equation based on the full width at half intensity of the XRD peaks. TG-DTA traces were recorded out in air on a PerkinElmer Diamond thermal analysis system with a heating rate of 10 C/ min. Elemental analysis was performed using a Shimadzu ICPS7500 inductively coupled plasma emission spectrometer (ICPES). Samples were dried at 80 C for 24 h prior to analysis, and solutions were prepared by dissolving the samples in dilute hydrochloric acid (1:1). Solid-state 27Al MAS NMR spectra were measured on a Bruker AV300 spectrometer operating at 78.20 MHz with a pulse width of 0.5 s, spinning rate of 8000 Hz, and an acquisition delay of 0.5 μs between successive pulses to avoid saturation effects. The UV blocking properties as well as the band gap of asprepared materials were evaluated as described in previous references29 by measuring the absorption of the samples in the wavelength range from 800 to 280 nm. The as-synthesized samples were finely ground and dispersed in ethanol at a concentration of about 0.1 g L-1 and then ultrasonicated at room temperature for 30 min. A transparent colloidal solution was obtained. The UV-visible spectra of the above solutions were recorded on a Shimadzu UV-2501PC spectrometer using a quartz cell (1 cm path length), with pure ethanol as a blank.

Results and Discussion (19) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (20) Sun, X. W.; Huang, J. Z.; Wang, J. X.; Xu, Z. Nano Lett. 2008, 8, 1219. (21) Yang, H. G.; Zeng, H. C. J. Am. Chem. Soc. 2005, 127, 270. (22) Barreca, D.; Comini, E.; Ferrucci, A. P.; Gasparotto, A.; Maccato, C.; Maragno, C.; Sberveglieri, G.; Tondello, E. Chem. Mater. 2007, 19, 5642. (23) Polarz, S.; Neues, F.; van den Berg, M. W. E.; Gr€ unert, W.; Khodeir, L. J. Am. Chem. Soc. 2005, 127, 12028. (24) Yan, J.; Fang, X. S.; Zhang, L. D.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 8, 2794. (25) Zou, L.; Li, F.; Xiang, X.; Evans, D. G.; Duan, X. Chem. Mater. 2006, 18, 5852. (26) Liu, J. P.; Li, Y. Y.; Huang, X. T.; Li, G. Y.; Li, Z. K. Adv. Funct. Mater. 2008, 18, 1448. (27) Zhao, Y.; Li, F.; Zhang, R.; Evans, D. G.; Duan, X. Chem. Mater. 2002, 14, 4286. (28) van der Laag, N. J.; Snel, M. D.; Magusin, P. C. M. M.; De With, G. J. Eur. Ceram. Soc. 2004, 24, 2417.

With regard to the TEM characterization of ZnAlLDH and the corresponding products obtained by calcination at different temperatures, it should be emphasized that the MMO material is capable of reforming the original LDH layered structure upon contact with ambient moisture and traces of CO2. Therefore, in order to acquire accurate structural information, a high-vacuum environment was employed to store the MMO materials before the TEM characterization was carried out. The morphology and structure of the ZnAl-LDH precursor (29) (a) Zhang, D. S.; Fu, H. X.; Shi, L. Y.; Pan, C. S.; Li, Q.; Chu, Y. L.; Yu, W. J. Inorg. Chem. 2007, 46, 2446. (b) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H. J. Phys. Chem. B 2003, 107, 10159.

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Figure 1. TEM images and the corresponding SAED patterns (inset) of the ZnAl-LDH precursor.

Figure 2. XRD pattern of the ZnAl-LDH precursor.

were characterized by TEM and selected-area electron diffraction (SAED). The TEM image of the ZnAl-LDH precursor (Figure 1a) shows a typical hexagonal nanoplatelet with the edge length being about 150-200 nm. The TEM image also indicates the thin nature and high aspect ratio (ratio of the edge length to the thickness) of the ZnAl-LDH platelet. The SAED pattern (shown in the inset of Figure 1a) indicates that the ZnAl-LDH nanoplatelets are highly crystalline; the spots can be indexed to a hexagonal structure. The XRD pattern of the ZnAlLDH precursor (Figure 2) exhibits the characteristic features of layered materials, with narrow, symmetric, strong lines at low 2θ values and weaker, less symmetric lines at high 2θ values. The (003), (006) and (009) reflections are found at ∼11.7, 23.7, and 34.7 2θ and the two (110) and (113) reflections between 60 and 63 2θ can be clearly distinguished. On the basis of ICP and TG analysis results (see Figure S1 in the Supporting Information), the proposed chemical composition of the precursor is [Zn0.64Al0.36(OH)2](CO3)0.18 3 0.60H2O, indicating that the Zn/Al ratio in the precursor is similar to that in the initial synthesis mixture. Figure 3 shows the TEM images of the sample obtained after calcination of the LDH precursor at 200 C. As shown in the inset of Figure 3a, no set of spots can be seen in the SAED image;in contrast to the SAED pattern of the ZnAl-LDH precursor;which confirms that the

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structure of the LDH has been destroyed, resulting in an essentially amorphous phase. Although the platelet-like morphology of the LDH precursor has been maintained, the roughness of the surface has increased. In the magnified image (Figure 3b), some crystalline nanoparticles can be clearly observed uniformly embedded in an amorphous matrix; the particle size falls in a narrow range of 2-5 nm. Figure 3c shows a typical HRTEM image of the nanoparticles which exhibit an interplanar distance of 0.25 nm. This interplanar distance can be ascribed to the (1011) plane of hexagonal ZnO and a corresponding weak broad reflection (at ∼36 2θ) can be seen in the XRD pattern of the material heated at this temperature (Figure 4). The weak broad reflections located at ∼10, ∼25, and ∼60 2θ in this XRD pattern confirm that most of the ZnAl-LDH precursor has already decomposed into an amorphous phase. All these results confirm that decomposition of the ZnAl-LDH precursor results in nucleation of ZnO, and that the ZnO nuclei are dispersed homogeneously in the remaining amorphous oxide phase. This conclusion is further confirmed by the (HR)TEM images of the samples calcined at 250 C. As shown in Figures 5a and 5b, the amount of crystalline nanoparticles increased on raising the temperature, with the particle size being mostly in the range 4-7 nm. The nanoparticles remain uniformly distributed in the amorphous matrix and no aggregation was observed. The HRTEM image (Figure 5b) also shows the (1011) planes of hexagonal ZnO, suggesting that with increasing calcination temperature, the continuous release of Zn2þ from the amorphous oxide phase forms more ZnO nanoparticles, which are homogeneously dispersed in the amorphous phase. When the temperature was increased to 300 C, dehydroxylation of the brucite-like layers along with decomposition of the interlayer carbonate anion occurred (as confirmed by the TG-DTA curves in Figure S1 in the Supporting Information). The platelet-like morphology was well maintained (Figure 6a) and the surface roughness was further increased. Another remarkable structural feature of the MMO obtained by calcination at 300 C is the oriented growth of the ZnO. Structurally, ZnO has three groups of fast growth directions:30 Æ2110æ (([2110], ([1210], and ([1120]); Æ0110æ (([0110], ([1010], and ([1100]); and ([0001]. Each preferential growth direction usually results in a specific morphology. For instance, growth predominantly following the Æ0001æ orientation results in wire or rodlike ZnO structures,31 whereas growth in the Æ2110æ direction results in ZnO nanobelts.32 In our case, the SAED pattern of the plateletlike sample (Figure 6b) displays hexagonally arranged bright spots; this is due to preferred orientation and growth of the ZnO phase occurring along specific crystallographic directions. The peripheral facets intersecting one another at approximately 120 can be identified as {1010} planes and the corresponding zone axis is determined to be [0001]. (30) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, 829. (31) (a) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnsom, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. Adv. Funct. Mater. 2002, 12, 323. (b) Wu, J. J.; Liu, S. C. Adv. Mater. 2002, 14, 215. (c) Gao, P. X.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1315. (32) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.

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Figure 3. (a) TEM image and the corresponding SAED patterns (inset) of the MMO sample obtained after calcination at 200 C; (b) magnified TEM and (c) HRTEM images of the MMO sample.

Figure 5. (a) TEM image and (b) HRTEM image of the MMO sample obtained after calcination of the ZnAl-LDH precursor at 250 C.

Figure 4. XRD patterns of MMO samples calcined at temperatures ranging from (a) 200 to (g) 800 C, in steps of 100 C. Patterns of ZnO (hexagonal wurtzite structure, JCPDF 36-1451) and cubic ZnAl2O4 spinel (JCPDF 06-559) are included for comparison.

This indicates that the growth of ZnO by decomposition of the plate-like LDH precursor predominantly follows the Æ1010æ orientation; that is, except for the dominant lateral Æ0001æ surfaces, all the fast growth fronts were

covered by Æ1010æ facets (as illustrated in Figure 6c). The magnified image of the sample (Figure 6d) shows that, during the preferred orientation growth process, the ZnO nuclei expand in two dimensions and then merge to form a continuous phase. At the same time, the amount of amorphous phase decreases substantially during this process and small areas become isolated by the continuous ZnO phase. The HRTEM image (Figure 6e) demonstrates the highly crystalline character and obvious preferred orientation of the continuous ZnO phase; that is, only the lattice spacings of (1011) and (0002) planes of the wurtzite ZnO structure with an interfacial angle of ∼60 can be observed. We believe that this oriented growth mechanism for the ZnO is closely

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Figure 7. (a) TEM image and the SAED pattern (inset) and (b) HRTEM image of the MMO sample obtained after calcination of ZnAl-LDH at 500 C.

Figure 6. (a) TEM image and (b) the corresponding SAED pattern of the MMO sample obtained by calcination of ZnAl-LDH at 300 C; (c) (I) schematic illustration of the preferential growth orientation of the ZnO phase; (II, II) ball-and-stick models of wurtzite ZnO along [0001] and [1010] directions; (d-f) magnified TEM, HRTEM, and representative HAADF images, respectively, of the MMO sample.

associated with the high aspect ratio of the LDH precursor platelets. In order to describe the distribution of the ZnO phase more directly, a high-angle annular darkfield (HAADF) image was taken along the Æ1010æ diffraction in the SAED patterns of the ZnO, as shown in Figure 6f. In the HAADF image, the ZnO phase (the bright part) and the amorphous phase (the dark part) are distributed homogeneously over the whole platelet and no aggregates of ZnO are observed. It has been suggested8 that the uniform arrangement at an atomic-level of the MII and MIII cations in the layers of the LDH precursor contributes to the homogeneous distribution of the different phases formed during the calcination process. All these results indicate that thermal treatment at high temperature leads to an increase in crystallinity and preferred orientation growth of the ZnO phase, with the amount of amorphous oxide phase becoming significantly reduced. Correspondingly, in the XRD pattern of the sample calcined at 300 C, no reflections of the LDH precursor can be observed, and broad peaks which roughly coincide with those of a mixed oxide phase with a ZnO-like structure become stronger. The preferred orientation of the ZnO phase can also be observed in the XRD pattern

(Figure 4): at 300 C, the (0002) peak located at ∼34 2θ is the strongest reflection, and with the further increase in calcination temperature to 400 C, the ratio of the intensities of the (0002):(1010) peaks increases to 2.48, which is considerably larger than the corresponding value for the bulk ZnO material (0.73), and indicative of oriented growth of the ZnO phase in the Æ1010æ direction. Calcination of LDHs at even higher temperatures is known to give oxides of the divalent metal uniformly mixed with spinels.33 Figure 7 shows the TEM images of the sample calcined at 500 C. It can be observed that the surface of the nanoplatelet has become even rougher with further crystallization of ZnO and the formation of spinel ZnAl2O4 during the calcination of the LDH precursor (Figure 7a). The HRTEM image (Figure 7b) suggests that an amorphous phase no longer exists and that the crystallinity of the ZnO has further increased at this temperature. The (1011) and (0002) lattice spacings of ZnO can be observed over the whole platelet, which indicates the ZnO has now become the major phase in the MMO material. The formation of ZnAl2O4 nanoparticles is also confirmed by the HRTEM image. In small areas within the continuous ZnO phase, interplanar distances of 0.29 and 0.17 nm can be seen, corresponding to the (220) and (422) planes of cubic spinel ZnAl2O4. The particle size of the ZnAl2O4 is ∼5 nm. Correspondingly, in the XRD pattern of the samples calcined at 500 C (Figure 4), new peaks characteristic of a ZnAl2O4 spinel phase are also observed in addition to those of the ZnO phase. The calculated average particle sizes of ZnO and ZnAl2O4 are ∼4.9 and ∼4.7 nm respectively, which are in good accordance with the TEM results. However, no SAED pattern for the ZnAl2O4 phase is observed (see the inset in Figure 7a), which can be attributed to the low concentration and poor crystallinity of the small ZnAl2O4 nanoparticles in the MMO matrix at this temperature. The peaks due to ZnO and spinel ZnAl2O4 phases in the XRD patterns (Figure 4) of the MMO material become increasingly sharper as the calcination temperature is increased from 500 to 800 C, indicative of an increase in their crystallite sizes; the mean particle sizes of ZnO and ZnAl2O4 at 800 C are 18.1 and 19.3 nm, respectively. The TEM images of the ZnO/ZnAl2O4 composite materials (33) (a) Sun, G.; Sun, L.; Wen, H.; Jia, Z.; Huang, K.; Hu, C. J. Phys. Chem. B 2006, 110, 13375. (b) Kannan, S.; Venkov, T.; Hadjiibanov, K.; Kn€ozinger, H. Langmuir 2004, 20, 730. (c) Ruano-Casero, R. J.; Perez-Bernal, M. E.; Rives, V. Z. Anorg. Allg. Chem. 2005, 631, 2142.

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Figure 8. TEM images of the MMO samples obtained at (a) 700 C and (b) 800 C.

obtained at 700 and 800 C are given in Figure 8. As shown in Figure 8a, the macroscopic morphology of the material obtained at 700 C is significantly different from that observed at lower temperatures; the platelet is now obviously composed of nanoparticles some 5-10 nm in size. In the TEM image of the ZnO/ZnAl2O4 composite material obtained at 800 C (Figure 8b), it can be seen that the platelet-like structure has been totally transformed into a spherical morphology, and the estimated particle size (∼20 nm) is much larger than that of the samples obtained at lower temperatures. This indicates that there is partial sintering of the ZnO and ZnAl2O4 phases during the calcination process at 800 C. Such nucleation and oriented crystal growth of ZnO also can be observed during the decomposition of ZnAlLDH precursors with different Zn/Al ratios. Panels a and b in Figure S2 in the Supporting Information show the TEM images of ZnO-based MMO materials derived by calcination at 200 C of ZnAl-LDH precursors with different Zn/Al ratios. The amount of amorphous phase shows an obvious decrease as the Zn/Al ratio in the ZnAlLDH precursors increases to 3:1 and 4:1; the SAED patterns of the ZnO nanoparticles (insets in c and d in Figure S2 in the Supporting Information) indicate that the samples are polycrystalline ZnO with the wurtzite structure. The spacings in the corresponding HRTEM images (Figures 2c and 2d) can be ascribed to the (1011) planes of ZnO. When the calcination temperature was increased to 300 C, the oriented growth of the ZnO phase is apparent in both HRTEM images and SAED patterns (panels e and f in Figure S2 in the Supporting Information). The formation of ZnAl2O4 nanoparticles which are embedded in the oriented ZnO matrix also takes place upon calcining these ZnAl-LDH precursors at 500 C (see Figure S3 in the Supporting Information). Eventually, the macroscopic morphology of the nanoplatelets breaks down and spherical particles with a size of ∼20 nm are obtained. The results also indicate the sintering of ZnO and ZnAl2O4 phases during the thermal treatment of the different ZnAl-LDH precursors at 800 C (see Figure S4 in the Supporting Information). The behavior of the ZnAl-LDH precursors is therefore clearly essentially independent of the Zn/Al ratio in the material. Previous work in our group25 utilized 27Al MAS NMR spectroscopy to study the variation in the site occupancy of 27Al in ZnAl2O4 obtained by calcination of ZnAl-LDH at high temperatures in the range 500-800 C, as well as

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in the materials obtained by subsequent alkaline leaching of the ZnO. The ZnO-based MMO materials obtained by calcination of ZnAl-LDH over the whole temperature range (room temperature to 800 C) in our experiments were characterized using 27Al MAS NMR, as shown in Figure S5 in the Supporting Information. For the LDH precursor, the single resonance observed at around 10 ppm is consistent with Al located in octahedral coordination sites.34 On calcination at 200 C, along with the partial dehydroxylation of the layers and decomposition of the interlayer carbonate anions with evolution of CO2, some Al(OH)6 octahedra in the layers of the ZnAl-LDH precursor were destroyed and the liberated Al3þ cations migrated into the gallery space and adopted a tetrahedral coordination with O atoms as shown by the resonance at around 70 ppm. When the calcination temperature was raised to 400 C, the occupancy of tetrahedral sites by Al3þ cations showed an obvious increase. This suggests that most of the Al3þ ions in the amorphous phase have now migrated to the ZnO-like lattice and adopted tetrahedral coordination with O atoms. This is consistent with the TEM results, which indicated that the content of the amorphous phase decreased significantly and more ZnOlike structure formed at this temperature. For the bulk ZnAl2O4 spinel material, which has a facecentered cubic oxygen sublattice with two lattice sites available for cation occupancy, all the Zn2þ cations are located in the tetrahedral sites, whereas the Al3þ cations occupy the octahedral sites; this is the normal spinel structure. When the particle size is decreased to the nanoscale, however, the cation occupancy can vary because of surface effects and structure defects and therefore the formula of the ZnAl2O4 spinel may be written as (Zn1-γAlγ)[ZnγAl2-γ]O4, where the round and square brackets denote cation sites with tetrahedral and octahedral coordination, respectively, and γ represents the degree of inversion (γ=0 for a normal spinel). The XRD and HRTEM results show that calcination of the ZnAlLDH precursor above 500 C gives rise to the formation of ZnAl2O4 nanoparticles. However, in the 27Al MAS NMR spectra, the relative intensities of the peaks corresponding to tetrahedral sites (∼75 ppm) and the shoulder at ∼45 ppm are very similar to those for the MMO materials obtained at 300 and 400 C. This can be attributed to the low content and crystallinity of the ZnAl2O4. That is, most of the Al3þ ions are still located in the ZnO lattice structure in a tetrahedral coordination environment. Furthermore, based on our previous work,25 the nanoscale ZnAl2O4 has a high value of γ compared with that for the bulk (γ = 0), and a significant number of Al3þ ions in the spinel have tetrahedral coordination. With a further increase in temperature, more Al3þ ions are released from the ZnO-like structure forming ZnAl2O4 spinel and the crystallinity of the spinel also gradually improves, and therefore, the intensities of peaks in the (34) (a) Fripiat, J. J.; Alvarez, L. J.; Sanchez, J. S. Appl. Catal., A 2001, 215, 91. (b) Pecharroman, C.; Sobrados, I.; Iglesias, J. E.; GonzalezCarre~no, T.; Sanz, J. J. Phys. Chem. B 1999, 103, 6160.

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Figure 9. Schematic illustration of the mechanism of transformation from the ZnAl-LDH precursor to the ZnO-based MMO material at different temperatures: First Stage represents the dehydroxylation of the LDH precursor; Second Stage represents the nucleation and growth in the preferred orientation along the Æ1010æ direction of the ZnO phase, and the gradual disappearance of the amorphous phase; Third Stage represents the formation of ZnAl2O4 particles and the aggregation of ZnO and ZnAl2O4 phases.

NMR spectra associated with tetrahedral coordination decrease monotonically with increasing calcination temperature. In addition, for the ZnO-based MMO materials obtained at temperatures ranging from 300 to 600 C, a shoulder at ∼50 ppm is clearly observed in the range corresponding to five-coordinate aluminum. It has been suggested that decomposition of complexes with bridging bidentate anions (CO32- in our case) and metal cations in the interlayer galleries results in such five-coordinate species.35 For the samples calcined at 700 and 800 C, the decreasing intensity of the tetrahedral peaks and the doublet structure of the peak for octahedral coordination of Al3þ ions both indicate the highly crystalline nature of ZnAl2O4 obtained by calcination at these higher temperatures.28 On the basis of the above study of the structural and chemical evolution of the system by TEM coupled with the other complementary techniques, a suggested mechanism for the transformation from the ZnAl-LDH precursor to the intermediate ZnO-based MMO, and then to the final ZnO/ZnAl2O4 nanocomposite is presented in Figure 9. The main stages can be summarized as follows: (i) during the first stage, from room temperature to about 180 C, dehydroxylation of the LDH precursor occurs with retention of the layered structure; (ii) during the second stage, nucleation of the ZnO occurs with the destruction of the structure of the LDH, and ZnO nuclei are formed and become homogeneously dispersed in the amorphous oxide phase; thermal treatment at higher temperatures leads to the preferred orientation growth of the ZnO structure along the Æ1010æ direction. During this process, the crystallinity of the ZnO increases significantly and the amount of the amorphous phase decreases to almost zero; (iii) in the third stage, ZnAl2O4 nanoparticles are formed at 500 C and become dispersed in the continuous ZnO phase; the growth and sintering (35) Velu, S.; Rankumar, V.; Narayanan, N.; Swamy, C. S. J. Mater. Sci. 1997, 32, 957.

Figure 10. (a) UV-visible spectra for the ZnO-based MMO materials obtained by calcination at the temperatures indicated, commercial ZnO, and the physical mixture ZA-C1; and (b) band gap energies of ZnO-based MMO materials obtained by calcination at the temperatures indicated.

of both the ZnO and ZnAl2O4 phases can be observed at higher temperatures, and eventually, the platelet-like morphology of the LDH is totally destroyed at 800 C and the final nanocomposite is composed of spherical ZnO/ZnAl2O4 nanoparticles with a size of about 20 nm. Figure 10 shows the typical UV-visible spectra of the ZnO-based MMO materials obtained by calcination at different temperature. The spectra indicate the variation in band gap absorption edges of the materials with the structural and compositional evolution. As shown in Figure 10a, a continuous wide absorption band located below 400 nm gradually appears with increasing temperature, and this suggests that these ZnO-based MMO materials can serve as excellent UV-blocking materials. Although ZnAl-LDHs intercalated with different organic UV-absorbing molecules have previously been shown to have good UV-blocking properties,36 there have been no studies of the UV-blocking properties of the materials obtained by thermal decomposition of simple inorganic ZnAl-LDH precursors. It is well-known that ZnO is a (36) (a) Perioli, L.; Nocchetti, M.; Ambrogi, V.; Latterini, L.; Rossi, C.; Costantino, U. Microporous Mesoporous Mater. 2008, 107, 180. (b) He, Q.; Sato, Y. T. J. Phys. Chem. Solids 2004, 65, 395.

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strong UV absorber, not only in the UVB (290-320 nm) but also in the UVA (320-400 nm) range.37 Compared with a commercial ZnO material (curve i in Figure 10a), the higher visible light transmittance and higher UVblocking absorption of ZnO-based MMO materials obtained by decomposition of ZnAl-LDH precursors at temperatures ranging from 200 to 400 C can be attributed to the high dispersion of the ZnO phase within the amorphous aluminum oxide phase. ZnAl2O4 is a useful material and can be utilized as a catalyst, dielectric, optical material, and even as a transparent conductor.38 Compared with ZnO (Eg = ∼3.37 eV), the optical properties of ZnAl2O4 (Eg = ∼3.8 eV) make it a better candidate for a UV absorber.39 In our case, the formation and the increasing proportion of ZnAl2O4 in the final composite material have beneficial effects on the UV-absorbing properties of the final material, as can be seen in Figure 10a. When the calcination temperature was increased from 500 to 800 C, the absorption in both the UVA and UVB ranges of the ZnO/ZnAl2O4 samples increases markedly. This can be attributed to the enhanced cooperative effect of the mixed wide-band gap semiconductor. A small shoulder located at ∼370 nm can be observed in the spectrum of the ZnO/ ZnAl2O4 sample obtained at 800 C, which can be associated with electronic excitations between filled O 2p orbitals and empty Zn 4s orbitals in ZnAl2O4.40 For comparison, ZA-C1 was obtained by physically mixing ZnAl2O4 (prepared by a coprecipitation method and then calcined at 800 C) with commercial ZnO, and its XRD pattern and SEM image are shown in Figures S6 and S7 in the Supporting Information. The optical properties of this mixture are very different from those of the ZnO/ ZnAl2O4 composite materials obtained by decomposition of the ZnAl-LDH precursor: as can be seen in Figure 10a, the absorption of ZA-C1 in the UVA and UVB ranges is very low, indicating that ZA-C1 shows very weak UVblocking properties. Meanwhile, ZA-C1 also shows lower transmittance in the visible light range. These results highlight that the structural features of the ZnO/ZnAl2O4 composite material obtained by decomposition of the ZnAl-LDH precursor have beneficial effects on its UVblocking properties. It should be emphasized that, compared with the ZA-C1 mixture, the distribution of particle size in the ZnO/ZnAl2O4 composite material obtained from the ZnAl-LDH precursor is monodisperse and the ZnAl2O4 particles are homogeneously dispersed in the network of ZnO nanoparticles;rather than being simply mixed with them;which is a consequence of them both being derived by direct decomposition of the LDH. We (37) (a) Wang, R. H.; Xin, J. H.; Tao, X. M. Inorg. Chem. 2005, 44, 3926. (b) Lu, J.; Ng, K. M.; Yang, S. H. Ind. Eng. Chem. Res. 2008, 47, 1095. (c) Luo, Y. S.; Yang, J. P.; Dai, X. J.; Yang, Y.; Fu, S. Y. J. Phys. Chem. C 2009, 113, 9406. (38) (a) Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharis, M.; Gosele, U. Nat. Mater. 2006, 5, 627. (b) Wang, Y.; Liao, Q.; Lei, H.; Zhang, X. P.; Ai, X. C.; Zhang, J. P.; Wu, K. Adv. Mater. 2006, 18, 943. (39) Mathur, S.; Veith, M.; Haas, M.; Shen, H.; Lecerf, N.; Huch, V. J. Am. Ceram. Soc. 2001, 84, 1921. (40) Sampath, S. K.; Cordaro, J. F. J. Am. Ceram. Soc. 1998, 81, 649.

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believe that both these factors are prerequisites for the improved UV-blocking performance and high transmittance in the visible range. Moreover, because the calcination products of ZnAlLDH precursors have been employed as photocatalysts in the degradation of several organic compounds,41 it is of interest to investigate the variation of the band gap values in the series of ZnO-based MMO materials. The calculated band gaps of the materials are shown in Figure 10b, and the results indicate that all the materials are semiconductors. For the sample derived from the ZnAl-LDH precursor on heating below 200 C, the calculated band gap is 3.43 eV, which is higher than that of the pure ZnO material (3.37 eV). Such an increase in band gap can be attributed to the quantum confinement effect when the size of ZnO particle is reduced to the nanoscale,42 and to the coupling between the ZnO and amorphous alumina phases. With the evolution of the structure and composition with increasing calcination temperature, the band gap of the ZnObased MMO materials decreases and such a decrease is in good agreement with the red-shift in the UV-visible absorption. For the ZnO/ZnAl2O4 MMO obtained by calcination at 800 C, the band gap is 3.11 eV, lower than for pure ZnO, which could be due to the coupling between the ZnO and ZnAl2O4 in the final composite. All these results indicate that the optical and semiconductor properties of ZnO-based MMO materials can be tuned by tailoring the compositional and structural features of the materials by varying the calcination temperature. Conclusions The HRTEM technique combined with SAED, in situ XRD, solid-state 27Al MAS NMR, and TG-DTA techniques can be employed to study the thermal decomposition of ZnAl-LDHs. A structural model describing the decomposition mechanism has been proposed. The layered structure of the LDH precursor was maintained up to ∼180 C, and above this temperature, destruction of the LDH precursor structure with concomitant nucleation of ZnO occurred. Calcination at higher temperatures (∼300 C) led to the preferred orientation of ZnO along the Æ1010æ direction and the two-dimensional expansion of the ZnO phase in the platelet-like structure; correspondingly, the amount of the amorphous phase gradually decreased. The formation of ZnAl2O4 nanoparticles in the continuous ZnO phase and the subsequent aggregation of the two phases were successively observed on increasing the temperature from 500 to 800 C. The structural and composition evolution from ZnAl-LDH to the ZnO/ZnAl2O4 composite material had significant effects on the UV-absorbing and semiconductor properties. The final ZnAl2O4-containing composite material exhibited higher absorbance in the UVA and UVB ranges (41) (a) Zhao, Y. F.; Wei, M.; Lu, J.; Wang, Z. L.; Duan, X. ACS Nano 2009, 12, 4009. (b) Valente, J. S.; Tzompantzi, F.; Prince, J.; Cortez, J. G. H.; Gomez, R. Appl. Catal. B: Environ. 2009, 90, 330. (c) Seftel, E. M.; Popovici, E.; Mertens, M.; De Witte, K.; Van Tendeloo, G.; Cool, P.; Vansant, E. F. Microporous Mesoporous Mater. 2008, 113, 296. (42) Chang, S. M.; Doong, R. A. J. Phys. Chem. B 2004, 108, 18098.

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than materials obtained by calcination at lower temperatures. Furthermore, the ZnO/ZnAl2O4 nanocomposite also showed better absorbance in the UVA and UVB ranges than either a pure ZnO phase or a similar nanocomposite formed by physical mixing of ZnO with ZnAl2O4 fabricated by a coprecipitation method. Furthermore, with the evolution of structure and composition of the ZnO-based MMO materials, the band gap decreased from 3.43 to 3.11 eV.

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Acknowledgment. We acknowledge generous financial support from the National Natural Science Foundation of China, the 973 Program (2009CB939802), the Program for New Century Excellent Talents in Universities (NCET-070055), and the Fundamental Research Funds for the Central Universities ZZ0916). Supporting Information Available: Additional figures (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.