Zn−Al Layered Double

J. Phys. Chem. B 2006, 110, 21865-21872

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Facile and Large-Scale Production of ZnO/Zn-Al Layered Double Hydroxide Hierarchical Heterostructures Jinping Liu,*,† Xintang Huang,*,† Yuanyuan Li,† K. M. Sulieman,† Xiang He,‡ and Fenglou Sun‡ Department of Physics, Central China Normal UniVersity, Wuhan 430079, P. R. China, and Plasma Institute, South-Central UniVersity for Nationalities, Wuhan 430074, P.R. China ReceiVed: July 15, 2006; In Final Form: August 24, 2006

ZnO/Zn-Al layered double hydroxide (ZnO/Zn-Al LDH) hierarchical architecture, a new type of ZnObased heterostructure, has been synthesized directly on an Al substrate via a facile solution phase process. The firecracker-like heterostructures consist of uniform ZnO nanorods orderly standing at the edges of twodimensional (2D) surfaces of Zn-Al LDH nanoplatelets. Experimental result obtained from the early growth stage indicates that the underlying Zn-Al LDH nanoplatelet arrays are well constructed with their (00l) planes perpendicular to the surface of Al substrate. We propose that the “edge effect” of Zn-Al LDH and the “lattice match” between ZnO and Zn-Al LDH are vital to the growth of such heterostructures. The effects of total solution volume and NH3‚H2O concentration on the formation of heterostructures are investigated. It is found that other LDH-based complex structures can also be achieved controllably by varying the mentioned experimental factors. Our work is the first demonstration of fabricating intricate ZnO/Zn-Al LDH heterostructures as well as well-defined Zn-Al LDH arrays on an Al substrate, for which several promising applications such as optoelectronics, biosensors, and catalysis can be envisioned.

Introduction Over the past few years, much effort has been directed to the synthesis of nanoscale materials as well as nanodevices fabricated from them, because of their unique properties as compared to the bulk materials.1,2 Many types of nanostructures with one-dimensional (1D) and two-dimensional (2D) morphologies have been successfully realized, giving rise to a variety of important technological applications which are derived from the low dimensionality combined with the quantum confinement effect. Assembling these nanostructures into the desired wellordered architectures, in particular, is highly desired and widely studied.3 For example, various interesting structures, such as propellers, combs, urchins, dendrites, ellipsoids, and saws, have been synthesized using functional oxides, such as ZnO, CuO, and MgO.4,5 These complex and hierarchical nanostructures can be considered as nanoscale building blocks for future optoelectronic devices and systems. More importantly, hierarchical heterostructures composed of different components can be integrated into a wide variety of nanodevices with diverse functions.6a To date, Si-based heterostructures have been successfully explored by several groups.6,7 Like silicon, other semiconductive species such as ZnO have received particular interest because of their potential use in electronic, information, and biological technologies currently being enjoyed in everyday life.8 ZnO belongs to the wurtzite structure with preferential growth directions that can be utilized to prepare, in a controlled manner, a variety of high-purity nanostructures with excellent performance.8 In particular, ZnO * To whom correspondence should be addressed. Fax: +86-02767861185; e-mail: [email protected] (X.H.); ljpphyccnu@ mails.ccnu.edu.cn (J.L.). † Central China Normal University. ‡ South-Central University for Nationalities.

has a direct wide band gap (3.37 eV) and a fairly high exciton binding energy (60 meV), which makes it a good candidate for blue-ultraviolet lasing at room temperature.9a In addition, it has also been demonstrated to be quite effective in dye-sensitized solar cells,9b photonic crystals,9c and even biodevices because of its biocompatibility.9d As a result of rapid advancements in synthetic strategies, some kinds of ZnO-based heterostructures such as In2O3/ZnO, ZnO/Zn0.8Mg0.2O, ZnO/SnO2, ZnO/CNTs, SiC/ZnO, and GaN/ZnO have been fabricated in recent years.10 Most of these nanostructures constructed by two 1D components were typically prepared by a chemical vapor-phase process conducted at high temperatures. It is envisaged that these heterostructures could find applications in a variety of fields such as field emission, optoelectronics, sensors, cells, and so forth. However, the creation of new ZnO-based heterostructures under mild conditions remains a huge challenge for the development of nanoscience and nanotechnology.10a,e It is well-known that Zn-Al LDH is one of the representative layered double hydroxides (LDHs) that have a general formula [M(II)1-xM′(III)x(OH)2](An-)x/n‚yH2O (M ) Zn, Mg, Fe, Co, Ni; M′ ) Al, Cr, Ga; A ) CO32-, NO32-, Cl-).11a LDHs are a family of inorganic layered materials composed of positively charged metal hydroxide layers and interlayer anions and have recently attracted increasing attention owing to their potential applications in wide areas, such as cellular drug/gene delivery, electrochemical biosensors, nanocomposite, catalysis, and adsorption.11 Very recently, some efforts have been directed to the preparation of colloidal LDH suspensions12 and oriented films13 to enhance their performance in currently existing applications. In this article, novel hierarchical heterostructures that consist of 1D uniform ZnO nanorods orderly standing at the edges of 2D surfaces of Zn-Al LDH nanoplatelets are synthesized via

10.1021/jp064487v CCC: $33.50 © 2006 American Chemical Society Published on Web 10/05/2006

21866 J. Phys. Chem. B, Vol. 110, No. 43, 2006 SCHEME 1: A Schematic Diagram of the Preparation Procedurea

Liu et al.

2Al + 2OH- + 6H2O ) 2Al(OH)4- + 3H2

(1)

Zn2+ + 4OH- ) [Zn(OH)4]2-

(2)

Al(OH)4- + [Zn(OH)4]2- + CO32- + H2O ) Zn-Al LDHs (3) [Zn(NH3)4]2+ + 4OH- ) [ZnO2]2- + 4NH3 + 2H2O [ZnO2]2- + H2O ) ZnO + 2OH-

a

Zn-Al LDH nanoplatelet arrays first grow on the surface of Al substrate through an interfacial reaction. ZnO 1D nanorods then deposit on the Zn-Al LDH nanostructures via a heterogeneous nucleation and subsequent anisotropic crystal growth.

a facile solution-based route on a large scale. Aluminum (Al) substrate is employed in the synthetic process to provide Al3+ for the growth of Zn-Al LDHs and to collect relatively ordered nanostructures rather than random powders. The underlying ZnAl LDHs are grown with their (00l) planes perpendicular to the surface of Al substrate. The growth conditions such as the volume of reaction solution and the concentration of NH3‚H2O are correlated to affect the heterostructure formation. Control of the surface morphology, composition, and structure of the LDH-related film formed on the Al substrate is demonstrated for the first time. In addition to the ZnO/Zn-Al LDH heterostructure, densely packed ZnO on the LDHs, 3D urchinlike ZnO clusters on the LDHs, and well-defined Zn-Al LDH arrays constructed by a large number of interconnected nanoplatelets can also be achieved successfully. Experimental Section The formation of LDHs is the first step to obtain ZnO/ZnAl LDH hierarchical heterostructures. Conventionally, Zn-Al LDHs are prepared by the coprecipitation of solutions of Zn2+ and Al3+ salts with alkali, followed by aging at a certain temperature for a period of time. In that case, the obtained products are usually aggregates composed of many plateletlike 2D LDH nanocrystallites, without preferential orientation. Herein, Al substrate was first introduced to the experiments to overcome this problem. Well-aligned Zn-Al LDHs can be produced at the initial growth stage, as will be discussed later. The solution-based method used to prepare ZnO/Zn-Al-LDH heterostructure is fairly simple and reproducible, as demonstrated in our previous work.4h Typically, 150 mL of 0.065 M Zn(CH3COO)2 aqueous solution was prepared in a two-neck flask (equipped with a reflux condenser) under magnetic stirring. A clean Al substrate (30 × 30 × 0.15 mm) was then suspended in the prepared solution. After this, 50 mL concentrated (1.83 M) NH3‚H2O was added dropwise into the above Zn2+contained solution. The final solution was subsequently heated to 60 °C and was kept reacting at this constant temperature under stirring for 24 h. After the reaction was finished, the Al substrate was taken out of the solution, was washed with distilled water, and was dried at 60 °C for several hours. ZnO/Zn-Al-LDHs were formed tightly on the surface of the substrate. The basic idea for our synthesis is first interfacial reaction on Al surface and then heterogeneous nucleation and growth of ZnO, as schematically displayed in Scheme 1. These reactions are listed below.

(4) (5)

The products were characterized using powder X-ray diffraction (Cu KR radiation; λ ) 1.5418 Å), transmission electron microscopy (TEM and HRTEM, JEM-2010FEF), and scanning electron microscopy (SEM, JSM-6700F) equipped with an energy-dispersive X-ray spectrometer (EDS). Thermogravimetric (TG) and differential thermal (DTA) analyses were carried out on an SDT600 apparatus with a heating rate of 10 °C min-1 in N2 atmosphere. Room-temperature photoluminescence (PL) spectra were recorded on a JY-Labram spectrometer with a continuous wave He-Cd laser focused at ∼2 µm as the exciting source at 325 nm. Results and Discussion The general morphology of the product is shown in Figure 1A, B and in Figure S1 in the Supporting Information, which reveal that large-scale firecracker-like hierarchical structures are formed on the substrate. The medium magnification in Figure 1B clearly shows that most of these nanostructures exhibit 2-fold structural symmetry with a blurry “line” at the center and parallel nanorod arrays on both sides. Additionally, some sheetlike structures can also be observed, as indicated by arrows. Close SEM examination of the product reveals that the so-called lines are actually the edges of 2D nanoplatelets produced on the Al substrate. In general, the lengths and diameters of the nanorods are in the range of 500-700 nm and 90-110 nm, respectively. Further enlarged images of the hierarchical structures in Figure 1C-H have demonstrated several structural characteristics: (i) Occasionally, nanorod arrays can just grow on one side of the 2D nanoplatelets (Figure 1C). (ii) In addition to single row (Figure 1D, E), multiple rows of nanorods (Figure 1C, H) on the surface of the nanoplatelets can be seen. (iii) Some hierarchical structures possessing underdeveloped nanorods can be detected (Figure 1E). The photograph in Figure 1F clearly shows the discontinuous distribution of underdeveloped nanorods (with smaller lengths) at the edge of nanoplatelet surface as well as the intimate combination between the 1D rod and 2D platelet. An X-ray diffraction (XRD) spectrum of the as-synthesized hierarchical structure is shown in Figure 2A. For comparison purpose, the XRD pattern of the pure Al substrate was also recorded, as shown in Figure 2B. From careful analyses, the hierarchical structures can be determined to be the mixture of Zn6Al2(OH)16CO3‚4H2O (JCPDS 38-0486; space group R3hm, a ) 3.076 Å, c ) 22.80 Å) and ZnO (JCPDS 36-1451). The diffraction lines for these two materials are indicated by different symbols in Figure 2A. Especially for Zn6Al2(OH)16CO3‚4H2O, the two strongest diffraction lines are (003) and (006), confirming the layer structure of LDH materials.11d,12a,b Figure 2C is the enlarged view of the XRD pattern of hierarchical structure in the range of 25-50°, from which three main diffraction lines of wurtzite ZnO can be clearly observed. EDS spectrum of the complex structures provided in the Supporting Information

Facile and Large-Scale Production of ZnO/Zn-Al LDH

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Figure 1. SEM images of the heterostructures. (A and B) Low magnification. Many firecracker-like structures can be observed. The arrows shown in Figure 1B indicate the existence of 2D nanoplatelets. (C) An enlarged image showing nanorod bundles standing on one side of the edge of a nanoplatelet. (D) A platelet with both sides of the edge decorated with nanorod arrays. (E) Several underdeveloped heterostructures including shorter nanorods. (F) Enlarged image of the area marked by a square in Figure 1E. (G and H) Multiple rows of intimately combined nanorods.

(Figure S2) gives another evidence for the existence of Zn6Al2(OH)16CO3‚4H2O. To understand the assembly process of the novel hierarchical structures, the product obtained at the early growth stage was characterized. Figure 3A shows a representative SEM image of the product on Al substrate collected after 5 h reaction. Surprisingly, well-defined arrays of 2D interconnected nanoplatelets can be observed. These nanoplatelets are hexagonal with an average of 100 nm in thickness and several microns in lateral dimension. The corresponding XRD spectrum displayed in Figure 3B indicates that these 2D nanostructures are pure Zn6Al2(OH)16CO3‚4H2O. No diffraction lines from ZnO can be detected. Evidently, the as-prepared Zn-Al LDH product is of high quality in terms of morphology, size, uniformity, and crystallinity. The hexagonal morphology of the particles is thought to have developed naturally as a result of the crystallographic habit, that is, rhombohedral symmetry. In combination with the aforementioned results, the nanorods observed in Figure 1 are ZnO, and the present hierarchical structures can be generally classified as ZnO/Zn-Al LDH heterostructures. The above results are in agreement with our experimental concept described in Scheme 1. The final heterostructure consists of 1D ZnO nanorods orderly standing at the edges of 2D surfaces of Zn-Al LDH nanoplatelets. The morphologies of subunit ZnO and Zn-Al LDH indeed accord with their intrinsic crystallographic structures and growth habits. Thermogravimetric (TG) and differential thermal analyses (DTA) of the obtained heterostructures were also carried out to confirm their structural property. ZnO cannot undergo weight loss while heating. As shown in Figure 4, the heterostructure mainly undergoes a

weight loss in two steps when the temperature increases. Associated with this loss, two characteristic temperatures (T1: 130 °C, T2: 237 °C) can be observed in its DTA curve. Structurally, LDHs consist of cationic brucitelike layers in which divalent ions are partially substituted by trivalent ones and exchangeable anions and water molecules in the interlayer to balance the positive charges. From this viewpoint, the first weak endothermic DTA effect recorded around 130 °C corresponds to the removal of weakly bonded water molecules.12a This effect is immediately followed by a second endothermic effect, centered at 237 °C and extending up to ca. 350 °C, which is attributed to decomposition. Decomposition involves the simultaneous deanation (loss of intercalated CO32-) and dehydroxylation of the layers. The above two processes lead to the collapse of the layered structure, in accordance with previous results.11d,12a Considerable weight loss can still be observed in the TG above 350 °C. This weight loss extending to high temperatures was also observed previously11d and may be due to the slow release of residual gas (produced in the second step) from the micropores of the oxide residue on sintering. Accordingly, the observed weight loss of weakly bonded water and the weight loss corresponding to decomposition are ca. 8.0% (expected weight loss ) 8.5%, calculated from Zn6Al2(OH)16CO3‚4H2O) and 25.4% (expected weight loss ) 22.2%), respectively. The structure of the heterostructure was further studied by TEM. Figure 5A and B shows the TEM images of ZnO nanorods at different magnifications. After scraped from the substrate and ultrasonicated in ethanol, the ordered structure can still be observed. The SAED pattern obtained from a single nanorod (indicated by a circle) is shown in the insert of Figure

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Figure 2. (A and B) XRD results of ZnO/Zn-Al LDHs and Al substrate, respectively. (C) XRD pattern of the heterostructures in the range of 25-50 degrees (indicated by a red bracket in Figure 2A).

5A and B. It can be indexed to the single-crystal wurtzite ZnO with the growth orientation along [001] (c axis). HRTEM images (Figure 5C and Figure S3 in the Supporting Information) further demonstrate the single-crystal nature of ZnO nanorods. The lattice interplanar spacing along the length direction is determined to be 2.6 Å, corresponding to the (002) plane of hexagonal ZnO. From HRTEM images, it can be seen that these nanorods are actually constructed by fused nanoparticles with an average size of 4 nm. Microscopic grain boundaries and structure defects between some of the subunit particles can also be observed, indicating that the structure formation may follow the “imperfectly oriented attachment” mechanism.4h,i Figure 5D shows the TEM image of a fraction of Zn-Al LDH platelet, from which the angles of 120° between adjacent sides can be identified. The SAED pattern (inserted picture) of the platelet exhibits hexagonally arranged spots, confirming their singlecrystal nature. The Zn-AL LDH nanostructures are very sensitive to the electron beam illumination. When we tended to obtain their HRTEM images, the structure was rapidly damaged and was transformed into many tiny amorphous particles (Zn(Al)O: aluminum in zincite11f), possibly because of the heat-induced collapse of the layered structure. In the present work, the hierarchical heterostructures are designed in a one-pot synthesis. However, as shown in Figure 3, Zn-Al LDHs are first obtained on the Al substrate. That is, two major stages are involved in our synthetic process. (1) The growth of well-defined arrays of 2D LDHs is the first stage. We believe that dissolving Al surface should be a prerequisite to start the nucleation and growth of LDHs. Al(OH)4- is first

Liu et al.

Figure 3. (A) The arrays of Zn-Al LDH nanoplatelets obtained at the initial growth stage (5 h). (B) XRD result of these nanostructures showing ZnO cannot be detected.

Figure 4. TG and DTA traces of ZnO/Zn-Al LDHs recorded in N2 atmosphere.

formed at the interface by the reaction between OH- and Al (eq 1), while [Zn(OH)4]2- is generated in solution (eq 2). The next reaction between Al(OH)4- and [Zn(OH)4]2- (eq 3) will readily give rise to Zn-Al LDHs. Understandably, the high nucleation density correlated to the high concentration of Al(OH)4- released from the Al substrate in the interfacial regions is very important for the formation of such oriented nanoplatelets. During the growth of LDHs, one nanoplatelet can be affected by others around it and the growth is physically limited, benefiting the overlap and upright arrangement of the growing crystals.14 On the other hand, the nature of LDHs is the direct driving force for the formation of continuous porous arrays. One characteristic of the LDH aggregates is a houseof-cards structure typical of edge-to-face particle interactions.12 This type of structure results in interparticle porosity that is

Facile and Large-Scale Production of ZnO/Zn-Al LDH

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Figure 5. (A) Low-magnification TEM image of ZnO nanorods separated from the heterostructures. The insert is the SAED pattern taken from one nanorod marked by a circle. (B) A typical enlarged TEM image of ZnO nanorods. (C) HRTEM image of ZnO nanorod. (D) TEM image of a fraction of hexagonal Zn-Al LDH. The corresponding SAED is shown in the insert.

Figure 6. SEM images of densely packed ZnO nanorods grown on the Zn-Al LDHs: (A) Low magnification image; (B) high magnification image. (C) The corresponding XRD result, indicating that the dominant diffraction lines belong to ZnO and Al substrate.

characteristic of most clay materials. (2) The heteronucleation and growth of ZnO nanorods is the second stage. The as-formed LDHs can act as supports and frameworks to grow well-ordered ZnO nanorods. The chemical reactions have been shown in eqs 4 and 5. As is well-known, the hexagonal ZnO c plane is 6-fold symmetry with a lattice constant a ) 3.249 Å, and the rhombohedral Zn-Al LDH has a lattice constant a ) 3.076 Å. The calculated lattice mismatch between these two materials on the a-axis is ∼5%. This small lattice mismatch favors the nucleation of ZnO with its (002) plane parallel to the basal plane of Zn-Al LDH. Thus, ZnO crystals subsequently grow along the fastest growth orientation,4h,14 the direction, and finally become 1D nanorods standing vertically on the 2D surfaces of Zn-Al LDHs. We note that, from the SEM images,

all ZnO nanorods grow at the edges of 2D Zn-Al LDH surfaces rather than on the whole surfaces. To explain this interesting result, we should refer to the “edge effect” of the LDHs. As recently reported by Gursky et al.,12c atoms on the surfaces and edges of LDHs are not fully coordinated, and high reactivity is expected for these surfaces and edges. In small particles, the chemistry is dominated by the surface atoms, as they represent a greater percentage of the total structure. However, as the particles become larger, such as several microns observed in the present work, the lack of coordination has a greater effect at the edges.12c Thus, ZnO preferentially nucleates at the edges of the 2D surfaces. Although some exceptions can be detected, most of the formed hierarchical heterostructures display uniform

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Figure 7. Room-temperature PL spectra of the heterostructures and densely packed ZnO nanorods on Al substrates.

shapes with limited rows of ZnO nanorods at the edges of 2D Zn-Al LDH nanoplatelets (usually one row on each side). The separation of the two growth stages plays a crucial role in the formation of well-constructed heterostructures. It is noticed that two competing reactions can occur with regard to [Zn(OH)4]2-/[Zn(NH3)4]2+ in our synthetic process. [Zn(OH)4]2-/ [Zn(NH3)4]2+ can either react with Al(OH)4- and CO32- to form Zn-Al LDH or react with OH- to form ZnO. The successful separation of the two growth stages herein benefits from the continuous release of Al3+ from Al substrate and thus the high concentration of Al(OH)4-. Al(OH)4- may preferentially react with [Zn(OH)4]2- and transform to LDHs, which was previously proven to be thermodynamically favored.11a,11b,13a As evidenced by the SEM, only the growth of Zn-Al LDHs can be observed

Liu et al. at the initial stage. We propose that a large quantity of Al(OH)4consumes nearly most of the [Zn(OH)4]2- during this period. As the reaction time increases, the concentration of Al(OH)4decreases and the reaction between [Zn(OH)4]2- and OH- for the growth of ZnO dominates accordingly, producing welldefined heterostructures. At the second growth stage, the preformed LDHs might also be further crystallized. Next, we study the effect of solution volume on product morphology. Figure 6A and B and Figure S4 in the Supporting Information show the typical product grown with a total solution volume of 150 mL (the absolute quantities of reactants are kept unchanged). In this case, densely packed ZnO nanorods are formed on the Zn-Al LDHs. The presence of Zn-Al LDHs below the ZnO nanorods is proven by the XRD result shown in Figure 6C, although the quantity is very small (EDS result shows carbon element cannot be detected, Figure S5 in the Supporting Information). With a relative smaller volume, it seems that the nucleation and growth of ZnO nanorods can be accelerated, possibly because of the higher concentrations of [Zn(OH)4]2- and [ZnO2]2-. By contrast, the reaction between [Zn(OH)4]2- and Al(OH)4- should be suppressed, and a large number of ZnO nanorods rather than Zn-Al LDHs are formed. The room-temperature PL spectra of heterostructures and densely packed ZnO nanorods are shown in Figure 7. Both of the two nanostructures exhibit a strong band-edge emission at ca. 383 nm resulting from free-exciton annihilation,4f,4h and they exhibit a weak and broad green emission centered at ca. 550 nm. The UV emission intensity of densely packed ZnO is larger than that of heterostructures because of the larger quantity of ZnO in the former sample. It is generally accepted that the green emission results from the recombination of electrons with holes trapped in singly ionized oxygen vacancies (Vo+).4f-h Since the visible emission is usually considered to be produced during

Figure 8. (A) SEM image of Zn-Al LDH nanostructures obtained after 24 h reaction using 1.25 M ammonia. Insert is the cross-sectional picture. (B) Low-magnification SEM image of ZnO/Zn-Al LDH nanostructures attained after 24 h reaction using 2.50 M ammonia. (C) An enlarged SEM image of ZnO urchinlike 3D nanostructure. Insert shows a single nanorod exhibiting a hexagonal cross section. (D) Magnified SEM image of Zn-Al LDHs marked by a white circle in Figure 8B. The scale bars in the inserts of Figure 8A, C, and D are 1 µm, 100 nm, and 500 nm, respectively.

Facile and Large-Scale Production of ZnO/Zn-Al LDH ZnO preparation and post-treatment, in the present case, the assembly of ZnO nanoparticles into 1D nanorods should give rise to oxygen vacancies and defects in the structure. Similarly, Gui et al.15 reported a visible light emission in the PL spectrum of ZnO nanoribbon and claimed that the vacancies were induced by the oriented texture particles in the assembles. On the other hand, the interactions/interfaces between ZnO and Zn-Al LDHs might affect the defect concentration and distribution in ZnO crystals, although Zn-Al LDHs could not emit any luminescence. We have further demonstrated that lower concentration of NH3‚H2O can produce arrays of 2D Zn-Al LDH nanoplatelets, and much higher concentration of NH3‚H2O can fabricate 3D urchinlike ZnO clusters on the LDHs. No well-defined heterostructures are formed under these conditions. Figure 8A shows the 2D Zn-Al LDH nanoplatelets grown with 1.25 M NH3‚ H2O for 24 h. It can be seen that the as-prepared platelet has larger thickness but smaller lateral dimension compared with that shown in Figure 3A. With few OH- in solution, the reaction rate at the interface slows down, giving rise to a failure to separate the mentioned two growth stages, and thus, the epitaxial growth of ZnO is not favored. We only obtain ZnO short nanorods in the solution. Figure 8B reveals the low-magnification SEM image of ZnO 3D clusters on the Zn-Al LDHs, prepared by using 2.50 M NH3‚H2O. An individual ZnO urchinlike cluster and the enlarged image of Zn-Al LDHs are demonstrated in Figure 8C and D, respectively. As can be seen, the nanorods in urchinlike cluster have an average diameter of 150 nm and lengths in the range of 5-8 µm. In addition, the cross section of the rods is obviously hexagonal (insert of Figure 8C), which is different from that of the short nanorods shown in Figure 1. With more OH- in the synthetic system, the anisotropic growth of ZnO as well as the branched growth from a central nucleus directing the formation of ZnO 3D urchins can be efficiently promoted, as reported before.4e-g The ZnAl LDHs obtained under this condition seem to be slightly flexible, and the hexagonal shape of the platelet is not obvious (Figure 8D). Regardless of the exact morphology of the ZnAl LDH porous arrays obtained in our experiments (Figure 3A, Figure 8A, and Figure 8D), these oriented 2D nanoplatelets that have seldom been reported will enable the development of new applications for LDH films, such as supports for catalysts of high efficiency, environmental sensors, intercellular drug delivery, and so forth.12c,13a We further expect that they are potential precursors for ZnO-based functional ceramics, such as Zn-Al spinel. As mentioned earlier, Zn-Al LDHs are typically prepared by coprecipitation of Zn2+ and Al3+ in OH--contained solution. We use Al substrate herein instead of Al3+ to conduct the experiment and to achieve oriented heterostructures and wellcontrolled LDH arrays. It is obvious that the formation of ZnAl LDHs in the present work belongs to surface precipitation, which is more complex than the coprecipitation and strongly relates to the interfacial reactions as discussed. Other in-depth experiments have shown that changing the Zn2+ species using different raw materials such as Zn(NO3)2 and ZnCl2 can give similar results described here. Thus, the Al substrate is extensively useful for interfacial growth of intriguing nanoarchitectures. Conclusions In summary, we have successfully synthesized a new kind of ZnO-based heterostructure, that is, ZnO/Zn-Al LDH hierarchical architecture, on an Al substrate via a facile solution

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