Hierarchical Construction of ZnO Architectures Promoted by

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Hierarchical Construction of ZnO Architectures Promoted by Heterogeneous Nucleation Dong-Feng Zhang, Ling-Dong Sun,* Jing Zhang, Zheng-Guang Yan, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3609–3615

ReceiVed February 5, 2008; ReVised Manuscript ReceiVed May 20, 2008

ABSTRACT: In this work, ZnO hierarchical architectures are successfully fabricated by concentration-controlled heterogeneous nucleation behavior through a one-pot solution method, free of organic structure-directing agents or pretreated substrates. A twostage nucleation-growth process should account for the assembly process. First, the ZnO flowerlike plate aggregates formed with fast nucleation-growth kinetics. Then, these plate aggregates served as substrates to induce secondary heterogeneous nucleations under appropriate supersaturation degree. Ostwald-ripening assisted oriented attachment was believed to play a key role in the growth behavior. It provides an approach for the construction of hierarchical architectures with the advantage of precluding alien species. Raman scattering and room-temperature photoluminescence studies showed high consistency with respect to the crystallization improvement at elevated temperatures, which further showed that optical methods possess great potential in probing the microstructure of ZnO crystals. Introduction Over the past decade, assemblies at the nanoscale have motivated great interest due to their diverse properties and the enormous potential in nanodevice fabrication.1 Several strategies have been employed to assemble primary building blocks into desired architectures. For example, surface tension,2 capillary effects,3 electric and magnetic forces,4 hydrophobic interactions,5 and templates assisted routes6 can now be used to arrange the nanoparticles into one- or two-dimensional arrays or threedimensional stacking architectures. In most of these cases, narrow size distribution and high dispersibility are the prerequisites for assembling. Moreover, the primary nanoparticles were normally brought together by physical interactions, that is, there was no direct crystal lattice fusion among them. To meet the future challenges, attention was concentrated recently on nanocomposites with complex architectures. Despite achievements in core-shell structures with spherical, rodlike, or beltlike morphologies,7 a general scheme is still lacking for the fabrication of hierarchical-structured composites. The basic idea for the construction of hierarchical structure involves multistep nucleation-growth processes. Lowering the interfacial energy barrier was the central consideration for heterogeneous nucleation. Organic additives, called capping agents or structure-directing agents, were proven efficient in tuning the size and shape of nanocrystals,8 mainly by manipulating the surface activity. Therefore, these organic additives could help to introduce secondary nucleation sites on initial structures by reducing the interfacial activation energy. Recently, Liu et al. employed this idea to achieve rotorlike hierarchical ZnO arrays in the presence of bifunctional diaminoalkane.9 By exerting the different selective adsorption behaviors of citrate and diaminopropane on primary ZnO crystals, Zhang et al. alternated the hierarchical growth of secondary and tertiary complex structures.10 Epitaxial growth is another way of lowering the interfacial barriers. Substrates with small lattice mismatch to the overlayer materials have been extensively chosen to guide the assembly of one-dimensional (1D) nanoar* Corresponding authors. Phone: +86-10-62754179; e-mail: [email protected] (C.-H.Y.); [email protected] (L.-D.S.).

rays.11 For example, by taking advantage of their isomorphism and excellent lattice match, Vassierres et al. successfully got ZnO microrod arrays with GaN and InGaN as substrates.11a Similarly, highly oriented ZnO nanorod arrays were also induced by the preformed ZnO nanoparticle film as a buffer layer.11c Recently, a few reports emerged on the fabrication of heterostructured nanocomposites based on the heterogeneous nucleation and subsequential epitaxial growth mechanism.12 For example, Zeng’s group reported several TiO2-based hierarchical nanocomposites, including TiO2/MoO3,12a TiO2/TiO2/ H2Ti5O11 · H2O, ZnO/H2Ti5O11 · H2O, ZnO/TiO2/H2Ti5O11 · H2O, and ZnO/TiO2.12b We successfully assembled SnO2 nanorod arrays onto the {110} planes of the R-Fe2O3 nanotubes.12c With the advantage of not introducing alien species such as Au, in the vapor-liquid-solid (VLS) growth of GaP,13 crystal epitaxial growth should be a good alternative for the construction of nanocomposites with expected architectures. According to classical crystallization theories, the supersaturation degree is a significant parameter affecting the nucleation process, that is, homogeneous or heterogeneous nucleation. Homogeneous precipitation is favored under a high supersaturation degree, while heterogeneous nucleation growth would be favored in a lower supersaturation region. In principle, the primarily formed structure could serve as substrate to initiate the secondary heterogeneous nucleation on the condition that the remaining concentration is sufficiently high to initiate nucleation and the interfacial energy of nucleus/substrate is lower than that of particle/solution. However, the research on such in situ hierarchical organization is limited probably due to the complicated kinetic processes. In this paper, we report that a two-stage nucleation-growth process is feasible for ZnO simply by adjusting the precursor concentration, where multistep injection or organic/inorganic addictives generally employed in heterogeneous growth is not necessary. Under appropriate concentrations, flowerlike plate aggregates formed in the first nucleation-growth stage serve as substrates to induce the secondary heterogeneous nucleation. Eventually, ZnO hierarchical structures with nanorod arrays assembled on the petals of the flowerlike plate aggregates form in a large scale. The detailed growth mechanism is discussed.

10.1021/cg800143x CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

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Figure 1. SEM images of the samples prepared at 60 °C for 6 h under different precursor concentrations. (a) 0.017 mol · L-1, (b) 0.025 mol · L-1, and (c) 0.032 mol · L-1.

The room-temperature photoluminescent and Raman spectra of the hierarchical structures are also studied. Experimental Section All the chemicals were of analytical grade and used as purchased without further purification. The hierarchical structure was fabricated by a hydrothermal method. Typically, the precursor Zn(OH)42- (MZnAc2/ MOH- ) 1:10) with a given volume was added into distilled water to make a total volume of 15 mL. After ultrasonic dispersing for several minutes, the mixture was transferred into a 25 mL Teflon-lined autoclave and subsequently heated at a given temperature for 5-10 h. After the autoclave was cooled to room temperature naturally, the resulting precipitates were collected by centrifugation and decanting, washed several times with distilled water, and finally dried in a vacuum at 60-70 °C for 5 h for further characterization. Scanning electron microscopy (SEM) observations were carried out with DB-235 focused ion beam (FIB) system operated at an acceleration voltage of 15 kV. High-resolution transmission electron microscopy (HRTEM) characterizations were performed with a Philips Tecnai F30 FEG-TEM operated at 300 kV. The X-ray diffraction (XRD) pattern was recorded on a powder sample with a Rigaku D/max-2000 diffractometer employing Cu KR radiation (λKR1) 1.54056 Å) at a scanning rate of 0.02° s-1 ranging from 15 to 90°. Room-temperature PL spectra were measured on a Jobin Yvon LabRAM HR800 spectrometer using He-Cd 325 nm laser as excitation source. Raman spectra were collected on a Jobin Yvon LabRAM HR800 spectrometer with micro focusing mode, ZnO samples which is pressed into pellets were excited with Ar+ 488 nm line and He-Cd 325 nm laser as excitation sources, which were focused through an Olympus 100× and UV 40× objective lens, respectively. The accuracy was better than 1 cm-1 and all measurements were performed at room temperature.

Results and Discussion Controlled Construction of Hierarchical ZnO Assembly. Our previous study indicated that flowerlike nanoplate aggregates are the kinetically stable form of wurtzite ZnO.14 It therefore occurred to us that if we could separate the whole process into two sequential nucleation-growth stages, it is possible to assemble ZnO nanocrystals into hierarchical structures through an epitaxial growth process with the nanoplates serving as substrates. Such a scheme requires the nanoplates to form instantly. As generally accepted, the formation of ZnO in aqueous solution would experience hydrolysis and subsequent condensation process, which can be simplified as the following two reactions:

Zn2+ + 4OH- f Zn(OH)24

(1)

Zn(OH)24 f ZnO + H2O + 2OH

(2)

2+

-

Thus, free Zn and relative excessive OH should accelerate the growth. For the sake of understanding, we carried out the experiment in aqueous solution with ZnAc2 as the Zn2+ source

Figure 2. XRD pattern of the hierarchical assembly fabricated at 60 °C for 6 h.

and NaOH as the hydrolysis-condensation promoter, where no additional organic species were introduced. Systematical study indicated that the morphology of ZnO could be switched from flowerlike nanoplate aggregates to flowerlike hierarchical structures by simply adjusting the concentration of the Zn(OH)42-. Figure 1a-c shows the typical SEM images of the products obtained at 60 °C for 6 h under different precursor concentrations. When the concentration was as low as 0.017 mol · L-1 (Figure 1a), flowerlike plate aggregates dominated the products, where secondary particles could barely be identified. After increasing the concentration to 0.025 mol · L-1 (Figure 1b), sparse, tiny nanoparticles grew onto the surface of the plates. As the concentration was further increased to 0.032 mol · L-1 (Figure 1c), dense and organized nanorods stood onto the surfaces of the plates. Besides the secondary nanocrystals, the primary plates also exhibited differences. The plates became thicker with increased precursor concentrations. The different diffusion rates may be responsible for the thickness variation. As the diffusion is faster under higher concentrations, the plates would be thicker with a faster growth rate. XRD characterization (Figure 2) indicated that the hierarchical structures consisted of pure hexagonal ZnO with wurtzite structure (JCPDS card No. 36-1451), which indicated that the secondary nanorods were of the same phase with the primary plates. A low-magnification SEM survey (Figure 3a) demonstrates a high yield of the assembly with structure uniformity. Closer observations (Figure 3c,d) of different parts from an individual hierarchical structure (Figure 3b) show that the plates were composed of distorted hexangular prism morphology with sawteeth-like structure at the heads. More detailed structural information is shown in Supporting Information (Figure S1). Secondary nanorod arrays stood perpendicular to all six prismy side faces. The adjacent secondary arrays of nanorods as shown

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Figure 3. SEM images of the hierarchical structure assembled at 60 °C for 6 h. (a) survey, (b) an individual, (c, d) closer observations.

in Figure 3d possess an angle of ca. 60°, in accordance with the angle between {100} planes of wurtzite ZnO. Combined with the geometry of the plates, it implied that the plates might be formed by the fusion of hexagonal ZnO nanorods perpendicular to the axial direction. HRTEM and selected-area electron diffraction (SAED) characterizations further confirmed the above analysis. As revealed in the SEM observations, the configuration of the assembly is rather complex, which toughen up the aligning of the electron beam. Thus, ultrasound was used to get separated plates and secondary nanorods. Figure 4a,b shows the typical TEM images and the corresponding SAED pattern of a separated plate, respectively. It indicated that the plate is of single crystallinity with sawteeth-like structure at the head. The zone axis of SAED pattern was determined as and the sawteeth were parallel to the [0001] direction of ZnO. Since the electron beam was most likely aligned perpendicular to the flat surface, it can be concluded that the flat surfaces of the plates belong to the {100} crystallographic group of wurtzite ZnO. A well-developed thermal equilibrium wurtzite ZnO exhibits hexagonal prism surrounded by six {100} planes as columnar facets. Thus, it is structurally reasonable to assume that the plates with distorted hexagonal prism structure were formed from the fusion of ZnO nanorods, as shown in the inset of Figure 4c. Figure 4c shows the corresponding HRTEM images recorded from the ends of the plate as depicted in the white frame in Figure 4a. The spacing between two adjacent lattice planes parallel to the sawteeth is 0.26 nm, coincident with (0002) plane of wurtzite ZnO. Figure 4d is a typical TEM image of the secondary nanorod. The corresponding SAED pattern (Figure 4e) with zone axis and the HRTEM image (Figure 4f) indicated that the nanorods grew along [0001] direction with high crystallinity. Combined with the geometry of the hierarchical assembly, the structure characterization results revealed that the interfacial crystallographic orientation relationship is (0001) plane of the secondary nanorod parallel to the {100} planes of the substrate plates. Because of the fast growth, it is hard to catch the detailed morphology evolvement under 60 °C. Since the morphology

Figure 4. (a) TEM, (b) SAED, and (c) HRTEM image of an isolated petal; (d) TEM, (e) SAED, and (f) HRTEM image of a secondary nanorod.

of ZnO obtained at room temperature was the same as that obtained under 60 °C, we performed the preparation at room temperature to slow down the formation rate, keeping the precursor concentration at 0.032 mol · L-1. SEM observation illustrated that the flowerlike morphology began to be rudimentary even with a reaction time as short as 10 min (Figure 5a), showing a fast formation rate. High-magnification SEM images indicated that at this stage, the flowerlike structure was indeed aggregates of fibrillar nanocrystals (Figure 5b). When the reaction was prolonged to 30 min, individual fibers were no longer observable, while plates with rough surfaces formed (Figure 5c). As shown in Figure 5d, sporadically distributed small nanoparticles appeared on the surface of the plates. When the reaction time was further extended to 1 h, the secondary nanoparticles became longer, denser, and more regular in shape, and the surface of the plates became smooth while the sawteeth still existed at the ends (Figure 5e,f). Both the elongating of the secondary nanorods and the smoothing of the plates suggested that Ostwald-ripening accompanied the whole growth process. Based on the above discussions, we believe that the hierarchical assembly experienced a two-stage nucleation-growth process. The whole assembling process was demonstrated in Scheme 1. It has been proven that relative higher OH- concentrations can accelerate the dehydration rate of hydroxides.15 The excessive OH- ions promoted the burst of the homogeneous nucleation (Scheme 1a). Because of the intrinsic anisotropic characters of hexagonal ZnO, one-dimensional nanostructures, such as the fibrillar nanocrystals found at room temperature, became the preferential crystal forms. The intense interaction between Zn2+ and the surface-adsorbed OH- induced the aggregation of the

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Figure 5. Morphology evolvement of the hierarchical assembly at room temperature. (a) 10 min, (c) 30 min, (e) 1.0 h. (b), (d), and (f) are the corresponding high-magnified SEM images, respectively.

1D nanostructures (Scheme 1b). The diffusion-limited aggregation theory16 may help to clarify the flowerlike aggregation style. It is concluded that when the formation rate of growth unit far exceeds that of diffusion, fractural aggregation is facilitated. Obviously, acceleration of the diffusion rate would decrease the fractural degree. We performed the control experiments with other conditions identical but under vigorous stirring. SEM observations (see Supporting Information, Figure S2) revealed that it was rods, instead of hierarchical assemblies, that dominated the products. In fact, flowerlike aggregations were frequently observed for oxides, such as Cu2O,17 ZnO,18 Bi2WO6,19 etc., although different mechanisms were employed to explain the aggregation process. In the proceeding of nanomaterials research, an orientedattachment mechanism is proposed to explain the observed

Zhang et al.

growth behavior of nanocrystals.20,21 It views the formation of bigger crystals as a result of attachment and coalescent of smaller ones, while the conventional Ostwald-ripening effect depicts the crystal growth in a way in which bigger crystals consume the smaller ones. It is predicted that orientedattachment among ZnO nanocrystals could efficiently decrease the total energy of the system,21 then the fusion among the adjacent nanofibers becomes energetically favorable, which produces the plates in the flowerlike aggregates in turn (Scheme 1c). According to the HRTEM and SAED characterization results, the fusion of primary ZnO nanocrystals should occur in a side-by-side manner. In fact, the side-by-side fusion among 1D ZnO nanostructures was also observed by other groups.22 It is easy to imagine that many defects must result during the attachment, as evidenced by the rough surfaces and irregular shape of the plates in Figure 5c,d. As observed by SEM characterization (Figure 5e,f), both the secondary nanorods and the primary plates become more smooth and regular with the reaction proceeding. We believe the Ostwald-ripening had worked simultaneously with the oriented-attachment to remedy the defects, and thus the rough surfaces caused by the fusion of the nanorods were filled and leveled. As for the sawteeth at the heads of the plates, two factors should be considered. First, the typical crystallography of wurtzite ZnO is of pyramidal structure,23 in which the top part is sharper than that of the basal. Hence, the sawteeth on the head of plates consist of aligned pyramidal heads. Second, although the fusion of primary ZnO nanocrystals occurred in an oriented manner, the slightly mismatch and defects are inevitable. It makes the fusion of the adjacent plates incomplete. The flowerlike aggregates grow fast and could serve as the substrate for the subsequent heterogeneous nucleation. From the viewpoint of the interfacial energy, {100} facets of wurtzite ZnO have been extensively used to hierarchically assemble secondary ZnO nanorods. After the first stage of nucleationgrowth, if the remaining concentration were high enough to initiate the heterogeneous nucleation, hierarchical structures would form (Scheme 1d). The inset in Scheme 1d shows the geometry model of the hierarchical structure and the orientation relationship between the plates and the secondary nanorods. The concentration-dependence experiments (as shown in Figure 1) provided vivid evidence for this prediction. Therefore, different from the formation of the branched nanocrystals (such as multipod, star-shaped, and nanothorn structures) via a one-stage nucleation-growth process, the as-prepared hierarchical architecture in this work experienced a two-stage nucleation-growth process. Recently, sequential nucleation and growth strategy has been successfully employed to fabricate complex structured micro/nanocrystals.9,10,12 Generally, preformed nanocrystals should disperse in the solution containing the required ions.

Scheme 1. Illustration of the Hierarchical Organization Process

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Figure 6. SEM images of the hierarchical structure assembled under different temperatures. (a) 30 °C, (b) 60 °C, and (c) 100 °C.

Figure 7. Raman scattering of the hierarchical assemblies obtained at different temperatures (prepared at 1. 30 °C, 2. 60 °C, 3. 100 °C) excited by (a) Ar+ 488 nm, and (b) He-Cd 325 nm laser beam, respectively. The inset of panel (a) demonstrated the E1(LO)/E2(high) with the temperature.

Then, with the assistance of structural-directing agents to reduce the interfacial activation energy, site-specific nucleation-growth processes occurred on the surface of primary nanostructure, which finally produced the hierarchical structures. In our case, the secondary nucleation was initiated by simply adjusting the supersaturation degree, where neither preformed nanocrystals nor organic addictives were necessary. It may provide a new approach for the designed synthesis of hierarchical architectures with the advantage of precluding alien species. The influence of the reaction temperature on the morphology of the products was also investigated. As displayed in Figure 6a-c, either the shape of the plates or the secondary nanorods became more regular with increasing temperature, although the character of the morphology was not changed. The SEM image of the assembly obtained at 100 °C depicted the same distorted hexagonal prism structure of the plates, which looked clearer, implicating the crystallinity improvement. Room-Temperature Raman and PL Studies. Raman scattering is a powerful tool commonly employed to study the microstructure of materials. With Ar+ 488 nm laser beam as the excitation source, Raman scattering behavior of the products fabricated under different temperatures was studied. As shown in Figure 7a, the samples exhibit similar scattering patterns. Based on the scattering modes of the bulk ZnO,24 the bands at 381, 439, and 581 cm-1 can be assigned to A1 symmetry with the TO mode, the nonpolar optical phonons high-E2 mode, and E1 symmetry with the LO mode, respectively. Besides these first-order scatterings, an additional feature with frequency of ca. 332 cm-1 was observed, which can be assigned to the second-order Raman scattering arising from zone-boundary phonons 2-E2(M).25 By comparing the Raman spectra of the three samples, it was found that the intensity of E2 (high) increases with the fabrication temperature, while it is quite the reverse for that of the E1 (LO) mode. It is generally accepted

that the E2 (high) mode is the characteristic scattering originating from the lattice vibration of ZnO, while E1 (LO) is believed to relate to the formation of defects in ZnO. Therefore, the lower the ratio of E1 (LO) to E2 (high) is, the less the oxygen vacancies or Zn interstitials present. The decreased ratio of E1 (LO) to E2 (high) (as illustrated in the inset of Figure 7a) agrees well with the shape regularity in the SEM observations (Figure 6a-c). When samples were excited with He-Cd 325 nm laser beam, however, the Raman spectra exhibited totally different behaviors. As shown in Figure 7b, three resonant scattering bands appeared with frequency shifts at the multiples of the 1LO zone-center. It is believed that resonant Raman scattering occurs when the energy of the incoming or scattered phonons matches the real electronic states of the ZnO nanostructures. Multiphonon scattering processes were also studied previously for ZnO bulk materials.24b Scott observed 8 LO lines in bulk ZnO and explained the phonon scattering line width by the equation λ(nLO) ) 9n (cm-1).25a However, in our case, only three resonant bands are observed centered at ca. 563, 1129, and 1729 cm-1 with much broader line width of 41, 86, and 241 cm-1, respectively. By Lorentzian fitting of 1LO phonon, the full width at half-maximum (fwhm) of the samples obtained at 30, 60, and 100 °C are found to be 55.84 ( 2.96, 41.67 ( 1.14, and 39.20 ( 1.39 cm-1, respectively (Figure S3, Supporting Information). Since the line width broadening is believed to originate from the oxygen deficiency of the ZnO assembly, the narrowing line width with elevated reaction temperatures indicates the improved crystallinity. In addition, different from the reports in the literature,26 the intensity of the 2LO resonance scattering is almost not enhanced, implying the weak resonance enhancement effect in the as-prepared ZnO hierarchical assembly. The influence of temperature on the crystallization of the hierarchical structure is also reflected from the room temperature

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References

Figure 8. Room temperature PL spectra of the hierarchical assemblies obtained at different temperatures with He-Cd 325 nm laser line as the excitation source.

PL studies with He-Cd laser (325 nm) excitation. All the PL spectra (Figure 8) of the three samples consisted of an UV emission band centered at ca. 385 nm and an orange emission band at ∼567 nm. With the increased preparation temperature, the UV emission was intensified while the orange one was lowered. Since the UV band is attributed to the near-band edge emission, while the orange one generally originates from the transition between photogenerated holes and singly ionized oxygen vacancies,27 the PL results provided more evidence for the crystallinity improvement with the temperature. The highly coincident trends in the Raman and PL results demonstrated that the optical properties have great potential in probing the microstructure of nanomaterials. Conclusions By simply adjusting the concentration of the precursors, we successfully fabricated the ZnO hierarchical structure with secondary nanorod arrays standing perpendicularly to the petals of flowerlike aggregates. Based on the detailed structural characterization and the understanding of the nucleation-growth kinetic, a two-stage nucleation-growth process could be responsible for the hierarchically assembly behavior. First, the flowerlike plate aggregates were produced with fast nucleation-growth kinetics. Then, the primarily formed plate aggregates served as the substrates to induce the secondary heterogeneous nucleation. Ostwald-ripening assisted oriented attachment plays a crucial role in the whole process. It provides an approach for the purposeful construction of hierarchical architectures with the advantage of not introducing alien species. Raman scattering and room temperature PL studies showed highly consistent results with respect to the crystallinity improvement with temperature, which further showed that optical signal possess great potential in probing the microstructure of ZnO. The special hierarchical structure and the simple and low-cost preparation procedure may promote the assembly as potential candidates of the photocatalyst and superhydrophobic/hydrophilic materials. Acknowledgment. This work is supported by the NSFC (20221101, 20671005, and 20423005) and MOE of China (NCET-06-0010). Supporting Information Available: Closer SEM observation of the ZnO hierarchical structures, SEM image of the product obtained under vigorously stirring, and Lorentzian fitting of the Raman spectra of the hierarchical structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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