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Aging-Induced Self-Assembly of Zn/ZnO Treelike Nanostructures from Nanoparticles and Enhanced Visible Emission Haibo Zeng,* Peisheng Liu, Weiping Cai,* Xueli Cao, and Shikuan Yang Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1092-1097

ReceiVed October 15, 2006; ReVised Manuscript ReceiVed April 9, 2007

ABSTRACT: The self-assembly formation of Zn/ZnO treelike nanostructures was achieved by simply aging the corresponding stable colloidal solution, produced by laser ablation in liquid, at room temperature without any further heating treatment. The selfassembled treelike structures consist of oriented attached nanoparticles with small misorientations and greatly improved crystallinity and exhibit significantly enhanced visible emission with two peaks in the blue and green regions, accompanying the decrease of the plasma resonance absorption of the core-part Zn nanocrystal. The self-assembly mechanism was attributed to the imperfect oriented attachment. The small misorientations among the attached nanoparticles were observed, and their accumulation guided the selfassembly formation of the multibranched nanostructures. As compensation for the insufficient driving force, adequate colloid stability and aging time were crucial for the accomplishment of the self-assembly process. This work could deepen the understanding of the nanocrystal self-assembly behavior, especially in the very “soft” environment, and the effect of the self-assembly process on the optical properties. 1. Introduction Nanocrystalline materials have attracted great attention in many fields due to their novel electronic, magnetic, optical, chemical, and mechanical properties.1 Among them, semiconductor and metal nanocrystals have been the most intensively studied because of their quantum confinement effects on electrons and surface plasmon, size- and shape-dependent photoemission, and photoabsorption properties.2-5 As an important wide-band-gap semiconductor, wurtzite ZnO has a bandgap energy of 3.37 eV and exciton binding energy of 60 meV at room temperature and hence possesses many important applications in electronic and optical devices, especially optoelectronic applications, such as in the ultraviolet (UV)/blue lasing media.6,7 Therefore, the controlled synthesis of ZnO nanostructures, such as nanocrystals, nanowires, nanobelts,8-14 and other complex nanoarchitectures,15 has been extensively studied. Also, zinc nanostructures have been fabricated by many methods.16-18 On the other hand, semiconductor-metal composite nanostructures have shown their attractive capability in improvement of catalytic and sensing properties and tunable luminescence.19-21 The Zn/ZnO core/shell heteronanobelts and nanocables have been synthesized by the solid-vapor process and pyrolysis method, respectively.22-24 Pulsed laser ablation of a metal target in liquid media has gained intensive attention in forming interesting nanostructures and in the study of the dynamical process among laser-solidliquid interactions.25-27 Liang and co-workers first reported the charging-directed formation of an interesting composite structure, ZnDS, and ZnO nanoparticles by laser ablation of Zn in SDS solution and pure water using a laser wavelength of the third harmonic of 355 nm.25,26 Recently, adopting a similar method, we successfully prepared Zn/ZnO core/shell nanoparticles by changing the laser wavelength to 1064 nm.28 Further works proved that these nanoparticles can be self-assembled into treelike nanostructures after aging of the stable colloidal solution for enough time at room temperature without any * To whom correspondence should be addressed. [email protected] (H.Z.), [email protected] (W.C.).

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further heating treatment. Aging leads to a crystalline improvement of Zn/ZnO composites in addition to a treelike assembly. Such treelike nanostructures exhibit obvious depression of the Zn surface plasmon resonance and great enhancement of the ZnO visible photoluminescence (PL). The self-assembly of micro- and nanostructures is of great significance in nanodevice integration.15 This work may facilitate further understanding of the nanocrystal self-assembly behaviors, especially in the very “soft” environment, and the aging effect on the defect-related metastable properties, which has been reported as one of the most important characteristics of the nanomaterials obtained by nonequilibrium methods.28-31 The details are reported in this paper. 2. Experimental Section The nanoparticle colloidal solution was prepared by laser ablation of a metal target in an aqueous solution with sodium dodecyl sulfate (SDS; 99.5%), as previously reported in detail.23-30 Briefly, a zinc plate (99.99%) was fixed on a bracket in a glass vessel filled with 10 mL of 0.0001-0.1 M SDS aqueous solution, which was continuously stirred. The plate was located 4 mm under the solution surface and then was ablated for 30 min by the first harmonic of a Nd:YAG pulsed laser (wavelength 1064 nm, frequency 10 Hz, pulse duration 10 ns) with a power of 70 mJ/pulse and a spot size of about 2 mm diameter. After laser ablation in a liquid, the colloidal solution was airproofed to avoid the evaporation-induced change of the colloidal stability and then was stored in a homeothermic chest at 20 °C for different times (from two weeks to two months). The colloidal suspensions with and without aging were centrifuged at 14000 rpm. The obtained products were ultrasonically rinsed with ethanol several times to remove the covered surfactant and then draft-dried at room temperature. X-ray diffraction (XRD) (Philips X’Pert using Cu KR line 0.15419 nm) was conducted directly on these powder samples. The roomtemperature PL spectra of the obtained powders were measured on the FLS920 PL spectrophotometer with a Xe lamp. For transmission electron microscopy (TEM) (JEM-200CX) examinations, the powder samples were ultrasonically redispersed in ethanol, and then a droplet of the solution was dropped onto a carbon-coated copper grid. The optical absorption spectra of the as-prepared and aged colloidal solutions were recorded on a Cary 5E UV-vis-IR spectrometer after ultrasonic dispersion and three dilutions.

10.1021/cg0607147 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007

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Figure 1. XRD patterns of the as-prepared nanoparticles synthesized by laser ablation in 0.05 M SDS solution (a) and the product after two months of aging (b). The inset shows the normative X-ray diffraction patterns of the sample before and after aging, the using Si(001) (PDF number 790590) diffraction line for calibration. The departures of the ZnO(100) diffraction peak were indexed, indicating the obvious lattice distortion.

3. Results 3.1. Characterizations. The as-prepared colloidal solution with 0.05 M SDS is deep-yellow in color and can be stable for more than 1 month without any sedimentation at 20 °C, in agreement with that reported previously.28 The XRD pattern of the as-prepared sample without aging is shown in Figure 1a. There are two sets of diffraction peaks corresponding to wurtzite ZnO and metal Zn, respectively, indicating the Zn/ZnO dual components. As previously reported,28 the as-prepared sample consists of about 20 nm core/shell-structured Zn/ZnO nanoparticles, which coincides with the TEM image in Figure 2a. After aging of the stable nanoparticle colloid for 2 months, some black flocculent sediment was found at the bottom of the container and the color of the colloidal solution was greatly reduced. Although the XRD pattern is still composed of diffraction peaks of wurtzite ZnO and metal Zn, the relative intensity of the peaks shows some marked changes, as seen in Figure 1b. Obviously, the relative intensity of the ZnO increased while that of the Zn decreased, which means that the aging process has induced the reduction of Zn or increase of the wurtzite ZnO, indicating that Zn nanocrystals in the core part have been partially oxidized. In addition, through Si(001) calibration, obvious departures of the ZnO(100) diffraction peak from the normal position (0.060° and 0.038° for the as-prepared and aged products, respectively) were observed as shown in the inset, indicating the obvious lattice distortion, which should be attributed to the highly concentrated defects such as interstitial zinc and oxygen vacancies produced in the laser ablation process and subsequent oxidation process. TEM observation has revealed that the nanoparticles were in an isolated state before aging, but after aging, most of these separated particles have been assembled into treelike nanostructures, as presented in parts a and b, respectively, of Figure 2. The assembled products are of rough surface and multibranch structure, and their sizes are much larger than those of the primal nanoparticles (the diameters of the branches are about 100 nm). Besides the treelike nanostructures, only a few unassembled nanoparticles remain to be isolated yet; the product yield (the rate of the assembled nanoparticles to the total number) is about 90% from the stable colloid. Surprisingly, such a self-assembly phenomenon has never been found in our previous mass TEM observations of the as-prepared ZnO and Zn/ZnO nanoparticles by laser ablation in a liquid without aging.28

Figure 2. TEM images of the as-prepared (a) and aged (b) product from a stable 0.05 M SDS nanoparticle colloid (the inset in (a) shows the SAED pattern of several particles). SAED patterns for the unassembled nanoparticle (c) and the edge region (d) of the selfassembled treelike nanostructures.

The selected-area electron diffraction (SAED) patterns, corresponding to the isolated nanoparticle and the edge of the assembled treelike nanostructures, are shown in parts c and d, respectively, of Figure 2. The isolated nanoparticles exhibit the patterns of a typical ZnO nanocrystal, meaning that they have been completely oxidized from the primal Zn/ZnO core/shell nanoparticle after the aging process. However, the SAED patterns of the treelike nanostructures are quite different. First, there are two sets of diffraction spot patterns corresponding to Zn and ZnO, indicating the Zn/ZnO dual components in the treelike nanostructures. Second, each diffraction spot has been elongated into a short bar composed of several points, indicating that there are some small misorientations among the attached nanocrystals in the electronic-beam-sized zone of TEM. Third,

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Figure 3. Treelike nanostructures from a relatively stable 0.1 M SDS nanoparticle colloid after two months of aging: TEM image (a), SAED pattern from the edge region (b), and local magnified image corresponding to the frame area (c).

the pattern composed of the nearest diffraction spots is not a regular hexagon, but has a vertex angle of 81° (instead of 60° as we usually expect). According to the crystallography analyses, the diffraction pattern is indexed as Figure 2d.32 Besides, the slight distortion of the diffraction pattern could be ascribed to the lattice distortion to some extent caused by the laser-induced nonequilibrium process, including high temperature, high pressure, and ultrarapid reactive quenching.28 This lattice distortion has been confirmed by the normative XRD as shown in the inset of Figure 1. Obviously, the formation of the treelike nanostructures is not simple particle-aggregation but aginginduced self-assembly. To verify the universality of the self-assembly formation of such treelike nanostructures from a nanoparticle colloid, similar aging experiments were done for the relatively stable 0.1 M SDS colloids, as previously mentioned, which are stable for about three weeks. The results are shown in Figure 3. Just as we anticipated, treelike nanostructures were also found to be formed from the comparatively stable nanoparticle colloid. The characteristics of the treelike structures are very similar to those of the structures from the 0.05 M SDS nanoparticle colloid, including the treelike morphology as shown in Figure 3a and the SAED specialties in Figure 3b. Figure 3c shows the local magnified image of the top part, from which we can see that the nanoparticles have been compactly attached to each other and no obvious holes on the trunks, which demonstrates that three-dimensional self-assembly has taken place. These results verify the universality of the formation of such self-assembled treelike nanostructures and their specialties in a stable colloid. However, there are still some differences between the products from 0.05 and 0.1 M SDS nanoparticle colloids, including a smaller size, slightly worse crystallinity, and lower product yield (about 75%) from the 0.1 M SDS colloid, which should all be related to the difference in the colloid stability. 3.2. Influence Factors. Further experiments indicated that the stability of colloidal solutions (or SDS concentration) and aging time are crucial to form such treelike structures and that the stable colloidal solution is beneficial to the formation of the treelike structures. Figure 4 shows the results of the product obtained from the unstable 0.001 M SDS solution with the same experimental conditions as those of the stable 0.05 M SDS solution. This sample is stable only for several days. It can be seen that no treelike nanostructures formed, and only some uncompact spherical aggregates were observed (see Figure 4a). Figure 4b shows the SAED pattern of the nanospheres, in which the typical Zn and ZnO diffraction rings are observed and are

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Figure 4. Nanospheres from an unstable 0.001 M SDS solution after an adequate aging time: TEM image (a) and SAED pattern (b).

Figure 5. TEM image of the networklike nanostructures from a stable 0.05 M SDS nanoparticle colloid after a deficient aging time (one month).

very similar to those of the primal core/shell nanoparticles (inset in Figure 2a), indicating the completely random orientation among the nanocrystals in the aggregated nanospheres. These features are quite different from the assembled treelike nanostructures formed in the stable nanoparticle colloid. Also, an adequate aging time is necessary to obtain the treelike nanostructures in the stable nanoparticle colloid. If the aging time is too short, the treelike structures will not be formed completely. Figure 5 shows the morphology of the sample prepared from the stable colloid with 0.05 M SDS after a deficient aging time, only one month. We can see many nanoparticles have been assembled into networklike structures comprised of linked nanospheres. Compared with the treelike nanostructures aged for an adequate time (two months), there exist more unassembled nanoparticles in this sample. These features apparently show the mediate state of the formation of the treelike nanostructures. These two experiments demonstrate that the colloidal stability and the aging time are important to the self-assembly formation of the treelike nanostructures in the nanoparticle colloid. 3.3. Optical Measurements. The optical absorption spectra of as-prepared and aged colloidal solutions are shown in Figure 6. As we previously reported, the as-prepared sample with Zn/ ZnO nanoparticles exhibits a strong UV absorption peak at 242 nm due to the surface plasma resonance (SPR) of the nanosized Zn cores.28 Recently, similar results have also been obtained by J. K. Lee et al.33 After two months of aging, the Zn SPR decreases significantly, indicating a big reduction of the Zn amount and occurrence of aqueous oxidation of Zn during aging. This is in agreement with the XRD results mentioned above

Aging-Induced Assembly of Zn/ZnO Nanostructures

Figure 6. Optical absorption spectra of the as-prepared colloidal solution with 0.05 M SDS before (a) and after (b) two months of aging.

Figure 7. PL spectra of the as-prepared sample from a 0.05 M SDS solution before (a) and after (b) two months of aging (excited at 370 nm). The dashed curves are the Gaussian fitting results of (b). The curve in the upper left corner is the excitation spectrum monitored at 452 nm.

and shows a very convenient examination method for Zn nanostructures by the optical absorption measurement. Figure 7 presents the corresponding PL spectra of both samples, together with the excitation spectrum and the Gaussian fitting curves. For the sample without aging, there is only one very weak blue emission around 452 nm. After aging, however, this band is greatly enhanced about 10 times, together with the appearance of another strong green emission around 515 nm. Such greatly enhanced visible emissions may be related to the aging process and would be in favor of the optical applications of nanostructured ZnO. 4. Discussion 4.1. Formation of Treelike Structures. The formation of core/shell-structured Zn/ZnO nanoparticles has been demonstrated in our previous study28 and will not be discussed here in detail. Briefly, it can be attributed to the competition between aqueous oxidation and surfactant protection of Zn clusters, which are produced in the high-temperature and high-pressure zinc plasma on the solid-liquid interface quickly after the interaction between the pulsed laser and the metal target. On the basis of the facts that all samples were ultrasonically rinsed more than five times to clean SDS by centrifugation and

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ultrasonically redispersed before TEM observation and that such a self-assembly phenomenon was not found in our previous mass TEM observations of the as-prepared ZnO and Zn/ZnO nanoparticles without aging under quite similar conditions,28 the treelike nanostructures were formed in the colloidal solution during the aging process, but not on the carbon-coated copper grid during solvent evaporation during preparation of the TEM sample. Recently, many nanostructures, especially complex nanostructures, have been fabricated through a self-assembly process called “oriented attachment” or “imperfect oriented attachment”, such as one-dimensional TiO2,34 ZnS,35 and ZnO36 nanorods, 2D R-MoO3 layered nanostructures,37 3D CuO nanodandelions37 and hollow SnO2 octahedra,38 PdSe zigzag nanowires and nanorings,39 and CaCO3 superstructures.40 In these self-assembly processes, the small nanoblocks self-attach to build a bigger nanostructured assembly by “self-adjusting” and by “lattice fusion” or so-called oriented attachment on the specific crystallographic interfaces.34-39 In the imperfect oriented attachment, there is usually a small misorientation at the interface, which is expected to induce the formation of complex polytypic and polymorphic nanostructures.34 Usually, such attachment occurs in hydrothermal conditions, which provide the flexible environment for the small nanoblocks to self-adjust the orientations and the driving force for the lattice fusion.34-36 The formation of the treelike structures in this study can be attributed to an imperfect oriented attachment-like process during aging. In our case, although the aging time is much longer than that of the reported oriented attachment processes, several common features of imperfect oriented attachment growth34-39 have been found, such as the coalescence of the initial Zn/ZnO nanoparticles in a liquid environment and the small misorientation among the attached particles, whose accumulation leads to the branched nanostructures and hence the treelike structures. The detailed assembly process can be described as follows. The core/shell-structured nanoparticles in our experiments can be stabilized through the surface adsorption of C12H25SO4- (DS 28 and then move together with -) ions in the colloidal solution each other during initial aging through the collision coalescence among the nanoparticles. Further aging will lead to imperfect oriented attachment among the touching particles, which gradually adjust their relative orientations due to the minimization requirement of energy. However, small misorientations among the touching particles remain because of the deficient driving force in the low-temperature aging, inducing slightly elongated electron diffraction spots as shown in Figure 2d. It is just the accumulation of such misorientations that results in the branched nanostructures. Obviously, according to such a formation mechanism, the stability of the colloidal solution and an adequate aging time are reasonably crucial, since coalescence of the nanoparticles and adjustment of the particles’ orientation only occur in the suspended state and need adequate time. Otherwise, if we use a very low SDS concentration, the colloidal solution is less stable and subsequent aging cannot induce the oriented attachment but aggregation. Also, short aging of the stable colloidal solution cannot form the treelike nanostructures. These observations are in good agreement with our further examination experiments, as shown in Figures 4 and 5. For further confirmation, microstructural examination was conducted for the treelike nanostructures. Figure 8a shows the high-resolution TEM observation of one segment in the treelike nanostructures. We can see the nanoparticles are surely attached together. Misorientation between two attached nanoparticles is observed as typically shown in Figure 8b. Furthermore, there

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Figure 8. (a) HRTEM image of one segment in the treelike nanostructure. (b) and (c) are the enlarged images corresponding to frames I and II in (a), which clearly demonstrate the imperfect oriented attachment among nanoparticles and the epitaxial relation between the core and shell.

exists an epitaxial orientation relation between the Zn core and ZnO shell (see Figure 8c). This is the reason for the coexistence of two sets of diffraction spot patterns with the nearly same orientation from Zn and ZnO in SAED of the treelike nanostructures (see Figure 2d). These microstructural observations clearly demonstrate the imperfect oriented attachment among the nanoparticles. In addition, the ultrarapid and nonequilibrium formation of the core/shell nanoparticles in the laser ablation process leads to the high activity of the particle surfaces with a great deal of suspended bonds, which will greatly facilitate the self-assembly process in room-temperature aging. Finally, from HRTEM, it can be seen that the improvement of the crystallinity mainly comes from the aging-induced ripening of ZnO lattices and the interfacial attachments. Such imperfect oriented attachment in the very soft conditions indicates a new method to fabricate highly ordered nanostructures from nanoparticles by laser ablation in a liquid and the subsequent hermetical aging process and could deepen our understanding of the nanocrystal self-assembly behaviors. 4.2. Enhancement of Visible Emissions. Recently, some interesting visible photoluminescence has been observed in ZnO nanostructures.41 In our previous work, blue and violet emissions were observed in laser-ablation-formed ZnO and Zn/ZnO nanoparticles,28,42 respectively. We have tried to attribute them to the electron transition from interstitial zinc to the valence band by annealing and EPR investigation and think the green emission is for the electron transition from the conduction band to the oxygen vacancies as most researchers think. In the present PL results, the weak blue emission was observed in the asprepared nanocrystals, and the strong blue and green emissions

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were observed in the aged treelike structures. According to our experimental process, the blue emission seems to relate to the defects in the as-prepared nanocrystal produced in the laserinduced extreme conditions, which very likely is interstitial zinc formed due to the highly nonequilibrium features of the ultrarapid laser reactive quenching synthesis.28,31,32,43 However, the green emission is obviously related to the defects formed in the subsequent partial oxidation process, which should be oxygen vacancies.15,44 In this aspect, the aging-induced visible emission change is in favor of the mechanism attribution25 for the defect-related emission in ZnO nanostructures. Further systemic investigation about the defect-related emission will be published later. The enhancement of the visible emissions of the treelike nanostructures is likely associated with the following two factors. The first one is that the self-assembly of the isolated nanoparticles can greatly reduce, by interface attachment at the atomic level, the density of the surface trapping states, which have been reported to trap excited electrons and availably depress the electron radioactive transition.46,47 Obviously, such a reduction will improve the emission efficiency. Another factor is the increase of the total defect concentration, mainly due to the increase of the oxygen vacancies with the aging process, which leads to partial oxidation of Zn nanocores. The existence of highly concentrated defects in the as-prepared and assembled treelike nanostructures is in agreement with the obvious lattice distortion from normative XRD (the interstitial zinc has a greater contribution to the lattice distortion). The subsequently produced ZnO in the treelike nanostructures during the aging process is different from the initial ZnO in the cor/shell nanoparticles from laser ablation. The former should have a higher oxygen vacancy concentration due to the incomplete oxidation. Therefore,, it is reasonable that the aging-induced increase of oxygen vacancies leads to the strong green emission band. Such aging-induced enhancement of visible emissions could be beneficial to the optical applications of nanostructured ZnO. 5. Summary In summary, we have demonstrated the self-assembly formation of treelike nanostructures containing Zn/ZnO core/shell nanoparticles after aging of the Zn/ZnO colloidal solution by laser ablation in a liquid at room temperature without any further heating conditions. The formation of the treelike nanostructures is attributed to the imperfect oriented attachment of the nanoparticles in the colloidal solution during aging. The stability of the colloidal solution and adequate aging time are crucial to formation of the treelike nanostructures due to the coalescence of the nanoparticles and adjustment of the particles’ orientation. In the optical properties, the treelike nanostructures exhibit a depression of the Zn SPR due to the partial oxidation and a great enhancement of the visible emission because of the reduction of the surface trap states and increase of the oxygen vacancy concentration. This study provides a very soft method to fabricate some highly ordered and self-assembled nanostructures by combining laser ablation in a liquid and the hermetical aging process. This work could facilitate the further understanding of the nanocrystal self-assembly, especially in the very soft conditions, and improve the optical applications of nanostructured ZnO. Acknowledgment. This project is financially supported by the Major State Research Program of China (Grant No. 2006CB300402) and National Natural Science Foundation of China (Grant No. 10604055).

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