Attachment-Driven Morphology Evolvement of Rectangular ZnO

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J. Phys. Chem. B 2005, 109, 8786-8790

Attachment-Driven Morphology Evolvement of Rectangular ZnO Nanowires Dong-Feng Zhang, Ling-Dong Sun,* Jia-Lu Yin, Chun-Hua Yan,* and Rong-Ming Wang State Key Lab of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, and Electron Microscopy Laboratory, Peking UniVersity, Beijing, 100871, China ReceiVed: February 4, 2005; In Final Form: February 25, 2005

The rectangular cross-sectional ZnO nanowires were synthesized in a solution method. An attachment-driven growth mechanism was proposed for the morphology evolvement of ZnO nanocrystals from nanoparticles to nanoplates and eventually to nanowires. Due to the pileup attachment of the nanoplates to recrystallize into nanowires, unique one-dimensional (1D) ZnO nanowires with the rectangular cross section were obtained, which is different from those nanowires in the previous reports. It is the first time the evidence that “oriented attachment” can occur not only for nanoparticles but also for nanoplates was obtained, suggesting that “oriented attachment” is an intrinsic behavior for nanosized materials. According to the growth model proposed based on the direct TEM observations, ZnO nanocrystals can be easily controlled as nanoparticles, nanoplates, or nanowires by tuning the synthetic parameters.

1. Introduction Bottom-up technique plays an important role in the fabrication of nanostructured materials due to its potential in simultaneously controlling the size, morphology, and dispersivity of nanocrystals.1 Among numerous preparation techniques, solution-based synthesis has been exploited toward this goal for many years and demonstrated as a promising alternative to lithography routes. Though in principle, crystal formation in such solutionbased synthesis is chaotic, a few theoretical works were proposed to rationalize the mechanism of such processes over the past few years. The rationales help us to understand the “magic” type of synthesis and will guide us toward our goal more efficiently. Size and shape controllable semiconductor nanostructures were achieved based on the understanding of the nucleation and growth process. For example, the solutionliquid-solid (SLS) mechanism proposed by Buhro and coworkers is one of the successful models that explained the formation of III-V nanofibers via an organometallic reaction in hydrocarbon solvents.2-5 Alivisatos and Peng et al. investigated the morphology carving strategy of CdSe nanocrystals.6-9 Recently, oriented attachment based modes10-14 were proposed based on the crystallization process that occurred naturally for the nanosized materials. It was demonstrated that nanoparticles can join together either with perfect aligned lattice planes by sharing specific crystallographic orientation or with dislocations by slight mis-orientation at the interface, requiring only similar surface symmetries10 and a nearly zero-kinetic barrier.14 The oriented attachment model differs from the Ostwald ripening theory in that the latter depicts crystal growth in a way that bigger crystals consume the smaller ones, whereas the former one views the formation of bigger crystals as a result of attachment and coalescence of smaller particles. Recent studies suggested that, in addition to size and dimension, the cross-section also has a “shape effect” on the properties of nanomaterials.15 The successful access to belt* Corresponding authors. Phone and Fax: 86-10-62754179. E-mail: [email protected].

morphological nanomaterials15-17 indicated that the cross-section configuration could be tailored. ZnO, with novel optical and electrical properties, is a good candidate to study the relationship between the properties and the geometry configurations. Rectangular cross-sectional ZnO has been achieved by the thermal evaporation method.16 However, for most of the solution-based synthesis, cross-section control is still difficult and ZnO usually exhibits the hexagonal prismatic or cylindrical morphology, consistent with the crystal structure. In this paper, we reported the preparation of rectangular crosssectional ZnO in a solution. The growth mechanism and the morphology evolvement were studied systematically. We revealed that oriented attachment could occur not only in zerodimensional (0D) nanoparticles, but also in two-dimensional (2D) nanoplates under hydrothermal environment. It was shown that the eventual formation of nanowire embraced three major steps over the entire process, i.e., the dehydration of the precursors, the diffusion-limited attachment of nanoparticles into nanoplates, and the pileup attachment and fusion of the nanoplates into rectangular cross-sectional nanowires: dehydration oriented attachment ZnO Zn(OH)42-98 nanoparticles 98 pileup and fusion ZnO ZnO nanosheets 98 nanowires

During the process, Ostwald ripening also worked to help the nanocrystals crystallize perfectly. On the basis of this model, one could expect that the morphology of ZnO nanocrystals can be easily controlled as 1D or 2D. 2. Experimental Section Typical syntheses were carried out in a reverse microemulsion system. Sodium dodecyl sulfate (SDS) (1.44 g) was dissolved in a solvent consisting of 10.2 mL of heptane and 3.0 mL of hexanol. Then an appropriate amount of Zn(OH)42- solution (V0.5mol/L,ZnAc2:V5.0mol/L,NaOH ) 1:1) was added into the above mixture. After several minutes of ultrasonic dispersion, the formed microemulsion was transferred into a 25 mL Teflon-

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Rectangular ZnO Nanowires

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Figure 2. (a) Time-dependent (reacting at 120 °C) and (b) temperaturedependent (reacting for 4 h) weight percentage of product A (ZnO nanowires).

Figure 1. TEM and SEM images of ZnO nanocrystals formed at 180 °C for 4 h: (a, c) products A and (b, d) B. The inset of Figure 1c shows the typical rectangular cross-section feature of the nanowires.

lined autoclave and heated to a given temperature ranging from 80 to 180 °C for 15 min to 6 h. The solution was allowed to cool to room temperature naturally. The directly precipitated product (denoted as A) was collected by centrifugation. Ethanol was added into the supernatant until no more precipitate formed, producing an additional product (denoted as B), which was also collected by centrifugation. Both kinds of precipitates (A and B) were washed several times with absolute ethanol and distilled water. Finally, the obtained products were dried in a vacuum at 60-70 °C for 5 h. The product of the whole formation process was monitored by scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, and confirmed by high-resolution TEM (HRTEM) and X-ray diffraction (XRD) characterizations. 3. Results and Discussion As discussed in the Experimental Section, the product can be separated into two parts (A and B). A was directly deposited from the solution, while B was precipitated by adding ethanol to the supernatant. SEM and TEM observation results indicated that A generally displayed as nanowires (Figure 1a,c) whereas B displayed as the dendritic aggregations of nanoplates (Figure 1b,d). From high-magnified SEM observation, one can identify clearly the rectangular cross-section feature of the nanowires as shown in the inset of Figure 1c. The extensive rectangular cross-section feature of the nanowires is further stated in Figure S1. The ratio of type A to type B products was time and temperature dependent. Retaining the total amount of product by fixing the Zn(OH)42- precursor volume constant and keeping the reaction at 120 °C, the respective weight of precipitates A and B was recorded with reaction time from 15 min to 6 h. Repeating the process three times, the average weight percentage of A (nanowires) versus time was plotted in Figure 2a. It can be seen that the weight percentage of A increased with time, demonstrating an increased ratio of A to B. Similarly, the temperature-dependent weight percentage curve of A was obtained (Figure 2b) with temperatures ranging from 80 to 180 °C, which exhibited a remarkable increase of A with temperature, whereas the total yield of the product from 15 min to 6

Figure 3. TEM images of ZnO nanocrystals at different stages of the growth process: (a) uniform ZnO nanoparticles formed by the confinement of reverse micelles, (b) flocky-like aggregates of the nanoparticles, (c) nanoplates formed by the reorganization of the nanoparticles, (d) intermediates of nanoplates and nanowires, (e) nanoplates evolved into nanowires by the pileup-attachment, and (f) dispersed nanowires with rectangular cross-section.

h showed no obvious change. It is suggested that product B, which appeared as nanoplates, was an intermediate in the formation of the nanowires. The morphology evolvement of the products provided detailed information on the crystallization process. When the reaction time was shorter than 15 min, there was no product directly precipitated, i.e. no A formed in the system. By adding ethanol, B was obtained. TEM observation showed that except for a few dispersed nanoparticles (Figure 3a), B was dominated by a flocky-sphere shape as shown in Figure 3b (the low magnified TEM image is shown in Figure S2). Prolonging the reaction time to 30 min, besides B, ca. 20 wt % of product was obtained as A. At this stage, B had evolved its morphology into flowerlike aggregates of nanoplates (Figure 3c) with improved crystallization. It is noticeable that the intermediates of nanoplate and nanowire aggregates can be observed as shown in Figure 3d, while the typical A behaved as nanowire aggregates (Figure 3e) and/or isolated nanowires (Figure 3f). Further prolonging the reaction to 12 h, nanowires were found as the only product. Monodispersed individual nanowires can be achieved by ultrasonication. The trend of morphological evolution indicated that nanoparticles and nanoplates were the intermediates in the formation of nanowires. The corresponding XRD patterns of as-prepared ZnO nanoplates (B) and nanowires (A) are shown in the upper parts of Figure 4, parts a and b. All the diffraction peaks can be indexed well to the pure hexagonal phase ZnO with wurtzite structure (JCPDS card No. 36-1451). Compared with the standard XRD pattern of bulk ZnO (the lower parts of Figure 4), the intensity of the (002) peak was enhanced for ZnO nanosheets, whereas

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Figure 4. (Upper parts) XRD patterns of ZnO nanoplates (a) and nanowires (b) obtained at 120 °C for 0.5-6 h and (lower parts) the standard XRD patterns of bulk ZnO (JCPDS No. 36-1451).

Figure 5. (a) TEM and (b) HRTEM images of a nanoplate, which is not well-crystallized, and (c) TEM and (d) HRTEM images of a typical nanowire formed in the same stage; imprints of the nanoplates still can be identified as indicated by the arrows.

the (100) peak was intensified for ZnO nanowires. The difference of peak intensity between nanowires and nanoplates may originate from their anisotropy and different orientations. HRTEM characterizations further evidenced the growth process from nanoparticles to nanoplates and eventually to nanowires. In the early stage of reaction (first 30 min or so), the nanowires and nanoplates were not crystallized well. A typical HRTEM image (Figure 5b) of a nanoplate (Figure 5a) depicted that the nanoplate was formed by nanoparticles with diameters of 4-6 nm in the “oriented attachment” manner. It can be seen that particles I and II had “fused” into each other with perfect crystallographic orientation, whereas dislocations formed among particles II-III and III-IV, respectively. The spacing between two adjacent lattice fringes of the nanoparticles was 2.63 Å, coincident with the (0002) plane of the wurtzite ZnO, indicating that the attachment occurred either parallel or perpendicular to the c-axis. On the edge of the nanowire (Figure 5c), some not well-merged fragments still remained. The corresponding HRTEM image (Figure 5d) revealed that the nanowire was actually formed by the pileup of the nanoplates layer by layer perpendicular to the c-axis as identified from the lattice fringe and imprint of the edges. Compared with the products with shorter reaction time, the crystallinity of both nanoplates and nanowires had been greatly improved after prolonging the reaction time to 1 h. Figure 6a

Figure 6. (a) High-magnification TEM image of ZnO nanoplates, (b) the HRTEM image, and (c) the corresponding FFT pattern recorded in the frame of the nanoplate with zone axis 〈1h100〉. The inset of panel b is an enlarged image showing the resulting dislocation formed during the “oriented attachment”. (d) The high-magnification TEM image of a ZnO nanowire, (e) the HRTEM image, and (f) the corresponding FFT pattern recorded in the frame of the nanowire.

was a TEM image of such a nanoplate. The HRTEM image (Figure 6b) with zone axis 〈1h100〉 and the corresponding fast Fourier transform (FFT) pattern (Figure 6c) demonstrated that the nanoplate had developed into a single crystal growing along the c-axis, the same direction as that of the preferred attachment of the nanoparticles. Since the electron beam is most likely irradiating perpendicular to the flat surface of the nanoplate, its flat surface can be assigned as the (1h100) plane of wurtzite ZnO. Moreover, as proposed in the literature,10 dislocations were also found as shown in the inset of Figure 6b. Parts d and e of Figure 6 are TEM and HRTEM images of a ZnO nanowire obtained simultaneously with the nanoplate in Figure 6a. Compared with Figure 5d, it seemed that the nanoplates had merged into each other more tightly, whereas the dislocations formed by the slightly mismatched pileup of the nanoplates still could be seen from the edge of the nanowire as shown in Figure 6e. Combined with the FFT technique (Figure 6f), it is found that the nanowire had the same diffraction pattern as that of the nanoplate, i.e. growing along the c-axis in length and [112h0] direction in width

Rectangular ZnO Nanowires with 〈1h100〉 as the zone axis, attesting further that the nanowire was formed by the nanoplates piling up along the [1h100] direction. Besides the reaction time, the overall Zn(OH)42- concentration also influenced the morphology of as-prepared ZnO. Under the similar condition, when the volume of Zn(OH)42- was reduced to 0.9 mL, the products preferred to be nanoparticles instead of nanoplates or nanowires. To identify the crucial contribution of reverse micelles, experiments were performed in various pure solvent systems. The obtained ZnO unexceptionally exhibited idealized hexagonal crystallographic morphology (Figure S3), which further confirmed our suggestion that, compared with the ordinary solution system, the reverse micelle system is an Ostwald-ripening unfavorable one.18 We believed that the morphology of the product was neither confined by the shape of the micelle nor directed by surfactant-metal ion pairs, but determined by the nucleation and growth control of the reverse microemulsion system, as well as the intrinsic isotropic property of hexagonal ZnO. The detailed growth mechanism will be discussed in the following part. Because the dehydration temperature of Zn(OH)2 is as low as 100 °C under hydrothermal condition,19 thermally induced homogeneous nucleation burst out as temperature increased. Confined by the reverse micelles, uniform ZnO nanoparticles formed. The formation rate of the crystallite unit far exceeded its dissolution and diffusion rate in the microemulsion system. Meanwhile, with the temperature and pressure increasing steadily, it became easier for the micelles to coalesce.20-22 Thus, the resulting nanoparticles would experience a rapid aggregation process. The sphere-like form was the primary choice for the aggregation owing to its low surface energy. When the precursor solution was reduced to 0.9 mL, the efficient collision among ZnO nanoparticles dropped prominently due to the relative lower ZnO nanoparticle concentration and high stabilization of the microemulsion by the low salt concentration.23 Consequently, the products were inclined to remain as nanoparticles. Due to the intrinsic anisotropic property of hexagonal ZnO, the aggregated nanoparticles would rearrange themselves and orientedly attach to each other to decrease the energy of the system. For wurtzite ZnO, the (0001) plane is the highest energy plane in crystal, thus attaching along the c-axis became energetically favorable. On the other hand, because the Brownian motion in the hydrothermal system is intense, the attachment between nanoparticles was not limited only to along the [0001] direction. To match the crystals, the nanoparticles attaching perpendicular to the c-axis was another preferred choice. Thus nanoplates growing along [0002] in length and [112h0] in width were obtained as evidenced by HRTEM characterization (Figure 6b). Weller et al. observed a similar trend and claimed that ZnO nanoparticles were inclined to align like a wall.13 However, they did not obtain or observe individual nanoplates. Because of the fast growth rate of ZnO nanocrystals, the concentration of ZnO nanoparticles decreased sharply due to the formation of nanoplates. With sufficient thermal energy provided by the hydrothermal system, the resulting nanoplates also experienced an “attachment” process in a manner perpendicular to the c-axis. This behavior is reasonable because, on one hand, the surface energy of an individual nanoplate was quite high with two exposed flat planes, thus they tended to pile up perpendicularly to the flat surface to decrease the surface energy by greatly reducing exposed areas; on the other hand, with the well-matched crystal lattice and active surface, the adjacent nanoplates were prone to “fuse” to each other driven

J. Phys. Chem. B, Vol. 109, No. 18, 2005 8789 SCHEME 1: Summary of the Formation Process of Rectangular Cross-Sectional ZnO Nanowires

by the gaining of free energy and lattice-free energy.13 This unique process occurring for the nanoplates eventually led to the formation of rectangular cross-sectional ZnO nanowires. It can be foreseen that there should exist many stacking faults or dislocations during the recrystallization from the nanoplates to nanowires, which were confirmed by the HRTEM characterization as shown in Figure 6e. We believed that oriented attachment was not the only way to proceed with growth, Ostwald ripening also contributed to perfect crystals by eliminating the blemishes formed during the conjugation with greatly dropped precursor concentration. Several reports implied that the nanoplate is a sub-stable state for hexagonal structured crystals.24-26 They regarded rolling as the mechanism for the conversion from nanoplates to nanowires. However, no evidence was shown to support this hypothesis from our observations. As mentioned above, the SEM and HRTEM characterizations clearly demonstrated that the ZnO nanowires had experienced a pileup and fusion process. In addition, if the nanowires were formed by a rolling mechanism, the cross-section should be circular or hexagonal, which conflicted with our SEM observations that ZnO nanowires exhibited rectangular cross-sections (inset of Figure 1c). Clear evidence of piled intermediates was also obtained for PbO nanocrystals fabricated by a similar procedure (Figure S4). On the basis of the above discussions, we believed that the mechanism shown in Scheme 1 described the hypothesis for the formation of rectangular ZnO nanowires. According to this model, if we could balance the formation rate of ZnO nanoparticles with their depleting rate by forming the nanoplates, the concentration of fresh ZnO nanoparticles would remain relatively high and steady. Thus the morphology of the products would be maintained as nanoplates instead of nanowires. Since the relative higher OH- ion concentration would accelerate the dehydration rate of hydroxides,18 we chose Zn(en)32-, ZnAc2, and ZnCl2 solutions as precursors without additional OH- ions to lower the formation rate of ZnO nanoparticles. As expected, only nanoplates were obtained (Figure S5) even at 180 °C for 20 h. 4. Conclusions Rectangular cross-sectional ZnO nanowires were fabricated in a microemulsion-mediated hydrothermal system. On the basis of SEM, TEM, HRTEM observations, and XRD characterization, an oriented attachment-driven growth mechanism was proposed for ZnO nanocrystals evolved from nanoparticles to nanoplates and eventually to nanowires. The reverse micelle was believed to have functioned as microreactors to confine ZnO nanoparticles. Owing to the anisotropic property of wurtzite ZnO, the resulting uniform ZnO nanoparticles experienced an oriented attachment process to lower the system energy, which led to the formation of dendritic nanoplate-aggregations. With sufficient energy provided by the hydrothermal system, the adjacent nanoplates piled up and fused each other to further minimize the energy. The pileup growth manner eventually led to the formation of rectangular cross-sectional ZnO nanowires.

8790 J. Phys. Chem. B, Vol. 109, No. 18, 2005 We believed that the oriented attachment based mechanism dominated the growth when the concentration of the precursors (nanoparticles for the nanoplates, and nanoplates for the nanowires) was high, while the Ostwald ripening process facilitated the perfection of the crystals when the concentration of precursor was lowered. The reaction time and temperature mainly impacted the crystallinity of the products, while the kind of Zn2+-containing precursors affected the morphology of the products. On the basis of this mechanism, ZnO nanocrystals can be easily controlled as 1D or 2D morphology. Acknowledgment. This work is supported by the NSFC (Nos. 10374006, 20221101, and 20423005) and the Founder Foundation of PKU. Supporting Information Available: SEM images of the rectangular cross-sectional ZnO nanowires and the intermediates of PbO nanorods, low-magnification TEM images of ZnO nanoplate aggregations, and TEM images of the ZnO nanocrystal formed with different precursors and in different solvents. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791. (3) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang, M. Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc. 1997, 119, 2172.

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