Growth of flower-like ZnO via surfactant-free hydrothermal synthesis on ITO substrate at low temperature

Abstract

Without any surfactant, rod-like and three kinds of flower-like ZnO microstructures were synthesized on indium-doped tin oxide (ITO) glass substrates through a simple and environmentally-benign hydrothermal process at 70 °C. The result indicated that rod-like ZnO would be transformed into flower-like ZnO microstructures with decreasing the concentration of sodium hydroxide. The ends, numbers and diameters of the petals of flower-like ZnO varied greatly by modulating the concentration of sodium hydroxide. The secondary nucleation and growth phenomena of ZnO were observed. Time-dependent experiments results indicate that the flower-like ZnO formed in a short period of time. The evolution of morphology and size of ZnO microstructures depended on the reaction time. The amounts and diameters of the petals of flower-like ZnO changed with increasing reaction time. On the basis of our observations and the mechanism proposed previously, the possible growth mechanism for flower-like ZnO was proposed.

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1 Introduction

It is well known that the physical and chemical properties of inorganic nanomaterials depend not only on their composition but also on their structure, phase, morphology, and spatial arrangement.¹ Therefore, the important role of morphology related properties of nanostructures has stimulated tremendous efforts in the design and synthesis of nanomaterials with special morphology.² The control over morphology has rapidly become a hot topic of research with development of new materials because of their unique physical, chemical and biological properties and potential applications in advanced functional materials.^3,4

Zinc oxide, ZnO, is one of the most important semiconductors with a direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature. It has been recognized as one of the most important semiconductor materials in scientific research and technological applications. In recent years, ZnO nanostructures have been paid considerable attention due to their rich morphologies, and potential applications. Compared with other semiconductors, ZnO has advantages including nontoxicity, thermal stability, irradiation resistance, and flexibility to form different nanostructures, which expedites its potential wide applications in biosensors,⁵ ultraviolet nanolasers,⁶ photodetectors,⁷ solar cells,⁸ gas sensors,^9,10 surface acoustic wave devices,¹¹ ceramics,¹² and nanogenerators.¹³ Hence, ZnO has attracted considerable research interest due to its special properties and wide applications.

Among this research, the control over size and morphology of nanometer and micrometer ZnO semiconductors represents a great challenge to realize the design of novel functional devices. This is because optical and electronic properties of ZnO semiconductors, which finally determine practical applications, can be modulated by varying their size and morphology.¹⁴ For this reason, ZnO with different morphologies, including nanobelts, nanorods, firecracker-shaped, nanowires, nanobridges, nanonails, and nanowhiskers,^15–21 have been prepared by different methods.

The synthesis of ZnO is related to many academic subjects, such as physics, chemistry, and materials chemistry etc. On the basis of conventional preparation methods, many methods have been developed, mainly involving hydrolysis in polyol media,²² chemical precipitation,²³ microwave heating,²⁴ templating,²⁵ thermal oxidation processes²⁶ and hydrothermal syntheses.^27,28 Among these methods, the hydrothermal technique has been widely utilized to synthesize inorganic nanomaterials at temperatures generally below 220 °C.²⁹ The hydrothermal process has several advantages over other growth processes, such as the use of simple equipment, catalyst-free growth, low cost, large surface area for particles, environmentally benign and less hazardous.³⁰ However, various organic additives, such as template or surfactant, are commonly involved during hydrothermal process. Therefore, self-assembly of nanoparticles into the three-dimension structured morphologies and hierarchical architectures in the absence of any surfactants, template supports and structure-directing reagents still remains a tremendous challenge.³¹ Recently, flower-like ZnO nanostructures have been successfully synthesized by various methods.^3,32–37 However, in these methods, surfactant or organic solvents were used, which are harmful to health and the environment. On the other hand, some methods also need multiple steps, complicated processes or expensive equipment.³¹ It is worth mentioning that Li and Wang have fabricated ZnO hierarchical microstructures with uniform flower-like morphology on a large scale through a template and surfactant-free low-temperature (80 °C) aqueous solution route.³⁸ Liu and coworkers synthesized flower-like ZnO microcrystals by employing Zn(NO₃)₂·6H₂O and NH₃·H₂O with stirring for 24 h at 60 °C.³⁹ Zhang et al. also fabricated flower-like ZnO nanostructures by an organic-free hydrothermal process at 120, 160 and 200 °C for 20 h.⁴⁰ The flower-like ZnO nanostructures consisting of sword-like nanorods 60–200 nm in width and several micrometres in length were obtained at 200 and 160 °C.

Indium tin oxide (ITO) glass substrate exhibits excellent properties, such as electrically conductive and optically transparent, high vis-NIR light transmission, uniform transmission homogeneity and reflection in the infrared range, which makes it one of the most suitable substrate for preparing thin films.^41,42 Pradhan et al. fabricated ZnO nanospikes and nanopillars on ITO substrate via a simple and direct electrodeposition technique at 70 °C.⁴³ Xu and coworkers synthesized rod arrays of ZnO on ITO substrate by aqueous chemical growth.⁴⁴ However, self-assembly of flower-like ZnO superstructures on ITO substrates via hydrothermal method without any surfactant are rarely reported.

Compared with the hydrothermal process, which was used to fabricate flower-like ZnO in recent years, ZnO was prepared without any surfactant on ITO substrate under low-temperature (70 °C) in our work. On the basis of the above considerations, our work prepared flower-like ZnO microstructures on a large scale on ITO substrates via hydrothermal approach without using any organic solvent or surfactant under low-temperature (70 °C) and compared with the sample without ITO substrate. The as-prepared flower-like ZnO microstructures are built by many nanorods or nanopencils and well-crystalline structures. In addition, its growth process and possible mechanism are discussed in some detail.

2 Experimental

2.1 Sample preparation

All chemical reagents supplied by the Chinese National Medicine Group in our experiments were of analytical grade and used without further purification. Commercial ITO slide glass was cut into 10 × 10 mm² slides as substrates for the direct deposition of ZnO and then carefully cleaned in ethanol and deionized water. The typical experimental details are described as follows: 40 mL aqueous solutions of 0.035 M Zn(CH₃COO)₂·2H₂O and 0.45 M NaOH were prepared using distilled water with stirring. The mixture precursor solutions were added to a 100 mL Teflon-lined stainless steel autoclave. A piece of pretreated ITO glass was then put into the precursor solution before sealing. The autoclave was kept at 70 °C for 24 h. Finally, the ITO glass was taken out of the solution, rinsed with distilled water several times and dried in air. Other experiments with different amounts of NaOH were performed. Additionally, time-dependent experiments were conducted to study the growth mechanism of the ZnO structures. Samples were withdrawn from the autoclave at different time. To investigate the effect of ITO glass, ZnO samples were prepared without using an ITO substrate.

2.2 Characterization

To identify the crystalline phase and structure, X-ray diffraction (XRD) patterns of samples were recorded on a multipurpose X-ray diffraction system (D8 ADVANCE, Bruker) with a Cu-Kα radiation source. The morphology of samples was observed using a QUANTA FEG 250 scanning electron microscope (SEM) equipped with an EDS system for elemental analysis (X-MAX50, Oxford).

3 Results and discussion

3.1 Variation of ZnO morphologies with sodium hydroxide

Fig. 1 displays the SEM images of the samples prepared using the typical procedure. Different amounts of sodium hydroxide are essential to the morphologies of the ZnO structures. It can be seen from Fig. 1A–B that randomly distributed ZnO rod-like structures with diameters in the range of 0.5–1.3 μm and average length of 7 μm (aspect ratio ∼8) are prepared when using 0.65 M sodium hydroxide. With 0.55 M sodium hydroxide, flower-like ZnO structures are observed (Fig. 1C–D). A close observation in Fig. 1D indicates that a single flower is composed of hexagonal rod-like crystals of 300–400 nm in diameter and a length of 3.5 μm (aspect ratio ∼10) radiated from the center. When the amount of sodium hydroxide was decreased to 0.45 M, ZnO bundles obtained vividly resemble natural flowers in shape (Fig. 1E–F). However, it is worth mentioning that a single flower mainly consists of pencil-like crystals with diameters of 300 nm and average length of 3 μm (aspect ratio ∼10) radiating from the center. When the concentration of sodium hydroxide was further decreased to 0.35 M (Fig. 1G–H), many flower-like structures ZnO are observed on the ITO substrate. These flowers are made of pencil-like rods with typical diameter of about 0.8 μm and length of 2 μm (aspect ratio ∼2.5). It is interesting that there are some very tiny nanorods on the lateral faces of single pencil-like rods, which makes the ZnO look like flowers with thorns. The nanorods grow almost vertically on the lateral faces of pencil-like rods (Fig. 1H), which would be induced by secondary nucleation. The secondary nucleation is also found during fabricating gold mesoparticles.⁴⁵ In addition, as Sounart et al. reported, nuclei that are aligned in the most energetically favorable orientation (∼ 80°) grow at the expense of the misaligned nuclei. This is energetically favored because it reduces the interfacial stress at the primary ZnO surface.⁴⁶

Fig. 1 Typical SEM images of ZnO prepared on ITO substrate: (A, B) 0.65 M NaOH; (C, D) 0.55 M NaOH; (E, F) 0.45 M NaOH; and (G, H) 0.35 M NaOH.

To know the effects of ITO substrate upon the ZnO growth, a experiment with 0.45 M sodium hydroxide and without ITO substrate has been carried out. Fig. 2 shows the SEM images of the ZnO samples. The result indicates that irregular flower-like ZnO were created when the ITO substrate was removed from autoclave during hydrothermal synthesis. Compared with the sample using 0.45 M sodium hydroxide on ITO substrate (Fig. 1E–F), the uniformity and symmetry of the flower-like ZnO without using ITO substrate became different. It can be easily found that flower shapes of the sample without the substrate are irregular and nonuniform. Fig. 2B indicated the diameters of petals are also nonuniform and range from about 100 nm to 1 μm. However, the petals of sample grown on ITO substrate are very uniform (Fig. 1E–F), which is attributed to the effect of substrate. Generally, it is widely recognized that during initial growth, interactions between substrate and ZnO particles play an important role in nucleation.⁴⁷ Because of a small lattice mismatch (3%) between the neighboring oxygen–oxygen (O–O) distance on the close-packed ITO (111) and ZnO (0001) plane as well as O dangling bonds on the ITO layer surface,⁴⁸ the initial nucleation and the subsequent growth of flower-like ZnO with uniformity and symmetry benefit from an ITO substrate. Therefore, it was concluded that the uniform flower-like ZnO grown on ITO depended on the formation of initial nuclei. The crystallinity and orientations of the substrate have important influences on the ZnO growth.

Fig. 2 Typical SEM images of ZnO without using ITO substrate, 0.45 M NaOH.

To confirm the formation of ZnO during a hydrothermal synthesis, an X-ray diffraction experiment of the ZnO microstructures prepared from 0.65 M NaOH precursor was carried out. Fig. 3 shows a typical XRD pattern of the ZnO nanostructures prepared from 0.65 M NaOH precursor. All diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO crystal (JCPDS No. 36-1451), no characteristic peaks of other impurities were detected in the pattern. The sharp diffraction peaks indicate the good crystallinity of the prepared crystals. It is noted that the relative intensities of the peaks differ from the standard pattern of the bulk material, which would be caused by preferred orientation and distribution of the ZnO crystals on the substrate surface.⁴⁹

Fig. 3 XRD pattern of the ZnO sample prepared from precursor with 0.65 M NaOH.

3.2 Variation of the ends of petals of flower-like ZnO with sodium hydroxide

Laudise et al. claimed that the growth of crystals is related to the relative growth rate of different crystal facets and the difference in the growth rates of various crystal facets results in a different outlook of the crystallite.⁵⁰ Single rod-like or pencil-like ZnO unique growth direction is explained by the favorable energy along a specific growth direction.⁵¹ The hexagonal prismatic morphology and the facet outlook of the pencil-like rods are caused by the different growth rates of the crystalline faces.⁵² In general, a larger ZnO crystal is a polar crystal whose positive polar plane is rich in Zn and the negative polar plane is rich in O. The negatively charged Zn(OH)₄²⁻ growth units of ZnO preferably adsorb on the positive polar plane (0001). During hydrothermal process, the complex Zn(OH)₄²⁻, which leads to the different growth rate of planes shown in following: V(0001) > V(1̄011̄) > V(1̄010) > V(1̄011) > V(0001̄).⁵³ It is well known that the more rapid the growth rate, the quicker the disappearance of the plane. Accordingly, the (0001) plane, the most rapid growth rate plane, disappears in the hydrothermal process when the concentration of sodium hydroxide was decreased from 0.55 M to 0.45 M and 0.35 M (Fig. 1C–H), which leads to the pencil shape at the end of the c axis, shown in Fig. 4.

Fig. 4 Growth sketch of larger ZnO crystal from hexagonal end to pencil-like end.

3.3 Variation of diameters and numbers of petals of flower-like ZnO with sodium hydroxide

Fig. 5 shows high-magnification SEM images of the flower-like ZnO prepared with 0.35 M, 0.45 M and 0.55 M NaOH, which usually are composed of dozens of petals with all the petals originating from the same core in a highly symmetric fashion. In addition, we found that the average diameter of the single petal of flower-like ZnO prepared with 0.35 M NaOH is evidently larger than those of ZnO flower fabricated with 0.45 M and 0.55 M NaOH (Fig. 5A–B). And, it is obvious that the petals of ZnO flowers fabricated with 0.35 M NaOH are fewer in number than those of ZnO flowers when the concentration of NaOH is 0.45 M and 0.55 M (Fig. 5C–D), which will be analyzed later.

Fig. 5 High-magnification SEM images of flower-like ZnO prepared on ITO glasses: (A) 0.55 M NaOH; (B) 0.45 M NaOH; (C, D) 0.35 M NaOH.

3.4 Variation of morphology with concentration of Zn²⁺ and OH⁻

According to the above results it can be seen that the amount of NaOH solution is the key to ZnO growth. In order to know the influence of the concentration of Zn²⁺ and OH⁻ on morphology, experiments lowering the concentration of Zn²⁺ and OH⁻ were performed. The sample with 0.45 M NaOH was investigated. The concentration of two samples were reduced to one half (0.225 M NaOH) and one fifth (0.09 M NaOH), respectively, while keeping the ratio of Zn²⁺ and OH⁻ the same as the sample with 0.45 M NaOH. Fig. 6 shows the morphologies of different concentration of Zn²⁺ and OH⁻. Fig. 6A–B indicated that some flower-like ZnO with a few petals are formed upon lowering the concentration of Zn²⁺ and OH⁻ to one half. Obviously, there are a few petals of flowers composed of many thin rods. The thin rods are held together and are not separate. When the Zn²⁺ and OH⁻ concentration of sample was reduced to one fifth, it can be seen that many cauliflower-like ZnO are fabricated (Fig. 6C–D). The diameters of the flower-like ZnO are not uniform and range from 1.3 to 5 μm when lowering the concentration. Compared with the sample of 0.45 M NaOH (Fig. 1E–F), it can be easily found that the shapes of flowers are not uniform when lowering the concentration of Zn²⁺ and OH⁻, which should be attributed to the shortage of raw materials in the later stage of growth.

Fig. 6 SEM images of lowering the concentration of Zn²⁺ and OH⁻: (A, B) 0.225 M NaOH; (C, D) 0.09 M NaOH.

3.5 Growth process

In order to reveal the growth mechanism of such flower-like structures, time dependent experiments of the sample with 0.45 M sodium hydroxide are studied in detail. The growth process of ZnO can be clearly observed by changing the hydrothermal reaction time (Fig. 7). When the reaction time is 5 min, plenty of nuclei with a diameter of about 460 nm preferentially are formed and it can be clearly seen that one nucleus is composed of some smaller nuclei (inset in Fig. 7A). Then, when the time is extended to 8 min, the diameters of nuclei increase to about 850 nm, and the aggregated nuclei slowly become dispersed (Fig. 7B). By increasing the reaction time to 10 min, the nuclei continue to grow and the diameters grow to about 1.3 μm (Fig. 7C). In addition, it can be found that the nuclei also consist of some small nuclei. Using the reaction time of 20 min, some flower-like ZnO with diameters of about 2–2.3 μm are formed (Fig. 7D). However, the petals of flowers are very few and wide. The sizes of the petals are not uniform and the largest diameter of petals can reach about 1 μm. However, it can be clearly seen that each petal is composed of many thin and small rods. When the reaction time is 30 min, the wide petals break into narrow ones with one end attached to the starting nucleus. The diameters of flower-like ZnO are about 2–2.6 μm on ITO substrate (Fig. 7E). The well-defined flower-like ZnO consists of many leaf-like petals with sharp ends and the largest diameter of petals is about 0.4 μm and the numbers of petals increases. When the reaction time is prolonged to 1 h, the flower-like ZnO with a diameter about 1.2–3 μm can be observed on ITO substrate (Fig. 7F). However, the largest diameter of petals stays nearly the same. Using the reaction time of 2 h, more flower-like structures are observed (Fig. 7G) and some flowers grow on other flowers. The numbers of petals increases and the diameter of the widest petal does not decrease again. When the reaction time is extended to 4 h, much more flower-like microstructures have been formed (Fig. 7H). The sizes (about 1.5–3 μm) of flowers do not increase significantly. However, it is especially noted that the largest diameter of petals reduced to about 0.3 μm and is very close to final average diameter of petals (24 h). When the time is increased to 8 h, the diameters of products increase to about 3–4 μm and become uniform (Fig. 7I). The diameters of petals become more uniform and the numbers of petals increases. Then, when the reaction time is 12 h, there are a large number of flower-like ZnO with diameters of about 3–5 μm (Fig. 7J). By increasing the reaction time to 24 h, the diameters of the large quantity of flower-like ZnO increase to 4–5 μm and the wider petals are rarely observed (Fig. 7 K). The diameters of the petals become very uniform and the average diameter of petals is 0.3 μm.

Fig. 7 High-magnified SEM images of the products obtained with different reaction time: (A) 5 min; (B) 8 min; (C) 10 min; (D) 20 min; (E) 30 min; (F) 1 h; (G) 2 h; (H) 4 h; (I) 8 h; (J) 12 h; and (K) 24 h.

On the basis of our experiments, it can be concluded that flower-like ZnO could be fabricated when the reaction time is 20 min. The small sizes of flower-like ZnO obtained grow into bigger ones after 8 h, which causes the uniformity of diameters of flower-like ZnO and of petals. In conclusion, further prolonging the reaction time makes the flower-like ZnO structures more defined and sizes more uniform, which arises from the split of bigger aggregates into smaller ones to satisfy the spatial requirements of the crystal growth during the “Ostwald ripening” process.⁵⁴

The time-dependent experiments confirm that flower-like ZnO are formed in a short time (Fig. 7 D), which means that nucleation and growth occur rapidly in our system. However, the evolution of flower shape would be ascribed to base erosion.

3.6 Possible growth mechanism of flower-like ZnO

As is known, ZnO is a polar crystal that exhibits a basal positive polar plane (0001) and a negative polar plane (0001̄), which are rich in Zn and O, respectively. These polar planes with surface dipoles are thermodynamically unstable and have higher growth rates to reduce their surface energy.⁵⁵ In the hydrothermal process, the hydroxyl groups are able to bind to metal cations through coordination or electrostatic interactions.⁵⁶ That is, the absorbed Zn²⁺ ions react with OH⁻ groups to form the growth unit of Zn(OH)₄²⁻ (eqn (1)–(2)). Then, Zn(OH)₄²⁻ forms ZnO by dehydrating (eqn (3)). Therefore, the Zn(OH)₄²⁻ precursor plays an important role in determining the morphology of ZnO crystallites due to the concentration of OH⁻ in the reaction solution, which should be a key factor for controlling the growth rate of different crystal faces and thus lead to the formation of an anisotropic particle such as rod-like or flower-like morphology. Thus, the basicity is a key factor for a controllable synthesis, especially for the morphologies of typical flower-like ZnO.

Zn²⁺ + 2OH⁻ → Zn(OH)₂ (1)

Zn(OH)₂ + 2OH⁻ → Zn(OH)₄²⁻ (2)

Zn(OH)₄²⁻ → ZnO + 2H₂O + 2OH⁻ (3)

As mentioned above, Zn²⁺ reacts with NaOH to produce a growth unit of Zn(OH)₄²⁻. Then, very tiny ZnO particles are formed under the hydrothermal conditions. Owing to the concentration of Zn²⁺ in every reaction system remaining constant (0.035 M), when the concentration of NaOH is 0.65 M, both nucleation and crystal growth are relatively fast. A lot of ZnO nuclei are formed due to fast nucleation rates. At the initial stage, large amounts of ZnO nuclei and growth unit Zn(OH)₄²⁻ are formed rapidly at the same time. After that, lots of growth units are preferentially supplied for the c-axis direction of every nucleus, which causes the generation of numerous ZnO rods randomly distributed (Fig. 1A–B). Furthermore, the size of the rods tends to be uniform upon the erosion effect of extra base as reported.⁵⁷ However, it is occasionally found that there is also flower-like ZnO obtained (Fig. 8). As for the formation mechanism of flower-like ZnO, it will be discussed subsequently.

Fig. 8 SEM image of flower-like ZnO prepared with 0.65 M NaOH on ITO substrate.

Previously, a number of works have described the formation of flower-like ZnO by means of nucleation and growth as well as anisotropy.^3,33,58,59 It is believed that the flower-like ZnO architectures are preferably formed from slow nucleation and growth.⁵⁸

On the basis of the above observations (Fig. 7) and the mechanism proposed previously, the formation mechanism of the flower-like ZnO superstructures can be proposed. The possible growth route of these flower-like ZnO is schematically shown in Fig. 9. Firstly, at low temperature (70 °C), when the concentration of sodium hydroxide is decreased from 0.65 M to 0.45 M (0.55 M and 0.35 M), both the nucleation rate and crystal growth of the ZnO crystal became relatively slower than growth with 0.65 M sodium hydroxide. In general, slow crystallization is required to form products with a thermodynamically stable structure because the crystallizing partners would aggregate and follow the lowest-energy path.⁵⁸ At the beginning of the reaction, a suitable amount of ZnO nuclei generate at step A. However, due to the driving forces of high surface energy, electrostatic force and so on, some ZnO nuclei can easy to aggregate together (at stage B). At step C during this process, a burst of initial homogeneous nucleation occurred and the growth units were directly incorporated into ZnO crystallites under the given conditions, which gave birth to the formation of cauliflower-like ZnO during the initial growth stage. In order to minimize the total surface energy, the petal ends of cauliflower-like ZnO would have sphere-like morphology and the diameters of petals are large and nonuniform. After that, fast growth of ZnO in the c-axis causes a situation of metastability, which is thermodynamically favorable for a large crystal to small one. Then, the size of the nanorods tends to be uniform at the erosion effect of extra base as reported at step D.^60,61 Following, they would tend to further decrease their surface energy, which would provide active sites for secondary heterogeneous nucleation and growth. Thus, the secondary small petals would grow out continuously from the surface of the primary ones. Because of the intrinsic anisotropic character of hexagonal ZnO, the petals grow along the c-axis at step E. As the reaction proceeded, the crystal split can not happen due to the concentration of Zn²⁺ and OH⁻ being lower than the critical level. Thus, at step F, the largest diameter of petals continues to reduce to a certain value in the reaction system and does not decrease evidently since the certain reaction time. Then, the small and thin petals continue to grow and more and more petals form with prolonging reaction time. Finally, the diameters of the petals become very uniform upon the effect of Ostwald ripening⁶² and the flower-like ZnO superstructures were constructed in the aqueous solution at step G. Therefore, when the concentration of sodium hydroxide decreased from 0.45 M to 0.35 M, the wide petals can not become narrow ones due to the low concentration of OH⁻, so the average diameter of petals is large (Fig. 5C–D) and it can be easily found that some petals are composed of few rods.

Fig. 9 Schematic diagram of the possible growth process for flower-like ZnO structures.

4 Conclusions

In summary, rod-like and three kinds of well-defined flower-like ZnO microstructures have been synthesized on ITO substrate via a hydrothermal method without any surfactant at low temperature. Sodium hydroxide plays an important role in this system. The morphologies, petal ends, diameters and petal numbers of flower-like ZnO vary with the concentration of sodium hydroxide. Compared with the sample without using substrate, the flower-shapes of ZnO possess higher symmetry and uniformity due to the ITO substrate. Time-dependent experiments show that flower-like ZnO were formed in a short time and the size evolution of flower-like ZnO is dependent on the reaction time. It is obvious that the amounts of petals increased and the wide petals broke into narrow ones with increasing reaction time. Finally, the size of the petals tends to be uniform when the reaction time is 24 h. On the basis of these experiments, a possible growth mechanism was proposed for elucidating formation of flower-like ZnO microstructures. Besides, this simple, low-cost and environmentally-benign method is expected to allow the large-scale fabrication on substrate, which has wide applications in many aspects.

Acknowledgements

This work was supported in part by the Program for Taishan Scholars of Shandong Province Government, projects from the National Natural Science Foundation of China (21071061), the Natural Science Foundation of Shandong Province (ZR2010EZ001 and ZR2009FM072), and Outstanding Young Scientists Foundation Grant of Shandong Province (BS2010CL004).

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