Studies on the PEG-Assisted Hydrothermal Synthesis and Growth

DOI: 10.1021/cg900327v

Studies on the PEG-Assisted Hydrothermal Synthesis and Growth Mechanism of ZnO Microrod and Mesoporous Microsphere Arrays on the Substrate

2010, Vol. 10 1500–1507

Yingjie Feng,† Mei Zhang,‡ Min Guo,‡ and Xidong Wang*,† †

Department of Energy and Resources Engineering, Peking University, Beijing 100871, P. R. China, and Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, P. R. China

‡

Received March 20, 2009; Revised Manuscript Received December 11, 2009

ABSTRACT: With the introduction of poly ethylene glycol (PEG) (10000), relatively well dispersed and oriented ZnO microrod arrays and ZnO microsphere arrays were successfully synthesized on unmodified indium tin oxide (ITO) substrate by a hydrothermal method. The growth behaviors of the two different kinds of ZnO arrays were experimentally investigated with variations of PEG addition and the precursor solution’s concentration. The PEG-assisted growth mechanism of ZnO microrod arrays and ZnO microsphere arrays has also been carefully discussed. Both the dissolved state of PEG and the interaction between PEG and ZnO crystalline grains were found to play important roles in the fabrication of the two different ZnO structure arrays. The research on PEG-assisted growth mechanism for these two kinds of ZnO structure arrays will provide more theoretical references for preparations of ZnO one-dimensional rod arrays and other kinds of assembled structures on the substrate.

1. Introduction ZnO has been attracting intensive interest not only because of its excellent electrical and optical performances but also because of its various morphologies.1,2 A wide variety of morphologies of ZnO, such as nanowire,3 nanobelt,4 nanoplate,5 nanotube,6 have been prepared. In addition, some more unique structures such as nanorings,7 hierarchy structures,8 ringlike nanosheets,9 mesoporous polyhedral cages and shells,10 and hollow hemispheres/spheres11 have also been successfully synthesized. As the material’s properties are hugely determined by its structure,12 many efforts have been devoted to the development of methods for the controllable preparation of desired structures of ZnO. Compared with other complicated methods, an additives-assisted solution approach has been generally accepted as the most effective and convenient way for preparation of some novel morphologies and large-scale productions.13 Because of the oriented attachment of additives, the aspect ratio of one- or twodimensional (1-D or 2-D) ZnO crystals could be well controlled.14 And ZnO nanowires, nanoplates, or nanotubes have been fabricated.15,16 Besides, surfactant-assisted arrangement of nanoscale building blocks, such as nanoparticles,17 nanorods,18,19 nanotubes,20 nanosheets,21 or other composites,22 into various three-dimensional (3-D) self-assembled ZnO structures with a controllable size and morphology has also become a hot topic. Poly ethylene glycol (PEG), as a polymer surfactant, has been used to fabricate many different morphologies of ZnO.23 For the synthesis of 1-D ZnO nanorods or nanowires, Li et al.24 have prepared ZnO nanowires or nanorods in the presence of PEG with different molecular weights and analyzed the mechanism from the point of ZnO self-growth behavior and energy decrease. In addition, for the fabrication of assembled ZnO structures, some previous literature reports proved that the oriented attachment of the primary nanoparticles, *To whom correspondence should be addressed. E-mail: xidong@ pku.edu.cn; tel: 86-10-82529083. pubs.acs.org/crystal

Published on Web 02/24/2010

nanorods, or nanosheets and effective removal of appropriate organic ligands at interfaces might form the assembled ordered structures.25-29 By the introduction of PEG, Ding and Zhou et al. reported the assembled spherical structure of unique 1-D ZnO nanorods, solid nanocones,20 and microspheric organization of multilayered ZnO nanosheets.30 Yuan et al. prepared ZnO micronuts,31 and Zhang reported self-assembled monocrystalline ZnO nanorod bundles.32 With the presence of PEG, Liu et al. also reported self-assembled unusual ZnO ellipsoids33 consisting of nanoparticles and rods. Although a number of novel structures have been obtained with PEG, studies on the PEG-assisted growth mechanism are not sufficient, and further investigations about the growth behaviors are still necessary. Additionally, most published work is focused on the products prepared in solution, while the products on the substrates which actually possess more promising applications were rarely reported. In this paper, two kinds of ZnO arrays are successfully prepared on unmodified indium tin oxide (ITO) glass substrate by a PEG-assisted hydrothermal synthesis strategy. One is ordered and dispersed ZnO microrods arrays, and the other is a novel kind of ZnO mesoporous microsphere array. Evolution of the morphologies was studied by adjusting the addition of PEG and precursor solution’s concentration. Through time-dependent experiments, growth behaviors and mechanism for the two kinds of ZnO structures arrays were systematically investigated and discussed. 2. Experimental Section 2.1. Materials. All chemicals (Beijing chemical Co. Ltd.) were of analytical reagent grade and used without further purification. All the aqueous solutions were prepared using distilled water. ITO (10 Ω/cm2) glass plates were used as substrates and were cleaned by standard procedures prior to use. 2.2. Synthesis Process. The precursor solutions were prepared by mixing 15 mL of zinc nitrate Zn(NO3)2 3 6H2O with 15 mL of hexamethylenetetramine (C6H12N4) and then adding PEG (10000) into the precursor solutions. The hydrothermal growth was carried out at 95 °C for 4 h in autoclaves by immersing the unmodified ITO r 2010 American Chemical Society

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Figure 1. (a) SEM image of ZnO rods array obtained with the addition of 0.02 g of PEG and the concentration of precursor solution (0.1 M) for 4 h at 95 °C; (b) SEM image of a comparative experiment without PEG. The inset in (a) is a magnified image of the ZnO nanorod arrays, scale bar is 2 μm; the inset in (b) is a magnified image of the ZnO nanorods cluster, scale bar is 2 μm.

Figure 3. SEM images of ZnO rods obtained with the addition of PEG (a) 0.005 g; (b) 0.01 g; (c) 0.02 g; and (d) 0.04 g and the same concentration of precursor solution (0.1 M) for 4 h. The inset in (a), (b), and (c) are the magnified images of the ZnO nanorods obtained with different PEG addition, and the scale bar is 2 μm.

Figure 2. XRD results of ZnO rods array. glass substrates in precursor solutions (30 mL). When the reaction time was reached, the autoclaves were cooled to room temperature, and then the samples were taken out and washed with deionized water and then dried in air. To confirm the influence of the addition of PEG and precursor solution’s concentration on the formation of the structure of ZnO, the addition of PEG and concentration of the precursor solution were changed to meet the experiments’ requirements. Meanwhile, a comparative experiment without surfactant was also carried out. In order to study the growth mechanism of the ZnO structures, the time-dependent experiments were carried out for different growth times. 2.3. Characterization. The morphology and size of as-prepared ZnO structures were characterized using scanning electron microscopy (SEM) (Zeiss Supra-55 operated at 5 or 15 kV). X-ray diffraction (XRD) analysis was performed with a Rigaku Dmax2000 diffractometer using Cu KR radiation. Transmission electron microscope (TEM) observations and energy-dispersive spectroscopy (EDS) were performed with a Tecnai G2 F20 at 200 kV and H-9000 NAR at 300 kV.

3. Results and Discussion SEM images of ZnO rods array obtained by hydrothermal growth at 95 °C for 4 h with and without PEG (10000) respectively on the substrate are shown in Figure 1. With the addition of 0.02 g of PEG, as illustrated in Figure 1a, relatively well ordered and dispersed ZnO rod arrays were successfully prepared on ITO glass substrate with no seed layers. An X-ray diffraction pattern of the as-synthesized ZnO rods array is shown in Figure 2, and the diffraction peaks can be perfectly indexed to the wurtzite-type ZnO (JCPDS no. 79-2205).

The strong and sharp diffraction peak of the (002) crystal face indicates a good orientation of the sample. From the enlarged picture in Figure 1a inset, it can also be identified that each rod grows independently, which is very different from the comparative experiment without PEG illustrated in Figure 1b. And seen from Figure 1b, lots of bundles of ZnO rods distribute on the substrate. Obviously, it is the presence of PEG that brought significant improvements in dispersion and orientation of the ZnO rods array. Therefore, to further investigate the effects of PEG on the growth of the dispersed ZnO rods array, more systematical experiments were carried out below. 3.1. Variation of ZnO Morphologies with PEG Addition. Figure 3 demonstrates the results with different additions of PEG while other conditions were kept constant. From Figures 1 and 3, it has been known that when the addition of PEG was 0.02 g, relatively well dispersed and oriented ZnO rods array could be obtained. However, when the addition of PEG was 0.005 or 0.01 g, no dispersed ZnO rods were obtained with 0.02 g of PEG nor ZnO rod bundles of comparative experiments, but stacks of ZnO rods formed. Different from the bundles of ZnO rods obtained in comparative experiments (Figure 1b), the obtained ZnO rods, shown in Figure 3a,b, grow in a stack independently and do not share a common nucleation site. When the addition of PEG increased, the number of rods contained in each stack decreased and the dispersibility was improved. However, when the addition of PEG reached 0.04 g, as shown in Figure 3d, the density of rods decreased sharply and the orientation became poorer. 3.2. Variation of ZnO Morphologies with Concentration of Precursor Solution. From the above results and discussion, it is indicated that appropriate addition of PEG is a key factor for the dispersion of a ZnO rods array on unmodified substrate. According to our experiments, the best appropriate addition of PEG is around 0.02 g. Furthermore, besides the addition of PEG, other influencing factors were also investigated by adjusting the concentration of precursor solution with PEG kept constant at 0.02 g. Figure 4a,b shows that when the precursor solution’s concentration decreased to 0.05 or 0.06 M, dispersed rods could also be obtained, but the density of the rods became lower and the orientation was

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Figure 4. SEM images with different concentrations of precursor solution at (a) 0.05 M; (b) 0.06 M; (c) 0.08 M; (d) 0.09 M while keeping the addition of PEG at 0.02 g; (e) the enlarged SEM image of (d). The inset image in (d) shows the magnified ZnO microsphere, scale bar is 5 μm.

Figure 5. SEM images obtained at different growth times: (a) 20 min; (b) 1.5 h; (c) 2 h; (d) 3 h; and (e) enlarged SEM image at 1.5 h; (f) EDS result of products at 20 min characterized by Tecnai G2 F20 with the addition of 0.02 g of PEG and the concentration of precursor solution 0.1 M. The inset image in (a) demonstrates the magnified coiled ZnO nanowires, scale bar is 200 nm; the inset image in (e) shows intermedium ZnO at the growth time of 1.5h, scale bar is 1 μm.

poorer. When the concentration of precursor solution reached 0.08 M, as illustrated in Figure 4c, no rods but only some small spheres with a diameter of about 100 nm could be observed. And when the concentration of precursor solution reached 0.09 M, as shown in Figure 4d, a totally different morphology of ZnO mesoporous microsphere arrays were

fabricated and well distributed in a large area. From the high magnification SEM image of Figure 4e, it could be identified that the microsphere is actually composed of assembled and connected nanosheets. The microsphere’s diameter is approximately 10 μm and the thickness of the nanosheets ranges between 4 and 5 nm. Obviously, this kind of structure

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Figure 8. Schematic illustration of the growth mechanism for the ZnO rods array.

Figure 6. TEM images obtained at different growth times: (a) 20 min, (c) 1.5 h, (e) 3 h; (b, d) and (f) are HRTEM images of (a), (c), and (e), respectively. The inset in (c) is the TEM image of the bottom end of intermedium ZnO rod shown in (c).

mechanism should be further clarified. And the systematical experiments and discussion are given below. 3.3. Investigations on the Growth Mechanism of Well Distributed ZnO Rods Array. In our research, two kinds of ZnO arrays were obtained, rods array and mesoporous microspheres array. Their growth mechanisms were also studied respectively. First, for the dispersed ZnO rods array, a careful time-dependent experiment was carried out. Figure 5a shows that at the initial growth period of 20 min, numerous flocculated agglomerates began to form on the substrate. The flocculated agglomerates were characterized by TEM and high resolution TEM (HRTEM), and the images are shown in Figure 6a,b. It is found that the flocculate agglomerate is the entangled nanowires composed of ZnO crystal grains whose diameter ranges from 3 to 4 nm. And the EDS result (Figure 5f) is in agreement with the results of TEM and HREM. To our knowledge, PEG, as a long chain nonionic surfactant, has hydrophilic -O- and hydrophobic -CH2-CH2- radicals on the long chains. PEG can be well dissolved in water, and the large amount of oxygen atoms on PEG chains will easily combine with metal ions24,31,33 to form the flocculated agglomerates. According to our experiments, it can be deduced that ZnO crystalline grains will be preferential to nucleate on PEG rather than on ITO substrate. The reaction equations are summarized as follows: ðCH2 Þ6 N4 þ H2 O f HCHO þ NH3 H2 O f NH4þ þ OH ð1Þ

Figure 7. Variation of length of ZnO rods array with growth time.

has a relatively higher specific surface area, which promises potential applications. As can be seen from the above experimental results, besides addition of PEG, the concentration of precursor solution is also an important factor for the fabrication of as-synthesized ZnO structures. Actually, it is the synergic effects between addition of PEG and concentration of precursor solution that lead to evolution of dispersibility and morphology transformation of ZnO structures prepared on the substrate. Therefore, the specific PEG-assisted growth

Zn2þ þ 4OH - ¼ ½ZnðOHÞ4 2 -

ð2Þ

½ZnðOHÞ4 2 - ¼ ZnðOHÞ2 þ 2OH -

ð3Þ

ZnðOHÞ2 ¼ ZnO þ H2 O

ð4Þ

In previous literature reports, Yuan et al. and Konar et al. have investigated the interaction between PEG and zinc species by TEM and dynamic light scattering (DLS) methods and proved that the dissolved PEG chains are in a coiled state in solution.31,15 On the basis of our experimental results, it is believed that the small ZnO crystal grains will be enwrapped into the coil of PEG and form agglomerates. According to the theory of crystal growth thermodynamics in the oversaturated solution, numerous small ZnO grains will be dissolved or recrystallized at a thermodynamic balance state. The larger crystalline grains, with lower specific surface energy, are usually more stable than the smaller ones with higher specific surface energy. Actually, only the large

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Figure 9. SEM images of ZnO microsphere structure arrays obtained with the addition of PEG (a) 0.016 g; (b) 0.018 g; (c) 0.020 g; (d) 0.022 g; (e) 0.024 g, and 0.09 M precursor concentration for 4 h. The inset images show the magnified ZnO microspheres obtained with different PEG addition, the scale bar: (a) 5 μm, (b) 2 μm, (c) 2 μm, (d) 5 μm, (e) 5 μm.

Figure 10. XRD results of obtained products annealed at 550 °C for 0.5 h.

crystalline grains, whose size is larger than the critical size, could grow up spontaneously, while the smaller ones whose size is under the critical size will redissolve into solution. On this level, in each coil of PEG, a dissolution-crystallization process of ZnO crystalline grains exists. And the PEG coils here just provide assistance to crystallization of ZnO crystals and the coalescence of the adjacent crystalline grains. During the initial period, the larger grains and the coalesced ones will grow continuously, while the small ones will be dissolved. When the growth time reached 1.5 h, as illustrated in the SEM image Figure 5b, some larger or the coalesced grains began to grow and crystallize into hexagonal nanoplates. Figure 5e demonstrates an enlarged picture of hexagonal nanoplates and agglomerate of entangled nanowires. At this stage, both the hexagonal ZnO nanoplates with self-growth behavior and entangled nanowires agglomerate coexist together. The TEM and HRTEM images, Figure 6c,d, of the products obtained at 1.5 h demonstrate that some small grains and an amorphous substance still attach on the large crystal’s surface.

Figure 11. (a) TEM image, (b) HRTEM image, and (c) electron diffraction pattern of ZnO nanoplate. The inset in (c) is the magnified electron diffraction pattern.

As we discussed above, it is known that the interaction between PEG and zinc species could accelerate the nucleation, and the coiled agglomerate could give assistance to the crystallization of ZnO crystal grains. Afterward, the dispersed large crystals on the substrate will grow continuously into nanorods, and the smaller crystal grains, with a higher specific surface energy, will dissolve into the solution again and eventually diffuse to the surface of the large ones and reprecipitate on them. As the growth time was prolonged to 2 and 3 h, shown in Figure 5c,d, it could be observed the synthesized ZnO nanoplates grew into 1-D ZnO rods and were well distributed on the substrate in a large area. TEM and HRTEM images of Figure 6e,f also show that the large ZnO grains, with self-growth behavior, will preferentially grow along the c-axis into ZnO rods. In addition, interaction between PEG and zinc species in the agglomerate could not only accelerate the nucleation and

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Figure 12. (a) Variations of diameter and density of ZnO microspheres array with the addition of PEG. (b) Variations of diameter and density of ZnO microspheres array with growth time with the addition of 0.020 g of PEG.

crystallization of ZnO crystal grains, but also influence the dispersibility of the ZnO rods array. When the PEG was introduced, the agglomerates composed of PEG and ZnO crystalline grains will absorb and distribute on the substrate, which will result in the corresponding well dispersed ZnO nanorods on the substrate. In order to clarify the specific growth process further, the statistical analysis of length variation with growth time was made and is shown in Figure 7. From the plot, it is identified that during the growth period from 20 min to 1.5 h, the length of ZnO rods increased slowly. However, after 1.5 h, ZnO rods grew very quickly. The quantitative analysis result is consistent with what we discussed above, and the growth process could be divided into two stages: (1) the nucleation and crystallization of ZnO crystal grains; (2) the preferential growth along the c-axis. The whole formation process of the ZnO rods array is illustrated in Figure 8. 3.4. Investigations on the Growth Mechanism of ZnO Mesoporous Microspheres Array. About the novel kind of ZnO mesoporous microspheres array fabricated with 0.09 M precursor solution and 0.02 g of PEG, structure evolutions were also investigated with variation of PEG addition around 0.02 g while the precursor solution was kept at 0.09 M. SEM images of Figure 9 present that the ZnO structures evolved from aggregated nanoplates to mesoporous microspheres and then to the coexistence of ZnO rods and microspheres. All the products were characterized by X-ray diffraction methods which is shown in Figure 10, and all peaks were perfectly indexed to the wurtzite-type ZnO (JCPDS no. 79-2205). Figure 9a shows that when the addition of PEG is 0.016 g, numerous clusters consisted of interconnected uniform hexagonal nanoplates distributed on the substrates. The average cluster’s diameter is about 15 μm. The products have also been studied by TEM and HRTEM which are shown in Figure 11. From the HRTEM image of Figure 11b, it can be observed that some little crystals attach on the surface of the nanoplate. When the PEG addition reached 0.018 g (Figure 9b), mesoporous spherical structures formed, and some nanowires could also be observed on the substrate. Then, with the addition of PEG further increasing to 0.020 and 0.022 g, the microsphere structures have grown bigger as shown in Figure 9c,d, and the dispersion of microspheres on the

Figure 13. SEM images of ZnO mesoporous microspheres array obtained by time-dependent experiments: (a) 20 min; (b) 45 min; (c) 1.5 h; (d) 3 h. The inset images respectively demonstrate the magnified intermedium morphologies of ZnO microspheres at different growth times, the scale bar: (a) 50 nm, (b) 1 μm, (c) 1 μm, (d) 2 μm.

substrate is improved. However, when the PEG addition reached 0.024 g, seen from Figure 9e, besides ZnO microspheres, ZnO rods formed again. And the density of the micropheres decreased and diameter was a little smaller. Statistical variations of density and diameter of ZnO microspheres with the addition of PEG around 0.02 g at 4 h are shown in Figure 12a. From the plots, it can be seen that the trend of the two variations with the addition of PEG is totally opposite. When the addition is 0.022 g, the size of synthesized ZnO microspheres is the largest while the density is the lowest. The above results indicate that the addition of PEG for the formation of ZnO microspheres is so rigorous that subtle changes could result in huge differences in morphology. In order to reveal the growth mechanism of the ZnO microspheres, a time-dependent experiment was also carried out. And the results, with the addition of 0.02 g of PEG, are shown in Figure 13. At a growth time of 20 min, some flexible and interconnected thin sheets began to form as shown in Figure 13a. When the growth time reached 45 min shown in Figure 13b, some primary microspheres were observed and distributed on the substrate. Their average diameter is only

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Figure 14. (a) TEM image and (b) HRTEM image of ZnO mesoporous microsphere obtained at 1 h; (c) TEM image; (d) TEM image (annealed at 550 °C for 2 h); and (e) HRTEM image of ZnO microspheres prepared at 4 h.

about 2 μm and the distribution of the diameters is not uniform. Further, when the growth time reached 1.5 h illustrated in Figure 13c, larger and integrated microspheres were prepared, and the mean diameter is about 5 μm. At a growth time of 3 h, seen from Figure 13d, the average diameter of ZnO microsphere reached 7 μm. Besides, the density and diameter variations of microspheres with growth time were also calculated and shown in Figure 12b. According to the theory of crystal growth thermodynamics, during the period from 45 min to 3 h, lots of small crystalline grains dissolved and only the larger ones grew bigger, which caused the density drop quickly. And from 3 h to 4 h, the density changed slightly. From the TEM and HRTEM images of microspheres obtained at 1 h and 4 h shown in Figure 14, it is found that each sheet of ZnO microspheres is actually composed of many different oriented small crystal grains. And with the growth time increasing, the crystallization also improved. From the above discussion, it is known that due to the interaction between PEG and zinc species, numerous little ZnO crystals will nucleate on PEG chains and grow further. However, compared with the PEG-assisted formation of the ZnO rods array, different molar ratios of PEG/Zn2þ will result in different interactions between PEG and ZnO crystal grains, which will also lead to different crystallizations and coalescence of ZnO crystalline grains and even the final morphology. The proper PEG addition with a certain molar ratio of PEG/Zn2þ and other influencing factors combine together to form the sheetlike structures which further assemble into the mesoporous microsphere with the lowest energy. From the ZnO rods array to microspheres array, from nanorods cluster to nanoplates cluster, it proves that the interaction between PEG and zinc speices and the followed process of recrystallization play important roles in the formation of ZnO morphologies. With the variations of the addition of PEG and the molar ratio of PEG/Zn2þ, the dissolved state of PEG and the interaction of PEG and Zn ions will be different, which also result in different

attachment stability and the orientation of the nucleation and further affect the final morphologies of ZnO from the point of the minimum energy of the crystal growth process. 4. Conclusion In this paper, by a PEG-assisted hydrothermal method, we synthesized well-distributed and ordered 1-D ZnO rod arrays and a novel kind of ZnO mesporous microsphere arrays on unmodified ITO substrate. Their growth behaviors were carefully investigated respectively. The dissolved state of PEG and the interaction of PEG and zinc species were found to play important roles in the fabrication of those structures. With the variations of addition of PEG and the molar ratio of PEG/ Zn2þ, the dissolved state of PEG and the interaction of PEG and zinc species will be different, which also results in different attachment stability and the orientation of the nucleation, and further affect the crystallization of ZnO crystalline grains and the final morphologies of ZnO. The studies will provide more theoretical references for the preparation of ZnO 1-D rods arrays and other kinds of assembled structures on the substrate. Acknowledgment. The authors would like to thank the National Science Foundation of China for the financial support (Nos. 50772004, 50872011) and National Basic Research Program of China (973 Program: 2007CB613608).

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Studies on the PEG-Assisted Hydrothermal Synthesis and Growth

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