17848
J. Phys. Chem. B 2006, 110, 17848-17853
Simple Solvothermal Route To Synthesize ZnO Nanosheets, Nanonails, and Well-Aligned Nanorod Arrays Soumitra Kar,* Apurba Dev, and Subhadra Chaudhuri DST Unit on Nano Science and Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: May 16, 2006; In Final Form: July 18, 2006
ZnO nanosheets, nanonails, and well-aligned nanorods were fabricated on Zn foils by a solvothermal approach using ethanol as the solvent. A lower synthesis temperature and a shorter time period favor the formation of nanosheets. By optimizing the synthesis temperature and time period, ZnO nanonails with a hexagonal cap and a long stem could be produced. A higher temperature was not favorable to produce uniform and smooth nanorods. Well-aligned ZnO nanorod arrays were produced with diameters within 100-250 nm and lengths up to ∼6 µm when NaOH was added to the solvent. By optimizing the reaction parameters, the morphology, size, and orientation of the nanoforms could be tailored. The ZnO nanorods exhibit an excitonic strong UV emission and a defect-related broad green emission at room temperature. The defect-related green emission band decreased with the improvement of the degree of alignment of the nanorods.
Introduction Zinc oxide is an important semiconductor with a wide band gap energy (3.3 eV) and a large exciton binding energy (60 meV) at room temperature. The exciton binding energy of ZnO is much higher than the thermal energy at room temperature (26 meV), and it is also much higher than those of other prospective materials such as ZnSe (22 meV), ZnS (40 meV), and GaN (25 meV), which make it one of the outstanding semiconductors for lasing. ZnO is a very promising alternative to tin-doped indium oxide (ITO) in flat panel displays.1,2 ZnO is also important for its wide range of applications as sensors,3,4 photocatalysts,5 light-emitting diodes and laser diodes in the UV-vis range,6-8 etc. ZnO 1-D nanoforms are found to be highly suitable for nanoscale applications such as roomtemperature lasers,9 field effect transistors,10 etc. 1-D nanostructures, specially aligned ZnO nanoforms, are especially important for their enhanced optical properties.9,10 Aligned ZnO nanostructures were fabricated by a high-temperature thermal evaporation process.9-11 Though evaporation and condensation processes are favored for their simplicity and high-quality products, they are generally constrained by the requirement of high temperature and use of a catalyst that can produce unintentional defect levels. Hence, low-temperature and versatile synthetic processes are of great importance for the realization of devices based on these nanostructures. Solution-based approaches for the synthesis of oriented ZnO nanorods is very attractive for their lower growth temperature and potential for large-scale production. Recent reports by several research groups12-15 have opened up new opportunities in this direction. Moreover, a precise control over the size, shape, and orientation of ZnO 1-D nanostructures is crucial before they find practical applications in promising areas. Here, we report the fabrication of ZnO nanosheets, nanonails, and well-aligned nanorod arrays, on zinc foils by a simple * To whom correspondence should be addressed. Phone: +91-033-24734971. Fax: +91-033-2473-2805. E-mail:
[email protected].
solvothermal approach. The photoluminescence properties of these ZnO nanostructures are also reported. Experimental Section For the synthesis of ZnO nanoforms, a closed cylindrical Teflon-lined stainless steel chamber with 110 mL capacity was used. All the reagents used in the experiments were of analytical grade, and no further purification was done. Zn foils with 1 cm2 surface area and 0.5 mm thickness were used as the source as well as the substrates. The foils were first mechanically polished with fine emery papers followed by cleaning in acetone and water by ultrasonic treatment. Unless mentioned specifically, four zinc foils were taken with 60 mL of ethanol in the Teflon chamber, whereas in some of the experiments one Zn foil was also used to investigate the effect of the amount of the source. The sealed chamber was then kept in a furnace at 170-230 °C for 3-12 h. In another set of experiments an appropriate amount of NaOH was added to the ethanol to adjust the pH level to 10.6, keeping all other parameters unchanged. Experiments were also carried out with pH values of 8.6 and 12.2 at 200 °C for 12 h. After the desired time period, the system was allowed to cool naturally. The foils collected from the reaction vessel were washed with water several times and dried in air. The products were characterized by X-ray diffractometry (XRD; Seifert 3000P) with Cu KR radiation, and the compositional analysis was done by energy-dispersive analysis of X-rays (EDAX; Kevex, Delta Class I). Microstructures of the nanoforms were studied by scanning electron microscopy (SEM; Hitachi S-2300) and transmission electron microscopy (TEM; JEOL 2010). The high-resolution transmission electron microscopy (HRTEM) images and the typical selected area electron diffraction (SAED) patterns of the ZnO nanoforms were also recorded. Optical absorption spectra of the products, dispersed in spectroscopic grade ethanol, were recorded with a spectrophotometer (Hitachi U 3410). Photoluminescence (PL) measurements were carried out at room temperature with a luminescence spectrometer (Hitachi, F2500), using 300 nm as the excitation wavelength.
10.1021/jp0629902 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006
ZnO Nanosheet, Nanonail, and Nanorod Synthesis
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17849
Figure 3. EDAX pattern of a representative ZnO nanorod sample.
Figure 1. XRD pattern of the ZnO nanoforms synthesized in ethanol.
Figure 2. XRD pattern of two representative samples synthesized in ethanol using NaOH.
Results and Discussion The crystal structure of the ZnO nanostructures prepared by the solvothermal process was studied by XRD. Figure 1 shows the XRD pattern of the ZnO nanoforms produced in ethanol in absence of NaOH. The diffraction peaks positioned at 2θ values of 31.76°, 34.42°, 36.25°, 47.53°, 56.6°, and 62.8° can be indexed to the hexagonal wurtzite phase of zinc oxide (JCPDS card no. 36-1451). Diffraction peaks corresponding to crystalline zinc appear in the XRD patterns due to the bare zinc substrate as a result of partial conversion of the zinc surface. Figure 2 shows the XRD pattern of the samples synthesized with NaOH (pH 10.6). The XRD patterns of the sample synthesized at 170 °C for 12 h and 200 °C for 3 h are quite identical, and one of the patterns is shown in Figure 2. This XRD pattern indicated that the peaks corresponding to the ZnO phase are quite prominent compared to the Zn-related peaks, whereas all other samples synthesized at 200 °C for 5 and 12 h show identical XRD peaks. The XRD pattern reveals that, with the use of NaOH, the intensity of the (002) peak increased significantly, indicating the preferred orientation of the ZnO nanoforms along this direction. The composition of the nanorods was analyzed by an EDAX study (Figure 3) revealing the presence of Zn and O as the only elementary components with slight oxygen deficiency (Zn:O ≈ 1:1 atomic ratio) in all the samples.
Figure 4. SEM images of the products synthesized at different temperatures for 12 h using pure ethanol as the solvent: (a) 170 °C, (a) 200 °C, (c) 230 °C.
The morphologies of the products deposited on the Zn substrates after 12 h at different temperatures are shown in Figure 4. Figure 4a shows the SEM image of the products deposited on the Zn foils at 170 °C revealing the formation of flower-like assemblies of ZnO nanonails separated by interconnected and vertically aligned nanosheets. The diameters of the caps of these ZnO nails varied within 50-100 nm, and their lengths were ∼200-500 nm. Figure 4b shows the formation of long quasi-aligned ZnO nanorods at 200 °C. The diameters of the ZnO nanorods varied within 100-250 nm, and their lengths were ∼5 µm. When the temperature was raised to 230 °C, similar kinds of quasi-aligned ZnO nanorods with a flat hexagonal top were produced (Figure 4c). A few nanoparticles were also attached to the side facets of the ZnO nanorods. With a view to investigate the growth mechanism, the experiments were also carried out in pure ethanol at 200 °C for shorter periods of time and also with fewer Zn foils. Figure 5a shows
17850 J. Phys. Chem. B, Vol. 110, No. 36, 2006
Figure 5. SEM images of the products deposited at 200 °C using pure ethanol as the solvent with (a) four Zn foils for 3 h, (b) four Zn foils for 5 h, and (c) one Zn foil for 12 h.
the formation of ZnO nanosheets after 3 h of deposition time. The widths of the nanosheets varied within 500-750 nm. When the deposition time was increased to 5 h, nail-like ZnO 1-D nanoforms were obtained (Figure 5b). The nanostructures possessed a flattened hexagonal cap and a long stem. The diameters of the hexagonal caps varied within 150-250 nm, and their length were ∼500-800 nm. When the experiment was carried out in ethanol with a single piece of Zn foil for 12 h, uniform nanosheets with widths of ∼150-200 nm were obtained along with a few larger sheets (Figure 5c). Experiments were also carried out in the presence of NaOH (pH 10.6), as it is well-known that the OH- ions help the formation of ZnO 1-D nanoforms in the solution phase. Figure 6a shows the tilted view of the product obtained after 3 h at 200 °C. The SEM image reveals the formation of ZnO nanorods perpendicular to the surface of Zn foils. Figure 6b shows the top view of the products obtained after 5 h of deposition at 200 °C revealing the formation of densely packed uniform nanorod arrays. The image in the inset reveals the hexagonal crosssectional view of the nanorods. The diameters of the nanorods varied within 50-150 nm. When the experiment was carried out for 12 h at 200 °C with a single piece of Zn foil, a wellseparated nanorod array was formed (Figure 6c), whereas highly dense nanorod arrays were formed when the synthesis was repeated with four Zn foils with all other experimental parameters identical to those of the above experiment (Figure 6d). The diameters of the nanorods varied within 100-200 nm. As it is known that the pH value has a great influence on the structures of materials, we have performed several experiments for 12 h at 200 °C using the normal amount, four pieces, of Zn foil with different pH values. It was observed that, with the
Kar et al.
Figure 6. SEM images of the products deposited at 200 °C in ethanol using NaOH (pH 10.6) with (a) four Zn foils for 3 h, (b) four Zn foils for 5 h, (c) one Zn foil for 12 h, and (d) four Zn foils for 12 h. The image in the inset of part b depicts the hexagonal top view of the nanorods. Parts e and f depict the SEM images of the products obtained at 200 °C with pH values 8.6 and 12.3, respectively.
addition of NaOH, i.e., an increase in the pH level (10.6), the growth of the nanorods was rather uncontrolled and nonaligned nanorods with larger diameters were produced. Figure 6f shows the SEM image of one such nonaligned nanorod produced at pH 12.2. The nanorods possessed a diameter of ∼500 nm. The morphology and crystal structure of the products were also characterized by TEM studies. Figure 7a shows the HRTEM image recorded at one edge of one representative ZnO sheet. The measured lattice spacing of 0.512 nm represents the (0001) plane of wurtzite ZnO. The selected area diffraction pattern recorded over the same position of the ZnO sheet also reveals the (0001) orientation of the sheet. These images indicated that the growth direction of the nucleus of the 1-D ZnO nanoforms, i.e., the nanosheets, was along the (0001) direction. Figure 7b shows the nail-like morphology of the product obtained after 5 h of deposition at 200 °C without using NaOH. Figure 7c shows the formation of long ZnO nanorods at 200 °C when the deposition was carried out for 12 h in the presence of NaOH. Figure 7d depicts the HRTEM image of a ZnO nanorod. The measured spacing between the lattice fringes was 5.12 Å corresponding to the (0001) lattice plane of wurtzite ZnO. The growth direction of the nanorod was perpendicular to this plane, i.e., (0001). The fast Fourier transformation of the HRTEM image, shown in the inset, also confirmed the growth direction. The growth mechanism could be well understood on the basis of the following reactions and the crystal habits of wurtzite ZnO as presented in the schematic diagram in Figure 8. It is well-
ZnO Nanosheet, Nanonail, and Nanorod Synthesis
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17851
Figure 7. TEM image of the products deposited in ethanol: (a) HRTEM image of a ZnO sheet along with the SAED pattern in the inset, (b) without using NaOH at 200 °C for 5 h, (c) using NaOH at 200 °C for 12 h, (d) HRTEM image of a single nanorod indicating the growth direction to be (0001). The FFT pattern shown in the inset confirmed the growth direction of the nanorods.
Figure 8. Schematic view of the growth mechanism of the ZnO nanoforms.
known16 that, in the solution-based approaches to grow ZnO nanoforms, the basic growth unit is [Zn(OH)4]2-. In the solvothermal condition, i.e., at high temperature and elevated pressure, the Zn foils produced Zn+ ions, which reacted with the OH- ions to produce [Zn(OH)4]2-. These [Zn(OH)4]2- ions decompose to produce ZnO molecular species.15
Zn f Zn2+ + (OH-) f Zn(OH)42- f ZnO
(1)
These ZnO species form ZnO seeds. The growth of the ZnO nanostructures can be explained on the basis of the schematic view presented in Figure 8. These ZnO seeds grew in dimension to form a hexagonal planar nucleus. The hexagonal wurtzite ZnO with a polar structure can be described as a hexagonal close packing (HCP) of oxygen and zinc atoms in point group 3m and space group P63mc with zinc atoms in tetrahedral sites. Thus, the crystal habit of wurtzite ZnO exhibits well-defined crystallographic faces, i.e., polar-terminated (0001) and nonpolar low-symmetry (1010) faces (and C6V symmetric ones). In the case of wurtzite ZnO crystals the zinc-terminated (0001) planes are active and promote 1-D growth.17 Thus, one face of the hexagonal sheet is Zn rich and forms the (0001) planes, whereas the opposite face is the (000-1) plane. Thus, the ZnO crystals are polar in nature, and the Zn-rich positive (0001) surface being more reactive than the oxygen-rich negative (000-1) surface
can attract new ZnO species or the opposite ionic species to its surface to promote anisotropic growth along the (0001) direction. The six side facets are generally bound by the (10-10) family of planes as shown in Figure 8. From the crystal habit of wurtzite ZnO it is well-known that the growth rates of the different family of planes follow the sequence (0001) > (10-11) > (10-10). Thus, normally ZnO columnar structures bound by six (1010) facets are grown along the (0001) direction, but under certain circumstances the growth along the (0001) direction disappears and the planes having higher Miller indices and lower specific surface energy such as (10-11) planes become preferred, resulting in the needle-like stem of the nanonails.11 When the reaction was carried out at lower temperature or for a shorter period of time in pure ethanol, the presence of a smaller amount of OH- caused the reaction to proceed much more slowly, and hence, the initial nucleus had sufficient time to grow in the lateral directions, resulting in the formation of nanosheets (Figures 4a and 5a). When the reaction was carried out for 12 h at 170 °C, slow but continuous reaction for the long period resulted in the formation of a few 1-D nanoforms along with the sheets. Due to the increase in the lateral dimensions these nanosheets penetrate through each other, forming a wall-like architecture. When the reaction was carried out for 5 h at 200 °C, the initial steady reaction favored the longitudinal growth, but as soon as the furnace was switched off, the lowering of the temperature reduced the production of the new ZnO species, causing a reduction in the lateral dimensions, giving the products a nail-like morphology. When the reaction was continued for a sufficient time (12 h) at 200 °C, the longitudinal growth continued to form the ZnO nanorod arrays. At a synthesis temperature of 230 °C even after the furnace was turned off, large quantities of ZnO species were present in the solvent. These ZnO species could not be absorbed by the ZnO nanorod growth front due to the lowering of the temperature; instead they formed nanoparticles on the surface of the nanorods. When the number of Zn foils was reduced, the production rate of Zn+ ions was also quite low, and as a result ZnO sheets were formed (Figure 5c). In the presence of NaOH, the presence of the OHions in high concentration induces longitudinal growth along the (0001) direction, and to maintain the lowest possible energy, six low-energy facets appear, preventing the lateral growth. Hence, the diameters of the products remain less than those of the products obtained in the absence of the NaOH. As the amount of the NaOH, i.e., the pH level of the ethanol, was increased slowly, the increase in the concentration of the OHions increased the production of the growth unit [Zn(OH)4]2-, resulting in the formation of more and more densely packed nanorod arrays. For the growth of uniform 1-D nanoforms, a steady reaction is necessary, but beyond a certain pH limit, i.e., at higher pH values (12.2), the reaction occurs vigorously, and as a result growth occurs in both the lateral and longitudinal directions in an uncontrolled manner, resulting in the formation of thick nonaligned ZnO nanorods. Thus, the results discussed above revealed that by regulating the experimental parameters such as the synthesis temperature, time period, number of the source Zn foils, and presence of NaOH in the solvent, it was possible to control the morphology, size, degree of orientation, and density of the 1-D ZnO nanoforms. For specific application these are essential parameters in ZnO. ZnO nanosheets have been previously synthesized by a chemical bath deposition method and utilized in dyesensitized solar cells.18 ZnO nanonails were previously synthe-
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Figure 9. Room-temperature photoluminescence properties of the ZnO 1-D nanoforms synthesized (i) at different experimental conditions with (pH 10.6) and without using NaOH and (ii) at 200 °C for 12 h using different concentrations of NaOH.
sized by a high-temperature-based thermal evaporation process,11,19 but to the best of our knowledge, this type of ZnO nanoform has not been synthesized so far by using any solutionbased approach. On the other hand, ZnO nanorod arrays have been synthesized previously on Zn foils by using an amineassisted15 or H2O2- and NaOH-assisted solvothermal14 approach. The morphological control, uniformity, density of the nanorods, and degree of orientation could be controlled in a much better way by the process reported here compared to those of the earlier reports. Figure 9 shows the room-temperature PL spectra of 1-D ZnO nanoforms with 300 nm excitation wavelength. Figure 9a depicts the PL spectra of the ZnO samples synthesized at different experimental conditions in the absence and presence of NaOH (pH 10.6), whereas Figure 9b shows the PL spectra of the samples synthesized at 200 °C for 12 h with four Zn foils at different concentrations of NaOH. The PL spectra show a strong UV emission peak and a broad green emission peak at 3.21 and 2.52 eV, respectively. The UV emission originated from the excitonic recombination corresponding to the band edge emission of ZnO.11,15 The origin of the green luminescence from the undoped ZnO is associated with the intrinsic defect centers such as an oxygen vacancy (VO), a zinc vacancy (VZn), interstitial zinc (Zni), interstitial oxygen (Oi), or antisite oxygen (OZn). Though the origin of the green emission is generally referred to the deep level or trapped state emission, there is no universally accepted mechanism. There are a few hypotheses to explain the origin of the green emission. The commonly cited reason is that the green emission originates due to the radiative
Kar et al. recombination of a photogenerated hole with an electron occupying the oxygen vacancy.20-22 Dijken et al.23 reported that the green luminescence might be due to the transition from the conduction band to the deeply trapped hole. The donor-acceptor transitions are also reported24,25 to be the origin of green emission. Complex defects involving transition from the interstitial zinc to a deep acceptor level such as an oxygen vacancy is another reason reported behind the green emission.26 Lin et al.27 have reported that antisite oxygen (OZn) could also induce green emission from ZnO. It was observed that the intensity of the green emission decreased with the addition of NaOH. At pH 10.6, the intensity of the defect-related green emission gradually decreased with an increase in reaction time. The PL spectra shown in Figure 9b reveal that, under identical experimental conditions, the intensity of the green emission decreased with an increase in the amount of NaOH, i.e., the pH level. Thus, on the basis of these observations we propose that the presence of the strong green emission band in the PL spectra of the sample, synthesized in pure ethanol, could be attributed to the lack of OH- ions. These OH- ions are the source of oxygen in the ZnO nanoforms. Thus, in the absence of NaOH, the OH- ion concentration was quite low, and as a result, lots of oxygen vacancies or interstitial Zn centers could be formed in the resulting ZnO nanoforms. These defect centers acted as the origin of the green emission. Even in the presence of NaOH the intensity of the green emission band decreased with increasing reaction time. As the reaction time was increased, more and more OH- ions participated in the reaction, reducing the defect centers, which in turn reduced the intensity of the defect-related emission. A decrease in the intensity of the defectrelated green emission band from the ZnO nanoforms synthesized with increasing pH values, keeping all other experimental conditions unchanged, was an indication of the fact that the defect-related emission was indeed associated with the oxygen vacancies. Thus, this result indicated that by using the proper amount of NaOH in the reaction medium one can regulate the degree of alignment as well as the optical quality of the ZnO nanorod arrays. Conclusions In summary, we have fabricated different forms of nanocrystalline ZnO such as nanosheets, nanonails, and aligned nanorod arrays by a solvothermal approach. The morphology, size, and alignment of the 1-D ZnO nanoforms could be controlled by varying different experimental parameters such as the synthesis temperature, time, and presence of NaOH. The degree of alignment of the nanorods improves with the use of NaOH, and the density of the nanorods could be controlled by varying the number of Zn foils. These nanorod arrays exhibited a strong UV emission and a broad green emission at room temperature. The intensity of the green emission band decreased substantially with the use of NaOH and an increase in the reaction time. Acknowledgment. We are thankful to the Department of Science and Technology (DST), Government of India, for financial assistance during the tenure of the work. A.D. is grateful to DST, Government of India, for the award of a fellowship. References and Notes (1) Banerjee, D.; Jo, S. H.; Ren, Z. F. AdV. Mater. 2004, 16, 2028. (2) Li, S. Y.; Lin, P.; Lee, C. Y.; Tseng, T. Y. J. Appl. Phys. 2004, 95, 3711.
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J. Phys. Chem. B, Vol. 110, No. 36, 2006 17853 (15) Dev, A.; Kar, S.; Chakrabarty, S.; Chaudhuri, S. Nanotechnology 2006, 17, 1533. (16) Li, P.; Wei, Y.; Liu, H.; Wang, X. Chem. Commun. 2004, 2856. (17) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (18) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. AdV. Mater. 2005, 17, 2091. (19) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano. Lett. 2003, 3, 235. (20) Dai, Y.; Zhang, Y.; Li, Q. K.; Nan, C. W. Chem. Phys. Lett. 2002, 358, 83. (21) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (22) Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13, 113. (23) Van Dijken, A.; Meulenkamp, E.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 1715. (24) Studenikin, S. A.; Cocivera, M. J. Appl. Phys. 2002, 91, 5060. (25) Reynolds, D. C.; Look, D. C.; Jogai, B. J. Appl. Phys. 2001, 89, 6189. (26) Korsunska, N. O.; Borkovska, L. V.; Bulakh, B. M.; Khomenkova L. Y.; Kushnirenko, V. I.; Markevich, I. V. J. Lumin. 2003, 102, 733. (27) Lin, B.; Fu, Z.; Jia, Y. Appl. Phys. Lett. 2001, 79, 943.
