Structural Characterization and Luminescence of Porous Single

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J. Phys. Chem. C 2008, 112, 240-246

Structural Characterization and Luminescence of Porous Single Crystalline ZnO Nanodisks with Sponge-like Morphology B. Reeja-Jayan,† E. De la Rosa,‡ S. Sepulveda-Guzman, R. A. Rodriguez,§ and M. Jose Yacaman* Department of Chemical Engineering and Texas Materials Institute, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: August 15, 2007; In Final Form: October 11, 2007

We report the synthesis of porous single crystalline ZnO nanodisks with sponge-like morphology through a wet chemical approach. To our best knowledge, this is the first report about highly porous single crystalline nanodisks of ZnO with an average diameter of ∼100 nm. The ZnO nanodisks exhibit strong visible (bluegreen) light emission on UV excitation. Scanning Transmission Electron Microcopy (STEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Selected Area Electron Diffraction (SAED) were performed to confirm that the nanodisks are single crystalline and porous in nature. The porosity of the nanodisks gives them the sponge-like appearance. Energy Dispersive X-ray Spectrometry (EDS) and Electron Energy Loss Spectrometry (EELS) analysis of the nanodisks together with high-resolution electron microscopy and photoluminescence measurements were used to determine the cause of the visible emission and its relation to the sponge-like morphology and growth mechanism. The larger surface area to volume ratio of these sponge-like nanostructures makes them very attractive for applications like biochemical sensors and solar cells.

1. Introduction Zinc oxide nanostructures have attracted much attention in recent years because of the strong dependence of their optical properties on the crystallite size and morphology. This makes ZnO a promising material for high-performance photonics applications. The properties of ZnO like wide band gap (3.37 eV), high breakdown strength, large excitonic binding energy (60meV), high refractive index, and high recombination efficiency make it a good candidate for use in electro-optic applications like high-efficiency LEDs and LASERs. Other potential applications in the future nano-optoelectronic industry include sensors, luminescent phosphor for displays, photonic crystals, solar cells, field emitter, etc., to mention a few. The characteristic emission band of ZnO is centered at 390 nm, and this UV emission is attributed to the free exciton recombination. Green (520 nm) and orange (620 nm) emission bands have been reported in both nano- and bulk crystals. The former has been associated to the recombination of a photogenerated hole with the single ionized charge state of the single ionized oxygen vacancy on or very close to the surface of the particle, and the second band has been associated to excess of oxygen inside the particle. 1,2 Such defects are produced during the synthesis process and are found to be influenced by parameters like reaction temperature and presence of oxygen during the synthesis process.3 There is a lot of interest in studying and understanding the visible emission from ZnO nanostructures for possible applications in solid-state lighting and displays.4 As * To whom correspondence should be addressed. Phone: (512) 2329111. Fax: (512) 475-8090. E-mail: [email protected]. † Department of Electrical and Computer Engineering, The University of Texas at Austin. ‡ On sabbatical from Centro de Investigaciones en Optica, Leo ´ n, Gto. 37150 Me´xico. § Universidad de Guadalajara.

ZnO is a biocompatible material,5 it is also possible to use the ZnO nanostructures as biochemical sensors and as an inorganic dye for labeling cells, taking advantage of their strong visible light emission. On account of these interesting and possibly lucrative applications, ZnO nanostructures are now a widely studied topic. A plethora of ZnO nanostructures have already been synthesized and characterized in detail. Of these, the most commonly reported morphologies are nanoparticles,6 nanorods,7 nanowires,8 nanotubes,9 and nanoflowers.10 More recently, nanodisks, nanoplates, nanospheres, and hemispheres11-17 have also been reported. In all cases the particle size is ∼1 µm with a thickness of ∼100 nm. The nanodisks and nanoplates are interesting nanostructures which have been studied for their superior photocatalytic properties.12,13 The morphology reported in this paper differs from earlier reports of ZnO nanodisks. In this case, the average diameter of the nanodisks is ∼100 nm, and they are the result of an aligned assembly of small nanocrystals. As a result, each nanodisk is single crystalline and porous in nature. This gives them a sponge-like resemblance. To our best knowledge, the structural characterization and luminescence of such morphology has not been reported before. The porous sponge-like morphology is interesting to study for several applications.18-20 The high porosity and resultant surface roughness of the sponge-like nanostructure produces larger surface area than its solid counterpart with a smooth surface. This makes sponge-like nanodisks very attractive for use in making efficient biochemical sensors with large reaction areas.21 A film of such sponges can also be used as the active region in devices like dye-sensitized solar cells, which require a large area of contact between the dye and the inorganic nanocrystals for efficient light capture and charge separation.22 In this work, we report the synthesis, structural characterization, and luminescence of sponge-like ZnO nanodisks that

10.1021/jp0765704 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007

Porous Single Crystalline ZnO Nanodisks

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present strong visible (blue-green) light emission. The nanodisks were found to be porous and very good quality single crystals. Structural characterization of nanodisks was done by electron microscopy, EDS, and EELS. The strong luminescence was explained in terms of defects induced by oxygen deficiency and the high porosity of the sponge-like nanodisks. 2. Experimental Procedures All reagents including zinc acetate dihydrate (Zn(CH3CO2)2· 2H2O) and sodium hydroxide (NaOH) were analytical grade purchased from Aldrich and used as received without further purification. A solution of 50 mL of ethanol with zinc acetate at 0.01 M was prepared and placed in an oil bath at 50 °C. In a separate beaker, a solution of 40 mL of ethanol and 10 mL of NaOH in deionized water at 0.03 M was prepared. NaOH solution was added gradually to zinc acetate solution, and precipitation occurred immediately. The pH of the final solution was 9.5. The solution was removed from the oil bath immediately after this. The precipitated powder was separated by centrifugation and washed with ethanol to remove all reaction wastes. The cleaned powder was suspended in ethanol and stored in a glass vial. After several hours, the large particles settle down at the base of the vial and the smaller particles remain suspended as a colloidal dispersion. This transparent colloid was separated into another vial by decantation and stored to be used for characterization. The nanostructures in the colloidal dispersion were analyzed by Scanning Transmission Electron Microscopy (STEM), HighResolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED), Electron Energy Loss Spectroscopy (EELS), and Energy Dispersive X-ray Spectrometry (EDS) performed in the Philips Tecnai F20 and JEOL 2010F at 200 kV. Samples suspended in ethanol at room temperature were dispersed with ultrasonic stirring, and then aliquots of the solution were dropped on 3 mm diameter ultrathin carbon copper grids. The photoluminescence (PL) characterization was performed at room temperature on a Jovin Yvon fluorometer under 350 nm excitation from a 75 W UV Xe lamp. The emission spectrum presented here is corrected for cuvette, solvent emission, and electronic noise. 3. Results and Discussion 3.1. Structural Characterization (STEM, TEM, and SAED). The general aspects of the morphology of the sponge-like ZnO nanodisks in the colloidal solution were analyzed using STEM and low-resolution TEM. Figure 1a and b shows STEM images of the ZnO nanodisks. The images revealed that the nanoparticles have both irregular shape and surface. The average diameter of the nanodisk is ∼100 nm based on size measurements taken from 100 nanodisks. We found that these nanodisks resemble sea sponges that attach themselves to hard surfaces along the ocean floor. The differences in the brightness observed in the STEM images could be related to several parameters such as the differences in composition and/or mass distribution.23 In our case, there is no other phase besides ZnO which can produce the dark areas in the structure. However, it is possible that differences in thickness created by big holes or depressions on the surface results in a different contrast. This reasoning was further supported by a line-scan microanalysis, and the electron profile is shown in Figure 1c. The intensity increases in the area limited by the particle and decreases in the dark regions where pores (holes) are present. This proves that the nanodisks are porous in nature. It is found that while most holes are

Figure 1. (a and b) STEM image of sea sponge-like ZnO nanodisks. (c) Depth profile of a single pore marked in nanodisk shown in b.

shallow, there are some holes that go right through the volume of the nanodisk. In order to study the structure of the sponge-like nanodisks, TEM analysis was done. The nanodisk appears to be an aggregate of smaller nanoparticles as the image depicts two regions of contrast: bright and dark; see Figure 2a. However, the differences in the contrast in TEM are mainly related to the differences in the thickness of the sample. Therefore, the bright regions of the TEM image (dark regions of STEM image) correspond to holes or pores in the nanodisk. Thus, the TEM images further confirm that the nanodisks are porous in nature. The HRTEM image of the nanodisks depicts very clear crossed fringes with an interplanar spacing of 0.29 nm, corresponding to the (010) lattice spacing of ZnO; see Figure 2b. A noise filtered image of the fringes is shown in Figure 2c. The lattice resolved image is a pattern of fringes running over regions of bright and dark intensity. The fringes in some regions of the image appear out of focus. We believe this is an effect of depth of focus in the HRTEM image mode and that these fringes are produced by the crystal planes in the base and walls of the pores

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Figure 2. (a) Low-resolution TEM image of ZnO nanodisk. (b) High-resolution TEM image of ZnO nanodisk. (c) Lattice resolved TEM image. (d) SAED pattern of a single nanodisk.

Figure 3. (a) TEM image of a group of nanodisks. (b) Electron diffraction pattern from the whole group of nanodisks. (c and e) SAED patterns from different portions of the single nanodisk shown in d.

in the nanodisk. It is also observed that the lattice fringes are found to curve slightly in some regions. This further indicates that the surface of the nanodisk is very irregular. The fast Fourier transform (FFT) of the filtered fringes, not shown here, produces

a well-defined hexagonal pattern characteristic of the wurzite structure of ZnO, suggesting that the nanodisks are single crystalline in nature. This hypothesis was confirmed by the analysis of SAED patterns, which for a single nanodisk

Porous Single Crystalline ZnO Nanodisks

Figure 4. Schematic diagram showing different crystal growth mechanisms. It is proposed that sponge-like nanodisks are the result of the oriented attachment growth process.

corresponds to a hexagonal array of spots typical of the [0001] zone axis orientation of ZnO, as shown in Figure 2d. The same result was obtained for two different regions on a nanodisk as shown in Figure 3c-e. While the SAED pattern from a single nanodisk exhibits a good hexagonal pattern, the SAED pattern from a group of nanodisks (Figure 3b) shows a lot more spots characteristic of a polycrystalline structure. These observations confirm that each nanodisk is a single crystal oriented along the [0001] zone axis and a collection of such nanodisks has a mosaic type of structure. The fact that individual ZnO nanodisks are both porous and single crystalline makes them a very interesting new nanostruc-

J. Phys. Chem. C, Vol. 112, No. 1, 2008 243 ture. The mechanism for formation of the sponge-like ZnO nanodisks can be explained by the oriented attachment process. This can be described as the self-assembly of adjoining particles in such a way that there is a tendency of the grains during coalescence to rotate with respect to each other until a lowindex crystallographic orientation is achieved.11 Unlike the Oswald ripening process for crystal growth in which the larger crystal grows at the expense of smaller crystals in the solution, the oriented attachment process produces bigger crystals by attachment and coalescence of smaller crystals. A schematic diagram of different crystal growth mechanisms is shown in Figure 4. The nucleated seeds first grow into small nanoparticles. After this, there are three possible processes that can happen. We propose that one of these processes, namely, oriented attachment, results in the self-assembled sponge-like nanodisks. The initial nucleation process is very rapid, and the nanoparticles formed will try to minimize surface energy by attaching to larger surfaces. Therefore, it is difficult to image the growth process during the early stages. However, we can find indications of oriented attachment by studying the fully formed nanodisks. Such a situation is seen in Figure 5, which is a TEM image of a particle attaching to an already formed nanodisk. A difference in contrast clearly indicates the interface between the particle and the nanodisk. Identical FFT patterns (shown as insets) are obtained from three regions in the image, namely, the particle, nandisk, and interface between them. This confirms that the particle attaches to the nanodisk in such a way that it forms a single crystal.24 Such coalescence of particles to form single crystals is a classic signature of oriented attachment. We believe that in the case of ZnO the driving force for oriented attachment is the interaction between polar surfaces and nonpolar surfaces of the interacting particles. The fact that all crystallites show a low index direction in the composite nanodisk suggests that the

Figure 5. TEM image of a particle attaching to a nanodisk. Insets show the FFT obtained from the particle, nanodisk, and interface between them.

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Figure 6. Emission spectra of ZnO nanodisks under 350 nm excitation. Inset shows a picture of the fluorescence from the colloidal dispersion.

interface is a low-energy boundary.25 Therefore, this result suggests that an oriented attachment process is taking place along the (010) planes of the nanodisk in Figure 5. In addition, the irregular surface of the nanodisks also suggests that the growth mechanism is by oriented attachment because in terms of morphology of the crystals produced, the Ostwald ripening process creates regular shaped structures (like hemispheres, spheres) with smooth edges, while oriented attachment results in structures with irregular shapes and visible facets.11 The interesting point is that in spite of such imperfections observed in the HRTEM image of a nanodisk, the SAED pattern (Figure 2d) suggests that each nanodisk is a single crystal of ZnO. We believe that these observations rule out other processes like Oswald ripening and confirm that the highly porous single crystalline nanodisks were self-assembled through oriented attachment of extremely small nanoparticles in a solution synthesis process. Other groups have also observed the self-assembly of ZnO nanostructures from nanoparticles and nanoplates through the oriented attachment mechanism.11,26,27 Oriented attachment becomes relevant when particles are free to move about, like in solution synthesis environments. This is also a reason why oriented attachment is found to occur in nature.25,28,29 3.2. Optical Properties of Nanodisks. The sponge-like ZnO nanodisks are found to exhibit strong blue-green emission on UV illumination. Figure 6 depicts the image of such emission in colloidal solution and the photoluminescence spectrum under 350 nm excitation. The weak emission band centered at 390 nm corresponds to the characteristic near band gap emission associated to free recombination of excitons and is in correspondence with the emission band reported in the literature. The visible emission band is the result of the overlapping of at

Reeja-Jayan et al. least three signals: two well-defined peaks centered at 512 and 550 nm and one shoulder at 590 nm. The resulting band is centered at 548 nm and red shifted compared to the typical green band centered at 510 nm and blue shifted compared to the yellow-orange band centered at 620 nm.1,2 However, the presence of green and yellow-orange emission is clear from the spectrum. The ratio between different components gives the final visual color of the colloidal solution. The origin of visible emission is controversial, and different hypotheses have been proposed to explain both the green and the orange emission bands. The most accepted explanation for green emission has been associated to oxygen vacancies and vacancies related defects.1,2,30-33 Changes in the peak wavelength have been associated to changes in the environment of the vacancy. The orange emission has been associated to an excess of oxygen.2,30 It has recently been proposed that the presence of Zn(OH)2 on the surface is responsible for both green and orange emission.34 In other words, visible emission could be produced by the combination of different kinds of defects, depending on the specific preparation method. It is possible that in the case of the porous nanodisks the oriented attachment growth process would have resulted in the creation of many defects in the structure, especially on the surface. These surface defects can be the reason for the observed visible (blue-green) emission. In order to relate the observed visible luminescence to the presence of defects in the structure of the sponge-like nanodisks, compositional analysis was performed using EELS and EDS techniques. Figure 7 b and c depicts the EELS zinc and oxygen L (high-energy) edge images obtained by conducting elemental mapping on one of the nanodisks. EELS analysis of the nanodisks was done in TEM mode. The three-window method is used in this case, wherein two images are first acquired in front of the characteristic L edge (pre-edge images) and a background correction is applied on the image acquired at the characteristic energy loss (post-edge image).35 The bright regions on these images correspond to the presence of zinc and oxygen, respectively. It appears as though there are several dark regions on the nanodisk, which indicates an absence or lower concentration of zinc. This observation is yet another confirmation that these regions correspond to pores on the nanodisk and is in agreement with the results obtained by STEM analysis. From the oxygen map image it appears that oxygen is present in these pores as well, suggesting the presence of oxygen adsorbed to the surface. However, this is a very qualitative observation. For a better understanding of the ratio of zinc and oxygen present in the sponge-like nanodisks, we performed an EDS line scan across a single nanodisk in STEM mode as shown in Figure 8. The concentration profile of zinc and oxygen based on the line scan was plotted and is shown in Figure 8b. The intensities of both Zn and O are higher in the area limited by the particle boundaries. However, there are fluctuations in the profile, and both intensities decrease in the dark regions of the nanodisk. This is in agreement with the analysis done by STEM and

Figure 7. EELS spectral imaging of nanodisk: (a) TEM image, (b) zinc L edge map, and (c) oxygen L edge map.

Porous Single Crystalline ZnO Nanodisks

J. Phys. Chem. C, Vol. 112, No. 1, 2008 245 with sponge-like morphology have been self-assembled from ZnO nanoparticles by oriented attachment during solution synthesis. To the best of our knowledge, this is the first report of such a nanostructure in ZnO. Photoluminescence spectrum, EELS, and EDS analyses indicate that the assembly process creates oxygen vacancies in the structure. This in turn causes the strong visible (blue-green) emission from the nanodisks. The larger surface to volume ratio of these porous ZnO nanodisks makes them attractive for use as biochemical sensors and solar cells. Acknowledgment. The authors are indebted to DARPA (HR011-06-1-005), the Welch Foundation (Grant F-1615), and the ICNAM. We thank Drs. J. P. Zhou and X. Gao for TEM assistance. References and Notes

Figure 8. (a) EDS line scan across a single nanodisk in STEM mode. (b) Concentration profile of Zn and O based on line scan.

Figure 9. EDS mapping of single nanodisk: (a) STEM image of single nanodisk, (b) Zn-L map, (c) Zn-K map, and (d) O-K map.

HRTEM and supports the conclusion that dark regions correspond to pores in the nanodisk. The line scan also revealed that the Zn and O intensities do not match. The O intensity is lower than Zn, which implies a deficiency or vacancy of oxygen in the nanodisk. In addition, EDS mapping analysis (Figure 9) of a complete nanodisk indicates the absence of any other impurities, suggesting that the observed luminescence is produced only by the oxygen vacancy defects. The strong intensity of the visible emission suggests that the porous nature of nanodisks produces a very high concentration of these defects. 4. Conclusions On the basis of STEM, HRTEM, EELS, and EDS analyses it is proposed that porous single crystalline nanodisks of ZnO

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