Growth Mechanism and Photonic Behaviours of Nanoporous ZnO Microcheerios S. Li,*,† Z. W. Li,‡ Y. Y. Tay,† J. Armellin,† and W. Gao‡ School of Materials Science and Engineering, The UniVersity of New South Wales, New South Wales 2052 Australia, and Department of Chemical and Materials Engineering, The UniVersity of Auckland, New Zealand
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1623–1627
ReceiVed October 27, 2007; ReVised Manuscript ReceiVed January 28, 2008
ABSTRACT: Nanoporous ZnO microcheerios were fabricated in a controlled fashion by oxidizing the DC sputtered metallic Zn crystals. The growth mechanism and photonic behavior of the materials with such a nano/microstructure were investigated and characterized in detail. It was demonstrated that the large surface energy caused by the growth steps around the equator of the metallic Zn crystals with a bi-hexagonal pyramid structure is essential for the growth of the cheerios. In particular, the nanoporous structure acts as a scaffold to provide an enormous surface area for further oxidation, resulting in composition gradients that drive Zn atoms from the core of crystals to the surface of the scaffold via the ZnO/Zn interface by self-diffusing. Such oriented diffusion behavior gives rise to the formation of ZnO microcheerios, which consist of nanowhiskers with radial orientations, having an interesting photonic behavior. As a result a high efficiency exciton emission and a visible emission that covers a wide span of wavelengths from 450 to 900 nm are observed. It is demonstrated that the visible spectra consist of five individual emissions dominated by various mechanisms. Such emissions can also be tuned precisely by manipulating the defect chemistry and the size of nanopores with the controlled synthesis process. Introduction ZnO has been the source of great scientific interest, both toward the understanding and exploitation of its intrinsic properties and its performance in optoelectronic applications.1,2 In particular, it has a highly diverse range of nanostructural configurations, including quantum dots, nanowires, nanobelts, nanorings, nanosprings, nanobows, nanohelices, nanoprisms, nanocages, and more.3–5 These nanostructural materials have been explored for their applications in sensors, transducers, field effect transistors, resonators, and cantilevers at the nanoscale.6,7 In general, nanostructured ZnO materials are mainly synthesized by vapor-solid/vapor–liquid–solid processes, wet chemical or sol–gel processing. Through careful manipulation of processing parameters, the materials with particular nanostructures could easily be prepared to meet the specific requirements for practical applications. However, no matter what synthesis methods are used, the oxidation process is the key determinant of the final configuration of structures and also the resultant performance of the materials.8,9 In this work, a new configuration of ZnO microcheerios with a nanoporous structure is reported. Such nanoporous materials are of particular interest on account of their photonic properties. Through an understanding of the formation mechanism and the origin of intrinsic photonic behaviors, the properties of the materials can be precisely tuned by manipulating defect chemistry, thus satisfying the specific requirements for applications. These findings may also provide new insights into the synthesis of nanoporous structures of ZnO for novel applications. Experimental Section Metallic Zn films were deposited on glass substrates by an unbalanced magnetron sputtering system in an argon atmosphere of 5 mTorr. The DC power forwarded to the target was of a density of 0.5 W/cm2, and the deposition time used was 2 h. The as-deposited metallic Zn * To whom correspondence should be addressed. † The University of New South Wales. ‡ The University of Auckland.
film was subsequently oxidized in ambient atmosphere at 380 °C. The microstructures of the as-prepared materials were characterized using field emission scanning electron microscopy and high-resolution transmission electron microscopy. Photoluminescence (PL) was measured by an accent rapid photoluminescence mapping system with an excitation wavelength of 325 nm (He-Cd laser source). X-ray photoelectron spectroscopy (XPS) was performed in VG ESCALAB 220i-XL spectrometer using a monochromatized Al K alpha X-ray source (hV ) 1486.6 eV) with 20 eV pass energy. The results are calibrated by the Adventious Carbon C 1s peak at 285 eV. High resolution spectra were recorded in 0.1 eV measurement step for respective core level bind energies (BE) with a very good signal-tonoise ratio.
Results and Discussion The surface morphology of the as-deposited metallic Zn film in Figure 1a shows that the film consists of a number of Zn crystals with hexagonal bipyramids structure. The smooth facets of these structures occur with equatorial growth steps and protuberances at the ends of the crystals. These structural features suggest that the crystals might grow initially with a flip protopyramid pattern (an upside down protopyramid) until the increase of surface energy (∆GSE) was comparable with the system energy reduction (∆GCG) caused by crystal growth. The rapid increase in surface energy impedes the continuation of crystal growth along the flip protopyramid pattern, which triggers shrinkage of the cross-section area of the protopyramid (Figure 1b). At the same time, the substantial reduction of system energy caused by the crystal growth promotes further crystal growth, driving an increase in volume and cross-sectional area. The subtle competition between the increasing surface energy and the reduction of system energy results in a pattern of growth steps. This competition ceases when the reduction of system energy eventually dominates the system and the crystal grows in a protopyramid pattern, resulting in the smooth facets, as shown in Figure 1b. It is noted that the higher surface energy instigated by the growth steps made the area with this particular feature much more active than the other parts of the
10.1021/cg701056q CCC: $40.75 2008 American Chemical Society Published on Web 04/11/2008
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Figure 1. (a) SEM surface morphology of an as-deposited metallic Zn film shows the crystals having a bi-protopyramid shape with a size of ∼1.5 µm in diameter and ∼3 µm in length. (b) Schematic illustration shows that the subtle competition between the increasing surface energy and the reduction of system energy results in a pattern of growth steps.
Figure 2. Schematic illustration shows that the ZnO forms through the intergrowth of the 36 planar net of O intercalated underneath the 36 net of Zn. With this growing mechanism, the formation of ZnO expands the volume of the Zn crystals by 56.44% instantaneously.
crystal. This provides an essential reaction platform for producing a unique structure of ZnO in the subsequent oxidation process. During oxidation, oxygen atoms first react with Zn atoms in the growth step area, forming ZnO around the equator to lower the energy of the system. In a crystallo-chemical sense, ZnO can be described as the intergrowth of the 36 planar net of O intercalated underneath the 36 net of Zn as shown in Figure 2. The Zn 36 net is the basis of hcp packing and corresponds to the parent Zn metal structure. The lattice parameters of
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Figure 3. Porous nanostructure of ZnO formed around a hexagonal Zn crystal from (a) front view and (b) top view angles.
hexagonal Zn are a ) 0.266490 nm and c ) 0.494680 nm, while ZnO has greater lattice parameters of a (0.324927 nm) and c (0.520544 nm). The formation of ZnO expands the volume of the entire single crystalline Zn particle by 56.44% instantaneously, resulting in the porous nanostructure shown in Figure 3. It is discernible that a ZnO ring with nanoporous structure formed around the equator of a hexagonal Zn crystal through a reaction between the metallic Zn in this particular area and the surrounding oxygen. It is also believed that the nanoporous ZnO acts as a scaffold, providing (1) massive surface area to accommodate the oxygen atoms and (2) a platform with massive surface area for the further oxidation. This unique nanostructure makes the localized oxygen content per unit volume significantly higher than other locations. Such oxygen gradients drive the surrounding metallic Zn atoms to diffuse along the frame of scaffold to react with the oxygen on its surface. This process consumes a number of Zn atoms to cause three-dimensional concentration gradients in two ways: (1) a radial gradient from the central axis of the hexagonal Zn crystals via the interface of ZnO/Zn to the edge of the nanoporous ZnO ring, and (2) a longitudinal gradient from two ends of the Zn crystals toward the equator plane of the crystal, as shown in Figure 4. These gradients construct the diffusion paths of the metallic Zn atoms and direct their motion, thus facilitating the nanoporous ZnO ring growth along the radial direction but restricting its growth along the thickness. These gradients continually drive the metallic Zn atoms to diffuse onto the edge of the scaffold via the ZnO/Zn interface, further consuming the metallic Zn to form
Nanoporous ZnO Microcheerios
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Figure 4. Schematic illustration shows the formation mechanism of ZnO cheerios through the movement of metallic Zn atoms in the constructed self-diffusion paths.
Figure 6. (a) Cross-section morphology of ZnO cheerio shows that the cheerios is an integration of nanowhiskers oriented along the radial directions. (b) The high resolution TEM image showing the nanowhiskers of 50 nm grew along the [001] direction.
Figure 5. (a) ZnO cheerios formed from the deposited metallic Zn film. (b) ZnO “rose”.
ZnO. This process may result in an instantaneous volume expansion, thus presenting the radial growth of the ZnO scaffold. The progress of such a reaction eventually ceases when the metallic Zn is completely consumed, forming a number of ZnO microcheerios with diameter of ∼5 µm and thickness of ∼1 µm, as shown in Figure 5a. This formation mechanism gives rise to the unique nano/macroporous morphology that is different from the densely packed doughnut-like ZnO prepared by the solvothermal route.10 Although ZnO sometimes nucleates on the smooth facets of metallic Zn (Figure 3b), the larger surface energy on the scaffold, which has higher potential for oxidation, restricts the growth of ZnO on the facets. In some cases, a few adjacent metallic Zn crystals join together and react with oxygen to form rose-shaped ZnO, as shown in Figure 5b. It is interesting to note that ZnO microcheerios consist of a number of nanowhiskers as shown in the cross-sectional
morphology of a microcheerio in Figure 6a. From this figure, the hole produced by the ZnO formation through the complete consumption of the metallic Zn crystal is discernible, further supporting the formation mechanism of ZnO microcheerios described above. The out-growing nanowhiskers show that the microcheerios grew along the radial direction rather than the thickness direction during the oxidation process. High resolution TEM images in Figure 6b show that the (100) planes of ZnO are parallel to the axis of nanowhisker, indicating that the nanowhiskers (of diameter 50 nm) grew along the [001] direction. The dipolarity appearing in other nanostructures of ZnO such as nanobelts and nanorods may not be in evidence in the microcheerios due to the integration of nanowhiskers. It is still not clear whether the individual whisker displays dipolarity and whether the charge repulsion between whiskers is the cause of the resultant porous nanostructure. These lines of inquiry will be further investigated in the near future. The distinctive nanoporous structure of ZnO microcheerios results in an interesting photonic property as shown in Figure 7a. The PL spectra of the as-prepared materials consist of a highly efficient exciton emission and a visible emission with over a wide span of wavelengths from 450 to 900 nm. The visible spectra have a FWHM of 200 nm, which could be deconvoluted into five individual spectra with peak values of 513 nm (2.41 eV), 560 nm (2.22 eV), 609 nm (2.03 eV), 675 nm (1.84 eV), and 769 nm (1.61 eV), respectively. Such spectra are the result of five characteristic mechanisms. In general, the
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Figure 7. (a) The photoluminescence spectra of ZnO cheerios; (b) XPS spectra showing Zn 3d core level binding energy of the as-prepared cheerios has the same value as the reference ZnO with purity of 99.999%.
UV emission of 375 nm and the red emission of 609 nm are the intrinsic PL properties of ZnO which also appear in the reference ZnO (99.999% purity). It is believed that the singly ionized oxygen vacancy (V0•) is responsible for the emission of 513 nm. This has been demonstrated by purging hydrogen to create the oxygen vacancies through the reduction processing of ZnO at 900 °C.11 On the other hand, the emission at ∼670 nm is frequently observed in the ZnO prepared by physical vaporization deposition (PVD) or sintering techniques in which the materials are usually subjected to a high temperature processing.12–15 The common feature of these materials is having Zn vaporization during the process. Such a vaporization results in a change of the stoichiometric ratio from Zn:O ) 1: 1 to Zn:O ) (1 - x): 1 (x > 0), exhibiting an oxygen-rich nature. This suggests that a similar emission of 675 nm observed in the ZnO cheerios might also be the consequence of excess oxygen. However, different from the materials prepared with deposition and sintering techniques, the ZnO cheerios formed from oxidation of metallic Zn at 380 °C, which is much lower than the Zn melting temperature of 420 °C. At such a low processing temperature the Zn vaporization would not occur. On the other hand, the enormous surface area and space of the scaffold in
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the cheerios provide a unique platform for the reaction of oxygen and Zn atoms. The purging oxygen flow continually replenishes the reacted oxygen and refills the space of the scaffold through surface absorption even when the reaction is completed by entire consumption of the metallic Zn, causing excess oxygen in the ZnO cheerios. In general, the excess lattice oxygen (OZn) acts as an acceptor to introduce an empty level with energy of ∼1.5 eV into the band of ZnO.16 This results in the emission with a wavelength of 663 nm (Eg ) 3.37-1.5 eV). The difference between the theoretical emission of 663 nm and the observed emission of 675 nm in ZnO cheerios may be caused by two mechanisms: (1) some crystallographic imperfections observed in Figure 6b, such as the dislocations induced by the rapid growth of ZnO cheerios which may result in lattice distortion to change the crystal field, thereby slightly increasing the energy level of the acceptor; and (2) the intensive surface absorption of oxygen may cause the electron redistribution to adjust the density of states on the surface. This would modify the bandgap energy on the surface layer. The combination of such energy modifications may cause the acceptor level to increase from 1.50 to 1.53 eV, producing a red emission with wavelength of 675 nm (Eg ) 3.37-1.53 eV) as shown in Figure 7a. Similarly, interstitial oxygen (Oi) may also be formed as excess oxygen during the process. A small amount of modification bandgap energy from 2.28 eV17 to 2.21 eV may also be caused by the aforementioned mechanisms. It is still not clear how the oxygen vacancies (VO), with the resultant weak emission of 769 nm (Eg ) 3.37-1.62 eV) are formed in the ZnO microcheerios in the oxidation environment. However, the total amount of the defects is very low, so the XPS could not determine the difference between the as-prepared cheerios and the reference ZnO with a purity of 99.999%, as shown in Figure 7b. This is also evidenced by the high efficiency exciton emission shown in Figure 7a. Normally the intensity of the exciton emission highly depends on the concentration of intrinsic defects, such as zinc and oxygen vacancies, interstitial zinc and oxygen as well as excess lattice oxygen.18 The higher concentration results in a stronger depression of UV emission. This phenomenon is frequently observed in the materials fabricated by PVD and chemical vaporization deposition techniques, in which the intrinsic defects with high concentrations were induced by Zn evaporation in a high temperature process. The high efficiency exciton emission of ZnO cheerios implies that the concentration of intrinsic defects in the as-prepared material is limited. On the other hand, ZnO is a biocompatible material. The ZnO microcheerios consist of nanopores and macropores (2-3 µm) that can act as scaffolds to promote cell adhesion and proliferation, releasing drug in a controlled fashion.19 This material can be sterilized using ultraviolet light and water or ethanol for hospital applications. The size of macropores can also be precisely controlled by manipulating the grain size of metallic Zn during the deposition process while varying the oxidation parameters could produce nanopores with different sizes. This allows the porous size of the scaffolds to be fine-tuned to suit the requirements of biomedical applications, such as bone replacement, drug release, stenting, photocatalysis, photocatalytic sterilization of surgical and food processing, etc. Conclusion In summary, nanoporous ZnO microcheerios, a unique configuration of ZnO, has been successfully fabricated by an oxidation process. It has been demonstrated that the large surface energy caused by the growth steps around the equator of the
Nanoporous ZnO Microcheerios
metallic Zn crystals with a bi-hexagonal pyramid structure is essential for the growth of the cheerios. The spontaneous formation of ZnO through the insertion of an oxygen network into the hexagonal lattice of metallic Zn during the oxidation process produces a nanoporous structure. This nanoporous structure acts as a scaffold to provide enormous surface area for further oxidation. This process results in composition gradients that drive Zn atoms from the core of the crystals to the surface of the scaffold via the ZnO/Zn interface by selfdiffusing. Such an oriented diffusion behavior gives rise to the formation of ZnO microcheerios, which consist of nanowhiskers with radial orientations, having UV and red emissions. This photoluminescent behavior is distinctive, with a high efficient exciton emission and relative strong visible emission. The spectra of the visible emission, which cover a wide span of wavelengths from 450 to 900 nm, consist of five individual emissions dominated by various mechanisms. The experimental results demonstrated that the chemical variation, such as oxygen content and its ionic configurations, may play an important role in dominating the photonic behavior of the materials. Such emissions can be tuned precisely by manipulating the defect chemistry and the size of nanopores with the controlled synthesis process. The scaffolds created by the nano/macropores in the microcheerios can also act as multifunctional biomaterials for medical applications. Acknowledgment. This work was financially supported by Australian Research Council Discovery Program DP 0665539 and Royal Society of New Zealand Marsden Fund.
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