Controlled Hydrothermal Synthesis of Zirconium Oxide Nanostructures

Department of Chemistry, University of Miami, Coral Gables, Florida 33124 ... Department of Physics and Astronomy, Louisiana State University, Baton R...
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DOI: 10.1021/cg800711m

Controlled Hydrothermal Synthesis of Zirconium Oxide Nanostructures and Their Optical Properties

2009, Vol. 9 3874–3880

Latha Kumari and W. Z. Li* Department of Physics, Florida International University, Miami, Florida 33199

J. M. Xu and R. M. Leblanc Department of Chemistry, University of Miami, Coral Gables, Florida 33124

D. Z. Wang Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467

Yi Li, Haizhong Guo, and Jiandi Zhang Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803 Received July 3, 2008; Revised Manuscript Received June 22, 2009

ABSTRACT: Zirconium oxide (ZrO2 or zirconia) nanostructures were synthesized by a hydrothermal route. Surface morphology analysis depicts the formation of various zirconia nanostructures at different synthesis conditions. X-ray diffraction examination demonstrates that the as-synthesized zirconia is of pure monoclinic phase (m-ZrO2). High resolution transmission electron microscopy (HRTEM) further confirms the high crystalline feature of the m-ZrO2 nanostructures. X-ray photoelectron spectroscopy (XPS) core-level spectra of Zr 3d and O 1s for the ZrO2 nanostructures have been studied to understand further the electronic states and chemical environment of the Zr and O atoms in ZrO2 for different synthesis conditions. XPS results also indicate the existence of oxygen defects and zirconia suboxides which affect the structural and optical properties of zirconia nanostructures. The nanostructures show UV-vis absorption band around 290 nm at room temperature. The band gap energy is determined, in the range of 2.5-3.8 eV for zirconia nanostructures synthesized at various conditions. A broad emission band with maximum intensity at around 400 nm is observed in the photoluminescence (PL) spectra of zirconia nanostructures at room temperature depicting the violet emission, which can be attributed to the ionized oxygen vacancy in the material. 1. Introduction Zirconia is a very interesting material in both fundamental study and application-oriented research. ZrO2 has been extensively investigated for a variety of applications related to its valuable chemical, physical, optical, dielectric, and mechanical properties, including high melting point, high resistance to thermal shock, good chemical stability, low electrical conductivity, high dielectric constant, excellent wear resistance, and biocompatibility.1-3 Zirconia is classified as a wide band gap semiconductor and tends to become more conductive with increasing temperatures. Zirconia has diverse practical applications in fuel-cell technology,4 catalysis,5 protective coating for optical mirrors and filters,1 nanoelectronics and microelectronics,6 ceramic biomaterials,2 thermoluminescence UV dosimeters,7 and preparation of piezoelectric, electrooptic, dielectric, and nanocomposite materials.3,8,9 The optical properties10 and especially the photoluminescence (PL) properties of ZrO2 have been seldom reported, although PL has already been observed in a ZrO2 sol and nanoparticle systems.11 There has been an increasing interest in the application of ZrO2 nanoparticles for photonics systems due to their enhanced luminescent properties associated with their small size.12 Luminescent materials have been utilized widely in applications, such as cathode ray tubes, *Author to whom any correspondence should be addressed. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 07/23/2009

fluorescence lamps, vacuum fluorescent display devices, color plasma display panels, and electroluminescent flat-panel displays.13 There is also a strong commercial desire to produce efficient and lasing blue-light-emitting diodes and short-wavelength laser diodes by exploiting this type of nanomaterial. Recently, it has been demonstrated that wide-band gap oxide nanostructures with short-wavelength PL emission may be used as compact disk (CD) read-heads.14,15 The performance of the zirconia-based devices considerably depends on the crystal structure of zirconia. Pure zirconia exists in three polymorphic phases at different temperatures. It has a cubic structure at high temperatures >2370 °C, a tetragonal structure at intermediate temperatures between 1150 and 2370 °C, and a monoclinic structure at low temperatures 200 °C), which is seldom reported.26 We present the synthesis of pure m-ZrO2 nanostructures with controlled morphology and high crystallinity. Structural analysis and optical studies of the nanostructures are performed by various characterization techniques. 2. Experimental Procedures 2.1. Synthesis of Zirconia Nanostructures. ZrO2 nanostructures were synthesized by a simple hydrothermal route. The starting materials used for the synthesis were zirconyl nitrate hydrate (ZrO(NO3)2 3 xH2O) and sodium hydroxide (NaOH). All the chemicals were analytic grade reagents (Fisher Scientific) and used without further purification. Experimental details are as follows: first, 0.5 M of ZrO(NO3)2 3 xH2O and 5 M of NaOH solutions were prepared in distilled water. Next, NaOH solution was slowly added to the ZrO(NO3)2 3 xH2O solution under manual stirring followed by sonication for 30 min to obtain homogeneous solution. After that, 10 mL of the above solution was loaded into a 20 mL Teflon-lined autoclave, which was subsequently filled with 2 mL of absolute ethanol. Finally, the autoclave was sealed and maintained at different temperatures, 200 and 250 °C for 24-72 h (hydrothermal treatment time, tH). It was then allowed to cool down to room temperature naturally. The precipitates were filtered, washed with distilled water to remove the soluble nitrates and with ethanol to reduce agglomeration, and later dried for 1 h at 80 °C. The white colored material so processed was later used for various characterizations. The materials synthesized at 200 °C for 24 and 72 h are correspondingly termed as sample A and sample B, whereas the materials synthesized at 250 °C for 24 and 48 h are respectively named as sample C and sample D, in the following text. 2.2. Characterization. Surface morphology analysis of the ZrO2 nanostructures was performed by a field emission scanning electron microscope (SEM, JEOL JSM-6330F) equipped with energy-dispersive X-ray spectroscopy (EDS, Thermo electron Corp.) and was operated at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images,

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and selected-area electron diffraction (SAED) patterns were obtained from a JEOL-2010F apparatus employing an accelerating voltage of 200 kV. For TEM analysis, the product was ultrasonically dispersed in ethanol and then spread on carbon-coated copper grids. Structural analysis was carried out by employing an X-ray diffractometer (D-8 Bruker-AXS) equipped with a Cu KR radiation source (λ = 1.5406 A˚) and a two-dimensional area detector. UVvis spectra were obtained from Perkin-Elmer Lambda 900 UV/vis/ NIR spectrometer and the photoluminescence spectra were recorded from SPEX FluoroLog spectrofluorometer (Horiba, Jobin Yvon). For the spectroscopic analysis, nanomaterials were dispersed in NaOH solution at room temperature and taken into a quartz cell. X-ray photoelectron spectroscopy experiment has been conducted in a UHV chamber with pressure of 5  10-9 torr at room temperature. The XPS spectra were measured by using a monochromatic Al KR1 X-ray source. The combined energy resolution of both electron analyzer and radiation source is ∼0.16 eV. The binding energy of the measured core levels of the zirconia are acquired together with the binding energy of Au 4f core level spectra simultaneously from the sample substrate. A binding energy of 83.56 eV of Au 4f7/2 has been used for the calibration. For the XPS measurements, ZrO2 powder was ultrasonically dispersed in isopropyl alcohol and deposited on the Au coated silicon substrate.

3. Results and Discussion 3.1. Surface Morphology. Figure 1a-d shows the highmagnification SEM images of the ZrO2 samples A-D, respectively. Sample A (200 °C, 24 h) shows highly uniform monodispersed rice grain-like nanostructures with a narrowshape distribution. These nanostructures are about 450 nm long and 50-140 nm wide. ZrO2 nanomaterials synthesized at 200 °C for 48 h showed almost similar surface morphology as compared to the sample A, but with a small increase in the particle size. Sample B (200 °C, 72 h) presents quite a different surface morphology compared to sample A. It shows fibrous nanostructures with bundled nanorods of length around 450-900 nm and width about 60-140 nm. Apart from these fibrous nanostructures, sample B also consists of spherical nanoparticles of 4-5 nm wide. SEM analysis indicates that the growth of rice grain-like nanostructure almost saturates for reaction duration of 48 h. However, with an increase in synthesis time to 72 h, the as-synthesized material shows different particle morphology. Indeed, the formation of two different kinds of nanostructures at the same synthesis condition can be expected due to the new growth mechanism activated at a long reaction time (72 h). However, the exact reason for the growth is not well understood at present. In samples C and D (250 °C, 24 and 48 h, respectively), hexagonal nanodiscs with clear facets and narrow size distribution are observed. The nanodiscs are about 250 nm long, 150 nm in width, and around 40 nm in thickness. An appreciable change in the shape of the nanodiscs is not observed for samples C and D synthesized at different hydrothermal treatment times, except for a slight width increase of about 7 nm of the nanodiscs in sample D. On the other hand, sample C shows the tendency to grow flat nanorods of about 130-300 nm in width, about 40 nm in thickness and 1.2-2.3 μm in length. EDS spectra (not shown) confirms that the ZrO2 nanostructures consist of only two elements, O and Zr, with approximate atomic concentration ratio of 2:1. From the SEM images, it can be concluded that at the hydrothermal treatment temperature of 250 °C, which is considered as the highest working temperature using a Teflon-lined autoclave, the morphology of the ZrO2 nanomaterials almost saturates at the reaction duration of 48 h. Hence, the synthesis duration higher than

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Figure 1. SEM images of ZrO2 nanostructures. (a) Sample A: 200 °C, 24 h; (b) sample B: 200 °C, 72 h; (c) sample C: 250 °C, 24 h; and (d) sample D: 250 °C, 48 h. Inset of (c) shows the growth of flat nanorods in sample C. The scale bar is 500 nm.

Figure 2. XRD profiles of (a) sample A, (b) sample B, (c) sample C, and (d) sample D. The diffraction peaks are indexed to the monoclinic phase of zirconia.

48 h at 250 °C is not expected to account for the formation of nanostructures with a well-defined shape and size. SEM analysis indicates that the hydrothermal synthesis conditions, such as temperature and time, play a very important role in the growth of various nanostructures. Our detailed studies have shown that a temperature of g200 °C and time of g24 h in the hydrothermal process are suitable for obtaining high-purity zirconia nanostructures.27 3.2. X-ray Diffraction Analysis. XRD profiles of ZrO2 nanostructures for samples A-D are shown in Figure 2. The diffraction peaks in the spectra are indexed as monoclinic (baddeleyite) ZrO2 with lattice constants, a = 0.5313 nm, b = 0.5213 nm, c = 0.5147 nm, and β = 99.22o and are in good agreement with those of the standard data (JCPDS 37-1484).28,29 Almost all the diffraction peaks are comparable with the standard data and are significantly sharp, hence indicating the purity and high crystallinity of the synthesized ZrO2 materials. The strongest diffraction peak at around 28.2° corresponds to the (111) plane. XRD patterns of sample A, sample C, and sample D repeat all the diffraction

peaks with similar characteristics, whereas sample B also shows all the diffraction peaks but with less intensity. It is noted that the peak at 17.5° corresponding to the (001) plane is missing for sample B, which does not have any clear explanation at present. The peak shift and decreased intensity of the diffraction peaks for sample B can be assigned to the presence of lattice defects or distortions or existence of zirconium suboxides. In the previous reports on ZrO2 samples synthesized by hydrothermal techniques, the as-synthesized products are of mainly tetragonal or cubic or mixed phases,26,28,29 and they attained monoclinic phase only after calcination or annealing at higher temperatures (>1000 °C).29 The formation of the nanostructured m-ZrO2 with single and pure crystalline phase as discussed in our work signifies the importance of the present synthesis technique in obtaining ZrO2 nanoparticles with a narrow size distribution, high purity, uniform composition, and high crystallinity.25 3.3. Structural Analysis by TEM. The structural information of the ZrO2 nanostructures is analyzed by TEM techniques. TEM images in Figure 3a for sample A indicates the formation of dumbbell-like grains (∼140 nm wide) with a tendency to grow into nanorods of about 20 nm in width at the edges. The HRTEM image (bottom inset of Figure 3a) was obtained at the region marked with a rectangular open box, and it shows a well-resolved lattice fringe pattern indicating the single-crystalline-like phase and high crystallinity of the synthesized material. The fringes are separated by 0.312 nm, which agrees well with the interplanar spacing (standard d-spacing, dstd ∼ 0.316 nm) of the (111) planes of m-ZrO2. The clear reflection spots in the SAED pattern (top inset of Figure 3a) for sample A are indexed to various planes corresponding to m-ZrO2, hence confirming the high purity and crystallinity of the synthesized ZrO2 material. Figure 3b represents the TEM image of sample B showing both the bundled spindle-like nanofibers (∼600 nm long) attached with the spherical nanoparticles (∼5 nm of diameter). The HRTEM image of the ZrO2 nanofiber (left inset of Figure 3b) and spherical nanoparticle

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Figure 4. XPS core-level spectrum of (a) Zr 3d and (b) O 1s for sample A through D. Inset in panel (a) represents the photoemission spectrum of Au 4f, which was used as the reference for XPS binding energy calibration.

Figure 3. TEM images of three samples. (a) Sample A, the top inset is the SAED pattern, and the bottom inset is the HRTEM image corresponding to the region marked by the open box; (b) sample B, the left inset is the HRTEM image of the fibrous nanorod, and the right inset is the HRTEM image of the spherical nanoparticle; (c) sample C, the left inset is the HRTEM image of the nanodisc recorded at the area marked by open box A, and the right inset is the HRTEM image of the nanorod obtained at the region indicated by open box B.

(right inset of Figure 3b) show well-resolved lattice fringes with a spacing of 0.322 and 0.315 nm for nanofiber and nanoparticle, respectively, suggesting the high crystallinity and purity of the synthesized nanomaterials. Figure 3c is the TEM image of the sample C, showing both the hexagonal nanodiscs (∼150 nm wide) and flat nanorods (∼140 nm wide and 620 nm long). The growth direction of the ZrO2 nanostructures follows along the [111] direction.30 HRTEM images of hexagonal nanodiscs (left inset of Figure 3c) and nanorods (right inset of Figure 3c) were obtained at the region indicated by open boxes A and B, respectively. These HRTEM images show well-resolved lattice fringes separated by 0.316 nm as shown in the insets of Figure 3c. Above results highlight the advantage of the present synthesis technique in forming nanostructures of

different shapes, sizes, and high crystallinity under slightly different conditions. 3.4. X-ray Photoelectron Spectroscopy. XPS measurements give information about the changes in the electronic states and chemical environment of Zr and O atoms in ZrO2. XPS spectra of the ZrO2 nanostructures synthesized at various hydrothermal reaction conditions were measured at room temperature. Figure 4a shows the Zr 3d photoemission spectra of the samples A through D, and the corresponding Zr 3d5/2 binding energies (BE) are 186.5, 186.6, 185.5, and 187.2 eV. The Zr 3d3/2 peak shows a BE maximum at around 188, 189.6, 189, and 189 eV for samples A, B, C, and D, respectively. The doublets of Zr 3d line shape, Zr 3d5/2 and Zr 3d3/2 peaks maintain a distance of about 2.5 eV between them.31 Inset of Figure 4a shows the Au 4f photoemission spectrum with Au 4f7/2 BE of 83.56 eV which is used as a calibration standard. Figure 4b shows the O 1s photoemission spectra of the samples A through D, which have BE values of 533, 532, 533.3, and 533.8 eV, respectively. From the photoemission core-level spectra of the Zr 3d and O 1s, it is evident that the ZrO2 nanostructures (samples A, B, C, and D) show a BE shift of around 2.5 eV as compared to that of pure ZrO2 (Zr 3d, B.E 182.9 eV, and O 1s, BE 531 eV).31 Earlier reports32-37 discussed that it was difficult to eliminate the effect of electrostatic charges completely because the zirconium oxides are not electronic but are an ionic conductor. Sometimes the increase in the BE of a core-electron is correlated not only to the change in the ionic valences induced by the chemical environment,32 but also to the change in the polarization and lattice energies for dielectric materials.33 As the charging effect induces energy shift, one can consider the zirconia film as the dielectric of a planar capacitor with a leak resistance.34,35 However, for

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samples with large film thicknesses, the capacitor charge would become important. Previous work by Okubo et al. reported a charge shift of 2.9 eV in the Zr 3d BE of a sol-gel ZrO2 film of 1 μm thickness,36 which is close to the energy shift observed in our samples. The BE shift of about 10 eV on to the higher energy side of XPS spectrum of nanocrystalline Y2O3-stabilized zirconia was also attributed to the charging effects.37 It has been noted from Figure 4a that the Zr 3d peak for samples C and D shift to higher BE and also show sharp features with enhanced intensity as compared to that of sample A, which can be related to the increasing particle size induced by increased synthesis temperature.34,35 The narrowing of Zr 3d peak indicates the more uniform nature of the zirconium chemical state in ZrO2.31 The Zr 3d photoemission spectra for samples A, B, C, and D show a shoulder on the low BE side which can correspond to the zirconium suboxide component (ZrOx, 0 < x < 2).31 The existence of suboxide component (with oxidation state less than þ 4) suggests the occurrence of some reduction in the ZrO2 sample, and this reduced suboxide state of zirconium is reported to have a Zr 3d5/2 BE at about 1.5 eV lower than that of ZrO2.31 The sample B shows quite different XPS features as compared to that of samples A, C, and D (Figure 4a). The Zr 3d photoemission spectra for sample B shows one prominent peak (187.3 eV) and two shoulders (184.7 and 189.6 eV) with reduced intensity and broad peak. The Zr 3d binding energy of 187.3 and 189.6 eV corresponds to doublet peaks of Zr 3d5/2 and Zr 3d3/2, respectively. The BE shift to higher energy and the shoulder peak on the lower binding energy side can arise due to the existence of suboxide of zirconium.31 The O 1s line shapes (Figure 4b) for samples C and D show a shift to a higher BE side with appreciable increase in intensity as compared to that of sample A. A shoulder on the high BE side (∼535 eV) is also observed for the O 1s peak which can be attributed to adsorbed oxygen38 or surface hydroxyl species and/or adsorbed water species present as contaminants at the surface.39 An earlier report by Galtayries et al.40 suggested that the decomposed O 1s peak with BE 530.1 eV, 531.7, 533.2, and 532.3 eV can be assigned to oxygen in the oxide network,39 oxygen attributed to hydroxyl surface species or defects at the surface,41 oxygen attributed to molecular water,41 and an oxide component from transition metal oxide at the interface, respectively. The oxidation rates for some transition metals, such as zirconium, depend on the rate of transport of O2- through the oxide samples,42 which occurs especially by the migration of oxygen vacancies. The suppression of O 1s line shape and existence of multi peaks (532 and 535.5 eV, respectively) for sample B with respect to the intense O 1s line shape observed for sample A (Figure 4b) can be related to the presence of surface oxygen defects and a new type of oxygen group. XPS results suggest the possible existence of surface defect states and disorders in sample B. These observations are in accordance with the surface morphology and XRD analysis of sample B, which indicated reduced crystallinity and contrast surface morphology, respectively. 3.5. UV-vis Absorption. UV-vis spectra were acquired in the wavelength region of 250-700 nm for samples A-D as shown in Figure 5a,b. In Figure 4a, optical absorption spectrum of sample A shows a strong and prominent absorption peak at around 288 nm (4.30 eV in photon energy), whereas the sample B has a weak and broad shoulder peak at about 290 nm but depicts strong absorption intensity

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Figure 5. UV-vis absorption spectra of zirconia nanostructures. (a) Sample A and sample B, inset (i) and (ii) represent the (Rhγ)2 vs E (eV) plot for sample A and sample B, respectively; and (b) sample C and sample D, inset shows the (Rhγ)2 vs E (eV) plot for sample C and sample D, respectively.

especially in the low wavelength region. The absorption peak in the low wavelength region can arise due to the valence band to conduction band transition.43 An absorption band near 290 nm at room temperature in the case of the monoclinic lattice can be attributed to the interstitial Zr3þ ions.44 Inset (i) and (ii) of Figure 5a shows (Rhγ)2 vs energy plot for sample A and sample B, respectively. The direct energy band gaps of 3.80 and 2.53 eV are obtained respectively for sample A and sample B. A drastic reduction in Eg for sample B should arise neither from a quantum size effect nor from crystal structure variation but rather from surface trap states or point defects,45 the existence of which is evident from the structural and XPS analysis. The enhanced absorption observed in the visible region (400-700 nm) for sample B can also be attributed to the presence of defect states. The UV-vis spectra of sample C and sample D show strong absorption peaks at 292 nm (corresponding to 4.25 eV in photon energy) and 290 nm (corresponding to 4.28 eV in photon energy), respectively, as presented in Figure 5b. However, the absorption peaks are at lower energy as compared to the reported optical band gap of 5.00 eV for bulk ZrO2.11 The result indicates that there is still contribution from extrinsic states toward the absorption in this region. Besides the strong absorption peak around 290 nm, a small shoulder at around 375 nm (3.30 eV in photon energy) and a broad and weak peak centered at around 500 nm (∼2.50 eV in photon energy) are also observed in the absorption spectra. The absorption spectrum features a weak absorption in the near UV and visible region, and this is most likely a result of transitions involving extrinsic states such as surface trap states or defect states, that is, oxygen vacancies or interstitials, Zr vacancies or interstitials.11 The

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Figure 6. PL spectra of (a) sample A and sample B, and (b) sample C and sample D obtained at excitation wavelengths of 290 and 300 nm, respectively.

large amount of surface defects can exist on the as-prepared ZrO2 nanostructures due to their high surface area.46 The direct band gap energies for sample C and sample D are determined by plotting (Rhγ)2 vs E (inset of Figure 5b) and are found to be 3.50 and 3.65 eV, respectively.45 The monoclinic nanophase zirconia synthesized by a solution route showed an absorption in the 250-350 nm range with estimated optical band gap of 3.6 eV,45 which is in close agreement with the results in the present work. A slight increase in Eg observed for sample D (synthesized for 42 h) can be attributed to the decrease of the defect states, which is related to the high synthesis time. A reduced energy band gap for sample C as compared to sample A synthesized for similar reaction duration at two different temperatures can be assigned to the quantum size effect.22 3.6. Photoluminescence. In general, emission spectra of metal oxides can be divided into two broad categories: the near-band-edge (NBE) UV emission and deep-level (DL) defect related visible emissions.43,47 Photoluminescence spectra were obtained in the wavelength range, 330-520 nm for ZrO2 nanostructures synthesized at various conditions. The PL spectra for sample A and sample B acquired with two excitation wavelengths, 290 and 300 nm, are shown in Figure 6a. For each sample, while the luminescence intensity changes appreciably with excitation wavelength, the luminescence peak position and band shape remained about the same for different excitations. This indicates that the luminescence involves the same initial and final states even though the excitation wavelength is varied. For sample A, the excitation wavelengths of 290 and 300 nm produce emission peaks at 385 and 387 nm, respectively, in the UV region at room temperature. This result indicates that the PL emission comes from the zirconia nanomaterials and not from other impurities as the UV emission can be related to the transitions involving free excitons. The observed PL

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emission peak of ZrO2 nanostructures at 387 nm is in good agreement with that of ZrO2 nanoparticles.11 Furthermore, the photoluminescence excitation is in agreement with the UV-vis absorption (λmax ∼ 290 nm). A broad emission band observed for zirconia nanomaterials at room temperature depicting the violet emission can be attributed to the ionized oxygen vacancy in the material. The PL spectra of sample B excited at a wavelength of 290 and 300 nm give rise to emission peaks at 402 and 404 nm, respectively, at the interface of the UV and visible regions corresponding to violet emission. PL spectra of sample B shows enhanced intensity as compared to that of sample A. The increase in the intensity and red shift of UV emission band for sample B at room temperature can be attributed to the existence of defect states or surface trap states which is in tune with the structural analysis and UVvis absorption,43 where the structural analysis revealed the presence of impurities and, the significant decrease in the optical band gap confirmed the existence of defect states. There are oxygen vacancies in ZrO2 crystals, and the oxygen vacancies can induce the formation of new energy levels in the band gap region. The UV emission in zirconia nanomaterials can be correlated to the NBE transitions and attributed to the high crystal quality of the sample. The broad emission band which also extends to the visible region is attributed to the singly ionized oxygen vacancies in ZrO2 nanomaterials, and the emission results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy. The main source of defects centers or point defects is oxygen vacancies/interstitial, Zr vacancies/interstitials, which are expected to exist on the surface of ZrO2 nanostructures due to their high surface area, given to their low dimensional structure.46 However, the presence of zirconium suboxides and defect states in the ZrO2 samples have been confirmed by XPS analysis. Figure 6b shows the PL emission spectra of sample C and sample D. The emission spectra centered at 410 and 414 nm were obtained when the sample C was excited with light of 290 and 300 nm wavelength, respectively. Even though the PL spectra exhibit a small change in the intensity and peak position at different excitation wavelengths, the band shape is still retained. For sample D, the excitation wavelength of 290 and 300 nm, respectively, produces emission peaks at 407 and 408 nm, which is in the vicinity of the visible region with the peak assigned to violet emission. The high intensity and red shift of PL band for sample C as compared to that of sample D at different excitation wavelengths can be due to the presence of surface defect states, such as oxygen vacancies. This result can also be related to the short synthesis time for sample C (250 °C, 24 h) as compared to sample D (250 °C, 48 h). The shifting of emission band to longer wavelength region in samples C and D as compared to those of samples A and B is sometimes also attributed to the quantum size effects originating from the reduced particle size in samples C and D. High crystal quality and quantum confinement in the nanostructures are two factors favoring the increase of the intensity of UV emission at room temperature. The wideband gap ZrO2 nanostructures with short-wavelength PL emission can find application in light emitting devices. 4. Conclusions In summary, ZrO2 nanostructures were synthesized by a hydrothermal route using zirconium salt as the starting

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material. Surface morphology analysis confirms the synthesis of various zirconia nanostructures at different hydrothermal treatment conditions. XRD, HRTEM, and SAED studies show that the synthesized ZrO2 material is of pure monoclinic phase. The present technique provides an efficient route for the synthesis of m-ZrO2 nanostructures with controlled morphology, high purity, uniform composition, and high crystallinity. UV-vis spectra for various zirconia nanostructures show a sharp absorption peak centered at about 290 nm; energy band gaps of 2.5-3.8 eV are determined from the absorption spectra. PL spectra show broad emission peaks at the interface of UV and visible regions (violet emission) which can be assigned to the ionized oxygen vacancy in the material. ZrO2 nanostructures with wide-band gap and short-wavelength luminescence emission can serve as a better luminescent material for photonic applications. XPS core-level spectra of Zr 3d and O 1s reveal the effect of synthesis conditions on the electronic states and chemical environment of Zr and O atoms in ZrO2. XPS results also indicate the presence of zirconia suboxides and oxygen vacancies/defects in the zirconia nanostructures. Acknowledgment. W.Z.L. acknowledges the support by the National Science Foundation under grant DMR0548061. J.Z. and W.Z.L. thank the support by the US DOD Grant No. W911NF-07-1-0532.

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