Rational Hydrothermal Route to Monodisperse Hf - American

Technology, Meiji UniVersity, 1-1-1 Higashimita, Tama-ku, Kawasaki 214-8571, Japan. ReceiVed: NoVember 7, 2007; In Final Form: January 16, 2008...
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J. Phys. Chem. C 2008, 112, 4884-4891

Rational Hydrothermal Route to Monodisperse Hf1-xEuxO2-x/2 Solid Solution Nanocrystals Takaaki Taniguchi,*,†,‡ Naonori Sakamoto,§ Tomoaki Watanabe,| Nobuhiro Matsushita,† and Masahiro Yoshimura† Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Laboratoire de Physico-Chimie des Mate´ riaux Luminescents-UMR 5620, UniVersite´ Claude Bernard Lyon 1, 10 rue Ampe` re, Villeurbanne 69622, France, Faculty of Engineering, Shizuoka UniVersity, 3-5-1 Johoku, Hamamatsu 432-8561, Japan, and Department of Applied Chemistry, School of Science and Technology, Meiji UniVersity, 1-1-1 Higashimita, Tama-ku, Kawasaki 214-8571, Japan ReceiVed: NoVember 7, 2007; In Final Form: January 16, 2008

Highly dispersed and nearly uniformly sized Hf1-xEuxO2-x/2 (x ) 0.05, 0.1, and 0.15) solid solution nanoparticles with particle sizes between 2.2 and 5.2 nm were produced via the hydrothermal treatment of a metal-oleate complex precursor at 200 °C for 6 h. The obtained nanocrystals were characterized by IR, EDX, XRD, Raman scattering, TEM, and PL spectroscopy. It was demonstrated that the proposed synthetic technique led to the formation of a Hf1-xEuxO2-x/2 solid solution with a thermodynamically metastable phase (tetragonal HfO2 phase). Pure HfO2 nanocrystals were rod-like in shape with a monoclinic phase, while the doping of HfO2 by Eu3+ resulted in the nearly spherically shaped nanocrystals. The PL spectroscopy showed significant quenching of Eu3+ 5Do f 7Fn radiative transitions for the as-prepared samples, whereas the annealed sample at 400 °C for 5 h showed clear red luminescence.

1. Introduction Current intensive research on the synthesis of inorganic nanoparticles is motivated not only by interests in fundamental science but also by the potential applications of nanoparticles in industry.1,2 Initial material studies of monodispersed nanoparticles focused on the exploration of the synthesis and structural and physical properties of nanoparticles based on CdSe and related II-VI semiconductors that already have found numerous applications in the area of electronics and optics.3,4 Recently, metal oxide nanoparticles with a uniform shape and narrow size distribution also have found interesting uses in a wide range of applications, such as chemiresistors,5 catalysis,6 electroceramics,7 and biological applications.8 To tailor their physical and chemical properties, the doping of metal oxides with various metal ions plays a significant role due to the large influence on the crystal structure and electronic properties. For example, doped oxides such as CGO (Gd3+ doped CeO2),9 YSZ (Y2O3 doped ZrO2),10 and ITO (SnO2 doped In2O3)11 have shown excellent chemical and/or physical properties and are applied in industry. Other than the previously mentioned oxides, Hf1-xEuxO2-x/2 is an attractive solid solution oxide material for its potential use in advanced electronic and optical applications. HfO2 is considered to be a promising socalled high-k gate dielectric material (k is the relative dielectric constant) for the replacement of SiO2 and SiON in the metal oxide-semiconductor field effect transistor (MOSFET).12 Although the monoclinic phase with lowest k value is thermodynamically stable up to 1720 K for pure HfO2, doping HfO2 with * Corresponding author. Tel.: +33-(0)4-72-43-12-15; [email protected]. † Tokyo Institute of Technology. ‡ Universite ´ Claude Bernard Lyon 1. § Shizuoka University. | Meiji University.

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an appropriate amount of M3+ can stabilize the tetragonal or cubic phases with higher k values at ambient conditions;13,14 the theoretically predicted k value is ∼16, ∼70, and ∼29 for monoclinic, tetragonal, and cubic HfO2, respectively.15 Another interesting feature relating to the preparation of doped HfO2 is the high refractive index and low phonon energy of HfO2-based materials, which exhibit great potential for photonic applications such as planer waveguides16,17 and phosphors.18-20 A number of chemical approaches such as sol-gel,21 polymer-based,22 flame,23 and hydrothermal methods24 have been employed for the preparation of doped metal oxide nanoparticles. However, the resulting nanoparticles are usually agglomerated in the sub-micrometer size range, and they also are not particularly dispersible in most solvents due to the high surface energy of the nanosized particles. To overcome this drawback, thermal decomposition methods have been studied intensively. These methods can yield surfactant coated metal oxide nanoparticles by non-hydrolytic condensation of metalsalt and/or metal-surfactant complexes in an organic medium. However, the precise control of the chemical composition is still beyond the capability of these routes. For example, the final chemical compositions (Zr/Ti ratios) of the ZrO2-TiO2 solid solution synthesized by the thermal decomposition method significantly differ from those of the precursors, owing to the higher reactivity of titanium alkoxide and tetrachloride than the Zr reactants.25 In particular, the synthesis of hetero-valence ion doped oxide nanoparticles with sizes less than 10 nm has been very difficult so far. Therefore, the exploration of a novel synthetic route to non-agglomerated metal oxide nanoparticles with a controlled size distribution and tailored chemical composition remains an extremely important task. In the present study, we designed and demonstrated a novel hydrolytic route, wherein a metal-oleate complex precursor was subjected to hydrothermal treatment for the preparation of Hf1-xEuxO2-x/2 nanoparticles with controlled chemical composi-

10.1021/jp710673y CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

Hf1-xEuxO2-x/2 Solid Solution Nanocrystals

Figure 1. Experimental flowchart of hydrothermal synthesis.

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Figure 2. Infrared spectra of the nanocrystalline 0%Eu/HfO2 sample (a) and hafnium oleate precursor (b).

tions. Detailed characterizations by EDX, XRD, Raman scattering, TEM, and IR spectroscopy show that the proposed method successfully yields oleate-stabilized Hf1-xEuxO2-x/2 nanoparticles (x ) 0.05, 0.10, and 0.15) with designed Hf/Eu ratios. Also notable is the utilization of inexpensive and less toxic aqueous solutions in all synthetic steps throughout the procedure, which makes the proposed method even more attractive for application in industry. 2. Experimental Procedures 2.1. Starting Materials. HfOCl2‚8H2O (Daiichi Kigenso Kagaku Kogyo, 99.5%), EuCl3‚6H2O (Aldrich, 99.9%), sodium oleate C17H33COONa (Wako, analytical grade), ammonia 25 wt. % aqueous solution (Wako), and acetone (Wako, 99.5%) were used as received. Distilled water was utilized for the preparation of the aqueous solutions and for washing. 2.2. Hydrothermal Syntheses of Pure and Eu3+ Doped HfO2 Nanoparticles. Un-doped/Eu3+ doped HfO2 nanoparticles were prepared hydrothermally (shown schematically in Figure 1). First, hafnium dichloride oxide octahydrate (3.5 mmol) was dissolved in distilled water (12.5 mL). Then, the sodium oleate (3.5 mmol dissolved in 12.5 mL of distilled water) was added to this solution under vigorous stirring. Immediately following this addition, a white suspension based on the hafnium-oleate complex was formed. Finally, the resulting suspension along with 10 mL of ammonia (25 wt. % aqueous solution) was placed in a polytetrafluoroethylene (PTFE) Teflon vessel (volume 40 cm3), and the vessel was capped by a PTFE Teflon cover. The vessel was placed inside a stainless steel autoclave. The autoclave was sealed and kept at 200 °C for 6 h under autogenous pressure. Mixed precursors of HfOCl2‚8H2O and EuCl3‚6H2O with Eu/ (Hf + Eu) equal to 5.0, 10.0, and 15.0% (total 3.5 mmol) were used to synthesize the Eu doped HfO2 with a Eu content of 5.0, 10.0, and 15.0 mol %, respectively. The other experimental procedures were kept, and the conditions were unchanged. (Throughout this work, a set of acronyms is used: 0%Eu/HfO2, 5%Eu/HfO2, 10%Eu/HfO2, and 15%Eu/HfO2. The numbers indicate the mol % of europium content in the nanoparticles.) The product of the hydrothermal process was collected by centrifugation (at 5000 rpm for 30 min), washed twice with distilled water, and then dried at 150 °C for 6 h in air. The as-produced nanocrystals were dispersible in various non-polar organic solvents, such as benzene, hexane, toluene, etc.

Figure 3. X-ray powder diffraction patterns of pure and Eu3+ doped hafnia nanoparticles synthesized by hydrothermal methods. Tick marks below the patterns correspond to the positions of the Bragg reflections expected for the monoclinic (red) and tetragonal (blue) HfO2 (JCPDS nos. 34-104 and 8-342, respectively).

To investigate the influence of a dopant (Eu3+) on the phase composition of hafnia, heat treatment of the synthesized Eu3+ doped HfO2 nanoparticles was carried out at 400, 600, 800, and 1000 °C in air. The treatment involved 2 h of heating up to the final temperature and 5 h of isothermal annealing. 2.3. Characterization. The crystalline products were characterized by powder XRD using a Rigaku RINT 2000 diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54178 Å). Data were collected in the 2θ range of 20-80°, with a scan speed of 0.5°/min and a 0.02° step width. The size of the hafnia crystallites was calculated by means of the Debye-Scherrer formula from the broadening of the (200) XRD reflection after KR2 correction. The average contents of the hafnium and europium were quantified by EDX using a Hitachi S-4500 microscope operating at 15 kV. The collected spectra were analyzed using the Quantex software program. TEM, ED, and HRTEM investigations were performed using a Hitachi H 9000 NAR microscope operating at 300 kV. For TEM measurements, one drop of the colloidal nanoparticle solution in benzene (Wako, 99.5%) was deposited onto a holey

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Figure 4. Volume fraction of monoclinic phase (Vm) in the Hf1-xEuxO2-x/2 solid solution nanoparticles (x ) 0.05, 0.1, and 0.15) as a function of the temperatures of calcination for 5 h.

Taniguchi et al.

Figure 6. Angular regions of the powder X-ray diffraction patterns of 5%Eu/HfO2 and 15%Eu/HfO2 samples measured with Al (cubic, a ) 4.0494 Å) as an internal standard (the peak of standard shown by the asterisk).

The PL spectra of the dried samples were measured using a PerkinElmer LS 55 spectrofluorometer with a Xe lamp. To analyze the surface effect on the luminescence properties of hydrothermally prepared Hf1-xEuxO2-x/2 nanocrystals, acetone washed samples were also investigated by PL spectroscopy. For this, hydrothermally prepared nanoparticles were repeatedly washed with acetone and dried at 150 °C for 6 h. 3. Results and Discussion

Figure 5. X-ray powder diffraction patterns from the sample 10%Eu/ HfO2 annealed at 400, 600, 800, and 1000 °C for 5 h in air. Tick marks below the patterns correspond to the positions of the Bragg reflections expected for the monoclinic (red) and tetragonal (blue) HfO2 (JCPDS nos. 34-104 and 8-342, respectively).

carbon grid. Calculated electron diffraction patterns by fast Fourier transform (FFT) were obtained using the ImageJ software program. Raman spectroscopy was carried out on a Jobin Yvon T64000 spectrometer with a visible laser (λ ) 514.5 nm) as the excitation wavelength at room temperature. The resolution was 1 cm-1. The scattered light was collected in the back-scattering geometry using a liquid nitrogen-cooled charge-coupled device (CCD) detector. All measurements were carried out under a microscope (the laser spot diameter was estimated to be between 1 and 2 µm). The system was operated with an output power of 50 mW. The room-temperature diffuse reflectance IR Fourier transform spectra were recorded on a Jeol JIR-7000 spectrometer with a resolution of 4 cm-1 and scans of 10 times. The precursors (metal-oleate complexes) as well as crystalline products (4 mg) were mechanically ground with potassium bromide powder (200 mg, KBr for IR, Wako) in a mortar and pestle to a fine consistency and subjected to IR analysis. For the background spectrum, finely ground KBr powder was used.

3.1. Synthesis. A novel hydrolytic pathway was investigated to synthesize monodisperse Hf1-xEuxO2-x/2 solid solution nanoparticles. Although the idea of the proposed approach is generally based on the non-hydrolytic thermal decompositioncondensation of a metal-oleate complex to form oleatestabilized metal oxide nanoparticles, a hydrolytic condensation of the metal-oleate complex was employed in the current study. The advantages of the proposed technique are a high purity and inexpensive precursors, molecular-scale mixing of the components in aqueous medium, as well as control of the coprecipitation process of the several metal sources via pH, which can lead to the homogeneity of the hydrothermal products with a high isotropy of physical, morphological, and chemical properties. The three-step reaction was designed as follows: (1) In the first step, Hf/Eu-oleate complexes were formed by the addition of C17H33COONa to a Hf/Eu aqueous solution by partial exchange of the coordinated ligand with oleate. (2) Subsequently, Hf/Eu-oleate complexes were hydrolyzed and condensed through double hydroxy bridges, -M-(OH)2-M- (M ) Hf or Eu), by the addition of a base. An extended network containing uniformly mixed Eu3+ and Hf4+ species on the atomic level was formed during the condensation process. (3) Finally, hydrothermal treatment led to the formation of Hf1-xEuxO2-x/2 by means of an oxolation resulting in -MO-M- bridges. During condensation, nucleation, crystallization, and crystal growth, the products were dispersed by oleate located at the terminal metal centers. The colloidal stability in benzene supported this strategy; the resultant nanoparticles easily could be dispersed to be transparent colloidal sols in non-polar solvents due to their hydrophobic nature. This observation shows that the hydrolytic condensation of the metal-oleate complex in the aqueous solution could lead to the formation of oleate-stabilized nanoparticles. IR spectroscopy was performed to investigate the interaction of oleate with the metal centers in the Hf-oleate complex and at the

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Figure 7. Raman spectra of the hydrothermally produced and subsequently annealed at 400 °C for 5 h 0%Eu/HfO2 (a), 5%Eu/HfO2 (b), 10%Eu/HfO2 (c), and 15%Eu/HfO2 (d) and at 800 °C for 5 h 15%Eu/HfO2 (e) samples. The peaks labeled as T and M correspond to the possible Raman bands for the tetragonal and monoclinic HfO2 phase, respectively, in 15%Eu/HfO2 annealed at 800 °C for 5 h.

nanocrystalline surface. Figure 2 shows IR spectra for the hafnium-oleic complex and hydrothermally prepared HfO2 nanocrystals. In both spectra, typical bands corresponding to the carboxylic chain of oleate clearly were observed.26 A series of bands in the region 2840-2970 cm-1 are attributable to the symmetric and asymmetric stretching of the CH2 groups and the terminal CH3 group. In addition, a band attributable to the stretching of the olefinic CH group was observed around 3006 cm-1. In the 1200-1750 cm-1 region, a strong band in the vicinity of 1458 cm-1, indicative of CH2 deformation, was detected. The remaining bands in the 1000-2000 cm-1 region give information about the terminal carboxyl group in oleic acid/ oleate. In both spectra, a band at 1556 cm-1 attributed to the asymmetric COO- stretching was clearly detected, whereas the peak around 1710-1780 cm-1 corresponding to the CdO stretch shows a weak intensity. This observation indicates that the chemisorbed oleate effectively prevents the agglomeration of intermediate compounds during the condensation process and subsequently plays a surfactant role on the surface of the hydrothermally prepared nanoparticles. We note that a broad adsorption corresponding to an OH stretching band was also observed at 3500 cm-1 for the IR spectra of HfO2 nanoparticles, indicating that the particles were not covered fully by oleate and that hydroxyl ligands also are located at the nanocrystalline surface. The EDX analysis supports the hypothesis that the present method allows for the precise control of the atomic ratio of Hf/Eu. The average atomic ratios of europium to hafnium were established to be ∼0.063 for sample 5%Eu/HfO2, ∼0.103 for sample 10%Eu/HfO2, and ∼0.144 for sample 15%Eu/HfO2. This is in good agreement with the initial molar ratios used in the hydrothermal synthesis. 3.2. Structural Studies. Powder XRD was performed to confirm the formation of the solid solution, Hf1-xEuxO2-x/2. The XRD patterns from the set of samples prepared by hydrothermal treatment of a metal-oleate complex precursor at 200 °C for 6 h are presented in Figure 3. All samples show typical XRD patterns for nanosized crystallites with low intensities and broad peaks. According to the XRD data from the un-doped HfO2 sample, 0%Eu/HfO2, the visible peaks belong to the monoclinic HfO2 phase (JCPDS no. 34-104). In contrast, the XRD patterns for all the Eu3+ doped samples correspond to tetragonal (JCPDS no. 8-342) or cubic HfO2 phases;14 the small admixture of a monoclinic modification was also detected by XRD for the

Figure 8. TEM images of the 0%Eu/HfO2 sample prepared by hydrothermal methods: (a) low-magnification TEM image, (b) typical ED pattern, and (c) representative HRTEM image.

5%Eu/HfO2 sample. Peaks related to the presence of separate Eu-containing phases such as Eu2O3 or Eu(OH)3 were not detected for the Eu3+ doped samples. The metastable modifications (tetragonal or cubic phases) of Eu3+ doped HfO2 samples were stable at moderate conditions for at least 1 year but were transformed to the thermodynamically stable monoclinic phase by post-calcination. Figure 4 shows the estimated volume fractions of the monoclinic phase, Vm, at different calcination temperatures and concentrations of Eu3+. This value was obtained by the following equations:27

Vm ) 1.31Xm/(1 + 0.31Xm)

(1)

Xm ) [Im(-111) + Im(111)]/[It(111) + Im(-111) + Im(111)] (2) where It(111), Im(111), and Im(-111) represent the integrated intensity of the tetragonal (111) and monoclinic [(111) and (-111)] peaks observed in XRD patterns for the doped samples annealed under certain conditions (see Figure 5 as well as Figures S1 and S2). In this analysis, all the reflections were assumed as tetragonal or monoclinic phases since it is not possible to conclusively identify the structure as tetragonal or cubic from the broad reflections. As can be seen clearly in Figure 4, the volume fractions of the monoclinic phase systematically

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Figure 9. TEM images of the 10%Eu/HfO2 sample prepared by hydrothermal methods: (a) low-magnification TEM image, (b) typical ED pattern, (c) representative HRTEM image, and (d) corresponding FFT pattern of the HRTEM image.

decreased with increasing the Eu3+ concentration. The metastable phase in samples 10%Eu/HfO2 and 15%Eu/HfO2 could be maintained up to calcinations temperatures of 600 and 800 °C, respectively. This tendency (the enhancement of the thermal stability of the metastable phase with increasing dopant concentration) has been commonly observed for HfO2- and ZrO2-based metastable solid solutions.14,28 It is notable that the tetragonal ZrO2 and HfO2 phases have sometimes appeared as metastable phases without doping the divalent or trivalent metal ions;29,30 this is due to the lower surface free energy of the tetragonal or cubic polymorphs as compared to that of the monoclinic polymorph.31 However, the thermal stability of the tetragonal phase of pure HfO2 is rather poor. According to the literature, Vm was established to be approximately 30% after annealing of the tetragonal HfO2 nanocrystals at 600 °C for 1 h.30 This value is higher than that of the 5%Eu/HfO2 sample (13.6%), where even a longer calcination time of 5 h was applied. This result shows a remarkable thermal stability of doped HfO2 nanoparticles prepared in the current study. At this point, the formation of a solid solution based on the tetragonal and/or cubic HfO2 structure through partial substitution of Hf by Eu in the HfO2 matrix is the most probable scenario. This possibility could be tested further by careful analysis of XRD data. The tetragonal symmetry of the space

Taniguchi et al. group P42/nmc (No. 137) was assumed for all doped samples. To improve the accuracy of the phase analysis, an internal standard, Al (cubic, a ) 4.0494 Å), was used. The ionic radius of Eu3+ in an octahedral environment is 0.95 Å, while the ionic radius of Hf4+ in an octahedral environment is 0.78 Å. Therefore, an increase of the unit cell volume would be expected when the Hf4+ sites are partially substituted by Eu3+. This is demonstrated in Figure 6, where the (220) and (311) diffraction peaks of tetragonal hafnia are shown for the Eu3+ doped samples. It can be seen in Figure 6 that the (220) and (311) peaks for the 15%Eu/HfO2 sample are slightly shifted to lower angles as compared to the 5%Eu/HfO2 sample, indicating a larger d-spacing in sample 15%Eu/HfO2 than that in sample 5%Eu/HfO2. The incorporation of Eu3+ in the structure of hafnia should result in small distortions of the MO6 octahedra and thus lead to the aforementioned changes in d-spacing. The influence of doping concentration on grain size also was detected. The average crystallite sizes calculated using the Debye-Scherrer formula were found to be 5.1 nm for sample 5%Eu/HfO2, 4.2 nm for sample 10%Eu/HfO2, and 2.3 nm for sample 15%Eu/HfO2. The crystallite size of Eu doped HfO2 samples decreased from 5.1 to 2.3 nm with a nominal increasing of europium content. The decrease in particle size may be attributed to an increase in the number of Eu-O-Hf bonds in the doped samples, suppressing the growth of crystal grains. This, in fact, has been studied for various metal oxide systems doped with hetero-elements.9,32,33 The detectable changes in the thermal stability of the metastable phase, the lattice parameters, and the grain sizes of the resultant nanocrystals with increasing Eu3+ concentration strongly indicate the formation of a solid solution rather than physical mixtures of pure HfO2 and Eubased phases. However, distinguishing between the metastable tetragonal and the cubic phases from the XRD patterns of the nanocrystalline samples is difficult. Although the tetragonal and cubic phases can be distinguished from the splitting of the cubic Bragg index (400) into tetragonal (400)/(004) for the bulk samples,34 the tetragonal distortion was not detectable because of significant line broadening due to the grain size effect. To better understand the crystalline phases of hydrothermally produced Hf1-xEuxO2-x/2 nanocrystals, the samples were furthermore investigated by Raman spectroscopy. This technique can detect distortion of the oxygen sub-lattice in the tetragonal phase due to its high sensitivity to polarized oxygen ions.34 According to the selection rules, the monoclinic (P21/c) and tetragonal phase (P42/nmc) should have 18 (9Ag + 9Bg) and six (1Ag + 2Bg + 3Eg) Raman active modes, respectively, while the cubic phase (Fm3m) has only one Raman active mode (T2g). To remove background luminescence coming from adsorbed organic molecules (vide infra), samples were annealed at 400 °C for 5 h prior to Raman investigation. It should be noted that this procedure did not lead to any significant changes in phase composition and crystalline size, as determined by XRD data (Figures 5, S1, and S2). Figure 7 shows the Raman spectra from annealed 0%Eu/ HfO2, 5%Eu/HfO2, 10%Eu/HfO2, and 15%Eu/HfO2 samples as well as one spectrum from the 15%Eu/HfO2 sample annealed at 800 °C for 5 h. The bands observed in the spectrum of sample 0%Eu/HfO2 can be assigned to the (9Ag + 9Bg) modes of monoclinic HfO2,35 supporting the result of XRD analysis. Although XRD showed that the tetragonal or cubic phase was dominant in the 5%Eu/HfO2 sample, bands corresponding to the monoclinic phase were strongly detected in the Raman spectra for 5%Eu/HfO2. On the other hand, two broad bands around 450 and 600 cm-1 were detected in the spectra of

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Figure 10. TEM images with typical ED patterns of the (a) 5%Eu/HfO2 and (b) 15%Eu/HfO2 samples prepared by hydrothermal methods.

10%Eu/HfO2 and 15%Eu/HfO2. These Raman bands were much broader than those for bulk HfO2-Y2O3 reported by Fujimori,34 likely due to the size effect that commonly is observed in the Raman spectra of oxide nanocrystals.36,37 Additionally, we measured the Raman scattering of the 15%Eu/ HfO2 sample annealed at 800 °C to assist in the assignment of the two broad bands observed in 10%Eu/HfO2 and 15%Eu/ HfO2. As shown in Figure 7e, three distinctive Raman bands, labeled as T1, T2, and T3, were detected at around 240, 480, and 680 cm-1, respectively. The peak positions, bands, and shapes of these bands are in close agreement with the characteristic Raman bands seen in tetragonal Y3+ doped HfO2;34 therefore, it would be expected that the 15%Eu/ HfO2 sample annealed at 800 °C should exhibit a tetragonal phase rather than a cubic one. Notably, the position of two detectable bands in the Raman spectra of the 10%Eu/HfO2 and 15%Eu/HfO2 samples annealed at 400 °C are very close to the T2 and T3 bands. This suggests that the tetragonal phase corresponds to the dominant crystalline phase for hydrothermally prepared Hf1-xEuxO2-x/2 nanocrystals. Similar tendencies supporting our interpretation of the Raman spectra have been reported by Gosh et al., where distinct Raman bands corresponding to tetragonal ZrO2-Y2O3 were detected after sintering, whereas the nanocrystalline samples predominantly exhibit two Raman bands at around 450 and 630 cm-1.38 3.3. Microscopic Studies. Figure 8 shows TEM images of the 0%Eu/HfO2 sample obtained by hydrothermal treatment of a hafnium-oleate complex precursor. Low-magnification TEM observation confirmed the formation of crystalline nanoparticles with a branched rod-like shape (Figure 8a). Electron diffraction studies from the 0%Eu/HfO2 sample show a distinct ring pattern, typical for a nanocrystal clustering (Figure 8b).

The rings of the ED pattern can be fully indexed in the monoclinic P21/a space group, using the HfO2 unit cell parameters of JCPDS no. 34-104. The ED analysis for the phase composition is in good agreement with the data obtained by XRD and Raman scattering. The lattice spacing value of 2.8 Å in the high-resolution TEM image (Figure 8c) was in close accordance with the (111) lattice plane of the monoclinic HfO2 phase. Figure 9 shows the TEM data for a 10%Eu/HfO2 sample. From the low-magnification micrograph, Figure 9a, one can observe that the synthesized particles appear quasi-spherical in shape and are nearly monodisperse. From a detailed particle size analysis of 100 particles (see Figure S3), the overall particle size was found to be 4.3 nm. The line-broadening analysis of the crystallite size (4.2 nm) is in good agreement with the data obtained by TEM (4.3 nm). The particles should, therefore, be considered as a single domain. The electron diffraction pattern (Figure 9b) from the 10%Eu/HfO2 sample closely resembles the tetragonal HfO2 patterns. At the same time, neither of the rings corresponding to secondary Eu-related phases nor the monoclinic HfO2 phase are present, supporting a solid solution formation. From the HRTEM image (Figure 9c), one can clearly distinguish lattice fringes on the individual particle, indicating that the particle is highly crystalline. The surface of this nanocrystal was found to be free of any amorphous or secondary phase. The corresponding Fourier transform pattern of the single nanoparticle is shown in Figure 9d. This pattern clearly indicates the single-crystalline nature of the isolated nanoparticle, which was subsequently confirmed to be a common feature of all nanocrystals observed in the sample. The samples 5%Eu/HfO2 and 15%Eu/HfO2 also show a single-crystalline/phase nature. Figure 10 shows low-magnification TEM images of the

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Figure 11. PL spectra (excited at 265 nm) for hydrothermally prepared 10%Eu/HfO2 nanoparticles without calcination.

Figure 12. PL spectra (excited at 265 nm) and PL excitation spectra (monitored at 611 nm) for 10%Eu/HfO2 calcined at 400 °C for 5 h.

aforementioned samples. The average particle sizes of samples 5%Eu/HfO2 and 15%Eu/HfO2 calculated from HRTEM image are 5.2 and 2.2 nm (see Figure S3), in good agreement with the crystallite sizes calculated form XRD data, 5.1 and 2.3 nm, respectively. Indeed, ED rings are indexed to the same position as those from the high-temperature phase HfO2. Interestingly, TEM observations revealed a concomitant change in the crystalline phase and shape of HfO2 nanocrystals doped with Eu3+. Tang et al. explain that the spherical to rodlike transition of the isolated ZrO2-HfO2 binary nanocrystal morphology in hydrolytic sol-gel reactions is related to a monoclinic-tetragonal phase transition.30 In the current case, doping HfO2 with Eu3+ should decrease the driving force for the tetragonal to monoclinic phase transition, which leads to inhibition of the spherical to rod-like morphological transition of the nanocrystals accompanying the phase transition. As well as the metastable phase of the Eu3+ doped nanocrystals, stabilization of the spherical morphology by doping with Eu3+ should be beneficial for the scope of their application. Spherical nanoparticles are considered to be preferable for the fabrication of dense films, composites, and ceramics due to the high packing density. 3.4. Luminescence Properties. Figure 11 shows the roomtemperature PL spectra of hydrothermally synthesized 10%Eu/ HfO2 powders: (a) as-prepared and (b) washed by acetone. The bands at 590, 613.5, and 696.5 nm assigned to the 5D0 to 7F1, 7F , and 7F transitions can be observed in the PL spectra. 2 4

Taniguchi et al. Additionally, a broad luminescence band covering the UV to visible region is also detected. This band is strongly linked to the surface state of the nanoparticles because the washing procedure has obviously altered the band features. Previous studies report that the defects in HfO2 and ZrO2 show broad luminescence in this range,39-41 indicating that the detected broad band originates from electron-hole recombination via a surface defect center. Strong and sharp bands corresponding to 5D f 7F (n ) 1-5) transitions are detected in the PL spectra 0 n for the sample annealed at 400 °C for 5 h (see Figure 12). The PL excitation spectrum at a monitored wavelength of 611 nm consists of some lines in the range of 350-500 nm, corresponding to the characteristic excitation bands of Eu3+ from the direct transitions within the 4f6 shell of Eu3+.42 The excitation spectrum also shows a broad band in the UV range. This band corresponds to a charge transfer from the coordination anions (O2-) to the rare-earth ions (Eu3+) and closely resembles the spectrum of tetragonal ZrO2 due to the analogous crystal and electronic band structures of these two oxides.15,43 We note that a lower emission intensity by a factor of 20 is observed in the as-prepared sample as compared to that of the sample annealed at 400 °C. Since the crystalline phase and grain size show no remarkable change after annealing at 400 °C based on XRD characterization, it can be concluded that the overall textural properties are not the origin of the quenching of 5D0 f 7Fn radiative transition in the as-prepared nanoparticles. Owing to the tiny dimensions of the Hf1-xEuxO2-x/2 nanoparticles, it is possible to speculate that surface states would play a considerable role in luminescence. The surface defect state and the adsorbates (OH and oleate) detected by PL and IR spectroscopy, respectively, and their large surface to volume ratio can give rise to many non-radiative pathways of the nanoparticles. 4. Conclusion In this work, we reported a novel hydrothermal route to monodispersed Hf1-xEuxO2-x/2 nanocrystals by hydrothermal methods at 200 °C using a metal-oleate complex as a precursor. Structural properties investigated by XRD and Raman spectroscopy revealed that the doping of Eu3+ into HfO2 stabilized a metastable tetragonal phase at ambient temperature at least up to 15 mol % Eu3+ concentration. HRTEM observation revealed that doping HfO2 with Eu3+ resulted in a concomitant change in the crystalline phase and shape of the resultant nanoparticles. Although monoclinic HfO2 nanoparticles are branched rod-like shapes, Hf1-xEuxO2-x/2 nanocrystals exhibit a rather spherical shape with a tetragonal form. The size of the resultant Hf1-xEuxO2-x/2 nanocrystals was decreased from 5.2 to 2.2 nm when the Eu3+ concentration was varied from 5 to 15 mol %. Significant quenching of the PL properties was observed in the as-prepared samples, whereas the sample annealed at 400 °C showed strong luminescence from 5D0 f 7F transitions. A hydrolytic condensation process using metaln oleate complex in aqueous solution was successfully employed to synthesize oleate-stabilized metal oxide nanoparticles. A significant advantage of the current approach is that it can simultaneously perform surface stabilization through oleate complexation and also allow compositional control of the solid solution nanocrystals. Moreover, the reactions could be carried out in inexpensive and less toxic aqueous solutions. We will continue to investigate this unique approach for the development of an environmentally conscious synthesis of metal oxide nanoparticles with a controlled size and chemical composition.

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