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J. Phys. Chem. C 2007, 111, 3300-3307
Phase Transformation and Photoluminescence Properties of Nanocrystalline ZrO2 Powders Prepared via the Pechini-type Sol-Gel Process Cuikun Lin, Cuimiao Zhang, and Jun Lin* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: October 9, 2006; In Final Form: December 20, 2006
Nanocrystalline ZrO2 fine powders were prepared via the Pechini-type sol-gel process followed by annealing from 500 to 1000 °C. The obtained ZrO2 samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), and photoluminescence spectra (PL), respectively. The phase transition process from tetragonal (T) to monoclinic (M) was observed for the nanocrystalline ZrO2 powders in the annealing process, accompanied by the change of their photoluminescence properties. The 500 °C annealed ZrO2 powder with tetragonal structure shows an intense whitish blue emission (λmax ) 425 nm) with a wide range of excitation (230-400 nm). This emission decreased in intensity after being annealed at 600 °C (T + M-ZrO2) and disappeared at 700 (T + M-ZrO2), 800 (T + M-ZrO2), and 900 °C (M-ZrO2). After further annealing at 1000 °C (M-ZrO2), a strong blue-green emission appeared again (λmax ) 470 nm). Based on spectral analysis and EPR results, the whitish blue emission (425 nm) and blue-green emission (470 nm) can be ascribed to interstitial carbon impurities (Ci) in the tetragonal ZrO2 and oxygen vacancies (VO) in the monoclinic ZrO2, respectively.
I. Introduction As an important ceramic material, ZrO2 has widespread potential applicability in the fields of structural materials, solidstate electrolytes, thermal barrier coatings, electro-optical materials, gas sensing, corrosion resistance, and catalysis.1 Zirconia (ZrO2) exhibits three crystallographic phases with increasing temperature at normal atmospheric pressure: the monoclinic phase, from room temperature to 1175 °C; the tetragonal phase, from 1175 to 2370 °C; and the cubic phase, from 2370 to 2750 °C (melting point).2 Most of the properties of ZrO2 depend strongly on the phase formation and transition. Seal and co-workers did extensive research on the stabilization of the metastable-tetragonal phase in undoped and yttria-doped (nanocrystalline) ZrO2 and gave a comprehensive review on the mechanisms of tetragonal phase stabilization at room temperature in nanocrystalline (1 µm).2 It is now well recognized that the mechanical, electrical, chemical, as well as catalytic properties of zirconia can be improved by using nanocrystalline instead of conventional micrometer-sized zirconia.2b Nanocrystalline ZrO2 powders and colloids have been prepared by quite a few methods, including the sol-gel process, forced hydrolysis, hydrothermal method, thermal decomposition, microwave irradiation, and two-phase process.3 Among them, the sol-gel process is one of the most frequently utilized methods for the preparation of (doped and undoped) nanocrystalline ZrO2.4 In most of the above cases, the sol-gel precursors for ZrO2 are zirconium(IV) alkoxides, which suffer from high cost, unavailability, toxicity, and fast hydrolysis rate (thus difficult in controlling the homogeneity of different components during experimental processes). These disadvantages can be avoided by the Pechini-type sol-gel * Corresponding author. E-mail: jlin @ciac.jl.cn.
process to a great extent. As an alternative to the alkoxidesbased sol-gel process, the Pechini-type sol-gel process (also known as and called a polymerizable-complex technique) is well known and extensively used for the synthesis of homogeneous (especially multicomponent) metal oxide materials.5 This method includes a combined process of metal complex formation and in situ polymerization. Normally, an R-hydroxycarboxylic acid such as citric acid (CA) is used to form stable metal complexes, and their polyesterification with a polyhydroxy alcohol (such as ethylene glycol, polyethylene glycol) forms a polymeric resin. Immobilization of metal complexes in such rigid organic polymer networks reduces segregation of particular metals, ensuring compositional homogeneity. The calcination of the polymeric resin at a moderate temperature (500-1000 °C) then generates pure phase (multicomponent) metal oxides.5 The Pechini-type sol-gel process has been used for the synthesis of electric and magnetic materials rather extensively, including ferroelectric and capacitor materials, superconducting materials, and photocatalytic materials.5 The improved material properties for the Pechini-type sol-gel process with respect to the other methods (such as solid-state reaction method, amorphous citrate method) have been demonstrated for the synthesis of superconductors and photocatalysts.5c In the past 5 years, we have extended the application of the Pechini-type sol-gel process to the systematic synthesis of various kinds of optical materials, including luminescent powders and thin films, core-shell structured phosphor, and pigment materials.6 Although the mechanical, electric, and thermal properties of ZrO2 have been well studied, there is lack of an extensive investigation on the optical (luminescent) properties of ZrO2, especially a clear clarification for the luminescent mechanisms in ZrO2.7 Furthermore, the luminescent properties of ZrO2 seem to depend strongly on the preparation methods. For example, the tetragonal ZrO2 nanoparticles synthesized by the microwave
10.1021/jp066615l CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007
Properties of Nanocrystalline ZrO2 Powders irradiation process show sharp emission peaks at 402, 420, and 459 nm under 254 nm excitation and a broad-band emission at 608 nm under 412 nm excitation;3j while two-phase processsynthesized colloidal zirconia nanocrystals exhibit a broad-band emission centered at 365 nm under 250 nm UV excitation.3i In both cases, the luminescence intensity of ZrO2 nanocrystals seemed to be very weak, and no detailed luminescence mechanisms were reported. So, it would be of great interest and significance to prepare nanocrystalline ZrO2 by other methods and investigate their luminescence properties together with their mechanisms, to see if some novel and strong and useful luminescence can be obtained from the nanocrystalline ZrO2. Although nanocrystalline ZrO2-related materials have been prepared by the Pechini-type sol-gel process, the luminescence study on them was neglected.8 It has been reported that sol-gel-derived SiO2-based materials, including SiO2 gels and organic/inorganic hybrid silicones, show strong luminescence from the blue to red spectral region, which are potentially used as a kind of environmentally friendly luminescent material without expensive or toxic metal elements as activators.9 Recently, we found that wet chemical route (glycerol and polyethylene glycol as additives, analogous to the Pechini-type sol-gel)-derived BPO4 (isomorphous with SiO2) exhibits a weak purple-colored emission under UV irradiation, and doping it with 6 mol % Ba2+ produces a material that emits an efficient bluish white light.6b Considering the above situations, it would be of great interest to prepare nanocrystalline ZrO2 powders by the Pechini-type sol-gel process and investigate their luminescence properties as a function of annealing temperature. On one hand, it would be possible to find some useful luminescent materials via this investigation; on the other hand, it is also helpful to understand and correlate the phase transition with the luminescence properties of the nanocrystalline ZrO2 prepared in such a process. Accordingly, in this paper, we prepared nanocrystalline ZrO2 fine powders by a facile Pechini-type sol-gel process and investigated their phase transformation and luminescent properties in detail. It is interesting to find that, after being annealed at 500 °C, the nanocrystalline ZrO2 crystallized with tetragonal structure exhibits an intense whitish blue emission centered at 425 nm under a wide range of UV light excitation (230-400 nm), making it suitably excited by different light sources. With the increase of the annealing temperature from 600 to 1000 °C, phase transformation from tetragonal to monoclinic occurred, and the emission from this material decreased (600 °C) and disappeared (700-900 °C), but an intense blue-green emission came out again after being annealed at 1000 °C (monoclinic). XRD, FT-IR, FESEM, TEM, and EPR were employed to characterize the samples, and possible mechanisms have been proposed to explain the observed luminescent phenomena. II. Experimental Section Preparation. ZrO2 phosphors were prepared via the Pechinitype sol-gel process.5,6 First, 10 mmol of ZrOCl2‚8H2O (analytical reagent ) A. R., Shanghai Guoyao Chemical Co., Ltd.) was dissolved in an aqueous solution containing citric acid (C6H8O7‚H2O, A. R.) as the chelating agent for the metal Zr4+ ions under vigorous stirring, and then the solution was mixed with a H2O-glycerol (V:V ) 1:4) solution. The molar ratio of the metal Zr4+ ions to citric acid was 1:2. Subsequently, 5 g of polyethylene glycol (PEG, molecular weight ) 20 000, A. R.) was added as cross-linking agent. The solution was stirred for 1 h to form a sol. The obtained sol was preheated at 400 °C for 3 h, fully ground, and annealed at 500-1000 °C for 3 h in air
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3301
Figure 1. XRD patterns of ZrO2 samples annealed at 500 (ZOSG500, a), 600 (ZOSG600, b), 700 (ZOSG700, c), 800 (ZOSG800, d), 900 (ZOSG900, e), and 1000 °C (ZOSG1000, f), respectively. The standard JCPDS card data for tetragonal and monoclinic ZrO2 are provided as references.
to produce the final samples. For comparison, 10 mmol of ZrOCl2‚8H2O (analytical reagent ) A. R., Beijing Chemical Co., Ltd.) was directly annealed at 500 °C for 3 h in air to obtain ZrO2 (solid-state process, SS) powder samples. We denote the final samples as follows: ZOSGx series for the samples prepared via the sol-gel process, where x is the annealing temperature; ZOSS500 for the sample prepared via the solidstate reaction, where 500 is the annealing temperature. Characterization. The X-ray diffraction (XRD) of powder samples was examined on a Rigaku-Dmax 2500 diffractometer using Cu Ka radiation (λ ) 0.15405 nm). Fourier transform infrared (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL JXA-840) and a transmission electron microscope (JEOL-2010, 200 kV). The excitation and emission spectra were taken on an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using 290 nm lasers (pulse width ) 4 ns) as the excitation source (Continuum Sunlite OPO). Electron paramagnetic resonance (EPR) spectra were taken on the JESFE3AX electronic spin resonance spectrophotometer. All of the measurements were performed at room temperature. The band structures of the defective ZrO2 were calculated using the CASTEP code (version 3.0, Accelrys) based on the density functional theory (DFT). III. Results and Discussion Phase Transformation and Morphology. XRD. The XRD patterns of ZrO2 samples prepared via the Pechini-type solgel process (SG) annealed from 500 to 1000 °C are shown in Figure 1. For ZOSG500 (Figure 1a), some broad peaks at 2θ ) 30.2°, 50.3°, 60.2° are present, which are assigned to (011), (112), (121) reflections of tetragonal ZrO2 (T-ZrO2), respectively. For ZOSG600 (Figure 1b), other weak peaks belonging to the (-111), (111), (200) reflections of monoclinic ZrO2 (MZrO2) have been observed. This indicates that phase transforma-
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TABLE 1: Size of the Nanocrystallite, Phase Transformation, and Luminescence Properties for ZrO2 Samples as a Function of Annealing Temperature sample ZOSG 500
ZOSG 600
ZOSG 700
ZOSG 800
ZOSG 900
ZOSG 1000
crystallite size (nm) phase of ZrO2
9.5 T (100%)
12.5 T (90%) M (10%)
17.4 T (80%) M (20%)
28.2 T (35%) M (65%)
29.6 M (100%)
47.4 M (100%)
luminescent properties peak position (nm) integrated intensity emission color
425 4.3 × 105 whitish blue
425 1.5 × 105 whitish blue
0
0
0
470 4.5 × 105 blue-green
tion of ZrO2 from tetragonal (T) to monoclinic (M) begins to take place around 600 °C. However, it should be mentioned that the ZOSG600 sample is basically similar to the ZOSG500 sample; only a small amount of monoclinic phase can be found, which implies a dominant tetragonal phase for the ZOSG600 sample. With the increase of annealing temperature (to 700800 °C, Figure 1c,d), the peaks of M-ZrO2 turn stronger, while those of T-ZrO2 become weaker, indicative of further phase transformation from tetragonal to monoclinic. After being annealed at 900 °C, the peaks from tetragonal ZrO2 cannot be detected, suggesting a complete transformation from T-ZrO2 to M-ZrO2 at this stage. In general, the nanocrystallite size can be estimated from the Scherrer formula: Dhkl ) Kλ/(β cos θ), where λ is the X-ray wavelength (0.15405 nm), β is the fullwidth at half-maximum (in radian), θ is the diffraction angle, K is a constant (0.89), and Dhkl means the size along (hkl) direction.8c The size of the nanocrystallite, phase transformation, and luminescence properties for ZrO2 as a function of annealing temperature are summarized in Table 1. In monoclinic ZrO2, which is stable at lower temperatures, the coordination number of Zr4+ cations is 7, whereas that in tetragonal and cubic-ZrO2 is 8. The strong covalent nature of the Zr-O bond favors a seven-fold coordination number, and, as a result, monoclinic ZrO2 is observed to be thermodynamically stable at lower temperatures (specifically at room temperature).2d The stability of the tetragonal phase in the bulk ZrO2 has been attributed to the presence of oxygen ion vacancies (induced by doping tri-, tetra-, and pentavalent impurities in the ZrO2 lattice), while in the case of nanocrystalline ZrO2 (
