Surfactant-Induced Nonhydrolytic Synthesis of Phase-Pure ZrO2

Article pubs.acs.org/cm

Surfactant-Induced Nonhydrolytic Synthesis of Phase-Pure ZrO2 Nanoparticles from Metal−Organic and Oxocluster Precursors Mahmoud A. Sliem,† Diedrich A. Schmidt,‡ Angélique Bétard,† Suresh Babu Kalidindi,† Silvia Gross,§ Martina Havenith,‡ Anjana Devi,† and Roland A. Fischer*,† †

Lehrstuhl für Anorganische Chemie II, Ruhr Universität Bochum, Universitätstrasse 150, 44801 Bochum, Germany Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, Universitätstrasse 150, 44780 Bochum, Germany § ISTM-CNR, Dipartimento di Scienze Chimiche, University of Padova, and INSTM, UdR Padova, via Marzolo, 1, 35131 Padova, Italy ‡

S Supporting Information *

ABSTRACT: Nonhydrolytic/non-sol−gel pyrolytic synthesis technique, as a convenient method, was applied to synthesize zirconium oxide nanoparticles (ZrO2 NPs). Pyrolysis of either the mononuclear keto ester/alkoxide complex zirconium bis(isopropoxide)bis(tert-butylacetoacetate) [Zr(OiPr)2(tbaoac)2] (I) or the oligonuclear oxocluster compound [Zr6(OH)4O4(OMc)12] (II, Mc = methacrylate) generated ZrO2 NPs at moderate conditions of 300−400 °C. Trioctylamine, stearic acid, and/or oleic acid, which act as both solvents and stabilizing agents, were used. Under the adopted process conditions, the stabilizing agent oleic acid plays a vital role in determining the phase of as-synthesized colloidal ZrO2 nanoparticles, which yield the high-temperature tetragonal phase at moderate conditions of 335 °C. Those as-synthesized samples that contained both monoclinic and tetragonal ZrO2 phases (depending on the choice of the surfactant) were transformed into pure tetragonal phase at 1000 °C. An unambiguous phase determination of ZrO2 nanoparticles was carried out by the combination of powder X-ray diffraction (XRD) and Raman spectroscopy. Furthermore, the samples were analyzed by transmission electron microscopy (TEM), ultraviolet−visible (UV−vis) and photoluminescence (PL) spectroscopy, dynamic light scattering (DLS), and Fourier transform infrared (FT-IR) spectroscopy to elucidate the structure, chemical composition, and morphology of the obtained nanoparticles. Also, the phase transformations of the as-synthesized ZrO2 nanoparticles upon annealing were followed via Raman spectroscopy. KEYWORDS: ZrO2 nanoparticles, pyrolysis, metal−organic precursor, stabilizers, tetragonal phase, Raman spectroscopy microwave irradiation,10 nonhydrolytic sol−gel,11 thermal decomposition,12 and laser ablation in liquid,13 among others.14−16 Even though synthesis of zirconia nanoparticles is widely established in literature, the as-synthesized materials obtained by these methods typically are amorphous or mixedphase zirconia nanoparticles. Further annealing at high temperature is required to improve the crystallinity of the particles. The final particles, after annealing, are collected as an agglomerated powder that affects or limits the handling and further processing of such nanocrystalline materials in many interesting applications, which require stabilized zirconia in a colloidal state. The challenge for nano-ZrO2 synthesis by wet chemical methods lies in tuning the reaction parameters (precursors, temperature, solvent, and type of stabilizer) for preparation of phase-pure, nonagglomerated crystalline ZrO2 nanoparticles as a stable colloidal solution. With regard to stabilizing the high-temperature phases of ziconia nanocolloids, very few attempts have been made. Large quantities (5 g per

1. INTRODUCTION Among various functional transition metal oxides, zirconia (ZrO2) is a unique ceramic semiconductor that possesses bifunctional surface properties originating from the presence of both acidic and basic sites.1,2 This inherent property is very important for zirconia as a heterogeneous catalyst and/or catalyst support for a variety of chemical reactions.3 Other applications of ZrO2 include development of solid oxide fuel cells4 and oxygen sensors.5 Bulk zirconia exists in three crystallographic (bulk) phases: monoclinic (m-ZrO2), tetragonal (t-ZrO2), and cubic (c-ZrO2) phases. The m-ZrO2 is thermodynamically stable from room temperature up to 1100 °C and transforms to t-ZrO2 in the temperature range 1100− 2370 °C and to c-ZrO2 above 2370 °C.6 Since the tetragonal phase of ZrO2 is more valuable for technological applications than the room-temperature monoclinic phase, many attempts have been made to stabilize the high-temperature phase (tZrO2) at ambient conditions. They include reducing the particle size to nanometer dimension7 or doping with cations such as Mg2+, Ca2+, Y3+, and Sc3+.8 ZrO2 nanoparticles have been prepared via a range of different methods that include chemical vapor deposition,9 © 2012 American Chemical Society

Received: April 11, 2012 Revised: October 4, 2012 Published: October 18, 2012 4274

dx.doi.org/10.1021/cm301128a | Chem. Mater. 2012, 24, 4274−4282

Chemistry of Materials

Article

= methacrylate) was synthesized starting from zirconium butoxide and methacrylic acid as reported in ref 19 by using an alkoxide:acid molar ratio of 4:1. Analytical data obtained for precursors I and II matched those of the literature preparative routes.9b,19a 2.2. Characterization. UV−vis measurements were performed on a Lambda 9 UV−vis−NIR Perkin-Elmer spectrometer. A diluted solution of the nanoparticles in n-hexane or in toluene was prepared and transferred to a quartz glass Suprasil cuvette and measured in the absorption mode, with hexane or toluene as a reference. Photoluminescence (PL) measurements were carried out on the same particle dispersion as for the UV−vis measurements on a SPEX FluoroMax-2 Jobin Yvon spectrometer. Raman analysis was carried out on a WITec (Ulm, Germany) CRM 300ARS confocal Raman microscope using a frequency-doubled Nd:YAG laser emitting at 532 nm as the excitation source. The laser power is 43 mW for synthesized ZrO2 nanoparticles and 5 mW for annealed samples. The laser radiation is coupled into a Zeiss microscope via a polarization-preserving single-mode fiber. Before passing through a holographic band-pass filter, the laser beam is collimated via an achromatic lens. It is then focused on the sample with a 20× Nikon E Plan 0.4 NA objective. The Rayleigh scattered light is blocked by the holographic band-pass filter as well as a laserline edge filter. Raman scattered light is detected by a back-illuminated 1024 × 127 pixel charge-coupled device (CCD) camera operating at −60 °C. A dispersive-type spectrometer with a 600 lines/mm grating resulted in a spectral resolution of 3 cm−1. Measurements were performed on thin films obtained by solution deposition on silicon wafers. X-ray diffraction (XRD) measurements were performed on the X’PertPRO PanAnalytical equipment (Bragg−Brentano geometry with automatic divergence slits, position-sensitive detector, continuous mode, room temperature, Cu Kα radiation, the range of 2θ = 20−80°, at steps of 0.0197°, with accumulation time of 1000 s/point). Fourier transform infrared (FTIR) spectra were recorded inside a glovebox under Ar atmosphere on a Bruker Alpha-P FT-IR instrument in the attenuated total reflectance (ATR) geometry with a diamond ATR unit. The morphology of the samples was studied on a Hitachi H-8100 transmission electron microscope (TEM) operating at accelerating voltages up to 200 kV with a single-crystal LaB6 filament. Chemical compositions of the samples were analyzed by energy-dispersive X-ray spectroscopy (EDX) on an EDX system attached to the TEM. The samples were prepared by placing a drop of a dilute solution of a sample in toluene on a carbon-coated copper grid and allowing it to drying at room temperature. Dynamic light scattering (DLS) measurements were performed on a fixed scattering angle Zetasizer Nano-S system (Malvern Instruments Ltd., Malvern, U.K.) with a He−Ne gas laser (λ = 635 nm). Samples were measured in a low-volume disposable sizing cuvette. The light scattering was detected at 173° and collected in automatic mode, typically requiring measurement duration of 90 s. The resulting data were analyzed by use of the “Zetasizer Nano software” Version 6.10 (Malvern Instruments Ltd., Malvern, U.K.). The viscosity of the sample and dispersant was measured by use of an automated micro viscometer (AMVn, Anton Paar) at 25.0 °C. Thermogravimetric analysis (TGA) was carried out on a Seiko model TG/DTA 6300S11 instrument. The measurements were performed in air at ambient pressure. Approximately 10 mg of the sample was filled in aluminum crucibles with a circular opening (diameter of 5 mm). The heating rate was 5 °C·min−1. 2.3. Synthesis of ZrO2 Nanoparticles. The nanoparticle synthesis procedure employs the thermolysis of suitable metal− organic precursors for the desired material in a hot organic liquid medium.17 The reaction pathways are shown in Scheme 1. A 100 mL two-necked flask was filled with the appropriate amount of surfactant (HDA, DPE/STA, TOA/STA, or TOA/OLA) and then heated to different temperatures for injection of the precursor solution (T = 250, 300, or 335 °C; determined internally by use of a thermocouple connected to a temperature controller). Accordingly, a sample of 0.5 g (1 mmol) of [Zr(OiPr)2(tbaoac)2] (I), dissolved in a minimal amount

batch) of highly monodisperse tetragonal zirconia nanocrystals (capped with TOPO, trisoctylphosphane oxide) were successfully obtained by Hyeon and co-workers.11 by applying a nonhydrolytic sol−gel process using the reaction of [Zr(OiPr)4] and ZrCl4 at temperatures above 300 °C. Phase-pure cubic ZrO2 nanocrystals was synthesized by Davar and coworkers. 1 2 through thermolysis of bisaqua-tris(2hydroxyacetophenato)zirconium(IV)nitrate in a mixture of oleylamine and triphenylphosphine as coordinating solvents. We have been engaged in the related synthesis of various metal, metal oxide, metal/metal oxide composite, magnetic, and semiconductor nanomaterials by nonaqueous thermolysis of metal−organic precursors.17 In particular, we investigated photoassisted deposition18 of organometallic precursors on the surface of metal oxide nanoparticles in different organic coordinating solvents aiming at metal/metal oxide composite nanoparticles suited for use as colloidal catalysts. Herein, we report on a surfactant-induced, nonhydrolytic approach for the selective synthesis of t-ZrO2 nanocolloids by thermolysis of tailored ZrO2 precursors at comparably low temperatures, 335 °C, which procedure is especially designed for potential applications of the nanoparticles in colloidal catalysis. We compared the solution pyrolysis of the mixed ligand keto ester/ alkoxide complex [Zr(OiPr)2(tbaoac)2] (I) and zirconium oxocluster [Zr6(OH)4O4(OMc)12] (II) in the presence of trioctylamine, stearic acid, and/or oleic acid (as both organic medium and stabilizing agent). Both precursors yield ZrO2 nanoparticles. Oleic acid plays a crucial role in controlling the phase of the as-synthesized ZrO2 nanoparticles. The pyrolysis of precursor I as well as precursor II in the presence of oleic acid resulted in formation of phase-pure t-ZrO2 at relatively low temperatures (335−400 °C). The phase identity was determined by powder X-ray diffraction (XRD) and Raman spectroscopy. However, pyrolysis of I in the absence of oleic acid resulted in formation of mixed-phase particles, t- and mZrO2. Annealing of the isolated mixed-phase particles at elevated temperature (1000 °C) resulted in the formation of t-ZrO2 phase, as expected. That is, thermal decomposition of tailored ZrO2 precursors with an all-O coordination sphere in different coordinating solvents/surfactant mixtures can be applied to control the formation of the crystalline phase of the resulting ZrO2 particles. Consequently, a convenient synthesis of colloidal ZrO2 nanoparticles of monocrystalline tetragonal phase, as required for the mentioned applications, may be derived from the results outlined below.

2. EXPERIMENTAL SECTION 2.1. Materials. Toluene and n-hexane solvents were of analytical grade, degassed and dried by use of an MBraun solvent purification system. Methanol and ethanol were dried by distillation over magnesium turnings and stored in a carefully sealed, dry flask. Zirconium isopropoxide/2-propanol complex [Zr(OPri)4·(HOPri)], tert-butyl acetoacetate, stearic acid (STA), and oleic acid (OLA) were purchased from Sigma−Aldrich. Trioctylamine (TOA), diphenyl ether (DPE), and hexadecylamine (HDA) were purchased from Acros. All chemicals, except solvents, were used as received. The ZrO2 precursor compound [Zr(OiPr)2(tbaoac)2] (I) was prepared according to the procedure described previously.9b Briefly, 1.66 mL (10 mmol) of tertbutyl acetoacetate was added to 1.94 g (5 mmol) of Zr(OPri)4·(HOPri) in 30 mL of n-hexane. The reaction mixture was refluxed for 90 min with subsequent removal of the solvent under reduced pressure to give a sticky white product. For purification, the product was recrystallized from saturated n-hexane at −30 °C. The alternative ZrO2 precursor compound [Zr6(OH)4O4(OMc)12] (II, Mc 4275



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Scheme 1. Thermal Decomposition Synthesis of ZrO2 Nanoparticles Starting from Precursors I and II

Table 2. Calcination at Different Temperatures of Sample 1aa calcination temp, °C

time, h

phase structure (from XRD)

600 800 1000

5 5 5

t-ZrO2 t-ZrO2 t-ZrO2

a

phase structure (from Raman)

crystallite domain size (from XRD patterns), nm

t/m-ZrO2 t/m-ZrO2 t-ZrO2

2.3 3.5 4.6

Sample 1a was obtained at 250 °C.

250 °C, in order to determine and optimize the best conditions for nucleation and growth processes of ZrO2 nanoparticles (Table 1). Upon thermolysis of I in a hot solution of HDA or DPE/STA at 250 °C, no change in the color of the solution was observed and no reflections in the XRD pattern of the solidified waxy reaction mixture were detected, indicating that neither HDA nor DPE/STA was the right medium for nucleation of nanocrystalline ZrO2 material from precursor I. However, when TOA/STA or TOA/OLA was used as coordinating solvent, the color of the solution changed to yellow and then to yellowish brown after injection of precursor I at 250, 300, and 335 °C, which probably indicates the generation of nanoparticles. Wide-angle X-ray diffraction (WXRD) patterns of the isolated ZrO2 particles (purified and dried powder samples) prepared under different conditions as a function of surfactants are presented in Figure 1. The XRD pattern of ZrO2 particles

of toluene, was injected into a hot mixture of 3.8 mL (8.6 mmol) of TOA and 0.85 g (3 mmol) of STA as the coordinating (surfactantlike) solvents under vigorous stirring at 250 °C (sample 1a), at 300 °C (sample 1b), and at 335 °C (sample 1c). Sample 1d resulted from thermolysis of 0.5 g of precursor I in 3.8 mL of TOA and 1 mL (3 mmol) of OLA at 335 °C. Alternatively, 1.7 g (1 mmol) of [Zr6(OH)4O4(OMc)12] (II) dissolved in a minimal amount of toluene was injected into a hot mixture of 3.8 mL of TOA and 1 mL of OLA at 400 °C (sample 2). In all cases, the reaction mixtures were kept at the stated temperatures for 30 min for nucleation and growth of ZrO2 nanomaterials under vigorous stirring and wet-Ar atmosphere (Table 1). Before injection, the reaction mixture was colorless. However, a couple of minutes after injection, it changed to a light yellow and then yellowish brown, which is a visible indication for nanoparticle generation (the color change may be associated with some pyrolysis of the organic components). In all cases, the mixture was allowed to cool down to room temperature and the product was precipitated by adding an excess amount of methanol. The ZrO2 particles were isolated by centrifugation, redispersed in a small amount of toluene, and flocculated again with methanol. The purified end product was obtained in the form of a white powder and was dried at 60 °C under dynamic vacuum. It could be easily redispersed in nonpolar organic solvents like n-hexane and toluene for characterization (colorless colloidal solution). Sample 1a (powder), obtained from the thermolysis of precursor I at 250 °C, was selected and annealed at different temperatures: 600, 800, and 1000 °C for 5 h (Table 2).

Figure 1. XRD patterns of as-synthesized ZrO2 samples by thermolysis of [Zr(OiPr)2(tbaoac)2] (I) in a mixture of TOA and STA at 335 °C (sample 1c, spectrum A) and a mixture of TOA and OLA at 335 °C (sample 1d, spectrum B). Patterns A and B are offset from each other.

3. RESULTS AND DISCUSSION 3.1. Synthesis and ZrO2 Nanoparticle Characterization. Precursor I was investigated in great detail for metal−organic chemical vapor deposition (MOCVD) of ZrO2 materials and was reported to be completely decomposed at about 240 °C, as clearly shown from TG analysis.9b Thus, it is concluded that I should be a very suitable precursor for wet chemical (thermolytic) synthesis of nanocrystalline ZrO2. Accordingly, thermolysis of I was carried out in various coordinating solvents at different temperatures, starting with

(sample 1c) prepared by thermolysis of 1 mmol of precursor I in a mixture of 8.6 mmol of trioctylamine (TOA) and 3 mmol of stearic acid (STA) at 335 °C for 30 min is shown in Figure 1, spectrum A, and is assigned to a mixture of monoclinic and tetragonal phases. When the reactions were carried out at 250 and 300 °C (samples 1a and 1b), mixed-phase XRD patterns resulted (see Supporting Information, Figure S1). However, the

Table 1. Experimental Conditions for Preparing ZrO2 Nanoparticles from Precursors I and II sample

precusor (1 mmol)

stabilizers (mmol)

temp, °C

time, min

phase (methods of detection)

mean diameter, nm

estimated particle size, nm

1a 1b 1c 1d 2

I I I I II

TOA/STA (8.6/3) TOA/STA (8.6/3) TOA/STA (8.6/3) TOA/OLA (8.6/3) TOA/OLA (8.6/3)

250 300 335 335 400

30 30 30 30 180

t/m (XRD + Raman) t/m (XRD + Raman) t/m (XRD + Raman) t (XRD + Raman) t (XRD)

6.5 10 28

2 5.5 23.5

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cm−1 is going to sharply decrease to background and does not show any structure. The “peaklike structure” that appears at 152 cm−1 is probably not the actual peak, but a convolution of the tail of the peak with the cutoff filter, which together make this look like a peak. We see indication of the 152 cm−1 peak, but due to the cutoff of the edge filter, we cannot fully resolve this peak. Different from precursor I, precursor II already mimics a nucleation center for ZrO2, as it represent an oligonuclear zirconium oxo cluster core structure. Thus, it is quite interesting to compare precursors I and II for selective tZrO2 nanocolloid synthesis. Thermal decomposition of [Zr6(OH)4O4(OMc)12] (II) was studied by thermogravimetry (TG) in air. As shown in Figure S3 (Supporting Information), a three-step TG profile was observed at about 118, 170, and 410 °C, corresponding to weight loss of 10%, 22%, and 56.5%, respectively. The TG solid-state thermolysis product of 43.5(±0.5)% residual weight is in excellent agreement with the value calculated for a quantitative conversion of II into ZrO2 (43.5%). Due to the instability of the coordinating solvents (TOA and OLA) above 400 °C, the solution thermolysis of II is carried out at a maximum of 400 °C for about 3 h at the same molar concentration and choice of surfactants as in the case of precursor I discussed above. The XRD pattern of ZrO2 nanoparticles (sample 2) prepared from thermolysis of precursor II is shown in Figure S4 (Supporting Information). The reflections at 2θ values 30.2°, 49.7°, and 60.2° of the XRD pattern in Figure S4 (Supporting Information) are assigned to the pure tetragonal phase of ZrO2.11,13 These data demonstrate the successful synthesis and stabilization of t-ZrO2 phase nanoparticles via the solution thermolysis method at relatively moderate conditions, irrespective of the choice of precursors I or II (but depending on the appropriate choice of solvent/surfactant). However, zirconium oxocluster precursor II obviously does not show any particular advantage with respect to the mononuclear precursor [Zr(OiPr)2(tbaoac)2] (I). Therefore we focus our discussion on structural and compositional characterizations of the nanoparticle samples derived from precursor I and then present the optical properties of both types of samples derived from I and II. The TEM micrographs of ZrO2 nanoparticles of samples 1a and 1b derived from precursor I are shown in Figure 3. The agglomeration shown in the images is probably due to the TEM specimen preparation technique, where the particles are deposited on a copper grid and the solvent is subsequently dried, thus allowing this agglomeration. The elemental composition of the prepared ZrO2 material is confirmed by EDX spectroscopy (Figure S5, Supporting Information). In order to confirm the dispersion of the prepared ZrO2 nanoparticles obtained from pyrolysis of [Zr(OiPr)2(tbaoac)2] (I) at three different temperatures (samples 1a−1c), DLS analysis was performed on a highly diluted hexane suspension of the ZrO2 particles. DLS provides the mean hydrodynamic radii of the particles, including the organic shell capping agent, by fitting the experimentally observed autocorrelation function to a theoretical function that contains the diffusion coefficient D and the radius r. Figure 4 shows the experimentally observed autocorrelation function of the number size distribution plot from the DLS analysis. The maximum of the curves is equal to the characteristic diameters of the main particles in the solution prepared at different three temperatures. Spectra A and B in

replacement of STA with OLA (oleic acid) during thermolysis under similar conditions gave sample 1d, whose XRD pattern shows broad reflections assigned to tetragonal ZrO2 particles (Figure 1, spectrum B). These data indicate the contribution of OLA in directing the crystallinity of ZrO2 particles to monocrystalline phase.10 The powder XRD patterns for cand t-ZrO2 are very similar; therefore it is not possible to establish the phase identity and purity from these XRD data. However, with Raman spectroscopy (Figures 2 and 9) the presence of pure tetragonal phase for sample 1d13,20 was unambiguously confirmed. A metastable tetragonal phase of high surface area may be present in pure zirconium dioxide when nonequilibrium techniques of synthesis are applied.21 Under these conditions, differences in the specific surface energies of the monoclinic and tetragonal forms may play a remarkable role in the stability criteria. Moreover, it is very well established in the literature that c-ZrO2 crystallizes as a face-centered cubic (fcc) structure with Zr surrounded by eight oxygen atoms, whereas the t-ZrO2 phase has the same coordination number but a longer c-axis. The cubic to tetragonal phase transition of ZrO2 is associated with displacement of the oxygen atoms from the ideal anion sites in the fluorite structure. Since Raman spectroscopy is highly sensitive to the polarizability of the oxygen ions, it is suited to distinguish between c- and t- ZrO2. As shown in Figure 2, spectrum A, the Raman peaks at wavenumbers 175, 185, 213, 330, and 371 cm−1 are the

Figure 2. Raman spectra of ZrO2 particles produced by decomposition of [Zr(OiPr)2(tbaoac)2] (I) in a mixture of TOA/STA (sample 1c, spectrum A) and TOA/OLA (sample 1d, spectrum B) at 335 °C. A Nd:YAG laser (λex = 532 nm) was utilized with a power of about 43 mW.

characteristic bands of m-ZrO2, and the Raman peaks at 469 and 630 cm−1 are assigned to t-ZrO2.13,22 Both t- and m-ZrO2 were produced by thermolysis of precursor I in a mixture of TOA and STA at 335 °C (sample 1c) and also at 250 °C (sample 1a; Figure S2, Supporting Information). However, sample 1d, which was obtained by thermolysis of the same precursor I in a mixture of TOA and OLA at the same temperature, is mainly t-ZrO2, as can be deduced from the characteristic Raman bands at 252, 427, and 620 cm−1 (Figure 2, spectrum B).13 In all these cases, the Raman spectra nicely corroborate the XRD data. The edge filter in the Raman system has a cutoff at about 150 cm−1, which means that the Raman intensity below about 150 4277



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Figure 3. TEM images of ZrO2 nanoparticles obtained by thermolysis of [Zr(OiPr)2(tbaoac)2] (I) in TOA and STA at (a) 250 °C (sample 1a) and (b) 300 °C (sample 1b). Scale bar is equal to 50 nm.

Figure 5. FTIR spectra of stearic acid (spectrum A) and as-prepared nanoparticles stearate@ZrO2 (spectrum B, sample 1a). The surface modification of ZrO2 nanoparticles with stearate was confirmed by FTIR measurements. Spectra A and B are offset from each other.

Figure 4. Number size distribution plot from DLS analysis of a hexane solution of stearate@ZrO2 nanoparticles prepared by pyrolysis of [Zr(OiPr)2(tbaoac)2] (I) at 250 (sample 1a, spectrum A), 300 (sample 1b, spectrum B), and 335 °C (sample 1c, spectrum C). A He−Ne gas laser (λ = 635 nm) was used.

1584 and 1480 cm−1 which are assigned to asymmetric and symmetric stretching vibrations of the COO− group of the stearate stabilizer in contact to the ZrO2 surface. The characteristic absorption bands below 600 cm−1 are referred to Zr−O−Zr vibrations.12 The presence of the C−N stretching vibration at about 1256 cm−1 (Figure 5, spectrum B) predicts the stabilization of the surface of ZrO2 with TOA together with stearic acid.12,17a 3.2. Optical Properties of ZrO2 Nanoparticles. The optical properties of the as-prepared ZrO2 nanoparticles were studied by absorption and fluorescence spectroscopy. It is wellknown that ZrO2 material is a direct band-gap insulator with two band-to-band transitions. The absorption behavior exhibits weak absorption in the near-UV and visible region. This arises from transitions involving extrinsic states, such as surface trap or defect states.24 Figure 6 illustrates the absorption spectra and a representative PL spectrum of the ZrO2 nanoparticles of samples 1a, 1b, and 1c. The absorption maxima around 310 nm (4 eV), 308 nm (4.02 eV), and 284 nm (4.4 eV) correspond to ZrO2 particles of samples 1a, 1b, and 1c, respectively. All absorption maxima are lower in energy as compared to the reported optical band gap of 5.0 eV for bulk ZrO2.24 This

Figure 4 are almost symmetric, suggesting that the maximum of these spectra is close to the mean value of the particles’ diameters. Spectrum C of Figure 4 shows a more significant asymmetric broadening in the number size distribution, which is a result of particle preparation at high temperature (335 °C). The number size distribution displays only one major peak for each sample centered at hydrodynamic diameters = 6.5, 10, and 28 nm, which correspond to the ZrO2 particles of samples 1a, 1b, and 1c, respectively. The stearic acid molecules attached on the surface of the particles have a mean length of about 2.32 nm,23 which leads to the estimation that the mean particle diameters should be about 2, 5.5, and 23.5 nm for samples 1a, 1b, and 1c, respectively. The absence of any other peaks at higher hydrodynamic radii suggests the absence of significant particle agglomeration at these conditions of dilution. Figure 5 shows the FTIR spectra of free stearic acid (spectrum A) and the as-prepared ZrO2 nanoparticles (sample 1a, spectrum B) stabilized with the stearate capping agent. The FTIR spectrum of stearate@ZrO2 NPs shows the absence of the CO vibration peak at 1710 cm−1, which is characteristic for the free carboxylic acid, and the appearance of two peaks at 4278



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[Zr6(OH)4O4(OMc)12] (II) in TOA/OLA at 400 °C. The absorption maximum is around 340 nm (3.6 eV), while the representative broad fluorescence band is centered at 377 nm (3.3 eV), upon excitation at 325 nm. The optical properties, in terms of absorption and PL measurements of the obtained ZrO2 nanoparticles, are similar to the as-prepared nanoparticles (samples 1) from the pyrolysis of [Zr(OiPr)2(tbaoac)2] (I), and this indicates the contribution from extrinsic states toward absorption and emission in the optical regions of measurements. It is known that most metal oxide nanoparticles show very weak or even no detectable photoluminescence at room temperature.25 However, strong luminescence is observed from all colloidal ZrO2 nanoparticles described herein. This observation demonstrates the high quality of the obtained ZrO2 nanoparticles in terms of protection and stabilization of the particle surface by the stabilizers used in this study. 3.3. Annealing and Phase Transitions of As-Prepared ZrO2 Nanoparticles. Most of the reported, crystalline, pureand mixed-phase ZrO2 nanoparticles were obtained af ter annealing of the as-prepared primary particles at high temperature and were then collected as agglomerated powder. This procedure affects or even limits the handling of such materials in many interesting applications requiring stabilized zirconia in colloidal state.10,12,13 In the present work, crystalline zirconia nanoparticles of both pure (t-ZrO2) and mixed (t/mZrO2) phase character were directly obtained as dispersion colloids at comparably low temperatures, which show long-term stability (no precipitation/agglomeration) resulting from highly efficient carboxylate stabilizing agents. In some of the experiments referring to samples 1a−1c derived from precursor I discussed above, the obtained asprepared ZrO2 nanoparticles were of mixed phase (depending on the conditions). In order to obtain pure, single-phase particles from these particular samples, an annealing step was used. ZrO2 nanoparticles of sample 1a were annealed at four different temperatures (400, 600, 800, and 1000 °C), and the resulting phases were examined by powder XRD and Raman spectroscopy measurements. Figure 8 shows the XRD

Figure 6. (A−C) UV−vis absorption spectra of the as-prepared ZrO2 nanoparticles (A, sample 1a; B, sample 1b; C, sample 1c) by thermolysis of [Zr(OiPr)2(tbaoac)2] (I) in TOA/STA at 250 (sample 1a), 300 (sample 1b), and 335 °C (sample 1c). (D) Photoluminescence (PL) spectrum of the particles of sample 1a in n-hexane (excited at 385 nm).

clearly indicates the contribution from extrinsic states toward absorption in these regions. Photoluminescence (PL) spectra were measured with several excitation wavelengths between 275 and 380 nm. The fluorescence intensities changed to some extent with excitation wavelength, but the band structure and fluorescence peak position remained about the same for excitations below 370 nm. This indicates that the fluorescence involves the same initial and final states, even as the excitation wavelength was varied from 275 to 370 nm, suggesting fast relaxation from the final state that is reached after photoexcitation to the states where the fluorescence originates. Figure 6 shows one representative broad fluorescence band (spectrum D) of sample 1a centered around 400 nm (3.1 eV) upon excitation at 365 nm. The absorption spectra B and C of samples 1b and 1c are more or less similar to spectrum A of sample 1a. The broad nature of the spectra and the substantial red shift of the peak position, as compared to the bulk band gap, strongly indicate that the fluorescence involves extrinsic states.24 Absorption and emission spectra of ZrO2 particles of sample 1d are shown in Figure S6 (Supporting Information) and behave like ZrO2 particles of samples 1a−1c. Figure 7 shows the absorption (A) and PL spectra (B) of ZrO2 nanoparticles of sample 2 prepared by thermolysis of

Figure 8. XRD patterns of ZrO2 nanoparticles of sample 1a after calcination for 5 h at 600, 800, and 1000 °C. The indexing is given for the tetragonal phase only (the peak positions for cubic phase are almost indistinguishable, so the corresponding indexing is omitted).

diffraction patterns of ZrO2 particles of sample 1a after calcination at different temperatures. From the XRD data, the crystallite sizes (Dc) of the annealed ZrO2 particles of sample 1a were calculated to be 2.3, 3.5, and 4.6 nm at 600, 800, and 1000 °C, respectively, by use of the Debye−Scherrer equation26 for (202) reflection.

Figure 7. Absorption spectrum (A) and photoluminescence spectrum (B) of ZrO2 nanoparticles (excited at 325 nm) prepared by thermolysis of [Zr6(OH)4O4(OMc)12] (II) in TOA/OLA at 400 °C (sample 2). 4279



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Figure 9. (A) Raman spectra of ZrO2 nanoparticles of sample 1a (obtained from precursor I) after calcinations for 5 h at (A) 400, (B) 600, (C) 800, and (D) 1000 °C. A Nd:YAG laser (λex = 532 nm) was used with a power of about 5 mW.

As expected, with increasing annealing temperatures, the diffraction peaks become more intense and sharper due to an increase in the crystallite size. Three observed peaks with 2θ values of 30.2°, 49.7°, and 60.2° correspond to the (111), (202), and (301) diffraction peaks of crystalline ZrO2, respectively. All reflections of the XRD pattern in Figure 8 can be assigned to the standard reflection peaks of phase-pure tetragonal or cubic ZrO2. Again, Raman spectroscopy is a very useful tool to distinguish between c- and t- ZrO2. The Raman spectra of the annealed ZrO2 nanoparticles (sample 1a) are given in Figure 9. The Raman bands corresponding to monoclinic and tetragonal phases are identified as m and t, respectively. As evident from Figure 9, the mixed phase is pronounced up to 800 °C and the ratio of tetragonal phase increases with increasing temperature. The pure tetragonal phase becomes predominant only at 1000 °C (D, Figure 9). Upon annealing the nanoparticles to higher temperatures, they should change phase from monoclinic to tetragonal, but the tetragonal peaks of A are much sharper (and at different Raman shift) with respect to the tetragonal peaks of D. It is possible that the difference in Raman peaks assigned to t-ZrO2 is due to a mixed phase coexisting within the same nanoparticle. As a result, the “bulklike” peaks of t-ZrO2 will be modified by the local crystal field of the m-ZrO2 phase, and this could be the reason for the difference.13 The UV−vis absorption spectra of the ZrO2 particles annealed at different temperatures and the representative PL spectrum are shown in Figure 10. In the case of annealed particles, the contribution from extrinsic states is more pronounced because of the absence of stabilizers and an increase of the defect density on their surfaces. The most important point deduced from the annealing studies is that the ZrO2 particle size remains in the nanometer regime even at temperatures up to 1000 °C, which should allow the application of ZrO2 nanoparticles (as colloidal solution) in the field of catalysis with interesting properties (Table 2). Furthermore, studies now in progress show that the prepared ZrO2 particles of high surface area are particularly suitable for the photoassisted deposition of metals.27,28 These findings

Figure 10. (A−C) UV−vis absorption spectra of ZrO2 particles of sample 1a after calcination at 600 (A; 290 nm, 4.3 eV), 800 (B; 290 nm, 4.3 eV), and 1000 °C (C; 302 nm, 4.1 eV). (D) Photoluminescence (PL) spectrum of the particles in n-hexane (excited at 325 nm) after calcination at 1000 °C (360 nm, 3.4 eV).

prompted us to continue the current study toward preparation and evaluation of Cu−ZrO2 nanocolloids following a procedure similar to our previous case study on Cu-ZnO and Cu−TiO2.18 This step will be followed by examination of Cu-ZrO2, with its alternative ZrO2 supported material for ZnO, in methanol synthesis as a nanocolloidal binary catalyst in the future.

4. CONCLUSION ZrO2 nanoparticles have been synthesized by thermolysis of two kinds of precursors, namely, [Zr(OiPr)2(tbaoac)2] (I) and [Zr6(OH)4O4(OMc)12] (II), in different organic solution mixtures byased on trioctylamine (TOA) at various temperatures (250−400 °C). Precursor I represents an optimized, mononuclear MOCVD precursor for ZrO2, and precursor II is a typical example of a methacrylate-ligand-stabilized zirconium oxide molecular cluster. Our comparative studies demonstrates that choice of the stabilizer, oleic acid (OLA) versus stearic acid (STA), allows control of the phase composition of the resulting ZrO2 nanoparticles, irrespective of the nature and structure of the chosen molecular ZrO2 precursors. Obviously, it is quite convenient to use an established MOCVD precursor also for 4280



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Egypt Government and membership in the Graduate College Chemical Processes at Oxide Surfaces associated with SFB 558. M.H. acknowledges funding from the BMBF under Grant BMBF 05 KS7PC2. D.A.S. acknowledges support within the BMBF-funded project.

wet chemical nanoparticle synthesis, if there is no significant advantage to move to more sophisticated zirconium oxide clusters. It turned out that [Zr(OiPr)2(tbaoac)2] (I) is the precursor of choice. The solution thermolysis of I at temperatures as low as 250 °C is convenient and can certainly be scaled up and utilized for large-scale production of ZrO2 nanocolloids, in particular phase-pure t-ZrO2. Thermolysis of both precursors I and II in the presence of OLA resulted in the formation of colloidal phase-pure t-ZrO2 nanoparticles at comparably low temperatures (300−400 °C). By changing the solvent/stabilizer combination, the assynthesized particles derived from I were of mixed phase. These mixed-phase samples yield phase-pure tetragonal ZrO2 nanoparticles by annealing at elevated temperature (1000 °C). The formation of both mixed-phase and phase-pure tetragonal particles was established by powder XRD and Raman spectroscopy. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and the optical properties (strong photoluminescence) of the resulting ZrO2 nanoparticles suggest that the nanoparticles are of narrow size distribution and exhibit a relatively low density of surface trap states. One interesting field in which ZrO2 nanoparticles and the nonhydrolytic (avoiding sol−gel) preparation method based on suited metal−organic precursors/surfactant combinations can possibly be useful is the in situ synthesis of colloidal catalysts. Herein, nano-ZrO2 serves as an interacting support for active metals and the employed ZrO2 precursor, solvent, and stabilizers must match stringent criteria (for example, free of catalytic poision, such as halides).29 Catalytic methanol synthesis may be an example where zirconia is interesting as an alternative support material for Cu/ZrO2 for its long-term stability as compared to Cu/ZnO. The photophysical properties of the prepared ZrO2 nanocolloids of pure and mixed phase suggest their suitability for the photoassisted deposition of active metals (like Cu) at the particle surface, which gives the opportunity for preparation of Cu/ZrO2 nanocolloidal binary catalysts (or other M/ZrO2 composites) with a controlled amount of Cu (or M), similar to our previous work on Cu/ ZnO and Cu/TiO2.18

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ASSOCIATED CONTENT

S Supporting Information *

Additional text and six figures showing characterization of ZrO2 nanoparticles by means of UV−vis, photoluminescence, EDX, powder XRD, and Raman spectroscopy and TG analysis of precursor II (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone +49 (0)234 32-24174; fax +49 (0)234 32-14174; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Research Center on Metal−Support Interaction in Heterogeneous Catalysis (SFB-558, TP B10) and the Research Department Interfacial Systems Chemistry (RD IFSC) established at Ruhr University Bochum. M.A.S. is grateful for a fellowship of the 4281



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Surfactant-Induced Nonhydrolytic Synthesis of Phase-Pure ZrO2

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