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TiO2- and CeO2-Based Biphasic CoreShell Nanoparticles with Tunable Core Sizes and Shell Thicknesses Deleon N. Corr^ea,† Juliana M. de Souza e Silva,† Elias B. Santos,† Fernando A. Sigoli,† Antonio G. Souza Filho,‡ and Italo O. Mazali*,† † ‡
Institute of Chemistry, University of Campinas—UNICAMP, Campinas, SP, Brazil Department of Physics, Federal University of Ceara, Fortaleza, CE, Brazil ABSTRACT: A layer-by-layer methodology was used for synthesizing CeO2/TiO2 and TiO2/ CeO2 coreshell nanoparticles supported in Vycor glass pores. The layers were deposited by cerium- or titanium-based metalloorganic precursor decomposition. Sequential depositions promoted linear mass increases of the Vycor pieces and a linear decrease of both total pore size and total surface area, confirmed by N2 adsorptiondesorption isotherms. Alternation in the metalloorganic precursors used results in the formation of spherelike nanoparticles (as observed by HRTEM) with coreshell architecture. Raman spectroscopy data showed that CeO2 is crystallized in the fluorite structure and TiO2 in the anatase phase. Shifts in the frequency and changes in line width of TiO2 Eg and CeO2 T2g Raman bands were used for monitoring changes in core size and shell thickness based on the quantum size effect and on the phonon confinement theory. Our results show that nanoparticle core sizes and the shell thicknesses can be tuned by changing the number of depositions used in the synthesis process.
’ INTRODUCTION Semiconductor oxide materials are used in different technologies, such as gas sensors,1 photocatalysts,2 and batteries.3 Their synthesis on the nanoscale expands their application range, due to the possibility to tune electronic and chemical properties by engineering their particle sizes and morphologies. New functionalities may arise with the preparation of hybrid nanoparticles (which are generally defined as nanomaterials that contain more than one component4) owing to the synergism between the different constituents. An interesting example of the synergism between two semiconductor oxides is the interaction between CeO2 and TiO2: CeO2 stabilizes TiO2, thus inhibiting the phase transition from anatase to rutile under high temperatures.5 Additionally, ceria, which is generally not considered as a photoactive material, shows cooperative effects with TiO2 and improves its photocatalytic activity.6,7 Regarding the use of semiconductors for the photodegradation of organic molecules, TiO2 is by far the most widely studied catalyst owing to its low cost, low toxicity, and high physical and chemical stabilities. However, the limited efficiency of titania while using natural solar energy has restricted its use. In this context, the synthesis of a titania- and ceria-based hybrid nanostructure seems to be a promising approach to overcome this barrier, since well-coupled CeO2TiO2 hybrids have different surface structures and band gaps with respect to pure TiO2, resulting in a special electron-transfer process that facilitates the separation of the electronhole pairs and improves titania photoactivity.8 Yang et al.8 and Lin and Xiaoming6 synthesized, respectively, CeO2 nanorodTiO2 nanotube and CeO2/TiO2 coreshell hybrid nanostructures, which are expected to present high photoactivity, while Alessandri et al.7 verified the significantly higher activity of CeO2/TiO2 core/shell inverse r 2011 American Chemical Society
opals when compared with pure TiO2 similar system. Semiconductorbased hybrid nanostructures with improved properties can be engineered by combining theoretical predictions of the final physical properties with adequate synthesis procedures to realize the proposed nanostructure, followed by structural characterization to check whether the nanostructure actually obtained corresponds to the desired one.3 Regarding these aspects, adequate theoretical models for the large amount of new nanostructured systems prepared help in the correct interpretation of the experimental results. One model which is widely used for the determination of nanoparticle size is the phonon confinement model, which takes into consideration the asymmetric broadening and shift of the Raman line that occurs for confined structures.9,10 In this work, we present a novel synthesis method for the preparation of CeO2- and TiO2-based coreshell nanoparticles with controllable core sizes and shell thicknesses. We propose the use of the phonon confinement model allied to the Raman spectroscopy as an adequate tool for the elucidation of the materials obtained, which present compositions that can be inverted (CeO2/TiO2 and TiO2/CeO2) and can be prepared with variable core sizes and shell thicknesses.
’ EXPERIMENTAL METHODS Porous Vycor Glass Monoliths Preparation. The porous Vycor glass (PVG) was purchased from Corning Glass Co. It is presented as cylinders that are 10 cm long with a 0.5 cm diameter. Received: January 18, 2011 Revised: April 14, 2011 Published: May 11, 2011 10380
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Figure 1. Schematic diagram of IDC procedure illustrating the preparation of PVG/xTiO2yCeO2.
The cylinders were cut into monoliths with 0.5 cm diameters and 0.1 cm thicknesses. In order to remove impurities, the PVG monoliths were subjected to ultrasound treatment during 5 min in 1.0 mol L1 aqueous HCl solution and 5 min in acetone. Afterward, the monoliths were thermally treated at 393 K for 2 h followed by treatment at 1023 K for 72 h in order to remove organic impurities. CoreShell Nanoparticles Syntheses. All reactants were used as received. For the preparation of coreshell nanoparticles supported inside the PVG pores presenting TiO2 in the core and CeO2 at the shell, the clean PVG monoliths were immersed in a 0.75 mol L1 solution of titanium(IV) di(n-propoxy)di(2-ethylhexanoate), hereafter named Ti(OnPr)2(hex)2, in hexane. This titania precursor Ti(OnPr)2(hex)2 was synthesized following the procedure described elsewhere.11 After 8 h of immersion in the Ti(OnPr)2(hex)2 solution, the PVG pieces were copiously washed with hexane to remove precursor adsorbed on the external surface. The PVG pieces impregnated with Ti(OnPr)2(hex)2 were thermally treated at 1023 K under static air for 8 h, after a heating rate of 5 K min1, to decompose the metalloorganic precursor. This procedure was named as one impregnation decomposition cycle (IDC) and was repeated 3, 5, or 7 times. PVG monoliths were weighed immediately after their removal from the muffle at 373 K after each IDC and TiO2 incorporation was determined by weight difference. All TiO2 containing PVG monoliths, independent of the number of IDCs, were submitted to a total of 80 h of thermal treatment at 1023 K. PVG monoliths containing TiO2 were subsequently submitted to IDC with cerium(III) 2-ethylhexanoate, from now on named Ce(hex)3 (48% excess of 2-ethylhexanoic acid and 3% of butyl carbitol) purchased from Strem Chemicals. The procedure used was the same as described for Ti(OnPr)2(hex)2, only changing the final treatment temperature, which was 923 K for the ceriumbased metalloorganic precursor. The final materials were named PVG/xTiO2yCeO2, with x corresponding to the IDC number with Ti(OnPr)2(hex)2, followed by y times IDC with Ce(hex)3, where x and y are 3, 5, or 7. Coreshell nanoparticles composed of CeO2 in the core and TiO2 in the shell were prepared using the same procedure, alternating the metalloorganic precursors, resulting in materials named PVG/xCeO2yTiO2, with x and y equal to 3, 5, or 7. A general scheme reporting an outline of the main steps of the above procedure is presented in Figure 1. Characterizations. Sorption data for nitrogen at liquid nitrogen temperature (77 K) were obtained for unground monoliths with 0.5 cm in diameter and 0.1 cm in thickness weighing about 50 mg using Quantachrome Autosorb1 equipment (Quantachrome Instruments, Boynton Beach, FL) in the relative pressure range P/P0 from 106 to 1. The samples were outgassed for at least 2 h at 423 K prior to the adsorption analysis. Specific surface area was determined by the BET method. Pore size distributions were
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calculated by the BarrettJoynerHalenda method using the adsorption branch.12 PVG monoliths after successive IDC were analyzed by X-ray fluorescence spectroscopy using a Shimadzu XRF1800. Powder X-ray diffraction (PXRD) measurements were obtained for the slightly pressed powders placed in stainless steel sample holders at room temperature at the D12A-XRD1 beamline at the Brazilian Synchrotron Light Laboratory (LNLS— Laboratorio Nacional de Luz Síncrotron, Campinas, Brazil) (λ = 1.24 Å) or in a Shimadzu XRD-7000 using Cu KR radiation (λ = 1.54 Å) operating at 40 kV and 30 mA and using divergence, scattering, and reception slits of 1.0°, 1.0°, and 0.3 mm, respectively. For crystallite size determination by the Scherrer equation, analyses performed in the Shimadzu XRD-7000 were done with 10 s acquisition time, 0.01° 2θ step sizes were used. Raman spectra were recorded with a Renishaw Raman Imaging Microprobe System 3000 spectrometer, equipped with a microscope and spectral resolution of 2 cm1, using a HeNe laser (632.8 nm, 8 mW). Samples were analyzed at the fracture of broken PVG pieces. High-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM 3010 microscope operating at 300 kV (1.7 Å point resolution) at the Brazilian Syncrotron Laboratory (LNLS—Laboratorio Nacional de Luz Síncrotron). PVG pieces were ground, suspended in water, subjected to ultrasound, and then dropped onto a carbon-coated copper grid.
’ RESULTS AND DISCUSSION The cumulative mass increase observed for the synthesis of PVG/5CeO25TiO2 and PVG/5TiO25CeO2 are presented to illustrate changes in the PVG piece masses after several IDC with Ti(OnPr)2(hex)2 and Ce(hex)3. The use of two different metalloorganic precursors results in two different slopes for mass increment as a function of IDC number (Figure 2). Each IDC with CeO2 precursor results in more than 1% mass increase, whereas for each IDC with Ti(OnPr)2(hex)2 this number reaches only 0.5%. Samples were analyzed by XRF, and the semiconductor mass percentage increase after each IDC is in agreement (considering the analyses errors) with results presented in Figure 2. Differences in the molar mass of the metalloorganic precursors and in the respective oxides formed after their thermal degradation may explain the less prominent mass increase observed after IDC with Ti(OnPr)2(hex)2 in comparison with Ce(hex)3. Considering that a certain mass of PVG is impregnated with one mol of metalloorganic precursor, 1 mol of oxide is obtained after its thermal decomposition. In this context, for Ti(OnPr)2(hex)2 (452.46 g mol1), 79.88 g of TiO2 is obtained, while for Ce(hex)3 (317.38 g mol1), 172.12 g of CeO2 is formed. This means that only 18% of the Ti (IV) metalloorganic precursor is transformed into oxide, while, for Ce(hex)3, this amount is much higher, equal to 54%, and explains the more prominent mass increases observed for CeO2. The porous structures of pure PVG and PVG after IDC with Ti(OnPr)2(hex)2 and Ce(hex)3 were analyzed by N2 adsorption desorption isotherms (Figure 3). Experimental data for the adsorptiondesorption equilibrium for the system pure PVGnitrogen at 77 K is presented (Figure 3). The isotherm for PVG exhibits a type 4 adsorption with a hysteresis loop of type H2 according to the IUPAC classification13 which is associated with the occurrence of percolation effects14 in materials consisting of disordered porous structures and broad pore size 10381
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Figure 3. N2 adsorptiondesorption isotherms for (A) PVG/5CeO2yTiO2 with y equal to 0, 3, 5, or 7 and (B) for the PVG/5TiO2yCeO2 with y equal to 0, 3, 5 or 7.
Figure 2. Mass gain for (A) PVG/5CeO25TiO2 and (B) PVG/ 5TiO25CeO2 as functions of IDC number with both TiO2 and CeO2 metalloorganic precursors.
distributions.15,16 Table 1 lists the specific surface area and the pore size distribution of pure PVG. Comparison of the N2 adsorptiondesorption isotherms of pure PVG with those related to PVG/5CeO2xTiO2 and PVG/ 5TiO2yCeO2 systems with x and y = 0, 3, 5, or 7 (Figure 3, A and B, respectively) indicated that the isotherm profile is conserved even after sequential impregnationdecomposition cycles with CeO2 or with TiO2. These results suggest that there is no relevant obstruction of the host pore structure up to the number of IDC preformed in this work. However, with the increase of number of IDC, the isotherm plateau is reached with a lower amount of adsorbed gas. In order to analyze the effect of this change on the textural properties of the solids, the specific surface area and the pore size distribution of all materials are compared (Table 1). For all samples, the specific surface area and the pore size distribution values tend to decrease as the number of IDC increases, thus implying that CeO2 and TiO2 are confined inside the PVG pores and that the porous PVG matrix network is preserved. Capillarity process rules the diffusion of the Ti(IV) and Ce(III) metalloorganic precursors into the PVG pores, as already reported in the literature for the incorporation of neutral molecules in the PVG.1719 At this stage, the precursor promotes coverage of the porous surface, establishing some nucleation sites due to its interaction with silanols (SiOH) present on the matrix surface. During the decomposition process, dispersed
Table 1. Textural Analyses of Pure PVG and TiO2- and CeO2Containing PVG material
PVG
specific surface area,
total pore volume,
SBET (m2 g1)
Vp (cm3 g1)
124
0.21
PVG/5CeO2 PVG/5CeO23TiO2
99 70
0.16 0.13
PVG/5CeO25TiO2
64
0.12
PVG/5CeO27TiO2
61
0.11
PVG/5TiO2
89
0.17
PVG/5TiO23CeO2
81
0.16
PVG/5TiO25CeO2
78
0.15
PVG/5TiO27CeO2
76
0.14
nanocrystals are formed on these nucleation sites and they remain anchored to the matrix through SiOTi20 and SiOCe bonds.21 As already described in some papers,17,21 PVG plays a fundamental role in controlling the coalescence process and nanoparticles formed inside the porous structure are dispersed and stabilized. With successive IDC, the exceeding interfacial free energy is expected to be minimized by nanoparticle growth. Therefore, instead of forming new nucleation sites, IDC with different metalloorganic precursors should preferentially result in coreshell nanoparticles. XRD analyses of PVG/xCeO2 samples (with x equal to 3, 5 or 7) (Figure 4A) corroborate this hypothesis. Pure PVG (Figure 4A(a)) presents a large reflection related to the amorphous halo of silica. Three large signals observed at 28.5°, 48.0°, and 57.0° 2θ are related to (111), (220), and (311) CeO2 crystal planes (JCPDS 34-0394). The Gaussian fitting of 10382
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Figure 4. XRD pattern of (A) PVG/xCeO2 samples, with x equal to 3, 5, or 7 (λ = 1.54 Å). Analyses of signal used for crystallite size determination in the inset: (B) (a) TiO2 anatase, (b) pure PVG, and (c) PVG/5TiO2 (λ = 1.24 Å); (C) (a) PVG/5TiO2, (b) PVG/5TiO25CeO2, and (c) PVG/5TiO27CeO2 (λ = 1.24 Å); and (D) of (a) PVG/5CeO2 and (b) PVG/5CeO25TiO2.
the (220) reflection was used for crystallite size determination by the Scherrer equation (eq 1). D ¼ kλ=B cos θ
ð1Þ
where D is the crystallite size, k is a shape constant equal to 0.9 for spherical crystallites, λ is the X-ray source wavelength (Cu = 1.54 Å), and B and θ are the full width at half-maximum (fwhm) of the diffraction peak and the Bragg diffraction angle, respectively. The results obtained indicate that CeO2 is under the nanometric regime. Ceria nanoparticle size increases are observed with IDC number increase: for x = 3, the diameter of the CeO2 nanoparticles is 4.9 nm and for x = 5 and 7, it is 5.1 and 7.6 nm, respectively. The same analysis was performed for PVG/xTiO2 samples; however, no conclusive data was obtained (Figure 4B). The most intense anatase peak (Figure 4B(a)) occurs at 20.4° 2θ and is related to the (101) plane. PVG/5TiO2 (Figure 4B(c)) presents a large signal centered at 17.3° 2θ related to the PVG amorphous halo, also observed for pure PVG (Figure 4B(b)). It is important to underline that the signal observed at 17.3° 2θ for PVG/5TiO2 is asymmetric and suggests the presence of a peak at 20° 2θ superimposed onto the amorphous halo. This assumption is reinforced by the presence of low-intensity signals at 30.5°, 38.0°, 45.5°, and 49.7° 2θ, suggesting that titania in the anatase phase is present in PVG/5TiO2 according to the file JCPDS 21-1272. PVG/5TiO2 was subjected to 5 and to 7 IDC with CeO2. XRD analyses of the PVG/5TiO25CeO2 and PVG/5TiO27CeO2 samples (Figure 4C) present intense peaks at 18° 2θ related to the PVG amorphous halo. Those bicomponent systems (Figure 4C, (b) and (c)) present four more peaks located at 22.9°, 26.5°, 37.8°, and 44.8° 2θ related to CeO2 (JCPDS 34-0394). Those peaks are more intense in the PVG/5TiO27CeO2, in agreement with the
higher amount of CeO2 in this sample. The IDC with CeO2 metalloorganic precursor over PVG/5TiO2 could result in the formation of pure CeO2 nanoparticles or in the coverage of the preformed TiO2 nanoparticles, resulting in coreshell biphasic systems. The CeO2 crystallite size determined by the Scherrer equation indicates that there is an increase in CeO2 size upon IDC (3.6 and 4.0 nm for PVG/5TiO25CeO2 and PVG/5TiO27CeO2, respectively). Comparing the CeO2 sizes in the PVG/5CeO2 and PVG/5TiO25CeO2 systems, one realizes that the CeO2 layer is smaller in PVG/5TiO25CeO2, suggesting that CeO2 is covering the preformed TiO2 nanoparticles. XRD analysis of PVG/5CeO2 and PVG/5CeO25TiO2 samples (Figure 4D) does not give enough information for TiO2 size determination. However, the comparison between these diffraction patterns suggests some structural differences between these systems. Anatase presents a peak at 25° 2θ, which is observed as a shoulder in the PVG/5CeO25TiO2 diffraction pattern and confirms the presence of TiO2 in this system. PVG/5CeO2xTiO2 (y = 3, 5, or 7) were analyzed by Raman spectroscopy. For TiO2 and CeO2, the Raman spectrum can be used for obtaining structural information and may be used as a versatile tool for crystal phase identification. TiO2 anatase phase has a tetragonal structure containing 12 atoms per unit cell. Factor group analysis predict six Raman-active modes (A1g þ 2B1g þ 3Eg) which are observed at 144 cm1 (Eg), 197 cm1 (Eg), 399 cm1 (B1g), 513 cm1 (A1g), 519 cm1 (B1g), and 639 cm1 (Eg) for bulk anatase.2224 CeO2 has a fluorite structure with space group Fm3m and has a simpler vibrational spectrum with one infrared active phonon (T1u symmetry) and only one Raman-active phonon which is located at 464 cm1 (T2g symmetry) for bulk CeO2.21,25,26 Raman analysis of nanostructured materials, where the effects of frequency shift and asymmetric broadening of the Raman 10383
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The Journal of Physical Chemistry C bands play an important role, can be very useful for a systematic characterization of nanoparticles. In ideal single crystals, firstorder Raman spectroscopy can be used to observe only the optical phonons close to the Brillouin zone center. However, for nanometric systems, which have interrupted translational periodicity, the phonon propagation gets interrupted at the nanoparticle boundary and, consequently, the phonon can get reflected from the nanoparticle limits, thus remaining confined within the crystal.27 For nanoparticles with spherical morphology, this confinement results in the appearance of several discrete spheroidal and torsional modes, which are labeled using the angular momentum quantum number and the branch number. The consequences of phonon confinement are that the frequency of an acoustic Raman mode is scaled as the inverse of the dimension of particles and, therefore, has often been used for estimation of the average nanoparticle size. This is particularly important for the low-frequency region. TiO2 and CeO2 are two examples where the frequency and the line width of Ramanactive modes change as the particle size varies, and thus the Raman spectrum can be used for estimating average particle size.21,22 Pure PVG presents no active modes in the Raman spectrum.22 Bulk anatase TiO2 presents an Eg related band at 144 cm1,22 whereas bulk CeO2 presents a T2g related band at 464 cm1.21 Under the nanometric regime, those TiO2 and CeO2 Raman bands suffer shifts and changes in signal profile, such as broadening and decreases in intensity. It is important to underline that the changes in nanoparticle size result in different Raman shifts and this depends on the phonon dispersion relation for the phonon branch to which the Raman mode belong. The TiO2 Eg band shifts to lower frequencies (energy upshift) as crystallite size increases,22 whereas the CeO2 T2g band shifts to higher frequencies (energy downshift) as nanoparticle size increases.21 The first set of samples analyzed in this work is composed of PVG/7CeO2yTiO2 (y = 3, 5, or 7) (Figure 5). For all PVG/ 7CeO2yTiO2 samples, the CeO2 T2g Raman mode is almost constant at 460 cm1. However, the frequency is shifted as compared with bulk CeO2, which is observed at 464 cm1. This is a size-induced phenomenon observed in nanostructured materials and it has been described for CeO2 nanoparticles supported in PVG based on the phonon confinement model21 presented above. On the other hand, the TiO2 Eg mode for PVG/7CeO2yTiO2 samples (Figure 5A) presents a frequency upshift as the IDC number increases. This difference in the Raman spectra is attributed to the effect of TiO2 size variation upon IDC, as predicted by the quantum size effect. For comparison, one may recall the result obtained for TiO2 nanoparticles prepared inside PVG pores using the same IDC methodology and the same number of IDC. For PVG/xTiO2 (x = 3, 5, or 7), the Raman Eg mode experiences an energy upshift as the number of IDC increases, suggesting nanoparticle size growth:17 for PVG/ 3TiO2, the Raman Eg mode is located at 157 cm1 and for PVG/5TiO2 and PVG/7TiO2 the Eg band occurs at 153 and 150 cm1, respectively.28 The Eg Raman band positions for PVG/7CeO2yTiO2, equal to 163, 158, and 155 cm1 for y = 3, 5, and 7, respectively, also present energy upshifts as the number of IDC increases. However, the comparison between the absolute frequency values observed for PVG/7CeO23TiO2 and for PVG/ 3TiO2 (163 and 157 cm1, respectively) shows differences in TiO2 nanoparticle size in these samples and suggests that TiO2 grows over the preformed CeO2 nanoparticles, forming
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Figure 5. Raman spectra of PVG/xCeO2yTiO2: (A) PVG/7CeO2yTiO2, (B) PVG/5CeO2yTiO2, and (C) PVG/3CeO2yTiO2.
nanoparticles with coreshell architecture supported inside PVG. Considering previous studies on isolated systems having TiO2 or CeO2 grown inside PVG,21,22 which, based on Raman and HRTEM analyses, point to the growing of preformed particles after each IDC, these results also signal changes in shell thickness as a result of IDC number. In fact, these studies were used to calibrate the Raman results presented here, since the exact size for TiO2 and CeO2 obtained after a given number of IDC is known, as well as how the Raman spectra would behave. The next set of samples correspond to nanoparticles prepared by 5 IDC with cerium-containing precursor and a variable IDC number with the titanium-containing metalloorganic precursor, resulting in PVG/5CeO2yTiO2 (y = 3, 5, or 7). As observed for 10384
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Figure 7. Raman shifts obtained for PVG/xCeO2yTiO2 (where x and y are equal to 3, 5, or 7).
Figure 6. Illustration of (A) nanoparticle core and shell sizes and (B) Raman shift for nanoparticles having cores with variable sizes.
the PVG/7CeO2yTiO2, the PVG/5CeO2yTiO2 Raman spectra (Figure 5B) show that the CeO2 T2g mode related band is again constant at 460 cm1. The TiO2 Eg mode, located at 161 cm1 for PVG/5CeO23TiO2, suffers a decrease in frequency upon increased IDC, which is consistent with the strong phonon confinement in these nanostructures. Qualitatively, this shift suggests an increase in the TiO2 shell thickness upon IDC and supports the coreshell formation assumption. PVG/3CeO2yTiO2 (y = 3, 5, or 7) constitutes the third set of samples analyzed by Raman spectroscopy (Figure 5C). For PVG/3CeO2yTiO2, as observed for the other systems previously discussed, an Eg band shift to higher energies is observed when y changes from 3 to 5 or 7, thus suggesting an increase in the shell thickness as the IDC number increases. With same number of IDC, CeO2 nanoparticles are expected to have the same size. However, for PVG/3CeO2yTiO2, variations observed in the T2g band position are probably related to deviations in particle size observed only for this system, which may result from the small size of these CeO2 nanoparticles formed after a low number of IDC. Also, the signal-to-noise ratio is low and there is a considerable error in the determination of the peak position. A summary of the systems discussed above is presented schematically in Figure 6A. Keeping the core size invariable (x = 5) and progressively increasing the number of IDC for the preparation of the shell (y = 3, 5, or 7), one can observe that the shell thickness increases as the number of IDC increases. By varying the core size and maintaining the number of IDC for the
Figure 8. (A,B) HRTEM micrographs of sample PVG/5CeO215TiO2 with the corresponding Fourier transform of each crystalline area (B). There is an error of (5% in the spacing values.
shell synthesis constant, one can observe a variation in the shell thickness. Those variations were examined by Raman spectroscopy (Figure 5) and are represented in Figure 6B. With regard to bulk TiO2, a shift (ω (cm1)) in the Raman spectrum is observed when TiO2 is presented as a nanoparticle. For TiO2 nanoparticles prepared by 5 IDC (PVG/5TiO2), this shift is equal to 5 cm1. When those 5 IDC are performed over CeO2 nanoparticles, a coreshell system is obtained (PVG/3CeO25TiO2) and a more pronounced shift in the Eg band is observed. By increasing 10385
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Figure 9. Raman spectra of (A) all PVG/xTiO2yCeO2 samples with x and y = 3, 5, or 7, (B) PVG/xTiO25CeO2 samples with x = 0, 3, 5 or 7, (C) PVG/ 7TiO25CeO2, PVG/5CeO27TiO2, and PVG/5CeO2 samples, and (D) PVG/3CeO23TiO2, PVG/3CeO2, and PVG/3TiO23CeO2 samples.
the CeO2 core size, (PVG/xCeO25TiO2, with x = 5 or 7), a thinner shell is expected to be formed and a larger Raman shift, of 12 cm1 with respect to bulk TiO2, supports this assumption (Figure 4D). All Raman spectroscopy results obtained for the PVG/ xCeO2yTiO2 (where x and y are equal to 3, 5, or 7) system are summarized (Figure 7). For the systems under study, varying the x coefficient and keeping the y coefficient fixed, it is possible to observe changes in the TiO2 band position, which is located at 144 cm1 for bulk TiO2: higher x values result in Eg bands located at lower frequencies (Figure 7). In fact, this is predicted by the phonon confinement theory and expected for coreshell architecture under a nanometric regime, since the larger the core (with higher volume and surface area) the smaller is the thickness of the shell prepared for the same number of IDC with the Ti(IV) metalloorganic precursor (Figure 6). The T2g band tends to be almost constant, as observed when the x coefficient is fixed and the y coefficient assumes variable numbers, indicating that the presence of a TiO2 shell over the CeO2 core does not affect the T2g band. The changes observed for the PVG/3CeO2yTiO2 system probably result from a nonmonomodal core size distribution. The results discussed above are qualitative evidence that core and shell consist of hierarchically nanostructured biphasic units, with CeO2 comprising the core, which is covered by a TiO2 shell. As suggested by the Raman band shifts, core and shell present variable sizes depending on the number of IDC performed with the respective metalloorganic precursor. HRTEM analyses were performed in order to observe the morphology of the obtained nanoparticles. Coreshell nanostructuring was not observed in the HRTEM images, since the shell thicknesses were probably too small to be distinguished from the core and/or may be noncrystalline. An alternative to limitations brought by the shell thickness was to increase the number of IDC used to prepare the shell. Therefore, a sample prepared with 5 IDC with CeO2 metalloorganic precursor and
15 IDC with TiO2precursor, PVG/5CeO215TiO2, was analyzed by HRTEM. Particles with nanometric size and spherelike morphology embedded in an amorphous matrix are observed in the typical images of the PVG/5CeO215TiO2 sample (Figure 8A). Under higher magnification (Figure 8B), different crystalline domains are clearly observed. The indexation of the (111) and (200) lattice planes in the particle core can be attributed to the CeO2 with cubic structure, corroborating XRD and Raman data. In the shell, a (101) lattice plane shows spacing of 3.52 Å, typically observed for anatase. This result supports the discussion over the formation of coreshell nanoparticles based on the Raman results previously shown. PVG/xTiO2yCeO2 samples, with x and y = 3, 5, or 7, were analyzed by Raman spectroscopy (Figure 9). Comparing all samples (Figure 9A), one can observe that if x is constant and equal to 5 or 7, the Eg mode of the TiO2 related band is invariable and occurs at 148 cm1 (x = 5) and 152 cm1 (x = 7). For PVG/ 3TiO2yCeO2, the Eg band is not observed. The position of the CeO2 T2g band for the PVG/xTiO25CeO2 samples, with x = 0, 3, 5, or 7, were compared (Figure 9B). For PVG/5CeO2, the T2g band shifts to lower frequencies with respect to bulk CeO2 (occurring at 464 cm1), as predicted by the phonon confinement model for CeO2 particles under a nanometric regime.29 Also for PVG/3TiO25CeO2, an important shift regarding this band is observed, which is expected considering that CeO2 grows over the preformed TiO2 nanoparticles and results in a thin shell. T2g band suppression is observed when IDC with Ce(hex)3 is performed over TiO2 nanoparticles supported in Vycor pores: an increase in the TiO2 core size results in a decrease in the CeO2 T2g band intensity up to a limit (noticed for PVG/ 7TiO25CeO2), when the band located at ca. 450 cm1 is completely suppressed and three new bands appear at 365, 403, and 475 cm1. It is interesting to note how the T2g band behaves when CeO2 composes the core or is located as the shell of the nanoparticles 10386
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The Journal of Physical Chemistry C supported in PVG (Figure 9, C and D). Pure CeO2 nanoparticles prepared after three or five IDC with Ti(OnPr)2(hex)2 present a relatively intense band at ca. 455 cm1. Its intensity increases after covering the CeO2 nanoparticles with a TiO2 shell, as observed for PVG/5CeO27TiO2 (Figure 9C) and for PVG/ 3CeO23TiO2 (Figure 9D). However, inverting the core and the shell composition and forming PVG/5TiO27CeO2 and PVG/ 3TiO23CeO2 results in CeO2 T2g band suppression. Shift and reduction in this band intensity is expected considering that the CeO2 shell is thin, but variations in the phonon relaxation with nanoparticle size, signal broadening associated with larger nanoparticle size distribution, defects (caused by oxygen vacancies and nonstoichiometry), strain, and phonon confinement must also be taken into account in these Raman-spectrum changes.29 The comparison of the results obtained when core and shell composition is inverted suggests that the high concentration of surface defects on CeO2 affects the Raman spectra. It shows the stabilization of the CeO2 T2g band when this oxide composes the nanoparticle core, and its distortion and shift when CeO2 composes the shell. Those modifications are spectroscopic evidence that CeO2 at the core results in CeO2 structural defect stabilization, whereas CeO2 as the shell induces surface defect maximization.
’ CONCLUSION A novel approach for the synthesis of spherical TiO2- and CeO2-containing coreshell nanoparticles supported inside Vycor porous structure is described. The experimental results indicate the ability to verify size variations in the coreshell structures formed by CeO2 and TiO2 by Raman spectroscopy allied to the phonon confinement theory. It was demonstrated that the IDC methodology allows tuning the size of the core and the shell thickness of the obtained spherical nanoparticles. Results suggest that, when CeO2 is located in the core, surface defects are minimized by the presence of a TiO2 shell. However, the opposite effect is observed when CeO2 is located as the shell. In this case, the CeO2 T2g mode Raman band is highly distorted and, if the shell is too thin, it suffers suppression in the 465 cm1 region and three other bands are observed in the range from 360 to 480 cm1. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ55-19-3521-3164. Fax: þ55-19-3521-3023. E-mail:
[email protected].
ARTICLE
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’ ACKNOWLEDGMENT Financial support from FAPESP and CNPq is gratefully acknowledged. The authors thank the C2NANO-Center of Nanoscience and Nanotechnology/MCT for the HRTEM measurements and Prof. C. H. Collins (IQ-UNICAMP, Campinas, Brazil) for English revision. This is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/FAPESP). ’ REFERENCES (1) Fine, G. F.; Cavanagh, L. M.; Afonja, A.; Binions, R. Sensors 2010, 10, 5469. (2) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol. C 2008, 9, 1. 10387
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