ARTICLE pubs.acs.org/crystal
Crystalline SnO2 Nanoparticles Size Probed by Eu3+ Luminescence Mathias Strauss, Thalita A. Destefani, Fernando A. Sigoli, and Italo O. Mazali* Functional Materials Laboratory, Institute of Chemistry, State University of Campinas (UNICAMP), P.O. Box 6154, Zip Code 13083-970, Campinas -SP, Brazil ABSTRACT: Size-controlled europium(III)-doped SnO2 nanoparticles dispersed inside of porous Vycor glass (PVG) were synthesized using the impregnation and decomposition cycle (IDC) method. XRD, DRS, and TEM analyses proved that the observed cumulative mass gain after each IDC is associated to a controlled growing of the SnO2 nanoparticles. Europium(III) emission spectra were acquired for all samples and clear differences on the relative intensity of the 5 D0f7F2/5D0f7F1 transitions were observed for different SnO2 nanoparticle sizes. The changes in the europium(III) emission spectra could be correlated with the increase of nanoparticle sizes. The smaller superficial nanoparticle area decreases the amount of europium(III) at the surface, where it can be located in different environments over distorted symmetry sites, compared to the crystal lattice sites, where the ions probably are located, when the particles become bigger with no changes in their crystallinity degree. The linear plot between the asymmetric intensity ratio of the 5D0f7F2/5D0f7F1 transitions and the particles area/volume ratio calculated from the XRD Scherrer data and particle size frequency counts (∼1/d) confirmed this behavior.
’ INTRODUCTION Tin oxide has applications as solar cells,1,2 conductive electrodes, gas sensors,3 and catalyst supports. This wide band gap semiconductor has size-dependent linear and nonlinear properties4,5 due to quantum confinement effects, and therefore particle size determination is extremely important in the use of the SnO2 in photonics applications, for example. Nanomaterials have been extensively studied for the most diverse uses, such as catalysis, gas-sensing, displays, photonics, medicine, magnetic devices, materials improvement, and others. Nanoparticles of semiconductor oxides are frequently used in several of these applications. These materials are known for having their structural, optical, and electronic properties changed due to particle size variations. The practical application of unsupported nanomaterials have some drawbacks, like the agglomeration or particle growth (Ostwald ripening)6,7 induced by high surface energies and elevated costs of filtration/separation processes. In order to overcome these problems, the development of nanoparticles supported inside of porous solids8 11 is being used to disperse and impart high mechanical and thermal stability to those materials. In some cases, extra beneficial properties due to synergistic effects between the host matrix and the nanoparticles are observed. Several techniques have been used to evaluate the size of semiconductor oxide nanoparticles. Weibel et al. have arranged some of these characterization techniques into three main groups:12 (i) Direct observation of the nanoparticles; the most common case is the use of transmission electron microscopy images on direct particle size evaluation. (ii) Measurement of particle coherence length; where the crystallite sizes can be calculated by X-ray powder diffraction results using the Scherrer r 2011 American Chemical Society
equation and (iii) Particle size determination using thermodynamic properties to get particle surface area values relating to their sizes. Normally gas adsorption techniques and liquid intrusion are used for this data acquisition. A fourth characterization technique uses nanomaterials interaction with light to get information about the particle sizes. Size reduction down to B€ohr radii confines the electronic and vibrational wave functions,13 which results in unique properties that may be used in determination of nanoparticle sizes. Raman spectroscopy has been used with success for the determination of semiconductor properties of oxidic nanomaterials. In this case, the quantum confinement of the vibrational mode phenomena is described by the Phonon Confinament Model (PCM) introduced by Richter et al.14 The PCM explains very well the energy shift of a determined vibrational mode, the enlargements and also the asymmetries of the Raman spectra peaks of nanomaterials due to size variations in one or more directions.14,15 UV vis diffuse reflectance spectroscopy (DRS) is part of the fourth group. In this case, the particle size can be calculated using the DRS spectra of semiconductor nanoparticles in suspension or dispersed in a solid matrix. The band gap values (Eg*), calculated from the DRS spectrum, can be described as functions of the particle or crystallite radii (r), Eg* = f(r), assuming a spherical geometry and using the Effective Mass Approximation Model (EMAM).16 18 It is frequently observed that the particle size values do not converge when different characterization tools are used. These differences may be related to instrumental limitations or to the Received: June 9, 2011 Revised: August 5, 2011 Published: August 29, 2011 4511
dx.doi.org/10.1021/cg2007292 | Cryst. Growth Des. 2011, 11, 4511–4516
Crystal Growth & Design models approximations, so that the use of one or another technique may take into account its advantages to get the true particle size value. Data about the area/volume ratio (A/V) of nanoparticles in the determination of its size (d), where a proportion of 1/d can be used is scarce in the literature. Having in mind this relation, the present work proposes the use of the ratio between the surface and the bulk substitutional site number in equally crystalline SnO2 nanoparticles monitored by Eu3+ luminescence spectra to probe particles size related to XRD and TEM calculated values. Several studies in the literature indicate that the emission of europium(III) doped- SnO2 nanoparticles may be intensified due to efficient energy transfer from the conduction band of this semiconductor.19,20 Several applications are the target of those materials, like microlasers, multicolor displays and luminescent labels. Variations in the emission spectra observed in these studies have been attributed to (i) different SnO2 crystallinity degrees, (ii) different europium concentrations, (iii) Eu2O3 phase segregation and/or particle size effects. In most of them, one or more of these effects may be acting together, interfering in the analysis of the emission spectra. The present work used the impregnation and decomposition cycles methodology21 to synthesize size controlled europium(III) doped-SnO2 nanoparticles (1% Eu/Sn molar ratio) inside Porous Vycor Glass (PVG). This synthesis allow the evaluation of the particle size effect on the Eu3+ emission spectra isolated from the other parameters cited above, since this synthetic approach leads to equally doped materials with different particle sizes and to crystalline SnO2 with no Eu2O3 phase segregation. Having no other effects acting on the emission spectra, a relationship between the asymmetric ratio from the Eu3+ emission spectra (ratio between the intensity of 5 D0 f 7F2 and 5D0 f 7F1 transitions) and the nanoparticle A/V ratio (∼1/d, d = particle size) was established, opening the opportunity to probe the particle size of europium doped-SnO2 nanoparticles using its luminescence emission spectra.
’ EXPERIMENTAL SECTION To obtain SnO2:Eu3+/PVG, a solution of tin(IV) 2-ethylhexanoate (0.75 mol L 1) with 1.0% (Eu/Sn molar ratio) of europium(III) 2-ethylhexanoate in hexane was impregnated into previously treated porous Vycor glass (PVG) disks (5 1 mm, diameter thickness) for 8 h at ambient temperature. Thereafter, the impregnated PVG disks were copiously washed with hexane to remove the precursor adsorbed on the external surface. The metalorganic precursor impregnated into the PVG pores was decomposed at 1023 K (5 K min 1) under static air for 8 h. This procedure is called one impregnation decomposition cycle (IDC). This IDC procedure was repeated to obtain samples with 2, 3, 5, and 7 IDC. All samples, independent of the number of IDC, were submitted to the same thermal treatment for a total of 56 h at 1023 K. The samples in this work are designated as xSnO2:Eu3+/PVG were x indicates the number of IDC to which the PVG monolith was submitted (x = 1, 2, 3, 5 or 7). A bulk sample (SnO2:Eu3+/Bulk) was synthesized by decomposition of the precursor solution at 1023 K (5 K min 1) under static air for 56 h. Samples were characterized using a Shimadzu diffractometer model XRD7000 with CuKα radiation (λ = 1.5406 Å). The continuous scanning diffractograms were acquired from 10 to 70° (2θ) at a rate of 2° (2θ) min 1, and the fixed time scanning mode diffractograms from 48° to 58° (2θ) with 0.01° (2θ) steps and 10 s of acquisition time. All analyses were performed at room temperature and using 0.5°, 0.5°, and 0.3 mm for output, divergence, and reception slits, respectively.
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Figure 1. Cumulative mass gain curve of the PVG monoliths after each IDC for the xSnO2:Eu3+/PVG materials (x = 1, 2, 3, 5, or 7). A CARY 5 spectrophotometer with an integrating sphere spectral collector was used for diffuse reflectance UV/vis spectra acquisition from 200 to 800 nm with a scanning rate of 400 nm min 1. A quartz sample holder was used and BaSO4 was used as standard for background correction. Transmission electron microscopy images were obtained on a JEOL JEM 2100 ARP and a Zeiss-Libra 120 microscope. The PVG pieces containing the SnO2:Eu3+ nanoparticles were grinded and sieved (100 mesh), the resulting powder was dispersed in water by sonication and than supported on carbon-coated 3 mm and 300 mesh copper grids. Particle size frequency counts were performed manually for 500 particles for the samples with 3 and 7 IDC using the Gatan Image Tool. The emission spectra of the europium doped-SnO2 nanoparticles were acquired in a Horiba Jobin Yvon spectrofluorimeter, FL3 222, with excitation at 260 nm using a 450 W xenon lamp as the excitation source and front face acquisition mode. All analyses were performed using 0.5 nm steps and integration times of 0.3 s. Excitation monochromator slits were set at 1.0 mm while the emission monochromator slits were set at 0.5 mm. The intensity of all spectra was normalized to the maxima of the 5D0 f 7F1 transition.
’ RESULTS AND DISCUSSION The cumulative mass gain curve (Figure 1) was obtained by weighting the PVG disks after each impregnation decomposition cycle (IDC), so that the given cumulative mass gain values are relative to the pristine PVG initial mass. After each IDC it can be observed that there is a mass increment that can be attributed to SnO2 particles growing inside of the mesoporous system of the PVG, as previously observed by Mazali and co-workers.21 It is important to underline that this controllable mass gain behavior is an essential issue for the possibility of controlling the desired oxide mass in the glass and, probably, its particle size. The Figure 2a shows the X-ray powder diffraction (XRD) patterns of the xSnO2:Eu3+/PVG (x = 1, 2, 3, 5, or 7 IDC) acquired using the step scanning mode (range of 48 58°, 2θ), and the Figure 2b shows the continuous scanning mode diffractograms (10 70°, 2θ) of the 7SnO2:Eu3+/PVG and SnO2:Eu3+/Bulk samples compared to the standards SnO2 (JCPDF 41 1445), Eu2Sn2O7 (JCPDF 70 1698), and Eu2O3 (JCPDF 34 392) XRD patterns. It can be observed that the synthesized tin oxide is in its rutile form with a tetragonal unit cell system and P42/mmm space group. Other crystalline phases, like Eu2O3 and/or Eu2Sn2O7, are not formed since any additional peaks besides those of the SnO2 cassiterite phase are not observed. The low europium precursor concentration, high homogeneity of the precursors solution and temperature used during the 4512
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Crystal Growth & Design
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Figure 3. Eu3+-doped SnO2 nanoparticle sizes (D, nm) of the xSnO2: Eu3+/PVG materials (x = 1, 2, 3, 5, or 7 IDC) as functions of IDC number.
Figure 2. (a) XRD patterns (48 58°, 2θ) of the xSnO2:Eu3+/PVG samples (x = 1, 2, 3, 5, or 7 IDC) and (b) XRD patterns from 10 to 70° (2θ) of the 7SnO2:Eu3+/PVG and SnO2:Eu3+/bulk samples compared to the SnO2, Eu2Sn2O7 and Eu2O3 standard pattern. The indexed peaks are related to the reflection planes of tetragonal rutile-SnO2, and the peaks marked with * and + are highlighting the amorphous halo and the peak used for Scherrer equations calculations, respectively.
IDC process make the formation of extra phases highly unfavorable. In particular, the europium stannate phase is not expected to be present in the samples because this compound is synthesized from a solid state reaction between SnO2 and Eu2O3 in temperatures (between 1723 and 1823 K) much higher than used during the IDC.22 Additionally, it is verified that the XRD peaks get narrower after each IDC, confirming that the mass increase, observed at the cumulative mass gain curve (Figure 1), is associated to the nanoparticle growing process, leading to larger particles after each synthesis step. At this point, it must be underlined that the variations in the full width at half-maximum (fwhm) values of the XRD peaks are attributed exclusively to size effects since all the samples were thermally treated at the same temperature (1023 K) for exactly the same heating time (56 h), regardless of the IDC number, resulting in SnO2 nanoparticles that are expected to have almost thermodynamically the same crystallinity degree with different particle sizes. The Eu3+-doped SnO2 nanoparticles sizes (D, nm) were calculated using the Scherrer equation (D = Kλ/βsenθ) using the shape factor K = 0.89 and the λ = 1.5406 Å (CuKα1 source). The position and fwhm of the peak at ∼51.8° (2θ) was used for the calculations because it is the most intense peak that has no influence from the vitreous halo of the PVG matrix. The results are presented in Figure 3. It is clear that the SnO2 nanocrystals gradually grow and that the final size depends on the number of IDC to which the PVG monoliths were submitted. This feature is extremely interesting because it confirms that the IDC methodology may be used as a synthesis protocol to obtain size controllable SnO2 nanoparticles.
Figure 4. Diffuse reflectance spectra of the xSnO2:Eu3+/PVG (x = 1, 2, 3, 5, or 7 IDC) and SnO2:Eu3+/bulk samples.
Complementary, the Eu3+-doped SnO2 nanoparticles may be dispersed inside the mesoporous system of the porous glass since that they are smaller (
