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Author's personal copy Materials Science and Engineering B 172 (2010) 68–71
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Structural study of spray dried silica–germanate nanoparticles Diana-Louisa Trandafir ∗ , R.V.F. Turcu, S. Simon Babes-Bolyai University, Physics Faculty & Institute of Interdisciplinary Research in Bio-Nano-Sciences, 400084 Cluj-Napoca, Romania
a r t i c l e
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Article history: Received 30 November 2009 Received in revised form 8 April 2010 Accepted 21 April 2010 PACS: 81.07.−b 76.30.−v 61.05.Qr 82.56.Vb Keywords: Nanostructure Sol–gel Spray dried 29 Si MAS-NMR Gd3+ EPR
a b s t r a c t Noncrystalline materials belonging to 0.005Gd2 O3 ·0.995[(1 − x)SiO2 ·xGeO2 ] system, with Si/Ge ratio ranging from 1:1 to 1:4 (x = 0.5; 0.667; 0.75; 0.8) were prepared by sol–gel and spray drying method. The samples were characterized by X-ray diffraction, scanning electron microscopy, electron paramagnetic resonance and nuclear magnetic resonance. The XRD patterns indicate that all prepared samples have an amorphous structure. The SEM imagines show that the samples are quite spherical conglomerates of amorphous nanoparticles. Gd3+ EPR spectra recorded from the gadolinium doped silica–germanate samples are specific for noncrystalline systems and show that gadolinium ions are mainly clustered on the nanoparticles surface. Solid-state 29 Si MAS-NMR results indicate the Si/Ge ratio depending presence of Q2 , Q3 and Q4 units in the silica–germanate samples, and a less polymerized silica network with increasing of germanium content. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Silicate systems containing different rare earth oxides, obtained by sol–gel techniques [1], are important in various fields of technology including laser and optical waveguides, in telecommunication applications, microelectronics and catalysis [2,3], due to their high chemical durability and thermal stability [4]. This technique has been successfully used for synthesis of SiO2 –GeO2 thin films, for planar waveguide application [5–7] or for obtaining large surface area xerogels or aerogels [8]. The sol–gel procedure only is quite difficult to be used for synthesis of silica high germanates, so it is necessary to be combined with another one, like spray drying. Spray drying is a widely used manufacturing process which transforms the aerosol phase in dry particles. The technology has been applied in many areas, including the food, pharmaceutical, ceramic, polymer and chemical industries [9–11]. Spray drying is a well-established technique but it remains an active field of innovation, driven by the ever increasing demand for more sophisticated particles. Hollow, low-density particles with controlled surface morphology, particles with functional layers, or particles comprising smaller subunits such as nanoparticles or defined voids, have
∗ Corresponding author. E-mail addresses: diana.trandafi
[email protected], dianalouisa trandafi
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[email protected] (D.-L. Trandafir). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.04.018
been produced. Depending on the preparation method and material used, microspheres show a typical size distribution which often deviates from the ideal mono-sized. The aim of this study is to characterize the structure of noncrystalline compounds of SiO2 –GeO2 system doped with gadolinium, prepared by sol–gel method and than spray dried. The combined techniques offered the possibility to prepare noncrystalline samples with high germanium content, at low temperature. The gadolinium has been added in order to cheek the rare earths distribution and local structure around them in these materials.
2. Experimental 2.1. Samples preparation The investigated samples belong to the 0.005Gd2 O3 ·0.995 [(1 − x)SiO2 ·xGeO2 ] system, with Si/Ge ratio from 1:1 to 1:4 (x = 0.5; 0.667; 0.75; 0.8). They were prepared by spray drying of sol–gels obtained from aqueous solution of Si(OC2 H5 )4 (TEOS), Ge(OC2 H5 )4 (TEOG) and Gd(NO3 )3 ·6H2 O of analytical purity grade. TEOS was mixed with ethanol and then a small amount of hydrochloric acid (HCl) was added as catalyst and stirred for about 1 h at room temperature. The molar ratio of TEOS:ethanol:HCl was 1.5:1.5:1. The resulted pH of the solution was ∼3–3.5. Germanium sols were prepared simply by mixing the desired amount of germanium alkoxide with ethanol, because the germanium alkoxide
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used in this work, Ge(OC2 H5 )4 , is extremely sensitive to humidity and it must be modified before use, in order to reduce its rate of hydrolysis. Hence, the first step for this component is the procedure of alcohol interchange [12]. The molar ratio of TEOG:ethanol was 1:5. The Gd(NO3 )3 ·6H2 O was dissolved in ethanol at a molar ratio of 1:40. The multi-component sol was obtained by mixing the three sols under vigorous stirring for about 1 h and then the resulting sol–gel was sprayed, using a Buchi-290 Mini Spray-dryer with two-fluid nozzle. Spray nozzle had a nozzle tip of 1.4 mm. The flow type is co-current with mixing of air and liquid at the nozzle head. The air spray flow rate was varied between 45 and 55 l/h and the aspirator rate were kept constant at 95%. The inlet temperature was controlled at 95 ◦ C for all samples. The outlet temperature was determined by the inlet temperature and relative factors such as air and liquid flow rates, varying between 43 and 54 ◦ C. All processing parameters for the sol–gel and spray drying procedure were the same for all prepared samples. 2.2. Techniques Fig. 1. The XRD patterns of the spray dried silica–germanate samples.
The structure of the samples was examined by X-ray diffraction with a LabX XRD-6000 Shimadzu diffractometer using Cu K␣ ( = 1.5405 Å) radiation. The measurements were performed in 2 geometry, with a scanning speed of 2◦ /min. The operation voltage was 40 kV and the current was 30 mA. The size distribution and the surface morphology of the dried particles were analysed by scanning electron microscopy (SEM) with a Jeol JSM 5510LV microscope. The EPR measurements were performed on powder samples using an ADANI X-band EPR spectrometer, in the magnetic field range 700–4700 G, at room temperature. The 29 Si MAS-NMR spectra were recorded on solid sample at room temperature, using a Bruker AVANCE 400 MHz Ultra Shield spectrometer, at a spinning speed of 11 kHz. The samples were centred-packed in zirconium rotors to minimize the effect of rf field inhomogeneity. The NMR signal of 29 Si nucleus of tetramethylsilane was used as reference signal for 29 Si MAS-NMR spectra.
germanate network, were they had a much distorted environment [16]. This broadening is naturally ascribed to dipole–dipole interaction between Gd3+ ions but also to the local disorder [17–20]. The larger lines expected for such a low concentration (0.5 mol% Gd2 O3 ), considering that the gadolinium would be homogeneously distributed in the entire sample volume, support the assumption of its distribution on the nanoparticles surface. In this case the mean distance between the gadolinium ions is shorter than for uniform bulk distribution and they are experiencing stronger mag-
3. Results and discussion 3.1. XRD The X-ray powder diffraction patterns (Fig. 1) prove the noncrystalinity of the samples [7,13–15]. The existence of small nanocrystals, of few nanometres size, is suggested by the weak features on XRD patterns, around 38◦ and 64◦ , better evidenced for the samples with higher germanium content. 3.2. SEM The SEM imagines show that the samples are quite spherical conglomerates (Fig. 2a) of amorphous nanoparticles (Fig. 2b). 3.3. EPR The structural order around Gd3+ ions and their magnetic interactions were investigated by electron paramagnetic resonance. The presence of Gd3+ in the samples inform on the structural changes occurred in their vicinity function on matrix composition and would be very useful for characterizing the sites distribution of other nonparamagnetic rare earths introduced in the same matrices. The EPR spectra of the investigated samples are presented in Fig. 3. The spectra consist of relative large line with g = 2.0, denoting that in their surrounding the Gd3+ ions are experiencing weak crystal fields, that means they are not included in the silicate or
Fig. 2. SEM imagines of the 1:2 sample.
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Fig. 3. The EPR spectra of 0.005Gd2 O3 ·0.995[(1 − x)SiO2 ·xGeO2 ] samples.
Fig. 5. The 29 Si MAS-NMR spectra of the 1:4 sample (experimental and simulated).
Fig. 6. Composition dependence of Qn units fraction.
3.4. NMR The 29 Si MAS-NMR experimental spectra are presented in Fig. 4. The NMR spectra consist of large featureless line extended on the region −90 to −120 ppm. In fully dense, amorphous SiO2 all the silicon would be expected to occur in Q4 units, but it was showed [21] that, due to the presence of OH groups, some Q3 species are also present. The deconvolution of 29 Si MAS-NMR spectra was made using the DmFit program [22]. Due to the unresolved lines in the experimental spectra, the deconvolution uncertainty is quite important,
Fig. 4. The 29 Si MAS-NMR spectra of 0.005Gd2 O3 ·0.995[(1 − x)SiO2 ·xGeO2 ] samples.
netic interactions. The observed differences in the shape of the lines, depending on the sample composition, show that this distribution on the nanoparticles surface is not uniform, being much clustered for the samples with 1:2 and 1:3 Si/Ge ratio, where the exchange interaction effect is clearly evidenced. Table 1 Qn units fraction (%), corresponding chemical shift (ppm) and line width (ppm). Si/Ge
Qn Q2
1:1 1:2 1:3 1:4
Q3
Q4
Chemical shift (ppm)
Fraction (%)
Width (ppm)
Chemical shift (ppm)
Fraction (%)
Width (ppm)
Chemical shift (ppm)
Fraction (%)
Width (ppm)
– – −92.0 −94.9
– – 5 8
– – 61.5 13.4
−99.0 −100.0 −99.7 −99.1
22 17 13 10
13.7 12.3 9.7 9.0
−107.2 −108.3 −108.5 −108.9
78 83 82 82
12.6 11.8 11.0 11.9
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but as illustrated in Fig. 5 the samples contain three types of Si environments which can be easily attributed to the following structural units: Q4 (silica-type tetrahedral with four bridging oxygens), Q3 (one non-bridging oxygen) and Q2 (two non-bridging oxygens) [21,23–26]. The Qn fractions, the chemical shift and the width of the lines obtained from the deconvolution of the experimental spectra are summarised in Table 1. By increasing the germanium content from 1:1 to 1:2 Si/Ge ratio, the fraction of Q4 units slightly increases, and further is kept relatively constant in 1:3 and 1:4 samples (Fig. 6). However, the increasing Si/Ge ratio determines a higher number of non-bridging oxygens in the SiO4 tetrahedra, that leads to the occurrence of less connected Q2 units. The weak depolimerization of the silica network in the region of higher germanium content takes place especially at the expense of Q3 units. 4. Conclusions Noncrystalline and nanostructured compounds of SiO2 –GeO2 system doped with gadolinium have been prepared by sol–gel and spray drying method, in the high germanium compositional range, with Si/Ge ratio between 1:1 and 1:4. All sample were conglomerates of amorphous nanoparticles, small features on the X-ray diffractions patterns denoting also the existence of some nanocrystals of few nanometres size. In the silica–germanate nanoparticles the Gd3+ ions are mainly distributed at their surface, as reflected by the shape of EPR lines typical for noncrystalline samples with high gadolinium concentration. The silica network is formed by Q2 , Q3 and Q4 structural units and it becomes less polymerized when the germanium content increases, as result of Q2 units fraction increase at the expense of Q3 units. Acknowledgment The present work was supported by the National University Research Council—CNCSIS, Romania, under PN II, ID-566/2007 project.
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