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Novel approach to the synthesis of core–shell particles by sacrificial polymer shell method
Materials Letters 64 (2010) 2265–2268
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Novel approach to the synthesis of core–shell particles by sacrificial polymer shell method Paola Fabbri a, Mehdi Ghahari b,c,⁎, Francesco Pilati a, Touradj Ebadzadeh b, Roya Aghababazadeh d,c, Ghasem Kavei b a
Dipartimento di Ingegneria dei Materiali e dell'Ambiente, Università degli Studi di Modena e Reggio Emilia, Via Vignolese 905/A, 41100 Modena, Italy Ceramic Department, Materials and Energy Research Center, Karaj, Iran Inorganic Pigments and Glazes Department, Institute of Colorants, Paints and Coatings, Tehran, Iran d Iran Composites Institute, Iran University of Science and Technology, Iran b c
a r t i c l e
i n f o
Article history: Received 26 May 2010 Accepted 9 July 2010 Available online 16 July 2010 Keywords: Sol–gel preparation Microstructure Core–shell particles Nanocoating
a b s t r a c t In this work a new approach has been developed for the synthesis of SiO2@Y2O3 particles with core–shell structure. The method is based on the synthesis of a covalently bonded sacrificial polymer shell grown onto silica particles. It is suitable to promote and stabilize the adsorption of different ions, namely Yttrium from its nitrate solution. After calcination and consequent elimination of the sacrificial polymer shell, the SiO2@Y2O3 core–shell particles are obtained. Results reveal that the shell thickness of these core–shell particles is higher and more uniform than that of particles prepared without sacrificial polymer shell. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Core–shell architectures in particles offer interesting and synergistic possibilities which are different from the properties of the single component materials: by changing the structure, composition and particles size, the magnetic, optical, mechanical, thermal, electrical, and electro-optical properties could be under the control [1,2]. Many synthetic procedures have been applied to the preparation of such core shell particles, including spray pyrolysis [3], precipitation [4] and the sol–gel process [5]. Up to now, several investigations have been published on silica particles coated by different materials [6,7] as far as SiO2@Y2O3:Eu3+ is concerned, the majority of the papers [8,9] are focused on the effect played by shell thickness, number of coating processes, silica particle size and annealing temperature, on the intensity of luminescence. However, the controlled coating of a silica core with a homogeneous Y2O3 shell without agglomeration remains a challenge for scientists who are interested in this topic [5,10]. The aim of the present work is therefore to find a new procedure to promote the Yttria shell onto silica particles, characterized by enhanced uniformity, in order to decrease the number of coating processes.
2.1. Materials
⁎ Corresponding author. Inorganic Pigments and Glazes Department, Institute of Colorants, Paints and Coatings, Tehran, Iran. Tel.: + 98 21 22956126; fax: + 98 21 22947537. E-mail address: [email protected] (M. Ghahari). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.033
High purity raw materials for the synthesis of silica particles (tetraethyloxsilane 99% (TEOS), absolute ethanol (EtOH), ammonia 28% (NH3·H2O)), Yttria complex (Y2O3 (99.99%) and HNO3), and polymer shell (3-(trimethoxysilyl) propyl methacrylate (MPS), methacryl oxyethyl isocyanate (MOI), tetrahydrofuran, THF and 2-2′ azobisisobutyronitrile (AIBN)), were all obtained from Merck and Sigma-Aldrich (Milan, Italy).
2.2. Synthesis of SiO2@Y2O3 particles Monodisperse spherical silica dispersions were prepared according to Stöber procedure [11] using a mixture of EtOH, H2O and NH3·H2O, following the so-called sol–gel process at low temperature. For each experiment EtOH, H2O (5.5 M) and NH3·H2O (2.7 M) were mixed in a flask. Then TEOS (0.225 M) was added quickly and the reaction mixture was vigorously stirred at 38 °C for 10–12 min. After 12 min from starting the reaction of TEOS, the coupling agent MPS, used to introduce vinyl groups on the silica surfaces, was added to the reaction slurry (0.33 mmol/mmol TEOS), and mixing was maintained for 45 min. Three washing steps with fresh EtOH and H2O, and a final drying at 80 °C under vacuum were done. Following this procedure, monodispersed silica spheres functionalized with MPS, with average diameters 240 ± 11 nm were obtained.
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P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
As a further step, the polymer shell was grown onto MPSfunctionalized silica by free radical polymerization. In a typical preparation, 0.1 g of MPS-functionalized silica were dispersed in 4 ml of THF and then AIBN (4 wt.% with respect to the monomer) was added as initiator. After 2.5 h of mixing at room temperature to promote AIBN adsorption onto Si-MPS particles, the suspension was centrifuged and the residual reintroduced in 5 ml of fresh THF along with 0.1 g of MOI monomer. The polymerization was carried out under N2 atmosphere at 60 °C for 24 h. After polymerization, the silica particles with the polymer shell (silica@MOI) were centrifuged and washed 3 times with fresh THF in order to eliminate any possible trace of residual unreacted monomer. A certain amount (2 g) of Y2O3 was dissolved in nitric acid to form Y(NO3)3 solution (YN). Silica@MOI particles (0.1 g) were dispersed in 20 mL aqueous solution containing yttrium nitrate (16.6 × 10−2 mol/L) in order to promote the sequestering of the Y3+ ions by the functional organic layer. Vigorous mechanical stirring was applied between 24 and 90 h, at a constant temperature of 50 °C. After mixing, the solution was centrifuged and the powder dried under vacuum. Then the powder was calcined at 750 °C for 2 h in air atmosphere to complete elimination of the sacrificial polymer shell and to get SiO2@Y2O3 particles. Fig.1 illustrates the whole synthesis process of those particles. 2.3. Characterization techniques Identification of the polymer and Yttria shell grown onto the silica particles was done by transmission electron microscopy using a JEM 2010 instrument (JEOL Oxford Instruments, UK). The crystal structure of samples was studied by means of X-rays diffraction (XRD, D500 instrument, Siemens, DE). Fourier-transform infrared spectroscopy (FT-IR) was performed with an Avatar 330 FT-IR spectrometer (Thermo Nicolet) equipped with a ZnSe crystal. 3. Results and discussion In this study, MPS was chosen as coupling agent since it can form covalent bonds both with the silica substrate and an organic moiety, because it carries terminal C=C double bonds reactive in the radical
polymerization of the MOI monomer. By the presence of these reactive moieties onto the inorganic cores, reaction of the coupling agent with growing polymer chains could be then expected, leading to chemically bonded polymer chains at the silica surface. The detailed reaction of functionalization of silica with MPS has been explained by Philipse and Vrij [12]. The polymerization of MOI starting from the vinyl groups that functionalize the surface of the silica particles, leads to a variation in the surface properties, wettability and dispersability of the particles due to the presence of an organic shell that make them dispersable in organic solvents, as required from further steps of the overall process. The FT-IR spectra of the pristine silica (dried) before functionalization with MPS is shown in Fig. 2a, where the absorption bands of OH (3400 cm− 1), H2O (1629 cm− 1), Si–O–Si (1099 cm− 1), and Si–OH (947 cm− 1) groups are clearly visible. MPS-functionalized silica particles present a peak at 1710 cm−1 in their spectrum (Fig. 2b), related to the C=O vibration band which is in agreement with its parallel orientation to the SiO2 surface that arises from hydrogen bonding between the carbonyl oxygen and surface silanols [13]. The successful preparation of the core–shell SiO2@MOI particles was firstly confirmed by FT-IR measurements, here reported in Fig. 2c, where peaks related to carbonyl (1724 cm−1), isocyanate (2274 cm−1) and the C–H stretching vibration (2957 cm−1) are clearly visible. A certain amount of isocyanate hydrolyzed to give amine and amide groups, revealed by peaks at 1558 and 3397 cm−1, respectively. Furthermore, the intensity of adsorption bands of silica (Si–O–Si asymmetric stretching vibration at 1000–1100 cm− 1, Si–OH asymmetric bending and stretching vibration at 850–960 cm− 1) is reduced and shifted in comparison to bare silica. The peak located at 1659 cm− 1 is assigned to the formation of urea linkages [14] between MOI chains grafted onto silica surfaces. The morphologies of silica particles after MOI grafting and creation of the polymer shell are shown in Fig. 3a. TEM image of SiO2@MOI shows grey zones of low contrast around the spherical particles. These grey zones are attributed to the grafted MOI domains, indicating that the resulting SiO2@MOI composite particles are of core–shell morphologies with a thick shell of polymer grafted onto the surface and SiO2 in the core. The average thickness of the polymer shell can be estimated around 50 nm.
Fig. 1. Schematic procedure for the synthesis of SiO2@Y2O3 core shell particles by sacrificial polymer shell method.
P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
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Fig. 2. FT-IR spectra of silica particles before (a), after functionalization with MPS (b) and after copolymerization with MOI (c).
Fig. 4. XRD patterns of pure Y2O3 and SiO2@Y2O3 particles prepared with or without the sacrificial MOI-polymer shell, at different mixing time with YN.
TEM micrographs, here reported in Fig. 3 for SiO2@Y2O3 core–shell particles prepared without (b) and with (c and d) polymer shell, revealed that the use of the sacrificial polymer layer allows the formation of a more homogeneous and thicker layer of Yttria onto the silica cores (10–12 nm instead of 3–5 nm obtained using unmodified silica cores), confirming that MOI-polymer act as adsorbent media on the silica core thanks to its amine, amide and urea groups which during the mixing with YN attract and stabilize Y3+ ions. Furthermore, the thermal removal of the sacrificial polymer shell does not affect the adhesion of the Yttria shell to the silica core. The XRD patterns of SiO2@Y2O3 particles prepared with or without the sacrificial MOI-polymer shell are reported in Fig. 4. All samples
were calcinated at 750 °C and the weight of them was equal. As it can be noticed, the position of all the main characteristic peaks (at 2θ values of 29.160, 48.541 and 57.9) in the samples matches those of the reference pattern (JCPDS reference No. 83-927). However there are differences in the intensity of peaks, which increase by increasing the mixing time of the particles with YN. 4. Conclusions A novel route has been described for the synthesis of core shell particles through grafting the poly (methacryl oxyethyl isocyanate) chains on silica surface. These silica particles having a highly
Fig. 3. TEM micrographs of SiO2@MOI (a) SiO2@Y2O3 core–shell particles prepared without (b) and with (c and d) sacrificial polymer shell method after 66 h mixing with YN and then calcination at 750 °C.
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P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
homogeneous polymer shell (thickness 50 nm) have been mixed for different hours with solutions of Y3+ ions in order to promote their sequestering. Calcination at 750 °C induced the complete thermal degradation of the sacrificial polymer shell, and the achievement of a highly homogeneous Y2O3 shell (thickness 12 nm) deposited onto the silica cores. The luminescent core shell particles such as SiO2@Y2O3:Eu3+ can be synthesized by applying this method. References [1] Liu G, Hong G, Sun D. J Colloid Interface Sci 2004;278:133–8. [2] Li G, Wang Z, Quan Z, Liu X, Yu M, Wang R, et al. Surf Sci 2006;600:3321–6. [3] Jung KY, Lee DY, Kang YC, Park HD. J Lumin 2003;105:127–30.
[4] Jimg X, Ireland TG, Gibbons C, Barber DJ, Silver J, Vecht A, et al. J Electrochem Soc 1999;146:4546–51. [5] Wang H, Yu M, Lin C, Liu X, Lin J. J Phys Chem 2007;111:11223–30. [6] Pa J, Du F. Mater Lett 2009;63:2126–8. [7] Lin C, Wang H, Kong D, Yu M, Liu X, Wang Z, et al. Eur J Inorg Chem 2006;18: 3667–75. [8] Yoo HS, Im WB, Kim SW, Kwon BH, Jeon DY. J Lumin 2010;130:153–6. [9] Feng H, Chen Y, Tang F, Ren J. Mater Lett 2006;60:737–40. [10] Fu YX, Sun YH. J Alloys Compd 2009;471:190–6. [11] Stober W, Fink A, Bohn E. J Colloid Interface Sci 1968;26:62–9. [12] Philipse AP, Vrij A. J Colloid Interface Sci 1989;128:121–36. [13] Freris I, Cristofori D, Riello P, Benedetti A. J Colloid Interface Sci 2009;331:351–5. [14] Mishra AK, Chattopadhyay DK, Sreedhar B, Raju KVSN. Prog Org Coat 2006;55: 231–43.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Novel approach to the synthesis of core–shell particles by sacrificial polymer shell method Paola Fabbri a, Mehdi Ghahari b,c,⁎, Francesco Pilati a, Touradj Ebadzadeh b, Roya Aghababazadeh d,c, Ghasem Kavei b a
Dipartimento di Ingegneria dei Materiali e dell'Ambiente, Università degli Studi di Modena e Reggio Emilia, Via Vignolese 905/A, 41100 Modena, Italy Ceramic Department, Materials and Energy Research Center, Karaj, Iran Inorganic Pigments and Glazes Department, Institute of Colorants, Paints and Coatings, Tehran, Iran d Iran Composites Institute, Iran University of Science and Technology, Iran b c
a r t i c l e
i n f o
Article history: Received 26 May 2010 Accepted 9 July 2010 Available online 16 July 2010 Keywords: Sol–gel preparation Microstructure Core–shell particles Nanocoating
a b s t r a c t In this work a new approach has been developed for the synthesis of SiO2@Y2O3 particles with core–shell structure. The method is based on the synthesis of a covalently bonded sacrificial polymer shell grown onto silica particles. It is suitable to promote and stabilize the adsorption of different ions, namely Yttrium from its nitrate solution. After calcination and consequent elimination of the sacrificial polymer shell, the SiO2@Y2O3 core–shell particles are obtained. Results reveal that the shell thickness of these core–shell particles is higher and more uniform than that of particles prepared without sacrificial polymer shell. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Core–shell architectures in particles offer interesting and synergistic possibilities which are different from the properties of the single component materials: by changing the structure, composition and particles size, the magnetic, optical, mechanical, thermal, electrical, and electro-optical properties could be under the control [1,2]. Many synthetic procedures have been applied to the preparation of such core shell particles, including spray pyrolysis [3], precipitation [4] and the sol–gel process [5]. Up to now, several investigations have been published on silica particles coated by different materials [6,7] as far as SiO2@Y2O3:Eu3+ is concerned, the majority of the papers [8,9] are focused on the effect played by shell thickness, number of coating processes, silica particle size and annealing temperature, on the intensity of luminescence. However, the controlled coating of a silica core with a homogeneous Y2O3 shell without agglomeration remains a challenge for scientists who are interested in this topic [5,10]. The aim of the present work is therefore to find a new procedure to promote the Yttria shell onto silica particles, characterized by enhanced uniformity, in order to decrease the number of coating processes.
2.1. Materials
⁎ Corresponding author. Inorganic Pigments and Glazes Department, Institute of Colorants, Paints and Coatings, Tehran, Iran. Tel.: + 98 21 22956126; fax: + 98 21 22947537. E-mail address: [email protected] (M. Ghahari). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.033
High purity raw materials for the synthesis of silica particles (tetraethyloxsilane 99% (TEOS), absolute ethanol (EtOH), ammonia 28% (NH3·H2O)), Yttria complex (Y2O3 (99.99%) and HNO3), and polymer shell (3-(trimethoxysilyl) propyl methacrylate (MPS), methacryl oxyethyl isocyanate (MOI), tetrahydrofuran, THF and 2-2′ azobisisobutyronitrile (AIBN)), were all obtained from Merck and Sigma-Aldrich (Milan, Italy).
2.2. Synthesis of SiO2@Y2O3 particles Monodisperse spherical silica dispersions were prepared according to Stöber procedure [11] using a mixture of EtOH, H2O and NH3·H2O, following the so-called sol–gel process at low temperature. For each experiment EtOH, H2O (5.5 M) and NH3·H2O (2.7 M) were mixed in a flask. Then TEOS (0.225 M) was added quickly and the reaction mixture was vigorously stirred at 38 °C for 10–12 min. After 12 min from starting the reaction of TEOS, the coupling agent MPS, used to introduce vinyl groups on the silica surfaces, was added to the reaction slurry (0.33 mmol/mmol TEOS), and mixing was maintained for 45 min. Three washing steps with fresh EtOH and H2O, and a final drying at 80 °C under vacuum were done. Following this procedure, monodispersed silica spheres functionalized with MPS, with average diameters 240 ± 11 nm were obtained.
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P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
As a further step, the polymer shell was grown onto MPSfunctionalized silica by free radical polymerization. In a typical preparation, 0.1 g of MPS-functionalized silica were dispersed in 4 ml of THF and then AIBN (4 wt.% with respect to the monomer) was added as initiator. After 2.5 h of mixing at room temperature to promote AIBN adsorption onto Si-MPS particles, the suspension was centrifuged and the residual reintroduced in 5 ml of fresh THF along with 0.1 g of MOI monomer. The polymerization was carried out under N2 atmosphere at 60 °C for 24 h. After polymerization, the silica particles with the polymer shell (silica@MOI) were centrifuged and washed 3 times with fresh THF in order to eliminate any possible trace of residual unreacted monomer. A certain amount (2 g) of Y2O3 was dissolved in nitric acid to form Y(NO3)3 solution (YN). Silica@MOI particles (0.1 g) were dispersed in 20 mL aqueous solution containing yttrium nitrate (16.6 × 10−2 mol/L) in order to promote the sequestering of the Y3+ ions by the functional organic layer. Vigorous mechanical stirring was applied between 24 and 90 h, at a constant temperature of 50 °C. After mixing, the solution was centrifuged and the powder dried under vacuum. Then the powder was calcined at 750 °C for 2 h in air atmosphere to complete elimination of the sacrificial polymer shell and to get SiO2@Y2O3 particles. Fig.1 illustrates the whole synthesis process of those particles. 2.3. Characterization techniques Identification of the polymer and Yttria shell grown onto the silica particles was done by transmission electron microscopy using a JEM 2010 instrument (JEOL Oxford Instruments, UK). The crystal structure of samples was studied by means of X-rays diffraction (XRD, D500 instrument, Siemens, DE). Fourier-transform infrared spectroscopy (FT-IR) was performed with an Avatar 330 FT-IR spectrometer (Thermo Nicolet) equipped with a ZnSe crystal. 3. Results and discussion In this study, MPS was chosen as coupling agent since it can form covalent bonds both with the silica substrate and an organic moiety, because it carries terminal C=C double bonds reactive in the radical
polymerization of the MOI monomer. By the presence of these reactive moieties onto the inorganic cores, reaction of the coupling agent with growing polymer chains could be then expected, leading to chemically bonded polymer chains at the silica surface. The detailed reaction of functionalization of silica with MPS has been explained by Philipse and Vrij [12]. The polymerization of MOI starting from the vinyl groups that functionalize the surface of the silica particles, leads to a variation in the surface properties, wettability and dispersability of the particles due to the presence of an organic shell that make them dispersable in organic solvents, as required from further steps of the overall process. The FT-IR spectra of the pristine silica (dried) before functionalization with MPS is shown in Fig. 2a, where the absorption bands of OH (3400 cm− 1), H2O (1629 cm− 1), Si–O–Si (1099 cm− 1), and Si–OH (947 cm− 1) groups are clearly visible. MPS-functionalized silica particles present a peak at 1710 cm−1 in their spectrum (Fig. 2b), related to the C=O vibration band which is in agreement with its parallel orientation to the SiO2 surface that arises from hydrogen bonding between the carbonyl oxygen and surface silanols [13]. The successful preparation of the core–shell SiO2@MOI particles was firstly confirmed by FT-IR measurements, here reported in Fig. 2c, where peaks related to carbonyl (1724 cm−1), isocyanate (2274 cm−1) and the C–H stretching vibration (2957 cm−1) are clearly visible. A certain amount of isocyanate hydrolyzed to give amine and amide groups, revealed by peaks at 1558 and 3397 cm−1, respectively. Furthermore, the intensity of adsorption bands of silica (Si–O–Si asymmetric stretching vibration at 1000–1100 cm− 1, Si–OH asymmetric bending and stretching vibration at 850–960 cm− 1) is reduced and shifted in comparison to bare silica. The peak located at 1659 cm− 1 is assigned to the formation of urea linkages [14] between MOI chains grafted onto silica surfaces. The morphologies of silica particles after MOI grafting and creation of the polymer shell are shown in Fig. 3a. TEM image of SiO2@MOI shows grey zones of low contrast around the spherical particles. These grey zones are attributed to the grafted MOI domains, indicating that the resulting SiO2@MOI composite particles are of core–shell morphologies with a thick shell of polymer grafted onto the surface and SiO2 in the core. The average thickness of the polymer shell can be estimated around 50 nm.
Fig. 1. Schematic procedure for the synthesis of SiO2@Y2O3 core shell particles by sacrificial polymer shell method.
P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
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Fig. 2. FT-IR spectra of silica particles before (a), after functionalization with MPS (b) and after copolymerization with MOI (c).
Fig. 4. XRD patterns of pure Y2O3 and SiO2@Y2O3 particles prepared with or without the sacrificial MOI-polymer shell, at different mixing time with YN.
TEM micrographs, here reported in Fig. 3 for SiO2@Y2O3 core–shell particles prepared without (b) and with (c and d) polymer shell, revealed that the use of the sacrificial polymer layer allows the formation of a more homogeneous and thicker layer of Yttria onto the silica cores (10–12 nm instead of 3–5 nm obtained using unmodified silica cores), confirming that MOI-polymer act as adsorbent media on the silica core thanks to its amine, amide and urea groups which during the mixing with YN attract and stabilize Y3+ ions. Furthermore, the thermal removal of the sacrificial polymer shell does not affect the adhesion of the Yttria shell to the silica core. The XRD patterns of SiO2@Y2O3 particles prepared with or without the sacrificial MOI-polymer shell are reported in Fig. 4. All samples
were calcinated at 750 °C and the weight of them was equal. As it can be noticed, the position of all the main characteristic peaks (at 2θ values of 29.160, 48.541 and 57.9) in the samples matches those of the reference pattern (JCPDS reference No. 83-927). However there are differences in the intensity of peaks, which increase by increasing the mixing time of the particles with YN. 4. Conclusions A novel route has been described for the synthesis of core shell particles through grafting the poly (methacryl oxyethyl isocyanate) chains on silica surface. These silica particles having a highly
Fig. 3. TEM micrographs of SiO2@MOI (a) SiO2@Y2O3 core–shell particles prepared without (b) and with (c and d) sacrificial polymer shell method after 66 h mixing with YN and then calcination at 750 °C.
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P. Fabbri et al. / Materials Letters 64 (2010) 2265–2268
homogeneous polymer shell (thickness 50 nm) have been mixed for different hours with solutions of Y3+ ions in order to promote their sequestering. Calcination at 750 °C induced the complete thermal degradation of the sacrificial polymer shell, and the achievement of a highly homogeneous Y2O3 shell (thickness 12 nm) deposited onto the silica cores. The luminescent core shell particles such as SiO2@Y2O3:Eu3+ can be synthesized by applying this method. References [1] Liu G, Hong G, Sun D. J Colloid Interface Sci 2004;278:133–8. [2] Li G, Wang Z, Quan Z, Liu X, Yu M, Wang R, et al. Surf Sci 2006;600:3321–6. [3] Jung KY, Lee DY, Kang YC, Park HD. J Lumin 2003;105:127–30.
[4] Jimg X, Ireland TG, Gibbons C, Barber DJ, Silver J, Vecht A, et al. J Electrochem Soc 1999;146:4546–51. [5] Wang H, Yu M, Lin C, Liu X, Lin J. J Phys Chem 2007;111:11223–30. [6] Pa J, Du F. Mater Lett 2009;63:2126–8. [7] Lin C, Wang H, Kong D, Yu M, Liu X, Wang Z, et al. Eur J Inorg Chem 2006;18: 3667–75. [8] Yoo HS, Im WB, Kim SW, Kwon BH, Jeon DY. J Lumin 2010;130:153–6. [9] Feng H, Chen Y, Tang F, Ren J. Mater Lett 2006;60:737–40. [10] Fu YX, Sun YH. J Alloys Compd 2009;471:190–6. [11] Stober W, Fink A, Bohn E. J Colloid Interface Sci 1968;26:62–9. [12] Philipse AP, Vrij A. J Colloid Interface Sci 1989;128:121–36. [13] Freris I, Cristofori D, Riello P, Benedetti A. J Colloid Interface Sci 2009;331:351–5. [14] Mishra AK, Chattopadhyay DK, Sreedhar B, Raju KVSN. Prog Org Coat 2006;55: 231–43.