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Effect of La doping on microstructure of SnO2 nanopowders prepared by co-precipitation method
Journal of Non-Crystalline Solids 357 (2011) 1172–1176
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids 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 / j n o n c r y s o l
Effect of La doping on microstructure of SnO2 nanopowders prepared by co-precipitation method Chong Fu a,b,⁎, Junbo Wang b, Minge Yang b, Xiaolei Su b, Jie Xu b, Bailing Jiang a a b
School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, P.R. China College of Mechanical and Electronic Engineering, Xi'an Polytechnic University, Xi'an 710048, P.R. China
a r t i c l e
i n f o
Article history: Received 17 November 2009 Received in revised form 27 September 2010 Available online 20 November 2010 Keywords: La-doped SnO2; Nanopowder; Monolayer dispersion; Chemical co-precipitation
a b s t r a c t A series of La-doped SnO2 nanopowders with various dopant concentrations were prepared by chemical coprecipitation technique, and the nanopowders prepared were characterized by differential scanning calorimeter (DSC), thermo-gravimetric (TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that La doping can obviously prevent the growth of nanosized SnO2 crystallites. When the La concentration reaches and surpasses 5 at.%, SnO2 crystallite size reaches a minimum value and remains almost constant. With the increase of La concentration, La tends to dissolve in the bulk phase of SnO2 to form solid solution below 10 at.% addition and then starts to disperse onto the surface of the solid solution as a monolayer above 10 at.%. The effect of La doping on hindering crystallite growth can be attributed to the solute drag and lattice distortion resulting from La dissolving in the bulk phase of SnO2 to form solid solution, rather than the monolayer of La on the surfaces of the SnO2 powders. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The synthesis of nanocrystalline SnO2 is of great technological and scientific interest owing to the use of these powders as gas sensors, catalysts and electrode materials [1–6]. However, it is very difficult to maintain the nanostructure of SnO2 when it is subjected to heat treatment. The heat treatment steps are fundamental to achieve an optimal combination of mechanical, catalytic and electronic properties. The usage of dopants to control surface properties of SnO2 nanopowders is useful to obtain desirable nanostructures with controlled and predictable macro properties [7]. It has been proved that many additives are quite effective to stabilize the crystalline SnO2 during calcination and the sensing operation and catalytic process at high temperature [8]. These additives can form a solid solution in the bulk [9,10] or a monolayer on the surface of major phase crystallites [11,12], or an own phase. Xie and Tang et al. [13] suggested that a great many oxides and salts can disperse spontaneously onto the surfaces of supports to form a monolayer or submonolayer, because in these cases the monolayer is a thermodynamically stable form. Gao et al. [11] reported that many oxides, such as NiO, CuO, ZnO, Bi2O3, MoO3, Cr2O3 and Sb2O3 can disperse onto the surface of SnO2 as a monolayer by impregnation method. Synchronously, Leite and coworkers [14–17] had carried out a ⁎ Corresponding author. School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, P.R. China. Tel.: + 86 29 82330319; fax: + 86 29 82330353. E-mail address: [email protected] (C. Fu).
series of studies to investigate the catalytic activity for the methanol decomposition, synthesis of methyl vinyl ketone and microstructure in nanoparticles of 5 mol% rare earth-doped (Ce, La, or Y) SnO2 prepared by the polymeric precursor method. They considered that the formation of a segregation layer of rare earth cations on the particle surface during heat treatments play an important role in inhibiting particle growth and improving the thermal stability of SnO2 nanopowders. Nevertheless, a de-mixing process occurs on the surface at thermal treatment temperatures higher than a certain extent, resulting in a two-phase material, and decreasing thermal stability of powders. However, up to now, the effect of the concentration of rare earth dopant on microstructure of SnO2 has not been reported systematically. Furthermore, the microstructure of nanoparticles has been found to be deeply dependent on the material's synthesis method. There have been large quantities of reports about the phenomena of monolayer dispersion of SnO2-based powders [11,12,14–17], but the preparation methods in these reports mainly focus on the impregnation and polymeric precursor method. The aim of the present work is to study systematically the microstructure of SnO2 powders doped with different La dopant concentrations synthesized by chemical coprecipitation technique. 2. Experimental details The undoped and La doped SnO2 powders were synthesized by the chemical co-precipitation method [18]. The aqueous solution of SnCl4, LaCl3 (both cation concentrations were 0.025 mol/L) and a measured amount of decentralization materials (polyethylene glycol) were
0022-3093/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.10.019
C. Fu et al. / Journal of Non-Crystalline Solids 357 (2011) 1172–1176
mixed together according to the [La]/[Sn] molar ratio of 0, 1, 3, 5, 10, 15, and 20 at.%, respectively, and ammonia solution was slowly added to the stirred solution to achieve a complete precipitation of stannic hydroxide until the pH reached 11. The resulting hydroxide was filtered and washed using deionized water up to disappearance of all the chloride ions (AgNO3 test) and dried at 80 °C for 4–6 h. Subsequently, the powder batches were grinded and calcined in air at 400 °C for 1 h. The thermal behaviors of hydroxides were examined by differential scanning calorimeter and thermo-gravimetric analysis (DSC-TG, Netzsch STA449C). The XRD analysis was performed on a Shimadzu 7000 diffractometer, using Cu Kα radiation (λ = 0.15418 nm), at 40 kV and 40 mA. The 2θ scan range was 15–80°, with a step size of 0.02° and a resolution of 0.01°. Crystallite sizes were estimated on the (110) peak by Scherrer equation. TEM was carried out on a JEOL JEM3010 electron microscope operating at 300 keV with 0.17 nm point resolution. For TEM observations, nanopowders were ultrasonically dispersed in ethanol and deposited on amorphous holey carbon membranes. The X-ray photoelectron spectra (XPS) were taken using the Axis ultra spectrometer at 10− 9 Torr using Al Kα radiation (1486.7 eV). The binding energy was calibrated using the peak of C1s (284.8 eV). The concentrations of the surface elements were calculated using the system's database after subtracting the background counts. 3. Results Fig. 1 shows DSC-TG curves of Sn(OH)4 and SnLa(OH)4, respectively. From Fig. 1(a), it can be seen that the endothermic peak of Sn (OH)4 at 70 °C can be attributed to the removal of surface absorptive
Fig. 1. DSC-TG curves of the hydroxides: (a) Sn(OH)4 and (b) SnLa(OH)4.
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Fig. 2. XRD patterns of La doped SnO2 as a function of the dopant concentration.
water of the precursor powders. The sharp peak of heat release at 345.7 °C can be explained as the decomposition of Sn(OH)4. With TG curve, it is known that the final temperature for the complete decomposition of Sn(OH)4 into SnO2 is about 345.7 °C. Fig. 1(b) shows DSC-TG curves of SnLa(OH)4 with different La dopant concentrations. The peaks of heat release at 348.7 and 354.3 °C, respectively, indicate that the powders of SnLa(OH)4 are decomposed into oxide. There is no significant weight loss corresponding to the peaks of heat release at 758.4 and 764.5 °C. It can be considered that La atoms are de-solubilized from the SnO2 matrix and form to La2O3. The peaks of heat release at 934.7 and 905.2 °C correspond to synthesis reaction of SnO2 and La2O3, and the formation of La2Sn2O7 [19]. The XRD patterns of the undoped and La doped SnO2 nanopowders are shown in Fig. 2. It reveals that only the cassiterite phase is observed and no secondary phases are detected, even with the highest La dopant concentration. It can also be found in Fig. 2 that the diffraction peaks (110), (101) and (221) become weaker and broader gradually with the increase of the dopant concentration. The average crystallite sizes of SnO2 nanopowders with various La doping concentrations are shown in Fig. 3. The average crystallite size is 5.2 nm for undoped SnO2, and it decreases to 2.2 nm significantly when the La dopant concentration is 5 at.%. It demonstrates that the doping of La can obviously prevent the growing-up of SnO2 crystallites. However, as the concentration of La reaches and surpasses 5 at.%, SnO2 crystallite size reaches a minimum value and remains almost constant. TEM images of samples are shown in Fig. 4. It can be observed that agglomerates are formed by a superposition and junction of particles. With the aid of selected area electron diffraction pattern (SAED) it is
Fig. 3. The mean crystallite size of La doped SnO2 versus the dopant concentration.
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Fig. 4. TEM micrograph of La doped SnO2 powders as a function of the dopant concentration: (a) 0 at.%; (b) 5 at.%; (c) 10 at.%; (d) 20 at.%.
determined that the powders have crystallized in the cassiterite phase and the lattice fringes can be clearly seen from high-resolution TEM (HRTEM). The XPS survey spectra show that the samples of undoped and 1 at.% La-doped contain only Sn, O elements and a trace amount of carbon, while the La element is detected at dopant concentration higher than 3 at.%. Figs. 5 and 6 present the high-resolution XPS spectra of Sn3d and La3d, respectively. As seen from Fig. 5, the binding energies of the Sn3d5/2 and Sn3d3/2 for undoped sample are 486.8 eV and 495.3 eV, respectively,
Fig. 5. XPS spectra for the Sn3d of La doped SnO2 as a function of the dopant concentration.
which are agreement well with the data of SnO2 in the literature [20]. The results of binding energies of Sn3d for all the doped samples are 486.4–486.6 eV and 494.9–495.1 eV, respectively, according well with those reported by the others [21,22], and it also indicates that the valency of Sn is + 4 in La doped SnO2 nanopowders, whatever dopant concentration is. The displacement in the position of Sn3d peaks in relation to the pure particle suggests that the dopants are strongly incorporated into the Sn–O matrix and Sn–O–La bond is formed in nanoparticles [15], since XPS peak displacement relates to the chemical environment. In Fig. 6, the core level spectra of La3d of samples were curve fitted utilizing Gaussion–Lorentzian functions. The La3d spectra show two double peaks and the energy loss peaks appearing on the high energy sides of the 3d5/2 and 3d3/2 peaks are satellite peaks. The XPS results show that La is in the +3 oxidation state. This is confirmed by the La3d5/2 binding energy position at 834.6–835.2 eV, the value of the spin-orbit splitting (16.7–16.9 eV) and the presence of intense lines of the “shake-up satellites.” The similar La3d5/2 binding energy values have been reported in elsewhere [23]. Fig. 7 shows the molar ratios of [La]/[Sn] for the La-doped SnO2 samples measured by XPS with different La doping concentration. To obtain these data, the ratio [La]/[Sn] was calculated based on the total peak area of La3d and Sn3d with the background already subtracted. When the dopant concentration is lower than 10 at.%, the ratio between [La] and [Sn] slowly grows with the increase of the dopant concentration, thereafter it increases very rapidly at dopant concentration exceeding 10 at.%. Such a similar phenomenon has already been observed in ZrO2–Al2O3 system [24], and reveals that, the solid solution is formed at La concentration below 10 at.%, whereas La may disperse on the surface of SnO 2 as a monolayer at dopant concentration above 10 at.%.
C. Fu et al. / Journal of Non-Crystalline Solids 357 (2011) 1172–1176
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Fig. 6. XPS spectra for the La3d of La doped SnO2 as a function of the dopant concentration: (a) 5 at.%; (b) 10 at.%; (c) 15 at.%; (d) 20 at.%.
4. Discussion As mentioned above, when the dopant concentration is below 5 at.%, the crystallite size of SnO2 decreases quickly with the increasing of La concentration. As the concentration of La approaches to 5 at.%, the increase of the size of crystalline SnO2 is suppressed to a great extent, and it reaches a minimum value and does not change any more. Considering that particle growth is dependent of the particle boundary motion [25]. There are two distinct ways can be used to prevent the particle growth, one by the reduction of the thermodynamic driving
force (surface energy) and another by slow down of particle boundary mobility[26]. For the solid solution systems, the addition of dopant can cause the solute drag (who causes a decrease in particle mobility) and lattice distortion, which may contribute to the inhibition of growing-up of SnO2 crystallites. On the other hand, for the monolayer-dispersed systems, the formation of the monolayer also may produce two beneficial effects. The first can contribute to a decreased surface energy, acting on the driving force. The second will segregate the SnO2 nanoparticles from each other before the particles have grown into bigger ones, its boundary motion will be suppressed, and the thermal stability of the doped system can be improved. In the present work, for the La-doped SnO2 nanopowders prepared by chemical co-precipitation method, it is considered that La doping can inhibit the growth of nanosized SnO2 crystallites, which can be mainly attributed to solute drag and lattice distortion rather than the monolayer of La on the surfaces of SnO2 powders.
5. Conclusion
Fig. 7. [La]/[Sn] ratio for La doped SnO2 versus the dopant concentration.
The influence of La doping concentration on microstructure of SnO2 nanoparticles was studied. The results show that La doping can obviously prevent the growth of nanosized SnO2 crystallites at La dopant concentrations are less than 5 at.%. As La concentration below 10 at.%, La tends to dissolve in the bulk phase of SnO2 to form solid solution, whereas La may disperse on the surface of SnO2 as a monolayer at dopant concentration above 10 at.%. La doping for hindering crystallite growth can be attributed to the solute drag and lattice distortion resulting from La dissolving in the bulk phase of SnO2 to form solid solution.
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Acknowledgments The authors gratefully acknowledge the financial support of the Science and Technology Department of Shannxi of China (grant 2007K06-09), the Education Office, Shannxi of China (grants 09JK446 and 07JK259), the National Natural Scientific Foundation of China (grant 51002113), and the Research Projects of Science and Technology of Xi'an Polytechnic University of China (grant 2010JC10). References [1] M. Rumyantseva, V. Kovalenko, A. Gaskov, E. Makshina, V. Yuschenko, et al., Sens. Actuators, B 118 (2006) 208. [2] T. Jinkawa, G. Sakai, J. Tamaki, N. Miura, N. Yamazoe, J. Mol. Catal. A: Chem. 155 (2000) 193. [3] J.G. Nam, H. Choi, S.H. Kim, K.H. Song, S.C. Park, Scr. Mater. 44 (2001) 2047. [4] F. Morazzoni, C. Canevali, N. Chiodini, et al., Chem. Mater. 13 (2001) 4355. [5] A. Circra, A. Vila, A. Cornct, J.R. Morantc, Mater. Sci. Eng., C 15 (2001) 203. [6] B. Satyavathi, A.N. Patwari, M.B. Rao, Appl. Catal., A 246 (2003) 151. [7] H.R. Ricardo, J.P. Gilberto, G. Douglas, Appl. Surf. Sci. 253 (2007) 4581. [8] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators, B 3 (1991) 147. [9] M.N. Rumyantseva, V.V. Kovalenko, A.M. Gaskov, T. Pagnier, et al., Sens. Actuators, B 109 (2005) 64.
[10] J.Z. Jiang, R. Lin, K. Nielsen, S. Mørup, K. Dam-Johansen, R. Clasen, J. Phys. D Appl. Phys. 30 (1997) 1459. [11] Y. Gao, H. Zhao, B. Zhao, J. Mater. Sci. 35 (2000) 917. [12] R.H.R. Castro, P. Hidalgo, R. Muccillo, et al., Appl. Surf. Sci. 214 (2003) 172. [13] Y.C. Xie, Y.Q. Tang, Adv. Catal. 37 (1990) 1. [14] E.R. Leite, A.P. Maciel, I.T. Weber, P.N. Lisboa-Filho, et al., Adv. Mater. 14 (2002) 905. [15] I.T. Weber, A.P. Maciel, P.N. Lisboa-Filho, E. Longo, E.R. Leite, Nano Lett. 2 (2002) 969. [16] N.L.V. Carreño, A.P. Maciel, E.R. Leite, P.N. Lisboa-Filho, et al., Sens. Actuators, B 86 (2002) 185. [17] N.L.V. Carreño, H.V. Fajardo, A.E. Maciel, A. Valentini, F.M. Pontes, et al., J. Mol. Catal. A: Chem. 207 (2004) 91. [18] J. Wang, M. Yang, Y. Li, et al., J. Non-Cryst. Solids 351 (2005) 228. [19] M.G. Yang, J.B. Wang, L.C. Chen, B.J. Ding, Mod. Chem. Ind. 26 (2006) 1798 (In Chinese). [20] J.A. Taylor, G.M. Lancaster, J.W. Rabalais, J. Electron. Spectrosc. Relat. Phenom. 13 (1978) 435. [21] L. Yan, J.S. Pan, C.K. Ong, Mater. Sci. Eng., B 128 (2006) 34. [22] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, G. Czempik, J. Szuber, Thin Solid Films 490 (2005) 36. [23] M.V. Bukhtiyarova, A.S. Ivanova, L.M. Plyasova, G.S. Litvak, Appl. Catal., A 357 (2009) 193. [24] J. Liang, H.Z. Huang, Y.C. Xie, Acta Phys. Chim. Sin. 19 (2003) 308 (In Chinese). [25] Y.M. Chiang, D.P. Birnie-III, W.D. Kingery, John Wiley & Sons, New York, 1997. [26] A.P. Maciel, P.N. Lisboa-Filho, E.R. Leite, C.O. Paiva-Santos, et al., J. Eur. Ceram. Soc. 23 (2003) 707.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids 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 / j n o n c r y s o l
Effect of La doping on microstructure of SnO2 nanopowders prepared by co-precipitation method Chong Fu a,b,⁎, Junbo Wang b, Minge Yang b, Xiaolei Su b, Jie Xu b, Bailing Jiang a a b
School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, P.R. China College of Mechanical and Electronic Engineering, Xi'an Polytechnic University, Xi'an 710048, P.R. China
a r t i c l e
i n f o
Article history: Received 17 November 2009 Received in revised form 27 September 2010 Available online 20 November 2010 Keywords: La-doped SnO2; Nanopowder; Monolayer dispersion; Chemical co-precipitation
a b s t r a c t A series of La-doped SnO2 nanopowders with various dopant concentrations were prepared by chemical coprecipitation technique, and the nanopowders prepared were characterized by differential scanning calorimeter (DSC), thermo-gravimetric (TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that La doping can obviously prevent the growth of nanosized SnO2 crystallites. When the La concentration reaches and surpasses 5 at.%, SnO2 crystallite size reaches a minimum value and remains almost constant. With the increase of La concentration, La tends to dissolve in the bulk phase of SnO2 to form solid solution below 10 at.% addition and then starts to disperse onto the surface of the solid solution as a monolayer above 10 at.%. The effect of La doping on hindering crystallite growth can be attributed to the solute drag and lattice distortion resulting from La dissolving in the bulk phase of SnO2 to form solid solution, rather than the monolayer of La on the surfaces of the SnO2 powders. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The synthesis of nanocrystalline SnO2 is of great technological and scientific interest owing to the use of these powders as gas sensors, catalysts and electrode materials [1–6]. However, it is very difficult to maintain the nanostructure of SnO2 when it is subjected to heat treatment. The heat treatment steps are fundamental to achieve an optimal combination of mechanical, catalytic and electronic properties. The usage of dopants to control surface properties of SnO2 nanopowders is useful to obtain desirable nanostructures with controlled and predictable macro properties [7]. It has been proved that many additives are quite effective to stabilize the crystalline SnO2 during calcination and the sensing operation and catalytic process at high temperature [8]. These additives can form a solid solution in the bulk [9,10] or a monolayer on the surface of major phase crystallites [11,12], or an own phase. Xie and Tang et al. [13] suggested that a great many oxides and salts can disperse spontaneously onto the surfaces of supports to form a monolayer or submonolayer, because in these cases the monolayer is a thermodynamically stable form. Gao et al. [11] reported that many oxides, such as NiO, CuO, ZnO, Bi2O3, MoO3, Cr2O3 and Sb2O3 can disperse onto the surface of SnO2 as a monolayer by impregnation method. Synchronously, Leite and coworkers [14–17] had carried out a ⁎ Corresponding author. School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048, P.R. China. Tel.: + 86 29 82330319; fax: + 86 29 82330353. E-mail address: [email protected] (C. Fu).
series of studies to investigate the catalytic activity for the methanol decomposition, synthesis of methyl vinyl ketone and microstructure in nanoparticles of 5 mol% rare earth-doped (Ce, La, or Y) SnO2 prepared by the polymeric precursor method. They considered that the formation of a segregation layer of rare earth cations on the particle surface during heat treatments play an important role in inhibiting particle growth and improving the thermal stability of SnO2 nanopowders. Nevertheless, a de-mixing process occurs on the surface at thermal treatment temperatures higher than a certain extent, resulting in a two-phase material, and decreasing thermal stability of powders. However, up to now, the effect of the concentration of rare earth dopant on microstructure of SnO2 has not been reported systematically. Furthermore, the microstructure of nanoparticles has been found to be deeply dependent on the material's synthesis method. There have been large quantities of reports about the phenomena of monolayer dispersion of SnO2-based powders [11,12,14–17], but the preparation methods in these reports mainly focus on the impregnation and polymeric precursor method. The aim of the present work is to study systematically the microstructure of SnO2 powders doped with different La dopant concentrations synthesized by chemical coprecipitation technique. 2. Experimental details The undoped and La doped SnO2 powders were synthesized by the chemical co-precipitation method [18]. The aqueous solution of SnCl4, LaCl3 (both cation concentrations were 0.025 mol/L) and a measured amount of decentralization materials (polyethylene glycol) were
0022-3093/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.10.019
C. Fu et al. / Journal of Non-Crystalline Solids 357 (2011) 1172–1176
mixed together according to the [La]/[Sn] molar ratio of 0, 1, 3, 5, 10, 15, and 20 at.%, respectively, and ammonia solution was slowly added to the stirred solution to achieve a complete precipitation of stannic hydroxide until the pH reached 11. The resulting hydroxide was filtered and washed using deionized water up to disappearance of all the chloride ions (AgNO3 test) and dried at 80 °C for 4–6 h. Subsequently, the powder batches were grinded and calcined in air at 400 °C for 1 h. The thermal behaviors of hydroxides were examined by differential scanning calorimeter and thermo-gravimetric analysis (DSC-TG, Netzsch STA449C). The XRD analysis was performed on a Shimadzu 7000 diffractometer, using Cu Kα radiation (λ = 0.15418 nm), at 40 kV and 40 mA. The 2θ scan range was 15–80°, with a step size of 0.02° and a resolution of 0.01°. Crystallite sizes were estimated on the (110) peak by Scherrer equation. TEM was carried out on a JEOL JEM3010 electron microscope operating at 300 keV with 0.17 nm point resolution. For TEM observations, nanopowders were ultrasonically dispersed in ethanol and deposited on amorphous holey carbon membranes. The X-ray photoelectron spectra (XPS) were taken using the Axis ultra spectrometer at 10− 9 Torr using Al Kα radiation (1486.7 eV). The binding energy was calibrated using the peak of C1s (284.8 eV). The concentrations of the surface elements were calculated using the system's database after subtracting the background counts. 3. Results Fig. 1 shows DSC-TG curves of Sn(OH)4 and SnLa(OH)4, respectively. From Fig. 1(a), it can be seen that the endothermic peak of Sn (OH)4 at 70 °C can be attributed to the removal of surface absorptive
Fig. 1. DSC-TG curves of the hydroxides: (a) Sn(OH)4 and (b) SnLa(OH)4.
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Fig. 2. XRD patterns of La doped SnO2 as a function of the dopant concentration.
water of the precursor powders. The sharp peak of heat release at 345.7 °C can be explained as the decomposition of Sn(OH)4. With TG curve, it is known that the final temperature for the complete decomposition of Sn(OH)4 into SnO2 is about 345.7 °C. Fig. 1(b) shows DSC-TG curves of SnLa(OH)4 with different La dopant concentrations. The peaks of heat release at 348.7 and 354.3 °C, respectively, indicate that the powders of SnLa(OH)4 are decomposed into oxide. There is no significant weight loss corresponding to the peaks of heat release at 758.4 and 764.5 °C. It can be considered that La atoms are de-solubilized from the SnO2 matrix and form to La2O3. The peaks of heat release at 934.7 and 905.2 °C correspond to synthesis reaction of SnO2 and La2O3, and the formation of La2Sn2O7 [19]. The XRD patterns of the undoped and La doped SnO2 nanopowders are shown in Fig. 2. It reveals that only the cassiterite phase is observed and no secondary phases are detected, even with the highest La dopant concentration. It can also be found in Fig. 2 that the diffraction peaks (110), (101) and (221) become weaker and broader gradually with the increase of the dopant concentration. The average crystallite sizes of SnO2 nanopowders with various La doping concentrations are shown in Fig. 3. The average crystallite size is 5.2 nm for undoped SnO2, and it decreases to 2.2 nm significantly when the La dopant concentration is 5 at.%. It demonstrates that the doping of La can obviously prevent the growing-up of SnO2 crystallites. However, as the concentration of La reaches and surpasses 5 at.%, SnO2 crystallite size reaches a minimum value and remains almost constant. TEM images of samples are shown in Fig. 4. It can be observed that agglomerates are formed by a superposition and junction of particles. With the aid of selected area electron diffraction pattern (SAED) it is
Fig. 3. The mean crystallite size of La doped SnO2 versus the dopant concentration.
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Fig. 4. TEM micrograph of La doped SnO2 powders as a function of the dopant concentration: (a) 0 at.%; (b) 5 at.%; (c) 10 at.%; (d) 20 at.%.
determined that the powders have crystallized in the cassiterite phase and the lattice fringes can be clearly seen from high-resolution TEM (HRTEM). The XPS survey spectra show that the samples of undoped and 1 at.% La-doped contain only Sn, O elements and a trace amount of carbon, while the La element is detected at dopant concentration higher than 3 at.%. Figs. 5 and 6 present the high-resolution XPS spectra of Sn3d and La3d, respectively. As seen from Fig. 5, the binding energies of the Sn3d5/2 and Sn3d3/2 for undoped sample are 486.8 eV and 495.3 eV, respectively,
Fig. 5. XPS spectra for the Sn3d of La doped SnO2 as a function of the dopant concentration.
which are agreement well with the data of SnO2 in the literature [20]. The results of binding energies of Sn3d for all the doped samples are 486.4–486.6 eV and 494.9–495.1 eV, respectively, according well with those reported by the others [21,22], and it also indicates that the valency of Sn is + 4 in La doped SnO2 nanopowders, whatever dopant concentration is. The displacement in the position of Sn3d peaks in relation to the pure particle suggests that the dopants are strongly incorporated into the Sn–O matrix and Sn–O–La bond is formed in nanoparticles [15], since XPS peak displacement relates to the chemical environment. In Fig. 6, the core level spectra of La3d of samples were curve fitted utilizing Gaussion–Lorentzian functions. The La3d spectra show two double peaks and the energy loss peaks appearing on the high energy sides of the 3d5/2 and 3d3/2 peaks are satellite peaks. The XPS results show that La is in the +3 oxidation state. This is confirmed by the La3d5/2 binding energy position at 834.6–835.2 eV, the value of the spin-orbit splitting (16.7–16.9 eV) and the presence of intense lines of the “shake-up satellites.” The similar La3d5/2 binding energy values have been reported in elsewhere [23]. Fig. 7 shows the molar ratios of [La]/[Sn] for the La-doped SnO2 samples measured by XPS with different La doping concentration. To obtain these data, the ratio [La]/[Sn] was calculated based on the total peak area of La3d and Sn3d with the background already subtracted. When the dopant concentration is lower than 10 at.%, the ratio between [La] and [Sn] slowly grows with the increase of the dopant concentration, thereafter it increases very rapidly at dopant concentration exceeding 10 at.%. Such a similar phenomenon has already been observed in ZrO2–Al2O3 system [24], and reveals that, the solid solution is formed at La concentration below 10 at.%, whereas La may disperse on the surface of SnO 2 as a monolayer at dopant concentration above 10 at.%.
C. Fu et al. / Journal of Non-Crystalline Solids 357 (2011) 1172–1176
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Fig. 6. XPS spectra for the La3d of La doped SnO2 as a function of the dopant concentration: (a) 5 at.%; (b) 10 at.%; (c) 15 at.%; (d) 20 at.%.
4. Discussion As mentioned above, when the dopant concentration is below 5 at.%, the crystallite size of SnO2 decreases quickly with the increasing of La concentration. As the concentration of La approaches to 5 at.%, the increase of the size of crystalline SnO2 is suppressed to a great extent, and it reaches a minimum value and does not change any more. Considering that particle growth is dependent of the particle boundary motion [25]. There are two distinct ways can be used to prevent the particle growth, one by the reduction of the thermodynamic driving
force (surface energy) and another by slow down of particle boundary mobility[26]. For the solid solution systems, the addition of dopant can cause the solute drag (who causes a decrease in particle mobility) and lattice distortion, which may contribute to the inhibition of growing-up of SnO2 crystallites. On the other hand, for the monolayer-dispersed systems, the formation of the monolayer also may produce two beneficial effects. The first can contribute to a decreased surface energy, acting on the driving force. The second will segregate the SnO2 nanoparticles from each other before the particles have grown into bigger ones, its boundary motion will be suppressed, and the thermal stability of the doped system can be improved. In the present work, for the La-doped SnO2 nanopowders prepared by chemical co-precipitation method, it is considered that La doping can inhibit the growth of nanosized SnO2 crystallites, which can be mainly attributed to solute drag and lattice distortion rather than the monolayer of La on the surfaces of SnO2 powders.
5. Conclusion
Fig. 7. [La]/[Sn] ratio for La doped SnO2 versus the dopant concentration.
The influence of La doping concentration on microstructure of SnO2 nanoparticles was studied. The results show that La doping can obviously prevent the growth of nanosized SnO2 crystallites at La dopant concentrations are less than 5 at.%. As La concentration below 10 at.%, La tends to dissolve in the bulk phase of SnO2 to form solid solution, whereas La may disperse on the surface of SnO2 as a monolayer at dopant concentration above 10 at.%. La doping for hindering crystallite growth can be attributed to the solute drag and lattice distortion resulting from La dissolving in the bulk phase of SnO2 to form solid solution.
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Acknowledgments The authors gratefully acknowledge the financial support of the Science and Technology Department of Shannxi of China (grant 2007K06-09), the Education Office, Shannxi of China (grants 09JK446 and 07JK259), the National Natural Scientific Foundation of China (grant 51002113), and the Research Projects of Science and Technology of Xi'an Polytechnic University of China (grant 2010JC10). References [1] M. Rumyantseva, V. Kovalenko, A. Gaskov, E. Makshina, V. Yuschenko, et al., Sens. Actuators, B 118 (2006) 208. [2] T. Jinkawa, G. Sakai, J. Tamaki, N. Miura, N. Yamazoe, J. Mol. Catal. A: Chem. 155 (2000) 193. [3] J.G. Nam, H. Choi, S.H. Kim, K.H. Song, S.C. Park, Scr. Mater. 44 (2001) 2047. [4] F. Morazzoni, C. Canevali, N. Chiodini, et al., Chem. Mater. 13 (2001) 4355. [5] A. Circra, A. Vila, A. Cornct, J.R. Morantc, Mater. Sci. Eng., C 15 (2001) 203. [6] B. Satyavathi, A.N. Patwari, M.B. Rao, Appl. Catal., A 246 (2003) 151. [7] H.R. Ricardo, J.P. Gilberto, G. Douglas, Appl. Surf. Sci. 253 (2007) 4581. [8] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators, B 3 (1991) 147. [9] M.N. Rumyantseva, V.V. Kovalenko, A.M. Gaskov, T. Pagnier, et al., Sens. Actuators, B 109 (2005) 64.
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