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Preparation of indium tin oxide (ITO) nanoparticles by DC arc plasma
Surface & Coatings Technology 205 (2010) S201–S205
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Preparation of indium tin oxide (ITO) nanoparticles by DC arc plasma Dong-Wook Kim, Dong-Wha Park ⁎ Department of Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma ( RIC-ETTP ), INHA University, 253 Yonghyun-dong, Nam-gu, Incheon, 402-751, Korea
a r t i c l e
i n f o
Available online 3 August 2010 Keywords: Plasma Indium tin oxide Nanoparticles
a b s t r a c t Tin-doped indium oxide (ITO) nanoparticles were prepared by DC arc plasma. The mixture of In(OH)3 and SnCl2 as precursors was injected into the arc plasma through a powder feeder and vaporized by the arc plasma. The ITO nanoparticles were synthesized through the rapid quenching of the thermal plasma processing and did not require post-treatment. The doped level of the synthesized particles was controlled by varying the molar ratio of SnCl2 in the powder feeder. The obtained nanoparticles were characterized by X-ray diffractometer (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and four-probe analysis. The XRD results showed a shift of the peak, a broadening of the peak width and the decrease of the peak intensity as the doping level increased. The doping amount of tin was analyzed by ICP-OES. XPS results presented that most of the tin was incorporated into the indium oxide lattice without the formation of any tin oxide. The particle size ranged from 5 nm to 25 nm. The faceted morphologies of the prepared ITO nanoparticles could be observed using TEM. Additionally, the transmittances and the electrical conductivities of the prepared ITO were measured to evaluate the properties as a transparent conductive oxide. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The importance of transparent conductive oxide (TCO) has grown over the last few years as the display industry has been growing. The most widely used TCO is tin-doped indium oxide (ITO), which has high electrical conductivity and high transmittance in the visible light region [1]. When substituted for an In3+ ion into an indium oxide (In2O3) lattice, Sn4+ ion donates free electrons which improve electrical conductivity. It is known that the solubility of Sn in In2O3 is about 6–8% [2]. The excess doping of tin over the solubility limit causes a decrease in carrier concentration which decreases electrical conductivity and forms an indium tin compound oxide (In2SnO5 and In4Sn3O12) [3–7]. Currently, studies on indium tin compounds which have both low indium content and ITO's properties have been performed due to the high cost of indium [8]. Recent interest about ITO nanoparticles is growing. ITO films have been traditionally prepared by sputtering method using a target which is composed of In2O3 and SnO2 known as a hard sintering compact. To improve the sintered density, high temperature and high pressure sintering methods such as hot pressing (HP) or hot isostatic pressing (HIP) are widely used. However, these methods cost too much. Hence, sintering under atmospheric pressure has been performed to reduce cost and requires a decrease in particle size.
⁎ Corresponding author. Tel.: +82 32 874 3785; fax: +82 32 872 0959. E-mail address: [email protected] (D.-W. Park). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.078
Also, the sputtering process requires an annealing step at a temperature over 700 °C to enhance crystallinity and electrical conductivity, which is not adequate for materials that cannot withstand high temperature [9]. Accordingly, studies on the deposition of ITO films using ITO nanoparticles without an annealing process at high temperature such as ink-jet printing have been performed [10]. Generally, ITO nanoparticles are prepared by a liquid phase method such as sol-gel method or hydrothermal method or co-precipitation process which requires post-treatments [6,11–15]. We successfully synthesized ITO nanoparticles by vapor phase method using a thermal plasma process which utilizes a plasma jet as the thermal source. In this process, raw materials are vaporized using a plasma jet and form thermodynamically stable materials. The synthesized materials are cooled by a rapid quenching system and obtained as a form of nanopowder [16,17]. In this experiment, ITO nanoparticles were prepared using thermal plasma at atmospheric pressure and the doping amounts of Sn were controlled by varying the molar ratio of Sn in raw materials when injected. Additionally, the phase formation and the characterization of the synthesized nanoparticles were analyzed. 2. Experiments ITO nanoparticles were synthesized by a DC arc plasma system using indium hydroxide [In(OH)3, 99.99%; Aldrich Co.] and tin (II) chloride anhydrous (SnCl2, +99.99%, Aldrich Co.) as precursors. Fig. 1 shows a schematic diagram of the DC plasma system. The power of the
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Fig. 1. Schematic diagram of DC arc plasma system.
plasma was 9 kW (300 A, 30 V) and the mixture gas of the argon and the nitrogen was used as the plasma gas. Also, the experiment was performed at the near-atmosphere pressure. The mixture of In(OH)3 and SnCl2 was injected into the plasma flame through a powder feeder using argon as a carrier gas, which evaporated due to the high temperature of the thermal plasma. As experimental variables, the molar ratio of SnCl2 in the powder feeder was varied from 0 at.% to 20 at.% to control the doping level of Sn. Plasma gas was a mixture of argon and nitrogen. Oxygen was injected to the nozzle of the plasma torch at 3 l/min. The details of the experimental condition and operating variables are summarized in Tables 1 and 2. Prepared ITO nanoparticles were collected at the reaction tube wall. The phase formation of the obtained ITO nanoparticles was verified using X-ray diffractometer (XRD, DMAX 2500, Rigaku Co.) and X-ray Photoelectron Spectroscopy (XPS, K-Alpha; Thermo Scientific). The amount of doped Sn was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 3700DV, Perkin Elmer). Morphology and the particle size of the obtained particles were analyzed via transmission electron microscopy (TEM, JEM-2100F, Jeol Co.). Optical transmittances were investigated using UV-Vis spectroscopy (Lambda 25, Perkin Elmer). Additionally, each sample was pressed into a 13 mm diameter pellet and then the electrical conductivities were measured through four-probe (MCP-T610, Mitsubishi Chemical).
3. Results and discussion Fig. 2 shows the XRD patterns of the synthesized ITO nanoparticles. Most of the peaks display a cubic bixbyite indium oxide (PDF #01071-2195) and any tin oxide peaks were not observed. The shift of peaks was observed as the molar ratio of Sn increase. It is clear between sample 1 and sample 2. The 2θ of (222) plane changed from 30.66° in sample 1 to 30.56° in sample 2. In sample 4 and sample 5, the broadening of peaks was observed. Parent et al. [5] reported that doping tin into indium oxide induces some changes in the diffraction patterns: the width of the peaks increases and the position of the peaks are shifted to lower angles. The ionic radius of tin is 0.69 Å, which is smaller than that of indium(r = 0.94 Å), which implies that tin could penetrate easily to the indium oxide lattice. The clear shift of the diffraction peaks between sample 1 and sample 2 can be explained by Bragg's law and selected area electron diffraction (SAED) patterns. Fig. 3 shows SAED patterns of sample 1 and the sample 2. The lattice distance (d) is obtained by the following equations; Lλ = Rd, where L is the camera length, λ is the wavelength of electrons and R is the radius of the diffraction ring. Lλ (9.285 nm2), called a camera constant, is measured using gold standards. The
Table 1 Operating conditions for the synthesis of ITO nanoparticles. Plasma current and power
300 A and 9 kW
Pressure Plasma gas (Ar + N2)
750 torr Ar : 15 l/min N2 : 2 l/min O2 : 3 l/min Ar : 1 l/min 0.3 g/min
Reaction gas Carrier gas Feeding rate of precursors
Table 2 Molar ratio of SnCl2 in the mixture of In(OH)3 and SnCl2. Sample Raw materials
In(OH)3 (mol%) SnCl2 (mol%)
1
2
3
4
5
100 0
94 6
88 12
84 16
80 20
Fig. 2. XRD patterns of the synthesized ITO nanoparticles (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, (e) sample 5.
D.-W. Kim, D.-W. Park / Surface & Coatings Technology 205 (2010) S201–S205
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Table 3 The calculated results using SAED patterns and Bragg's law.
R (the radius of the diffraction ring, nm) d (the lattice distance, Å) Calculated 2θ of (222) plane Practical 2θ of (222) plane
Fig. 3. The SAED patterns of (a) sample 1 and (b) sample 2.
diameter of the (222) diffraction ring of sample 1 and the sample 2 is 63.79 nm and 63.49 nm, respectively. So the radius of the (222) diffraction ring of sample 1 and sample 2 is 31.895 nm and 31.745 nm, respectively. Therefore, the lattice distance of (222) plane of sample 1 and sample 2 is 2.910 Å and 2.925 Å, respectively. These results are similar to the value of PDF card (PDF #01-071-2195, d = 2.9205 Å). The 2θ of the diffraction peak is obtained using Bragg's law. Bragg's law is described by the following equation; nλ = 2dsinθ, where n is an integer determined by the order given, λ is the wavelength of the X-ray (for Cu-Kα, λ = 1.5418 Å), d is the lattice distance and θ is the angle between the incident ray and the scattering planes. The calculated results shows that the 2θ of (222) plane of sample 1 and sample 2 is 30.72° and 30.54°, which explains the shift of the diffraction peaks by comparison with the experimental results [the practical 2θ of (222) plane of sample 1 is 30.66° and sample 2 is 30.56° in the experiment]. The calculated results are summarized in Table 3. In sample 5, some extra peaks observed around the main indium oxide peaks are displayed. PDF card (PDF #01-088-0773) implies the diffraction peaks of In4Sn3O12 appeared around the main peaks of
Sample 1
Sample 2
31.895 2.910 30.72° 30.66°
31.745 2.925 30.54° 30.56°
In2O3. As In2O3 is converted to In4Sn3O12, the structure of lattice is altered from cubic to rhombohedral. This change of the lattice structure made further lattice plane and suppressed the growth of the cubic In2O3 lattice. In Fig. 2, the decrease of peak intensity is not clearly observed in samples 2 and 3. This means that Sn ions were completely solvated in the indium oxide. The doping amounts of Sn and the electrical conductivities of each sample are summarized in Table 4. The doping amounts of tin from samples 2 and 3 are 4.97 mol% and 7.86 mol%, respectively. It is known that the maximum solubility of tin in indium oxide is ~10 mol%. Thus, Sn over the solubility in samples 4 and 5 leads to the presence of some extra peaks around the In2O3 peaks in Fig. 2, forming In4Sn3O12. The electrical conductivities of each sample pressed into the form of a 13 mm diameter pellet were measured and their results are shown in Table 4. Sample 1 (undoped In2O3) revealed a relatively lower conductivity than other samples. As the doping amount was increased, the electrical conductivities increased since the substituted Sn4+ donates free electrons. However, an increase in conductivity was not observed in sample 5, as In4Sn3O12 no longer donates any more free electrons. On the other hand, sample 5 revealed similar electrical conductivity with sample 4 despite the formation of indium tin compound oxide. Accordingly, it is considered that the formation of indium tin compound oxide does not mean the interruption of free electron donation because most of the indium tin compound oxide was synthesized from the excess Sn over the solubility. To confirm the bonding state of In and Sn, XPS analysis was conducted for the prepared ITO. Fig. 4(a) displays XPS analysis of samples 1, 3 and 5 for In3d bond. The peak for In3d5/2 of sample 1 was located at a binding energy of 444.58 eV. Meanwhile, those of samples 3 and 5 were located at the binding energies of 445.38 eV and 445.18 eV respectively. The binding energy of A in AzMsOt (an AmOn oxide will be substantially more ionic than MxOy) is usually larger than that for A in AmOn [18]. Because the relative covalency of M will make A in AzMsOt more ionic than that for A in AmOn. In case of In4Sn3O12, In2O3 is more ionic than SnO2, which implies that the binding energy of In in In4Sn3O12 is larger than that for In in In2O3. As expected, the binding energy of In3d5/2 in sample 5 is larger than that in sample 1 because of the formation of In4Sn3O12. The binding energy of In3d5/2 in sample 3 is slightly larger than in sample 5. It seems that the electrons donated from Sn4+ in In2O3 make In more ionic than that in In4Sn3O12. Conversely, the binding energy of Sn is shifted to the low energy region as ITO converts to In4Sn3O12. Fig. 4(b) shows that the binding energy of Sn3d5/2 was shifted from 487.38 eV (sample 3) to 487.08 eV (sample 5). A 0.005 g sample was suspended in 30 ml of distilled water and the transmittances of suspensions were measured at the wavelength region Table 4 Characterization of the synthesized ITO nanoparticles. Amount of Sn measured by ICP-OES (mol%) Electrical conductivity (S/cm) Sample 1 0 Sample 2 4.97 Sample 3 7.86 Sample 4 13.53 Sample 5 23.42
1.04 × 10−4 1.50 × 10−1 7.75 17.55 16.31
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Fig. 6. TEM images of the obtained ITO nanoparticles.
ITOs in this spectral region. It is considered that Sn4+ ion substituted for In3+ ions disturbed the transmission of near-UV and UV light. Fig. 6 displays TEM image of the synthesized ITO nanoparticles. Regardless of the content of Sn, the morphologies of the obtained nanoparticles were cubic with a size distribution ranging from 5 nm to 25 nm. 4. Conclusions
Fig. 4. XPS analysis of the synthesized nanoparticles for (a) In3d bond of samples 1, 3 and 5 and (b) Sn3d bond of samples 3 and 5.
within the range of from 300 to 900 nm. This was performed for each sample. The results of the measurement are shown in Fig. 5. Sample 1 did not appear to decrease the transmittance in the near-UV spectral region. Meanwhile, the transmittance of the tin-doped In2O3 was relatively lower than pure In2O3 in the near-UV spectral region. Especially sample 5, which showed lower transmittance than other
Indium oxide nanoparticles and tin-doped indium tin oxide (ITO) nanoparticles were synthesized using DC arc plasma. Raw materials were In(OH)3 and SnCl2 and were injected into a plasma flame using a powder feeder. The doped level was controlled by changing the molar ratio of the precursors. The Sn contents of the synthesized ITO were analyzed by ICP-OES. To confirm the phase formation, XRD and XPS analyses were performed. The particle size ranged from 5 nm to 25 nm and the morphology of the powder was cubic. The transmittances and the electrical conductivities were measured to verify the effect of the Sn doping level on the optical and the electrical properties. The formation of the second phase such as In2SnO5 and In4Sn3O12 was observed from the XRD patterns as the Sn contents exceeded the solubility of Sn in In2O3. Also, a shift and broadening of the XRD peaks by doping Sn were observed. The XPS results showed the transition of the binding energy of the In3d5/2 and the presence of the Sn–O bond. ITOs showed a relatively lower transmittance in the near-UV spectra region compared to the undoped In2O3. The ITO with Sn 13.53 mol% revealed the best electrical conductivity. However, the more doped ITO with Sn 23.42 mol% did not show an increase in the conductivity because In2SnO5 and In4Sn3O12 did not donate free electrons. Acknowledgement This work was supported by the Regional Innovation Center for Environmental Technology of Thermal Plasma (ETTP) at Inha University designated by MKE (2010). References
Fig. 5. UV-vis transmittance spectra of the synthesized ITO nanoparticles.
[1] J.C.C. Fan, F.J. Bachner, J. Electrochem. Soc. 122 (1975) 1719. [2] W.G. Haines, R.H. Bube, J. Appl. Phys. 49 (1) (1978) 223. [3] M.B. Varfolomeev, A.S. Mironova, F.Kh. Chibirova, V.E. Plyushchev, Inorg. Mater. (Eng Transl.) 12 (1975) 1926. [4] J.L. Bates, C.W. Gritten, D.D. Marchant, J.E. Garnier, Am. Ceram. Soc. Bull. 65 (1986) 673.
D.-W. Kim, D.-W. Park / Surface & Coatings Technology 205 (2010) S201–S205 [5] Ph. Parent, H. Dexpert, G. Tourillon, J.-M. Grimal, J. Electrochem. Soc. 139 (no.1) (1992) 276. [6] S.M. Kim, K.H. Seo, J.H. Lee, J.J. Kim, H.Y. Lee, J.S. Lee, J. Eur. Ceram. Soc. 26 (2006) 73. [7] William J. Heward, Douglas J. Swenson, J. Mater. Sci. 42 (2007) 7135. [8] T. Minami, Y. Takeda, S. Takata, T. Kakumu, Thin Solid Films 308 (1997) 13. [9] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 83 (1997) 2631. [10] G. Bühler, D. Thölmann, C. Feldmann, Adv. Mater. 19 (2007) 2224. [11] K. Yanagisawa, C. Udawatte, J. Mater. Res. 15 (2000) 1404. [12] B.C. Kim, J.H. Lee, J.J. Kim, T. Ikegami, Mater. Lett. 52 (2002) 114.
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[13] H. Xu, G. Zhu, H. Zhou, J. Am. Ceram. Soc. 88 (no.4) (2005) 986. [14] M.K. Jeon, M. Kang, Mater. Lett. 62 (2008) 676. [15] R.A. Gilstrap Jr., C.J. Capozzi, C.G. Carson, R.A. Gerhardt, C.J. Summers, Adv. Mater. 20 (2008) 4163. [16] S.M. Oh, D.W. Park, J. Koran Ind. Eng. Chem. 16 (no.3) (2003) 132. [17] D.W. Jung, D.W. Park, Appl. Surf. Sci. 255 (2009) 5409. [18] Tery L. Barr, Modern ESCA: The Principles and Practice of X-ray Photoelectron Spectroscopy, CRC Press, Inc, 1994.
Contents lists available at ScienceDirect
Surface & Coatings Technology 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 / s u r f c o a t
Preparation of indium tin oxide (ITO) nanoparticles by DC arc plasma Dong-Wook Kim, Dong-Wha Park ⁎ Department of Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma ( RIC-ETTP ), INHA University, 253 Yonghyun-dong, Nam-gu, Incheon, 402-751, Korea
a r t i c l e
i n f o
Available online 3 August 2010 Keywords: Plasma Indium tin oxide Nanoparticles
a b s t r a c t Tin-doped indium oxide (ITO) nanoparticles were prepared by DC arc plasma. The mixture of In(OH)3 and SnCl2 as precursors was injected into the arc plasma through a powder feeder and vaporized by the arc plasma. The ITO nanoparticles were synthesized through the rapid quenching of the thermal plasma processing and did not require post-treatment. The doped level of the synthesized particles was controlled by varying the molar ratio of SnCl2 in the powder feeder. The obtained nanoparticles were characterized by X-ray diffractometer (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and four-probe analysis. The XRD results showed a shift of the peak, a broadening of the peak width and the decrease of the peak intensity as the doping level increased. The doping amount of tin was analyzed by ICP-OES. XPS results presented that most of the tin was incorporated into the indium oxide lattice without the formation of any tin oxide. The particle size ranged from 5 nm to 25 nm. The faceted morphologies of the prepared ITO nanoparticles could be observed using TEM. Additionally, the transmittances and the electrical conductivities of the prepared ITO were measured to evaluate the properties as a transparent conductive oxide. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The importance of transparent conductive oxide (TCO) has grown over the last few years as the display industry has been growing. The most widely used TCO is tin-doped indium oxide (ITO), which has high electrical conductivity and high transmittance in the visible light region [1]. When substituted for an In3+ ion into an indium oxide (In2O3) lattice, Sn4+ ion donates free electrons which improve electrical conductivity. It is known that the solubility of Sn in In2O3 is about 6–8% [2]. The excess doping of tin over the solubility limit causes a decrease in carrier concentration which decreases electrical conductivity and forms an indium tin compound oxide (In2SnO5 and In4Sn3O12) [3–7]. Currently, studies on indium tin compounds which have both low indium content and ITO's properties have been performed due to the high cost of indium [8]. Recent interest about ITO nanoparticles is growing. ITO films have been traditionally prepared by sputtering method using a target which is composed of In2O3 and SnO2 known as a hard sintering compact. To improve the sintered density, high temperature and high pressure sintering methods such as hot pressing (HP) or hot isostatic pressing (HIP) are widely used. However, these methods cost too much. Hence, sintering under atmospheric pressure has been performed to reduce cost and requires a decrease in particle size.
⁎ Corresponding author. Tel.: +82 32 874 3785; fax: +82 32 872 0959. E-mail address: [email protected] (D.-W. Park). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.078
Also, the sputtering process requires an annealing step at a temperature over 700 °C to enhance crystallinity and electrical conductivity, which is not adequate for materials that cannot withstand high temperature [9]. Accordingly, studies on the deposition of ITO films using ITO nanoparticles without an annealing process at high temperature such as ink-jet printing have been performed [10]. Generally, ITO nanoparticles are prepared by a liquid phase method such as sol-gel method or hydrothermal method or co-precipitation process which requires post-treatments [6,11–15]. We successfully synthesized ITO nanoparticles by vapor phase method using a thermal plasma process which utilizes a plasma jet as the thermal source. In this process, raw materials are vaporized using a plasma jet and form thermodynamically stable materials. The synthesized materials are cooled by a rapid quenching system and obtained as a form of nanopowder [16,17]. In this experiment, ITO nanoparticles were prepared using thermal plasma at atmospheric pressure and the doping amounts of Sn were controlled by varying the molar ratio of Sn in raw materials when injected. Additionally, the phase formation and the characterization of the synthesized nanoparticles were analyzed. 2. Experiments ITO nanoparticles were synthesized by a DC arc plasma system using indium hydroxide [In(OH)3, 99.99%; Aldrich Co.] and tin (II) chloride anhydrous (SnCl2, +99.99%, Aldrich Co.) as precursors. Fig. 1 shows a schematic diagram of the DC plasma system. The power of the
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D.-W. Kim, D.-W. Park / Surface & Coatings Technology 205 (2010) S201–S205
Fig. 1. Schematic diagram of DC arc plasma system.
plasma was 9 kW (300 A, 30 V) and the mixture gas of the argon and the nitrogen was used as the plasma gas. Also, the experiment was performed at the near-atmosphere pressure. The mixture of In(OH)3 and SnCl2 was injected into the plasma flame through a powder feeder using argon as a carrier gas, which evaporated due to the high temperature of the thermal plasma. As experimental variables, the molar ratio of SnCl2 in the powder feeder was varied from 0 at.% to 20 at.% to control the doping level of Sn. Plasma gas was a mixture of argon and nitrogen. Oxygen was injected to the nozzle of the plasma torch at 3 l/min. The details of the experimental condition and operating variables are summarized in Tables 1 and 2. Prepared ITO nanoparticles were collected at the reaction tube wall. The phase formation of the obtained ITO nanoparticles was verified using X-ray diffractometer (XRD, DMAX 2500, Rigaku Co.) and X-ray Photoelectron Spectroscopy (XPS, K-Alpha; Thermo Scientific). The amount of doped Sn was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 3700DV, Perkin Elmer). Morphology and the particle size of the obtained particles were analyzed via transmission electron microscopy (TEM, JEM-2100F, Jeol Co.). Optical transmittances were investigated using UV-Vis spectroscopy (Lambda 25, Perkin Elmer). Additionally, each sample was pressed into a 13 mm diameter pellet and then the electrical conductivities were measured through four-probe (MCP-T610, Mitsubishi Chemical).
3. Results and discussion Fig. 2 shows the XRD patterns of the synthesized ITO nanoparticles. Most of the peaks display a cubic bixbyite indium oxide (PDF #01071-2195) and any tin oxide peaks were not observed. The shift of peaks was observed as the molar ratio of Sn increase. It is clear between sample 1 and sample 2. The 2θ of (222) plane changed from 30.66° in sample 1 to 30.56° in sample 2. In sample 4 and sample 5, the broadening of peaks was observed. Parent et al. [5] reported that doping tin into indium oxide induces some changes in the diffraction patterns: the width of the peaks increases and the position of the peaks are shifted to lower angles. The ionic radius of tin is 0.69 Å, which is smaller than that of indium(r = 0.94 Å), which implies that tin could penetrate easily to the indium oxide lattice. The clear shift of the diffraction peaks between sample 1 and sample 2 can be explained by Bragg's law and selected area electron diffraction (SAED) patterns. Fig. 3 shows SAED patterns of sample 1 and the sample 2. The lattice distance (d) is obtained by the following equations; Lλ = Rd, where L is the camera length, λ is the wavelength of electrons and R is the radius of the diffraction ring. Lλ (9.285 nm2), called a camera constant, is measured using gold standards. The
Table 1 Operating conditions for the synthesis of ITO nanoparticles. Plasma current and power
300 A and 9 kW
Pressure Plasma gas (Ar + N2)
750 torr Ar : 15 l/min N2 : 2 l/min O2 : 3 l/min Ar : 1 l/min 0.3 g/min
Reaction gas Carrier gas Feeding rate of precursors
Table 2 Molar ratio of SnCl2 in the mixture of In(OH)3 and SnCl2. Sample Raw materials
In(OH)3 (mol%) SnCl2 (mol%)
1
2
3
4
5
100 0
94 6
88 12
84 16
80 20
Fig. 2. XRD patterns of the synthesized ITO nanoparticles (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4, (e) sample 5.
D.-W. Kim, D.-W. Park / Surface & Coatings Technology 205 (2010) S201–S205
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Table 3 The calculated results using SAED patterns and Bragg's law.
R (the radius of the diffraction ring, nm) d (the lattice distance, Å) Calculated 2θ of (222) plane Practical 2θ of (222) plane
Fig. 3. The SAED patterns of (a) sample 1 and (b) sample 2.
diameter of the (222) diffraction ring of sample 1 and the sample 2 is 63.79 nm and 63.49 nm, respectively. So the radius of the (222) diffraction ring of sample 1 and sample 2 is 31.895 nm and 31.745 nm, respectively. Therefore, the lattice distance of (222) plane of sample 1 and sample 2 is 2.910 Å and 2.925 Å, respectively. These results are similar to the value of PDF card (PDF #01-071-2195, d = 2.9205 Å). The 2θ of the diffraction peak is obtained using Bragg's law. Bragg's law is described by the following equation; nλ = 2dsinθ, where n is an integer determined by the order given, λ is the wavelength of the X-ray (for Cu-Kα, λ = 1.5418 Å), d is the lattice distance and θ is the angle between the incident ray and the scattering planes. The calculated results shows that the 2θ of (222) plane of sample 1 and sample 2 is 30.72° and 30.54°, which explains the shift of the diffraction peaks by comparison with the experimental results [the practical 2θ of (222) plane of sample 1 is 30.66° and sample 2 is 30.56° in the experiment]. The calculated results are summarized in Table 3. In sample 5, some extra peaks observed around the main indium oxide peaks are displayed. PDF card (PDF #01-088-0773) implies the diffraction peaks of In4Sn3O12 appeared around the main peaks of
Sample 1
Sample 2
31.895 2.910 30.72° 30.66°
31.745 2.925 30.54° 30.56°
In2O3. As In2O3 is converted to In4Sn3O12, the structure of lattice is altered from cubic to rhombohedral. This change of the lattice structure made further lattice plane and suppressed the growth of the cubic In2O3 lattice. In Fig. 2, the decrease of peak intensity is not clearly observed in samples 2 and 3. This means that Sn ions were completely solvated in the indium oxide. The doping amounts of Sn and the electrical conductivities of each sample are summarized in Table 4. The doping amounts of tin from samples 2 and 3 are 4.97 mol% and 7.86 mol%, respectively. It is known that the maximum solubility of tin in indium oxide is ~10 mol%. Thus, Sn over the solubility in samples 4 and 5 leads to the presence of some extra peaks around the In2O3 peaks in Fig. 2, forming In4Sn3O12. The electrical conductivities of each sample pressed into the form of a 13 mm diameter pellet were measured and their results are shown in Table 4. Sample 1 (undoped In2O3) revealed a relatively lower conductivity than other samples. As the doping amount was increased, the electrical conductivities increased since the substituted Sn4+ donates free electrons. However, an increase in conductivity was not observed in sample 5, as In4Sn3O12 no longer donates any more free electrons. On the other hand, sample 5 revealed similar electrical conductivity with sample 4 despite the formation of indium tin compound oxide. Accordingly, it is considered that the formation of indium tin compound oxide does not mean the interruption of free electron donation because most of the indium tin compound oxide was synthesized from the excess Sn over the solubility. To confirm the bonding state of In and Sn, XPS analysis was conducted for the prepared ITO. Fig. 4(a) displays XPS analysis of samples 1, 3 and 5 for In3d bond. The peak for In3d5/2 of sample 1 was located at a binding energy of 444.58 eV. Meanwhile, those of samples 3 and 5 were located at the binding energies of 445.38 eV and 445.18 eV respectively. The binding energy of A in AzMsOt (an AmOn oxide will be substantially more ionic than MxOy) is usually larger than that for A in AmOn [18]. Because the relative covalency of M will make A in AzMsOt more ionic than that for A in AmOn. In case of In4Sn3O12, In2O3 is more ionic than SnO2, which implies that the binding energy of In in In4Sn3O12 is larger than that for In in In2O3. As expected, the binding energy of In3d5/2 in sample 5 is larger than that in sample 1 because of the formation of In4Sn3O12. The binding energy of In3d5/2 in sample 3 is slightly larger than in sample 5. It seems that the electrons donated from Sn4+ in In2O3 make In more ionic than that in In4Sn3O12. Conversely, the binding energy of Sn is shifted to the low energy region as ITO converts to In4Sn3O12. Fig. 4(b) shows that the binding energy of Sn3d5/2 was shifted from 487.38 eV (sample 3) to 487.08 eV (sample 5). A 0.005 g sample was suspended in 30 ml of distilled water and the transmittances of suspensions were measured at the wavelength region Table 4 Characterization of the synthesized ITO nanoparticles. Amount of Sn measured by ICP-OES (mol%) Electrical conductivity (S/cm) Sample 1 0 Sample 2 4.97 Sample 3 7.86 Sample 4 13.53 Sample 5 23.42
1.04 × 10−4 1.50 × 10−1 7.75 17.55 16.31
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Fig. 6. TEM images of the obtained ITO nanoparticles.
ITOs in this spectral region. It is considered that Sn4+ ion substituted for In3+ ions disturbed the transmission of near-UV and UV light. Fig. 6 displays TEM image of the synthesized ITO nanoparticles. Regardless of the content of Sn, the morphologies of the obtained nanoparticles were cubic with a size distribution ranging from 5 nm to 25 nm. 4. Conclusions
Fig. 4. XPS analysis of the synthesized nanoparticles for (a) In3d bond of samples 1, 3 and 5 and (b) Sn3d bond of samples 3 and 5.
within the range of from 300 to 900 nm. This was performed for each sample. The results of the measurement are shown in Fig. 5. Sample 1 did not appear to decrease the transmittance in the near-UV spectral region. Meanwhile, the transmittance of the tin-doped In2O3 was relatively lower than pure In2O3 in the near-UV spectral region. Especially sample 5, which showed lower transmittance than other
Indium oxide nanoparticles and tin-doped indium tin oxide (ITO) nanoparticles were synthesized using DC arc plasma. Raw materials were In(OH)3 and SnCl2 and were injected into a plasma flame using a powder feeder. The doped level was controlled by changing the molar ratio of the precursors. The Sn contents of the synthesized ITO were analyzed by ICP-OES. To confirm the phase formation, XRD and XPS analyses were performed. The particle size ranged from 5 nm to 25 nm and the morphology of the powder was cubic. The transmittances and the electrical conductivities were measured to verify the effect of the Sn doping level on the optical and the electrical properties. The formation of the second phase such as In2SnO5 and In4Sn3O12 was observed from the XRD patterns as the Sn contents exceeded the solubility of Sn in In2O3. Also, a shift and broadening of the XRD peaks by doping Sn were observed. The XPS results showed the transition of the binding energy of the In3d5/2 and the presence of the Sn–O bond. ITOs showed a relatively lower transmittance in the near-UV spectra region compared to the undoped In2O3. The ITO with Sn 13.53 mol% revealed the best electrical conductivity. However, the more doped ITO with Sn 23.42 mol% did not show an increase in the conductivity because In2SnO5 and In4Sn3O12 did not donate free electrons. Acknowledgement This work was supported by the Regional Innovation Center for Environmental Technology of Thermal Plasma (ETTP) at Inha University designated by MKE (2010). References
Fig. 5. UV-vis transmittance spectra of the synthesized ITO nanoparticles.
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