pubs.acs.org/Langmuir © 2010 American Chemical Society
Nonaqueous Synthesis and Photoluminescence of ITO Nanoparticles Zhaoyong Sun,† Jibao He,‡ Amar Kumbhar,§ and Jiye Fang*,† †
Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, Coordinated Instrumentation Facility, Tulane University, New Orleans, Louisiana 70118, and §Chapel Hill Analytical and Nanofabrication Laboratory, Institute of Advanced Materials, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599 ‡
Received September 3, 2009. Revised Manuscript Received January 18, 2010 SnO2 has successfully been doped into octahedral In2O3 nanoparticles using a high-temperature nonaqueous reaction. The resultant ITO nanoparticles exhibit a particle/crystal decrease in size, sphericity in morphology, and enhancement in photoluminescence.
Introduction Transparent conducting oxides (TCOs) are important materials due to their applications in optical/optoelectronic devices such as displays, organic light-emitting diodes (OLEDs), solar cells,1,2 functional glass,3-6 and energy efficient windows.7-9 Indium tin oxide (ITO) is the most widely used TCO material because of its excellent characteristics in conductivity and optical transparency as well as its good surface feature.7,10-12 One of the best examples for the ITO applications is its contribution in solar cell development.13-15 Traditionally, ITO is directly produced in the form of thin films using various methods such as magnetron sputtering, chemical vapor deposition, spray pyrolysis, vacuum evaporation, and pulsed laser deposition.16-21 In consideration of the apparent weakness in the above-mentioned processing approaches with respect to the influence of substrate structures, less control of growth, *Corresponding author. E-mail:
[email protected]. (1) Kobayashi, H.; Ishida, T.; Nakato, Y.; Tsubomura, H. J. Appl. Phys. 1991, 69, 1736–1743. (2) Martinez, M. A.; Herrero, J.; Gutierrez, M. T. Thin Solid Films 1995, 269, 80–84. (3) Gazotti, W. A.; Casalbore-Miceli, G.; Geri, A.; Berlin, A.; Paoli, M. A. d. Adv. Mater. 1998, 10, 1522–1525. (4) Pichot, F.; Ferrere, S.; Pitts, R. J.; Gregg, B. A. J. Electrochem. Soc. 1999, 146, 4324–4326. (5) Xu, C.; Liu, L.; Legenski, S. E.; Ning, D.; Taya, M. J. Mater. Res. 2004, 19, 2072–2080. (6) Granqvist, C. G.; Hulta˚ker, A. Thin Solid Films 2002, 411, 1–5. (7) Lewis, B. G.; Paine, D. C. MRS Bull. 2000, 25, 22–27. (8) Gordon, R. G. MRS Bull. 2000, 25, 52–57. (9) Ziegler, J. P.; Howard, B. M. Electrochem. Soc. Interface 1994, 3, 27–30. (10) Wang, R. X.; Djurisic, A. B.; Beling, C. D.; Fung, S. Indium Tin Oxide (ITO) Thin Films: Fabrication, Properties, Post-deposition Treatments and Applications; Nova Science Publishers: New York, 2005. (11) Li, Z. Q.; Lin, J. J. J. Appl. Phys. 2004, 96, 5918–5920. (12) Ginley, D. S.; Bright, C. MRS Bull. 2000, 25, 15–21. (13) Coutts, T. J.; Li, X.; Gessert, T. A.; M. W. Wanlass, M. W. Proc. SPIE; Int. Soc. Opt. Eng. 1989, 1144, 466–475. (14) Sholin, V.; Breeze, A. J.; Anderson, I. E.; Sahoo, Y.; Reddy, D.; Carter, S. A. Sol. Energy Mater. Sol. Cells 2008, 92, 1706–1711. (15) Tong, S. W.; Zhang, C. F.; Jiang, C. Y.; Liu, G.; Ling, Q. D.; Kang, E. T.; Chan, D. S. H.; Zhu, C. Chem. Phys. Lett. 2008, 453, 73–76. (16) Ni, J.; Yan, H.; Wang, A.; Yang, Y.; Stern, C. L.; Metz, A. W.; Jin, S.; Wang, L.; Marks, T. J.; Ireland, J. R.; Kannewurf, C. R. J. Am. Chem. Soc. 2005, 127, 5613–5624. (17) Canhola, P.; Martins, N.; Raniero, L.; Pereira, S.; Fortunato, E.; Ferreira, I.; Martins, R. Thin Solid Films 2005, 487, 271–276. (18) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1–46. (19) Zheng, J. P.; Kwok, H. S. Appl. Phys. Lett. 1993, 63, 1–3. (20) Maruyama, T.; Fukui, K. J. Appl. Phys. 1991, 70, 3848–3851. (21) Frank, G.; Kostlin, H. Appl. Phys. A: Mater. Sci. Process. 1982, 27, 197– 206.
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and expensive equipment required, solution-processing of ITO thin film is an alternative approach which attracts a high interest although a post-heating-treatment is still required.22-27 In this “soft” route, wet-chemical deposition of ITO nanoparticles (NPs) is a simple pathway to fabricate the thin film.22,28-30 The key step is to produce NPs with a high quality including high crystallinity, homogeneous composition, and uniform particle size. More importantly, it is believed that ITO NPs are directly involved in photodevices, and they seem to be a significant component in dye-sensitized solar cells. For example, it was recently reported that incorporation of ITO NPs into TiO2 thin film resulted in an enhanced light-conversion efficiency.31 Although it has been demonstrated that nonaqueous synthesis of NPs is a powerful method to accomplish the preparation of state-of-the-art metal oxide NPs,32-36 few groups37-39 have applied this strategy to synthesis of ITO NPs and/or to improve the NP quality. One of the excellent preliminary contributions from Ba et al.37 was to synthesize ITO NPs using indium acetylacetonate and tin tert-butoxide in benzyl alcohol at high temperature, although it took two days for completion of (22) Ederth, J.; Heszler, P.; Hultaker, A.; Niklasson, G. A.; Granqvist, C. G. Thin Solid Films 2003, 445, 199–206. (23) Minami, T. Semicond. Sci. Technol. 2005, 20, S35–S44. (24) Aegerter, M. A.; Puetz, J.; Gasparro, G.; Al-Dahoudi, N. Opt. Mater. 2004, 26, 155–162. (25) Kundu, S.; Biswas, P. K. Chem. Phys. Lett. 2006, 432, 508–512. (26) Epifani, M.; Diaz, R.; Arbiol, J.; Siciliano, P.; Morante, J. R. Chem. Mater. 2006, 18, 840–846. (27) Alam, M. J.; Cameron, D. C. Thin Solid Films 2002, 420-421, 76–82. (28) Goebert, C.; Nonninger, R.; Aegerter, M. A.; Schmidt, H. Thin Solid Films 1999, 351, 79–84. (29) Ederth, J.; Johnsson, P.; Niklasson, G. A.; Hoel, A.; Hultaker, A.; Heszler, P.; Granqvist, C. G.; Doom, A. R. v.; Jongerius, M. J.; Burgard, D. Phys. Rev. B 2003, 68, 155410/1–10. (30) Al-Dahoudi, N.; Aegerter, M. A. J. Sol-Gel Sci. Technol. 2003, 26, 693–697. (31) Chou, T. P.; Zhang, Q.; Russo, B.; Cao, G. J. Nanophoton. 2008, 2, 023511. (32) Tang, J.; Redl, F.; Zhu, Y.; Siegrist, T.; Brus, L. E.; Steigerwald, M. L. Nano Lett. 2005, 5, 543–548. (33) Niederberger, M.; Garnweitner, G.; Pinna, N.; Neri, G. Prog. Solid State Chem. 2005, 33, 59–70. (34) Garnweitner, G.; Niederberger, M. J. Am. Ceram. Soc. 2006, 89, 1801– 1808. (35) Niederberger, M.; Garnweitner, G. Chem.;Eur. J. 2006, 12, 7282–7302. (36) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798–12801. (37) Ba, J.; Rohlfing, D. F.; Feldhoff, A.; Brezesinski, T.; Djerdj, I.; Wark, M.; Niederberger, M. Chem. Mater. 2006, 18, 2848–2854. (38) Neri, G.; Bonavita, A.; Micali, G.; Rizzo, G.; Pinna, N.; Niederberger, M.; Ba, J. Thin Solid Films 2007, 515, 8637–8640. (39) Buehler, G.; Thoelmann, D.; Feldmann, C. Adv. Mater. 2007, 19, 2224– 2227.
Published on Web 02/08/2010
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the reaction. Recently, B€uhler et al. introduced a microwaveassisted synthesis of ITO NPs in ionic liquids (ILs).39 Nevertheless, halide contamination from the precursors and broad particle size distribution are still the issues to be improved. Most recently, Kanehara et al. successfully synthesized ITO NPs in n-octyl ether using indium(III) acetate-tin(II) 2-ethylhexanoate at high temperature in the presence of n-octanoic acid and oleylamine.40 Here we report a facile synthesis of ITO NPs in octadecene with relatively high uniformity in both the size and the shape as well as the improved photoluminescence from their colloidal suspensions.
Table 1. Preparation Inputs and Analyzed Compositions of the ITO Samples ITO-A 0.060 g In(ac)3 input 0.004 g Sn(ac)4 input 2.50 wt % SnO2 wt % by ICP-MS analysisa 2.75 wt % SnO2 wt % by EDS on TEMa a 2.15 wt % SnO2 wt % by EDS on SEM a wt % = massSnO2/(massSnO2 þ massIn2O3).
ITO-B 0.060 g 0.007 g 4.50 wt % 6.09 wt % 5.85 wt %
Experimental Section Materials. Indium(III) acetate (In(ac)3, 99.99%), oleic acid (C9H18dC8H15COOH, 90%), oleylamine (C9C18dC9H17NH2, 70%), and octadecene (C16H33CHdCH2, 90%) were purchased from Sigma-Aldrich without further purification. Tin(IV) acetate (Sn(ac)4, 99.99%) was obtained from Alfa Aesar as received. Anhydrous ethanol (200 proof) and anhydrous hexane (98.5%) were from AAPER and BDH, respectively. Synthesis. The synthesis of ITO NPs is similar to the preparation of In2O3 NPs.41 Typically, 0.20 mmol of In(ac)3, 0.60 mL of oleic acid, 0.80 mL of oleylamine, and various amounts of Sn(ac)4 were introduced into a condenser-equipped three-neck roundbottom flask containing 5.0 mL of octadecene. The standard airfree technique was used. The mixture was vigorously stirred and heated to 120 °C under argon purge. The temperature was kept for 20 min and then rapidly raised to 320 °C at an increasing rate of 15 °C/min, and the system was refluxed for an additional 40 min under an argon stream. During this period of reflux time, the transparent yellow mixture turned green gradually. The resultant colloids were subsequently cooled down to room temperature, isolated by adding a sufficient amount of anhydrous ethanol, and then followed by centrifugation to collect the NPs. The yielded blue precipitates can be easily redispersed in anhydrous hexane, forming suspensions of ITO NPs with a light blue color. Characterization of Nanoparticles. The morphology and phase structure of In2O3 and ITO NPs were characterized using transmission electron microscopes (TEM) (JEOL 2010F and Hitachi 9500) and an X-ray diffractometer (PANalytical X’pert system). Compositions of the products were analyzed using an inductively coupled plasma mass spectroscopic (ICP-MS) method (Department of Geosciences, University of Houston), energy dispersive X-ray spectroscopy (EDS) on a TEM (JEOL 2010F), and EDS on a scanning electron microscope (SEM) (Carl Zeiss Supra 55 VP). The photoluminescence spectra and optical density were recorded on a Cary Eclipse spectrometer (Varian Inc.) and Genesys 20 spectrophometer (ThermoSpectronic Inc.), respectively. The optical absorption spectra were collected from a Cary 50 Bio UV-vis spectrophotometer (Varian Inc.).
Results and Discussion In our strategy, ITO NPs were typically prepared through a thermal decomposition of indium(III) acetate and tin(IV) acetate in octadecene (as a nonpolar solvent) with vigorous agitation at 320 °C, in the presence of oleic acid and oleylamine as capping ligands under an argon stream. The input weight ratio of Sn-/Inprecursors was varied so as to tune the real composition in ITO NPs (Table 1). Two typical samples, designated as ITO-A and ITO-B, were studied. On the basis of the ICP-MS characterization, the SnO2 constituents in these ITO NPs (in wt %, that is, wt % = (40) Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T. J. Am. Chem. Soc. 2009, 131, 17736–17737. (41) (a) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem. Soc. 2005, 127, 5276–5277. (b) Sun, Z.; Kumbhar, A.; Sun, K.; Liu, Q.; Fang, J. Chem. Commun. 2008, 1920–1922.
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Figure 1. Photographs of hexane suspensions of In2O3 and ITO nanoparticles. From left to right, the samples are In2O3, ITO-A and ITO-B as detailed below.
massSnO2/(massSnO2 þ massIn2O3)) were determined as 2.50 and 4.50 wt %, respectively. As shown in Table 1 and in Figures S1 and S2 (Supporting Information), both types of EDS data closely match the ICP-MS results. For comparison, pristine In2O3 NPs were also prepared using a reported method.41 As demonstrated in Figure 1, all of the NP suspensions of ITO in hexane show a light blue color. Their absorption spectra are presented in Figure S3 (Supporting Information), revealing that the absorption maxima are slightly blue-shifted with increasing tin content. As discussed later, this can be attributed to the Burstein-Moss effect, indicating that the doped tin leads to a partial filling of the conduction band and accordingly a widening of the optically observed bandgap of ITO.42-44 Also, the deeper color in sample ITO-B implies the incorporation of a higher concentration of charge carriers in the conduction band.45 To examine the particle shape and size, all of the as-prepared In2O3/ITO NPs were investigated using a TEM, and their images are presented in Figure 2 as well as in the Supporting Information (Figure S4), in which relatively dilute particle suspensions were used to prepare the TEM specimen. According to the size histogram determined from their TEM images, the average particle sizes for In2O3, ITO-A, and ITO-B were calculated as ∼12, ∼5.6, and ∼5.2 nm, respectively (Figure S5 (42) Hamberg, I.; Granqvist, C. G.; Berggren, K.-F.; Sernelius, B. E.; Engstr€om, L. Phys. Rev. B 1984, 30, 3240–3249. (43) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123. (44) Choi, S.-I.; Nam, K. M.; Park, B. K.; Seo, W. S.; Park, J. T. Chem. Mater. 2008, 20, 2609–2611. (45) Li, S.; Qiao, X.; Chen, J.; Wang, H.; Jia, F.; Qiu, X. J. Cryst. Growth 2006, 289, 151–156.
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Figure 2. TEM images of as-prepared nanoparticles: (a) In2O3, (b) ITO-A, and (c) ITO-B. Data bars represent 50 nm.
Figure 3. TEM characteristics of sample ITO-B: (a) low-magnification image, (b) high-resolution image, and (c) selected area electron diffraction (negative) pattern.
in Supporting Information). Figure 3 shows the TEM images of sample ITO-B (∼5.6 nm) as an example, revealing a relatively narrow size-distribution (Figure 3a) as well as high crystallinity 4248 DOI: 10.1021/la903316b
(Figure 3b,c). A high-resolution image (Figure 3b) also demonstrates that each particle contains only one domain without any internal grain boundaries, indicating not only a single crystalline Langmuir 2010, 26(6), 4246–4250
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Figure 5. Photoluminescence spectra of In2O3 and ITO samples in
Figure 4. XRD patterns of as-prepared In2O3, ITO-A, and ITO-B nanoparticles deposited on a polished Si wafer. The standard diffraction lines shown on the bottom are from a standard In2O3 ICDD PDF card (no. 06-0416).
nature of each particle but also a homogeneous doping on lattice level. Debye-Scherrer rings (Figure 3c) in the selected area electron diffraction (SAED) pattern can be attributed to (222), (400), (440), and (622) diffraction planes. This is consistent with the observation of X-ray diffraction (XRD) traces (vide infra). XRD patterns of these samples were recorded and are presented in Figure 4. As depicted, all of the detectable diffraction peaks in the XRD trace of pure In2O3 can be indexed to those obtained from body-centered-cubic In2O3 (refer to ICDD PDF card no. 06-0416). The intensity enhancement on peak (222) indicates the presence of octahedral NPs (refer to Figure 2a) and highly ordered deposition of these NPs on the substrate by their {111} facets.46 On the basis of the studied samples, doping of SnO2 into In2O3 results in neither an XRD pattern shift nor an additional pattern of SnO2, implying that the Sn ions may be distributed inside the In2O3 lattices rather than existence as an individual phase of tin oxide. Additional evidence to support this inference is that the intensity of (222) peak “decreased” down to the standard order of strength when the SnO2 dopants were introduced, suggesting that ITO NPs have lost their octahedral morphology. This was actually verified by the direct shape observation in TEM projection images (Figure 2). Meanwhile, crystalline sizes of three samples were estimated by applying the Scherrer’s equation47 to the line broadening of the (222) peaks, i.e. D ¼ 57:3kλ=ðβ cos θÞ where k is particle shape factor (taken as 0.9), λ the wavelength of β the calibrated half-intensity Cu KR1 radiation (1.540 56 A), width (fwhm) of the selected (222) diffraction peak (in degrees) of these XRD patterns, θ the Bragg angle (half of the peak position angle), D the crystallite size (in angstroms), and the coefficient “57.3” is used to convert β from degrees to radians. From this equation, the average sizes of crystallites for samples In2O3, ITO-A, and ITO-B were estimated as 11.2, 5.3, and 5.0 nm, respectively. For each sample, the crystalline size estimated from its XRD pattern is consistent with the size obtained on the basis of the TEM projection image very much, revealing that (46) Lu, W.; Liu, Q.; Sun, Z.; He, J.; Ezeolu, C.; Fang, J. J. Am. Chem. Soc. 2008, 130, 6983–6991. (47) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; Addison-Wesley: Reading, MA, 1978.
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hexane excited at λex = 370 nm (optical density of all samples at 370 nm were adjusted as ∼0.48).
particles in all three samples are single-domain. On the other hand, both TEM and XRD show an order of size decrease in sequence of In2O3, ITO-A, and ITO-B. This observation of decrease in the crystal size with increasing dopant content of SnO2 may be attributed to the increase in the number of lattice defects.48 During the ITO NP synthesis, a sustained color change of the mixture was observed. At the beginning, an optically clear solution quickly turned from light yellow to orange-yellow, implying the formation of In2O3 NPs with an initial incorporation of Sn dopant atoms, whereas an appearance of a green color in a few minutes indicates a formation of ITO with high carrier concentration.49,50 It is commonly believed that the color change was caused by a microstructure change. Although the photoluminescence (PL) of nanostructured ITO has been studied by a few groups,25,48,51 the emission mechanism is still ambiguous. It was typically attributed to the bound and free exciton emission as well as oxygen vacancy/oxygen-indium vacancy pairs.25,51,52 To the best of our knowledge, the quantitative relationship between PL intensity of ITO NP colloids and the SnO2 content in ITO specimen has not been reported so far. As shown in Figure 5, excitingly, we realize that the PL intensity of ITO NP colloids was greatly enhanced by the tin doping and increased with the content of SnO2. (The optical density was adjusted to ∼0.48 for all of the samples by tuning their colloidal concentrations prior to the PL study.) For all three samples, the position of the PL peak was determined at the same position, 423 nm, when 370 nm was used as the excitation wavelength. However, the PL intensity increases while increasing the SnO2 content from 0 to 4.50 wt % in In2O3. For wide bandgap semiconductors, doping in them often induces dramatic changes in their electrical and optical properties.53,54 In2O3, as a matrix crystal of ITO, is considered as a wide bandgap (Eg ∼ 3.6 eV) transparent semiconductor. Therefore, the optical properties of ITO NPs should be related to the energy level structures of In2O3 NPs. As illustrated in Figure S6 adopted from the literature,48 it is well-known that a pure In2O3 crystal belongs to cubic bixbyite-type structure (Ia3 space group) with (48) Quaas, M.; Eggs, C.; Wulff, H. Thin Solid Films 1998, 332, 277–281. (49) Cilstrap, R. A., Jr.; Capozzi, C. J.; Carson, C. G.; Gerhardt, R. A.; Summers, C. J. Adv. Mater. 2008, 20, 4163–4166. (50) Buhler, G.; Tholmann, D.; feldmann, C. Adv. Mater. 2007, 19, 2224–2227. (51) Kundu, S.; Biswas, P. K. Chem. Phys. Lett. 2005, 414, 107–110. (52) Su, Y.; Li, S.; Chen, Y.; Xu, L.; Zhou, Q.; Peng, B.; Yin, S.; Feng, Y. Mater. Lett. 2007, 61, 3818–3821. (53) Sernelius, B. E.; Berggren, K. F.; Jin, Z. C.; Hamberg, I.; Granqvist, C. G. Phys. Rev. B 1988, 37, 10244–10248. (54) Sanon, G.; Rup, R.; Mansingh, A. Phys. Rev. B 1991, 44, 5672–5680.
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two nonequivalent “In” sites. In this structure, only threequarters of the tetrahedral sites are filled with oxygen. Each “In1” is coordinated 6-fold in a regular oxygen octahedron, whereas each “In2” is surrounded by six oxygen atoms at three different distances. When the Sn content is lower than 5 wt %, Sn(IV) ions will predominantly occupy the “In2” positions. This will result in a large repulsive force arising from the additional positive charge of the tin cations. To balance this extra charge, free electrons have to be released into the conduction band of In2O3, increasing carrier concentration and allowing their easy ionization even at room temperature.37 As a result, the carrier concentration of the electron-hole pairs will be increased. When such doped structure is excited with a light, the PL emission should be greatly enhanced. Therefore, it is reasonable to attribute the PL emission at 423 nm to a bandedge emission. In addition, the increase of the charge carrier density resulting from an incorporation of tin into In2O3 nanocrystals could result in the formation of tin defects in In2O3 nanocrystals, which can also be described with Kroger-Vink notation Sn 3 In. That means a tin ion resides on an indium lattice site in In2O3 crystal with singular positive charge. This consequently results in higher conductivity for this material, which is well-known.
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Conclusions In summary, a facile nonaqueous approach was successfully used to synthesize highly crystalline ITO NPs in which the tin component was successfully incorporated into the In2O3 lattice. The resultant ITO NPs are quasi-spherical and monodisperse. Their average particle sizes decrease from ∼12 to ∼5.2 nm (TEMbased) or from ∼11 to 5.0 nm (XRD-based) with increasing the SnO2 content from 0 to 4.50 wt % on the basis of our present experimental conditions. Furthermore, the PL emission intensity enhances with increasing the SnO2 content in the ITO NPs, which may be attributed to a band-edge emission of the ITO NPs. This optical behavior of as-prepared ITO NPs implies a potential application such as for optical devices, especially for new generation solar cells. Acknowledgment. This work was supported by the NSF (DMR-0731382) and Binghamton University. Supporting Information Available: EDS spectra, optical absorption spectra, TEM images of ITO nanoparticles with low concentrations, size histograms, and the cubic bixbyite structure of ITO. This material is available free of charge via the Internet at http://pubs.acs.org.
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