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Synthesis of indium tin oxide (ITO) nanoparticles in supercritical methanol
Accepted Manuscript Title: Synthesis of indium tin oxide (ITO) nanoparticles in supercritical methanol Author: Bonggeun Shong Naechul Shin Young-Ho Lee Ki Ho Ahn Youn-Woo Lee PII: DOI: Reference:
S0896-8446(16)30039-0 http://dx.doi.org/doi:10.1016/j.supflu.2016.03.002 SUPFLU 3580
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
16-1-2016 1-3-2016 4-3-2016
Please cite this article as: B. Shong, N. Shin, Y.-H. Lee, K.H. Ahn, Y.-W. Lee, Synthesis of indium tin oxide (ITO) nanoparticles in supercritical methanol, The Journal of Supercritical Fluids (2016), http://dx.doi.org/10.1016/j.supflu.2016.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract
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*Highlights
Highlights
Cubic indium oxide and indium tin oxide (ITO) nanoparticles are synthesized.
Supercritical solvothermal process is applied to inorganic precursors in methanol.
The synthesis can be completed in less than 2 minutes at a temperature of
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The products are highly crystalline without aging or calcination.
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250 ℃.
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Synthesis of indium tin oxide (ITO)
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nanoparticles in supercritical methanol
School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul
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Bonggeun Shong,ab Naechul Shin,ac Young-Ho Lee,a Ki Ho Ahn,a and Youn-Woo Lee a*
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea Department of Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu,
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Daejeon 34134, South Korea
Department of Chemical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon
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22212, South Korea
* Corresponding author.
Phone: +82-2-880-1883, FAX: +82-2-883-9124, e-mail: [email protected]
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Abstract Indium oxide (In2O3) and indium tin oxide (ITO; Sn-doped In2O3) nanoparticles are synthesized via a solvothermal method using surfactant-free supercritical methanol. X-ray
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diffraction analysis of the products indicates that crystalline indium oxide is formed within 2
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minutes at temperatures as low as 250°C. Transmission electron microscopy shows that the
particles are single crystals and have a cubic shape with an edge length of about 20 nm. The
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crystallite size of the particles calculated from X-ray diffraction is consistent with the particle size. The electrical resistivity and lattice parameters of the synthesized ITO nanoparticles
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Keywords
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vary as a function of tin content.
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hydrothermal; solvothermal; ITO; In2O3; nanostructure; nanocrystal;
1. Introduction
As an n-type transparent conducting oxide (TCO) with high electrical conductivity
and optical transparency, indium tin oxide (tin-doped indium oxide, ITO) has been used widely for various optoelectronic applications, such as thin film transistors and solar cells [1; 2]. However, the price of indium has fluctuated markedly due to the recent growth of these industries, especially because the element is relatively scarce in the earth’s crust [3]. Possible alternative transparent electrode materials are being investigated, but much remains to be done before they can replace ITO [4]. ITO is typically deposited as a thin film through sputtering or spray pyrolysis, during which a large fraction of the indium is wasted. As an alternative, methods that print ITO nanoparticle-dispersed ink directly on a target are being investigated. Such printing method allows thin TCO films to be deposited with less waste, 2 Page 4 of 20
requires less expensive equipment, and allows deposition on temperature-sensitive substrates such as polymers that are unable to go through post-deposition firing [5-7]. Therefore, it is desirable to develop appropriate synthetic methods to produce nanoscale and crystalline ITO nanoparticles.
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The synthesis of ITO and indium oxide nanoparticles has been explored via multiple
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routes. Solvothermal reactions can be used to synthesize colloidal indium oxide quantum dots, but often require expensive, toxic solvents and organometallic indium salts [8-12].
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Using inorganic indium salts, which are less expensive and easier to handle [13], In2O3 can be produced via two-step processes in which In(OH)3 or InOOH nanoparticles are initially
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prepared using co-precipitation [14-18] or subcritical hydrothermal/solvothermal methods
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[19-23], and then calcined at temperatures above 400°C. One-step solvothermal methods with inorganic precursors at lower temperatures have also been reported [24-33], but the desirable
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body-centered cubic (bcc) phase is formed only after hours to days of heating, which is a
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common timescale for all reported processes. Interestingly, some researchers have reported that indium oxide nanoparticles are formed via solvothermal synthesis using ethanol mixed with surfactants at temperatures as low as 120°C [34; 35], suggesting that short-chain alcohols can be promising solvent for synthesis of ITO nanoparticles in benign and efficient conditions. The low price of methanol makes it an economically suitable choice for large scale processing.
Supercritical hydrothermal and solvothermal synthesis is being investigated as a way
to produce various inorganic nanomaterials [36; 37]. Simple alcohols and water are promising reaction media for the supercritical synthesis of nanoparticles, because they are inexpensive, relatively less toxic, and have solvent properties that can be adjusted easily, such as density, transport coefficients, solubility, and permittivity [38; 39]. This allows to control the size, morphology, and structure of the particles in the supercritical solvothermal method. 3 Page 5 of 20
In addition, supercritical methods often produce inorganic nanomaterials that are highly crystalline without further processing, and also they can be readily adapted to continuousflow systems. Recently, the synthesis of ITO nanoparticles with a supercritical hydrothermal method using water with reaction times on the order of seconds and reaction temperatures as
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high as 400°C was demonstrated [40; 41]. If methanol were used as the solvent instead of
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water, ITO nanoparticles could be synthesized under more benign processing conditions due to its lower critical temperature and pressure (239°C, 81 bar) than those of water (374°C, 221
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bar) [42], but still with short reaction times.
In this work, highly crystalline ITO nanoparticles were synthesized via a single-step
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supercritical solvothermal method using surfactant-free methanol as the reaction medium.
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Cubic nanocrystals with size of 20 nm formed in less than 2 minutes at temperatures as low as 250°C, without either pre-synthesis aging or post-synthesis calcination. The effects of
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reaction time and temperature on particle crystallization were investigated. Variation in the
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electrical resistivity and lattice parameters of the products as a function of tin content was also evaluated. A mechanism for the rapid evolution of the products is suggested.
2. Material and methods
All chemicals used were of analytical reagent grade (Samchun Pure Chemical, Korea)
and as received without further purification. Methanol solutions of indium chloride (InCl3·nH2O, n ≈ 3.5) and tin chloride (SnCl4·5H2O) were mixed at the desired ratio to give a total metal ion concentration of 0.2 M. Then, a stoichiometric amount of 0.6 M sodium hydroxide (NaOH) solution was added to the mixture, resulting in precipitation of a white precursor with a concentration equivalent to 0.1 M ITO. Without aging, the mixture was transferred to a custom-built cylindrical SUS316 batch reactor with an inner volume of 18 mL and inner diameter of ca. 2 cm. The volume of the mixture transferred to the reactor was 4 Page 6 of 20
calculated from the density of supercritical methanol at the intended temperature and pressure [42; 43]. Two glass beads with d = 5 mm were added to the reactor to help agitation. Then, the reactor was sealed and maintained at a preset temperature (250–400°C) with stirring in a molten salt bath for the desired time (1–20 min), and quickly quenched in a water bath. The
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product was collected and washed with distilled water and methanol in dispersion and
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centrifugation steps. Finally, the particles were dried in a vacuum oven kept at 60°C for 24 hours.
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X–ray diffraction (XRD) patterns were obtained using a Rigaku D/Max-3C diffractometer equipped with a rotating anode and a Cu-Kα radiation source. No refinement
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was applied processing the XRD data. Transmission electron microscopy (TEM) images were
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acquired with a JEOL JEM-3010 transmission electron microscope equipped with a Gatan MSC-794 digital camera. The elemental composition of the samples was analyzed with a
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Perkin Elmer Optima 4300 DV inductively coupled plasma atomic emission spectrometer
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(ICP-AES). The conductivities of the nanoparticles were determined for pressed pellets (13 mm diameter) at room temperature using a four-point probe method with a Keithley 2400 sourcemeter.
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3. Results and discussion
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Figure 1. XRD patterns of the products, after (a) 60 s, (b) 80 s, (c) 100 s, and (d) 120 s
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immersion in the salt bath of 300 °C. The crystal planes of bcc-In2O3 (JCPDS 06-0416) are
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indicated next to the peaks.
Figure 1 shows the evolution of the XRD pattern of indium oxide nanoparticles as a
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function of the time of the batch reactor immersed in the salt bath at 300°C. Temporal
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evolution of the product was investigated without doping with tin, as a small amount of tin has little effect on the crystallization of ITO in the technologically desirable composition range [15]. Indeed, in the XRD patterns of Sn-doped samples, the positions and relative intensities of the patterns were the same as those of pure indium oxide with little variation, indicating a change in the lattice parameters of the same crystal structure. Until 60 s after the reactor was immersed in the molten salt (a), the product was amorphous, only showing broad features around 2θ = 34° and 55°. Similar patterns are observed for the precipitate without thermal treatment in this study (not shown) and also in previous solvothermal syntheses, which do not correspond to any known crystal structure in the In–Sn–O–H system [9]. Bodycentered cubic In2O3 (JCPDS 06-0416) started to develop after 80 s (b), and the sample showed the highly crystalline XRD pattern of bcc-In2O3 after 120 s (d). The sizes of the
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crystallites were estimated using the Scherrer equation from the positions of three highintensity XRD peaks corresponding to the (222), (400), (440), and (622) lattice planes. The particle size from the XRD data averaged 21 ± 6 nm, and did not show clear dependence on
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the process conditions.
Figure 2. (a-d) TEM images of the products: (a) indium oxide, 80 s at 300 °C, (b) indium oxide, 180 s at 300 °C, (c) indium oxide, 300 s at 250 °C, and (d) ITO (tin content = 9.5 wt %), 180 s at 300 °C. (e) High-resolution TEM image of a single indium oxide nanoparticle, 180 s at 300 °C. (f) Photograph of synthesized particles dispersed in methanol, with tin contents of 0.0 %, 5.0 %, and 9.5 %, from left to right.
Figure 2 shows TEM images of the products made using different process conditions 7 Page 9 of 20
(a-d). Only the amorphous phase and no crystalline material were observed in samples with process times shorter than 80 s (not shown), consistent with the XRD data. This material could be In(OH)3 or InOOH precursor formed in the heated solution still in the liquid phase before it enters the supercritical regime (see figure 4). After the reactor had been immersed in
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the salt bath for 80 s (a), emergence of cubic particles could be identified, still surrounded by
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remaining amorphous material. The fraction of the crystalline particles gradually increased with the process temperature, and by 180 s of immersion in the salt bath (b), the product
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consisted mostly of cubic nanocrystals with slightly rounded edges, and almost no amorphous material remained. No significant change in product morphology or size distribution was
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observed thereafter. Therefore, it can be suggested that the formation of particles was
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completed less than 180 s after the process began, and the synthesized nanoparticles were not subjected to sintering. A similar product was obtained at 250°C (c), which is 150°C below the
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reaction temperature required using supercritical water [40; 41] but above the critical point of
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methanol. The morphology and size distribution of tin-doped samples (d) did not differ significantly from those of pure In2O3 products. The high-resolution TEM image in Figure 2(e) indicates that the product has several lattice planes with high crystallinity. No grain boundary is observed in any of the particles, suggesting that the synthesized nanoparticles are single crystals. The average size (edge length) of the particles observed in TEM images of crystalline samples was 16 ± 5 nm (averaged over 400 particles), similar to the size of the crystallites based on XRD (21 ± 6 nm).
No apparent change in product morphology was observed for salt bath temperatures ranging from 250 to 400°C, nor for the pressure range of the supercritical methanol in the reactor from 200 to 300 bar (not shown). Such insensitivity of the product properties to the processing conditions, as well as the rapid formation of a highly crystalline indium oxide structure, can be explained by the general mechanism of the supercritical solvothermal 8 Page 10 of 20
synthesis of inorganic materials [44]. Solvents containing metal ions abruptly lose most of their solvation power when they enter a supercritical phase. Therefore, any inorganic solutes in the solution quickly precipitate as ultrafine, possibly monodisperse, particles. Nevertheless, this temperature is lower than the temperatures required for sintering the particles, preserving
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the initial morphology of the crystals after crystallization. In case of ITO in methanol,
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supercritical expansion of the solvent and the resulting formation of nanoparticles would
occur simultaneously with dehydration of the precursor, so that nanoparticles in cubic In2O3
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phase can form. The reaction scheme for the dehydration can be either (eq 1), or
2 InOOH → In2O3 + H2O
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2 In(OH)3 → In2O3 + 3 H2O
(eq 2),
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according to the stoichiometry of the precursor.
Figure 3. Electrical resistivity (black circles) and lattice constants (red crosses) of the products versus tin content. The lines are drawn to guide the eyes.
Figure 3 shows the electrical resistivity and lattice constants of the ITO nanoparticles as functions of tin content. The resistivity was minimal near 10 weight percent of tin, confirming reported findings [45]. The lattice parameter increased from 1.014 nm for pure In2O3 to 1.016–1.017 nm for tin concentration of ~5 %, and then saturates afterwards, which also corresponds to the literature [46]. The minimum observed resistivity of 0.64 Ω·cm is 9 Page 11 of 20
comparable to those reported for pellets of ITO nanoparticles without annealing [8; 29]; note that annealing at high temperature would further lower the resistivity of the film made of nanoparticles [5]. In addition, figure 2(f) shows that, while pure indium oxide nanoparticles were white, the Sn-doped products had bluer color depending on tin content. This coloration
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of ITO according to doping with Sn is attributed to the increase in the charge carrier
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concentration or point-defect density [47; 48], both of which are due to the incorporation of substitutional Sn in the In2O3 lattice [45]. The decreasing resistivity of ITO nanoparticles as a
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function of tin content is often related to a gradually deepening blue color of the products [8; 10; 11; 16; 17; 24]. Moreover, it has been reported that, while ITO materials are yellowish
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after oxidative calcination, they become blue-green upon annealing under a reducing
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atmosphere with a simultaneous decrease in electrical resistivity [16; 18; 45]. In our synthetic process, the reducing atmosphere of supercritical methanol [25; 43] would have resulted in an
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high electrical conductivity.
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in-situ reduction of the products, manifested as the bluish color of the products and possibly
Figure 4. Temperature profile of the reactor immersed in salt bath at 300 °C as a function of time.
Since the synthesis was carried out in high-pressure stainless steel batch reactors,
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which have a high heat capacity and were initially kept at room temperature, there was a temporal delay in the rise in temperature inside the reactor upon immersion in the salt bath. We measured the temperature profile of an empty reactor immersed in a salt bath at a temperature of 300°C (Figure 4). It took 60 s for the inside of the reactor to reach 239°C, the
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critical temperature of methanol; therefore, under these experimental conditions, the heated
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solution would have remained in a liquid phase for up to 1 minute, and subsequently
expanded to the supercritical phase. Another 60 s was required for the reactor to reach 290°C.
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If a continuous-flow process enabling fast heating of the sample were to be optimized, the residence time required for the formation of ITO crystals might be much shorter than
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currently observed [40; 41]. Rapid heating may have an additional benefit of lowering the
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process temperature, as it was recently reported that the nucleation temperature of In2O3 is lowered by increasing the heating rate of the solvothermal reactor [33]. It also should be
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noted that the pressure inside the reactor increased after reaction times longer than 10
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minutes, indicating the formation of gaseous byproducts. Although the composition of the byproducts was not analyzed, nanostructured indium oxide has high catalytic activity in decomposition of methanol [49]. Therefore, prolonged reaction times should be avoided when producing ITO nanoparticles with the current method.
4. Conclusions
Within short reaction times of less than 2 minutes, cubic ITO nanoparticles measuring
about 20 nm were synthesized in supercritical methanol without any surfactant. The electrical and structural properties of the synthesized nanoparticles as a function of Sn content are comparable to those of reported ITO nanoparticles. The temperature required for the formation of cubic In2O3 phase with this solvothermal method is 250°C, which is lower than when water is used as the solvent, resulting in more benign processing conditions for the 11 Page 13 of 20
synthesis of this material. Adaptation of current method in a continuous flow system, as well as analysis and control of the impurities, are suggested as future work.
Conflict of interest statement
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The authors confirm that there are no conflicts of interest regarding this paper.
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S0896-8446(16)30039-0 http://dx.doi.org/doi:10.1016/j.supflu.2016.03.002 SUPFLU 3580
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
16-1-2016 1-3-2016 4-3-2016
Please cite this article as: B. Shong, N. Shin, Y.-H. Lee, K.H. Ahn, Y.-W. Lee, Synthesis of indium tin oxide (ITO) nanoparticles in supercritical methanol, The Journal of Supercritical Fluids (2016), http://dx.doi.org/10.1016/j.supflu.2016.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract
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*Highlights
Highlights
Cubic indium oxide and indium tin oxide (ITO) nanoparticles are synthesized.
Supercritical solvothermal process is applied to inorganic precursors in methanol.
The synthesis can be completed in less than 2 minutes at a temperature of
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The products are highly crystalline without aging or calcination.
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250 ℃.
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Synthesis of indium tin oxide (ITO)
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nanoparticles in supercritical methanol
School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul
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a
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Bonggeun Shong,ab Naechul Shin,ac Young-Ho Lee,a Ki Ho Ahn,a and Youn-Woo Lee a*
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea Department of Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu,
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Daejeon 34134, South Korea
Department of Chemical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon
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22212, South Korea
* Corresponding author.
Phone: +82-2-880-1883, FAX: +82-2-883-9124, e-mail: [email protected]
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Abstract Indium oxide (In2O3) and indium tin oxide (ITO; Sn-doped In2O3) nanoparticles are synthesized via a solvothermal method using surfactant-free supercritical methanol. X-ray
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diffraction analysis of the products indicates that crystalline indium oxide is formed within 2
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minutes at temperatures as low as 250°C. Transmission electron microscopy shows that the
particles are single crystals and have a cubic shape with an edge length of about 20 nm. The
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crystallite size of the particles calculated from X-ray diffraction is consistent with the particle size. The electrical resistivity and lattice parameters of the synthesized ITO nanoparticles
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Keywords
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vary as a function of tin content.
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hydrothermal; solvothermal; ITO; In2O3; nanostructure; nanocrystal;
1. Introduction
As an n-type transparent conducting oxide (TCO) with high electrical conductivity
and optical transparency, indium tin oxide (tin-doped indium oxide, ITO) has been used widely for various optoelectronic applications, such as thin film transistors and solar cells [1; 2]. However, the price of indium has fluctuated markedly due to the recent growth of these industries, especially because the element is relatively scarce in the earth’s crust [3]. Possible alternative transparent electrode materials are being investigated, but much remains to be done before they can replace ITO [4]. ITO is typically deposited as a thin film through sputtering or spray pyrolysis, during which a large fraction of the indium is wasted. As an alternative, methods that print ITO nanoparticle-dispersed ink directly on a target are being investigated. Such printing method allows thin TCO films to be deposited with less waste, 2 Page 4 of 20
requires less expensive equipment, and allows deposition on temperature-sensitive substrates such as polymers that are unable to go through post-deposition firing [5-7]. Therefore, it is desirable to develop appropriate synthetic methods to produce nanoscale and crystalline ITO nanoparticles.
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The synthesis of ITO and indium oxide nanoparticles has been explored via multiple
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routes. Solvothermal reactions can be used to synthesize colloidal indium oxide quantum dots, but often require expensive, toxic solvents and organometallic indium salts [8-12].
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Using inorganic indium salts, which are less expensive and easier to handle [13], In2O3 can be produced via two-step processes in which In(OH)3 or InOOH nanoparticles are initially
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prepared using co-precipitation [14-18] or subcritical hydrothermal/solvothermal methods
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[19-23], and then calcined at temperatures above 400°C. One-step solvothermal methods with inorganic precursors at lower temperatures have also been reported [24-33], but the desirable
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body-centered cubic (bcc) phase is formed only after hours to days of heating, which is a
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common timescale for all reported processes. Interestingly, some researchers have reported that indium oxide nanoparticles are formed via solvothermal synthesis using ethanol mixed with surfactants at temperatures as low as 120°C [34; 35], suggesting that short-chain alcohols can be promising solvent for synthesis of ITO nanoparticles in benign and efficient conditions. The low price of methanol makes it an economically suitable choice for large scale processing.
Supercritical hydrothermal and solvothermal synthesis is being investigated as a way
to produce various inorganic nanomaterials [36; 37]. Simple alcohols and water are promising reaction media for the supercritical synthesis of nanoparticles, because they are inexpensive, relatively less toxic, and have solvent properties that can be adjusted easily, such as density, transport coefficients, solubility, and permittivity [38; 39]. This allows to control the size, morphology, and structure of the particles in the supercritical solvothermal method. 3 Page 5 of 20
In addition, supercritical methods often produce inorganic nanomaterials that are highly crystalline without further processing, and also they can be readily adapted to continuousflow systems. Recently, the synthesis of ITO nanoparticles with a supercritical hydrothermal method using water with reaction times on the order of seconds and reaction temperatures as
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high as 400°C was demonstrated [40; 41]. If methanol were used as the solvent instead of
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water, ITO nanoparticles could be synthesized under more benign processing conditions due to its lower critical temperature and pressure (239°C, 81 bar) than those of water (374°C, 221
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bar) [42], but still with short reaction times.
In this work, highly crystalline ITO nanoparticles were synthesized via a single-step
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supercritical solvothermal method using surfactant-free methanol as the reaction medium.
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Cubic nanocrystals with size of 20 nm formed in less than 2 minutes at temperatures as low as 250°C, without either pre-synthesis aging or post-synthesis calcination. The effects of
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reaction time and temperature on particle crystallization were investigated. Variation in the
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electrical resistivity and lattice parameters of the products as a function of tin content was also evaluated. A mechanism for the rapid evolution of the products is suggested.
2. Material and methods
All chemicals used were of analytical reagent grade (Samchun Pure Chemical, Korea)
and as received without further purification. Methanol solutions of indium chloride (InCl3·nH2O, n ≈ 3.5) and tin chloride (SnCl4·5H2O) were mixed at the desired ratio to give a total metal ion concentration of 0.2 M. Then, a stoichiometric amount of 0.6 M sodium hydroxide (NaOH) solution was added to the mixture, resulting in precipitation of a white precursor with a concentration equivalent to 0.1 M ITO. Without aging, the mixture was transferred to a custom-built cylindrical SUS316 batch reactor with an inner volume of 18 mL and inner diameter of ca. 2 cm. The volume of the mixture transferred to the reactor was 4 Page 6 of 20
calculated from the density of supercritical methanol at the intended temperature and pressure [42; 43]. Two glass beads with d = 5 mm were added to the reactor to help agitation. Then, the reactor was sealed and maintained at a preset temperature (250–400°C) with stirring in a molten salt bath for the desired time (1–20 min), and quickly quenched in a water bath. The
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product was collected and washed with distilled water and methanol in dispersion and
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centrifugation steps. Finally, the particles were dried in a vacuum oven kept at 60°C for 24 hours.
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X–ray diffraction (XRD) patterns were obtained using a Rigaku D/Max-3C diffractometer equipped with a rotating anode and a Cu-Kα radiation source. No refinement
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was applied processing the XRD data. Transmission electron microscopy (TEM) images were
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acquired with a JEOL JEM-3010 transmission electron microscope equipped with a Gatan MSC-794 digital camera. The elemental composition of the samples was analyzed with a
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Perkin Elmer Optima 4300 DV inductively coupled plasma atomic emission spectrometer
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(ICP-AES). The conductivities of the nanoparticles were determined for pressed pellets (13 mm diameter) at room temperature using a four-point probe method with a Keithley 2400 sourcemeter.
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3. Results and discussion
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Figure 1. XRD patterns of the products, after (a) 60 s, (b) 80 s, (c) 100 s, and (d) 120 s
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immersion in the salt bath of 300 °C. The crystal planes of bcc-In2O3 (JCPDS 06-0416) are
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indicated next to the peaks.
Figure 1 shows the evolution of the XRD pattern of indium oxide nanoparticles as a
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function of the time of the batch reactor immersed in the salt bath at 300°C. Temporal
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evolution of the product was investigated without doping with tin, as a small amount of tin has little effect on the crystallization of ITO in the technologically desirable composition range [15]. Indeed, in the XRD patterns of Sn-doped samples, the positions and relative intensities of the patterns were the same as those of pure indium oxide with little variation, indicating a change in the lattice parameters of the same crystal structure. Until 60 s after the reactor was immersed in the molten salt (a), the product was amorphous, only showing broad features around 2θ = 34° and 55°. Similar patterns are observed for the precipitate without thermal treatment in this study (not shown) and also in previous solvothermal syntheses, which do not correspond to any known crystal structure in the In–Sn–O–H system [9]. Bodycentered cubic In2O3 (JCPDS 06-0416) started to develop after 80 s (b), and the sample showed the highly crystalline XRD pattern of bcc-In2O3 after 120 s (d). The sizes of the
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crystallites were estimated using the Scherrer equation from the positions of three highintensity XRD peaks corresponding to the (222), (400), (440), and (622) lattice planes. The particle size from the XRD data averaged 21 ± 6 nm, and did not show clear dependence on
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the process conditions.
Figure 2. (a-d) TEM images of the products: (a) indium oxide, 80 s at 300 °C, (b) indium oxide, 180 s at 300 °C, (c) indium oxide, 300 s at 250 °C, and (d) ITO (tin content = 9.5 wt %), 180 s at 300 °C. (e) High-resolution TEM image of a single indium oxide nanoparticle, 180 s at 300 °C. (f) Photograph of synthesized particles dispersed in methanol, with tin contents of 0.0 %, 5.0 %, and 9.5 %, from left to right.
Figure 2 shows TEM images of the products made using different process conditions 7 Page 9 of 20
(a-d). Only the amorphous phase and no crystalline material were observed in samples with process times shorter than 80 s (not shown), consistent with the XRD data. This material could be In(OH)3 or InOOH precursor formed in the heated solution still in the liquid phase before it enters the supercritical regime (see figure 4). After the reactor had been immersed in
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the salt bath for 80 s (a), emergence of cubic particles could be identified, still surrounded by
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remaining amorphous material. The fraction of the crystalline particles gradually increased with the process temperature, and by 180 s of immersion in the salt bath (b), the product
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consisted mostly of cubic nanocrystals with slightly rounded edges, and almost no amorphous material remained. No significant change in product morphology or size distribution was
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observed thereafter. Therefore, it can be suggested that the formation of particles was
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completed less than 180 s after the process began, and the synthesized nanoparticles were not subjected to sintering. A similar product was obtained at 250°C (c), which is 150°C below the
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reaction temperature required using supercritical water [40; 41] but above the critical point of
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methanol. The morphology and size distribution of tin-doped samples (d) did not differ significantly from those of pure In2O3 products. The high-resolution TEM image in Figure 2(e) indicates that the product has several lattice planes with high crystallinity. No grain boundary is observed in any of the particles, suggesting that the synthesized nanoparticles are single crystals. The average size (edge length) of the particles observed in TEM images of crystalline samples was 16 ± 5 nm (averaged over 400 particles), similar to the size of the crystallites based on XRD (21 ± 6 nm).
No apparent change in product morphology was observed for salt bath temperatures ranging from 250 to 400°C, nor for the pressure range of the supercritical methanol in the reactor from 200 to 300 bar (not shown). Such insensitivity of the product properties to the processing conditions, as well as the rapid formation of a highly crystalline indium oxide structure, can be explained by the general mechanism of the supercritical solvothermal 8 Page 10 of 20
synthesis of inorganic materials [44]. Solvents containing metal ions abruptly lose most of their solvation power when they enter a supercritical phase. Therefore, any inorganic solutes in the solution quickly precipitate as ultrafine, possibly monodisperse, particles. Nevertheless, this temperature is lower than the temperatures required for sintering the particles, preserving
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the initial morphology of the crystals after crystallization. In case of ITO in methanol,
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supercritical expansion of the solvent and the resulting formation of nanoparticles would
occur simultaneously with dehydration of the precursor, so that nanoparticles in cubic In2O3
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phase can form. The reaction scheme for the dehydration can be either (eq 1), or
2 InOOH → In2O3 + H2O
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2 In(OH)3 → In2O3 + 3 H2O
(eq 2),
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according to the stoichiometry of the precursor.
Figure 3. Electrical resistivity (black circles) and lattice constants (red crosses) of the products versus tin content. The lines are drawn to guide the eyes.
Figure 3 shows the electrical resistivity and lattice constants of the ITO nanoparticles as functions of tin content. The resistivity was minimal near 10 weight percent of tin, confirming reported findings [45]. The lattice parameter increased from 1.014 nm for pure In2O3 to 1.016–1.017 nm for tin concentration of ~5 %, and then saturates afterwards, which also corresponds to the literature [46]. The minimum observed resistivity of 0.64 Ω·cm is 9 Page 11 of 20
comparable to those reported for pellets of ITO nanoparticles without annealing [8; 29]; note that annealing at high temperature would further lower the resistivity of the film made of nanoparticles [5]. In addition, figure 2(f) shows that, while pure indium oxide nanoparticles were white, the Sn-doped products had bluer color depending on tin content. This coloration
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of ITO according to doping with Sn is attributed to the increase in the charge carrier
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concentration or point-defect density [47; 48], both of which are due to the incorporation of substitutional Sn in the In2O3 lattice [45]. The decreasing resistivity of ITO nanoparticles as a
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function of tin content is often related to a gradually deepening blue color of the products [8; 10; 11; 16; 17; 24]. Moreover, it has been reported that, while ITO materials are yellowish
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after oxidative calcination, they become blue-green upon annealing under a reducing
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atmosphere with a simultaneous decrease in electrical resistivity [16; 18; 45]. In our synthetic process, the reducing atmosphere of supercritical methanol [25; 43] would have resulted in an
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high electrical conductivity.
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in-situ reduction of the products, manifested as the bluish color of the products and possibly
Figure 4. Temperature profile of the reactor immersed in salt bath at 300 °C as a function of time.
Since the synthesis was carried out in high-pressure stainless steel batch reactors,
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which have a high heat capacity and were initially kept at room temperature, there was a temporal delay in the rise in temperature inside the reactor upon immersion in the salt bath. We measured the temperature profile of an empty reactor immersed in a salt bath at a temperature of 300°C (Figure 4). It took 60 s for the inside of the reactor to reach 239°C, the
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critical temperature of methanol; therefore, under these experimental conditions, the heated
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solution would have remained in a liquid phase for up to 1 minute, and subsequently
expanded to the supercritical phase. Another 60 s was required for the reactor to reach 290°C.
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If a continuous-flow process enabling fast heating of the sample were to be optimized, the residence time required for the formation of ITO crystals might be much shorter than
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currently observed [40; 41]. Rapid heating may have an additional benefit of lowering the
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process temperature, as it was recently reported that the nucleation temperature of In2O3 is lowered by increasing the heating rate of the solvothermal reactor [33]. It also should be
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noted that the pressure inside the reactor increased after reaction times longer than 10
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minutes, indicating the formation of gaseous byproducts. Although the composition of the byproducts was not analyzed, nanostructured indium oxide has high catalytic activity in decomposition of methanol [49]. Therefore, prolonged reaction times should be avoided when producing ITO nanoparticles with the current method.
4. Conclusions
Within short reaction times of less than 2 minutes, cubic ITO nanoparticles measuring
about 20 nm were synthesized in supercritical methanol without any surfactant. The electrical and structural properties of the synthesized nanoparticles as a function of Sn content are comparable to those of reported ITO nanoparticles. The temperature required for the formation of cubic In2O3 phase with this solvothermal method is 250°C, which is lower than when water is used as the solvent, resulting in more benign processing conditions for the 11 Page 13 of 20
synthesis of this material. Adaptation of current method in a continuous flow system, as well as analysis and control of the impurities, are suggested as future work.
Conflict of interest statement
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The authors confirm that there are no conflicts of interest regarding this paper.
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