Evaporation-Driven Deposition of ITO Thin Films from Aqueous

May 16, 2017 - (5, 6) Despite the high demand for ITO thin film materials, a vapor phase sputtering process is required for making ITO thin films with...
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Evaporation-Driven Deposition of ITO Thin Films from Aqueous Solutions with Low-Speed Dip-Coating Technique Takashi Ito, Hiroaki Uchiyama,* and Hiromitsu Kozuka Department of Chemistry and Materials Engineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan S Supporting Information *

ABSTRACT: We suggest a novel wet coating process for preparing indium tin oxide (ITO) films from simple solutions containing only metal salts and water via evaporation-driven film deposition during low-speed dip coating. Homogeneous ITO precursor films were deposited on silica glass substrates from the aqueous solutions containing In(NO3)3·3H2O and SnCl4·5H2O by dip coating at substrate withdrawal speeds of 0.20−0.50 cm min−1 and then crystallized by the heat treatment at 500−800 °C for 10−60 min under N2 gas flow of 0.5 L min−1. The ITO films heated at 600 °C for 30 min had a high optical transparency in the visible range and a good electrical conductivity. Multiple-coating ITO films obtained with five-times dip coating exhibited the lowest sheet (ρS) and volume (ρV) resistivities of 188 Ω sq−1 and 4.23 × 10−3 Ω cm, respectively.

1. INTRODUCTION Indium tin oxide (ITO) is the most commonly known transparent conductive material because of the low electrical resistivity and the high optical transparency in the visible range.1−16 Nowadays, ITO coating films are widely utilized in the practical fields such as displays,1−3 solar cells,4 and gas sensors.5,6 Despite the high demand for ITO thin film materials, a vapor phase sputtering process is required for making ITO thin films with high transparency and high electrical conductivity, which raises the production cost.17 Therefore, an alternative coating technique for achieving more cheaply and easily ITO thin film materials has been desired. The wet coating process is a favorable method to obtain ITO thin films at cheaper prices due to the simple and inexpensive facilities.11−16 Especially, aqueous solutions of metal salts are one of the ideal precursors to reduce the production cost of ITO materials via solution routes because water is a cheap and manageable solvent. However, the high surface tension of water (72 mN m−1) provides a poor wettability of many substrates such as glasses, ceramics, and metals, inhibiting the homogeneous film formation from aqueous coating solutions. Recently, a low-speed dip-coating technique has been suggested for making metal oxide thin films from metal salt aqueous solutions.18−21 The schematic illustration of the film deposition during low-speed dip coating is shown in Figure 1. When the substrate withdrawal speed of dip coating is fully low (below 1.0 cm min−1), the evaporation of solvents from the coating layer becomes faster than the motion of the substrates. During such low-speed dip coating, the solvent evaporates from the edge of the meniscus, and then the solutes locally deposit there, resulting in the formation of dried coating layer on the © 2017 American Chemical Society

Figure 1. Schematic illustration of the film deposition during lowspeed dip coating.

substrate. The coating process accompanied by the evaporation-driven deposition of solutes would hinder the aqueous solution from gathering together to form droplets, achieving the homogeneous deposition of a film on the substrate. Previously, we succeeded in preparing SnO2,20 TiO2,20 and WO321 thin films from SnCl4, TiOSO4, and (NH4)10W12O41 aqueous solutions, respectively, by low-speed dip coating, where any other additives were not used for modifying the wettability of the coating solutions. The novel dip-coating technique enables the fabrication of metal oxide thin films from Received: March 10, 2017 Revised: April 30, 2017 Published: May 16, 2017 5314

DOI: 10.1021/acs.langmuir.7b00823 Langmuir 2017, 33, 5314−5320

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The thickness of the film samples was measured with a contact probe surface profilometer (SE-3500K31, Kosaka Laboratory, Tokyo, Japan). A part of the as-deposited precursor film on the silica glass substrate was scraped off with a surgical knife, followed by the drying and heat treatment of the film samples. The level difference between the coated part and the scraped part was regarded as the film thickness. The surface roughness of the films was also measured using the profilometer. The surface roughness parameter, Ra (calculated average roughness), was automatically calculated from the surface profile. The sheet (ρS) and volume (ρV) resistivities of the ITO heat-treated films on silica glass substrates were evaluated by the four-point prove method. The electrical resistance (R) of the film samples was measured using a digital multimeter (7555, Yokogawa Electric Co., Tokyo, Japan) equipped with a four-point probe (MCP-TP06, Mitsubishi Chemical Analytech Co., Mie, Japan). The sheet resistivity (ρS) was calculated from R and the resistivity correction factor ( f) by ρS = R × f. The volume resistivity (ρV) was calculated from ρS and the film thickness (t) by ρV = ρS × t.

simple solutions containing only metal salts and water with common and inexpensive equipment, providing a low-cost coating process. In this work, we attempted to prepare ITO thin films by lowspeed dip coating from aqueous solutions and to evaluate their electrical resistance. The precursor films were made from simple coating solutions prepared by mixing In(NO3)3·3H2O, SnCl4·5H2O, and water, and then ITO films were obtained by heating the precursor films in a N2 atmosphere. The influence of the substrate withdrawal speed and the Sn/In mole ratio in the coating solutions on the formation of ITO precursor films was systematically investigated. Moreover, we discussed the effect of the heat-treatment temperature and time and the multiple coating on the electrical resistance of the ITO film products.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Influence of the Substrate Withdrawal Speed on the Film Formation. The influence of the substrate withdrawal speed on the formation of coating layer was studied for the ITO precursor films prepared from the aqueous coating solutions of r ([SnCl4·5H2O]/[In(NO3)3·3H2O]) = 0.12 ([In(NO3)3·3H2O] = 0.20 M and [SnCl4·5H2O] = 0.024 M), where the films were deposited on silica glass substrates with one-time dip coating at 0.050−1.0 cm min−1. In the case of low-speed dip coating, the evaporation-driven deposition of solutes progresses with increasing coating temperature and with decreasing substrate withdrawal speed.18−21 The dip coating at room temperature did not result in the formation of coating layer on the substrates, irrespective of substrate withdrawal speed. On the other hand, ITO precursor films were obtained by the low-speed dip coating at 60 °C in a thermostatic oven. Table 1 shows the appearance and thickness of the ITO

2.1. Preparation of ITO Thin Films by Low-Speed Dip Coating. 2.1 g of In(NO3)3·3H2O (Wako Pure Chemical Industries, Osaka, Japan) and 0−0.32 g of SnCl4·5H2O (Wako Pure Chemical Industries, Osaka, Japan) were added and dissolved in 30 cm3 of purified water under stirring at room temperature, resulting in transparent solutions of [In(NO3)3·3H2O] = 0.20 M and [SnCl4· 5H2O] = 0−0.030 M. Here, the Sn/In mole ratio in the aqueous solutions was defined as r (r = [SnCl4·5H2O]/[In(NO3)3·3H2O] = 0− 0.15). After filtering by using a syringe with a polypropylene membrane filter of 0.45 μm in pore size, the resultant solutions served as coating solutions. ITO precursor films were deposited on silica glass (20 mm × 40 mm × 0.85 mm) and Si(100) substrates (20 mm × 40 mm × 0.5 mm) (cleaning method of the substrates is shown in the Supporting Information) by dip coating at substrate withdrawal speeds of 0.050− 1.0 cm min−1 using a dip-coater (Portable Dip Coater DT-0001, SDI, Kyoto, Japan). The dip coating was done in a thermostatic oven to keep the coating temperature (i.e., the temperature of coating solutions, substrates, and atmosphere) at 60 °C. Before the dip coating, the coating solutions and substrates were heated at 60 °C for 30 min in the thermostatic oven. ITO films were obtained from the precursor films deposited on silica glass substrates by heating at 500− 800 °C for 10−60 min under N2 gas flow of 0.5 L min−1, where the precursor films were directly inserted to a tube electrical furnace held at the prescribed temperature. The dip coating and the heating were cycled 1−6 times, and the resultant films were served for the measurements. 2.2. Characterization. The appearance of the ITO precursor and heat-treated films was observed with an optical microscope (KH-1300, HiROX, Tokyo, Japan). The surface morphology of the products was investigated using a field emission scanning electron microscope (FESEM) (Model JSM-6500F, JEOL, Tokyo, Japan). The crystalline phase of the precursor and heat-treated films was analyzed using an Xray diffractometer (Model Rint-Ultima III, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.540 56 Å), where the X-ray diffraction (XRD) measurement was performed at 40 kV and 40 mA at an incident angle of 0.5°. Fourier transform infrared (FT-IR) spectra were recorded for the ITO precursor films deposited on Si(100) substrates using an FTIR spectrophotometer (FT/IR-410, Jasco, Tokyo, Japan), where an Si(100) substrate was used as the reference. Optical transparency of the ITO heat-treated films was evaluated using an optical spectrometer (V-570, JASCO, Tokyo, Japan), where a silica glass substrate was used as the reference. The chemical compositions of the ITO heat-treated films were identified by X-ray photoelectron spectroscopy (XPS). The XPS measurement was performed using an X-ray photoelectron spectrometer (PHI5000 Versa Probe, ULVAC-PHI, Chigasaki, Japan) with a monochromatic Al Kα X-ray source, where a charge neutralizer was used to counter the surface charging. The composition depth profiles of the ITO films were collected by the XPS measurement after every 15 s sputtering by Ar+ ions, where the etching rate by the sputtering was ca. 20 nm min−1.

Table 1. Appearance and Thickness of the ITO Precursor Films Deposited by Dip Coating of Various Substrate Withdrawal Speedsa precursor films substrate withdrawal speed (cm min−1)

film formation

0.050 0.20 0.30 0.50 1.0

yes yes yes yes nob

cracking

thickness (nm)

yes no no no

126.0 95.0 87.7 78.7

a The precursor films were obtained from the coating solutions of r = 0.12 with one-time dip coating in a thermostatic oven at 60 °C. bITO precursor films could not be visually observed on the substrates.

precursor films. Film formation on the whole substrate was visually confirmed below 0.50 cm min−1. The film thickness became larger with decreasing substrate withdrawal speed, leading to the cracking of films at 0.050 cm min−1. Noncracking ITO precursor films below 100 nm in thickness were achieved between 0.20 and 0.50 cm min−1, as seen in Figure 2. The precursor films were found to be amorphous (Supporting Information Figure S1). The formation of ITO precursor films was also confirmed by the IR absorption analysis. Figure 3 shows the IR absorption spectra of the precursor films deposited on Si(100) substrates from the coating solutions of r = 0.12 at 0.050−1.0 cm min−1. 5315

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The absorption bands at around 460, 670, 1100, 1650, and 3400 cm−1 were found in the spectra of the film samples deposited below 0.50 cm min−1. The broad bands at 1650 and 3400 cm−1 are assigned to the bending vibrations of adsorbed H2O and the stretching vibrations of O−H, respectively. The broad bands at around 1100 cm−1 could be attributed to the vibrations of In−OH22 and Sn−OH groups.23 The absorption bands at 460 and 670 cm−1 may be the vibrations of In−O6,12,14 and Sn−O24,25 bonds, respectively. These absorption peaks attributed to OH groups, Sn−O bonds, and In−O bonds suggest the deposition of Sn(OH)4 and In(OH)3 on the substrates. Moreover, the absorption band at around 1360 cm−1 attributed to the vibrations of NO3− ions was detected in the spectra, which might indicate that nitrate species such as In(NO3)3 and Sn(NO3)4 slightly deposited on the substrate. The formation of ITO precursor films with decreasing substrate withdrawal speed could be caused by the evaporationdriven film deposition during low-speed dip coating. As mentioned in the Introduction, the solvents preferentially evaporate from the edge of the meniscus during low-speed dip coating, where the deposition of the solutes locally progresses, resulting in the formation of coating layer on the substrate.18−21 In the present case, In3+ and Sn4+ compounds were deduced to deposit at the meniscus from the aqueous solutions containing In(NO3)3 and SnCl4 during dip coating below 0.50 cm min−1, resulting in the formation of ITO precursor films on the substrate. However, an excessive deposition of precursor films led to the increase in the film thickness, resulting in the cracking of films (Table 1). These results suggest that the homogeneous film deposition from aqueous solutions needs moderate substrate withdrawal speeds. 3.2. Influence of the Sn/In Mole Ratio on the Film Formation and the Electrical Resistance. ITO precursor films were prepared on silica glass substrates from the coating solutions with various Sn/In mole ratio (r = 0−0.15) ([In(NO3)3·3H2O] = 0.20 M and [SnCl4·5H2O] = 0−0.030 M) with 1e-time dip coating at 0.20 cm min−1, followed by the heat treatment at 600 °C for 10 min under N2 gas flow of 0.5 L min−1. The precursor films were not obtained from the solutions containing only In(NO3)3 (r = 0), while the addition of SnCl4 (r = 0.030−0.15) led to the film formation on the whole substrate (Table 2). ITO heat-treated films were obtained from the precursor films of r = 0.030−0.15 by the heating in a N2 atmosphere. The Sn/In mole ratio in the heattreated films, r′, evaluated by the XPS measurement is shown in Table 2. The r′ values increased with increasing SnCl4 contents in the coating solutions (r). Moreover, the composition depth

Figure 2. Optical micrographs of the ITO precursor films prepared from the aqueous solution of r = 0.12 with one-time dip coating at 0.20 cm min−1.

Figure 3. IR absorption spectra of the ITO precursor films deposited on Si(100) substrates from the aqueous solution of r = 0.12 with onetime dip coating at 0.050−1.0 cm min−1.

Table 2. Thickness, Appearance, and Electrical Resistance of the ITO Films Obtained from the Coating Solutions of Various Sn/In Mole Ratios heat-treated filmsb r 0 0.030 0.050 0.12 0.15

film formationa

r′c

cracking

thickness (nm)

ρS (Ω sq−1)

0.02 0.04 0.09 0.11

no no no no

36.6 41.4 72.2 81.1

8730 5670 2400 6980

ρV (Ω cm)

d

no yes yes yes yes

3.19 2.35 1.73 5.66

× × × ×

10−2 10−2 10−2 10−2

The precursor films were obtained with one-time dip coating at 0.20 cm min−1 in a thermostatic oven at 60 °C. bThe heat treatment of the films was performed at 600 °C for 10 min under N2 gas flow of 0.5 L min−1. cThe Sn/In mole ratio in the heat-treated films, r′, was evaluated by XPS measurement after sputtering by Ar+ ions for 30 s (etching rate: ca. 20 nm min−1). dITO precursor films could not be visually observed on the substrates. a

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Langmuir profiles measured by etching with Ar+ sputtering show that the Sn contents at the inside of films were higher than the surface (Figure 4). These results suggest that Sn4+ compounds could

In2O3 phase (Figure 5b), indicating the substitution of Sn4+ ions for In3+ ions. Since the ionic radius of Sn4+ is smaller than that of In3+, the substitution of Sn4+ ions could shrink the bixbyite lattice. The electrical resistance of the ITO heat-treated films of r = 0.030−0.15 was evaluated using a four-point probe technique. The sheet (ρS) and volume (ρV) resistivities of the ITO heattreated films decreased with increasing r from 0.030 to 0.12 (Table 2), which could be caused by the increase in carrier density due to the replacement of In3+ by Sn4+ in bixbyite lattice.16,26 The ITO film of r = 0.12 exhibited the lowest ρS and ρV of 2400 Ω sq−1 and 1.73 × 10−2 Ω cm, respectively. The ρS and ρV values of the heat-treated films were 3 orders of magnitude higher than those of the ITO films prepared by vapor phase sputtering processes (ρS: ca. 10 Ω sq−1; ρV: < 10−4 Ω cm),8−10,17 which could be attributed to the low electron mobility due to the porous structure (Figure S2). The further increase in r from 0.12 to 0.15 increased ρS and ρV (Table 2). The addition of a large amount of Sn4+ ions into In2O3 is reported to lead to the formation of the lattice defects composed of two Sn4+ ions, providing lower electron mobility.27 Thus, the depression of the electrical conductivity over r = 0.15 might be attributed to the decrease in electron mobility due to the lattice defects consisting of Sn4+ ions. 3.3. Influence of the Heat-Treatment Temperature and Time on the Electrical Resistance. Table 3 shows the appearance, thickness, and electrical resistance of the ITO heattreated films obtained by heating at 500−800 °C for 10−60 min under N2 gas flow of 0.5 L min−1, where the ITO precursor films were prepared on silica glass substrates from the coating solutions of r = 0.12 with one-time dip coating at 0.20 cm min−1. The heat-treated films were confirmed to consist of bixbyite In2O3 phase, irrespective of the heat-treatment temperature (Figure S3a) and time (Figure S3b). The thickness of the heat-treated films decreased with increasing heattreatment temperature and time (Table 3), which indicates the densification of the films. The high-temperature (800 °C) and ling-time (60 min) heating resulted in the cracking of films (Table 3).

Figure 4. Composition depth profiles obtained by X-ray photoelectron spectroscopy measurements for the ITO heat-treated films prepared from the aqueous solution of r = 0.030−0.15.

first nucleate on the substrate, which involved the coprecipitation of In3+ species, resulting in the formation of the ITO precursor films. The thickness of the heat-treated films increased with increasing r, where cracks were not observed for the all films (r = 0.030−0.15) even after the heating. The porous structure consisting of fine grains below 50 nm in diameter was observed for the heat-treated films irrespective of the Sn/In mole ratio (Figure S2). Figure 5 shows the XRD patterns of the heattreated films obtained at r = 0.030−0.15. Bixbyite In2O3 phase was detected for the all heat-treated films (Figure 5a). The diffraction peaks of the ITO heat-treated films of r = 0.030− 0.15 slightly shifted to higher angles than those of the pure

Figure 5. XRD patterns of the ITO heat-treated films prepared from the aqueous solution of r = 0.030−0.15: (a) from 20 to 60° and (b) from 28 to 33°. 5317

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Langmuir Table 3. Thickness, Appearance, and Electrical Resistance of the ITO Films Heated at Various Heat-Treatment Conditionsa heat-treatment conditions

heat-treated filmsb

temp (°C)

heating time (min)

cracking

500 600 700 800 600 600 600

10 10 10 10 10 30 60

no no no yes no no yes

thickness (nm)

ρS (Ω sq−1)

ρV (Ω cm)

77.0 72.2 53.0

3270 2400 3930

2.52 × 10−2 1.73 × 10−2 2.08 × 10−2

72.2 45.1

2400 1090

1.73 × 10−2 0.49 × 10−2

The precursor films were obtained from the coating solutions of r = 0.12 with one-time dip coating at 0.20 cm min−1 in a thermostatic oven at 60 °C. bThe heat treatment of the films was performed under N2 gas flow of 0.5 L min−1. a

The electrical resistance was evaluated for the crack-free ITO heat-treated films heated at 500−700 °C for 10−30 min. The increase in heat-treatment temperature from 500 to 600 °C and time from 10 to 30 min provided lower ρS and ρV (Table 3), which could be attributed to the improvement of the electron mobility due to the progress of the densification and crystallization. The densification and crystallization during the heat treatment may have reduced the lattice distortion and the oxygen vacancy, resulting in the higher electron mobility. On the other hand, the heating at 700 °C for 10 min showed higher resistivity than 600 °C (Table 3). The high-temperature heating might drastically reduce the oxygen vacancy in ITO lattice, which decreased the carrier density in the films, resulting in the higher ρS and ρV.28 3.4. Preparation of Multiple-Coating ITO Films and Their Optical and Electrical Properties. Multiple-coating ITO films were prepared by repeating the dip coating and the heat treatment, and their optical and electrical properties were evaluated. Here, ITO precursor films were prepared on silica glass substrates from the coating solutions of r = 0.12 with dip coating at 0.20 cm min−1, followed by the heat treatment at 600 °C for 30 min under N2 gas flow of 0.5 L min−1. As seen in Table 4, six-times coating resulted in the cracking of films. Crack-free, transparent ITO films were obtained by five-times dip coating (Table 4 and Figure 6a), and fine grains below 50 nm in diameter were observed in the films (Figure 6b). The

Figure 6. (a) Optical micrograph and (b) SEM image of the ITO heattreated films prepared with three-times dip coating and (c) UV−vis transmission spectra of the ITO films prepared with (1−5)-times dip coating.

thickness of the multiple-coating films was calculated from that of one-time dip coating, which almost agrees with the results of the SEM cross-sectional images (Figure S4). The surface roughness parameter, Ra (calculated average roughness), of the five-times coating film was ca. 15 nm, indicating a flat surface. The ITO films thus obtained showed high optical transmittance over 80% at the visible wavelengths of 400−800 nm (Figure 6c). The sheet resistivity decreased with increasing coating time (Table 4), while the volume resistivity was almost unchanged (Table 4). The decrease in the ρS was thought to result from the increase in film thickness. The five-times coating film exhibited the lowest ρS and ρV of 188 Ω sq−1 and 4.23 × 10−3 Ω cm, respectively (Table 4). The ρV value is 1 order of magnitude higher than those reported so far for ITO films prepared by ordinary dip coating (ρV: (2−8) × 10−4 Ω cm).29−34 The amount of the residual NO3− and Cl− ions was evaluated by the XPS analysis for the five-times coating film. Nitrogen was not detected, while a small amount of Cl (ca. 0.85 atom %) was contained in the five-times coating ITO film. The residual chlorine might affect the electrical conductivity of the ITO film product. Figure 7 demonstrates the electrical

Table 4. Thickness, Appearance, and Electrical Resistance of the Multiple-Coating ITO Filmsa heat-treated filmsb coating times

cracking

thicknessc (nm)

ρS (Ω sq−1)

1 2 3 4 5 6

no no no no no yes

45.1 90.2 135.3 180.4 225.5

1090 567 305 262 188

ρV (Ω cm) 4.92 5.11 4.13 4.72 4.23

× × × × ×

10−3 10−3 10−3 10−3 10−3

The precursor films were obtained from the coating solutions of r = 0.12 with dip coating at 0.20 cm min−1 in a thermostatic oven at 60 °C. bThe heat treatment of the films was performed at 600 °C for 30 min under N2 gas flow of 0.5 L min−1. cThe thickness of the multiplecoating films was calculated from that of one-time dip coating. a

Figure 7. Demonstration of electrical conductivity of the ITO heattreated films. 5318

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Langmuir conductivity of the ITO films obtained with three-times dip coating, where an LED lamp is shining by an electric current through the film. These results suggest that the ITO films obtained from aqueous solutions by low-speed dip coating work as transparent conductive film materials.

(3) Oh, C. S.; Lee, S. M.; Kim, E. H.; Lee, E. W.; Park, L. S. ElectroOptical Properties of Index Matched ITO-PET Film for Touch Panel Application. Mol. Cryst. Liq. Cryst. 2012, 568, 32−37. (4) Armstrong, N. R.; Carter, C.; Donley, C.; Simmonds, A.; Lee, P.; Brumbach, M.; Kippelen, B.; Domercq, B.; Yoo, S. Y. Interface modification of ITO thin films: organic photovoltaic cells. Thin Solid Films 2003, 445 (2), 342−352. (5) Patel, N. G.; Patel, P. D.; Vaishnav, V. S. Indium tin oxide (ITO) thin film gas sensor for detection of methanol at room temperature. Sens. Actuators, B 2003, 96 (1−2), 180−189. (6) Yadav, B. C.; Agrahari, K.; Singh, S.; Yadav, T. P. Fabrication and characterization of nanostructured indium tin oxide film and its application as humidity and gas sensors. J. Mater. Sci.: Mater. Electron. 2016, 27 (5), 4172−4179. (7) Bender, M.; Seelig, W.; Daube, C.; Frankenberger, H.; Ocker, B.; Stollenwerk, J. Dependence of oxygen flow on optical and electrical properties of DC-magnetron sputtered ITO films. Thin Solid Films 1998, 326 (1−2), 72−77. (8) Bhagwat, S.; Howson, R. P. Use of the magnetron-sputtering technique for the control of the properties of indium tin oxide thin films. Surf. Coat. Technol. 1999, 111 (2−3), 163−171. (9) Granqvist, C. G.; Hultaker, A. Transparent and conducting ITO films: new developments and applications. Thin Solid Films 2002, 411 (1), 1−5. (10) Hu, Y. L.; Diao, X. G.; Wang, C.; Hao, W. C.; Wang, T. M. Effects of heat treatment on properties of ITO films prepared by rf magnetron sputtering. Vacuum 2004, 75 (2), 183−188. (11) Alam, M. J.; Cameron, D. C. Optical and electrical properties of transparent conductive ITO thin films deposited by sol-gel process. Thin Solid Films 2000, 377-378, 455−459. (12) Celik, E.; Aybarc, U.; Ebeoglugil, M. F.; Birlik, I.; Culha, O. ITO films on glass substrate by sol-gel technique: synthesis, characterization and optical properties. J. Sol-Gel Sci. Technol. 2009, 50 (3), 337−347. (13) Duta, M.; Anastasescu, M.; Calderon-Moreno, J. M.; Predoana, L.; Preda, S.; Nicolescu, M.; Stroescu, H.; Bratan, V.; Dascalu, I.; Aperathitis, E.; Modreanu, M.; Zaharescu, M.; Gartner, M. Sol-gel versus sputtering indium tin oxide films as transparent conducting oxide materials. J. Mater. Sci.: Mater. Electron. 2016, 27 (5), 4913− 4922. (14) Jafan, M. M. H.; Zamani-Meymian, M. R.; Rahimi, R.; Rabbani, M. Effect of pyrolysis temperature on the electrical, optical, structural, and morphological properties of ITO thin films prepared by a sol-gel spin coating process. Microelectron. Eng. 2014, 130, 40−45. (15) Kim, S. S.; Choi, S. Y.; Park, C. G.; Jin, H. W. Transparent conductive ITO thin films through the sol-gel process using metal salts. Thin Solid Films 1999, 347 (1−2), 155−160. (16) Li, Z.; Ren, D. Fabrication and structure characterization of ITO transparent conducting film by sol-gel technique. Trans. Nonferrous Met. Soc. China 2007, 17 (3), 665−668. (17) Ye, S. R.; Rathmell, A. R.; Chen, Z. F.; Stewart, I. E.; Wiley, B. J. Metal Nanowire Networks: The Next Generation of Transparent Conductors. Adv. Mater. 2014, 26 (39), 6670−6687. (18) Grosso, D. How to exploit the full potential of the dip-coating process to better control film formation. J. Mater. Chem. 2011, 21 (43), 17033−17038. (19) Krins, N.; Faustini, M.; Louis, B.; Grosso, D. Thick and CrackFree Nanocrystalline Mesoporous TiO2 Films Obtained by Capillary Coating from Aqueous Solutions. Chem. Mater. 2010, 22 (23), 6218− 6220. (20) Uchiyama, H.; Ito, T.; Sasaki, R.; Kozuka, H. Preparation of metal oxide thin films from organic-additive-free aqueous solutions by low-speed dip-coating. RSC Adv. 2015, 5 (26), 20371−20375. (21) Uchiyama, H.; Igarashi, S.; Kozuka, H. Evaporation-Driven Deposition of WO3 Thin Films from Organic-Additive-Free Aqueous Solutions by Low-Speed Dip Coating and Their Photoelectrochemical Properties. Langmuir 2016, 32 (13), 3116−3121. (22) Karthik Kannan, S.; Thirnavukkarasu, P.; Jayaprakash, R.; Chandrasekaran, J.; Mohanraj, V. Transition of nanocrystalline

4. CONCLUSION We prepared ITO films by low-speed dip coating from the aqueous solutions containing In(NO3)3·3H2O and SnCl4· 5H 2 O. The ITO precursor films were obtained via evaporation-driven deposition during dip coating, followed by the heat treatment at 500−800 °C for 10−60 min under N2 gas flow of 0.5 L min−1. The electrical resistance of the ITO heattreated films depended on the heating temperature and time, where the films heated at 600 °C for 30 min had a good electrical conductivity. The ITO films obtained with five-times dip coating exhibited low sheet (ρS) and volume (ρV) resistivities of 188 Ω sq−1 and 4.23 × 10−3 Ω cm, respectively, and high optical transmittance over 80% in the visible range, which indicates that the film products obtained from aqueous coating solutions by low-speed dip coating can be applied as transparent conductive film materials. Such a synthetic route that realized ITO films from simple solutions composed only of metal salts and water has not been reported so far. The lowspeed dip-coating technique would allow us to make ITO thin films at cheaper prices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00823. XRD pattern of the ITO precursor films prepared from the aqueous solution of R = 0.12 with one-time dip coating at 0.20 cm min−1 (Figure S1), SEM images of the ITO heat-treated films prepared from the aqueous solution of R = 0.030−0.15 (Figure S2), XRD patterns of the ITO films heated at 500−800 °C for 10−60 min under N2 gas flow of 0.5 L min−1 (Figure S3), and SEM cross-sectional image of the ITO heat-treated films prepared with four-times dip coating (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.U.) E-mail [email protected]; Ph +81-6-6368-1121 ext 6131; Fax +81-6-6388-8797. ORCID

Hiroaki Uchiyama: 0000-0001-9337-6418 Hiromitsu Kozuka: 0000-0002-3385-215X Notes

The authors declare no competing financial interest.

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REFERENCES

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