Article pubs.acs.org/Langmuir
Evaporation-Driven Deposition of WO3 Thin Films from OrganicAdditive-Free Aqueous Solutions by Low-Speed Dip Coating and Their Photoelectrochemical Properties Hiroaki Uchiyama,* Seishirou Igarashi, 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 prepared tungsten trioxide (WO3) photoelectrode films from organic-additive-free aqueous solutions by a low-speed dip-coating technique. The evaporation-driven deposition of the solutes occurred at the meniscus during lowspeed dip coating, resulting in the formation of coating layer on the substrate. Homogeneous WO3 precursor films were obtained from (NH4)10W12O41·5H2O aqueous solutions and found to be crystallized to monoclinic WO3 films by the heat treatment at 400−700 °C. All the films showed a photoanodic response irrespective of the heat treatment temperature, where a good photoelectrochemical stability was observed for those heated over 500 °C. The highest photoanodic performance was observed for the WO3 film heated at 700 °C, where the IPCE (incident photon-to-current efficiency) was 36.2% and 4.6% at 300 and 400 nm, respectively.
1. INTRODUCTION Aqueous solutions containing metal ions are environmentally friendly precursors for metal oxide film materials because water is not inflammable or toxic and has a lower volatility. The replacement of the precursor solutions containing potentially harmful solvents by aqueous ones is favorable in industrial fields. However, the surface tension of water is very high (72 mN m−1), which disturbs the wetting of many substrates such as glasses, ceramics, and metals by aqueous solutions, inhibiting the formation of homogeneous coating layer on the substrates. Addition of organic polymers into aqueous solutions is well known as a good method to modify the low wettability. The authors improved the low wettability with organic additives such as poly(vinylpyrrolidone), poly(vinylacetamide), and poly(acrylamide), where TiO2 and ZrO2 thin films were obtained by dip coating from aqueous solutions containing Ti(SO4)2 and ZrOCl2, respectively.1 Jia et al. succeeded the preparation of metal oxide films from aqueous solutions by the addition of poly(ethylenimine) with ethylenediaminetetraacetic acid, where the organic additive suppresses the undesirable reactions of metal ions and keeps the moderate viscosity of the solutions, leading to the homogeneous film formation.2−12 On the other hand, we have addressed the film deposition from aqueous solutions with another strategy without organic additives. Homogeneous film formation from organic-additivefree aqueous solutions can be achieved by a low-speed dipcoating technique. Figure 1 shows the schematic illustration of the film deposition during low-speed dip coating. In the case of dip coating of extremely low substrate withdrawal speeds (below 1.0 cm min−1), the solvent evaporation from coating layer becomes faster than the motion of the substrates. During © XXXX American Chemical Society
Figure 1. Schematic illustration of the evaporation-driven deposition during low-speed dip coating.
the low-speed dip coating, the solvent evaporation preferentially progresses at the edge of meniscus, and then the deposition of the solutes locally progresses there, leading to the formation of dried coating layer. Such evaporation-driven deposition prevents aqueous solutions with high surface tension from gathering together to form droplets, providing homogeneous coating layer on the substrates.13 The formation of films from aqueous solutions of metal salts by low-speed dip coating was first reported by Krins et al.14 They prepared mesoporous anatase TiO2 films from aqueous solutions containing TiCl4 and an organic polymer template (F127), finding that the rapid solvent evaporation at higher temperatures provide larger film Received: February 1, 2016 Revised: March 14, 2016
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DOI: 10.1021/acs.langmuir.6b00377 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Table 1. pH Value and Appearance of the Coating Solutions and Deposition Area and Appearance of the Precursor Films precursor filmsa
coating solutions [(NH4)10W12O41·5H2O] (x) (mM)
pH of HCl solutions
pH
appearance
deposition area
appearance
3.75
1.8 2.0 2.2 1.2 1.4 1.6 1.7 1.8 2.0 1.3 1.5 1.7
2.8 4.2 5.0 1.6 2.1 3.6 4.4 4.7 5.0 2.6 4.3 5.0
cloudy transparent transparent cloudy cloudy transparent transparent transparent transparent cloudy transparent transparent
partial area partial area partial area whole area whole area whole area whole area partial area partial area whole area whole area partial area
dendritic layer smooth layer plate-like precipitates dendritic layer dendritic layer smooth layer smooth layer plate-like precipitates plate-like precipitates dendritic layer smooth layer plate-like precipitates
7.57
11.4
a
Film deposition was done on a soda-lime glass substrate by dip coating at 0.02 cm min−1 in thermostatic oven at 60 °C.
thickness.14 Previously, we attempted to prepare SnO2 and TiO2 thin films from organic-additive-free aqueous solutions.13 Homogeneous SnO2 and TiO2 precursor films were obtained from SnCl4 and TiOSO4 aqueous solutions, respectively, and the precursors were crystallized without cracking by the heat treatment. The SnO2 and TiO2 thin films thus obtained exhibited high optical transparency in the visible range. The low-speed dip coating can be performed for any kind of metal oxides if metal salt aqueous solutions can be obtained as coating solutions. Such fabrication method of metal oxide thin films from simple solutions containing only water and metal salts has promise as a low-cost and resource-saving coating process. For the development of the low-speed dip coating technique as a novel coating method for aqueous solutions, the film products need to sufficiently work as practical devices such as sensors, electrodes, and photocatalysts. However, the device properties of the metal oxide films obtained by low-speed dip coating were not intensively evaluated in our previous work. Thus, in this work, we attempted to prepare WO3 photoelectrode films from organic-additive-free aqueous solutions by low-speed dip coating and to evaluate their photoelectrochemical properties. WO3 is attractive photoanode materials due to the photoelectrochemical stability in aqueous solutions and the relatively small band gap (ca. 2.5 eV) that can respond to the visible light.15−21 Here, WO3 thin films were obtained by lowspeed dip coating from simple coating solutions prepared by mixing of (NH4)10W12O41·5H2O and HCl aqueous solutions. We investigated the effect of the pH value and (NH4)10W12O41· 5H 2 O concentration of the coating solutions on the evaporation-driven deposition of WO3 precursor films during low-speed dip coating and then evaluated the photoanodic performance of the WO3 film products.
1.0 mm) and FTO (fluorine-doped tin oxide) glass substrates (20 mm × 40 mm × 1.0 mm) by dip coating, where the substrates were withdrawn at 0.02 and 0.03 cm min−1. The coating temperature, i.e., the temperature of substrates, solutions, and atmosphere, was kept at 30 and 60 °C, where the solutions and substrates were heated at the prescribed temperature for 10 min in the thermostatic oven before the dip coating. WO3 films were obtained from the precursor films deposited on FTO glass substrates by drying at 100 °C for 12 h and subsequent heat treatment at 200−700 °C for 10 min in air, where the dried precursor films were directly transferred to an electric furnace held at the prescribed temperature. 2.2. Characterization. Microscopic observation was made on the thin film samples using an optical microscope (KH-1300, HiROX, Tokyo, Japan). The microstructure of the thin films was observed using a scanning electron microscope (SEM) (Model JSM-6510, JEOL, Tokyo, Japan). The crystalline phases were identified by X-ray diffraction (XRD) measurement by ordinary 2θ/θ mode using an Xray diffractometer (Model Rint 2550 V, Rigaku, Tokyo, Japan) with Cu Kα radiation operated at 40 kV and 300 mA. Optical transmission spectra were measured on the film samples using an optical spectrometer (V-570, JASCO, Tokyo, Japan), where a FTO glass substrate was used as the reference. Film thickness was measured using a contact probe surface profilometer (SE-3500K31, Kosaka Laboratory, Tokyo, Japan). A part of the thin film was scraped off with a surgical knife immediately after the film deposition, and the level difference between the coated part and the scraped part was measured after drying and heat treatment. The thickness was measured for the heat-treated films deposited on FTO glass substrates. 2.3. Measurement of the Photoanodic Properties. Photoanodic properties of the WO3 films were evaluated in a three-electrode cell using a potentiostat (HZ-7000, Hokuto Denko, Osaka, Japan) consisting of the film electrode sample, a platinized Pt electrode, and a saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively, and of a buffer solution of pH 7, an aqueous solution of 0.2 M Na2B4O7, 0.14 M H2SO4, and 0.3 M Na2SO4, as the supporting electrolyte. For measuring the current−potential curves, the potential of the working electrode was scanned from −0.2 to 1.4 V vs SCE at a rate of 20 mV s−1. A 500 W xenon lamp (model UXL-500-D-0, XB-50101AAA, UI-502Q, Ushio Denki, Tokyo, Japan) was used as the white light source where the light intensity was reduced to 110−120 mW at a wavelength of 550 nm using an ND filter. The film was illuminated for 4 s, and then, the light was turned off for 4 s. The light turning on and off were repeated during the scanning of the working electrode potential. Action spectra of the films were measured at 1.0 V vs SCE, where the xenon lamp light was monochromatized using a monochromator (SPG-100s, Shimadzu, Kyoto, Japan). The intensity of the monochromatized light was measured using a power meter (NOVA,
2. EXPERIMENTAL SECTION 2.1. Preparation of WO3 Thin Films by Low-Speed Dip Coating. HCl aqueous solutions of pH 1.2−2.2 were prepared as the solvents of coating solutions by diluting ca. 36.0 mass % hydrochroric acid (Wako Pure Chemical Industries, Osaka, Japan) with purified water. 0.52−1.6 g of (NH4)10W12O41·5H2O (Wako Pure Chemical Industries, Osaka, Japan) was added and dissolved in 44 cm3 of the HCl solutions under stirring at 60 °C. After stirring at 60 °C for 1 h, the resultant solutions served as coating solutions ([(NH4)10W12O41· 5H2O] (x) = 3.75−11.4 mM). Low-speed dip coating was performed using a dip-coater (Portable Dip Coater DT-0001, SDI, Kyoto, Japan) in a thermostatic oven. Precursor films were deposited on soda-lime glass (20 mm × 40 mm × B
DOI: 10.1021/acs.langmuir.6b00377 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir PD300-UV, Ophir Japan, Saitama, Japan), which was ca. 23 μW at a wavelength of 500 nm. For this measurement, the film was first illuminated for 10 s, and then the light was turned off. The difference in current before and after turning off the light was taken as the photocurrent. Quantum efficiency, IPCE (incident photon-to-current efficiency), was calculated from photocurrent and incident light intensity.
solutions of pH over 4.7 resulted in the partial deposition (Table 1 and Figure 2e,f), which may be due to the lower and higher solubility of tungsten species in aqueous solutions,22,23 respectively. The higher (NH4)10W12O41·5H2O concentration (7.57−11.4 mM) and lower pH value (
