Article pubs.acs.org/JPCB
Characterization and Film Properties of Electrophoretically Deposited Nanosheets of Anionic Titanate and Cationic MgAlLayered Double Hydroxide Atsunori Matsuda,* Hisatoshi Sakamoto, Mohd Arif Bin Mohd Nor, Go Kawamura, and Hiroyuki Muto Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan ABSTRACT: Anionic hydrated titanate (HnTiOm: HTO) nanosheets and cationic magnesium−aluminum layered double hydroxide (Mg−Al LDH) nanosheets were electrophoretically deposited on positively and negatively charged indium tin oxide (ITO)-coated glass substrates, respectively. The HTO nanosheets and Mg−Al LDH nanosheets obtained were identified in neutral water as H2Ti4O9·nH2O with a ζpotential of −23 mV and Mg6Al2(OH)18·4.5H2O with a ζpotential of +41 mV, respectively. Dense and smooth HTO and Mg−Al LDH films with layered structures with thicknesses of about 10−15 μm were prepared in 300 s at 7.5 V by electrophoretic deposition (EPD) from the nanosheet suspensions. Both EPD HTO and LDH films showed elasticity because of their layered laminate structures. The HTO thick films demonstrated large adsorption properties and high photocatalytic activity, while the Mg−Al LDH thick films showed relatively high ionic conductivity of 10−5 S cm−1 at 80 °C and 80% relative humidity.
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INTRODUCTION Electrophoretic deposition (EPD) is a simple and cost-effective process that enables us to obtain uniform thick films under ambient pressure and temperature in comparison with conventional physical vapor deposition (PVD) using vacuum system and aerosol deposition (AED) using high-pressure carrier gas. This technique can be widely applied to produce coatings for substrates for use as electrodes of various shapes and allows us to produce laminated composite materials composed of ceramics, polymers, dyes, and metals.1−3 EPD is an effective method to prepare nanostructured thick films of more than 1 μm in thickness and has been widely used for the fabrication of functional coatings of charged colloidal materials such as nanoparticles, nanotubes, and nanosheets.4−6 When we use nanosheets for EPD, the resulting EPD films consisting of the nanosheet materials may show better adhesion without cracking in the films when compared with EPD films based on particles. Various types of two-dimensional nanosheets have been synthesized by ion exchange and delamination of layered alkali titanates, layered double hydroxides, layered niobates, and α-zirconium phosphate.7−10 Hydrated titanate (HTO) nanosheets show high photocatalytic activity, distinctive solid acidity, good ion exchange properties, and large intercalation capacity because of their high specific surface area, high activity, and ultrathin dimensions. The synthesis, structures, and characteristics of several types of nanosheets derived from HTOs such as H2Ti3O7, H2Ti4O9, and H2Ti5O11 have been reported.11−13 Thin flakes and porous aggregates of HTO can be obtained from colloidal nanosheet suspensions by centrifugation and drying, whereas the © 2012 American Chemical Society
immobilization of HTO nanosheets on substrates, that is, the formation of thin or thick films of these HTO nanosheets on substrates, is essential for practical applications. For the EPD of HTO, highly oriented titania nanosheet films were prepared on Pt substrates under optimal conditions.14 An interesting UV− visible light-sensitive energy conversion system design using methyl viologen-intercalated titania nanosheet films prepared by EPD on indium tin oxide (ITO)-coated glass substrates has been reported.15 We have also reported the preparation of layered thick films from a surface-modified HTO (H2Ti4O9) nanosheet colloidal suspension by EPD.16 The H2Ti4O9 nanosheet is composed of corrugated ribbons of edge-sharing TiO6 octahedral units, which join at the corners to form stepped sheets separated by H+ ions in an intersheet. Layered double hydroxides (LDHs) have attracted considerable attention for applications including catalysts, adsorbent materials and inorganic fillers for organic−inorganic composites. Several types of LDH, including so-called hydrotalcite clays such as Mg−Al LDH, Zn−Al LDH, and Co−Al LDH, have been reported.8,17,18 Among these materials, Mg−Al LDH and its nanosheets were found to be promising as a solid electrolyte for direct methanol fuel cells to prevent methanol crossover.19−21For the EPD of Mg−Al LDH, oriented films were fabricated on aluminum substrates and were used to Special Issue: Electrophoretic Deposition Received: July 2, 2012 Revised: November 13, 2012 Published: December 3, 2012 1724
dx.doi.org/10.1021/jp306538q | J. Phys. Chem. B 2013, 117, 1724−1730
The Journal of Physical Chemistry B
Article
remove heavy metal ions and anionic dyes from aqueous solutions.22 A novel controlled molecular release based on highly oriented LDH nanoplates on ITO/glass substrates was also reported.23 In the present study, film formation using negatively charged HTO nanosheets and positively charged Mg−Al LDH nanosheets in water by EPD has been investigated and the structures of the resulting EPD nanosheet films have been evaluated. The mechanical properties of the EPD nanosheet films have also been evaluated and compared using a nanoindentation system. In addition, the photocatalytic activity and ionic conductivity of these EPD nanosheet films have been discussed.
EPD films. The penetration of a Berkovich indenter (radius, R = 50 μm) was controlled to be less than 10% of the film thickness using a piezo actuator. The resolution for control of the penetration depth via the piezo actuator was 0.04 nm. The penetration depth h was measured using twin capacitive gap sensors and the indentation load P was monitored using a load cell. The indentation tests were conducted on the EPD films on ITO-coated glass substrates at a penetration rate of 50 nm/s. The indentation contact behavior was well characterized by the load P versus penetration depth h hysteresis during the loading and subsequent unloading processes. The relationship between the indentation load P and h is expressed by
EXPERIMENTAL SECTION Electrophoretic Deposition. An HTO colloidal suspension consisting of tetratitanate (H2Ti4O9) nanosheets (2.6 wt %, pH = 7.6) was supplied by Otsuka Chemical Co., Ltd., Osaka, Japan. The HTO suspension was prepared from layered titanates by exfoliation using N,N-dimethylethanolamine (DMEA), which was then neutralized with boric acid and washed with deionized water (this process is the subject of Japanese Unexamined Patent Application Publication No. P2009−161678). The layered titanate was synthesized from TiO2, K2CO3, KCl, and LiOH at 1020 °C and was treated with 10% sulfuric acid. The layered titanate obtained was then dispersed in 3.5% hydrochloric acid, filtrated, and repeatedly washed with pure water before the exfoliation process using DMEA. A colloidal suspension of Mg−Al LDH acetate nanosheets (1.0 wt %, pH = 9.8) was supplied by Tayca Co., Osaka, Japan. The Mg−Al LDH nanosheets were prepared from hydrotalcite in a Mg(CH3COO)2 aqueous solution. The Mg−Al LDH nanosheets obtained were separated by centrifugation, dried at 90 °C, pulverized, and then dispersed in pure water (this process is the subject of Japanese Unexamined Patent Application Publication No. P2006− 274385). The thicknesses of the HTO and Mg−Al LDH nanosheets were measured using an atomic force microscope (AFM, NPX 200, SII Nano Technology Inc.). The surface charges of the HTO and Mg−Al LDH nanosheets in deionized water were measured using a ζ-potential analyzer (ELS-Z1NS, Otsuka Electronics Co., Ltd.). An average of five measurements at the stationary level was taken for each point. ITO-coated glass substrates (
