Synthesis and Electrochemistry of Layered Double Hydroxide Thin Films by Layer-by-Layer Assembly Method
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0988-QQ06-05
Synthesis and Electrochemistry of Layered Double Hydroxide Thin Films by Layer-byLayer Assembly Method Ugur Unal, Ozge Altuntasoglu, Shintaro Ida, and Yasumichi Matsumoto Department of Nano Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto, 860-8555, Japan
ABSTRACT In this study, we have investigated the synthesis of several Layered Double Hydroxides (LDHs), such as Ni-Ga, Mg-Ga and Ni-Al LDHs with hydrothermal method. Total metal content and ratio of the hydrolysis agent in the reaction solution have found to be determining factors for the crystallinity and morphology of LDH products. In addition, we were able to obtain highly crystalline LDHs in a relatively short reaction times, when the reaction was carried out under stirring at moderately higher temperatures. Delamination of LDHs was carried out in formamide and delamination was confirmed by the images obtained from Atomic Force Microscope (AFM). The observed thickness of sheets was in agreement with the theoretical thickness of LDH layers. Delaminated single nanosize sheets were deposited on a substrate by Layer-by-Layer Assembly (LBL) method, which utilizes the electrostatic attraction principles between oppositely charged units. UV-vis spectroscopy of the films showed that deposition of intercalated LDH thin films with LBL was successfu. Electrochemical properties of the films were also investigated in this study. Cyclic voltammograms of the films revealed that LDH layers and intercalated species are electrochemically active. The redox peaks of Ni/Ni2+ couple were observed for Ni2+ containing LDHs in the region of -0.45 V vs Ag/AgCl. In conclusion, we have demonstrated in this study that various LDH nanosize sheets can be intercalated with anionic species in a Layer-by-Layer deposition manner. In addition, LDH thin films prepared by LBL method showed electrochemical response. INTRODUCTION Layered double hydroxides (LDHs) are formed from Brucite –like layers with positive charge and have a general formula of [M2+1-xM3+x(OH)2][An-x/n mH2O] [1]. The positive charge is resulted from the isomorphously partially replaced divalent metal cations (M2+) with trivalent ones (M3+) and is compensated by interlayer anionic species (An-x/n) such as CO32-, NO3- and Cl-.
There are various preparation methods for LDHs. Most widely used method is coprecipitation of metals in an aqueous solution at high pH values mostly adjusted with slow addition of NaOH [2]. Among other methods are urea hydrolysis [3], hydrothermal [4] and solgel techniques [5]. Coprecipitation method is a multistep process and may result in very small LDH crystals. In addition, coprecipitation in the presence of NaOH is fast and the process less controllable when compared to homogenous precipitation with the hydrolysis of urea. Hydrothermal synthesis of LDHs can be carried out in the presence of urea and alternatively Hexamethylenetetramine (HMTA) [4]. The advantage of HMTA is that it does not release any CO32-, which is formed during the hydrolysis of urea. Layers of LD
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