Layer-by-layer assembly and electrochemical properties of sandwiched film of manganese oxide nanosheet and carbon nanotube Huajun Zheng a,b,c , Fengqiu Tang c , Yi Jia c , Lianzhou Wang c, * , Yuchun Chen c , Melvin Lim c , Lei Zhang c , Gaoqing (Max) Lu c, * a Zhijiang college, Zhejiang University of Technology, Hangzhou 310024, PR China b State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, PR China c ARC Centre of Excellence for Functional Nanomaterials, University of Queensland, St. Lucia, Brisbane QLD 4072, Australia ARTICLE INFO Article history: Received 21 September 2008 Accepted 2 February 2009 Available online 10 February 2009 ABSTRACT Sandwiched film of MnO 2 nanosheet (MONS) and multi-walled carbon nanotube (MWCNT) was assembled by using the layer-by-layer method, based on electrostatic interaction of positively-charged poly (diallyl dimethyl ammonium chloride) and negatively-charged MONS and MWCNT. Ultraviolet–visible spectroscopy is used to probe the dynamic growth of multilayer film, exhibiting progressive enhancement of optical absorption due to the assembly of MONS and MWCNT. Thus, the assembled sandwiched film was characterized using scanning electron microscopy and X-ray photoelectron spectra. The multilayer film electrode presents excellent electrochemical capacitance properties, which were also highly dependent upon the deposition sequence and the order of structural components in sandwiched film. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double- layer capacitors) or fast surface redox reactions (pseudo- capacitors). They can complement or replace batteries in elec- trical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the devel- opment of advanced nanostructured materials [1]. Manga- nese oxide is a promising electrode material for pseudo- capacitors in replacing expensive ruthenium oxide due to low cost and little toxicity [2,3]. Toupin et al. predicted theo- retically a high specific capacitance (SC) of 1370 F/g for MnO 2 -based supercapacitor electrodes [4]. However, practi- cally, these oxides generally show poorer SC, ca. one-fifth or one-sixth of the above value. Major limitations of MnO 2 -based materials are their poor electronic conductivity and densely packed structures with limited accessible surface area, which significantly affect their electrochemical activity. One key strategy in improving the SC is the morphological and crystal- lographic design of manganese oxides at nanometer-scale including the fabrication of nanoparticle, nanowire, nano- sheet, hollow nanosphere and nanoporous materials. A num- ber of studies have reported that a capacitance value of nanoscale manganese oxides can be improved by enhancing the surface area [5–11]. More recently, a layered compound of K 0.45 MnO 2 was successfully exfoliated to colloidal nano- sheets of MnO 2 [12–15]. The obtained MnO 2 film showed good 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.02.004 * Corresponding authors: Fax: +61 7 33656074. E-mail addresses: zhenghj@zjut.edu.cn (H. Zheng), l.wang@uq.edu.au (L. Wang), maxlu@uq.edu.au (G. (Max) Lu). CARBON 47 (2009) 1534 – 1542 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon