Low substrate temperature synthesis of carbon nanowalls by ultrasonic spray pyrolysis Jianhui Zhang a , Ishwor Khatri a, ⁎, Naoki Kishi a , Sharif M. Mominuzzaman b , Tetsuo Soga a , Takashi Jimbo a a Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555, Japan b Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh abstract article info Article history: Received 9 October 2009 Received in revised form 1 February 2011 Accepted 1 February 2011 Available online 12 February 2011 Keywords: Ultrasonic spray pyrolysis Carbon nanowalls Ethanol Scanning electron microscopy Raman spectroscopy Low temperature In this paper, we report the synthesis of two-dimensional wall like carbon nanostructures (i.e. carbon nanowalls) by ultrasonic spray pyrolysis of ethanol and fullerene mixture. At higher temperature carbon nanofibers were formed on the substrate placed at the center of the reactor tube, whereas carbon nanowalls were observed on the substrate placed downstream of the tube below 100 °C. Spaces between the nanowalls changed with distance of the substrates from the furnace. Qualitative analysis of materials was performed using scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. © 2011 Elsevier B.V. All rights reserved. 1. Introduction One-dimensional self-assembled carbon nanostructures like car- bon nanotubes (CNTs) [1] and carbon nanofibers (CNFs) [2] have potential applications in the electrochemical devices [3], field emitters [4], sensors [5] etc. Similarly, two-dimensional carbon structures such as carbon nanoflakes [6], carbon nanoflowers [7], carbon nanosheets [8], carbon nanohorns [9], and carbon nanowalls (CNWs) are also expected to be applied to optoelectronic devices such as electron field emitter and capacitor. CNWs are chemically stable, have mechanical strength with large surface area and high aspect ratio. Thus, many researchers are attracted towards CNWs for its possible application in electronic devices. CNWs have a wall structure consisting of graphene, which stands on a substrate as vertically aligned carbon sheets with an average thickness of several nanometers [10]. Different methods have been developed to synthesize CNWs. Chemical vapor deposition (CVD) is one of the promising ways to synthesize nanostructured carbon materials. CNWs have been synthesized by plasma enhanced CVD [11,12], hot-wire CVD [13], dc-plasma enhanced CVD (dc-PECVD) [14], rf-PECVD [15] and so on. Wang et al. [16] reported the synthesis of freestanding graphite sheets with thickness less than 1 nm using inductively coupled rf PECVD employing methane and hydrogen mixture. Among varieties of fabrication methods for carbon nanostructures, spray pyrolysis is a promising technique due to its feasibility and low cost. Recently, we reported the synthesis of CNTs and CNFs by an ultrasonic spray pyrolysis, which is similar to the spray pyrolysis method, but it is equipped with an ultrasonic nebulizer to generate mist of carbon source [17–21]. It is a simple and convenient method that does not require expensive gasses, vacuum pump, reducing agent and pre-prepared catalyst for the synthesis of nanostructured carbon materials. In this paper, we report the synthesis of CNWs by ultrasonic spray pyrolysis at low substrate temperature. CNWs were obtained on silicon substrate placed downstream of the electric furnace. Graphene layers are clearly observed at the edge of the CNW, indicating the graphitized structure of the CNWs. 2. Experimental details Detail description about the system is given by Khatri et al. [17]. In our previous studies we used a large quartz tube (100 cm long and 75 mm diameter) with substrates placed at the center. Here, a small quartz tube (25 mm diameter and 500 mm long) was used with substrates placed at different positions, as shown in Fig. 1(a). Fig. 1(b) shows the temperature of the quartz tube at different positions. Parameters such as reactor temperature gas flow, relative concentration of carbon sources were investigated and adjusted. We obtained good results at a 100 sccm flow of nitrogen. Silicon substrates of size 20 mm×20 mm were cleaned in acetone and methanol by ultra sonication, washed with deionized water and finally dried using Thin Solid Films 519 (2011) 4162–4165 ⁎ Corresponding author. Tel.: + 81 52 735 5532; fax: + 81 52 735 7120. E-mail address: ishwor_nep2000@yahoo.com (I. Khatri). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.006 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf