Spontaneous charging of single-walled carbon nanotubes in the gas phase David Gonzalez a , Albert G. Nasibulin a , Sergey D. Shandakov a,1 , Hua Jiang b , Paula Queipo a , Esko I. Kauppinen a,b, * a NanoMaterials Group, Laboratory of Physics and Center for New Materials, Helsinki University of Technology, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland b VTT Processes, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland Received 6 April 2006; accepted 7 April 2006 The discovery of the spontaneous charging of single- walled carbon nanotubes (CNTs) during their aerosol (floating catalyst) synthesis is reported. The charging phe- nomenon was surprisingly correlated to the process of the CNT bundle formation in the gas phase, while individual tubes remained electrically neutral. CNTs were synthesized using two different methods de- scribed in detail elsewhere [1,2]. In the first method, ferro- cene was vaporized by passing room temperature CO (with a flow rate of 300 cm 3 /min) through a cartridge filled with ferrocene powder [1]. The flow containing ferrocene vapor (0.7 Pa) was introduced into the high temperature zone of the ceramic tube reactor through a water-cooling probe and mixed with an additional CO flow (100 cm 3 /min). The reactor temperature was fixed from 1000 °C to 1150 °C. In the hot wire generator (HWG) method for the synthe- sis of CNTs, Fe particles were produced from a resistively heated catalyst wire in a H 2 /Ar (with a 7/93 mol ratio) flow (400 cm 3 /min) and, subsequently, introduced into a cera- mic tubular reactor, mixed with a carbon monoxide (CO) flow of 400 cm 3 /min, and heated to induce CNT formation [2]. Additionally, 12 cm 3 /min of CO 2 was introduced into the reactor as an etching agent to maintain the conditions for the growth of CNTs [3]. Experiments were carried out at reactor temperatures from 700 °C to 900 °C. The aerosol product was collected downstream of the reactor on a holey carbon coated copper grid, using an electrostatic pre- cipitator, for transmission electron microscopy (TEM) observations. In order to determine the charging of CNTs in the gas phase, we used a differential mobility analyzer (DMA) [1]. A DMA is a standard tool in the field of aerosol science for the determination of the electrical mobility distribution of charged aerosol particles [4]. Commonly, for the DMA measurements, a radioactive bipolar charger is used for the artificial charging of the aerosol. However, we have ob- served that the CNTs synthesized in both ferrocene and HWG systems were naturally electrically charged. In order to remove the charged CNT fraction from the gas phase, we used a 10 cm long electrostatic filter (ESF) with an elec- tric field of 4000 V/cm. Aerosol size distribution measure- ments of charged and non-charged CNTs showed that almost all the CNTs (up to 99%) coming from the reactor were charged (Table 1). Furthermore, CNTs were found to be charged both positively and negatively. Importantly, TEM observations of the CNTs produced in the ferrocene (Fig. 1a and b) and HWG (Fig. 1c and d) systems showed that the nanotubes were single-walled and clearly aggre- gated in bundles. From the experimental results, it can be speculated that the charging phenomenon is directly correlated to the pro- cess of formation of bundles. In order to examine this hypothesis, we collected neutral CNTs produced in the HWG reactor by filtering out charged nanotubes at the reactor outlet by using the ESF. Experimental conditions were selected to maintain a small concentration of CNTs and, thereby, to minimize their bundling. This was accom- plished by reducing the heating power applied to the wire, which, in turn, results in lowering the concentration of the introduced catalyst clusters and, subsequently, CNTs grown in the gas phase. Surprisingly, TEM observation 0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.04.021 * Corresponding author. Address: NanoMaterials Group, Laboratory of Physics and Center for New Materials, Helsinki University of Technology, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland. Fax: +358 20 722 7021. E-mail address: esko.kauppinen@vtt.fi (E.I. Kauppinen). 1 On leave from Kemerovo State University, Kemerovo, Russia. www.elsevier.com/locate/carbon Carbon 44 (2006) 2102–2104