Fabrication and electrochemical behavior of vertically-aligned carbon nanotube electrodes covalently attached to p-type silicon via a thioester linkage
Zihan Poh, Benjamin S. Flavel, Cameron J. Shearer, Joseph G. Shapter, Amanda V. Ellis ⁎
Flinders University, School of Chemistry, Physics and Earth Sciences, GPO Box 2100, Adelaide, SA, 5001, Australia a r t i c l e i n f o a b s t r a c t
Article history:
Received 7 October 2008
Accepted 23 December 2008
Available online 6 January 2009
Keywords:
Carbon nanotubes
Thioester
Surfaces
Nanomaterials
Sensors
Silicon
Vertically-aligned single-walled carbon nanotubes (SWCNTs) (VACNTs) are becoming increasingly recognized due to their fast electron transfer rates. However, the chemistry available for further functionalizing these electrodes is limited. Here we describe a new approach to the fabrication of VACNTs. SWCNTs were covalently attached to a p-type silicon (100) (Si) wafer surface using a thioester linkage in which the nanotubes were firstly acid treated and then, in the presence of a hydroxylated Si wafer surface, reacted with phosphorus pentasulfide (a mild electrophilic catalyst). The novel nanostructure was characterized using atomic force microscopy (AFM) showing vertical alignment with FTIR spectroscopy indicating pendant thiocarboxylic acid groups for further reaction. In addition, electrochemical properties using cyclic voltammetry indicate that the electrodes have excellent electrochemical properties with an electron transfer rate of 2.98×10−3 cm s−1.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The direct patterned growth of aligned carbon nanotube (CNT) arrays via chemical vapour deposition (CVD) has been widely explored with successful deposition of well-aligned tubes now readily achievable
[1]. However, the very nature of CVD deposition has limitations in regards to large-scale applications due to the high growth temperatures required [2]. This makes positioning CNTs onto temperature sensitive substrates, such as those required for integrated circuits
(ICs), impossible. An additional limitation of CVD is the apparent lack of covalent bonding between the deposited tubes and the underlying substrate. Recently, work by Yu et al. [3] has focussed on the covalent attachment of aligned nanotubes onto silicon substrates through wetchemical deposition. This method involves the formation of an ester linkage between acid treated CNTs containing surface carboxyl moieties and hydroxylated SiO2 surfaces using dicyclohexylcarbodiimide
(DCC) [3]. Successful fabrication of vertically-aligned shortened single-walled CNT architectures onto a silicon (100) substrate was achieved [3]. This approach has numerous potential applications such as silicon-based electro-chemical and bio-electrochemical sensors, and solar cells, although the chemistry is somewhat limiting.
Ellis and colleagues [4,5] have recently publishedwork on two new techniques for the functionalization of CNTs with thioester moieties.
The most facile [4] of which involves firstly functionalizing pristine
CNTs with hydroxyl (–OH) and carboxyl (–COOH) groups by wet chemical oxidation in concentrated H2SO4/HNO3 [3,6]. By utilizing this acid treatment technique the newly formed hydroxyl and carboxyl moieties can be readily derivatized to thioester groups using a mild but versatile electrophilic catalyst, phosphorus pentasulfide (P4S10)
[4]. P4S10 provides an electrophilic single-step pathway for the preparation of thiols, thiocarboxylic esters and thioesters when carboxyl and carbonyl groups are reacted with thiols or alcohols in varying mole ratios [7,8].
To date there has been no use of this catalyst to form thioester bonds between a hydroxylated silicon substrate and acid treated carbon nanotubes. The motivation for this report has been to create a reaction pathway in which SWCNTs are vertically attached to a p-type silicon surface via a thioester linkage, which can be used for unique electrochemistry applications and subsequent derivatization of the nanotube sidewalls using unique thio-chemistry.
2. Experimental
Fig.1 shows a schematic representation of the fabrication of SWCNTs covalently attached to p-doped silicon via a thioester linkage. The mechanism in Fig. 1 shows an ideal case where the nanotubes have attached at their open ends via thioester linkages to give verticallyaligned nanotube arrays. The carboxyl groups on the SWCNTs are created through initial acid treatment (Fig.1 Step 1a) and are in a higher proportion at the open ends [9]. The electrophilic phosphorus center of
P4S10 activates the newly formed carboxylic acid groups via Fig.1barrow.
Silanol (Si–OH) groups on the silicon surface are initially converted to thiols (Si–SH) on reactionwith the P4S10 via Fig.1a arrow. InFig.1 Step 2 the thiolated silicon surface then undergoes nucleophilic attack by the
P4S10 activated carboxyl group on the SWCNTs to generate the thioester linkage. Materials Letters 63 (2009) 757–760
⁎ Corresponding author. Tel.: +61 8 8201 3104; fax: +61 8 8201 2904.
E-mail address: Amanda.ellis@flinders.edu.au (A.V. Ellis).
0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.043 Contents lists available at ScienceDirect
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