Gas Sensing Mechanism of Gold Nanoparticles Decorated Single- Walled Carbon Nanotubes Syed Mubeen, a Jae-Hong Lim, a Aarti Srirangarajan, b Ashok Mulchandani, a Marc A. Deshusses , c Nosang V. Myung* a a Department of Chemical and Environmental Engineering and Center for Nanoscale Science and Engineering; University of California-Riverside; Riverside, CA 92521, USA b Nanoelectronics, MIMOS Berhad, Kuala Lumpur, Malaysia c Department of Civil and Environmental Engineering, Duke University, Durham, NC 27708, USA *e-mail: myung@engr.ucr.edu Received: June 3, 2011; & Accepted: July 31, 2011 Abstract Metal nanoparticles decorated single-walled carbon nanotubes (SWNTs) can lead to considerable enhancement in sensing performance towards different gas analytes, however the sensing mechanism was not clearly elucidated. The detailed sensing mechanism of hybrid gold-SWNT nanostructures toward hydrogen sulfide was investigated using field effect transistor (FET) transfer characteristics. At low H 2 S concentrations ( 100 ppb V ), FET transfer charac- teristics show that the gold nanoparticles at the surface of SWNTs acted as nano-Schottky barriers to predominately modulate transconductance upon exposure to unfunctionalized SWNTs on gold electrodes which showed little or no response upon exposure. Although the sensitivity of Au/SWNT toward H 2 S was strongly dependent upon the size and number of gold nanoparticles, the sensing mechanism was independent of it. Keywords: Electrodeposition, Single-walled carbon nanotube, Hydrogen sulfide, Sensors, Field effect transistors, Nanoparticles DOI: 10.1002/elan.201100299 1 Introduction Since the first demonstration of single-walled carbon nanotubes (SWNTs) as highly sensitive gas sensors [1], there have been numerous studies to understand the in- teraction between carbon nanotubes and gas molecules including NH 3 , NO 2 , CO, H 2 O, CH 4 ,O 2 , etc. [2–19]. The possible sensing mechanisms include electrostatic gating [1], interaction with pre-adsorbed oxygen species [3], charge transfer from adsorbed gas species to carbon nanotubes [4, 5], alteration of the electrode work function which lead to change in the carrier mobility due to forma- tion or removal of Schottky barrier [12, 15, 20], etc. Al- though significant progress has been made in understand- ing the sensing mechanism of pristine SWNTs towards gas molecules, the operation/sensing mode of surface modified/functionalized SWNTs still remains ambiguous [14, 18, 21–27] . While many covalent and noncovalent methods have been employed to functionalize SWNTs with various materials including polymers, metal oxides, and metals [25], metal nanoparticles decorated SWNTs have been extensively studied and have shown to cause great enhancement in sensing performance. Kong et al. first demonstrated that surface modification of SWNTs with palladium nanoparticles resulted in superior sensitiv- ity toward hydrogen [26]. They attributed the response to changes in the work function of palladium nanoparticles upon exposure to hydrogen. By comparing aluminum nanoparticles decorated on SWNT to decoration on con- tacts, Kim et al. found that sensitivity of contact decorat- ed devices towards NH 3 and NO 2 markedly increased compared to SWNT decorated devices, suggesting that the higher sensitivity is mainly attributed to Schottky bar- rier modulation between contacts/electrodes and SWNTs [27]. Kauffman et al. [24] electrochemically decorated SWNTs with different metal nanoparticles and exposed them to NO x . They concluded that the sensor response is a combination of both charge transfer at the SWNT-metal nanoparticles and Schottky barrier modulation between SWNTs and electrodes and it is a function of the individ- ual work function of metal deposited. Recently, we dem- onstrated that electrochemically decorated palladium and gold nanoparticles on SWNTs resulted in a 0.4 % per ppm V and 0.07 % per ppb V detection toward H 2 and H 2 S, respectively, while carboxylated SWNTs showed no or very little response towards these analytes [18, 28], again indicating that the sensing enhancement is due to the presence of metal nanoparticles. However, a number of questions remain as far as the sensing mechanism of these hybrid nanostructures is concerned. In this report, the sensing mechanism for gold nanoparticles decorated SWNTs towards very low concentrations of H2S (2–100 ppb V ) was systematically investigated using field effect transistor (FET) measurements at room temperature. Gold was selected as both functional metal nanoparticles and contact/electrode material for SWNTs to differentiate Electroanalysis 2011, 23, No. 11, 2687 – 2692  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2687 Full Paper