Real-time detection of chlorine gas using Ni/Si shell/core nanowires
- Dong-Jin Lee†1,
- Kwang Heo†2,
- Hyungwoo Lee3,
- Joon-Hyung Jin1,
- Hochan Chang1,
- Minjun Park1,
- Han-Bo-Ram Lee4,
- Hyungjun Kim5 and
- Byung Yang Lee1Email author
© Lee et al.; licensee Springer. 2015
Received: 27 October 2014
Accepted: 3 January 2015
Published: 28 January 2015
We demonstrate the selective adsorption of Ni/Si shell/core nanowires (Ni-Si NWs) with a Ni outer shell and a Si inner core on molecularly patterned substrates and their application to sensors for the detection of chlorine gas, a toxic halogen gas. The molecularly patterned substrates consisted of polar SiO2 regions and nonpolar regions of self-assembled monolayers of octadecyltrichlorosilane (OTS). The NWs showed selective adsorption on the polar SiO2 regions, avoiding assembly on the nonpolar OTS regions. Utilizing these assembled Ni-Si NWs, we demonstrate a sensor for the detection of chlorine gas. The utilization of Ni-Si NWs resulted in a much larger sensor response of approximately 23% to 5 ppm of chlorine gas compared to bare Ni NWs, due to the increased surface-to-volume ratio of the Ni-Si shell/core structure. We expect that our sensor will be utilized in the future for the real-time detection of halogen gases including chlorine with high sensitivity and fast response.
Nanoscale hybrid structures of metals and semiconductors have attracted a lot of attention due to their exotic properties [1,2]. For example, metal-coated semiconducting nanowires (NWs) have demonstrated remarkable optical, mechanical, and electronic properties, enabling their application to areas such as field effect transistors and optical waveguides [3-5]. Biochemical sensors using metal–semiconductor hybrid structures have been also a source of growing interest due to their high sensitivity and integration capability [6-8]. Among metal-based materials, nickel and its oxides have been utilized in the detection of ammonia [9,10], glucose , and cigarette smoke . In this paper, we demonstrate the utilization of metal-coated Si NWs with Ni/Si shell/core structures (Ni-Si NWs) as sensor transducers for the detection of chlorine (Cl2) gas, which is a toxic halogen gas. It is worth noting that not much study has been performed on Cl2 gas sensors using nickel oxide materials except for nickel ferrite structure , mainly due to its relatively low sensitivity. In this work, we introduced a NW with a Ni-Si core/shell structure and demonstrated that a nickel oxide-based structure with high surface-to-volume ratio can be used for the room temperature real-time detection of chlorine gas. The sensor transducer was prepared by selective adsorption of Ni-Si NWs on molecularly patterned substrates without any functionalization of the NWs. The molecularly patterned substrates consisted of polar SiO2 regions and nonpolar octadecyltrichlorosilane (OTS). The NWs were selectively adsorbed on the polar surface regions, avoiding assembly on the nonpolar OTS regions. To utilize these assembled Ni-Si NWs for practical applications, we demonstrated a sensor for the detection of Cl2 gas, which is a toxic material generated during many chemical reactions and manufacturing processes [13-15]. Because it can be dangerous when inhaled, a rapid and reliable detection method of Cl2 gas is highly demanded . The utilization of Ni-Si NWs resulted in a much larger sensor response to Cl2 gas compared to bare Ni NWs, due to the increased surface-to-volume ratio of the sensor transducer. We expect that our sensor can be used in the future for the real-time detection of halogen gas including chlorine with high sensitivity and fast response.
Preparation of Ni-Si NWs
Single-crystalline Si NWs were first grown via chemical vapor deposition (CVD) process with Au catalysts [17,18]. The surface of CVD-grown Si NWs was usually coated with native oxide, and their surface properties varied depending on the growth condition and degree of surface contamination . The Ni thin film was then deposited by the atomic layer deposition (ALD) process using bis(dimethylamino-2-methyl-2-butoxo)nickel (Ni(dmamb)2) as the precursor and NH3 gas as the reactant. This resulted in Ni/Si shell/core structures with uniform Ni thickness of 20 nm. The Ni precursor in a steel bubbler kept at 70°C was carried with Ar gas at a rate of 50 sccm into the main chamber. NH3 reactant gas was injected to the chamber at a rate of 400 sccm. The substrate was maintained at 300°C. One ALD cycle consisted of 4 s of Ni(dmamb)2 precursor exposure, 1 s of Ar purging, 6 s of NH3 gas reactant exposure, and 1 s of Ar purging. The saturated growth rate was 0.64 Å/cycle.
Preparation of Ni NWs
The bare Ni NWs were grown by the electrodeposition method using anodic aluminum oxide (AAO) substrates as templates [19,20]. To explain briefly, a 400-nm silver (Ag) layer was thermally deposited on one side of the AAO filters with a pore size of approximately 80 nm (Synkera Technologies, Longmont, CO, USA). Afterwards, an additional Ag (Techni Silver 1025, Technic. Inc., Anaheim, CA, USA) film was electrochemically deposited at −0.8 V vs Ag/AgCl for 4 C using a potentiostat (Reference 600, Gamry Instruments Inc., Warminster, PA, USA). Ni (Nickel Sulfamate RTU, Technic. Inc.) was then deposited at −0.85 V vs Ag/AgCl for 6 C. This resulted in Ni NWs with an average length of 20 μm. After Ni growth, the Ag was removed with 4:1:1 (v/v) solution of methanol, hydrogen peroxide, and ammonium hydroxide. The AAO was removed in 3 M sodium hydroxide solution. Finally, the NWs were rinsed repeatedly with deionized water.
NW self-assembly and device fabrication
A Ni-Si NW solution (approximately 107 ml−1) was first prepared by placing the ALD-processed substrate in deionized (DI) water and applying sonication for 2 min. The assembly process of Ni-Si NWs is similar to the previous assembly methods . First, nonpolar and polar regions were created on solid substrates by patterning self-assembled monolayers (SAMs) of OTS on a SiO2 substrate. For nonpolar regions, methyl-terminated OTS SAM was patterned via photolithography , while leaving some bare SiO2 regions. The methyl-terminated OTS SAM worked as a neutral region. The SiO2 surface worked as the polar region in DI water due to the hydroxyl groups (−OH) on the SiO2 surface. Ni had an isoelectric point pI of 3.5 to 4 in water [21,22], taking a weak negative surface charge in normal DI water (pH 5.5). When the molecularly patterned substrates were exposed to the Ni-Si NW suspension, the NWs were selectively adsorbed onto the polar SiO2 regions due to van der Waals force, avoiding the nonpolar regions. The substrate was then rinsed with DI water to remove weakly adhered NWs. For device fabrication, we deposited Ti/Au (10 nm/50 nm) via thermal evaporation followed by a lift-off process to fabricate the metal electrodes.
The sensing experiments were performed using a homemade gas flowing system consisting of a source gas, flow meters, a N2 carrier gas, a chamber, and electrical leads. The Ni-Si NW sensor transducer was placed in the closed chamber, and the NW devices were connected to a computer-controlled two-channel sourcemeter (Keithley 2636A, Keithley Instruments Inc., Cleveland, OH, USA) in a two-probe configuration. Then, Cl2 gas of known concentrations (5 and 20 ppm) was sequentially injected into the chamber while simultaneously monitoring the current change of the Ni-Si NW and Ni NW devices. The applied voltage bias was 1 V for the Ni-Si NW and 0.01 V for the Ni NW-based sensors.
Results and discussion
We have demonstrated the selective adsorption of Ni-Si NWs on molecularly patterned substrates and their application to sensors for the detection of Cl2 gas. The Ni-Si NWs have a larger surface-to-volume ratio compared to that of Ni NWs, which makes them more advantageous in detecting Cl2 gas. The Ni-Si NW sensor showed the real-time detection to Cl2 gas with high sensitivity and fast response time. We expect that our Ni-Si NWs can be utilized in the future as an integrated platform for sensor applications.
This project was supported by the Basic Science Research Program through the National Research Foundation (NRF; grant number 2013R1A1A1010802) and in part by Korea Ministry of Environment as ‘The Environmental Health Action Program’ (grant number: ARQ201303173002) and as ‘The Converging Technology Program’ (grant numbers: 2013001650001, ARQ201403075001).
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