課題番号 ：S-20-SH-0014
利用形態 ：共同研究型支援
利用課題名（日本語） ：
Program Title (English) ：Enhancement Sensitivity and Selectivity of Ammonium Hydroxide Using Nitrogen-Doped Double-Walled Carbon Nanotubes
利用者名（日本語） ：
Username (English) ：W.Muangrat1,2), M.Obata2), Y.Hashimoto2)
所属名（日本語） ：
Affiliation (English) ：1) Materials Engineering Program, Burapha University, 2) Shinshu University.

１．概要（Summary ）
We successfully fabricated the high sensitivity and selectivity of NH4OH sensor using nitrogen-doped double-walled carbon nanotubes (N-DWCNTs) as sensing material. The fabricated sensor from N-DWCNTs exhibited a 4-fold improvement in the sensor response to ammonium hydroxide (NH4OH). The improvement of sensitivity and selectivity can be explained by nitrogen site in N-DWCNTs. These results suggest that nitrogen doping in double-walled carbon nanotubes (DWCNTs) is a promising approach for the improvement of the sensitivity and selectivity for NH4OH detection.
２．実験（Experimental）
DWCNTs were synthesized by chemical vapor deposition (CVD) using mixed homogenous ethanol-ferrocene-thiophene solution. The CVD system was raised up to 1300 °C under Ar gas with a flow rate of 500 sccm. The mixed solution was injected into reaction zone by micropumping at a pumping speed of 0.04 mLmin-1, which was introduced by H2 gas at a flowrate of 1000 sccm as carrier gas. The synthesis time was carried out for 20 min. As-synthesized sample was further purified by acid and air-annealing treatments repeatedly for 2 times. For comparison, the N-DWCNTs was synthesized at the same condition by using ethanol-urea (95:5) mixture solution as carbon and nitrogen sources. The nanostructure, crystallinity and chemical composition and state were characterized by transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). To fabricate gas sensor, DWCNTs and N-DWCNTs were separately sonicated with ethanol solution at the concentration of 0.01 mgmL-1 for 120 min. A 1000 µL of DWCNTs and N-DWCNTs in ethanol were separately dropped on to the printed circuit board consisting of an interdigitated Cu/Au electrode. During drop casting the substrate was heated at 90 °C to remove the solvent in the sample material. All sensors were placed into the chamber and then N2 gas was introduced into the detection chamber at a flow rate of 500 sccm for 10 min. Acetone, ethanol and NH4OH vapors were separately injected into the chamber by control the volume of solution and monitoring its electrical resistance for 10 min. The concentration of all vapors was 500 ppm. To recover, the fabricated sensors were recovered by N2 gas at a flow rate of 500 sccm for 10 min. The sensor response (SR) was defined by the equation: SR = [(RVapor-RN2)/ RN2]×100, where RVapor and RN2 are the electrical resistance of sensor before and after vapor exposure, respectively.
３．結果と考察（Results and Discussion）
Figure 1(a-b) shows TEM images of DWCNT and N-DWCNT bundles. TEM images reveal the DWCNTs in a bundle structure. The average diameter of DWCNTs and N-DWCNTs were 1.78±0.31 and 1.68±0.27 nm, respectively. The results show that average diameter of N-DWCNTs is narrower than that of DWCNTs, which implies that the nitrogen doping plays role in decrease of diameter of N-DWCNTs.
Raman spectroscopy was employed to characterize the structure and crystallinity of DWCNTs and N-DWCNTs. The four significant Raman peaks: radial breathing mode (RBM) at ~150-270 cm-1, disordered carbon-derived at ~1340 cm-1, graphitic structure-derived G-band at ~1590 cm-1 and second-order of D-band-derived at ~2670 cm-1. The intensity ratio between the D- and G-bands (ID/IG) ratio of DWCNTs and N-DWCNTs were 0.021 and 0.030, respectively. The ID/IG value of N-DWCNTs were higher than that of DWCNTs, indicating the lower crystallinity and purity of N-DWCNTs due to nitrogen doping induced structural damage.
XPS was used to analyze the chemical composition and chemical state of DWCNTs and N-DWCNTs. The spectra of DWCNTs and N-DWCNTs consist of strong C1s and O1s peaks at ~284 and ~532 eV, respectively. For the C1s peak, there are four different carbon group in the XPS spectrum: sp2 C=C at 284.8 eV, C-O bands at 286.2 eV and shake-up satellite line at 290.5 eV. For the O1s peak, there are three different oxygen groups in the XPS spectrum: C=O at 532.6 eV and C-O at 533.9 eV. In the case of N-DWCNTs, there are four types of nitrogen; pyridinic at 399.7 eV and graphitic at 401.4 eV. The total atomic percentage of N-DWCNTs is approximately 0.90 at.%.
Figure 2(a-c) show sensor responses as a function of time of DWCNTs and N-DWCNTs exposure to ethanol, acetone and NH4OH vapors at 500 ppm. The electrical resistance of DWCNTs increased upon ethanol, acetone and NH4OH exposure and decreased after replacing vapor with N2 gas. The sensor response of fabricated sensor from N-DWCNTs to NH4OH vapor greatly improved by 4.1-fold compare to that of undoped DWCNTs, whereas the fabricated sensor from N-DWCNTs is sensitive to acetone and ethanol vapors similar with undoped DWCNTs. These results imply that the improvement of sensitivity and selectivity of N-DWCNTs to NH4OH could be attributed by nitrogen-doping in N-DWCNTs. The advantage of the gas sensor based on N-DWCNTs proposed in this work is a promising approach for effective NH4OH detection.
４．その他・特記事項（Others）
なし。
５．論文・学会発表（Publication/Presentation）
(1) W.Muangrat, W. Wongwiriyapan, S. Morimoto and Y. Hashimoto, Graphene nanosheets-grafted double-walled carbon nanotube hybrid nanostructures by two-step chemical vapor deposition and their application for ethanol detection, 9 (2019) p.p.1-9.
６．関連特許（Patent）
なし。