課題番号 ：S-19-SH—0014
利用形態 ：共同研究型支援
利用課題名（日本語） ：
Program Title (English) ：Low-Temperature Synthesis and Electrochemical Characterization of Highly-Branched Vertical Graphene
利用者名（日本語） ：王　志朋1), 緒方　啓典2)
Username (English) ：Zhipeng. Wang1), Hironori Ogata2)
所属名（日本語） ：1) 江西師範大学, 2) 法政大学
Affiliation (English) ：1) Jiangxi Normal University, 2) Hosei University

１．概要（Summary ）
Highly-branched vertical graphenes (HBVGs), one type of VG films, consist of individual vertical graphene sheets, in which two sides were attached with many smaller isolated sheets, as shown in Figure 1a. The HBVG films were usually fabricated by rf or microwave PECVD approaches with changing the ratios of reaction gases or extending the growth time. They can be utilized for the electrodes of supercapacitors and biosensors with excellent performances, even better than other type VGs. For example, Shang et al. reported that the original graphene nanoflakes demonstrated fast electron-transfer kinetics for the Fe(CN)63-/4- redox system and excellent electrocatalytic activity for detecting dopamine with high selectivity and sensitivity, which was comparable with the wavy VGs. The redox couple of Fe(CN)63-/4- solution showed a small anodic peak to cathodic peak potential (ΔEP) of 61.5 mV at a scan rate of 10 mV s-1, which is close to the ideal value of 59 mV. However, the ΔEP of 56 mV was obtained for the HBVGs at the same condition. It means that the VGs with many branches exhibit large degree of porosity that enables the diffusive or convective transport of electroactive reagents within the electrode, thus increasing the electron transfer rate and providing high surface area. However, the effect of the porosity on the electrochemical characteristics of HBVGs has hardly reported so far. In addition, high growth temperatures for synthesizing the HBVG films are necessary, usually higher than 600 oC by rf-PECVD techniques. Thus, it is a challenge to realize the low-temperature synthesis of the HBVG by PECVD methods.
Previously, it was reported that we could fabricate the carbon nanosheet films by microwave PECVD (MPECVD) technique at the temperature below 500 oC using CH4 gas in Ar plasma, which has a low electron temperature at low working pressures. In this work, we will employ the MPECVD technique to synthesize the HBVG films at relatively low temperatures, and will investigate their porosity and electrochemical properties including electron transfer kinetics and sensing applications.

２．実験（Experimental）
The HBVG and common VG films were synthesized directly on substrates without any catalyst by MPECVD with the sources of CH4, Ar and H2 gases at the power of 500 W, which was equipped with a rectangular waveguide to couple the microwave (2.45 GHz) through a quartz tube for generating the plasma. The Cu foil (10 μm in thickness, Nilaco) was cut into 20 mm × 20 mm pieces as substrates. The growth processes were carried out with the mixture of Ar and H2 gases at first to generate plasma for heating the substrate up to 400 oC. Then, H2 gas was closed and CH4 gas was introduced into the reaction chamber. The diverse morphological structures can be controlled by the flux ratio of CH4 to Ar during the MPECVD process. The samples were deposited for 30 min at 400-500 oC.
The morphology and microstructure of the samples were performed by scanning (SEM, S-4500, Hitachi) and transmission (TEM, JEM-2100F, JEOL) electron microscopies, respectively. The crystalline structure was analyzed by a RENISHAW inVia Raman spectroscope using the laser with a wavelength of 633 nm. The chemical composition of the CNSs was determined by a PHI 5600 X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) measurements were carried out at a CHI 600C electrochemical workstation assembled with a Pt wire as counter electrode, a Ag/AgCl(3 M KCl) electrode as reference electrode, and bare Pt or the modified Pt electrode as working electrode.

３．結果と考察（Results and Discussion）

Figure 1 (a) Diagram of HBVGs, (b) Top view SEM image of HBVGs, (c) low- and (d) high-magnification TEM images of HBVGs.

Figure 2 (a) CV patterns of 5 mM Fe(CN)63-/4- in 1 M KCl measured at HBVG-modified Pt electrode with different scan rates of 10, 20, 30, 40, 50, 100 and 200 Vs-1. (b) The respective anode and cathode peak currents vs. square root of scan rates. (c) CV responses of 5 mM Fe(CN)63-/4- in 1 M KCl measured at the HBVG-, VG-modified Pt electrode. Scan rate: 100 mV s-1.

４．その他・特記事項（Others）
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５．論文・学会発表（Publication/Presentation）
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６．関連特許（Patent）
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