以電化學阻抗譜法分析石墨烯與水介面的電路模型

Abstract

石墨烯是一種由碳原子以六角型排列而成的二維材料，具有良好的電性、導熱性與機械特性。基於其極高的表面積和載子遷移率，石墨烯常被用作感測器的感測材料。這些感測器中，包含了以液態閘極石墨烯電晶體(liquid-gated graphene field effect transistor)為基礎的溶液感測器。由於其靈敏度與石墨烯-水的介面特性有關，本篇論文希望以電化學阻抗譜法(electrochemical impedance spectroscopy, EIS)量測石墨烯-去離子水的阻抗曲線，建立該介面的等效電路模型，並討論電路模型中每個元件所代表的意義。 本實驗所使用的石墨烯是以化學氣相沉積法(chemical vapor deposition, CVD)在銅箔上合成，再轉移到二氧化矽晶圓破片上。由於元件尺寸較小，電化電槽的待測溶液會以打洞的二甲基矽氧烷聚合體(polydimethylsiloxane, PDMS)片盛裝於石墨烯電極表面，並透過eDAQ leakless Ag/AgCl 參考電極定義溶液電壓。本次實驗以BioLogic SP-150恆電位儀進行循環伏安法(cyclic voltammetry, CV)及EIS的量測，並以BioLogic EC-Lab®軟體進行EIS曲線的擬合。 石墨烯的ESI曲線在奈奎斯特圖(Nyquist plot)上呈現出兩個半圓，我們透過改變水溶液及電及表面的阻抗的實驗，顯示高頻半圓對應到的是溶液阻抗、而低頻半圓對應到電極表面阻抗。在進行EIS阻抗曲線的擬合後，我們發現由於本實驗的水溶液為去離子水，沒有添加輔助電解質(supporting electrolyte)，溶液阻抗中出現明顯的質量傳輸(mass-transport)限制效應。在擬合結果中，我們發現石墨烯的電極介面電容值只有金電極的1/65倍，透過計算可發現石墨烯極低的介面電容來自於石墨烯本身的量子電容(quantum capacitance)效應。 在未來的研究裡，我們希望能透過電化學方法觀測石墨烯表面第一水層的表現。然而在前面的實驗中我們得知量測石墨烯介面電容時電容值會被量子電容主導，使得水中的電雙層電容效應較難被觀測，因此我們未來的量測中應以多層石墨烯作為樣本或施加偏壓以降低量子電容的主導性，並利用量測到的電雙層電容特性來佐證石墨烯表面水分子結構的相關模擬研究。

Graphene is a 2D carbon hexagonal lattice that possesses extraordinary electrical, thermal and mechanical properties. Due to its remarkable carrier mobility and large surface area, it has been used as sensing layers of solution sensors based on liquid-gated graphene field effect transistor (GFET). The sensitivity of a liquid-gated GFET depends on the properties of the graphene-water interface. This work focuses on building an electrical model of the interface between single-layer graphene (SLG) and deionized (DI) water for electrochemical impedance spectroscopy (EIS) analysis and discusses the meaning of each element. Graphene used in our experiments was grown by chemical vapor deposition (CVD) and transferred to Si/SiO2 substrates which were pre-deposited with Ti/Au pads. The solution of the electrochemical cell was confined on the electrodes by polydimethylsiloxane(PDMS) pieces. Solution potential was applied through an eDAQ leakless Ag/AgCl reference electrode suspended above the substrate. Cyclic voltammetry (CV) and EIS measurement were conducted with BioLogic SP-150 potentiostat. The EIS results were fit with proposed electric circuit models by BioLogic EC-Lab® Software. The EIS curve of graphene showed two semicircles in the Nyquist plot, which suggested that the circuit model of impedance should base on two RC parallel circuits connected in series. By modifying surface and solution impedance, we identified that the high-frequency semicircle corresponds to solution impedance, while the low-frequency one relates to interface impedance. Through impedance fitting, we found that the impedance of DI water possessed a large mass-transport-limitation characteristic due to the absence of supporting electrolytes. The impedance fitting result showed that surface capacitance Cs of SLG working electrode was about 65 times smaller than gold working electrode and found to meet the theoretic value of SLG quantum capacitance CQ. In the future, we want to look into the first-water-layer behavior at the graphene surface through electrochemical methods. Since the surface impedance has been dominated by CQ in SLG electrodes, we have to use multilayer graphene or apply a back-gate voltage to enlarge CQ value and bring out the effect of electrical double layer capacitance Cdl. We hope to use the experimental value of Cdl to back up other simulation works.