Electrochemical interfacial capacitance in multilayer graphene sheets: Dependence on number of stacking layers
Introduction
Graphene-monolayer of carbon atoms arranged in a honeycomb lattice – is a prevalent building block in many carbon materials [1], [2]. As a novel model for theoretical study, unusual electronic properties have been discovered in graphene. Previous studies have shown the dependence of the electronic structure of graphene on its number of layers [3], [4], [5]. Most of these results fall in the topic of nanoelectronics. Electrochemistry, however, fails to play its role until recently [6], [7]. Electrochemical interfacial capacitance, defined as the capacitance per surface area (F m−2), is a function of electric double layer capacitance, which is determined by the electronic structure of specified electrode. Insights into the relationships between number of layers and interfacial capacitance of graphene sheets can provide electrochemical approaches to study the electronic structure of graphene and optimize the capacitance of graphene materials as supercapacitor electrode.
Up to date, many efforts have been devoted to the interfacial capacitance of carbon electrodes. The higher interfacial capacitance on edge plane than basal plane of bulk graphite was observed years ago [8]. New insights are recently thrown into the effects of surface chemistry (boron [9], oxygen [10], nitrogen [11], [12] and phosphorus [13]) and micropore size on interfacial capacitance of diverse carbon electrodes [14], [15], [16]. Besides, the interfacial capacitance of porous carbon electrode is limited as the pore wall thickness is reduced [17]. We herein address the dependence of interfacial capacitance on number of layers of multilayer graphene sheets.
Section snippets
Experimental
Reduced graphene oxide sheets were chemically exfoliated from natural flake graphite powder (Sinopharm Chemical Reagent Co. Ltd.) [18]. Post-treatments in argon (g), hydrogen (g), ammonia (g) and hydrazine (l) were performed to modify the surface chemical environment of reduced graphene oxide sheets and hence tailor the stacking thickness. Gaseous treatments were conducted at 400 °C for 4 h, while liquid treatment was refluxed at 100 °C for 24 h. The samples are denoted as r-GO (reduced graphene
Surface chemistry and physical texture characterizations
XPS reveals a decreased oxygen atomic concentration in the sequence of r-GO > GAr ∼ GHy > GH > GN (Table 1). Hydrazine is considered to unlikely reduce carboxylic groups but can react with lactones and anhydrides to form hydrazides and with quinones to form hydrazones [19], [20]. However, it is believed that only the formation of hydrazone can remove oxygen [19]. Oxygen groups can be removed by thermal treatment [21]. Ammonia can be thermally decomposed to yield atomic hydrogen for oxygen removal [22].
Conclusions
In this contribution, we engineered the number of layers of graphene sheets by selective treatments. The number of layers of graphene was determined according to specific surface area. The interfacial capacitance of multilayer graphene sheets is found to depend on the number of layers. This result is attributed to the dependence of space charge layer capacitance of graphene on the number of layers, where the two factors of screening length and stacking thickness play dominantly. This work opens
Acknowledgement
This work was supported by the NSFC grants (Nos. 50872136, 90606008, and 50632040), MOST of China (No. 2006CB932703), and Chinese Academy of Sciences (No. KJCX2-YW-M01).
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