Description
Electrochemical Synthesis of Graphene Nanomaterials from Aqueous Solution
The decorative oxygen functional groups of GO such as hydroxyl, carboxyl, or epoxides make it hydrophilic and easily dispersible in water [7]. Therefore, many aqueous electrolytes including NaCl [25], Na3PO4 [12], Na2SO4 [26], NaNO3 [16], PBS [10], KNO3 [27], and KCl [28] aqueous solutions have been investi-gated as mediums for electrochemical reduction of GO. The removal of these functional groups by electrochemical reduction was generally reported as partial and the recovery of sp2 structure by healing of the sp3 defects in GO is still a chal-lenge [10] requiring thus further treatments of the electroreduced GO materials.
The strategies to fabricate graphene nanomaterials by electrochemical method followed different routes as depending on the necessity of a substrate or the steps involved.
Surface-Mediated Electrodeposition of Graphene Nanomaterials
Graphene sheets have the ability to strongly adhere to various substrates due to their high van der Waals attraction, large surface area, and flatness [27]. As dictated by the application envisioned for such materials, the electrodeposition of graphene on various substrates was performed by either direct mode or by two-step electrosynthesis.
Direct Electrodeposition Hilder et al. [30] studied the effect of conductivity of deposition medium direct electrochemical reduction of GO from aqueous suspension by applying reduction voltages exceeding −1.0 to −1.2 V. According to the tests carried out, he observed that the optimum conductivity values for the deposition to produce ranged 4–25 mScm−1 irrespective of system parameters such as concentration and history of the GO sample or electrolyte.
On the basis of pulse voltammetry and cyclic voltammetry measurements, Guo et al. [10] described how hydroxyl and epoxide groups on GO sheets can be reduced more quickly at an applied potential of −1.5 to 0 V compared with −1.3 to 0 V. Similarly, a constant potential reduction at −1.2 V was carried out by Peng et al.[16] to reduce GO film and fabricate a capacitor with the electrochemically rGO which achieved a specific capacitance of 128 Fg−1.
Strongly alkaline solutions were reported as effective for the deoxygenation of GO [29]. The electrodeposition of rGO from GO suspension by cyclic voltammetry technique in 6 molL−1 KOH aqueous solution in the range of 0.9–0 V (vs. Hg/HgO) showed an O/C ratio as low as 1.29 %. This material showed good electrochemical performance upon cycling. After 3,000 cycles, the electrode preserved 99 % of its initial capacitance of 152 Fg−1 at a current density of 5 Ag−1.
Chen et al. [31] directly deposited graphene nanosheets onto a glassy carbon electrode (GCE) through cyclic voltammetric (CV) reduction of a GO colloidal solution. Prior to CV deposition, the GO material was exfoliated in a phosphate buf-fer solution by ultrasonication and further subjected to N2 bubbling. The irreversible electrochemical reduction of GO was identified by the presence of a cathodic peak at −1 V in the CV curves (Fig. 1), while the other cathodic and anodic peaks were attributed to active oxygen-containing groups on graphene planes that were too stable to be reduced by the CV method. The charge-transfer property of the rGO electrode evaluated with the help of carbon-material surface chemistry-sensitive redox probe Fe(CN)63−/4− showed increased electrochemical active sites after depo-sition of rGO on GCE. The rGO-modified GCE showed high-resolution capacity to hydroquinone and catechol, therefore indicating graphene as a promising sensing platform for isomer determination.